# Explaining allene IR stretching frequency

So I was comparing the IR stretch frequency of ethyne, ethene and allene. Of course triple bond is stiffer than double bond so ethyne has a higher stretch frequency. The problem then is where allene sits when compared to these two. I guess it should be the lowest because I just think of the cumulated double bonds as serial spring, which should give lower spring stiffness than only one double bond in ethene. Obviously, I'm wrong... It turns out allene IR stretch frequency is in the middle of ethene and ethyne. I wonder how might we explain that. Thanks!

• There are two normal modes for the along-axis stretch in allene. Are you thinking about the right one? – Zhe May 10 '20 at 3:25
• Can you expand on that a bit? I’m not sure which two mode you are talking about – Macrophage May 10 '20 at 3:26
• The symmetrical stretching mode won't participate in IR spectroscopy just like that of $CO_2$ but the asymmetrical stretching mode would. – Suraj S May 10 '20 at 6:31
• @SurajS okay, but I’m asking about how to explain the frequency – Macrophage May 10 '20 at 6:32
• @SurajS we don’t cover stretching mode in the intro org class – Macrophage May 10 '20 at 6:32