Spatial Magnetic Properties Research Articles

Simple and Effective Identification of Local 6π- and Global [4n + 2] Aromaticity of Macrocyclic Conjugated Hydrocarbons by 1H/13C Chemical Shifts and the Corresponding Ring Current Effect.

Structures, 1H/13C chemical shifts, and the ring current effects (spatial magnetic properties: through-space NMR shieldings [TSNMRSs]) of various π-conjugated macrocyclic hydrocarbons and the corresponding charged analogues have been calculated at the B3LYP/6-311G(d,p) theory level using the GIAO perturbation method and employing the nucleus-independent chemical shift (NICS) characterization. The spatial magnetic properties (TSNMRS) are visualized as iso-chemical shielding surfaces (ICSSs) of various size and direction and together with especially the δ(1H)/ppm chemical shifts employed to unequivocally qualify and quantify local 6π-aromaticity of individual benzenoid building blocks and the global ([4n + 2], n > 1) aromaticity of the macrocyclic ring.

1H and 13C NMR spectra of infinitene and the ring current effect of the aromatic molecule.

The spatial magnetic properties (through-space NMR shieldings-TSNMRSs-actually the ring current effect in 1H NMR spectroscopy) of the recently synthesized infinitene (the helically twisted [12]circulene) 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. Both 1H and 13C chemical shifts of infinitene and the aromaticity of this esthetically very appealing molecule have been studied subject to the ring current effect thus obtained. This spatial magnetic response property of TSNMRSs dominates the different magnitude of 1H and 13C chemical shifts, especially in the cross-over section of infinitene, which is unequivocally classified as an aromatic molecule based on the deshielding belt of its ring current effect. Differences in aromaticity of infinitene compared with isolated benzene can also be qualified and quantified on the magnetic criterion.

Identification and quantification of local antiaromaticity in polycyclic aromatic hydrocarbons (PAHs) based on the magnetic criterion.

The spatial magnetic properties, through-space NMR shieldings (TSNMRSs, actually the ring current effect in 1H NMR spectroscopy), of a selection of entirely antiaromatic and aromatic polycyclic conjugated hydrocarbons (PCHs), and aromatic PCHs with antiaromatic components, have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSSs) of various sizes and directions. Using both in-plane and above/below-plane ICSS data, polycyclic aromatic hydrocarbons can be readily distinguished from polycyclic antiaromatic ones, even when antiaromatic components are present in the polycyclic aromatic hydrocarbons (PAHs). These antiaromatic zones can also be attributed to internal components of the in-plane deshielding belt present in aromatic compounds and possible partial antiaromatic ring current effects in the same place. This makes it possible to unequivocally confirm correctly assigned or adjust incorrectly assigned antiaromaticity of individual rings in the same molecule.

Carbones (-C2--), carbenes (-C:-) and carbodications (-C2+-) on the magnetic criterion.

The spatial magnetic properties, particularly the through-space NMR shieldings (TSNMRSs, the anisotropy effect in 1H NMR spectroscopy) of carbenes, carbones and carbodication (carbo2+) compounds (with and without stabilization by NMe2 π-donation) and those of a number of carbo2+ analogues 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 sizes and directions. TSNMRSs prove the electronic structure of carbo2+ compounds to be completely different from those of carbenes and carbones, preferring both the π-electron distribution and the structure of allenes/cumulenes despite the central carbon atom being the most electrophilic centre.

Carbones - A Classification on the Magnetic Criterion.

Carbones (carbodiphosphoranes, bent allenes and chalcogen-stabilized carbones) bear the same resonance contributor X+ -C2- -Y+ (X+ , Y+ =PR3 + , CR2 + , SR2 + , SeR2 + , S+ R2 =NR) and exhibit unique bonding and donating properties at the central carbon atom. A classification is given on basis of both the geometry and the magnetic properties (13 C chemical shift of the central carbon atom and the spatial magnetic properties, through-space NMR shieldings (TS NMRSs), actually the anisotropy effect or the ring current effect of aromatic species). TS NMRS values have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The synergy of geometry (linear or bent, orthogonal or twisted structures) and NMR characteristics (extend of the high field shift of the central carbon atom, anisotropy effect of the allene-like C=C double bonds or the ball-like anisotropy effect of carbone-like central carbon atom) provides a comprehensive picture of the dominating resonance contributor.

