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Through the reactions of 1- or 2-naphthol and 4,5-dihydro-3H-benz[c]azepine or 6,7-dihydrothieno[3,2-c]pyridine, new aminonaphthol derivatives were prepared. The syntheses were extended by using N-containing naphthol analogues such as 5-hydroxyisoquinoline and 6-hydroxyquinoline. The ring closures of the novel bifunctional compounds were also achieved, resulting in new naphth[2,1-e][1,3]oxazines, naphth[1,2-e][1,3]oxazines, isoquinolino[5,6-e][1,3]oxazines and quinolino[5,6-e][1,3]oxazines. H-1 NMR spectra of the target heterocycles 16, 20 and 21 were sufficiently resolved to indentify the present stereochemistry; therefore, beside computed structures, spatial experimental (dipolar coupling-NOE) and computed (ring current effect of the naphthyl moiety-TSNMRS) NMR studies were employed. The studied heterocycles exist exclusively as S(14b),R(N), R(14b),S(N), and S(16b)S(N) isomers, respectively. The flexible moieties of the studied compounds prefer. (C) 2016 Elsevier Ltd. All rights reserved.
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.
A set of structures encompassing 1-(9-acridinyl)thiosemicarbazide and its 2-methyl derivative together with their various tautomeric structures; the 5-membered ring 1,3-thiazolidin-4-one products resulting from the reaction of 1- (9-acridinyl)thiosemicarbazide and its 2-methyl derivative with dimethyl acetylenedicarboxylate (DMAD) together with the alternative 6-membered ring isomeric reaction products as well as other potential isomeric structures; and the 6- membered ring 1,3-thiazin-4-one product resulting from the reaction of 2-methyl-1-(9-acridinyl)thiosemicarbazide with methyl propiolate (MP) together with the alternative 5-membered ring isomeric reaction product were all extensively studied by molecular modeling calculations using DFT at the B3LYP/6-31G(d,p) level of theory. The ring-chain tautomerism of the thiosemicarbazides, the regio- and stereoselectivity of the reactions, the adopted conformations and E/Z configurations of the products, the prototropic tautomerism of all the compounds, and the reasons for the predominance of the s-cis conformation of the Z configuration of the 1,3-thiazolidin-4-one product in particular were all extensively analyzed. Comparison of the modeled structures were also made to the 1,3-thiazolidin-4-one and 1,3-thiazin-4-one structures of the methyl derivative as well as 1-(9-acridinyl)thiosemicarbazide available from X-ray crystallographic analysis. Tactics utilizing spectroscopic methods {1R frequencies (nu) and NMR chemical shifts (delta), scalar coupling constants (J), and NOEs (eta)} in conjunction with molecular modeling calculations of the spectral parameters (frequency calculations (v) and NMR 6 using the GIAO method and J by calculation of the Fermi contact term) were evaluated in terms of proving 5- or 6-membered ring formation.
The manuscript describes the phytochemical investigation of the roots, leaves and stem bark of Millettia lasiantha resulting in the isolation of twelve compounds including two new isomeric isoflavones lascoumestan and las-coumaronochromone. The structures of the new compounds were determined using different spectroscopic techniques.
A set of structures encompassing 1-(9-acridinyl)thiosemicarbazide and its 2-methyl derivative together with their various tautomeric structures; the 5-membered ring 1,3-thiazolidin-4-one products resulting from the reaction of 1- (9-acridinyl)thiosemicarbazide and its 2-methyl derivative with dimethyl acetylenedicarboxylate (DMAD) together with the alternative 6-membered ring isomeric reaction products as well as other potential isomeric structures; and the 6- membered ring 1,3-thiazin-4-one product resulting from the reaction of 2-methyl-1-(9-acridinyl)thiosemicarbazide with methyl propiolate (MP) together with the alternative 5-membered ring isomeric reaction product were all extensively studied by molecular modeling calculations using DFT at the B3LYP/6-31G(d,p) level of theory. The ring-chain tautomerism of the thiosemicarbazides, the regio- and stereoselectivity of the reactions, the adopted conformations and E/Z configurations of the products, the prototropic tautomerism of all the compounds, and the reasons for the predominance of the s-cis conformation of the Z configuration of the 1,3-thiazolidin-4-one product in particular were all extensively analyzed. Comparison of the modeled structures were also made to the 1,3-thiazolidin-4-one and 1,3-thiazin-4-one structures of the methyl derivative as well as 1-(9-acridinyl)thiosemicarbazide available from X-ray crystallographic analysis. Tactics utilizing spectroscopic methods {IR frequencies (;) and NMR chemical shifts (;), scalar coupling constants (J), and NOEs (;)} in conjunction with molecular modeling calculations of the spectral parameters {frequency calculations (;) and NMR ; using the GIAO method and J by calculation of the Fermi contact term} were evaluated in terms of proving 5- or 6-membered ring formation.
