Shinichi Yamabe

Molecular orbital calculations of the electronic structure of borazane

An ab initio molecular orbital calculation of the addition compound of boron hydride and ammonia has been carried out. By expanding the molecular orbitals of the addition compound in terms of the molecular orbitals of boron hydride and of ammonia in a separated state, the origin of charge transfer and bond formation between these two species in the interaction has been studied. A configuration analysis of the wavefunction of the addition compound has been performed.

Mechanism of thermal 2+2 cycloaddition reactions between electron-donors and electron-acceptors : Reactions of heteronuclear cycloaddends, CO groups

Abstract The 2 + 2 cycloaddition and “ene” mechanisms previously proposed for electron-accepting homonuclear cycloaddends have been found to hold for heteronuclear ones. Only a difference recognized between them consists in the relative stability of intermediate species. We think the proposed mechanism including its variations can cover thermal 2 + 2 cycloaddition reactions between donors and acceptors and “ene” reactions.

An MO-theoretical calculation of solvent effect upon the NH3 + HF = NH4F reaction

An ab initia molecular orbital calculation was done as to a reacting system, NH3+HF=NH4F, with the inclusion of the solvent effect as the origin of dipolar field. The reactants were assumed to stay in dimers, (NH3)2 and (HF)2, in advance to the reaction, and the respective partners of two reactants were regarded as point dipoles. The system was stabilized to some extent by two dipoles adopted. A study of configuration analysis on this system was made with and without the dipoles. Their effect was found to be favorable for proceeding of the reaction.

Atmospheric pressure chemical ionization of explosives using alternating current corona discharge ion source: APCI of explosives using ac corona discharge ion

The high-sensitive detection of explosives is of great importance for social security and safety. In this work, the ion source for atmospheric pressure chemical ionization/mass spectrometry using alternating current corona discharge was newly designed for the analysis of explosives. An electromolded fine capillary with 115 µm inner diameter and 12 mm long was used for the inlet of the mass spectrometer. The flow rate of air through this capillary was 41 ml/min. Stable corona discharge could be maintained with the position of the discharge needle tip as close as 1 mm to the inlet capillary without causing the arc discharge. Explosives dissolved in 0.5 µl methanol were injected to the ion source. The limits of detection for five explosives with 50 pg or lower were achieved. In the ion/molecule reactions of trinitrotoluene (TNT), the discharge products of NOx (-) (x = 2,3), O3 and HNO3 originating from plasma-excited air were suggested to contribute to the formation of [TNT - H](-) (m/z 226), [TNT - NO](-) (m/z 197) and [TNT - NO + HNO3 ](-) (m/z 260), respectively. Formation processes of these ions were traced by density functional theory calculations. Copyright

An MO study of bridge bonds in B2H6

An ab initio molecular orbital calculation is carried out on diborane (B2H6). Regarding B2H6 as an interacting system of two BH3, we investigate the charge density in the area between these BH3 within the framework of configuration analysis. The charge transfer interaction is found to be most important for the proper description of the bridged three-center bonds in B2H6.

SN1‐SN2 and SN2‐SN3 mechanistic changes revealed by transition states of the hydrolyses of benzyl chlorides and benzenesulfonyl chlorides

Hydrolysis reactions of benzyl chlorides and benzenesulfonyl chlorides were theoretically investigated with the density functional theory method, where the water molecules are explicitly considered. For the hydrolysis of benzyl chlorides (para‐ZC6H4CH2Cl), the number of water molecules (n) slightly influences the transition‐state (TS) structure. However, the para‐substituent (Z) of the phenyl group significantly changes the reaction process from the stepwise (SN1) to the concerted (SN2) pathway when it changes from the typical electron‐donating group (EDG) to the typical electron‐withdrawing one (EWG). The EDG stabilizes the carbocation (MeOC6H4CH2+), which in turn makes the SN1 mechanism more favorable and vice versa. For the hydrolysis of benzenesulfonyl chlorides (para‐ZC6H4SO2Cl), both the Z group and n influence the TS structure. For the combination of the large n value (n > 9) and EDG, the SN2 mechanism was preferred. Conversely, for the combination of the small n value and EWG, the SN3 one was more favorable.

