Measurement of the sign and the magnitude of heteronuclear coupling constants from spin-state-edited J-cross-polarization NMR experiments

New spin-state-selective (S3) NMR pulse sequences exclusively applying cross-polarization schemes to achieve optimum homonuclear and heteronuclear 1H-X coherence transfer are reported for the simple and accurate measurement of the magnitude and sign of heteronuclear coupling constants for samples at natural abundance. The proposed spin-edited HCP-TOCSY experiments are based on clean heteronuclear S3 excitation, generated by simultaneous co-addition of two independent in-phase and anti-phase components created during the mixing heteronuclear J-cross-polarization (HCP) step, which is finally transferred to other protons by a conventional homonuclear TOCSY mechanism. Selective 1D and non-selective 2D approaches for the easy determination of long-range proton-carbon and proton-nitrogen coupling constants on any protonated and non-protonated heteronuclei are presented and discussed for several organic molecules.     

Measurement of coupling constants in symmetrical spin systems using a full multiple-step cross-polarization-driven NMR pulse scheme

New NMR pulse schemes completely driven under homonuclear and heteronuclear cross-polarization conditions are proposed for the study and the measurement of coupling constants in symmetrical molecules in solution. The appropriate superimposition of independent magnetization components can afford several spin-selective multiplet patterns that are suitable for the determination of the magnitude and the sign of proton-proton and proton-carbon coupling constants with optimum sensitivity levels. A detailed product operator formalism analysis for the proposed doubly selective 1D and nonselective 2D HCP-TOCSY versions is provided and experimental verification for the configurational analysis of symmetric olefinic systems having chemical equivalence is demonstrated.     

Accurate measurement of JHH in overlapped signals by a TOCSY‐edited SERF Experiment

Selective refocusing (GSERF or the recent PSYCHEDELIC) experiments were originally designed to determine all proton–proton coupling constants (J_HH) for a selected proton resonance. They work for isolated signals on which selective excitation can be successfully applied but, as it happens in other selective experiments, fail for overlapped signals. To circumvent this limitation, a doubly‐selective TOCSY‐GSERF scheme is presented for the measurement of J_HH in protons resonating in crowded regions. This new experiment takes advantage of the editing features of an initial TOCSY transfer to uncover hidden resonances that become accessible to perform the subsequent frequency‐selective refocusing. Copyright © 2016 John Wiley & Sons, Ltd.     

BASHD‐J‐resolved‐COSY: a new method for measuring proton‐proton spin coupling constants of multiplet signals

An effective pulse sequence for measuring H–H coupling constants, named BASHD‐J‐resolved‐COSY, has been developed. In the spin systems such as –CH_A–CH_B(CH₃)–CH_C–, a methine proton H_B splits into a multiplet owing to several vicinal couplings, resulting in attenuation of its cross‐peak intensity. Therefore, the measurements of ³J_H–H with respect to H_B are generally difficult in the E‐COSY‐type experiments. With the aim of accurate measurements of ³J_H‐H in such a spin system, we have developed a new pulse sequence, which selectively decouples the secondary methyl group. The proposed pulse sequence provides the simplified cross‐peak patterns, which are suitable for reliable measurements of ³J_H‐H in a complicated natural product. Copyright © 2012 John Wiley & Sons, Ltd.     

<Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR spectra of some drimanic sesquiterpenoids

One- and two-dimensional homo- and heteronuclear correlation proton, carbon, proton—proton, and proton—carbon NMR spectra of fifteen drimanic sesquiterpenoids: 11,12-dibromodrima-5,8-dien-7-one, drim-8-en-7-one, 11-hydroxydrim-8-en-7-one, 11,12-dihydroxydrim-8-en-7-one, 11-hydroxy-11,12-epoxydrim-8-en-7-one, 11-hydroxy-11,12-epoxydrim-8-en-7-one, 8,9-epoxydriman-7-one, 8,9-epoxydriman-7-ol, 11,12-diacetoxydrim-8-en-7-ol, drimane-7,8,11-triol, 7,8-isopropylidenedioxydriman-11-al, 9, 11-dihydroxydrim-7-en-6-one, drimane-7,8,9-triol, drimane-7,8,11-triol, and drim-8-ene-7,11,12-triol were studied. The proton and carbon chemical shifts were assigned.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2589–2594, December, 2004.     

