Determination of the singlet/triplet branching ratio in the photodissociation of ketene

Abstract

The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm- ' below, 56, 110,200,325,425,1107, 1435, 1720, and 2500 cm-' above the singlet threshold (30 116.2 cm-'), are measured in a supersonic free jet of ketene. The CO(v!' = 0) rotational distributions at 56, 110,200,325, and 425 cm-' are bimodal. The peaks at low J's, which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described by phase space theory (PST). For CO( u" = 0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm-'), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm-' below the singlet threshold. The singlet yield shows a rapid increase in the low excess energy region (O-300 cm -'>, and a slower increase above. The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields. The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region. The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory. At 2500 cm -' excess energy, the CO( u" = 1) rotational distribution is obtained, and the ratio of CO( 0" = 1) to CO(u" = 0) products for the singlet channel is measured to be 0.045 f 0.017. This ratio is close to the variational Rice-Ramsberger-Kassei-Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016. 1954

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation

“…The experimental data at 355.1 nm can be interpreted with the aid of the schematic potential energy diagram shown in Figure . By analogy to ketene photodissociation, which has been studied extensively both experimentally , and theoretically, the initially prepared, electronically excited (S 1 ) dimethylketene can undergo internal conversion to the ground singlet state, S 0 , which subsequently undergoes CC bond fission to produce ground state singlet dimethylcarbene (denoted as CH 3 CCH 3 ) plus CO with no potential energy barrier expected in excess of the bond dissociation energy.…”

Direct Observation of Ethylidene (CH3CH), the Elusive High-Energy Isomer of Ethylene

“…Products from the second channel are very weakly detectable at m / e = 28 using 8.8 eV photoionization, but they are observed at 9.9 eV at m / e = 28 and also yield very strong daughter ion signals at m / e = 26. By analogy to the well-studied production of singlet methylene from ketene photodissociation, , this second channel is attributed to formation of electronically excited singlet ethylidene. The singlet and triplet ethylidene channels both contribute to all of the TOF spectra in Figure ; only the relative contributions differ.…”

“…The near-UV photodissociation of ketene (CH 2 CO) producing both singlet and triplet methylene (CH 2 ) plus carbon monoxide (CO) has been studied extensively, both theoretically and experimentally. , By analogy, electronic excitation of methylketene might be expected to produce CH 3 CH + CO. Early kinetics studies revealed that C 2 H 4 + CO were among the primary stable products and that ethylidene is likely formed initially .…”

A Velocity Map Ion-imaging Study on Ketene Photodissociation at 218 nm

“…Ketene was prepared by thermal decomposition of acetic anhydride in a quartz oven at 650 o C, and purified by distilling 3 times from 196 to 77 K [10].…”

“…While a recent comparison of the corresponding features in the ultraviolet absorption spectra of both CH 2 CO and CD 2 CO suggests that the C=O stretching υ 2 mode is also a major active mode for the ns Rydberg series [7]. Most of the previous studies of the dissociation dynamics of ketene concentrated on the dissociation from 1 A 2 and 3 A 2 excited states following excitation between 300−360 nm [8][9][10][11][12]. The triplet state 3 A 2 correlates to CH 2 (X 3 B 1 )+CO(X 1 Σ + ) products with a threshold of 28250 cm −1 above the zero point energy of the ground state of ketene molecule.…”