| The photochemistry ofπ-bonded hydrocarbons-ethylene and its mono-substitutions has induced great experimental and theoretical interests. Acrylonitrile (cyanoethylene, vinyl cyanide, 2-propenitrile,), as the simplest cyano-substituted alkene, deserves great interest and detailed investigation in its unimolecular dissociation processes after photoexcitation. The mono-substitution of ethylene by cyanogen group instead of single atom is much interesting as the C≡N triple and the C=C double bonds constitute aπ,π-conjugated system, and some innovations can be expected. In particular, to make clear the photodissociation mechanism of acrylonitrile has drawn the attentions of astronomers. The cyano-substituted molecules found in Titan's atmosphere provide hints for the origin of life. They may produce or be produced by acrylonitrile. In addition, acrylonitrile was detected in the circumstellar envelope around an AGB star last year, indicating its existence outside dark clouds.Stationary points (minima and first order saddle points) for acrylonitrile in the lowest three electronic states (S0, T1, and 1π2πC≡N*) have been optimized with the complete active space self-consistent field (CASSCF) method employing the Dunning correlation consistent triple-zeta (cc-pVTZ) basis set. Equilibrium geometries in the 1π2π1* and 1πC≡Nπ1* states were endeavored but failed to locate, and the two states tend to decay through conical intersections according to our calculations. Energies of all critical points have been recomputed with the multiconfigurational second-order perturbation (CASPT2) method, which includes dynamic correlation effects. To connect the transition states with reactants or products, the minimum energy pathway (MEP) calculations were performed at the CASSCF level of theory. The vertical excitation energies have been calculated at the CASPT2//CASSCF level of theory based on the S0 minimum structure, with zero point energy (ZPE) correction.All the stationary points have been searched using an active space of 14 electrons and 12 orbitals. The (14,12) active space includes all the sixπandπ* on C=C and C≡N, the fourσandσ* on the C1?C2 and C?H which involved the interested dissociations, and the lone pair electrons occupied orbital on nitrogen atom. An extra orbital, determined to be the combination of two involved orbitals: C-Hσand N 2s, is also included in the active space. All the calculations mentioned above were carried out using the MOLCAS 6.2 program. A state average CASSCF (8,7) approach with the cc-pVTZ basic set was adopted to explore the crossing points by employing GAUSSIAN 03 software package.The equilibrium structure of ground-state acrylonitrile was obtained both with and without Cs symmetry maintaining nearly the same geometry parameters, in agreement with previous experiments. It is notable that the C1?C2 distance is obviously shorter than the typical C?C bond length, which implies the existence of a conjugation interaction between N≡C and C=C. Calculations for other structures hereafter were carried out with no symmetry constraint.The vertical excitation energies for the five lowest singlet excited states were calculated without symmetry constraint, employing state average CASSCF method followed by MS-CASPT2 reevaluation. Through calculation, we assigned theπC≡N→π1* transition which hasn't been assigned in previous experimental treatment. We compare the experimental peaks with our calculated transitions, and found the counterpart for each of them. It is the first time the experimental spectrum is theoretically explained.In the ground state, two dissociation channels and one radical migration process have been investigated, all of which can find references in previous calculations by Derecskei-Kovacs and North. The CN dissociate is a reaction without potential barrier above endothermicity, and the calculated energy for product is only 0.09 eV above the previous calculation. In the HCN elimination process, the leaving CN radical approach H1 atom to form a three-center transition state, and finally dissociate into CH2C + HCN. For the latter channel, our calculations provide a transition state that is close with previous investigation in structure and energy. The channel for CN radical to migrate was also calculated. The transition state for this process is only 0.03 eV higher than that of the previous calculation and can undergo further H2 and HCN eliminations.Searching for stationary points in the lowest singlet excited state were carried out starting from different structures and a minimum was finally located on the 1π2πC≡N* surface. The adiabatic excitation energy is 5.74 eV when taking the Smin energy as reference, deviate only slightly from the experimental