LUMINESCENT AND PHYSICAL-CHEMICAL PROPERTIES OF. BENZAMIDES. V. Ya. Artyukhov and A. V. Morev. UDC 539.196:541.14. The effect of a halogen ...
Russian Physics Journal, Vol. 46, No. 5, 2003
QUANTUM-CHEMICAL INVESTIGATION OF SPECTRALLUMINESCENT AND PHYSICAL-CHEMICAL PROPERTIES OF BENZAMIDES V. Ya. Artyukhov and A. V. Morev
UDC 539.196:541.14
The effect of a halogen atom in orthoposition of a phenyl radical on the spectral, geometrical, and photophysical characteristics as well as on the proton-donor and proton-acceptor properties of the 2-Cl- and 2-Br-benzamides was studied by electron spectroscopy and quantum chemistry. The intersystem crossing is shown to be the major decay pathway for lower singlet states. A comparative analysis of the bond populations of free molecules of benzamide and its halogen derivatives suggests that NH bonds of an amino group are nonequivalent in the amides under study. INTRODUCTION The use of a quantum-chemical method – intermediate neglect of differential overlap (INDO) with spectroscopic parametrization [1] – for studying spectral-luminescent and photophysical properties and intermolecular interactions is of practical interest since it is a component part of a complex experimental and theoretical approach to the problems of photonics of organic molecules [2]. In [3, 4], the effect of the position and number of a chlorine atom on the spectral, geometrical, and photophysical properties of free molecules of chloroanilines is studied in detail. It is shown that the long-wave absorption band of chloroanilines is formed by several electronic transitions of varying nature and intensity. The C−Cl bonds that weaken under excitation, participate in the electronically excited states of the πσ*-type. According to the quantum-chemical calculations, fluorescence quenching of chloroanilines is due to the fact that the intersystem crossing is more effective than the rate of radiation decay of the S1 state. Amides are chosen as an object of investigation owing to their wide use in medicine [5]. An introduction of a polar carbonyl group to the space between an amino group and a phenyl radical brings about changes in the dynamic and electrooptical equivalence of the NH bonds of a benzamide amino group [6], which determines different proton-donor properties of hydrogen atoms of the amino group. The presence of several functional groups possessing clearly pronounced proton-acceptor properties in benzamides allows these molecules to be considered as proton acceptors in the intermolecular H-bond. These facts open new aspects in qualitative agreement of proton-donor and proton-acceptor properties of amides and anilines. The objective of the work is to study the structural features of benzamide, 2-Cl- , and 2-Br-benzamide molecules by electron spectroscopy and quantum chemistry and to compare their different physical-chemical properties. EXPERIMENTAL TECHNIQUE AND CALCULATION PROCEDURE The absorption spectra of benzamide and its halogen derivatives were recorded by means of an SF26 spectrophotometer. Isooctane was used as a solvent. To find the maxima of the overlapping absorption bands, use was made of the method of the second derivative [7]. The accuracy of determination of the absorption band maxima was ±1 nm. The oscillator strengths of electronic transitions were not found due to poor solubility of benzamides in nonpolar solvents.
V. D. Kuznetsov Siberian Physical-Technical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 49–53, May, 2003. Original article submitted October 25, 2002. 488
1064-8887/03/4605-0488$25.00 2003 Plenum Publishing Corporation
TABLE 1. Characteristics of Electronic Transitions for Free Molecules of Benzamides Compound
Benzamide
2-Сl-benzamide
2-Br-benzamide
State S1(nπ*) S2(ππ*) S3(ππ*) S1(nπ*) S2(ππ*) S3(nσ*) S4(nσ*) S5(ππ*) S1(nπ*) S2(ππ*) S3(nσ*) S4(nσ*) S5(ππ*)
Energy, cm–1 Calc. 33360 37910 44960 33660 36930 38350 41970 43920 33850 36740 38200 42410 43720
Exp. 37310 44540 36290
42920 35970
42830
fcalc
Polarization
0.001 0.005 0.181 0.001 0.008 0.005 0.001 0.171 0.001 0.011 0.006 0.001 0.147
Z X X Z Y Z Z X, Y Z Y Z Z Y, X
We have calculated the energies of singlet and triplet states, oscillator strengths of electronic transitions, chemical bond populations, atomic charges, and other characteristics for free molecules of amides by the INDO method with spectroscopic parametrization [1]. The rate constants of intramolecular physical processes (internal and singlet-triplet conversions) were assessed following [2]. The method of molecular electrostatic potential (MEP) [2] was used to evaluate proton-acceptor properties of different functional groups of benzamide and its halogen derivatives. The method helps to calculate the electrostatic nuclear interaction energy and electronic distribution of molecules with a positive point charge put at a given point of space surrounding the molecule. The data on the geometry of benzamides of interest were taken from [6, 8]. The plane of the molecules was in the XY plane, with the major axis coinciding with the X axis of the Cartesian coordinate system. RESULTS AND DISCUSSION Table 1 gives the results of calculation of benzamide, 2-Cl-, and 2-Br-benzamides, and Fig. 1 − energy-level diagrams of electronically excited states of these molecules. It is worthwhile to start