Semiempirical andab initio calculations of the full configuration interaction using iterated Krylov’s spaces

The algorithm proposed previously for calculating the full configuration interaction using the variation matrix of the wave operator involves the numerical solution of the corresponding incomplete eigenvalue problem based on iterated Krylov’s subspaces. In practice, that means using the multistep gradient method as a special version of the Lanczos method. The high efficiency of this algorithm, which can readily be used in personal computer calculations, is proved by particular ab initio calculations of the full configuration interaction for the helium and beryllium atoms as well as by semiempirical calculations of π-shells for naphthalene and diphenylene. The algorithm is of particular assistance in obtaining numerous excited states, which are used for determining various spectral sums (polarizability, van der Waals interaction constants, and photoionization cross sections). Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 1, pp. 14–22, January–February, 1997.     

Photoelectron spectra andab initio calculations of the electronic structure of amino-n-sulfenyl chlorides

The electronic structure of some amino-N-sulfenyl chlorides and related compounds is studied by photoelectron spectroscopy and ab initio calculations. Similar values of IP(n _S ) and IP(n _N ) and the total energy minimum indicate that in stable conformations the n _S and n _N orbitals are orthogonal. These conformers are characterized by an effective n_N-σ _S-Cl ^* interaction. The relationship between the intramolecular shift of charge and ionization potential values and the spatial structure of amino-N-sulfenyl chlorides is analyzed. A. E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 69–73, March–April, 1994. Translated by L. Smolina     

Semiempirical hyperfine structure and ab initio isotope shift predictions in Zr II

The fine structure of the even‐parity low configurations has been reanalyzed by simultaneous parameterization of the one‐ and two‐body interactions for the model space (4d + 5s)³. Using the calculated eigenfunctions, the magnetic‐dipole A hyperfine constants for the whole 37 existing levels of the model space were predicted and compared partially to those obtained using relativistic configuration‐interaction approach. Moreover, volume shifts (VS) and specific mass shifts (SMS) of numerous configurations of singly ionised zirconium are deduced by means of ab initio estimates combined with a few experimental isotope shift data available in literature: VS(4d¹5s²) = 840 MHz, VS(4d³) = ?649 MHz, VS(4d¹5p²) = ?387 MHz, VS(5s²5p¹) = 1250 MHz, and SMS(4d¹5s²) = ?634 MHz, SMS(4d³) = 484 MHz, SMS(5s²5p¹) = ?1459 MHz, referred to 4d²5s for the pair Zr⁹⁰–Zr⁹². © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Gas phase electron diffraction analysis andab initio calculations of the molecular structure of 3,3-dimethyl-1-oxa-3-silacyclopentane

3,3-Dimethyl-1-oxa-3-silacyclopentane was synthesized, and its molecular structure was determined by gas phase electron diffractometry using ab initio calculations (GAUSSIAN-92, RHF method, 6-31G^* basis set). The six models derived from the gas phase electron diffraction analysis equally adequately (R≈3%) describe experimental data, whereas quantum chemical calculations lead to only one (O-envelope) conformer. The stereochemical data and the results of ab initio calculations confirm that the O-envelope is the best conformation; its geometrical parameters are as follows (bond lengths r_a in Å, and angles in degrees; total error in parentheses in units of the least significant digit): r(Si-C_cyc) 1.891(3) and 1.904; r(Si-C_Me) 1.877 and 1.879; r(C-C) 1.556(7); r(C-O) 1.431(3) and 1.445; r(C_cyc-H) 1.11(2); r(C_Me-H) 1.10(2); ϕ(O−C−C−Si) −33.0(14); ∠C_cycSiC_cyc 92.2(5); −33.0(14); ∠C_MeSiC_Me 107.2(8); ∠SiCC 102.6(6); ∠CCO 109.9(11); ∠HC_cycH 110.5(47); ∠SiC_MeH 111.6(14). Method for the synthesis of 3,3-dimethyl-1-oxa-3-silacyclopentane is given, and generalized notation for five-membered ring confor-mations is suggested. Moscow State University. Tromse University, Norway. Irkutsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 689–707, July–August, 1996. Translated by L. Smolina     

Alkylhydrazides and their protonated forms: Physical methods andab initio calculations

The basicity of hydrazides of the highest aliphatic carboxylic acids RC(O)NHNH₂ (R = C_nH_2n+1,n = 5-12) has been studied by potentiometric titration, and IR and¹H NMR spectroscopy.Ab initio Hartree-Fork calculations using the 6–31G^* basis set with full optimization of geometry were carried out on the simplest acy1hydrazines and their possible protonated forms. Based on these calculations, and the 1R and¹H NMR spectra, the tautomerism of alkylhydrazides and the structures of their protonated forms are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2645–2649, November, 1996.     

