Theoretical investigation of the conformation, acidity, basicity and ...

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Theoretical investigation of the conformation, acidity, basicity and hydrogen bonding ability of halogenated ethersw Wiktor Zierkiewicz,*a Danuta Michalskaa and The´re`se Zeegers-Huyskens*b Received 12th April 2010, Accepted 22nd July 2010 DOI: 10.1039/c0cp00192a MP2/6-311++G(d,p) calculations have been carried out to investigate the conformation, protonation and the hydrogen bonding interactions with water of several halogenated ethers (CH3OCH2Cl, CH2ClOCH2Cl, CH3OCHCl2, CHFClOCHF2). The optimized geometries, n(CH) harmonic vibrational frequencies and the SAPT decomposition of the interaction energies are studied. The interaction with one water molecule gives several stable structures characterized by OwHw  O and CH  Ow hydrogen bonds or by O  Cl halogen bonding. The MP2/CBS calculated binding energies of different complexes between the halogenated ethers and water vary between 1.7 and 7.7 kcal mol1. The energies of these structures are discussed as a function of the proton affinity of the ethers and the deprotonation enthalpy of the CH bonds. The contraction of the CH bonds and blue shifts of the corresponding stretching vibrations in the O-protonated ethers and their O  HwOw complexes are compared. A natural bond orbital analysis has revealed that substitution of the H atoms by one or several halogen atoms has a great influence on the hyperconjugative effects from the two non-equivalent O lone pairs to relevant antibonding orbitals, and the subsequent geometry of the hydrogen bonded complexes.

1. Introduction It is well known that general anaesthetics act by perturbing intermolecular interactions such as van der Waals complexes without breaking or forming covalent bonds.1–5 Their action depends on specific binding with proteins and therefore the knowledge of their conformational properties and structural properties is very important. Knowledge of their hydrogen bonding ability is also of prime importance. The conformation of ethers and halogenated ethers has been discussed in several works6–13 and has been shown to depend on the delocalization of the lone pairs of the O atom to the s*(CX) orbitals within the molecules. Other effects such as the attraction or repulsion between non-bonded atoms can also influence the relative stability of the conformers.13 On the other hand, the complexing ability of ethers and their fluorinated derivatives has been discussed in several experimental or theoretical works.14–20 The hydrogen bonding properties are related to the presence of CH groups that can act as proton donors; the presence of electron withdrawing groups such F or Cl atoms tend to make the CH bonds more acidic and increase their hydrogen bonding ability. In contrast, the presence of these electron-attracting substituents decreases the proton acceptor ability of the O atom. From these considerations, it can be anticipated that the complexing ability of ether derivatives will depend on a a

Faculty of Chemistry, Wroc!aw University of Technology, Wybrzez˙e Wyspian´skiego 27, 50-370 Wroc!aw, Poland. E-mail: [email protected] b Department of Chemistry, University of Leuven, 200F Celestijnenlaan, 3001, Heverlee, Belgium. E-mail: [email protected] w Electronic supplementary information (ESI) available: The selected NBO parameters (orbital occupancies, s-orbital character, energy E2 of donor–acceptor interactions) in isolated and O-protonated halogenated ethers (Tables 1S to 5S). See DOI: 10.1039/c0cp00192a

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subtle balance between the acidity and the basicity of the atoms or groups involved in the hydrogen bonding interaction. The present work deals with a theoretical study of the conformation and complexing ability of halogenated ethers (CH3OCH2Cl, CH2ClOCH2Cl, CH3OCHCl2, CHFClOCHF2). These molecules can be considered as models for more complex systems such as enflurane (CHFCl–CF2–O–CHF2) or isoflurane (CF3–CHCl–O–CHF2) which are used as highly volatile narcotic gases. This work is arranged as follows. In the first part, the conformation of the ethers and the deprotonation enthalpy of the CH bonds is discussed. In the second part, the protonation of the molecules is investigated. In the last part, the interaction of the halogenated ethers with one water molecule is discussed. Our study involves the optimized geometries, harmonic frequencies of relevant vibrational modes along with a natural bond orbital analysis and the SAPT decomposition of the interaction energies. It must be noticed that except for the unsubstituted dimethyl ether, no experimental data on the acidity, basicity and hydrogen bonding ability of these molecules have been reported in the literature.

