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.


1: InChI=1/C15H18Br3ClO3/c1-8-14(18)12-6-13(17)15(22-12)7-10(19)11(21-15)5-9(20-8)3-2-4-16/h3-4,8-14H,5-7H2,1H3/t2-,8-,9+,10+,11-,12+,13-,14-,15-/m0/s1

2: InChI=1/C15H19Br2ClO3/c1-9-14(17)12-4-5-15(20-12)8-11(18)13(21-15)7-10(19-9)3-2-6-16/h3,6,9-14H,4-5,7-8H2,1H3/t2-,9-,10+,11+,12+,13-,14-,15-/m0/s1

3: InChI=1/C15H20Br3ClO3/c1-8-15(18)14-6-10(17)13(22-14)7-11(19)12(20)5-9(21-8)3-2-4-16/h3-4,8-15,20H,5-7H2,1H3/t2-,8-,9+,10-,11+,12-,13-,14+,15-/m0/s1