J . Phys. Chem. 1988, 92, 4892-4898
4892
a-
CH, I
6
7
and D3 conformers; (2) a shallow potential was found for twisting of the methylene groups. Similar results have been obtained for XP(=CH2)2 (X = H or C1) in the present work. However, the present investigation indicates somewhat stronger phosphoruscarbon * bonding in the case of bis(methy1ene)phosphoranes. Phosphorus d orbitals play a significant role in accepting electron density from the methylene groups either as pure d orbitals (as in HP(=CH2),) or as pd hybrid orbitals (as in C1P(=CH2),).
This is somewhat reminiscent of the bonding in transition-metal carbene complexes,30 although in these latter cases d-orbital participation is obviously much more pronounced. Finally, an examination of the relative energies of 1-3 reveals the cyclic phosphirane structure to be the most stable isomer, whereas the open bis(methy1ene)phosphorane structure is the least stable isomer. Acknowledgment. We thank the National Science Foundation and the Robert A. Welch Foundation for financial support. (30) References for both experimental and theoretical rotational barriers for transition-metal carbenes may be found in: (a) Dobbs, K. D.; Hehre, W. J. J . Comput. Chem. 1987,8, 861. (b) Dobbs, K. D.; Hehre, W. J. J. Comput. Chem. 1987,8, 880.
Photoelectron Spectra and Theoretical Studies of Bonding in Strained Quinolines' William R. Moomaw,* Daniel A. Kleier: J. Hodge Markgraf, John W. Thoman, Jr., Department of Chemistry, Williams College, Williamstown, Massachusetts 01 267
and J. Neil A. Ridyard Perkin-Elmer Ltd., Beaconsfeld HP9 1 QA. England (Received: January 22, 1988)
The effect of ring strain on the bonding, ionization potentials, and basicities of quinoline and pyridine is explored. A systematic analysis of the photoelectron spectra of alkyl- and cycloalkylquinolinesand a comparison with the corresponding naphthalenes confirm the assignment of the third ionization potential of quinoline at 9.4 eV to the n-orbital. This assignment is supported by all-electron,molecular orbital calculations using the PRDDO (partial retention of diatomic differential overlap) approximation. These calculations reveal extensive bent-bond character involving the carbons at the cyclobutene ring juncture. Geometry optimization predicts a significant decrease in the CNC bond angle from 117' in quinoline to 1 1 2 O in cyclobuta[b]quinoline, making this the smallest CNC bond angle reported to date for a planar aromatic heterocycle. Surprisingly, both photoelectron spectroscopy data and the calculations show that ring strain produces no more net effect on the n-ionization potential than does 2,3-dimethyl substitution. This result is counter to the predictions of a strain-induced orbital rehybridization model but is well accounted for by the offsetting effects of lone-pair hybridization arising from the smaller CNC angle and the bent-bond consequences for ring strain. Delocalization of the n-orbital onto the methylene groups also appears to play a role. A comparison of our theoretical and gas-phase results with solution data demonstrates that orbital models do not fully account for the effect of ring strain on measured solution basicities.
Introduction The development of photoelectron spectroscopy (PES) has made possible the direct experimental study of many important aspects of chemical bonding. In particular, the technique has been used extensively to measure the ionization potentials (IP) of aromatic hydrocarbons and their nitrogen analogues and to correlate these with theoretical calculations of n-, u-, and ?r-orbital energies based upon Koopmans' t h e ~ r e m . ~It is not always easy, however, to correlate a particular observed ionization potential in the photoelectron spectrum to a specific orbital. As a result, a number of conflicting assignments of n- and *-ionizations, even for such but this problem has molecules as pyridine, have (1) (a) Strained Heterocyclic Systems. 10. Part 9: Markgraf, J. H.; Antin, J. H.;Walker, F. J.; Blatchly, R. A. J . Org. Chem. 1979, 44, 3261. (b) Preliminary reports of this work were presented at the 29th (1974) and 36th (1981) Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH; abstracts MH 12 and MH 10, respectively. (2) Dreyfus Teacher-Scholar 1979-1981. A portion of this work was carried out by D.A.K. at the Theoretical Division, MS 569, Los Alamos National Laboratory. Present address: Agricultural Products Department, E. I. du Pont de Nemours and Co., Inc., Experimental Station, Wilmington, DE 19898. (3) Kooprnans, T. Physica 1933, 1 , 104. (4) (a) Al-Joboury, M. I.; Turner, D. W. J. Chem. Soc. 1964, 4434. (b) Turner, D. W. Tetrahedron Lett. 1967, 3419. (5) Dewar, M. J. S.; Worley, S. D. J. Chem. Phys. 1969, 51, 263.
