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NMR Spectroscopy
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  1. NMR Spectroscopy CHEM 212

  2. Introduction to Spectroscopy Spectroscopy is the study of the interaction of matter with the electromagnetic spectrum • Electromagnetic radiation displays the properties of both particles and waves • This “packet” of wave and particle properties is called a photon The term “photon” is implied to mean a small, massless particle that contains a small wave-packet of EM radiation/light • The energy E component of a photon is proportional to the frequency n E = hn The constant of proportionality is Plank’s constant, h

  3. Introduction to Spectroscopy • Because the speed of light (c ) is constant, the frequency (n) (number of cycles of the wave per second) can complete in the same time, must be inversely proportional to how long the oscillation is, or wavelength (l): • Amplitude describes the wave height, or strength of the oscillation • Because the atomic particles in matter also exhibit wave and particle properties (though opposite in how much) EM radiation can interact with matter in two ways: • Collision – particle-to-particle – energy is lost as heat and movement • Coupling – the wave property of the radiation matches the wave property of the particle and “couple” to the next higher quantum mechanical energy level c ___ hc ___ n =  E = hn = l l

  4. Introduction to Spectroscopy • Remember atoms and molecules are quantum mechanical particles • Where a photon is a wave with some particle character, matter is made of particles with some wave character – wave/particle duality • As a result of this, the energy of these particles can only exist at discrete energies – we say these energy levels are quantized • It is easy to understand if we visualize the “wave” property of matter as an oscillating string in a box—only certain “energy levels”can exist as the string is bound at both ends: Energy

  5. The Spectroscopic Process 2. Absorption:Molecule takes on the quantum energy of a photon that matches the energy of a transition and becomes excited excited state 5. Detection: Photons that are reemitted and detected by the spectrometer correspond to quantum mechanical energy levels of the molecule hn 4. Relaxation 3. Excitation Energy rest state rest state 1. Irradiation: Molecule is bombarded with photons of various frequencies over the range desired hn hn hn 5

  6. Types of Spectroscopy g-rays X-rays UV Visible IR Microwave Radio Frequency, n (Hz) ~1017 ~1015 ~1013 ~1010 ~105 Wavelength, l ~0.01 nm 10 nm 1000 nm 0.01 cm 100 m Energy (kcal/mol) > 300 300-30 300-30 ~10-4 ~10-6 6

  7. Basis of NMR Spectroscopy Nuclear Spin States • The sub-atomic particles within atomic nuclei possess a spin quantum number just like electrons • Just as when using Hund’s rules to fill atomic orbitals with electrons, nucleons must each have a unique set of quantum numbers • The total spin quantum number of a nucleus is a physical constant, I • For each nucleus, the total number of spin states allowed is given by the equation: 2I + 1

  8. Basis of NMR Spectroscopy • Observe that for atoms with no net nuclear spin, there are zero allowed spin states • Nuclear Magnetic Resonance can only occur where there are allowed spin states • Note that two nuclei, prevalent in organic compounds have allowed nuclear spin states – 1H and 13C, while two others do not 12C and 16O

  9. Basis of NMR Spectroscopy Nuclear Magnetic Moments • A nucleus contains protons, which each bear a +1 charge • If the nucleus has a net nuclear spin, and an odd number of protons, the rotation of the nucleus will generate a magnetic field along the axis of rotation • Thus, a nucleus has a magnetic moment, m, generated by its charge and spin • A hydrogen atom with its lone proton making up the nucleus, can have two possible spin states—degeneratein energy m H H I = +½ I = -½ m

  10. Creating Non-Degenerate Nuclei

  11. Nuclear Magnetic Resonance

  12. Nuclear Magnetic Resonance • For the 1H nucleus (proton) this resonance condition occurs at low energy (lots of noise) unless a very large magnetic field is applied • Early NMR spectrometers used a large permanent magnet with a field of 1.4 Tesla—protons undergo resonance at 60 MHz (1 MHz = 106 Hz) • Modern instruments use a large superconducting magnet—our NMR operates at 9.4 T where proton resonance occurs at 400 MHz • In short, higher field gives cleaner spectra and allows longer and more detailed experiments to be performed

