CH Stretch Vibrations Research Articles

Nonstatistical Unimolecular Decay of the CH2OO Criegee Intermediate in the Tunneling Regime.

Unimolecular decay of the formaldehyde oxide (CH2OO) Criegee intermediate proceeds via a 1,3 ring-closure pathway to dioxirane and subsequent rearrangement and/or dissociation to many products including hydroxyl (OH) radicals that are detected. Vibrational activation of jet-cooled CH2OO with two quanta of CH stretch (17-18 kcal mol-1) leads to unimolecular decay at an energy significantly below the transition state barrier of 19.46 ± 0.25 kcal mol-1, refined utilizing a high-level electronic structure method HEAT-345(Q)Λ. The observed unimolecular decay rate of 1.6 ± 0.4 × 106 s-1 is 2 orders of magnitude slower than that predicted by statistical unimolecular reaction theory using several different models for quantum mechanical tunneling. The nonstatistical behavior originates from excitation of a CH stretch vibration that is orthogonal to the heavy atom motions along the reaction coordinate and slow intramolecular vibrational energy redistribution due to the sparse density of states.

Quantitative and Sensitive Mid-Infrared Frequency Modulation Detection of HCN behind Shock Waves

Despite its key role for the study and modeling of nitrogen chemistry and NOx formation in combustion processes, HCN has only rarely been detected under high-temperature conditions. Here, we demonstrate quantitative detection of HCN behind incident and reflected shock waves using a novel sensitive single-tone mid-infrared frequency modulation (mid-IR-FM) detection scheme. The temperature-dependent pressure broadening of the P(26) line in the fundamental CH stretch vibration band was investigated in the temperature range 670K≤T≤1460K, yielding a pressure broadening coefficient for argon of 2γAr296K=(0.093±0.007)cm−1atm−1 and a temperature exponent of nAr=0.67±0.07. The sensitivity of the detection scheme was characterized by means of an Allan analysis, showing that HCN detection on the ppm mixing ratio level is possible at typical shock wave conditions. In order to demonstrate the capability of mid-IR-FM spectroscopy for future high-temperature reaction kinetic studies, we also report the first successful measurement of a reactive HCN decay profile induced by its reaction with oxygen atoms.

Influence of Vibrational Excitation on the Reaction of F- with CH3I: Spectator Mode Behavior, Enhancement, and Suppression.

Detailed insight into chemical reaction dynamics can be obtained by probing the effect of mode-specific vibrational excitation. Suppression or enhancement of reactivity is possible as is already known from the Polanyi rules. In the reaction F– + CH3I, we found vibrational enhancement, suppression, and spectator mode dynamics in the four different reaction channels. For this system we have probed the influence of symmetric CH-stretching vibration over a collision energy range of 0.7–2.3 eV. Proton transfer is significantly enhanced, while for the nucleophilic substitution channel the spectator mode dynamics at lower collision energies unexpectedly move toward enhancement at higher collision energies. In contrast, for two halide abstraction channels, forming FI– and FHI–, we found an overall suppression, which stems mainly from a suppression of the FHI– product. We compare these results to quasiclassical trajectory calculations and with the sudden vector projection model.

Proton transfer dynamics modified by CH-stretching excitation.

Gaining insight how specific rovibrational states influence reaction kinetics and dynamics is a fundamental goal of physical chemistry. Purely statistical approaches often fail to predict the influence of a specific state on the reaction outcome, evident in a great number of both experimental and theoretical studies. Most detailed insight in atomistic reaction mechanisms is achieved using accurate collision experiments and high level dynamics calculations. For ion-molecule reactions such experiments are scarce. Here we show the influence of symmetric CH-stretching vibration on the rate and dynamics of proton transfer in the reaction of F- + CH3I. We find a pronounced shift in the reaction dynamics for excited reactions from indirect to preferred direct dynamics at higher collision energy. Moreover, excited reactions occur at larger impact parameters. Finally, we compare vibrational excitation with collision energy and find that vibration is overall more efficient in promoting reactivity, which agrees with recent theoretical calculations.

