Reflection Absorption Infrared Spectroscopy Characterization of SAM Formation from 8-Mercapto-N-(phenethyl)octanamide Thiols with Phe Ring and Amide Groups.

Multifunctional amide-containing self-assembled monolayers (SAMs) provide prospects for the construction of interfaces with required physicochemical properties and distinctive stability. In this study, we report the synthesis of amide-containing thiols with terminal phenylalanine (Phe) ring functionality (HS(CH2)7CONH(CH2)2C6H5) and the characterization of the formation of SAMs from these thiols on gold by reflection absorption infrared spectroscopy (RAIRS). For reliable assignments of vibrational bands, ring deuterated analogs were synthesized and studied as well. Adsorption time induced changes in Amide-II band frequency and relative intensity of Amide-II/Amide-I bands revealed two-state sigmoidal form dependence with a transition inflection points at 2.2 ± 0.5 and 4.7 ± 0.5 min, respectively. The transition from initial (disordered) to final (hydrogen-bonded, ordered) structure resulted in increased Amide-II frequency from 1548 to 1557 cm-1, which is diagnostic for a strongly hydrogen-bonded amide network in trans conformation. However, the lateral interactions between the alkyl chains were found to be somewhat reduced when compared with well-ordered alkane thiol monolayers.

Interaction of 4-imidazolemethanol with a copper electrode revealed by isotope-edited SERS and theoretical modeling.

Adsorption of 4-imidazolemethanol (ImMeOH) on a copper electrode has been investigated by in situ isotope-edited (H/D and (63)Cu/(65)Cu) surface enhanced Raman spectroscopy (SERS) in aqueous solutions at physiological pH (7.0) in a potential window from -0.500 to -1.100 V. Theoretical modeling by DFT calculations at the B3LYP/6-311++G(d,p) level for light atoms and LANL2DZ with ECP for copper atoms have been employed for the interpretation of experimental data. The copper surface was modeled by a cluster of 6 atoms. It was found that the imidazole ring adopts Tautomer-I form in the adsorbed state and coordinates with the Cu surface through the N3 atom. Linear potential-dependence of ν(C4=C5) mode with the slope of (15 ± 1) cm(-1) V(-1) was experimentally observed. The imidazole ring mode near 1492 cm(-1) primarily due to ν(C2-N3) + ß(C2H) vibration has also showed a considerable decrease in frequency at more negative electrode potentials with the slope of (9 ± 2) cm(-1) V(-1). Both modes can be used as sensitive probes for analysis of interaction of the imidazole ring with the metal surface. In agreement with experimental data theoretical modeling has predicted higher stability of surface bound Tautomer-I compared with Tautomer-II. The formation of a covalent bond between the metal and adsorbate was experimentally evidenced by metal isotopic ((63)Cu/(65)Cu) frequency shift of ν(Cu-N) mode at 222 cm(-1), combined with theoretical modeling of the surface complex.

Time-temperature superposition for kinetic mapping of solventless autocatalytic addition of diisocyanates and macrodiols.

The migration of chemicals from polyurethane (PUR) is a concern in many applications, such as adhesives for food packaging. Low molecular weight catalysts, which are prone to migration, need to be eliminated from PUR, in particular those containing Sn or other metals. This is difficult partly due to many uncertainties of autocatalytic polyaddition between isocyanates and polyols. Hexamethylene and tolylene diisocyanates, HDI and TDI, are often reacted with macrodiols to produce prepolymers for PUR. This study measures isocyanate contents during the polyaddition of HDI and TDI with excess macrodiols. Ester-based macrodiols were reacted between 60 °C and 90 °C using 1 : 0.3 and 1 : 0.5 molar ratios to form OH-terminated prepolymers. Time-temperature superposition (TTS) was used to process the values of unreacted isocyanate fractions from several temperatures. Presumed activation energies and kinetic data scatter implied that polymerization has a distinct initial phase of conversion of unreacted diisocyanate into monofunctional adducts by addition to macrodiols, followed by more complex processes. Utilization of the activation energy from the initial phase and TTS application might allow the prediction of kinetic trends without the need for a large volume of accurate data. Such kinetic mapping should be useful for developing catalyst-free PUR with low levels of migrating chemicals.

SERS observation of soft C-H vibrational mode of bifunctional alkanethiol molecules adsorbed at Au and Ag electrodes.

An unusually low frequency (near 2820 cm(-1)) C-H stretching band of methylene groups was detected using the SERS technique at the initial stage of formation of self-assembled monolayers on Au and Ag substrates from bifunctional alkanethiol molecules containing indole and phenol ring terminal groups. Based on the sensitivity of the band parameters to the nature of the substrate, potential and temperature, the observed band was attributed to the vibration of -CH(2) groups directly contacting the metal surface.

Infrared and Raman bands of phytantriol as markers of hydrogen bonding and interchain interaction.

Hydrogen bonding and interchain interactions in phytantriol, 3,7,11,15-tetramethyl-1,2,3-hexadecanetriol, have been studied by Fourier transform infrared (FT-IR) and Raman spectroscopies. Assignments of the bands were performed based on the OH/OD isotopic substitution, molecular modeling, and measurements of polarized Raman spectra. Marker bands were evaluated from the temperature-dependent spectral changes. It is shown that Raman spectroscopy provides sensitive markers, namely I(delta(CH2))/I(deltas(CH3)), tau(CH)2, I(nus(CH3)(FR))/I(nus(CH2)), and nus(CH2) for probing the interactions between the hydrocarbon chains. Hydrogen bonding interaction might be studied through the difference Raman spectroscopy by the analysis of polarized band at 811 cm-1. Relationship is found between the frequencies of IR bands at 883-873 and 1097-1086 cm-1, associated with the vibrations localized at the primary COH site, and the frequencies of OH stretching mode, making these bands specific markers in the analysis of hydrogen bonding. Evaluated marker bands may be of utility to probe the interchain and hydrogen bonding interaction of phytantriol with guest molecules in the practically important aqueous liquid-crystalline phases of this lipid.

