Why is the IR spectrum inverted?

Question

Christian Anderson · Accepted Answer

It's arbitrary.

You can feel free to invert it again and switch between absorbance ("right-side-up") and %transmittance ("upside-down") before you print one out.

It really doesn't matter whether the peaks are facing downwards or upwards, or whether the wavenumbers are left-to-right or right-to-left; it communicates the same information in slightly different ways.

Honestly, I've actually submitted lab reports where I've used absorbance or %transmittance, and there were no complaints either way (just as long as I didn't do both in the same report).

As for the wavenumbers going right to left, it's just a common convention. There's no real reason to do that, given that it's been done both ways.

What matters is what the peaks look like. In other words:

how strong/weak they are
how narrow/wide they are
where they are horizontally (their vibrational frequency)
because that's how you identify which bond stretches or bends those rotational or vibrational peaks correspond to.

You just need to know what the peak looks like, not which way it's facing.

Brooklyn Barber · Answer

undefined The infrared (IR) spectrum is inverted because it represents the absorption of energy at different wavelengths by a sample. In IR spectroscopy, the y-axis represents the percent transmittance or absorbance of light by the sample, while the x-axis represents the frequency or wavelength of light. Absorption peaks appear as downward spikes because they indicate regions where the sample absorbs energy, resulting in less light being transmitted through it. Therefore, the IR spectrum is inverted compared to a normal graph where increasing values typically rise upwards on the y-axis.