BKB Nuclear Magnetic Resonance 2024ppt.pptx

Nuclear Magnetic Resonance

1. Nuclear Magnetic Resonance Spectroscopy Bedanta Kr Borah Department of Chemistry D. R. College, golaghat Assam

2. Chapter 13 Spectroscopy overview Spectroscopy relies on the absorption of energy to gain information about the atoms in or structure of molecules NMR – magnetic spin of certain nuclei IR – bond vibrational energy UV/VIS – electron transition energy MS - kinetic energy mass/charge ratio

3. Chapter 13 The absorption of radio–frequency radiation by the nuclei of atoms is termed as Nuclear Magnetic Resonance. n.m.r. spectroscopy depends on the magnetic properties of the nucleus of an element. n.m.r. differs from other spectroscopy in that difference in energy states being examined are created by a magnetic field Motion involved in n.m.r. is that of nuclear spin

4. Chapter 13 NMR is the most powerful tool available for organic structure determination. It is used to study a wide variety of nuclei: 1 H 13 C 15 N 19 F 31 P =>

5. Earth magnetic field 2X 10-5 Tesla

6. Chapter 13 NMR certain isotopes have magnetic spins 1 H, 13 C, 14 F, 31 P, etc.

7. Basic principle Spinning of magnetic nucleus Odd protons & /odd neutrons I = Non zero (Whole number / Fraction) I = ½ is the most important (H-1, C-13, F-19, P-31) Magnetic Moment (2.79, 0.7, 2.63, 1.13) Natural Abundance (99.98, 1.1, 100, 100) Sensitive & Insensitive Nuclues

8. Chapter 13 Nuclear Spin A nucleus with an odd atomic number or an odd mass number has a nuclear spin. The spinning charged nucleus generates a magnetic field. =>

9. Chapter 13 External Magnetic Field When placed in an external field, spinning protons act like tiny bar magnets. =>

10. Element 1 H 2 H 12 C 13 C 14 N 16 O 17 O 19 F Nuclear Spin Quantum No 1/2 1 0 1/2 1 0 5/2 1/ 2 ( I ) No. of Spin 2 3 0 2 3 0 6 2 States Spin Quantum Numbers of Some Common Nuclei Elements with either odd mass or odd atomic number have the property of nuclear “spin”. The number of spin states is 2 I + 1, where I is the spin quantum number. The most abundant isotopes of C and O do not have spin.

11. Chapter 13 An electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2 this spinning charge creates an associated magnetic field in effect, an electron behaves as if it is a tiny bar magnet and has what is called a magnetic moment The same effect holds for certain atomic nuclei any atomic nucleus that has an odd mass number, an odd atomic number, or both also has a spin and a resulting nuclear magnetic moment the allowed nuclear spin states are determined by the spin quantum number, I , of the nucleus

12. Chapter 13 NUCLEAR SPIN STATES - HYDROGEN NUCLEUS + 1/2 - 1/2 The two states are equivalent in energy in the absence of a magnetic or an electric field. + + The spin of the positively charged nucleus generates a magnetic moment vector, m . m m TWO SPIN STATES

13. Chapter 13 within a collection of 1 H and 13 C atoms, nuclear spins are completely random in orientation when placed in a strong external magnetic field of strength B , however, interaction between nuclear spins and the applied magnetic field is quantized, with the result that only certain orientations of nuclear magnetic moments are allowed

14. Chapter 13 Nuclear Spin Energy Levels B o +1/2 -1/2 In a strong magnetic field (B o ) the two spin states differ in energy. aligned unaligned N S

15. B o D E + 1/2 - 1/2 = kB o = h n degenerate at B o = 0 increasing magnetic field strength THE ENERGY SEPARATION DEPENDS ON B o

16. Chapter 13 Absorption of Energy B o +1/2 -1/2 +1/2 -1/2 D E = h n D E quantized Radiofrequency Applied Field Aligned Opposed

17. Chapter 13 Two Energy States The magnetic fields of the spinning nuclei will align either with the external field, or against the field. A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. =>

