SQ—SQ experiment for determination of relative signs of small nJ(29Si,13C) couplings in a wide variety of silicon compounds

A modification of double quantum–zero quantum (DQ—ZQ) experiment termed single‐quantum–single‐quantum (SQ—SQ) experiment is proposed for the determination of relative signs and magnitudes of coupling constants. The modification replaces the multiple‐quantum evolution period by two synchronously incremented single‐quantum periods. Similarly to DQ—ZQ experiment, the sequence requires only two coupling constants that share one nucleus, the one to be measured and a reference one. This allows application to a larger variety of molecular fragments than traditional 2D sequences producing E.COSY or TROSY pattern. The SQ—SQ experiment eliminates the effects of some other couplings during t₁, thereby simplifying the 2D pattern and increasing the signal intensity in comparison with DQ—ZQ experiment. The presented sequence is particularly designed for the determination of silicon–carbon coupling constants across several bonds at natural abundance using silicon–hydrogen couplings as the sign reference. The signs of silicon–carbon couplings across two and three bonds in dimethyl(phenoxy)silane which cannot be detected by traditional methods and which have not yet been determined are established by the SQ—SQ method here: ²J(Si,C) = +2.2 Hz and ³J(Si,C) = ?1.7 Hz. Copyright © 2009 John Wiley & Sons, Ltd.     

HCSE method for detection of small carbon–carbon couplings and their signs,comparison with SLAP pulse sequence

Performance of homonuclear coupling sign edited (HCSE) experiment applied to detection of signed carbon–carbon couplings is discussed using a set of already measured samples of nine monosubstituted benzenes. It is shown that coupling sign detection is insensitive to the settings of carbon–carbon polarization transfer delays. The HCSE spectra of ten from the total of 43 measured carbon–carbon couplings were considerably influenced by relaxations and proton–proton strong couplings. These effects are quantitatively discussed. The results of HCSE and SLAP experiments are compared. It is shown that the two methods may complement each other in detection of signed carbon–carbon couplings. Copyright © 2013 John Wiley & Sons, Ltd.     

Improved pulse sequences for measurement of small long‐range couplings between heteronuclei (29Si?13C) at natural abundance

The gradient pulse sequences for measurement of small long‐range couplings between heteronuclei (²⁹Si? ¹³C) in natural abundance reported to date (INEPT‐(Si,C)gCOSY and INEPT‐(Si,C,Si)HMQC) suffer from significant signal loss when these nuclei (²⁹Si, ¹³C) are coupled through one‐bond couplings to protons. This negative effect can be completely eliminated by using non‐gradient versions (INEPT‐(Si,C)COSY) or by switching proton decoupling off during gradient pulses (modified INEPT‐(Si,C,Si)gHMQC pulse sequence). The beneficial effects of these two approaches on the quality of the spectra are demonstrated here. Copyright © 2009 John Wiley & Sons, Ltd.     

Geminal 2J(29Si‐O‐29Si) couplings in oligosiloxanes and their relation to direct 1J(29Si‐13C) couplings

Absolute values of (79) geminal ²J(²⁹Si‐O‐²⁹Si) couplings were measured in an extensive series of (55) unstrained siloxanes dissolved in chloroform‐d. Signs of ²J(²⁹Si‐O‐²⁹Si) in some (9) silicon hydrides were determined relative to ¹J(²⁹Si‐¹H) which are known to be negative. It is supposed that positive sign of the ²J(²⁹Si‐O‐²⁹Si) coupling found in all studied hydrides is common to all siloxanes. Theoretical calculations for simple model compounds failed to reproduce this sign and so their predictions of bond length and angle dependences cannot be taken as reliable. Useful empirical correlations were found between the ²J(²⁹Si‐O‐²⁹Si) couplings on one side and the total number m of oxygen atoms bonded to the silicon atoms, sum of ²⁹Si chemical shifts or product of ¹J(²⁹Si‐¹³C) couplings on the other side. The significance of these correlations is briefly discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

Suppressing one‐bond homonuclear 13C,13C scalar couplings in the J‐HMBC NMR experiment: application to 13C site‐specifically labeled oligosaccharides

