^{1,a)}, Hue Minh Thi Nguyen

^{2}, Rehab M. I. Elsamra

^{3}, Minh Tho Nguyen

^{3}and Jozef Peeters

^{3}

### Abstract

The rate coefficient of the gas-phase reaction has been experimentally determined over the temperature range using a pulsed laser photolysis-chemiluminescence (PLP-CL) technique. Ethynyl radicals were generated by pulsed photolysis of in the presence of vapor and buffer gas at . The relative concentration of radicals was monitored as a function of time using a chemiluminescence method. The rate constant determinations for were , , and . The error in the only other measurement of this rate constant is also discussed. We have also characterized the reactiontheoretically using quantum chemical computations. The relevant portion of the potential energy surface of in its doublet electronic ground state has been investigated using density functional theory and molecular orbital computations at the unrestricted coupled-cluster level of theory that incorporates all single and double excitations plus perturbative corrections for the triple excitations, along with the basis set and using optimized geometries. Five isomers, six dissociation products, and sixteen transition structures were characterized. The results confirm that the hydrogen abstraction producing is the most facile reaction channel. For this channel, refined computations using and complete-active-space second-order perturbation theory/complete-active-space self-consistent-field theory (CASPT2/CASSCF) [B. O. Roos, Adv. Chem. Phys.69, 399 (1987)] using the contracted atomic natural orbitals basis set (ANO-L) [J. Almlöf and P. R. Taylor, J. Chem. Phys.86, 4070 (1987)] were performed, yielding zero-point energy-corrected potential energy barriers of and , respectively. Transition-state theoryrate constant calculations, based on the UCCSD(T) and CASPT2/CASSCF computations that also include H-atom tunneling and a hindered internal rotation, are in perfect agreement with the experimental values. Considering both our experimental and theoretical determinations, the rate constant can best be expressed, in modified Arrhenius form as for the range . Thus, at temperatures above , reaction of with is predicted to be one of the dominant reactions in hydrocarbon combustion.

The authors are indebted to the Flemish Fund for Scientific Research, *FWO-Vlaanderen* (postdoctoral fellowship, research project) and the KULeuven Research Council (GOA program, doctoral scholarship) for continuing financial support.

I. INTRODUCTION

II. EXPERIMENTAL SECTION

A. Apparatus and experimental methods

B. Experimental results

III. QUANTUM CHEMICAL CALCULATIONS

A. Reaction channels from

B. Processes on other portions of the potential energy surface

C. Refined calculations for the hydrogen abstraction

IV. TRANSITION STATE THEORY CALCULATIONS

V. DISCUSSION

VI. CONCLUSIONS

### Key Topics

- Hydrogen reactions
- 56.0
- Reaction rate constants
- 45.0
- Water vapor
- 16.0
- Photodissociation
- 12.0
- Chemical reactions
- 11.0

## Figures

Experimental time profile of —fitted to a single-exponential function—following dissociation of in the presence of , and at a total pressure of .

Experimental time profile of —fitted to a single-exponential function—following dissociation of in the presence of , and at a total pressure of .

Variation of pseudo-first-order decay rates with at and at a total pressure of .

Variation of pseudo-first-order decay rates with at and at a total pressure of .

Variation of pseudo-first-order decay rates with at and at a total pressure of .

Variation of pseudo-first-order decay rates with at and at a total pressure of .

Variation of pseudo-first-order decay rates with at and at a total pressure of .

Variation of pseudo-first-order decay rates with at and at a total pressure of .

Arrhenius plot of the experimental rate constants (circles) determined for the reaction in the present study. Also shown are our TST calculations [Eq. (9)] applied to the -optimized structures of , , and . The bottom two lines of the key refer to TST calculations using the barrier height calculated at the level with ZPE, the top two lines refer to TST calculations based on the barrier height. For each pair of lines in the key, the upper one refers to calculations that treat the HOHC torsional motion as a vibration , the lower one treats this motion as a hindered internal rotation ( and barrier to rotation of ).

Arrhenius plot of the experimental rate constants (circles) determined for the reaction in the present study. Also shown are our TST calculations [Eq. (9)] applied to the -optimized structures of , , and . The bottom two lines of the key refer to TST calculations using the barrier height calculated at the level with ZPE, the top two lines refer to TST calculations based on the barrier height. For each pair of lines in the key, the upper one refers to calculations that treat the HOHC torsional motion as a vibration , the lower one treats this motion as a hindered internal rotation ( and barrier to rotation of ).

Schematic potential energy profiles showing the different channels related to the reaction of ethynyl radical with water. Relative energies, in kJ/mol, were obtained using calculations. Selected optimized geometries are also displayed. Bond lengths are given in angstroms and bond angles in degrees.

Schematic potential energy profiles showing the different channels related to the reaction of ethynyl radical with water. Relative energies, in kJ/mol, were obtained using calculations. Selected optimized geometries are also displayed. Bond lengths are given in angstroms and bond angles in degrees.

Schematic potential energy profiles showing the interconversions between different isomers. Relative energies, in kJ/mol, were obtained using calculations. Selected optimized geometries are also displayed. Bond lengths are given in angstroms and bond angles in degrees.

Schematic potential energy profiles showing the interconversions between different isomers. Relative energies, in kJ/mol, were obtained using calculations. Selected optimized geometries are also displayed. Bond lengths are given in angstroms and bond angles in degrees.

Schematic potential energy profiles showing the interconversions between different isomers, in particular the reaction. Relative energies, in kJ/mol, were obtained using calculations. Selected optimized geometries are also displayed. Bond lengths are given in angstroms and bond angles in degrees.

Schematic potential energy profiles showing the interconversions between different isomers, in particular the reaction. Relative energies, in kJ/mol, were obtained using calculations. Selected optimized geometries are also displayed. Bond lengths are given in angstroms and bond angles in degrees.

Geometry of the transition structure for hydrogen abstraction of the reaction using different levels of theory. Entries are (upper), , , and (lower). Bond lengths are given in angstroms and bond angles in degrees.

Geometry of the transition structure for hydrogen abstraction of the reaction using different levels of theory. Entries are (upper), , , and (lower). Bond lengths are given in angstroms and bond angles in degrees.

## Tables

Relative energies (kJ/mol) of all stationary points considered at two levels of theory.

Relative energies (kJ/mol) of all stationary points considered at two levels of theory.

Calculated total energy (hartree), zero-point energy (kJ/mol), and relative energy (kJ/mol) of the , , and TS abstraction .

Calculated total energy (hartree), zero-point energy (kJ/mol), and relative energy (kJ/mol) of the , , and TS abstraction .

Calculated vibrational frequencies for the critical species at the level of theory.

Calculated vibrational frequencies for the critical species at the level of theory.

Transition-state-theory calculations of the rate constant for the reaction based on or CASSPT2/CASSCF single-point energies and critical structures computed at the level.

Transition-state-theory calculations of the rate constant for the reaction based on or CASSPT2/CASSCF single-point energies and critical structures computed at the level.

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