Guglielmo Monaco

Ring Current Model and Anisotropic Magnetic Response of Cyclopropane

Three-dimensional models of the quantum mechanical current density, induced in the electron cloud of the cyclopropane molecule by a uniform magnetic field applied either along the C3 or the C2 symmetry axes (indicated by B∥ and B⊥, respectively), have been constructed via extended calculations. These models of near Hartree−Fock quality, previously shown to provide a good agreement between computed and observed values of magnetic tensors, have been used to interpret the magnitude of the diagonal components of susceptibility (χ), nuclear shielding of carbon (σC) and hydrogen (σH), and shielding at the center of mass (σCM). The source of the exceptionally large in-plane component σ⊥CM, dominating the anomalous average σavCM, is shown to be a strong delocalized current flowing around the methylene moieties and the noncyclic CH2−CH2 fragment. The total current strength for a magnetic field applied in the direction of a C2 symmetry axis is 15.7 nA/T, approximately 1.5 times larger than that calculated for B∥. T...

Beyond NICS: estimation of the magnetotropicity of inorganic unsaturated planar rings

A simple classical model of magnetic-field induced electron flow is used to evaluate the ring current strength for a few inorganic monocyclic compounds: B(3)H(3)N(3), B(3)H(3)O(3), P(6), N(6), Si(6)H(6), N, Al and H(6). It is shown that, for these neutral and charged systems, sustaining delocalized electron currents in the presence of a magnetic field B(ext) orthogonal to the σ(h) plane, the out-of-plane component of the nuclear magnetic shielding along the central axis is connected to the out-of-plane magnetizability by a simple equation, involving the radius of an average loop of current. A novel estimate of this effective radius is provided. Reliable ring current susceptibilities (that is, current strengths) can be evaluated by a simple relationship, using the out-of-plane components of nuclear shielding and magnetizability tensors. The accuracy of the current susceptibilities calculated by the classical model is established by comparison with corresponding ab initio estimates obtained by integrating the quantum mechanical current-density vector field. The out-of-plane components of nuclear shielding and magnetizability are both strongly biased by the molecular geometry. Their combined use to estimate the ring current susceptibility offers a quantifier of magnetotropicity more reliable than (i) the ξ(∥) out-of-plane component of magnetizability, (ii) the σ(∥)(CM) out-of-plane component of the magnetic shielding at the center of mass, widely reported as NICS(∥)(0) = -σ(∥)(CM). The inadequacy of these commonly adopted magnetotropicity measures is demonstrated by comparing a set of related molecules, C(6)H(6) and Si(6)H(6), N(6) and P(6).

Absolute configuration assignment of inherently chiral calix[4]arenes using DFT calculations of chiroptical properties.

The racemate of an inherently chiral meta-substituted calix[4]arene derivative 3 has been resolved via enantioselective HPLC. Measured optical rotation dispersion and electronic circular dichroism have been compared with DFT theoretical predictions. The comparison indicates that the absolute configuration of the dextrorotatory enantiomer (+)-3 is cS. The procedure has been successfully tested against a literature precedent confirming the validity of the method.

On the additivity of current density in polycyclic aromatic hydrocarbons

Calculations of the pi current density for polycyclic aromatic hydrocarbons placed in a uniform magnetic field reveal in some cases a substantial localization on subunits. This localization can be anticipated either for molecules with a factorizable Kekule count K, in light of some theoretical models of ring currents, or for system with proper symmetry, in light of magnetic group theory. We have addressed the problem of whether the localization is compatible with a description of the current density field as a sum of current density fields, studying the sum of two purely rotational fields. When this general model is specialized with the parameters taken from benzene ring current, it turns out that two corotating purely rotational fields separated by a distance comparable to a chemical bond must be separated by a saddle point. We have looked for the occurrence of this criterion in K-factorizable molecules, chosen according to a novel corollary to Kasteleyns theorem, in coronenes, which have patterns localized by symmetry and in some further systems reported in literature. For those systems already described to have an additive current density pattern, the separating bonds do host saddle points, which are thus effective signatures of additivity.