Site‐specific ¹³C isotope labeling is a useful approach that allows for the measurement of homonuclear ¹³C,¹³C coupling constants. For three site‐specifically labeled oligosaccharides, it is demonstrated that using the J‐HMBC experiment for measuring heteronuclear long‐range coupling constants is problematical for the carbons adjacent to the spin label. By incorporating either a selective inversion pulse or a constant‐time element in the pulse sequence, the interference from one‐bond ¹³C,¹³C scalar couplings is suppressed, allowing the coupling constants of interest to be measured without complications. Experimental spectra are compared with spectra of a nonlabeled compound as well as with simulated spectra. The work extends the use of the J‐HMBC experiments to site‐specifically labeled molecules, thereby increasing the number of coupling constants that can be obtained from a single preparation of a molecule. Copyright © 2014 John Wiley & Sons, Ltd.     

Q.E.COSY: determining sign and size of small deuterium residual quadrupolar couplings using an extended E.COSY principle

Residual quadrupolar couplings contain important structural information comparable with residual dipolar couplings. However, the measurement of sign and size of especially small residual quadrupolar couplings is difficult. Here, we present an extension of the E.COSY principle to spin systems consisting of a Spin 1 coupled to a spin ½ nucleus, which allows the determination of the sign of the quadrupolar coupling of the Spin 1 nucleus relative to the heteronuclear coupling between the spins. The so‐called Q.E.COSY approach is demonstrated with its sign‐sensitivity using variable angle NMR, stretched gels and liquid crystalline phases applied to various CD and CD₃ groups. Especially the sign‐sensitive measurement of residual quadrupolar couplings that remain unresolved in conventional deuterium 1D spectra is shown. Copyright © 2016 John Wiley & Sons, Ltd.     

2D correlation spectra edited by the sign of relative coupling constant

Modifications (CSEc and CSEh) of recently published SQSQc and SQSQh pulse sequences are proposed and tested on detection of small (～2 Hz) signed silicon-carbon coupling constants. The new sequences increase signal intensity by simplifying the spectra. The signals are about four times stronger than in SQSQc or SQSQh spectra, achieving the sensitivity of E.COSY-type experiment. The information about sign and magnitude of the coupling is preserved. CSEc and CSEh spectra for two silicon compounds are presented and compared. The two new sequences allow editing of heteronuclear correlation spectra according to the sign of the selected heteronuclear coupling constants.     

HyperBIRD: A Sensitivity‐Enhanced Approach to Collecting Homonuclear‐Decoupled Proton NMR Spectra

Samples prepared following dissolution dynamic nuclear polarization (DNP) enable the detection of NMR spectra from low‐γ nuclei with outstanding sensitivity, yet have limited use for the enhancement of abundant species like ¹H nuclei. Small‐ and intermediate‐sized molecules, however, show strong heteronuclear cross‐relaxation effects: spontaneous processes with an inherent isotopic selectivity, whereby only the ¹³C‐bonded protons receive a polarization enhancement. These effects are here combined with a recently developed method that delivers homonuclear‐decoupled ¹H spectra in natural abundance samples based on heteronuclear couplings to these same, ¹³C‐bonded nuclei. This results in the HyperBIRD methodology; a single‐shot combination of these two effects that can simultaneously simplify and resolve complex, congested ¹H NMR spectra with many overlapping spin multiplets, while achieving 50–100 times sensitivity enhancements over conventional thermal counterparts.     

29Si-13C spin-spin couplings over Si-O-Carom link

29Si-13C couplings were measured in para substituted silylated phenols, X--C6H4--O--SiR1R2R3 (X = NO2, CF3, Cl, F, H, CH3, CH3O). The SiR1R2R3 silyl groups included trimethylsilyl (Si(CH3)3, TMS), tert-butyldimethylsilyl (Si(CH3)2C(CH3)3, TBDMS), dimethylsilyl (SiH(CH3)2, DMS), and tert- butyldiphenylsilyl (Si(C6H5)2C(CH3)3, TBDPS). Previously developed (Si,C,Si)gHMQC methods and narrow 29Si lines allowed the determination of coupling constants over up to five bonds. Besides the number of intervening bonds between the silicon and carbon atoms, all the measurable couplings depend also on the nature of the substituents on the silicon. The two- and three-bond couplings are not affected by ring substitution in the para position. These properties render the 29Si-13C couplings suitable for line assignment in the spectra of silylated polyphenols. The experimental results are in reasonable agreement with theoretical calculations. The calculations show, in agreement with the data reported in the literature for couplings between other nuclei, that the two-bond and three-bond couplings, which are of similar magnitudes, are of opposite signs. If the signs of these geminal and vicinal couplings could be determined experimentally, they would greatly facilitate the line assignment. The four- and five-bond couplings are affected by the substituent X in a nontrivial manner.     

