Thermal Corrections (thermal + correction)

Distribution by Scientific Domains

Selected Abstracts

Keto-enol tautomerism in linear and cyclic ,-diketones: A DFT study in vacuo and in solution

Giuliano Alagona
Abstract DFT geometry optimizations have been performed at the B3LYP/6-31G* level in the gas phase and at the IEF-PCM/B3LYP/6-31G* level in tetrahydrofuran (THF) and aqueous solutions using scaled radii for the diketo and ketoenol forms of acetylacetone and cyclohexanedione. To evaluate basis set effects, starting from the aforementioned minima, the 6-311++G** optimized structures have been obtained. A number of complexes of both systems including one explicit water molecule have been considered up to the B3LYP/6-311++G** level, for cyclohexanedione taking into account the B3LYP/6-31G* basis set superposition errors as well. The diketo,ketoenol interconversion mechanisms have been investigated at the B3LYP/6-31G* level in vacuo. Interestingly, the geometric constraint due to the presence of the ring facilitates the description of the reaction mechanism in cyclohexanedione. Despite the very different flexibility of the two systems that in the case of acetylacetone prevents a straightforward interconversion of the diketo to the most stable of its ketoenol forms, both reactions occur with a very high barrier (about 62,63 kcal/mol), unaffected by continuum solvents, that decreases by 2.5,3.5 kcal/mol after the inclusion of thermal corrections. The barriers are almost halved, becoming ,31,35 kcal/mol, for the addition of a single water molecule according to various model reaction paths. Thermal corrections are limited (0.8,1.6 kcal/mol) for those adducts. The formation of a 1,1-diol, explored in the case of acetylacetone, might facilitate the obtainment of the most stable diketo conformation, featuring the carbonyl groups in distinct orientations. Inclusion of dispersion and basis set effects via the G2MP2 protocol does not alter the relative stability of both system tautomers. In contrast, the G2MP2 interconversion barriers for the isolated systems in vacuo are close to the B3LYP ones, whereas they turn out to be somewhat higher than the free energy-based B3LYP barriers in the presence of a catalytic water molecule. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]

OH hydrogen abstraction reactions from alanine and glycine: A quantum mechanical approach

Annia Galano
Abstract Density functional theory (B3LYP and BHandHLYP) and unrestricted second-order Møller,Plesset (MP2) calculations have been performed using 3-21G, 6-31G(d,p), and 6-311 G(2d,2p) basis sets, to study the OH hydrogen abstraction reaction from alanine and glycine. The structures of the different stationary points are discussed. Ring-like structures are found for all the transition states. Reaction profiles are modeled including the formation of prereactive complexes, and very low or negative net energy barriers are obtained depending on the method and on the reacting site. ZPE and thermal corrections to the energy for all the species, and BSSE corrections for B3LYP activation energies are included. A complex mechanism involving the formation of a prereactive complex is proposed, and the rate coefficients for the overall reactions are calculated using classical transition state theory. The predicted values of the rate coefficients are 3.54×108 L,mol,1,s,1 for glycine and 1.38×109 L,mol,1,s,1 for alanine. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1138,1153, 2001 [source]

Quantum and thermal effects in the double exchange ferromagnet

N. Shannon
Abstract The physics of the ferromagnetic phase of the "double exchange" model has been widely discussed in the context of the CMR manganites. Usually, the double exchange ferromagnet is treated is classically, by mapping it onto an effective Heisenberg model. However this mapping does not permit a correct treatment of quantum or thermal fluctuation effects, and the results obtained lack many of the interesting features seen in experiments on the manganites. Here we outline a new analytic approach to systematically evaluating quantum and thermal corrections to the magnetic and electronic properties of the double exchange ferromagnet. [source]

Distinct Hydroxy-Radical-Induced Damage of 3,-Uridine Monophosphate in RNA: A Theoretical Study

bo Zhang Dr.
Abstract Cutting ties: Strand scission and base release in hydroxy-radical adducts of 3,-uridine monophosphate (UMP) have been explored by using density functional theory. The presence of the ribose 2,-OH group and the resultant formation of low-barrier hydrogen bonds with oxygen atoms of the 3,-phosphate linkage are highly important for hydrogen transfer and the subsequent bond-breakage reactions (see picture). RNA strand scission and base release in 3,-uridine monophosphate (UMP), induced by OH radical addition to uracil, is studied at the DFT B3LYP/6-31+G(d,p) level in the gas phase and in solution. In particular, the mechanism of hydrogen-atom transfer subsequent to radical formation, from C2, on the sugar to the C6 site on the base, is explored. The barriers of (C2,-)H2,a abstraction by the C6 radical site range from 11.2 to 20.0,kcal,mol,1 in the gas phase and 14.1 to 21.0,kcal,mol,1 in aqueous solution, indicating that the local surrounding governs the hydrogen-abstraction reaction in a stereoselective way. The calculated N1C1, (N1,glycosidic bond) and ,-phosphate bond strengths show that homolytic and heterolytic bond-breaking processes are largely favored in each case, respectively. The barrier for ,-phosphate bond rupture is approximately 3.2,4.0,kcal,mol,1 and is preferred by 8,12,kcal,mol,1 over N1,glycosidic bond cleavage in both the gas phase and solution. The ,-phosphate bond-rupture reactions are exothermal in the gas phase and solution, whereas N1C1, bond-rupture reactions require both solvation and thermal corrections at 298,K to be energetically favored. The presence of the ribose 2,-OH group and its formation of low-barrier hydrogen bonds with oxygen atoms of the 3,-phosphate linkage are highly important for hydrogen transfer and the subsequent bond-breakage reactions. [source]