Synthesis, molecular structure, vibrational and theoretical studies of a ...

Journal of Molecular Structure 1171 (2018) 771e785

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Synthesis, molecular structure, vibrational and theoretical studies of a new non-centrosymmetric organic sulphate with promising NLO properties n b, Noureddine Issaoui c, *, Thierry Roisnel d, Cherifa Ben M'leh a, Silvia Antonia Branda a Houda Marouani a

Laboratory of Material Chemistry, Faculty of Sciences of Bizerte, University of Carthage, Bizerte, Tunisia tedra de Química General, Instituto de Química Inorga nica, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucuma n, Ayacucho Ca n, Tucuma n, Argentina 471, 4000, San Miguel de Tucuma c Quantum Physics Laboratory, Faculty of Sciences, University of Monastir, Monastir 5079, Tunisia d Centre de Diffractom etrie X, UMR 6226 CNRS, Unit e Sciences Chimiques de Rennes, Universit e de Rennes I, 263 Avenue du G en eral Leclerc, 35042 Rennes, France b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 May 2018 Received in revised form 9 June 2018 Accepted 11 June 2018 Available online 20 June 2018

The new non-centrosymmetric inorganic-organic hybrid material tetrakis(2,6-dimethylpiperazine-1,4diium) tetrakis(sulfate) dihydrate (DMPS) has been prepared and crystallized in the noncentrosymmetric space group Cc of the monoclinic system with the crystallographic parameters: a ¼ 25.044 (4) Å, b ¼ 12.1046 (18) Å, c ¼ 14.075 (2) Å, b ¼ 107.289 (5) , V ¼ 4074.0 (10) Å3 and Z ¼ 4. The 2 tetrahedron (S(1)O2 4 ) and (S(4)O4 ) anions are connected to water molecules forming as infinite chains 4 type C 4(12) along the c-axis in y ¼ ¼ and ¾. The 2,6-dimethylpiperazine-1,4-diium cations are correlated to the inorganic part by means of NeH/O and CeH/O hydrogen bonds. The diprotonated piperazine ring endorses a chair conformation, with the methyl groups placing in an equatorial site. Intermolecular contacts in the crystal structure were computed by Hirshfeld surface examination. Infrared spectrum proves the presence of the functional groups in the synthesised salt and different coordination modes are observed in the sulfate groups. Optical propriety affirms a noteworthy band gap energy signifying steadiness of the title compound. Furthermore, the polarizability and first order hyperpolarizability of the title molecule were calculated and interpreted. Subsequently, theoretical calculations indicate that first order hyperpolarizability is 13 times greater than urea. Moreover, the thermodynamic properties (heat capacity, entropy, and enthalpy) of the title compound at different temperatures were calculated in gas phase. Additionally, two neutral, one anionic and the hydrated species experimentally observed by Xray diffraction were identified in the vibrational infrared spectrum of DMPS combining the experimental spectrum with the corresponding predicted by using DFT calculations and the scaled quantum mechanical force field (SQMFF) procedure. © 2018 Elsevier B.V. All rights reserved.

Keywords: Hybrids X-ray diffraction IR Hirshfeld surface NLO DFT calculations

1. Introduction Piperazine and its derivatives have been extensively considered in many fields such as complexant agent, analytic reactif, pharmaceutic products and biologic substance for some metabolism [1]. These materials have attracted great attention for scientific study and many research because of their optical, electronic and

* Corresponding author. E-mail address: [email protected] (N. Issaoui). https://doi.org/10.1016/j.molstruc.2018.06.041 0022-2860/© 2018 Elsevier B.V. All rights reserved.

magnetism. Piperazine and substituted piperazines play an important role to prepared drug in manufacturing for example antibacterial and antiallergenic. Piperazine and its derivatives can be used in industrial field like insecticides, antioxidants and corrosion inhibitors. The combination of amine with oxoanions akin to sulfate can create noncentrosymmetric cell materials with important nonlinear optical coefficients, specific physical and chemical properties. Non-linear optics (NLO) has been a fast upward scientific field nowadays. It is based on the occurrence associated to the interaction of strong coherent beam radiation with mater.

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C. Ben M'leh et al. / Journal of Molecular Structure 1171 (2018) 771e785

Developments in the field of non-linear optics hold promise for important applications in optical communications, signal processing and data storage technology etc [2]. Hence, the presence of sulfate ions in their structures with its different coordination modes has influence on the stereochemistry and their properties. Besides, the knowledge of the modes de coordination that can present these sulfate groups, for instance, bidentate and/or monodentate, are of great interest and useful to identify these species in different medium by using the vibrational spectroscopy, as reported in the literature [3e6]. In this context and in the framework of our study of the crystal packing containing the 2,6-dimethylpiperazine-1,4-diox cation [7], single crystals of a novel sulfate, (C6H16N2)4(SO4)4$2H2O were synthesized and characterized by using the techniques such as Xray diffraction analysis, Fourier transform infrared spectroscopy (FT-IR) and UV spectral analysis. The steady molecular structure of DMPS in the ground state was optimized and the structural parameters were computed by Gaussian 09 program package at B3LYP level with standard 631G(d) basis set. HOMO-LUMO, electrostatic potential surface (ESP) and some global descriptors were theoretically analyzed by using that level of theory. First-order hyperpolarizability calculations and thermodynamic calculations were as well prepared using the optimized structure. In addition, the vibrational assignments for four different species experimentally observed in DMPS, which are two neutral, one anionic and one hydrated species were performed and their force fields are reported by using that level of theory. The vibrational studies for the species studied of DMPS reveal the strong dependence of the coordination modes of the sulfate groups on positions and intensities of the bands predicted in the spectra. 2. Experimental details 2.1. Synthesis of DMPS An aqueous solution of H2SO4 (2 mmol, C ¼ 0.2 M, 99.999%) was added dropwise to 2 mmol of 2,6-dimethylpiperzine (97%) dissolved in 10 mL of ethanol. All reactants were purchased from Sigma-Aldrich and used without additional purification. The resulting solution was stirred continually for 1 h and then allowed to evaporate at room temperature until the crystallization of colourless prisms of DMPS more than two weeks. The chemical equation can be written as follow: 25 C

4C6 C14 N2 þ 4H2 SO2 þ 2H2 O! ½C6 H16 N2 4 ðSO4 Þ4 $2H2 O Crystallographic data for the structure study described in this work have been deposited in the Cambridge Crystallographic Data centre as supplementary materials N CCDC 1527550. Copies of the data can be obtained, free of charge, from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 2.2. Investigation techniques 2.2.1. X-ray crystallography A single crystal of DMPS was elected under a polarized microscope and mounted on a glass fiber. The data collections were assembled by means of an APEXII, Bruker-AXS diffractometer with MoKa radiation (l ¼ 0.71073 Å). The empirical absorption corrections were skilled using the multi-scan technique using the SADABS program [8]. The structure was solved by direct methods using the SIR97 program [9] and then refined with full-matrix least-square methods based on F2 (SHELXL-97) [10] with the help of the WINGX program [11]. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters.

