Basic IR and NMR spectroscopy questions

Discussion in 'Chemistry' started by cnidocyte, Nov 20, 2010.

  1. cnidocyte Registered Senior Member

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    15
    The idea of quantisation of energy is confusing me. For example in IR, the bond will only absorb energy that is identical to the energy of the bonds bending or stretching vibrations. Why is this and what does the bond do with that new energy?

    Then NMR, the proton will only absorb the exact amount of energy required to jump to the higher energy spin state. Why is this? Why doesn't it absorb some energy and make a partial jump towards the higher spin state?
     
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  3. Communist Hamster Cricetulus griseus leninus Valued Senior Member

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    Well, get used to quantisation. It just is.

    A) It absorbs the quantum of energy and the molecule moves up one vibrational energy level, ie the bond stretches at a higher frequency. The frequencies at which bonds vibrate (ie bend/stretch, like a pendulum) are discrete.

    B) You can't have a partial jump towards another spin state, they are discrete. You can be in one or the other but not in between.
     
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  5. cnidocyte Registered Senior Member

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    I don't get it. Lets say theres 2 different bonds in the molecule, 1 vibrates at a frequency of 2 and the other, a frequency of 5. Lets say I do an IR sweep from 1 to 10. The molecule will absorb at frequencies 2 and 5 but what happens to the bonds? When the 1st bond absorbs radation of frequency 2 does it step up and start vibrating at frequency 4? Likewise does the 2nd bond step up to frequency 10?

    Also why don't 2 photons of frequency 4 = 1 photon of frequency 8?
     
    Last edited: Nov 20, 2010
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  7. Trippy ALEA IACTA EST Staff Member

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    Think of it this way.
    The chemical bond is a guitar string.
    You can only make a guitar string vibrate in certain modes, and you need to apply enough energy to it to get it there.

    Maybe not the best analogy, however...
     
  8. eddanco Registered Senior Member

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    12
    That's the beauty of quantization. Without quantization, spectroscopy would be difficult, if not impossible.

    The premise of spectroscopy is the interaction between electromagnetic energy and matter. Quantization, very loosely, means only CERTAIN "types" (i.e. frequencies) of electromagnetic radiation can interact with a certain aspect of matter. The frequency of the interacting EM energy is called the resonant frequency.

    For example, electromagnetic energy in the radio wave (low frequency) part of the spectrum can interact with an NMR-active nuclei under the influence of an external magnetic field. Likewise, infrared energy can interact with the bonds of molecules and visible "light" energy can interact with the loose pi electrons in molecules.

    These various aspects of matter can exist only in certain "states". For example, in NMR, the nuclei can exist in either a high energy (antiparallel to external magnetic field) or low energy (parallel) state. The "type" of electromagnetic energy required to switch between states is very specific.

    For example, to excite the pi electrons in chlorophyll from the low energy to high energy state, you need electromagnetic energy of a certain frequency.

    Taking several disparate photons and "adding" them up to get the same energy as the resonant frequency simply won't work because the frequencies of each photon will not match the resonance frequency.

    Without quantization, the idea of a resonant frequency is moot. Because THEN, you can take various photons, add them up (that is launch them all the same time against electrons/nuclei/bonds) and you can excite electrons/nuclei/bonds, etc. as long as you pass a certain energy "threshold".
     
  9. Peter Kinnon Registered Member

    Messages:
    10
    I suspect the last explanation may be way over your head.
    So to keep it simple:

    Photons of a pure single color all jiggle at the same rate.
    A photon of a pure single red will jiggle more slowly than one from the green part of the spectrum, and that, in turn jiggles more slowly than one in the blue.

    One, two, or a hundred similar photons of the same kind, still jiggle at the same speed - they don't "add up" to make another color.

    Now to resonance: You may be familiar with soldiers breaking step before crossing a bridge?
    The idea is that if their marching rate happens to match the natural rate at which the bridge bounces up and down the bridge may absorb sufficient energy to cause the bridge to break.

    Or how it is possible for an opera singer to hit just the exact note that corresponds to the natural vibration rate of a delicate glass and cause that to absorb energy sufficient for it to shatter?

    That is exactly the kind of thing that is happening in IR spectroscopy. Photons that jiggle even slower than red ones are sent through the sample. If the particular frequency of those photons happen to correspond to the natural bend/stretch rate of the bonds then the energy of the photons is absorbed.

    What is the result? Well, if the photon intensity is high enough, just like the bridge or the glass, the bonds will break, destroying the compound. For spectroscopy, lower intensities are used (less photons) so that the bonds merely bend/stretch more vigorously. This corresponds to an increase in temperature. Comparison of the number of photons sent into the sample with those transmitted through or reflected from the sample gives a measure of the amount of energy absorbed at that frequency. Various bonds having characteristic resonant frequencies allows identification.
     

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