Raman acids

The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

First, it is possible to excite a chromophore corresponding to the active site, and detennine which modes interact with it. Second, by using UV excitation, the amino acids with phenyl rings (tryptophan and tyrosine, and a small contribution from phenylalanine) can be selectively excited [4], The frequency shifts in the resonance Raman spectrum associated with them provide infomiation on their enviromnent. 

The Raman spectrum of nitric acid shows two weak bands at 1050 and 1400 cm. By comparison with the spectra of isolated nitronium salts ( 2.3.1), these bonds were attributed to the nitrate and nitronium ion respectively. Solutions of dinitrogen pentoxide in nitric acid show these bands , but not those characteristic of the covalent anhydride , indicating that the self-dehydration of nitric acid does not lead to molecular dinitrogen pentoxide. Later work on the Raman spectrum indicates that at —15 °C the concentrations of nitrate and nitronium ion are 0-37 mol 1 and 0 34 mol 1 , respectively. The infra-red spectrum of nitric acid shows absorption bands characteristic of the nitronium ion. The equivalence of the concentrations of nitronium and nitrate ions argues against the importance of the following equilibrium ... 

The state of aqueous solutions of nitric acid In strongly acidic solutions water is a weaker base than its behaviour in dilute solutions would predict, for it is almost unprotonated in concentrated nitric acid, and only partially protonated in concentrated sulphuric acid. The addition of water to nitric acid affects the equilibrium leading to the formation of the nitronium and nitrate ions ( 2.2.1). The intensity of the peak in the Raman spectrum associated with the nitronium ion decreases with the progressive addition of water, and the peak is absent from the spectrum of solutions containing more than about 5% of water a similar effect has been observed in the infra-red spectrum. ... 

In equimolar mixtures of nitric acid and water a monohydrate is formed whose Raman spectrum has been observed. There is no evidence for the existence of appreciable concentrations of the nitric acidium ion in aqueous nitric acid. 

The two absorption bands, at 1050 and 1400 cm , which appear in the Raman spectra of solutions of nitric acid in concentrated sulphuric acid are not attributable to either of the acid molecules. In oleum the lower band appears at 1075-1095 cm. That these bands seemed to correspond to those in the spectra of anhydrous nitric acid and solid dinitrogen pentoxide caused some confusion in the assignment of the spectrum. The situation was resolved by examining the Raman spectra of solutions of nitric acid in perchloric or selenic acids , in which the strong absorption at 1400 cm is not accompanied by absorption at about 1050 cm . Thus, the band at 1400 cm arises from the nitronium ion, and the band at about 1050 cm can be attributed in the cases of nitric acid and solid dinitrogen pentoxide to the nitrate ion formed according to the following schemes ... 

Raman spectroscopy provides the easiest way of estimating the concentration of nitronium ions in different media ( 2.4.1). The concentration, determined by infra-red spectroscopy, of nitronium ions in nitric acid was increased markedly by the addition of sulphuric acid. ... 

Raman spectroscopy I c.i.so showed that the addition of up to io% of water does not affect the concentration of nitronium ions further dilution reduces the concentration of this species, which is not detectable in solutions containing < 85 % sulphuric acid. The introduction of... 

THE STATE OF NITRIC ACID IN INERT ORGANIC SOLVENTS The absence of ions in mixtures of acetic acid and nitric acid is shown by their poor electrical conductivity and the Raman spectra of solutions in acetic acid, nitromethane, and chloroform show only the absorptions of the solvent and molecular nitric acid the bands corresponding to the nitronium and nitrate ions cannot be detected. -... 

The relative abilities of nitromethane, sulpholan, and acetic acid to support the ionisation of nitric acid to nitronium ions are closely similar to their efficiencies as solvents in nitration. Raman spectroscopy showed that for a given concentration of mixed acid (i i nitric and sulphuric acids) the concentration of nitronium ions in these three solvents varied in the order nitromethane > sulpholan > acetic acid. The concentration of mixed acid needed to permit the spectroscopic detection of nitronimn ions was 25 %, 50 % and 60 % in the three solvents, respectively (see 4.4.3). 

Solutions of dinitrogen tetroxide (the mixed anhydride of nitric and nitrous acids) in sulphuric acid are nitrating agents ( 4.3.2), and there is no doubt that the effective reagent is the nitronium ion. Its formation has been demonstrated by Raman spectroscopy and by cryoscopy ... 

Solutions of dinitrogen pentoxide in nitric acid or sulphuric acid exhibit absorptions in the Raman spectrum at 1050 and 1400 cm with intensities proportional to the stoichiometric concentration of dinitrogen pentoxide, showing that in these media the ionization of dinitrogen pentoxide is complete. Concentrated solutions in water (mole fraction of NgOg > 0-5) show some ionization to nitrate and nitronium ion. Dinitrogen pentoxide is not ionized in solutions in carbon tetrachloride, chloroform or nitromethane. ... 

Nitration in the presence of strong acids or Lewis acids Solutions of dinitrogen pentoxide in sulphuric acid nitrate 1,3-dimethyl-benzene-4,6-disulphonic acid twice as fast as a solution of the same molar concentration of nitric acid. This is consistent with Raman spectroscopic and cryoscopic data, which establish the following ionisation ... 

In aqueous solutions of sulphuric (< 50%) and perchloric acid (< 45 %) nitrous acid is present predominantly in the molecular form, although some dehydration to dinitrogen trioxide does occur.In solutions contairdng more than 60 % and 65 % of perchloric and sulphuric acid respectively, the stoichiometric concentration of nitrous acid is present entirely as the nitrosonium ion (see the discussion of dinitrogen trioxide 4.1). Evidence for the formation of this ion comes from the occurrence of an absorption band in the Raman spectrum almost identical with the relevant absorption observed in crystalline nitrosonium perchlorate. Under conditions in which molecular nitrous... 