All methine, methylene and methyl hydrogen atoms were included geometrically and refined using a “riding model” [with Uiso(H) ¼ 1.2Ueq(C-metylene atoms) and 1.5Ueq(C-methyl atoms)]. The ammonium hydrogen atoms were located from difference Fourier maps and refined isotropically. The refinement converged with acceptable final agreement factors (R(F2) ¼ 0.030 and wR(F2) ¼ 0.074). The details of crystallographic data and structural refinement and of title compound are assembled in Table 1. An ORTEP [11] depiction of the molecular arrangement is exposed in Fig. 1. 2.2.2. IR and UV measurements IR spectrum of sample in KBr pellets was registered at room temperature in the 4000 - 400 cm1 range with a NICOLET IR 200 FT-IR infrared spectrometer by using a spectral resolution of 4 cm1. UVeVis spectrum of sample at a concentration of 10 mM (1 105 M) in aqueous solution was registered at room temperature on a Perkin-Elmer Lambda 19 spectrophotometer between 200 and 800 nm with a spectral resolution of 2 nm. 3. Computational details The investigation of the coordination modes that present the (SO2 4 ) sulfate and (HSO4 ) bisulfate groups is very important to perform the vibrational analyses of DMPS because the normal internal coordinates and the vibration modes observed for monodentate groups are different from those observed for bidentate ones and, for these reasons, the vibration modes for monodentate and bidentate coordinations are expected in different wavenumbers regions [3e6]. Hence, in this work taking into account the experimental structure determined by X-ray diffraction, four different species of DMPS were theoretically optimized by using the hybrid B3LYP/6-31G* level of theory and the Gaussian program Revision A-02 [12]. Thus, from the experimental structure were taken two neutral and one anionic species where the sulfate groups present different coordination modes are presented in Fig. S1 while in Fig. S2 can be seen the theoretical structure for the hydrated species. First, two neutral species formed, one with only a cation and an anion bisulfate containing bidentate coodination, named neutral I and, the other one named neutral II containing a bisulfate group with monodentate coordination (see Fig. S1). Then, we considered one anionic species, named anion I, formed by a cation and two bisulfate anions, where one of these groups presents bidentate coordination and the other one, monodentate, as can be seen in Fig. S1. Finally, the hydrated species can be seen in Fig. S2 which is the neutral species with four cations and four anions and two water molecules. Obviously, the natures of the reached stationary points for all structures were checked by using the vibrational wavenumbers resulting all with positive frequencies. The scaled quantum mechanical force field (SQMFF) approach methodology and the Molvib program were employed to calculate the force fields for those two neutral and the anionic species of DMPS [13,14]. Here, the normal internal coordinates for bisulfate groups with monodentate coordination were built considering C2v symmetry while for those groups with bidentate coordination with C3v symmetry, as reported for species containing similar groups [3e6]. Then, the vibrational assignments for neutral and anionic species were performed by using the Potential Energy Distribution (PED) contributions 10% and by comparison of their predicted vibrational spectra with that experimental infrared spectrum observed for DPMS in solid state. In the case of the hydrated species of DPMS the assignments were performed with the aid of the GaussView program due to elevate number of vibration modes expected [15]. The predicted Raman spectra for those four species, expressed in intensities, are also reported after the conversion of

C. Ben M'leh et al. / Journal of Molecular Structure 1171 (2018) 771e785 Table 1 Crystal data and experimental parameters used for the intensity data collection strategy and final results of the structure determination. Temperature Empirical formula Formula weight (g mol1) Crystal system Space group a b c

b Z V F(000) m(Mo Ka) Index ranges Reflections collected Independent reflections Reflections with I ˃ 2s(I) (sinq/l)max (Å1) Rint Absorption correction: multi-scan Refined parameters R[F2 ˃ 2s(F2] wR(F2) Goodness of fit Drmax ¼ 0.24 e Å3

150 K (C6H16N2)4(SO4)4$2H2O 885.10 monoclinic Cc 25.044 (4) Å 12.1046 (18) Å 14.075 (2) Å 107.289 (5) 4 4074.0 (10) Å3 1904 0.31 mm1 32 h 32, 15 k 15, 18 l 18 18853 7945 7551 0.649 0.030 Tmin ¼ 0.819, Tmax ¼ 0.925 555 0.030 0.074 1.05 Drmin ¼ 0.45 e Å3

activities to intensities by recognized equations [16,17]. Additionally, the frontier orbitals and electrostatic potential surface (ESP) analyses were theoretically computed with the Gaussian program [12]. 4. Results and discussion 4.1. Description of the structure The asymmetric unit of the synthesized compound, 2 4C6H16N2þ contains four independent 2,62 .4SO4 .2H2O dimethylpiperzine-1,4-diium dications, four independent sulfate

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anions and two independent water molecules (Fig. 1). Note that the experimental structure the sulfate groups are present as bisulfate in environmental tetrahedral. 2 In the structure, the independent S(1)O2 4 and S(4)O4 anions are hydrogen bonded through a C44 (12) chain pattern spreading in the c direction in y ¼ ¼ and ¾ involving the two independent water molecules, forming as a polyanion ½ðSO4 Þ2 ðH2 OÞ2 2n n . The diprotonated 2,6-dimethylpiperazine are allied to the polyanions by manifold bifurcated NeH/O(O,O) hydrogen bonding and weak CeH/O interactions, generating a 3D packing, along with donoracceptor distances varying in the range 2.671 (2) - 3.474 (2) Å (Fig. 2). Geometrical features of the sulfate anions spread respectively within the ranges 1.4591(17) e 1.4903(15) Å and 107.63(8) e 111.83(10) (Table 2). These values are close to those reported other related salts containing sufate anion [18e20]. The interatomic bond lengths and angles of the 2,6-dimethylpiperazin-1,4-dium dications not demonstrate noteworthy divergence to those described in another similar structure [7]. The CeC bond lengths range from 1.512(3) to 1.528(3) Å and the NeC distances, the CeNeC and CeCeN angles are similar and lie within the ranges 1.481(3) e 1.504(3) Å, and 108,34(16) e 113,46(14) , respectively. The cyclic diamines adopt a chair conformation with the methyl group occupying the equatorial position with puckering parameter [21]: Q ¼ 0.5641 Å, q ¼ 0 and 4 ¼ 65 for N1C2C3N4C5C6 ring, Q ¼ 0.5739 Å, q ¼ 2 and 4 ¼ 0 for N11C12C13N14C15C16 ring, Q ¼ 0.5811 Å, q ¼ 177 and 4 ¼ 99 for N21C22C23N24C25C26 ring and Q ¼ 0.5729 Å, q ¼ 1 and 4 ¼ 31 for N31C32C33N34C35C36 ring. The crystal structure of DMPS is maintained by complicated 3D H-bond network as shown in Table 3. The sulfate oxygen atoms are concerned in hydrogen bonding as acceptors while the protonated amine groups are wholly donors. Within the configuration there are three kinds of hydrogen bonds, OeH/O, NeH/O and CeH/O. A three-dimensional network is afterward shaped. The experimental structure determined by X-ray diffraction shows clearly the presence of bisulfate groups with monodentate and bidentate coordination modes in environmental tetrahedral. Table S1 shows

Fig. 1. ORTEP drawing of [C6H16N2]4(SO4)4$2H2O with the atom-labeling scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms are represented as small spheres of arbitrary radii and Hydrogen bonds are shown as dashed lines.

