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

Abstract

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 Δ_J, and the nuclear hyperfine coupling constants χ_aa(Br_x) and ½{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 Br₂ 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) 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···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(ϕ) was a double-minimum function of ϕ 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 ϕ_e = ± 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, χ_aa(Br_x) (x = i or o) interpreted in the approximation of the Townes–Dailey model led to estimates of the electronic redistribution on complex formation. Fractions δ_i = 0.040(4) and δ_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 = 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.