1632
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
pected that this emitting state has a lower energy than that reached by the less hydrated basic ground state resulting in the observed red shift of the acidic relative to the basic emission spectrum. The excess in hydration of the ground state of the acid form relative to that of the basic form would also be expected to lead to a lowering of the quantum ~ i e l d ,in~ qualitative ,~~ agreement with observations. However, there is evidence for an additional effect which contributes to the lowering of the quantum yield. The fact that specific pairs of acidic and basic spectra corresponding to different values of a of the butyl or pentyl copolymer may be superposed, as shown in Figure 6, indicates that the ultimately emitting basic excited state reached by the probe from its acidic ground state at one conformation of the copolymer is identical with that reached from its basic ground state at a certain other, less compact, conformation of the copolymer. However, the need for the scaling factors indicates that the observed quantum yields are substantially lower for the fluorescence originating from the ground state acid than from the ground state basic form, the lowering for the pentyl being twice as great as that for the butyl copolymer. Apparently the spectra originating from the acid form undergo some quenching process which becomes increasingly pronounced with alkyl group size. This alkyl group size effect appears to be present even if the copolymers are in their random coil states. We have already seen in Figure 5, that while the emission spectra of the acidic and basic forms of the unconjugated probe coincide, the attachment of the probe to the ethyl copolymer causes a substantial lowering of the acid fluorescence relative to the basic one. Furthermore, the acid emission spectrum of the butyl copolymer a t a = 0.85 where the polyion exists in its random coil form is seen in the upper part of Figure 5 to be still lower than that of the ethyl copolymer, while the basic emission spectrum of this copolymer in the same state, i.e., when a = 0.85, is seen in Figure 1 to be very close to that of the free probe. This quenching effect which the alkyl groups have on
M. Leuchs and G. Zundel
the fluorescence of the acidic form of the dansyl group and which increases with their size has apparently not been reported previously. Whether it is due to the interference of the alkyl groups with the deprotonization of the excited dansyl groups or whether the alkyl groups facilitate some competitive nonradiative deenergization process cannot be determined with the experimental evidence currently available.
References and Notes (1) The support of this research by grants from the United States Public Health Service (Grant GM 12307) and from the Mobil Research and Development Corporation are gratefully acknowledged. This work constitutes part of a thesis presented by M. S. Schlesinger to Rutgers University in partial fulfillment of the requirements for the R.D. degree. Part of this paper was presented at the 172nd National Meeting of the American Chemical Society, San Francisco, Calif., Aug 1976. (2) Louis Bevier Fellow, Rutgers University, 1975-1976. (3) R. F. Steiner and H. Edelhoch, Chem. Rev., 62, 457 (1962). (4) G. Weber, Biochemistry, 51, 155 (1952). (5) R. F. Chen, Arch. Biochem. Biophys., 120, 609 (1967). (6) U. P. Strauss and G. Vesnaver, J. Phys. Chem., 79, 2426 (1975). (7) P. L. Dubin and U. P. Strauss, J . Phys. Chem., 74, 2842 (1970). (8) U. P. Strauss and G. Vesnaver, J . Phys. Chem., 79, 1558 (1975). (9) H. T. S. Britton, "Hydrogen Ions", Van Nostrand, New York, N.Y., 1932, p 225. (10) U. P. Strauss and M. Schlesinger, J . Phys. Chem., 82, 571 (1978). (1 1) G. C. Guilbault, "Practical Fluorescence", Marcel Dekker, New York, N.Y.. 1973. DD 71-72. (12) C. A: Parker,'";Photoluminescenceof Solutions", Elsevier, Amsterdam, 1968, pp 220-226. (13) The units of F , and F A in this paper are identical. However, they differ from those used in a previous publication' for values of F by a factor of approximately 3.5. (14) G. Vesnaver, unpublished results. (15) Since all of the studies involving rF focussed on the behavior of the basic species, the excitation wavelength chosen was always 330 nm where the acidic species has zero absorbance. Under these conditions an inspection of eq 1 shows that for the calculation of rF it is not necessary to know the values of the absorbance ABand of the correction factors g, s, and x . Thus the determination of rF is vety convenient and requires only a corrected emission spectrum. (16) A. J. Begala and U. P. Strauss, J . Phys. Chem., 76, 254 (1972). (17) P. Lane and U. P. Strauss, Adv. Chem. Ser., No. 142, 31 (1975). (18) A. Rosengart and U. P. Strauss, unpublished results. (19) D. Lagunoff and P. Ottolenghi, C . R . Trav. Lab. Carlsberg, 35, 63 (1965). (20) E. Lippert, Z . Elektrochem., 61, 962 (1967).
