NMR Chemical Shifts. 1. The Role of Relative Atomic Orbital Phase in Determining the Sign of the Paramagnetic Terms:  ClF, CH₃F, CH₃NH₃⁺, FNH₃⁺, and HC⋮CF

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, CH 3 F, CH 3 NH 3 +, FNH 3 +, and FC⋮CH. The main paramagnetic contributions arose from the π and π‘ molecular orbitals (MOs) interacting with the σ* MO. The π interaction leads to deshielding in all cases. However, for the π‘ 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.