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aniline complex in gas phase
Chemical Physics Letters 446 (2007) 199–205 www.elsevier.com/locate/cplett
Ab initio and TDDFT investigations on charge transfer transition for the o-chloranil/aniline complex in gas phase Sumanta Bhattacharya
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Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India Received 29 June 2007; in final form 3 August 2007 Available online 15 August 2007
Abstract The present Letter deals with the theoretical investigations of the o-chloranil/aniline complex in gas phase employing ab initio and time-dependent density functional theory methods. The dipole moment vector is directed from aniline to the o-chloranil in o-chloranil/aniline complex. The two O atoms of o-chloranil are found to be oriented towards the –NH2 group of aniline and C@O bond length increases upon complexation with aniline. The charge transfer (CT) transition energy of the o-chloranil/aniline complex corroborates fairly well with the reported experimental value. Frontier molecular orbital calculations reveal a strong propensity of photo-induced electron transfer in o-chloranil/aniline CT complex. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Since Mulliken presented the well-known theory [1] of the charge transfer (CT) interaction between electron donor and acceptor, it has been successfully and widely applied to many interesting research subjects. CT complexes are being regarded as important materials for use as organic superconductors [2–4], photo-catalysts [5] and dendrimers [6]. They are also finding application in solar energy storage [7,8], mechanism of drug action [9,10], non-linear optical activity [11], surface chemistry [12] and molecular recognition [13,14]. With the imminence of computer programs for various theoretical calculations like ab initio, density functional theory (DFT) etc., it has become a current trend [15–17] to treat such complexes by these methods. According to Mulliken’s theory [1], chemical hunch is needed to define a ‘donor’ and ‘acceptor’ molecule in a CT complex. However, elucidation of electronic charge distribution in an adduct of two different molecules by the help of ab initio and DFT calculations
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0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.08.041
enable us to characterize the donor and acceptor molecule, directly. Thus, no pre-assumption is necessary for such case. As a result of this, researchers have paid their attention to do explicit theoretical calculations for various CT complexes [18,19]. For example, Reiling et al. have presented results of comparative ab initio calculations on the iodine molecule and the pyridine/I2 CT complex employing various kinds basis sets [18]. Full geometry optimization of the benzene/ICl complex has been carried out [19] in the ground state of the complex in gas phase and the interaction energies have been estimated theoretically at the Hartree–Fock (HF) and the second order Møller–Plesset perturbation theory levels and also by using the DFT with hybrid functionals within symmetry constraints (B3LYP). However, most of the available spectral data on CT transition energies of electron donor–acceptor complexes are the collection of experimental results, and so it is desirable to estimate the CT transition energies by theoretical calculations. The present work aims at carrying out such a study by ab initio and time-dependent density functional (TDDFT) theory on the electron donor–acceptor (EDA) complex of o-chloranil with aniline, for which the experimental CT transition energy is already known [20].
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2. Computational details All the computational calculations have been done with the GAUSSIAN 03W, GassView 3.09 and SPARTAN software implemented on a Pentium computer. CT transition energy of the complex in gas phase was calculated by the restricted configuration interaction singlets (RCIS) method using the ground state optimized geometry at the respective levels of theory. DFT calculations have been performed by using combination of the Becke’s three parameter hybrid [21] exchange potential with the correlation function of Lee et al. [22] (B3LYP). In DFT calculations, other than B3LYP, I have also performed MPW1PW91, B3PW91, PBEPBE and HCTE levels of theoretical calculations in present investigations. The basis set for all the calculations (i.e., ab initio and DFT) used in the present work is 3-21G and for molecular electrostatic potential (MEP) calculation, I have preferred 6-31G. 3. Results and discussions
ing the O atoms and originated from the carbon atoms containing the Cl atom. However, these parameters suffer appreciable changes when o-chloranil is complexed with aniline. The important structural parameters and dipole moments of the o-chloranil/aniline complex in its ground state as obtained after optimization using these two levels of theory are provided in Table 1. The optimized structures of the o-chloranil/aniline complex in B3LYP and HF levels of theory are shown in Figs. 1 and 2S, respectively. Like previous, the bond lengths value between various atoms of o-chloranil, after complexation with aniline, do not show any sort of difference between B3LYP (Table 1) and HF methods (Table 1S). The most important point to be mentioned here is that the oxygen end of the o-chloranil moiety in the complex is always directed towards the – NH2 group of aniline ring in all the levels of theory. Even when the complex is prepared by placing the Cl atom of ochloranil in front of the aniline, the optimization process turns the o-chloranil molecule in such a way that brings the O atom to the front of the –NH2 group. It is further observed that in all the levels of calculation, the C@O bond
3.1. Results of ground state optimization in gas phase ˚ ) of o-chloranil The optimized bond length values (in A in gas phase, calculated by DFT (B3LYP) and HF methods are summarized in Table 1 and 1S, respectively (Fig. 1S). It has been observed that there is no such difference in HF and B3LYP methods regarding bond length between various atoms of o-chloranil. Mulliken’s electronic charge on the various C atom, e.g., C1, C2, C3, C4, C5 and C6, of o-chloranil molecule in gas phase are computed to be 0.525 (0.418), 0.525 (0.418), 0.232 ( 0.150), 0.152 ( 0.078), 0.152 ( 0.078) and 0.232 ( 0.150) a.u., for HF {DFT (B3LYP)} calculations. The numerical values of Mulliken’s electronic charge in both the O atom are 0.477 and 0.381 a.u. according to HF and DFT (B3LYP) calculations, respectively. The ground state dipole moment, calculated by HF and DFT (B3LYP) methods, are 3.21 and 3.30D, respectively, where the dipole moment vector is pointing towards the carbon end containTable 1 Optimized bond length and dipole moment values of o-chloranil and ochloranil/aniline complex after B3LYP calculations in ground state Parameter
o-Chloranil
o-Chloranil/aniline complex
Bond length
1.2032 1.2032 1.7773 1.7773 1.7831 1.7831 1.4706 1.3159 1.5195 1.4706 1.3159 1.4687 2.2901
1.2040 1.2040 1.7786 1.7786 1.7836 1.7836 1.4684 1.5197 1.3156 1.4678 1.3156 1.4684 1.0040
r(C2–O12) r(C3–O11) r(C1–Cl9) r(C4–Cl10) r(C6–Cl8) r(C5–Cl7) r(C1–C2) r(C1–C6) r(C2–C3) r(C3–C4) r(C4–C5) r(C5–C6) Dipole moment (D)
Fig. 1. B3LYP optimized ground state geometric structure of the ochloranil/aniline complex in the gas phase; (a) side-view, (b) front view and (c) top view.
