Experiment and theory at the convergence limit: accurate equilibrium structure of picolinic acid by gas-phase electron diffraction and coupled-cluster computations

Abstract

The accurate molecular structure of picolinic acid has been determined from experimental data and computed at the coupled cluster level of theory. Only one conformer with the O[double bond, length as m-dash]C-C-N and H-O-C[double bond, length as m-dash]O fragments in antiperiplanar (ap) positions, ap-ap, has been detected under conditions of the gas-phase electron diffraction (GED) experiment (Tnozzle = 375(3) K). The semiexperimental equilibrium structure, rsee, of this conformer has been derived from the GED data taking into account the anharmonic vibrational effects estimated from the ab initio force field. The equilibrium structures of the two lowest-energy conformers, ap-ap and ap-sp (with the synperiplanar H-O-C[double bond, length as m-dash]O fragment), have been fully optimized at the CCSD(T)_ae level of theory in conjunction with the triple-ζ basis set (cc-pwCVTZ). The quality of the optimized structures has been improved due to extrapolation to the quadruple-ζ basis set. The high accuracy of both GED determination and CCSD(T) computations has been disclosed by a correct comparison of structures having the same physical meaning. The ap-ap conformer has been found to be stabilized by the relatively strong NH-O hydrogen bond of 1.973(27) Å (GED) and predicted to be lower in energy by 16 kJ mol-1 with respect to the ap-sp conformer without a hydrogen bond. The influence of this bond on the structure of picolinic acid has been analyzed within the Natural Bond Orbital model. The possibility of the decarboxylation of picolinic acid has been considered in the GED analysis, but no significant amounts of pyridine and carbon dioxide could be detected. To reveal the structural changes reflecting the mesomeric and inductive effects due to the carboxylic substituent, the accurate structure of pyridine has been also computed at the CCSD(T)_ae level with basis sets from triple- to 5-ζ quality. The comprehensive structure computations for pyridine as well as for carbon dioxide have been used to examine the convergence with respect to the basis set size.

Accurate single crystal and gas-phase molecular structures of acenaphthene: a starting point in the search for the longest C–C bond

“…Thus for Ace our conservative expectations for the possible accuracy and standard deviations of the ae-CCSD(T)/cc-pwCVTZ level are of the order 0.005 Å and 0.5 degrees for bond lengths and angles, respectively. This seems to be compatible with conclusions in systematic investigations by Vogt, see for example 71 and references therein. Another way for the assessment of theory is the direct comparison of experimental data with respective models based on theoretical values.…”

Molecular Structure of Pyrazinamide: A Critical Assessment of Modern Gas Electron Diffraction Data from Three Laboratories

“…If the structure from the ae-CCSD­(T)/cc-pwCVTZ calculation can be taken as an approximate reference, the accuracy of the refined parameters is about 0.003 Å for bond lengths and 0.2° for angles. This is in good agreement with results of systematic investigations of Vogt et al; see for example refs – . The electronic structure of PZA is relatively simple and can be accurately calculated using single-reference coupled cluster theory, as the calculated value (0.012) of the T 1 diagnostic showed.…”

Determination of the semiexperimental equilibrium structure of 2-acetylthiophene in the presence of methyl internal rotation and substituent effects compared to thiophene

“…Additional terms taking into account the A–E splittings mentioned previously strongly perturb the structural parameters, notably the three rotational constants used for structure determination purposes. Though it is known that substitution on an aromatic ring changes the structural parameters such as bond lengths and bond angles at the substituted position, 21–25 obtaining this information in the case of 2AT is challenging as highly precise structural parameters are required, which in turn needs an appropriate Hamiltonian model to treat the methyl internal rotation.…”