Ab Initio Molecular Orbital Research Articles

Ab Initio Molecular Orbital Study of the Insertion of H2 into POSS Compounds 2: The Substituent Effect and Larger Cages

The insertion of H2 molecules into the substituted cubic oligomeric silsesquioxanes (POSS), R8T8, [RSiO1.5]8; R = F, Li, CH3, and SiH3, was investigated in comparison with the parent H compound, [HSiO1.5]8, at the HF and second order perturbation theory (MP2) methods. Also investigated were the stabilities of isomers of larger size of POSS, T14 ([HSiO1.5]14), and T16 ([HSiO1.5]16), and the same reaction for them. Furthermore, the possibility of the encapsulation of multiple H2 molecules is discussed for T14 and T16.

Stereodynamics and Outer Valence Ionic States of Ferrocene in Collisional Ionization with a He*(23S) Metastable Atom by Two-Dimensional Penning Ionization Electron Spectroscopy

Outer valence ionic states of ferrocene were investigated by means of Penning ionization electron spectroscopy upon collision with metastable He*(2(3)S) excited atoms. By two-dimensional measurement combining electron spectroscopy and collision-energy-resolved technique of the metastable atomic beam, ionic-state-resolved measurements of collision energy dependence of partial Penning ionization cross sections (CEDPICS) were carried out. Since the partial Penning ionization cross sections can be connected with spatial extension of corresponding molecular orbitals (MOs) outside the boundary surface for the collision with He* atoms, different slope values of CEDPICS were related with stereodynamics in Penning ionization as well as anisotropic interaction around the ionization region: attractive interaction around ligand pi orbitals and repulsive interaction around ligand sigma orbtials and the metal atom. The observed negative collision energy dependence of ionization cross sections for the first ((2)E(2)') and second ((2)A(1)') ionic states was consistent with configuration interactions suggested by ab initio MO calculations [J. Chem. Phys. 2002, 117, 6533], which is due to the strong electron correlation effects beyond the one-electron MO description.

Theoretical Studies of Metallo (Li and Na)−Ene Reaction Mechanisms

The reaction mechanisms of allyl-lithium and allyl-sodium with ethylene were studied by ab initio molecular orbital (MO) methods. The reaction mechanisms were analyzed by a CiLC-IRC method on the basis of ab initio CASSCF MOs. The ene reaction pathways of allyl-Li and allyl-Na with ethylene were located. The complex between allyl-metal and ethylene for both systems is found in the first step of the reaction, and then the metal migration and new C-C bond formation occur synchronously through the transition state. The complexation energies are -13.2 and -9.6 kcal/mol for Li and Na systems, respectively. The activation energy barriers from the reactants are 3.5 kcal/mol for the Li system and 2.0 kcal/mol for the Na system at the MRMP2 calculation level. These barriers are significantly lower than that of the ene reaction of propene with ethylene as the parent reaction. The CiLC-IRC analysis shows that the reaction of allyl-metal with ethylene is a concerted ene reaction mechanism, not a metal catalysis and/or a stepwise reaction.

Quaternary Ammonium Room-Temperature Ionic Liquid Including an Oxygen Atom in Side Chain/Lithium Salt Binary Electrolytes: Ab Initio Molecular Orbital Calculations of Interactions between Ions

Interactions of the lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) complex with N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium (DEME), 1-ethyl-3-methylimidazolium (EMIM) cations, neutral diethylether (DEE), and the DEMETFSA complex were studied by ab initio molecular orbital calculations. An interaction energy potential calculated for the DEME cation with the LiTFSA complex has a minimum when the Li atom has contact with the oxygen atom of DEME cation, while potentials for the EMIM cation with the LiTFSA complex are always repulsive. The MP2/6-311G**//HF/6-311G** level interaction energy calculated for the DEME cation with the LiTFSA complex was -18.4 kcal/mol. The interaction energy for the neutral DEE with the LiTFSA complex was larger (-21.1 kcal/mol). The interaction energy for the DEMETFSA complex with LiTFSA complex is greater (-23.2 kcal/mol). The electrostatic and induction interactions are the major source of the attraction in the two systems. The substantial attraction between the DEME cation and the LiTFSA complex suggests that the interaction between the Li cation and the oxygen atom of DEME cation plays important roles in determining the mobility of the Li cation in DEME-based room temperature ionic liquids.

