Non-empirical calculations on the electronic spectrum of butadiene

Ab initio SCF and CI calculations of the electronic spectrum of butadiene are reported which employ a gaussian basis of double zeta quality augmented by diffuse 3s and 3p functions. Good agreement is obtained with experimental details of this spectrum, both for π→π* and certain Rydberg transitions, and it is concluded that the important NV₁ and NV₂ absorption systems both involve diffuse upper states of ¹B_u symmetry.     

Ab initio MO Calculations of benzene + TCNE and naphthalene + TCNE complexes with STO-3G π-split basis set

Ab initio SCF MO calculations have been carried out on benzene + TCNE (tetracyanoethylene) and naphthalene + TCNE complexes with the STO -3G, STO -3G π-split (STO -3G for π orbitals and a split basis for π orbitals), and 4–31G basis sets. The interaction energy, gross charges, dipole moment, and the electron density in the middle plane of the complexes have also been evaluated. The STO -3G π-split basis set is appropriate for the calculation of large π–π stacking complexes from two points of view, production of reliable results and ease of computations. The approximation scheme based on the semiorthogonalized orgitals is revealed to be very efficient to save CPU time and storage in such calculations. The stable conformation and the charge-transfer interaction of the two complexes are discussed on the basis of the calculated quantities.     

An ab initio study of intermolecular hydrogen bonding between small peptide fragments

Several small peptide fragments are investigated with ab initio (Hartree-Fock) calculations, using Gaussian basis sets. Complexation energies, net atomic charges, and optimum geometries are obtained. The geometries predicted by the STO -6G, and 6–31G* basis sets are quite similar, whereas the binding energies obtaiend by the 6–31G calculations are higher than those obtained with STO -6G and 6–31G* basis sets.     

An ab initio calculation of 14n quadrupole coupling constants

Ab initio SCF-MO calculations of ¹⁴N quadrupole coupling constants are reported for HCN, HNC, CH₃CN, CH₃NC, NH₃, NH₂NH₂, FCN, N₂O, (CN)₂, BrCN, pyridine and pyrazine. There is excellent correlation between calculation and experiment yielding Q = 1.503 ± 0.159 × 10^?26 cm² for the ¹⁴N nuclear quadrupole moment. Dunning sp basis sets are more than adequate for such calculations, STO/4G basis sets yielding almost identical results for pyridine and pyrazine. Unsuccessful attempts were made to correlate coupling constants with electronic population analysis indices.     

Acetic acid monomer: Ab initio study,barrier to proton tunnelling,and infrared assignment

The barrier to tunnelling of the carboxyl proton in monomeric acetic acid is found to be 96.7 kcal/mole in an ab initio study, which is not compatible with an earlier interpretation of the strong observed in the infrared matrix spectra of monomeric acetic acid and deuterated species. A new explanation is suggested for the anharmonic effects, and a vibrational assignment, supported by normal coordinate calculations is proposed.The ab initio SCF calculations were carried out with two different Gaussian basis sets for comparison. The values obtained for the methyl torsional barrier, for the energy difference between the trans OCOH and the more stable cis OCOH conformtion, and for the OH-torsional force constant (0.15 and 0.19 mdyn/Å) are in good agreement with earlier results and/or with experimental data. The main anharmonicities observed in the spectra are apparently due to Fermi resonance involving CO stretching and COH bending fundamentals and combination tones of skeletal breathing and bending motions.     

Calculation of the isotropic nuclear spin spin coupling constants for H2O,NH3 and CH4

Isotropic nuclear spin spin coupling constants have been evaluated for all possible couplings in H₂O, NH₃ and CH₄ by two different double perturbation techniques. All calculations were performed employing bases of detors constructed with SCF canonical and, for H₂O, localized orbitals, produced by extended STO basis set calculations.     

AB initio calculations of 2H and 14N quadrupolar coupling constants in hydrogen bonded dimers

SCF MO calculations at the 6-31G^** level of approximation are reported for ²H and ¹⁴N electric field gradients in HCN?HCN, HCN?HF, and CH₃CN?HF dimers, with emphasis on the configurational dependence of these quantities in (HCN)₂. In comparison with available experimental nuclear quadrupolar coupling constants, the calculated values for the monomers and dimers exhibit an accuracy of ≈ 10%, which is comparable to that of other spectroscopic parameters. The implications of hydrogen bonding for quadrupolar spin-lattice relaxation rates are briefly discussed.     

