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Two-dimensional Raman correlation analysis of benzophenone
Journal of Molecular Structure 929 (2009) 200–206
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Two-dimensional Raman correlation analysis of benzophenone Pinaky Sett a,*, Joydeep Chowdhury b, Prabal Kumar Mallick c a b c
Department of Physics, Gobardanga Hindu College, 24-Parganas(N) 743 273, India Department of Physics, Sammilani Mahavidyalaya, E.M. Bypass, Kolkata 700 075, India Department of Physics, University of Burdwan, Golpbag, Burdwan 713 104, India
a r t i c l e
i n f o
Article history: Received 7 February 2009 Received in revised form 22 April 2009 Accepted 22 April 2009 Available online 3 May 2009 Keywords: Benzophenone Vibrational spectroscopy Quantum chemical calculation Raman excitation profile Two-dimensional correlation analysis
a b s t r a c t Effect of varying excitation wavelength on the Raman spectral intensity of benzophenone molecule has been examined. The overlapped bands are resolved using two-dimensional correlation method. The symmetric and asymmetric pair associated with a particular type of vibration is highly asynchronously correlated. Evidently no corresponding cross peaks are present in the synchronous spectra. This finding indicates that the succession of change of amplitude of symmetric and asymmetric vibrations related with that specific mode is not simultaneous but completely chronological. Interaction of C@O group with the surroundings may be responsible for the appearance of a relatively lower wavenumber peak correspond to new type of C@O stretching vibration. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Two-dimensional (2D) correlation spectroscopy is now routinely used as a technical tool to analyze traditional one-dimensional spectra [1–13]. The key features of this method are: simplification of complicated spectra consisting of overlapped peaks, enhancement of spectral resolution by spreading peaks over the second dimension and extraction of interaction and dynamic information of functional group of the system [1–13]. By means of such powerful technique, the variation of Raman spectral intensities of some stretching and in-plane angle bending modes of Benzophenone molecule with the change in excitation frequencies have been investigated in this article. Benzophenone (BOP) is an essential compound in aromatic photochemistry and as well as in organic synthesis. It is commonly used as a constituent of synthetic perfumes. For the manufacture of adhesives, dyes, pesticides and drugs BOP is a major chemical. It can also act as optical filters or deactivate substrate molecules that have been excited by light for the protection of polymers and organic substances. These different industrial applications of BOP have attracted us to investigate the spectroscopic properties of BOP molecule. The molecule of our interest (i.e. BOP) belongs to C2 symmetry [14]. The phenyl moieties are placed at two wings of the carbonyl
* Corresponding author. Tel.: +91 94333 02465. E-mail address: [email protected] (P. Sett). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.04.034
group making an angle about 60° between the planes of the two rings. The loan pair electrons of oxygen are especially suitable for electron transfer mechanisms associated with different types of photophysical and photochemical processes. However, for better understanding of the photophysical and photochemical behavior of the molecule, a detailed knowledge of deformation of molecular excited state geometry with respect to that of ground state is essential [14–23]. In this context, extensive investigation on Raman excitation profile study is found to be very helpful in getting structural insights and many other interesting properties of molecules (e.g. symmetries of several excited states). The detailed Raman excitation profile study of the BOP molecule has been reported in our earlier communication [14]. In the previous article [14] the excited state properties of this double ring (phenyl) compound were explained only on the basis of intensity variation of strong Raman bands with the excitation wavelengths. But at that time, no such information had been collected for the relatively weak Raman bands because of the poor signal to noise ratio. To overcome this problem the 2D-correlation spectroscopy has been exploited to get valuable information associated with the variation of weak Raman intensity with the excitation frequency. Again according to the earlier publication, the vibrational signatures of some Raman bands were appearing degenerate, as the weak Raman peaks were completely hidden under the relevant envelopes of relatively strong neighbouring bands. Although the quantum chemical calculation has already removed this degeneracy, the 2D-correlation spectroscopic investigation has been carried out to verify the reality.
201
P. Sett et al. / Journal of Molecular Structure 929 (2009) 200–206 Table 1 Some selected vibrational wavenumbers (in cm
1
) of BOP molecule.
RAMAN*
IR*
Scaled DFT*
996 (ha) 997 (la) 1027 (la) 1029 (ha) 1142 1159 (la) 1159 (ha) 1584 (ha) 1585 (la) 1605 (ha) 1607 (la) 1689
Solid
CHCl3
CCl4
1003 (vs)
1000 (vs) P
1000 (vs) P
998 (vs)
1027 (s)
1026 (m) P
1026 (m) P
1027 (s)
1149 (s) 1164 (m)
1150 (m) P 1158 (w) P
1149 (m) P 1157 (w) P
1150 (m) 1160 (m)
1574 (m)
1578 (w)
1577 (w)
1575 (s)
1592 (vs)
1597 (vs) P
1597 (vs) P
1594 (s)
1653 (vs)
1655 (s) P
1660 (m) P
1651 (vs)
2D-Correlation
Assignment
CHCl3
CCl4
1000 1003 1025 1028 1152 1160
1000 1002 1025
1579
1577
1598 1601 1654 1659
1595 1599 1652 1661
1148 1156
Ring breathing (asym) Ring breathing (sym) CH in-plane bending (sym) CH in-plane bending (asym) X-sensitive stretching (sym) CH in-plane bending (asym) CH in-plane bending (sym) CC stretching (sym) CC stretching (asym) CC stretching (sym) CC stretching (asym) CO stretching (restricted) CO stretching (free)
The abbreviations are: vs, very strong; s, strong; m, medium; w, weak; ha, high activity; la, low activity; P, polarised; X, substitute. Ref. [14]
*
3. Calculation The strongest Raman bands, 667 and 461 cm 1 of CHCl3 and CCl4, respectively, are used as reference lines to estimate the relative intensities of the Raman bands of the titled compound, BOP. For the measurement of Raman excitation profiles, the Stokes intensity of each band is normalized relative to that for the excitation wavelength 514.5 nm. The excitation wavelength dependence of the intensity of the solvent band, used as internal standard, has also been taken into consideration. The detailed scheme of normalization of Raman signals has been described elsewhere [20–22]. The geometry optimization and vibrational wavenumber calculation in the ground state of BOP is done by density functional level of theory (DFT) using Gaussian 98 program [24] for Windows. The Pople split valance basis set 6-311G (d, p) in conjunction with B3LYP functional are used for the DFT computation. Gauss View 3.0 program is utilized for the visual inspection of the normal modes of the molecule. Animations of different vibrations help us to distinguish symmetric and asymmetric modes. 2D-correlation analysis is performed using 2D Shige version 1.3 software [25], developed in Professor Y. Ozaki’s laboratory (Kwansei Gakuin University, Sanda, Japan). Prior to 2D analysis, 5-point smoothing for all Raman spectra is carried out after base line correction in order to remove the spectral artifacts. The data matrix is arranged in the order from higher to lower excitation wavelengths. Second-derivative spectra have been obtained by the simplified least-squares method of Savitzky and Golay [26]. The derivative spectrum, especially, looks noisy since noise differences between adjacent data points are amplified by taking derivatives. Thus to improve the performance, final spectra are obtained after base line correction, normalization and 11-point smoothing of the original Raman spectra.
