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6 INTERVIEW: PROFESSOR PAUL VON RAGUÉ SCHLEYER
FUNDAMENTALS OF ORGANIC CHEMISTRY
Schleyer quickly became enamored with the power of ab initio computations
to tackle interesting organic problems. His enthusiasm for computational chemistry eventually led to his decision to move to Erlangen—they offered unlimited
(24/7) computer time, while Princeton’s counteroffer was just 2 h of computer
time per week. He left Erlangen in 1998 due to enforced retirement. However,
his adjunct status at the University of Georgia allowed for a smooth transition
back to the United States, where he now enjoys a very productive collaborative
relationship with Professor Fritz Schaefer.
Perhaps the problem that best represents how Schleyer exploits the power
of ab initio computational chemistry is the question of how to define and
measure aromaticity. Schleyer’s interest in the concept of aromaticity spans his
entire career. He was drawn to this problem because of the pervasive nature
of aromaticity across organic chemistry. Schleyer describes his motivation:
“Aromaticity is a central theme of organic chemistry. It is re-examined by each
generation of chemists. Changing technology permits that re-examination to
occur.” His direct involvement came about by Kutzelnigg’s development of
a computer code to calculate chemical shifts. Schleyer began the use of this
program in the 1980s and applied it first to structural problems. His group
“discovered in this manner many experimental structures that were incorrect.”
To assess aromaticity, Schleyer first computed the lithium chemical shifts in
complexes formed between lithium cation and the hydrocarbon of interest. The
lithium cation would typically reside above the aromatic ring and its chemical
shift would be affected by the magnetic field of the ring. While this met with
some success, Schleyer was frustrated by the fact that lithium was often not
positioned especially near the ring, let alone in the center of the ring. This led
to the development of NICS, where the virtual chemical shift can be computed
at any point in space. Schleyer advocated using the geometric center of the ring,
then later a point 1 Å above the ring center.
Over time, Schleyer came to refine the use of NICS, advocating an examination of NICS values on a grid of points. His most recent paper posits using just
the component of the chemical shift tensor perpendicular to the ring evaluated
at the center of the ring. This evolution reflects Schleyer’s continuing pursuit
of a simple measure of aromaticity. “Our endeavor from the beginning was to
select one NICS point that we could say characterizes the compound,” Schleyer
says. “The problem is that chemists want a number which they can associate
with a phenomenon rather than a picture. The problem with NICS was that it
was not soundly based conceptually from the beginning because cyclic electron
delocalization-induced ring current was not expressed solely perpendicular to
the ring. It’s only that component which is related to aromaticity.”
The majority of our discussion revolved around the definition of aromaticity.
Schleyer argues that “aromaticity can be defined perfectly well. It is the manifestation of cyclic electron delocalization which is expressed in various ways.
The problem with aromaticity comes in its quantitative definition. How big is
INTERVIEW: PROFESSOR PAUL VON RAGUÉ SCHLEYER
the aromaticity of a particular molecule? We can answer this using some properties. One of my objectives is to see whether these various quantities are related
to one another. That, I think, is still an open question.”
Schleyer further detailed this thought, “The difficulty in writing about aromaticity is that it is encrusted by two centuries of tradition, which you cannot
avoid. You have to stress the interplay of the phenomena. Energetic properties
are most important, but you need to keep in mind that aromaticity is only 5
percent of the total energy. But if you want to get as close to the phenomenon
as possible, then one has to go to the property most closely related, which is
magnetic properties.” This is why he focuses upon the use of NICS as an aromaticity measure. He is quite confident in his new NICS measure employing the
perpendicular component of the chemical shift tensor. “This new criteria is very
satisfactory,” he says. “Most people who propose alternative measures do not
do the careful step of evaluating them against some basic standard. We evaluate
against aromatic stabilization energies.”
Schleyer notes that his evaluation of the ASE of benzene is larger than many
other estimates. This results from the fact that, in his opinion, “all traditional
equations for its determination use tainted molecules. Cyclohexene is tainted
by hyperconjugation of about 10 kcal mol−1 . Even cyclohexane is very tainted,
in this case by 1,3-interactions.” An analogous complaint can be made about the
methods Schleyer himself employs: NICS is evaluated at some arbitrary point
or arbitrary set of points, the block-diagonalized “cyclohexatriene” molecule is
a gedanken molecule. When pressed on what then to use as a reference that is
not ‘tainted’, Schleyer made this trenchant comment: “What we are trying to
measure is virtual. Aromaticity, like almost all concepts in organic chemistry,
is virtual. They’re not measurable. You can’t measure atomic charges within
a molecule. Hyperconjugation, electronegativity, everything is in this sort of
virtual category. Chemists live in a virtual world. But science moves to higher
degrees of refinement.” Despite its inherent ‘virtual’ nature, “Aromaticity has
this 200 year history. Chemists are interested in the unusual stability and reactivity associated with aromatic molecules. The term survives, and remains an
enormously fruitful area of research.”
His interest in the annulenes is a natural extension of the quest for understanding aromaticity. Schleyer was particularly drawn to -annulene because it can
express the same D6h symmetry as does benzene. His computed chemical shifts
for the D6h structure differed significantly from the experimental values, indicating that the structure was clearly wrong. “It was an amazing computational
exercise,” Schleyer mused, “because practically every level you used to optimize the geometry gave a different structure. MP2 overshot the aromaticity, HF
and B3LYP undershot it. Empirically, we had to find a level that worked. This
was not very intellectually satisfying but was a pragmatic solution.” Schleyer
expected a lot of flak from crystallographers about this result, but in fact none
occurred. He hopes that the X-ray structure will be redone at some point.
FUNDAMENTALS OF ORGANIC CHEMISTRY
Reflecting on the progress of computational chemistry, Schleyer recalls that
“physical organic chemists were actually antagonistic toward computational
chemistry at the beginning. One of my friends said that he thought I had gone
mad. In addition, most theoreticians disdained me as a black-box user.” In those
early years as a computational chemist, Schleyer felt disenfranchised from
the physical organic chemistry community. Only slowly has he felt accepted
back into this camp. “Physical organic chemists have adopted computational
chemistry; perhaps, I hope to think, due to my example demonstrating what
can be done. If you can show people that you can compute chemical properties,
like chemical shifts to an accuracy that is useful, computed structures that are
better than experiment, then they get the word sooner or later that maybe you’d
better do some calculations.” In fact, Schleyer considers this to be his greatest
contribution to science—demonstrating by his own example the importance of
computational chemistry toward solving relevant chemical problems. He cites
his role in helping to establish the Journal of Computational Chemistry in both
giving name to the discipline and stature to its practitioners.
Schleyer looks to the future of computational chemistry residing in the
breadth of the periodic table. “Computational work has concentrated on one
element, namely carbon,” Schleyer says. “The rest of the periodic table is
waiting to be explored.” On the other hand, he is dismayed by the state of
research at universities. In his opinion, “the function of universities is to do
pure research, not to do applied research. Pure research will not be carried out
at any other location.” Schleyer sums up his position this way—“Pure research
is like putting money in the bank. Applied research is taking the money out.”
According to this motto, Schleyer’s account is very much in the black.
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