Ethylenes triplet state

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Molecular orbital calculations on ethylene indicate that the lowest energy excited singlet and triplet states have a twisted geometry.(2) This geometry helps minimize electron-electron repulsion. Figure 9.1 gives the calculated... 

Figure 9.1. Potential energy diagram for the electronic states of ethylene N, ground state (w)2 r(3Biu) first excited triplet state (mr ) V, first excited singlet state (wir ) Z, two-electron excitation (w )2. For the ion C2H + R and R, Rydberg states, /, / ground and excited states. [From Ref. 2(c).]...

The situation changes if one of the oxygen atoms is replaced by a chlorine. Limberg could show that the product of the addition of Mn03Cl to ethylene is more stable in the triplet state and that the product distribution can be explained in terms of reaction channels [47]. 

From the state correlation diagram based on Fig. 9 it can be seen that the triplet state of methylene and the ground state of ethylene correlate with... 

These results can be summarized as follows The triplet carbene ( 5i) adds nonstereospecifically because its complex and a ground state ethylene correlate with the triplet state of an excited trimethylene configuration, which has no barriers to rotation around terminal bonds. 

Substituent effects on the electron-transfer processes between pyrrolidinofullerenes and tetrakis(dimethylamino)ethylene (TDAE) were studied in both the ground state and excited triplet state. ° Equilibrium constants and rate constants for forward and backward electron-transfer processes in the ground state, in addition to rate constants of the forward electron transfer in the excited triplet state were measured. 

Figure 2.44. Charge density difference plotted in a plane containing the metal atoms and the carbon skeleton of the ethylene molecule. The difference is taken between interacting and non-interacting molecules and metal cluster for the adsorbed cases. For the gas phase molecule (top), the difference between the singlet and triplet state is shown. From Ref. [85].

When an alkyl or aryl ketone, or an aryl aldehyde, reacts with an alkyl-substituted ethylene, or with an electron-rich alkene such as a vinyl ether, the mechanism involves attack by the (n,n triplet state of the ketone on ground-state alkene to generate a 1,4-biradical that subsequently cyclizes. The orientation of addition is in keeping with this proposal, since the major product is formed by way of the more stable of the possible biradicals, as seen for benzophenone and 2-melhylpropene (4.64). As would be expected for a triplet-state reaction, the stereoselectivity is low, and benzophenone gives the same mixture of stereoisomers when it reacts with either trans or... 

Irradiation of asingle crystal of deuterated and undeuterated 6-methyl-l,2,4-triazine-3,5-dione (13) with y- or X-rays afforded radicals, the EPR spectra of which were recorded by Horak and Schoffa (71MI21901). The 19-line spectrum of the deuterated radical indicates interaction of the unpaired electron with two equivalent protons, and two non-equivalent nitrogen atoms. The EPR spectra of metastable triplet states of 6-methyl-l,2,4-triazine-3,5-dione (12) in ethylene glycol/water glass at -196 °C were recorded by Shulman and Rahn (66JCP(45)2940). It was found that the lowest triplet state is a 7r- 7r state. 

The primary products of the Hg(3P,) atom sensitized reaction with olefins are electronically excited molecules. For ethylene the process is the production of electronically excited ethylene, C2HJ, probably in the triplet state r/3Bi at about 3.56 eV (16). 

Deactivation processes competing with fluorescence are mainly nonradiative deactivation to the S0 state (IC) and nonradiative transition to a triplet state (intersystem crossing, ISC). Photochemical products are often formed from this triplet state. Important photochemical reactions are the E—yZ isomerization of ethylene, the oxidation of pyrazoline to pyrazole, and the dimerization of cou-marins. 

CHa + CH2— C2H4.—-The coplanar approach of two methylenes to form ethylene was investigated by Basch268 using the MCSCF method. The states of bent methylene that correlate with the ground state of ethylene are the triplet states. It is found that for two closed-shell singlet-state methylenes, the reaction path is purely repulsive. 

Hammond and co-workers have obtained evidence for chemical reactions of several triplet states in solution using triplet energy transfer. They have shown (a) triplet ethylenes and stilbenes undergo cis-trans isomerizations,41 42 (b) triplet ethyl pyruvate decomposes to give acetaldehyde and carbon monoxide,40 and that (c) triplet azomethane decomposes to give nitrogen and methylene (probably in a triplet state).47... 

Prom the results presented in Ref. [132] the absence of observed phosphorescence in the short polyenes is understood as a combination of small T — So transition probability and vibrational quenching. Because of the similarity of results for all triplet state quantities of the ethylene, butadiene and hexatriene molecules investigated here, one can propose that these arguments also hold as explanation for the lack of phosphorescence in the longer polyenes [132]. 

Response theory describes the S-T transition probabilities in unsaturated hydrocarbons quite well more than 99 % of the So - Xi transition intensity is out-of-plane polarized in agreement with experiment for aromatics in ethylene, butadiene and naphthalene the y spin-sublevel of the T state is the most active one, where y is the long in-plane axis of the molecules [134,132]. The main difference between the triplet states of aromatic and aliphatic compounds is the lack of phosphorescence for the latter. We have related this to the fact that polyenes also lack fluorescence (or have very weak fluorescence). This have been explained from the effective quenching of singlet excited (tr r ) states, which is an inherent property for the short polyenes. Our results suggest that this situation also prevails for the lowest triplet states. 

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