A novel dynamic exciton expression based on the ab initio MO CI based quantum master equation approach

We propose a novel dynamic exciton expression based on the quantum master equation approach using the ab initio molecular orbital (MO) singly excited configuration interaction (CI) method developed in our previous paper [M. Nakano, M. Takahata, S. Yamada, R. Kishi, T. Nitta, K. Yamaguchi, J. Chem. Phys. 120 (2004) 2359]. This expression is derived from the partition of polarization density in the configuration basis into the electron and hole contributions, and can describe both the coherent and incoherent dynamics of electron and hole density distributions, e.g., dynamic electric polarization, exciton recurrence and exciton migration.

Nonadiabatic electron wavepacket dynamics of molecules in an intense optical field: An ab initio electronic state study

A theory of quantum electron wavepacket dynamics that nonadiabatically couples with classical nuclear motions in intense optical fields is studied. The formalism is intended to track the laser-driven electron wavepackets in terms of the linear combination of configuration-state functions generated with ab initio molecular orbitals. Beginning with the total quantum Hamiltonian for electrons and nuclei in the vector potential of classical electromagnetic field, we reduce the Hamiltonian into a mixed quantum-classical representation by replacing the quantum nuclear momentum operators with the classical counterparts. This framework gives equations of motion for electron wavepackets in an intense laser field through the time dependent variational principle. On the other hand, a generalization of the Newtonian equations provides a matrix form of forces acting on the nuclei for nonadiabatic dynamics. A mean-field approximation to the force matrix reduces this higher order formalism to the semiclassical Ehrenfest theory in intense optical fields. To bring these theories into a practical quantum chemical package for general molecules, we have implemented the relevant ab initio algorithms in it. Some numerical results in the level of the semiclassical Ehrenfest-type theory with explicit use of the nuclear kinematic (derivative) coupling and the velocity form for the optical interaction are presented.

Dissociation mechanisms of cluster ions resolved using ab-initio molecular orbital calculations

We have examined the cluster-size effect on fragments in the collision-induced dissociation (CID) of small boron clusters (Bn+, n≦8) by means of ab-initio molecular orbital (MO) calculation, for conditions representative of low-Z cationic clusters in plasma. Single cations B+ (leaving Bn-10) are predominantly fragmented from small clusters Bn+ (n<5) by low-energy collisions (<10eV) with Xe atoms. With increasing the size n, beyond n=5, Bn-1+ (leaving B0) clusters are observed preferentially. This alternating trend in experiment has not been quantitatively explained by previous ab-initio MO calculations. We used a density functional method called B3LYP and basis functions 6-31+G∗∗ including diffuse functions, and have succeeded in reproducing the alternation of the competing two dissociation processes as cluster size increases. In addition, the accuracy of ionization potentials and binding energies were also improved.

Conformational Structure of Gaseous 3-Chloropropanoyl Chloride by Electron Diffraction, Normal Coordinate Analysis, and ab Initio Molecular Orbital, and Density Functional Theory Calculations

