Adverbs of frequency include words like “always,” “usually,” “sometimes,” “often,” “never,” etc. They are often misplaced with respect to the verbs that they modify. These adverbs come before the main verb:

• “The enzyme cleaves usually the bond when…” WRONG
• “The enzyme usually cleaves the bond when…” RIGHT

However, if the verb contains a derivative of to be, the adverb comes after the to be (and before the main verb):

• “The sulfonyl usually is deprotonated…” WRONG
• “The sulfonyl is usually deprotonated…” RIGHT