J. Phys. Chem. 1986, 90, 1541-1547
1541
Electronic Structure and Classification of Electronic Transitions in Some Parapyridinophanes Antoni K. Wisor and Leszek Czuchajowski* Department of Organic Chemistry, Silesian University, Katowice 40006, Poland, and Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: August 28, 1985; In Final Form: October 9, 1985)
The combined CNDO/S and transition density matrix methods reproduced very well the UV spectra of [2]paracyclo[2](2,5)pyridinophane and of four isomers of [2,2](2,5)pyridinophane, as well as the spectrum of [2.2]paracyclophane, the latter compound considered as a reference, not a heterocyclic phane. The excimer transitions were characterized by localization numbers showing the contributing exciton-resonanceand charge-resonancestates. The transannular effect, which influences the third KR* band most strongly, decreases in the order of pseudo-geminal and pseudo-para [2.2](2,5)pyridinophanes (identical with [2.2]paracyclophane)> [2]paracyclo[2](2,5)pyridinophane > pseudo-ortho and pseudo-meta [2.2](2,5)pyridinophanes. Both n-a* transitions are totally localized on pyridine ring(s), the localization number of the excitation being almost identical for all pyridinophanes under consideration.
Introduction Since the synthesis of [2.2](2,6)pyridinophane by Baker et a1.l the knowledge of that group of heterophanes has rapidly expanded.2 UV spectroscopy, so useful in the explanation of the electronic structure of phanes as layered compounds, as far as pyridinophanes are concerned, provided only limited data and no information concerning electronic transitions. Pyridine is isoelectronic with benzene and so are pyridinophanes with regard to cyclophanes. However, while the changes of UV spectrum of pyridine when compared to the benzene spectrum concern only a small enhancement of the long wavelength absorption which represents the a-a* transition ( a band) and the new na* band covered by the former, a similar analogy between the pyridinophanes and cyclophanes cannot be expected. This is due to the interaction in pyridinophanes between the lone pair electrons on the nitrogen atom and the a-electron system in the ring,3 which appears in addition to the normal transannular aelectron interaction between the layered rings. The present paper describes the influence of these two types of interactions on the electronic structure of [2]paracyclo[2](2,5)pyridinophane, 1, and [2.2](2,5)pyridinophane, existing as the pseudo-geminal, ps-g, 2, pseudo-para, ps-p, 3, pseudo-ortho, ps-0, 4, and pseudo-meta, ps-m, 5, isomers, Figure 1. The discussion takes advantage of the application of the transition density matrix D,4 which had been used for the first time in the interpretation of electronic transitions in heterophanes. Theoretical Approach The appearance in pyridinophanes of an additional interaction between N : and the a-aromatic sextet makes the decision concerning the choice of a calculation method a very important one. Of the semiemirical methods originating directly from the Hartree-Fock-Roothaan method, the all valence electron approximation, AVE, seemed to be preferable to the a-electron approximation. The latter PPP method was successfully applied to cyclophane~;~-~ however, when applied to systems containing an ~~
( I ) Baker, W.; Buggle, K . M.; McOmie, J. F. W.; Watkins, 0. A. M. J . Chem. Soc. 1958, 3594. (2) Majestic, V. K.; Newkome, G. R. Top. Curr. Chem. 1982, 106, 79. (3) Bernardi, F.; Colonna, F. P.; Dembech, P.; Distefano, G. Chem. Phys. Lett. 1975, 36, 539. (4) (a) McWeeny, R.; Sutcliffe, B. T. Methods of Molecular Quantum Mechanics; Academic: London, 1969. (b) Luzanov, A. V. Usp. Khim. 1980, 49, 2086 (Engl. transl.: Russ. Chem. Rev. 1980, 49, 1033). ( 5 ) Wisor, A. K.; Czuchajowski, L., manuscript in preparation. (6) (a) Koutecky, J.; Paldus, J. Collect. Czech. Chem. Commun. 1962, 27, 599. (b) Tetrahedron 1963, 19, Suppl. 2, 201. (c) Theor. Chim. Acta (Berlin) 1963, I , 268.
