Vibrational spectroscopic characteristics of =C-H center dot center dot center dot O and N-H center dot center dot center dot pi interaction in crystalline N-(2,6-dimethylphenyl)-5-methylisoxazole-3-carboxamide

Infrared and Raman spectra have been scrutinised for features reflecting the presence of a very short =C-H ... O contact in crystalline N-(2,6-dimethylphenyl)-5-methylisoxazole-3-carboxamide (D2624). A tentative assignment of infrared and Raman bands is presented for the pure and partly deuterated compound measured in the solid state and in the non-polar solution. The intensity increase of the =C-4-H stretching vibration upon changing the matrix from a solution to the crystalline state, is the most prominent feature particularly in infrared. The temperature sensitivity was determined for the =C-4-H and N-H stretching vibration; the wavenumber-temperature coefficients (WTc) are significant but small, 0,0137 and 0.0143 cm(-1) K-1, respectively. The presence of an N-H ...pi interaction in the crystalline phase is established by mimicking this type of interaction in solution. Infrared measurements at temperatures above the melting point show that an intermolecular N-H ... O=C amide bond is present in the liquid phase. Upon cooling (crystallisation) this typical energetically favourable amide bond is disrupted as a result of several competing attractive forces (intermolecular =C-H ... O and N-H ...pi interaction, intramolecular N8-H ... N2) and the effect of steric shielding of the amide functionality by the 2,6-dimethylphenyl group.