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Fernando Sancassan - One of the best experts on this subject based on the ideXlab platform.
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A theoretical and NMR Lanthanide-Induced Shift (LIS) investigation of the conformations of lactams.
Magnetic resonance in chemistry : MRC, 2017Co-Authors: Raymond J. Abraham, Marco Filippi, Giovanni Petrillo, Paola Piaggio, Chiara Vladiskovic, Fernando SancassanAbstract:Molecular mechanics (MM) with MMFF94 and MMX force-fields and ab initio (RHF/6-31G*,RHF/6-311G** and B3LYP/6-311G** ) calculations are used with LIS to investigate the conformations of N-methyl-2-pyrrolidone 1, N-methyl-2-piperidone 2, e−caprolactam 3, γ-valerolactam (1,5-dimethyl-2-pyrrolidone) 4, 2-azetidinone 5, 4-methyl azetidinone 6, 4-phenyl azetidinone 7, and N-methyl-4-phenyl azetidinone 8. The Yb(fod)3 paramagnetic Induced Shifts of all the 1H and 13C nuclei are measured and the corresponding diamagnetic complexation Shifts obtained by the addition of Lu(fod)3. The complexation model (two-, three- or four-site) used depends on the relative rates of the processes involved. The amide inversion is the same order as that of the five and six membered lactam rings and much faster than the Lanthanide complexation and the inversion of the seven membered ring. Both MM and ab initio calculations give an envelope conformation for 1 with C-4 out of the ring plane in agreement with the LIS analysis. For the piperidone ring of 2 the half-chair is calculated as the most stable form. The LIS analysis confirms this but cannot exclude a small amount ( 1.5%). In 4-phenyl azetidinone 7 the MMFF94 geometry with 60% of the axial conformer gave Rcryst 1.2% but the other geometries Rcryst > 1.5%. In contrast the N-methyl-4-phenyl-2-azetidinone 8 gave good agreement for all the geometries. The butterfly conformation gave Rcryst 1.1% for 80% of the axial conformer and the planar geometries Rcryst 0.98%. The LIS results confirm the ab initio and MM optimised geometries but the conformer energies at times differ from the calculated values. They also differ considerably from the corresponding values for the lactones studied previously and possible reasons for this are discussed.
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eMagRes - Lanthanide Induced Shifts (LIS) in Structural and Conformational Analysis
Encyclopedia of Magnetic Resonance, 2011Co-Authors: Raymond J. Abraham, Fernando SancassanAbstract:The uses and applications of Lanthanide Induced Shifts (LIS) in structural and conformational analysis are reviewed. The theoretical basis of LIS including contact, pseudocontact, and complexation Shifts is considered and a reliable procedure for evaluating the pseudocontact Shift at any nucleus is given. The simplified complexation models used in the LIRAS (Lanthanide Induced relaxation and Shifts) suite of programs are described and their applications to structural analysis are discussed. This includes lutetium versus lanthanum in probing diamagnetic Shifts, the conditions required for evaluating molecular geometries by this technique, and the development of the LISA (Lanthanide Induced Shift analysis) suite to take account of the diamagnetic Shifts in a more complete analysis. Some illustrative examples of the use of this technique are given. The solution conformation of bicyclo [3,1,0] hexan-3-one was determined as a flattened boat conformation. The molecular geometries of a number of 4-substituted and 2,6-disubstituted benzaldehydes, acetophenones, and benzoates were deduced and considered with respect to theoretical studies. The results showed clearly that LIS can be used to directly test the molecular geometries. The solution conformations of some α,β-unsaturated carbonyl compounds (aldehydes, ketones, and esters) were investigated and the s-cis–s-trans conformer energies were obtained. The conformations of some aromatic SO compounds were obtained and compared with theoretical predictions. Also reference is given to the use of LIS in alcohols, lactones, quinolines, cyclic amines, 1,3-dioxanes, and epoxides. Keywords: Lanthanide Induced Shifts; contact Shifts; pseudo contact Shifts; diamagnetic Shifts; LIRAS (Lanthanide Induced relaxation and Shifts); LISA (Lanthanide Induced Shift analysis)
