Indian Journal of Chemistry Vol. 25A, January 1986, pp.zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 7-14 Molecular Orbital Study of Some Aromatic N-Oxide Systems RITA CHADHA Department of Chemistry, University of Delhi, Delhi 110007zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP Received 6 October 1983; revised and accepted 26 July 1985 The effect of substituents on the 7l-electronic charge distributions in a set of monosubstituted pyridine N-oxides has been analyzed by means of Pariser-Parr-Pople (PPP) calculations. The electronic charge distribution in the ring has been used to predict the electrophilic and nucleophilic reactivities of these systems. The UV spectra of some N-oxide systems have also been calculated and compared with the experimental ones. The 7l-electroniccharges, bond orders and energies of molecular orbitals have been correlated with experimental quantities such as the proton magnetic resonance chemical shifts, infrared stretching frequencies and polarographic half-wave reduction potentials. The unrestricted-Hartree-Fock method of Amos and Snyder has been used to calculate the spin density distributions in some N-oxide radical anions. Empirical relations between the spin densities and experimental ESR hyperfine splitting constants have been derived. The N-oxides form a class of compounds which not only find applications in synthetic organic chemistry, but are known for their biological activity'. Many potent antibiotics contain the N-oxide group. Pyridine N-oxides show marked catalytic activity in a number of diverse reactions; their reactivity pattern is also of considerable theoretical importance. In view of this, we have considered it worthwhile to theoretically calculate the electronic properties of aromatic N- oxides with a view to understanding the structure and reactivity of these compounds. Some molecular orbital calculations on N-oxides are available". Method of Calculation The Pariser-Parr-Pople (PPP) method.' -5 was used for all n-electron calculations on ground state neutral molecules. Nine configurations obtained by single electron excitation from three HOMOs to three LUMOs were considered in configuration interaction calculations. Allring distances were assumed to' be 1.40 A. TheC-CH3' C-OCH3,C-OH,C-NH2' C - Cl and N + - 0 - bond distances were taken as 1.51, 1.36,1.36,1.46, 1.69 and 1.24A, respectively. Incyano substituted N-oxides, the C' - C bond distance in the 3 C - C' == N fragment was taken as 1.42 A, while the C' - N distance in - C' == N was taken as 1.16 A. The one-centre two-electron repulsion integrals (Ypp) were calculated using the Pariser" approximation. Two-centre electron repulsion integrals were calcu- lated by Hie method of Mataga and Nishimoto", All core resonance integrals (fJrJ were estimated using Linderberg's relation. Orbital exponents (Zr) needed for the evaluation of overlap gradients were taken from the literature". Hetero-atom (X) model? was used for the methyl group and parameters for the pseudo- hetero atom X were taken from the work of Ray and Narasimhan! 0. VESCF calculations!! were also performed on some representative systems. The unrestricted Hartree-Fock (UHF) method of Amos and Snyder! 2 was used for calculating the spin density distributions. The parameters employed for the calculation 13 were the same as those used in the PPP calculations. Results and Discussion The dipolar N-oxide group (3 N+ -0 -) can act both as an electron donor and as an electron acceptor. The positively charged nitrogen and the negatively charged oxygen have tendencies to withdraw electrons from and donate to the n-electron system respectively. The extent to which both these effects operate depends on the nature of the other substituents. Thus, the net electron density at the dipolar N-oxide group, i.e. (q N+ +qo~ gives an indication of the electron withdrawing or electron accepting ability of the group. If a substituent (X) is present in the ring, the rr- electronic charge (q,J on the substituent gives a measure of the electron donating or accepting ability of the substituent. In Table 1, the quantities (q N+ +qo~, qringand qx are given for a number ofN-oxide systems. The N-oxide group contributes three electrons towards conjugation: one from N + and two from 0-. In the parent pyridine N-oxide (qw+qo~ > 3, indicating that in the parent compound the N- oxide group acts as a better electron acceptor than an electron donor, since it accepts 0.07 units of electron density from the carbon atoms in the ring. A comparison of substituent electronic charges (qJ for amino, methoxy and chloro groups indicates the order of electron donating abilities of these substituents as OCH 3 > NH 2> Cl. For each of these substituents, the efficiency of electron donation to the 7