Double-Resonance Experiments

Transcription

1 Double-Resonance Eperiments The aim - to simplify complicated spectra by eliminating J-couplings. omonuclear Decoupling A double resonance eperiment is carried out using a second rf source B 2 in addition to the rf transmitter B 1 used for the detection of the spectrum. omonuclear spin pair AX with a scalar J AX 0: if we observe the A nucleus while irradiating selectively the X nucleus, the scalar coupling J AX vanishes. The necessary strength of the B 2 field: γb 2 /2π > 2J AX. In terms of the vector model: the frequency ν 2 = γb 2 /2π coincides with the Larmor frequency of the X nucleus B eff = B 2, i.e., the vector µ X precesses around B 2 and thus around ais. The nuclear spin vectors I A and I X are then quantized along the z- and the -ais respectively. They are orthogonal and their scalar product (E = J AX I A I X ) is therefore zero (the scalar spin-spin coupling vanishes). For simple spectra, the results are straightforward: record two spectra, one with and another without selective decoupling and check which multiplets change. For more crowded spectra, it is common to take the difference between the two spectr a in order to observe changes that would otherwise be difficult to detect. O B2 O O N O ppm ppm ppm NMR Spectroscopy 1

2 Main problems: (i) suffers from difficulties associated with selective irradiatiation of only those spins which are desired; (ii) Bloch-Siegert shift: ν ' A γ B ν A + 8π 2 ( ν ν ) A 2 2 X If several spin-decoupling eperiments are desired, it is generally more time-efficient to perform a twodimensional COSY eperiment in order to establish connectivities. eteronuclear Decoupling The frequency difference ν 2 - ν 1 lies in the Mz range. The second field B 2 is produced by a separate transmitter. 1{ 14 N} NMR N 4 + A curly bracket is used to indicate which nucleus is being decoupled during the eperiment, e.g. 13C{1} indicates a 13C spectrum acquired with 1 decoupling. Can be used for simplifying spectra complicated by heteronuclear J-couplings (e.g., J F or J P ) or for removing line broadening caused by quadrupolar nuclei (e.g., 14 N). 1 NMR N 4 + Broadband decoupling Broadband decoupling is used in order to eliminate the coupling between two nuclei completely. For eample, broadband 1 decoupling while acquiring 13 C spectrum leads to singlets for each 13 C environment and a substantial increase in the sensitivity of the eperiment. Previously broadband decoupling techniques were based on the combined use of high-power decoupling and frequency modulation of the B 2 frequency with a randomly varying waveform (proton noise decoupling). Generally, this results in an inefficient broadband decoupling and leads to loss of both sensitivity and resolution. Broadband decoupling techniques based upon the spin flip eperiment (MLEV, Malcolm Levitt): 1 π τ π τ π τ π τ π τ C π/2 Acquire... NMR Spectroscopy 2

3 180 ( 1 ) -J/2 +J/2 +J/2 -J/2 On Off τ 2τ ADC A train of π pulses will be susceptible to the errors due to pulse length inaccuracy, since they will accumulate. The non-ideal π pulse can be replaced with a composite pulse. Composite π y pulse: π π y Eamples of composite pulse decoupling techniques: WALTZ-16 and GARP The linewidth achievable in WALTZ C NMR spectra is ~ z (suitable for detecting small effects such as 35/37 Cl isotope effects of a few ppb) conventional modulation technique; composite pulse decoupling kz Broadband decoupling of nuclei with large chemical shift ranges ( 19 F, 31 P) with 1 observation becomes much more feasible on using such decoupling techniques as GARP. The effective decoupling bandwidth: WALTZ-16 Gated decoupling GARP-1 2B 2 (B 2 5 kz) 5B 2 (slightly greater residual broadening of the lines) Disadvantages of 13 C{ 1 } spectra: (i) the nuclear Overhauser effect (NOE) leads to intensity distortions and the integration of these spectra becomes questionable; (ii) the elimination of line splittings prevents the measurement of 13 C, 1 coupling constants. NMR Spectroscopy 3

4 The application of a method known as gated decoupling eperiment successfully handles spin-spin coupling and intensity distortions. In the first case, the decoupler is switched on during acquisition, but switched off during the remaining time before the net pulse. The 13 C- 1 coupling is thus eliminated without producing NOE, as the integrals depend mainly on the intensities of the initial points of the time domain function. B 1 NOE Alternatively, relaation reagents (e.g., chromium acetylacetonate) can be used to eliminate unwanted NOEs. Because of their paramagnetic moment, relaation reagents provide an efficient mechanism for 13 C spin-lattice relaation that suppresses dipolar relaation, which is the main phenomenon leading to NOEs. B 2 In the second case, the decoupler is gated: it is on during the delay time between different pulses and switched off during acquisition. FID FID B 1 NOE B 2 As a result, the spectra are not decoupled and most of the NOE is preserved. This provides higher sensitivity for measurements of J( 13 C, 1 ) couplings. NMR Spectroscopy 4

5 Eample of signal multiplicities in a proton-coupled 13 C NMR spectrum 13 C, 1 coupling constants in pyridine C-3 2 J(4) 2 z 3 J(5) 6 z 2J(2) 7 z 1J(3) 166 z C-4 C-3 C ppm 3J(2) 4 z 1 J(4) 163 z ppm N C-2 2 J(3) 2 z 3J(4) 7 z 3J(6) 12 z 1 J(2) 177 z C ppm NMR Spectroscopy 5

6 Polarization Transfer The aim - to increase sensitivity by transferring favourable NMR properties of the proton to another nucleus. N β / N α = ep (- E / kt) 1 - E / kt = 1 - γh B 0 / kt n = N α N β n γh B 0 / kt Selective population inversion A two-spin system 1-13 C (e.g., in CCl 3 ) h c C 2 h+ c 1 {INVERTED} - h+ c C 2 - h- c 1 2 h+ c C 1 h c 2 h- c h+ c C C 1 13C γ 4 γ C h 4 c C +1 : +1 3 : +5 Relative intensities for the X multiplet of an AX n group: at Boltzmann distribution after selective population inversion n NMR Spectroscopy 6

7 INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) 1 : π τ π τ 2 13 C: π τ = 1 / (4 1 J C ) y Acquire Non-selective pulses are used to put pairs of proton transitions into antiphase /(4J) 180 1/(4J) 90 y The proton magnrtisation vectors are prepared in antiphase: the α vector along +z and the β vector along -z (as in SPI). This corresponds to inverted 1 spin populations with the ecess population in the upper level. A 1 13 C polarization transfer occurs and a fourfold (γ /γ C ) enhancement in 13 C signal-to-noise ratio is achieved. As relaation times T 1 ( 1 ) are shorter than T 1 ( 13 C), there is a further gain in the rate at which signals can be accumulated. DEPT (Distortionless Enhancement by Polarization Transfer) J 2J ± y 2J 1 : π θ ( Decouple ) 13 C: π ± Acquire( ) θ ±y a pulse of variable length. To separate C, C 2 and C 3 subspectra, eperiments with θ = π/4, π/2 and 3π/4 are performed. NMR Spectroscopy 7