Designing ring-current patterns: [10,5]-coronene, a circulene with inverted rim and hub currents.

In an axial magnetic field, coronene, corannulene and kekulene support disjoint paratropic-hub and diatropic-rim ring currents. Bond-order and orbital arguments suggest [10,5]-coronene, C30H10, comprising 10 fused pentagons around a central decagon, as a system that should support an inverted diatropic-hub/paratropic-rim pattern of induced currents. The proposal is verified by ipsocentric ab initio mapping of currents, reproduced with the economical pseudo-pi method, thus validating a powerful toolkit for design of (induced) molecular magnets.

Determination of 13C/12C carbon isotope ratio.

Isotopomers 12CO2 and 13CO2 absorbed into polystyrene films provide narrow, sharp, and well-resolved IR absorption bands for the nu3 antisymmetric stretching mode. This is exploited to set up an inexpensive FT-IR-based method for the measurement of the carbon isotope ratio. Accuracy of 2.5 per thousand delta13C units is readily achieved already at a low resolution of 2 cm(-1).

Harmonic and anharmonic contributions to parity-violating vibrational frequency difference between enantiomers of chiral molecules.

A general perturbative procedure for the computation of harmonic and anharmonic contributions to parity-violating vibrational shifts is introduced and applied to PHBrF and AsHBrF. The results point out the importance of both diagonal and off-diagonal anharmonic contributions and indicate that some parity-violating shift of AsHBrF approaches the resolution forecasted for next generation experiments. The proposed approach is sufficiently general and computationally effective to allow studies of similar and larger molecular systems.

Theoretical determination of parity-violating vibrational frequency differences between the enantiomers of chiral molecules.

A perturbation treatment has been used to compute the leading first- and second-order parity-violating corrections to the vibrational energy levels of a chiral molecule. Assuming the molecular equilibrium geometry as expansion point of both parity-violating and parity-conserving potential-energy surfaces, it is shown that these corrections, i.e., harmonic and anharmonic contributions, are of the same order of magnitude and that none of them can be neglected for a realistic determination of vibrational frequency differences. Numerical tests based on ab initio MP2 force fields and quantum-relativistic calculations of the parity-violating potential for each normal mode of PHBrF and AsHBrF molecules confirm this conclusion. In particular, it is shown that a normal mode of AsHBrF is characterized by one of the largest vibrational frequency difference ever found--the prediction is approximately 0.1 Hz--only one order of magnitude less than the presumed resolution limit of current experimental investigations.

Local aromaticity of the six-membered rings in pyracylene. A difficult case for the NICS indicator of aromaticity.

In this work, we have analyzed the local aromaticity of the six-membered rings (6-MRs) of planar and pyramidalized pyracylene species through the structurally based harmonic oscillator model of aromaticity (HOMA), the electronically based para-delocalization index (PDI), and the magnetic-based nucleus independent chemical shift (NICS) measurements, as well as with maps of ring current density. According to ring currents and PDI and HOMA indicators of aromaticity, there is a small reduction of local aromaticity in the 6-MRs of pyracylene with a bending of the molecule. In the case of NICS, the results depend on whether the NICS value is calculated at the center of the ring (NICS(0)) or at 1 A above (NICS(1)(out)) or below (NICS(1)(in)) the ring plane. While NICS(1)(out) values also indicate a slight decrease of aromaticity with bending, NICS(0) and NICS(1)(in) wrongly point out a large increase of aromaticity upon distortion. We have demonstrated that the NICS(0) reduction in the 6-MRs of pyracylene upon bending is due to (a) a strong reduction of the paratropic currents in 5-MRs and (b) the fact that, due to the distortion, the paratropic currents point their effects in other directions.

Ab initio calculation of optical rotatory dispersion (ORD) curves: a simple and reliable approach to the assignment of the molecular absolute configuration.

In this paper, both Hartree-Fock (HF) and density functional theory (DFT) methods have been used to make ab initio calculations of the optical rotatory power of selected molecules at several wavelengths; that is, part of the optical rotatory dispersion (ORD) curve has been predicted. This approach constitutes a new, simple, and reliable method to assign the molecular absolute configuration, at least for rigid molecules such as those studied in the present work. In fact, in this way, it is possible to overcome the difficulties connected to some relevant cases, in particular that of (-)-beta-pinene, for which even a very high-level (DFT/B3LYP/6-311++G(2d,2p)) calculation affords the wrong sign of the optical rotation at 633 nm. On the contrary, the predicted ORD curve, even using small basis sets, reproduces (below 400 nm) the experimental trend well, allowing for the correct configurational assignment. This result clearly shows that to have a reliable configurational assignment the comparison between experimental and predicted rotation values must be carried out at different wavelengths and not at a single frequency. The reason for this is that working at wavelengths approaching the absorption maximum the [alpha](lambda) values become larger and their prediction becomes more reliable. Coupling the use of an inexpensive instrument (a polarimeter working at a few wavelengths) with the use of a DFT-calculation package can also allow the experimental organic chemist to arrive, quickly and reliably, at the assignment of the molecular absolute configuration.

Are ring currents still useful to rationalize the benzene proton magnetic shielding?

[structure: see text] The conventional interpretation of proton NMR chemical shifts is supported by large basis set ab initio quantum mechanical calculations. The benzene protons are predicted to lie within the deshielding zone defined in terms of the out-of-plane magnetic shielding domain. However, ring currents by themselves are not sufficient to account quantitatively for the observed benzene proton downfield chemical shift. sigma-Electron contributions must also be taken into account. The conventional explanation for the ethyne proton chemical shift is valid.

Assignment of the molecular absolute configuration through the ab initio Hartree-Fock calculation of the optical rotation: can the circular dichroism data help in reducing basis set requirements?

In this paper the calculation of the optical rotation (OR) of some rigid organic molecules, using the Hartree-Fock method with small (6-31G, DZP) basis sets, has been studied thoroughly to carefully evaluate the scope and limitations of this method, previously introduced by other authors. Calculations on test molecules (compounds 1-13) together with a careful analysis of their CD spectra allow a simple criterion for the reliability of this approach to be formulated: for unsaturated and/or aromatic (i.e., absorbing in the near-UV region) molecules, if the [alpha](D) is quantitatively determined by the lowest energy Cotton effect (at wavelengths >220 nm), then the HF/6-31G result is reliable. The usefulness of this method for the experimental organic chemist has been further demonstrated because the OR (sign and order of magnitude) of compounds 14-19 (i.e., large molecules having considerable interest in organic chemistry), which fulfill the above criterion and for which an extended basis set treatment is not feasible owing to their size, is correctly predicted.