2. Computational methodology Full geometry optimizations were performed for the isolated molecules of the following ethers: CH3OCH3; CH3OCH2Cl; CH2ClOCH2Cl; CH3OCHCl2 and CHFClOCHF2. Subsequently, the optimized geometries, vibrational harmonic frequencies and infrared intensities were calculated for the most stable conformers of these ethers and their water complexes. In these calculations the MP2 method21 with the 6-311++G(d,p) basis set22,23 was used. Phys. Chem. Chem. Phys., 2010, 12, 13681–13691

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The interaction energy between the ethers and one water molecule was determined as the difference between the total energy of the complex and the energies of isolated monomers, obtained by the complete basis set (CBS) limit calculations at the MP2 level of theory (abbreviated as DEMP2/CBS). The CBS limit energies were derived by the separate extrapolation of the Hartree–Fock energy and the MP2 correlation energy using the cc-pVTZ and cc-pVQZ basis sets.24,25 The two-point extrapolation method of Halkier et al.26,27 was employed. To get detailed insight into the nature of the bonding in all complexes investigated, symmetry adapted perturbation theory SAPT28 calculations were carried out at the MP2/aug-cc-pVDZ level of theory using the MP2/6-311++G(d,p) optimized geometries. The proton affinity (PA) as well as the deprotonation energy (DPE) were calculated as the negative enthalpy (PA) and enthalpy (DPE) of reactions (1) and (2), respectively, assuming the standard conditions in the gas phase. B(g) + H(g)+ - BH(g)+ PA = DH298

(1)

where B = isolated ether molecule, AH(g) - A(g) + H(g)+ DPE = DH298

(2)

where AH = isolated ether molecule. A natural bond orbital (NBO) analysis29 was performed at both the MP2 and B3LYP levels of theory using the 6-311++G(d,p) basis set. The hyperconjugative interaction energies between the lone pairs of O and different sigma antibonding orbitals were estimated from the second order perturbation theory: E ð2Þ ¼ nsLP

hsLP jFjs i F2  ¼ nsLP sLPs es  esLP DE

ð3Þ

where hsLP|F|s*i is the Fock matrix element between the lone pair (sLP) and s* antibonding orbitals (NBOs), es* and esLP are the energies of s* and sLP orbitals, and nsLP is the population of the donor lone pair orbital.29 The SAPT calculations were carried out with the MOLPRO 2006 package,30 while all the other computations were performed with the Gaussian 03 set of programs.31

3. Results and discussion 3.1 Conformation of the halogenated ethers and acidity of the CH bonds In ether derivatives with the general formula ROCH2X, the gauche (synclinal) conformation is predicted to be more stable than the trans (antiperiplanar) structure.32–35 The structures of the most stable conformers of the isolated: CH3OCH3 (I); CH3OCH2Cl (II); CH2ClOCH2Cl (III); CH3OCHCl2 (IV) and CHFClOCHF2 (V) molecules are shown in Fig. 1. Relevant distances and selected NBO data are listed in Tables 1 and 2. The remaining NBO results are given in the Supplementary Information.w As stated in earlier works,6–13 the conformation of ethers is largely dominated by the anomeric effects or, in other words, by the hyperconjugation taking place from the two O lone pairs to the s*(CH) orbitals. The hyperconjugation energy is 13682

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markedly larger for the CH bonds in the gauche position than for the CH bonds in the trans position. In CH3OCH3 (I), the C1H6 distance is shorter than the C1H4 distance. Our results indicate that the E(2) value from the LP(2)O (having a pure p hybridization) to the s*(C1H6) orbital is zero, as expected. On the other hand, the E(2) value from LP(2)O to the s*(C1H4) orbital is equal to 8.49 kcal mol1 (Table 2). Further, in complexes II, III, IV and V, there is a very large delocalization from the LP(2)O to the s*(CCl) orbitals (between 17.25 and 23.07 kcal mol1) and a weaker delocalization from the LP(1)O to the s*(CCl) orbitals (between 0.66 and 4.43 kcal mol1). Another effect contributing to the stability of disubstituted derivatives is an electrostatic interaction between the non-bonded H and Cl atoms in cis-orientation, the H4  Cl7 (in III) and the H7  Cl4 (in IV) distances are relatively short, 2.718 and 2.731 A˚, respectively (see Fig. 1). In all the molecules, except for CHFClOCHF2 (V), the C1O2C3H9 atoms are coplanar, the dihedral angle d(1, 2, 3, 9) has values between 173 and 1801. However, in V, the C1O2C3H9 dihedral angle is equal to 131 but the other dihedral angles d(1, 2, 3, 7) and d(1, 2, 3, 8) are equal to 134 and 1091. This conformation does not correspond to a gauche (synclinal), or to a trans (antiperiplanar) conformation. A possible explanation for this special conformation is an electrostatic repulsion between the negatively charged F5 and Cl4 atoms implanted on the C1 atom and between the F7 and F8 atoms bonded to the C3 atom. Further, this conformation may also be favoured by an electrostatic interaction between the H9 atom and the F5 or Cl4 atoms, the intramolecular H9  F5 and H9  Cl4 distances being relatively short, 2.575 and 2.738 A˚, respectively. According to the NBO results, the s-character of the C at the H atoms has a tendency to increase with halogen substitution at the C atom or, in other words, with the positive charges on the H atoms in the CH bonds, in agreement with the Bent’s rule.36 For the C1H6 bond as for example, the s-character increases from 25.6% (I) to 28.9% (V), while the s*(C1H6) occupancy increases from 0.008 (I) to 0.037e (V) (see Table 1S in the ESIw). This suggests that the C1H6 bond length is largely dominated by the hybridization of the C atom, in agreement with recent theoretical studies.37,38 It is also worth noting that in agreement with the shortening of the CH bond resulting from halogen substitution, the corresponding n(CH) vibrational frequencies (indicated in Table 1) are predicted to increase. The average of the n(CH3) vibrational modes in I is equal to 3101 cm1. These frequencies increase up to 3233 cm1 n(C1H6) and 3205 cm1 n(C3H9) in V. As expected, the DPE decrease with the number on halogen atoms implanted on the C atoms. In agreement with the previous considerations, the DPE of the CH bond in the gauche position is larger than the DPE of the CH bond in the trans position. In V, however, the DPE of the C1H6 bond in the trans position is smaller than the DPE of the C3H9 bond in the gauche position. This may be due to the special structure of this molecule, as previously mentioned. A close inspection of the data shows that there is a rough correlation between the DPE values and the % s-character at the C atom.39 This journal is