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conformational analysis part 31 1 a theoretical and Lanthanide Induced Shift lis investigation of the conformations of some epoxides
Journal of The Chemical Society-perkin Transactions 1, 1999Co-Authors: Raymond J. Abraham, Fernando Sancassan, Irene Castellazzi, Timothy A D SmithAbstract:An improved LIS technique, using Yb(fod)3 to obtain the paramagnetic Induced Shifts of all the spin ½ nuclei in the molecule, together with complexation Shifts obtained by the use of Lu(fod)3, has been used to investigate the conformations of a group of epoxides. These are cis (1) and trans (2) stilbene oxide, cyclopentene oxide (3), cyclohexene oxide (4), cycloheptene oxide (5), propene oxide (6) and styrene oxide (7).The LIRAS3 complexation model involving two symmetric lone pairs on the oxygen atom was used for the symmetric compounds but, for the unsymmetric compounds, a more complex unsymmetric complexation model (HARDER) was found to be necessary. The calculated LIS for styrene oxide and cis- and trans-stilbene oxide were in excellent agreement with the observed data for both the molecular mechanics (MM) and the ab initio geometries with the phenyl ring dihedral angles optimised.In styrene oxide and trans-stilbene oxide the phenyl rings are approximately perpendicular to the oxirane ring, in agreement with the conformation in the solid state and with the theoretical calculations. In cis-stilbene oxide steric repulsions between the phenyl rings splay them apart so that they are now exo to the oxirane ring. Again the LIS analysis is in good agreement with the theoretical calculations.Both the LIS data and the modelling studies agree that cyclopentene oxide is in a boat conformation with an angle of pucker of ca. 30° and that cyclohexene oxide is in a half-chair conformation with C4 and C5 displaced from the ring plane.The LIS analysis of cycloheptene oxide gave good agreement for two equilibrating chair conformations with an endo/exo ratio of 70∶30, in excellent agreement with low temperature NMR data.The accurate reproduction of the LIS data provides an unambiguous method of assigning the proton chemical Shifts of the individual methylene protons in the cyclic epoxides, which are not easily available by any other technique.
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conformational analysis part 30 the conformational analysis of some lactones by the Lanthanide Induced Shift lis technique
Journal of The Chemical Society-perkin Transactions 1, 1997Co-Authors: Raymond J. Abraham, Giovanni Petrillo, Alessandro Ghersi, Fernando SancassanAbstract:An improved LIS technique, using Yb(fod)3to obtain the paramagnetic Induced Shifts of all the spin 1/2 nuclei in the molecule, together with complexation Shifts obtained by the use of Lu(fod)3, has been used to investigate the conformations of several lactones. The appropriate complexation model was obtained by investigations on the planar well-defined structures of isocoumarin (1) and coumarin (2). This complexation model was then used to investigate the conformations and conformational equilibria in 3,4-dihydrocoumarin (3), β-butyrolactone (4), γ-valerolactone (5), δ-valerolactone (6) and Iµ-caprolactone (7).3,4-Dihydrocoumarin is puckered with both C2 and C3 displaced from the benzene ring plane. β-Butyrolactone is planar. γ-Valerolactone interconverts between the two envelope conformations with C4 out of the plane of the other ring atoms with 70% of the conformer with a pseudo-equatorial methyl group. For δ-valerolactone the two interconverting conformations are the half-chair and the boat form and analysis of the data suggests that there is ca. 20% of the boat form. In Iµ-caprolactone the LIS data gives a well-defined minimum for 100% of the chair form with no other significantly populated conformer. The LIS results agree with both the ab initio and MM optimised geometries and the observed and calculated conformer energies are in reasonable agreement to give ΔG (ax–eq) 0.6 kcal mol-1 for γ-valerolactone, ΔG (boat–half-chair) 0.9 kcal mol-1 for δ-valerolactone and ΔG (boat–chair) > 3.5 kcal mol-1 for Iµ-caprolactone.
