Why can single bonds rotate?
1 Answer
Because the orbitals that form
The rotation of a particular single bond does not change the identity of the orbital in question used to make the bond, so the bond itself does not change.
That means we still retain the same molecule after a full
(Note that rotating double bonds is not possible, and rotating an entire molecule about the internuclear axis is not rotating the bond, but the molecule itself, which doesn't count in any case.)
SINGLE BONDS IN RELATION TO HEAD-ON ORBITAL OVERLAP
Single bonds are really the result of one sigma (
So, let's consider two orbitals that are capable of overlapping head-on to generate a
where
#sigma_(2p_z)# is the molecular orbital that formed from the linear combination of the two#2p_z# atomic orbitals.
ORBITAL SYMMETRY DICTATES ABILITY TO ROTATE THE BOND
You can see that rotating one of these orbitals about the internuclear axis (the
This is an important feature of a
A linear transformation preserves the properties of vector addition and scalar multiplication.
This may seem trivial, but it ensures that the two orbitals can continue overlapping as they have been in a LCAO and generate the same molecular orbital upon that overlap.
(It's one of those "do I really need to care" moments in chemistry, but that's the true reason why.)
CONCLUSION
Overall...
If we rotate a single bond about the internuclear axis, if either orbital participating in the bond becomes a different orbital (such as a