Technology An "investigative scan" feature is incorporated in ICAMS. Such a scan is run automatically whenever an alarm condition occurs and the mass spectrometer data are stored on a floppy disk for separate interpretation. An investigative scan also can be initiated on demand or at preset times. This feature is especially useful, the IBM researchers say, for investigating complaints about odors and for indicating the presence of unknown chemicals. Extensive calibration work has been run on ICAMS in an effort to ensure system accuracy and reliability. For a calibration, a known concentration of each of the chemicals (in the vapor state) to be monitored is diluted with air and the dilution drawn through a sampling port to the analyzer. There, a comparison is made between the known amount delivered and the amount reported
by the system. About an hour is needed to run a set of single calibration points for all of the chemicals being monitored. A diagnostics routine monitors a signal from each of the ICAMS subsystems and, during normal operation, runs automatically after each cycle of the 50 sampling ports. If there is a malfunction in either subsystem, the fault is highlighted on the video display terminal in a reverse format. More features are in store for the future. Marcote notes that a great deal of flexibility has been added to the original ICAMS design. Besides an increase to a capacity of 25 chemicals, the system will include the capability for nonsequential monitoring and adjustable alarm levels at each port. There is also the possibility of an improved detection system. •
Binuclear platinum complex can be photocatalyst
cohols examined so far, 2-propanol works best. But cyclohexanol, ethanol, and methanol also evolve hydrogen gas in the presence of the platinum complex and light. The alcohols react directly with the excited platinum complex, and in the case of 2-propanol, the complex has a turnover number exceeding 400 at a rate of 1.75 M _ 1 over a threehour time period. Structurally, the complex resembles other transition-metal-bridged photocatalysts, such as binuclear rhodium-bridged photocatalysts and phosphine-bridged palladium dimers. However, it doesn't seem to catalyze hydrogen formation in the same way these other complexes do. In the case of the platinum complex, the photocatalytic reaction appears to proceed through a bimolecular reaction. Roundhill proposes that the first step is the abstraction of the secondary methine hydrogen from the alcohol by the complex, followed by propagation steps leading to the formation of acetone and the dihydrogen-substituted platinum complex. Hydrogen is removed from the complex by reductive elimination, completing the catalytic cycle. O
The chemicals to be monitored can be programed for each of the 50 sampling points. About eight seconds are required for analysis of the chemicals at each location, making the cycle time to return to any one sample point about eight minutes. The instrument's accuracy for the eight-minute cycle time is ±10% of the alarm set point. The system activates two different kinds of alarms if the tolerance level for any chemical at any location is exceeded. The lower alarm points are the levels, as recommended by the American Conference of Governmental & Industrial Hygienists, that should not be exceeded in the workplace for any 15-minute period. The upper alarm levels are set at 10% of the low explosive limits for flammable chemicals and at other reasonable indicator levels for nonflammable chemicals.
SCIENCE
A binuclear platinum(II) complex alkyl halides such as methyl iodide that has attracted the attention of to give axially disubstituted platchemists for several years because inum(III)-platinum(III) complexes, of its interesting spectroscopic prop- thermal addition was not possible erties also may be of interest as a using aryl halides. However, aryl photocatalyst. D. Max Roundhill, bromides and iodides add to the professor of chemistry at Tulane excited state complex. This reaction University, has shown that the com- can be followed with photoinduced plex, tetrakis(ju-pyrophosphito)di- substitution of the halide, so that platinum(II), reacts in its excited the dibromide, for example, can be state with 2-propanol to form ace- made either by direct reaction of tone and evolve hydrogen. It also can the platinum complex with an aryl participate in a photoinduced sub- bromide or by reaction of the comstitution reaction with aryl halides plex with an aryl iodide followed [/. Am. Chem. Soc, 107,4354 (1985)]. by photosubstitution of bromide ion The complex has attracted the at- for iodide ion. tention of a number of organome"We believe this reaction is the tallic chemists for several years be- first example of aryl halide addicause it has a relatively long-lived tion of platinum(II)," Roundhill says. phosphorescence (nearly 10 micro- "Usually the addition of aryl haseconds) resulting from the relaxa- lides to platinum compounds oction of a triplet excited state. The curs only with the electron-rich alrelatively long lifetime of this excit- kylphosphine complexes of platied triplet state suggested to Round- num(0)," he explains. "By using the hill that it might be available to excited state as reductant, we have participate in chemical reactions. modified the reaction chemistry to Roundhill began by investigating that expected of the metal center in the complex's ability to react with a lower formal oxidation state." The complex also can serve as a organic halides. He found that although the ground-state complex photocatalyst for the evolution of reacts thermally with halogens and hydrogen from alcohols. Of the al32
August 12, 1985 C&EN
