Resonance Raman examination of the two lowest amide ππ* excited states

✍ Scribed by Sanford A. Asher; Zhenhuan Chi; Pusheng Li

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 204 KB
• Volume: 29
• Category: Article
• ISSN: 0377-0486
• DOI: 10.1002/(sici)1097-4555(199810/11)29:10/11<927::aid-jrs334>3.0.co;2-r

✦ Synopsis

The UV Raman spectra of N-methylacetamide (NMA) were measured in and between the visible H 2 O D 2 O spectral region and the UV region down to 184 nm. The Raman excitation proÐles indicate that all of the amide bands are resonance enhanced by the Ðrst dipole-allowed p Ç p* transition at 190 nm. The Raman excitation proÐle data indicate the presence of destructive interference from the second allowed transition at ca. 165 nm. For example, the amide I and amide Iº bands disappear in the 184 nm UV Raman spectra and the relative intensities of the amide II and III bands change. The Ðrst allowed electronic transition at ca. 190 nm occurs to an excited pp* state where the major geometry change, relative to the ground state, involves a dominating CÈN bond elongation, with smaller CÈC and NÈC bond contractions, and a more modest CxO bond elongation. The CÈN groundstate double bond character may decrease to the point where free rotation may occur. This may explain the previously observed monophotonic photoisomerization of trans-NMA to cis-NMA. The second allowed transition at ca. 165 nm occurs to an excited state whose major geometry change involves CxO bond elongation. ( 1998 John Wiley & Sons, Ltd.