A new version of the two-dimensional INADEQUATE experiment was designed for detection of small couplings between equivalent carbon atoms separated in the molecule by several bonds, where other techniques fail due to rich line splitting and mutual peak cancellation in many molecules. As the proposed method is suitable for detection of couplings in strongly coupled systems in general, we propose the name STRONG INADEQUATE in the paper. Similar to other methods for detection of couplings between equivalent carbons, the STRONG INADEQUATE experiment utilizes one-bond carbon-proton coupling for creation of the effective chemical shift differences. The STRONG INADEQUATE experiment works superbly for n JCC , where n ≥ 3. Then the F1 pattern is reduced to a simple antiphase doublet with n JCC separation, and this pattern is also preserved when a symmetrical HC···C'H' system is coupled to other protons. Even in the measurement of 2 JCC couplings, the STRONG INADEQUATE experiment generates a much simpler pattern than the original pulse sequences for measurement of couplings between equivalent carbons.

• Klíčová slova
• 13C, INADEQUATE, NMR, carbon-carbon coupling, equivalent carbons, strong coupling,
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

The approach to the measurement of one-bond indirect spin-spin coupling constants between equivalent nuclei was revisited. The analytical formulas for development of the density matrix of strongly coupled symmetrical HC-C'H' spin systems were derived and the optimal duration of polarization delay in the original 2QHMBC pulse sequence is discussed. Based on the analytical formulas a new version of a robust indirect detection experiment, called SYMONA (SYmmetrical MOlecules Natural Abundance double-quantum experiment), was proposed for carbon-carbon coupling constants detection in symmetrical molecules. Additionally, application of the SYMONA experiment to more complicated spin systems than isolated HC-C'H' is discussed.

• Klíčová slova
• 2QHMBC, Carbon-carbon coupling, Equivalent carbons, NMR, Proton detection, SYMONA, Strong coupling,
• Publikační typ
• časopisecké články MeSH

Contrary to common expectations, multiple-spin coherences containing products of proton and heteronucleus operators (e.g. Hu Cx , u = x, y, z) can produce not only sidebands but also noticeable centerband NMR signals of the heteronucleus during acquisition under 1H broadband decoupling. Such centerband signals of low abundant heteronuclei can be sources of relatively strong unexpected artifacts in NMR experiments that aim to detect very weak signals from much less-abundant isotopomers, e.g. 13C-13C ones. These findings lead to a new design of Sign Labeled Polarization Transfer (SLAP) pulse sequence (MSS-SLAP) with improved suppression of centerband peaks that are because of singly, e.g. 13C, labeled molecules (parent peaks). The MSS-SLAP experiment and its MSS-BIRD-SLAP variant are compared with a few older SLAP versions.

• Klíčová slova
• 13C, 13C-13C coupling constants, BIRD, NMR, SLAP, composite decoupling, double-quantum filter, signs of coupling constants,
• Publikační typ
• časopisecké články MeSH

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.

• Klíčová slova
• 13C, 1H, HCSE, INADEQUATE, NMR, SLAP, carbon–carbon coupling, coupling sign, monosubstituted benzenes, strong coupling,
• Publikační typ
• časopisecké články MeSH

Signed values of all intra-ring (2,3,4) J(C,C) couplings in nine monosubstituted benzenes (C6H5-X where X = F, Cl, Br, CH3, OCH3, Si(CH3)3, C≡N, NO, NO2) are experimentally determined as well as nine couplings to substituent carbons. It is confirmed that while all the vicinal intra-ring (3)J(C,C) are positive and all geminal (2)J(C2,C4) are negative, both signs are found for geminal (2)J(C1,C3) couplings. All the determined signs agree with those already predicted by theoretical calculations.

• MeSH
• benzenové deriváty chemie   MeSH
• magnetická rezonanční spektroskopie normy   MeSH
• referenční standardy MeSH
• uhlík chemie   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• benzenové deriváty MeSH
• uhlík MeSH

Infinite dilution (29)Si and (13)C NMR chemical shifts were determined from concentration dependencies of the shifts in dilute chloroform and acetone solutions of para substituted O-silylated phenols, 4-R-C6 H4 -O-SiR'2 R″ (R = Me, MeO, H, F, Cl, NMe2, NH2, and CF3), where the silyl part included groups of different sizes: dimethylsilyl (R' = Me, R″ = H), trimethylsilyl (R' = R″ = Me), tert-butyldimethylsilyl (R' = Me, R″ = CMe3), and tert-butyldiphenylsilyl (R' = C6 H5, R″ = CMe3). Dependencies of silicon and C-1 carbon chemical shifts on Hammett substituent constants are discussed. It is shown that the substituent sensitivity of these chemical shifts is reduced by association with chloroform, the reduction being proportional to the solvent accessible surface of the oxygen atom in the Si-O-C link.

• Klíčová slova
• 13C NMR, 1H NMR, 29Si NMR, Hammett-type dependence, chemical shifts, solvent accessible surface, substituent effects,
• Publikační typ
• časopisecké články MeSH

The proposed homonuclear coupling sign edited (HCSE) experiment can detect signed homonuclear couplings between low abundant nuclei like (13)C, (29)Si and (15)N in linear spin systems, that is, in systems where two nuclei are coupled by the measured coupling, and one of them is coupled by a second coupling to a nucleus of different kind. The third nucleus is usually high abundant hydrogen. Two spectra are measured during the HCSE experiment. Their weighed sum and difference yield two other spectra, one containing peaks coupled only by positive measured couplings and the other having peaks coupled by negative measured couplings. The usual E.COSY-type experiment requires all three couplings in the three spin system (triangular spin system) and not only two couplings as the HCSE experiment. The experiment was successfully tested on known carbon-carbon and silicon-silicon two bond couplings. A set of six simple siloxanes with |(2) J(Si-O-Si)| couplings ranging from 0.5 to 9.0 Hz was measured for the first time, and all the couplings were found to be positive.

• Klíčová slova
• 13C NMR, 29Si NMR, CSE, linear spin system, radiation dumping, reduced coupling, sign of J(C,C), sign of J(Si,Si), signed coupling,
• Publikační typ
• časopisecké články MeSH

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.

• Publikační typ
• časopisecké články MeSH

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.

• Publikační typ
• časopisecké články MeSH

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.

• Publikační typ
• časopisecké články MeSH