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ACCOUNTS of

CHEMICAL RESEARCH EDITOR JOAN S. VALENTINE ASSOCIATE EDITORS Paul F. Barbara Christopher S. Foote Joseph F. Bunnett Editor 1966-1986 EDITORIAL ADVISORY BOARD Anna C. Balazs Allen J. Bard Fred Basolo Jeremy M. Berg Jerome Berson John I. Brauman Joseph F. Bunnett R. Graham Cooks F. Albert Cotton Mark D. Erion Graham R. Fleming Jean Fréchet Barbara J. Garrison Joshua Lederberg Jean-Marie Lehn Nathan S. Lewis Charles Lieber Fred W. McLafferty Thomas J. Meyer Josef Michl Anne B. Myers Kyriacos C. Nicolaou Ryoji Noyori Gerard F. R. Parkin J. C. Scaiano E. W. Schlag Peter G. Schultz Jan Sjövall Richard E. Smalley John C. Tully Mark E. Vol'pin Robert L. Whetten George M. Whitesides R. Stanley Williams John T. Yates, Jr. Richard N. Zare PUBLICATIONS DIVISION Robert H. Marks, Director Journal Publishing Operations: Charles R. Bertsch, Director Editorial Office: Mary E. Scanlan, Manager; Kathleen E. Duffy, Journals Editing Manager; Teresa M. Schleifer, Assistant Editor

Accounts of Chemical Research (ISSN 0001-4842) is published monthly by the American Chemical Society a t 1155 16th St., N.W., Washington, DC 20036. Second-class postage paid at Washington, DC, and additional mailing offices. POSTMASTER: Send address changes to Accounts of Chemical Research, Member & Subscriber Services, P.O. Box 3337, Columbus, OH 43210. Canadian GST Reg. No. 127571347. Printed in the USA. American Chemical Society 1155 16th St., N.W. Washington, DC 20036 (202) 872-4600 TDD (202) 872-8733 FAX (202) 872-4615 J o u r n a l P u b l i s h i n g Operations American Chemical Society 2540 Olentangy River Road P.O. Box 3330 Columbus, OH 43210 (614) 447-3665 or (614) 447-3600, Ext. 3171 TELEX 6842086; FAX (614) 447-3745 Member & Subscriber Services American Chemical Society P.O. Box 3337 Columbus, OH 43210 (614) 447-3776; FAX (614) 447-3671 (800) 333-9511

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Editorial Information Accounts of Chemical Research publishes concise, critical reviews of research areas currently under active investigation. Most articles are written by scientists active in the area reviewed. Reviews may be concerned in large part with work in the author's own laboratory, providing t h a t r e l e v a n t c o n t r i b u t i o n s by o t h e r investigators are mentioned sufficiently to place the author's research in perspective. Also published are occasional articles in which t h e a u t h o r offers a critical assessment of a subject that has become somewhat muddled or even controversial, and proposes unifying concepts to clarify it. Instructions for authors are contained in the "Statement of Policies and Procedures" printed in the first issue of each

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