Question

If the spin of the electrons in the excited state are parallel, it is called as:

• A. Doublet state
• B. Triplet state
• C. Singlet state
• D. Parallel state

Hint:

If the excited-state electron spins are parallel, the system is in a triplet state. If they are paired (opposite), the system is in a singlet state.

Answer 1

The Correct Option is B

Spin states in photochemistry are named using the multiplicity value \( 2S + 1 \), where \( S \) is the total spin quantum number of the electrons involved. Applying this to each option shows which one fits a parallel-spin pair.

1. Doublet state: A multiplicity of 2 comes from a single unpaired electron with \( S = \frac{1}{2} \), which is a different situation from a pair of excited electrons.
2. Triplet state: Two parallel spins give \( S = 1 \), so \( 2S + 1 = 3 \), a triplet. Parallel spins also mean the electrons occupy separate orbitals, which lowers electron-electron repulsion and makes the triplet state lower in energy than the corresponding singlet.
3. Singlet state: Paired, opposite spins give \( S = 0 \), so \( 2S + 1 = 1 \), which is the reverse spin arrangement of what the question describes.
4. Parallel state: There is no multiplicity value or spectroscopic term attached to this phrase, so it is not a real classification.

Working through the multiplicity formula confirms that parallel electron spins in an excited state correspond to \( S = 1 \), the triplet state. The correct answer is Triplet state.