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Ab initio molecular orbital calculations at the MP4(SDTQ)/6-31G∗//RHF/6-31G∗ level of theory were used to obtain detailed insight into the reaction profile on the Mg controlled 1,3-cycloaddition of nitriles oxide with allylic alcohols. It was confirmed that the complex formation reduces the HOMO–LUMO energy gaps of the two reactants. The propensity correlates well to calculated activation energies of the cycloaddition. The syn-stereoselectivity depends on the steric repulsion between the vinyl fragment and the substituent at the α-position of the alcohol or alkoxide. THF or Et2O in the reaction mixture prevent the coordination of R1CNO from forming the Mg intermediate and result in deccelerating the cycloaddition. The results obtained in the present study well explain an important role of the Mg2+ ion in the 1,3-dipolar cycloadditions of nitrile oxides with allylic alcohols.

A substitution on 2,2-difluorovinylic carbon was investigated by using ab initio molecular orbital calculations. Three feasible mechanisms, which are the SN1-like, the SN2-type and the addition-elimination mechanisms, were ex- amined for a model borate, 2,2-difluoro-1-mesyloxyvinyl(trimethyl)borate. Four TSs were obtained depending on the position of Li+ around the vinylborate although activation energies in the gas phase are rather high (ca. 30–40 kcal mol−1) in comparison with that expected from the experimental conditions. It was confirmed at the SCRF-IPCM calculations that the solvent effect reduces the acti- vation energy of one SN2-type mechanism very much (4. l kcal mol−1 at the B3LYP/6-31+G∗//RHF/6-31+G/s* level of theory) while those for the other mechanisms do not change very much. Therefore, the SN2-type mechanism is applicable to the substitution reaction observed for the vinylborate.

In a previous paper (J. Am. Chem. Soc. 1991, 113, 4738), we reported that there was a linear relationship between the activation free energies ΔG⧧ of the Menshutkin reaction of CH3I and the proton ...

Ab initio molecular orbital (MO) study on the inversion process of six trimethylene bridges between two degenerate C 6h conformers of [3 6 ](1,2,3,4,5,6)cyclophane suggests that the inversion occurs in a stepwise manner rather than a synchronous mechanism.

The alkaline hydrolysis of amides, N-methyl acetamide 1, acetanilide 2, and N-acetyl imidazole 3, was investigated by use of ab initio molecular orbital calculations. A detailed analysis was performed for two possible mechanisms, Path 1 and 2. Path 1, similar to ester hydrolysis, releases CH3COOH and RNH whereas Path 2 contains the RNH group that extracts the H atom from the OH fragment in the tetrahedral intermediate. In this mechanism, carboxylate ion and an amine directly form. It was ascertained that Path 2 is a better route to decompose the TD intermediate than Path 1 for 1 and 2 in the gas phase. The stability of the leaving group RNH determines which route, Path 1 or 2, is preferred in the gas phase. The solvent effect was estimated for the two mechanisms by using the SCRF calculations of the IPCM model.

Vibration frequencies of the NCO stretching mode were calculated for model molecules of the copper ion-exchanged ZSM-5 catalytic system, Cu(OH 2 ) 3 X n + (X = NCO, NCO − and HNCO, n = 1 or 2) by use of ab initio molecular orbital calculations. The complexes, Al(OH) 3 (NCO), Al(OH) 3 (NCO − ) and Al(OH) 3 (OCNH) were adopted as the models of the zeolite framework. According to the present calculations, the absorption bands observed at 2189 cm −1 and 2251 cm −1 are assigned to the stretching vibration of the NCO − coordinating to the copper ion and to the aluminum in the zeolite (ZSM-5) framework, respectively. This result is consistent with the experimental assignment.

