Elucidating the Gas Phase Thermochemistry and the H‑Atom Abstraction Reactions of Triethyl Phosphite

Triethyl phosphite (TEPI), an organophosphorus compound, offers
potential applications in flame-retardant materials, organic synthesis,
homogeneous catalysis, agrochemicals, and pharmaceutical intermediate
production. However, TEPI has received little attention compared to
more extensively studied phosphates and phosphonates, with its thermal
decomposition and chemical reactivity, particularly under combustion
and high-temperature conditions, remaining largely unexplored. This
study addresses that gap by analyzing the thermochemical properties
and reaction kinetics of TEPI to clarify its combustion behavior and
support the accurate modeling of its reaction pathways. In this study,
the M06-2X/6-311++G­(d,p) level of theory was used for geometry optimization,
vibrational frequency calculations, and dihedral scans. The single-point
energies (SPEs) of TEPI and its five radicals were calculated at the
MP2/cc-pVXZ (X = D, T, or Q) and CCSD­(T)/cc-pVXZ (X = D, T) levels
of theory. We applied complete basis set (CBS) extrapolation to these
energies to improve the accuracy and approximate the basis set limit.
The bond dissociation energies (BDEs) of TEPI were calculated using
single-point energies (SPEs) corrected with zero-point energies (ZPEs),
as well as total energies at zero Kelvin (TEZK) obtained from an average
of composite methods, including G3B3, G2, CBS-QB3, and G3B3. The thermochemical
properties of TEPI and the rate constants for hydrogen atom abstraction
(HAA) reactions with molecular oxygen, O2, and various
radicals: •H (hydrogen), •OH (hydroxyl), •CH3 (methyl), CH3O• (methoxy), and HO2• (hydroperoxyl)
were calculated using the Master Equation System Solver (MESS). The
computed rate constants were further correlated with the corresponding
energy barrier heights to elucidate their relationship. The results
show that HAA from the secondary hydrogen site is more favorable than
from the primary site, with the highest reaction rates observed for •H and •OH abstractions. With a kinetic
mechanism still developing, additional reaction pathways such as hydrogen
atom transfer and scission of various single bonds were estimated
using rate constants derived by analogy. Furthermore, sensitivity
analysis of the ignition delay time (IDT) confirmed the significance
of HAA reactions, which control the initial consumption of TEPI. However,
further refinement of the kinetic mechanism and experimental validation
are necessary to fully confirm these reaction pathways and establish
a robust model for TEPI’s behavior under practical conditions.
These findings offer fundamental insights and quantitative kinetic
parameters into TEPI’s reactivity, which serve as inputs for
constructing detailed chemical kinetic models, thereby offering a
quantitative basis for predicting its combustion behavior and optimizing
its performance in flame-retardant applications.