William Klemperer
Encyclopedia
William A. Klemperer is a prominent and accomplished American
chemist
who was one of the most influential Chemical Physicists
and Molecular Spectroscopists
in the second half of the 20'th century. Klemperer is most widely known for: (1)Introducing Molecular beam
methods into Chemical Physics Research. (2) 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. (3) Pioneering Astrochemistry
, including developing the first gas phase chemical models of cold molecular clouds that predicted an abundance of the molecular HCO+ molecular ion that was later confirmed by Radio Astronomy
.
. 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 is still active in both research and teaching.
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 that 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[E. Herbst and W. Klemperer (1973). "The Formation and Depletion of Molecules in Dense Interstellar Clouds", The Astrophysical Journal 185, 505.] 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 Klemperer 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.,[R.C. Woods, T.A. Dixon, R.J. Saykally
, and P.G. Szanto (1975). "Laboratory Microwave Spectrum of HCO+", Physical Review Letters 35, 1269.]) 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. Klemperer 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 Klemperer’s “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,[T.R. Dyke, B.J. Howard and W. Klemperer (1972). "Radio Frequency and Microwave Spectrum of the Hydrogen Fluoride Dimer: A Nonrigid Molecule", Journal of Chemical Physics 56, 2442.] 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.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
chemist
Chemist
A chemist is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties such as density and acidity. Chemists carefully describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms...
who was one of the most influential Chemical Physicists
Chemical physics
Chemical physics is a subdiscipline of chemistry and physics that investigates physicochemical phenomena using techniques from atomic and molecular physics and condensed matter physics; it is the branch of physics that studies chemical processes from the point of view of physics...
and Molecular Spectroscopists
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
in the second half of the 20'th century. Klemperer is most widely known for: (1)Introducing Molecular beam
Molecular beam
A molecular beam is produced by allowing a gas at higher pressure to expand through a small orifice into a chamber at lower pressure to form a beam of particles moving at approximately equal velocities, with very few collisions between the particles...
methods into Chemical Physics Research. (2) Greatly increasing the understanding of nonbonding interactions
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
between atoms and molecules through development of the microwave spectroscopy of Van der Waals molecules formed in supersonic expansions. (3) Pioneering Astrochemistry
Astrochemistry
Astrochemistry is the study of the abundance and reactions of chemical elements and molecules in the universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium...
, including developing the first gas phase chemical models of cold molecular clouds that predicted an abundance of the molecular HCO+ molecular ion that was later confirmed by Radio Astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
.
Biography
Bill Klemperer was born in New York City in 1927 and was raised there and in New Rochelle. His parents were both Physicians. Bill graduated from New Rochelle High school in 1944 and then enlisted in the U.S. Navy Air Corps, where he trained as a tail gunnerTail gunner
A tail gunner or rear gunner is a crewman on a military aircraft who functions as a gunner defending against enemy fighter attacks from the rear, or "tail", of the plane. The tail gunner operates a flexible machine gun emplacement on either the top or tail end of the aircraft with a generally...
. He obtained an A.B. from Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
in 1950, majoring in Chemistry, and then headed to the University of California, Berkeley
University of California, Berkeley
The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...
, where in early 1954 he obtained a Ph.D. in Physical Chemistry under the direction of George C. Pimentel
George C. Pimentel
George Claude Pimentel was the inventor of the chemical laser. He also developed the modern technique of matrix isolation in low-temperature chemistry. In theoretical chemistry, he proposed the three-centre four-electron bond which is now accepted as the best simple model of hypervalent...
. After one semester as Instructor at Berkeley, Bill returned to Harvard in July, 1954. Klemperer's initial appointment was an Instructor of Analytical Chemistry
Analytical chemistry
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...
, 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
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...
. 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 is still active in both research and teaching.
Science
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
Quantum chemistry
Quantum chemistry is a branch of chemistry whose primary focus is the application of quantum mechanics in physical models and experiments of chemical systems...
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 that 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
Stark effect
The Stark effect is the shifting and splitting of spectral lines of atoms and molecules due to presence of an external static electric field. The amount of splitting and or shifting is called the Stark splitting or Stark shift. In general one distinguishes first- and second-order Stark effects...
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[E. Herbst and W. Klemperer (1973). "The Formation and Depletion of Molecules in Dense Interstellar Clouds", The Astrophysical Journal 185, 505.] 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 Klemperer 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.,[R.C. Woods, T.A. Dixon, R.J. Saykally
Richard J. Saykally
Richard J. Saykally is an American chemist. He is currently a professor at the University of California, Berkeley. He has received numerous awards and accolades for advanced research on the molecular characteristics of water.-Career:...