Novel piperidine-fused benzoxazino- and
quinazolinonaphthoxazines-synthesis and conformational study
(2012)
The reactions of 1-(amino(2-hydroxyphenyl)methyl)-2-naphthol (3) and 1-(amino(2-aminophenyl) methyl)-2-naphthol (6) with glutardialdehyde resulted in the formation of piperidine-fused benzox-azinonaphthoxazine 4 and quinazolinonaphthoxazine 7, respectively, both in diastereopure form. The full conformational search protocols of 4 and 7 were successfully carried out by NMR spectroscopy and accompanying molecular modelling; the global minimum-energy conformers of all diastereomers were computed, and the assignments of the most stable stereoisomers, G(tct)(1) for 4 and G(tct)(1) for 7, were corroborated by spatial NOE information relating to the H-7a-H-10a-H-15b and H,H coupling patterns of the protons in the flexible part of the piperidine moiety. Additionally, mass spectrometric fragmentation was investigated in collision-induced dissociation experiments. The elemental compositions of the ions were determined by accurate mass measurements. (C) 2012 Elsevier Ltd. All rights reserved.
Synthesis and conformational analysis of new naphth[1,2-e][1,3]oxazino[3,4-c]quinazoline derivatives
(2011)
A new highly functionalized aminonaphthol derivative, 1-(amino(2-aminophenyl)methyl)-2-naphthol (4), was synthesized by the reaction of 2-naphthol, 2-nitrobenzaldehyde and tert-butyl carbamate or benzyl carbamate, followed by reduction and/or removal of the protecting group. The aminonaphthol derivative thus obtained was converted in ring-closure reactions with formaldehyde. benzaldehyde and/or phosgene to the corresponding naphth[1,2-e][1,3]oxazino[3,4-c]quinazoline derivatives. The conformational analysis of some derivatives by NMR spectroscopy and accompanying molecular modelling are also reported.
Syntheses and conformational analyses of new naphth[1,2-e][1,3]oxazino[3,2-c] quinazolin-13-ones
(2012)
The syntheses of naphth[1,2-e][1,3]oxazino[3,2-c]quinazolin-13-one derivatives (3a-f) were achieved by the solvent-free heating of benzyloxycarbonyl-protected intermediates (2a-f) with MeONa. For intermediates 2a-f, prepared by the reactions of substituted aminonaphthols with benzyl N-(2-formylphenyl)carbamate, not only the expected trans ring form B and chain form A(1), but also the rearranged chain form A(2) as a new tautomer were detected in DMSO at room temperature. The quantity of A(2) in the tautomeric mixture was changed with time.
Conformational analyses of the target heterocycles 3a-f by NMR spectroscopy and accompanying theoretical calculations at the DFT level of theory revealed that the oxazine ring preferred a twisted chair conformation and the quinazolone ring was planar. Besides the conformations, both the configurations at C-7a and C-15 and the preferred rotamers of the 1-naphthyl substituent at C-15 were assigned, which allowed evaluation of the aryl substituent-dependent steric hindrance in this part of the molecules. Configurational assignments were corroborated by quantifying the ring current effect of 15-aryl in terms of spatial NICS.
Synthesis and conformational analysis of new naphth[1,2-e][1,3]oxazino[3,4-c]quinazoline derivatives
(2011)
Through-space NMR shieldings were calculated for trinuclear metal-carbonyl compounds [M-3(CO)(12)] (M = Fe, Ru, Os), employing the nucleus-independent chemical shift approach. The through-space shieldings were visualized as a contour plot of iso-chemical shielding surfaces, and were applied to quantify the overall anisotropic effect of the carbonyl groups, as well as to identify the influence of the transition metal on the scopes of the corresponding anisotropy cones. The shielding surfaces show that the anisotropic effect of the carbonyl groups at equatorial positions changes depending on the metal. This effect was associated with pi-backdonation from the metal to the carbonyl groups in that position, in agreement with geometric data as well as calculated NMR parameters. Therefore, visualization of the through-space NMR shieldings of trinuclear metal-carbonyl compounds of group 8 is able to reflect the distinct arrangements of the carbonyl groups in these organometallic compounds.
