Journal of Molecular Structure-theochem最新文献

Nucleophilic epoxide ring opening reactions can be effectively carried out with BF₃ but not with BH₃. This has been traced to the formation of an intermediate with an intramolecular dihydrogen bond which prefers to yield a species with frustrated Lewis acid–base pair after liberating H₂ molecule in the BH₃ catalyzed system. Quantum chemical analysis indicates that the H₂ liberation is more favorable by 37.8 kcal/mol in comparison to the desired aminoalcohol formation.

Systematic conformational searches have been performed for various species of L-Threonine (L-Thr) and L-allo-Threonine (L-allo-Thr) in gas phase. The trial structures were generated by allowing for all combinations of internal single-bond rotamers. The trial structures were optimized at the HF/3-21G(d) level and then subjected to further optimization at the B3LYP/6-311++G(d, p) level. A total of 74 canonical neutral, 12 protonated, 22 deprotonated and 4 secondary deprotonated conformers are found for the L-Thr species, while 75 canonical neutral, 14 protonated, 22 deprotonated, and 6 secondary deprotonated conformers are found for the L-allo-Thr species. However, no stable zwitterionic structures of isolated threonine may exist. The conformational energies are determined by the MP2/6-311+G(2df, p) calculations. The intramolecular hydrogen bonds were characterized. The equilibrium conformer distributions at different temperatures are shown. The proton affinities, gas phase basicities, proton dissociation energies and gas phase acidities are determined and compared with the experiments. The conformations of threonine hydrated with one water molecule were also systematically searched. The zwitterionic structures may be stabilized by one water molecule, but their energies are relatively high. The infrared spectra of the most stable structures of all species are presented.

The thermal Curtius rearrangement of benzoyl azide in the presence of Lewis acids has been studied by DFT (PBE/TZ2P) method. The complexation of Lewis acids (BF₃, AlCl₃, SbCl₅) with benzoyl azide leads to the formation of 1:1 and 1:2 stable complexes with interaction of catalysts with O and N atoms of carbonyl azide group. The potential energy surfaces of the catalytic rearrangement have been calculated for each complex and the relation between the complexes and the transition states on potential energy surface have been established by IRC calculation. The energy barriers for catalytic reactions are significantly lower in the most cases in comparison with an uncatalyzed reaction. The activation energy is decreasing in the range of Lewis acids AlCl₃ > SbCl₅ > BF₃ and it correlates with the decreasing of Lewis acids strength. The Mulliken bond population analysis has been done for three compounds RCON₃ (R = H, Me, Ph) and for their complexes, and for all corresponding transition states using the B3LYP/6-311G∗ approximation. The interaction of Lewis acids with carbonyl azide group causes the decreasing of N1N2 bond strength and it helps the thermal Curtius rearrangement to proceed.

A study of the structure, electronic, and optical properties of new indolo[3,2-b]carbazoles is reported. Geometry optimizations of the ground state of the derivatives were carried out using the density functional theory (DFT) with the B3LYP functional and the 6-31G∗ basis set. Molecules disubstituted with phenyl or thiophene units at molecular ends were found nonplanar in their electronic ground states (S₀), whereas indolocarbazoles having phenylenevinylenes at molecular ends are nearly planar. The electronic excitation transitions of the indolocarbazoles were investigated using the time-dependent (TD) DFT method performed on the ground state optimized geometries. For all the derivatives, excitation to the S₁ state corresponds mainly to LUMO ← HOMO transition, whereas the second electronic transition mainly originates from the LUMO ← HOMO − 1 excitation. The excitation energies are found in fair agreement with the absorption energies of the indolocarbazoles. The optimization (relaxation) of the first singlet excited electronic state (S₁) has been done using the restricted configuration interaction (singles) (RCIS/6-31G∗) approach. The electronically excited geometries favor a more quinoidic type structure. Emission energies have been obtained from TDDFT calculations performed on the S₁ optimized geometries and are in fair agreement with experimental data obtained from fluorescence spectra. The change from phenyl to thiophene rings as well as the incorporation of vinyl units between the phenyl and the indocarbazole moieties induce a significant decrease in the excitation and emission energies.

Results given by the new formula for the standard perfect-gas enthalpy of formation, $\Delta H_{\text{f}}^{\circ }={\sum }_{\text{K}}F\left(\text{K}\right)+\text{ZPE}+H_T-H_0-{\sum }_{k<l}{\epsilon }_{\mathrm{kl}}-(\text{CNE}-E_{\text{nb}})$, are compared with experiment. $F\left(\text{K}\right)\text{,}F\left(\text{L}\right)\text{,}\dots$ are fixed parameters of chemical groups K, L, etc. and ${\epsilon }_{\mathrm{kl}}$ is the intrinsic energy of the link between K and L. $Z=\text{ZPE}+H_T-H_0$ is the familiar sum of zero-point + heat-content energies and CNE accounts for the fact that the fragments K, L, etc. are not individually electroneutral in their host molecule. $E_{\text{nb}}$ stands for nonbonded interactions between the fragments. The reduction of the 3-fragment formula applicable to molecules written K-[CH(X)]-L to get its equivalent for the 2-fragment form [CHK(X)]-L reveals a most useful relationship between the functions $F$[CH(X)] and $F$[${\text{CH}}_2$(X)]: the latter is usually easy to obtain, but it is $F$[CH(X)] that is actually required in the 3-fragment problem K-[CH(X)]-L. Fragments of this form, already known for $\text{X}={\text{CH}}_3$, were successfully tested for $\text{X}={\text{C}}_2{\text{H}}_5\text{,}n-{\text{C}}_3{\text{H}}_7\text{,}{\text{C}}_6$