Paramagnetic ring current effects in anti-aromatic structures subject to substitution/annelation quantified by spatial magnetic properties (TSNMRS)

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of the typically anti-aromatic cyclopentadienyl cation, cyclobutadiene, pentalene, s-indacene and of substituted/annelated analogues of the latter structures have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to visualize and quantify the dia(para)magnetic ring current effects in the studied compounds. The interplay of dia(para)magnetic ring current effects due to substitution/annelation caused by heavy exo-cyclic n,π-electron delocalization can be qualified.     

π-Electron ring currents and magnetic properties of porphyrin molecules in the MO LCAO SCF method

The coupled variant of double-parameter perturbation theory in the MO LCAO SCF method in the London approximation has been used for the calculation of π-electron current distributions in the molecules of porphin and its derivatives. The chemical shifts of¹H-NMR have been computed on the basis of calculations of ring currents and charge distributions. It is shown that π-electron ring currents are responsible for the dominant contribution to the shielding of protons. The theoretical and experimental values of proton chemical shifts are in a good agreement. Chemical shifts of the¹³C and¹⁵N nuclei have also been estimated. Two aromaticity scales are proposed for the compounds under study based on the calculations of the π-electron contribution to the diamagnetic susceptibility and of π-electron currents, respectively.     

Molecular conformations and π-hydrogen bonds in anti- and syn-binuclear Rh(I) complexes of as-indacene-diide: a computational study

Ab initio calculations are employed to interpret the different conformations in mono- and bi-nuclear Rh(I) derivatives of as-indacene, observed by X-ray crystallography. In particular, we discuss the quite unusual COD (1,5-cyclooctadiene) group orientation in the homo-bimetallic complex anti-{2,7-dimethyl-as-indacene-diide-[Rh(COD)]₂}, which is related to the metal hapticity and is stabilised by the presence of π-hydrogen bonds between olefin protons of COD and the π-electron cloud of the six-membered ring of the bridging ligand. Finally, the structure of syn-{2,7-dimethyl-as-indacene-diide-[Rh(COD)]₂} is treated where mainly steric constraints appear to control the spatial disposition of the ancillary ligands. Second-order perturbative natural bond orbital (NBO) analysis provides a meaningful picture of non-covalent intramolecular C-H?C(π) interactions.     

(Anti)aromaticity of dehydroannulenes of various ring size proved by the ring current effect in 1H NMR spectra

The spatial magnetic properties (Through-Space NMR Shieldings—TSNMRS) of already synthesized dehydro[n]annulenes of various ring size (from C₁₂ to C₂₀) have been computed, visualized as Isochemical Shielding Surfaces (ICSS) of various size and direction, and were examined subject to present (anti)aromaticity. For this purpose the thus quantified ring current effect of the macro cycles on proximate protons in proton NMR spectra was employed.     

Preferential site of solvation of the furan ring in acidic solvents

A study of the correlation of solvent effects on the chemical shifts of the methyl protons of sylvan, toluene and α-picoline led to the conclusion that the preferential site of solvation of the furan ring in acidic solvents is the π-electron cloud and not the n-electron orbital of the oxygen atom.     

Synthesis and properties of azobenzocrown ethers with π-electron donor, or π-electron donor and π-electron acceptor group(s) on benzene ring(s)

New azobenzocrown ethers of differentiated size and with substituted benzene residues have been synthesized. These crown ethers possess π-electron donor, or π-electron donor-π-electron acceptor pair of functional group(s) in benzene ring(s) in the para position to azo-grouping. Their metal ion complexation abilities in solution were studied using UV-vis spectrophotometry. The X-ray structure of a 19-membered crown ether with 4-dimethylamino-4′-nitroazobenzene fragment has been solved.     

