- Photodissociation of a bicyclic azoalkane: Time-resolved coherent anti-stokes Raman spectroscopy studies of vapor-phase 2,3-diazabicyclo[2.2.1]hept-2-ene
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The photodissociative mechanism of vapor-phase 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) has been studied with nanosecond-regime transient spectroscopic methods. Following excitation of the vibrationless S1 level at 338.5 nm, data from time-resolved CARS (a vibrational spectroscopy) show the appearance rate for formation of N2 to be 4 × 107 s-1. This value is significantly slower than the 5 × 108 s-1 principal component observed in S1 fluorescence decay, establishing that the dissociating state is not S1. CARS measurements on the nascent N2 photofragments reveal a vibrational distribution (84% v = 0, 12% v = 1) very similar to that observed earlier for the nitrogen formed in the stepwise photodissociation of azomethane. This result and the low level of nascent rotational excitation suggest that dissociation into N2 plus 1,3-cyclopentanediyl biradical occurs from an excited state of the diazenyl biradical that has a linear CNN bond angle. Transient CARS probing has also revealed the subsequent appearance of bicyclo[2.1.0]pentane formed by ring closure of the 1,3-cyclopentanediyl biradicals. Formation kinetics of this ring closure product shows a single first-order component with a rate coefficient of approximately 5.1 × 106 s-1. This observation implies that S1 excitation of vapor-phase DBH produces 1,3-cyclopentanediyl biradicals only in their ground triplet state. Mechanistic differences between the gas-phase photochemistries of DBH and acyclic azoalkanes are attributed to a low-lying excited state of the diazenyl biradical that becomes accessible in DBH through the release of ring strain energy.
- Adams, J. Stephen,Weisman, R. Bruce,Engel, Paul S.
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- UV Laser Photochemistry: Triplet Biradical Trapping Efficiencies and Lifetimes
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The trapping of triplet 1,3-cyclopentadiyl (1a) and the triplet 1,4-biradical 2-cyclopent-2-enyl (6) with molecular oxygen has been studied quantitatively.The lifetime of 1a has been found to range between 720 and 900 ns and that of 6 between 53 and 94 ns depending on the solvent.The biradical 1a was found to be trapped in essentially quantitative yield with no indication of oxygen-catalyzed intersystem crossing.In contrast, 6 was found to be trapped in 54-79percent yield with the residual biradical quenching being due to oxygen-catalyzed intersystem crossing.These and other biradical trapping data have been correlated with 1,3-biradical geometries and found to be in accord with Salem's rules for spin-orbit coupling in triplet biradicals.
- Adam, Waldemar,Hannemann, Klaus,Wilson, R. Marshall
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- Time-resolved infrared studies of triplet 1,3-cyclopentanediyl
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Triplet-sensitized photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (1) in argon- or oxygen- saturated acetonitrile-d3 solutions results in the formation of bicyclo[2.1.0]pentane (3), a ring closure product arising from an intermediate 1,3-cyclopentanediyl triplet biradical (2). Time-resolved infrared (TRIR) spectroscopy was used to monitor the kinetics of bicyclopentane 3 production. This analysis provides a measurement of the triplet biradical lifetime and an estimate of the bimolecular reaction rate between biradical 2 and oxygen, both in good agreement with previous investigations. Our studies also indicate that certain IR bands due to 3 in the C-H stretching region overlap with corresponding bands in biradical 2. This interpretation is supported by computational investigations. Copyright
- Showalter, Brett M.,Bentz, Timothy C.,Ryzhkov, Lev R.,Hadad, Christopher M.,Toscano, John P.
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- Photochemistry of the Azoalkanes 2,3-Diazabicyclohept-2-ene and Spirodiazabicyclohept-2-ene>: On the Questions of One-Bond vs. Two-Bond Cleavage during the Denitrogenation, Cyclization vs. Rearrangement of the 1,3-Diradicals, and Double Inversion.