S. Bhattacharya / Chemical Physics Letters 446 (2007) 199–205
length increases on complexation, and is situated above a C–C bond connecting the –NH2 group of aniline. The most remarkable feature is that the –NH2 group of aniline is inclined towards the O atom of o-chloranil moiety. The dihedral angle consisting of N24, C19, C1(C6) and O12(O11) plane in o-chloranil/aniline complex is 33.73250 as determined by both DFT (B3LYP) and HF calculations (Fig. 2S(a)). The major component of the dipole moment vector of the complex is directed along the positive direction of the X-axis according to the Gaussian convention. Furthermore, the Mulliken’s electronic charge on the O atom of the complex in its ground state is found to be more negative than that obtained in the isolated o-chloranil molecule at the two levels of theory. For example, the Mulliken’s electronic charges on the two O atoms of o-chloranil in o-chloranil/aniline complex are 0.493 a.u. and 0.400 a.u. in HF and DFT (B3LYP) calculations, respectively. This indicates that appreciable amount of electronic charge has been transferred from aniline to o-chloranil in o-chloranil/aniline complex. Apart from that, the nitrogen atom of aniline molecule suffers considerable reduction in electronic charge as it becomes 0.865 a.u. after complexation with o-chloranil compared to uncomplexed form, i.e., 0.960 a.u., in HF. DFT (B3LYP) calculation also reveals that Mulliken’s electronic charge on N atom of aniline ( 0.836 a.u.) becomes more positive ( 0.778 a.u.) which is a clear indication of ground state electronic interaction between these two moieties. B3LYP calculation estimated the sum of the Mulliken’s electronic charges on the aniline and o-chloranil moiety of the complex as 0.012 and 0.012, respectively. This phenomenon indicates that with respect to aniline, the ochloranil moiety is negatively charged, and about 2.0% of the charge of an electron has been transferred from aniline to o-chloranil in the ground state of the complex. HF calculation also accredits me to estimate the sum of Mulliken’s electronic charge on aniline (0.002) and o-chloranil ( 0.002) moiety of o-chloranil/aniline complex, which substantiates the results of DFT calculations. 3.2. Calculation of CT transition energy in gas phase RCIS and time-dependent SCF calculations by both HF and DFT on o-chloranil at various levels of theory, have been carried out using STO 3-21G basis set. Results are shown in Table 2. The transitions, which are found to be the components of maintaining degeneracy and perfectly polarized with respect to x-, y- and z-directions, have been considered in the calculations. The other calculated transitions of o-chloranil containing higher energy (greater than 5 eV) are irrelevant to the present investigations. Transition energies of the o-chloranil/aniline complex along with CT absorption maxima and oscillator strengths have been computed by RCIS method at the six levels of theory, namely, HF, B3LYP, B3PW91, MPW1PW91, PBEPBE and HCTH. All the results are summarized in Table 3 in which the main transition energy of the complex has been
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Table 2 Absorption maximum, excitation energy and oscillator strength of ochloranil: results of configuration interaction and TDDFT calculations in gas phase Method
Absorption maximum (nm)
Transition energy (eV)
Oscillator strength
HF B3LYP B3PW91 MPW1PW91 PBEPBE HCTH
271.6 446.5 449.1 429.6 553.3 551.0
4.5655 2.7771 2.7607 2.8857 2.2406 2.2499
0.2447 0.0518 0.0514 0.0572 0.0316 0.0319
Table 3 CT absorption maximum, excitation energy and oscillator strength of ochloranil/aniline complex: results of configuration interaction and TDDFT calculations in gas phase Method
CT absorption maximum (nm)
Transition energy (eV)
Oscillator strength
HF B3LYP B3PW91 MPW1PW91 PBEPBE HCTH
386.2 562.3 567.5 548.0 598.7 608.6
3.2104 2.2049 2.1845 2.2625 2.0710 2.0372
0.0128 0.0038 0.0040 0.0046 0.0033 0.0018
shown. The characteristic electronic transitions of aniline are known to belong to near UV region and are not considered. It has been observed that the degeneracy of the ochloranil transitions is lifted by complexation with aniline causing reduction of the symmetry. The new transition should be assigned as a CT transition from aniline to ochloranil which can also be governed by proper monitoring of the cartesian components of the ground-to-excited state transition electric dipole moments. Close inspection of the dihedral angle consisting of N24, C19, C1(C6) and O12(O11) plane in o-chloranil/aniline complex in its excited state reveals that it becomes 33.73700 as obtained by both DFT (B3LYP) and HF calculations (Fig. 2S(b)). The angle of inclination of both the oxygen atoms of ochloranil towards nitrogen atom of aniline in o-chloranil/ aniline complex, in excited state, gains 0.00450 as compared to its ground state geometry. This is a clear evidence of favorable orientation of o-chloranil and aniline molecules behind the CT transition. B3LYP optimized excited state geometric structure of o-chloranil/aniline complex is shown in Fig. 2. Experimental CT absorption maxima and transition energy of the o-chloranil/aniline complex is already being reported in CCl4 medium [20]. Table 3 reveals that CT absorption maxima estimated by HCTH formalism (608.6 nm) corroborates excellently with the reported experimental value (607 nm). From Table 3, it is also observed that HF method does not correspond well to the experimentally observed CT transition energy since electron correlation effects are not properly taken into account in HF theory. The other methods like B3LYP,
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Fig. 2. B3LYP optimized geometry of the CT excited state for the ochloranil/aniline complex in gas phase.
