Toomas Tamm

Calculations on indium and thallium cyclopentadienyls. Metal–metal interactions and possible new species

The possibility of an attractive interaction between two closed-shell molecules containing In(I) or Tl(I) was studied using relativistic pseudopotentials and correlated abinitio methods ranging from MP2 to CCSD(T). The results for the simple test system (TlH)2 qualitatively agree with earlier ones by Schwerdtfeger (Inorg. Chem., 1991, 30, 1660). The calculated dimerization energy is -20 kJ mol-1. The results for the more realistic [M(η5-Cp)]2 model (M=In, Tl; Cp=cyclopentadiene, C5H5) yield a weaker attraction of about - 12 and - 16 kJ mol-1, respectively. The M–M secondary bond lengths are somewhat longer and the M–M–Cp angles more acute than in the dimers found in solids. This is attributed to the crystal effects. The calculated structures for [M(η5-Cp)2]- anions agree with experimental ones for the known M=Tl case. Predicted structures are given for the [M(η5-Cp2)]+ cations and for a hypothetical M(η5-Cp)3.

Calculations of hydrated titanium ion complexes: structure and influence of the first two coordination spheres

Abstract Density-functional studies of titanium cation complexes with 6 and 18 water molecules were carried out. The charges +2, +3 and +4 were all considered. The lowest-energy minimum of the trivalent hexahydrated cation may have S6 symmetry. The inclusion of the second coordination sphere leads to two geometrically distinct types of minima. Hydration energies of the Ti2+ and Ti3+ are calculated with 13–86 kJ/mol accuracy. The hydration energy of the Ti4+ ion is estimated at 7800 kJ/mol.

Application of the QSPR approach to the boiling points of azeotropes.

CODESSA Pro derivative descriptors were calculated for a data set of 426 azeotropic mixtures by the centroid approximation and the weighted-contribution-factor approximation. The two approximations produced almost identical four-descriptor QSPR models relating the structural characteristic of the individual components of azeotropes to the azeotropic boiling points. These models were supported by internal and external validations. The descriptors contributing to the QSPR models are directly related to the three components of the enthalpy (heat) of vaporization.

Organocatalytic asymmetric addition of malonates to unsaturated 1,4-diketones

Summary The organocatalytic Michael addition of malonates to symmetric unsaturated 1,4-diketones catalyzed by thiourea and squaramide derivatives with Cinchona alkaloids afforded the formation of a new C–C bond in high yields (up to 98%) and enantiomeric purities (up to 93%). The absolute configuration of the product was suggested from comparison of the experimental and calculated VCD spectra of the reaction product 3a.

Stability and Conformation of Polycopper-Thiolate Clusters Studied by Density Functional Approach

Quantum chemical studies of biologically relevant copper thiolate clusters can afford unique information about energetic principles of their formation and structure, which is important for understanding the basic principles of their formation and functioning in biological systems. In the current study, we used quantum chemical methods for the investigation of the structure and stability of Cu(x)S(y)-type clusters that serve as models for different copper thiolate clusters or for their intermediates in a variety of copper proteins. Density functional theory based modeling was performed including solvent effects for water and protein-like environments. Thermodynamic parameters (DeltaH, DeltaS, DeltaG) were calculated in order to assess the effect of thermal contributions to the formation energies of various copper thiolate clusters. The all-tricoordinated polycopper thiolate cluster [Cu4(SMe)6]2- turned out to be the most stable structure among the calculated ones. This result is in agreement with the prevalence of this type of clusters in various copper proteins with no sequence homology that contain six cysteine residues. The cooperativity of formation of [Cu4(SMe)6]2- can be inferred from the significant energy differences between intermediary clusters. Among tetrathiolate structures, [Cu2(SMe)4]2- was the most stable one. This cluster is also found in many copper proteins. Influence of slight structural perturbations on the energetics of copper thiolate clusters is also analyzed and discussed.

2-Substituted agelasine analogs : synthesis and biological activity, and structure and reactivity of synthetic intermediates

