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Chem-806 Identification of organic and inorganic compounds by advance NMR techniques. Tool box 2D-NMR: Homonuclear 2D-NMR: Heteronuclear 3D-NMR. Parameter Consideration. Receptivity Spin Quantum Number Resonance Frequency Sensitivity Natural abundance Frequency Shift Absolute Frequency
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Chem-806Identification of organic and inorganic compounds by advance NMR techniques Tool box 2D-NMR: Homonuclear 2D-NMR: Heteronuclear 3D-NMR
Parameter Consideration • Receptivity • Spin Quantum Number • Resonance Frequency • Sensitivity • Natural abundance • Frequency Shift • Absolute Frequency • Relative Chemical Shift Scale (Referencing) • Relaxation – T1 and T2 • Definition • Mechanisms • Measurement • NOE
gB0 n0 = 2p Multinuclear NMR At Bo=2.35 Tesla Nuclei Spin I Frequency % natural abundance Z • 11H ½ 100.00 99.98 • 613C ½ 25.15 1.108 • 714N 1 7.23 99.935 • 15N ½ 10.136 0. 365 • 919F ½ 94.103 100 • 1531P ½ 40.43 100 When sampling a nuclei, following parameters shoud be considered: • Sensitivity • Natural abundance • Relaxation time : T1 recycle time, T2 acquisition Time • Influence of H-decoupler: {NOE and J}
Sensitivity and Receptivity H H R = 0.83 R = C F • Magnetic Field ( g) • Population excess ( g) • Magnetic field induced in receiver coil ( g) The sensitivity of a nuclei depends on: Sensitivity = k * gx3 * Ix(Ix +1) e.g. g(13C)/g(1H) = 1/4 13Cless sensitive than proton (64 less) Receptivity Rx = ax * Sensitivity Where ax = natural abundance e.g. Relative receptivity of 1H and 13C aC * gC3 0.01 * (25)3 = = 1.6 * 10-4 aH * gH3 1 * (100)3
T1 considerations z z z z z Mo 5 T1 ..t t y y y y y x x x x x Spin lattice relaxation time T1 “lifetime” of First Order Rate process Mo 90x My Recovery of the magnetization along the Z axis • the type of nuclei • State of the sample Magnitude of T1 is highly dependant on : T1 governs the efficiency of the NMR experiment : recycle time For 1H in solution T1 can be 0.01 to 100 sec. For low g nuclei – spin ½ - relaxation can be much longer!
Dinitrobenzene: T1 180 90 t H4/H6 H2 H5
180 90 t Inversion recovery : 13C t = 50 s D1= 5T1 t = 6 s t = 3 s t = 1.5 s t = 0.03 s T1 = tnull / ln2 = 1.443 * tnull e.g. C2 => T1 = 4.3 s (tnull=3 )
One approach of reducing relaxation time is by the addition of paramagnetic relaxation reagent (Chromium III acetylacetonate => Cr(acac)3) Helping relaxation With Cr(acac)3 1s delay, 30o pulse Without Cr(acac)3
Intensity vs Pulse Interval PW=90o AQ D1 NS Mo z z 90x My t M(t) 1 * T1 0.63 M0 y y x x 2 * T1 0.86 M0 3 * T1 0.95 M0 4 * T1 0.98 M0 5 * T1 0.99 M0 10 * T1 0.99995 M0 Mz t t = pulse interval = D1 + AQ M(t) = Mo(1-e-t/T1)
Optimum recycle delay (pulse interval) with 90` pulse PW=90o AQ D1 NS t Sensitivity .1 T1 .3 .2 T1 .41 .5 T1 .56 .75 T1 .61 1 T1 .63 1.26 T1 .64 1.5 T1 .63 2 T1 .55 Total experiment time fixed t = pulse interval = D1 + AQ During the experiment, t (D1+AQ) is repeated for NS Optimum delay
Optimum angle with D1 < T1 PW<90o AQ D1 NS z z y y x x D1 = 0 t = pulse interval = AQ aE T1(t=1) t Mo M PWx Short T1 100 T190o .01 10 T190o .1 2.5 T186.3o .4 1.5 T177.1o .67 Optimum angle Ernst angle 1. T168.4o 1 a = cos-1 et/T1 0.5 T152.7o 2 0.25 T138.8o 4 0.1 T125.2o 10 Long T1 0.01 T18.1o 100
Sensitivity curves for different pulse and different delays and relaxation time
CPMG used to get rid of broad signals 90 180 t t Polystyrene (50,000) + camphor t = 1.5 ms
O1 SW SW and Memory size The spectral window (SW) and the carrier offset (O1) are chosen to match entire spectra (to avoid Fold-over and aliasing) For a given SW, the time (Dwell time) between 2 data point is defined by the Nyquist theorem (1/2SW). The total number of data point (TD) acquired is related to the Acquisition time AQ(DW*TD) The digital resolution depends on the window and on the number of points placed in that window TD = 2 * SW * AQ Digital resolution = 2 * SW/ TD = 1/AQ Sharp lines have long FID (long T2*), broad peaks have short FID (short T2*), AQ ~ 3 * T2
O1 SW r.f. DIGITALLIZATION Digitallization: Convert FID (Volt/Time) in Digital form Digitallization process is limited by: • Accuracy • Speed Carrier OffsetorTransmitter Offsetor “O1” is the frequency of the irradiating field. It is also the “Reference” or “Rotating Frame” frequency The “Window” or “Spectral Width” also called “SW” define the range of frequencies that can be measured _1__2*SW The Sampling Rate => 2 Points/Cycle Dwell Time = DW=
If Maximum Frequency to be sampled is fmax = SW We must sample at a rate no less than 2 * SWsec. Digital Resolution The amount of memory limit the accuracy of the signal to be recorded For a given # of memory(# Points -> TD (time domain)), one obtain: NP(real) and NP (Imaginary) 22 Digital Resolution = D.R. = Df (Separation between 2 points) D.R. = 2 * SWNP
Example At 200 MHz If: SW=2000 Hz (10 ppm) TD = 16,000 points (16K) What is the Digital Resolution: D.R. = 2*SW/TD = 4000 / 16,000 = 1 / 4 = 0.25 Hz What is the Acquisition Time AQ: AQ = TD * DW = TD / (2 * SW) = 4 seconds D.R. = 1 / AQ = 2 * SW / TD
Heteronuclear nOe For nuclei having positive g : (e.g. 13C) Decoupling proton can produce higher signals due to nOe. Enhancement is dependant on motion and distance between interacting nuclei As a consequence, quaternary carbons are much smaller than protonated carbons
Heteronuclear nOe • Can yield higher positive signal for nuclei with positive g (e.g. 13C) • For nuclei with negative g(e.g. 29Si, 15N) can yield larger (negative) signals or for partial T1DD can null the signal!!! e.g. 15N {1H} S =A0+NOE= -4 S =A0+NOE= 0 S =A0+NOE= -1.5 50% NOE 20% NOE Without NOE S = A0 =1 100% NOE NOE=-1 NOE=-2.5 gH ~ -5 NOEmax 2 gN
Delay as a sequence building block J J - 2 2 x x x x dA AX Spin system A=1H, X=13C, JAX For each isochromats the distance they run in the xy frame during a delay t is: JAX Distance = Frequency (cycles/sec) * delay (sec) Distance = 2p * (+/-) J/2 * t t=0 t=1/2J t=1/J Delay t=1/4J p/2 (90o) p (180o) 0 Dist. p/4 (45o) J= 10 Hz t (1/2J) = 0.05 s J=100 Hz t (1/2J) = 0.005 s J=140 Hz t (1/2J) = 0.00357 s
