The
weakly bound complex H2S···Br2 was detected and characterised experimentally through observation of
its ground-state rotational spectrum. The isotopomers H2S···79Br79Br, H2S···81Br79Br, H2S···81Br81Br,
H2S···79Br81Br, D2S···79Br79Br, D2S···81Br79Br, HDS···79Br79Br and HDS···81Br79Br were investigated by using
a pulsed-jet, Fourier-transform microwave spectrometer fitted with a fast-mixing nozzle to preclude any
chemical reaction between H2S and Br2. The rotational and centrifugal distortion constants ½(B
+
C) and ΔJ, and the nuclear hyperfine coupling constants χaa(Brx) and ½{Mbb(Brx) +
Mcc(Brx)}, where x = i (inner) or o (outer), were
determined in each case. Interpretations of these spectroscopic constants yielded a number of conclusions
about the nature of the complex. The geometry was established to be of Cs symmetry, with the Br2 subunit
lying approximately perpendicular to the plane of the H2S nuclei and forming a bromine bond to S. The distance r(S···Bri) = 3.1785(1) Å and the angle ϕ
= 98.54(8)° between the C2 axis of H2S and the Br2 internuclear
axis were obtained under the assumption of unperturbed monomer geometries and collinear S···Bri–Bro
nuclei. Ab initio calculations conducted at the aug-cc-pVDZ/MP2 level of theory confirmed this perpendicular
geometry and demonstrated that the potential energy V(ϕ) was a double-minimum function of ϕ with a barrier
of height V(0) = 830(60) cm−1 at the planar C2v
conformation separating the two equivalent Cs
minima
at ϕe
=
± 91(3)°. This PE function is characterised by nearly degenerate pairs of vibrational energy levels (the
separation of
= 0 and 1 corresponding to an inversion frequency of only ≈0.9 MHz), in agreement with
the experimental conclusion that H2S···Br2 has a permanently pyramidal configuration at S. The Br nuclear
quadrupole coupling constants, χaa(Brx) (x = i or o) interpreted in the approximation of the Townes–Dailey
model led to estimates of the electronic redistribution on complex formation. Fractions δi
= 0.040(4)
and δp
= 0.067(2) of an electron were shown to be transferred from S to Bri and from Bri to Bro, respectively, implying
a net loss of 0.027e at Bri. Comparisons of the properties in the two series of complexes
H2S···XY (XY = F2, Cl2, Br2, ClF, BrCl or ICl) and H2S···HX (X = F, Cl or Br) revealed a parallelism which reinforces
the notion of a halogen bond in
the former series
that is the analogue
of the more familiar hydrogen bond in the latter.