Electron diffraction studies of supersonic jets. IV. Conformational cooling of n‐butane

Abstract

Expansions through small tapered nozzles (∼10−2 cm inlet diameter) have produced conformational cooling of gas phase n‐butane to estimated conformational temperatures as low as 180 K. Relaxation into the lower energy trans form was seen with neat butane and with addition of up to ∼30% helium or neon. Thin plate nozzles of comparable diameter do not seem to produce the same effects, presumably because the more rapid cooling they bring about is accompanied by many fewer collisions. Conformational analyses carefully checked for and took into account butane cluster scattering, which if present and ignored, artificially increases the apparent trans mole fraction. At higher concentrations of monatomic carrier gas the cluster scattering becomes strong enough to interfere seriously with the determination of conformational composition. Analysis of the present data and a reanalysis of earlier, conventional, gas electron diffraction data both gave the room temperature trans mole fraction as 64% (3σ=9%) in agreement with a recent spectroscopic inference of 68%.