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Hyperthermal collisions of ground-state atomic oxygen with H2O have been investigated, with special attention paid to the H-atom elimination reaction, O(3P) H2O(X 1A1) → HO2(2A‘) H(2S). This reaction was observed in a crossed-beams experiment, and the relative excitation function in the region around its energy threshold (50−80 kcal mol-1) was measured. Direct dynamics calculations were also performed at two levels of theory, B3LYP/6-31G(d,p) and MP2/6-31G(d,p). The shape of the B3LYP excitation function closely matches that of the experiment. The calculations provided a detailed description of the dynamics and revealed a striking dependence of the reaction mechanism on collision energy, where the cross section rises from a threshold near 60 kcal mol-1to a peak at ∼115 kcal mol-1and then decreases at higher energies as secondary dissociation of the internally excited HO2product becomes dominant. The calculations show that the cross section for H-atom elimination (O H2O → HO2H) is about 10−25% that of the H-atom abstraction (O H2O → OH OH) cross section for collision energies in the 70−160 kcal mol-1range. [ABSTRACT FROM AUTHOR]

The results are reported of mass-spectrometric studies of the nucleobases adenine 1h (1, R = H), guanine 2h, and cytosine 3h. The protonated nucleobases are generated by electrospray ionization of adenosine 1r (1, R = ribose), guanosine 2r, and deoxycytidine 3d (3, R = deoxyribose) and their fragmentations were studied with tandem mass spectrometry. In contrast to previous EI-MS studies of the nucleobases, NH3 elimination does present a major path for the fragmentations of the ions [1h + H]+, [2h + H]+, and [3h + H]+. The ion [2h + H − NH3]+ also was generated from the acyclic precursor 5-cyanoamino-4-oxomethylene-dihydroimidazole 13h and from the thioether derivative 14h of 2h (NH2 replaced by MeS). The analyses of the modes of initial fragmentation is supported by density functional theoretical studies. Conjugate acids 15–55 were studied to determine site preferences for the protonations of 1h, 2h, 3h, 13h, and 14h. The proton affinity of the amino group hardly ever is the substrate’s best protonation site, and possible mechanisms for NH3 elimination are discussed in which the amino group serves as the dissociative protonation site. The results provide semi-direct experimental evidence for the existence of the pyrimidine ring-opened cations that we had proposed on the basis of theoretical studies as intermediates in nitrosative nucleobase deamination. [Copyright &y& Elsevier]

The transition state for -hydride elimination in alkyl groups on the Pt(111) surface has been probed by studying the effects of fluorine substitution on the barriers to -hydride elimination, %@mt;sys@%%@ital@%E%@rsf@%%@sx@%H%@be@%‡%@sxx@%%@mx@%. Four different fluoroalkyl groups have been formed on the Pt(111) surface by dissociative adsorption of four fluoroalkyl iodides:  RCH2CH2−I (R CF3, CF3CH2, and CF3CF2) and (CF3)2CHCH2−I. In the absence of preadsorbed hydrogen, fluoroalkyl groups on the Pt(111) surface dehydrogenate via -hydride elimination to form unsaturated fluorocarbons and deposit hydrogen atoms onto the surface. Those hydrogen atoms then hydrogenate the remaining fluoroalkyl groups to produce fluoroalkanes that desorb rapidly from the surface. The kinetics of hydrogenation and fluoroalkane desorption are rate limited by the -hydride elimination step and thus serve as measures of the kinetics of -hydride elimination. The field effects of the fluorinated substituents increase the barriers to -hydride elimination with a reaction constant of F 19 ± 2 kJ/mol. The interpretation of this effect is that the -carbon atom in the transition state is cationic, RC···H‡, with respect to the reactant. The field effect of the fluorinated substituent energetically destabilizes the electron deficient -carbon atom in the transition state. This is consistent with observations made on the Cu(111) surface; however, the substituent effect is significantly smaller on the Pt(111) surface. On the Pt(111) surface, the transition state for -hydride elimination is less polarized with respect to the initial state alkyl group than on the Cu(111) surface. [ABSTRACT FROM AUTHOR]

Photodissociation of fluorine-substituted benzenes, including 1,3-difluorobenzene, 1,2,4-trifluorobenzene, 1,2,4,5-tetrafluorobenzene, and pentafluorobenzene, at 193 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. HF elimination was found to be the major dissociation channel for all of these molecules. Small amounts of photofragments of C6H3F2 and C6H2F3 from 1,3-difluorobenzene and 1,2,4-trifluorobenzene, respectively, were also observed. They correspond to the minor dissociation channel of hydrogen elimination. Dissociation rates and fragment translational energy distributions obtained from experimental measurements suggest that HF and hydrogen elimination reactions occur in the ground electronic state. The potential energy surface obtained from ab initio calculations indicates that the four-center reaction in the ground electronic state is the major dissociation mechanism for the HF eliminations. A comparison with the RRKM calculation has been made. [ABSTRACT FROM AUTHOR]

