3He NMR studies on helium–pyrrole,helium–indole,and helium–carbazole systems: a new tool for following chemistry of heterocyclic compounds

The ³He nuclear magnetic shieldings were calculated for free helium atom and He–pyrrole, He–indole, and He–carbazole complexes. Several levels of theory, including Hartree–Fock (HF), Second‐order Møller‐Plesset Perturbation Theory (MP2), and Density Functional Theory (DFT) (VSXC, M062X, APFD, BHandHLYP, and mPW1PW91), combined with polarization‐consistent pcS‐2 and aug‐pcS‐2 basis sets were employed. Gauge‐including atomic orbital (GIAO) calculated ³He nuclear magnetic shieldings reproduced accurately previously reported theoretical values for helium gas. ³He nuclear magnetic shieldings and energy changes as result of single helium atom approaching to the five‐membered ring of pyrrole, indole, and carbazole were tested. It was observed that ³He NMR parameters of single helium atom, calculated at various levels of theory (HF, MP2, and DFT) are sensitive to the presence of heteroatomic rings. The helium atom was insensitive to the studied molecules at distances above 5 Å. Our results, obtained with BHandHLYP method, predicted fairly accurately the He–pyrrole plane separation of 3.15 Å (close to 3.24 Å, calculated by MP2) and yielded a sizable ³He NMR chemical shift (about ?1.5 ppm). The changes of calculated nucleus‐independent chemical shifts (NICS) with the distance above the rings showed a very similar pattern to helium‐3 NMR chemical shift. The ring currents above the five‐membered rings were seen by helium magnetic probe to about 5 Å above the ring planes verified by the calculated NICS index. Copyright © 2014 John Wiley & Sons, Ltd.     

Local aromaticity in polyacenes manifested by individual proton and carbon shieldings: DFT mapping of aromaticity

Exponential dependencies between locally calculated geometric and magnetic indexes of aromaticity, harmonic oscillator model of aromaticity (HOMA) and nucleus independent chemical shifts (NICS)(0), NICS(1) and NICS(1)zz, and the number of conjugated benzene rings in linear acenes, from benzene to decacene were observed at B3LYP/6-311+G** level of theory. Correlations between HOMA and NICS indexes showed exponential dependencies and were fitted with simple three-parameter function. Similar correlations between both indexes of aromaticity and proton and carbon nuclear isotropic shieldings of individual acene rings were observed. Contrary to proton data, the predicted ¹³C nuclear isotropic shieldings of carbon atoms belonging to inner rings in polyacenes were less shielded, indicating lower aromaticity and therefore, higher reactivity.     

3He NMR: from free gas to its encapsulation in fullerene

The ³He nuclear magnetic shieldings were calculated for single helium atom, its dimer, simple models of fullerene cages (He@C_n), and single wall carbon nanotubes. The performances of several levels of theory (HF, MP2, DFT‐VSXC, CCSD, CCSD(T), and CCSDT) were tested. Two sets of polarization‐consistent basis sets were used (pcS‐n and aug‐pcS‐n), and an estimate of ³He nuclear magnetic shieldings in the complete basis set limit using a two‐parameter fit was established. Theoretical ³He results reproduced accurately previously reported theoretical values for helium gas, dimer, and helium probe inside several fullerene cages. Excellent agreement with experimental values was achieved. ³He nuclear magnetic shieldings of single helium atom approaching various points of benzene ring were tested, and an impact of ³He confinement within fullerene cages of different size on the ³He chemical shift was determined. Copyright © 2013 John Wiley & Sons, Ltd.     