Semiempirical versusab initio calculations of molecular properties

A comparative study of the structural parameters in some strained spirohydrocarbons is presented. The theoretical values provided by the STO-3G, MINDO/3 and IMOA methods are in reasonable agreement with each other and with available experimental data offering thus a posteriori justification of the employed approximate schemes.     

IR spectroscopic andab initio quantum-chemical study of the products of hydrolysis of silicon tetrafluoride at low water concentrations

According to the results of IR spectroscopic study and quantum-chemical calculations, hydroxo derivatives SiF_4-x(OH)_x are formed in the course of hydrolysis of silicon tetrafluoride in the presence of small amount of water along with hexafluorodisiloxane. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 92–96, January, 1999.     

Parametric analysis andab initio calculation of isotope shifts in rhenium I

The isotope shift in the arc spectrum of rhenium was studied in 12 lines for the highly enriched isotopes¹⁸⁵Re and¹⁸⁷Re by means of a photoelectric recording Fabry-Perot spectrometer with digital data processing. The observed shifts together with results from earlier studies were analyzed by means of the parametric method. The field shift difference between the terms 5d ⁶(⁵ D)6s ⁶ D and⁴ D was found to be 40 (5) mK, showing the influence of second-order effects. The parameterz _5d , displaying the magnitude of spin-dependent effects in 5d ⁶6s, was found to be 1.3 (8) mK. The experimental data are compared with results from non-relativistic Hartree-Fock calculations. The calculated electron densities describe the measured isotope shifts with good accuracy.     

Semiempirical study of the conformation of tetroxane and its halogenated derivatives

We have made a conformational study of tetroxane and its bi- and tetra-halogenated derivatives by means of the semiempirical AM1 and PM3 molecular orbital methods. The results are compared with previous semiempirical and ab initio studies as well as with available experimental data. We have found that in every case the AM1 method underestimates the peroxidic O-O bond length by approximately 12%, while the PM3 procedure describes such a bond in a better way. The degree of accuracy of these two semiempirical methods is discussed in relation with the tetroxane structure.     

An ab initio study of the guanidinium fragments in tetrodotoxin and saxitoxin

SCF Hartree–Fock calculations are used to predict properties of the guanidinium fragment present in tetrodotoxin and saxitoxin, toxins known to block the sodium channels of the nerve cell membrane. The results, in terms of net atomic charges and geometries, are compared to similar calculation results obtained for the Na⁺–H₂O complex.     

Ab initio and molecular mechanics study of n-phenyl phthalimide and its crystal structure

Ab initio molecular orbital calculations are reported on the energetics for torsional motion of N-phenyl phthalimide using 3-21G, 6-31G, and 6-31G^** basis sets and incorporating electron correlation effects for selected geometries. With the largest basis set, a minimum energy is found for a torsion angle of 59.2°. Atomic charges are assigned to the molecules on the basis of a least-squares fit to the molecular electrostatic potential. This information is then used in molecular mechanics calculations of the crystal structure, where the calculated unit cell parameters are in good agreement with those observed experimentally.     

Trinitromethyl anion. An ab initio MO study of its structure

Several conformers of nitroform anion are studied with ab initio MO methods.     

A comparison of the results of PCILO andab initio SCF calculations for the molecules glycine,cysteine and N-acetyl-glycine

Conformational energy maps for glycine, cysteine and N-acetyl-glycine obtained using PCILO andab initio SCF calculations are compared.     

The 1:1 glycine zwitterion-water complex: An ab initio electronic structure study

More than a dozen stationary points on the potential energy surface for the 1:1 glycine zwitterion—water complex have been investigated at Hartree-Fock or MP2 levels of theory with basis sets ranging from split valence (4-31G) to split valence plus polarization and diffuse function (6–31 + + G**) quality. Only one true minimum (GLYZWM, C₁ symmetry) could be located on the potential energy surface. GLYZWM features a bridged water molecule acting as both a hydrogen bond acceptor and donor with the NH₃⁻ and CO₂⁻ units of the glycine zwitterion. The total hydrogen bond energy in GLYZWM is computed as 16 kcal/mol (MP2/6–31 ++ G** // 6–31 ++ G**, including corrections for basis set superpositions errors). The computed vibrational frequencies and normal mode forms of the GLYZWM complex resemble in many cases experimental assignments made for the glycine zwitterion in bulk water on the basis of Raman spectroscopy. © 1996 by John Wiley & Sons, Inc.     

