Quantum chemical and conventional transition-state theory calculations of rate constants for the NO3 plus alkane reaction

The reaction paths of NO3 with methane, ethane, propane, and isobutane have been modeled using accurate ab initio (MP2) and hybrid DFT (BHandHLYP) methods with large basis sets (6-311g(d,p)). The energies of the optimized geometries were recalculated with the CCSD(T) method. Rate constants were obtained with the conventional transition-state theory (CTST). For propane and isobutane, in addition to the respective secondary and tertiary H-abstraction channels, abstraction of primary hydrogen atoms was also considered. Taking into account the internal rotations in the partition functions is shown to be essential for the determination of the preexponential parameters. This correction has a strong influence on the transition state partition function of the primary channel of isobutane, producing a noticeable increase in the preexponential factor and an almost perfect agreement with the experimental values. The calculated rate constants for tertiary and primary H-abstractions are 2.28 x 10(4) and 3.41 x 10(4) L mol(-1) s(-1), respectively, and the overall rate coefficient is 5.69 x 10(4) L mol(-1) s(-1). In contrast, the rate constant for the primary H-abstraction in propane is about 1 order of magnitude lower than that of the secondary channel, a 4.71 x 10(3) value versus 4.47 x 10(4) L mol(-1) s(-1). The calculated rate constants for methane (2.52 L mol(-1) s(-1)) and ethane (4.94 x 10(3) L mol(-1) s(-1)) reproduce remarkably well the experimental results. The tunnel effect is shown to be a very important factor for methane and ethane, and especially for the primary H-abstractions in propane and isobutane, the tunneling factor is about 5 times the one for the tertiary abstraction.