  13. Origin of the Chemical Shift

  14. The Proton (1H) NMR Spectrum

  15. The 1H NMR Spectrum • A reference compound is needed—one that is inert and does not interfere with other resonances • Chemists chose a compound with a large number of highly shielded protons—tetramethylsilane (TMS) • No matter what spectrometer is used the resonance for the protons on this compound is set to d 0.00

  16. The 1H NMR Spectrum • The chemical shift for a given proton is in frequency units (Hz) • This value will change depending on the B0 of the particular spectrometer • By reporting the NMR absorption as a fraction of the NMR operating frequency, we get units, ppm, that are independent of the spectrometer

  17. The 1H NMR Spectrum • We need to consider four aspects of a 1H spectrum: • Number of signals • Position of signals • Intensity of signals. • Spin-spin splitting of signals

  18. The Number of Signals • The number of NMR signals equals the number of different types of protons in a compound • Protons in different environments give different NMR signals • Equivalent protons give the same NMR signal

  19. The Number of Signals • To determine if two protons are chemically equivalent, substitute “X” for that each respective hydrogen in the compound and compare the structures • If the two structures are fully superimposible (identical) the two hydrogens are chemically equivalent; if the two structures are different the two hydrogens were not equivalent • A simple example: p-xylene Same Compound

  20. The Number of Signals • Examples Important: To determine equivalent protons in cycloalkanes and alkenes, always draw all bonds to show specific stereochemistry:

  21. The Number of Signals • In comparing two H atoms on a ring or double bond, two protons are equivalent only if they are cis or trans to the same groups.

  22. The Number of Signals • Proton equivalency in cycloalkanes can be determined similarly:

  23. The Number of Signals • Enantiotopic Protons – when substitution of two H atoms by Z forms enantiomers: • The two H atoms are equivalent and give the same NMR signal • These two atoms are called enantiotopic

  24. The Number of Signals • Diastereotopic Protons - when substitution of two H atoms by Z forms diastereomers • The two H atoms are not equivalent and give two NMR signals • These two atoms are called diastereotopic

  25. Chemical Shift – Position of Signals • Remember:

  26. Chemical Shift – Position of Signals • The less shielded the nucleus becomes, the more of the applied magnetic field (B0) it feels • This deshielded nucleus experiences a higher magnetic field strength, to it needs a higher frequency to achieve resonance • Higher frequency is to the left in an NMR spectrum, toward higher chemical shift—so deshielding shifts an absorption downfield  Downfield, deshielded Upfield, shielded 

  27. Chemical Shift – Position of Signals • There are three principle effects that contribute to local diamagnetic shielding: • Electronegativity • Hybridization • Proton acidity/exchange

  28. Chemical Shift – Position of Signals • Electronegative groups comprise most organic functionalities: -F -Cl -Br -I -OH -OR -NH2 -NHR -NR2 -NH3+ -C=O -NO2 -NO -SO3H -PO3H2 -SH -Ph -C=C and most others In all cases, the inductive WD of electrons of these groups decreases the electron density in the C-H covalent bond – proton is deshielded – signal more downfield of TMS

  29. Chemical Shift – Position of Signals • Protons bound to carbons bearing electron withdrawing groups are deshielded based on the magnitude of the withdrawing effect – Pauling electronegativity:

  30. Chemical Shift – Position of Signals • The magnitude of the deshielding effect is cumulative: As more chlorines are added d becomes larger • The magnitude of the deshielding effect is reduced by distance, as the inductive model suggests

  31. Chemical Shift – Position of Signals Hybridization • Increasing s-character (sp3 sp2  sp) pulls e- density closer to nucleus effectively raising electronegativity of the carbon the H atoms are bound to – a deshielding effect • We would assume that H atoms on sp carbons should be well downfield (high d) and those on sp3 carbons should be upfield (low d)

  32. Chemical Shift – Position of Signals • What we observe is slightly different: Chemists refer to this observation as magnetic anisotropy

  33. Chemical Shift – Position of Signals • Magnetic Anisotropy – Aromatic Protons • In a magnetic field, the six  electrons in benzene circulate around the ring creating a ring current. • The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons. • The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance. Thus they are deshielded and absorb downfield.

