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A surge of progress in both laser spectroscopy experiments and theoretical dynamics methods has facilitated new, highly detailed studies of water clusters. The geometrical structures and hydrogen-bond tunneling pathways of the water trimer, tetramer, pentamer, and hexamer systems have recently been characterized with global analysis of potential surfaces, diffusion Monte Carlo calculations, and far-infrared laser vibration-rotation tunneling spectroscopy. Results from these and other studies are yielding important insights into the cooperativity effects in hydrogen bonding, aqueous solvation, and hydrogen-bond network rearrangement dynamics, which promise to enhance our understanding of solid and liquid water behavior.

Far-infrared laser vibration-rotation tunneling spectroscopy was used to measure an intermolecular vibration (81.1 91 98 wave numbers) of the isolated water (D,O) pentamer. Rotational analysis supports the chiral, slightly puckered ring structure predicted by theory. The experimentally deduced interoxygen separations for the water clusters up to the pentamer showed exponential convergence toward the corresponding distance in bulk phase water.

Tunable terahertz laser vibration-rotation-tunneling spectroscopy has been employed to characterize the structure and hydrogen bond network rearrangement dynamics of a cage form of the water hexamer having eight hydrogen bonds. The isolated clusters are produced in a pulsed supersonic slit jet. Striking similarities are found between the structure and the average interoxygen distance R O-O (2.82 Å) of the hexamer cage and those of the basic unit of ice VI. The hybrid perpendicular band of b-and c-types is observed near 2.491 THz (83.03 cm -1 ) and rationalized to originate from the torsional motions of the two single-donor singleacceptor monomers about their donor hydrogen bonds, thereby causing changes in the dipole moments from each monomer to be orthogonal to each other as well as to be perpendicular to the approximate symmetry a-axis. Triplet spectral patterns accompanying each rovibrational transition with line spacings of 1.9 MHz and intensity ratios of 9:6:1 are attributed to the degenerate quantum tunneling that involves the exchange of protons within two similar monomers of the cluster. The Stark effect of the degenerate asymmetry doublets of K a g 3 has been analyzed to yield vibration-and (J,K a )-dependent electric dipole moment components ranging from 1.82 to 2.07 D along the a-axis of this near prolate rotor. The selection rules establish that this dipole moment component preserves the sign upon vibrational excitation. A reasonable agreement is found between the dipole measurement and the results of a model calculation using an iterated induction expansion including the quadrupole-induced dipole. The same model has also been applied to extract the individual monomer dipole moments for the dimer and the cage and cyclic hexamers. The trends of two molecular propertiessthe contraction of the R O-O distance and enhancement of the average individual monomer dipole with increasing cluster size up to the cyclic hexamer-are found to converge exponentially to the bulk phase values. In both cases, the cage properties deviate from the trends established by the dimer and cyclic clusters.

Shining light on diamond particles makes them MRI-“bright,” opening avenues for room temperature hyperpolarized liquids.

The detailed analysis of a parallel vibration−rotation-tunneling (VRT) band of the isolated cyclic perdeuterated (d 10) water pentamer measured near 2.4 THz (81.2 cm-1) is presented. The vibrationally averaged rotational constants correspond rigorously to those of a quasiplanar (C 5 h ) oblate top whereas the equilibrium cyclic structure is predicted to be slightly asymmetric due to puckering of the oxygen framework and uneven distribution of the free O−D bonds above and below the ring. The vibrational averaging which underlies the symmetric top behavior, the absence of a first-order Stark effect, as well as the origin of the observed intermolecular vibration, is consequently rationalized using a five-dimensional model of the pseudorotation, analogous to that established for the water trimer. Pseudorotation is induced by the nearly barrierless “flipping” of the monomers about their donor hydrogen bonds and by accompanying hydrogen bond network puckering motions. The observed vibration is tentatively assigned to the k (pseudorotational quantum number) = 5 (upper) ← 0 pseudorotational transition classified under the cyclic molecular symmetry group G10 (isomorphic to C 5 h ); new transitions are also predicted from this model. The donor (bifurcation) tunneling responsible for the spectral splittings observed in each pseudorotational state in the water trimer does not produce observable splittings in the pentamer-d 10, but is predicted to do so for the normal isotopic pentamer-h 10. The experimentally deduced inter-oxygen separations for water clusters up to the pentamer (for which the vibrationally averaged result is R O - O = 2.76 Å) as a function of the cluster size exhibit exponential contraction toward the corresponding distance in ice Ih.