Easily Polarizable Hydrogen Bonds in Aqueous Solutions of Acids. Nitric Acid Martin Leuchs and Georg Zundel" Physikalisch-ChemischesInsfitut der Universifat Muchen, Theresienstrasse 4 1, 0-8000Munchen 2, West Germany (Received February 17, 1978) Publication costs assisted by the Physikaiisch-Chemisches Instifur der Universifat Muchen
Aqueous nitric acid solutions were studied as function of concentration by IR spectroscopy. The degree of dissociation was determined from the absorbance of N020H bands. Easily polarizable hydrogen bonds are indicated by IR continua. In the case of very highly concentrated solutions (water:acid mole ratio n < 1)easily polarizable acid-water hydrogen bonds are formed: N020H-OH2 (I) N03--H+OH2 (11). For n = 1 the weight of proton boundary structure I1 amounts only to 0.13, Le., the energy surface in these hydrogen bonds is an asymmetrical double minimum energy surface with its deeper well at the anion. With increasing dilution the weight of proton boundary structure I1 increases and more and more protons are present in easily polarizable hydrogen bonds of H502+groupings. Complete removal of the protons from the anions is, however, only observed above n. = 12. The position of the water bands shows that the 0 atoms of N020H are only weak hydrogen bond acceptors while those of the NO3- anions are comparatively stronger acceptors.
I. Introduction Nitric acid was studied by IR spectroscopy in 1953 by Bethell and Sheppard.l,2 Since then, IR methods for studying strong acids have improved considerably, and more exact knowledge on the solvation of the excess proton
has been obtained (ref 3, pp 753 ff). In aqueous solutions of strong acids, the Proton is Present in H502' grouPings, which are imbeded in a network of other water molecules, and the hydrogen bond in these groupings is easily POl a r i ~ a b l e . ~Not - ~ only hydrogen bonds of type (BH-*B)+
0022-3654/78/2082-1632$01.00/00 1978 American Chemical Society
Polarizable Hydrogen Bonds in Aqueous Solutions of Acids
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978 1633 0.0
wave number
....
n
10
[
a
GO
1
0
10
54
3
I
I 1
1
0
1
2 1
'
1
'
1.0
n
26
crn-l
Figure 1. Spectra of aqueous nitric acid solutions (layer thickness 9.5 pm, 293 K): (a) - 23.7 M ( n = O.l), -e-. 20.3M (n = 0.53),--16.0 M (n = 1.34),-- 13.3M (n = 2.13); (b) - 9.8 M (n= 3.79),- . - e 2.4M (n = 21.8), 1.2 M (n = 45.3),-- H,O, -..-e.NaNO, solution. m
05
Figure 3. (a) Molarity of protons removed from the anion MH+in aqueous HNO, solutions depending on the molarity of water MHZ0 and the mole ratio n = MH20/M"03. (b) Weights of the boundary structures I and I1 depending on MHPO and n. 00
10,
I
-0-0
+'oe,;\ ,
,
,
,
,
10
",.
