Electronic structures, vibrational spectra, and revised assignment of aniline and its radical cation: Theoretical study

Wojciechowski, Piotr M.; Zierkiewicz, Wiktor; Michalska, Danuta; Hobza, Pavel

doi:doi:10.1063/1.1574788

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Near-infrared spectroscopy of H3+ above the barrier to linearity

The first H3+ transitions above the barrier to linearity have been observed in absorption in the near infrared using a highly sensitive dual-beam, double-modulation technique with bidirectional optical multipassing. A total of 22 rovibrational transitions of H3+ have been detected and assigned to the fourth and fifth overtone and combination bands (5ν21, 5ν25, 2ν1+2ν22, 3ν1+ν21, ν1+4ν22, 2ν1+3ν21, and 6ν22). These transitions, which are more than 4600 times weaker than the fundamental band, prob...

Electronic structure of the π-bonded Al–C₂H₄ complex: Characterization of the ground and low-lying excited states

The equilibrium properties of the π-bonded Al–ethylene complex in its ground state are calculated by coupled-cluster theory. Significant changes in the geometry of the ethylene molecule upon complexation (elongation of the CC bond, pyramidalization of the CH2 groups) are consistent with the formation of a chemical bond between fragments. The overall interaction is rather weak because bonding is derived from the overlap between: (i) a singly occupied p orbital of Al and the antibonding π∗ orbital...

Comprehensive studies of the molecular and electronic structures, vibrational frequencies, and infrared and Raman intensities of the aniline radical cation, C₆H₅NH2+ have been performed by using the unrestricted density functional (UB3LYP) and second-order Møller–Plesset (UMP2) methods with the extended 6-311++G(df,pd) basis set. For comparison, analogous calculations were carried out for the closed-shell neutral aniline. The studies provided detailed insight into the bonding changes that take place in aniline upon ionization. The natural bond orbital (NBO) analysis has revealed that the pπ-radical conjugative interactions are of prime importance in stabilizing the planar, quinoid-type structure of the aniline radical cation. It is shown that the natural charges calculated for aniline are consistent with the chemical properties of this molecule (an ortho- and para-directing power of the NH₂ group in electrophilic substitutions), whereas Mulliken charges are not reliable. The theoretical vibrational frequencies of aniline, calculated by the B3LYP method, show excellent agreement with the available experimental data. In contrast, the MP2 method is deficient in predicting the frequencies of several modes in aniline, despite the use of the extended basis set in calculations. The frequencies of aniline radical cation, calculated at the UB3LYP/6-311++G(df,pd) level, are in very good agreement with the recently reported experimental data from zero kinetic energy photoelectron and infrared depletion spectroscopic studies. The clear- cut assignment of the IR and Raman spectra of the investigated molecules has been made on the basis of the calculated potential energy distributions. Several bands in the spectra have been reassigned. It is shown that ionization of aniline can be easily identified by the appearance of the very strong band at about 1490 cm⁻¹, in the Raman spectrum. The redshift of the N–H stretching frequencies and the blueshift of the C–H stretching frequencies are observed in aniline, upon ionization. As revealed by NBO analysis, the frequency shifts can be correlated with the increase of electron density (ED) on the antibonding orbitals (σNH∗) and decrease of ED on σCH∗, respectively. These effects are associated with a weakening of N–H bonds and strengthening of C–H bonds in the aniline radical cation. The simulated theoretical Raman and infrared spectra of aniline and its radical cation, reported in this work, can be used in further spectroscopic studies of their van der Waals clusters and hydrogen bonded complexes. © 2003 American Institute of Physics.