Experimental determination of relationship between intramolecular bond lengths and their stretching vibrational frequencies of N,N-dimethylformamide and acetonitrile molecules in the liquid state

Abstract

Abstract Time-of-flight (TOF) neutron diffraction measurements have been carried out on pure liquid deuterated N,N-dimethylformamide (DMF-d7), acetonitrile (AN-d3), and their concentrated LiTFSA solutions to investigate the effect of intermolecular interaction to intramolecular bond lengths, rCO, of the DMF and rCN of the AN molecules in the liquid state. Intramolecular parameters of DMF and AN are determined by least-squares fitting analyses of neutron total interference terms observed in the high-Q region. Attenuated total reflection (ATR) IR spectra have been measured for pure liquid DMF, AN, and their LiTFSA solutions of natural abundances to determine the average C=O and C≡N stretching vibrational frequencies (νCO and νCN) of the DMF and AN molecules, respectively. The relationships between rCO and νCO for DMF, and between rCN and νCN for AN, are well approximated by linear functions. The value, dνCO/drCO = −3,400 ± 1,600 cm−1 Å−1 has been derived for DMF from the slope of the fitted function. dνCN/drCN = −7,000 ± 3,000 cm−1 Å−1 has been obtained for AN. It has been revealed that the C = O bond length of the DMF and the C≡N one of the AN molecules are sensitively affected by intermolecular interaction.