2744
Gamba, Malatesta. Morosi, Oliva, and Sirnonetta
(a) 8. R . Smith and J. J. Pieroni, Can. J. Chem.. 42, 2209 (1964); (b) S. Fujii and J. E, Willard, J. Phys. Chem.. 74, 4313 (1970): (c) H. Tsujikawa, K. Fueki, and Z. Kuri, J. Chem. Phys., 47, 256 (7967). R . I-efebvre and J. Maruani,J. Chem. Phys.. 42. 1480 (1965). The hyperfine tensor for the cy proton coupling is well established. For exampie, CH&OOH in malonic acid gives the principal elements of -32.9, -20.1, and -10.8 G with the isotropic component of -22.2 G. [ A . Horsfield, J. R . Morton, and D. H. Whiffen, Moi. Phys.. 1, 475 (1961).] The three tensor elements obtained from the simulation for our radical are reduced by factors of 0.61, 0.62, and 0.60, respecrively, because of larger delocalization of the spin density in our radical. J. R. Boiton, "Radical ions," E. T. Kaiser and L. Kevari, Ed., interscience, New York. N. Y., 1968, p 10. J. W. Wells,J. Chem. Phys.. 52,4062 (1970). The carboxyl anion in an irradiated single crys?al of alanine14 has an exceptionaliy large OH coupling (ao N 15 G). However, our recent study by ENDOR has made it clear that this is due to the conformation of the OH bond, which !s out of the COO plane (J. Chem. Phys., in press).
(14) (a) A. Minegishi, Y . Shinohara, and G . Meshituka, Buii. Chern. SOC. Jap., 40, 1549 (1967); (b) I. Miyagawa, N. Tamura, and J. W. Cook, Jr., J. Chem. Phys., 51, 3520 (1969); (c) J, W. Sinclair and M. W. Hanna, J. Phys. Chem., 71, 84 (1967); (d) J. W. Sinclair and M. W. Hanna, J. Chem. Phys.. 50,2125 (1969). (15) J. A. Pople and D. L. Beveridge, "Approximate Molecular Orbital Theory," McGraw-Hili. New York. N. Y., 1970, p 166. (16) Y. Chatani, Y. Sakata. and I . Nitta, J. Poiym. Sci., Part B-1. 419 (1963). (17) H. Fischer,Z. Naturforsch. A , 19, 866 (1964). (18) (a) F. A. Rushworth. Proc. Roy. SOC.,Ser. A , 222, 526 (1954); ( b ) J. G. Aston, "Physics and Chemistry of the Organic Solid State,'' D. Fox, Ed., Wiley, New York, N. Y., 1963, p 564. (19) E. R. Andrew, J. Phys. Chern. Soiids, 18, 9 (1961). (20) B. Eda and M. Iwasaki, J. Chem. Phys., 55, 3442 (1971). (21) M. Iwasaki, K. Minakata, K. Nunome. and K, Tagami, J. Chem. Fhys.. 57, 3187 (1972). (22) K. Minakataand M. iwasaki, J. Chem. Phys.. 57, 4758 (1972). (23) T. Shida and W. H . Harnill, J , Chem. Phys., 44. 4372 (1966); S . Arai and L. M. Dorfman, ibid., 41, 2190 (1964): M. C. SaLcer, S . Arai, and L. M. Dorfrnan, ibid.,42, 708 (1965).
Aldo Gamba. ~~~c~~~~ ~ a l a t e $Gabriele ~~, Morosi. Cesare Oliva, and Massirno Simonetta" C. N . R. Center for fhe Study of StructurelReactivity Relations and lnstifute of Physical Chemistry, Unwersity of Miian. Milan. ltaiy (Received February 9, 1973)
The esr and uv spectra of 2 - , 3-, and 4-nitropyridine and 2 - , 3-, and 4-nitropyridine N-oxide anion radicals in solution have been measured as well as the uv spectra of 2 - , 3-, and 4-nitropyridine N-oxide neutral molecules in the same so!vent. The results have been interpreted by application of a number of semiempirical quantum mechanical methods. By comparison of experimental and calculated data an assignment of visible and ultraviolet absorption bands to electronic transitions and of hyperfine splitting (hfs) coupling constants to magnetic nuclei is proposed.
