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If the average separation between the adjacent lines in the rotational IR spectrum of CO molecule is 3.844 cm\(^{-1}\), then the value of rotational constant is :

  • A.7.688 cm\(^{-1}\)
  • B.1.922 cm\(^{-1}\)
  • C.3.844 cm\(^{-1}\)
  • D.5.766 cm\(^{-1}\)

Answer: B

The rotational constant is \(B = 1.922\) cm\(^{-1}\).

For a rigid rotor the rotational levels are \(F(J) = BJ(J + 1)\), and the allowed transitions \(J \rightarrow J + 1\) give lines at

\[ \bar{\nu} = 2B(J + 1) \]

so adjacent lines are separated by 2B. Hence

\[ B = \frac{3.844}{2} = 1.922\ \text{cm}^{-1} \]

Taking the spacing itself as B gives 3.844, twice the spacing gives 7.688, and 5.766 is 3B; none is the rotational constant.

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