2 Chem questions
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.
typo correction & amendment for Q1
... and there are FOUR TYPES of vibrations --- scissoring, twisting, asymmetrical stretching and symmetrical stretching ...
Thx, but some questions about IR spectroscopy also raised.
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
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?
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?
- ?Lv 79 years agoFavorite Answer
Well, in fact a lot of information can be found on Wikipedia:
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：
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.