Low-pressure barrier discharge ion source using air as a carrier gas and its application to the analysis of drugs and explosives

In this work, a low-pressure air dielectric-barrier discharge (DBD) ion source using a capillary with the inner diameter of 0.115 and 12 mm long applicable to miniaturized mass spectrometers was developed. The analytes, trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), 1,3,5,7-tetranitroperhydro-1,3,5,7-tetrazocine (HMX), pentaerythritol tetranitrate (PETN), nitroglycerine (NG), hexamethylene triperoxide diamine (HMTD), caffeine, cocaine and morphine, introduced through the capillary, were ionized by a low-pressure air DBD. The ion source pressures were changed by using various sizes of the ion sampling orifice. The signal intensities of those analytes showed marked pressure dependence. TNT was detected with higher sensitivity at lower pressure but vice versa for other analytes. For all analytes, a marked signal enhancement was observed when a grounded cylindrical mesh electrode was installed in the DBD ion source. Among nine analytes, RDX, HMX, NG and PETN could be detected as cluster ions [analyte + NO3 ](-) even at low pressure and high temperature up to 180 °C. The detection indicates that these cluster ions are stable enough to survive under present experimental conditions. The unexpectedly high stabilities of these cluster ions were verified by density functional theory calculation.

An aniline dication-like transition state in the Bamberger rearrangement.

Summary A Bamberger rearrangement of N-phenylhydroxylamine, Ph–N(OH)H, to p-aminophenol was investigated by DFT calculations for the first time. The nitrenium ion, C6H5–NH+, suggested and seemingly established as an intermediate was calculated to be absent owing to the high nucleophilicity of the water cluster around it. First, a reaction of the monoprotonated system, Ph–N(OH)H + H3O+(H2O)n (n = 4 and 14) was examined. However, the rate-determining transition states involving proton transfers were calculated to have much larger activation energies than the experimental one. Second, a reaction of the diprotonated system, Ph–N(OH)H + (H3O+)2(H2O)13, was traced. An activation energy similar to the experimental one was obtained. A new mechanism of the rearrangement including the aniline dication-like transition state was proposed.

A new intermediate in the Prins reaction.

Summary Two Prins reactions were investigated by the use of DFT calculations. A model composed of R–CH=CH2 + H3O+(H2O)13 + (H2C=O)2, R = Me and Ph, was adopted to trace reaction paths. For both alkenes, the concerted path forming 1,3-diols was obtained as the rate determining step (TS1). TS stands for a transition state. From the 1,3-diol, a bimolecular elimination (TS2) leads to the allylic alcohol as the first channel. In the second channel, the 1,3-diol was converted via TS3 into an unprecedented hemiacetal intermediate, HO–CH2–O–CH(R)–CH2–CH2–OH. This intermediate undergoes ring closure (TS4), affording the 1,3-dioxane product. The intermediate is of almost the same stability as the product, and two species were suggested to be in a state of equilibrium. While the geometry of TS1 appears to be forwarded to that of a carbocation intermediate, the cation disappeared through the enlargement of the water cluster. Dynamical calculations of a classical trajectory using the atom-centered density matrix propagation molecular dynamics model on the four TSs were carried out, and results of IRC calculations were confirmed by them.

A DFT study on proton transfers in hydrolysis reactions of phosphate dianion and sulfate monoanion

B3LYP calculations were carried out on hydrolysis reactions of monosubstituted(R) phosphate dianion and sulfate monoanion. In the reacting system, water clusters (H2O)22 and (H2O)35 are included to trace reaction paths. For both P and S substrates with R = methyl group, elementary processes were calculated. While the phosphate undergoes the substitution at the phosphorus, the sulfate does at the methyl carbon. For the S substrate with R = neopentyl group, the product tert‐amyl alcohol was found to be formed via a dyotropic rearrangement from the neopentyl alcohol intermediate. For R = aryl groups, transition‐state geometries were calculated to be similar between P and S substrates. Calculated activation energies are in good agreement with experimental values. After the rate‐determining transition state of the substitution, the hydronium ion H3O+ is formed at the third water molecule. It was suggested that alkyl and aryl substrates are of the different reactivity of the hydrolysis.