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7 INTERVIEW: PROFESSOR WESTON THATCHER BORDEN

7 INTERVIEW: PROFESSOR WESTON THATCHER BORDEN

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INTERVIEW: PROFESSOR WESTON THATCHER BORDEN



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papers and to one of Borden’s major contributions to theoretical chemistry:

the Borden–Davidson rules for predicting the ground states of diradicals.

Borden considers the research done with Davidson to be the best of his many

collaborations because he learned so much from Davidson. In particular,

Borden acquired a much more mathematical way of thinking about chemical

problems, while in turn he believes that Davidson acquired a more intuitive

chemical sense.

The success of the Borden–Davidson rules directly led to enormously fruitful collaboration collaborations with Professors Jerry Berson, Carl Lineberger,

and Matt Platz. About the last of these collaborations, Borden says jokingly,

“It’s all Matt’s fault!” Platz had been concerned about size of the singlet–triplet

gap in phenylnitrene. Impressed with Borden’s success in computing this energy

difference in a number of hydrocarbons, Platz asked Borden to tackle the phenylnitrene problem. Borden admits with some embarrassment that his own lack of

familiarity with nitrene chemistry resulted in Platz’s having to continue to ask

for a number of years. More out of a desire to finally placate Platz than anything

else, Borden agreed to compute the singlet–triplet gap of phenylnitrene. This

eventually led to a realization of the unusual wavefunction of singlet phenylnitrene, as discussed in Section 5.2. Borden says, “Even after this initial discovery,

Platz was not satisfied and continued to drive our collaboration forward. Every

computational result would trigger further questions from Matt.” Eventually,

Borden’s calculations led to a novel two-step mechanism that helped to rationalize the chemistry of phenylnitrene. Platz has told Borden that this mechanism

would not have been found without the help of the calculations.

Although Platz was certainly the driving force behind their collaboration,

occasionally Borden did suggest a new direction. For example, the Borden

group’s calculations of the effects of cyano substituents on the phenylnitrene

rearrangement were completed prior to Platz’ experimental tests of the

computational predictions. “It is more fun to make the prediction before

the experiment is done,” said Borden. This attitude results from the familiar

complaint of experimentalists about computational chemists: “All you ever do

is predict things that are already known!”

Borden’s interest in the Cope rearrangement dates back to 1973 when, as

an Assistant Professor at Harvard, Professor William von Eggers Doering

approached Borden with the question about substituents on this rearrangement.

Borden utilized an orbital correlation diagram to support an argument for a

continuum transition state model. This conclusion was in direct opposition to

Professor Michael Dewar contention of two distinct mechanisms. To this day,

Borden can still vividly recollect the reviews to his submitted JACS paper. The

first, signed by Professor Howard Zimmerman, was supportive but warned that

Dewar will not approve. The second anonymous review read: The publication of

this manuscript will only serve to provide the gladiatorial spectacle of Borden

being torn to shreds in print. The article never was published. Nonetheless, as

discussed in Section 3.2, the calculations of Houk and Borden and Doering’s



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experiments firmly established the variable (chameleonic) nature of the Cope

transition state.

Thus began Borden’s long involvement with the Cope rearrangement. It

included a sabbatical trip to Japan on a Guggenheim Fellowship to collaborate

with Professor Morokuma, which, as a side effect, fomented Borden’s

passionate interest in Japanese culture.

He has worked on the Cope rearrangement with many students and in

particular two long-time associates, Dr. David Feller and Dr. Dave Hrovat.

Their CASSCF(6,6)/3-21G computations located a single synchronous

transition state. Borden recalls the dismay and almost sense of betrayal he felt

when Davidson’s larger MCSCF results confirmed Dewar’s finding of two

distinct transition states. However, the poor activation energy predicted by this

method inspired Borden to realize the need for both nondynamic and dynamic

correlation in order to properly describe the Cope transition state. Ultimately,

Borden’s CASPT2 computations settled the matter in favor of the concerted

mechanism. The CASPT2 calculations gave an enthalpy of activation that was

in excellent agreement with the experimental value that had been measured by

Doering. Borden says, “When we saw the CASPT2 activation enthalpy, we

knew immediately that CASPT2 had gotten the Cope rearrangement right.”