18. Chapter 13 Nuclear Spins in B In an applied field strength of 7.05T, which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for 1H is approximately 0.120 J (0.0286 cal)/mol, which corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz) 13C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz)

19. h n = B o h g 2 p constants frequency field strength Stronger magnetic fields (B o ) cause the instrument to operate at higher frequencies ( n ). NMR Field Strength 1 H Operating Frequency 60 Mhz 100 MHz 300 MHz 7.05 T 2.35 T 1.41 T n = ( K) B o

20. Chapter 13 B o D E + 1/2 - 1/2 = kB o = h n degenerate at B o = 0 increasing magnetic field strength THE ENERGY SEPARATION DEPENDS ON B o

21. Chapter 13  E and Magnet Strength Energy difference is proportional to the magnetic field strength.  E = h =  h B 2 Gyromagnetic ratio, , is a constant for each nucleus (26,753 s -1 gauss -1 for H). In a 14,092 gauss field, a 60 MHz photon is required to flip a proton. Low energy, radio frequency. =>

22. Chapter 13 Nuclear Magnetic Resonance If the precessing nucleus is irradiated with electromagnetic radiation of the same frequency as the rate of precession, the two frequencies couple, energy is absorbed, and the nuclear spin is flipped from spin state +1/2 (with the applied field) to -1/2 (against the applied field)

23. Chapter 13 N S w Nuclei precess at frequency w when placed in a strong magnetic field. h n If n = w then energy will be absorbed and the spin will invert. NUCLEAR MAGNETIC RESONANCE NMR RADIOFREQUENCY 40 - 600 MHz

24. Chapter 13 Nuclear Magnetic Resonance the origin of nuclear magnetic “resonance

25. Chapter 13 Nuclear Magnetic Resonance Resonance : in NMR spectroscopy, resonance is the absorption of electromagnetic radiation by a precessing nucleus and the resulting “flip” of its nuclear spin from a lower energy state to a higher energy state The instrument used to detect this coupling of precession frequency and electromagnetic radiation records it as a signal signal: a recording in an NMR spectrum of a nuclear magnetic resonance

26. Chapter 13 The strength of the NMR signal depends on the Population Difference of the two spin states resonance induced emission excess population Radiation induces both upward and downward transitions. For a net positive signal there must be an excess of spins in the lower state. Saturation = equal populations = no signal POPULATION AND SIGNAL STRENGTH

27. Chapter 13 The NMR Spectrometer =>

28. Chapter 13 Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a radio-frequency detector The sample is dissolved in a solvent, most commonly CDCl 3 or D 2 O, and placed in a sample tube which is then suspended in the magnetic field and set spinning Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds

29. Chapter 13 NMR Signals The number of signals shows how many different kinds of protons are present. The location of the signals shows how shielded or deshielded the proton is. The intensity of the signal shows the number of protons of that type. Signal splitting shows the number of protons on adjacent atoms. =>

30. Chapter 13 Magnetic Shielding If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. But protons are surrounded by electrons that shield them from the external field. Circulating electrons create an induced magnetic field that opposes the external magnetic field. =>

31. Chapter 13 shielding Electrons shield the “spin” nuclei from the magnetic field Different chemical environments lead to different amounts of sheilding

32. Chapter 13 if we were dealing with 1 H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1 H would produce a signal for all 1 H. The same is true of 13 C nuclei but hydrogens in organic molecules are not isolated from all other atoms; they are surrounded by electrons, which are caused to circulate by the presence of the applied field the circulation of electrons around a nucleus in an applied field is called diamagnetic current and the nuclear shielding resulting from it is called diamagnetic shielding

33. Chapter 13 the difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small the difference in resonance frequencies for hydrogens in CH 3 Cl compared to CH 3 F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency

34. Fortunately, different types of protons precess at different rates in the same magnetic field. N S 59.999995 MHz 59.999820 MHz 59.999700 MHz h n 60 MHz To cause absorption of the incoming 60 MHz the magnetic field strength, B o , must be increased to a different value for each type of proton. B o = 1.41 Tesla Differences are very small, in the parts per million range. EXAMPLE:

35. Chapter 13 Shielded Protons Magnetic field strength must be increased for a shielded proton to flip at the same frequency. =>

