OH-stretching Vibration Research Articles

State‐to‐state studies of intramolecular dynamics and unimolecular reactions

AbstractDouble‐resonance vibrational overtone excitation spectra of small molecules, in combination with laser‐induced fluorescence product detection, provide information on both their energetics and intramolecular energy redistribution dynamics. Application of this approach to measure OH stretch overtone spectra of HOOH and NH2OH near their dissociation threshold provides estimates of the O‐O and N‐O bond energies of 17051.8±4 cm−1 and 21620±20 cm−1 respectively.In symmetrical molecules, such as HOOH, the double‐resonance excitation technique allows us to prepare molecules in either pure OH stretch overtone levels or combination levels in which the excitation energy is divided between two identical OH stretch oscillators. Comparison of the resulting vibrational overtone linewidths indicates no clear difference between the pure OH stretch and the local‐local combination levels. However, comparison of the energy transfer rates from pure OH stretch vibrations and those in which one quantum of energy is put in a low frequency mode reveals faster rates from the latter. This observation clearly indicates that coupling strengths depend markedly on the vibrational character of the coupling partner and not simply on the total energy.Comparison of the OH stretch overtone spectra of the structurally similar molecules HOOH, NH2OH, and CH3OH reveal similar rates of vibrational energy redistribution, despite the difference in the density of states in these molecules. This observation reinforces the notion that energy flow between the OH stretch and dark background states is not statistical but rather is controlled by coupling to a small subset of background states.

Ab initio molecular dynamics calculations to study catalysis

The modern versions of the density functional theory (DFT), especially those using the generalized gradient approximation (GGA), have reached (almost) chemical accuracy and thus can be applied to study problems of real chemical interest such as catalysis. The important equations for the DFT, the local density approximation (LDA), and GGA are given. The full-potential linearized augmented plane wave method (LAPW) is used to check the accuracy of GGA in solids. The basic concepts of the ab initio molecular dynamics (MD) method by Car and Parrinello and its recent implementation using the projector augmented Wave (PAW) method which use a similar augmentation as LAPW are described. PAW applications to ferrocene and beryllocene are summarized, which illustrate that vibrational or fluxional behavior are well described. Sodalite, an aluminosilicate, is discussed as a generic zeolite in comparison with gmelinite. A study of the dynamics of such a system allows the determination of, e.g., the proton stretch vibrations which can be related to infrared spectra. This is illustrated for the OH stretch vibration of the acid site in silicon-rich sodalite. With this methodology, we are able to study the interaction of methanol trapped inside the cage structure of silicon-rich sodalite and to gain new insight into crucial steps of catalytic reactions, namely, the hydrogen-bonding and the possible protonation in this system, or a proton-exchange reaction. The strategies for parallelizing the PAW code are outlined. © 1997 John Wiley & Sons, Inc.

Conformational analysis of alkoxyalcohols A combined IR and molecular mechanics study

The conformational behaviour of one branched and three linear alkoxyalcohols has been investigated by IR spectroscopy and molecular mechanics (MM). The OH-stretching region (3750–3150 cm-1) has been studied in dilute apolar solution. In the MM calculations, for each compound all unique staggered conformations have been analysed, both with and without contributions to the steric energy from rotational and vibrational energy and entropy. The corrections enhance the amount of non-bonded conformers and significantly improve the agreement between theoretical and experimental populations. Calculated H-bond geometries are seen to be related with observed OH-stretching vibrations. The agreement between theoretical and experimental frequency shifts (ΔOH, i.e. the difference between free and bonded) is poor, especially for short H-bond contacts, thus making calculated OH-stretching frequencies unsuitable for assignment purposes.

Structures of size-selected hydrogen-bonded phenol-(H 2O) n clusters in S 0, S 1 and ion

The structures of the size-selected hydrogen-bonded phenol-(H 2O) n clusters were investigated by analysing the OH stretching vibrational spectra in S 0, S 1 and the ionic states. Several spectroscopic methods were applied to measure the vibrational spectra of the clusters; Ionization detected IR (IDIR) and fluorescence detected IR (FDIR) spectroscopies for the neutral clusters, and trapped ion infrared multiphoton dissociation spectroscopy for the ionic clusters. In S 0, characteristic red shifts of the OH stretch vibrations upon hydrogen bond (H-bond) formation were observed and it was confirmed that the most stable clusters for 2 ≤ n ≤ 4 have the ring-form structure. The observed OH stretching vibrations fit well with the calculated ones which were obtained by Watanabe and Iwata with ab initio molecular orbital calculations. In S 1, the reduction of the frequency of the OH stretch vibrations upon H-bond was found to be much larger compared to that in S 0. The result demonstrates the evidence that a proton of the phenolic OH group is more attracted to H 2O (proton acceptor), that is, an acidity of phenol increases in S 1. The increase of the acidity in S 1 leads a drastic distortion of the ring structure of the phenol-(H 2O) 2 cluster, which is the smallest ring-form cluster. In the ionic cluster, an indication of the proton transfer was obtained for the clusters with n ≥ 3 by the observation of the OH stretch vibration characteristic to the hydrated hydronium ions.

