CH Stretch Vibrations Research Articles

Contrasting patterns of coupling between the CH stretches and the large-amplitude motions in the molecules, CH3NH2, CH3OH2+ and CH3CH2[rad

In each of the title molecules, torsional and inversion tunneling occurs between six equivalent minima. The coupling between these large-amplitude motions and the CH stretch vibrations stems from the variation of the CH stretching force constants as a function of the torsional and inversion coordinates. First principles 2D maps of the variation of the CH-stretch force constants are reported. Although the torsional barriers differ by more than a factor of 20, the torsion–inversion–vibration coupling is similar for two of the species, CH3NH2 and CH3CH2, but is much weaker and qualitatively different in form for the third species, CH3OH2+.

Discrimination between oral cancer and healthy tissue based on water content determined by Raman spectroscopy.

Tumor-positive resection margins are a major problem in oral cancer surgery. High-wavenumber Raman spectroscopy is a reliable technique to determine the water content of tissues, which may contribute to differentiate between tumor and healthy tissue. The aim of this study was to examine the use of Raman spectroscopy to differentiate tumor from surrounding healthy tissue in oral squamous cell carcinoma. From 14 patients undergoing tongue resection for squamous cell carcinoma, the water content was determined at 170 locations on freshly excised tongue specimens using the Raman bands of the OH-stretching vibrations (3350-3550 cm(-1)) and of the CH-stretching vibrations (2910-2965 cm(-1)). The results were correlated with histopathological assessment of hematoxylin and eosin stained thin tissue sections obtained from the Raman measurement locations. The water content values from squamous cell carcinoma measurements were significantly higher than from surrounding healthy tissue (p-value < 0.0001). Tumor tissue could be detected with a sensitivity of 99% and a specificity of 92% using a cutoff water content value of 69%. Because the Raman measurements are fast and can be carried out on freshly excised tissue without any tissue preparation, this finding signifies an important step toward the development of an intraoperative tool for tumor resection guidance with the aim of enabling oncological radical surgery and improvement of patient outcome.

Broadband two dimensional infrared spectroscopy of cyclic amide 2-Pyrrolidinone.

In the past one-and-a-half decade there has been a significant methodological and technological development of two dimensional infrared (2DIR) spectroscopy, which unfolds many underlying physical and chemical processes of complex molecules, especially for biological molecules. Due to the extreme technical difficulties and non-uniform performance of ultrafast laser, so far, the method has been mostly applied to a small spectral region. A rather simple experimental methodology is presented here which is able to cover a broad spectral range from 1500 cm(-1) to 3500 cm(-1) to explore the molecular structure of cyclic amide, 2-Pyrrolidinone, via the time-resolved coupling of CO, CH and NH stretch vibrations. The signature of the coherent as well as incoherent coupling has been found. The amide-I band is incoherently coupled to CH and NH stretch vibrations and acts as an acceptor mode for vibrational energy relaxation from CH and NH stretch vibrations.

Communication: conical intersections between vibrationally adiabatic surfaces in methanol.

A set of seven conical intersections (CI's) in methanol between vibrationally adiabatic surfaces is reported. The intersecting surfaces represent the energies of the two asymmetric CH stretch vibrations regarded as adiabatic functions of the torsion and COH bend angles. The ab initio data are well described by an extended Zwanziger and Grant (E ⊗ e) model [J. W. Zwanziger and E. R. Grant, J. Chem. Phys. 87, 2954 (1987)] that might also be regarded as an extension of the XHL model [L.-H. Xu, J. T. Hougen, and R. M. Lees, J. Mol. Spectrosc. 293-294, 38 (2013)]. The CI's illuminate the role of geometric phase in methanol. More generally, they suggest the importance of energy transfer processes localized near the CI's.

Stimulated Raman spectral microscope using synchronized Er- and Yb-fiber lasers

We report our development of a stimulated Raman scattering (SRS) spectral microscope employing synchronized fiber-laser (FL)-based pulse sources, which generate both pump and Stokes pulses. The pump pulses are obtained through the second-harmonic generation of Er-FL pulses. The high-frequency noise in the photocurrent of the pump pulses is successfully suppressed to the shot-noise limit by the collinear balanced detection (CBD) technique using a compact fiber delay line. Fast wavelength-tunable Stokes pulses are generated through the spectral filtering of broadband Yb-FL pulses and post amplification. To achieve a wide wavelength tunability, we introduce a double-pass spectral filter and confirm that the wavelength tunability can be extended to ∼280 cm−1, while the wavelength tunability with a single-pass spectral filter is ∼190 cm−1. The spectral filter is also used to compensate for the group delay dispersion (GDD) of Stokes pulses, which is induced by fiber amplifiers. We show that the negative dispersion can be introduced to the pulses simply by shifting the position of the diffraction grating in the spectral filter. Although the wavelength tunability is slightly decreased to 230 cm−1 after the compensation, it is sufficient for covering the CH-stretching vibration region. The SRS imaging and microspectroscopy of polymer beads and HeLa cells are successfully demonstrated.

