3s Rydberg and Cationic States of Pyrazine Studied by Photoelectron Spectroscopy (original) (raw)
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Comments on the Rydberg spectrum of pyrazine
Journal of Molecular Spectroscopy, 1972
The vacuum ultraviolet spectrum of pyrazine vapor is analyzed in terms of two Rydberg series leading to the same ionization potential, namely, 9.28 f 0.01 eV. The absence of interference effects in the spectrum is inconsistent with predictions of previous theories, and the discrepancy is related to configuration interaction between the Rydberg states and isoenergetic valence states. It is suggested that configuration interaction is a general phenomenon and can reasonably explain the contradictions between experiment and theories based on a one-electron approximation of Rydberg states for large molecules, especially aromatic hydrocarbons and their nitrogen heterocycle analogues.
Journal of the Serbian Chemical Society, 2019
Derivatives of dipole transition moments between spin-orbit coupled (SOC) multireference configuration interaction wave functions have been used in conjunction with vibrational frequencies from density functional theories to compute vibronic S 1 ←S 0 (1 1 B 3u ←1 1 A g) and T 1 ←S 0 (1 3 B 3u ← 1 1 A g) absorption spectra in Herzberg-Teller approximation. The experimentally known spectra are well reproduced. The calculations reveal unexpectedly small spin-orbit couplings between the 1 3 B 3u (3 nπ*) state and nearby optically bright 1 B 2u (1 ππ*) states, thus explaining the absence of the 1 1g0 1b (1 10a0 ν) fundamental in the vib-rational fine-structure of the T 1 ←S 0 transition. Adiabatically, two triplet states are found below the S 1 state. The out-of-plane distorted T 2 minimum results from a pseudo Jahn-Teller interaction between two 3 ππ* states of B 1u and B 2u symmetry. At the D 2h-symmetric S 0 and S 1 minimum geometries, the latter states are located well above S 1. The S 1 and T 2 potentials intersect at geo-metries far away from the Franck-Condon region. This explains the apparently contradictory results that the linewidth in the higher energy regime above the T 1 ←S 0 origin suddenly broadens while no trace of a second triplet state, located energetically below the S 1 origin, could be identified in phosphorescence excitation spectra of the ultracold isolated pyrazine molecule.
J Phys Chem a, 1999
We evaluate using a range of ab initio and density-functional approaches the vibrational frequencies, including correction for diagonal anharmonicity, of the lowest triplet state of pyrazine T 1 ( 3 B 3u ); less extensive calculations are also performed for the ground state, three excited singlet states, and five other triplet states. The results indicate that CASSCF-based methods are cumbersome to apply to molecules of this size, with no practicable CASSCF methodology producing a continuous potential energy surface for T 1 . While CASPT2 (and also MRCI) methods can correct for erroneous CASSCF state energies, they are not capable of removing the effects of erroneous CASSCF conical intersections. Density-functional schemes, and in particular B3LYP, provide the best qualitative and quantitative results, with the time-dependent approximation to density-functional theory providing results comparable with those from direct evaluation. An overview of vibronic coupling theory is presented and used to demonstrate the relative strengths and weaknesses of these Born-Oppenheimer calculations compared to the traditional diabatic vibronic coupling calculations of Fischer. In particular, for T 1 ( 3 B 3u ), the current assignments of the strongly vibronically active modes ν 4 , ν 5 , and ν 10a are readily verified, vibronic activity is predicted for ν 12 , the anomalous behavior of ν 16a and ν 16b is reproduced, and ν 8a is reassigned. by using multiconfigurational wave functions such as valencebond wave functions 4,6 or state-averaged complete-active-space SCF (CASSCF) wave functions, 23 with (typically) a doublewell potential resulting. Dynamic electron correlation favors high-symmetry configurations, however, and after a multireference configuration-interaction (MRCI) calculation based on the CASSCF wave function, a single-well potential is obtained, as is observed experimentally. 23 Hence, the level of theory, and the level of caution, necessary to satisfactorily complete an excited-state vibrational analysis is appreciable.
Spectrum of the molecular eigenstates of pyrazine
Chemical Physics Letters, 1982
The molecular eigenstate spectrum belonging to the P and R branches of the 1 B3 u (0-0) electronic transition of pyrazine-h 4 was recorded with a 200 kHz wide laser in a supersonic nozzle at a temperature of ~-1 K. The square of the Fourier transform of the amplitude spectrum yields the quantum beats in the fluorescence decay that have been reported before.
Quantum dynamics of the photostability of pyrazine
Physical chemistry chemical physics : PCCP, 2015
We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state using multireference electronic structure and quantum dynamics calculations. We construct a quadratic vibronic coupling Hamiltonian, including the four lowest electronic states and ten vibrational modes, by fitting to more than 5000 ab initio points. We then use this model to simulate the non-adiabatic excited state dynamics of the molecule using the multi-configuration time-dependent Hartree method. On the basis of these calculations, we propose a new mechanism for this decay process involving a conical intersection between the Au(nπ*) state and the ground state. After excitation to the B2u(ππ*) state, the molecule decays to both the B3u(nπ*) and Au(nπ*) states on an ultrashort timescale of approximately 20 fs. The radiationless decay to the ground state then occurs from the Au(nπ*) state on a much longer timescale.
Theoretical study on S1(1B3u) state electronic structure and absorption spectrum of pyrazine
Science in China Series B: Chemistry, 2008
Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S 0 (1 A g)) and the first (S 1 (1 B 3u)) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorption spectrum of S 1 (1 B 3u) state of pyrazine. The assignment of main vibronic transitions is made for S 1 (1 B 3u) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode ν 6a. For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S 1 (1 B 3u) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S 1 (1 B 3u) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode ν 10a. The calculated S 1 (1 B 3u) absorption spectrum is in reasonable agreement with the experimental spectra.
The Journal of Chemical Physics, 1999
The molecular dynamics of pyrazine after excitation to the S 2 electronic state is investigated using the S 2 absorption spectrum as a benchmark. We first present a realistic model Hamiltonian including all 24 vibrational modes of the pyrazine molecule. Using this model, we determined the potential energy surfaces of the lowest two excited states, S 1 and S 2 , which are strongly coupled to each other. We then treated the nuclear motion of all 24 vibrational modes using the multiconfiguration time-dependent Hartree ͑MCTDH͒ wave packet propagation method. This method obtains results of good accuracy with acceptable computational effort for such a large system. The calculated spectrum is in good agreement with the experimental one. Furthermore, our results shed light on the role of the 20 modes which are only weakly coupled to the system, and demonstrate that essential physical features, such as symmetries, have to be considered when one wants to treat the molecular dynamics of pyrazine realistically.
Localized excitations and the geometry of the ¹n. pi. * excited states of pyrazine
J Am Chem Soc, 1982
Previous theoretical work has shown that the lowest excited singlet state of pyrazine, the nn* lBgu state, is best described in terms of interacting excitations localized on each nitrogen. The present work refines the localized excitation model and considers its implications for the geometry of the lB3, state. Hartree-Fock calculations show that the best single configuration description of the nn* state has broken (IBJ symmetry with the excitation strongly localized at one end of the molcule. If the symmetry-restricted HF result is used for reference, this localization describes an important correlation effect. The excited-state geometry was probed using configuration interaction wave functions based on the symmetry-restricted orbitals, as well as properly symmetrized "valence-bond" wave functions based on the broken symmetry solutions. Both descriptions lead to a very flat potential for a bl, vibrational mode. This mode reduces the molecular geometry from DZh to Ch. We present spectroscopic evidence of our own and of other workers which is consistent with such a flat potential.