Abstract
Raman excitation profiles of several normal modes of 2-benzoylpyridine were measured, and the structural changes encountered on excitations, excited state symmetries, and vibronic couplings among various excited electronic states of the molecule were investigated. Vibrational spectroscopic studies of the molecule were done in detail, and critical investigation on the electronic spectra of the molecule was also carried out. It is shown that the experimentally allowed transitions, corresponding to the band around 262 and 238 nm, occur to the excited states, where the major geometry changes involve both ring CC/CN and CO stretching vibrations. An excited state lying around 185 nm above the ground state was also found to play an important role in the scattering process. All necessary and valuable quantum chemical calculations accompany the presented spectral studies.
Keywords
INTRODUCTION
Aza aromatic and carbonyl compounds have certain elusive photophysical,1–12 photochemical,13–15 and biological properties, 16 which has attracted the attention of much research in these fields. Distinguishing features of the photophysical and photochemical properties of aza compounds with respect to their hydrocarbon analogs arise due to the presence of some low-lying n,π* states in the immediate neighborhood of the lowest singlet and triplet π, π* states. The case becomes more complicated, but somewhat more interesting, if along with the nonbonding (n-) electrons of the ring nitrogen atom(s), nonbonding electrons are also present in other atoms of the substituent group(s) to the ring. 17 Conformational properties of such molecules depend not only on the nature of the substituent and the surrounding environment, but also on the position of the substituent with respect to the ring heteroatom. Photophysical and photochemical behaviors of such molecules can be understood with much elegance if knowledge of their geometrical and electronic structural properties in the ground and excited electronic states is acquired.18,19
Critical study of vibrational spectroscopy has the potential to yield important information related to structural and conformational properties of aromatic molecules if used in conjunction with necessary quantum chemical calculation.19–22 The exploration of potential energy surfaces has received focused attention to describe the effects of nuclear motions on the properties of the excited electronic states of the concerned molecules.23–25 Comparative studies of the potential energy surfaces of different excited states with respect to that of the ground state are very supportive in this regard. There is a decrease in the ratio of the intensity contributions of Franck-Condon (FC) and Herzberg-Teller (HT) scatterings when there is an increase in the excitation distance from the resonance with an allowed electronic transition. For such excitation, intensity contributions from several electronic states might be important. So studies of Raman excitation profiles (REPs) of different normal vibrations in this region might be helpful to extract information related to structural and other important aspects that are necessary for understanding many photophysical and photochemical behaviors of the molecules. The study of the FC envelope helps to determine the conformational changes encountered by molecules on excitation to higher electronic states. On the other hand, HT scattering may unravel many interesting features of excited electronic states, such as natures, symmetries, vibronic couplings, decay mechanisms, etc.17–19
Like some other N- and S-containing compounds, 2-benzoylpyridine (2-BOP) has been found to exhibit the corrosion inhibition properties of mild steel in hydrochloric acid solution to a good degree. 26 In addition to this, the molecule 2-BOP has been found to be an important complexation agent for the spectroscopic determination of a great variety of metals.27–29 So, vibrational and electronic spectra of the molecule have been critically analyzed in the present work. Moreover, the scattering mechanisms have also been investigated to get insight into the excited state dynamics of the molecule. The present investigation is mainly divided into three parts. The first part is comprised of detailed studies of the infrared and polarized Raman spectra of 2-BOP and the identification of different normal modes, which is essential for the analyses of the REPs. The second part contains an critical investigation of the electronic spectra of the molecule. All these spectral studies are accompanied by necessary and valuable quantum chemical calculations (QCCs). The third part involves the analyses of REPs of several normal modes to get insight into the properties of the molecule in different electronic states. In this communication, experimentally measured relative intensities have been simulated satisfactorily using the theoretical profiles determined by the sum-over-states method based on pertinent FC and HT (vibronic coupling) terms. In addition to these, theoretical calculations have also been extended to get an idea about the possible sites of adsorption of the molecule to silver hydrosol.
The reference lines 267, 373, and 674 cm−1 for CHCl3 and 220, 324, and 466 cm−1 for CCl4 were used as internal standards. The same procedure, as described in earlier publications30,31 and the references therein, were followed here to determine the variation of relative intensity of different Raman bands with the exciting wavelength for each reference line. For the measurement of REPs, the Raman intensity of each band was normalized relative to that for the exciting wavelength at 514.5 nm, and the excitation frequency dependence of the intensity of the solvent band, used as internal standard, was taken into consideration. The average of the results found on the basis of the measurements with respect to the three internal standards in each solution was compared with the theoretical curves.
