J . Phys. Chem. 1985, 89, 2860-2864
2860
cannot oscillate. The nonlinear term--k4,!xz-makes this decoupling impossible and makes the system a true threedimensional one. The fifth equation of the "Oregonator", although linear, provides similar coupling between the equations and makes the oscillations possible. Comparison of the model with the oregonator model shows that
both have the same number of species-3; the number of parameters is six (five rate constants and a stoichiometric factor) in the Oregonator case and only four in the present one. In contrast to the Oregonator model, the present one will not show any interesting features unless the flow is present. Registry No. Ce, 7440-45-1; Br03-, 15541-45-4.
A Matrix Isolation Study of the Benzene-Water Interaction Anders Engdahl and Bengt Nelander* Division of Thermochemistry, University of Lund, Chemical Center, S-221 00 Lund, Sweden (Received: November 14, 1984)
The infrared spectra of 1:1 complexes between C6Ha,C6D6, or C6H5Dand HzO, D20,or HDO in solid argon have been obtained. The results indicate that the water molecule is hydrogen bonded to a flat side of the benzene ring, with considerable freedom to move.
Introduction The water-benzene interaction was the subject of a recent theoretical study.' The method used is based on standard molecular orbital methods, but involves several new procedures designed to minimize the errors involved in such calculations and to simplify them so that more complicated and chemically interesting systems can be studied. There seems to be no experimental study published of the 1:l benzenewater interaction which could be compared with the theoretical work. We have therefore extended our matrix isolation studies of molecular complexes involving water2-I0 to include the benzene-water complex. Our results indicate that water is hydrogen bonded to the benzene *-orbital system with H D O forming a D bond, similar to what we found for the water-ethylene complex.'O The complex shifts of the OH (OD) fundamentals of water indicate that the dissociation energies of the benzene-water and ethylene-water complexes are approximately equal. The larger size of the ?relectron system of benzene, compared to ethylene, seems to give the water molecule significant freedom to move. The calculated potential energy surface is rather flat around the potential energy minimum. The observed spectra suggest that the water molecule undergoes a significant and probably complex motion relative to the benzene ring. In particular HzOand D 2 0 appear to switch rapidly between their two possible hydrogen bonds. Summarizing, our results agree with the theoretical calculation insofar as they can be compared. Experimental Section The gas mixtures were prepared by standard manometric techniques and sprayed on a CsI window kept at 17 K in a cryostat cooled by an Air Products CS208 refrigeration system. The deposition rate was =9 mmol/h. The deposition system has been (1) G. Karlstram, P. Linse, A. Wallqvist, and B. Jansson, J . Am. Chem. SOC.,105, 3777 (1983).
(2) L. Fredin and B. Nelander, J . Mol. Struct., 16, 217 (1973). (3) L. Fredin, B. Nelander. and G. RibbegArd, Chem. Scr., 7 , 1 1 (1975). (4) L. Fredin, B. Nelander, and G. Ribbegird, J. Chem. Phys., 66,4065, 4073 (1977). ( 5 ) B. Nelander, J . Chem. Phys., 69, 3870 (1978). (6) B. Nelander, J. Chem. Phys., 72, 77 (1980). (7) B. Nelander and L. Nord, J . Phys. Chem., 86,4375 (1982). ( 8 ) L. Nord, J . Mol. Strucf., 96, 27 (1982). (9) A. Engdahl and B. Nelander, Chem. Phys. Letf., 100, 129 (1983). (10) A. Engdahl and B. Nelander, Chem. Phys. Lett., 113, 49 (1985).
0022-3654/85/2089-2860$01.50/0
TABLE I: Water Fundamentals of the Water-Benzene Complex (Ar, 17 K) (cm-')
H20"
HOHCnHn
D20
Y,
3638.0 3639.4 3640.1
3620.1 3616.9 3615.9 (sh) 3612.3
2657.7 2658.5 2657.9
2646.5 2644.0 2642.7 2641.7
2710.0 2709.4 2707.4
~2
1589.1 1591.1 1589.5
1594.4 1598.9 1600.9 1601.7 1609 (s)
1174.6 1175.7 1174.8
1182.2 1181.4 1191.1
1398.8 1399.3 (s) 1400 1400.5 1396.8 1403.5 (s) 1405.5
3734.3
3705 (s) 3714.0 3716.0 (s)
2771.1 2766 2770.7
2743 (s) 3687.3 2755.0 3685.7 3688.4
~j
3732.6
DODGH,
HDO HODGH, 2685.5 2684.1 2681.1 2679.9 2676.1
3693.4 (s) 3686.1
"he three values per fundamental given under the respective monomer are the VR origin (ref 14) and the positions of the nonrotating monomer (ref 14) and the benzene-induced nonrotating monomer bands. s, observed only after subtraction of water absorption; sh shoulder.
