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Title: Ab initio dynamics and photoionization mass spectrometry reveal ion-molecule pathways from ionized acetylene clusters to benzene cation

Abstract

The growth mechanism of hydrocarbons in ionizing environments, such as the interstellar medium (ISM), and some combustion conditions remains incompletely understood. Ab initio molecular dynamics (AIMD) simulations and molecular beam vacuum-UV (VUV) photoionization mass spectrometry experiments were performed to understand the ion-molecule growth mechanism of small acetylene clusters (up to hexamers). A dramatic dependence of product distribution on the ionization conditions is demonstrated experimentally and understood from simulations. The products change from reactive fragmentation products in a higher temperature, higher density gas regime toward a very cold collision-free cluster regime that is dominated by products whose empirical formula is (C 2H 2) n +, just like ionized acetylene clusters. The fragmentation products result from reactive ion- molecule collisions in a comparatively higher pressure and temperature regime followed by unimolecular decomposition. The isolated ionized clusters display rich dynamics that contain bonded C 4H 4 + and C 6H 6 + structures solvated with one or more neutral acetylene molecules. Such species contain large amounts ( > 2 eV) of excess internal energy. The role of the solvent acetylene molecules is to affect the barrier crossing dynamics in the potential energy surface (PES) between (C 2H 2) n + isomers and providemore » evaporative cooling to dissipate the excess internal energy and stabilize products including the aromatic ring of the benzene cation. Formation of the benzene cation is demonstrated in AIMD simulations of acetylene clusters with n > 3, as well as other metastable C 6H 6 + isomers. Lastly, these results suggest a path for aromatic ring formation in cold acetylene-rich environments such as parts of the ISM.« less

Authors:
 [1];  [2];  [2];  [2];  [2]; ORCiD logo [2]; ORCiD logo [1]
  1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Division; Univ. of California, Berkeley, CA (United States). Dept. of Chemistry
  2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Division
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); National Aeronautics and Space Administration (NASA)
OSTI Identifier:
1355949
Alternate Identifier(s):
OSTI ID: 1379856
Grant/Contract Number:
AC02-05CH11231
Resource Type:
Journal Article: Published Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 114; Journal Issue: 21; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; ion-molecule reactions; polycyclic aromatic hydrocarbons; molecular dynamics; quantum chemistry; photoionization mass spectrometry

Citation Formats

Stein, Tamar, Bandyopadhyay, Biswajit, Troy, Tyler P., Fang, Yigang, Kostko, Oleg, Ahmed, Musahid, and Head-Gordon, Martin. Ab initio dynamics and photoionization mass spectrometry reveal ion-molecule pathways from ionized acetylene clusters to benzene cation. United States: N. p., 2017. Web. doi:10.1073/pnas.1616464114.
Stein, Tamar, Bandyopadhyay, Biswajit, Troy, Tyler P., Fang, Yigang, Kostko, Oleg, Ahmed, Musahid, & Head-Gordon, Martin. Ab initio dynamics and photoionization mass spectrometry reveal ion-molecule pathways from ionized acetylene clusters to benzene cation. United States. doi:10.1073/pnas.1616464114.
Stein, Tamar, Bandyopadhyay, Biswajit, Troy, Tyler P., Fang, Yigang, Kostko, Oleg, Ahmed, Musahid, and Head-Gordon, Martin. Mon . "Ab initio dynamics and photoionization mass spectrometry reveal ion-molecule pathways from ionized acetylene clusters to benzene cation". United States. doi:10.1073/pnas.1616464114.
@article{osti_1355949,
title = {Ab initio dynamics and photoionization mass spectrometry reveal ion-molecule pathways from ionized acetylene clusters to benzene cation},
author = {Stein, Tamar and Bandyopadhyay, Biswajit and Troy, Tyler P. and Fang, Yigang and Kostko, Oleg and Ahmed, Musahid and Head-Gordon, Martin},
abstractNote = {The growth mechanism of hydrocarbons in ionizing environments, such as the interstellar medium (ISM), and some combustion conditions remains incompletely understood. Ab initio molecular dynamics (AIMD) simulations and molecular beam vacuum-UV (VUV) photoionization mass spectrometry experiments were performed to understand the ion-molecule growth mechanism of small acetylene clusters (up to hexamers). A dramatic dependence of product distribution on the ionization conditions is demonstrated experimentally and understood from simulations. The products change from reactive fragmentation products in a higher temperature, higher density gas regime toward a very cold collision-free cluster regime that is dominated by products whose empirical formula is (C2H2)n+, just like ionized acetylene clusters. The fragmentation products result from reactive ion- molecule collisions in a comparatively higher pressure and temperature regime followed by unimolecular decomposition. The isolated ionized clusters display rich dynamics that contain bonded C4H4+ and C6H6+ structures solvated with one or more neutral acetylene molecules. Such species contain large amounts ( > 2 eV) of excess internal energy. The role of the solvent acetylene molecules is to affect the barrier crossing dynamics in the potential energy surface (PES) between (C2H2)n+ isomers and provide evaporative cooling to dissipate the excess internal energy and stabilize products including the aromatic ring of the benzene cation. Formation of the benzene cation is demonstrated in AIMD simulations of acetylene clusters with n > 3, as well as other metastable C6H6+ isomers. Lastly, these results suggest a path for aromatic ring formation in cold acetylene-rich environments such as parts of the ISM.},
doi = {10.1073/pnas.1616464114},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 21,
volume = 114,
place = {United States},
year = {Mon May 08 00:00:00 EDT 2017},
month = {Mon May 08 00:00:00 EDT 2017}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1073/pnas.1616464114

