J. H. van der Waals

Paramagnetic resonance in phosphorescent aromatic hydrocarbons

Electron paramagnetic resonance spectra are given of glassy solutions of triphenylene, 1, 3, 5-triphenylbenzene, coronene and naphthalene excited into their lowest triplet state by ultra-violet irradiation at liquid-nitrogen temperature. For each substance two spectra were measured: one with the r.f. magnetic field parallel to the constant field, the other with the fields at right angles. A method is suggested for analysing the spectra of those molecules having a trigonal symmetry axis and values of the zero-field splitting parameter are derived. A complete treatment for molecules of lower symmetry has not been given, but it is shown that our results on a glassy solution of naphthalene are in agreement with those obtained by Hutchison and Mangum with single crystals.

Paramagnetic resonance in phosphorescent aromatic hydrocarbons. I: Naphthalene

Recently Hutchison and Mangum have observed electron spin resonance in the lowest triplet state of naphthalene. An analysis of their results is given on the basis of spin-spin interaction between the two unpaired electrons. In addition to the lines observed by these authors another transition, which corresponds to Δm = ±2 in the atomic case, must occur at low field strength. This transition has been observed in a mixed crystal as well as in rigid glass solution.

Vibronic interaction in the lower electronic states of benzene: I. Review of the static problem and solution of the pseudo-cylindrical vibronic equations

In the present paper the pseudo Jahn-Teller interaction between the four lower electronic states of benzene is considered, with special reference to the mixing of the 3 B 1u and 3 E 1u states through the e 2g carbon-carbon stretching mode. First a review is given of the pioneering work of Liehr on the static problem, where the ‘electronic Hamiltonian’ is solved as a function of fixed nuclear coordinates. Results of a specific calculation are presented. Subsequently, the equations for the vibronic problem are set up by generalizing the methods proposed by Moffitt et al. and Longuet-Higgins et al. for the dynamic Jahn-Teller effect. These equations are then solved in the first (‘pseudo-cylindrical’) approximation in which only the strong coupling linear in the displacements between the B 1u and E 1u states is considered. The results are discussed with reference to known properties of the phosphorescent state of benzene.

The statistics of the adsorption of rod-shaped molecules in connection with the stability of certain colloidal dispersions

Abstract The adsorption of rod-shaped molecules from a solution onto an adsorbent is studied. Expressions for the adsorption isotherm and surface tension are given using the model of a localized monolayer, and allowing for the interaction between the adsorbed molecules due to steric hinderance. Examination of the adsorption isotherm reveals that its slope becomes extremely small at high concentrations, long before the surface is completely covered. The same statistical technique is then applied to the model of two parallel adsorbing planes. When the planes are brought closer together the free energy of the system rises due to the interaction of the two adsorbed layers; an expression for the free energy of repulsion is given. Comparison with some experimental data shows that this effect must be responsible for the stabilization of certain colloidal dispersions in non-polar media.

Zero-field splitting of the lowest triplet state of some aromatic hydrocarbons: Calculation and comparison with experiment

It is investigated how far the zero-field splitting of the lowest triplet state of aromatic hydrocarbons can quantitatively be accounted for with a π-electron representation of the wave function. The calculations are based on triplet SCF-MOs, as obtained from an extension by Colpa of Parisers formalism to open-shell systems. Interaction with all π-electron configurations that are either singly or doubly excited relative to the lowest triplet configuration is included. The pertinent integrals over 2pz Slater AOs have been evaluated by numerical integration, with a result almost identical to that obtained by Gouterman and Moffitt for Gaussian AOs. When the calculated values of the parameters D and E in the spin Hamiltonian for naphthalene, anthracene and phenanthrene are compared with experiment, those of D are found to be 15–20 per cent too high, whereas those of E—though lying in the right order—are considerably too large. The contribution to the splitting parameters due to spin polarization of the σ-...

Phosphorescence and spin polarization

Intersystem-crossing in aromatic molecules is caused by spin-orbit coupling. This mechanism is expected to be highly selective for a specific spin-component of each triplet state. Experimental evidence is given for quinoxaline proving that spin polarization indeed occurs when this molecule is optically excited into its lowest triplet state. A remarkable coherence phenomenon, reminiscent of level-crossing situations in atomic spectroscopy, seems to be observed following flash excitation in a magnetic field.

Optical pumping in an organic crystal: quinoxaline in durene

Experiments have been performed to determine the path of entry into and exit from the phosphorescent triplet state T 0 of quinoxaline in a durene host. First of all the decay of phosphorescence after flash excitation was followed at 4·2 and 1·34°k. It was found that for both perdeutero- and perhydroquinoxaline the lifetime is shortened by a factor of about three when the temperature is lowered from 4·2 to 1·34°k. At 1·34°k relaxation between the spin components (i.e. re-orientation of the triplet spin angular momentum) is slow relative to the decay, and the observed reduction in lifetime indicates that entry into and exit from T 0 are through the same spin component. Similar decay experiments were then carried out at 1·34°k in a 10 kg magnetic field or in a somewhat weaker field so chosen that the effect of microwave saturation of one of the E.S.R. transitions between the components could be observed. From the results it follows that on intersystem crossing the molecules enter the manifold T 0 through the...

Thermodynamic properties of gas hydrates

A statistical mechanical description based on the Lennard-Jones and Devonshire model has been applied to the gas hydrates. The theory enables one to calculate dissociation pressures, compositions and heats of dissociation of several such gas hydrates. The agreement between the calctilated values and the experimental data at present available supports the theory.

Proton magnetic resonance of aromatic carbonium ions

N.M.R. spectra of aromatic carbonium ions obtained by dissolving highly basic aromatic hydrocarbons in CF3COOH+H2O.BF3 or HF+BF3 were measured. The resonance of the protons in the CH2 group in the carbonium ion was detected. It was concluded that this configuration, formed by the addition of the proton to the most reactive carbon atom in the hydrocarbon, has an ‘aliphatic’ character. In most instances the N.M.R. spectra of the carbonium ions are blurred by exchange reactions. The character of these reactions is discussed.

The effect of deuterium and chlorine substitution on triplet → singlet transition probabilities in naphthalene

The effect of substitution in aromatic molecules on the radiative transition probability from the triplet phosphores cent state (T) to the ground (S/sup 0/) state, was measured by irradiating a glassy solution of the phosphorescent substance, at 77 deg K, in a microwave cavity. The relative concentration of molecules in their triplet state was determined from the (integrated) intensity of the magnetic resonance signal. After the exciting light was switched off the corresponding phosphorescence intensity and decay time were measured by means of a photomultiplier and a recording instrument. Preliminary results on naphthalene, deuteronaphthalene, and beta - chloronaphthalene are tabulated. (L.N.N.)