Experimental Detection and Properties of H2O⋅⋅⋅AgCl and H2S⋅⋅⋅AgCl by Rotational Spectroscopy

Abstract

We report the first generation and characterization of two simple compounds formed by the interaction of either H2O or H2S with AgCl, namely H2O···Ag Cl and H2S···Ag Cl. They were observed in the gas phase by rotational spectroscopy. The AgCl is produced by laser ablation of metallic silver in the presence of CCl4 and then picks up an H2O or H2S molecule. AgCl is known, from interpretation of its Cl nuclear quadrupole coupling constant, to have a fractional ionic character of approximately 0.7, so that it has significant ionpair character. The interaction of AgCl with H2Omolecules, and in particular one H2O molecule, is of fundamental chemical interest because the H2O molecules have the opportunity to interact with an incipient Ag ion. How does this interaction differ from those of H2O with the less polar, covalent Lewis acids HCl and ClF (fractional ionic characters of 0.25 and 0.35)? The hydrogenand halogen-bonded complexes H2O···H Cl and H2O···Cl F have each been investigated in the gas phase by rotational spectroscopy as part of an extensive systematic program. H2O···H Cl and H2O···Cl F have equilibrium geometries of Cs symmetry, [2,3] with a pyramidal configuration at O when HCl or ClF forms either a hydrogen or halogen bond to that atom (see Figure 1). In each case, however, there is a low potential-energy barrier to the planar, C2v geometry so that even in the zero-point state the molecule is inverting and is effectively planar. On the other hand, in the analogous pair of complexes H2S···H Cl and H2S···Cl F the potential energy barrier is high enough to ensure that each has a permanently pyramidal configuration at S, with HCl or ClF forming a weak bond to S at approximately 908 to the H2S subunit, as shown in Figure 1. These angular geometries, and those of many other hydrogenand halogen-bonded complexes, can be rationalized by means of some simple empirical rules. For both H2O and H2S acting as Lewis bases, the electrophilic ends H and Cl of the weakly polar molecules HCl and ClF, respectively, are assumed to seek the axis of a nonbonding electron pair carried by the base. A further question is: Are the angular geometries of the resulting molecules H2Y···Ag Cl isomorphic with those of H2Y···H Cl and H2Y···Cl F (Y=O or S), indicating that the empirical rules are also obeyed when AgCl is the Lewis acid? All B···M X (B=CO, M=Cu, Ag, Au) have a linear arrangement similar to those in OC···H Cl and OC···Cl F. Thus, in all these systems, the Lewis acid attaches along the axis of the n-pair on C. A linear geometry was also found in N2···Cu F. Observed rotational transitions of both H2O···Ag Cl and H2S···Ag Cl were characteristic of a nearly prolate asymmetric rotor of large A value having only a-type transitions, which exhibit Cl nuclear quadrupole hyperfine structure. For H2O···Ag Cl, R-branch K 1= 1 transitions of the type (J+ 1)1,J+1 !J1,J and (J+ 1)1,J !J1,J 1 were observed in addition to the (J+ 1)0,J+1 !J0,J series and for a given J were more intense than those having K 1= 0. This observation confirms that the molecule has a pair of equivalent H nuclei exchanged by a rotationC2 about the a axis and therefore that the equilibrium geometry of H2O···Ag Cl is either C2V planar at equilibrium or Cs pyramidal but with a potential-energy barrier to planarity low enough that the v= 0 and 1 states associated with the motion that inverts the configuration at the O atom are well separated. For H2S···Ag Cl only the (J+ 1)0,J+1 !J0,J series could be detected despite a careful search, an observation consistent with a pyramidal configuration at the S atom and no inversion on the microwave timescale. The reason for the different behavior with respect to inversion is presumably that H2S···Ag Cl is more strongly bound than H2O···Ag Cl (see below) and that there is a much larger angle between the n-electron pairs on S than on O. The result is a higher and wider barrier to inversion in H2S···Ag Cl. The usual arguments show that for H2O···Ag Cl in the ground state the K 1= 1 levels occur in combination with the three symmetric proton spin functions while theK 1= 0 levels combine with the single antisymmetric function. Moreover, population transfer from K 1= 1 levels into K 1= 0 levels during the supersonic expansion is hindered by a collisional propensity rule which forbids triplet state (K 1= 1) to Figure 1. Angular geometries of several H2Y M X compounds; d indicates the local C2 axis of the H2O or H2S subunit.