0022-3654/88/2092-4892$01.50/0
recently been resolved by using polarized multiphoton laser techniques." In quinoline and several other azanaphthalenes, the n-ionization either is assigned to a small shoulder on a larger band or is totally buried under other bands and assumed to lie in that region of the PES.5J2-'4 Following Robin and co-~orkers,'~ van den Ham and van der Meer have utilized the large differences (6) Heilbronner, E.; Hornung, V.; Bock, H.; Alt, H. Angew. Chem., Int. Ed. Engl. 1969, 8, 524. (7) Baker, A. D.; Betteridge, D.; Kemp, N. R.; Kirby, R. E. J. Chem. Soc., Chem. Commun. 1970, 286. (8) (a) Gleiter, R.; Heilbronner, E.; Hornung, V. Angew. Chem., Int. Ed. Engl. 1970.9, 901. (b) Gleiter, R.; Heilbronner, E.; Hornung, V. Helo. Chim. Acta 1972, 55, 255. (9) Heilbronner, E.; Hornung, V.; Pinkerton, F. H.; Thames, S . F. Helu. Chim. Acta 1972, 55, 289. (10) (a) Palmer, M. H.; Gaskell, A. J.; Findlay, R. H. Tetrahedron Lett. 1973, 4659. (b) Palmer, M. H.; Gaskell, A. J.; Findlay, R. H. J . Chem. SOC., Perkin Trans. 2 1974, 118. ( 1 1) Berg, J. 0.; Parker, D. H.; El-Sayed, M. A. Chem. Phys. Lett. 1978, 56, 411. (12) Eland, J. H. D.; Danby, C. J. Z. Naturforsch., A : Phys., Phys. Chem., Kosmophys. 1968, 23, 355. ( 1 3) van den Ham, D. M. W.; van der Meer, D. Chem. Phys. Lett. 1972, 15, 549. (14) Brogli, F.; Heilbronner, E.; Kobayashi, T. Helu. Chim.Acta 1972, 55, 274. (15) (a) Brundle, C. R.; Robin, M. B.; Kuebler, N. A,; Basch, H. J . Am. Chem. Soc. 1972, 94, 1451. (b) Brundle, C. R.; Robin, M. B.; Kuebler, N. A. J . Am. Chem. SOC.1972, 94, 1466.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4893
Bonding in Strained Quinolines
t
4 e
,
?#
.
,
.
,
,
.
,
,
.
,
,
t
n,
.
.
J.
.
.
.
.
.
.
.
.
.
.
.
4
ELECTRON V O L T S
c
75
R,
. . ' a5. . Po. . 9s. . . . . . .
Bo
100
105
110
ELECTRON VOLTS
Figure 1. Photoelectron spectra, proposed ionization potential assignments, and structural formulas of the nine molecules studied. Spectra are energy corrected and, except for 7 and 8, normalized in intensity on the a3band above 10 eV.
in the substituent effect of fluorine to study a and 0 (or n) ionization potentials in quin01ine.l~ The principal purpose of the present study is to explore the influence of ring strain on chemical bonding, molecular geometry, and ionization potentials of nitrogen heterocycles. The effect of ring strain on a-electron IPS has been reported by Heilbronner and co-workers,1618 and more recently the influence of fused ring strain on the n and a IPS of pyrazine was investigated by Dewey et al.19 It was hoped that ring strain might provide a simple means of differentially shifting a and 0 (especially n) ionization potentials in a manner that would help in their identification. Finally, it is of interest to see if the n-ionization potentials correlate in any direct way with the solution-phase basicities observed for the series of methyl and cycloalkyl quinolines. In this work we have remeasured for comparison the IPS and PES of naphthalene (l), (16) Brogli, F.; Giovannini, E.; Heilbronner, E.; Schurter, R. Chem. Ber. 1973, 106,961.
(17) Clary, D. C.; Lewis, A. A.; Morland, D.; Murrell, J. N.; Heilbronner, E. J . Chem. SOC.,Faraday Trans. 2 1974, 70, 1889. (18) Heilbronner, E.; Hoshi, T.; von Rosenberg, J. L.; Hafner, K. Nouv. J . Chem. 1977, 1 , 105. (19) Dewey, H. J.; Miller, R. D.; Michl, J. J. Am. Chem. SOC.1982, 104, 5298.
2,3-dimethylnaphthalene (2), 2,3-dihydro-lH-cyclopenta[b]naphthalene (3), 1,2-dihydrocyclobuta[b]naphthalene(4), and quinoline (5) and report for the first time the IPSand PES of 2,3-dimethylquinoline (6),2,3-dihydro-lH-cyclopenta[b]quinoline (7), 6,7-dihydrocyclobuta[g]quinoline (S), and 1,2-dihydrocyclobuta[b]quinoline ( 9 ) ; structural formulas are presented in Figure 1. We have utilized all-electron, PRDDO, a b initio molecular orbital calculations to support the assignments of the IPS and to elucidate the effect of ring strain on bonding in these molecules. Photoelectron Spectra (PES) The photoelectron spectra are presented in Figure 1. The spectra and IPS of 1-5 agree with those published e l ~ e w h e r e . ' ~ ~ ' ~ . ~ ~ We will first briefly describe the naphthalene series and then deal with the more difficult task of analyzing the quinoline spectra. In the region below 11 eV three distinct electronic bands appear in the PES of all four naphthalenes. These bands have been unambiguously assigned to the first three a-ionizations (as, a4, and a3).I8 As can be seen in Table I and Figure 2, the elec(20) Santiago, C.; Gandour, R. W.; Houk, K. N.; Nutakul, W.; Gravey, W. E.; Thummel, R. P. J . Am. Chem. SOC.1978, 100, 3730.
