C13 NMR

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By: anjookamboj (24 month(s) ago)

Hi Prashant... Very good presentation. could you please mail me a copy this presentation to [email protected]

By: pr.prashant (110 month(s) ago)

I have mailed it..just check it out...All the best...

By: vannathankandi (110 month(s) ago)

hi prasanth.... It is a very good presentation . Could u plz mail me a copy to [email protected]

By: vannathankandi (110 month(s) ago)

hi prasanth.... It is a very good presentation . Could u plz mail me a copy to [email protected]

By: pr.prashant (110 month(s) ago)

I have sent it ..just check it out...:-)

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C13 NMR Spectroscopy:

C 13 NMR Spectroscopy By Prashant J Patel (Department of pharmaceutical technology) Indukaka Ipcowala College of Pharmacy New vidhyanagar,anand

Why C13-NMR is required though we have H1-NMR?:

Why C 13 -NMR is required though we have H 1 -NMR?

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1 H nmr spectroscopy - The powerful and useful tool a tool for structural analysis. Useless when portions of a molecule lack C-H bonds , no information is forthcoming. Ex: polychlorinated compounds such as chlordane , polycarbonyl compounds such as croconic acid , and compounds incorporating triple bonds (structures below, orange colored carbons).

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Even when numerous C-H groups are present, an unambiguous interpretation of a proton nmr spectrum may not be possible. The following three pairs of isomers (A & B) which display similar proton nmr spectra. Although a careful determination of chemical shifts should permit the first pair of compounds (blue box) to be distinguished, the second and third cases (red & green boxes) might be difficult to identify by proton nmr alone.

Natural Abundance :

Natural Abundance Since the major isotope of carbon ( 12 C) has no spin, this option seems unrealistic. Fortunately , 1.1% of elemental carbon is the 13 C isotope, which has a spin I = 1/2, so possible to conduct a carbon nmr experiment. It is worth noting here, that if much higher abundances of 13 C were naturally present in all carbon compounds, proton nmr would become much more complicated due to large one-bond coupling of 13 C and 1 H.

Many obstacles needed to be overcome before carbon nmr emerged as a routine tool ::

As noted, the abundance of 13 C in a sample is very low (1.1%), so higher sample concentrations are needed . The 13 C nucleus is over fifty times less sensitive than a proton in the nmr experiment, adding to the previous difficulty. Hydrogen atoms bonded to a 13 C atom split its nmr signal by 130 to 270 Hz, 1 H- 13 C splitting is o vercome by using an instrumental technique that decouples the proton-carbon interactions, so that every peak in a 13 C NMR spectrum appears as a singlet. Many obstacles needed to be overcome before carbon nmr emerged as a routine tool :

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The two features of a 13 C NMR spectrum that provide the most structural information are the number of signals observed and the chemical shifts of those signals .

13C NMR—Number of Signals:

13 C NMR—Number of Signals The number of signals in a 13 C spectrum gives the number of different types of carbon atoms in a molecule. Because 13 C NMR signals are not split, the number of signals equals the number of lines in the 13 C spectrum. In contrast to the 1 H NMR situation, peak intensity is not proportional to the number of absorbing carbons, so 13 C NMR signals are not integrated.

13C NMR—Position of Signals:

13 C NMR—Position of Signals In contrast to the small range of chemical shifts in 1 H NMR (1-10 ppm usually), 13 C NMR absorptions occur over a much broader range (0-220 ppm). The chemical shifts of carbon atoms in 13 C NMR depend on the same effects as the chemical shifts of protons in 1 H NMR.

13C Chemical shifts are mainly most affected by::

13 C Chemical shifts are mainly most affected by: Electronegativity of groups attached to the Carbon Hybridization state of Carbon sp 3 hybridized carbon is more shielded than sp 2 sp hybridized carbon is more shielded than sp 2 , but less shielded than sp 3 Anisotropy All affect 13 C Chemical shifts in nearly same fashion as they affect 1 H chemical shift

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Types of Carbons (CH 3 ) 3 CH CH 4 CH 3 CH 3 CH 3 CH 2 CH 3 (CH 3 ) 4 C primary secondary tertiary quaternary Classification Chemical shift, d 1 H 13 C 0.2 0.9 1.3 1.7 -2 8 16 25 28 Replacing H by C (more electronegative) deshields C to which it is attached.

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Electronegativity effects on CH 3 CH 3 F CH 4 CH 3 NH 2 CH 3 OH Chemical shift, d 1 H 0.2 2.5 3.4 4.3 13 C -2 27 50 75

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Electronegativity effects and chain length Chemical shift, d Cl CH 2 CH 2 CH 2 CH 2 CH 3 45 33 29 22 14 Deshielding effect of Cl decreases as number of bonds between Cl and C increases.

