# NMR analysis regarding cis-1-isopropyl-3-phenylcyclohexane

I have come across a question in my textbook:

I understand the explanation for the first compound. However, for the second structure, I am not understanding why the 2 methyl groups are diastereotopic. Is there not free rotation about the $$\ce{C-C}$$ bond, connecting the $$\ce{CH(CH3)2}$$ group to the ring such that the 2 methyl groups become equivalent? The answer key makes it seem as though there is no rotation and the methyl groups are fixed in place. I feel as if I am misunderstanding the question. Please help me clear up the confusion.

• Substitution is frequently the easiest approach. Replace a hydrogen on each of those two methyl groups with a deuterium to create two new compounds? What is the relationship between those two compounds? Identical? Hydrogen atoms are homotopic. Enantiomers? Enantiopic. Diastereomers? Diastereotopic. – Zhe Nov 23 '18 at 19:04