Clemens Woywod

Characterization of the S1–S2 conical intersection in pyrazine using ab initio multiconfiguration self‐consistent‐field and multireference configuration‐interaction methods

Potential‐energy surfaces of the three lowest singlet states of pyrazine have been calculated as a function of ab initio determined ground‐state normal coordinates, using complete‐active‐space self‐consistent‐field (CASSCF) and multireference configuration interaction (MRCI) techniques. The conical intersection of the S1 and S2 adiabatic potential‐energy surfaces has been mapped out in selected subspaces spanned by the most relevant vibrational coordinates. A unitary transformation from the adiabatic to a quasidiabatic electronic representation is performed, which eliminates the rapid variations of the wave functions responsible for the singularity of the nonadiabatic coupling element. Transition‐dipole‐moment functions have been obtained in the adiabatic and in the diabatic representation. The leading coefficients of the Taylor expansion of the diabatic potential‐energy and transition‐dipole‐moment surfaces in terms of ground‐state normal coordinates at the reference geometry have been obtained at the CA...

Ab initio investigation of potential‐energy surfaces involved in the photophysics of benzene and pyrazine

Potential‐energy surfaces of the lowest singlet and triplet excited states of benzene and pyrazine have been calculated using complete‐active‐space self‐consistent‐field and multireference configuration interaction (MRCI) techniques. We have focused our attention on the saddle points and surface intersections associated with the reaction path to a biradical form called prefulvene. The barrier heights separating the prefulvenic minimum from the minimum of the planar aromatic form on the ππ* excited singlet surface and on the ground‐state surface have been estimated by large‐scale MRCI calculations. The conical intersection of the lowest ππ* excited singlet surface with the S0 surface has been mapped out in two dimensions, the reaction coordinate to prefulvene and the coordinate of maximum coupling perpendicular to it. The relevance of these ab initio potential‐energy data for the interpretation of photophysical relaxation pathways in benzene and pyrazine (‘‘channel‐three’’ effect) is discussed.

Diabatic CASSCF orbitals and wavefunctions

Abstract It is shown that diabatic CASSCF orbitals can conveniently be obtained via a block-diagonalization of the Fock matrix in a frozen-orbital representation. In a second step, diabatic N-electron wavefunctions are constructed by block-diagonalization of the Hamiltonian matrix in the representation of configuration-state functions. As a demonstration of the method, two-dimensional diabatic surfaces of the 1A2 and 1B1 excited states of ozone are reported.

Resonance Raman spectroscopy of the S1 and S2 states of pyrazine: Experiment and first principles calculation of spectra

New experimental and theoretical data on the resonance Raman (RR) spectroscopy of the S1 and S2 states of pyrazine are presented. Based on recent ab initio CASSCF (complete‐ active‐space‐self‐consistent‐field) and MRCI (multireference configuration interaction) calculations of Woywod et al. [J. Chem. Phys. 100, 1400 (1994)], we construct a vibronic coupling model of the conically intersecting S1 and S2 states of pyrazine, which includes the seven most relevant vibrational degrees of freedom of the molecule. Employing a time‐dependent approach that treats the intramolecular couplings in a nonperturbative manner, we calculate RR cross sections for this model, taking explicitly into account the nonseparability of all vibrational modes. The combination of high‐level ab initio calculations and multimode propagation techniques makes it possible, for the first time, to make first‐principles predictions of RR spectra for vibronically coupled electronic states of an aromatic molecule. The theoretical data are comp...

Photodissociation of ozone in the Chappuis band. I. Electronic structure calculations

Potential-energy surfaces of the 1 1A′, 1 1A′′, and 2 1A′′ states of ozone and corresponding transition-dipole-moment surfaces have been computed as a function of the two bond distances and the bond angle. The calculations are based on the complete-active-space self-consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) electronic-structure models. For the calculations of the 1A′′ surfaces, which exhibit a conical intersection, a diabatic representation has been constructed, employing a direct diabatization method implemented at the CASSCF level. The slow variation of the diabatic potentials and transition dipole moments with nuclear geometry allows us to perform the ab initio calculations on a widely spaced grid. The complete potential-energy and transition-dipole-moment surfaces are then efficiently obtained by interpolation. This procedure leads to very significant savings of computing time compared to the mapping of the rapidly varying potentials and derivative co...

