Select one of the below sets in which all the molecules show a microwave

rotational spectrum?

H2, CH4, SF6

HCl, CH3Cl, CH2Cl2, H2O

H2, HCl, CH4, CH3Cl

CH2Cl2, H2O, SF6

The correct set is HCl, CH3Cl, CH2Cl2, H2O. All these molecules possess a permanent electric dipole moment, essential for interacting with microwave radiation during rotational transitions.

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Microwave Spectroscopy Basics

Microwave rotational spectroscopy detects transitions between rotational energy levels in gaseous molecules. A molecule shows a spectrum only if it has a permanent dipole moment, enabling dipole-radiation coupling. Symmetric nonpolar molecules lack this, remaining microwave inactive.

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Option Analysis

• H2, CH4, SF6: H2 is homonuclear diatomic with no dipole. CH4 has tetrahedral symmetry canceling dipoles. SF6’s octahedral symmetry yields zero net dipole. None show spectra.

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• HCl, CH3Cl, CH2Cl2, H2O: HCl is polar diatomic. CH3Cl and CH2Cl2 have C-Cl bonds creating dipoles. H2O’s bent structure gives a dipole. All exhibit spectra.

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• H2, HCl, CH4, CH3Cl: H2 and CH4 lack dipoles; HCl and CH3Cl have them. Not all qualify.

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• CH2Cl2, H2O, SF6: CH2Cl2 and H2O are polar; SF6 is not. Incomplete set.

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Key Takeaway for Chemistry Exams

Focus on polarity: heteronuclear diatomics and asymmetric polyatomics succeed. This rule applies in gas-phase studies for bond lengths via rotational constants.