Experimental Measurements and AB Initio Calculations of Chemical Shieldings for Some Selenium and Cadmium Compounds.
Abstract
In this research, some fundamental aspects of ^{77}Se and ^ {113}Cd chemical shieldings have been investigated. Based on the gas phase measurement of ^{77}Se chemical shift and the nuclear spin rotation coupling constant in OCSe, the absolute ^{77}Se shielding for the commonly used standard (60% (v/v) solution of (CH _3)_2Se, in DCCl_3 ) was established as 1845 +/- 65 ppm. This value was utilized in the solid state ^{77}Se NMR study of CSe _2. Because of the very inefficient spin lattice relaxation mechanism, the ^{77 }Se chemical shift component along the parallel axis, delta_{//}, of CSe_2 could not be determined directly. Instead, it was derived from the ab initio calculated result of the diamagnetic term combined with the absolute shielding scale. The chemical shift anisotropy, Deltadelta(^{77}Se), and the ratio between Deltadelta(^ {77}Se) and Deltadelta( ^{13}C) are both in good agreement with the values reported with other methods. The ^{77}Se shielding surface of H _2Se in terms of symmetry coordinates was constructed with GIAO calculations. The large isotope effect induced by deuteration in H_2Se was qualitatively discussed and closely reproduced. The ^{77}Se and ^ {113}Cd chemical shifts as well as absolute shieldings were calculated with GIAO method at both the SCF and MP2 levels for a variety of compounds rm (H_2Se, CH_3SeH, (CH _3)_2Se, OCSe, CSe_2, SeF_6, (CH_3)_2Cd and rm (C_2H_5)_2Cd). A comparison of the calculated results with the experimental gas phase chemical shifts indicated that the relative shift between rm (CH_3)_2Cd and rm (C_2H_5)_2Cd can be predicted within several ppm at the SCF level with f-type polarization functions added to the basis set, while the relative shifts among the selenium compounds, rm H_2Se, CH_3SeH (CH_3)_2Se and OCSe, can only be reproduced at the MP2 level. On the other hand, the relative shifts of CSe_2 and SeF _6 cannot be computed to the same satisfactory accuracy even with the second order electron correlation included. The calculated absolute shieldings also agree reasonably well with the experimental results.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- January 1995
- Bibcode:
- 1995PhDT........87C
- Keywords:
-
- Chemistry: Physical; Physics: Molecular