PHOTOIONIZATION STUDIES OF INTERNALLY REACTIVE SMALL CLUSTERS - VANDERWAALS COMPLEXES OF 1,3-BUTADIENE WITH SULFUR-DIOXIDE

Efficiency functions for ions from the title complexes were measured using a photoionization mass spectrometer in which a molecular beam of target complexes, formed by a skimmed and recollimated free jet expansion of 1,3-butadiene + SO2, was made to intersect a tunable VUV beam from Brookhaven's 750-MeV electron storage ring. The dissociation energies at 0 K of 1,3-C4H6-SO2 and (1,3-C4H6-SO2)+ were measured to be 3.24 ± 0.48 and 3.00 ± 0.68 kcal mol-1, respectively. For 3-sulfolene, the condensation product, the ionization potential was measured to be 10.073 ± 0.006 eV, while the appearance potential of its fragment C4H6+ is 10.076 ± 0.029 eV, the same within the experimental uncertainty. However, the sulfolene parent ion is unstable with respect to disintegration into butadiene ion and sulfur dioxide by 0.38 ± 0.03 eV. The ion C4H6SO+ is produced in very low yield from the heterodimer but in much higher yields from heterotrimers, showing that it is born excited and is stabilized by the presence of a third “solvent” molecule. Its onset from the heterodimer is 15.90 ± 0.10 eV, about2.3 eV higher than the threshold, and 1.0 eV into the continuum for butadiene ion plus SO. Possible mechanisms for this surprising reaction are discussed. © 1990, American Chemical Society. All rights reserved.