Rare letter pairs

I have been looking at the gaps in the pairs of letters that begin Biblical Hebrew root words.  I started with the two great gutturals aleph (a) and ayin (y) here. 

In this post I will pick the first gap: the combination beth-peh (bp) and its reversal pb. These two combinations never occur together in a root. The only place the letter pair bp occurs is when the root begins with p and it is used with the preposition b (in, on, against, with, when and other glosses). Here are some examples:
בפאת in quarter Amos 3:12
בפריו for his fruit Song 8:11
בפיהם in their mouths Psalms 59:8
בפניהם in their presence Joshua 21:44
בפרש will spread Psalms 68:15
ובפרשיו and his cavaliers Exodus 14:17
בפרעה in Pharaoh Exodus 14:17
בפקודיך by your precepts Psalms 119:15
בפלגות streams Job 20:17
בפני in my face Job 16:8
בפידו in its disaster Job 30:24
בפעלם in their work Job 24:5
בפתע sudden Numbers 6:9
בפרע disheveled were Judges 5:2
בפתאם sudden 2 Chronicles 29:36

Psalms 68:15 is a curious one, the only verb I noticed in the translation - and it is clearly not quite a 'future' action but an infinitive. Notice the root prw. It has several homonyms. In this case I probably should not have dropped the preposition. Perhaps b could be acting as a temporal when (it sometimes does). One day I will revisit this psalm.

But without being sidetracked, the initial b in all these words is not part of the root and is a separate initial syllable. b-p seems to be rejected as a letter pair for Biblical Hebrew roots and words.