Modern proton‐detected 1D 1H–15N NMR experiments. Application to the measurement of 1H,15N coupling constants at natural abundance

Non‐selective and selective versions of several proton‐detected 1D NMR experiments to be applied to ¹⁵N are proposed. Clean, artifact‐free 1D spectra are easily obtained by the effective coherence selection by pulsed‐field gradients and the attainable sensitivity is maximized using modern pulse schemes. Despite the low sensitivity inherent to ¹⁵N NMR spectroscopy, the successful application of these experiments is demonstrated for resonance assignments and accurate measurement of both one‐bond and long‐range proton–nitrogen coupling constants on a model tripeptide at natural abundance. Copyright © 2001 John Wiley & Sons, Ltd.     

Selective J‐resolved‐HMQC‐1 and ‐2, new methods for measuring proton–proton coupling constants in strongly coupled spin systems

Efficient pulse sequences for measuring ¹H–¹H coupling constants (J_HH) in strongly coupled spin systems, named selective J‐resolved‐HMQC‐1 and ‐2, have been developed. In the strongly coupled spin systems such as ‐CH₂‐CH_A(OH)‐CH_B(OH)‐CH₂‐, measurements of ³J_HAHB are generally difficult owing to the complicated splitting caused by the adjacent CH₂ protons. For easier and accurate measurements of ³J_HAHB in such a spin system, a selective excitation pulse is incorporated into the J‐resolved HMQC pulse sequence. In the proposed methods, only two strongly coupled protons, H_A and H_B which are excited by a selective pulse, are observed as J‐resolved HMQC signals. The cross peaks of H_A and H_B appear as doublets owing to ³J_HAHB along the F₁ dimension in the selective J‐resolved HMQC‐1 and ‐2 experiments. The efficiency of the proposed pulse sequences has been demonstrated in application to the stereochemical studies of the complicated natural product, monazomycin. Copyright © 2011 John Wiley & Sons, Ltd.     

Complete assignment of the 1H and 13C NMR spectra of some N‐benzyl‐(piperidin or pyrrolidin)‐purines

The ¹H and ¹³C shifts of six N‐benzyl‐(piperidin or pyrrolidin)‐purines were fully assigned by a combination of HSQC and HMBC experiments. The ¹H,¹H coupling constants were also determined. Copyright © 2002 John Wiley & Sons, Ltd.     

Determination of 31P,31P coupling constants in cyclotriphosphazenes and their influence on 1H and 13C NMR spectra of phosphorus substituents

¹H and ¹³C NMR spectra of symmetrically substituted cyclotriphosphazenes exhibit second‐order effects. The influence of the ³¹P,³¹P coupling constants between ring phosphorus atoms on these effects was studied. Some values of this coupling constant between phosphorus bearing identical substituents were measured using ¹³C satellites of the ³¹P signals or by introduction of a chiral substituent on the third phosphorus atom. Copyright © 2003 John Wiley & Sons, Ltd.     

Identification of bound waters in the solution structure of ribonuclease T1 using the double pulsed field gradient spin‐echo NMR technique for selective water excitation

Novel pulse sequences incorporating the double pulsed field gradient spin‐echo technique are presented that have particular use in identifying macromolecular bound water. The use of these sequences is illustrated using ribonuclease T₁. Five amide protons cross‐relaxing with bound water protons were observed. Examination of the crystal structure revealed that all of these amide protons donate hydrogen bonds or are in close proximity to water molecules with very low temperature factors, indicating that these amide protons are highly correlated with the bound water molecules. This method rapidly provides reliable information for characterizing macromolecular bound water molecules. Copyright © 2002 John Wiley & Sons, Ltd.     

Study of stereochemical dependences of the direct13C−1H spin-spin coupling constants in vinylpyridines by the quantum-chemical INDO-RPA method

The stereochemical dependences of the direct¹³C−¹H spin-spin coupling constants (SSCC) in the vinyl group of vinylpyridines studied by the quantum-chemical INDO-RPA method indicate that the steric closeness of the hydrogen atom to the nitrogen atom causes an increase in the corresponding direct¹³C−¹H SSCC and a decrease in the analogous constant of the geminal hydrogen atom. The results obtained qualitatively agree with the data ofab initio calculations for the simplest systems and the experiment. At the same time, an essential quantitative overestimation of the real effect takes place. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 265–267, February, 1997.     

