Nucleophilicity of both normal (1R) and abnormal (2R) N-heterocyclic carbene (NHC); every 2R showing a higher nucleophilicity than its corresponding 1R isomer (R = H, methyl, ethyl, i-propyl, and t-butyl).
Steric effects are probed on the multiplicity, stability, and reactivity of normal 2,4‐disubstituted and abnormal 2,5‐disubstituted diaminocarbenes 1R‐3R 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 (1R) and abnormal (1′R), one observes cyclic normal (2R) and abnormal (2′R) as well as cyclic‐unsaturated normal (3R) and abnormal (3′R) exhibit a decrease in nucleophilicity (N) and proton affinity (PA) in going from 1R → 2R → 3R, 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 (ΔES‐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 (ΔEH‐L) increase in going from 1R → 2R → 3R, 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 1R and 1′R, as well as 3R 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.
Anticancer character of gold cluster has been indicated through its free radical scavenging properties. This is in contrast to its free radical promoting ability suggested by other workers. Here, we address this controversy by probing the stabilizing effects of Au 3 cluster on RO • vs its impacts on RO-H bond dissociation enthalpy, at B3LYP/ LACVP+* level (R═H, methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, and phenyl). In the presence of Au 3 cluster, bond dissociation enthalpy of O-H bond and the spin density at the RO • oxygen are reduced dramatically. These are clear evidences for both the Au 3 facilitation of the RO-H bond breakage and its scavenging of RO • radical. Since O-Au anchoring bond is responsible for the interaction of Au 3 cluster and ROH (or RO • ), its nature was interpreted by means of the quantum theory of atoms in molecules and the natural bond orbital. The results indicate that O-Au bond is stronger and has more covalent character in RO • -Au 3 than in ROH-Au 3 .The interaction of Au 3 cluster with RO • is 1.5 to 3 times more than that with ROH. As a result, gold cluster scavenging property appears more prominent than its free radical initiation activity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.