The relative intensity of each Raman band of frequency (ν a ) corresponding to an exciting radiation of frequency (νexc) is given by
where all the intensities (I) are measured in terms of the number of photons per second per scan. Thus ν 3 dependence was taken into consideration, ν514.5 is the frequency of the exciting radiation of wavelength 514.5 nm, and νIS is the frequency of the reference Raman band used as internal standard. Theoretical calculations of relative Raman intensities are described in the section on Raman excitation profiles.
RESULTS AND DISCUSSION
Equilibrium geometries of 2-BOP in internal coordinate system.
Optimized ground-state geometries of 2-BOP molecule for (
In a previous investigation, Kolev et al. 29 considered the symmetry group of the molecule to be CS. But the lowest symmetry (C1) found from the present QCC is also supported using the polarization characteristics of the observed Raman bands. The molecule, 2-BOP, possesses 63 normal modes of vibration. Obviously, under C1 symmetry all the vibrations are expected to be both Raman and infrared (IR) active. The observed Raman spectra of the sample in different environments (Fig. 2) along with their IR counterparts are listed in Table II. Theoretical bands, calculated using the DFT method, have also been included in this table in separate columns. Note that the calculated wavenumbers represent vibrational signatures of the molecule in its gas phase. So the wavenumbers of the molecule observed experimentally in the solid phase and in solutions may differ to some extent from the calculated ones. Moreover, the calculated harmonic force constants and frequencies are usually found higher than the corresponding entities determined from the experimental data, due to a combination of electron correlation effects and basis set deficiencies. This is the reason for using scaling factors for both the functionals, B3LYP and B3PW91. The force constants and therefore the corresponding wavenumbers are scaled down using uniform scaling factors (0.968 and 0.969) for the cis and trans conformations, respectively, using B3LYP and 0.9645 (for both conformers) in the case of B3PW91 functionals. It has been found that the B3LYP functional yields results where the wavenumber matching between the calculated and observed values is better, and so these results are only given in Table II. Small disagreements between the theoretical and experimental results may be attributed to anharmonicities and also to the general tendency of quantum chemical methods to overestimate the force constants at the exact equilibrium geometry.
Vibrational assignments of different Raman and infrared bands (cm−1) of 2-BOP.
Scaling factor for cis is 0.968. Percentage error in wavenumber calculation is 1.32% (8.14 cm−1).
ν, stretching; α, in-plane ring bending; β, in-plane angle bending; δ, in-plane substitute bending; γ, wagging; φ, torsion; CX, carbon atom of carbonyl group.
Scaling factor for trans is 0.969. Percentage error in wavenumber calculation is 1.28% (7.90 cm−1).
Raman spectra of 2-BOP with excitation wavelength 514.5 nm; (
To get a complete picture of the normal modes, a comparative study was made with BP, 18 2,2′-DPK, 19 and 4-BOP. 35 Available literature concerning vibrational assignments of similar types of molecules30,31,38–56 was also consulted. Previously, Chowdhury et al. 54 made the assignment job for some selected modes, although Kolev et al. 29 made a complete assignment of different normal modes of the molecule earlier. Still, during theoretical calculation and Raman excitation profile studies, it was felt that reassignments of a good number of modes were necessary, and so the entire assignment job had to be made afresh.
