General Synonyms:
* Attribute
* Ascribe
* Assign
* Charge
* Credit
* Lay (as in "lay the blame on")
* Place
* Put down to
Synonyms with a negative connotation:
* Accuse
* Blame
* Charge (especially when referring to a crime)
* Indict
* Pin (as in "pin the blame on")
Synonyms with a neutral connotation:
* Connect
* Link
* Relate
* Associate
* Contribute
The best synonym for "impute" will depend on the specific context in which you are using it.