Electronic Structure of Six-Membered N-Heterocyclic Carbenes and Its Heavier Analogues: Reactivity of the Lone Pair versus the Exocyclic Double Bond

Electronic structure of the six-membered N-heterocyclic carbene, silylene, germylene, and stannylene having an exocyclic double bond at the C3 carbon atom as well as the relative reactivity of the lone-pair on the divalent group 14 element and the exocyclic double bond have been studied at the BP86 level of theory with a TZVPP basis set. The geometrical parameters, NICS values, and NBO population analysis indicate that these molecules can be best described as the localized structure <b>1X</b>_{<i><b>a</b></i>}, where a <i>trans</i>-butadiene (C1–C2–C3–C4) unit is connected with diaminocarbene (N1–X–N2) via N-atoms having a little contribution from the delocalized structure <b>1X</b>_{<i><b>b</b></i>}. The proton affinity at X is higher than at C4 for <b>1C</b>, and a reverse trend is observed for the heavier analogues. Hence, the lone pair on a heavier divalent Group 14 element is less reactive than the exocyclic double bond. This is consistent with the argument that, even though the parent six-membered carbene and its heavier analogues are nonaromatic in nature, the controlled and targeted protonation can lead to either the aromatic system <b>3X</b> having a lone pair on X or the nonaromatic system <b>2X</b> with readily polarizable C3–C4 π-bond. The energetics for the reaction with BH₃ and W­(CO)₆ further suggest that both the lone pair of Group 14 element and the exocyclic double bond can act as Lewis basic positions, although the reaction at one of the Lewis basic positions in <b>1X</b> does not considerably influence the reactivity at the other. The protonation and adduct formation with BH₃ and W­(CO)₅ at X lead to nonaromatic systems whereas similar reactions at C4 lead to aromatic systems due to π-bond polarization at C3–C4. The degree of polarization of the C3–C4 π-bond is maximum in the protonated adduct and reduces in the complexes formed with BH₃ and W­(CO)₅

Ke garne? (What can one do?.: How people 'on the ground' perceived the incomplete improvised mortuary rituals at Pashupatinath after the earthquake in Nepal, 2015

Triggered by ongoing debates about Hindu traditionalism and ritualism versus adaptation, attenuation and secularization, we estimated that studying ritual behavior in the extremely deprived situation right after the disaster would teach us something fundamental about basis human needs, basic cultural skills, as well as basic ritual repertoire

Transition between [R]- and [S]-Stereoisomers without Bond Breaking

We for the first time shown that transition between (R) and (S) stereoisomers via a planar transition state or an intermediate structure without having to break a bond is possible. Rigorous theoretical calculations have been used to study this novel phenomenon and to characterize the energetic, structure, dynamic and kinetic properties