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Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Click to edit Master title style,13-,1,OrganicChemistry,WilliamH.Brown,ChristopherS.Foote,BrentL.Iverson,OrganicChemistryWilliamH.Brown,Nuclear Magnetic Resonance,Chapter 13,Nuclear Magnetic ResonanceChap,Molecular Spectroscopy,Nuclear magnetic resonance (NMR) spectroscopy,:,a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of,hydrogen atoms using,1,H-NMR spectroscopy,carbon atoms using,13,C-NMR spectroscopy,phosphorus atoms using,31,P-NMR spectroscopy,Molecular SpectroscopyNuclear,Nuclear Spin States,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,Nuclear Spin StatesAn electron,Nuclear Spin States,a nucleus with spin quantum number,I,has,2,I,+ 1,spin states; if,I,= 1/2, there are two allowed spin states,Table 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds,Nuclear Spin Statesa nucleus w,Nuclear Spins in B,0,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,0, 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,Nuclear Spins in B0within a co,Nuclear Spins in B,0,for,1,H and,13,C, only two orientations are allowed,Nuclear Spins in B0for 1H and,Nuclear Spins in B,0,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,1,H is approximately 0.120 J (0.0286 cal)/mol, which corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz),13,C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz),Nuclear Spins in B0In an appli,Nuclear Spin in B,0,the energy difference between allowed spin states increases linearly with applied field strength,values shown here are for,1,H nuclei,Nuclear Spin in B0the energy d,Nuclear Magnetic Resonance,when nuclei with a spin quantum number of 1/2 are placed in an applied field, a small majority of nuclear spins are aligned with the applied field in the lower energy state,the nucleus begins to precess and traces out a cone-shaped surface, in much the same way a spinning top or gyroscope traces out a cone-shaped surface as it precesses in the earths gravitational field,we express the rate of precession as a frequency in hertz,Nuclear Magnetic Resonancewhen,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),Nuclear Magnetic ResonanceIf t,Nuclear Magnetic Resonance,Figure 13.3 the origin of nuclear magnetic “resonance,Nuclear Magnetic ResonanceFigu,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,Nuclear Magnetic ResonanceReso,Nuclear Magnetic Resonance,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,Nuclear Magnetic Resonanceif w,Nuclear Magnetic Resonance,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,Nuclear Magnetic Resonancethe,Nuclear Magnetic Resonance,signals are measured relative to the signal of the reference compound tetramethylsilane (TMS),for a,1,H-NMR spectrum, signals are reported by their shift from the 12 H signal in TMS,for a,13,C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS,Chemical shift (,):,the shift in ppm of an NMR signal from the signal of TMS,Nuclear Magnetic Resonancesign,NMR Spectrometer,NMR Spectrometer,NMR Spectrometer,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,NMR SpectrometerEssentials of,NMR Spectrum,1,H-NMR spectrum of methyl acetate,Downfield:,the shift of an NMR signal to the left on the chart paper,Upfield:,the shift of an NMR signal to the right on the chart paper,NMR Spectrum1H-NMR spectrum of,Equivalent Hydrogens,Equivalent hydrogens:,have the same chemical environment,a molecule with 1 set of equivalent hydrogens gives 1 NMR signal,Equivalent HydrogensEquivalent,Equivalent Hydrogens,a molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set,Equivalent Hydrogensa molecule,Signal Areas,Relative areas of signals are proportional to the number of H giving rise to each signal,Modern NMR spectrometers electronically integrate and record the relative area of each signal,Signal AreasRelative areas of,Chemical,Shifts,1,H-NMR,Chemical,Chemical Shift -,1,H-NMR,Chemical Shift - 1H-NMR,Chemical Shift,Depends on (1) electronegativity of nearby atoms, (2) the hybridization of adjacent atoms, and (3) diamagnetic effects from adjacent pi bonds,Electronegativity,Chemical ShiftDepends