An electrostatic model for hydrogen bonds in alcohols

We have measured the Raman spectra of liquid methanol at temperatures between 50° and –77°C. The weak O–H stretching bands appear, under amplification, more and more asymmetric as the temperature is lowered. They can be decomposed into three Gaussian components centered at about 3220, 3310, and 3400 cm^–1. The former, predominant at low temperature, corresponds to single, linear hydrogen bonds (LHB) between two molecules. The other two are assigned to branched hydrogen bonds, respectively bifurcated (BHB), between three molecules, and trifurcated (THB), between four molecules. We conclude that the molecular structure of liquid alcohols is not chain-like, as presumed so far, but a three-dimensional network featuring a mixture of single (LBH), and multiple hydrogen bonds (BHB, and THB). They are mainly electrostatic in nature, their relative proportions and geometry governed by the packing conditions for minimum energy. They form distinct trimers and tetramers in dilute solutions of alcohols in inert solvents and frozen matrices, and the latter even in the vapor.Deceased December 25, 1987.     

New method and results of Raman studies of non-diffusional reorientational dynamics of molecules in the nematic phase

The method of differential Raman spectroscopy is developed to the case of uniaxial liquid crystals, increasing the precision of the determination of relative broadening and splitting of the polarized components of Raman bands in liquid crystals. With the help of the new technique and on the basis of a more general treatment of rotational broadening of Raman bands, some typical liquid crystal materials, namely, 4-pentyl-4'-cyanobiphenyl (5CB) and trans-4-pentyl-(4-cyanophenyl)cyclohexane (5PCH) have been studied. It is shown that for a particular form of intramolecular vibration (for example the cyano stretching mode), it is possible to determine all independent orientational autocorrelation functions without applying model considerations of the rotational motion in the nematic phase. The deviations of the results of these studies from a simple diffusional model of orientational relaxation in the short-time limit are discussed.     

Raman spectroscopy of potassium acetate-intercalated kaolinites at liquid nitrogen temperature

Raman microscopy has been used to study low and high defect kaolinites and their potassium acetate intercalated complexes at 298 and 77 K. Raman spectroscopy shows significant differences in the spectra of the hydroxyl-stretching region of the two types of kaolinites, which is also reflected in the spectroscopy of the hydroxyl-stretching region of the intercalation complexes. Additional bands to the normally observed kaolinite hydroxyl stretching frequencies are observed for the low and high defect kaolinites at 3605 and 3602 cm(-1) at 298 K. Upon cooling to liquid nitrogen temperature, these bands are observed at 3607 and 3604 cm(-1), thus indicating a weakening of the hydrogen bond formed between the inner surface hydroxyls and the acetate ion. Upon cooling to liquid nitrogen temperature, the frequency of the inner hydroxyls shifted to lower frequencies. Collection of Raman spectra at liquid nitrogen temperature did not give better band separation compared to the room temperature spectra as the bands increased in width and shifted closer together.     

Raman spectroscopy of urea and urea-intercalated kaolinites at 77 K

The Raman spectra of urea and urea-intercalated kaolinites have been recorded at 77 K using a Renishaw Raman microprobe equipped with liquid nitrogen cooled microscope stage. The NH2 stretching modes of urea were observed as four bands at 3250, 3321, 3355 and 3425 cm(-1) at 77 K. These four bands are attributed to a change in conformation upon cooling to liquid nitrogen temperature. Upon intercalation of urea into both low and high defect kaolinites, only two bands were observed near 3390 and 3410 cm(-1). This is explained by hydrogen bonding between the amine groups of urea and oxygen atoms of the siloxane layer of kaolinite with only one urea conformation. When the intercalated low defect kaolinite was cooled to 77 K, the bands near 3700 cm(-1) attributed to the stretching modes of the inner surface hydroxyls disappeared and a new band was observed at 3615 cm(-1). This is explained by the breaking of hydrogen bonds involving OH groups of the gibbsite-like layer and formation of new bonds to the C=O group of the intercalated urea. Thus it is suggested that at low temperatures two kinds of hydrogen bonds are formed by urea molecules in urea-intercalated kaolinite.     

