Solvent spectra

The infra-red spectra of the trimethyl, dimethyl- and dimethylethyl-carbonium salts in excess antimony pentaduoride are shown in Figs. 4a, b, and c. The IRTRAN cells used are not transparent below 770 cm , thus obscuring the 650 cm SblY absorption which would, however, be overlapped by the solvent SbFs absorption. The broad, intense absorption band which appears in all the spectra near 1550 cm is present in the solvent spectrum. It was found to be dependent on the purity of the SbFs, but the nature of the impurity was not established. It should also be mentioned that Deno found an intense absorption at 1533 cm in cyclohexenyl cations thus, secondary carbonium ions formed from the reaction with olefins (which arise from deprotonation) could add to this broad absorption. 

Comparison of the complete spectra of the solvent (10 mM NaClO in water) and of the solutions (MYKO 63 or MYKO 63-DNA in the solvent) showed that the region around 1850 cm can be considered to have nearly zero Raman intensity at least for the lower wave numbers. Thus, for each spectrum, a horizontal base-line is drawn from this point, then the solvent spectrum is subtracted taking into account a coefficient determined from the compound concentration of the solution in order to keep the horizontal base-line unchanged for the new spectrum. 

Figure 26 compares the spectrum of free MYKO 63 with that of MYKO bound to calf thymus DNA. Both spectra have been normalized to the same Raman scattering intensity and drug concentration. The spectrum of free MYKO 63 was obtained by subtracting the solvent spectrum from the MYKO solution spectrum. The spectrum of the bound drug was obtained by a two-step process (i) the solvent spectrum was subtracted both from the spectrum of the MYKO-DNA complex and from the DNA solution spectrum (ii) subtraction of these two new spectra was... 

The spectrum of free SOAz was obtained by subtracting the solvent spectrum from the SOAz solution spectrum. The spectrum of the bound drug was obtained... 

The differences in selection rules between Raman and IR spectroscopy define ideal situations for each. Raman spectroscopy performs well on compounds with double or triple bonds, different isomers, sulfur-containing and symmetric species. In contrast to IR, the Raman spectrum of water is extremely weak so direct measurements on aqueous systems are easy to do. In general, quality IR spectra cannot be collected from samples in highly polar solvents because the solvent spectrum overwhelms that of the sample typically the Raman spectrum of the solvent is weak enough not to present similar problems. 

Figure 2. Overlay of the solution ATR, dry ATR, and dry IRRAS spectra of 0.12 lysozyme. The solvent spectrum has been subtracted from the solution ATR spectrum.

Figure 3. A series of spectra of 100 ppm subtilisin BPN from 6 to 180 minutes after injection into the CIRcle cell. The solvent spectrum has been subtracted.

Figure 4. Adsorption of 190 ppm lysozyme onto hydrophobic and hydrophilic Ge IREs as determined by the amide II band absorbance after subtraction of the solvent spectrum.

One of the most common uses of subtraction techniques is to remove a solvent spectrum from the spectrum of a solution. In most every case there is a solute-solvent interaction, and the spectra of all components in the solution have changed from those of the pure compounds. The changes can occur in band frequencies, intensities, and shapes. The user may gain some insight by performing the subtraction, but the spectra will be distorted. Examples of typical distortions are shown in Fig. 5-7 (6). The top spectrum is a mixture of EDTA and water, and the second spectrum is that of pure water. Two substractions are shown. The first was based on removal of the water band... 

Infra-red Spectrophotometers can be single- or double-beam instruments, or both facilities may be available. In the double-beam mode, the beam of radiation passes through both the sample and a reference path (or reference cell). The use of a reference cell enables compensation to be made for unwanted absorption, e.g. from solvents. Alternatively, an instrument fitted with computing facilities can record and remember the solvent spectrum and then subtract it from a subsequent spectrum obtained with the same cell. 

There are applications, where best results can be obtained without using a baseline (McClure, 1987). If solvent and sample bands overlap, the solvent bands can be compensated for by placing a matching cell in the reference beam, filled with pure solvent only. Using modem computer assisted instrumentation, this compensation can also be performed mathematically by subtracting the solvent spectrum from that of the dissolved sample. 

