We normally use 'that' with negative forms of 'doubt':
[FONT="]"With some verbs expressing in themselves lack of certainty it appears that THAT and
IF can both be used, with little
practical difference in meaning between them:[/FONT]
[FONT="]
1. I doubt if he was there.[/FONT][FONT="]
2.I doubt that he was there.[/FONT]
[FONT="]In [1] the uncertainty about his being there may be stronger than in [2]; only the context of the utterance can make this clear. The difference is not important for practical purposes.[/FONT]
[FONT="]With
absence of doubt we return
to certainty, and THAT is now the only appropriate choice:[/FONT]
[FONT="] 3. I don't doubt that he succeeded."[/FONT]
http://www.gramorak.com/Articles/If.pdf