UV RESONANCE RAMAN-SPECTROSCOPY OF CIS-AMIDES

UV resonance Raman (UVRR) spectroscopy with 210 nm excitation was used to study the cis-amide bonds of caprolactam (CAP) and cis-N-methylace tamide (NMA) in aqueous solution. The UVRR spectrum of CAP is dominate d by an intense stretching mode at 1492 cm(-1), designated amide cII, which is assigned to a localized CN stretching vibration. The amide cI mode, described as a C=O stretch, can be located at 1621 cm(-1) using Fourier transform IR spectroscopy, Photoisomerization of NMA with hig h laser power, followed by subtraction of the trans-NMA spectrum, is u sed to generate the isolated UVRR spectrum of cis-NMA and its (C)CD3, (N)CD3 and (C,N)CD3 methyl isotopomers. The spectrum of cis-NMA is als o dominated by an intense amide cII band at 1495 cm(-1), and a weak ba nd at 1623 +/- 3 cm(-1) can be assigned to amide cI. The amide cll mod e is strongly mixed with vibrations of the (N)CH3 group, as revealed b y its 24 cm(-1) downshift upon (N)CD, substitution, but no isotopic se nsitivity is observed for (C)CD3 substitution. Studies of CAP in vario us solvents show linear correlations between amide cI and cII wavenumb ers and the solvent acceptor number, where amide cI downshifts and ami de cII upshifts in hydrogen bonding solvents. However, the slope of th e wavenumber dependence for the cis-amide modes of CAP is smaller than for the corresponding tuans-amide modes of NMA and cannot be attribut ed to the differences in mode composition. This behavior is ascribed t o different orientations of the ground-state dipole for amides in tran s and cis configurations. Finally, the selective enhancement of cis-am ide modes at 210 nm excitation was applied to monitor the increase in equilibrium population of cis-NMA with increasing temperature, A Van't Hoff plot of the spectral data to calculate Delta H for cis-trans iso merization yielded values of 1.9 +/- 0.2 kcal mol(-1) from peak bights and 1.7 +/- 0.2 kcal mol(-1) from peak areas (1 kcal = 4.180 kJ), whi ch are in good agreement with previously published theoretical and exp erimental results.