Solvent polarity the SPP scale

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 

A brief analysis of these SPP data allows one to draw several interesting eonelusions from struetnral effeets on solvent polarity, namely  

---

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 ... 

A seleetion of SPP values is eolleeted in Table 7-5 for nearly the same set of solvents as given in Table 7-4. Whereas eyelohexane is often used as a nonpolar referenee solvent in other solvent seales, the SPP seale shows a eonsiderable gap between eyelohexane and the gas phase (0.557 units), whieh is nearly as wide as that between eyelohexane and polar hexafiuoro-2-propanol (0.457 units). Therefore, the ehoiee of eyelohexane as a reference solvent to define a solvent scale has been called into question [335]. For a comparison of the SPP scale with Kamlet and Taft s k scale, see reference [338]. 

The probe 2-(dimethylamino)-7-nitrofluorene (54) (see Scheme 2) was proposed by Catalan and coworkers59 as the basis of the SPP scale of solvent polarity/polarizability (in conjunction with 2-fluoro-7-nitrofluorene (55)). On the assumption that the band shapes are the same for these two probes, they cancel out in the difference AvSpp = v(54)-v(55). The normalized SPP scale was then defined in equation 13 ... 

Catalan (1995) has developed a set of polarity parameters known as the solvent bipolarity-polarizability (SPP) scale. Like the tt scales, the SPP parameters are based on the abihty of the solvent to shift the positions of absorption bands in a test molecule used as a probe. The effect, known as solvatochromism, utilizes 2-(N,Af-dimethylamino)-7-nitrofluorene (DMANF) by measuring the shift in the absorption spectrum as a series of solvents is used. The value of SPP for each solvent is calculated from the relationship... 

The general SPP scale of solvent dipolarity/polarizability and the specific SB and SA scales of solvent HBA basicity and HBD acidity, respectively, are orthogonal to one another and they can be used in the correlation analysis of solvent effects in single- or, in combination with the others, in two- or three-parameter correlation equations, depending on the solvent-influenced process under consideration see also Section 7.7. Examples of the correlation analysis of a variety of other solvent-dependent processes by means of SPP, SB, and SA values, including those used for the introduction of other solvent polarity parameters, can be found in references [335-337, 340-342]. In particular, comparisons with Kamlet and Taft s n scale [340] and Winstein and Grunwald s Y scale [341] have been made. 

Reported scales for describing solvent polarity [f(8,n), Jt " and S ], basicity (DN and P ) and acidity (AN and a ) were previously analyzed against our SPP, SA and SB scales in the originating references, so no fiirther comment is made here. 

The scale is based on the solvatochromic behavior of an n — jt electronic transition, so it reflects a high hypsochromic sensitivity to solvent acidity (specific solvation between the lone pair involved in the elecironic transition and solvent acidity is lost when one electron in the pair is electronically excited). An n —> Jt electronic transition is also known to decrease the polarity of the chromophore concerned, so an increase in SPP for the solvent will cause a hypsochromic shift in the elecironic transition. 

Recently, Sapre et al. showed that a plot of the maximum fluorescence of Neutral Red (NR) against the solvent polarity function Af is clearly bilinear (see Figure 10.3.8). Accordingly, they concluded that, in solvents with Af > 0.37, the emitting state changes to a much more polar, ICT state. The analysis of this spectroscopic data in the light of our scales reveals that, in fact, the solvatoehromism of NR is normal, albeit dependent not only on the polarity of the solvent (SPP), but also on its acidity (SA) (see Figure 10.3.9) ... 

A brief review has been presented of the correlation analysis of solvolysis rates 50 years later, i.e. since Grunwald and Winstein proposed their eponymous equation in 1948.111 -pije authors then propose a method of correlation analysis involving multiple regression on solvent scales SPP (polarity-polarizability), SA (acidity) and SB (basicity). These scales are based on the solvatochromism of suitable probes and were initially for pure (i.e. one-component) solvents, but have now been extended to binary solvent mixtures. This enabled the authors to present a correlation for the solvolysis rate constants of r-butyl chloride in 27 pure solvents and 147 binary solvent mixtures, having a correlation coefficient r = 0.990 and a standard error of the estimate s = 0.40. The most important term in the equation is that involving SPP next comes... 

---