Structural study of smectic A phases in homologous series of N-alkylpyridinium alkylsulphates

The mesomorphic properties of 27 homologues of the N-alkylpyridinium alkylsulphate series were studied by X-ray diffraction and dilatometry. All of these compounds exhibit single smectic A phases whose layer spacings are between 0.64 and 0.8 times the molecular length. Polarizing optical microscopy observation shows that these peculiar layer spacings are not due to the tilting of the molecules; a new packing model is proposed to explain the particular structure of these thermotropic ionic liquid crystal phases.     

Structural study of smectic A phases in homologous series of N-alkylpyridinium alkylsulphates

The mesomorphic properties of 27 homologues of the N-alkylpyridinium alkylsulphate series were studied by X-ray diffraction and dilatometry. All of these compounds exhibit single smectic A phases whose layer spacings are between 0.64 and 0.8 times the molecular length. Polarizing optical microscopy observation shows that these peculiar layer spacings are not due to the tilting of the molecules; a new packing model is proposed to explain the particular structure of these thermotropic ionic liquid crystal phases.     

Non-empirical LCAO-MO-SCF calculations of the electronic structure of SiF2

Non-empirical LCAO-MO-SCF calculations on SiF₂ using two Gaussian basis sets are reported. The larger basis set gives a calculated geometry in good agreement with experiment. The effect on the energy and population analysis of optimization of the Si 3d exponent was investigated. 3d orbitals are found to be much less important in the bonding than in the isoelectronic molecule SO₂.     

Non-empirical calculations on the conformational structure of azobenzene and benzylideneaniline

The conformational structures of cis- and trans-azobenzene and benzylideneaniline have been investigated by means of ab initio SCF calculations. Contrary to semiempirical results, the equilibrium molecular geometries are correctly accounted for in the non-empirical SCF-formalism. Trans-azobenzene is found to be planar, or at least peri-planar, while the phenyl rings of the cis-isomer are twisted by 56° out-of-plane. Both isomers of benzylideneaniline are non-planar, with rotational angles θ₁ (C-N) = 48°, θ₂(C-C) = 0° and θ₁ = θ₂ = 75° for the trans and cis form, respectively. Trans-azobenzene is calculated to be more stable by 10.4 kcal mol^?1 than the cis isomer, which is in good accord with the experimental value of 10 kcal mol^?1. The energy of isomerization of benzylideneaniline amounts to 13.0 kcal mol^?1.     

Non-empirical calculations of HF oligomers

The Gibbs free energies of oligomers of hydrogen fluoride are calculated using the ab-initio molecular orbital method and by statistical mechanics. The equilibrium of gaseous hydrogen fluoride is discussed.     

Application of quantum chemical calculations of C NMR chemical shifts to quinoxaline structure determination

Comparison of experimental and theoretical (GIAO DFT) ¹³C NMR chemical shifts allows the reliable assignment of isomeric structures of heteroaromatic compounds. This methodology was applied to establish the structures of isomeric quinoxalines. A modern 1D NOE technique permitted independent proof of the proposed structures.     

Non-empirical calculations on the rydberg states of ethylene

Different methods for representing the upper orbitals in Rydberg transitions are tested by means of a series of ab initio SCF and CI calculations for ethylene and various properties for such diffuse united-atom species are reported. Calculated transition energies indicates the maximum separation between individual components of the (π,3d) Rydberg species to be in the 0.1-0.2 range, with similarly small energy separations being obtained for the corresponding (π, 3p) states     

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.     

Non-empirical calculations on the conformation and hyperfine structure of the silyl radical influence of vibrational effects

An ab-initio spin-restricted SCF and perturbative configuration interaction study of the silyl radical is presented. The vibrational dependence of isotropic coupling constants is investigated using double-zeta and double-zeta plus polarization basis sets. The calculations predict a nearly tetrahedral geometry for the radical with an inversion barrier of 5.85 kcal/mol. The vibrational treatment leads to coupling constants (α_Si = ?190.20 G;α_H = +5.03 G) in excellent agreement with experiment.     

