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2-Thiobarbituric acid (TBA) coated gold nanoparticles (average diameter = 5.90 nm) were produced and studied by several experimental and theoretical methods. As part of this study, the molecular structure of TBA tautomers in the solid, in polar solutions, and adsorbed onto gold nanoparticles was studied. The resolution of this complicated system (10 possible isomers) was accomplished with the aid of experimental (IR, UV−vis, and NMR) and theoretical (DFT and MP2) methods. The general conclusion is that there are two preeminent isomers, N1 and N10, with different stabilities in different media. N1, the keto−thione tautomer, is the most stable in gas phase (ΔG o 298 ≈ 8−9 kcal/mol lower than the second-most stable isomer, depending on the method of calculation used). However, experimental spectroscopic data supported by the theoretical calculations strongly suggest an equilibrium between the tautomers N1 and N10 in methanol solution, where enolization of one keto group is produced by proton transfer from the methylene group, which is more acidic than the NH groups. With the use of the polarizable continuum method for simulating solvents, N10 is predicted to be even more stable than N1 by ΔG o 298 ≈ 1 kcal/mol in methanol. On the other hand, the IR spectrum of the solid can be best explained by assuming that only N10 is present, a fact also supported by the observation that the IR spectrum of TBA absorbed onto gold nanoparticles can be explained by a larger ratio of [N10]/[N1] than that present in methanolic solution. Isomerization of N1⇋ N10 can be explained by intervention of the solvent, proceeding faster in methanol solutions than in DMSO, where it is nevertheless observed after a time, according to the 13C NMR spectra. Our experiments support absorption of TBA onto gold nanoparticles through S−Au and N−Au interactions, with the preeminence of a N10-like enol structure. The experiments also demonstrate that the synthesized TBA-coated gold nanoparticles can autoassociate by hydrogen bonding to form larger structures. This same H-bonding capacity also assures that these coated nanoparticles act as thistles toward proteins in solution, binding them strongly, presumably not by chemical reaction but by a network of hydrogen bonds.

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A simple composite scheme is presented and benchmarked against the 38 reactions in Truhlar's HTBH38/08 and NHTBH38/08 databases. Mean unsigned deviation (MUD) for the complete set of 68 independent barriers is 0.40 kcal mol À1 , compared to 1.31 kcal mol À1 for G4 and 1.62 kcal mol À1 for the M06-2X-D3 method. The MUD of the new scheme on the barriers in the DBH24/08 subset (12 out of the 38 reactions in the other sets) is 0.27 kcal mol À1 , better than that obtained at the expensive CCSD(T,full)/aug-cc-pCV(T+d)Z level (0.46 kcal mol À1 ) and comparable to the most exact (and costly) Wn calculations (MUD = 0.14 kcal mol À1 ). The method was further tested against a subset of reactions, for which the geometry and energies of all species were determined at the much more demanding CCSD(T)-F12//pVQZ-F12 level. The SVECV-f12 procedure on this database results in RMSE and MUD values of only 0.21 and 0.16 kcal mol À1 .