有机光谱化学分析 (25).pdf
红外光谱介绍 有机波谱分析 Organic Spectroscopic Analysis Introduction to Infrared Spectroscopy(IR)Infrared Spectroscopy(I)-introduction NMR Molecular vibrations Valence band and shallow electronic levels(atoms)core level electrons of atoms Radio Waves Infrared,Raman UV-vis absorption UV photoemission Fluorescence X-ray photoemission(XPS,ESCA)Auger Electron(AES)IR-introductin Typical IR Spectra The ordinate(纵坐标纵坐标)is linear in transmittance.The abscissa(横坐标横坐标)is linear in wavenumber(cm-1).Where does IR Absorption come from?1.The absorption of IR radiation by a sample is due to changes in the vibrational energy statesof a molecule.2.The pattern of absorption as a function of wavelength is called an IR spectrum.Dipole Changes during Vibration As a polar molecule(such CO)vibrates,a regular fluctuation in dipole moment occurs,and a field is established that can interact with the electrical field associated with radiation.If the frequency of the radiation exactly matches a natural vibrational frequency of the molecule,absorption occurs.Similarly,the rotation of asymmetric molecules around their centers of mass results in a periodic dipole fluctuation that can interact with IR radiation.Where does IR Absorption come from?Dipole Changes during Vibration Where does IR Absorption come from?Molecular vibration Quantum Treatment of Vibrations 1()22hkE =(m1+m2)/(m1 m2)Mechanical Model of Vibrations A disturbance of one of these masses along the axis of the spring results in a vibration called a simple harmonic motion(简谐振动简谐振动).Where:k=force constant(力常数力常数)indicating the strength of the bond;m1 and m2 are the masses of the two atoms m At What Frequency Will Absorption Occur?Harmonic Oscillator Model m Harmonic and Anharmonic Oscillator In the mechanical model,the energy curve take the harmonic form.In the more accurate quantum treatment,the energy curve must take the anharmonic form.Note that the harmonic and anharmonic curves are nearly alike at low potential energies.This fact accounts for the success of the approximate methods described.Absorption Frequency?Energy difference in adjacent levelsEnergy difference in adjacent levels（相邻能级的（相邻能级的能量差）能量差）:2hkEEh1()22hkE1113072kkc112kc =(m1+m2)/(m1 m2)Stretching Frequency Example Calculate the approximate wavenumber of the stretching vibration of a carbonyl group C=O.m1=2.010-26kg;m2=2.710-26kg=(m1+m2)/(m1m2)=1.110-26kg The force constant(k)for the typical double bond is about 1103N/m.=1600 cm-1 The carbonyl stretching band is found experimentally to be in the region of 1600 to 1800 cm-1.112kcMolecular Vibration Modes Bending H O H HHHHScissoring 1450 cm-1 Rocking 720 cm-1 HHHHWagging 1350 cm-1 Twisting 1250 cm-1 Types of Molecular Vibrations Vibrational modes of H2O molecule IR Spectrometer and Experiment IR Spectrometer Sample Preparation for Infrared Spectroscopy Samples for infrared spectroscopy can be prepared in a number of ways:1.Solids-Solid samples can be ground with potassium bromide(KBr)to form a very fine powder.This powder is then pressed into a thin pellet(KBr disk)which can be analyzed.2.Liquids-For liquid samples,the easiest is to place one drop of sample between two plates of NaCl,forming a thin film.3.Solutions can also be analyzed in a cell made from NaCl(or other IR-transparent material)which can be filled with liquid.IR windows：The sample cell window must be infrared transparent!GasDirectly induced to sample inlet.
