Theoretical calculations of the thermal decomposition kinetics of several tert-nitroalkanes in the gas phase
Introduction
A thorough survey and systematic description on the gas-phase thermal decomposition of aliphatic nitro compounds were reported by Nazin and Manelis [1]. This investigation described the pyrolysis kinetics of several types of nitro compounds such as mononitroalkanes, polynitroalkanes, unsaturated nitroalkanes, aromatic nitro compounds, halonitroalkanes, and other nitro molecules with different substituents. The experimental results of this work suggested [1] two principal mechanisms for the decomposition in the gas phase:
- 1.
The molecular mechanism proceeds through a five-membered cyclic transition state to the give the corresponding olefin and HNO2 (reaction (1)).
The molecular decomposition of the nitroalkanes requires the presence of a C–H bond adjacent to the C–NO2 group. These reactions are known to take place at moderate temperatures (up to 300 °C).
- 2.
Owing to the reduced strength of the C–N bond of substrates, with a large number of halo and nitro groups, the decomposition reaction of this type of nitro compounds proceeds via a free radical mechanism (reaction (2)).
At high temperatures, reaction (2) is dominant due to a higher pre-exponential factor in the expression of its rate coefficient.
The results of log A and Ea of the gas-phase pyrolysis of three tertiary nitroalkanes reported by Nazin and Manelis [1] are given in Table 1. From the experimental values of log A and Ea, we have calculated the rate coefficients and the thermodynamic parameters shown in Table 1. From these informations, the present work aimed at examining the theoretical studies of the elimination kinetics of 2-methyl-2-nitropropane, 2-methyl-2-nitrobutane, and 2,3-dimethyl-2-nitrobutane at MP2 and DFT levels of theory. The purpose is to estimate the kinetic and thermodynamic parameters, the characterization of the potential energy surface (PES), and to consider or support the nature of the molecular mechanism of these decomposition reactions.
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Computational methods and models
Electronic structure calculations were carried out by employing Møller–Plesset perturbation MP2/6-31G(d), and DFT B3LYP/6-31G(d), and B3PW91/6-31(d) levels as implemented in Gaussian 98W [2]. The Berny analytical gradient optimization routines were used. The requested convergence on the density matrix was 10−9 atomic units; the threshold value for maximum displacement was 0.0018 Å, and that for the maximum force was 0.00045 Hartree/Bohr. The nature of stationary points was established by
Kinetic and thermodynamic parameters
The kinetic and thermodynamic parameters for the gas-phase elimination of nitroalkanes were calculated using MP2/6-31G(d), and DFT B3LYP/6-31G(d), B3PW91/6-31G(d). Results are described in Table 1. Comparison with experimental values shows better agreement at B3PW91/6-31G(d) level of theory. The reaction barrier decreases as the nitroalkanes is more ramified. Log A has been used to suggest the type of TS according to Benson [6]. Values of log A between 13.63 and 13.30 insinuate a five-membered
Conclusions
The elimination of nitrous acid from tertiary nitroalkanes has been studied at various levels of theory. The most satisfactory results were obtained with B3PW91/6-31G(d). The mechanism for tertiary nitroalkanes is concerted and non-synchronic. The synchronicity parameter Sy of these nitro compounds was found to be approximately 0.89. The TS structure is a five-membered cyclic structure. The determining factor for the reaction is the breaking of C2–N3 bond.
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