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Fig. 1 Structures of the most stable conformers of the isolated CH3OCH3 (I), CH3OCH2Cl (II), CH2ClOCH2Cl (III), CH3OCHCl2 (IV) and CHFClOCHF2 (V) molecules. Distances are in angstroms, angles in degrees. Results from MP2/6-311++G(d,p) calculations. Table 1 Distances (r in A˚), deprotonation enthalpies (DPE, kcal mol1) of the CH bonds and n(CH) stretching vibrational frequencies (cm1) in isolated halogenated ethers. Results of MP2/6-311++G(d,p) calculations I

II

III

IV

V

Distances r(C1H4) 1.099 r(C1H6) 1.091 r(C3H7) 1.099 r(C3H8) 1.099 r(C3H9) 1.091 r(C1Cl4) — r(C1Cl5) — r(C1F5) —

1.095 1.087 1.097 1.094 1.089 — 1.808 —

1.091 1.086 — 1.091 1.086 — 1.794 —

— 1.086 1.092 1.092 1.088 — 1.795 —

— 1.085 — — 1.089 1.779

DPE C1H4 C1H6 C3H7 C3H9

392.5 395.4 399.7 403.1

384.3 387.1 384.3 387.1

— 376.4 397.1 397.6

— 366.7 — 373.3

c

c

413.1 413.6 413.1 413.6

n(CH) a

a

b

3235, 3139 3184 3091 3214, 3091 3028 3207, 3143, 3056e

nCH3 (C1) group.

3.2

b

b

nCHHCl. c nC1H6.

d

3215

1.362

nC3H9. e nCH3 (C3).

Fig. 2 illustrates the structures of the O-protonated molecules. Relevant CH, CX and CO distances in the O-protonated species are indicated in Table 3. This Table also reports the PA of the O and Cl atoms. Selected NBO parameters are listed in the ESI (Tables 2S and 3Sw). c

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LP(1)Os*(C1H4) s*(C1H6) s*(C3H7) s*(C3H8) s*(C3H9) s*(C1Cl5) s*(C1F5) s*(C3F7) LP(2)O s*(C1H4) s*(C1H6) s*(C3H8) s*(C1Cl5) s*(C1F5) s*(C3F7) a

I

II

III

IV

V

0.98 3.31 0.93 0.98 3.31 — — —

3.62 3.61 2.18 0.00 2.99 0.66 — —

1.61 3.92 — 1.61 3.92 2.28 — —

— 5.21 0.76 0.76 3.28 4.43 — —

— 4.39 — — 3.27 3.47a 2.68 3.53

8.49 0.00 8.49 — — —

5.57 0.93 8.24 23.07 — —

6.60 0.00 6.60 17.25 — —

— 0.00 6.60 17.91 — —

— 0.00 — 14.88b 15.93 8.71

E(2) from LP(1)O to s*(C1Cl4).

b

E(2) from LP(2)O to s*(C1Cl4).

3233 3205d

Protonation of the halogenated ethers

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Table 2 E(2) energies (kcal mol1) from two lone pairs of O to the s* orbitals in the isolated molecules

Protonation of cyclic and acyclic ethers usually results in a marked elongation of the CO bond along with an increase of the COC angle.13,18,33,40,41 The same effect is observed in the molecules shown in Fig. 2. The C1O2C3 angle varies between 116.41 for I(H+) and 119.81 for V(H+), while for the corresponding neutral molecules this angle is smaller by ca. 6 to 41. The C1O2C3H+ dihedral angle takes values between 126.6 and 131.31. As indicated in Fig. 2, in protonated V(H+), the H9  F5 and H9  Cl4 distances are markedly longer than the corresponding distances in the neutral molecule (V). Phys. Chem. Chem. Phys., 2010, 12, 13681–13691

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Fig. 2 Structures of O-protonated I(H+), II(H+), III(H+), IV(H+), V(H+) species. Distances are in angstroms, angles in degrees. Results from MP2/6-311++G(d,p) calculations.

