Electron diffraction studies of supersonic jets. III. Clusters of n-butane

Expansions of n-butane from small nozzles were observed to generate large clusters, provided that enough helium or neon carrier gas accompanied the butane to remove the heat of condensation rapidly and to produce low temperatures. Although expansions of neat n-butane led to substantial conformational cooling under some circumstances, clusters were never seen without carrier gas. Both thin-plate and tapered glass nozzles yielded clusters. Diffraction patterns of the clusters are characteristic of a liquidlike structure perhaps as cold as 100 K. No evidence for inclusion of carrier gas atoms in the clusters was found. Cluster scattering signals are stronger and show less noise than those which could be expected from bulk liquid by x-ray or neutron diffraction. Their interference terms are dominated by nearest neighbor intermolecular C...H contacts. Information about the distribution of these contacts has not yet been deduced in computer simulations of liquid butane but should soon become available for comparison.