NHC Carbene-Metal Complex Ligand Binding Energies.

The ligand binding energies (LBEs) of N-heterocyclic carbenes (NHCs) and CH2 and CF2 adducts with group 1, 2, 10, and 11 metals and complexes with metals from these groups are predicted at the coupled cluster CCSD(T) level of theory by using density functional theory optimized geometries. The differences in LBEs as a function of the metal and the types of bonding interactions as well as the type of carbene are described. The bonding between the alkali cations and alkaline earth dications is predominantly ionic with a linear correlation between the LBEs and the cation hardness. In contrast, the bonding behaviors of the group 10 and 11 metals and metal complexes have only a weak, indirect correlation between the LBEs and the metal hardness. The difference in bonding behavior between the groups of metals arises due to the accessibility of electron donation between the ligand and the metal in the transition metal complexes, which results in more covalent-like bonding behavior. The presence of the methyl groups on the NHC nitrogen results in only slightly more delocalized charge from the metal onto the ring, but there is significant redistribution of the charge on the ring. Saturation of the NHC ring had a much smaller effect on how the charge was distributed on the ring. The analysis of the bonding behavior of NHCs with various metal groups enables improved understanding of carbene-metal interactions to inform rational design of NHC-based systems.

Bond Dissociation Energies of Carbene-Carbene and Carbene-Main Group Adducts.

A range of carbene structures and their adducts with one another and with a selection of small-molecule electrophiles and nucleophiles were examined at the composite correlated molecular orbital theory G3MP2 level to explore ground-state "carbenic" structures, their stabilities, and reactivities. Differences between carbene general classification as a singlet electrophilic carbene or singlet nucleophilic carbene and their given reactivity are discussed. A key quantity is the carbon-carbon bond dissociation energy for carbene dimers or the carbene-adduct dissociation energy for other species. The carbene dimer bond dissociation energies span a wide range from 10 to 170 kcal/mol. The hydrogenation energies and singlet-triplet splitting were found to correlate best with the carbene's self-dimerization energy, whereas other descriptors do not. The proton and fluoride affinities of the carbenes alone prove inadequate for classifying reactivity among classes of carbenes. The self-dimerization bond dissociation energy, hydrogenation energy, and singlet-triplet splitting of various carbenes, despite sometimes large differences in proton affinity and other indicators of reactivity, provide usable metrics to correlate substantial amounts of thermodynamic and kinetic (reactivity) information regarding these structures.

Shape of population interfaces as an indicator of mutational instability in coexisting cell populations.

Cellular populations such as avascular tumors and microbial biofilms may 'invade' or grow into surrounding populations. The invading population is often comprised of a heterogeneous mixture of cells with varying growth rates. The population may also exhibit mutational instabilities, such as a heavy deleterious mutation load in a cancerous growth. We study the dynamics of a heterogeneous, mutating population competing with a surrounding homogeneous population, as one might find in a cancerous invasion of healthy tissue. We find that the shape of the population interface serves as an indicator for the evolutionary dynamics within the heterogeneous population. In particular, invasion front undulations become enhanced when the invading population is near a mutational meltdown transition or when the surrounding 'bystander' population is barely able to reinvade the mutating population. We characterize these interface undulations and the effective fitness of the heterogeneous population in one- and two-dimensional systems.