On a Perturbed Hartree-Fock Study of the Magnetic Susceptibility and the 13C and 17O NMR Shielding Constants in Formaldehyde with Slater Basis Sets

Abstract

The theoretical study of Flygare et al. on one-electron properties of the formaldehyde molecule is extended to include the paramagnetic susceptibility and the 13C and 17O NMR shielding constants and to investigate their sensitivity to Slater basis-set quality. The Gaussian transform method is used to evaluate the necessary integrals. Using the fully coupled perturbed Hartree-Fock formalism, we find that improvements in the wavefunction produce significant changes in most components of the second-order properties. The best shielding calculations, obtained with a double-zeta Slater set, are within the rather large experimental uncertainties, whereas the agreement between the theoretical and experimental susceptibility results is less satisfactory. The complete paramagnetic tensors are analyzed in terms of MO and AO contributions. Comparisons are made with other ab initio calculations and with simplified forms of the theory.

Calculations of proton magnetic shielding constants in polyatomic molecules

“…We see that the conditions (12) are fulfilled with satisfactory accuracy for basis sets IV and V. It is interesting to note that, even if we are concerned here with proton shielding constants, the computed values of quantities like (7) and (10) are sensibly affected by the characteristics of the basis functions on the heavy atom. This can be easily understood: the operator (L/y3)H' owing to its form, weights heavily on the proton, but large contributions (sometimes the largest ones) also arise from the charge distribution surrounding the electron-rich heavy atom.…”

“…(9) give important information on the reliability of computed values: from (9) the conditions for exact gauge invariance of total CHF shielding are (12) hence the deviation from (12) gives an idea of the quality of a calculation in the light of the conclusions previously drawn. 24…”

“…At any rate, it can be reasonably argued that the theoretical predictions of diamagnetic shielding are, probably, more accurate For sets IV and V the difference a(c. m.)a(H) is appreciably reduced as can be observed on the tables. More quantitative considerations can be made on the basis of the gauge invariance conditions (12), strictly valid for complete sets of expansion, e. g. , deviations from conditions (12) measure, in a certain sense, the departure of a given basis set from completeness with respect to operators (3) and (4).…”