Table 3 Distances (A˚), vibrational frequencies n(CH) and n(OH+) (cm1) in protonated halogenated ethers. Proton affinity (PA) of the O and Cl atoms (kcal mol1). The numbers in parentheses indicate the differences between the protonated and isolated molecules. Calculations performed at the MP2/6-311++G(d,p) level

Distances r(C1H4) r(C1H6) r(C3H8) r(C3H9) r(C1Cl4) r(C1Cl5) r(C3X)a r(C1O2) r(O2C3) r(OH+) n(CH) Dn n(OH+) PA(O) PA(Cl)

I(H+)

II(H+)

III(H+)

IV(H+)

V(H+)

1.087(0.013) 1.087(0.003) 1.089(0.010) 1.087(0.003) — — — 1.488(+0.077) 1.488(+0.077) 0.973 3270, 3256, 3125b (+156)g 3745 189.9 —

1.087(0.008) 1.087(0.000) 1.088(0.006) 1.087(0.002) — 1.732(0.076) — 1.492(+0.116) 1.495(+0.073) 0.976 3261b, 3171c (+64)g 3698 180.9 178.9

1.088 (0.003) 1.088(+0.002) 1.086(0.005) 1.087(+0.001) — 1.718(0.076) 1.732(0.062) 1.518(+0.125) 1.479(+0.086) 0.978 3256, 3160c (+21)g 3678 171.6 170.9

— 1.088(+0.002) — 1.085(0.003) 1.719(0.076) 1.733(0.062) — 1.524(+0.165) 1.500(+0.069) 0.977 3207d (8)d 3691 173.6 184.1

— 1.089(+0.003) — 1.087(0.002) 1.709(0.007) — 1.308 (0.006) 1.523(+0.151) 1.532(+0.145) 0.982 3251e, 3212f (+46)e, (22)f 3633 155.3 162.5

a X = Cl7 for III, X = F7 for V. n(CH2) vibrations.

b

n(CH3) vibrations. c C1HHCl group.

In the protonated ethers, the LP(2) of the O atom is tied up in the OH+ bond, as a consequence, the hyperconjugation from this lone pair to the different s* orbitals of the molecules vanishes. Larger effects are expected for the CH bonds in the gauche position. The data of Table 3 show that in the protonated I(H+) and II(H+) molecules, the contraction of the CH bonds in the gauche position (C1H4 or C3H8) is 13684

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d

CHClCl group. e C3H9. f C1H6.

g

Average shift of the n(CH3) or

markedly larger than the contraction of the C1H6 or C3H9 bonds in the trans position. The small contraction of the C1H6 or C3H9 bonds can be explained by a decrease of the hyperconjugation from the LP(1)O to the corresponding s*(CH) orbitals (Table 2S in the ESIw). It is also worth noticing that protonation leads to an asymmetry of the molecules. This can be illustrated in the This journal is

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case of molecule III which is totally symmetrical in the neutral form. The two CO bond lengths and the H8  Cl5 and H4  Cl7 bond lengths are both equal. In the protonated III(H+) molecule, two CO distances and the H8  Cl5 and H6  Cl7 distances become different. Further, in the neutral form, the C–Cl distances in the two CHHCl groups are the same (1.794 A˚), whereas in the protonated form, the C1Cl5 and C3Cl7 distances are shortened to 1.718 and 1.732 A˚, respectively. In agreement with the variation of the CH distances, the n(CH3) or n(CH2Cl) vibrations are blue-shifted by 150 and 64 cm1 in I(H+) or III(H+). In protonated V(H+), the n(C3H9) vibration is blue-shifted by 46 cm1 but the n(C1H6) vibration is red-shifted by 22 cm1, in agreement with the small elongation of the C1H6 bond. The PA values for O-protonation range between 189.9 and 155.3 kcal mol1 and, as expected, they decrease with the number of halogen atoms implanted of the ether derivatives. The PA of V is very similar to the PA of enflurane (154.5 kcal mol1).42 The PAs for Cl-protonation range between 184.1 and 162.5 kcal mol1. It must be noticed that protonation at the Cl atoms leads to a cleavage of the CCl bond and formation of an ion-dipole complex. The same feature has been observed for the enflurane molecule where protonation at the F atoms leads to a cleavage of the CF bond and formation of an ion–dipole complex.42 Similarly, protonation at the F atom of F2NH also leads to a break down of the NF bond.43 As expected from the basicity of the molecules, the OH+ distances slightly increase from 0.973 to 0.982 A˚ and the n(OH+) decrease from 3745 to 3633 cm1 upon halogen substitution. 3.3

