Science
IR instrument combination gives reflectance spectrum of spot area of acrylic polymer % Transmittance 100 I 90 80 70 60 50 40
Spectrum of a 4Q-;ftm spot of pojymethyl methacrylate on silicon wafer, coliected with 4 crrr 1 resolution using an IBM in struments IR microscope on an IBM IB/ 38 FHft spectrophotometer
30
20 10 Ι ι ι 4U00
3500
3000
J_
J
2500
2000
I t
L—L 1500
1000
Wave number, cm"1
Phοtomicrograph (magnification 300X, 1 cm = 33μπ\) of polymethyl methacrylate on silicon wafer corresponds to spectrum above t u n g s t e n needle. IR microscopy showed the substance to be sodium carboxymethylcellulose. This find ing helped to identify the coating as a cosmetic that the suspect used. To identify specks of spray paint in a hit-and-run case, Paleneik used a pyrolysis technique that McCrone Associates developed. Using this technique, the IR spectrum of a paint speck pyrolyzed in a melting-point capillary tube, extracted, and dried 16
December 9, 1985 C&EN
showed the presence of bisphenol A. This helped in tracing the paint to the car in the case. Detective Kubic described sever al cases he had investigated with police officers. In one case, red fi bers were found near the body of a man shot to death in his driveway. IR microscopy showed that the fi bers came from a rug laid in the back seat of a car parked nearby. On taking up the rug and a ply wood floor from the back seat, offi cers found more red fibers beneath. They proved to come not from the rug but from a red ski cap that a suspect wore. These findings linked the suspect to the car and the car to the scene of the slaying. In a hit-and-run case, Kubic and his partners had to trace the make and model of the car from a speck of paint. Light microscopy showed that the car had been repainted. Us ing a tungsten needle, the investi gators pried a layer of original black paint from between the white top coat and a brown primer. IR mi croscopy of the black paint identi fied it and pointed to the kind of car involved. In another case, a child riding a
bicycle was hit by a car, and the Mineola criminologists looked for evidence to implicate a particular car. IR microscopy showed that a smear of blue paint on one of the car's rubber bumper guards had come from the bicycle, and that a bit of resin on the front of the car hood had come from a cap on one of the bicycle handlebar grips. The IR microscope attachments in troduced at the symposium by Analect and IBM have binocular, stereoscopic eyepieces to view sam ples in visible light. Moving an out side lever selects for viewing under visible light or for recording of transmission or reflectance IR spec tra. A user may move metal plates with pinholes of different diame ters over samples to mask areas whose IR spectra are to be deter mined. For such samples as fibers, rectangular slits are adjustable for widths and lengths. The Analect attachment, not in cluding an FTIR spectrometer, is priced at $23,000 and the IBM at $35,000. Also, for $17,000, Analect offers an attachment for transmis sion spectroscopy only. D
Limits of phosphorus hypervalency explored A quirky molecule first made about 10 years ago at Iowa State Universi ty is helping chemists there ex plore the limits of hypervalency in nonmetallic elements such as phosphorus. The molecule is an amine phosphate in which three of the phosphate oxygens each are con nected to the same nitrogen via a two-carbon bridge. Nomenclature fans would call it 2,8,9-trioxa-5-azal-phosphabicyclo[3.3.3]undecane 1oxide. When graduate student Dean S. Milbrath prepared it as part of his doctoral thesis research for chemis try professor John G. Verkade, nei ther one realized what he had stum bled onto. But molecular orbital cal culations done on this molecule in collaboration with professor Henk M. Buck and coworkers at Eindhoven University of Technology in the Netherlands revealed an intriguing possibility: The nitrogen atom might
be able to donate some electron density to the four-coordinate phosphorus atom, forming an axial or transannular bond; in the process, the phosphorus would become hypervalent. With a little prodding, says Verkade, that's what the molecule seems to do. For example, when former graduate student Leslie E. Carpenter II treated the amine phosphate with boron trifluoride, he discovered that the electron-deficient boron binds to the phosphoryl oxygen—not, as one might expect, to the nitrogen. In addition, a dative bond from the nitrogen to the phosphorus forms, creating a neutral, tricyclic paddle-wheel structure called a phosphatrane [/. Am. Chem. Soc, 107, 7084 (1985)]. In the process, the geometry around the phosphorus shifts from four-coordinate tetrahedral to five-coordinate trigonal bipyramidal. The phosphorus, by strict definition, has become hypervalent because it is surrounded by 10 electrons, rather than its usual octet, and it coordinates to five groups, rather than the usual three or four. Carpenter and Verkade have demonstrated that other Lewis acids, by virtue of their electron-withdrawing ability, also can trigger the collapse of the amine phosphate to a phosphatrane. This happens, for instance, when a triethylsilyl group is attached to the phosphoryl oxygen. Even more unexpected, Carpenter found that by using an excess of the silylating reagent, he could hitch a second triethylsilyl moiety to the oxygen, forming a dication (positive charges on oxygen and nitrogen). Working with the Eindhoven group, the Iowa State researchers also generated cationic phosphatranes simply by protonating or diprotonating the phosphoryl oxygen. More surprises were in store when Carpenter treated the amine phosphate with anhydrous phosphoric acid. A white solid immediately precipitated out of solution. A 31 P nuclear magnetic resonance spectrum of the solid product, as well as other evidence, suggests a novel "doublecage" adduct in which an H 2 P0 4 ~ moiety is hydrogen-bonded to a hydroxy phosphatrane cation.
duct's extreme insolubility, Verkade notes. Although the trigonal bipyramidal structure of other phosphatranes already has been established by x-ray methods, these new derivatives created by Carpenter cry out for crystal structures. But the prospects for some of them don't look very promising. The trifluoroboratooxy derivative, which hasn't been isolated, is "probably a lost cause," in Verkade's words, "because the BF3 group dissociates too easily." The phosphoric acid adduct appears to be too insoluble for crystallization. The silylated phosphatranes, on the other hand, offer some hope because they have been isolated as an apparently microcrystalline mixture. Verkade thinks other silyl derivatives may be easier to crystallize in pure form. One of his new coworkers is now tackling the challenge, with continuing support from the National Science Foundation.
This solid-state NMR spectrum is the only concrete evidence Verkade has so far to support the proposed structure of the adduct. That's because the adduct is so insoluble that Carpenter, now a postdoctoral research associate at Rensselaer Polytechnic Institute, never was able to crystallize it and get an x-ray structure. The one solvent that does dissolve the adduct—dimethylsulfoxide—also dissociates it into the reactants used to make it, Verkade says. The Iowa State professor readily admits that the pleasing symmetry of the proposed structure is speculation on his part, but, he adds, "it's not bad speculation because it explains why the stuff is so insoluble." In the proposed structure, the bond dipole moments all point in the same direction, and, w h e n summed, give a molecular dipole moment of about 10.7 debye units. Such an extraordinarily large dipole moment could explain the ad-
React to this... HOOCCH2 - CH - NH - (CH2COOH)2 NaS-C-N(CH2COONa)2
COOH
SOLUBLE DITHIOCARBAMATES
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HOOCCH2-N-CH2COOH IMINODIACETIC ACID (IDA) CH,-C
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COOCH2-CH CH 2 -N (CH2COOH)2 OH CHELATING POLYMERS
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17
THE CHOICE IS EASY
Science Bicyclic molecule collapses to hypervalent tricyclics
Amine phosphate
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