Showing papers in "Chemical Society Reviews in 1994"

Tetrathiafulvalenes as building blocks in supramolecular chemistry II

Abstract: This account highlights developments in the field of supramolecular tetrathiafulvalene (TTF) chemistry. Progress in synthetic TTF chemistry during the past decade has allowed the preparation of a number of elaborate molecular systems based on host–guest interactions being able to act as either sensors, molecular switches or mediators for specific reactions. Among these systems interlocked architectures such as rotaxanes and catenanes make a significant contribution.

Solvent structure and perturbations in solutions of chemical and biological importance

Abstract: Much of chemistry and biology happens in an aqueous environment. Although the water is thought to play a significant role in the various processes, our understanding of these solvent effects at the molecular level is limited. Recent developments In neutron scattering are changing this: we can now begin to see directly how different solutes are hydrated, and how the solute perturbs its nearby solvent. This article discusses recent results on the hydration of both polar and non-polar groups, and tentatively concludes that the classical

The hydrides of aluminium, gallium, indium, and thallium: a re-evaluation

Abstract: boron gives little hint of the comparative wasteland making up much of the hydride estate of the heavier Group 13 elements., At a recent count2 about 100 binary boranes are now known, typically as discrete molecules remarkable for their stoicheiometries and structures which have done much to challenge and reshape our understanding of chemical bonding at large. By contrast, aluminium forms only one binary hydride stable under normal conditions as a polymeric solid, [AIH,],, the a-form of which is isostructural with AlF,, featuring 6-coordinate aluminium atom^.^^^ Attempts to prepare the analogous gallium compound have a chequered hist~ry,~ and it has taken nearly 50 years from the first reported sighting to establish the true credentials of gallane, [GaH,],,6 which now emerges as showing obvious affinities to diborane in the vapour state (i.e. n = 2) while being relatively short-lived under normal conditions. Despite some claims, however, it is unlikely that the hydrides [JnH,], and [TlH,], have yet materialized. In this account we review the current status of the hydrides formed by the Group 13 metals aluminium, gallium, indium, and thallium. Coordinatively saturated derivatives like MH, (M = Al, Ga, In, or T1) and Me,N. MH, (M = A1 or Ga) having been known for some year^,^?^ we are concerned primarily with the parent hydrides, [MH,], (m = 1, 2, or 3; n = 1, 2...), and related unsaturated derivatives. The last category includes species with more than one Group 13 element, for example tetrahydroborate derivatives like Al(BH,), and H,Ga(BH,),-, (m = 1 or 2) and tetraborane(l0) derivatives like 2-R2MB,H, (M = Al, R = Me; M = Ga, R = H or Me). It is appropriate first to consider the physical properties of the binary hydrides [MH,],. Hence it is possible to identify not only feasible methods of synthesizing compounds with M-H bonds, but also the origins of the thermal lability and reactivity besetting such compounds. 1.1 Theoretical Modelling Exploration of the Group 13 metal hydrides has been spurred by the greatly enhanced sophistication of modern computational methods which now admit the use of relatively elaborate basis sets, as well as making due allowance for factors like configuration interaction and relativistic correction^.^ Where comparisons can be made, such calculations typically yield dimensions and energetics which reproduce closely the experimental findings, and in some cases improve upon those findings. Such is the case, for example, with the monohydride molecules MH (M = B, Al, Ga, In, or Tl), which are short-lived under normal condition^.^,^ Accordingly we can place some trust in such results to anticipate the likely equilibrium molecular structures, vibrational properties, and binding energies of Group 13 hydrides, including numerous species whose existence has yet to be authenticated. Just what inferences are to be drawn will be discussed in Section 2.

The diagnosis of concerted organic mechanisms

Abstract: the concept of concertedness is paramount in all discussions of reaction mechanisms. The criterion that a concerted mechanism has a single transition state and hence possesses no intermediates enables the unambiguous diagnosis of these mechanisms. This review describes current experimental techniques which provide unequivocal evidence against processes involving intermediates; such techniques therefore indicate concertedness. Contemporary evidence is discussed for reactions, such as displacements at unsaturated centres, not hitherto regarded as capable of supporting concerted mechanisms.