DFT calculation and Raman excitation profile studies of benzophenone molecule

Vibrational Spectroscopy 44 (2007) 331–342 www.elsevier.com/locate/vibspec

DFT calculation and Raman excitation profile studies of benzophenone molecule P. Sett a,*, T. Misra b, S. Chattopadhyay c, A.K. De d, P.K. Mallick b a

Department of Physics, Gobardanga Hindu College, North 24-Parganas 743273, West Bengal, India b Department of Physics, University of Burdwan, Golapbag, Burdwan 713104, West Bengal, India c Department of Spectroscopy, Indian Association for the Cultivation of Science, Calcutta 700032, India d Department of Engineering Science, Haldia Institute of Technology, Haldia 721657, West Bengal, India Received 25 September 2006; received in revised form 30 January 2007; accepted 12 February 2007 Available online 24 February 2007

Abstract Excitation profiles of different Raman bands of benzophenone molecule have been critically analysed. Structural and symmetry properties of the molecule in different electronic states have been investigated. The possibility of existence of an excited electronic state in the region bellow 200 nm has been explored. Calculations on isolated molecule in gas phase have been performed with the use of density functional theory to correlate the observed vibrational spectra. The time dependent density functional theory has used to determine the singlet excitation energies. The optimized structural parameters have also been computed. # 2007 Elsevier B.V. All rights reserved. Keywords: Vibrational spectra; Raman excitation profile; Sum over states method; Excited electronic states; Molecular configuration; Quantum chemical calculation

1. Introduction Photophysical and photochemical properties can be understood with much elegance if a good knowledge of the geometrical and electronic structural behaviour of molecules in their ground and excited electronic states is acquired [1–9]. Electronic spectroscopy can provide both quantitative and qualitative information regarding geometries of the molecules in the relevant electronic states. The study of Franck–Condon envelop can offer insights into the geometrical distortions which occur upon electronic distortion. Again both normal and resonance Raman spectroscopy are very powerful tools in studying the structural properties of molecules in the ground and excited electronic states [10–12]. Analysis of Raman Excitation Profile (REP) for resonance excitation bridges the gap between electronic (absorption and emission) spectroscopy on one hand and ordinary Raman Spectroscopy on the other

* Corresponding author. Tel.: +91 33 2532 1728. E-mail addresses: [email protected] (P. Sett), [email protected] (A.K. De), [email protected] (P.K. Mallick). 0924-2031/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2007.02.004

hand. But in case of non-resonant excitation, contributions from several electronic states to Raman intensities might be significant. For such kind of excitation, extensive investigations of REPs are found to be very helpful in getting structural insights and in studying many other interesting properties of molecules in different excited electronic states [13–20]. For resonance excitation, there exist some theories [10– 12,1–28], which explain the observed REPs in terms of several parametric values of the resonating and its nearby states, as in such cases, Raman intensities are governed by contributions from those states. In the present work, attention has been focused to measure and study the excitation wavelength dependence of several fundamental normal modes of vibration of benzophenone (BOP) molecule as a part of our ongoing investigation on the vibrational dynamics of different compounds containing two aromatic rings. As Raman excitation lies in a region not close to any excited electronic state, the contributions from several electronic states might be important in the scattering process [29–31]. So in this communication, experimental findings have been found to be simulated satisfactorily by using sum-over-states (SOS) method based on pertinent Franck-Condon and vibronic (Herzberg–Teller)

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coupling terms [13–20]. Previously Kamei et al. [32] also showed that the confirmation of BOP is affected by electronic excitation from the ground electronic state. Besides ab initio and density functional theoretical (DFT) calculations have been successfully extended which corroborates the experimental observations regarding assignment normal modes and ground and excited molecular geometry. 2. Experimental and computational procedure 2.1. Chemicals BOP is supplied by Aldrich Chemical Company, USA with purity grade 99% and was used as such. Spectroscopic grade solvents cyclohexane, carbon-tetrachloride and chloroform are purchased from SLR, India. Concentrations of the solutions are maintained around 1 M for Raman spectra and 106 M to 102 M for the electronic absorption spectra.

computed on the ground state geometry, specially in the study of the solvent effect [35–37]; thus TD-DFT method is used with B3LYP function and 6-311G(d,p) basis set for vertical excitation energy of electronic spectra. Calculations are performed for vacuum/gas phase, and cyclohexane environment. To simulate the solvent effect the IEFPCM (Polarization Continuum model) model is used [38–41]. For singly excited electronic spectra, configuration interactions (CI) are carried out by 10  10 occupied and unoccupied molecular orbital. For revealing the nature of excited states, BOP is treated by CIS method using 3–21 basis set. Molecular co-ordinates in ground excited electronic states are obtained from minimum energy geometry using Chem-Office molecular modeling software. Using Gauss View 3.0 and Molekel 4.2 program visual inspection of different normal modes animation has been made. 3. Results and discussions 3.1. Vibrational analysis