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The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
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NAPHTHALENES
OUINOLINES Exparlmnt
Theory
Eaperimonl
Thwry
(9.C
5
8 7. p
10.0
c Y Y
E 4
9
5
\
7.
f
0.c
7.
0
G
N
L
= , .
7 \
....-
0
-L/-f== *s 8.0
t
2
3
4
1
2
3
s
4
8
7
8
0
YOLECULE
YOLECULE
Figure 2. Comparison of experimental and theoretical ionization potentials of the alkylnaphthalenes and -quinolines studied TABLE I: Experimental (PES) and Theoretical (Koopmans' Theorem) Ionization Potentials and Assignments (in electronvolts) and Solution pK, Values
1 (naphthalene) 2 (2,3-dimethylnaphthalene) 3 (2,3-dihydro-J H-cyclopenta[b]naphthalene) 4 (1,2-dihydrocyclobuta[b]naphthalene) 5 (quinoline) 6 (2,3-dimethylquinoline) 7 (2,3-dihydro-JH-cyclopenta[blquinoline) 8 (6,7-dihydrocyclobuta[g]quinoline) 9 (1,2-dihydrocyclobuta[b]quinoline)
n
=4
=5
exptl 8.13 7.93 7.8 3 7.92 8.62 8.32 8.28 8.33 8.36
theory 7.91 7.77 7.80 7.89 8.46 8.21 8.25 8.37 8.35
theory 8.97 8.62 8.61 8.71 9.35 8.96 8.97 9.06 9.03
exptl 8.98 8.54 8.45 8.57 9.18 8.71 8.65 8.82 8.75
=3
exptl
theory
9.39 9.11 9.05 9.22 9.15
10.26 10.05 10.05 10.16 10.11
exptl 10.08 9.84 9.82 9.93 10.63 10.21 10.33 10.59 10.41
theory 10.83 10.64 10.66 10.73 11.23 10.96 10.98 11.13 11.09
pK,
5.06' 5.99* 5.45b 5.45c 3.99c
"Streuli, C. A. Anal. Chem. 1958, 30, 997. bReference 26. 'Reference 33. tron-releasing alkyl groups lower the first and third IPS ( a 5 and r3)about equally and the a4ionization by about twice as much.2' This is consistent with the magnitudes of the p-a atomic orbital coefficients in the 2,3-positions of naphthalene (Figure 3). The strained cyclobutane ring (4)lowers the IP of a4by the same amount as cyclopentane (3) and 2,3-dimethyl(2) substitution but lowers the IP of a3and as only about one-half as much. The quinoline PES is seen in Figure 1 to be similar to naphthalene but is shifted to higher energy and shows much greater intensity in the region of the second IP. The first band at 8.62 eV has been previously assigned to the adiabatic as i ~ n i z a t i o n , ' ~ and the second and third as 0.155 f 0.01 eV (1290 cm-') vibrations, somewhat lower in frequency than found in the first naphthalene IP, 0. 1g5A 0.01 eV (1 570 cm-'). The band at 10.63 eV shows six vibronic bands spaced 0.07 f 0.01 eV (560 cm-') apart, identical with the frequency observed in the n3 ionization of na~htha1ene.l~ The extra intensity between 9 and 10 eV strongly implies the presence of two ionizations in this region. Our theoretical calculations and comparison with the napthalene and quinoline derivatives support Heilbronner's assignment of the strong feature a t 9.18 eV as the a4 ionization and the broad shoulder at ca. 9.4 eV to the n40nization.l~Reversing the order of this assignment requires all three PIPS to shift about equally between naphthalene and quinoline. The presence of a node (21) PBrkBnyi, C.; Levitt, B. W.; Levitt, L. S. Chem. Ind. (London) 1977, 356.
,438
I
-428
I
-.008
,077
-.350
,330 l3sd
[.023) 774
:294
-,422
7134
-.262
(350) 7-f3
Figure 3. Atomic coefficients for the three highest filled n molecular orbitals of quinoline and naphthalene calculated by the PRDDO approximation. The coefficients of naphthalene are given in parentheses and may be related to the other carbon atoms by symmetry. passing through the nitrogen atom in r4(Figure 3) requires that it be shifted less than the other two a-orbitals. The PES of all the alkyl- and cycloalkylquinoline derivatives are similar to each other in appearance. While the integrated area of the bands below 10 eV (where one expects to find the n and
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4895
Bonding in Strained Quinolines first two a-ionizations) is virtually unchanged from quinoline, the band shape is significantly altered. In place of the five spectral features seen in quinoline, only four appear in the derivatives. Following the strong origin band of the lowest ionization in all of the quinoline derivatives are two weaker and broader bands which appear initially to be vibronic structure. The spacing between the first and second bands is 0.16-0.19 eV, except for 8, where it is 0.21 eV. However, the spacing between the second and third bands of the quinoline derivatives is, in every case, larger by 0.04-0.07 eV. Such a large positive anharmonicity appears to lie outside the limits of error (0.02eV) and is inconsistent with the regularity of the spacing (fO.O1 eV) observed for quinoline and all of the naphthalenes. We therefore assign the third band to the second r-ionization potential, a4. The only remaining spectral feature in this region is a broad shoulder between 9.05 and 9.25eV, which we assign to the n-ionization. Reversing the assignment would cause the quinoline ionization potential for a4 to shift away from as and toward a3upon alkyl and cycloalkyl substitution, in contrast to what is observed for the naphthalenes. Our proposed assignments also agree with the order of our theoretically calculated ionization potentials. Assigning the third band in the spectrum to a vibronic band of the first ionization potential would leave us short one electronic ionization potential and with an unexplained positive anharmonicity. Finally, the shape of the broad shoulders is similar to the assigned n-ionization in quinoline. What is surprising about this assignment is the similarity of the n-ionization potential for the cyclobuta[b] derivative to that of the other three alkylquinolines. The band above 10 eV is readily assigned as the third a-ionization. In none of the derivatives does it retain any of the vibronic structure seen in quinoline or naphthalene. In 8 it is buried under a broad continuum of higher lying u and/or a-ionizations.
