THE MICROWAVE SPECTRUM AND MOLECULAR STRUCTURE OF CHF2Cl

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Abstract
Sufficient Q- and R-branch rotational transitions have been measured in the microwave spectrum of the asymmetric top molecule CHF2Cl to determine the moments of inertia Ia, Ib, and Ic for each of the three isotopic species C12HF2Cl35, C12HF2Cl37, and C13HF2Cl35. The value of Ia+Ic−Ib, which should depend only upon the out-of-plane co-ordinate of the fluorine atoms, is sufficiently constant to give a very accurate value for the co-ordinate of 1.08524 ± 0.00006 Å but involves some variations which limit the accuracy of determining molecular structures by isotopic substitutions. The components of the nuclear electric quadrupole coupling dyadics in the principal axes systems for the moments of inertia ellipsoids have been calculated accurately from the hyperfine structure of the C12HF2Cl35,37 spectra. In both cases, eQVaa+eQVbb+eQVcc = 0 and the ratio of any component for Cl35 to the same component for Cl37 is equal to 1.269 within the experimental error. The low-field Stark splitting of the 00,0 → 11,0 transition indicates a dipole moment component μc = 1.5 debyes while the absence of a-type transitions indicates a value for μa, which is less than 0.1 μc. The C—Cl bond length and the angle between this bond and the a-axis have been calculated by Kraitchman's formulae for isotopic substitution to be 1.747 ± 0.002 Å and 14.3 ± 0.2°. This angle and the values of eQVaa, eQVbb, and eQVcc lead to eQVzz = eQq = −71.5 ± 0.3 Mc/s and quadrupole coupling asymmetry parameter η = −0.012 ± 0.006, which indicates very little π-electron character. If the C—H bond length is assumed to be 1.095 ±.015 Å and the angle H—C—Cl is assumed to be 108 ± 1°, then the calculated values for the C—F bond length and for the angle F—C—F are 1.350 ±.003 Å and 107.0 ± 0.4°, a value considerably less than the tetrahedral angle. The values for bond lengths, bond angles, and nuclear electric quadrupole coupling components are compared with those in other halogenated methanes.