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conformational analysis part 29 1 the conformationalanalysis of 2 substituted fluoro and trifluoromethyl benzaldehydes acetophenones and methyl benzoates by the Lanthanide Induced Shift lis technique
Journal of The Chemical Society-perkin Transactions 1, 1997Co-Authors: Raymond J. Abraham, Mark Edgar, Simone Angioloni, Fernando SancassanAbstract:An improved LIS technique, using, Yb(fod)3 to obtain the paramagnetic Induced Shifts of all the spin ½ nuclei in the molecule, together with complexation Shifts obtained by the use of Lu(fod)3 has been used to investigate conformational isomerism in 2-fluorobenzaldehyde 1, 2-fluoroacetophenone 2, methyl 2-fluorobenzoate 3 and the corresponding 2-trifluoromethyl compounds 4, 5 and 6. The use of fluorine LIS in these molecules was first established by analysis of the LIS in 4-fluorobenzaldehyde 7 and 4-trifluoromethylbenzaldehyde 8 in which conformational isomerism is not possible and confirmed in subsequent analyses. It is shown that 19F LIS may be used with the same degree of confidence as the corresponding 1H and 13C LIS in theses molecules. Analysis of the LIS data was considered together with ab initio, modelling and solvation calculations to provide a comprehensive account of the conformer geometries and energies for these compounds in a variety of solvents. The 2-fluoro compounds are all planar, with the trans (CO···F) conformer always more stable, in 1 and 2 predominating in all but very polar solvents. In the corresponding 2-trifluoromethyl compounds both the cis and trans conformers of the aldehyde 4 are planar with the trans form predominating, but the ketone 5 is essentially in one orthogonal conformation and the ester 6 interconverting between two nonplanar conformations with the trans conformer predominant.
Raymond J. Abraham - One of the best experts on this subject based on the ideXlab platform.
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A theoretical and NMR Lanthanide-Induced Shift (LIS) investigation of the conformations of lactams.
Magnetic resonance in chemistry : MRC, 2017Co-Authors: Raymond J. Abraham, Marco Filippi, Giovanni Petrillo, Paola Piaggio, Chiara Vladiskovic, Fernando SancassanAbstract:Molecular mechanics (MM) with MMFF94 and MMX force-fields and ab initio (RHF/6-31G*,RHF/6-311G** and B3LYP/6-311G** ) calculations are used with LIS to investigate the conformations of N-methyl-2-pyrrolidone 1, N-methyl-2-piperidone 2, e−caprolactam 3, γ-valerolactam (1,5-dimethyl-2-pyrrolidone) 4, 2-azetidinone 5, 4-methyl azetidinone 6, 4-phenyl azetidinone 7, and N-methyl-4-phenyl azetidinone 8. The Yb(fod)3 paramagnetic Induced Shifts of all the 1H and 13C nuclei are measured and the corresponding diamagnetic complexation Shifts obtained by the addition of Lu(fod)3. The complexation model (two-, three- or four-site) used depends on the relative rates of the processes involved. The amide inversion is the same order as that of the five and six membered lactam rings and much faster than the Lanthanide complexation and the inversion of the seven membered ring. Both MM and ab initio calculations give an envelope conformation for 1 with C-4 out of the ring plane in agreement with the LIS analysis. For the piperidone ring of 2 the half-chair is calculated as the most stable form. The LIS analysis confirms this but cannot exclude a small amount ( 1.5%). In 4-phenyl azetidinone 7 the MMFF94 geometry with 60% of the axial conformer gave Rcryst 1.2% but the other geometries Rcryst > 1.5%. In contrast the N-methyl-4-phenyl-2-azetidinone 8 gave good agreement for all the geometries. The butterfly conformation gave Rcryst 1.1% for 80% of the axial conformer and the planar geometries Rcryst 0.98%. The LIS results confirm the ab initio and MM optimised geometries but the conformer energies at times differ from the calculated values. They also differ considerably from the corresponding values for the lactones studied previously and possible reasons for this are discussed.
