Sub-Doppler RQ-branch spectra and ab initio dipole derivative calculation for the ν11 (au) CH2-wagging mode of 1,3-butadiene

The infrared spectrum of the ν11 (au) out-of-plane CH2-wagging vibrational mode of the environmentally important 1,3-butadiene molecule has been investigated at sub-Doppler resolution with a CO2-laser/microwave-sideband spectrometer in order to fully resolve the rotational structure in a number of compact RQ-branch heads. The center frequencies of over 90 saturation Lamb dips in the 11 μm region have been measured with an estimated absolute accuracy of 200 kHz. The new data have been combined with previous Fourier transform results to refine the parameters in the molecular Hamiltonian. A value of |dμc/dQ11|=0.3135 debye has been obtained for the transition dipole derivative for the ν11 mode on the basis of ab initio calculations.