Characterization of OH-stretching vibration in HOD based on the fluctuation model of hydrogen bonds and derived structural data

The HOD system has been studied by the fluctuation model. A development of this model is applied and the idea about the expression of the V-structure of water by a Gibbs canonic ensemble is supported. The temperature independence of the local-average energy E( v) of hydrogen bonds of the type OH … OD and the function of state degeneracy W( v) is proved on the basis of the developed model and our previous infrared-derived data about the statistical distribution of hydrogen-bonded OH oscillators P( v; T) at 10 different temperatures in the range 22.0–86.1°C. Both the probability distribution P( R O…O; T) of the local-equilibrium distances R O…O between neighbouring oxygen atoms and local-average energy E( R O…O) of hydrogen bonds have been found. The application of the model to the HOD system gives the temperature dependencies of some of the most important parameters of the probability distribution P( R O…O; T) and molar absorptivity ϵ OH( v i T). All functions studied are expressed in analytical form.

Characterization of OH-stretching vibration in HOD based on the fluctuation model of hydrogen bonds and derived structural data

The HOD system has been studied by the fluctuation model. A development of this model is applied and the idea about the expression of the V-structure of water by a Gibbs canonic ensemble is supported. The temperature independence of the local-average energy E( v ̃ ) of hydrogen bonds of the type OH ⋯ OD and the function of state degeneracy W( v ̃ ) is proved on the basis of the developed model and our previous infrared-derived data about the statistical distribution of hydrogen-bonded OH oscillators P( v ̃ ; T) at 10 different temperatures in the range 22.0–86.1°C. Both the probability distribution P( R ̃ O…O ; T) of the local-equilibrium distances R ̃ O…O between neighbouring oxygen atoms and local-average energy E( R ̃ O…O ) of hydrogen bonds have been found. The application of the model to the HOD system gives the temperature dependencies of some of the most important parameters of the probability distribution P( R ̃ O…O ; T) and molar absorptivity ϵ OH( v; T). All functions studied are expressed in analytical form.

Bimolecular Interactions in Et3SiOH:Base:CCl4 Hydrogen-Bonded Solutions Studied by Deactivation of the "Free" OH-Stretch Vibration

Bimolecular Interactions in Et3SiOH:Base:CCl4 Hydrogen-Bonded Solutions Studied by Deactivation of the "Free" OH-Stretch Vibration

Acidity of aluminophosphate structures. Part 2.—Incorporation of cobalt into CHA and AFI by microwave synthesis

Samples of CoAPO/CoAPSO-44 and CoAPO-5 have been prepared using cyclohexylamine as templating agent and a microwave power system for the crystallization. Cobalt substitutes for aluminium. The Co/Al ratios are in the range of 0.05 to 0.3 for the structure 44 and Co/Al = 0.06 for CoAPO-5. The crystallinity of the samples was studied by X-ray diffraction (XRD), IR spectroscopy, optical microscopy, thermal analysis and adsorption measurements and compared with the results for SAPO-44 and SAPO-5. The thermal stability of the samples decreases strongly as a function of the cobalt content.The Co2+ incorporation-generated acid sites were investigated by diffuse reflectance IR Fourier transform (DRIFT) spectroscopy in the OH-stretching vibration region and by the adsorption of probe molecules (NH3 and CO), calorimetric measurements of the differential molar heat of NH3 sorption and temperature-programmed desorption (TPD) of ammonia. From the results of calorimetry and TPD of ammonia it follows that each cobalt atom inserted into a tetrahedrally coordinated framework position creates one acid site. Concerning the nature of acid sites, an equilibrium between bridging hydroxy groups, paired centres of Lewis sites and –POH groups is proposed.The formation of Co3+ in framework positions could not be confirmed. Thermal analysis combined with mass spectrometry indicate the addition of molecular oxygen in calcined CoAPO-44.

Raman Study of OH-Stretching Vibrations in Kaolinite at Low Temperature

In recent years there has been an increasing interest in the study of intercalated catalysts. Most of these studies are based on Raman and infrared (IR) spectra to explain the properties of the intercalated layer silicates, and many of them are studies of intercalated compounds in kaolinite. The vibrational bands of the hydroxyl groups in kaolinite are very sensitive probes for distinguishing between the kaolinite surface and guest molecules such as hydrazine molecules. In order to explain the change of OH stretching vibrations of the kaolinite through the intercalation processes, it is necessary to make a precise study of their behavior in pure kaolinite.