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eMagRes - Lanthanide Induced Shifts (LIS) in Structural and Conformational Analysis
Encyclopedia of Magnetic Resonance, 2011Co-Authors: Raymond J. Abraham, Fernando SancassanAbstract:The uses and applications of Lanthanide Induced Shifts (LIS) in structural and conformational analysis are reviewed. The theoretical basis of LIS including contact, pseudocontact, and complexation Shifts is considered and a reliable procedure for evaluating the pseudocontact Shift at any nucleus is given. The simplified complexation models used in the LIRAS (Lanthanide Induced relaxation and Shifts) suite of programs are described and their applications to structural analysis are discussed. This includes lutetium versus lanthanum in probing diamagnetic Shifts, the conditions required for evaluating molecular geometries by this technique, and the development of the LISA (Lanthanide Induced Shift analysis) suite to take account of the diamagnetic Shifts in a more complete analysis. Some illustrative examples of the use of this technique are given. The solution conformation of bicyclo [3,1,0] hexan-3-one was determined as a flattened boat conformation. The molecular geometries of a number of 4-substituted and 2,6-disubstituted benzaldehydes, acetophenones, and benzoates were deduced and considered with respect to theoretical studies. The results showed clearly that LIS can be used to directly test the molecular geometries. The solution conformations of some α,β-unsaturated carbonyl compounds (aldehydes, ketones, and esters) were investigated and the s-cis–s-trans conformer energies were obtained. The conformations of some aromatic SO compounds were obtained and compared with theoretical predictions. Also reference is given to the use of LIS in alcohols, lactones, quinolines, cyclic amines, 1,3-dioxanes, and epoxides. Keywords: Lanthanide Induced Shifts; contact Shifts; pseudo contact Shifts; diamagnetic Shifts; LIRAS (Lanthanide Induced relaxation and Shifts); LISA (Lanthanide Induced Shift analysis)
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conformational analysis part 31 1 a theoretical and Lanthanide Induced Shift lis investigation of the conformations of some epoxides
Journal of The Chemical Society-perkin Transactions 1, 1999Co-Authors: Raymond J. Abraham, Fernando Sancassan, Irene Castellazzi, Timothy A D SmithAbstract:An improved LIS technique, using Yb(fod)3 to obtain the paramagnetic Induced Shifts of all the spin ½ nuclei in the molecule, together with complexation Shifts obtained by the use of Lu(fod)3, has been used to investigate the conformations of a group of epoxides. These are cis (1) and trans (2) stilbene oxide, cyclopentene oxide (3), cyclohexene oxide (4), cycloheptene oxide (5), propene oxide (6) and styrene oxide (7).The LIRAS3 complexation model involving two symmetric lone pairs on the oxygen atom was used for the symmetric compounds but, for the unsymmetric compounds, a more complex unsymmetric complexation model (HARDER) was found to be necessary. The calculated LIS for styrene oxide and cis- and trans-stilbene oxide were in excellent agreement with the observed data for both the molecular mechanics (MM) and the ab initio geometries with the phenyl ring dihedral angles optimised.In styrene oxide and trans-stilbene oxide the phenyl rings are approximately perpendicular to the oxirane ring, in agreement with the conformation in the solid state and with the theoretical calculations. In cis-stilbene oxide steric repulsions between the phenyl rings splay them apart so that they are now exo to the oxirane ring. Again the LIS analysis is in good agreement with the theoretical calculations.Both the LIS data and the modelling studies agree that cyclopentene oxide is in a boat conformation with an angle of pucker of ca. 30° and that cyclohexene oxide is in a half-chair conformation with C4 and C5 displaced from the ring plane.The LIS analysis of cycloheptene oxide gave good agreement for two equilibrating chair conformations with an endo/exo ratio of 70∶30, in excellent agreement with low temperature NMR data.The accurate reproduction of the LIS data provides an unambiguous method of assigning the proton chemical Shifts of the individual methylene protons in the cyclic epoxides, which are not easily available by any other technique.