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Corrrelation chart for C13-NMR chemical shift(ppm)

Spin-Spin Splitting:

Spin-Spin Splitting Homonuclear spin-spin splitting: Because of its low natural abundance there is a low probability of finding two C 13 atoms next to each other in a single molecule. C 13 -C 13 coupling negligible. Hetronucler spin-spin splitting: C 13 will magnetically couple with attached protons and adjacent protons . N+1 rule is obeyed.

Off-Resonance Decoupling:

Off-Resonance Decoupling 13 C nuclei are split only by the protons attached directly to them. The N + 1 rule applies: a carbon with N number of protons gives a signal with N + 1 peaks .

13C Off-resonance decoupled spectrum :

13 C Off-resonance decoupled spectrum

Proton-decoupled spectra:

A common method used in determining a carbon-C 13 NMR spectrum is to irradiate all of the hydrogen nuclei in the molecules at the same time the carbon resonances are being measured. Thins required a second radiofrequency(RF) source (the decoupler) tuned to the frequency of the hydrogen nuclei, while the primary RF source is tuned to the C 13 frequency. Proton-decoupled spectra

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In this method the hydrogen nuclei are “saturated”, a situation where there are as many downward as there are upward transition, all occurring rapidly. During time the C-13 spectrum is being determined, the hydrogen nuclei cycle rapidly between their two spin state (+1/2 and -1/2) and the carbon nuclei see an average coupling (i.e. zero) to the hydrogen. The hydrogen are said to be coupled from the carbon-13 nuclei. You no longer see multiples for the c13 resonances. Each carbon gives a singlet, and the spectrum is easier to interpret.

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When we obtain a proton-decoupled c13 spectrum, the intensities of many of the carbon resonances increase significantly above those observed on a proton-coupled experiments. Carbon atoms with hydrogen atoms directly attached are enhanced the most, and the enhancement increases as more hydrogen are attached. This efface is called the Nuclear Over Hauser enhancement (NOE). Shown when two different type of atoms are irradiated while NMR spectroscopy of other type is determined. The effect can be either positive or negative, depending on which atom types are involved. In case od c-13 interacting with H-1 the effect is positive.so, Intensities of signals increases. Nuclear Over Hauser enhancement effect

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Magnetogyric ratio of nucleus being irradiated Magnetogyric ratio of nucleus being observed

1H & 13C NMR: 1,1,2-trichloropropane :

1 H & 13 C NMR: 1,1,2-trichloropropane

DEPT spectra (Distortionless Enhancement by Polarization Transfer):

Useful method for determining the presence of primary, secondary and tertiary carbon atoms. The DEPT experiment differentiates between CH, CH 2 and CH 3 groups by variation of the selection angle parameter (the tip angle of the final 1 H pulse. 45° angle gives all carbons with attached protons (regardless of number) in phase 90° angle gives only CH groups, the others being suppressed 135° angle gives all CH and CH 3 in a phase opposite to CH 2 Signals from quaternary carbons and other carbons with no attached protons are always absent (due to the lack of attached protons. DEPT spectra (Distortionless Enhancement by Polarization Transfer)

DEPT Spectrum:

Chemical shift (  , ppm) 0 20 40 60 80 100 120 140 160 180 200 DEPT Spectrum CH CH CH CH 2 CH 2 CH 2 CH 3 CCH 2 CH 2 CH 2 CH 3 O CH and CH 3 unaffected C and C=O nulled CH 2 inverted

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blue box- cyclohexane and2,3-dimethyl-2-butene single sharp resonance signal in the proton nmr spectrum (the former at δ 1.43 ppm and the latter at 1.64 ppm). carbon nmr spectrum :- cyclohexane displays a single signal at δ 27.1 ppm, generated by the equivalent ring carbon atoms (colored blue) and isomeric alkene shows two signals 1) at δ 20.4 ppm from the methyl carbons (colored brown) (2)at 123.5 ppm (typical of the green colored sp 2 hybrid carbon atoms)

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The C 8 H 10 isomers in the center (red) box have pairs of homotopic carbons and hydrogens, so symmetry should simplify their nmr spectra. The fulvene (isomer A) has five structurally different groups of carbon atoms (colored brown, magenta, orange, blue and green respectively) and should display five 13 C nmr signals (one near 20 ppm and the other four greater than 100 ppm).

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ortho -xylene (isomer B) will have a proton nmr very similar to isomer A, it should only display four 13 C nmr signals, originating from the four different groups of carbon atoms (colored brown, blue, orange and green). The methyl carbon signal will appear at high field (near 20 ppm), and the aromatic ring carbons will all give signals having δ > 100 ppm. Finally, the last isomeric pair, quinones A & B in the green box, are easily distinguished by carbon nmr. Isomer A displays only four carbon nmr signals (δ 15.4, 133.4, 145.8 & 187.9 ppm); whereas, isomer B displays five signals (δ 15.9, 133.3, 145.8, 187.5 & 188.1 ppm), the additional signal coming from the non-identity of the two carbonyl carbon atoms (one colored orange and the other magenta).

Thank You:

Thank You

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