C−H Stretch Modes as a Probe of H-Bonding in Methanol-Containing Clusters

An approximation to the vibrational coupled-cluster method for CH-stretching of large molecules: application to naphthalene and anthracene

We propose an approximation to the vibrational coupled-cluster method (VCCM) to describe the CH-stretching region of the vibrational spectrum of large molecules. The vibrational modes of a molecule are divided into two sets: the target set and the bath set. The target set includes the CH stretches and the modes that are strongly coupled with the CH stretches and/or involve strong Fermi resonances with a CH stretch fundamental. The rest of the modes are in the bath set. First, the effective harmonic oscillator (EHO) approximation is invoked for the whole system to obtain the zeroth-order frequencies and modified potentials. The effects of interaction between the bath set and the target sets are included in the modified potential from the EHO calculation. The VCCM equations are constructed with the modified potential from the EHO calculations and for the target set only. The transition energies and intensities are calculated using such a truncated VCCM approximation. The proposed method is applied to calculate the IR spectra of naphthalene and anthracene. The results with three different criteria for selecting the modes in the target set are compared with the experimental IR spectra.

Understanding of the C-H stretch region of infra-red spectroscopy: an analysis of the final state wavefunctions

The nature of the wavefunctions associated with the final states in the CH stretching region of several medium sized molecules is analysed. The number of optically bright transitions is much larger than the number of CH oscillators present in the molecule, and they are spread over a range of about 300 cm^-1. Several of them are clustered together within about 5 cm^-1 with near equal intensities. The final states of all these transitions are superpositions of multiple zeroth order states. In almost all of such superpositions, no single zeroth order state has more than 50% weight. Several multiquantum states, with three to four quanta of excitation dominate the final states, with the CH chromophore contributing only a small weightage. Thus the band structure of the CH stretch region is due to several optically bright transitions whose final states are superpositions of low frequency multiquantum states with the CH chromophore contributing only a small weight to make them spectroscopically active.

Liquid/Vapor Interface of Dimethyl Carbonate-Methanol Binary Mixtures Investigated by Sum Frequency Generation Vibrational Spectroscopy and Molecular Dynamics Simulation

In the present work, the dimethyl carbonate (DMC)-methanol binary mixture was used as a benchmark system to study the molecular structures of the liquid/vapor interface of organic-organic mixtures by sum frequency generation vibrational spectroscopy (SFG-VS) and molecular dynamics (MD) simulations. It was discovered that both the methanol and DMC molecules are anisotropically oriented at the surface, yielding strong SFG-VS signals in the C-H stretching frequency range for both molecules. The detailed analyses of the spectroscopic and MD data reveal that the increase of the methanol bulk concentrations reduces the orientational order of the methyl groups for both the interfacial DMC and methanol molecules but does not significantly affect the orientations of the carbonyl group in DMC. Moreover, no obvious correlations were found between the room-temperature orientations of the surface molecules and the azeotropic mole fraction. The present work paves the road for future investigations on the molecular structures of the liquid/vapor interfaces of other organic-organic mixtures, especially those that are important in industrial separations.

Enrichment of methanol inside pNIPAM gels in the cononsolvency-induced collapse

From Raman, we determined an enrichment of methanol inside the polymer in the cononsolvency-induced collapse and donor-type hydrogen-bonding of methanol with pNIPAM.

Probe of Alcohol Structures in the Gas and Liquid States Using C⁻H Stretching Raman Spectroscopy

Vibrational spectroscopy is a powerful tool for probing molecular structures and dynamics since it offers a unique fingerprint that allows molecular identification. One of important aspects of applying vibrational spectroscopy is to develop the probes that can characterize the related properties of molecules such as the conformation and intermolecular interaction. Many examples of vibrational probes have appeared in the literature, including the azide group (⁻N₃), amide group (⁻CONH₂), nitrile groups (⁻CN), hydroxyl group (⁻OH), ⁻CH group and so on. Among these probes, the ⁻CH group is an excellent one since it is ubiquitous in organic and biological molecules and the C⁻H stretching vibrational spectrum is extraordinarily sensitive to the local molecular environment. However, one challenge encountered in the application of C⁻H probes arises from the difficulty in the accurate assignment due to spectral congestion in the C⁻H stretching region. In this paper, recent advances in the complete assignment of C⁻H stretching spectra of aliphatic alcohols and the utility of C⁻H vibration as a probe of the conformation and weak intermolecular interaction are outlined. These results fully demonstrated the potential of the ⁻CH chemical group as a molecular probe.

