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