Effect of the cyano group in the HAVA mechanism for PAHs formation: a reaction force analysis for the reaction between benzonitrile (C6H5CN) and vinylacetylene

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ABSTRACT Due to the importance and fascination that arises from processes involving Polycyclic Aromatic Hydrocarbon (PAHs) in the interstellar medium (ISM), in this work quantum calculations were used to study the HAVA mechanism (Hydrogen Abstraction Vinylacetylene Addition) involved in the PAHs formation from the benzonitrile (${\rm C_6H_5CN}$), molecule recently discovered in the ISM, and vinylacetylene (${\rm C_4H_4}$). The reaction force analysis was used to obtain information about the mechanisms in terms of structural and electronic rearrangements presents in the formation of naphthalene, 1-naphthonitrile, 2-naphthonitrile, and 3-ethynylisoquinoline. Due to the presence of the cyano group on the aromatic ring for the formation of 1-naphthonitrile and 2-naphthonitrile, these reactions: (a) follow the HAVA mechanism, showing the same structural nature of the submerged energy barriers and (b) result consistent with the fact that both molecules have recently been found in TMC-1, where benzonitrile and vinylacetylene are also present. The mesomeric and inductive effect of the cyano group on the aromatic ring contributes to lowering the energy barrier when the radical is in the ortho position on the aromatic ring with respect to the cyano group. This decrease contributes greatly to the work of electronic reordering. Although the formation of 3-ethynylisoquinoline (N-Heterocycle) does not follow the HAVA mechanism and involves positive barriers at the conditions of TMC-1, important astrochemical and astrobiological implications can be derived from this result, since N-containing heterocycles, have been detected in carbonaceous chondrites would be providing interesting connection between chemistry in space and the origin of life on Earth.