Correlation between the out-of-plane components of magnetizability and central magnetic shielding in unsaturated cyclic molecules

A simple classical model of magnetic-field induced π-electron flow is discussed, showing that the contribution to the σ(∥) out-of-plane component of the virtual magnetic shielding provided by π-ring currents, at points P along the C(n) axis of cyclic planar unsaturated hydrocarbons C(n)H(n) with D(nh) symmetry, in the presence of a magnetic field B(ext) at right angles to the σ(h) plane, is, with good approximation, connected with the π-electron contribution to the out-of-plane component of the magnetizability, ξ(∥). The relationship is σ(∥)(h) = -(μ(0)/2π)(s(2) + h(2))(-3/2)ξ(∥), where s is the distance of a C nucleus from the center of the carbon ring, and h is the distance of P from σ(h). The ring current susceptibility, that is, the strength of the π currents, expressed in nA/T (nano ampe?re per tesla) within the SI system of units, is given by ?I/?B(ext) = -ξ(∥)/(πs(2)), which can be used as a reliable virtual measure of magnetotropicity and relative π-electron mobility in isoelectronic systems. Criteria for the practicality of the proposed ring current model are discussed.     

Magnetic orientation of 1,3,5-triphenyl-6-oxoverdazyl radical crystals

The magnetic orientation has been studied for paramagnetic organic radical crystals 1,3,5-triphenyl-6-oxoverdazyl and 1,5-di-p-tolyl-3-phenyl-6-oxoverdazyl in magnetic fields of 2-80 kOe at temperatures of 77-343 K. The X-ray diffraction measurement has revealed that the crystals are oriented with the crystallographic c axis perpendicular to the field. The anisotropic diamagnetic susceptibility arising from the benzene rings has been estimated for the crystals along the principal magnetic chi 1, chi 2, and chi 3 axes. (The chi 1 axis is at a small angle to the a axis in the monoclinic ac plane, and the chi 3 axis is along the b axis.) Since the paramagnetic susceptibility originating from the verdazyl ring is isotropic (though a large absolute value), it is shown that the magnetic orientation occurs by the anisotropy of the diamagnetic susceptibility in the crystals. The diamagnetic susceptibility is found to have a relation of chi 2 < chi 1 < chi 3 < 0.     

Quantification of the (anti)aromaticity of fulvenes subject to ring size

Tria-, penta-, hepta- and nonafulvenes (1-4) have been studied theoretically at the MP2 ab initio level of theory. For the global minimum structures, the occupation of the bonding π_CC orbital of the exocyclic CC double bond, obtained by NBO analysis, quantitatively proves π-electron delocalization which can reveal partial 2-, 6- and 10-π-electron aromaticity, and 4-, 8- and 12-π-electron antiaromaticity of the ring moieties. Beside the corresponding occupation number, this conjugation was quantified by the length of the exocyclic CC double bond whilst the (anti)aromaticity of the ring moieties of 1-4 was visualized and quantified by through space NMR shielding surfaces (TSNMRS).     

Additives atomares punktdipol-(APUDI)-modell zur berechnung der 1h-NMR-werte von benzoiden kohlenwasserstoffen

Our empirical atomic point dipole (APUDI) model simply allows the calculation of ring-current produced chemical shifts of the ring protons of 17 planar benzenoid hydrocarbons with high accuracy. It simulates the effect of the anisotropy of diamagnetic susceptibility of the π-systems by assumption of two effective atomic magnetic point dipoles, located perpendicular above and below each carbon atom of the π-system at 70 pm, the distance of the maximum of π-electron density. Application of the McConnell equation on each of these point dipoles leads after summation for each ring proton j to a corresponding geometry factor GF_j which was correlated by linear least squares method with 160 literature values of experimental chemical shifts leading to a value of (- 12.5± 0.7) × 10^-36m³, the effective anisotropy of diamagnetic susceptibility per atomic point dipole with a correlation coefficient of 0.943. The chemical shift values may be obtained in the APUDI-model by the least squares equation δ^calc_j p me>= - 12.5 × 10^-36 GF_j - 5.16. This empirically determined slope may be interpreted as the average value of the z-component of the experimental diamagnetic susceptibility per atomic point dipole of these benzenoid aromatic compounds. Besides the simple applicability the APUDI-model yields superior results for the chemical shifts of sterically crowded protons in comparison to many known ring current models.     

