The G values of Allerhand and Schleyer

Allerhand and Schleyer formd that a proportionality exists between the stretching frequencies of vibrators of the type X=0 (with X = C, N, P or S) and the corresponding stretching If equencies in situations involving hydrogen bonds of the X-H- B type in a variety of solvents. They thought these results to be indicative that the solvents studied interacted in a non-specific manner with both types of vibrator, in contradiction with the specific interaction-only approach advocated by Bellamy et al. They used this information to construct a solvent polarity scale that they called the G scale . Allerhand and Schleyer used an empirical linear fi ee-energy equation to define G  

The Et(30) scale is based on the extremely solvatochromic character of 2,6-diphenyl-4-(2,4,6-triphenyl-l-pyrido)phenoxide (1) and is defined by the position (in kcal mol ) of the maximum of the first absorption band for this dye, which has marked intramolecular charge-transfer coimotations and gives rise to an excited electronic state that is much less dipolar than the groxmd state(see scheme III). This results in strong hypso- 

On this Ex(30) scale, pine solvents take values fi om 30.7 kcal mol for tetramethylsilane (TMS) to 63.1 kcal mol for water. These data allow one to easily normal- 

The corresponding values (between 0 and 1) allow one to rank all solvents studied. 

This is no doubt the most comprehensive solvent scale (it encompasses more than 300 solvents) and also the most widely used at present. The probe (1) exhibits solubility problems in non-polar solvents that the authors have overcome by using a tert-butyl derivative (2). One serious hindrance to the use of this type of probe is its high basicity (pK = 8.64 ), which raises problems with acid solvents. 

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Figure 10.3.5. Plot of Allerhand and Schleyer s G val- Figure 10.3.6. Plot of Reichardt s Ep(30) values vs. the...

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