Question about organic spectroscopy (IR)

[Rujano]

Homework Statement

Hi. I'm starting to read about IR spectroscopy and I have a question. I know that there are 2 kinds of vibrational modes in molecules: bending and stretching, so in a molecule like water for example, how can I tell which one of the vibrational does it have?

What I mean is that I also know that there different types of modes within the bending category (scissoring, rocking, wagging and twisting), so which one of them is the right one (in the book that I'm reading it says that it can only have scissoring... why?)?

Thanks in advance!

 

[HeavyMetal]

I don't think your book is right, but I could be wrong.

A non-linear molecule with three atoms should have 3 degrees of freedom, because 3n-6 = 3(3)-6 = 3, where n = number of atoms involved in the molecule.