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# Association of Cl with C _{2} H _{2} by unified variable-reaction-coordinate and reaction-path variational transition-state theory

## Abstract

Barrierless unimolecular association reactions are prominent in atmospheric and combustion mechanisms but are challenging for both experiment and kinetics theory. A key datum for understanding the pressure dependence of association and dissociation reactions is the high-pressure limit, but this is often available experimentally only by extrapolation. Here we calculate the high-pressure limit for the addition of a chlorine atom to acetylene molecule (Cl + C _{2} H _{2} →C _{2} H _{2} Cl). This reaction has outer and inner transition states in series; the outer transition state is barrierless, and it is necessary to use different theoretical frameworks to treat the two kinds of transition state. Here we study the reaction in the high-pressure limit using multifaceted variable-reaction-coordinate variational transition-state theory (VRC-VTST) at the outer transition state and reaction-path variational transition state theory (RP-VTST) at the inner turning point; then we combine the results with the canonical unified statistical (CUS) theory. The calculations are based on a density functional validated against the W3X-L method, which is based on coupled cluster theory with single, double, and triple excitations and a quasiperturbative treatment of connected quadruple excitations [CCSDT(Q)], and the computed rate constants are in good agreement with some of the experimentalmore »

- Authors:

- Publication Date:

- Sponsoring Org.:
- USDOE

- OSTI Identifier:
- 1602557

- Grant/Contract Number:
- [SC0015997; 51536002]

- Resource Type:
- Published Article

- Journal Name:
- Proceedings of the National Academy of Sciences of the United States of America

- Additional Journal Information:
- [Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Volume: 117 Journal Issue: 11]; Journal ID: ISSN 0027-8424

- Publisher:
- Proceedings of the National Academy of Sciences

- Country of Publication:
- United States

- Language:
- English

### Citation Formats

```
Zhang, Linyao, Truhlar, Donald G., and Sun, Shaozeng. Association of Cl with C 2 H 2 by unified variable-reaction-coordinate and reaction-path variational transition-state theory. United States: N. p., 2020.
Web. doi:10.1073/pnas.1920018117.
```

```
Zhang, Linyao, Truhlar, Donald G., & Sun, Shaozeng. Association of Cl with C 2 H 2 by unified variable-reaction-coordinate and reaction-path variational transition-state theory. United States. doi:10.1073/pnas.1920018117.
```

```
Zhang, Linyao, Truhlar, Donald G., and Sun, Shaozeng. Mon .
"Association of Cl with C 2 H 2 by unified variable-reaction-coordinate and reaction-path variational transition-state theory". United States. doi:10.1073/pnas.1920018117.
```

```
@article{osti_1602557,
```

title = {Association of Cl with C 2 H 2 by unified variable-reaction-coordinate and reaction-path variational transition-state theory},

author = {Zhang, Linyao and Truhlar, Donald G. and Sun, Shaozeng},

abstractNote = {Barrierless unimolecular association reactions are prominent in atmospheric and combustion mechanisms but are challenging for both experiment and kinetics theory. A key datum for understanding the pressure dependence of association and dissociation reactions is the high-pressure limit, but this is often available experimentally only by extrapolation. Here we calculate the high-pressure limit for the addition of a chlorine atom to acetylene molecule (Cl + C 2 H 2 →C 2 H 2 Cl). This reaction has outer and inner transition states in series; the outer transition state is barrierless, and it is necessary to use different theoretical frameworks to treat the two kinds of transition state. Here we study the reaction in the high-pressure limit using multifaceted variable-reaction-coordinate variational transition-state theory (VRC-VTST) at the outer transition state and reaction-path variational transition state theory (RP-VTST) at the inner turning point; then we combine the results with the canonical unified statistical (CUS) theory. The calculations are based on a density functional validated against the W3X-L method, which is based on coupled cluster theory with single, double, and triple excitations and a quasiperturbative treatment of connected quadruple excitations [CCSDT(Q)], and the computed rate constants are in good agreement with some of the experimental results. The chlorovinyl (C 2 H 2 Cl) adduct has two isomers that are equilibrium structures of a double-well C≡C–H bending potential. Two procedures are used to calculate the vibrational partition function of chlorovinyl; one treats the two isomers separately and the other solves the anharmonic energy levels of the double well. We use these results to calculate the standard-state free energy and equilibrium constant of the reaction.},

doi = {10.1073/pnas.1920018117},

journal = {Proceedings of the National Academy of Sciences of the United States of America},

number = [11],

volume = [117],

place = {United States},

year = {2020},

month = {3}

}

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