The influence of small monovalent cations on neighbouring N…HO hydrogen bonds

The influence of small monovalent metal ions on hydrogen bonds of the OH…N type is studied with the example Li⁺/H₂ONH₃. The net stabilization effect is discussed and compared with the OH…O system. The applicability of the semi-empirical CNDO/2 method and ab initio calculations with extended and minimal basis sets is investigated.     

A theoretical study of the strcture of (NO)2

Ab initio SCF calculations on the cis form of nitric oxide dimer, (NO)₂, indicate that the NN bond distance is 1.74 Å, the NO bond distance is 1.16 Å, and the angle NNO is 107°.     

Ab initio SCF and MRD CI studies of the FHCl− ion

The equilibrium geometry and hydrogen-bonding energy of the heterobihalide ion FHCl^? have been calculated by ab initio SCF and MRD CI methods using an AO basis set of near Hartree-Fock quality. In the most stable (linear) conformation of this ion, the equilibrium F Cl/FH distances are predicted to be 5.657/1.754, 5.453/L800 and 5.437/1.801 bohr by SCF, MRD CI and full CI (estimated) calculations respectively. A second minimum, which is of extremely small depth and corresponds to the hydrogen atom near the chlorine atom, begins to appear in the potential surface at an FCl distance of about 6.0 bohr. The hydrogen-bonding energy of FHCl^? lies in the range 18–22 kcal/mol.     

Density functional theory study of nitrogen‐14 nuclear quadrupole coupling parameters of L‐histidine: hydrogen‐bonded system

The calculations of nitrogen‐14 nuclear quadrupole parameters, nuclear quadrupole coupling constant, χ, and asymmetry parameter, η, of L‐His were done in two distinct environments: one as a free fully optimized molecule, an isolated molecule with the geometrical parameters taken from X‐ray, and the other in the orthorhombic and monoclinic solid states. The most probable interacting molecules with the central molecule in the crystalline phase were considered in the hexameric clusters to include hydrogen‐bonding effects in the calculations. The computations were performed with PW91P86/6‐31++G** and B3LYP6‐31++G** methods using the Gaussian 98 program. The good agreement between the nitrogen‐14 quadrupole parameters of the free His and imidazole molecules with their microwave available data demonstrates that the applied level of theory and the 6‐31++G** basis set are suitable to obtain reliable electric field gradient values. In the solid state, the shifts of quadrupole coupling parameters from the monomer to the solid phase are reasonably well reproduced for the amino and imino sites of imidazole ring in a hexameric cluster. That implies the fact that the hexameric cluster worked effectively to generate the results which are compatible with the experiment. The quadrupole coupling constant values of –N⁺H₃ group are in fair agreement with the experiment. This discrepancy is due to the absences of vibrational effects and the rotation of –N⁺H₃ group around N–C(α) bond. Copyright © 2012 John Wiley & Sons, Ltd.     

The influence of the solvent on the conformational energy differences due to the anomeric effect

The solvation free energy ΔG^sol of molecules exhibiting the anomeric effect is computed in an approach that considers a continuous distribution for the solvent. A partition of ΔG^sol into separately evaluated contributions confirms that changes in the energy of the systems due to changes in conformation of the solute are ruled by the electrostatic contribution. A comparison with the “exact” values indicates that the approximate expression for the electrostatic contribution to ΔG^sol are not accurate enough to permit a proper modeling of the solvent influence on the anomeric effect. The systems are composed of methanediol, methoxymethanol, dimethoxymethane, and 2-methoxytetrahydropyran in carbon tetrachloride, chloroform, acetone, and water. The calculations have been performed at the SCF level with the STO -3G and 4–31G basis sets.     

Proton affinity correlations for alkyl chlorides

Ab initio SCF calculations with minimal STO-3G and extended 44-31G basis sets have been performed on the simple alkyl chlorides, HCl to t-BuCl, and their protonated analogs. MINDO/3 calculations are also reported for these species and a variety of cyclic and bicyclic chlorides. The much closer agreement with experiment for STO-3G proton affinities than for 44-31G values is in sharp contrast to the results for first-row bases. An excellent correlation is found between both the STO-3G and MINDO/3 proton affinities and the charge on the CIH fragment in RCIH⁺. For the acyclic chlorides, correlations of PA's with the polar substituent constant, σ*, and IP's are also reasonable. In addition, the calculated carbonium ion-HCl interaction energy for t-BuClH⁺ indicates that protonated tertiary chlorides are no more than marginally stable in the gas phase.     