Magnitude of Finite‐Nucleus‐Size Effects in Relativistic Density Functional Computations of Indirect NMR Nuclear Spin–Spin Coupling Constants

A spherical Gaussian nuclear charge distribution model has been implemented for spin‐free (scalar) and two‐component (spin–orbit) relativistic density functional calculations of indirect NMR nuclear spin–spin coupling (J‐coupling) constants. The finite nuclear volume effects on the hyperfine integrals are quite pronounced and as a consequence they noticeably alter coupling constants involving heavy NMR nuclei such as W, Pt, Hg, Tl, and Pb. Typically, the isotropic J‐couplings are reduced in magnitude by about 10 to 15 % for couplings between one of the heaviest NMR nuclei and a light atomic ligand, and even more so for couplings between two heavy atoms. For a subset of the systems studied, viz. the Hg atom, Hg₂²⁺, and Tl? X where X=Br, I, the basis set convergence of the hyperfine integrals and the coupling constants was monitored. For the Hg atom, numerical and basis set calculations of the electron density and the 1s and 6s orbital hyperfine integrals are directly compared. The coupling anisotropies of TlBr and TlI increase by about 2 % due to finite‐nucleus effects.     

SQSQh: 1H‐detected SQ–SQ experiment for determination of signed silicon–carbon coupling constants

A new variant of SQ–SQ pulse sequence (SQSQh) for relative sign determination and detection of small silicon–carbon couplings over more than one bond is presented. In the SQSQh sequence, proton detection replaces carbon detection used in the original SQ–SQ pulse sequence (SQSQc). The theoretical gain in sensitivity was experimentally tested on two samples (trimethylsiloxyethane, 1, and 1,2,4‐tris(trimethylsiloxy)benzene, 2), the experimentally found gain provided by the SQSQh over the SQSQc method varied between 6 and 8. The method can be applied to linear spin systems, i.e. to systems where the silicon is coupled to the carbon in question and to any hydrogen not necessarily bonded to the carbon. Copyright © 2010 John Wiley & Sons, Ltd.     

J‐compensated PGSE: an improved NMR diffusion experiment with fewer phase distortions

Peak distortion caused by homonuclear ¹H J‐coupling is a major problem in many spin‐echo‐based experiments such as pulsed gradient spin‐echo (PGSE) experiments. Although peak phase distortions can be lessened by the incorporation of anti‐phase purging sequences, the sensitivity is substantially decreased. Techniques for lessening the effect of homonuclear J‐coupling evolution in spin‐echo‐based experiments have been investigated. Two potentially useful candidates include a J‐compensated inversion sequence that is efficient over a wide range of J‐coupling values and a pulse sequence that refocuses homonuclear J‐evolution during the spin‐echo. The latter was found to work superbly on samples containing two spin (AX or AB) systems and still provided significant advantage over the standard method on samples containing more complicated spin systems. Implementation of this J‐refocusing technique into a PGSE‐type experiment (J‐PGSE) leads to dramatic improvement of spectra and easier data analysis. The J‐PGSE sequence should find applications in many diffusion studies where the PGSE‐type method is required and should be a viable alternative to PGSTE especially in dilute samples due to its enhanced sensitivity. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis,NMR spectroscopic characterization and structure of a divinyldisilazane‐(triphenylphosphine)platinum(0) complex: observation of isotope‐induced chemical shifts 1Δ12/13C(195Pt)

