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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 recalls.

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 . “[10]-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 [10]-annulene. Never being a

great fan of DFT or perturbation methods, he does find a sense of satisfaction

that these fail to properly treat [10]-annulene and that the coupled cluster method

arrives at the proper answer. One technique utilized in tackling the [10]-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!



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.”


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



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



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



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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