INVERSE BOND-LENGTH BOND-ANGLE RELATIONSHIPS .6. AN AB-INITIO SURVEY OF BEHAVIORAL TYPES

The correlation between a bond angle and the length of an adjacent bon d has been modeled in molecular mechanics programs by introduction of a simple negative stretch-bend interaction constant. We have investiga ted the validity of this approach by ab initio calculations at the Har tree Fock 6-31G level for 20 molecules of the following types: AH2X2 (A = C, Si; X = F, Cl), AX3 (A = N, P) and AX2 (A = O, S; X = H, CH3, F, Cl). The bond length/bond angle (A-X/angle X-A-X) relationships fou nd are of two different types: purely inverse and having a minimum. A set of experimental data (25 molecules) for the P-F/angle F-P-F case i s in good agreement with our calculations performed for the PF3 molecu le. Equilibrium structures of the molecules studied are compared with available experimental data.