An improved algorithm has been designed to characterize ligand interactions in organometallic and coordination complexes in terms of the percentage of the metal coordination sphere shielded by a given ligand. The computations for ligand solid angles are performed numerically and employ introduced atomic radii that are larger than covalent but smaller than van der Waals radii. This approach enables facile evaluation of steric congestion in the metal coordination sphere, quantification of unfavorable interligand contacts, and in some cases prediction of the complex composition or ligand coordination on purely geometrical grounds.

Peptoids, or oligomers of N-substituted glycines, are a class of foldamers that have shown extraordinary functional potential since their inception nearly two decades ago. However, the generation of well-defined peptoid secondary structures remains a difficult task. This challenge is due, in part, to the lack of a thorough understanding of peptoid sequence-structure relationships and consequently, an incomplete understanding of the peptoid folding process. We seek to delineate sequence-structure relationships through the systematic study of noncovalent interactions in peptoids and the design of novel amide side chains capable of such interactions. Herein, we report the synthesis and detailed structural analysis of a series of (S)-N-(1-naphthylethyl)glycine (Ns1npe) peptoid homooligomers by X-ray crystallography, NMR and circular dichroism (CD) spectroscopy. Four of these peptoids were found to adopt well-defined structures in the solid state, with dihedral angles similar to those observed in polyproline type I (PPI) peptide helices and in peptoids with α-chiral side chains. The X-ray crystal structure of a representative Ns1npe tetramer revealed an all cis-amide helix, with approximately three residues per turn, and a helical pitch of approximately 6.0 Å. 2D-NMR analysis of the length-dependent Ns1npe series showed that these peptoids have very high overall backbone amide Kcis/trans values in acetonitrile, indicative of conformationally homogeneous structures in solution. Additionally, CD spectroscopy studies of the Ns1npe homooligomers in acetonitrile and methanol revealed a striking length-dependent increase in ellipticity per amide. These Ns1npe helices represent the most robust peptoid helices to be reported, and the incorporation of (S)-N-(1-naphthylethyl)glycines provides a new approach for the generation of stable helical structure in this important class of foldamers.

Carbonyl–carbonyl interactions between adjacent backbone amides have been implicated in the conformational stability of proteins. By combining experimental and computational approaches, we show that relevant amidic carbonyl groups associate through an n→π* donor–acceptor interaction with an energy of at least 0.27 kcal/mol. The n→π* interaction between two thioamides is 3-fold stronger than between two oxoamides due to increased overlap and reduced energy difference between the donor and acceptor orbitals. This result suggests that backbone thioamide incorporation could stabilize protein structures. Finally, we demonstrate that intimate carbonyl interactions are described more completely as donor–acceptor orbital interactions rather than dipole–dipole interactions.

With six polymorphs coexisting at room temperature, 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY) is the top system in the current Cambridge Structural Database (Feb. 2005) for the number of polymorphs of solved crystal structures. Here we report two new ROY polymorphs, Y04 and YT04, and the crystal structure of YT04. Y04 is a metastable polymorph that tends to crystallize first from a melt at room temperature, and YT04 is a product of solid-state transformation of Y04. Despite its late discovery, YT04 is the densest among the polymorphs at 25 degrees C and likely the second most stable at 0 K. The conformation of ROY in YT04 is similar to those in the other two yellow polymorphs (Y and YN) but significantly different from those in the orange and red colored polymorphs (ON, OP, ORP, and R). Having escaped years of solution crystallization in several laboratories, Y04 and YT04 exemplify polymorphs that are likely missed by solvent-based screening and discovered through alternative routes.

The stability and formation of a perovskite structure is dictated by the Goldschmidt tolerance factor as a general geometric guideline. The tolerance factor has limited the choice of cations (A) in 3D lead iodide perovskites (APbI 3 ), an intriguing class of semiconductors for high-performance photovoltaics and optoelectronics. Here, we show the tolerance factor requirement is relaxed in 2D Ruddlesden–Popper (RP) perovskites, enabling the incorporation of a variety of larger cations beyond the methylammonium (MA), formamidinium, and cesium ions in the lead iodide perovskite cages for the first time. This is unequivocally confirmed with the single-crystal X-ray structure of newly synthesized guanidinium (GA)-based ( n -C 6 H 13 NH 3 ) 2 (GA)Pb 2 I 7 , which exhibits significantly enlarged and distorted perovskite cage containing sterically constrained GA cation. Structural comparison with ( n -C 6 H 13 NH 3 ) 2 (MA)Pb 2 I 7 reveals that the structural stabilization originates from the mitigation of strain accumulation and self-adjustable strain-balancing in 2D RP structures. Furthermore, spectroscopic studies show a large A cation significantly influences carrier dynamics and exciton–phonon interactions through modulating the inorganic sublattice. These results enrich the diverse families of perovskite materials, provide new insights into the mechanistic role of A-site cations on their physical properties, and have implications to solar device studies using engineered perovskite thin films incorporating such large organic cations.