a
Intensity
Raman spectra of BOP in CHCl3 and CCl4 solutions of concentration around 1 M have been recorded with Spex Raman spectrometer (RAMLOG) system, fitted with Ar+ ion laser using 514.5, 501.7, 496.5, 488.0, 476.5 and 457.9 nm as an exciting wavelength. The accuracy of wavenumber measurements is ±1 cm 1 for strong Raman bands and slightly less for the others (see reference [14] for detail).
analyses have been applied to two independent sets of data. The first set is constructed from the spectra of BOP in CHCl3 solution and the second set from those in CCl4 solution. The experimentally observed and theoretically calculated wavenumbers, associated with 2D-correlation information of some selected vibrational modes, are assigned in Table 1. The Raman spectra of BOP in CHCl3 and CCl4 solutions at different excitation wavelengths are presented in Fig. 1. Synchronous 2D-correlation spectra from the first set, plotted as a contour map in the wavenumber ranges of 1680–1570 cm 1 (sec-
[vi] [v] [iv] [iii] [ii] [i]
1000
1100
1200 1300 1400 1500 -1 Raman Wavenumber (cm )
1600
1700
b
Intensity
2. Experimental
[vi] [v] [iv] [iii] [ii] [i]
1000
4. Discussion In order to explore the excitation wavelength dependent variation of the intensity of the ring breathing mode, 2D-correlation
1100
1200
1300 1400 1500 -1 Wavenumber (cm )
1600
1700
Fig. 1. The Excitation frequency dependent Raman spectra of BOP molecule in (a) CHCl3 and (b) CCl4 environment. Incident laser wavelengths are [i] 514.5, [ii] 501.7, [iii] 496.5, [iv] 488.0, [v] 496.5 and [vi] 457.9 nm.
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P. Sett et al. / Journal of Molecular Structure 929 (2009) 200–206
tion-I) and 1170–980 cm 1 (section-II), are presented in Fig. 2(a). In the section-I of synchronous map, the diagonal autopeaks appear at wavenumbers 1659 and 1600 cm 1 with a positive cross peak at (1659, 1600) cm 1. As previously mentioned [14] the first band is assigned as the C@O stretching mode, whereas the second one is corresponded to a pair of in plane ring deformation (i.e. CC stretching) vibrations associated with the two phenyl ring moieties. Though the 2D-correlation spectrum yields greater resolution than conventional 1D spectrum, the synchronous picture is unable to remove the frequency degeneracy. Evidently the quantum chemical calculation [14] shows that the band is almost degenerate. The appearance of the cross peak in the synchronous spectrum, due to simultaneous changes in the spectral intensities, suggests that strong correlation exists between two stretching modes in CHCl3 environment. Again the sign of the peak indicates that intensities of both the bands are increasing with the increase of excitation frequency. It is noteworthy to point out that more or less same type of excitation profile has been followed by each of these modes [14]. For both cases the excited electronic state at 248 nm (1La band) plays an important role in intensity enhancement. There is no autopeak generated from the relatively weak band at 1578 cm 1, another CC stretching vibration, probably because of its very small variation in intensity. However, the existence of an autopeak at 1579 cm 1 may be apprehended by constructing a correlation square through the autopeak at 1600 cm 1, positive cross
peak at (1600, 1579) cm 1 and the mirror image of this cross peak about the diagonal. The asynchronous spectrum represents sequential changes of the spectral intensities measured for the two wavenumbers, which generate the respective asynchronous cross peaks. As depicted in Fig. 2(b), intense negative cross peak appears at (1601, 1598) cm 1 in section-I. Thus asymmetric and symmetric CC stretching vibrations, not readily noticeable in the synchronous spectrum separately are now clearly distinguished in the asynchronous spectrum. Another very weak positive peak is noticed at (1657, 1601) cm 1 analogous to the positive cross peak of the synchronous spectrum. The sign rule [3,13] reveals that the variation in spectral intensity at X-axis wavenumber occurs before the variation in the spectral intensity at Y-axis wavenumber. Since the data matrix has been arranged from higher to lower incident laser wavelength, then the intensity enhancement of CC stretching vibration (1601 cm 1) occurs at higher excitation frequency than that of C@O stretching mode. From the sign of the asynchronous cross peak at (1601, 1579) cm 1, it appears that the sequence of intensity variation with the decrease of excitation wavelength occurs in the order from 1579 to 1601 cm 1. There are three other positive cross peaks at (1654, 1002), (1654, 1148) and (1654, 1601) cm 1. Surprisingly, the peak at 1654 cm 1, as has been resolved from the correlation spectrum, was not observed before. The presence of the new peak implies the existence of a new kind
Fig. 2. (a) Synchronous and (b) asynchronous 2D-Raman correlation spectra generated from excitation wavelength dependent spectral intensity variations of ring breathing modes of BOP molecule in CHCl3 environment. The shaded areas represent the negative correlation intensity. Average Raman spectrum is added at top.
