Letters: July 2026

Pauling’s paradigm not at large

I read with interest the article ‘Molecules with largest dipole moments aren’t due to electronegativity differences’, which notes that ‘some of the trends seen in diatomics may not translate directly to more complex chemistry’.

One familiar trend in diatomics is that homonuclear molecules have no dipole moment. That trend can be violated strikingly in larger homonuclear molecules, as shown in ‘Partial ionic character beyond the Pauling paradigm: metal nanoparticles’.

That article found that silver clusters can exhibit substantial dipole moments despite consisting of atoms with identical electronegativity. For example, a formaldehyde-like structure of Ag₄ has a dipole moment of 3.4D, roughly one and a half times larger than that of formaldehyde.

The paper reported studies of homonuclear silver clusters with up to 18 atoms and found dipole moments as large as 4.65D. The uneven charge distributions were correlated with coordination numbers, and the study found that ‘peaky’ atoms on irregular nanoparticle surfaces have more positive partial charges.

The atoms with more positive partial charges were found to be energetically more favourable sites for CO adsorption. Thus, the breakdown of the Pauling electronegativity model has direct implications for understanding catalysis by metal nanoparticles, surface reactivity and nanoparticle structure–function relationships.

Don Truhlar FRSC
Via email

Curious crystals

The article on ‘disappearing polymorphs’ referred to the famous case of ritonavir. Actually, the first discussion on the phenomenon was some 10 years earlier in a litigation regarding cefadroxil monohydrate.

I was an expert witness in that case, which was essentially an argument about the crystal structure of Form1 that had been patented earlier but later could not be crystallised in the same structure. I remember long arguments about whether one x-ray powder pattern was the same as another, and about the ‘seeding hypothesis’. There were discussions about the last breath of Henry VIII!

Initially, I did not believe this was possible but later changed my view when I saw the seeding effect in my own office. The method from the old patent was used to make cefadroxil in the chemistry department at Oxford, being very careful to keep all possible seeds of the new form out of the preparation. I was given some vials of a solution with white powder at the bottom. X-ray diffraction indicated this was not the new form. However, I also saw crystals of the new form appear on the surface of the liquid – seeding from the air happened in front of my eyes.

Mike Glazer FRSC
University of Oxford, UK

Stamping chemistry’s legacy

As a chemist and keen philatelist with an interest in the history of chemistry, I am struck by how selectively Britain’s chemical heritage is represented on its postage stamps. 

Royal Mail has rightly honoured figures such as Michael Faraday, Robert Boyle, Ernest Rutherford and Dorothy Hodgkin. Together, they offer a compact philatelic snapshot of Britain’s contribution to chemical science, spanning early experimental chemistry, electrochemistry, atomic structure and 20th century structural analysis.

Yet several foundational British chemists remain absent. Humphry Davy, John Dalton and Joseph Priestley all played central roles in the development of modern chemistry: Davy through the electrochemical isolation of metals, Dalton through atomic theory and Priestley through his work on gases. Their achievements helped shape what we now regard as classical chemical science.

What makes this omission especially striking is that these scientists have been commemorated elsewhere. Priestley appears on stamps from the US and the Maldives, while Dalton has been honoured by Madagascar and the Marshall Islands. It is curious that British chemists have found philatelic recognition abroad while remaining absent from British issues.

The current philatelic record therefore presents only a partial picture of Britain’s chemical legacy. Better commemoration of our great chemists on stamps might also help inspire a younger generation to consider chemistry as a future profession. Many years ago, I was myself inspired to pursue chemistry by learning about the achievements of great chemists, and stamps can be one effective way of introducing their stories to a wider audience.

Vitaliy Khutoryanskiy FRSC
Via email

No lie

Re: ‘We cannot lie our way to the truth’ by Derek Lowe:

Amen.

Dorothy Feigl FRSC
Notre Dame, Indiana, US

One half-drop

In ‘One extra drop’ Asmin Beren Aydin narrates how one mistake in the laboratory changed outlook of her life. Yes, precision in analytical chemistry is often learned not from instruments, but from moments of careful human judgement.

Sixty years ago, I had one such experience while performing an acid-base titration for the first time in my life, in a pre-university level laboratory experiment. After the professor explained the theory and methodology to carry out the acid-base titration, I obtained a pilot reading. Then, while I was carrying out the experiment to get an accurate end point, the pilot reading was guiding me to be careful in observing and adding the last drop.

I knew from my intuition that it needed one more drop. I was about to add it and at that moment my professor, who was watching me experimenting, came near to me. I told him that I might need one drop more. The professor authoritatively told me ‘No’. I was a bit taken aback. He told me that I needed half a drop.

He asked me to fetch a sharp shaving blade. He released one drop on his left-hand forefinger and with the help of the blade he cut the drop. He then dropped the half portion in the conical flask and lo! colour change occurred.

That lesson has remained with me ever since, and I continue to pass it on to my students today as a reminder that chemistry is as much about judgement as it is about technique. Many of my students are in the profession furthering this message. 

Koyar Sanlo Rane FRSC
Rani Channamma University, India