Bond angle variations in XH<inf>3</inf> [X = N, P, As, Sb, Bi]: The critical role of Rydberg orbitals exposed using a diabatic state model

Reimers, Jeffrey R., McKemmish, Laura K., McKenzie, Ross H. and Hush, Noel S. (2015) Bond angle variations in XH<inf>3</inf> [X = N, P, As, Sb, Bi]: The critical role of Rydberg orbitals exposed using a diabatic state model. Physical Chemistry Chemical Physics, 17 38: 24618-24640. doi:10.1039/c5cp02237a


Author Reimers, Jeffrey R.
McKemmish, Laura K.
McKenzie, Ross H.
Hush, Noel S.
Title Bond angle variations in XH3 [X = N, P, As, Sb, Bi]: The critical role of Rydberg orbitals exposed using a diabatic state model
Journal name Physical Chemistry Chemical Physics   Check publisher's open access policy
ISSN 1463-9076
1463-9084
Publication date 2015-07-08
Year available 2015
Sub-type Article (original research)
DOI 10.1039/c5cp02237a
Open Access Status Not Open Access
Volume 17
Issue 38
Start page 24618
End page 24640
Total pages 23
Place of publication Cambridge, United Kingdom
Publisher Royal Society of Chemistry
Language eng
Abstract Ammonia adopts sp(3) hybridization (HNH bond angle 108 degrees) whereas the other members of the XH3 series PH3, AsH3, SbH3, and BiH3 instead prefer octahedral bond angles of 90-93 degrees. 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 sigma* 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 degrees. The multiple states and associated multiple conical intersection seams controlling the ground-state structure of XH3 renormalize this to acos[3sin(2)(2(1/2)atan(1/2))/2 - 1/2] = 86.7 degrees. Depending on the ratio of the resonance energy to the reorganization energy, equilibrium angles can vary from these limiting values up to 120 degrees, 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 sigma* orbital swap, allowing Rydbergization to compresses s* 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.
Formatted abstract
Ammonia adopts sp3 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 sin2(21/2atan(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.
Keyword Chemistry, Physical
Physics, Atomic, Molecular & Chemical
Chemistry
Physics
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Mathematics and Physics
Official 2016 Collection
 
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