Imperial College London

Henry Rzepa

In an [earlier
post](http://www.ch.ic.ac.uk/rzepa/blog/?p=1587){target="_blank"}, I
re-visited the conformational analysis of cyclohexane by looking at the
vibrations of the entirely planar form (of D~6h~ symmetry). The method
also gave interesting results for the larger cyclo-octane ring. How
about a larger leap into the unknown?

Let us proceed as follows. One fun game to play in chemistry is to
invoke i*so-electronic* substitutions. In this case, we can subtitute a
nitrogen and a boron atom for a pair of carbons. Thrice invoked, it
leads to a molecule known as cyclotriborazane.

::: {#attachment_1664 .wp-caption .aligncenter style="width: 299px"}
[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/02/B3N3H12.jpg "B3N3H12"){.size-full
.wp-image-1664 loading="lazy" decoding="async"
aria-describedby="caption-attachment-1664" width="289"
height="198"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/02/B3N3H12.jpg)

Cyclotriborazane.
:::

This species is in fact very well known, and a crystal-structure was
determined some time ago (DOI:
[10.1021/ja00786a022](http://dx.doi.org/10.1021/ja00786a022){target="_blank"}).
It is worth considering some of its properties.

1.  The species is crystalline, and sublimes rather than melts. Contrast
    this with the iso-electronic cyclohexane, which melts at around 6C
    (itself a surprisingly high value).
2.  The parent H~3~BNH~3~ also has a very high melting point of \> 100C,
    which is attributed to an extensive array of so-called
    **dihydrogen** bonds in the crystal lattice, in which a positively
    charged hydrogen deriving from an NH is attracted to a negatively
    charged hydrogen deriving from a BH. Such **dihydrogen** bonds have
    been shown to be quite strong due to this electrostatic interaction,
    and are responsible for the extraordinary elevation of the melting
    point compared to the iso-electronic ethane.
3.  The chair form of cyclotriborazane aligns the three hydrogens shown
    in either blue or red in the axial positions. The three red
    hydrogens might be expected to be all negatively charged, and the
    three blue ones positively charged. So in the chair conformation,
    might we expected the electrostatic repulsions between either the
    blue or the red hydrogens to destabilize these axial positions, and
    hence perhaps even destabilize the chair conformation itself?

The crystal structure however shows clearly that the chair is still the
favoured conformation. Equally intriguing, one might expect the three
blue hydrogens to stack up to attract electrostatically to the three red
hydrogens. But you can see from the crystal packing if you activate the
model below that this does not happen!

::: {#attachment_1667 .wp-caption .aligncenter style="width: 273px"}
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/02/cyclotriborazene-xray.jpg "cyclotriborazene-xray"){.size-full
.wp-image-1667 loading="lazy" decoding="async"
aria-describedby="caption-attachment-1667"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('black');jmolApplet([450,450],'load wp-content/uploads/2010/02/10.1021_ja00786a022_FUZPAR.cif;');"
width="263" height="151"}

Cyclotriborazane Crystal structure. Click for 3D
:::

What of the vibrational analysis, conducted as it was for cyclohexane
itself (DOI:
[10042/to-4170](http://hdl.handle.net/10042/to-4170){target="_blank"}).
Well, just as before, for the planar geometry, three imaginary modes are
calculated (A~2~", E") and just as before, they distort the geometry in
the direction of a chair (C~s~ symmetry), a C~2~-disymmetric twist boat
(with a predicted optical rotation of -54°) and a boat respectively (the
latter, as before, being a transition state connecting the two
C~2~-enantiomers).

::: {#attachment_1670 .wp-caption .aligncenter style="width: 341px"}
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/02/cyclotriborazene-chair.jpg "cyclotriborazene-chair"){.size-full
.wp-image-1670 loading="lazy" decoding="async"
aria-describedby="caption-attachment-1670"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('black');jmolApplet([450,450],'load wp-content/uploads/2010/02/FUZPAR.log;frame 17;zoom 5;moveto 4 0 2 0 90 90;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 6;animation mode loop;');"
width="331" height="247"}

Planar cyclotriborazane distorting to chair.
:::

But here we get a surprise! According to the B3LYP/6-311G(d,p) model,
the final resting energy for the chair is almost the same (indeed 0.2
kcal/mol higher in free energy) as the twist-boat. Perhaps that blue/red
repulsion did have an effect after all! If you look at the calculated
structure, you can indeed see that the blue/red hydrogens are
splayed-out, avoiding each other!

::: {#attachment_1675 .wp-caption .aligncenter style="width: 275px"}
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/02/cyclotriborazene-chair1.jpg "cyclotriborazene-chair1"){.size-full
.wp-image-1675 loading="lazy" decoding="async"
aria-describedby="caption-attachment-1675"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('black');jmolApplet([450,450],'load wp-content/uploads/2010/02/cyclotriborazene-cs.cml;');"
width="265" height="217"}

Calculated geometry of the chair form of cyclotriborazane
:::

This is one of those molecules where one might have expected surprises.
In the end, it is surprising at how similar cyclotriborazane is to its
iso-electronic cousin cyclohexane.

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
:::


In an earlier post, I re-visited the conformational analysis of cyclohexane by looking at the vibrations of the entirely planar form (of D
<sub>
 6h
</sub>
symmetry). The method also gave interesting results for the larger cyclo-octane ring. How about a larger leap into the unknown? Let us proceed as follows. One fun game to play in chemistry is to invoke i
<em>
 so-electronic
</em>
substitutions.

Messages de Rogue Scholar

Conformational analysis of cyclotriborazane