In order to avoid effects caused by the sample itself, quantitative analyses should be carried out of dilute solutions. The bands which are selected for analysis should be isolated and preferably be free of interference with the solvent or other components. If overlapping cannot be avoided, the bands of the solvent can be compensated for by placing a matching solvent-filled cell into the reference beam, or by subtracting the solvent spectrum by computer. In recording a spectrum of the empty cell, interferences occur due to the reflection of radiation at the surface of the window material inside the cell. The number and wavenumbers of the maxima and minima of the interference fringes in the spectrum (see Fig. 5.1-6) gives the cell thickness ... 

Whether background correction needs to be applied depends on the separation system employed. If the instrument is balanced properly, then, for isocratic separations, the solvent background will be eliminated by the built-in automatic subtraction of the solvent spectrum, as present at the beginning of the analysis, from all recorded spectra. For gradient separations, background corrections will have to be applied after the analysis. 

The effect of solvents on the interactions between the cation and the anion for a 7.6 mole % Na-SPS in THF, DMF, DMSO, and a mixed solvent of 95% toluene and 5% methanol (w/v) is summarized in Table II. The samples used were gels containing about 50% polymer and in each case the solvent spectrum was subtracted from the solution spectrum. 

A gaseous, liquid or solid sample solutions can be handled by subtraction of the solvent spectrum, with aqueous solution acceptable if using Raman. 

Spectral subtraction can be applied in numerous applications and can be used for the -data collected for solutions. In order to obtain the spectrum of a solution it is necessary to record spectra of both the solution and the solvent on its own. The solvent spectrum may then be subtracted fron the solution spectrum. There are two approaches — you may use the solvent alone in the cell as your reference in a double-beam dispersive experiment, or you may record the solvent spectrum separately and then subtract the latter if you are using a single rbeam instrument. 

Figure 7. ATR FTIR spectra of the styrene acrylic acid copolymer in the 2200-1000-cm region. A is the aqueous solvent spectrum. B through E are the difference spectra of various concentrations of styrene acrylic acid minus solvent. Reproduced with permission from ref. 19. Copyright 1987 Applied...

Figure 16. Spectra of natural rubber cross-linked with 25 phr ROOR. Spectrum (A) swollen in benzene to equilibrium swelling. Spectrum obtained under conditions of NFT experiment. Spectrum (B) same sample as (A), obtained under CP-MASS. The asterisk marks resonance of benzene solvent. Spectrum (C) the difference between (A-B).

Fig. 4.10 (A) and (B) show H spectra recorded at 500 MHz of a solution of 4-methoxy indole in 50 50 (v/v) D2O (containing 0.1% trifluoroacetic acid-dj) and methanol. Spectrum (A) shows the spectrum very soon after dissolution - the indole NH and 3 protons have already exchanged with solvent. Spectrum (B), obtained after a weekend in solution, shows the resonance of proton 5 has been completely exchanged. The 5 proton signal in (A) forms the X part of a tightly coupled ABX spin system.

Totally unanticipatable are the effects of spectral abnormalities. These are usually the result of slight changes in the raw material composition and/or the manufacturing process. These changes may manifest themselves as extra shoulders, plateaus, or in extreme instances, maxima in the spectrum. More aggravating still are aromatic contaminants in a solvent that create broad-wavelength, low-absorbance peaks in the solvent spectrum over the 250-280 nm range. 

MO calculation of spin densities McLachlan method plot of a, vs. dielectric constant of solvents. Spectrum reproduced. 

Spectrum reproduced plot of vs. a for p-substituent afg = n.801 —1.621 it values in 10 solvents. Spectrum reproduced deuterated analogues also measured. ... 

Further evidence in support of the general growth model just described is derived from IR studies of the A" = 0.3 lithium borate sol. Figure 36 compares the FTIR difference spectrum (sol spectrum minus solvent spectrum) of the w = 0.3 sol after 24 hours of aging in 100% RH water vapor to the FTIR spectrum of trimethoxyboroxine (63) ... 

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