CH Stretch Region Research Articles

In-Contact IR Spectroscopy of Hydrocarbon Lubricants

This paper reviews the application of infrared (IR) spectroscopy to in-contact lubrication studies, details improvements to the method and reports initial results from the study of a simple hydrocarbon fluid. IR spectra have been taken from the Hertzian region of a sliding contact lubricated by poly-α-olefin fluids. Single reflection sampling was used with a spot size of 100 μm square. The spectral range was limited to the CH stretch region of 3100--2700 cm−1. The results show that it is possible to obtain good quality IR spectra from hydrocarbon films of less than 50 nm. Spectra sampled from the contact zone show peak intensity changes, frequency shifts and shape changes. Analysis of these results indicate that in the high pressure region there is an increase in gauche defects in the alkyl chain implying a more globular molecule with a lower volume but higher energy. In thin lubricant films (<100 nm) there is evidence of fluid structuring close to the metal surface.

Coherent Vibrational Quantum Beats as a Probe of Langmuir−Blodgett Monolayers

We combine frequency- and femtosecond time-domain measurements of vibrational coherences for spectroscopic characterization of surface monolayer films, utilizing 3-wave mixing as the surface-selective technique. Frequency-domain spectra in the CH-stretch region are obtained by infrared + visible sum frequency generation (SFG). Time-domain coherences are measured using SFG free induction decay (SFG-FID), where a 75 fs IR pulse excites several vibrational modes and a delayed 40 fs visible pulse probes the oscillating surface polarization. A unified framework based on optical Bloch equations is used to simultaneously analyze the time- and frequency-domain data. We compare molecular organization of monolayers in different two-dimensional phases. Highly ordered films transferred at high surface pressure are dominated by two transitions in the frequency domain, CH3 symmetric stretch (2875 cm(-1)) and CH3-Fermi resonance with bend overtone (2935 cm(-1)), and a coherent quantum beat in the time-domain at the difference frequency (approximately 540 fs period). At lower surface pressure, relative amplitudes change and additional transitions emerge (CH3 asymmetric stretch and CH2 modes), indicating changes in molecular orientation and onset of disorder. Information redundancy in the combined frequency- and time-domain data allows more accurate determination of the spectral parameters than purely frequency- or time-domain techniques.

First-Principle DFT and MP2 Modeling of Infrared Reflection−Absorption Spectra of Oriented Helical Ethylene Glycol Oligomers

First-principle modeling is used to obtain a comprehensive understanding of infrared reflection absorption (RA) spectra of helical oligo(ethylene glycol) (OEG) containing self-assembled monolayers (SAMs). Highly ordered SAMs of methyl-terminated 1-thiaoligo(ethylene glycols) [HS(CH2CH2O)(n)CH3, n = 5, 6] on gold recently became accessible for systematic infrared analyses [Vanderah et al., Langmuir, 2003, 19, 3752]. We utilized the quoted experimental data to validate the first-principle modeling of infrared RA spectra of HS(CH2CH2O)(5,6)CH3 obtained by (i) DFT methods with gradient corrections (using different basis sets, including 6-311++G) and (ii) HF method followed by a Møller-Plesset (MP2) correlation energy correction. In focus are fundamental modes in the fingerprint and CH-stretching regions. The frequencies and relative intensities in the calculated spectra for a single molecule are unambiguously identified with the bands observed in the experimental RA spectra of the corresponding SAMs. In addition to confirming our earlier assignment of the dominating peak in the CH-stretching region to CH2 asymmetric stretching vibrations, all other spectral features observed in that region have received an interpretation consistent (but not in all cases coinciding) with previous investigations. The obtained results provide an improved understanding of the orientation and conformation of the molecular building blocks within OEG-containing assemblies, which, in our opinion, is crucial for being able to predict the folding and phase characteristics and interaction of OEG-SAMs with water and proteins.

