Proton, deuterium, and tritium nuclear magnetic resonance of intramolecular hydrogen bonds. Isotope effects and the shape of the potential energy function

Abstract

The difference in chemical behavior of the three isotopes of hydrogen is mainly caused by the difference in their masses, which in turn influences the vibrational motions and the zero-point vibrational energies. Thus, from a measurement of an isotope effect in some property, one can make deductions about the shape of potential energy surfaces. This has been extensively exploited for elucidating chemical reaction paths from kinetic data.1 Here we report the usefulness of , 2H, and 3H NMR as a means of studying the shape of the potential energy surface in the vicinity of the equilibrium position.Primary isotope effects2 of the NMR chemical shifts of 2H and 3H in organic compounds are usually very small, in the case of 3H usually <0.03 ppm for tritium bound to sp2-or sp3-type carbons.3-4 Exceptions to this general rule are systems with relatively strong hydrogen bonds, for which deuterium isotope effects, ( , 2H), as large as 0.6 ppm have been found.5-7 We have previously reported the results of a systematic NMR study of the deuterium isotope effect, ( , 2H), in a series of systems all containing hydrogen bonds of the type O-H-O.6 A correlation between the isotope effect and the value of the chemical shift, 5(1H), was observed for the hydrogen-bonded nucleus.The only conceivable source of isotope effects on chemical shifts within the Born-Oppenheimer approximation lies in the isotopes mass dependence of vibrational motion. The quantities that determine the magnitude are the vibrational wave functions, f, for the ground state (and possibly low-lying excited vibrational states) and the variation of the chemical shift, ,

Ab initio study of the structure of the hydrogen maleate anion

“…1) have usually been considered as paradigms of symmetric hydrogen bonding (for review, see ref. 4), and for the former this notion has been supported experimentally by X-ray (5) and neutron diffraction (6) studies (which, while not locating the position of the hydrogen atom, have shown that the heavy atom skeleton is symmetric, implying that the hydrogen bond also should be symmetric) and by nrnr experiments (7,8).…”

Hydrogen bond dynamics in the active site of photoactive yellow protein

“…Because of anharmonicity in the shape of the hydrogen bond potential energy well, the lower zero-point energy for deuterium results in a shorter O-D bond than the original O-H bond by 0.01-0.02 Å and a lengthening of the O⅐⅐⅐O distance by a similar magnitude (7,8,23,30). These geometric changes are readily detected by NMR for small molecules in aprotic organic solvents (8,31,32).…”

Spin–spin coupling between hydroxyl and aldehydic protons in some salicylaldehyde derivatives. Correlation with the hydroxyl proton chemical shift

“…This attitude is certainly valid for the keto-en01 tautomerization observed in the Schiff bases derived from salicylaldehyde (9). However, it is unlikely that tautomerization or an (asymmetric) double minimum potential with a low barrier exists in 1 (10). Substitution of the hydroxyl proton by deuterium and tritium nuclei, together with the measurement of I their chemical shifts (primary isotope effects), demonstrates this situation for 1.…”