Anharmonic vibrational spectroscopic investigation of malonaldehyde

Anharmonic IR spectra of H-bonded and non-H-bonded conformers of malonaldehyde (MA) and its isotopomers MA-D6D8 and MA-D7D9 have been computed by the Vibrational-Self-Consistent-Field (VSCF) and the correlation-corrected-VSCF (CC-VSCF) techniques using ab initio MP2/6-31G*(+p) potential energies. The agreement between the experimental and calculated frequencies is significantly improved to within 2-3%. Anharmonic contributions are substantial especially for νOH of the H-bonded form, by reducing the harmonic value by more than 500 cm-1. The effect is less important in the non-H-bonded form. The νOH stretching mode is strongly coupled with the ν3 mode (essentially νCH7) and with the in-plane and out-of-plane OH bending deformations. H-bond formation and deuteration batochromically shift νOH by an amount which is influenced by the anharmonic terms, the major contribution arising from coupling between modes. The comparison with the νOH mode of some other H-bonded systems suggests that anharmonic correction follows H-bonding strength