,
-,,
2o
M
Figure 2. Degree of dissociation a plotted against the molarity Mand the mole ratio n = MH20/MHNH3: (+) results from Young.20
(B-HB)+ are, however, easily polarizable but also proton transfer hydrogen bonds of the type B1H-.B2 + B1--H+B2, where B1 # B2, Le., hydrogen bonds with double minimum energy surfaces. The polarizability of these bonds is also indicated in the infrared spectra by strong continuous a b s ~ r p t i o n . ~ -Such ~ bonds may be hydrogen bonds formed between acid OH groups and water molecules. The following pK, values of nitric acid were determined by various authors: 1.64,1° 1.34,111.38,121.44,131.40,14and 1.37.15 All these values are in the range 1.25-1.75. With regard to the size of these pK, values of nitric acid, the hydrogen bond between the OH group of this acid and water should be largely symmetrical. Hence aqueous nitric acid systems should be especially suitable for studying molecular processes involving the removal of a proton from the anion due to the influence of water molecules.
11. Results and Discussion Figure 1 shows selected spectra of aqueous nitric acid solutions. The spectrum of pure water, and the spectrum of an aqueous sodium salt solution, in the 1500-1200-~m-~ region, are given for comparison. The assignment of the IR bands has already been discussed by various authors.16-ls In highly concentrated solutions the bands of N 0 2 0 H are observed, YJNO)~= 1670 cm-l, v,(N02) = 1300 cm-l, and u(N-0) = 925 cm-l, and a bending vibration of the whole molecule at 765 cm-l. In dilute solutions, u,, of the NO, ion is found as a doublet in the region 1450-1250 cm-l. The doublet structure of this band is due to the removal of degeneracy of this vibration caused by interactions with the environment (ref 19, pp 37 f f and p 172). Degree of Dissociation. As described in the Experimental Section, the degree of dissociation is determined from the band a t 765 cm-l, and plotted in Figure 2 as a function of the molarity, M, and as function of n, the number of water molecules per acid molecule. For
Figure 4. Continuous absorption at 1900 cm-' depending on the molarity of water MHPO and on n. This wavenumber value is chosen for the evaluation since the influence of band wings is smallest at this wavenumber value.
comparison, values determined by Young20using Raman spectroscopy are also given in Figure 2 by crosses. These data agree well with our results. Complete dissociation only occurs at concentrations less than 4 M, Le., when n = 12. The spectroscopically observed dissociated acid molecules are not only those from which the protons have been removed and are present as H502+,but also those with which the proton is present in the proton boundary structure I1 (see below) of the acid-water hydrogen bond. In Figure 3a the concentration of protons which have been removed from the anion, MH+,is plotted as a function of the molarity of water and of n. The concentration MH+ increases in a proportion greater than the molarity of water: This observation is explained in the following. Water-Acid Ratio n C 1. When water is added to NOPOH a strong continuous absorbance arises beginning at the u(OH) bands of the water molecules and extending toward smaller wave numbers (Figure la). Simultaneously the OH stretching vibration of the N 0 2 0 H molecules vanishes at 3340 cm-l. Figure 4 shows that the absorbance of this continuum increases for n > 1 in proportion to the molarity of water. This continuum indicates that easily polarizable hydrogen bonds are The bonds formed are acid-water hydrogen bonds. Thus in these hydrogen bonds a double minimum surface is p r e ~ e n t ~ - ~ and these hydrogen bonds must be described by the two proton boundary structures: NO,OH...OH, I
NO,-..*H+OH,
I1
For n C 1the weights of the proton boundary structures of this hydrogen bond can be determined. Under these conditions the number of hydrogen bonds formed is the
1634
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
Figure 5. Continuum at 1900 cm-' depending on the mole fraction of and the molarity Mof nitric acid. the water x , the ratio n = MH20/M,,ld,
