C. Maclean

Emission spectra of naphthalene, anthracene and naphthacene oriented in stretched polyethylene and in shpolskii matrices

Abstract Emission spectra of naphthalene, anthracene and naphthacene, oriented in stretched polyethylene sheets, have been studied. Absolute polarization of the bands and symmetry assignments of the vibrational modes are deduced and progressions of bands are drawn up. A comparison with other polarization investigation is given. To support the interpretation of the film spectra the molecules have also been measured (20 K) in suitable crystalline solvents (Shpolsku matries). Discrepancies with the literature are discussed. The anomalous behavior of the environment multiplet structure of naphthacene in n-nonane is described.

Magnetic field induced alignment effects in 2H NMR spectra

Abstract Magnetic field induced line splittings have been recorded in the 2 H NMR spectra of perdeuteropyrene and perdeuteronaphthalene dissolved in diethylether. The splittings originate from a small alignment, caused by the combined action of the anisotropic diamagnetic susceptibility χ and the magnetic field H . They are proportional to the anisotropy in χ and to H 2 . These effects provide a new method to study this anisotropy and the influence thereon of intermolecular interactions. Since the splittings are dependent on the directions of the C–D axes relative to the principal axes of χ, they constitute a potential source of stereochemical information.

The structure of the ion pairs of the carbanion of indene with alkali metal cations

Abstract NMR spectra have been measured of the Li+, Na+ and K+ ion pairs of the indenyl carbanion in 1,2-dimethoxyethane and tetrahydrofuran as a function of temperature. The changes of the chemical shifts are explained in terms of the detailed structure of the ion pairs. The results in both solvents strongly suggest that in indenyl-Li+ the counterion is predominantly located over the six-membered ring. In THF the preferred position of the cations Na+ and K+ in the contact ion pairs seems to be the five-membered ring.

Fluorescence and phosphorescence polarization of molecules oriented in stretched polymers. General description

Abstract A description is given of an experimental technique used to measure fluorescence and phosphorescence polarization spectra of molecules oriented in a stretched polymer sheet. This method of orienting molecules is applied to diphenylhexatriene, a molecule with a large long-axis-short-axis ratio and to coronene, a molecule with a three-fold axis of symmetry. The spectra observed indicate that both compounds are oriented considerably. The fluorescence of the first molecule is long-axis polarized; the phosphorescence of coronene is predominantly polarized out-of-plane. The merits of the stretched film technique are discussed.

13C NMR spectroscopy of the carbanion of indene as a function of temperature and counterion

Abstract The ion pairs of indenyl-lithium and -sodium in dimethoxyethane have been investigated by 13C NMR as a function of temperature. The results are in line with optical absorption measurements on these systems. Furthermore, they support the interpretation of the temperature dependence of the 1H chemical shifts in terms of a “direct” effect (a polarization of the carbon-proton bonds) and an “indirect” effect (a redistribution of the excess negative charge in the anion) caused by the cation. The present paper demonstrates that 13C NMR is very appropriate to study cation-anion interactions.

Some characteristic features of shpolskii spectra. The key and hole rule for shpolskii systems

Abstract The key and hole rule for Shpolskii systems has been re-examined by studying fluorescence spectra of naphthalene, anthracene and naphthacene in several n -alkane polycrystals (20 K). It is demonstrated that the key and hole rule has no significance for the systems under study. From our results and literature data the Shpolskii effect appears to be a rather general phenomenon for aromatic hydrocarbons in frozen n -alkanes, provided a correct concentration is chosen and a proper cooling rate is applied.

Phosphorescence polarisation of aromatic compounds oriented in stretched polyethylene films

Abstract Phosphorescence polarisation measurements are reported for a number of planar aromatic compounds in stretched polyethylene. The phosphorescence is to about 80% polarised perpendicular to the molecular plane. In the in-plane intensity second order Herzberg-Teller coupling terms may be at least competitive with three-center ππ spin-orbit integrals, in agreement with suggestions of Friedrich, Metz and Dorr.

Electric field effects in NMR

Abstract Electric field effects have been detected in the 14 N NMR spectrum of nitrobenzene. Application of an electric field induces molecular alignment in the liquid. The quadrupolar interaction is then not averaged out and line splitting results. From these line splittings it is deduced that the Lorentz- and Onsager local field models are not valid for nitrobenzene.

Spectroscopic investigations of photolysis products of organic compounds in Shpolskii matrices. Application to acenaphthene and monomethylnaphthalenes

Abstract It is shown that Shpolskii matrices can be very suitable to acquire quasilinear spectra of trapped unstable species. Photolysis of acenaphthene and monomethylnaphthalenes in Shpolskii matrices yields photoproducts exhibiting highly resolved spectra. It is proposed that in addition to the acenaphthenyl radical the 1-acenaphthenium cation is formed, whereas photolysis of monomethylnaphthalenes produces the methylnaphthyl radicals, the methylnaphthenium cations and the benzocycloheptenium cation.

Site-selection polarization measurements on tetracene in stretched polyethylene films

Abstract The polarization of individual vibronic bands in fluorescence spectra can be established with site-selection experiments using stretched polymer films. This is illustrated with an investigation on the 19000–20000 cm −1 region of the fluorescence of tetracene. The merits and possible applications of the method are discussed.