What are the Differences between bending and stretching?

2 Answers
Apr 19, 2018

see below


stretching is the variation of the bond distance caused by a vibration. It can occur if a molecula has at least 2 atoms. Bending is the variation of the angle of the bond caused by a vibration. For having bending you must have a molecula with at least 3 atoms (for example #CO_2 or H_2O#)

Jul 26, 2018

Here are the key differences.

#ul("Motion"" ""Rel. Frequency"" "" ""Desc."" "" "" "" "" ")#
#"Str."" "" "color(white)(/)"Higher"" "" "" "" "" ""Bond length varies"#
#"Bend"" "" ""Lower"" "" "" "" "" ""Bond angle varies"#

Visual examples are shown below for stretching and bending of #"H"_2"O"#. Together they are called the vibrational modes of water:


You can see that bends are often on the order of #tildenu = 400 - 1600 "cm"^(-1)#, while stretches are often on the order of #tildenu = 1600 - 4000 "cm"^(-1)#.

Bends are therefore found in the so-called fingerprint region of the IR spectrum (the right-hand side, lower frequency).