TECNICHE DI ALTA E BASSA RISOLUZIONE IN NMR STATO SOLIDO

Transcription

1 TECNICHE DI ALTA E BASSA RISOLUZIONE IN NMR STATO SOLIDO Roberto Gobetto Dipartimento di Chimica I.F.M., Università di Torino, Via P. Giuria 7, Torino

2 Chemical Shift Anisotropy s q Chemical shift depends on orientation Spectra from powdered samples are sums over individual crystallite orientations: (Shape reflects probability of particular orientation) axial symmetry (h = 0)

3 Dipolar Interaction For an isolated pair in a crystal dd = 0 ± ¾ D (3cos 2-1) D AX = (μ 0 /4π)(h/4π 2 )γ A γ X /(r AX ) 3 in frequency units B 0 q r d ( 3cos 2 q -1) 2 µ 3 r D α ( 3cos 2-1) Through space interaction between magnetic nuclei Potential direct information about geometry = 0 Many nuclei in a powder B 0 = 45 = 90

4 FID and SPECTRA of Liquid versus Solid: LIQUID s FT 4 khz at 0 =400 MHz SOLID FT s Solids have shorter FID decay (larger linewidths) Impossible to observe chemical shift differences in inequivalent nuclei

5 Linewidth is reduced since CSA and Dipolar Interactions are averaged MOTION IN THE SOLID STATE Different Spin-Lattice Relaxation Times and T 1 dependence with the temperature

6 Effect of motion on the CSA Chemical shift of powder sample effect of the motion Asymmetric Rigid s 33 s 22 Axially symmetric Fast C 6 Rotaton s 11 Isotropic motion s iso = 1/3 (s 11 +s 22 +s 33 ) s s iso s 11 s 22 s 33, s

7 Effect of motion on the Dipolar Interaction Temperature Dependence Crystal Lattice Mobility Changes with Temperature Changes in bond rotations Large changes in line-shape depending on mobility in lattice Rotation about C-N bond Rotation of NMe 3 Whole molecule rotates and diffuse within crystal

8 Effect of motion Motional on the effect Proton-Proton on the linewidth Dipolar Interaction Linewidth

9 Dynamic information from NMR Relaxation Motion effect on the relaxation time 1/T 1 = C[(t c /(1+ 2 t c2 )+4t c /(1+4 2 t c2 )] Arrhenius type activation law t c =t 0 exp (E a /RT)

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11 HOW TO OBTAIN HIGH RESOLUTION SOLID STATE NMR SPECTRA? Direct observation of a dilute nucleus ( 13 C) in the presence of an abundant nucleus ( 1 H) PROBLEMS 1. HIGH DIPOLAR INTERACTION 2. CHEMICAL SHIFT ANISOTROPY 3. LONG RELAXATION TIMES SOLUTION TO PROBLEMS HIGH POWER PROTON DECOUPLING MAGIC ANGLE SPINNING CROSS-POLARIZATION

12 High power proton decoupling Alanine H 3 C O OH COOH * * CH CH 3 Solid State NH 2 Solution Solution state Solid state high power decoupling COOH CH CH 3 Applying same techniques like in solution

13 Magic Angle Spinning (MAS) B 0 A s iso B s iso The idea consists in reproducing the effects of the brownian motion occurring in liquids by fast macroscopic rotation of the sample......but why a magic angle (54.74 ) between the magnetic field and the spinning axis should (and indeed does) work?

14 HOW TO REMOVE DIPOLAR AND CSA EFFECT FAST MAGIC ANGLE SPINNING Average to zero the geometric term of the Hamiltonian (3cos 2 1) 3cos 1 = 2 2 3cos 1 ( 3cos 1) 2 2 B 0 r = cos 2 1 = 0

15 Chemical Shift upon sample spinning (,f) =gb 0 [ 1 s iso ½ D(3 cos 2 1 hsin 2 cos 2 f)] B 0 s, = 0 s xx S 2 2 Under rotation = (t) and f = f(t) + y PAS n 2 sin 2 +s zz PAS cos 2 ) = gb 0 s iso D 3 os n 2 os 2f (

16 Let s consider the sample rotating about an axis, in the frame of which the orientation of the magnetic field B 0 is described by the polar angles a and. By choosing s zz in the rotor reference system B 0 y R a z R a(t) = a (t) = r t x R Isotropic term Anisotropic term, no time dependent, = 0 when a = s zz = s iso + ½(3cos 2 a 1)(s zzr s iso ) + sin 2 a[½ (s zzr s yyr )cos(2 R t)+s xyr sin(2 R t)+sin(2a)[s xzr cos( R t)+s yzr sin( R t)] Time dependent term, negligible when R >> D, static linewidth

