Comprehensive exploration of the C3H8N radicals potential energy surface

Abstract

C3 amines (C3H9N) are the simplest aliphatic amine subgroup encompassing primary, secondary, and tertiary isomers, playing a key role in understanding nitrogen migration during biomass thermal conversion. Despite extensive computational studies on the H abstraction rates of C3 amines, a systematic investigation of C3H8N radical reactions remains missing. The lack of data undermines the theoretical foundation for developing C3 amines kinetic sub-model. In this work, a comprehensive high-level quantum chemical study is performed on 12 C3H8N radicals derived from n-propylamine (NPA), iso-propylamine (IPA), N-methyl ethylamine (MEA), and trimethylamine (TMA), systematically exploring β-scission, intramolecular H-shift, and cyclization reaction channels. Geometry optimizations and frequency calculations are carried out at the revDSD-PBEP86-D3BJ/cc-pVTZ level. Single-point energies are refined at the CCSD(T)-F12b level with complete basis set extrapolation, core-valence correlation, and relativistic corrections. Pressure- and temperature-dependent rate constants are obtained via RRKM/master equation simulations using the MESS code. Results show that β-scission of C-C and C-N bonds dominate the consumption of C3H8N radicals. The 1,4 H shift is also another important channel for the straight-chain radicals with a primary radical center. Based on reactant structure, 12 key β-scission reactions are classified into 3 groups and analyzed individually: NP (straight-chain, primary C/N radical center), NS (straight-chain, secondary C/N radical center), and BP (branched-chain, primary C/N radical center). For NP radicals, the rate constants of primary-amine-derived radicals are closer to each other than those from secondary amines. An opposite trend is observed for NS radicals. For BP radicals, a central tertiary nitrogen atom leads to lower rate constants than a central tertiary carbon. It is found that the rate constants of the reaction that produce an alkene and a nitrogen centered radical are the fastest among the reactions in every group. The calculated rate constants are used to update a comprehensive C3 amine kinetic model, significantly improving predictions of key intermediates and products in NPA, IPA, and MEA pyrolysis. This work provides a systematic and reliable kinetic database for a C3H8N radical sub-model and lays a foundation for understanding nitrogen-migration mechanisms in biomass thermal conversion.