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

作者：Adcock William^*

来源：Journal of Physical Organic Chemistry, 2011, 24(6): 492-498.

DOI：10.1002/poc.1791

• B3LYP/6-311+G(2d, p)
• DFT-GIAO calculations
• electronic effects
• NBO analysis
• O-17 chemical shifts
• regression analysis
• substituent effects

O-17 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, SiMe3, Li, O-, and NH3+) 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 O-17 SCS and polar field and group electronegativity substituent constants (sigma(F) and sigma(chi), respectively) and also the NBO derivedmolecular parameters (oxygen natural charge, Q(n), occupation numbers of the oxygen lone pairs, n(o), and occupancy of the C-O antibonding orbital, sigma* (CO)(occup)). In the case of the alcohols (1-6, Y = OH) the O-17 SCS appear to be governed predominantly by the sigma(chi) effect of the substituent. Furthermore, the key determining NBO parameters appear to be n(o) and sigma* (CO)(occup). Unlike the alcohols, the calculated O-17 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.

• 出版日期2011-6