( ( In ( a ( unimolecular reaction ) ) ) ( , ( ( a ( single molecule ) ) ( ( rearranges ( atoms ( forming ( different molecules ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (DT a) (NN unimolecular) (NN reaction))) (, ,) (NP (DT a) (JJ single) (NN molecule)) (VP (VBZ rearranges) (NP (NP (NNS atoms)) (VP (VBG forming) (NP (JJ different) (NNS molecules))))) (. .)))
In a unimolecular reaction, a single molecule rearranges atoms forming different molecules.
(ROOT (S (ADVP (RB Replace)) (NP (NP (DT another)) (PP (IN in) (NP (DT a) (NN molecule)))) (VP (VBZ happens) (PP (TO to) (NP (NNS atoms))) (PP (IN during) (NP (DT a) (NN substitution) (NN reaction)))) (. .)))
Replace another in a molecule happens to atoms during a substitution reaction.