Photodissociation of triplet and singlet states of the CCO radical

Abstract

The triplet and singlet states of the ketenylidene ͑CCO͒ radical are investigated using fast radical beam photofragment translational spectroscopy, in which CCO is generated by laser photodetachment of CCO Ϫ and subsequently photodissociated, and anion photoelectron spectroscopy. In the photodissociation experiment, two bands in which the upper state of CCO predissociates are studied. Photodissociation from excitation of the à 3 ⌸-X 3 ⌺ Ϫ band in CCO is observed from 16 666-23 529 cm Ϫ1 ; resonances are observed and assigned to excited vibrational levels involving all three vibrational modes. We also report the first observation of the c 1 ⌸-ã 1 ⌬ band in CCO. Here, the ã 1 ⌬ state of CCO is generated by laser photodetachment at higher photon energy than was used to generate the X 3 ⌺ state. The c 1 ⌸ state is approximately located by photoelectron spectroscopy of CCO Ϫ , and the photodissociation experiment shows that the origin of the c 1 ⌸-ã 1 ⌬ band occurs around 17 170 cm Ϫ1. Kinetic-energy release spectra from both bands yield accurate values for the C-CO bond dissociation energy and heat of formation of CCO: D 0 ͑C-CO͒ϭ2.24Ϯ0.02 eV (51.7Ϯ0.5 kcal/mol) and ⌬H f ,298 0 ͑CCO͒ϭ4.04Ϯ0.02 eV (91.1 Ϯ0.5 kcal/mol). Although the translational-energy distributions resulting from excitation to the à 3 ⌸ and c 1 ⌸ states are clearly nonstatistical, consideration of the potential-energy surfaces indicates that dissociation from both states occurs via radiationless transitions to the X 3 ⌺ Ϫ state.

Ab initio chemical kinetics for the reactions of N2 with singlet and triplet C2O radicals

“…2), the heats of reaction for the products of NCO + CN, 3 CNN + CO and 3 NCN + CO are 45.9, 18.1 and À10.7 kcal/mol. They are in excellent agreement with the experimental values, 45.8, 17.7 and À10.7 kcal/mol based on the known experimental heats of formation (at 0 K) of CCO (91.1 ± 0.5 kcal/mol) [26]; NCO (31.4 ± 0.7 kcal/mol) [29]; CNN (136 ± 5 kcal/mol) [30]; CN (105.5 ± 2 kcal/mol kcal/mol) [31]; NCN (107.6 ± 3.2 kcal/mol) [32] and CO (À27.2 ± 0.04) [31].…”

“…Our calculated singlet CCO lies18.6 kcal/mol above the triplet CCO; the singlet-triplet energy difference is slightly higher than the experimental value 15.1 kcal/mol [26]. For the singlet reaction, CCO and N 2 initially form a very loose complex (S-P1) without a well-defined transition as shown in Fig.…”

Classical treatment of molecular collisions: striking improvement of the description of recoil energy distributions using Gaussian weighted trajectories

“…The threshold behaviour at E T Ϸ 0.05 eV, also seen experimentally by Neumark and co-workers in the reaction C 2 O → C͑ 3 P͒ +CO͑ 1 ⌺ + ͒, 16 is not reproduced by the GWQCT calculations. However, such a behaviour is not seen experimentally in the reaction NO 2 → O͑ 3 P 2 ͒ + NO; 17,18 despite strong similarities between this process and the previous ones, the measured recoil energy distribution shows a sharp increase from E T Ϸ 0 eV, in agreement with the present calculations.…”

Product Channels of the CN + HCNO Reaction

“…Many product channels are thermodynamically possible: Thermochemical information has been obtained from standard tables 24 as well as other literature for the heats of formation of HCNO, NCO, 25 HCNN, 26 HCCO, 27 NCCO, 28 and CCO. 29 In our previous experiment, 12 we detected NO products, presumably from channel 1a, and upon the addition of NO reagent, we detected CO, CO 2 , and N 2 O secondary products, which we attributed to the following secondary chemistry:…”