Accurate Determination of Equilibrium Structure of 3-Aminophthalonitrile by Gas Electron Diffraction and Coupled-Cluster Computations: Structural Effects Due to Intramolecular Charge Transfer.

ABSTRACT

For the first time, the molecular structure of 3-aminophthalonitrile with unique electronic properties has been determined by the gas electron diffraction (GED) method supported by a mass spectrometric analysis of the gas phase. Moreover, it has been optimized at the high-level quantum-chemical coupled-cluster theory, CCSD(T), in conjunction with the triple-ζ basis set. The equilibrium structure has been determined from the GED data taking into account harmonic and anharmonic vibrational corrections estimated from the quantum-chemical force field (up to cubic terms). The computed CCSD(T) structure has been corrected for the core-core and core-valence electron-correlation effects estimated at the MP2 level and extrapolated to the basis set of quadruple-ζ quality. A remarkable agreement between the experimental and theoretical equilibrium structural parameters (bond lengths and angles) points to a high accuracy of both the molecular structure and applied theories. The high accuracy of structure computations and experimental determination allows the observation of structural changes due to the intramolecular charge transfer predicted by natural bond orbital (NBO) calculations. According to the NBO analysis, the amino group is the electron-donating substituent, whereas the nitrile groups are able to withdraw the π electrons from the benzene ring. Noticeable variations in the structural parameters are also explained by the interaction of σ → π* orbitals of the nearest C≡N and N-H bonds.