High pressure Raman spectroscopy of H2O-CH3OH mixtures

Complex intra-molecular interactions and the hydrogen-bonding network in H2O-volatile mixtures play critical roles in many dynamics processes in physical chemistry, biology, and Earth and planetary sciences. We used high pressure Raman spectroscopy to study the pressure evolution of vibrational frequencies and bonding behavior in H2O-CH3OH mixtures. We found that the presence of low CH3OH content in H2O increases the transition pressure where water crystallizes to ice VI, but does not significantly change the pressure where ice VI transforms to ice VII. Furthermore, the stiffening rates of C-H stretching frequencies dω/dP in CH3OH significantly decrease upon the crystallization of water, and the softening rates of the O-H stretching frequencies of ice VII are suppressed over a narrow pressure range, after which the frequencies of these modes shift with pressure in ways similar to pure CH3OH and ice VII, respectively. Such complex pressure evolution of Raman frequencies along with pronounced variations in Raman intensities of CH3OH within the sample, and the hysteresis of the water-ice VI phase transition suggest pressure-induced segregation of low content CH3OH from ice VII. These findings indicate the significant influence of volatiles on the crystallization of sub-surface ocean and thermal evolution within large icy planets and satellites.

The water-hydrophobic interface: neutral and charged solute adsorption at fluorocarbon and hydrocarbon self-assembled monolayers (SAMs)

Adsorption of small molecular solutes in an aqueous solution to a soft hydrophobic surface is a topic relevant to many fields. In biological and industrial systems, the interfacial environment is often complex, containing an array of salts and organic compounds in the solution phase. Additionally, the surface itself can have a complex structure that can interact in unpredictable ways with small solutes in its vicinity. In this work, we studied model adsorption processes on hydrocarbon and fluorocarbon self-assembled monolayers by using vibrational sum frequency spectroscopy, with methanol and butylammonium chloride as adsorbates. The results indicate that differences in surface functionality have a significant impact on the organization of adsorbed organic species at hydrophobic surfaces.

Environmental chemistry at vapor/water interfaces: insights from vibrational sum frequency generation spectroscopy

The chemistry that occurs at surfaces has been an intense area of study for many years owing to its complexity and importance in describing a wide range of physical phenomena. The vapor/water interface is particularly interesting from an environmental chemistry perspective as this surface plays host to a wide range of chemistries that influence atmospheric and geochemical interactions. The application of vibrational sum frequency generation (VSFG), an inherently surface-specific, even-order nonlinear optical spectroscopy, enables the direct interrogation of various vapor/aqueous interfaces to elucidate the behavior and reaction of chemical species within the surface regime. In this review we discuss the application of VSFG to the study of a variety of atmospherically important systems at the vapor/aqueous interface. Chemical systems presented include inorganic ionic solutions prevalent in aqueous marine aerosols, small molecular solutes, and long-chain fatty acids relevant to fat-coated aerosols. The ability of VSFG to probe both the organization and reactions that may occur for these systems is highlighted. A future perspective toward the application of VSFG to the study of environmental interfaces is also provided.

Identification of spectroscopic patterns of CH...O H-bonds in proteins

Ab initio calculations are used to identify characteristics of vibrational and NMR spectra that signal the involvement of a protein backbone in a CH...O H-bond and that distinguish this sort of interaction from other H-bonds in which a protein might participate. Glycine and alanine dipeptides, in both their C7 and C5 minimum-energy structures, are paired with formamide in a number of different H-bonding arrangements. The CH...O H-bond is characterized by a small contraction of the C-H bond length, along with a blue shift in its stretching frequency, accompanied by an intensification of this vibrational band. In the context of NMR spectra, the bridging CH proton's chemical shift is moved downfield by 1-2 ppm. The aforementioned features are not produced by other H-bonds in which the protein backbone might participate, such as NH proton donation or accepting a proton via the peptide C=O.

