C NMR Spectroscopy C NMR. C Transition Energy

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1 NMR NMR Spectroscopy is the most abundant natural isotope of carbon, but has a nuclear spin I = 0, rendering it unobservable by NMR. Limited to the observation of the nucleus which constitutes only.% of naturally occurring carbon. Transition Energy Nucleus γ (0 rad/tesla sec). Field strength B 0 (Tesla).00 Frequency ν (Mz). The magnetogyric ratio, γ, for the is. compared to. for. Remember the resonance condition for a nucleus is given by: ν = (γ/π)b 0 9 F If the gyromagnetic ratio is lowered, the E is also lowered. Where a spectrum using a. T magnet is observed at 0 Mz, a spectrum is observed at Mz roughly times less energetic. Boltzmann: N upper /N lower = e - E/k T = e 98 K the ratio is,000,000 /,000,00

2 NMR The combined effects of smaller excess populations in the lower energy state, low natural abundance, and slow relaxation rates result in a signal that is typically 000 times weaker than that observed for. With FT instruments, this is not a problem simply take more scans! (recall S/N increases as the square root of the number of scans). Fourier Transform NMR Radio-frequency pulse given. Nuclei absorb energy and precess (spin) like little tops. A complex signal is produced, then decays as the nuclei lose energy. Free induction decay is converted to spectrum. scans on a -0 mg sample will give a good spectrum, scans on a 0 mg sample will give a good spectrum. NMR hem low abundance a single molecule will have at most only one atom however, we are sampling a very large number of molecules, even in a 0 mg sample! thus our sampling will see a at every position in the molecule! TUTORIAL PM

3 Shielding spectra are typically recorded from 0 0 ppm; with the zero being the methyl carbon in TMS (much wider range than spectra!) nuclei are shielded or deshielded (EMIAL SIFT) due to the same factors as for NMR.. Electron withdrawing ability (by inductance or resonance) of nearby groups.. ybridization.. Electron current effects. NMR hemical Shifts Several functionalities appear directly on NMR which are not visible in NMR: - Quaternary carbons sp carbon - ipso carbons -EWG - arbonyl carbons alkyne carbons 0 carbonyl carbons alkene carbons aromatic carbons downfield δ (ppm) upfield deshielded shielded higher E lower E 0 0 Si 0.0 arbonyl arbon hemical Shifts Spin-Spin oupling in NMR anhydrides acid chlorides amides esters nitriles omonuclear coupling of - is possible in theory. owever, due to the low natural abundance of, it is rare to find two s in the same molecule, let alone adjacent to one another. No need to consider - coupling except for enrichment studies! aldehydes carboxylic acids eteronuclear coupling between and the atoms attached to them is observed ( abundance ~99%). conj. ketones ketones Because the atoms are directly attached, the coupling constants ( J)are large, typically 00-0 z When such spectra are observed, they are referred to as proton coupled spectra (or non-decoupled spectra).

4 Splitting NMR Spectrum The splitting follows the simple N+ rule: quaternary methine methylene methyl singlet doublet triplet quartet The multiplet analysis gives useful information, but there are two major limitations: ) If the signal is weak (common) the outer peaks of the multiplet may be lost in the noise of the spectrum. ) Due to the large J-constants, the multiplets quickly begin to overlap and become congested. Proton-oupled Effect of oupling Decoupling oupled Three equal intensity lines at ppm Dl solvent - D coupling To simplify the spectrum, and to increase the intensity of the observed signals, a decoupler is used to remove the spin effects of the nucleus. A second RF generator irradiates at the resonance frequency causing the saturation effectively averaging all their spin states to zero. channel- ν pulse channel FID oupling can cause NMR spectra to become very complicated (convoluted) quite easily.

5 Proton Decoupled Spectrum Effect of Decoupling oupled { } Decoupled NMR Spectra NMR Intensities Due to signal enhancement and spectral simplification, spectra are usually reported as decoupled. Each chemically unique carbon in the molecule gives rise to a single peak. Of course chemically equivalent carbons contribute to the same peak! The number of different signals (peaks) indicates the number of different kinds of carbon. The location (chemical shift) indicates the type of functional group. Peak areas (~heights) are NOT proportional to number of carbons. arbon atoms with more hydrogens give stronger signals, due to more efficient relaxation (transfer of spin to the hydrogens). owever, peak areas (~heights) can be compared within the same type of carbons (e.g. methyls)

