Three-dimensional models of the quantum mechanical current density, induced in\ud the electron cloud of the cyclopropane molecule by a uniform magnetic field applied either along\ud the C3 or the C2 symmetry axes (indicated by B| and B⊥, respectively), have been constructed\ud via extended calculations. These models of near Hartree-Fock quality, previously shown to\ud provide a good agreement between computed and observed values of magnetic tensors, have\ud been used to interpret the magnitude of the diagonal components of susceptibility (), nuclear\ud shielding of carbon (σC) and hydrogen (σH), and shielding at the center of mass (σCM). The\ud source of the exceptionally large in-plane component σ⊥\ud CM, dominating the anomalous average\ud σav\ud CM, is shown to be a strong delocalized current flowing around the methylene moieties and\ud the noncyclic CH2-CH2 fragment. The total current strength for a magnetic field applied in the\ud direction of a C2 symmetry axis is 15.7 nA/T, approximately 1.5 times larger than that calculated\ud for B|. The largest component of the susceptibility is instead the out-of-plane |, which depends\ud on the intensity of the σ-electron currents and on the entire area enclosed within the loops that\ud they form about the C3 axis, all over its length. In a magnetic field perpendicular to the plane of\ud the carbon atoms, both H and C nuclei sit inside diatropic whirlpools, flowing within the sp3\ud hybrid orbital which form the C-H bonds and extending for several bohrs above and below the\ud σh plane. The average values and the anisotropy of carbon and proton shieldings are strongly\ud biased by the diamagnetic shift of the out-of-plane tensor components partially determined by\ud these vortices. The current density model of cyclopropane is revised according to these findings

Using long-range corrected density functional theory, the relationships between the electronic, magnetic, and nonlinear optical properties are drawn for two families of organic compounds, the dicyclopenta-fused acenes (DPAs) and the polyacenes (PAs), containing up to N = 12 fused rings. First, the longitudinal second hyperpolarizability (γ) of singlet DPAs is significantly enhanced with increasing system size, in comparison to PAs. This behavior is associated with an increase in the longitudinal spin polarization between the terminal five-membered rings of DPAs and is consistent with previous studies where γ is maximized for intermediate diradical character. The size dependence of the diradical character is also found to cause a hump in the γ/N evolution for singlet DPAs around N = 8. In fact, in the case of singlet PAs, the diradical characters y(0) and y(1), the various magnetic properties and the γ/N values vary monotonically with N, whereas for singlet DPAs, the shielding, the magnetizability, and the γ/N values exhibit extrema near N = 8 due to the appearance of transversal spin polarization in the middle six-membered rings in addition to the longitudinal spin polarization between the terminal five-membered rings. Moreover, it is shown that for singlet DPAs the longitudinal spin polarization (characterized by y(0)) is associated with the antiaromaticity (N ≤ 3) and the slight- or non-aromaticity (N ≥ 4) of the terminal five-membered rings, whereas the appearance of transversal spin-polarization (characterized by y(1)) is associated with the decrease in the aromaticity in the inner six-membered rings as shown for large PAs. Therefore, the exceptional behaviors in singlet DPAs for small N (N < 9) are caused by the increase in diradical character y(0) correlated with the anti-aromaticity or the slight-/non-aromaticity of terminal rings and the corresponding emergence of a global aromatic character. Such a relationship between the aromaticity/antiaromaticity and the diradical character is useful for designing real open-shell NLO molecules through the control of their diradical characters.

The behavior of benzene, 1,2-dihydro-1,2-azaborine, and borazine under a magnetic perturbation is theoretically evaluated to assess how the 1,2dihydro-1,2-azaborine molecule displays an intermediate character between its hydrocarbon (benzene) and fully boro-nitrogen (borazine) parents. The use of tridimensional visualization tools confirms that "single-value" magnetic properties do not allow one to properly evaluate the whole magnetic behavior and the "magnetic π-aromaticity" of these compounds, since the σ-electron response is also modified due to the introduction of polar B-N bond(s) and has a significant influence on the various magnetic properties.

A joint theoretical-experimental investigation has been carried out to unravel the details of the complexation of cations by fluoroionophores based on coumarin 343 and to interpret the modifications in the ligand and also in the coumarin structural, electronic, magnetic, and vibrational properties. It is confirmed that C343-dea (1) complexes the cations by both the lactone and the amide oxygen atoms whereas for C343-crown (2) and C343-dibenzocrown (3), the cations are complexed by the oxygen atoms of the lactone as well as those of the crown ligand. These complexations induce geometric modifications, which are delocalized over the coumarin backbone and are related to electronic reorganizations that modify the spectroscopic signatures. This paper analyzes these signatures and shows how they are related as well as how they can be used to monitor the complexation process. Upon complexation, the UV-visible absorption spectra display a bathochromic shift of the most intense electronic transition; this shift is generally larger for the most flexible compound 1 as well as when complexing divalent cations. NMR spectra bear many signatures of the complexation, of which the most remarkable ones are the large shielding of C(1) and the large deshieldings of C(9) and C(16). Additional makers of complexation are highlighted in the IR vibrational spectra, in particular the bands associated with the lactone and amide CO vibrations, which are downshifted when the corresponding CO is involved in the complexation mode and, otherwise, upshifted. A high degree of consistency characterizes the different geometrical, electronic, magnetic, and vibrational signatures, which substantiates the assignment of the modes of complexation in 1-3. In addition, the agreement between the experimental data and the theoretical values is rather satisfactory, in that it at least enables us to interpret the spectral signatures.

Three-dimensional models of the quantum-mechanical current density J(B) , induced in the electron cloud of the C(10)H(10) pentaprismane molecule by a magnetic field B applied along the C(5) (a C(2)) symmetry axis, orthogonal to the pentagonal (a rectangular) face, and denoted by B(‖) (B(⊥)), have been constructed. Predictions of near Hartree-Fock quality are reported for the diagonal components of magnetic tensors, magnetizability (ξ), nuclear shielding of carbon (σ(C)) and hydrogen (σ(H)), and virtual shielding at the center of mass (σ(CM)). The complicated spatial features of the induced electronic current-density field have been rationalized and compactly described via stagnation graphs that elucidate the details of its topological structure. A representation of J(B) is obtained by three-dimensional perspective plots and by planar maps visualizing phase portraits of electron flow in a series of molecular domains. Both streamline J(B) /|J(B) | and modulus |J(B) | are analyzed. These graphic tools illustrate the competition between diatropic and paratropic regimes which determine the magnitude of various components of magnetizability and magnetic shielding of hydrogen and carbon nuclei. Shielding density maps show that the differential Biot-Savart law explains magnetic shielding at hydrogen and carbon nuclei, and virtual shielding at ring and cage centers. Similarities and/or contrasting ring current effects on magnetotropicity are discussed by a comparison with triprismane C(6)H(6) and cubane C(8)H(8) .