require名词Organic Spectroscopic Analysis
Summary of Key Points
General Principles
1. The energy of atoms and the molecules (and so the difference in energy between these levels) have discrete values (quanta).
2.The wavenumber of a transition is inversely proportional to the wavelength (ν=1/λ).
3. The relationship between the energy of a transition and the frequency is given by ΔE= hν or ΔE=hc/λ or ΔE=hcν, where h is Planck’s constant. The energy of a particular transition is, therefore, proportional to the frequency or wavenumber, and inversely proportional to the wavelength.
4. NMR transitions correspond to wavelengths in the radiowave region of the spectrum, vibrational transition correspond to wavelengths in the IR region, and electronic transition to the UV-Vis region.
5. The number of double bond equivalents corresponds to the difference between the molecular formula and that for the saturated acyclic parent compound. Each DBE (double bond or ring) results in the subtraction of 2 hydrogens or halogens from the molecular formula of this parent structure.
Ultraviolet and Visible Spectra
1. UV-Vis spectroscopy involves the promotion of electrons from bonding or non-bonding orbitals to anti-bonding orbitals.
2. The UV spectrum arises from absorption of UV or visible light of the appropriate energy for a particular electronic transition. The absorbance (A) at any wavelength is calculated from the intensity of light transmitted through a solution of the sample in solvent (I) compared to the intensity of light transmitted through the solvent alone (I0): A= log10 (I0/I).
3. The molar absorptivity(ε）of an absorption maximum (λmax)gives an indication of the probability of that particular electronic transition and is characteristic for a given molecule. It
can be calculated using Beer-Lamberts law to relate absorbance(A) to the molar absorptivity(ε),the concentration(c, mol·L-1) and the path length(l, cm):A=εcl.
4. The most useful UV absorbances are those of conjugated organic molecules, which arise from non-bonding and π-orbitals. Systems with a higher degree of conjugation have absorption bands with increased intensities and large ε values; the longer the chromophore, the longer the wavelength of the absorption maximum.
5. The chromophore of aromatic systems is by conjugation with s substituent, e.g. those with π-electrons or lone pairs of electrons, in a predictable manner.
Infrared Spectroscopy
1. IR spectroscopy involves the study of transitions between the vibrational energy levels of a molecule and the interaction of the oscillating electric vector of the IR light with the oscillating dipole moment the molecule.
2. The frequency of the vibrational can be described by the Hooke’s Law, which describes t
he relationship between the wavenumber (ν), the strength of the bond (force constant, κ) and the reduced mass μ:
or
4. The position of an IR band is affected by many factors, including hydrogen bonding and conjugation, which both result in a lowering of the stretching frequency (wavenumber) for a vibration, and ring strain, which results in an increase in the stretching frequency (wavenumber). Intramolecular hydrogen bonding is independent of concentration, while intermolecular hydrogen bonding is dependent upon concentration.
5. The 1600-1000cm-1 region is known as the fingerprint region, and the vibrations which occur in this region usually involve the whole molecular skeleton. The fingerprint region is virtually unique to a given molecule, so two unknown samples which are believed to the same should have identical absorptions in this region.
6. The 1000-666cm-1 region corresponds to the bending vibrations of C-H bonds in unsatur
ated systems, and can often be used to determine the substitution pattern of aromatic rings.
Nuclear Magnetic Resonance
1. Nuclei with I≠0 can adopt certain allowed orientations in an external magnetic field and NMR transitions (between the energy levels for these orientations) require radiofrequency irradiation. The nuclei can adopt (2I+1) orientations in the magnetic field and the NMR selection rule states that transitions with Δm1≠±1 are allowed. The relationship between the magnetic field and the frequency of the transition is given by ν=Beff/2π.
2. Nuclei in molecules are affected by the local electron density and their chemical shift, the ratio of frequency at which an NMR transition occurs to the strength of the main field as measured by the frequency at which protons resonate, is given by:
  3. Peak splitting patterns indicate the number of spin active (I≠0) nuclei in the molecule which are close to the nucleus. Chemically and magnetically equivalent protons do not couple to one another. Spins-spin coupling is usually restricted in 1H NMR to three bonds between the interacting nuclei and can be predicted by the method of successive splitting, in which we take advantage of the fact that each spin 1/2 nucleus will split the peak due to its non-equivalent neighbor into a doublet of separation J(Hz). A useful general rule is that if we have n protons coupling to the nucleus of interest, the signal for that nucleus will have (n+1) lines.

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