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.

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.

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.