The multiple bond character of the carbon−boron bond in boron trapped N-heterocyclic carbenes (NHCs) and cyclic(alkyl)(amino)carbenes (CAACs) on the magnetic criterion

Geometry, 11B, 13C chemical shifts and the spatial magnetic properties (Through-Space NMR Shieldings − TSNMRS) of both cations and anions of boron-trapped N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) and of the corresponding diborane/diborene/diboryne dis-carbene adducts have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept; the TSNMRS results are visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The ICSS of the TSNMRS (actually the anisotropy effects measurable in 1H NMR spectroscopy) are employed to qualify and quantify the present multiple bond character of the Carbene−Boron bond in the trapped NHCs and CAACs. Results are confirmed by bond length and 11B/13C chemical shift variations. Thus the partial multiple bond character of the Carbene−Boron bond cannot be expressed by the arrow of weak, much longer dative bonds and should be omitted as in other covalent lone pair-π or triel bonds.

Phosphorus stabilization of the carbene function in P-analogues of non-cyclic carbenes, N-heterocyclic carbenes and cyclic(alkyl)-(amino)carbenes − an assessment on basis of geometry, 13C, 31P chemical shifts and the anisotropy effects of the carbene electron deficient centres

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of the P-analogues of non-cyclic, N-heterocyclic carbenes (NHCs) and cyclic (alkyl) (amino)carbenes (CAACs) have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values (actually the anisotropy effects experimentally measurable in 1H NMR spectroscopy) are employed to qualify and quantify the position of the present mesomeric equilibria (P-carbenes ↔ P-ylides ↔ P-acetylenes/P-allenes). Results are confirmed by geometry (bond angles and bond lengths), the 31P and the 13C chemical shifts of the electron-deficient carbene centres. It was found out that the π-donor and σ-acceptor power of phosphorus in planar and not pyramidalized PR2 substituents to stabilize P-carbenes (compared with NR, OR, SR and Hal substituents) is only minor, but complete PC bonds along sole ylide-like mesomeric contributors are existing. A phosphorus carbene stabilization mechanism, not yet specified, in addition or different to π-donation, is badly needed.

Cyclazines‐Structure and Aromaticity or Antiaromaticity on the Magnetic Criterion

AbstractStructure and spatial magnetic properties, through‐space NMR shieldings (TSNMRSs), of all ten cycl[2.2.2]azine to cycl[4.4.4]azine, hetero‐analogues and the corresponding hydrocarbons have been calculated at the B3LYP/6‐311G(d,p) theory level using the GIAO perturbation method and employing the nucleus independent chemical shift (NICS) concept. The TSNMRS values (actually, the ring current effect as measurable in1H NMR spectroscopy) are visualized as iso‐chemical‐shielding surfaces (ICSS) of various size and direction, and employed to readily qualify and quantify the degree of (anti)aromaticity. Results are confirmed by NMR [δ(1H)/ppm, δ(15N)/ppm] and geometry (planar, twisted, bow‐shaped) data. The cyclazinesN[2.2.2]−up toN[2.4.4]−are planar or at most slightly bowl‐shaped and, due to coherent peripheral ring currents (except inN[2.3.3]−,N[2.3.4],N[3.3.4]+andN[2.4.4]+), develop aromaticity or anti‐aromaticity of the whole molecules dependent on the number of peripheral conjugated π electrons. The cyclazinesN[2.3.3]−,N[2.3.4],N[3.3.4]+andN[2.4.4]+develop two ring currents of different direction within the same molecule, in which the dominating ring current proves to be paratropic (inN[3.3.4]+diatropic) including the nodalNpzlone pair into the conjugation. The residual cyclazinesN[3.4.4],N[4.4.4]−andN[4.4.4]+are heavily twisted and, therefore, are not developing peripheral or diverse ring currents. The TSNMRS information about cyclazines and the parent tricyclic annulene analogues is congruent subject to structure and number of peripheral or internal conjugated π electrons, the corresponding (anti)aromaticity is in unequivocal accordance with Hückel's rule.

Quantification of σ-Acceptor and π-Donor Stabilization in O, S and Hal Analogues of N-Heterocyclic Carbenes (NHCs) on the Magnetic Criterion.