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  • The growth mechanism of hydrocarbons in ionizing environments, such as the interstellar medium (ISM), and some combustion conditions remains incompletely understood. Ab initio molecular dynamics (AIMD) simulations and molecular beam vacuum-UV (VUV) photoionization mass spectrometry experiments were performed to understand the ion-molecule growth mechanism of small acetylene clusters (up to hexamers). A dramatic dependence of product distribution on the ionization conditions is demonstrated experimentally and understood from simulations. The products change from reactive fragmentation products in a higher temperature, higher density gas regime toward a very cold collision-free cluster regime that is dominated by products whose empirical formula is (Cmore » 2H 2) n +, just like ionized acetylene clusters. The fragmentation products result from reactive ion- molecule collisions in a comparatively higher pressure and temperature regime followed by unimolecular decomposition. The isolated ionized clusters display rich dynamics that contain bonded C 4H 4 + and C 6H 6 + structures solvated with one or more neutral acetylene molecules. Such species contain large amounts ( > 2 eV) of excess internal energy. The role of the solvent acetylene molecules is to affect the barrier crossing dynamics in the potential energy surface (PES) between (C 2H 2) n + isomers and provide evaporative cooling to dissipate the excess internal energy and stabilize products including the aromatic ring of the benzene cation. Formation of the benzene cation is demonstrated in AIMD simulations of acetylene clusters with n > 3, as well as other metastable C 6H 6 + isomers. Lastly, these results suggest a path for aromatic ring formation in cold acetylene-rich environments such as parts of the ISM.« less
  • Small cationic and anionic clusters of lithium formate were generated by electrospray ionization and their fragmentations were studied by tandem mass spectrometry. Singly as well as multiply charged clusters were formed with the general formulae, (HCOOLi)nLi+, (HCOOLi)nLimm+, (HCOOLi)nHCOO- and (HCOOLi)n(HCOO)mm-. Several magic number cluster ions were observed in both the positive and negative ion modes although more predominant in the positive ion mode with (HCOOLi)3Li+ being the most abundant and stable cluster ions. Fragmentations of singly charged clusters proceed first by the loss of a dimer unit ((HCOOLi)2) followed by sequential loss of monomer units (HCOOLi). In the case ofmore » positive cluster ions, all fragmentations lead to the magic cluster (HCOOLi)3Li+ at higher collision energies which later fragments to dimer and monomer ions in lower abundance. Quantum mechanical calculations performed for smaller cluster ions showed that the trimer ion has a closed ring structure similar to the phenalenylium structure with three closed rings connected to the lithium ion. Further additions of monomer units result in similar symmetric structures for hexamer and nonamer cluster ions. Thermochemical calculations show that trimer cluster ion is relatively more stable than neighboring cluster ions, supporting the experimental observation of a magic number cluster with enhanced stability.« less
  • Small cationic and anionic clusters of lithium formate were generated by electrospray ionization and their fragmentations were studied by tandem mass spectrometry (collision-induced dissociation with N{sub 2}). Singly as well as multiply charged clusters were formed in both positive and negative ion modes with the general formulae, (HCOOLi){sub n}Li{sup +}, (HCOOLi){sub n}Li{sub m}{sup m+}, (HCOOLi){sub n}HCOO{sup −}, and (HCOOLi){sub n}(HCOO){sub m}{sup m−}. Several magic number cluster (MNC) ions were observed in both the positive and negative ion modes although more predominant in the positive ion mode with (HCOOLi){sub 3}Li{sup +} being the most abundant and stable cluster ion. Fragmentations ofmore » singly charged positive clusters proceed first by the loss of a dimer unit ((HCOOLi){sub 2}) followed by the loss of monomer units (HCOOLi) although the former remains the dominant dissociation process. In the case of positive cluster ions, all fragmentations lead to the magic cluster (HCOOLi){sub 3}Li{sup +} as the most abundant fragment ion at higher collision energies which then fragments further to dimer and monomer ions at lower abundances. In the negative ion mode, however, singly charged clusters dissociated via sequential loss of monomer units. Multiply charged clusters in both positive and negative ion modes dissociated mainly via Coulomb repulsion. Quantum chemical calculations performed for smaller cluster ions showed that the trimer ion has a closed ring structure similar to the phenalenylium structure with three closed rings connected to the central lithium ion. Further additions of monomer units result in similar symmetric structures for hexamer and nonamer cluster ions. Thermochemical calculations show that trimer cluster ion is relatively more stable than neighboring cluster ions, supporting the experimental observation of a magic number cluster with enhanced stability.« less
  • We present the results of a detailed operando XAFS and density functional theory (DFT) based ab initio molecular dynamics (AIMD) investigation of the proposed mechanism of dehydrogenation of dimethylaminoborane (DMAB) by a homogeneous Rh4 cluster catalyst. Our AIMD simulations reveal that the previously proposed Rh structures are highly fluxional exhibiting both metal cluster and ligand isomerizations and dissociaton which can only be accounted for by a examining finite temperature ensemble as generated by AIMD. It is found that a highly fluxional species Rh4((H2BNMe2)82+ is fully compatible with operando XAFS measurements which suggest that this species may be the catalyst restingmore » state. Based on this assignment we propose a catalytic mechanism for DMAB dehydrogenation which exhibits a maximum energy barrier of 24 kcal/mol, which is half that observed for the uncatalyzed thermal reaction. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program, and was performed in part using the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a DOE national scientific user facility located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. Department of Energy.« less