The molecular structure of gaseous 1,4-dioxane: An electron-diffraction reinvestigation aided by theoretical calculations

The structure of the molecule 1,4-dioxane (DIOX) has some features in common with other ring systems previously studied in this laboratory. In contrast to 1,4-cyclohexanedione, however, which consists both of a twisted boat form of D2 symmetry and a chair form of C2h symmetry, DIOX was reported, in two much earlier studies, to exist only as the chair form. The results of our work are in agreement with the earlier conclusions that gaseous DIOX exists either entirely, or essentially entirely (less than a few percent) in the chair form. Our work is much more extensive than the previous studies, and, aided by high-level theoretical molecular orbital- and normal-coordinate calculations, yielded the following bond distances (rg/Å) and bond angles (∠α/deg). 〈C–H〉 = 1.104 (4), C–O = 1.420 (2), C–C = 1.514 (4), 〈C–C–H〉 = 105.4 (55), H–C–H = 108.0 (26), C–C–O = 111.1 (3), C–O–C = 110.9 (10). The “flap” angle — the angle by which the COC plane is tilted up from the plane of the four carbon atoms — is equal to 50.6 (7)°. The structure is discussed and compared with the previous work and with predictions from theory.