Microwave Absorption and Molecular Structure in Liquids. XLII. Molecular and Group Rotation in Aromatic Methoxy and Ethoxy Compounds<sup>1,2</sup>

Microwave dielectric constants and losses, and viscosities (η) have been obtained for acetophenone, anisole, bromobenzene, and p-dimethoxybenzene in n-heptane, n-hexadecane, paraffin oil, and three n-heptane + paraffin oil mixtures between 15 and 60 °C. The dielectric results have been analyzed for mean relaxation times (τ0). Plots of ln τ0T against ln η for bromobenzene are compared with those for the other solutes in an attempt to determine the effect of contributions from intramolecular relaxation processes. The results suggest that measurements employing several solvents at one temperature provide a better insight into mechanisms of dipole reorientation than those with one solvent at several temperatures.

Section: Discussionsupporting

confidence: 76%

Section: Mean Relaxation Times ( T~) mentioning

confidence: 99%

Macroscopic viscosity and the dielectric relaxation of some substituted benzenes

Srivastava¹,

Crossley²

1976

“…A number of previous studies has been carried out on aromatic methoxy compounds (2)(3)(4), and a detailed study of four such compounds is now reported. Serious discrepancies exist amongst different workers with respect to analysis of the anisole data which justify its re-examination.…”

Section: Introductionmentioning

confidence: 99%

Dielectric studies. Part XXV. Methoxy group rotation in anisole and three para-substituted anisoles

Farmer¹,

Walker²

1969

The dielectric absorption at several microwave frequencies of anisole, p-methylanisole, and p-bromoanisole in the solvent p-xylene, and p-diniethoxybenzene in the solvent cyclohexane has been investigated at 4 to 6 temperatures. Anisole, p-methylanisole, and p-dimethoxybenzene were all found to relax mainly by methoxy group rotation, whereas the relaxation process in p-bromoanisole was very largely niolecular relaxation. In the literature, considerable divergence exists for the analyses of the dielectric data of anisole and substituted anisoles into contributions from two relaxation times and the magnitude of the weight factors governing each relaxation process. Such divergencies have been explored and justifiable analyses established for these systems where the weight factors governing the relaxation processes are shown to be roughly of the same order as those estimated from group nioment data. The weight factor for molecular relaxation in the pure liquid appears considerably greater than that for a dilute solution of it in a non-polar solvent.

“…Further, the free energies of activation estimated at 333 K for this solute in polystyrene and o-terphenyl, 12 and 14 kJ mol-' respectively, are virtually identical. Grubb and Smyth favoured a small contribution from the molecular process (C, -0.15) for 1,4-dimethoxybenzene in Nujol and also in decalin solution (14). A con~parison of the relaxation times a t 333 K in benzene (11 -0.5 cP), r , = 4.6 ps with that in o-terphenyl solution (q -25 cP), r , = 19.7 ps, strongly supports contributions from both group and molecular processes.…”

Section: 4-dil71ethoqbel1zenr a D 44'-dir~1etl1oq~oipi1e11~~1mentioning

confidence: 99%

“…Ho~fever, dielectric studies on sim~lar solutes in non-polar solvents (14,15) gave values of -6 kJ mol-' T h~s significant d~fference in the cnthalp~cs of activation for the acetyl and methoxy groups bet\$een these t~ o d~fferent dlelectr~c approaches merited further study. The molecules chosen fol investigation mere of the type ,X-aro-,y Y matic->(' ; such molecules have a common feature in that the fixed component of the inain dipole along the main principal axis is zero: thus, if inolecular relaxatioll is to be detected, it has to result fro111 a (temporary) stable isomer which has a dipole component perpendicular to this long axis (e.g.…”

Section: Introductionmentioning

confidence: 99%

Molecular and group relaxation studies in parasubstituted benzenes in various media

Crossley¹,

Shukla²,

Tay³

et al. 1979

Dielectric absorption studies have been made on molecules of the type [Formula: see text][Formula: see text] where X—Y is —OCH3, —COCH3, —CH2Cl, and —CH2CN. The molecules have been dissolved in (a) a polystyrene matrix and (b) in the viscous liquid o-terphenyl. For 1,4-dimethoxybenzene the group process dominates in both media, and estimates have been made of the activation enthalpy for group relaxation. For 1,4-dimethoxybenzene in benzene, decalin, Nujol, and o-terphenyl solutions, however, there appears to be a small contribution from the molecular process. No such contribution was detected in the matrix studies on this solute molecule. For 1,4-diacetylbenzene, 1,4-bis(chloromethyl)benzene, and 1,4-bis(cyanomethyl)benzene in o-terphenyl solution it again seems that the group and molecular processes overlap. However, in polystyrene the enthalpies of activation which emerge are characteristic of the group relaxation process as determined by the nmr approach. Altogether, the work illustrates that o-terphenyl may be a useful solvent in helping to decide whether a solute has contributions from an intramolecular motion.