36. Chapter 13 Protons in a Molecule Depending on their chemical environment, protons in a molecule are shielded by different amounts. =>

37. Chapter 13 The NMR Graph =>

38. Chapter 13 signals are measured relative to the signal of the reference compound tetramethylsilane (TMS) for a 1H-NMR spectrum, signals are reported by their shift from the 12 H signal in TMS for a 13C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS

39. Chapter 13 Tetramethylsilane TMS is added to the sample. Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero. Organic protons absorb downfield (to the left) of the TMS signal. =>

40. Chapter 13 CHEMICAL SHIFT

41. Chapter 13 Chemical Shift Measured in parts per million. Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (Hz). Same value for 60, 100, or 300 MHz machine. Called the delta scale. =>

42. Chapter 13 PEAKS ARE MEASURED RELATIVE TO TMS TMS shift in Hz tetramethylsilane “TMS” reference compound n Rather than measure the exact resonance position of a peak, we measure how far downfield it is shifted from TMS. Highly shielded protons appear way upfield. Chemists originally thought no other compound would come at a higher field than TMS. downfield

43. Chapter 13 chemical shift ( d) = shift in Hz spectrometer frequency in MHz = ppm This division gives a number independent of the instrument used. parts per million THE CHEMICAL SHIFT The shifts from TMS in Hz are bigger in higher field instruments (300 MHz, 500 MHz) than they are in the lower field instruments (100 MHz, 60 MHz). We can adjust the shift to a field-independent value, the “chemical shift” in the following way: A particular proton in a given molecule will always come at the same chemical shift (constant value).

44. Chapter 13 the difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small the difference in resonance frequencies for hydrogens in CH 3 Cl compared to CH 3 F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency

45. FACTORS FOR CHEMICAL SHIFT ELECTRONEGETIVITY DIAMAGNETIC ANISOTROPIC EFFECT HYDROGEN BONDING

46. Electronegativity Dependence of Chemical Shift Compound CH 3 X Element X Electronegativity of X Chemical shift d CH 3 F CH 3 OH CH 3 Cl CH 3 Br CH 3 I CH 4 (CH 3 ) 4 Si F O Cl Br I H Si 4.0 3.5 3.1 2.8 2.5 2.1 1.8 4.26 3.40 3.05 2.68 2.16 0.23 0 Dependence of the Chemical Shift of CH 3 X on the Element X deshielding increases with the electronegativity of atom X TMS most deshielded

47. Chapter 13 Equivalent Hydrogens Equivalent hydrogens: have the same chemical environment a molecule with 1 set of equivalent hydrogens gives 1 NMR signal

48. Chapter 13 Equivalent Hydrogens a molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set

49. Chapter 13 h n = B o g 2 p constants frequency field strength Stronger magnetic fields (B o ) cause the instrument to operate at higher frequencies ( n ). REMEMBER FROM OUR EARLIER DISCUSSION NMR Field Strength 1 H Operating Frequency 60 Mhz 100 MHz 300 MHz 7.05 T 2.35 T 1.41 T n = ( K) B o

50. Chapter 13 TMS shift in Hz n downfield The shift observed for a given proton in Hz also depends on the frequency of the instrument used. Higher frequencies = larger shifts in Hz. HIGHER FREQUENCIES GIVE LARGER SHIFTS

51. Chapter 13 1 2 3 4 5 6 7 ppm Hz Equivalent of 1 ppm 1 H Operating Frequency 60 Mhz 60 Hz 100 MHz 100 Hz 300 MHz 300 Hz HERZ EQUIVALENCE OF 1 PPM Each ppm unit represents either a 1 ppm change in B o (magnetic field strength, Tesla) or a 1 ppm change in the precessional frequency (MHz). 1 part per million of n MHz is n Hz n MHz = n Hz 1 10 6 ( ) What does a ppm represent?

