Probe homomorph

CHARACTERIZATION OF A MOLECULAR ENVIRONMENT WITH THE AID OF THE PROBE/HOMOMORPH MODEL... 

Replacement of the acid proton (N-H) by a methyl group in this molecular structure has the same side effects as a result, the compound will be similarly sensitive to the polarity and acidity of the medium, but not to its basicity owing to the absence of an acid site. Consequently, l-methyl-5-nitroindoline (MNI, 10) possesses the required properties for use as a homomorph of 5-nitroindoline in order to construct our solvent basicity scale (SB). The suitability of this probe/homomorph couple is consistent with theoretical MP2/6-31G data. Thus, both the probe and its homomorph exhibit the same sensitivity to solvent polar-ity/polarizability because of their similar dipole moments (p i = 7.13, = 7.31 D) and... 

These results show that an oxygen lone pair in (11) is basic enough, as a result of hydrogen bonding, for (11) to be used as an acidity probe in UV-Vis spectroscopy thus, the absorption maximum for TBSB shifts by about 1000 cm from 1-decanol to ethanol, whereas that for DTBSB shifts by only 300 cm. This sensitivity to acidity, and the structural likeness of the probe and its homomorph -which must endow them with a similar sensitivity to solvent basicity and dipolarity/polarizability-, suggest that the two compounds make an appropriate probe-homomorph couple for developing a pure solvent acidity scale. For this purpose, the first visible absorption band for these compounds exhibits quite an in-... 

The SA value of an acid solvent as determined using the DETZ probe is calculated using the expression established from the information provided by a series of solvents of increased acidity measurable by the TBSB/DTBSB probe/homomorph couple (viz. 1,2-butanediol, 1,3,-butanediol, ethanol, methanol and hexafluoro-2-propanol) this gives rise to eq. [10.3.14], which, in principle, should only be used to determine SA for solvents with Avr,FT7 values above 100 cm" ... 

A solvatochromic scale, based on the ultraviolet-visible, rather than the infrared, spectral band of suitable probes is that based on the Kamlet -Taft P parameter. This is again an averaged quantity, for which the wavenumber shifts of several protic indicators relative to structurally similar but aprotic homomorphs are used (Kamlet et al. 1983 Kamlet and Taft 1976). It is assumed that the nonspecific effect of a solvent on the protic probe is the same as that on the aprotic one, and that it can be expressed in terms of the n parameter for the solvent, so that the donicity of the solvent, if it is a Lewis base, causes the difference between the responses of the two probes towards the solvent. The probes originally employed were 4-nitrophenol (vs 4-nitroanisole) and 4-nitroaniline (vs 4-nitroN,N-diethylaniline), but once a n scale is known, the need for the specific aprotic homomorph values no longer exists, since the general expression ... 

Finally, the ambitious approach of Catalan et al. to introduce complete new comprehensive scales of solvent dipolarity/polarizabihty [SPP scale), solvent basicity SB scale), and solvent acidity SA scale) must be mentioned [296, 335-337]. These three UV/Vis spectroscopic scales are based on carefully selected positively solvatochromic and homomorphic pairs of probe dyes and include values for about 200 organic solvents for a recent review, see reference [296]. The molecular structures of the three pairs of homomorphic indicator dyes proposed are as follows ... 

Kauffman and coworkers118 119 tried to fit the solvatochromic shifts of l-(9-anthryl)-3-(4-/V,/V-dimcthylanilino)propanc (83), relative to the hydrocarbon homomorph with the dimethylamino group replaced by H, to the dielectric non-ideality of solvent mixtures involving hexane with ethanol, tetrahydrofuran and dichloromethane. The shifts were not linear with the mole fraction of the polar component, and Suppan s theory of dielectric enrichment was applied to the data. It was found that the dielectric enrichment that can be calculated from the relative permittivities of the components and of the mixtures is not sufficient to account for the observed solvatochromic shifts, but that preferential solvation of the probe by the polar component is superimposed on this dielectric effect. Earlier,... 

The fluorescent probe 4-aminophthalimide (63) as well as its /V,/V-diethyl homomorph (89) were used by Samanta and coworkers148 149 to study the polarity and fluorescence dynamics in l-alkyl-3-methylimidazolium salts. The alkyl groups were variously ethyl, butyl and octyl and the anions were nitrate, tetrafluoroborate, hexafluorophosphate and bis(triflyl)imide. Similar conclusions were reached concerning the effects of alkyl chain length and anion as those obtained by Carmichael and Seddon142 using Nile Red. The solvation dynamics were found to depend on the viscosity of the media. Further use of 63 in l-butyl-3-methylimidazolium hexafluorophosphate was reported by Ingram and... 

Compound 4, which displays large positive solvatochromic shifts and good solubility in a wide range of solvents, was proposed as a 7i probe.Another example involves the homomorphic pair 2-(dimethylamino)-7-nitrofluorene (S)/2-fluoro-7-nitrofluorene that were sug-... 

All probes that meet the previous requirements are not necessarily good solvent probes, however in fact, the sensitivity of the probe may be the result of various types of interaction with the solvent and the results difficult to generalize as a consequence. A probe suitable for determining a solvent effect such as polarity, acidity or basicity should be highly sensitive to the interaction concerned but scarcely responsive to other interactions so that any unwanted contributions will be negligible. However, eonstrueting a pure solvent scale also entails offsetting these side effects, which, as shown later on, raises die need to use a homomorph of the probe. 

The same approach can be used to design a suitable acidity probe. The previous comments on the basicity probe also apply here however, there is the added difficulty that the basic site used to probe solvent basicity caimot be blocked so easily. The situation is made much more complex by the fact that, for example, if the basic site used is a ketone group or a pyridine-like nitrogen, then no homomorph can be obtained by blocking the site. The solu-... 

However, the change in structure of the first absorption band for DMANF in passing from non-polar solvents to polar solvents and the potential contaminating effect of solvent acidity on the position of this band entails introducing a homomorph for the probe in order to offset the detrimental effects of these factors on the estimation of solvent polarities. 

The homomorph to be used should essentially possess the same structure as the probe, viz. a nitro group at position 7 ensuring the occurrence of the same type of interaction with the solvents and an electron-releasing group at position 2 ensuring similar, through weaker, interactions with the nitro function at 7 in order to obtain a lower dipole moment relative to... 

Obviously, the differenee between the solvatochromism of DMANF and FNF will cancel many of the spurious effects involved in measurements of solvent polarity. Because the envelopes of the first absorption bands for FNF and DMANF are identical (see Figure 1 in ref 15), one of the most common sources of error in polarity scales is thus avoided. The polarity of a solvent on the SPP scale is given by the difference between the solvatochromism of the probe DMANF and its homomorph FNF [Av(solvent) = VpNp -Vdmanf] and can be evaluated on a fixed scale from 0 for the gas phase (i.e., the absence of solvent) to 1 for DMSO, using the following equation ... 

Table 10.3.1 gives flic SA values for a wide variety of C-H, N-H and 0-H acid solvents. Data were all obtained from measurements made in our laboratory. The homomorph was insoluble in some solvents, which therefore could not be measured, so they where assumed not to interact specifically and assigned a zero SA value if the probe exhibited a structured spectrum in fliem. 

---