The complex H3N⋅⋅⋅Br2 characterized in the gas phase by rotational spectroscopy

The ground-state rotational spectra of four isotopomers of the complex H3N...Br2 have been observed using pulsed-nozzle, FT microwave spectroscopy. Reaction of NH3 and Br2 was avoided by using a fast-mixing nozzle. The isotopomers H315N...79Br2, H315N...81Br79Br, H315N...79Br81Br, and H315N...81Br2 exhibited symmetric-top type spectra which were analyzed to give rotational constants B0, the centrifugal distortional constants DJ and DJK, the Br-nuclear quadrupole coupling constants χ(Brx) (x=i, inner or o, outer), and the Br spin-rotational coupling constants Mbb (Brx). The distances r(N...Bri)=2.72(2) Å and r(Br–Br)=2.335(10) Å in the complex were estimated from the rotational constants while the intermolecular stretching force constant kσ=18.5(5) N m−1 was determined from DJ. An interpretation of the magnitude of the χ(Brx) values and the difference Δχ(Br)=χ(Bri)−χ(Bro) allows the conclusion that the extent of intermolecular charge transfer is probably small and that a polarization of Br2 equivalent to the transfer of ∼0.1e from Bri to Bro accompanies formation of the complex. The properties of the series of complexes H3N...XY, where XY=Cl2, BrCl, and Br2 are compared.