Rotation of CO2 bonds and triple bonds.

[khanhhung2512]

I read in several websites that triple bonds cannot rotate freely.
However, I've also read in the book "Chemical Principles" the following lines: "Various types of evidence suggest that the electron density around the two C-O bonds in CO2 is actually cylindrically symmetric—that is, the electron density is homogeneous all around the O-C-O molecular axis."
That intrigues me. Now I don't see any problems preventing the two C-O bonds to rotate like single bonds.
Additionally, since the way CO2 bonds is quite similar to triple bond, will triple bond be cylindrically symmetric too? Can triple bond rotate like single bond?
Btw, how can people know whether linear configurations like triple bond or CO2 can rotate around the bond?
Thanks very much.

 

[DrDu]

I read in several websites that triple bonds cannot rotate freely.

That's kind of nonsense, as, at least with linear molecules like N2 or HCCH there is no way to decide whether it has rotated or not.
There are many metal compounds with triple bonds which aren't linear molecules, e.g. dimolybdenum compounds, see:
http://books.google.com/books/about/Multiple_Bonds_Between_Metal_Atoms.html?id=rYqKC74r_rIC
In these compounds, rotation around the axis should be observable and isomerisation energies be measurable.

 

[khanhhung2512]

That's kind of nonsense, as, at least with linear molecules like N2 or HCCH there is no way to decide whether it has rotated or not.
There are many metal compounds with triple bonds which aren't linear molecules, e.g. dimolybdenum compounds, see:
http://books.google.com/books/about/Multiple_Bonds_Between_Metal_Atoms.html?id=rYqKC74r_rIC
In these compounds, rotation around the axis should be observable and isomerisation energies be measurable.

Thanks.

Additionally, since the way CO2 bonds is quite similar to triple bond, will triple bond be cylindrically symmetric too?

There's still one part in my questions. Is triple bond cylindrically symmetric?

 

[DrClaude]

There's still one part in my questions. Is triple bond cylindrically symmetric?

In a linear molecule, it has to be, since if you take the bond axis to be z, you still have an arbitrary choice as to where x and y point. In other cases, there might be some symmetry breaking, but I suspect that the electrons participating in the triple bond will be nearly isotropically distributed around the bond axis.

 

[DrDu]

In this context it is maybe interesting, that even a double bond may be rotationally symmetric, e.g. like the one in $\mathrm{^1\Delta}$ singlet oxygen.