CALCULATION OF DIAMAGNETIC SHIELDING IN MOLECULES

The average diamagnetic shielding at a nucleus can be computed from the free-atom shieldings plus a knowledge of the molecular structure. The validity of computing this molecular quantity from free-atom values is related to the isoelectronic principle or the negligible change in dissociation energy with change in nuclear charge (total numbers of electrons and internuclear distances remain constant). The theory is also applied to the determination of the signs of nuclear spin-rotation constants. The signs of the proton spin-rotation constants in methane and acetylene are shown to be positive.