Complex formation between catalysts,alcohols, and isocyanates in the preparation of urethanes

The mechanism of the catalyzed reaction between alcohols and isocyanates was investigated by means of NMR, infrared, and ultraviolet spectroscopy. The shift of the ? OH proton resonance in the NMR spectra indicated the existence of a 1 : 1 complex in the system dibutyltin dilaurate (DBTDL)–1-methoxy-2-propanol. Complex formation was also observed when lead naphthenate or triethylamine (TEA) were substituted for the DBTDL. Mixtures of the DBTDL–TEA catalysts caused a shift of the ? OH proton resonance greater than that observed for either catalyst alone. This correlates with the synergistic effect noted when preparing urethanes with a mixture of these catalysts. No direct evidence of alcohol–catalyst complex formation could be obtained by infrared spectroscopy. Efforts were also made to detect complex formation in mixtures of phenyl isocyanate and catalysts. These complexes could not be detected by NMR, infrared, or ultraviolet spectroscopy.     

Vibrational analysis of I2*- x nCO2 clusters (n = 1-10): a first principle study on microsolvation

Structure, stability, and vibrational IR and Raman spectra of I(2)(*-) x nCO(2) clusters (n = 1-10) are reported based on first-principle electronic structure calculations. Several close-lying minimum energy structures are predicted for these solvated clusters following the quasi Newton-Raphson procedure of geometry optimization. Search strategy based on Monte-Carlo simulated annealing is also applied to find out the global minimum energy structures of these clusters. Successive addition of solvent CO(2) molecules to the negatively charged diatomic solute, I(2)(*-), is fairly symmetrical. Energy parameters of these solvated clusters are calculated following second-order Moller-Plesset perturbation (MP2) as well as coupled cluster theory with 6-311+G(d) set of basis function (I atom is treated with 6-311G(d) set of basis function). The excess electron in these solvated clusters is observed to be localized mainly over the two I atoms. Average interaction energy between the anionic solute, I(2)(*-), and a solvent CO(2) molecule is approximately 129 meV in I(2)(*-) x nCO(2) clusters, and the average interaction energy between two solvent CO(2) molecules is approximately 85 meV in the case of neutral (CO(2))(n) clusters at MP2 level of theory. IR spectra show similar features in all these solvated clusters, depicting a strong band at approximately 2330 cm(-1) for C-O stretching and a weak band at approximately 650 cm(-1) for CO(2) bending modes. Degeneracy of the bending mode of a free solvent CO(2) unit gets lifted when it interacts with the charged solute I(2)(*-) to form a molecular cluster because of the change in structure of solvent CO(2) units. The vibrational band at the bending region of CO(2) in the Raman spectra of these anionic clusters shows a characteristic feature for the formation of I(2)(*-) x nCO(2) clusters showing a Raman band at approximately 650 cm(-1).     

Infrared and molecular-dynamics studies of the rotational dynamics of water highly diluted in supercritical CO2

Far-infrared (FIR) and mid-infrared (MIR) profiles of D2O infinitely dilute in supercritical CO2 have been studied using molecular-dynamics simulations. For this purpose, we have proposed an intermolecular potential model taking implicitly into account electron donor-acceptor (EDA) interactions between water and CO2 evaluated from ab initio calculations of the intermolecular potential-energy surface (IPS). Interaction-induced dipole mechanisms have been also taken into account in addition to the water permanent dipole to evaluate the simulated FIR profiles of water and CO2 polarizable molecules. They were found to play a minor role in the genesis of the FIR profiles of water/CO2 under supercritical conditions. The analysis of the reorientational dynamics of D2O shows that the rotational dynamics of water is weakly anisotropic due to the EDA interactions which affect more specifically the reorientational motions of the C2 symmetry axis of solute. These results have been used to assess the contribution of the vibrational relaxation in the experimental mid-infrared profiles associated with the nu1 symmetric and nu3 antisymmetric stretching and nu2 bending modes of D2O. It was found that the rotational dynamics mainly contribute to the broadening of the infrared (IR) profiles. Nevertheless, the vibrational processes play a role in the frequency shifts of the band centers and the relative intensity enhancements of the nu1 and nu3 modes of D2O. In particular, the EDA interactions between water and CO2 lead to the appearance of a well-defined IR band of the nu1 mode of D2O. Finally, a comparison with another model taking only into account dipole-quadrupole electrostatic interactions between water and CO2 molecules clearly reveals that EDA interactions have to be considered to reproduce both MIR and FIR measurements. From this point of view CO2 can be classified on a hydrophilic solvent scale based upon the solubility criterion as an intermediate solvent between "inert" xenon and carbon tetrachloride.     

