In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: These findings show that the brain - dissociated state induced by pentobarbital is formed with the participation of the mechanisms of information perception .
### Output Text:
These findings show that the brain - dissociated state induced by <span class="chemical">pentobarbital</span> is formed with the participation of the mechanisms of information perception . 