Torsion–Inversion Tunneling Patterns in the CH-Stretch Vibrationally Excited States of the G12 Family of Molecules Including Methylamine

Two torsion-inversion tunneling models (models I and II) are reported for the CH-stretch vibrationally excited states in the G12 family of molecules. The torsion and inversion tunneling parameters, h(2v) and h(3v), respectively, are combined with low-order coupling terms involving the CH-stretch vibrations. Model I is a group theoretical treatment starting from the symmetric rotor methyl CH-stretch vibrations; model II is an internal coordinate model including the local-local CH-stretch coupling. Each model yields predicted torsion-inversion tunneling patterns of the four symmetry species, A, B, E1, and E2, in the CH-stretch excited states. Although the predicted tunneling patterns for the symmetric CH-stretch excited state are the same as for the ground state, inverted tunneling patterns are predicted for the asymmetric CH-stretches. The qualitative tunneling patterns predicted are independent of the model type and of the particular coupling terms considered. In model I, the magnitudes of the tunneling splittings in the two asymmetric CH-stretch excited states are equal to half of that in the ground state, but in model II, they differ when the tunneling rate is fast. The model predictions are compared across the series of molecules methanol, methylamine, 2-methylmalonaldehyde, and 5-methyltropolone and to the available experimental data.

Anharmonic coupling of the CH-stretch and CH-bend vibrations of chloroform as studied by near-infrared electroabsorption spectroscopy

Combination bands that involve CH- or OH-stretch vibrations appear in the near-infrared (NIR) region (4000-10 000 cm(-1)). Because they arise from anharmonic coupling between the component fundamentals, detailed analysis of the frequency and intensity of NIR combination bands allows one to elucidate the mechanisms behind the vibrational coupling in the condensed phase in terms of mechanical and electrical anharmonicities. Nevertheless, little has been studied, in particular experimentally, on the origin of the combination band intensity. Here, we show that NIR electroabsorption (EA) spectroscopy, which directly probes the effects of an externally applied electric field on a combination band, can shed new light on anharmonic vibrational coupling through determination of the direction of the transition moment for the combination band. We studied the combination band of the CH-stretch (ν(1)) and CH-bend (ν(4)) modes of liquid chloroform. The electric-field induced absorbance change of the ν(1) + ν(4) combination band caused by reorientation of the chloroform molecule was measured at various χ angles, where χ is the angle between the direction of the applied electric field and the polarization of the incident IR light. We were able to detect an absorbance change as small as 5 × 10(-8) for the combination band. Using the NIR EA spectra of the combination band together with those of the CH-stretch and bend fundamentals, the angle between the transition moment for the combination band and the permanent dipole moment was determined experimentally for the first time to be (79 ± 14)°. The present investigation indicates that the contribution of the CH-stretch mode to the mechanical anharmonicity is minor and that the CH-bend mode plays a dominant role in the mechanical part of the vibrational coupling between the two fundamentals. Furthermore, density functional theory calculations show that both the mechanical anharmonicity of the CH-bend mode and the electrical anharmonicity may contribute equally to the anharmonic coupling.

Chemically‐specific dual/differential CARS micro‐spectroscopy of saturated and unsaturated lipid droplets

We have investigated the ability of dual-frequency Coherent Antistokes Raman Scattering (D-CARS) micro-spectroscopy, based on femtosecond pulses (100 fs or 5 fs) spectrally focussed by glass dispersion, to distinguish the chemical composition of micron-sized lipid droplets consisting of different triglycerides types (poly-unsaturated glyceryl trilinolenate, mono-unsaturated glyceryl trioleate and saturated glyceryl tricaprylate and glyceryl tristearate) in a rapid and label-free way. A systematic comparison of Raman spectra with CARS and D-CARS spectra was used to identify D-CARS spectral signatures which distinguish the disordered poly-unsaturated lipids from the more ordered saturated ones both in the CH-stretch vibration region and in the fingerprint region, without the need for lengthy CARS multiplex acquisition and analysis. D-CARS images of the lipid droplets at few selected wavenumbers clearly resolved the lipid composition differences, and exemplify the potential of this technique for label-free chemically selective rapid imaging of cytosolic lipid droplets in living cells.