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conformational analysis part 30 the conformational analysis of some lactones by the Lanthanide Induced Shift lis technique
Journal of The Chemical Society-perkin Transactions 1, 1997Co-Authors: Raymond J. Abraham, Giovanni Petrillo, Alessandro Ghersi, Fernando SancassanAbstract:An improved LIS technique, using Yb(fod)3to obtain the paramagnetic Induced Shifts of all the spin 1/2 nuclei in the molecule, together with complexation Shifts obtained by the use of Lu(fod)3, has been used to investigate the conformations of several lactones. The appropriate complexation model was obtained by investigations on the planar well-defined structures of isocoumarin (1) and coumarin (2). This complexation model was then used to investigate the conformations and conformational equilibria in 3,4-dihydrocoumarin (3), β-butyrolactone (4), γ-valerolactone (5), δ-valerolactone (6) and Iµ-caprolactone (7).3,4-Dihydrocoumarin is puckered with both C2 and C3 displaced from the benzene ring plane. β-Butyrolactone is planar. γ-Valerolactone interconverts between the two envelope conformations with C4 out of the plane of the other ring atoms with 70% of the conformer with a pseudo-equatorial methyl group. For δ-valerolactone the two interconverting conformations are the half-chair and the boat form and analysis of the data suggests that there is ca. 20% of the boat form. In Iµ-caprolactone the LIS data gives a well-defined minimum for 100% of the chair form with no other significantly populated conformer. The LIS results agree with both the ab initio and MM optimised geometries and the observed and calculated conformer energies are in reasonable agreement to give ΔG (ax–eq) 0.6 kcal mol-1 for γ-valerolactone, ΔG (boat–half-chair) 0.9 kcal mol-1 for δ-valerolactone and ΔG (boat–chair) > 3.5 kcal mol-1 for Iµ-caprolactone.
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conformational analysis part 29 1 the conformationalanalysis of 2 substituted fluoro and trifluoromethyl benzaldehydes acetophenones and methyl benzoates by the Lanthanide Induced Shift lis technique
Journal of The Chemical Society-perkin Transactions 1, 1997Co-Authors: Raymond J. Abraham, Mark Edgar, Simone Angioloni, Fernando SancassanAbstract:An improved LIS technique, using, Yb(fod)3 to obtain the paramagnetic Induced Shifts of all the spin ½ nuclei in the molecule, together with complexation Shifts obtained by the use of Lu(fod)3 has been used to investigate conformational isomerism in 2-fluorobenzaldehyde 1, 2-fluoroacetophenone 2, methyl 2-fluorobenzoate 3 and the corresponding 2-trifluoromethyl compounds 4, 5 and 6. The use of fluorine LIS in these molecules was first established by analysis of the LIS in 4-fluorobenzaldehyde 7 and 4-trifluoromethylbenzaldehyde 8 in which conformational isomerism is not possible and confirmed in subsequent analyses. It is shown that 19F LIS may be used with the same degree of confidence as the corresponding 1H and 13C LIS in theses molecules. Analysis of the LIS data was considered together with ab initio, modelling and solvation calculations to provide a comprehensive account of the conformer geometries and energies for these compounds in a variety of solvents. The 2-fluoro compounds are all planar, with the trans (CO···F) conformer always more stable, in 1 and 2 predominating in all but very polar solvents. In the corresponding 2-trifluoromethyl compounds both the cis and trans conformers of the aldehyde 4 are planar with the trans form predominating, but the ketone 5 is essentially in one orthogonal conformation and the ester 6 interconverting between two nonplanar conformations with the trans conformer predominant.
Niko S Radulovic - One of the best experts on this subject based on the ideXlab platform.
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Lanthanide Induced Shift reagents enable the structural elucidation of natural products in inseparable complex mixtures the case of elemenal from inula helenium l asteraceae
RSC Advances, 2015Co-Authors: Marija S Gencic, Niko S RadulovicAbstract:The use of Lanthanide complexes for resolving intricate NMR signals and, in the case of chiral ligands, for determining enantiomeric excess has progressively decreased in the last 30 years. Recently, a sesquiterpene aldehyde from Inula helenium with a possible potent antistaphylococcal activity remained unidentified due to the impossibility of separating the compound from its complex matrix available in very low amounts (ca. 5 mg). Detailed analyses of 1D and 2D NMR spectra of this original complex sample allowed access to a very limited amount of structural data for the unknown aldehyde. We decided to investigate the potential usefulness of Lanthanide-Induced Shift reagents for the resolution and assigning of overlapped 1H NMR signals originating from different components of this complex mixture (i.e. for a qualitative analysis). The incremental addition of tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)europium(III) (Eu(fod)3) led to a simplification of the NMR spectra in terms of signal overlap and removal of chemical Shift degeneracy, allowing the mining of crucial data from the Shifted NMR spectra. 2D NMR spectra (1H–1H–COSY, NOESY, HSQC and HMBC) of the sample mixed with Eu(fod)3 proved to be particularly valuable in this respect. The obtained additional information revealed that the compound in question was a rare sesquiterpene – elemenal (elema-1,3,11(13)-trien-12-al). Therefore, herein we report on a new chromatography-free methodology that could be of value in structure elucidation of unknown compounds even if they are not available in a pure state.
