Knowledge of the ratios between different polarizability betai'j'k' tensor elements of a chemical group in a molecule is crucial for quantitative interpretation and polarization analysis of its sum frequency generation vibrational spectroscopy (SFG-VS) spectrum at interface. The bond additivity model (BAM) or the hyperpolarizability derivative model along with experimentally obtained Raman depolarization ratios has been widely used to obtain such tensor ratios for the CH3, CH2, and CH groups. Successfully, such treatment can quantitatively reproduce the intensity polarization dependence in SFG-VS spectra for the symmetric (SS) and asymmetric (AS) stretching modes of CH3 and CH2 groups, respectively. However, the relative intensities between the SS and AS modes usually do not agree with each other within this model even for some of the simplest molecular systems, such as the air/methanol interface. This fact certainly has cast uncertainties on the effectiveness and conclusions based on the BAM. One of such examples is that the AS mode of CH3 group has never been observed in SFG-VS spectra from the air/methanol interface, while this AS mode is usually very strong for SFG-VS spectra from the air/ethanol interface, other short chain alcohol, as well as long chain surfactants. In order to answer these questions, an empirical approach from known Raman and IR spectra is used to make corrections to the BAM. With the corrected ratios between the betai'j'k' tensor elements of the SS and AS modes, all features in the SFG-VS spectra of the air/methanol and air/ethanol interfaces can be quantitatively interpreted. This empirical approach not only provides new understandings of the effectiveness and limitations of the bond additivity model but also provides a practical way for its application in SFG-VS studies of molecular interfaces.
Azido-modified aromatic amino acids have been used as powerful infrared probes for the site-specific detection of proteins because of their large transition dipole strengths. However, their complex absorption profiles hinder their wider application. The complicated absorption profile of 4-azido-l-phenylalanine (pNPhe) in isopropanol was identified and attributed to accidental Fermi resonances (FRs) by means of linear absorption and two-dimensional (2D) IR spectroscopies. The 2D IR results of pNPhe in HO and DO further demonstrate that the FRs are distinctively influenced by the hydrogen-bonding environment. Under the influence of FRs, the 2D IR shape is distorted, indicating that pNPhe is not a good candidate in spectral diffusion studies. A three-state model and first-principles calculations were used to analyze unperturbed energy levels, unveiling the FRs between the azide asymmetric stretching band and two combination bands. Furthermore, the anharmonic frequency calculations suggest that changing the substitution position of the azide group from para to meta can effectively modulate the FRs by reducing the coupling strength. This work provides a deep understanding of the FRs in azido-modified aromatic amino acids and sheds light on the modification of azido-modified amino acids for wider utilization as vibrational probes.
Recently, Ag-Ag2S hybrid nanostructures have attracted a great deal of attention due to their enhanced chemical and thermal stability, in addition to their morphology- and composition-dependent tunable local surface plasmon resonances. Although Ag-Ag2S nanostructures can be synthesized via sulfidation of as-prepared anisotropic Ag nanoparticles, this process is poorly understood, often leading to materials with anomalous compositions, sizes, and shapes and, consequently, optical properties. In this work, we use theory and experiment to investigate the structural and plasmonic evolution of Ag-Ag2S nanoprisms during the sulfidation of Ag precursors. The previously observed red-shifted extinction of the Ag-Ag2S hybrid nanoprism as sulfidation occurs contradicts theoretical predictions, indicating that the reaction does not just occur at the prism tips as previously speculated. Our experiments show that sulfidation can induce either blue or red shifts in the extinction of the dipole plasmon mode, depending on reaction conditions. By elucidating the correlation with the final structure and morphology of the synthesized Ag-Ag2S nanoprisms, we find that, depending on the reaction conditions, sulfidation occurs on the prism tips and/or the (111) surfaces, leading to a core(Ag)-anisotropic shell(Ag2S) prism nanostructure. Additionally, we demonstrate that the direction of the shift in the dipole plasmon is a function of the relative amounts of Ag2S at the prism tips and Ag2S shell thickness around the prism.
Laboratory low-pressure cold plasma discharges, which are used to simulate some hot core environments in the star-forming region, have been investigated by employing single-photon vacuum ultraviolet ( VUV) photoionization mass spectrometry. Enols with two to four carbon atoms were detected in plasma discharges of alcohols, indicating that enols can result from alcohol destruction induced by ultraviolet and cosmic radiation and accelerated electrons that are abundant in the interstellar medium. This observation, together with the detection of ethenol toward Sgr B2, suggests that larger enols, such as propenols and butenols, should be in the search list of potential molecular species to be identified in interstellar space. The laboratory experiment presented here shows that VUV photoionization sampling of plasma discharges is a valuable method for guiding the search for new interstellar molecules and helping to understand the transformation mechanism of molecular species of astrochemical significance.
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