P. J. Bray

Bio: P. J. Bray is an academic researcher. The author has an hindex of 1, co-authored 1 publications receiving 93 citations.

Estimates of Hammett's Sigma Values from Quadrupole Resonance Studies

Abstract: A linear correlation exists between the Cl35 pure quadrupole resonance frequency in substituted chlorobenzenes and the Hammett parameter sigma for the substituent. The relation is expressed as f=1.024σ+34.826 Mcps. For the 52 frequencies used in determining this relation, the correlation coefficient is r = 0.96 and the root‐mean‐square deviation of the experimentally determined f values from those predicted by the equation is 360 kcps.A proper ortho sigma parameter for use in the f vs σ correlation is apparently the value derived from the Hammett relation σ = lnK/K0 in which K and K0 are the equilibrium constants for ionization of the substituted and unsubstituted benzoic acid in water at 25°C. This parameter is linearly proportional to the polar substituent σ* of Taft for the ortho substituent.In the many multiply substituted chlorobenzenes investigated, the proper total sigma value has been taken as the sum of the individual parameters. Few cases are apparent in which the linear relation of f to σ is se...

A Critical Compilation of Substituent Constants

Abstract: The practical usefulness of a correlation equation as well as its significance in theory strongly depend on its range of validity, not only as it is defined but also as its use is restricted in practice by the number of parameters actually known. Hence the tables of parameters are of primary importance and reflect the actual state of a given field. In this chapter attention is focused on constants characterizing the substituent, which are mainly denoted by the letter σ with various subscripts and superscripts, but sometimes also by other symbols (E s, v). The correlation equation most frequently has the form (10.1). For the most common equations of this type $$y = \rho _0 + \sum\limits_{i = 1}^A {\rho _i \sigma _i } $$ (10.1) (Hammett, Taft, Yukawa-Tsuno) many tabulations of σ constants are available (e.g. Refs. 1–13), but authors usually select from the literature only one preferred value for each substituent and the extent of the table is always a compromise between two demands: reliability and completeness. Actually neither can be fully satisfied, so that the user cannot really rely on all the data on the one hand, and must always search for additional data on the other. The problem is more urgent than with the tabulation of any other constants, since the individual σ values are not only of varying precision but may even be defined and calculated in quite different ways. It follows that the main problem of this chapter will be the clear definition of how a given σ value was obtained.

Induction Studies in Several Groups of Halogen‐Containing Organic Compounds by Their Cl35, Br79, or Br81 Pure Quadrupole Resonance Spectra

Abstract: Induction studies have been carried out in several groups of halogen‐containing organic compounds by means of the halogen pure quadrupole spectra In several straight‐chain compounds inductive effects have been examined as a function of the length of the chain in some monohalogenated‐paraffin hydrocarbons and as a function of the number of carbon atoms separating halogen atoms attached to either end of dihalogenated‐paraffin hydrocarbons The effects of the hybridization of the carbon atom have also been examined in some aliphatic hydrocarbons Chlorine pure quadrupole resonance frequencies from chlorine atoms bonded to atoms other than carbon, such as silicon, nitrogen, and sulfur, have been measured, and the effects of these different atoms upon the Cl35 resonance frequencies are qualitatively discussedInductive effects have also been examined in different groups of aromatic compounds The inductive effect in several chlorobenzyl compounds is found to be about ¾ of what it is in the corresponding chlorobenzene compounds as determined from a comparison of the Cl35 resonance frequencies Definite discrepancies have been observed between the Cl35 spectra obtained from several aromatic compounds which have molecular structures similar to anthraquinone and the Cl35 frequencies that can be predicted from data obtained from chlorobenzene compounds and Hammett σ valuesEmploying the linear relationship between Cl35 resonance frequency and Hammett σ values, several Cl35 frequencies were predicted and compared to the measured frequencies in some chlorobenzene compounds Some unknown σ values have also been predicted A linear relationship between Br81 resonance frequencies and Hammett σ values has been developed from measured bromine quadrupole frequencies Several halogen frequencies from some unrelated compounds have been measured and are tabulated in this paper

Lack of Charge Transfer in Aromatic Charge—Transfer Complexes

Abstract: Halogen pure electrical quadrupole resonance frequencies have been measured in the three 1:1 complexes, CBr4·p‐xylene, CCl4·p‐xylene, and Br2·benzene. From an examination of the measured frequencies of the complexes and the halogen‐containing compounds (CBr, CCl4, and Br2) together with information from the x‐ray structure determinations for two of the complexes, it has been concluded that little if any charge transfer exists in the ground state of these complexes.

Quadrupole Coupling of Solid Cyanogen Chloride

Abstract: The nuclear quadrupole resonances of N14 and Cl35 have been observed in solid ClCN. In the course of study of the N14 resonances a detailed study was made of the shape of the negative wings which occur when Zeeman modulation and phase sensitive detection are used. The shape of these wings can reveal whether two resonances are due to two nonequivalent positions of the N14 nuclei, or one position with a nonzero asymmetry parameter. A theoretical expression has been derived for the shape of the wings which agrees with the experimental results. The coupling constants of the Cl35 and N14 nuclei have been interpreted in terms of a set of hybrid structures.

Correlations between 35Cl Nuclear Quadrupole Resonance (NQR) Frequencies of Chlorobenzene Derivatives and Characteristic Reaction Parameters of the Substituents

Abstract: The 35Cl NQR frequencies (at 77°K) of a number of chlorobenzene derivatives have been measured. The frequencies (including additional data from the literature) have been correlated with characteristic parameters, such as Taft's σI and σR. It is found that the Bray–Barnes relation can be somewhat improved by introducing a parameter κ: ν = ν0 + ∑ iκi. The following relations are found: (a) the effects of more than one substituent on the benzene are additive; (b) steric effects (o‐substituents) are not important for the NQR frequencies; (c) a correlation between the NQR frequency ν and Taft's σI and σR (correlation coefficient r = 0.976) shows the importance of the inductive effect on NQR. The mesomeric effect plays only a minor role in 35Cl NQR in benzene derivatives. Unknown σI values may be gained from NQR measurements; for the evaluation of σR, the determination of the chemical shift of 19F NMR seems to be the better method.