From chaos to order: Chain-length dependence of the free energy of formation of meso-tetraalkylporphyrin self-assembled monolayer polymorphs

© 2016 American Chemical Society. We demonstrate that systematic errors can be reduced and physical insight gained through investigation of the dependence of free energies for meso-tetraalkylporphyrin self-assembled monolayers (SAMs) polymorphism on the alkyl chain length m. These SAMs form on highly ordered pyrolytic graphite (HOPG) from organic solution, displaying manifold densities and atomic structures. SAMs with m = 11-19 are investigated experimentally while those with m = 6-28 are simulated using density-functional theory (DFT). It is shown that, for m = 15 or more, the alkyl chains crystallize to dominate SAM structure. Meso-tetraalkylporphyrin SAMs of length less than 11 have never been observed, a presumed effect of inadequate surface attraction. Instead, we show that free energies of SAM formation actually enhance as the chain length decreases. The inability to image regular SAMs stems from the appearance of many polymorphic forms of similar free energy, preventing SAM ordering. We also demonstrate a significant odd/even effect in SAM structure arising from packing anomalies. Comparison of the chain-length dependence of formation free energies allows the critical dispersion interactions between molecules, solvent, and substrate to be directly examined. Interpretation of the STM data combined with measured enthalpies indicates that Grimme's D3 explicit-dispersion correction and the implicit solvent correction of Floris, Tomasi and Pascual Ahuir are both quantitatively accurate and very well balanced to each other

Genetic evidence for multiple invasions of subterranean termites into Canada

Modern quantum chemical electronic structure methods typically applied to localized chemical bonding are developed to predict atomic structures and free energies for meso-Tetraalkylporphyrin self-Assembled monolayer (SAM) polymorph formation from organic solution on highly ordered pyrolytic graphite surfaces. Large polymorphdependent dispersion-induced substrate-molecule interactions (e.g., -100 kcal mol-1 to -150 kcal mol-1 for tetratrisdecylporphyrin) are found to drive SAM formation, opposed nearly completely by large polymorph-dependent dispersion-induced solvent interactions (70- 110 kcal mol-1) and entropy effects (25-40 kcal mol-1 at 298 K) favoring dissolution. Dielectric continuum models of the solvent are used, facilitating consideration of many possible SAM polymorphs, along with quantum mechanical/molecular mechanical and dispersion- corrected density functional theory calculations. These predict and interpret newly measured and existing high-resolution scanning tunnelling microscopy images of SAM structure, rationalizing polymorph formation conditions. A wide range of molecular condensed matter properties at room temperature now appear suitable for prediction and analysis using electronic structure calculations

Atomic-resolution kinked structure of an alkylporphyrin on highly ordered pyrolytic graphite

The atomic structure of the chains of an alkyl porphyrin (5,10,15,20-tetranonadecylporphyrin) self-assembled monolayer (SAM) at the solid/liquid interface of highly ordered pyrolytic graphite (HOPG) and 1-phenyloctane is resolved using calibrated scanning tunneling microscopy (STM), density functional theory (DFT) image simulations, and ONIOM-based geometry optimizations. While atomic structures are often readily determined for porphyrin SAMs, the determination of the structure of alkyl-chain connections has not previously been possible. A graphical calibration procedure is introduced, allowing accurate observation of SAM lattice parameters, and, of the many possible atomic structures modeled, only the lowest-energy structure obtained was found to predict the observed lattice parameters and image topography. Hydrogen atoms are shown to provide the conduit for the tunneling current through the alkyl chains. © 2010 American Chemical Society

Polymorphism in porphyrin monolayers: the relation between adsorption configuration and molecular conformation

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New Insights on the Reversible Lithiation Mechanism of TiO 2

International audienceOperando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) measurements provide new insights on the mechanism of lithium insertion into TiO2(B). The investigation of the evolution of electronic, long-range, and local structure during electrochemical cycling indicates a purely monophasic insertion mechanism upon lithium insertion, while global and local structure are only slightly modified. While XRD reflects an anisotropic lattice expansion, EXAFS reveals a wide distribution of Ti–O bond length, in line with the presence of two distinct distorted octahedral Ti environments, in agreement with previous DFT calculations. Upon lithium insertion, these Ti–O coordination shells undergo significant modifications which are enhanced once the insertion of 0.4 Li is exceeded, connoting a two regime process that is in good agreement with the electrochemical signature of this material. DFT calculations and local chemical bond analyses were coupled with experimental results, thus providing additional insights into the structural response of TiO2(B) upon lithiation