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Chem-806 Identification of organic and inorganic compounds by advance NMR techniques

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##### Chem-806 Identification of organic and inorganic compounds by advance NMR techniques

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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**Homonuclear 2D correlation techniques**• Through Bond: nJHH (scalar coupling) COSY : COrrelated SpectroscopY Directly coupled neighbors Relay-COSY : RELAY-COrrelated SpectroscopY Directly coupled neighbors and protons coupled to the coupled neighbors (relay transfer) TOCSY : TOtal Correlation SpectroscopY Directly coupled neighbors and protons coupled to the coupled neighbors (More efficient than Relay) • Through Space: Distance NOESY : NOE SpectroscopY ROESY : ROE SpectroscopYNOE in Rotating frame**Processing for Absolute value COSY: Sine-belland Pseudo-echo**shaping**2D-COSY of glucose derivative**5 4 6 a/b 2 1 3 5 4**C4H8O**CH3 – CH2 – CH2 – OH**C4H8O: COSY**CH3 CH2 OH CH3 – CH2 – CH2 – OH CH2**C5H8O2 I= 5 – 8/2 +1 = 2**CH2 - O CH2 - O O-C=O**CH2 - O**c d b a C5H8O2 4 b CH2 3 c CH2 a CH2 d CH2 C=O 2 O 5**3H**C8H16O : I = 8 – 16/2 + 1 = 1 2H 3H 4H 2H CH2 – CO – CH2 2H Ketone C=O**Me-1**5 4 3 2 C8H16O Me1 – CH22 – CH23 – CH24 CH25 C=O 5 Me-8 CH27 3 CH27/5 Me8**C11H20O4**X 2 3H 3H CH2 – CH3 2H 2H O – CH2 – CH3 O-C O-C=O**C11H20O4**CH2 – CH3 O – CH2 – CH3 O = C –O – CH2 – CH3 CH2 – CH3 CH3– CH2 C O = C –O – CH2 – CH3 Only missing quaternary carbon**COSY-90: disaccharide**H2 H1 H3 H5a H5b H4/2 H3? H1 H2**COSY-45**J1,2 => negative J1,3 J2,3 => positive**COSY-45**If we consider cross peak 2/1Passive couplings – involving passive nuclei 3 - (J1,3 and J2,3 ) have same Sign (positive slant) If we consider cross peak 3/1Passive couplings – involving passive nuclei2 - (J1,2 and J2,3 ) have different Sign (negative slant)**COSY45: H2 and H4 overlap at ~ 4.7 ppm. Cross-peak in**COSY-45 allow to assign H3 and H5a/b unambiguously as H4 is coupled to a geminal pair => different sign in J 2/4 5a 5b 3/5a/5b? 2 1**H4**H2 H3 H6a/b H5**H5’**H3’ H4 H2’ H2 H3 H4’ H6a H6b H5 Deshielded multiplet H1’ H6a/b H5 H6a’ H6b’ H4’**DQCOSY: Active couplingisantiphaseandthere isno intensity on**central peak of a triplet (aspositiveandnegativepeaks cancel out)**Multiple Relay-COSY:As we have longer relay sequence =>**relaxation during transfer step attenuate the signal and Magnetization get weaker**NOE and distance**1 F (tc) NOE r6 NOE is a consequence of cross-relaxation between 2 spins close to each other in space. NOE is a consequence of modulation of the Dipole-Dipole couplingby motion of the molecule in solution. The NOE intensity is related to the internuclear distance r and is a function of the correlation timetc**Relaxation and tumbling rate**Relaxation is caused by fluctuating magnetic field generated by neighboring dipole. If the rate at which the fluctuation occur in the transverse plane matches the frequency of double quantum transition, positive NOE will be observed. If the fluctuation is slower, zero quantum transition will produce negative NOE.**NOE is related to dipole relaxation**• For small molecules, translation and rotational motion occur at high frequency. • The vector linking two nuclei rijchange orientation more frequently in small molecule than in larger molecule (Small molecules tumble at rates around 1011 Hz, Larger molecules such as proteins tumble at rates around 107 Hz), dissipating the energy between different spin states • In small molecules the frequency of motion can occur frequently at Larmor frequency o and twice 2xo(W2) dissipating energy between single quantum and double quantum state. (this produce Positive NOE) • In Large molecule, only low frequency transition like Zero quantum (W0) can dissipate energy (aibj biaj ) (Negative NOE)**NOE: applying gB2 to the A of an AX spin system**bb bb X2 A2 ba ba ab ab A1 X1 aa aa X1 Dp = 2 X1 Dp = 2 X2 Dp = 2 X2 Dp = 2 X2 A2 {A} A1 X1 Immediately after irradiation, there is NO change in the intensity of X Turning on the Decoupler do not change population of the X transition**NOE: relaxation with double quantum pathway W2 probability**(positive NOE) bb bb X2 A2 X2 A2 {A} ba ba ab ab A1 A1 X1 X1 aa aa X1 Dp = 2 X2 Dp = 2 bb bb X2 A2 X2 A2 ba ba ab ab A1 A1 X1 X1 aa aa X1 Dp = 3 X2 Dp = 3 delay … T1 Dec. continue W2 After W2 relaxation, there is a net increase in the intensity of X (50%) Relaxation takes time to establish a new equilibrium: T1 process**NOE: Relaxation with zero quantum pathway W0 probability**(negative NOE) bb bb X2 A2 X2 A2 {A} ba ba ab ab A1 A1 X1 X1 aa aa X1 Dp = 2 X2 Dp = 2 bb bb X2 A2 X2 A2 ba ba ab ab A1 A1 X1 X1 aa aa X1 Dp = 1 X2 Dp = 1 delay … T1 Dec. continue W0 W0 After W0 relaxation, there is a net decrease in the intensity of X (50%) negative NOE Relaxation takes time to establish a new equilibrium: T1 process**NOE: summary of relaxation pathways** bb X2 A2 ba ab A1 X1 aa W1: probability of single quantum relaxation do not create nOe W2 W0 A new population ditribution is generated by relaxation through dipole-dipole relaxation : double quantum and zero quantum pathway W2 and W0 If W2 is efficient (small molecule – fast motion large frequency ) Level increase level increase also with decoupler continuing W2 pathway yield positive nOe If W0 is efficient (large molecule – slow motion small freq. Diff.) Level increase level increase also with decoupler continuing W0 pathway yield negative nOe**Maximum NOE vs correlation time (nuclei interacting with 1H)**hC = 4/2 = 2 0.5 Fast tumbling maximum NOE : Line intensity : Ii = 1 + hi {H} hN = 10/-2 = -5 wtc ~1 Large molecule Small molecule**NOE vs Distance and motion**If B is irradiated, nucleus A should show the largest NOE (closest to nucleus B) (the relative distances are shown as A to B = 1, B to C and C to D = 2). Nucleus C is relaxed by nucleus Das well as B, so it shows a smaller NOE. Nucleus D has an indirect NOE from nucleus B. Indirect effects usually give rise to negative NOEs. Note that as the tumbling rate decreases all other parameters become irrelevant and the NOEs tend towards -100%. The notation fA{B} means the NOE enhancement of spin A when spin B is saturated.**NOE difference: nOe-d**Me Cl C C Me H d1 AQ irr Dec on frq irr d1 AQ Dec off frq NOE is a kinetic effect: need delay ~ T1 It take time to develop It takes time to decay control nOe difference**Choosing a structure by nOe**{OH} {OMe} H3 H6 H5