Tautomer identification troubles: The molecular structure of itaconic and citraconic anhydride revealed by rotational spectroscopy

Microwave spectra of citraconic anhydride and its tautomer itaconic anhydride have been recorded in a frequency range of 6 – 18 GHz. Both a- and b-type transitions were observed for both tautomers, while c-type transitions could only be observed for the E torsional symmetry state of citraconic anhydride. For both molecules, a molecular substitution structure, rS, was obtained by measurements of mono-substituted 13C isotopologues in natural abundance. For citraconic anhydride, 18O isotopologues were also observed and the V3 barrier to internal rotation has been determined at 326.5153(61) cm−1. In addition to the microwave spectra, a gas-phase study of isomerisation between the tautomers was carried out, which was assisted by theoretical transition state calculations, employing a variety of different density functionals as well as the wavefunction based Møller-Plesset perturbation theory, MP2, and coupled cluster methods, CCSD(T)-F12c and DCSD-F12b. These were also used to benchmark the experimentally determined rS structures and V3 barrier of rotation in citraconic anhydride. Via theoretical ground state vibrational calculations, semi-experimental equilibrium structure, rSE0→e, were derived for each theoretical method and were compared to the coupled cluster equilibrium structures, re. In addition, mass dependent r(1) m and r(2)m fits were conducted to obtain approximate re structures. Using the determined structures we can revise a previous study that misidentified citraconic anhydride as itaconic anhydride.