Introduction The electronic structure of the ground and excited states of pyridine. pyridine N-oxide, then mononitro derivatives. and the Corresponding anion radicals has been the subject of several investigations, both experimental and theoretical (see ref 1 10 and references therein), the most frequently used techniques being uv and esr spectroscopy. However, the esr spectra of home of these radicals had never been measured, and the same is true for the uv spectra. It seems worthwhile collecting esr and uv spectra of all of these radicals in the same conditions. particularly in the same solvent. together with the uv spectra of the parent neutral molecules. Many theoretical methods are available for the interpretation of these experimental data. some of which are unfortunately of no practical use for our systems, due to tbeir complexity. We tried some of the most well-established semiempirical approaches including the g / 7 r approximation. The aim of the present work is to verify the possibility of obtaining for each anion radical a good correlation between experimental data for all the measured observables, t h a t is hfs coupling constants, electronic transition enerThe Journai of Phys:cai Chemistry, Voi. 77, No. 23, 7973
gies and probabilities, and the Corresponding theoretical results obtained by solution of just one eigenvalue-eigenvector problem. Also transition energies and probabilities for the neutral molecules were calculated in each method by using the same approximations and parametrization as for the corresponding anions. The numbering and reference axes for the neutral molecules are shown in Figure 1. The corresponding radicals will be identified with the same numbers primed
Methods of Calculation and Parameters
Anion Radicals. Calculations were carried out by means of semiempirical methods based on the a / a approximation and by means of the INDO method,ll which includes all valence electrons. Owing to the lack of experimental data, geometries for ions were assumed equal to those of parent neutral molecules. The experimental (planar) geometries were used for the heavy atom skeletons in pyridine12 and pyridine Noxide. In the case of pyridine N-oxide the suggested weighted average13 for bond lengths and bond angles of the two crystallographically independent molecules, contained in the unit cell, was used: K O = 135 A; N-C2 = 1.34 A, cz-C3 z.= C3-c* = 1.37 #; c5C4c3 = 118"; c4c3c2
2745
Uv and Esr Spectra of Nitropyridines
0 0 0
0
'3
t
t
@
0
Figure 1. Numbering of molecules and reference axes a
a
2
NITROPYRIOINE
rI
b
b
Figure 2. Experimental ( E and simulate8 ( b (J. Heinzer, "Least Squares Fitting of Isotropic Multiline Esr Spectra," Quantum Chemistry Program Exchange X P E ) , Indiana L iversity, Bloomington, Ind.) esr spectra of anion radicals of nitropyridine isomers. The modulation amplitude prc uces negligible distortion as fields between 0.75 X and 5 X l o M 4G were used for widths between 0.2 and 0.4 G (C. P. Poole, Jr. "Electron Spin Resonance," Chapter 10-E, Interscience, New York, N . Y., 1967). TABLE I: Polarographic Data Molecule
2-Nitropyridine 3-Nitropyridine 4-Nitropyridine 2-Nitropyridine N-oxide 3 - N itropyridine N-oxide 4- N itropyr id ine N-oxide
-El,? (V) vs. sce 1.08 1.06 0.92 0.96
0.83 0.87
= 120"30'; C&N = 119"30'. All C-H bonds were as-' sumed in the ring plane bisecting external ring angles,