described." The benzene concentration was varied between 1:140 and 1:1200, and the water concentration between 1:150 and 1:300. Benzene (Fisher B255) was degassed and used without further purification. Benzene-d6(CIBA, 99.5% D) was degassed and used without further purification. Benzene-d (Merck Sharpe and Dohme, 98% D) was degassed and used without further purification. Water was doubly distilled and degassed. DzO (99.5% D) was degassed. An equilibrium mixture containing approximately equal amount of HzO and DzO was used to obtain HDO spectra. Argon (L'Air Liquide 99.9995%) was passed through a glass spiral in 02.* All spectra were run on a Bruker 113v FTIR at 0.5-cm-' resolution. In one experiment, a spectrum was also run at 0.25-cm-I resolution but no band seemed to be sharp enough to warrant the use of the higher resolution.
Nomenclature In complexes of the type studied here, the intramolecular vibrations of the complex-forming molecules are only slightly shifted from their unperturbed positions. Therefore, in order to simplify (11) L. Fredin, Chem. Scr., 5, 193 (1974).
0 1985 American Chemical Society
Benzene-Water Interaction 0.15
-0.00s
-
0.3
.,
k% -0.005
-t--t 3606 3616 3624 WAVENUMBERS CH-1
0.2
0.05
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2861
1600 1608 1616 WAVENUMBERS CH-1
n
I -0.005 + 2632 2640 2848 WAVENUMBERS CH-I
WAVENUH6ERS CH-1
-0.005 1176 1164 1182 WAVENUMBERS cn-1
1392 1400 1406 WAVENUMBERS CH-1
Figure 1. The vI and v2 fundamentals of water complexed to benzene. Upper C U r V W Ar/H20 = 153, Ar/CgH6 5 300, Ar 32.1 "01. Left: v1(HOH.C6H6). Right: u2(HOH.C6H6).Middle curves: Ar/D20 = 150, AT/C& = 146, Ar = 31.3 mmol; Left: v1(DOD*C6H6). Right: Y ~ ( D O D * C ~ Lower H ~ ) . CUrVeS: Ar/total Water = 102, Ar/C& = 144, Ar = 32.1 "01; Left: vI(HOD-C6H6).Right: v2(HOD.C6H6).The
lower right-hand curve was obtained by subtraction of the water absorption. The water dimer bands did not subtract perfectly, but left two sharp negative peaks at 1397.7 and 1402.4 cm-I. the notation, the perturbed ith fundamental of A in a complex with B will be denoted as vi(A.B). be and aq will be used when it is not necessary to specify the isotopic composition of the benzene or the water molecule involved. The fundamentals of benzene are as numbered by Herzberg.I2 The fundamental of C6D6 and C6H5D are numbered analogously by comparing the data ,of Brodersen and Langseth13 with that of Herzberg.I2
Assignment Water Fundamental Region. The spectrum of water in solid argon has been studied in great detail by Ayers and P ~ l l i n . ' ~Our spectra agree with theirs and our assignments of the water absorptions are identical with theirs. In the following, only bands induced by the presence of benzene will be discussed. In the v 1 region of H 2 0 , a structured band appears between 3620 and 3612 cm-' (Figure 1). Its concentration dependency clearly shows that it is due to a 1:l water-benzene complex. Note in particular that the relative intensities of the components of the band are concentration independent. We therefore assign the whole group Of peaks to Yl(HOHC6H6). A similar group of peaks appears when CsD6 is used instead of C6H6; the only difference is that the 3612.3-cm-I (c6H6)peak shifts to 361 1.9 cm-' (C6D6). With C6H5D,the shifts are below 0.1 cm-'. In addition to this band, a weak band appears at 3640.1 cm-I, close to the 3638.4-cm-I band assigned to nonrotating H20 in ref 14. We observe similar bands, close to or coinciding with the bands of nonrotating water14 in all fundamental regions of H 2 0 , D20, and HDO (Table I). We assign these bands to water molecules, trapped so close to a benzene (approximately within 12 A) that the quadrupole field of benzene is able to stop the water rotation. Note that, in most regions, we observe both the nonrotating monomer band of ref 14 and the benzene-induced band. With D20, vl(DOD.be) is observed in the interval 2641-2647 cm-l. Its shape is similar to that of vl(HOH.be) (Figure 1). Also (12) G. Herzberg, "Electronic Spectra and Electronic Structure of Polyatomic Molecules", van Nostrand, Princeton, 1966. (13) S.Brodersen and A. Langseth, Mat. Fys. Skr. Dan. Vidensk. Selsk., 1, 1 (1956); 1, 7 (1959). (14) G . P. Ayers and A. D. E. Pullin, Spectrochim. Acta, Port A , 32, 1629 (1976).
