RHF-SCF 3-21G calculations are reported for the 1Σ+/1A1, 3A′ and 1,3A″ states of simple substituted nitriles YCN (Y = F, Cl, CH3 and CF3) and their Markownikoff (M) cations (YCNH+), and the 1A′ and 1,3A″ states of their anti-Markownikoff (aM) cations (YCHN+). Equilibrium electronic structures and the mechanism of adiabatic protonation are described qualitatively via Lewis/resonance schematic representations. Calculated proton affinites (PAs) suggest that relative to YCN(1Σ+/1A1), YCN(1,3A″) is of enhanced basicity with respect to both C and N protonation, especially in the case of the former. A graphical representation of the ordered pairs (PA(YCN*(M)), PA(YCN*(aM)) enables one to conclude the following: (1) irrespective of both regiocenter (M/aM) and state (S0(1Σ+/1A1)/T2(3A″)/S1(1A″)) the PAs for YCN decrease with respect to Y in the order CH3>H>Cl>F>CF3; (2) for S0 the dependence of PA (M) and PA (aM) on substituent is such that for all Y, PA (M) > PA (aM); (3) for T2, although PA (aM) > PA (M) for all Y, in a relative sense regioselectivity appears to be independent of substituent: (4) for S1, unlike S0 and T2, the thermodynamically preferred site of protonation is highly substituent dependent; and (5) in a relative way, S0 and S1, exhibit similar dependencies of PA (M) and PA (aM) on Y. Critical comparison is made with available theoretical and experimental results.
ASJC Scopus subject areas
- Condensed Matter Physics
- Physical and Theoretical Chemistry