Application of proton magnetic resonance to rotational isomerism in halotoluene derivatives. II. α,α,2,4,6-Pentachlorotoluene

Abstract

At temperatures below -40 "C the proton magnetic resonance of a,a,2,4,6-pentachlorotoluene in toluene-d8 or methylcyclohexane is an ABX spectrum where X is the methine proton. The C-H bond of the dichloromethyl group lies in the plane of the ring and its proton couples to one ring proton only. As the temperature increases the mean lifetime before interconversion of the "mirror image" isomers decreases and at 70 "C the proton spectrum is A2X. A density matrix formalism is used to calculate the line shape as a function of the lifetime before exchange. The free energy barrier to rotation of the dichloromethyl group is 15.0 + 0.1 kcal/mole in both solvents. In toluene-d8 the entropy of activation is near zero. The high energy transition state is very likely one in which the C-CI bonds of the ring and the dichloromethyl groups are eclipsed. Canadian Journal of Chemistry, 48, 1558 (1970) IntroductionExtensive nuclear magnetic resonance (n.m.r.) studies of the rotational isomers of halogenated ethanes are described in the recent literature (1-4). Reliable thermodynamic parameters for the isomers in their ground state as well as rotational barriers are only obtainable if the n.m.r. spectra of the individual isomers can be observed (1, 2). In common with the halogenated ethanes, toluene derivatives present the problem of rotation about an essentially single carbonxarbon bond. In toluene itself the sixfold barrier to rotation of the methyl group lies well below 200 cal/mole in the gaseous and solid states (5, 7) but it has become clear that in toluene derivatives containing halogen atoms both in the methyl groups and in the ortho ring positions (7),3 the barrier to rotation can be large enough for convenient study by dynamic n.m.r. (d.n.m.r.) methods (4).In this laboratory a series of such compounds are under investigation. Previous studies were confined to fluoromethyl toluene derivatives in which the n.m.r. spectra of individual rotamers were not observed (8). In this paper we describe the analytical techniques involved in our studies and report the barriers to rotation about the C-C bond of a,a,2,4,6-pentachlorotoluene (PCT) in toluene-d8 (C,D8) and in methyl cyclohexane (C6HllCH,).

Application of proton magnetic resonance to rotational isomerism in halotoluene derivatives. III. α,α-Dichloro-2,4,6-tribromotoluene

“…In perchlorobutadiene the 100 MHz proton spectrum consisted of a tightly coupled ABC spectrum at 30 OC. However, in toluene-d8 the 100 MHz proton spectrum is ABX, very similar to the spectrum for PCT at -40 OC (1). It was previously shown that the intrinsic temperature dependence of the internal ring proton shift of PCT in toluene-d8 can and must be found in order to obtain reliable exchange rates from fitted spectra.…”

“…The spectra themselves are very similar to those of PCT presented in ref. 1. Table 1 gives the spectral parameters in the region of slow exchange.…”

“…The spectra were repeatedly calibrated by reading oscillator frequencies in the frequency sweep mode. The homogeneity of the external magnetic field was monitored by the appearance of the methine proton peaks (Hx), whose widths are independent of exchange times (1). Over the temperature range used here, the homogeneity could be accurately controlled to give an effective TZ = 1.72 f 0.12 s. The internal ring proton shift was measured as a function of temperature in the region of slow exchange (10-35 "C).…”

“…The previous paper (1) in this series presented a detailed description of a proton magnetic resonance (p.m.r.) study of hindered rotation of the dichloromethyl group in cr,cr,2,4,6-pentachlorotoluene (PCT).…”

Conformational dependence of long-range spin-spin coupling constants in 2,6-dichlorobenzylfluoride

“…From studies on a,a,2,6-tetrachloro-and a,a,2,6-pentachlorotoluene (6,10) it is known that this five-bond coupling is conformation dependent. Thus in 2, s 0.5 1iz whereas J>nHx < 0.10 HZ, indicating a preponderant o electron contribution to J :~~~~~.…”