- Strong Short-Range Cooperativity in Hydrogen-Bond Chains
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Chains of hydrogen bonds such as those found in water and proteins are often presumed to be more stable than the sum of the individual H bonds. However, the energetics of cooperativity are complicated by solvent effects and the dynamics of intermolecular interactions, meaning that information on cooperativity typically is derived from theory or indirect structural data. Herein, we present direct measurements of energetic cooperativity in an experimental system in which the geometry and the number of H bonds in a chain were systematically controlled. Strikingly, we found that adding a second H-bond donor to form a chain can almost double the strength of the terminal H bond, while further extensions have little effect. The experimental observations add weight to computations which have suggested that strong, but short-range cooperative effects may occur in H-bond chains.
- Dominelli-Whiteley, Nicholas,Brown, James J.,Muchowska, Kamila B.,Mati, Ioulia K.,Adam, Catherine,Hubbard, Thomas A.,Elmi, Alex,Brown, Alisdair J.,Bell, Ian A. W.,Cockroft, Scott L.
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supporting information
p. 7658 - 7662
(2017/06/13)
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- Electronic influences in phosphinesulfonato palladium(II) polymerization catalysts
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To study the influence of electronics on catalytic polymerization properties independent from sterics, phosphinesulfonato Pd(II) complexes bearing remotely located substituents on the nonchelating P-bound aryls [κ2-(P,O)-(4-R-2-anisyl)2PC6H 4SO2O]Pd(Me)(dmso) (1a-e-dmso: 1a, R = CF3; 1b, R = Cl; 1c, R = H; 1d, R = CH3; 1e, R = OCH3) were prepared. The electron-poor complex 1a-dmso (4-CF3) undergoes the fastest insertion of methyl acrylate (MA) and is the most active for ethylene polymerization. The polyethylene molecular weight increases by a factor of 2 for the more electron rich complex 1e-dmso (4-OCH3) (Mn = 17 × 103 vs 8 × 103 for 1a-dmso (4-CF 3)). MA/ethylene copolymerization experiments revealed that the MA incorporation ratio and copolymer molecular weights are largely independent of the electronic nature of the remote substituents. These trends were further confirmed by studies of two mixed P-aryl/-alkyl complexes 1f-dmso ([κ2-(2,4,6-(OMe)3C6H2)( tBu)PC6H4SO2O]Pd(Me)(dmso)) and 1g-dmso ([κ2-(C6H5)(tBu) PC6H4SO2O]Pd(Me)(dmso)). In ethylene/MA copolymerization, 1f-dmso affords a significantly higher molecular weight polymer with reasonable MA incorporation (Mn = 12 × 10 3 and 7.7 mol % MA) and activities similar to those observed for complexes 1a-e-dmso.
- Wucher, Philipp,Goldbach, Verena,Mecking, Stefan
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supporting information
p. 4516 - 4522
(2013/09/23)
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