in this case there is a small shift of the low wavenumber peak between C6H6 and C6D6 (0.2 Cm-I). vl(HOD.be) has a shape which differs significantly from that of vl(HOH.be) or vl(DOD.be) (Figure 1). Instead of three approximately equally intense peaks, there is one dominating peak and a few weaker maxima on its high wavenumber side. The position of the main peak shifts 0.4 cm-I between C & , and C6D6 (lowest). v2(HOH.be) appears as a structured band at ca. 1601 cm-I (Figure 1). Its concentration dependency is the same as that of vl(HOH.be). The band has a low wavenumber satellite at 1594.4 cm-'. Also, the presence of benzene changes the shape of the 1610-cm-' water band (due to an overlap between u2(HOH.0H2) and a monomer bandI4). Subtraction of the water absorption invariably gives a peak a t 1609 cm-I with an intensity between one-half and one-third of the main peak at 1601 cm-'. We assign the 1609-cm-I peak, the 1594-cm-I peak, and the 1601-cm-I band to v2(HOH-be). The assignment of the 1609-cm-I peak must be regarded as tentative, since it is next to impossible to measure the concentration dependency of bands which are observable only after subtraction of monomer bands. With D20, benzene induces a structured band at 1182.2 cm-I with a relatively clear satellite at 1191.1 cm-I (Figure 1). This observation supports the assignment of the 1609-cm-I peak to v2(HOH.be). The low wavenumber component in the H 2 0 case may have a counterpart at 1173 cm-I in the D 2 0 case. With HDO, the benzeneinduced absorption overlaps the dimer bands of HDO. The main peak is visible as a shoulder on the u2(HOD-OHD) band. Subtraction of the HDO absorption gives a strong band at 1399.3 cm-', with several weaker satellite bands on its high wavenumber side (Figure 1). The peak at 1396.8 cm-I (in Figure 1) is assigned to nonrotating HDO. We have no decisive argument for the last assignment; its only advantage is that the band assigned to v2(HOD.be) then has a shape quite similar to that of vl(HOD.be). u3(HOH-C6H6)is observed as a satellite peak at 3714.0 cm-' on the combined water monomer and dimer band at 3712 cm-I. Subtraction of the water absorption gives a strong band with its maximum at 3714.0 cm-', a sharp satellite peak at 3716.0 cm-l, and a weak, broad peak at 3705 cm-I. v3(DOD.be) is a sharp, strong band at 2755.0 cm-'. Also in this case the subtraction of the water absorption suggests the presence of a weak, broad component at 2743 cm-I but, as far as we can see, no sharp satellite corresponding to the 3716.0-cm-I peak in the H 2 0 case. The v3(HOD) region is more problematic. Before subtraction of the H D O adsorption, a relatively weak, sharp band at 3688.4 cm-I and a broad, even weaker band at 3686.1 cm-I are the only benzene-induced absorptions observed. Subtraction of the HDO absorption makes it likely that the v3(HOD-aq) band at 3693.4 cm-I has a significant benzene-induced contribution. We tentatively assign this latter peak to v3(HOD.be) and the 3688.4-cm-I band to nonrotating HDO. The weak, broad band at 3686.1 cm-I may correspond to the weak broad component of the v,(HOH.be) and v3(DOD-be) bands. Benzene Absorption Regions. When water is introduced in a benzene-containing argon matrix, the strong hydrogen out-of-plane bending fundamental, ~4(A2,,),gets a high wavenumber satellite band, with a concentration dependency parallel to v 1 and v2(HOH.C6H6). We assign this band to V4(C&&.HOH). The shape of the Y ~ ( C ~ H ~ * Hband O H )changes only slightly when D 2 0 is used instead of H20. When the water concentration is increased, the satellite peak at 683.5 cm-I is red-shifted a few tenths of a wavenumber. This shift is probably caused by a band due to a ternary complex C6H6(H20)2on the low wavenumber side of the 683.5-cm-l peak. In the same concentration range, we observe bands in the OH-stretching region which are probsbly due to C6H6.(H20)2by analogy with the observations in the ethylenewater case.I0 For the three isotopic benzenes studied, C6H6, C6D6, and C6H5D, the v4 band of the benzene-water complex has one dominating peak and two (C&, C6H5D)or three (C&) satellite
2862
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 8.3
T
WAVENUHBERS CH-I
0.3
-
WAVENUMBERS CM-1 Figure 2. The temperature dependence of u,(HOH.C6H6). Ar/H20 = 150, Ar/C6H6 = 143, Ar = 32.7 mmol. Upper curve: T = 17 K. Lower curve: T = 12 K.
