A new mechanism for spin--lattice relaxation of heavy nuclei in the solid state: 207Pb relaxation in lead nitrate.

A detailed investigation of the spin-lattice relaxation time, T1, for 207Pb in solid lead nitrate has been undertaken in an effort to understand the mechanism of relaxation. The results show that the 207Pb T1 is independent of magnetic field strength and inversely proportional to the square of the temperature. These are signatures of relaxation by a spin-phonon Raman scattering mechanism. Nuclear spin-lattice relaxation in solid lead salts is more efficient for sites with smaller magnetic shielding anisotropy. A coupling mechanism is proposed whereby phonons create a local magnetic field by modulating the valence electron shell motion relative to the nuclear/electron core. Literature data suggest that spin-phonon scattering is a common relaxation pathway for other spin-1/2 heavy nuclei in solids.

Modeling 2hJiso(N, N) in nucleic acid base pairs: ab initio characterization of the 2hJ(N, N) tensor in the methyleneimine dimer as a function of hydrogen bond geometry.

The observation of 2hJiso(N, N) coupling has prompted considerable interest in this phenomenon from experimentalists and theoreticians due to the potential these couplings hold for the determination of secondary and tertiary structure in biologically important molecules. Here, we present an ab initio (MCSCF) study of the complete 2hJ(N, N) tensor for a model methyleneimine dimer system as a function of (i) the N-N separation, rNN, and (ii) the hydrogen bond angle, theta. This simple system models the 2hJ(N, N) tensor of nucleic acid base pairs. Results indicate that although the Fermi-contact mechanism dominates 2hJiso(N, N), the coupling tensor is anisotropic due to contributions from the Fermi-contact spin-dipolar cross term. The variation in 2hJiso(N, N) as a function of rNN is fit to an exponential decay. The influence of theta on the coupling constant is less pronounced but must be considered if experimental coupling constants are to be used for quantitative structure determination. Our results for this simple model system demonstrate that 2hJiso(N, N) is a valuable probe of hydrogen bonding in nucleic acid base pairs.

Anisotropy in the 31P, 63/65Cu indirect spin-spin coupling and 31P nuclear shielding tensors of linear copper(I) phosphines.

Phosphorus-31 CP/MAS NMR spectra of two linear bis(tribenzylphosphine)cuprate(I) salts reveal significant anisotropy in the one-bond 31P, 63,65Cu J tensors. The parallel and perpendicular components of 1J(31P, 63Cu), for example, are +2.0 and +1.3 kHz, respectively. The well-characterized space group and point symmetry in one of these compounds greatly simplifies the 31P lineshape analysis, thus dispensing with the need to make assumptions about the interaction tensors involved. This offers an unique opportunity to study DeltaJ by calculating dipolar coupling constants from known internuclear distances, and by using zero-field nuclear quadrupole resonance spectroscopy to obtain the magnitudes of the 63/65Cu nuclear quadrupole coupling constants, CQ. Copper-63 CQ values in these complexes exceed 80 MHz, proving to be the largest reported for copper(I) phosphines. Phosphorus-31 NMR spectra of non-spinning samples at three applied magnetic fields are also presented, along with lineshape calculations based on full-matrix Zeeman-quadrupolar Hamiltonian diagonalization. It is shown that exact 31P lineshape calculations provide the relative signs of CQ, the isotropic J-coupling, and the effective dipolar coupling constant. This appears to be the first unambiguous determination of Delta1J(31P, 63/65Cu).

Spinning sidebands in slow-magic-angle-spinning NMR spectra arising from tightly J-coupled spin pairs.

Complex spinning sidebands are observed in magic-angle-spinning (MAS) NMR spectra arising from isolated tightly J-coupled spin pairs under slow spinning conditions. Such spinning sidebands are sensitive to the magnitude and relative orientation of the chemical-shift tensors, the dipolar-coupling tensor, and the sign of the indirect spin-spin (J) coupling. We show that it is possible to extract information concerning such NMR parameters from an analysis of the observed spinning sidebands. As an example, numerical simulations are carried out to reproduce observed 31P MAS NMR spectra of a phosphole tetramer (1) and o-bis(diphenylphosphino)benzene (2), so that invaluable information concerning the orientations of the phosphorus chemical-shift tensors and the sign of J(31P, 31P) can be deduced. Simulations are carried out by numerically evaluating the spin-density matrix of the spin system.

Phosphorus chemical shift tensors in dithiadiphosphetane disulfides determined by solid-state 31P nuclear magnetic resonance.

