Hammett equation spectroscopic correlations

The impressive success of the Hammett equation in correlating literally hundreds of observed properties (17) (e.g., rate and equilibrium constants, spectroscopic properties, etc.) may be attributable to the multitude of interaction mechanisms that is implicitly embedded in the values of a. The validity of the separability and additivity axioms used in the derivation of extra-thermodynamic relationships is confirmed by the ability to separate experimentally multiple interaction mechanisms (e.g., inductive and resonance (19, 20, 21), polar and steric (10), enthalpic and entropic (22)). This separation fostered significant progress in the application of quantitative structure-activity relationships to the study of chemical mechanisms. For these relationships can now be expressed in terms of more basic properties of the molecules under study. 

Haloform reaction, 237, 296 Halogenation alkanes, 300, 323 alkenes, 179,186, 313 benzene, 138,316 ketones, 295 Hammett equation, 362 additional parameters, 374, 388, 395 derivation of, 362 deviations from, 375 empirical nature of, 395 implications of, 394 reaction pathway, and, 375 solvent effects and, 388 spectroscopic correlations, 392 standard reaction for, 362, 395 steric effects and, 361, 383 thermodynamic implications of, 394 Hammett plots, 359 change in rate-limiting step and, 383 change in reaction pathway and, 378... 

The numerous reactions and physical properties that can be correlated with electronic substituent constants suggest that they reflect a basic mechanism for the effect of the substituent on the reaction or the property. Many reactions were correlated by the Hammett equation, such as electrophilic (4, 31, 32, 33), nucleophilic (4, 34) and radical (35, 36, 37). Spectroscopic properties, such as infrared and ultraviolet absorption frequencies and intensities (38). nuclear magnetic resonance shifts of various nuclei, e.g., -H, F, (39), and mass spectroscopic ionization... 

Attempts were made to apply Hammett benzoic acid-based Om and ap constants to the correlation analysis of spectroscopic data. Some significant correlations were obtained, but many of the correlations were rather poor, trends rather than precise relationships. Success in this area was found to involve the separation of inductive and resonance effects and the application of the dual substituent-parameter (DSP) equation (Section II.B). Indeed the development of the DSP equation became closely connected with the correlation analysis of NMR shielding of substituted fluorobenzenes at an early stage, around I957IO6 0 appijg(j extensively to F NMR data , and within a few years... 

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