same as the number of water molecules present and proton boundary structure I1 is equal to the concentration of protons which have been removed from the anion. Hence from the curve in Figure 3a, the weights of the two boundary structures can be obtained. These weights are plotted in Figure 3b as a function of n. This figure shows that the weight of proton boundary I1 increases slightly with n. For n = 1the weight of structure I is 0.87 and that of structure 11, 0.13. Hence, as expected, with regard to the continuum, a double minimum energy surface is present in these hydrogen bonds, whereby a t these very high concentrations the deeper well is at the anion. The increase of the weight of proton boundary structure I1 with increasing water content is in good agreement with results from other systems, since increasing polarity of the environment generally increases the weight of the polar proton boundary This increase of the weight of structure I1 with increasing n shows that the well of the energy surface at the water molecule is lowered with increasing n. Simultaneously the N-0 bond gains a little more double bond character as indicated by the shift of v(N-0) from 925 to 940 cm-l. The water molecules in the acid-water groupings cause the following bands: a strong stretching vibration at 3580 cm-' and a scissor vibration at 1615 cm-l, the latter observed as a shoulder on the slope of the intense band of v,,(NOz) a t 1670 cm-l. The very high position of the stretching and the very low position of the scissor vibration of the water molecules demonstrate that the OH groups of these HzO molecules form only very weak hydrogen bonds. The acceptors of these hydrogen bonds are preferentially 0 atoms of the nondissociated NOzOH molecules hence these 0 atoms are only very weak acceptors. It is known that water molecules, the OH groups of which form only weak hydrogen bonds, cause a relatively strong antisymmetrical stretching vibration at larger, and a weak symmetrical one at lower wavenumbers.21i22 Therefore, the band observed at 3580 cm-l is probably the antisymmetrical, and the weak band observed at 3380 cm-l probably the symmetrical stretching vibration of these H 2 0 molecules. Water-Acid Ratio n > 1. Figure 3a shows that also with n > 1,MH+,the concentration of protons which have been removed from the anion, increases much more rapidly than n. This is caused by two factors: first, as already discussed, in the acid-water hydrogen bond, the weight of the polar proton boundary structure NOp.H+OHz increases due to the influence of additional water molecule^;^-^ and secondly, more and more protons transfer into the network of the water molecules, i.e., the protons form H5OZ+ groupings. In Figure 5 the absorbance of the continuum is plotted as a function of the mole fraction, x , n, and the molarity, M . It can be seen that the absorbance of the continuum increases with n up to a maximum at about n = 2.5 and
M. Leuchs and G. Zundel
then decreases. The intensity increase for n > 1 has two causes. First, due to the influence of water molecules the energy surfaces, which for n < 1 are largely asymmetrical double minimum energy surfaces with the deeper well at the anion, become more and more symmetrical due to the influence of the additional water molecules and hence the intensity of the continuum increases, as demonstrated earlier with other Secondly, the polarizable hydrogen bonds in H502+,increasingly formed, give a relatively large contribution to the continuum. For n > 2.5 the intensity decreases, since the concentration of polarizable hydrogen bonds decreases. This dilution effect overcompensates for n > 2.5 the above discussed effects causing an increase of the continuum. For n = 12, i.e., in the 3.9 M solution, all protons are removed from the anions. The absorbance of the continuum at 1900 cm-l per proton amounts to 0.14. This is the same value as observed with an aqueous HC1 solution under comparable conditions. This shows that in the 3.9 M solution most of the protons are present in easily polarizable