Assessment of Ring Current Models for Monocycles

Interpretation of both measured and computed values of chemical shift in (poly)cyclic molecules is widely based on ring current models (RCMs). Few improvements have been considered to date for the reference RCM, consisting of an infinitely thin circular loop of current (ICLOC). In this paper, six analytical RCMs (three of which are proposed for the first time) have been discussed, and they have been graded by comparing their ability to reproduce the π and σ contributions to the ab initio ring current strengths and to the scans of the parallel component of the magnetic shielding (σ∥ = -NICS∥) for a set of 33 organic and inorganic monocycles. For π currents, two vertically displaced ICLOCs (ICLOC2 model) are the preferred choice to have a very good reproduction of the scans, while the toroidal widening of the loops, proposed long ago by Farnum and Wilcox, is unable to give a significant improvement. For σ currents, the best model is the novel ICLOC2C, formed by two concentric ICLOCs. An off-plane extremum in the NICS∥ scan is not a general indication for the presence or absence of π-aromaticity. In agreement with the models proposed here, such an extremum is absent in the NICS∥,π scans of large rings; however, it characterizes most of the NICS∥,π scans of small rings and can also appear in NICS∥,σ scans.

Three contra-rotating currents from a rational design of polycyclic aromatic hydrocarbons: altan-corannulene and altan-coronene.

Both the ab initio expression of the current density within the ipsocentric approach and conjugated circuit models indicate that placing an unsaturated hydrocarbon inside a [4n]annulene, in such a way that outgoing C-H bonds are substituted by C-C bonds to alternating carbon atoms of the annulene, leads to an homologue altan-molecule whose perimeter is expected to preserve the paratropic circulation of the parent annulene. Computations of current on the novel altan-corannulene and altan-coronene revealed unprecedented patterns of three contra-rotating paratropic/diatropic/paratropic circulations. Graph-theoretical methods have been used to highlight the peculiar topology of these altan-molecules.

Different zeroes of interaction energies as the cause of opposite results on the stabilizing nature of C-H···O intramolecular interactions.

The interaction energy of the C-H···O intramolecular interaction is estimated by several methods for a large group of systems possessing a quasi-cyclic six-membered ring. In the case of the geometry corrected method (GCM), the related rotamers method (RRM), and Espinosas method (EM), the linear correlations between interaction energies and the electron density at the bond critical point have close slopes. The first and the last two methods yield almost systematically opposite results concerning the stabilizing/destabilizing character of the interaction, and their main difference is their zero of the interaction energy. An investigation on the limitations of reference energies has led to the introduction of the geometry corrected related rotamers method (GCRRM), estimating both stabilizing and destabilizing C-H···O interactions. An extension of EM is proposed.

Quantitative Indicators of Bond Current Susceptibility

A novel method is presented for the determination of bond current susceptibilities from ab initio calculations. Integration over domains in a plane perpendicular to the bond investigated allows defining contributions to the bond current susceptibilities for different orientation of the inducing magnetic field. Moreover the ipsocentric method has been used to separate the current susceptibility in σ and π contributions as well as in terms of individual spectral contributions. Examples of application of the method to benzene, ethylene and some simple linear molecules are reported.

Delocalized currents without a ring of bonded atoms: strong delocalized electron currents induced by magnetic fields in noncyclic molecules.

Some noncyclic small molecules, electrically neutral or charged, sustain interatomic electronic currents in the presence of a stationary, spatially uniform magnetic field. The existence of fairly large delocalized electron flow is demonstrated in H₂O, BH₃, NH₃, CH₄, CH₃-CH₃, H₃O⁺, CH₃⁺, and NH₄⁺, by plots of quantum mechanical current density. Convincing quantitative evidence is arrived at by current strengths, defined via a flux integral of the ab initio current density. Application of a simple ring current model shows that the delocalized current strengths account for the out-of-plane component of the magnetic shielding tensor along the symmetry axis. A definition of delocalized electron current as a current flowing along a closed loop containing three or more atoms is discussed.

Analysis of the Nucleus-Independent Chemical Shifts of [10]Cyclophenacene: Is It an Aromatic or Antiaromatic Molecule?

[10]Cyclophenacene is an important synthetic target that shows a pair of nucleus-independent chemical shift (NICS) values for the center of mass and six-membered rings typical of an aromatic species. This is found in contrast with the global paratropic current density induced by a magnetic field parallel to the main symmetry axis. This apparent contradiction has been analyzed by studying the tensor character of the magnetic response. It turns out that the molecule displays two characters, one paratropic (antiaromatic) and another one diatropic (aromatic), depending on the orientation of the inducing magnetic field. The paratropic response, which cannot be recognized from the NICS values, is associated with a well-defined destabilization of the belt closure, as witnessed by homodesmotic reactions. A scalar measure of magnetic aromaticity, the field-independent current strength, has been introduced, which allows us to reach the conclusion that [10]cyclophenacene is indeed an aromatic molecule, although it is significantly affected by the paratropic response.