Knowledge Bank

The observed Raman frequencies for L- alanine, L-alanine-[$ND3$], L-alanine-[$CDCD3$], and L- alanine-[$D7$] were used to calculate a valence force field consisting of 28 primary symmetry force constants and 6 interaction constants. The final refinement, in which 5 primary force constants were constrained to fixed values, resulted in an average error of just $9cm-1$ ($\sim 1.3$) for the 127 observed frequencies of the four isotopically substituted molecules. The calculated potential energy distribution for normal alanine shows that the vibrations above $1420cm-1$, as well as those below $700cm-1$, may be properly described as group frequencies. By contrast, the remaining modes in the middle frequency range -- namely, the four $NH3+$ and $CH3$ rocks, the three skeletal stretches and the $COO-$ deformation and “symmetric” stretch--are very strongly coupled to one another.