Question

In the ¹H-NMR spectrum of the following molecule, the signal of proton H_a appears as:

• A. singlet
• B. triplet
• C. quintet
• D. quartet

Hint:

Deuterium has a nuclear spin \( I = 1 \), so a single proton coupled to two deuterium atoms shows a quintet in the \(^1\)H-NMR spectrum. Use the formula \( 2nI + 1 \) to determine multiplicity.

Answer 1

The Correct Option is C

The key to solving this problem is understanding the splitting pattern in ¹H NMR due to coupling with neighboring nuclei. Proton \( H_a \) is adjacent to two deuterium atoms (D).

Important points:

• Deuterium (D) has a nuclear spin \( I = 1 \).
• The NMR signal of a proton coupled to \( n \) equivalent nuclei with spin \( I \) will be split into \( (2nI + 1) \) lines.
• Here, \( n = 2 \) deuterons and \( I = 1 \), so:

\[ \text{Multiplicity} = 2 \times 2 \times 1 + 1 = 5 \Rightarrow \text{quintet} \]

Hence, \( H_a \) appears as a quintet.

\[ \boxed{\text{The signal of } H_a \text{ appears as a quintet due to coupling with two deuterium atoms (} I = 1 \text{)}} \]