NMR is the most powerful structure determination tool available to organic chemists.

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1 Nuclear Magnetic esonance (NM) Spectrometry NM is the most powerful structure determination tool available to organic chemists. An NM spectrum provides information about: 1. The number of atoms of a given element in a molecule 2. Each atom's electronic environment 3. Each atom's relationship with neighboring atoms Principles of NM spectroscopy Atoms such as 1 H, 13 C, 19 F, and 31 P have nuclei with a spin of +1/2 or -1/2 and therefore can behave like magnets. In the presence of a magnetic field, these nuclei orient themselves so their own magnetic fields are aligned with the field (α-state) or against the field (β-state). Electromagnetic radiation with the correct energy can cause the nuclei in the α- state to "flip" to the higher-energy β-state. As the nuclei "relax" and return to the α-state, they release the energy, and can "resonate" back & forth between the two states. The energy difference between the two states is related to the strength of the applied magnetic field (H 0 ) and the gyromagnetic ratio (γ) of a given nucleus: Proton ( 1 H) NM: H = hν = h γ H 0 2π The typical applied magnetic field requires an electromagnetic radiation source in the radiofrequency range (rf radiation) to cause a hydrogen nucleus or "proton" to resonate. Our "low field" (Anasazi) NM has magnetic field = 1.4 Tesla it uses a rf source of 60 MHz to cause protons to resonate. Our "High field" (Bruker) NM operates at a field of about 7 T the rf source operates at 300 MHz for protons/ 75 MHz for 13 C nuclei

2 Generating an NM spectrum The exact energy ( E) required for each proton to resonate depends on its environment. Protons "feel" less effective magnetic field when they are shielded by surrounding electrons so it takes higher field strength to make them respond If the applied magnetic field is varied over constant frequency ("sweeping" the field), the energy difference between spin states changes. When the applied field causes E of a nucleus to match the frequency of the rf source, the molecule absorbs energy and a signal is produced. Every proton or set of equivalent protons generates a signal at a given field strength The spectrum we get is a plot of energy absorptions vs. applied magnetic field: The exact magnetic field strengths are difficult to measure, so signals are referenced to a standard compound which absorbs at a relatively high field strength: TMS = tetramethylsilane Each signal's field strength is called its "chemical shift" The units of chemical shift are usually given in parts per million: δ (ppm) = distance downfield from TMS in Hz frequency of spectrometer in MHz

3 What affects the CHEMICAL SHIFT at which a proton's signal appears? ENVIONMENT! Signals from shielded nuclei appear upfield, deshielded nuclei appear downfield Protons located close to electron-withdrawing or delocalized groups are deshielded O atoms, N atoms, halogens C = O groups aromatic rings and multiple bonds (magnetic anisotropy = magnetic field induced by π electrons) Protons in electron-dense (but not delocalized) groups are shielded CH 2 (methylene) and CH 3 (methyl) groups Protons bonded to electropositive elements How many signals will a molecule produce? Chemically equivalent protons (in the same group) will produce a single signal. The number of different signals = the number of groups of equivalent protons in the structure How many different types of proton signals are expected for these examples? H 3 C CH 3 OH CH 3 H 3 C O O H 2 C CH3 Cl O CH 3

4 or ester 13 C NM chemical shifts: Aldehydes &Ketones Carboxylic acids & esters

5 Determining the number of protons giving the signal: INTEGATION With NM, unlike I, peak areas are directly proportional to the number of atoms giving rise to the signal (the number of equivalent H s in the structure) The spectrometer will integrate the signal to give a ratio which is either expressed in numbers or as a step integral of a given height: From the heights or numbers, a whole-number ratio can be obtained which tells us how many protons gave rise to each signal. The total of the ratio should equal the number of protons in the whole molecule

6 The effect of neighboring atoms: SPLITTING PATTENS and SPIN-COUPLING A major difference between the spectra of bromoethane and methyl 2,2- dimethylpropanoate (previous page): The signals for bromoethane protons are split into sets of peaks Splitting of signals by neighboring protons: Protons on adjacent C atoms have magnetic spins that either add to or subtract from the effective magnetic field felt by the protons giving rise to the signal The effect: the signal is split into N + 1 peaks (where N = number of neighboring H) Most splitting occurs due to interactions between protons 3 σ bonds away; some long-range splitting may occur when a pi bond is present When there are no protons on the adjacent atoms, the signal is a single peak (singlet) Typical splitting patterns: Doublet Triplet Quartet "Multiplets" "Doublet of doublets": splitting from two nonequivalent neighbors Compare: I C H 2 H 2 C CH 3 H 3 C CH 3