The spatial magnetic properties, through-space NMR shieldings (TSNMRSs), of stable O, S and Hal analogues of N-heterocyclic carbenes (NHCs) have been calculated using the GIAO perturbation method employing the nucleus-independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSSs) of various sizes and directions. The TSNMRS values (actually the anisotropy effects measurable in 1H NMR spectroscopy) are employed to qualify and quantify the position of the present mesomeric equilibria (carbenes ↔ ylides). The results are confirmed by geometry (bond angles and bond lengths), IR spectra, UV spectra, and 13C chemical shifts of the electron-deficient carbon centers.

Intramolecular carbene stabilization via 3c,2e bonding on basis of the magnetic criterion

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of bent cyclobutylcarbene 8, 1,2-diboretane-3-ylidene 9, and some carbene analogues of boron 14–18 as most intriguing examples of carbenes, which can be stabilized as homoaromatic systems with 3c,2e bonding, have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values (actually, ring current effect/anisotropy effects as measurable in 1H NMR spectroscopy) are employed to qualify and quantify the degree of present 3c,2e-homoaromaticity. Results are confirmed by geometry (bond angles and bond lengths) and spectroscopic data, the δ(11B)/ppm data and the 13C chemical shifts of the carbene electron-deficient centre.

Dative or coordinative carbon−boron bond in boron trapped N-heterocyclic carbenes (NHCs)? An answer given on the magnetic criterion

Abstract The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of isolated as well as B–C bond length varied model compounds (BR3 trapped NHCs) have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values (actually the anisotropy effects measurable in 1H NMR spectroscopy) are employed to qualify and quantify the present dative vs. coordinative bond character of the boron−carbon bond in the trapped NHCs. Results are confirmed by bond lengths and 11B/13C chemical shift variations in the BR3 trapped NHCs.

Quantum-Chemical Study of Cobalt Complexes with o-Quinones Modified with Silicon Triangulene Derivatives

Computational modeling (DFT UB3LYP/6-311++G(d,p)) of the spatial structure, energy parameters, and magnetic properties of the adducts of fluorinated cobalt(II) bis-diketonates with o‑benzoquinones containing hydrosilicon triangulene moieties has been performed for the first time. Compounds potentially capable of thermally initiated redox-isomeric rearrangements have been revealed. It has been demonstrated that extension of the polycyclic moiety has no effect on the energy differences between the electromers, but leads to an increase in the total spin of the system. The presence of triangulene radical fragments and exchange coupling are responsible for paramagnetism of all the studied molecules.

Bent Allenes or Di-1,3-betaines-An Answer Given on the Magnetic Criterion.

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of bent allene 1, the corresponding C-extended 1,3-butadiene derivative 2, and a number of related compounds 3-20 have been calculated using the gauge-independent atomic orbital perturbation method, employing the nucleus-independent chemical shift concept and visualized as isochemical shielding surfaces of various sizes and directions. Prior to that, both structures and 13C chemical shifts were calculated and compared with available experimental bond lengths and δ(13C)/ppm values (also, as a quality criterion for the computed structures). Bond lengths, the δ(13C)/ppm, and the TSNMRS values are employed to qualify and quantify the electronic structure of the studied compounds in terms of dative or classical electron-sharing bonds.

The 13 C chemical shift and the anisotropy effect of the carbene electron-deficient centre: Simple means to characterize the electron distribution of carbenes.

Both the 13 C chemical shift and the calculated anisotropy effect (spatial magnetic properties) of the electron-deficient centre of stable, crystalline, and structurally characterized carbenes have been employed to unequivocally characterize potential resonance contributors to the present mesomerism (carbene, ylide, betaine, and zwitter ion) and to determine quantitatively the electron deficiency of the corresponding carbene carbon atom. Prior to that, both structures and 13 C chemical shifts were calculated and compared with the experimental δ(13 C)/ppm values and geometry parameters (as a quality criterion for obtained structures).

Benzyne − an acetylene- or cumulene-like electronic structure?

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of benzyne 1 and analogues (benzene 2, 1,2,3-cyclohexatriene 3, cyclohexen-3-yne 4, cyclohexen-4-yne 5, cyclohexyne 6) 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 could be employed to compare the diatropic ring current effects of benzene and benzyne, and, when compared with the spatial magnetic properties of the analogues, to answer the question whether the benzyne electronic structure is more acetylene- or cumulene-like, supported by structural data and δ(13C)/ppm values.

