On the sign of the dipole moment derivatives in CH3X (X = F, Cl, Br, I)

It is shown in this paper that the principal methods for reducing the indeterminacy of the sign of (δ M/δ$\text{O}$_i) (isotropic invariance of M, quantum mechanical calculations, determination of (δM/δ$\text{O}$_i) via high resolution data or the Stark effect) lead, in the case of CH₃X (X = F, Cl, Br, I) molecules, to contradictory conclusions. The discrepancies are illustrated and discussed. It is also pointed out that either a revision of the previous results or a calculation of electro-optical parameters on a set of chemically similar molecules including CH₃X could possibly lead to a unique determination of the sign.