Local aromaticity mapping in the vicinity of planar and nonplanar molecules

We report on nucleus-independent magnetic shielding (NICS) scans over the centers of six- and five-membered rings in selected metal phthalocyanines (MPc) and fullerene C60 for more accurate characterization of local aromaticity in these compounds. Detailed tests were conducted on model aromatic molecules including benzene, pyrrole, indole, isoindole, and carbazole and subsequently applied to H₂Pc, ZnPc, Al(OH)Pc, and CuPc. Similar behavior of three selected magnetic probes, Bq, ³He, and ⁷Li⁺, approaching perpendicularly the ring centers, was observed. For better visualization of shielding zone over the centers of aromatic rings, we introduced a simple mathematical procedure: the first and second derivatives of scan curves with respect to magnetic probe position enabled their additional examination. It allowed an easier localization of curve minimum and discrimination between areas in space varying by the magnetic field magnitude and to illustrate local aromaticity of two different kinds of rings in MPc with better resolution. Our results supported earlier reports on very low aromaticity indexes of pyrrole ring incorporated into MPc and significant aromaticity of the central macrocycle. No direct dependence between harmonic oscillator model of aromaticity and NICS was observed. Instead, a correlation between position of scan curve minimum and its magnitude were observed. In addition, the NICS values and ³He chemical shifts in the middle of neutral C60 and C60⁶⁻ anion agreed well with the reported experimental NMR values for He@C60 and He@C60⁶⁻.     

Summation of nucleus independent chemical shifts as a measure of aromaticity

When the nucleus independent chemical shifts, NICS(1)zz, for a set of aromatic and antiaromatic hydrocarbons are summed, they show an excellent linear relationship with the magnetic susceptibility exaltation, Lambda, for neutral, cationic, and monoanionic species. Aromatic and antiaromatic dianions show a similar relationship but with a different slope. However, when both Lambda and the summation of NICS(1)zz are divided by the area of the ring squared, the vast majority of the aromatic and antiaromatic species fall on the same line, indicating that both NICS and Lambda are affected by the size of the ring. The species that deviate slightly from the line are a few of the anionic compounds, which may be a result of the difficulties of calculating magnetic properties of anions. This is the first report of the relationship of NICS to ring area. In addition, the excellent correlation between Lambda and the summation of NICS(1)zz demonstrates that summation of NICS(1)zz values for individual ring systems of polycyclic ring systems to give a measure of the aromaticity of the entire system is justified. By extension, the excellent correlation also serves to demonstrate that the NICS(1)zz values for individual ring systems are reliable measures of local aromaticity/antiaromaticity. Finally, the excellent correlation between experimental shifts and the 13C NMR shifts calculated with density functional theory, B3LYP/6-311+g(d,p), serves as an indirect validation of the accuracy of the NICS(1)zz calculated by the same method and basis set.     

The Laplacian of electron density versus NICSzz scan: measuring magnetic aromaticity among molecules with different atom types

The electron density versus NICS(zz) (the out-of-plane component of nucleus-independent chemical shifts (NICS)) scan for assessing magnetic aromaticity among similar molecules with different ring sizes is improved by scanning the Laplacian of electron density versus NICS(zz) to include molecules containing different types of atoms. It is demonstrated that the new approach not only reproduces the results of the previous method but also surpasses that in the case of species with different atom types. The relative positions of curves in the plots of the Laplacian of electron density versus NICS(zz) correlate well with the ring current intensities of these molecules both near and far from the ring planes of the considered molecules. Accordingly, relative magnetic aromaticity of a number of planar hydrocarbons and a group of double aromatic metallic/semimetallic species are studied and discussed.     

Quantum‐chemical analyses of aromaticity,UV spectra,and NMR chemical shifts in plumbacyclopentadienylidenes stabilized by Lewis bases

