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 insight into the details of the bonding. It also led Klemperer to recognize the potential of
molecular beams in spectroscopy, and in particular the use of the
electric resonance technique to address fundamental problems in
structural chemistry. Klemperer introduced the technique of supersonic cooling as a spectroscopic tool, which has increased the intensity of molecular beams and also simplified the spectra. 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 to 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 the
Planck 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 ==