Out of the six-ring angle bending modes (αCCC) of the two rings, four were assigned unambiguously (Table II) following the PEDs. However, in assigning the trigonal bending modes, some difficulties were encountered, and therefore animated views of the normal modes were also examined. These modes were assigned to the strongly polarized Raman bands at 993 cm−1 (for the pyridyl ring) and 1000 cm−1 (for the phenyl ring). Between the CC and CC/CN stretching modes, four lying in the regions 1400–1475 cm−1 and 1550–1600 cm−1 (which are derived from benzene/pyridine normal modes 19a, 19b and 8a, 8b) were easily identified. After careful examination of both PEDs and animated views of the corresponding normal modes, it was felt that assignments of the Kekule modes and the two CH in-plane bending modes of the two rings around 1300 cm−1 have to be made very cautiously. Accordingly, two strong infrared bands at 1282 and 1316 cm−1 were assigned to the CH-bending in-plane modes of the pyridyl and phenyl rings, respectively. The Kekule mode was effectively assigned to the weak Raman band at 1302 cm−1 for the phenyl ring. For the pyridyl ring, this mode was reassigned at 1249/1269 cm−1 for the cis/trans isomer on the basis of the theoretical calculation. One point to be mentioned here is that, unlike Kolev et al., 29 two bands were not found very close to 1320 cm−1. Some problems arose in assigning the breathing modes of the two rings. So, in addition to the PEDs, here also the animated views of the normal modes were consulted. On the basis of this dual searching, a strong polarized Raman band at 1048 cm−1 and a weak Raman band at 927 cm−1 have been assigned as the ring breathing modes of the phenyl and pyridyl rings, respectively. Based on the theoretical calculation, the assignment job in this region was made cautiously, because here the PEDs are mixed up intricately. The assignments of the other ring modes are in fair agreement with those of similar molecules30,31,38–41,43–55 and also in accordance with Varsanyi's work. 56
Substituent sensitive CCX-bending modes were assigned in accordance with the previous works of similar molecules.30,31,38–41,43–56 But in assigning the CCX-stretching modes, some problems were encountered. However, both by examining the PEDs and by viewing the normal mode animation, the two CCX-stretching modes were assigned to the Raman frequencies at 286 and 1241 cm−1; the corresponding calculated values were found to be 279 and 1243 cm−1. Such a low value of the wavenumber of one of the stretching modes (i.e., of the pyridyl ring or symmetric stretching) remains unexplained at this stage. But both PED and normal mode animation compelled us to assign them as such. However, one point is noteworthy in this connection. The CCX distances linking the pyridyl and the phenyl groups with the carbonyl group are much different (1.517 and 1.498 Å) in both the forms of this isomer than the corresponding bond distances in 4-BOP (1.507 and 1.497 Å). 35 The corresponding bond distances are 1.508 and 1.509 Å in 2,2′-DPK 19 and 1.502 and 1.502 Å in BP. 18
Chowdhury et al. 54 and Kolev et al. 29 previously assigned the medium strong polarized Raman band at 575 cm−1 as the in-plane bending mode of the C=O bond. But the present theoretical study establishes this band as the angle (CCXC) bending mode. Conformer dependent in-plane C=O bending vibration was assigned to the weak Raman wavenumber at 355 and 385 cm−1 for cis and trans isomers, respectively. By critical overview of the PED and normal mode animation, the C=O wagging mode was assigned to the strong infrared frequency at 818 cm−1, which seems quite high in magnitude compared with the corresponding wavenumbers in other aromatic molecules having carbonyl group as a substituent.
Average errors of the calculations for the B3LYP functional with the 6-311G(d,p) basis set are 1.32% (8.14 cm−1) and 1.28% (7.90 cm−1) for the two conformers, cis and trans, respectively.
Since the energy difference between the two isomers is less than 20 kJ/mole, it is not possible here to isolate them quantitatively, and this is also not essential in the interest of the present investigation. However, the concomitance of Raman bands of both cis and trans isomers signifies the presence of both the forms of the molecule in the solid state and also in CCl4 and CHCl3 environments.
Electronic absorption spectra of 2-BOP.
See text and Table IV.
Experimental electronic absorption bands and theoretical vertical one-electron excitation energies of 2-BOP (trans configuration).
Experiment is performed in Methylcyclohexane solution.
Electronic absorption spectra of 2-BOP (
To make an in-depth study of the nature of the ultraviolet absorption spectra, the low-energy electronic excited states of 2-BOP were calculated at the B3LYP/6-311G(d,p) level using TD-DFT approach on the previously optimized ground-state molecular geometry (for the trans structure) of the molecule. Transition energies and oscillator strengths are listed in Table IV along with the description of absorptions expressed in terms of dominant one-electronic vertical transitions. The TD-DFT method predicts appreciable shifts of energies of singlet states in going from gas phase to different solvent phases (see Table IV). Based on the computational work, it has been found that the highest occupied molecular orbital (HOMO) is number 48 and the lowest occupied molecular orbital (LUMO) is number 49. LUMO and other higher orbitals (LUMO+1 and LUMO+3) of the molecule were found to be of π-nature and spread over the two rings, whereas LUMO+2 is mainly confined to the phenyl ring (Fig. 4). In the case of LUMO, the π-bonding orbital also encloses the carbonyl group; however, the next three higher orbitals above LUMO do not dominantly enclose the carbonyl group, i.e., the atom positions 7 and 23 have practically zero coefficients, but those of the atoms in the ring moieties have large coefficients in the molecular orbitals of the respective linear combinations of atomic orbitals (LCAO-MOs). On the other hand, HOMO and to some extent HOMO–1 to HOMO–4 have significant contributions both from the nonbonding orbitals and also from the π-clouds. The other orbital, HOMO–5, is found to exhibit π-orbital characteristics localized in the pyridyl ring.
