The Quantum Chemistry Study Of Ultraviolet Spectroscopy On Disubstitued Benzene Compound Isomers

Disubstitued benzene compounds,as important intermediates for fine chemicals,are widely used in pharmaceutical industry,chemical industry,food industry and agriculture.However,due to the isomerism,their physical and chemical properties are very similar between each isomer,their absorption spectrums overlap seriously.At present,simple operation and cheap UV spectroscopy combined with chemometrics used for the simultaneous determination of isomer mixture have grown in the related literature in domestic and overseas.But the root of the serious ultraviolet spectral overlap of these isomers is not clear.This paper used the theory of quantum chemistry to discover the reason of the serious ultraviolet spectral overlap of these isomers from microscopic,which would provide sufficient theoretical basis for scientific experiments.Quantum chemistry,as a branch of theory chemistry,is used the basic principle and method of quantum mechanics to research the microscopic structure of the materials,properties and the relationship between the structure and performance.In order to build a good theoretical basis for the simultaneous determination of isomers of disubstituted benzene compounds by chemometrics-ultraviolet spectrophotometry,density functional theory was used to explore the reason of the serious ultraviolet spectral overlap of these disubstitued benzene compound isomers from the molecular structure description,the front rail analysis,atomic charge configuration and UV belonging multiple aspects.The conclusion of this topic has four mainly aspects:1.The benzene compounds conjugate system of molecular is reflected by ultraviolet spectrum,and the ultraviolet spectrum calculation is based on the molecular structure,which means that the ultraviolet spectral differences of isomers are due to the differences of molecular structure:?1?The influence of different kinds of substituent on the benzene ring is different.Methoxyl,methyl,hydroxyl,fluorine-based,chlorine-based,bromine-based and amino-group,whose electronegativity is greater than the H atom,have inductive effect on benzene ring.Their lone pair electrons have p-? conjugate action with benzene formation.Nitro-group,aldehyde and carboxyl have ?-? conjugate action with benzene formation.All these cause different degree of distortion of benzene ring structure.Mainly displays in:?1?The bond length of benzene ring are unequal.Methoxy,methyl,hydroxyl,fluorine-based,aldehyde carboxyl and cyano-group could make the adjacent carbon-carbon bonds in benzene ring increase.Bromine-based makes the adjacent carbon-carbon bonds in benzene ring decrease.?2?The bond angle of benzene ring is unequal.Angle connected to the substituent is greater or less than 120°.?3?The asymmetric effect between amino-group and benzene makes the benzene ring not coplanar.The benzene ring of o-nitrobenzaldehyde,o-phthalaldehyde and o-carboxybenzaldehyde are not coplanar due to the near substituent group.?2?For one kind of disubstitued benzene compound,different substitution site makes different molecular structure.Ortho substitution compounds with two adjacent substituent groups,are different from meta-position and para-position compounds due to steric effects or intramolecular hydrogen bonding.Para-position compounds are easier to form conjugate system with benzene ring and the other substituent,which leads to the bound averageness and energy reduction.For example: H-O intramolecular hydrogen bonding is existence in o-nitrobenzaldehyde,o-phthalaldehyde and o-carboxybenzaldehyde,leading to the different structure from meta-position and para-position compounds.Their benzene ring and two substituents have rotated,which makes the whole molecules not coplanar.Fluorine-based,chlorine-based,methoxyl oxygen and nitro oxygen in o-fluoroaniline,o-chloroaniline,o-anisidine and o-nitroaniline can form intramolecular hydrogen bonding with amine hydrogen,which leads to two of its amino N-H unequal.2.The UV spectrum of the compound includes the position of the absorption peak and the absorption intensity.The position of the absorption peak depends on the energy level of the electron transition,and the energy difference is derived from the energy between the transitions.There exist differences between isomers despite of their similar structures.The energy of the same orbitals is similar but not identical.So the transition energy gaps are not identical and the corresponding UV absorption wavelengths are different.The different electron transition intensity leads to the different ultraviolet absorption intensity.Therefore,the difference of electron transition energy gaps and the difference of transition intensity of isomers are the fundamental reasons for the difference of UV spectrum.3.The structures of the isomers are similar,resulting in similar electron cloud distribution of the conjugate system,and the energy of the same transition orbital is similar.At the same time,the HOMO,HOMO-1 and HOMO-2 energy levels of the isomers of the disubstituted benzaldehyde compounds are very similar in the range of-8eV to-7eV?nitrobenzaldehyde is-8.5eV to-7.7eV?.The LUMO and LUMO+1 energy levels of the isomers of the two-substituted aniline compounds are very similar,and almost all of them are in the range of-0.8eV to 0eV?nitroaniline differ greatly,-3eV to-0.6eV?.The LUMO and LUMO+ 1 energy levels of the isomers of chlorotoluene,chlorophenol and bromotoluene are very similar and are in the range of-1eV to-0.5eV.Since the energy occupied by the front line or the orbit is so close,even if the transition types of the compound ars different,the energy required for the transition is also very close,which further leads to the overlapping of the UV spectrum of the isomer.4.The difference in the charge distribution between the o-,m-and p-compounds can support the difference in the UV spectra of the three:?1?The substitution of different substituents for the charge reduction of the C atoms on the benzene ring is arranged in size: —F>—OCH3? —OH>—Cl?—CH₃>—NO₂>—Br>—CHO>—COOH>—CN;?2?Substituents are different,which mainly leads to a decrease in the charge value of the benzene ring C atoms connected to the substituents.