on (1) e,Chemical Shift,Hybridization of adjacent atoms,Chemical ShiftHybridization of,Chemical Shift,Diamagnetic effects of pi bonds,a carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield (to the right) to a smaller,value,a carbon-carbon double bond deshields vinylic hydrogens and shifts their signal downfield (to the left) to a larger,value,Chemical ShiftDiamagnetic effe,Chemical Shift,magnetic induction in the pi bonds of a carbon-carbon triple bond (Fig 13.9),Chemical Shiftmagnetic inducti,Chemical Shift,magnetic induction in the pi bond of a carbon-carbon double bond (Fig 13.10),Chemical Shiftmagnetic inducti,Chemical Shift,magnetic induction of the pi electrons in an aromatic ring (Fig. 13.11),Chemical Shiftmagnetic inducti,Signal Splitting; the (,n,+ 1) Rule,Peak:,the units into which an NMR signal is split; doublet, triplet, quartet, etc.,Signal splitting:,splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens,(,n,+ 1) rule:,if a hydrogen has,n,hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its,1,H-NMR signal is split into (,n,+ 1) peaks,Signal Splitting; the (n + 1),Signal Splitting (n + 1),1,H-NMR spectrum of 1,1-dichloroethane,Signal Splitting (n + 1)1H-NMR,Signal Splitting (n + 1),Problem,:,predict the number of,1,H-NMR signals and the splitting pattern of each,Signal Splitting (n + 1)Probl,Origins of Signal Splitting,Signal coupling:,an interaction in which the nuclear spins of adjacent atoms influence each other and lead to the splitting of NMR signals,Coupling constant (J):,the separation on an NMR spectrum (in hertz) between adjacent peaks in a multiplet;,a quantitative measure of the influence of the spin-spin coupling with adjacent nuclei,Origins of Signal SplittingSig,Origins of Signal Splitting,Origins of Signal Splitting,Origins of Signal Splitting,because splitting patterns from spectra taken at 300 MHz and higher are often difficult to see, it is common to retrace certain signals in expanded form,1,H-NMR spectrum of 3-pentanone; scale expansion shows the triplet quartet pattern more clearly,Origins of Signal Splittingbec,Coupling Constants,Coupling constant (J):,the distance between peaks in a split signal, expressed in hertz,the value is a quantitative measure of the magnetic interaction of nuclei whose spins are coupled,Coupling ConstantsCoupling con,Origins of Signal Splitting,Origins of Signal Splitting,Signal Splitting,Pascals Triangle,as illustrated by the highlighted entries, each entry is the sum of the values immediately above it to the left and the right,Signal SplittingPascals Trian,Physical Basis for (,n,+ 1) Rule,Coupling of nuclear spins is mediated through intervening bonds,H atoms with more than three bonds between them generally do not exhibit noticeable coupling,for H atoms three bonds apart, the coupling is referred to as vicinal coupling,Physical Basis for (n + 1) Rul,Coupling Constants,an important factor in vicinal coupling is the angle,a,between the C-H sigma bonds and whether or not it is fixed,coupling is a maximum when,a,is 0 and 180; it is a minimum when,a,is 90,Coupling Constantsan important,More Complex Splitting Patterns,thus far, we have concentrated on spin-spin coupling with only one other nonequivalent set of H atoms,more complex splittings arise when a set of H atoms couples to more than one set H atoms,a tree diagram shows that when H,b,is adjacent to nonequivalent H,a,on one side and H,c,on the other, the resulting coupling gives rise to a doublet of doublets,More Complex Splitting Pattern,More Complex Splitting Patterns,if H,c,is a set of two equivalent H, then the observed splitting is a doublet of triplets,More Complex Splitting Pattern,More Complex Splitting Patterns,because the angle between C-H bond determines the extent of coupling, bond rotation is a key parameter,in molecules with relatively free rotation about C-C sigma bonds, H atoms bonded to the same carbon in CH,3,and CH,2,groups generally are equivalent,if there is restricted rotation, as in alkenes and cyclic structures, H atoms bonded to the same carbon may not be equivalent,nonequivalent H on the same carbon will couple and cause signal