CH-Stretching Overtone Spectroscopy of 1,1,1,2-Tetrafluoroethane

We have recorded the vibrational absorption spectrum of 1,1,1,2-tetrafluoroethane (HFC-134a) in the fundamental and first five CH-stretching overtone regions with the use of Fourier transform infrared, dispersive long-path, intracavity laser photoacoustic, and cavity ringdown spectroscopies. We compare our measured total oscillator strengths in each region with intensities calculated using an anharmonic oscillator local mode model. We calculate intensities with 1D, 2D, and 3D Hamiltonians, including one or two CH stretches and two CH stretches with the HCH bending mode, respectively. The dipole moment function is calculated ab initio with self-consistent-field Hartree-Fock and density functional theories combined with double- and triple-zeta-quality basis sets. We find that the basis set choice affects the total intensity more than the choice of the Hamiltonian. We achieve agreement between the calculated and measured total intensities of approximately a factor of 2 or better for the fundamental and first five overtones.

Vibrational Excitons in CH-Stretching Fundamental and Overtone Vibrational Circular Dichroism Spectra

A set of vibrational circular dichroism (VCD) spectra in the CH-stretching fundamental region for about twenty compounds belonging to the class of essential oils was empirically analyzed by the use of a sort of vibrational exciton mechanism, involving three centers. Through a general formula applicable to many coupled dipole oscillators, the rotational strengths of the previously identified vibrational excitons are evaluated. The results are then critically reviewed by the use of recent ab initio methodology, as applied to selected molecules of the original set. Further insight is gained by model calculations adding up the contribution of the coupling between electric dipole moments associated with normal mode behavior and that of the polarizability from polarizable groups. The former part is responsible for the excitonic behavior of the VCD spectra. For the same selected molecules we have also investigated whether some excitonic behavior is taking place in the second overtone region, and have concluded that this is not the case.

Infrared spectra of the Cl––C2H4 and Br––C2H4 anion dimers

Infrared spectra of mass-selected Cl- -C2H4 and Br- -C2H4 complexes are recorded in the vicinity of the ethylene CH stretching vibrations (2700-3300 cm(-1) using vibrational predissociation spectroscopy. Spectra of both complexes exhibit 6 prominent peaks in the CH stretch region. Comparison with calculated frequencies reveal that the 4 higher frequency bands are associated with CH stretching modes of the C2H4 subunit, while the 2 weaker bands are assigned as overtone or combinations bands gaining intensity through interaction with the CH stretches. Ab initio calculations at the MP2/aug-cc-pVDZ level suggest that C2H4 preferentially forms a single linear H-bond with Cl- and Br- although a planar bifurcated configuration lies only slightly higher in energy (by 110 and 16 cm(-1), respectively). One-dimensional potential energy curves describing the in-plane intermolecular bending motion are developed which are used to determine the corresponding vibrational energies and wavefunctions. Experimental and theoretical results suggest that in their ground vibrational state the Cl- -C2H4 and Br- -C2H4 complexes are localized in the single H-bonded configuration, but that with the addition of modest amounts of internal energy, the in-plane bending wavefunction also has significant amplitude in the bifurcated structure.

Infrared Spectroscopy of Size-Selected Benzene−Water Cluster Cations [C6H6−(H2O)n]+ (n = 1−23): Hydrogen Bond Network Evolution and Microscopic Hydrophobicity

Infrared (IR) spectra of size-selected benzene−water cluster cations ([C6H6−(H2O)n]+, n = 1−23) were measured in the OH and CH stretch regions to investigate the cluster structures, especially at large sizes. In the size range of n = 4−23, the IR spectra show features almost identical to those of protonated water cluster cations H+(H2O)n, consistent with the occurrence of the intracluster proton-transfer reaction from the benzene moiety to the water moiety, as suggested in the previous IR and electronic spectroscopic studies of the small-sized clusters. The structure of the protonated water moiety was found to be almost the same as that of H+(H2O)n up to n = 23, including the characteristic three-dimensional cage structure at n = 21. On the basis of the IR spectra, it was demonstrated that the phenyl radical participates in the direct solvation of the H3O+ core in the clusters of n ≲ 10. In the larger sized clusters, while the protonated water moiety forms the exclusive hydrogen bond network, the phenyl r...