data of 5.88 eV. Seen from the structure, the Smin has a tendency to form a five-center transition state from which HNC can be yielded. This channel was proved to experience two steps by our calculations. In the first step, 1π2πC≡N* and S0 intersection (CI3) occurs after a transition state, and the molecule reaches an intermediate on the lower surface. The second step, which happens in the ground state, is the cleavage of C1?C2 bond to generate HCCH(X1Σ+g ) + HNC(X1Σ+) through a transition state. The reaction happens only at wavelength shorter than 181 nm. We also investigated CN dissociation on 1π2πC≡N* surface, which has the energy barrier of 7.10 eV and generates CH2CH(X2A′) + CN(A2П). It is accessible when induced by photon with wavelength shorter than 175 nm.The optimization for T1 minimum resulted in a near Cs structure, with the H2 and H3 atoms out of the molecular quasi symmetry plane. The C2?C3 bond elongates to 1.47 ? from an original typical double bond length of 1.34 ? in S0. Judging from the structure of the T1 minimum and proved by calculation, 1,1 elimination through a three-center transition state to yield HCN is superior to 1,2 elimination. The dissociation of HCN is found to be a two-step process. In the first step, the T1 minimum achieves a local minimum via a three-center transition state which lies 4.89 eV above S0 minimum. While in the second step, the local minimum dissociate directly into the product of HCN + CH2=C(3B2), which is 5.64 eV. In fact, the CH2=C(3B2) was detected in Fahr and Laufer's experiment. By our calculation, the CN dissociation is a process without transition state at the CASPT2 level of theory, with the product CH2CH + CN energy of 5.55 eV.In the intersystem crossing points locating we only found T1/S0, which occurs interestingly at the immediacy of T1 minimum at the CASSCF(8,7) level. The spin-orbital coupling constant at T1/S0 was calculated to be 233.52 cm-1, relatively large; thus a high probability for intersystem crossing via T1/S0 can be expected. We can conclude that molecules which populate T1 mostly come from intersystem crossing of the ground-state molecules.Local minima is found in the 1π2πC≡N* state and fail to be locate on both 1πC≡Nπ1* and 1π2π1* surfaces, indicating the lack of local minima preceding conical intersections in the latter two states. Optimizing start from the FC point of 1π2π1* state, we found CI1 crossing point for 1π2π1* and 1πC≡Nπ1* intersection. Another conical intersection point, CI2, where ground state and the lowest excited singlet state-1π2π1* intersect was also located. Thus, we concluded that for molecule which initially populates either the 1πC≡Nπ1* or 1π2π1* state, the most probable route should be: 1πC≡Nπ1*(1π2π1*)(FC)→CI1→CI2→S0Three issues should be addressed for the molecule on its funneling to the ground state from CI2: 1. along the direction of gradient difference at CI2, the molecule can evolve to the minimum on S0 surface. Since adequate internal energy is accumulated, it would undergo further dissociations to yield CH2C(1A1) + HCN(X1Σ+) or CH2CH(X2A′) + CN(X2Σ+). 2. Based on the fact that the C3?C2?C1 angles at CI2 and S0-TS-CN-mig are close to each other, the molecule can evolve along the negative orientation of the derivative coupling vector, making possible the path of CN radical transfer from C2 to C3 following IC. This path is also energetically accessible at 193 nm absorption. 3. Moving along the positive direction of the derivative coupling vector, the C3?C2?C1 angle become larger and the ground-state molecule would reach T1 minimum structure, and a subsequent intersystem crossing from S0 to T1 is expected, considering the significant spin-orbital coupling constant. It's energetically accessible as T1/S0 lies 1.72 eV below CI2.Among the three channels mentioned above, molecular evolving to S0 minimum on funneling is energetically most favorable. On both the S0 and 1π2πC≡N* surfaces, especially the S0 where reactions mainly take place, the CN radical dissociations need more energy than the HCN eliminating channels. In the T1 state, transition state energy for CN elimination is higher than that of the HCN elimination. Both indicate the CN dissociation a minor process. Our calculations consist well with the experimentally detected low quantum yield of CN.Our results can be compared with experimental spectrum in two ways: 1. the calculatedπ2→π1* and n→π2* transitions correspond to the two bands in the 5.5–8 eV region of the absorption spectrum; 2. the 193 nm photo-energy is higher than HCN and CN dissociation barriers in S0 and T1 states by our calculations, and can well explain the diffuse nature of the absorption band.
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