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RECEIVED for review July 3, 1989. Accepted September 20, 1989. This work was supported, in part; by grants from the NationalScience Foundation (CHE-g704024)and the Robert A. Welch Foundation (F-1108).
Simulation of Carbon- 13 Nuclear Magnetic Resonance Spectra of Linear Cyclic Aromatic Compounds Abigail S. Barber and Gary W. Small*
Department of Chemistry, T h e University of Iowa, Iowa City, Iowa 52242
Huckel molecular orbital theory and molecular mechanics calculations are used to derlve structural parameters that allow carbon-13nuclear magnetk resonance spectra of hear cyclk aromatic compounds to be modeled. Wlth a set of 32 substituted benzenes, naphthalenes, and anthracenes, three models are derived that allow complete spectra to be simulated to an average error of 0.509 ppm. The accuracy of the simulations Is keyed by the development of a serles of electronic structural parameters based on free valence and autopolarlzabillty, two parameters derived from the molecular orbital calculations. The performance of the computed models is evaluated In detail, and they are subsequently applied to the slmulatlon of spectra of compounds not included In the model development work. The predictive ability of the models is judged to be excellent based on an analysis of these simulated spectra.
INTRODUCTION In carbon-13 nuclear magnetic resonance spectroscopy (13C NMR), spectrum simulation techniques have come into use as a means to verify chemical shift assignments or to substantiate a proposed structure for an unknown. Aromatic compounds are a challenge to most spectrum simulation methods due to the unique manner in which the aromatic system influences chemical shifts. Unlike carbons in saturated systems, the chemical shifts of aromatic carbons can be influenced by chemical structural effects a t large interatomic distances. Four approaches can be used to simulate or model chemical shifts in aromatic compounds: (1) the direct calculation of chemical shifts based on the theoretical principles of magnetic shielding (I-B), (2) the calculation of chemical shifts based on substituent effects (9),(3) the derivation of empirical models that relate structural characteristics to chemical shifts (IO-IB), and (4) the retrieval of chemical shifts from a spectral database in which each stored chemical shift is indexed to an encoded representation of the corresponding carbon atom environment (19). Each of these methods has limitations. The theoretical methods are computationally intensive, and reported errors between actual and predicted chemical shifts have often exceeded 20 ppm. The approach based on substituent effects is limited in that only short-range inductive
effects on chemical shifts are typically encoded. Steric and long-range effects, often important in aromatic systems, are seldom included. Empirical modeling studies have produced highly accurate chemical shift predictions (errors
