IPA: /ˌmɒnəʊməˈkjʊlərli/
KK: /mɑnəˈmoʊkjələrli/
In a way that relates to a single molecule or involves only one molecule.
The reaction occurred monomolecularly, meaning it involved only one type of molecule.
Monomolecularly → It is formed from "mono-" (meaning one) and "molecular" (from Latin "molecula", meaning a small mass or particle) and the suffix "-ly" (meaning in a manner). The word describes something that occurs in a manner involving one molecule.
Think of 'mono-' meaning one and 'molecular' referring to small particles, so 'monomolecularly' means in a way that involves just one small particle.