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The thermolysis and 350-nm and 185-nm photolyses of the azoalkanes 2,3-diazabicyclohept-2-ene (1a) and spirodiazabicyclohept-2-ene> (1b) have been investigated.The exo/endo stereochemistry in the bicyclopentanes 2a,b and in the rearranged olefin 3b was determined by deuteration experiments using 5,6-exo-dideuterioazoalkanes 1a,b-d2.Whereas thermal and direct photochemical (350 nm; n, ?*) denitrogenation of azoalkane 1a-d2 led exclusively (>99percent) to bicyclopentane 2a-d2 with preferential (1.54,2.94) double inversion, the triplet-sensitized photolysis afforded nearly complete stereoequlibration.In 185-nm denitrogenetion an unexpectedly high exo/endo ratio (3.1) for bicyclopentane 2a-d2 was found, besides isomerization to cyclopentene 2a-d2.Similar results were obtained in the denitrogenation of spiroazoalkane 1b-d2, which exhibited exo stereochemical preferences in both photoproducts spiropentane-5,1'-cyclopropane> 2b-d2 and bicyclohept-1-ene 3b-d2.The 350-nm photolysis of azoalkane 1b-d2 gave preferential formation of exo-spirobicyclopentane 2b-d2 and exo-olefin 3b-d2 whereas triplet-sensitized decomposition yielded almost complete loss of stereochemical preference in the olefin 3b-d2.The 185-nm photolysis of azoalkane 1b-d2 showed similar behavior compared with the azoalkane 1a, eg., at high exo/endo ratio in spirobicyclopentane 2b-d2.Also olefin 3b was formed with complete stereoequilibration.These diverse experimental results are discussed in terms of one-bond vs. two-bond cleavage processes leading to the diazenyl diradicals D'?,? and D'?,? in the case of low-energy activation (350-nm photolysis and thermolysis) and 1,3-cyclopentadiyls D?,? and D?,? on high-energy activation (185-nm activation).The relatively high degree of double inversion in the corresponding bicyclopentanes and the formation of rearranged cycloalkenes in the 185-nm photodenitrogenation is presumably a direct consequence of concerted two-bond cleavage via the formation of 1D?,? and zwitterionic states of the 1,3-diradical.A Salem diagram for one-bond and two-bond denitrogenation was most helpful in rationalizing these results mechanistically.
- Adam, Waldemar,Oppenlaender, Thomas,Zang, Gerald
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- Determination of the Enthalpy and Reaction Volume Changes of Organic Photoreactions Using Photoacoustic Calorimetry
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Photoacoustic calorimetry (PAC) can be used to measure both the thermal and reaction volume changes for photoinitiated reactions.The photoreactions of 2,3-diazabicyclohept-2-ene (DBH), diphenylcyclopropenone (DPC), and trans-stilbene (TS) are investigated by PAC.The resolution of these experimental volume changes is accomplished by either a temperature dependence or a binary solvent mixture method.The thermal volume changes yield the enthalpies of reaction in solution, which can be compared to literature values.In two cases (DPH and DPC), the values are moreendothermic than those predicted from gas-phase heats of formation.The differences can possibly be attributed to differential solvation of the reactants and products in the polar solvents employed.Absolute reaction volume changes for the photoreactions are also obtained for the photoreactions.PAC is a useful alternative technique to pressure-dependence studies to obtain this information.These volume changes can further be time-resolved to provide kinetic information about the photoprocesses.
- Herman, Michael S.,Goodman, Joshua L.
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- Dynamics of the Thermal Decomposition of 2,3-Diazabicyclo[2.2.1]hept-2-ene
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The dynamics of the thermal decomposition of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) have been investigated over the temperature range 900-1400 K. A tunable continuous wave CO laser was used to follow the vibrational energy content of the coupled CO-N2 system from the initial to the equilibrium conditions. It has been shown that the N2 is born with very little vibrational energy, while the bicyclo[2.1.0]pentane is born with an excess over the equilibrium distribution. These results provide clear evidence against the concerted decomposition mechanism.
- Simpson,Wilson,Adam
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- Viscosity dependence of the denitrogenation quantum yield in azoalkane photolysis: Experimental evidence for reversible formation of the diazenyl diradical
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(equation presented) Experimental evidence is reported for the reversible formation of the singlet diazenyl diradical (1DZ), photolytically generated from the structurally elaborate DBH-type azoalkane. Reversiblity of the 1DZ formation manifests itself through the decrease of the photodenitrogenation quantum yield over a ca. 40-fold viscosity variation (from 0.5 to 19.3 cP). This viscosity behavior is interpreted in terms of frictional effects on the competitive reaction modes of the diazenyl diradical.
