Why is the IR spectrum inverted?

The concept of an "inverted" IR spectrum usually refers to the display of peaks and valleys in an infrared (IR) spectrum. In IR spectroscopy, molecules absorb infrared radiation at specific frequencies corresponding to the vibrational modes of their chemical bonds.

When you observe an IR spectrum, you typically see peaks and valleys. Peaks represent absorption of infrared radiation, while valleys represent regions where little or no absorption occurs.

The "inverted" appearance occurs because IR spectra are often plotted with transmittance (or % transmittance) on the y-axis, which measures the fraction of light that passes through the sample relative to the initial intensity, rather than absorbance. Absorbance, which measures the logarithm of the ratio of incident light intensity to transmitted light intensity, would typically be plotted in an upward direction, with peaks representing higher absorbance.

Transmittance, on the other hand, is plotted inversely, so that peaks represent regions where less light is transmitted, corresponding to absorption by the sample. This can give the appearance of an "inverted" spectrum, where peaks represent absorption rather than transmission.

The inversion is simply a convention in how the data is presented graphically, chosen to make interpretation more intuitive, especially for beginners in the field of spectroscopy. It facilitates easier comparison with other spectroscopic techniques like UV-Vis spectroscopy, where peaks do indeed represent absorption.