Vibrational Polarization Spectroscopy of CH Stretching Modes of the Methylene Group at the Vapor/Liquid Interfaces with Sum Frequency Generation

Abstract

In this paper we report detailed examples of the surface sum frequency generation vibrational spectroscopy (SFG-VS) as a polarization spectroscopic technique for vibrational spectral band assignment and orientational analysis for molecular groups at the interfaces. Surface sum frequency generation vibrational spectroscopy (SFG-VS) has been widely used as an important spectroscopy probe for chemical bonding, structural conformation and molecular interactions of both fundamentally and technologically important interfaces. However, the potential for SFG-VS as a polarization spectroscopic technique is yet to be fully explored. In IR and Raman studies, polarized spectroscopy (PS) can provide information on molecular symmetries, which is necessary for accurate vibrational band assignment in complex chemical environments. We shall show that SFG-vibrational polarization spectroscopy (VPS) is the polarization spectroscopic tool on this purpose for the interface, along with its advantage of submonolayer sensitivity. This ability of SFG-VPS comes from the fact that vibrational bands from different symmetry types do not have strongest peak intensities in the same polarization configuration of the SFG-VPS spectra, because molecular groups are aligned or partially aligned at the interface. We chose to study the SFG-VPS of three diols, namely, ethylene glycol, 1,3-propanediol, and 1,5-pentanediol, at the vapor/liquid interfaces as model systems for the methylene-only molecules. The polarization analysis of the SFG spectra resulted in few explicit polarization selection rules or guidelines for the assignment of the CH stretching modes of the methylene groups. These results could be used to provide clarifications for some of the disagreements and controversies still exist in the literatures. These results can also shed light on the IR and Raman studies on methylene group in the bulk condensed phases.

Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

“…47 The real difficulty lies on the fact that quantitative analysis and interpretation of the SFG-VS, as well as SHG, data had been not as well developed and widely performed until recently. 25,35,48,49,50,51,52,53,54,55,56,57,58 Therefore, conclusions in many previous reports on the investigations of air/water interface, as well as other liquid interfaces, with SFG-VS are subjected to different interpretations.…”

“…The notations and the experiment geometry have been described in detail previously. 35,50 χ (2) ef f for the four generally used independent polarization combinations can be deduced from the 7 nonzero macroscopic susceptibility tensors for an achiral rotationally isotropic interface (C ∞v ). 35,50 …”

“…In addition, we also demonstrated that a set of polarization selection rules (or guidelines) in SFG-VS can be developed for vibrational spectrum assignment through symmetry analysis of the SFG-VS spectral features. 50,51 This latter approach is extremely useful for discerning complex SFG-VS spectrum with unidentified or controversial assignments. Recently, based on polarization analysis, Ostroverkhov et al demonstrated a phase-sensitive interference analysis of SFG polarization spectra of water/quartz interface.…”

“…49,50,51,52,53,54,55,56,57,58 In these works, we not only developed methodology for quantitative polarization and experimental configuration analysis in SFG-VS and SHG, we also tested accuracy and sensitivity of some of the methodology. We have applied them to elucidated the anti-parallel double layered structure and thermodynamics of some organic liquid aqueous solution interfaces.…”

Revealing structural properties of the marine nanolayer from vibrational sum frequency generation spectra

“…The observed spectral signatures with three strong CH bands are very typical for the SFG response of alkyl groups and hence indicate the presence of organic surfactants possessing lipid chains. Measured peak positions can be readily assigned to the symmetric stretch resonances of terminal methyl groups (r + mode, ≈2875 cm −1 ), the corresponding Fermi resonance with its bending mode (r FR + , ≈2950 cm −1 ), and the symmetric stretch mode of methylene groups (CH 2 ) at gauche defects (d + , ≈2850 cm −1 ) [Zhuang et al, 1999;Lu et al, 2004]. For all trans-(i.e., straight) alkyl chains, SFG intensity of CH 2 stretch vibrations is known to be nearly absent due to inner inversion symmetry of the opposing CH 2 groups.…”