P. Sett et al. / Journal of Molecular Structure 929 (2009) 200–206
203
Fig. 2 (continued)
of C@O bond in the system. As is well known, if the C@O bond interacts with its surroundings, the wavenumber of the C@O stretching vibration may shift to a lower frequency. In synchronous contour plot of section-II [Fig. 2(a)] two autopeaks are located at 1152 and 1002 cm 1. The wavenumbers are assigned as substitute sensitive stretching and ring breathing mode, respectively. The connection of two autopeaks with the detectable cross peaks provides four correlation squares which suggest the probable existence of other diagonal peaks at 1160 and 1028 cm 1 related with different CH in-plane bending vibrations. Although these bands do not come into sight on the diagonal of the synchronous spectra, most likely due to poor intensity variation, their influence is realized definitely in the off-diagonal region of section-I & II. The asynchronous picture [Fig. 2(b)] provides more useful information than the synchronous one. The synchronous band at 1002 cm 1 splits into two components which appear at coordinates 1003 and 1000 cm 1 associated with symmetric and asymmetric ring breathing modes, respectively. Other distinguishable cross peaks are (1028, 1000) and (1025, 1003) cm 1 with negative and positive correlation coefficients, respectively. According to the previous quantum chemical calculation the lower and higher wavenumbers (i.e. 1025 and 1028 cm 1) are assigned as symmetric and asymmetric CH in-plane bending vibrations. It is a clear evidence of strong correlation of symmetric–symmetric and asymmetric–asymmetric vibrations. Furthermore, the cross peaks in the region of coordinates (1680–1570, 1170–980) cm 1 reflects
the dominant correlation of the respective bands. In this region also symmetric–symmetric and asymmetric–asymmetric correlations have been found between CC stretching and ring breathing modes. Fig. 3(a) and 3(b) display synchronous and asynchronous 2Dcorrelation spectra, respectively, from second data set. Four autopeaks are shown in synchronous spectrum at 1661, 1597, 1148 and 1000 cm 1 whose band positions and intensity are almost same as set one. Along with these bands other two weak but definite peaks are also identified at 1156 and 1025 cm 1 as in CHCl3 environment. The cross peaks of the synchronous spectrum are all positive and order of strength is comparable to that of first data set. In the asynchronous plot, the wavenumber 1577 cm 1 forms cross peaks with 1661 and 1597 cm 1. Examining the signs and the values of asynchronous parameters the sequence of intensity variation with changing excitation wavelength may be inferred as: 1577 ? 1597 ? 1661 cm 1. Once more the asynchronous view [specifically the cross peaks at (1599, 1595) and (1002, 1000) cm 1] establishes the splitting of symmetric and asymmetric CC stretching deformation bands centered at 1597 and 1000 cm 1, respectively. New cross peaks at (1661, 1652) appears due to two different kind of C@O vibrations: lower one probably arises, as said earlier, due to interaction of the C@O bond with its surroundings and other one may be assigned as free C@O stretching vibration. Baseline fluctuation may be responsible for the appearance of other weak cross peaks, as they often
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P. Sett et al. / Journal of Molecular Structure 929 (2009) 200–206
Fig. 3. (a) Synchronous and (b) asynchronous 2D-Raman correlation spectra generated from excitation wavelength dependent spectral intensity variations of ring breathing modes of BOP molecule in CCl4 environment. The shaded areas represent the negative correlation intensity. Average Raman spectrum is added at top.
cause artifacts particularly in asynchronous spectra [13]. In fact the CCl4 asynchronous spectrum is more complicated than the CHCl3 one. It is rather unlikely that symmetric and asymmetric CC stretching modes near 1000 or 1600 cm 1 share the asynchronous peaks. Particularly the peaks near 1600 cm 1 are debatable as they show roughly butterfly pattern in the asynchronous representations [Fig. 2(b) and 3(b)]. Since shifts of the corresponding synchronous peaks in this region with the change in the excitation wavelength may also be responsible for such kind of band appearances. Thus in order to check whether two independent bands do really exist in this region, the second-derivative method has been used as well. Generally, the second-derivative spectra could enhance the spectral resolution by amplifying the tiny differences in spectra [27]. The second-derivative spectra of BOP in CHCl3 and CCl4 solutions are shown in Fig. 4(a) and 4(b), respectively. Appearance