Comparison of the acidic properties of ZSM-5 zeolites isomorphously substituted by Ga, In, B and Fe

The acidic properties of isomorphously substituted MFI-type zeolites prepared and pretreated in the same way were studied to obtain an exact gradation of the intrinsic acidity caused by the incorporation of Ga, Fe, In and B instead of Al into the framework. The nature, strength and concentration of the acidic sites were studied by temperature-programmed desorption of ammonia (TPDA) and Fourier transform infrared spectroscopy. The assignment of the two peaks observed in the TPDA plots of all zeolite samples required the additional registration of the decomposition plot of the ammonium form in the cases of In-ZSM-5 and B-ZSM-5. A temperature program with two ramps and an intermediate isothermal hold was used to separate the low-temperature peak from the high-temperature peak. In this way, the Brönsted sites could be determined more clearly on samples containing a high portion of non-framework species that inhibit the egress of ammonia from the pores. The nature and the strength of the acidic sites were spectroscopically characterized by recording the stretching vibration of the bridged OH groups of Brönsted sites and the spectra of pyridine adsorbed on Brönsted and Lewis sites. The intrinsic acidity of the Brönsted sites on In-ZSM-5 could be clearly classified to be intermediate between the acid strength of B- and Fe-ZSM-5. A relation could be revealed between the frequencies of the OH-stretching vibration and the temperature of the peak maxima of ammonia desorption from the Brönsted sites of the isomorphously substituted ZSM-5 samples.

Vibrational spectra of water molecules at quartz/water interfaces.

Optical sum-frequency generation was used to study OH stretch vibrations of water molecules at fused quartz-water interfaces. The results indicate that orientations and bond ordering of interfacial water molecules are strongly affected by electrostatic interaction and hydrogen bonding of the molecules with the quartz surface.

Vibrational circular dichroism of the OH-stretching vibration in 2,2′-dihydroxy-1,1′-binaphthyl

Vibrational circular dichroism (VCD) spectra in the near-IR region of (R)-2,2′-dihydroxy-1,1′-binaphthyl have been obtained in various organic solvents. It is found that the VCD feature of the OH-stretching vibration exhibits a remarkable change with solvent polarity reflecting different states of the hydrogen bond. A strong negative VCD band accompanying a weak positive band on the low-wavenumber side is observed for the OH-stretching vibration in dilute CCl4 solution. The strongly negative-biased bisignate feature is interpreted as the sum of the conservative bisignate couplet and the negative monosignate term. The strong negative VCD is attributed to the dynamic polarization mechanism due to the large anisotropy of polarizability of the naphthalene ring in the intramolecular hydrogen-bond structure. The bisignate feature is found to originate in the three-centre dynamic polarization coupling mechanism as well as the classical electric dipole–dipole coupling mechanism in the degenerate coupled-oscillator system.

Corrections to the Baseline Distortions in the OH-Stretch Region of Aqueous Solutions

The intensity gains or intensity losses of the OH-stretch vibrations and their changes of band shapes as observed in IR or Raman spectra of dilute aqueous solutions of carboxylic acids, amino acids, and amines were able to be simulated. The difference spectra of the type {sample solution} – {standard} x {empirical factor} displayed essentially flat baselines throughout the OH-stretch region of isotropic Raman scattering. Peaks of the solute spectra which had been hidden by the OH-stretch contour emerged from the background. At concentrations below 1 M, pure water was the standard. Distortions of the isotropic Raman spectra at higher solute concentrations (1 M to 4 M) could be mimicked by phosphoric acid or sulfuric acid solutions as standards. The influence of solutes on the reorientational motions of water molecules made baseline corrections of anisotropic Raman scattering and IR absorption of the more concentrated solutions difficult, if not impossible.

Raman scattering and water structure in nonionic amphiphile solutions.

The structure of water in aqueous solutions of polyoxyethylene nonionic amphiphiles ${\mathrm{C}}_{10}$${\mathrm{E}}_{5}$, where ${\mathrm{E}}_{5}$ denotes ${\mathrm{H}}_{21}$(${\mathrm{OCH}}_{2}$${\mathrm{CH}}_{2}$${)}_{5}$OH, is studied by Raman scattering along an isothermal path crossing the isotropic one-phase region from 0 to 1 amphiphile volume fraction \ensuremath{\varphi}. The isotropic OH stretching-vibration spectrum points out a water behavior that depends on the amphiphile concentration. The data analysis is performed using the bond-percolation model developed for water. The interpretation of scattering data leads to the following structural picture for the water in the systems: for \ensuremath{\varphi} lower than 0.75, water is partially bound to the oxyethylene groups of the amphiphile; above \ensuremath{\varphi}=0.75 all water present in the system is bound. Analogously to the behavior of water in confined geometries, the structure of bound water presents a local fourfold-coordinated environment.

