Prague NMR activities

Transcription

1 Polarised Nucleon Targets for Europe, 3rd meeting, Rech 26 Prague NMR activities H. Štěpánková, J. Englich, J. Kohout, M. Pfeffer, J. Štěpánek J. Černá, V. Chlan, K. Kouřil, V. Procházka E. Bunyatova * Faculty of Mathematics and Physics, Charles University Prague, Czech Republic * Joint Institute for Nuclear Research Dubna, Russia NMR in alcohols with TEMPO, liquid state (analysis of 1 H, 13 C relaxations in ethanol +TEMPO solutions) On the way to lower temperatures (solid state)

2 Interactions between electron and nuclear spins: - are known to achieve high polarization of the nuclear spin system in processes of dynamic nuclear polarization - give rise to effective relaxation mechanisms for excited nuclear spins Stable nitroxyl radicals: - spin labels for ESR experiments - paramagnetic probe in NMR Ethanol: interesting from the point of view of - molecular structure and dynamics, - forming and properties of intermolecular hydrogen bonds in polar liquids (water, simple alcohols)

3 NMR spectra and spin-lattice relaxations T 1 in the liquid state - first experimental results reported previously NMR BRUKER AVANCE 5 pulse spectrometer B external = 11.7 T ( 5 MHz for 1 H, 125 MHz for 13 C) six samples with -1.5 wt% of TEMPO in CH 3 CH 2 OH temperature range K NMR of ethanol: 1 H spectra: 3 signals 13 C spectra: 2 signals

4 Spectra 1 H at 21K all samples FWHM (Hz) CH3 CH2 OH Linewidth Shift 5 δ (ppm) 4 3 δ (CH2) - δ(ch3) δ (OH) - δ(ch3) TEMPO Conc. (wt%)

5 Spectra 1 H sample #2/2 (.5wt%) at various temperatures FWHM (Hz) Linewidth CH3 CH2 OH Shift 5 δ (ppm) 4 3 δ (CH2) - δ(ch3) δ (OH) - δ(ch3) Temperature (K)

6 Spectra 13 C at 21K all samples CH 2 CH 3 FWHM (Hz) Linewidth CH3 CH Shift 39.4 δ (CH2) - δ(ch3) δ (ppm) TEMPO Conc. (wt%)

7 Spectra 13 C sample #4/2 (1.5wt%) at various temperatures FWHM (Hz) Linewidth CH3 CH Shift 39.4 δ (CH2) - δ(ch3) δ (ppm) Temperature (K)

8 Relaxation rate R 1 = 1/T 1 Strong dependence of proton and carbon relaxation rates on temperature, maximal OH-proton relaxation rate at ~2K.5 wt% of TEMPO R 1 (s -1 ) H (OH) 1 H (CH2 ) 1 H (CH3 ) 13 C (CH2 ) 13 C (CH3 ) Temperature (K)

9 Relaxation rate R 1 = 1/T 1 Strong and linear dependence of proton and carbon relaxation rates of ethanol on TEMPO concentration 21 K R 1 (s -1 ) H (OH) 1 H (CH2 ) 1 H (CH3 ) 13 C (CH2 ) 13 C (CH3 ) x TEMPO concentration (wt %)

10 Relaxation enhancement k 1 dr 1 /d(concentration) k 1 (s -1 M -1 ) H (OH) Temperature (K) k 1 (s -1 M -1 ) 45 1 H (CH 2 ) H (CH 3 ) Temperature (K) k 1 (s -1 M -1 ) C (CH 2 ) C (CH 3 ) Temperature (K) The most pronounced effect of doping is seen for OH protons a role of hydrogen bonds between OH group of ethanol and the oxygen of TEMPO

11 Mechanisms of nuclear relaxation enhancement Fluctuating magnetic interactions between nuclear (ethanol) and electron (TEMPO) spins: - Dipolar interaction modified by motion translational diffusion rotational diffusion (of complex) - Contact interaction Particular models; approximations. Concentration, temperature, field dependences. (NMRD... dispersion of nuclear magnetic relaxation rates)

12 Translational diffusion Force free (FF) model for motion (excluded volume). Long electron spin correlation time π γ S γ I Na S( S + 1) k1 = ( 7J ( ωs ) + 3J ( ωi )), 45 1d D where the spectral density function is given by J(ω): J ( z) = ; zn = 2ωn τ D 1+ z + z z 8+ z z z 81+ z z translational diffusion correlation time τ D = d D D = Dethanol + DTEMPO, Di = kb T 6π ai η Di... translation - diffusion coefficients d... distance of the closest approach ai...size of the i - molecule (its dynamic radius) η... viscosity of the liquid ( τ is proportional D toη/t)

