Properties of the halogen-bonded complex H2S•••Br2 established by rotational spectroscopy and ab initio calculations

The weakly bound complex H2S . . . Br-2 was detected and characterised experimentally through observation of its ground-state rotational spectrum. The isotopomers H2S . . . (BrBr)-Br-79-Br-79, H2S . . . (BrBr)-Br-81-Br-79, H2S . . . (BrBr)-Br-81-Br-81, H2S . . . (BrBr)-Br-79-Br-81, D2S . . . (BrBr)-Br-79-Br-79, D2S . . . (BrBr)-Br-81-Br-79, HDS . . . (BrBr)-Br-79-Br-79 and HDS . . . (BrBr)-Br-81-Br-79 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 Br-2. The rotational and centrifugal distortion constants 1/2(B+C) and Delta (J), and the nuclear hyperfine coupling constants chi (aa)(Br-x) and 1/2{M-bb(Br-x) + M-cc(Br-x)}, 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 C-s symmetry, with the Br-2 subunit lying approximately perpendicular to the plane of the H2S nuclei and forming a bromine bond to S. The distance r(S . . . Br-i) = 3.1785(1) Angstrom and the angle phi = 98.54(8)degrees between the C-2 axis of H2S and the Br-2 internuclear axis were obtained under the assumption of unperturbed monomer geometries and collinear S . . . Br-i-Br-o 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 (phi) was a double-minimum function of phi with a barrier of height V(0) = 830(60) cm(-1) at the planar C-2v conformation separating the two equivalent C-s minima at phi (e) = +/-91(3)degrees. This PE function is characterised by nearly degenerate pairs of vibrational energy levels (the separation of v = 0 and 1 corresponding to an inversion frequency of only approximate to0.9 MHz), in agreement with the experimental conclusion that H2S . . . Br-2 has a permanently pyramidal configuration at S. The Br nuclear quadrupole coupling constants, chi aa(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 delta (i) = 0.040(4) and delta (p) = 0.067(2) of an electron were shown to be transferred from S to Br-i and from Br-i to Br-o, respectively, implying a net loss of 0.027e at Br-i. Comparisons of the properties in the two series of complexes H2S . . . XY (XY = F-2, Cl-2, Br-2, 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.