In a recent letter (J. Phys. Chem. A, 2001, 105,1), we argued that, although all major thermochemical tables recommend a value of ∆H°f 0 (OH) based on a spectroscopic approach, the correct value is 0.5 kcal/mol lower as determined from an ion cycle. In this paper, we expand upon and augment both the experimental and theoretical arguments presented in the letter. In particular, three separate experiments (mass-selected photoionization measurements, pulsed-field-ionization photoelectron spectroscopy measurements, and photoelectron-photoion coincidence measurements) utilizing the positive ion cycle to derive the O-H bond energy are shown to converge to a consensus value of the appearance energy AE 0 (OH(18.116 2 ( 0.003 0 eV). With the most accurate currently available zero kinetic energy photoionization value for the ionization energy IE(OH) ) 104989 ( 2 cm -1 , corroborated by a number of photoelectron measurements, this leads to D 0 (H-OH) ) 41128 ( 24 cm -1 ) 117.59 ( 0.07 kcal/mol. This corresponds to ∆H f0 (OH) ) 8.85 ( 0.07 kcal/mol and implies D 0 (OH) ) 35593 ( 24 cm -1 ) 101.76 ( 0.07 kcal/mol. These results are completely supported by the most sophisticated theoretical calculations ever performed on the H x O system, CCSD(T)/aug-cc-pVnZ, n ) Q, 5, 6, and 7, extrapolated to the CBS limit and including corrections for core-valence effects, scalar relativistic effects, incomplete correlation recovery, and diagonal Born-Oppenheimer corrections. These calculations have an estimated theoretical error of e0.2 kcal/mol based on basis set convergence properties. They reproduce the experimental results for dissociation energies, atomization energies, and ionization energies for the H x O system to within 0.0-0.2 kcal/mol. In contrast, the previously accepted values of the two successive bond dissociation energies of water differ from the current values by 0.5 kcal/mol. These values were derived from the spectroscopic determinations of D 0 (OH) using a very short Birge-Sponer extrapolation on OH/OD A 1 Σ + . However, on the basis of a calculation of the A state potential energy curve (with a multireference single and double excitation wave function and an augcc-pV5Z basis set) and an exhaustive reanalyzis of the original measured data on both the A and B states of OH, the Birge-Sponer extrapolation can be demonstrated to significantly underestimate the bond dissociation energy, although only the last vibrational level was not observed experimentally. The recommended values of this paper affect a large number of other thermochemical quantities which directly or indirectly rely on or refer to D 0 (H-OH), D 0 (OH), or ∆H°f(OH). This is illustrated by an analysis of several reaction enthalpies, deprotonation enthalpies, and proton affinities.

A new method called adaptive force matching (AFM) has been developed that is capable of producing high quality force fields for condensed phase simulations. This procedure involves the parametrization of force fields to reproduce ab initio forces obtained from condensed phase quantum-mechanics/molecular-mechanics (QM/MM) calculations. During the procedure, the MM part of the QM/MM is iteratively improved so as to approach ab initio quality. In this work, the AFM method has been tested to parametrize force fields for liquid water so that the resulting force fields reproduce forces calculated using the ab initio MP2 and the Kohn-Sham density functional theory with the Becke-Lee-Yang-Parr (BLYP) and Becke three-parameter LYP (B3LYP) exchange correlation functionals. The AFM force fields generated in this work are very simple to evaluate and are supported by most molecular dynamics (MD) codes. At the same time, the quality of the forces predicted by the AFM force fields rivals that of very expensive ab initio calculations and are found to successfully reproduce many experimental properties. The site-site radial distribution functions (RDFs) obtained from MD simulations using the force field generated from the BLYP functional through AFM compare favorably with the previously published RDFs from Car-Parrinello MD simulations with the same functional. Technical aspects of AFM such as the optimal QM cluster size, optimal basis set, and optimal QM method to be used with the AFM procedure are discussed in this paper.

We used the atomic force microscope to manipulate and unfold individual molecules of the titin I27 domain and reconstructed its free energy surface using Jarzynski's equality. The free energy surface for both stretching and unfolding was reconstructed using an exact formula that relates the nonequilibrium work fluctuations to the molecular free energy. In addition, the unfolding free energy barrier, i.e., the activation energy, was directly obtained from experimental data for the first time. This Letter demonstrates that Jarzynski's equality can be used to analyze nonequilibrium single-molecule experiments, and to obtain the free energy surfaces for molecular systems, including interactions for which only nonequilibrium work can be measured.

