Benzene, crystal structure dimer

The benzene crystal also gives us an example of an experiment that can be carried out by calculation but that cannot be carried out in the laboratory (Such experiments are sometimes said to have been carried out in silico. ) One would predict that if there were no bond dipoles, the benzene crystal structure of type (A) would be more stable than that of (B), and molecular mechanics calculations on the dimers agree. If we start with a vertical stack of plates for the benzene structure, we can study the... 

Rh(OEP)H reacts with CNR (R = Me, n-Bu,) to give the adduct Rh(OEP)-(H)CNR (which has no parallel in CO chemistry) which then slowly transforms to the formimidoyl insertion product, Rh(OEP)C(H)=NR. The dimer Rh(OEP))2 reacts with CNAr (Ar = 2.6-Cf,H3Mc2) in aqueous benzene to give the carbamoyl product. Rh(OEP)C(0)NHAr (characterized by an X-ray crystal structure) together with the hydride, which it.self reacts further with the isocyanide. This is suggc.sted to form via a cationic carbene intermediate, formed by attack of HiO on coordinated CNAr in concert with disproportionation to Rh(III) and Rh(l). 

Photoirradiation of both neat and benzene solutions of 2-cyclohexenone (66b) gives a complex mixture of photodimers [40]. However, photoirradiation of a 1 1 complex of 66b with the chiral host (S,S)-(-)-l,4-bis[3-(o-chlorophenyl)-3-hydroxy-3-phenylprop-l-ynyl]benzene (167) in the solid state (Scheme 24) gave (-)-anf/-head-to-head dimer 168 of 46% ee in 75% yield [40]. This reaction was found to proceed in a single crystal-to-single crystal manner. The mechanism of the reaction was studied by X-ray crystal structural analysis [41]. 

These new silyl complexes are monomeric in benzene solution, but are dimeric in the solid state. The crystal structure of the samarium analogue (Figure 1) shows that dimers form via intermolecular Sm-CH3-Si interactions. The Sm-Si distance, 3.052 (8) A, is to our knowledge the longest metal-silicon distance known. As with other complexes that display Ln-CH3-Si interactions in the solid state, evidence for these interactions in solution is not observed in NMR spectra. The [H NMR chemical shifts for the neodymium... 

The related tertiary phosphines PMe3 and PMe2Ph may also be met-alated by BunLi(tmeda), giving phosphinomethanide complexes that are shown by cryoscopy to be monomeric in benzene solution, but that crystallography confirms are dimeric in the solid state (126). Both complexes crystallize as dimers, [Li(CH2PMeR)(tmeda)]2 [R = Me (40), Ph (41)], with a similar structure to that of 39, containing a (CPLi)2 core. However, whereas 40 adopts a chair conformation, 41 is closer to a boat conformation with the phenyl groups in equatorial... 

A series of alkylzinc alkoxides in which the alkoxide is an a-substituted 2-pyridinemeth-anol group has been prepared. Based on aggregation-state studies by cryoscopy in benzene, it appeared that depending on the particular a-substituent these compounds exist in solution either as dimers or as trimers. An X-ray crystal structure determination of one of these compounds, i.e. Mc3SiCH2ZnOCH2-2-Pyr, showed that this compound exists in the solid state as a cyclic tetramer. The four alkylzinc alkoxide units are linked via... 

The X-ray crystal structure of complexes such as XXII shows that the bridge phosphorus is much more strained than that of classical, analogous endo-phosphole dimers. When complex XXII (M = Mo) is treated with sulfur in benzene at 50 °C, the exo-dimeric phosphole sulfide (XXIII) is obtained. 

Three examples of nitrile-stabilized enolates have been described by Boche et al. Two of these structures incorporate the anion of phenylacetonitrile. Hie TMEDA-solvated dimer (178) crystallizes out of benzene solution however, the mixed nitrile anion LDA-(TMEDA)2 complex (179) is obtained when excess LDA is present. This latter complex has often been mistaken as a geminal d anion since it frequently gives products that appear to arise from a dianion. The crystal structure of the anion l-cyano-2,2-dimethylcyclopropyllithium (180) consists of an infinite polymer (181) that is solvated by THF. Interestingly, there are C— Li contacts in this structure and the carbanionic carbon remains tetrahedral. 

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