Solution-phase conformations of N-acetyl-N'-methyl-L-alaninamide from vibrational Raman optical activity

Experimental and ab initio theoretical vibrational Raman optical activity (VROA) spectra are presented for N-acetyl-N'-methyl-L-alaninamide (NANMLA), in order to determine the predominant conformations of this molecule in the solution phase. The experimental spectra were obtained in three different solvents, CHCl3, H2O, and D2O. The ab initio VROA spectra were predicted for nine different conformations of NANMLA optimized with the 6-31G* basis set. The vibrational frequencies and normal modes for all nine conformers were also obtained with the 6-31G* basis set. From a comparision of the predicted and observed VROA spectral patterns, it is suggested that there is reasonable support for the presence of C-7,C-eq-C-5, C-7,C-eq, and alpha(R) conformers in aqueous solution and for C-7,C-eq-C-5 and alpha(R) conformers in chloroform solutions.