Experimental and theoretical investigations of the anti-selective nucleophilic addition induced by BF 3 to 3-formyl-Δ 2 -isoxazolines are described. Ab initio molecular orbital (MO) calculations show that the 3-formyl-Δ 2 -isoxazoline-BF 3 complex prefers s-trans conformation to s-cis conformation. Nucleophiles attack the stable s-trans conformer from the opposite side of C(4) substituent, giving anti-adducts selectively. The BO bond lengths and the stabilization energies suggest that the features of the complex resemble those of aliphatic aldehyde-BF 3 complexes. Introduction of substituent to allyl tin improves the anti-selectivity from 87/13 to 98/2. These results suggest that the reaction proceeds via anti-periplanar transition state.

A systematic investigation of the cyclic ethers and amines was conducted using molecular orbital calculations to assess their possibility as building blocks of novel functional molecules. A method was developed to quantitatively calculate the Free-O and Comp-O radii which correspond to the hole sizes of free and complexed crown compounds. These radii show the linear relationship with the number of atoms in crown ring for both calculated and observed coordinates. On the other hand, the Li-X (XO and N) lengths are not dependent on the number of atoms in the crown ring. While the ΔEstab's of the crown ethers show a clear dependence on the number of the crown ring atoms, the values does not vary significantly among the cyclic amines. The MNDO method gave reasonable results by chance.

Ab initio molecular orbital calculations at the MP4(SDTQ)/6-31G∗//RHF/6-31G∗ level of theory were used to obtain detailed insight into the reaction profile on the Mg controlled 1,3-cycloaddition of nitriles oxide with allylic alcohols. It was confirmed that the complex formation reduces the HOMO–LUMO energy gaps of the two reactants. The propensity correlates well to calculated activation energies of the cycloaddition. The syn-stereoselectivity depends on the steric repulsion between the vinyl fragment and the substituent at the α-position of the alcohol or alkoxide. THF or Et2O in the reaction mixture prevent the coordination of R1CNO from forming the Mg intermediate and result in deccelerating the cycloaddition. The results obtained in the present study well explain an important role of the Mg2+ ion in the 1,3-dipolar cycloadditions of nitrile oxides with allylic alcohols.

The effects of axial ligands on the ground-state geometries, electronic structures and the characteristic optical properties of iron phthalocyanine and its derivatives, FePc and FePcL(n) (L = pyridine (Py) and cyanide (CN(-)); n = 1, 2), were investigated using the density functional theory (DFT) method. The geometries of FePc with a triplet spin state and of FePc(Py), FePc(Py)(2), FePc(CN(-)) and FePc(CN(-))(2) with singlet spin states were optimized under D(4h), C(2v), D(2h), C(4v), and D(4h) molecular symmetries, respectively. The highest occupied molecular orbitals (HOMOs) of FePc, FePc(Py), FePc(Py)(2), and FePc(CN(-)) are pi-type orbitals, which have no contribution from the p(z) atomic orbitals of all nitrogen atoms, whereas the HOMO of FePc(CN(-))(2) is the 7e(g) orbital, which has contributions from the d(xz) and the d(yz) orbitals of the Fe atom mixing with the pi-orbitals of the axial CN(-) ligands. The time-dependent (TD) DFT method gives many optically allowed excitations for FePc, FePc(Py), FePc(Py)(2), FePc(CN(-)), and FePc(CN(-))(2) in the UV-VIS region. Our calculated bands corresponded well with the experimental results. In FePc(Py)(2), the metal-ligand charge transfer (MLCT) transitions from the metal d to the axial-ligand pi*-type orbitals contributed to the B band region. In FePc(CN(-))(2), the MLCT transitions from the metal d to the Pc-ring pi*-type orbitals contributed mainly to the first B band region, but those from the metal d to the axial-ligand pi*-type orbitals did not appear in the energy regions of the Q and B bands. Thus, the axial ligands caused a spectral change in FePc through orbital mixing.

