ISSN:
0377-0486
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
UV resonance Raman (UVRR) spectroscopy with 210 nm excitation was used to study the cis-amide bonds of caprolactam (CAP) and cis-N-methylacetamide (NMA) in aqueous solution. The UVRR spectrum of CAP is dominated 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 high laser power, followed by subtraction of the trans-NMA spectrum, is used 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 also dominated by an intense amide cII band at 1495 cm-1, and a weak band at 1623 + 3 cm-1 can be assigned to amide cI. The amide cII mode is strongly mixed with vibrations of the (N)CH3 group, as revealed by its 24 cm-1 downshift upon (N)CD3 substitution, but no isotopic sensitivity is observed for (C)CD3 substitution. Studies of CAP in various solvents show linear correlations between amide cI and cII wavenumbers and the solvent acceptor number, where amide cI downshifts and amide cII upshifts in hydrogen bonding solvents. However, the slope of the wavenumber dependence for the cis-amide modes of CAP is smaller than for the corresponding trans-amide modes of NMA and cannot be attributed to the differences in mode composition. This behavior is ascribed to different orientations of the ground-state dipole for amides in trans and cis configurations. Finally, the selective enhancement of cis-amide 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 ΔH for cis-trans isomerization yielded values of 1.9 + 0.2 kcal mol-1 from peak heights and 1.7 + 0.2 kcal mol-1 from peak areas (1 kcal = 4.180 kJ), which are in good agreement with previously published theoretical and experimental results.
Additional Material:
10 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/jrs.1250260838