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! Fig. 2. Projection along the b axis of atomic arrangement of [C6H16N2]4(SO4)4$2H2O.

that there is a little energy difference between the monodentate and bidentate coordination modes and, that the dipole moment and volume values are higher for the hydrated species than the other ones. For these reasons, these two coordination modes should be later considered in the vibrational analysis. 4.2. Hirshfeld surface investigation With the objectif to specify the varied intermolecular connections, Hirshfeld surface (HS) [22] and their coupled fingerprint plots (FP) [23] were measured by means of Crystal Explorer 3.1 [24]. The Hirshfeld dnorm surfaces of DMPS are exposed in Fig. 3. Here, we calculate approximately the intermolecular interactions, which are shown in Fig. 4. The above investigation can be done by quantitative calculation of 2D FP plots throughout the CrystalExplorer program. This figure represents the major contributor's contacts on the HS specifically H / H and O / H/H / O contacts. For the compound, H/H contacts appear as asymmetrically scattered points covering a large region of the two-dimensional FP maps with a single broad peak at de ¼ di ~ 1.2 Å and a most significant percentage contributions of 54.7%. The O/H/H/O contacts, which are attributed to OeH/O, NeH/O and CeH/O hydrogenbonding interactions, appear as two sharp symmetric spikes in the two-dimensional FP maps with a prominent long spike at de þ di ~ 1.7 Å. This value is fewer than the sum of van der Waals radii of hydrogen (1.09 Å) and oxygen (1.52 Å) atoms; it affirms that the inter-contact is measured as being close contact. They have the 45.3% contribution to the total HS. The intermolecular interactions were additional evaluated by using the enrichment ratio, ER [25], tabulated in Table 4. The main contribution to the HS is from H/H contacts and their ER value is close to unity (ERH … H ¼ 0.91). This shows that the contributions from dispersive forces are significant in the molecule. Besides, the group 2,6-dimethylpiperazine-1,4-diium is vacant of double and triple bonds. This chemical edifice has a large number of hydrogen atoms on their surface (SH ¼ 77.35%), The ER value for O/H/H/O contacts, 1.29, shows the susceptibility to form OeH/O, NeH/O and CeH/O interactions. Such illustration study for intermolecular contacts is logical with those observed by crystallography investigation. 4.3. HOMOeLUMO analysis HOMO and LUMO are two types of molecular orbitals very

useful to predict the reactivities of different species by using their energy differences typically known as gap [26]. In frontier molecular orbital theory, HOMO and LUMO are identified as frontier orbitals and the energy difference between these two frontiers orbitals can also characterize the molecular chemical stability. The HOMO and LUMO values are determined for the four species of DMPS by using the B3LYP/6-31G(d) level of theory and their graphics can be easily observed by using Gauss View program [15]. In Table 5 are summarized the frontier orbitals and gap values for the four species studied of DMPS. The surfaces for the frontier orbital corresponding to the hydrated species are shown in Fig. 4. It is observed that the highest occupied orbitals are localized on the organic cation but the lowest unoccupied orbital's is located on the sulfate anions and water molecules. The lowest value of energy gap observed for the hydrated species, of 5.8897 eV, clearly indicates that these species is the most reactive than the other ones while the high value observed for the neutral I species of 7.8699 eV imply high kinetic stability and low chemical reactivity [27]. Here, the gap value calculated for the hydrated species is comparable to those obtained for p-xylylenediaminium bis(nitrate) (5.7108 eV) [28] and for pyridonium cation (6.0063 eV) [29] while the value for the anionic species of DMPS is similar to those calculated for the [BMIm] cation of 1-buthyl-3-methyl imidazolium nitrate ionic liquid (6.5835 eV) [31] and for the toxic saxitoxin species (6.5287 eV) [29]. 4.4. Global reactivity descriptors In this work, the global chemical descriptors, such as the hardness (h), softness (S), chemical potential (m), electronegativity (c), electrophilicity index (u) and nucleophilicity index were also calculated for the molecule of the title and their neutral and anionic species by using HOMO and LUMO energy values and adequate equations in order to predict the behaviours of these species of DMPS, as suggested in the literature [28e31]. Thus, the hardness h: h ¼ (I̶ A)/2 where A is the ionization potential and I is the electron affinity [ A ¼ -ELUMO, I ¼ -EHOMO ], the chemical potential m,: m ¼ (I þ A)/2; the softness, S: S ¼ 1/2h; the electronegativity c: c ¼ (I þ A)/2 and, the electrophilicity index u: u ¼ m2/2h were computed for the four species of DMPS. The values obtained for those species of DMPS are observed in Table 5 together with the ionization potential I and electron affinity A calculated for the hydrated species. The high gap value in the bidentate neutral I species indicates that this species is the less reactive with higher h, low S


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Table 2 Principal intermolecular distances (Å) and bond angles ( ) in (C6H16N2)4(SO4)4$2H2O. S1 S1 S1 S1 S2 S2 S2 S2 S3 S3 S3 S3 S4 S4 S4 S4

e e e e e e e e e e e e e e e e

O1 O2 O3 O4 O11 O12 O13 O14 O21 O22 O23 O24 O31 O32 O33 O34

N1 e C6 N1 e C2 C2 e C7 C2 e C3 C3 e N4 N4 e C5 C5 e C6 C6 e C8 N11 e C16 N11 e C12 C12 e C17 C12 e C13 C13 e N14 N14 e C15 C15 e C16 C16 e C18 N21 e C26 N21 e C22 C22 e C27 C22 e C23 C23 e N24 N24 e C25 C25 e C26 C26 e C28 N31 e C36 N31 e C32 C32 e C37 C32 e C33 C33 e N34 N34 e C35 C35 e C36 C36 e C38

1.4903(15) 1.4644(16) 1.4774(16) 1.4726(13) 1.4806(12) 1.4745(16) 1.4602(18) 1.4793(16) 1.4806(13) 1.4813(16) 1.4758(15) 1.4591(17) 1.4755(15) 1.4796(16) 1.4811(14) 1.4702(16)

1.504(3) 1.504(2) 1.512(3) 1.520(3) 1.494(3) 1.490(3) 1.521(3) 1.514(3) 1.491(3) 1.501(2) 1.514(3) 1.527(2) 1.484(3) 1.491(3) 1.524(3) 1.519(3) 1.498(2) 1.503(2) 1.522(3) 1.522(3) 1.488(3) 1.496(3) 1.516(3) 1.527(3) 1.496(3) 1.497(2) 1.512(3) 1.520(3) 1.481(3) 1.497(3) 1.528(3) 1.517(3)

HW1A - OW1 e HW1B HW2A - OW2 e HW2B

and higher values of electrophilicity and nucleophilicity. Evidently, the low reactivity is related with the bidentate coordination of sulfate anion probably because their structure in this case is more restricted. Another very important observation is the lower electrophilicity observed in the hydrated species when it is compared with the neutral II species and the anionic ones. In this case, the