hydrogen bonds of H502+groupings. For n > 1, the OH stretching vibration of the water molecules at 3580 cm-l and the scissor vibration at 1615 cm-' vanish. The stretching vibration of the water molecules is observed under these conditions at 3390 cm-l and the scissor vibration is masked by v,(NO,). The position of the stretching vibration shows that the water molecules form relatively strong hydrogen bonds to 0 atoms of the NO3- ions. Thus the 0 atoms of the NO3- ions are much stronger hydrogen bond acceptors than those of N 0 2 0 H . With increasing dilution, the maximum of the water stretching vibration shifts, as expected from 3390 to 3450 cm-l, the position of this band in pure water. The removal of the deneracy of v,, of the NO3- ions in aqueous solutions has been already discussed by various a ~ t h o r s . ~For ~ -summary ~~ of this work see the review of HesteraZ6In the acid solutions this band is observed as doublet in the region 1450-1250 cm-'. The splitting due to removal of degeneracy decreases from 140 cm-l in the 13.5 M, to 50 cm-l in the 1 M solution. Hence the local symmetry at the NO3- ions changes, i.e., the environment of the NO< ions becomes more symmetrical with increasing dilution. 111. Experimental Section
Materials and Methods. The spectra were recorded a t 293 f 1 K with a Perkin-Elmer double-beam spectrophotometer, Model 325. The air in the spectrophotometer was dried and made free of C02 by ventilation through silica gel and sodium asbestos. Nitric acid, analytical grade, was bought from Merck AG, Darmstadt. The same cell was used as in ref 7, with the exception that silicon windows were applied because of their acid resistance. With regard to the wedge-shaped layer the absorbance was corrected as described in ref 7. The mean layer thickness of the cell was 9.5 Km. The absorbance of the continuum at 1900 cm-l was determined as described in ref 27 refering to the same background absorbance a t 4700 cm-l. To determine the degree of dissociation, the intensity of the y vibration of the nondissociated NOzOH molecules at 765 cm-l was plotted against the acid molarity. The intensity of this band in the pure acid yields the factor needed to obtain the concentration of nondissociated molecules in each solution. This procedure is justified by the fact that the strong band of the anions at 1400 cm-' is no longer observed in the spectrum of the pure acid. This shows that self-dissociation can be neglected within experimental error.
ESR Studies of Vanadyl Acetylacetonate in Tetrahydrofuran
IV. Conclusions In the region n > 1 easily polarizable acid-water hydrogen bonds are formed as indicated by a strong IR continuum: NO,OH**.OH,t NO,-.**H+OH, I I1
At n = 1the weight of proton boundary structure I is 0.87 and that of structure I1 is 0.13. Hence under these conditions an asymmetrical double minimum energy surface with the deeper well a t the anion is present in these hydrogen bonds. With increasing n, the weight of the polar proton boundary structure increases due to the influence of the additional water molecules, and protons transfer in the network of hydrogen bonds formed between water molecules, i.e., they are now present in easily polarizable hydrogen bonds of H5O2+ groupings. Complete dissociation, however, does not occur below n = 12, Le., before 12 water molecules are available per acid molecule. The 0 atoms of N 0 2 0 H are very weak acceptors. Therefore, for n > 1 the OH stretching vibration of the water molecules is observed a t 3580 cm-l and the scissor vibration a t 1615 cm-l. In contrast, the 0 atoms of NO3are relatively strong hydrogen bond acceptors, therefore with increasing n, the OH stretching vibration of the water molecules is found a t 3390 cm-l.
Note Added i n Proof. A similar study of nitric acid by J. Potier and A. Potier will appear soon ( J . R a m a n Spectrosc.). Acknowledgment. Our thanks are due to the Deutsche Forschungsgemeinschaft and to the Fonds der Chemischen
The Journal of Physical Chemistry, Vol. 82, No.
14, 1978 1635
Industrie for providing the facilities for this work.
