Substituent effect on structure and surface activity of N‐methylpyridinium salts studied by FT‐IR, FT‐RS, SERS and DFT calculations

Abstract

The substituent effect on structure and surface activity of mono-and disubstituted N-methylpyridinium salts was investigated by means of Raman, infrared and surface-enhanced Raman spectroscopy (SERS). The significant differences observed in Raman and infrared spectra have been correlated with marker bands assigned to in-plane and out-of-plane vibrations, respectively. This vibrational analysis, complemented by quantum chemical calculations (B3LYP/ 6-311++G(d,p)) was a basis for investigation of the surface activity of the studied compounds. Significant differences in their SERS spectra related to the enhancement mechanism and adsorbate orientation have been observed and analyzed.

SERS of the Positive Charge Bearing Pyridinium Ring Terminated Self-Assembled Monolayers: Structure and Bonding Spectral Markers

“… a Based on references ,,,− and calculations. b Calculated with the DFT method using the B3LYP functional and 6-31++G­(d,p) basis set for C, H, N, and S atoms, and LANL2DZ with ECP for Au and Ag atoms with hydrocarbon chain in trans conformation. c Wilson vibration number of the benzene ring and vibration symmetry. Abbreviations: ν, stretching; ν s , symmetric stretching; ν as , asymmetric stretching; δ, deformation; β, in-plane bending; γ, out-of-plane bending; r, rocking; t, twisting; s, strong; vs, very strong; m, middle; w, weak; sh, shoulder; p, polarized; n.d., not determined; n.a., not applicable; T, trans; G, gauche; FR, Fermi resonance. …”

“…The calculated spectra are also displayed in Figure . Assignments of the bands based on previous studies of similar compounds ,,,− and our own DFT calculations as well as experimental depolarization ratio measurements are displayed in Table .…”

“…Surface-enhanced Raman spectroscopy (SERS) is one of the most sensitive vibrational spectroscopic tools for probing the bonding, structure, and orientation of adsorbed molecules at certain nanostructured metals (mostly Au, Ag, and Cu) with molecular specificity and submonolayer sensitivity. − In particular, low frequency metal–adsorbate vibrational modes provide possibility for direct monitoring of the interaction of molecules with surfaces, − while C–C and C–S stretching vibrations provide information on ordering and gauche/trans conformation of adsorbed alkanethiols. − Previous SERS studies of molecules containing pyridinium group revealed the importance of ion-pairing at the interface. ,,,− It was demonstrated that N -methylpyridinium adsorbs at silver electrode interface in the presence of iodide ions due to formation of ion pairs . Analysis of relative intensities of ring SERS bands coupled with quantum chemical calculations and surface-selection rules revealed orientation of pyridinium ring with respect to the electrode surface. ,, Recent SERS study of N -methylpyridinium and its ring-substituted derivatives on colloidal silver surface revealed enhancement of the in-plane ring bending modes for 1-methylpyridinium chloride (1-MP), while enhancement of out-of-plane ring modes was observed in the case of adsorbed 1,4-dimethylpyridinium (1,4-DMP) chloride . Such difference was explained by more vertical ring orientation of 1-MP with respect to the electrode surface.…”

“…32 Analysis of relative intensities of ring SERS bands coupled with quantum chemical calculations and surface-selection rules revealed orientation of pyridinium ring with respect to the electrode surface. 32,33,35 Recent SERS study of N-methylpyridinium and its ring-substituted derivatives on colloidal silver surface revealed enhancement of the in-plane ring bending modes for 1-methylpyridinium chloride (1-MP), while enhancement of out-of-plane ring modes was observed in the case of adsorbed 1,4-dimethylpyridinium (1,4-DMP) chloride. 34 Such difference was explained by more vertical ring orientation of 1-MP with respect to the electrode surface.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part VIII

“…Simulations of the induced electromagnetic fields on the Pt films show the same trend as the observed Raman intensity with the feature size . Marzec and colleagues studied substituent effects on the structure and surface activity of N‐methylpyridinium salts by FT‐IR, FT‐RS, SERS, and DFT calculations . Pointurier and Marie used micro‐Raman spectrometry coupled with scanning electron microscopy for the first time to determine the chemical form of uranium compounds in micrometer‐size particles .…”