DFT Studies Of The Geometries And Vibrational Spectra For Strychnine And Related Molecules

The theoretical studies of geometries and vibrational spectra for complex organic molecules can provide significant information about understanding and predicting the infrared spectra, molecular structures, electronics configurations, and chemical bonds.Usually, a complete infrared spectrum is difficult to determine and assign by experimental methods. Quantum chemistry theoretical study can optimize the geometries, calculate the vibrational force fields ,vibrational frequencies and infrared intensities of the target molecule. In this paper, we used the density function theories method B3LYP and large basis sets to calculate and assign the vibrational spectra of important molecules 2-fluoro-5-bromopyridine, urea, uric acid, strychnine, strychnine nitrate and icajine, and the following conclusions are drawn out:1. The optimized geometries, harmonic force fields and infrared spectra of the 2-fluoro-5-bromopyridine molecule were calculated by density functional theory B3LYP method with 6-311++G(2df,2pd) basis set. The theoretical force field was scaled using the scaled quantum mechanical scheme. The error of frequencies was found to be 14 cm-1. The assignment of the vibrational fundamentals for the 2-fluoro-5-bromopyridine molecule was also performed according to the potential energy distributions.2.On the other hand, the optimized geometries, harmonic force fields, infrared frequencies and intensities of the urea and uric acid moleculewere calculated by density functional theory BLYP and B3LYP respectively with 6-311++G(2df,2pd) basis set. The theoretical force field was scaled using the scaled quantum mechanical scheme with a set of scaling factors. The error of frequencies was found to be 17 cm-1 for urea and 27 cm-1 for uric acid. The assignment of the fundamentals for urea and its deuterated isotopemer CO(ND2)2 were performed according to the potential energy distributions, and the same were done for uric acid .3. In addition, density functional theory B3LYP calculations using 6-31G* basis set were carried out to study the molecular structure, force fields, vibration spectra of the strychnine, strychnine nitrate, icajine molecules. The optimized geometries, harmonic force fields, vibrational frequencies, IR intensities have been obtained. A set of scaling factor for calculated frequencies was constituted for strychnine and translated to strychnine nitrate arid icajine to predict their vibrational spectra. Normal mode analysis were performed to assign the vibration fundamental frequencies for strychnine, strychnine nitrate and icajine. According to the assignments,we predict and compare the vibrational spectra of strychnine, strychnine nitrate and icajine. The error of frequencies was found to be 19 cm-1 for strychnine.The computational freaquencies are in good agreement with the observed results.