Proton solvent dependence

Annelation of a benzene ring on to the [Z>] faee of the heteroeyelie ring does not have any pronouneed effeet upon the ehemieal shifts of the heteroeyelie protons (cf. Table 8). The rather unexpeeted heteroatom sequenee for shifts to progressively lower field for both H-2 and H-3 remains NHsolvent dependent and as in pyrrole it is also eoupled to the ring protons with Ji,2 = 2.4Hz and Ji,3 = 2.1 Hz. The assignment of the benzenoid protons H-5 and H-6 has eaused some eonfusion in the literature as they have almost... 

Absorption and emission spectra of six 2-substituted imidazo[4,5-/]quinolines (R = H, Me, CH2Ph, Ph, 2-Py, R = H CH2Ph, R = Ph) were studied in various solvents. These studies revealed a solvent-independent, substituent-dependent character of the title compounds. They also exhibited bathochromic shifts in acidic and basic solutions. The phenyl group in the 2-position is in complete conjugation with the imidazoquinoline moiety. The fluorescence spectra of the compounds exhibited a solvent dependency, and, on changing to polar solvents, bathochromic shifts occur. Anomalous bathochromic shifts in water, acidic solution, and a new emission band in methanol are attributed to the protonated imidazoquinoline in the excited state. Basic solutions quench fluorescence (87IJC187). 

Several investigations into the photochemistry of eucarvone have shown that upon irradiation it isomerizes to a mixture of products whose composition is solvent-dependent (Biichi and Burgess, I960 Hurst and Whitham, 1963 Shuster et al., 1964 Shuster and Sussman, 1970 Takino and Hart, 1970 Hart and Takino, 1971). Ionic intermediates have been invoked in the case of polar solvents (Chapman, 1963). Irradiation of protonated eucarvone 58 in fluorosulfonic acid seems to... 

Spectral data of these alkaloids are presented in the review works (4,8) but do not include data for bicucullinidine (110), which was discovered in 1981 (113-116). In the IR spectra of these compounds the carbonyl region generally consists of three bands. The first one is placed at 1675-1670 cm-1 and the latter two around 1625-1590 cm-1. The amino acid nature of these compounds is demonstrated by the presence of an NH band (2350 cm-1) found in the IR spectrum of bicucullinine (108) (117), as well as by the solvent-dependent position of the N(CH3)2 group in the H-NMR spectra. For instance, in the spectrum of bicucullinine (108) run in basic aqueous solution it can be found at <52.08 (118), in DMSO-d6 at <52.69 (113,116), and in CFjCOOD at <53.13 (117,119). Moreover, in H-NMR spectra the influence of the C-l carbonyl group on the chemical shift of H-8 can be observed. This proton falls in its deshielding zone and is shifted downfield around 1 ppm compared to the absorption of H-8 in spectra of monoketo acids. 

Chou PT, Martinez ML, Clements JH (1993) The observation of solvent-dependent proton-transfer/charge-transfer lasers from 4 -diethylamino-3-hydroxyflavone. Chem Phys Lett 204 395-399... 

Gaines (15) has previously described the use of merocyanine dyes as a nonaqueous means of determining Bronstcd acid concentration. Merocyanine dyes are protonated by strong acids to produce protonated dye which has a distinct visible absorption (Figure 1). The unprotonated dye form (3) has a solvent dependent visible absorption maxima. The present studies were performed in acetonitrile or dichloromcthane solvent where absorption maxima were at 576 nm and 610 nm respectively. The absorbance of the protonated form (4) is relatively unaffected by choice of solvent and is clearly separable from the absorbance of the free dye. The extinction coefficient of the free dye is quite large (71,000 in dichloromethane) which allows determination of small amounts of acid such as 10 6 mmol with an average error of less than 10%. 

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

TABLE 2.2. Solvent Dependence of the Substituent Effects upon Enthalpy Change for Proton Transfer a... 

Amino groups may act not only as proton acceptor, but also as proton donor. Acidic N—H protons interact with basic solvents. In these cases an ortho-nitro group in an aniline system competes with the solvent by an internal hydrogen bond66, as depicted in 12. The stretching frequencies (by IR spectra in carbon tetrachloride) of vnh of complexes between A-methylaniline or diphenylamine (and some nitro-anilines66) and solvents depend on the proton accepting ability of the solvent (which is a moderate base)67. The frequency shifts are linearly related to the solvent s donor number (DN)3. 

McLauchlan, K. A., Reeves, L. W., Schaefer, T. The Sign, Temperature and Solvent Dependence of the Proton Coupling Constant and Chemical Shift in a-Chloroacrylo-nitrile. Can. J. Chem. 44, 1473 (1966). 

Bates, P., Cawley, S., Danyluk, S. S. Solvent Dependence of Proton-Proton Coupling Constants in Substituted Vinyl Silanes. J. Chem. Phys. 40, 2415 (1964). 

In conjugated molecules one or other of the possible protonation sites may be more or less favoured by solvation effects and for this reason sites of protonation are often solvent dependent. In some instances, similar stability of two possible cations results in tautomeric equilibria and these too may be solvent dependent. Just as solute-solvent interactions have an effect on the relative stability of two possible cations formed from a conjugated molecule, so in solid salts stability relationships depend on the mode of packing of ions, which determines interactions with the nearest neighbours. Therefore the types of cation observed in solid salts are not necessarily the most stable ones in solution. 

The teirperature dependencies of the chemical shift values for both Cl and C4 were determined in four different solvents (water, dimethyl sulfoxide, methanol and dioxane) and are shown in Figures 8 and 9. The resonance for Cl at 298 C varied from 101.6 ppm in D2O to 104.0 ppm in methanol. The resonance for C4 at the same temperature varied from 75.3 ppm in dimethyl sulfoxide to 78.3 ppm in methanol. The most pronounced tenperature dependence is observed in water and dioxane, where Cl and C4 signals varied from 101.4 ppm to 101.9 ppm (Cl, water, 278-358 K) and from 75.7 ppm to 76.5 ppm (C4, dioxane, 288-360 K), respectively. Thus, both tenperature and solvent dependence of C shifts indicate different conformational behavior of the molecule at various physico-chemico conditions. This feature is manifested even more clearly by the dependencies of the three-bond proton-carbon J and J coupling constants (< ) - Hl -Cl -04-C4 and f = H4-C4-04-C1 ) which are plotted against tenperature in Figures 10 and 11. 

Figure 8. Temperature and concentration dependence of the solvent proton-solvent proton relaxation rate (%),20% (O), 10% (X), 0%.

Tables 3-2 and 3-3 summarize the infrared and proton-NMR (nuclear magnetic resonance) spectroscopic properties of alcohols and ethers. In the proton NMR, the oxygen atom is deshielding. Phenols and alcohols rapidly exchange protons so their NMR spectra are solvent dependant. The alcohol and ether groups don t have any characteristics absorptions in UV-vis spectra.

Solvatochromic species have a solvent-dependent electronic transition. This method is inapplicable to acidic solvents that would protonate the 0 of the betaine. Thus the... 

Related to these electronic factors, some examples of stereospecific coupling are known. In 7 the geminal protons on phosphorus are non-equivalent and exhibit slightly different couplings to phosphorus, both of which are temperature and solvent dependent.(1969,51) A clearer example is given by the recent work of Mikolajczyk(1969,52) on the two isomers 8 and 9. 

TABLE 2.3. Solvent Dependence of the Snbstitnent Effects upon Rate Constants for Proton Transfer ... 

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