Centrifugal Distortion Effects in Asymmetric Rotor Molecules. I. Quadratic Potential Constants and Average Structure of Oxygen Difluoride from the Ground-State Rotational Spectrum

The microwave spectrum of oxygen difluoride has been thoroughly investigated in the frequency regions 8 to 40 and 49 to 60 kMc. A total of 83 absorption lines have been assigned as ground-state rotational transitions in which the total angular momentum J ranges from 1 to 39. These data provide the means for a critical test of the Kivelson—Wilson theory of centrifugal distortion. It is found that the K—W energy expression only approximately accounts for the measurements. The K—W formula obtains when (1) separation of rotation and vibration is carried out to second-order of approximation, and (2) when the quartic (P4) distortion terms in the resulting effective rotational Hamiltonian are treated by first-order perturbation theory. It is found that both of these approximations must be carried one order higher to obtain a satisfactory fit of the observed rotational frequencies of OF2. Analysis of the rotational spectrum by the method of least squares yields the following ground-state rotational parameters (Mc): A=58782.630, B=10896.431, C=9167.412, τxxxx=—0.0949559, τzzzz=—6.615402, τxxzz=0.399611, τxzxz=—0.162486, HJK=—2.2184×10—6, and HKJ=3.1073×10—6, where HJK and HJK are the coefficients of semiempirical P6 distortion terms in the effective rotational Hamiltonian. The quartic distortion constants yield vibrational frequencies which agree with the observed values to within 4% or better. By combining microwave data with the observed vibrational frequencies the following quadratic potential constants are obtained: fr=3.9499 mdyn/Åfrr=0.8059 mdyn/Å,fα/r2=0.7237 mdyn/Å,frα/r=0.1373 mdyn/Å.Using this potential function with formulas derived by Laurie and Herschbach the average structure of OF2 in the ground vibrational state is found to be 〈r〉=1.4124 Å, 〈α〉=103∘10′,where r is the OF distance, and α the FOF angle.