Photoacoustic spectra and modes of vibration of TNT and RDX at CO2 laser wavelengths

IR spectra in the 'Finger Print' spectral range has great importance in qualitative and quantitative analysis of explosives like trinitrotoluene (TNT) and cyclotrimethyltrinitramine (RDX). Highly resolved IR bands of these compounds have been recorded in the 9.6 and 10.6 microm regions of CO2 laser. TNT and RDX are large molecules each having 21 atoms and it is very difficult to assign the modes of vibrations by comparison with those in other molecules making the vibrational assignments of observed bands a difficult task. The ab initio quantum chemical calculation is used for determining the molecular geometries and modes of vibration of these molecules with a view to assign their normal modes in the high resolution vibrational photoacoustic spectra. These assignments are very reliable in view of the good agreement between the observed and calculated frequencies of deuterated TNT.     

Spectroscopic and Structural Study of the 1:2 Aluminium(III) Complex of Quercetin

Quercetin (3, 3’, 4’, 5, 7-pentahydroxyflavone) is one of the most common flavonols present in nature. The complexation of Al(III) by various flavonoids has been suggested to reduce the overload of aluminum in the diet, a metal which has been implicated…     

Vibrational Spectroscopy in Supercritical Fluids: From Analysis and Hydrogen Bonding to Polymers and Synthesis

Supercritical fluids are beginning to be used widely in chemistry. Applications range from extraction and chromatography in analytical chemistry to solvents for reaction chemistry and preparation of new materials. Spectroscopic monitoring is important in much of supercritical chemistry, and vibrational spectroscopy is particularly useful in this context because the vibrational spectrum of a given molecule is usually quite sensitive to the environment of that molecule. Thus, vibrational spectra are excellent probes of conditions within the fluid. In this review, we describe a variety of techniques and cells for IR and Raman spectroscopy in supercritical fluids and illustrate the breadth of applications in supercritical fluids. The examples include: the use of supercritical Xe as a spectroscopically transparent solvent for chemistry and for supercritical fluid chromatography with FTIR detection of analytes; Raman spectroscopy as a monitor for gases dissolved in supercritical CO₂; the effect of solvent density on hydrogen bonding in supercritical fluids and the formation of reverse micelles; IR as a monitor for the supercritical impregnation/extraction of polymers and the reactions of organometallic compounds impreganated into polymers; reactions of organometallic compounds in supercritical fluids; and finally, the use of miniature flow reactors for laboratory-scale preparative chemistry. Overall, our aim is to provide a starting point from which individual readers can judge whether such measurements might usefully be applied to their own particular problems.     

An IR study of (CO2)(+)n (n=3-8) cluster ions in the 1000-3800 cm-1 region

Infrared photodissociation (IRPD) spectra of carbon dioxide cluster ions, (CO(2))(n) (+) with n=3-8, are measured in the 1000-3800 cm(-1) region. IR bands assignable to solvent CO(2) molecules are observed at positions close to the vibrational frequencies of neutral CO(2) [1290 and 1400 cm(-1) (nu(1) and 2nu(2)), 2350 cm(-1) (nu(3)), and 3610 and 3713 cm(-1) (nu(1)+nu(3) and 2nu(2)+nu(3))]. The ion core in (CO(2))(n) (+) shows several IR bands in the 1200-1350, 2100-2200, and 3250-3500 cm(-1) regions. On the basis of previous IR studies in solid Ne and quantum chemical calculations, these bands are ascribed to the C(2)O(4) (+) ion, which has a semicovalent bond between the CO(2) components. The number of the bands and the bandwidth of the IRPD spectra drastically change with an increase in the cluster size up to n=6, which is ascribed to the symmetry change of (CO(2))(n) (+) by the solvation of CO(2) molecules and a full occupation of the first solvation shell at n=6.     

