Halogen bonding in mono- and dihydrated halobenzene.

Density functional theory calculations were performed on halogen-bonded and hydrogen-bonded systems consisting of a halobenzene (XPh; X = F, Cl, Br, I, and At) and one or two water molecules, using the M06-2X density functional with the 6-31+G(d) (for C, H, F, Cl, and Br) and aug-cc-pVDZ-PP (for I, At) basis sets. The counterpoise procedure was performed to counteract the effect of basis set superposition error. The results show halogen bonds form in the XPh-H2 O system when X > Cl. There is a trend toward stronger halogen bonding as the halogen group is descended, as assessed by interaction energy and X•••Ow internuclear separation (where Ow is the water oxygen). For all XPh-H2 O systems hydrogen-bonded systems exist, containing a combination of CH•••Ow and Ow Hw •••X hydrogen bonds. For all systems except X = At the X•••Hw hydrogen-bonding interaction is stronger than the X•••Ow halogen bond. In the XPh-(H2 O)2 system halogen bonds form only for X > Br. The two water molecules prefer to form a water dimer, either located around the CH bond (for X = Br, At, and I) or located above the benzene ring (for all halogens). Thus, even in the absence of competing strong interactions, halogen bonds may not form for the lighter halogens due to (1) competition from cooperative weak interactions such as CH•••O and OH•••X hydrogen bonds, or (2) if the formation of the halogen bond would preclude the formation of a water dimer. © 2018 Wiley Periodicals, Inc.

Self-Assembled Monolayers of Oligophenylenecarboxylic Acids on Silver Formed at the Liquid-Solid Interface.

A series of para-oligophenylene mono- and dicarboxylic acids (R-(C6H4)nCOOH, n = 1-3, R = H,COOH) was studied. Adsorbed on Au(111)/mica modified by an underpotential deposited bilayer of Ag, the self-assembled monolayers (SAMs) were analyzed by near-edge X-ray absorption fine structure spectroscopy, X-ray photoelectron spectroscopy, and scanning tunneling microscopy. In all cases SAMs are formed with molecules adopting an upright orientation and anchored to the substrate by a carboxylate. Except benzoic acid, all SAMs could be imaged at molecular resolution, which revealed highly crystalline layers with a dense molecular packing. The structures of the SAMs are described by a rectangular (5 × âˆš3) unit cell for the prevailing phase of the monocarboxylic acids and an oblique ([Formula: see text]) unit cell for the dicarboxylic acids, thus evidencing a pronounced influence of the second COOH moiety on the SAM structure. Density functional theory calculations suggest that hydrogen bonding between the SAM-terminating COOH moieties accounts for the difference. Contrasting other classes of SAMs, the systems studied here are determined by intermolecular interactions whereas molecule-substrate interactions play a secondary role. Thus, eliminating problems arising from the mismatch between the molecular and the substrate lattices, coordinatively bonded carboxylic acids on silver should provide considerable flexibility in the design of SAM structures.

Competition between hydrogen and halogen bonding in halogenated 1-methyluracil: Water systems.

The competition between hydrogen- and halogen-bonding interactions in complexes of 5-halogenated 1-methyluracil (XmU; X = F, Cl, Br, I, or At) with one or two water molecules in the binding region between C5-X and C4=O4 is investigated with M06-2X/6-31+G(d). In the singly-hydrated systems, the water molecule forms a hydrogen bond with C4=O4 for all halogens, whereas structures with a halogen bond between the water oxygen and C5-X exist only for X = Br, I, and At. Structures with two waters forming a bridge between C4=O and C5-X (through hydrogen- and halogen-bonding interactions) exist for all halogens except F. The absence of a halogen-bonded structure in singly-hydrated ClmU is therefore attributed to the competing hydrogen-bonding interaction with C4=O4. The halogen-bond angle in the doubly-hydrated structures (150-160°) is far from the expected linearity of halogen bonds, indicating that significantly non-linear halogen bonds may exist in complex environments with competing interactions.

New class of metal bound molecular switches involving H-tautomerism.

A potential end-point in the miniaturization of electronic devices lies in the field of molecular electronics, where molecules perform the function of single components. To date, hydrogen tautomerism in unimolecular switches has been restricted to the central macrocycle of porphyrin-type molecules. The present work reveals how H-tautomerism is the mechanism for switching in substituted quinone derivatives, a novel class of molecules with a different chemical structure. We hence reveal that the previous restrictions applying to tautomeric molecular switches bound to a surface are not valid in general. The activation energy of switching in a prototypical quinone derivative is determined using inelastic electron tunneling. Through computational modeling, we show that the mechanism underlying this process is tautomerization of protons belonging to two amino groups. This switching property is retained upon functionalization by the addition of side groups, meaning that the switch can be chemically modified to fit specific applications.