Vibronic absorption spectrum of the ethylene dimer

Vibronic coupling theory is used to construct the vibronic absorption spectrum of the ethylene dimer. It is shown that in this case an extended four-parameter form of the vibronic Hamiltonian should be considered. In addition to the commonly used three vibronic parameters, the difference between the ground and excited state force constants of the monomer is taken into account. Numerical calculations were performed for the dimer geometry resembling that of norbornadiene. Some comments on the interpretation of the absorption spectrum of norbornadiene are made.     

Extent of the sudden polarization effect for long polyenes: an analytical study

The sudden polarization effect is studied for a series of dodecahexenes, by means of minimal basis set ab initio calculations, followed by limited configuration interaction. In order to understand the somewhat surprising results, the CI process is modeled and an analytical formula for calculating the polarization is demonstrated Simple rules are then stated which govern the polarization within the restricted open shell + limited CI framework.     

Quantification of sudden light-induced polarization in bacteriorhodopsin by optical rectification

Upon population of its excited state, the retinal chromophore in the membrane protein bacteriorhodopsin (bR) undergoes a sudden (less than approximately 10 fs) change in dipole moment, Deltamu, that can be visualized in a direct way by optical rectification of a broadband visible femtosecond light pulse to the infrared but has not been quantified in this way. Here we show that a transparent thick AgGaS2 crystal delivers infrared radiation with the same spectral profile as bR and is a suitable reference for quantifying conversion efficiency. Using this reference, we estimate the projection of Deltamu on the membrane normal at 11 D, corresponding to the displacement of a full charge over approximately half the length of the retinal chromophore. This result may help to evaluate models describing the interplay between the initial polarization change and the subsequent isomerization of the retinal.     

A study of the electronic spectrum of halogen derivatives of ethylene

The spectra of tetrachloroethylene, monofluoroethylene, gem-, cis- and trans-difluoroethylene and trifluoroethylene were studied in the region 150–250 nm in the gas, solid and Kr matrix. The valence character of the π å π^* transition in the various compounds was verified and the Rydberg transitions were identified. The lowest energy band in the fluoroethylene was found to be a Rydberg transition. The vibronic structure of the π å π^* transition in tetrachloroethylene indicates that the excited state geometry is planar. The existence of a π å σ^* transition at lower energy than the π å π^* transition was verified for cis-difluoroethylene and trifluoroethylene.     

Substituent effect on the sudden polarization in simply substituted ethylenes

The sudden polarization effect occurring in the excited states of simply-substituted ethylenes is analyzed in both molecular orbital and valence bond terms for the substituents R = CH₃, F, HO and NH₂. Ab initio level computations (4—31G + 100 × 100 CI) indicate that at 90° twist the ground states of all these structures are diradical in character with very little substituent effect on the ground state cis—trans isomerization barrier. On the other hand, the energies of both the Z₁ and Z₂ states are strongly affected by the substituent. In all cases the Z₁ states have a nearly pure R---C⁺H---C⁻H₂ zwitterionic character. The stabilization energies of the Z₁ states as compared to ethylene roughly correspond to the order expected from the analogous carbonium ions with the exception of the fluoro substituent.     

Theoretical study of diels-alder reactions between substituted derivatives of butadiene and ethylene

The potential surfaces for the butadiene + ethylene and l-hydroxybutadiene+acrolein reactions, have been calculated (MINDO/2). A simplified study of the potential surface for several reactions between monosubstituted derivatives of butadiene and ethylene has also been done (MINDO/3). From the results obtained, the endo-exo ratio, the regioselectivity, the influence of substituents on the reaction rate and the mechanism of the process are discussed.     

Density functional study of Diels-Alder reactions between cyclopentadiene and substituted derivatives of ethylene

Diels-Alder reactions of cyclopentadiene with ethylene, acrylonitrile, and acrolein were theoretically studied using density functional methods. The results obtained were compared with those corresponding to ab initio methods. Density functional methods, using the B-LYP nonlocal functional, or hybrid methods, yield energy barriers in excellent agreement with the experimental data. For the reactions with substituted derivatives of ethylene, the results show that acrylonitrile reacts with cyclopentadiene with virtually no stereoselectivity, while for acrolein, the formation of the endo product is predicted to be the most favorable from the values of activation Gibbs energies. © 1997 John Wiley & Sons, Inc.     

Alkylation of pyrimidine derivatives with ethylene chlorohydrin

Alkylation of 6-methyluracil, 5-fluorouracil, uracil-5-ammonium sulfate, 5-hydroxyuracil, 5-hydroxy-6-methyluracil, 3,6-dimethyl-5-hydroxyuracil, and 1,3,6-trimethyl-5-hydroxyuracil with ethylene chlorohydrin in water-alcohol medium in the presence of KOH was investigated.     

