J. Phys. Chem. 1982, 86,5133-5135
sensitive to the nature of radial structural nonhomogeneity. Clearly, this opens up further interesting possibilities of obtaining structural information about the diffusion medium, which will be considered in a future paper. Acknowledgment. The present work was sponsored by the Hellenic Atomic Energy Commission and is part of a
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Ph.D. Thesis submitted by C.S. to the University of Athens. Thanks are due to the National Research Foundation for a scholarship to C.S., to Professor Th. Yannakopoulos for his active interest and for sponsoring the thesis, to Dr. S. Philipakis for his help with the X-ray diffraction measurements, and to Professor P. Theocaris for making his densitometer available to us.
Electronic Structure of Molecules and Infrared Continua Caused by Intramolecular Hydrogen Bonds with Great Proton Polarizability Bogumli Brrezinski Institute of Chemistry, A. Mickiewicz University, 60-780 Poznafi, Poland
and Georg Zundei' Institute of Physical Chemistry, University of Munich, D8000 Munich 2, West Germany (Received: December 16, 1981)
2-Quinuclidinecarboxylicacid N-oxide (compound 1) and 3-methyl-2-pyridinecarboxylicacid N-oxide (compound 2) are studied by IR and NMR spectroscopy. With both compounds, strong, short intramolecular hydrogen bonds are formed. Both proton-limiting structures OH...ON --t -O.--H+ONhave noticeable weight. Only with compound 1, but not with compound 2, is an intense IR continuum observed. Thus, only if the hydrogen-bond donor and acceptor groupings are not electronically conjugated do IR continua occur, since, when they are conjugated, the charge fluctuation connected with the proton fluctuation is compensated by an electron flux in the opposite direction within the molecule. Thus, not only the proton potential but also the dependence of the dipole moment on the vibrational coordinate of the proton is decisive for the large transition moments and the proton polarizability of hydrogen bonds, and thus for the occurrence of infrared continua. On the basis of these results it is explained that, with a large number of compounds forming intramolecular hydrogen bonds, described in the literature, neither a continuum nor OH or NH stretching vibration bands are observed.
Introduction Various authors1* studied the IR spectra of compounds forming intramolecular hydrogen bonds in which the hydrogen-bond donor and acceptor groupings are electronically conjugated. Perhydroxyquinolines were studied by Josien et a1.l and by Hadii and Sheppard,2hydroxy and ~
~
~~~
~~~~~
(1)M. L. Josien, N. Fuson, J. M. Lebas, and T. M. Gregory, J . Chem. Phys., 21,331 (1953). (2)D.Hadii and N. Sheppard, Trans. Faraday SOC., 50,911(1954). (3)S. Trofimenko, J. Am. Chem. SOC.,85,1357 (1963). (4)K. Hafner, E. A. Kramer, H. Musso, and G. Ploss, Chem. Ber., 97, 2066 (1964). (5)H. L. Ammon and U. Muller. Tetrahedron. 1437 (1974). (6) K. Kuratani, M. Tsuboi, and T. Shimanouchi, B d l . Chem. SOC. Jpn., 25, 250 (1952). (7)B. E. Bryant, J. C. Pariant, and W. C. Fernelius, J . O m . Chem., 19,1884 (1954). (8) W. J. Linn and W. M. Shavkey, J . Am. Chem. SOC., 79,4970(1957). (9)S.Bratan-Mayer and F. Strohbusch, Z.Naturforsch. B , 31, 1106 (1976). (10)M. L. Porte, H. S. Gutowski, and J. Moyer, J. Am. Chem. SOC., 82,5057 (1960). (11)R. Blinc and D. Hadii, J . Chem. SOC., 4536 (1958). (12)A. Bigotto, V. Galosso, and G. Alti, Spectrochim. Acta, Part A, 27,1659 (1971). (13)E.0.Schlemper,C. H. Walter, and S. J. La Placa, J. Chem. Phys., 54,3990 (1971). (14)M. Szafran and B. Brzezinski in "Synthesis, Structure and Properties of Heterocyclic Compounds", W. Wiewi6rowski, Ed., UAM PoznG, 1975,p 219.
