Chemistry 143 Problem Set #3 Problem 1 The 20 MHz 13C NMR spectrum of cis-decalin is shown at right. The five peaks found in the spectrum at -20 °C have equal areas. (a) Show how the chair inversion of cis-decalin produces a coalescence of signals. Explain why the peak at 37 ppm remains sharp throughout the range of temperatures. (b) Determine which peaks are coalescing with each other and using Δν , calculate the barrier to inversion. Due April 24, 2015 Problem 2 The 500 MHz 1H NMR spectrum of 1,4 dioxane is shows a singlet at 3.53 ppm. When the spectrum is amplified (shown below), two multiplets become observable as shown below. a) What is the origin of these multiplets and multiplicities? Draw a structure of the species. b) What information can you obtain from this spectrum? Problem 3. Presented below are temperature dependent 25 MHz proton noise-decoupled 13C NMR spectra of 9-chloromethyltriptycene. The four downfield peaks, A, B, C and D belong to carbons 15-20. (a) Explain the molecular process responsible for these variable temperature spectra. This will necessitate an explanation of the relative intensities of peaks A, B, C and D. (b) Which peak is undergoing coalescence with peak A ? (c) Give a rough estimate (within 5 kcal/mole) for the activation energy of the process displayed in these spectra. Problem 4. Breen and Stephan have recently described the synthesis and characterization of the zirconocene phosphide complex 3 (Scheme 1) (Organometallics, 1996, 15, 4223.) An Xray structure shows a pyramidal phosphorus atom bearing the bulky 2,4,6-tri-tert-butylphenyl group (Figure 1). The 1H NMR spectra shown in Figure 2 were obtained at four temperatures (cyclopentadienyl resonances and t-butyl groups only). (a) Explain why three t-butyl resonances are found near 1.5 ppm at 233 K. Suggest a molecular process which would cause coalescence of the t-butyl groups. (b) Explain why two Cp resonances are found at 233K. Suggest a molecular process which would cause coalescence of the CP groups. (c) If the same molecular process were responsible for both (a) and (b) above, would the pair of signals for the Cp protons coalesce at the same temperature as the pair of signals for the t-butyl groups? (d) The authors suggest that two separate dynamic processes are required to produce the ‡ variable temperature results found for compound 3. Calculate a ΔG value using first the Cp resonances and then using the tBu resonances. Do you agree or disagree with the authors’ claim? Ar P Cl Cp2Zr 1 Me Cp2Zr Ar H Me 2 Figure 1. X-Ray Crystal Structure of 3. P Cp2Zr H NHPh 3 Figure 2. Variable Temperature 1H NMR (300 MHz) spectrum for 3. Problem 5. Widenhofer, et al. have studied the dynamic NMR behavior of the cationic gold complex shown below. (a) Using the known three dimesional structure of allenes (non-planar), draw the expected structure of the gold-allene complex (choose from the two coordination modes below) and explain why the methyl groups on the allene are inequivalent. (b) Describe the process responsible for the temperature-dependence of the spectra and using the 1H (500 MHz) and 13C NMR (125 MHz) of the methyl resonances shown, calculate the activation energy for the dynamic process involved. CMe2 C P Au+ CH2 CH2 C P Au+ CMe2