The molecular and conformational structures of 3-chloropropanoyl chloride (CH(2)Cl-CH(2)-C(=O)Cl) have been studied by using gas-phase electron diffraction (GED) data obtained at 22 degrees C (295 K) and ab initio molecular orbital (MO) and density functional theory (DFT) calculations up to the levels of MP4(SDQ) and B3LYP using larger basis sets. Normal coordinate calculations (NCA) taking into account nonlinear vibrational effects were also used in the analyses. The title compound may have up to four low-energy conformers in the gas phase, labeled according to the position of each of the two chlorine atoms in relation to the CCC propanoyl backbone, labeling the carbonyl chlorine torsion angle first: AA, AG, GG, and GA; where A is anti (ideal C-C-C-Cl torsion angle of approximately 180 degrees) and G is gauche (ideal C-C-C-Cl torsion angle of approximately 60 degrees). It has been judged from the experimental GED data and the theoretical calculations, as well as from previously published infrared (IR) studies on the molecule in both the liquid phase and in argon-trapped matrices at 10 K, that the gas phase consists of a mixture of at least three conformers: AA (most stable), AG, and GG, with the possibility of a smaller contribution (<10%) from the higher-energy GA form. The GA conformer cannot be ruled out by the GED experimental data. Relevant structural parameter values obtained from the GED least-squares refinements, with calculated ab initio MO MP2/6-31+G(2d,p) values used as constraints, were as follows (AA values with estimated 2sigma uncertainties): Bond lengths (r(h1)): r(C-C(=O)) = 1.505(4) A, r(C-CH(2)Cl) = 1.520(4) A, r(C=O) = 1.197(4) A, r(C(=O)-Cl) = 1.789(3) A, and r(C-Cl) = 1.782(3) A. Bond angles (angle(h1)): angle CCC = 111.5(11) degrees , angle CCO = 127.0(5) degrees, angle CC(O)Cl = 112.5(3) degrees, and angle CCCl = 110.3(3) degrees. Torsion angles (phi(C-C) = phi(ClCCC)): for AA, phi(1)(C-C(O)) = phi(2)(C-CH(2)Cl) = 180 degrees (assumed for true C(s) symmetry); for AG, phi(1)(C-C(O)) = -140(5) degrees, phi(2)(C-CH(2)Cl) = 76(13) degrees; for GG, phi(1)(C-C(O)) = 46(8) degrees, phi(2)(C-CH(2)Cl) = 77(14) degrees; for GA, phi(1)(C-C(O)) = 67.9 degrees (assumed), phi(2)(C-CH(2)Cl) = 177.8 degrees (assumed). The non-AA conformers all have chiral C(1) symmetry with twice the statistical weight (multiplicity) of C(s). The MP2/6-31+G(2d,p) calculated composition (%) based on the zero-point energy (ZPE) corrected energy differences, and the statistical weights for conformers: AA/AG/GG/GA = 28/35/28/9 was assumed in the final GED refinement. The more recent literature concerning the title molecule, as well as for several related molecules, has been examined and a survey has been attempted in the present article. The new experimental results for 3-chloropropanoyl chloride are discussed and compared with the previously published findings.

Solvation effect on resonance structure: Extracting valence bond-like character from molecular orbitals

Solvation effect on the electronic structure of small molecules is examined using a combination of the RISM-SCF and an analysis, in which resonance structures and their weights are extracted from ab initio molecular orbitals. The present method provides us chemical intuitions on solvation effect from modern quantum computational results. The illustrative results on hydrogen halides and water molecules in an aqueous solution are presented.

Theoretical study on the second hyperpolarizabilities of tetrathiafulvalene (TTF) and tetrathiapentalene (TTP) using highly correlated ab initio MO and the density functional theory methods

We investigate the static second hyperpolarizabilities ( γ) of tetrathiafulvalene (TTF) and tetrathiapentalene (TTP) using the ab initio molecular orbital (MO) method. The cationic radical states of these systems (TTF + and TTP + ) are expected to have large negative γ values, which are rare in organic systems, based on our classification rule of γ. It turns out that at the higher-order electron correlation level TTP + gives a large negative γ value though TTF + gives a positive γ. We also investigate the applicability of the density functional theory (DFT) methods to the calculation of the γ values for these systems. By tuning the mixing parameter of DFT/HF exchange term, a DFT method turns out to semiquantitatively reproduce the γ values of TTP and TTP + at the higher-order electron correlation method, i.e., CCSD(T), while fail in reproducing the γ value of TTF + . This suggests the necessity of further improvement in correlation functional for obtaining reliable γ values of charged radical states.

