Journal of Molecular Structure, 53 (1979) 15--24 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands A STUDY OF INTERMOLECULAR HYDROGEN BONDING IN FORMAMIDE BY VIBRATIONAL SPECTROSCOPY DEREK J. GARDINER Chemistry Department, The Polytechnic, EUison Place, Newcastle-upon-Tyne NE1 8ST (Gt. Britain) ALISTAIR J. LEES and BRIAN P. STRAUGHAN Department of Inorganic Chemistry, University of Newcastle-upon-Tyne NE1 7RU (Gt. Britain) (Received 13 July 1978; in final form 11 October 1978) ABSTRACT Intermolecular hydrogen bonding in formamide has been studied by an analysis of the N--H stretching regions in both the infrared and Raman spectra of solid and liquid formamide and for solutions of formamide in nitromethane and liquid sulphur dioxide. The spectra indicate that on melting the linked C,h dimers of the solid break up to form smaller units of linked dimers having Cs symmetry. In solution the dimeric structure is dis- rupted to the monomerform with increasing dilution. A simple valence force field calcu- lation has also been carried out and indicates extensive mixing of the low frequency vibrations. INTRODUCTION The use of infrared and Raman spectroscopy to study solutions and non- aqueous solvents is well established [1]. We have previously investigated liquid ammonia both as a pure liquid [2] and as a solvent for electrolytesystems [3--6]. We have now extended our intereststo pure liquid formamide which is also expected to be an effectivesolvent for electrolytesdue to its high dielectricconstant (109.5 at 25°C) [7]. Some preliminary resultsfrom Raman difference spectroscopy support this view [8]. Formamide is the simplest molecule which may be linked by hydrogen bonds between C--O and N--H groups. Several vibrationalspectra for the solid [9], liquid [10, 11], solution [12--14] and gaseous [15, 16] phases of formamide as well as microwave studies [17--19] and some normal coordinate calculations [9, 10] have been reported. However, the nature of the intermole- cular interactions present in the liquid and a fullanalysis of the complex (N--H) stretching region require further clarification.This work investigatesthe inter- actions present in liquid and solid formamide and in solutions of formamide by an analysis of the (N--H) stretching region in both the infrared and Raman spectra.