49
Electronic Structure of Nitrostyrene Molecules (5) D. W. Herlocker, R. S. Drago, and V. I. Meek, lnorg. Chem., 5, 2009 (1966). (6) R. Whyman, W. E. Hatfield, and J. S. Paschal, lnorg. Chim. Acta, 1, 113 (1967). (7) D. W. Meek, R. S. Drago, and T. S. Piper, lnorg. Chem., 1, 285 (1962). (8) R. Sharma, T. P. Das, and R. Orbach, Phys. Rev., 155, 338 (1967). (9) D. L. Kepert, D. Taylor, and A. H. White, J. Chem. Soc., Dalton Trans., 670 (1973). (IO) R. L. Carlin, J. Roitman, M. Dankleff, and J. 0. Edwards, lnorg. Chem., 1, 182 (1962). (11) B. Bleaney and D. J. E. Ingrarn, Proc. Roy. Soc., Ser. A, 205, 336
(1951). (12) G. Burns, J. Appi. Phys., 32, 2048 (1961). (13) G.W. Strauss, J. Chem. Phys., 40, 1988 (1964). (14) G. J. Long, G. M. Wolterrnann, and J. R. Wasson, unpublished results. (15) 0. Matarnura, J. Phys. Soc. Jap., 14, 108 (1959). (16) M. Vijayan and M. Viswamitra, Acta CrYStallOgr., 21,522 (1966). (17) G. M. Woiterrnann and J. R. Wasson, J. Phys. Chem., 77, 945 (1973). (18) R. S. Title, Phys. Rev., 136, 623 (1963). (19) C. Kikuchi and G. Azarbayejani, J. Phys.Soc. Jap., Suppl. E-7, 2503 (1962).
Geometry and Electronic Structure of Nitrostyrene Molecules and Anions M. Barzaghi, A. Gamba, G. Morosi, and M. Simonetta* C. N.R. Center for the Study of Structure/Reactivity Relations and Institute of Physical Chemistry, University of Miian, 20733 Milan, ltaiy (Received May 22, 7973)
Ultraviolet spectra and dipole moments for neutral molecules of 0-, m-, and p-nitrostyrenes, and esr and uv spectra for the corresponding anion radicals, have been determined. Anion radicals were obtained by electrolytic reduction in liquid ammonia, acetonitrile, and dimethyl sulfoxide. Hfs coupling constants have been assigned through a comparison with related compounds and on the basis of calculations based on INDO and PPP methods. The dipole moments of the excited states associated to the most relevant transitions have been determined through the McRae theory of solvent shift of absorption bands. Ground- and excited-state properties have been studied by means of semiempirical (PPP and CNDO-CI) and ab initio (STO-3G) methods. Information on the conformations of the three isomers was obtained on the basis of a study of INDO energies us. geometrical parameters.
Introduction
Following a research program on spectroscopic behavior of anion radicals containing the nitro group, we presented in recent paperslJ for a number of nitro compounds some experimental data including hyperfine splitting constants and electronic transition energies for anion radicals, as well as transition energies and probabilities for the parent neutral molecules. Their interpretation was substantiated by means of theoretical calculations. In this paper the same experimental and theoretical work has been extended to nitrostyrenes, whose spectroscopic properties have scarcely been i n ~ e s t i g a t e d .The ~ following experimental data for 0-, rn-, and p-nitrostyrenes have been investigated: uv spectra and dipole moments for neutral molecules, and uv and esr spectra for anion radicals. The related observations have been obtained by means of Pariser and Parr calculations, but for situations in which the validity of U / T approximation seemed questionable, results have been supplemented by means of calculations based on “all valence electrons” methods. The ground state of neutral molecules has been also studied by ab initio MO calculations. Owing to the lack of experimental data on the geometry, the energy dependence on the twist angle of vinyl and nitro groups around the bonds to the benzene ring has been determined by means of INDO calculations. In particular, the variation of hyperfine splitting constants with conformation has been examined. The numbering of
atoms in the molecules and the corresponding ions is shown in Figure 1. Experimental Section Materials. 2-Nitrostyrene was prepared by decarboxilation of o-nitrocynnamic a ~ i d ; ~the , ~product, , ~ a pale yellow oil, was purified by chromatography on silica column, using n-hexane as an eluent. A mass spectrum confirmed the purity of the sample. 3- and 4-Nitrostyrene were highpurity K & K Labs., Inc. products. Solvents. Solvents were Carlo Erba commercial products for spectrophotometry. Acetonitrile (ACN) and dimethyl sulfoxide (DMS) used for radical solutions were further purified following the prescriptions given in ref 7 and 8-10, respectively. Tetraethylammonium perchlorate (TEAP) was a Carlo Erba product for polarography. Liquid ammonia was carefully purified in an ad hoc apparatus described in ref 11. Anion Radical Preparation and Measurements. Anion radicals were obtained in vacuum cells by controlled potential electrolysis using as solvents ACN, DMS, and liquid ammonia. The technical details and the vacuum apparatus, when ACN and DMS have been used as solvents, were previously described.lJ% Reduction potentials for each compound were evaluated from polarographic curves recorded at room temperature in ACN, using a saturated calomel electrode (sce) as an external reference, and the corresponding half-wave potentials are collected in Table I. The Journal of Physical Chemistry, Voi. 78, No. 7, 1974
50
M. Barzaghi, A. Gamba, G. Morosi, and M. Simonetta
' 0
I
0
0
Figure 1. Topology of nitrostyrenes and numbering of atoms.
