No abstract

High field strength MRI demonstrates lateral substantia nigra pars compacta abnormalities in early Parkinson disease (PD) consistent with increased iron content and corresponding to the known distribution of neuronal loss occurring in this disorder. This may ultimately provide an imaging marker for disease progression in PD, although longitudinal studies are required.

A series of organic hydrochloride salts has been investigated using solid-state 35 Cl and 37 Cl NMR spectroscopy at applied magnetic field strengths of 9.4 and 18.8 T. Magic-angle spinning, static Hahn-echo, and quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) echo experiments have been applied to investigate the chlorine electric field gradient (EFG) and chemical shift (CS) tensors for L-tyrosine hydrochloride, L-cysteine methyl ester hydrochloride, L-cysteine ethyl ester hydrochloride, quinuclidine hydrochloride, and tris sarcosine calcium chloride. Chlorine-35 nuclear quadrupolar coupling constants for these compounds range from 2.23 to 5.25 MHz, and isotropic chemical shifts range from approximately 9 to 53 ppm relative to the chloride ion in aqueous solution. The results demonstrate the feasibility and benefits of high-field 35/37 Cl NMR studies of organic chloride salts. A discussion of the data in the context of the known X-ray or neutron diffraction structures for these compounds suggests that the chlorine EFG tensor is a valuable probe of hydrogen bonding to the chloride ion. Because the anisotropies of the CS tensors are rather small, precise determination of the chlorine CS tensors proved to be challenging and was only feasible for L-cysteine ethyl ester hydrochloride, where the span, Ω, was found to be 47 ( 4 ppm. This represents the first determination of Ω(Cl) from a powder sample. Results of ab initio calculations of the chlorine EFG and CS tensors in L-tyrosine hydrochloride are presented and compared with the experimental data.

Despite the large known chemical shift (CS) range for boron and the large number of 11 B NMR studies of glasses, no boron CS tensors have been characterized to date. We report the application of solid-state NMR techniques at moderate (9.4 T) and high (17.63 T) applied magnetic field strengths to the characterization of the boron CS tensors in trimesitylborane (BMes 3 ) and triphenyl borate (B(OPh) 3 ). The boron CS tensor of the former compound exhibits a remarkably large span, Ω ) 121 ( 1 ppm, which encompasses the known range of isotropic chemical shifts for tricoordinate boron compounds. Conversely, the effect of the boron CS tensor on the 11 B NMR spectra of B(OPh) 3 is difficult to observe and quantify even at field strengths as high as 17.63 T; we find Ω e 10 ppm. This marked difference in the boron nuclear magnetic shielding tensors is reproduced accurately by a series of ab initio and DFT calculations with a range of basis sets. The difference is rationalized in the context of Ramsey's theory of nuclear magnetic shielding by considering contributions to the paramagnetic shielding in the tricoordinate boron plane. Differences in the in-plane shielding tensor components for the molecules considered are a result of variations in the effectiveness of the mixing of occupied σ orbitals with virtual π orbitals under the influence of an applied magnetic field. A similar explanation has been invoked to rationalize 13 C isotropic chemical shifts in classical and nonclassical carbocations. We also report experimental and calculated boron nuclear quadrupolar coupling constants and asymmetry parameters for BMes 3 and B(OPh) 3 . A combination of experimental and theoretical results provides the orientation of the CS and electric field gradient tensors in the molecular framework.