Steric effects on normal and abnormal acyclic, cyclic-saturated, and cyclic-unsaturated diaminocarbenes using DFT method

Steric effects are probed on the multiplicity, stability, and reactivity of normal 2,4-disubstituted and abnormal 2,5-disubstituted diaminocarbenes 1 _R -3 _R and 1′ _R -3′ _R , respectively, at B3LYP/6-311++G ^** level (R = H, Me, Et, i-Pr, t-Bu). All optimized structures appear with singlet ground states. In comparison with acyclic normal (1 _R ) and abnormal (1′ _R ), one observes cyclic normal (2 _R ) and abnormal (2′ _R ) as well as cyclic-unsaturated normal (3 _R ) and abnormal (3′ _R ) exhibit a decrease in nucleophilicity (N) and proton affinity (PA) in going from 1 _R  → 2 _R  → 3 _R , and in going from 1′ _R  → 2′ _R  → 3′ _R (except PA in 3′ _R ). On the account of higher reactivity and lower stability of 3′ _R , PA increases in going from 2′ _R to 3′ _R . The singlet-triplet energy gaps (ΔE _S-T ) and the energy difference between the highest occupied molecular orbital (HOMO) energy and the lowest unoccupied molecular orbital (LUMO) energy of the compound, HOMO-LUMO energy gaps (ΔE _H-L ) increase in going from 1 _R  → 2 _R  → 3 _R , and in going from 1′ _R to 2′ _R . In contrast, the stability of abnormal species decreases in going from the cyclic-saturated 2′ _R , to cyclic-unsaturated diaminocarbenes 3′ _R . Isodesmic reactions indicate that both normal and abnormal diaminocarbenes 1 _R and 1′ _R , as well as 3 _R and 3′ _R become more stable in the presence of heteroatoms. The abnormal ones become more stable than the corresponding normal carbenes. The larger the substituent (R) the greater is the stability effect of the heteroatom.