Electron density contours for selected molecular orbitals of 2-BOP.
The first transition (n → π*) is calculated at the wavelength 369.69 nm in vacuum and at 367.45/361.06 nm in CHX/EtOH surroundings with very negligible oscillator strength. In all cases the vertical one-electron excitation can be mainly described using the HOMO → LUMO transition. This band corresponds with the weak band observed on the longer wavelength side of the electronic absorption spectra with the band maximum found at 360 nm in the MCH environment.
The next calculated band is found at 302.81 nm in vacuum where the significant transition is associated with (HOMO–1) → LUMO. In CHX solvent the peak is slightly blue-shifted (to 302.62 nm) and in the EtOH environment it is slightly red-shifted to 303.10 nm. The theoretical calculation also shows that the next band toward the lower wavelength region is observed at 286.51 nm in gas phase and it exhibits very small or practically no shift in going from CHX to EtOH environments. Here also several transitions are involved. But the major contributions come from the transitions (HOMO–4, HOMO–1) → LUMO. From the calculated F-values (0.0012 and 0.0086), both the bands (302.81 and 286.51 nm, respectively), are assigned as π → π* bands. They appear to be the exciton split components of the 1 Lb band where the chief transitions come from 44 → 49 and 47 → 49.
The next three calculated bands are found at 278.96, 263.07, and 254.52 nm in gas phase. The first one exhibits practically no solvent shift. High F-values compelled us to assign this as a π → π* band (associated mainly with the 46 → 49 orbital transition). The next band appearing from the HOMO → LUMO+1 transition exhibits prominent blueshift in going from the nonpolar to polar solvent with low F-value. So this corresponds with an n → π* transition. The remaining one exhibits a prominent redshift in EtOH solution, where the chief orbital contributor is 45 → 49. This also corresponds with a π → π* band. The two π → π* bands (the first and the third) of this group are assigned as the exciton split components of the 1 La band. Such an exciton splitting of the 1 La band has also been observed in 4-BOP 35 and 2,2′-DPK. 19 These three bands could not be resolved and appeared as a broad band around 262/263 nm in the observed spectra in MCH/EtOH solutions.
The next three bands are calculated at 237.13, 234.32, and 231.50 nm in the gaseous state. The first and the third of these bands exhibit solvent blueshift in alcoholic environments. So they are assigned as bands appearing from n → π* transitions. The remaining (second) one, on the other hand, exhibits redshift in the alcohol solution, and so the corresponding transition is assigned as a π → π*. The experimentally observed prominent band at 238 nm in MCH solution corresponds with this group of three bands having dominant contributions from 43 → 49, 47 → 50, and 48 → 51 orbital transition. The transitions, 47 → 50 and 48 → 51, redistribute the π-electronic cloud in the two rings. On the other hand the orbital transition 43 → 49 not only transfers electron density from the pyridyl ring to the phenyl ring including the carbon atom (CX) of the carbonyl group, but also redistributes it in the pyridyl ring.
There are four bands calculated at 187.66, 185.59, 182.67, and 181.70 nm in the gas phase. These bands exhibit different orbital transitions 44 → 52, 45 → 52, 46 → 52, and 47 → 52. By examining these orbital transitions and calculated oscillator strengths, all these bands are assigned as π → π* bands. In these orbital transitions, the charges are redistributed in the two rings. This group of bands (around 185 nm) has been found to play an important role in the scattering mechanism to be discussed in the next section.
