Knowledge Bank

We have continued calculations of dipole moments of polyatomic molecules, as a function of their nuclear configuration, using the Pople and Segal CNDO approximate $method1$ to obtain the molecular wave functions. Earlier $tests2,3$ have encouraged some confidence in the calculated results. Here we report calculated dipole moment derivatives for 1,1-difluoroethylene and cis 1,2-difluoroethylene and illustrate their application to the interpretation of the experimental infrared intensity data of Kagel and $Overend.4$ These experimental results have been extremely difficult to interpret. For example, a typical symmetry class gives four to six possible sets of values for the dipole moment derivatives $(\partial \mu /\partial Rj$ where $Rj$ is a symmetry coordinate) which are consistent with all the experimental data, even from isotopically substituted molecules, depending upon the choice of relative signs for $\partial \mu /\partial Qi$. Thus, interpretation of these data provides a real challenge for the theory. Use of these calculated dipole moment functions has been most encouraging and appears to be a major step forward in the interpretation of such experimental infrared intensity measurements.