discussing photophysical processes in these compounds with a benzamide molecule. It follows from the calculation (Fig. 1) that the lower singlet state in this molecule is the state of the nπ-type. An analysis of the expansion of molecular orbitals (MO) in terms of atomic orbitals (AO) shows that the n-orbital is largely localized on carbonyl oxygen. The S0→S2 and S0→S3 transitions are due to the MO of the π-type and are clearly pronounced π→π* transitions. An intensive absorption of benzamide results from the allowed S0→S3 transition (Table 1) followed by the internal conversion (Fig. 1). As a result, the molecule is in the S2(ππ*) state. The excitation of S0→S2 will lead to occupying the T4 state by means of the ST-conversion. T4 decays by the internal conversion T4→T3. The T3 state is of the ππ* orbital nature, therefore it will decay by the TS-conversion T4(ππ*)→S1(nπ*) more effectively than by the internal conversion T3→T1. The calculations suggest that the singlet-triplet conversion is the main decay pathway for the S1 state. An analysis of the MO of 2-Cl- and 2-Br-benzamides and comparison with the MO of benzamide show that the atomic orbitals of chlorine and bromine participate in the formation of the upper populated π-MО and third free σ*-МО. This brings about singlet and triplet states of the πσ*-type, whose σ*-orbital is partially localized on the С−Hal ( S3 and T3 states) bond, in the electron absorption spectra of free molecules of halogen derivatives of benzamide. It is seen from Table 1 that the substitution by chlorine and bromine in benzamide results in the shift into the longwavelength region. As in the benzamide absorption spectrum, the absorption band intensity of halogen derivatives at 489
E⋅103, cm–1 46 S3(ππ)
1010
44
T6(ππ) 42
kr=2,6⋅108
10
40
10
36 kr=5,1⋅106 34
9
10
10
11
S3(πσ) T4(ππ) 10 T3(ππ) S2(ππ)
107
12
10
S5(ππ) 1010
5⋅108
32 1⋅1010 30 28
9
7⋅109 0
kr=4,4⋅105
T1(ππ)
H
O
T5(ππ) 1013 T4(ππ)
2⋅108
108 T3(πσ) 9⋅108 1010 T2(nπ) 1⋅1010
kr=3,4⋅105
T1(ππ) H
N C
1010
1010
T3(πσ) S1(nπ) 108 T2(nπ) kr=1,0⋅107
2⋅109
1012
107
H
N C
S3(πσ)
108
1⋅108
H O
1011 T6(πσ)
T5(ππ) S2(ππ) 1013 T4(ππ)
kr=8,2⋅106
T7(ππ)
1011
1010
9
T2(nπ) T1(ππ)
kr=7,3⋅105
S1(nπ)
109
T6(πσ)
1010
10 108
1011
11
107
10
S1(nπ)
T7(ππ) S4(πσ)
T5(πσ) S4(πσ)
10
38 S2(ππ)
S5(ππ)
H Cl
O
N C
H Br
Fig. 1. Energy-level diagram of electronically excited states and rate constants of photoprocesses (s–1) in the molecules of amides under study (kr is the rate constant of radiation decay). 260−270 nm is due to the S0→S2(ππ*) transition. A certain contribution to the intensity of this band is made by the electronic transition to the S3(πσ*) state, whose oscillator strength is about an order of magnitude lower than that of the S0→S2(ππ*) transition. The absorption intensity in the middle region is largely formed by the S0→S5(ππ*) state much in the nature, energy, and polarization with the S0→S3(ππ*) transition in the nonsubstituted benzamide (Table 1). Let us consider the influence of substitution on the dynamics of photophysical processes. The calculations show that the contribution of AO of halogen atoms to MO participating in the formation of the S3 and T3 states of halogensubstituted benzamides results in the formation of new deactivation pathways absent in benzamide − S2(ππ*)→T3(πσ*) and T5(πσ*)→T3(πσ*) conversion. An increase in the efficiency of going of excited molecules to the triplet-state channel results from an increase in the excitation energy, which allows us to assume a decrease in the quantum yield of fluorescence in 2Cl- and 2-Br-benzamides even in the absence of phototransformations . Of interest is the NH-bond population of the amino group (electron density, РNH) of the N−H bonds [2], responsible for the strength of the NH bond of the benzamide amino group (Table 2), since primary amides in solutions, in contrast to anilines, form two types of 1 : 1 H-bonded complexes with proton acceptors. The difference in the complexes is due to the nonequivalence of hydrogen atoms of an amino group, this nonequivalence, in turn, resulting from an asymmetric position of the NH bonds of the amide amino group with respect to the rest of the molecule. In this case, one of the bonds undergoes polarization effect of a carbonyl group (cis-NH bond), and the other experiences no action of the kind (trans-NH bond). The calculations also suggest that the trans-NH bond in different electron states of the benzamides under
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TABLE 2. Population of Benzamide Molecules in Different Electron States Compound Benzamide
2-Сl-benzamide
2-Br-benzamide
State S0 S1(nπ*) S2(ππ*) S0 S1(nπ*) S2(ππ*) S0 S1(nπ*) S2(ππ*)
Trans-NH-bond 0.720 0.722 0.720 0.724 0.727 0.724 0.723 0.726 0.723
Population of the РАВ bond, е Cis-NH-bond С=O 0.713 0.857 0.716 0.859 0.713 0.822 0.715 0.845 0.719 0.840 0.715 0.812 0.715 0.848 0.719 0.830 0.715 0.812
C−Hal
0.445 0.433 0.460 0.496 0.480 0.517
TABLE 3. The MEP Values (kJ/mol) for Benzamide Molecules in Different Electronic States Compound
Atom
Benzamide
O O Сl O Br
2-Сl-benzamide 2-Br-benzamide
S0 −722 −702 −62 −670 −60
S1(nπ*) −36 −104 −99
State S2(ππ*) −786 −761 −34 −745 −29
S3(ππ*) or S5(ππ*) −788 −777 −33 −756 −28
Note. The S3(ππ*) state refers to benzamide and the S5(ππ*) state to 2-Сl- and 2-Br-benzamides.