A new isoflavone, 8-prenylmilldrone (1), and four new rotenoids, oblarotenoids A-D (2-5), along with nine known compounds (6-14), were isolated from the CH2Cl2/CH3OH (1:1) extract of the leaves of Millettia oblata ssp. teitensis by chromatographic separation. The purified compounds were identified by NMR spectroscopic and mass spectrometric analyses, whereas the absolute configurations of the rotenoids were established on the basis of chiroptical data and in some cases by single-crystal X-ray crystallography. Maximaisoflavone J (11) and oblarotenoid C (4) showed weak activity against the human breast cancer cell line MDA-MB-231 with IC50 values of 33.3 and 93.8 mu M, respectively.
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 delta(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.
A new functional group, the hydroxy group, was inserted into a Betti base by reaction with salicylaldehyde, and the naphthoxazine derivatives thus obtained were converted by ring-closure reactions with formaldehyde, acetaldehyde, propionaldehyde or phosgene to the corresponding naphth[1',2':5,6][1,3]oxazino[3,2-c][1,3]benzoxazine derivatives. Further, the conformational analysis of these polycyclic compounds by NMR spectroscopy and an accompanying molecular modelling are reported; especially, both quantitative anisotropic ring current effects of the aromatic moieties in these compounds and steric substituent effects were employed to determine the stereochemistry of the naphthoxazinobenzoxazine derivatives.
(3)J(C,H) coupling constants via a sulfur atom in two series of compounds, both including a sulfide, a sulfoxide and a sulfone, were detected experimentally and calculated by quantum mechanical methods. In the first series (1-3) the coupling between a hydrogen, bonded to an Sp(3) carbon, and an Sp(2) carbon is treated; the second series (4- 6) deals with the coupling between a hydrogen, bonded to an Sp3 carbon, and an Sp3 carbon. Different pulse sequences (broadband HMBC, SelJres, 1D HSQMBC, J-HMBC-2, selective J-resolved long-range experiment and IMPEACH-MBC) proved to be useful in determining the long-range (3)J(C,H) coupling constants. However, the dynamic behaviour of two of the compounds (4 and 6) led to weighted averages of the two coupling constants expected (concerning equatorial and axial positions of the corresponding hydrogens). DFT calculations proved to be useful to calculate not only the (3)J(C,H) coupling constants but also the different contributions of FC, PSO, DSO and SD terms; the calculation of the Fermi contact term (FC) was found to be sufficient for the correct estimation of (3)J(C,H) coupling constants. Copyright (C) 2004 John Wiley Sons, Ltd
The synthesis of a series of 1H-pyrazolo[3,4-b]quinoxalines (flavazoles) by acylation, alkylation, halogenation, and aminomethylation of the parent compound is reported and their structure is investigated by H-1, C-13 and N-15 NMR spectroscopy. The restricted rotation about the partial C, N double bond of the N-acyl derivatives 7-10 is studied by dynamic NMR spectroscopy and the barriers to rotation are determined. In order to assign unequivocally the 15 N chemical shifts of N-4 and N-9, in case of 3-substituted flavazoles, exemplary the H-1, C-13, and N-15 NMR chemical shifts of 34, 35, and 39 are also theoretically calculated by quantum chemical methods [ab initio at different levels of theory (HF/6-3G* and B3LYP/6-31G*)]. (C) 2005 Elsevier Ltd. All rights reserved
Through the cyclization of 1-(;-hydroxynaphthyl)-1,2,3,4-tetrahydroisoquinoline and 1-(;- hydroxynaphthyl)-1,2,3,4-tetrahydroisoquinoline with formaldehyde, phosgene, p-nitrobenzaldehyde or p-chlorophenyl isothiocyanate, 8-substituted 10,11-dihydro-8H,15bH-naphth[1,2-e][1,3]oxazino[4,3-a]isoquinolines (3 and 4) and 10,11- dihydro-8H,15bH-naphth[2,1-e][1,3]oxazino[4,3-a]isoquinolines (15 and 16) were prepared. Conformational analysis of both the piperidine and the 1,3-oxazine moieties of these heterocycles by NMR spectroscopy and an accompanying theoretical study revealed that these two conformationally flexible six-membered ring moieties prefer twisted chair conformers.
