රසායන විද්‍යාව අර්බුදයේ
පාසලේ දී හරි විශ්වවිද්‍යාලයෙ දි හරි රසායන විද්‍යාව මගේ ප්‍රිතම විෂය වුණේ නැහැ. එහෙත් බෙන්සීන් අණුවේ ව්‍යුහය ගැන සිහින මැවූ කෙක්‍යුලේ මට වැදගත් වුණා. නාථ දෙවියන් පිළිබඳ මෙරට වි්ද්වත් බුද්ධිමත් තාර්කික විද්‍යාඥයන් කී කතා අනාථ කිරීමට කෙක්‍යුලේ හා රාමනුජන් සමත් වුණා. අදහස් ලැබෙන්නේ කාගෙන් ද කෙසේ ද ආදිය පිළිබඳ ව කිසිම අදහසක් මෙරට විද්‍යාඥයන්ට හා අනෙක් බුද්ධිමතුන්ට නැත්තේ ඔවුන්ට කිසිම අදහසක් නොඑන නිසා.
රසායන විද්‍යාවේ භෞතික විද්‍යාවේ මෙන් ප්‍රවාද අර්බුද එතරම් සඳහන් වෙන්නේ නැහැ. එහෙත් කෙක්‍යුලේගේ බෙන්සීන් අණුව අවුරුදු දෙසියකට පමණ පසුව අර්බුදයට ගිහින්. මේ ඒ පිළිබඳ ව ජූලි 15 නිවු සයන්ටිස්ට් සඟරාවේ පළ වූ ලිපියක්. මෙය අරුන් සිද්ධාර්ථන් දෙමළ ජාතිවාදය අර්බුදයකට පත්කර ඇති අන්දමට විවිධ විද්වතුන්ගේ හා හා වලට ලක්වෙන එකක් නැහැ. එහෙත් රසායන විද්‍යාව ගැන උනන්දුවක් දක්වන අයට සමහරවිට මෙය වැදගත් වේවි. කොහොමටත් සකර්බර්ග්ට මෙය වැදගත්.
මේ ලිපිය පළ කිරීමෙන් මා කියන්නේ බටහිර විද්‍යාවේ පරම සත්‍යය කියා දෙයක් නොමැති බවත් ඊනියා සත්‍යය හඹා යෑමක් නැති බවත් එහි ඇත්තේත් නිරීක්‍ෂණ විස්තර කිරීමට වැඩ කරන කතා ගෙතීම පමණක් බවත්. මෙරට විද්‍යාඥයන්ට කරන්න පුළුවන මේ අලින්නෙ අපේ වැඩ කියමින් පාරම්බෑම පමණයි. .
One of chemistry's most crucial concepts is in crisis - can we fix it?
Our understanding of aromaticity, a
concept that underpins life itself, has been thrown into chaos. But from the
ashes have risen powerful new tools including supercharged solar cells and a
Jekyll-and-Hyde material powered by "antiaromaticity"
10 July 2023
In 1862, a chemist
nodded off in front of a fire and began to dream. August Kekulé had been
pondering the most pressing question in his field at the time: what was the
chemical structure of a curious compound called benzene? As Kekulé slumbered,
the atoms danced in his mind and organised themselves into a vision of a snake
eating its own tail. That was it! The carbon atoms of benzene were joined
together in a ring.
It may not have
quite the drama of Archimedes running through the streets naked, but, in its
own way, it was a eureka moment. Benzene turned out to be the archetype of a
compound with a property called aromaticity, now recognised as among the most
important ideas in chemistry. Instead of standard bonds, aromatic molecules
contain dynamic, ring-shaped bonding that endows them with incredible stability
and a raft of other handy properties. Two-thirds of known molecules are
aromatic, including components of DNA and proteins, the building blocks of life.
Recently, though,
the dream has turned into a nightmare. We have been discovering new kinds of
aromaticity everywhere – to the point where there are dozens of competing
definitions. It may be one of the most important concepts in chemistry, but no one can agree on what it
means any more. It isn’t all bad news, though. The confusion has prompted
chemists to think deeply about aromaticity and this has led to a raft of new
ideas about how we can put it to use.
Chemistry is the
science of atoms, bonds and molecules and the craft of cajoling those molecules
to react with one another. Today, the fruit of that labour is all around us, in
the form of the wondrous substances that make up everything from clothes to
toothpaste. And the ground rules of chemistry are well established. We know,
for instance, that basic chemical bonds involve two atoms sharing a pair of
electrons.
But there are
molecules that don’t play by the usual rules. The biggest group of these first
came to light in 1825, when Michael Faraday was experimenting with the
compressed gas used to light the lamps at the Royal Institution in London where
he worked. He managed to isolate a sweet-smelling substance called benzene.
