Discussion
In the following table, the left column represents the rigid-rotor type and the right column shows a set of molecules.
| P. Symmetric rotor (oblate) | 1. SiH4 |
| Q. Symmetric rotor (prolate) | 2. CH3Cl |
| R. Spherical rotor | 3. C6H6 |
| S. Asymmetric rotor | 4. CH3OH |
| 5. CO2 |
The CORRECT match is
Answer: B
The correct match is P-3, Q-2, R-1, S-4.
- Benzene, C6H6, is a flat symmetric top: the two in-plane moments are equal and the moment about the six-fold axis is the largest, so it is an oblate symmetric rotor.
- CH3Cl has a three-fold axis along C-Cl; the moment about that axis is the smallest and the other two are equal and larger, a prolate symmetric rotor.
- SiH4 is tetrahedral, with all three moments equal: a spherical rotor.
- CH3OH has no axis of three-fold or higher symmetry, so all three moments differ: an asymmetric rotor.
CO2 is linear and is the extra item. Options that call SiH4 oblate, benzene spherical, or CO2 or CH3OH prolate misclassify at least one molecule.
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