A Theoretical study of 13C NMR chemical shifts in a series of rigid bicyclo[ m.n.o.] alkanes

Abstract

The INDO-FPT calculated 13C NMR shielding constants, using GIAO's and at the London approximation, were analysed in a series of rigid bicyclo[ m.n.o.] alkanes ( m = 2, 3, 4; n = 2, 3; o = 1, 2), and adamantane. The trends in the experimental 13C chemical shifts values in this series of compounds are reasonably reproduced, and improved upon by the correction of the < r −3> integrals with the use of Mulliken populations. The discrepancy observed between the experimental and calculated 13C chemical shifts of the C(2) carbon atoms in bicyclo[2.2.1]heptane and bicyclo[3.2.1]octane, and the C(7) carbon atom in bicyclo[4.2.1]nonane, can be explained on the basis of a changed charge distribution resulting from nonbonded interactions in these highly strained five-membered rings, which, in turn, affect the one-center paramagnetic contribution to the shielding constant. The problem can be circumvented for C(2) in bicyclo[2.2.1]heptane and bicyclo[3.2.1]octane by regarding a more rigorous method for calculating charge distributions within molecules than the Mulliken population analysis.