PHOTODECOMPOSITION OF CH2O CD2O CHDO AND CH2O-CD2O MIXTURES AT XENON FLASH LAMP INTENSITIES

The volume and composition of the gaseous products of the photolyses of the pure formaldehydes, CH2O, CD2O, and CHDO, and mixtures of CH2O and CD2O were determined in experiments at various xenon flash lamp intensities, pressures of aldehyde, temperatures, and at selected wavelength regions of the aldehyde absorption. The data are consistent with the occurrence of two primary photodissociative processes, CH2o* → H + HCO (I) and CH2O* → H2 + CO (II), and the dominance of radical-radical reactions following process I for these conditions. In CH2O1 φI + φII ≅ 1.0 over the entire absorption band; for CD2O and CHDO the sum is less than unity at the long-wavelength region. From the H2, HD, and D2, product distribution in CH2O-CD2O mixture photolyses and from the photolyses of CHDO, it was found that the relative efficiencies of processes I and II show a marked dependence on the wavelength of absorbed light; øIøII varies from near zero at the longest wavelengths (3550 Å) to a value over unity at the shortest wavelengths employed in this work (2800 Å). On the basis of certain reasonable assumptions concerning the mechanism of the secondary reactions, estimates were derived for øI and øII for each major absorption band for CH2O and CD2O. The temperature and pressure independence of the øIøII estimates from the photolyses limited to the long-wavelength bands of formaldehyde shows that process III, CH2O* + CH2O → CH2OH + HCO, is unimportant for these conditions. Although process II is the dominant mode of formaldehyde photodecomposition for these conditions, process I does occur measurably on the absorption of light by formaldehyde vapor in the band near 3385 Å. From these observations it is concluded that DHCO-H ≤ 85 kcal/mole. © 1969, American Chemical Society. All rights reserved.