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1.
When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S2(π,π*) state, certain unimolecular reactions can be observed to compete with S2 → S1 internal conversion. These reactions do not occur from the S1(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8 , 9 → 32 + 33 ), γ → α formylacetal migration (cf. 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*-induced transformations is placed energetically higher than the S1(n,π*) state, and it is either identical with the thermally equilibrated S2(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*-induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones. 相似文献
2.
The two components of the dual phosphorescence of 1-indanone ( 1 ) and six related ketones ( 2–7 ) possess different excitation spectra exhibiting the vibrational progression characteristic of the S0 → S1 (n, π*) transition (shorter-lived emission) and two bands of the S0 → S2 and 3 (π,π*) 0–0 transitions, respectively. The most favorable intersystem crossing routes are S1 (n, π*) → T (n, π*) and S2,3 (π*) → T (π, π*). Internal conversion to S1 competes more effectively with S (π, π*) → T (π, π*) intersystem crossing only from higher vibrational levels of the S2 and S3 states. 相似文献
3.
The α,β-unsatured ketone 10α-testosterone has been reported previously [6] to photoisomerize in t-butanol solution to the β,γ-unsaturated ketone. The irradiation had been carried out using a high-pressure mercury lamp in a quartz vessel. For structural reasons this double bond shift cannot proceed through a photoenolization mechanism involving an intramolecular hydrogen transfer from the γ-position to the enone oxygen as has been suggested to operate in several formally analogous cases of aliphatic enone isomerizations. In the present reinvestigation, O-acetyl 10α-testosterone ( 1 ) was used, employing selectively either excitation of its n → π* (with wavelengths > 300 nm) or its π → π* absorption band (with 253,7 nm). In t-butanol solution the doublebond shift 1 → 2 could be effected with π→* excitation only. Experiments in deuterated solvent (t-BuOD) resulted in deuterium in corporation in both the δ5-ketone in the C(4)-position, cf.( 3 ) and in the conjugated ketone. These results indicate that the reactions is initiated either in the, Sπ,π* state or in a high vibrational mode of the S0 or tππ*state. n→ π* Excitation of 1 in t-butanol gave essentially no over-all chemical change, while in benzene solution it resulted again in a double bond isomerization ( 1 → 2 ). In analogy to results with similar enones [28] under identical conditions the deconjugation in benzene may be the consequence of an intermolecular hydrogen abstraction of the Tn,π* excited state of the enone. Another specifically π →π* induced photoreaction was observed on irradiation of the β, γ-unsaturated ketone 2 in t-BuOD with 253,7 nm. The olefinic hydrogen at C-6 of 2 was exchanged with deuterium and, to a small extent, isomerization to the conjugated ketone 1 with concomitant deuterium incorporation occurred. It is concluded that from the higher excited state of the β, γ-unsaturated ketone, but not from its Sn,π* state, an activation mode of the double bond is accessible to effect D+ addition at C-6 followed by deprotonation to 4 and to deuterated 1 , respectively. 相似文献
4.
4a-(Z-1-Propenyl)-bicyclo[4.4.0]dec-1(8a)-en-2-one ( 6 ) and 4a-(Z-1-propenyl)-bicyclo[4.4.0]deca-1(8a), 7-dien-2-one ( 17 ) undergo an intramolecular hydrogen transfer from the methyl group of the propenyl substitutent to the α-carbon atom of the enone group, and cyclization to the [4.4.3]propellane derivatives 9 and 30 , respectively, when excited in the π → π* wavelength region. The quantum yield for (Z)- 6 → 9 under optimum conditions is 0.29 at 254 nm. These reactions occur specifically from the S2 (π,π*) state, competing with the S2 → T decay. The triplet reactions of 6 are E–Z double-bond isomerization, double-bond shift to (E,Z)- 8 , and rearrangement to (E)- 10 . Further investigations concern some structural limitations in the scope of the reaction type 6 → 9 and enone S2 reactivity in general. 相似文献
5.
By CNDO-CI calculations we have found that dicarbonyl compounds exhibit only two n → π* transitions in the visible or near UV. region, instead of four as expected from simpler MO-models. The dominant features of the long-wavelength electronic spectra may be characterized by the relative energy of the two n and the two lowest π* orbitals. In general we distinguish between three cases:
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6.
