A self consistent reaction field study has been carried out at the MINDO/3 level, in order to explain the decomposition of the species in solution.Publicado on line en 2014.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Castro, Eduardo AlbertoA Self Consistent Reaction Field Calculation of the Fluorobenzonitrile Radical Ions

Villar, Hugo O.

Zeitschrift für Naturforschung A

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Notizen 1339A S e lf  C on sisten t R eaction  Field C alcu lation  o f  the F luorobenzonitrile R adical IonsHugo O. Villar and Eduardo A. CastroInstituto de Investigaciones Fisicoquimicas Teöricas y Aplicadas INIFTA*, Divisön Quimica Teörica, Casilla de Correo 16, Sucursal 4, (1900) La Plata, ArgentinaZ. Naturforsch. 41a, 1339 -  1340 (1986); received June 18, 1986A self consistent reaction field study has been carried out at the M INDO/3 level, in order to explain the decom­position of the species in solution.In previous articles we have studied the cleavage reactions of aromatic radical anions [1 -2 ] , We have shown the dependence o f the decomposition rate on the position o f an orbital crossing point [2]. If the singly occupied molecular orbital (SOMO) is o* the radical anion is unstable, while if  it is n* it is stable. Eventually, the crossing o f these two orbitals allows for the decomposition o f a stable radical anion, and the energy to reach this crossing point from the equilibrium governs the reaction rate. The anti­bonding character o f the umpaired electron should be beared in mind. This means that the system is stable while the electron density excess due to the extra electron is spread over several centres (pi character), so that the destabilization is distributed over the bonds associated to them. A soon as the electron density becomes localized to a certain extent (sigma character), a larger destabilization is attached to the bond on which the electronic density is mainly localized [3]. The majority o f these reac­tions is explained with such a simple scheme.The isomers o f fluorobenzonitrile radical, how­ever, show a particular behaviour [4], The p-fluoro- benzonitrile radical yields fluoride and p-benzo- nitrile radical, while the metal derivative yields cyanide and m-fluorobenzene radical. This fact could not be explained in vacuum energetic terms (as usually used in standard molecular orbital calculations), since in both cases fluorine cleavage is favoured by 10 kcal/m ol.Reprint requests to Dr. E. A. Castro, INIFTA, Divisiön Quimica Teörica, Sucursal 4, Casilla de Correo 16, (1900) La Plata, Argentina.* Facultad de Ciencias Exactas, Universidad Nacional de La Plata.The reaction field techniques provide a deeper insight in reactions which depend on the environ­ment. A self consistent reaction field scheme pro­posed by Tapia and Constanciel [5] was applied on the M INDO/3 algorithm [6], so that we were able to carry out calculations for the systems under con­sideration, using the geometrical parameters opti­mized at the vacuum level. This method has widely been used in the literature in M IN D O /3 [7] as well as CNDO /S-CI [8] calculations. We have used the charges obtained by this procedure to calculate electrostatic potential maps. Joining the positions o f  the minima of the electrostatic potential in several planes to the molecular one, we obtained the trajec­tory the cation would follow if  its interaction with the radical anion were merely electrostatic, which is generally true in our case.The results for the para derivative are shown in Figure 1. The full line indicates the vacuum level result, while the dotted one shows the path for an environment with a dielectric constant o f  10. The trajectory lies in the molecular symmetry plane in both cases, showing that the interaction with the fluorine atom is dominant. The derivative shows a completely different behaviour (Figure 2). The vacuum level displays a similar behaviour to that observed for the para isomer. The environment, however, brings about an interaction with the nitrile group.z I '5 \ \\  \\ \1 \\\I \  .______]o_ü\ \/ iIFig. 1. Trajectories (axis units: A) along the minimum of electrostatic potential generated by p-fluorobenzonitrile radical anion. Full line: vacuum level, dotted line: con­tinuous dielectric with a dielectric constant of 10. The trajectories stay in the symmetry plane of the system.0340-4811 /  8 6  /  1100-1339 $ 01.30/0. -  Please order a reprint rather than making your own copy.1340 NotizenFig. 2. As Fig. 1, but for m-fluorobenzonitrile radical anion. The trajectories do not stay in planes.The presence o f the cation favours the probability of localizing the antibonding electron on the same side o f the molecule than the counter ion, weaken­ing the bonding between the nearest prospective nucleofuge group and the aromatic moiety. A similar conclusion is reached as regard the products. The final state is certainly more stable when the cation is directly interacting with the anion formed as a consequence o f the decom position than the one reached if the counter ion were interacting with the radical formed. The experimental measurementswere carried out in solvents with moderate dielectric constants, as the one considered in this note. The values of the dielectric constant do not substantially affect the conclusions presented here, except for the para isomer in a medium with a large dielectric constant, where the ion remains in the neighbour­hood o f the nitrile group.The meta derivative behaviour is explained tak­ing into account that the charge defect on the carbon atom bonded to the fluorine is larger than that of the carbon atom attached to the cyanide. A similar behaviour was found for neutral molecules[9], where the positions the absolute minima depend on the dielectric constant o f the media considered.This note proves once more [10] the ability o f the reaction field techniques, in particular that in refer­ence [5], to deal with experimental situations where simple vacuum level calculations are not sufficient.AcknowledgementTo Dr. R. A. Rossi for helpful discussions. To CESPI (UNLP) for the computational facilities. This research project is financially supported by the Universidad Nacional de La Plata, the Consejo Nacional de Investigaciones Cientificas y Tecnicas and the Comisiön de Investigaciones Cientificas de la Provincia de Buenos Aires.[1] H. O. Villar, E. A. Castro, and R. A. Rossi, Can. J. Chem. 60,2525 (1982).[2] H. O. Villar, E. A. Castro, and R. A. Rossi, Z. Natur- forsch. 39 a, 49 (1984).[3] R. J. Houser, D. E. Bartak, and M. D. Hawley, J. Amer. Chem. Soc. 95,6033 (1973).[4] R. Constanciel and O. Tapia, Theor. Chim. Acta 48, 75(1978).[5] R. C. Bingham, M. J. S. Dewar, and D. H. Lo, J. Amer. Chem. Soc. 97,1285 (1975).[6] See for instance an interesting analysis in: G. Klopman and P. Andreozzi, Theor. Chim. Acta 55, 77 (1980).[7] A. Botrel, A. le Beuze, P. Jacques, and H. Strub, J. Chem. Soc. Faraday Trans. 2 80, 1235 (1984).[8] H. O. Villar and E. A. Castro, Croat. Chim. Acta 58, 153 (1985).[9] See for instance: J. Angyän, M. Allavena, M. Picard, A. Potier, and O. Tapia, J. Chem. Phys. 77,4723 (1982).Nachdruck —  auch auszugsweise —  nur mit schriftlicher Genehm igung des Verlages gestattet Verantwortlich für den Inhalt: A. KLEMM Satz und Druck: Konrad Triltsch. Würzburg

A Self Consistent Reaction Field Calculation of the Fluorobenzonitrile Radical Ions

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A Self Consistent Reaction Field Calculation of the Fluorobenzonitrile Radical Ions

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