For a nucleus with a

By Dale Cole,2014-04-04 22:02
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For a nucleus with a spin of 1/2, there are two allowed orientations of the nucleus; parallel to the field (low energy) and against the field (high energy).



2003~~ 2004学年 I学期

    院(系、所、部)化学与环境学院有机化学研究所 有机化学

     有机化学(双语教学) 化学教育



     Organic Chemistry

    2003 09 01

    授课题目(教学章节或主题):第八章.现代物理实授课类型 验方法在有机化学中的应用(Spectroscopic

    Methods of Structure Determination

    授课时间 10周第37-42


    和质谱的基本理论。掌握并能解析红外光谱,紫外光谱,核磁共振谱。 教学内容(包括基本内容、重点、难点):


    和质谱的基本识谱方法。掌握并能解析红外光谱,紫外光谱,核磁共振谱。 难点是解析氢核磁共振谱谱图。

    The region of the infrared spectrum which is of greatest interest to organic

    chemists is the wavelength range 2.5 to 15 micrometers (?. In practice, units

    -1proportional to frequency, (wave number in units of cm) rather than wavelength, are commonly used and the region 2.5 to 15 ?corresponds to approximately 4000

    -1to 600 cm.

    Absorption of radiation in this region by a typical organic molecule results

    in the excitation of vibrational, rotational and bending modes, while the

    molecule itself remains in its electronic ground state. Movie files

    demonstrating vibrational and bending modes for water (HO) are available by 2clicking on the icons shown below:

    Symmetric Stretch Asymmetric Stretch Symmetric Bend Molecular asymmetry is a requirement for excitation by infrared radiation and

    fully symmetric molecules do not display absorbances in this region unless

    asymmetric stretching or bending transitions are possible. For the purpose of routine organic structure determination, using a battery of spectroscopic methods, the most important absorptions in the infrared region

    are the simple stretching vibrations. For simple systems, these can be

approximated by considering the atoms as point masses, linked by a 'spring'

    having a force constant k and following Hooke's Law. Using this simple

    approximation, the equation shown below can be utilized to approximate the

    -1characteristic stretching frequency (in cm) of two atoms of masses m and m, 2

    linked by a bond with a force constant k:

where ?= mm/(m+m) (termed the 'reduced mass'), and c is the velocity of light. 1212

    The stretching vibrations of typical organic molecules tend to fall within distinct regions of the infrared spectrum, as shown below:

    -1? 3700 - 2500 cm: X-H stretching (X = C, N, O, S)

    -1? 2300 - 2000 cm: CX stretching (X = C or N)

    -1? 1900 - 1500 cm: CX stretching (X = C, N, O)

    -1? 1300 - 800 cm: C-X stretching (X = C, N, O)

    -1Since most organic molecules have single bonds, the region below 1500 cm can

    become quite complex and is often referred to as the 'fingerprint region': that

    is, if you are dealing with an unknown molecule which has the same 'fingerprint'

    in this region, that is considered evidence that the two molecules may be identical.

    -1Because of the complexity of the region below 1500 cm, in this review, we will focus on functional group stretching bands in the higher frequency region. You

    should note that for many of these bands, the IR spectrum may give equivocal structural information; quite often the absence of a band is as informative as the presence of a particular band.

Nuclei of isotopes which possess an odd number of protons, an odd number of

    neutrons, or both, exhibit mechanical spin phenomena which are associated with

    angular momentum. This angular momentum is characterized by a nuclear spin

    quantum number, I such that,

    1/n, where n is an integer 0,1,2,3...etc. I = 2

    Those nuclei for which I = 0 do not possess spin angular momentum and do not

    1216exhibit magnetic resonance phenomena. The nuclei of C and O fall into this

    1119133115214category. Nuclei for which I = / include H, F, C, P and N, while H and N 2

    have I = 1.

    Since atomic nuclei are associated with charge, a spinning nucleus generates

    a small electric current and has a finite magnetic field associated with it.

    The magnetic dipole, ? of the nucleus varies with each element.