A Comparative Study of Overtone CH-Stretching Vibrational Circular Dichroism Spectra of Fenchone and Camphor

Near-infrared vibrational circular dichroism (VCD) spectra in the region 1500−850 nm are recorded for both enantiomers of camphor and fenchone using a home-built dispersive instrument. Two CH-stretching overtone regions and two CH-stretching/HCH-bending combination regions are investigated. The near-infrared VCD data are examined and compared with the corresponding previously published data of (3R)-(+)-methylcyclopentanone and (3R)-(+)-methylcyclohexanone. The absorption spectra of both the fundamental and overtone regions are also considered, and the fundamental CH-stretching VCD data are taken into account, where possible. An empirical assignment of the spectra is first considered, with special regard to the second CH-stretching overtone region. This region is then investigated by a heuristic approach based on DFT calculations, using the hybrid functional B3LYP at the 6-31G** basis set level. Subsequently rotational strengths have been evaluated by use of a classical model based on the contribution of p...

Surface Structural Characterization of Protein- and Polymer-Modified Polystyrene Microspheres by Infrared−Visible Sum Frequency Generation Vibrational Spectroscopy and Scanning Force Microscopy

Structural investigations of bare and surface-modified polystyrene microspheres (beads) have been carried out by infrared-visible sum frequency generation (SFG) vibrational spectroscopy and scanning force microscopy (SFM). Bead surfaces have been modified by either the covalent linking of immunoglobulin G (IgG) and bovine serum albumin (BSA) or the nonspecific adsorption of a Pluronic surfactant. After surface modification with protein, SFG signals in the aliphatic CH-stretch region are detected at both the buffer/bead and air/bead interfaces, indicating that some amino acid residues in proteins adopt preferred orientations. SFG results indicate that the hydrophobic poly(propylene glycol) moieties in the Pluronic order when adsorbed onto the bead, at both the buffer/bead and air/bead interfaces, whereas hydrophilic poly(ethylene glycol) groups align to a lesser extent. SFG spectra also show that the phenyl rings of bare polystyrene beads in contact with air or buffer are ordered, with a dipole component directed along the surface normal, but become less ordered after the adsorption of either proteins or the polymer. Molecular orientation and ordering at the bead surface affect its hydrophobicity and aggregation behavior. SFM results reveal the formation of nonuniform islands when bare beads with more hydrophobic character are spun-cast onto a silica substrate. In the presence of adsorbed protein, a hexagonal packing of beads, with some defects, is observed, depending on the bulk pH and the type of attached protein. Adsorbed Pluronic causes the beads to aggregate in a disordered fashion, as compared to the behavior of bare and protein-modified beads.

Structures of F - -(CH4)n and Cl - -(CH4)n (n = 1,2) Anion Clusters Elucidated through Ab Initio Calculations and Infrared Spectra

Ab initio calculations are performed at the MP2/aug-cc-pVTZ level for F−-CH4 and Cl−-CH4, to show that the dimers have C3v symmetry with the CH4 sub-unit attached to the halide anion by a single hydrogen bond. This geometry is consistent with infrared spectra of F−-CH4 and Cl−-CH4 recorded in the CH-stretch region. The calculations also indicate substantial anharmonicity in the H-bonded CH stretch of F−-CH4. Infrared spectra of the F−-(CH4)2 and Cl−-(CH4)2 trimer clusters are consistent with structures that have two equivalent CH4 sub-units H-bonded to the halide core. Additional bands in the F−-(CH4)2 spectrum are assigned as transitions to CH4 bending overtone and combination levels, gaining infrared intensity from Fermi interaction with the H-bonded CH stretch.

