Photoacoustic Raman Spectroscopy Research Articles

Dependence of Intramolecular Hydrogen Bond on Conformational Flexibility in Linear Aminoalcohols.

Intramolecular hydrogen bonds (H-bonds) are abundant in physicochemical and biological processes. The strength of such interaction is governed by a subtle balance between conformational flexibility and steric effect that are often hard to predict. Herein, using linear aminoalcohols NH2(CH2)nOH (n = 2-5) as a model system, we demonstrated the dependence of intramolecular H-bond on the backbone chain length. With sensitive photoacoustic Raman spectroscopy (PARS), the gas-phase Raman spectra of aminoalcohols were measured in both N-H and O-H stretching regions at 298 and 338 K and explained with the aid of quantum chemistry calculations. For n = 2-4, two conformers corresponding to the O-H···N intramolecular H-bond and free OH were identified, whereas for n = 5, only the free-OH conformer was identified. Compared to free OH, a striking spectral dependence was observed for the intramolecular H-bonded conformer. According to the red shift of the OH-bonded band, the strongest intramolecular H-bond yields in n = 4, but the favorable chain length to form an intramolecular hydrogen bond at room temperature was observed in n = 3, which corresponds to a six-membered-ring in 3-aminopropanol. This is in good agreement with statistical analysis from the Cambridge Structural Database (CSD) that the intramolecular hydrogen bond is preferred when the six-membered ring is formed. Furthermore, combined with the calculated thermodynamic data at the MP2/aug-cc-pVTZ//M062X/6-311++G(d,p) level, the origin of decrease in intramolecular hydrogen-bond formation was ascribed to an unfavorable negative entropy contribution when the backbone chain is further getting longer, which results in the calculated Gibbs free energy optimum changing with increasing temperature from n = 4 (0-200 K) to n = 3 (200-400 K) and to n = 2 (above 400 K). These results will provide new insight into the nature of intramolecular hydrogen bonds at the molecular level and the application of intramolecular hydrogen bonds in rational drug design and supramolecular assembly.

New spectral assignment of n‐propanol in the C―H stretching region

The C―H stretching vibration serves as an important probe for characterizing molecular structures and properties of hydrocarbons. In this work, we present a detailed study on gas‐phase Raman spectrum of n‐propanol in the C―H stretching region using stimulated photoacoustic Raman spectroscopy. A complete assignment was carried out with the aid of quantum chemistry calculations and depolarization ratio measurement as well as isotope substitutions, i.e. CH3CD2CD2OH, CD3CH2CD2OH and CD3CD2CH2OH. It is shown that the spectra of three C―H groups of n‐propanol overlap each other because of Fermi resonance coupling and different molecular conformations, leading to complex features that were not determined previously. In addition, the comparisons between the spectra of three isotopologues reveal that the C―H vibrations at different sites of carbon chain exhibit different sensitivity to conformational change of n‐propanol. The CH3 stretching vibration at terminated γ‐carbon is not sensitive whereas the CH2 stretching vibrations at both α‐carbon and β‐carbon atoms are sensitive. Furthermore, Raman spectra of liquid propanol recorded by conventional spontaneous Raman technique are reassigned on the basis of gas‐phase analysis. Copyright © 2016 John Wiley & Sons, Ltd.

Cβ-H stretching vibration as a new probe for conformation of n-propanol in gaseous and liquid states.

The development of potential probes to identify molecular conformation is essential in organic and biological chemistry. In this work, we investigated a site-specific C-H stretching vibration as a conformational probe for a model compound, 1,1,3,3,3-deuterated n-propanol (CD3CH2CD2OH), using stimulated photoacoustic Raman spectroscopy in the gas phase and conventional spontaneous Raman spectroscopy in the liquid state. Along with quantum chemistry calculations, the experiment shows that the CH2 symmetric stretching mode at the β-carbon position is very sensitive to the conformational structure of n-propanol and can serve as a new probe for all five of its conformers. Compared with the O-H stretching vibration, a well-established conformational sensor for n-propanol, the Cβ-H stretching vibration presented here shows better conformational resolution in the liquid state. Furthermore, using this probe, we investigated the conformational preference of n-propanol in pure liquid and in dilute water solution. It is revealed that in pure liquid, n-propanol molecules prefer the trans-OH conformation, and in dilute water solution, this preference is enhanced, indicating that the water molecules play a role of further stabilizing the trans-OH n-propanol conformers. This leads to conformational evolution that n-propanol molecules with gauche-OH structure are transferred to the trans-OH structure upon diluting with water. These results not only provide important information on structures of n-propanol in different environments, but also demonstrate the potential of the C-H stretching vibration as a new tool for conformational analysis. This is especially important when considering that hydrocarbon chains are structural units in organic and biological molecules.

