Centrifugal Distortion in Asymmetric Top Molecules. I. Ordinary Formaldehyde,H2C12O

A semiclassical approach is used to develop a simple expression for the centrifugal distortion correction in asymmetric top molecules. The general expression for the shift of any given energy level involves five experimentally determined distortion coefficients and a knowledge of the dependence of the term value as a function of asymmetry. A useful simplified expression for the frequency correction in $\Delta J=0\mathrm{}$ transitions involves only two effective rigid rotor parameters and four distortion coefficients. The method is applied to the microwave spectrum of ${\mathrm{H}}_2$ ${\mathrm{C}}^{12}$O; the results show excellent internal consistency. The resulting rigid-rotor parameters are: $a=282,106\mathrm{}$ Mc/sec, $b=38,834\mathrm{}$ Mc/sec, and $c=34,004\mathrm{}$ Mc/sec. The electric dipole moment is determined to be 2.31±0.04 debye and the line breadth parameter as 97±10 microns Hg per Mc/sec.