Solid-state13C chemical shift tensors in terpenes. Part I. Spectroscopic methods and chemical shift structure correlations in caryophyllene oxide

Abstract

Principal values of the 13C chemical shift tensor were obtained for the 15 carbons of solid caryophyllene oxide using an improved PHORMAT NMR analysis. The improvements include TIGER processing and improved proton decoupling. TIGER is an alternative to Fourier methods and shortens 2D data collection by incorporating information from a high-resolution isotropic 1D FID to allow accurate processing of even severely truncated 2D evolution FIDs. In caryophyllene oxide, data collection required less than 1 day, giving significant time savings over comparable 2D Fourier methods. Experimental principal values were assigned with high statistical confidence to specific carbons by comparing them with corresponding calculated values. Correctly assigned values were used to evaluate five different tensor calculation methods. For caryophyllene oxide, the B3PW91 method gave the best correlation with experimental principal values with an RMS error of 2.3 ppm. Refinement of x-ray positions for hydrogens was shown to improve the calculated RMS error by a factor of >2. Calculated tensors can be used to provide principal value orientations in the three methyl groups of caryophyllene oxide. One of the perpendicular component, δ⊥, is found to exhibit the largest shift variation and dominates the methyl shifts. Sterically unfavorable non-bonded interactions between proximate hydrogens are shown to correlate with this large upfield shift in the δ⊥ component. © 1998 John Wiley & Sons, Ltd.