Scalar Relaxation of the Second Kind — A Potential Source of Information on the Dynamics of Molecular Movements. 2. Magnetic Dipole Moments and Magnetic Shielding of Bromine Nuclei

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• 作者：Adam Gryff-Keller ; Sergey Molchanov ; Artur Wodyński
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：January 9, 2014
• 年：2014
• 卷：118
• 期：1
• 页码：128-133
• 全文大小：232K
• ISSN：1520-5215

文摘

In this paper, we continue the exploration of possibilities, limitations, and methodological problems of the studies based on measurements of the nuclear spin relaxation rates running via the scalar relaxation of the second kind (SC2) mechanism. The attention has been focused on the ¹³C鈥?sup>79Br and ¹³C鈥?sup>81Br systems in organic bromo compounds, which are characterized by exceptionally small differences of Larmor frequencies, 螖蠅_CBr, of the coupled nuclei. This unique property enables experimental observation of longitudinal SC2 relaxation of ¹³C nuclei, which makes investigation of the SC2 relaxation rates an attractive experimental method of determination of spin鈥搒pin coupling constants and relaxation rates of quadrupole bromine nuclei, both types of parameters being hardly accessible by direct measurements. A careful examination of the methodology used in SC2 relaxation studies of carbon鈥揵romine systems reveals, however, some disturbing facts that could burden the results with systematic inaccuracies. Namely, the way of calculating the Larmor frequency differences between ¹³C and bromine isotopes, 螖蠅CBr, may cause some reservations. In this work, the values of ⁷⁹Br and ⁸¹Br magnetogyric ratios have been rechecked using bromine NMR data for the KBr路Kryptofix 222 complex in acetonitrile solution and the results of the advanced calculations of the magnetic shielding of the bromine nucleus in the Br^{鈥?/sup> anion. Moreover, it has been pointed out that in the case of 13C鈥?sup>79}Br, the magnetic shielding of the bromine nucleus in the investigated molecule must not be neglected during the calculation of the 螖蠅_CBr parameter. Some recommendations concerning the exploitation of available theoretical methods to calculate bromine shielding constants for bromo compounds have also been formulated, keeping in mind relativistic effects.