Gauge invariance and average excitation energy approximation in the theory of nuclear magnetic shielding

The effects of three-body non-additivities on the most stable structures of Ar⁺ _n (3 ≤ n ≤ 27) clusters have been investigated in an extended diatomics-in-molecules (DIM) approach. Non-additive interaction terms for both the neutral and the ionic trimer have been included explicitly in the DIM Hamiltonian. Structural rearrangement is observed for a number of cluster sizes due to induced dipole—induced dipole interactions.     

Cooperative and anticooperative mixed trimers of HCl and methanol

In the hydrogen-bonded complex between two HCl molecules and one methanol unit, there are two isomers with similar energies. A cyclic isomer profits from hydrogen bond cooperativity, whereas a converging structure, in which both HCl molecules bind to the lone pairs of methanol, profits from the good acceptor quality of methanol. The latter compensates for anticooperative three-body forces, which arise from the competition of the HCl units for acceptor electron density. This theoretical prediction is supported by FT-IR spectra of supersonic jet expansions, in which both mixed trimers are tentatively assigned along with the mixed dimer of HCl and methanol based on their O–H and Cl–H stretching bands.     

Interaction in the ternary complexes of HCl-methanol-X, X = H2O or NH3: Ab initio calculations and on-the-fly molecular dynamics

Dynamics, structures, energetics, and vibrational spectra of the ternary complexes of hydrogen chloride with either methanol and water or methanol and ammonia were investigated by on-the-fly molecular dynamics and ab initio and density functional theory (DFT) with aug-cc-pvDZ basis sets. Addition of CH3OH to the HCl-NH3 system catalyzes the proton transfer from HCl to NH3. However, the dynamics of the system show that the proton is not localized on NH3; rather, it is shared between N and Cl.     

Control of the length of microfibers

Uniform polymeric microfibers of prescribed lengths were synthesized in microfluidic devices using two different approaches--valve actuation and pulses of ultraviolet (UV) light. The more versatile valve approach was employed to demonstrate control of the length of the microfiber as a function of the frequency of valve actuation.     

Transfer of electron density as a result of hydrogen bond formation— A case of charged complexes

Previous investigation of transfer of electron density accompanying hydrogen bond formation has been extended to complexes between positively charged donors and neutral acceptors, as well as to the complexes between a neutral donor and a negatively charged acceptor molecules. The amount of transferred electron density from acceptor to donor for the charged complexes may be adequately described by the same exponential dependence on the equilibrium distance between the hydrogen atom and the nearest atom of the acceptor molecule as it was found for neutral complexes. Relation of the H‐bond energy to electron density at the H‐bond critical point was dependent on the sign of Laplacian of the electron density. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Exact derivation of the single-excitation model in the Hartree-Fock perturbation theory

Neutron diffraction studies have been made for D₂O water in two high surface area silica samples, Spherisorb and Gasil. The results show that the structural arrangement is similar to that of bulk water and that the modification due to the interface is restricted to a range of the order of 10 Å or less. Measurements also indicate that the dispersed water undergoes significant structural changes with temperature variation but the behaviour is similar to that of the bulk liquid.

The differences observed in the diffraction patterns can be partially described in terms of diffraction broadening due to the small volume of the dispersed water, but there remains a significant variation which appears to be related to the hydrogen bonding interactions.     

Unlinked cluster effects in limited CI calculations of molecular properties

The unlinked cluster effects in limited CI calculations of dipole moments and polarizabilities are estimated by using the approximate corrections due to Davidson and Siegbahn. The results obtained for FH, H₂O, NH₃, and CH₄ indicate that the limited CI results for molecular electric properties need to be corrected for the erratic treatment of unlinked clusters.     

Near-degeneracy effects and efficiency of MBPT calculations of molecular properties. Polarizability curve of H2

The polarizability curve of H₂ is calculated by using the finite-field perturbation method. All self-consistency effects are accounted for at the HF level and many-body perturbation theory (MBPT) is used to evaluate the correlation contributions. Using a single HF determinant as a reference in MBPT calculations makes the near-degeneracy effects of essential importance on increasing the interatomic distance. Nevertheless, applying the MBPT scheme with appropriate fourth-order terms gives nearly exact values of both components of the polarizability tensor for interatomic distances up to ≈3.6 au.     

Polymeric adducts of rhodium(II) tetraacetate with aliphatic diamines: natural abundance 13C and 15N CPMAS NMR investigations

Complexation properties of dimeric rhodium(II) tetracarboxylates have been utilised in chemistry, spectroscopy and organic synthesis. Particularly, the combination of these rhodium salts with multifunctional ligands results in the formation of coordination polymers, and these are of interest because of their gas‐occlusion properties. In the present work, the polymeric adducts of rhodium(II) tetraacetate with flexible ligands exhibiting conformational variety, ethane‐1,2‐diamine, propane‐1,3‐diamine and their N,N′‐dimethyl‐ and N,N,N′,N′‐tetramethyl derivatives, have been investigated by means of elemental analysis, ¹³C CPMAS NMR, ¹⁵N CPMAS NMR and density functional theory modelling. Elemental analysis and NMR spectra indicated the axial coordination mode and regular structures of (1 : 1)_n oligomeric chains in the case of adducts of ethane‐1,2‐diamine, N,N′‐dimethylethane‐1,2‐diamine N,N,N′,N′‐tetramethylethane‐1,2‐diamine and N,N,N′,N′‐tetramethylpropane‐1,3‐diamine. Propane‐1,3‐diamine and N,N′‐dimethylpropane‐1,3‐diamine tended to form heterogeneous materials, composed of oligomeric (1 : 1)_n chains and the additive of dirhodium units containing equatorially bonded ligands. Experimental findings have been supported by density functional theory modelling of some hypothetical structures and gauge‐invariant atomic orbital calculations of NMR chemical shifts. Copyright © 2013 John Wiley & Sons, Ltd.     

Explaining the derivation and functioning of the exact infinite-order two-component Dirac theory in terms of the Hückel model

The transition from the four-component Dirac theory to the exact two-component formalism is considered by using a simple algebraic model of the Dirac hamiltonian. This model is found to correspond to the four-center Hückel matrix and permits to replace the complex operator algebra by very easy matrix operations. This mapping of operators onto number matrices shows how the exact two-component relativistic hamiltonians are derived. It also explains certain conceptual aspects of the relation between four-component and two-component hamiltonians and their eigenfunctions. The generalized Hückel model of a four-center heteroatomic π-electron system can be used to analytically analyze the essential features of a variety of different exact and approximate two-component methods of relativistic quantum chemistry. This article is dedicated to Professor Tadeusz Marek Krygowski on the occasion of his 70th birthday.