Low-power Decoupling Research Articles

Low-power decoupling in fast magic-angle spinning NMR

We investigate the possibility of a low-power rf-irradiation approach to heteronuclear spin decoupling in solid-state NMR under high-frequency magic-angle sample spinning. Decoupling is achieved by applying an rf-field with an amplitude corresponding to a precession frequency much lower than the spinning frequency. This leads to a reversal of the averaging processes compared to normal high-power continuous-wave decoupling. Such an approach becomes increasingly interesting with increasing MAS spinning frequency. In rigid solids, low-power decoupling becomes competitive above about 40 kHz MAS frequency.

Selective Decoupling

Low-power broadband decoupling sequences WALTZ-16 and GARP-1 generate large far-from-resonance frequency modulations which preclude selectivity. The framework developed to construct these broadband sequences is modified to permit selective spin decoupling. Selective-decoupling sequences are created from shaped 90° pulses combined consecutively using WALTZ permutations and supercycle symmetry while shaped 180° pulses are combined in supercycle symmetry to make inversion-based decoupling sequences. Simulations and experiments compare the decoupling bandwidth, frequency selectivity, and quality of near-resonance decoupling for broadband and selective-decoupling sequences.

Efficiency of heteronuclear broadband decoupling and homonuclear J cross polarization analyzed on two time scales

Decoupling sequences can be evaluated, as shown in Waugh's theory, with a J scaling factor on a long time scale. The efficiency of low-power decoupling, however, must be determined by cycling sidebands, as well as by the J scaling factor, when the sampling is not synchronized with the decoupling cycle. We introduced, therefore, another scaling factor which characterizes the decoupling on a short time scale. It is also shown that these scaling factors are useful for evaluating the efficiency of homonuclear J cross polarization. We clarified criteria of the factors suitable for various J coupling constants and chemical-shift ranges. Typical decoupling sequences were analyzed using the two scaling factors.

Conformational studies of d(m5CpGpm5CpG) and d(CpGpCpG) by 1H and 31P NMR.

Exhaustive conformational studies of d(CpG)2 and d(m5CpG)2, two convenient targets for DNA bisintercalating drugs, have been carried out by 1H and 31P NMR in low salt concentration and in the presence of 30% ethanol. Unambiguous 31P assignments of the B for are obtained with low-power heteronuclear decoupling experiments, while 31P assignments in the Z form are obtained by two-dimensional homonuclear chemical exchange experiments. The 31P chemical shifts and 3JH3'P coupling constants studied at various temperatures in methylated and non-methylated tetranucleotides, are interpreted as resulting from conformational differences between the compounds. These features are corroborated by homonuclear proton nuclear Overhauser effect experiments showing the steric role of the 5-methylcytosine in the induction of an alternating B form in d(m5CpG)2.

Further Study on the Distribution of Substituent Group in Cellulose Acetate by 13C{1H}NMR Analysis: Assignment of Carbonyl Carbon Peaks

An attempt was made to determine an unsolved problem in peak assignment of carbonyl carbon region in 13C NMR spectra for cellulose acetate. For this purpose, Kamide and Okajima’s method for determination of the distribution of acetyl group among three hydroxyl groups in a glucopyranose unit was reexamined carefully using a 200 MHz FT-NMR spectrometer, adequate and the most reasonable assignment at present was made on the carbonyl carbon peaks in NMR spectra, applying a low-power selective decoupling method to acetyl methyl proton located at specific carbon position. All three NMR peaks in the carbonyl carbon region were assigned to be attributed to the acetyl groups located at C6, C3, and C2 positions, from the low magnetic field side. The previous Kamide and Okajima’s assignment on C3 and C2 peaks was found to be erroneoulsy made by neglecting an overlapping of the three peaks due to low resolving power in the 1H and 13C NMR spectrometer used by them. The new and complete assignments were in good agreement with those by Miyamoto et al. The method of Miyamoto et al. was examined in detail in the light of the validity of the assignment of carbonyl carbon peaks for not fully substituted cellulose acetate.

