Assignment of the Ultraviolet Mystery Band of Olefins

Robin, M. B.; Hart, Robert R.; Kuebler, N. A.

doi:10.1063/1.1726944

Cited by 98 publications

(13 citation statements)

References 27 publications

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“…The gas- and solution-phase UV absorption spectra of 1,2-dimethylcyclobutene ( 1 ; Figure ) and the differences between them are typical of the UV spectra of tetralkyl-substituted alkenes. − The gas-phase spectrum shows the structured long wavelength absorption band, with vibronic spacings on the order of 1400−1500 cm -1 , which is typical of alkene π,R(3s) absorptions, along with a more intense band below 200 nm which can be assigned to the valence (π,π*) absorption. The intensity and fine structure associated with the long wavelength absorption band is reduced in the solution-phase spectrum, and the absorption is shifted to shorter wavelengths, consistent with the Rydberg assignment in the gas-phase spectrum.…”

Section: Discussionmentioning

confidence: 99%

“…There are at least two excited singlet states accessible in alkylcyclobutenes by direct irradiation in the 185−230-nm spectral range, and both have been proposed to contribute directly to the photochemistry. , The UV spectra of 1,2-disubstituted alkylcyclobutenes are typical of tetrasubstituted alkenes, showing a prominent (π,π*) absorption band in the 185−195-nm range and, in the gas phase, a finely structured band at longer wavelengths (200−230 nm) due to π,R(3s) absorptions. ,− In condensed phases, the position of the Rydberg absorption band shifts to slightly shorter wavelengths, the fine structure is absent, and the band is substantially reduced in intensity. This character, and the fact that its position correlates with the vertical π-ionization potential of the alkene, are characteristic of Rydberg absorptions…”

Section: Introductionmentioning

confidence: 99%

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Stereospecific (Conrotatory) Photochemical Ring Opening of Alkylcyclobutenes in the Gas Phase and in Solution. Ring Opening from the Rydberg Excited State or by Hot Ground State Reaction?

Leigh

Cook

1999

J. Org. Chem.

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The photochemistry of 1,2-dimethylcyclobutene and cis- and trans-1,2,3,4-tetramethylcyclobutene has been studied in the gas phase (1 atm; SF6 buffer) and in hydrocarbon solvents with 193-, 214-, and 228-nm light sources. The major products are the isomeric dienes from electrocyclic ring opening and 2-butyne + alkene (ethylene or E-/Z- 2-butene) due to formal [2+2]-cycloreversion. The total yields of dienes relative to 2-butyne are generally higher in the gas phase than in solution but decrease with increasing excitation wavelength under both sets of conditions. In the case of cis-1,2,3,4-tetramethylcyclobutene, 228-nm photolysis results in the stereospecific formation of E,Z-3,4-dimethyl-2,4-hexadienethe isomer corresponding to ring opening by the thermally allowed (conrotatory) electrocyclic pathwayin both the gas phase and solution. All three diene isomers are obtained upon 228-nm photolysis of trans-1,2,3,4-tetramethylcyclobutene, but control experiments suggest that the thermally allowed isomers (E,E- and Z,Z-3,4-dimethyl-2,3-hexadiene) are probably the primary products in this case as well. The results are consistent with cycloreversion resulting from excitation of the low-lying π,R(3s) singlet state and with ring opening proceeding by at least two different mechanisms depending on excitation wavelength. The first, which dominates at short wavelengths, is thought to involve direct reaction of the second excited singlet (π,π*) state of the cyclobutene. The second mechanism, which dominates at long wavelengths, is proposed to ensue either directly from the lowest energy (Rydberg) state or from upper vibrational levels of the ground state, populated by internal conversion from this excited state.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Stereospecific (Conrotatory) Photochemical Ring Opening of Alkylcyclobutenes in the Gas Phase and in Solution. Ring Opening from the Rydberg Excited State or by Hot Ground State Reaction?

Leigh

Cook

1999

J. Org. Chem.

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“…Optimized geometries for the two transition states are shown in Figure 2 and reveal a very late transition state (40) for the silacyclic rearrangement, and a very early transition state (41) for the carbocyclic system. At the HF/(6-31G(d) level 40 is, barely, an energy maximum 53.5 kcal/ mol above 32, yielding a miniscule 0.4 kcal/mol barrier for carbene 33 to' return to 32.…”

Section: Resultsmentioning

confidence: 99%

Thermally-Induced 1,2-Shifts To Convert Olefins to Carbenes: Does Silicon Do It? If So, Why Not Carbon?

Barton¹,

Lin²,

Ijadi-Maghsoodi³

et al. 1995

J. Am. Chem. Soc.

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Thermal isomerization of olefins to carbenes via a 1,2-silyl shift was examined by both experiment and theory. No evidence of this rearrangement was found for acyclic vinylsilanes, nor could electronic assistance by silicon be identified in cis, trans isomerizations. Serendipitous synthesis of a 2,4-dimethylene-1,3-disilacyclobutane allowed a kinetic examination of its gas-phase, thermal ring expansion to a 2-methylene-1,3-disilacyclopentene. The Arrhenius parameters (log A = 12.48, Eact = 54.09 kcallmol) are the first to be reported for an olefin-to-carbene rearrangement. The analogous all-carbon system failed to ring expand. Ab initio calculations revealed that this was opposite to any predictions which would be made from ring strain considerations. Calculations showed that for silyl migration the transition state was late and was actually the carbene, while for carbon migration the TS was early and considerably higher in energy than the resulting carbene. The 2-methylene-1-silacyclobutane rearrangement (ref 5) was reexamined to find that reversible ring opening to a 1,4-diradical occurred at temperatures below those required to ring expand via a carbene TS. Disciplines Chemistry CommentsReprinted (adapted) Abstract: Thermal isomerization of olefins to carbenes via a 1,2-silyl shift was examined by both experiment and theory. No evidence of this rearrangement was found for acyclic vinylsilanes, nor could electronic assistance by silicon be identified in cis, trans isomerizations. Serendipitous synthesis of a 2,4-dimethylene-1,3-disilacyclobutane allowed a kinetic examination of its gas-phase, thermal ring expansion to a 2-methylene-1,3-disilacyclopentene.The Arrhenius parameters (log A = 12.48, Eact = 54.09 kcallmol) are the first to be reported for an olefin-to-carbene rearrangement. The analogous all-carbon system failed to ring expand. Ab initio calculations revealed that this was opposite to any predictions which would be made from ring strain considerations. Calculations showed that for silyl migration the transition state was late and was actually the carbene, while for carbon migration the TS was early and considerably higher in energy than the resulting carbene. The 2-methylene-1-silacyclobutane rearrangement (ref 5) was reexamined to find that reversible ring opening to a 1,4-diradical occurred at temperatures below those required to ring expand via a carbene TS.

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“…In the prototype ethylene molecule, the 3s Rydberg band is just 10 000 cm À1 higher in frequency [21] than the origin of the valence V N transition. In methylated [20,22] and fluorinated [20,23] ethylenes, the 3s Rydberg band is observed at a lower frequency than the V N transition. It is reasonable to expect a similarly red-shifted Rydberg band for vinylamine, which is isoelectronic with fluoroethylene.…”

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confidence: 92%