17 Increasing Spinning Speed Magic Angle Spinning (MAS) Impact of Spinning Speeds at MAS O 13 C NMR of glycine powder H 2 N glycine OH Similar to Solution Spectrum Number of lines are reduced with increase in spinning speed as it approaches static line-width Lines are separated by spinning speed Powder Pattern Angew. Chem. Int. Ed. 2002, 41,

18 What happens when the condition R >> D is not fulfilled? Two different cases Inhomogeneous Hamiltonians, for example chemical shift, heteronuclear interaction, two-spin homonuclear interaction Homogeneous Hamiltonians Multi-spin homonuclear interaction

19 Inhomogeneous Hamiltonians Isotropic term (s iso ) and the time-dependent term contribute to the spectrum. Time-dependent term gives narrow lines at frequencies that differ from the central, isotropic signal by an integer multiple of the spinning frequency. CH These lines are called spinning sidebands (ssb). 3 H 3 C CH 3 n ssb - n iso = nn R, n = ±1, ±2,... H 3 C CH 3 CH 3 static isotrop peak lines are narrowed even at very low spinning rates the intensity of the spinning sidebands reflects that of the static pattern the spinning sidebands can be easily distinguished from the isotropic signal by recording the spectrum at different spinning rates: only the position of the isotropic line is not affected by the spinning rate MAS rate 0.5 khz 1.6 khz 2.1 khz

20 EFFECT OF MAS ON HOMOGENEOUS HAMILTONIAN R = 0 Broad peak R D Broad peak R D /4 Isotropic peak + spinning sidebands R D Isotropic peak

21 FLIP-FLOP EXCHANGE flip-flop term (Î + Ŝ - + Î - Ŝ + ): exchanges the spin states of reciprocal spins at constant energy t a a a interchange of the spin state during time t At intermediate spinning speed the spin state is not constant in the rotor period The homonuclear dipolar interaction cannot be averaged by MAS at intermediate spinning speed

22 Homogeneous Hamiltonians 1 H spectra of gluten the narrower line gives rise to a high-resolution spectrum when R >D static two components are present with linewidths of 2.5 and 25 khz. there is substantially no effect of MAS on the spectrum for R < D the broader line partially splits into very broad sidebands at high spinning rates due to the presence of a partial inhomogeneous character of the homonuclear dipolar Hamiltonians, ascribable to two-spins interactions MAS is not sufficient to get high resolution spectra when homogeneous static linewidths are present larger than the currently available MAS frequencies.

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24 MAGIC ANGLE SPINNING Sample Rotor Rotors Air Supply dry air Spinning air MAS probes Spinning rate read-out Pneumatic unit (air and spinning rate control)

25 Magic Angle Spinning (MAS) Spin Samples at o to reduce line-width Spinning speed must be greater than static linewidth to be studied (powder pattern width) Normal speed limit is 35 khz rotor at MAS Sample holder rotor Sample holder at MAS MAS probe

26 Probe MAS standard bore stator flip mechanism BN stator RF coil bearing gas inlet RF electronics

27 Solenoid coil in MAS probe

28 How to insert or eject sample

29 φ(mm) Rotors Temp Range Max speed Volume/amount 7 ZrO l /360mg Si 3 N Si 3 N l/ 75mg Caps Kel-F Torlon pmma

30 C. Dybowski; R. L. Lichter NMR Spectroscopy Techniques Ed. M. Dekker, inc N.Y.(1987)

31 MAS Setting B 0 = 54.7 HOOC-CH 2 -NH 2 J. Shaefer; E. O. Stejskal Topics in 13 C NMR Spectroscopy, Ed. G. C. Levy, Vol. 3, Chap. 4, (1979)

32 SPINNING SIDEBAND ANALYSIS with the STARS PROGRAM for the 119 Sn spectrum of CaSn(EDTA) Fitted Observed Calculated values: s 11,s 22,s 33

33 CROSS-POLARIZATION RD PULSE SEQUENCE 90 x spin-lock y decoupling y I ( 1 H) CP g I B 1I = g S B 1 S S ( n X) FID contact time (10-3 s) (P15) acquisition time (10-1 s) (AQ) relaxation delay (s) (D1)