Interaction between the halogenated ethers and water

3.3.1 Structures and binding energies. Several stable structures are found on the potential energy surface of the complexes between the investigated halogenated ethers and water. Among all of stable 18 structures three types of complexes have been distinguished, and they are denoted as a, b and c. In the ‘‘a’’ type complex, water is bonded to the oxygen atom of ether. In the ‘‘b’’ type complex, water binds to other atoms of ether through intermolecular Ow  H and (Hw  X) bonds (there are usually two such structures, denoted as 1 and 2). In the ‘‘c’’ type complex, water is involved in intermolecular halogen bond (Ow  Cl). These structures are illustrated in Fig. 3–5. The MP2/CBS binding energies and the SAPT decomposition of the energies are given in Table 4. The most stable complex between dimethylether and water (Ia) is found when the OH bond of water interacts with the O atom of ether.18,20 The calculated MP2/CBS stabilization energy of this complex equals 7.71 kcal mol1. Among six a-type complexes, the IVa2 (Fig. 4) is the weakest one, the interaction energy is 5.33 kcal mol1 (Table 4). Of the b-type complexes the most stable is IIIb2 (Fig. 4). In this complex, the water molecule is involved in two intermolecular hydrogen bonds, H4  Ow and Hw  Cl7. The MP2/CBS stabilization energy of this complex equals 5.86 kcal mol1. A stable complex of dimethylether (Ib), This journal is

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not described in previous works, is characterized by a C1H4  Ow interaction. The large H4  O distance of 2.870 A˚ is at the limit of a classical hydrogen bond, and the binding energy is low (2.24 kcal mol1). The weak proton donor ability of the CH bond has also been suggested by the fact that despite its large DPE (413 kcal mol1), it is able to form weak cyclic homodimers characterized by CH  O hydrogen bonds.18,43–45 In this cyclic dimer stabilized by three hydrogen bonds, the intermolecular distances calculated at the MP2/6-311++G(d,p) level are between 2.52 and 2.60 A˚ and the binding energy including the BSSE-correction is equal to 2.30 kcal mol1.45 The SAPT analysis (Table 4) shows that in the a- and b-type complexes, the electrostatic term represents about 60% of the total attraction forces, while the induction and dispersion components represent about 8 and 30%, respectively. The c-type complexes provide interesting examples of Ow  Cl halogen bonding. It is increasingly recognized that halogen bonding occurs in various biological systems and can be utilized effectively in anaesthetics.5,46 The Ow  Cl interaction is rather weak and the binding energy varies between 1.67 and 2.64 kcal mol1. The Ow  Cl distances are relatively long, between 3.145 and 3.038 A˚. As follows from Table 4, the contribution of the dispersion term to the total attraction force is about 40%, which is nearly twice as large as in the a-type complexes. The electrostatic and induction term represents about 50 and 10%, respectively. Let us mention that a similar O  Cl intermolecular bond has been recently found for the interaction between dimethylether and halothane, the O  Cl distance being equal to 2.99 A˚ and the binding energy 2.60 kcal mol1.46 The next step is to discuss the bonding trends as a function of the acidity and basicity of the centres involved in the interaction. The results of the present work show that when the PA of the O atoms of the ethers ranges between 189.9 and 173.6 kcal mol1 (Table 3), stable structures characterized by OwHw  O bonds (Ia, IIa, IVa2) are predicted. When the PA of the ether derivative is as low as 155 kcal mol1, only a cyclic structure involving the O atom can be formed (Va). It must be remembered that these PA values are still larger than the PA value of the O atom of water (165 kcal mol1). The binding energies (7.71, 6.74 and 5.33 kcal mol1) are ordered according to the PA of the ethers (189.9, 180,9 and 173.6 kcal mol1). The same remark also holds for the other parameters such as the intermolecular distances, the elongation of the OwHw bond, the red shift of the n(OwHw) vibration and the charge transfer from the ether to the water molecule. When the DPE of the CH bond varies between 413.1 and 366.7 kcal mol1 (Table 1) the ether derivative and water can be bonded by CH  Ow hydrogen bonds. This is the case of structures Ib, IIb1, IVb1, Vb1 and Vb2. The binding energies vary between 2.24 and 6.31 kcal mol1 and the intermolecular H  Ow distances decrease from 2.87 to 2.234 A˚. The binding energies are ordered according to the DPE values and follow the equation: EHB = 0.087 DPE + 38.2 (r = 0.963)

(1)

Let us mention that for the carbon-to-carbon proton transfer reaction in substituted carbanions, (CH  C), the slope of Phys. Chem. Chem. Phys., 2010, 12, 13681–13691

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Fig. 3 Structures of complexes of I and II with water. Distances are in angstroms, angles in degrees. The dotted lines indicate the interatomic distances between the Ow or Hw atoms and other atoms (distances smaller than the sum of the corresponding van der Waals radii). Calculations performed at the MP2/6-311++G(d,p) level.

eqn (1) corresponding to higher binding energies is larger (0.12).47 When the DPE values range between 366.7 and 387.1 kcal mol1 and the PA values between 155.3 and 173.6 kcal mol1 cyclic complexes are formed, wherein both water and ethers act simultaneously as both donor and acceptor (IIIa, IVa1, Va). These complexes are characterized by very similar binding energies ranging between 6 and 6.5 kcal mol1. Of these cyclic complexes, the most stable (Va) is formed at the O atom of the ether derivative having the lowest PA and the less stable complex (IIIa) is formed at the O atom having the highest PA. The same remark also holds for the cyclic structures involving the Cl atom. The most stable complex is formed at the Cl atom having the lowest PA and the lowest DPE. This suggests that the acidity of the CH bond is the main factor in determining the hydrogen bond energies of these closed structures. This results agrees with those reported in previous works showing that the acidity of the proton donor is the main factor in determining the hydrogen bond energies.47–52 3.3.2 Variations of the CH and OH distances, vibrational frequencies and NBO data. In order to have a better insight into their characteristic properties, the different complexes will be discussed separately. 13686

