Keiji Morokuma

Molecular Orbital Studies of Hydrogen Bonds. III. C=O···H–O Hydrogen Bond in H2CO···H2O and H2CO···2H2O

Ab initio LCAO–MO–SCF calculation for H2CO···H2O is carried out with a minimal Slater basis set. The most stable conformation has an O···H distance of 1.89 A with <C=O···H=− 64° and a stabilization energy of 3.5 kcal/mole, about a half of that for H2O···H2O. Nonlinear and π hydrogen bonds, H2CO···2H2O and the O···H–C hydrogen bond in H2O···HCHO, are also studied. An energy decomposition scheme is proposed and applied to H2CO···H2O and H2O···H2O. In the latter the electrostatic energy 8.0 kcal/mole, the exchange repulsion − 9.9 kcal/mole, the polarization and dispersion energy 0.3 kcal/mole, and the delocalization energy 8.2 kcal/mole are in good agreement with Coulsons estimates.

The intrinsic reaction coordinate. An ab initio calculation for HNC→HCN and H−+CH4→CH4+H−

A practical method of calculating the intrinsic reaction coordinate starting at a saddle point is proposed. The method has been used in combination with the analytical evaluation of the energy gradient for the calculation of the reaction coordinate on an ab initio potential energy surface. The reaction coordinates are obtained for the HNC to HCN isomerization and the SN2 exchange reaction involving H−+CH4→CH4+H−.

Efficient determination and characterization of transition states using ab-initio methods

Abstract The gradient of the potential energy with respect to nuclear coordinates has been calculated using ab-initio single determinant molecular orbital methods. The calculated gradient is used together with very efficient minimization methods to locate and characterize transition states on many-dimensional potential energy surfaces. Previously such methods have only been applied to semi-empirical potential functions. Although the calculation of the gradient in addition to the energy increases the computational time by about a factor of four, we have demonstrated the feasibility of these calculations by locating the transition state for the model rearrangement of HNC to HCN using both minimal (STO-3G) and split valence shell (4-31G) basis sets. Further use of such methods in the direct application of ab-initio wavefunctions to dynamical investigations is discussed.

Extended Hartree-Fock theory for excited states

Abstract An extended Hartree-Fock method for excited states is proposed. Starting from the ground state Hartree-Fock molecular orbitals and allowing mixing within occupied and vacant subspaces, respectively, we minimize the energy of a single configuration excited state. This wavefunction satisfies the orthogonality and Brillouin theorem not only with the ground state but also with other excited states. One of the major advantages of the method is the common set of MOs for the ground and excited states. By making an additional assumption this method reduces to Huzinagas scheme. A few sample calculations are presented.

Molecular Orbital Studies of Hydrogen Bonds: Dimeric H2O with the Slater Minimal Basis Set

Ab initio LCAO–MO–SCF calculation for the dimeric H2O system is carried out with a minimal Slater basis set with exponents optimized for H2O. The most stable linear dimer is found to have an O···H distance of 1.80 A, with the proton‐acceptor molecule perpendicular to the donor molecule and bent by 54° trans with respect to the end OH bond of the donor. The stabilization energy calculated is about 6.55 kcal/mole. The changes in the length and the stretching force constant of the donor O–H bond are also discussed. A population analysis shows that the stabilization at a larger O···H distance (>2.3 A) is essentially electrostatic, while at a smaller distance, the charge transfer becomes increasingly important.

A simple model of solvation within the molecular orbital theory

A simple model of solvation within the molecular orbital method is proposed whereby the effect of solvent molecules is simulated by the inclusion of fractional point charges at the solvent atomic centers. The method is applied to three solvation problems: the hydration of Li+ and F− and the solvation effect on the interaction between NH3 and HF. The results of the first two calculations indicate that the point charge model is capable of reliably predicting solvation energies. The calculations for H3N···HF demonstrate that the hydration has a profound effect on the potential energy surface favoring a proton transfer structure H3NH+···F−.

Theoretical studies of carbonyl photochemistry. I. ab initio potential energy surfaces for the photodissociation H2CO*→H + HCO

Abstract Potential energy surfaces for the ground state S 0 , and first excited triplet T 1 , and singlet S 1 states have been calculated for the photodissociation H 2 CO * →H + HCO using a minimal zeta SCF MO CI procedure. As the CH distance | D | increases, S 0 monotonically increases in energy to form the ground state of H + HCO; departure of the H atom in the HCO plane is preferred. The minimum energy path on the T 1 potential surface passes through a saddlepoint near | D | ≈ 1.9 A and goes to the ground state of the products; D is almost perpendicular to the HCO plane in the region of the saddlepoint. Probable mechanisms for the reaction are discussed in light of these calculations and related experimental work.

Electronic Structures of Linear Polymers. II. Formulation and CNDO/2 Calculation for Polyethylene and Poly(tetrafluoroethylene)

The Hartree–Fock procedure for infinite linear polymers is formulated by the use of the cyclic boundary condition. An actual calculation using this procedure is carried out for the various conformations of infinite polyethylene and poly(tetrafluoroethylene) in the semiempirical CNDO/2 SCF approximation. Polyethylene has an energy minimum at the C–C rotational angle ω = 180° or for the trans zigzag conformation, while poly (tetrafluoroethylene) has a minimum around ω = 162°, a slightly twisted trans zigzag conformation. Both are in excellent agreement with experimental results for the single crystals. Both polymers, experimentally good electric insulators, are calculated to have a large energy gap between the highest‐filled and the lowest‐vacant band. The segmental and atomic energy contributions and charge distributions are discussed in detail in comparison with alkanes and as functions of the conformational angle ω.

Energy Decomposition Analysis of Molecular Interactions

In the theoretical study of molecular interactions, ab initio molecular orbital calculations have been applied successfully in predicting the binding energy and the geometry of intermolecular complexes.1–3 In what is called the supermolecule method the entire complex is considered as a supermolecule, and the calculated energy difference between the supermolecule and the monomers is the binding energy. In order to facilitate an interpretation of the results, methods have been proposed4–7 to decompose the interaction energy into physically meaningful energy components such as electrostatic, polarization, charge transfer, and exchange energies. Analyses of many molecular complexes along these lines have provided the basis for elucidating the origin or the nature of various molecular interactions such as hydrogen bonds and electron donor-acceptor complexes.

Molecular orbital studies of hydrogen bonds

The ground state, the lowest singlet and triplet n-π* states, and the lowest triplet π-π* state of the formic acid monomer and dimer are studied with the ab initio molecular orbital theory. The two-configuration electron-hole potential method is used for calculations of excited states of dimers. The potential energy curves for the symmetrical simultaneous movement of two bridging protons are studied for all of the states. The barrier of the proton transfer in the ground state is found to be the smallest of the states studied. The association energy is analyzed in terms of various components.