Rate constant dependence on the size of aldehydes in the NO3+ aldehydes reaction. An explanation via quantum chemical calculations and CTST

The reactions of NO3 with formaldehyde, acetaldehyde, propanal, n-butanal, and isobutanal have been modeled using accurate ab initio and hybrid DFT me thods with large basis sets. The results clearly indicate that the reaction is a simple aldehydic H atom abstraction; no adduct was found to support t he idea of a complex mechanism. Alternative hydrogen abstractions were mode led for the alpha carbon hydrogen atoms and for the C beta of n-butanal; th e differences in activation energies ruled out the possibility that competi tive abstraction could be responsible for the anomalous increase of the rat e constants with the size of aldehydes. The anomalous behavior was found to be a consequence of the preexponential factor increase, due to the enlarge ment of the internal rotation partition functions with the size of the alde hydes. The reaction rate constants, calculated using the conventional trans ition-state theory as applied to a proposed simple mechanism, reproduce rem arkably well the reported experimental results. Consideration of the intern al rotation partition functions is shown to be essential for the determinat ion of the preexponential parameters and thus for the correct calculation o f the rate constants. The tunneling correction was found negligible due to the features of the transition vector.