Spectra of Solvents and Reference Compounds

The maxima of the spectra of the eluted reference compound, diguaiacylmethane, and of the reaction product are located at almost identical wavelengths for both the neutral solvent and ionization Ac curve, as shown in Table VII. Furthermore, in both cases, the locations of the maxima of the ionization Ac curves agree well with the locations of the maxima of the ionization Ac curve of an ethanolic diguaiacylmethane solution (Table VII). Thus, paper chromatography and spectral analyses indicate that diguaiacylmethane is a product of the reaction of vanillyl alcohol with sodium hydroxide under aqueous reaction conditions. 

After the reaction product at the Rf value of 0.40 was eluted with water from the chromatogram, its spectrum in neutral solvent and its ionization Ac curve were recorded. The neutral spectrum and the ionization Ac curve of bis-4-hydroxy-3,5-dimethoxyphenylmethane in 47.5% ethanol were also recorded the wavelengths of the ultraviolet absorption maxima of the two compounds—eluted product and reference compound—are given in Table II. The spectra of the compounds possess maxima at nearly identical wavelengths. Thus, paper chromatography and ultraviolet spectroscopy indicate positively that bis-4-hydroxy-3,5-dimethoxyphenylmethane is a product of the reaction of syringyl alcohol with alkali in aqueous solution. 

Chemical shifts in NMR spectra are usually reported relative to the peak position of tetramethylsilane (TMS) added to the sample as an internal reference. If TMS is not sufficiently soluble, a capillary with TMS may be used as external reference. In this case, owing to the different volume susceptibilities, the local magnetic fields in solvent and reference differ, and the peak position of the reference must be corrected. For a D2O solution in a cylindrical sample and neat TMS in a capillary, the correction amounts to +0.68 and -0.34 ppm for superconducting and electromagnets, respectively. These values must be subtracted from the chemical shifts relative to external TMS if its position is set to 0.00 ppm. Alternatively, secondary references with (CH3)3SiCH2 groups may be used. The following spectra of two such secondary reference compounds in D2O were measured at 500 MHz with TMS as external reference. Chemical shifts are reported in ppm relative to TMS upon correction for the difference in the volume susceptibilities of D2O and TMS. As a result, the peak for the external TMS appears at 0.68 ppm. 

These compounds usually give many of the reactions characteristic of phenols and were long considered to exist completely in the hydroxy form (see, for example, reference 42). It has been noted that the ultraviolet spectra of aqueous ethanolic solutions of hydroxy-acridines varied with changes in the composition of the solvent, and this phenomenon has been interpreted in terms of the equilibria 132 133 and 134 135. Some compounds of these types show... 

The least problematic issues are UV spectral changes as a function of different solvents between the reference and the test sample. Solvent effects on UV spectra in solvents of decreased dielectric constant compared with water parallel solvent effects on apparent pKa. The changes are most marked for acids, for example, leading to a numerical increase of up to two pKa units - an apparent decrease in the acidity of the carboxylic acid. Effects on bases are considerably less. The apparent pKa of a base in a reduced dielectric constant solvent might be up to about half a pKa unit numerically lower (less basic). The UV spectra of neutral compounds... 

Oxidation of these model sulfur compounds was studied without solvent to investigate the chemical structure of the products using S K-edge XANES. A solvent free tri-phase (organic/ H202aq./catalyst) was used under the described conditions. Figure 1 shows the XANES spectra from the organic and aqueous phases as well as reference materials. The thiophene oxidized to thiophene-sesquioxide [3a,4,7,7a-tetrahydro-4,7-epithiobenzo[b]-thiophene 1,1.8-trioxide ] first.. The sesquioxide solid precipitated from the solvent free reaction mixture and was identified by NMR, IR and C,H,S elemental analytical. The sesquioxide oxidized to sulfate. 2-MT and 2,5 DMT also oxidized to... 

Absorption studies of the model compound in polar and nonpolar solvents support the finding that the dimer is present in a form which allows for close proximity between the two porphyrin rings. The absorption spectra of the dimer and the monomer are shown in Fig. 9. The spectra in methanol show a significant blue-shift of the Soret peak and a small red-shift in the visible bands for the diporphyrin, consistent with the spectral properties of previously synthesized cofacial diporphyrins(16,17) wherein exciton interactions can take place. ( 18) In methylene chloride, the Soret blue shift appears to be much less (<5 nm with reference to 4). 

For the reference compound 4, two different parts in the transient spectrum were found (data not shown) a negative signal (480...600 nm) resulting from a combined ground state bleaching and excited state absorption, and second positive part (>600 nm) due to excited state absorption. Both decay on a ns time scale. Close inspection of the spectra reveils another process with a time constant of 10 ps, which in accordance with previous results [1] is attributed to vibrational/solvent relaxation. 

CTTS transitions in coordination compounds result in a radial movement of electron density from the metal to the surrounding solution medium. The energies of these transitions generally are very sensitive to environmental parameters such as solvent polarity, temperature and the presence of salts.104 This sensitivity has been used in a diagnostic sense to identify CTTS bands in the spectra of anionic cyanide complexes105 and 1,2-dithiolene complexes of Ni, Pd and Pt.106 Hydrated cations such as Cr2+(aq) and Fe2+ (aqj exhibit absorption bands that are sometimes referred to as CTTS in character. Since the solvent occupies the first coordination sphere of the metal, however, the distinction between CTTS and CTTL transitions in these systems becomes obscured. 

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