B3PW91, MPW1PW91 and PBEPBE reproduce close values to the experimental one. However, all of them overestimate the experimental CT transition energy in the range of 0.027–1.1664 eV. Very recently, Tiwari et al. [23] have estimated the CT transition energy for benzene/I2 complex in both gas phase and in CCl4 medium. However, the calculated CT absorption maximum deviates with the experimentally observed value by 45–80 nm. Thus, the CT absorption maxima, elucidated in present theoretical investigations by various theoretical approximations, are much more reliable compared to Tiwari et al. [23]. 3.3. HOMO–LUMO calculations for o-chloranil/aniline complex in both ground and excited states It was already being reported that CT complexes play an important role for the possible application of solar cells due to an efficient photo-induced electron transfer (PET) behavior [24,25]. The PET process can be qualitatively understood by looking into the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Various HOMOs and the LUMOs of the o-chloranil/aniline complex in ground state are shown in Figs. 3 and 4, respectively, applying DFT method at B3LYP level. It may be mentioned that validity of molecular orbitals generated by DFT methods is already being recognized [26]. The accuracy of the above mentioned method, especially at the B3LYP level, was very recently demonstrated by Schaefer and co-workers on electron affinities of aromatic compounds [27] and the HOMO and LUMO orbital shapes obtained by B3LYP have been successfully applied to explain the PET properties [28,29]. In the o-chloranil/aniline complex, HOMO shows that the electrons are localized at the aniline, while LUMO shows that the electrons are precisely positioned at ochloranil. On the other hand, the second LUMO (LUMO2) is higher in energy by 0.080 eV (in ground state). It is also seen from Figs. 3 and 4 that for o-chloranil/aniline CT complex, the HOMO and LUMO + 6 are localized on the donor side (i.e., aniline). This implies that upon photoexcitation, the electron transfer is very much feasible from the aniline moiety to the o-chloranil in o-chloranil/aniline
Fig. 3. HOMO and HOMO n, where n = 1–6 for the o-chloranil/aniline complex at different electronic states; (a) HOMO, (b) HOMO 1 (c) HOMO 2, (d) HOMO 3, (e) HOMO 4, (f) HOMO 5 and (g) HOMO 6.
CT complex. The energies of HOMO, HOMO n, LUMO and LUMO + n, where n = 1–6, for o-chloranil, aniline and o-chloranil/aniline complex in both ground and excited states are provided in Table 4 and 2S, respectively. It is interesting to note that the LUMO energy levels of the CT complex compare well with the LUMO energy level of o-chloranil, while the HOMO energy levels of the complex are close to the HOMO energy levels of the aniline. This tendency for localization of the frontier orbitals of o-chloranil/aniline system is very much similar to other electron donor–acceptor composites [30,31]. From the above discussions, it is quite clear that orbital interaction
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Fig. 4. LUMO and LUMO + n, where n = 1–6 for the o-chioianil/aniline complex at different electronic states; (a) LUMO, (b) LUMO + 1, (c) LUMO + 2 (d) LUMO + 3, (e) LUMO + 4, (f) LUMO + 5 and (g) LUMO + 6.
energy arises mainly from CT between occupied and unoccupied orbitals. This result suggests that correlation of the relative orientation of the acceptor and donor molecules in the o-chloranil/aniline CT complex through examination of their molecular electrostatic potential maps (MEPs) will be useful. The MEPs of the o-chloranil, aniline and their complex have been calculated at the B3LYP/6-31G level at the optimized geometries shown in Fig. 5 with SPARTAN ’06. The MEP plot for o-chloranil (Fig. 5a) shows that the positive electrostatic potential (shown in blue1) corresponds to the center regions of the six-membered ring. Along the C@O bonds, regions of negative potential (shown in red)
1 For interpretation of the references in colour in Fig. 5, the reader is referred to the web version of this article.
are noticeable, although the extent of electron density is less due to presence of four chlorine atoms in the opposite side of the molecule. For the aniline, the negative potential is mainly associated with the nitrogen atom and the center of the benzene ring due to presence of p-electrons. However, Fig. 5b and c reveals while in the ground state, there is a feeble electrostatic interaction between aniline and ochloranil, it is much more pronounced in the excited state of the complex. It has been observed that other than the N atom of –NH2 group, the electron density around the p-electron reached center of the aniline moiety interacts strongly with the o-chloranil unit of o-chloranil/aniline complex. This result is consistent with the notion that ochloranil is generally accepted as a good electron acceptor in forming EDA and/CT complexes with various donors [32,33].
