Molecules Most new laboratory in- struments are developed to satisfy a demand, usually at the same time the demand occurs. On rare oc- and s Microwaves casions, an instrument whose unique capabilities promise to advance the state-of-the-art in a particular branch of science makes an appearance so far ahead of a clear demand for it that its immediate commercial value can be questioned. Precisely such an instrument is the Hewlett-Packard Model 8400B Microwave Spectrometer. It fits all descriptions of a technological and scientific breakthrough, although it is much closer to homein the area of current and useful application than the preceding discourse might indicate. In simplest operational terms, the Microwave Spectrometer looks into the molecular structure of a compound by measuring its absorption frequencies during an X, R, or K band sweep. It makes molecular determinations by using the microwave to measure changes in rotational energy levels in a molecule. Because differ- ences exist in the geometry of individual molecular species, the microwave spectrum for an individual molecule is characteristic for — that species. A logical objection, if you’re up on your species, is that most compounds would present a tremendous number of absorption peaks. True. But with the 8400B it is relatively easy to differentiate spectra of two different species because of the inherent high resolution of microwave spectroscopy, in conjunction with an accurate means of measuring microwave frequencies. In terms of its application, the Microwave Spectrometer provides a means of measuring the total amount of information available from gas-phase microwave spectroscopy absorption lines—fre- quency, intensity, line width or relaxation rates. This, in turn, per- mits researchers to delve into such areas as molecule identification, molecular concentration, bond distance, bond angle, molecular vibrational levels, barriers hindering internal rotation, equilibrium constants, molecular collision rates, and reaction kinetics. Precisely where the Microwave Spectrometerfits into the pattern of modern chemistry is still being studied, but early indications show it may well establish patterns of its own. Based on a recent experiment it has already carved one niche—and an important field of study for the microwave spectroscopist—in the detection and quantitative determination of components in a complex, gaseous, molecular mixture differing only in isotopic composition. (The ex- periment was to determinethe relative concentration of C''HsC'CH to C4H,CC**H in C’* enriched methyl acetylene.) Such experi- ments are published as regularly as they occur in H-P’s newest publication, Molecules and Microwaves, a copy of which awaits your request to Hewlett-Packard, 1501 Page Mill Road, Palo Alto, ’ California 94304. Europe: 54 Route des Acacias, Geneva. HEWLETTjp; PACKARD ANALYTICAL INSTRUMENTS 071A 1259