Vibrational solvatochromism and electrochromism of infrared probe molecules containing C≡O, C≡N, C=O, or C-F vibrational chromophore

Solvatochromic vibrational frequency shifts of a few different infrared (IR) probe molecules have been studied by carrying out quantum chemistry calculations for a number of their water clusters. We are particularly focused on the vibrational solvatochromic and electrochromic effects on the CO, CN, and CF stretch modes in carbon monoxide, acetone, 4-cyanopyridine, p-tolunitrile, fluorobenzene, and 3-fluoropyridine. Using multiple interaction site antenna model, we show that their solvatochromic vibrational frequency shifts can be successfully described by considering spatially nonuniform electrostatic potential generated by the surrounding water molecules. It turns out that the CO and CF stretch mode frequencies are approximately proportional to the solvent electric field projected onto the bond axes, whereas the vibrational frequencies of the nitrile stretch mode in 4-cyanopyridine and p-tolunitrile are not. Consequently, it is confirmed that the vibrational Stark tuning rates of the CO and CF stretching modes can be directly used to describe their solvatochromic frequency shifts in condensed phases. However, the nitrile stretch mode frequency shift induced by solvent electrostatic potential appears to be more complicated than its electrochromic phenomenon. To examine the validity of the distributed interaction site model for solvatochromic frequency shifts of these vibrational chromophores, we thus calculated the vibrational Stark tuning rates of the CO, CN, and CF stretch modes and found that they are in good agreement with the experimental results found in literatures. This confirms that a collection of properly chosen distributed interaction sites can be an excellent electric antenna sensing local electrostatics that affects on vibrational frequencies of IR probe modes.     

Electron donor-acceptor interactions in ethanol-CO2 mixtures: an ab initio molecular dynamics study of supercritical carbon dioxide

The nature of interactions between ethanol and carbon dioxide has been characterized using simulations via the Car-Parrinello molecular dynamics (CPMD) method. Optimized geometries and energetics of free-standing ethanol-CO2 clusters exhibit evidence for a relatively more stable electron donor-acceptor (EDA) complex between these two species rather than a hydrogen-bonded configuration. This fact has also been confirmed by the higher formation rate of the EDA complex in supercritical carbon dioxide-ethanol mixtures. The probability density distribution of CO2 molecules around ethanol in the supercritical state shows two high probability regions along the direction of the lone pairs on the oxygen atom of ethanol. The EDA interaction between ethanol and CO2 as well as that between CO2 molecules themselves leads to significant deviations from linearity in the geometry of the CO2 molecule. The vibrational spectra of carbon dioxide obtained from the atomic velocity correlation functions in the bulk system as well as from isolated complexes show splitting of the nu2 bending mode that arises largely from CO2-CO2 interactions, with ethanol contributing only marginally because of its low concentration in the present study. The stretching frequency of the hydroxyl group of ethanol is shifted to lower frequencies in the bulk mixture when compared to its gas-phase value, in agreement with experiments.     

Ab initio studies of solvent effect on molecular conformation and vibrational spectra of diacetamide

The conformational equilibria and vibrational spectra of diacetamide have been investigated by ab initio molecular orbital studies using the basis sets 6-31g(d,p) and 6-31++g(d,p) at Hartree-Fock and MP2 levels. The vibrational spectra of diacetamide have been satisfactorily interpreted taking into consideration the agreement between the calculated harmonic vibrational frequencies, infrared and Raman band intensities and shifts in deuterated molecules with those observed. The solvent effects were investigated by the self-consistent reaction field (SCRF) theory. The effect of solvent on the conformational equilibria and vibrational spectra is discussed. The calculated changes in the geometry and vibrational spectra on going from the gas phase to the solvent medium are in accord with the increasing weight of the dipolar resonance structure of the amide group in more polar solvents.     

不同方法制备的Cu－Co低碳醇合成催化剂的比较研究Ⅰ．吸附态CO的红外光谱

本文并行考察了用溶剂化金属原子分散（ＳＭＡＤ）、浸渍、共沉淀三种方法制备的Ｃｕ－Ｃｏ催化剂的一氧化碳加氢反应（５６３Ｋ，６ＭＰａ，Ｈ２／ＣＯ＝２）性能及吸附态ＣＯ的红外光谱．结果表明：（１）三种催化剂上反应产物均为Ｃ１－Ｃ５正构醇及正构烃，总醇的选择性依下列次序递增：ＳＭＡＤ＜浸渍＜共沉淀；（２）表面低配位钻中心上多重吸附态ＣＯ的红外吸收峰的面积分数，对不同催化剂的变化规律，与醇选择性的变化一致；（３）高温还原和焙烧均使醇选择性下降，同时使表面低配位Ｃｏ０中心减少．据此讨论了ＣＯ插入中心，活性结构及制备方法的影响．     

Vibrational spectra of partially oriented molecules having two conformers in nematic and isotropic solutions: furfural and 2-chlorobenzaldehyde

Vibrational analysis of the two conformers of furfural and 2-chlorobenzaldehyde has been carried out on the basis of their IR and Raman spectra measured in isotropic and anisotropic (nematic liquid crystalline) solvent. The average orientation of the individual conformers in the nematic solvent has been determined by means of a recently developed approach for low symmetry planar molecules using DFT calculations of the vibrational transitions moments. The complex shape of the carbonyl band additionally split into several components is interpreted as an effect of Fermi resonance.     

