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Professor David Craig was interviewed in 1998 for the Interviews with Australian scientists series. By viewing the interviews in this series, or reading the transcripts and extracts, your students can begin to appreciate Australia's contribution to the growth of scientific knowledge.
The following summary of Craig's career sets the context for the extract chosen for these teachers notes. The extract covers his early scientific interest in understanding molecular orbital theory. Use the focus questions that accompany the extract to promote discussion among your students.
David Craig was born in 1919 in Sydney. He was introduced to the application of quantum mechanics to chemical bonding at the University of Sydney, where he completed a BSc(Hons) in 1940 and an MSc in 1941. From 1942 to 1944, his scientific career was interrupted by war service as an aide to General Sir Iven MacKay. In 1944 he returned to the University of Sydney as a lecturer in physical chemistry.
In 1946 Craig moved to University College, London, where as a Turner and Newall Research Fellow he worked on the theory of the excited states of benzene. Immediately on completion of his PhD in 1949 he was appointed lecturer at University College, where he continued his pioneering work on the inclusion of configuration interaction in molecular orbital theory. This lectureship continued until 1952.
Craig again returned to the University of Sydney in 1952, to be the first Professor of Physical Chemistry. In 1956 he went back to University College, London, this time to be Professor of Theoretical Chemistry. During his time there his principal research interest was investigating the spectroscopic properties of molecular crystals and he also supervised experimental spectroscopy work.
In 1967 Craig came back to Australia when he was appointed Foundation Professor of Physical and Theoretical Chemistry at the Research School of Chemistry, Australian National University (ANU), a position he held until his retirement in 1984. At the ANU he served as Dean of the Research School of Chemistry from 1970 to 1973 and has been an Emeritus Professor there since 1984.
Craig has received many honours during his long and distinguished career. His distinctions include honorary doctorates from the Universities of Sydney and Bologna, Fellowship of the Royal Society of London (1968) and Fellowship of the Australian Academy of Science (1969), where he served as president from 1990 to 1994. He was made Officer of the Order of Australia in 1985 for his services to Australian science. He has received the H G Smith Memorial Medal (1972) and the Leighton Memorial Medal (1991), both from the Royal Australian Chemical Institute.
His achievements as one of Australia’s most eminent chemists is recognised by the establishment in 1999 of the David Craig Medal by the Australian Academy of Science awarded annually to an active researcher for contributions to chemistry of a high order. Also named in his honour is the David Craig Wing of the Research School of Chemistry, opened at the ANU in 1995.
Applying molecular orbital theory
What was your main scientific interest during that spell in University College?
When I was doing my doctorate, knowing the interest in the department in benzene I worked on the theory of the excited states of benzene – although they had been experimentally characterised to a degree, theoretically there was no proper account of them. First I enlarged the existing method of valence-bonds, and added a different kind of structure in which there were separated plus and minus charges as well as ordinary covalent bonds. I got from that a reasonable picture of the excited levels of benzene, of which two by that time had been roughly characterised so I had a kind of test.
But then I got interested in the serious discrepancy between that theoretical picture of benzene and the molecular orbital picture which had been developed beforehand by Mulliken, amongst others. Working through some work that had been done in 1938 on the molecular orbital theory in which the parameters were all calculated instead of being empirically fitted to experiment, as was the pattern of the time, I was struck that there was something very odd about the results. The separations between the states were sometimes smaller than the coupling integrals between them, so that it really made no sense to claim that those states were physically real. What you had to do was to work it out again, including all that interaction between configurations, and that’s what started me off on the business of configuration interaction. Once that was done, it all began to fit into place – not with great numerical precision, but with agreement in the ordering of the states, which was the first thing you had to look for.
Next I had an interesting, enjoyable period of working with Ian Ross, who was one of my first PhD students and also had worked with me a bit in Sydney so we were very comfortable working together. We wanted to improve the integrals that went into these computations – technically it’s the change from two carbon centres to three and four carbon centres that you’ve got to accommodate. Then I had a letter from Robert Parr, an American who was working with Mulliken in Chicago, saying that they were working with these integrals and he’d got a few himself, and would we like to team up and do a joint trans-Atlantic effort, in which we’d recalculate the whole business, using Parr’s integrals and ours? We agreed to do that. I’d never met him, didn’t meet him for 10 years, but we had a good correspondence and published a joint paper which really for the first time got sensible agreement between theory and experiment.
To extract the roots of a six by six determinant for this work, as we had to do, was a formidable task using the mechanical calculators of the time and was really a full day’s work. And to allow for mistakes it all had to be done twice – we did it once in London, and Parr repeated everything in the US before we could accept that it was right. Now, of course, we can do it in microseconds.
An edited transcript of the full interview can be found at http://www.science.org.au/scientists/interviews/c/dc.
Focus questions
Select activities that are most appropriate for your lesson plan or add your own. You can also encourage students to identify key issues in the preceding extract and devise their own questions or topics for discussion.
chemistry
covalent bonding
electron configuration
excited states
molecular orbitals
theoretical calculation
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