Abstract
Factors affecting the kinetic isotope effects (KIEs) of the gas-phase SN2 reactions and their temperature dependence have been analyzed using the ion-molecule collision theory and the transition state theory (TST). The quantum-mechanical tunneling effects were also considered using the canonical variational theory with small curvature tunneling (CVT/SCT). We have benchmarked a few ab initio and density functional theory (DFT) methods for their performance in predicting the deuterium KIEs against eleven experimental values. The results showed that the MP2/aug-cc-pVDZ method gave the most accurate prediction overall. The slight inverse deuterium KIEs usually observed for the gas-phase SN2 reactions at room temperature were due to the balance of the normal rotational contribution and the significant inverse vibrational contribution. Since the vibrational contribution is a sensitive function of temperature while the rotation contribution is temperature independent, the KIEs are thus also temperature dependent. For SN2 reactions with appreciable barrier heights, the tunneling effects were predicted to contribute significantly both to the rate constants and to the carbon-13, and carbon-14 KIEs, which suggested important carbon atom tunneling at and below room temperature.
Highlights
If the reaction is dominated by tunneling effects, the predicted kinetic isotope effects (KIEs) by canonical variational theory with small curvature tunneling (CVT/small-curvature tunneling (SCT)) theory could be significantly smaller since the tunneling effects are less sensitive to the barrier heights
An important point here is that for very fast SN2 reactions, the observed KIEs do not reflect the properties of the transition states involved or the reaction paths on which the systems follow, since in such cases the bottlenecks are located very early in the entrance channels
We have shown in this study that how the kinetic isotope effects (KIEs) of the gas-phase SN2 reactions can be realistically modeled
Summary
Bimolecular nucleophilic substitution (SN2) reactions are ubiquitous in organic chemistry and they have been extensively studied in the last two decades [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. It is noted that in the (low-pressure) gas-phase reaction, which is our focus in the current study, the barrier heights defined above determine the overall rate constants if the transition state is the sole bottleneck. The rates of high-barrier (approximately higher than 3 kcal/mol or with rate constants lower than 10−12 cm3·molecule−1·s−1) gas-phase SN2 reactions are too slow to be measured reliably It is expected, the quantum mechanical tunneling effects may play an important role in determining the rate constants at lower temperature since the widths of the barriers of SN2 reactions tend to be narrow due to the deep energy wells of the ion-dipole complexes before and after the central energy barriers along the reaction paths. Tunneling contribution to the deuterium, 13C-, and 14C-KIEs of the SN2 reactions will be modeled for the reaction of CN− + CH3OCl in the gas phase
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