engleski

Photochemistry of formamide and its protonated forms – An ab initio surface hopping dynamics simulations

Photoinduced fragmentation of charged small polypeptides has been intensively investigated in recent years due to their importance in ‘ soft’ ionization methods such as MALDI and ESI. These studies revealed that photofragmentation patterns depend strongly on the peptide composition, on electronic state excited by the photon (excitation of aromatic amino acid residues at 266 nm or amide chromophore of the peptide backbone at 193 or 157 nm), on the mass analyzer employed and its time regime. In this study, the protonated formamide will be used as a model molecule for analysis of photofragmentation of protonated peptide group which does not incorporate an aromatic chromophore and in which excitation to low lying excited states takes place primarily at the C=O double bond of the amide moiety. The calculations were carried out using the multireference configuration interaction with singles and doubles (MR-CISD) method while dynamics were simulated by employing the mixed quantum-classical direct trajectory method with surface hopping based on multi-configurational self-consistent wave functions. The photodeactivation from the first excited singlet state in O-protonated formamide resembled those found for the second valence excited state of the parent molecule. Two photodissociation processes were found: the C-N (major) and C-O (minor) dissociations with very short lifetimes (33 fs). Similarly, the major process for photodecomposition in the first excited state of N-protonated formamide resembles that for the parent formamide, involving C-N dissociation with a lifetime around 390 fs. However, 55% of trajectories remained undissociated and undeactivated until 1000 fs, indicating existence of other deactivation processes on a longer time scale.

photofragmentation; protonated formamide; excited states; dynamics

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