Weak Hydrogen Bonds Make a Difference: Dimers of Jet-Cooled Halogenated Ethanols

Hydrogen-bonded clusters of fluorinated and chlorinated ethanols exhibit rich isomerism in terms of monomer conformation, secondary contacts between the OH and CH groups and the halogen atoms, hydrogen bond topology, chirality recognition and acceptor lone electron pair choice. By expanding the six alcohols involving one to three fluorine or chlorine atoms at the methyl group in a supersonic slit jet expansion and by probing their monomer, dimer and trimer IR spectra between 800 and 4000 cm−1, this isomerism is unravelled in substantial detail. Argon relaxation experiments and complementary cluster Raman spectroscopy provide further information on the individual dimer conformations and on trimer assignments. Energy sequences, helicity- and topology-dependent OH red-shifts, differences between fluorine and chlorine, the influence of dispersion-like interactions and halogen number trends are uncovered and compared to systematic quantum-chemical calculations up to MP2/6–311+G* level. The experimental data provide rigorous reference values for an accurate and balanced quantum-mechanical description of weak hydrogen bond interactions to halogens in the presence of a strong hydrogen bond between oxygen atoms.

IR Spectroscopy of Microsolvated Aromatic Cluster Ions: Ionization-Induced Switch in Aromatic Molecule–Solvent Recognition

IR spectroscopy, mass spectrometry, and quantum chemical calculations are employed to characterize the intermolecular interaction of a variety of aromatic cations (A+) with several types of solvents. For this purpose, isolated ionic complexes of the type A+–L n , in which A+ is microsolvated by a controlled number (n) of ligands (L), are prepared in a supersonic plasma expansion, and their spectra are obtained by IR photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. Two prototypes of aromatic ion–solvent recognition are considered: (i) microsolvation of acidic aromatic cations in a nonpolar hydrophobic solvent and (ii) microsolvation of bare aromatic hydrocarbon cations in a polar hydrophilic solvent. The analysis of the IRPD spectra of A+–L dimers provides detailed information about the intermolecular interaction between the aromatic ion and the neutral solvent, such as ion–ligand binding energies, the competition between different intermolecular binding motifs (H-bonds, π-bonds, charge–dipole bonds), and its dependence on chemical properties of both the A+ cation and the solvent type L. IRPD spectra of larger A+–L n clusters yield detailed insight into the cluster growth process, including the formation of structural isomers, the competition between ion–solvent and solvent–solvent interactions, and the degree of (non)cooperativity of the intermolecular interactions as a function of solvent type and degree of solvation. The systematic A+–L n cluster studies are shown to reveal valuable new information about fundamental chemical properties of the bare A+ cation, such as proton affinity, acidity, and reactivity. Because of the additional attraction arising from the excess charge, the interaction in the A+–L n cation clusters differs largely from that in the corresponding neutral A–L n clusters with respect to both the interaction strength and the most stable structure, implying in most cases an ionization-induced switch in the preferred aromatic molecule–solvent recognition motif. This process causes severe limitations for the spectroscopic characterization of ion–ligand complexes using popular photoionization techniques, due to the restrictions imposed by the Franck–Condon principle. The present study circumvents these limitations by employing an electron impact cluster ion source for A+–L n generation, which generates predominantly the most stable isomer of a given cluster ion independent of its geometry.

Cooperativity of hydrogen-bonded networks in 7-azaindole(CH3OH)n (n=2,3) clusters evidenced by IR-UV ion-dip spectroscopy and natural bond orbital analysis

IR-UV ion-dip spectra of the 7-azaindole (7AI)(CH(3)OH)(n) (n=1-3) clusters have been measured in the hydrogen-bonded NH and OH stretching regions to investigate the stable structures of 7AI(CH(3)OH)(n) (n=1-3) in the S(0) state and the cooperativity of the H-bonding interactions in the H-bonded networks. The comparison of the IR-UV ion-dip spectra with IR spectra obtained by quantum chemistry calculations shows that 7AI(CH(3)OH)(n) (n=1-3) have cyclic H-bonded structures, where the NH group and the heteroaromatic N atom of 7AI act as the proton donor and proton acceptor, respectively. The H-bonded OH stretch fundamental of 7AI(CH(3)OH)(2) is remarkably redshifted from the corresponding fundamental of (CH(3)OH)(2) by 286 cm(-1), which is an experimental manifestation of the cooperativity in H-bonding interaction. Similarly, two localized OH fundamentals of 7AI(CH(3)OH)(3) also exhibit large redshifts. The cooperativity of 7AI(CH(3)OH)(n) (n=2,3) is successfully explained by the donor-acceptor electron delocalization interactions between the lone-pair orbital in the proton acceptor and the antibonding orbital in the proton donor in natural bond orbital (NBO) analyses.