Niko S. Radulović - One of the best experts on this subject based on the ideXlab platform.
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Lanthanide-Induced Shift reagents enable the structural elucidation of natural products in inseparable complex mixtures – the case of elemenal from Inula helenium L. (Asteraceae)
RSC Advances, 2015Co-Authors: Marija S. Genčić, Niko S. RadulovićAbstract:The use of Lanthanide complexes for resolving intricate NMR signals and, in the case of chiral ligands, for determining enantiomeric excess has progressively decreased in the last 30 years. Recently, a sesquiterpene aldehyde from Inula helenium with a possible potent antistaphylococcal activity remained unidentified due to the impossibility of separating the compound from its complex matrix available in very low amounts (ca. 5 mg). Detailed analyses of 1D and 2D NMR spectra of this original complex sample allowed access to a very limited amount of structural data for the unknown aldehyde. We decided to investigate the potential usefulness of Lanthanide-Induced Shift reagents for the resolution and assigning of overlapped 1H NMR signals originating from different components of this complex mixture (i.e. for a qualitative analysis). The incremental addition of tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)europium(III) (Eu(fod)3) led to a simplification of the NMR spectra in terms of signal overlap and removal of chemical Shift degeneracy, allowing the mining of crucial data from the Shifted NMR spectra. 2D NMR spectra (1H–1H–COSY, NOESY, HSQC and HMBC) of the sample mixed with Eu(fod)3 proved to be particularly valuable in this respect. The obtained additional information revealed that the compound in question was a rare sesquiterpene – elemenal (elema-1,3,11(13)-trien-12-al). Therefore, herein we report on a new chromatography-free methodology that could be of value in structure elucidation of unknown compounds even if they are not available in a pure state.
Marija S Gencic - One of the best experts on this subject based on the ideXlab platform.
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Lanthanide Induced Shift reagents enable the structural elucidation of natural products in inseparable complex mixtures the case of elemenal from inula helenium l asteraceae
RSC Advances, 2015Co-Authors: Marija S Gencic, Niko S RadulovicAbstract:The use of Lanthanide complexes for resolving intricate NMR signals and, in the case of chiral ligands, for determining enantiomeric excess has progressively decreased in the last 30 years. Recently, a sesquiterpene aldehyde from Inula helenium with a possible potent antistaphylococcal activity remained unidentified due to the impossibility of separating the compound from its complex matrix available in very low amounts (ca. 5 mg). Detailed analyses of 1D and 2D NMR spectra of this original complex sample allowed access to a very limited amount of structural data for the unknown aldehyde. We decided to investigate the potential usefulness of Lanthanide-Induced Shift reagents for the resolution and assigning of overlapped 1H NMR signals originating from different components of this complex mixture (i.e. for a qualitative analysis). The incremental addition of tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)europium(III) (Eu(fod)3) led to a simplification of the NMR spectra in terms of signal overlap and removal of chemical Shift degeneracy, allowing the mining of crucial data from the Shifted NMR spectra. 2D NMR spectra (1H–1H–COSY, NOESY, HSQC and HMBC) of the sample mixed with Eu(fod)3 proved to be particularly valuable in this respect. The obtained additional information revealed that the compound in question was a rare sesquiterpene – elemenal (elema-1,3,11(13)-trien-12-al). Therefore, herein we report on a new chromatography-free methodology that could be of value in structure elucidation of unknown compounds even if they are not available in a pure state.
