Transient Charge Transfer in Gaseous Rubidium—Benzene Collisions

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Abstract
The interaction that arises during gas‐phase rubidium‐atom—benzene‐molecule collisions has been investigated through Rb87 hyperfine frequency measurements which utilize optical double resonance. An unusually large hyperfine‐splitting shift of —137 000 cps/cm Hg of benzene vapor is observed. In contrast to previous studies with other buffer gases, the shift is too large to be explained by a mechanism involving either Pauli exclusion or van der Waals forces. When the shift is interpreted in terms of an adiabatic collision model following the theory of Foley for the pressure broadening of spectral lines, it is seen that the unpaired‐electron density at the Rb nucleus is reduced about 8% during an average collision. This behavior is attributed to a partial and transient electron transfer to the benzene ring. A comparison with ESR studies of liquid media where the alkali valence electron resides dominantly on the aromatic ring illustrates the importance of the solvent in facilitating electron transfer.