Acetylene, orbitals

For a second example, let s return to acetylene on Pt(lll), specifically in the twofold and fourfold geometries.29 In the twofold geometry, we saw earlier (from the decomposition of the DOS) that the most important acetylene orbitals were irff and it. These point toward the surface. Not surprisingly, their major interaction is with the surface z2 band. But t and 7r interact preferentially with different parts of the band, picking out those metal surface orbitals which have nodal patterns similar to those of the adsorbate. Diagram 116 shows this in the twofold geometry at hand the trff orbital interacts better with the bottom of the surface z2 band and the ir0 with the top of that band. 

The location of a substituent in the orf/ o-position to the yne branch of the benzoannelated enediynes affects the reaction kinetics of Bergman cyclization [234-236]. In all cases, there is an acceleration of the cycloaromatization process that is associated with the stabilization of the transition state through the interaction of acetylene orbitals with the nearest substituent (Scheme 3.11) [234],... 

We conclude this introduction to hydrocarbons by describing the orbital hybridization model of bonding m ethylene and acetylene parents of the alkene and alkyne families respectively... 

Because each carbon m acetylene is bonded to two other atoms the orbital hybridization model requires each carbon to have two equivalent orbitals available for CT bonds as outlined m Figure 2 19 According to this model the carbon 2s orbital and one of Its 2p orbitals combine to generate two sp hybrid orbitals each of which has 50% s character and 50% p character These two sp orbitals share a common axis but their major lobes are oriented at an angle of 180° to each other Two of the original 2p orbitals remain unhybridized... 

As portrayed m Figure 2 20 the two carbons of acetylene are connected to each other by a 2sp-2sp cr bond and each is attached to a hydrogen substituent by a 2sp-ls CT bond The unhybndized 2p orbitals on one carbon overlap with their counterparts on the other to form two rr bonds The carbon-carbon triple bond m acetylene is viewed as a multiple bond of the ct + rr + rr type... 

Section 2 21 Carbon is sp hybridized m acetylene and the triple bond is of the ct + Tt + Tt type The 2s orbital and one of the 2p orbitals combine to give two equivalent sp orbitals that have their axes m a straight line A ct bond between the two carbons is supplemented by two tr bonds formed by overlap of the remaining half filled p orbitals... 

An easy way to keep track of the effect of the s character of carbon is to associ ate It with electronegativity As the s character of carbon increases so does that carbon s electronegativity (the electrons m the bond involving that orbital are closer to carbon) The hydrogens m C—H bonds behave as if they are attached to an increasingly more electronegative carbon m the series ethane ethylene acetylene... 

The acidity mcreases as carbon becomes more electronegative Ionization of acetylene gives an anion m which the unshared electron pair occupies an orbital with 50% s character... 

In the corresponding ionizations of ethylene and ethane the unshared pair occupies an orbital with 33% sp ) and 25% sp ) s character respectively Terminal alkynes (RC=CH) resemble acetylene m acidity... 

Acetylene is linear and alkynes have a linear geometry of their X—C=C—Y units The carbon-carbon triple bond m alkynes is com posed of a CT and two tt components The triply bonded carbons are sp hybridized The ct component of the triple bond contains two electrons m an orbital generated by the overlap of sp hybndized orbitals on adja cent carbons Each of these carbons also has two 2p orbitals which over lap m parrs so as to give two tt orbitals each of which contains two electrons... 

Many of the reactions in which acetylene participates, as well as many properties of acetylene, can be understood in terms of the stmcture and bonding of acetylene. Acetylene is a linear molecule in which two of the atomic orbitals on the carbon are sp hybridized and two are involved in 7T bonds. The lengths and energies of the C—H O bonds and C=C<7 + 27t bonds are as follows ... 

In the third type of hybridisation of the valence electrons of carbon, two linear 2sp orbitals are formed leaving two unhybridised 2p orbitals. Linear a bonds are formed by overlap of the sp hybrid orbitals with orbitals of neighbouring atoms, as in the molecule ethyne (acetylene) C2H2, Fig. 1, A3. The unhybridised p orbitals of the carbon atoms overlap to form two n bonds the bonds formed between two C atoms in this way are represented as Csp Csp, or simply as C C. 

This stereoelectronic requirement would lead to a large distortion of the normal geometry of a five-membered ring and introduce strain. It is this distortion and strain that disfavor the 5-endo-trig cyclization. In contrast, 5-endo-dig cychzation is feasible because the acetylenic system provides an orbital that is available for a nearly planar mode of approach. 

It should be noted that CASSCF methods inherently tend to give an unbalanced description, since all the electron correlation recovered is in die active space, but none in the inactive space, or between the active and inactive electrons. This is not a problem if all the valence electrons are included in the active space, but this is only possible for small systems. If only part of die valence electrons are included in the active space, the CASSCF methods tend to overestimate the importance of biradical structures. Consider for example acetylene where the hydrogens have been bent 60° away from hnearity (this may be considered a model for ort/zo-benzyne). The in-plane jt-orbital now acquires significant biradical character. The true structure may be described as a hnear combination of the three configurations shown in Figure 4.11. 

Table 4.4 Natural orbital occupation numbers for the distorted acetylene model in Figure 4.11. Only the occupation numbers for the six central orbitals are shown...

When two sp-hybridized carbon atoms approach each other, sp hybrid orbitals on each carbon overlap head-on to form a strong sp-sp a bond. In addition, the pz orbitals from each carbon form a pz-pz it bond by sideways overlap and the py orbitals overlap similarly to form a py-py tt bond. The net effect is the sharing of six electrons and formation of a carbon-carbon triple bond. The two remaining sp hybrid orbitals each form a bond with hydrogen to complete the acetylene molecule (Figure 1.16). 

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