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Table 4 HOMO–LUMO energies for o-chloranil, aniline and o-chloranil/aniline CT complex in ground stat; the orbital numbers are writen in parenthesis Name of the orbital
HOMO HOMO 1 HOMO 2 HOMO 3 HOMO 4 HOMO 5 HOMO 6 LUMO LUMO+1 LUMO+2 LUMO+3 LUMO+4 LUMO+5 LUMO+6
Orbital energy (eV) O-Chloranil
Aniline
0.285 (60) 0.293 (59) 0.326 (58) 0.331 (57) 0.348 (56) 0.348 (55) 0.365 (54) 0.164 (6l) 0.O82 (62) 0.038 (63) 0.037 (64) 0.O22 (65) 0.019 (66) 0.043 (67)
0.216 0.245 0.249 0.346 0.356 0.363 0.424 0.008 0.012 0.117 0.128 0.161 0.171 0.188
O-Chloranil/aniline CT complex (25) (24) (23) (22) (21) (20) (19) (26) (27) (28) (29) (30) (31) (32)
0.209 (85) 0.261 (84) 0.272 (83) 0.278 (82) 0.313 (81) 0.316 (80) 0.322 (79) 0.148 (86) 0.068 (87) 0.025 (88) 0.O23 (89) 0.009 (90) 0.000 (91) 0.018 (92)
Values of various bond moments for the o-chloranil/aniline complex have been calculated by both the HF and TDDFT methods. One typical bond moment data for such complex, calculated by TDDFT (HCTH) method, has been given in Table 5. The bond moment data for the same complex obtained by other methods, e.g., HF, TDDFT (B3LYP), TDDFT (B3PW91), TDDFT (MPW1PW91) and TDDFT (PBEPBE), are given in supplementary materials. It is known that charged species can have a dipole moment and even higher (quadrupole, octapole, etc.) moments. However, according to quantum mechanics, only the highest non-zero ‘pole’ is independent of the chosen origin. This means that for ions, only the monopole (overall charge) is independent of the origin. For most neutral molecules or complexes, the dipole is independent of the origin. Thus, for neutral complex that have a zero dipole moment by symmetry, the quadrupole moment is the highest, non-zero ‘pole’. However, there are probably molecules out there that have a zero quadrupole moment,
Fig. 5. MEPs of (a) only o-chloranil and o-chloranil/aniline CT complex in (b) ground and (c) excited states.
making the octapole moment independent of origin. In the present case the various bond moments shown in Table 5 and Tables 3S–7S give a good view of separation and distribution of charges within the complex molecule which is accounted by the CT phenomena at excited state.
Table 5 Values of various bond moments of o-chloranil/aniline complex calculated by TDDFT (HCTH) method Quadrupole moment (Debye Ang) XX = 130.1947 XY = 0.0010
YY = 134.7264 XZ = 2.6818
Traceless Quadrupole moment (Debye Ang) XX = 3.5926 YY = 0.9390 XY = 0.0010 XZ = 2.6818
ZZ = 136.4410 YZ = 0.0001 ZZ = 2.6536 YZ = 0.0001
Octapole moment (Debye Ang2) XXX = 9.9520 XXY = 0.0018 YYZ = 14.2955
YYY = 0.0040 XXZ = 29.4751 XYZ = 0.0050
ZZZ = 9.9697 XZZ = 21.3526
XYY = 10.5744 YZZ = 0.0031
Hexadecapole moment (Debye Ang3) XXXX = 2892.9830 XXXZ = 214.6878 ZZZY = 0.0051 XXYZ = 0.0252
YYYY = 2497.9069 YYYX = 0.0005 XXYY = 879.7963 YYXZ = 14.9629
ZZZZ = 1691.2990 YYYZ = 0.0028 XXZZ = 809.0379 ZZXY = 0.0047
XXXY = 0.0108 ZZZX = 66.7522 YYZZ = 716.6756
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4. Conclusions The following conclusions can be drawn from the foregoing discussions: (1) Ab initio and TDDFT methods can be utilized successfully to enumerate the CT transition energy for the o-chloranil/aniline molecular complex in gas phase. (2) The CT absorption maxima estimated by HCTH method in TDDFT calculation correlates superbly with the experimental finding. (3) Theoretically determined CT transition energy obtained by HF method does not agree well with the experimentally observed value. However, B3LYP, B3PW91, MPW1PW91 and PBEPBE levels of theory have replicated the experimental value closely, the final one has been found to be more perfect in case of o-chloranil/aniline CT complex. (4) Frontier molecular orbital calculations reveal that there is a conceivable opportunity of electron transfer from aniline to o-chloranil in o-chloranil/aniline CT complex. (5) The relative orientations of the acceptor (i.e., ochloranil) with respect to the donor (i.e., aniline) in the o-chloranil/aniline CT complex is mirrored in the MEPs of the donor/acceptor complex. (6) The results excogitated from the present investigations indicate that o-chloranil/aniline CT complex could be a potential candidate for the construction of novel photo-voltaic devices.