Retraction Note: Analysis of the dynamics of laser induced plume propagation from liquid matrix using fast photography

Glycerol, a liquid matrix material for matrix-assisted laser desorption ionization mass spectrometry, was irradiated by a tunable pulsed infrared laser at wavelengths of 2.80 μ.m, 2.94 μ.m, 3.10 μ.m and 3.50 μ.m, covering the OH and CH stretch vibrations. A fast photography system was introduced to analyze the dynamic process of plume propagation induced by laser ablation up to 1000 μ.s of the delay time. Propagation distance of the plume front was measured and the corresponding velocities were calculated; they varied with the wavelength and decreased with the delay time. At the tunable wavelength of the peak of the OH absorption (3.0 μ.m), theoretical calculations indicate that energy deposition from the pulsed laser is in the regime of stress confinement. The mode of energy deposition depends on the wavelength of the OH vibration and its distance from the absorption maxima. However, stages after a 10 μ.s delay at various wavelengths show a certain similarity in the distance of plume propagation, which can be well fitted by a drag model.

IR Spectra of Hydrogen-Bonded Complexes of Trifluoroacetic Acid with Acetone and Diethyl Ether in the Gas Phase. Interaction between CH and OH Stretching Vibrations.

The spectra of complexes of trifluoroacetic acid (TFA) with acetone and diethyl ether (DEE) and their perdeuterated isotopologues were extracted from the spectra of the mixture of the compounds recorded at room temperature. The ν(OH) bands of the complexes with protiated and deuterated acetone notably differ from each other, whereas these ν(OH) bands are practically not affected by the deuteration of DEE. An assumption about the interaction of CH and OH groups in the (CH3)-C═O···HO fragment is made. According to density functional theory calculations, complexes of TFA with both acetone and DEE have a cyclic structure with one strong ═O···HO hydrogen bond and one weak CH···O═ bond. The structural, spectroscopic, and electronic properties indicate an essential role of weak bonds in the total complexation energy of the systems studied.

Spectral fingerprinting of decellularized heart valve scaffolds.

Decellularized heart valves hold promise for their use as bioscaffolds in cardiovascular surgery. Quality assessment of heart valves after decellularization processing and/or storage is time consuming and destructive. Fourier transform infrared spectroscopy (FTIR) allows rapid non-invasive assessment of biomolecular structures in tissues. In this study, IR-spectra taken from different layers of the pulmonary artery trunk and leaflet tissues of decellularized porcine heart valves were compared with those of pure collagen and elastin, the main protein components in these tissues. In addition, spectral changes associated with aging and oxidative damage were investigated. Infrared absorbance spectra of the arteria intima and media layer were found to be very similar, whereas distinct differences were observed when compared with spectra of the externa layer. In the latter, the shape of the CH-stretching vibration region (3050-2800 cm-1) resembled that of pure collagen. Also, pronounced νCOOH and amide-II bands and a relatively high content of α-helical structures in the externa layer indicated the presence of collagen in this layer. The externa layer of the artery appeared to be sensitive to collagenase treatment, whereas the media and intima layer were particularly affected by elastase and not by collagenase treatment. Protein conformational changes after treatment with collagenase were observed in all three layers. Collagenase treatment completely degraded the leaflet tissue sections. Spectra were also collected from scaffolds after 2 and 12 weeks storage at 37 °C, and after induced oxidative damage. Spectral changes related to aging and oxidative damage were particularly evident in the CH-stretching region, whereas the shape of the amide-I band, reflecting the overall protein secondary structure, remained unaltered.

Simultaneous dual-channel stimulated Raman scattering microscopy demultiplexed at distinct modulation frequencies.

To increase the information per pixel in stimulated Raman scattering (SRS) microscopy as well as to correct from artifacts, it is valuable to acquire images at two different Raman shifts. We present a three-color SRS approach acquiring two perfectly registered SRS images where both pump beams are modulated at distinct frequencies while demodulating the Stokes beam. Our implementation uses two optical parametric oscillators that can be tuned to an almost arbitrary energy difference of Raman shifts, allowing investigation of fingerprint resonances simultaneously to CH-stretch vibrations.

Structural sensitivity of C[sbnd]H vibrational band in methyl benzoate

The CH vibrational bands of methyl benzoate are studied to understand its coupling pattern with other vibrational bands of the biological molecule. This will facilitate to understand the biological structure and dynamics in spectroscopic as well as in microscopic study. Due to the congested spectroscopic pattern, near degeneracy, and strong anharmonicity of the CH stretch vibrations, assignment of the CH vibrational frequencies are often misleading. Anharmonic vibrational frequency calculation with multidimensional potential energy surface interprets the CH vibrational spectra more accurately. In this article we have presented the importance of multidimensional potential energy surface in anharmonic vibrational frequency calculation and discuss the unexpected red shift of asymmetric CH stretch vibration of methyl group. The CD stretch vibrational band which is splitted to double peaks due to the Fermi resonance is also discussed here.

