CALCULATION OF NMR CHEMICAL-SHIFTS - THE 3RD DIMENSION OF QUANTUM-CHEMISTRY

NMR chemical shift calculations provide the basis for an intensive col laboration between quantum chemists and experimentalists. Calculated s hift data can be used to describe the magnetic properties of a molecul e, to identify unknown compounds by comparison of experimental and the oretical shift values, to determine equilibrium geometries, to investi gate conformational changes, to elucidate the mechanism of molecular r earrangements, to determine solvent effects on NMR data, to identify c omplexation or coordination of soluted molecules by solvent molecules, to detect electronic structure changes caused by the medium, and to d escribe chemical bonding. This is demonstrated by three examples, name ly the determination of the equilibrium structure of the homotropylium cation, the description of BH3NH3 in solution or condensed phases, an d the investigation of stannyl cation complexes in solution. IGLO calc ulations of C-13, B-11, N-15, and Sn-119 chemical shifts with DZ+P or TZ+P basis sets lead to the following results: (1) The homotropylium c ation possesses an equilibrium 1,7 distance of 2 Angstrom that is indi cative of strong through-space interactions and, as a consequence, hom oaromatic character. (2) In solution, the charge transfer from NH3 to BH3 is increased, which leads to a decrease of the BN bond length, an increase of the dipole moment, and a shielding of both the B and the N nucleus. The experimental delta(B-11) and delta(N-15) values can be r eproduced when the geometry effect and the direct solvent effect are i ncluded in the shift calculations. (3) Stannyl cations form strongly-b ounded coordination complexes with solvent molecules (binding energy: greater than or equal to 50 kcal/mol) that make the cation properties, in particular delta(Sn-119) values, similar to those of covalently-bo unded stannyl compounds. An experimental detection of stannyl cations in solution by NMR spectroscopy should only be possible by extensive s olvent variations.