34 CROSS POLARIZATION: PULSE SEQUENCE 1 H Spinlocking High CW Power decoup Decoupling x y y -x 13 C Cross polarization 1 2 z B o M o H 90 pulse on 1 H z 1) x y Spin lock on 1 H and CP X M o H B rf Y Z Z 2) M o H X B 1H Y B 1C X M H Y γ H B 1H = γ C B 1C

35 1 H Spinlocking High CW Power decoup Decoupling x y y -x CROSS - POLARIZATION 13 C Cross polarization 1 H 90 o pulse generates xy magnetization (B 1H ) Spin-lock pulse keeps magnetization in xy plane precessing at: g H B 1H /2p Hz 13 C pulse generates xy magnetization that precesses at: g C B 1C /2p Hz Polarization transfer occurs if: g H B 1H /2p Hz = g C B 1C /2p Hz Hartmann Hahn matching condition Polarization transfer 1 H g H B 1H /2p g C B 1C /2 p 1 H a 13 C 13 C a DE = g h B o / 2p

36 10-6 s Cross Polarization (CP) cnctc at d s 10-1 s 10 s Hartmann-Hahn matching condition: γ H B 1H = γ C B 1C Contact time: 1-10 ms Gain in S/N: γ H / γ C ~ 4 Recycle delay ~ 5 x T 1 ( 1 H) leads to further gain in S/N E. O. Stejska; J. D. Memory High Resolution NMR in the Solid State. Ed. Oxford University Press (1994)

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38 Solution and Solid state NMR Alanine H 3 C O OH COOH * * CH CH 3 CPMAS Solid State NH 2 rotation at 5 KHz Solution high power decoupling CH CH 3 COOH applying same techniques like in solution

39 H OH 13 C- CPMAS HO HO H H H O OH H OH H O H O OH OH H OH Spinning rate 4000 Hz ppm

40 The concept of Spin Temperature

41 Spin Temperature in CP Experiment 1 H Spinlocking High CW Power decoup Decoupling x y y -x 0) At the beginning 13 C 0 Cross polarization 1 2 P - P + = e għb 0 kt L Where T L = laboratory temperature Boltzmann Law 1) Immediately after 90 pulse on 1 H and spin-locking in the rotating frame 1 H P - P + = e għb 0 kt L 90 x P - P + = e għb 1H kt H T H = T L B 1H T H << T L B 0 13 C No 13 C magnetization in the plane x y, then T C =

42 2) During cross polarization the two spin systems exchange energy between 1 H spin system having low spin temperature and with an high reservoir and 13 C spin system having high spin temperature and low reservoir T eq =T CP = T H B 1H T C = T H = T L = B 0 T L g C B 1C = T C g H B 0 g C g H Where T C is the 13 C spin temperature for direct excitation 1 H T 1ρ LATTICE T CP 13 C

43 Spin Temperature in CP Experiment 1 H Spinlocking CW decoup ling High Power Decoupling x y y -x 13 C Cross polarization At the beginning T H =T L The cross polarization starts with a spin lock on abundant proton spins. When the field on the proton spin is B 1H the hydrogen spins are initially cooled to the temperature Q H = B 1H T L B 0 When the Hartmann-Hahn condition is fulfilled: g I B 1H = g S B 1C the energy splittings of the two spin species are equal. The flip-flop transitions tend to equalize the spin temperature of both the I and the S spin systems. Before the CP contact we assume that the 13 C spins are saturated Q 13C = By assuming a long CP contact both spin systems reach the same final spin temperature: Q HFin = Q CFin = Q Fin

44 Before cross polarization the total energy of the spin systems in their rotating frames is - C H B 2 1H - C C B 2 1C E H + E C = + = - C H B 1H 2 Q Hin Q Cin Q Hin After equilibrium has been reached - C H B 2 1H - C C B 2 1C E H + E C = + Q fin Q fin Because the energy during cross polarization is maintained Q fin Q 1H N C ½ (½ + 1) = 1 + = 1 + N H ½ (½ + 1) since N C << N H Due to cross polarization the 13 C spin temperature went from to a value very close to the initial low 1 H spin temperature

45 CROSS-POLARISATION DYNAMICS limiting intensity T 1ρ (H) T CP contact time t M( t) = A exp exp t + T T ( H) CP 1

46 DISCRIMINATION by SHORT-CONTACT CP for a homogeneous sample Reason: Cross polarisation occurs by dipolar interactions, with strengths proportional to 1/ r CH 3

47 QUANTITATIVE SOLID-STATE NMR for a heterogeneous system showing the effect of long contact times Case considered: Two types of carbon, A & B, in equal amounts but in different spatial regions with no proton spin diffusion between them.