Analysis of Active-Site Distribution in ZSM-5 Crystals by Infrared Microscopy

The acid Site distribution over large ZSM-5 crystals was analyzed using FTIR microscopy. Results of the analysis of the OH-stretching vibrations and of vibrations of pyridine adsorbed on Brønsted and Lewis acid sites were found to be in good agreement. The resolution which can be obtained using this novel technique is about 10-20 μm. This limits the application to large crystals as analyzed in this study. Comparison of the results of IR microscopy with aluminum distributions obtained by electron microprobe analysis revealed that IR microscopy cannot resolve inhomogeneities on a scale smaller than the above-stated limits. The results of both methods on a larger scale, however. are in very good agreement. Using a combination of both techniques, it is possible to obtain a clearer picture of the distribution of acid sites in large zeolite crystals, where electron microprobing gives the higher resolution, and IR microscopy allows the determination of the types of aluminum site present in the zeolite spatially resolved.

Erratum

Abstract other minerals, e.g. topaz, on irradiation. It has been proved [3, 4] that in case of topaz the OH-stretching vibrations change on irradia-tion, which, according to [5], is related to the crystal defects produced by irradiation. The Fe 2+ ions pres-ent in topaz are ionized to Fe 3+ ions on irradiation,

Correlation between the Radiation Induced Colours and the OH-stretching Vibrations of Amazonite

Abstract The changes of colour and the OH-stretching vibrations of amazonites (K-Feldspar) on irradiation have been investigated. It is concluded that the changes are caused by radiation induced crystal defects.

Vibrational and orientational relaxation of monomeric water molecules in liquids

A polarization resolved infrared double resonance experiment with picosecond pulses has been used to measure the vibrational and orientational relaxation times of monomeric water and heavy water molecules in different organic solvents after excitation of the antisymmetric ν3 vibration. In all investigated systems a fast (but definitely time resolved) equilibration among the OH (OD) stretch vibrations was found, followed by a rather slow decay of this ensemble. Furthermore hints to a medium lived intermediate state, most likely the bending vibration, are reported. The principal relaxation scheme is very similar to that of water vapor. Comparing H2O and D2O in different solvents significant differences were found, which can be understood at least semiquantitatively. The orientational relaxation times of H2O and D2O differ by a factor of 2, which can be explained by the influence of weak hydrogen bonds of different strength for H2O and D2O on the orientational relaxation. Additionally interesting aspects of the transient spectra are discussed.

Self-association of octan-1-ol in the pure liquid state and in decane solutions as observed by viscosity, self-diffusion, nuclear magnetic resonance and near-infrared spectroscopy measurements

The self-association of octan-1-ol in the pure liquid state and in decane solutions has been studied by near-infrared spectroscopic observation over the temperature range 298.1–348.1 K and also by viscosity, self-diffusion and NMR measurements. The absorption bands assigned to the hydroxy group are superimposed on several bands attributed to the methylene groups belonging to both the alcohol and decane molecules. A superior technique using a computer was adopted for the cancellation of the absorption bands due to the methylene groups so as to obtain the net OH absorption bands. The analysis of the sharp band at 1410 nm (the first overtone band of the OH-stretching vibration attributed to the free OH-monomer and partly to the OH-polymer) leads to the conclusion that the mean degree of association n(i.e. the average size of the multimer in monomeric units) in the polymer of octan-1-ol is ca. four, both in the liquid and in decane solutions under the temperature conditions given above. From the arguments about the aggregated species, a cyclic tetramer was proposed as the associated alcohol species in the liquid and in decane solutions. The association equilibrium constant and other thermodynamic properties such as changes in enthalpy, entropy and Gibbs energy for tetramer formation were also determined.

Vibrational overtone spectroscopy of the 4νOH+νOH′ combination level of HOOH v i a sequential local mode–local mode excitation

Sequential pumping of the local OH stretch vibrations in hydrogen peroxide using infrared-optical double resonance permits spectroscopic access to the 4νOH+νOH′ combination level. Analysis of the rotationally resolved vibrational overtone spectra generated by this technique determines approximate rotational constants for this level and a value of 17 051.8±3.4 cm−1 for the O–O bond dissociation energy. The linewidths of individual zeroth-order rotational transitions increase sharply with increasing K and change from smooth Lorentzian profiles to clumps of individual lines. The K dependence of the clump widths suggests that an a-axis Coriolis interaction is the primary coupling mechanism between the zeroth-order bright state and dark bath states. As a function of increasing J, each clump coalesces into a smooth Lorentzian profile. We interpret this J dependence in terms of a model that includes rotationally induced vibrational coupling among zeroth-order dark states.