NQR studies

Studies have considered the effect of crystallinity on the performance of CR adhesives (97), on segmental mobiUty as determined by nqr studies (101), on strain induced property changes (102), and on relaxation processes (103). 

Also known for some time is a phase transition at low temperature (111K), observed in studies with various methods (NQR, elasticity measurement by ultrasound, Raman spectrometry) 112 temperature-dependent neutron diffraction showed the phase transition to be caused by an antiphase rotation of adjacent anions around the threefold axis ([111] in the cubic cell) and to lower the symmetry from cubic to rhombohedral (Ric). As shown by inelastic neutron scattering, this phase transition is driven by a low-frequency rotatory soft mode (0.288 THz 9.61 cm / 298 K) 113 a more recent NQR study revealed a small hysteresis and hence first-order character of this transition.114 This rhombohedral structure is adopted by Rb2Hg(CN)4 already at room temperature (rav(Hg—C) 218.6, rav(C—N) 114.0 pm for two independent cyano groups), and the analogous phase transition to the cubic structure occurs at 398 K.115... 

In the past, the boron anomaly had generally been attributed to a rapid decrease in the relative proportion of BO4 groups at 15-20 molar percent alkali oxide. However, more recent NMR and NQR studies show that the concentration of BO4 groups continues to increase up to about 40 molar percent alkali oxide and then drops off to near zero by about 70 molar percent. This means that the boron anomaly does not correlate with a sudden disappearance of BO4 groups, as previously supposed. 

A thorough NQR study ( Nb, 35C1, 81Br, 127I) reflects the environmental difference between bridging and terminal halides. The latter resonate at lower frequencies, which was considered to mean that the relative amount of n bonding from halide to metal is much larger for X, than for Xb. 

NQR studies on (Me4N)2[(Nb6Cli2)Cl6] revealed only the Clb resonance. It was suggested that the Cl, resonance may have occurred outside the observed range (3 to 33 MHz) as a result of a high ionic character of the Nb—Cl, bonds. 

As in the case of cytosine, several NMR and NQR studies were performed in search of the predominating tautomeric structures of uracil and thymine and their nucleotides and nucleosides. Investigation of PMR spectra of these compounds in nonaqueous solvents, such as dimethyl sulfoxide, localized the mobile protons in a number of 5- and 6-substituted uracils.59,61,328 These and similar studies63,85,329,330 indicated that dilactam structure 32 predominates in uracil compounds in aqueous and nonaqueous solutions as well as in the solid state. Proton and N-15 magnetic resonance spectra of several pyrimidines85 confirmed the diketo structure usually ascribed to uracil. 

The vibrational spectra,79,155 160 197Au Mossbauer spectra75,74,78 and NQR spectra78,79 of [AuC12]- and the bromo and iodo analogues have been studied. The NQR study indicated that the AuCl bonds in [AuC121- have —68% ionic character,78,79 but theoretical studies indicate greater covalent character., 64... 

The rotational motion of the ammonium ions in ammonium trifluorozincate has been studied over the temperature range 298-1 K by 3H NMR methods.62 Halogen NQR studies of [MX4]2-and [MX3] salts (M = Cd or Zn X = Cl, Br or I) have been reported.63... 

A considerable number of NQR studies have been made on zinc and cadmium complexes, for example, the 35C1, 81Br and 127I NQR spectra of a number of CdX2-polyether complexes have been reported and indicate that the compounds are dimeric in solution, with symmetrical halogen bridges between the metal atoms.69 A number of amino acids and peptide complexes of cadmium(II) have been investigated by 14N NQR spectroscopy.70... 

Iodine and bromine NQR studies on the complexes MX2L2 (M = Zn, Cd or Hg L = nitrogen heterocycle) show that the covalency of the M—L bond decreases in the order Cd>Zn> Hg.482 NMR, X-ray photoelectron studies and extended Hiickel calculations on HgCl2py2 indicate a moderate amount of electron transfer to the metal from pyridine relative to ZnCl2py2 filled mercury d orbitals do not participate in bonding.483... 

On the basis of 127I nuclear quadrupole resonance (NQR) studies of I31 AICI, , Corbett and co-workers785 have predicted a bond angle of 97° between the two bonding orbitals on the central atom. X-ray crystal structures of I31 AICI and I5 SbF6 have been obtained. In the latter salt, two cations are weakly bound to a... 

I, 373—380 (1965). NQR Study of Substituent Induction in organochloro-silanes. Theoretical and Experimental Chemistry 7, 241—246 (1965). 

Vibrational spectral studies have suggested that Me2AsCl3 and Ph2AsCl3 have a trigonal-bipyramidal structure with methyl or phenyl groups in equatorial positions (XXXV) (154). These results are in complete agreement with the results of 35C1-NQR studies (48). 

A 35C1-NQR study (48) indicates that R3ASCI2 (R = Me or Ph), unlike the phosphorus analogs, possesses a trigonal-bipyramidal structure in... 

Le Cointe, M., Gallier, J., Cailleau, H., Gourdji, M., Peneau, A. and Guibe, L. (1995b). Cl NQR study on TTF-CA crystals symmetry lowering and hysteresis at the neutral-to-ionic transition. Solid Stale Commun., 94, 455-9. [197]... 

No N nuclear quadrupole resonance (NQR) studies have been reported for transition metal nitrosyl complexes. This technique is expected to show interesting differences between linear and bent nitro-syls since the values of the quadrupole coupling constant and the asymmetry parameter depend on the relative populations of the nitrogen s,p, py, andpz orbitals. NQR spectroscopy has been used to study organic and inorganic nitroso compounds, and it is found that both... 

NQR studies have been made on a number of coordination complexes containing other nitrogen ligands such as o-aminobenzoic acid, pyridine, imidazole, cyanide, and thiocyanate 98). For the latter ligand it was possible to distinguish between N and S bonded ligands. 

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