52. Chapter 13 NMR Correlation Chart 12 11 10 9 8 7 6 5 4 3 2 1 -OH -NH CH 2 F CH 2 Cl CH 2 Br CH 2 I CH 2 O CH 2 NO 2 CH 2 Ar CH 2 NR 2 CH 2 S C C-H C=C-CH 2 CH 2 -C- O C-CH-C C C-CH 2 -C C-CH 3 RCOOH RCHO C=C H TMS CHCl 3 , d (ppm) DOWNFIELD UPFIELD DESHIELDED SHIELDED Ranges can be defined for different general types of protons. This chart is general, the next slide is more definite.

53. Chapter 13 Location of Signals More electronegative atoms deshield more and give larger shift values. Effect decreases with distance. Additional electronegative atoms cause increase in chemical shift. =>

54. Chapter 13 Typical Values =>

55. Chapter 13 Aromatic Protons, 7-8 =>

56. Chapter 13 Vinyl Protons, 5-6 =>

57. Chapter 13 Acetylenic Protons, 2.5 =>

58. Chapter 13 Aldehyde Proton, 9-10 => Electronegative oxygen atom

59. Chapter 13 O-H and N-H Signals Chemical shift depends on concentration. Hydrogen bonding in concentrated solutions deshield the protons, so signal is around 3.5 for N-H and 4.5 for O-H. Proton exchanges between the molecules broaden the peak. =>

60. Chapter 13 Carboxylic Acid Proton, 10+ =>

61. Chapter 13 Number of Signals Equivalent hydrogens have the same chemical shift. =>

62. Chapter 13 Intensity of Signals The area under each peak is proportional to the number of protons. Shown by integral trace. =>

63. Chapter 13 How Many Hydrogens? When the molecular formula is known, each integral rise can be assigned to a particular number of hydrogens. =>

64. Chapter 13 SPIN-SPIN SPLITTING

65. Chapter 13 THE ORIGIN OF SPIN-SPIN SPLITTING HOW IT HAPPENS

66. Chapter 13 C C H H C C H H A A upfield downfield B o THE CHEMICAL SHIFT OF PROTON H A IS AFFECTED BY THE SPIN OF ITS NEIGHBORS 50 % of molecules 50 % of molecules At any given time about half of the molecules in solution will have spin +1/2 and the other half will have spin -1/2. aligned with B o opposed to B o neighbor aligned neighbor opposed +1/2 -1/2

67. Chapter 13 n + 1 RULE

68. Chapter 13 1,1,2-Trichloroethane integral = 2 integral = 1 Where do these multiples come from ? ….. interaction with neighbors

69. Chapter 13 two neighbors n+1 = 3 triplet one neighbor n+1 = 2 doublet singlet doublet triplet quartet quintet sextet septet MULTIPLETS this hydrogen’s peak is split by its two neighbors these hydrogens are split by their single neighbor

70. Chapter 13 C C H H C C H H one neighbor n+1 = 2 doublet one neighbor n+1 = 2 doublet SPIN ARRANGEMENTS yellow spins blue spins The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.

71. Chapter 13 two neighbors n+1 = 3 triplet one neighbor n+1 = 2 doublet SPIN ARRANGEMENTS methylene spins methine spins

72. Chapter 13 three neighbors n+1 = 4 quartet two neighbors n+1 = 3 triplet SPIN ARRANGEMENTS C C H H H H H C C H H H H H methyl spins methylene spins

73. Chapter 13 Often a group of hydrogens will appear as a multiplet rather than as a single peak. SPIN-SPIN SPLITTING Multiplets are named as follows: Singlet Quintet Doublet Septet Triplet Octet Quartet Nonet This happens because of interaction with neighboring hydrogens and is called SPIN-SPIN SPLITTING .

74. Chapter 13 integral = 2 integral = 1 triplet doublet 1,1,2-Trichloroethane The two kinds of hydrogens do not appear as single peaks, rather there is a “triplet” and a “doublet”. The sub peaks are due to spin-spin splitting and are predicted by the n+1 rule.

