郑州大学有机化学.ppt
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1、第九章第九章 测定有机化合物结构的物理方法测定有机化合物结构的物理方法波谱学波谱学(Spectroscopy)基础基础Organic structures can be determined accurately and quickly by spectroscopyThe electromagnetic spectrumX-rays(X-射线射线)give bond lengths and anglesNuclear magnetic resonance(核磁共振核磁共振)tells us about the carbon skeleton of the moleculeInfrared s
2、pectroscopy(红外光谱红外光谱)tells us about the types of bond in a moleculeMass spectrometry(质谱)(质谱)determine mass of molecule and atomic compositionuX-ray crystallography has its limitationsX-ray crystallography works by the scattering of X-rays from electrons and requires crystalline solids.If an organic
3、compound is a liquid or is a solid but does not form good crystals,its structure cannot be determined in this way.uX-ray is the final appealdraw saturated carbon chains as zig-zagsand not in straight lines with 90or 180bond anglesHO2C(CH2)4 CO2Hhexanedioic acidshape of hexanedioic acidX-ray crystal
4、structure of hexanedioic acidInfrared spectroscopyInfrared(IR)spectroscopy detects the stretching and bending of bonds rather than any property of the atoms themselves.It is particularly good at detecting the stretching of unsymmetrical bonds of the kind found in functional groups such as OH,C=O,NH2
5、,and NO2.When the carbon skeleton of a molecule vibrates,all the bonds stretch and relax in combination and these absorptions are unhelpful.However some bonds stretch essentially independently of the rest of the molecule.This occurs if the bond is either:much stronger or weaker than others nearby,or
6、 between atoms that are much heavier or lighter than their neighboursf-force constant(more or less the bond strength)-frequence of vibration-reduced massThere are four important regions of the infrared spectrumThe XH region distinguishes CH,NH,and OH bondsThe reduced masses of the CH,NH,and OH combi
7、nations are all about the same.Any difference between the positions of the IR bands of these bonds must then be due to bond strength.4000 3000 2000 1500 1000IR bands for bonds to hydrogenBond Reduced mass IR frequency,cm-1 Bond strength,KJ mol-1C-H 12/13=0.92 2900-3200 CH4:440N-H 14/15=0.93 3300-340
8、0 NH3:450O-H 16/17=0.94 3500-3600a H2O:500a When not hydrogen bonded.Bonds to hydrogenO-HN-HC-HTriple bondsCCCNDouble bondsC=CC=OSingle bondsC-OC-FC-ClThe OH bands occur at higher frequency,sometimes as a sharp absorption at about 3600 cm1.More often,you will see a broad absorption at anywhere from
9、3500 to 2900 cm1.This is because OH groups form strong hydrogen bonds that vary in length and strength.The sharp absorption at 3600 cm1 is the non-hydrogen-bonded OH and the lower the absorption the stronger the H bond.NH,NH2O-HThe double bond region is the most important in IR spectraIn the double
10、bond region,there are three important absorptions,those of the carbonyl(C=O),alkene(C=C),and nitro(NO2)groups.All give rise to sharp bands:C=O to one strong(i.e.intense)band anywhere between 1900 and 1500 cm1;C=C to one weak band at about 1640 cm1;and NO2 to two strong(intense)bands in the mid-1500s
11、 and mid-1300s cm1.SummaryThe strength of an IR absorption depends on dipole momentThe strength of an IR absorption varies with the change of dipole moment when the bond is stretched.If the bond is perfectly symmetrical,there is no change in dipole moment and there is no IR absorption.Obviously,the
12、C=C bond is less polar than either C=O or N=O and is weaker in the IR.Position of band depends on:reduced mass of atoms light atoms gives high frequency bond strength strong bonds gives high frequency Strength of band depends on:change in dipole moment large dipole moment gives strong absorptionWidt