Structures, properties and nature of DMSO–XY (XY = ClF and BrF) complexes: Redshift and blueshift of S[dbnd]O stretch

The DMSO–XY (XY=ClF and BrF) complexes have been investigated with quantum chemical calculations. In general, two minima complexes were found, one with an O···X halogen bond and the other one with a S···X halogen bond. The former is more stable than the latter. Additionally, one first order saddle point complex was also observed. The interaction energies in the S complexes suffer a prominent influence from the calculation methods. At the CCSD(T)/aug-cc-pVDZ level, the interaction energies are calculated to be −9.19 and −12.73kcal/mol for the Cl and Br global minima, respectively. Both complexes have also been evidenced to be stable at room temperature. The SO stretch vibration exhibits a red shift at the global minimum but a blue shift at the local minimum, whereas the CSC and CH stretch vibrations move to high frequency in both cases. The energy decomposition analyses indicate that the electrostatic interaction plays a dominant role in stabilizing these halogen-bonded complexes.

Revealing structural properties of the marine nanolayer from vibrational sum frequency generation spectra

Natural nanolayers originating from sea surface and subsurface water samples collected in the Baltic Sea have been investigated using surface-sensitive vibrational sum frequency generation (VSFG) spectroscopy. Distinct spectral signatures of CH and OH bond stretch vibrations have been detected at wavenumbers ranging from 2700 to 3900 cm(-1). Measured water-air interface spectra as well as observed signal intensity trends are discussed in terms of composition and structure of the natural organic nanolayer. Reasoning was based on the comparison with reference spectra, spectral trends inferred from previous VSFG studies, reported average composition of dissolved organic matter in seawater, and simplified assumption that surfactants can be classified as soluble (wet) and insoluble (dry) surfactants. Wet surfactants have been found to be dominant, and often lipid-like compounds form a very dense surfactant nanolayer. Supported by comparison spectra of xanthan gum solutions, the observed VSFG spectral signatures were tentatively assigned to lipopolysaccharides or other lipid-like compounds embedded in colloidal matrices of polymeric material. In addition, VSFG spectra of a polluted harbor water sample and a water sample covered with diesel oil are reported.

Hyperspectral data processing for chemoselective multiplex coherent anti-Stokes Raman scattering microscopy of unknown samples

Multiplex coherent anti-Stokes Raman scattering (MCARS) provides labeling free and fast characterization of materials and biological samples in nonlinear microscopy. In spite of its success, remaining challenges regarding the data analysis for chemoselective imaging still have to be solved. In general, image contrast has been realized by using only one spectral feature directly taken from the unprocessed raw data. This procedure is limited to strong and well separated Raman resonances like the saturated CH-stretching vibration of lipids in the case of biological samples. In order to overcome this limitation, we present a new method of MCARS data processing that exploits the whole measured spectrum to disentangle overlapping contributions of different (bio-) chemical components. Our "two-step" approach is based on the combination of imaginary part extraction followed by global fitting of the hyperspectral data set. Previous knowledge about the sample, e.g., pure spectra of the individual components is no longer necessary. The result is a highly contrasted image, where the patterns and differences between the sample components can be represented in different colors. We successfully applied this method to complex structured polymer samples and biological tissues.

Calibration of mid-infrared transmission measurements for hydrocarbon detection and propane concentration measurements in harsh environments by using a fiber optical sensor

The local fuel concentration at the ignition position is a crucial parameter, especially for spark ignited engines. Hydrocarbon concentration can be determined by infrared transmission measurements because of the attenuation of infrared radiation in the 2940 cm −1 frequency range due to the excitation of fundamental CH-stretch vibrations of the molecules. In-cylinder measurements can be done by modified spark plugs with integrated absorption paths. A tungsten halide lamp was used in the experiments as an infrared light source. There is a non-linear relation between measured transmissions and hydrocarbon densities. So far, infrared transmission measurements have to be calibrated on absolute density values by using various hydrocarbon test gases with different concentrations. Furthermore in-cylinder transmission measurements can be calibrated at homogeneous engine operation conditions. In this case, a well mixed air/fuel composition is expected at the end of the compression stroke. Both time-consuming calibration processes lead to a new calibration procedure for band integrated transmission measurements which is presented in this study. The non-linear correlations between band integrated transmissions and absorber densities were calculated by using the absorption cross section as a spectral characteristic of absorbing molecules and the filter transmission as the spectral influence of the experimental setup. The differences between measurement and calculation can be corrected by a single measurement using a test gas with known hydrocarbon concentration. The calibration procedure was verified on temperature and pressure influences by measurements using a heatable optical cell at pressures up to 1800 kPa and within a temperature range from 298 to 473 K. Finally, the calibration procedure was adapted from cell measurements to an infrared fiber optical sensor which was used for in-cylinder measurements. An adapter for the spark plug bore enables a parallel use of a flame-ionization-detector and the optical sensor. Simultaneous propane concentration measurements were made in a motored engine with both measurement systems.

