Theoretical model calculations of the absolute proton affinities of benzonitrile, nitroso- and nitrobenzene

Abstract

Protonation in benzonitrile ( 1), nitrosobenzene ( 2) and nitrobenzene ( 3) is examined by using the MP2(fc)/6-31G ∗∗//HF/6-31G ∗ + ZPE(HF/6-31G ∗) theoretical model. It is shown that the studied systems are protonated at heteroatoms: nitrogen in 1 and 2 and oxygen in 3. The absolute proton affinities (PAs) are in good accordance with the available measured data. However, PAs of nitrogen and oxygen atoms in 2 are relatively close implying that much more accurate calculations are necessary for an ultimate assignment. Analysis of the descriptors of covalent bonding reveals that the heteroatomic protonation increases the resonance interaction between substituent groups and the benzene moiety. The ring protonation, on the other hand, is energetically less favorable. It yields lower PAs than in the parent benzene molecule. Deactivation of the aromatic fragment toward electrophilic (proton) attack is rationalized by the antagonism between the two π-bond localization patterns: one caused by the electron withdrawing substituents CN, NO and NO 2 and the other arising from protonation and the subsequent formation of a sp 3 center. The ipso protonation is the least favorable in all three cases since the attacked C(1) atom has a depleted electron density due to the highly electronegative nearest neighbor (inductive effect). In addition, the resonance interaction between the benzene ring and the substituent are practically switched off, since protonation at the ipso position bends the substituent's covalent bond out of the molecular plane leading to substantial ring puckering of the aromatic moiety at the same time.