References and Notes D. E. Bethell and N. Sheppard, J. Chim. Phys., 50, C72 (1953). D. E. Bethell and N. Sheppard, J . Chem. Phys., 21, 1421 (1953). G. Zundel in "The Hydrogen Bond-Recent Developments in Theory and Experiments", P. Schuster, G. Zundel, and C. Sandorfy, Ed., Vol. 11, North Holland Publishing Co., Amsterdam, 1976. E. G. Weidemann and G. Zundel, 2.Naturforsch. A , 25, 627 (1970). R. Janoschek, E. G. Weidemann, H. Pfeiffer, and G. Zundel, J . Am. Chem. SOC.,94, 2387 (1972). R. Janoschek, E. G. Weidemann, and G. Zundel, J . Chem. SOC., Faraday Trans. 2 , 69, 505 (1973). R. Lindemann and 0. Zundel, J . Chem. SOC.,Faraday Trans. 2, 73, 788 (1977). M. Matthies and G. Zundel, Biochem. Biophys. Res. Commun., 74, 831 (1977). G. Zundel and A. Nagyrevi, J . Phys. Chem., 82, 685 (1978). A. Albert and E. P. Serjeant, "Ionization Constants of Acids and Bases", Wiley, New York, N.Y., 1962. D'Ans-Lax, Taschenbuch Vol. I,3rd ed, Springer, Heidelberg, 1964. W. L. Marshall and R. Slusher, J. Inorg. Nucl. Chem.,37, 1191 (1975). P. Lumme, P. Lahermo, and J. Tummavuori, Acta Chem. Scand., 19, 2175 (1965). C. G. Hood and C. A. Reilly, J . Chem. Phys., 32, 127 (1960). H. A. McKay, Trans. Faraday SOC.,52, 1568 (1956). 0. Redlich and L. E. Nielsen, J. Am. Chem. Soc., 65, 654 (1943). 0. Redlich and J. Bigeleisen, J. Am. Chem. SOC.,67, 893 (1945). H. Cohn, C. K. Ingold, and H. G. Poole, J. Chem. Soc., 4272 (1952). G. Zundel, "Hydration and Intermolecular Interaction-Infrared Investigations of Polyelectrolyte Membranes", Academic Press, New York, N.Y., 1969 and Mir, Moscov, 1972. T. F. Young, L. F. Meranville, and H. M. Smith in "The Structure of Electrolytic Solutions", W. Hamer, Ed., Wiley, New York, N.Y., 1959. D. Schloberg and W. Luck, Spectrosc. Left., 10, 613 (1977). D. Schioberg and W. Luck, in press. H. Lee and J. K. Wilmhurst, Aust. J . Chem., 17, 943 (1964). H. Brintzinger and R. E. Hester, Inorg. Chem., 5, 980 (1966). R. E. Hester and W. E. L. Grossmann, Inorg. Chem., 5, 1308 (1966). R. E. Hester, Annual Report for 1969, Vol. 66, Chemical Society, London, 1970, p 79. D. Schioberg and G. Zundel, Can. J . Chem., 54, 2193 (1976).
Electron Spin Resonance Studies of Translational and Reorientational Motions of Vanady! Acetylacetonate in Tetrahydrofuran Myong-Ku Ahn' and D. E. Ormond Department of Chemistry, Indiana State University, Terre Haute, Indiana 47809 (Received March 16, 1978) Publication costs assisted by the Petroleum Research Fund
The reorientational motion and the translational motion of vanadyl acetylacetonate in tetrahydrofuran have been studied simultaneously by combining a capillary diffusion method with ESR spectroscopy. The ESR intensity change as a function of time gives the translational diffusion coefficient D = 0.86 X cm2/s, and ESR line widths analysis gives the reorientational correlation time r2 = 13 ps. These values are used to obtain Kivelson's parameter K = 0.49. No concentration dependence of D or r2 is observed at the dilute limit.
Introduction Paramagnetic vanadyl acetylacetonate (VOAA) is recognized as a model compound to study reorientational motion in low viscosity s01vents.l~This is because electron spin resonance (ESR) spectra of VOAA solutions contain eight hyperfine lines due to the 712 nuclear spin of 51Vand the width variations of these lines are readily interpreted in terms of the reorientational motion of VOAA molecules. The reorientational correlation time, 72, thus obtained can be expressed by the modified Debye expression1
0022-3654/78/2082-1635$01 .OO/O
where ro is the hydrodynamic radius of an equivalent spherical volume, k is the Boltzmann constant, T is the absolute temperature, and q is the shear viscosity of the fluid. The dimensionless factor K , introduced by McClung and K i v e l ~ o n ,depends ~ upon solvent but is seemingly independent of temperature and viscosity for a particular solution.6 Theoretical consideration by Kivelson, Kivelson, and Oppenheim7suggests that K is proportional to the ratio of mean square intermolecular torque and mean square force on the paramagnetic solute molecule. The derivation of eq 1 is based on the theory of low Reynolds number hydrodynamics.s K = 1 corresponds to 0 1978 American Chemical Society
