Conformational Study of the Structure of dibenzo-18-crown-6. Comparison with 18-crown-6.

We report for the first time the conformational analysis of dibenzo-18-crown-6, db18c6. The conformational search was carried out using the CONFLEX conformational search method of cyclic molecules. Energies were calculated for the low-lying predicted conformations at different levels of theory up to the G3MP2 level. At the G3MP2 level, the predicted ground state (GS) conformation was more stable than the experimental conformation by only 1.60kcal/mol. Strong similarity was found between the GS structure and experimental conformations of db18c6 and 18-crown-6, 18c6. The GS and experimental conformations of db18c6 are non-planar. This allows db18c6 to exist in optically active enantiomers. Similar to 18c6, it was concluded that the db18c6 structure is stabilized by intramolecular hydrogen bond. We also performed the computations for the water and chloroform solution phase, where the same conformation was predicted as the GS conformation.

Vibrational analysis of dibenzo-18-crown-6. Effect of dispersion correction on the calculated vibrational spectra.

We report for the first time a detailed vibrational analysis of dibenzo-18-crown-6, db18c6. The experimental IR and Raman spectra of db18c6 were measured. The assignment of the fundamental vibrational frequencies of db18c6 was aided by using scaled quantum mechanical force fields calculated at the B3LYP/6-311G** and CAM-B3LYP/6-311G** levels. Comparison between the experimental and calculated spectra of some of the important conformations of db18c6 led to the conclusion that db18c6 in the solid phase exists in a C2 conformation that is similar to that predicted by X-ray, for also the solid phase. The effect of inclusion of the atom pair-wise dispersion correction to the B3LYP method, known as the B3LYP-D3 method, on the calculated IR and Raman spectra of db18c6 at the B3LYP level was also investigated. It was concluded that the effect of inclusion of the dispersion correction on the calculated vibrational frequencies and intensities is negligible.

Conformational analysis of 9-crown-3, 9-thiacrown-3 and 9-azacrown-3.

Crown ethers are an important class of molecules with wide applications. Crown ethers are large ring flexible molecules with a large number of possible conformations. In the current manuscript, we report the conformational analysis results of one of the smallest crown ether systems, namely, 9-crown-3, 9c3, 9-thiacrown-3, 9t3, and 9-azacrown-3, 9a3. The conformational search is performed using the CONFLEX conformational search method utilizing the MMFF94s force field. 8, 11 and 62 conformations were predicted for 9c3, 9t3 and 9a3, respectively. The ab initio computations were performed at B3LYP and MP2 levels using the 6-311G** basis set. For the accurate prediction of the ground state conformation and the energy order of the low-lying energy conformations, the computations were performed at the G4 level for some selected conformations. Factors affecting the stability of different conformations of 9c3, 9t3 and 9a3 are discussed. These factors are also discussed with respect to larger crown ether systems. The ab initio computations were performed also for water and chloroform as solution phases. The same ground state predicted in the gas phase was also predicted in the solution water and chloroform phases.

Conformational study of the structure of 18-thiacrown-6.

Conformational analysis was performed for 18-thiacrown-6 (18t6) using the CONFLEX method and the MMFF94s force field. Computations were performed for some of the low energy conformations at the HF, B3LYP, CAM-B3LYP, M06, M06L, M062x, M06HF and MP2 levels. The computations were also performed using the DFT-D3 method along with the TPSS and PBE functionals. The study predicted a new C2 conformation as the ground state conformation of 18t6. This new C2 conformation is more stable than the experimentally known solid state conformation by 4.7 kcal/mol at the MP2/6-311G** level. This conformation has all of the SCCS dihedral angles adopt exodentate structure. However, the experimentally known conformation of the solid phase has two of the SCCS dihedral angles violating this exodentate rule. It was concluded that for 18t6 stability a linear dihedral SCCS angle requirement is more important than a gauche CSCC dihedral angle requirement.

Experimental and theoretical study of the vibrational spectra of 12-thiacrown-4 and 18-thiacrown-6, evaluation of the performance of the different anharmonic force fields compared to the scaled quantum mechanical force fields.

