Does divorce occur when a husband imagines divorcing his wife and utters it during an argument with her?

Divorce does not occur unless the husband intends to utter it, either with an explicit phrase or an implicit one coupled with the intention of effecting it. What you mentioned indicates no intention to utter the divorce, but rather it slipped from your tongue in a manner resembling delirium, and therefore, no divorce occurs through it. Even if we were to assume you intended to utter it and it was not delirium, your statement: "at that time, I will tell her 'you are divorced'" is a promise of divorce, and a promise of divorce does not obligate anything. So, in any case, divorce has not occurred based on what you mentioned.