An investigation of the hydrogen-bonded dimer H3N⋅⋅⋅HBr by pulsed-nozzle, Fourier-transform microwave spectroscopy of ammonium bromide vapor

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Abstract
The ground‐state rotational spectrum of a dimer of ammonia and hydrogen bromide has been detected by using the technique of Fourier‐transform microwave spectroscopy in a Fabry–Perot cavity to examine a supersonically expanded gas pulse composed of ammonium bromide vapor entrained in argon. The spectroscopic constants B0, DJ, DJK, χ(14N), χ(79Br), and χ(81Br) have been determined (where appropriate) for the four symmetric‐top type isotopic species (14NH3, H79Br), (14NH3, H81Br), (15NH3, H79Br), and (15NH3, H81Br) and for the first of these the values are as follows: B0=3226.862(1) MHz, DJ=9.0(2) kHz, DJK=142.2(6) kHz, χ(14N)=−3.183(8) MHz, and χ(79Br)=361.245(6) MHz. The spectroscopic constants have been interpreted in terms of a hydrogen‐bonded dimer of C3v symmetry, having r(N⋅⋅⋅Br)=3.255 Å and the hydrogen‐bond stretching force constant kσ=13.4 N m−1. A detailed analysis has demonstrated that χ(79Br) is consistent with a model of the dimer in which only a small electrical rearrangement in the HBr subunit occurs on dimer formation (as opposed to proton transfer to NH3) and that this can be viewed as the transfer of ∼0.le from H into the 4pz orbital of Br.