, and P.G. Szanto (1975). "Laboratory Microwave Spectrum of HCO+", Physical Review Letters 35, 1269.]) 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. Klemperer 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 Klemperer’s “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
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
was the first hydrogen bonded complex to be studied by these new techniques,[T.R. Dyke, B.J. Howard and W. Klemperer (1972). "Radio Frequency and Microwave Spectrum of the Hydrogen Fluoride Dimer: A Nonrigid Molecule", Journal of Chemical Physics 56, 2442.] 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
Quantum tunnelling
Quantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the sun, and has important...
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.
Awards
Bill Klemperer has had many awards and honors, which include- Elected to the American Academy of Arts and SciencesAmerican Academy of Arts and SciencesThe American Academy of Arts and Sciences is an independent policy research center that conducts multidisciplinary studies of complex and emerging problems. The Academy’s elected members are leaders in the academic disciplines, the arts, business, and public affairs.James Bowdoin, John Adams, and...
, 1963 - Elected to the National Academy of Science, 1969
- John Price Wetherill Medal, awarded by the Franklin InstituteFranklin InstituteThe Franklin Institute is a museum in Philadelphia, Pennsylvania, and one of the oldest centers of science education and development in the United States, dating to 1824. The Institute also houses the Benjamin Franklin National Memorial.-History:On February 5, 1824, Samuel Vaughn Merrick and...
, 1978 - Irving Langmuir AwardIrving Langmuir AwardThe Irving Langmuir Award in Chemical Physics is awarded annually, in even years by the American Chemical Society and in odd years by the American Physical Society. The award is meant to recognize and encourage outstanding interdisciplinary research in chemistry and physics, in the spirit of Irving...
, awarded by the American Chemical SocietyAmerican Chemical SocietyThe American Chemical Society is a scientific society based in the United States that supports scientific inquiry in the field of chemistry. Founded in 1876 at New York University, the ACS currently has more than 161,000 members at all degree-levels and in all fields of chemistry, chemical...
, 1980 - The Distinguished Service Medal, awarded by the U.S. National Science Foundation, 1981
- The Earle K. Plyler Prize for Molecular SpectroscopyEarle K. Plyler Prize for Molecular SpectroscopyThe Earle K. Plyler Prize for Molecular Spectroscopy and Dynamics is a prize that has been awarded annually by the American Physical Society since 1977. The recipient is chosen for "notable contributions to the field of molecular spectroscopy and dynamics". The prize is named after Earle K...
, awarded by the American Physical SocietyAmerican Physical SocietyThe American Physical Society is the world's second largest organization of physicists, behind the Deutsche Physikalische Gesellschaft. The Society publishes more than a dozen scientific journals, including the world renowned Physical Review and Physical Review Letters, and organizes more than 20...
, 1983 - The Bomem-Michelson Award for the advancement of the field of vibrational spectroscopy. awarded by the Coblentz SocietyCoblentz SocietyThe Coblentz Society is an American organization promoting and fostering the understanding and variety of applications of all types of molecular or vibrational spectroscopy founded in 1954. It's the oldest such organization in the United States....
, 1990 - Inaugural George C. Pimentel Memorial Lecturer, Chemistry Department, UC Berkeley. 1991-2.
- The Remsen Award from the Maryland Section of the American Chemical Society, 1992
- The Peter Debye AwardPeter Debye AwardThe Peter Debye Award in Physical Chemistry is awarded annually by the American Chemical Society to encourage and reward outstanding research in Physical Chemistry. The award is granted without regard to age or nationality.-Past recipients:*2011 Louis E. Brus...
in Physical Chemistry, awarded by the American Chemical Society, 1994 - The Faraday Medal and Lectureship from the Royal Society of ChemistryRoyal Society of ChemistryThe Royal Society of Chemistry is a learned society in the United Kingdom with the goal of "advancing the chemical sciences." It was formed in 1980 from the merger of the Chemical Society, the Royal Institute of Chemistry, the Faraday Society and the Society for Analytical Chemistry with a new...
(England), 1995 - Honorary Doctor of Science from the University of ChicagoUniversity of ChicagoThe University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
, 1996 - Honorary Citizen of Toulouse, France, 2000
- E. Bright Wilson Award in SpectroscopyE. Bright Wilson Award in SpectroscopyThe E. Bright Wilson Award in Spectroscopy is awarded annually by the American Chemical Society to recognize outstanding accomplishments in fundamental or applied spectroscopy in chemistry. It was first awarded in 1997 and was named in honor of the American Physical Chemist and Spectroscopy...
from the American Chemical Society, 2001