The observed bands around 360, 262, and 238 and the one calculated around 185 nm correspond with the transitions appearing from the ground state |G〉 to different excited states |S0〉, |S2〉, |S3〉, and |S4〉, i.e., G → S0, G → S2, G → S3, and G → S4, respectively.
where K is a constant, ν0 is the exciting frequency, ν a is the frequency of the ath mode, and αρ,σ is the ρσth component of the scattering (polarizability) tensor (ρ, σ = x, y, z). νIG = h−1[EI0 – EG0]; |I〉 and |G〉 are, respectively, excited and ground electronic states; EI0 and EG0 are the energies of the lowest vibrational states of the respective electronic states; and |J〉 is another excited electronic state. Δ i a is the displacement of the potential minimum of the Ith electronic state with respect to the ground state along the concerned (Qath) normal coordinate. FA and FB are given by
where the symbols have their usual meaning. The first term in the square bracket in Eq. 2 corresponds to the A term (FC), and the second term corresponds to the B term (HT/vibronic coupling). The contribution of the A term depends on the molecular distortion Δ i a and that of the B term depends on the matrix ha IJ, which mixes the two excited states |I〉 and |J〉 through the ath normal mode gradient of the electronic Hamiltonian (H) at the equilibrium configuration. Thus the nonzero A term contribution implies a displacement of the excited state potential minimum along the concerned normal coordinate with respect to the ground state. Moreover, the change in curvature of PES suggests that the concerned normal mode has different vibrational frequencies in the ground and excited states. Symmetry considerations require that such a displacement in the Franck–Condon enhancement mechanism can occur only for totally symmetric vibrational modes, and no mode that is not totally symmetric can be expected to undergo Franck–Condon scattering process.
The B term involves vibronic (HT) coupling of the excited state |I〉 with another excited state |J〉. For the matrix element 〈I|∂H/∂Qa|J〉0 to be nonvanishing, the irreducible representation of the vibrational fundamental with normal coordinate Qa (also of ∂H/∂Qa) must be contained in the direct product of the irreducible representations of the states |I〉 and |J〉. Thus, the B term can be nonzero for both totally symmetric and not totally symmetric fundamentals. Even if the states |I〉 and |J〉 are nearer to each other, the above rule has to be followed in order to have a nonvanishing value of the coupling element 〈I|∂H/∂Qa|J〉0. In such cases the molecule will be distorted along Qa. On the other hand, the electric dipole moment selection rule affirms that for totally symmetric vibration, these two favorable excited states always belong to the same symmetry species and the PESs repel. This repulsion gives rise to change in the shape of the respective PESs and therefore in the vibrational frequencies. 22 The strength of the coupling depends on the energy gap between the coupled states. In extreme cases, for very strong coupling, the curvature of the PES of the lower curve may be inverted, and double minima may arise. In such a case, both totally and not totally symmetric vibrations may be responsible for vibronic coupling provided the symmetry property of the product, mentioned above, is satisfied.
Generally, the effect of the B term is smaller than that of the A term. The relative contribution of the B term is found to increase as the excitation wavelength is more and more away from the band arising from an allowed electronic transition. In the present case since the exciting radiations are not in the region of resonance, the contributions of both the terms may be important. Theoretical A and B term scattering intensities of different bands are normalized relative to those for the exciting wavelength at 514.5 nm according to Eq. 2. These theoretical relative intensities are compared with the experimental ones determined from Eq. 1.
Both the calculated and observed REPs of several normal modes of vibration of 2-BOP molecule are exhibited in Fig. 5. The observed profiles in chloroform and carbon-tetrachloride solutions at room temperature (around 30°C) are presented using symbols. The theoretically determined REPs, i.e., the diagonal (A term) and the off-diagonal (B term) contributions from different excited electronic states, and also the (νo – ν a ) 3 dependence for classical contributions, are represented using solid curves accordingly marked to indicate the contributors. Another point worth mentioning here is that since the n → π* transition (around 360 nm) is very weak, no significant contribution has been found from this state (S0), and hence they are not shown in Fig. 5. This is also the case for the 1 Lb band. Moreover, the diagonal contribution from the state S3 and the off-diagonal contribution from the pair of states S2 and S4 lie so close to each other that it becomes very difficult to resolve their individual contributions in the REP studies (Fig. 5). The effect of the line width on REPs has been checked before,38–42 but no significant effect has been observed. For each mode, both the experimental and theoretical intensities are normalized relative to the respective intensities for the excitation radiation at 514.5 nm (19 435 cm−1). The accuracy in the measurements of intensities of different Raman bands varies, but REP values are confined within the error ±5%.
