I know that some verbs take interrogative clauses ("I know where they are") while others don't (* "I believe where they are"). The verb "sure" is kind of like "believe", but unlike "believe" it seems to allow interrogatives under some circumstances, such as under negation:
*I am sure whether it's raining.
I am not sure whether it's raining.
To be clear, this is about interrogative clauses in general, not just 'whether' ones. Consider also:
I am not sure why the floor is wet.
*I am sure why the floor is wet.
I am not sure why this is so. Are there any explanations of this?