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when the ligand atoms are arranged in a nearly collinear fashion, solution structural determinations such as those pertaining to the ligand fluxionality and kinetics, ligation of simple well-defined molecules to biological molecules in solution, etc. It is clear that such structural information involving the distant, normally nonbonding, shells can greatly facilitate either interpretation of the mode of binding of known ligands or identification of the unknown ligands. These applications will be subjects of future publications.Acknowledgment. I am indebted to Dr. P. A. Lee (Bell Laboratories) for his generous assistance and consultations throughout the course of this work. I am also grateful to Dr. J. Rowe (Bell Laboratories) and Dr. C. J. Powell (NBS) for helpful discussions on electron mean free path. Thanks also go to Dr. B. M. Kincaid, A. Simons, and B. Chambers for their assistance in programming and Drs. J. Miller and S. Lippard for samples of 2 and 3, respectively. The experimental EXAFS measurements were made at the Stanford Synchrotron Radiation Laboratory with the financial support of the National Science Foundation under Contract DMR 77-27489 in cooperation with the U.S. Department of Energy.Tables I-IV contain scattering amplitude and phase functions for carbon and oxygen and the least-squares refined fitting parameters for AB and ABC fits. Figures A and B contain best AB fits and parameter correlations for 1-3 (15 pages). Ordering information is given on any current masthead page. Abstract:The infrared photochemistry of vinylcyclcpropane has been comprehensively investigated. Irradiation of vinylcyclopropane at relatively low pressures with the partially focused output of a C 0 2 TEA laser leads to a mixture of the C5 products cyclopentene, cyclopentadiene, 1,4-pentadiene, and cis-and trans-1,3-pentadiene. The composition of the product mixture as well as the total product yields are a sensitive function of experimental parameters. The effects of bath gas pressure, laser power, laser intensity, laser frequency, and number of pulses have been systematically examined. A simple physical picture of the multiphoton activation and resulting decomposition is developed. RRKM theory is employed to calculate energy-dependent unimolecular reaction rates. The model is quite successful in rationalizing these data, providing good evidence for the qualitative validity of a rate equation description of infrared multiphoton dissociation.

Monozygotic twinning has been associated with a variety of vascular disruptive defects including congenital hydranencephaly/porencephaly. Data involving 24 cases of congenital hydranencephaly/porencephaly associated with twinning are reported. In these cases, the finding of a preponderance of monozygotic twins and the common association of a deceased co-twin support the hypothesis of a vascular disruptive etiology. These defects are presumed to be secondary to embolic phenomena or thromboplastin release from the deceased co-twin to the survivor via the vascular interconnections of a conjoined monochorionic placenta. In all cases of hydranencephaly/porencephaly, a careful examination of the placenta and membranes for evidence of a deceased co-twin is warranted prior to providing recurrence risk counseling.

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