The 1-(p-anisyl)-2-(triisopropylsilyl)vinyl cation 1 and the 1-(p-anisyl)vinyl cation 2 were generated in solution and characterized by NMR spectroscopy. Ab initio molecular orbital calculations using the 6-31 G basis set were performed for cation 2, the 1-(p-anisyl)-2-methylvinyl cation 3, and the 1-(p-anisyl)-2-silylvinyl cation 4, serving as a model cation for 1, to elucidate the importance of α-π aryl stabilization and β-σ bond stabilization in 1-(p-anisyl)vinyl cations with various β substituents. For comparison, the p-anisylmethyl cation 5 was also calculated

The Michael addition of alkoxide anions to nitro olefins is studied in terms of ab initio and MNDO molecular orbital calculations. We have also done experiments using nitro olefins with the same substituents calculated here. It is proposed that the stereoselectivity of the reaction is due to the endo alkoxy effect, which is structural, and the electronic effect in nitronate anion intermediates. The effect does not exist in intermediates of conjugate addition of simple alkyl group to nitro olefines. The difference depends on whether the atom in the γ-position possesses lone pair orbitals or not. High stereoselectivity was observed in the Michael addition to nitro olefins with the bulky substituents of the α-carbon. This is due to the difficulty in rotating the alkoxy fragment about the Cα-Cβ axis

A recent paper by Lui et al. [Z. Liu, X. Zhang, Y. Zhang, J. Jiang, Spectrochim. Acta A 67 (2007) 1232] reported on the theoretical investigations of the fully optimized geometries and electronic structures of iron (II) phthalocyanine (FePc) with the singlet spin state carried out with the restricted density functional theory (DFT) method, where the B3LYP functional was adopted for the exchange-correlation term; however, the triplet spin state was experimentally reported, and we also obtained the triplet spin state by the unrestricted DFT calculations.

This paper describes a new facial procedure to substitute a tosyloxy group in 2-(tosyloxy)alkanoate with SH(-) to yield 2-mercaptoalkanoate on the basis of ab initio MO calculations. Combination of substrate and solvent effects can control both reactivity and selectivity of reaction for 2-(tosyloxy)-3-alkoxycarboxylic acid which gave 2-mercapto-3-alkoxycarboxylic acid in good yield while its ethyl ester gave alpha,beta-unsaturated carboxylate ester as a main product. The difference of carboxylate moiety in the substrate causes remarkable change in reactivity and selectivity. To clarify origin of the difference, ab initio MO calculations in the gas phase and in DMF have been carried out. The solvent effect was considered at RHF/6-31+G with the IPCM-SCRF model. It was confirmed that the substrate with an ester fragment prefers the E1cB to the S(N)2 mechanism. In the transition state of the S(N)2 mechanism with a carboxylate ion fragment, the nucleophile SH(-) locates far from the reaction center due to the electrostatic repulsion between the COO(-) fragment and SH(-). This repulsion causes high activation barrier in the gas phase while polar solvent can reduce the barrier height. Therefore, reaction conditions can control reactivity of carboxylic acid. On the basis of analysis of the MO calculations, subsequent experiments were designed for a new dianion system to synthesize 2-pyrimidinylthio carboxylic acid from 2-tosyloxy carboxylate. We succeeded in developing a new facile method that the two reactions for thioether carried out in a one-pot procedure in excellent yield.

