Vibrational studies of polar aprotic molecule in aromatic chemical and isotropic solvents: experimental and quantum chemical investigations Ganesh Upadhyay, Pradeep Upadhyaya and Th. Gomti Devi* Raman spectroscopic technique has been used to study the intermolecular interactions and dynamics of S¼O, C―H and CSC stretching modes of dimethyl sulfoxide (DMSO) in binary mixtures using methyl benzene (MBN) and deuterated methyl benzene (MBNd) aromatic solvents. The Raman band of S¼O stretching mode has been deconvoluted into four distinct bands for neat DMSO as well as in binary mixtures. Deconvoluted bands in neat DMSO were assigned as monomer, cyclic out-of-phase, cyclic in-phase and chain dimers having peak wavenumbers 1069.10, 1056.60, 1041.50 and 1027.30 cm À1 respectively. Peak wave- number of S¼O stretching mode shows red shift, while peak wavenumbers of C―H and CSC stretching modes show blue shift with the increase in solvent concentration. The vibrational relaxation phenomena for all the stretching modes have been studied as a function of solvent concentration. Quantum-chemical calculations have been carried out to gain more insight into the self-association of DMSO and in interacting environment with the solvents using ab initio and density functional theory method. The ab initio basis set is HF/6-31 + G (d, p) for the interacting system. The hydrogen bond complexes of DMSO with MBN and MBNd using IEF-PCM model have been calculated using B3LYP functional and 6-31 + G(d,p) basis sets. Theoretical calculations have been compared with the experimental findings and we obtained good coherence of the results. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: vibrational relaxation; bandwidth; isotropic; peak wavenumber; hydrogen bonding Introduction The polarized Raman spectroscopic technique is one of the best ways to investigate the molecular interaction between the solute and solvent molecules in binary liquid mixture system. It provides the information regarding the structural changes of molecules occurring in liquid binary mixtures because of molecular interac- tions. There is spectral change, change in wavenumber, bandwidth, intensity etc. of the reference vibrational Raman band of solute because of the perturbation induced by the solvent on further dilution of it. [1–6] The change in bandwidth provides the dynamical information while peak wavenumber shift provides the static information. These changes in binary mixture are because of the result of change in molecular structure, electronic structure, dipole moment respectively. [1–13] In binary liquid mixture, the compo- nents of solvent may interact with the component of solute either by specific, such as hydrogen bonding and self-association or non-specific, such as dipole–dipole, dipole-induced dipole, dispersion interactions etc. [12,13] The quantum chemical calcula- tions may boost the experimental findings obtained by using Raman technique and help to explain the experimental results. Recently, experimental and quantum theoretical investigations of the vibrational relaxation dynamics of various molecules in liquid (condensed) phase have attracted a considerable attention. The quantum chemical calculations such as ab initio, DFT theoretical calculations etc. have been employed to provide the better explanations of the experimental findings obtained by using Raman technique. [13–19] A brief review of the work done on DMSO molecule by various researchers has been provided in our previous papers. [16,17] In our previous papers, [6,16] a systematic Raman study was made for DMSO using aliphatic chemical and deuterated solvents such as chloroform (CLF), chloroform-d (CLFd), acetonitrile (ACN) and acetonitrile-d3 (ACNd) solvents. In the study we obtained very inter- esting information about the molecular dynamics in the interacting systems. To have better understanding about the influence of solvent in the interacting environment, aromatic chemical and deuterated solvents such as methyl benzene (MBN) and deuterated methyl benzene (MBNd) have been used to explore further investi- gations about the molecular dynamics of DMSO in the interacting systems. To the best of our knowledge, limited study has been done for DMSO at different chemical and deuterated aromatic solvents. Present study deals with the interacting nature of S¼O, C―H and CSC stretching modes of DMSO in binary mixtures using methyl benzene (MBN) and deuterated methyl benzene (MBNd) * Correspondence to: Th. Gomti Devi, Department of Physics, North Eastern Regional Institute of Science and Technology, Arunachal Pradesh-791109, India. E-mail: devigomti@yahoo.co.in Department of Physics, North Eastern Regional Institute of Science and Technology, Arunachal Pradesh, 791109, India J. Raman Spectrosc. 2015, 46, 1291–1302 Copyright © 2015 John Wiley & Sons, Ltd. Research article Received: 17 April 2015 Revised: 23 June 2015 Accepted: 30 June 2015 Published online in Wiley Online Library: 3 August 2015 (wileyonlinelibrary.com) DOI 10.1002/jrs.4759 1291