Refine
Has Fulltext
- no (299)
Year of publication
Document Type
- Article (299) (remove)
Is part of the Bibliography
- yes (299) (remove)
Keywords
- Conformational analysis (14)
- NMR spectroscopy (9)
- conformational analysis (9)
- NICS (8)
- Theoretical calculations (8)
- Through-space NMR shieldings (TSNMRS) (8)
- Ring current effect (7)
- Anisotropy effect (6)
- NMR (6)
- Aromaticity (5)
- 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)
- 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)
- 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)
- 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)
- 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)
- Chelatoaromaticity (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)
- 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)
- 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)
- Stereochemistry (1)
- Steric effects (1)
- Steric hindrance (1)
- Steric substituent constant (1)
- Substituent chemical shifts (1)
- Substituent effects (1)
- Sulfoxide (1)
- Tautomerism (1)
- Tetraoxo[8]circulenes (1)
- Thienopyridine (1)
- Through -space NMR (1)
- Through-space NMR (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
Both the structure and intramolecular flexibility of a series of aza crown ethers were studied by experimental NMR and theoretical molecular modeling. The stoichiometries of complexation to the anions H2PO4- and resulting complex stabilities were determined by experimental NMR (1H, 31P) titration and, in addition, the structure and mobility changes of the aza crown ethers upon complexation were also examined.
The conformational equilibria of 3-methyl-3-silathiane 5, 3-fluoro-3-methyl-3-silathiane 6 and 1-fluoro-1- methyl-1-silacyclohexane 7 have been studied using low temperature 13C NMR spectroscopy and theoretical calculations. The conformer ratio at 103;K was measured to be about 5ax:5eq;=;15:85, 6ax:6eq;=;50:50 and 7ax:7eq;=;25:75. The equatorial preference of the methyl group in 5 (0.35;kcal;mol;1) is much less than in 3-methylthiane 9 (1.40;kcal;mol;1) but somewhat greater than in 1-methyl-1- silacyclohexane 1 (0.23;kcal;mol;1). Compounds 5-7 have low barriers to ring inversion: 5.65 (ax;;;eq) and 6.0 (eq;;;ax) kcal mol;1 (5), 4.6 (6), 5.1 (Meax;;;Meeq) and 5.4 (Meeq;;;Meax) kcal;mol;1 (7). Steric effects cannot explain the observed conformational preferences, like equal population of the two conformers of 6, or different conformer ratio for 5 and 7. Actually, by employing the NBO analysis, in particular, considering the second order perturbation energies, vicinal stereoelectronic interactions between the Si-X and adjacent C-H, C-S, and C-C bonds proved responsible.
Structures, C-13 chemical shifts, and the occupation quotients of anti-bonding pi* and bonding pi orbitals of the C C triple bond along a series of push-pull alkynes (p)X-C6H4 C(O)-C C-NH-C6H4-Y(P) (X,Y= H, Me, OMe, NMe2, NO2, COMe, COOMe, F, Cl, Br) were computed at the DFT level (B3LYP/6-311G**) of theory. Both the stereochemistry (cis/trans-isomers) by steric twist and the push-pull character by both C-13 chemical shift differences (Delta delta(C C)) and the occupation quotient (pi(C C)/pi(C C)) were studied; the latter two parameters can be readily employed to precisely quantify the push-pull effect in alkynes. (C) 2014 Elsevier B.V. All rights reserved.
