[Twickel]

Hello,

I am having trouble identifying molecules which will have a center of inversion and which ones will not. Water ( H₂O) has all its vibrations appear in IR spectroscopy, I am not sure if this molecule has a center of inversion or not.  

One of my lecturers said, if it has a axis of symmetry ( so cut it in half with a line vertical down the middle of the molecule. and the two halves should be the same but opposite) then it has a center of inversion, so then I assume that water has no Raman signals but I do not think that is the case.

now using the definition, An inversion occurs through a centre of symmetry, denoted i. (This operation involves taking each point in the object in a straight line to the inversion centre and then moving it an equal distance out the other side. i.e. if we consider the inversion centre to lie at (0,0,0) , then this operation turns a general point (x,y,z) into (-x,-y,-z). Spheres and cubes both have centres of inversion, as does a benzene molecule (a regular hexagon). Molecules such as water, ammonia and methane do not.

I do not see why water and ammonia do not have a center of inversion ( please refer to image drawn)

Why does CH₂I₂ have a center of inversion?

[juanrga]

Hello,

I am having trouble identifying molecules which will have a center of inversion and which ones will not. Water ( H₂O) has all its vibrations appear in IR spectroscopy, I am not sure if this molecule has a center of inversion or not.  

One of my lecturers said, if it has a axis of symmetry ( so cut it in half with a line vertical down the middle of the molecule. and the two halves should be the same but opposite) then it has a center of inversion, so then I assume that water has no Raman signals but I do not think that is the case.

now using the definition, An inversion occurs through a centre of symmetry, denoted i. (This operation involves taking each point in the object in a straight line to the inversion centre and then moving it an equal distance out the other side. i.e. if we consider the inversion centre to lie at (0,0,0) , then this operation turns a general point (x,y,z) into (-x,-y,-z). Spheres and cubes both have centres of inversion, as does a benzene molecule (a regular hexagon). Molecules such as water, ammonia and methane do not.

I do not see why water and ammonia do not have a center of inversion ( please refer to image drawn)

Why does CH₂I₂ have a center of inversion?

A molecule has a centre of symmetry when, for any atom in the molecule, an identical atom diametrically opposite this centre an equal distance from it.

[Twickel]

Ok, so why does CH2I2 have a center of inversion if, there is no iodine directly opposite the left iodine and vice versa?

[sjb]

.
One of my lecturers said, if it has a axis of symmetry ( so cut it in half with a line vertical down the middle of the molecule. and the two halves should be the same but opposite) then it has a center of inversion, so then I assume that water has no Raman signals but I do not think that is the case
.

If you are cutting this in half as you describe, this is not an axis, but a plane of symmetry

[Twickel]

Thankyou, I have scrapped that method now.

What I would like to know is, how does CH2I2have a center of inversion, if directly opposite either a hydrogen or iodine, you do not find the same atom?

[juanrga]

Ok, so why does CH2I2 have a center of inversion if, there is no iodine directly opposite the left iodine and vice versa?

CH₂I₂, as methane CH₄ has no center of inversion.