Experimental and Theoretical Analysis of the Vibrational Spectra and Theoretical Study of the Structures of 3,6-Dichloropyridazine and 3,4,5-Trichloropyridazine

Abstract

The infrared and Raman spectra of 3,6-dichloropyridazine and 3,4,5-trichloropyridazine were recorded. These spectra were taken in the solid phase and with various solvents (HCCl3, CS2, CCl4), giving wavenumbers and relative intensities of the bands and the qualitative Raman depolarization ratios. Conventional ab initio methods at the Hartree−Fock and MP2 levels using the 3-21G*, 6-31G*, 6-31G**, and 6-311G** basis sets, as well as density functional theory (DFT) using the 6-31G* basis functions and the BLYP and B3LYP functionals, were used to predict the geometries and to calculate the harmonic frequencies and force fields of these molecules. In addition, coupled cluster calculations, i.e., CCSD and CCSD(T), using the 6-311G** basis set were included to evaluate the geometries of pyridazine and 3,6-dichloropyridazine, analyzing in detail the structure of these systems. To fit the calculated wavenumbers to the experimental ones, the B3LYP/6-31G* force field was selected to be scaled. The following two sets of scale factors were tested: (1) a standard set derived from a training set of 30 organic molecules (set A), and (2) a set directly calibrated from the pyridazine molecule (set B). The calculations indicate that pyridazine and its chlorinated derivatives show less aromatic character than expected. The B3LYP exchange-correlation functional with the 6-31G* basis functions gave the most reliable structural parameters for both molecules. The a priori scaled spectra were sufficiently decisive to get a complete assignment of the vibrational spectra. Both series of scale factors led to similar results. However, although the standard set (set A) proved to be an useful tool to carry out the initial assignments, further detailed analysis of the spectra required specific empirical parameters derived from the pyridazine molecule. This was especially true for the most congested spectral areas.

Experimental and ab Initio Equilibrium Structure and Harmonic Force Field of 1,2,5-Oxadiazole

“…DFT potentials give similar scale factors but all of them are included on the interval 0.91-0.96. This behaviour of the scale factor values for both DFT and MP2 force fields have been observed in other ring molecules and also in branched systems (50). Much of the difference in the scale factors in the three sets comes from the different geometries at which they were calculated.…”

Self-Consistency versus “Best-Fit” Approaches in Understanding the Structure of Metal Nitrosyl Complexes

“…Instead, here we follow a different approach, which increases only modestly the computer effort. It is the comparison of experimental and theoretical vibrational stretching frequencies that has previously been used for structural characterization of many compounds. − We show in what follows that the structures of RuX(CO)(NO)L 2 + (X = NCMe, H - , and CO) can be conclusively assigned by correlating the experimental and theoretical nitrosyl vibrational frequencies for the whole RuX(CO)(NO)L 2 + family.…”

Synthesis and reactivity of a 1,4-dihydropyrazine derivative

“…The reaction was monitored by TLC, (diethyl ether/n-hexane, 1:1) and when no starting material nor Boc-DSer(O-toluenesulfinyl)-OMe was detected (z72 h), the solution was filtered and evaporated at reduced pressure (the same procedure but heating the reaction mixture under reflux allowed the reaction to be complete in 12 h). The residue obtained was partitioned between 200 cm 3 of diethyl ether and 100 cm 3 (6 equiv). After reacting for approximately 3 days the same work-up as described above was carried out to give compound 2 (78% yield).…”

“…Diazines constitute an important class of heterocyclic compounds present in several natural occurring compounds. Pyrazines are found in the luminescent chromophores of certain marine organisms, 1 in cephalostatins isolated from Cephalodiscus Gilchrist which are powerful anticancer agents, 2 in the fungal metabolite aspergillic acid 3 and in foods as potent flavour components. 4 1,4-Dihydropyrazine is an important structural unit of certain redox-active biological molecules such as flavin coenzymes and several marine luciferines.…”