Effects of hydration on the structure, vibrational wavenumbers, vibrational force field and resonance raman intensities of N-methylacetamide

✍ Scribed by Hajime Torii; Tomoaki Tatsumi; Mitsuo Tasumi

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 305 KB
• Volume: 29
• Category: Article
• ISSN: 0377-0486
• DOI: 10.1002/(sici)1097-4555(199806)29:6<537::aid-jrs272>3.0.co;2-x

✦ Synopsis

The e †ects of hydration on the structure and vibrational force Ðeld of the peptide group were examined by performing ab initio molecular orbital (MO) calculations on N-methylacetamide (NMA) and its clusters with up to three water molecules. The dielectric solvent e †ect was taken into account by the self-consistent reaction Ðeld (SCRF) method. It is shown that the wavenumbers of NMA are strongly a †ected by both the dielectric e †ect and formation of hydrogen bonds. The vibrational force Ðeld of NMA in aqueous solution is well described by taking into account the hydrogen bonding of three water molecules and the dielectric e †ect of the surrounding water solvent. The wavenumber shifts of the amide I, II and III bands induced by formation of hydrogen bonds are approximately additive. There is a strong correlation among the CxO stretching force constant, the CxO bond length and the amide I wavenumber. Calculated changes in the structural parameters induced by hydrogen-bond formation are consistent with changes in the resonance Raman intensities of the amide I, II and III bands. The changes in the CÈN and CxO bond lengths induced by hydrogen-bond formation in the excited electronic (2 1Aº) state are opposite from those in the ground electronic (1 1Aº) state. These opposite structural changes between the two electronic states are rationalized by using a simple Hamiltonian based on a two-state model, which describes the vibronic interaction between the two states involving a mode consisting of a linear combination of the CxO stretching and the CÈN contraction.