The phosphorus chemical shift tensors of two dithiadiphosphetane disulfides, 2,4-bis(4-methoxyphenyl)-1,3-dithia-2,4-diphosphetane-2,4-di sulfide (1) and 2,4-bis(methylthio)-1,3-dithia-2,4-diphosphetane-2,4-disu lfi de (2), have been characterized by solid-state 31P nuclear magnetic resonance (NMR) measurements. The weak homonuclear dipolar interaction between the two 31P nuclei in each of these compounds has a significant influence on the 31P NMR line shapes of static powder samples. From the weak dipolar splittings it is possible to deduce the orientation of the phosphorus chemical shift tensor. In both compounds, the intermediate components of the phosphorus chemical shift tensors, delta 22, were found to be perpendicular to the plane defined by the two phosphorus atoms and the two terminal sulphur atoms in the molecule, S=P ... P=S. The smallest shift components, delta 33, were found to deviate approximately 14 degrees from the P=S bond direction.

Observation of nitrogen-14, carbon-13 indirect spin-spin coupling in high-resolution 13C CP/MAS spectra of solids.

The 13C CP/MAS NMR spectrum of [(n-C3H7)4N][Cd(SCN)3], 1, indicates the presence of three non-equivalent thiocyanate ligands, in agreement with the results of a recent single-crystal X-ray diffraction study. Examination of the 13C MAS line shapes allows direct measurement of the indirect spin-spin coupling constants, 1J(14N, 13C) = 16 +/- 1 Hz and 2J(111/113Cd, 13C) = 75 +/- 5 Hz, for the unique N-bonded thiocyanate ligand. This is the first reported measurement of 1J(14N, 13C) and 2J(111/113Cd, 13C) in the solid state. Possible reasons for the failure to observe 1J(14N, 13C) values in previous high-resolution 13C CP/MAS NMR studies are summarized.

Phosphorus-31 NMR studies of stabilized phosphorus ylids in the solid state.

Phosphorus-31 powder NMR spectra and high-resolution MAS spectra have been obtained for a number of stabilized phosphorus ylids under conditions of high-power proton decoupling and cross-polarization. The 31P CP/MAS spectra are compared to those obtained from isotropic solutions. The variation of chemical shift anisotropy and of the principal components of the 31P chemical shift tensor determined from 31P powder NMR line shapes are discussed in terms of the relative importance of accepted valence bond resonance structures. The results indicate that the invariance of the isotropic chemical shift, delta iso, observed in previous 31P NMR investigations of phosphorus ylids in solution is due to fortuitous cancellation of opposing changes in the principal components, delta 11 and delta 33, of the 31P chemical shift tensor. The 31P dipolar NMR powder spectrum of a typical stabilized ylid, (C6H5)3(31)P-13CHC(O)OCH2CH3, is analyzed in order to obtain the orientation of the 31P chemical shift tensor with respect to the 31P-13C alpha dipolar vector.

Natural abundance 13C nuclear magnetic resonance studies of live cestodes.

1. Natural abundance carbon-13 nmr spectra of several intact cestodes have been obtained. 2. All spectra show peaks assignable to triglycerides and the N(CH3)3 carbons of the choline moiety. 3. The olefinic region of the 13C nmr spectra indicated that the cestode larvae Mesocestoides corti and Echinococcus multilocularis have a larger concentration of polyunsaturated fatty acids than Hymenolepis adults. 4. Mobile fragments of glycogen were detected in all species studied, but its apparent concentration in individual cestodes was highly variable.

Application of long-range 13C,H nuclear spin-spin coupling constants in the study of imidazole tautomerism in L-histidine, and related compounds.

Three-bond 13C,H nuclear spin-spin coupling constants can be used to estimate the relative populations of the two possible tautomers of the imidazole ring in compounds containing this moiety. The mole fraction, XII, of the 1-H tautomer (i.e. the tautomer having a proton bound to the nitrogen atom at position 1 of the ring, where the ring is numbered according to IUPAC-IUB convention) can be calculated from the following equation: XII congruent to 1.705--0.164 3J(C5,H2)obs. Estimates of the predominant tautomer in L-histamine (both the neutral molecule and the monocation), and the dipeptide L-histidyl-L-glutamic acid indicate that the 1-H form is favored in basic aqueous solution. The results are compared with those obtained by other methods, and show that the present technique can provide a simple, rapid means of determining tautomeric equilibrium in imidazole derivatives.

A nuclear magnetic resonance study of the metal binding sites in bacitracin.

Carbon-13 nuclear magnetic resonance spectroscopy has been used to identify sites in bacitracin which bind Cu2+ and Mn2+. Results are presented which implicate the free carboxyl groups of the aspartic and glutamic acid residues and the imidazole ring of the histidine residue as metal complexation sites between pH 6 and 8. Evidence is presented which also indicates that the thiazoline ring of bacitracin binds Mn2+. Bacitracin does not bind Cu2+ or Mn2+ at pH values of 2.5 or less.

pH-dependence of 13C chemical shifts and 13C,H coupling constants in imidazole and L-histidine.

The pH-dependence of selected 13C chemical shifts reflects the state of ionization of the imidazole ring in both imidazole and L-histidine. Titration of the amino and carboxyl groups of histidine also perturbs the shifts. The coupling constants 1J (13C(2),H) and 1J (13C(5),H) for both compounds also vary with pH, but in L-histidine these constants are relatively insensitive to the titration of groups outside the imidazole ring.