The structure of a molecule can change considerably as its energy and thus its electron distribution within the time-domain of dynamic relaxation varies. Based on comparison of approriate measured data of related compounds and supported by quantum chemical calculations, therefore, charge-perturbed and/or sterically overcrowded molecules can be designed. Their preparation, handling, and structural characterization, frequently under extreme and especially largely aprotic conditions, provides some surprises. New structural principles become evident and old-fashioned ones are confirmed. Thus the contact-ion aggregates that form on ultrasonically supported reduction of unsaturated hydrocarbons with sodium metal partly contain dibenzene sodium sandwiches. Vicinal dimethylamino substituents or isoelectronic isopropyl groups cause steric overcrowding and facilitate oxidation to molecular cations by energetically favorable delocalization of the generated positive charge. Molecules and molecular ions in which an even number of II electrons are distributed over a D skeleton containing an odd number of centers preferentially form cyanine subunits. This is demonstrated by the novel ethene dication and dianion salts with central C-C single bonds and molecular halves twisted relative to each other. Altogether in two years well over 50 structures have been determined. Much has been learned from them, especially about electron transfer and contact ion-pair formation in aprotic solvents. Nevertheless, we had to realize that answers to many questions, above all "what crystallizes, how, and why", are still out of reach."ActuuNy, everything is more comjdicuted".calculation of an approximate one-dimensional MNDO energy hypersurface, in which only one of the total of (3n -6) = 30 degrees freedom for the molecule containing n = 12 centers, namely the dihedral angle w(C-SCCS-C) (Scheme 2), is considered. 550@ VCH Verluxsgesellschaft mbH, W-6940 Weinheinz. 1992 OS7O-OS33~92jOSOS-OSSO $3.50+ .2SjO Angew. Chem. Int. Ed. Engl. 31 (1992) 550-581 The Structures of Radical Cations R,NNR;@ in SolutionThe oxidation of silyl-substituted hydrazine derivatives by the oxygen-free and selective one-electron-transfer oxidation reagent AICI,/CH,Cl,[41 yields their radial cations['a' 4 9 61 in their doublet ground states [Eq. (a)].

Die Struktur eines Molekiils kann sich mit seiner Energie und damit seiner Elektronenverteilung innerhalb der Zeitskala dynamischer Relaxation betrlchtlich andern. Ausgehend von Vergleichen geeigneter MeDdaten verwandter Verbindungen und unterstiitzt durch quantenchemische Naherungsrechnungen lassen sich daher ladungsgestorte undloder raumlich iiberfiillte Molekiile entwerfen, deren Synthese, Handhabung und Strukturbestimmung -unter oft extremen und vor allem weitgehend aprotischen Bedingungen -zu manchen Uberraschungen fuhrt. Dabei werden neue Strukturprinzipien deutlich und altbekannte bestatigt. So enthalten einige der bei ultraschallgeforderter Reduktion ungesattigter Kohlenwasserstoffe mit Natriummetall entstehenden Kontaktionenaggregate von Molekiilanionen DibenzolnatriumSandwich-Einheiten. Vicinale Dimethylaminosubstituenten bewirken wie die isoelektronischen Isopropylgruppen sterische Uberfiillung und erleichtern die Oxidation zu Molekiilkationen durch energetisch giinstige Delokalisierung der erzeugten positiven Ladung. Molekiile und Molekiilionen, in denen sich eine gerade Anzahl von .n-Elektronen iiber ein ungeradzahliges cr-Geriist verteilt, bilden bevorzugt Cyanin-Untereinheiten aus; dies belegen auch die erstmals isolierten Salze von Ethen-Dikationen und Ethen-Dianionen mit C-C-Einfachbindung und gegeneinander verdrillten Molekiilhalften. -Insgesamt konnten in zwei Jahren iiber 50 unbekannte Strukturen bestimmt werden. Aus ihnen haben wir vie1 gelernt, insbesondere iiber Elektronentransfer und Kontaktionenpaar-Bildung in aprotischen Losungen. Wir mu& ten aber auch erkennen, daI3 eine Beantwortung vieler Fragen -darunter vor allem ,,was kristallisiert wie und warum?" -noch in weiter Ferne liegt. Eigentlich ist alles vie1 komplizierterVorwort einer Frankfurter Doktorarbeit Ausgangspunkt : Strukturabschatzungen aus MeBdaten und naherungsweisen Energieh yperflachen-BerechnungenAuch der iiberwiegend praparativ engagierte Chemiker profitiert erheblich, wenn er seine Spektren zusatzlich unter dem Gesichtspunkt ,,Molekiilzustands-Fingerabdriicke" betrachtet"]. Je ein Beispiel fur Gasphase und Losung sollen Vorgehen und Erkenntnisgewinn verdeutlichen. Die Struktur von H,CS-CEC-SCH, in der GasphaseDie Gasphasenthermolyse von Bis(methy1thio)cyclobuten-l,2-dion bei 720 K verlauft nach Photoelektronen(PE)-

using [2 b] obtained from the extinction coefficient of 2 b and absorbance after the pulse. A half-life ( t l , J of roughly 310 ms was determined for 2 b from the decay curve, while the lifetime based on k, was estimated to be 555 -t 62 ms. Similar measurements for the carbenes 2 a and 2c gave values of k,, k , , and tijz (z) for 2 a of 7.3 +1.5x lo5 M -~s -' , 3.1 0.4s-', and 210 (323 42) ms, respectively, while 2c decayed unimolecularly with k i = 6.9 & 0.5 s -l (z =I45 +10ms).These results indicate that when four methyl groups are introduced at the meta positions of the ortho-methylated diphenylcarbene, the carbenic center is more effectively blocked by the four ortho methyl groups presumably owing to the buttressing effect exerted by the meta substituents. Didurylcarbene is found to be some five orders of magnitude longer lived than the parent diphenylcarbene (2 ps in cyclohexane) .[l4I However, when two more methyl groups are introduced at the para positions, the carbenic center is forced to react with the orlho methyl groups, which are brought much closer to the carbene, and the carbene is shorter lived.Support is lent to this explanation by semiempirical calculations. Optimized geometries calculated for a series of polymethylated phenylcarbenes with the programm PM3-ROHF/ CI (4 x 4)" ' I indicate that the distances between carbenic carbon and o-methyl carbon atom for mesityl-. duryl-, and pentamethylphenylcarbenes are 2.8461, 2.7942, and 2.7778 pm, respectively.The present study reveals that polymethylated diphenylcarbenes are exceptionally long-lived for diarylcarbenes, although they are not persistent enough to be isolable. The buttressing effects effectively strengthen the ortho effect that protects the reactive center. This strategy may prove successful in the preparation of a "bottleable" triplet carbene.

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