O2 e S1 e O4 O2 e S1 e O3 O4 e S1 e O3 O2 e S1 e O1 O4 e S1 e O1 O3 e S1 e O1 O13 e S2 e O12 O13 e S2 e O14 O12 e S2 e O14 O13 e S2 e O11 O12 e S2 e O11 O14 e S2 e O11 O24 e S3 e O23 O24 e S3 e O21 O23 e S3 e O21 O24 e S3 e O22 O23 e S3 e O22 O21 e S3 e O22 O34 e S4 e O31 O34 e S4 e O32 O31 e S4 e O32 O34 e S4 e O33 O31 e S4 e O33 O32 e S4 e O33 C6 e N1 e C2 N1 e C2 e C7 N1 e C2e C3 C7 e C2 e C3 N4 e C3 e C2 C5 e N4 e C3 N4 e C5 e C6 N1 e C6 e C8 N1 e C6 e C5 C8 e C6 e C5 C16 e N11 e C12 N11 e C12 e C17 N11 e C12 e C13 C17 e C12 e C13 N14 e C13 e C12 C13 e N14 e C15 N14 e C15 e C16 N11 C16 C18 N11 e C16 e C15 C18 e C16 e C15 C26 e N21 e C22 N21 e C22 e C27 N21 e C22 e C23 C27 e C22 e C23 N24 e C23 e C22 C23 e N24 e C25 N24 e C25 e C26 N21 e C26 e C25 N21 e C26 e C28 C25 e C26 e C28 C36 e N31 e C32 N31 e C32 e C37 N31 e C32 e C33 C37 e C32 e C33 N34 e C33 e C32 C33 e N34 e C35 N34 e C35 e C36 N31 e C36 e C38 N31 e C36 e C35 C38 e C36 e C35 107(4) 99(3)

111.83(10) 108.72(10) 110.29(9) 108.69(10) 107.63(8) 109.64(9) 108.13(12) 110.41(11) 109.58(11) 111.00(10) 109.69(9) 108.03(8) 110.09(11) 111.57(10) 108.43(8) 107.84(11) 109.84(10) 109.06(9) 110.52(9) 109.08(10) 108.63(10) 108.96(9) 110.30(10) 109.33(9) 113.46(14) 109.45(15) 108.88(15) 110.65(17) 110.93(16) 111.48(15) 111.56(16) 109.80(16) 110.11(16) 110.06(17) 112.40(15) 109.61(15) 108.92(15) 111.91(16) 110.70(16) 112.76(16) 110.87(16) 109.11(16) 108.82(16) 111.44(16) 112.23(14) 109.60(16) 109.20(16) 110.91(19) 110.83(17) 111.16(15) 110.10(16) 108.34(16) 109.40(16) 111.26(18) 112.70(15) 108.83(16) 109.16(15) 111.88(17) 111.06(16) 112.47(16) 110.01(16) 109.20(16) 109.26(16) 112.18(17)

water molecules form H bonds that could probably justify the decreasing of the electrophilicity values while perhaps the similarity in the nucleophilicity values for the neutral II and hydrated species could be related to the monodentate coordination modes that present the sulfate groups in both species. This way, there are more donor O atoms in sulfate anions available to react with

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4.6. Non-linear optical properties

Table 3 Hydrogen-bonds geometry (Å, ) in (C6H16N2)4(SO4)4$2H2O. DdH$$$A

DdH

H$$$A

D$$$A

OW1dHW1A$$$O31i OW1dHW1B/O2 OW2dHW2A$$$O32ii OW2dHW2B/O3 N1eH1A$$$O12iii N1eH1B/O31 N4eH4A$$$O22iv N4eH4A$$$O24iv N4eH4B/O1 N11eH11A$$$O21 N11eH11B/O14 N14eH14A$$$O4 N14eH14B/O32 N21eH21A$$$O22 N21eH21B/O34v N21eH21B/O33v N24eH24A$$$O13 N24eH24A$$$O12 N24eH24B/O1vi N31eH31A$$$O11vii N31eH31B/O23 N34eH34B/O3vii N34eH34B/O2vii N34eH34A$$$O33 C2eH2/O4 C3eH3A$$$O11iii C3eH3B/O13viii C5eH5A$$$O34ii C5eH5B/O11iii C7eH7C/O34 C12eH12/O3 C13eH13B/O23 C15eH15A$$$O23 C22eH22/O2vi C23eH23A$$$O21 C25eH25A$$$O24ii C25eH25B/O21 C32dH32$$$OW1ii C33eH33B/O14vii C35eH35A$$$O14vii C36eH36/O32

0.87 0.92 0.82 0.85 0.95 0.92 0.88 0.88 0.90 0.97 0.91 0.77 0.89 0.97 0.91 0.91 0.88 0.88 0.91 1.00 0.92 0.86 0.86 0.82 1.00 0.99 0.99 0.99 0.99 0.98 1.00 0.99 0.99 1.00 0.99 0.99 0.99 1.00 0.99 0.99 1.00

2.10 1.89 2.22 2.07 1.81 1.85 1.97 2.45 1.87 1.76 1.74 1.90 1.85 1.80 1.88 2.50 1.97 2.52 1.91 1.75 1.76 1.98 2.47 1.86 2.51 2.46 2.41 2.43 2.43 2.44 2.48 2.40 2.54 2.53 2.49 2.25 2.46 2.59 2.60 2.39 2.28

2.938 2.789 2.975 2.890 2.741 2.767 2.821 3.126 2.743 2.728 2.654 2.668 2.712 2.731 2.780 3.135 2.805 3.244 2.765 2.743 2.671 2.806 3.153 2.671 3.406 3.319 3.343 3.338 3.291 3.380 3.474 3.270 3.375 3.374 3.353 3.218 3.336 3.210 3.428 3.275 3.237

(5) (5) (3) (3) (2) (2) (3) (3) (3) (2) (2) (3) (3) (3) (3) (3) (3) (3) (3) (2) (2) (3) (3) (3)

(6) (5) (4) (4) (2) (2) (3) (3) (3) (2) (2) (3) (3) (3) (3) (3) (3) (3) (3) (2) (2) (3) (3) (3)

DdH$$$A (3) (3) (3) (2) (2) (2) (2) (3) (2) (2) (2) (2) (2) (2) (2) (2) (3) (3) (3) (2) (2) (2) (3) (2) (2) (2) (3) (3) (3) (3) (2) (2) (3) (3) (3) (3) (3) (3) (3) (3) (2)

161 (5) 165 (5) 152 (3) 162 (3) 167 (2) 170 (2) 162 (2) 134 (2) 164 (2) 171 (2) 177 (2) 175 (3) 164 (2) 163 (2) 168 (2) 126.8 (19) 157 (2) 140 (2) 156 (2) 170 (2) 169 (2) 159 (3) 137 (2) 171 (3) 149.1 145.5 157.1 151.7 145.3 160.1 171.0 146.9 141.4 142.2 145.1 165.0 147.5 119.8 141.2 148.6 160.2

Symmetry codes: (i) x, yþ1, z1/2; (ii) x, yþ1, zþ1/2; (iii) x1/2, yþ1/2, z; (iv) x1/2, y1/2, z; (v) xþ1/2, yþ3/2, zþ1/2; (vi) xþ1/2, yþ1/2, zþ1/2; (vii) x, yþ1, z; (viii) x1/2, yþ1/2, z1/2.

electrophilic reagents. 4.5. Thermodynamic properties The thermodynamic functions such as enthalpy changes (H), entropies (S) and heat capacity at a constant pressure (Cp) were determined for the title compound using B3LYP/6-31G(d) at different temperatures 100e1000 K and as observed in Table 6. It can be shown that these thermodynamic functions increase with temperature as predicted that the molecular vibrational intensities change with rise of temperature. The correlation equations between thermodynamic functions (heat capacity, entropy, enthalpy change) and temperature are 0.977, 0.993 and 0.973 respectively (Fig. 5). The corresponding fitting equations are as follow:

C ¼ 13:6935 þ 0:46636T 2:00436 104 T2

(1)

S ¼ 235:25067 þ 0:52891T 1:29758 104 T2

(2)

H ¼ 4:38517 þ 0:06306T 1:24201 104 T2

(3)

The vibrational zero-point energy calculated of compound is 657.74615 kJ/mol and shown in Table 7.

