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Abstract
Indole, being a biologically relevant and abundant chromophore, is a prime molecule of interest both in experimental and computational research. In previous works, the nature of the dissociative states for indole and indole derivatives were predicted. In this work, the N-H photodissociation dynamics of indole have been studied using nonadiabatic quantum dynamics. First, the important vibrational modes responsible for the photodissociation were detected using vibrational analysis. Potential energy cuts (PECs) along important vibrational modes have been calculated using the complete active space self-consistent field (CASPT2) method with (10,9) active space. The multi-mode multi-state model vibronic Hamiltonian is constructed by the parameters obtained from the fitting of ab initio PECs, including Morse and harmonic functions, and vibronic couplings. Nonadiabatic quantum dynamics is performed using the multi-configuration time-dependent Hartree (MCTDH) method, considering four electronic states and four vibrational modes. N-H stretching mode, Q42 turns out to be the most important of all modes, showing an anharmonic, dissociative π-π* state at the third excited state. The out-of-plane C-N-H bending i.e. mode Q3 is found to facilitate the dissociation process. Additionally, effects of inclusion of Q8 and Q17, which are mixed motion modes with contribution from N-H displacement, were also assessed. Two timescales obtained from the population dynamics, 37 fs and 383 fs, are attributed to internal conversion from the second excited La state to the π-π* states and photodissociation at the π-π*, respectively, and are in good agreement with timescales obtained from femtosecond time-resolved ionization experiments. After 500 fs of dynamic propagation, around 55% reaction probability for N-H fission in indole is recorded. This study of photofission of indole is crucial for understanding photodynamics of similar large molecules with high accuracy.
Supplementary materials
Title
Supporting Information
Description
Optimized geometries for ground and relevant excited states, frequencies for vibrational modes, coupling parameters for the model Hamiltonian, potential energy surface along mode Q$_{42}$ and Q$_3$, details of the parameters used in MCTDH dynamics, wavefunction density plots at L$_a$ and L$_b$ excited states and ground states, properties for convergence check and effect of CAP position are reported in the supporting information.
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