( ( A ( HYDROGEN BOND ) ) ( ( is ( ( the attraction ) ( of ( ( a ( hydrogen atom ) ) ( of ( ( one molecule ) ( to ( ( an atom ) ( of ( another molecule ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NNP HYDROGEN) (NN BOND)) (VP (VBZ is) (NP (NP (DT the) (NN attraction)) (PP (IN of) (NP (NP (DT a) (NN hydrogen) (NN atom)) (PP (IN of) (NP (NP (CD one) (NN molecule)) (PP (TO to) (NP (NP (DT an) (NN atom)) (PP (IN of) (NP (DT another) (NN molecule))))))))))) (. .)))
A HYDROGEN BOND is the attraction of a hydrogen atom of one molecule to an atom of another molecule.
(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.