DFT CALCULATIONS OF THE 'LONE PAIR' EFFECT - A TOOL FOR THE CHEMIST TO PREDICT MOLECULAR DISTORTIONS?

摘要

The steric "lone pair" effect is a well-known phenomenon in chemistry. Not much is known about the chemical and structural criteria, however, on which it depends, and whether a distortion from the high symmetry structure actually occurs, and to which extent. The fast availability of reliable results by Density Functional Theory calculations provides a sound basis for attacking this problem. We analyse the energetic, steric and bonding properties of molecules AX_3 (A: N to Bi; X = H, F to Ⅰ). The application of a vibronic coupling approach of the PSJT type (D_(3h) → C_(3v) transition: A_1' directX α_2"directX A_2" interaction) yields some valuable rules, from which two are mentioned. The vibronic coupling constant and the vibronic coupling energy, which predominantly determines the total D_(3h) → C_(3v) energy change, vary according to: F > H > Cl > Br > Ⅰ (A: N to Bi) ; N > P > As > Sb > Bi (X: H, F), the dependence on A being only small or not present (X: Cl to Ⅰ). Apparently the hardest molecules are the most susceptible to vibronic coupling, while a roughly inverse trend is observed for the extent of the angular distortion, where the influence of the force constant becomes distinct. Here the softest molecules such as Sb(Bi)Br_3 exhibit the largest and NH_3 the smallest deviations from D_(3h). We further note that the vibronic coupling energy approximately equals the hardness difference η(C_(3v)) - η(D_(3h)), while the corresponding electro-negativity difference does not characterise the lone pair influence. The dependence on the coordination number is studied for AX_(3+δ)~(δ-) ~ entities (δ = -1, 1, 2, 3); one may broadly state, that - in agreement with structural data -the tendency towards lone pair distortions decreases with increasing numbers of ligands.