6 Example: Ethanol Example: -bromohexane O Br Example: cyclohexane Example: cyclohexene

7 Example:,-cyclohexadiene Example:,-cyclohexadiene Example: m-nitrotoluene hemical Shift Predictions O N Examining a large set of chemical shift data has allowed the development of empirical rules or substituent parameters to allow chemical shift predictions for most commonly encountered situations. Example: the carbon atoms of a substituted benzene ring. Benzene itself single peak at 8. ppm Add to this value substituent increments which depend on the chemical nature of the substituent and where it is on the ring relative to the carbon whose shift is being predicted. 8

8 Aromatic Substituent Parameters ON = 8. + ( )ipso + (NO )meta = = 8.9 ppm = 8. + ( )ortho + (NO )ortho = (-.) =. ppm 9 Example: m-nitrotoluene O N alc d Obs d Example: p-ydroxyacetophenone O O alc d Obs d

9 Shift Predictions Alkyls Example: bromocyclopentane an also make predictions for alkyl groups Br alc d Obs d..9. Base value: use unsubstituted hydrocarbon NMR Intensities Nuclear Overhauser Enhancement (NOE) Peak areas (~heights) are NOT proportional to number of carbons. arbon atoms with more hydrogens give stronger signals, due to more efficient relaxation (transfer of spin to the hydrogens). owever, peak areas (~heights) can be compared within the same type of carbons (e.g. methyls) A phenomenon observed with proton-decoupled -NMR is that the intensity of the signal for a given increases versus the proton-coupled spectrum roughly proportional to the number of protons attached. The degree of this signal enhancement is called the Nuclear Overhauser Enhancement (NOE). This effect is general, and appears anytime when one of two types of atoms is irradiated, while the spectrum of the other is observed. In this case, while the population is irradiated to saturation, the is observed. ere: a heteronuclear effect. 9

10 NOE The effect can be a positive or negative one, but for the case of -, the effect is positive The maximum enhancement is given by: NOE max = (γ irradiated) (γ observed) This value is what is added to the observed intensity in the coupled spectrum to give the intensity observed in the decoupled spectrum: total predicted intensity = + NOE max NOE For, NOE = ½ (./.8) =.988 A maximum enhancement of almost 00% is possible. NOE operates in both directions nuclei (if decoupled) would enhance the signal of however, this signal would be weak due to the low abundance of. Because NOE for operates in the opposite direction (a rare nuclei always bound to an abundant one) it is a useful probe into structural assignments. The NOE effect is very short-range, falling off as /r the distance between the nuclei. Origin of NOE Origins of NOE An isolated two spin system between a single carbon and single hydrogen atom N Quantum mechanics dictates that allowed transitions involve only one change of spin at a time single quantum transitions N The effects of coupling are left out for simplicty Shown are the four combinations of spin states of these two nuclei, N - N N The allowed transitions are shown in red N N The two energy states where both are spin up or spin down are the lowest and highest energy states N N The mixed states are roughly degenerate in energy 0

11 Origins of NOE Origins of NOE Let the equilibrium population of the two degenerate states be B The N level would be higher than B by a small amount, δ The N level would be lower than B by a the same amount, δ The signal for a in this case would be proportional to δ at equilibrium The two transitions are N N and N N N N N N When a decoupler is used, the populations are disturbed from their equilibrium values Relaxation processes restore these disturbed populations to their equilibrium values One such process is a doublequantum transition, where both the and nuclei relax simultaneously (blue line) This leak in the upper state enhances the population of the lower energy state for carbon the excess population is larger and the signal intensifies N double N quantum transition N N NOE NOE NOE: an example of cross-polarization, polarization of spin states of one type of nucleus causes a polarization of the spin states of another nucleus. A heteronuclear NOE effect is always observed in normal decoupled spectra. Total NOE for a given increases with number of nearby s. Thus intensities of signals are generally: > > > Difference NOE effect is quite general. an also be applied in a homonuclear sense, i.e. { } Difference

12 NOE Example: m-nitrotoluene Depends on cross-polarization of spin states. an tell us what nuclei are close together. In contrast to J-coupling (spin-spin) which operates through the bonding electrons, NOE is a through-space effect. O N Thus NOE can tell us about the proximity of atoms which are separated by many bonds, e.g. proteins, RNA, DNA Example: benzonitrile N Very weak: no attached s No NOE effect!