Accurate Measurement of Raman Depolarization Ratio in Gaseous CO2

The Raman depolarization ratios of gaseous CO2 in the spectral range of 1240–1430 cm−1 are determined with a sensitive photoacoustic Raman spectroscopy, and more accurate data compared to the literature results are presented. The precision of the obtained depolarization ratio is achieved by measuring and fitting the dependence of the PARS signal intensity on the cross angle between the polarizations of two incident laser beams.

Overlapping spectral features and new assignment of 2‐propanol in the C–H stretching region

The vibrational spectra of gaseous and liquid 2‐propanol in the C–H stretching region of 2800 ~ 3100 cm−1 were investigated by polarized photoacoustic Raman spectroscopy and conventional Raman spectroscopy, respectively. Using two deuterated samples, that is, CH3CDOHCH3 and CD3CHOHCD3, the overlapping spectral features between the CH and CH3 groups were identified. With the aid of depolarization ratio measurements and density functional theory calculations, a new spectral assignment was presented. In the gas phase, the band at 2884 cm−1 was assigned to the overlapping of one CH3 Fermi resonance mode and a CH stretching of gauche conformer. The bands at 2917 and 2933 cm−1 were assigned to another two CH3 Fermi resonance modes, but the latter includes weak contribution from CH stretching of trans conformer. The bands at 2950 and 2983 cm−1 were assigned to CH3 symmetric and antisymmetric stretching, respectively. The spectral features of liquid 2‐propanol are similar to those in the gas phase except for the blue shift of CH and the red shift of CH3 band positions, which can be attributed to the intermolecular interaction in the liquid state. The new assignments not only clarify the confusions in previous studies from different spectral methods but also provide the reliable groundwork on spectral application of 2‐propanol in the futures. Copyright © 2014 John Wiley & Sons, Ltd.

Complete Raman Spectral Assignment of Methanol in the C–H Stretching Region

In this work, the Raman spectrum of gaseous methanol in the C-H stretching region was investigated by polarized Photoacoustic Raman spectroscopy (PARS). On the basis of the depolarization ratio measurement and density functional theory (DFT) calculations, a complete spectral assignment has been presented. The band at ~2845 cm(-1) was assigned to CH3 symmetric stretching, the bands at ~2925 and ~2955 cm(-1) were assigned to two Fermi resonance modes of CH3 bending overtones, and the bands at ~2961 and ~3000 cm(-1) were assigned to out-of-plane and in-plane vibrations of splitting CH3 antisymmetric stretching. Such assignments can clarify the confusions among the previous spectral studies from the different experimental methods and be confirmed by the Raman spectrum of liquid methanol. Furthermore, the large splitting of 39 cm(-1) between two antisymmetric stretching in gaseous methanol was ascribed to the strong coupling between CH3 and OH groups within methanol molecule because it decreased rapidly in other long-chain alcohol, such as CH3CD2OH.

Modeling of photoacoustic Raman spectroscopy with dissipation

Photoacoustic Raman spectroscopy (PARS) is a technique used to identify chemical species mixed in a gas or liquid based on their pattern of vibrational energy levels. Raman spectroscopy differs from the more familiar absorption spectroscopy by using a nonlinear two-photon process that can be more sensitive to small differences in vibrational energy levels. Thus it can detect defect sites in solid-state optical materials, or low concentrations of chemical species in gases. The Raman scattering process generates acoustic pulses that can be detected with a microphone. In this talk we present an overview of PARS and present an updated model that includes dissipation for the production of acoustic pulses from the energy deposited in the medium during the Raman scattering process. We also show some preliminary measurements of the process for Raman conversion in hydrogen. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Modeling of photoacoustic Raman spectroscopy.

Photoacoustic Raman spectroscopy (PARS) is a technique used to identify chemical species mixed in a gas or liquid based on their pattern of vibrational energy levels. Raman spectroscopy differs from the more familiar absorption spectroscopy by using a nonlinear two-photon process that can be more sensitive to small differences in vibrational energy levels. Thus, it can detect defect sites in solid-state optical materials or low concentrations of chemical species in gases. The Raman scattering process generates acoustic pulses that can be detected with a microphone. In this talk, we present an overview of PARS and present a model of the production of these acoustic pulses from the energy deposited in the medium during the Raman scattering process. We will discuss the different types of PARS (cw, pulsed, and resonant) and their relative advantages. Finally, we describe an experimental platform that we plan to use for validating the model. [This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.]

