Vibrational Properties of Hydrogen Astatide, HAt

Astatine (Z = 85) is the heaviest member of the elements of group 17 of the Periodic System (halogens). About 20 isotopes are known, and some of them have been identified as short-lived products in the natural radioactive series. The longest-lived isotope is 210At with a half-life of only 8.3 hours. Consequently, macroscopic quantities of this element cannot be accumulated. Our knowledge on its chemistry is mainly based on tracer studies, which show that it behaves in the manner one would expect, by extrapolation from the other halogens [1, 2]. Hydrogen astatide, HAt, is therefore the heaviest hydrogen halide. Although its formation has been detected mass spectroscopically [1], no other experimental studies on its physical and chemical behavior could be undertaken. Notwithstanding, in recent years a number of theoretical studies, has been performed on this molecule [3 – 5]. Therefore, it seems interesting to explore with more detail its vibrational properties and to establish correlations with the other hydrogen halides. The bond distance, dissociation energy and harmonic frequency for HAt have been calculated on the basis of different theoretical frameworks and at different degrees of sophistication, using relativistic and non-relativistic approaches [3 – 5]. For most of our purposes the knowledge of the harmonic frequency is of central importance. A thorough analysis of the available data shows that the most reasonable values lie between 2155 and 2185 cm−1 [3 – 5]. Force constants calculated with these two extreme figures only differ by about 3%. Thus, in all the calculations we have used