- Adam, Waldemar,Corley, David A.,Trofimov, Alexei V.,White, Rick C.
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- Transition-metal-promoted chemoselective photoreactions at the cucurbituril rim
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When included in a supramolecular barrel with transition-metal ions as lids, bicyclic azoalkanes undergo phase-selective photolysis to afford new photoproducts and photoproduct distributions. In the presence of the macrocycle cucurbit[7]uril and Ag+ ions, 2,3-diazabicyclo[2.2.1]hept-2-ene forms a ternary host-guest inclusion complex in which the cations are coordinated to the carbonyl rims of the host. Direct photolysis of this ternary complex provides cyclopentene as a new photoproduct.
- Koner, Apurba L.,Marquez, Cesar,Dickman, Michael H.,Nau, Werner M.
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supporting information; experimental part
p. 545 - 548
(2011/03/16)
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- On the Mechanism of the Benzophenone-Sensitized Photolysis of 2,3-Diazabicyclohept-2-ene in the Laser Jet: Evidence for Intermolecular Triplet Diradical Reactions
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The benzophenone-sensitized laser jet photolysis of 2,3-diazabicyclohept-2-ene (1) affords, besides the previously reported cyclopentene and housane (2), also cyclopentane, cyclopentadiene, and the dimers bicyclopent-2-en-1-yl (7), 3-cyclopentylcyclopent-1-ene (8), and 1,1'-bicyclopentyl (9).As a model reaction, the pyrolysis of dimer 8 at 600 deg C/ 20 Torr leads to the other dimers 7 and 9 together with cyclopentadiene, cyclopentene, and traces of cyclopentane.Control experiments showed that H abstraction by the cyclopentane-1,3-diyl diradical (3) from cyclohexene (as model substrate for cyclopentene) and addition to housane (2) with formation of diradical 6 are unlikely pathways.Instead, the product data available can be best explained in terms of an intermolecular disproportionation of two diradicals 3 to give the cyclopent-2-en-1-yl (4) and cyclopentyl (5) radical pair, which is subsequently converted to the observed products by in-cage and out-of-cage coupling and H transfer reactions.Such intermolecular diradical chemistry becomes feasible due to the high steady-state concentrations (ca. micromolar) generated in the laser jet.Two-photon processes take place, but are of subordinate importance. - Key Words: Laser jet/ Azoalkane/ Diradical/ Radical coupling
- Adam, Waldemar,Finzel, Ralf,Walther, Barbara
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p. 2137 - 2142
(2007/10/02)
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- Preparation of Bicycloalkanes by Electrochemical Reduction of 1,3-Dibromocycloalkanes
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Good yields of bicyclo pentane (17) and bicyclo-hexane(3) are obtained on electroreduction of the corresponding 1,3-dibromocycloalkanes.The results do not depend upon the stereochemistry of the starting material (14 vs. 15 or 11 vs. 12).Key Words: Electrochemical reduction / Bicycloalkanes / Cycloalkanes, 1,3-dibromo / Hunsdiecker reaction / 1,3-Cycloalkanedicarboxylic acids
- Hoffmann, Joachim,Voss, Juergen
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p. 1415 - 1420
(2007/10/02)
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- Trapping of Cyclopentanediyl and Trimethylenemethane Triplet Diradicals with the Nitroxide 1,1,3,3-Tetramethyl-1,3-dihydroisoindolin-2-yloxyl
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Nitroxide trapping constitutes a convenient and effective alternative to dioxygen for the detection of triplet diradical intermediates.Thus, photolysis of the azoalkanes 1a-c in the presence of the nitroxide 1,1,3,3-tetramethyl-1,3-dihydroisoindolin-2-yloxyl produced the bisalkoxyamines 3a-c by trapping of the transient triplet diradicals 5a-c.The resulting bis-adducts 3a-c were fully characterized, and their regio- and stereochemistry were established on the basis of spectral and X-ray data for trans-3a and trans-3b.The novel bis-azoalkane 1d was prepared andits photochemical loss of nitrogen studied in the presence of the above nitroxide or dioxygen as scavengers.In the case of the nitroxide, the tetrakis adduct 3d was obtained, tentatively assigned in view of its thermal instability, while with dioxygen the stable bis-peroxide 4d was isolated and rigorously characterized.Instead of concurrent double denitrogenation to afford the high-spin non-Kekule species 5d or its low-spin quinoid diradical 5d', stepwise loss of dinitrogen and trapping is proposed to be the pathway to these products.
