Vibrational Properties of Bismutine, BiH3

Bismutine, BiH3, is the least stable of the group15 trihydrides. F. Paneth first detected it in minute traces in 1918 using a radiochemical technique involving Bi2Mg3 [1]. The best preparation method seems to be the one proposed by Amberger in 1961, based on the disproportionation of CH3BiH2 at −45 ◦C[̃2]. This relatively complex synthesis has recently been successfully reproduced and allowed to obtain the vibrational and rotational spectra of BiH3 [3]. Using the spectroscopic and structural data reported in this study, we have now performed the calculation of the most important vibrational properties and thermodynamic functions of this molecule. For comparative purposes, similar calculations have also been made for SbH3, the immediately adjacent lower-mass trihydride in group-15 of the Periodic System. It is expected that on going from NH3 to BiH3 an increase of the p-orbital character in the three bonding orbitals occurs, generating a stepwise diminution of the respective bond angle [4]. This expectation is clearly fulfilled, as BiH3 presents the lowest bond angle (90.48◦) [3], even lower as that of SbH3 (91.55◦) [5]. This means that in the vibrational ground state BiH3 is an oblate symmetric top molecule (Ia = Ib < Ic) close to a spherical top (Ia = Ib = Ic), even more than SbH3. A parameter that can be used to quantify the spherical character of an oblate top molecule is the relation between the rotational constants C and B, i.e., γ = (Cc – Bc)/1/2(Cc + Bc) [5]. Bismutine with γ = −0.015 represents the most quasi-spherical oblate top so far investigated, whereas for stibine γ = −0.052.