We carried out a series of zeroth‐order regular approximation (ZORA)‐density functional theory (DFT) and ZORA‐time‐dependent (TD)‐DFT calculations for molecular geometries, NMR chemical shifts, nucleus‐independent chemical shifts (NICS), and electronic transition energies of plumbacyclopentadienylidenes stabilized by several Lewis bases, (Ph)₂(^tBuMe₂Si)₂C₄PbL₁L₂ (L₁, L₂ = tetrahydrofuran, Pyridine, N‐heterocyclic carbene), and their model molecules. We mainly discussed the Lewis‐base effect on the aromaticity of these complexes. The NICS was used to examine the aromaticity. The NICS values showed that the aromaticity of these complexes increases when the donation from the Lewis bases to Pb becomes large. This trend seems to be reasonable when the 4n‐Huckel rule is applied to the fractional π‐electron number. The calculated ¹³C‐ and ²⁰⁷Pb‐NMR chemical shifts and the calculated UV transition energies reasonably reproduced the experimental trends. We found a specific relationship between the ¹³C‐NMR chemical shifts and the transition energies. As we expected, the relativistic effect was essential to reproduce a trend not only in the ²⁰⁷Pb‐NMR chemical shifts and J[Pb‐C] but also in the ¹³C‐NMR chemical shifts of carbons adjacent to the lead atom. © 2014 Wiley Periodicals, Inc.     

Transannular pi-pi interactions in janusenes and in related rigid systems with cofacial aromatic rings; gauging aromaticity in the hydrocarbons and in model carbocations; a DFT study

The utility of NICS (nuclear independent chemical shift) as a probe for detecting/sensing variation in aromaticity due to transannular pi-pi interactions in janusene , a [3.3]orthocyclophane having two cofacial benzene rings within van der Waals distance, its tetrafluoro- and octafluoro-derivatives and , and in tropiliojanusene was studied by DFT at the B3LYP/6-31G(d) level. The related hydrocarbons and with a buried double-bond and their carbocations were also included in this study. Whereas NICS(0) and NICS(1) are rather insensitive to transannular interactions, computed NICS(1)(zz), values are larger and more negative for both pi-decks in the interannular space and this is consistent with increased transannular pi-pi interactions in the cofacial rings, previously shown in these systems via spectroscopic studies (UV and NMR), and by electrophilic chemistry. Transannular effects in , , and were also probed by examining the forms of HOMO-LUMOs. Attempts to measure donor-accepter interactions between electron rich/electron poor cofacial decks via NICS (1)(zz) through substituent effects proved unsuccessful, resulting in only very small changes. Protonation of the double-bond buried in between the two pi-decks in and results in internally pi-stabilized carbocations that exhibit more negative NICS(1) and NICS(1)(zz) values in the interannular space. GIAO NMR data were computed for the neutral hydrocarbons and their derived carbocations, as a guiding tool for planned experimental studies.     

The NICS‐XY‐Scan: Identification of Local and Global Ring Currents in Multi‐Ring Systems

Nucleus‐independent chemical shift (NICS)‐based methods are very popular for the determination of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the determination of the aromaticity and antiaromaticity of molecules and ions, both qualitatively and quantitatively. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chemical shift. However, the source of the field cannot be determined by NICS. Thus, in a multi‐ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this paper, we present a NICS‐based method for the determination of local and global ring currents in conjugated multi‐ring systems. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a constant height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICS_πZZ, but it is shown that at a height of 1.7 Å above the molecular plane, NICS_ZZ provides the same qualitative picture. This method, namely the NICS‐XY‐scan, gives information equivalent to that obtained through current density analysis methods, and in some cases, provides even more details.     

The Influence of Benzo‐Fusion on the Aromatic Pathways and Ring Currents of Cycl[3.2.2]azine

The effect of benzene ring fusion on the aromaticity of cycl[3.2.2]azine was studied by calculating topological resonance energy (TRE), the percentage topological resonance energy (%TRE), and magnetic resonance energy (MRE). The bond resonance energy (BRE) and circuit resonance energy (CRE) indices were used to estimate the local aromaticity. Our BRE and CRE results show that the central nitrogen atom plays a significant role both in the global and local aromaticity of the compounds in our study, and contrary to what has been reported in the literature, none of these compounds are peripheral π‐electronic systems. In the case of benzene ring‐fused derivatives, benzene ring(s) exhibit relatively larger local aromaticity and reduce the local aromaticity of the central portion of cycl[3.2.2]azine to a level comparable to a corresponding non‐benzene fused parent system. Ring current results predict that a strong diamagnetic current flows around the whole molecular perimeter. The diatropic bond current results, as computed here, are in good agreement with the observed ¹H‐NMR chemical shifts of these compounds.     