2.2. Instrumentation The electronic absorption spectra are recorded by UNICAM UV 500 UV–vis spectrophotometer. The infrared spectra are taken in thin film on a previously calibrated Perkin Elmer Model 783 infrared spectrophotometer. The resolution of the infrared bands is about 2 cm1 for sharp bands and slightly less for the broader ones. Raman spectra of the samples in pure form and in chloroform and carbon tetrachloride solutions are monitored with a Spex Raman Spectrophotometer (RAMALOG) system at room temperature (around 30 8C), fitted with an Ar+ ion laser using 514.5, 501.7, 496.5, 488.0, 476.5 and 457.9 nm as exciting wavelengths, interfaced with the computer in photon counting mode. For other critical details of experiment Refs. [18,19] may be consulted. 2.3. Theoretical calculations

The vibrational analysis of BOP molecule was previously done by Volovsek et al. [42]. But while analyzing the Raman spectra for various excitation wavelengths and the infrared spectra of the molecule, certain ambiguities are encountered. So the task has been taken up to analyse the polarized Raman spectra of the molecule in different environments (Fig. 2) along with their infrared counterpart and thus to carry out the assignment job afresh and also to compare it with the previous one [42]. Polarised Raman spectra of BOP in the pure form and also in different solutions (chloroform and carbon-tetrachloride) are recorded. The relevant spectral data including their infrared counterpart and computed wavenumbers are presented in Table 1 along with the assignments of different wavenumbers to different normal modes. It is to be emphasized that the calculated frequencies represent vibrational signatures of

The geometry optimization and vibrational wavenumber calculation in the ground state of the BOP (Fig. 1) is done both by DFT and RHF method with B3LYP function and 6311G(d,p) basis set of Gaussian-98 Package [33,34]. As time dependent density functional theory (TD-DFT) is able to detect accurate absorption wavelengths at a relatively small computer time which correspond to vertical electronic transitions

Fig. 1. Optimized ground state geometry (DFT) of benzophenone (BOP) molecule with atom numbering.

Fig. 2. Polarised Raman spectra of BOP with excitation wavelength 514.5 nm, where asterisks indicate respective solvent bands and the sign ‘x’ denotes the plasma line. [A] for pure substance, [B] in CHCl3 solution and [C] in CCl4 solution.

P. Sett et al. / Vibrational Spectroscopy 44 (2007) 331–342

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Table 1 Vibrational assignment of BOP (wavenumbers in cm1)a Species [1]

Raman In solid phase [2]

A

Infra-red spectra [5] In CHCl3 solution [3]

In CCl4 solution [4]

Scaled DFT frequency [6]

Assignment [7]

641 717 917 942 996 997 1027 1159 1159 1487 1490 1605 1607 3057 3058 3065 3068 3070 3071

aCCC (I) aCCC (II) aCCC (I) aCCC (II) Ring breathing (I, II)

b

A1 c

A2 b

Bb B1 c

637 (w) 724 (s) 918 (vvw) 940 (vw) 1003 (vvs)

722 (m) P

720 (m) P

940 (vvw) 1000 (vvs) P

947 (vvw) 1000 (vvs) P

918 (vs) 944 (vs) 998 (s)

1027 (s) 1164 (m)

1026 (vvs) P 1158 (m) P

1026 (vvs) P 1157 (m) P

1027 (s) 1160 (m)

1490 (w)

1486 (w)

1485 (w)

1492 (w)

1600 (vs)

1603 (vvs) P

1603 (vvs) P

1594 (s)

3059 (s)

3057 (sh)

3061 (sh)

3060 (m)

3062 (m)

3070 (s) 3082 (sh)

3071 (sh) 3085 (w)

3068 (sh)

404 411 810 848

401 (vvw) D 409 (vvw) D

400 (vvw) 409 (vvw)

851 (vvw)

849 (vvw)

624 (w) D

617 (w) D

(m) (m) (vw) (w)

623 (m)

637 (vvs)

3088 (m)

1076 (w) 1174 (w, sh) 1192 (w) 1313 (vw)

(m) (vvw) (vs) (m) (m)

618 (w, sh) 1076 (s)

1178 (vvw)

1179 (w)

1175 (m) 1317 (m, sh)

1322 (vw)

B2 c

402 412 813 842 973

1322 (vvs)

1447 (w)

1447 (w)

1447 (w)

1447 (vs)

1574 (m)

1578 (w)

1577 (w)

1575 (s)

3033 (w)

3037 (w)

3032 (w)

3032 (m)

3056 (m)

3053 (w)

3050 (m)

3054 (s)

440 (m)

436 (w)

435 (s) 694 (vvs)