0022-3654/82/2086-5133$0 1.2510
amino derivatives of fulvenes by T r ~ f i m e n k oby , ~ Hafner et and by Ammon and Muller,5 hydroxy and amino derivatives of tropolenes by Kuratani et al.? by Bryant et al.,7 and by Linn and Shavkey; and rubazonic acid by Bratan-Mayer and S t r o h b u ~ c h . ~With all these compounds, with which the hydrogen-bond donor and acceptor groupings are electronically conjugated, no continuum or bands due to OH or NH stretching vibrations could be observed. Only with a very large layer thickness was a weak increase of the background in the region below 3000 cm-', compared with deuterated compounds, found. Compounds forming intramolecular hydrogen bonds in which the hydrogen-bond donor and acceptor groupings are partially electronically conjugated, i.e., in which the electronic conjugation is limited to a certain degree, were studied in ref 10-15. o-Hydroxy derivatives of benzene compounds were studied by Porte et al.,1° nickel and copper dimethylglyoximates by Blinc and Hadii," by Bigotto et a1.,12 and by Schlemper et al.,13and derivatives of picolinic acid N-oxides by Szafran and Brzezinski.14J5 In all these cases, no band of an OH stretching vibration is observed, but the absorbance of the background below 3000 cm-l is slightly increased. This increase of the absorbance can be recognized by comparison with the spec(15)M. Szafran and B. Brzezinski, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 18,247 (1970);20,735 (1972).
0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86,No. 26, 1982 ____
ao
Brzezinski and Zundel
1
Wave number cm-' Figure 1. I R spectra of compounds 1 (-)
and 2 (-
- -).
T A B L E I: Chemical Shifts ( p p m ) of the OH Protons in C o m p o u n d s 1 and 2
concn, compd mol dm-3 0.1
1 2
0.2
0.3
0.4
0.5
0.6
18.78 18.78 18.79 18.77 18.78 18.78 18.64 18.64 18.64 18.65 18.64 18.65
trum of the deuterated compounds. Compounds forming intramolecular hydrogen bonds in which the hydrogen-bond donor and acceptor groupings are not electronically conjugated were studied in ref 1619. Aksnes and Bergensen16studied diphenylphosphineoxides of hydroxyalkenes and Brzezinski and Z~ndel"-'~investigated a large number of compounds with structurally symmetrical hydrogen bonds of the types B+H...B + Be. .H+B and BH.. .B- * -B.. .HB and with hydrogen bonds of the type BIH. .B2 * B1-. .H+B2in which both proton-limiting structures have considerable weight. With all these compounds, continuous absorptions are observed in the IR spectra. These continua are observed in different wavenumber regions as a function of the mean length of the hydrogen bonds.18 These IR continua are caused by the great proton polarizability of hydrogen bonds of this since, due to this polarizability, they interact strongly with their solvent environment. Herewith, for the occurrence of the continua the induced dipole interaction of the hydrogen bonds with the local fields as well as the interaction of the proton transitions with other vibrations, particularly intermolecular ones,21!22are of significance. Theoretical considerations have ~ h 0 ~ nthat ~ large ~ . ~proton ~ 1 polar~ ~ izabilities may occur when double-minimum proton potentials or broad flat potentials are present in hydrogen bonds. Usually, the fluctuation of the proton is connected with a considerable charge fluctuation. Large transition moments occur which are responsible for the great proton polarizability as well as for the high intensity of the IR contin~a.~~~~~ It was shown in Ref 18 with intramolecular hydrogen bonds of medium length, formed in o-(dimethylamino)and 0-[ (dimethy1amino)methyllbenzoic acids and 8-
-
(16) G. Aksnea and K. Bergensen, Acta Chim. Scand., 18,1586 (1964). (17) B. Brzezinski and G. Zundel, Chem. Phys. Lett., 44, 521 (1976); 61,315 (1979); 70,55 (1980);2.Phys. Chem. (Frankfurt am Main), 111, 31 (1978);J. Phys. Chem., 83,1787 (1979);J. Mol. Struct., 68,315 (1980); Can. J. Chem. 59,786 (1981);J. Chem. Soc., Faraday Trans. 2,77,1101 (1981). (18) B. Brzezinski and G. Zundel, Chem. Phys. Lett., 75, 500 (1980). (19) B. Brzezinski and G . Zundel, J. Mol. Struct., 72, 9 (1981). (20) E. G. Weidemann and G . Zundel, 2. Naturforsch. A , 25, 627 (1970). (21) R. Janoschek, E. G . Weidemann, H. Pfeiffer, and G . Zundel, J . Am. Chem. SOC.,94, 2387 (1972). (22) G . Zundel in "The Hydrogen Bond, Recent Developments in Theory and Experiments", Vol. 11, P. Schuster, G . Zundel, and C. Sandorfy, Eds., North-Holland Publishing Co., Amsterdam, 1976, Chapter 15. (23) A. Hayd, E. G . Weidemann, and G . Zundel, J . Chem. Phys., 70, 86 (1979). (24) R. Janoschek, E. G. Weidemann, and G . Zundel, J . Chem. SOC., Faraday Trans. 2, 69, 505 (1973).
