Knowledge Bank

Studies of the $PF3$-Ar system resulted in some six transitions which could not be assigned to that species. Since they appeared with either Ar and Ne carrier gases they were suspected to be a $PF3$ dimer and this possibility was mentioned at previous conferences. We have discovered that the transitions are considerably enhanced when water is added to the raregas-$PF3$ mixture. Twelve transitions have now been assigned and the rotational constants are plausible for a $PF3$-water dimer separated by $3.24\backslash AA(Rcm)$. The transitions appear as unequal doublets presumably split by tunneling of the water. The dipole moment of the complex is 2.28 D. This suggests that the water and $PF3$ dipoles make an angle of about $110\circ$ in the complex (assuming no polarization of either species). A symmetric top spectrum was also found in the $PF3-Ne-H2O$ system. Moments of inertia and mixing experiments indicate that this is the $PF3-Ne$ complex. $Rcm$ for this dimer is 3.38 {\AA}. Unlike $PF3$-Ar and $PF3$-Kr in which the rare-gas atom lies over a $PF2$ face, the neon atom lies on the $PF3$ symmetry axis. A simple van der Waals radius hard sphere model puts the neon atom over the $F3$ face. The dipole moment of the complex is about 1.0 D, similar to free $PF3$.