Quadrupole Resonance Spectrum of Chloranil and Its Hexamethylbenzene Complex

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.

Nuclear quadrupole resonance study of phase transition in tetrachloro‐p ‐benzoquinone (chloranil)

Abstract: The phase transition in chloranil crystal has been studied by measurements of the 35Cl nuclear quadrupole resonance frequency and the spin‐lattice relaxation time. The splitting of the resonance line in the low temperature phase was shown to behave as an order parameter of the phase transition. The experimental order parameter and the anomalous behavior of the transition probability (1/T1) near the transition point were interpreted satisfactorily in terms of the librational soft‐mode theory which assumes anharmonic coupling between molecular librational modes in the crystal.

Temperature Dependence of the Zeeman Effect in the Nuclear Quadrupole Resonance in Chloranil

Abstract: The nuclear quadrupole resonance of Cl35 and its Zeeman effect have been observed in chloranil between 78° and 373°K. Effects of thermal expansion appear negligible hence the Bayer theory is used with the measured temperature dependence of the pure quadrupole frequencies and the field‐gradient asymmetries to yield room‐temperature rigid‐body libration amplitudes of 〈θx2〉 = 18×10—3 rad2, 〈θy2〉 = 〈θz2〉 = 0, where x is along the molecular normal. A broad phase transition centered near 100°K is examined. The results suggest that the potential well V(θx) has a double minimum and that the transition occurs when the molecules have sufficient torsional energy to override the barrier.

Transient EPR emission spectra of a free radical trapped in a single crystal of chloranil

Abstract: The EPR spectrum of a free trapped in single crystals of chloranil changes its phase from absorption to emission when the crystal is illuminated by visible light. The time evolution of the EPR signal is discussed in terms of the interaction between the doublet species and triplet excitons produced by light excitation.

Pure Nuclear Quadrupole Spectra of Chlorine and Antimony Isotopes in Solids

Abstract: Experimental measurements on nuclear quadrupole resonances of chlorine and antimony isotopes in solids have been made to an accuracy of about 0.001 percent. The results are compared in detail with theoretical results for (1) nuclear quadrupole interaction, (2) interaction between quadrupole coupling and thermal vibrations, and (3) effects of a nuclear hexadecapole. The ratio $\frac{{(\mathrm{eQq})}_{{\mathrm{Cl}}^{35}}}{{(\mathrm{eQq})}_{{\mathrm{Cl}}^{37}}}$ varies between 1.268736 and 1.268973 while $\frac{{(\mathrm{eQq})}_{{\mathrm{Sb}}^{123}}}{{(\mathrm{eQq})}_{{\mathrm{Sb}}^{121}}}$ varies from 1.274714 to 1.274770. These variations may be attributed to zero-point vibrations and to thermal vibrations, so that no clear evidence is found for nuclear polarization by surrounding electric fields. For $p$-dichlorobenzene, the temperature coefficient of the coupling constant of Cl and the variation of isotopic coupling ratio with temperature are shown to be in fair quantitative agreement with the extension of Bayer's theory of vibrational effects. Relaxation times, Zeeman effects, and certain effects due to crystal structure are examined. Small discrepancies in the measured ratios of frequencies of transitions in ${\mathrm{Sb}}^{121}$ and ${\mathrm{Sb}}^{123}$, which can be attributed to nuclear hexadecapole interactions are found. These indicate a hexadecapole coupling constant in ${\mathrm{Sb}}^{123}$ of 24 kc/sec and a ratio of the ${\mathrm{Sb}}^{123}$ hexadecapole coupling to that of ${\mathrm{Sb}}^{121}$ of 0.8\ifmmode\pm\else\textpm\fi{}0.3. A convenient high-sensitivity circuit for observation of nuclear resonances in solids has been developed and is discussed.

Correlation of Hammett's σ-Values with Electron Densities Calculated by Molecular Orbital Theory

Abstract: It is shown that Hammett's σ‐values can be correlated with electron densities calculated by the MO LCAO method. Since Coulomb and exchange integrals for atoms other than carbon are not accurately known, an absolute calculation of σ's is not possible. Nevertheless, σ‐values permit derivation of a self‐consistent and reasonable set of parameters which can be used to calculate reasonable values for absorption frequencies, mesomeric moments, and orienting power of the substituents.

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...