of a weak band near 1592/1590 cm 1 along with the strong one at 1599/ 1597 cm 1 (in CHCl3/CCl4) becomes more evident in the spectra corresponding to excitation wavelength 457.9 nm. On the other hand in butterfly or angel (distorted butterfly) pattern, the asynchronous peak cluster consists of a pair of elongated cross peaks of opposing signs located very close to the diagonal [2]. Close to the main pair of elongated cross peaks, there also exists another set of weaker cross peaks, which are confined to smaller parts and located slightly away from the diagonal [2]. But even readjustment of contour level does not reveal any existence of such elongated or relatively weak confined cross peaks pairs. Thus in fact
the category of the pattern is neither really a butterfly type (related with band position shift) nor an angel one (connected with band position shift coupled with intensity change). Certainly such a typical characteristic cluster pattern is generated due to intensity changes of multiple bands. 5. Conclusion In this study generalized 2D-Raman correlation spectroscopy is successfully utilized to get some valuable information regarding vibrational properties of BOP molecule. Such type of study helps us to remove the quasi-degeneracy of different Raman bands. Thus this methodology may be utilized as an inbuilt program with Raman spectrophotometer to get high resolution spectra. At the same time here it also confirms better symmetric–symmetric and asymmetric–asymmetric asynchronous correlation between ring breathing mode and other ring CC stretching or CH in-plane bending vibrations. Consequently symmetric–asymmetric correlations of the ring breathing mode with other ones appear to be restricted. But strong asynchronous correlations exist between symmetric and asymmetric pair of different modes. The fact is pointed out that in both environments (i.e. in CHCl3 and in CCl4 solutions) the synchronous correlation characters of all the modes are almost same, whereas the asynchronous behaviors of them are quite different. All cross peaks in synchronous spectra are positive which signify increase of Raman band intensities with the decrease of incident wavenumber. Here it
205
P. Sett et al. / Journal of Molecular Structure 929 (2009) 200–206
Fig. 3 (continued)
a
b
[v]
[iv]
[iii]
[ii]
[vi]
Second Derivative of Intensity
Second Derivative of Intensity
[vi]
[v]
[iv]
[iii]
[ii]
[i]
[i]
1500
1550
1600
1650
1700
Raman Wavenumber (cm -1)
1500
1550
1600
1650
1700
Raman Wavenumber (cm -1)
Fig. 4. Second-derivative spectra in (a) CHCl3 and (b) CCl4 solvents. Incident laser wavelengths are [i] 514.5, [ii] 501.7, [iii] 496.5, [iv] 488.0, [v] 496.5 and [vi] 457.9 nm.
is worth mentioning that the C@O vibration is strongly affected by the solvents. Along with the free C@O stretching vibration, another kind of C@O stretching vibration with lower wavenumber
appears probably due to interaction of the C@O group with the surroundings, although the correlation characters of them are definitely solvent dependent. Both types of C@O vibration are
206
P. Sett et al. / Journal of Molecular Structure 929 (2009) 200–206
asynchronously correlated with symmetric ring breathing mode in CHCl3 environment, whereas no such evidence is found in CCl4 solution. Furthermore the sign of CC and C@O stretching off-diagonal correlation peak intensity is opposite in these two asynchronous Fig. 2(b) and 3(b). Thus in other words, sequence of intensity variation for these two normal modes in the spectra is just opposite in CHCl3 and CCl4 solutions. Strong synchronous cross intensity between these bands may be due to similar kind of intensity contribution from excited electronic states through Frank–Condon A-term as was observed earlier [14] from their excited profile studies. References [1] I. Noda, Appl. Spectrosc. 47 (1993) 1329. [2] I. Noda, Y. Ozaki, Two-dimensional Correlation Spectroscopy Applications in Vibrational and Optical Spectroscopy, John Wiley & Sons, Ltd., 2004. [3] M. Thomas, H.H. Richardson, Vibrational Spectrosc. 24 (2000) 137. [4] Y.M. Jung, B.C. Matusewicz, Y. Ozaki, J. Phys. Chem. B 104 (2000) 7812. [5] I. Noda, C. Marcott, J. Phys. Chem. A 106 (2002) 3371. [6] Y.M. Jung, Bull. Korean Chem. Soc. 24 (2003) 1243. [7] Y.M. Jung, B.C. Matusewicz, S.B. Kim, J. Phys. Chem. B 108 (2004) 13008. [8] I. Noda, Vibrational Spectrosc. 36 (2004) 143. [9] A.A. Moore, M.L. Jacobson, N. Belabas, K.L. Rowlen, D.M. Jonas, J. Am. Chem. Soc. 127 (2005) 7292. [10] I. Noda, J. Mol. Struct. 799 (2006) 2. [11] Y.M. Jung, I.S. Yang, J. Mol. Struct. 799 (2006) 173. [12] C. Mello, A.E.M. Crotti, R.V. Lourenço, W.R. Cunha, J. Mol. Struct. 799 (2006) 141.
