An investigation of the hydrogenhyphen;bonded dimer H3Nsdot;sdot;sdot;HBr by pulsedhyphen;nozzle, Fourierhyphen;transform microwave spectroscopy of ammonium bromide vapor

摘要

The groundhyphen;state rotational spectrum of a dimer of ammonia and hydrogen bromide has been detected by using the technique of Fourierhyphen;transform microwave spectroscopy in a Fabryndash;Perot cavity to examine a supersonically expanded gas pulse composed of ammonium bromide vapor entrained in argon. The spectroscopic constantsB0,DJ,DJK, khgr;(14N), khgr;(79Br), and khgr;(81Br) have been determined (where appropriate) for the four symmetrichyphen;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, khgr;(14N)=minus;3.183(8) MHz, and khgr;(79Br)=361.245(6) MHz. The spectroscopic constants have been interpreted in terms of a hydrogenhyphen;bonded dimer ofC3vsymmetry, havingr(Nsdot;sdot;sdot;Br)=3.255 Aring; and the hydrogenhyphen;bond stretching force constantksgr;=13.4 Nthinsp;mminus;1. A detailed analysis has demonstrated that khgr;(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 sim;0.lefrom H into the 4pzorbital of Br.