Substituent solution, comparison

As with many other classes of pigments, all DPP pigments show a bathochromic shift of the maximum absorption in the solid state with respect to the maximum absorption in solution. The solid state absorption maxima of DPP depend strongly on the nature and position of the substituent. In comparison with the unsubstituted DPP 2, the of m,m substituted DPP often show a hypsochromic shift and the Knax of p,p substituted DPP a bathochromic shift. 

Having considered how solvents can affect the reactivities of molecules in solution, let us consider some of the special features that arise in the gas phase, where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, because most theoretical calculations do not treat solvent effects, experimental data from the gas phase are the most appropriate basis for comparison with theoretical results. Frequently, quite different trends in substituent effects are seen when systems in the gas phase are compared to similar systems in solution. 

A comparison of phenol acidity in DMSO versus the gas phase also shows an attenuation of substituent effects, but not nearly as much as in water. Whereas the effect of ubstituents on AG for deprotonation in aqueous solution is about one-sixth that in the gas phase, the ratio for DMSO is about one-third. This result points to hydrogen bonding of the phenolate anion by water as the major difference in the solvating properties of water and DMSO. ... 

Table 4.13. Comparison of Substituent Contributions to Phenol Ionization in the Gas Phase and Solution" ...

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas-phase basicity for a series of substituted acetophenones and methyl benzoates. It was foimd that scnsitivtiy of the free energy to substituent changes was about four times that in solution, as measured by the comparison of A( for each substituent. The gas-phase data for both series were correlated by the Yukawa-Tsuno equation. For both series, the p value was about 12. However, the parameter r" ", which reflects the contribution of extra resonance effects, was greater in the acetophenone series than in the methyl benzoate series. This can be attributed to the substantial resonance stabilization provided by the methoxy group in the esters, which diminishes the extent of conjugation with the substituents. 

In a similar manner, the diffusion of hexane into dichloromethane solutions containing mixtures of the alkylammonium salts of bromide and the olefinic acceptors o-CA and TCNE result in the formation of brown-red crystals [23]. X-ray analysis reveals the (1 1) complex of bromide with o-CA, in which the anion is located over the center of the C - C bond of the acceptor moiety (Fig. 15b) and Br - C contacts are shortened by as much as 0.6 A relative to the sum of van der Waals radii (Table 3). In bromide complexes with TCNE, the location of the anion relative to the acceptor is variable. In fact, a 2 1 complex [(Br )2,TCNE] is isolated in which both anions reside over the olefinic bond when the tetraethylammonium salt of bromide is used. In comparison, if the tetrapropyl- or tetrabutylammonium salts of the same anion are employed, the (1 1) complexes [Br ,TCNE] are formed in which the bromide donors are shifted toward the cyano substituents (Fig. 15a). In both cases however, the short intermolecular separations that are characteris-... 

The structure of the dimers from mero-substituted derivatives was initially determined by comparison of the observed and calculated dipole moments. For a head-to-tail dimer the dipole moments resulting from the 9 and 9 substituents should cancel each other and the resultant dipole moment should be essentially zero. For a head-to-head arrangement the dipoles would be in the same direction and the resultant should be considerably greater than zero. The dimers produced upon irradiation of 9-chloro and 9-bromoanthracene solutions were observed to be 0.36 and 0.60 D, respectively. Since these values are much less than expected for a head-to-head arrangement for these derivatives (3.8 D), it was concluded that both of these dimers were formed in a head-to-tail configuration/30 ... 

The major activity in gas-phase studies now depends on the use of modem techniques such as ion cyclotron resonance (ICR). Thus, as already mentioned (Section ELD). Fujio, Mclver and Taft131 measured the gas-phase acidities, relative to phenol, of 38 meta- or para-substituted phenols by the ICR equilibrium constant method, and their results for +R substituents led them to suggest that such substituents in aqueous solution exerted solvation-assisted resonance effects. It was later163 shown by comparison of gas-phase acidities of phenols with acidities of phenols in solution in DMSO that solvation-assisted resonance effects could also occur even when the solvent did not have hydrogen-bond donor properties. Indeed for p-NC>2 and certain other substituents these effects appeared to be larger than in aqueous solution. 

The dependence of this phenomenon on temperature and concentration has been studied in detail (70,71,87) and treated mathematically (87). In principle any compound capable of self-association might be capable of self-induced nonequivalence. These cases should be sufficient to suggest due caution on the part of those who would establish the identity of a racemate (e.g., a synthetic natural product ), by comparison of its NMR spectrum with that of the naturally derived optically pure substance. This phenomenon is not restricted to solutes with aromatic substituents, as evidenced by Table 12. Self-induced nonequivalence may be eliminated by addition of polar solvents or by dilution of the sample. Under these conditions, as has been shown for dihydroquinine (14), spectra of racemic, optically pure, and enriched material become identical. 

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

COMPARISON OF SUBSTITUENT GROUP ROTATION IN DILUTE SOLUTION... 

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