Heats Of Formation ΔH Research Articles

Reactions of some cyclic ketene acetals with maleic anhydride and acrylonitrile

The reactions of 9-methyl-4-methylidene-3,5-dioxabicyclo[5.4.0]undecane with maleic anhydride and acrylonitrile were studied by NMR spectroscopy and were found to give 1:1 molecular complexes with equilibrium constants of 0.24±0.06 and 0.14±0.04 l mol−1, respectively. Analysis of the calculated (AM1) heats of formation ΔH, differences in the dipole moments Δμ, frequencies and intensities of stretching vibrations of the donor and acceptors, and charge distribution showed formation of π-π, mixed π-π-H, and H complexes with partial charge transfer. These complexes can be used in copolymerization of the above monomers.

Mass spectrometric study of the ionization and dissociation of sulphur hexafluoride by monoenergetic electron impact

The ionization and fragmentation of SF6 under electron impact has been examined by means of a trochoidal electron velocity selector and a mass spectrometer. The ionization efficiency curves of the positive ions were recorded and the appearance energies of different electronic states of these ions were determined. The experimental results were combined with auxiliary thermochemical data to evaluate the dissociation energies D°°(F5S–F) = 4.01 eV, D°°(F4S–2F) = 6.39 eV, D°°(F3S–3F) = 9.98 eV, the ionization energy of SF5, = 11.27 eV and heats of formation ΔH°f(SF5+) = 1.81 eV, ΔH°f(SF4+) = 4.12 eV, ΔH°f(SF3+) = 4.11 eV and ΔH°f(SF3) = − 5.13 eV. © 1998 John Wiley & Sons, Ltd.

Substituenteneffekte auf die CC‐Bindungsstärke, 14. Kinetische und thermodynamische Stabilität von 2,3‐Bis(dialkylamino)‐1,4‐diketonen — Stabilisierungsenergie capto‐dativ substituierter a‐Dialkylamino‐a‐Carbonylalkyl‐Radikale

Substituent Effects on the Strength of CC Bonds, 14[1]. — Kinetic and Thermodynamic Stability of 2,3‐Bis(dialkylamino)‐1,4‐diketones — Energy of Stabilization of α‐Dialkylamino α‐Carbonylalkyl Radicals with Capto‐dative SubstituentsProf. Dr. H.‐G. Viehe zum 65. Geburtstag gewidmet. The equilibrium constants and rate constants for the dissociation of the 2,3‐bis(dialkylamino)‐1,4‐diketone diastereomers meso‐ and DL‐7a and 7b were measured over a temperature range of 40°C. From the enthalpies of dissociation ΔHDiss and enthalpies of activation ΔH≠ and the strain enthalpies of 7 the bond dissociation enthalpies BDE(CC) of 7 were determined. By comparison with the dissociation enthalpies of Ct‐Ct alkanes[21] the change of these BDEs(CC) by the cap‐to‐dative substitution was determined to be 85.4 kJ mol−1 (20.4 kcal mol−1). The heats of formation ΔH (g) of a series of amino ketones 8 were determined from their heats of combustion and their heats of evaporation. From the ΔH(g) values in combination with MM2 calculations of their strain enthalpies strain‐free increments CHn[N, CO, C2 ‐ n] with n With n = 0, 1, 2 were derived and geminal interaction enthalpies in the ground states were obtained thereof. The radical stabilization enthalpy RSE of 6 was deduced from the ΔBDE(CC) values and the ground state effect to be 73.6 kJ mol−1 (17.6 kcal mol−1). From these data and the radical stabilization enthalpies RSE of α‐aminoalkyl radicals (4.2 kJ mol−1) and α‐carbonyl radicals (28.9 kJ mol−1) a synergetic radical stabilization enthalpy of 40.5 kJ mol−1 (9.7 kcal mol−1) is deduced. This number combines “extra” resonance stabilization and general inductive or anomeric geminal substituent interaction in the radicals. The crystal structure of meso‐7a has been determined by X‐ray diffraction methods.

Thermolabile Kohlenwasserstoffe, 31 Stereoselektive Bildung und Spaltung der Dimeren des 1‐(5,6,7,8‐Tetrahydro‐1‐naphyl)neopentyl‐Radikals

Thermolabile Hydrocarbons, 31. – Stereoselective Formation and Cleavage of the Dimers of the 1‐(5,6,7,8‐Tetrahydro‐1‐naphthyl)neopentyl RadicalThe 1,2‐diaryl‐1,2‐di‐tert‐butylethanes meso‐ and DL‐6 are synthesised by partial catalytic hydrogenation of the parent dinaphthylethanes 5. The crystal structure of meso‐6 was obtained experimentally and calculated by molecular mechanics methods (MM2). It is shown, that 1‐(5,6,7,8‐tetrahydro‐1‐naphthyl)neopentyl radical 8 forms its dimers 6 with high stereoselectivity, e.g. DL‐6: meso‐6 = 45 (at ‐20°C) and 7.07 (at 100°C). The selectivity was measured over a range of 300 K by using several radical precursors. The difference of the enthalpy of activation has been derived for the two dimerisation reactions: δH†dim(DL – meso) = –2.8 ± 0.2 kcal/mol. The cleavage of DL‐6 and of meso‐6 into 8 was measured kinetically, and the enthalpies of activation δH†dis = 46.2 ± 0.6 (DL‐6) and 52.6 ± 1.3 kcal/mol (meso‐6) and the entropies of activation δ†dis = 8.4 ± 0.6 (DL‐6) and 20.0 ± 2.5 e.u. (meso‐6) have been obtained. A complete thermodynamic cycle is constructed by using the calculated (MM2) heats of formation δH°f = –27.6 (DL‐6) and –30.0 kcal/mol (meso‐6). Thus, the diastereomer (DL‐6), which is formed preferentially, appears to be the thermodynamically and kinetically less stable one. It turns out, that the high stereoselectivity of the dimerisation of 8, compared to the parent 1‐phenyl‐neopentyl radical (2a), is mainly caused by the steric repulsions between the approaching radicals.