Properties of the halogen-bonded complex H2S center dot center dot center dot Br-2 established by rotational spectroscopy and ab initio calculations

The weakly bound complex H2S . . . Br-2 was detected and characterised expe rimentally through observation of its ground-state rotational spectrum. The isotopomers H2S . . . (BrBr)-Br-79-Br-79, H2S . . . (BrBr)-Br-81-Br-79, H2 S . . . (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, Four ier-transform microwave spectrometer fitted with a fast-mixing nozzle to pr eclude any chemical reaction between H2S and Br-2. The rotational and centr ifugal distortion constants 1/2(B+C) and Delta (J), and the nuclear hyperfi ne coupling constants chi (aa)(Br-x) and 1/2{M-bb(Br-x) + M-cc(Br-x)}, wher e 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 th e 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 ax is 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 con firmed this perpendicular geometry and demonstrated that the potential ener gy 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 equi valent C-s minima at phi (e) = +/-91(3)degrees. This PE function is charact erised by nearly degenerate pairs of vibrational energy levels (the separat ion of v = 0 and 1 corresponding to an inversion frequency of only approxim ate to0.9 MHz), in agreement with the experimental conclusion that H2S . . . Br-2 has a permanently pyramidal configuration at S. The Br nuclear quadr upole coupling constants, chi aa(Brx)(x = i or o) interpreted in the approx imation of the Townes-Dailey model led to estimates of the electronic redis tribution 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. Co mparisons 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) re vealed a parallelism which reinforces the notion of a halogen bond in the f ormer series that is the analogue of the more familiar hydrogen bond in the latter.