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

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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, 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, rSE 0→e, were derived for each theoretical method and were compared to the coupled cluster equilibrium structures, re. In addition, mass dependent rm (1) and rm (2) 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.
Funding Information
  • Deutsche Forschungsgemeinschaft (389479699/GRK2455, 405832858, GR 1344, OB 535/1-1)

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