Rotational spectrum, ring-puckering vibration and ab initio calculations on 1,1-difluorocyclobutane

The rotational spectra of the ground and first five ring-puckering excited states of 1,1-difluorocyclobutane have been investigated in the centimeter- (8-40 GHz) and millimeter-wave regions (96-106 and 144-159 GHz). Analysis of the spectra for the pairs of coupled vibrational states v(p)=0/1 and v(p )=2/3 has been carried out using two-state Hamiltonians, yielding accurate rotational and centrifugal distortion constants, vibration-rotation couplin g parameters, and the energy spacings Delta E-01 and Delta E-23. The spectr um for the v(p)=4 to v(p)=6 ring-puckering states was satisfactorily accoun ted for in terms of effective semi-rigid Hamiltonians for each vibrational state. A double minimum ring-puckering potential function with a barrier to ring inversion of 231(4) cm(-1) has been obtained from the analysis of the vibrational dependence of the rotational constants and inversion splitting s Delta E-01 and Delta E-23. The calculated variation of the quartic centri fugal distortion constants with the ring-puckering quantum number reproduce s satisfactorily the experimental trends, confirming the validity of the de rived potential function. Ab initio calculations with HF, MP2, and B3LYP de nsity functional hybrid methods have been carried out for this molecule usi ng different basis sets. The experimental and ab initio potential functions , coupling terms, and ring-puckering dynamical parameters are compared. Fin ally, an ab initio near-equilibrium structure is presented. (C) 1999 Americ an Institute of Physics. [S0021-9606(99)31037-0].