Tadashi Tokuhiro

Theoretical studies of heavy‐atom magnetic shielding in some small polyatomic molecules

Perturbed Hartree‐Fock calculations are performed on the molecules CH4, C2H2, C2H4, C2H6, CH3F, CH3OH, H2O, NH3, and C6H6 to obtain the heavy‐atom NMR shielding constants and magnetic susceptibilities. Slater‐type atomic orbitals (STOs) are used as the basis set. Various aspects of this model, including the dependence on basis‐set size and on the origin of the vector potential, are examined. The results are compared with the available experimental data and with other ab initio and semiempirical treatments.

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

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.

Modulation of Spin Echoes in Multi‐Half‐Spin Systems. I. Closed Formulas of Carr–Purcell Spin Echoes in Several AaBXx Systems

A density‐matrix approach to derive the effects of chemical shift and both homo‐ and heteronuclear spin coupling in a molecule on the Carr–Purcell spin‐echo (CPSE) train is developed. Closed formulas are derived for several multi‐half‐spin systems of the “AaBXx” type where a and x are arbitrary numbers of spins, and X is either the same nuclear species as A and B or different. As a special case of this type, the formulas are also obtained for both homo‐ and heteronuclear AA′XX′ systems. In heteronuclear multi‐half‐spin systems, the general equation is derived as a function of all the coupling constants. Features of the modulation patterns in “AaBXx” systems are discussed and the following relation is obtained: By introducing “effective internal chemical shifts” the modulation pattern of AaB resonance in a heteronuclear AaBXx system can be related to that in the homonuclear AaB system. Modulation patterns in homonuclear “AaBXx” systems are not summations of those in simpler spin systems and these facts ari...