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.

GRAPHICAL ABSTRACT

KEYWORDS:

• BOMD
• AIMD
• RRKM
• strong field chemistry
• nonstatistical dynamics

Acknowledgements

We thank the Wayne State University computing grid for the computer time.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by a grant from National Science Foundation (CHE1464450).