The anisotropic effect of a proximally introduced ethynyl group on the chemical shifts of H-4 and C-4 of the phenanthrene skeleton was calculated using GIAO-HF/NICS methodology. The anisotropic effect, long considered to be the source of the considerable downfield shift of H-4 in 11-ethynylphenanthrene in comparison to the chemical shift value of the corresponding proton in phenanthrene, was determined to be only negligible in magnitude on the basis of these calculations. Partitioning of the natural chemical shieldings of H-4 and C-4 by the NCS-NBO method into various contributions from the C-C and C-H bonds present in each molecule revealed that steric compression was able to account for the large downfield shifts of both H-4 and C-4 in 11-ethynylphenanthrene relative to phenanthrene. Thus, the substituent effect is almost totally permeated by this latter interaction and not by the aforementioned process, which was previously presumed to be the sole underlying cause
The ring current effects of aromatic moieties and the anisotropic effects of the C=O and C-X (X = C, N, S) bonds and of the NH=C(NH2)-NH- moiety in the side chains of amino acid residues of proteins were ab initio calculated based on nuclear independent chemical shieldings as employed by P.v.R. Schleyer. Hereby, quantitative information about the spatial extension, sign and scope of the corresponding ring current/anisotropic effects was obtained and they were visualized as iso-chemical-shielding-surfaces. Examining this quantitative information compared with experimental NMR chemical shifts, the role of the corresponding amino acid residues in binding substrates in the binding site of enzymes was studied. (C) 2004 Elsevier B.V. All rights reserved
Through space NMR shieldings of aromatic (benzene, mono-substituted and annelated benzenes, ferrocene, [14]- and [18]-annulenes, phenylenes and tetra- to heptahelicene) and anti-aromatic molecules (cyclobutadiene and pentalene) were assessed by ab initio molecular-orbital calculations. Employing the nucleus-independent chemical shifts (NICS) concept, these through space NMR shieldings were visualized as iso-chemical-shielding surfaces (ICSSs) and can be applied quantitatively to determine the stereochemistry of proximal nuclei. In addition, the distances in Å at ICSS values of ±0.1 ppm in-plane and perpendicular-to-center of the aromatic ring system were employed as a simple means to compare and estimate qualitatively the aromaticity of the systems at hand.
The H-1 and C-13 NMR spectra of a number of push-pull alkenes were recorded and the C-13 chemical shifts calculated employing the GIAO perturbation method. Of the various levels of theory tried, MP2 calculations with a triple- zeta-valence basis set were found to be the most effective for providing reliable results. The effect of the solvent was also considered but only by single-point calculations. Generally, the agreement between the experimental and theoretically calculated C-13 chemical shifts was good with only the carbons of the carbonyl, thiocarbonyl, and cyano groups deviating significantly. The substituents on the different sides of the central C=C partial double bond were classified qualitatively with respect to their donor (S,S < S,N < N,N) and acceptor properties (CdropN < C=O < C=S) and according to the ring size on the donor side (6 < 7 < 5). The geometries of both the ground (GS) and transition states (TS) of the restricted rotation about the central C=C partial double bond were also calculated at the HF and MP2 levels of theory and the free energy differences compared with the barriers to rotation determined experimentally by dynamic NMR spectroscopy. Structural differences between the various push-pull alkenes were reproduced well, but the barriers to rotation were generally overestimated theoretically. Nevertheless, by correlating the barriers to rotation and the length of the central C=C partial double bonds, the push-pull alkenes could be classified with respect to the amount of hydrogen bonding present, the extent of donor-acceptor interactions (the push-pull effect), and the level of steric hindrance within the molecules. Finally, by means of NBO analysis of a set of model push-pull alkenes (acceptors: - CdropN, -CH=O, and -CH=S; donors: S, O, and NH), the occupation numbers of the bonding pi orbitals of the central C=C partial double bond were shown to quantitatively describe the acceptor powers of the substituents and the corresponding occupation numbers of the antibonding pi* orbital the donor powers of the substituents. Thus, for the first time an estimation of both the acceptor and the donor properties of the substituents attached to the push-pull double bond have been separately quantified. Furthermore, both the balance between strong donor/weak acceptor substituents (and vice versa) and the additional influences on the barriers to rotation (hydrogen bonding and steric hindrance in the GSs and TSs) could be differentiated
The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of benzenoid and quinoid tautomeric structures such as benzodifurantrione and phenazine-type molecules have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept of Paul von Rague Schleyer and visualized as iso- chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to quantify and visualize the partial aromaticity of the studied compounds. In the case of the surprisingly more stable quinoid tautomers, the aromaticity-synonymous with stability due to the conjugation of p electrons and lone pairs-was not found to be particularly reduced.
The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of benzenoid and quinoid tautomeric structures such as benzodifurantrione and phenazine-type molecules have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept of Paul von Rague Schleyer and visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to quantify and visualize the partial aromaticity of the studied compounds. In the case of the surprisingly more stable quinoid tautomers, the aromaticity-synonymous with stability due to the conjugation of p electrons and lone pairs-was not found to be particularly reduced.
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 CIAO 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,pi-electron delocalization can be qualified.