13、h of band depends on:hydrogen bonding strong H bond gives wide peakAbsorptions in IR spectra1.指出下列哪对化合物可用红外光谱区别指出下列哪对化合物可用红外光谱区别和和和和和和和和abcd核磁共振核磁共振 NMR1951年年 Arnold 发现乙醇的发现乙醇的NMR信号及其与信号及其与结构的关系结构的关系1945年年 Purcell和和 Bloch发现核磁共振现象,发现核磁共振现象,他们获得他们获得1952年年Nobel物理奖物理奖1953年年 Varian公司试制了第一台公司试制了第一台NMR仪器仪器
14、Richard R.ErnstThe Nobel Prize in Chemistry 1991 for his contributions to the development of the methodology of high resolution nuclear magnetic resonance(NMR)spectroscopy凡是自旋量子数凡是自旋量子数I 0的原子核,都可发生核磁共振的原子核,都可发生核磁共振质量数质量数和和原子序数同时为偶数的原子核原子序数同时为偶数的原子核,其自旋量子数为零,没有磁矩,其自旋量子数为零,没有磁矩,所以所以不发生核磁共振不发生核磁共振Nucle
15、ar magnetic resonanceNuclear magnetic resonance(NMR)allows us to detect atomic nuclei and say what sort of environment they are in,within their molecule.An NMR machineMagnetic Resonance ImagingNMR uses a strong magnetic field,and radio wavesI)some atomic nuclei act like tiny compass needles and have
16、 different energy levels when placed in a magnetic field.II)The energy levels of atomic nucleus are quantized,and there are only certain specific energy levels it can adopt.This is like allowing our compass needle to point,say,only north or south.III)Some nuclei(e.g.,carbon-12)do not interact with a
17、 magnetic field at all and cannot be observed in an NMR machine.The nuclei we shall be looking at,1H and 13C,do interact and have just two different energy levels.When we apply a magnetic field to these nuclei,they can either align themselves with it,which would be the lowest energy state,or they ca
18、n align themselves against the field,which is higher in energy.IV)Radio-waves are used to flip the nucleus from the lower energy state to the higher state.When the nuclei fall back down to the lower energy level,energy was given out,which was detected.How does it work?intensityfrequency(in parts per
19、 million,p.p.m.)100 MHz spectrumSolvent peakTypes of NMR spectrometers:1.Sweep(CW)NMR spectrometers2.Fourier Transform(FT)NMR spectrometersScales of NMRMagnetic field:telsa.The earths magnetic field has a field strength of 2 x 10-5 telas.A typical magnet used in an NMR machine has a field strength o
20、f between 2 and 10 telsa.Radio-wave frequency:MHz.Rather than referring to the strength of the magnet in tesla,chemists usually just refer to its operating frequency.A 9.4 T NMR machine is referred to as a 400 MHz spectrometer since that is the frequency in this strength field at which the protons i
21、n the reference sample resonate;other nuclei,for example 13C,would resonate at a different frequency,but the strength is arbitrarily quoted in terms of the proton operating frequency.Why do chemically distinct nuclei absorb energy at different frequencies?The variation in frequency for different car
22、bon atoms must mean that the energy jump from nucleus-aligned-with to nucleus-aligned-against the applied magnetic field must be different for each type of carbon atom.The reason there are different types of carbon atom is that their nuclei experience a magnetic field that is not quite the same as t
23、he magnetic field that we apply.Each nucleus is surrounded by electrons,and in a magnetic field these will set up a tiny electric current.This current will set up its own magnetic field,which will oppose the magnetic field that we apply.The electrons are said to shield the nucleus from the external
24、magnetic field.If the electron distribution varies from 13C atom to 13C atom,so does the local magnetic field,and so does the resonating frequency of the 13C nuclei.The variation in frequency is known as the chemical shift.Why do the chemical shift scale appear in parts per million(p.p.m.),not in ma
25、gnetic field units or frequency units?The exact frequency at which the nucleus resonates depends on the external applied magnetic field.This means that,if the sample is run on a machine with a different magnetic field,it will resonate at a different frequency.It would make life very difficult if we
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