1,721,195 research outputs found
Explicitly correlated coupled cluster calculations of the dissociation energies and barriers to concerted hydrogen exchange of (HF)n oligomers (n=2,3,4,5)
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Basis-set convergence of correlated calculations on water
No full textThe basis-set convergence of the electronic correlation energy in the water molecule is investigated at the second-order Mà ̧ller-Plesset level and at the coupled-cluster singles-and-doubles level with and without perturbative triples corrections applied. The basis-set limits of the correlation energy are established to within 2 mEhby means of (1) extrapolations from sequences of calculations using correlation-consistent basis sets and (2) from explicitly correlated calculations employing terms linear in the interelectronic distances rij. For the extrapolations to the basis-set limit of the correlation energies, fits of the form a + bX-3(where X is two for double-zeta sets, three for triple-zeta sets, etc.) are found to be useful. CCSD(T) calculations involving as many as 492 atomic orbitals are reported. © 1997 American Institute of Physics
Origin of the Argon Nanocoating Shift in the OH Stretching Fundamental of n-Propanol: A Combined Experimental and Quantum Chemical Study
No full textSupersonic jet Raman spectroscopy reveals an increasing bathochromic shift of the OH stretching vibration in the most stable conformation of propanol with increasing extent of argon nanocoating. It falls short of the bulk matrix limit of 17 cm(-1) even at large nozzle distances. Quantum chemical harmonic frequency calculations up to the CCSD(T) level show that this shift cannot be accounted for by individual Ar atoms or even a first solvation layer but instead requires several layers of Ar atoms around the molecule to be explained. It is shown that the stability of Ar-propanol clusters correlates with the number of close O and C contacts to the Ar and that bathochromic shifts are largely caused by backbone solvation. Hydrogen-bonding OH solvation tends to slightly increase the OH stretching frequency but is very sensitive to the computational level.Fonds der Chemischen Industrie; DFG [Su 121/2
Tethering for Selective Synthesis of 2,2′-Biphenols : the Acetal Method
No full text2,2'-Biphenols are a large and diverse group of compounds with exceptional properties both as ligands and bioactive agents. Traditional methods for their synthesis by oxidative dimerisation are often problematic and lead to mixtures of ortho- and para-connected regioisomers. To compound these issues, an intermolecular dimerisation strategy is often inappropriate for the synthesis of heterodimers. The ‘acetal method’ provides a solution for these problems: stepwise tethering of two monomeric phenols enables heterodimer synthesis, enforces ortho regioselectivity and allows relatively facile and selective intramolecular reactions to take place. The resulting dibenzo[1,3]dioxepines have been analysed by quantum chemical calculations to obtain information about the activation barrier for ring flip between the enantiomers. Hydrolytic removal of the dioxepine acetal unit revealed the 2,2′-biphenol target
Spin flipping in ring-coupled-cluster-doubles theory
No full textWe report a critical analysis and comparison of a variety of random-phase-approximation (RPA) based
approaches to determine the electronic ground-state energy. Interrelations between RPA variants are
examined by numerical example with particular attention paid to the role of spin-flipped excitations
and the behaviour of the adiabatic-connection integrands where appropriate. In general, it is found that
RPA variants that include Hartree–Fock exchange contributions are unsuitable as generally applicable
methods for the determination of correlation energies. Of the remaining methods only the direct RPA
and RPA with second-order screened exchange are recommended for general use
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