Background Hand, foot, and mouth disease (HFMD) is an acute viral illness commonly caused by coxsackievirus A16 (CV-A16) and enterovirus 71 infections. Recently, atypical HFMD has been reported in association with CV-A6, an uncommon enterovirus strain. Objective To describe the clinical features of atypical HFMD associated with CV-A6 infection and its diagnostic laboratory evaluation. Methods Patients presenting to our institution with history and examination suggestive of atypical HFMD from January 2012 to July 2012 were identified. Morphology and distribution of mucocutaneous lesions were recorded. Enterovirus infection was assessed by reverse transcriptase polymerase chain reaction of biologic specimens. Enterovirus type was determined by viral capsid protein 1 gene sequencing. Results Two adults and 3 children with atypical HFMD were identified. Four of 5 patients exhibited widespread cutaneous lesions. In 2 patients with a prior history of atopic dermatitis, accentuation in areas of dermatitis was noted. Associated systemic symptoms prompted 4 of 5 patients to seek emergency care, and both adults were hospitalized for diagnostic evaluation. Infection with CV-A6 was confirmed in all patients. Limitations This study is a case series from a single institution. Conclusion Consideration of the expanded range of cutaneous findings in atypical HFMD caused by CV-A6 infection may assist clinicians in diagnosis and management.

Dynamic Nuclear Polarization (DNP) has enabled enormous gains in magnetic resonance signals and led to vastly accelerated NMR/MRI imaging and spectroscopy. Unlike conventional cw-techniques, DNP methods that exploit the full electron spectrum are appealing since they allow direct participation of all electrons in the hyperpolarization process. Such methods typically entail sweeps of microwave radiation over the broad electron linewidth to excite DNP, but are often inefficient because the sweeps, constrained by adiabaticity requirements, are slow. In this paper we develop a technique to overcome the DNP bottlenecks set by the slow sweeps, employing a swept microwave frequency comb that increases the effective number of polarization transfer events while respecting adiabaticity constraints. This allows a multiplicative gain in DNP enhancement, scaling with the number of comb frequencies and limited only by the hyperfine-mediated electron linewidth. We demonstrate the technique for the optical hyperpolarization of 13 C nuclei in powdered microdiamonds at low fields, increasing the DNP enhancement from 30 to 100 measured with respect to the thermal signal at 7T. For low concentrations of broad linewidth electron radicals, e.g. TEMPO, these multiplicative gains could exceed an order of magnitude.Introduction: -Dynamic nuclear polarization (DNP) -the process of polarizing (cooling) nuclear spins to a spin temperature far lower than the lattice temperature [1, 2] -has emerged as a technological breakthrough that serves as the starting point for a wide-range of applications, including signal enhanced spectroscopy [3,4] and imaging [5] and for state initialization in quantum information processing and metrology [6,7]. Indeed, magnetic resonance (NMR and MRI) signals from hyperpolarized nuclear spins can be enhanced by several orders of magnitude allowing enormous gains, even approaching a million-fold, in experimental averaging time. This has opened up avenues for the sensitive probing of phenomena, species and surfaces [8], whose detection would otherwise have remained intractable.In its simplest manifestation DNP involves the use of electrons whose polarization is transferred to the nuclear spins via microwave irradiation [10], allowing a polarization enhancement ε γ e /γ n , where γ e,n are the gyromagnetic ratios of the electron and nuclear spins respectively. Resonant polarization transfer between electron and nuclear spin is achieved via microwave excitation. Depending on the concentrations of the electron and nuclear spins in the insulating solid, the transfer can be mediated by thermal mixing, the cross effect, the solid effect and even the Overhauser effect. However several common (e.g. nitroxide based) electron polarizing agents have large ganisotropy and severely inhomogeneously broadened electronic linewidths that scale rapidly with field and can be as broad as 0.5GHz at high fields (>3T) [9,[11][12][13]. This broadening limits the number of spins contributing to the resonant energy exchange at a particular mi...