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

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, r_CO, of the DMF and r_CN 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 (ν_CO and ν_CN) of the DMF and AN molecules, respectively. The relationships between r_CO and ν_CO for DMF, and between r_CN and ν_CN for AN, are well approximated by linear functions. The value, dν_CO/dr_CO = −3,400 ± 1,600 cm⁻¹ Å⁻¹ has been derived for DMF from the slope of the fitted function. dν_CN/dr_CN = −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.