| Abstract
| - The energetic, steric, and bonding properties of molecules AX3 (A=N to Bi; X=H, F to I) are analyzed usingdensity functional theory. It is found that the “lone pair” in the initial D3h geometry is of central atom pzcharacter for the NX3 and AH3 molecules, whereas it possesses s symmetry in all other cases − here generallywith a strong delocalization toward the ligands. The stabilization of the distorted C3v geometry is due mainlyto covalency effects, whereas steric interaction forces according to the Gillespie−Nyholm model do not seemto play a significant role. The application of the conventional vibronic pseudo Jahn−Teller coupling approach(PJT), here for the D3h→C3v transition [A1‘⊗(α2‘ ‘ + α1‘)⊗A2‘ ‘ interaction], is an appropriate means for inorganicchemists to predict trends for the extent of distortion and for the corresponding energy gain. The vibroniccoupling constants and the vibronic stabilization energies, which mainly determine the total D3h→C3v energygain, vary according to the sequences F > H > Cl > Br > I (A: N to Bi), and N > P > As > Sb > Bi(X: H,F), the dependence on A being only small or not present (X: Cl to I). Thus, the hardest molecules arethe most susceptible to vibronic coupling, the latter energy being approximately imaged by the hardnessdifference η(C3v) − η(D3h). A roughly inverse trend is observed if the extent of the angular distortion τα fromD3h to C3v symmetry is considered; here, the softest molecules such as Sb(Bi)Br3 exhibit the largest and NH3the smallest deviations from D3h geometry. The different sequences for τα are due to the strong influence ofthe force constant, which represents the C3v→D3h restoring energy. It is remarkable that the vibronic couplingenergy is strongly correlated with the chemical hardness η (an observable quantity), while the stabilizationenergy for the D3h→C3v transition is not directly reflected by η, in contrast to what is generally called the“principle of maximum hardness”.
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