[13] A. Wesełucha-Birczyn´ska, J. Mol. Struct. 826 (2007) 96. [14] P. Sett, T. Mishra, S. Chattopadhyaya, A.K. De, P.K. Mallick, Vibrational Spectrosc. 44 (2007) 331. [15] C. Dubroca, P. Lazaro, Chem. Phys. Lett. 24 (1974) 49. [16] S. Chakraborty, S.K. Sarkar, P.K. Mallick, Chem. Phys. Lett. 187 (1991) 93. [17] S.C. Shim, D.W. Kim, M.S. Keins, J. Photochem. Photobiol. 56A (1991) 227. [18] P. Jana, T. Ganguly, S.K. Sarkar, A. Mitra, P.K. Mallick, J. Photochem. Photobiol. 94A (1996) 113. [19] S.K. Pal, T. Sahu, T. Mishra, P.K. Mallick, M.A. Paddon-Row, T. Ganguly, J. Phys. Chem. 108A (2004) 10395. [20] P. Sett, S. Chattopadhyaya, P.K. Mallick, Chem. Phys. Lett. 331 (2000) 215. [21] P. Sett, A.K. De, S. Chattopadhyaya, P.K. Mallick, Chem. Phys. 276 (2002) 211. [22] T. Mishra, A.K. De, S. Chattopadhyaya, P.K. Mallick, P. Sett, Spectrochim. Acta 61A (2004) 767. [23] P. Sett, T. Misra, J. Chowdhury, M. Ghosh, S. Chattopadhyay, S. Sarkar, P.K. Mallick, J. Chem. Phys. 128 (2008) 144507. [24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski Jr., J.A Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J. Foresman, J.V. Bortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, J. Komaromi, I. Cioslowski, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M.H. Gordon, E.S. Replogle, J.A. Pople, Gaussian 98, Gaussian Inc., Pittsburgh, PA, 1998. [25] 2Dshige (c) Shigeaki Morita, Kwansei Gakuin University, 2004–2005. [26] A. Savitsky, M.J.E. Golay, Anal. Chem. 36 (1964) 1627. [27] A. Watanabe, S. Morita, S. Kokot, M. Matsubara, K. Fukai, Y. Ozaki, J. Mol. Struct. 799 (2006) 102.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Two-dimensional Raman correlation analysis of benzophenone Pinaky Sett a,*, Joydeep Chowdhury b, Prabal Kumar Mallick c a b c
Department of Physics, Gobardanga Hindu College, 24-Parganas(N) 743 273, India Department of Physics, Sammilani Mahavidyalaya, E.M. Bypass, Kolkata 700 075, India Department of Physics, University of Burdwan, Golpbag, Burdwan 713 104, India
a r t i c l e
i n f o
Article history: Received 7 February 2009 Received in revised form 22 April 2009 Accepted 22 April 2009 Available online 3 May 2009 Keywords: Benzophenone Vibrational spectroscopy Quantum chemical calculation Raman excitation profile Two-dimensional correlation analysis
a b s t r a c t Effect of varying excitation wavelength on the Raman spectral intensity of benzophenone molecule has been examined. The overlapped bands are resolved using two-dimensional correlation method. The symmetric and asymmetric pair associated with a particular type of vibration is highly asynchronously correlated. Evidently no corresponding cross peaks are present in the synchronous spectra. This finding indicates that the succession of change of amplitude of symmetric and asymmetric vibrations related with that specific mode is not simultaneous but completely chronological. Interaction of C@O group with the surroundings may be responsible for the appearance of a relatively lower wavenumber peak correspond to new type of C@O stretching vibration. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Two-dimensional (2D) correlation spectroscopy is now routinely used as a technical tool to analyze traditional one-dimensional spectra [1–13]. The key features of this method are: simplification of complicated spectra consisting of overlapped peaks, enhancement of spectral resolution by spreading peaks over the second dimension and extraction of interaction and dynamic information of functional group of the system [1–13]. By means of such powerful technique, the variation of Raman spectral intensities of some stretching and in-plane angle bending modes of Benzophenone molecule with the change in excitation frequencies have been investigated in this article. Benzophenone (BOP) is an essential compound in aromatic photochemistry and as well as in organic synthesis. It is commonly used as a constituent of synthetic perfumes. For the manufacture of adhesives, dyes, pesticides and drugs BOP is a major chemical. It can also act as optical filters or deactivate substrate molecules that have been excited by light for the protection of polymers and organic substances. These different industrial applications of BOP have attracted us to investigate the spectroscopic properties of BOP molecule. The molecule of our interest (i.e. BOP) belongs to C2 symmetry [14]. The phenyl moieties are placed at two wings of the carbonyl
* Corresponding author. Tel.: +91 94333 02465. E-mail address: [email protected] (P. Sett). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.04.034
group making an angle about 60° between the planes of the two rings. The loan pair electrons of oxygen are especially suitable for electron transfer mechanisms associated with different types of photophysical and photochemical processes. However, for better understanding of the photophysical and photochemical behavior of the molecule, a detailed knowledge of deformation of molecular excited state geometry with respect to that of ground state is essential [14–23]. In this context, extensive investigation on Raman excitation profile study is found to be very helpful in getting structural insights and many other interesting properties of molecules (e.g. symmetries of several excited states). The detailed Raman excitation profile study of the BOP molecule has been reported in our earlier communication [14]. In the previous article [14] the excited state properties of this double ring (phenyl) compound were explained only on the basis of intensity variation of strong Raman bands with the excitation wavelengths. But at that time, no such information had been collected for the relatively weak Raman bands because of the poor signal to noise ratio. To overcome this problem the 2D-correlation spectroscopy has been exploited to get valuable information associated with the variation of weak Raman intensity with the excitation frequency. Again according to the earlier publication, the vibrational signatures of some Raman bands were appearing degenerate, as the weak Raman peaks were completely hidden under the relevant envelopes of relatively strong neighbouring bands. Although the quantum chemical calculation has already removed this degeneracy, the 2D-correlation spectroscopic investigation has been carried out to verify the reality.
201
P. Sett et al. / Journal of Molecular Structure 929 (2009) 200–206 Table 1 Some selected vibrational wavenumbers (in cm
1
) of BOP molecule.