with a bond length equal to 1.08 A. For nitro derivatives the same geometries of the rings as for unsubstituted compounds were used. The nitro group was assumed coplanar with the ring, with the following geometry: O N 0 = 124"; N - 0 = 1.21 A; C-N = 1.48 and bisecting external ring angle. In the scope of a a/* approximation, restricted LCISCF methods, in versions given by Pople and LonguetHiggins (P)I5and by Roothaan (R)l6 were taken into consideration, as well as the McLachlan (M) method.17 Starting HMO's were evaluated with the parametrization suggested by Rieger and Fraenkell8 and t h a t adopted by Janzen and Happ for the N 0 gr0up;g for the ring
-
The Journal O f Physical Chemistry. Voi. 77. No. 23, 7973
2746
Gamba, Malatesta, Morosi, Oliva, and Simonetta
TABLE II: Calculated (P and R Methods) Spin
ensities and Proton Hfs Coflstants and Experimental Coupling Constantsa
____ ____ Anion radical
Posltlon
1’
N 2 3 4 N 3 4
2’
5 6 N(-Nod
3‘
N 2 4
5 6
4’
5’
N(-N~z) N 2 3 N(-NOz) N(N-0) 2 3 4 N (N+O)
3 4 5 6 N(-NOz)
7‘
N (N-0) 2 4
5 6
8’
N (-N02) N (N-0) 2 3 N(-N02)
Theoretical ____ P method -___
_ _ _ _ _ I
aic
Pi
0.34553 0.10727 0.00835 0.42324 0,19238 0.07615 -0.01015 0.15542 - 0 06210 0.35847 -0.03899 0.09704 0.2071 9 -0.06758 0.20404 0.38375 0.18351 -0.03064 0.13348 0.34948 0.23120 0.1 3761 -0.01 131 0.38087 0.12892 0.03996 -0.03053 0.08615 - 0 06652 0.23426 -0.01 81 5 0.08022 0.19986 -0.06193 0.1 9034 0.4271 4 0.13495 0.0396 7 0.10438 0.30542
-
-2.496 -0 194 --9.849
- 1.772 0.236 -3.616 1445
-2.258 - 4 a21 1.572 -4.748
0 713 -3.106
-3.202 0 263 -8.862 --0.930 0.710 -2,005 1.548
- 1.867 -4.650 1441 -4.429
0.923 -2.429
nitrogen in nitropyridines the following parameters were assumed: d = 0.5 and y c x = 3 .Q.Ig Energy parameters adopted through PPP calculations are available.20 Two-center Coulomb repulsion integrals were calculated adopting the Pariser and Parr approximation.21 Prescriptions t o evaluate the integrals at distances less than 2.80 A were given elsewhere.22 In CI calculations we considered interaction of ground configuration with all singly excited configurations of types A, B, C,, and CS: according to the definitions reported in ref 23. For details on the calculation of energy and oscillator strength matrix elements for doublets, based on Longuet-Higgins and Pople and Roothaan procedures, see ref 21 and 23. INDO calculations were performed using the program by D o b o ~ h , ~modified * to include the calculation of the spin densities and hfs coupling constants from wave functions purified of the quartet c o m p 0 n e n t . ~ 5 % ~ ~
0.34572 0.10659 0.00868 0.42374 0.!8476 0.07804 -0.007’14 0.14908 0.05302 0.371 70 -0,03403 0.09831 0.19934 0.05469 0.19098 0.40151 0.17080 - 0.02392 0.13355 0.36318 0.23074 0.13656 -0.01054 0.38243 0.14196 0.03237 -0,0191 1 0 07883 -0.04675 0.17631 -0.01888 0.0931 7 0.19243 -0.04845 0.1 7990 0.44466 0.13663 -0.03247 0.10582 0.30325
-
-
aH = -23.3
p(
Exptlb
a
PL
* In gauss. In acetonitrile (present work) except when otherwise stated. In dimethylformamide.5
The Journal of Physical Chemistry, Vol. 77, No. 23. 7973
_.___l_l
R method
(G).
6.28e 3.55 0.82 9.70 1.893 2.728 0.591 3.777 0.866 8.184 1.313 3,183 4.433 7.091 3.643 9.327 2.473 0.451 3.033 7.817 10.91f 3.01 0.44
-2.526 0.206 - 10.042
-
- 1.849 0.169
- 3.533 1.257
- 2.330 -4.724 1.296 -4.526
0.567 -3.165
-3.236 0.250 9.063
-
8.51 3.76 1.43 1.13 3.76 2.65 7.30 0.91 5 3.354 4.322 2.767 3.501 7.752 4.380
-0.767 0.453 1.868 1.108
-
-2.208 -4.560 1.148 -4.263
1.180
0.770 -2.508
aH = -23.7 pc“ { G ) .
3.187 6.240 e
In liquid NH3.36
Xeutral Molecules. Singlet-singlet excitation energies and probabilities were evaluated by means of the standard PPP method with the same parameters used for ions, including all singly excited configurations in the CI treatment. For better confidence in the assignment of bands to electronic transitions, it seemed worthwhile comparing PPP excitation energies and probabilities with those obtained by the “Molecules in Molecules” (MIM) method.27 The results for nitropyridines can be found in the literature.2 MIM calcularions for nitropyridine N-oxides were performed following the same prescriptions as given in ref 2 for nitropyridine derivatives. Geometry. two-center Couiomb integrals, and 8 resonance integrals are the same as used in PPP calculations. All the starting data necessary for MIM calculations are available.20 T o verify if the transition energies bbtained by the two different methods are consistent and to interpret the re-
2747
Uv and Esr Spectra of Nitropyridines TABLE Ill: Calculated ( M Method) Spin Densities and Hfs Coupling Constants and Experimental Valuesa
Anion radical
Position
Spin density
Calculated coupling constantb
1'
N
0.34694 0.13049 -0.01403 0.42015 0.16380 0.08966 - 0.01334 0.14685 - 0.05094 0.21834 -0.04393 0.1 1259 0.16222 -0.05222 0.1 a832 0.23389 0.16834 -0.01819 0.1 1a21 0.20635 0.28693 0.13809 -0.01720 0.38393 0.13603 0.10120 - 0.02438 0.14578 -0.05222 0.21369 -0.03415 0.1 2369 0.161 64 -0.05062 0.1 9074 0.23456 0.14624 -0.02777 0,11730 0.20419
6.3W -3.52 0.38 -11.34 2.12e -2.42 0.36 -3.96 1.38 7.345 - 1.52@ -3.04 -4.38 1.41 -5.08 9.10f 1.95e 0.49 -3.19 6.861 8.72;g 10.14h -3.73 0.46 10.37 4.66;g 4.78h -2.06 0.66 -3.94 1.41 7.50f -3.05;g - l . l g h -3.34 -4.36 1.37 -5.15 9.14f 5.19:g 5.14h 0.75 -3.17 7.14
2 3 4 2'
3'
N
3 4 5 6 N (-NOz) N 2 4 5 6 N (-NO?) N 2 3 N (-NO21 N (N-+O) 2 3 4 N (N+O)
3
7'
8'
4 5 6 N (-NO21 N(N4O) 2 4 5 6 N (-NOz) N (NW+O) 2 3 N (-NOz)
Measured coupling constante
6.2ad 3.55 0.82 9.70 1 .a93 2.72% 0.591 3.777 0.866 8.184 1.313 3.183 3.643 1.091 4.433 9.327 2.473 0.451 3.033 7.817 10.91' 3.01 0.44 8.51 3.76 2.65 1.13 3.76 1.48 7.30 0.915 3.354 3.501 2.767 4.322 7.752 4.38 1.18 3.19 6.24
aCoupling constants in gauss. b a H = -27.0p~'.~ i n acetonitrile except when otherwise stated. In liquid NH3.36e aN = f 1 3 . 1 p x x f (pczir pcBT). aNIN02) = f99.0pN" r 35.8(2pr~').'~ g a N ' i + + O= ) 42.57~~ -18.98~0" - 6.66(pC2' pc6r).5 h a N l N - + " I = 35.61pN" PO' from P. B. Ayscough and F. P. Sargent, J. Chem. SOC.B, 907 (1966). In dirnethylf~rrnamide.~
+
+
6.960.93-
sults of PPP calculations directly in the language of locally excited and charge-transfer configurations, a configurational analysis28 was carried out.