704 764 857 934 990

(vvw) (m) (w) (vw) (w)

987 (w)

989 (w)

703 765 865 935 991

(vvs) (vs) (m) (vs) (sh)

405 415 816 846 971 975

618 621 1080 1084 1175 1180 1306 1306

bCH (I, II) bCH (I, II) nCC (I, II) nCC (I, II) nCH (I. II) nCH (I, II) nCH (I) nCH (II) wCC wCC gCH gCH gCH

(I) (II) (I) (II) (I, II)

aCCC (I, II) bCH (I, II) bCH (I) bCH (II) nCC (I, II)

1325 1326 1446 1446 1584 1585 3036 3036 3054 3054

bCH (I, II)

439 441 686 700 701 765 850 935 991 991

wCC (I, II)

nCC (I, II) nCC (I, II) nCH (I, II) nCH (I, II)

wCC (I, II) gCH gCH gCH gCH gCH

(I) (II) (I) (II) (I, II)

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Table 1 (Continued ) Species [1]

Raman In solid phase [2]

X-sens.

a

c

Scaled DFT frequency [6]

Assignment [7]

564 1150 1279 1651

43 61 92 134 209 231 282 371 562 1142 1258 1689

(CX (I) (CX (II) gCX (I) gCO gCX (II) bCX (I) bCX (II) bCO dCCC nCX (I) nCX (II) nCO

In CCl4 solution [4]

68 (m) 111 (m) 128/150 (m) 218 (sh) 226 (m) 286 (m) 374 (vw) 563 (m) 1149(s) 1276 (m) 1655 (vs)

b

In CHCl3 solution [3]

Infra-red spectra [5]

151 (sh) D 217 (w) D

147 (sh) D

287 (w) P

286 (w) P

566 (w) P 1150 (m) P 1279 (w) 1662 (vvs) P

564 (m) P 1149 (m) P 1276 (w) 1668 (vvs) P

(w, sh) (m) (vs) (vvs)

Calculation is performed using DFT method with B3LYP function and 6-311G(d,p) basis set. Considering the whole molecule belonging to C2 symmetry. Considering the molecule as substituted benzene belonging to C2V symmetry.

molecule in its gas phase. Hence, the experimentally observed spectrum of the solid and solution may differ to some extent from the calculated spectrum. In the DFT calculation, the B3LYP function tends to overestimate the wavenumbers of the fundamental modes compared to the experimentally observed values. In order to obtain a considerably better agreement with the experimental data, scaling factor has to be used. Therefore, DFT vibrational wavenumbers presented in Table 1 have been uniformly scaled down by the scaling factor of magnitude 0.9792 [43]. Moreover, the ab initio (RHF) calculation typically predicts vibrational wavenumbers larger than the observed experimental values though the respective scaling factor has been used. Thus the data with RHF calculations are not included in Table 1. As a consequence, the DFT method yields the better results in terms of wavenumbers. The observed slight disagreement between the theory and the experiment could be a consequence of the anharmonicity and of the general tendency of the quantum chemical methods to overestimate the force constants at the exact equilibrium geometry [44]. Nevertheless, after applying the scaling factor the theoretical calculation reproduce the experimental data well. In assigning the vibrational frequencies of BOP molecule, the visual inspection of the normal modes animated from the output files of DFT calculations using Gauss View 3.0 and Molekel 4.2 program have been conducted. The availability of the literature concerning the vibrational assignments of this [42] and related molecules [13–19] have also been consulted. In order to simplify the assignment, the molecule is considered to be a mono substituted Benzene C6H5X, where the substituent (X) contains another phenyl ring (f) with one hydrogen atom replaced by a carbonyl group (C O). Since these two phenyl rings are equivalent, they are designated as ring (I) and ring (II). Thus overall 66 normal modes of vibration of this molecule may be considered to be comprised of 60 normal modes arising from these two phenyl rings, three modes associated with the carbonyl group, one angle bending mode (df-C-f) and two torsion modes (tf-C). Corresponding modes of the two rings may differ in wave numbers and the magnitude of these splitting will depend on the strength of interaction between different parts