Tetramethyldivinyldisilazane‐(triphenylphosphine)platinum(0) was prepared, characterized in solid state by X‐ray crystallography and in solution by multinuclear magnetic resonance spectroscopy (¹H, ¹³C, ¹⁵N, ²⁹Si, ³¹P and ¹⁹⁵Pt NMR). Numerous signs of spin–spin coupling constants were determined by two‐dimensional heteronuclear shift correlations (HETCOR) and two‐dimensional ¹H/¹H COSY experiments. Isotope‐induced chemical shifts ¹Δ^12/13C(¹⁹⁵Pt) were measured from ¹⁹⁵Pt NMR spectra of the title compound as well as of other Pt(0), Pt(II) and Pt(IV) compounds for comparison. In contrast to other heavy nuclei such as ¹⁹⁹Hg or ²⁰⁷Pb, the “normal” shifts of the heavy isotopomers to low frequencies are found, covering a range of >500 ppb. Copyright © 2013 John Wiley & Sons, Ltd.     

29Si-13C spin-spin couplings over an Si-O-Csp3 link

(29)Si-(13)C spin-spin couplings over one, two, and three bonds as well as other NMR parameters [delta((29)Si), delta((13)C), delta((1)H), (1)J((13)C-(1)H), and (2)J((29)Si-C-(1)H)] were calculated and measured for a series of trimethylsilylated alcohols of the types Me(3)Si-O-(CH(2))(n)CH(3) and Me(3)Si-O-CH(3-n)R(n)(n = 0-3; R = Me, Ph, or Vi). The signs of the coupling constants determined for selected compounds can likely be extended to all such compounds, as supported by theoretical calculations. Similar to couplings between other pairs of nuclei, the 2-bond and 3-bond (29)Si-O-(13)C couplings are of opposite signs ((2)J > 0 and (3)J < 0), and their relative magnitudes depend on the extent of branching at the alpha-carbon.     

Relative signs of 29Si-13C couplings

Two pulse sequences applicable to the determination of relative signs of coupling constants, gHSQC-RELAY(P) and gHSQC-RELAY(D), were developed and tested. These sequences are suitable for determination of relative signs of long-range coupling constants (<2 Hz) between two heteronuclei of low abundance (such as (29)Si and (13)C), and are applicable even to cases in which one of the heteronuclei ((29)Si) does not exhibit coupling with some of the detected protons ((1)H). The two sequences differ in the manner in which they suppress undesirable homonuclear coherence transfers. Each of the sequences can be combined with an isotope filter for better suppression of the centerlines arising from more abundant NMR-inactive isotopes. The sequences were tested on ethoxytrimethylsilane and (E)-(buta-1,3-dienyloxy)trimethylsilane, and we conclude that (2)J((29)Si-O-(13)C) is positive while (3)J((29)Si-O-C-(13)C) is negative in both compounds.     

J‐Edited Pure Shift NMR for the Facile Measurement of nJHH for Specific Protons

We report a novel 1D J‐edited pure shift NMR experiment (J‐PSHIFT) that was constructed from a pseudo 2D experiment for the direct measurement of proton–proton scalar couplings. The experiment gives homonuclear broad‐band ¹H‐decoupled ¹H NMR spectra, which provide a single peak for chemically distinct protons, and only retain the homonuclear‐scalar‐coupled doublet pattern at the chemical‐shift positions of the protons in the coupled network of a specific proton. This permits the direct and unambiguous measurement of the magnitudes of the couplings. The incorporation of a 1D selective correlation spectroscopy (COSY)/ total correlation spectroscopy (TOCSY) block in lieu of the initial selective pulse, results in the exclusive detection of the correlated spectrum of a specific proton.     

Simultaneous measurement of J(HH) and two different nJ(CH) coupling constants from a single multiply edited 2D cross‐peak

Three different J‐editing methods (IPAP, E.COSY and J‐resolved) are implemented in a single NMR experiment to provide spin‐state‐edited 2D cross‐peaks from which a simultaneous measurement of different homonuclear and heteronuclear coupling constants can be performed. A new J‐selHSQMBC‐IPAP experiment is proposed for the independent measurement of two different ⁿJ(CH) coupling constants along the F2 and F1 dimensions of the same 2D cross‐peak. In addition, the E.COSY pattern provides additional information about the magnitude and relative sign between J(HH) and ⁿJ(CH) coupling constants. Copyright © 2013 John Wiley & Sons, Ltd.     