Using DFT/B3LYP/6-31G(d), we calculated the total dipole moment m, the linear polarizability a and the first hyperpolarizability b. An extension of Taylor series of total dipole moment m has been presented to determine the non linear optical response of an isolated molecule in an electric field Ei(u), Resulting from this field:

m ¼ m0 þ aij Ej þ bijk Ej Ek þ …

(4)

Where m0: the permanent dipole moment

bijk: the first hyperpolarizability The linear polarizability is determined as:

a¼

1 ðaxx þ ayy þ azzÞ 3

(5)

Using the following equation, the components of the first hyperpolarizability can be calculated by

bi ¼ biii þ

1 X bijj þ bjij þ bjji ; ði#jÞ 3

(6)

The magnitude of the first hyperpolarizability tensor can be determined using the x, y and z components

b¼

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi

b2x þ b2y þ b2z

(7)

The polarization values atot and the hyperpolarization btot are calculated using B3LYP/6-31G (d) in atomic units (a.u) and are transformed into electrostatic units (esu) (aij: 1 a.u ¼ 0.1482 1024; bijk: 1 a.u ¼ 8.6393 1033 esu). The results of the moment of the electric dipole moment mi (i ¼ x,y,z) of our compound studied are mentioned in the Table 7. The dipole moment found is equal to 12.8428 D. The table shown the most important dipole moment values are observed for the component mx and my. The latter have values equal to 0.223219632 for mx and 4.83241327 for my. The value of mz for the z direction is equal to 1.45885052. Furthermore, the polarizability atot is assimilate to 46.7570 1024 esu. Similarly, it can be seen from this table that the value of the first hyperpolarizability btot is equal to 13 times with respect to the reference crystal KDP (bKDP ¼ 6.85 1031 esu). This latter contributes to the stabilization of crystalline structure and also allows the amelioration of hyperpolarisability and susceptibility. The title compound has the first hyperpolarizability btot and the state dipole moment m are measured to be 10.0880 1031 esu (13 times more than those of urea 0.07763 1031) and 12.8428 Debye respectively. According to the literature published recently, it can be concluded that this material admits important properties in nonlinear optics. It is a good material for NLO applications [32]. 4.7. Molecular electrostatic potential (MEP) The surface of the electrostatic potential is sometimes used to known the probable nucleophilic and electrophilic reaction sites present in the species and, in this case, taking into account the presence of different H bond's types experimentally observed in the structure of DMPS, where clearly are involved the sulfate and NH2 groups, it is useful and necessary to known that surface mapped to predict exactly the donor and acceptor sites of H bonds. In particular, the qualitative interpretation of reactions electrophiles and nucleophiles due to the presence of NeH/O and CeH/O bonds interactions observed in DMPS can be easily predicted by the


777

Fig. 3. Fingerprint plots of all (a), O/H/H/O (b) and H/H (c) contacts: Surfaces to the side highlight the relevant surface patches associated with the specific contacts, with dnorm mapped in the same manner.

Fig. 4. The frontier molecular orbitals of (C6H16N2)4(SO4)4$2H2O molecule.

778


Table 4 Enrichment ratio of different inter-contact and percentage of each atom on the surface Hirshfeld in (C6H16N2)4(SO4)4$2H2O. Enrichment

H

O

H O

0.91

1.29 0.00

% Surface

77.35

22.65

Table 5 Calculated energy values of (C6H16N2)4(SO4)4$2H2O by B3LYP/6-31G(d) method. Energies (eV) Frontier orbitals

Neutral I

Neutral II

Anionic

Hydrated

EHOMO (eV) ELUMO (eV) jEHOMO-ELUMOj Gap (eV)

6.5679 1.3020 7.8699

6.4124 0.8832 7.2956

2.5781 3.8714 6.4495

5.5733 0.3164 5.8897

Ionization potential A

1.0593

Electron affinity I

6.8091

Descriptors 2.6483 3.2884 2.6483 0.1888 2.0416 8.7087

c m h S

u Е

1.8850 3.6632 1.8850 0.2653 3.5594 6.9051

1.8842 3.6586 1.8842 0.2654 3.5520 6.8935

1.8863 3.6582 1.8863 0.2651 3.5473 6.9005

different colorations in those surfaces [33]. The graphic representation of the potentials created in space around a molecule by its nuclei and its electrons. The blue colour indicates the strong attraction of donor H bond sites whereas the red colour shows a region of strong repulsion acceptor of H bond and a green colour indicates the neutral zone. The surface of the electrostatic potentials of the title compound is shown in Fig. 6. The highest potential is marked at the level of the oxygen atoms of the sulfate groups because these are hydrophilic atoms acceptor of H bond while the most positive potential is observed at the level of the hydrogen atoms, as expected due to its higher donor capacity. These results contribute to the formation of those hydrogen bonds observed between the two organic cations and inorganic anions. The surfaces of the electrostatic potentials are traced in the region between 9.469 e2 (red) and 9.469 e2 u.a. (blue).

Table 6 Thermodynamic properties. T (K)

S (J/mol.K)

Cp (J/mol.K)

H (kJ/mol)

100.00 150.00 200.00 250.00 298.15 300.00 350.00 400.00 450.00 500.00 550.00 600.00 650.00 700.00 750.00 800.00 850.00 900.00 950.00 1000.00

764.945 986.499 1186.063 1371.423 1541.194 1547.586 1717.375 1882.061 2042.068 2197.467 2348.237 2494.380 2635.952 2773.062 2905.852 3034.487 3159.144 3279.999 3397.228 3510.998

472.795 626.861 765.703 900.626 1030.957 1035.960 1169.815 1298.544 1419.344 1530.876 1632.967 1726.125 1811.156 1888.929 1960.260 2025.864 2086.351 2142.239 2193.965 2241.908

26.838 54.419 89.270 130.929 177.431 179.343 234.501 296.238 364.222 438.017 517.152 601.165 689.629 782.160 878.415 978.090 1080.916 1186.649 1295.070 1405.982

Fig. 5. The correlation graphics of temperature of thermodynamic functions for (C6H16N2)4(SO4)4$2H2O.