Vibrational spectroscopy of cyclohexylaminoglutethimide in low-temperature matrices, solutions and the solid state

The vibrational spectra of cyclohexylaminoglutethimide (CHAG) molecules were studied in low-temperature Ar and N₂ matrices and in CCl₄, CHCl₃, CS₂ and CH₃CN solutions (at different concentrations). It was found that increasing the solute concentration in CCl₄, CS₂ and CHCl₃ solutions leads to the formation of dimers. On the other hand, in CH₃CN solution only the solute–solvent complexes occur.

The molecular structure and theoretical IR and Raman spectra of CHAG were predicted with the use of ab initio RHF/6-31G^** and density functional B3LYP/6-31G^** quantum mechanical methods. A comparative analysis of theoretical and experimental spectra of monomeric (isolated) species has led to the assignment of most of the absorption bands in terms of CHAG normal modes.

The IR and Raman spectra of crystalline samples (solid film and KBr pellet) were recorded as well. A reliable assignment of the IR and Raman spectra of crystalline CHAG was obtained with the help of theoretically simulated spectra and the results of our earlier investigations of aminoglutethimide and glutethimide vibrational spectra.     

C-H stretching vibrations of methyl, methylene and methine groups at the vapor/alcohol (N = 1-8) interfaces

In IR and Raman spectral studies, the congestion of the vibrational modes in the C-H stretching region between 2800 and 3000 cm(-1) has complicated spectral assignment, conformational analysis, and structural and dynamics studies, even with quite a few of the simplest molecules. To resolve these issues, polarized spectra measurement on a well aligned sample is generally required. Because the liquid interface is generally ordered and molecularly thin, and sum frequency generation vibrational spectroscopy (SFG-VS) is an intrinsically coherent polarization spectroscopy, SFG-VS can be used for discerning details in vibrational spectra of the interfacial molecules. Here we show that, from systematic molecular symmetry and SFG-VS polarization analysis, a set of polarization selection rules could be developed for explicit assignment of the SFG vibrational spectra of the C-H stretching modes. These polarization selection rules helped assignment of the SFG-VS spectra of vapor/alcohol (n = 1-8) interfaces with unprecedented details. Previous approach on assignment of these spectra relied on IR and Raman spectral assignment, and they were not able to give such detailed assignment of the SFG vibrational spectra. Sometimes inappropriate assignment was made, and consequently misleading conclusions on interfacial structure, conformation and even dynamics were reached. With these polarization rules in addition to knowledge from IR and Raman studies, new structural information and understanding of the molecular interactions at these interfaces were obtained, and some new spectral features for the C-H stretching modes were also identified. Generally speaking, these new features can be applied to IR and Raman spectroscopic studies in the condensed phase. Therefore, the advancement on vibrational spectra assignment may find broad applications in the related fields using IR and Raman as vibrational spectroscopic tools.     

Low-energy vibrations of the group 10 metal monocarbonyl MCO (M = Ni, Pd, and Pt): rotational spectroscopy and force field analysis

The rotational spectra of NiCO and PdCO in the ground and ν(2) excited vibrational states were observed by employing a source-modulated microwave spectrometer. The NiCO and PdCO molecules were generated in a free space cell by the sputtering reaction of nickel and palladium sheets, respectively, lining the inner surface of a stainless steel cathode with a dc glow plasma of CO and Ar. The molecular constants of NiCO and PdCO were determined by least-squares analysis. By force field analysis for the molecular constants of not only NiCO and PdCO but also of PtCO as previously reported, the harmonic force constants were determined for these three group 10 metal monocarbonyls. The vibrational wavenumbers derived for the lower M-C stretching vibrations were in good agreement with those obtained from the IR spectra in noble gas matrices and those predicted by several quantum chemical calculations published in the past. The bending vibrational wavenumbers derived by the force field analysis were also consistent with most quantum chemical calculations previously reported, but showed systematic discrepancies from the matrix IR values by about 40 cm(-1), even after reassignment (ν(2) band → 2ν(2) band) of the matrix IR spectra of PdCO and PtCO.     