- Adam, Waldemar,Bottle, Steven E.,Finzel, Ralf,Kammel, Thomas,Peters, Eva-Maria,et al.
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p. 982 - 988
(2007/10/02)
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- Benzophenone-sensitized two-photon chemistry of azoalkanes in the "laser/ liquid jet": Evidence for photoinduced hydrogen 1,2-shift in 1,3-cyclopentadiyl triplet diradicals
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While the triplet-sensitized photolysis of the bicyclicazoalkanes 1 led on denitrogenation to the corresponding bicycloalkanes 2, the laser/liquid jet photochemistry gave additionally also the cyclopentenes 3 by hydrogen 1,2-shift; the amount of the latter increased with increasing lifetime of the intermediary cyclopentadiyl triplet diradicals 4, which suggests that two-photon chemistry may operate.
- Adam, Waldemar,Finzel, Ralf,Kita, Fumio
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p. 2211 - 2214
(2007/10/02)
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- BENZOPHENONE-SENSITIZED PHOTOLYSIS OF THE AZOALKANE DIAZABICYCLO(2.2.1)HEPT-2-ENE (DBH) : TRAPPING OF THE 1,3-CYCLOPENTADIYL TRIPLET DIRADICAL BY A NITROXIDE.
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The production and isolation of bis-alkoxyamines derived from the trapping of a 1,3-diradical is reported; the trapping of diradicals by nitroxides represents a new method in the identification and characterisation of these transient species.
- Adam, Waldemar,Bottle, Steven E.
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p. 1405 - 1408
(2007/10/02)
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- Ring opening and hydrogen atom transfer trapping of the bicyclo[2.1.0]pent-2-yl radical
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Relative rate constants (kr/kH) for ring opening of the bicyclo[2.1.0]pent-2-yl radical (1) to the cyclopent-3-enyl radical and trapping of radical 1 with t-BuSH, PhSH, and PhSeH in solvent THF were measured at temperatures between -78 and 50 °C. The hydrogen atom donors reacted more rapidly with radical 1 than with the cyclopropylcarbinyl radical (6). Rate constants for ring opening of 1 (kr) could be obtained by estimating the values of kH via Marcus theory. From initial kH values for reactions with radical 6, new kH values were calculated for increasingly exergonic reactions until the derived kr values from the three trapping agents agreed with one another and an extrapolated value of kr from Tempo trapping of 1. The results suggest that hydrogen atom transfer reactions with 1 were about 3 kcal/mol more exergonic than reactions with 6. Arrhenius functions for ring opening of 1 averaged log (ks/s-1) = 13.0 - 5.2/2.3RT; the value of kr at 25 °C is 1.5 × 109 s-1. Trapping studies of 1 and 6 with 2,6-dimethylthiophenol indicated that no special steric effects were present in hydrogen atom transfers to 1. However, highly stereoselective trapping of 1 was observed in reactions with ArSD with endo-bicyclo[2.1.0]pentane-2-d predominating, and the rate constant for decarboxylation of the endo-bicyclo[2.1.0]pentane-2-carboxy radical (endo-3) at -78 °C apparently was greater than that for decarboxylation of exo-3. The stereochemical results are ascribed to a stereoelectronic effect between the C1-C4 bond and endo-C2-X bonds of bicyclo[2.1.0]pentanes that weakens endo-C2-X bonds.
- Newcomb, Martin,Manek, M. Beata,Glenn, Anne G.
