(IUCr) The Low-temperature And High-pressure Crystal Structures Of ...

1. Introduction

In the mono-alcohols (ROH) there is competition between the packing requirements of the relatively bulky R group and the demand for the small hydroxyl groups to be sufficiently close for hydrogen bonding to occur. Brock & Duncan (1994) have described the general features of the packing motifs adopted by mono-alcohols. They found that if the molecules containing the hydroxyl groups are relatively `thin' (by Brock and Duncan's terminology) then they can form catemers where the molecules are symmetry-related by either a glide plane or a 21-screw axis so that the molecules form an approximately coplanar alternating sequence about the central chain of hydrogen bonds. For bulkier R groups, steric hindrance often prohibits the molecules adopting this simple arrangement and, instead, these systems often form chains about three-, four- or sixfold screw axes, or adopt crystal structures with more than one molecule in the asymmetric unit. If the R group is particularly bulky, then the molecules may no longer form hydrogen-bonded chains or catemers, but cyclic dimer, trimer, tetramer or hexamer rings can be created.

In our recent high-pressure structural studies of phenol (Allan et al., 2002) and its halogenated derivatives 2-chloro­phenol and 4-fluorophenol (Oswald et al., 2005), we have observed a clear change in the nature of the R-group packing behaviour. All three systems form crystal structures at ambient pressure, characterized by the formation of hydrogen-bonding schemes associated with bulky R groups. Both phenol and 2-chlorophenol form crystal structures where the molecules are hydrogen bonded into pseudo-threefold chains. The ambient-pressure structure of 4-fluorophenol has a markedly different packing arrangement with the molecules hydrogen bonding to form hexamer rings about threefold rotoinversion sites. After the application of pressure, however, all three systems form crystal structures with the molecules disposed along chains that are generated by 21 screw axes. In effect, pressure has transformed the packing behaviour of the phenyl and halophenyl groups from having characteristics more closely associated with bulky groups to those more typical of small groups.

Here we report the crystal structure of the high-pressure phase of cyclobutanol (C4H7OH) along with the crystal structure of its low-temperature phase, which, to the best of our knowledge, has not been reported previously. The low-temperature crystal structure (space group Aba2, Z′  =  2) is composed of pseudo-threefold hydrogen-bonded molecular catemers which lie parallel to the crystallographic a-axis. At high-pressure, the crystal symmetry changes to Pna21 (Z′  =  1) and the molecular catemers, which are generated by the a-glide symmetry, adopt a pseudo-twofold arrangement. This structural behaviour parallels what we have observed previously for phenol and 2-chlorophenol and is in agreement with the favouring of a small-group packing behaviour under pressure.