Table 1. Calculated Gas-Phase Proton Affinities (PA, kcal/mol) and Changes in NICS (ΔNICS(1)zz, ppm) upon Protonation of Benzene and Cyclooctatetraene (COT) in the S0, S1, and T1 Statesa.
| S0 |
S1 |
T1 |
||||
|---|---|---|---|---|---|---|
| compound | PA | ΔNICS(1)zz | PA | ΔNICS(1)zz | PA | ΔNICS(1)zz |
| benzene | 183.1 | 16.0 | 214.7 (C2v, i)b | –55.6 | 217.0 (C2v, i)b | –28.3 (C2v, i)b |
| (181.3)d | 232.1 (Cs, HA)b | 218.4 (Cs, HA)b | –48.2 (Cs)b | |||
| (207.3)d | ||||||
| COT | 220.8 (Cs, HA)c | –6.7c | 182.8 | –18.7 | 195.0 | 15.3 |
Proton affinities at (U)- and (TD-)B3LYP levels and ΔNICS(1)zz values at CASSCF//B3LYP. NICS(1)zz at CASSCF//B3LYP, B3LYP//B3LYP, and CASSCF//CASSCF levels are given in Table S34. For further computational details and additional discussion, including electronic aromaticity index (MCI and FLU), see Table S36 and section 7.9 in the Supporting Information. ΔNICS(1)zz = NICS(1; non-protonated)zz – NICS(1; protonated)zz.
Benzenium cation in the T1 and S1 states has homoaromatic (HA) minimum (Cs), but the values for PA and ΔNICS related to the proton addition to the planar benzene are also given and indicated as “(C2v, i)” (i = imaginary frequency). The NICS(1)zz value of the Cs-symmetric S1 structure cannot be determined as it is too close to the conical intersection.
The COT and COTH+ minima are strongly puckered, and COTH+ has homoaromatic (HA) character. NICS(1)zz values for both puckered and planar structures are given in Table S34.
Experimental values from ref (48).