John A. S. Smith

The mechanism of proton transfer in carboxylic acid dimers as studied by 17O quadrupole double resonance

Abstract If the 17 O quadrupole double resonance results at room temperature for a number of hydrogen-bonded dimeric carboxylic acids, and β-oxalic acid, are interpreted in terms of a proton disorder model, it can be shown that the observed variations are satisfactorily explained in terms of fast concerted two-proton jumps, O 1 ue5f8Hue5f8O 2 → O 1 ue5f8Hue5f8O 2 within the dimer, but are inconsistent with rotational disorder. Similar arguments are used to show that the (HCO 3 ) 2 2− dimer in KHCO 3 is also significantly disordered at room temperature by the same mechanism.

14N Quadrupole coupling tensors in solid pyrazole, 1H-1,2,4-triazole, and 1H-tetrazole: Theory and experiment

Abstract New 14N quadrupole double resonance measurements of pyrazole, 1H-1,2,4-triazole and 1H-tetrazole have led to complete and self-consistent sets of frequencies at all the 14N sites, from which the quadrupole coupling constants and asymmetry parameters have been deduced. The results are compared with the calculations by ab initio SCF MO methods for a trimeric molecular unit with the same geometry as found in the solid state. As a consequence, most if not all the 14N quadrupole coupling tensors can be assigned to particular nitrogen atoms within the three molecules, and their sign and orientation determined. Large changes are observed at the hydrogen bonding NH and -N = nuclei relative to their quadrupole coupling tensors measured in the gas phase by microwave spectroscopy, which are closely paralleled by the theoretically calculated differences between trimer and monomer. They can be interpreted in terms of a simple electron transfer model where almost equal charges are transferred from 2pφ to 2pσ at the nitrogen atom of the NH group and from 2 atom.

Calculation of the 14N and 2H quadrupole coupling tensor in trimeric imidazole

Abstract The 14 N and 2 H quadrupole coupling tensors at both nitrogen and at all hydrogen sites, have been calculated by ab initio SCF MO methods, in a trimeric imidazole molecule having the geometry found in the solid state. The 14 N changes produced at N(1)H by hydrogen bonding are correctly reproduced to a noteworthy degree of precision, as is the 2 H quadrupole interaction at the same site. The changes in the values of the three principal components of the tensor, which have been inferred from NQR measurements when imidazole passes from the gas to the solid state, are supported by the calculations, both for N(1)H and N(3).

17 O and 2H quadrupole double resonance in some carboxylic acid dimers

17 n O quadrupole coupling constants and asymmetry parameters for the C17O and C—17OH nuclei in benzoic acid, eleven substituted derivatives and β-oxalic acid have been determined at 291 K by double resonance techniques together with the 2H values for the hydrogen-bonding deuteron. Many of these compounds form planar, or nearly planar, cyclic hydrogen-bonded units in the solid state, and the maximum principal components of the 17O quadrupole coupling tensor, e2qzzQ/h, lie probably in this plane, with the minimum component, e2qxxQ/h, perpendicular to the plane. The changes in these two components, Δqxx and Δqzz, due to substituents display an almost linear relationship to each other with a ratio Δqzz/Δqxx of ca.–0.5 for C—17OH. This relationship can be understood if the changes in the 17O quadrupole coupling constants are predominantly due to variations in the pπ oxygen populations, with smaller but proportional changes in pσ. Such changes can be brought about by a variety of effects, including molecular vibration, and their special effects on the C—17OH and C17O quadrupole coupling tensors may be related to the fixed geometry which these tensors tend to assume in the bonding system.

Deuteron quadrupole resonance studies. Part 8.—1H/2H double resonance in some inorganic hydroxy compounds

2 n H quadrupole resonance frequencies have been detected in partially deuterated H2SeO3, HIO3, B(OH)3, Te(OH)6, KHSO4, NaHCO3 and KHCO3 using double resonance by level crossing (d.r.l.c.) techniques. Te(OH)6 shows a complex multiplet which has been tentatively assigned using a combination of Zeeman measurements, double irradiation and double quantum methods.The quadrupole coupling constants have been correlated with the H … O hydrogen bond lengths derived from neutron diffraction data, and deviations are attributed to differences in π-bonding to the oxygen atom.

17 O and 2H quadrupole double resonance in some acid carboxylates

An 17O and 2H quadrupole double resonance study has been made of a number of strong O—H—O hydrogen bonds in acid carboxylates and related compounds and an analysis of the 17O⋯1H dipolar structure in one (potassium hydrogen maleate). The compounds selected show a range of hydrogen-bond structures varying from symmetric and centred to asymmetric and non-centred; with certain assumptions, it is possible to follow the consequent changes in sign and orientation of the 17O quadrupole coupling tensor. The arguments are based on the proposal that the 17O quadrupole coupling constant is negative in symmetric hydrogen bonds, its orientation being related to that of the 17O tensor in the C—17OH group in carboxylic acids by a rotation of 25 ° with respect to the O—H direction. The behaviour of the 2H quadrupole coupling constant and asymmetry parameter in strong O—H—O hydrogen bonds is discussed in terms of the position of the hydrogen atom.

17 O and 2H quadrupole double resonance in potassium hydrogen maleate and related compounds

2 n H, 17O, 39K and 85,87Rb quadrupole resonance signals have been recorded in potassium and rubidium hydrogen maleates and several related compounds by double resonance methods. The 17O signals were detected in natural abundance by a novel technique of high sensitivity based on the “solid effect” and thermal mixing. The symmetric hydrogen bonds in potassium hydrogen maleate and chloromaleate are shown to have characteristic values of the 17O and 2H quadrupole coupling constants which can be used to identify this kind of bonding in other related compounds. The orientation of the quadrupole tensor within the bonding system is also discussed.

Experimental and theoretical studies of the 14N quadrupole tensors in 1H-1,2,3-benzotriazole

Abstract The 14N quadrupole tensor elements have been determined for the title compound by a combination of double resonance level crossing, and quadrupole dip spectroscopy. Three of the four sites in the lattice were assigned. Analysis of the spectra was assisted by consideration of 1,2,3-triazole-4,5-dicarboxylic acid, which from its spectra appears to be in the 1H-form. Combination of these results with ab initio studies of (a) the equilibrium geometry of the monomeric 1H- and 2H-tautomers and (b) cluster studies at the 1H-benzotriazole positions, allowed a firm assignment of the quadrupole parameters at the three main chemical sites, although the different lattice positions in the asymmetric unit could not be identified by these means.

Deuteron quadrupole resonance studies. Part 9.—1H/2H, 23Na double resonance in some anhydrous and hydrated hydroxides

2 n H double-resonance signals have been detected at room temperature in a number of partially-deuterated hydroxides of alkali metals and alkaline earths and some of their monohydrates. The 2H quadrupole coupling constants of the OH– ion are correlated with the stretching force constants. 2H signals from the water molecules have been observed at 293 K only in LiOH·H2O; they show a marked structure due to a combination of dipolar coupling together with the “isotope effect” arising from hydrogen bonding to the OH– ion. 23Na quadrupole resonance was also detected in both NaOH and NaOH·H2O by an interesting variation of the normal double-resonance level-crossing technique.

A theoretical and experimental study of 14n quadrupole coupling in maleic hydrazide

Abstract Theoretical calculations have been performed of the 14 N quadrupole coupling tensor in maleic hydrazide, both as monomer and as an N-H⋯O hydrogen-bonded dimer at the orientation found in the crystal; the latter are in reasonable agreement with the experimental results obtained by double resonance and cross-relaxation spectroscopy, and reveal small changes in electron density due to the hydrogen bonding.