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Title: [Cu(aq)] 2+ is structurally plastic and the axially elongated octahedron goes missing

High resolution (k = 18 Å –1 or k = 17 Å –1) copper K-edge EXAFS and MXAN (Minuit X-ray Absorption Near Edge) analyses have been used to investigate the structure of dissolved [Cu(aq)] 2+ in 1,3-propanediol (1,3-P) or 1,5-pentanediol (1,5-P) aqueous frozen glasses. EXAFS analysis invariably found a single axially asymmetric 6-coordinate (CN6) site, with 4×O eq = 1.97 Å, O ax1 = 2.22 Å, and O ax2 = 2.34 Å, plus a second-shell of 4×O water = 3.6 Å. However, MXAN analysis revealed that [Cu(aq)] 2+ occupies both square pyramidal (CN5) and axially asymmetric CN6 structures. The square pyramid included 4×H 2O = 1.95 Å and 1×H 2O = 2.23 Å. The CN6 sites included either a capped, near perfect, square pyramid with 5×H 2O = 1.94 ± 0.04 Å and H 2O ax = 2.22 Å (in 1,3-P) or a split axial configuration with 4×H 2O = 1.94, H 2O ax1 = 2.14 Å, and H 2O ax2 = 2.28 Å (in 1,5-P). The CN6 sites also included an 8-H 2O second-shell near 3.7 Å, which was undetectable about the strictly pyramidal sites. Equatorial angles averaging 94° ± 5° indicated significant departures from tetragonal planarity. MXAN assessmentmore » of the solution structure of [Cu(aq)] 2+ in 1,5-P prior to freezing revealed the same structures as previously found in aqueous 1M HClO 4, which have become axially compressed in the frozen glasses. [Cu(aq)] 2+ in liquid and frozen solutions is dominated by a 5-coordinate square pyramid, but with split axial CN6 appearing in the frozen glasses. Among these phases, the Cu–O axial distances vary across 1 Å, and the equatorial angles depart significantly from the square plane. Although all these structures remove the d x2–y2, d z2 degeneracy, no structure can be described as a Jahn-Teller (JT) axially elongated octahedron. The JT-octahedral description for dissolved [Cu(aq)] 2+ should thus be abandoned in favor of square pyramidal [Cu(H 2O) 5] 2+. The revised ligand environments have bearing on questions of the Cu(i)/Cu(ii) self-exchange rate and on the mechanism for ligand exchange with bulk water. In conclusion, the plasticity of dissolved Cu(ii) complex ions falsifies the foundational assumption of the rack-induced bonding theory of blue copper proteins and obviates any need for a thermodynamically implausible protein constraint.« less
Authors:
ORCiD logo [1] ;  [2] ;  [3]
  1. Stanford Univ., Stanford, CA (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States)
  2. Lab. Nazionali di Frascati-INFN, Frascati (Italy)
  3. Stanford Univ., Stanford, CA (United States); Intel Corp., Hillsboro, OR (United States)
Publication Date:
Grant/Contract Number:
AC02-76SF00515; P41GM103393
Type:
Accepted Manuscript
Journal Name:
Journal of Chemical Physics
Additional Journal Information:
Journal Volume: 148; Journal Issue: 20; Journal ID: ISSN 0021-9606
Publisher:
American Institute of Physics (AIP)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
OSTI Identifier:
1458530