The dynamics of hydrogen atoms in the hydrogen bonds of carboxylic acid dimers

Abstract

The techniques of pulsed NMR spectroscopy and inelastic neutron scattering (INS) have been employed to study the coordinated motion of hydrogen atoms in the hydrogen bonds of carboxylic acid dimers. The proton spin-lattice relaxation time T1 has been measured at two frequencies in diglycolic acid and suberic acid and the reorientation rate in diglycolic acid has been measured directly by quasi-elastic neutron scattering. Furthermore T1 data have been collected on benzoic acid, terephthalic acid and malonic acid at different spectrometer frequencies to those in earlier published data. The NMR and INS data have been successfully fitted to a new model proposed by Skinner and Trommsdorff (1988) in which the low temperature dynamics, under the influence of an asymmetric double-minimum potential, are dominated by phonon-assisted tunnelling while that at high temperature follows an Arrhenius rate law. Fitting the T1 data at two different frequencies simultaneously with the INS data restricts the possible values of the parameters in the theory to a narrow range. The values of the parameters, particularly the barrier heights and asymmetry, are discussed in terms of the known molecular structures. Further discussion centres upon some new NMR data of the proton dipolar relaxation time T1D in terephthalic acid and benzoic acid.

Molecular dynamics in ferroelectric 4-aminopyridinium tetrachloroantimonate(III), [4-NH2C5H4NH][SbCl4]

“…(4) and (5) are as follows: E a =30.9 kJ mol À1 , t 0 =1 Â 10 À14 s and C=1.6 Â 10 9 s À2 . It should be noted that these parameters are in an excellent agreement with those obtained for the bromine analogue [27]. According to them we have modified equation.…”

Quantum tunnelling in the hydrogen bond

“…This promoted significant progress in the interpretation of T 1 data, particularly that recorded on conventional NMR spectrometers using fixed magnetic field. Horsewill and Aibout [33] studied diglycolic acid, suberic acid, benzoic acid, terephthalic acid and malonic acid achieving a good fit with experimental T 1 data. In similar 1 H T 1 studies, incoherent tunnelling has been observed in 2,5-dinitrobenzoic acid [34] and dodecanoic acid [35].…”

“…Some of the early papers [10,13,33] were conducted on less well-characterised samples and display small differences in T 1 compared with more recent studies [41,42,[44][45][46] that have been conducted on very pure, repeatedly zone-refined material.…”

Detailed Characterization of the Dynamics of Ibuprofen in the Solid State by a Multi‐Technique NMR Approach

“…The molecular moiety thought responsible for this motion is therefore the dimeric structure (COOH) 2 , which is known to be present from X-ray diffraction studies. [14] It is well known that in cyclic dimers of many carboxylic acids the two hydrogenbonded protons may undergo fast simultaneous jumps along the hydrogen bonds [17] and a slower p-flip of the whole dimeric structure about its long axis. [18] The parameters obtained from the analysis for the a motion (see Table 3) are compatible with the p-flip of the dimeric structure, based on data previously reported in the literature.…”

“…[18] The parameters obtained from the analysis for the a motion (see Table 3) are compatible with the p-flip of the dimeric structure, based on data previously reported in the literature. [17,18] …”