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Table 5 reports relevant CH distances and the OwHw distances in complexes characterized by an OwHw  O interaction (Ia, IIa and IVa2). This Table reports also the intermolecular hyperconjugation energies occurring from these lone pairs to the s*(OwHw) orbitals of water. Owing to the partial occupancy of one of the lone pair of the O atom of the ethers, smaller perturbations than in the protonated systems are expected. Our results show that the formation of the OwHw  O hydrogen bonds results in a contraction of all the CH bonds in the gauche position and the contraction of the bonds in the trans position being very small. Interestingly, in Ia, the intermolecular hyperconjugation energy to the s*(OwHw) orbital is larger when taking place from the LP(2)O (6.11 kcal mol1) than when occurring from the LP(1)O (3.87 kcal mol1). A reverse trend is observed for the two other complexes where the intermolecular energy transfer from the LP(2)O to the s*(OwHw) orbital of water is nearly zero. The large difference between the complexes is also illustrated by the values of the C1O2C3Hw dihedral angles which is equal to 127.81 in Ia, 178.61 in IIa and 172.31 in IVa2. In accordance with the contraction of the CH bonds, the n(CH) vibrations are blue-shifted by 6 to 25 cm1 at the exception of the n(C1H6) vibration in IVa2 which is red-shifted by 4 cm1. This red shift probably results from the small elongation of the C1H6 bond. This journal is

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Fig. 4 Structures of complexes of III and IV with water. Distances are in angstroms, angles in degrees. The dotted lines indicate the interatomic distances between the Ow or Hw atoms and other atoms. Calculations performed at the MP2/6-311++G(d,p) level.

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Fig. 5 Structures of complexes of V with water. Distances are in angstroms, angles in degrees. The dotted lines indicate the interatomic distances between the Ow or Hw atoms and other atoms. Calculations performed at the MP2/6-311++G(d,p) level.

Table 4 SAPT decomposition of the interaction energies between ethers and water (in kcal mol1) calculated at the MP2/aug-cc-pVDZ level (DESAPT) and interaction energies calculated at the MP2/CBS level (DEMP2/CBS)

Ia Ib IIa IIb1 IIb2 IIIa IIIb1 IIIb2 IIIc IVa1 IVa2 IVb1 IVb2 IVc Va Vb1 Vb2 Vc

E(elec.)

E(exch.)

E(ind.)

E(disp.)

DESAPT

DEMP2/CBS

10.11 1.29 7.78 2.17 4.85 6.42 4.78 5.69 0.90 6.85 6.22 2.49 3.71 1.49 6.58 5.75 5.38 2.24

9.68 1.54 7.39 1.96 4.24 6.48 4.08 5.63 1.81 6.67 6.80 2.18 3.86 2.21 5.16 4.28 4.49 2.35

1.88 0.21 1.31 0.27 0.70 1.04 0.61 0.82 0.27 1.05 1.20 0.30 0.67 0.32 0.78 0.66 0.65 0.36

2.83 1.17 2.67 1.31 2.17 2.67 2.09 2.63 1.08 2.80 2.50 1.39 2.47 1.22 2.43 1.96 2.20 1.25

5.14 1.13 4.37 1.79 3.48 3.65 3.40 3.51 0.44 4.03 3.11 2.00 2.99 0.82 4.61 4.09 3.74 1.49

7.71 2.24 6.74 3.01 5.52 6.01 5.28 5.86 1.67 6.37 5.33 3.35 5.46 2.09 6.50 5.63 5.49 2.64

It must be noticed that blue or red shifts can be predicted by comparing the equilibrium structure and vibrational data of a given complex with its protonated counterpart.53 This condition based on a positive or negative intramolecular coupling mechanism53 is fulfilled in the present case. Indeed, in the three OwHw  O complexes, the CH bond is not involved in the interaction and there is a contraction of this bond smaller by a factor between 4 and 6 than in the protonated species. Table 6 lists the CH distances, n(CH) vibrational frequencies for the complexes Ib, IIb1, IVb1, Vb1 and Vb2 where the ether 13688