Electric Field-Induced Changes in Lipids Investigated by Modulated Excitation FTIR Spectroscopy

The effect of electric fields on dry oriented multibilayers of dimyristoylphosphatidylcholine (DMPC) was investigated by transmission Fourier transform infrared electric field modulated excitation (E-ME) spectroscopy. A periodic rectangular electric potential (0–150 V, 1.25 Hz, 28.4°C ± 0.2°C) was applied across the sample. To discriminate electric field-induced effects from possible temperature-induced effects resulting from a current flow (<1 pA) across the sample, corresponding temperature-modulated excitation (T-ME) measurements within the temperature uncertainty limits of ±0.2°C at 28.4°C were performed. T-ME induced reversible gauche defects in the hydrocarbon chains, whereas E-ME resulted in reversible compression of dry DMPC bilayers. Periodic variation of the tilt angle of the hydrocarbon chains is suggested. The degree of absorbance modulation in the CH-stretching region was found to be in the order of 1:700, corresponding to a variation of the bilayer thickness of Δ z = 0.0054 nm. Using a series connection of capacitors as equivalent circuit of the cell resulted in E = (1.2 ± 0.7) × 10 7 V/m for the electric field in DMPC. Young's elasticity modulus of DMPC could be calculated to be E ⊥ = 2.2 × 10 6 Pa ± 1.8 × 10 6 Pa, which is in good agreement with published data obtained by electric field-dependent capacitance measurements.

PM FTIRRAS Studies of Potential-Controlled Transformations of a Monolayer and a Bilayer of 4-Pentadecylpyridine, a Model Surfactant, Adsorbed on a Au(111) Electrode Surface

Polarization modulation Fourier transform infrared reflection−absorption spectroscopy (PM FTIRRAS) has been combined with electrochemistry to monitor the potential-induced transformations of a monolayer and a bilayer formed by 4-pentadecylpyridine (C15−4Py), a model amphiphilic compound, at a Au(111) electrode surface. The optical constants for a solution of randomly oriented C15−4Py molecules have been determined and used to calculate the integrated band intensities for a monolayer and bilayer of randomly oriented molecules. The orientation of the hydrophobic and hydrophilic parts of the molecule with respect to the surface normal was determined from the ratio of the intensity of experimental to calculated bands. The CH stretch region of the spectra was used to determine the tilt angle between hydrocarbon chains and the surface normal. The tilt angle varied with potential. The minimum value of the tilt angle was 16° for the monolayer and 27° for the bilayer. In the monolayer, the headgroup of the C15−4Py...

Vibrational spectra of diethylsulfoxide

FT IR and Raman spectroscopic studies of pure diethylsulfoxide (DESO) in the liquid and in the solid states and its solutions in various solvents have been performed. Analysis of SO- and CH-stretching regions in a wide range of concentration shows that the bands may be fitted satisfactorily by considering seven components. In addition, fundamental frequencies have been assigned using ab initio calculations at the RHF/3–21G* levels. The results obtained confirm a viewpoint on a self-associative structure of DESO, and support the hypothesis of the existence of different types of intermolecular associates including both dipole–dipole and hydrogen bonding mechanisms.

Orientation and vibrational relaxation of acetonitrile at a liquid:solid interface, observed by sum-frequency spectroscopy

The vibrational spectrum of acetonitrile at the liquid:solid interface with yttrium aluminium garnet is investigated in the CN- and CH-stretching region, using two-color pump–probe sum-frequency spectroscopy in internal total reflexion geometry. An average tilt angle of the molecular symmetry axis with the surface normal of ≅53° is determined suggesting weak interaction with the surface. Measuring the coherent excitation of the symmetric CH 3-transition yields a dephasing time of T 2=420±100 fs. An effective population lifetime of 5.9±1.8 ps and a faster energy redistribution with a time constant of 1.8±1.0 ps are found in the CH-stretching region. Ground state recovery of the molecules at the interface occurs slowly with 150±50 ps, quite different to the bulk liquid.

