2H quadrupole coupling

A correlation between the 2H quadrupole coupling constant e2qQ/h and O-H stretching frequency v0h (with e2qQ/h proportional to v0h2) was reported by... 

Attempts were also made to correlate the magnitudes of 2H quadrupole coupling constants to hydrogen bond lengths [66-68]. Initially, a (rH...0) 3 dependence of e2qQlh was suggested [67] and an empirical relationship of the form... 

Simple correlations have been established between isotropic H chemical shifts and 0 H and 0---0 distances in 0-H---0 hydrogen bonds for a variety of organic and inorganic solids. Correlations between isotropic H chemical shift and O--0 distance, as well as between 2H quadrupole coupling constant and O O distance, have also been reported [75]. 

As in the case of 2H quadrupole coupling constants discussed above, this relationship is supported by the bond polarisation theory. Furthermore, a linear relationship between SH and the 2H quadrupole coupling constant was reported [70] ... 

In addition, basic quantum mechanical calculations have shown that the change in isotropic H chemical shift (SH) due to hydrogen bond formation can be attributed primarily to O-H bond polarisation [80]. Similarly, the change in 2H quadrupole coupling constant is also expected to be caused by O-H bond polarisation. It would therefore be interesting to explore correlations between SH and the O-H bond length (r0 H) and correlations between e2qQlhand r0 H> as r0. H... 

Fig. 21. The pulse sequence for the 2H DQ experiment, which separates -spinning sideband patterns arising from the 2H quadrupole coupling according to 2H DQ isotropic chemical shifts.32...

Fig. 24. Calculated 2H quadrupole echo and MAS NMR spectra for a two-site reorientation35 such that the 2H quadrupole coupling tensor unique principal axis moves through 106°, i.e. the motion appropriate for the two-site motion of the methyl groups in deuterated DMS. The simulations assume an inhomogeneous symmetric log-Gaussian distribution of correlation times with a mean correlation time of 5 x 10 5s and a standard deviation ranging from 0 to 3 decades, (a) Quadrupole echo spectra with echo delay time t = 30 /is. (b) MAS spectra.

In presence of molecular motion the NMR line shape will change. A particularly simple situation arises, if the motion is rapid on timescale defined by the inverse width of the spectrum in absence of motion 6 1. In this fast exchange limit, which in 2H NMR is reached for correlation times tc < 1CT7 s, the motion leads to a partially averaged quadrupole coupling and valuable information about the type of motion can directly be obtained from analysis of the resulting line shapes. The NMR frequency is then given by... 

The temperature dependent T data are shown in Fig. 9. 7j values decrease from 28 ms at 21°C with increasing temperature, and show a minimum of 6.4 ms at 80° C. These results indicate the presence of the motion with a Larmor frequency of 30 MHz at this temperature. This minimum was found to be attributed to the flipping motion of a phenyl ring from the result of our other experiments discussed in later section.13 The jump rates of the flipping motion were estimated with a two-site jump model that a C-2H bond jumps between two equivalent sites separated by 180°, and that the angle made by the C-2H bond and the rotational axis is 60°. The quadrupole coupling constant of 180 kHz and the asymmetry parameter approximated to zero were used in the calculation. The calculated values for fitting with the... 

A 3H and 2H NMR study of single crystals of [Ag(NH3)2][Ag(ONO)2] has been reported.32 Rapid reorientational motions of the ammine groups around their C3 axes were found. The orientations of the C3 axes within the crystal corresponded with the Ag—N bond directions of the [H3N—Ag—NH3]+ unit. The deuterium quadrupole coupling constant was determined and found to be identical to that of solid ND3. It was concluded that the electronic configuration and the geometric structure of ammonia were changed only very slightly upon coordination to silver ions. 

While the nuclei 3H and 13C relax predominantly by the DD mechanism, relaxation of a quadrupole nucleus such as deuterium essentially involves fluctuating fields arising from interaction between the quadrupole moment and the electrical field gradient at the quadrupole nucleus [16]. If the molecular motion is sufficiently fast (decreasing branch of the correlation function, Fig. 3.20), the 2H spin-lattice relaxation time is inversely proportional to the square of the quadrupole coupling constant e2q Q/H of deuterium and the effective correlation time [16] ... 

Lowenstein and Igner86 have determined the quadrupole coupling tensor of 33S in dimethyl sulphone, both in liquid crystalline solvents and in CHCI3 solutions. In solution, an approximate value of xc has been measured from 2H y and Tj. 33S y values are as follows 1.63MHz for rj — 0 and 1.41 MHz for q — 1. In liquid crystals, x is about 1.8 MHz. These values seem to be in agreement within... 

Fig. 14. The pulse sequence for recording the double-quantum 2H experiment.37 The entire experiment is conducted under magic-angle spinning. This two-dimensional experiment separates 2H spinning sideband patterns (or alternatively, static-like 2H quadrupole powder patterns) according to the 2H double-quantum chemical shift, so improving the resolution over a single-quantum experiment. In addition, the doublequantum transition frequency has no contribution from quadrupole coupling (to first order) so, the double-quantum spectrum is not complicated by spinning sidebands. Details of molecular motion are then extracted from the separated 2H spinning sideband patterns by simulation.37 All pulses in the sequence are 90° pulses with the phases shown (the first two pulses are phase cycled to select double-quantum coherence in q). The r delay is of the order 10 gs. The q period is usually rotor-synchronized.

Further support for the non-centred enol hydrogen bond has come from 13C and 2H spin-lattice relaxation times of AA and 2H2AA from which deuterium quadrupole coupling constants are derived. These are explained in terms of a double minimum potential well67. These same workers also discuss the broadness of the <5(OHO) signal and conclude that it is due to bond lengthening rather than contributions from trans enol forms. The minimum line width for AA achieved by exhaustive drying was 0.5 Hz. 

Correlations between 2H quadrupole interaction parameters and hydrogen bond geometry have also been considered for situations other than 0-H---0 hydrogen bonds. For example, solid state 2H NMR spectra of 2H labelled amino acids, peptides and polypeptides were measured over a wide temperature range [74]. From spectral simulations based on dynamic 2H NMR theory, parameters such as the 2H quadrupolar coupling constant and asymmetry parameter were determined, and relationships between these NMR parameters and the hydrogen... 

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