HNCO + h.nu.(193.3 nm) .fwdarw. H + NCO: Center-of-Mass Translational Energy Distribution, Reaction Dynamics, and D0(H-NCO)

Zhang, Jingsong; Dulligan, M.; Wittig, C.

doi:10.1021/j100019a030

Cited by 76 publications

(86 citation statements)

References 19 publications

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“…Figure 2 25 which means that the fast direct dissociation pathway is open at relatively short photolysis wavelength. The change in recoil anisotropies around 210.02 nm shows a transition region from a mechanism dominated by IC followed by decomposition via the lower surface S 0 to a mechanism dominated by direct dissociation on the S 1 surface.…”

Section: ■ Experimental Methodsmentioning

confidence: 95%

Competition between Direct and Indirect Dissociation Pathways in Ultraviolet Photodissociation of HNCO

Dorenkamp

et al. 2013

J. Phys. Chem. A

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Photodissociation dynamics of HNCO at photolysis wavelengths between 200 and 240 nm have been studied using the Hatom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. At low photon energy excitation, the product translational energy distribution is nearly statistical and the angular distribution is isotropic, which is consistent with an indirect dissociation mechanism, i.e., internal conversion from S 1 to S 0 surface and dissociation on S 0 surface. As the photon energy increases, a direct dissociation pathway on S 1 surface opens up. The product translational energy distribution appears to be quite nonstatistical and the product angular distribution is anisotropic. The fraction of direct dissociation pathway is determined to be 36 ± 5% at 202.67 nm photolysis. Vibrational structures are observed in both direct and indirect dissociation pathways, which can be assigned to the NCO bending mode excitation with some stretching excitation. ■ INTRODUCTIONPhotodissociation of isocyanic acid (HNCO) has attracted much attention over the last thirty years because of its importance in combustion and atmospheric chemistry. In addition, HNCO can serve as a benchmark system for understanding the multiple decomposition pathways in a simple four-atom molecule, such as internal conversion (IC), intersystem crossing (ISC), and direct dissociation processes. The first UV absorption band of HNCO is known to extend from 280 nm to wavelengths shorter than 200 nm. It has been analyzed by Dixon and Kirby 1 and by Rabalais et al. 2,3 and assigned to an S 1 ( 1 A″) ← S 0 ( 1 A′) transition to the first singlet state. Photodissociation dynamics of HNCO in the S 1 state have been revealed by dozens of experimental and theoretical studies and are found to be quite complicated, because at least three electronic states (S 0 , S 1 , and T 1 ) participate in the dissociation. 4−11 There are three thermodynamically allowed dissociation channels from the S 1 state:30060 25 cm The energetically lowest energy channel 1 is a spin-forbidden dissociation pathway and has only recently been observed by direct detection of 3 NH in 260−217 nm photolysis. Zyrianov et al. 7 suggested that channel 1 requires ISC by which HNCO eventually reaches the triplet state T 1 and then dissociates. Theoretical studies 12 showed that the dissociation of this channel follows both the IC and ISC processes from S 1 to S 0 and then from S 0 to T 1 . Although its exact quantum yield has not been determined, channel 1 appears to be the major channel in the region just above the threshold of channel 2. 13,14 The dynamics of channel 3 have been extensively studied at several different wavelengths. [15][16][17][18][19][20] measured the 1 NH state distribution by using laser-induced fluorescence (LIF) following 193.3 nm photodissociation. They found that NH(a 1 Δ) was formed to be predominantly in v = 0, with rotational excitation accounting for about 8% of the available energy. Fujimot...

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Section: ■ Experimental Methodsmentioning

confidence: 95%

Competition between Direct and Indirect Dissociation Pathways in Ultraviolet Photodissociation of HNCO

Dorenkamp

et al. 2013

J. Phys. Chem. A

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“…In the VUV region, two HNCO dissociation channels are allowed, leading respectively to H˙+ NCO˙(reaction 1) and HN + CO (reaction 2) (Zhang et al 1995;Brownsword et al 1997). Reaction 1 is a source of H atoms which can play a role in the production of new species.…”

Section: Ocnmentioning

confidence: 99%

Tentative identification of urea and formamide in ISO-SWS infrared spectra of interstellar ices

Raunier

Chiavassa

Duvernay

et al. 2004

A&A

127

146

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Abstract. Laboratory experiments involving vacuum ultraviolet (VUV) irradiation of solid isocyanic acid (HNCO) at 10 K, followed by infrared spectroscopy (FTIR), are used to interpret the complex spectra associated with Interstellar Medium (ISM) dust grains, particularly the spectra associated with the icy phase observed toward dense molecular clouds. The comparison of the infrared spectra of the photolysis products with spectra recorded from the protostellar source NGC 7538 IRS9 shows that the "unexplained" 1700 cm −1 feature can be attributed to the contribution of several species H 2 CO (formaldehyde), HCONH 2 (formamide) and H 2 NCONH 2 (urea) mixed with H 2 O as the main contributor. Urea, formaldehyde and NH− (ammonium cyanate) may also contribute to a band at 1470 cm −1 , widely observed in many protostellar infrared sources and which remains up to now poorly explained in numerous ISO-SWS spectra. Isocyanic acid could be a precursor of formamide and urea in interstellar ices.

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“…21,[38][39][40][41] The first of these was Okabe's observation of the threshold for emission from electronically excited NCO (A 2 ⌺ ϩ ) in the vacuum ultraviolet photolysis of HNCO, which gave an upper limit D ͑H-NCO͒р112.9 Ϯ0.2 kcal/mol. 38 An ab initio calculation of the energetics of the N-H bond dissociation by Allen and co-workers yielded a value of D 0 ͑H-NCO͒ϭ110.5 kcal/mol, 39,40 consistent with Okabe's limit.…”

Section: A the N-h Bond Enthalpy D 0 (H-nco)mentioning

confidence: 99%

The HNCO heat of formation and the N–H and C–N bond enthalpies from initial state selected photodissociation

Brown

Berghout

Crim

1996

The Journal of Chemical Physics

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HNCO + h.nu.(193.3 nm) .fwdarw. H + NCO: Center-of-Mass Translational Energy Distribution, Reaction Dynamics, and D0(H-NCO)

Cited by 76 publications

References 19 publications

Competition between Direct and Indirect Dissociation Pathways in Ultraviolet Photodissociation of HNCO

Competition between Direct and Indirect Dissociation Pathways in Ultraviolet Photodissociation of HNCO

Tentative identification of urea and formamide in ISO-SWS infrared spectra of interstellar ices

The HNCO heat of formation and the N–H and C–N bond enthalpies from initial state selected photodissociation

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