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Title: Ultrafast Photodissociation Dynamics of Nitromethane

Abstract

Nitromethane (NM), a high explosive (HE) with low sensitivity, is known to undergo photolysis upon ultraviolet (UV) irradiation. The optical transparency, homogeneity, and extensive study of NM make it an ideal system for studying photodissociation mechanisms in conventional HE materials. The photochemical processes involved in the decomposition of NM could be applied to the future design of controllable photoactive HE materials. In this work, the photodecomposition of NM from the nπ* state excited at 266 nm is being investigated on the femtosecond time scale. UV femtosecond transient absorption (TA) spectroscopy and excited state femtosecond stimulated Raman spectroscopy (FSRS) are combined with nonadiabatic excited state molecular dynamics (NA-ESMD) simulations to provide a unified picture of NM photodecomposition. The FSRS spectrum of the photoproduct exhibits peaks in the NO 2 region and slightly shifted C–N vibrational peaks pointing to methyl nitrite formation as the dominant photoproduct. A total photolysis quantum yield of 0.27 and an nπ* state lifetime of ~20 fs were predicted from NA-ESMD simulations. Predicted time scales revealed that NO 2 dissociation occurs in 81 ± 4 fs and methyl nitrite formation is much slower having a time scale of 452 ± 9 fs corresponding to the excited state absorptionmore » feature with a decay of 480 ± 17 fs observed in the TA spectrum. Lastly, although simulations predict C–N bond cleavage as the primary photochemical process, the relative time scales are consistent with isomerization occurring via NO 2 dissociation and subsequent rebinding of the methyl radical and nitrogen dioxide.« less

Authors:
ORCiD logo [1]; ORCiD logo [1];  [1];  [1];  [1]; ORCiD logo [1];  [1]; ORCiD logo [1]
  1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1479945
Report Number(s):
LA-UR-15-27850
Journal ID: ISSN 1089-5639
Grant/Contract Number:  
AC52-06NA25396
Resource Type:
Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory
Additional Journal Information:
Journal Volume: 120; Journal Issue: 4; Journal ID: ISSN 1089-5639
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; nitromethane; photochemistry; photodissociation dynamics; quantum yield

Citation Formats

Nelson, Tammie Renee, Bjorgaard, Josiah August, Greenfield, Margo Torello, Bolme, Cindy, Brown, Katie, Mcgrane, Shawn David, Scharff, Robert Jason, and Tretiak, Sergei. Ultrafast Photodissociation Dynamics of Nitromethane. United States: N. p., 2016. Web. doi:10.1021/acs.jpca.5b09776.
Nelson, Tammie Renee, Bjorgaard, Josiah August, Greenfield, Margo Torello, Bolme, Cindy, Brown, Katie, Mcgrane, Shawn David, Scharff, Robert Jason, & Tretiak, Sergei. Ultrafast Photodissociation Dynamics of Nitromethane. United States. doi:10.1021/acs.jpca.5b09776.
Nelson, Tammie Renee, Bjorgaard, Josiah August, Greenfield, Margo Torello, Bolme, Cindy, Brown, Katie, Mcgrane, Shawn David, Scharff, Robert Jason, and Tretiak, Sergei. Wed . "Ultrafast Photodissociation Dynamics of Nitromethane". United States. doi:10.1021/acs.jpca.5b09776. https://www.osti.gov/servlets/purl/1479945.
@article{osti_1479945,
title = {Ultrafast Photodissociation Dynamics of Nitromethane},
author = {Nelson, Tammie Renee and Bjorgaard, Josiah August and Greenfield, Margo Torello and Bolme, Cindy and Brown, Katie and Mcgrane, Shawn David and Scharff, Robert Jason and Tretiak, Sergei},
abstractNote = {Nitromethane (NM), a high explosive (HE) with low sensitivity, is known to undergo photolysis upon ultraviolet (UV) irradiation. The optical transparency, homogeneity, and extensive study of NM make it an ideal system for studying photodissociation mechanisms in conventional HE materials. The photochemical processes involved in the decomposition of NM could be applied to the future design of controllable photoactive HE materials. In this work, the photodecomposition of NM from the nπ* state excited at 266 nm is being investigated on the femtosecond time scale. UV femtosecond transient absorption (TA) spectroscopy and excited state femtosecond stimulated Raman spectroscopy (FSRS) are combined with nonadiabatic excited state molecular dynamics (NA-ESMD) simulations to provide a unified picture of NM photodecomposition. The FSRS spectrum of the photoproduct exhibits peaks in the NO2 region and slightly shifted C–N vibrational peaks pointing to methyl nitrite formation as the dominant photoproduct. A total photolysis quantum yield of 0.27 and an nπ* state lifetime of ~20 fs were predicted from NA-ESMD simulations. Predicted time scales revealed that NO2 dissociation occurs in 81 ± 4 fs and methyl nitrite formation is much slower having a time scale of 452 ± 9 fs corresponding to the excited state absorption feature with a decay of 480 ± 17 fs observed in the TA spectrum. Lastly, although simulations predict C–N bond cleavage as the primary photochemical process, the relative time scales are consistent with isomerization occurring via NO2 dissociation and subsequent rebinding of the methyl radical and nitrogen dioxide.},
doi = {10.1021/acs.jpca.5b09776},
journal = {Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory},
number = 4,
volume = 120,
place = {United States},
year = {2016},
month = {1}
}

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