Polarised Nucleon Targets for Europe, 2nd meeting, Bochum 2005

Transcription

1 Polarised Nucleon Targets for Europe, nd meeting, Bochum Temperature dependence of nuclear spin-lattice relaxations in liquid ethanol with dissolved TEMPO radicals H. Štěpánková, J. Englich, J. Kohout, M. Pfeffer J.Černá, V.Chlan, V.Procházka E. Bunyatova * Faculty of Mathematics and Physics, Charles University Prague, Czech Rep. * Joint Institute for Nuclear Research Dubna, Russia Introduction, NMR in ethanol Experimental results (spectra and relaxations) Summary criteria: Introduction Proton/neutron target materials alcohols + stable nitroxyl radicals polarization as high as possible X relaxation rate as long as possible technical requirements (T, B, chemical stability) Current study C H C H C H C H C C C H ethanol + TEMPO C H N * C H O NMR spectra and spin-lattice relaxation in the liquid state System is interesting from the point of view of hydrogen bonding (molecular structure and dynamics of water and simple alcohols) 1

2 Introduction Nuclear magnetic resonance (NMR) Nucleus with nonzero total angular momentum (spin) nonzero magnetic dipolar momentum I = ħ I ( gyromagnetic ratio) In a static magnetic field B z energy depends on z Zeeman splitting I+1 equidistant levels I = 1/ I z = -1/ (for > ) E E = B B = B I z = 1/ Radiofrequency field induces transitions if rf = B (Larmor frequency) Resonating nucleus = probe sensitive to the local static field B Introduction High resolution NMR in liquids 1 H spectra 1 C spectra Chemical shift - different resonant frequencies from nuclei in nonequivalent positions in a molecule due to the different shielding of external field by electrons J-coupling - splits lines into multiplets due to the neighbour nuclei coupled via chemical bonds Direct dipol-dipol interaction between nuclei - not seen in the spectra of liquids (averaged to zero) Linewidth (homogeneous)~1/t Chemical exchange - may broaden the lines or reduce the number of lines Paramagnetic atom in the neighbourhood may cause a shift due to the contact Fermi interaction (dipolar interaction in liquids is averaged to zero)

3 Spin-lattice (T 1 ) relaxation Introduction High resolution NMR in liquids Relaxation is induced by fluctuations b(t) of local fields: K( ) = < b x (t).b x (t+ ) > ~ < b x (t) > exp (- / c ) T 1-1 is determined by the spectral density of K( ) at, ( ) J( ) ~ c / (1 + c ) c correlation time Minimum T 1 at c ~1/ ~1/B T c left extreme narrowing higher flexibility shorter c longer relaxation Introduction High resolution NMR in liquids Various relaxation mechanisms Diamagnetic samples: direct dipolar interactions Relaxation due to electron spin: dipolar interaction Fermi contact interaction Electron relaxation seen by nucleus - electronic correlation time s Various kinds of atomic and molecular motion: rotation ( r ) modulates dipolar relaxation translational diffusion ( t ) no adduct formation - modifies both contact and dipolar relaxation, non Lorentzian J chemical exchange ( m ) mean time for the adduct of the two chemical moieties Anisotropic motion. Internal motions. Concentration, temperature, field dependences

4 High resolution NMR in ethanol CH CH OH 1 H spectra: signals 1 C spectra: signals Introduction Line splittings due to the J - coupling - dependence on ph - chemical exchange Introduction High resolution NMR in ethanol CH CH OH ethanol + water ethanol in CCl (non polar solvent) neutral solution acidic solution

5 Introduction High resolution NMR in ethanol CH CH OH Hydrogen bonds, donor/accepor Formation of clusters (linear/cyclic) similarly as in water, but more simple (not the extended D clusters common to water), lifetime of the O-H covalent bond (and probably also the hydrogen bond) is relatively long B 1 Pulsed NMR experiments - spectra FID = free induction decay Distribution of Larmor frequencies, inequivalent sites of resonating nuclei 1,,,... z t 1 t / T u ( t ) ~ N e cos( t ) u(t) i i i t i isochromates FOURIER TRANSFORM x y FID SPECTRUM

6 Pulsed NMR experiments - spin-lattice relaxation (T 1 ) Inversion recovery / t 1 t Proton relaxations 1 H Carbon relaxations 1 C { 1 H} I(t 1 ) For individual spectral lines: I(t 1 ) = I( ) (1-A.exp(- t 1 /T 1 )) ~T 1 / ln t 1 Experimental NMR BRUKER AVANCE spectrometer B external = 11.7 T ( MHz for 1 H, 1 MHz for 1 C) Ethanol absolute for analysis, >99.9%, FM.7, Merck TEMPO =,,, tetramethylpiperidin-1-yloxy, 9%,FM1., Aldrich degassed samples, sealed, Ar atmosphere C H C H C H C H C C C H N C H C H * Sample TEMPO (wt%) HO (mol %) #/. #1/.. #/.. #/.17. #/.. #/ 1..9 (. wt% of TEMPO~ mm) O

7 Spectra 1 H at 9K all samples Linewidth OH CH CH FWHM (Hz) 1 1 CH CH OH Shift (ppm) (CH) - (CH) (OH) - CH) TEMPO Conc. (wt%) Spectra 1 H at 1K all samples FWHM (Hz) CH CH OH Linewidth Shift (ppm) (CH) - (CH) (OH) - CH) TEMPO Conc. (wt%) 7

8 Spectra 1 C at 1K all samples CH CH FWHM (Hz) Linewidth CH CH Shift 9. (CH) - (CH) (ppm) TEMPO Conc. (wt%) Spectra 1 H sample #/ (.wt%) at various temperatures FWHM (Hz) 1 1 Linewidth CH CH OH 1 Shift (ppm) (CH) - (CH) (OH) - CH) 1

9 Spectra 1 H sample #/ (1. wt%) at various temperatures FWHM (Hz) 7 1 Linewidth CH CH OH 1 Shift (ppm) (CH) - (CH) (OH) - CH) 1 Spectra 1 C sample #/ (1.wt%) at various temperatures FWHM (Hz) 1 Linewidth CH CH 1 Shift 9. (CH) - (CH) (ppm)

10 Inversion recovery - #/ (.wt%) at 1K 1 H CH CH OH T 1 values 1 H OH.1 s CH. s CH.9 s 1 C CH 1. s CH.9 s 1 sample: #/ wt%

11 1 sample: #1/.wt% sample: #/.wt%

12 1 sample: #/.17wt% sample: #/.wt%

13 1 sample: #/ 1.wt% wt% wt%

14 K TEMPO Conc. (wt%) 7 1 7K TEMPO Conc. (wt%) TEMPO Conc. (wt%) TEMPO Conc. (wt%) Summary Strong and linear dependence of proton and carbon relaxation rates T 1-1 of ethanol on TEMPO concentration Strong dependence of relaxation rates T 1-1 on temperature, maximal proton relaxation rate at ~K Besides the relaxation rates the doping affects also chemical shift and linewidth 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 C H C H C H C H C C C H N C H C H * O 1

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