High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3 μm region: role of hydrogenation and alkylation

Aim. We aim to elucidate the spectral changes in the 3 μm region that result from chemical changes in the molecular periphery of polycyclic aromatic hydrocarbons (PAHs) with extra hydrogens (H-PAHs) and methyl groups (Me-PAHs). Methods. Advanced laser spectroscopic techniques combined with mass spectrometry were applied on supersonically cooled 1,2,3,4-tetrahydronaphthalene, 9,10-dihydroanthracene, 9,10-dihydrophenanthrene, 1,2,3,6,7,8-hexahydropyrene, 9-methylanthracene, and 9,10-dimethylanthracene, allowing us to record mass-selective and conformationally selective absorption spectra of the aromatic, aliphatic, and alkyl CH-stretches in the 3.175 − 3.636 µm region with laser-limited resolution. We compared the experimental absorption spectra with standard harmonic calculations and with second-order vibrational perturbation theory anharmonic calculations that use the SPECTRO program for treating resonances. Results. We show that anharmonicity plays an important if not dominant role, affecting not only aromatic, but also aliphatic and alkyl CH-stretch vibrations. The experimental high-resolution data lead to the conclusion that the variation in Me- and H-PAHs composition might well account for the observed variations in the 3 μm emission spectra of carbon-rich and star-forming regions. Our laboratory studies also suggest that heavily hydrogenated PAHs form a significant fraction of the carriers of IR emission in regions in which an anomalously strong 3 μm plateau is observed.

Modeling the CH Stretch/Torsion/Rotation Couplings in Methyl Peroxy (CH3OO).

The manifestations of CH stretch/torsion/rotation coupling in the region of the CH stretch fundamentals are explored in the CH3OO radical. Following our earlier study of the fundamental in the totally symmetric CH stretch (the ν2 fundamental), this work focuses on the other two CH stretch fundamentals, ν1 and ν9, which would be degenerate in the absence of a barrier in the potential along the methyl torsion coordinate. The simplest model, which assumes a decoupling of the CH stretch vibrations from the torsion, fails to reproduce several important features of the spectrum. Specifically, the absence of a strong peak around the origin of the ν1 fundamental and broadening of the strong peak near the origin in the observed spectrum of the ν9 fundamental are not captured by this model. The origins of these features are explored through two more sophisticated treatments of the torsion/CH stretch couplings. In the first, a four-dimensional potential based on the three CH stretches and the torsion is developed and shown to reproduce both of these features. On the basis of the results of these calculations, the calculated parameters are adjusted to simulate the recorded spectrum. To further explore the torsion/CH stretch couplings in CH3OO, a 9-state model Hamiltonian is developed and discussed. The implications of various types of couplings on the observed energy level patterns are also discussed.

Microstructure, thermal properties and crystallinity of amadumbe starch nanocrystals

Amadumbe (Colocasia esculenta), commonly known as taro is a tropical tuber that produces starch-rich underground corms. In this study, the physicochemical properties of starch nanocrystals (SNC) prepared by acid hydrolysis of amadumbe starches were investigated. Two varieties of amadumbe corms were used for starch extraction. Amadumbe starches produced substantially high yield (25%) of SNC’s. These nanocrystals appeared as aggregated and individual particles and possessed square-like platelet morphology with size: 50–100nm. FTIR revealed high peak intensities corresponding to OH stretch, CH stretch and H2O bending vibrations for SNCs compared to their native starch counterparts. Both the native starch and SNC exhibited the A–type crystalline pattern. However, amadumbe SNCs showed higher degree of crystallinity and slightly reduced melting temperatures than their native starches. Amadumbe SNCs presented similar thermal decomposition property as their native starches. Amadumbe starch nanocrystals may have potential application in biocomposite films due to their square-like platelet morphology.

Infrared spectroscopy of the helium solvated cyclopentadienyl radical in the CH stretch region.

