Assignment of the vibrational spectrum of l-cysteine

- Periodic density functional theory of the polymorphic forms of l-cysteine.
- A weak dihydrogen bond in the gauche conformer of the monoclinic form was found.
- Comparison of observed and calculated neutron spectra shows good agreement.

Ab initio calculations of the complete unit cell of l-cysteine for both the orthorhombic and monoclinic polymorphs have been carried out. The results suggest the presence of a previously unrecognised, weak dihydrogen bond of a novel type: S-H···N-H in the gauche conformer of the monoclinic polymorph. Comparison of the calculated transition energies to those observed in the infrared, Raman and inelastic neutron scattering spectra of the orthorhombic form shows excellent agreement, as does the simulated INS spectra to that experimentally measured. The assignments are in general agreement with those in the literature but differ in detail. The strong intermolecular interactions present make the use of periodic-DFT essential in order to correctly assign the spectra. The need for, and the complementarity of, all three types of vibrational spectra: infrared, Raman and INS is clearly demonstrated.