Imperial College London

Henry Rzepa

In [several posts](https://www.ch.imperial.ac.uk/rzepa/blog/?p=17205) a
year or so ago I considered various suggestions for the most polar
neutral molecules, as measured by the dipole moment. A record had been
claimed\[cite\]10.1002/anie.201508249\[/cite\] for a synthesized
molecule of \~14.1±0.7D. I pushed this to a calculated 21.7D for an
admittedly hypothetical and unsynthesized molecule. Here I propose a new
family of compounds which have the potential to extend the dipole moment
for a formally neutral molecule up still further.

[![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/sb1-page001.svg){.aligncenter
.size-large .wp-image-19561 decoding="async"
width="400"}](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/sb1-page001.svg)

These molecules derive from a well-known class of molecule known as
ortho-quinomethides. If the methide part is substituted with an electron
donating substituent such as an amino group in **3**, a push-pull
opportunity now arises, which is strongly driven by aromatisation of the
quinomethide ring. This allows one to design "neutral" molecules such as
**1** and **2**, which now contain respectively two and three rings that
will be aromatised by the process. The aromatisation stabilization
energy is balanced of course by an opposing increase in energy resulting
from charge separation. You can observe that partially aromatising three
independent rings as in **2** can drive a great deal of charge
separation. One may indeed wonder how much charge separation can be
sustained before a triplet instability occurs, driving the molecule back
to being neutral. In the case of **2**, the wavefunction is in fact
stable to such an open shell state, but higher homologues may not be. An
aspect worth investing!

|                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| 1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  | 2                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| ![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/sb-581x1024.jpg){.size-large .wp-image-19555 decoding="async" width="200" srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/sb-581x1024.jpg 581w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/sb-170x300.jpg 170w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/sb-768x1353.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/sb.jpg 801w" sizes="(max-width: 581px) 100vw, 581px"} | ![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/ssbase-425x1024.jpg){.size-large .wp-image-19552 decoding="async" width="200" srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/ssbase-425x1024.jpg 425w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/ssbase-125x300.jpg 125w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/ssbase.jpg 696w" sizes="(max-width: 425px) 100vw, 425px"} |
| DM 12.3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | DM 31.5                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| DOI: [10.14469/hpc/4004](https://doi.org/10.14469/hpc/4004)                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | DOI: [10.14469/hpc/4059](https://doi.org/10.14469/hpc/4059)                                                                                                                                                                                                                                                                                                                                                                                                    |

Molecule **1** does have some precedent in
**3**\[cite\]10.1002/ejoc.200900056\[/cite\] but this system exists as a
phenol, having abstracted a proton from an acid and leaving behind the
acid anion, as per below for **1**. Any attempts to deprotonate this
phenol with a superstrong base were unreported.

[![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/sb2-page001.svg){.aligncenter
.size-large .wp-image-19563 decoding="async"
width="400"}](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/04/sb2-page001.svg)

Unsurprisingly therefore, molecules such as **1** and **2** could be
regarded as even more highly potent bases than **3**, driven again by
further aromatisation. The properties of such a potential superbase will
be investigated in the next post.

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


In several posts a year or so ago I considered various suggestions for the most polar neutral molecules, as measured by the dipole moment. A record had been claimed[cite]10.1002/anie.201508249[/cite] for a synthesized molecule of ~14.1±0.7D. I pushed this to a calculated 21.7D for an admittedly hypothetical and unsynthesized molecule.

A record polarity for a neutral compound?

A bond index (BI) approximately measures the totals of the bond orders
at any given atom in a molecule. Here I ponder what the maximum values
might be for elements with filled valence shells.

Following Lewis in 1916\[cite\]10.1021/ja02261a002\[/cite\] who proposed
that the full valence shell for main group elements should be 2 (for the
first two elements) and 8 (the "*octet*"), Bohr
(1922\[cite\]10.1007/BF01326955\[/cite\]), Langmuir
(1919-1921\[cite\]10.1126/science.54.1386.59\[/cite\]) and Bury
(1921\[cite\]10.1021/ja01440a023\[/cite\]) extended this rule to include
18 (the transition series) and 32 (the lanthanides and actinides). If we
assume no contributions from higher Rydberg shells (thus 3s, 3p, 3d for
carbon etc) and an electron pair model for orbital population (which
amounts to the single-determinantal model), then the maximum bond index
for hydrogen (and helium) would be 1, it would be 4 for main group
elements, and then what?

For the special case of hydrogen, I have [previously
identified](https://www.ch.imperial.ac.uk/rzepa/blog/?p=19347) (for a
hypothetical species) a bond index of 1.33, due mostly to a high Rydberg
occupancy of 1.19e. The more normal BI is \<1.0, as noted for this
[hexacoordinated
hydride](https://www.ch.imperial.ac.uk/rzepa/blog/?p=19383) system. My
current estimate for the maximum bond index for main group elements is
\<4.5. Thus for SF~6~, it has the value of \~4.33 and that includes a
modest occupancy of Rydberg shells of 0.36e = 0.18 BI. Exclude these and
it is close to 4.

Move on from group 16 to group 6 and you get compounds such
as Me~4~CrCrMe~4~^4-^ or ReMe~8~^2- ^where the metal bond indices are
\~6.5.^‡^ Compounds such as Cr(Me)~6~ (BI = 5.6)** ** and W(Me)~6 ~(BI =
6.1) are rather lower. This is a long way from 18/2 = 9. The lanthanides
and actinides\[cite\]10.1002/9781118688304.ch15\[/cite\] are unlikely to
reveal many large BIs (32/2= 16 maximum value) since they are [often
ionic](https://www.ch.imperial.ac.uk/rzepa/blog/?p=3977) and the
wavefunctions may be too complex to allow a simple index such as a BI to
be safely computed.

So if we are hunting for record BIs, the transition elements are the
place to hunt. Can a BI of 6.5 be beaten? Can it even approach 9, its
maximum value? Does anyone know of candidate molecules? 

------------------------------------------------------------------------

^‡^FAIR Data doi:
[10.14469/hpc/3352](https://doi.org/10.14469/hpc/3352).

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rel="author"}
:::


A bond index (BI) approximately measures the totals of the bond orders at any given atom in a molecule. Here I ponder what the maximum values might be for elements with filled valence shells.

Messaggi di Rogue Scholar

A record polarity for a neutral compound?

What are the highest bond indices for main group and transition group elements?