In principle, IR spectroscopy can be used to measure the populations of diastereomeric conformers that are in rapid equilibrium. However, the use of IR spectroscopy suffers from the necessity to obtain independent measures of the extinction coefficients for pertinent bands in a given conformer. Common assumptions are that extinction coefficients are equal for different conformers and independent of temperature. We have examined the NMR spectra of four halocyclohexanes over a temperature range where the axial-to-equatorial equilibration is slow on the NMR chemical exchange time scale. Using complete line-shape analysis, we carefully determined the variation of the equilibrium constant with temperature (allowing calculation of thermodynamic parameters). Measurements of the IR spectra over the same temperature range enabled the calculation of the relative IR extinction coefficients for the C-X bond stretch in the axial and equatorial conformers. The vibrational extinction coefficients ratios are indeed not unity and exhibit a slight temperature dependence. Based on the IR data and on our measurements of the one-bond scalar coupling of the methine carbon to the proton, there are indications that axial carbon-halogen and carbon-hydrogen bonds are weaker than the equatorial counterparts.

Examination of the 1H DNMR spectrum of 5 (4% v/v in CH2CHCI) at -37.0 °C (Figure 1) revealed a singlet resonance for the tert-butyl protons (6 1.045) consistent with rapid tert-butyl rotation on the DNMR time scale. At temperatures below -37.0 °C, the tert-butyl resonance broadens and is separated at -111.9 °C (Figure 1) into three singlet resonances at 0.980 (3 ), 1.046 (3 H), and 1.129 (3 H) consistent with slow tert-butyl rotation on the DNMR time scale and the symmetry experienced by a static tertbutyl (eq 1). Total DNMR line shape analyses at various

The "P('H) DNMR spectra of a series of rhodium(1) square-planar (d8) complexes of the type rrans-[RhX(CO)L,]. in which L = (t-C4H9)2PCH3, or (t-C4H9)2P(C6H~) and X = C1, Br, or I show changes consistent with slowing intramolecular conformational exchange. The 31P(lH) NMR spectrum of each complex at slow exchange can be interpreted in terms of different conformers. DNMR line-shape analyses of the exchange-broadened spectra speak for a specific stepwise interconversion among the various conformers rather than a nonselective exchange pathway. Such a stepwise itinerary is consistent with simple internal rotation about the rhodium-phosphorus bonds.Previous research in our laboratory has shed some light on the intramolecular stereodynamics of tertiary phosphines2 and bis(ph~sphines).~ In addition, we have examined the effects of complexation on carbon-phosphorus bond rotation in tertiary phosphine boranes4 and LM(C0)5 complexes (L = tertiary phosphine; M = Cr, Mo, W).s This work led us to examine the dynamic stereochemistry of a series of transbis(phosphine) halocarbonylrhodium( I) systems.Planar four-coordinate Rh(1) complexes play key roles in catalysis of the hydroformylation reaction (e.g., Rh [P-(c6H5)3]2(cO)cl) and the homogeneous hydrogenation of alkenes and acetylenes (e.g., Rh[P(C6H5)3]3C1).6 Solution conformational preferences and conformational exchange rates may be factors in determining the stereospecificity of asymmetric induction by related complexes.' Many studies of rhodium complexes using techniques such as electronic spectroscopy,* 'H and 31P nuclear magnetic resonance spectroscopy? infrared spectroscopy? and X-ray diffractionlo have shed light on the gross structure and the nature of metal-ligand bonding in these systems. There have been reports concerning "dynamic" N M R (DNMR) studies of ligand exchange and restricted internal rotation for complexes of various metals including Rh( I). l , I 2 However, for metal-phosphine complexes, there is generally a dearth of information regarding (a) University of Vermont; Alfred P. Sloan Research Fellow; Camille and Henry Dreyfus Teacher-Scholar. (b) Worcester Polytechnic Institute. (c) E. I. du Pont de Nemours and Co. (d) University of Michigan. Bushweller, C. H.; Brunelle, J. A. J. Am. Chem. SOC. 1973, 95, 5949. Brunelle, J. A,; Bushweller, C. H.; English, A. D. J. Phys. Chem. 1976, 80, 2598. Bushweller, C. H.; Brunelle, J. A. Tetrahedron Lerr. 1974, 893. Bushweller, C. H.; Lourandos, M. Z. Inorg. Chem. 1974, 13, 2514. (a) Livingstone, S. E. Compr. Inorg. Chem. 1973,3, 1237. (b) Osborn, J. A.; Jardine, F. H.; Young, F. H.; Wilkinson, G. J . Chem. SOC. A 1966, 1711. Tolman, C. A. Chem. Reo. 1977, 77, 313. Brady, R.; Flynn, B. R.; Geoffroy, G. L.; Gray, H. B.; Peone, J.; Vaska, L. Inorg. Chem. 1976, 15, 1485. (a) Brown, T. H.; Green, P. J. J . Am. Chem. Soc. 1970, 92,2359. (b) Brown, T. H.; Green, P. J. Ibid. 1%9,91 3378. (c) Mann, B. E.; Shaw, B. L.; Slade, R. M. J. Chem. SOC. A 1971, 2976. (d) Tolman, C. A.; Meakin, P. A.; Lindner, D. L.; Jesson, J. P. Meak...

The variable temperature 1H n.m.r. spectra of neopentyl bromide and iodide from -150 to -170 "C show changes in the respective t-butyl resonances consistent with slowing t-butyl rotation on the n.m.r. time scale.

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