Chemical shift quadrupolar correlation

Vosegaard and Massiot [144] showed that it is possible to create a high-resolution 2D spectrum correlating the chemical shifts with the second-order quadrupolar lineshapes from several spectra recorded at different magnetic field strengths using a projection-reconstruction method called chemical shift-quadrupolar projection-reconstruction of one-dimensional spectra (CQ-PRODI). 

Chemical shifts may correlate with quadrupolar linewidths (if AE terms are similar, or vary consistently), since a is related to the NQCC, and the linewidth to (NQCC). Examples have been reported in chlorine and in manganese resonance. 

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

The technique can be applied to both spinning and static samples, taking advantage of the fact that chemical shifts are proportional to the Larmor frequency while second-order quadrupole couplings are inversely proportional to the Larmor frequency (9). Thus, the spectrum of the CT can be seen as the projection along a specific angle a of a 2D chemical shift/second-order quadrupolar correlation spectrum. The angle a. for a spectrum acquired at u>o is defined as... 

Isotropic Chemical Shift and Second-Order Quadrupolar Induced Shift Correlations The MQDOR Method... 

From a very general outlook, one can argue that the NMR studies of quadrupolar nuclei present in nanoscale semiconductors should offer a more incisive look into the chemical and electronic structure than do studies of spin-1/2 nuclei. The rationale is that quadrupolar nuclei can report on the same chemical, hyperftne, or Knight shifts and dipolar or indirect couplings as observed for spin-1/2 nuclei, but also provide an additional dimension of information in terms of the NQCC and associated EFGs. Although not yet reported, DFT calculations of both chemical shifts and NQCC values for the same nuclei in nano-semiconductors should provide a more stringent comparison of theoretical and experimental results, particularly if the two parameters can be correlated experimentally, as seems feasible. 

Chemists pay much less attention to the NMR relaxation rates than to the coupling constants and chemical shifts. From the point of view of the NMR spectroscopist, however, the relaxation characteristics are far more basic, and may mean the difference between the observation or not of a signal. For the quadrupolar nucleides such as 14N the relaxation characteristics are dominated by the quadrupole relaxation. This is shown by the absence of any nuclear Overhauser effect for the 14N ammonium ion despite its high symmetry, which ensures that the quadrupole relaxation is minimized. Relaxation properties are governed by motional characteristics normally represented by a correlation time, or several translational, overall rotational and internal rotational, and thus are very different for solids, liquids and solutions. 

Recent applications of X/Y correlation techniques to measure chemical shifts of low- , spin-1/2 transition metal nuclei other than rhodium focused mainly on the analysis of silver and tungsten complexes in addition, the use of 13C-relayed H/195Pt correlations for the stereochemical analysis of some platinum complexes was explored.44 A report on the application of 31P/51V correlation spectroscopy demonstrated further the possibility of indirect detection of rapidly relaxing quadrupolar metal nuclei and the feasibility of the... 

There have been a number of publications on correlation or separation experiments involving quadrupolar nuclei. Using switching angle spinning (SAS), the correlation of quadrupolar and chemical shift interactions between the two spinning axes can be established to extract the quadrupolar and chemical shift tensors as well as their relative orientations.199... 

In multidimensional NMR studies of organic compounds, 2H, 13C and 31P are suitable probe nuclei.3,4,6 For these nuclei, the time evolution of the spin system is simple due to 7 1 and the strengths of the quadrupolar or chemical shift interactions exceed the dipole-dipole couplings so that single-particle correlation functions can be measured. On the other hand, the situation is less favorable for applications on solid-ion conductors. Here, the nuclei associated with the mobile ions often exhibit I> 1 and, hence, a complicated evolution of the spin system requires elaborate pulse sequences.197 199 Further, strong dipolar interactions often hamper straightforward analysis of the data. Nevertheless, it was shown that 6Li, 7Li and 9Be are useful to characterize ion dynamics in crystalline ion conductors by means of 2D NMR in frequency and time domain.200 204 For example, small translational diffusion coefficients D 1 O-20 m2/s became accessible in 7Li NMR stimulated-echo studies.201... 

In concluding this review it must be noted that there are many other techniques that are being utilized to increase our understanding about the structure of synthetically important carbanions. A partial listing of these techniques would include the theoretical approaches taken by Schleyer, Streitweiser, Houk and others and classical spectroscopic techniques. There exist also a number of useful NMR techniques in addition to the 2D-HOESY method previously mentioned. These NMR techniques include analysis of chemical shifts, Li- N spin-spin splitting, Li quadrupolar coupling o and rapid injection which has proven useful as a technique for structural investigations of aliphatic carba-nions. Last, but certainly not least, the excellent thermochemical measurements recently reported by Arnett and coworkers serve to correlate the solid state structural studies with solution species. A... 

Closely related to the measurement of HMQC spectra of I S , spin systems is the indirect detection of quadrupolar nuclei. While the standard HMQC sequence of Fig. 3(c) proved feasible for indirect detection of spin-3/2 nuclei such as "B and Ni, a theoretical analysis of the behaviour of spin systems with I(S) = 1 led to suggest a modified HMQC scheme (Fig. 4(a)) for systems with a single spin-1 S nucleus. Deviations from the standard scheme include the use of a shorter defocusing delay (4 /(I,S)) and of an additional 90°(I) pulse prior to t,. Application of the sequence was demonstrated for the case of a C D spin system. In addition, it was shown that correlation spectra of I Sm spin systems with different numbers of spin-1 S-nuclei can be obtained with an HSQC sequence, albeit by paying the price that the correlation signals appear at twice their real chemical shift in FI. 

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