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8 Spin coupling and structure When an NM signal is split by neighboring protons, the distance between each peak in the split is called the coupling constant (J). Coupling constants will be equal for coupled signals (see Fig ) Magnitude is independent of spectrometer frequency; normal range J = 0-15 Hz In high-resolution spectrum peaks appear closer because more Hz/ppm J decreases as distance, number of bonds between coupled protons increases Presence of π bonds increases J J values can provide structural information Tree diagrams (Fig ) can be used to show the relationship between coupling constants and the observed splitting pattern for a particular signal When the coupling constants between the protons giving rise to the signal and each set of adjacent protons are similar, the N + 1 rule is obeyed. When coupling constants are sufficiently different, each neighboring group splits the signal separately (Fig 13.19): a doublet of doublets is observed.

9 13 C NM For determination of complex molecular structure, 13 C NM is very useful Information obtained from a 13 C NM: The number of peaks = the number of exactly equivalent C atoms in the molecule Chemical shift values for each C depend on its environment, much like H Electron-withdrawing groups shift signals downfield. The presence of certain functional groups such as C=O, C=C, OH, O, aromatic may be confirmed by characteristic chemical shifts Some important differences between 13 C and 1 H NM spectra: The range of chemical shifts is much broader; 220 ppm compared to 12 ppm for 1 H. Signals are less likely to overlap. The % abundance of 13 C is much less than that of 1 H, so signals are much weaker. Fourier transform operations add thousands of scans to get a single spectrum. The area under the peak is not proportional to the number of atoms giving rise to it. Certain carbons typically give rise to weaker signals (C=O, C=C) Most 13 C spectra are spin-decoupled (splitting from protons is removed) and each signal is a sharp single peak with no splitting.

10 Getting a bit more out of a 13 C spectrum: 13 C-DEPT experiments DEPT = distortionless enhancement by polarization transfer A series of 13 C spectra are obtained on a single sample, varying the angles of the rf energy pulse so that only certain types of carbons give signals. C with attached H have much stronger signals due to association with the protons One can determine whether a signal in the 13 C spectrum comes from a CH 3, CH 2, CH or quarternary C (C q give no DEPT signal) CH 2 signal arises from: CH signals are given by: H C H CH 2 C H H C O H C aromatic C A quarternary C signal arises from: C C O C O C Y Interpreting DEPT spectra: In the lab (Anasazi 60 MHz NM): stackplot of DEPT-45, DEPT-90 & DEPT- 135 The DEPT-135 spectrum shows the CH 2 carbon signals pointing down and the CH and CH 3 signals pointing up

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12 Two-dimensional NM spectra: COSY and HETCO For large, complex molecules, 1-D spectra are useful only if the compound is previously known and the chemical shifts are assigned to atoms in structure based on literature values. 2-D spectra generate signals that identify atoms that are spin-coupled together and can therefore provide more structural data. The most commonly used 2-D programs are: 1 H 1 H shift correlated spectra (COSY) 13 C 1 H heteronuclear correlated spectra (HETCO) In both types of spectra, a contour plot is used to display the key signals, which come from correlation between signals located along the x and y axes. COSY spectra: COSY signals come from protons that are located on adjacent carbons and are splitting each other s signal. A COSY spectrum shows the 1 H NM spectrum along both x and y axes. Spots that appear off the diagonal (cross-peaks) can be traced back to peaks at corresponding chemical shift values on each axis to find correlated protons HETCO spectra: HETCO signals come from carbon atoms and their attached protons The 13 C spectrum is displayed along the x-axis and the 1 H along the y-axis Each spot can be traced back to a carbon signal and its correlated proton signal Using a combination of 13 C-DEPT, COSY and HETCO with molecular formula allows you to find the best molecular structure and make accurate assignments for each signal.

13 CH 2 1) BH 3, THF 2) H 2 O 2, OH- CH 3 or CH 2? OH OH

14

15

16 13.57: C 8 H 9 Br

17 C 4 H 9 Br C 4 H 8 Cl 2

18 Comparison of 13 C NM of aliphatic and aromatic compounds

19 C 5 H 10 O

20 C 8 H 10 O

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