Accurate Structure\nand Spectroscopic Properties of\nAzulene and Its Derivatives by Means of Pisa Composite Schemes and\nVibrational Perturbation Theory to Second Order

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