Vibrationally selective radiative and non-radiative transitions in gaseous hydrogen molecules

An efficient vibrationally selective technique to build-up the v″ = 1 vibrational levels in gaseous hydrogen is demonstrated using stimulated Raman pumping (SRP). Both photo-acoustic Raman spectroscopy (PARS) and coherent anti-Stokes Raman spectroscopy (CARS) are used to study non-radiative and radiative ( v″ = 0 and v″ = 1) transitions in gaseous H 2 molecules. The population fraction in the v″ = 1 vibrational level has been estimated using combined photo-acoustic and coherent anti-Stokes Raman spectroscopy with stimulated Raman pumping.

A novel demonstration of photoacoustic Raman spectroscopy with combined stimulated Raman pumping in H 2 molecule

We present the experimental demonstration of a novel, efficient, and vibrational selective technique to prepare population in vibrational level v″ = 1 using the stimulated Raman pumping. Photoacoustic Raman signal has been studied in non-radiative transitions in the molecule H 2 ( v″ = 0) and ( v″ = 1). The population fraction in the v″ = 1 level can be estimated by using combined photoacoustic Raman spectroscopy with stimulated Raman pumping for the first time.

Precise measurement of the depolarization ratio from photoacoustic Raman spectroscopy

AbstractA new method for the accurate determination of the Raman depolarization ratio is reported with an improved setup for photoacoustic Raman spectroscopy (PARS). The precise measurement is achieved by measuring the dependence of the acoustic signal intensity on the cross‐angle between the polarizations of two incident laser beams. We demonstrate this sensitive and simple method with several gaseous molecules, such as CH4 and H2. The measured results of depolarization ratios agree well with the theoretical values with an upper error limit of ± 0.005, which is comparable to that with polarization‐resolved CARS spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.

New C−H Stretching Vibrational Spectral Features in the Raman Spectra of Gaseous and Liquid Ethanol

Traditionally, the Raman spectrum of ethanol in the C−H vibrational stretching region between 2800 cm-1 and 3100 cm-1 had been assigned as three bands of the −CH2 symmetric stretching, the −CH3 symmetric stretching, and the −CH3 antisymmetric stretching. In this report, new Raman spectral features were observed for ethanol and two deuterated ethanols, that is, CD3CH2OH and CH3CD2OH, in both gaseous and liquid phases with polarized photoacoustic Raman spectroscopy (PARS) as well as normal Raman spectroscopy. With the aid of depolarization ratio measurements and quantum chemical calculations, different assignments were presented to the complex spectral features in the ethanol Raman spectra. In the gaseous ethanol spectra, the band at ∼2882 cm-1 was assigned to the overlapping symmetric stretching vibrational modes of both −CH2 and −CH3; the band at ∼2938 cm-1 was assigned to the two symmetric −CH3 Fermi resonance modes and the weak −CH2 antisymmetric stretching mode; and the band at ∼2983 cm-1 was assigned ...

Thermodynamic and noise considerations for the detection of microscopic particles in a gas by photoacoustic Raman spectroscopy

We develop a simple thermodynamic model to describe the heat transfer mechanisms and generation of acoustic waves in photoacoustic Raman spectroscopy by small particulate suspensions in a gas. Using Langevin methods to describe the thermal noise we study the signal and noise properties, and from the noise equivalent power we determine the minimum number density of the suspended particles that can be detected. We find that for some relevant cases, as few as 100 particles per cubic meter can be detected.

Nonlinear Raman spectroscopy without tunable laser for sensitive gas detection in the atmosphere

A new technique for photo-acoustic Raman spectroscopy (PARS) and coherent anti-Stokes Raman spectroscopy (CARS) is proposed and demonstrated for the detection of H 2 and CH 4 at atmospheric pressure. Conventionally, these types of nonlinear Raman spectroscopy require two lasers whose frequency difference is tuned to the Raman frequency. In the proposed scheme, only a pulsed Nd:YAG laser is used as a pumping source, and a Raman shifter filled with the same gas to be detected is combined. This allows automatic generation of the Raman-shifted radiation. In the case of CH 4, the measurement with the optimized scheme shows that detection limits up to 1 ppm for PARS and 15 ppm for CARS are achieved. The proposed PARS technique allows the measurement of the CH 4 concentration in the natural air. Although the sensitivity of CARS is lower than that of PARS, the signal to noise ratio (S/N) for higher concentrations is better.

Detection of Trace Molecules in the Atmosphere by Nonlinear Raman Spectroscopy without a Tunable Laser.

A new technique for nonlinear Raman spectroscopy is proposed and discussed theoretically in terms of the detection of gas molecules in atmospheric pressure. In the proposed scheme, only a pulsed Nd: YAG laser is used as a pumping source, and instead of a tunable laser, a Raman shifter filled with the sametype of gas as that to be detected automatically generates the Raman shifted radiation. The detection limits of H2for SRGS (Stimulated Raman Gain Spectroscopy), PARS (Photo-Acoustic Raman Spectroscopy), and CARS (Coherent Anti-Stokes Raman Spectroscopy) are demonstrated experimentally, and the ppm order detection limit can be achieved by PARS. Although the sensitivity of CARS is smaller than that of PARS, the signal tonoise ratio in higher concentration is more effective.