Simultaneous measurement of N?H and Cα?Hα coupling constants in proteins

We present a pulse sequence for the simultaneous measurement of N? H and Cα? Hα couplings in double‐labeled proteins from 2D spectra. The proposed sequence, a modification of the HN(CO)CA experiment, combines the J‐modulation method and the IPAP scheme. The couplings can be readily retrieved from a series of 2D ¹⁵N? ¹H correlation spectra, differing in the time point at which a ¹H 180° pulse is applied. This induces an intensity modulation of the ¹⁵N? ¹H correlation peaks with the Cα? Hα coupling. The Cα? Hα coupling is then obtained by fitting the observed intensities to the modulation equation. The N? H coupling is measured in each member of the set from peak‐to‐peak separations in the IPAP subspectra. The pulse sequence is experimentally verified with a sample of ¹⁵N/¹³C‐enriched ubiquitin. Copyright © 2009 John Wiley & Sons, Ltd.     

Configurational assignment in alkyl‐branched sugars via the geminal C,H coupling constants

The measurement of the magnitude and sign of ²J(C,H) couplings offers a reliable way to determine the absolute configuration at a carbon center in a fixed cyclic system. A decrease of the dihedral angle ? in the O—C_A—C_B—H fragment always leads to a change of the ²J(C_A,H_B) coupling to more negative values, independent of the type and position of substituents at the two carbon centers. The orientations of the two substituents at C‐3 of the epimeric pair 1 and 2 were determined unambiguously through the measurement of the geminal coupling constants between C‐3 and the hydrogen atoms at C‐2 and C‐4. In particular, ²J(C‐3,H‐2ax) with ?1.5 Hz, ? = 174° in 1 and ?6.6 Hz, ? = 47° in 2 , and ²J(C‐3,H‐4) with +1.5 Hz, ? = 175° in 1 and ?4.7 Hz, ? = 49° in 2 showed the greatest differences between the two epimers. Both couplings therefore allow the determination of the absolute configuration at C‐3. It should be noted, however, that the size of the coupling constants can be different for dihedral angles of nearly identical size, when there are different numbers of electronegative substituents on the two coupling pathways, i.e. no O‐substituent at C‐2, but one axial O‐substituent at C‐4. It becomes clear that it is not sufficient to measure the magnitude of ²J coupling constants only, but that the sign of the geminal coupling is needed to identify the absolute configuration at a chiral center. The coupling of C‐3 with H‐2eq is not useful for the determination of the configuration at C‐3, as the similarity of the dihedral angles ? (O—C‐3—C‐2—H‐2eq) (57° in 1 and 70° in 2 ) leads to identical coupling constants (?6.1 Hz) for both epimers. Copyright © 2003 John Wiley & Sons, Ltd.     

A critical evaluation of heteronuclear TOCSY (HEHAHA) experiments for 1H,6Li spin pairs

Heteronuclear TOCSY (HEHAHA) experiments for ¹H,⁶Li spin pairs in organolithium compounds with adjacent strongly coupled ¹H,¹H spin systems showed unexpected cross peak behaviour: for n‐butyllithium ¹H,⁶Li cross peaks were completely missing, whereas for the dimer of (Z)‐2‐lithio‐1‐(o‐lithiophenyl)ethane, a cross peak for remote protons was observed even at very short mixing times. It was assumed that strong magnetization transfer within the proton spin systems was responsible for these results, which prevented unambiguous chemical shift assignments. Selective experiments with the ⁶Li,¹H‐HET‐PLUSH‐TACSY sequence then showed the expected ⁶Li,¹H cross peaks for the transfer via the directly coupled ¹H and ⁶Li nuclei. For n‐butyllithium transfer to H(C_α) via an unresolved heteronuclear coupling constant below 0.1 Hz is unambiguously observed. Cross peaks in the 2D ⁶Li,¹H‐HET‐PLUSH‐TACSY spectra for the dimer of (Z)‐2‐lithio‐1‐(o‐lithiophenyl)ethane are readily explained by the measured coupling network and the corresponding active mixing conditions. Copyright © 2014 John Wiley & Sons, Ltd.