Hiroshi Yamataka

Ab initio molecular dynamics simulations on the hydrolysis of methyl chloride with explicit consideration of three water molecules

Ab initio molecular dynamics (MD) simulations were performed on the hydrolysis of methyl chloride (CH3Cl) with explicit consideration of three water molecules to understand the dynamics of the reaction and the role of solvent molecules in SN2 reactions in solution. The simulations clearly showed the existence and dynamical characteristics of two nearly concerted proton transfers involving the attacking water molecule and the solvent water molecules on the way to formation of the products. Observation of these proton transfers points clearly to the need for an explicit quantum chemical treatment of at least a few solvent water molecules to describe methyl chloride solvolysis.

One transition state leading to two product states: ab initio molecular dynamics simulations of the reaction of formaldehyde radical anion and methyl chloride

Ab initio molecular dynamics (MD) simulations for the reaction of formaldehyde radical anion and methyl chloride indicate that trajectories starting from a well-characterized single transition state reach either an electron-transfer (ET) product or a C-substituted SN2 product. The two kinds of trajectories have different characteristics. Trajectories which lead to the SN2 product state are simple, with C–C bond formation and C–Cl bond breaking essentially completed within 50 fs. By contrast, trajectories leading to the ET product are more complex with a sudden electron reorganization taking place around 15–30 fs; the major bonding changes and electron and spin reorganization are completed after 250 fs.

Analysis of borderline substitution/electron transfer pathways from direct ab initio MD simulations

Abstract Ab initio molecular dynamics simulations were carried out for the borderline reaction pathways in the reaction of CH2O − with CH3Cl. The simulations reveal distinctive features of three types of mechanisms passing through the SN2-like transition state (TS): (i) a direct formation of SN2 products, (ii) a direct formation of ET products, and (iii) a two-step formation of ET products via the SN2 valley. The direct formation of the ET product through the SN2-like TS appears to be more favorable at higher temperatures. The two-step process depends on the amount of energy that goes into the C–C stretching mode.

AN AB INITIO MO STUDY ON THE HYDROLYSIS OF METHYL CHLORIDE WITH EXPLICIT CONSIDERATION OF 13 WATER MOLECULES

Abstract Ab initio MO calculations (HF/3-21G, HF/6-31G, HF/6-31+G* and MP2/6-31+G*) were carried out on the hydrolysis of CH 3 Cl in which up to 13 water solvent molecules were explicitly considered. For n ⩾3, three important stationary points ( cmp1 , TS , and cmp2 ) were detected in the course of the reaction. The calculations for the n =13 system at the HF/6-31+G* level reproduced the experimental activation enthalpy and the secondary deuterium kinetic isotope effect. The two reacting bond lengths in the transition state are 1.975 A (O–C) and 2.500 A (C–Cl), and CH 3 Cl is surrounded by 13 water molecules without any apparent vacant space. The proton transfer from the attacking water to the water cluster occurs after TS is reached.

Critical assessment of the hybrid QM/MM-pol-vib approach: Small water clusters using polarizable flexible water potentials

The authors report a systematic study of the structure of small water clusters, up to pentamers, using a hybrid quantum-mechanics/molecular-mechanics approach with polarizable flexible water-interaction potentials in conjunction with HF SCF wave functions. The model is denoted QM/MM-pol-vib. For each optimized QM cluster, QM water molecules were replaced one at a time with MM-pol-vib water molecules and reoptimized the cluster structure. The hybrid structures and energies were found to reproduce well their full QM counterparts. This finding indicates that the first hydration shell of solvation obtained with such a model is described at a semiquantitative level of accuracy. The model should prove useful in modeling aqueous reactions. The efficient computational strategy adopted for coupling the polarizable response of the solvent with the solute wave function calculation was outlined. Energy gradients for the solute and the solvent molecules are also efficiently calculated.