(internal co-ordinates) of the two rings. For a few modes these splitting are so small that they may be considered as quasi degenerate and for the other modes, significant splitting is observed. Any way these thirty normal modes of C6H5X molecule may be distributed among different symmetry species of the point groups (C2V) as 11A1 + 3A2 + 6B1 + 10B2. Of the in phase and out of phase planar triangular modes in mono substituted benzene molecules, one is found around 1000 cm1 and the wavenumber of the other mode is generally found at a lower value from that of the parent molecule (benzene), depending on the nature of the substituent. These two modes are observed as polarized bands in the Raman spectra. Some authors assigned the band around 1000 cm1 to the in phase (ring breathing) mode and the other to the out of phase (in plane angle bending aCCC) mode and the reverse assignments were done by others. On the basis of our previous work [45] related with the calculation of wavenumbers of different normal modes, the former assignment has been followed. The strong polarized Raman band at 1003 cm1 is assigned as the unique component of the ring breathing mode of the two phenyl rings, instead of the split components at 1002 and 948 cm1 as assigned by Volovsek et al. [42]. Another weak band (at 990 cm1) appears in this region. This vibration is thought to be an out of plane wagging (gCH) mode, as our theoretical calculation shows that two out of plane wags (at 991 cm1) exist in this region. The assignments of the other ring stretching (nCC) and in plane angle bending (bCH) modes are well in accordance with the present calculation and also with the previous work [42]. As shown in the Table 1, none of the above mentioned band shows appreciable splitting except one bCH mode of A1 symmetry species, which shows separation of about 18 cm1. But the significant amount of splitting is observed for symmetric and asymmetric vibrations of all Xsensitive modes i.e. stretching (nCX), in plane bending (bCX) and out of plane bending (gCX) modes. Some difficulties have been faced to assign one of the three ring CCC angle bending (aCCC) modes. Volovsek et al. [42] previously assigned the band around 940 cm1 as an in plane ring breathing mode. But we have reassigned this band as the

P. Sett et al. / Vibrational Spectroscopy 44 (2007) 331–342

out of phase planar triangular mode with its corresponding component at 918 cm1 for the other ring as suggested by our theoretical calculation. This type of assignment is also supported by the result of normal coordinate calculation of some substituted benzene molecules by Gambi et al. [46]. Other two aCCC vibrations under A1 and B2 symmetry species lie in the same region as proposed by Versanyi [47] for mono-heavysubstituents in the phenyl ring. Most of the assignments of the out of plane ring distortion modes are in accordance with the previous works [13– 19,42,45–47]. However, unlike previous study [42], some of the bands show significant splitting. One point noteworthy to mention here is that along with the assignments of the split components of the X-sensitive mode (10B) and those of five gCH modes, Volovsek et al. [42] assigned two wavenumbers 144 and 147 cm1 as PhC out of plane bending modes. Obviously some modifications regarding these assignments are needed. It is not unwise to think that the IR wavenumber at 144 cm1 is the counterpart of the Raman frequency at 147 cm1, which, in the present study, is observed at 150 cm1. According to visual inspection and examination of the nature of the normal mode of the relevant vibration, this wavenumber is corresponded with C O out of plane vibration. Again, in a similar way, the wavenumber 374 cm1 with its strong IR counterpart at 369 cm1 is assigned as the bCO mode and the wavenumber 563 cm1 is correlated with the CPhCCPh in plane angle bending dCCC mode owing to better fit of the theoretical calculation. The Cartesian displacements of the atoms for some substitution sensitive vibrations, calculated from DFT, are shown in Fig. 3. Kamei et al. [32], in their study of the np* state of jet-cooled Benzophenone, assigned the vibrations at 60 and 100 cm1 to

335

the X-sensitive torsional mode in the excited state. The corresponding ground state vibrational frequencies were calculated by Blazevic and Colombo [48] to be 45 and 50 cm1. The present theoretical calculation also yields nearly the same result. Since a good number of ring modes including all the Xsensitive modes exhibit significant splitting, it is expected that the two phenyl rings are not oriented evenly with respect to the carbonyl unit. Most probably the rings make an angle between themselves which is in accordance with the theoretical calculation. As mentioned above, to make the assignment job simple so far the molecule is considered as mono substituted benzene belong to C2V symmetry, where this substituent group contains a phenyl ring with a hydrogen atom replaced by a carbonyl group. It can be found from the assignment Table 1 that most of the A2 vibrations appear with significant intensities in the infra spectra which are expected to be absent as per group theoretical point of view. Definitely this indicates that the symmetry of the molecule as a whole must be lower than that of C2V. Moreover unlike diphenylamine [18] no out of plane phenyl ring mode exhibits Raman band which is polarized. So all these considerations encourage us to consider the molecule as belonging more towards C2. 3.2. Dipole moment Dipole moment for BOP is calculated from the stable geometry for ground state obtained by using TD-DFT with B3LYP/6-311G(d,p) basis set and presented in Table 2. The solvation effect was simulated using continuum model

Fig. 3. Cartesian displacements and calculated (B3LYP/6-311G(d,p)) wavenumbers of some X-sensitive vibrational modes of BOP. The number in the parentheses referred to the experimental value of the assigned band.

