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: After 180 mg of diazepam i . v . he remained tremulous with muscle rigidity and clenched jaws .
### Output Text:
After 180 mg of <span class="chemical">diazepam</span> i . v . he remained tremulous with muscle rigidity and clenched jaws . 