Consider the following series of molecules: benzene, methylbenzene, trifluoromethylbenzene, benzonitrile, and nitrobenzene in which the substituents para to the C atom of interest are H, CH₃, CF₃, CN, and NO₂, respectively.

(a) Use the computational method of your choice to calculate the net charge at the C atom para to these substituents in this series of organic molecules.

(b) It is found empirically that the ¹³C chemical shift of the para C atom increases in the order: methylbenzene, benzene, trifluoromethylbenzene, benzonitrile, nitrobenzene. Is there a correlation between the behaviour of the ¹³C chemical shift and the computed net charge on the ¹³C atom?

(c) The ¹³C chemical shifts of the para C atoms in each of the molecules that you examined computationally are as follows:

Is there a linear correlation between net charge and ¹³C chemical shift of the para C atom in this series of molecules?

(d) If you did find a correlation in part (c), explain the physical origins of the correlation.

Transcribed Image Text:

Substituent 8 CH3 128.4 H 128.5 CF3 128.9 CN 129.1 NO₂ 129.4