Figure 2. I R spectra of nondeuterated (-) compounds 1 (A) and 2 (B).
and deuterated (- - -)
hydroxy- and 8-(hydroxymethy1)quinolineN-oxides, that IR continua are observed with these compounds when the hydrogen-bond donor and acceptor groupings are not electronically conjugated, whereas only in the region of the fundamentals is a weak increase of the background observed when these groupings are conjugated. In this publication, compounds with relatively strong short hydrogen bonds are studied, in which the hydrogen-bond donor and acceptor groupings are either conjugated or not conjugated.
Results and Discussion Acetonitrile-d3solutions of 2-quinuclidinecarboxylicacid N-oxide (compound 1) and 3-methyl-2-pyridinecarboxylic
1
2
acid N-oxide (compound 2) are studied by IR and NMR spectroscopy. In Figure 1 the IR spectra, and in Table I the NMR data, are given. The large chemical shift of the carboxylic acid proton shows that strong hydrogen bonds are formed with both compounds. The chemical shift of the carboxylic proton is completely independent of the c o n c e n t r a t i ~ n . ~Thus, ~?~ these hydrogen bonds are formed intramolecularly. The chemical shifts of the hydrogen-bonded protons of both compounds are very similar; thus, the intramolecular hydrogen bonds in both compounds are of comparable strength. Figure 1 shows that with compound 1, i.e., with the compound in which the donor and acceptor groupings are not electronically conjugated, a continuous absorption is observed which is particularly intense in the region 1700-800 cm-I, showing a bandlike maximum at about 1050 cm-l. This continuum demonstrates that the intramolecular hydrogen bonds formed show great proton pol a r i ~ a b i l i t y . ~Thus, ~ both proton-limiting structures OH.. -ON + 0-. .H+ON in these hydrogen bonds have (25) L. Eberson and S. Forsen, J. Phys. Chem., 64, 767 (1960). (26) M. Foreman in "Nuclear Magnetic Resonance", Vol. V., Alden Press, Oxford, 1976, p 298.
The Journal of Physical Chemistry, Vol. 86, No. 26, 1982 5135
Intramolecular Hydrogen Bonds
TABLE 11: Characteristic IR Absorption Bands (cm-' ) of Compounds 1 and 2 and Their Deuterated Analogues compd 1
compd 2
H
D
H
D
1708 1680 1600-1300
1730
1710sh 1683 1600-1300
1728
1588 1335
assignments u(C=O)
Yas(co*-'~ a 6 (OH)
1590 1340
u,(CO,-)
6(OD)
Both vibrations coupled by Fermi resonance.
