Substituent Effects on the C-C Bond Strength, 15[1] Geminal Substituent Effects, 7[2]. Thermochemistry and Thermal Decomposition of Alkyl-substituted Tricyanomethyl Compounds

The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile ( 10a ) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile ( 10b ) decomposed heterolytically into carbenium ions and (CN)₃C⁻ anions, while 9-methyl-9-(tricyanomethyl)fluorene ( 10c ) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH ( 10c ) = 155· 2 kJ mol⁻¹, ΔS ( 10c ) = 58· 5 J mol⁻¹ K⁻¹]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile ( 2 ) (422.45 kJ mol⁻¹), 2,2-dicyanohexanenitrile ( 3 ) (349.74 kJ mol⁻¹), 2,2-dicyano-3-phenylpropionitrile ( 4 ) (540.75 kJ mol⁻¹), 2-butyl-2-methylhexanentrile ( 5 ) (-133.20 kJ mol⁻¹), 2,2-dimethylpentanenitrile ( 6 ) (-45.78 kJ mol⁻¹), and 2-methylbutyronitrile ( 7 ) (2.44 kJ mol⁻¹) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1 RSE ( 1 ) = -18 kJ mol⁻¹ was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol⁻¹) and α,α-dicyanoalkyl (27 kJ mol⁻¹) radicals, which were recalculated from bond homolysis measurements^[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2).     

Thermochemical study of aliphatic and phenyl-substituted geminal diacetates

The standard enthalpies of combustion _c H ^o of aliphatic diacetates1 and aromatic diacetates2 were measured calorimetrically. The enthalpies of vaporization _vap H ^o or sublimation _sub H ^o of1 and2 were obtained from the temperature function of the vapor pressure measured in a flow system. From _f H ^o(g) of1 and2 new values of group increments for the estimation of standard enthalpies of formation of these classes of compounds were derived. The geminal interaction energy between the geminal acyloxy groups shows no anomeric stabilization.Geminal Substituent Effects, Part 12, for part 11 see Ref. 7.     

Melting of icosahedral gold nanoclusters from molecular dynamics simulations

Molecular dynamics simulations show that gold clusters with about 600-3000 atoms crystallize into a Mackay icosahedron upon cooling from the liquid. A detailed surface analysis shows that the facets on the surface of the Mackay icosahedral gold clusters soften but do not premelt below the bulk melting temperature. This softening is found to be due to the increasing mobility of vertex and edge atoms with temperature, which leads to inter-layer and intra-layer diffusion, and a shrinkage of the average facet size, so that the average shape of the cluster is nearly spherical at melting.     

Structural Studies of Bis(cyclopentadienyl)molybdenum‐Amino Acid Complexes

The crystal structures of molybdocene‐amino acid compounds of the type [Cp₂Mo^IV(κN, κO‐AA)]⁺Cl^—·xH₂O with AA = D ‐phenylalaninato (x = 1.5), DL ‐leucinato (x = 2) and DL ‐valinato (x = 1) have been determined (Cp = η⁵‐C₅H₅). The compounds feature an almost planar, five‐membered chelate ring of the aminocarboxylate moiety (deprotonated amino acid) with the molybdenum atom. In the phenylalaninato complex π‐stacking between the phenyl rings is found. The complexes were proven kinetically stable at pH < 1 for at least 24 h.     

