Although the accuracy of computed tomographic imaging is better than that of plain radiographs, the difference does not reach statistical significance. Postoperative use of plain radiographs remains a reliable method for evaluation of pedicle screw insertion in the absence of neurologic deficit.

The products and the kinetics of the thermolysis of the title compounds 3 and 5 were investigated. Like the trans-isomers, the cis-azo compounds also undergo homolytic decomposition without accompanying cis-trans-isomerization. The observed structure-reactivity relationships are discussed. On irradiation of trans-tetra-tert-butylazomethane (trans-5) at 60 -120°C in benzene or chlorobenzene an almost quantitative yield of 1,1,2,2-tetra-tertbutylethane is obtained. Aliphntische Amverbindungen, XV ' ) cis-und trtuts-Tetracyclopropyl-und Tetra-tert-butylazomethanDie Produkte und die Kinetik der Thermolysen der Titelverbindungen 3 und 5 wurden untersucht. Wie die trans-Isomeren, so zerfallen auch die cis-Azoverbindungen in Radikale ohne begleitende cis-trans-Isomerisierung. Der EinfluD der Strukturvariation auf die Reaktivitat wird diskutiert. Durch Bestrahlen von trans-Tetra-tert-butylazomethan (trans-5) bei 60-120°C in Benzol oder Chlorbenzol la& sich fast quantitativ 1,1,2,2-Tetra-tert-butylethan darstellen.The cis-trans-isomerizations and the thermolysis reactions of tertiary azoakanes have been investigated much more thoroughly than those of secondary ones 1*). This is due to an additional complicating side reaction of the latter, the tautomerization to hydrazones 2 3). RzCH-N=N-CHRzR~CH-NH-N=CRZ 1 2Through our attempts to prepare 1,1,2,2-tetra-tert-butylethaneg and 1,1,2,2-tetracyclopropylethane ' z 6 ) the title azo compounds became available. The observation that they were not very sensitive to tautomerization stimulated our interest in the investigation of their thermal decomposition reactions. SynthesisA mixture of cis-and trans-l,1,1',1'-tetracyclopropylazomethane (3) was obtained6) from dicyclopropyl ketone via the azine, followed by reduction to the hydrazine and subsequent air oxidation.

Die Enthalpie-und Entropiedifferenz zwischen meso-und ~~-2,3-Dimethoxy-2,3-diphenylbernsteinsauredinitril (2) wurde aus der Temperaturabhangigkeit des thermisch uber reversible Bildung von a-Cyan-a-methoxybenzyl-Radikalen (3) eingestellten Gleichgewichts bestimmt und durch Kraftfeldrechnungen bestatigt. Die Aktivierungsenthalpien der Thermolyse von meso-und DL-2 bestatigen diese Differenz ebenfalls und stimmen uberein mit der ESR-spektroskopisch bestirnmten Dissoziationsenthalpie von 2 in 3. Die Radikale 3 sind demnach nicht kinetisch stabilisiert. Aus der Diskrepanz zwischen AS * von 2 und der Dissoziationsentropie A S wird auf primare Bildung sandwichartiger Radikalpaarkomplexe im DissoziationsprozeR geschlossen. Aus den Aktivierungsparametern der Thermolyse und den Spannungsenthalpien der Diastereomeren 2 sowie aus der experimentell bestimmten C,e, -H-Bindungsenthalpie von a-Methoxybenzylcyanid (1) ergibt sich kein Hinweis fur eine uber das Additive hinausgehende capto-dative Stabilisierung der Radikale 3. Substituent Effects on the CC-Bond Strength, 5 l) Kinetics and Thermochemistry of the Homolytic Dissociation of meso-and ~,~-2,3-Dimethoxy-2,3-diphenylsuccinonitrile The differences in enthalpy and entropy between meso-and ~~-2,3-dimethoxy-2,3-diphenylsuccinonitrile (2) were deduced from the equilibrium constants of their interconversion at elevated temperatures via reversible dissociation into a-cyano-a-methoxybenzyl radicals (3). They were reproduced by force field calculations and by the difference in activation enthalpy of dissociation of meso-and DL-2. The identity of A H * (2) and A H for the dissociation process 2 + 3 as obtained by ESR spectroscopy proves that the radicals 3 have no kinetic stabilization. From the discrepancy between AS * (2) and the entropy of dissociation AS (2 + 3) it is concluded that sandwich radical pair complexes are intermediates in the dissociation process of 2. From the relationships between AG * of thermolysis and the strain enthalpies H, of 2 and from the experimentally determined Cf,,, -H bond enthalpy of a-methoxybenzyl cyanide (1) it is concluded that no particular capto-dative stability effect exists for the radicals 3.In jiingster Zeit wurde vor allem durch die Arbeiten von Viehe2) auf die in mancher Hinsicht ungewohnliche Reaktivitat sogenannter ,,capto-dativer" Radikale aufmerk-0 Verlag Chemie GmbH, D

No abstract

No abstract