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

Abstract

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, r_S, 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 r_S structures and V₃ barrier of rotation in citraconic anhydride. Via theoretical ground state vibrational calculations, semi-experimental equilibrium structure, r^SE_0→e, were derived for each theoretical method and were compared to the coupled cluster equilibrium structures, r_e. In addition, mass dependent r_m⁽¹⁾ and r_m⁽²⁾ fits were conducted to obtain approximate r_e structures. Using the determined structures we can revise a previous study that misidentified citraconic anhydride as itaconic anhydride.