Ab initio molecular orbital study of the mechanism of photodissociation of formamide

Abstract

In the present Letter, the reaction pathways for H₂NCHO decomposition in the ground and excited states are traced with the use of the MP2 and CASSCF energy gradient techniques. The intersection point of the S₁ and T₁ surfaces are determined by means of the state-average CASSCF optimization. The n→π^∗ excitation leads to the first excited singlet state, where a radical pair of NH₂ and CHO is formed. Then a hydrogen atom transfers from the CHO radical to the NH₂ radical, producing CO and NH₃ in the ground state. This is in good agreement with recent experimental findings. The calculated results also show that H₂ and HNCO can be formed along a two-step T₁ pathway. A recent experimental study on the photochemistry of the H₂NCHO suggested that H₂ elimination from H₂NCHO takes place on the T₁ surface.

Introduction

Formamide contains a prototype HN–CO peptide linkage. As an especially important model in the study of biological macromolecules, numerous experimental and theoretical investigations have been devoted to its ground-state structure and barrier to rotation around the C–N bond, which has been reviewed in previous studies 1, 2, 3, 4, 5, 6. Either the planar or non-planar structure of formamide has been reported in these studies. There are also numerous experimental 7, 8, 9, 10and theoretical 11, 12, 13, 14investigations on the electronic excitation of formamide, which have been reviewed in the previous studies 1, 5.

The pyrolysis and photolysis of formamide have received less attention from both experiment and theory. Early in 1974, the pyrolysis of formamide vapor was investigated by Back and Boden [15]. CO and NH₃ were observed to be the major products, while H₂ is the minor one. Rates of production of CO and H₂ were approximately first order at pressures ranging from 6 to 33 Torr at temperatures from 262 to 500°C. Kakumoto et al. [16]have studied the thermal decomposition of formamide diluted in Ar from shock waves over the temperature ranging from 1690 to 2180 K. The rate was monitored by means of the IR emission of the carbon monoxide produced. As part of their investigations, an ab initio study on decomposition of NH₂COH into CO+NH₃ was performed at the HF level with the 3-21G, 4-31G and 6-31G basis sets. The rate constants of this reaction in low- and high-pressure limits were determined on the basis of the experimental data and the results from ab initio calculations. Boden and Back 15, 17also investigated the photolysis of formamide. Products detected were CO, H₂ and NH₃ with quantum yields at 150°C of 0.8, 0.6 and 0.2, respectively. It was deduced that there were three major primary processes, forming the products of NH₂+CO+H, HNCHO+H and NH₃+CO, respectively. Another 193-nm-induced photolysis of formamide in matrices have been recently reported by Lundell et al. [5]. They suggested that in solid Ar, the n–π^∗ excitation leads to the first exited singlet state, where the radical pair NH₂⋯CHO is formed. The hot CHO radical transfers hydrogen to the NH₂ radical, forming CO+NH₃. In Xe, the n–π^∗ excitation occurs as well with 193 nm photolysis. However, when the radical pair is formed in the excited state, a strong external heavy-atom effect due to Xe is present, which induces intersystem crossing to a triplet surface, yielding HNCO+H₂. In order to test the validity of the mechanism proposed above, Lundell and co-workers have carried out ab initio studies on the potential energy profiles of NH₂CHO dissociating into CHO+NH₂ in the low-lying electronic states. Since only the C–N bond distance was scanned, with all other structural parameters kept at ground-state equilibrium values, their potential curves are only qualitatively reliable.

The characterization of the mechanism of a photochemical reaction requires detailed knowledge of ground and excited-state reaction pathways and potential crossing regions where the system decays from one state to another. In the present Letter, the possible dissociation pathways of formamide

$$\text{H}{}_2\text{NCHO}\text{→}\text{NH}{}_3\text{+CO}\text{,}$$

$$\text{ →}\text{HNCO+H}{}_2\text{,}$$

$$\text{ →}\text{NH}{}_2\text{+CHO}\text{,}$$

$$\text{ →}\text{HNCHO+H}\text{,}$$

$$\text{ →}\text{H}{}_2\text{NCO+H}$$

in the ground, first excited singlet, and lowest triplet states, have been investigated with more advanced quantum chemical techniques. The crossing points of the different potential energy surfaces were determined using the state-averaged CASSCF method. The mechanism leading to different photoproducts was characterized on the basis of the potential energy profiles and their crossing points.