T e t r a e t h y l a m m o n i u m perchlorate (TEAP) was a Carlo Erba product for polarography. Preparation and Measurements for A n i o n Radicals. Anion radicals were prepared in vacuum cells by cnnExperimental Section trolled potential electrolysis following external generation Materials. 2-Nitropyridine was prepared by oxidation of (EG) t e c h n i q ~ e . ~Technical ~>~5 details and the vacuum 2-aminopyridine (Fluka);29 m p 71" (ethanol); (lit.2971"). apparatus have been described previously.22 34 The EG 3 - N i t r o p y i d i n e was obtained by oxidation of 3-aminopyritechnique presents the advantage of reaching almost comdine30 and separated on a silica column; mp 36" (ethaneplete reduction and allows the measurements in parallel e prepared ethyl acetate); (lit.30 35-36'). 4 - N i t r o p ~ r i d i n was of esr and uv spectra of radicals. which give more confiby reduction of 4-nitropyridine N-oxide ( F l ~ k a ) m ; ~p ~ dence on the assignment of observed electronic spectra to 47.5" (ligroin); (lit.31 47.5"). 8-Nitropyridine N-oxide was anion radicals. prepared by oxidation of 2-aminopyridine ( F l ~ k a ) m ;~ p~ Reduction potentials for each compound were evaluated 86" (ethanol); (lit.32 85-86'). 3 - N i t r o p ~ r i d i n eN-oxide was from polarographic curves recorded in acetonitrile at room prepared by oxidation of 3-aminopyridine (Fluka);33 mp temperature, and the half-wave potentials are collected in 173" (ethanol); (lit.33 172-173'). 4-Nitrop? ridine N-oxide Table I. A multipurpose AMEL Model 463 polarograph was a Fluka product, purified from acetone; m p 159". .4cwas used. etonitrile was Merck UVASOL 16/66. I t was further puriEsr spectra were obtained with a Varian 4502 Xfied following a previously described procedure.34 band spectrometer with a 100-kHz field modulation. ElecThe Journal of Physical Chemistry, Vol. 77, No. 23, 7973
2748
Gamba, Malatesta, Morosi, Oliva, and Simonetta
TABLE IV: Spin Densities and Hfs Coupling Constants Calculated by INDO Method, and Experimental Values for Magnetic Nuclei in Pyridine, Nitropyridines, and Pyridine N-Oxide Calcd [A]" Anion radical
Position
1'
N
Calcd [B]" a,
P1
( S 2 )= 0.7862
2 3 4
3'
N (-N+O)
2 3 4
0.0211 - 0.0070 0.0007 - 0.0162
8.022 -3.759 0.365 -8.739 (S2) = 0.7815 0.0076 2.876 -0.0049 -2.641 0.0027 1.437 -0.0062 -3.323 0.0031 1.671 0.0194 7.349 (S2) = 0.7770 -0.0040 - 1.535 -0.0052 -2.781 -0.0063 -3.384 0.0033 1.785 -0.0056 -3.028 0.0234 8.876 (S2) = 0.7871 0.0096 3.660 0.0022 1.210 -0.0060 -3.225 0.0172 6.526 (S2) = 0.7796 0.0270 10.261 -0.0053 -2,883 0.0009 0.472 -0.0126 -6.827
a,
PI
( S 2 ) = 0.7503
0.0071 -0.0023 0.0002 -0.0054
The Journal of Physical Chemistry. Vol. 77, No. 23, 7973
8.024
- 1.643
0.161 -3.816 ( S 2 ) = 0.7506 0.0025 2.860 -0.0016 -1.158 0.0009 0.632 -0.0020 - 1.456 0.0010 0.736 0.0065 7.342 (Sz\ = 0.7504 -0.0013 -1,516 -0.0017 -1.219 -0.0021 - 1.481 0.001 1 0.785 -0.0019 -1.327 0.0079 8.880 ( S 2 ) = 0.7506 0.0032 3.641 0.0007 0.533 -0.0021 -1.781 0.0058 6.514 (S2) = 0.7504 0.0091 10.278 -0.0018 - 1.261 0.0003 0.209 -0.0042 -2.985
a Calcd [A]: INDO wave function: calcd [B]: INDO wave function after quartet spin component annihilation. stated. In hexarnethylphosphoramide.' In liquid e In dimethylf~rmarnide.~
Figure 3. Experimental (a) and simulated (b) esr spectra for 2-nitropyridine N-oxide.
