J. zyxwvutsrqp Phys. Chem. 1995,99, 17825-17831 17825 zyxwvutsrqp Vibrational Analysis of trans-Azobenzene D. R. Armstrong, J. Clarkson; and W. E. Smith* Department of Pure and Applied Chemistry, University of Strathclyde, 125 Cathedral Street, Glasgow GI XL, Scotland, U.K. Received: April 12, 1995; zyxwvutsrq In Final Form: September 11, 1 9 9 9 Molecular orbital calculations were performed to determine the normal modes and vibrational energies of azobenzene. A semiempirical calculation using the PM3 Hamiltonian and an ab zyxw initio calculation carried out at the SCF level using the 6-3 1G basis set gave unsatisfactory predictions especially for vibrations dominated by azo atom displacements. High-level electron correlation ab inifio calculations carried out at the MP2 level improved the fit with experiment but the choice of basis set was found to be critical. When the basis set for the nitrogens of the zyxwvutsr azo group was changed to the 6-31+G(d) basis set, the calculation gave a satisfactory fit. Normal-mode diagrams and energies are presented, and assignments to experimentally observed vibrational energies of azobenzene are made. The main azo stretch, YIO, observed at 1440 cm-I, is theoretically predicted at 1450 cm-I. The calculation correctly predicts an increase in frequency in the azo stretch mode upon deuteration of the phenyl rings. Coupling of several phenyl modes with azo vibrations are revealed by the calculation, in agreement with previous assignments of the vibrational spectra of azobenzene and azobenzene derivatives. The calculation indicates why certain in-plane stretching frequencies give rise to relatively intense Raman and resonance Raman scattering. In Raman scattering, the modes giving rise to the strongest scattering involve displacements along the N-N and C-N bonds. The same modes give intense resonance Raman scattering with the stretches along the azo bond providing the greatest intensity. Introduction Azobenzene is the simplest example of a wide range of theoretically and practically important molecules containing the azo chromophore. Vibrational spectroscopy is often used to elucidate the in situ properties of these molecules. For example, there are recent studies on cis-trans isomerization and the acid base equilibria of azo comp~undsl-~ and in addition, resonance Raman spectroscopy has been used to study azobenzene derivatives which show tautomerism between the azo and hydrazo structures and therefore are of value as indicator^.^,^ Thus, a normal-mode calculation would be of value in providing an improved understanding of the nature of the vibrations in azoben~ene.~-l Raman scattering, resonance Raman scattering, and infrared spectroscopy of trans-azobenzene have been studied by several groups.8-'' There have also been a few surface-enhanced Raman scattering ~ t u d i e s . ' ~ - ' ~ The assignment of the azo bands has been aided by the use of isotopically substituted deriva- tive~~.'~-'~ and by studying the depolarization ratios of the Raman lines.'* In particular, Gruger et al. have published a series of papers detailing the vibrational spectra of trans-azo- ben~ene,~ cis-azobenzene, and cis-az~xybenzene'~ and also trans-azoxybenzene,20 making use of isotopically substituted derivatives. The azo stretch was assigned to the 1470 cm-' Raman band and there was evidence that it is coupled to phenyl mode 19alb. They also found evidence that other phenyl modes, 12 or 1 and 6a, couple to azo vibrations. However, recent resonance zyxwvutsrq Raman studies support a previous assignment of the mode at 1440 cm-' to the azo stretch.21-23 The other main Raman active azo band, the N=N-Ph symmetric bend, has been assigned to the 1143 cm-' band in these studies. The coupling of the azo vibration with the phenyl modes, especially the C-H Current address: Department of Biochemistry, Case Western Reserve zyxwvuts * To whom correspondence should be addressed. @ Abstract published in zyxwvutsrqpon Advance ACS Abstracts, November 15, 1995. University, 10900 Euclid Ave., Cleveland, OH 44106-4935. 0022-365419512099- 17825$09.00/0 bending modes such as 9a, is believed to contribute to the difficulty in assigning the azo trans-Azobenzene has two absorption bands in the visible and near-W regions, which are assigned to zyx n* - n and n* - n transitions, respectively. The lower energy band occurs near 450 nm and is symmetry forbidden and less intense than the intense peak near 320 nm which is symmetry allowed.',2 The 320 nm band is shifted towards the visible region by the addition of electron withdrawing or electron-donating groups on the phenyl rings.5 Initial observations of the intensity enhancement of the high-energy Raman bands of azobenzene when the excitation wavelength is changed from the red to the blue were interpreted as a resonance effect involving the n* - n tran~iti0n.I~ However, resonance Raman profiles (REPS) of azobenzene2' obtained from carbon tetrachloride in the wave- length range 361-494 indicate that the intensities are due to a preresonance effect involving the n* - n transition. Further studies by the same group concluded that the high- energy symmetric vibrations derive intensity from the n* - n electronic transiti~n.",~~ However, detailed studies of the REPS of azobenzene on the low-energy side of the n* - n electronic transition revealed evidence for a contribution to the Raman intensity from this absorption f eat~re.~~.~~ Several groups have commented on the absence of any complete normal-mode calculations giving a precise description of the vibrations of azobenzene . The conformation of trans-azobenzene in solution is not known precisely. It is thought to be either planar and of C2h symmetry or slightly twisted from planarity and of Ci symmetry. The crystal structure shows a slight twist from planarity, but this could be due to crystal packing forces.27 A twist of approximately 30" around the N-Ph bonds has been detected by gas electron diffraction.** Kellerer et al. found that they could not detect any depolarized Raman peaks in the Raman spectra of azobenzene and its isotopically substituted deriva- tives." They concluded that azobenzene has Ci molecular 0 1995 American Chemical Society