4,6-Dimethyl-α-pyrone: a matrix isolation study of the photochemical generation of conjugated ketene, Dewar valence isomer and 1,3-dimethyl-cyclobutadiene (original) (raw)
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Physical Chemistry Chemical Physics, 2004
A combined matrix isolation FTIR and molecular orbital study on the photochemistry of alpha-pyrone (AP) was undertaken. The most efficient of the observed photoprocesses was the Norrish type I, ring opening reaction leading to conjugated ketene. Upon irradiation of matrix isolated AP with UV (l > 285 nm) light, rapid formation of Z isomers of the aldehyde-ketene was observed. After a few minutes of irradiation the IR bands assigned to these photoproducts stopped growing. Further irradiation generated, by excited state internal rotations around the central C=C bond in the Z forms, new aldehyde-ketene isomers, which were assigned to the E forms of the conjugated ketene. These forms were identified in the present work for the first time. Upon subsequent UV (l > 337 nm) irradiation, the Z aldehyde-ketene isomers reverted back to the closed-ring, initial form of AP, while the E forms did not react further. The ring opening reaction occurring upon UV (l > 285 nm) irradiation was accompanied by very slow valence isomerization of AP to the Dewar form (2-oxa-3-oxobicyclo[2.2.0]hex-5-ene; OOBH). Observation of cyclobutadiene, produced by shorter wavelength UV irradiation (l > 235 nm) of OOBH, was also achieved in the present study. The structure of the cage confined complex of cyclobutadiene with CO 2 was theoretically investigated. The possibility of formation of 4-formyl-2-cyclobutene-1-one, by photochemical ring closure of aldehyde-ketene isomers with the central C=C bond and the ketene group in s-cis position with respect to each other is also discussed. PCCP www.rsc.org/pccp R E S E A R C H P A P E R T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4 P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 9 2 9 -9 3 7 929
Chemical Physics Letters, 2006
Photochemical transformations of methyl coumalate have been studied by matrix-isolation technique. Two photoreactions were induced by UV (k > 295 nm) light: isomerisation to the Dewar form and a-bond cleavage leading to the open-ring aldehyde-ketene. The first photoprocess was found to be strongly dominating. Experimental evidence of photoreversibility of both photoisomerisation processes has been provided. Upon k > 200 nm irradiation, the photochemically formed monomeric Dewar species underwent decarboxylation, with production of methoxycarbonyl-substituted cyclobutadiene. This antiaromatic photoproduct was experimentally observed for the first time. All the photoproduced species were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-311++G(d,p) level.
Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy, 2016
The structure, infrared spectrum, barrier to internal rotation, and photochemistry of 4-pyridinecarboxaldehyde (4PCA) were studied by low-temperature (10K) matrix isolation infrared spectroscopy and quantum chemical calculations undertaken at both Moller-Plesset to second order (MP2) and density functional theory (DFT/B3LYP) levels of approximation. The molecule has a planar structure (Cs point group), with MP2/6-311++G(d,p) predicted internal rotation barrier of 26.6kJmol(-1), which is slightly smaller than that of benzaldehyde (~30kJmol(-1)), thus indicating a less important electron charge delocalization from the aromatic ring to the aldehyde moiety in 4PCA than in benzaldehyde. A complete assignment of the infrared spectrum of 4PCA isolated in an argon matrix has been done for the whole 4000-400cm(-1) spectral range, improving over previously reported data. Both the geometric parameters and vibrational frequencies of the aldehyde group reveal the relevance in this molecule of th...
Infrared Spectra and Photochemistry of Matrix-Isolated Pyrrole-2-carbaldehyde
The Journal of Physical Chemistry A, 2010
Monomeric pyrrole-2-carbaldehyde (P2C) was isolated in low-temperature argon and xenon matrices, and its UV-induced photochemistry was studied. The structures of the reagent as well as the reaction photoproducts were characterized by FTIR spectroscopy. Interpretation of the experimental results was assisted by theoretical calculations carried out at the MP2 and DFT (B3LYP) levels with the 6-311++G(d,p) basis set. The compound can assume two conformations, cis and trans, regarding the orientation of the N-C-CdO dihedral angle. The cis form is the conformational ground state, being more stable than the trans by ca. 15 kJ mol-1. The relative stability of the two conformers was analyzed based on the comparison of their structures and using the natural bond orbital method. In agreement with the calculations, only the signature of the cis conformer was found in the experimental FTIR spectra of matrix-isolated P2C monomers. UV irradiation (λ > 235 nm) readily converts the cis-P2C into the trans-P2C form, and a photostationary equilibrium is established where the [cis]/[trans] ratio is ca. 3.3:1 in both Ar and Xe. Upon prolonged irradiation, P2C slowly undergoes photolysis to [pyrrole + CO]. In matrices, pyrrole and CO form associates of different geometry, which could be characterized based on their vibrational signatures.
