Transmission of polar substituent effects across the cubane ring system: 19F substituent chemical shifts (SCS) of 4‐substituted (X) cub‐1‐yl fluorides revisited |
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Authors: | William Adcock |
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Institution: | School of Chemistry, Physics, and Earth Sciences, Flinders University, Adelaide, SA 5001, Australia |
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Abstract: | 19F NMR shieldings of 4‐substituted (X) cub‐1‐yl fluorides ( 4 ) for a set of substituents (X?H, NO2, CN, NC, CF3, COOH, F, Cl, HO, NH2, CH3, Si(CH3)3, Li, O? and NH ) covering a wide range of electronic substituent effects were calculated using the DFT‐GIAO theoretical model. The level of theory, B3LYP/6‐311+G(2d,p), provided 19F substituent chemical shifts (SCS) in good agreement with experimental values where known. By means of NBO analysis, various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated 19F SCS and polar field, resonance and group electronegativity substituent constants (σF, σR and σx, respectively) and also the NBO derived molecular parameters (fluorine natural charges (Qn), electron occupancies on fluorine of lone pairs (nF) and occupation number of the C? F antibonding orbital (σCF*)). The key determining parameters appear to be nF and σCF*(occup). Both factors are a function of the electrostatic field influence of the substituent (σF effect) but are counteractive in their influence on the shifts. No evidence for a significant resonance effect influence on the shifts could be identified. Copyright © 2009 John Wiley & Sons, Ltd. |
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Keywords: | cubane 19F chemical shifts substituent effects electronic effects DFT‐GIAO B3LYP/6‐311+G(2d p) NBO regression analysis |
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