A New Approach For The Calculation Of N-15 Chemical Shifts Of Cyclic CompoundsAuthor(s): Emad A.S.Al-Hyali
Two sets of additive parameters used previously for the calculation of C- 13 chemical shifts ofmono and poly sixmember cycloalkanes are applied for the prediction of N-15 chemical shifts of some methyl piperidine derivatives. These sets are based on two principles; The p-character (substituents electronic effect) of the atom under consideration and the steric effect that reflect the spatial arrangement of the molecule. Additional parameter, namely the ring angle at the studied atom, is added to the other parameters to define the structural distortion that due to large steric interactions. The p-character in set (1) is represented by the number of carbon atoms of primary, secondary, tertiary, and quaternary types at ï¡-position to the nitrogen, while in set (2) is introduced as the partial electronic charge. The steric effect is expressed in terms of the actual number of the interacting proton- proton and/or proton-loan pair in both sets. Quantum mechanical and regression analysis methods are employed for this treatment. The results showed that, both sets are good for prediction of N-15 chemical shifts. The parameters of set (2) are found to be more consistent with the theory of nuclear magnetic resonance(NMR) spectroscopy, therefore, they are considered for further studies. The Study also included derivation of a common set of parameters for the calculation of C-13 and N-15 chemical shifts of cyclic systems. The derived parameters are tested by the estimation of the chemical shifts of other systems. Deviations are noticed only in positions that involved in large steric interactions.