Borden considers his group’s work on the importance of the inclusion of

dynamic electron correlation in many different types of problems to be his most

significant contribution to computational methodology. In addition, his work on

the Cope rearrangement and artifactual symmetry breaking taught him to question the applicability of routine methods to what first appears to be a routine

problem but one that later turns out to require high level calculations.

Borden said, “For example, first we thought our CASSCF(2,2) calculations on the Cope rearrangement should have worked. What we failed to

realize initially is that CASSCF(2,2) is prejudiced, because it can describe

cyclohexane-1,4-diyl, but not the other diradical extreme of two allyl radicals

for the Cope transition structure. Then, we thought that CASSCF(6,6) would

give the correct answer, but didn’t realize the need to include dynamic

correlation.” The Cope rearrangement opened his eyes to the dangers afoot in

computational chemistry. “You could get a wrong answer really easily. It is an

interesting problem to challenge methodology.”

The last collaboration we discussed was the work on the response of the

Cope transition state to substituents. Doering independently contacted Professor

Ken Houk and Borden to get them interested in the problem. Both subsequently

did pursue the project, only to discover that they were both presenting similar

results at a Bartlet session at a Reaction Mechanisms Conference. Following the

conference, they began to truly collaborate, working together on calculations,

interpretations and writing.

A major influence that Houk had on Borden was insisting on the importance

of including lot of figures in the article. Borden claims Houk taught him that

“organic chemists look at the pictures; they don’t look at the tables.”



INTERVIEW: PROFESSOR WESTON THATCHER BORDEN



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I asked Borden, “After working on the Cope rearrangement for more than 30

years, what do you think still remains to be done?” He responded, “Though it is

always dangerous to reply to such a question about future research, I have told

Ken Houk that I have published my last paper on the Cope.”

Borden emphasizes the importance of experiment as part of his work.

The ratio of his experimental to computational emphasis has shifted over his

career, from mostly experiment when he began his career at Harvard to mostly

computation during his later years at Washington, to exclusively computation

at North Texas. Nevertheless, he still regards himself as an experimentalist,

“My group used to do experiments in the laboratory. Now we do them on the

computer.”

His students have always been encouraged (if not forced) to engage in both

experimental and computational research studies. He believes this approach “is

great training for a theoretician to know the kinds of questions that experimentalists want answered. And it’s wonderful for an experimentalist to be able to say,

‘Gee, maybe we should do a calculation to understand an experimental result’.

The synergy between experiments and calculations is extraordinary.”

Borden has seen the evolution of the discipline of computational chemistry

from being the object of friendly ridicule by experimentalists to being a full

partner with experimentation. He points to the methylene problem (Section 5.1)

and discrepancies in the heats of formation of the isomeric benzynes (Section

5.5) as examples of where computations identified the existence of errors in the

interpretation of well-regarded experiments. Nonetheless, he notes that some

experimentalists regard computations as second-rate science, partly because calculations are relatively easy to do, partly because (bad) calculations can yield

bad results, and partly because many computational chemists still seem only to

offer explanations, without also providing predictions.

It is Borden’s belief that computational methodology is now good enough to

address most problems in organic chemistry. However, he remains somewhat

wary about density functional methods, characterizing them as “AM1 for the

21st century.” Borden says, “Functionals that work well for most problems, can

unexpectedly fail. The same is, of course, also true for ab initio methods. However, ab initio calculations can systematically be improved, by expanding the

basis set and/or including more electron correlation; whereas, there is no systematic way to improve upon a DFT result. Although my group certainly uses

DFT methods for many problems, whenever possible, we try to validate the

results by comparison with those obtained from ab initio calculations.”

Borden points to Professor Roald Hoffmann as a great influence on the way

he does science. “Roald’s early papers, which relied exclusively on Extended

Hückel theory, taught me that critical interpretation of calculations is even more

important than their numerical accuracy.” The key to Borden’s success in computational chemistry can be summarized by his statement: “You need to ask the

right question and use the right tools to get the right answer. However, the goal

of calculations is not just numbers; it’s the interpretation that really matters.”



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Borden’s career is marked by the many research problems that he has tackled

with just this approach.



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