On the definition of the atomic charge. Relationship between 13C NMR chemical shifts, dipole moments, and charges in saturated hydrocarbons

The problem of defining a reliable quantum mechanical charge by comparison with one-electron properties is analyzed, and it is stressed that properties involving the virtual space are not suited to that end. Particular attention is devoted to the relationship between charges and chemical shifts for the case of saturated hydrocarbons. A simple explanation of the Grant and Paul a effect is suggested, which can also account for the modified population analysis proposed by Flisz´ar. Moreover the vexata quaestio of the direction of the C—H bond dipole moment has been reexamined. The awkward theoretical prediction Cq—Hy. can be reconciled with the one based on experimental data and electronegativities Cy—Hq. if one considers that the former is determined by an hydridization contribution to the dipole moment, which tend to cancel in a summation over all the bonds formed by each atom.