Vibrational spectra of 3-phenylpropionic acid and L-phenylalanine

The Scaled Quantum Mechanical Force Field (SQMFF) methodology of Pulay has been applied to the DFT-computed force field of the 3-phenylpropionate (3PP) and L-phenylalaninate (PHE) anions in order to assign the respective vibrational spectra. The scale factors of simple molecules, e.g. benzene, glycine and α-alanine, have been transferred to 3PP and PHE yielding a satisfactory agreement between the computed and the experimental frequencies for both molecules. These results provide a fairly good example of the usefulness of the SQMFF methodology to study the vibrational spectra of complex systems.