Competitive resonance at the carbonyl group as visualized by the natural bond orbital analysis
(1997)
The minima on the potential energy surface of 1,2-bis(o-carboxyphenoxy) ethane (CPE) molecule in its electronic ground state were searched by a molecular dynamics simulation performed with MM2 force field. For each of the found minimum-energy conformers, the corresponding equilibrium geometry, charge distribution, HOMO-LUMO energy gap, force field, vibrational normal modes and associated IR and Raman spectral data were determined by means of the density functional theory (DFT) based electronic structure calculations carried out by using B3LYP method and various Pople- style basis sets. The obtained theoretical data confirmed the significant effects of the intra- and inter-molecular hydrogen bonding interactions on the conformational structure, force field, and group vibrations of the molecule. The same data have also revealed that two of the determined stable conformers, both of which exhibit pseudo-crown structure, are considerably more favorable in energy to the others and accordingly provide the major c ntribution to the experimental spectra of CPE. In the light of the improved vibrational spectral data obtained within the "SQM FF" methodology and "Dual Scale Factors" approach for the monomer and dimer forms of these two conformers, a reliable assignment of the fundamental bands observed in the experimental room-temperature IR and Raman spectra of the molecule was given, and the sensitivities of its group vibratb20s to conformation, substitution and dimerization were discussed.
Based on the nucleus-independent chemical shift (NICS) concept, isotropic magnetic shielding values have been computed along the three Cartesian axes for ethene, cyclobutadiene, benzene, naphthalene, and benzocyclobutadiene, starting from the molecular/ring center up to 10;Å away. These through-space NMR spectroscopic shielding (TSNMRS) values, which reflect the anisotropic effects, have been broken down into contributions from localized- and canonical molecular orbitals (LMOs and CMOs); these contributions revealed that the proton NMR spectroscopic chemical shifts of nuclei that are spatially close to the C=C double bond or the aromatic ring should not be explained in terms of the conventionally accepted ;-electron shielding/deshielding effects. In fact, these effects followed the predictions only for the antiaromatic cyclobutadiene ring.
Novel piperidine-fused benzoxazino- and quinazolinonaphthoxazines-synthesis and conformational study
(2012)
The reactions of 1-(amino(2-hydroxyphenyl)methyl)-2-naphthol (3) and 1-(amino(2-aminophenyl)methyl)-2-naphthol (6) with glutardialdehyde resulted in the formation of piperidine-fused benzoxazinonaphthoxazine 4 and quinazolinonaphthoxazine 7, respectively, both in diastereopure form. The full conformational search protocols of 4 and 7 were successfully carried out by NMR spectroscopy and accompanying molecular modelling; the global minimum-energy conformers of all diastereomers were computed, and the assignments of the most stable stereoisomers, Gtct1 for 4 and Gtct1 for 7, were corroborated by spatial NOE information relating to the H7a-H10a-H15b and H,H coupling patterns of the protons in the flexible part of the piperidine moiety. Additionally, mass spectrometric fragmentation was investigated in collision-induced dissociation experiments. The elemental compositions of the ions were determined by accurate mass measurements.
Syntheses and conformational analyses of new naphth[1,2-e][1,3]oxazino[3,2-c]quinazolin-13-ones
(2012)
The syntheses of naphth[1,2-e][1,3]oxazino[3,2-c]quinazolin-13-one derivatives (3a-f) were achieved by the solvent-free heating of benzyloxycarbonyl-protected intermediates (2a-f) with MeONa. For intermediates 2a-f, prepared by the reactions of substituted aminonaphthols with benzyl N-(2-formylphenyl)carbamate, not only the expected trans ring form B and chain form A1, but also the rearranged chain form A2 as a new tautomer were detected in DMSO at room temperature. The quantity of A2 in the tautomeric mixture was changed with time. Conformational analyses of the target heterocycles 3a-f by NMR spectroscopy and accompanying theoretical calculations at the DFT level of theory revealed that the oxazine ring preferred a twisted chair conformation and the quinazolone ring was planar. Besides the conformations, both the configurations at C-7a and C-15 and the preferred rotamers of the 1-naphthyl substituent at C-15 were assigned, which allowed evaluation of the aryl substituent-dependent steric hindrance in this part of the molecules. Configurational assignments were corroborated by quantifying the ring current effect of 15-aryl in terms of spatial NICS.
