Zum Radikalcharakter des Bis‐Dibenzo[a.i]Fluorenylidens

Abstract

Messungen des Paramagnetismus zeigen, daß beim Bis-dibenzo[a.i]fluorenyliden bei Zimmertemperatur neben dem diamagnetischen Grundzustand ein paramagnetischer Zustand im Gleichgewicht vorhanden ist. Es wird angenommen, daß der paramagnetische Zustand ein Biradikalzustand mit senkrecht aufeinanderstehenden Molekelhälften ist. Der Energieunterschied beider Formen beträgt 3–4 kcal/Mol. Bei einer Reihe von Radikalreaktionen wirkt Bis-dibenzo[a.i]fluorenyliden als starker Inhibitor

A Thermally Populated, Perpendicularly Twisted Alkene Triplet Diradical

“…Thed ata described above imply that J ex should be determined at as high at emperature as possible,b ut not above 500 K, and using the shortest practical measuring time in the higher temperature range.Estimates of J ex and hence of the singlet-triplet splitting obtained from intensity measurements in the lower temperature range are less accurate. Franzen and Joschek [6] used small intensity differences in the visible spectrum in the range 293-423 Ktoestimate an energy difference of 4kcal mol À1 for 1 in toluene solution. Thevalue of 3.4 kcal mol À1 reported by Lenoir et al was calculated for the gas phase.…”

“…Asingle-line ESR spectrum ascribed to 1 has been reported, and the energy difference between the singlet 1 1 (S 0 )a nd the triplet 3 1 (T 1 ) has been estimated to be approximately 4kcal mol À1 based on changes in the UV/Vis spectrum (l max = 623 nm) in the 293-123 Kr ange. [6] TheX -ray crystal structure of 1( 0 )f eatures atorsion angle of 528 8 between the two nearly planar halves of the molecule. [7] Thec entral C13=C13' distance is 140.8 pm.…”

Observation of a Thermally Accessible Triplet State Resulting from Rotation around a Main‐Group π Bond

“…Experiment and theory suggest at hermal equilibrium between 1 and 2 with av ery lows inglet-triplet energy gap of only 7.3 kcal mol À1 .Rotation around the p-C=Cdouble bond, leaving the s-(C= C) bond intact, is one of the most fundamental processes in chemistry and is important in many chemical and biological transformations.C onsequently,i th as intrigued chemists for decades and has been studied extensively both experimentally and theoretically. [1][2][3][4][5][6][7][8][9] One approach to investigate this rotation process is to study tetrasubstituted olefins bearing large substituents that force the C = Cd ouble bond out of its preferred planar geometry,t hus reducing the energy (and temperature) required for rotation, [10][11][12][13][14][15][16][17][18] which is about 60 kcal mol À1 for H 2 C=CH 2 . [19] Some of these highly twisted C=Cd ouble bonds exhibit ac hemical behavior suggesting that they possess some diradical character;h owever, generally diradicals were not observed spectroscopically.…”

“…[19] Some of these highly twisted C=Cd ouble bonds exhibit ac hemical behavior suggesting that they possess some diradical character;h owever, generally diradicals were not observed spectroscopically. [15][16][17][18] In one case,t hat of bis-[dibenzo[a.i]-fluorenylidene],i tw as claimed that the singlet ground state and the diradical triplet state resulting from rotation around the C = Cd ouble bond exist in equilibrium, but spectroscopic evidence was not provided. [15][16] Disilenes,R 2 Si=SiR 2 ,t he heavier analogs of alkenes,p resent an attractive opportunity to observe triplet diradicals because the barrier to rotation around Si=Si pdouble bonds is about 15-25 kcal mol À1 , [20][21][22][23][24][25] significantly lower than around C = C p-bonds.C onsequently,d isilenes are more easily torsionally twisted than alkenes; [25][26][27][28] for example,t etrakis(di-tert-butylmethylsilyl)disilene (1)s hows av ery large torsional distortion around the Si=Si double bond, with an average SiÀSi=SiÀSi dihedral angle (q)o f 55.98 8.…”

Fulvenes and Thermochromic Ethylenes. Part 70 Syntheses in the Dibiphenylene‐Ethene (Bifluorenylidene) Series

“…In acetone as solvent the amount of V (X = CH 3 ) increased to 18%; the colorless product could in this case be isolated and identified by mixed mp (see below). Finally, the method proposed by Wanscheidt [19,6,7] was applied, viz. the dehydrogenation of V (X =CH 3 ) with silver acetate in refluxing pyridine.…”