Frustrated 3-Dimensional Quantum Spin Liquid in CuHpCl

Inelastic neutron scattering measurements are reported for the quantum antiferromagnetic material Cu_2(C_5H_12N_2)_2Cl_4 (CuHpCl). The magnetic excitation spectrum forms a band extending from 0.9 meV to 1.4 meV. The spectrum contains two modes that disperse throughout the a-c plane of the monoclinic unit cell with less dispersion along the unique b-axis. Simple arguments based on the measured dispersion relations and the crystal structure show that a spin ladder model is inappropriate for describing CuHpCl. Instead, it is proposed that hydrogen bond mediated exchange interactions between the bi-nuclear molecular units yield a three-dimensional interacting spin system with a recurrent triangular motif similar to the Shastry-Sutherland Model (SSM). Model independent analysis based on the first moment sum rule shows that at least four distinct spin pairs are strongly correlated and that two of these, including the dimer bond of the corresponding SSM, are magnetically frustrated. These results show that CuHpCl should be classified as a frustration induced three dimensional quantum spin liquid.Comment: 13 pages, 17 figures (Color) ReSubmitted to Phys. Rev. B 9/21/2001 resubmission has new content email comments to [email protected] or [email protected]

"PROTON SPONGES": A RIGID ORGANIC SCAFFOLD TO REVEAL THE QUANTUM STRUCTURE OF THE INTRAMOLECULAR PROTON BOND

Author Institution: Yale University, P. O. Box 208107, New Haven, CT, 06520; Johns Hopkins University, 3400 North Charles Street, Baltimore, MD, 21218; Brock University, St. Catherines, ON, Canada L2S 3A1Spectroscopic analysis of systems containing charged hydrogen bonds (e.g. the Zundel ion, $\mathrm{H}_{5}\mathrm{O}_{2}^{+}$) in a vibrationally cold regime is useful in decongesting numerous anharmonic features common to room temperature measurements.[Roscioli, J. R.; et. al. Science 2007] This approach has been extended to conjugate acids of the "Proton Sponge" family of organic compounds, which contain strong intramolecular hydrogen bonds between proton donor (D) and acceptor (A) groups at the 1- and 8-positions. By performing $\mathrm{H}_2/\mathrm{D}_2$ vibrational predissociation spectroscopy on cryogenically cooled ions, we explore how the proximity and spatial orientation of D and A moieties relates to the spectroscopic signature of the shared proton. In the cases studied ($\mathrm{D = Me_{2}N-H^{+}; A = OH, O(C=O)Ph}$), we observe strong anharmonic couplings between the shared proton and dark states that persist at these cryogenic temperatures. This leads to intense NH stretching features throughout the nominal CH stretching region ($2800-3000 \mathrm{cm}^{-1}$). Isotopic substitution has verified that the oscillator strength of these broad features is driven by NH stretching. Furthermore, the study of A = O(C=O)Ph has provided a spectroscopic snapshot of the shared proton at work as an active catalytic moiety fostering ester hydrolysis by first order acylium fission ($\mathrm{A_{AC}1}$). This is apparent by the high frequency carbonyl stretch at $1792\ \mathrm{cm}^{-1}$, which is a consequence of the strong hydrogen bond to the ether-ester oxygen atom. Thus, these "Proton Sponges" are useful model systems that unearth the quantum structure and reactivity of shared proton interactions in organic compounds

Interaction of a C–F Bond with the π-System of a CC Bond or “Head On” with a Proximate C–H Bond

We describe the synthesis and preliminary study of two molecules, in which a fluorine atom is positioned proximately above the π-orbitals of a CC bond or else wherein a C–F bond interacts in a “head on” fashion with a proximate C–H bond. The spectroscopic characteristics of these unusual interactions are documented, X-ray crystallographic analyses are reported, and theoretical calculations are employed to support the observed spectroscopy

Search for a Symmetrical C–F–C Fluoronium Ion in Solution: Kinetic Isotope Effects, Synthetic Labeling, and Computational, Solvent, and Rate Studies

