ISSN:
0749-1581
Keywords:
Intramolecular hydrogen bonding
;
17O NMR
;
Ab initio MO calculations
;
o-Anisic acid
;
o-Anisamide
;
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The 17O chemical shifts of the methoxy groups of o-anisic acid (1) and o-anisamide (2) were measured in various solvents at 40°C. The methoxy resonances were displaced to low frequency in CDCl3 relative to CD3CN and CD3OD solutions. This shielding can be attributed to intramolecular hydrogen bonding in the non-polar solvent. The percentage of hydrogen bonding was determined by 1H NMR spectroscopy. The hydrogen bond-induced 17O chemical shifts of the methoxy groups were evaluated as --10 and -3 ppm in 1 and 2, respectively. Their sensitivity to hydrogen bonding is therefore much smaller than that of carbonyl groups. Ab initio calculations at the STO-3G level were used to determine the geometry of the hydrogen bonds and their influence on the total energy of the molecules. The intramolecular hydrogen bond energy of 1 was evaluated as 2.18 kcal mol-1. Hydrogen bonding in 1 and 2 resulted in an increase in both the σ- and π-electron densities at the methoxy oxygen atoms. Arguments are presented which show that the variation in atomic charges is the principal origin of the low-frequency 17O chemical shifts.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1260260312
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