RAMAN*
IR*
Scaled DFT*
996 (ha) 997 (la) 1027 (la) 1029 (ha) 1142 1159 (la) 1159 (ha) 1584 (ha) 1585 (la) 1605 (ha) 1607 (la) 1689
Solid
CHCl3
CCl4
1003 (vs)
1000 (vs) P
1000 (vs) P
998 (vs)
1027 (s)
1026 (m) P
1026 (m) P
1027 (s)
1149 (s) 1164 (m)
1150 (m) P 1158 (w) P
1149 (m) P 1157 (w) P
1150 (m) 1160 (m)
1574 (m)
1578 (w)
1577 (w)
1575 (s)
1592 (vs)
1597 (vs) P
1597 (vs) P
1594 (s)
1653 (vs)
1655 (s) P
1660 (m) P
1651 (vs)
2D-Correlation
Assignment
CHCl3
CCl4
1000 1003 1025 1028 1152 1160
1000 1002 1025
1579
1577
1598 1601 1654 1659
1595 1599 1652 1661
1148 1156
Ring breathing (asym) Ring breathing (sym) CH in-plane bending (sym) CH in-plane bending (asym) X-sensitive stretching (sym) CH in-plane bending (asym) CH in-plane bending (sym) CC stretching (sym) CC stretching (asym) CC stretching (sym) CC stretching (asym) CO stretching (restricted) CO stretching (free)
The abbreviations are: vs, very strong; s, strong; m, medium; w, weak; ha, high activity; la, low activity; P, polarised; X, substitute. Ref. [14]
*
3. Calculation The strongest Raman bands, 667 and 461 cm 1 of CHCl3 and CCl4, respectively, are used as reference lines to estimate the relative intensities of the Raman bands of the titled compound, BOP. For the measurement of Raman excitation profiles, the Stokes intensity of each band is normalized relative to that for the excitation wavelength 514.5 nm. The excitation wavelength dependence of the intensity of the solvent band, used as internal standard, has also been taken into consideration. The detailed scheme of normalization of Raman signals has been described elsewhere [20–22]. The geometry optimization and vibrational wavenumber calculation in the ground state of BOP is done by density functional level of theory (DFT) using Gaussian 98 program [24] for Windows. The Pople split valance basis set 6-311G (d, p) in conjunction with B3LYP functional are used for the DFT computation. Gauss View 3.0 program is utilized for the visual inspection of the normal modes of the molecule. Animations of different vibrations help us to distinguish symmetric and asymmetric modes. 2D-correlation analysis is performed using 2D Shige version 1.3 software [25], developed in Professor Y. Ozaki’s laboratory (Kwansei Gakuin University, Sanda, Japan). Prior to 2D analysis, 5-point smoothing for all Raman spectra is carried out after base line correction in order to remove the spectral artifacts. The data matrix is arranged in the order from higher to lower excitation wavelengths. Second-derivative spectra have been obtained by the simplified least-squares method of Savitzky and Golay [26]. The derivative spectrum, especially, looks noisy since noise differences between adjacent data points are amplified by taking derivatives. Thus to improve the performance, final spectra are obtained after base line correction, normalization and 11-point smoothing of the original Raman spectra.
a
Intensity
Raman spectra of BOP in CHCl3 and CCl4 solutions of concentration around 1 M have been recorded with Spex Raman spectrometer (RAMLOG) system, fitted with Ar+ ion laser using 514.5, 501.7, 496.5, 488.0, 476.5 and 457.9 nm as an exciting wavelength. The accuracy of wavenumber measurements is ±1 cm 1 for strong Raman bands and slightly less for the others (see reference [14] for detail).
analyses have been applied to two independent sets of data. The first set is constructed from the spectra of BOP in CHCl3 solution and the second set from those in CCl4 solution. The experimentally observed and theoretically calculated wavenumbers, associated with 2D-correlation information of some selected vibrational modes, are assigned in Table 1. The Raman spectra of BOP in CHCl3 and CCl4 solutions at different excitation wavelengths are presented in Fig. 1. Synchronous 2D-correlation spectra from the first set, plotted as a contour map in the wavenumber ranges of 1680–1570 cm 1 (sec-
[vi] [v] [iv] [iii] [ii] [i]
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4. Discussion In order to explore the excitation wavelength dependent variation of the intensity of the ring breathing mode, 2D-correlation
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Fig. 1. The Excitation frequency dependent Raman spectra of BOP molecule in (a) CHCl3 and (b) CCl4 environment. Incident laser wavelengths are [i] 514.5, [ii] 501.7, [iii] 496.5, [iv] 488.0, [v] 496.5 and [vi] 457.9 nm.
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tion-I) and 1170–980 cm 1 (section-II), are presented in Fig. 2(a). In the section-I of synchronous map, the diagonal autopeaks appear at wavenumbers 1659 and 1600 cm 1 with a positive cross peak at (1659, 1600) cm 1. As previously mentioned [14] the first band is assigned as the C@O stretching mode, whereas the second one is corresponded to a pair of in plane ring deformation (i.e. CC stretching) vibrations associated with the two phenyl ring moieties. Though the 2D-correlation spectrum yields greater resolution than conventional 1D spectrum, the synchronous picture is unable to remove the frequency degeneracy. Evidently the quantum chemical calculation [14] shows that the band is almost degenerate. The appearance of the cross peak in the synchronous spectrum, due to simultaneous changes in the spectral intensities, suggests that strong correlation exists between two stretching modes in CHCl3 environment. Again the sign of the peak indicates that intensities of both the bands are increasing with the increase of excitation frequency. It is noteworthy to point out that more or less same type of excitation profile has been followed by each of these modes [14]. For both cases the excited electronic state at 248 nm (1La band) plays an important role in intensity enhancement. There is no autopeak generated from the relatively weak band at 1578 cm 1, another CC stretching vibration, probably because of its very small variation in intensity. However, the existence of an autopeak at 1579 cm 1 may be apprehended by constructing a correlation square through the autopeak at 1600 cm 1, positive cross
peak at (1600, 1579) cm 1 and the mirror image of this cross peak about the diagonal. The asynchronous spectrum represents sequential changes of the spectral intensities measured for the two wavenumbers, which generate the respective asynchronous cross peaks. As depicted in Fig. 2(b), intense negative cross peak appears at (1601, 1598) cm 1 in section-I. Thus asymmetric and symmetric CC stretching vibrations, not readily noticeable in the synchronous spectrum separately are now clearly distinguished in the asynchronous spectrum. Another very weak positive peak is noticed at (1657, 1601) cm 1 analogous to the positive cross peak of the synchronous spectrum. The sign rule [3,13] reveals that the variation in spectral intensity at X-axis wavenumber occurs before the variation in the spectral intensity at Y-axis wavenumber. Since the data matrix has been arranged from higher to lower incident laser wavelength, then the intensity enhancement of CC stretching vibration (1601 cm 1) occurs at higher excitation frequency than that of C@O stretching mode. From the sign of the asynchronous cross peak at (1601, 1579) cm 1, it appears that the sequence of intensity variation with the decrease of excitation wavelength occurs in the order from 1579 to 1601 cm 1. There are three other positive cross peaks at (1654, 1002), (1654, 1148) and (1654, 1601) cm 1. Surprisingly, the peak at 1654 cm 1, as has been resolved from the correlation spectrum, was not observed before. The presence of the new peak implies the existence of a new kind
Fig. 2. (a) Synchronous and (b) asynchronous 2D-Raman correlation spectra generated from excitation wavelength dependent spectral intensity variations of ring breathing modes of BOP molecule in CHCl3 environment. The shaded areas represent the negative correlation intensity. Average Raman spectrum is added at top.