NMR spectroscopic and theoretical structural analysis of 5,5-disubstituted hydantoins in solution
(1997)
NMR spectroscopic and theoretical structural analysis of 5,5-disubstituted hydantoins in solution
(1997)
The conformational equilibrium of the axial/equatorial conformers of 4-methylene-cyclohexyl pivalate is studied by dynamic NMR spectroscopy in a methylene chloride/freon mixture. At 153K, the ring interconversion gets slow on the nuclear magnetic resonance timescale, the conformational equilibrium (-G degrees) can be examined, and the barrier to ring interconversion (G(#)) can be determined. The structural influence of sp(2) hybridization on both G degrees and G(#) of the cyclohexyl moiety can be quantified.
The esters of 4-hydroxy-cyclohexanone and a series of carboxylic acids R-COOH with R of different electronic and steric influence (R=Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, sec-Bu, t-Bu, CF3, CH2Cl, CHCl2, CCl3, CH2Br, CHBr2, and CBr3) were synthesized and the conformational equilibria studied by 1H and 13C NMR spectroscopy at 103 K and at 295 K, respectively. The geometry of optimized structures of the axial/equatorial chair conformers was computed at the ab initio MO and DFT levels of theory. Only one preferred conformation was obtained for the axial and the equatorial conformer as well. When comparing the conformational equilibria of the cyclohexanone esters with those of the corresponding cyclohexyl esters a certain polarity contribution of the cyclohexanone framework was revealed, which is independent of the substituent effects and increases the stability of the axial conformers by a constant amount.
The esters of 4-hydroxy-cyclohexanone and a series of carboxylic acids R-COOH with R of different electronic and steric influence (R=Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, sec-Bu, t-Bu, CF3, CH2Cl, CHCl2, CCl3, CH2Br, CHBr2, and CBr3) were synthesized and the conformational equilibria studied by H-1 and C-13 NMR spectroscopy at 103 K and at 295 K, respectively. The geometry of optimized structures of the axial 'equatorial chair conformers was computed at the ab initio MO and DFT levels of theory. Only one preferred conformation was obtained for the axial and the equatorial conformer as well. When comparing the conformational equilibria of the cyclohexanone esters with those of the corresponding cyclohexyl esters a certain polarity contribution of the cyclohexanone framework was revealed, which is independent of the substituent effects and increases the stability of the axial conformers by a constant amount.
Configuration and stereodynamics of exo/endoisomeric push-pull alkenes of pentadiene structure
(1998)
NMR spectroscopic and ab-initio MO study of sterically hindered 2,3-disubstituted quinoxalines
(1999)
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.
Endohedral and external through-space NMR shieldings (TSNMRS) and the magnetic susceptibilities of the fullerene carbon cages of C50, C60, C60-6, C70, and C70-6 were assessed by ab initio molecular orbital calculations. Employing the nucleus-independent chemical shift (NICS) concept, these TSNMRS were visualized as isochemical shielding surfaces (ICSS) and were applied to quantitatively estimate either the aromaticity or the anti-aromaticity on the fullerene surface pertaining to the five- or six-membered ring moieties and the shielding of any nuclei enclosed within the carbon cages. Differences between the NICSs calculated at the center of the fullerene carbon cages and the experimental chemical shifts of encapsulated NMR-active nuclei as well as experimental shieldings observed for different encapsulated nuclei were able to be understood readily for the first time.
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.
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.
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 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.
The spatial magnetic properties (Through Space NMR Shieldings - TSNMRS) of a number of Y-shaped structures possessing 4n+2 pi-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. (C) 2016 Elsevier Ltd. All rights reserved.
