ROTATIONAL SPECTRA OF A SERIES OF POLYCYCLIC AROMATIC HYDROCARBONS

A theoretical study of the rotational spectra of a number of polycyclic aromatic hydrocarbons (PAHs) was carried out on the basis of quantum-chemical calculations by PRIRODA program in the PBE/3ζ approximation. Calculations are given for neutral molecules, their radical anions and radical cations in the rigid top approximation. Twelve neutral PAHs under study have no dipole moment and are not of interest for rotational spectroscopy. Thirteen neutral PAHs have a dipole moment not exceeding 0.09 Debye and, under certain conditions, can be studied in the microwave region. The remaining six compounds are PAHs with methyl and phenyl substituents, their dipole moments are 0.32-0.65 Debye, which makes it possible to study their microwave spectrum. For radical ions, the situation with the dipole moment is as follows: if a neutral molecule does not have a dipole moment, then the corresponding radical ion does not have it either; if the dipole moment of a neutral molecule is nonzero, then for radical ions it increases at least several times. For example, in the benzo [b] chrysene radical anion, the μa-component component of the dipole moment increases by 250 times, the μb-component by 7 times, and the total dipole moment by about 100 times. This effect is more pronounced for radical anions than for radical cations. In this case, it becomes possible to detect the spectrum of if not a neutral molecule, then at least one of its charge states. For a number of compounds, the patterns of various spectroscopic parameters depending on the number of carbon atoms in PAHs were found. In the B3LYP/6-31G(d,p) approximation, quartic centrifugal distortion constants were calculated for five compounds, and their effect on the rotational spectrum was estimated: for many compounds under study, they can have a significant effect on the microwave spectrum. With an increase in PAHs, the centrifugal constants decrease indicating an increase in the rigidity of the molecules. Thus, substituted neutral PAHs, as well as a number of radical ions, may be of interest for experimental studies in laboratory conditions and in space.