Appendix A. Supplementary data Optimized bond length and dipole moment values of ochloranil and o-chloranil/aniline complex after HF calculation in ground state, HOMO–LUMO energies for ochloranil, aniline and o-chloranil/aniline CT complex in excited state, B3LYP optimized geometric structure of ochloranil in gas phase and demonstration of dihedral angle between the plane of o-chloranil and aniline in both ground and excited states are provided as Tables 1S and 2S and Figs. 1S and 2S, respectively. The bond moment data of the o-chloranil/aniline complex obtained by various methods, e.g., HF, TDDFT (B3LYP), TDDFT (B3PW91), TDDFT (MPW1PW91) and TDDFT (PBEPBE), are given as Tables 3S, 4S, 5S, 6S and 7S, respectively. These materials are provided as supplementary material. Supplementary data associated with this article can be found, in online version of Chemical Physics Letters. Supplementary
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Ab initio and TDDFT investigations on charge transfer transition for the o-chloranil/aniline complex in gas phase Sumanta Bhattacharya
*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India Received 29 June 2007; in final form 3 August 2007 Available online 15 August 2007
Abstract The present Letter deals with the theoretical investigations of the o-chloranil/aniline complex in gas phase employing ab initio and time-dependent density functional theory methods. The dipole moment vector is directed from aniline to the o-chloranil in o-chloranil/aniline complex. The two O atoms of o-chloranil are found to be oriented towards the –NH2 group of aniline and C@O bond length increases upon complexation with aniline. The charge transfer (CT) transition energy of the o-chloranil/aniline complex corroborates fairly well with the reported experimental value. Frontier molecular orbital calculations reveal a strong propensity of photo-induced electron transfer in o-chloranil/aniline CT complex. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Since Mulliken presented the well-known theory [1] of the charge transfer (CT) interaction between electron donor and acceptor, it has been successfully and widely applied to many interesting research subjects. CT complexes are being regarded as important materials for use as organic superconductors [2–4], photo-catalysts [5] and dendrimers [6]. They are also finding application in solar energy storage [7,8], mechanism of drug action [9,10], non-linear optical activity [11], surface chemistry [12] and molecular recognition [13,14]. With the imminence of computer programs for various theoretical calculations like ab initio, density functional theory (DFT) etc., it has become a current trend [15–17] to treat such complexes by these methods. According to Mulliken’s theory [1], chemical hunch is needed to define a ‘donor’ and ‘acceptor’ molecule in a CT complex. However, elucidation of electronic charge distribution in an adduct of two different molecules by the help of ab initio and DFT calculations
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enable us to characterize the donor and acceptor molecule, directly. Thus, no pre-assumption is necessary for such case. As a result of this, researchers have paid their attention to do explicit theoretical calculations for various CT complexes [18,19]. For example, Reiling et al. have presented results of comparative ab initio calculations on the iodine molecule and the pyridine/I2 CT complex employing various kinds basis sets [18]. Full geometry optimization of the benzene/ICl complex has been carried out [19] in the ground state of the complex in gas phase and the interaction energies have been estimated theoretically at the Hartree–Fock (HF) and the second order Møller–Plesset perturbation theory levels and also by using the DFT with hybrid functionals within symmetry constraints (B3LYP). However, most of the available spectral data on CT transition energies of electron donor–acceptor complexes are the collection of experimental results, and so it is desirable to estimate the CT transition energies by theoretical calculations. The present work aims at carrying out such a study by ab initio and time-dependent density functional (TDDFT) theory on the electron donor–acceptor (EDA) complex of o-chloranil with aniline, for which the experimental CT transition energy is already known [20].
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2. Computational details All the computational calculations have been done with the GAUSSIAN 03W, GassView 3.09 and SPARTAN software implemented on a Pentium computer. CT transition energy of the complex in gas phase was calculated by the restricted configuration interaction singlets (RCIS) method using the ground state optimized geometry at the respective levels of theory. DFT calculations have been performed by using combination of the Becke’s three parameter hybrid [21] exchange potential with the correlation function of Lee et al. [22] (B3LYP). In DFT calculations, other than B3LYP, I have also performed MPW1PW91, B3PW91, PBEPBE and HCTE levels of theoretical calculations in present investigations. The basis set for all the calculations (i.e., ab initio and DFT) used in the present work is 3-21G and for molecular electrostatic potential (MEP) calculation, I have preferred 6-31G. 3. Results and discussions
ing the O atoms and originated from the carbon atoms containing the Cl atom. However, these parameters suffer appreciable changes when o-chloranil is complexed with aniline. The important structural parameters and dipole moments of the o-chloranil/aniline complex in its ground state as obtained after optimization using these two levels of theory are provided in Table 1. The optimized structures of the o-chloranil/aniline complex in B3LYP and HF levels of theory are shown in Figs. 1 and 2S, respectively. Like previous, the bond lengths value between various atoms of o-chloranil, after complexation with aniline, do not show any sort of difference between B3LYP (Table 1) and HF methods (Table 1S). The most important point to be mentioned here is that the oxygen end of the o-chloranil moiety in the complex is always directed towards the – NH2 group of aniline ring in all the levels of theory. Even when the complex is prepared by placing the Cl atom of ochloranil in front of the aniline, the optimization process turns the o-chloranil molecule in such a way that brings the O atom to the front of the –NH2 group. It is further observed that in all the levels of calculation, the C@O bond
3.1. Results of ground state optimization in gas phase ˚ ) of o-chloranil The optimized bond length values (in A in gas phase, calculated by DFT (B3LYP) and HF methods are summarized in Table 1 and 1S, respectively (Fig. 1S). It has been observed that there is no such difference in HF and B3LYP methods regarding bond length between various atoms of o-chloranil. Mulliken’s electronic charge on the various C atom, e.g., C1, C2, C3, C4, C5 and C6, of o-chloranil molecule in gas phase are computed to be 0.525 (0.418), 0.525 (0.418), 0.232 ( 0.150), 0.152 ( 0.078), 0.152 ( 0.078) and 0.232 ( 0.150) a.u., for HF {DFT (B3LYP)} calculations. The numerical values of Mulliken’s electronic charge in both the O atom are 0.477 and 0.381 a.u. according to HF and DFT (B3LYP) calculations, respectively. The ground state dipole moment, calculated by HF and DFT (B3LYP) methods, are 3.21 and 3.30D, respectively, where the dipole moment vector is pointing towards the carbon end containTable 1 Optimized bond length and dipole moment values of o-chloranil and ochloranil/aniline complex after B3LYP calculations in ground state Parameter
o-Chloranil
o-Chloranil/aniline complex
Bond length
1.2032 1.2032 1.7773 1.7773 1.7831 1.7831 1.4706 1.3159 1.5195 1.4706 1.3159 1.4687 2.2901
1.2040 1.2040 1.7786 1.7786 1.7836 1.7836 1.4684 1.5197 1.3156 1.4678 1.3156 1.4684 1.0040
r(C2–O12) r(C3–O11) r(C1–Cl9) r(C4–Cl10) r(C6–Cl8) r(C5–Cl7) r(C1–C2) r(C1–C6) r(C2–C3) r(C3–C4) r(C4–C5) r(C5–C6) Dipole moment (D)
Fig. 1. B3LYP optimized ground state geometric structure of the ochloranil/aniline complex in the gas phase; (a) side-view, (b) front view and (c) top view.