considerable weight. The observed dependence of the intensity of the continuum on the wavenumber, and particularly the maximum in the region around lo00 cm-l due to the fundamental transition of the proton, is characteristic for short hydrogen bonds.23 In contrast to this result, with compound 2, only an extremely weak rise of the background is observed. In this compound the hydrogen-bond donor and acceptor groupings are, however, electronically conjugated. The comparison of the results with compounds 1 and 2 demonstrates that the continuum vanishes to a large extent, also with relatively short hydrogen bonds, when the hydrogen-bond donor and acceptor groupings are electronically conjugated. These results are explained by the fact that, if the acceptor and donor groups are conjugated, the charge fluctuation connected with the proton movement is compensated by an electron fluctuation through the molecule in the same direction. Thus, the moments of the proton transitions, and the proton polarizability, become very small. In Figure 2 the IR spectra of these compounds are compared with those of their deuterated analogues. Figure 2 shows that the continuum is still observed with intramolecular deuterium bonds, but it is less intense and shifted toward smaller wavenumbers. The most important bands are collected in Table 11. In Figure 2A, with compound 1,the v(C=O) band caused by the nonpolar limiting structure is observed at 1708 cm-'. Furthermore, an intense band is found at 1680 cm-I and a very broad band in the region 1600-1300 cm-' with various Evans holes. After deuteration, the very broad band in the region 1600-1300 cm-' is shifted and found with a maximum at 1335 cm-'. The very broad band observed in the nondeuterated sample is mainly caused by the 6(OH) vibration, whereas the band observed at 1335 cm-' is caused by the 6(OD) vibration. After deuteration, the band of v,(COz-) of the polar proton-limiting structure is observed at 1588 cm-'. The fact that this band is shifted in the nondeuterated compound toward 1680 cm-' demonstrates that this vibration shows Fermi resonance with the 6(OH) vibration when the compound is not deuterated. The fact that a band of v(C=O) as well as a band of v,(CO,-) are observed demonstrates that both protonlimiting structures of the OH...ON + O-...H+ON bonds have noticeable weight. The shift of u(C=O) from 1708 toward 1730 cm-I with deuteration, and the intensity increase, demonstrate that the deuteron is more tightly bound to the carboxylic acid group than the proton and that the residence time of the deuteron at this group is greater than that of the proton. In every case, the energy levels of the deuteron are positioned deeper in the potential well than those of the proton. Hence, the residence time of the deuteron becomes
greater in the deeper well of the double-minimum potential. Thus, the intensity increase of v(C=O) with deuteration demonstrates that the deeper well of the double-minimum proton potential in this intramolecular hydrogen bond is present at the carboxylic group. An analogous conclusion can be drawn from the bands of compound 2.
Conclusions With the short intramolecular OH. .ON + 0-. -H+ON bonds both proton-limiting structures show noticeable weight. An IR continuous absorption, however, is only observed if the hydrogen-bond donor and acceptor groupings are not electronically conjugated. Thus, only under this condition do the hydrogen bonds show great proton polarizability. This result is in good agreement with results obtained with medium long hydrogen bonds (formed in o-(dimethylamino)- and o-[(dimethylamino)methyllbenzoic acid and in 8-hydroxy- and 8-(hydroxymethy1)quinolineN-oxidesl*). No continuous absorption is observed since no charge fluctuation is connected with the proton fluctuation if the electrons may fluctuate within the molecule in the same direction. This result explains that, with the compounds studied in ref 1-9, no continuum or band due to a OH or NH stretching vibration is found, since, with all these molecules, the hydrogen-bond donor and acceptor groupings are electronically conjugated. Furthermore, this result explains that, with the compounds studied in ref 10-15, instead of OH stretching vibration bands, only very weak background absorptions below 3000 cm-' are observed, since with these compounds the charge fluctuation caused by the proton fluctuation is partially compensated by the electron flux within the molecules. With the compounds studied in ref 16-19, the hydrogen-bond donor and acceptor groupings are not electronically conjugated, and thus IR continua are observed, indicating large transition moments of the proton transitions and great -proton polarizabilities. This is in good agreement with theory121,27 since the intensities of the absorption, as well as the proton polarizabilities, are determined not only by the proton potentials but also by the dependence of the dipole moment on the vibrational coordinate.
-
.
Experimental Section The preparation of the compounds as well as the analytical data are given in ref 28. The deuterated compounds were prepared by repeated recrystallization from CD,OD. All IR spectra were recorded in CD&N solutions and in the region 1100-900 cm-' in CDC1, solutions. The NMR spectra were recorded with a Varian HA 100 spectrometer in CD,CN solutions, calibrated against MelSi as internal standard. The IR spectra were obtained from 0.3 M (CD3CN or CDC1,) solutions by using a cell with NaCl windows (layer thickness 0.1 mm). All IR spectra were recorded with a. 325 spectrophotometer (Bodenseewerk Perkin-Elmer, Uberlingen).
Acknowledgment. Our thanks are due to the Deutsche E'orschungsgemeinschaft and the Fonds der Chemischen Industrie for their support of this work. (27) G. Herzberg, "Infrared and Raman Spectra", Van Nostrand, Princeton, NJ, 1960. (28) B. Brzezinski, in preparation.
