The mixed cation salts, (dimethylammonium)(3,5-dimethylpyridinium)CuX4 (X = Cl, Br), henceforth (DMA)-(35DMP)CuX4, are new examples of spin-ladders based on nonbonded halide center dot center dot center dot halide interactions between CuX(4)(2-)anions. In these structures, double rows of the CuX42- anions are sheathed by the 35DMP(+) cations, while the edges are capped by the DMA(+) cations. For the Br salt, the Br center dot center dot center dot Br contacts that define the rungs of the ladder are 4.017 angstrom in length, while those that define the rails are 3.983 angstrom. For the Cl salt, the corresponding lengths are 3.967 and 4.045 angstrom. The susceptibility data for the Br salt exhibits a maximum at similar to 5.5 K, and fitting the data to the spin 1/2 antiferromagnetic ladder model yields 2J(rail)/k = -7.95 K and 2J(rung)/k = -4.07 K. The exchange coupling is much weaker in the Cl salt, no maximum in chi is observed down to 1.8 K, and the corresponding exchange constants are -1.59 and -1.25 K, respectively. An analysis is made of the structural factors involved in the J(rung) pathway.
Multiple-Turnover Isotopic Labeling of Fmoc- and Boc-Protected Amino Acids with Oxygen Isotopes
摘要:
An efficient method for the selective isotopic labeling of carboxylic acids is reported. By reacting an amino acid with excess carbodiimide and (OH2)-O-18, a kinetically enhanced multiple turnover reaction provides the O-18-labeled product in high yield and excellent isotopic enrichment. This reaction is fully compatible with standard Fmoc, Boc, Trt, and OtBu protecting groups and provides a means to selectively label the alpha-carboxylic acids of functionalized amino acids with stable oxygen isotopes.
<sup>17</sup>O MAS NMR Correlation Spectroscopy at High Magnetic Fields
作者:Eric G. Keeler、Vladimir K. Michaelis、Michael T. Colvin、Ivan Hung、Peter L. Gor’kov、Timothy A. Cross、Zhehong Gan、Robert G. Griffin
DOI:10.1021/jacs.7b08989
日期:2017.12.13
FMOC-l-leucine and FMOC-l-valine, and a dipeptide, N-acetyl-l-valyl-l-leucine (N-Ac-VL), were studied via one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy. Utilizing 17O magic-angle spinning (MAS) NMR at multiple magnetic fields (17.6-35.2 T/750-1500 MHz for 1H) the 17O quadrupolar and chemical shift parameters were determined for the two oxygen sites of each FMOC-protected
The crystal structures of 21 new pyridine-based Pb(II)-bromides show that there is a complex relationship between geometrical, steric and electronic aspects in the formation of N−H⋅⋅⋅Br bridges. The interactions also influence the linkage pattern of the PbBrx polyhedra and vice versa.
Nucleophile-Assisted Alkene Activation: Olefins Alone Are Often Incompetent
作者:Kumar Dilip Ashtekar、Mathew Vetticatt、Roozbeh Yousefi、James E. Jackson、Babak Borhan
DOI:10.1021/jacs.6b02877
日期:2016.7.6
strategies for design of new reactions. Alkene reactivity is seen to span the range from the often overgeneralized "sophomore textbook" image of stepwise electrophilic attack on the alkene and subsequent nucleophilic bond formation, to the nucleophile-assisted alkene activation (NAAA) cases where electron donation from the nucleophilic addition partner activates the alkene for electrophilic attack. By highlighting
作者:Firas Awwadi、Roger D. Willett、Brendan Twamley、Ryan Schneider、Christopher P. Landee
DOI:10.1021/ic800905e
日期:2008.10.20
The mixed cation salts, (dimethylammonium)(3,5-dimethylpyridinium)CuX4 (X = Cl, Br), henceforth (DMA)-(35DMP)CuX4, are new examples of spin-ladders based on nonbonded halide center dot center dot center dot halide interactions between CuX(4)(2-)anions. In these structures, double rows of the CuX42- anions are sheathed by the 35DMP(+) cations, while the edges are capped by the DMA(+) cations. For the Br salt, the Br center dot center dot center dot Br contacts that define the rungs of the ladder are 4.017 angstrom in length, while those that define the rails are 3.983 angstrom. For the Cl salt, the corresponding lengths are 3.967 and 4.045 angstrom. The susceptibility data for the Br salt exhibits a maximum at similar to 5.5 K, and fitting the data to the spin 1/2 antiferromagnetic ladder model yields 2J(rail)/k = -7.95 K and 2J(rung)/k = -4.07 K. The exchange coupling is much weaker in the Cl salt, no maximum in chi is observed down to 1.8 K, and the corresponding exchange constants are -1.59 and -1.25 K, respectively. An analysis is made of the structural factors involved in the J(rung) pathway.