TABLE I: Half-wave Potentials Measured in ACN and Tomes Reversibility Relationship" for the First Polarographic Wave
a
Isomer
-E112. v us. 8ce
Para Meta Ortho
1.078 1,164 1.151
Ell4
- Eair. V
0.065 0.067 0.075
J. Tomes, Collect. Czech. Chem. Commun., 9, 12,S1,150 (1937).
The use of liquid ammonia as solvent required a particular apparatus, described in ref 11 and 13. The optimized conditions to obtain well-resolved spectra were (a) reduction potentials of -28 V, (b) temperatures ranging between -60 and -80", and (c) concentration of the substrate ca. M with an equivalent concentration of supporting electrolyte (TEAP) . Esr spectra were recorded using a Varian 4500-10A Xband spectrometer with a 100-kHz magnetic field modulation. Electronic absorption spectra were measured with a Beckman DK-2A spectrophotometer. Polarographic measurements were obtained with a multipurpose Amel Model 463 polarograph. Neutral Molecules Measurements. The electronic uv spectra in 21 different solvents were measured with a Beckman DK-2A spectrophotometer. Recording was carried out in a purified nitrogen stream (oxygen content
(IO) S. F. Nelsen, B. M. Trost, and D. H. Evans, J. Amer. Chem. SOC., 89,3034 (1967). (11) M. Ciaii, Thesis. University of Milan, 1972. (12) S. Wawzonek and M. E. Runner, J. Electrochem. Soc., 99, 457 (1952). (13) D. H. Levy and R. J. Myers, J. Chem. Phys., 41,1062(1964). (14) E. A. Guggenhelrn, Trans. Faraday. SOC., 45,714 (1949). (15) J. W. Smith, Electric Dipole Moments." Butterworths, London, 1955. (16) (a) J. A. Pople, D. L. Beveridge, and P. A. Dobosh, J. Chem. Pbys., 47, 2026,(1967);(b) J . A, Pople, D. L. Beveridge, and P. A. Dobosh, J. Amer. Chem. SOC., 90,4201 (1968). (17) R. Pariser, J. Chem. Phys., 24,250 (1956). (18)J. Del Bene and H. H. Jaffe, J. Chem. Phys., 48,1807 (1968). (19) P. H. Rieger and G. K. Fraenkel, J. Chem. Phys., 39,609 (1963). (20) R. L. Ellis, G. Kuehnlenz, and H. H. Jaffe, Theor. Chim. Acta, 28, 131 (1972). (21) N. Mataga and K. Nishimoto, 2. Phys. Chem. (Frankfurt am Main), 13,140 (1957). (22) C. C. J. Roothaan, Rev. Mod. Phys., 32,179 (1960). (23) R. Zahradnik and P . &sky, J. Phys. Chem., 74, 1235 (1970). (24) W. J. Hehre, R. F. Stewart, and J. A . Pople, J, Chem. Phys., 51, 2657 (1969). (25) E. G. McRae, J. Phys. Chem., 61,562(1957). (26) H. C. Longuet-Higgins and J. A. Pople, J. Chem. Phys., 27, 192 (19571. (27) W l W: Robertson, A. D. King, and 0. E. Weigang, J. Chem. Phys., 35,464(1961). (28) T. Kubota and M. Yamakawa, Bull. Chem. SOC.Jap., 35, 555 (1962);40,1600 (1967);41,1046 (1968). (29) J. Heinzer, "Least-Squares Fitting of isotropic Multiline Esr Spectra," Quantum Chemistry Program Exchange (QCPE) No. 197,Indiana University, Bioomington, Ind.
References and Notes (1) A. Gamba, V. Malatesta, G. Morosi, and M. Simonetta, J. Phys. Chem., 76,3960 (1972).
(30) T. Fujinaga, Y. Deguchi, and K. Umemoto, Bull. Chem. SOC.Jap., 37,822 (1964). (31) A. Carrington and A. D. McLachian, "introduction to Magnetic Resonance," Int. Ed., Harper, New York, N. Y., 1967,p 83.
*
Octahedral d4, d6 Ligand Field Spin-Orbit Energy Level Diagrams E. Konig* and S. Kremer institute of Physical Chemistry il,University of Erlangen-Numberg, 0-8520Erlangen, West Germany (Received March 12, 1973; Revised Manuscript Received August 27, 1973)
The complete ligand field Coulomb-repulsion spin-orbit interaction matrices have been computed for the d4, d6 electron configurations in a field of octahedral symmetry. Correct energy level diagrams are presented.
Introduction Complete octahedral ligand field spin-orbit interaction matrices for the d4, ds electron configurations have been reported both by Schroederl in the strong-field coupling The Journal of Pbysical Chemistry, Vol. 78, No. 1, 1974
scheme and by Dunn and Li2 in the weak-field coupling ~ c h e m e . 3 -In ~ the configurations d2, d8 and d3, d7, "complete" energy level diagrams have been c o n s t r ~ c t e don~ ~ ~ the basis of corresponding matrices and these have been