Core correlating basis functions for elements 31–118

Gaussian functions for correlation of all core shells of elements from Z = 31 to Z = 118 have been optimized in relativistic singles and doubles CI calculations, performed on the shell of highest angular momentum for each principal quantum number. The SCF functions were derived from the double-zeta, triple-zeta, and quadruple-zeta basis sets previously optimized by the author. Only those Gaussian functions that are not represented in the SCF basis sets were optimized. The functions are available from the Dirac program web site, .     

Molecular integrals in the generalized hylleraas–CI method

In the generalized Hylleraas–CI method, the original correlation factor r^v_ij is multiplied by a Gaussian geminal. Using the approach of generating functions, the general formulas of molecular integrals in this method are derived over Cartesian Gaussian orbitals. From differentiations of the generating functions, the expanding length in the incomplete Gamma functions is reduced, and some cancellations presented in other approaches are avoided. Preliminary calculations for H₂ and H₂—H₂ systems are carried out over STO -3G basis. The results are encouraging.     

Performance of the elongation method with larger basis sets

The elongation method proposed by Imamura serves as a theoretical model for polymerization processes. It can now be used together with larger basis sets, Hartree–Fock and density functional methods from the Gaussian 94 package with direct self‐consistent field (SCF). This allows electronic structure calculation of elongating clusters with an efficiency superior to full cluster calculations and a precision superior to previous versions of our elongation method. Performance and accuracy compared with full cluster calculations on a regular polymer using the BLYP/6‐31G(d, p) method. Interaction energies of water and hydrogen fluoride polymers of increasing length are compared between HF, BLYP methods and 4‐31G, 6‐31G(d, p) basis sets: Diffuse and polarization functions have a large influence on the interaction energy on both polymers. Local density of states are calculated for different cluster lengths. They are in good agreement with full cluster calculations. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 35–47, 1999     

The effect of metal ion bonding to amides on the character of the CN bond of the ligand molecule

The influence of various metal ions on the character of the amide CN bond has been studied by means of NMR line shape analysis of the changes in the activation energy for internal rotation about this bond. All cations investigated raise the barrier, the increase was found to be approximately linearly correlated to the inverse radii of the ions. MO SCF calculations with small gaussian basis sets have been employed for the theoretical discussion of these results. Theoretical and experimental results concerning the ion influence on the CN bond are in satisfactory agreement. The changes in the electronic structure of the amide molecule and the character of the CN bond are illustrated using localized orbitals and electron density maps.     

Molecular orbital studies of dinitrogen tetroxide and related molecules

Hartree-Fock LCAO MO calculations for N₂O₄ have been performed in a basis of symmetry orbitals formed from a minimal Slater basis set. Effects of rounding and truncation errors were minimized by the use of the symmetry basis, which also allowed the order or tilling of molecular orbitals to be specified independently of orbital energies. Convergence difficulties were overcome by combined use of the conjugate gradients method and Roothaan's iterative procedure; the method of steepest descents was less effective than either of these. Multicentre ‘non-NDDO’ two-electron integrals were evaluated by the gaussian expansion technique. The wavefunction obtained for the lowest state is NN antibonding, largely as a result of the filling of the 6b_1u antibonding sigma orbital in preference to the 6a_g bonding sigma orbital. There is only a small amount of NN pi-bonding. A bond energy analysis shows that the lowest state is markedly stabilized by NNO three-centre interactions.     

An ab initio method for approximation of the frozen molecular fragment

An ab initio method for calculation on many-electron molecular systems with the approximation of the inactive part of a molecule by frozen molecular fragment is presented. In the following method the SCF calculations are performed in two series. First the molecular orbitals resulting from the first SCF calculation (modest basis set) are localized. In the second SCF run, the basis set is extended for the active part of the molecule, while molecular orbitals of the inactive part, selected from the localized set, are kept frozen. The results are in good agreement with the extended basis set calculation.