A new Approach For the Calculation of C-13 Chemical Shifts of Some Methyl Derivatives of Cyclohexanols | ||
| Tikrit Journal of Pure Science | ||
| Article 1, Volume 17, Issue 2, June 2012, Pages 62-75 | ||
| Authors | ||
| Emad A.S. AL-Hyali; Aburahman K.AL.Taie; Hemin M.K.M.AL-Teleko | ||
| Abstract | ||
| This work is devoted for the calculation of C-13 chemical shifts for a number of methyl cyclohexanol compounds. Five sets of additively parameters have been suggested for this purpose .These parameters are based on two principles ; the electronic effect that a certain substituent experience on the carbon under consideration and the steric effect resulted from spatial arrangement of the molecule . The former effect is represented by various forms such as the number of carbon atoms at α,β,γ and δ position with respect to the carbon consideration ,the number of primary , secondary ,tertiary and quaternary carbons at α -position and the position of OH group with respect to a certain carbon .These parameters are then replaced by single parameter , the electronic charge ,which is calculated by the Austen (AM1) and Hartree Fock (HF) methods . The steric factor is introduced in terms of proton interaction between the protons exist on the consider carbon and protons on the other carbons (H..H) of the molecule which was presented by various forms. The presence of two lone pairs of electrons on the OH group was the reason of adding another type of proton interactions namely the proton – lone pair (H ..Lp) interactions .All the suggested sets of parameters predicted successfully the chemical shifts of the studied compounds . Deviations are noticed only on carbon of those involved in large steric interactions . The best results obtained were by using 5 set in which the p-character is described by Mulliken charge calculated by HF method and the spatial interactions are represented proton (H..H and H..Lp) . In this set the number of parameters is reduced from (15) (used in the first fourth sets) to (8) parameters . These parameters have physical meaning consistent with the theory of C-13 nmr chemical shifts . | ||
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