Martin Demuth Christopher O. Bender Slivia E. Braslavsky Helmut Grner Ulrich Burger Walter Amrein Kurt Schaffner 《Helvetica chimica acta》1979,62(3):847-851
8-Benzoyl-9-deuterio-naphtho [de-2.3.4]bicyclo [3.2.2]nona-2,6,8-triene ( 1 ) rearranged quantitatively in a photochemical di-π-methane-type process to 2-, 6-, and 9-deuteriated 1-benzoyl-naphtho [de-2.3.4]tricyclo [4.3.0.02,9]nona-2, 6-diene ( 8a–c ). The phenylhydroxymethyl analogue 2 underwent a similar regioselective rearrangement to 9a–c . The rearrangement 1 → 8a–c is proposed to proceed along three reaction paths evolving from two primary photochemical processes of naphthylvinyl and vinyl-vinyl bonding ( 1 → 3 + 6 ). Evidence for a competition between several paths and involvement of biradical intermediates derives from changes in the isotopomeric composition with temperature, and from laser flash detection (λexc 353 nm) of a transient. The dependence of the quantum yield for product formation from 1 on excitation wavelength and sensitizer triplet energy leads to the conclusion that reaction to the primary biradicals occurs directly from the S1 (n, π*) and T2 (n, π* ) states, and that reaction from T1 (π, π*) and from S2 (π, π*) proceed either directly or via T2. 相似文献
7.
Jean Gloor Grald Bernardinelli Raymond Gerdil Kurt Schaffner 《Helvetica chimica acta》1973,56(7):2520-2523
Irradiations at 254 nm of the α,β-unsaturated γ-dimethoxy-methyl ketone 7 in iso-octane and t-butyl alcohol afforded in a specifically π→π*induced process and in high chemical yield the epimeric products 9 and 10 . These products were not formed on n→π* excitation of 7 at > 340 nm, but triplet energy transfer to 1,3-cyclohexadiene could be observed. Photolyses of the hexadeuterio analog 7-d6 at 254 nm led to the fully deuteriated products (cf. 9-d6 ) in both solvents, with stereospecific incorporation of a deuterium atom in position C(1α). The structures of 9 and 10 were determined by an X-ray diffraction analysis of 9 and chemical correlations of the two products. The structural constraints in 7 demand a hitherto unprecedented direct transfer of a methoxyl hydrogen to the α-carbon of the excited enone and formation of intermediate 8 . 相似文献
8.
Herbert Eichenberger Kentaro Tsutsumi Guy de Weck Hans Richard Wolf 《Helvetica chimica acta》1980,63(6):1499-1519
Photochemistry of Conjugated δ-Keto-enones and β,γ,δ,?-Unsaturated Ketones On 1π,π*-excitation the δ-keto-enones 5–8 are isomerized to compounds B ( 18 , 22 , 26 , 28 ) via 1,3-acyl shift and to compounds C ( 19 , 23 , 27 , 29 ) via 1,2-acyl shift, whereas the β,γ,δ,?-unsaturated ketone 9 gives the isomers 32 and 33 by 1,2-and 1,5-acyl shift, respectively. Furthermore, isomerization of 6 to 24 , dimerization of 8 to 30 and addition of methanol to 8 ( 8 → 31 ) is observed. Unlike 7 and 8 the acyclic ketones 5 , 6 and 9 undergo photodecarbonylation on 1π,π*-excitation ( 5 → 20 , 21 ; 6 → 20 , 25 ; (E)- 9 → 35–38 ). Evidence is given, that the conversion to B as well as the photodecarbonylation of 5,6 and 9 arise from an excited singulet state, but the conversion to C as well as the dimerization of 8 from the T1-state. 相似文献
9.
