Search

Abstract: Ab initio calculations have been performed for both H-atom and Cl-atom abstraction reactions by the hydroxyl radical. Geometry optimization and vibrational frequencies analysis were performed for reactants, transition state and products at the MP2(full)/cc-pVTZ level of theory. On the basis of the ab initio data, the rate constants have been deduced over a wide temperature range, 200–2000 K, using conventional transition state theory, including Wigner tunneling correction and the hindered rotor approximation. Calculated transition state rate constants with Wigner tunneling correction are in good agreement with experimental results. The tunneling contribution becomes important at ambient temperatures, resulting in the exhibited non-Arrhenius behavior of the rate constant. [Copyright &y& Elsevier]

Abstract: In this work, magnetometry and EPR spectroscopy studies on the binuclear Mn(II) complex [Mn2{(OPPh2)2N}4] (1) established antiferromagnetic interactions between the two S =5/2 Mn(II) centers being at 3.378Å from each other. The magnitude of the J AB coupling constant, in the framework of the formalism, was determined to be −4.5±0.5cm−1, by the temperature dependence of the magnetic susceptibility of 1. This value is comparable with the one of the previously studied [Mn2{(OPPh2)[OP(OEt)2]N}4(H2O)2] complex (−3.5cm−1), which bears similar type of ligands. The antiferromagnetic nature of the coupling within the binuclear Mn2O4 core of 1 was unequivocally confirmed also by EPR studies on frozen CH2Cl2 solutions at various temperatures. Density functional broken-symmetry calculations confirmed the above experimental findings and provided J AB values in close agreement with the experiment. Comparisons with literature binuclear Co(II) or Mn(II) complexes were made. [Copyright &y& Elsevier]

Abstract: Ab initio calculations were conducted for both reaction pathways of the CH3Br and OH system. Geometry optimization and vibrational frequencies analysis were performed for reactants, pre-reaction complex, transition states, post-reaction complex and products at the MP2(full)/6-311+G(d,p), MP2(full)/6-311+G(3df,2pd) and MP2(full)/cc-pVTZ levels of theory. Energetic results were further refined by CCSD(T,full) calculations, using optimized geometric parameters of the MP2(full)/cc-pVTZ level, i.e., CCSD(T,full)/cc-pVTZ//MP2(full)/cc-pVTZ. Based on MP2(full)/cc-pVTZ and CCSD(T,full)/cc-pVTZ//MP2(full)/cc-pVTZ data, rate constants were deduced in the 200–2000K temperature range, using conventional transition state theory and Wigner tunneling correction. Calculated rate constants, including tunneling corrections, were found to be in excellent agreement with experimental results. [Copyright &y& Elsevier]

Dimethyl sulfoxide (DMSO) has a significant, multi-faceted role in medicine, pharmacy, and biology as well as in biophysical chemistry and catalysis. Its physical properties and impact on biomolecular structures still attract major scientific interest, especially the interactions of DMSO with biomolecular functional groups. In the present study, we shed light on the "isolated" carboxylic (-COOH) and amide (-NH) interactions in neat DMSO via 1 H NMR studies along with extensive theoretical approaches, i.e. molecular dynamics (MD) simulations, density functional theory (DFT), and ab initio calculations, applied on model compounds (i.e. acetic and benzoic acid, ethyl acetamidocyanoacetate). Both experimental and theoretical results show excellent agreement, thereby permitting the calculation of the association constants between the studied compounds and DMSO molecules. Our coupled MD simulations, DFT and ab initio calculations, and NMR spectroscopy results indicated that complex formation is entropically driven and DMSO molecules undergo multiple strong interactions with the studied molecules, particularly with the -COOH groups. The combined experimental and theoretical techniques unraveled the interactions of DMSO with the most abundant functional groups of peptides (i.e. peptide bonds, side chain and terminal carboxyl groups) in high detail, providing significant insights on the underlying thermodynamics driving these interactions. Moreover, the developed methodology for the analysis of the simulation results could serve as a template for future thermodynamic and kinetic studies of similar systems.

The one photon and the two photon S(1)<--S(0) spectra of jet-cooled p-ethynyltoluene have been measured for the first time, and a detailed vibronic analysis for both spectra has been attained. Mass analyzed resonance enhanced multiphoton ionization spectroscopy is the employed technique. In the one photon spectrum, the allowed component (origin and Franck-Condon bands) is much weaker than the forbidden component, and the same mechanisms as in the one photon spectrum of phenylacetylene are observed. The methyl torsional transitions are active. The 0(0) (0) band is at 35 483 cm(-1). The two photon spectrum is very strong and bears a resemblance to the two photon spectrum of phenylacetylene. The potential barrier of the methyl rotor in the S(1) state has been determined as V(6)=-12 cm(-1) with B(CH(3) )=5.55 cm(-1). Ab initio calculations, MP2(full)/cc-pVTZ and CAS/cc-pVTZ, have been implemented for the geometry optimization and the normal mode vibration computation in the S(0) and S(1) states.