Ultra-high-resolution NMR. III. Performance of WALTZ-16 proton decoupling

In a recent report (1) we used the nonprotonated carbon of toluene to demonstrate the feasibility of ultra-high-resolution NMR. We achieved an instrumental broadening (Win) of as little as 6.6 mHz (1). However, a nonprotonated carbon does not place a severe requirement on proton decoupling efficiency. In this Communication we show that WALTZ16 proton decoupling, developed by Freeman and co-workers (2, 3), is an extraordinarily effective low-power proton decoupling method for ultra-high-resolution NMR. It should be noted that the experimental tests of WALTZ-16 performance reported by Shaka et al. (Fig. 3 of Ref. (2)) were done under conditions of 0.25 Hz line broadening and are therefore not necessarily indicative that WALTZ-16 is suitable for ultra-high-resolution NMR. We need a low-power decoupling method that yields 5 mHz or less residual broadening of r3C resonances for the whole typical range of ‘H chemical-shift offsets relative to the ‘H carrier frequency. Before the development of highly efficient low-power proton-decoupling methods in recent years it would have been impossible to obtain ultra-high-resolution 13C NMR spectra, for two reasons. First, even with high-power (b10 W) ‘H irradiation, older proton-decoupling schemes such as random-noise modulation (4) and square-wave modulation (5) left large residual unresolved 13C-‘H splittings. Second, we have found that even if the residual broadening had been absent, temperature gradients in the sample resulting from uneven sample heating by the high decoupling power would have caused enough of a chemical-shift gradient in the sample to nullify ultra-high resolution. Detailed measurements of temperature gradients as a function of gas flow rate, decoupling power, and other variables will be presented in a separate report. To determine the limits imposed by ‘H decoupling on the linewidths of 13C resonances under ultra-high-resolution conditions, we examine here the effect of offset of the ‘H carrier frequency on the linewidth of the methyl resonance of acetone (Fig. 1), recorded with WALTZ16 decoupling. The experiments were done on a slightly modified (I) Nicolet NT-200 NMR spectrometer at a 13C resonance frequency of 50.3 MHz, with a standard 12 mm probe and a sample volume of 4.2 ml. Standard singlepulse 13C excitation was used. A hard-wired WALTZ16 d&al control box, purchased from Bio-Magnetic Instruments of West Lafayette, Indiana, was used to create the required 180” phase shifts in the ‘H decoupler circuit of the Nicolet NT-200 NMR spectrometer. In our sample (see caption of Fig. 1) lJ& = 126.85 Hz and 4JCH = 1.50

13C spin diffusion of adamantane

Two-dimensional exchange spectroscopy of natural abundance 13C–13C spin diffusion in solid adamantane illustrates the influence that 13C–1H dipole–dipole coupling exerts on 13C spin diffusion by determining spectral overlap in the 13C system. 2D 13C spectra were obtained for several values of mixing time τm and compared with spectra calculated in the limit of nearest-neighbor coupling. Good agreement is obtained for short τm, during which the equilibration of neighboring spins dominates. For longer τm, slower spin diffusion that is not acounted for by the simple model is seen; after nearest-neighbor spins equilibrate, communication over larger distances produces further mixing. It is possible to modify spin diffusion rates by altering experimental conditions, e.g., magic-angle spinning, low-power 1H decoupling, or spin locking 13C in the rotating frame during τm.

Complex formation of platinum(II) and rhodium(III) ions with aminated silica surfaces as studied by c-13 nmr spectroscopy

Complex formation of Pt(II) and Rh(III) ions with aminated silica surfaces (NH 2CH 2CH 2CH 2Si≡, NH 2CH 2CH 2NHCH 2CH 2CH 2Si≡) has been analyzed by C-13 NMR spectroscopy in both the suspended state (in water; ordinary high-resolution NMR spectrometer with low-power 1H decoupling) and the dry state (with CP-MAS techniques). New peaks assignable to Pt(II)-coordinated species appeared for the diamine silica in contact with aqueous K 2PtCl 4 solutions, where bis-chelate formation was suggested on the basis that the peaks of uncoordinated species were converted completely to those of coordinated ones at the [Pt]/[ligand] ratio of 0.5; bis-chelate structure was also indicated for RhCl 3/diamine silica by diffuse reflectance spectra. While coordinated species did not give high-resolution signals in other cases, they were found detectable with CP-MAS techniques, although the peaks were not fully resolved at 25 MHz. The implications of the associated protonation shifts of uncoordinated ligands are also discussed.

1H–13C n.m.r. chemical shift correlation

A 1H–13C n.m.r. correlation map is obtained by low-power heteronuclear decoupling, with carbon signal acquisition under proton broad-band decoupling in order to suppress off-resonance effects.