Potential energy surface and unimolecular dynamics of stretched n-butane

Abstract

The potential energy surface (PES) and unimolecular reaction dynamics of stretched n-butane are investigated, as a model for a stretched "normal" alkane or straight chain polymer. The nature of the PES for stretched n-butane depends on the extent of stretching. If it is less than that required to reach the inflection points in the C[Single Bond]C stretch potentials and the C[Single Bond]C torsions are considered free rotors, there is only one potential energy minimum, with each bond elongated. However, for stretching past these inflection points, the PES has three minima and each has one bond longer than the other two, i.e., C[Single Bond]C[Single Bond]C[Single Bond]C, C[Single Bond]C[Single Bond]C[Single Bond]C, and C[Single Bond]C[Single Bond]C[Single Bond]C. There are three transition states (TSs) connecting these minima. A linear alkane, consisting of n carbon atoms and stretched past its C[Single Bond]C inflection points, has (n-1) minima and (n-1)(n-2)2 TSs connecting them. For stretching less than that required to reach the C[Single Bond]C inflection points, the only unimolecular pathways are dissociations to form the C+C[Single Bond]C[Single Bond]C, C[Single Bond]C+C[Single Bond]C, and C[Single Bond]C[Single Bond]C+C products. However, with stretching past the C[Single Bond]C inflection points, isomerizations between the three potential energy minima may also occur. The relative importance of isomerization versus dissociation depends on the relative size of their barriers. For slight stretching past the C[Single Bond]C inflection points, the isomerization barriers are much lower than those for dissociation and relaxation between the minima is much faster than dissociation. Thus, the molecule samples these minima randomly during its dissociation, with a density of states that comprises the complete PES. With extensive stretching past the inflection points, isomerizations between the potential energy minima do not occur and only dissociation for the excited minima occurs, e.g., C[Single Bond]C[Single Bond]C[Single Bond]C-->C+C[Single Bond]C[Single Bond]C. For intermediate stretching past the C[Single Bond]C inflection points, the rates for the isomerization and dissociation pathways are competitive and both must be considered in modeling the dissociation kinetics. Microcanonical chemical dynamics simulations are performed to study the unimolecular kinetics of n-butane in three stretched configurations: Stretched less than the C[Single Bond]C inflection point; stretched slightly beyond the C[Single Bond]C inflection point; and stretched significantly beyond the C[Single Bond]C inflection point. The resulting unimolecular dynamics were found to be in excellent agreement with RRKM theory. Frequency nu factors, determined by fitting the trajectory unimolecular rate constants to the classical harmonic RRKM rate constant expression, depend upon the extent of stretching and vary from 1.0 x 10(12)-8.4 x 10(16) s(-1). For a molecule with a large number of vibrational degrees of freedom and high excess energy, it is shown that the classical harmonic RRKM and classical harmonic transition state theory rate constants, k(E) and k(T), are equivalent.