Vibrational Sum Frequency Generation Spectra Research Articles

Uptake and Surface Reaction of Methanol by Sulfuric Acid Solutions Investigated by Vibrational Sum Frequency Generation and Raman Spectroscopies

The uptake of methanol at the air-liquid interface of 0-96.5 wt % sulfuric acid (H2SO4) solutions has been observed directly using vibrational sum frequency generation (VSFG) spectroscopy. As the concentration of H2SO4 increases, the VSFG spectra reveal a surface reaction between methanol and H2SO4 to form methyl hydrogen sulfate. The surface is saturated with the methyl species after 15 min. The uptake of methyl species into the solutions by Raman spectroscopy was also observed and occurred on a much longer time scale. This suggests that uptake of methanol by sulfuric acid solutions is diffusion-limited.

Surface and bulk structure of poly-(lactic acid) films studied by vibrational sum frequency generation spectroscopy

The surface and bulk structure of various poly-(lactic acid) (PLA) films were examined by vibrational sum frequency generation (VSFG). In contrast to linear infrared spectroscopy, the VSFG spectra show significantly different characteristics in the CH stretching region. For crystalline PLLA we observed strong surface and bulk signals, which were caused by both the order and the symmetry of the space group. Amorphous PLLA and racemically composed PDLLA were found to consist of a relatively disordered bulk. Contrary to the bulk, all surfaces studied generated VSFG signal caused by the broken symmetry at the polymer/air interface.

Making “Sense” of DNA

Polarization-resolved vibrational sum frequency generation spectra of glass surfaces functionalized with single-stranded and duplex DNA oligonucleotides are presented. These spectra yield detailed conformational information of surface-bound oligonucleotides, including the chirality of the stereogenic centers and the chirality of the duplex structure.

Vibrational Sum Frequency Generation Spectroscopy of Dodecanethiol on Metal Nanoparticles

We report studies of metal nanoparticle surfaces using vibrational sum frequency generation (SFG) spectroscopy. SFG is demonstrated as a sensitive and information-rich probe of nanostructured surfaces. Detection of SFG spectra from a few percent of a monolayer of nanoparticles on a transparent substrate was achieved, corresponding to as few as 105 isolated particles within the laser beam spot. A new effect arises when the nanoparticle size approaches the molecular scale: the dependence of the molecular conformation on the geometry of the substrate. Conformation of the dodecane chain of the ligand shows systematic variation with the particle diameter in the 1.8−25 nm range. More gauche defects are observed on smaller particles, judged from the relative intensities of the CH2 and CH3 stretch transitions in SFG spectra. Similar behavior observed for both gold and silver particles suggests a nanoscale geometric packing effect, whence more volume is available to the chain on a curved surface than a flat surface. Drying-mediated aggregation of gold nanoparticles results in an enhancement of the SFG signal, enabling vibrational SFG spectra of single submicron size aggregates to be obtained. These are different from spectra of isolated particles, indicating a vibrational mode-selective enhancement mechanism.

An empirical approach to the bond additivity model in quantitative interpretation of sum frequency generation vibrational spectra

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.

Rubbing-induced anisotropy of long alkyl side chains at polyimide surfaces

Molecular organization at polyimide surfaces used as alignment layers in liquid crystal displays was investigated using vibrational sum frequency generation (SFG) spectroscopy. We focus on the orientation of the long alkyl side groups at the polymer surface using polarization-selected SFG spectra of the CH(3)- and CH(2)-stretch modes of the side chain. Mechanical rubbing and baking, an accepted industrial procedure used to produce pretilt of the liquid crystal, was found to induce pronounced azimuthal anisotropy in the orientational distribution of the alkyl side chains. Orientational analysis of the SFG vibrational spectra in terms of the azimuthal and tilt angles (in and out of plane, respectively) of the alkyl side chains shows their preferential tilt along the rubbing direction, with the azimuthal distribution narrower for stronger rubbed polymer samples.

