有机化学英文chapter13.pptx

Nuclear Magnetic ResonanceChapter 13Chapter 13第1页/共74页Molecular Spectroscopy Nuclear magnetic resonance(NMR)spectroscopyNuclear 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 1H-NMR spectroscopycarbon atoms using 13C-NMR spectroscopyphosphorus atoms using 31P-NMR spectroscopy第2页/共74页Nuclear Spin StatesAn 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 fieldin effect,an electron behaves as if it is a tiny bar magnet and has what is called a magnetic momentThe same effect holds for certain atomic nucleiany atomic nucleus that has an odd mass number,an odd atomic number,or both also has a spin and a resulting nuclear magnetic momentthe allowed nuclear spin states are determined by the spin quantum number,I,of the nucleus第3页/共74页Nuclear Spin Statesa nucleus with spin quantum number I I has 2 2I I +1 +1 spin states;if I =1/2,there are two allowed spin statesTable 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds第4页/共74页Nuclear Spins in B0within a collection of 1H and 13C atoms,nuclear spins are completely random in orientationwhen placed in a strong external magnetic field of strength B0,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 第5页/共74页Nuclear Spins in B0for 1H and 13C,only two orientations are allowed第6页/共74页Nuclear Spins in B0In 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)第7页/共74页Nuclear Spin in B0the energy difference between allowed spin states increases linearly with applied field strength values shown here are for 1H nuclei第8页/共74页Nuclear Magnetic Resonancewhen 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 statethe 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 fieldwe express the rate of precession as a frequency in hertz第9页/共74页Nuclear Magnetic ResonanceIf 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)第10页/共74页Nuclear Magnetic ResonanceFigure 13.3 the origin of nuclear magnetic“resonance 第11页/共74页Nuclear Magnetic Resonance ResonanceResonance: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 stateThe instrument used to detect this coupling of precession frequency and electromagnetic radiation records it as a signal signal:signal:a recording in an NMR spectrum of a nuclear magnetic resonance第12页/共74页Nuclear Magnetic Resonanceif we were dealing with 1H nuclei isolated from all other atoms and electrons,any combination of applied field and radiation that produces a signal for one 1H would produce a signal for all 1H.The same is true of 13C nucleibut 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 fieldthe circulation of electrons around a nucleus in an applied field is called diamagneticdiamagnetic currentcurrent and the nuclear shielding resulting from it is called diamagnetic shieldingdiamagnetic shielding第13页/共74页Nuclear Magnetic Resonancethe difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very smallthe difference in resonance frequencies for hydrogens in CH3Cl compared to CH3F under an applied field of 7.05T is only 360 Hz,which is 1.2 parts per million(ppm)compared with the irradiating frequency 第14页/共74页Nuclear Magnetic Resonancesignals 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 TMSfor a 13C-NMR spectrum,signals are reported by their shift from the 4 C signal in TMS Chemical shift(Chemical shift():):the shift in ppm of an NMR signal from the signal of TMS第15页/共74页NMR Spectrometer第16页/共74页NMR SpectrometerEssentials of an NMR spectrometer are a powerful magnet,a radio-frequency generator,and a radio-frequency detectorThe sample is dissolved in a solvent,most commonly CDCl3 or D2O,and placed in a sample tube which is then suspended in the magnetic field and set spinningUsing a Fourier transform NMR(FT-NMR)spectrometer,a spectrum can be recorded in about 2 seconds第17页/共74页NMR Spectrum1H-NMR spectrum of methyl acetate Downfield:Downfield:the shift of an NMR signal to the left on the chart paper Upfield:Upfield:the shift of an NMR signal to the right on the chart paper第18页/共74页Equivalent Hydrogens Equivalent hydrogens:Equivalent hydrogens:have the same chemical environmenta molecule with 1 set of equivalent hydrogens gives 1 NMR signal第19页/共74页Equivalent Hydrogensa molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set第20页/共74页Signal