OH Dimer Research Articles

A Study of C=O…HO and OH…OH (Dimer, Trimer, and Oligomer) Hydrogen Bonding in a Poly(4-vinylphenol) 30%/Poly(methyl methacrylate) 70% Blend and its Thermal Behavior Using Near-Infrared Spectroscopy and Infrared Spectroscopy.

Inter- and intramolecular hydrogen bonding and their temperature-dependent changes in a poly(4-vinylphenol)/poly(methyl methacrylate)(PVPh 30%/PMMA 70%) blend were investigated using near-infrared (NIR) and infrared (IR) spectroscopy. Band assignments of the fundamentals and first overtones of the OH stretching mode of a free OH group and OH groups in C=O···HO and OH···OH (dimer, trimer, and oligomer) hydrogen bonding of PVPh 30%/PMMA 70% were carried out by comparison between its NIR and IR spectra and comparison with NIR and IR spectra of phenol. The comparison of the NIR spectra of the PVPh 30%/PMMA 70% blend (hereafter, we denote it as PVPh30%) with the corresponding IR spectra reveals that to observe bands arising from the free OH and OH···OH dimer, which is a weaker hydrogen bonding, NIR is better while to investigate bands originating from OH groups in the OH···O=C and OH···OH (oligomer) hydrogen bonds, which are stronger hydrogen bonding, IR is better. Thus, a combination of IR and NIR spectroscopy has provided convincing results for the hydrogen bonding of PVPh30%. The relative intensity of the two bands at 7058 and 6921 cm-1 (I7058/I6921) due to the first overtones of the OH stretching modes of the free OH group and the OH group in the dimer, respectively, increases significantly above 90 °C, which is close to Tg of PVPh. In concomitance with the intensity increase in the relative intensity of the free OH band, the intensity of a band at 1706 cm-1 due to the C=O stretching mode of the C=O···HO hydrogen bond of PVPh30% decreases above 90°C. These results suggest that above the Tg of PVPh the C=O···HO hydrogen bond is broken gradually and that the free OH increases. Of note is that below Tg the intensities of NIR bands due to the OH first overtones of free OH group and OH groups in the OH···OH dimer gain intensity in parallel with temperature increase, and above Tg the intensity of the band derived from the OH···OH group increases linearly much slower than that of the band due to the free OH. Moreover, a band due to an OH···OH oligomer decreases linearly. Hence, it is very likely that the OH···OH oligomers dissociate into free OH groups. Anharmonicity of O-H bonds, which is sensitive to a hydrogen bond, was estimated for the free OH and OH bonds in the C=O···HO and OH···OH (dimer, trimer, and oligomer) hydrogen bonding by comparison between the NIR and IR spectra in the OH stretching band regions.

Synergetic mediation of reduced graphene oxide and Cu(II) on the oxidation of 2-naphthol in water

Reduced graphene oxide (rGO) is one of the most widely used carbon nanomaterials. When it is released into the environment, rGO can markedly affect the transformation of many pollutants, and change their fate and risk. In this work, the synergetic effects of rGO and Cu(II) on the oxidation of 2-naphthol were examined in water in the dark. It was found that the coexistence of rGO and Cu(II) significantly promoted the oxidation of 2-naphthol. Corresponding products were identified as the coupling oligomers of 2-naphthol (dimer, trimer and tetramer) and hydroxylated compounds (OH-2-naphthol, OH-dimer, di–OH–dimer and naphthoquinone derivatives). In the oxidation reaction, rGO played dual roles, i.e. adsorbent and electron-transfer mediator. rGO firstly adsorbed Cu(II) and 2-naphthol on its surface, and then transferred electrons from 2-naphthol to Cu(II) to yield 2-naphthol radicals and Cu(I). 2-Naphthol radicals coupled to each other to form different oligomers of 2-naphthol. Cu(I) was re-oxidized back to Cu(II) by dissolved oxygen, which sustained the continuous oxidation of 2-naphthol. During the autoxidation of Cu(I), reactive oxygen species were generated, which further reacted with 2-naphthol to form hydroxylated products. These findings provide new insights into the risk assessment of rGO and 2-naphthol in aquatic environments.

