Ethylene, bond angles molecular orbitals

Recall that in the VSEPR model a double bond acts as one effective pair. Thus in the ethylene molecule each carbon is surrounded by three effective pairs. This model requires a trigonal planar arrangement with bond angles of 120 degrees. What orbitals do the carbon atoms use in this molecule The molecular geometry requires a planar set of orbitals at angles of 120 degrees. Since... 

A theoretical treatment of 1,2-oxathietane indicates planarity with aS-0 bond length of 1.669 A and a C-S-0 angle of 100.6°. The electronic spectrum was calculated. The character of the HOMO is largely that of the sulfur 3p orbital. A CNDO molecular orbital study of the retrocycloaddition of 1,2-oxathietane 2-oxide to sulfur dioxide and ethylene shows that strong heteroatom asymmetry lifts the stereoelectronic requirement that the thermal fragmentation occur by a suprafacial-antarafacial path. ... 

Since each carbon is bonded to three other atoms, it uses sp orbitals (as in ethylene. Sec. 5.2). Overlap of these orbitals with each other and with the s orbitals of five hydrogen atoms gives the molecular skeleton shown in Fig. 6.5, with all bond angles 120. In addition, each carbon atom has a p orbital which, as we know, consists of two equal lobes, one lying above and the other lying below the plane of the q bonds it is occupied by a single electron. 

Ethylene exhibits a non-steric barrier to rotation about the molecular axis. The conventional explanation ascribes this effect to the lateral tt overlap of parallel p orbitals with spin pairing. The triple bond in acetylene is conventionally defined in terms of one sp rr-bond and two rr-bonds at right angles to each other. Triple bonds should therefore be torsionally even more rigid than double bonds. The curious fact... 

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