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.

Introduction

Fumaric and maleic acid, being the trans and cis isomers of butenedioic acid, respectively, are the simplest dicarboxylic acids with a CC double bond. Besides being a prototype dicarboxylic acid, fumaric acid is of practical interest due to its applications in medicine (e.g. as anti-tumoral agent, an inhibitor of transaminase reactions) and in polymer chemistry [1], [2]. The accurate determination of its molecular constants is important for a fundamental understanding of its chemical properties.

Previously, the molecular structure of fumaric acid was studied twice by gas-phase electron diffraction (GED). In the earlier study at 533 K [3], the structural analysis was carried out, assuming a single conformer with both CO bonds in antiperiplanar (ap) positions relative to the CC bond (s-trans arrangements, see conformer III in Fig. 1). However, the recent GED reanalysis [4] has shown that, in the gas-phase at 480 K, fumaric acid constitutes a mixture of three conformers in approximately equal amounts. These conformers exhibit either two s-cis (I), or two s-trans (III), or one s-cis and one s-trans (II) OCCC arrangements (see Fig. 1). The same conformational composition of fumaric acid was revealed in the analysis of infrared (IR) vibrational spectra observed in an Ar matrix and assigned with use of scaled harmonic frequencies from the B3LYP/6-31G(d,p) calculations [5]. Recent MP2/cc-pVTZ calculations [4], predict six stable conformations for fumaric acid. The low-energy conformers have both OCOH fragments in synperiplanar (sp) positions, while the high-energy conformers have one or two OCOH groups in ap positions. The relative energies of the three low-energy conformers I–III were calculated to be 0 kJ mol⁻¹, 1.5 kJ mol⁻¹ and 2.7 kJ mol⁻¹ (MP2/cc-pVQZ), respectively [4]. According to this ab initio prediction, the conformers exist in a ratio of I:II:III = 33:48:19 at 480 K, whereas the high-energy conformers are practically absent at this temperature. Since the conformers I and III, both belonging to the point group C_2h, have no permanent dipole moments, only the most abundant conformer II (point group C_s) exhibits a rotational spectrum and can thus be detected by microwave (MW) spectroscopy.

The aim of the present work is a high-resolution study of the vibrational ground-state rotational spectra of fumaric acid by supersonic jet expansion Fourier-transform (FT) MW spectroscopy while employing high-level ab initio calculations to deduce semi-experimental equilibrium constants.