This was confusing stuff, chemically speaking. Besides its sweet smell, the
molecule’s defining characteristic was its strong bonding, which made it
unexpectedly stable. Soon, other molecules with this aberrant behaviour were
found and they became collectively known as “aromatics” in a nod to the
characteristic aroma of several founding members.
By careful
experimentation, chemists gleaned that benzene molecules were composed of six
carbon atoms and six hydrogen atoms. That was odd, since most compounds of
hydrogen and carbon had roughly twice as many hydrogen atoms as carbon ones.
And so, for almost 40 years, the structure of benzene remained enigmatic –
until Kekulé published his ring idea a few years after he had dreamed it up.
Later generations of
researchers deduced that Kekulé was more or less right. Benzene is a flat,
hexagon-shaped ring of six carbon atoms with a hydrogen atom attached to each.
Crucially, not all the electrons in the molecule are shared between two atoms.
Some, known as pi electrons, are “delocalised” over the entire ring, meaning
they can flow around it. It is this factor that gives benzene its famous
stability. Other aromatics, which might have different sized rings, for
example, also have these delocalised electrons.
Hückel’s rule
A century or so ago,
then, everything seemed to be falling neatly into place. A molecule was
aromatic if it was flat, made of carbon atoms arranged in a ring structure, and
contained the right number of pi electrons. The “right number” was defined by
Hückel’s rule, named after chemist Eric Hückel, who worked on the idea in 1931.
Technically, the rule says a ring of atoms is aromatic if it is flat and has 4n
+ 2 delocalised electrons (where n is any positive whole number). In practice,
this means rings made of six, 10 or 14 carbon atoms, for example, can be
aromatic, but rings of say four, eight or 12 can’t.
Alas, things
wouldn’t remain so straightforward. “In the second 100 years of aromaticity
research, there was this explosive development in what we understand as
aromaticity,” says computational chemist Judy Wu at the University of Houston,
Texas.
It was quickly
realised, for instance, that one of the carbon atoms in benzene could be
swapped for one of a few other elements, such as nitrogen, without losing its
stable aromatic character. Flatness was another factor to be questioned. In
2003, Rainer Herges at the University of Kiel,
Germany, and his colleagues created
a molecule shaped like a Möbius strip – a twisted
ring that is far from flat – that nonetheless had those delocalised electrons.
This Möbius molecule also trampled on Hückel’s rule. The twist in the ring
skews the orbits of the electrons so that, in these systems, rings with 4n
delocalised electrons are stable.
Do you even need a
ring? One of the more recent actors on the aromaticity stage is buckminsterfullerene, a molecule that
looks like a rough sphere made of hexagons and pentagons, akin to a soccer
ball. When Harry Kroto discovered it in 1985, he suggested it might be the
first spherical aromatic molecule. He was close, says Miquel Solà at the University of Girona,
Spain. The molecule itself “is not particularly aromatic, but if you remove 10
of its electrons, it is very aromatic”. With that magic number of delocalised
electrons, “buckyballs” gain a stabilising coating of these charged particles
that circulate in a sphere rather than a ring. Andreas Hirsch at the University of
Erlangen-Nuremberg in Germany and his colleagues devised a set of rules to capture this aromaticity subtype in
2000.
But whether these
unusual molecules are truly aromatic is to some extent a matter of opinion.
“Aromaticity is not like melting point, there is no single, direct experiment I
can do to measure how aromatic a molecule is,” says Solà.
That said, one
indirect method for defining aromaticity quantitatively has gained some
traction. If you put an aromatic molecule in a magnetic field, the delocalised
electrons will start to circulate around the ring, generating a tiny magnetic
field. This then affects how other nearby atoms show up when measured using a
technique called nuclear magnetic resonance spectroscopy.
It is usually too
finicky to measure this effect in practice. But in 1996, Paul Schleyer and his
colleagues at the University of Erlangen-Nuremberg developed a computational
method that simulates the experiment
and spits out a single value that indicates how aromatic any given molecule is.
It is a neat trick that has helped bring some order to the debates around
aromaticity. But it is just a simulation, not a direct observation. And for
very complex molecules even the simulations cease to work.
Meanwhile, reports
of unusual aromaticity have kept coming. In 2022, Stefanie Dehnen at the University of Marburg,
Germany, and her colleagues claimed to have found a new form of this bonding in a
prism-shaped cluster of bismuth atoms. It was bold, given that this molecule
had no carbon atoms at all or any pi orbitals. Dehnen’s reasoning was that the
cluster’s electrons were being spread across the molecule in an orbit that had
a different geometry to the classic pi orbitals and this explained its unusual
stability. “I’m not particularly sure that this claim is correct,” says Solà,
who has co-written a counter-argument to the claim with Dariusz Szczepanik
at Jagiellonian University in KrakÓw, Poland.
