Complex formation of HCONH2 in CH3OH environment and investigation of linewidth changes of ν(CO) stretching and NH2 bending modes

The isotropic part of the Raman bands corresponding to NH2 bending and ν(CO) stretching modes of formamide (HCONH2) at ∼1593 and 1668 cm−1, respectively, in neat HCONH2 as well as in binary mixtures with methanol (CH3OH) were reinvestigated. Variations of their linewidths exclusively with mole fractions of HCONH2, in the range C = 0.1–0.9 were studied. The linewidth variation of the NH2 bending mode shows a departure from the trend expected on the basis of concentration fluctuation model and this has been explained using a recently suggested empirical model by invoking the concept of microviscosities of the solute, HCONH2 and the solvent, CH3OH. The other peak at ∼1668 cm−1 shows a peculiar variation of the linewidth with concentration having two minima at C = 0.8 and 0.4, which have been explained in terms of formation of hydrogen bonded complexes, NH2HCO⋯HOCH3, and NH2HCO⋯(HOCH3)2 and the two phenomena, namely motional narrowing and diffusion dynamics being simultaneously operative. The equilibrium constants have been evaluated from the spectral data and their variation with total molar concentration has been presented.