21 research outputs found
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, ) 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 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 (), 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 (). 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 (). This is apparent by the high frequency carbonyl stretch at , 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 CāF Bond with the Ļ-System of a Cī»C Bond or āHead Onā with a Proximate CāH Bond
We describe the synthesis and preliminary study of two
molecules,
in which a fluorine atom is positioned proximately above the Ļ-orbitals
of a Cī»C bond or else wherein a CāF bond interacts in
a āhead onā fashion with a proximate CāH 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 CāFāC 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 [CāFāC]<sup>+</sup> 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<sub>N</sub><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<sub>N</sub>1 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 Ī¼-hydrido bridging.
After extensive testing, we can make the remarkable conclusion that
our system reacts solely through a ātunableā S<sub>N</sub>1 mechanism involving a fluoronium ion intermediate. Alternative
scenarios, such as S<sub>N</sub>2 reactivity, do not occur even under
forced conditions where they should be highly favored
Interaction of a CāF Bond with the Ļ-System of a Cī»C Bond or āHead Onā with a Proximate CāH Bond
We describe the synthesis and preliminary study of two
molecules,
in which a fluorine atom is positioned proximately above the Ļ-orbitals
of a Cī»C bond or else wherein a CāF bond interacts in
a āhead onā fashion with a proximate CāH 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 CāF Bond with the Ļ-System of a Cī»C Bond or āHead Onā with a Proximate CāH Bond
We describe the synthesis and preliminary study of two
molecules,
in which a fluorine atom is positioned proximately above the Ļ-orbitals
of a Cī»C bond or else wherein a CāF bond interacts in
a āhead onā fashion with a proximate CāH 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 Ī±āHelix NāCaps 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 NĪ“1 atom of histidine and both the backbone
NāH and side-chain OĪ³-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 <sup>2h</sup><i>J</i><sub>NN</sub> HBCs supports that
Thr NāHĀ·Ā·Ā·NĪ“1 His H-bonds within internal
repeats are stronger (ā¼4 Hz) than in the solvent exposed C-terminal
AR (ā¼2 Hz). In agreement, p<i>K</i><sub>a</sub> 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 <sup>2h</sup><i>J</i> and <sup>1h</sup><i>J</i> couplings are dominated by the Fermi contact interaction.
Finally, a Thr-to-Val replacement, which eliminates the Thr OĪ³-HĀ·Ā·Ā·NĪ“1
His H-bond and decreases protein stability, results in a 25% increase
in <sup>2h</sup><i>J</i><sub>NN</sub>, attributed to optimization
of the Val NāHĀ·Ā·Ā·NĪ“1 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