splitting,this type of coupling is called,geminal coupling,More Complex Splitting Pattern,More Complex Splitting Patterns,in ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent,More Complex Splitting Pattern,More Complex Splitting Patterns,a tree diagram for the complex coupling of the three vinylic hydrogens in ethyl propenoate,More Complex Splitting Pattern,More Complex Splitting Patterns,cyclic structures often have restricted rotation about their C-C bonds and have constrained conformations,as a result, two H atoms on a CH,2,group can be nonequivalent, leading to complex splitting,More Complex Splitting Pattern,More Complex Splitting Patterns,a tree diagram for the complex coupling in 2-methyl-2-vinyloxirane,More Complex Splitting Pattern,More Complex Splitting Patterns,Complex coupling in flexible molecules,coupling in molecules with unrestricted bond rotation often gives only,m,+,n,+ I peaks,that is, the number of peaks for a signal is the number of adjacent hydrogens + 1, no matter how many different sets of equivalent H atoms that represents,the explanation is that bond rotation averages the coupling constants throughout molecules with freely rotation bonds and tends to make them similar; for example in the 6- to 8-Hz range for H atoms on freely rotating,sp,3,hybridized C atoms,More Complex Splitting Pattern,More Complex Splitting Patterns,simplification of signal splitting occurs when coupling constants are the same,More Complex Splitting Pattern,More Complex Splitting Patterns,an example of peak overlap occurs in the spectrum of 1-chloro-3-iodopropane,the central CH,2,has the possibility for 9 peaks (a triplet of triplets) but because J,ab,and J,bc,are so similar, only 4 + 1 = 5 peaks are distinguishable,More Complex Splitting Pattern,Stereochemistry & Topicity,Depending on the symmetry of a molecule, otherwise equivalent hydrogens may be,homotopic,enantiotopic,diastereotopic,The simplest way to visualize topicity is to substitute an atom or group by an isotope; is the resulting compound,the same as its mirror image,different from its mirror image,are diastereomers possible,Stereochemistry & TopicityDepe,Stereochemistry & Topicity,Homotopic atoms or groups,homotopic atoms or groups have identical chemical shifts under all conditions,Achiral,H,C,H,C,l,C,l,H,C,D,C,l,C,l,Dichloro-,methane,(achiral),Substitution does not,produce a stereocenter;,therefore hydrogens,are homotopic.,Substitute,one H by D,Achiral,H,C,H,C,l,C,l,H,C,D,C,l,C,l,Dichloro-,methane,(achiral),Substitution does not,produce a stereocenter;,therefore hydrogens,are homotopic.,Substitute,one H by D,Stereochemistry & TopicityHomo,Stereochemistry & Topicity,Enantiotopic groups,enantiotopic atoms or groups have identical chemical shifts in achiral environments,they have different chemical shifts in chiral environments,Chiral,H,C,H,C,l,F,H,C,D,C,l,F,Chlorofluoro-,methane,(achiral),Substitute,one H by D,Substitution produces a,stereocenter;,therefore, hydrogens are,enantiotopic. Both,hydrogens are prochiral;,one is pro-R-chiral, the,other is pro-S-chiral.,Chiral,H,C,H,C,l,F,H,C,D,C,l,F,Chlorofluoro-,methane,(achiral),Substitute,one H by D,Substitution produces a,stereocenter;,therefore, hydrogens are,enantiotopic. Both,hydrogens are prochiral;,one is pro-R-chiral, the,other is pro-S-chiral.,Stereochemistry & TopicityEnan,Stereochemistry & Topicity,Diastereotopic groups,H atoms on C-3 of 2-butanol are diastereotopic,substitution by deuterium creates a chiral center,because there is already a chiral center in the molecule, diastereomers are now possible,diastereotopic hydrogens have different chemical shifts under all conditions,Stereochemistry & TopicityDias,Stereochemistry & Topicity,The methyl groups on carbon 3 of 3-methyl-2-butanol are diastereotopic,if a methyl hydrogen of carbon 4 is substituted by deuterium, a new chiral center is created,because there is already one chiral center, diastereomers are now possible,protons of the methyl groups on carbon 3 have different chemical shifts,O,H,3-Methyl-2-butanol,Stereochemistry & TopicityThe,Stereochemistry and Topicity,1,H-NMR spectrum of 3-methyl-2-butanol,the methyl groups