2-Substituted N-methoxy-9-methyl-9H-purin-6-amines were synthesized either from their corresponding 6-chloro-9-methyl-9H-purines or 2-chloro-N-methoxy-9-methyl-9H-purin-6-amine. Great diversity in the amino/imino tautomeric ratios was observed and calculated based on 1H NMR. The tautomers were identified by 1D and 2D 1H, 13C, and 15N NMR techniques, and showed significant variation both in 13C and 15N shift values. Comparison of the tautomeric ratios with Hammett F values revealed that as the field/inductive withdrawing abilities of the 2-substituent increased, the ratio of amino:imino tautomers was shifted toward the amino tautomer. Computational chemistry exposed the significance of hydrogen bonding between solvent and the compound in question to reach accurate predictions for tautomeric ratios. B3LYP/def2-TZVP density functional theory (DFT) calculations resulted in quantitatively more accurate predictions than when employing the less expensive BP86 functional. N-7-Alkylation of the 2-substituted N-methoxy-9-methyl-9H-purin-6-amines showed that when the field/inductive withdrawing ability of the 2-substituent reached a certain point the reactivity drastically dropped. This correlated with the atomic charges on N-7 calculated using a natural bond orbital (NBO) analysis. Biological screening of the final 2-substituted agelasine analogs indicated that the introduction of a methyl group in the 2-position is advantageous for antimycobacterial and antiprotozoal activity, and that an amino function may improve activity against several cancer cell lines.

Theoretical prediction and assignment of vicinal 1H-1H coupling constants of diastereomeric 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes.

Spin–spin coupling constants between nuclei in NMR spectroscopy reflect their spatial arrangement. A number of calculation methods, applying different levels of theory, have been developed to support the stereochemical assignment of novel compounds. Nevertheless, revisions of the assignment of structures in the literature are not rare. In the present work, the reliability of the calculation methods amenable for a theoretical prediction of spin–spin coupling constants of vicinal protons to support correct stereochemical assignment of substitution at five‐membered rings of 3‐alkoxy‐6,7‐epoxy‐2‐oxabicyclo[3.3.0]octanes was studied. Experimental 3J(H,H) coupling constants were compared with the coupling constants calculated for all possible diastereomers. The fully quantum chemical approach provided theoretical 3J(H,H) coupling constants with an absolute deviation of no more than 1.1 Hz for 91% of the experimentally studied coupled spins, whereas the methods without quantum chemical geometry optimization resulted in completely unreliable predictions. Consequently, for a reliable stereochemical assignment of small and medium size molecules, the protocol for calculating the coupling constants based on the results of the quantum chemical geometry optimization is recommended. Copyright

Possible high-pressure structures of sulfur trioxide

Calculations with the linearized augmented plane wave method indicate that several high-density forms of sulfur trioxide should be accessible at pressures above 29 GPa, with densities up to 1.7 times larger than the presently known forms of solid SO3.

Using theoretical descriptors to model solvent effects in the isomerization of cis‐stilbene

Experimental observations of the photoinduced excited-state lifetime of cis-stilbene have shown a distinct dependence on solvation processes. The rate of decay, dominated by a cis-trans isomerization, is more rapid in polar solvents than in nonpolar solvents. Linear solvation energy relationship (LSER) techniques show that this can be explained in terms of polarity and polarizability parameters for the solvent. Theoretical linear solvation energy relationship (TLSER) analysis shows that this can be explained in terms of solvent polarizability and electrostatic basicity. New TLSER descriptors based on calculated solvent bond diplole parameters are also successful in describing this solvent dependence. Solvent-dependent dipole moments are calculated for an approximate stilbene transition-state geometry using the polarizable continuum model (PCM), which suggests the usefulness of a more detailed study of this photoisomerization process using current solvation theory and computational techniques.

Synthesis of Superhard Lightweight Composites and Improvement of Their Properties via Chemical Processing

Superhard lightweight composites were prepared by self-propagating high-temperature synthesis (SHS), attrition milling, chemical leaching in concentrated HCl and/or HNO3 acids and spark plasma sintering (SPS) under N gas pressure. Such processing features were conducted for production of the superhard B13C2, B11.72C3.28, c-BN and c-BC2N chemical compounds in the B4C(67 wt%)-Al/WC-Co/Cu-based composite. The materials were studied by X-ray diffraction, scanning electron microscope equipped with energy-dispersive spectrometer system (SEM-EDS) and also with microindentation and nanoindentation of composites. During SHS, the B4C was partly transformed into boron-rich B13C2 boron carbide in quantity of ~60 wt%. The c-BC2N content in the composite was ~12 wt% after heat treatment of SHS-composite under nitrogen gas flow at temperature of 850 °C for 2 h. Subsequently, the disintegrated and attrition-milled powdered SHS-composite was chemically leached, and as result, the soft Al-containing compounds were reduced from ~27 wt% to ~4 wt%. During SPS, the boron-rich B13C2 boron carbide was transformed into carbon-rich B11.72C3.28 boron carbide in quantity of ~87 wt%. The maximal Vickers microhardness of superhard composites was ~4400 HV1.0 (~46 GPa). The composite has high thermal stability and chemical inertness.