Accurate Heats of Formation of the “Arduengo-Type” Carbene and Various Adducts Including H2 from ab Initio Molecular Orbital Theory

The heats of formation of saturated and unsaturated diaminocarbenes (imadazol(in)-2-ylidenes) have been calculated by using high levels of ab initio electronic structure theory. The calculations were done at the coupled cluster level through noniterative triple excitations with augmented correlation consistent basis sets up through quadruple. In addition, four other corrections were applied to the frozen core atomization energies: (1) a zero point vibrational correction; (2) a core/valence correlation correction; (3) a scalar relativistic correction; (4) a first-order atomic spin-orbit correction. The value of DeltaHf( 298) for the unsaturated carbene 1 is calculated to be 56.4 kcal/mol. The value of DeltaHf( 298) for the unsaturated triplet carbene (3)1 is calculated to be 142.8 kcal/mol, giving a singlet-triplet splitting of 86.4 kcal/mol. Addition of a proton to 1 forms 3 with DeltaHf( 298)(3) = 171.6 kcal/mol with a proton affinity for 1 of 250.5 kcal/mol at 298 K. Addition of a hydrogen atom to 1 forms 4 with DeltaHf( 298)(4) = 72.7 kcal/mol and a C-H bond energy of 35.8 kcal/mol at 298 K. Addition of H- to 1 gives 5 with DeltaHf( 298)(5) = 81.2 kcal/mol and 5 is not stable with respect to loss of an electron to form 4. Addition of H2 to the carbene center forms 6 with DeltaHf( 298)(6) = 41.5 kcal/mol and a heat of hydrogenation at 298 K of -14.9 kcal/mol. The value of DeltaHf( 298) for the saturated carbene 7 (obtained by adding H2 to the C=C bond of 1) is 47.4 kcal/mol. Hydrogenation of 7 to form the fully saturated imidazolidine, 8, gives DeltaHf( 298)(8) = 14.8 kcal/mol and a heat of hydrogenation at 298 K of -32.6 kcal/mol. The estimated error bars for the calculated heats of formation are +/-1.0 kcal/mol.

Ab Initio Molecular Orbital Study of the Reactivity of Active Alkyl Groups. VII. Solvent Effects on the Formation of Enolate Isomers from 2-Butanone with Methoxide Anion in Methanol

The mechanism of the deprotonation of 2-butanone (1) with methoxide anion (2) was studied by ab initio molecular orbital (MO) methods. Calculations of the thermodynamic stabilities of each complex and the regioselectivity of the reaction were performed using a static isodensity surface polarized continuum model (IPCM) which takes the solvent effect into consideration. The calculated energies of the complexes lead ultimately to the conclusion that the major deprotonation pathway in protic solvents is dependent upon thermodynamically stable complexes with small activation energies under equilibrium control.

Ab Initio Integrated Multi-Center Molecular Orbitals Method for Large Cluster Systems: Total Energy and Normal Vibration

A new computational scheme integrating multi-center ab initio molecular orbitals for determining total energy and normal vibration of large cluster systems is presented. This method can be used to treat large cluster systems such as solvents by quantum mechanics. The geometry parameters, the total energies, the relative energies, and the normal vibrations for four models of water cluster, the hydrated hydronium ion complex, and the transition state of proton transfer are calculated by the present method and are compared with those obtained by the full ab initio MO method. The results agree very well. The scheme proposed in this article is also intended to be used in modeling computer cluster systems using parallel algorithms.

Ab Initio Molecular Orbital and Density Functional Studies on the Solvolysis of Sarin and O,S-Dimethyl Methylphosphonothiolate, a VX-like Compound

[reaction: see text] Potential energy surfaces for the alkaline hydrolysis of sarin and O,S-dimethyl methylphosphonothiolate, a VX model compound, and the perhydrolysis of the latter have been computed at the MP2/6-31+G(d)//mPW1K/MIDI! level of theory. The effect of aqueous solvation was accounted for via the integral equation formalism polarizable continuum model (IEF-PCM) at the HF/6-31+G(d) level. Excellent agreement with the experimental enthalpy of activation for alkaline hydrolysis of sarin was found. For the alkaline hydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that the P-O and P-S bond cleavage processes are kinetically competitive but that the products of P-S bond cleavage are thermodynamically favored. For the perhydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that P-O bond cleavage is not kinetically competitive with P-S bond cleavage. In both cases, the data support initial formation of trigonal bipyramidal intermediates and demonstrate kinetic selectivity for nucleophilic attack on the face opposite the more apicophilic methoxide ligand.

