Deuterium Quadrupole Constants Research Articles

Influence of vibrational motion on solid state line shapes and NMR relaxation

The influence of vibrational motion on bond lengths and quadrupole constants obtained from dipolar and quadrupolar solid state line shapes is considered. It is shown that such motions average both the magnitude and the orientation of the intrinsic interaction tensor. Explicit expressions for the effective coupling constants that can be conveniently evaluated using the results of a normal mode analysis are derived. When the vibrationally averaged interaction tensor is axially symmetric, it is shown that the effect of vibrational motion on relaxation can be rigorously incorporated into an effective coupling constant which is formally identical to the one that determines the line shape. Illustrative calculations for several alkanes, in both the gas and solid phases, are presented. The relative contributions of stretching and bending vibrations and the effect of anharmonicity on the effective C–H bond lengths and deuterium quadrupole constants are examined. The influence of vibrational averaging on the magnitude of the dipolar coupling is shown to be essentially independent of the nature of the molecule and its environment and to be quite small (i.e., for C–H bonds, the coupling constant decreases by 3% and hence the effective bond length increases by 1%). Vibrational averaging of the orientation of the dipolar interaction vector, on the other hand, depends on the size of the molecule and its environment because of the predominant role played by low frequency bending and torsional modes. The implication of these results for the value of the effective internuclear distance that should be used for the interpretation of the dipolar relaxation experiments is considered.

2D Ligand ENDOR and TRIPLE resonance studies of Cu(II)-doped DTGS single crystals

The deuteron hyperfine and quadrupole coupling tensors in the bis(glycinato)-Cu(II) ((ND2CH2COO)2Cu) complex in ferroelectric triglycine sulfate (DTGS) single crystals were measured by ENDOR spectroscopy. These data indicate that the molecular symmetry of the Cu complex is C1. From TRIPLE resonance experiments the signs of the isotropic coupling constants were determined. Structural information was obtained from the principal values of the quadrupole coupling tensors of the deuterium atoms and their direction cosines. Using the relationship between the deuterium quadrupole constant and ND bond lengths, the positions of the deuterons in bis(glycinato)-Cu(II) were estimated. The negative signs of the isotropic hyperfine coupling constants were explained by spin polarization of the deuteron 1s orbitals. The values of the anisotropic hyperfine coupling constants are discussed in the framework of conventional molecular-orbital theory and can be understood quite satisfactorily.

Beam maser spectroscopy on CD3CN

The J=1→0 rotational transitions of CD3CN were measured using a two-cavity molecular beam maser spectrometer. This spectrometer employs the Ramsey method of separated oscillating fields to obtain very high resolutions and allows precise measurements of the hyperfine structure splittings. The nitrogen quadrupole coupling constant is eqaaQ(N)=−4229.2±0.6 kHz and the deuterium quadrupole constant is eqaaQ(D)=−55.1±0.4 kHz. Assuming cylinderical symmetry about the C–D bond axis we obtain the C–D bond-axis deuterium quadrupole coupling strength of eqzzQ(D)=169.6±4.5 kHz. The b-axis rotational constant, and nitrogen and deuterium spin-rotation interaction strengths are also reported.

Structural study of sulfamic acid at 4.2 K by ENDOR-detected NMR

The electron–nuclear double resonance method (ENDOR) is used to obtain greatly enhanced NMR signals from the distant deuterium nuclei of perdeuterated sulfamic acid (ND3SO3) in single crystals at 4.2 K. The structural information is obtained by measuring the quadrupole coupling tensors of the deuterium atoms and their direction cosines. There is very good agreement between the N–D bond angles derived from these ENDOR-detected NMR (EDNMR) results and bond angles from neutron diffraction data at 78 K. Relationships between the deuterium quadrupole constants and N–D and D⋅⋅⋅O bond lengths are derived and discussed.

Deuterium and nitrogen pure quadrupole resonance in amino acids. II

Further research has been carried out on amino acids using pure quadrupole resonance with spin mixing by level crossing. Both deuterium and nitrogen nuclei were investigated. Two new techniques associated with the spin mixing method are outlined. The new deuterium data for N +−D…O hydrogen bonds are shown to be consistent with an empirical relation between the electric field gradient parameter e 2 qQ and the D…O length described in an earlier paper. The deuterium e 2 qQ values for N +−D…Cl − bonds are found to follow a similar relation, e 2 qQ/ h = 239 − (728/ R 3) kHz. Evidence is presented which suggests that deuterium quadrupole constants in N−D…O bonds involving trigonally hybridized nitrogen conform to the relation e 2 qQ/ h = 282 − (572/ R 3) kHz, where 282 is the e 2 qQ for a free ND 3, molecule and the coefficient of 1 R 3 is the same as that found for N +−D…O bonds where the nitrogen is in a tetrahedral configuration. Where possible, the new nitrogen data are predicted from deuterium e 2 qQ values using a model presented earlier, and the implications of that model are discussed.