One is the familiar acetylene structure and the other is isoelectronic with form
ID: 821547 • Letter: O
Question
One is the familiar acetylene structure and the other is isoelectronic with formaldehyde, CH2O. Indicate how it is possible to distinguish these two structure using CC stretching frequencies and using the number of vibrations expected in the IR spectrum of these molecules. Use formaldehyde to determine the number of vibrations expected for the second structure; molecules with the same symmetry have the same number of IR active vibrations.
Explanation / Answer
In the molecule the triple bond-C-H bond strecthing frequency occurs at 3100-3300cm-1 and the C-triple bond-C strecthing frequency occurs at 2100-2300cm-1..
Whereas in the second molecule the C=C strecthing frequency occurs at 1550-1700cm-1....
Linear molecules have the 3N-5 fundamental vibrations..
As the acetylene is the linear molecule it has 3N-5 fundamental vibrations...
where N = number of atoms in the molecule..
thus, acetylene has 4 aotms in it..
the number of fundamental vibrations = 3N-5 = 3x4 - 5 = 12 -7 = 5
thus, there are 5 fundamental vibrations in the acetylene molecule...
In the second molecule, the number of atoms are 4... but it is a non-linear molecule..
the number of vibrations for the non-linear moleucle = 3N-6..
thus number of vibrations = 3N-6 = 3x4 - 6 = 12-6 = 6
therefore, there are 6 vibrations in the second molecule...
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