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H-atom Transfer (h-atom + transfer)

Distribution by Scientific Domains

Chemistry	60%

Selected Abstracts

Multidimensional Reaction Coordinate for the Excited-state H-atom Transfer in Perylene Quinones: Importance of the 7-Membered Ring in Hypocrellins A and B

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2000
Anindya Datta
ABSTRACT The excited-state intramolecular H-atom transfer reactions of hypocrellins B and A are compared by using time-resolved absorption and fluorescence upconversion techniques. The hypocrellin B photophysics are well described by a simple model involving one ground-state species and excited-state forward and reverse H-atom transfer with a nonfluorescent excited state. We suggest that excited-state conformational changes are coupled to the H-atom transfer in hypocrellin B just as gauche/anti changes are coupled to the H-atom transfer in hypocrellin A. [source]

Rearrangements in Model Peptide-Type Radicals via Intramolecular Hydrogen-Atom Transfer

HELVETICA CHIMICA ACTA, Issue 10 2006
Damian Moran
Abstract Intramolecular H-atom transfer in model peptide-type radicals was investigated with high-level quantum-chemistry calculations. Examination of 1,2-, 1,3-, 1,5-, and 1,6[C,,,N]-H shifts, 1,4- and 1,7[C,,,C]-H shifts, and 1,4[N,,,N]-H shifts (Scheme,1), was carried out with a number of theoretical methods. In the first place, the performance of UB3-LYP (with the 6-31G(d), 6-31G(2df,p), and 6-311+G(d,p) basis sets) and UMP2 (with the 6-31G(d) basis set) was assessed for the determination of radical geometries. We found that there is only a small basis-set dependence for the UB3-LYP structures, and geometries optimized with UB3-LYP/6-31G(d) are generally sufficient for use in conjunction with high-level composite methods in the determination of improved H-transfer thermochemistry. Methods assessed in this regard include the high-level composite methods, G3(MP2)-RAD, CBS-QB3, and G3//B3-LYP, as well as the density-functional methods B3-LYP, MPWB1K, and BMK in association with the 6-31+G(d,p) and 6-311++G(3df,3pd) basis sets. The high-level methods give results that are close to one another, while the recently developed functionals MPWB1K and BMK provide cost-effective alternatives. For the systems considered, the transformation of an N-centered radical to a C-centered radical is always exothermic (by 25,kJ,,,mol,1 or more), and this can lead to quite modest barrier heights of less than 60,kJ,,,mol,1 (specifically for 1,5[C,,,N]-H and 1,6[C,,,N]-H shifts). H-Migration barriers appear to decrease as the ring size in the transition structure (TS) increases, with a lowering of the barrier being found, for example when moving from a rearrangement proceeding via a four-membered-ring TS (e.g., the 1,3[C,,,N]-H shift, CH3C(O)NH.,,,.CH2C(O)NH2) to a rearrangement proceeding via a six-membered-ring TS (e.g., the 1,5[C,,,N]-H shift, .NHCH2C(O)NHCH3,,,NH2CH2C(O)NHCH2.). [source]

Multidimensional Reaction Coordinate for the Excited-state H-atom Transfer in Perylene Quinones: Importance of the 7-Membered Ring in Hypocrellins A and B

Hydrogen bonding and ,,, interactions in 1-benzofuran-2,3-dicarboxylic acid and its 1:1 cocrystals with pyridine, phenazine and 1,4-phenylenediamine

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2009
Hatem M. Titi
The structure of 1-benzofuran-2,3-dicarboxylic acid (BFDC), C10H6O5, (I), exhibits an intramolecular hydrogen bond between one ,COOH group and the other, while the second carboxyl function is involved in intermolecular hydrogen bonding to neighbouring species. The latter results in the formation of flat one-dimensional hydrogen-bonded chains in the crystal structure, which are ,,, stacked along the normal to the plane of the molecular framework, forming a layered structure. 1:1 Cocrystallization of BFDC with pyridine, phenazine and 1,4-phenylenediamine is associated with H-atom transfer from BFDC to the base and charge-assisted hydrogen bonding between the BFDC, monoanion and the corresponding ammonium species, while preserving, in all cases, the intramolecular hydrogen bond between the carboxyl and carboxylate functions. The pyridinium 2-carboxylato-1-benzofuran-3-carboxylic acid, C5H6N+·C10H5O5,, (II), and phenazinium 3-carboxylato-1-benzofuran-2-carboxylic acid, C12H9N2+·C10H5O5,, (III), adducts form discrete hydrogen-bonded ion-pair entities. In the corresponding crystal structures, the two components are arranged in either segregated or mixed ,,, stacks, respectively. On the other hand, the structure of 4-aminoanilinium 2-carboxylato-1-benzofuran-3-carboxylic acid, C6H9N2+·C10H5O5,, (IV), exhibits an intermolecular hydrogen-bonding network with three-dimensional connectivity. Moreover, this fourth structure exhibits induction of supramolecular chirality by the extended hydrogen bonding, leading to a helical arrangement of the interacting moieties around 21 screw axes. The significance of this study is that it presents the first crystallographic characterization of pure BFDC, and manifestation of its cocrystallization with a variety of weakly basic amine molecules. It confirms the tendency of BFDC to preserve its intramolecular hydrogen bond and to prefer a monoanionic form in supramolecular association with other components. The aromaticity of the flat benzofuran residue plays an important role in directing either homo- or heteromolecular ,,, stacking in the first three structures, while the occurrence of a chiral architecture directed by multiple hydrogen bonding is the dominant feature in the fourth. [source]

Hydrogen-bonded structures of the isomeric 2-, 3- and 4-carbamoylpyridinium hydrogen chloranilates

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2009
Kazuma Gotoh
In the three isomeric salts, all C6H7N2O+·C6HCl2O4,, of chloranilic acid (2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone) with 2-, 3- and 4-carbamoylpyridine, namely, 2-carbamoylpyridinium hydrogen chloranilate (systematic name: 2-carbamoylpyridinium 2,5-dichloro-4-hydroxy-3,6-dioxocyclohexa-1,4-dienolate), (I), 3-carbamoylpyridinium hydrogen chloranilate, (II), and 4-carbamoylpyridinium hydrogen chloranilate, (III), acid,base interactions involving H-atom transfer are observed. The shortest interactions between the cation and the anion in (I) and (II) are pyridinium N,H...(O,O) bifurcated hydrogen bonds, which act as the primary intermolecular interaction in each crystal structure. In (III), an amide N,H...(O,O) bifurcated hydrogen bond, which is much weaker than the bifurcated hydrogen bonds in (I) and (II), connects the cation and the anion. [source]