Matti Hotokka

Solvent effects in enantioselective hydrogenation of 1-phenyl-1,2-propanedione

Abstract Solvent effects in enantioselective hydrogenation of 1-phenyl-1,2-propanedione ( A ) were investigated in a batch reactor over a cinchonidine modified Pt/Al 2 O 3 catalyst. The effect of different solvents, binary solvent mixtures and solvent dielectric constant on regio- and enantioselectivity as well as on the hydrogenation rate were studied. The hydrogen solubility in different solvents and the dielectic constants of solvent mixtures were measured. The highest enantiomeric excesses (ee) of ( R )-1-hydroxy-1-phenylpropanone ( B ) (65%) were obtained in toluene. The ee decreased non-linearly with an increasing solvent dielectric constant being close to zero in methanol. The role of the reactant conformation in different solvents was evaluated by quantum chemical calculations and the role of the Open(3) conformer of the modifier, cinchonidine was discussed. The dependence of ee on the dielectric constant could not solely be attributed to the abundance of the Open(3) conformer of cinchonidine in the liquid phase. A possible involvement of additional factors was proposed and discussed. The non-linear dependence of the ee on the dielectric constant was included in a kinetic model to describe quantitatively the variation of the ee in different solvents.

COMPUTER MODELLING OF THE 3-DIMENSIONAL STRUCTURES OF THE CYANOBACTERIAL HEPATOTOXINS MICROCYSTIN-LR AND NODULARIN

The 3-dimensional structures of two cyanobacterial hepatotoxins microcystin-LR, a cyclic heptapeptide and nodularin, a cyclic pentapeptide, and the novel amino acid ADDA (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid) were constructed, and optimized using the CHEM-X molecular mechanics program. The peptide rings were planar and of rectangular shape. Optimized ADDA formed a U-shape and a difference in the orientation of ADDA with respect to the peptide ring of the two hepatotoxins was observed.

An ab initio study of bonding trends in the series BO33−, CO32−, NO3− and O4(D3h)

Abstract Hartree-Fock 6-31G* calculations of the bond lengths and vibrational frequencies of the title series show a bond-length minimum and frequency maximum at nitrate. The same trends are observed with STO-3G calculations, in contrast to earlier work by Jubert and Varetti. The high-lying, trigonal planar tetraoxygen is found to be locally stable. CASSCF calculations on O 4 (D 3h ) predict a bond length of 132 pm and a heat of formation 7 (1) eV above 2O 2 and 3 (1) eV below 4O( 3 P), in good agreement with the EXRHF2 calculations of Roeggen and Wisloff Nielsen.

On bonding in transition-metal helide ions

CASSCF calculations are reported for the low-lying states of TiHe2+ and the lowest bound state of VHe3+. The computed dissociation energies D 0 are 0·16 and 0·69 eV and the equilibrium distances R 0 213 and 189 pm, respectively. The experimentally observed but metastable VHe3+ is estimated to have a very long lifetime. Corresponding calculations on AlHe3+ give D 0 and R 0 of 1·19 eV and 167 pm, respectively. Numerical one-centre expansion calculations on HeH+, BeHe++ and TiHe++ give much worse results than previously obtained for hydrides.

Relativistically parameterized extended Hückel calculations. VII. Nuclear spin—spin coupling tensors and densities of states for cluster models of CdTe, HgTe and PbTe

Abstract Relativistically parameterized extended Huckel (REX) calculations of spin—spin coupling tensors 1 J(MTe) for cluster models of CdTe, HgTe and PbTe are reported. The relativistic equivalent of Ramseys theory is used. Electronic densities of states are obtained as a by-product. Two assignments of the experimental NMR data are shown to be possible. The REX results support the original Nolle one for PbTe and the alternative ones for CdTe and HgTe. The increase of Δ J is attributed to relativistic effects, arising from the M( ns )—Te(5p 1 2 ) AO combination, recently discussed by Pyykko and Wiesenfeld. A principle is proposed that both K and Δ K are positive for systems with dominant bonding-to-antibonding excitations, a case likely for half-filled valence shells. Nearly empty or nearly full valence shells with dominant bonding-to-bonding or antibonding-to-antibonding excitations should lead to negative K and Δ K . Double-zeta REX radial parameters are reported for ZnHg and OPo.

