IR spectroscopy as a probe for C–H⋯X hydrogen bonded supramolecular synthons

Abstract

Weak hydrogen bonds of the type C–H⋯X (X: N, O, S and halogens) have evoked considerable interest over the years, especially in the context of crystal engineering. However, association patterns of weak hydrogen bonds are generally difficult to characterize, and yet the identification of such patterns is of interest, especially in high throughput work or where single crystal X-ray analysis is difficult or impossible. To obtain structural information on such assemblies, we describe here a five step IR spectroscopic method that identifies supramolecular synthons in weak hydrogen bonded dimer assemblies, bifurcated systems, and π-electron mediated synthons. The synthons studied here contain C–H groups as hydrogen bond donors. The method involves: (i) identifying simple compounds/cocrystals/salts that contain the hydrogen bonded dimer synthon of interest or linear hydrogen bonded assemblies between the same functionalities; (ii) scanning infrared (IR) spectra of the compounds; (iii) identifying characteristic spectral differences between dimer and linear; (iv) assigning identified bands as marker bands for identification of the supramolecular synthon, and finally (v) identifying synthons in compounds whose crystal structures are not known. The method has been effectively implemented for assemblies involving dimer/linear weak hydrogen bonds in nitrobenzenes (C–H⋯O–NO), nitro-dimethylamino compounds (NMe₂⋯O₂N), chalcones (C–H⋯OC), benzonitriles (C–H⋯NC) and fluorobenzoic acids (C–H⋯F–C). Two other special cases of C–H⋯π and N–H⋯π synthons were studied in which the band shape of the C–H stretch in hydrocarbons and the N–H deformation in aminobenzenes was examined.