NMR chemical shifts.: 1.: The role of relative atomic orbital phase in determining the sign of the paramagnetic terms:: ClF, CH3F, CH3NH3+, FNH3+, and HCCF

The shielding of several nuclei for molecules in magnetic fields was examined via B3LYP/6-311+G(3df, 3pd) calculations using the MP2/6-31G* geometries. Both the gauge-invariant atomic orbital (GIAO) and individual gauges for atoms in molecules (IGAIM) methods were employed, and the molecules were the isoelectronic series F-2, ClF, CH3F, CH3NH3+, FNH3+, and FC=CH. The main paramagnetic contributions arose from the pi and pi' molecular orbitals (MOs) interacting with the sigma* MO. The pi interaction leads to deshielding in all cases. However, for the pi' MO in which the two p atomic orbitals have opposite phase, one nucleus is deshielded but the other is shielded. This appears to be a general phenomenon.