13C chemical shifts of the push;pull oligoalkynes Don-(C;C)n-Acc (n = 1;4; Don = morpholino; Acc = COMe, COOMe) were computed at the DFT (B3LYP/6-311+G(d,p) level of theory compared with the experimental ; values and the agreement employed as a measure of quality for the underlying structures. For the global minima structures, the occupation quotients of antibonding ;* and bonding ; orbitals (;*C;C/ ;C;C) and the bond lengths (dC;C) of the various C;C triple bonds were also computed and correlated to each other. The linear dependence obtained for the two parameters dC;C and ;*C;C/ ;C;C quantifies changes in ;-delocalization induced by the push;pull effect of the substituents and 1,3-conjugation (1,3,5- and 1,3,5,7-, respectively) of the C;C bonds in the oligoalkynes studied. A critical comparison of the push;pull effect, attenuated with increasing n, and the conjugative stabilization of the oligoalkynes, increasing with n, as concluded from dC;C and ;*C;C/;C;C of the oligoalkynes and the reference compounds Me-(C;C)n-Me, Don-(C;C)n-Me, and Me-(C;C)n-Acc), respectively (Don = morpholino; Acc = COMe, COOMe), is affiliated.
The through space NMR shielding (TSNMRS) values of two tricyclobutabenzene (TCBB) derivatives 2, of the corresponding hexamethylene and hexaoxo TCBB derivatives 3, of [4n]annuleno[4n + 2]annulene 5 and of its tricyclobutadiene parent compound 4 have been ab initio calculated by the GIAO perturbation method employing the nucleus- independent chemical shift (NICS) concept of Paul von Ragué Schleyer, and visualized as iso-chemical shielding surfaces (ICSS). TSNMRS values can be successfully employed to quantify and visualize the aromaticity of the central, and in 5 also of the terminal benzene ring moieties.
Prototypes for homoaromaticity in cations, neutral molecules, and anions are theoretically studied at the MP2 level of theory. For the global minimum structures on the potential energy surface both 1H/13C chemical shifts and spatial magnetic properties as through space NMR shieldings (TSNMRS) were calculated by the GIAO perturbation method. The TSNMRS are visualized as iso-chemical-shielding surfaces (ICSS) of different sign and size. Coincident experimental and computed 1H/13C chemical shifts afforded the possibility to decide from the TSNMRSs at hand on both the existence and the size of homoaromaticity in the molecules studied.
The through space NMR shieldings (TSNMRS) of dodecahedrane C20H20, of the isomeric hydrocarbons C20H12, of the ions C20H122+ and C20H122- of the fluxional fullerene C20 and of its dication C202+ have been ab initio calculated employing the NICS concept on basis of MP2/6-31G* geometries and visualized as iso-chemical-shielding/deshielding surfaces (ICSSs). TSNMRS values were employed to study the exohedral magnetic properties of the compounds studied. Hereby, the curved It-conjugation in the compounds studied could be quantified.
Identification of benzenoid and quinonoid structures by through-space NMR shieldings (TSNMRS)
(2010)
Rotation about the single bond adjoining the aryl and fluorene moieties in 9-arylfluorenes can be frozen out on the NMR timescale if methyl groups are located at either one or both of the ortho positions of the aryl substituent. In the ground-state of these rotamers, the planes of the aryl and fluorene moieties are perpendicular to each other and the methyl protons are either shielded or deshielded, respectively, due to the ringe current effect of the fluorene moiety. This anisotropic effect on the 1H chemical shifts of the methyl protons has been quantified on the basis of through- space NMR shieldings (TSNMRS) and subsequently _______ compared with the experimentally observed chemical shift differences _____. In this context, the experimental anisotropic effects of functional groups in the !H NMR have proven to quantitatively be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS). Differences between _______ and _____ were, for the first time, also quantified as arising from steric compression.
The spatial magnetic properties (Through-Space NMR Shieldings-TSNMRS) of already synthesized dehydro[n]annulenes of various ring size (from C-12 to C-20) have been computed, visualized as Isochemical Shielding Surfaces (ICSS) of various size and direction, and were examined subject to present (anti)aromaticity. For this purpose the thus quantified ring current effect of the macro cycles on proximate protons in proton NMR spectra was employed.
The spatial magnetic properties (through-space NMR shieldings, or TSNMRSs) of the antiaromatic 9-oxaanthracene anion 12(-) and of the corresponding 9-dimeric dianion 11(2-) have been calculated by the gauge-invariant atomic orbitals (GIAO) perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSSs) of various size and direction. The TSNMRS values, thus obtained, can be employed to indicate antiaromaticity by paratropic ring currents of the anionic compounds of 11(2-) and 12(-) studied and other neutral and ionic antiaromatic molecules from previous studies because anisotropic effects of functional groups in H-1 NMR spectra have quantitatively proven to be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS).