AROMATIC NITRO-SUBSTITUTED ANION-RADICALS
To evaluate these reactions energetically we need t o know E(CH2),and this value is uncertain. Early work on rnethanelg has suggested that E(CH2) = -0.95 eV but recent thermochemical data in conjunction with a redetermination of A (CH,-) from CH46 suggest 0.4 eV. Using this value then reacE(CH,) 2 0.9 tions 31 and 32 have minimum appearance potentials of 6.8 h 0.8 and 11.2 f 0.7 eV, respectively. Reaction 32 may therefore be discounted and both appearances are probably due to reaction 31, the first onset possibly occurring m-ith a small amount of excess energy. Since the major onset occurs with about 2.3 eV of excess energy it is possible that the neutral CH2is formed in an excited state, possibly the triplet state.
2591 (cl) Thermochemical Data. The folloming heats of formation at 298°K have been used in calculations (all values in eV): I: (0.8); C (7.4); H (2.3); CH (6.2); CHZ (4.1); CHs (1.4); CzH (4.9); CzH4 (0.5); H F (-2.8); CF (2.9); CFz (-1.6); CHF (1.3); CFzH (-3.0); CHzF (-0.3); CzF (2.9); CzFz (-3.2); C, (8.6); CzHF (0.1); CHzCFz (-4.3). The heats of formation of CHF and CzHF have been assumed to be the mean of CFZ and CHe and of CzFz and CzH2,respectively; all other values are taken from ref 23. (23) J . L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron, K. Draxl, and F. H . Field, NSRDS-NBS, 26 (1969).
Electronic Spectra of Ion-Radicals and Their Molecular Orbital Interpretation. I.
Aromatic Nitro-Substituted Anion-Radicals
by Tadamasa Shida* and Suehiro Iwata The Institute of Phwical and Chemical Research, Wako-shi, Saitama, Japan
(Received December 31, 1970)
Publication costs assisted by The Institute of Physical. and Chemical Research
A number of hitherto unknown electronic spectra have been observed for aromatic nitro-substituted anion-
radicals produced in yirradiated rigid solutions at 77°K and are compared with an SCF-MO calculation. It is found that the strong character of the nitro group has a decisive influence on the whole electronic states of the ions and that the transitions can often be characterized clearly with simple excitation modes such as charge resonance and charge transfer between nitro groups and between nitro and hydrocarbon constituents. Systematic effects of alkylation and of twisting the nitro group are found for many cases and are accounted for consistently by the theory.
Introduction Although quantum-mechanical calculations have had remarkable success in explaining esr spectra of ionradicals, the interplay b e h e e n theory and experiment on the excited electronic states of the radicals is rather slim. This is partly because of insufficiency of the experimental data. The observation of the electronic spectra is not easy in general owing to the high reactivity of the radicals under the usual experimental conditions. We have established a method to produce ion-radicals by y-ray irradiation of glassy solutions frozen at 77°K. Unambiguous spectra of the elusive radicals can be measured reliably for a variety of substances.”2 Utilizing the technique, we have undertaken a systematic study on the electronic state of radicals. Recently Zahradnik and Carsky3 have begun similar studies on
odd-electron systems. I n our work emphasis will be placed on the presentation of informative experimental data as well as on the exploration of molecular orbital theory for the open-shell molecule. I n this paper we present the results of the study on aromatic nitro anions. The reasons for choosing the system are as follows. (1) Compared with the ions of aromatic hydrocarbons, the system containing heteroatoms has been less frequently studied. (2) The existence of a multitude of nitro compounds enables us to compare isomers and related derivatives. (3) There are plenty
(1) W. H. Hamill, “Radical Ions,” E. T. Kaiser and L. Kevan, Ed., Interscience, New York, N. Y., 1968, p 321. (2) T. Shida, J . Phys. Chem., 73, 4311 (1969). (3) R . Zahradnik and P. Carsky, ibid., 74, 1235, 1240, 1249 (1970), and succeeding papers.
The Journal of Physical Chemistry, Vol. 76, N o . 17,1971
TADAMASA SHIDAAND SUEHIRO IWATA
2592 of esr data of the anions to be used for the test of validity of the theory employed. It is found that the anions show very characteristic spectral features according to the number of nitro substituents, the position of substitution, and the additional substitution by alkyl groups. These results have been successfully explained by an SCF-110 calculation. Theoretical Section The standard restricted SCF-CI procedure4 was programmed using FORTRAN IV. For each symmetry the total number of 40 configurations \vas chosen out of the first 100 lowenergy configurations of one- and twoelectron excitation^.^ Of necessity, the molecular configuration of the ions \vas constructed with the standard bond lengths and angles of the neutral molecules. Unless otherwise stated, all the T electron systems were taken as planar. For the ions having sterically hindered functional groups, eg., the anion of o-dinitrobenzene, the skew angle was chosen rather arbitrarily. As a check of our computer program we carried out calculations for several aromatic hydrocarbon anions adopting the Pariser-Parr approximation and obtained very close agreement with the result of Carsky and Zahradnik.3 Similar calculations for the nitro compounds, however, yielded a serious disagreement with experiment when we fixed the one-center integrals as in eq 1 and 2 and changed the resonance integrals PC-X and ON-o within a tolerable range
w,= -I, Y, =
I , - A,
(1) (2)
where I , and A , are the valence state ionization potential and the electron affinity of the ?r electron. A possible effect of charge heterogeneity in anion radicals on the one-center integrals has been examined critically and an explicit dependence of the integrals upon the density of electrons on each atom has been derived. This is a variation of Brown and Heffernan’s variable electronegativity methodj6 and the details will appear elsewhere.’ I n effect, however, the heterogeneity of charge distribution itself does not account a great deal for the discrepancy between the calculated and experimental results. However, the agreement mas achieved by subtracting 1.5 eV and 2.0 eV from the one-center core integrals of nitrogen and oxygen atoms, respectively. Although the origin of the necessitated adjustment of the core integrals is not known at present, the systematic agreement with the experiment for all the ions studied seems to indicate that the parametrization may be justified by reasons yet to be found. The two-center Coulomb repulsion integral was estimated using the Kishimoto-Nataga approximation for planar molecules, the extrapolation being made to the one-center repulsion integral modified by our treatment. For nonplanar molecules we assumed declined pairs of The Journal of Physical Chemistry, Vol. 7 6 , N o . 17, 1971
spheres and applied the Kishimoto-Nataga formula t o each combination of spheres. Experimental Section
It is knosvn that the pure glassy Z-methyltetrahydrofuran (NTHF) a t 77°K shous on 7 irradiation a strong absorption of Y ~ ,1.2~p. ~The absorption is ascribed to the electron metastably trapped in the amorphous matrix. When the ether glass has been doped n i t h solutes prior to irradiation, the electron band is replaced by the absorption band of solute anions. At solute concentrations of 10-100 m M ,practically all the electrons liberated by irradiation are scavenged by the solute to form the anion, and clean absorption spectra of the solute anion are observed for longer wavelengths, the visible-near-infrared regions. For the near-uv-uv regions the spectra are less clean-cut because the positive ion of the matrix molecule, the counterpart product of the electron, decomposes to radical fragments which absorb in this spectral region. Moreover, most solute molecules absorb in the uv regions, so that the initial absorption of sample is too high to observe the optical change caused b j the irradiation. Despite this, \!e attempted to extend the measurement of the spectra of solute anions t o shorter TI avelengths by reducing the solute concentration and the cell thickness to 0.5 mm. As the initial concentration was reduced, electrons ere not completely scavenged and a residual absorption of the trapped electron appeared along nith the absorption of the solute anion. The electron band, however, was easily photobleached by the red light of a tungsten lamp (A > 700 mp); thereby the absorption of solute anion was enhanced by the electrons released from the trap. Since the yield of the electron per unit energy absorbed is known to be G = 2.55 for JITHB,*the yield of the solute anion is also equated to G = 2.35 b? the assumption that all the electrons are scavenged at sufficiently high solute concentrations (10-100 mM). Therefore, the measurement of the total energy absorbed and the optical density of the anion give the extinction coefficient ( E ) of the ion. All the E values in this work have been obtained on the basis of G(e-) = 2.55 and the dosimetry using the Fricke solution. Other details of the optical measurement as well as the irradiation procedure have been described else~vhere.~A Cary spectrophotometer, Alodel 14 KI, was used which was particularly suitable for the measurement of absorptions at longer wavelengths. (4) H. C. Longuet-Higgins and J . A . Pople, Proc. Phys. Soc., London, Sect. A , 68, 591 (19%).
( 5 ) We wish t o thank M r . S. Katsurnata for his thorough check of the matrix element involving the two-electron excitation and of our computer program. (6) It. D. Brown and 11.L . Heffernan, Trans. Faraday Soc., 54, 757 (1958). (7) S. Iwata and T. Ghida, to be published.
Table I : Xitrobcnzene
kK
17.1 20.6
z I1
28.6 31.4 ?A.r,
(5
ti e
Polariration"
__ ____ -_Ca]cd Oscillator strength
?I
0,00:3
3'
0.07G 0.075 0 . 123 0 . l!),i
?/
z z
ol,sli.
Character'
(0 -+ 7) pure CT - 0.40 ( 5 -P G ) (3 -., 6 ) pure back CT (6 8) - 0.40 (,j 6) -0.,51 (-5-P 6) 0 . 6 3 (6 -+ 9)
0.!):3 0.83 0.94 -0.38
(6 + 8) -+
+
-
kK
+ 0.66 (4
-
7)
17.0 21.2 27 sh 30.6
: i i ~the loiig : i i i d thc short axes (if iiitrot)eiizelie, respecatively. * Oiily coiifiguratioiis whose cwefficielit i i i the total wave T :iiid fuiictioii cxcccd 0.3 arc s h o w i i i thc tables. The numbers i i i pareiitheses refer to the orbitals in Figwe 2. The clectroiiic configurstioii (4 --+ 7) hns t,wo doublet states. The wipriined refers to (4-P 7 ) = I/d2[14671 - 146711, while the primed refers t o (4-+ 7)' = I / d(i[-214671 14G7/ 146711. This i , i i l e ILpplies to the other rases also. The iiotatioii CT stands for t,he charge transfer from ttw nitro group t o t h e hydrocarboil t:oiistitueiit. The ti'aiisfer of the reverse directioii will be cdled back CT. '1
+
+
Results and Discussion ( I ) Nl'frobrnzrne. 1:igurc. 1 sho\vs the spectrum of tiitroberizene ariiori produced in thc )ITHI: matrix at' 77°K. T h e band at about 21 I i I i can be idcnt,ified \vit'h thc tr:tnsition of A,, 40,; -435 nm report'ed by Chambers and Ad!ims,8 and by Ticmula and Siodaag The othcr hinds, not I i n o \ ~ ~ previously, i :ire also ascribcd to t hc anion because of t'hc gcncral agrccmcrit, \vith the cnlculatcd rcsult shown in the figures and in t,ha accomp:inying T:iblc I. Although thc ion produced i n the prescncct of :illi:tli metal cations is I
