A comparative study of the scaled vibrational frequencies in the local anesthetics procaine, tetracaine and propoxycaine by means of semiempirical methods: AM1, PM3 and SAM1

Femtosecond photodecarbonylation of photo-ODIBO studied by stimulated Raman spectroscopy and density functional theory

Photo-oxa-dibenzocyclooctyne (Photo-ODIBO) undergoes photodecarbonylation under UV excitation to its bright S₂ state, forming a highly reactive cyclooctyne, ODIBO. Following 321 nm excitation with sub-50 fs actinic pulses, the excited state evolution and cyclopropenone bond cleavage with CO release were characterized using femtosecond stimulated Raman spectroscopy and time-dependent density functional theory Raman calculations. Analysis of the photo-ODIBO S₂ CO Raman band revealed multi-exponential intensity, peak splitting and frequency-shift dynamics. This suggests a stepwise cleavage of the two C-C bonds in the cyclopropenone structure that is completed within <300 fs after excitation. Evidence of intramolecular vibrational relaxation on the S₂ state, concurrent with photodecarbonylation, with dynamics matching previous electronic transient absorption spectroscopy, was also observed. This confirms an excited state, as opposed to ground state, photodecarbonylation mechanism resulting in a vibronically excited photoproduct, ODIBO.

Ultrafast transient absorption spectroscopy of the photodecarbonylation of photo-oxadibenzocyclooctyne (photo-ODIBO)

The ultrafast dynamics of photo-OxaDiBenzocycloOctyne (photo-ODIBO) photo-dissociation was studied using femtosecond transient absorption spectroscopy. Steady-state UV-Vis, time-dependent density functional theory, and 350 nm and 321 nm transient absorption studies are reported. Photo-ODIBO excitation with 321 nm and 350 nm light-induced photodecarbonylation of the cyclopropenone functional group results in the formation of ODIBO. The presence of the photoproduct was confirmed by the results of steady-state photolysis experiments and the observation of absorption signatures of ODIBO in the photo-ODIBO transient absorption spectra. Analysis of the latter revealed the underlying photochemical mechanisms and associated time constants, following excitation of the samples. The dynamics show a multi-exponential decay process, following the dissociation of photo-ODIBO into an excited state of the photoproduct ODIBO within <294 fs after 321 nm excitation. 350 nm excitation, on the other hand, is shown to produce ground state ODIBO via an intermediate species. Additional transient absorption measurements were performed directly on the photoproduct ODIBO to help distinguish spectral signatures associated with these processes.

Vibrational frequencies and structure of 2-thiouracil by Hartree-Fock, post-Hartree-Fock and density functional methods

Vibrational study of the biomolecule 2-thiouracil was carried out. Ab initio and density functional calculations were performed to assign the experimental spectra. A comparison with the uracil molecule was made, and specific scale factors were deduced and employed in the predicted frequencies of 2-thiouracil. Several scaling procedures were used. The geometry structure of the molecule was determined. The effect of sulfur substitution at C2 position in the uracil molecule, on the N1-H and N3-H frequencies and intensities reflects changes in proton donor abilities of these groups. Calculations with the 6-31 G** basis set with HF and DFT methods appear in general to be useful for interpretation of the general features of the IR and Raman spectra of the molecule. Using specific scale factors a very small error was obtained. The use of these specific scale factors resolve and correct some of the controversial assignments in the literature.

A semiempirical study on the electronic structure of 10-deacetylbaccatin-III

We performed a conformational and electronic analysis for 10-deacetylbaccatin-III (DBAC) using well-known semiempirical methods (parametric method 3 (PM3) and Zerner's intermediate neglect of differential overlap (ZINDO)) coupled to the concepts of total and local density of states (LDOS). Our results indicate that regions presented by paclitaxel (Taxol) as important for the biological activity can be traced out by the electronic features present in DBAC. These molecules differ only by a phenylisoserine side chain. Compared to paclitaxel, DBAC has a simpler structure in terms of molecular size and number of degrees of freedom (d.f.). This makes DBAC a good candidate for a preliminary investigation of the taxoid family. Our results question the importance of the oxetane group, which seems to be consistent with recent experimental data.