Vibronic interactions in s-triazine spectra

The two-photon absorption spectrum of s-triazine has been measured using multiphoton ionization. A vibronic coupling scheme, comprising the first excited ¹E″ state coupled by the ν₆(e′) vibration to the ¹A₂″ and ¹A₁″ states, successfully reproduces the essential features of the absorption, fluorescence and two-photon absorption spectra.     

Vibronic calculations in aromatic hydrocarbons

On the basis of a simple composite approach involving semi-empirical MO and valence force field methods, it is demonstrated that vibronic band intensities can be calculated in good quantitative accuracy not only for benzene in contrast to some earlier work but also for larger systems.     

Vibronic coupling activity in benzene

The absorption spectra of benzene-h₆, -d₃ and -d₆ have been remeasured in the region of the hot band transitions 1⁰₁6⁰₁ and 8⁰₁. Whereas for the first an intense doublet is found, for the latter two only a single intense peak is observed. This is taken as evidence of the inactivity of ν″₈ in inducing intensity in contradiction to some recent calculations. The strong doublet for benzene-h₆ has been attributed to Fermi resonance.     

Vibronic interaction in one-dimensional polymer

Asymptotic properties of the “attractive” vibronic coupling in a one-dimensional polymer are analyzed. The strength of the vibronic coupling grows linearly with the size of polymer, which may relate to the softening mechanism of Coulombic repulsive interaction between a Cooper pair of superconducting electrons.     

Vibronic coupling in the excited cationic states of ethylene: simulation of the photoelectron spectrum between 12 and 18 eV

The effect of vibronic coupling on structure and spectroscopy is investigated in the excited cationic states of ethylene. It is found from equation of motion coupled cluster singles and doubles method for ionization potential electronic structure calculations in a triple-zeta plus double polarization basis set that ethylene in its third (B (2)A(g)) and fourth (C (2)B(2u)) ionized states does not have a stable minimum-energy geometry. The potential-energy surfaces of these states are energetically distinct and well separated at the ground-state geometry of ethylene, but in a geometry optimization as the structure of the ion relaxes, these surfaces end up in conical intersections and finally in the stable equilibrium geometry of the second ionized state (A (2)B(3g)). The topology of the potential-energy surfaces can be clearly understood using a vibronic model Hamiltonian. Furthermore, by diagonalizing this model Hamiltonian, the photoelectron spectrum of ethylene corresponding to the second, third, and fourth ionized states (12-18 eV) is simulated. Spectra from vibronic simulations including up to quartic coupling constants and using various normal-mode basis sets are compared to those from vertical Franck-Condon simulations to understand the importance of vibronic coupling and nonadiabatic effects and to examine the influence of individual normal modes on the spectrum.     

Chalcogen-containing analogs of ethylene glycol and its derivatives

Ethylene chlorohydrin when reacted with elemental chalcogens or dimethyl dichalcogenides in the hydrazine hydrate-alkali system forms chalcogen-containing analogs of ethylene glycol and its derivatives (dichalcogenated β-diglycols, chalcogenated ethanols, and chalcogenated methyl cellosolves).     

Theoretical study of mechanism of extraction reaction between silylene carbene and its derivatives and ethylene oxide

The mechanism of the oxide extraction reaction between singlet silylene carbene and its derivatives [X₂Si = C: (X = H, F, Cl, CH₃)] and ethylene oxide has been investigated with density functional theory, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by B3LYP/6‐311G(d,p) method. From the potential energy profile, it can be predicted that the reaction pathway of this kind consists two steps, the first step is the two reactants firstly form an intermediate (INT) through a barrier‐free exothermic reaction; the second step is the INT then generates a product via a transition state (TS). This kind reaction has similar mechanism, when the silylene carbene and its derivatives [X₂Si = C: (X = H, F, Cl, CH₃)] and ethylene oxide close to each other, the shift of 2p lone electron pair of O in ethylene oxide to the 2p unoccupied orbital of C in X₂Si = C: gives a p → p donor–acceptor bond, thereby leading to the formation of INT. As the p → p donor–acceptor bond continues to strengthen (that is, the C? O bond continues to shorten), the INT generates product (P + C₂H₄) via TS. It is the substituent electronegativity, which mainly affects the extraction reactions. When the substituent electronegativity is greater, the energy barrier is lower, and the reaction rate is greater. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Vibronic transitions in single metalloporphyrins.

Structural and electronic properties of single zinc etioporphyrin molecules adsorbed on Al2O3/NiAl(110) were probed by a low-temperature scanning tunneling microscope (STM). Scanning tunneling spectroscopy (STS) revealed progressions of spectral features corresponding to the vibronic states of individual molecules that depend strongly on the molecular conformations. Vibronic features observed by STS were compared with the results from fluorescence induced by tunneling electrons (tunneling-induced fluorescence, TIF).