Raman scattering studies of the Ba2MnWO6 and Sr2MnWO6 double perovskites

Polycrystalline Ba2MnWO6 (BMW) and Sr2MnWO6 (SMW) samples were studied between 80 and 1200 K by Raman scattering spectroscopy. In the case of BMW (space group Fmm), four Raman active vibrational modes, predicted by factor group analysis, were identified. Raman scattering studies with different wavelengths revealed a resonant bands between 300 and 800 cm-1. The origin of these bands was related to the Franck-Condon process. Line broadening versus temperature and phonon frequency were studied, and a qualitative explanation was proposed. SMW samples had considerably more complex Raman spectra. It was found that SMW transformed from tetragonal (room-temperature space group P42/n) to the cubic phase between 670 and 690 K; the phase transition temperature was dependent on sample preparation conditions, and it was considerably lower than in the case of large grain size powders. The role of grain size in phase transition is discussed. Mn ions were found to have a crucial role in the lattice dynamics of both materials.     

高岭石结构缺陷的 1H MAS NMR和Raman研究

The structure default of kaolinites was characterized with ¹H MAS NMR and Raman spectra. Although the HI indexes of Suzhou and Maoming kaolinite are similar, their ¹H MAS NMR and Raman spectra are very different. ¹H MAS NMR showed that the hydroxyl proton chemical shifts of Suzhou kaolinite are in the higher field and with larger different between the inner surface hydroxyls protons and inner hydroxyls proton chemical shifts than Maoming kaolinite. Raman spectra showed that the surface hydroxyls stretching vibration bands of Suzhou kaolinite are in the high frequency region, and the half height widths of the bands are 7.0～14 cm^-1. The area ratio S_z/(S_z+S_A), where S_Z and S_A are the areas of bands 3685 cm^-1 and 3695 cm^-1 respectively, is 0.23. But the surface hydroxyls stretching vibration bands of Maoming kaolinite are in the low frequency region, and the half height widths of the bands are 8.9～15.1 cm^-1. The area ratio S_z/(S_z+S_A) is 0.77. Those data proved that Suzhou kaolinite has lower structure default than Maoming kaolinite and ¹H MAS NMR and Raman spectra are effective method for study of kaolinite structure default.     

Effect of water on the formamide-intercalation of kaolinite

The molecular structures of low defect kaolinite completely intercalated with formamide and formamide-water mixtures have been determined using a combination of X-ray diffraction, thermoanalytical techniques, DRIFT and Raman spectroscopy. Expansion of the kaolinite to 10.09 A was observed with subtle differences whether the kaolinite was expanded with formamide or formamide-water mixtures. Thermal analysis showed that greater amounts of formamide could be intercalated into the kaolinite in the presence of water. New infrared bands were observed for the formamide intercalated kaolinites at 3648, 3630 and 3606 cm(-1). These bands are attributed to the hydroxyl stretching frequencies of the inner surface hydroxyls hydrogen bonded to formamide with water, formamide and interlamellar water. Bands were observed at similar positions in the Raman spectrum. At liquid nitrogen temperature, the 3630 cm(-1) Raman band separated into two bands at 3633 and 3625 cm(-1). DRIFT spectra showed the hydroxyl deformation mode at 905 cm(-1). Changes in the molecular structure of the formamide are observed through both the NH stretching vibrations and the amide 1 and 2 bands. Upon intercalation of kaolinite with formamide, bands are observed at 3460, 3344, 3248 and 3167 cm(-1) attributed to the NH stretching vibration of the NH involved with hydrogen bonded to the oxygens of the kaolinite siloxane surface. In the DRIFT spectra of the formamide intercalated kaolinites bands are observed at 1700 and 1671 cm(-1) and are attributed to the amide 1 and amide 2 vibrations.     

Raman spectroscopic study of the rare earth sesquisulfides

Raman spectra have been measured on carefully synthesized and characterized sesquisulfides of the rare earth ions plus yttrium and scandium. Six structure-types are represented. The Raman spectra are diagnostic of the structure-types. Raman line widths indicate structural disorder only in the defect gamma-type structure. High wavenumber bands shift with ionic radius of the rare earths and only slightly with cation coordination number.     

