karbonylstretch
The term "karbonylstretch" is not a recognized scientific or technical term in standard chemistry or physics literature. It appears to be a neologism or a misspelling. However, if interpreted as a combination of "karbonyl" (likely a misspelling of carbonyl) and "stretch," it could refer to the stretching vibration of a carbonyl group (C=O) within a molecule, as observed in spectroscopy.
A carbonyl group is a functional group with the chemical formula C=O. This double bond is polar