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

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

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...

Estimates of Hammett's Sigma Values from Quadrupole Resonance Studies

Quadrupole coupling data obtained from microwave spectroscopy are utilized in assessing double‐bond character in planar molecules such as the vinyl halides. The approximations involved and the effect of structural uncertainties have been considered in detail. An uncertainty of 20% or less is estimated for the calculated values of δ, the number of halogen py electrons lost as a result of double bonding. Application of the method to vinyl chloride and vinyl iodide yields δ values of 0.06 and 0.03, respectively. A revised value of −72 Mc for (eQq)bond in vinyl chloride is reported. The present results are compared with conclusions based on other molecular properties.

Quadrupole Coupling and Bond Character in the Vinyl Halides

The microwave spectra of four isotopic species of chlorine azide, ClN3, have been observed in the frequency range 26–48 GHz. From the measured values of the rotational constants, the molecule has been deduced to be planar with the following internuclear parameters: r(Cl–N(1)) = 1.745 ± 0.005 A, r(N(1)–N(2)) = 1.252 ± 0.010 A, r(N(2)–N(3)) = 1.133 ± 0.010 A, ∠ClN(1)N(2) = 108°40′ ± 30′, ∠N(1)N(2)N(3) = 171°56′ ± 30′, with Cl and N(3) trans. Values for the chlorine quadrupole coupling constants were obtained for all isotopic species, and for the most abundant, 35Cl14N3, were transformed into the principal field gradient axis system. The values obtained are consistent with the deduced structure.

Microwave Spectrum and Structure of Chlorine Azide

The optically detected magnetic resonance (ODMR) of para‐dichlorobenzene in its first 3ππ* state is reported. The structures of the three zero‐field transitions are interpreted in terms of a Hamiltonian incorporating the electron spin—spin, chlorine nuclear quadrupole, and chlorine hyperfine interactions. It is concluded from an analysis of the spin‐Hamiltonian parameters that the triplet spin density in the chlorine out‐of‐plane orbitals is small, the chlorine electric field gradient is significantly reduced in the 3ππ* state as compared to the value for the ground state, and that the molecule most likely is distorted.

Lowest Triplet State of Substituted Benzenes. I. The Optically Detected Magnetic Resonance of para‐Dichlorobenzene

The π electron loss of a chlorine atom conjugated with a resonating system can be obtained at least approximately from experimental measurements of the asymmetry of the chlorine nuclear electric quadrupole interaction. Using molecular orbital theory the π electron loss ρ can be simply expressed in terms of the chlorine Coulomb integral, the carbon‐chlorine resonance integral, and the energies and wave functions of the system with which the chlorine atom is conjugated. Comparison of theoretical and experimental values of ρ show that the carbon‐chlorine resonance integral is about one‐third of the carbon‐carbon resonance integral. The basic conclusion is that chlorine atoms in general resonate only weakly with attached aromatic systems. Most of the effects previously ascribed to double bond character can alternatively be explained by an increased electronegativity of the carbon sp2 orbital.

R. Bersohn

Papers

Double‐Bond Character of Conjugated Carbon‐Chlorine Bonds