13C NMR spectra study of ring interaction in the dipyridyls andα,α,α-tripyridyl

Conclusions ¹³C NMR spectroscopy has been used to study ring interaction of dipyridyl isomers. It has been shown that there is a direct proportionality between the change in the paramagnetic contribution to the shielding constant and the interplanar angle determined from the PMR spectra. It has also been shown that the interaction between the O-CH bond electrons and the-electron system of the neighboring ring can affect the magnetic parameters of the substituted C^* atom.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1787–1790, August, 1977.     

Protonenresonanz-Spektren des CycIohexa-3,5-dien-1-on-Systems (ortho-Dienone)

Proton magnetic resonance spectra of a scries of ortho-dienones have been analyzed using multiple resonance and computer techniques. The sequence of the chemical shifts are in agreement with the calculated π-electron distribution. The coupling constants of the ring protons are of like sign, whereas those of the methyl protons with the ring protons alternate in sign with the number of intervening bonds.     

Spectroscopy and stereochemistry of the optically active copper(II), cobalt(II) and nickel(II) complexes with Schiff bases N,N′-(1R,2R)-(−)-1,2-cyclohexylenebis(3-methylbenzylideneiminato) and N,N′-(1R,2R)-(−)-1,2-cyclohexylenebis(5-methylbenzylideneiminato)

Schiff bases obtained from N,N′-(1R,2R)-1,2-cyclohexanediamine and 2-hydroxy-3-methylbenzaldehyde, 2-hydroxy-5-methylbenzaldehyde, have been used as ligands for copper(II), cobalt(II) and nickel(II). The complexes were characterized with UV–Vis, circular dichroism (CD), infrared, diamagnetic and paramagnetic ¹H NMR spectroscopy. CD spectra revealed exciton coupled π→π* transitions. Assignments of LMCT and d–d transitions in CD spectra of Ni(II), Co(II) and Cu(II) complexes is proposed. CD data are characteristic for central ion tetrahedral distortion from the planarity and λ conformation of the cyclohexane ring. ¹H NMR of Ni(II) complexes exhibited significant coordination shifts of CHN and ring protons which are in the closest proximity to Ni(II). The ¹H NMR paramagnetic spectra of Co(II) complexes revealed the most upfield shifted resonance at −60 ppm assigned to CHN and −28 ppm to hydrogen atom at C(5′) of the phenyl ring. Results of spectral analyses suggest central ions in a distorted square-planar geometry with N₂O₂ chromofore group.     

Disclosure of Ground-State Zimmerman-Möbius Aromaticity in the Radical Anion of [6]Helicene and Evidence for 4π Periodic Aromatic Ring Currents in a Molecular “Metallic” Möbius Strip

In 1966, Zimmerman proposed a type of Möbius aromaticity that involves through-space electron delocalization; it has since been widely applied to explain reactivity in pericyclic reactions, but is considered to be limited to transition-state structures. Although the easily accessible hexahelicene radical anion has been known for more than half a century, it was overlooked that it exhibits a ground-state minimum and robust Zimmerman-Möbius aromaticity in its central noose-like opening, becoming, hence, the oldest existing Möbius aromatic system and with smallest Möbius cycle known. Despite its overall aromatic stabilization energy of 13.6 kcal mol⁻¹ (at B3LYP/6-311+G**), the radical also features a strong, globally induced paramagnetic ring current along its outer edge. Exclusive global paramagnetic currents can also be found in other fully delocalized radical anions of 4N+2 π-electron aromatic polycyclic benzenoid hydrocarbons (PAH), thus questioning the established magnetic criterion of antiaromaticity. As an example of a PAH with nontrivial topology, we studied a novel Möbius[16]cyclacene that has a non-orientable surface manifold and a stable closed-shell singlet ground state at several density functional theory levels. Its metallic monoanion radical (0.0095 eV band gap at HSE06/6-31G* level) is also wave-function stable and displays an unusual 4π-periodic, magnetically induced ring current (reminiscent of the transformation behaviour of spinors under spatial rotation), thus indicating the existence of a new, Hückel-rule-evading type of aromaticity.     