Synthesis, structure and quantum chemical calculations on p-trifluoromethylphenyl thioacid amide

The title compound of p-trifluoromethylphenyl thioacid amide has been synthesized in one step and characterized by elemental analysis, UV and X-ray single crystal diffraction. Ab initio calculations indicate that both HF/6-311G^** and B3LYP/6-311G^** methods can reproduce the title compound well. Electronic absorption spectra calculated by the time-dependent density functional theory (TD-DFT) show that the two absorption bands are mainly derived from the contribution of bands π → π^*. Thermodynamic properties of the title compound have been predicted based on the optimized structure. The calculation of the second order optical nonlinearity also has been carried out, and the molecular hyperpolarizability is 2.31770 × 10⁻³⁰ esu.     

Electronic and geometric structure of isomers of nitric acid. DFT quantum chemical calculations

Based on the B3LYP/6-311++G(3df,3pd) density functional method, quantum chemical calculations of the electronic structure, geometry, and thermodynamic parameters of eight isomers of nitric acid (three known isomers in the form of peroxynitrous acid ONOOH and five new isomers in the form of oxo-conformation OON(H)O) are presented in the work. The molecular structure of each isomer is characterized by a local minimum on the potential energy hypersurface of the HNO₃ molecular system and corresponds to one of its stationary states. A theoretical study of the reactivity of nitric acid oxo-isomers characterized for the first time can provide adequate explanation for experiments on the autocatalytic use of nitric acid vapors in binding molecular nitrogen. The results obtained can be a direction for developing principally new methods to bind atmospheric nitrogen and activate methane, which are fundamental problems in chemical science and technology.     

Molecular structure of noradrenaline according to gas-phase electron diffraction data and quantum chemical calculations

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Molecular structure of carphedon as studied by gas electron diffraction and quantum chemical calculations

The gas-phase structure and conformational properties of carphedon (C₁₂H₁₄N₂O₂, phenylpiracetam, 2-oxo-4-phenyl-1-pyrrolidineacetamide) have been determined by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 with 6-31G and cc-pVDZ basis sets). Since quantum chemical calculations demonstrate that the orientation of the acetamide group is fixed by a strong intramolecular N–H(amide)···O(pyrrolidone) hydrogen bond, the number of possible conformers is reduced considerably. Depending on the conformation of the pyrrolidine ring, envelope with out-of-plane C4 atom and acetamide group on the same side of the plane (“+”) or envelope with C4 and acetamide group on opposite sides (“?”), and on the orientation of the phenyl ring, axial (Ax), or equatorial (Eq), four relevant conformations, Ax?, Ax+, Eq?, and Eq+, exist. According to both quantum chemical methods (B3LYP and MP2 with cc-pVDZ basis sets) these four conformers differ by less than 2 kcal/mol in free energies. However, the two methods predict different relative free energies. The GED data were analyzed with different models. With a single-conformer model the best fit of the experimental GED intensities (agreement factor R _f = 4 %) is obtained with the Ax+ conformer. Using a two-conformer model the fit improves considerable for a 50(11):50(11) mixture of Ax? and Eq+ conformers (R _f = 2.7 %). No further improvement is obtained with a three-conformer model and large uncertainties for relative contributions occur. The geometric parameters of gaseous carphedon are compared with those in the crystal phase, where two molecules are connected by two intermolecular N–H···O hydrogen bonds, and with gas-phase values of piracetam.     

Molecular structure and conformations of benzenesulfonamide: gas electron diffraction and quantum chemical calculations

The molecular structure and conformational properties of benzenesulfonamide, C6H5SO2NH2, were studied by gas electron diffraction (GED) and quantum chemical methods (MP2 and B3LYP with different basis sets). The calculations predict the presence of two stable conformers with the NH2 group eclipsing or staggering the SO2 group. The eclipsed form is predicted to be favored by about 0.5 kcal/mol. According to GED, the saturated vapor over solid benzenesulfonamide at a temperature of 150(5) degrees C consists of the eclipsed conformer. The GED intensities, however, possess a very low sensitivity toward the vapor composition, and contributions of the anti conformer of up to 75% (at the 0.05 level of significance) or up to 55% (at the 0.25 level of significance) cannot be excluded. The molecule possesses C(sS) symmetry with the S-N bond perpendicular to the ring plane.     