1,2-Dibromoethyl-trichlorosilane (CH 2Br [sbnd]CHBr [sbnd]SiCl 3): conformational structure and vibrational properties by gas-phase electron diffraction, infrared and Raman spectroscopy, and ab initio molecular orbital and density functional theory calculations

The molecular structure and conformational properties of 1,2-dibromoethyl-trichlorosilane (CH 2Br CHBr SiCl 3) have been investigated using gas-phase electron diffraction (GED) data recorded at a temperature of 100 °C, together with ab initio molecular orbital (MO) and density functional theory (DFT) calculations, infrared (IR) and Raman spectroscopy in the liquid and solid phases, and normal coordinate analysis (NCA). The molecule exists in the gas- and liquid phases as a mixture of three conformers, gauche(−) [G(−)], with a refined torsion angle ϕ(BrCCBr) = −71(6)°, anti [A], with a torsion angle ϕ(BrCCBr) ≈ −170°, and gauche(+) [G(+)], with a torsion angle ϕ(BrCCBr) ≈ +70°. The second torsion angle of importance, the rotation about the C Si bond, has been refined to a value of +175(13)°. Torsion angles were only refined for the more abundant G(−) conformer. In the solid phase, only the G(−) conformer was observed. The temperature-dependent Raman spectra have provided an estimate of the relative conformational entropies, Δ S. The obtained composition from GED refinements was (%) G(−)/A/G(+) = 64(27)/23(13)/13(18) (values with estimated 2 σ uncertainties), giving a conformational stability order in agreement with both the Raman enthalpy measurements and the ab initio MO and DFT calculations using the 6-311G(d) basis set and scaled zero-point energies. Relevant structural parameter values obtained from the GED refinements (with the ab initio HF values used as constraints) were as follows (G(−) values with estimated 2 σ uncertainties): bond lengths ( r g): r(C C) = 1.501(18) Å, r(Si C) = 1.865(15) Å, r(〈C Br〉) = 1.965(8) Å (average), r(〈Si Cl〉) = 2.028(3) Å (average). Bond angles (∠ α): ∠CCSi = 114.1(33)°, ∠C 1C 2Br = 114.0(21)°, ∠〈CSiCl〉 = 109.6(7)° (average). Experimental IR/Raman and obtained vibrational wavenumbers based on both the unscaled, fixed-scaled as well as the scale-refined quantum-mechanical force fields [HF/6-311G(d)] are presented. The results are discussed and compared with some similar molecules from the literature.

Ab Initio Molecular Orbital Study of the Reactivity of Active Alkyl Groups. VI. Modified Reaction Model for the Elimination Process of Nitrosation Reaction

The mechanisms of nitrosation of acetone through sodium enolate [CH3COCH2]- Na+ (1) or naked enolate [CH3COCH2]- (2) with tert-butyl nitrite (CH3)3CONO (3) were studied using ab initio molecular orbital (MO) methods. When the modified complex model was used in the elimination process, our results demonstrated the predominant formation of E-1-hydroxy-imino-2-oxo-propane CH3COCH=NOH (4E), in which a counter-cation of the base catalyst did not participate during the reaction. On the other hand, participation of the counter-cation during the reaction contributed to the formation of the Z-isomer of 4 (4Z).

Isotope Effects in the Reaction of Benzo[h]quinoline N-Oxides with Methylsulfinyl Carbanion Examined by ab Initio Molecular Orbital Methods

The reactions of benzo[h]quinoline N-oxide with methylsulfinyl carbanion and deuterated methylsulfinyl carbanion, respectively, were studied theoretically. Differences in yield between these reactions were explained using ab initio molecular orbital methods by considering the zero-point energy correction and the barrier penetration effect. In these reactions, two transition states affected the total reaction rate. The hydrogen- or deuterium-transfer step played a significant role, accounting for the difference in reaction rates.

