Spectroscopic Scales

Laser evaporation of clusters will not be a popular technique, it is costly in equipment, demanding in know-how, and probably limited to a spectroscopic scale. Nevertheless, it has already shown itself... 

More recently, in 1989, Allen developed his spectroscopic scale, xs, where he equated the electronegativity of an atom to its configuration energy (CE)... 

Finally, the perturbation induced on the stretching mode of adsorbed CO (expressed as the difference AD between the stretching frequency of CO in the gas phase and CO in the adsorbed state) is proportional to the electric field E at the equilibrium distance from the adsorbing site (55,56). It will be shown that the AD (CO) a E relationship can be used to establish a spectroscopic scale of the polarizing tendencies of the positive centers at surfaces of ionic solids and, indirectly, to construct a resonable scale of their acidic strengths. The spectrum of CO adsorbed on NaCl films (Fig. 3), consisting of several bands, can be used to show that fourfold coordinated Na+ ions on steps have enhanced polarizing character toward CO. 

The solvent, temperature, pressure, ionophore, and substituent effects on the UV/ Vis spectra of the pyridinium A -phenolate betaine dyes indicate the extreme sensitivity of this class of compounds to small changes in the environment. Their behaviour may be eompared to that of the Princess and the Pea in one of H. C. Andersen s fairy-tales [76] . Their utility for setting up linear Gibbs energy relationships is demonstrated by the fact that the same betaine dye can be used for establishing kinetic and spectroscopic scales of substituents cf. Figs. 7-1 and 7-2) as well as a spectroscopic scale of solvent polarity cf. Table 7-3). 

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 ... 

For technical reasons, phenols are convenient reference hydrogen-bond donors for hydrogen-bonding studies. We present below their use for constructing thermodynamic and spectroscopic scales of hydrogen-bond basicity. These scales are either solute scales when the phenol and the base are dissolved in an inert solvent, or solvent scales when the phenol is studied in the pure base. In the latter case, methods such as the solvatochromic comparison method or the calorimetric pure base method have been developed to unravel the hydrogen-bond contribution to the overall solvent effect. 

There is no inconsistency in the EN of a free atom being different from that of an atom in a valence state. Scales such as Mulliken s and the recently developed spectroscopic scale" show that the absolute and Pauling-like scales can be commensurable. Since the applications are so different, it is not a meaningful question to ask which scale is more correct. Each scale is more correct in its own area of use. 

We shall demonstrate this for Arnold s EPR-spectroscopic scale (ax, Table 3) [36, 39], which relates the radical substituent constant ((7rad) to the benzylic a spin densities (p) according to Eq. (17), in which px and p refer to the substituted and unsubstituted benzyl radicals. Since the a hyperfine coupling constants (a) are linearily related to the spin densities by the McConnell equation [36,38], that is, ax oc px (cf. Section III.B), the coupling constants may be used instead of the spin densities [Eq. (18)]. According to Eq. (8) (pA = pB for the symmetrically substituted cases), the D value is proportional to the square of the spin density and a trAD scale may be defined by taking the square root of the ratio of the D values instead... 

In conclusion, it can be said that the outstanding sensitivity of the spectral absorption and chemical reactivity of our pyridinium-N-phenoxide betaine dyes to small changes in solvent, temperature, pressure, and substituents makes these dyes a very useful class of compounds. They are not only solvatochromic compounds, but exhibit also the phenomena of thermo and piezo-solvatochromism. Their use for setting up different reaction series in order to get Linear Free-Energy Relationships has been demonstrated by the fact that the same betaine dye can be used not only for the introduction of a spectroscopic solvent polarity scale, the so-called E i-scale, but also for the establishment of kinetic and spectroscopic scales of substituents. 

All chromophores are affected by the polarity of the bulk environment to some extent. Some indicators exhibit more pronounced solvatochromatic behaviour, however, showing a pronounced shift in the position and sometimes intensity of an electronic absorption or emission band when the polarity of the medium is changed. The classic example is Reichardt s dye, which shows such an exceptionally large solvatochromatic shift (Amax shifts from 810 nm in diphenyl ether to 453 nm in water) that it was used to establish the commonly used Et(30) spectroscopic scale of solvent polarity [33]. 

Laurence C, Queignec-Cabanetos M, Dziembowska T, (Jueignec R, Wojtkowiak B (1981) 1-Iodoacetylenes. 1. Spectroscopic evidence of their complexes with Lewis bases. A spectroscopic scale of soft basicity. J Am Chem Soc 103 2567... 

A few scales of Lewis affinity and some spectroscopic scales of Lewis basicity (see below) have been constructed by carrying out the reaction A - - B AB on a dilute solution of the acid in pure, liquid base as solvent. This pure base method will be studied in Chapter 4. It gives solvent basicity scales which are not strictly equivalent to solute basicity scales measured on a dilute solution of the acid and the base in an inert solvent. 