Experimental and theoretical IR,Raman, NMR spectra of 2‐, 3‐, and 4‐nitrobenzoic acids

The influence of the position of nitro group toward the carboxylic group on the vibration structure of the molecule was estimated. Optimized geometrical structures were calculated (HF, B3PW91, B3LYP). Experimental and theoretical FT‐IR, FT‐Raman, and nuclear magnetic resonance (NMR) spectra of the title compounds were recorded and analyzed. The most important vibrational bands of nitro and carboxyl groups and the benzene ring were assigned. Wavenumbers and intensities for the three acids studied were compared and discussed. Data of chemical shifts in ¹H and ¹³C NMR spectra of 2‐, 3‐, and 4‐nitrobenzoic acids were analyzed in comparison with benzoic acid molecule. The calculated parameters are compared with experimental characteristics of these molecules. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Long-distance structural consequences of H-bonding. How H-bonding affects aromaticity of the ring in variously substituted aniline/anilinium/anilide complexes with bases and acids

The aromaticity of the ring in variously substituted aniline/anilinium/anilide derivatives in their H-bonded complexes with various Broensted acids and bases was a subject of an analysis based on 332 experimental geometries retrieved from the Cambridge Structural Database and geometries optimized at the B3LYP/6-311+G** and MP2/aug-cc-pVDZ levels of theory. Ab initio modeling was applied to the para-substituted aniline, anilinium cation, and anilide anion derivatives (X = NO, NO2, CN, CHO, H, CH3, OCH3, and OH) and their H-bonded complexes (only for X = NO, NO2, CHO, H, and OH) with B (B = F- and CN-) or HB (HB = HF and HCN). In both cases, the harmonic oscillator model of aromaticity index (HOMA) was used, whereas for computational geometries, additionally, the magnetism-based indices NICS, NICS(1), and NICS(1)zz were also applied (NICS = nucleus-independent chemical shift). There is an equivalent prediction of aromaticity by NICSs and HOMA and approximate monotonic dependences of HOMA and NICS on the C-N bond length. The strongest changes in aromaticity estimated by HOMA and NICSs were found for aniline derivatives with NH2...B and anilide derivatives without and with NH-...HB interactions. The changes observed for two other kinds of interactions, NH2...HB and NH3+...base (for anilinium cations), are much smaller. For all four kinds of interactions, the relationships between ipso-bond angle, mean ipso-ortho bond length, and C-N bond length follow the Bent-Walsh rule.     

Density Functional Calculations of the Anisotropic Effects of Borazine and 1,3,2,4‐Diazadiboretidine

On the basis of the nucleus‐independent chemical shift (NICS) concept, the anisotropic effects of two inorganic rings, namely, borazine and planar 1,3,2,4‐diazadiboretidine, are quantitatively calculated and visualized as isochemical shielding surfaces (ICSSs). Dissection of magnetic shielding values along the three Cartesian axes into contributions from σ and π bonds by the natural chemical shielding–natural bond orbital (NCS–NBO) method revealed that their appearance is not a simple reflection of the extent of (anti)aromaticity.     

Electronic Structures and Spectroscopic Characters of Modified Oligo(alkylenedioxypyrrole)

Polyalkylenedioxypyrrole (PADOP) exhibited an excellent conductivity experimentally. A series of oligomers for the electron‐rich monomer alkylenedioxypyrrole (ADOP) were designed in order to study properties of PADOP. The structures of these oligomers were optimized using density function theory (DFT) at B3LYP/6‐31G(d) level. The energy gaps and thermal stabilities of the oligomers were decreased when the chain lengths were increased. These properties were also decreased with the enlargement of the neighboring substituted rings. The ¹³C nuclear magnetic resonance (NMR) spectra and nucleus independent chemical shifts (NICS) of the oligomers were calculated at B3LYP/6‐31G(d) level. The chemical shifts at δ 96.1 of the linking carbon atoms in the dimer of 3,4‐methylenedioxypyrrole (MDOP) were moved downfield relative to those at δ 89.5 of the same carbon atoms in the monomer of MDOP. The aromaticity of the central pyrrole ring in the oligomers is improved with the enlargement of the neighboring substituted rings.     

