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spectroscopy (sp?)

Discussion in 'MCAT Discussions' started by rCubed, Aug 14, 2002.

  1. rCubed

    rCubed taiko master 10+ Year Member

    Jul 14, 2002
    are there certain values for NMR we're supposed to know?

    and also, besides the C==O stretch and the O--H stretch, is there something else we should know for IR?

    as for mass spec...all i know is that it seperates on basis of mass. i remember going over it in orgo class, but all i can remember is that certain functional groups break certain bonds showing certain characteristic peaks/readings...very vague...u guys don't think i need to know anything else about it, right?
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  3. Biffer

    Biffer The good times doc 10+ Year Member

    Dec 21, 2001
    maybe also Triple bonded carbons.. I believe its a small absorption around 2200.. correct me if I'm wrong.

  4. WingMD

    WingMD Junior Member 7+ Year Member

    Aug 13, 2002
    From what I have in my notes for IR:

    O-H 3600-3300
    C=O 1750
    C triple bond C 2200

    For NMR, don't have to memorize certain values, but just look for certain trends. Like if a H is next to a highly electronegative atom, it will have a higher NMR value than a H that is next to an electron donating atom.

    I am not sure for mass spects. I don't think we have to worry too much about that.
  5. Mudd

    Mudd Charlatan & Trouble Maker 7+ Year Member

    Jun 5, 2002
    relocated again
    They have expected people to know that protons on benzene have an HNMR sgnal around 7 ppm.

    As far as IR goes, know O-H is broad and around 3500 cm-1 and that C=O is sharp and around 1700cm-1.

    Weaker bonds take less energy to break and stretch an equal distance, so for comparable bonds, IR values can be approxiamted.

    For instance, C=C is weaker than C=O, so it has a slightly less energetic absorbance in the IR (around 1600 cm-1). Knowing two peaks and having good logic can fill in the table for you.

    As fas as mass spec goes, know that it gets the molecular mass and isotopic composition. The fragmentation pattern is likely beyond the scope of the MCAT.
  6. ukdoc74

    ukdoc74 Junior Member 10+ Year Member

    Aug 14, 2002
    Don't forget NH3...It has a very sharp absorbance at around 3400-3500 i.e compared to broad for -OH

    Also, for the NMR you have to know which side is downfield and upfield in reference to TMS(i'll give you a clue it is the opposite of DNA upstream/downstream). Also, if a functional group shields or deshields a proton.

    Lastly, the effect of D20 has on proton signals.
  7. limit

    limit Molesting my inner-child 10+ Year Member

    Jun 21, 2000
    New York City
    We have a winner, ladies and gentlemen:

    The most helpful post of the week !!! :rolleyes:
  8. Mr. Z

    Mr. Z Senior Member 7+ Year Member

    Apr 19, 2002
    deuterium doesn't show up on an NMR does it? so it has no effect on peak splitting?

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