1-Boryl-3,4-dimethylphosphole trimer: Synthesis, crystal structure and quantum chemical calculations

A novel cycloadduct of 1-boryl-3,4-dimethylphosphole was prepared by reaction of 3,4-dimethylphospholyl anion with monobromoborane-methylsulfide complex (CH₃)₂S · BH₂Br at −60 °C. It was characterized as a six-membered trimer by spectroscopic means, and its structure confirmed by an X-ray crystal analysis and quantum chemical calculations. Density functional theory calculations (B3LYP) showed that the cyclic trimer is by far more stable than the monomer, dimers or open-chain forms. Various molecular and spectroscopic properties of the borylphosphole monomer and trimer were evaluated. In particular, the changes of the ³¹P NMR chemical shifts upon oligomerization were examined. The six-membered ring was demonstrated to exist preferentially in a chair-like conformation. Computed NMR chemical shifts (¹H, ¹³C and a lesser extent ³¹P) appear to be a highly sensitive analytical tool for distinguishing ring conformations having only small energy differences.     

Ureases: quantum chemical calculations on cluster models

Herein, we present results from a computational study of dinickel complexes that are relevant to the catalytic hydrolysis of urea exerted by the urease enzymes. The B3LYP density functional is used to characterize the equilibrium geometry, electronic and magnetic properties, and energies for a series of realistic complexes modeling the active site of ureases. The analysis of the theoretical results gives new insight into the structure, substrate binding, and catalytic mechanism. The water bridge between the two Ni(II) ions observed in the crystallographic structures of the ureases was assigned to a hydroxide bridge in agreement with the observed small antiferromagnetic coupling. Both monodentate and bidentate urea-bound complexes, in which urea had favorable orientations for catalysis, were characterized. Finally, two reaction mechanisms were investigated starting from the monodentate and bidentate urea-bound complexes, respectively. Both a Ni1...Ni2 bridging hydroxide and a Ni2-bound water molecule play crucial roles in the two mechanisms.     

Conformational properties and electronic structure of tetrahydrotetrazines studied by photoelectron spectroscopy and quantum chemical calculations

The photoelectron (PE) spectra of tetrahydro-1,2,3,4-tetrazines 1 and 2 and tetrahydro-1,2,4,5-tetrazines 3–5 have been recorded and their conformations have been investigated by ab initio SCF calculations. While v-tetrazine2 is planar, tetrazines 1 and 3–5 each possess two low-energy conformations, according to ab initio HF and Becke3LYP methods. Attempts to assign ionization potentials to molecular orbitals obtained by semiempirical PM3 calculations indicate that this method is not suited for the compounds studied. Best results were obtained when the ab initio hybrid method Becke3LYP of the density functional theory was employed. Two conformers of 1 and 3–5 are present in the gas phase and their PE spectra are superimposed one upon the other. For v-tetrazine1, ionizations arising from half-chair and unsymmetrical boat conformers have similar energies and cannot be separated in the PE spectrum. For s-tetrazine3, on the other hand, the spectrum clearly shows different ionizations of both half-chairs, 3ee and 3ae.     

Ab initio quantum chemical calculations of a cluster C8H12

Using unrestricted Hartree–Fock–Roothaan approximation and density functional theory on 6‐31G* basis, ab initio calculation cluster C₈H₁₂ has been carried out. It was shown as a result of calculations that a ground state for the conformation given is a septet state. This cluster can be used for investigation of the kinematic mechanism of magnetic exchange and magnetic ordering in polyradicals. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002