Long-wave Raman spectra of some normal alcohols

Long-wave Raman spectra of some normal alcohols (from n-pentanol to n-decanol) in the liquid phase were registered. The regularities in the dependencies of Raman bands frequencies on the number of carbon atoms in the hydrocarbon chain were deduced. The calculations of Raman spectra of the studied molecules, their equilibrium structures and possible conformers were carried out in the approximation B3LYP/cc-pVDZ. These results in combination with the analysis of literature data allowed to explain the observed regularities in Raman band positions in the spectral range of 200–600 сm⁻¹ and their shifts upon increasing length of the chains. It was found that the plane configurations dominate in the liquid phase for molecules with short- and moderate-chain lengths. The elongation of the chain leads to the decrease of the fraction of plane conformers and in n-decanol the plane structure is completely absent.     

Polarized vibrational studies of bisguanidinium hydrogenphosphate monohydrate

Detailed vibrational studies (FTIR and Raman on powder samples, polarized FTIR microscope on a small single crystal, polarized FTIR using Bruker reflection unit on a single crystal and polarized Raman) have been carried out. Vibrational spectra are discussed in relation to the crystal structure published previously. In this crystal a network of hydrogen bonds link water molecules, guanidinium cations and hydrogenphosphate ions. The 13 different hydrogen bonds in G2HP crystal structure are detected. On the basis of detailed vibrational studies the detailed assignment of observed bands was made. Calorimetric (DSC) studies have been performed, but no phase transition was found in the temperature range 100-350 K.     

Time-domain theoretical analysis of the noncoincidence effect, diagonal frequency shift, and the extent of delocalization of the C=O stretching mode of acetone/dimethyl sulfoxide binary liquid mixtures

A time-domain method for simulating vibrational band profiles that simultaneously takes into account both the diagonal and off-diagonal effects is developed and applied to the C=O stretching bands of neat liquid acetone and the acetone/dimethyl sulfoxide (DMSO) binary liquid mixtures. By using this method, it is possible to examine the influence of liquid dynamics on the noncoincidence effect (NCE), which arises from the off-diagonal vibrational interactions, as well as the frequency shifts and band broadening, which are related to both the diagonal and off-diagonal effects. It is shown that the simulations for the C=O stretching bands of acetone in acetone/DMSO binary liquid mixtures on the basis of this method can reproduce the experimentally observed concave curvature of the concentration dependence of the NCE and the unusually large frequency shift of the anisotropic Raman band. The widths of the infrared, isotropic Raman, and anisotropic Raman bands calculated for neat liquid acetone are also in good agreement with those observed. Based on these calculations, the extent of delocalization of the C=O stretching vibrational motions is examined by referring to two quantitative measures of this property, one calculated in the frequency domain and the other in the time domain. It is shown that the extent of delocalization gets larger as the mole fraction of acetone increases, the C=O stretching vibrations being delocalized over a few tens of molecules in neat liquid acetone. It is also shown that the extent of delocalization is related to the quantity called NCE detectability, which is the ratio between the magnitude of NCE and the bandwidth. It is therefore suggested that the extent of delocalization of vibrational motions may be estimated from observable features of Raman band profiles.     

Molecular mechanisms of cryoprotection in aqueous proline: light scattering and molecular dynamics simulations