Molecular Orbital Calculations A rational assignment of ionization states to the features in the measured PES requires a model that can satisfactorily account for trends in IPS with substitution. To this end we have performed all-electron, molecular orbital calculations on pyridine and compounds 1-9 using the partial retention of diatomic differential overlap (PRDDO) approximati~n.~Z~~ The calculations employed a minimal basis set of Slater-type orbitals with standard Slater exponents, except that an exponent of 1.20was used for hydrogen. The molecular orbitals and their associated eigenvalues are analyzed to determine the effects of ring strain and substitution on chemical bonding, the IPS, and proton affinities. We use Koopmans' theorem3 to correlate the canonical molecular orbital energies with the n and highest three PIPS. Results are presented and compared with experiment in Tables 1-111 and in Figure 2. It was proposed by Streitwieser that fusion of a strained ring onto an aromatic system induces rehybridization of the u orbitals.24 High p character is presumed for the two u atomic orbitals of the fused aryl carbon, C2, which are involved in the strained ring.2s Hence, the remaining orbital which bonds to N (or C,) should have higher s character (i.e., be more electronegative). To maximize overlap with this hybrid, the orbital on N (or C,) would need increased p character, which would increase the s character in the n- (or C I H ) orbital. For 1,2-dihydrocyclobuta[b]quinoline (9), this model predicts an increase in the nitrogen lone-pair IP and a decrease in basicity relative to quinoline.26 The strain-induced orbital rehybridization model presumes that hybrid orbitals in a u bond are directed along the axis joining the (22) Halgren, T. A.; Lipscomb, W. N. J. Chem. Phys. 1973, 58, 1569. (23) Halgren, T. A.; Kleier, D. A.; Hall, J. H., Jr.; Brown, L. D.; Lipscomb, W. N. J. Am. Chem. SOC.1978, 100, 6595. (24) Streitwieser, A., Jr.; Ziegler, G. R.; Mowery, P. C.; Lewis, A.; Lawler, R. G. J. Am. Chem. SOC.1968, 90, 1357. (25) The following numbering system is used throughout this work:
ab 8
1
(26) Markgraf, J. H.; Scott, W. L. J. Chem. Soc., Chem. Commun. 1967,
296.
two bonded centers. However, several theoretical calculations strongly support a "bent-bond" orbital description in which orbitals essentially retain their hybridization and do not follow the debetween the formation angles of the n u ~ l e i . ~To ~ distinguish ,~~ induced orbital rehybridization model and the bent-bond model, we have carried out PRDDO calculations on pyridine as a function of the C C H angle, cy, and the C N C angle, p:
So as to better compare the bond angles and percent s character with the two proposed models, we have transformed the canonical u orbitals into their localized counterparts using the Boys criterion.29,30 This procedure has been shown to give a description that is very close to the valence bond hybrid orbitals used in both models.28 Analysis of PES data in terms of localized orbitals has a significant disadvantage: the localized orbitals cannot be directly associated with IPS since they are not eigenfunctions of the effective one-electron Hamiltonian. Nevertheless, they are related to the canonical orbitals by a unitary transformation. As such, changes in the orbital eigenvalue spectrum brought about by alterations in molecular structure should be correlated with changes in the localized orbitals. Specifically, the canonical orbital which consists predominantly of the nitrogen lone pair might be expected to respond to molecular deformations in a manner similar to that of the corresponding localized molecular orbital. Indeed, we observe that the change in s character of the canonical lone-pair orbital that is brought about by molecular deformation is nearly identical with that for the localized orbital of which it predominantly consists. Thus we choose to describe the response of the electronic structure to molecular deformations in terms of localized orbitals. For analysis purposes, the localized orbitals have the advantage of being directly related to the concept upon which the bent-bond and rehybridization models are based. A seen in Table 11, closing the a angle from 118' to 88' has a relatively small effect on the