Acidichloride

From Memory Alpha, the free Star Trek reference.

Acidichloride is a gas that is toxic to most humanoids.

While under the effects of hyper-evolution in 2372, the alveoli in Tom Paris's lungs changed so that he could only breathe a mixture of 80% nitrogen and 20% acidichloride. (VOY: "Threshold")

There is a similar sounding real-world compound, acetyl chloride, but it is synthetic (not appearing in nature) and is a liquid at standard temperature and pressure.