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The mechanical behavior of nanobubbles represents their physical essence and has been thought to be closely related to their mysteriously long lifetimes. However, it is difficult to measure the mechanical properties of nanobubbles by conventional atomic force microscopy (AFM). In this paper, nanobubbles were investigated via a novel AFM imaging mode, PeakForce Quantitative Nano-Mechanics (PF-QNM), at the interface of water and highly oriented pyrolytic graphite (HOPG). High resolution images of the nanobubbles in true-contact were achieved by PF-QNM and compared with those obtained by tapping mode AFM (TM-AFM) in the same area. From the force curves simultaneously captured during the PF-QNM imaging processes, the stiffness of the nanobubbles was derived and mapped, ranging usually from 60 to 120 pN nm À1 , indicating that the gas-water interface of nanobubbles has similar mechanical properties to those of microbubbles. Interestingly, a size dependence of the stiffness was found and the small nanobubbles had a higher stiffness.

A novel nanohybrid of hyaluronic acid (HA)-decorated graphene oxide (GO) was fabricated as a targeted and pH-responsive drug delivery system for controlling the release of anticancer drug doxorubicin (DOX) for tumor therapy. For the preparation, DOX was first loaded onto GO nanocarriers via π-π stacking and hydrogen-bonding interactions, and then it was decorated with HA to produce HA-GO-DOX nanohybrids via H-bonding interactions. In this strategy, HA served as both a targeting moiety and a hydrophilic group, making the as-prepared nanohybrids targeting, stable, and disperse. A high loading efficiency (42.9%) of DOX on the nanohybrids was also obtained. Cumulative DOX release from HA-GO-DOX was faster in pH 5.3 phosphate-buffered saline solution than that in pH 7.4, providing the basis for pH-response DOX release in the slightly acidic environment of tumor cells, while the much-slower DOX release from HA-GO-DOX than DOX showed the sustained drug-release capability of the nanohybrids. Fluorescent images of cellular uptake and cell viability analysis studies illustrated that these HA-GO-DOX nanohybrids significantly enhanced DOX accumulation in HA-targeted HepG2 cancer cells compared to HA-nontargeted RBMEC cells and subsequently induced selective cytotoxicity to HepG2 cells. In vivo antitumor efficiency of HA-GO-DOX nanohybrids showed obviously enhanced tumor inhibition rate for H22 hepatic cancer cell-bearing mice compared with free DOX and the GO-DOX formulation. These studies suggest that the HA-GO-DOX nanohybrids have potential clinical applications for anticancer drug delivery.

The formation of methyl cation (CH 3 ϩ ) from methane (CH 4 ) has been investigated in high resolution using the newly perfected pulsed field ionization photoelectron-photoion coincidence ͑PFI-PEPICO͒ scheme. The PFI-PEPICO data reveal that fragmentation of CH 4 in high-n Rydberg states occurs at energies above the dissociation threshold prior to pulsed field ionization. The crossover point of the breakdown curves is found to depend strongly on the Stark field in the ion source and thus traditional simulation procedures based on such a feature for ion dissociation energy determination are not appropriate in PFI-PEPICO studies. We show that for a prompt dissociation process, the disappearance energy of the parent molecule provides an accurate measure of the 0 K ion dissociation threshold, as that for CH 3 ϩ from CH 4 is 14.323Ϯ0.001 eV.

The pulsed-field ionization zero-electron kinetic-energy ͑PFI-ZEKE͒ threshold photoionization spectrum of NO 2 from 9.58 to 20 eV is obtained using vacuum ultraviolet synchrotron radiation by means of the Chemical Dynamics Beamline at the Lawrence Berkeley National Laboratory Advanced Light Source. The high resolution afforded by PFI threshold discrimination yields new or refined spectroscopic constants for a number of known excited states of the cation, including the first estimate of the A rotational constant in the a 3 B 2 state, as well as new fundamental frequencies for the A 1 A 2 and B 1 B 2 states, a precise determination of the singlet-triplet splitting in the c 3 B 1 -C 1 B 1 complex and the first observations of the states, d 3 A 1 and D 1 B 2 . Most significantly, ZEKE photoelectron detection resolves vibrational structure in the linear X 1 ⌺ g ϩ ground state of NO 2 ϩ . Vibrational positions in the first electron volt of the spectrum are found to conform with the predictions of a Hamiltonian that includes Fermi resonance and other anharmonic terms derived from earlier multiresonant laser spectroscopic experiments on the lower bending excited states.