The rotational Zeeman effect in fluorobenzene is reinvestigated with a resolution improved by a factor of almost five to give more accurate g-tensor elements, magnetic susceptibility anisotropics and molecular electric quadrupole moments. The results fit into the pictures of a linear dependence of the out of plane molecular electric quadrupole moment, Qcc, on the number of fluorine substituents and of a linear correlation between the nonlocal (ring current) susceptibility, Χccnonlocal, and the CNDO/2-π-electron density alternation. They lead to a gasphase molecular electric quadrupole moment in benzene, Qcc,benzene = - (28.4 ± 4.7) · 10-40 Cm2 which is slightly less negative than the value deduced from electric field-gradient birefringence experiments on dilute benzene solutions with carbon tetrachloride as solvent. A detailed description of the high resolution microwave spectrometer is given in the appendix.
The rotational Zeeman effect of the most abundant isotopic species of pyrazole and of imidazole has been studied under AM=Q and AM = ± 1 selection rules. With field close to 1.9 Tesla, the nuclear Zeeman effect uncouples the rotational angular momentum and the spins of the two nonequivalent l4 N quadrupole nuclei. The observed ^-tensor elements are g aa = -0.07498 (14), g hh = -0.12531(13), and g cc = 0.06346 (12) The so-called nonlocal (n-ring current) contributions to the out of plane components of the susceptibility tensor, / f n°nlocal , derived as differences between the observed susceptibilities and values calculated from additivity rules for local atom susceptibilities, are compared to those derived earlier for other aromatic five membered ring molecules.
The highfield, linear and quadratic Zeeman effect has been observed in Acrylonitrile (Vinyl Cyanide). The Zeeman multiplets are complicated by the presence of the 14N nuclear quadrupole coupling, however the 14N nuclear Zeeman effect effectively uncouples 14N spin from the rotational angular momentum. This uncoupling was used to derive improved molecular electric dipole moments from the Stark-shifts in the 211 ← 202 and 312 ← 303 rotational transitions observed under ΔMj=0 selection rule in the presence of a high magnetic field. They are |μa| = 3.815(12)D and |μb| = 0.894(68)D, respectively. From the zero field hfs multiplets, observed under high resolution conditions, improved 14N quadrupole coupling constants were obtained: χaaN = - 3.7800(21) MHZ, χbbN - χccN = - 0.4200(89) MHz. They are discussed with reference to the structure of the Nitrile group. From the Zeeman splittings of the 101 ← 000, 110 ← 101, 211 ← 202 and 312 ← 303 rotational transitions observed under ΔMj = 0 and ΔMj = ± 1 selection rule, the diagonal elements of the molecular g-tensor and the anisotropics in the diagonal elements of the molecular magnetic susceptibility tensor were obtained as gaa = -0.17901(33), gbb, = - 0.04585(17), gcc = -0.01820(16) and 2 χaa - χbb - χcc = - 7.22(25) 10-6 erg G-2 mole-1 and 2χbb - χcc - χaa = + 15.90(31) 10-6 erg G-2 mole-1.They are discussed with reference to the molecular electric quadrupole moment, the paramagnetic and diamagnetic contributions to the molecular susceptibilities, and the second moments of the electronic charge distribution. The susceptibility data are also used to derive the magnetic susceptibility tensor contribution of the Nitrile group.
BackgroundPost processing for brain spectra has a great influence on the fit quality of individual spectra, as well as on the reproducibility of results from comparable spectra. This investigation used pairs of spectra, identical in system parameters, position and time assumed to differ only in noise. The metabolite amplitudes of fitted time domain spectroscopic data were tested on reproducibility for the main brain metabolites.MethodsProton spectra of white matter brain tissue were acquired with a short spin echo time of 30 ms and a moderate repetition time of 1500 ms at 1.5 T. The pairs were investigated with one time domain post-processing algorithm using different parameters. The number of metabolites, the use of prior knowledge, base line parameters and common or individual damping were varied to evaluate the best reproducibility.ResultsThe protocols with most reproducible amplitudes for N-acetylaspartate, creatine, choline, myo-inositol and the combined Glx line of glutamate and glutamine in lesion free white matter have the following common features: common damping of the main metabolites, a baseline using only the points of the first 10 ms, no additional lipid/macromolecule lines and Glx is taken as the sum of separately fitted glutamate and glutamine. This parameter set is different to the one delivering the best individual fit results.DiscussionAll spectra were acquired in “lesion free” (no lesion signs found in MR imaging) white matter. Spectra of brain lesions, for example tumors, can be drastically different. Thus the results are limited to lesion free brain tissue. Nevertheless the application to studies is broad, because small alterations in brain biochemistry of lesion free areas had been detected nearby tumors, in patients with multiple sclerosis, drug abuse or psychiatric disorders.ConclusionMain metabolite amplitudes inside healthy brain can be quantified with a normalized root mean square deviation around 5 % using CH3 of creatine as reference. Only the reproducibility of myo-inositol is roughly twice as bad. The reproducibility should be similar using other references like internal or external water for an absolute concentration evaluation and are not influenced by relaxation corrections with literature values.
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