Identification of overlapping features in the sum frequency generation vibrational spectra of air/ethanol interface

The stretching vibrations of the –CH 3 and –CH 2– groups not only overlap but also interfere with each other in the Sum-Frequency Generation (SFG) vibrational spectra of the air/ethanol interface. Using deuterated ethanol samples, i.e., CH 3CD 2OH and CD 3CH 2OH, the C–H stretching vibrational modes are clearly assigned for the SFG vibrational spectra of their air/liquid interfaces according to the polarization selection rules. It is found that the CH 3-ss and CH 2-ss modes overlap in the ∼2875 cm −1 peak, the CH 3-as and CH 2-ss-Fermi modes overlap in the ∼2970 cm −1 peak, and the ∼2930 cm −1 peak belongs to the CH 3-ss-Fermi mode, respectively.

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

Here we report a detailed study on spectroscopy, structure, and orientational distribution, as well as orientational motion, of water molecules at the air/water interface, investigated with sum frequency generation vibrational spectroscopy (SFG-VS). Quantitative polarization and experimental configuration analyses of the SFG data in different polarizations with four sets of experimental configurations can shed new light on our present understanding of the air/water interface. Firstly, we concluded that the orientational motion of the interfacial water molecules can only be in a limited angular range, instead of rapidly varying over a broad angular range in the vibrational relaxation time as suggested previously. Secondly, because different vibrational modes of different molecular species at the interface has different symmetry properties, polarization and symmetry analyses of the SFG-VS spectral features can help the assignment of the SFG-VS spectra peaks to different interfacial species. These analyses concluded that the narrow 3693 cm(-1) and broad 3550 cm(-1) peaks belong to C(infinityv) symmetry, while the broad 3250 and 3450 cm(-1) peaks belong to the symmetric stretching modes with C2v symmetry. Thus, the 3693 cm(-1) peak is assigned to the free OH, the 3550 cm(-1) peak is assigned to the singly hydrogen-bonded OH stretching mode, and the 3250 and 3450 cm(-1) peaks are assigned to interfacial water molecules as two hydrogen donors for hydrogen bonding (with C2v symmetry), respectively. Thirdly, analysis of the SFG-VS spectra concluded that the singly hydrogen-bonded water molecules at the air/water interface have their dipole vector directed almost parallel to the interface and is with a very narrow orientational distribution. The doubly hydrogen-bonded donor water molecules have their dipole vector pointing away from the liquid phase.

Interaction of self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiols with water studied by vibrational sum-frequency generation

Vibrational sum-frequency generation (VSFG) was used to investigate the conformational changes in self-assembled monolayers (SAMs) of (1-mercaptoundec-11-yl) hexa(ethylene glycol) monomethylether (EG6-OMe) on gold when exposed to liquid water. VSFG spectra of the EG6-OMe SAMs were recorded before, during, and after exposure of the films to water and after a subsequent evacuation step. While in contact with water the entire ethylene glycol chains are found in a random, solvated state, after removal from the fluid water molecules remain absorbed only at the terminal groups of the film giving rise to distinct conformational changes. After evacuation, the structure of the EG6-OMe SAM reverts to its original state, indicating that water has been removed from the monolayer. Our findings support recent ab initio calculations and Monte Carlo simulations on the interaction of ethylene glycol-terminated monolayers with water.

Detection of chiral sum frequency generation vibrational spectra of proteins and peptides at interfaces in situ.

In this work, we demonstrate the feasibility to collect off-electronic resonance chiral sum frequency generation (SFG) vibrational spectra from interfacial proteins and peptides at the solid/liquid interface in situ. It is difficult to directly detect a chiral SFG vibrational spectrum from interfacial fibrinogen molecules. By adopting an interference enhancement method, such a chiral SFG vibrational spectrum can be deduced from interference spectra between the normal achiral spectrum and the chiral spectrum. We found that the chiral SFG vibrational spectrum of interfacial fibrinogen was mainly contributed by the beta-sheet structure. For a beta-sheet peptide tachyplesin I, which may be quite ordered at the solid/liquid interface, chiral SFG vibrational spectra can be collected directly. We believe that these chiral signals are mainly contributed by electric dipole contributions, which can dominate the chiroptical responses of uniaxial systems. For the first time, to our knowledge, this work indicates that the off-electronic resonance SFG technique is sensitive enough to collect chiral SFG vibrational spectra of interfacial proteins and peptides, providing more structural information to elucidate interfacial protein and peptide structures.

