The Complete Structure of Maleic Anhydride by DRM Microwave Spectroscopy

Abstract

Abstract Double resonance modulation (DRM) microwave spectroscopy has been used for the study of rotational spectra of normal and 3,4-d2 maleic anhydride, and for the investigation of the iso­ topic species spectra due naturally occurring 13C-and 18O-species of the aforementioned two parent forms. From the experimental data the complete structures of the normal and the 3,4-d2-species were deduced independently to yield the following, averaged bond lengths (in Å) and angles (in degrees) for this planar, C2v-symmetrical molecule: C-O =1.3876 C-O-C = 108.06 C=O = 1.1962 O-C-C = 108.07 C-C = 1.4849 C-C=C = 107.90 C=C = 1.3331 O-C=0 = 122.32 C-H = 1.0791 C=C-H =129.99 The compatibility of these averaged parameters with individual values obtained for the normal and the 3,4-d2 species is better than 0.0004 Å and 0.01° for distances and angles, respectively. The internal consistency of the rS parameters is discussed, and the vibrational contributions to the change in the moments of inertia under isotopic substitution of carbon and oxygen atoms near a principal axis are deduced. The present structure data are compared with previous results by diffraction methods, and the bond lengths in maleic anhydride are discussed in relation to those in chemically similar substances.