2 Chem questions

1. bonds

In a molecule, according to the book, when bonds exists, they vibrates

and there are three typrs of vibrations --- scissoring, asymmetrical stretch and symmetrical stretch

But, as far as I know, there are also something called strain --- angle strain, torsional strain and steric strain.

I want to ask whether they are the same or some relevant concepts?

2. infrared spectroscopy

I want to know the principal and operation procedures, in detail, of the infrared spectrophtometer in mearsuring the transmittance of infrared radiation.

It is not mentioned in notes/books.

Update:

typo correction & amendment for Q1

... and there are FOUR TYPES of vibrations --- scissoring, twisting, asymmetrical stretching and symmetrical stretching ...

Update 2:

Thx, but some questions about IR spectroscopy also raised.

Update 3:

For each molecule, there is fingerprint region, which is at 400~500cm-1 and is unique to any molecule

But in your answer, it said the anhydrous salt plates used for treating liquid absorb IR of less than 650cm-1; or for solid, nujol may be used which have major peaks at 1370 and 1450cm-1

Update 4:

So, wouldn't the fingerprint region be distracted?

Besides, from wikipedia, the fingerprint region actually are most single bonds vibrations (e.g. C-C and C-H)

Does that means the transmittance of IR at that range reflects the existence of certain single bonds?

Update 5:

If yes, wouldn't the molecules of structural isomers(e.g. propan-1-ol and propan-2-ol) have same fingerprint region as they have same number of C-H, C-C and C-O bonds?

1 Answer

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  • ?
    Lv 7
    9 years ago
    Favorite Answer

    Well, in fact a lot of information can be found on Wikipedia:

    http://en.wikipedia.org/wiki/Infrared_spectrometry

    圖片參考:http://imgcld.yimg.com/8/n/HA00075708/o/7012012500...

    圖片參考:http://imgcld.yimg.com/8/n/HA00075708/o/7012012500...

    http://en.wikipedia.org/wiki/Cyclodecapentaene

    2012-02-05 14:12:24 補充:

    1. Sorry for some inaccurate info, as I'm not doing org. chem. and use no IR spect.; when one doesn't use sth he will ofrget what he has learnt...

    upon checking, KBr absorbs above ~30000nm, i.e. absorbs <350cm^-1 .

    2012-02-05 14:15:29 補充:

    and, nujol does absorb at 1370 and 1450cm-1.

    but so what? they're from th methyl group and -CH2- (methylene) group, which many, many componds have.

    2012-02-05 14:16:20 補充:

    The presence of them in IR spect can prove little, while the presence of -CH3 & -CH2- can be determined by NMR accurately -- no. of C, chem. environment, adjacient to what atoms / groups etc.

    2012-02-05 14:19:23 補充:

    2.

    yes, it can show presence / absence of some bonds.

    however, the absorption peaks could deviate a little from others ue to difference in molecular shape, arrangement of bonds & groups etc.

    2012-02-05 14:22:07 補充:

    imagine you're shaking a coconut tree. the vibration mode, freq. etc. of shaking could be different depending on the position of the coconut.

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