SOLID-STATE C-13 CHEMICAL-SHIFT TENSORS IN TERPENES - PART I - SPECTROSCOPIC METHODS AND CHEMICAL-SHIFT STRUCTURE CORRELATIONS IN CARYOPHYLLENE OXIDE

Principal values of the C-13 chemical shift tensor were obtained for t he 15 carbons of solid caryophyllene oxide using an improved PHORMAT N MR analysis. The improvements include TIGER processing and improved pr oton decoupling. TIGER is an alternative to Fourier methods and shorte ns 2D data collection by incorporating information from a high-resolut ion isotropic ID FID to allow accurate processing of even severely tru ncated 2D evolution FIDs. In caryophyllene oxide, data collection requ ired less than 1 day, giving significant time savings over comparable 2D Fourier methods. Experimental principal values were assigned with h igh statistical confidence to specific carbons by comparing them with corresponding calculated values. Correctly assigned values were used t o evaluate five different tensor calculation methods. For caryophyllen e oxide, the B3PW91 method gave the best correlation with experimental principal values with an RMS error of 2.3 ppm. Refinement of x-ray po sitions for hydrogens was shown to improve the calculated RMS error by a factor of >2. Calculated tensors can be used to provide principal v alue orientations in the three methyl groups of caryophyllene oxide. O ne of the perpendicular component, delta(perpendicular to), is found t o exhibit the largest shift variation and dominates the methyl shifts. Sterically unfavorable non-bonded interactions between proximate hydr ogens are shown to correlate with this large upheld shift in the delta (perpendicular to) component. (C) 1998 John Wiley & Sons, Ltd.