The Organic Refractory Material in the Diffuse Interstellar Medium: Mid‐Infrared Spectroscopic Constraints

This is an analysis of the 4000¨1000 cm~1 (2.5¨10 km) region of the spectrum of diUuse interstellar medium (DISM) dust compared with the spectra of 13 materials produced in the laboratory which serve as analogs to the interstellar material. The organic signatures of extragalactic dust, carbonaceous chondritic material, and E. coli bacteria are also presented because these have been discussed in the literature as relevant to the diUuse interstellar medium. Spectral analysis of the DISM allows us to place signi—cant constraints on the applicability of proposed candidate materials. The spectra of candidate materials are evaluated using four spectral characteristics based on the interstellar data: (i) comparisons of the pro—le and subpeak positions of the 2940 cm~1 (3.4 km) aliphatic CH stretching-mode band, (ii) the ratio of the optical depth (O.D.) of the aliphatic CH stretch to the O.D. of the OH stretch near 3200 cm~1 (3.1 km), (iii) the ratio of the O.D. of the aliphatic CH stretch to the O.D. of the carbonyl band near 1700 cm~1 (5.9 km), and (iv) the ratio of the O.D. of the aliphatic CH stretch feature to the O.D. of the CH deformation modes near 1470 cm~1 (6.8 km) and 1370 cm~1 (7.25 km). We conclude that the organic refractory material in the diUuse interstellar medium is predominantly hydrocarbon in nature, possessing little nitrogen or oxygen, with the carbon distributed between the aromatic and aliphatic forms. Long alkane chains with n much greater than 4 or 5 are not major constituents H 3 Cw(CH 2 ) n w of this material. Comparisons to laboratory analogs indicate the DISM organic material resembles plasma processed pure hydrocarbon residues much more so than energetically processed ice residues. This result is consistent with a birth site for the carrier of the 3.4 km band in the out—ow region of evolved carbon stars. The organic material extracted from the Murchison carbonaceous meteorite and the spectrum of E. coli bacteria reveal spectral features in the 5¨10 km region that are absent in the DISM. Although the presence of unaltered circumstellar components in the Murchison meteorite has been established through several lines of evidence, it is unclear whether or not the aliphatic component which gives rise to the 3.4 km band is in that category. Considering the complete 2¨10 km wavelength region, there is no spectral evidence for a biological origin of the 3.4 km interstellar absorption band. The similarity of the aliphatic CH stretch region of dust from our own Galaxy compared with that of distant galaxies suggests that the organic component of the ISM is widespread and may be an important universal reservoir of prebiotic organic carbon. )

Matrix isolation study of the interaction of excited neon atoms with allene and propyne: Infrared spectra of H2CCCH2+ and H2CCCH−

When a Ne:allene or a Ne:propyne sample was codeposited at approximately 5 K with a sample of pure neon that had been excited in a microwave discharge to provide a 16.6–16.85 eV energy source, prominent new infrared absorptions which can be assigned to the H2CCCH2+ cation appeared. Also present in the allene experiments were two absorptions which can be assigned to H2CCCH−. In the propyne experiments, an absorption is tentatively attributed to the strongest infrared fundamental of CH3CCH+. The structures and vibrational fundamentals obtained from density functional and ab initio calculations for various isotopomers of H2CCCH2+, CH3CCH+, cyc-C3H3+, H2CCCH+, and H2CCCH− are given. The infrared absorption pattern of the cation common to the allene and propyne experiments matches that of the predominant gas-phase product, cyc-C3H3+, reasonably well, except for the presence of extra peaks in the CH-stretching region. However, comparison of the results of experiments on isotopically substituted samples with the calculated spectra excludes that assignment and supports the identification of the neon-matrix product as H2CCCH2+. It is suggested that collisions with the excess of neon atoms in the sampling region rapidly remove excess energy from the initially formed allene and propyne cations, inhibiting the loss of an H atom from those two species.