Solvent effects on the local structure of p-nitroaniline in supercritical water and supercritical alcohols

Raman spectra of p-nitroaniline in supercritical water and supercritical alcohols were measured, and the effects of solvents on the NO 2 and NH 2 stretching modes were investigated. The intensity and frequency of the NO 2 stretching mode significantly changed as a function of the solvent density and temperature. The frequency of the NO 2 stretching mode correlated with the absorption peak energy of the S 1<--S 0 transition. On the other hand, the vibrational frequency of the NH 2 stretching mode did not correlate with the absorption peak shift, although it had a large frequency shift as a function of the density. The correlation between the NO 2 frequency and absorption peak energy suggested that the solvent effects of supercritical water and supercritical alcohols were similar to those for nonpolar solvents. The density functional calculation using the polarizable continuum model and p-nitroaniline-water clusters qualitatively reproduced the density dependence of the NO 2 stretching mode as well as the solvent polarity dependence. Detailed vibrational analysis revealed that the coupling between the NO 2 and C-NH 2 vibrational motions at the harmonic level has an important effect on the intensity and frequency shift of the NO 2 stretching mode. The frequency shift of the NH 2 stretching mode correlated with the degree of hydrogen bonding between the solvent molecules estimated from NMR measurements [Hoffmann M. M.; Conradi, M. S. J. Phys. Chem. B. 1998, 102, 263]. The existence of intermolecular hydrogen bonding around the NH 2 group was demonstrated even at low-density conditions.     

Structure of CO2 adsorbed on the KCl(100) surface

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.     

Vibrational analysis of free and hydrogen bonded complexes of nicotinamide and picolinamide

Harmonic and anharmonic vibrations of free nicotinamide (NIA) and picolinamide (PIA) molecules together with their hydrogen bonded complexes H₂O–NIA and H₂O–PIA have been studied by means of density functional method. The calculation results of the vibrational spectra of free molecules have been investigated and are compared to the available experimental spectra. The vibrational wavenumbers of both molecules have also been calculated by polarizable continuum model (PCM) that represents the solvent as a polarizable continuum and places the solute in a cavity within the solvent (water is chosen as the solvent in this study). The results of PCM calculations and the H₂O–NIA, H₂O–PIA complexes, are used to investigate the H-bonding interactions of both molecules with the water molecule. The harmonic wavenumbers have been scaled by proper factors obtained by comparing the observed versus calculated wavenumbers and it is shown that anharmonic corrections on the vibrational spectra provided a better agreement between the observed and calculated wavenumbers compared to the results obtained by scaling factor method.     

Supercritical carbon dioxide: an inert solvent for catalytic hydrogenation?

Various surface species originating from the reaction between CO2 and H2 over Al2O3-supported Pt, Pd, Rh, and Ru model catalysts were investigated by attenuated total reflection infrared (ATR-IR) spectroscopy under high-pressure conditions. Two different spectroscopic cells were used: a variable-volume view cell equipped with ATR-crystal and transmission IR windows (batch reactor) and a continuous-flow cell also equipped with a reflection element for ATR-IR spectroscopy. The study corroborated that CO formation from dense CO2 in the presence of hydrogen occurs over all Pt-group metals commonly used in heterogeneous catalytic hydrogenations in supercritical CO2 (scCO2). In the batch reactor cell, formation of CO was detected on all metals at 50 and 90 degrees C, with the highest rate on Pt. Additional surface species were observed on Pt/Al2O3 at 150 bar under static conditions. It seems that further reaction of CO with hydrogen is facilitated by the higher surface concentration at higher pressure. In the continuous-flow cell, CO coverage on Pt/Al2O3 was less prominent than that in the batch reactor cell. A transient experiment in the continuous-flow cell additionally revealed CO formation on Pt/Al2O3 at 120 bar after switching the feed from a H2-ethane to a H2-CO2 mixture. The in situ ATR-IR measurements indicate that CO formation in CO2-H2 mixtures is normally a minor side reaction during hydrogenation reactions on Pt-group metal catalysts, and dense ("supercritical") CO2 may be considered as a relatively "inert" solvent in many practical applications. However, blocking of specific sites on the metal surface by CO and consecutive products can affect structure sensitive hydrogenation reactions and may be at the origin of unexpected shifts in the product distribution.