Thursday, November 15, 2012

Experimental and Theoretical Investigation of 1JCC and nJCC Coupling Constants in Strychnine

Williamson, R. T.; Buevich, A. V.; Martin, G. E. Org. Letters 2012, 14, 5098-5101 (Paywall)
Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry with permission

The use of computed NMR coupling constants is starting to grow. In a previous post I discussed a general study by Rablen and Bally on methods for computing JHH coupling constants. Now Williamson reports methods to experimentally obtain 1 JCC and 3JCC coupling constants.1 These were obtained for strychnine. He then computed the coupling constants in two steps. Using the B3LYP/6-31G(d) optimized geometry, first the Fermi contact contribution was computed at B3LYP/6-31+G(d,p) by uncontracting the basis set and adding an additional tighter set of polarization functions. Second, the remaining terms (spin-dipolar, paramagnetic spin-orbit and diamagnetic spin-orbit coupling) were computed with the 6-31+Gd,p) set without modifications. The two computed terms were added to give the final estimate.

A plot of the experimental vs. the DFT computed 1 JCC and 3JCC coupling constants shows an excellent linear relation, with correlation coefficient of 0.9986 and a slope of 0.98. The mean absolute deviation for the computed and experimental 1 JCC and 3JCC coupling constants is 1.0 Hz and 0.4 Hz, respectively, both well within the experimental error. I expect that computed NMR spectra will continue to be a growth area, especially for structural identification.

References

(1) Williamson, R. T.; Buevich, A. V.; Martin, G. E. "Experimental and Theoretical Investigation of 1JCC andnJCC Coupling Constants in Strychnine," Org. Letters 201214, 5098-5101, DOI: 10.1021/ol302366s

InChIs

strychnine:
InChI=1S/C21H22N2O2/c24-18-10-16-19-13-9-17-21(6-7-22(17)11-12(13)5-8-25-16)14-3-1-2-4-15(14)23(18)20(19)21/h1-5,13,16-17,19-20H,6-11H2/t13-,16-,17-,19-,20-,21+/m0/s1
InChIKey=QMGVPVSNSZLJIA-FVWCLLPLSA-N