8321
The time scale tau ( c ) and the activation energy of PF ( 6 ) ( - ) rotation in this RTIL are found to be comparable with those observed in ordinary alkali and ammonium salts despite the large counterion size and low melting point of the former .
 Chemical: PF ( 6 ) ( - ); Chemical: ammonium salts
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
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Answer:
 Chemical: PF ( 6 ) ( - ); Chemical: ammonium salts