Rotational spectra and structures of small clusters: the Ar4-H/DF pentamers

The 40Ar4–HF and –DF clusters have been identified and characterized by their microwave rotational spectra using a Flygare–Balle FT spectrometer with a pulsed supersonic nozzle to generate and detect the clusters. The observed J=2→3 to 6→7 transitions have fine structure limited to K=0, ±3 and ±6, indicative of symmetric tops with a threefold axis of symmetry produced by I=0 nuclei. For Ar4–HF the rotational constants B0, DJ, and DJK are 623.4539(1) MHz, 1.062(1) kHz, and 0.611(31) kHz, respectively; and for Ar4–DF, 618.5846(1) MHz, 1.030(1) kHz, and 0.665(4) kHz. The results show that the clusters consist of the trigonal Ar3–H/DF tetramer reported earlier, with a fourth argon on the back side of the Ar3 group to form a tetrahedral or near tetrahedral Ar4. In both clusters the H/DF lies along the threefold axis of the Ar3 group, with the H/D end pointed at its face. If one assumes an Ar–Ar distance of 3.85 Å for both clusters, the Ar3 plane to HF c.m. distance for Ar4–HF with a tetrahedral Ar4 is 2.713 Å compared to 2.735 Å for Ar3–HF. The nuclear hyperfine structure of the rotational transitions shows that the H/DF experiences large amplitude bending vibrations with respect to the C3 symmetry axis. The average amplitude for HF (40.4°) is comparable with that reported for the smaller Arm–HF complexes (m=1, 2, 3). However, for DF the apparent 25.1 (1.6)° amplitude found is 10° less than that in the smaller complexes. The difference is attributed to an increased electric field gradient at the D in the pentamer. A possibly related feature is an Ar4 c.m. to F distance which is 0.013 Å longer for Ar4–DF than for Ar4–HF.