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Table 2 Calculated dipole moment of benzophenone molecule in different environment Dipole moment in ground state (Debye) Vacuum Cyclohexane Acetonitrile

2.9861 3.5141 4.141

Table 3 Electronic absorption spectra of benzophenone molecule in cyclohexane solution Peak positions (nm/cm1)

Transition

Band system

G ! S3

1

Ba/b band

210 nm

G ! S2 G ! S1

1

La band Lb band

248 nm 282 nm

G ! S0

np* band

30,836 cm1 29,886 cm1 28,846 cm1 27,675 cm1 26,405 cm1

1

(324.3 nm) (334.6 nm) (346.7 nm) (361.3 nm) (378.7 nm)

Vibronic analysis

Fig. 4. Absorption spectra of BOP in cyclohexane environment at concentration 104 M, and in inset at 102 M.

(0 + 4)  1270 (0 + 3)  1270 (0 + 2)  1270 0 + 1270 00

(es = 2.02, PI = 0.2) and 4.14 Debye in polar solvent acetonitrile (es = 37.5, PI = 5.8) [es and PI being dipole constant and polarity index, respectively]. The gradual increase of dipole moment with polarity signifies the charge delocalization due to solvation and depends on polarity. The increase in dipole moment also facilitates the charge transfer probability in the ground state.

IEF-PCM (polarization continuum model) for vertical excitation energy incorporated in the Gaussian-98 package. The value of dipole moment in the ground state is 2.99 Debye in vacuum, 3.51 Debye in non-polar solvent cyclohexane Table 4 Simulation of excitation wavelengths and oscillatory strengths of BOP moleculea Vacuum Excitation

Cyclohexane CI expansion co-efficient

Wavelength (nm)

Oscillator strength

Excitation

CI expansion co-efficient

Wavelength (nm)

Oscillator strength

48 ! 49

0.616

346.80

0.0003

44 ! 49 48 ! 49

0.222 0.604

343.26 (361)*

0.0003

46 ! 49

0.643

270.35

0.0029

46 ! 49 47 ! 49

0.529 0.393

272.28 (282)*

0.0028

45 ! 49

0.619

266.81

0.0017

45 ! 49

0.641

268.68

0.0028

44 ! 49 47 ! 49

0.351 0.481

264.75

0.0346

44 ! 49 46 ! 49 47 ! 49

0.351 0.347 0.390

266.30

0.0340

44 ! 49 47 ! 49

0.444 0.370

247.58

0.0160

44 ! 49 47 ! 49

0.457 0.316

248.14 (248)*

0.0164

48 ! 50

0.663

232.36

0.0005

48 ! 50

0.649

229.73

0.0005

48 ! 51

0.617

226.80

0.0007

47 ! 50 48 ! 51

0.209 0.559

224.93

0.0006

45 ! 51 48 ! 51 48 ! 52

0.258 0.203 0.551

220.81

0.0003

45 ! 51 48 ! 51 48 ! 52

0.276 0.312 0.456

218.93

0.0002

44 ! 50 45 ! 51 46 ! 50 48 ! 52

0.226 0.286 0.255 0.385

217.06

0.0005

215.24

0.0010

47 ! 50 48 ! 52

0.227 0.466 0.260 0.219 0.391 0.226 0.215

0.0052

0.234 0.300 0.355

45 ! 50 45 ! 51 46 ! 50 47 ! 50 47 ! 52

214.14 (210)*

45 ! 50 46 ! 50 47 ! 50

214.31

0.0058

Only CI expansion coefficients with absolute value > 0.2 are included in the table. Asterisk (*) indicates the experimental absorption peak). a Calculation is performed using TDDFT method with B3LYP function and 6-311G(d,p) basis set.

P. Sett et al. / Vibrational Spectroscopy 44 (2007) 331–342

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Table 5 Equilibrium geometry of DPA in internal coordinate systema Ground state (G) Ring 1 Atomic distance (in A0) DCC D(C1(C2) D(C2(C3) D(C3(C4) D(C4(C5) D(C5(C6) D(C6(C1)

Excited state (S0)

Excited state (S1)

Excited state (S2)

Excited state (S3)

1.393 1.396 1.389 1.401 1.401 1.393

1.384 1.387 1.377 1.393 1.391 1.382

1.384 1.387 1.378 1.393 1.392 1.383

1.436 1.391 1.371 1.489 1.418 1.369

1.366 1.387 1.447 1.390 1.393 1.440

DCH D(C1(H15) D(C2(H16) D(C3(H17) D(C4(H18) D(C6(H19)

1.084 1.084 1.084 1.083 1.083

1.072 1.072 1.072 1.069 1.071

1.072 1.072 1.072 1.070 1.071

1.072 1.071 1.071 1.069 1.068

1.070 1.070 1.071 1.070 1.064

DCX D(C5(C13)

1.502

1.461

1.484

1.434

1.492

Atomic angle ACCC A(C1C2C3) A(C2C3C4) A(C3C4C5) A(C4C5C6) A(C5C6C1) A(C6C1C2)