4.8. Vibrational study The experimental structure determined by X-ray diffraction shows clearly the presence of bisulfate groups with monodentate and bidentate coordination modes in an environmental tetrahedral. Hence, these two coordination modes should be considered in this vibrational analysis. Fig. 7 shows the FT-IR spectrum of DMPS in the solid phase compared with the corresponding predicted for those four species studied by using the B3LYP/6-31G* level of theory. Here, it is observed that the positions and intensities of all bands predicted in the IR spectra of those neutral and anionic species of DMPS are in agreement with the observed in the experimental one especially in the 4000-2000 cm1 region. On the other hand, the IR bands predicted for the anionic species justify the intense bands observed in the experimental spectrum in the 2000-400 cm1 region. Note that the typical IR bands associated with the water molecules are clearly justified by the hydrated species, as expected because only in the structure of this species were considered the water molecules. All neutral, anionic and hydrated species molecules were optimized with C1 symmetry and, hence, all bands should present activity in both IR and Raman spectra. For the two neutral species are expected 81 normal vibration modes while for the anionic species 99 normal vibration modes are expected. Due to the elevated number of vibration modes expected for the hydrated species (360) their vibrational assignments was performed with the aid of the GaussView program [15] while for the other species

Table 7 Calculated thermodynamic parameters (at 298.15 K) for (C6H16N2)4(SO4)4$2H2O with DFT/B3LYP method using 6-31G (d) basis set. Thermodynamic parameters

values

SCF energy E (u.a) Zero-point vibrational energy (kcal mol1) Rotational constants (GHz) A B C Thermal energy (kcal mol1) Specific heat at constant volume Cv (cal mol1 K1) Entropy S (cal mol1 K1) Dipole moment (Debye)

4340.20743402 657.74615 0.05515 0.03019 0.02299 699.561 244.418 368.340 12.8428


779

Fig. 6. The molecular electrostatic potential surface (MEP) of (C6H16N2)4(SO4)4$2H2O.

the SQMFF methodology was applied to perform their complete assignments. The differences between the experimental and calculated vibrational for all studied species are summarized in Table 8 with detailed vibrations and assignments. The predicted Raman spectra for the neutral, anionic and hydrated species of DMPS are shown in Fig. 8. In this section, it is necessary to clarify that despite the sulfate ions are involved in environmental tetrahedral the Td symmetry was not considered because the O atoms of sulfate groups are linked to H atoms forming HSO 4 groups where the four O atoms are asymmetric and, as a consequence they are not equivalent. Hence, in bisulfate groups with monodentate coordination the C3v symmetry was considered while in bidentate coordination of that group the C2v symmetry was considered. The group of intense bands observed between 2728 and 2188 cm1 can be associated to the stretching modes corresponding to OHe–N bonds because these modes are clearly predicted by B3LYP/6-31G* calculations. Below we present a discussion of assignments only for the most important groups of the studied species.

4.8.1. NH2 and NeH modes The antisymmetrical and symmetrical stretching modes of NH2 group are normally assigned between 3518 and 3093 cm1, as in saxitoxin and azetazolamide species [29,34]. Hence, in the two neutral and anionic species, the antisymmetrical stretching modes are predicted at lower wavenumbers than the corresponding symmetrical ones, thus, the SQM calculations predicted the symmetrical stretching modes at 2978 and 2967 cm1 and, in particular in the neutral monodentate species these modes are observed coupled with other stretching modes while the antisymmetric stretching modes are predicted at 2845 cm1. In the anionic species, these two stretching modes are predicted at 3260 and 2859 cm1 while in the hydrated species the symmetrical modes can be easily assigned to the IR band of medium intensity at 3373 cm1, as predicted by calculations. Here, the symmetry of these modes cannot be confirmed because the experimental Raman spectrum was not recorded. The NeH stretching modes in the neutral, anionic and hydrated species of DMPS were predicted in the same regions, hence the bands between 3518 and 3373 cm1 can be assigned to these vibration modes, as reported for other species [29,30]. The SQM calculations predicted the NH2 deformation modes for both neutral species strongly coupled with NHeO deformation modes at lower wavenumbers (1492-1447 cm1), therefore, the bands

observed between 1945 and 1447 cm1 can be associated to these vibration modes. In the anionic and hydrated species, the NH2 deformation modes are predicted species at 1726 and 1657 cm1, respectively, hence, the bands at 1945 and 1634 cm1 can also be attributed to these modes. The rocking and waging are predicted in

Fig. 7. The experimental and calculated infrared spectra for the neutral, anionic and hydrated species of [C6H16N2]4(SO4)4$2H2O.

780


Table 8 Observed and calculated wavenumbers (cm1) and assignments of the neutral, anionic and hydrated species of (C6H16N2)4(SO4)4$2H2O. Exp.

Neutrala

Anionica

Monodentate C3v c

IR

b

SQM

Assignment

Bidentate C2v a

SQMb

Assignmenta

3518 m 3472 m 3472 m 3373 m 3097sh

3097sh 3060sh 3020sh 3000s 2953s

2880 m 2800sh 2728vs 2670vs 2586s 2500vs 2448vs 2256s 2188 m 1986s 1945 m 1634s

3575 3373 3018 3018 3013 3012 2978 2975 2967 2955 2954 2941 2940 2845 2823 2819

1912

Hydrated.2(H2O)a

nO28-H24 nN9-H10 naCH3(C16) naCH3(C20) naCH3(C20) naCH3(C16) dsN-HeO,nsNH2 naCH2(C6) nsNH2 dsN-H-OnsNH2 nC4-H5nC14-H15 nsCH3(C16) nsCH3(C20) naNH2 nsCH2(C6) nsCH2(C11)naCH2(C11)

dsNHO

3575 3373 3018 3018 3013 3012 2978 2975 2967 2955 2954 2941 2940 2845 2823 2819

1907

nO28-H24 nN9-H10 naCH3(C16) naCH3(C20) naCH3(C20) naCH3(C16) naCH2(C11) naCH2(C6) nsNH2 nC14-H15 nC4-H5 nsCH3(C16) nsCH3(C20) naNH2 nsCH2(C6) nsCH2(C11)

SQMb

Assignmenta

Calc.c

Assignmenta

3579 3357

nO28-H24 nN9-H10

3646 3498

3280 3260 3021 3008 3008 3003 3000 2996 2971 2952 2933 2932 2859 2826 2822

nO32-H30 nsNH2 nC14-H15 naCH3(C20) naCH3(C16) naCH3(C16)

3444 3345 3234 3176 3146 3142 3112 3094 3058 2956 2954 2953 2913

nO-H (H2O) nO-H(H2O) nN-H nN-H nO-H nN-H nsNH2 naCH3 nN-He-OH naCH2,nN-H,nC-H nC-H naCH3 naCH2 naCH2 naCH2 nO-He–N

2687 2635 2572 2410

nO-He–N nO-He–N nO-He–N nO-He–N

1742 1726 1573

dH2O dNH2 dNHO

1559 1542 1506

dNHO daCH3 bN-H

aCH2(C11)

naCH3(C20) naCH2(C6) nC4-H5 nsCH3(C16) nsCH3(C20) naNH2 nsCH2(C6) nsCH2(C11)

dsNHO 1657

dNH2 dsNH—O

1634s 1500sh 1492

dNH2 dsNHO

1492

dNH2dsNHO

1502

wagNH2

1500sh 1467s 1467s

1480 1469 1464

wagNH2 dNH2 dsNHO dCH2(C6) daCH3(C16) daCH3(C20)dCH2(C11) daCH3(C16)

1480 1469 1464

1488 1471 1462

rNH2 daCH3(C16) dCH2(C6) daCH3(C16)

1494

dCH2

1467s 1467s

1460 1457 1454 1447

bN9-H10 dNH2 dsNHO dNH2 dsNHO dNH2 dsNHO

1460 1457 1454 1447

dNH2dsNHO dCH2(C6) dCH2(C11) daCH3(C16) daCH3(C20) daCH3(C16) bN9-H10 dNH2dsNHO dNH2dsNHO dNH2dsNHO

