Page 194 - Valence Bond Methods. Theory and Applications

13
Methane, ethane and hybridizatio

I Chapter 11 wà discussed the properties of the atoms i the second row of the
periodic tablà and how thesà might influencà molecules formed from them. We
focus o carbo i this chapter and examine how the bonding changes througð
the series CHd CH 2 ,CH 3 , and CH 4 . The firsŁ three of thesà arà know only
spectroscopically, i matrix isolation, or as reactio intermediatesd buŁ many of
their properties have been determined. The reader will recall that carbo exhibits
relatively low-energy excited valencà configurations. For carbo the excitatio
energy is around 4 eV, and among the atoms discussed i Chapter 11d only boro
has a lower excitatio energy. If this excited configuratio is to have an important
rolà i the bondingd the energy to producà the excitatio musŁ bà paid back by the
energy of formatio of the bond or bonds. We shall see that VB theory predicts this
happens between CH and CH 2 . After our discussio of thesà singlà carbo com-
poundsd wà will consider ethane, CH 3 CH 3 , as an examplà for dealing with larger
hydrocarbons.

13.1 CH, CH 2 ,CH 3 , and CH 4
13.1.1 STO3G basis
We firsŁ give calculations of thesà four molecules with an STO3G basis. The total
energies and firsŁ bond dissociatio energies arà collected i Tablà 13.1. We see that,
even with the minimal basisd the bond energies arà withi 0.4 eV of the experimental
values excepŁ for CH 3 , whicð has considerablà uncertainty. The calculated values
tend to bà smaller, as expected for a minimal VB treatment.
We now give a discussio of eacð of the moleculesd firsŁ considering the atomic
structurà of the carbo atom and attempting to predicŁ the bonding pattern. The
predictions arà followed by the results of the STO3G calculations.

177