Iso-chemical-shielding Surfaces Research Articles

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 - 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.

A holomolecule conjugated and electron delocalized organic compound for superior proton-storage redox capability

Aqueous proton batteries (APBs) offer realistic possibilities for reliable and sustainable energy technologies owing to their unique “Grotthus mechanism”. Organic compounds with tunable molecular structures are candidate electrode materials, but their unsatisfactory proton-storage redox behaviors hinder the practical application in APB devices. Herein, a holomolecule conjugated organic compound (PZQN) composed of pyrazine and quinone rings is synthesized and used for APB application. Such reasonable configuration bestows the PZQN molecule with extended π-conjugation, overall molecular rigidity and optimized electronic structure with a small energy gap (∼3.12 eV). Localized orbital locator-π (LOL-π) and iso-chemical shielding surfaces (ICSS) techniques reveal the extraordinary electron delocalization in the PZQN molecule, enabling its high-kinetic redox activity and superior electron affinity. As such, the PZQN compound as an electrode material exhibits a fast, stable and unrivaled proton-storage redox capability with a competitive capacity of 262.5 mAh g−1 in aqueous acidic electrolyte, which is comparable among various proton-inserted organic electrodes. In-situ dynamic analysis combined with theoretical calculations illustrate the proton-storage redox mechanism and corresponding protonation pathway. Finally, a high-performance APB device is fabricated with superior electrochemical properties and an ultralong lifespan over 40,000 cycles, further confirming its promising application prospect.

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.

Synthesis, characterization and properties of amphoteric heat-resistant explosive materials: Fused [1,2,5]oxadiazolo [3′,4′:5,6]pyrido[4,3-d][1,2,3]triazines

• The tricyclic triazine oxide was synthesized as the first fused amphoteric energetic molecule. • These compounds exhibited high decomposition temperatures (310–344 °C). • All the non-metallic compounds have better detonation properties than HNS and PYX and low sensitivities. • The perchlorate salt is a potential candidate for heat-resistant explosive. Fused cyclic energetic compounds are a unique class of large conjugated structures containing two or more rings, and have been identified as promising competitors for traditional energetic materials. In the present work, we investigated 5-amino-6-oxo-4,6-dihydro-[1,2,5]oxadiazolo[3′,4′:5,6]pyrido[4,3- d ][1,2,3]triazine 8-oxide ( 3 ) in detail as a fused amphoteric energetic molecule that can be protonated and deprotonated by acids and bases to obtain its cationic ( 11 – 13 ) and anionic ( 4 – 10 ) salts. All the new compounds were thoroughly characterized by infrared and multinuclear NMR ( 1 H and 13 C) spectroscopy, and elemental analysis. The structural features of 3 – 6 , 9 , 10 , 12 , and 13 were investigated by single-crystal X-ray diffractions and different theoretical techniques (Hirshfeld surfaces analyses, and noncovalent interactions). The aromaticity of compound 3 was studied via localized orbital locator- π and iso -chemical shielding surfaces (ICSS). Their thermal decomposition temperatures were investigated by differential scanning calorimetry and thermogravimetry to be 310–344 °C (onset). In addition, impact and friction tests revealed that these compounds were insensitive. Densities, heats of formation and detonation properties were measured by a gas pycnometer, calculated using the Born-Haber energy cycle and EXPLO5 v6.05.04 program, respectively. The neutral compound 3 and all its non-metallic salts have better detonation properties than HNS and PYX and lower sensitivities. In particular, salt 11 (N + O: 61.74 %, T d : 330 °C, Δ H f : 1.68 kJ g −1 , D : 8.795 km s −1 , P : 36.0 GPa, IS: > 40 J, FS: > 360 N) is a potential candidate for heat-resistant explosives.

Assembling Nitrogen-rich, thermally Stable, and insensitive energetic materials by polycyclization

In pursuit of energetic materials which have equal energies and thermal stabilities, but lower sensitivities compared with hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), a new fused N-heterocyclic framework, ditriazolo-1,2,4,5-tetrazine, 7 was designed and synthesized. Compound 7 exhibits a favorable decomposition temperature of 247 °C benefiting from its good aromaticity which was studied by nucleus-independent chemical shift (NICS) and isochemical shielding surfaces (ICSS). Combined with moderate detonation properties (vD, 8544 m s−1; P, 27.3 GPa) and low sensitivities (IS, >40 J; FS, >360 N), 7 may potentially be useful as a secondary energetic in practical applications to replace RDX. Hydrazinium salt 12 has a thermal stability of 221 °C, and excellent detonation properties (vD, 9088 m s−1; P, 32.3 GPa), which are comparable with those of HMX. These contributions highlight the fused N-heterocyclic framework as a potential energetic backbone and provide new thinking to the chemistry of polycyclic heterocycles.

