Here’s another interesting application of computed NMR spectra to resolve the structure of natural products. Braddock and Rzepa have examined obtusallenes V (1), VI (2) and VII (3).1 The geometries were optimized at mPW1PW91/6-31G(d,p) and the chemical shifts were obtained at this level and using the aug-cc-pVDZ basis set. The larger basis reduces the error and no statistical correction need be applied. The coordinates of these compounds are available through this web-enhanced object of the paper.
The confusion in these structures relates to the position of the halide attachments. For 1 and 2, the problem is which halide (Br or Cl) is at C-7 and C-13. The original structures proposed had these halogens switched from what I’ve drawn, and the correlation between the computed chemical shifts for these original structures and the experiment shows significant deviation: a mean deviation of 1.42 ppm for 1 and 1.67 ppm for 2. Using the structures shown above, along with switching the assigned 13C chemical shifts gives much better agreement between the computed and experimental values; the mean deviation is 1.15 ppm for both 1 and 2. Unfortunately the stereochemistry about the allene cannot be determined using NMR – the two different isomers have similar chemical shifts. Similarly, the structure of 3 is predicted as shown above, though the experiment reported only some of the chemical shifts so some uncertainty remains.
(1) Braddock, D. C.; Rzepa, H. S., "Structural Reassignment of Obtusallenes V, VI, and VII by GIAO-Based Density Functional Prediction," J. Nat. Prod., 2008, DOI: 10.1021/np0705918.
InChIKey = PVIUYMGCQVXTIT-JUHTWQEGBT
InChIKey = WPEZFVRVOYPLJW-LXJGPXSEBA
InChIKey = QTZNVLUNNGQAFG-SOAHCKLOBC