1460 1457 1453 1444

daCH3(C20) daCH3(C20) daCH3(C16) dCH2(C6)

1484

bN-H

1455 1445

rC-H dsCH3

1435 1424

dCH2(C11) wagCH2(C6) rC4-H5 daCH3(C16) r0 C14eH15 r0 C4eH5 dsCH3(C20)

1422

dS-OeH

1405

dS-OeH

1391

dNHO

1386

rC-H

1374 1352 1331

rCH2 dNHO rC-H,rNH2

1306

rNH2

rCH2(C11) dS31O32H30 twO-S(M) naS-O2 rCH2(C6) r0 CH3(C20)

1304 1281

wagCH2 naSO3

1256

rCH3

1202

naSO3

1189 1160 1153 1139 1137 1133

naSO3 tR1 tR1 naSO3 naSO3 naSO3

1447sh

wagCH2(C11) 1447sh 1422 m

1429

1422 m

1419

1390 m

1396

1390 m

1386

1361 m 1330sh

1280w 1232w 1196w 1196w 1142s 1142s 1142s

1113sh

1378 1345 1328

wagCH2(C11) wagCH2(C6) rNH2 daCH3(C20) dsCH3(C20) dsCH3(C16) dsCH3(C20) dsCH3(C16) dNH2 dsN-HeO dNH2 dsN-HeO rC14-H15 rC4-H5

1378 1345 1328

1318

r0 C14eH15 r0 C4eH5

1318

1289 1280

rCH2(C11) wagCH2(C6) wagCH2(C11) naSO3

1289 1280

1267

1429 1419 1396 1386

wagCH2(C11) wagCH2(C6) rNH2 daCH3(C20) wagNH2

dsCH3(C20) dsCH3(C16) dsCH3(C20) dsCH3(C16) dNH2dsN-HeO dNH2dsN-HeO rC14-H15 rC4-H5 r0 C14eH15 r0 C4eH5 rCH2(C11)

1266

wagCH2(C6) wagCH2(C11) dsNHO dNH2

1404 1388 1384 1369 1363 1321 1316 1298 1289

dsCH3(C16) rC14-H15 dsCH3(C20) rC14-H15 rC4-H5 dS31O32H30 twO-S(M) dS31O32H30

1192

r CH3(C16)

1183

r CH3(C16) r0 CH3(C20) rCH2(C6)

1253 1196 1194

1183

rCH2(C6)

1189

naSO3

1144 1127 1122 1113

t R1 nN9-C11 dNH2 dsNHO dS-OeH

1144 1126 1121 1113

tR1 nN9-C11 dS-OeH bR1

1151 1130 1124 1115

tR1 rCH3(C20) naS ¼ O2 nN9-C11 naSO3nC4-C6

1193

0

0


781

Table 8 (continued ) Exp.

Neutrala

Anionica

Hydrated.2(H2O)a

Monodentate C3v

Bidentate C2v

IRc

SQMb

Assignmenta

SQMb

Assignmenta

SQMb

Assignmenta

Calc.c

Assignmenta

1113sh

1109

rCH3(C20) rCH3(C16)

1109

rCH3(C20) rCH3(C16)

1103

b R1

1124

dS-OeH

1099 1088 1073

r CH3(C20) dS25O28H24 r0 CH3(C16) rCH3(C20)

1074

dNHO

1066 1030 1016

nC-C gN-H nsSO3

998 986 974 960

nsSO3 nN-C b R1 tS-OH

933

nC-C

1063vs 1063vs 1063vs 1046sh 1046sh 1046sh

1080

naSO3 1078

naS ¼ O2

1071

r0 CH3(C16) r0 CH3(C20)

1071

r0 CH3(C20)

1020

nC14-C20

1020

993

nN9-C6 nN9-C11

993

nC14-C20 nC4-C16 nN9-C6 nN9-C11

1018 992

0

nC14-C20 nC4-C16 dH10N9C11 dH10N9C6

988sh 969sh

963

nsSO3

962

naS ¼ O2

926 m

941 931

nN1-C4nC4-C6 nC4-C16

941 931

nN1-C4nN1-C14 nC4-C16

887

nN9-C6

887

nN9-C6

843sh

873

twCH2(C6) twCH2(C11)

873

843sh

865

gN9-H10

865

twCH2(C6) twCH2(C11) gN9-H10

965 959 937 932

naS ¼ O2 nsSO3 twCH2(C6) nN1-C14 nN1-C4

879 877 875

nN9-C6 twCH2(C11) dH10N9C11 dH10N9C6

910 885

nN-C tS-OH

863

tS-OH, gN-H

822 811

twNH2 nC11-C14

852 834 816

nC-C,nN-C twNH2 nS-O

766

rN9-H30

802 766

tS-OH nS-O

727 714 683

naS-O2 nS31-O32 dN9H30O32

553 544 544 519 506 504 485 477

dO ¼ S]O twS ¼ O2 dO ¼ S]O

743 700 685 608 595 571 555

wagH2O rH2O rH2O twNH2 b R1 daSO3 daSO3

wags ¼ O2 daSO3(S31) daSO3(S31) dsSO3(S31) dC16C4N1

547 513 507

twH2O twH2O twH2O,tR1

484

b R2

469

dC20C14N1

431 416 397 374 360 358 355 322

b R2 b R3 dO-SeO r0 SO3(S31) r0 SO3(S31) rSO3(S31) rSO3(S31) r S ¼ O2 t R2

440 430 410 387

b R2 b R3 daSO3 daSO3

361

t R2

259 235 195 188 157 138 129 109 100 96 83

dC11C14C20 naNH—O twCH3(C20) twCH3(C16) tR3 r0 N9eH30 nN9-H30 t R1 nsNH—O tSO28H24 twO-S(M) daNH—O twO-S(M) twSO3(S31)

354 329 270 264 252 234 205 198 160 152 133

t R2 rH2O dCCC twCH3 dCCC twCH3 dCCC twCH3 nO—HO(H2O) OeHe–N OeHe–N

64

twSO3

926 m

816

nN1-C14nC11-C14

816

803 748

twNH2 nO26-H3gN9-H10

804 748

740

nS25-O28

739

nC4-C6 nC11-C14 twNH2 nO29-H2 gN9-H10 naS-O2naS-O2

558 550

daSO3 bR1

557 550

dO ¼ S]O bR1gN9-H10

547 523 510

daSO3 dsSO3 daNHO

520 512 500

wagSO2 tR1 twSO2

473w

479

dC16C4N1 dC20C14N1

479

dC16C4N1 dC20C14N1

431w

431 422

bR2 bR3

431 422

bR2 bR3

394

daSO3 r0 SO3 rSO3

393

dO-SeO

359 357 323 270 262

daSO3 dC11C14C20 daSO3 tR2tR1 dC6C4C16 dC11C14C20 nO29-H2 dsNHO

359 355 323 270 261

dC11C14C20 rSO2 tR2 dC6C4C16 nO26-H3 dsNHO

786w 749sh

720sh 620s 591s 565sh 565sh 565sh 565sh 521sh 508 m

403w

204 196 159 155 129

twCH3(C16) twCH3(C20) tR3 nO26-H3 nO29-H2

204 196 159 154 128

twCH3(C16) twCH3(C20) tR3 nO29-H2 nO26-H3

68

dNH2

67

dNH2

73

(continued on next page)

782


Table 8 (continued ) Exp.

