Dielectric relaxation processes of some dialkyl disulfides

Abstract

. J. Chem. 61, 2107 (1983).Dielectric relaxation studies have been performed for dimethyl, diethyl, di-isopropyl, di-n-butyl, di-tert-butyl, and di-n-octyl disulfides in a polystyrene matrix. For the last five compounds the absorption appears in two temperature regions. The relaxation data for the higher temperature regions are consistent with intramolecular rotation around the S-S bond with accompanying flexibility in the alkyl chains. The enthalpy barrier to this relaxation process is -29 kJ mol-' and is the trclr~s (lower) barrier. This value is typical for all six dialkyl disulfides. The most likely interpretation of the low temperature process is that there is segmental relaxation of the alkyl chains which also involves movement of the main dipoles (S-S and S-C).On a effect& des Ctudes de relaxation dielectrique sur les disulfures de mkthyle, d'kthyle, d'isopropyle, de n-butyle, dc tert-butyle et de n-octyle dans un support de polystyrene. Pour les 5 derniers composes I'absorption se manifeste dans deux zones de temperature. Les donnees de relaxation pour la region de haute temperature sont en accord avec une rotation intramoleculaire autour de la liaison S-S avec une flexibilitC dans la chaine alkylCe. La barriere d'enthalpie de ce processus de relaxation est de 29 kJ mol-' environ et c'est la barriere trans (la plus basse). Cette valeur est typique pour tous les six disulfures d'alkyle. L'interpretation la plus courante de processus a basse tempkrature prkconise une relaxation segmentaire des chaines d'alkyies qui implique Cgalement Ie mouvement des dipoles principaux (S-S et S-C).[Traduit par le journal] IntroductionThe existence of a barrier to rotation around the S-S link of a disulfide was established at least two decades ago, but experimentally determined barrier heights span quite a range. The rotation was frequently treated on a two-fold energy barrier model although in many measured values the cis and trans barriers were not separately identified. Two of the earlier studies on infrared spectra of dialkyl disulfides were those of Thompson and Trotter (1) and of Sheppard (2). At about the same time Scott and co-workers (3) reported the vibrational assignments of dimethyl disulfide (3a), diethyl disulfide (3b), and dimethyl di-n-propyl disulfide (3c), together with the collection of early data of infrared and Raman frequencies for thermodynamic studies. Detailed infrared and Raman spectra of di-tert-butyl disulfide were measured later by Scott and co-workers (3d). These investigations showed the presence of some rotational freedom about the S-S link. Through the use of calculated and observed entropy and heat capacity, an effective barrier height of 28.4 kJ mol-I (3c) for the internal rotation around the S-S bond of dimethyl disulfide was deduced. El-Sabban and Scott (3e), using infrared data, reported a sim-