48 Resolution of crystallographic independent sites

49 Resolution of crystallographic independent sites in Si(OSi) 4 Systems 29 Si of ZSM 5 29 Si of high crystalline ZSM 5 after shimmimg optimization Simulated spectrum C.A. Fyfe, Nature 1987, 363, non-equivalent independent sites in ZSM-5

50 Identification of Polymorphs Oxybuprocain: local anesthetic drug 12 O NH O 9 Cl C CPMAS 17 NH O form II C11 C3 C4 C6 C1 C5 C2 C14/16 C12C13 C7 C9 C10 C15/17 C8 form I ppm U.J. Griesser et al. Crystal Growth & Design, 2008, 8, 44

51 Information on local environments: Crystallographic Asymmetric Units One or more than one molecule in the unit cell? H 2 N O O O NH Cl 13 Oxybuprocaine hydrochloride (ophtalmic drug for eye-drop formulations) 13 C - CPMAS DIPOLAR DEPHASING Form II, two independent molecules in the crystal cell Form I, one molecule in the crystal cell

52 Information on local environments: Crystallographic Asymmetric Units Form II 13 C-CPMAS

53 Local Environment and Disorder Chemical shift is very sensitive to small differences of the local environment. Structural disorder may afford two different signals for a specific molecular environment in the molecule. O O Disorder p-formyl-trans-cinnamic acid H H O O H O H OH O OH O OH O OH

54 Local Environment and Disorder -form Disorder 70% : 30% O H COOH C CPMAS S. Meejoo et al. Helvetica Chimica Acta 2003, 86, 1467

55 Static or Dynamic Disorder Unit Cell of the 3:2 PHENOL: TPPO adduct disorder of one phenol molecule Variable temperature 31 P CPMAS Experimental Simulated ½ oxygen atom phosphorus atom D.C. Apperley et al. PCCP 2000, 2, 3511

56 Solid State Conversion [S,S]-Ethambutol dihydrochloride antituberculosis drug 5 4 HO Cl - H 2 N N H2 Cl - OH 80 C Form II >74 C ~74 C 50 C Form I J.M. Rubin-Preminger et al., Crystal Design & Growth 2004, 4, 431

57 Weak Interactions MALONIC ACID DABCO ADDUCT Difference between COO - and 13 C chemical shifts 13 C CSA COOH 2-10 ppm COO - COOH COO - COOH COOH/COO - Gobetto R, Chierotti,M.R.. et al. Chem. Eur. J. 2003, 9,

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59 Weak Interactions SUPRAMOLECULAR ADDUCTS BETWEEN DABCO AND DICARBOXYLIC ACIDS OF DIFFERENT CHAINS 1. C3 1. C4 1. C5 1. C6 1. C7 1. C8 1. C9

60 Weak Interactions Gobetto R., Chierotti M.R. et al. Chem. Mater. 2005, 17, N CPMAS for detection of Hydrogen bond Isot. Natural abundance Sp Magnetogyric ratio Relative receptivity 1 H % ½ * C 1.1 % ½ 67.2 * * N 99.6 % * * N 0.37 % ½ * *10-6 Free nitrogen d ppm 15 N chemical shifts Protonated nitrogen + N H O - d 1 6 ppm Nitrogen involved in hydrogen bond N H O d -2-9 ppm

61 d 15 N calculated Weak Interactions 15 N CPMAS for detection of Hydrogen bond 15 N chemical shift vs DFT calculation DFT vs NMR: 15 N chemical shift N + H O N H O Free N Gaussian G(2d,p) Experimental d 15 N

62 Weak Interactions 15 N CPMAS for detection of Hydrogen bond N-O DISTANCE vs 15 N CHEMICAL SHIFT Gobetto R, Chierotti, M.R.. et al. Chem. Eur. J. 2005, 11,

63 CONCLUSIONS LOW RESOLUTION SOLID STATE NMR TECHNIQUES AFFORD INFORMATION ON: SOLID STATE DYNAMICS HIGH RESOLUTION SOLID STATE NMR SPECTRA OBTAINED BY CPMAS AFFORD INFORMATION ON: SOLID STRUCTURE AND DYNAMICS LOCAL STRUCTURE LOCAL DISORDER POLYMORPHIC FORMS SOLID REACTIONS INTRA AND INTERMOLECULAR INTERACTIONS