Relationship between calculated NMR data and intermolecular hydrogen bond properties in X-pyridine⋯HF

The effect of different substituents in para and metapositions on the NMR data of X-pyridine?HF complex has been studied at B3LYP/6-311++G(d,p) level of theory. The relationship between NMR data and electron donation of substituents has been investigated. The results of topological properties of electron charge density calculated using atoms in molecules (AIM) analysis can be used to predict some NMR data. The magnetism-based indices, nucleus independent chemical shift NICS(1) and its z component NICS(1)_ZZ, were used to investigate the ring aromaticity changes on complexation. A linear correlation between Hammett coefficients and some NMR data could be found with a good correlation coefficient.     

The role of the 5f orbitals in bonding, aromaticity, and reactivity of planar isocyclic and heterocyclic uranium clusters

The molecular and electronic structures, stabilities, bonding features and magnetic properties of prototypical planar isocyclic cyclo-U n X n ( n = 3, 4; X = O, NH) and heterocyclic cyclo-U n (mu 2-X) n ( n = 3, 4; X = C, CH, NH) clusters as well as the E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] molecules including a planar tetracoordinate element E (ptE) and pentacoordinate U (ppU) at the ring centers, respectively, have been thoroughly investigated by means of electronic structure calculation methods at the DFT level. It was shown that 5f orbitals play a key role in the bonding of these f-block metal systems significantly contributing to the cyclic electron delocalization and the associated magnetic diatropic (magnetic aromaticity) response. The aromaticity of the perfectly planar cyclo-U n X n ( n = 3, 4; X = O, NH), cyclo-U n (mu 2-X) n ( n = 3, 4; X = C, CH, NH), E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] clusters was verified by an efficient and simple criterion in probing the aromaticity/antiaromaticity of a molecule, that of the nucleus-independent chemical shift, NICS(0), NICS(1), NICS zz (0) and the most refined NICS zz (1) index in conjunction with the NICS scan profiles. Natural bond orbital analyses provided a clear picture of the bonding pattern in the planar isocyclic and heterocyclic uranium clusters and revealed the features that stabilize the ptE's inside the six- and eight-member uranacycle rings. The ptE's benefit from a considerable electron transfer from the surrounding uranium atoms in the E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] clusters justifying the high occupancy of the np orbitals of the central atom E.     

全金属负离子团簇Ga42-、In42-芳香性的磁特征与多重芳香性

The optimized geometries, frequencies, and total electronic energies of two all-metal dianionic clusters Ga42- , In42- are calculated at the B3LYP, B3PW91, and MP2 levels of theory. There are two stable structures for each Ga42- , In42- species. For Ga42- , In42- species the square isomers are the most stable. On the basis of these computed stable structures we focus on two magnetic properties: magnetic susceptibility anisotropy and nucleus-independent chemical shift (NICS) for the square planar Ga42- , In42- isomers, which are calculated with B3LYP and HF methods. The computed results of NICS show that the square planar Ga42- , In42- isomers possess strong aromaticity. The detailed molecular orbital analysis for the two isomers further reveals that the two square planar Ga42- , In42- isomers have multiple-fold aromaticity: one delocalized π MOs and two delocalized σ MOs, which play important role in explaining the special stability of these all-metal square clusters.     

Magnetic properties and aromaticity of o-, m-, and p-benzyne

The relative aromaticities of the three singlet benzyne isomers, 1,2-, 1,3-, and 1,4-didehydrobenzenes have been evaluated with a series of aromaticity indicators, including magnetic susceptibility anisotropies and exaltations, nucleus-independent chemical shifts (NICS), and aromatic stabilization energies (all evaluated at the DFT level), as well as valence-bond Pauling resonance energies. Most of the criteria point to the o-benzyne相似文献