75. Chapter 13 EXCEPTIONS TO THE N+1 RULE IMPORTANT Protons that are equivalent by symmetry usually do not split one another no splitting if x=y no splitting if x=y 1) 2) Protons in the same group usually do not split one another or more detail later

76. Chapter 13 3) The n+1 rule applies principally to protons in aliphatic (saturated) chains or on saturated rings. EXCEPTIONS TO THE N+1 RULE or but does not apply (in the simple way shown here) to protons on double bonds or on benzene rings. NO NO YES YES

77. Chapter 13 SOME COMMON PATTERNS

78. Chapter 13 SOME COMMON SPLITTING PATTERNS ( x = y ) ( x = y )

79. Chapter 13 SOME EXAMPLE SPECTRA WITH SPLITTING

80. Chapter 13 NMR Spectrum of Bromoethane

81. Chapter 13 NMR Spectrum of 2-Nitropropane 1:6:15:20:16:6:1 in higher multiplets the outer peaks are often nearly lost in the baseline

82. Chapter 13 NMR Spectrum of Acetaldehyde offset = 2.0 ppm

83. Chapter 13 INTENSITIES OF MULTIPLET PEAKS PASCAL’S TRIANGLE

84. Chapter 13 1 2 1 PASCAL’S TRIANGLE 1 1 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 1 1 6 15 20 15 6 1 1 7 21 35 35 21 7 1 singlet doublet triplet quartet quintet sextet septet octet The interior entries are the sums of the two numbers immediately above. Intensities of multiplet peaks

85. Chapter 13 THE COUPLING CONSTANT

86. Chapter 13 J J J J J THE COUPLING CONSTANT The coupling constant is the distance J (measured in Hz) between the peaks in a multiplet. J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet. J

87. Chapter 13 100 MHz 200 MHz 1 2 3 4 5 6 1 2 3 100 Hz 200 Hz 200 Hz 400 Hz J = 7.5 Hz J = 7.5 Hz 7.5 Hz 7.5 Hz Coupling constants are constant - they do not change at different field strengths The shift is dependant on the field ppm FIELD COMPARISON Separation is larger

88. Chapter 13 100 MHz 200 MHz 1 2 3 4 5 6 1 2 3 100 Hz 200 Hz J = 7.5 Hz J = 7.5 Hz ppm 4 200 Hz 400 Hz 5 6 J = 7.5 Hz Note the compression of multiplets in the 200 MHz spectrum when it is plotted on the same scale as the 100 MHz spectrum instead of on a chart which is twice as wide. Separation is larger

89. Chapter 13 1 2 3 1 2 3 100 MHz 200 MHz Why buy a higher field instrument? Spectra are simplified! Overlapping multiplets are separated. Second-order effects are minimized. 1 2 3 50 MHz J = 7.5 Hz J = 7.5 Hz J = 7.5 Hz

90. Chapter 13 NOTATION FOR COUPLING CONSTANTS The most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms. This is sometimes called vicinal coupling. It is designated 3 J since three bonds intervene between the two hydrogens. Another type of coupling that can also occur in special cases is 2 J or geminal coupling Geminal coupling does not occur when the two hydrogens are equivalent due to rotations around the other two bonds. ( most often 2 J = 0 ) 3 J 2 J

91. Chapter 13 Couplings larger than 2 J or 3 J also exist, but operate only in special situations. Couplings larger than 3 J (e.g., 4 J, 5 J, etc) are usually called “long-range coupling.” C C C H H 4 J , for instance, occurs mainly when the hydrogens are forced to adopt this “W” conformation (as in bicyclic compounds). LONG RANGE COUPLINGS

92. Chapter 13 6 to 8 Hz 11 to 18 Hz 6 to 15 Hz 0 to 5 Hz three bond 3 J two bond 2 J three bond 3 J three bond 3 J SOME REPRESENTATIVE COUPLING CONSTANTS H ax ,H ax = 8 to 14 H ax ,H eq = 0 to 7 H eq ,H eq = 0 to 5 three bond 3 J trans cis geminal vicinal

93. Chapter 13 4 to 10 Hz 0 to 3 Hz four bond 4 J three bond 3 J 0 to 3 Hz four bond 4 J cis trans 6 to 12 Hz 4 to 8 Hz three bond 3 J Couplings that occur at distances greater than three bonds are called long-range couplings and they are usually small (<3 Hz) and frequently nonexistent (0 Hz).