Irradiation in the n→π* absorption band of the α,β-unsaturated γ,δ-epoxyketone 5 in ethanol at ?65° exclusively afforded the rearranged ene-dione 13 , whereas at + 24° under otherwise unchanged reaction conditions or upon triplet sensitization with Michler's ketone and with acetophenone at + 24° essentially identical mixtures of 13 (major product), 14 , and 15 were obtained. Selective π→π* excitation of 5 at ?78° and + 24° led to similar product patterns. The 9β,10β-epimeric epoxyketone 7 selectively isomerized to 14 and 15 at + 24° and n → π* or π → π* excitation. Neither the epoxyketones 5 and 7 nor the photoproducts 13–15 were photochemically interconverted. In separate photolyses each of the latter gave the double bond isomers 16 , 18 , and 19 , respectively. Cleavage of 13 to the dienone aldehyde 17 competed with the double bond shift ( → 16 ) when photolyzed in alcoholic solvents instead of benzene. The selective transformations 5 → 13 (at ?65° and n → π* excitation) and 7 → 14 + 15 are attributed to stereoelectronic factors facilitating the skeletal rearrangements of the diradicals 53 and 55 , the likely primary photoproducts resulting from epoxide cleavage in the triplet-excited compounds 5 and 7 , via the transition states 54 , 56 , and 57 . The loss of selectivity in product formation from 5 at higher temperature and n → π* excitation or triplet sensitization is explicable in terms of radical dissociation into 58 and 59 increasingly participating at the secondary thermal transformations of 53 . The similar effect of π → π* excitation even at ?78° indicates that some of the π,π* singlet energy may become available as thermal activation energy. It is further suggested that the considerably lesser ring strain in 14 and 15 , as compared with 13 , is responsible that selectivity in product formation from 7 is maintained also at +24° and at π → π* excitation. 相似文献
10.
The photochemistry of the conjugated cyclohexenones O-acetyl testosterone ( 1 ) and 10-methyl-Δ1,9-octalone-(2) ( 24 ) has been investigated in detail. The choice of reaction paths of both ketones depends strongly on the solvent used. In t-butanol, a photostationary equilibrium 1 ? 3 is reached which is depleted solely by the parallel rearrangement 1 → 5 (Chart 1; for earlier results on these reactions see [2a] [6] [7]). In benzene, double bond shift 1 → 16 (Chart 3) occurs instead, which is due to hydrogen abstraction from a ground-state ketone by the oxygen of an excited ketone as the primary photochemical process. In toluene, the major reaction is solvent incorporation ( 1 → 17 , Chart 4) through hydrogen addition to the β-carbon of the enone, accompanied by double bond shift and formation of saturated dihydroketone as the minor reactions. Contrary in part to an earlier report [19], the photochemical transformation of the bicyclic enoné 24 exhibit a similar solvent dependence. The corresponding products 25 – 29 are summarized in Chart 5 and Table 1. Sensitization and quenching experiments established the triplet nature of the above reactions of 1 and 24 . Based on STERN -VOLMER analyses of the quenching data (cf. Figures 2, 4–8, and Table 3), rearrangement, double bond reduction and toluene addition are attributed to one triplet state of the enones which is assigned tentatively as 3(π, π*) state, and the double bond shift is attributed to another triplet assigned as 3(n, π*) state (cf. Figure 9). The stereospecific rearrangement of the 1α-deuterated ketone 2 to the 4β-deuterio isomer 4 shows the reaction to proceed with retention at C-1 and inversion at C-10. The 4-substituted testosterone derivatives 33 – 36 (Chart 8) were found to be much less reactive in general than 1 . In particular, 4-methyl ketone 33 remains essentially unchanged on irradiation in t-butanol, benzene and toluene. 相似文献
11.
Photolysis of Conjugated Epoxy-dienes Direct and sensitized excitation of the (E)-β-ionylidene-epoxides 1 and 4 leads to different types of isomerizations. Thus photocycloelimination to the cyclopropene-ketones 2 and 6 is only achieved by 1(π, π*)-excitation (λ=254 nm), whereas 3(π, π*)-excitation (λ > 280 nm, acetone) gives selective C(1′), O-cleavage of the oxirane ( 1 → 7 – 10 and 4 → 11 – 13 ). In contrast to 1 the twofold methylsubstituted epoxy-diene 4 shows mainly (E/Z)-isomerization ( 4 → 5 ) on both 1(π, π*)- and 3(π, π*)-excitation while the isomerizations 4 → 6 and 4 → 11 – 13 are minor processes, only. 相似文献
12.
