Dimerisation of nitrile oxides: a quantum-chemical study

Abstract

The [3 + 2] and [3 + 3] cyclodimerisation processes of small nitrile oxides, XCNO (X = F, Cl, Br, CN, CH(3)) are investigated by ab initio coupled cluster theory at the CCSD, CCSD(T) and MR-AQCC levels for the first time. The favoured dimerisation process is a multi-step reaction to furoxans (1,2,5-oxadiazole-2-oxides) involving dinitrosoalkene-like intermediates with diradical character. The rate determining step for all but the F-species is the first, corresponding to the C-C bond formation. The kinetic energy barrier depends on the nature of the substituent X, generally increasing with decreasing electronegativity and increasing pi-donor ability of the substituent: F (DeltaG(298) = 0 kJ mol(-1)) < Cl (72) < Br (90) < CH(3) (104) < CN (114) (MR-AQCC(2,2)//UB3LYP/cc-pVTZ). Following initial C-C bond formation, three possible dinitrosoethylene diradical pathways are explored. Two of them are new, and one of them is a low-energy three-step path with implications for cycloreversion, tautomerism and detection of dinitrosoethylene intermediates. Alternative one-step, concerted [3 + 2] and [3 + 3] cyclodimerisation processes leading to 1,2,4-oxadiazole-4-oxides and 1,4,2,5-dioxadiazines have kinetic energy barriers around 100-240 kJ mol(-1) (CCSD//B3LYP), some 1.6 to 2.5 times higher than those leading to furoxans, supporting the experimental observations of furoxan formation as nitrile oxide loss channels during storage, trapping/re-vaporisation and reactions of nitrile oxides. Potential polymerisation initiation processes for NCCNO, involving the 1,2-dipolar NC substituent are also explored.

Theoretical studies of furoxan-based energetic nitrogen-rich compounds

“…This gradient-corrected hybrid functional is well known for the proper reproduction of molecular geometries and vibrational frequencies [34]. A comparison with experimental data for furoxan supports these findings [35]. Therefore, in this work, zero-point vibrational energies were calculated at this level.…”

Structure, Stability, and Cycloaddition Reactions of Nitrile Selenides

“…The (U)B3LYP method includes measurements of nondynamical and dynamical electron correlation, and it has been shown to provide a good description of geometries for singlet diradical species in cyclization reactions. [8][9][10][11] Our calculations have indicated that a small (2,2) active space is sufficient to describe the two basic configurations in MR-AQCC calculations. It is important to note that the MR-AQCC(2,2)//UB3LYP method provides a reliable and computationally feasible route to calculate the energetics of nitrile oxide dimerisation, as reported.…”

“…It is important to note that the MR-AQCC(2,2)//UB3LYP method provides a reliable and computationally feasible route to calculate the energetics of nitrile oxide dimerisation, as reported. [9] The previous work also included tests involving a larger active space and various basis sets. Therefore, MR-AQCC (2,2)//(U)B3LYP/cc-pVTZ calculations were performed for investigating bimolecular reactions; energies of closed-shell singlet reacting species, namely, nitrile selenides, nitriles, ethenes, and ethynes, were calculated at the SR-AQCC// B3LYP/cc-pVTZ level.…”

Reaction Mechanism of 3,4‐Dinitrofuroxan Formation from Glyoxime: Dehydrogenation and Cyclization of Oxime

“…So it is necessary to remove the spin contamination. Previous investigations indicated that using (U)DFT with the hybrid B3LYP functional method can provide a good description of the geometry of cyclization reactions, in particular those that involve singlet diradical species . The CASSCF single‐point energy calculations based on (U)B3LYP‐optimized geometries have been performed to investigate the dimerization reaction mechanism of nitrile oxides .…”