On the structure and dynamics of secondary n-alkyl cations

摘要

A variety of computational studies was undertaken to examine and establish the relative importanceof open versus closed structures for unbranched secondary n-alkyl cations. First, the PW91 level ofdensity functional theory was used to optimize over 20 minimum-energy structures of sec-pentyl,sec-hexyl, and sec-heptyl ions, demonstrating that closed structures are more stable than open oneson the potential energy surface (PES). Second, PW91 was used with a theoretical Andersenthermostat to perform a molecular dynamics simulation (150 ps) of C_9H_(19)~+at a typical catalytictemperature of 800 K, demonstrating that the structure preference is inverted on the free-energysurface. Third, both quantum (rigid-rotor/harmonic oscillator) and classical partition functions wereused to demonstrate that the simulated structure-opening at catalytic temperatures is due to thefloppiness of the open forms, which improves its free energy by both lowering its zero-pointvibrational energy and increasing its molecular entropy. The particular conformer of the preferredopen form (at 800 K) is dependent on length of alkyl ion, with pentyl ions preferring syn/antistructures but longer ions preferring open-clinal ones. These results, plus an additional set of PESoptimized structures from an alternative level of theory (MP2/ 6-31G(d,p)), are used to discuss thelikely nature of secondary n-alkyl ions.