a
b
Fig. 2. Maps of the benzamide MEP in the electronic S0 (а) and S1 (b) states. study is stronger than the cis-NH bond (Table 2). It should be noted that electronic excitation causes weakening of a carbonyl bond. An analysis of the calculation results of the MEP in different electronically excited states of the molecules provides the data on changes in the proton-acceptor properties of molecules, which can be a basis for determination of the Hcomplex geometries in different excited states. The MEP value is the integral characteristic of the molecule. However, one can speak about the proton-acceptor properties of the atom, keeping in mind the localization of the MEP minima nearby a certain atom, this localization reflecting some features of a nonuniform distribution of electron density in the molecule. The calculations suggest that electron excitation significantly changes acceptor properties of different fragments of the molecules under study (Table 3). Substitution by chlorine and bromine decreases the proton-acceptor properties of an oxygen atom of the benzamide carbonyl group. Figure 2 shows isolines of the benzamide molecule in the ground and first excited state. Note that the MEP minimum of the oxygen atom is in the plane of the ring and has asymmetric isolines (Fig. 2a). The estimation revealed that
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this minimum is shifted by 0.25−0.30 A relative to the axis of the carbonyl group to the amino-group side in all amides studied. This is accounted for by the integral nature of the MEP and allowance for not only proton-acceptor properties of the oxygen atom itself but also the effect of the nitrogen atom of the amino group on these properties. This suggests that the equilibrium between trans- and cis- 1:1 H-bonded complexes of amides with proton acceptors will be shifted to the weaker complex with the trans-NH-bond, though the cis-NH-bond has a higher proton-donor properties (Table 3) due to asymmetry of a fairly deep minimum observed (steric hindrance). In the S1 state, this potential minimum of the carbonyl group is drastically decreased and distributed over the phenyl ring (Fig. 2b). The proton-acceptor properties of the phenyl ring amount to the maximum values in the S1 state and equal −226, −203, and −193 kJ/mol for the amides under study. Thus, hydrogen bonding with a proton-donor solvent will go largely through carbonyl groups for the molecules in the state of the ππ*-type, and in the S1 state − through phenyl rings and halogen atoms. The work was supported by the Ministry of Education of Russia (Grant Е02–3.2–448). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
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V. Ya. Artyukhov and A. I. Galeeva, Sov. Phys. J., No. 11, 949–952 (1986). G. V. Mayer, V. Ya. Artyukhov, O. K. Basyl’, et al., Electronically Excited States and Photochemistry of Organic Compounds [in Russian], Nauka, Novosibirsk (1997). V. Ya. Artyukhov, A. V. Morev, Yu. P. Morozova, and B. V. Korolev, Russ. Phys. J., No. 1, 48–52 (2003). V. Ya. Artyukhov, A. V. Morev, Yu. P. Morozova, and V. A. Pomogaev, Russ. Phys. J., No. 12, 1203–1207 (2002). M. D. Mashkonskii, Medical products, [in Russian], Meditsina , Moscow (1988). A. V. Morev, Investigation of Intermolecular Interactions of Amines in the H-Bond Complexes by Means of IR Absorption Spectra [in Russian], Vector Book, Tyumen’ (2001). O. V. Sverdlova, Electronic Spectra in Organic Chemistry [in Russian], Khimiya, Leningrad (1985). A. I. Kitaigorodskii, P. M. Zorkii, and V. K. Bel’skii, The Structure of Organic Matter [in Russian], Nauka, Moscow (1982).