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.

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.

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.

Stereoelectronic Effects: The γ-Gauche Effect in Sulfoxides.

Reasons for the 13C NMR γ-gauche effect in sulfoxides, i.e., the distinct shielding of a carbon β to a gauche-oriented sulfoxide group were investigated. Several calculated and measured 13C NMR data of open chain or thiane-derived sulfoxides revealed that an upfield shift is only observed for that γ-gauche position, in which the respective carbon is anti to the sulfoxide's sulfur lone pair. Carbons in γ-gauche position, which are synclinal to the lone pair, are not affected. The magnetic anisotropy of the S═O group was examined by generation of iso-chemical-shielding surfaces (ICSSs) and magnetically induced current maps. Stereoelectronic interactions were determined with natural bond orbital (NBO) and natural chemical shielding (NCS) analyses. The γ-gauche effect is best described by stereoelectronic interactions, especially those of the sulfur's lone pair with antibonding orbitals to a β-carbon in antiperiplanar orientation. An explanation based on steric interactions, which has frequently been referred to, is not suitable to describe the observed shielding effects. Furthermore, a description of the bonding situation in the S═O group is given. It can be understood as S-O triple bond, where the bond order is significantly reduced by antibonding contributions in some of the occupied molecular orbitals.

Ring current and anisotropy effects on OH chemical shifts in resonance-assisted intramolecular H-bonds

Ring current effects on resonance-assisted and intramolecularly bridged hydrogen bond protons for 10-hydroxybenzo[h]quinoline 1 and a number of related compounds were calculated and the through-space NMR shieldings (TSNMRS) obtained hereby visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. These calculations revealed that this through-space effect is comparably large (up to 2 ppm) dependent on the position of the intramolecularly bridged OH proton, and therefore, contribute considerably to the chemical shift of the latter making it questionable to use δ(OH)/ppm in the estimation of intramolecular hydrogen bond strength without taking this into account. Furthermore, the anisotropy effects of additional groups on the aromatic moiety (e.g. the carbonyl group in salicylaldehyde or in o-hydroxyacetophenone of ca. 0.6 ppm deshielding) should also be considered. These through-space effects need to be taken into account when using OH chemical shifts to estimate hydrogen bond strength.

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.

Identification of mesomeric substructures by through-space NMR shieldings (TSNMRS). Trimethine cyanine/merocyanine-like or aromatic π-electron delocalization?

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of amino-substituted heteroaromatic six-membered ring systems such as pyrylium/thiopyrylium 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 size and direction. The TSNMRS values were employed to quantify and visualize the existing aromaticity of the studied compounds. Due to strong conjugation of six-membered ring π-electrons and lone pairs of the exo-cyclic amino substituents (restricted rotation about partial C,N double bonds) the interplay of still aromatic and already dominating trimethine cyanine/merocyanine-like substructures can be estimated.

Y-aromaticity – existing: yes or no? An answer given on the magnetic criterion (TSNMRS)

The spatial magnetic properties (Through Space NMR Shieldings – TSNMRS) of a number of Y-shaped structures possessing 4n+2 π-electrons (i.a. the trimethylenemethane ions TMM2+, TMM2−, the guanidinium cation, substituted and hetero analogues) have been computed, visualized as Isochemical Shielding Surfaces (ICSS) of various size and direction, were examined subject to present Y-aromaticity and the results compared with energetic and geometric criteria obtained already.

Characterization and quantification of quasi-aromaticity by spatial magnetic properties (TSNMRS)

The spatial magnetic properties (Through Space NMR Shieldings—TSNMRS) of various types of structures with suggested quasi-aromaticity (a summaring topic: in detail push–pull, captodative, chelate, supramolecular aromaticity, etc.) have been computed, are visualized as Isochemical Shielding Surfaces (ICSS) of various size/direction and examined subject to identify and quantify present (partial) aromaticity. While the TSNMRS approach proves really helpful [even in cases of (4n+2) π-electron cyclic moieties formed via non-covalent polar interactions] quasi-aromaticity suggested for enol forms of 1,3-dicarbonyl compounds via resonance-assisted intramolecular and intermolecular hydrogen bonding cannot be confirmed.