RESEARCH
EPR Technique Shows Ground State Triplets EPR with rigid glass sampling detects stable triplet state in diphenylmethylene, nitrenes
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TRIPLETS. Diagram shows energy levels and transitions in triplet state molecules. Dr. C. A. Hutchison and Dr. B. W. Mangum of the University of Chicago were the first to use electron paramagnetic resonance (EPR) to observe the triplet state; they found the Am r= 1 transitions, using dilute single crystals. Dr. J. H. van der Waals and Dr. M. S. de Groot of the Royal Dutch Shell Laboratories, Amsterdam, were the first to use the rigid glass technique. They observed the |Am : = 2 transition when the phosphorescent species was randomly oriented in a rigid glass. Using a rigid glass technique, Bell Telephone Laboratories scientists have observed the Am = 1 transitions in addition to the |Am| = 2 transition in triplet state molecules. Thus their technique combines many of the features of the other two procedures
EARLY STEP. R. M. R. Cramer (left) and W. A. Yager place a liquid sample — a perfluorokerosene solution of an organic compound—in a dewar in early step of electron paramagnetic resonance (EPR) technique
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With the aid of a rigid glass sampling technique, Bell Telephone Laboratories scientists have used electron paramagnetic resonance (EPR) to detect the stable triplet state in diphenylmethylene and nitrenes. Bell workers have found EPR absorptions corresponding to the [Am | = 1 transitions as well as the previously observed EPR absorptions for the |Am| = 2 transition with randomly oriented triplets. In finding evidence for both |Am| = 1 and |Am| = 2 transitions, Bell's W. A. Yager, Dr. E. Wasserman, and R. M. R. Cramer have shown that use of the EPR method can be greatly extended in studying triplet molecules in biological and chemical systems [/. Chem. Phys., 37, 1148, (1962)]. In the EPR technique used by the Bell scientists, they dissolve a small amount of the organic species to be studied in a solvent. Then they freeze the solution to a glass in a cryogenic dewar at liquid nitrogen temperatures. The Bell workers place the dewar containing the sample in a microwave transmission cavity placed between the poles of an electromagnet. Next, they irradiate the sample with ultraviolet light. The light energy excites the molecule, inducing an electron to move to a higher, previously unoccupied orbital. Since the unpaired electrons are now in different energy levels, one of them may flip its spin orientation. The resulting triplet state can last a comparatively long time (several seconds) before the excited electron is flipped back and falls to the lower (ground state) orbital. Microwave energy of particular frequency can induce transitions between the three possible states of a triplet molecule in the magnetic field. In the technique used by the Bell workers, molecules are randomly frozen in the solvent. Transitions are detected in those molecules whose molecular axes are approximately parallel to the external magnetic field.
EPR Probes Structure of Magnetic Molecules and Free Radicals In electron paramagnetic resonance (EPR) work scientists study the structure of magnetic molecules and free radicals by exposing samples simultaneously to microwave radiation and a varying magnetic field. As the magnetic molecule absorbs radiation, its unpaired electrons change their orientations with respect to the field. Considering the magnetic field strength at which absorption of radiation occurs, scientists get information about the position of the electrons in the molecule. Electrons in a stable organic molecule are usually paired. The directions of the spin axes are antiparallel, and their magnetic spin moments cancel each other. When irradiated with light, an electron may hop to a higher, previously unoccupied orbital. When the electron falls back to the lower (ground) state, light is sometimes emitted (fluorescence). However, the two electrons in the separate orbitals may become unpaired, giving rise to a molecule which has two unpaired electrons. This excited state usually has a longer lifetime, and when the electron falls back to the ground state, light is emitted (phosphorescence). When the molecule's excited state phosphoresces, the molecule has two unpaired electrons. In a strong magnetic field they can orient their axes either parallel to each other in the direction of the field; parallel in the direction opposite to the field; or parallel in the direction perpendicular to the field. The three variations yield different energy levels. Because of the three possible spin orientations, the excited molecule with the unpaired electrons is said to be in a triplet state.
T h e Bell scientists vary t h e m a g netic field s t r e n g t h slowly. At certain field s t r e n g t h s t h e electrons " r e s o n a t e " a n d a b s o r b t h e m i c r o w a v e e n e r g y as t h e y flip their orientations. The m i c r o w a v e f r e q u e n c y at w h i c h electrons flip is affected b y t h e a v e r a g e distance b e t w e e n electrons. T h u s t h e resonance signal's position with respect to t h e m a g n e t i c field s t r e n g t h tells m u c h a b o u t t h e relative positions of t h e electrons in t h e excited triplet state m o l e c u l e . Dr. C. A. H u t c h i s o n a n d D r . B. W . M a n g u m of t h e University of C h i c a g o w e r e t h e first to u s e E P R to observe p h o s p h o r e s c e n t m a t e r i a l a n d obtain detailed information of t h e triplet state molecule's electronic s t r u c t u r e . In their classic s t u d y t h e y p r e p a r e d a dilute single crystal of t h e m o l e c u l e a n d tested it at various orientations of the crystal axes in t h e m a g n e t i c field. W h e n a dilute single crystal can b e o b t a i n e d , this m e t h o d is still t h e m o s t a c c u r a t e available. F o r s o m e t i m e t h e single crystal m e t h o d w a s t h o u g h t to b e t h e only satisfactory m e t h o d for t h e d e t e c t i o n of triplet states. H o w e v e r , D r . J. H .