Internal conversion funnel in benzene and pyrazine: adiabatic and diabatic representation

Abstract The two lowest singlet surfaces of benzene and pyrazine exhibit a low-lying conical intersection on the reaction path to a biradical form called prefulvene. The potential-energy surfaces have been calculated with the complete-active-space self-consistent-field method. The conical intersection has been mapped out in two dimensions, the reaction coordinate to prefulvene and the coordinate of maximal interstate coupling. Quasi-diabatic potential-energy surfaces have been obtained via a block-diagonalization of the CI matrix. The ab initio diabatic potentials reveal that the conical intersection is locally well described by a simple linear model in both benzene and pyrazine.

Photodissociation of ozone in the Chappuis band. II. Time-dependent wave-packet calculations and interpretation of diffuse vibrational structures

We present time-dependent wave-packet calculations describing the photodissociation of ozone in the Chappuis band, which evolves in the two lowest states of 1A′′ symmetry. The calculations are performed in the diabatic representation and include the coupling between the two relevant states. All three nuclear degrees of freedom are taken into account. The two potential-energy surfaces, the coupling potential, and the two transition-dipole-moment functions with the electronic ground state have been calculated previously by ab initio methods [Woywod et al., J. Chem. Phys. 107, 7282 (1997)]. The coupling between the two diabatic states is exceedingly strong, resulting in very fast dissociation into O+O2 on the time scale of only one symmetric stretch period. A small portion of the initially created wave packet is temporarily trapped leading to three tiny recurrences, which reflect basically symmetric stretch motion plus some amount of bending motion. The experimentally observed diffuse vibrational structures ...

Theoretical investigation of Jahn-Teller and pseudo-Jahn-Teller interactions in the ammonia cation

The spectroscopic and dynamic aspects of Jahn–Teller and pseudo-Jahn–Teller interactions in the ammonia cation are investigated within an ab initio based vibronic-coupling model approach. Multireference second-order perturbation theory (CASPT2) has been employed to obtain the potential energies of the ground state and the first excited state of NH3+ as a function of symmetry-coordinate displacements. Vibronic-coupling parameters determining the Franck–Condon, Jahn–Teller, and pseudo-Jahn–Teller activity of the normal modes have been obtained from the ab initio data. The vibronic structures of the X 2A1 and A 2E photoelectron bands of ammonia have been calculated by numerical diagonalization of the vibronic Hamiltonian matrix. All six vibrational degrees of freedom are taken into account. The effects of Jahn–Teller and pseudo-Jahn–Teller interactions on the band shape of the A 2E photoelectron band are analyzed. The calculation of the time-dependent population probability of the A 2E state reveals a radia...

Remarkable impact of intermode couplings on multimode vibronic dynamics: the photoelectron spectrum of CH3F

Electronic and nuclear motions on intersecting potential energy surfaces are often intricately mixed and the spectrum can become very complex. Here we choose the strongly coupled Jahn-Teller system CH3F+ as a prototype example, and establish the importance of intermode coupling terms on multimode vibronic dynamics. The theoretical approach consists of a full second-order diabatic vibronic Hamiltonian, constructed from high-quality electronic structure calculations. Our results compare amazingly well with the experimental data. This highlights the success of the present theoretical approach in explaining the complex structure of vibronic spectra, ubiquitous in molecular systems.

Theoretical study of the photoelectron spectrum of allene

Multi-mode dynamical E⊗B Jahn-Teller coupling effects in the X 2E and A 2E states of the allene cation are systematically investigated. Ab initio electronic potential energies of the X 2E, A 2E and B 2B2 states of C3H+4 have been obtained as a function of normal coordinates of A1, B1 and B2 symmetry. The potential-energy functions reveal the Jahn-Teller activity (B1, B2 symmetry) and Condon activity (A1 symmetry) of the normal modes. It is shown that the recently published (Yang et al. Chem. Phys. Letters 171 (1990) 9) high-resolution photoelectron spectrum of the X 2E band of allene can quantitatively be interpreted in terms of Jahn-Teller coupling involving the mode ν4 (torsion) and the antisymmetric CC stretching mode ν6. The vibronic fine structure of the A 2E band reported by Yang et al. is shown to arise primarily from the excitation of the Jahn-Teller active antisymmetric HCH bending mode ν7 and the totally symmetric HCH bending mode ν2.