title: A comparative study of the infrared and Raman spectra of aniline and o-, m-, p-phenylenediamine isomers.
abstract: The structural stabilities of o-, m- and p-phenylenediamine (PDA) isomers were investigated by DFT-B3LYP and ab initio MP2 calculations with the 6-311G(**) basis set. From the calculations the three isomers were predicted to exist predominantly in an anti (transoid) structure. In the o-isomer, the syn (cisoid) form is calculated to turn to the anti (transoid) form with the two HNCC torsional angles of about 44 and 10° and the NH2 inversion barrier of 3-4 kcal/mol. The CCNH torsional angles in the m-PDA and p-PDA isomers were calculated to be about 25-26° as compared to 20° in aniline. A comparison of the Raman spectra of the three PDA-s with those of aniline shows the high sensitivity of the ring breathing mode to the nature of substituents in the aniline ring. The vibrational wavenumbers were computed at the DFT-B3LYP for aniline and the o-, m- and p-PDA isomers for the purpose of comparison. Complete vibrational assignments were made on the basis of normal coordinate analyses and potential energy distributions for aniline and the o-, m- and p-PDA molecules.
authors.affiliation: Department of Chemistry, King Fahd University of Peteroleum & Minerals, Dhahran 31261, Saudi Arabia. hbadawi@kfupm.edu.sa
authors.forename: Hassan M
authors.initials: HM
authors.lastname: Badawi
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Förner
authors.forename: Shaikh A
authors.initials: SA
authors.lastname: Ali
mesh_terms: D000814:Aniline Compounds / Q000737:chemistry*; D007536:Isomerism; D008958:Models, Molecular; D010655:Phenylenediamines / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013059:Spectrum Analysis, Raman
chemical_list: D000814:Aniline Compounds; D010655:Phenylenediamines; C034193:1,2-diaminobenzene; C008381:3-phenylenediamine; C023650:aniline; C029728:4-phenylenediamine