Refine
Has Fulltext
- no (307) (remove)
Year of publication
Document Type
- Article (299)
- Monograph/Edited Volume (3)
- Other (3)
- Conference Proceeding (1)
- Review (1)
Is part of the Bibliography
- yes (307) (remove)
Keywords
- Conformational analysis (14)
- NMR spectroscopy (9)
- conformational analysis (9)
- NICS (8)
- Theoretical calculations (8)
- Through-space NMR shieldings (TSNMRS) (8)
- Anisotropy effect (7)
- Ring current effect (7)
- Aromaticity (6)
- NMR (6)
- DFT calculations (5)
- Dynamic NMR (5)
- Quantum chemical calculations (5)
- quantum chemical calculations (5)
- ICSS (4)
- TSNMRS (4)
- Conformational equilibrium (3)
- GIAO (3)
- Gas phase electron diffraction (3)
- NBO analysis (3)
- dynamic NMR (3)
- (Anti)aromaticity (2)
- Anisotropic effect (2)
- Barrier to ring inversion (2)
- Chelatoaromaticity (2)
- DFT (2)
- Density functional calculations (2)
- Dynamic NMR spectroscopy (2)
- H-1 NMR (2)
- Iso-chemical-shielding surfaces (ICSS) (2)
- NHCs (2)
- Push-pull character (2)
- Push-pull effect (2)
- Stereochemistry (2)
- Steric effect (2)
- Taft equation (2)
- anisotropic effects (2)
- aromaticity (2)
- low-temperature NMR spectroscopy (2)
- modified Mannich reaction (2)
- shieldings (TSNMRS) (2)
- (1)H NMR (1)
- (13)C NMR (1)
- (TSNMRS) (1)
- 1,1-dimethyl-1,2,3,4-tetrahydrosiline (1)
- 1,2,4-Dithiazole (1)
- 1,2-Dithiole (1)
- 1,2-diboretane-3-ylidene (1)
- 1,3-Azasilinanes (1)
- 1,3-Dimethyl-3-phenyl-1,3-azasilinane (1)
- 1,3-Oxasilinanes (1)
- 1,4,2-Oxazasilinanes (1)
- 1-(Dimethylamino)-1-phenyl-1-silacyclohexane (1)
- 1-Methylthio-1-phenyl-1-silacyclohexane (1)
- 2 (1)
- 2,2-Disubstituted adamantane derivatives (1)
- 2-Alkylidene-4-oxothiazolidine (1)
- 2-Substituted adamantane derivatives (1)
- 3,4-Dihydroisoquinoline (1)
- 3,4-dihydro-2H-pyran (1)
- 3,4-dihydro-2H-thiopyran (1)
- 3-Fluoro-3-methyl-3-silatetrahydropyran (1)
- 3-Silatetrahydropyrans (1)
- 3-silathianes (1)
- 3c,2e-bonding (1)
- 4,4-dimethyl-3,4-dihydro-2H-1,4-thiasiline (1)
- 4-Oxothiazolidine (1)
- 4-Substituted cyclohexanones (1)
- 4-methylene-cyclohexyl pivalate (1)
- 4-silapiperidines (1)
- 4-silathianes (1)
- 6-disilamorpholines (1)
- 9-Arylfluorenes (1)
- A-values of COOAr on cyclohexane (1)
- ALTONA equation (1)
- ATR-FTIR (1)
- Ab initio MO computations (1)
- Additivity of conformational energies (1)
- Aminonaphthol (1)
- Aminonaphthols (1)
- Annelation effect (1)
- Anserine (1)
- Anti-aromaticity (1)
- Anticancer (1)
- Antileishmanial (1)
- Antiplasmodial (1)
- Aromatic or quinonoid (1)
- Assignment of stereochemistry (1)
- Asteraceae (1)
- B,N heterocycles (1)
- B3LYP/6-31+G(d,p) calculations (1)
- B3LYP/6-311++G** (1)
- Barrier to rotation about C-N bond (1)
- Benzazepine (1)
- Benzenoid structure (1)
- Benzenoid structures (1)
- Benzoic acid esters (1)
- Benzyne-allene or cumulene-like structure (1)
- Betaines (1)
- Binding pocket position (1)
- C-13 (1)
- C-13 NMR (1)
- C-13 NMR spectroscopy (1)
- C-13 chemical shift (1)
- C-13 chemical shift difference Delta delta(C C) (1)
- CAACs (1)
- CH center dot center dot center dot O hydrogen bonds (1)
- Carbamoyl tetrazoles (1)
- Carbene or zwitterions (1)
- Carbenes (1)
- Carbohydrates (1)
- Carvotacetones (1)
- Chiral dopants (1)
- Condensed thiazolidines (1)
- Conformation analysis (1)
- Conformational equilibria (1)
- Copper Metal Complexes (1)
- Cyanine/merocyanine-like structures (1)
- Cyclazines (1)
- Cyclobutylcarbene (1)
- Cyclohexyl esters (1)
- DFT and MP2 calculations (1)
- DFT and MP2 simulation (1)
- DFT calculation (1)
- DFT structural study (1)
- DFT theoretical calculations (1)