hybridization and hence direction of the atomic orbitals of the fused aryl carbon, C2. Therefore, very large bond-angle deviations are observed between the vector to the centroid of the hybrid orbital and the line joining the atomic centers as cy approaches 90'. For an angle of 92.8', which approximates the expected geometry of 1,2-dihydrocyclobuta[b]pyridine, the strained hybrid on C2 that forms the C2-H bond shows a deviation of 16.0'. The C2 hybrid forming the C2-C3 bond deviates by 7.5'. Somewhat to our surprise the s character for the nitrogen lone-pair orbital actually decreases by approximately 2% for this value of cy, and the I P drops by nearly 0.3 eV. The pyridine results are consistent with our calculations for 9 in which the parent quinoline framework of 6 was used. With these constraints the calculated I P for the lone-pair orbital of strained 9 was actually 0.27eV lower than for unstrained 6,rather than higher as predicted by the induced rehybridization model.24*26 These calculations strongly suggest that this model is much less useful than the bent-bond description. To account for the insensitivity of the n-IP to ring strain, we propose an alternative model for the effect of strain on the lone pair that emphasizes the response of the n-orbital to changes in geometry caused by fusion of a strained ring. To test the dependence of I P on bond angle, we varied the C N C angle, p, of pyridine (Table 11). Decreasing the CNC angle from 122' to 1 1'1 increases the s character of the n-orbital from 49.6 to 57.4%. In response to this change in hybridization, the lone-pair IP rises (27) (a) Coulson, C. A,; Moffitt, W. E. Philos. Mug. 1949, 40, 1. (b) Coulson, C. A,; Goodwin, T. H. J. Chem. SOC.1962, 2851. (c) Coulson, C. A.; Goodwin, T. H. J . Chem. SOC.1963, 3161. (d) Peters, D. Tetrahedron 1963, 19, 1539. (28) Chipman, D. M.; Palke, W. E.; Kirtman, B. J. Am. Chem. SOC.1980, 102, 3377. (29) Foster, J. M.; Boys, S. F. Rev. Mod. Phys. 1960, 32, 300. (30) Kleier, D. A.; Halgren, T. A.; Hall, J. H., Jr.; Lipscomb, W. N. J. Chem. Phys. 1974, 61, 3905.
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The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
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TABLE II: Summary of Theoretical Calculations for Pyridine: Dependence of Orbital Hybridization, Lone-Pair IP, and Proton Affinity' on the Angles a and B
A
B
C
IP for
PA for
pb
lone pair, eV
lone pair,
ab
118.0 113.4 108.7 102.1 97.5 92.8 88.1 122.8' 121.5 120.0 118.5 117.1 117.5d 91.6'
116.5 116.5 116.5 116.5 116.5 116.5 116.5 122.1 119.3 116.5 113.6 110.8 117.0d 111.4'
10.32 10.28 10.23 10.18 10.12 10.06 10.01 9.97 10.13 10.33 10.53 10.70 10.22 10.29
10.88 10.90 10.91 10.93 10.94 10.96 10.97 11.36 11.oo 10.88 10.74 10.62 10.88 10.76
eV
%s
N hybrid in C2-N bond angle %s dev
53.5 53.1 52.7 52.2 51.9 51.7 51.5 49.6 51.7 53.7 55.6 57.4 53.8 56.5
27.2 27.3 27.6 27.9 28.1 28.2 28.4 28.8 27.9 27.1 26.2 25.5 26.8 26.5
lone pair
7.17 6.75 6.32 5.74 5.40 5.07 4.77 10.43 8.90 7.35 5.70 4.1 1 8.11 3.06
C, hybrid in C2-N
C2 hybrid in C2-H
C2 hybrid in C2-C,
% S
bond angle dev
% S
bond angle dev
% S
bond angle dev
41.8 42.4 42.9 43.4 43.7 43.8 43.9 40.7 41.1 41.6 42.0 42.4 42.0 44.1
4.32 2.88 1.43 -0.54 -1.77 -2.95 -4.03 3.62 4.31 4.96 5.56 6.19 4.85 -2.05
37.9 37.1 36.3 35.2 34.5 33.9 33.4 37.6 38.0 38.3 38.6 39.0 38.2 35.0
0.82 2.17 3.54 5.36 6.46 7.49 8.31 2.32 1.07 .22 1.58 2.91 .53 9.34
38.4 38.7 39.0 39.5 39.9 40.4 40.9 40.4 39.3 38.3 37.2 36.2 39.4 39.7
0.14 3.02 5.94 10.04 13.00 16.05 19.23 1.06 1.11 1.12 1.14 1.16 0.23 15.16
"Proton affinity is calculated as the difference in energy between the neutral and protonated pyridine. bAll angles and angle deviations are expressed in degrees. cThe values of a were chosen so as to minimize the energy as 6 was systematically varied. dThese angles are identical with the corresponding angles in geometry-optimized quinoline. e These angles are nearly identical with the corresponding angles in geometry optimized cyclobuta [b]quinoline from 9.97 to 10.70 eV and the calculated proton affinities drop. We thus posit that the lone-pair IP in the strained quinolines is more sensitive to changes in the C N C angles than to changes in the hybridization of the fused aryl carbons.
