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7 INTERVIEW: PROFESSOR HENRY “FRITZ” SCHAEFER
DIRADICALS AND CARBENES
wrong. Schaefer then “went to talk to wiser heads, like Yuan Lee and Brad
Moore. Their response was ‘You made a splash on triplet methylene, so what
if you got the singlet wrong’.” For 6 months, Schaefer was convinced that his
calculation must be wrong, but he didn’t know why. So he returned to the problem with a vengeance, along with many others. “Methylene became a cause,”
Schaefer’s improved calculations continued to indicate a gap of only about
10 kcal mol−1 , and Goddard’s calculation indicated the possible presence of hot
bands in the PES spectrum. Schaefer recalled a conversation with Lineberger
at that time (about 1977–1978): “These preprints all show the computational
results indicating a much smaller singlet-triplet gap than your experiments.
Aren’t you going to be embarrassed about this? Lineberger said, I’m not worried
about that at all. In fact, I’m feeling pretty good about it. Before, yours was
the only career I destroyed. Now, I’m taking down all of quantum chemistry!”
Lineberger did eventually retract, but only after achieving a much better experimental design. Schaefer continues to admire Lineberger’s resolve and faith in
his own research, and finished with the comment “All of us are still friends.”
Schaefer holds to his assertion made in his Science article that methylene
remains the paradigm for computational chemistry. He notes that one could
point to Kolos’ paper on H2 or Davidson’s doctoral thesis that demonstrated
a double minimum in an excited state of H2 . But he maintains the prominence
of methylene: “Ours was the first non-linear triatomic. Organic chemists were
interested in methylene; lots of experiments were available.” The computational
studies on methylene corrected two serious interpretations of experimental data,
and clearly demonstrated that computational chemistry can hold its own with
experiment. Because of the impact of this work, Schaefer considers his methylene studies to be his most important contribution to science. “We were at the
right place, at the right time. There’s a brashness about it that set it apart.”
Schaefer has a long interest in aromaticity, including studies of paracylophanes and N6 . “-annulene just seemed obvious. I read Masamune’s paper
and said ‘We could probably help these guys out’.” He was surprised, however,
in the difficulties that occurred in dealing with -annulene. Never being a
great fan of DFT or perturbation methods, he does find a sense of satisfaction
that these fail to properly treat -annulene and that the coupled cluster method
arrives at the proper answer. One technique utilized in tackling the -annulene
story has piqued Schaefer’s interest. He was very much taken by Monte Carlo
and other broad search techniques, whereby “you put in the atoms and it goes
and finds everything.” He calls this ‘mindless chemistry’ and has used this term
as the title of a collaborative paper with Professor Schleyer describing stochastic searches used to locate all local energy structures of a series of pentatomic
molecules. This paper has resonated within the chemistry community. Though
rejected by the Journal of the American Chemical Society, it was published in
2006 by the Journal of Physical Chemistry A and was that journal’s most frequently accessed paper for the first half of 2006!
INTERVIEW: PROFESSOR PETER R. SCHREINER
Collaborations have played a major role in his scientific pursuits, but
he points to the very asynchronous nature of such collaborations with
experimentalists. Early on, experimentalists would have nothing to do with
him or his computations—“They were all skeptics.” Nowadays, Schaefer is
frequently contacted by experimentalists looking for computational assistance
or confirmations. However, experimentalists are much less receptive to his
calls for suggested experiments based upon his computations. He believes
that this reluctance is not due to mistrust of the computations, but rather to
the fundamental differences in the nature of doing computations and doing
experiments. “Experimentalists have several tricks in their hats and that’s what
they know how to do.” If the suggested chemistry is not within that scope,
they hesitate to take on the suggestion. “Computational chemistry has much
greater coverage. We don’t have universal coverage but it’s heading that way.”
He qualifies this statement, “A spectroscopist can measure something to 10−4
wavenumbers. They may not be exactly sure of what they are measuring, but it
is what it is. And we’re not going to compute anything to 10-4 wavenumbers.”
Schaefer has few regrets when it comes to missed scientific opportunities.
The one in particular that he notes is C60 . He met with Professor Rick Smalley
days after the discovery of C60 , but didn’t get excited about it.
