Estimation of Hammett sigma constants of substituted benzenes through accurate densityfunctional calculation of coreelectron binding energy shifts
Abstract
For substituted benzenes such as (pFC6H4Z), Linderberg et al. 1 demonstrated the validity of an equation similar to: ∆CEBE ≈ κσ, where ∆CEBE is the difference in coreelectron binding energies (CEBEs) of the fluorinated carbon in pFC6H4Z and that in FC6H5, the parameter κ is a function of the type of reaction, and σ is the Hammett substituent (σ) constant. In this work, CEBEs of ring carbon atoms for a series of para disubstituted molecules pFC6H4Z were first calculated using Density Functional Theory (DFT) with the scheme ∆EKS (PW86PW91)/TZP+Crel//HF/631G*. An average absolute deviation of 0.13 eV from experiment was obtained for the CEBEs. Then we performed a linear regression analysis in the form of Y = A+B*X for a plot of Hammett σp constants against calculated shifts ∆CEBEs (in eV) for the fluorinated carbon. The results were: A = 0.08 and B = 1.01, with correlation coefficient R = 0.973, standard deviation = 0.12, and P < 0.0001. The intercept A of the fitted line, close to zero, shows that the Hammett σp constant is proportional to the calculated ∆CEBEs. On the other hand, the slope B of the straight line gives an estimate of the parameter κ. Similar statistical correlations were obtained for the carbon atoms ortho and meta to the substituent Z.
 Publication:

International Journal of Quantum Chemistry
 Pub Date:
 2005
 DOI:
 10.1002/qua.20533
 Bibcode:
 2005IJQC..103..509T
 Keywords:

 ∆CEBE;
 Hammett σconstant;
 DFT;
 disubstitued benzenes FC6H4Z