Ortho-Quinone Methide Driven Synthesis of New O,N- or N,N-Heterocycles.

To synthesize functionalized Mannich bases that can serve two different types of ortho‐quinone methide (o‐QM) intermediates, 2‐naphthol and 6‐hydroxyquinoline were reacted with salicylic aldehyde in the presence of morpholine. The Mannich bases that can form o‐QM and aza‐o‐QM were also synthesized by mixing 2‐naphthol, 2‐nitrobenzaldehyde, and morpholine followed by reduction of the nitro group. The highly functionalized aminonaphthol derivatives were then tested in [4+2] cycloaddition with different cyclic imines. The reaction proved to be both regio‐ and diastereoselective. In all cases, only one reaction product was obtained. Detailed structural analyses of the new polyheterocycles as well as conformational studies including DFT modelling were performed. The relative stability of o‐QMs/aza‐o‐QM were also calculated, and the regioselectivity of the reactions could be explained only when the cycloaddition started from aminodiol 4. It was summarized that starting from diaminonaphthol 25, the regioselectivity of the reaction is driven by the higher nucleophilicity of the amino group compared with the hydroxy group. 12H‐benzo[a]xanthen‐12‐one (11), formed via o‐QM formation, was isolated as a side product. The proton NMR spectrum of 11 proved to be very unique from NMR point of view. The reason for the extreme low‐field position of proton H‐1 could be accounted for by theoretical calculation of structure and spatial magnetic properties of the compound in combination of ring current effects of the aromatic moieties and steric compression within the heavily hindered H(1)‐C(1)‐C(12b)‐C(12a)‐C(12)=O structural fragment.

Benzenium Ion: Aromatic as the π-Complex or Antiaromatic as the σ-Complex Being Somewhat Similar to the Cyclopentadienyl Cation.

The spatial magnetic properties, through-space NMR shieldings (TSNMRSs), of the benzenium cation (C6H7+) 1 and of ±I/M-substituted analogues C6H6X+ 3-8 [X = -Me, -CF3, -NH2, -NO2, -NO, -SiH3] have been calculated using the gauge-independent atomic orbital perturbation method employing the nucleus-independent chemical shift concept, and iso-chemical-shielding surfaces of various sizes and directions have been observed. The TSNMRS values were employed to compare the spatial magnetic properties (TSNMRS) of benzene and the benzenium ion 1 and then further compared with analogues 3-8, to answer the question whether the electronic structures of 1 and 3-8 are still similar to those of aromatic species or somewhat similar to the antiaromatic cyclopentadienyl cation 2, supported by structural data and δ(13C)/ppm values.

Is the term “Carbene” justified for remote N-heterocyclic carbenes (r-NHCs) and abnormal N-heterocyclic carbenes (aNHCs/MICs)?

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of typical N-heterocyclic carbenes NHCs, r-NHCs, a-NHCs and MICs 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. Prior to that both structures and 13C chemical shifts were calculated and in case of isolated carbenes the computed δ(13C)/ppm values compared (as a quality criterion for obtained structures) with the experimental ones. The TSNMRS values of the studied carbenes, which are in mesomeric equilibrium with zwitterionic (ylide/betaine/mesoionic) resonance contributors, are employed to qualify and quantify the present electronic structure and if the term carbene is still justified to denote the compounds studied. The results, thus obtained from spatial magnetic properties (TSNMRS), are compared with the geometry of the compounds, the corresponding WIBERG's bond index values, and the 13C chemical shifts especially of the carbene electron-deficient centre.

Stable Carbenes or Betaines?

The anisotropy effect in 1H NMR spectroscopy can be readily employed to indicate the position of carbene/betaine mesomeric equilibria. NR2 substituted carbene/betaines tend to adopt betaine structures, whereas in the absence of NR2 substituents, the betaine structures cannot stabilise the structure through both π‐donation effects of the NMe2 groups and the electronegativity of the nitrogen atoms, and the corresponding carbene‐like structures are preferred. These conclusions are supported by calculated bond orders and δ(13C)/ppm values. The spatial magnetic properties of isonitriles and carbon monoxide, which can be counted as stable carbenes or, at least, as carbene‐analogues, also exist as stable betaine structures, which is again supported by structural and magnetic properties.

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.