Hydrogen atom abstraction from Piperazine by hydroxyl radical: a theoretical investigation

ABSTRACT

Dual level of quantum mechanical calculations have been carried out for hydrogen abstraction from Piperazine [HN(CH₂CH₂)₂NH] initiated by OH radical. Geometry optimisation and frequency calculations of all species involved in the titled reaction have been performed at M06-2X/6-31+G(d,p) level of theory. For the accuracy in the thermochemistry and kinetics data, single-point energy calculations have been further carried out at coupled cluster CCSD(T) method along with 6-311G(d,p) basis set. An energy profile diagram for the reaction has been plotted along with pre-reactive and post-reactive complexes at entrance and exit channels. Intrinsic reaction coordinates (IRCs) calculations have been performed for identification of real transition states that connect it via reactant to product. Our result shows that the H-atom abstraction takes place from the C–H position of Piperazine. The rate constant is calculated using canonical transition state theory (CTST) is found to be 2.86 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ which is in good agreement with the reported experimental rate constant (2.38 ± 0.28) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 298 K. We have also reported rate constant for the temperature range 300–500 K. Using group-balance isodesmic reaction, the standard enthalpies of formation for Piperazine and product radicals generated by hydrogen abstraction are reported. The branching ratios for both reaction channel (i.e. H-abstraction from –CH₂ and –NH position of Piperazine) are found to be 93% and 7%, respectively. The calculated atmospheric life time of Piperazine is found to be 0.97 hour.

KEYWORDS:

• Piperazine
• rate constant
• branching ratios
• M06-2X

Acknowledgments

NKG is thankful to University Grant Commission (UGC), New Delhi for providing Dr. D. S. Kothari Post- Doctoral Fellowship (Award letter no: F.4-2/2006(BSR)/CH/14-15/0217). Plaban J. Sarma is also thankful to Tezpur University for providing institutional fellowship.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the University Grant Commission (UGC), New Delhi [grant number F.4-2/2006(BSR)/CH/14-15/0217].