Vacuum ultraviolet absorption spectra of the bromomethanes

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Abstract
The vapor phase vacuum ultraviolet absorption spectra of methyl bromide, methylene bromide, bromoform, and carbon tetrabromide are reported for the region 40 000−90 000 cm−1 (250−110 nm). Many of the Rydberg and intravalent absorptions of the bromine electrons in the nonbonding p orbitals, which are perpendicular to the C−Br bonding axes, and the σ*←σ (C−Br) absorptions of the compounds have been assigned. A correlation of the molecular Rydberg series of methyl bromide to series in krypton, which has made possible the assignment of the 4d and 4f Rydberg absorptions, is presented. Expanded vibrational progressions of the first ns and np Rydberg transitions of methylbromide are presented. The analysis of these progressions indicates that transitions to the 5s and 5p Rydberg orbitals bring about a Jahn−Teller distortion of the molecule. Three ns Rydberg series of methylene bromide were assigned which converge on the ionization potentials of 10.629±0.010, 10.839±0.010, and 11.261±0.010 eV. The states formed upon ionization are assigned as the b2, b1, and a1, respectively, as the ns transitions of the a2 combination of bromine p orbitals are forbidden in C2v symmetry. Other absorptions of methylene bromide, as well as the majority of the first ns, np, and nd molecular Rydberg assignments of bromoform and carbon tetrabromide were made using the term values of the bands with respect to the ionization potentials of the molecules. It was found that the averaged ns term values of the bromomethanes vary slightly through the series, while the np term values were increased in methylene bromide and bromoform. In each spectrum the assignment of the σ*←σ (C−Br) bands was made based on an empirically derived relation.

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