    When a spinning nucleus is placed in a magnetic field, the nuclear magnet

    experiences a torque which tends to align it with the external field. For a

    1nucleus with a spin of /, there are two allowed orientations of the nucleus; 2

    parallel to the field (low energy) and against the field (high energy). Since

    the parallel orientation is lower in energy, this state is slightly more

    populated than the anti-parallel, high energy state. (Figure 1)

If the oriented nuclei are now irradiated with electromagnetic radiation of

    the proper frequency, the lower energy state will absorb a quantum of energy

    and spin-flip to the high energy state. When this spin transition occurs, the nuclei are said to be in resonance with the applied radiation, hence the name

    nuclear magnetic resonance.

    The amount of electromagnetic radiation necessary for resonance depends on both

    the strength of the external magnetic field and on the characteristics of the nucleus being examined. The nucleus of the proton, placed in 14,100 gauss field,

    undergoes resonance when irradiated with radiation in the 60 MHz range

    (microwave radiation); higher magnetic fields, such as those common in

    superconducting magnets, require higher energy radiation and give a

    correspondingly higher resolution.

In mass spectrometry, a substance is bombarded with an electron beam having

    sufficient energy to fragment the molecule. The positive fragments which are

    produced (cations and radical cations) are accelerated in a vacuum through a

    magnetic field and are sorted on the basis of mass-to-charge ratio. Since the

    bulk of the ions produced in the mass spectrometer carry a unit positive charge,

    the value m/e is equivalent to the molecular weight of the fragment. The analysis

    of mass spectroscopy information involves the re-assembling of fragments,

    working backwards to generate the original molecule. A schematic representation

    of a mass spectrometer is shown below:

A very low concentration of sample molecules is allowed to leak into the

    ionization chamber (which is under a very high vacuum) where they are bombarded

    by a high-energy electron beam. The molecules fragment and the positive ions

    produced are accelerated through a charged array into an analyzing tube. The

    path of the charged molecules is bent by an applied magnetic field. Ions having

    low mass (low momentum) will be deflected most by this field and will collide

    with the walls of the analyzer. Likewise, high momentum ions will not be

    deflected enough and will also collide with the analyzer wall. Ions having the

    proper mass-to-charge ratio, however, will follow the path of the analyzer,

    exit through the slit and collide with the Collector. This generates an electric

    current, which is then amplified and detected. By varying the strength of the

    magnetic field, the mass-to-charge ratio which is analyzed can be continuously varied.

    The output of the mass spectrometer shows a plot of relative intensity vs the

    mass-to-charge ratio (m/e). The most intense peak in the spectrum is termed the and all others are reported relative to it's intensity. The peaks

    themselves are typically very sharp, and are often simply represented as

vertical lines.

    The process of fragmentation follows simple and predictable chemical pathways

    and the ions which are formed will reflect the most stable cations and radical

    cations which that molecule can form. The highest molecular weight peak observed

    in a spectrum will typically represent the parent molecule, minus an electron, and is termed the (M+). Generally, small peaks are also observed

    above the calculated molecular weight due to the natural isotopic abundance

    132C, H, etc. Many molecules with especially labile protons do not display of

    molecular ions; an example of this is alcohols, where the highest molecular

    weight peak occurs at m/e one less than the molecular ion (m-1). Fragments can be identified by their mass-to-charge ratio, but it is often more informative

    to identify them by the mass which has been lost. That is, loss of a methyl

    group will generate a peak at m-15; loss of an ethyl, m-29, etc. The mass spectrum of toluene (methyl benzene) is shown below. The spectrum

    displays a strong molecular ion at m/e = 92, small m+1 and m+2 peaks, a base peak at m/e = 91 and an assortment of minor peaks m/e = 65 and below.

    The molecular ion, again, represents loss of an electron and the peaks above the molecular ion are due to isotopic abundance. The base peak in toluene is

    due to loss of a hydrogen atom to form the relatively stable benzyl cation.

    This is thought to undergo rearrangement to form the very stable tropylium

    cation, and this strong peak at m/e = 91 is a hallmark of compounds containing

    a benzyl unit. The minor peak at m/e = 65 represents loss of neutral acetylene

    from the tropylium ion and the minor peaks below this arise from more complex



    思考题、讨论题、作业:(601面,Additional problems;14.19-14.32)


    1. Solomons, Organic Chemistry, fifth adition 2. Oxford; Organic Chemistry

    3. 北京大学, 有机化

    4.南京大学, 有机化学,(上,下) 5.邢其毅,有机化学, (上,下) 6.有机化合物波谱解析;姚新生 主编,中国医药科技出版社




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