94. Chapter 13 Spin-Spin Splitting Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field. This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly up field when the external field is opposed. All possibilities exist, so signal is split. =>

95. Chapter 13 1,1,2-Tribromoethane Nonequivalent protons on adjacent carbons. =>

96. Chapter 13 Doublet: 1 Adjacent Proton =>

97. Chapter 13 Triplet: 2 Adjacent Protons =>

98. Chapter 13 Range of Magnetic Coupling Equivalent protons do not split each other. Protons bonded to the same carbon will split each other only if they are not equivalent. Protons on adjacent carbons normally will couple. Protons separated by four or more bonds will not couple. =>

99. Chapter 13 Splitting for Ethyl Groups =>

100. Chapter 13 Splitting for Isopropyl Groups =>

101. Chapter 13 Values for Coupling Constants =>

102. Chapter 13 Splitting Tree =>

103. Chapter 13 Spectrum for Styrene =>

104. Chapter 13 Stereochemical Nonequivalence Usually, two protons on the same C are equivalent and do not split each other. If the replacement of each of the protons of a -CH 2 group with an imaginary “Z” gives stereoisomers, then the protons are non-equivalent and will split each other. =>

105. Chapter 13 Some Nonequivalent Protons =>

106. Chapter 13 Time Dependence Molecules are tumbling relative to the magnetic field, so NMR is an averaged spectrum of all the orientations. Axial and equatorial protons on cyclohexane interconvert so rapidly that they give a single signal. Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule. =>

107. Chapter 13 Hydroxyl Proton Ultrapure samples of ethanol show splitting. Ethanol with a small amount of acidic or basic impurities will not show splitting. =>

108. Chapter 13 N-H Proton Moderate rate of exchange. Peak may be broad. =>

109. Chapter 13 Identifying the O-H or N-H Peak Chemical shift will depend on concentration and solvent. To verify that a particular peak is due to O-H or N-H, shake the sample with D 2 O Deuterium will exchange with the O-H or N-H protons. On a second NMR spectrum the peak will be absent, or much less intense. =>

110. Chapter 13 Carbon-13 12 C has no magnetic spin. 13 C has a magnetic spin, but is only 1% of the carbon in a sample. The gyromagnetic ratio of 13 C is one-fourth of that of 1 H. Signals are weak, getting lost in noise. Hundreds of spectra are taken, averaged. =>

111. Chapter 13 Fourier Transform NMR Nuclei in a magnetic field are given a radio-frequency pulse close to their resonance frequency. The 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. =>

112. Chapter 13 Hydrogen and Carbon Chemical Shifts =>

113. Chapter 13 Combined 13 C and 1 H Spectra =>

114. Chapter 13 Differences in 13 C Technique Resonance frequency is ~ one-fourth, 15.1 MHz instead of 60 MHz. Peak areas are not proportional to number of carbons. Carbon atoms with more hydrogens absorb more strongly. =>

115. Chapter 13 Spin-Spin Splitting It is unlikely that a 13 C would be adjacent to another 13 C, so splitting by carbon is negligible. 13 C will magnetically couple with attached protons and adjacent protons. These complex splitting patterns are difficult to interpret. =>

116. Chapter 13 Proton Spin Decoupling To simplify the spectrum, protons are continuously irradiated with “noise,” so they are rapidly flipping. The carbon nuclei see an average of all the possible proton spin states. Thus, each different kind of carbon gives a single, unsplit peak. =>

117. Chapter 13 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. =>

118. Chapter 13 Interpreting 13 C NMR The number of different signals indicates the number of different kinds of carbon. The location (chemical shift) indicates the type of functional group. The peak area indicates the numbers of carbons (if integrated). The splitting pattern of off-resonance decoupled spectrum indicates the number of protons attached to the carbon. =>

119. Chapter 13 Two 13 C NMR Spectra =>

120. Chapter 13 MRI Magnetic resonance imaging, noninvasive “Nuclear” is omitted because of public’s fear that it would be radioactive. Only protons in one plane can be in resonance at one time. Computer puts together “slices” to get 3D. Tumors readily detected. =>

121. Chapter 13 End of Chapter 13