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Comprehensive studies of the molecular and electronic structures, vibrational frequencies, and infrared and Raman intensities of the aniline radical cation, C 6 H 5 NH 2 ϩ have been performed by using the unrestricted density functional ͑UB3LYP͒ and second-order Møller-Plesset ͑UMP2͒ methods with the extended 6-311ϩϩG͑df,pd͒ 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 ͑NBO͒ analysis has revealed that the p-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 NH 2 group in electrophilic substitutions͒, 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/ 6-311ϩϩG͑df,pd͒ 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 Ϫ1 , 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 ͑ED͒ on the antibonding orbitals (NH *) and decrease of ED on CH * , 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.

ABSTRACT:A comparison of eight density functional models for predicting the molecular structures, vibrational frequencies, infrared intensities, and Raman scattering activities of platinum(II) antitumor drugs, cisplatin and carboplatin, is reported. Methods examined include the pure density functional protocols (G96LYP, G96PW91, modified mPWPW and original PW91PW91), one-parameter hybrid approaches (mPW1PW and mPW1LYP), and three-parameter hybrid models (B3LYP and B3PW91), as well as the HF and MP2 levels of theory. Different effective core potentials (ECPs) and several basis sets are considered. The theoretical results are discussed and compared with the experimental data. It is remarkable that the mPW1PW protocol introduced by Adamo and Barone [J Chem Phys 1998, 108, 664], is clearly superior to all the remaining density functional methods (including B3LYP). The geometry and vibrational frequencies of cisplatin and carboplatin calculated with the mPW1PW method, and the ECP of Hay and Wadt (LanL2DZ basis set) are in better agreement with experiment than those obtained with the MP2 method. The use of more elaborated ECP and the enlargements of basis sets do not significantly improve the results. A clear-cut assignments of the platinum-ligand vibrations in cisplatin and carboplatin are presented. It is concluded that mPW1PW is the new reliable method, which can be used in predicting molecular structures and vibrational spectra of large coordination compounds containing platinum(II).

This work deals with a theoretical study of the acidity and basicity of the amino-oxo, amino-hydroxy, and imino-oxo tautomers (including their rotamers) along with their interaction with one water molecule. The calculations are carried out using the DFT/B3LYP functional combined with the 6-31++G(d,p) or 6-311++G-(d,p) basis sets. The proton affinities (PA) of the O and N atoms and the deprotonation enthalpies (DPE) of the OH and NH bonds of the cytosine tautomers are calculated as well. The results suggest that the aminooxo tautomer may be the most stable form in the gas phase. The optimized geometries, binding energies, and harmonic vibrational frequencies of the cyclic structures of the monohydrated cytosine tautomers are calculated. Complex formation results in a moderate change of the pyramidal character of the amino group. For the cyclic CdO‚‚‚HO‚‚‚HN structures, the binding energies depend on the PA and DPE of the sites involved in the interaction. The perturbations of selected vibrational modes such as the stretching vibrations and the inversion mode of the amino group along with the blue shift of the NH stretching vibration in the imino-oxo complexes are discussed. The natural bond orbital analysis shows that there is an increase of the occupancy of the σ* antibonding orbitals of the proton donor groups involved in the interaction.

The molecular structures of p-chlorophenol and p-bromophenol have been calculated with the MP2, DFT-(hybrid), and HF methods using the extended 6-311++G(df, pd) basis set. The geometrical parameters of p-ClPh and pBrPh in the gas phase have not been reported as yet. The results show that substitution of phenol with σ-electron-withdrawing groups (Br and Cl) leads to small shortening of the C-C and C-O bonds and small changes in the CCC angles. The structural changes of the phenol ring are governed mainly by the electronegativity of the para-substituent and, to a lesser extend, by resonance factors. The FT-IR spectra of p-ClPh, p-BrPh, and their OD counterparts were measured in CCl 4 and cyclohexane solutions in the frequency range of 3700-400 cm -1 , and the integrated infrared intensities were determined. The theoretical harmonic frequencies and infrared intensities were calculated for all the molecules using the DFT and HF methods. The best overall agreement between the calculated and experimental spectra has been obtained at the B3LYP/ 6-311++G(df,pd) level. A clear-cut vibrational assignment is made on the basis of the calculated potential energy distribution (PED). The effect of the p-Cl and p-Br substituents upon the characteristic phenolic frequencies and infrared intensities is discussed; in particular, it is shown that both the OH and OD torsional frequencies are related to the nature of the substituent.

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The infrared spectra of phenol and phenol-OD are thoroughly reinvestigated, to resolve the contradictory assignment of some vibrations. The harmonic frequencies, integrated IR intensities, and potential energy distribution (PED) have been calculated by the B3LYP method with the 6-311++G(df,pd) basis set. The Fourier transform infrared (FT-IR) spectra of phenol and phenol-OD have been measured in carbon tetrachloride and cyclohexane solutions, in the frequency range 3700-400 cm -1 , and the experimental integrated infrared intensities are reported. On the basis of the results obtained, the detailed assignment of all the fundamental modes of Ph-OH and Ph-OD are presented. The study demonstrates that density functional B3LYP is clearly superior to the ab initio Hartree-Fock (HF) and second-order Möller-Plesset (MP2) methods in reliable prediction of the vibrational spectra of phenol. In particular, it is shown that scaling of the B3LYP-calculated frequencies of the CH and OH(OD) stretching vibrations by the scaling factor, derived by Baker et al. [J. Phys. Chem. A 1998, 102, 1412 gives excellent agreement between theoretical and experimental frequencies of these vibrations. Detailed theoretical investigations are performed for these troublesome normal modes in phenol and benzene, which show the largest deviations between the MP2-predicted frequencies and the experimental ones. It has been demonstrated that these modes have almost identical atomic displacements and potential energy distributions in both the molecules. The electron correlation effects and basis set dependences are examined, and the nature of these problematical vibrations in aromatic molecules is discussed.

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