Knowledge Bank

An approximato potential function method based on a combination of molecular orbital calculations and limited vibrational data has been developed. In the MOCIC (Molecular Orbital Constraint of interaction Coordinates) method interaction displacement $coordinates1$ are first evaluated using a molecular orbital calculation. The Interaction displacement coordinates are then constrained in a least-squares evaluation of the general quadratic compliance function, where the calculated frequencies of vibration of the normal isotopic molecule are fitted to the observed frequencies. In effect, the constraint of the various Interaction displacement coordinate, (i), fixes the relationship between primary and interaction compliants $Cij$ and $Cjj$. Thus, a completely general quadratic compliant function may be obtained using vibrational data of only one isotopic species. The MOCIC method has been applied to several molecules, (including $CH4,CF4,BF3,NF3,C2H4,C2H6$, and $C6H6$) using the semi-empirical molecular orbital method MNDO (Modified Neglect of Diatomic Overlap).$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ The quadratic potential functions so obtained are in remarkable agreement with those potential functions determined uniquely from vibrational data.