Aldo Domenicano and the structural chemistry of substituent effects: a personal tribute
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- Hargittai, I. Struct Chem (2013) 24: 735. doi:10.1007/s11224-012-0162-4
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Aldo Domenicano has been a provider and interpreter of structural information on substituent effects, especially in benzene derivatives. He started in X-ray crystallography, continued in gas-phase electron diffraction, and eventually added quantum chemical computations to his arsenal of tools of research. He was a charter member of the Editorial Board of Structural Chemistry; he has served the community in other ways as well, such as with directing international schools and editing monographs. He has also been an esteemed colleague and faithful friend.
“To understand the properties of molecules,
not only must you know their structures,
but you must know them accurately.”
Linus Pauling (attributed)
He studied at the chemistry school of the University of Rome, known formerly as Università degli Studi di Roma “La Sapienza”, nowadays, Sapienza—Università di Roma, and often referred to in short simply as Sapienza. His career started at the same university. Alessandro Vaciago was his mentor not only during his graduate studies, but for years, following graduation, when Aldo was a budding scientist. Alessandro was a man of the world and helped Aldo look around beyond the narrow specialization that modern research often forces scientists in.
Aldo did some post-graduate studies with Ronald S. Nyholm at University College London in 1970, not long before Nyholm’s tragic death in 1971. When Aldo embarked on his new project, which would become his life motif, the structural variations of substituted benzene derivatives, Vaciago recommended to him to contact Charles A. Coulson of Oxford. Aldo visited Coulson a couple of times in the early 1970s, and a fruitful interaction ensued between them. Aldo’s first publications in his chosen area of research were co-produced with Coulson and Vaciago [1, 2].
Aldo has been a professor of chemistry at the University of L’Aquila, without giving up his home in Rome where he and his wife and co-worker, Anna Rita Campanelli, live. Anna Rita works at the chemistry department of the Sapienza, Aldo’s alma mater, so their family life and their respective laboratories are interwoven on more than one level.
The substituted benzene derivatives proved to be a fortunate choice of inquiry not only because they represent a fundamental class of organic substances, but also because the geometrical variations upon substitution proved to be possible to measure and possible to interpret in a meaningful manner, provided that the data were sufficiently accurate. Aldo has successfully dealt with the basic problem of distinguishing between precision and accuracy for every experimental and computational technique he has employed. He has become an early and influential advocate of the importance of taking the physical meaning of structural information into account when assessing the uncertainties of structural studies, regardless whether they originate from experiment, and which one, or from computation.
Aldo has always had a deep sense of history and he likes to point out that Keidel and Bauer were the first who noticed that the geometry of the benzene ring undergoes a change upon substitution when they determined the structure of phenylsilane by gas-phase electron diffraction . The structural chemistry community was slow in catching up, and the next studies were about a decade later. Aldo’s publications have meticulously recorded the progress in the field, never failing to give credit to his predecessors in the literature. Aldo’s initial studies were by X-ray crystallography, but eventually he realized that he should turn to gas-phase electron diffraction if he wanted unambiguous information about the geometrical variations he was seeking. One reason was the need to collect data referring to internuclear distances rather than the rather vague concept of interatomic distances. The other reason was to acquire information on the structure of free molecules, which is determined solely by intramolecular interactions. These considerations, of course, could not have meant much a few decades before, but with increasing accuracy requirements as well as possibilities, this was the right direction to develop his investigations.
We in Budapest were very lucky that Aldo chose us in the late 1970s-early 1980s to learn the technique of gas-phase electron diffraction and, eventually, to build up a most successful and rewarding cooperation of joint studies. By then, the Budapest group had been in the field for more than a decade. Aldo knew that our group had introduced technological innovations in our experiments and we were striving for maximum possible precision in our structure determinations. We were always open to cooperation and to taking up challenges of interesting structural puzzles. Aldo was always open to interactions and he built up an extensive cooperation with the crystallographers of the CNR Institute of Structural Chemistry as well.
The first time Aldo arrived in Budapest for a few months, it was winter, but all of us warmed up quickly to a fruitful exchange of ideas and experience. What followed was a most productive joint venture, which has so far produced well over thirty joint research papers in refereed periodicals and two edited monographs. Below I present a brief survey of the most characteristic topics of these joint publications, singling out only some representative references.
