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?
1H 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).
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 Since the major isotope of carbon (12C) has no spin, this option seems unrealistic. Fortunately, 1.1% of elemental carbon is the 13C 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 13C were naturally present in all carbon compounds, proton nmr would become much more complicated due to large one-bond coupling of 13C and 1H.
Many obstacles needed to be overcome before carbon nmr emerged as a routine tool : As noted, the abundance of 13C in a sample is very low (1.1%), so higher sample concentrations are needed. The 13C nucleus is over fifty times less sensitive than a proton in the nmr experiment, adding to the previous difficulty. Hydrogen atoms bonded to a 13C atom split its nmr signal by 130 to 270 Hz, 1H-13C splitting is overcome by using an instrumental technique that decouples the proton-carbon interactions, so that every peak in a 13C NMR spectrum appears as a singlet.
The two features of a 13C 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 SignalsThe number of signals in a 13C spectrum gives the numberof different types of carbon atoms in a molecule.Because 13C NMR signals are not split, the number ofsignals equals the number of lines in the 13C spectrum.In contrast to the 1H NMR situation, peak intensity is notproportional to the number of absorbing carbons, so 13CNMR signals are not integrated.
13C NMR—Position of SignalsIn contrast to the small range of chemical shifts in 1HNMR (1-10 ppm usually), 13C NMR absorptions occur over amuch broader range (0-220 ppm).The chemical shifts of carbon atoms in 13C NMR dependon the same effects as the chemical shifts of protons in 1HNMR.
13C Chemical shifts are mainly most affected by: Electronegativity of groups attached to the Carbon Hybridization state of Carbon sp3 hybridized carbon is more shielded than sp2 sp hybridized carbon is more shielded than sp2, but less shielded than sp3 Anisotropy All affect 13C Chemical shifts in nearly same fashion as they affect 1H chemical shift
Types of Carbons Classification Chemical shift, 1H 13CCH4 0.2 -2CH3CH3 primary 0.9 8CH3CH2CH3 secondary 1.3 16(CH3)3CH tertiary 1.7 25(CH3)4C quaternary 28Replacing H by C (more electronegative) deshieldsC to which it is attached.
Electronegativity effects and chain length Cl CH2 CH2 CH2 CH2 CH3Chemical 45 33 29 22 14shift, Deshielding effect of Cl decreases as number of bonds between Cl and C increases.
Corrrelation chart for C13-NMR chemical shift(ppm)
Spin-Spin Splitting Homonuclear spin-spin splitting: Because of its low natural abundance there is alow probability of finding two C13 atoms next to eachother in a single molecule. C13-C13 coupling negligible. Hetronucler spin-spin splitting: C13 will magnetically couple with attachedprotons and adjacent protons. N+1 rule is obeyed.
Off-Resonance Decoupling 13C 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.
Proton-decoupled spectra A common method used in determining a carbon-C13 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 C13 frequency.
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.
Nuclear Over Hauser enhancement effect 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.
Magnetogyricratio of nucleusbeing irradiated Magnetogyric ratio of nucleus being observed
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, CH2 and CH3 groups by variation of the selection angle parameter (the tip angle of the final 1H 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 CH3 in a phase opposite to CH2 Signals from quaternary carbons and other carbons with no attached protons are always absent (due to the lack of attached protons.
DEPT Spectrum O CCH2CH2CH2CH3 CH CH CH3 CHCH and CH3 unaffectedC and C=O nulledCH2 inverted CH2 CH2 CH2 200 180 160 140 120 100 80 60 40 20 0 Chemical shift ( , ppm)
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 sp2 hybrid carbon atoms)
The C8H10 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 13C nmr signals (one near 20 ppm and the other four greater than 100 ppm).
ortho-xylene (isomer B) will have a proton nmr very similar to isomerA, it should only display four 13C nmr signals, originating from the fourdifferent groups of carbon atoms (colored brown, blue, orange and green).The methyl carbon signal will appear at high field (near 20 ppm), and thearomatic ring carbons will all give signals having δ > 100 ppm.Finally, the last isomeric pair, quinones A & B in the green box, are easilydistinguished by carbon nmr. Isomer A displays only four carbon nmrsignals (δ 15.4, 133.4, 145.8 & 187.9 ppm); whereas, isomer B displays fivesignals (δ 15.9, 133.3, 145.8, 187.5 & 188.1 ppm), the additional signalcoming from the non-identity of the two carbonyl carbon atoms (onecolored orange and the other magenta).