besides vs if not

besides

adv
  • Used to emphasize an additional point, especially an important or stronger reason; moreover; furthermore. 

  • Otherwise; else. 

  • Also; in addition. 

prep
  • In addition to. 

  • Other than; except for; instead of. 

  • Beside, next to. 

if not

adv
  • Used to link a semantically weaker word to a semantically stronger word following the adverb, both sharing the same basic meaning, indicating the increased likelihood of the former. 

How often have the words besides and if not occurred in a corpus of books? (source: Google Ngram Viewer )