Measured and calculated Raman excitation profiles (REPs) for 2-BOP. The symbols (* and o) indicate the measured profiles in CHCl3 and CCl4 solutions, respectively. ν3 denotes classical calculated profiles considering (ν
a
As mentioned earlier, the strong and highly polarized Raman band at 730 cm−1 corresponds to the ring angle bending mode (αCCC/CCN/CNC) of the pyridyl ring. Figure 5 shows that none of the experimentally observed excited electronic states (S2 or S3) contributes to this mode through A or B term of the scattering tensor in each of the two solvents (CHCl3 and CCl4). Actually, classical wavenumber dependence is most suitable in this case. This means that the mode is getting intensity contributions from the electronic states lying high in the energy scale.
There are four strong and polarized Raman bands around 993, 1000, 1027, and 1045 cm−1. Out of these, the first two are the triangular modes of the pyridyl and phenyl rings, respectively, the last one is the pyridyl ring breathing mode (νCC/CN), and the rest is the CH-bending mode (having significant contributions from the ring stretching vibrations) of the phenyl ring. REPs of all of the four bands are more or less similar in nature, though none of the experimentally observed excited electronic states (S2 or S3) is found to be a dominant contributor. Classical contribution also does not agree very well with the experimental observation. The observed profiles of these modes lie between the profiles of the classical contribution and that of the electronic state S3. If a transition wavelength λIG is chosen around 185 nm, lying in the UV region, the intensity contribution from the relevant state (S4) becomes very worthy. In all the cases, the chief contributor is this theoretically calculated electronic state S4 through the diagonal A term. This means that equilibrium geometries of both the rings are significantly changed in this state, since we know that the intensity contribution to a Raman band from an excited electronic state through the A term is proportional to the square of the displacement of the potential energy minimum of that state with respect to that of the ground state18,19,30,31,38–42 along the concerned normal coordinate (Eq. 2). Here, some interesting observations have been made. In all cases, it has been found that the measured REP data are depleted for the excitation wavelength 457.9 nm, especially for the second vibration of wavenumber 1000 cm−1. Two probable interpretations can be given to these observations. First, some destructive interference between the A term (contribution from the state S4) and the B term (contribution due to interaction(s) between the S4 state with other nearby state(s)) may take place in this region of excitation. Alternatively, a higher electronic state lying above S4 might be the better contributor.
One of the ring stretching (νCC/CN) vibrations appears at 1581 cm−1. The REP of this vibration indicates that the excited states S2 and S3 are the chief contributors through the A term. Thus the change of geometry of the molecule is expected in these states as a result of distortion of ring geometry (Δℜ) related to the shift parameter (ΔIa) according to the relation ΔIa = L−1Δℜ, where L is the transformation matrix from normal to internal coordinate system. In addition to this, the B term contribution from the electronic states S2 and S3 may also be important. This means that there is efficient vibronic interaction between the states through this mode. Again the existence of strong vibronic coupling among the states suggests that the symmetries of the relevant states remain the same.
It was possible to measure the REPs for two CC-stretching vibrations of the phenyl ring, which appear around 1432 and 1596 cm−1. Here also the diagonal contributions from the states, S2 and S3, are most effective. This indicates that along these modes the potential minima of the molecule undergo appreciable displacements due to excitation from the ground to these states (S2 and S3). These bands also get favorable off-diagonal (B term) contributions from the states S2 and S3, which implies the existence of effective vibronic coupling between the states through these modes.
The wavenumber of the C=O stretching mode of the said molecule lies around 1666 cm−1. Critical examination of REP of this mode indicates that the favorable contributor is the B term, which mixes the states S2 and S3. Apart from this contribution, this mode also gets a possible A term contribution from the state S2. This indicates that some charge transfer characteristic is mixed up with this state, which is also substantiated using theoretical calculation of wavenumber for vertical one-electronic molecular orbital transition (Table IV).
The chief orbital transitions associated with the S2 band (262 nm) are 45 → 49 and 46 → 49. In these transitions, the electronic charge cloud goes chiefly from the phenyl ring to the region containing the atom C7 of the carbonyl group, which strengthens the bonds C6C7/C7C8. For a similar reason, these transitions weaken the bonds C8C9/C10C11/C11C12/C8C13. As a result, the shortening of the bonds C6C7/C7C8 and extension of the bonds C8C9/C10C11/C11C12/C8C13 are expected. In the pyridyl ring, the contraction of the bond C1C2 and extension of the bonds C3C4/C6C1 may also be expected due to electronic charge redistribution associated with these transitions (45→49 and 46 → 49). This observation is also in compliance with the REP studies of CC, CC/CN stretching vibrations of the phenyl (1432 and 1596 cm−1) and pyridyl (1581 cm−1) rings.