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Fig. 2 (continued)
of C@O bond in the system. As is well known, if the C@O bond interacts with its surroundings, the wavenumber of the C@O stretching vibration may shift to a lower frequency. In synchronous contour plot of section-II [Fig. 2(a)] two autopeaks are located at 1152 and 1002 cm 1. The wavenumbers are assigned as substitute sensitive stretching and ring breathing mode, respectively. The connection of two autopeaks with the detectable cross peaks provides four correlation squares which suggest the probable existence of other diagonal peaks at 1160 and 1028 cm 1 related with different CH in-plane bending vibrations. Although these bands do not come into sight on the diagonal of the synchronous spectra, most likely due to poor intensity variation, their influence is realized definitely in the off-diagonal region of section-I & II. The asynchronous picture [Fig. 2(b)] provides more useful information than the synchronous one. The synchronous band at 1002 cm 1 splits into two components which appear at coordinates 1003 and 1000 cm 1 associated with symmetric and asymmetric ring breathing modes, respectively. Other distinguishable cross peaks are (1028, 1000) and (1025, 1003) cm 1 with negative and positive correlation coefficients, respectively. According to the previous quantum chemical calculation the lower and higher wavenumbers (i.e. 1025 and 1028 cm 1) are assigned as symmetric and asymmetric CH in-plane bending vibrations. It is a clear evidence of strong correlation of symmetric–symmetric and asymmetric–asymmetric vibrations. Furthermore, the cross peaks in the region of coordinates (1680–1570, 1170–980) cm 1 reflects
the dominant correlation of the respective bands. In this region also symmetric–symmetric and asymmetric–asymmetric correlations have been found between CC stretching and ring breathing modes. Fig. 3(a) and 3(b) display synchronous and asynchronous 2Dcorrelation spectra, respectively, from second data set. Four autopeaks are shown in synchronous spectrum at 1661, 1597, 1148 and 1000 cm 1 whose band positions and intensity are almost same as set one. Along with these bands other two weak but definite peaks are also identified at 1156 and 1025 cm 1 as in CHCl3 environment. The cross peaks of the synchronous spectrum are all positive and order of strength is comparable to that of first data set. In the asynchronous plot, the wavenumber 1577 cm 1 forms cross peaks with 1661 and 1597 cm 1. Examining the signs and the values of asynchronous parameters the sequence of intensity variation with changing excitation wavelength may be inferred as: 1577 ? 1597 ? 1661 cm 1. Once more the asynchronous view [specifically the cross peaks at (1599, 1595) and (1002, 1000) cm 1] establishes the splitting of symmetric and asymmetric CC stretching deformation bands centered at 1597 and 1000 cm 1, respectively. New cross peaks at (1661, 1652) appears due to two different kind of C@O vibrations: lower one probably arises, as said earlier, due to interaction of the C@O bond with its surroundings and other one may be assigned as free C@O stretching vibration. Baseline fluctuation may be responsible for the appearance of other weak cross peaks, as they often
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Fig. 3. (a) Synchronous and (b) asynchronous 2D-Raman correlation spectra generated from excitation wavelength dependent spectral intensity variations of ring breathing modes of BOP molecule in CCl4 environment. The shaded areas represent the negative correlation intensity. Average Raman spectrum is added at top.
cause artifacts particularly in asynchronous spectra [13]. In fact the CCl4 asynchronous spectrum is more complicated than the CHCl3 one. It is rather unlikely that symmetric and asymmetric CC stretching modes near 1000 or 1600 cm 1 share the asynchronous peaks. Particularly the peaks near 1600 cm 1 are debatable as they show roughly butterfly pattern in the asynchronous representations [Fig. 2(b) and 3(b)]. Since shifts of the corresponding synchronous peaks in this region with the change in the excitation wavelength may also be responsible for such kind of band appearances. Thus in order to check whether two independent bands do really exist in this region, the second-derivative method has been used as well. Generally, the second-derivative spectra could enhance the spectral resolution by amplifying the tiny differences in spectra [27]. The second-derivative spectra of BOP in CHCl3 and CCl4 solutions are shown in Fig. 4(a) and 4(b), respectively. Appearance of a weak band near 1592/1590 cm 1 along with the strong one at 1599/ 1597 cm 1 (in CHCl3/CCl4) becomes more evident in the spectra corresponding to excitation wavelength 457.9 nm. On the other hand in butterfly or angel (distorted butterfly) pattern, the asynchronous peak cluster consists of a pair of elongated cross peaks of opposing signs located very close to the diagonal [2]. Close to the main pair of elongated cross peaks, there also exists another set of weaker cross peaks, which are confined to smaller parts and located slightly away from the diagonal [2]. But even readjustment of contour level does not reveal any existence of such elongated or relatively weak confined cross peaks pairs. Thus in fact
the category of the pattern is neither really a butterfly type (related with band position shift) nor an angel one (connected with band position shift coupled with intensity change). Certainly such a typical characteristic cluster pattern is generated due to intensity changes of multiple bands. 5. Conclusion In this study generalized 2D-Raman correlation spectroscopy is successfully utilized to get some valuable information regarding vibrational properties of BOP molecule. Such type of study helps us to remove the quasi-degeneracy of different Raman bands. Thus this methodology may be utilized as an inbuilt program with Raman spectrophotometer to get high resolution spectra. At the same time here it also confirms better symmetric–symmetric and asymmetric–asymmetric asynchronous correlation between ring breathing mode and other ring CC stretching or CH in-plane bending vibrations. Consequently symmetric–asymmetric correlations of the ring breathing mode with other ones appear to be restricted. But strong asynchronous correlations exist between symmetric and asymmetric pair of different modes. The fact is pointed out that in both environments (i.e. in CHCl3 and in CCl4 solutions) the synchronous correlation characters of all the modes are almost same, whereas the asynchronous behaviors of them are quite different. All cross peaks in synchronous spectra are positive which signify increase of Raman band intensities with the decrease of incident wavenumber. Here it
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Fig. 3 (continued)
a
b
[v]
[iv]
[iii]
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Fig. 4. Second-derivative spectra in (a) CHCl3 and (b) CCl4 solvents. Incident laser wavelengths are [i] 514.5, [ii] 501.7, [iii] 496.5, [iv] 488.0, [v] 496.5 and [vi] 457.9 nm.