119.90 120.00 120.50 119.10 120.30 120.10

119.70 120.10 120.60 119.00 119.90 120.30

119.70 120.10 120.70 118.80 120.40 120.20

119.50 120.10 120.90 117.10 119.90 122.00

118.80 120.50 121.00 117.30 121.40 120.90

ACCH A(C6C1H15) A(C2C1H15) A(C1C2H16) A(C3C2H16) A(C2C3H17) A(C4C3H17) A(C3C4H18) A(C5C4H18) A(C5C6H19) A(C1C6H19)

119.80 120.10 120.00 120.00 120.00 119.90 121.10 118.40 120.00 119.70

119.60 120.10 120.10 120.20 120.10 119.80 121.30 118.20 120.20 119.50

119.70 120.00 120.10 120.20 120.00 119.90 121.00 118.30 120.10 119.50

119.70 118.30 119.40 121.00 120.00 119.80 123.40 115.60 119.80 120.20

119.00 119.00 120.40 120.80 119.30 120.20 122.10 116.90 119.10 119.50

ACCX A(C4C5C13) A(C6C5C13)

117.90 122.90

117.00 123.80

116.90 124.10

114.30 128.50

115.30 127.00

Ring 2 Atomic distance (in A0) DCC D(C7(C8) D(C8(C9) D(C9(C10) D(C10(C11) D(C11(C12) D(C12(C7)

1.401 1.389 1.396 1.393 1.393 1.401

1.460 1.355 1.387 1.422 1.351 1.410

1.489 1.371 1.391 1.436 1.369 1.418

1.393 1.378 1.387 1.384 1.383 1.392

1.501 1.362 1.381 1.419 1.420 1.420

DCH D(C8(H20) D(C9(H21) D(C10(H22) D(C11(H23) D(C12(H24)

1.083 1.084 1.084 1.084 1.083

1.075 1.071 1.070 1.071 1.069

1.069 1.071 1.070 1.072 1.068

1.070 1.072 1.072 1.072 1.071

1.067 1.069 1.068 1.069 1.062

DCX D(C7(C13)

1.502

1.360

1.435

1.484

1.473

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P. Sett et al. / Vibrational Spectroscopy 44 (2007) 331–342

Table 5 (Continued ) Ground state (G)

Excited state (S0)

Excited state (S1)

Excited state (S2)

Excited state (S3)

Atomic angle ACCC A(C7C8C9) A(C8C9C10) A(C9C10C11) A(C10C11C12) A(C11C12C7) A(C12C7C8)

120.50 120.00 119.90 120.10 120.30 119.10

120.30 119.90 120.00 121.60 119.30 118.60

120.90 120.10 119.50 122.00 119.90 117.10

120.70 120.10 120.00 120.20 120.40 118.80

121.00 120.50 119.70 115.90 120.70 115.90

ACCH A(C7C8H20) A(C9C8H20) A(C8C9H21) A(C10C9H21) A(C9C10H22) A(C11C10H22) A(C10C11H23) A(C12C11H23) A(C11C12H24) A(C7C12H24)

118.40 121.10 119.90 120.00 120.00 120.00 120.10 119.80 119.70 120.00

115.90 123.80 119.90 120.20 120.70 119.30 118.70 119.80 120.80 119.90

115.60 123.50 119.80 120.00 121.00 119.40 118.30 119.70 120.20 119.80

118.30 121.00 120.00 120.00 120.20 120.10 120.00 119.70 119.50 120.10

116.30 119.80 119.80 119.70 120.90 119.40 118.10 119.70 119.50 119.50

ACCX A(C8C7C13) A(C12C7C13)

117.90 122.90

113.00 128.30

114.30 128.50

116.90 124.10

114.50 129.30

Substitute Atomic distance (in A0) DCO D(C13(O14) Atomic angle ACCC A(C5C13C7) ACCO A(C5C13O14) A(C7C13O14)

1.219

1.1632

1.256

1.256

1.305

120.30

124.70

121.60

121.70

122.10

119.80 119.80

120.70 114.50

119.90 118.50

118.40 119.80

117.70 120.20

a

Ground state optimization is performed using DFT method with B3LYP function and 6-311G(d,p) basis set whereas to get optimized geometries of different excited states CIS method is employed with 3–21 basis set.