Ari Lahti Matti Hotokka Kari Neuvonen Gunnar Karlstrm 《International journal of quantum chemistry》1999,72(1):25-37
The photochemical cis/trans isomerization of urocanic acid (UCA, (E)‐3‐(1′H‐imidazol‐4′‐yl)propenoic acid) was investigated using complete active space SCF (CASSCF) ab initio calculations. The singlet ground state and the triplet and the singlet manifolds of the lowest‐lying π→π* (HOMO→LUMO) excitation of the neutral and the anionic UCA were calculated using the 6‐31G* and the 6‐31+G* basis sets, respectively. The torsional barrier of the double bond of the propenoic acid moiety in UCA is observed to be considerably lower in the T1 and S1 excited states of the neutral UCA and in the T1 but not in the S1 excited state of the anionic UCA, as compared to the S0 state of the respective protonation form. The cis‐isomer of both the neutral and the anionic UCA is lower in energy than the trans‐isomer in the S0, T1, and S1 states. This energy difference is larger in the excited states than in the ground state, probably due to strengthening of the intramolecular hydrogen bond of cis‐UCA as the molecule is excited. The results of the calculations, interpreted in terms of the idea that UCA is deprotonated upon electronic excitation, led to construction of a new model for the photoisomerization mechanisms of UCA. According to this model, the trans‐to‐cis isomerization proceeds via both the triplet and the singlet manifolds in the deprotonated form of UCA. This isomerization may occur in the S0 state of the neutral UCA as well. The cis‐to‐trans isomerization is suggested to proceed only in the S0 state of the neutral UCA. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 25–37, 1999 相似文献
13.
J. Pitarch‐Ruiz S. Evangelisti D. Maynau 《International journal of quantum chemistry》2005,101(3):325-333
Localized orbitals have recently been employed in large ab initio calculations, but their use has generally been restricted to ground‐state problems. In this work, we analyze the molecular orbitals of the excited states, optimized with a recently proposed local procedure. This method produces local orbitals of the CAS–SCF type, which permits its application to the study of excited states. In particular, we focus on the π→π* triplet excited state in polyenes, calculated using a 2/2 CAS space which includes two electrons in one π and one π* orbitals. In small polyenes, these two singly occupied active orbitals are delocalized all along the molecule. The extent of the delocalization is analyzed by studying polyenes of increasing size. Different polyenes have been studied, going from C14H16 to the C70H72 polyene. The relation of the π→π* excitation with the cation and anion systems is also discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
14.
Bruno Frei Herbert Eichenberger Beat Von Wartburg Hans Richard Wolf Oskar Jeger 《Helvetica chimica acta》1977,60(8):2968-3006
Photoinduced Vinylogous β-Cleavage of Epoxy-enones of the Ionone Series The photochemistry of the α,β-unsaturated γ,δ-epoxy-enones 1–3 is determined by: (i) C(γ)-O-scission of the epoxide (vinylogous β-cleavage of Type A); (ii) C(γ)-C(δ)-cleavage of the oxirane (vinylogous β-cleavage of Type B); (iii) (E/Z)-isomerization of the enone chromophore. In contrast, 4 with tertiary C(β) shows no Type B cleavage. Type A cleavage is induced both by n,π*- and π,π*-excitation and arises probably from the T1-state, but Type B cleavage is observed only on π,π*-excitation and represents presumably a S2-reaction. On Type A cleavage 1–4 undergo 1,2-alkyl-shifts to 1,5-dicarbonyl compounds ( 15–18, 25–28, 34 and 35 ) or rearrange to dihydrofuranes ( 7 and 30 ). The isomerization 1→7 proceeds by a stereoselective [1,3]-sigmatropic shift. On Type B cleavage 1–3 isomerize to a bicyclic enol-ether ( 8, 29 ) or to a monocyclic enol-ether ( 9 ; product of a homosigmatropic [1,5]-shift) or undergo fragmentation to isomers such as allenes 10, 22 and 31 or cyclopropenes 11 and 21 . The non-isolated, unstable (Z)-epoxy-enones 14, 19, 24 and 38 isomerize by fragmentation to the furanes 12, 23, 33 and 39 respectively, on contact with traces of acid or by heating. However, for 19 and 4 , Type B cleavage may lead to the furanes 23 and 39 . On UV. irradiation of the epoxy-enone 4 the initially formed (E/Z)-isomers 34 and 35 yield on π,π*-excitation the enones 37 and 40 by a vinylogous β-fragmentation. In addition, on n,π*-excitation 34 isomerizes to 35 , which decarbonylates exclusively to the enone 37 . The reactions of 1–4 with BF3 · O(C2H5)2 were also studied (see appendix). The epoxy-enones 1 and 2 isomerize by an 1,2-alkyl shift in good yield to the 1,4-dicarbonyl compounds 79 and 81 , whereas 3 gives the 1,4-diketone 83 , and in small amounts the 1,5-diketone 84 . On the other hand, 4 is converted to the fluorohydroxy-enone 85 and to the 1,5-dicarbonyl product 34 , the only isomer in this series which is identical with one of the photoproducts. 相似文献
15.
