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Free H_2 Rotation vs Jahn−Teller Constraints in the Nonclassical Trigonal (TPB)Co−H_2 Complex

Gunderson, William A. and Suess, Daniel L. M. and Fong, Henry and Wang, Xiaoping and Hoffmann, Christina M. and Cutsail, George E., III and Peters, Jonas C. and Hoffman, Brian M. (2014) Free H_2 Rotation vs Jahn−Teller Constraints in the Nonclassical Trigonal (TPB)Co−H_2 Complex. Journal of the American Chemical Society, 136 (42). pp. 14998-15009. ISSN 0002-7863.

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Proton exchange within the M–H_2 moiety of (TPB)Co(H_2) (Co–H_2; TPB = B(o-C_6H_4PiPr_2)_3) by 2-fold rotation about the M–H_2 axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiP^(iPr)_3)Fe(H_2) (Fe–H_2) (SiP^(iPr)_3 = [Si(o-C_6H_4PiPr_2)_3]). The g-values for Co–H_2 and Fe–H_2 show that both have the Jahn–Teller (JT)-active ^2E ground state (idealized C_3 symmetry) with doubly degenerate frontier orbitals, (e)^3 = [|m_L ± 2>]^3 = [x^2 – y^2, xy]^3, but with stronger linear vibronic coupling for Co–H_2. The observation of ^1H ENDOR signals from the Co–HD complex, ^2H signals from the Co–D_2/HD complexes, but no ^1H signals from the Co–H_2 complex establishes that H_2 undergoes proton exchange at 2 K through rotation around the Co–H_2 axis, which introduces a quantum-statistical (Pauli-principle) requirement that the overall nuclear wave function be antisymmetric to exchange of identical protons (I = 1/2; Fermions), symmetric for identical deuterons (I = 1; Bosons). Analysis of the 1-D rotor problem indicates that Co–H_2 exhibits rotor-like behavior in solution because the underlying C_3 molecular symmetry combined with H_2 exchange creates a dominant 6-fold barrier to H_2 rotation. Fe–H_2 instead shows H_2 localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe(3d)→ H_2(σ^*) π-backbonding that becomes dependent on the H_2 orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L_2–M–E axis (E = Si for Fe–H_2; E = B for Co–H_2). Notably, the isotropic ^1H/^2H hyperfine coupling to the diatomic of Co–H_2 is nearly 4-fold smaller than for Fe–H_2.

Item Type:Article
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URLURL TypeDescription Information
Wang, Xiaoping0000-0001-7143-8112
Peters, Jonas C.0000-0002-6610-4414
Hoffman, Brian M.0000-0002-3100-0746
Additional Information:© 2014 American Chemical Society. Received: August 7, 2014. Published: September 22, 2014. Publication Date (Web): September 22, 2014. This work is dedicated to Prof. Harden M. McConnell, whose recognition of the role of quantum-statistical considerations in paramagnetic resonance spectroscopies occupies but a small place among his numerous scientific contributions, on the occasion of his 87th birthday. We thank Mrs. Junhong (Helen) He, Oak Ridge National Laboratory, for assistance with mounting crystal for neutron diffraction. This work was supported by the National Science Foundation (MCB 1118613, BMH; the NSF Center CHE-1305124, JCP; DGE-0824162, GEC). Research conducted at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy, under Contract No. DE-AC05-00OR22725 with UT Battelle, LLC.
Group:CCI Solar Fuels
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Department of Energy (DOE)DE-AC05-00OR22725
Issue or Number:42
Record Number:CaltechAUTHORS:20141202-135435499
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Official Citation:Free H2 Rotation vs Jahn–Teller Constraints in the Nonclassical Trigonal (TPB)Co–H2 Complex William A. Gunderson, Daniel L. M. Suess, Henry Fong, Xiaoping Wang, Christina M. Hoffmann, George E. Cutsail III, Jonas C. Peters, and Brian M. Hoffman Journal of the American Chemical Society 2014 136 (42), 14998-15009
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:52282
Deposited By: Ruth Sustaita
Deposited On:02 Dec 2014 22:59
Last Modified:09 Mar 2020 13:19

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