The S(N)i mechanism, which was previously proposed for the isomerization of 1-acylaziridines to the oxazolines, was reexamined theoretically by performing molecular orbital (MO) calculations of 1-formylaziridine and its derivatives as model compounds and experimentally by using 1(R)-[alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl]-2(S)-methylaziridine (5). At the MP2/6-31G//RHF/6-31G level, the activation energy was estimated to be 38.9 kcal mol(-1) for the S(N)i mechanism in which N-protonated 1-formylaziridine 8a(NH(+)) isomerizes to the N-protonated oxazoline 9a(NH(+)). Intrinsic reaction coordinate calculations showed that this reaction proceeds with retention of the ring carbon configuration. Methyl substitution in the aziridine ring reduces the activation energy by 10 kcal mol(-1). The ring closure of N-(2-chloroethyl)formamide (10a) to the oxazoline, which is a model reaction of the rate-determining step for the addition-elimination mechanism, was estimated to have an activation energy of 45.4 kcal mol(-1). The results of these MO calculations are consistent with the observation that the isomerization of the acylaziridine 5 to the oxazoline 6 is facilitated in the presence of weak nucleophiles such as with BF(3).OEt(2) while the formation of 6 is very slow in the presence of stronger nucleophiles such as p-toluenesulfonate. Both theoretical and experimental results suggest that the S(N)i mechanism explains well the isomerization of (R,S)-5 to the oxazoline with BF(3).OEt(2) in refluxing benzene.

The vinyl cation with a ethynyl fragment, C4H3+ is investigated with ab initio molecular orbital calculations. Two geometrical isomers of C4H3+, open chain and triangular geometries are calculated in order to ascertain the existence of the triangular isomer. The 4–31G* and MP3/6–31G*//4–31G* level calculations confirm that the latter is more stable than the former. However, the activation energy for intra-conversion is considered to be rather small. The calculated results are analyzed by comparing with experimental results. The relation between molecular vibration and reaction is analyzed by dynamic electron transfer.

The characteristics of Ca2+-binding sites and of the structural reorganization induced by Ca2+-binding in storage proteins and ion carriers are being studied as models for molecular mechanisms in Ca2+ channels and in Ca2+-dependent modulatory proteins. A first step in the study was the development of energy parameters for Ca2+ compatible with those in the CHARMM package of computer simulation software. Such parameters were obtained from an analytical fit to the potential surface for [(Ca)(OCH2)4]2+ calculated with an ab initio molecular orbital method. The resulting parametrization was tested for the hexapeptide cyclo-(Pro-Gly)3, and a 75 residue long calcium binding protein from bovine intestine (ICaBP). The geometrical parameters calculated for the hexapeptide and its 2:1 complex with Ca2+ were in good agreement with experimental data from crystallography and nmr. Similarly, the structure of ICaBP optimized with CHARMM using the new Ca2+ parameters showed good agreement with the x-ray structure both in the local structures of the calcium-binding sites and in the overall shape of the protein.

Electronic structures of different d configurations have been studied in relation to the facility of protonation to dinitrogen ligand in Cr(0) and Co(I) complexes. The d6 Cr complex has much better MO's for protonation than the d8 Co complex. This difference largely affects the protonation energies and the reduction of nitrogen molecule in these complexes.

The vinyl cation with a ethynyl fragment, C4H3+ is investigated with ab initio molecular orbital calculations. Two geometrical isomers of C4H3+, open chain and triangular geometries are calculated in order to ascertain the existence of the triangular isomer. The 4–31G* and MP3/6–31G*//4–31G* level calculations confirm that the latter is more stable than the former. However, the activation energy for intra-conversion is considered to be rather small. The calculated results are analyzed by comparing with experimental results. The relation between molecular vibration and reaction is analyzed by dynamic electron transfer.

The general dynamic aspect of electron density of a molecular system is studied on the basis of the general equation of the electron orbital which is formulated for the dynamic study of electronic motion. The newly defined electron orbital incorporates the dynamics of molecular vibration into the electronic structures. In this scheme, the change of electron distribution caused by excitation of vibrational state is defined as the ‘‘dynamic electron transfer.’’ The dynamic electron density is found to have the remarkable ‘‘additive’’ property. The time‐dependent aspect of the dynamic electron redistribution is also analyzed on the basis of the ‘‘coherent state.’’ The new method relates the classical vibrational amplitude to the quantum number of the vibrational state. As a preliminary application of the present treatment, the dynamic electron densities of H2, HD, HT, HF, and HCl molecules are calculated by use of ab initio molecular orbital method.