Exptlb
6.28c 3.14c 0.88c 9.10c
6.28d 3.55d 0.82d 9.70d
1.893 2.728 0.591 3.777 0.866 8.184 1.313 3.183 4.433 1.091 3.643 9.327 2.473 0.451 3.033 7.817 10.91" 3.0Ie 0.44" 8.51e
In acetonitrile except when otherwise
Figure 4. Experimental (a) and simulated (b) spectra tor 3-nitropyridine N-oxide.
2749
Uv and Esr Spectra of Nitropyridines TABLE V: Doublet-Doublet Transition Energy, Oscillator Strength, and Polarization for Electronic Bands in Pyridine, Pyridine N-Oxide, and Their Nitro Derivatives Anion Radicals Calcd ( R method) Anion radical
A€, eV
f
1‘
0.555
6.20
a In acetonitrile except when otherwise stated. Under vacuum: L. W, Pickett, M. E. Corning, M.Wieder, D, A. Sernenow, and J. M.Buckley, J. Amer. Chem. SOC.,75, 1618 (1958). K. K. Innes, J. P. Birne, and I. G. Ross, J. Mol. Specfrosc., 22, 125 (1967). The inversion of the first two transitions is In n-hexane. Polarization and Stark effect measurements on the justified by means of the analysis of the excited states wave functions, see Table V I electronic origin of the transition (at 3.87 eV) show that the transition is polarized in the moiecular plane along the short axis;"' the first two calculated transitions are predicted in the inverse order. g Reference 6. In n-pentane.
t i ~ n is, ~in perfect agreement with our assignments. Nitrogen coupling constants were evaiuated from M spin dens;-ties using well known relationships given as footnotes in Table 111. Since we could not find in the literature a generally accepted relationship between P and R spin densities and coupling constants for the different kinds of nitrogen atoms present in our molecules and we had few experimental data to obtain significant values of the necessary constants through a correlation, we did not calcuThe Journa! of PhysJCa!Chemistry, Vol. 77, No 23, 7973
*
late nitrogen coupling constants from spin densities obtained by the P and R methods. In the case of proton nuclei the coupling constants were assigned on the basis of ca!culated spin densities. The calculated proton coupling constants were obtained by means of the McConnell relationship. When I? and R spin densities were used, the constant Q was evaluated through a linear regression procedure on the experimental a11 values. The best Q's are equal to -23.3 and -23.7 G, re-
2751
Uv and Esr Spectra of Nitropyridines
TABLE VII: Major Contributing (%) Configurationsa in PPP and MIM Wave Functions of Nitropyridines and Nitropyridine N-Oxides for Excited Statesb - Experimentally Found According to Our Assignments -_______ MIM
PPP Configurations
2
3
4
6
7
8
4.903 4.888 6.381 4.674 4.989 6.31 1 4.280 4.981 6.274 3.241 3.974 4.843 5.293 2.927 3.948 5.016 5.281 6.334 3.698 5.084 6.180
0.06 0.20 0.56 0.16 0.12 0.58 0.14 0.12 0.83 0.19 0.04 0.59 0.21
0.05 0.26 0.57 0.29 0.17 0.65 0.25 0.08
T ~ ~ ( 4 3 . 5L)i;( 2 8 . 0 ) ; Lz(11.6) T3A(44.5); Lp(22.5): Li(8.4) L3 (66.0) ; L4 (7.0) Li(57.0); T ~ ~ ( 1 3 . 2 ) L ~ ( 4 2 . 5 ) ;L i ( 1 3 . 6 ) ; T3*(10.5); T2A(9.5) Lz(26.2); T3A (18.5) Li(62.0); T ~ ~ ( 2 2 . 0 ) L2 (54.0) ; T2* (17.4) ; L3 (7.3) Lz(31.2); T ~ ~ ( 2 3 . 0 L3(14.5) ); T4A(38.8); L i ( 2 4 . 8 ) Lz(41.8); L i ( 1 6 . 9 ) Lz(20.O); T4A(15.4); Ls(13.0); Ll(12.0) Lj(23.6); L3(18.5); T~,*(15.8); T3*(14.6) L i (48.8); T4A(30.3) Lz (75.5) T4*(17,5); T3*(15.4); Lj (14.0); L3(11 .O) TdA(26.2): T3A(21.3): L i ( 1 7 . 1 ) Ls(52.1); TzA(14.0) ; T4'(3.6) L ~ ( 5 7 . 2 ) ;T4*(19.3) L3 (49.5) ; T3A (1 5.6) L.4(1 6.4)
AE,eV
f
4.729 5.519 6.452 4.863 5.336 6.512 4.504 5.888 6.500 3.088 4.042 4.985 5.733 3.447 4.282 5.534 6,043 6.393 3.881 5,191 6.512
0.09 0.14 0.23 0.08 0.22 0.63 0.04
0.07 0.72
0.09 0.18 0.37 0.36 0.01 0.29
0.55 9.28 0.28 0.43 0.52 0.26
