5 research outputs found

    Systematic Parametrization of Polarizable Force Fields from Quantum Chemistry Data

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    We introduce ForceBalance, a method and free software package for systematic force field optimization with the ability to parametrize a wide variety of functional forms using flexible combinations of reference data. We outline several important challenges in force field development and how they are addressed in ForceBalance, and present an example calculation where these methods are applied to develop a highly accurate polarizable water model. ForceBalance is available for free download at https://simtk.org/home/forcebalance

    Role of Molecular Dipoles in Charge Transport across Large Area Molecular Junctions Delineated Using Isomorphic Self-Assembled Monolayers

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    Delineating the role of dipoles in large area junctions that are based on self-assembled monolayers (SAMs) is challenging due to molecular tilt, surface defects, and interchain coupling among other features. To mitigate SAM-based effects in study of dipoles, we investigated tunneling rates across carboranesisostructural molecules that orient along the surface normal on Au (but bear different dipole moments) without changing the thickness, packing density, or morphology of the SAM. Using the Au-SAM//Ga<sub>2</sub>O<sub>3</sub>-EGaIn junction (where “//” = physisorption, “–” = chemisorption, and EGaIn is eutectic gallium–indium), we observe that molecules with dipole moments oriented along the surface normal (with dipole moment, <i>p</i> = 4.1D for both M9 and 1O2) gave lower currents than when the dipole is orthogonal (<i>p</i> = 1.1 D, M1) at ±0.5 V applied bias. Similarly, from transition voltage spectroscopy, the transition voltages, <i>V</i><sub>T</sub> (volt), are significantly different. (0.5, 0.43, and 0.4 V for M1, M9, and 1O2, respectively). We infer that the magnitude and direction of a dipole moments significantly affect the rate of charge transport across large area junctions with Δ log|J| ≅ 0.4 per Debye. This difference is largely due to effect of the dipole moment on the molecule-electrode coupling strength, Γ, hence effect of dipoles is likely to manifest in the contact resistance, <i>J</i><sub>o</sub>, although in conformational flexible molecules field-induced effects are expected

    Limits to the Effect of Substrate Roughness or Smoothness on the Odd–Even Effect in Wetting Properties of <i>n</i>‑Alkanethiolate Monolayers

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    This study investigates the effect of roughness on interfacial properties of an <i>n</i>-alkanethiolate self-assembled monolayer (SAM) and uses hydrophobicity to demonstrate the existence of upper and lower limits. This article also sheds light on the origin of the previously unexplained gradual increase in contact angles with increases in the size of the molecule making the SAM. We prepared Au surfaces with a root-mean-square (RMS) roughness of ∼0.2–0.5 nm and compared the wetting properties of <i>n</i>-alkanethiolate (C<sub>10</sub>–C<sub>16</sub>) SAMs fabricated on these surfaces. Static contact angles, θ<sub>s</sub>, formed between the SAM and water, diethylene glycol, and hexadecane showed an odd–even effect irrespective of the solvent properties. The average differences in subsequent SAM<sup>E</sup> and SAM<sup>O</sup> are Δθ<sub>s|<i>n </i> – (<i>n</i>+1)|</sub> ≈ 1.7° (<i>n</i> = even) and Δθ<sub>s|<i>n </i>– (<i>n</i>+1)|</sub> ≈ 3.1° (<i>n</i> = odd). A gradual increase in θ<sub>s</sub> with increasing length of the molecule was observed, with values ranging from water 104.7–110.7° (overall Δθ<sub>s</sub> = 6.0° while for the evens Δθ<sub>s</sub><sup>E</sup> = 4.4° and odds Δθ<sub>s</sub><sup>O</sup> = 3.5°) to diethylene glycol 72.9–80.4° (overall Δθ<sub>s</sub> = 7.5° while for the evens Δθ<sub>s</sub><sup>E</sup> = 2.9° and odds Δθ<sub>s</sub><sup>O</sup> = 2.4°) and hexadecane 40.4–49.4° (overall Δθ<sub>s</sub> = 9.0° while for the evens Δθ<sub>s</sub><sup>E</sup> = 3.7° and odds Δθ<sub>s</sub><sup>O</sup> = 2.1°). This article establishes that the gradual increase in θ<sub>s</sub> with increasing molecular size in SAMs is due to asymmetry in the zigzag oscillation in the odd–even effect. Comparison of the magnitude and proportion differences in this asymmetry allows us to establish the reduction in interfacial dispersive forces, due to increasing SAM crystallinity with increasing molecular size, as the origin of this asymmetry. By comparing the dependence of θ<sub>s</sub> on surface roughness we infer that (i) RMS roughness ≈ 1 nm is a theoretical limit beyond which the odd–even effect cannot be observed and (ii) on a hypothetically flat surface the maximum difference in hydrophobicity, as expressed in θ<sub>s</sub>, is ∼3°

    Investigating the Dearomative Rearrangement of Biaryl Phosphine-Ligated Pd(II) Complexes

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    A series of monoligated L·Pd<sup>II</sup>(Ar)­X complexes (L = dialkyl biaryl phosphine) have been prepared and studied in an effort to better understand an unusual dearomative rearrangement previously documented in these systems. Experimental and theoretical evidence suggest a concerted process involving the unprecedented Pd<sup>II</sup>-mediated insertion of an aryl group into an unactivated arene