Ab initio and spectroscopic examination of butadienyl radicals

Spectroscopic and theoretical studies were carried out on the butadienyl radicals. These radicals were obtained by electric discharge from the precursor butadiene and almost instantly cooled to cryogenic temperatures in a supersonic jet. The spectroscopic method used to study them is frequency modulation enhanced magnetic rotation spectroscopy, a very sensitive technique, selective to paramagnetic species. Signal-to-noise calculations predict that the radicals should be observable in the supersonic jet. However, no transitions were detected in the frequency range scanned. Detector noise and insufficient laser power also hampered the experiments.;Ab initio calculations were carried out to guide the experimental studies. Along with the butadienyl radicals, the other most stable C4H5 radicals were studied, because several radical fragments can be generated by electrical discharge from the same precursor. Geometry optimizations were carried out at the UHF, B3LYP, MP2, CISD, QCISD, MCSCF(7,7) and MCSCF(9,9) levels of theory. Single-point energies were evaluated at the MP4, CCSD(T), MCSCF(11,11) and multireference CISD levels. Disparities up to 70 kJ mol−1 were found between relative energies predicted by single- and multireference methods for the same isomer. Comparison to experimental values suggests that the multireference methods inadequately model the relative correlation energy. Relative energies and hyperfine constants obtained at the QCISD level are in agreement with available experimental data and empirical estimates. Predicted vibrational transition frequencies and intensities, dipole moments, Fermi contact hyperfine constants and potential energy curves are presented.