Interpretation of Sum Frequency Generation Vibrational Spectra of Interfacial Proteins by the Thin Film Model

To interpret sum frequency generation (SFG) vibrational spectra collected from adsorbed protein molecules using structures of the interfacial proteins, the thin film model has been evaluated both theoretically and experimentally. The calculations indicate that nonlocal contributions to SFG spectra of interfacial proteins usually can be ignored. The experimental results indicate that for some polarization combinations, such as ssp and sps, local field variations across the adsorbed protein layer will not substantially affect SFG signals. Therefore, SFG spectra collected from adsorbed protein molecules are generally dominated by the electric-dipole contribution. From such spectra, quantitative structural or conformation information about interfacial proteins can be deduced using the thin film model. As an example, we analyzed the orientation distribution of the methyl groups of BSA molecules at different interfaces.

Adsorption of 4-cyanopyridine on Au(111): ab initio calculations and SFG measurements

The adsorption of 4-cyanopyridine (4-CP) on a gold single crystal electrode in sodium perchlorate supporting electrolyte has been investigated by voltammetry, sum frequency generation (SFG) and ab initio calculations. Combination of in situ vibrational SFG spectra and DFT, MP2 and MP2 + SQM calculations allows us to obtain fundamental information on the behavior of adsorbed 4-CP as a function of the electrode potential. The configuration of the adsorbed 4-CP is dependent on the electrode potential. 4-CP is adsorbed perpendicular to the electrode surface through the pyridine nitrogen at negative potential, and through the nitrile nitrogen at positive potential. In the intermediate potential range, the molecule is adsorbed parallel to the electrode surface, through the π electrons of the aromatic ring.

Vibrons in a disordered monolayer: Application to the determination of the infrared and sum frequency generation spectra of12COand13COisotopic mixtures adsorbed on NaCl(001)

The internal dynamics of a disordered monolayer formed by a mixture of ${}^{12}\mathrm{CO}$ and ${}^{13}\mathrm{CO}$ isotopes is studied using a description in terms of vibron propagation and scattering by isotopic defects randomly distributed in the layer. The spectral densities connected to the infrared and sum frequency generation vibrational spectra of the disordered monolayer are expressed in terms of the average Green propagator of the vibrons using a generalized t-matrix approximation to describe the collision of vibrons with the distributed defects. Our model can interpret the experimental infrared spectra of CO monolayers adsorbed on NaCl(100) and explain the behavior of the linewidth with temperature and isotopic concentration. Differences in the infrared and sum frequency generation line shapes and intensities are discussed. Nonlinear spectroscopy experiments on this system should evidence these features.

High pressure catalytic processes studied by infrared-visible sum frequency generation

The recent development of infrared-visible sum frequency generation (SFG), a surface-specific vibrational spectroscopy, has helped bridge the pressure gap between studies of heterogeneous catalysis under high vacuum and atmospheric pressure. This is achieved by in situ monitoring of surface species at high pressure via their SFG vibrational spectra and correlating the results with the simultaneously measured reaction rate using gas chromatography. Examples of systems studied include olefin hydrogenation and carbon monoxide oxidation over the (111) crystalline face of platinum. In these examples, the studies succeed in revealing the molecular details of the surface reactions. Identification of key intermediates and their concentrations has made it possible for the first time to calculate turn over rates per active surface species rather than just per exposed surface metal atom. In all cases, the key intermediate of the reaction is not detectable on the surface in UHV under similar temperatures.

Probing the Structure of Water Molecules at an Oil/Water Interface in the Presence of a Charged Soluble Surfactant through Isotopic Dilution Studies

