A DFT-GIAO and DFT-NBO study of polar substituent effects on NMR ¹⁷O chemical shifts in some rigid polycyclic alkanes

¹⁷O NMR shieldings of 3-substituted(X)bicyclo[1.1.1]pentan-1-ols (1, Y = OH), 4-substituted(X)bicyclo[2.2.2]octan-1-ols (2, Y = OH), 4-substituted(X)-bicyclo[2.2.1]heptan-1-ols (3, Y = OH), 4-substituted(X)-cuban- 1-ols (4, Y = OH) and exo- and endo- 6-substituted(X)exo-bicyclo[2.2.1]heptan-2- ols (5 and 6, Y = OH, respectively), as well as their conjugate bases (1-6, Y = O^-), for a set of substituents (X = H, NO₂, CN, NC, CF₃, COOH, F, Cl, OH, NH₂, CH₃, SiMe ₃, Li, O^-, and NH 3+) covering a wide range of electronic substituent effects were calculated using the DFT-GIAO theoretical model at the B3LYP/6-311 + G(2d, p) level of theory. By means of natural bond orbital (NBO) analysis various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated ¹⁷O SCS and polar field and group electronegativity substituent constants (σ_F and σ_χ, respectively) and also the NBO derived molecular parameters (oxygen natural charge, Q_n, occupation numbers of the oxygen lone pairs, n_o, and occupancy of the C-O antibonding orbital, σ*_CO(occup)). In the case of the alcohols (1-6, Y = OH) the ¹⁷O SCS appear to be governed predominantly by the σ_χ effect of the substituent. Furthermore, the key determining NBO parameters appear to be n_o and σ* _CO(occup). Unlike the alcohols, the calculated ¹⁷O SCS of the conjugate bases (1-6, Y = O^-), except for system 1, do not respond systematically to the electronic effects of the substituents. An analysis of the SCS of 1(Y = O^-) raises a significant conundrum with respect to their origin.