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1、红外光谱和选律 有机波谱分析 Organic Spectroscopic Analysis IR Spectrum and selection rule IR spectrum of CO2 A=lg I0/I=lg 1/T Typical IR Spectra Number of Vibrational Modes Nonlinear molecule with n atoms usually has 3n-6 fundamental vibrational modes.Linear molecule with n atoms usually has 3n-5 fundamental vib
2、rational modes.O=C=O;S=C=S Number of Vibrational Modes a molecule containing N atoms is said to have 3N degrees of freedom.3 degrees of freedom of the translational motion.23 degrees of freedom of the rotational motion.Number of vibrational modes:3N 6 or 3N 5.Vibration Degrees of Freedom and Selecti
3、on Rules of IR Practice:1)What are the Vibration Degrees of Freedom of H2O?(Answer:3)2)What are the Vibration Degrees of Freedom of CO2?(Answer:4)Vibrations of CO2 IR-Active and Inactive?A polar bond is usually IR-active.A nonpolar bond in a symmetrical molecule such as those homonuclear species of
4、O2,N2,or Cl2,will absorb weakly or not at all.A change in dipole of a vibration IR Selection Rule:For infrared the selection rule states that in order for the photon to be absorbed,the molecule must undergo a change in dipole moment.Vibration Spectroscopy Selection Rule:V1,2,3。At room temperature,th
5、e vast majority of molecules are in the vibrational ground state of V=0,so the main observed band is the transition from V=0 to the fundamental frequency absorption band of V=l.An Alkyne（炔烃）（炔烃）IR Spectrum Number of Experimental Peaks Four factors tend to produce fewer experimental peaks than would
6、be expected.1.No change in dipole of a particular vibration;2.The energies of two or more vibrations are identical or nearly identical;3.The absorption intensity is too low;4.The vibrational energy is in a wavelength beyond the range of the instrument.Number of Experimental Peaks Occasionally more peaks are found than are expected.For example,Overtone(倍频倍频)and Combination Bands(合频合频)Selection rule Selection rule 容許容許:v=1 overtone peakovertone peak：v=2 2 2 v=3 3 3 combination peak：1+2 difference peak(差频差频)：1-2 Overtone(倍频倍频)and Combination Bands(合频合频)1()22hkE