Wigner‐Smith delay times and the non‐Hermitian Hamiltonian for the HOCl molecule

AbstractWe construct the scattering matrix for a two‐dimensional model of a Cl atom scattering from an OH dimer. We show that the scattering matrix can be written in terms of a non‐Hermitian Hamiltonian whose complex energy eigenvalues can be used to compute Wigner‐Smith delay times for the Cl‐OH scattering process. We compute the delay times for a range of energies, and show that the scattering states with the longest delay times are strongly influenced by unstable periodic orbits in the classical dynamics.

Water-hydroxyl phases on an open metal surface: breaking the ice rules

Hydroxyl is a key reaction intermediate in many surface catalyzed redox reactions, yet establishing the phase diagram for water/hydroxyl adsorption on metal surfaces remains a considerable challenge for interfacial chemistry. While the structures formed on close packed metal surfaces have been discussed widely, the phase diagram on more reactive, open metal surfaces, is complex and the H-bonding structures are largely unknown. Based on scanning tunnelling microscopy and density functional theory calculations, we report the phase diagram for water/hydroxyl on Cu(110), providing a complete molecular description of the complex hydrogen bonding structures formed. Three distinct phases are observed as the temperature is decreased and the water/hydroxyl ratio increased: pure OH dimers, extended 1H2O:1OH chains, aligned along the close-packed Cu rows, and finally a distorted 2D hexagonal c(2 × 2) 2H2O:1OH network. None of these phases obey the conventional ‘ice rules’, instead their structures can be understood based on weak H donation by hydroxyl, which favours H-bonding structures dominated by water donation to hydroxyl, and competition between hydroxyl adsorption sites. Hydroxyl binds in the Cu bridge site in the 1D chain structures, but is displaced to the atop site in the 2D network in order to accommodate water in its preferred atop binding geometry. The adsorption site and stability of hydroxyl can therefore be tuned simply by changing the surface temperature and water content, giving a new insight as to how the open metal template influences the water/hydroxyl structures formed and the activity of hydroxyl.

Structure of propanoic acid dimers as studied by means of MIR and FIR spectroscopy

Infrared spectra of propanoic acid isolated in the neon and argon matrixes were obtained and studied with the aid of the DFT calculations. The geometric structures of the cyclic and open dimers of PA were elucidated. The calculations predict that among various types of the open dimers only one type with additional hydrogen bond between oxygen of hydroxyl group of a monomer and hydrogen of hydroxyl group of other monomer (OH dimer) is energetically favorable. Structure of this dimer differs from that of formic and acetic acids, where open dimers with the hydrogen bond between oxygen of hydroxyl group and hydrogen of terminal methylene group (CH dimer) are formed. This finding explains fact why formic and acetic acids in crystalline state form polymer chains while higher member of homological row of the monocarboxylic acids (starting from propanoic) acid crystallizes as cyclic dimers. Due to the low barrier between the open and cyclic dimers of propanoic acid the open dimer is experimentally observed only in the neon matrix formed by rapid freezing of the gaseous matrix mixture from 300 K down to 2.8 K. Similar structures were not observed in the conventional 15 K argon matrix.

Interaction energy and the shift in OH stretch frequency on hydrogen bonding for the H2O → H2O, CH3OH → H2O, and H2O → CH3OH dimers

The ability to use calculated OH frequencies to assign experimentally observed peaks in hydrogen bonded systems hinges on the accuracy of the calculation. Here we test the ability of several commonly employed model chemistries--HF, MP2, and several density functionals paired with the 6-31+G(d) and 6-311++G(d,p) basis sets--to calculate the interaction energy (D(e)) and shift in OH stretch fundamental frequency on dimerization (delta(nu)) for the H(2)O --> H(2)O, CH(3)OH --> H(2)O, and H(2)O --> CH(3)OH dimers (where for X --> Y, X is the hydrogen bond donor and Y the acceptor). We quantify the error in D(e) and delta(nu) by comparison to experiment and high level calculation and, using a simple model, evaluate how error in D(e) propagates to delta(nu). We find that B3LYP and MPWB1K perform best of the density functional methods studied, that their accuracy in calculating delta(nu) is approximately 30-50 cm(-1) and that correcting for error in D(e) does little to heighten agreement between the calculated and experimental delta(nu). Accuracy of calculated delta(nu) is also shown to vary as a function of hydrogen bond donor: while the PBE and TPSS functionals perform best in the calculation of delta(nu) for the CH(3)OH --> H(2)O dimer their performance is relatively poor in describing H(2)O --> H(2)O and H(2)O --> CH(3)OH.