Cyclopentadienyl radicals were produced by vacuum flash pyrolysis and trapped in superfluid helium nanodroplets. The infrared spectrum of the embedded radicals was recorded in the range 3020-3120 cm(-1). Three bands observed at 3056.3, 3086.9, and 3117.3 cm(-1) were assigned to the transitions from the ground state to the three levels of the quadratically Jahn-Teller distorted (i.e., second order Jahn-Teller active) CH stretch vibration with E1' symmetry. Accompanying ab initio calculations showed a vibrationally averaged symmetric equilibrium structure in agreement with the observed vibronic transitions. Our results confirm a symmetric configuration of the cyclopentadienyl (cp) radical that is dynamically Jahn-Teller distorted. Density functional theory calculations and infrared data further suggest that barrier-less reactions of two cp radicals occur within the droplets.

In-line determination of the degree of crosslinking of ethylene vinyl acetate in PV modules by Raman spectroscopy

In photovoltaics, the quality of the polymeric solar cell embedding material is of key importance for the long-term durability and performance of the PV modules. For the most common encapsulant, elastomeric ethylene vinyl acetate (EVA), the degree of crosslinking is a key issue in this respect. Raman spectroscopy was researched as a fully contact-free, truly non-destructive optical method to measure the degree of EVA crosslinking of bare EVA film samples as well as in-situ inside assembled PV modules. The method relies on evaluating subtle but characteristic spectral differences in the CH-stretching vibration region (3.000–2.800cm−1) that correlate directly, unambiguously and reliably to the crosslinking state of the EVA encapsulant. With measurement times below one second per analysis point, the method is suitable for real-time, in-line quality control. Using a self-referencing data evaluation approach, the method could be designed to be widely immune to real-world effects, including different sample reflectivity and instrument drifts. Involving the reaction kinetics of the curing reaction, the Raman data can be converted into crosslinking readings that equal the traditional, Soxhlet-derived gel content values. The method has been successfully validated for bare foils, glass–backsheet and glass–glass modules with three different EVA types and deployed in-line in a commercial PV module manufacturing line. Using Raman spectroscopy, it is thus possible to determine the degree of crosslinking of the EVA encapsulant inside assembled modules in-line and with high accuracy, even on moving modules.

Analysis of the dynamics of laser induced plume propagation from liquid matrix using fast photography

AbstractGlycerol, a liquid matrix material for matrix-assisted laser desorption ionization mass spectrometry, was irradiated by a tunable pulsed infrared laser at wavelengths of 2.80 μm, 2.94 μm, 3.10 μm and 3.50 μm, covering the OH and CH stretch vibrations. A fast photography system was introduced to analyze the dynamic process of plume propagation induced by laser ablation up to 1000 μs of the delay time. Propagation distance of the plume front was measured and the corresponding velocities were calculated; they varied with the wavelength and decreased with the delay time. At the tunable wavelength of the peak of the OH absorption (3.0 μm), theoretical calculations indicate that energy deposition from the pulsed laser is in the regime of stress confinement. The mode of energy deposition depends on the wavelength of the OH vibration and its distance from the absorption maxima. However, stages after a 10 μs delay at various wavelengths show a certain similarity in the distance of plume propagation, which can be well fitted by a drag model.

Modeling the CH Stretch Vibrational Spectroscopy of M(+)[Cyclohexane] (M = Li, Na, and K) Ions.

The CH stretch vibrations of M(+)[cyclohexane][Ar] (M = Li, Na, and K) cluster ions were theoretically modeled. Results were compared to the corresponding infrared photodissociation spectra of Patwari and Lisy [ J. Chem. Phys A 2007 , 111 , 7585 ]. The experimental spectra feature a substantial spread in CH stretch vibration frequencies due to the alkali metal cation binding to select hydrogens of cyclohexane. This spread was observed to increase with decreasing metal ion size. Exploring the potential energy landscape revealed the presence of three conformers whose energy minima lie within ∼1 kcal of each other. It was determined that in all conformers the metal ion interacts with three hydrogen atoms; these hydrogen atoms can be either equatorial or axial. The corresponding spectra for these conformers were obtained with a theoretical model Hamiltonian [ J. Chem. Phys. 2013 , 138 , 064308 ] that consists of local mode CH stretches bilinearly coupled to each other and Fermi coupled to lower frequency modes. Frequencies and coupling parameters were obtained from electronic structure calculations that were subsequently scaled on the basis of previous studies. Theoretical spectra of a single low energy conformer were found to match well with the experimental spectra. The relative frequency shifts with changing metal ion size were accurately modeled with parameters generated by using ωB97X-D/6-311++(2d,p) calculations.