3.3. Electronic absorption spectra The electronic absorption spectra of BOP in cyclohexane solution at room temperature (27 8C) are shown in Fig. 4. As appeared from this figure, the spectra are consisted mainly of four bands. The first one, appearing on the longer wavelength side, in the region between 300 and 400 nm side is a weak band system (emax  100, f  0.001). This band shows a structure and it appears only at high concentration (1.23  102 M). This band is assigned as the G ! S0 (np*) band [49]. A vibronic analysis of this system has been attempted and is shown in Table 3. In this band system about five peaks have been observed and the highest wavelength peak at 378.7 nm has been assigned as the 0–0 band (see Table 3). The entire spectra is analysed in terms of the appearance of a v/-progression (originating from v== ¼ 0) of an excited state fundamental of wavenumber around 1270 cm1. This wavenumber most probably corresponds to the nC O mode in the excited S0 (np*) state. Similar progression is found in the phosphorescence excitation spectra [49]. The appearance of the v= progression indicates that the molecule is distorted in the

relevant excited electronic state (designated as S0) along this mode. Such kind of effect is expected in the carbonyl compounds. In acetophenone molecule also, the G ! S0 (np*) band was found to have a vibrational structure and the 0–0 band was assigned at 27692 cm1 (361.1 nm) in 3-methyl pentane solution [49]. Unfortunately we could not measure REP (see below) of this mode owing to its weak intensity in the Raman spectra of the molecule in different solutions. The weak Raman intensity of this mode may be the result of weak intensity of the G ! S0 (np*) electronic band system. At the lower concentration (1.23  104 M) three other band systems appear around 282, 248 and 210 nm, the last two of which are found to have more or less even intensity. The band, around 282 nm, is comparatively much weaker and is assigned as the 1Lb band. Let us designate the corresponding excited states as S1. The two other band systems at 248 and 210 nm are therefore assigned as the 1La and 1Ba/b bands. Let us designate the corresponding excited states as S2 and S3. In diphenylamine [18] 1 La band is found around 286 nm. Thus it is natural to expect that the two rings are more non-planar in the present molecule than the rings of diphenylamine. This is well supported by the

P. Sett et al. / Vibrational Spectroscopy 44 (2007) 331–342

theoretical calculation, which exposes the fact that the angle between the ring planes of the molecule in the ground state is 600 and that in different excited states lies within 450–500. The computed electronic spectra of the molecule BOP in gas phase and in cyclohexane environment with the corresponding oscillator strength is presented in Table 4. The theory predicts that in both cases the bichromophore has C2 symmetry with non-planar structure. Such kind of observation also has been apprehended from the analysis of vibrational spectra. The optimized structure of the molecule in ground and different excited states are depicted in the Table 5.

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From the computation it is assigned that the HOMO is the molecular orbital numbers 48 with A symmetry and LUMO is the molecular orbital number 49 with the same A symmetry. In the theoretical simulation a dipole allowed transition is calculated at 343 nm. This calculation (for vertical transition) agrees well with the experimentally observed n ! p* band. In this particular basis set function, 1Lb state is correspond with that calculated at 272 nm. In order of increasing, the theoretically observed fifth and tenth excited singlet states are correlated with those associated the 1La and 1Ba/b band systems.

Fig. 5. Measured and calculated REPs for BOP. The symbols (* and o) indicate the measured profiles in CHCl3 and CCl4 solutions, respectively. n3 denotes classical calculated profiles considering (na–n0)3 dependence only. (I, J) Calculated profile considering contribution from SI and SJ electronic states. (I = J) denotes A-term contribution and (I 6¼ J) denotes B-term contribution and interference between two A-terms. The B-term contribution and the interference between two A-terms superpose with each other. All the (I, J) symbols attached with double arrowed line are in accordance with vertical positions of the respective curves.

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P. Sett et al. / Vibrational Spectroscopy 44 (2007) 331–342

Fig. 5. (Continued ).

3.4. Raman excitation profile (REP) Following the methodology as discussed earlier [13–19], the relative contribution of Albrecht’s A-term and B-term has been calculated separately to get theoretical REPs of different modes of vibration. The A term is generally found to be responsible for enhancing the totally symmetric vibrations for tuning the excitations to the region of an allowed electronic transition. For excitation not close to resonance, contribution from B-term is not unlikely. In such a case both totally and non totally symmetric modes may be responsible for vibronic coupling of one electronic state with another, provided the symmetry of the product of two electronic wave functions is same as that of the concerned normal mode. The relative contribution of B-term with respect to A-term is found to increases as the excitation wavenumber (¯n0 ) is further away from resonance with an allowed electronic transition. In the present case, as the excitation is away from resonance, the contributions of both the terms to Raman intensities may be imperative. Both the calculated (theoretical) and observed (experimental) REPs of several normal modes of vibration of the molecule are presented in Fig. 5. Two polarized Raman bands at 563 and 623 cm1 are assigned as aliphatic (dCCC) and aromatic (aCCC) angle bending modes. Raman excitation profiles (REPs) in Fig. 5 show that these two modes are getting major intensity contributions from the diagonal terms of the scattering tensor through the electronic state S3. (Regarding various EESs and their designations see the section of electronic absorption spectra). Generally aCCC modes have good amount of mixing with the in plane distortion of the phenyl ring [42]. Thus the above findings in the REPs are indication of the appreciable change in the ring dimensions in this electronic state. The validity of such conclusion is strongly supported by quantum chemical calculation. Table 5 also puts on view that the some of the ring angles [A(C4C5C6), A(C8C7C12) and A(C10C11C12)] are changed appreciably in S3 electronic state.

