Hydrogen/Deuterium Isotope Effects on the NMR Chemical Shifts and Geometries of Intermolecular Low-Barrier Hydrogen-Bonded Complexes

Abstract

In this paper we describe H/D isotope effects on the chemical shifts of intermolecular hydrogen-bonded complexes exhibiting low barriers for proton transfer, as a function of the position of the hydrogen bond proton. For this purpose, low-temperature (100−150 K) 1H, 2H, and 15N NMR experiments were performed on solutions of various protonated and deuterated acids AL (L = H, D) and pyridine-15 N (B) dissolved in a 2:1 mixture of CDClF2/CDF3. In this temperature range, the regime of slow proton and hydrogen bond exchange is reached, leading to resolved NMR lines for each hydrogen-bonded species as well as for different isotopic modifications. The experiments reveal the formation of 1:1, 2:1, and 3:1 complexes between AH(D) and B. The heteronuclear scalar 1H−15N coupling constants between the hydrogen bond proton and the 15N nucleus of pyridine show that the proton is gradually shifted from the acid to pyridine-15 N when the proton-donating power of the acid is increased. H/D isotope effects on the chemical shifts of the hydrogen-bonded hydrons (proton and deuteron) as well as on the 15N nuclei involved in the hydrogen bonds were measured for 1:1 and 2:1 complexes. A qualitative explanation concerning the origin of these low-barrier hydrogen bond isotope effects is proposed, from which interesting information concerning the hydron and heavy atom locations in single and coupled low-barrier hydrogen bonds can be derived. Several implications concerning the role of low-barrier hydrogen bonds in enzyme reactions are discussed.

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).…”

A review with comprehensive data on experimental indirect scalar NMR spin–spin coupling constants across hydrogen bonds

“…[55 -69] In 1996, Shenderovich et al studied the complexes formed by [ 15 N 1 ]pyridine and different acids, most of them carboxylic, at 100-150 K in a 2 : 1 mixture of CDClF 2 /CDF 3 (Freon mixture). [55] The equilibrium 12a/12b (Scheme 12) is shifted to the right when the strength of the acid increases (Table 15), increasing the value of 1 J NH from 0 to 90.8 Hz.…”

“…[55] q 1 corresponds to the deviation of the HB proton from the HB center as depicted in Scheme 13. If the HB is not linear and forms an AHB angle = 180…”

Measurement of Proton Tunneling in Short Hydrogen Bonds in Single Crystals of 3,5 Pyridinedicarboxylic Acid Using Nuclear Magnetic Resonance Spectroscopy

“…Incoherent tunneling, via excited vibrational states, dominates the rate of PT at all temperatures since the potential barrier, estimated to be 2000 K [23], is always significantly greater than kT. However, in short HBs, the direct observation of tunneling processes has, to our knowledge, never been reported despite many experimental [24][25][26][27][28] and theoretical [29][30][31][32][33][34][35][36] studies. is an almost perfect model system in which to study PT in a very short HB (Fig.…”