Dipolar tensors

It is worth mentioning that parameter p is insensitive, to first order approximation, to modulation of the residue-specific 15N chemical shift anisotropy tensor and/or dipolar interaction, as the (d2 + c2) term in the R) / R ratio is canceled out. The noncollinearity of the CSA and dipolar tensors will require corrections to Eqs. (10) and (12) for high degrees of rotational anisotropy (D /D > 1.5), as described in detail in Ref. [22]. 

Any small-amplitude motion is excluded for it could not lead to the large observed dipolar tensor reduction, as well as 90° flips about the 1,4 axis or... 

Rotational resonance has become a routine experiment and is now increasingly employed in practice. In addition to the conventional situations, its usefulness has been expanded recently. Costa et a/.146 found that while n = 1 rotational resonance can be used to determine the internuclear distance, the = 2 rotational resonance contains the information for determining the relative orientation of the chemical shift and dipolar tensors. 

Rotary resonance recoupling (R3) was used to measure chemical shift tensors,153 and the relative orientation of the shift and the dipolar tensors.154 It can also be used to conduct a 3D chemical shift correlation experiment to investigate molecular rearrangement.155 Another interesting application of the rotary resonance is to obtain an NMR spectrum with peaks at the principal values of the chemical shift tensor.156 The principle is that the spins in principal-value orientations experience an effective field of zero magnitude at the rotary resonance, hence their magnetization vectors do not oscillate while spins at other orientations may be dephased. 

Symmetry (referred to Fig. 3) Lower eigenvalue of the magnetic dipolar tensor Preferred spin direction angle with axes ... 

Explicit expressions for the G -parameters in terms of the Euler angles f2ab = aab,f ab,7ab) between the initial dipolar tensor frame and the rotor frame are also given in Appendix 1, as well as the distance dependence, 1 of the dipolar frequency coefficient ujab- The Gn,ab and gn,a coefficients satisfy... 

Fig. 42. The form of the DQ-DQ correlation experiment.68 DQ coherence is excited and reconverted using a /-encoding pulse sequence such as BABA. In the experiment, conducted under rapid MAS, t is set equal to t2 and the reorientation of a homonuclear dipolar tensor is monitored through the DQ rotor-encoded spinning sidebands that emerge from Fourier transforming in the t — t — t2 time domain.

An advantage in observing such dipolar spectra is that the dipolar tensors are virtually axially symmetric, i.e., two of the tensor elements are the same. 

Wetmore et al. report calculations on the N2 deprotonated cation with spin densities p(C8) = 0.19, p(N2) = 0.35 and p(N3) = 0.37. Here one sees excellent agreement with the same parameters determined experimentally (Table VII). This table also contains the dipolar tensors. There is also rather good agreement between the dipolar couplings determined from the experimental data, and the corresponding dipolar couplings determined from the DFT calculations. 

In VO(TPP) CHCly-toluene the measured hf values of the pyrrole protons are found to be 1.38 MHz and -0.26 MHz (in-plane) and -0.26 MHz (out-of-plane). The hfe tensor may be separated into a contact contribution = 0.29 MHz and an axial dipolar contribution A = 1.10 MHz which conesponds to a V-H distance of 5.24 A. The axial-ity of the dipolar tensor indicates that the spin density on the nitrogen ligands is very small in VO(TPP) (in agreement with the N hfs data), in contrast to Ag(TPP), where the unpaired electron is significantly delocalized and strongly rhombic pyrrole proton tensors have been observed. No interpretation of the N quadrupole interaction has been given. As found in VO(acac)2, the proton resonances in VO(TPP) show a considerable solvent dependence. Since interpretation of this effect is highly speculative at present, we shall not discuss this problem. 

Chan and Tycko ° have introduced a novel solid state NMR experiment that combines ROCS A (recoupling of chemical shift anisotropy) and C- H dipolar dephasing. Designed for uniformly labeled samples, this technique is shown to correlate the carbonyl shielding and the dipolar tensors, thereby pro-... 

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