The Infrared and Ultraviolet Spectra of Individual Conformational Isomers of Biomolecules: Tryptamine

Resonant ion-dip infrared (RIDIR) and UV−UV hole-burning spectroscopies are used to record the hydride stretch infrared spectra and S1←S0 ultraviolet spectra of each of seven conformational isomers of tryptamine free from interference from one another. The different conformations of the ethylamine side chain produce unique S1←S0 vibronic spectra, which can serve as the basis for RIDIR spectroscopy. The seven conformers possess unique spectral signatures in the alkyl CH stretch region of the infrared, which aid in the assignment of the observed transitions in the ultraviolet. Density functional theory (DFT) calculations of the structures, relative energies, and harmonic vibrational frequencies of nine low-energy minima are compared with the present and previous experimental data on tryptamine to assign the spectra of all seven conformers, all of which point the α carbon out of the plane of the indole ring. The nine conformers consist of all combinations of the three minimum-energy amino group positions (an...

Ultrafast vibrational energy redistribution within C–H and O–H stretching modes of liquid methanol

An ultrafast IR–Raman technique is used to study fast redistribution of vibrational energy within CH and OH stretching vibrations of liquid methanol. The instantaneous population distribution throughout the entire CH and OH stretch region (2700–3700 cm −1) is probed simultaneously. The fast redistribution produces a nonthermal population distribution within CH and OH stretching excitations. The nonequilibrium CH stretch population occurs because different CH stretching modes have different lifetimes. The nonequilibrium OH stretch population occurs when OH stretching excitations that are pumped by the laser on the red edge of the band undergo spectral diffusion toward the band center.

Infrared intensities. Use of the CH-stretching band intensity as a tool for evaluating the acidity of hydrogen atoms in hydrocarbons

The accurate study of the infrared spectra (both frequencies and intensities) of a large number of molecules and the decodification of their intensities in terms of parameters representing changes of charge distribution with vibration have provided standards that, at least in the case of molecules with an appreciable presence of CH bonds, may now be used to give information on the acidity of the hydrogen atoms even based only on the qualitative aspects of the infrared spectrum in the CH-stretching region. The charge on the hydrogen atom of a CH bond may change by hybridization of the carbon atom, by induction of polarization in the CH bond by a close electronegative atom, by backdonation into the CH bond of negative charge from any lone pair of an electronegative atom and with hyperconjugation of CH 3 and CH 2 with the electrons of double and triple bonds. The variation of the hydrogen charge is also accompanied by a change in the CH stretching frequencies, as reported in several works by McKean and coworkers. It has to be pointed out that the change in the CH stretching intensities is almost always more noticeable and often so large that it can be detected by the simple qualitative examination of the spectrum. We present here examples of such a qualitative analysis.

Theoretical and experimental studies for the interpretation of vibrational circular dichroism spectra in the CH-stretching overtone region

Two theoretical models for the interpretation of the existing data of CH-stretching overtones' vibrational circular dichroism data are presented. The first model is based on the quantum mechanical Van Vleck contact transformation theory and is applied to the full vibrational problem, the second is based on classical trajectories calculations, by which we treat a simplified three-degrees of freedom Hamiltonian. The latter allows one to derive a qualitative but efficacious picture of the behavior of coupled anharmonic oscillators. In this framework, we analyze the Poincare Surfaces of Section, and calculate the Fourier Cross Spectra of coupled CH-stretchings. Values for the harmonic frequencies and anharmonicities are derived from absorption spectra in the near infrared on partially deuterated compounds. The effect of large amplitude, low-frequency puckering or twisting modes on the ensemble of coupled CH-stretching is taken into account. Copyright 2000 Wiley-Liss, Inc.