An electronegative substituent tends to open somewhat the adjacent angle in the benzene ring, usually identified as the ipso angle; an electropositive substituent causes the opposite effect. When there is more than one substituent, the substituent impacts add up in a seemingly complicated manner, but it has been well established on the basis of a large amount of observations. We have determined through our joint efforts the molecular geometries of quite a few halogen-substituted and alkyl-substituted and similar benzene derivatives. They included fluoro , chloro  and bromobenzenes ; para-xylene , mesitylene , tert-butylbenzene , and ethynylbenzenes .
Further, similar studies included aniline  and para-diaminobenzene ; and para-methylsulphonylbenzoic acid and methylphenylsulphone  as well as para-benzenedithiol . Phenol  and other hydroxybenzenes  have also been studied and in this case the effects of intermolecular hydrogen bonding in the crystal were also examined. Terephthalaldehyde  and nitrobenzene  have also been investigated, including their conformational peculiarities. Several reports dealt with substituted derivatives with silicon in the substituent and not only the variations in the ring geometries, but also trends in silicon stereochemistry were uncovered [19–21].
Above, it has been mentioned that the electronegativity of the substituent has been observed to determine the variations in the ipso angle in the ring. Utilizing the reverse approach, accurately determined ring deformations provide a tool to estimate substituent electronegativity, which may be especially useful for gaining information about electronegativities of functional groups .
Extended structural studies on cyano  and isocyano  derivatives led to interesting comparisons of gaseous and crystalline molecular structures, and their variations, once again pointing to the importance of intermolecular interaction in the crystal.
Our joint efforts manifested themselves in organizing and co-directing two summer courses in the framework of the International School of Crystallography at the Ettore Majorana Center for Scientific Culture in Erice, Sicily. The first took place in the spring of 1985 with the title “Static and Dynamic Implications of Precise Structural Information.” There was a third co-director of the 1985 course, Peter Murray-Rust. The second course followed in the spring of 2001 with the title “Strength from Weakness: Structural Consequences of Weak Interactions in Molecules, Supermolecules, and Crystals.” Thanks to the efforts of the grand organizer of the School, Lodovico Riva di Sanseverino (1939–2010), these courses became a great success both for scientific instruction and as cultural events. With Lodovico’s passing away, Aldo and I lost a dear friend.
From each of these two courses an edited volume grew out. The first came out very slowly, long after the course, due to various organizational difficulties, but the contents did not suffer from these difficulties. We had also time to realize that the title of the volume should be somewhat different from the title of the course; in particular, the word precision should be replaced by accuracy. The resulting Accurate Molecular Structures has been well received, and Oxford University Press has kept it in print ever since . We were happy to see that our book was made into Volume No. 1 in the International Union of Crystallography series Monographs on Crystallography. The second book followed more quickly the course; it differs mainly from the first in its giving more emphasis to computational work and to the investigation of biological systems. Its title is identical with the title of the course . The first part of the title was meant to suggest that even weak interactions may be the source of important structural variations let alone when cooperative effects amplify their consequences.
Our numerous personal encounters over the years in Rome, L’Aquila, and Budapest never ceased to further enrich us scientifically, but these meetings gave also opportunities to engage in some exchanges at a broader level than just structural chemistry. Some of our joint “cultural” ventures received manifestation in publications about enantiomers in M.C. Escher’s drawings , a magic square discovered in the wall of a medieval church in Italy  and another in the Roman city Aquincum in Budapest . Perhaps, the most memorable one was our “discovery” of an ancient system of elements from the twelfth or thirteenth century in form of a fresco on the wall of the crypt beneath the cathedral in Anagni, some sixty kilometers southeast of Rome, off the Rome–Naples motorway .
Above, I have tried to give a glimpse into a segment of the oeuvre of our celebrant, the seventy-five-year old Aldo Domenicano, through our joint research. I wish you, Aldo, on behalf of the Editorial Board of Structural Chemistry many happy returns and a continuation of your fruitful and enjoyable life in which our science keeps taking an important share.