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length increases on complexation, and is situated above a C–C bond connecting the –NH2 group of aniline. The most remarkable feature is that the –NH2 group of aniline is inclined towards the O atom of o-chloranil moiety. The dihedral angle consisting of N24, C19, C1(C6) and O12(O11) plane in o-chloranil/aniline complex is 33.73250 as determined by both DFT (B3LYP) and HF calculations (Fig. 2S(a)). The major component of the dipole moment vector of the complex is directed along the positive direction of the X-axis according to the Gaussian convention. Furthermore, the Mulliken’s electronic charge on the O atom of the complex in its ground state is found to be more negative than that obtained in the isolated o-chloranil molecule at the two levels of theory. For example, the Mulliken’s electronic charges on the two O atoms of o-chloranil in o-chloranil/aniline complex are 0.493 a.u. and 0.400 a.u. in HF and DFT (B3LYP) calculations, respectively. This indicates that appreciable amount of electronic charge has been transferred from aniline to o-chloranil in o-chloranil/aniline complex. Apart from that, the nitrogen atom of aniline molecule suffers considerable reduction in electronic charge as it becomes 0.865 a.u. after complexation with o-chloranil compared to uncomplexed form, i.e., 0.960 a.u., in HF. DFT (B3LYP) calculation also reveals that Mulliken’s electronic charge on N atom of aniline ( 0.836 a.u.) becomes more positive ( 0.778 a.u.) which is a clear indication of ground state electronic interaction between these two moieties. B3LYP calculation estimated the sum of the Mulliken’s electronic charges on the aniline and o-chloranil moiety of the complex as 0.012 and 0.012, respectively. This phenomenon indicates that with respect to aniline, the ochloranil moiety is negatively charged, and about 2.0% of the charge of an electron has been transferred from aniline to o-chloranil in the ground state of the complex. HF calculation also accredits me to estimate the sum of Mulliken’s electronic charge on aniline (0.002) and o-chloranil ( 0.002) moiety of o-chloranil/aniline complex, which substantiates the results of DFT calculations. 3.2. Calculation of CT transition energy in gas phase RCIS and time-dependent SCF calculations by both HF and DFT on o-chloranil at various levels of theory, have been carried out using STO 3-21G basis set. Results are shown in Table 2. The transitions, which are found to be the components of maintaining degeneracy and perfectly polarized with respect to x-, y- and z-directions, have been considered in the calculations. The other calculated transitions of o-chloranil containing higher energy (greater than 5 eV) are irrelevant to the present investigations. Transition energies of the o-chloranil/aniline complex along with CT absorption maxima and oscillator strengths have been computed by RCIS method at the six levels of theory, namely, HF, B3LYP, B3PW91, MPW1PW91, PBEPBE and HCTH. All the results are summarized in Table 3 in which the main transition energy of the complex has been
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Table 2 Absorption maximum, excitation energy and oscillator strength of ochloranil: results of configuration interaction and TDDFT calculations in gas phase Method
Absorption maximum (nm)
Transition energy (eV)
Oscillator strength
HF B3LYP B3PW91 MPW1PW91 PBEPBE HCTH
271.6 446.5 449.1 429.6 553.3 551.0
4.5655 2.7771 2.7607 2.8857 2.2406 2.2499
0.2447 0.0518 0.0514 0.0572 0.0316 0.0319
Table 3 CT absorption maximum, excitation energy and oscillator strength of ochloranil/aniline complex: results of configuration interaction and TDDFT calculations in gas phase Method
CT absorption maximum (nm)
Transition energy (eV)
Oscillator strength
HF B3LYP B3PW91 MPW1PW91 PBEPBE HCTH
386.2 562.3 567.5 548.0 598.7 608.6
3.2104 2.2049 2.1845 2.2625 2.0710 2.0372
0.0128 0.0038 0.0040 0.0046 0.0033 0.0018
shown. The characteristic electronic transitions of aniline are known to belong to near UV region and are not considered. It has been observed that the degeneracy of the ochloranil transitions is lifted by complexation with aniline causing reduction of the symmetry. The new transition should be assigned as a CT transition from aniline to ochloranil which can also be governed by proper monitoring of the cartesian components of the ground-to-excited state transition electric dipole moments. Close inspection of the dihedral angle consisting of N24, C19, C1(C6) and O12(O11) plane in o-chloranil/aniline complex in its excited state reveals that it becomes 33.73700 as obtained by both DFT (B3LYP) and HF calculations (Fig. 2S(b)). The angle of inclination of both the oxygen atoms of ochloranil towards nitrogen atom of aniline in o-chloranil/ aniline complex, in excited state, gains 0.00450 as compared to its ground state geometry. This is a clear evidence of favorable orientation of o-chloranil and aniline molecules behind the CT transition. B3LYP optimized excited state geometric structure of o-chloranil/aniline complex is shown in Fig. 2. Experimental CT absorption maxima and transition energy of the o-chloranil/aniline complex is already being reported in CCl4 medium [20]. Table 3 reveals that CT absorption maxima estimated by HCTH formalism (608.6 nm) corroborates excellently with the reported experimental value (607 nm). From Table 3, it is also observed that HF method does not correspond well to the experimentally observed CT transition energy since electron correlation effects are not properly taken into account in HF theory. The other methods like B3LYP,
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Fig. 2. B3LYP optimized geometry of the CT excited state for the ochloranil/aniline complex in gas phase.