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and water molecules are held together by CH  Ow hydrogen bonds. No important variations of the parameters for the other CH bonds are predicted by our calculations. The results show that here we are dealing with typical blue-shifted hydrogen bonds,54 namely a contraction of the CH bonds, between 0.0016 and 0.0029 A˚, and blue shifts of the corresponding n(CH) vibration, ranging from 20 to 44 cm1. In the Vb2 complex, there is an infrared intensity decrease of the n(C3H9) vibration from 14.6 km mol1 in the isolated molecule to 4.3 km mol1 in the complex. In the Vb1 complex however, the infrared intensity of the n(C1H6) vibration increases slightly from 5.8 km mol1 to 8.0 km mol1 in the complex. Relevant characteristics of cyclic complexes where the OwHw bond of water acts as a proton donor and as a proton acceptor are listed in Table 7. The intermolecular Hw  O and (C)H6  Ow distances show that on going from the IIIa to Va complexes, the OwHw  O bond becomes weaker and the (C)H6  Ow bond becomes stronger. In agreement with these geometric data, the contraction of the C1H6 bond and the blue shift of the n(C1H6) vibration are larger in the Va complex. Further, the change of the OwHw  O hydrogen bond strength in Va is also illustrated by the smaller elongation of the OwHw bond and frequency shift of the corresponding stretching vibration along with the smaller hyperconjugation energy from the LP(1)O to the s*(OwHw) orbital. The charge transfer from the ether moiety to water is small in the two first complexes but completely vanishes in the Va complex. The C1O2C3Hw dihedral angles are also similar (between 173.1 and 177.51). For all the complexes where the OwHw bond of water is involved in the interaction, our calculations reveal a linear correlation between the elongation of the OwHw bond and the This journal is

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Table 5 CH and OwHw distances (A˚), vibrational frequencies (cm1) and relevant NBO data (E(2) in kcal mol1, natural charges in e for structures characterized by OH  O hydrogen bond. The numbers in parentheses indicate the differences between the complexes with water and the isolated molecules. Calculations performed at the MP2/6-311++G(d,p) level

r(C3H7) r(C3H8) r(OwHw) LP(1)O -s*(OwHw) LP(2)O -s*(OwHw) Sq(H2O) n(OH) n(CH)

a e

Ia

IIa

IVa2

1.097 (0.002) 1.098 (0.002) 0.969 (+0.010) 3.87 6.11 0.017 3974 (30) 3728 (157) 3192 (+10)a 3115 (+25) 3042 (+14)

1.096 (0.001) 1.094 (0.000) 0.966 (+0.006) 5.28 0.18 0.005 3968 (36) 3804 (81) 3222 (+8)b 3107 (+16)d

1.092 (0.001) 1.091 (0.001) 0.964 (+0.005) 5.26 0.10 0.007 3972 (32) 3822 (63) 3211 (4)c 3172 (+6)e

Average of the in-phase and out-of-phase vibrations. Average of the n(CH3) vibrations.

b

n(C1H4H6) vibrations. c n(C1H6) vibration.

d

Average of the n(CH3) vibrations.

Table 6 CH distances (A) and relevant NBO data (charges in e, E(2) in kcal mol1, vibrational frequencies in cm1) in CH  Ow complexes. The numbers in parentheses indicate the differences between the complexes with water and the isolated molecules. Calculations performed at the MP2/6-311++G(d,p) level

r(CH) Sq(H2O) n(CH) a

C1H5  Ow.

b

Iba

IIb1b

IVb1c

Vb1d

Vb2e

1.098 (0.002) 0.001 3202(+20)f

1.095 (0.002) 0.002 3218(+21)f

1.086 (0.002) 0.003 3247(+34)

1.028 (0.003) 0.002 3274(+41)

1.086 (0.003) 0.002 3249(+44)

C3H7  Ow. c C3H9  Ow.

d

C1H6  Ow. e C3H9  Ow. f Average of the CH3 frequencies.

Table 7 Distances (A˚), NBO data (charges in e, E(2) in kcal mol1, vibrational frequencies (in cm1) in cyclic OwHw  O  HC complexes. The numbers in parentheses indicate the differences between the the complexes with water and the isolated molecules. Calculations performed at the MP2/6-311++G(d,p) level IIIa r(C1H6) r(OwHw)

IVa1

1.086(0.000) 1.086(0.000) 1.084(0.002) 0.964(+0.005) 0.964(+0.004) 0.962(+0.002)

LP(1)O - s*(OwHw) 3.36 LP(2)O - s*(OwHw) 0.06 0.004

Sq(H2O)

a

3194(+7)

n(CH) a

Va

2.80 0

0.24 0

0.003

0

3225(+10)

3261(+28)

Average of the CH2 frequencies.

Fig. 6 Dr(OwHw) (A˚) as a function of E(2) [LPO - s*(OwHw)] (kcal mol1).

hyperconjugation energy from the two LPs of O to the s*(OwHw) orbital (Fig. 6): Dr(OwHw) = 8.0  104 [E(2)(LPO-s*(OwHw)] + 1.6  104 (r = 0.983)

(4)

Finally, it is interesting to compare the C1H6 distances and the corresponding blue shifts in the Vb1 and Va complexes, the C1H6 bond being involved in the interaction in both structures. In the Vb1 complex, where water acts as a proton acceptor, the C1H6 bond is contracted by 0.0026 A˚ and the corresponding blue shift is 41 cm1. In the Va complex, where both the water and ether molecules are acting as proton donor and proton acceptor, the contraction of the CH bond is 0.0015 A˚ and the corresponding blue shift is 28 cm1. This journal is

