Structural variability in transition metal oxide clusters: gas phase vibrational spectroscopy of V3O(6-8)+

We present gas phase vibrational spectra of the trinuclear vanadium oxide cations V(3)O(6)(+)·He(1-4), V(3)O(7)(+)·Ar(0,1), and V(3)O(8)(+)·Ar(0,2) between 350 and 1200 cm(-1). Cluster structures are assigned based on a comparison of the experimental and simulated IR spectra. The latter are derived from B3LYP/TZVP calculations on energetically low-lying isomers identified in a rigorous search of the respective configurational space, using higher level calculations when necessary. V(3)O(7)(+) has a cage-like structure of C(3v) symmetry. Removal or addition of an O-atom results in a substantial increase in the number of energetically low-lying structural isomers. V(3)O(8)(+) also exhibits the cage motif, but with an O(2) unit replacing one of the vanadyl oxygen atoms. A chain isomer is found to be most stable for V(3)O(6)(+). The binding of the rare gas atoms to V(3)O(6-8)(+) clusters is found to be strong, up to 55 kJ/mol for Ar, and markedly isomer-dependent, resulting in two interesting effects. First, for V(3)O(7)(+)·Ar and V(3)O(8)(+)·Ar an energetic reordering of the isomers compared to the bare ion is observed, making the ring motif the most stable one. Second, different isomers bind different number of rare gas atoms. We demonstrate how both effects can be exploited to isolate and assign the contributions from multiple isomers to the vibrational spectrum. The present results exemplify the structural variability of vanadium oxide clusters, in particular, the sensitivity of their structure on small perturbations in their environment.     

Carbohydrate molecular recognition: a spectroscopic investigation of carbohydrate-aromatic interactions

The physical basis of carbohydrate molecular recognition at aromatic protein binding sites is explored by creating molecular complexes between a series of selected monosaccharides and toluene (as a truncated model for phenylalanine). They are formed at low temperatures under molecular beam conditions, and detected and characterized through mass-selected, infrared ion depletion spectroscopy-a strategy which exploits the extraordinary sensitivity of their vibrational signatures to the local hydrogen-bonded environment of their OH groups. The trial set of carbohydrates, alpha- and beta-anomers of glucose, galactose and fucose, reflects ligand fragments in naturally occurring protein-carbohydrate complexes and also allows an investigation of the effect of systematic structural changes, including the shape and extent of 'apolar' patches on the pyranose ring, removal of the OH on the exocyclic hydroxymethyl group, and removal of the aglycon. Bound complexes invariably form, establishing the general existence of intrinsic intermolecular potential minima. In most of the cases explored, comparison between recorded and computed vibrational spectra of the bound and free carbohydrates in the absence of solvent water molecules reveal that dispersion forces involving CH-pi interactions, which promote little if any distortion of the bound carbohydrate, predominate although complexes bound through specific OH-pi hydrogen-bonded interactions have also been identified. Since the complexes form at low temperatures in the absence of water, entropic contributions associated with the reorganization of surrounding water molecules, the essence of the proposed 'hydrophobic interaction', cannot contribute and other modes of binding drive the recognition of sugars by aromatic residues. Excitingly, some of the proposed structures mirror those found in naturally occurring protein-carbohydrate binding sites.     

Competitive reactions among three monomers over a catalytic surface

We studied in this work a three-monomer reaction model on one- and two-dimensional lattices. We have taken different reactivity rates among pairs of monomers and the reaction between two selected monomers was forbidden. We have employed the mean field and the pair approximation to decouple the equations of motion for the densities of single and pairs of monomers. We found the stationary states and the phase diagram of the model. We have shown that, in two dimensions and within the pair approximation, there is a first-order transition line between active and poisoned steady states.     

4-Cyanopyridine and amide-N and amide-O linkage isomers of 4-pyridinecarboxamide on trans-chloro(1,4,8,11-tetraazacyclotetradecane)ruthenium(II/III)

The synthesis, UV-vis spectra, and electrochemical behavior of the nitrile-bonded trans-[Ru(II)Cl(cyclam)(4-NCpyH(+))](BF(4))(2) (4-Ncpy = 4-cyanopyridine; cyclam = 1,4,8,11-tetraazacyclotetradecane) and of trans-[Ru(III)Cl(cyclam)(NHC(O)-4-pyH(+))](2+) are described. The UV-vis spectrum of the Ru(II) nitrile complex shows a MLCT band at 548 nm at pH 1, which is shifted to 440 nm at pH approximately 6, for the unprotonated species. trans-[Ru(II)Cl(cyclam)(4-NCpyH(+))](2+) was electrolytically oxidized (+600 mV vs Ag/AgCl) at pH 1 to Ru(III), followed by hydrolysis (k = 0.25 s(-1)) of the coordinated nitrile to give trans-[Ru(III)Cl(cyclam)(NHC(O)-4-pyH(+))](2+), in which the amide is deprotonated and coordinated through nitrogen. The identity of the species is pH dependent, the nitrogen-bonded amide prevailing at low pH (< 7), but the oxygen-bonded amide is formed through linkage isomerization at higher pH (>8). Reduction of trans-[Ru(III)Cl(cyclam)(NHC(O)-4-pyH)](2+) in acidic media does not result in fast aquation (k = approximately 2.4 x 10(-5) s(-1)) as for other amides on ruthenium(II) pentaammine, but instead linkage isomerization occurs, resulting in the oxygen-bonded species, with an estimated rate constant of approximately 2 x 10(-2) s(-1), smaller than in the pentaammine analogues.     

Solvent interactions and conformational choice in a core N-glycan segment: gas phase conformation of the central, branching trimannose unit and its singly hydrated complex

The intrinsic conformational preferences and structures of the branched trimannoside, alpha-phenyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside (which contains the same carbohydrates found in a key subunit of the core pentasaccharide in N-glycans) and its singly hydrated complex, have been investigated in the gas phase isolated at low temperature in a molecular beam expansion. Conformational assignments of their infrared ion dip spectra, based on comparisons between experiment and ONIOM (B3LYP/6-31+G(d):HF/6-31G(d)) and single-point MP2 calculations have identified their preferred structures and relative energies. The unhydrated trimannoside populates a unique structure supported by two strong, central hydrogen bonds linking the central mannose unit (CM), and its two branches (3M and 6M) closely together, through a cooperative hydrogen-bonding network: OH4(CM)-->OH6(3M)-->OH6(6M). A closely bound structure is also retained in the singly hydrated oligosaccharide, with the water molecule bridging across the 3M and 6M branches to provide additional bonding. This structure contrasts sharply with the more open, entropically favored trimannoside structure determined in aqueous solution at 298 K. In principle this structure can be accessed from the isolated trimannoside structure by a simple conformational change, a twist about the alpha(1,3) glycosidic linkage, increasing the dihedral angle psi[C1(3M)-O3(3M)-C3(CM)-C2(CM)] from approximately 74 degrees to approximately 146 degrees to enable accommodation of a water molecule at the centrally bound site occupied by the hydroxymethyl group on the 3M ring and mediation of the water-linked hydrogen-bonded network: OH4(CM) -->OH(W)-->OH6(6M). The creation of a "water pocket" motif localized at the bisecting axis of the trimannoside is strikingly similar to the structure of more complex N-glycans in water, suggesting perhaps a general role for the "bisecting" OH4 group in the central (CM) mannose unit.