Fluorine alters molecular energy levels Φ CHICAGO U n m v e l i n s t h e elrecVUIIIUHUU
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molecule is a demanding chore. It may be made easier through the use of two types of photoelectron spectros copy: molecular photoelectron spec troscopy and electron spectroscopy for chemical analysis (ESCA). Applica tions of photoelectron spectroscopy were discussed at a symposium on that subject held jointly by the Division of Physical Chemistry and the Division of Analytical Chemistry. The effects of fluorination, for in stance, on the electronic structure of various molecules, determined by mo lecular photoelectron spectroscopy, and by ah initio molecular orbital cal culations, have been examined by Dr. C. R. Brundle and M. B. Robin of Bell Telephone Laboratories at Murray Hill, N.J., along with Harold Basch of Ford Scientific Laboratories at Dear born, Mich. Dr. William Jolly and Patricia Finn of the University of California and Lawrence Radiation Laboratory, Berkeley, and coworkers Richard Pear son and Jack Hollander of Lawrence Radiation Laboratory used ESCA to get core electron binding energies (ionization potentials) for gaseous ni trogen compounds. They used values obtained to compare the relative mer its of methods for calculating binding energies. ESCA uses soft x-rays to eject inner core electrons of atoms in molecules. The variation of inner shell ionization potentials is a function of the chemi cal environment of the atoms in a molecule. It is possible to identify atoms in nonequivalent positions and to correlate these variations, for exam ple, with oxidation numbers and oxi dation states. Molecular photoelec tron spectroscopy, which uses ultravio let radiation to eject electrons, provides detailed information on the electrons involved in chemical bonding. Dr. Brundle and coworkers com pared the photoelectron spectra of methane (CH 4 ), monofluoromethane ( C H , F ) , difluoromethane (CKLF.,), trifluoromethane (CHF ; i ), and tetrafluoromethane (CF 4 ). They found that the electronic structure of these molecules changes in a regular way through the series. Moreover, Dr. Brundle says, theoretical calculations successfully predict these changes. Fragmentation. Photoelectron spec troscopy has been used, for instance, to confirm suspected fragmentation processes that occur in a mass spec trometer. The CF 4 + molecular ion, for example, has never been observed in a mass spectrometer, Dr. Brundle 44 C&EN SEPT. 28, 1970
says, and was assumed to be totally unstable. A photoelectron spectrum, however, shows that the ion exists in several ionized states for at least 10~14 second. The electronic structure of buta diene has caused some consternation among theoretical chemists and others over whether a sigma energy level lies between the energies of the molecule's two outermost pi molecular orbitals. Photoelectron spectroscopy helped re solve the issue in what Dr. Brundle thinks is the first use of the method to determine a geometric structure as well. The inductive effect of a fluorine atom substituted on a planar molecule is fairly specific, he says, in that sigma ionization potentials become much higher while pi ionization potentials don't change much. So, he says, it should be possible to determine which are sigma and pi orbitals in butadiene by examining the spectrum of hexafluorobutadiene and seeing which ioni zation potentials are raised. He found sigma potentials went higher, one pi became smaller, the other greater. The only explanation for this, he says, is that the two double bonds became partly unconjugated by the molecule no longer remaining pla nar. A dihedral twisting angle of cis about 40° was estimated and later de termined by a group at Cornell Uni versity to be 47.6° from electron dif fraction data. The likely electron structure of butadiene, he says, has the pi levels separated by a sigma level. Core electrons. The relative mer its of theoretical and empirical meth ods of obtaining core electron binding
energies for gaseous nitrogen com pounds such as nitrogen dioxide, tetrafluorohydrazine, nitric oxide, ammonia, and dimethyl amine have been ex amined by Dr. Jolly and coworkers. Experimental values come from use of the ESCA technique. An atomic charge correlation method, Dr. Jolly says, gives only rough correlations with the binding energy. He used Pauling's method for calculating charges, which is based on the relation between the ionic char acter of a bond and the difference in the electronegativities of the atoms. A thermodynamic method, which gives a more accurate prediction ( ± 0.52 e.v.) than the atomic charge method (±1.62 e.v.), Dr. Jolly com ments, can be used only when heats of formation are known or can be esti mated. An empirical parameter method, he says, should be capable of predicting chemical shifts to about ± 0.2 e.v. In this method, chemical shifts are approximated by adding em pirically evaluated parameters char acteristic of attached atoms or groups. For example, when all hydrogen atoms of ammonia are replaced with methyl groups, the binding energy decreases 0.9 e.v. For monomethyl amine, the expected decrease of 0.3 e.v. is ac tually 0.4 e.v. For dimethyl amine the predicted decrease is 0.6 e.v.; actual decrease is 0.7 e.v. Few comparisons have been made between experimental and highly re fined molecular-orbital-calculated binding energies, Dr. Jolly says. For those that have been compared, he says, average disagreement with ex perimental values is ± 1.0 e.v.
Metabolism of man, animals compared by organ cultures ^^ΡΙΙΙΡΛΡΠ ^ n e °^ tne
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v ' w n l U H U U ficult assessments a research scientist is obliged to make is the effect on humans of long-term ex posure to an experimental compound. L. J. Sullivan, B. H. Chin, and C. P. Carpenter of Carnegie-Mellon Univer sity's Bushy Run laboratory, Pitts burgh, Pa., have proposed an in vitro technique for metabolic studies that they hope will lead to selection of the animal species most similar to man for long-term studies to predict chronic toxicity. Their findings, the result of work sponsored by Carnegie-Mellon and Union Carbide Corp., were pre
sented at a meeting of the probation ary Division of Pesticide Chemistry. Based on the assumption that chem ical interactions within biological sys tems would be reflected by metab olism, the Carnegie-Mellon group sought an in vitro technique for meta bolic studies based upon successful organ maintenance methods and chro matographic profiles. The technique qualitatively reflects in vivo metabolic processes of animals, including man, under prescribed conditions without re sorting to the risk of dosing humans. Previously published data on in vivo metabolism of carbaryl—a pesticide—