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This corresponds to the structural and spectroscopic ‘‘signature’’ discussed by Scheiner and Kar.55 When H2O acts as a proton acceptor in its interaction with H2CQO, the CH  Ow bond is contracted by 0.0031 A˚ and the blue shift is 52 cm1. When H2CQO and H2O act both as proton donor and acceptor forming a cyclic complex, the contraction of the CH bond decreases to 0.0019 A˚ and the blue shift is equal to 43 cm1. Further, when H2O interacts on the O atom of H2CQO, the CH bonds not involved in the interaction are contracted by 0.0015 A˚ and the corresponding blue shifts of the two n(CH2) vibrations are equal to 18 and 26 cm1.55 Although not discussed in ref. 55, this last effect may be ascribed to a decrease of the delocalization of the O LPs of H2CQO to the s*(CH) orbitals. Phys. Chem. Chem. Phys., 2010, 12, 13681–13691

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3.3.3

O  Cl interaction

The structures IIIc, IVc and Vc are an interesting case of O  Cl halogen bonding discussed in several theoretical reports.56–72 The studies of the electrostatic potentials of the halogen bonding systems show that the lone pairs of the halogen atom bound to the carbon form a belt of negative electrostatic potential around its central part leaving the outermost region positive, called s-hole.58,59 The halogen bonding was explained as a noncovalent interaction between a covalently bound halogen on one molecule and a negative site of another.62–65 Riley and Hobza have shown that the nature of the halogen bonds are dominated by the electrostatic and dispersion contributions of the interaction energy.66–68 Similar results were obtained in this work, see section 3.3.1. Palusiak stated that the HOMO/LUMO charge transfer and polarization are the dominant terms of the stabilization energy of halogen bonded complexes, based on the Kohn–Sham approach.69 According to our results, the overall charge transfer occurring from H2O to ethers in IIIc, IVc and Vc complexes is very weak (0.001, 0.002 and 0.002, respectively). From these results it is clear that the charge transfer can not be responsible for stabilization of these complexes. The results indicate that the interaction results in a very small increase of the polarity of the OwHw bond of water and a larger increase of the polarity of the C–Cl bond involved in the interaction, the Cl atom loosing ca. 0.03 e and the Cl atom gaining ca. 0.015 e, leading to an increase of the polarity of the Hw+Ow  Cl+C bonds. It is worth noticing that in the three complexes, the interaction with water results in a contraction of ca. 0.010 A˚ of the C–Cl bond involved in the Ow  C–Cl interaction. This should lead to a blue shift of the C–Cl vibration. Owing to strong coupling with other vibrational modes, the blue shift could not be predicted by our calculations. It must be noted that blue- as well as red-shifts have been predicted for halogen bonding70 and for the complex between dimethylether and trifluoromethyl chloride, a very weak blue shift of the C–Cl vibration has been observed experimentally.19 Further, an interesting study based on the electronic charge density has shown that for the interaction between water and mixed halogens, there is, for the O atom, a greater tendency of binding to the hydrogen atom instead of binding to the halogen atom,72 in good agreement with the results of the present work. It is also worth noting that the interaction between fluorinated dimethylethers (nF = 1 to 5) did not reveal any Ow  F halogen bonding.20 This is in good agreement with the s-hole theory, showing that the most powerful anaesthetics have strong positive potentials associated with their hydrogens, chlorines and bromines, but not fluorines.

4. Concluding remarks The present work deals with a theoretical study of the conformation and protonation of CH3OCH3 and halogenated ethers (CH3OCH2Cl, CH2ClOCH2Cl, CH3OCHCl2, CHFClOCHF2). The interaction between these ethers and one water molecule is investigated as well. The calculations carried out by the MP2 method with the 6-311++G(d,p) basis set include the optimization of the geometries, the SAPT 13690

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decomposition of the interaction energies along with a natural bond orbital analysis. The main conclusions of our work are the following ones: 1. The conformation of the investigated ethers is governed by anomeric effects and by electrostatic attraction or repulsion between non-bonded atoms. 2. Protonation at the O atom of the ethers results in large contraction of the CH bonds and blue shifts of the corresponding n(CH) vibrations. Protonation also induces an asymmetry of the CH2ClOCH2Cl molecule. 3. Several stable structures are found on the potential energy surface of the complexes between the ethers and water. Depending on the PA of the O atom and the DPE of the CH bond, the structures involving the O atom of the ethers as a proton acceptor, the CH bond of the ethers as proton donor and cyclic complexes, where water acts both as a proton donor and proton acceptor, are formed. Contraction of the CH bonds and blue shifts are predicted even when the CH bond is not involved in the interaction with water. 4. The MP2/CBS calculated binding energies of the halogenated ethers and water complexes vary between 1.7 and 7.7 kcal mol1. 5. Weak O  Cl halogen bonding between the O atom of water and the Cl atom of the ethers are also predicted by our calculations.

Acknowledgements This work was supported in part by Wroclaw University of Technology. The generous computer time from the Poznan Supercomputer and Networking Center as well as Wroclaw Supercomputer and Networking Center is acknowledged.

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