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Chowdhury PR, Kawade M, Patwari GN. Mechanistic variances in NO release: ortho vs. meta isomers of nitrophenol and nitroaniline. Chem Commun (Camb) 2024; 60:5431-5434. [PMID: 38686426 DOI: 10.1039/d4cc01497a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/02/2024]
Abstract
The NO release following 266 nm photolysis of ortho and meta isomers of nitrophenol and nitroaniline shows a bimodal translational energy distribution, wherein the slow and fast components originate from dynamics in the S0 and T1 states, respectively. The translational energy distribution profiles for any NO product state show a higher slow-to-fast (s/f) branching ratio for the ortho isomer in comparison with the meta isomer. The observed variation in the s/f branching ratio vis-à-vis the ortho and meta isomers is attributed to the presence of intramolecular hydrogen bonding between the ortho substituent and NO2 moiety, which favours the roaming mechanism.
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Affiliation(s)
- Prahlad Roy Chowdhury
- Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
| | - Monali Kawade
- Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
| | - G Naresh Patwari
- Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
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Lau N, Ghosh D, Bourne-Worster S, Kumar R, Whitaker WA, Heitland J, Davies JA, Karras G, Clark IP, Greetham GM, Worth GA, Orr-Ewing AJ, Fielding HH. Unraveling the Ultrafast Photochemical Dynamics of Nitrobenzene in Aqueous Solution. J Am Chem Soc 2024; 146:10407-10417. [PMID: 38572973 PMCID: PMC11027148 DOI: 10.1021/jacs.3c13826] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2023] [Revised: 03/09/2024] [Accepted: 03/11/2024] [Indexed: 04/05/2024]
Abstract
Nitroaromatic compounds are major constituents of the brown carbon aerosol particles in the troposphere that absorb near-ultraviolet (UV) and visible solar radiation and have a profound effect on the Earth's climate. The primary sources of brown carbon include biomass burning, forest fires, and residential burning of biofuels, and an important secondary source is photochemistry in aqueous cloud and fog droplets. Nitrobenzene is the smallest nitroaromatic molecule and a model for the photochemical behavior of larger nitroaromatic compounds. Despite the obvious importance of its droplet photochemistry to the atmospheric environment, there have not been any detailed studies of the ultrafast photochemical dynamics of nitrobenzene in aqueous solution. Here, we combine femtosecond transient absorption spectroscopy, time-resolved infrared spectroscopy, and quantum chemistry calculations to investigate the primary steps following the near-UV (λ ≥ 340 nm) photoexcitation of aqueous nitrobenzene. To understand the role of the surrounding water molecules in the photochemical dynamics of nitrobenzene, we compare the results of these investigations with analogous measurements in solutions of methanol, acetonitrile, and cyclohexane. We find that vibrational energy transfer to the aqueous environment quenches internal excitation, and therefore, unlike the gas phase, we do not observe any evidence for formation of photoproducts on timescales up to 500 ns. We also find that hydrogen bonding between nitrobenzene and surrounding water molecules slows the S1/S0 internal conversion process.
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Affiliation(s)
- Nicholas
A. Lau
- Department
of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
| | - Deborin Ghosh
- School
of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
| | | | - Rhea Kumar
- Department
of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
| | - William A. Whitaker
- School
of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
| | - Jonas Heitland
- Department
of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
| | - Julia A. Davies
- Department
of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
| | - Gabriel Karras
- Central
Laser Facility, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, U.K.
| | - Ian P. Clark
- Central
Laser Facility, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, U.K.
| | - Gregory M. Greetham
- Central
Laser Facility, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, U.K.
| | - Graham A. Worth
- Department
of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
| | - Andrew J. Orr-Ewing
- School
of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
| | - Helen H. Fielding
- Department
of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
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Giussani A, Worth GA. A First Proposal on the Nitrobenzene Photorelease Mechanism of NO 2 and Its Relation to NO Formation through a Roaming Mechanism. J Phys Chem Lett 2024; 15:2216-2221. [PMID: 38373198 PMCID: PMC10910573 DOI: 10.1021/acs.jpclett.3c03457] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2023] [Revised: 02/13/2024] [Accepted: 02/14/2024] [Indexed: 02/21/2024]
Abstract
Despite the fact that NO2 is considered to be the main photoproduct of nitrobenzene photochemistry, no mechanism has ever been proposed to rationalize its formation. NO photorelease is instead a more studied process, probably due to its application in the drug delivery sector and the study of roaming mechanisms. In this contribution, a photoinduced mechanism accounting for the formation of NO2 in nitrobenzene is theorized based on CASPT2, CASSCF, and DFT electronic structure calculations and CASSCF classical dynamics. A triplet nπ* state is shown to evolve toward C-NO2 dissociation, being, in fact, the only low-lying excited state favoring such a deformation. Along the triplet dissociation path, the possibility to decay to the singlet ground state results in the frustration of the dissociation and in the recombination of the fragments, either back to the nitro or the nitrite isomer. The thermal decomposition of the latter to NO constitutes globally a roaming mechanism of NO formation.
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Affiliation(s)
- Angelo Giussani
- Instituto
de Ciencia Molecular, Universitat de València, Apartado 22085, ES-46071 Valencia, Spain
| | - Graham A. Worth
- Department
of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
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