inseparable pair of electrons on the heteroatom,lOJ’an additional n-type orbital had to be considered. This gave the whole scheme a quasi-r-electron character which we found disadvantageous because of its approximate nature and the lack of precise substantiation of accepted assumptions. The decision concerning the use of the AVE approximation in this paper was based, first of all, on good simulation by its standard schemes (CNDO, INDO) of the UV spectra of not only benzene, pyridine, and their derivatives, but also of the [2.2]meta- and paracyclophanes and the pyridinophanes under consideration. This criterion had been best fulfilled by the C N D O / S scheme in Del Bene and Jaffe parametrization,l* modified by Ellis et al.,I3 in which the Nishimoto-Mataga approximationi4 was used to evaluate two-center Coulomb integrals. It has to be said that application by Duke et al.Is of a different parametrization to CNDO resulted in an excitation energy and intensity values which did not reproduce the UV spectrum of [2.2]paracyclophane as satisfactorily as the CNDO/S method used in this investigation. It is characteristic that the INDO/S methodt6applied to [2.2]paracyclophane gave completely erroneous results. In the C N D O / S method used here, mixing of singly excited configurations, CI- 1, was applied to generate the excited states, the choice of the latter being governed by an energy criterion. For benzene, pyridine, and their derivatives, 30 configurations were considered, while 50 configurations of the lowest energy were taken into consideration in case of the phanes. For a description of the electronic transitions the method of Luzanov” based on an investigationof the transition density matrix (7) (a) Vala, M. T., Jr.; Hillier, I . H.; Rice, S. A,; Jortner, J. J . Chem. Phys. 1966, 44, 2 3 . (b) Hillier, I. H.; Glass, L.; Rice, S. A . J . Chem. Phys. 1966, 45, 3015. (c) J . Am. Chem. Soc. 1966, 88, 5063. (8) Iwata, S.; Fuke, K.; Sasaki, M.; Nagakura, S.; Otsubo, T.; Misumi, S. J . Mol. Spectrosc. 1973, 46, 1. (9) (a) Czuchajowski, L.; Pietrzycki, W. J . Mol. Struct. 1978,47, 423. (b) Czuchajowski, L.; Wisor, A. K.; MaSlankiewicz, M. J. Monatsh. Chem. 1981, 112, 1175. (c) Wisor, A. K.; Czuchajowski, L. Ibid. 1983, 114, 1023. Wisor, A. K.; KuS, P.; Czuchajowski, L. Ibid. 1983, 114, 1213. (10) Fischer-Hjalmars. I.; Sundtmm, M. Acta Chem. Scand. 1968,22,607. (1 1) Luzanov, A. V.; Pedash, V. F. Opt. Spektrosk. 1977,43, 176 (Engl. transl.: Opt. Spectrosc. 1977, 43, 96). (12) Del Bene, J.; JaffE, H. H. J . Chem. Phys. 1968, 48, 1807. (1 3) Ellis, R. L.; Kuehnlenz, G.; J a m , H. H. Theor. Chim. Acta (Berlin) 1972, 26, 13 1. (14) Nishimoto, K.; Mataga, N. Z . Phys. Chem. (Frankfurt am Main) 1957, 12, 335. 1957, 13, 140. (15) Duke, C.B.; Lipiari, N. 0.;Salaneck, W. R.; Schein, L. B. J . Chem. Phys. 1975, 63, 1758. (16) Ridley, J.; Zerner, M. Theor. Chim. Acta (Berlin) 1973, 32, 1 1 1. (17) Luzanow, A. V.; Pedash, V. F. Teor. Eksp. Khim. 1979, 15, 436 (Engl. transl.: Theor. Exp. Chem. 1979, 15, 338).
