Toward triplet disilavinylidenes: A Hammett electronic survey for substituent effects on singlet-triplet energy gaps of silylenes by DFT

To reach for novel triplet silylenes, para-and meta-R-1,1-diphenyldisilavinylidenes with various electron donating and withdrawing groups (EDGs and EWGs, respectively) are compared and contrasted using the B3LYP/6-311++g** method, where para silylenes consist of 1(H), 2(CH3), 3(OH), 4(NH2), 5(F), 6(Cl), 7(CF3), 8(CHO), 9(NMe2), and 10(NO2), whereas their meta isomers are 2(CH3)', 3(OH)', 4(NH2)', 5(F)', 6(Cl)', 7(CF3)', 8(CHO)', 9(NMe2)', and 10(NO2)'. The overall trend for triplet tendency of the latter group based on their singlet-triplet energy gap (Delta Es-t) is 3(OH)' > 4(NH2)' > 1(H) > 2(CH3)' > 9(NMe2)' > 8(CHO)' > 5(F)' > 6(Cl)' > 7(CF3)' > 10(NO2)' while that for the former, para series is 9(NMe2) > 4(NH2) > 3(OH) > 2(CH3) > 5(F) > 6(Cl) > 1(H) > 7(CF3) > 8(CHO) > 10(NO2). Plotting disilavinylidenes Delta Es-t, against their corresponding Hammett substituent constants (sigma(p) or sigma(m)) shows rather significant rho factors of -2.07 and -1.42 with good correlations of R-2 = 0.98 and 0.63, respectively. The negative sign of the rho indicates that the more EDGs have higher impacts on stabilizing or reaching for the triplet ground states, displaying 4(NH2) and 9(NMe2) as two silylenes with triplet ground states. The EWGs increase electrophilicity (omega) making 10(NO2) (7.62 eV) the most electrophilic. In contrast, EDGs increase nucleophilicity (N) showing 9(NMe2) (5.03 eV) as the most nucleophilic silylene. Moreover, 9(NMe2) shows the highest proton affinity (PA = 388.13 kcal/mol), chemical potential (mu = -3.67 eV), energy of HOMO (E-HOMO = -4.46 eV), dipole moment (5.44 debye), and the lowest band gap (Delta EH-L = -1.59 eV), and electrophilicity (omega = 4.24 eV), with the lowest NBO charge on divalent Si atom (+0.129) among the scrutinized species.