Calculations of proton magnetic shielding constants in polyatomic molecules

Abstract

Proton magnetic shielding constants in H 2 0, NH 3 , and CH 4 molecules have been evaluated within the framework of coupled and uncoupled Hartree-Fock perturbation theories for the Fock-Dirac density matrix, employing several basis sets of gaugeless Gaussian functions. The results yielded by the largest basis sets are in good agreement with experiment, as well as other ab initio calculations. Some quantities, useful in testing the degree of gauge independence of computed screening constants and in evaluating this property for a gauge whatsoever, are also presented. Numerical tests demonstrate the basic soundness of the uncoupled approach to magnetic shielding proposed here, because of its higher accuracy with respect to other uncoupled schemes and its practicality for larger molecules. 832

Coupled-Hartree-Fock calculations of susceptibilities and magnetic shielding constants

“…H20 , NH, and CH 4 have been treated by Arrighini et al [5][6][7][8] R. H611er and H. Lishcka [9][10][11]. These investigations have made use of field independent basis sets, and thus are directly comparable to our results.…”

“…Guided by the experience obtained from the calculations on HF, H20 and CH 4 (to be described later) we performed only one calculation with a large basis set for NH a (see table 10). As far as agreement with experiment is concerned, we obtain similar results to Lazzeretti and Zanasi [10], e.g.…”

“…Rather large s and p basis sets and at least one or two d-sets on the heavy atoms are necessary for accurate calculations [4][5][6][7][8][9][10][11]. We have used 9s5p to lls7p basis sets and additional s and p functions.…”

“…In recent years several theoretical investigation on magnetic properties of molecules have been published [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. From these calculations it is clear that the situation is much more complicated than in the case of the computation of molecular energies in the absence of external fields.…”

Density functional theory investigation of3dtransition metal NMR shielding tensors in diamagnetic systems using the gauge-including projector augmented-wave method

“…[31] Nevertheless, only small molecules have been studied in such a way because of the prohibitive computational effort required. [32,33] GIAO, CSGT and IGAIM methods have been developed for molecular NMR calculations using localized basis sets. The difficulty associated with application of localized methods to extended systems was circumvented by the use of a cluster approximation.…”