- Dative vs. coordinative NHC -> BR3 bond (1)
- Dehydro[n]annulenes (1)
- Diastereomers assignment (1)
- Diastereoselectivity (1)
- Dual Scale Factors (1)
- Dual scale factors (1)
- Dynamic H-1-NMR (1)
- Electrostatic effects (1)
- F-19 (1)
- GIAO calculations (1)
- Gas-phase electron diffraction (1)
- Glycol podands (1)
- H-1 (1)
- H-1 NMR spectroscopy (1)
- Hammett-Brown plots (1)
- Hemiporphyrazines (1)
- Heterocycles (1)
- IR and Raman spectra (1)
- Iso-chemical shielding surfaces (ICSS) (1)
- Isothiocyanic acid (1)
- Low temperature NMR spectroscopy (1)
- Low-temperature C-13 and Si-29 NMR (1)
- Low-temperature NMR (1)
- Low-temperature d-NMR (1)
- M062X/6-311G** calculations (1)
- MP2 (1)
- MP2 and CCSD(T) calculations (1)
- Matrix IR spectrum (1)
- Mesomeric equilibrium of carbene/zwitterion (1)
- Mesomerism (1)
- Modified Mannich reaction (1)
- Molecular dynamics (1)
- Multiple NHC(CAAC)-Boron bonds (1)
- N-acetyl glucosamine derivatives (1)
- N-unsubstituted (primary)S-thiocarbamates (1)
- N-unsubstituted(primary)O-thiocarbamates (1)
- NBO and STERIC analyses (1)
- NBO/NCS analysis (1)
- Naphthoxazines (1)
- Naphthoxazinoquinazolines (1)
- Naphthoxazinoquinazolinones (1)
- Occupation quotient pi*/pi (1)
- Peripheral ring current (1)
- Polar effect (1)
- Polar substituent constant (1)
- Porphyrins (1)
- Push-pull alkynes (1)
- Push-pull allenes (1)
- Quantum Chemical Calculations (1)
- Quasi-aromaticity (1)
- Quinazolines (1)
- Quinonoid structure (1)
- Quotient method (1)
- RA-intramolecular hydrogen bond (1)
- Rearrangement to trithiaazapentalene (1)
- Residual dipolar couplings (1)
- Ring-current effect (1)
- SQM FF (1)
- SQM-FF (1)
- Salicylic acid (1)
- Silacyclohexanes (1)
- Silaheterocyclohexanes (1)
- Silica sulfuric acid (1)
- Simulation of H-1 NMR spectra (1)
- Solid acid (1)
- Solvent effects (1)
- Solvent-free (1)
- Spatial NICS (1)
- Sphaeranthus bullatus (1)
- Steric effects (1)
- Steric hindrance (1)
- Steric substituent constant (1)
- Substituent chemical shifts (1)
- Substituent effects (1)
- Sulfoxide (1)
- Supramolecular compounds (1)
- Tautomerism (1)
- Tetraoxo[8]circulenes (1)
- Thienopyridine (1)
- Through -space NMR (1)
- Through-space NMR (1)
- Through-space NMR shielding (TSNMRS) (1)
- Trithiapentalene (1)
- Trough-space NMR shieldings (TSNMRS) (1)
- Twisted double bonds (1)
- Vinylogous N-acyliminium ion (1)
- X-ray analysis (1)
- Y-aromaticity (1)
- Ylide (1)
- [4+2] cycloaddition (1)
- ab initio calculations (1)
- barrier to ring inversion (1)
- benzenoid structures (1)
- carbene electron deficiency (1)
- carbenes (1)
- cis,cis-Tricyclo[5.3.0.0(2,6)]dec-3-enes (1)
- computational chemistry (1)
- conformational equilibrium (1)
- cyclic imines (1)
- cycloaddition (1)
- density functional calculations (1)
- dielectric spectroscopy (1)
- dynamic NMR spectroscopy (1)
- endo-Mode cyclization (1)
- ephedrine/pseudoephedrine (1)
- exo-methylene conformational effect at cyclohexane (1)
- low temperature NMR spectroscopy (1)
- molecular structure (1)
- nucleus-independent chemical shift (1)
- nucleus-independent chemical shifts (NICS) (1)
- onformational analysis (1)
- ortho-quinone methide (o-QMs) (1)
- para-Nitro-pyridine N-oxides (1)
- pi interactions (1)
- pi-Electron delocalization (1)
- pi-Stacking (1)
- quinoid structures (1)
- restricted N-S rotation (1)
- silacyclohexanes (1)
- silapiperidines (1)
- siloxanes (1)
- spectroscopy (1)
- sulfimides (1)
- sulfur heterocycles (1)
- through space NMR shieldings (1)
- zwitterions (1)
Institute
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-it or triel bonds. & COPY; 2023 Elsevier Ltd. All rights reserved.