Differential Imaging of Biological Structures with Doubly-resonant Coherent Anti-stokes Raman Scattering (CARS)

Coherent Raman imaging techniques have seen a dramatic increase in activity over the past decade due to their promise to enable label-free optical imaging with high molecular specificity 1. The sensitivity of these techniques, however, is many orders of magnitude weaker than fluorescence, requiring milli-molar molecular concentrations 1,2. Here, we describe a technique that can enable the detection of weak or low concentrations of Raman-active molecules by amplifying their signal with that obtained from strong or abundant Raman scatterers. The interaction of short pulsed lasers in a biological sample generates a variety of coherent Raman scattering signals, each of which carry unique chemical information about the sample. Typically, only one of these signals, e.g. Coherent Anti-stokes Raman scattering (CARS), is used to generate an image while the others are discarded. However, when these other signals, including 3-color CARS and four-wave mixing (FWM), are collected and compared to the CARS signal, otherwise difficult to detect information can be extracted 3. For example, doubly-resonant CARS (DR-CARS) is the result of the constructive interference between two resonant signals 4. We demonstrate how tuning of the three lasers required to produce DR-CARS signals to the 2845 cm-1 CH stretch vibration in lipids and the 2120 cm-1 CD stretching vibration of a deuterated molecule (e.g. deuterated sugars, fatty acids, etc.) can be utilized to probe both Raman resonances simultaneously. Under these conditions, in addition to CARS signals from each resonance, a combined DR-CARS signal probing both is also generated. We demonstrate how detecting the difference between the DR-CARS signal and the amplifying signal from an abundant molecule's vibration can be used to enhance the sensitivity for the weaker signal. We further demonstrate that this approach even extends to applications where both signals are generated from different molecules, such that e.g. using the strong Raman signal of a solvent can enhance the weak Raman signal of a dilute solute.

Differential Imaging of Biological Structures with Doubly-resonant Coherent Anti-stokes Raman Scattering (CARS)

Coherent Raman imaging techniques have seen a dramatic increase in activity over the past decade due to their promise to enable label-free optical imaging with high molecular specificity 1. The sensitivity of these techniques, however, is many orders of magnitude weaker than fluorescence, requiring milli-molar molecular concentrations 1,2. Here, we describe a technique that can enable the detection of weak or low concentrations of Raman-active molecules by amplifying their signal with that obtained from strong or abundant Raman scatterers. The interaction of short pulsed lasers in a biological sample generates a variety of coherent Raman scattering signals, each of which carry unique chemical information about the sample. Typically, only one of these signals, e.g. Coherent Anti-stokes Raman scattering (CARS), is used to generate an image while the others are discarded. However, when these other signals, including 3-color CARS and four-wave mixing (FWM), are collected and compared to the CARS signal, otherwise difficult to detect information can be extracted 3. For example, doubly-resonant CARS (DR-CARS) is the result of the constructive interference between two resonant signals 4. We demonstrate how tuning of the three lasers required to produce DR-CARS signals to the 2845 cm-1 CH stretch vibration in lipids and the 2120 cm-1 CD stretching vibration of a deuterated molecule (e.g. deuterated sugars, fatty acids, etc.) can be utilized to probe both Raman resonances simultaneously. Under these conditions, in addition to CARS signals from each resonance, a combined DR-CARS signal probing both is also generated. We demonstrate how detecting the difference between the DR-CARS signal and the amplifying signal from an abundant molecule's vibration can be used to enhance the sensitivity for the weaker signal. We further demonstrate that this approach even extends to applications where both signals are generated from different molecules, such that e.g. using the strong Raman signal of a solvent can enhance the weak Raman signal of a dilute solute.

