Microwave rotational spectrum and ab initio equilibrium structure of fumaric acid: Anharmonicity bridging the molecular characterizations

Abstract

The rotational spectrum of fumaric acid was studied in a pulsed supersonic jet expansion using Fourier-transform microwave (FT-MW) spectroscopy. The ground-state rotational constants, centrifugal distortion constants and the electric dipole moment of the molecule were determined at high accuracy. Agreement of experimental values with those predicted by ab initio methods for the s-cis,s-trans conformer, one of the three plausible conformers in the gas-phase, has unambiguously confirmed the presence of this conformer in the cold jet. The semi-experimental equilibrium rotational constants B e ( i ) derived from the experimental values B 0 ( i ) and the rovibrational corrections from the MP2/cc-pVTZ cubic force field calculation are in excellent agreement with the high-level ab initio values. The ab initio geometries of low-energy conformers s-cis,s-cis, s-cis,s-trans, and s-trans,s-trans were optimized at the CCSD(T) level of theory. It was shown that the structural and spectroscopic parameters (i.e. bond lengths, rotational constants, and vibrational frequencies) from experimental and theoretical methods are compatible when anharmonic effects are taken into account.