Accurate Structure and Spectroscopic Properties of Azulene and Its Derivatives by Means of Pisa Composite Schemes and Vibrational Perturbation Theory to Second Order

The structural and spectroscopic properties in the gas phase of azulene and some of its N-bearing derivatives have been analyzed by a general computational strategy based on the recent Pisa composite schemes (PCSs). First of all, an accurate semiexperimental equilibrium structure has been derived for azulene and employed to validate the geometrical parameters delivered by different quantum chemical methods. Next, different isomerization energies (azulene to naphthalene, 1-aza-azulene to quinoline and to other isomers) have been computed by an explicitly correlated PCS version employing frozen natural orbitals. Accurate geometries have been obtained by a cheaper PCS variant based on a double-hybrid functional improved by one-parameter bond corrections, with the same functional providing also remarkable harmonic frequencies. The corresponding equilibrium rotational constants show average deviations within 0.1% from experimental results when taking into account anharmonic vibrational corrections obtained by a global hybrid functional. Therefore, reliable computational estimates have been produced for the rotational constants of several nitrogen derivatives (isomeric aza-azulenes and guaiazulene), whose non-negligible dipole moments could allow experimental microwave characterizations. An analogous approach delivers infrared spectra in remarkable agreement with their experimental counterparts for naphthalene, quinoline, and azulene, together with reliable predictions for the still-unknown spectrum of 1-aza-azulene. In addition to their intrinsic interest, the results of this paper further confirm that a very accurate yet robust and user-friendly tool is now available for aiding high-resolution spectroscopic studies of quite large systems of current technological and/or biological interest.