Buyong Ma

IMPORTANCE OF SELECTING PROPER BASIS SET IN QUANTUM MECHANICAL STUDIES OF POTENTIAL ENERGY SURFACES OF CARBOHYDRATES

An extensive quantum mechanical study of a water dimer suggests that the introduction of a diffuse function into the basis set, which significantly reduces the basis set superposition error (BSSE) in the hydrogen bonding energy calculation, is the key to better calculations of the potential energy surfaces of carbohydrates. This article examines the potential energy surfaces of selected d‐aldo‐ and d‐ketohexoses (a total of 82 conformers) by quantum mechanics (QM) and molecular mechanics (MM) methods. In contrast to the results with a smaller basis set (B3LYP/6‐31G** 5d), we found at the higher level calculation (B3LYP/6‐311++G(2d,2p)//B3LYP/6‐31G** 5d) that, in most cases, the furanose forms are less stable than the pyranose forms. These discrepancies are mainly due to the fact that intramolecular hydrogen bonding energies are overestimated in the lower level calculations. The higher level QM calculations of the potential energy surfaces of d‐aldo‐ and d‐ketohexoses now are more comparable to the MM3 results. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1593–1603, 1999

Molecular polarizabilities and induced dipole moments in molecular mechanics

Molecular polarizabilities may be divided into either atomic contributions or bond contributions. The common way to estimate molecular polarizabilities is to assign atomic or bond parameters for each atom or bond type to fit experimental or quantum mechanical results. In this study we have taken a different approach. A general formula based on MM3 force constants and bond lengths was used to compute bond polarizabilities and molecular polarizabilities. New parameters for polarizabilities are not required. A fair agreement between experimental and computed molecular polarizabilities was obtained, with a RMS deviation of 0.82 Å3 (11.7%) and signed average error of 0.01 Å3 for a broad selection of 57 molecules studied. Two methods, the many‐body interaction and the pair‐interaction approaches, have been used to study induced dipole moments using the bond polarizabilities estimated from the new formula. The pair‐interaction approximation, which involves much less computation than the many‐body interaction approach, gives a satisfactory representation of induced dipole interaction.

Spectroscopic constants and potential energy surfaces for the possible interstellar molecules A1NC and A1CN

Ab initio quantum mechanical methods were employed to study the A1NC and A1CN isomers, resulting in theoretical predictions of the equilibrium geometries, spectroscopic constants, dipole moments, relative energies, harmonic vibrational frequencies, transition state structures and the activation energy for the isomerization between the isomers. Basis sets as large as triple zeta plus double polarization plus f functions (TZ2P + f) and cc-pVQZ have been used with the self-consistent-field (SCF), configuration interaction including all single and double excitations (CISD), and coupled cluster including all single and double substitutions (CCSD) methods, as well as CCSD with the effects of connected triple excitations added perturbatively (CCSD(T)). The A1NC structure is predicted to be linear and lie about 5·5 kcal mol-1 lower in energy than the A1CN isomer at the TZ2P + f CCSD(T) level of theory. The activation energy for the isomerization from A1CN to A1NC is predicted to be 6 kcal mol-1. A1NC is more rigi...

Toward the observation of silanone (H2SiO) and hydroxysilylene (HSiOH) via microwave spectroscopy

Ab initio quantum mechanical methods were employed to study the H2SiO and HSiOH (both cis and trans) isomers, resulting in high‐level theoretical predictions of the equilibrium geometries, rotational constants, dipole moments, relative energies, vibrational frequencies, transition state structures, and the activation energy for the isomerization between the cis‐ and trans‐HSiOH isomers. Basis sets as large as triple zeta plus double polarization plus silicon and oxygen atom f and hydrogen atom d functions [TZ2P(f,d)] have been used with the self‐consistent‐field configuration interaction including all single and double excitations (CISD), and coupled cluster including all single and double substitutions (CCSD) methods, as well as CCSD with the effects of connected triple excitations added perturbatively [CCSD(T)]. Our predictions for the dipole moment components and geometry of silanone (H2SiO) were instrumental in its recent microwave spectroscopic identification (accompanying paper by Bogey and co‐worke...

Spectroscopic constants and potential energy surfaces for silanone (H2SiO), hydroxysilylene (HSiOH), the hydroxysilylene dimer, and the disilynyl radical (Si2H)

Ab initio quantum mechanical methods were employed to study the spectroscopic constants and potential energy surfaces of H2SiO, HSiOH, the HSiOH dimer, and the Si2H radical. Consideration of the spectroscopic constants of silanone, cis‐ and trans‐HSiOH and Si2H began with the TZ2P SCF level of theory. We predict a strongly bonded cis‐HSiOH dimer. The structure of the cis‐HSiOH dimer was optimized at the DZP SCF, DZP CISD, DZP+diff CISD and DZP MP2 levels. The hydrogen bond energy of the dimer is 14.8 kcal/mol at the DZP MP2 level and 12.0 kcal/mol at the DZP CCSD/DZP CISD level. The vibrational frequency of one Si–O bond stretch in the HSiOH dimer is 967 cm−1 at the DZP MP2 level, close to the 951 cm−1 and 986 cm−1 fundamentals observed experimentally for HxSiyOz aggregates. Therefore, it is possible that the HSiOH dimer has been observed in matrices. The potential energy surface of the Si2H radical was studied initially at the DZP CISD level. We found a bent Cs 2A″ Si2H structure which is 10.8 kcal/mol h...

