Computational Cogitation of C n @Al 12 Clusters Benjamin J. Irving* and Fedor Y. Naumkin [a] 1. Introduction Owing to their idiosyncratic sizes and shapes, with typically (at least) one dimension  100 nm, nanomaterials exhibit unique properties distinct from those shown in the bulk. [1, 2] Nanoparti- cles of both the theoretical and physical ilk have gained in- creasing attention due to the novel properties that arise as we progress from the molecular to macroscopic domain. [3, 4] The small size of nanoparticles goes hand in hand with a high sur- face-area-to-volume ratio, often leading to particle behaviour being dominated by the “unsaturated” surface shell atoms, rather than those held within the core: a trait that may be ex- ploited in the design of, for example, catalytic nanoparticles. Naturally, the (controllable) sizes of nanoparticles introduce quantum confinement effects, which play a crucial role in dic- tating cluster behaviour. [5] Indeed, manipulating the size of a nanoparticle can be just as important as altering its chemical composition when at- tempting to alter and enhance its properties. In this regard, perhaps the best-known size-dependent phenomenon is the tunable emission of fluorescent semiconductor nanoparticles, namely quantum dots. [6, 7] Due to the quantised states of electrons and holes in nano- clusters, they are commonly referred to as “pseudo-atoms” or “superatoms”. At specific cluster sizes, they seemingly exhibit the same properties as the corresponding individual atom. Pseudo-atoms containing a magic number of electrons (2, 8, 20, 40, 58…), that is, those with a closed-shell electronic con- figuration, naturally exhibit enhanced stability. [8] In conjunction with this a priori rationalisation of stability, one can readily ex- plain the behaviour of comparatively reactive pseudo-atoms, for example, pseudo-alkalis and pseudo-halogens, with refer- ence to their open-shell electronic configurations. Experimental and theoretical corroboration comes in the form of measura- ble/calculable electronic properties such as electron affinities and ionisation potentials, which reveal shell structures analo- gous to those of ordinary elements. [5] Indeed, superatoms with one more electron than a full shell can behave like an alkali metal (facile electron donation) whereas a cluster one electron short will exhibit a large electron affinity, that is, behave as a (super)halogen. [9] In addition to the commonplace magic numbers, alternative interpretations of cluster stability do exist. Moreover, it is not clear whether typical “magic” behaviour is universally applica- ble to heterogeneous clusters, such as those discussed herein. Free electrons in each cluster will occupy a new set of orbitals defined by the complete group of atoms, rather than on a reg- ular individual basis. As such, there will undoubtedly be devia- tions in the number of electrons required for forming a closed shell on moving between the unique clusters. Recent examples of atypical magic clusters include [Al (10–12) Cs (3–1) ]  anions, [10] in which doping or attachment of a few (Cs) atoms significantly alters the electronic structure (large molecular orbital splitting and reordering effectively renders the jellium model invalid) with magic behaviour rationalised through detailed analysis of molecular orbital perturbations, and clusters such as Pu@C 24 satisfying the 32-electron principle, that is, the number of va- lence electrons corresponding to fully occupied spdf subshells for the central metal atom. [11] Recent reviews of nanoparticles touch on aspects spanning the entire nanotechnology spectrum, from synthesis and char- acterisation to the applications of these unique materi- als. [3, 4, 12, 13] Synthesis techniques fall into one of three broad categories: condensation from vapour, chemical reaction and solid-state processes. Such methods allow for the synthesis of both pure and hybrid core–shell nanoparticles, [14–16] with the latter allowing for a greater extent of modification, for exam- ple, incorporating either a hydrophobic or hydrophilic coating, dependent on the desired (application-based) properties. [3] Core–shell (or more generally heterogeneous) nanoclusters gained serious attention in the 1980s, primarily in light of the fact that properties of the homogeneous sister nanoparticles could be enhanced, sometimes alongside the emergence of A variety of novel C n Al 12 core–shell nanoclusters have been in- vestigated using density functional calculations. A series of C n cores (n = 1–4) have been encapsulated by icosahedral Al 12 , with characteristic physical properties (energetics and stabili- ties, ionisation energies, electron affinities) calculated for each cluster. Other isomers, with the C n moiety bound externally to the Al 12 shell, have also been studied. For both series, a peak in stability was found for n(C) = 2, a characteristic that appears to be inextricably linked with the relaxation of the constituent parts upon dissociation. Analysis of trends for ionisation ener- gies and electron affinities includes evaluation of contributions from the carbon and aluminium components, which highlights the effects of composition and morphology on cluster proper- ties. [a] Dr. B. J. Irving, Dr. F. Y. Naumkin Faculty of Science, University of Ontario Institute of Technology Oshawa, ON, L1H 7K4 (Canada) E-mail : benjamin.irving@uoit.ca ChemPhysChem 2015, 16, 233 – 242  2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 233 Articles DOI: 10.1002/cphc.201402436