Sir:Progress in structure elucidation and synthesis of organic sub.stances usually has accompanied advances in organic functional group analysis. We herewith wish t o describe a method of direct analysis of the least functionalized site, the quaternary carbon center, and other nonprotonated carbon positions of complex organic compounds based on natural-abundance 13C high-resolution nuclear magnetic resonance spectroscopy using a noise off-resonance decoupling technique. l t 2 It has 13C H l3 C H3 1 400 Hz -(C) n Figure 1. Carbon-13 nmr spectra (15.074 MHz) of liquid HC-(0CHa)a under conditions of noise decoupling holding the noisemodulated side band at (a) 59.950687 MHz (9 scans), (b) 59.950267 MHz (17 scans), and (c) 59.949876 MHz (35 scans). The noise band width in each case was 300 Hz.enabled us t o determine the number of quaternary carbon units in a hydrocarbon framework, tertiary carbinols in polyfunctional systems, and disubstituted olefinic centers in a polyunsaturated material.(1) R. R. Ernst, J . Chem. Phj>s., 45, 3845 (1966).(2) Single-frequency off-resonance proton decoupling of 13C nmr spectra, an alternate method of identification of quaternary carbon sites as well as a procedure for the differentiation of variously substituted carbon centers, has been used extensively for the analysis of a variety of natural products (L. F. Johnson, private communication), steroids, monosaccharides, inositols, and terpenes (J. D. Roberts, private communication), and camphor and related substances (E. Wenkert, A. 0. Clouse, D. W. Cochran, and D. Doddrell, manuscript in preparation ; P. V. Demarco, D. Doddrell, and E . Wenkert, Chem. Commun., in press).(3) The method can be applied to many other problems, e.g., the determination of tertiary alcohols and tertiary mercaptans and their derivatives, the substitution pattern of aromatic nuclei, and the distinction, infer alia, between aldehydes and ketones, acetals and ketals, and mono-and disubstituted acetylenes, although in many instances such differentiation can be achieved from 13C chemical shift data alone. Unfortunately the chlorine and nitrogen quadrupoles make the method JCC" t I I I I I I 1 I , qb. 5 l o 6 5 Figure 2. Carbon-I3 nmr spectrum (15.074 MHz) of a carbon tetrachloride solution of cedrol (2) under noise resonance decouplingconditions after 65 scans; parts per million from CS2.Figure 3. Carbon-13 nmr spectrum (15.074 MHz) of the solution used in Figure 2 under noise off-resonance decoupling conditions after 32 scans; in parts per million from CS2. The noise-modulated side band was set ca. 800 Hz upfield that of Figure 2.The technique of noise resonance decoupling, i.e., double resonance with an incoherent radiofrequency field, has been demonstrated to produce simplified 13C nmr spectra revealing only singlets for all carbon unit^.^-^ Since residual broadening of the signals is related intimately to the coupling constants, inefficient decoupling, e.g., by application of an oscillating random radiofrequency field of low amplitude or by a shift of the band...