2-Polyfluoroalkylchromones. 12. Nitration of 5,7-dimethyl-2-polyhaloalkylchromones and complete assignment of signals in the 1H and 13C NMR spectra of 5,7-dimethyl-2-trifluoromethylchromone and its mono- and dinitro derivatives

The nitration of 5,7-dimethyl-2-polyhaloalkylchromones affords either 5,7-dimethyl-6-nitro- or 5,7-dimethyl-6,8-dinitro-2-polyhaloalkylchromones, depending on the reaction conditions. Signals in the ¹H and ¹³C NMR spectra of the sterically hindered chromones were completely assigned using the 2D NOESY, HETCOR, and COLOC spectra. The influence of nonplanar nitro groups on chemical shifts of carbon atoms was studied. Some spectral peculiarities of the peri-methyl group were revealed. The ¹H-¹H and ¹³C-¹H spin-spin coupling constants, including the extreme six-bond long-range coupling between the protons of the Me(5) group and H(8), were determined.     

Tautomerism and isotopic multiplets in the 13C NMR spectra of partially deuterated 3‐arylpyrimido[4,5‐c]pyridazine‐5,7(6H,8H)‐diones and their sulfur analogs—evidence for elucidation of the structure backbone and tautomeric forms

The tautomerism of the synthesized 3‐arylpyrimido[4,5‐c]pyridazine‐5,7(6H,8H)‐diones ( 1a–d ) and 3‐aryl‐7‐thioxo‐7,8‐dihydro‐6H‐pyrimido[4,5‐c]pyridazine‐5‐ones ( 2a–d ) was studied in dimethyl sulfoxide (DMSO)‐d₆. ¹H NMR spectra of 1a–d showed a clustered water molecule in the structure backbone that is attached by strong intermolecular H bonding. The relation between the temperature and H bonding of the clustered water molecule with 1a was also studied as representative. The relation between the electronegativity (χ) of the substituent on phenyl ring and the chemical shifts of clustered water protons in 1a–d was also studied. All of 1a–d and also 2d compounds existed in lactam ( I ) form, whereas 2a–c compounds have two distinguished tautomers in DMSO‐d₆ [lactam ( I ) and lactim ( II ) forms]. The solvent‐substrate proton exchange was examined in compounds 1a–d and 2a–d by adding one drop of D₂O. All compounds (except 1d ) showed proton/deuterium exchange of the clustered water protons in DMSO by adding one drop of D₂O. Some compounds (but not all of them) that are easily soluble in DMSO‐d₆ containing D₂O showed isotopic splitting (β‐isotope effect) in their ¹³C NMR spectra. Among them, compound 1a was the best evidence to help the spectral assignments and structure determination of predominant tautomer by carbon‐13 splitting (β‐isotope effect). Copyright © 2010 John Wiley & Sons, Ltd.     

Tautomeric behaviour and isotopic multiplets in the 13C NMR spectra of partially deuterated 5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones and 5‐arylazo‐2‐thioxo‐pyrimidine (1H,3H,5H)‐4,6‐diones—evidence for elucidation of tautomeric forms

NMR spectra of the synthesized azo dyes, 5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones (5a–g), 1,3‐dimethyl‐5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones (6a–g), and 5‐arylazo‐2‐thioxo‐pyrimidine (1H,3H,5H)‐4,6‐diones (7a–g) were studied in (CD₃)₂SO (three drops of CD₃OD were added into solutions of the dyes in two different concentrations). All dyes showed intramolecular hydrogen bonding. Dyes 5a–7a showed bifurcated intramolecular hydrogen bonds. Tautomeric behaviours of some of N‐methylated azo dyes (6a‐g) were studied in two different concentrations. The solvent–substrate proton exchange of dyes 5a–d, 6a and 7a–e was examined in presence of three drops of CD₃OD. The dyes which were soluble in (CD₃)₂SO containing CD₃OD showed isotopic splitting (β‐isotope effect) in the ¹³C NMR spectra. Copyright © 2009 John Wiley & Sons, Ltd.