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p. 949 - 958
(2007/10/02)
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- Homolytic Rearrangements of Bicyclohexane and Bicycloheptane
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Free radicals abstract hydrogen from both the bridge and bridgehead sites in bicyclohexane (4).The bicyclohexan-1-yl radical was observed by e.p.r. spectroscopy.The bicyclohexan-2-yl radical rearranges by stereoelectronically forbidden β-scission to give cyclohex-3-enyl radicals.Unlike other cyclobutanes, compound (4) undergoes an SH2 reaction with bromine atoms.Free radicals abstract hydrogen only from the methylene groups of the C5 ring in bicycloheptane (15a).The bicycloheptan-2-yl radicals were observed by e.p.r. spectroscopy, as was their rearrangement, by stereoelectronically allowed β-scission, to 2-(cyclopent-2-enyl)ethyl radicals.Bromine atoms abstract hydrogen from (15a) and no SH2 reaction was detected.The radicals and their rearrangements were studied by semi-empirical MINDO/3 and MNDO methods.
- Walton, John C.
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p. 1371 - 1376
(2007/10/02)
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- Deuterium-Hydrogen Exchange and Scrambling Reactions in the Pyrolysis of Labeled Cyclopentene. A Radical Mechanism
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Inter- and intraradical mechanism that promote deuterium-hydrogen scrambling in labeled cyclopentene-d1 are investigated at 800 K and 1200 K.Rate constants are estimated for each step and for possible competing side reactions.The fast radical bimolecular exchange at 800 K and unimolecaular scrambling at 1200 K of labeled cyclopentene are shown to be faster than the Woodward-Hoffman allowed 1,4 concerted molecular elimination of hydrogen.The low-energy estimate of 8.0 kcal/mol by Lewis of the difference between the allowed 1,4 and disallowed 1,2 channels can thus be explained.No conclusions can be drawn concerning the 1,2 channel.The estimated rates of radical reactions are in agreement with experiments on the addition of D2 to cyclopentadiene at 300 deg C which shows only cis, 3-5 addition and with pyrolysis experiments at 500 +/- 20 deg C which show about 5percent contribution of a higher than first-order radical reaction.The mechanism of this radical decomposition is given and its steps are explicitly evaluated.
- Kosnik, Kenneth G.,Benson, Sidney W.
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p. 2790 - 2795
(2007/10/02)
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- THE 185-NM PHOTOCHEMISTRY OF 2,3-DIAZABICYCLOHEPTENE AND OF ITS DENITROGENATION PRODUCTS BICYCLOPENTANE AND CYCLOPENTENE
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The 185-nm denitrogenation of 2,3-diazabicycloheptene (1) afforded bicyclo pentane (2) and cyclopentene (3) presumably via a "hot" cyclopentane-1,3-diyl diradical (8); 1,4-pentadiene (4) and methylenecyclobutane (5) were secondary products of the 185-nm photolysis of (2) and (3).
- Adam, Waldemar,Oppenlaender, Thomas
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p. 5391 - 5394
(2007/10/02)
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- DIRECT PHOTOLYSIS AT 185 NM OF CYCLOPENTENE AND 2-NORBORNENE. A NOVEL REACTION CHANNEL FOR ?, ?* EXCITED SINGLET ALKENE
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Upon direct irradiation at 185 nm, the title compounds undergo hydrogen abstraction from the solvent via a ?, ?* excited singlet state as well as skeletal rearrangement via a ?,R(3s) Rydberg excited state.
- Inoue, Yoshihisa,Mukai, Toshio,Hakushi,Tadao
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p. 1045 - 1048
(2007/10/02)
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- Kinetics of the Thermal Skeletal Inversion of Bicyclopentane and Methylbicyclopentanes
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cis-exo-2,3-Dideuteriobicyclopentane, cis-exo-2,3-dideuterio-1-methylbicyclopentane, and the endo and exo isomers of 5-methylbicyclopentane have been prepared; rate constants for the gas-phase equilibrations between these substrates and the corresponding ring-inverted isomers have been determined between 153 and 208 deg C.The parent hydrocarbon and 1-methylbicyclopentane have equal or nearly equal activation energies, 37.8 +/- 0.1 and 38.0 +/- 0.4 kcal/mol; the 2-methyl and 5-methyl systems have slightly higher Ea values, 38.7 to 39.2 kcal/mol.These data and related information from the literature are assessed in terms of electronic and steric substituent effects on ground-state bicyclopentanes and transition states leading to planar 1,3-cyclopentadiyl diradicals.
- Baldwin, John E.,Ollerenshaw, James
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p. 2116 - 2119
(2007/10/02)
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