is worth mentioning that the C@O vibration is strongly affected by the solvents. Along with the free C@O stretching vibration, another kind of C@O stretching vibration with lower wavenumber
appears probably due to interaction of the C@O group with the surroundings, although the correlation characters of them are definitely solvent dependent. Both types of C@O vibration are
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asynchronously correlated with symmetric ring breathing mode in CHCl3 environment, whereas no such evidence is found in CCl4 solution. Furthermore the sign of CC and C@O stretching off-diagonal correlation peak intensity is opposite in these two asynchronous Fig. 2(b) and 3(b). Thus in other words, sequence of intensity variation for these two normal modes in the spectra is just opposite in CHCl3 and CCl4 solutions. Strong synchronous cross intensity between these bands may be due to similar kind of intensity contribution from excited electronic states through Frank–Condon A-term as was observed earlier [14] from their excited profile studies. References [1] I. Noda, Appl. Spectrosc. 47 (1993) 1329. [2] I. Noda, Y. Ozaki, Two-dimensional Correlation Spectroscopy Applications in Vibrational and Optical Spectroscopy, John Wiley & Sons, Ltd., 2004. [3] M. Thomas, H.H. Richardson, Vibrational Spectrosc. 24 (2000) 137. [4] Y.M. Jung, B.C. Matusewicz, Y. Ozaki, J. Phys. Chem. B 104 (2000) 7812. [5] I. Noda, C. Marcott, J. Phys. Chem. A 106 (2002) 3371. [6] Y.M. Jung, Bull. Korean Chem. Soc. 24 (2003) 1243. [7] Y.M. Jung, B.C. Matusewicz, S.B. Kim, J. Phys. Chem. B 108 (2004) 13008. [8] I. Noda, Vibrational Spectrosc. 36 (2004) 143. [9] A.A. Moore, M.L. Jacobson, N. Belabas, K.L. Rowlen, D.M. Jonas, J. Am. Chem. Soc. 127 (2005) 7292. [10] I. Noda, J. Mol. Struct. 799 (2006) 2. [11] Y.M. Jung, I.S. Yang, J. Mol. Struct. 799 (2006) 173. [12] C. Mello, A.E.M. Crotti, R.V. Lourenço, W.R. Cunha, J. Mol. Struct. 799 (2006) 141.
[13] A. Wesełucha-Birczyn´ska, J. Mol. Struct. 826 (2007) 96. [14] P. Sett, T. Mishra, S. Chattopadhyaya, A.K. De, P.K. Mallick, Vibrational Spectrosc. 44 (2007) 331. [15] C. Dubroca, P. Lazaro, Chem. Phys. Lett. 24 (1974) 49. [16] S. Chakraborty, S.K. Sarkar, P.K. Mallick, Chem. Phys. Lett. 187 (1991) 93. [17] S.C. Shim, D.W. Kim, M.S. Keins, J. Photochem. Photobiol. 56A (1991) 227. [18] P. Jana, T. Ganguly, S.K. Sarkar, A. Mitra, P.K. Mallick, J. Photochem. Photobiol. 94A (1996) 113. [19] S.K. Pal, T. Sahu, T. Mishra, P.K. Mallick, M.A. Paddon-Row, T. Ganguly, J. Phys. Chem. 108A (2004) 10395. [20] P. Sett, S. Chattopadhyaya, P.K. Mallick, Chem. Phys. Lett. 331 (2000) 215. [21] P. Sett, A.K. De, S. Chattopadhyaya, P.K. Mallick, Chem. Phys. 276 (2002) 211. [22] T. Mishra, A.K. De, S. Chattopadhyaya, P.K. Mallick, P. Sett, Spectrochim. Acta 61A (2004) 767. [23] P. Sett, T. Misra, J. Chowdhury, M. Ghosh, S. Chattopadhyay, S. Sarkar, P.K. Mallick, J. Chem. Phys. 128 (2008) 144507. [24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski Jr., J.A Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J. Foresman, J.V. Bortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, J. Komaromi, I. Cioslowski, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M.H. Gordon, E.S. Replogle, J.A. Pople, Gaussian 98, Gaussian Inc., Pittsburgh, PA, 1998. [25] 2Dshige (c) Shigeaki Morita, Kwansei Gakuin University, 2004–2005. [26] A. Savitsky, M.J.E. Golay, Anal. Chem. 36 (1964) 1627. [27] A. Watanabe, S. Morita, S. Kokot, M. Matsubara, K. Fukai, Y. Ozaki, J. Mol. Struct. 799 (2006) 102.