In all, more than 45
subtypes of aromaticity have now been put forward, leaving chemists despairing.
In one recent review article, a number of them declared the ruckus around aromaticity had reached crisis point.
For some, it is time
to move on. We could keep expanding the definition of aromaticity, but that
would leave an already vague concept next to meaningless. This is certainly
Wu’s view. She says the question we need to focus on now is how to make use of
aromaticity. Ranana Gershoni-Poranne at the Technion – Israel
Institute of Technology shares that view.
Chemists have, of
course, made use of aromaticity to create strong and stable molecules for years
– but that barely scratches the surface of its useful properties. One neat
thing about aromatic molecules is that they can be joined together like beads
on a string. The delocalised electrons can then escape their rings and spread
out along the string. The result is a material that conducts electricity like a
wire, but is soft and stretchy. “They’re lightweight and made from atoms that
we have in abundance on Earth, rather than from metals that we might have a
limited supply of,” says Gershoni-Poranne.
But rather than
simply use them as stretchy wires, she and her colleagues hope to harness them
in a new kind of silicon-free computer chip. Computers require semiconductors,
materials whose electrical conductivity can easily be switched on and off.
“We’re using artificial intelligence machine-learning models to try to design better
semiconducting organic molecules,” she says. Her models are already beginning to identify aromatic structures with promising
semiconducting properties. Such systems might
be used, for example, as smart biosensors that could be placed in the body and
that biodegrade after use.
Aromaticity is also
being deployed in medicine. Aromatics are great at absorbing light and, if you
decorate the rings in different ways, for example by attaching different groups
of atoms, that changes the wavelength of light they absorb. This includes
wavelengths that penetrate the body. One application involves sealing a drug
inside an aromatic “photocage” that can be cracked open with light. Doctors
could shine light on a particular location, such as the site of a tumour, to
release the drug. In a recent animal study, cancers were
selectively targeted in this way.
Antiaromaticity
We have long known
that the superpower of aromaticity also has a dark side: antiaromaticity. Find
ways to manipulate this alter ego and it could help tackle one of the world’s
biggest problems: climate change.
Antiaromatic
molecules are rings with delocalised electrons, but in this case they are less,
not more, stable than expected. Take the simplest antiaromatic, a ring of four
carbon atoms called cyclobutadiene. It is so reactive that it took chemists 60
years to find a way to trap it and prove it could really exist.
If the extreme
reactivity of antiaromatics can be corralled – say by bonding them to their
aromatic counterparts – they can be useful. Increasingly, that is possible. In
2019, Jorge Juan Cabrera-Trujillo and Israel Fernández at the Complutense
University of Madrid in Spain ran calculations showing stabilised antiaromatics react rapidly with
carbon dioxide, offering a way to capture the
greenhouse gas.
We have also
recently begun to master the art of converting aromatic molecules into
antiaromatic ones, like turning Jekyll into Hyde. When an aromatic molecule
absorbs light of a suitable wavelength, the captured energy can kick one of its
delocalised electrons into a high-energy orbit, which makes the molecule
antiaromatic. When benzene becomes antiaromatic, it rearranges its bonds to
form a molecule that is under so much strain that it explodes at the slightest
scratch.
Aside from explosives, the power to flip a stable aromatic
molecule into an unstable antiaromatic one offers almost endless possibilities.
Chemists have already suggested using light-triggered antiaromaticity to break
down drug molecules in wastewater to eliminate their environmental damage, for
example, or as a controlled way to depolymerise plastics back into their
starting materials, ready for reuse.
By the same token,
molecules that are normally antiaromatic become aromatic when they absorb
light. This could be ideal for building a special type of solar cell that makes
use of a process called singlet fission. Singlet fission occurs when a molecule
that has just captured a photon of sunlight interacts with and passes some of
that captured energy to a neighbour. Molecules that become aromatic when they
absorb light will be more stable in that state and have a greater chance to
pass on energy. “With one excitation, you generate two excited states – two for
the price of one,” says Henrik Ottosson at Uppsala University in Sweden.
In other words, a singlet fission solar cell could generate almost 50 per cent
more power from the same amount of sunlight.
Two centuries after
benzene’s discovery, defining the concept of aromaticity looks more tricky than
ever. “I’m not sure if we will ever solve it,” says Gershoni-Poranne. But for
those interested in real-world applications, the new attention on aromaticity may
ultimately prove to be a winning situation.
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