Comparison of MO Calculations and PES for the Quinolines and Naphthalenes The orbital energies of the bicyclic molecules 1-9 were calculated by using the same methods described for pyridine. The geometries used in these calculations were determined as follows. All CH bond distances were held fixed at 1.09 A. The naphthalene geometry was taken from the X-ray s t r ~ c t u r e ,and ~ ' the CH axes were directed along the bisectors of the corresponding CCC angles. The structure of the parent framework was frozen for calculations on 2,3,and 4. The carbon atoms of all substituents were assumed to lie in the plane of the parent framework. For quinoline (5) and its cyclobuta[b] derivative (9) geometry optimization yielded the structures in Figure 4. For comparison we note that the calculated bond distances and angles for 9 agree reasonably well with those determined by X-ray crystallography (root mean square In par(rms) deviations of 0.020 A and 1.72', re~pectively).~~ ticular, our optimized C N C angle for 9 is 112.1', in excellent agreement with the experimental value of 112.5', the smallest such angle reported to date for an aromatic heterocycle. Efforts were also made to optimize critical geometric parameters in 7 and 8. For the dimethyl derivative, 6, methyl groups were simply substituted onto the framework of the optimized quinoline. The Koopmans' theorem IPSand those measured from the PES are shown in Figure 2 and are tabulated in Table I. One can readily see the general decrease in IPS as one proceeds from the parent compounds, 1 and 5, to any of the substituted derivatives. This is presumably due to the electron-releasing effect of the substituted alkyl groups relative to hydrogen and is in good agreement with the experimental PES. When one compares the substituted naphthalenes with the substituted quinolines, one is also struck by the similarity of the trends in n-ionization potentials. Addition of 0.38 eV to each 7-ionization potential for the naphthalenes reproduces the corresponding quinoline IPS with an rms deviation of only 0.05 eV. A summary of the relevant results from the molecular orbital localizations is presented in Table 111. Here we include the hybridizations of the fused aryl carbon (C,) and the adjacent nitrogen atom. The bond angle deviation of almost 16' in 9 clearly indicates the bent nature of the bonds that join the aryl carbons (31) Cruickshank, D. W. J. Acta Crystallogr. 1957, 10, 504. (32) Deroski, B. R.; Markgraf, J. H.; Ricci, J. S.,Jr. J . Heferocycl. Chem. 1983, 20, 1155.
Quinoline
Cyclobuta [b] Ouinoline Figure 4. Best optimized ground-state geometries of neutral quinoline and cyclobuta[b]quinolinecalculated by the PRDDO approximate molecular orbital method. Note the significant decrease in the CNC angle for the strained molecule.
with the aliphatic carbons of the fused four-membered ring. Note also that the p character in the C2 hybrid of the C2-C2' bond slightly decreases from sp1.61in 6 to in 9, Le., the percent s character increases by about 1.3%, contrary to the predictions of the strain-induced rehybridization model. Thus, in proceeding from the 2,3-dimethyl to the cyclobuta[b] derivative, the direction of this hybrid orbital on C2 changes by a relatively small amount compared with the altered direction of the C2-C2' internuclear axis. Therefore, the bond-angle deviation for the C2-C2' bond is found to increase monotonically from 3' in 2,3-dimethyIquinoline, 6,to 6' in cyclopenta[b]quinoline, 7,to 16' in cyclobuta[b]quinoline, 9. See Table 111. The large deviation in 9 is the value expected for a bent bond. The trends in lone-pair hybridization for the substituted quinolines are similar to the pyridine calculations, although the associated trends in IPSare modified by inductive effects. No significant change in lone-pair hybridization occurs in the 2,3dimethyl derivative even though the Koopmans' theorem IP de-
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4891
Bonding in Strained Quinolines TABLE III: Localized Molecular Orbital (LMO) Analysis for Alkyl-Substituted Quinolines N atom C2 atom
bond molecule 5 (quinoline)
%s
38.3
1.03
C2-N
26.4
7.64 42.5
5.11
Nlons pair
54.0
38.3
3.09
39.0
1.20 5.03
LMO C2-H c2-c3
6
(2,3-dimethylquinoline)
c2-c2' c2-c3
C2-N 7 (2,3-dihydro-IH-cyclo-
penta [blquinoline)
Nlonc p i r
c2-c2/ c2-c3
C2-N
8 (6,7-dihydrocyclobuta[g]quinoline)
Nlmc p i r
C,-H
Nlons pair
quinoline)
a
26.1 8.42 42.8 54.6 39.7
6.12
38.7 26.5 6.61 44.0 54.5
2.57
38.1
0.15
38.3
0.74
42.5
5.08
c;-c, C2-N
9 (1,2-dihydrocyclobuta[b]-
bond
angle angle de? % s de? 38.3 0.15
c2-c2'
26.4 7.42 54.2
3.88
C2-N
40.8 15.95 38.1 8.94 26.5 3.28 44.4 1.85
Nloncpair
55.9
c2-c3
Angles are in degrees.
