Bonding alternative conceptions

An alternative concept for the deposition of III-V material films, the so-called single-source precursor concept, was introduced by Cowley, Jones and others almost 15 years ago [10]. Typical group 13/15 single-source precursors such as Lewis acid-base adducts R3M - ER 3 or heterocycles [R2MER 2]% as shown in Scheme 1 contain the specific elements of the desired material connected by a stable chemical bond in a single molecule. 

We start with some biographical notes on Erich Huckel, in the context of which we also mention the merits of Otto Schmidt, the inventor of the free-electron model. The basic assumptions behind the HMO (Huckel Molecular Orbital) model are discussed, and those aspects of this model are reviewed that make it still a powerful tool in Theoretical Chemistry. We ask whether HMO should be regarded as semiempirical or parameter-free. We present closed solutions for special classes of molecules, review the important concept of alternant hydrocarbons and point out how useful perturbation theory within the HMO model is. We then come to bond alternation and the question whether the pi or the sigma bonds are responsible for bond delocalization in benzene and related molecules. Mobius hydrocarbons and diamagnetic ring currents are other topics. We come to optimistic conclusions as to the further role of the HMO model, not as an approximation for the solution of the Schrodinger equation, but as a way towards the understanding of some aspects of the Chemical Bond. 

By combining high-level ab initio calculations with high-resolution infrared spectroscopy, the equilibrium bond lengths in x-frans-butadiene have been determined to an unprecedented precision of 0.1 pm. The values found for the pair of n-electron delocalized double bonds and the delocalized central single bond are 133.8 and 135.4 pm, respectively. The data provide definitive structural evidence that validates the fundamental concepts of n-electron delocalization, conjugation, and bond alternation in organic chemistry. 

It is of vital interest for a wider applicability of CTCB to examine how these two mechanisms can be accommodated in OCT. In Section 3, we shall argue that the mutual decoupling status of several subsets of basis functions, manifesting itself by the absence of any external communications (bond orders) in the whole system, calls for the separate unit normalization of its input probabilities since such fragments constitute the mutually nonbonded (closed) building blocks of the molecular electronic structure. It will be demonstrated, using simple hydrides as an illustrative example, that the fulfillment of this requirement dramatically improves the agreement with the accepted chemical intuition and the alternative bond multiplicity concepts formulated in the MO theory. 

This alternative hybridization scheme explains how carbon can combine with four atoms in some of its compounds and with three other atoms in other compounds. You may be aware of the conventional way of depicting carbon as being tetravalent in all its compounds it is often stated that carbon always forms four bonds, but that sometimes, as in the case of ethylene, one of these may be a double bond. This concept of the multiple bond preserves the idea of tetravalent carbon while admitting the existence of molecules in which carbon is clearly combined with fewer than four other atoms. 

The momentum-space representation also proves particularly convenient for comparisons of the electron distributions of systems with different nuclear frameworks. Difference density plots in r-space are complicated by the different sets of nuclear positions. Such complications are absent in p-space and, in the case of polyenes [23], for example, momentum-space concepts have proved useful for examining the effects of bond alternation on the electron density - an important characteristic of such systems and of doped polyacetylene. 

The origins of the chemical shifts are probably not sufficiently well understood (as yet), to allow a quantitative discussion of aromatic character in the annulenes. If such a concept is considered meaningful it would probably best be defined in terms of the degree of bond alternation therein, which is of pivotal importance to the jr-electron properties (see Sections B and C). Apart from theoretical calculations, a number of physical methods have demonstrated their ability to estimate the extent of bond alternation in annulenes (crystallographic analysis, electronic/vibronic spectral analysis, diamagnetic anisotropy/susceptibility exaltation measurements and of course n.m.r.), see ref. > for a full discussion. (Furthermore the known correlation between n.m.r. vicinal coupling constants and carbon-carbon bond orders is of potential utility in any determination of bond alternation 65>). 

In considering each of these five themes, relevant literature on learners alternative conceptions about bonding is reviewed, and the teaching models and approaches considered to encourage such ideas are considered. The chapter considers recommendations for the way in which the topic should be taught to minimise the incidence of these alternative conceptions, and to encourage learners to develop models of chemical bonding which are more authentic, and which consequently have more explanatory power. 

A highly prevalent alternative conception for chemical bonding at secondary school and (to a lesser extent) the tertiary level is that continuous covalent or ionic lattices contain molecular species (Birk Kurtz, 1999 Butts Smith, 1987 de Posada, 1997 Peterson et al., 1989 Taber, 1994, 1998). Other research has revealed that some learners believe ionic substances such as sodium chloride possess covalent bonds (Peterson et al., 1989). 

There is some suggestion that these common alternative conceptions may survive university teaching, for example, among New Zealand chemistry graduates (see Coll Treagust, 2001). In the U.K., Oversby (1996) found that some of his post-graduate trainee chemistry teachers considered the alternative conceptions of the ionic bond reported by Taber (1994) to be an acceptable model of ionic bonding, i.e. ... 

Coll, R. K. and Taylor, N. (2001a) Alternative conceptions of chemical bonding amongst senior secondary and tertiary students Nature and origins. Teaching and Learning, 22(1), 48-60. 

There are many studies, cited throughout the book, into students alternative conceptions - those meanings associated with a given concept-label that are different from those used by scientists. Chapter 10 is a typical example, with many reports of enquiries into students understanding of bonding . The intention behind all such studies is that teaching activities intended to produce conceptual development are subsequently to be produced for use with students. 

Scionix has developed an alternative concept to forming eutectic-based ionic liquids which is to complex the anion of choline chloride with a hydrogen-bonding compound rather than a metal halide [21,22]. The ionic liquids allow electropolishing with high current efficiency (>80%), improved surface finish and improved corrosion resistance [23]. 

Somewhat more complicated than the metallic case is that of ionic compounds. Research shows us that students very commonly misunderstand ionic bonding. You can download a diagnostic task to identify where students hold common alternative conceptions about ionic bonding from the Royal Society of Chemistry website (see the Other resources section at the end of this chapter). 

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