Coupled Interaction

Infrared spectroscopy: Coupled Interact Interactions: ions: I- Coupling interactions due to stretching stretching vibrations:

This results from a mechanical coupling interaction between the oscillators. - It appears when: i) Two Two bond oscillators share a common atom and ii) Their individual oscillation frequencies are not widely different. For example carbon dioxide molecule which consists of two !"# bonds with a common carbon atom has two fundamental stretching vibrations an asymmetrical and a symmet symmetrica ricall stretch stretching ing mode. mode.

(N.B. It shows also bending vibrations at 667 cm -1  as has been

mentioned):

a) The symme symmetr trica icall stret stretchi ching ng mode: mode:

- It cons consis ists ts of an in-p in-pha hase se stre stretc tchi hing ng or cont contra ract ctin ing g of the the !"# !"# bond bondss . ← O=C=O   O=C=O ← - $bsorption occurs at a frequency lower than that observed for the carbonyl

group in an aliphatic %etone. - The symmetrical stretching mode produces no change in the dipole moment of  inactive& and can't be observed in I(  the the molec molecul ulee and and it is there therefo fore re &I! inactive

spectrum but it is easily observed in the (aman spectrum near *+, cm -.  N.B.  Band intensity in Raman sectra deends on bond olari!ability rather than molec"lar diole changes

b) In the asymmetrical stretching stretching mode"

The two !"# bonds stretch out of phase one !"# bond stretches as the other  contracts.

 O=C=O 

- The asymmetrical stretching mode is I! active since it produces a change in the dipole moment. -

The The absorp absorpti tion on of asymm asymmetr etrica icall stretc stretchi hing ng occu occurs rs at /*0, /*0, cm - i.e. at a higher  frequency than observed for a carbonyl group in aliphatic %etones.

-

This This diff differ eren ence ce in carb carbon ony yl abso absorp rpti tion on freq freque uenc ncie iess disp displa lay yed by the the carb carbon on dioxide molecule results from strong mechanical coupling or interaction.

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#the #therr exam exampl ples es for coup couplin ling g inte interac racti tion onss due due to stretc stretchi hing ng vibrat vibratio ions ns are the the following:

#) The two 123 stretching bands in the *+45-*,55 cm - region in primary amine

and primary amide spectra: $) $lso for the two !"# stretching bands in the 66-5/, cm - region for carboxylic

anhydride and imide spectra.  %) The two !23 stretching bands in the *,,,-/57, cm - region for both methylene

and methyl groups. II-Coupling interactions due to bending vibrations:

8ibrations resulting from bond angle changes frequently couple in a manner similar  .to stretching vibrations For example: The ring !23 out-of-plane bending frequencies of aromatic   .molecules depend on the number of ad9acent hydrogen atoms on the ring !oupling between the hydrogen atoms is affected by the bending of the !2! bond  .in the ring to which the hydrogen atoms are attached III-Coupling due to interactions bet&een stretching stretching and bending vibrations:

-

!oupli !oupling ng betwe between en stretch stretching ing and and bending bending vibr vibratio ations ns can occur occur if the stretc stretchin hing g

 bond forms one side of the changing changing angle. -

This This type type of intera interacti ction on can be be illustr illustrated ated by the absorp absorptio tion n of secon secondary dary acy acyclic clic amides which exist predominantly in the trans conformation. O N H

It sho shows ws stro strong ng abs absor orpt ptio ion n in the the 07 07**-0 00 0 cm cm - region.

 -

This absorpt absorption ion involv involves es coupling coupling of of the 123 bending bending and !21 stretchin stretching g

vibrations. ' (e can summarie summarie the re* re*uire uirement mentss for effective effective coupling coupling interact interaction ion as follo&s:

. The vibrations must be of the same symmetry species if interaction is to occur. /. trong coupling between stretching vibrations requires a common atom between the groups. *. Interaction is greatest when the coupled groups absorbs individually near the same frequency. +. !oupling between bending and stretching vibrations can occur if the stretching  bond forms one side of the changing angle. angle.

0. $ common common bond is required for coupling c oupling of bending vibrations. 7. !oupling is negligible when groups are separated by one or more carbon atoms. $s we have have seen seen in our discussio discussion n of coupli coupling ng interac interactio tions ns of two fundamen fundamental tal vibrational modes will produce two new modes of vibration with frequencies higher  and lower than that observed when interaction is absent. I+I+-Coupli Coupling ng due to intera interacti ctions ons bet&e bet&een en fun funda damen menta tall vibra vibratio tions ns and overtones ,ermi resonance):

;hen coupling occur between fundamental vibrations and overtones or combination tone vibrations. uch interaction is %nown as Fermi resonance. #ne example of Fermi resonance is afforded by the absorption pattern of carbon dioxide. $s we have have previo previously usly discussed discussed that the symmet symmetrica ricall stretch stretching ing band band of !"# !"# appears in the (aman spectrum near *+, cm -. $ctually there are two bands are observed: one at /67 cm - and one at *66 cm -. The splitting results from coupling  between the fundamental !"# stretching vibration near *+, cm - and the first overtone of the bending vibration.

(N.B. #he $"ndamental bending vibration occ"rs near 667 cm -1 and the $irst 

overtone near 1%%& cm-1).

- Fermi resonance is a common phenomenon in both I( and (aman spectra. It requires that a) The vibr vibrati ationa onall levels levels be of the the same symme symmetry try specie speciess and  b)

The interacting groups must be located in the molecule so that mechanical coupling is appreciable.