6

I am starting to learn (Visual) Group Theory and I saw this hourglass shaped molecule Ferrocene:

Ferrocene

I am wondering what group is described by its symmetries. On top of the five rotations I think there are vertical $v$ and horizontal $h$ flips. Mapping its Cayley Diagram gave me $4$ cycles with flips acting as bridges between them. I've also noticed that $hv = vh$ which is reminiscent of the Klein group $V_4$. Is there a name for this "hourglass group"? And I feel like it could be separated into subcomponents like $V_4$ and a Cycle group of five elements $C_5$? Is there a way to formalize this?

Thanks a lot ! Pierrick

Shaun
  • 44,997

1 Answers1

8

Yes!

The group $G$ should be $D_5\times \Bbb Z_2$, where $D_5$ is the dihedral group of order ten.

If $a$ is a rotation of $2\pi/5$ counterclockwise from a bird's-eye view, $b$ is a horizontal symmetry from that view, and $c$ is a flip about the centre, then we have

$$\begin{align} bab&=a^{-1},\\ a^5&=e,\\ b^2&=e, \\ c^2&=e,\\ ac&=ca,\\ bc&=cb, \end{align}$$

which give the presentation

$$\begin{align} D_5\times \Bbb Z_2&\cong\langle a,b\mid a^5, b^2, bab=a^{-1}\rangle \times\langle c\mid c^2\rangle \\ &\cong\langle a,b, c\mid a^5, b^2, c^2, bab=a^{-1},ac=ca, bc=cb\rangle \end{align}$$

by this standard result.

As we can see here (in particular, here, although it is very technical${}^\dagger$), we can use $\Bbb Z_2^2$ (i.e., $V_4$) and $\Bbb Z_5$, but we would have to use the semidirect product:

$$\boxed{G\cong \Bbb Z_5\rtimes\Bbb Z_2^2.}$$

Also, $G\cong D_{10}.$


$\dagger$: The notation $\Bbb Z_5\rtimes\Bbb Z_2^2$ has some ambiguity. Really, it should be

$$\color{red}{\boxed{G\cong\Bbb Z_5\rtimes_\varphi\Bbb Z_2^2,}}$$

where, if $\mathbb{Z}_5 = \langle x\mid x^5 \rangle$ and $\mathbb{Z}_2^2 = \langle y, z\mid y^2,z^2, yz=zy \rangle$, we have the homomorphism

$$ \varphi:\Bbb Z_2^2\to\Bbb Z_2\le {\rm Aut}(\Bbb Z_5)$$

with $\varphi(y)(x) = x^{-1}$ and $\varphi(z)(x) = x. $ (This determines the whole map by the homomorphism property.)

Shaun
  • 44,997
  • Absent any context, the notation $\Bbb Z_5\rtimes\Bbb Z_2^2$ is ambiguous, right? In order to know what group is meant by $\Bbb Z_5\rtimes\Bbb Z_2^2$, we need to know what the intended homomorphism $\Bbb Z_2^2 \to \text{Aut}(\Bbb Z_5)$ is. – Tanner Swett Jun 21 '23 at 18:34
  • 1
    Indeed, yes, @TannerSwett; that is the reason behind the "in particular, here, although it is very technical" - it gives the three semidirect products, including the direct product, and the $D_{10}$ one should be highlighted in the link as well as by the labelling. I'll add a note to that effect now. – Shaun Jun 21 '23 at 18:37
  • Is this better, @TannerSwett? – Shaun Jun 21 '23 at 19:00
  • *two $${}{}{}{}{}$$ . . . not three. – Shaun Jun 21 '23 at 19:43
  • 1
    Yes, thank you for adding that note! I remember that when I first encountered the semidirect product notation, I was very confused by the fact that the homomorphism was left unstated, so it's great to see that spelled out explicitly. – Tanner Swett Jun 22 '23 at 02:22