Quadrupole Contribution of C═O Vibrational Band in Sum Frequency Generation Spectra of Organic Carbonates

Abstract

Sum Frequency Generation (SFG) is usually governed by surface-selective signals of dipole origin, but it can also contain some bulk signals of quadrupole origin. In this work, we examined the dipole and quadrupole contributions in the CO stretching band of organic carbonate liquids with collaboration of heterodyne SFG measurement and theoretical analysis. As a result, we found that these spectra are substantially affected by the quadrupole contribution of the bulk, which resolved the discrepancy between the experimental and computational SFG spectra.

Bulk Response of Carboxylic Acid Solutions Observed with Surface Sum-Frequency Generation Spectroscopy

“…The positive sign of the Im[χ (2) ] response of the C=O stretch vibration indicates that the positive charge of the C=O group is closer to the surface than the negative charge, which implies that the C=O group points with its oxygen atom toward the bulk. 14 , 29 , 30 This finding is consistent with the observation that the methyl group of AA and the methine group of FA are oriented toward the air. The Im[χ (2) ] spectrum of the neat water surface shows a vibrational response centered at 1650 cm –1 ( Figure S1b ) that is assigned to the water bending mode.…”

“…In a cyclic dimer the two carbonyl vibrations have a nearly perfect antiparallel arrangement, meaning that the dipolar response vanishes, thus leaving only the quadrupolar response of the C=O vibrations. 14 A detailed description of the quadrupolar response of molecular vibrations can be found in refs ( 10 ), ( 13 ), and ( 38 − 41 ). As the quadrupolar response of a particular vibration is not surface specific, its amplitude is expected to be proportional to the bulk concentration of that vibration.…”

“…Recently, many studies challenged this basic understanding of VSFG and demonstrated that sum-frequency light can also be generated by a bulk quadrupolar response. 10 − 14 Clearly, the presence of such a response influences the surface selectivity and interpretation of VSFG spectra. 10 , 13 , 14 Previous intensity VSFG measurements of aqueous FA and AA show that the vibrational responses of the CH, OH, and C=O vibrations of acid molecules at the surface can already be observed at low concentrations (0.3 mol %).…”

“… 10 − 14 Clearly, the presence of such a response influences the surface selectivity and interpretation of VSFG spectra. 10 , 13 , 14 Previous intensity VSFG measurements of aqueous FA and AA show that the vibrational responses of the CH, OH, and C=O vibrations of acid molecules at the surface can already be observed at low concentrations (0.3 mol %). 15 − 17 These studies also reported that these spectral responses change in shape upon increasing the concentration, which was explained from the formation of complexes of acid molecules, such as the hydrated monomer, the linear dimer, and the cyclic dimer.…”

“…Beside surface tension measurements, atomic force microscopy (AFM), surface-enhanced Raman scattering (SERS), and X-ray photoelectron spectroscopy (XPS), vibrational sum-frequency generation spectroscopy (VSFG) is a powerful tool as a probe for microscopic structures at interfaces. , VSFG is highly surface sensitive and surface selective because VSFG is based on the second-order polarization of the system, which is prohibited in centrosymmetric media and therefore in most bulk phases. Recently, many studies challenged this basic understanding of VSFG and demonstrated that sum-frequency light can also be generated by a bulk quadrupolar response. − Clearly, the presence of such a response influences the surface selectivity and interpretation of VSFG spectra. ,, Previous intensity VSFG measurements of aqueous FA and AA show that the vibrational responses of the CH, OH, and CO vibrations of acid molecules at the surface can already be observed at low concentrations (0.3 mol %). − These studies also reported that these spectral responses change in shape upon increasing the concentration, which was explained from the formation of complexes of acid molecules, such as the hydrated monomer, the linear dimer, and the cyclic dimer. Here, we investigate the properties of FA and AA molecules at the surface of aqueous solutions with heterodyne detected vibrational sum-frequency generation (HD-VSFG).…”