Journal of Electron Spectroscopy and Related Phenomena, 1979
to report the photoelectron spectra of 2-oxa-blcyclo[2 2.01 hex-5-en-3-one (1) [ 21 and 2H-pyran-a-one (ar-pyrone) (2) [ 21 and the thermal behavior of these compounds up to 8OO*C under the condltlon of vmabletemperature photoelectron spectroscopy (VTPES) 133 . This study was tnggered by our mterest m the photoelectron spectra of cyclobutadlenes (e.g ,3 and 4) [ 4] , and evrdence that thermolysls of la&one (1) @ves cyclobutadlene (5) [5] (besrdes prevdmg amounts of cw-pyrone (2)) and heatmg of cw-pyrone mtermedlate ketene aldehydes 6) [6]. For sunllar photochemlcal work m solid matices the reader ti referred to ref. 7 1 2 3 4 5 6 * Part 81 of Theory and Apphcatlon of Photoelectron Spsctrokepy, for Part 80 lbe ref 1
J Am Chem Soc, 1979
Although the photochemical behavior of the carbonyl group has received much scrutiny in the past d e~a d e ,~.~ relatively little is known about the photochemistry of the small amount of enol tautomer which exists in equilibrium with the keto In an earlier reportS dealing with the photorearrangement of 4-phenyl-3-chromanone (1) to 4-phenyldihydrocoumarin (2),6 evidence was presented which demonstrated that the enol content can be an overriding factor in determining the quantum efficiency of a photoreaction. As part of our continuing studies dealing with enol photoc h e r n i~t r y ,~~~ we have undertaken an investigation of the excited state behavior of the 4-carbomethoxy-3-chromanone (3) system. W e now report that in extending our studies to this system, we have discovered an unusual solvent effect which controls the product distribution. In addition, we have uncovered a n unprecedented photodecarbonylation reaction which can best be explained by invoking the intermediacy of a transient cyclopropanone.
To clarify the cis-trans isomerization mechanism of simple alkenes on the triplet excited state surface, the photochemistry of acyclic and cyclic vinylketones with a pmethoxyacetophenone moiety as a built-in triplet sensitizer (1 and 2, respectively) was compared. When irradiated, ketone 1 produces its cis-isomer whereas ketone 2 does not yield any photoproducts. Laser flash photolysis of ketone 1 yields a transient spectrum with max ~ 400 nm ( ~ 125 ns). This transient is assigned to the first triplet excited state (T 1) of 1, which presumably decays to form a triplet biradical (1BR) that is shorter lived than the triplet ketone. In comparison, laser flash photolysis of 2 reveals two transients ( ~ 20 and 440 ns), which are assigned to T 1 of 2 and triplet biradical 2BR, respectively. Density functional theory (DFT) calculations support the characterization of the triplet excited states and the biradical intermediates formed upon irradiation
Journal of the American Chemical Society, 1971
Mechanistic studies on the photorearrangements of benzobicyclo[2.2.2]octadienone (1) and bicyclo-[2.2.2]octenone (5) are reported. The limiting quantum yields for the singlet reaction of 1 are: &is = 0.50; @nsphthslene = 0.45; ~4,5-benLobie,c~o[l.l.0100ta-2,1-dion.8~0~~ = 0.10. The limiting quantum yields for the acetophenonesensitized reaction are: adis = 0.14; anaphthslene = 0.01 ; ~b e n r o t r i o y e l o [ 3 . 3 . 0 , 0 2~8~~~t e n~3 . 0 n e = 0.12. A deuterium labeling experiment established the 1,2-acyl migration mechanism for the photosensitized rearrangement of 1. Finally, it was shown that the triplet reaction proceeded by a concerted, symmetry-allowed process by comparison with the photoisomerization of lactone 12.
Photochemistry of 1-(2,6-dichlorobenzyl)-1,4-dihydronicotinamide
Tetrahedron Letters, 1985
1,4,5,6-tetrahydropyridines Via and VIb were obtained. The investigation on the photochemistry of NADH and model compounds is centered around the electron-transfer processes and the reducing properties of these substances.' Few works are devoted to the photocycloadditions of 1,4-dihydropyridines which take place in nonoxidizing conditions, with high regio-and stereo-specificity: 1,4-dihydropyridine-3,5-dicarboxylate dimerizes giving head-to-tail-= and head-to-tail-anti dimers,2 1-benzyl-1,4-dihydronicotinamide (BNAH) give head-to-tail--dimer 3 or, in the presence of acrylonitrile, photoadducts across the 2-3 double bond. 4