Free proline amino acid is a natural cryoprotectant expressed by numerous organisms under low-temperature stress. Previous reports have suggested that complex assemblies underlie its functional properties. We investigate here aqueous proline solutions as a function of temperature using combinations of Raman spectroscopy, Rayleigh-Brillouin light scattering, and molecular dynamics simulations with the view to revealing the molecular origins of the mixtures' functionality as a cryoprotectant. The evolution of the Brillouin frequency shifts and line widths with temperature shows that, above a critical proline concentration, the water-like dynamics is suppressed and viscoelastic behavior emerges: Here, the Landau-Placzek ratio also shows a temperature-independent maximum arising from concentration fluctuations. Molecular dynamics simulations reveal that the water-water correlations in the mixtures depend much more weakly on temperature than does bulk water. By contrast, the water OH Raman bands exhibit strong red-shifts on cooling similar to those seen in ices; however, no evidence of ice lattice phonons is observed in the low-frequency spectrum. We attribute this primarily to enhanced proline-water hydrogen bonding. In general, the picture that emerges is that aqueous proline is a heterogeneous mixture on molecular length scales (characterized by significant concentration fluctuations rather than well-defined aggregates). Simulations reveal that proline also appears to suppress the normal dependence of water structure on temperature and preserves the ambient-temperature correlations even in very cold solutions. The water structure in cold proline solutions therefore appears to be similar to that at a higher effective temperature. This, coupled with the emergence of glassy dynamics offers a molecular explanation for the functional properties of proline as a cryoprotectant without the need to invoke previously proposed complex aggregates.     

Influence and correction of peak shift and band broadening observed by rank analysis on vibrational bands from variable-temperature measurements

Variable-temperature infrared (IR) spectra of cyclohexane and IR and Raman spectra of chlorocyclohexane have been investigated by graphic eigenvalue analysis. Thermal effects known as peak shift and band broadening combined with heteroscedastic noise in vibrational bands are found to have severe influence on the interpretation of the outcome of rank analysis. Methods for correction of frequency shifts and band broadening in the spectral profiles due to temperature variation are developed and tested.     

Effect of temperature and water content on the structure of 1,2-propanediol and 1,3-propanediol: Near-infrared spectroscopic study

Effect of temperature and water content on the structure of 1,2-propanediol (12PD) and 1,3-propanediol (13PD) in the liquid phase has been studied by Fourier-transform near-infrared (FT-NIR) spectroscopy. In addition, the spectra of both diols in CCl₄ solutions at various concentrations were measured. The experimental spectra were analyzed by two-dimensional (2D) correlation approach and chemometric methods. The present results give no evidence that 12PD form the intramolecular hydrogen bonding. In contrast, significant amounts of 13PD molecules in diluted CCl₄ solution is involved in the intramolecular hydrogen bonding. At higher concentrations the intramolecular hydrogen bonds are broken and replaced by the intermolecular ones. The structure of pure liquid propanediols is determined by the intermolecular hydrogen bonding. Unlike for monohydroxyl alcohols, addition of water to propanediols leads to faster temperature-induced breaking of the hydrogen-bonded associates. However, variation of water content at constant temperature does not influence the structure of both diols. In this respect behavior of propanediols is similar to that of the monohydric alcohols. The molecules of water in the mixtures are hydrogen bonded to the diols and act as a double proton donor. This bonding appears to be stronger than that in bulk water.     

High-pressure Raman study of vibrational relaxation in N2O

The vibrational widths of the ν₁ and ν₃ Raman bands of N₂O were determined at pressures ranging from 8 bar to 2 kbar and temperatures varied from 25 to 150°C. The different dephasing theories including motional narrowing collisional models and resonant vibrational energy transfer theory were tested. A comparison of the theoretical predictions with the experimental data indicates the resonance VV transfer represents the dominant broadening mechanism. The observed frequency shifts between isotropic and anisotropic components of the bands were interpreted in terms of dipole-dipole interactions in dense N₂O.     

Identification of Free OH and its Implication on Structural Changes of Liquid Water

The molecular structure of liquid water has been an outstanding issue for many years. The identification of free -OH holds the key in differentiating structure models for liquid water. By analyzing the relative changes of the intensity and depolarization ratio in temperature dependent Raman spectra, the occurrence of free -OH in liquid water is unambiguously de-termined. Furthermore, upon the increase of temperature from 5 oC to 85 oC, the structure of liquid water undergoes significant change, but the relative proportion of free -OH is con-siderably small and remains almost unchanged. This implies that the breaking of hydrogen bond from the tetrahedral structure prefers to occur at the site of the hydrogen acceptor. The energetic favoring of the structural change for liquid water is thus clearly revealed from experiments.     

