engleski

Theoretical study of formamide photodissociation using an ab initio on-the-fly surface-hopping approach

In recent years we have been concerned with exploring the effect of photoexcitation on basicity and metal cation affinity of formamide using MR-CISD computational methods. In this work we turned to characterization of its photodeactivation paths. This interest was not surprising, considering that formamide presents the simplest amide containing a prototype HNC=O peptide linkage. Simulations of the photodynamics of formamide were carried out by employing the semi-classical direct trajectory with surface hopping method. State averaging CASSCF(10, 8)/6-31G(d) over three singlet states have been applied for calculating excited state energies, gradients and nonadiabatic transition probabilities. The dynamics calculations from the first excited singlet state nO-pi* showed, in agreement with experiments and calculated potential energy curves, that the major process for photodecomposition is C-N dissociation (75% of trajectories) with lifetime around 420 fs. In 16% of trajectories, after excitation formamide moves to the minimum of this state by strong CO and CN stretching vibrations and is trapped there for a long time. In the second excited singlet state pi-pi* the C-N dissociation process is much faster than in nO-pi* state (less than 100 fs). Surprisingly, in 13% of trajectories C-O bond cleavage is noticed, too.

formamide photodissociation; dynamic symulations; on-the-fly surface-hopping approach

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