Supersonic jet studies of solvation effects on the spectroscopy and photophysics of 4-diethylaminopyridine

We present the results of spectroscopic and photophysical investigations of 4-diethylaminopyridine (DEAP) and its 1 : 1 complexes with a number of protic solvents such as water and various alcohols of different acidity isolated under supersonic jet conditions. While a double resonance vibrational spectroscopic method was employed to investigate the size and geometrical structure of jet-cooled clusters, laser-induced fluorescence spectroscopy was used to examine the changes of photophysics induced by complexation of DEAP with solvent molecule(s). The results obtained from ab initio calculations enable the assignment of geometries and of the vibrational spectra of the clusters in the OH-stretch region. The comparison of the experimental and calculated vibrational spectra indicates that the solvent molecule is hydrogen-bonded to the pyridine nitrogen atom. Dual luminescence is observed only for the complexes with alcohols of relatively strong acidity.

IR Spectroscopy of Hydrogen-Bonded 2-Fluoropyridine−Methanol Clusters

The electronic and infrared spectra of 2-fluoropyridine-methanol clusters were observed in a supersonic free jet. The structure of hydrogen-bonded clusters of 2-fluoropyridine with methanol was studied on the basis of the molecular orbital calculations. The IR spectra of 2-fluoropyridine-(CH3OH)n(n = 1-3) clusters were observed with a fluorescence-detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The structures of the clusters are similar to those observed for 2-fluoropyridine-(H2O)n (n = 1-3) clusters. The existence of weak hydrogen bond interaction through aromatic hydrogen was observed in the IR spectra. The theoretical calculation also supports the result. The vibrational frequencies of CH bonds in CH3 group are affected by hydrogen bond formation although these bonds do not directly relate to the hydrogen bond interaction. The B3LYP/6-311 ++G(d,p) calculations reproduce well the vibrational frequency of the hydrogen-bonded OH stretching vibrations. However, the calculated frequency of CH stretching vibration could not reproduce the IR spectra because of anharmonic interaction with closely lying overtone or combination bands for nu3 and nu9 vibrations. The vibrational shift of nu2 vibration is reproduced well with molecular orbital calculations. The calculation also shows that the frequency shift of nu2 vibration is closely related to the CH bond length at the trans position against the OH bond in hydrogen-bonded methanol.

Interfacial structures of methanol:water mixtures at a hydrophobic interface probed by sum-frequency vibrational spectroscopy

Sum-frequency vibrational spectroscopy was used to study interfacial structure of methanol:water mixtures at an octyltrichlorosilane-covered hydrophobic surface. Methanol was found to adsorb preferentially than water at the interface with its methyl group tilted from the surface normal by approximately 35 degrees for all methanol concentrations. Redshift of the methanol symmetric stretch mode, gradual disappearance of the water dangling-OH mode, and blueshifts of the dangling and liquidlike bonded-OH modes were also observed as the methanol concentration increased. They could be understood from the change of the interfacial hydrogen-bonding network associated with the change of surface composition.

Surfaces of Alcohol−Water Mixtures Studied by Sum-Frequency Generation Vibrational Spectroscopy

Sum-frequency generation vibrational spectroscopy was used to investigate the surface molecular structure of binary mixtures of water and alcohol (methanol, ethanol, and propanol) at the air/liquid interface. In this study, it is shown that the sum-frequency signal from the alcohol molecules in the CH-stretch vibration region is always larger for mixtures than that from pure alcohol. For example, the sum-frequency signal from a propanol mixture surface at a 0.1 bulk mole fraction was approximately 3 times larger than that from a pure propanol surface. However, the ratio between the sum-frequency signals taken at different polarization combinations was found to be constant within experimental errors as the bulk alcohol concentration was changed. This suggested that the orientation of surface alcohol molecules does not vary appreciably with the change of concentration and that the origin of the signal enhancement is mainly due to the increase in the surface number density of alcohol molecules contributing to the sum-frequency signal for the alcohol/water mixture as compared to the pure alcohol surface.