A BENT BOND MODEL FOR THE VIBRATIONAL FORCE CONSTANTS OF NON-PLANAR XY3 MOLECULES.

A model is developed for the vibrational force constants of non-planar XY sub-three molecules which includes terms for bent bonds. The stretching and non-bonded atom repulsion terms are the usual Urey-Bradley type while the bending force constant is replaced by a force constant for the angle between the bond forming orbitals and a term for the amount by which the bond is bent. These force constants are then transformed into the usual internal coordinates. For the molecule NF3 which has large forces between the non-bonded atoms, the computed vibrational force constants are consistent with the bent bond model assuming the bonds are bent by some 5 deg. although it is not possible to solve uniquely for all the adjustable parameters. Orbital following effects do not suffice for NF3 so there is some plausibility for this mode. For some XH3 molecules, whose force constants can be accounted for by orbital following effects and where non-bonded atom forces are small, there is no evidence for bent bonds, in accord with most chemists intuition. Author