Raman spectra of gas hydrates--differences and analogies to ice 1h and (gas saturated) water

It is generally accepted that Raman spectroscopic investigations of gas hydrates provide vital information regarding the structure of the hydrate, hydrate composition and cage occupancies, but most research is focused on the vibrational spectra of the guest molecules. We show that the shape and position of the Raman signals of the host molecules (H(2)O) also contain useful additional information. In this study, Raman spectra (200-4000 cm(-1)) of (mixed) gas hydrates with variable compositions and different structures are presented. The bands in the OH stretching region (3000-3800 cm(-1)), the O-H bending region (1600-1700 cm(-1)) and the O-O hydrogen bonded stretching region (100-400 cm(-1)) are compared with the corresponding bands in Raman spectra of ice Ih and liquid water. The interpretation of the differences and similarities with respect to the crystal structure and possible interactions between guest and host molecules are presented.     

Raman spectra of (NH4)3ZnCl4NO3 and (ND4)3ZnCl4NO3 between 295 and 60 K

Raman spectra from polycrystalline samples of (NH4)3ZnCl4NO3 and (ND4)3ZnCl4NO3 have been studied in the temperature range 60-295 K. Internal modes of both nitrate and tetrachlorozincate ions show expected band narrowing and intensification at lower temperature but no significant changes in frequency. Two bands in the lattice region of both compounds, assigned to nitrate ion libration and rocking, show linear increases in frequency with lowering temperature. The intensity of the libration mode shows a linear decrease with lowering temperature, but the intensity of the rocking mode is relatively insensitive to temperature change. Ammonium ion bands show greater structure at low temperature, suggesting differentiation between the two crystallographically distinct types of cation. The observed spectral changes are interpreted on the basis of increasing ordering and effectiveness of hydrogen bonds between ammonium ions and nitrate ions at low temperatures. The Raman spectra give no evidence of discontinuous changes in frequency or intensity, which would signal temperature-dependent transitions of the crystal structure. Unlike the related single-anion compounds NH4NO3 and (NH4)2ZnCl4, the room-temperature structure of (NH4)3ZnCl4NO3 and (ND4)3ZnCl4NO3 appears to persist at least to 60 K, being stabilized by increasingly ordered hydrogen bonding.     

Raman spectroscopy of likasite at 298 and 77 K

Raman spectroscopy at 298 and 77K has been used to study the structure of likasite, a naturally occurring basic copper(II) nitrate of formula Cu3NO3(OH)5.2H2O. An intense sharp band is observed at 3522 cm(-1) at 298 K which splits into two bands at 3522 and 3505 cm(-1) at 77 K and is assigned to the OH stretching mode. The two OH stretching bands at 3522 and 3505 provide estimates of the hydrogen bond distances of these units as 2.9315 and 2.9028 angstroms. The significance of this result is that equivalent OH units in the 298 K spectrum become two non-equivalent OH units at 77 K suggesting a structural change by cooling to liquid nitrogen temperature. A number of broad bands are observed in the 298 K spectrum at 3452, 3338, 3281 and 3040 cm(-1) assigned to H2O stretching vibrations with estimates of the hydrogen bond distances of 2.8231, 2.7639, 2.7358 and 2.6436 angstroms. Three sharp bands are observed at 77 K at 1052, 1050 and 1048 cm(-1) attributed to the nu1 symmetric stretching mode of the NO3 units. Only a single band at 1050 cm(-1) is observed at 298 K, suggesting the non-equivalence of the NO3 units at 77 K, confirming structural changes in likasite by cooling to 77 K.     

On the shape of the Q-branch of Raman scattering spectra in dense media. Comparison with experiment

The theory of the isotropic Raman line broadening is compared with gas and liquid experimental data. The effect of rotational structure narrowing with increasing density is confirmed. The vibrational relaxation (dephasing) contribution to the spectral broadening has been extracted, it being essential in liquid phase. It has been shown to be due to weak collisions and to increase proportionally to their frequency.