Photodissociation dynamics of the CNN free radical

Abstract

The spectroscopy and photodissociation dynamics of the à 3 ⌸ and B 3 ⌺ Ϫ states of the CNN radical have been investigated by fast beam photofragment translational spectroscopy. Vibronic transitions located more than 1000 cm Ϫ1 above the à 3 ⌸←X 3 ⌺ Ϫ origin were found to predissociate. Photofragment yield spectra for the B 3 ⌺ Ϫ ←X 3 ⌺ Ϫ band between 40 800 and 45 460 cm Ϫ1 display resolved vibrational progressions with peak spacing of Ϸ1000 cm Ϫ1 corresponding to symmetric stretch 1 0 n and combination band 1 0 n 3 0 1 progressions. Ground state products C(3 P)ϩN 2 were found to be the major photodissociation channel for both the à 3 ⌸ and B 3 ⌺ Ϫ states. The translational energy distributions for the à 3 ⌸ state are bimodal with high and low translational energy components. The distributions for the B 3 ⌺ Ϫ state reveal partially resolved vibrational structure for the N 2 photofragment and indicate extensive vibrational and rotational excitation of this fragment. These results suggest that bent geometries are involved in the dissociation mechanism and provide more accurate values: ⌬ f H 0 ͑CNN͒ϭ6.16Ϯ0.05 eV and ⌬ f H 298 ͑CNN͒ϭ6.15Ϯ0.05 eV. These values, coupled with recent D 0 ͑RH͒ and D 298 ͑RH͒ values from Clifford et al. ͓J. Phys. Chem. 102, 7100 ͑1998͔͒, yield ⌬ f H 0 ͑HCNN͒ϭ5.02Ϯ0.18 eV, ⌬ f H 298 ͑HCNN͒ϭ4.98Ϯ0.18 eV, ⌬ f H 0 ͑H 2 CNN͒ϭ3.09Ϯ0.21 eV, and ⌬ f H 0 ͑H 2 CNN͒ϭ3.09Ϯ0.21 eV.

Photodissociation dynamics of the HCNN radical

“…The value of D 0 obtained in this way is 1.139± 0.019 eV. Together with the heat of formation of CH, 6.149± 0.013 eV, 36,47 this yields a heat of formation, ⌬ f H 0 ͑HCNN͒ = 5.010± 0.023 eV, in agreement with the value of 5.02± 0.18 eV previously reported, 25 but with much less uncertainty. The values of E T max for channel ͑3͒ products ͑CH͑X 2 ⌸͒ +N 2 ͑X 1 ⌺ g + ͒͒ calculated from this value of D 0 are shown as dashed, vertical lines in Fig.…”

“…25 10,16 There is, however, an exit barrier of about 1.46 eV for dissociation of HCNN ͑X 2 AЉ͒ to HCN ͑X 1 ⌺ + ͒ +N ͑ 4 S͒ products, a process which, in addition, requires intersystem crossing. 16 The results of the present study reveal photofragmentation of HCNN following excitation at energies above 25 600 cm −1 , a range in which no previous spectroscopic studies have been performed on this radical.…”

“…There have been relatively few experimental studies of HCNN. Bise et al 25 determined the heat of formation, ⌬ f H 0 ͑HCNN͒ = 5.02± 0.18 eV, based on their experimentally determined value of ⌬ f H 0 ͑CNN͒ and the value of D 0 ͑H-CNN͒ for H atom loss previously determined by Clifford et al 26 Infrared spectra of HCNN and DCNN were measured in inert gas matrices following the photolysis of H 2 CNN and ͑H/D͒ 2 CNN with a mercury lamp, but the absorptions were not assigned. 27 Decay of the HCNN signal, attributed to photodissociation, was observed after longer irradiation with a mercury lamp; it was determined that photon energies below 16 000 cm −1 did not contribute to HCNN loss.…”

State-resolved translation energy distributions for NCO photodissociation

“…Although the mass resolution of these experiments hinders the separation of the two mass channels, in previous studies of NCN and CNN we observed the presence of a N 2 ϩC mass channel that was energetically overlapped with a CNϩN mass channel. 38,39 A broadening and shifting of the peaks in the mass spectrum from those of a single channel marked the presence of both channels. A similar trend was not observed in the mass spectrum of the products of NCO photodissociation, indicating that the contribution from the OϩCN mass channel is not substantially greater than the contribution from the NϩCO mass channel in the region of E T р0.71 eV.…”

A comparative study for elastic electron collisions on the isoelectronic CNN, NCN, and CCO radicals

“…7,8 The presence of NCN in that reactive environment was confirmed by the observation of a complicated system of bands near 3290 Å. 14 For instance, it has been appointed as an intermediate in the reaction of CH radicals with N 2 , CH͑X 2 ⌸͒ + N 2 ͑X 2 ⌺ g + ͒ → HCN͑X 1 ⌺ + ͒ + N͑ 4 S͒, providing a low energy path to nitrogen atom formation. 10 In addition, ultraviolet emission studies 11 have suggested that the NCN radical is present in the Comet Brorosen-Metcalf.…”