Computation of the acetone ultraviolet spectrum in gas phase and in aqueous solution by a mixed discrete/continuum model

The ultraviolet spectrum of acetone in vacuum and in aqueous solution has been computed by different quantum mechanical methods coupled to the polarizable continuum model (PCM) for describing bulk solvent effects. The results in vacuo show that the time-dependent density functional theory (TDDFT) approach using the PBEO functional reproduces quite well the result obtained at the CASPT2 level. Supermolecule computations confirm that water molecules belonging to the first shell of polar groups (here the carbonyl oxygen) must be explicitly included in the quantum mechanical treatment, whereas the effect of other solvent molecules (which is far from being negligible) can be reliably described by the PCM. Moreover, statistical averaging effects have been taken into account by performing canonical molecular dynamics (MD) simulations followed by TDDFT quantum mechanical computations on representative clusters of increasing dimensions immersed in a polarizable continuum. The results show that the combined MD/DFT/PCM approach is reliable and effective, although the performances of the force field used in the MD simulations must be further investigated.     

Finite perturbation theory-density functional theory calculation of the spin-dipolar contribution to NMR spin-spin coupling constants

In this work an implementation of the FPT-DFT approach for calculating the spin-dipolar contribution to NMR spin-spin coupling constants is presented. This method was tested in a set of small molecules, giving results in excellent agreement when comparing them with values taken from the literature, which were obtained with state-of-the-art calculations. To obtain an insight into the relative importance of the spin-dipolar contribution in unsaturated compounds, calculations of J(F,C), J(F,F) and J(F,H) couplings in 1,2-, 1,3-, and 1,4-difluorobenzenes were performed. An important spin-dipolar contribution to ³ J(F, F) and ⁵ J(F,F) was found, suggesting that this term might be important in some cases. When performing DFT calculations the non-singlet instabilities usually found in unsaturated compounds are overcome.     

Natural J coupling (NJC) analysis of the electron lone pair effect on NMR couplings: 2. The anomeric effects on 1 J (C,H) couplings and its dependence on solvent

The electronic origin of the influence of the anomeric effect (negative hyperconjugative interaction, NHI) on the Fermi contact (FC) term of ¹ J(C, H) couplings has been studied from a theoretical point of view at the DFT-B3LYP level. The HN=CH₂, molecule was chosen as the primary model compound, in which both FC ¹ J(C, H) couplings were decomposed into bond contributions with the natural J coupling dissection approach (NJC). Differences between the ¹ J (C, H)^FC couplings for C——H bonds in synperiplanar and antiperiplanar orientations with respect to the nitrogen non-bonding electron pair closely follow the experimental trend. They are made up chiefly of three NJC contributions: ‘bond’, ‘direct lone pair’ and the ‘carbon-core orbitals’. The NHI influence on these terms was studied by applying the natural bond orbital (NBO) deletion procedure to the charge transfer interaction into the antiperiplanar (C——H) antibond (n(N)→(C——H)?) prior to the NJC dissection calculation. The dielectric solvation effect on both the total FC terms and the respective NJC contributions was estimated by carrying out the calculations using the polarization continuum model. Inhibition of the anomeric effect is evident when the solvent polarity is increased. NHI saturates rapidly with increasing solvent dielectric. Specific solute-solvent interaction effects on ¹ J(C, H) couplings were estimated by evaluating molecular complex models of the form CH₂=HN…S (S = H₂O and DMSO).     

Special Boundary Integral Equations for Approximate Solution of Potential Problems in Three-dimensional Regions with Slender Cavities of Circular Cross-section

A special boundary integral method is developed for solvingpotential problems in a general three-dimensional region withslender internal cavities of circular cross-section. The solutionon the boundary of each cavity is assumed to be axisymmetricso that the surface integrals on a cavity boundary may be reducedto contour integrals along the centre line of the cavity. Specialintegral equations are introduced to determine the solutionalong each cavity. Although the contour integrals in these newequations are never singular, they have a nearly singular characterwhich gives them computational advantages similar to traditionalboundary integral equations with out the danger of ill-conditioningcaused by the strongly contrasting length scales introducedby the slender cavities. For the special case of parallel toroidalcavities, the method gives results with accuracy comparableto previous perturbation methods. The numerical characteristicsof the new integral equations are explored by solving the problemsof two perpendicular, interlocking tori and two perpendicular,finite cylindrical cavities, both in unbounded regions. Theequations exhibit excellent numerical characteristics over abroad range of parameters.     

ChemInform Abstract: Structure of the Ammonia-Boryl Radical (BH2NH3), an Inorganic Analogue of the Ethyl Radical. A Non Empirical Study

at the UHF-SCF level employing the 6-31*G basis set including complete geometry optimization on various possible conformations suggests that the distortion of the BH₂ group is higher than the corresponding distortion of the CH₂ group in the isoelectronic CH₂CH₃ and CH₂NH₃ radicals.