FOURIER-TRANSFORM NEAR-INFRARED SPECTRA OF N-METHYLACETAMIDE - DISSOCIATION AND THERMODYNAMIC PROPERTIES IN PURE LIQUID FORM AND IN CCL4 SOLUTIONS

FT-NIR spectra have been measured for N-methylacetamide (NMA) in pure liquid form and in carbon tetrachloride (CCl4) solutions over temperat ure ranges of 303-368 K and 283-343 K, respectively. For the pure liqu id, the FT-NIR spectra give one band due to a free NH group, at least five bands due to free end NH groups, and at least sis bands due to hy drogen-bonded NH groups in the region of the first overtone of the NH stretching mode, demonstrating the potential of FT-NIR spectroscopy fo r investigating the hydrogen-bonding of NMA. The temperature-dependent spectral changes of NMA in the pure liquid indicate that, with increa sing temperature, longer-chain oligomers of NMA dissociate into monome ric and dimeric species. For the CCl4 solutions, the intensity of a ba nd assigned to the first overtone of the NH stretching mode of the mon omeric NMA has been employed to study the degree of dissociation and t hermodynamic properties for the dissociation process. Below 4 x 10(-3) M, NMA exists completely in the monomer form, and from the absorbance -concentration plot, the molar absorption coefficient of the first ove rtone of the NH stretching mode has been obtained. The degree of disso ciation and dissociation equilibrium constant K have been calculated f or the solutions at various concentrations, and then they have been em ployed to evaluate the thermodynamic parameters (Delta H, Delta S, and Delta G) for the dissociation of hydrogen-bonded NMA into its monomer ic species. The results show that the contribution of the entropy (Del ta S) to the standard free energy (Delta G) becomes larger with a redu ction in the concentration. It seems very likely that in the dilute so lutions shorter-chain oligomers exist predominantly, and they easily d issociate into the monomeric species.