Photochemistry of Small Molecules - NIST Virtual Library

Photochemistry of Small MoleculesThe classic text book in photochemistry is that ofNoyes and Leighton, The Photochemistry of Gasespublished in 1941. In 1966, a comprehensive volume byCalvert and Pitts brought the field up to date, but it soonbecame obvious that the field of photochemistry wasexpanding too rapidly to be covered in a single comprehensivevolume. In fact, in the latter part of the 20thcentury organic photochemistry and physical photochemistryexisted as separate disciplines with largelyseparate constituencies. Organic photochemistry includedsuch molecules as methane (CH 4 ), whereas NOand other oxides of nitrogen are inorganic. The book byHideo Okabe, Photochemistry of Small Molecules [1],published in 1978, is so titled to convey the author’sintent to include both organic and inorganic molecules,but to direct his attention to simple molecules of bothgenres. The author’s definition of “small” as five orfewer atoms is arbitrary and, unfortunately, excludesmuch of his own seminal work on the photochemistry ofethane and propane, which contain eight and elevenatoms respectively.In his Photochemistry of Small Molecules Okabeprovides an elegant discussion of the spectroscopy ofatoms and molecules as well as of the rotational, vibrational,and electronic energy states. In photochemicalresearch, small molecules often have discrete electronicspectra, and any discussion of mechanisms of photochemicalreactions must be consistent with thesespectra. Accordingly, Okabe gives special attention tothe details of what was known about the spectra. Hesystematically presents chapters on photochemistry andspectroscopy of diatomic, triatomic, and polyatomicmolecules (i.e., four and five atom) in Chapters V, VIand VII of the book. His discussion of the basis ofphotochemical correlation rules and selection rules isvery thoroughly presented. His knowledge of the relevantspectroscopy is at a very high level, and his incorporationof spectroscopic aspects into his discussion ofthe photochemistry, if not unique, is certainly extraordinary.Okabe’s subject, the photochemistry of smallmolecules, came into prominence in the 1960-1980period just at the time when planetary modelers werestruggling to provide a quantitative understanding of thechemistry of the atmospheres of the giant planets andtheir moons. The key molecules were methane andammonia, and the electromagnetic radiation driving thechemistry was the Lyman alpha solar emission line inthe vacuum ultraviolet at 121.6 nm. It was Okabe and hisNBS colleagues who provided much of the hard sciencein support of this objective. The table had been set forthe writing of this scholarly book by the appearance in1964 of a review of the field of vacuum ultraviolet(VUV) photochemistry by McNesby and Okabe [2].The long term impact of Okabe’s work is illustrated byNobel Laureate Roald Hoffmann’s devoting a chapter inhis 1995 book [3] to Okabe’s 1961 paper on the photolysisof ethane [4] (“a text book case for the applicationof the scientific method”). Another Nobel prize winner,Yuan T. Lee, published a paper in 1999 [5] with hiscolleagues in which testimony is given that Okabe wasthe discoverer of molecular elimination of hydrogen inVUV photolysis of hydrocarbons. The Lee paper wasthe first in which molecular elimination from propanewas studied using very sophisticated techniques not evenimagined at the time of Okabe’s 1961 discovery. Leeand coworkers used 157 nm laser excitation of propaneand employed photofragment translational spectroscopyto measure directly the site-specific molecular eliminationof hydrogen. They determined that the eliminationof molecular hydrogen from the central carbon atomwas the dominant process. This is what Okabe concluded37 years earlier on the basis of experiments withdeuterated propanes [6,2].As a result of the book and his series of researchpapers, Okabe became widely known as a leader in thefield of vacuum ultraviolet photochemistry. There havebeen more than 600 (non-self) citations to Photochemistryof Small Molecules, and it continues to beregarded as the definitive text on this subject.Prior to Okabe’s work at NBS (which extended from1959 to 1983), little attention was given to photochemicalrequirements for measurements in the vacuumultraviolet, such as spectral purity of light sources,appropriate window materials, and the effect of temperatureon the transmittance of these materials. When hefirst joined the photochemistry group at NBS, he wasgiven the choice of a project to pursue, but was urged toconsider photochemistry in the vacuum ultraviolet andsoon made this commitment. He quickly recognized thenecessity of establishing the purity and monochromaticityof his light sources. Early attempts to study VUVphotochemistry with electroded resonance lamps wereplagued by outgassed impurities which made it impossibleto know the active wavelengths in the photolysis.Okabe’s meticulous methods produced the first high224

