Resonance constant definition

Definitions ctr, resonance constant cti, field constant a = or + ai. Reaction mole ratio of Ib/Ph-R/aluminum chloride = 1 20 0.1. 

Molecular Switches, p. 917 Molecular-Level Machines, p. 931 Nuclear Magnetic Resonance Spectroscopy, p. 981 Rotaxanes and Pseudorotaxanes, p. il94 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 Stability Constants Definition and Determination, p. 1260 The Template Effect, p. 1493 X-Ray Crystallography, p. 1586... 

A plot against Hammett s cr-constants of the logarithms of the rate constants for the solvolysis of a series of Mz-substituted dimethylphenylcarbinyl chlorides, in which compounds direct resonance interaction with the substituent is not possible, yielded a reasonably straight line and gave a value for the reaction constant (p) of — 4 54. Using this value of the reaction constant, and with the data for the rates of solvolysis, a new set of substituent parameters (cr+) was defined. The procedure described above for the definition of cr+, was adopted for... 

Many other definitions of an ortho substituent constant have been made Shorter has reviewed these. Charton analyzed Oo in terms of Oi and CTr, i.e., = a(Ti -I- fpoR, finding that the distribution of inductive and resonance effects (the ratio a/b) varies widely with the substituent and, therefore, that no general Oo scale is possible. Charton also subjected to analysis according to Eq. (7-47),... 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

Here q represents the coulomb energy of an electron occupying a definite p]h orbital in unsubstituted benzene its value has been estimated to be about —2.7 v. e. = —60 kcal./mole.5 /3 is a resonance integral between adjacent orbitals its value has been estimated to be about —0.85 v. e. = — 20 kcal./mole.6 Sk is a constant, the purpose of which is to allow for the different electron affinities of the different atoms. For Sk > 0, the... 

The definition used depends on the phenomenon under study. For instance, the intensity-averaged lifetime must be used for the calculation of an average colli-sional quenching constant, whereas in resonance energy transfer experiments, the amplitude-averaged decay time or lifetime must be used for the calculation of energy transfer efficiency (see Section 9.2.1). 

Equation (2) for a reaction giving a set of apparent substituent constants o can be rewritten in the form, (o- t/ ) = r a - cr ), where r is constant for the reaction regardless of substituents. As the increment of any o-from (f should be a reasonable measure of the resonance capability of the respective substituents, this proportionality represents a linear resonance energy relationship. The original form (Yukawa and Tsuno, 1959, 1965) using cr instead of (f in (2) has the same significance since the proportionality relation holds for the resonance increment (a - t/ ) or (a - cr). The definition of resonance substituent constants Ao by any set of (o— o") is arbitrary, and the definition of the r scale is also arbitrary. While the definition of the r = 0 scale by [Pg.269]

---