Intramolecular Vibrational Energy Redistribution in Aromatic Molecules of Type C6H5X (X = H, D, F, Cl, CH3, CF3)

Femtosecond IR pump UV probe spectroscopy was employed in the gas phase to study intramolecular vibrational energy redistribution (IVR) in benzene and five monosubstituted derivatives thereof. After selective excitation of the first overtone of the ring CH-stretch vibration, all molecules showed the same two-step redistribution dynamics characteristic for nonstatistical IVR. The nature of the substituent influences mainly the second, slower IVR component. The presence of an internal rotor does not alter the redistribution rate or pathway compared to that of a monatomic substituent of equal mass. Coupling order model calculations reflect the experimental trends well if the polyatomic substituents are regarded as decoupled from the intra-ring dynamics and modeled as point masses.

ChemInform Abstract: IR and Raman Study of Monohydrogenated Cyclopentenes-3-h1 and -4-h1: CH Stretching Vibration and Its Two First Overtones in Gas Phase.

ChemInform Abstract: IR and Raman Study of Monohydrogenated Cyclopentenes-3-h1 and -4-h1: CH Stretching Vibration and Its Two First Overtones in Gas Phase.

Quantum-chemical study and FTIR jet spectroscopy of CHCl3–NH3 association in the gas phase

High level ab initio quantum chemical calculations have been performed on the association of chloroform with ammonia in the gas phase (counterpoise corrected MP2 and coupled-cluster CCSD(T) calculations with 6-311++G(d,p) basis functions). Minimum energy equilibrium structures have been found for CHCl(3)-NH(3) dimer, CHCl(3)-(NH(3))(2) trimer and CHCl(3)-(NH(3))(3) tetramer. Association is characterised by a CHN hydrogen bond between a chloroform and an ammonia molecule, with further ammonia units attached by hydrogen bonds to ammonia and by an electrostatic NHCl interaction to chloroform. Cooperative effects provide additional stabilisation. The complexes exhibit characteristic shifts of vibrational bands and change of IR intensity; in particular there is a pronounced progressive shift of the CH-stretching vibration towards lower wavenumber with each unit of ammonia attached in the complex. The shift is accompanied by an up to 600 fold increase in IR intensity. The experimental FTIR jet spectra have provided firm evidence of CHCl(3)-NH(3) association, with the clearest effects seen in the region of the CH-stretching vibration. First tentative assignments have been made based on the dependence of relative intensities of cluster features on the concentration of monomers, and assignments have been corroborated by the quantum chemical calculations. The present combined ab initio and FTIR spectroscopy study reveals the structure and energetics of cluster formation and intermolecular bonding in CHCl(3)-NH(3) association.

Compact coherent anti-Stokes Raman scattering microscope based on a picosecond two-color Er:fiber laser system

We present a compact coherent anti-Stokes Raman scattering microscope based on a widely tunable picosecond Er:fiber laser. Intense and bandwidth-limited 1 ps pump pulses at a center wavelength of 775 nm are generated via frequency mixing within the broadband fundamental at 1.55 microm. Narrowband Stokes pulses are obtained by frequency shifting of solitons in a highly nonlinear bulk fiber and subsequent second-harmonic generation. The tuning range from 850 nm to 1100 nm gives access to vibrational resonances between 1150 cm(-1) and 3800 cm(-1). A first imaging application in the spectral region of CH stretch vibrations is demonstrated.

SH-Stretching Vibrational Spectra of Ethanethiol and tert-Butylthiol

Vibrational spectra of vapor-phase ethanethiol and tert-butylthiol have been recorded in the 1000 to 12,000 cm(-1) region. Both the gauche and trans conformers of ethanethiol are observed. The intensities of SH-stretching vibrations are found to be significantly weaker than the equivalent CH-stretching vibrations and the OH-stretching vibrations in the corresponding alcohols. The relative strength of the SH-stretching vibrations in ethanethiol and tert-butylthiol compared with the OH-stretching vibrations in the corresponding alcohols are approximately 0.2 in the fundamental region and approximately 0.03 in the overtone regions. The SH- and OH-stretching intensities have been modeled with an anharmonic oscillator local mode model with ab initio calculated dipole moment functions. The weak nature of SH-stretching transitions is shown to be a result of both low anharmonicity of the vibrational mode and relatively small dipole moment derivatives.

A picosecond time-resolved vibrational circular dichroism spectrometer

We present an experimental setup to detect transient vibrational circular dichroism signals. A femtosecond laser system is synchronized to a photoelastic modulator to produce alternating left- and right-handed circularly polarized mid-IR pulses at 1 kHz repetition rate. Transient changes in the circular dichroism of the CH-stretch vibrations of a cobalt-sparteine complex were probed in a proof-of-principle experiment and are clearly distinct from conventional transient absorption changes.