# Why can single bonds rotate?

##### 1 Answer

Because the orbitals that form **totally symmetric** about the internuclear axis, and the properties of vector addition and scalar multiplication of these orbitals is *preserved* during the bond rotation.

*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 **linear combination of atomic orbitals** (LCAO):

where

#sigma_(2p_z)# is the molecular orbital that formed from thelinear combinationof 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 *does not change their look at all* because they are *totally symmetric* about the

**This is an important feature** of a *different* upon rotation about the internuclear axis, it would **not** be what is known as a linear transformation.

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*

*vs. a*

*orbital), then we are either looking at a*

**different**bond or the bond**doesn't exist**.