Acetylene molecular orbitals

Ross, I. G., Trans. Faraday Soc. 48, 973, Calculations of the energy levels of acetylene by the method of antisymmetric molecular orbitals, including a—7r-interaction."... 

The facile thermal decomposition of the dimethyl and diethyl derivatives of (II) to nitrogen and carbene intermediates is emphasized by the readily discernible correlations between the reactant and product orbitals. On the other hand, the greater delocalization of the molecular orbitals of (I) may be a factor in its preference to rearrange, without decomposition, to methyl acetylene and allene. 

With regard to the different points of view outlined in (a), (b) and (c), it should be pointed out that these differences arise mainly from the use of localized (a, LMO), or canonical (CMO, b, and c) molecular orbitals. In principle LMOs and CMOs are equivalent and are related by a unitary transformation. This can be illustrated by the C=C bonding in acetylene. 

Acetylenes XCCY with n conjugated substituents, X and Y, on both carbon atoms have planar or perpendicular conformations. The substituents can be electron-donating or -accepting. The planar conformers are linear conjugate D-TI-D, D-IT-A, or A-IT-A systems whereas the perpendicular conformers are composed of ri-D and IT-A not in conjugation with each other. The orbital phase is continuous only in the planar conformations of D-TI-A (Scheme 23, cf. Scheme 4). The acetylenes with X=D (OR, NR ) and Y=A (RCO, ROCO) prefer planar conformations. When both substituents are electron-donating or accepting, the phase is discontinuous. The acetylenes then prefer perpendicular conformations. The predicted conformational preference was confirmed by ab initio molecular orbital calculations [32]. Diacetylenic molecules show similar conformational preference, which is, however, reduced as expected [32]. 

Figure 10-30 shows the constmction of the 2 -based molecular orbitals. One pair of MOs forms from the p orbitals that point toward each other along the bond axis. By convention, we label this as the z-axis. This end-on overlap gives Cp and

Fig. 10.2. Localized molecular orbitals of the complex (CP) between Me2CuLi-LiCI and acetylene.

Fig. 4. Molecular orbitals showing the hybridization of s and p orbitals, (a) Acetylene (C2H2, sp hybridization) (b) ethylene (C2H4, sp hybridization), and (c) methane (CH4, sp hybridization).

Note how we have resorted to another form of representation of the ethane, ethylene, and acetylene molecules here, representations that are probably familiar to you (see Section 1.1). These line drawings are simpler, much easier to draw, and clearly show how the atoms are bonded - we use a line to indicate the bonding molecular orbital. They do not show the difference between a and rr bonds, however. We also introduce here the way in which we can represent the tetrahedral array of bonds around carbon in a two-dimensional drawing. This is to use wedges and dots for bonds instead of lines. By convention, the wedge means the bond is coming towards you, out of the plane of the paper. The dotted bond means it is going away from you, behind the plane of the paper. We shall discuss stereochemical representations in more detail later (see Section 3.1). 

Chemists are familiar with the molecular orbitals of simple molecules. They recognize the c and n orbitals of acetylene, and readily associate these with the molecule s o and 7t bonds. 

Note, however, that even in such a simple case as this, molecular orbitals do not correspond one-to-one with bonds. For example, the highest-energy a orbital in acetylene is clearly made up of both CC and CH bonding components. The reason, as pointed out in Chapter 2, is that molecular orbitals are written as linear combinations of nuclear-centered basis functions, and will generally be completely delocalized over the entire nuclear skeleton. 

The main result that emerges from the discussions of particular eases is that it has proved possible to give a description of a molecule in terms of equivalent orbitals which are approximately localised, but which can be-transformed into delocalised molecular orbitals without any change in the value of the total wave function. The equivalent orbitals are closely associated with the interpretation of a chemical bond in the theory, for, in a saturated molecule, the equivalent orbitals are mainly localised about two atoms, or correspond to lone-pair electrons. Double and triple bonds in molecules such as ethylene and acetylene are represented as bent single bonds, although the rather less localised o-n description is equally valid. 

This reduces the energy of low-lying vacant molecular orbitals of free acetylene in this complex, as compared with analogous orbitals of free acetylene, and consequently the triple bond becomes more accessible to nucleophilic attack. As for nucleophiles, they become supernucleophiles in superbase media because of a sharp increase in their energy (76G817 77AP0133). 

The qualitative picture of o and k molecular orbitals can be extended to molecules with three or more atoms. Thus the double bond of ethylene, H2C=CH2, and the triple bond of acetylene, HC=CH, can be... 

The field of acetylene complex chemistry continues to develop rapidly and to yield novel discoveries. A number of recent reviews 1-10) covers various facets including preparation, structure, nature of bonding, stoichiometric and catalytic reactions, and specific aspects with particular metals. The first part of this account is confined to those facets associated with the nature of the interactions between acetylenes and transition metals and to the insertion reactions of complexes closely related to catalysis. Although only scattered data are available, attempts will be made to give a consistent interpretation of the reactivities of coordinated acetylene in terms of a qualitative molecular orbital picture. 

PE spectra of the two angle strained cyclopolyynes 1,5-cyclooctadiyne (35)177) and the twelve membered tetraacetylene (43)88) have been measured and interpreted with the aid of semiempirical and ab initio molecular orbital calculations. In both compounds evidence for through-space and through-bond interactions between the acetylene moieties has been found 88,177). The compound (43) has been described on the basis of these results as weakly antiaromatic88). 

Spectroscopic analysis revealed that the thermally initiated [3 + 2] polycycloaddition produced 1,4- and 1,5-substituted triazole isomers in an approximately 1 1 ratio. This ratio appears to be statistic and dependant on the bulkiness of the organic moieties. For example, hfr-r-P[30(4)-20] with butyl spacers contained slightly more 1,4-triazole isomers than did hb-r-P[30(6)-20] with hexyl spacers. This becomes clearer if we look at the proposed transition states a and b of the [3 + 2]-dipolar cycloaddition (Scheme 16). Because of their molecular orbital symmetry, the acetylene and azide functional groups arrange in two parallel planes, a so-called two-plane orientation complex [48], which facilitates a concerted ring formation. If the monomer fragment or the polymer branch ( ) attached to the functional groups are bulky, steric repulsion will come into play and transition state a will be... 

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