A theoretical study of the photodissociation mechanism of cyanoacetylene in its lowest singlet and triplet excited states

Cyanoacetylene (H5-C4≡C3-C2≡N1) is a minor constituent of the atmosphere of Titan, and its photochemistry plays an important role in the formation of the haze surrounding the satellite. In this paper, the complete active space self-consistent field (CASSCF) and multiconfigurational second-order perturbation (CASPT2) approaches are employed to investigate the photochemical processes for cyanoacetylene in its first singlet and triplet excited states with the cc-pVTZ basis set. Fissions of the C4-H5 and C2-C3 bonds in S₁ yield H(²S) + CCCN(A ²Π) and HCC(A ²Π) + CN(X ²Σ⁺), respectively. In T₁, the corresponding dissociation products are H(²S) + CCCN(X ²Σ⁺) and HCC(X ²Σ) + CN(X ²Σ⁺). At the CASPT2(14,13)//CASSCF(14,13) + ZPE level, the barriers for the adiabatic dissociation of the C4-H5 and C2-C3 bonds are 6.11 and 6.94 eVin S₁ and 5.71 and 6.39 eVin T₁, respectively, taking the energy of S₀ minimumas reference. Based on the calculated potential energy surfaces, the existence of a metastable excited molecule is anticipated upon 260-230 nm photoexcitation, which provides a probable approach for cyanoacetylene to polymerize. The internal conversion (IC) process through vibronic interaction followed by C4-H5 fission in the ground state is found to account for the observed diffuse character in the UV absorption spectrum below 240 nm.