Quantum Chemical Semiempirical Approach to the Structural and Thermodynamic Characteristics of Fluoroalkanols at the Air/Water Interface

In the framework of quantum chemical PM3 approximation, the geometrical structure and thermodynamic functions characteristics of the formation of monomers (n = 1-14, 34), dimers (n = 1-14, 34), and trimers and tetramers (n = 1-8) of fluoroalkanols with the composition C(n)F(2)(n+1)CH(2)CH(2)OH are calculated. It is shown that, in contrast to the fatty alcohols, which have a flat zigzag structure, the fluoroalkanol monomers are helical with an average backbone torsion angle equal to 162 degrees. For the minimum-energy structure of dimers, the self-organization of the molecules in a dimer was observed; that leads to an opposite alternation of the torsion angles corresponding to the matching atoms in the two molecules that form the dimer. This results in the fact that the most stable conformation of the dimer is the double helix. The lead (39.5 A) and diameter (7.3 A) of the double helix are determined from the calculations of C(34)F(69)CH(2)CH(2)OH dimers. Enthalpy, entropy, and Gibbs energy of the clusterization are shown to be linearly dependent on the length of the fluorinated chain. From the analysis of these thermodynamic quantities, it is concluded that dimerization of fluoroalkanols at the air/water interface takes place if the hydrocarbon link number exceeds 6, whereas for ordinary alcohols this characteristic number is 11. These calculated values agree with experimental data. The additive scheme for the evaluation of the clusterization free energies for arbitrary clusters is developed and applied to obtain the estimate of the Gibbs clusterization energy for infinitely large clusters.

Theory of the clean and hydrogenatedAl2O3(0001)−(1×1)surfaces

We present the results of a first principles investigation of the equilibrium properties of c-plane $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ surfaces. The stable structure for the $1\ifmmode\times\else\texttimes\fi{}1$ clean surface is Al terminated with a stoichiometric composition, while other terminations are unstable independent of surface preparation conditions. We discuss the implications of our results in the frame of possible extended reconstructions. For 1 monolayer of H coverage, we find that the preferred structure has OH dimers both perpendicular and nearly parallel to the surface. H-terminated surfaces may form in suitable preparation conditions. We discuss our results in terms of water adsorption and atomic layer epitaxy of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001).$

Electrochemistry and surface-enhanced Raman spectroscopy (SERS) of adsorbed chromium–bipyridyl complexes†

A series of bis-bipyridyl complexes, cis-[CrIII(bpy)2(L)(L′)] (L = OH, L′ = OH2 monomer; L = L = OH dimer; L = L = Cl; and L, L′ = polyacrylic acid) were studied at silver and glassy carbon electrodes by cyclic voltammetry. On reduction, slow electron transfer reactions giving adsorbed species are evident from the cyclic voltammograms. In situ surface-enhanced Raman spectroscopy (SERS) of the aquahydroxo and dimer complexes shows a broad, intense Cr–O vibration at around 550 cm–1. This arises as the exciting line (514.5 nm) is close to resonance with the lowest energy absorption band assigned to a ligand field transition (4A2g → 4T2g). At –0.4 V SERS spectra show features of both the normal Raman spectrum of [Cr(bpy)3]3+ and the colloidal SERS of [Cr(bpy)3]2+ and can be explained by the presence of partially-reduced adsorbed species. The electron is localized first on to the metal and then on to one of the bpy ligands as follows: [CrIII(bpy0)2X2]n+ + e →[CrII(bpy0)2X2](n–1)+ and [CrII(bpy0)2X2](n–1)+ + e →[CrII(bpy–)(bpy0)2X2](n–2)+. Although the SERS evidence suggested that the integrity of the complexes is maintained during the reaction, prolonged electrolysis leads to hydrolysis and/or substitution of the complexes. Thus [(PVP)2-Cr(bpy)2]3+ can be electrosynthesized by reduction of cis-[Cr(bpy)Cl2]+ at a PVP modified glassy carbon electrode and shows a characteristic peak at –0.32 V.