Janet E. Del Bene 《Journal of computational chemistry》1981,2(2):200-206
Hydrogen bonding of uracil with water in excited n → π* states has been investigated by means of ab initio SCF -CI calculations on uracil and water–uracil complexes. Two low-energy excited states arise from n → π* transitions in uracil. The first is due to excitation of the C4? O group, while the second is associated with excitation of the C2? O group. In the first n → π* state, hydrogen bonds at O4 are broken, so that the open water–uracil dimer at O4 dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N3? H and a cyclic structure at N3? H and O4 in the ground state, collapses to a different “wobble” dimer at N3? H and O2 in the excited state. The third dimer, a “wobble” dimer at N1? H and O2, remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O2 are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N1? H and O2 changes to an excited open dimer at N1? H. The “wobble” dimer at N3? H and O4 remains intact, and the open dimer at O4 is further stabilized upon excitation. Dimer blue shifts of n → π* bands are nearly additive in 2:1 and 3:1 water:uracil structures. The fates of the three 2:1 water:uracil trimers and the 3:1 water:uracil tetramer in the first and second n → π* states are determined by the fates of the corresponding excited dimers in these states. 相似文献
16.
R. Daudel H. Le Rouzo R. Cimiraglia J. Tomasi 《International journal of quantum chemistry》1978,13(4):537-552
17.
It is shown that the correlation of the π-ionization potentials of ethylene ( 1 ), butadiene ( 2 ) and trans-1,3,5-hexatriene ( 4 ) favours the orbital sequence π, π, σ in butadiene and π, π, σ, π in the hexatriene in excellent agreement with the results of SPINDO calculations. Within the experimental error the π-ionization potentials of cis-1,3,5-hexatriene ( 3 ) and trans-1,3,5-hexatriene ( 4 ) are the same. Methyl-substitution of 2 lowers the π-ionization potentials I1(π) and I2(π). For 1 and/or 4 substitution the difference I2(π)?I1(π) remains constant (≈ 2.5 eV). On the other hand 2 and/or 3 substitution leads to a smaller gap of I2(π)–I1(π) ≈ 1.6 to 2.0 eV without changing the mean π-ionization potential I (π) relative to the corresponding 1,4 substituted derivatives. This result is rationalized in terms of a through space interaction between double bond π-orbitals and non-bonded pseudo-π-orbitals of the substituting methyl groups. The reduced split I2(π)–I1(π) in cyclopentadiene is attributed to hyperconjugation across the methylene group. 相似文献
18.
Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the 1A1(π → π*) state. The 1A1(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing. 相似文献
19.
The syntheses of Co(π-all)(PF3)2(PPh3) complexes (π-all = π-allyl, anti-1-Me-π-allyl, syn-1-Me-π-allyl, 1,1-dimethyl-π-allyl, anti-1,2-diMe-π-allyl, syn,syn-1,3-diMe-π-allyl, 2Et-π-allyl, π-cyclooctenyl, h3-π-cycloheptadienyl) are described. 1H and 19F NMR data are presented and discussed in relation to the structures of the complexes. The compound Co(π-C5H9)(PF3)(PPh3)2 is also reported. Several of the π-allylic complexes are found to be active catalysts for the isomerisation of 1-octene to 2-octene under a hydrogen atmosphere. 相似文献
20.
Enantiomerically enriched samples of 1,2-dimethyl-3-(2H3)methyl-2-cyclopentenyl (1) and 1-(2H3)methyl-2,3-dimethyl-2-cyclopentenyl methyl ketones ( 2 ) have been irradiated at 313 nm in methanol in the temperature range +50 to ?45°. The 1,3-acetyl shift, which interconverts the two isomeric ketones, occurs with a small change in the enantiomeric composition and independently of temperature in the range studied. This change corresponds to an upper limit of approximately 20% reaction with racemization. It is proposed that reaction occurs from both the S1(n, π*) and T2(n, π*) excited states with stereospecific reaction from S1 (rapid primary geminate recombination of a singlet radical pair, with a possible contribution by a concerted 1,3 shift) dominating throughout the temperature range, but with the proportion of reaction from T2 increasing as the temperature is lowered. The racemization results from secondary geminate recombination of the singlet and triplet radical pairs and the random recombination of free radicals. Viscosity effects are proposed to explain the independence of the racemization on temperature. 相似文献