Ab initio molecular dynamics study of the reactions of allene cation induced by intense 7 micron laser pulses

ABSTRACT

The isomerisation and fragmentation of allene cation (H₂C=C=CH₂⁺) by short, intense laser pulses were simulated by Born-Oppenheimer molecular dynamics (BOMD) on the ground state potential energy surface using the B3LYP/6-31?+?G(d,p) level of theory and a 10 cycle 7?µm cosine squared pulse with a maximum field strength of 0.07?au. Laser fields polarised along the C=C=C axis deposits an average of 150?kcal/mol in the molecule, compared to only 25 and 51?kcal/mol for perpendicular polarisations. Approximately 90% of the trajectories with the field aligned with the C=C=C axis underwent one or more structural rearrangement steps to form H₂C=CH–CH⁺ (15%), H₃CCCH⁺ (4%), cyclopropene cation (6%), and allene cation with rearranged hydrogens and carbons (47%). In addition, a variety of fragments including H₂CCCH⁺?+?H (10%), c-C₃H₃⁺?+?H (7%), and HCCCH⁺?+?H₂ (2%) trajectories were produced after isomerisation. With the same amount of thermal energy, field-free BOMD shows good agreements with the BOMD with the field. However, RRKM calculations favour isomerisation to propyne cation and dissociation to HCCCH⁺?+?H₂. This suggests that for molecules in intense laser fields the energy in the intermediate isomers is not distributed statistically.