Highly accurate theoretical
values of formation enthalpies and
bond energies are crucial for reliable predictions of performance
and detonation-related phenomena of energetic materials (EM). However,
high-level ab initio calculations even for medium-sized important
EMs still remain a demanding challenge. In the present work, we studied
in detail the gas-phase thermochemistry of novel high-energy polynitro
derivatives of 5/6/5 structural frameworks comprised of fused 1,2,3,4,-tetrazine
and two 1,2,4-triazole or pyrazole rings. To this end, we proposed
and benchmarked a “bottom-up” approach. First, highly
accurate multilevel procedures W2-F12 and/or W1-F12 in conjunction
with the atomization energy approach were utilized for smaller species.
In turn, for medium-sized species (up to 24 non-H atoms), these values
were complemented with the enthalpies of isodesmic reactions calculated
using the recently proposed domain-based local pair natural orbital
(DLPNO) modifications of coupled cluster techniques. The benchmarks
on a number of atomization energies and enthalpies of isodesmic reactions
reveal that the DLPNO-CCSD(T)/aVQZ approach does not deteriorate the
quality of the W1-F12 and W2-F12 procedures and exhibits overall accuracy
close to “chemical” (∼1 kcal mol–1). We obtained a set of accurate and mutually consistent gas-phase
formation enthalpies for 12 energetic heterocyclic species. Among
them, the gas-phase formation enthalpy of 1,2,9,10-tetranitrodipyrazolo[1,5-d:5′,1′-f][1,2,3,4]tetrazine,
a novel promising EM, turned out to be Δf
H
gas
0 = 214.5 kcal mol–1, which is ∼12 kcal mol–1 higher than the best theoretical estimates available
in the literature. The formation enthalpy of another novel EM, a fused
tricyclic 1,2,3,4-tetrazine with two nitro-1,2,4-triazole moieties,
was predicted to be Δf
H
gas
0 = 213.5 kcal
mol–1, which is also ∼4 kcal mol–1 higher than the reported value. Apart from this, we considered the
thermodynamics of radical reactions (viz., C–NO2 bond scission) and the thermochemistry of the corresponding radicals.
The difference between DLPNO-CCSD(T)/aVQZ and CCSD(T)-F12/VTZ-F12
benchmark values did not exceed 1 kcal mol–1. In
a more general sense, the use of DLPNO-CCSD(T) in conjunction with
the bottom-up approach is promising for quantitative thermochemical
calculations of energetic materials composed of species up to several
dozens of CHNO atoms.