When asked to speculate as to what the computational chemistry landscape
might look like a decade from now, Schaefer became a bit pessimistic. He notes
that computer performance has stagnated recently, calling it “depressing that
PCs are about as fast now as they were two years ago”. He feels that parallel
processing will be the key to future progress, a prospect he finds discouraging
when contemplating the coding efforts that will be required to maximize parallel
Nonetheless, he continues to actively pursue new projects, welcoming new
students into the group and pushing forward. “Why do we study chemistry?” he
asks. “We want to know answers to qualitative questions. That is what distinguishes us from physics.”
5.8 INTERVIEW: PROFESSOR PETER R. SCHREINER
Interviewed September 6, 2012
Peter Schreiner is the Professor of Organic Chemistry at the JustusLiebig-University Giessen (Germany). He was introduced to computational
chemistry in graduate school at Erlangen working with Paul Schleyer. Wanting
to understand more about what the quantum chemical computer programs were
actually doing, he took a fellowship at the University of Georgia, where in
addition to pursuing an experimental project with Dick Hill, he worked with
Fritz Schaefer. Computational chemistry “just sucked me in,” he said. “The
DIRADICALS AND CARBENES
more you know, the better you can answer organic chemistry questions. It turns
out to be a great tool: you can ask questions about why this bond is a bit long
or this angle is odd and it doesn’t tell you the answer at first glance but as you
pour over it, there is more to see. Computational chemistry is such a creative
tool; it can trigger your thinking.”
Combining experiments and computations, Schreiner’s research has always
exhibited a symbiotic character. His research in carbenes exemplifies this interplay. Despite a great deal of previous work on carbenes, Schreiner discovered
that the chalcogene carbenes were not well known. Knowing that the simplest
example, hydroxymethylene, had not been made or characterized, he was sure
that its preparation would be difficult. “How does nature make carbenes?” he
asked. “The pyruvate cycle—CO2 extrusion from alpha-ketocarboxylic acids.
She figured it all out!” So, the group tried oxalic acid and “Bam! Extrusion gave
us dihydroxycarbene smoothly! The surprise was it was so simple.” But it was
not all to be so simple.
The next case they examined was the pyrolysis of glyoxylic acid, and hydroxymethylene was observed, but it disappeared overnight. This offered a clue as to
why it might not have been previously prepared. Schreiner thought that perhaps
someone might have done this previously but did not spectroscopically identify
it right away. “Let it sit too long and it’s gone!” he noted. After many repeated
tries to isolate hydroxymethylene, Schreiner had to face this problem. He discounted a matrix or a stereoelectronic effect because the molecule is too small.
He was at a loss for explaining the rate, but “then as a good organic chemist
and the rate doesn’t fit with what you’d expect, you say ‘something’s fishy and
it’s a tunneling effect.’” His students measured the rate of disappearance as 2 h,
which was way too fast for any normal reaction at 11 K. When they increased
the temperature to 20 K and the rate remained 2 h, “that sort of confirmed the
hunch,” he said. Worried that no one would accept tunneling through a barrier as
large as 30 kcal mol−1 , he contacted his colleague Wesley Allen, whom he had
met at the University of Georgia. Schreiner asked Allen to compute the proper
IRC, and from it the barrier height and width, and ultimately a tunneling rate.
Allen responded, “apart from the fact that you’re crazy, I’ll do it.” Of particular
importance is that Schreiner did not tell Allen the value of the experimental rate.
In about 3 weeks, Allen called Schreiner and told him that the computed rate
was 2 h! (Actually, Allen computed a rate of 122 min, “a bit more accuracy than
I needed,” said a sheepish Schreiner.) Both Schreiner and Allen were in shock:
this was a clear case of tunneling.
Next was the study of methylhydroxycarbene, which could potentially tunnel from either the C–H or O–H direction. The kinetic product is vinyl alcohol
while the thermodynamic product is acetaldehyde. Which direction would tunneling choose? The previous examples all had the tunneling selection through
the lower barrier. “We got lucky, and it tunnels through the higher barrier,” said
Schreiner. “I always thought of tunneling as a correction to the rate but it’s not.