Configurations L l ( 8 3 . 0 ) ; T3A(13.2) T3*(57.0); Lz(22.6); L I (1 2.5) Ls(31.4) ; T2A (29.7) ; L4( 19.6) Li(89.1); T3*(8.7) T3* (58.2): Lz( 16.1) ; Tz* (12.7) ; LI (6.1) Tz(38.4): L4(26.4); Ls(21.6); Lz(6.8) Li(82.1): T3"(17.6) Lz(52.3); T ~ ~ ( 3 8 . 6L3(7.9) ); La(46.1); T2A(32.3); L3(20.8) L j (54.0); T4*(35.4) Lz(52.1); L i ( 3 2 . 0 ) L2(37.6); TdA(36.4); L3(16 7); Li(9.2) L3(54.0) ; T4*(15.5) : L4( 13.7) ; T3* (1 1.8) Li(89.0); T4"(9.3) Lz(96.8) L3(66.0); T4A(23.8); L i ( 2 . 8 ) : T3*(2.5) L4(38.1); TdA(27.4); T3A(25.0); L i (3.6) TaA(35.0); L3(27.7); L4(24.O) Lz(86.5); T4*(13.0) L3 (85.5) ; T3* ( I 3.2) L4 (82.5)
a Confiaurations with weiaht less than 10% in both calculations, or with weight less than 1%, have been 2. L, for nhecules 6 7, and-8 from ref 20
omitted.
L, for molecules 2, 3, and 4 from ref
spectively. The corresponding plots of the proton coupling constants as a function of the spin density on contiguous carbon atom are shown in Figure 6. When M spin densities were used the Q constant was assumed equal to -27 G.9 Spin densities and coupling constants for pyridine, nitropyridines, and pyridine N-oxide calculated by the INDO method, before and after annihilation of the quartet component, are shown in Table IV. Calculations for nitropyridine N-oxides gave unsatisfactory results; for exI + I ample in the case of 4-nitropyridine N-oxide the coupling constants of the two different nitrogens are predicted in reverse order with respect to experimental e ~ i d e n c e . It~ . ~ ~ is probably due to the fact t h a t too many heteroatoms, and in different bonding situations, are present in these molecules: an ad hoc parametrization would be needed, but it would not be justified here owing to the paucity of available data. For considered pyridines INDO calculations without projection are in fair agreement with experiment. Annihilation of quartet components preserves the correct order of the proton constants, but the absolute values are generally -01 0.1 0.2 0.3 QC worse. Figure 6. Regression of theoretical spin densities at carbons on On the whole the values calculated by the different observed coupling constants of adjacent protons: a , spin densimethods (up to five in some cases) allow the prediction of ties calculated by R method: r = 0.929, n = 26; b, spin densithe coupling constants with confidence, since all the ties calculated by P method: r = 0.932, n = 26. methods agree almost in all cases. The only exceptions are found for the M method; in 3-nitropyridine aFI6 > aH4 according to the M method, while aH4 > a136 according to the INDO, P, and R methods; in 2-nitropyridine N-oxide with transition energies, oscillator strength, and polarizations, calculated by the R method for the same six radia ~> 3 aH6 according to the M method, while aH6 > a H 3according to the P and R methods, and in 3-nitropyridine cals as well as for pyridine and pyridine N-oxide anions, N-oxide a ~ 4> O H 6 according to the R and P methods, for which no experimental data are available. The experi4 to the M method. while aHG > a ~ according mental data for 3- and 4-nitropyridine anions obtained by Uu Spectra. We measured visible and uv spectra in acemetal reduction are also shown for the sake of comparitonitrile for the same six radicals for which esr spectra son. The assignment of experimental bands to electronic were taken. The results are shown in Table V together transitions was made taking into account both calculated
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The Journai of Physical Chetnisiry, Voi. 77, No. 23, 7973
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Garnba, Malatesta, Morosi, Oliva, and Sirnonetta