Besides direct measurement of the thermodynamic quantities K and AH discussed above, spectroscopic estimates of basicity (affinity) have been proposed. Their main attraction is the ease with which they can be carried out. Moreover, many systems that do not possess the physical properties needed for a thermodynamic study can be characterized by spectroscopic parameters. Spectroscopic scales of basicity (affinity) are based on the change of a spectrochemical property (NMR, UV-Vis, IR, etc.) of the Lewis acid upon complexation... 

One can attempt [169, 170, 173] to interpret basicity scales or spectroscopic scales of basicity with the ECW model by rewriting Equation 1.121 as... 

The application of Equation 1.126 to the thermodynamic functions of a variety of Lewis acid/base reactions shows [184] that their responses to the electrostatic/covalent character increase in the order —AS < —AH < —AG. A rather covalent character is found for the spectroscopic scale of hydrogen-bond basicity Ai (OH), and the sequence —AS < A v(OH) < —AH < —AG is observed for the formation of hydrogen bonds. 

Vmin, -Pvmin and 7s,min are closely related to the electrostatic, polarization and charge-transfer components, respectively, of the Morokuma decomposition of proton affinity. It has been shown [197] that the spectroscopic scale Ziv(OH) (related to methanol), the phenol affinity, the diiodine affinity and the proton affinity of 42 nitrogen, oxygen and sulfur bases can be correlated by the triple-scale Equation 1.133 ... 

Mathematically, the aqueous Brbnsted basicity scale corresponds to a one-column data matrix. In fact, several columns are needed to take into account the influence of the medium but two media have been studied most, water and the gas phase. So the two most filled, and consequently most used, columns are the p/fBH+ nd GB scales. In the field of proton affinity, no column attains the degree of completeness and reliability of the gas-phase PA column. There is no need for spectroscopic scales of Bronsted basicity since the thermodynamic scales are satisfactory. 

Second, there are in practice not many systems in which a Lewis reference acid reacts with a series of diversified bases for which a series of reliable equilibrium constants can be measured easily by known physicochemical techniques over a large range of values (that is, from possibly very large to possibly very low equilibrium constants), and in the same conditions of temperature and medium. This is why chemists have turned to the determination of spectroscopic scales of Lewis basicity (affinity). 

Fortunately, there are a number of theoretical, statistical and empirical reasons to believe that Lewis basicity (affinity) depends on a limited number of factors. From the quantum chemical point of view, the acid/base interaction energy can be partitioned into five terms (electrostatic, dispersion, polarization, charge transfer and exchange-repulsion). By a principal component analysis [184], 99% of the variance of an afflnity/basicity data matrix can be explained by three factors, the first two being by far the most important. A number of experimental affinity and basicity scales, and of spectroscopic scales of basicity, can be correlated by two parameters, using the EC or equations, or three quantum chemical descriptors of basicity [197]. However, these statistical and empirical approaches are limited to systems where steric effects and tt back-bonding are not important. 

The discovery of charge-transfer bands in the UV spectra of diiodine complexes (Benesi and Hildebrand, 1949) [202] and the development of the underlying theory (Mulliken, 1952) [31] initiated a wealth of thermodynamic and spectroscopic measurements on diiodine complexes, mainly in the period 1949-1980. Complementary measurements by Berthelot, Guiheneuf, Laurence et al. (1970-2002) and Abboud et al. (1973-2004) enabled a homogeneous scale of diiodine basicity to be constructed. In addition, recommended values of diiodine affinity have been compiled from the literature (Laurence, 2006), for comparison with the SbCls, BF3 and 4-FC6H4OH affinity scales. UV and/or IR shifts upon complexation of the acids h, ICl and ICN have also been systematically measured by Berthelot, Laurence, Nicolet et al. (1981-1985). These thermodynamic and spectroscopic scales will allow the recent concept of a halogen bond to be treated quantitatively. They can be found in Chapter 5. 

Berthelot, M., Grabowski, G. and Laurence, C. (1985) Spectroscopic scales of basicity. Effect of the medium. Spectrochim. Acta, 41 A, 657-660. 

Thermodynamic and Spectroscopic Scales of Hydrogen-Bond Basicity and Affinity... 

Similarly, UV spectroscopic scales have been defined from solvatochromic shifts upon hydrogen bonding of the electronic transitions of probes such as 4-nitroaniline and 4-nitrophenol. Among these scales, it is essential to make a distinction between solute and solvent scales. A solute scale is obtained when the HBD probe is smdied in dilute solutions of a series of bases in the same given solvent, whereas a solvent scale comes from measurements on binary systems of the HBD probe dissolved in the pure bases. 

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