Recently, we reported evidence for the generation of a symmetrical fluoronium ion (a [C–F–C]⁺ interaction) in solution from a cage-like precursor, relying heavily on a single isotopic-labeling experiment. Paraphrasing the axiom that a strong claim must be met by as much evidence as possible, we seek to expand upon our initial findings with comprehensive labeling studies, rate measurements, kinetic isotope effect (KIE) experiments, synthetic studies, and computations. We also chronicle the development of the system, our thought process, and how it evolved from a tantalizing indication of fluoronium ion assistance in a dibromination reaction to the final, optimized system. Our experiments show secondary KIE experiments that are fully consistent with a transition state involving fluorine participation; this is also confirmed by a significant remote isotope effect. Paired with DFT calculations, the KIE experiments are indicative of the trapping of a symmetrical intermediate. Additionally, starting with an epimeric <i>in</i>-triflate precursor that hydrolyzes through a putative frontside S_N<i>i</i> mechanism involving fluorine participation, KIE studies indicate that an identical intermediate is trapped (the fluoronium ion). Studies also show that the rate-determining step of the fluoronium forming S_N1 reaction can be changed on the basis of solvent and additives. We also report the synthesis of a nonfluorinated control substrate to measure a relative anchimeric role of the fluorine atom in hydrolysis versus μ-hydrido bridging. After extensive testing, we can make the remarkable conclusion that our system reacts solely through a “tunable” S_N1 mechanism involving a fluoronium ion intermediate. Alternative scenarios, such as S_N2 reactivity, do not occur even under forced conditions where they should be highly favored

Interaction of a C–F Bond with the π-System of a CC Bond or “Head On” with a Proximate C–H Bond

We describe the synthesis and preliminary study of two molecules, in which a fluorine atom is positioned proximately above the π-orbitals of a CC bond or else wherein a C–F bond interacts in a “head on” fashion with a proximate C–H bond. The spectroscopic characteristics of these unusual interactions are documented, X-ray crystallographic analyses are reported, and theoretical calculations are employed to support the observed spectroscopy

Interaction of a C–F Bond with the π-System of a CC Bond or “Head On” with a Proximate C–H Bond

We describe the synthesis and preliminary study of two molecules, in which a fluorine atom is positioned proximately above the π-orbitals of a CC bond or else wherein a C–F bond interacts in a “head on” fashion with a proximate C–H bond. The spectroscopic characteristics of these unusual interactions are documented, X-ray crystallographic analyses are reported, and theoretical calculations are employed to support the observed spectroscopy

Direct NMR Detection of Bifurcated Hydrogen Bonding in the α‑Helix N‑Caps of Ankyrin Repeat Proteins

In biomolecules, bifurcated H-bonds typically involve the interaction of two donor protons with the two lone pairs of oxygen. Here, we present direct NMR evidence for a bifurcated H-bonding arrangement involving <i>nitrogen</i> as the acceptor atom. Specifically, the H-bond network comprises the Nδ1 atom of histidine and both the backbone N–H and side-chain Oγ-H of threonine within the conserved TXXH motif of ankyrin repeat (AR) proteins. Identification of the H-bonding partners is achieved via solution NMR H-bond scalar coupling (HBC) and H/D isotope shift experiments. Quantitative determination of ^2h<i>J</i>_NN HBCs supports that Thr N–H···Nδ1 His H-bonds within internal repeats are stronger (∼4 Hz) than in the solvent exposed C-terminal AR (∼2 Hz). In agreement, p<i>K</i>_a values for the buried histidines bridging internal ARs are several units lower than those of the C-terminus. Quantum chemical calculations show that the relevant ^2h<i>J</i> and ^1h<i>J</i> couplings are dominated by the Fermi contact interaction. Finally, a Thr-to-Val replacement, which eliminates the Thr Oγ-H···Nδ1 His H-bond and decreases protein stability, results in a 25% increase in ^2h<i>J</i>_NN, attributed to optimization of the Val N–H···Nδ1 His H-bond. Overall, the results provide new insights into the H-bonding properties of histidine, a refined structural rationalization for the folding cooperativity of AR proteins, and a challenging benchmark for the calculation of HBCs