It is the rate.” Thus, was born the expression tunneling control. “Now we have
INTERVIEW: PROFESSOR PETER R. SCHREINER
several cases. It’s a general phenomenon, not unusual at all! Practicing organic
chemists should care about this.”
The outcome of this endeavor was really quite a surprise to Schreiner. “Tunneling was not something that I thought about. In fact it was so far back in my
mind I was hoping that I would never have to unbury it,” said Schreiner. He
noted that the theory of tunneling is rigorous but “so non-classically difficult for
a paper-and-pencil organic chemist to think about it. Qualitative models are not
at hand. And this is our goal now—that’s why we did the Hammett plots, to
give a handle on the qualitative understanding of the electronic effects.”
There has been a recent shift in how new projects are approached within
the Schreiner group—computations are now leading experiments. “We simply
can’t put in the effort for weeks to months to characterize these species without
knowing beforehand if the molecule will be interesting,” he noted. The group
is expanding into other areas in the pursuit of tunneling effects. “Who would
have thought about a JACS paper on benzoic acid in 2010?” he exclaimed.
“We have looked at many acids now and haven’t found one that does not
undergo tunneling-assisted conformational change. How is it possible this was
not broadly known?” He noted that all discussions on the conformations of
amino acids in space are “pointless since tunneling will be fast even at low
Schreiner continues to collaborate with Wesley Allen, who brings the ability to carry out very high level computations, which are especially valuable in
structure identification via vibrational frequency matching—computations that
require anharmonic corrections that Allen is expert in performing. They hope
to develop in the near future an add-on to the Gaussian and CFOUR computer
packages that will allow nonspecialists to do simple tunneling computations. A
“friendly competitor” group, led by Magyarfalvi in Hungary, has spent time in
Giessen and has now scooped Schreiner on tunneling in glycine. Schreiner takes
this as a good development: “You have to have good competitors. It keeps you on
your toes.” His other collaborations are with surface and solid-state physicists,
because “the questions they ask are so different.”
Schreiner’s interest in dispersion effects led to collaboration with Stefan
Grimme. Schreiner always felt uncomfortable with the usual textbook explanations for some basic organic facts, like the fact that branched alkanes are more
stable than linear ones. The culprit is dispersion. “Dispersion is hard to grasp
because it is so dispersed,” he complained. “Typically we just wipe it under
the carpet. The most blatant examples are the hydrocarbon dimers with very
long C—C bonds that you can heat to 300 ∘ C without decomposing. What
keeps them together? It’s a balance of things: repulsion and attraction.”
Schreiner discussed this problem with Grimme, and they joined forces on
the hexaphenylethane riddle. Ultimately, they found that “dispersion is a
huge component of the total binding energy. It’s so large you can no longer
overlook it.” He believes that we have just begun to recognize the importance
of dispersion. “With modern density functionals, we can cleanly dissect out the
DIRADICALS AND CARBENES
role of dispersion and now we can look at larger molecules—and dispersion
plays a larger role in larger molecules.”
Schreiner points to the C–H activation problem as one that has so far eluded
his group’s solution. “Methane to methanol conversion, if we manage, that that
would be an enormous breakthrough—and by manage I mean large scale with
simple affordable methods—but it’s such a difficult nut to crack,” he notes with
disappointment. “I didn’t think we would solve it, but I thought we would learn
something about it. You keep running into a wall and your head is bloody, you
heal and run again—but the wall isn’t getting any thinner.”
In thinking about the future of computational chemistry, Schreiner, like Stefan Grimme, noted the conformation and multiple isomer problems. He explains
that “we scribble a mechanism and come up with a feeling for the reactive conformation, but science is not just a feeling. So we have to probe large conformational spaces for large molecules, and what is missing are filters that would rule
out the conformations that are nonsense. How do we input our chemical knowledge into the optimization process?” What Schreiner is hoping for is a way of
incorporating chemical intuition into the computational chemistry toolset. Synthetic chemists seem to have this intuition—“K.C. Nicolaou, he knows how
to cut a large molecule into pieces in a few minutes; it’s amazing. How does
he know this?” Schreiner wistfully mused. “That logic coupled with quantum
mechanics would be amazing.”
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