Molecular length, monolayer density, and charge transport: lessons from Al-AlOx/alkyl-phosphonate/Hg junctions

A combined electronic transport-structure characterization of self-assembled monolayers (MLs) of alkyl-phosphonate (AP) chains on Al-AlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that "long" APs (C14 and C16) form much denser MLs than do "short" APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed "short" AP MLs attenuate the current relatively more efficiently than those with densely packed, "long" ones. Furthermore, "long" AP ML junctions showed strong bias variation of the length decay coefficient, β, while for "short" AP ML junctions β is nearly independent of bias. Therefore, even for these simple molecular systems made up of what are considered to be inert molecules, the tunneling distance cannot be varied independently of other electrical properties, as is commonly assumed.     

An experimental-theoretical analysis of protein adsorption on peptidomimetic polymer brushes

Surface-grafted water-soluble polymer brushes are being intensely investigated for preventing protein adsorption to improve biomedical device function, prevent marine fouling, and enable applications in biosensing and tissue engineering. In this contribution, we present an experimental-theoretical analysis of a peptidomimetic polymer brush system with regard to the critical brush density required for preventing protein adsorption at varying chain lengths. A mussel adhesive-inspired DOPA-Lys (DOPA = 3,4-dihydroxy-phenylalanine; Lys = lysine) pentapeptide surface grafting motif enabled aqueous deposition of our peptidomimetic polypeptoid brushes over a wide range of chain densities. Critical densities of 0.88 nm(-2) for a relatively short polypeptoid 10-mer to 0.42 nm(-2) for a 50-mer were identified from measurements of protein adsorption. The experiments were also compared with the protein adsorption isotherms predicted by a molecular theory. Excellent agreements in terms of both the polymer brush structure and the critical chain density were obtained. Furthermore, atomic force microscopy (AFM) imaging is shown to be useful in verifying the critical brush density for preventing protein adsorption. The present coanalysis of experimental and theoretical results demonstrates the significance of characterizing the critical brush density in evaluating the performance of an antifouling polymer brush system. The high fidelity of the agreement between the experiments and molecular theory also indicate that the theoretical approach presented can aid in the practical design of antifouling polymer brush systems.     

Nonheme iron-oxo and -superoxo reactivities: O2 binding and spin inversion probability matter

DFT calculated barriers for C-H activation of 1,4-cyclohexadiene by nonheme iron(IV)-oxo and iron(III)-superoxo species show that the experimental trends can be explained if the spin inversion probability of the TMC iron(IV)-oxo is assumed to be poor. Also, the TMC iron(III)-superoxo reaction proceeds with an endothermic O(2)-binding energy followed by an intrinsically reactive quintet state.     

Correlating DFT-Calculated Energy Barriers to Experiments in Nonheme Octahedral Fe(IV) O Species

The experimentally measured bimolecular reaction rate constant, k(2) , should in principle correlate with the theoretically calculated rate-limiting free energy barrier, ΔG(≠) , through the Eyring equation, but it fails quite often to do so due to the inability of current computational methods to account in a precise manner for all the factors contributing to ΔG(≠) . This is further aggravated by the exponential sensitivity of the Eyring equation to these factors. We have taken herein a pragmatic approach for C?H activation reactions of 1,4-cyclohexadiene with a variety of octahedral nonheme Fe(IV) O complexes. The approach consists of empirically determining two constants that would aid in predicting experimental k(2) values uniformly from theoretically calculated electronic energy (ΔE(≠) ) values. Shown in this study is the predictive power as well as insights into energy relationships in Fe(IV) O C?H activation reactions. We also find that the difference between ΔG(≠) and ΔE(≠) converges at slow reactions, in a manner suggestive of changes in the importance of the triplet spin state weight in the overall reaction.     

Identity SN2 reactions X- + CH3X --> XCH3 + X- (X=F, Cl, Br, and I) in vacuum and in aqueous solution: a valence bond study

The recently developed (L. Song, W. Wu, Q. Zhang, S. Shaik, J. Phys. Chem. A 2004, 108, 6017) valence bond method coupled with a polarized continuum model (VBPCM) has been applied to the identity SN2 reaction of halides in the gas phase and in aqueous solution. The barriers computed at the level of the breathing orbital VB method (P. C. Hiberty, J. P. Flament, E. Noizet, Chem. Phys. Lett. 1992, 189, 259), BOVB and VBPCM//BOVB, are comparable to CCSD(T) and CCSD(T)//PCM results and to experimentally derived barriers in solution (W. J. Albery, M. M. Kreevoy, Adv. Phys. Org. Chem. 1978, 16, 85). The reactivity parameters needed to apply the valence bond state correlation diagram (VBSCD) method (S. Shaik, J. Am. Chem. Soc. 1984, 106, 1227), were also determined by VB calculations. It has been shown that the reactivity parameters along with their semiempirical derivations provide a satisfactory qualitative and quantitative account of the barriers.