The binding of water to the carboxylate group in R-CO2- (R = H, CH3, NH2, OH, and F): An ab initio molecular orbital study

George D. Markham, Fox Chase Cancer Center
Mendel Trachtman, Thomas Jefferson University
Cindy L. Bock, Elizabethtown College
Charles W. Bock, Fox Chase Cancer Center


The results of extensive ab initio molecular orbital calculations are reported for the enthalpy, entropy and free energy changes of the decomposition of the cyclic and linear (anti and syn) conformers of the carboxylate monohydrates R-CO ·H O, where R = H, CH , NH , OH and F. Geometries have been optimized and frequency analyses performed at the MP2(FC)/6-31 + G** level in all cases, and at the MP2(FC)/6-311++G** level in selected cases. Where comparison is possible, the results from the two computational levels are consistent and yield computed reaction energetics within experimental uncertainty. These computational results support the conclusions from pulsed electron beam mass spectrometry (M. Meot-Ner, L.W. Sieck, J. Am. Chem. Soc. 108 (1986) 7525) that the gas-phase decomposition of the H-CO ·H O and CH -CO ·H O complexes at 500 K occurs from linear singly hydrogen-bonded structures rather than from cyclic doubly hydrogen-bonded structures. Furthermore, the results identify this linear structure as the anti conformer for both complexes. © 1998 Elsevier Science B.V. All rights reserved. 2 2 3 2 2 2 3 2 2 - - -