Raman signatures of strong and weak hydrogen bonds in binary mixtures of phenol with acetonitrile, benzene and orthodichlorobenzene Anurag Singh, Debraj Gangopadhyay, Rajib Nandi, Poornima Sharma and Ranjan K. Singh* The present study aims at investigating the effect of hydrogen bonds of phenol in binary mixtures of phenol with three solvents viz. acetonitrile, orthodichlorobenzene and benzene respectively in order of decreasing hydrogen bond strength. Raman spec- troscopy in correlation with density functional theory (DFT) calculations has led to a profound understanding of changes in struc- ture, energy, dipole moment and other physical and chemical properties of phenol pertaining to hydrogen bond formation in solution. The spectral variation in wavenumber and linewidth of ring deformation, ring stretching, C≡N stretching and C―H stretching modes have been analyzed in detail. The breaking of self association of phenol in solution and formation of strong or weak hydrogen bonds depending on the nature of the solvent has been discussed by comparing the Raman and DFT results for three different solvents. Copyright © 2016 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: phenol; acetonitrile; benzene; orthodichlorobenzene; hydrogen bonding; Raman spectroscopy; DFT Introduction Hydrogen bonding is a very fundamental phenomenon in under- standing of intermolecular interactions. The strong interactions of aromatic molecules with other aromatic or non-aromatic solvents often help to explore some unique properties of a refer- ence system. But sometimes the weak interactions with non- covalent aromatic molecules also play a significant role because of the involvement of π hydrogen bonding. [1–6] Phenol is a very important organic compound both chemically and biologically. Phenol has a hydroxyl group (OH) but it is acidic in nature be- cause of the influence of the resonance stabilization in aromatic ring. [7,8] It is frequently used as a parent molecule of many or- ganic materials and as an anti-bacterial and antiseptic material for the treatment of surgical instruments. [7] Phenol also has anti- oxidant activity in living organisms. [9–11] Depending upon the hydrogen bonding involved in a particular system, it acts as a pro- ton donor (Lewis acid) as well as a proton acceptor (Lewis base). [12] It exists in keto and enol tautomeric forms, but in solvent me- dium the enol form is the exclusive structure. [13] The strong O―H---O hydrogen bond exists between phenol molecules in crystalline form. [14] Kryachko et al. reported that phenol interacts with two acetoni- trile (ACN) molecules, which is the simplest organic nitrile commonly used in donor–acceptor reactions. [15] The C≡N fundamental mode of ACN is a very sensitive probe for interactions with cations and other acidic centers. [16–18] An infrared study of water–ACN mixtures performed by Jamroz et al. suggests the presence of strong hydro- gen bond interactions. [16] In addition to strong covalent interaction, a non-covalent aromatic solvent interaction must also be taken into account where the π electron acts as a proton donor. [17,18] The role of aromatic interactions in molecular recognition has been de- scribed by Hunter et al.. [19,20] Many studies have been performed to understand the weak interactions of phenol with benzene and substituted benzene derivatives. [21–24] Saggu et al. have done vibra- tional Stark measurement to study the weak interactions of phenol with substituted benzene derivatives, where the aromatic edge to face electrostatic interactions were studied and a linear response of OH bond of phenol to electric field was observed. [25] This re- sponse becomes nonlinear when O atom of phenol is replaced by some other electronegative atom (e.g. nitrogen or sulfur). [26] Re- cently, Nikolova et al. studied the interaction of substituted phenol with benzene using infrared spectroscopy and density functional theory (DFT). They showed that π hydrogen bond plays an impor- tant role in such systems. [27] Vibrational study, in particular Raman spectroscopy, is one of the most suitable techniques to understand hydrogen bonding. Concentration-dependent Raman study gives clear idea of hy- drogen bonding patterns in organic as well as aqueous solu- tions and helps to understand their solute–solvent interaction behavior. [28–39] In some recent studies by our group, strong hy- drogen bonding has been observed in aqueous solutions such as pyridine–water, pyrimidine–water and many other systems. [29,31,33] In pyridines/pyrimidine + H 2 O mixtures, at higher water concentration the original reference peak of solute completely vanishes. In the present study we focus on the strong as well as weak hydrogen bond interactions moving from ACN which makes strong hydrogen bond(s) with phenol to 1,2- Dichlorobenzene (orthodichlorobenzene or ODCB) and benzene * Correspondence to: Ranjan Kumar Singh, Department of Physics, Banaras Hindu University, Varanasi-221005, India. E-mail: ranjanksingh65@rediffmail.com Department of Physics, Banaras Hindu University, Varanasi 221005, India J. Raman Spectrosc. 2016, 47, 712–719 Copyright © 2016 John Wiley & Sons, Ltd. Research article Received: 13 August 2015 Revised: 17 November 2015 Accepted: 16 December 2015 Published online in Wiley Online Library: 24 January 2016 (wileyonlinelibrary.com) DOI 10.1002/jrs.4880 712