Neat spectrum

Liquids. A drop of a liquid organic compound is placed between a pair of polished sodium chloride or potassium bromide plates, referred to as salt plates. When the plates are squeezed gently, a thin liquid film forms between them. A spectrum determined by this method is referred to as a neat spectrum since no solvent is used. Salt plates break easily and are water soluble. Organic compounds analyzed by this technique must be free of water. The pair of plates is inserted into a holder which fits into the spectrometer. 

The simplest method of preparing the sample, if it is a liquid, is to place a thin layer of the liquid between two sodium chloride plates that have been ground flat and polished. This is the method of choice when you need to determine the infrared spectrum of a pure liquid. A spectrum determined by this method is referred to as a neat spectrum. No solvent is used. The polished plates are expensive because they are cut from a large, single crystal of sodium chloride. Salt plates break easily, and they are water soluble. 

Preparing the Sample. Silver chloride plates should be handled in the same way as salt plates. Unfortunately, they are smaller and thinner (about like a contact lens) than salt plates, and care must be taken not to lose them Remove them from the light-tight container with care. It is difficult to tell which side of the plate has the slight circular depression. Your instructor may have etched a letter on each plate to indicate which side is the flat one. To determine the infrared spectrum of a pure liquid (neat spectrum), select the flat side of each silver chloride plate. Insert the O-ring into the cell body as shown in Figure 25.3, place the plate into the cell body with the flat surface up, and add 1 drop or less of liquid to the plate. 

In hydrogen-bonded alcohols, there is a broad peak in the 1460-1600 nm region (6850 cm -6240 cm ), which has been generally attributed to the first overtone of the hydroxyl. This bonded OH peak appears to be broader as a first overtone than its fundamental, as was observed in a series of spectra taken at different temperatures. On the other hand, the nonbonded OH first overtone is stronger relative to the bonded species than it is in the fundamental, and can be readily detected even when not visible in the fundamental region. Figure 5.3 illustrates the differences between the bonded and nonbonded hydroxyls of methanol. The broad bonded OH is indistinct in the neat spectrum, whereas the nonbonded OH in a dilute carbon tetrachloride solution is very sharp and strong. 

In the dilute solution spectrum of methanol, three sharp peaks are observed superimposed on the broader continuum. These can be seen clearly in Figure 5.3. The sharp peaks have been assigned to OH stretch plus CH bending at 5090 cm" (1965 nm), OH stretch plus OH bending at 4960 cm" (2017 nm), and OH stretch plus CO stretch at 4710 cm" (2124 nm). There is also a second combination of OH-stretching and a different OH-bending mode at about 3970 cm (2520 nm). These sharp peaks are at different positions in other alcohols. In the neat spectrum, these peaks are seen as only one broad band centered at about 4770 cm" (2100 nm). In the mid-infrared, a diffuse OH association band related to deformation occurs at about 1420 cm" which can account for the 4770-cm near-infrared band (1420 cm" + 3350 cm" = 4770 cm". The diffuse mid-infrared band is said to disappear in dilute solutions of alcohols, where hydrogen... 

As the discussion of group frequencies continues, several other examples of the effect of intermolecular H bonding on the carbonyl group frequencies will be noted. (For the neat spectrum of hexanoic acid see Figure 7.11.)... 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

IR spectra can be recorded on a sample regardless of its physical state—solid liquid gas or dissolved m some solvent The spectrum m Eigure 13 31 was taken on the neat sample meaning the pure liquid A drop or two of hexane was placed between two sodium chloride disks through which the IR beam is passed Solids may be dis solved m a suitable solvent such as carbon tetrachloride or chloroform More commonly though a solid sample is mixed with potassium bromide and the mixture pressed into a thin wafer which is placed m the path of the IR beam... 

The purity of cyclobutanone was checked by gas chromatography on a 3.6-m. column containing 20% silicone SE 30 on chromosorb W at 65°. The infrared spectrum (neat) shows carbonyl absorption at 1779 cm. - the proton magnetic resonance spectrum (carbon tetrachloride) shows a multiplet at 8 2.00 and a triplet at S 3.05 in the ratio 1 2. 

The product is a mixture of at least two diasteriomers as indicated by its proton magnetic resonance spectrum (carbon tetrachloride) showing eight singlets at S 0.9-1.22 for a total of twelve methyl protons. Its ir spectrum (neat) exhibits absorption bands at 3440 and 1695 cm. 