  34. Chemical Shift – Position of Signals • Similarly this effect operates in alkenes:

  35. Chemical Shift – Position of Signals • In alkynes there are two perpendicular sets of p-electrons—the molecule orients with the field lengthwise—opposing B0shielding the terminal H atom

  36. Chemical Shift – Position of Signals

  37. Chemical Shift – Position of Signals

  38. Intensity of Signals—Integration • The area under an NMR signal is proportional to the number of absorbing protons • An NMR spectrometer automatically integrates the area under the peaks, and prints out a stepped curve (integral) on the spectrum • The height of each step is proportional to the area under the peak, which in turn is proportional to the number of absorbing protons • Modern NMR spectrometers automatically calculate and plot the value of each integral in arbitrary units • The ratio of integrals to one another gives the ratio of absorbing protons in a spectrum; note that this gives a ratio, and not the absolute number, of absorbing protons

  39. Intensity of Signals—Integration

  40. Intensity of Signals—Integration

  41. Spin-Spin Splitting • Consider the spectrum of ethyl alcohol: • Why does each resonance “split” into smaller peaks?

  42. Spin-Spin Splitting • The magnetic effects of nuclei in close proximity to those being observed have an effect on the local magnetic field, and therefore DE • Specifically, when proton is close enough to another proton, typically by being on an adjacent carbon (vicinal), it can “feel” the magnetic effects generated by that proton • On any one of the 108 of these molecules in a typical NMR sample, there is an equal statistical probability that the adjacent (vicinal) proton is either in the + ½ or – ½ spin state • If there is more than one proton on an adjacent carbon – all the statistical probabilities exist that each one is either + ½ or – ½ in spin • The summation of these effects over all of the observed nuclei in the sample is observed as the spin-spin splitting of resonances

  43. Spin-Spin Splitting • Recall, we are observing the frequency (E = hn) where a proton goes into resonance Any change in B0 will cause a change in energy at which the resonance condition will occur for a proton of a given chemical shift

  44. In solution we are not looking at a single molecule but about 108 • On some molecules the proton being observed may be next to another proton of spin + 1/2 :

  45. Spin-Spin Splitting • On some molecules the proton being observed may be next to another proton of spin – 1/2 :

  46. Spin-Spin Splitting • Observe what effect this has on an isolated ethyl group: • The two methylene Ha protons have three neighbors, Hb, on the adjacent methyl carbon • Each one of these hydrogens can be + ½ or – ½ , and since we are not looking at one molecule, but billions, we will observe all combinations

  47. Spin-Spin Splitting • The first possibility is that all three Hb protons have a + ½ spin; in this case the three protons combine to generate three small magnetic fields that aid B0 and deshield the protons – pushing the resonance for Ha slightly downfield (the magnetic field of a proton is tiny compared to B0) All 3 Hb protons + ½ resonance for Ha in absence of spin-spin splitting

  48. Spin-Spin Splitting • The second possibility is that two Hb protons have a + ½ spin and the third a - ½ ; in this case the two protons combine to enhance B0 and the other against it, a net deshielding; there are 3 different combinations that generate this state or or 2 Hb protons + ½ resonance for Ha in absence of spin-spin splitting

  49. Spin-Spin Splitting • The third possibility is that two Hb protons have a –½ spin and the third +½; here, the two protons combine to reduce B0 and the other enforce it, a net shielding effect; there are 3 different combinations that generate this state or or 2 Hb protons - ½ resonance for Ha in absence of spin-spin splitting

  50. Spin-Spin Splitting • The last possibility is that all three Hb protons have a – ½ spin; in this case the three protons combine to oppose B0, a net shielding effect; there is one combination that generates this state All 3 Hb protons - ½ resonance for Ha in absence of spin-spin splitting