Absorption Spectra of Contact-Charge-Transfer Bands and Photochemical Reactions of Simple Alkenes in the Cryogenic Oxygen Matrix

Article abstract of DOI:10.1021/j100290a015

The UV-vis absorption spectra and photochemistry of alkene-oxygen pairs in solid oxygen at 10 K have been studied.The contact-charge-transfer (CCT) bands well separated from the intrinsic absorption bands of isolated alkenes were recorded with the maxima at 218, 243, 234, 235, 263, and 287 nm for propene, 2-methylpropene (MP), cis-2-butene (CB), trans-2-butene (TB), 2-methyl-2-butene (MB), and 2,3-dimethyl-2-butene (DMB), respectively.The vertical transition energy correlated linearly with the ionization potential of the alkenes.Photochemical reaction products in the excitation within the CCT bands were studied by using FTIR.The reaction in the ion pair state can be classified into the following four pathways: (i) photooxygenation to give an oxygen adduct, which is observed only for DMB; (ii) cis-trans isomerization, which is the major reaction path for CB and TB, suggesting the crossing to the alkene triplet state; (iii) double-bond scission to give two corresponding carbonyl compounds, which is observed for all alkenes studied with energy threshold higher than the photooxygenation and cis-trans isomerization; (iv) formation of CO2, CO, and O3, which occurs at still shorter wavelength than the double-bond scission.In the case of ethene and propene, HO2 was also produced.Relevance of the present results to those in the CCT photochemistry in the oxygen-saturated organic liquid and on the semicinductor surfaces has been discussed.