IRc

Neutrala

Anionica

Hydrated.2(H2O)a

Monodentate C3v

Bidentate C2v

SQMb

Assignmenta

SQMb

Assignmenta

SQMb

Assignmenta

Calc.c

Assignmenta

55 44 37

tS-OH twSO3 nO29-H2 nO26-H3

55 44 37

tS-OH tsOH-S daNHO

54

twO-S(B)

dsNHO dNH2tOH-S

24

dsNHO dNH2taOH-StOH-S

tipH-O twSO3(S31) dN9H30O32 topH-O daNH—O

twSO3 tsOH-S tsOH-S

24

34 30 24 19

59 46 35 24

nO—HN

Abbreviations: n, stretching; wag, wagging; t, torsion; r, rocking; tw, twisting; d, deformation; a, antisymmetric; s, symmetric; M, monodentate; B, bidentate. a This work. b From scaled quantum mechanics force field B3LYP/6-31G* method c From B3LYP/6-31G* method.

different (1492e1331 cm1) regions in the neutral, anionic and hydrated species, where in the hydrated species are predicted at lower wavenumbers than the other ones, but the twisting modes are predicted in approximately the same (834e804 cm1) regions. Thus, these modes are assigned in accordance to the calculations and, as observed in other similar species [29,33]. Here, we observed that the different coordination modes predicted for the sulfate

groups have slight influence on the vibration modes, as can be seen in Table 8. 4.8.2. CH3 modes In the four species of DMPS, the different coordination modes predicted for the sulfate groups do not have influence on the vibration modes of CH3 groups, hence, the antisymmetric and

Fig. 8. The predicted Raman spectra for the neutral, anionic and hydrated species of [C6H16N2]4(SO4)4$2H2O by using B3LYP/6-31G* calculations.


symmetric stretching, deformation and rocking modes are assigned as predicted by SQM calculations to the IR bands at 3097/2953, 1500/1390 and 1232/1046 cm1, as detailed in Table 8. Here, the twisting modes for the four species of DMPS are predicted between 234 and 196 cm1 and, here, these modes were not assigned due to the absence of bands in these regions. 4.8.3. CH2 modes The vibration modes expected for these groups in the four species are not influenced by the different coordination modes of the sulfate groups. Thus, all vibration modes expected for the four species are predicted by SQM calculations in approximately the same regions, as detailed in Table 8. Therefore, the two antisymmetrical and symmetrical stretching, deformation, waging, rocking and twisting modes for these CH2 group can be assigned in the 3112/2819, 1495/1435, 1429/1280, 1289/1183 and 937/873 cm1 regions predicted by SQM calculations, as shown in Table 8. 4.8.4. Sulfate anion vibration The sulfate anion plays an important role in establishing hydrogen bonding with organic cations and maintaining the stability of crystalline structure. When these groups present bidentate coordinations, two S]O and SeO antisymmetric (naSO2) and symmetric stretching modes are expected while in a monodentate coordination two SeO (naSO3) antisymmetric and one SeO symmetric stretching modes are expected [3e6]. Hence, the IR weak

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bands at 1280 and 1232 cm1 and, the very strong bands at 1142 and 1063 cm1 together with the shoulders associated with that broad and intense band at1046, 988 and 969 cm1 can be easily associated to those stretching modes while the SeO stretching are predicted by SQM calculations between 740 and 714 cm1. The O] S]O deformation modes are predicted between 571 and 485 cm1 while the OeSeO deformation modes between 410 and 357 cm1, as reported for other similar compounds [3e6]. The rocking and twisting mode are predicted between 374 and 355 cm1 and between 73 and 30 cm1, respectively as indicate in Table 8. Obviously, these modes were not assigned. 4.8.5. The vibration of water molecule The OH stretching modes associated to the water molecules are predicted between 3646 and 3498 and, for this reason, these modes can be assigned to the bands at 3518 and 3472 cm1. In the hydrated species the two in-plane bending modes d(HeOeH) are predicted at 1742 cm1 and for this reason, the IR band at 1945 cm1 can be assigned to these vibration modes. The waging, rocking and twisting modes related to both water molecules are predicted in the lower wavenumbers, thus, the waging is predicted at 743 cm1 while the rocking and twisting at 700e685 and 547-507 cm1, respectively. Therefore, those modes are assigned in the regions predicted by calculations. The differences observed between the experimental vibrations and the theoretical ones obviously can be explained by the fact that

Fig. 9. Solution state UV/vis spectra of [C6H16N2]4(SO4)4$2H2O compared with the predicted for the hydrated anionic and neutral species in aqueous solution by using B3LYP/6-31G* level of theory.

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the experimental IR spectrum is carried out to in the solid phase while the theoretical calculations are performed for the isolated molecule in the gas phase. 5. Ultravioletevisible spectrum Fig. 9 shows the ultravioletevisible spectrum of DMPS in aqueous solution compared with the corresponding predicted for the hydrated species, anionic and neutral species in water by using Time-dependent DFT calculations (TD-DFT) at the 6-31G* level of theory with the Gaussian 09 program [12]. The experimental spectrum shows a very strong band at 282 nm with two shoulders at c.a. 315 and 360 nm. The most intense band can be quickly assigned to n/p* transitions due to the presence of auxochromes NH2 groups while the shoulders could probably have its origin in the SO2 4 groups, as reported by Benito el al [35]. The differences observed between the predicted and experimental spectra could be attributed to the presence of anions in solution because the experimental spectrum shows from the 200 nm the similar signature. 6. Conclusions In the present work, a new non-centrosymmetric inorganicorganic hybrid material tetrakis(2,6-dimethylpiperazine-1,4diium) tetrakis(sulfate) dihydrate (DMPS) was synthesized and characterized in the solid phase by using X-ray diffraction and infrared spectroscopy and by ultravioletevisible spectrum in aqueous solution. Here, two neutral species, anionic and hydrated species of DMPS were theoretically studied by using the hybrid B3LYP/6-31G* method in order to investigate the coordination modes that can present the sulfate ions in the structure of DMPS. This way, two different monodentate and bidentate coordination modes were proposed for the sulfate ions in those species. The vibrational analysis show clearly the influence of the different ccordination modes on the intensities and positions of the bands associated with the NH2 and sulfate groups. Besides, the broad and intense bands and shoulders observed between 1280 and 950 cm1 region are associated to stretching modes related to monodentate and bidentate sulfate groups while the group of intense bands between 2728 and 2188 cm1 are easily assigned to the different types of H bonds experimentally observed and theoretically predicted by calculations for the hydrated species. Here, the predicted Raman spectra for all species were also reported. The Hirshfeld surface analysis was employed to quantify the intermolecular interactions in the crystal structure. The graphic of frontier orbital for the hydrated species suggest that the highest occupied orbitals are localized on the organic cation while the lowest unoccupied orbital's is located on the sulfate anions and water molecules. The gap values evidence that the bidentate coordination of sulfate anion is associated to a low reactivity probably because the structure in this case is more restricted while the descriptors show that the lower electrophilicity observed in the hydrated species could be probably attributed to the H bonds formed by the water molecules. Acknowledgements The authors are grateful to the Tunisian Ministry of Higher Education and Scientific Research for his support and would like to thank Prof. Tom Sundius for his permission to use MOLVIB. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.molstruc.2018.06.041.

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