A five-step preparation of carbohydrate-based oxepines from hept-l-enitols is presented. The hept-l-enitols were subjected to silyl protection and hydroboration/oxidation to give the protected heptan-1-itols. Swern oxidation of the homologated alcohols followed by sequential acetal formation/ cyclization provided methyl 2-deoxyseptanosides that underwent elimination reactions to give the carbohydratebased oxepines. The new sequence is an alternative to the ring-closing metathesis for the synthesis of carbohydratebased oxepines from protected pyranose sugars. The product oxepines can serve as glycosyl donors in the synthesis of novel septanose carbohydrates. In addition, C-methylenealdehydo arabinofuranoside 16 was formed from 2-deoxyseptanoside 10 as the only product during protic acid mediated elimination reactions. This novel ring contraction complements other reported preparations of C-methylenaldehydo furanosides and underscores the acid-mediated reactivity introduced by competing eliminations between the C-1/C-2 and C-2/C-3 bonds. [ABSTRACT FROM AUTHOR]

The unimolecular fragmentation of metastable anisole dications is studied by tandem-mass spectrometry. Permutations of OH, OCH3, and NH2 substituents as well as extensive labeling experiments are used as probes to unravel the mechanisms of the various fragmentation pathways observed. The main channel corresponds to methyl ion elimination, which leads to an energetically favorable charge separation (“Coulomb explosion”). Loss of a neutral CO molecule forms another important branch, and the expelled CO has incorporated a carbon atom from the phenyl ring of anisole. Substitution of the anisole dication with OH, OCH3, and NH2 groups results in more specific fragmentations. Ortho- and para-substitutions lead to preferential methyl ion elimination, whereas meta-substitution favors CO loss. The substituent effects are also reflected in pronounced changes in the apparent kinetic isotope effects upon substitution of OCH3 by OCD3. On the basis of the various experimental findings and some explorative DFT calculations, possible mechanisms for the skeletal rearrangements prior to CO elimination are proposed. [ABSTRACT FROM AUTHOR]

A direct ab initio dynamics study is carried out for three channels of the unimolecular reaction of fluoromethoxy, CH2FO. The minimum energy paths (MEPs) of the three channels are computed with the MPW1K/6-31+G(d,p) method. The geometries, harmonic vibrational frequencies, energies, and enthalpies of all stationary points are calculated at the MPW1K and QCISD levels of theory in conjunction with the 6-31+G(d,p) basis set. The energies of stationary points and the points along MEP for each channel are further refined by using single-point multilevel energy calculations. The rate constants are evaluated by means of the conventional transition state theory (TST), the canonical varitional transition state theory (CVT), and the canonical varitional transition state theory with small-curvature tunneling correction (CVT/SCT) in the temperature range of 200-2500 K. The fitted Arrhenius expressions of the calculated CVT/SCT rate constants are k1CVT/SCT(T) = (1.26 × 106)T2.19e-7150.6/T s-1, k2CVT/SCT(T) = (3.68 × 102)T3.23e-12327.7/T s-1, k3CVT/SCT(T) = (9.97 × 10-26)T10.90e-3873.3/T s-1 for the H elimination, HF elimination, and isomerization channels, respectively. [ABSTRACT FROM AUTHOR]

In enzymes, multiple structural effects cooperatively lead to the high catalytic activity, while individually these effects can be small. The design of artificial enzymes requires the understanding and ability to manipulate such subtle effects. The 34E4 catalytic antibody, catalyzing Kemp elimination of 5-nitrobenzisoxazole, and its Glu50Asp (E50D) variant are the subject of the present investigation. This removal of only a methylene group yields an approximately 30-fold reduction in the rate for the catalyzed Kemp elimination. Here, the aim is to understand this difference in the catalytic performance. The mechanism of Kemp elimination catalyzed by 34E4 and the E50D mutant is elucidated using QM/MM Monte Carlo simulations and free energy perturbation theory. In both proteins, the reaction is shown to follow a single-step, concerted mechanism. In the mutant, the activation barrier rises by 2.4 kcal/mol, which corresponds to a 62-fold rate deceleration, which is in good agreement with the experimental data. The positions and functionality of the residues in the active site are monitored throughout the reaction. It is concluded that the looser contact with the base, shorter base-Asn58 contact, less favorable π-stacking with Trp91 in the transition state of the reaction, and different solvation pattern all contribute to the reduction of the reaction rate in the E50D variant of 34E4. [ABSTRACT FROM AUTHOR]

The radical reaction of tributyltin hydride with o-iodo-N- methylanilides derived from α-azido acids provides an excellent access to α-(aminocarbonyl)iminyl radicals through 1,5-hydrogen transfer reaction of initially formed aryl radicals followed by β-elimination of dinitrogen from ensuing α-azido-α-(aminocarbonyl)alkyl radicals. The outcoming iminyls display a peculiar tendency to form corresponding nitriles by β-elimination of aminocarbonyl radicals. [ABSTRACT FROM AUTHOR]