Dismiss Notice
Hey Guest! Check out the 3 MCAT Study Plan Options listed in the 'stickies' area at the top of the forums (BoomBoom, SN2ed, and MCATJelly). Let us know which you like best.

Also, we now offer a MCAT Test-Prep Exhibitions Forum where you can ask questions directly from the test-prep services.
Dismiss Notice
SDN members see fewer ads and full resolution images. Join our non-profit community!

spectroscopy (sp?)

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

  1. rCubed

    rCubed taiko master

    Joined:
    Jul 14, 2002
    Messages:
    416
    Likes Received:
    0
    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?
     
  2. Thread continues after this sponsor message. SDN Members do not see this ad.

  3. Biffer

    Biffer The good times doc

    Joined:
    Dec 21, 2001
    Messages:
    328
    Likes Received:
    0
    maybe also Triple bonded carbons.. I believe its a small absorption around 2200.. correct me if I'm wrong.

    later,
    biffer
     
  4. WingMD

    WingMD Junior Member

    Joined:
    Aug 13, 2002
    Messages:
    15
    Likes Received:
    0
    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

    Joined:
    Jun 5, 2002
    Messages:
    376
    Likes Received:
    2
    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

    Joined:
    Aug 14, 2002
    Messages:
    78
    Likes Received:
    1
    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

    Joined:
    Jun 21, 2000
    Messages:
    570
    Likes Received:
    1
    We have a winner, ladies and gentlemen:

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

    Mr. Z Senior Member

    Joined:
    Apr 19, 2002
    Messages:
    451
    Likes Received:
    0
    deuterium doesn't show up on an NMR does it? so it has no effect on peak splitting?
     

Share This Page