energies and intensities. Unfortunately with the technique used in the present work it is not possible to measure intensities. The agreement for the transition energies is satifactory but not excellent as may be expected for open shell systems. In all cases, however, the appearance of absorption in the visible region is correctly predicted. The differences in solvents and methods of preparation do not significantly influence the measured spectra (see the last column in Table V). In Table VI our experimental data (in one or two solvents) for the parent neutral molecules of the six nitrosubstituted anions are reported together with the results of PPP and MIM calculations. Experimental data from the literature and our PPP calculations for pyridine and pyridine N-oxide are also reported. For pyridine and pyridine N-oxide the interpretation by PPP calculations of the electronic bands in the uv region is correct if the fact is taken into account that this method (without an ad hoc parametrization) systematically underestimates the energy of the second transition of azines.38J9 As a consequence, in the case of pyridine N oxide, owing to the smaller energy difference, the first two transitions are predicted in reverse order. In this case, indeed. polarization measurements show that the first band is polarized in the molecular plane along the short axis.40 The same situation occurs with 2-nitropyridine and this fact can be confirmed by the configurational analysis of excited states calculated by the PPP method in terms of MIM configurations, as shown in Table VII. For the other molecules shown in the same table, a complete correspondence between PPP and MIM wave functions for excited states can be observed. On the whole MIM calculations agree better with experiment than PPP calculations as far as transition energies and oscillator strengths are concerned. The data reported in Table VI1 put into evidence the following points. In general no band can be classified as a pure chargetransfer or locally excited bands. but a mixing of configurations of the two types is always present to a different extent. The lowest energy locally excited configuration ril ( A 2 in the case of 4-nitropyridine N-oxide) and the lowest energy charge-transfer configuration (T3A in nitropyridines and TqAin nitropyridine N-oxides, respectively) give the major contribution to the first excited state for all molecules. We have assigned the first band of nitropyridine N-oxides to a K* *- T transition on the basis of the values of oscillator strengths (0.013, 0.012, and 0.413 for 2-, 3-, and 4-nitropyridine N-oxides, respectively), which are rather high for a T * n transition.41 The uv spectra of the three isomers recorded in a protic solvent (methanol) show a blue shift for all the bands, as is usual for the K* + K bands of aromatic N-oxides,G so that in this case the solvent effect cannot distinguish a K* *- T from a K* n transition. In conclusion, it may be said that semiempirical methods (for example, the PPP method) with one parametrization can be used for the description of the eiectronic structure of a series of molecules and radical anions in ground and excited states.
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?he Journal of Physical Chemistry, Vol. 77, No. 23, 1973
Acknowledgment. The authors are indebted to Dr. G. F. Tantardini for the configurational analysis program. Supplementarq Material Available. Parameters for PPP and MIM calculations will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 2OX reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-73-2744.
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