title: Bond angle variations in XH3 [X = N, P, As, Sb, Bi]: the critical role of Rydberg orbitals exposed using a diabatic state model.
abstract: Ammonia adopts sp(3) hybridization (HNH bond angle 108°) whereas the other members of the XH3 series PH3, AsH3, SbH3, and BiH3 instead prefer octahedral bond angles of 90-93°. We use a recently developed general diabatic description for closed-shell chemical reactions, expanded to include Rydberg states, to understand the geometry, spectroscopy and inversion reaction profile of these molecules, fitting its parameters to results from Equation of Motion Coupled-Cluster Singles and Doubles (EOM-CCSD) calculations using large basis sets. Bands observed in the one-photon absorption spectrum of NH3 at 18.3 eV, 30 eV, and 33 eV are reassigned from Rydberg (formally forbidden) double excitations to valence single-excitation resonances. Critical to the analysis is the inclusion of all three electronic states in which two electrons are placed in the lone-pair orbital n and/or the symmetric valence σ* antibonding orbital. An illustrative effective two-state diabatic model is also developed containing just three parameters: the resonance energy driving the high-symmetry planar structure, the reorganization energy opposing it, and HXH bond angle in the absence of resonance. The diabatic orbitals are identified as sp hybrids on X; for the radical cations XH3(+) for which only 2 electronic states and one conical intersection are involved, the principle of orbital following dictates that the bond angle in the absence of resonance is acos(-1/5) = 101.5°. The multiple states and associated multiple conical intersection seams controlling the ground-state structure of XH3 renormalize this to acos[3 sin(2)(2(1/2)atan(1/2))/2 - 1/2] = 86.7°. Depending on the ratio of the resonance energy to the reorganization energy, equilibrium angles can vary from these limiting values up to 120°, and the anomalously large bond angle in NH3 arises because the resonance energy is unexpectedly large. This occurs as the ordering of the lowest Rydberg orbital and the σ* orbital swap, allowing Rydbergization to compresses σ* to significantly increase the resonance energy. Failure of both the traditional and revised versions of the valence-shell electron-pair repulsion (VSEPR) theory to explain the ground-state structures in simple terms is attributed to exclusion of this key physical interaction.
authors.affiliation: International Centre for Quantum and Molecular Structure, College of Sciences, Shanghai University, Shanghai 200444, China.
authors.forename: Jeffrey R
authors.initials: JR
authors.lastname: Reimers
authors.forename: Laura K
authors.initials: LK
authors.lastname: McKemmish
authors.forename: Ross H
authors.initials: RH
authors.lastname: McKenzie
authors.forename: Noel S
authors.initials: NS
authors.lastname: Hush
mesh_terms: D000641:Ammonia / Q000737:chemistry*; D000965:Antimony / Q000737:chemistry*; D001152:Arsenicals / Q000737:chemistry*; D001729:Bismuth / Q000737:chemistry; D008956:Models, Chemical; D010758:Phosphorus / Q000737:chemistry; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D001152:Arsenicals; C021254:stibine; D010758:Phosphorus; D000641:Ammonia; D000965:Antimony; D001729:Bismuth; C006633:arsine