— Analytical Chemistry — Spectroscopy Chemistry Question
NMR Spectra
At room temperature, the NMR spectrum of cyclohexane shows only a single resonance signal. As the temperature of the sample is lowered, the sharp single signal broadens until at -61.0 °C it begins to split into two broaden signals. As the temperature is lowered further to -90 °C, each of the two signals begins to give a splitting pattern of its own. Explain the origin of these two families of signals.
[VISUAL]
Model Answer
A molecule can undergo fluxional process by interchanging two or more sites. If the rate of exchange is faster than the NMR time scale, the two different groups will appear at an average shift. As temperature decreases the rate becomes lower and separate shift can be obtained.
Rapid equilibration at room temperature between chair conformations leads to one peak. As one lowers the temperature, the interconversion is slowed down until, at temperatures below -66.7 °C, peaks due to the axial and equatorial hydrogens are observed. Axial and equatorial hydrogens have different chemical shifts under these conditions.
[VISUAL]
k at coalescence (at -61oC): kc = π∆ν/√2
In cis-1-bromo-4-tert-butylcyclohexane, the proton on carbon-1 is found to give resonance at 4.33 ppm. In the trans isomer, the resonance of the C1 hydrogen is at 3.63 ppm. Why do these compounds have different chemical shift values for the C1 hydrogen? Justify why this difference cannot be observed in the 4-bromomethylcyclohexanes except at very low temperatures.
Model Answer
The t-butyl-substituted rings are conformationally locked. The hydrogen at C1 has different chemical shifts, depending upon whether it is axial or equatorial. 4-Bromocyclohexanes are conformationally mobile. No difference between axial and equatorial hydrogens is observed until the rate of chair–chair interconversion is decreased by lowering the temperature.