Up for auction "Astrochemistry" William Klemperer Hand Signed First Day Cover Dated 1963. 

ES-7236E

William

A. Klemperer (October 6, 1927

– November 5, 2017) was an American chemist who was one of the most

influential chemical physicists and molecular spectroscopists in the second half of the 20th

century. Klemperer is most widely known for introducing molecular beam methods into chemical physics research,

greatly increasing the understanding of nonbonding interactions between

atoms and molecules through development of the microwave spectroscopy of van der Waals molecules formed

in supersonic expansions, pioneering astrochemistry, including developing the first gas phase

chemical models of cold molecular

clouds that predicted an abundance of the molecular HCO+ ion

that was later confirmed by radio astronomy. Bill Klemperer was born in New York City in

1927 and was raised there and in New Rochelle. His parents were both

Physicians. He graduated from New Rochelle High school in 1944 and then

enlisted in the U.S. Navy Air Corps, where

he trained as a tail gunner. He obtained

an A.B. from Harvard University in

1950, majoring in Chemistry, and then headed to the University of California,

Berkeley, where in early 1954 he obtained a Ph.D. in Physical

Chemistry under the direction of George C. Pimentel. After

one semester as instructor at Berkeley, Bill returned to Harvard in July 1954. Klemperer's

initial appointment was an instructor of analytical chemistry, but

he quickly rose through the ranks and was appointed full professor in 1965. He

has remained associated with Harvard Chemistry throughout his long career. He

spent 1968-69 on sabbatical with the Astronomers at Cambridge University and

1979-81 as Assistant Director for Mathematical and Physical Sciences at the

U.S. National Science

Foundation. He was a visiting scientist at Bell Laboratories during a time when it was the premier

industrial laboratory. Klemperer became an emeritus professor in 2002 but

remained active in both research and teaching. Klemperer's early work

concentrated on the infrared spectroscopy of small molecules that are only

stable in the gas phase at high temperatures. Among these are the alkali

halides, for many of which he obtained the first vibrational spectra. The work

provided basic structural data for many oxides and fluorides, and gave

remarkable insight into the details of the bonding. It also led Klemperer to

recognize the immense potential of molecular beams in spectroscopy, and in

particular the use of the electric resonance technique to address fundamental

problems in structural chemistry. An important result was his benchmark

measurement of the electric dipole moment of LiH, at a date when this was the

largest molecule for which quantum chemical calculations had any hope of getting

useful results in a sensible length of time. Klemperer has always been

enthusiastic about molecular beams; he writes: "Molecular beams are fun

for a chemist. They give one a sense of power." An

example of this is the use that Klemperer and his students made of electric

deflection methods to determine the polarities of a number of high temperature

species; the results were unexpected, and to everyone's surprise it turned out

that half the alkaline earth dihalides are polar, meaning they cannot be symmetric linear

molecules, contrary to the simple and widely taught models of ionic bonding.

Klemperer also provided precise dipole moments of excited electronic states

both by using the Stark effect in

electronic spectra and by using electric resonance spectroscopy of

metastable states of molecules. Klemperer introduced the technique

of supersonic cooling as a spectroscopic tool, which has dramatically increased the intensity

of molecular beams and also greatly simplified the spectra. This innovation has

been second only to the invention of the laser in its impact on high-resolution

spectroscopy.  Klemperer helped to found

the field of interstellar chemistry. In interstellar space, densities and

temperatures are extremely low, and all chemical reactions must be exothermic,

with no activation barriers. The chemistry is driven by ion-molecule reactions,

and Klemperer's modeling of those that occur in molecular clouds has led

to a remarkably detailed understanding of their rich highly non-equilibrium

chemistry. Klemperer assigned HCO+ as the carrier of the

mysterious but universal "X-ogen" radio-astronomical line at

89.6 GHz which had been reported by D. Buhl and L.E. Snyder. Klemperer

arrived at this prediction by taking the data seriously. The radio telescope

data showed an isolated transition with no hyperfine splitting; thus there were

no nuclei in the carrier of the signal with spin of one or greater nor was it a

free radical with a magnetic moment. HCN is an extremely stable molecule and

thus its isoelectronic analog, HCO+, whose structure and spectra

could be well predicted by analogy, would also be stable, linear, and have a

strong but sparse spectrum. Further, the chemical models he was developing

predicted that HCO+ would be one of the most abundant molecular

species. Laboratory spectra of HCO+ (taken later by Claude

Woods et al.,) proved him right and thereby demonstrated that

Herbst and Klemperer's models provided a predictive framework for our

understanding of interstellar chemistry. The greatest impact of Klemperer's

work has been in the study of intermolecular forces, a

field of fundamental importance for all of molecular- and nano-science. Before

Klemperer introduced spectroscopy with supersonic beams, the spectra of weakly

bound species were almost unknown, having been restricted to dimers of a few

very light systems. Scattering measurements provided precise intermolecular

potentials for atom–atom systems, but provided at best only limited information

on the anisotropy of atom–molecule potentials. He foresaw that he could

synthesize dimers of almost any pair of molecules he could dilute in his beam

and study their minimum energy structure in exquisite detail by rotational

spectroscopy. This was later extended to other spectral regions by Klemperer and

many others, and has qualitatively changed the questions that could be asked.

Nowadays it is routine for microwave and infrared spectroscopists to follow his

"two step synthesis" to obtain the spectrum of a weakly bound

complex: "Buy the components and expand." Klemperer quite literally

changed the study of the intermolecular forces between molecules from a

qualitative to a quantitative science. The dimer of hydrogen fluoride was the first hydrogen bonded complex

to be studied by these new techniques, and it was a puzzle. Instead of the simple

rigid-rotor spectrum, which would have produced a 1 – 0 transition at

12 GHz, the lowest frequency transition was observed at 19 GHz.

Arguing by analogy to the well known tunneling-inversion spectrum of ammonia, Klemperer

recognized that the key to understanding the spectrum was to recognize that HF

– HF was undergoing quantum tunnelling to

FH – FH, interchanging the roles of proton donor and acceptor. Each rotational

level was split into two tunneling states, with an energy separation equal to

the tunneling rate divided by Planck's constant. The

observed microwave transitions all involved a simultaneous change in rotational

and tunneling energy. The tunneling frequency is extremely sensitive to the

height and shape of the inter-conversion barrier, and thus samples the

potential in the classically forbidden regions. Resolved tunneling splittings

proved to be common in the spectra of weakly bound molecular dimers.