Photochemistry of ethylene: A multireference configuration interaction investigation of the excited-state energy surfaces (original) (raw)
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Nonempirical Calculations on Excited States: The Ethylene Molecule
The Journal of Chemical Physics, 1967
A series of nonempirical calculations are reported on the excited states of the ethylene molecule using a recent minimum basis set LCAO MO SCF wavefunction. For the lowest excited singlet state of ethylene (1 Bau) the coupling between the 1r electrons and u electrons is significant: the excitation energy being decreased from 11.98 to 10.17 eV and the oscillator strength from 1.03 to 0.73. This coupling has little effect on the triplet state. In the next higher approximation (the random-phase approximation) the excitation energy is further decreased to 9.44 eV and the transition moment to 0.51. With the use of accurate LCAO MO SCF wavefunctions, it is felt that the methods presented here will provide a basis for the theoretical interpretation of electronic spectra.
Ab initio molecular dynamics study of cis–trans photoisomerization in ethylene
Chemical Physics Letters, 1998
We have used ab initio multi-electronic state molecular dynamics to study the photoinduced cis-trans isomerization of ethylene. The initial motion on the excited state is a stretching of the C5C bond and the photoisomerization begins within ; 50 fs of optical excitation. Quenching to the ground electronic state is found to be ultrafast and proceeds from an ionic state via a conical intersection. Accessing the conical intersection requires pyramidalization of one of the methylene groups and this can happen only after energy is funneled from the twisting mode into the pyramidalization mode. q 1998 Elsevier Science B.V. All rights reserved. 0009-2614r98r$ -see front matter q 1998 Elsevier Science B.V. All rights reserved.
Semiempirical molecular dynamics investigation of the excited state lifetime of ethylene
Chemical Physics Letters, 2005
Semiempirical molecular dynamics with surface hopping was employed to investigate the lifetime of excited states of ethylene. Based on previous ab initio multireference configuration interaction results, a complete reparametrization of the AM1 semiempirical parameters was performed. Depending on the initial vertical excitation energy, lifetimes from 105 to 139 fs were found for the Vstate decay. Comparison to the pump-probe experiments was performed in order to explain the large differences between the theoretically and experimentally obtained lifetimes. The results show that probe energies of at least 7.4 eV should be employed to ionize the system for geometries close to the conical intersections.
The Journal of chemical physics, 2014
In this paper we report a new ground state potential energy surface for ethylene (ethene) C2H4 obtained from extended ab initio calculations. The coupled-cluster approach with the perturbative inclusion of the connected triple excitations CCSD(T) and correlation consistent polarized valence basis set cc-pVQZ was employed for computations of electronic ground state energies. The fit of the surface included 82,542 nuclear configurations using sixth order expansion in curvilinear symmetry-adapted coordinates involving 2236 parameters. A good convergence for variationally computed vibrational levels of the C2H4 molecule was obtained with a RMS(Obs.-Calc.) deviation of 2.7 cm(-1) for fundamental bands centers and 5.9 cm(-1) for vibrational bands up to 7800 cm(-1). Large scale vibrational and rotational calculations for (12)C2H4, (13)C2H4, and (12)C2D4 isotopologues were performed using this new surface. Energy levels for J = 20 up to 6000 cm(-1) are in a good agreement with observations....
Role of Rydberg States in the Photochemical Dynamics of Ethylene
The Journal of Physical Chemistry A, 2012
We use the ab initio multiple spawning method with potential energy surfaces and nonadiabatic coupling vectors computed from multistate multireference perturbation theory (MSPT2) to follow the dynamics of ethylene after photoexcitation. We introduce an analytic formulation for the nonadiabatic coupling vector in the context of MSPT2 calculations. We explicitly include the low-lying 3s Rydberg state which has been neglected in previous ab initio molecular dynamics studies of this process. We find that although the 3s Rydberg state lies below the optically bright ππ* state, little population gets trapped on this state. Instead, the 3s Rydberg state is largely a spectator in the photodynamics, with little effect on the quenching mechanism or excited state lifetime. We predict the time-resolved photoelectron spectrum for ethylene and point out the signature of Rydberg state involvement that should be easily observed.
High-Level Quantum Chemical Methods for the Study of Photochemical Processes
2005
Multireference configuration interaction calculations have been performed on the excited state energy surfaces of the methyleneimmonium cation using recently developed methods for the computation of analytic gradients and nonadiabatic coupling terms. Excited-state structures and minima on the crossing seam have been determined. It was found that the topology of the methyleneimmonium surfaces is qualitatively different from that of the isoelectronic ethylene. In the former case a conical intersection between the S 1 and ground states is found for the twisting motion around the CN bond, whereas a more complicated motion including pyramidalization and hydrogen-transfer is needed in case of ethylene.
J. Am Chem. Soc. 1983, 105, 5252-5257, 1983
The C2H42+ potential energy surface was examined by ab initio molecular orbital theory corrected for electron correlation by means of Mdler-Plesset perturbation theory to third order (MP3/6-31G**) using 6-31G* (and 3-21G) optimized geometries. The perpendicular (D2,,) ethylene dication, 1 (DZd), is the global and only singlet C2H42+ minimum with an estimated heat of formation of 654 kcal/mol. The rotational transition structure, 2 (D2J, is 28.1 kcal/mol higher in energy. This rotational barrier is remarkably large for such 14-electron species (compare HzBBH2 and H2B<H2+, 10.5 and 20.1 kcal/mol, respectively). The C, , ethylidene dication, 4, 21.3 kcal/mol higher in energy than 1, is probably not a minimum, but may facilitate hydrogen scrambling. Although 1 is unstable thermodynamically toward proton loss (by 16 kcal/mol), the barriers for deprotonation (and homolytic cleavage (into two CH2+ cations)) are 68.8 and about 88.4 kcal/mol, respectively. The transition structure for cleavage of 4 into CHSt and CH+ lies 86.5 kcal/mol above 1 in energy. These large bamers are consistent with the experimental observation of CZHd2+ in the gas phase.
Application of the MC SCF method to the π → π* excitation energies of ethylene
Chemical Physics, 1984
The MC SCF method is employed to cakulate the N-+ T and N + V rr-+ + verticaI excitation energies of ethylene. To obtain accurate excitation energies it is found to be necessary to utilize an expanded valence space containing two n and two ?I* orbitiils. Relatively small MC SCF calculations, allowing at most one-electron excitations from the sigma space, are found to yield excitatton energies and spatial extents of the exctted states in excelient agreement with the predictions of large multi-reference or tterative-natural-orbital CI calculations. These results show that within au MC SCF framework O-Q correlation is unimportanr for descnbing the +R + a* processes, We also conclude that the neglect of the effects of unlinked cluster terms in some of the CI calculattons may have introduced smail. but important, errors in the excitation energies and predictions of the spatial extent of the V state. ' John Smon Guggenhe& Fonndation Fellow. *-An exc5lleu~ review of the ethylene saga prior to 1974 is given by Robin in ref. 1lj.-=