We have investigated the photo-chemical reaction from tetra-tert-butylcyclobutadiene (TB-CBD) to tetra-tert-butyltetrahedrane (TB-THD) and its reverse thermo-chemical reaction processes in the ground state by using CASSCF and MRMP2 computational methods. According to our results, the initial step of the photochemical reaction is the HOMO-LUMO single-electron excitation (11B1 state) and the arrangement from TB-CBD to TB-THD occurs via the HOMO-LUMO double electron excited state (21A1 state). After the transition from the 11B1 to the 21A1 state, most TB-CBD molecules show only photo-physical property without any reactions because the final point of the minimum-energy-path (MEP) calculation at the MRMP2//CASSCF level is the S1/S0 conical intersection (ionic like structure), which results in turning back to TB-CBD in the S0 state. However, on the way to the final point of the MEP, it is possible for some TB-CBD to transit at another S1/S0 conical intersection (tetra radical like structure), which is related to the photoreaction from TB-CBD to TB-THD. On the other hand, two routes from TB-THD to TB-CBD were found in the S0 state. One is the route via bicyclodiradical transition state. The other is the ionic transition state. In both reaction paths, only one TS is there in contrast to the plural step reaction suggested previously.
(Comp. Theor. Chem., 969, 44-52 (2011))
We have explored the singlet ground state potential energy surface (S0 PES) between cyclobutadiene (CBD) and tetrahedrane (THD) looked down from S1/S0 conical intersections through multi-configuration self-consistent field theory. On the basis of the obtained S0 PES, we propose the revised process of the THD to CBD symmetry-forbidden reaction. According to the present result, the THD to CBD rearrangement occurs via plural steps similarly to previous suggestions, but via a tetra-radical species (instead of an endo-species), which is considered for the first time in this paper. Since the endo-species is significantly destabilized when hydrogen atoms are replaced by bulky substituents (such as tert-butyl group), the present one, where endo-species are not involved, would be realized in actual systems having bulky substituents.
(Tetrahedron, 66, 5212-5217 (2010))
We have located two conical intersections between the first singlet excited (S1) and singlet ground (S0) states of cyclobutadiene (CBD) using the complete active space self-consistent field (CASSCF) method. One is the ionic-structure S1/S0 conical intersection (CIionic), which was located by carrying out a minimum-energy-path calculation from the Franck-Condon point of the HOMO to LUMO double-electron excited state, and the other is the tetra-radical S1/S0 conical intersection (CItetra), which was located by exploring the S1/S0 degeneracy space. While CIionic is only involved in the automerization of CBD, CItetra is involved in not only the automerization but also the criss-cross reaction. It is possible for one of the highest constrained compounds, tetrahedrane, to be produced if S1 excited CBD undergoes a transition to the S0 state via the tetra-radical S1/S0 conical intersection. In this paper, we discuss the possibility that unsubstituted tetrahedrane can be produced by irradiating CBD.
(Chem. Phys., 371, 30-35 (2010))
We report the result of comparison between two reaction coordinates [on the potential energy surface of the first excited state (S1)] produced by CASSCF and these energies recalculated by MRMP2 in the Z to E photoisomerization of penta-2,4-dieniminium (PDI) as the minimal model of the retinal protonated Schiff base (RPSB). One coordinate is the S1 state minimum-energy-path (MEP) in mass-weighted coordinates from the S1 vertically excited point, where a strong hydrogen-out-of plane (HOOP) motion is not exhibited. The energy profile of the S1 MEP at the MRMP2//CASSCF level shows a barrier for the rotation around the reactive C-C and hits the S1/S0 degeneracy space where the central C-C-C-C dihedral angle is distorted by 65 degrees. The other coordinate is an S1 coordinate obtained by the relaxed scan strategy. The relaxed coordinate along the central C-C-C-C dihedral angle, which we call the HOOP coordinate, shows strong HOOP motion. According to the MRMP2//CASSCF calculation, there is no barrier on the HOOP coordinate. Furthermore, the S1 to S0 transition may be possible without the large skeletal deformation by HOOP motion because the HOOP coordinate encounters the S1/S0 degeneracy space where the central C-C-C-C dihedral angle is distorted by only 40 degrees. Consequently, if PDI is a suitable model molecule for the RPSB as often assumed, the 11-cis to all-trans photoisomerization is predicted to be accelerated by the HOOP motion.
(Phys. Chem. Chem. Phys., 11, 6406-6414 (2009))
We have characterized the degeneracy space (DS) between the ground (S0) state and the first excited (S1) state along the exocyclic methylene twist motion of fulvene, using our calculation strategy, i.e., a two-step procedure with CASSCF. The origin of the "cancellation error" on locating degeneracy points under geometrical constraints is analyzed, leading to a method to assess adequacy of the strategy. According to our estimation, these S1/S0 DPs are optimized for energy within 2.0 10-3 Eh Angstrom-1 (the value of root-mean-square). From the obtained S1/S0 DS, we provide some information about the exocyclic methylene rotation by 180 degrees.
(J. Chem. Theor. Comp., 4, 42-48 (2008))
Potential energy surfaces (PESs) of the maleic acid anion radical (MA-*: cis isomer)/fumaric acid anion radical (FA-*: trans isomer) system as a model system of their esters have been studied in detail using CASSCF method. The results suggest the following: The photoisomerization is initiated with the H-C-C-H dihedral angle distortion [hydrogen out of plain (HOOP) motion] on the D1 PES. The C-C-C-C dihedral angle distortion occurs on the D0 PES after the deactivation from D1 to D0. A large fraction of the net motion along the isomerization coordinate occurs on the D0 PES. The D0 state is responsible for the one-way nature of the photoisomerization.
(J. Phys. Chem. A, 110, 12276-12281 (2006))
Hula-twist (HT) motion of Z-penta-2,4-dieniminium (PDI) on the S1 surface is studied on the basis of complete active space selfconsistent field (CASSCF) theory. An S1/S0 crossing region (a segment of the S1/S0 conical intersection hyperline) along a HT coordinate (simultaneous rotation of the central double bond and an adjacent single bond) is characterized. One-dimensional relaxed scan calculation along the postulated HT coordinate shows no barrier up to the S1/S0 crossing region, suggesting the possible involvement of the HT process in a fast photochemical reaction in constrained states.
(Chem. Phys. Lett., 424, 374-378 (2006))