Spectroscopic and structural investigations of iron(III) isothiocyanates. A comparative theoretical and experimental study.

A combined experimental and theoretical study on the molecular structure and vibrational spectra of [Fe(NCS)](2+) complex in the aqueous solution at the pH∼2 ± 0.1 have been performed. Experimental Raman spectra of the iron(III) isothiocyanate with higher coordination number in the acidic aqueous solution have been analyzed. Molecular modeling of the iron(III) monoisothiocyanate complex was accomplished by the density functional theory (DFT) method using B3LYP and PBE1PBE functionals. Theoretical vibrational spectra of the iron(III) monoisothiocyanate were interpreted by means of the potential energy distributions (PEDs). The influence of different solvation models and position of SO4(2)(-) ligand vs. NCS(-) ligand upon its geometry and vibrational frequencies have been evaluated. The effect of H2O/D2O isotopic substitution on the experimental and calculated Raman spectra of iron(III) isothiocyanates has been examined. Procedures of Raman spectra subtraction have been applied for the extractions of weak and/or obscured Raman signals. As a result, the presence of bound SO4(2)(-) ion and water molecules in the first coordination sphere in the acidic aqueous iron(III) isothiocyanate solution was confirmed. The vibrational assignments for the investigated iron(III) isothiocyanates were proposed here for the first time.

Molecular structure of mercury(II) thiocyanate complexes based on DFT calculations and experimental UV-electron spectroscopy and Raman studies.

In this work, we report a combined experimental and theoretical study on molecular structure, vibrational and electronic spectra of [Hg(SCN)n](2-)(n) complexes (where n=2, 3, 4) in the aqueous solution. Molecular modeling of the mercury(II) complexes were done by the density functional theory (DFT) method using B3LYP functional with Stuttgart relativistic ECP 78MWB basis set for Hg and 6-311++G(d,p) basis set for all other atoms. The effect of different solvation models with explicit (ligand) and/or implicit water environment upon its geometry, vibrational frequencies and UV spectrum have been studied. The influence of H2O/D2O exchange on the experimental and calculated vibrational frequencies of studied complexes has been established. The double-peak character of the νHgS vibrational mode of the all analyzed mercury complexes and νCN mode of [Hg(SCN)3H2O](-) complex, respectively, were proposed here for the first time. The formation of four-coordinated Hg(II) complexes with thiocyanate and (or) water ligands was verified.

Raman spectroelectrochemical study of Meldola blue, adsorbed and electropolymerized at a gold electrode.

Surface-enhanced Raman spectroscopy (SERS) was used to probe the structure of adsorbed and electropolymerized Meldola blue (MB) films on roughened gold surfaces in solutions with pH 1.0 and 7.0 by using 785 nm excitation wavelength. Spectral bands were assigned based on density functional theory (DFT) calculations at B3LYP/6-311+G(2d, p) level. The most characteristic band of the oxidized MB form was found to be the breathing vibration of the central ring containing heteroatoms at 596 cm(-1). Based on a red shift of bands assigned to vibrations of double C=N(C(2)H(6)) bonds and adjacent ring C=C bonds in surface spectra as compared with solution 1 it was suggested that polymerization and interaction with an electrode surface proceed through these moieties. The presence of out-of-plane bands in SERS spectra was attributed to "flat" or slightly "tilted" orientation of aromatic rings at the interface. Potential-dependent spectral changes were followed by SERS spectroscopy. Raman spectra of the reduced MB form were obtained in both pH 1.0 and pH 7.0 solutions by analysis of the potential-difference SERS spectra. Reduced MB form can be recognized by characteristic bands near 1620, 1574, 1374, and 1234 cm(-1). By comparing the intensities of 1637 cm(-1) (oxidized MB form) and 1374 cm(-1) (reduced MB form) bands in experimental spectra of polymerized MB in pH 1.0 solution, a reduction-induced decrease by factor of 7 was estimated. A similar tendency in intensity changes showed calculations indicating that this effect is associated with reduction-induced changes in the molecular structure of the dye.

A study of cysteamine ionization in solution by Raman spectroscopy and theoretical modeling.

Different cysteamine (H2N-CH2-CH2-SH) ionization forms have been studied by polarized Raman spectroscopy in solutions prepared with H2O and D2O and by DFT calculations at the B3LYP/6-31++G(d,p) level. To account for solvation effects, we employed the integral equation formalism polarizable continuum model (IEFPCM) option and explicit water molecules. Calculated relative energies and Raman spectra revealed that gauche rotamers around the C-C bond are the most stable conformers in solution. The experimental pKa values and Raman spectra of various ionization forms were best predicted by using a model with three explicit water molecules and the IEFPCM option. In general, the use of IEFPCM tends to lower the calculated frequencies for a few bands, but in some cases (S-H stretching mode) this effect is expressed very strongly. Potential energy distribution (PED) analysis of gauche conformers of various cysteamine ionization forms provided the possibility of discriminating spectroscopically methylene groups adjacent to sulfur, (CH2)S, and nitrogen, (CH2)N, sites. In general, stretching and scissoring modes as well as wagging and twisting vibrations of the (CH2)N group were found to be at higher frequencies. The influence of ionization of SH and NH2 groups on the vibrational spectrum is discussed, and Raman markers for further amine group ionization studies are suggested.