van d e r W a a l s a n d D r . M . S. d e Groot of t h e Royal D u t c h Shell L a b o r a t o r i e s , A m s t e r d a m , found t h a t t h e | A m | = 2 transition could b e seen if t h e p h o s phorescent species was randomly oriented in a rigid glass. This w a s possible b e c a u s e this transition w a s less sensitive to molecular orientation t h a n t h e | A m | = 1 transitions, w h i c h occur b e t w e e n sublevels of t h e triplet. F o r m a n y years scientists s u s p e c t e d t h a t s o m e divalent c a r b o n molecules ( c a r b e n e s ) w e r e triplet state molecules in their g r o u n d states. H o w e v e r , n o one w a s able to p r o v e this b y E P R m e t h o d s , a l t h o u g h D r . G. H e r z b e r g of t h e N a t i o n a l R e s e a r c h Council of C a n a d a , O t t a w a , found b y using vacuum ultraviolet spectroscopy that m e t h y l e n e , C H 2 , h a s a triplet g r o u n d state. Using E P R with t h e rigid glass s a m p l i n g t e c h n i q u e , Bell's D r . R. W . M u r r a y , D r . A. M . Trozzolo, D r . W a s serman, a n d M r . Yager find physical e v i d e n c e for t h e triplet n a t u r e of diphenvlmethylene [JACS, 84, 3213 (1962)]. T h e Bell workers irradiate a dilute frozen solution of d i p h e n y l d i a z o m e t h -
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ane in Fluorolube or in Nujol. This expells a nitrogen molecule, leaving diphenylmethylene with two unpaired electrons. They then observe EPR absorptions at several magnetic fields. They detect EPR absorptions due to the |Am | == 1 transitions in addition to the |Am| = 2 transition, establishing that the diphenylmethylene molecule is a triplet. When they turn off the irradiating light, they find that lines remain the same for many hours. From this they deduce that the diphenylmethylene is already in its most stable form. In other words, the ground state of diphenylmethylene is a triplet which is stable in rigid media. Dr. Hutchison, Dr. G. L. Gloss, and Dr. W. Brandon of the University of Chicago have observed EPR absorptions in an irradiated dilute single crystal solid solution of diphenyldiazomethane. Their findings also indicate the presence of an oriented ground state triplet molecule, the Bell workers note. Using the same EPR rigid glass sampling technique, Bell's Dr. G. Smolinsky, Dr. Wasserman, and Mr. Yager have studied several nitrenes. These monovalent nitrogen compounds—sometimes termed imenes or azenes—are thought to be intermediates formed when organic azides produce primary or secondary amines and azo compounds by thermolysis and photolysis. They find evidence for the stable triplet state in nitrenes formed from four azides: phenyl, o-trifluoromethylphenyl, benzenesulfonyl, and ptoluenesulfonyl. Therefore, they conclude that these nitrenes also exist as ground state triplets [JACS, 84, 3220 (1962)]. The Bell group has also studied molecules containing two divalent carbons (dicarbenes) or two monovalent nitrogens (dinitrenes) with EPR, using the rigid glass sampling technique.
f-Butyl Hypochlorite Use Calls for Caution tert-Butyl hypochlorite—a reagent for the detection of peptides, nucleotides, and similar compounds on paper chromatograms—requires very careful handling to avoid explosive decomposition under relatively mild conditions, warns Dr. James C. Lewis of the U.S. Department of Agriculture's Western Utilization Research and Development Division, Albany, Calif.
A glass-sealed ampoule containing 10 g. of the commercially available material exploded violently after several minutes exposure to fluorescent and north window light, Dr. Lewis says. Ambient temperature was not above 25° C , he notes. tert-Butyl hypochlorite has been recommended by Dr. Robert H. Mazur, Bernice W. Ellis, and Dr. Peter S. Cammarata of G. D. Searle & Co. for the chlorination and subsequent detection of compounds containing susceptible nitrogen-hydrogen bonds on paper chromatograms [/. Biol. Chem., 237, 1619 (1962)]. Amino acids, proteins, purines, nucleic acids, vitamins, steroids, and many other types of compounds can be detected in 5 jxg. amounts or less. Dr. H. M. Teeter and Dr. E. W. Bell of USDA's Northern Utilization Research and Development Division, Peoria, 111., who proposed a technique for the laboratory synthesis of tertbutyl hypochlorite [Organic Synthesis, 32, 20 (1952)], recognized its lability. They noted that ultraviolet irradiation causes rapid decomposition with attendant rise in temperature. They also advise that, although customary room illumination does not induce noticeable decomposition, this might be sufficient over a prolonged exposure period to cause sealed ampoules to explode. Dr. Lewis stresses the importance of maintaining extreme caution at all times while using the material. This is particularly true for the samples of commercial product now available since there is evidence that they have undergone some decomposition, he adds.
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BRIEFS The total number of calculated wavelength standards of neutral germanium in the vacuum ultraviolet region with uncertainties estimated to be within 0.0008 A. has been increased to about 100 lines. This is the result of a cooperative study by the National Bureau of Standards and Purdue University. Twelve additional standards from singly ionized germanium in this region were also calculated. This study, by Victor Kaufman of NBS and K. L. Andrew of Purdue, extends the interferometric region of measurement of neutral germanium lines to 12069 A. OCT.
2 2,
1962
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