0022-3654/86/2090-1541$01.50/00 1986 American Chemical Society
1542
The Journal of Physical Chemistry, Vol. 90, iVo. 8, 1986
Wisor and Czuchajowski
TABLE I: Calculated and Observed Electron Transitions for Pyridine and 2S-Lutidine
pyridine symmetry
type
l’B, I’B2 ( R ) I’A, I ’ A , (p) 2‘A2 2’Bl ( 6 )
n-A*
T-H*
n-n-* T-T*
O-H*
H-T*
2,Slutidine
energy, nm exptD calcd 288 279.5 260 251.6 211.0 201 199.4 179.5 173.8
P calcd 0.004 0.055
expt“ 0.003 0.029
0
0.098
0.085
0
1.003 0.90
2‘A, (p’)
0.925
173.5
symmetry
energy, nm calcd expt“ 281.1 276 266.5 270 212.8
U-T*
1‘A 2’A ( a ) 3’A 4’A (p) 5‘A
H-H*
6’A (0)
177.1
7’A (p’) 8’A
175.9
n--K*
J-T*
n-T*
177 H-T*
type
T-H*
T-H* P-Y*
212.1
185.7 173.9
210
f” calcd
expt“
0.004
0.056 0.075
0.097 0.013 0.201
0.224
0.001 0.334
0.853 0.010
“Experimental data for pyridine and 2,j-lutidine were taken respectively from ref 29 and 23. */denotes the oscillator strength
3’
2
Figure 1. The cyclophanes under consideration: ( 1 ) [2]paracyclo[2](2,5)pyridinophane; (2)-(5) isomers of [2.2](2,5)pyridophane: (2) pseudo-geminal, (3) pseudo-para, (4) pseudo-ortho, (5) pseudo-meta; ( 6 ) [ 2.2jparacyclophane.
was used, which allows one to achieve a simple quantitative characterization of the localization of excitation. In this method the basic magnitude which characterizes participation of A 0 I p ) in the 14) 14,) excitation is the localization number L, (%). For any transition the sum of L, over all AO’s of the molecule equals, ex definitio, 100%. L, numbers are defined by the transition density matrix because the positive diagonal elements of D2 matrix in the A 0 basis are always considered as the contribution of atomic orbitals p into excitation18
-
Localization of excitation on the distinguished group of AO’s is defined as
-
Localization of transition 14) Id*) on a distinguished group of orbitals means that the natural orbitals In) and In’) are localized In’) which are the on it. The one-electron transitions In) components of the multielectron transitions 14) 14.) take place between them. Consequent application of the transition density matrix to the description of electronic transitions showed” that, when the whole set of AO’s is divided into a number of distinct groups A B + ... C, then
--
+
+
where LA represents the localization of excitation on the distinpartial localization guished group of AO’s A, and [,--the number-describes the probability of the excited electron residing on the group of orbitals A. Similarly, lA-B represents the so-called charge-transfer number describing the probability of charge transfer from the A to B group orbitals during the excitation. The advantage of these numbers consists of their natural relation to the observed charge transfer, i.e. to the change of electron density AqA on the distinguished group of orbitals A: IqA =
(IB-A BfA
-
[A-B)
(4)
The method provides several relevant details concerning the (18) Luzanov, A . V.; Sukhorukov, A. A,; Umanskii, V . E. Teor. Eksp. Khim. 1974, 10, 456 (Engl. transl.: Theor. Exp. Chem. 1974. 10. 354)
electronic structure of excited states which cannot be achieved by the usual orbital-configurational approach. This concerns, in particular, the case when a molecule consists of two isolated, identical moieties, A = B. Then, the wave function of the excited state represents either a function of the exciton-resonance (ER) or of the charge-resonance (CR) type. However, when a perceptible overlapping of electron shells of separated fragments of the molecule takes place, as happens in the phanes, excimer states appear which represent a combination of the ER and C R s t a t e ~ . l ’ . ~In~ the case under consideration, the sum of partial localization numbers l A lB represents the contribution of the ER states and the 1A-B + lB-Asum concerns that of the C R type in the state of excimeric excitation.2’ Previous applications of the above-mentioned method concerned only a-electron