Structural characterization of poly(diethylsiloxane) in the crystalline, liquid crystalline and isotropic phases by solid‐state 17O NMR spectroscopy and ab initio MO calculations

The structure of poly(diethylsiloxane) (PDES) has been characterized using solid-state NMR of (17)O. The sample studied had a weight-average molecular weight of 2.45 x 10(5). The sample was prepared by utilizing the cationic ring-opening polymerization of (17)O-enriched hexacyclotrisiloxane. Solid-state NMR of (17)O-enriched PDES was measured on the low-temperature beta(1) phase, the high-temperature beta(2) phase, the two-phase system consisting of the liquid crystal and isotropic liquid phase and the isotropic phase. From these data, the molecular structure and dynamics of PDES in the various phases were characterized via the chemical shifts of (17)O, and electric field gradient parameters were determined from NMR and ab initio molecular orbital (MO) calculations. In addition to the solid-state NMR of (1)H, (13)C and (29)Si previously reported on these samples, knowledge of the dynamic behavior of PDES as inferred from the NMR of (17)O in the present study was enhanced significantly. Further, the potential of combining the experimental NMR of (17)O with ab initio MO calculations to characterize the dynamics of polymers containing oxygen is demonstrated.

Atomic quadrupolar effect in intermolecular electrostatic interactions of chloroalkanes: the cases of chloroform and dichloromethane

The atomic quadrupolar effect in intermolecular electrostatic interactions is studied for chloroform and dichloromethane. From the fitting to the electrostatic potentials around these molecules obtained by ab initio molecular orbital (MO) calculations, atomic quadrupoles of the magnitude as large as Θ≅1.5 ea 0 2 are obtained for the chlorine atoms in these molecules. It is shown that a reasonably good fit to the electric field around the molecules can be obtained only by including those atomic quadrupoles, indicating that atomic quadrupoles are essential for correct representation of the intermolecular electrostatic interactions of these molecules. From the calculations of the radial distribution functions (rdfs) of liquid chloroform and dichloromethane by the Monte Carlo and molecular dynamics methods, it is shown that a significant atomic quadrupolar effect is seen in the rdfs of the C…C, C…H and H…H pairs. The intermolecular distances in the optimized structures of the dimers of chloroform and dichloromethane calculated by the ab initio MO method support the new features in the rdfs of the liquids that are seen upon including the effect of atomic quadrupoles. It is suggested that an atomic quadrupole has a significant effect on the angular (rather than radial) component of the location of the atoms of neighboring molecules.

Conformational Analysis of Poly(ethylene imine) and Its Model Compounds: Rotational and Inversional Isomerizations and Intramolecular and Intermolecular Hydrogen Bonds

Conformational characteristics of poly(ethylene imine) (PEI) have been investigated by a rotational isomeric state (RIS) analysis of ab initio molecular orbital (MO) calculations and 1H and 13C NMR experiments for a monomeric model compound, N,N‘-dimethylethylenediamine (di-MEDA). From the MO and NMR data, it was shown that the C−C and C−N bonds of di-MEDA have high gauche (71−93%) and trans (64−86%) preferences, respectively. Conformational energies of PEI were determined from the MO calculations for di-MEDA at the MP2/6-311++G(3df, 3pd)//HF/6-31G(d) level. The high gauche stability in the C−C bond was indicated to stem from a moderate and a weak intramolecular N−H···N hydrogen bonds; the interaction energies were evaluated as −1.54 and −0.58 kcal mol-1, respectively. The RIS scheme including rotational and inversional isomerizations was developed and applied to PEI to evaluate the chain dimension and diad probabilities. With the conformational energies determined as above, the characteristic ratio and meso-diad probability of PEI at 25 °C were calculated to be 2.9 and 0.63, respectively. In polar and protic solvents, the intramolecular hydrogen bonds are weakened, and consequently the PEI chain extends. Branching effects on the conformation were investigated from MO and NMR analysis for monomeric model compounds of branched PEI, N,N,N‘-trimethylethylenediamine and N,N,N‘,N‘-tetramethylethylenediamine; the gauche preference in the C−C bonds, due to the hydrogen bonds, is reduced with increasing number of methyl groups. Ab initio MO calculations were carried out for the double-stranded helix found in anhydrous PEI crystal. The PEI chain was indicated to adopt the isotactic form exclusively. The natural bond orbital analysis showed that intermolecular N−H···N hydrogen bonds are formed between paired chains of the double helix. The enthalpy of association per repeating unit was estimated to be −3.6 kcal mol-1 at the MP2/6-311+G(2d,p)//HF/6-31G(d) level.