The two triangular modes, in phase and out of phase, corresponding to the ring breathing and the in plane ring distortion (aCCC) modes, have been assigned to two strong and highly polarized Raman bands at 1003 and 723 cm1 (see Table 1). Some interesting observations have been made in the excitation profiles of these two modes which also show some solvent effect. The REP of the ring breathing mode shows that major contributions to Raman intensity in chloroform solution comes from the lowest curve (i.e. classical contributions). This means that the intensity contribution to this Raman band comes from electronic states lying very high in the energy scale. But in carbon-tetrachloride solutions, the REP lies between the classical curve and that arising from the diagonal contribution from the electronic state S3. Similar effect has been observed for the angle bending mode (720 cm1) in carbon-tetrachloride solution. The REP of this mode in chloroform solution shows that the major contribution to Raman intensity comes from the electronic state S3 through A-term. This is a clear indication of the distorted ring geometry in the S3 state. If a lIG (corresponding to n¯ IG ) is chosen around 180 nm, the diagonal contributions from this state to Raman intensities of these two bands in carbon-tetrachloride solution through A-term also become very fascinating. The relevant excited state is designated as S4. Similar distortion effect may be expected in this electronic state. For the two prominent, polarized Raman active bCH modes REPs have been measured. Of these, one with lowest wavenumber (1027 cm1) has a REP showing major contributions coming from the lowest lying theoretical curve (i.e. from the classical curve). This means that this mode is drawing intensity contribution from state(s) lying very high in the energy scale. Again the electronic state S3 is principally contributing to the remaining mode of wavenumber 1164 cm1 through A-term. Again, according to the Table 5, except angles associated with the two bCH modes (bC1H15 and bC4H18), all other CCH angle change lies within 10 in all excited states.

P. Sett et al. / Vibrational Spectroscopy 44 (2007) 331–342

The wavenumber at 1149 cm1 is depicted as X-sensitive stretching (nCX) mode. The REP of this mode indicates the solvent effect to some extent. In CHCl3 solution this vibration is getting intensity contribution from the excited electronic state S3. But in CCl4 solution major contributor is B-term, which couples the states S2 and S3. Again this B-term contribution implies that this mode is also effective in mixing the two electronic states S2 and S3. Table 5 also hints towards the fact that C–X bond lengths are also modified in S2 and S3 states with respect to the ground state. REPs have also been measured for a strong and polarized Raman band of wavenumber 1600 cm1 which corresponds to a nCC mode. The major intensity to this mode comes from the diagonal contribution of the electronic state S2 through A-term. The A-term contribution from the S2 state indicates that the molecule is appreciably distorted in this electronic state along this mode. On the other hand, theoretical calculation shows that the bond D(C4C5) are elongated about 0.088 A0 in this electronic state, while the other CC ring bond length changes are within 0.045 A0. Thus it is expected that this D(C4C5) bond may have major contribution in the potential energy distribution of this mode. The last mode for which the REP has been measured is the C O stretching mode (nCO) at 1655 cm1. REP of this mode shows that diagonal contribution from the state S2 through Aterm is most effective. So it is expected that this state experiences an effective change in the C O bond length on excitation from the ground to this electronic state. This anticipation is also consistent with the theoretical result (see Table 5). So it is not unwise to expect that the S2 state may have a good amount of charge transfer between the p-clouds of the rings and that of the ketone group. 4. Conclusion The purpose of the paper is to confirm theoretically the experimental findings about the molecular geometry. It has been observed from the theoretical calculation that the angle between two ring planes is about 600 in the ground state. Again in different excited electronic states this measurement lies within 450–500, although the relative orientations of two phenyl rings with respect to the carbonyl substitution vary in those states. Table 5 also depicts the planar geometry of carbonyl group (CPhC OCPh) though the relative angles among the relevant bonds are different in all these states. Further, according to REP measurements, geometry change of molecule is expected in the excited states S2 and S3 associated with the allowed transitions at 248 and 210 nm, respectively, since distortion of ring geometry (DR) is related with the shift parameter (DIa ) of the potential energy minima of the excited electronic state jIi with respect to that of ground state corresponding to ath normal mode of vibration following the relation DIa L1 DR, L being the transformation matrix from normal to internal coordinate system. This result (i.e. the deformed molecular structures in S2 and S3 states) is in conformity with the theoretical calculation. Another upshot of the paper is that DFT method provides accurate description of

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