The shape of the ring in trans-1,3-dibromocyclobutane

Abstract IR spectra of the title compound have been investigated as the vapour, in dilute solution, in various solid phases in a cryostat, and isolated in inert gas matrices. Raman spectra have been investigated using a Miller-Harney cell in the liquid and various solid phases. Evidence based on selective weakening of various peaks and, in the case of the planar ring, mutual exclusion will be presented for the following interpretation of the shape of the ring: vapour and liquid (puckered), crystal I (somewhat flattened), crystal II (planar), and crystal III (puckered). Support for the planar form from the ab initio calculations was provided by noting that, for five of the bands lying between 150 and 600 cm −1 , the calculated changes and directions of these changes on going from puckered to planar agreed well with those found experimentally.