B3PW91, MPW1PW91 and PBEPBE reproduce close values to the experimental one. However, all of them overestimate the experimental CT transition energy in the range of 0.027–1.1664 eV. Very recently, Tiwari et al. [23] have estimated the CT transition energy for benzene/I2 complex in both gas phase and in CCl4 medium. However, the calculated CT absorption maximum deviates with the experimentally observed value by 45–80 nm. Thus, the CT absorption maxima, elucidated in present theoretical investigations by various theoretical approximations, are much more reliable compared to Tiwari et al. [23]. 3.3. HOMO–LUMO calculations for o-chloranil/aniline complex in both ground and excited states It was already being reported that CT complexes play an important role for the possible application of solar cells due to an efficient photo-induced electron transfer (PET) behavior [24,25]. The PET process can be qualitatively understood by looking into the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Various HOMOs and the LUMOs of the o-chloranil/aniline complex in ground state are shown in Figs. 3 and 4, respectively, applying DFT method at B3LYP level. It may be mentioned that validity of molecular orbitals generated by DFT methods is already being recognized [26]. The accuracy of the above mentioned method, especially at the B3LYP level, was very recently demonstrated by Schaefer and co-workers on electron affinities of aromatic compounds [27] and the HOMO and LUMO orbital shapes obtained by B3LYP have been successfully applied to explain the PET properties [28,29]. In the o-chloranil/aniline complex, HOMO shows that the electrons are localized at the aniline, while LUMO shows that the electrons are precisely positioned at ochloranil. On the other hand, the second LUMO (LUMO2) is higher in energy by 0.080 eV (in ground state). It is also seen from Figs. 3 and 4 that for o-chloranil/aniline CT complex, the HOMO and LUMO + 6 are localized on the donor side (i.e., aniline). This implies that upon photoexcitation, the electron transfer is very much feasible from the aniline moiety to the o-chloranil in o-chloranil/aniline
Fig. 3. HOMO and HOMO n, where n = 1–6 for the o-chloranil/aniline complex at different electronic states; (a) HOMO, (b) HOMO 1 (c) HOMO 2, (d) HOMO 3, (e) HOMO 4, (f) HOMO 5 and (g) HOMO 6.
CT complex. The energies of HOMO, HOMO n, LUMO and LUMO + n, where n = 1–6, for o-chloranil, aniline and o-chloranil/aniline complex in both ground and excited states are provided in Table 4 and 2S, respectively. It is interesting to note that the LUMO energy levels of the CT complex compare well with the LUMO energy level of o-chloranil, while the HOMO energy levels of the complex are close to the HOMO energy levels of the aniline. This tendency for localization of the frontier orbitals of o-chloranil/aniline system is very much similar to other electron donor–acceptor composites [30,31]. From the above discussions, it is quite clear that orbital interaction
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Fig. 4. LUMO and LUMO + n, where n = 1–6 for the o-chioianil/aniline complex at different electronic states; (a) LUMO, (b) LUMO + 1, (c) LUMO + 2 (d) LUMO + 3, (e) LUMO + 4, (f) LUMO + 5 and (g) LUMO + 6.
energy arises mainly from CT between occupied and unoccupied orbitals. This result suggests that correlation of the relative orientation of the acceptor and donor molecules in the o-chloranil/aniline CT complex through examination of their molecular electrostatic potential maps (MEPs) will be useful. The MEPs of the o-chloranil, aniline and their complex have been calculated at the B3LYP/6-31G level at the optimized geometries shown in Fig. 5 with SPARTAN ’06. The MEP plot for o-chloranil (Fig. 5a) shows that the positive electrostatic potential (shown in blue1) corresponds to the center regions of the six-membered ring. Along the C@O bonds, regions of negative potential (shown in red)
1 For interpretation of the references in colour in Fig. 5, the reader is referred to the web version of this article.
are noticeable, although the extent of electron density is less due to presence of four chlorine atoms in the opposite side of the molecule. For the aniline, the negative potential is mainly associated with the nitrogen atom and the center of the benzene ring due to presence of p-electrons. However, Fig. 5b and c reveals while in the ground state, there is a feeble electrostatic interaction between aniline and ochloranil, it is much more pronounced in the excited state of the complex. It has been observed that other than the N atom of –NH2 group, the electron density around the p-electron reached center of the aniline moiety interacts strongly with the o-chloranil unit of o-chloranil/aniline complex. This result is consistent with the notion that ochloranil is generally accepted as a good electron acceptor in forming EDA and/CT complexes with various donors [32,33].