on carbon 3 are diastereotopic and appear as two doublets,Stereochemistry and Topicity1H,13,C-NMR Spectroscopy,Each nonequivalent,13,C gives a different signal,a,13,C signal is split by the,1,H bonded to it according to the (,n,+ 1) rule,coupling constants of 100-250 Hz are common, which means that there is often significant overlap between signals, and splitting patterns can be very difficult to determine,The most common mode of operation of a,13,C-NMR spectrometer is a hydrogen-decoupled mode,13C-NMR SpectroscopyEach noneq,13,C-NMR Spectroscopy,In a hydrogen-decoupled mode, a sample is irradiated with two different radio frequencies,one to excite all,13,C nuclei,a second broad spectrum of frequencies to cause all hydrogens in the molecule to undergo rapid transitions between their nuclear spin states,On the time scale of a,13,C-NMR spectrum, each hydrogen is in an average or effectively constant nuclear spin state, with the result that,1,H-,13,C spin-spin interactions are not observed; they are decoupled,13C-NMR SpectroscopyIn a hydro,13,C-NMR Spectroscopy,hydrogen-decoupled,13,C-NMR spectrum of 1-bromobutane,13C-NMR Spectroscopyhydrogen-d,Chemical Shift -,13,C-NMR,Chemical Shift - 13C-NMR,Chemical Shift -,13,C-NMR,Chemical Shift - 13C-NMR,The DEPT Method,In the hydrogen-decoupled mode, information on spin-spin coupling between,13,C and hydrogens bonded to it is lost,The DEPT method is an instrumental mode that provides a way to acquire this information,Distortionless Enhancement by Polarization Transfer,(,DEPT):,an NMR technique for distinguishing among,13,C signals for CH,3, CH,2, CH, and quaternary carbons,The DEPT MethodIn the hydrogen,The DEPT Method,The DEPT methods uses a complex series of pulses in both the,1,H and,13,C ranges, with the result that CH,3, CH,2, and CH signals exhibit different phases;,signals for CH,3,and CH carbons are recorded as positive signals,signals for CH,2,carbons are recorded as negative signals,quaternary carbons give no signal in the DEPT method,The DEPT MethodThe DEPT method,Isopentyl acetate,13C-NMR: (a) proton decoupled and (b) DEPT,Isopentyl acetate13C-NMR: (a),Interpreting NMR Spectra,Alkanes,1,H-NMR signals appear in the range of,0.8-1.7,13,C-NMR signals appear in the considerably wider range of,10-60,Alkenes,1,H-NMR signals appear in the range,4.6-5.7,1,H-NMR coupling constants are generally larger for,trans,vinylic hydrogens (J= 11-18 Hz) compared with,cis,vinylic hydrogens (J= 5-10 Hz),13,C-NMR signals for,sp,2,hybridized carbons appear in the range,100-160, which is downfield from the signals of,sp,3,hybridized carbons,Interpreting NMR SpectraAlkane,Interpreting NMR Spectra,1,H-NMR spectrum of vinyl acetate (Fig 13.33),Interpreting NMR Spectra1H-NMR,Interpreting NMR Spectra,Alcohols,1,H-NMR O-H chemical shifts often appears in the range,3.0-4.0, but may be as low as,0.5.,1,H-NMR chemical shifts of hydrogens on the carbon bearing the -OH group are deshielded by the electron-withdrawing inductive effect of the oxygen and appear in the range,3.0-4.0,Ethers,a distinctive feature in the,1,H-MNR spectra of ethers is the chemical shift,3.3-4.0, of hydrogens on carbon attached to the ether oxygen,Interpreting NMR SpectraAlcoho,Interpreting NMR Spectra,1,H-NMR spectrum of 1-propanol (Fig. 13.34),Interpreting NMR Spectra1H-NMR,Interpreting NMR Spectra,Aldehydes and ketones,1,H-NMR: aldehyde hydrogens appear at,9.5-10.1,1,H-NMR:,a,-hydrogens of aldehydes and ketones appear at,2.2-2.6,13,C-NMR: carbonyl carbons appear at,180-215,Amines,1,H-NMR: amine hydrogens appear at,0.5-5.0 depending on conditions,Interpreting NMR SpectraAldehy,Interpreting NMR Spectra,Carboxylic acids,1,H-NMR: carboxyl hydrogens appear at,10-13, lower than most any other hydrogens,13,C-NMR: carboxyl carbons in acids and esters appear at,160-180,Interpreting NMR SpectraCarbox,Interpreting NMR Spectra,Spectral Problem 1; molecular formula C,5,H,10,O,Interpreting NMR SpectraSpectr,Interpreting NMR Spectra,Spectral Problem 2; molecular formula C,7,H,14,O,Interpreting NMR SpectraSpectr,Nuclear,Magnetic Resonance,End Chapter 13,Nuclear,
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