Imperial College London

Henry Rzepa

An attosecond is 10^-18^s. The chemistry that takes place on this
timescale is called electron dynamics. For example, it is the time taken
for an electron to traverse the 1s orbit in a hydrogen atom. And
chemists are starting to manipulate electrons (and hence chemistry) on
this timescale; for example a recent article (DOI:
[10.1021/ja206193t](http://dx.doi.org/10.1021/ja206193t){target="_blank"})
describes how to control the electrons in benzene using attosecond laser
pulses.

[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/benzene.svg "benzene"){.aligncenter
.size-full .wp-image-5406
decoding="async"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/benzene.svg)

The diagram above is famously attributed to Kekulé, and we now teach
that this diagram represents resonance structures. This term implies
electron (rather than nuclear) dynamics, which results in us only being
able to observe an averaged structure with six equivalent C-C bonds.
Kekulé himself (see [this
review](http://dx.doi.org/10.1021/cr0300946){target="_blank"}) could
have no way of knowing what the timescale was that his representation
implied, but he certainly must have thought it was fast. What is rarely
mentioned in textbooks however is how fast it actually is. If the
timescale were to be less then say a nanosecond (10^-9^s) then we would
classify such a process not as a resonance but as a **valence
isomerism**. This does implicate nuclear motions, and such would be the
appropriate description for the isomerism of say cyclo-octatetraene,
which is famously slow enough to observe by NMR
(see [10.1039/P29920001951](http://dx.doi.org/10.1039/P29920001951%20){target="_blank"}).
If the timescale were to be a femtosecond (10^-15^s) this would
correspond to molecular vibrations, and benzene indeed has an observable
normal vibrational mode that corresponds more or less to the
representation above (the hydrogens also wag a bit). This mode even has
a name, the [Kekulé
mode](http://www.ch.imperial.ac.uk/rzepa/blog/?p=485){target="_blank"}.
But even this is not fast enough for the intent of the diagram above,
which describes what the electrons (and not any moving nuclei) are up
to.

The article I mentioned at the start, by Inga Ulusoy and Mathias Nest
probes exactly this aspect at the attosecond timescale. More
particularly, they look into how to shape an ultra-short laser pulse to
excite the electrons into excited states of benzene. This act destroys
the aromaticity of the molecule, and changes the electron dynamics in
the process. I should quote them here: "We have shown that by
controlling the electron dynamics we can selectively switch benzene into
nonaromatic target states. These target states exhibit an ultra-fast
bidirectional electron circulation around the ring system."

This is the chemistry in (a few) attosecond(s) that I titled this blog.
Whilst the article noted here is theoretical, there seems little doubt
that experimental studies of chemistry in an attosecond will became more
common, and who knows what surprises await us. Exciting times (sorry
about the pun).

I would conclude by mentioning the other extreme, chemistry in an
exasecond (10^18^s). This happens to correspond more or less to the age
of the universe! As it happens, it is not that difficult to come up with
chemical processes that occur on this timescale. Any (unimolecular)
process that has a free energy barrier larger than that inferred using
*e.g.* the equation Ln k/T = 23.76 -- ΔG/RT would fit. An example might
be the half life for the enantiomerisation of alanine (left to its own
devices, and not interfered with by e.g. catalysts).

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


An attosecond is 10
<sup>
 -18
</sup>
s. The chemistry that takes place on this timescale is called electron dynamics. For example, it is the time taken for an electron to traverse the 1s orbit in a hydrogen atom. And chemists are starting to manipulate electrons (and hence chemistry) on this timescale; for example a recent article (DOI: 10.1021/ja206193t) describes how to control the electrons in benzene using attosecond laser pulses.

Postagens de Rogue Scholar

Chemistry in an attosecond.