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Published October 13, 2005 | public
Journal Article

Intramolecular Hydrogen Bonding in Disubstituted Ethanes: General Considerations and Methodology in Quantum Mechanical Calculations of the Conformational Equilibria of Succinamate Monoanion

Abstract

The importance of intramolecular hydrogen bonding between the carboxylate oxygen and amide proton of succinamate anion has been investigated by quantum mechanical simulations as a function of solvent for comparison with conformational equilibria estimated by NMR spectroscopy. The focus is on those methodological considerations of general interest to the conformational equilibrium problem, which are also particularly relevant to the quantum calculations. The roughly planar symmetry of the amide and carboxylate substituents of succinamate anion and the possibility of intramolecular hydrogen-bond formation together suggest that the orientational degrees of freedom of the substituents could play an important role in the equilibrium of the CH_2−CH_2 torsion. To test this hypothesis, two-dimensional potential-energy surfaces (PESs) were mapped out from the quantum mechanical calculations, with coordinates corresponding to the CH_2−CH_2 torsional and amide group rotational degrees of freedom. The Boltzmann populations obtained from two-dimensional PESs and those obtained from a one-dimensional adiabatic surface for the CH_2−CH_2 torsion were compared with the experimental results. In these comparisons, the agreement of calculated gauche fractions with corresponding experimental values was checked, as well as the agreement between predicted coupling constants and those determined from experimental spectra. In polar protic and aprotic solvents, where highly polar trans conformations can be stabilized by dipole−dipole and hydrogen-bonding interactions with the solvent, the orientational degree of freedom of the amide substituent appears to play a sufficiently important role in the CH_2−CH_2 torsional equilibrium that it cannot be safely ignored in the simulations.

Additional Information

© 2005 American Chemical Society. Received: June 2, 2005. Publication Date (Web): September 17, 2005. Acknowledgment is made to the donors of the Petroleum Research Fund administered by the American Chemical Society, for support of this research. We are also deeply indebted to the National Science Foundation under grant CHE-0104273, the Summer Undergraduate Research Fellowship Program (SURF) at the California Institute of Technology, the Camille and Henry Dreyfus Foundation, Merck and Company, the E. I. Du Pont Company, and Dr. & Mrs. Chester M. McCloskey for their helpful financial assistance. D.R.K. is grateful for support of this research from a graduate fellowship from the Fannie and John Hertz Foundation. The computational resources at the MSC were provided by ARO-DURIP and ONR-DURIP. Other support for the MSC came from DOE, ONR, NSF, NIH, Chevron-Texaco, Nissan, Aventis, Berlex, Intel, and Beckman Institute.

Additional details

Created:
August 19, 2023
Modified:
October 24, 2023