Homonuclear interaction second-order

The Nb NMR spectra of a number of alkali and lead niobates have been acquired using MAS, DAS, MQMAS and two-dimensional nutation NMR to improve the resolution and determine values of the quadrupolar products Pq for these materials (Prasad et al. 2001). The 9.4 T Nb MAS NMR spectrum of LiNbOs spun at 25 kHz (Figure 10.16A) shows a lineshape dominated by the second-order quadrupolar interaction, with only marginal improvement in resolution at a field of 14.1 T. For most of the metal niobates, the second-order interaction is not removed by MAS alone, even at high magnetic fields and fast spinning speeds. The expected improvement in Nb resolution provided by DAS spectroscopy is offset by significant homonuclear... 

Experimental studies have shown that, in practice, the effect of second-order quadrupolar interactions can produce gross deviations from a Gaussian-type decay curve, reflecting the influence of higher moments on the time evolution behavior [ 11 ]. It has been shown, however, that dipolar information can still be obtained if the analysis is restricted to the initial curvature in the limit of short dipolar evolution times (2ti<200 ps). The validity of this approach has been tested recently for homonuclear Na- Na dipole-dipole interactions in crystalline solids, for which the M2 values are readily calculable from the known crystal structures [11]. 

In the last two equations the upper line is for the hetero- and the lower line for the homonuclear case. There are two specialities for quadrupolar nuclei (I> 1/2) (i) for strong quadrupolar interactions there are additional non-diagonal terms in the quadrupolar Hamiltonian which influence the spectra (ii) for half-integer quadrupolar nuclei, there exists a central transition (—1/2<-> +1/2) which is only in second order influenced by the quadrupolar interaction and thus an important observable for the chemical characterization of these nuclei. Details of these second order broadening are beyond the scope of this review and can be found for example in the recent reviews by Jerschow or Wimperis et alP ... 

There are two requirements for the use of multiple-pulse sequences to speed up the spin-diffusion process First, the scaling factor of the homonu-clear dipolar coupling should be as large as possible and, second, the effective spin lock has to be larger than the homonuclear dipolar interactions between the S-spins [19] to prevent loss of sum magnetization into dipolar order [15]. 

There has been considerable interest in the possibility of bound excited states arising from the interaction of unMe noble gas atoms. Jortner and co-workers have searched for second continue in radiolysis studies of noble gas mixtures in the solid and liquid phase and at pressures of the order of 1000 mmHg (1 mm Hg = 133 Nm" ) in the gas phase. In general, emission from the homo-nuclear molecules is favoured even for the minor component, but weak continua have been ascribed to ArKr and KrXe. These states are postulated to correlate with the excited state of the heavier atom and their weaker bonding relative to homonuclear molecules is clear from comparison of the wavelengths corresponding to peak intensity of the continua ... 

Observed interatomic distances for diatomic transition-element interactions are estimates of the fraction, d = 0.783 of nearest-neighbor approaches in the metals [5] and may be considerably in error in the present context, especially for the second transition series. Apart from first-order La2 and Ce2, with = 245 30kJmol homonuclear diatomics have weak interactions with an average Dx = 70 40kJmor in agreement with our estimates. Multiple bond orders, in general, are characterized by stepwise reduction of the first-order golden exponent, such that... 

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