We have employed vibrational sum frequency generation (VSFG) to obtain the first vibrational spectra of water molecules at an oil/water interface in the presence of a charged, soluble surfactant. By examining OH stretching modes that are highly sensitive to the local hydrogen-bonding environment, we have been able to compare the structure of interfacial water molecules with the structure of bulk water molecules determined from previous studies. From the VSFG spectra we infer that there is more extensive hydrogen bonding between neighboring water molecules at the CCl4/water interface as compared to water molecules in the bulk aqueous phase. The presence of a charged surfactant enhances the SF response in the OH stretching spectral region in a manner similar to what we have previously observed at the air/water interface. To further probe the hydrogen bonding of water molecules at the oil/water interface, we have employed VSFG to study mixed samples of H2O and D2O. As the mole fraction of H2O is decreased, the peak position of the ice-like OH stretching mode is blue-shifted by approximately 120 cm-1 and converges on the peak position of the uncoupled OH symmetric stretch observed in bulk ice studies. This shift in energy is discussed within the context of hydrogen bonding of the interfacial H2O and D2O molecules and the intermolecular uncoupling of the OH symmetric stretching vibration. Finally, we have obtained the first vibrational spectra of the OH stretching mode from uncoupled HOD molecules at the oil/water interface located at approximately 3460 cm-1.

Ordering of Interfacial Water Molecules at the Charged Air/Water Interface Observed by Vibrational Sum Frequency Generation

The molecular structure and orientation of interfacial water molecules at the air/water interface in the presence of soluble cationic and anionic surfactants has been characterized. We have employed vibrational sum frequency generation (VSFG) to probe the orientation of interfacial water molecules as a function of both bulk surfactant concentration and solution ion strength. The observed ordering of interfacial water molecules is manifested by an enhancement of the OH stretching modes in the VSFG spectra. We attribute the observed enhancement to an alignment of the interfacial water molecules induced by the large electrostatic field present at these charged interfaces. The nature of this enhancement is further explored by studying mixed (cationic and anionic) surfactant systems as well as surfactant systems at different ionic strengths. From these studies we find that the interfacial water molecules attain their highest degree of alignment at surface surfactant concentrations well below maximum surface co...

Vibrational sum-frequency generation spectroscopy of a homoepitaxially-grown diamond C(100) surface

Vibrational sum-frequency generation (VSFG) spectroscopy was applied to a study of the CH bonds on a homoepitaxially-grown diamond C(100) surface. The VSFG spectrum taken under the (p,p) polarization combination of visible and infrared excitation beams gave vibrational resonance peaks at 2910 and 2960 cm −1 along with a significant background (BG) signal, and the spectrum taken under the (s,p) combination had a peak at 2910 cm −1 and a negligibly weak BG signal. From analyses of the spectral features and polarization characteristics, the 2910 cm −1 band was assigned to the CH stretching vibration of the monohydride (CH) groups on a reconstructed C(100) 2 × 1 surface, and the 2960 cm −1 band to the anti-symmetric CH stretching vibration of the dihydride (CH 2) groups located at either the unreconstructed C(100) 1 × 1 island surface or step sites. The tilt angle of the CH group was estimated to be in the range of 30–60°, the large uncertainty originating from the lack of data on the Raman depolarization ratio. The tilt angle of the CH 2 group was estimated to be less than 40°.

The orientation analysis from polarization characteristics of the surface vibrational SFG spectrum of HCO 2 on MgO(001) surface

Application of vibrational SFG (sum frequency generation) spectroscopy has been made to the study of the adsorption product of formic acid(HCOOH) on clean MgO(001) surface. The observed (CH) frequency of 2870 cm −1 revealed that the adsorbed species are formate (HCOO −), and the analysis made on the polarization characteristics of signal intensity led to the conclusion that CH bonds stand perpendicular to the surface with rather broad distribution width of tilt angle: φ=0±30°. Formulas for the analysis have been presented.

Orientation analysis by simulation of vibrational sum frequency generation spectrum: CH stretching bands of the methyl group

A description is given of the procedure which simulates the spectral features of vibrational sum frequency generation (VSFG) spectrum by the ν(CH) stretching bands of the methyl group. Instead of assuming that α ζζ alone is significant as in previous expressions for the molecular hyperpolarizability tensor, we have introduced a parameter r=α ξξ /α ζζ where α ζζ and α ξξ are the parallel and perpendicular components, respectively, of the Raman tensor associated with CH bond. The statistical aspects of molecular orientation and finite width of an infrared light source were also incorporated. A model calculation is presented of the signals from two dimensionally arranged methyl groups to reveal the effects of the quantities on the spectral and angular profiles