The Microwave Spectra, Molecular Structures, And Dipole Moments Of Trans -difluoroethylene Ozonide And Methoxyethylene Ozonide.

Abstract

The rotational spectra of six isotopic species of trans-difluoroethylene ozonide (trans-3,5-difluoro-1,2,4-trioxolane, t-F$\sb2$Oz) and fifteen species of methoxyethylene ozonide (3-methoxy-1,2,4-trioxolane, MeO-Oz) were assigned. These included all of the possible species of both compounds singly deuterated on the ring, two $\sp{18}$O species of t-F$\sb2$Oz and six $\sp{18}$O species of MeO-Oz, one single $\sp{13}$C species of t-F$\sb2$Oz, and two single $\sp{13}$C species and the triply deuterated methyl group of MeO-Oz. The structures of t-F$\sb2$Oz and MeO-Oz were analyzed by both least-squares ($r\sb{\rm o}$) and Kraitchman's substitution ($r\sb{\rm s}$) methods. The preferred structures of both were taken to be the average of their $r\sb{\rm o}$ and $r\sb{\rm s}$ structural parameters with uncertainties sufficient to encompass the calculations. t-F$\sb2$Oz has an O$\sb{\rm p}$-O$\sb{\rm p}$ twist ring conformation with $C\sb2$ symmetry and diaxial fluorine substituents. The shortening of the C-O$\sb{\rm peroxy}$ bonds (1.368 A) relative to the C-O$\sb{\rm ether}$ bonds (1.401 A) is most apparent in this member of the fluoroozonide series. The dipole moment has been determined as 0.994(5) Debye. MeO-Oz has a peroxy envelope conformation and an axial methoxy substituent. The methyl group exhibits an unusual orientation anti to the ring hydrogen placing it over the ozonide ring. There is also a pronounced shortening of the C-OCH$\sb3$ bond (1.377 A) relative to C-O$\sb{\rm p}$ (1.411 A) and C-O$\sb{\rm e}$ (1.406 A). The dipole moment has been determined as 1.99(4) Debye. In both molecules the influence of the generalized anomeric effect is apparent. The short C-O$\sb{\rm p}$ and long C-F bonds in t-F$\sb2$Oz are consistent with anomeric interactions between the electron lone pairs of the peroxy oxygen atoms and the C-F $\sigma$-antibonding orbitals. The methyl group orientation and differentiation in C-O bond lengths in MeO-Oz are attributed to exo-anomeric interactions between the methoxy oxygen lone pairs and the $\sigma$-antibonding orbitals of the adjacent ring C-O bonds. These structural preferences are discussed in relation to other ozonide structures and carbohydrate-pyranose systems.