Analyzing Anisotropic Exchange in a Pentanuclear Os2Ni3 Complex

Spin Hamiltonian parameters of a pentanuclear Os Ni cyanometallate complex are derived from ab initio wave function based calculations, namely valence-type configuration interaction calculations with a complete active space including spin-orbit interaction (CASOCI) in a single-step procedure. While fits of experimental data performed so far could reproduce the data but the resulting parameters were not satisfactory, the parameters derived in the present work reproduce experimental data and at the same time have a reasonable size. The one-centre parameters (local matrices and single-ion zero field splitting tensors) are within an expected range, the anisotropic exchange parameters obtained in this work for an Os−Ni pair are not exceedingly large but determine the low-T part of the experimental χT curve. Exchange interactions (both isotropic and anisotropic) obtained from CASOCI have to be scaled by a factor of 2.5 to obtain agreement with experiment, a known deficiency of such types of calculation. After scaling the parameters, the isotropic Os−Ni exchange coupling constant is cm⁻¹ and the D parameter of the (nearly axial) anisotropic Os−Ni exchange is ⁻¹, so anisotropic exchange is larger in absolute size than isotropic exchange. The negative value of the isotropic J (indicating antiferromagnetic coupling) seemingly contradicts the large-temperature behaviour of the temperature dependent susceptibility curve, but this is caused by the negative g value of the Os centres. This negative g value is a universal feature of a pseudo-octahedral coordination with configuration and strong spin-orbit interaction. Knowing the size of these exchange interactions is important because Os(CN) is a versatile building block for the synthesis of / magnetic materials.     

Isolierung,Konstitution und Synthese der (R)-(−)-Eucominsäure

From the bulbs of Eucomis punctata L'Hérit. (Liliaceae) and of a hitherto undefined species of Eucomis a new optically active phenolic carboxylic acid, eucomic acid, was isolated. Structure 1 was assigned on the basis of chemical and spectral evidence. The absolute configuration of eucomic acid was determined by its correlation with piscidic acid ((2 R, 3 S)-2-(4′-hydroxybenzyl)-tartaric acid) ( 8 ). Consequently, eucomic acid is (R)-(?)-2-(4′-hydroxybenzyl)-malic acid ( 1 ). For the stereospecific synthesis, methyl cis-p-methoxybenzylidene-succinic acid ( 22 ) was transformed into the γ-lactone 24 which, by catalytic hydrogenolysis, yielded (±)-2-(4′-hydroxybenzyl)-malic acid 1-methyl ester ( 27 ). Resolution with (?)-quinine led to the enantiomeric acids 29 and 30 . The methyl ester of the levorotatory enantiomer 30 was identical with the dimethyl ester 3 of 4′-O-methyl-eucomic acid.     

Silylanions: inversion barriers and NMR chemical shifts

Alpha-substituent effects on inversion barriers and NMR chemical shifts have been studied on a set of silyl anions, [X(3-n)Y(n)Si](-) (X, Y=H, CH(3), and SiH(3)). The MP2/6-31+G* optimized structures show a pattern of increasing inversion barriers with augmenting numbers of methyl substituents. The highest barrier of 48.5 kcalmol(-1) is obtained for the (CH(3))(3)Si(-) ion. The silyl group displays the opposite effect by decreasing the inversion barrier to a minimum of 16.3 kcalmol(-1) in (SiH(3))(3)Si(-). The influence of counterions on these barriers is probed by addition of a lithium or potassium cation. In most cases, a decrease of the energy barriers with respect to the bare anions is observed. The (29)Si NMR chemical shifts calculated at the IGLO-DFT and GIAO-MP2 level of theory are also analyzed in view of the substituents and counterions.     

Substituent control of hydrogen bonding in palladium(II)-pyrazole complexes

Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.     

Structural phase transitions in ‘hcp’ C70 single-crystals

The thermal expansion of vapor-grownC ₇₀ single crystals ahs been investigated using high-resolution capacitance dilatometry from 5–380 K. Measurements were made both parallel and perpendicular to the hexagonalc-axis. Three first-order phase transitions which we associate with the consecutive disordering of theC ₇₀ molecules are observed upon heating at 280 K (long-axis spinning), 300 K (long-axis precession) and 355 K (quasi-free rotation), respectively. The highest-temperature transition exhibits a very large (50 K) thermal hysteresis. Powder and single-crystal X-ray diffraction show that the crystals are predominantly hexagonal-close-packed (HCP) with an idealc/a1.63 above 360 K andc/a1.84 at 295 K.