Experimental and DFT studies on the vibrational and electronic spectra of 4,5-dihydro-6-methyl-4-[(E)-(3-pyridinylmethylene)amino]-1,2,4-triazin-3(2H)-one

Vibrational and electronic spectral measurements were made for 4,5-dihydro-6-methyl-4-[(E)-(3-pyridinylmethylene)amino]-1,2,4-triazin-3(2H)-one (pymetrozine). Optimized geometrical structure and harmonic vibrational frequencies were computed by ab initio RHF, B-based DFT methods (BLYP, BP86 and BPW91) and B3-based DFT methods (B3LYP, B3P86 and B3PW91) using 6-311++G(d,p) basis set. Complete assignments of the observed spectra were proposed. The absorption spectra of the compound were computed both in gas-phase and in C2H5OH solution using TD-B3LYP/6-311++G(d,p) and PCM-B3LYP/6-311++G(d,p) approaches, respectively, the calculated results provide a good description of positions of the bands maxima in the observed electronic spectrum. The MEP calculation indicates that the most possible site for electrophilic attack is H23 and the most possible sites for nucleophilic attack are N5 and O19.

The Fourier-transform infrared and Raman spectra and UV–vis spectra of 4,5-dihydro-6-methyl-4-[(E)-(3-pyridinylmethylene)amino]-1,2,4-triazin-3(2H)-one (pymetrozine) have been recorded experimently and analyzed with the aid of B-based (BLYP, BP86 and BPW91) and B3-based (B3LYP, B3P86 and B3PW91) DFT methods.Figure optionsDownload as PowerPoint slideHighlights
► For 4,5-dihydro-6-methyl-4-[(E)-(3-pyridinylmethylene)amino]-1,2,4-triazin-3(2H)-one, the observed frequencies are reproduced well by the B-based DFT methods.
► The calculated band maximums at 219.8, 245.7 and 298.5 nm in gas phase (217.6, 246.4 and 313.4 nm in C2H5OH solution, respectively) provide a good description on positions of the band maximums in the observed electronic spectrum.
► The molecular electrostatic potential (MEP) predict the possible site of pymetrozine for electrophilic attack is H23 and the most possible sites for nucleophilic attack are N5 and O19.