13 Relaxation enhancement k 1 = dr 1 /d(concentration) k 1 (s -1 M -1 ) H (OH) Temperature (K) k 1 (s -1 M -1 ) H (CH 2 ) 1 H (CH 3 ) Temperature (K) k 1 (s -1 M -1 ) C (CH 2 ) C (CH 3 ) Temperature (K) We made a fit of k 1 (translation diffusion) for protons except the 1H(OH) and for carbon nuclei, using common free parameters. Reasonable agreement for relaxations of 13 C, 1 H(CH 2 ) and 1 H(CH 3 ) in the region of temperatures above ~ 21K. The optimized values of the free parameters d =.4 nm, ζ a ethanol a TEMPO / (a ethanol + a TEMPO ) =.12 nm. The diffusion-controlled regime could be dominant for the relaxation enhancement of carbon nuclei and protons in ethanol except the OH group.

14 Translational diffusion 1 8 correlation time of diffusion motion Comparison of fitted slopes k 1 for 5 MHz (dashed lines) and predicted for 2 MHz (solid lines) spectrometers τ D (ns) H - others (FF model) 13 C Temperature (K).8.6 function J(z) T M -1 /CTEMPO (s -1 M -1 ) 6 4 T M -1 /CTEMPO (s -1 M -1 ) 15 1 J (z) Temperature (K) Temperature (K) z

15 Rotational diffusion k 1 where J ( τ ( ω)... [ I]... R the γ 2 S 2 γ I n S( S 6 b [ I] spectral distance between the molar concentration of is proportional toη/t) ; + 1) ( 7J ( ω ) + 3J ( ω )) density function rotational diffusion correlation time τ b r = r τ R = ω τ R I S and the S I J(ω) R spins spins I =, is : 4πηa 3kT 3

16 Rotational and translation diffusion Fit, rotational + translational mechanisms of relaxation 1 H - OH 1 H - others 13 C T M -1 /CTEMPO (s -1 M -1 ) T M -1 /CTEMPO (s -1 M -1 ) CH 2 CH 3 T M -1 /CTEMPO (s -1 M -1 ) CH 2 CH Temperature (K) Temperature (K) Temperature (K) Anisotropic motion? Internal motions?

17 Outlook: Relaxation models Dependence on frequency Chemical shifts interpretation Other alcohols/radicals Non polar solvents Adding water Changing viscosity Deuterated solvents Technical questions

18 May/June 25 New NMR laboratories in a new pavilion Laboratory views

19 Lower temperatures (solid state) 2 MHz NMR spectrometer (homemade): new hardware components and software

20 Lower temperatures (solid state) 5 T cryomagnet, cryoshimms (homogeneity ~1-5 ) 57 mm bore

21 Lower temperatures (solid state) 5 T cryomagnet, cryoshimms (homogeneity ~1-5 ) 57 mm bore

22 Lower temperatures (solid state) Helium gas continuous flow cryostat Janis, 2-4K

23 Lower temperatures (solid state) Nitrogen liquid cryostat (77 K) Vakuum Praha

24 Lower temperatures (solid state) NMR tunable probe for protons designed, made and tested

25 Lower temperatures (solid state) NMR tunable probe for protons designed, made and tested

26 Lower temperatures (solid state) NMR tunable probe for protons designed, made and tested tuning matching saddle rf coil

27 Lower temperatures (solid state) Installation of a new cryomagnet 9.4 T (4 MHz) cryoshimms (homogeneity ~1-5 ) 52 mm bore

28 Lower temperatures (solid state) Test of the new facilities: 1 H NMR spectra in ethanol 3 Intensity 2 Decreasing temperature ppm 3 K, 2 MHz 29 K, high resolution spectrum 5 MHz

29 Lower temperatures (solid state) Outlook: Technical questions Cooling regime: complicated thermal properties of ethanol - polymorphic forms T melt =159 K crystal I (monoclinic) supercooled liquid T g =97 K glassy liquid (extremely rapid chilling); supercooled liquid T melt =127.5 K metastable crystal II (cubic, plastic,molecules rotate) T g=97 K glassy crystal II (molecules frozen at random orientation) Relaxation models, structural and motional dependent, embedded TEMPO Deuterated ethanols

30 Outlook: Lower temperatures (solid state) T. Eguchi et al.