CHF2Cl and CH3CF2Cl Detection by Coherent Anti-Stokes Raman Scattering and Photoacoustic Raman Spectroscopy

The coherent anti-Stokes Raman scattering (CARS) and photoacoustic Raman spectroscopy (PARS) spectra of gaseous CHF2Cl and CH3CF2Cl in the 3000 cm-1 region are reported for the first time. Several vibrational bands are observed related to C−H stretch fundamentals of CHF2Cl isotopomers, and to CH3 stretches and the first overtone of CH3 deformation of CH3CF2Cl. The simultaneous detection of CARS/PARS signals aids the assignment of the isotopomer bands. Both methods are appropriate for detection of these compounds and can be applied in chemical dynamics studies.

Raman spectroscopy of the N–C–O symmetric (ν3) and antisymmetric (ν2) stretch fundamentals in HNCO

We report the first gas-phase Raman spectra of the N–C–O stretching fundamentals in isocyanic acid. Using stimulated Raman excitation to prepare vibrationally excited molecules, we record spectra via two different techniques, photoacoustic Raman spectroscopy and action spectroscopy. The former detects the sound wave generated as the Stokes laser tunes through resonances and deposits heat in the gas sample. The latter detects transitions by photodissociating the vibrationally excited states prepared in the vibrational excitation step and detecting the photofragments by laser induced fluorescence. In analogy with the stretching modes in CO2, the N–C–O symmetric stretch (ν3) Raman fundamental in HNCO is strong while the antisymmetric stretch (ν2) is weak, although neither is symmetry forbidden. Both vibrational states are strongly perturbed. The symmetric stretch interacts with combination states that contain two quanta of bending excitation, and the antisymmetric stretch interacts with several different combination states. Both Raman spectra have strong QQ branch rotational structure in which the band origins for different K sublevels in this near-prolate symmetric top follow no simple pattern. Photodissociation of the vibrationally excited states demonstrates the influence of the initial state preparation on the rotational resonances, photofragment appearance thresholds, and Franck–Condon factors in the transition to a dissociative excited electronic state.

Raman spectroscopy of the ν1 N–H stretch fundamental in isocyanic acid (HNCO): State mixing probed by photoacoustic spectroscopy and by photodissociation of vibrationally excited states

We report the first gas-phase Raman spectrum of isocyanic acid. Using stimulated Raman excitation (SRE) to prepare vibrationally excited states, we detect transitions by both photoacoustic Raman spectroscopy (PARS) and action spectroscopy. In this paper we present results on the ν1 N–H stretch fundamental, leaving the spectra of the N–C–O symmetric and antisymmetric stretch modes for a separate publication. The Raman spectrum shows extensive state mixing in the ν1 fundamental, in agreement with previous infrared work. Measurement of the effective b-axis rotational constants for different mixed vibrational states in this near-prolate symmetric top limits the number of candidates for perturbing states and shows which vibrational modes participate. Double resonance photodissociation further probes the vibrational spectroscopy of isocyanic acid. The scheme is first to prepare a vibrationally excited state by SRE, then photodissociate only the molecules prepared in the first step, and finally probe the decomposition products by laser-induced fluorescence (LIF). An action spectrum, obtained by scanning the vibrational excitation laser (Stokes) wavelength with the photolysis laser wavelength fixed and the probe laser tuned to a LIF transition in one of the photofragments, is the key to unraveling the spectroscopy. The intensity differences between PARS and action spectrum transitions reveal the vibrational state mixing and provide the Franck–Condon factors for transitions to the excited electronic state.

High‐resolution photoacoustic Raman spectroscopy of gases

AbstractA high‐resolution photoacoustic Raman spectroscopy experiment is described. The resolution achieved by using two single‐mode pulsed lasers is about 0.0054 cm−1 (full width at half maximum intensity). The experiment was tested first on the v1/2v2 bands of CO2 and gave an increase of at least about one order of magnitude in the signal‐to‐noise ratio with respect to stimulated Raman spectroscopy at low pressure (ca. 10 Torr ≈ 1.3 kPa). The sensitivity is also demonstrated by the study of the weak hot band v1 + v2 — v2 of CO2. In both cases, the experimental line shape is well reproduced by taking into account Doppler and collisional effects. A comparison with CARS spectra was also made.

THEORETICAL ANALYSIS OF PHOTOACOUSTIC RAMAN SPECTROSCOPY

The theoretical analysis of photoacoustic Raman spectroscopy (PARS) is prese-nted The general expressions of PARS signal in solid and liquid samples are derived and the theoretical estimation is given.