CH-Stretching Vibrational Trends in Natural Gas Hydrates Studied by Quantum-Chemical Computations

Vibrational Raman spectroscopy of hydrocarbon CH-stretching vibrations is often used to study natural gas hydrates. In this work, CH-stretching vibrational Raman spectra of hydrocarbon molecules (CH4, C2H6, C3H6, C3H8, C4H8, i-C4H10, and n-C4H10) encapsulated in the water cages (D, ID, T, P, H, and I) of the sI, sII, sH, and sK crystal phases are derived from quantum-chemical computations at the ωB97X-D/6-311++G(2d,2p) level of theory. The trends of CH-stretching vibrational frequencies of hydrocarbon molecules in natural gas hydrates are found to follow the prediction by the “loose cage–tight cage” model: as the size of water cavity increases, the CH frequencies will first decrease and then increase until equal to that in the gas phase. In the “tight cage” situation, the frequency will be greater than in the gas phase; in the “loose cage” situation, the frequency will be smaller or asymptotic to that in the gas phase. Furthermore, the OH-stretching frequencies are sensitive to the H-bond configuration, a...

Reassignment of ν2,3 IR band of the allyl radical in liquid helium nanodroplets

We have recorded the IR-spectrum of the deuterated allyl radical in the frequency range of the CH stretch vibrations in liquid helium nanodroplets. Comparison to the allyl radical spectrum enabled us to make an unambiguous assignment. Based on these new experimental measurements, a reassignment of the ν2,3 IR bands was deemed necessary.

Low-Temperature hydrogenation of diamond nanoparticles using diffuse coplanar surface barrier discharge at atmospheric pressure

Due to an extraordinary combination of intrinsic properties of diamond nanoparticles (DNPs), there is an increased demand for their usage in various areas. While a broad range of surface termination is often done by wet-chemical, UV-irradiation or plasma treatments, DNPs hydrogenation is still an open issue. Thermal annealing or microwave plasma treatment, both known as high-temperature processes, do not offer satisfactory solutions for DNPs hydrogenation. Here, we report on a new approach for plasma-assisted DNPs hydrogenation at temperatures below 100 °C. As-received detonation DNPs with size about 5 nm were annealed (oxidized) in air at 450 °C for 30 min to reduce non-diamond carbon content. Then the annealed DNPs were plasma treated using atmospheric-pressure diffuse coplanar surface barrier discharge in pure hydrogen at powers densities of 1.8 and 3.1 W/cm2. While infrared spectra of annealed DNPs were dominated by oxygen containing functional groups, plasma-hydrogenated DNPs revealed increase of bands of C-H stretching vibrations in the region of 2800-3000 cm-1 with only 5 min of plasma treatment. Prolonged plasma treatments up to 60 min resulted in the increase of C-H stretching vibrations and a decrease of C-O-C groups in the spectral region of 1000–1300 cm-1.

Infrared rovibrational spectroscopy of OH-C2H2 in (4)He nanodroplets: Parity splitting due to partially quenched electronic angular momentum.

The T-shaped OH-C2H2 complex is formed in helium droplets via the sequential pick-up and solvation of the monomer fragments. Rovibrational spectra of the a-type OH stretch and b-type antisymmetric CH stretch vibrations contain resolved parity splitting that reveals the extent to which electronic angular momentum of the OH moiety is quenched upon complex formation. The energy difference between the spin-orbit coupled (2)B1 (A″) and (2)B2 (A') electronic states is determined spectroscopically to be 216 cm(-1) in helium droplets, which is 13 cm(-1) larger than in the gas phase [Marshall et al., J. Chem. Phys. 121, 5845 (2004)]. The effect of the helium is rationalized as a difference in the solvation free energies of the two electronic states. This interpretation is motivated by the separation between the Q(3/2) and R(3/2) transitions in the infrared spectrum of the helium-solvated (2)Π3/2 OH radical. Despite the expectation of a reduced rotational constant, the observed Q(3/2) to R(3/2) splitting is larger than in the gas phase by ≈0.3 cm(-1). This observation can be accounted for quantitatively by assuming the energetic separation between (2)Π3/2 and (2)Π1/2 manifolds is increased by ≈40 cm(-1) upon helium solvation.