Photoelectron spectra and their relativistic interpretation for gaseous bismuth trihalides

Abstract Photoelectron spectra are reported for BiCl 3 , BiBr 3 and BiI 3 . Assignments are obtained using the relativistically parameterized extended Huckel (REX) and perturbative Hartree—Fock—Slater methods. The halogen p-bands (4a 1 , 1a 2 , 4e, and 3e) resemble those in the lighter MX 3 molecules at (M = P− Sb). The bismuth—halogen sigma bond MOs (3a 1 and 2e) suffer a relativistic rehybridization into e 1 2 > e 3 2 > e 1 2 .

A THEORETICAL STUDY OF THE CONFORMERS OF TRANS- AND CIS-UROCANIC ACID

Abstract The geometry of both prototropic structures of trans-urocanic acid ((E)-3-(1H-imidazol-4(5)-yl)propenoic acid) and of the corresponding cis isomer [(E)-3-(1H-imidazol-4(5)-yl)propenoic acid] were studied by using quantum chemical ab initio and various semiempirical methods. A comparison of the computational methods shows that the AM1 Hamiltonian gives the best qualitative agreement with the 3-21G ab initio results for the potential energy surfaces of this class of molecules. The AM1 semiempirical method thus seems to be the best choice for locating the most interesting points on the potential energy surfaces of urocanic acid related molecules. The ab initio calculations can then be restricted to those regions of the surfaces. Both trans- and cis-urocanic acid are found to be planar. The most stable conformer of both trans-urocanic acid and cis-urocanic acid have the s-trans,s-cis conformation but they represent different prototropic structures. The barriers to torsion around all rotatable bonds are found to be quite high in both isomers of urocanic acid due to the π-bond conjugation which extends all over the molecules.

Quantum chemical potential energy surfaces for HXeCl

Abstract Relativistic pseudopotential calculations are reported for HXeCl at the CISD level. Potential energy surfaces for the singlet ground state and two excited singlet states are shown for the linear case. The two lowest triplet states are also shown. The potential energy curve for the bending motion is shown for the singlet ground state. The optimized ground state structure is linear, with R XeCl =267.4 pm and R XeH =175.8 pm. The depth of the minimum is 0.9 eV. This structure agrees favorably with the earlier reported nonrelativistic ab initio results. The second excited singlet state of the linear system also shows a minimum at R XeCl =249 pm and R XeH =267 pm. The depth of this minimum is 2.0 eV. The second triplet state shows a van der Waals minimum at R XeCl =320 pm and R XeH =360 pm with a depth of 0.02 eV. The harmonic fundamental frequencies for the ground state, calculated at the CISD level are 263 cm −1 for ν XeCl , 547 cm −1 for δ (doubly degenerate) and 1788 cm −1 for ν XeH . The corresponding scaled frequency for ν XeH agrees well with the experimental one.

Structure-stability relationships in unsaturated sulfur compounds VI. An ab initio study of the stable conformations of (E)- and (Z)-2-methylthio-, methylsulfinyl- and methylsulfonyl-2-butenes

Abstract The potential energy curves for the CS torsion of (E)- and (Z)-2-methylthio-, methylsulfinyl- and methylsulfonyl-2-butenes were calculated using ab initio methods (RHF/3-21G∗ and 6–31G∗). Each molecule had two energy minima symmetrically (ca. 60 °) of the s-trans (antiperiplanar) conformation. (E)-2-methylthio-2-butene also had a third local minimum in the s-cis (synperiplanar) conformation. In 2-methylthio-2-butene the Z isomer had ca. 2 kJ mol−1 lower energy than the E isomer. In the other two compounds the E isomer was the more stable one. The energy difference between the geometric isomers in 2-methylsulfinyl- and 2-methylsulfonyl-2-butene was 6–8 and 11–14 kJ mol−1, respectively.

A study of the stable conformations of methylvinyl sulfoxide and sulfone by ab initio calculations

Abstract The potential energy curves for the C-S torsion of methylvinyl sulfoxide and sulfone were calculated using ab initio methods (RHF/3-21G∗ and MP2/6-31G∗). In both cases two minima were found. In the case of methylvinyl sulfoxide the most stable conformation has the SO group cis to the double bond. The other stable conformation has the lone-pair electrons of the sulfur atom cis to the double bond. The energy difference between the two minima is 8–16 kJ mol−1 depending on the method used. In methylvinyl sulfone there are two equivalent minima with the SO group cis to the double bond.