The electronic transitions associated with the S3 band (238 nm), are chiefly 43→49, 47→50, and 48→51. In the first of these transitions, the charge cloud is found to shift from the pyridyl ring to cast over the phenyl ring. In addition to this, the π-electronic charge redistribution also takes place due to these transitions. These have an overall effect of strengthening and weakening the CC/CN bonds of the phenyl and pyridyl rings, which is also in agreement with the observed REPs of the CC/CN stretching modes (at 1432, 1581, and 1596 cm−1).
All the major transitions associated with the S4 band terminate to the molecular orbital (MO) 52 from the MOs 44, 45, 46, and 47. These transitions bring about an overall redistribution of the electronic charges over the two rings, which results in the elongation of the bonds (C8C9/C9C10 and C11C12/C12C13) of the phenyl ring. Extensions of the C3C4/C6C1 bonds of the pyridyl ring are also expected to arise from these transitions. These are consistent with the large intensities of the Raman bands at 993, 1000, 1027, and 1045 cm−1.
Atomic charges of 2-BOP in different environments.
The probable site of adsorption of the molecule on the silver surface in silver sol may be predicted from the estimation of atomic charge density. The higher the negative charge density on the atom, the greater its probability to act as an adsorption site on silver substrate. So the 2-BOP molecule may be adsorbed to the silver surface through the loan pair electrons of the nitrogen atoms, N1 and N5, of the cis and trans isomers, respectively, of the pyridine ring moiety and oxygen (O23) atom of the substituent carbonyl group. Theoretical results show that the negative charge densities are appreciable on both the nitrogen and the oxygen atoms. Therefore active involvements of both the atoms are expected in the adsorption process of 2-BOP by forming a coordination bond with the metal. In fact both Ag–N and Ag–O stretching vibrations are observed in the surface-enhanced Raman spectra of 2-BOP. 54 Inhibition of corrosion of mild steel in hydrochloric acid solution using 2-BOP has been observed. 26 This process of inhibition may be conducted through the adsorption process.
Dipole moments (in debye) of 2-BOP in different environments.
CONCLUSION
The purpose of this paper was to theoretically confirm the experimental findings of the electronic energy levels, vibrational signals, and molecular geometry of 2-BOP. The molecule is found to exist in two isomeric forms, trans and cis. The two rings in the trans isomer (with the angle between the ring planes being around 30°) are found to be more planar than the cis isomer (for which the above dihedral angle is 60°) and are also found to be more planar than BP, 2,2′-DPK or 4-BOP, for which the dihedral angles between the ring planes are 60°, 45°, and 50°, respectively. The orientations of the two rings with respect to the carbonyl group are such that the point group of the molecule is C1, although the geometry of the carbonyl group [C6 (C7=O) C8] is strictly planar. An extensive study on the spectral characteristics of 2-BOP has been carried out. All these spectral studies are accompanied by necessary and valuable QCC. Moreover, detailed analyses of the REPs of several normal modes have also been carried out to get insight into the properties of the molecule in different electronic states. DFT is also found to reproduce the singlet vertical excitation energies and is also compatible with the findings from the REP studies. From a comparative study of the REPs of the different normal modes of vibration and also from absorption spectra, it is seen that experimentally allowed transitions around 262 and 238 nm occur to the excited states S2 and S3, respectively, where the major geometry changes involve ring CC/CN and CO stretching vibrations. The major intensity contribution to the Raman bands corresponding to the phenyl and pyridyl ring triangular modes, pyridyl ring breathing mode, and phenyl CH angle bending (having good contributions also from the CC-stretching vibrations) mode comes from the S4 band, which has been found from REP studies to be present around 185 nm in conformity with the theoretical calculations. Besides these, the charge densities on the atoms in the two rings and the carbonyl group have been calculated to determine the possible sites of adsorption on the silver surface of silver sol and are found to be compatible with the experimental findings. This may also provide some clues to the metal corrosion inhibition properties of the molecule.
Footnotes
ACKNOWLEDGMENT
Pinaky Sett expresses thanks to UGC, Government of India, for financial support through the minor research project (MRP Project No. F. PSW-078/08-09).