The low temperature (95 K) NMR study of 1-Ph-1-t-Bu-silacyclohexane (1) showed the conformational equilibrium to be extremely one-sided toward thePh(ax),t-Bueq conformer. The barrier to interconversion has been measured (4.2-4.6 kcal/mol) and the conformational equilibrium [Delta nu = 1990.64 ppm (Si-29), 618.9 ppm (C-13), 1-Ph-ax:1-Pheq = (95.6-96.6%):(3.4-4.4%), K = 25 +/- 3, Delta G degrees = -RT ln K = 0.58-0.63 kcal/mol] analyzed. The assignment and quantification of the NMR signals is supported by MP2 and DFT calculations.
The synthesis of new phenanthr[9,10-e][1,3]oxazines was achieved by the direct coupling of 9-phenanthrol with cyclic imines in the modified aza-Friedel-Crafts reaction followed by the ring closure of the resulting bifunctional aminophenanthrols with formaldehyde. Aminophenanthrol-type Mannich bases were synthesised and transformed to phenanthr[9,10-e][1,3]oxazines via [4 + 2] cycloaddition. Detailed NMR structural analyses of the new polyheterocycles as well as conformational studies including Density Functional Theory (DFT) modelling were performed. The relative stability of ortho-quinone methides (o-QMs) was calculated, the geometries obtained were compared with the experimentally determined NMR structures, and thereby, the regioselectivity of the reactions has been assigned.
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 H-1 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 B-11/C-13 chemical shift variations in the BR3 trapped NHCs.
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 H-1 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 C-13 chemical shifts of the electron-deficient carbon centers.
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 H-1 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 delta(B-11)/ppm data and the C-13 chemical shifts of the carbene electron-deficient centre.
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 C-13 chemical shifts were calculated and compared with available experimental bond lengths and delta(C-13)/ppm values (also, as a quality criterion for the computed structures). Bond lengths, the delta(C-13)/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.
Both the C-13 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 C-13 chemical shifts were calculated and compared with the experimental delta(C-13)/ppm values and geometry parameters (as a quality criterion for obtained structures).
Structure 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 in H-1 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 [delta(H-1)/ppm, delta(N-15)/ppm] and geometry (planar, twisted, bow-shaped) data. The cyclazines N[2.2.2](-) up to N[2.4.4](-) are planar or at most slightly bowl-shaped and, due to coherent peripheral ring currents (except in N[2.3.3](-), N[2.3.4], N[3.3.4](+) and N[2.4.4](+)), develop aromaticity or anti-aromaticity of the whole molecules dependent on the number of peripheral conjugated pi electrons. The cyclazines N[2.3.3](-), N[2.3.4], N[3.3.4](+) and N[2.4.4](+) develop two ring currents of different direction within the same molecule, in which the dominating ring current proves to be paratropic (in N[3.3.4](+) diatropic) including the nodal N p(z) lone pair into the conjugation. The residual cyclazines N[3.4.4], N[4.4.4](-) and N[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 pi electrons, the corresponding (anti)aromaticity is in unequivocal accordance with Huckel's rule.
The molecular structure and conformational preferences of 1-phenyl-1-X-1-silacyclohexanes C5H10Si(Ph,X) (X = F (3), Cl (4)) were studied by gas-phase electron diffraction, low-temperature NMR spectroscopy, and high-level quantum chemical calculations. In the gas phase only three (3) and two (4) stable conformers differing in the axial or equatorial location of the phenyl group and the angle of rotation about the Si-C-ph bond (axi and axo denote the Ph group lying in or out of the X-Si-C-ph plane) contribute to the equilibrium. In 3 the ratio Ph-eq:Ph-axo:Ph-axi is 40(12):55(24):5 and 64:20:16 by experiment and theory, respectively. In 4 the ratio Ph-eq:Ph-axo is 79(15):21(15) and 71:29 by experiment and theory (M06-2X calculations), respectively. The gas-phase electron diffraction parameters are in good agreement with those obtained from theory at the M06-2X/aug-ccPVTZ and MP2/aug-cc-pVTZ levels. Unlike the case for M06-2X, MP2 calculations indicate that 3-Ph-eq conformer lies 0.5 kcal/mol higher than the 3-Ph-axo, conformer. As follows from QTAIM analysis, the phenyl group is more stable when it is located in the axial position but produces destabilization of the silacyclohexane ring: By low temperature NMR spectroscopy the six-membered ring interconversion could be frozen, at 103 K and the present conformational equilibria of 3 and 4 could be determined. The ratio of the conformers is 3-Ph-eq:3-Ph-ax = (75-77):(23-25) and 4-Ph-eq:4-Ph-ax = 82:18.