We report, to the best of our knowledge, for the first time the vibrational, IR and Raman, spectra of 12-thiacrown-4 (12t4) and 18-thiacrown-6 (18t6). To predict in what conformation 12t4 and 18t6 exist, for the vibrational analysis of both molecules and to assess the performance of the different computational methods for the accurate prediction of the vibrational frequencies of relatively large molecules, the computations were done using the harmonic and anharmonic force fields using the 6-31G* and 6-311G** basis sets. The computations were performed at the HF, B3LYP, CAM-B3LYP, BLYP, BP86, G96LYP, PBE1PBE, TPSSH and MP2 levels. Comparison was made between the calculated and experimental vibrational frequencies as indicated by the root-mean-square (rms) deviations, using either the unscaled and scaled harmonic vibrational frequencies and, unscaled, anharmonic vibrational frequencies. For the harmonic vibrational frequencies two scaling schemes were used. One uses one-scale-factor (1SF) scaling and the other uses 8SF scaling. In terms of the vibrational analysis of 12t4 and 18t6, the report confirms the solid state X-ray structure of D4 of 12t4 and C2 of 18t6. It is concluded that a lower rms deviation is obtained using 1SF scaled harmonic vibrational frequencies at even the HF/6-31G* level than using anharmonic vibrational frequencies at the MP2/6-311G** level. The CAM-B3LYP method showed some improvement over the traditional B3LYP method.

Electronic structure and properties of neutral, anionic and cationic silicon-nitrogen nanoclusters.

We performed a G3 investigation of the possible stable structures of silicon-nitrogen SinNm clusters where m = 1-4, n = 1-4, m + n = 2-5. We considered the neutral, anionic and cationic molecular species in the singlet, doublet and triplet states, as appropriate. For neutral clusters, our data confirm previous DFT and post Hartree-Fock findings. For charged clusters, our results represent predictions. Several molecular properties related to the experimental data, such as the electronic energy, equilibrium geometry, binding energy (BE), HOMO-LUMO gap (HLG), and spin contamination mathematical left angle bracket S2 mathematical right angle bracket were computed. We also derived the vertical electron attachment (VEA), the adiabatic electron affinity (AEA), and the vertical ionization energy (VIE), of the neutral clusters. Similar to their carbon-nitrogen counterparts, the lowest energy structures for neutral and cationic silicon-nitrogen clusters are found to be linear or quasilinear. In contrast, anionic silicon-nitrogen clusters tend to form 3D structures as the size of the cluster increases.

Computational and spectral investigation of 5,12-dihydro-5,12-ethanonaphthacene-13-carbaldehyde.

A conformational search of 5,12-dihydro-5,12-ethanonaphthacene-13-carbaldehyde predicted the presence of twelve conformations. The geometry of the twelve conformations established at the B3LYP/6-31G* level showed only six unique ones. Vibrational frequencies were calculated at the B3LYP/6-31G* level. The calculated vibrational frequencies enabled us to interpret the appearance of two bands corresponding to the C=O stretching mode of 5,12-dihydro-5,12-ethanonaphthacene-13-carbaldehyde. The first band corresponded to the 5,12-dihydro-5,12-ethanonaphthacene-13-carbaldehyde structure where the aldehyde group O atom was above the benzene or naphthalene ring. The other band was due to the O atom of the aldehyde group pointing out of the benzene or naphthalene ring.

A G3 study of the structure of carbon-nitrogen nanoclusters.

Possible structures of the carbon-nitrogen clusters of the form C(m)N(n) (m = 1-4, n = 1-4, m + n = 2-5) were predicted for the neutral, anion, and cation species in the singlet, doublet, and triplet states, whenever appropriate. The calculations were performed at the G3, MP2(fc)/6-311+G*, and B3LYP/6-311+G* levels of theory. Several molecular properties related to the experimental data--such as the electronic energy, equilibrium geometry, binding energy, HOMO-LUMO gap (HLG), and spin contamination --were calculated. In addition the vertical electron attachment, the adiabatic electron affinity, and vertical ionization energy, of the neutral clusters were calculated. Most of the predicted lowest energy structures were linear, whereas bent structures became more stable with the increase of the cluster size and increase of the number of the N atoms. In most of the predicted lowest energy structures, the N atom prefers the terminal position with acetylenic bond. The calculated BE of the predicted clusters increases with the increase of the cluster size for the neutral and cation clusters but decreases with the increase of the cluster size for the anion clusters. The predicted clusters are characterized by high HLG of about 11 eV on the average, with that of the anion clusters is smaller than that for the neutral and cation clusters. It is concluded then that the anion clusters are less stable than the corresponding neutral and cation clusters. Finally, the N(2) loss reaction is treated.