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Electronic States of Methylene from Ketene Photodissociation between 2 14 and 3 13 nm Vaclav P. Zabransky and Robert W. Carr, Jr.* Contribution from the Department of Chemical Engineering a n d Materials Science, University of Minnesota, Minneapolis. Minnesota 55455. Received M a y 19, 1975
Abstract: The photolysis of ketene in the presence of cis-2-butene was studied at a series of wavelengths from 3 13 to 250 nm ' A I transition. The effects of pressure and absorbed intensiof the ketene IAff IAI transition, and at 214 nm in the 'B2 ty at 285 and 214 nm were examined, and the effect of small amounts ( 4 mol %) of 0 2 was determined. The hydrocarbon reaction products formed between 250 and 313 nm are consistent with the presence of two reactive electronic states of methylene during photolysis, the ground state, X 3BI, and first excited state, I ' A I . Furthermore, the two states appear to be formed in approximately constant relative proportion over this wavelength region. At 214 nm an increase in the nonstereospecificity of the methyle_neaddition to the double bond in cis-2-butene can be attributed to the presence of the second excited state, CH2(b IBl). If b 'Bl is postulated to be formed from ketene at 214 nm, the results require that it react more rapidly with oxygen than with cis-2-butene. and also indicate that the X 3B, and I ' A I states of methylene are simultaneously present. +
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The photolysis of ketene between about 270 and 350-360 nm results in the formation of two electronic states of methylene, the 3B1 ground state and the 'A1 first excited state.'-4 At shorter wavelengths higher excited states of methylene are possible, and changing product yields at 249 nm have been suggested to provide evidence for participation of C H ~ ( I B I )the , second excited state.5 Experiments on ketene photolysis at 2 13.9 nm have been inconclusive concerning whether the reactive states of methylene are only the ground and first excited state, or whether C H ~ ( ' B I is ) also i n ~ o l v e d . ~ . ~ In this paper we report the results of a wavelength study of ketene photolysis in the presence of cis-2-butene between 200 and 313 nm. Hoffmann has predicted that C H ~ ( ' B I ) should add nonstereospecifically to olefins.* Since cis-2butene lacks a symmetry axis along the carbon-carbon double bond, and since its reactions with C H Z ( ~ B ~and ) C H ~ ( I A Ihave ) been extensively ~ t u d i e d it, ~was selected as the substrate in this work. HarrisonlO has recently reviewed experimental and theoretical work on the low-lying electronic states of methylene.
Experimental Section Apparatus. A conventional Pyrex vacuum apparatus, capable of Torr, was used. Teflon-glass valves were employed in to all portions where reaction products were handled. The 142-cm3 cylindrical reactor had planar Ultrasil quartz end windows, was 18.3 cm long, and had a 3.1 cm i.d. Ketene-cis-2-butene mixtures were photolyzed with three different light sources. (1) The emission from a medium pressure Zn arc was filtered with 4 cm of Cl2 ( I atm) to block radiation above 290 nm. (2) Emission from a 1000 W dc Xe arc lamp was passed through a Bausch and Lomb High Intensity grating monochromator, permitting selection of desired wavelength regions between 250 and 3 13 nm. The slits were set to pass a 5.0-nmband centered on the selected wavelength. (3) A high-pressure Hg arc, Osram HBO 500, was used with a threecompartment chemical filter containing a 2.5-cm path of a 55.2 g NiSOp6H20/100 cm3 H 2 0 solution and a 5-cm path of CI2 at 1 atm, for which the transmitted actinic wavelength was 265 nm. A higher intensity a t 265 nm was possible with this arrangement than with the Xe arc. Transmission spectra of arc-filter and arc-monochromator combinations were recorded photoelectrically with a 0.5-m Ebert-mounting scanning spectrometer, using IO p slits. Reactor absorbance was measured with an RCA 935 photocube. Reactants. Ketene was prepared by acetic anhydride pyrolysis, as previously d e ~ c r i b e d .After ~ several trap-to-trap distillations from -78 to -196 OC, and being pumped on at -127 OC, it contained only trace (