systems4s5in which, while analyzing the localization, the participation of only the atom or the molecular fragment was considered in the excitation. In the present paper the whole set of AO’s is divided into some individually distinguished groups. Based on the symmetry properties to these groups AO’s have been assigned, which represent the basis for MO’s of the n, K , u, T * , and u* type. In this way the transitions A-A* and u-u* could be ascribed to l A and the transitions n-a*, n-u*, u-K*, and T-CT*could be ascribed to As an example of this approach, pyridine had been considered earlier by the authors.22 AqA reflects the “flow” of electrons density during excitation. Because of the character of the compounds considered here some of the above-listed types of transitions can be localized on specific fragments of a molecule (local excited transition, LE) and some can be connected with intramolecular transfer (CT transitions). The calculations performed for parapyridinophanes took into consideration the electron spectra provided kindly by B~ekelheide.~~ Because 2J-lutidine can be considered as the structural unit of the investigated pyridinophanes, it was essential to extend calculations to that compound also and, in addition, to [2.2]paracyclophane, representing the analogue of parapyridinophanes. In case of these reference compounds the experimental UV spectra were taken from the following sources: 2 , 5 - l ~ t i d i n e ,[2.2]~~ p a r a c y ~ l o p h a n e .The ~ ~ ~parapyridinophane ~ geometry was taken from [2.2]para~yclophane.~~ In all pyridinophanes, 1.34 A was taken as the length of the C-N bond, as in pyridine.26
+
Results Pyridine and 2,5-Lutidine. The calculations showed all changes observed in the experimental spectrum of 2,Wutidine as compared to pyridine spectrum, see Table I: the small hipsochromic shift (19) Murrell, J. N.; Tanaka, J. Mol. Phys. 1964, 7 , 363. (20) Azumi, T.; Armstrong, A . T.; McGlynn, S. P. J . Chem. Phps. 1964, 4 1 , 3839. (21) Luzanov, A. V. Teor. Eksp. Khim. 1977, 13, 579 (Engl. transl.: Theor. Exp. Chem. 1977, 13, 433). (22) Wisor, A. K.; Czuchajowski, L.,submitted for publication. (23) Boekelheide, V., private communication. (24) Cram, D. J.; Allinger, N . L.; Steinberg, H. J . A m . Chem. Soc. 1954, 76, 6132. ( 2 5 ) (a) Brown, C. J . J . Chem. SOC.1953, 3265. (b) Lonsdale, K.; Milledge, J.; Rao, K. V . K. Proc. R. SOC.London, Ser. A 1960, 255, 82. (c) Hope, H.; Bernstein, J.; Trueblood, K. N. Acta Crystallogr., Sect. B 1972, 28, 1733. (26) Bak, B.; Hansen-Nygaard, L.; Rastrup-Andersen, J. J . Mol. Spectrosc. 1958, 2, 361
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1543
Electronic Transitions in Some Parapyridinophanes
TABLE II: Calculated and ObservedeElectron Transition for [2.2]Paracyclophane and [21Paracyclo[2]( 2,S)pyridinophane
[ 2.21paracyclophane type n-n* (ER, CR)" r-n* (ER, CR)
n-n* (ER, CR) n-n* (ER, CR) n-a* (ER, CR) n-n* (ER, CR) n-n* (CR, ER) n-n* (CR, ER) u-n* (LE)* n-n* (CR, ER) n-n* (CR, ER) u-n* (LE)
symmetry 192$ 1'B3"
[2]paracyclo[2]pyridinophane
energy, nm calcd expt 336.4 294.7 286.1 253.8 246.3 245.3 226.4 220.4 206.2 204.8 200.0 199.4
305, sh 286
244, sh
225
r" calcd 0 0.033 0 0 0 0.102 0.084 0.002 0.003 0.691 0.975 0
expt 0.004 0.007
0.105
type n-n* (ER, CR) n-n* (LE, pyridine) n-n* (ER, CR) n-n* (ER, CR) n-n* (LE, pyridine) n-n* (CR, ER) n-a* (ER, CR)
symmetry 1IA 2IA 3IA 4'A 5IA 6'A 7'A 8'A 9'A 10'A 11'A 12IA
(CR, ER) n-n* (CR, ER) n-n* (CR, ER) T-X* (CT)' n-n* (CR, ER) A-T*
0.760
energy, nm calcd expt 330.5 320.3 291.5 281.1 251.5 244.1 241.0 236.1 225.2 216.5 202.4 201.4
308 287
r" calcd
expt
0.005 0.002 0.058 0.007
0.020 0.037
0 240, sh
:Fi
Log E
2
2
215
0.019 0.119 0.006 0.034 0.033 0.484 0.265
0.106
0.422
ER, CR and CR, ER denote respectively the excimeric excited states with predominana of exciton resonance or charge resonance. LE denotes excitation in the aromatic ring.