Observable Enols of Anhydrides: Claimed Literature Systems, Calculations, and Predictions

The literature describing the observation of enols of carboxylic anhydrides and mixed carboxylic-sulfuric anhydrides was examined. In the phenylbutyric anhydride system, the alleged enol was shown to be ethylphenylketene, and the monoenol EtC(Ph)=C(OH)OC(=O)CH(Ph)Et (5) and the dienol (6) should not be observed according to calculations. Calculations also show that the claimed enols H2C=C(OH)OSO2Y, Y=SO, Ac (15) and the enol of 2H-pyran-2,6(3H)-dione (7) are too unstable to be observed. The bulky enols of β,β-ditipylacetic formic (35a) or trifluoroacetic (35b) anhydride were calculated to be unstable with pKEnol=7.7 (6.2). The suggestion that compounds with the 3-acyl or 3-aroyl-2H-pyran-2,6(3H)-dione skeleton are enolic was examined. In the solid state, all the known structures show that enolization takes place on C(5)=O. However, B3LYP/6-31G** calculations show that, for 3-acetyl-4-methyl-2H-pyran-2,6(3H)-dione (10, R1=Me, R2=H), which is completely enolic, the enol on the acetyl group (cf.12) is only 0.9u2005kcal/mol more stable than the enol on the anhydride (cf.11). Calculations also revealed that 3-(trifluoroacetyl)-2H-pyran-2,6(3H)-dione (28) should exist in nearly equal amounts of the enol of anhydride (cf.30) and the enol of the acyl group (cf. 29), whereas the enol of anhydride (cf. 32) is the only stable species for 3-(methoxycarbonyl)-2H-pyran-2,6(3H)-dione (31). Furan-2,5-diol (27) and 5-hydroxyfuran-2-one (26) are calculated not to give observable isomers of succinic anhydride (25) (pKEnol=30 and 18, resp.) in spite of the expected aromatic stabilization of 27. Surprisingly, the calculations reveal that the enol (NC)2C=C(OH)OCHO (38) is less stable than its tautomeric anhydride (37) (pKEnol=1.6). Comparison of calculated pKEnol values for (NC)2CHC(=O)X (41) and MeC(=O)X indicates that the assumption that substitution by two β-CN groups affects similarly all the systems regardless of X is incorrect. A pKEnol((NC)2CHC(=O)X) vs. pKEnol(MeC(=O)X) plot is linear for most substituents with severe and mild negative deviations, respectively, for X=NH2 and MeO. Appropriate isodesmic reactions have shown that the β,β-(CN)2 substitution increases the stabilization of the enol of amide (X=NH2) by 14.6u2005kcal/mol over that for the anhydride (X=OCHO), whereas the amide form is 7.1u2005kcal/mol less destabilized than for the anhydride. The pKEnol value for (MeOCO)2CHCOOCHO (43) is 3.6, i.e., stabilization by these β-electron-withdrawing groups is insufficient to make the enols observable.

An ab initio MO study on the hydrolysis of methyl chloride

Abstract The methyl chloride hydrolysis in aqueous solution is a typical S N 2 reaction. Ab initio MO calculations (HF/3-21G, HF/6-31G, HF/6-31+G*, HF/6-31++G** and MP2/6-31+G*) were carried out on the hydrolysis of CH 3 Cl, in which up to 13 solvent water molecules were explicitly considered. The precursor complex, transition state, product complex and final complex were detected for each of the systems. It was found that the attacking H 2 O molecule kept two hydrogen atoms at the transition state and the proton transfer from the attacking water to the water cluster began to occur after the transition state for each of the systems. The solute and solvent kinetic isotope effects (KIEs) were calculated for all the systems and were compared with the experimental KIEs. The calculated results for the system with the 13 water molecules reproduced the experimental energetics and various deuterium kinetic isotope effects quite well. In this system, CH 3 Cl is surrounded by 13 water molecules without any apparent vacant space. The 13 water molecule system produces a reasonable picture of the hydrolysis both in terms of its structure and energetics.

Origin of regioselectivity in electrophilic reaction of ambident enaldimines

Abstract The observed substituent-dependent regioselectivity (1,2- vs 1,4-addition) in the electrophilic reaction of ambident imines 1 – 3 is rationally understood by the relative magnitude of the LUMO coefficients of the imines.

Calculated amide/enol of amide energy differences for several interesting amide systems

The calculated pKEnol values for the enols nof amides Me2CHCON(Me)Ph and nXCH2CH(NH2)CONHMe (X = H,OH) are lower by only n1.1–2.7 units than that of acetamide, and therefore these enols nshould not display unusual stability.

α-tritium isotope effects in the menschutkin-type reaction with variable transition states

Abstract Secondary α-tritium isotope effects in the series of the Menschutkin-type reaction of benzyl benzenesulfonates with N,N -dimethylanilines were all small and varied only slightly; it was concluded that the transition states vary mainly in the parallel direction to the reaction coordinate.