Setsuo Taniguchi

A molecular orbital study on the reactivity of L-ascorbic acid towards OH radical

The reaction between L-ascorbic acid (AAH2) and OH radical has been investigated theoretically. The addition site on AAH2 and AAH– is found to be the olefinic carbon adjacent to the carbonyl group, with much more distinct selectivity for AAH–. Dehydration is found to occur readily not from the neutral ˙OH adduct AAH2OH˙, but from the anion adduct, AAHOH˙–, which leads to formation of the most probable key intermediate radical 17(ascorbate anion radical). The parent molecule of AAH2, triose reductone TRH2(2,3-dihydroxprop-2-enal), undergoes ˙OH addition and dehydration in a similar manner to AAH2. The addition site has been confirmed by comparison of MO energies for four possible transition states of the TRH2(ab initio and MNDO) and AAH2(MNDO) systems. The importance of the conjugate bases, AAH– and TRH–, for yielding the key radicals is suggested.

Proton Transfer Reactions in Oxygen-Containing Molecules and Ammonia Mixtures

Ion-molecule reactions leading to the formation of NH4+ in a number of mixtures of oxygen-containing molecules and ammonia have been studied using pulsedsource mass spectrometry. Some unique features in the reactivities of C2H4O+ and CHO+ ions from ethylene oxide have been found.

A theoretical study on the mechanism of oxidation of L-ascorbic acid

The oxidation of L-ascorbic acid (vitamin C) to dehydroascorbic acid is investigated by ab initio MO calculations. Their geometries together with intermediate species are optimized at the STO-3G level. The electron and spin densities are obtained with the 4-31G basis set. The five-membered (γ-lactone) ring is found to be planar during the oxidation process. Neutral and anion radicals are of the similar geometry but are of the different spin-density distribution. The stability of the ascorbate ion and the ascorbate anion radical is attributed to the ‘pseudo-aromaticity’ involved. The energy variation of the oxidation process of L-ascorbic acid is compared with that of triose reductone which is the simplest molecule having the same functional groups as the former compound.

A Mechanistic Study of a Gas-Phase Ion/Molecule Reaction between CH2NH2+ and CH4

A gas-phase ion/molecule reaction starting from (CH4+ and N2O)/(CH4 and N2O+) is studied using a pulsed-discharge high-pressure mass spectrometer and the ab initio MO calculation. Two dominant ions, C2H6N+ and C3H8N+, are recognized; they are formed by the methylation of CH4N+. The C2H6N+ is formed via the following two reactions:CH2NH2+(or CH3NH+)+CH4 → (CH3)2NH2+ (5)(CH3)2NH2+ → CH3NHCH2++H2 (7)The methylation is composed of the heterolytic CH3+–H− addition to the C=N bond of CH4N+ and the subsequent H2 elimination. The growth of the ion by the methylation is largely exothermic, while the H2 elimination has a large energy barrier on the potential energy surface.A gas-phase ion/molecule reaction starting from (CH4+ and N2O)/(CH4 and N2O+) is studied using a pulsed-discharge high-pressure mass spectrometer and the ab initio MO calculation. Two dominant ions, C2H6N+ and C3H8N+, are recognized; they are formed by the methylation of CH4N+. The C2H6N+ is formed via the following two reactions:CH2NH2+(or CH3NH+)+CH4 → (CH3)2NH2+ (5)(CH3)2NH2+ → CH3NHCH2++H2 (7)The methylation is composed of the heterolytic CH3+–H− addition to the C=N bond of CH4N+ and the subsequent H2 elimination. The growth of the ion by the methylation is largely exothermic, while the H2 elimination has a large energy barrier on the potential energy surface.

Five-membered ring intermediates in the gas-phase CN-+N2O reaction. A theoretical study

Abstract A gas-phase reaction between N 2 O and CN − is studied theoretically. All the intermediates and transition states of three reactions (a) N 2 O+CN − →NCO − +N 2 , (b)→N − 3 +CO and (c)→CNO − +N 2 are determined with RHF/6-31+G*. Two five-membered key intermediates, 3 and 7 , are found. At the MP4/6-31+G* level, Δ G * values are 141 kJ/mol for reaction (c) and 123 kJ/mol for reactions (a) and (b). Reactions (a) and (c) are predicted to be the likely channels.

Oxidation intermediates of borohydride and tetraphenylborate ions in aqueous solutions obtained by pulse radiolysis

Absorption spectra of one-electron oxidized intermediates (the H4B· and the Ph4B· radicals) were observed upon the oxidation of H4B– and Ph4B– by the azide radical (N3·), Br2[graphic omitted], or (SCN)2[graphic omitted] in plus-irradiated solutions; the spectra have absorption maxima at 400 nm, Iµ= 1800 dm3 mol–1 for H4B· and at 335 nm, Iµ= 8600 dm3 mol–1 for cm–1 for Ph4B·.

The entropy inhibition of a gas-phase SN2 reaction

The observed rate constant for proton transfer is much larger than that for the SN2 gas-phase reaction between protonated methanol and ammonia, which is reproduced reasonably by the theoretical Gibbs free energy change.

Micro-Analysis of Lithium by Isotope Dilution Method I. Studies on Standard Solutions.

The relative errors in the micro-analysis of lithium of known quantities under condition of different error factors were examined. Lithium of 10/sup -6/ to 10/sup -7/ order was determined with satisfactory accuracy. For the determination of lithium in 10/sup -7/ to 10/sup -8/ g order, the isotopic contamination exents considerable influence on the relative error. (auth)