Hydrogen bonding antibonding orbitals

Figure 1.28 Formation of the bonding molecular orbitals of ethyne from two sp-hybridized carbon atoms and two hydrogen atoms. (Antibonding orbitals are formed as well but these have been omitted for simplicity.)...

The frontier orbital of an electropilic reagent, HCl, is the LUMO or the antibonding orbital of the cr bond. The Is orbital energy (-13.6 eV) of hydrogen atom is higher than the 3p orbital energy (-15.1 eV) of chlorine atom [5]. The main component of is Is which mixes 3p out of phase. 

To get the molecular orbital of the hydrogen molecule, the orbital equations of the two atoms are combined. When the orbital equations are added together, the result is a bonding molecular orbital that extends over both atoms. Subtracting the orbital equations of the atoms produces an antibonding molecular orbital. This process is called the linear combination of atomic orbitals or LCAO. 

Figure 1.17 The hypothetical formation of the bonding molecular orbitals of ethane from two sp -hybridized carbon atoms and six hydrogen atoms. All of the bonds are sigma bonds. (Antibonding sigma molecular orbitals — are called a orbitals — are formed in each instance as well, but for simplicity these are not shown.)... 

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