In an excess of nitric acid, nitrous acid exists essentially as dinitrogen tetroxide which, in anhydrous nitric acid, is almost completely ionised. This is shown by measurements of electrical conductivity, and Raman and infra-red spectroscopy identify the ionic species... 

The use of i i mixed aeid in sulpholan and in aeetie acid was examined (table 4.1, eolumns (/)-(i))- The variation of the eoncentration of nitronium ions with the eoneentration of mixed aeids ([H2SO4] [HNO3], 1 1), in sulpholan (a), aeetie aeid (6), and nitromethane (c) are illustrated in fig. 4.1. The results for aeetie aeid and sulpholan were determined by Raman speetroseopy, and those for nitromethane from the infra-red speetra. 

Fig. 4.1. Variation of NO2+ ion concentration with the concentration of mixed acid (nitric sulphuric, i mole i mole) inorganic solvents (a) in sulpholan (6)in aceticacid (c) in nitromethane. Curves (a) and (6) were determined by Raman measurements using the 1400 cm band while curve (c) was derived from infra-red measurements on the 237s cm band. Unity on the NO2+ concentration scale was determined to be 5-6 molar ( 2S 8 weight %). (From Olah et...

When acetic anhydride was in excess over nitric acid, acetyl nitrate and acetic acid were the only products. When the concentration of nitric acid was greater than 90 moles %, dinitrogen pentoxide, present as (N02+)(N0a ), was the major product and there were only small traces of acetyl nitrate. With lower concentrations of nitric acid the products were acetic acid, acetyl nitrate and dinitrogen pentoxide, the latter species being present as covalent molecules in this organic medium. A mixture of z moles of nitric acid and i mole of acetic anhydride has the same Raman spectrum as a solution of i mole of dinitrogen pentoxide in 2 moles of acetic acid. 

Table 1 Hsts some of the physical properties of duoroboric acid. It is a strong acid in water, equal to most mineral acids in strength and has a p p o of —4.9 as compared to —4.3 for nitric acid (9). The duoroborate ion contains a neady tetrahedral boron atom with almost equidistant B—F bonds in the sohd state. Although lattice effects and hydrogen bonding distort the ion, the average B—F distance is 0.138 nm the F—B—F angles are neady the theoretical 109° (10,11). Raman spectra on molten, ie, Hquid NaBF agree with the symmetrical tetrahedral stmcture (12).

In superacidic media, the carbocationic iatermediates, which were long postulated to exist duting Friedel-Crafts type reactions (9—11) can be observed, and even isolated as salts. The stmctures of these carbocations have been studied ia high acidity—low nucleophilicity solvent systems usiag spectroscopic methods such as nmr, ir, Raman, esr, and x-ray crystallography. 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

Figure 1 indicates how the NO concentrations vary at 20°C as a function of the molar composition of the acid mixture these results were determined using Raman spectra readings. 

Benzene, toluene, and other aromatics that are easily nitrated can sometimes be nitrated using acids having zero NO/ concentrations (see Fig. 1). Two explanations for this are (/) NO/ is actually present but in concentrations too low to be measured by Raman spectra, and (2) NO/ is hydrated to form H2N0" 2> which is also a nitrating agent. 

In aqueous solution, all the sodium peroxoborates dissociate for the most part into boric acid, or its anion, and hydrogen peroxide. Peroxoborate species are also present in these solutions, depending on the pH and the concentration for the species type. The nature of these species has been extensively examined by classical physicochemical methods (13), by nmr, and by Raman spectroscopy (14—17). Both monomeric and polymeric species are usually present. There is some evidence (18) suggesting that these peroxoborates are more reactive than hydrogen peroxide alone under similar conditions. 

The dielectric constants of amino acid solutions are very high. Thek ionic dipolar structures confer special vibrational spectra (Raman, k), as well as characteristic properties (specific volumes, specific heats, electrostriction) (34). 

The possibihty of six-coordinate sihcon species in aqueous solution has been suggested. Raman studies have indicated, however, that monosilicic acid in solution contains a tetracoordinate sihcon species (37). 

The sodium hydroxide is titrated with HCl. In a thermometric titration (92), the sibcate solution is treated first with hydrochloric acid to measure Na20 and then with hydrofluoric acid to determine precipitated Si02. Lower sibca concentrations are measured with the sibcomolybdate colorimetric method or instmmental techniques. X-ray fluorescence, atomic absorption and plasma emission spectroscopies, ion-selective electrodes, and ion chromatography are utilized to detect principal components as weU as trace cationic and anionic impurities. Eourier transform infrared, ft-nmr, laser Raman, and x-ray... 

Several properties of the filler are important to the compounder (279). Properties that are frequentiy reported by fumed sihca manufacturers include the acidity of the filler, nitrogen adsorption, oil absorption, and particle size distribution (280,281). The adsorption techniques provide a measure of the surface area of the filler, whereas oil absorption is an indication of the stmcture of the filler (282). Measurement of the sdanol concentration is critical, and some techniques that are commonly used in the industry to estimate this parameter are the methyl red absorption and methanol wettabihty (273,274,277) tests. Other techniques include various spectroscopies, such as diffuse reflectance infrared spectroscopy (drift), inverse gas chromatography (igc), photoacoustic ir, nmr, Raman, and surface forces apparatus (277,283—290). 

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