AreasRelative areas of signals are proportional to the number of H giving rise to each signalModern NMR spectrometers electronically integrate and record the relative area of each signal第21页/共74页ChemicalChemicalShiftsShifts1 1H-NMRH-NMR第22页/共74页Chemical Shift-1H-NMR第23页/共74页Chemical ShiftDepends on(1)electronegativity of nearby atoms,(2)the hybridization of adjacent atoms,and(3)diamagnetic effects from adjacent pi bondsElectronegativity第24页/共74页Chemical ShiftHybridization of adjacent atoms第25页/共74页Chemical ShiftDiamagnetic effects of pi bondsa carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield(to the right)to a smaller valuea carbon-carbon double bond deshields vinylic hydrogens and shifts their signal downfield(to the left)to a larger value第26页/共74页Chemical Shiftmagnetic induction in the pi bonds of a carbon-carbon triple bond(Fig 13.9)第27页/共74页Chemical Shiftmagnetic induction in the pi bond of a carbon-carbon double bond(Fig 13.10)第28页/共74页Chemical Shiftmagnetic induction of the pi electrons in an aromatic ring(Fig.13.11)第29页/共74页Signal Splitting;the(n+1)Rule Peak:Peak:the units into which an NMR signal is split;doublet,triplet,quartet,etc.Signal splitting:Signal splitting:splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens(n n+1)rule:+1)rule:if a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s),its 1H-NMR signal is split into(n+1)peaks第30页/共74页Signal Splitting(n+1)1H-NMR spectrum of 1,1-dichloroethane第31页/共74页Signal Splitting(n+1)ProblemProblem:predict the number of 1H-NMR signals and the splitting pattern of each第32页/共74页Origins of Signal Splitting Signal coupling: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):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第33页/共74页Origins of Signal Splitting第34页/共74页Origins of Signal Splittingbecause splitting patterns from spectra taken at 300 MHz and higher are often difficult to see,it is common to retrace certain signals in expanded form1H-NMR spectrum of 3-pentanone;scale expansion shows the triplet quartet pattern more clearly第35页/共74页Coupling Constants Coupling constant(J):Coupling constant(J):the distance between peaks in a split signal,expressed in hertzthe value is a quantitative measure of the magnetic interaction of nuclei whose spins are coupled第36页/共74页Origins of Signal Splitting第37页/共74页Signal SplittingPascals Triangleas illustrated by the highlighted entries,each entry is the sum of the values immediately above it to the left and the right第38页/共74页Physical Basis for(n+1)RuleCoupling of nuclear spins is mediated through intervening bondsH atoms with more than three bonds between them generally do not exhibit noticeable couplingfor H atoms three bonds apart,the coupling is referred to as vicinal coupling第39页/共74页Coupling Constantsan important factor in vicinal coupling is the angle a between the C-H sigma bonds and whether or not it is fixedcoupling is a maximum when a is 0 and 180;it is a minimum when a is 90第40页/共74页More Complex Splitting Patternsthus far,we have concentrated on spin-spin coupling with only one other nonequivalent set of H atomsmore complex splittings arise when a set of H atoms couples to more than one set H atomsa tree diagram shows that when Hb is adjacent to nonequivalent Ha on one side and Hc on the other,the resulting coupling gives rise to a doublet of doublets 第41页/共74页More Complex Splitting Patternsif Hc is a set of two equivalent H,then the observed splitting is a doublet of triplets第42页/共74页More Complex Splitting Patternsbecause the angle between C-H bond determines the extent of coupling,bond rotation is a key parameterin molecules with relatively free rotation about C-C sigma bonds,H atoms bonded to the same carbon in CH3 and CH2 groups generally are equivalentif there is restricted rotation,as in alkenes and cyclic structures,H atoms bonded to the same carbon may not be equivalentnonequivalent H on the same carbon will couple and cause signal splittingthis type of coupling is called geminal couplinggeminal coupling第43页/共74页More Complex Splitting Patternsin ethyl propenoate,an unsymmetrical terminal alkene,the three vinylic hydrogens are nonequivalent 第44页/共74页More