creases by 0.2 eV due to the electron-releasing effect of the methyl groups. Little change is observed in the lone-pair hybridization as we proceed from 6 to 7,although the IP for the cyclopenta[b] compound is significantly lower than that of the 2,3-dimethyl derivative. This is presumably due to the enhanced inductive effect of the larger alkyl group. Of all quinoline derivatives studied, the lone-pair hybridization of 8 most closely resembles that of the parent, though the inductive effect of the remotely substituted ring of 8 is sufficient to lower the n-IP by 0.12 eV relative to the parent compound. A very small increase in the s character of the lone pair (1.9% relative to quinoline) accompanies fusion of a four-membered ring in the b position. This overall, slight increase in s character of the n orbital arises primarily from the decrease in the C N C angle, p, from 117' in 5 to 112' in 9. It is to a large extent offset by a decrease in s character associated with the a-bond-angle distortion at the cyclobutene ring juncture. The model calculations on pyridine show that the net effect of simultaneously varying the a and p angles from the quinoline geometry to that of cyclobuta[b]quinoline is to leave the hybridization of the n orbital nearly unchanged. The fact that we calculate and measure an IP for 9 that is comparable to unstrained 6 and lower than quinoline, rather than slightly higher as the increased s character would predict, appears to result from two additional factors. An analysis of the canonical lone-pair orbital of the cyclobuta[b] derivative reveals that this lone pair more than any other in the series is mixed with the framework u-orbitals. This extra delocalization lowers the IP, as does the inductive effect of the methylene groups. Our general IP trends are similar to, but less marked than, those calculated by Dewey et al. for pyrazine lone-pair I P dependence on the a and angles using INDO/S.l9 They report that decreasing a from 120' to 90' lowers the n+-IP by 1.0 eV (3 times the decrease for pyridine). Decreasing from 120° to 110' raises the n+-IP by 1.5 eV (5 times the increase for pyridine). Experimentally, the IP for a cyclobuta[b]pyrazine derivative is assigned to a PES band 0.7 eV lower than the one in 2,3-dimethylpyrazine. These authors invoke strain-induced orbital rehybridization as an explanation for this large decrease in I P but do not ascribe the effect to rehybridization of the lone pair, which would predict an increase in IP. Instead, they argue that strain induces increased p character in the C2 orbitals involved in the C2-H and C2-C3 bonds, raising the energy of these u molecular obitals (MOs). This allows for stronger mixing between n, and the C2-C3 u MOs, thereby raising the energy (lowering the IP) of n,. Our pyridine
-
calculation also shows a small decrease in s character of about 4%for the hybrid involved in the C2-C3 bent bond when a equals 9 3 O . However, we find the s character in the hybrid of the C2-H bond actually increases by 2%. This is contrary to the predictions of the strain-induced rehybridization model. Therefore although strain-enhanced, through-bond interaction of the lone-pair orbitals may be a contributing factor to the lowered pyrazine IP, orbital hybridization does not seem to be its principal cause. It appears that the bent-bond description and the decreased CNC bond angle provide the best explanation of the observed and calculated IPS of the cyclobuta[b] derivatives of pyrazine, pyridine, and quinoline.
Basicities and Proton Affinities Solution-phase basicities of the quinoline derivatives studied here have been previously measured, and their apparent pKa values are summarized in Table I.26p33 The increase in basicity by up to 1 pKa unit for 6, 7,and 8 relative to quinoline is seen to follow closely the PES, n-orbital ionization potentials. The sharp drop in basicity of 1,2-dihydrocyclobuta[b]quinoline( 9 ) to a value 1 pKa unit less than that of quinoline itself is not reflected in the gas-phase ionization potential data. More recently Aue et al. have found a similar decrease in the proton affinity for cyclobuta[ b l ~ y r i d i n e . Strain-induced ~~ orbital rehybridization has been used to explain these anomalous basicities in both the pyridineIg and quinoline series.26 Attempts to correlate IPS (from PES data or Koopmans' theorem) with aqueous basicities meet with several complications. First, differences in IPS are likely to be much larger than differences in aqueous basicities. Proton transfers in aqueous media may be viewed as occurring by an alternate pathway involving ionization of the base in the gas phase:
-
\z, 0"
+
+
+
H
// +
for which AHo = A H o , AH', AHo3 AH0,. The second step in this process involves ionization of the base. An increase in the I P of the lone pair increases AH2 and is thus expected to decrease both the proton affinity in the gas-phase (PA = -AH2 - AH3)and the aqueous-phase basicity of B (-AHo). However, an increase in IP may be opposed by a concomitant increase in hydrogen affinity35(algebraically large -AH3) so that the expected decrease in proton affinity (and aqueous basicity) with increasing I P may be damped. Furthermore, solvation of B: and H + is apparently greater than that of BH' (Le., -AHo, - AH', is negative) so that there is an additional damping of basicity in aqueous solution (-AHo = [PA - AHl - AH,]< PA).36 Finally, for there to be a linear correlation between gas-phase IPS and solution basicities (pK, values) several conditions must be met: AH',,AHo3,and AH0, must be constant for the series, or must cancel out, or must all have a linear dependence on IP. Also, since pKa values measure free energies and not enthalpies, it is necessary that the relative entropy changes in the solvation of the quinoline base and the quinolinium type ion be the same for all members of the series. These conditions are difficult to meet unless the compounds used to establish the linear correlation are very similar. A linear (33) Markgraf, J. H.; Antin, J. H.; Walker, F. J.; Blatchly, R. A. J . Org. Chem. 1979,44, 3261. (34) See ref 19, footnote 11. (35) Purcell, K. F.;Kotz, J. C. Inorganic Chemistry; W. B. Saunders: Philadelphia, 1977; pp 226-228. (36) Aue, D. H.; Webb, H. M.; Bowers, M. T.; Liotta, C. L.; Alexander, C. J.; Hopkins, H. P., Jr. J . Am. Chem. SOC.1976, 98, 854.