intensity rare gas resonance lamps with outputs ofessentially pure wavelength. He achieved this by use ofa scanning vacuum monochromator to establish thepurity of the resonance lines of xenon, krypton, andargon, the first time this powerful technique was used bya photochemist. The simplicity of the design of theresonance lamps is illustrated in Fig. 1. By powering thelight sources with a microwave generator and getteringthe impurities, light sources of spectacular chromaticpurity were produced (Fig. 2), which facilitated studiesof the photolysis of small molecules in the vacuumultraviolet.Fig. 1. Resonance lamp design.Fig. 2. Krypton resonance lamp with and without (----) getter.There was particular interest in the NBS group instudying the kinetics of the reactions of methyl (CH 3 )radicals. Virtually all such studies had been done byproducing CH 3 by near ultraviolet photolysis of suchlarge molecules as acetone. The difficulty always wasthe concurrent formation of large organic fragments accompanyingthe methyl radicals and the complicationsattending the resulting chemistry. The simple solution tothese problems seemed rather obvious—to photolyseethane (CH 3 -CH 3 ) since the conventional wisdomwas that the weaker C-C bond would break, producingnothing but methyl radicals. What Okabe learned veryquickly came as a tremendous surprise to him and to hiscolleagues. The conventional wisdom was wrong: thereaction product was almost entirely hydrogen ratherthan the expected methane. Further experiments withdeuterium strategically placed at various locations in themolecule gave still more surprising results—that themolecular hydrogen produced came nearly exclusivelyby molecular elimination from one end of the molecule,not one hydrogen atom from each carbon atom.This result was to change everyone’s thinking aboutVUV photochemistry of ethane and other simplehydrocarbons of planetary interest, because the majorplanetary light source, Lyman alpha at 121.6 nm, hadnearly the same frequency as the krypton resonance lineat 123.6 nm. The extension of this research to methanephotolysis, where H 2 was also found to be ejected,caused modelers of atmospheric dynamics of the giantplanets, which are rich in methane, to alter their models.This result attracted the attention of NASA, which thenpartially supported the NBS work for a number of years,with the objective not only to understand planetaryatmospheric dynamics but also to satisfy their wellknown interest in the origin of life in the solar system.In all cases the key molecules are water, methane,ammonia, and carbon dioxide (all “small molecules”)and ethane.Hideo Okabe was born 13 December 1923 inNagano-ken, Japan. During World War II he was anundergraduate at the University of Tokyo; he was deferredfrom military service because he was a sciencestudent. Needless to say, these were trying times for himas bombs fell all around on a regular basis. As a studenthe was twice bombed out of his rooming house. After aprotracted course of study during these ordeals, Okabecompleted the requirements for an undergraduate degreein engineering in 1947. The idea of coming to theUnited States seemed attractive in a milieu that wasdriven by the devastation of Japan, resulting in itsinability to support graduate education adequately. Hehad developed an interest in photochemistry duringthese years and ultimately applied for admission tothe University of Rochester to work with the dean ofacademic photochemists, W. Albert Noyes, Jr. Hideowas awarded the Ph.D. from the University of Rochesterin 1957, after which he received a post-doctoral appointmentat the National Research Council of Canadawith E. W. R. Steacie. Both Noyes and Steacie endorsedOkabe as one of their better students, and he was offeredand accepted an NBS staff position in 1959. Shortly225

after joining NBS he was granted a leave of absence toreturn to Japan for a short stay for the purpose of marryinghis bride, Tomoko Shoji. The newlyweds returned tothe United States and eventually he and Tomoko hadthree children. Okabe was a Visiting Scholar at BonnUniversity in 1963 and a Visiting Professor at TokyoInstitute of Technology in 1978, the year recognitioncame in the form of a Gold Medal from the U.S. Departmentof Commerce and the publication of Photochemistryof Small Molecules [1]. In 1983 Okabe’sposition at NBS was eliminated in a reorganization andhe opted for early retirement, although he remainedactive at NBS as a consultant for a number of years.A colleague, then at Howard University in Washingtonbut a former member of the NBS group, William M.Jackson, took him on as a senior researcher in hislaboratory. Okabe still works at Howard as ResearchProfessor of Chemistry in the Institute of AtmosphericResearch and has two remaining graduate students.Prepared by James R. McNesby with assistance fromRalph Klein.Bibliography[1] Hideo Okabe, Photochemistry of Small Molecules, John Wiley &Sons, New York (1978).[2] J. R. McNesby and H. Okabe, Vacuum Ultraviolet Photochemistry,Adv. Photochem. 3, 157-240 (1964).[3] R. Hoffmann, The Same And Not The Same, Columbia UniversityPress, New York (1995).[4] H. Okabe and J. R. McNesby, Vacuum Ultraviolet Photolysis ofEthane: Molecular Detachment of Hydrogen, J. Chem. Phys. 34,668-669 (1961).[5] S. M. Wu, J. J. Lin, Y. T. Lee and X. Yang, Site Specificity inMolecular Hydrogen Elimination From Photodissociation ofPropane at 157 nm, J. Chem. Phys. 111, 1793-1796 (1999).[6] H. Okabe and J. R. McNesby, Vacuum Ultraviolet Photochemistry.IV. Photolysis of Propane, J. Chem. Phys. 37, 1340-1346 (1962).226