Complex Splitting Patternsa tree diagram for the complex coupling of the three vinylic hydrogens in ethyl propenoate第45页/共74页More Complex Splitting Patternscyclic structures often have restricted rotation about their C-C bonds and have constrained conformationsas a result,two H atoms on a CH2 group can be nonequivalent,leading to complex splitting第46页/共74页More Complex Splitting Patternsa tree diagram for the complex coupling in 2-methyl-2-vinyloxirane第47页/共74页More Complex Splitting PatternsComplex coupling in flexible moleculescoupling in molecules with unrestricted bond rotation often gives only m+n+I peaksthat 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 representsthe 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 sp3 hybridized C atoms 第48页/共74页More Complex Splitting Patternssimplification of signal splitting occurs when coupling constants are the same第49页/共74页More Complex Splitting Patternsan example of peak overlap occurs in the spectrum of 1-chloro-3-iodopropanethe central CH2 has the possibility for 9 peaks(a triplet of triplets)but because Jab and Jbc are so similar,only 4+1=5 peaks are distinguishable第50页/共74页Stereochemistry&TopicityDepending on the symmetry of a molecule,otherwise equivalent hydrogens may behomotopicenantiotopicdiastereotopicThe simplest way to visualize topicity is to substitute an atom or group by an isotope;is the resulting compoundthe same as its mirror imagedifferent from its mirror imageare diastereomers possible第51页/共74页Stereochemistry&TopicityHomotopic atoms or groupshomotopic atoms or groups have identical chemical shifts under all conditionsAchiralHCHClClHCDClClDichloro-methane(achiral)Substitution does not produce a stereocenter;therefore hydrogensare homotopic.Substitute one H by DAchiralHCHClClHCDClClDichloro-methane(achiral)Substitution does not produce a stereocenter;therefore hydrogensare homotopic.Substitute one H by D第52页/共74页Stereochemistry&TopicityEnantiotopic groupsenantiotopic atoms or groups have identical chemical shifts in achiral environmentsthey have different chemical shifts in chiral environmentsChiralHCHClFHCDClFChlorofluoro-methane(achiral)Substitute one H by DSubstitution produces a stereocenter;therefore,hydrogens are enantiotopic.Both hydrogens are prochiral;one is pro-R-chiral,the other is pro-S-chiral.ChiralHCHClFHCDClFChlorofluoro-methane(achiral)Substitute one H by DSubstitution produces a stereocenter;therefore,hydrogens are enantiotopic.Both hydrogens are prochiral;one is pro-R-chiral,the other is pro-S-chiral.第53页/共74页Stereochemistry&TopicityDiastereotopic groupsH atoms on C-3 of 2-butanol are diastereotopicsubstitution by deuterium creates a chiral centerbecause there is already a chiral center in the molecule,diastereomers are now possiblediastereotopic hydrogens have different chemical shifts under all conditions第54页/共74页Stereochemistry&TopicityThe methyl groups on carbon 3 of 3-methyl-2-butanol are diastereotopicif a methyl hydrogen of carbon 4 is substituted by deuterium,a new chiral center is createdbecause there is already one chiral center,diastereomers are now possibleprotons of the methyl groups on carbon 3 have different chemical shiftsOH3-Methyl-2-butanol第55页/共74页Stereochemistry and Topicity1H-NMR spectrum of 3-methyl-2-butanolthe methyl groups on carbon 3 are diastereotopic and appear as two doublets第56页/共74页13C-NMR SpectroscopyEach nonequivalent 13C gives a different signala 13C signal is split by the 1H 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 determineThe most common mode of operation of a 13C-NMR spectrometer is a hydrogen-decoupled mode第57页/共74页13C-NMR SpectroscopyIn a hydrogen-decoupled mode,a sample is irradiated with two different radio frequenciesone to excite all 13C nucleia second broad spectrum of frequencies to cause all hydrogens in the molecule to undergo rapid transitions between their nuclear spin statesOn the time scale of a 13C-NMR spectrum,each hydrogen is in an average or effectively constant nuclear spin state,with the result that 1H-13C spin-spin interactions are not observed;they are decoupled第58页/共74页13C-NMR Spectroscopyhydrogen-decoupled 13C-NMR spectrum of 1-bromobutane第59页/共74页Chemical S