Synthesis and Structural Studies of Metal Complexes of the Biological Ligand 2-Quinaldic Acid: Utilization of the Polymer Pendant Analog PS-2-QA for Selective Aluminum Ion Removal from Aqueous Solution.

The synthetic reactions of quinaldic acid (2-QA, 1), a ligand with potential implications in Al(3+) ion biological transport and in pharmaceutical applications and of use for the removal and recovery of Al(3+) ions from environmental waste sites, were studied with tri- and divalent metal ions that encompass Al(3+), Fe(3+), Ga(3+), Zn(2+), Ni(2+), Mn(2+), and Co(2+). The Al(3+), Fe(3+), and Ga(3+) metal ion complexes, 2-4, of 1 were characterized by FT-IR, FAB/MS, NMR, and elemental analysis and provided the following structural formula with 1 of 2-QA(4)M(2)(&mgr;-OH)(2).S, where S = H(2)O or pyridine. In the case of the Ga(3+ )analog, 4.Py, the unequivocal &mgr;-OH dimer structure was determined by single-crystal X-ray analysis [space group, P1; a = 13.387(3) Å; b = 14.016(2) Å; c = 14.549(2) Å; alpha = 87.74(2) degrees; beta = 73.44(2) degrees; gamma = 82.61(2) degrees; Z = 2; volume = 2592.6 Å(3)]. A description of the X-ray crystal structure of 2-QA(4)Ga(2)(&mgr;-OH)(2).4 pyridine, 4.Py, will also be presented. The corresponding bis(2-QA) metal complexes of Zn(2+), Ni(2+), Mn(2+), and Co(2+), 5-8, were also studied and all provided a formula of 2-QA(2)M.1.5 H(2)O, which were also characterized by many of the above-mentioned spectroscopic techniques. The polymer-supported (PS) version of 1, PS-2-QA, bonded to modified, macroporous 6% cross-linked polystyrene-divinylbenzene beads, was synthesized by an electrophilic substitution reaction on the aromatic ring of a 2-QA derivative, ethyl 2-quinaldate, with the chloromethylated precursor, PS-CH(2)Cl, followed by subsequent ester hydroysis to the free PS-2-QA ligand. The PS-2-QA ligand was found to selectively remove Al(3+) ions from aqueous acidic solution (pH = 3-5) in the presence of other divalent metal ions, namely, Cu(2+), Zn(2+), Ni(2+), Mn(2+), and Co(2+) as well as a trivalent metal ion, Cr(3+).

Mechanisms of degradation of 2′-5′ oligoadenylates

We have studied the mechanisms of breakdown of 2′-5′ oligoadenylates. We monitored the time-courses of degradation of ppp(A2′p5′) nA (dimer to tetramer) and of 5′OH-(A2′p5′) nA (dimer to pentamer) in unfractionated L1210 cell extract. The 5′ triphosphorylated 2′-5′ oligoadenylates are converted by a phosphatase activity. However, 2′-5′ oligoadenylates are degraded mainly by phosphodiesterase activity which splits the 2′-5′ phosphodiester bond sequentially at the 2′ end to yield 5′ AMP and one-unit-shorter oligomers. The nonlinear least-squares curve-fitting program CONSAM was used to fit these kinetics and to determine the degradation rate constant of each oligomer. Trimers and tetramers, whether 5′ triphosphorylated or not, are degraded at the same rate, whereas 5′ triphosphorylated dimer is rapidly hydrolyzed and 5′-OH dimer is the most stable oligomer. The interaction between degradation enzymes and the substrate strongly depends on the presence of a 5′ phosphate group in the vicinity of the phosphodiester bond to be hydrolyzed; indeed, when this 5′ phosphate group is present, as in pp/pA2′p5′ A/or A2′/p5′ A2′p5′ A/, affinity is high and maximal velocity is low. Such a degradation pattern can control the concentration of 2′-5′ oligoadenylates active on RNAse L either by limiting their synthesis (5′ triphosphorylated dimer is the primer necessary for the formation of longer oligomers) and/or by converting them into inhibitory ( e.g., monophosphorylated trimer) or inactive ( e.g., nonphosphorylated oligomers) molecules.