Ab initio calculations on the electronic structure of the divalent lead-water complex

We studied the electronic structure of the Pb (2+)-4H 2O system. Analysis of the complex orbital evidenced no mixing between the 6s lone pair orbital of the lead and the 6p orbital components. Moreover, we found that the HOMO is widely described by the mixture of the 6p components with the 7s valence orbital of the lead. This orbital shows an important elliptical electron charge density around the lead ion and opposite the direction of the short lead-water bonds. From these results, we demonstrated that the hemidirected conformation of the Pb (2+)-4H 2O system could be easily explained by the shape of the electron charge density distribution of the HOMO rather than by the stereochemically active character of the 6s (2) lone pair of lead electrons.     

Semiempirical study on the electronic structure of antitumor drugs ellipticines, olivacines and isoellipticines

Ellipticine [5,11-dimethyl-6H-pyrido[4,3]carbazole] is a planar organic compound isolated from the plant extract of Ochrosia elliptica, with a high degree of antitumor and cytotoxic activity. Ellipticine and its derivatives have different modes of action, which may include intercalation or covalent binding to DNA and interference with the activity of topoisomerase II. In the present work we studied the electronic, geometrical and spectroscopic properties of 31 ellipticine derivatives and analogue compounds, using the semiempirical methods Parametric Method 3 and Zerner's Intermediate Neglect of Differential Overlap. The Zero Differential Overlap semiempirical Molecular Electrostatic Potential (MEP) was also calculated in order to interpret the electronic structure of the molecules. For the biologically tested ellipticines and olivacines we have observed a rule based on the dipole moment values and a selective distribution of active sites (coming from the MEP calculations) that can be used to identify the active molecules. These results provide a pattern which can be used to select potentially active molecules from the untested group of molecules and to design new ellipticine derivatives with improved antitumor activity.     

Equilibrium structure of the carbon dioxide-water complex in the gas phase: an ab initio and density functional study

High-level ab initio (MP2/6-311++G(2d,2p) geometry, Gaussian-2, MP4(SDTQ) and QCISD(T) binding energies) and density-functional (Becke3LYP/6-311++G(2df,2pd)) calculations have been performed on the charge-transfer complex between water and carbon dioxide. The complex appears to have two equivalent non-planar minima of C_s symmetry. Minima are separated by transition states with C₁ symmetry, whereas the totally planar structure with C_2v symmetry is a second-order transition state. All the critical points lie at approximately the same energy (less than 0.05 Kj mol⁻¹ difference). Therefore, the experimentally observable structure should be planar. The best equilibrium intermolecular distance for this complex calculated at the MP2/6-311++G(2d,2p) level is 2.800 Å. Our best estimate of the observable intermolecular distance (corrected for anharmonicity) is 2.84 Å, in agreement with the experimentally derived value of 2.836 Å. Our best estimate of the binding energy at the QCISD(T) level, taking into account the variation of the distance owing to anharmonicity and the use of more sophisticated theoretical treatments, is −12.0 ± 0.2 kJ mol⁻¹. Our best estimate of the barrier to internal rotation, also at the MP2/6-311++G(2d,2p) level, is 4.0 kJ mol⁻¹, outside the error limits of the experimental determination (3.64 ± 0.04 kJ mol⁻¹). Density functional theory at the level employed here gives an equilibrium intermolecular distance that is too large (2.857 Å), a binding energy that is too small (8.1 kJ mol⁻¹), attributable neither to geometry nor to the basis set, and also a barrier to internal rotation that is slightly too small (3.39 kJ mol⁻¹). The overall picture is, however, reasonably good.     

Semiempirical study of the electronic and optical properties of the Er(8-hydroxyquinolinate)3 complex

We use a simple quantum chemical semiempirical procedure to study the electronic properties of organic-lanthanide complexes, taking as a model system Er(8-hydroxyquinolinate)3. Among the problems inherent to such a study is the fact that the lanthanide ion has never been parametrized in any of the standard semiempirical Hamiltonians. To overcome this difficulty, the lanthanide ion is replaced by a different but somewhat similar parametrized ion, or merely by a point charge. Good agreement with experiment, where available, is obtained, particularly in the former case. In fact, the electronic properties of the complex (apart from the emission properties) are seen to be scarcely affected by the nature of the lanthanide ion itself, but the core interactions between the metal ion and the ligand units play a relevant role, also in the calculation of the excitation energies. In particular, the ordering and separation of both singlet and triplet excited states are affected. The main conclusion is that to describe in detail the mechanism of the energy-transfer process occurring in the complex it is essential to take into account the geometry relaxation effects in the excited states.     

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.