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Table 4 HOMO–LUMO energies for o-chloranil, aniline and o-chloranil/aniline CT complex in ground stat; the orbital numbers are writen in parenthesis Name of the orbital
HOMO HOMO 1 HOMO 2 HOMO 3 HOMO 4 HOMO 5 HOMO 6 LUMO LUMO+1 LUMO+2 LUMO+3 LUMO+4 LUMO+5 LUMO+6
Orbital energy (eV) O-Chloranil
Aniline
0.285 (60) 0.293 (59) 0.326 (58) 0.331 (57) 0.348 (56) 0.348 (55) 0.365 (54) 0.164 (6l) 0.O82 (62) 0.038 (63) 0.037 (64) 0.O22 (65) 0.019 (66) 0.043 (67)
0.216 0.245 0.249 0.346 0.356 0.363 0.424 0.008 0.012 0.117 0.128 0.161 0.171 0.188
O-Chloranil/aniline CT complex (25) (24) (23) (22) (21) (20) (19) (26) (27) (28) (29) (30) (31) (32)
0.209 (85) 0.261 (84) 0.272 (83) 0.278 (82) 0.313 (81) 0.316 (80) 0.322 (79) 0.148 (86) 0.068 (87) 0.025 (88) 0.O23 (89) 0.009 (90) 0.000 (91) 0.018 (92)
Values of various bond moments for the o-chloranil/aniline complex have been calculated by both the HF and TDDFT methods. One typical bond moment data for such complex, calculated by TDDFT (HCTH) method, has been given in Table 5. The bond moment data for the same complex obtained by other methods, e.g., HF, TDDFT (B3LYP), TDDFT (B3PW91), TDDFT (MPW1PW91) and TDDFT (PBEPBE), are given in supplementary materials. It is known that charged species can have a dipole moment and even higher (quadrupole, octapole, etc.) moments. However, according to quantum mechanics, only the highest non-zero ‘pole’ is independent of the chosen origin. This means that for ions, only the monopole (overall charge) is independent of the origin. For most neutral molecules or complexes, the dipole is independent of the origin. Thus, for neutral complex that have a zero dipole moment by symmetry, the quadrupole moment is the highest, non-zero ‘pole’. However, there are probably molecules out there that have a zero quadrupole moment,
Fig. 5. MEPs of (a) only o-chloranil and o-chloranil/aniline CT complex in (b) ground and (c) excited states.
making the octapole moment independent of origin. In the present case the various bond moments shown in Table 5 and Tables 3S–7S give a good view of separation and distribution of charges within the complex molecule which is accounted by the CT phenomena at excited state.
Table 5 Values of various bond moments of o-chloranil/aniline complex calculated by TDDFT (HCTH) method Quadrupole moment (Debye Ang) XX = 130.1947 XY = 0.0010
YY = 134.7264 XZ = 2.6818
Traceless Quadrupole moment (Debye Ang) XX = 3.5926 YY = 0.9390 XY = 0.0010 XZ = 2.6818
ZZ = 136.4410 YZ = 0.0001 ZZ = 2.6536 YZ = 0.0001
Octapole moment (Debye Ang2) XXX = 9.9520 XXY = 0.0018 YYZ = 14.2955
YYY = 0.0040 XXZ = 29.4751 XYZ = 0.0050
ZZZ = 9.9697 XZZ = 21.3526
XYY = 10.5744 YZZ = 0.0031
Hexadecapole moment (Debye Ang3) XXXX = 2892.9830 XXXZ = 214.6878 ZZZY = 0.0051 XXYZ = 0.0252
YYYY = 2497.9069 YYYX = 0.0005 XXYY = 879.7963 YYXZ = 14.9629
ZZZZ = 1691.2990 YYYZ = 0.0028 XXZZ = 809.0379 ZZXY = 0.0047
XXXY = 0.0108 ZZZX = 66.7522 YYZZ = 716.6756
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4. Conclusions The following conclusions can be drawn from the foregoing discussions: (1) Ab initio and TDDFT methods can be utilized successfully to enumerate the CT transition energy for the o-chloranil/aniline molecular complex in gas phase. (2) The CT absorption maxima estimated by HCTH method in TDDFT calculation correlates superbly with the experimental finding. (3) Theoretically determined CT transition energy obtained by HF method does not agree well with the experimentally observed value. However, B3LYP, B3PW91, MPW1PW91 and PBEPBE levels of theory have replicated the experimental value closely, the final one has been found to be more perfect in case of o-chloranil/aniline CT complex. (4) Frontier molecular orbital calculations reveal that there is a conceivable opportunity of electron transfer from aniline to o-chloranil in o-chloranil/aniline CT complex. (5) The relative orientations of the acceptor (i.e., ochloranil) with respect to the donor (i.e., aniline) in the o-chloranil/aniline CT complex is mirrored in the MEPs of the donor/acceptor complex. (6) The results excogitated from the present investigations indicate that o-chloranil/aniline CT complex could be a potential candidate for the construction of novel photo-voltaic devices.
Appendix A. Supplementary data Optimized bond length and dipole moment values of ochloranil and o-chloranil/aniline complex after HF calculation in ground state, HOMO–LUMO energies for ochloranil, aniline and o-chloranil/aniline CT complex in excited state, B3LYP optimized geometric structure of ochloranil in gas phase and demonstration of dihedral angle between the plane of o-chloranil and aniline in both ground and excited states are provided as Tables 1S and 2S and Figs. 1S and 2S, respectively. The bond moment data of the o-chloranil/aniline complex obtained by various methods, e.g., HF, TDDFT (B3LYP), TDDFT (B3PW91), TDDFT (MPW1PW91) and TDDFT (PBEPBE), are given as Tables 3S, 4S, 5S, 6S and 7S, respectively. These materials are provided as supplementary material. Supplementary data associated with this article can be found, in online version of Chemical Physics Letters. Supplementary
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