Photolytic mechanisms of hydroxylamine.

Jittima Thisuwan,Phorntep Promma,Kritsana Sagarik

doi:10.1039/c9ra10956k

Jittima Thisuwan, Phorntep Promma + Show 1 more

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https://doi.org/10.1039/c9ra10956k

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Abstract

The photodissociation of small molecules has been extensively studied because of the increase in environmental problems related to the atmosphere of the Earth. In this work, the photodissociation mechanisms of hydroxylamine (NH2OH) as a model molecule in its lowest singlet-excited (S1) state were systematically studied using the complete active space second-order perturbation theory (CASPT2) and transition state theory (TST). In particular, this study focused on nonradiative relaxation processes that convert the S0 → S1 excited-state molecule to its products in their respective electronic ground states. The potential energy curves obtained from relaxed scans suggest that O–H dissociation is the preferred process in the S1 state. For the N–O and N–H dissociation pathways, thermally excited precursors were hypothesized to form in the S0 state to circumvent O–H dissociation. Thus, S0 → S1 vertical excitations lead to transition structures in the S1 state, which fragment to their respective electronic-ground-state products. The thermodynamic and kinetic results confirmed the precursor hypothesis, showing that the exothermic energy caused by the formation of HNO and H2 is sufficient to generate such precursors in the S0 state. Additionally, the TST confirmed that unimolecular isomerization–dissociation is a two-step process that generates products effectively by direct photolysis of the corresponding covalent bonds. In particular, the process consists of O–H bond dissociation, followed by spontaneous isomerization and formation of H2 in its electronic ground state, resulting in the high quantum yield observed in the UV absorption experiments in the preferential formation of HNO and H2. The configuration interaction coefficients of the characteristic structures on the potential energy curves revealed considerable changes in the multiconfigurational character of the wavefunctions, especially for the transition structures. These are characterized by the development of Rydberg orbitals, being produced at the intersection of the S0 and S1 states. The present study highlights the effects of thermal selectivity and the multiconfigurational character of the wavefunctions on photodissociation. Because detailed information on the photolytic mechanisms of isolated NH2OH is limited both theoretically and experimentally, these results provide fundamental insight into unimolecular photodissociation, posing ground for future studies on related systems.

Highlights

The photodissociation of small molecules has been extensively studied both theoretically and experimentally because environmental problems related to the atmosphere of the Earth are increasing.1 Because it has O–H and N–H groups, as well as lone-pair electrons, hydroxylamine (NH2OH) has o en been employed as a prototypical molecule in mechanistic studies of gas-phase photodissociation processes.2–7 For isolated NH2OH, two types of unimolecular photodissociation mechanisms have been reported: (i) direct photolysis of the O–H, N–O, and N–HIn the direct photolysis pathway, ultraviolet (UV) absorption experiments have shown that the H-atom channel, in which two H atoms are generated with a quantum efficiency greater than one (1.7), is the preferred process at an absorption wavelength of 193 nm
The characteristic structures of NH2OH, identi ed on the S0 and S1 potential energy curves, are labeled with a three-character code as Gk-[l], Ek-[l]s, or Ek-[l]*, where G indicates a structure in the S0 state, E indicates one in the S1 state, and k indicates dissociation channels (1)–(7)
The photodissociation mechanisms of NH2OH in the lowest singlet-excited state were studied by ab initio calculations in the CASPT2(10,9)/aug-cc-pVDZ framework

Summary

Introduction

The photodissociation of small molecules has been extensively studied both theoretically and experimentally because environmental problems related to the atmosphere of the Earth are increasing.1 Because it has O–H and N–H groups, as well as lone-pair electrons, hydroxylamine (NH2OH) has o en been employed as a prototypical molecule in mechanistic studies of gas-phase photodissociation processes.. In the direct photolysis pathway, ultraviolet (UV) absorption experiments have shown that the H-atom channel, in which two H atoms are generated with a quantum efficiency greater than one (1.7), is the preferred process at an absorption wavelength of 193 nm In this pathway, N–O dissociation is a minor process, with a quantum efficiency of less than 0.1.5 Instead, photolysis by UV absorption at 240 nm leads mainly to the dissociation of N–O and formation of NH2 and OH in their electronic ground states.. N–O dissociation is a minor process, with a quantum efficiency of less than 0.1.5 Instead, photolysis by UV absorption at 240 nm leads mainly to the dissociation of N–O and formation of NH2 and OH in their electronic ground states. though the O–H dissociation was rst proposed, both O–H and N–O dissociation have been reported as primary processes (representing 60% and 40%, respectively) in the direct photolysis of NH2OH vapor at 298 K, because of the possible thermal decomposition. Paper

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