4898
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
correlation between pK, and IP has been reported for a series of substituted (but unstrained) pyridines3' The regression yields pKa = -6.801 70.7, where I is the ionization potential in electron volts. From the data in ref 37 we calculate an rms deviation of 0.67 pKa units for the calculated pKa. Thus, the expected rms error for the difference between any two pKa values calculated from the above regression is about 0.94 units. Our observed reversal of the correlation between IP and pK, for 5 and 9 is comparable in magnitude ( f l pKa unit) to the mean error in the pyridine regression. It is also known that the gas-phase basicity of pyridine is higher than that of ammonia but that this order is reversed in aqueous solution. We conclude that the n-orbital ionization potential of 1,2-dihydrocyclobuta[b]quinolineis not related in a simple fashion to its solution-phase basicity and that solution basicities are not accounted for strictly by orbital models.
+
Conc1usions The systematic changes in the photoelectron spectra of the series of quinoline and naphthalene derivatives support Heilbronner's assignment of the broad shoulder near 9.4 eV as the n-ionization potential of quin01ine.l~ In contrast to the large increase in nionization potentials upon fluorine s u b ~ t i t u t i o n , ' ~we~ 'see ~ only a slight decrease upon alkyl substitution. Most surprising is the finding that the n-ionization potential when a strained cyclobutane ring is fused adjacent to the nitrogen atom of quinoline is essentially the same as that for any other alkyl substituent. Theoretical calculations show that the ring strain by itself should slightly decrease the amount of s character in the n-orbital. When the quinoline framework is allowed to respond to ring-strain effects, the C N C bond angle closes down and the amount of s character increases. Our model calculations for pyridine show that these two effects nearly offset each other at the optimized angles for 1,2-dihydrocyclobuta[b]quinoline,9. Hence the inductive effect of the methylene groups accounts for most of the calculated and measured decrease of 0.25 eV in the n-orbital I P of 9 relative to quinoline. The lack of a large increase in the measured n-orbital I P and these theoretical findings argue that induced rehybridization within a rigid molecular framework is inadequate to explain the bonding of atoms adjacent to strained rings. We find a bent-bond model with little rehybridization more consistent with the observed ionization potentials. Both theory and experiment demonstrate that vapor-phase ionization potentials are essentially independent of the particular (37) Ramsey, B. G.; Walker, F. A. J. Am. Chem. SOC.1974, 96, 3314.
Moomaw et al. alkyl substituents. This behavior contrasts with the dependence of solution-phase basicities on substitution. Hence these observed basicities cannot generally be deduced from the electronic structure of the isolated molecules. This suggests that over the narrow range of basicities studied here ( - 2 pKa units) solvation or other entropy effects could be playing a dominant role in determining the ordering of solution basicities. Experimental Section Melting points were determined on a modified Hershberg apparatus with tota!-immersion Anschutz thermometers and are uncorrected. Materials. Compounds 1, 2, and 5 were purchased from Aldrich. Professor R. D. Rieke kindly provided a sample of 3.38 Compounds 4 and 7 were prepared as reported in the literature: 4, mp 86.6-87.0 "C (lit.39mp 86.5 "C); 7, mp 58.8-59.8 "C (lit.40 mp 59-60 "C). Compounds 6, 8, and 9 were available from 7 decomposed while in the previous s t ~ d i e s . ~ Compound ~,~~ spectrometer. Photoelectron Spectra. The photoelectron spectra were obtained by using a Perkin-Elmer PS-18 instrument. Sample vapor pressures were ca. 3-9 Pa. The fixed 150-pm-width ionization chamber slit was retained, but best fine structure could be obtained only with the exit slits adjusted to ca. 50-pm width. Calibration was performed in situ by admixture of a 1:l argon-xenon mixture (0.7 Pa) to the heated vapor in the ionization chamber. Argon resolution was 15-1 8 meV, and xenon resolution was 20-25 meV. Random variation of surface potentials due to the presence of chemical vapor was eliminated sufficiently to give a precision of &0.02 eV to the calibration. Acknowledgment. W.R.M. and D.A.K. thank the Camille and Henry Dreyfus Foundation for Dreyfus Teacher-Scholar Grants that helped support this research. The generous support of Williams College through faculty research awards and the resources of its Computer Center is gratefully acknowledged. We are indebted to Prof. E. Heilbronner for helpful comments and suggestions, and we thank Prof. Dewey for a preprint of the pyrazine results19 and Prof. Michl for valuable discussion. (38) Rieke, R. D.; Bales, S. E.; Meares, C. F.; Rieke, L. I.; Milliren, C. M. J . Org. Chem. 1974, 39, 2276. (39) Cava, M. P.; Shirley, R. L. J. Am. Chem. SOC.1960, 82, 654. (40) Borsche, W. Justus Liebigs Ann. Chem. 1910, 377, 7 0 . (41) Markgraf, J. H.; Katt, R. J.; Scott, W. L.; Shefrin, R. N. J. Org. Chem. 1969, 34, 4131.