Singlet-triplet energy separation and barrier for ring closure for trimethylenemethane and its complexes

Abstract The energy separations between the 3 A′ 2 ground state and the excited electronic 1 B 1 and 1 A 1 states of trimethylenemethane (TMM) have been studied using quantum mechanical methods. The 1 B 1 minimum TMM is predicted to lie 15.7 kcal/mol higher in energy than the 3 A′ 2 ground state at the TZ2P + f CISD + Q level. This is by far the highest level of theory employed to date to the vexing TMM singlet-triplet separation. The 1 A 1 minimum of TMM is 1.5 kcal/mol higher in energy than the 1 B 1 minimum at the DZP CISD + Q level. The transition state for ring closure of TMM to methylenecyclopropane has been located, with a barrier of 1.4 kcal/mol on the 1 A 1 surface at the DZP CISD + Q level. The above energies relative to the 3 A′ 2 ground state for isolated TMM are much higher than Dowds experimental maximum (7.8 kcal/mol) for the singlet-triplet separation. The discrepancy is discussed in the light of possible solvation effects. Direct evidence for the magnitude of the solvation effect has been obtained from a study of the TMM · methylenecyclopropane (TMM · MCP), TMM · H 2 CO, TMM · CO, and TMM · Ne complexes. There is essentially no change of the triplet-singlet energetics upon forming the TMM · Ne complex. However, the 3 A′ 2 - 1 A 1 energy difference is 0.24 kcal/mol lower in the TMM · H 2 CO, 0.20 kcal/mol lower for the TMM · MCP complex, and 0.10 kcal/mol lower in the TMM · CO complex than for isolated TMM, respectively, at the DZP CISD + Q level. There is no barrier for ring closure for the TMM · H 2 CO complex on the 1 A 1 surface at the DZP TC-CISD level, even though the barrier is 0.2 kcal/mol at the DZP TCSCF level. It has been found that the electronic states of TMM interact with surrounding molecules in the order: 1 B 1 3 A ′ 2 1 A 1 state.

THEORETICAL INVESTIGATION OF THE CA+-N2 AND CA2+-N2 COMPLEXES

Abstract The structures, harmonic frequencies and binding energies for the ground states of Ca + –N 2 and Ca 2+ –N 2 are computed at several theoretical levels, including Moller–Plesset second-order perturbation with full electron correlation and density functional theory. The charge–quadrupole interaction yields linear geometries as minima for both complexes, while the C 2v geometries are transition states. At the MP2 level, Ca + –N 2 has a dissociation energy ( D e ) of 5.3 kcal/mol and a Ca + –N bond distance of 2.78 A. Ca 2+ –N 2 has a dissociation energy ( D e ) of 23.8 kcal/mol and a shorter metal–ligand bond of 2.48 A.

Tetraethynylethylene, a molecule with four very short CC single bonds. Interpretation of the infrared spectrum

Abstract The (HCC) 2 CC(CCH) 2 molecule recently synthesized by Rubin, Knobler, and Diederich has been studied using ab initio molecular quantum mechanics. The experimental crystal structure shows significant distortions from D 2h symmetry, so the equilibrium geometry has been determined via the self-consistent-field method using a double zeta plus polarization basis set. The structure of the isolated molecule displays perfect D 2h symmetry. Theoretical vibrational frequencies and infrared intensities make possible an interpretation of the observed IR spectrum. Some attention is given to the molecular orbitals of this the first C 10 H 4 isomer to be prepared in the laboratory.

Fragmentation surface of triplet ketene

The photofragmentation of ketene to triplet methylene and carbon monoxide is a paradigm for unimolecular dissociation over an exit channel barrier. The geometric structures, quadratic force fields, and harmonicvibrational frequencies of the triplet ketene reactant, the 3B1 CH2+1Σ+ CO products, and both in-plane (CsII) and out-of-plane (CsI) transition states have been determined at the TZ(2d1f,2p) coupled-cluster singles and doubles (CCSD) level of theory. An unusual, shallow minimum at long range [R(C–C)=4.0 A] has also been discovered and characterized. A rigorous mapping and analytic parametrization has been performed of the TZ(2d1f,2p) CCSD intrinsic reaction paths connecting the CsII transition state to both the reactant and products. Final potential-energy functions along the entire reaction path have been determined with the aid of [(C,O)/H] atomic-orbital basis sets as large as [6s5p4d3f2g1h/5s4p3d2f1g] and electron correlation treatments as extensive as the coupled-cluster method through triple excitations [CCSDT or CCSD(T)]. The final theoretical curve is highlyanharmonic in the transition-state region, displaying a classical barrier of1045 cm-1, a critical C–C distance of 2.257 A, and a barrier frequency of321i cm-1. Effective barrier frequencies in the 100i cm-1 range which resultfrom RRKM modelling with tunnelling corrections of the observed steplike structure in the triplet ketene dissociation rate constant are thus shown to be physically untenable. Various implications of such abinitio predictions on unravelling the intricacies of the fragmentation dynamics are discussed.

CALCULATION OF RZ STRUCTURES FROM RS STRUCTURES

Experimental bond lengths are determined in a variety of ways. Gas phase electron diffraction experiments give the thermal average value of the internuclear distance (rg). Two kinds of operational bond lengths (r0 and rs) may be derived directly from rotational and rotation-vibrational spectroscopy. The distance between the equilibrium nuclear positions is called re, and that between the average nuclear positions at zero K is called rz. The relationship derived for diatomic molecules, rs = (rc + rz)2, was used to estimate the rs structures from molecular mechanics (MM3) calculations of rg, and subsequently rz and re bond lengths, for a group of 23 molecules. This relationship works well for polyatomic molecules. The root-mean-square (RMS) deviation between experimental rs bond lengths and those evaluated by the MM3 force field over this set of molecules is 0.0058 A, which is similar to the experimental uncertainty (RMS = 0.0066 A).