The C(ls) and 0(ls) spectra of polyphenylene ether (PPE) before and after evaporation of chromium onto the surface are shown in Fig. 23. C(ls) spectra of neat PPE consisted of two components, near 284.6 and 286.0 eV, that were assigned to carbon atoms in the benzene and methyl groups and in the ether groups, respectively. The 0(ls) spectrum consisted of a single peak, near 533.4 eV, that was assigned to the ether oxygen atoms. After evaporation of chromium, a new peak related to formation of Cr202 was observed near 531.0 eV. 

Unassociated alcohols show a fairly sharp absorption neat 3600 cm-1, whereas hydrogen-bonded alcohols show a broader absorption in the 3300 to 3400 cm-1 range. The hydrogen-bonded hydroxy) absorption appears at 3350 cm J in the IR spectrum of cyclohexanol (Figure 17.11). 

Photochemical [2h-2] cycloadditions of olefins occur with retention of configuration according to the Woodward-Hoffmarm rule [6,7], These are excited-state reactions in the delocalization band of the mechanistic spectrum. A striking example of the symmetry-allowed reaction was observed when the neat cis- and tran -butenes were irradiated (delocahzation band in Scheme 3) [8],... 

The absorption and fluorescence spectra of a neat film made of RdB-den-drimer are shown in Fig. 2. The absorption spectrum in visible-wavelength region was similar to that obtained from a solution of RdB with a concentration less than 0.1 mmol/1. Interpretation of the fluorescence in terms of the Frank-Condon mechanism indicated that the core RdB chromophore behaved with a site-isolation effect and had little interaction with the neighboring chro-... 

Fig. 11. Nonlinear scattering spectra for chloroform solution of nitro-azobenzene dendrimer (curve a) and neat chloroform solution (curve b). Inset is absorption spectrum of sample...

FIG. 9 Vibrational sum frequency spectrum in the OH mode region of the neat air-water interface at different temperature for the fundamental visible and infrared beams respectively s- and p-polar-ized and the SFG beam s-polarized. (From Ref. 120, copyright American Physical Society.)... 

Figure 3.29. Ps-TR spectrum of HPDP in H20/MeCN (1 1) (a) and neat MeCN (b) obtained with 267 nm excitation and a 400 nm probe wavelength at a 50 ps delay time. (Reprinted with permission from reference [49]. Copyright (2006) American Chemical Society.)...

Fig. 5.4.7 (a-c, e) Spatially resolved NMR spectra detected during AMS hydrogenation in a catalyst bed of 1-mm beads. Each spectrum corresponds to a voxel size of 2 x 0.17 x 0.33 mm3. Spectra in (a-c) correspond to the same radial position within the operating reactor and are detected in the top, middle and bottom parts of the reactor, respectively. Three spectra in (b, e) correspond to the same vertical position in the operating reactor, with the two spectra in (e) corresponding to voxels shifted by 1.3 mm to ether side of the voxel of the spectrum in (b). The two spectra in (e) are shifted vertically relative to each other for better presentation. The lower traces with narrow lines in (d, f) are experimental spectra detected for bulk neat AMS (d) and cumene (f), the upper traces in (d, f) were obtained by mathematically broadening the lines to 300 Hz. 

The first two reaction types often lead to the formation of stable end-products, but (c) and (d) lead to the formation of new carbocations to which the whole spectrum of reaction types is still open. Most of these possibilities are neatly illustrated in the reaction of 1-amino-propane (11) with sodium nitrite and dilute hydrochloric acid [the behaviour of diazonium cations, e.g. (12), will be discussed further below, p. 119] ... 

Hentz and Kenney-Wallace (1972, 1974) made a detailed study of esin 25 neat alcohols and three alkane solutions in 1-hexadecanol at 30° using a 5-ns electron pulse. Most data were new, but in some cases they confirmed earlier observations (Dorfman, 1965 Baxendale and Wardman, 1971). The authors found the spectrum fully developed at the end of the pulse, with no spectral change thereafter. The spectra are all broad, asymmetric, and structureless,... 

Tris(dimethylamino)arsine (d2o 1.1248 nd 1.4848)3 is a colorless liquid which is readily hydrolyzed to form arsenic (III) oxide and dimethylamine when brought into contact with water. The compound is soluble in ethers and hydrocarbons. The product is at least 99.5% pure (with respect to hydrogen-containing impurities) as evidenced by the single sharp peak at —2.533 p.p.m. (relative to tetramethylsilane) seen in the proton nuclear magnetic resonance spectrum of the neat liquid. 

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