Magnetic resonance experiments on the radical cation salt (fluoranthenyl)2+ PF6−. Correlation of proton-NMR,fluorine-NMR and ESR

The longitudinal relaxation rates of the protons and the fluorine were determined for the radical cation salt (fluoranthenyl)₂⁺PF₆^? as a function of the temperature. These data are compared with the temperature dependence of the pulsed ESR signal amplitude at the same Larmor frequency (44 MHz). Whereas the fluorines are relaxed mainly by the reorientational motion of the anions and by the interaction with fixed paramagnetic impurities, the protons are relaxed additionally above 150 K predominantly by highly mobile paramagnetic species, whose concentration could be determined directly via the signal amplitudes of the NMR and ESR signals in the same set up. The Pauli susceptibility χ_c^s = 3.1 ×10^?5 cm³/mole which is derived from this experiment and the Korringa relation (T_1H4^?)^?1 = constant of the proton relaxation leads to the assumption of a metal-like behavior of the salt above 183 K. The observation that the protons of the cation stacks, rather than the fluorines on the anions are relaxed by the mobile paramagnetic species favors the assumption of one-dimensional spin transport within the fluoranthenyl stacks, which is further supported by an ω^{$\text{?1}\text{2}$} dependence of T_1H^?1.     

Experimental and theoretical study of the induced paramagnetic ring-current in the 4-membered ring of biphenylene and related hydrocarbons

Ring current intensities and proton chemical shifts have been calculated for a series of condensed aromatic hydrocarbons including biphenylenes. The calculations show that an induced paramagnetic ring current occurs in the 4-membered ring of each of the biphenylenes studied and this effect accounts for the observed spectra. PMR measurements on 1-mono-, 2-mono- and 2,3,6,7-tetra-deuteriated biphenylene prove that the chemical shifts of the 1- and 2-protons are at δ_CDCl3 6.60 and 6.70 respectively and not vice versa as assumed by previous authors.     

Preparation of pyrrolo[3,2,1-jk]carbazole by nickel(0) mediated ring contraction of pyrrolo[3,2,1-kl]phenothiazine. Proton NMR and mass spectral studies

Pyrrolo[3,2,1-jk]carbazole 1a , the previously unreported 16π-electron parent compound, has been synthesized by treatment of pyrrolo[3,2,1-kl]phenothiazine 2a with a 1:1 mixture of bis(1,5-cyclooctadiene) nickel(0) and 2,2′-bipyridyl. An unequivocal assignment of the ¹H nmr spectrum of 1a was made by decoupling experiments and by comparison with the ¹H nmr spectrum of 1,9-dideuteriopyrrolo[3,2,1-jk]carbazole 1b . Metastable ion studies, exact mass measurements and the dideuterioderivative 1b were utilized to investigate the electron impact induced fragmentation of 1a .     

Canonical orbital contributions to the magnetic fields induced by global and local diatropic and paratropic ring currents

The induced magnetic field (IMF) of naphthalene, biphenyl, biphenylene, benzocyclobutadiene, and pentalene is dissected to contributions from the total π system, canonical π‐molecular orbitals (CMO), and HOMO→π* excitations, to evaluate and interpret relative global and local diatropicity and paratropicity. Maps of the IMF of the total π system reveal its relative strength and topology that corresponds to global and local diatropic and paratropic ring currents. The total π magnetic response is determined by this of canonical HOMOs and particularly by paratropic contributions of rotational excitations from HOMOs to unoccupied π * orbitals. Low energy excitations and similar nodal structure of HOMO and π * induce strong paratropic fields that dominate on antiaromatic rings. High energy excitations and different nodal structures lead to weak paratropic contributions of canonical HOMOs, which are overwhelmed by diatropic response of lower energy canonical orbitals in aromatic rings. CMO‐IMF analysis is found in agreement with ring current analysis. © 2017 Wiley Periodicals, Inc.     

A novel total synthesis of isocryptolepine based on a microwave-assisted tandem Curtius rearrangement and aza-electrocyclic reaction

A new entry to the total synthesis of isocryptolepine (cryptosanguinolentine), isolated from Cryptolepis sanguinolenta, was achieved by constructing a tetracyclic ring system through a microwave-assisted tandem Curtius rearrangement and electrocyclic reaction of an aza 6π-electron system. The tetracyclic lactam was converted to isocryptolepine in a four-step sequence.