Ultrafast photodissociation dynamics of 2-ethylpyrrole: adding insight to experiment with ab initio multiple cloning

The ultrafast photodissociation dynamics of 2-ethylpyrrole (2-EP) is simulated in a fully quantum manner on the S1 and S2 πσ* states by the ab initio multiple cloning (AIMC) method. AIMC treats electrons with accurate electronic structure methods "on the fly", and nuclear dynamics with wavefunction propagation via a basis set of Ehrenfest trajectory guided Gaussian wavepackets. Total kinetic energy release (TKER) spectra are produced, as well as velocity map images and N-H dissociation times. These are compared to results from time-resolved velocity map imaging studies, and the AIMC method is able to provide quantitative reproduction of experimental data, including dissociation times of 50-80 fs. Novel insight into the dissociation mechanism is then obtained, with the experimentally obtained time constant shown to be composed of two components. Firstly, there is a contribution in <50 fs from 2-EP molecules that have sufficient energy in the N-H stretch coordinate to dissociate almost immediately over the barrier, and this is followed by a second slower contribution from 2-EP molecules that must sample the potential energy surface before finding a way around the barrier to dissociate. This two component mechanism is not observed experimentally due to the temporal widths of the laser pulses obscuring the dynamics in the <50 fs window, and is shown for the first time via theory. Calculations are also performed on selectively deuterated 2-EP, demonstrating that AIMC is able to produce a kinetic isotope effect for the dissociation time constant, and correctly predict a shift to lower energy in the TKER spectrum. The S2 πσ* state is also shown to be unstable with respect to the S1 πσ* state, with the N-H dissociation proceeding along S1 when initially excited to S2. This work demonstrates that the combination of state of the art theory and experiments can provide unprecedented novel insight into the N-H dissociation mechanism, with the tantalising prospect of providing insight into more general heteroatom hydride bond dissociation.