▁flu or ide ▁in ▁a ▁solid ▁arg on ▁matrix . ▁H Ar Ar F ▁would ▁have ▁such ▁a ▁low ▁bar rier ▁to ▁decomposition ▁that ▁it ▁will ▁likely ▁never ▁be ▁observed . ▁However ▁H Be Ar F ▁is ▁predicted ▁to ▁be ▁more ▁stable ▁than ▁H Ar F . ▁ ▁U ran ium ▁comp ounds ▁C U O ▁in ▁a ▁solid ▁arg on ▁matrix ▁can ▁bind ▁one , ▁or ▁a ▁few ▁arg on ▁atoms ▁to ▁yield ▁C U O · Ar , ▁C U O · Ar 3 ▁or ▁C U O · Ar 4 . ▁C U O ▁itself ▁is ▁made ▁by ▁ev apor ating ▁u ran ium ▁atoms ▁into ▁carbon ▁mon ox ide . ▁U ran ium ▁acts ▁as ▁a ▁strong ▁Lewis ▁acid ▁in ▁C U O ▁and ▁forms ▁b onds ▁with ▁energ ies ▁of ▁about ▁ 3 . 2   k cal / m ol ▁( 1 3 . 4   k J / m ol ) ▁with ▁arg on . ▁The ▁arg on ▁acts ▁as ▁a ▁Lewis ▁base . ▁Its ▁electron ▁density ▁is ▁inserted ▁into ▁an ▁empty ▁ 6 d ▁orb ital ▁on ▁the ▁u ran ium ▁atom . ▁The ▁spectrum ▁of ▁C U O ▁is ▁changed ▁by ▁arg on ▁so ▁that ▁the ▁U − O ▁stretch ▁frequency ▁changes ▁from ▁ 8 7 2 . 2 ▁to ▁ 8 0 4 . 3   cm − 1 ▁and ▁the ▁U − C ▁stretch ▁frequency ▁from ▁ 1 0 4 7 . 3 ▁to ▁ 8 5 2 . 5   cm