Engdahl and Nelander subtraction, while for only the strongest component is observed. In both regions subtraction of the benzene adsorption leaves bands whose shape resembles that of vI3(C.&HOH) (Figure 4). Also in the v14(Elu) region, subtraction of the benzene adsorption H ) In leaves a band similar in shape to the V I ~ ( C ~ H ~ . H Oband. this case, the strongest component is very close to the ~ 1 benzene 4 ~14,monomer band, either on the low wavenumber side, (C,H$), or on the high wavenumber side C&6, V I ~ ~ ( C ~ H $ ) . Considering the possibilities of water-induced changes in the shape of the benzene band, we have refrained from assigning the major component of the band remaining after subtraction to a water complex. Y~~~(C.&SD*HOH) is observed at 928.3 cm-', close to V1gb(C6H5D;B,) at 923.9 cm-'. In the CH stretching region of C6H6subtraction of the benzene absorption suggests the presence of Y~~(C&-HOH)at 3042.2 cm-I. Similar bands are also observed for C6H$ and C6D6. The strong combination bands vI1 4- vI9 and u7 + vI9 of C6H6 have well-separated, water-induced satellites 9.0 and 7.4 cm-' above the C6H6 bands. Both bands have concentration dependencies which suggest that they are due to a C6H6.HOHcomplex and we assign them accordingly. The corresponding C,&*HOH bands were not observed, but for C6HSD,several combination bands have been observed in the 1700-2000-~m-~region (Table
peaks (Figure 3). The Vl(C,&aq) band seems to be rather sensitive to the isotopic composition of the water of the complex. In an experiment where HDO was used instead of HzO, the strongest component at 503.0 cm-I shifted to 502.7 cm-I and a 11). new maximum appeared at 503.6 c d . This change was probably Discussion due to an increase in the concentration of a ternary complex, since The results presented above (see also Tables I and 11) clearly the total concentration of water used was higher than in the H 2 0 show that water and benzene form a 1:l complex in solid argon. experiments. us(C6H5D;B2)is a strong band at 698.1 cm-I. In the presence Note in particular that the bands assigned to HOH.C6H6and DOD.C6H6do not change shape or get satellite bands when the of water it gets a satellite a t 699.5 cm-I which we assign to equilibrium mixture of H 2 0 and D 2 0 is used. If complexes of v~(C~H~D*HOH). the composition (H20)2C6H6were involved, the appearance of Vlo(C&*HOH) is observed at 1048.4 cm-' as a satellite on the (H20)(HDO)C6H6and (HDO)(D20)(C6H6)should lead to new very weak Vlo(C6H6) band at 1047.3 cm-l. VII~(C~H~D.HO isHa )Strong band a t 782.9 Cm-', 4.6 Cm-' bands in the H 2 0 and D 2 0 regions, respectively, similar to what was observed in ref 10. above vllb(C6HsDI;B2).It has weak satellites at 785.6 and 784.6 cm-', and its shape suggests the presence of even more components. The assignments of the shifted water fundamentals and benzene u13(C6H6;El,) is a very strong band with its maximum at 1483.3 out-of-plane hydrogen bands of the benzene-water complex seem cm-'. When water is added to the matrix, its shape changes straightforward; however, for the in-plane ring deformations, the slightly. Subtraction of the benzene absorption, using coefficients situation is more problematic. The shape of the benzene bands obtained from studies of all observed benzene-adsorption regions, may change somewhat due to the presence of a polar impurity gives a band with one strong peak at 1482.9 cm-' and a pair of in the matrix. The amount of benzene dimer relative to that of weaker components at 1480.8 and 1479.9 cm-'. The water-induced benzene monomer decreases slightly when water is introduced in the matrix (at constant benzene concentration). These two effects changes in the V13(C6D6) region are easier to see and the domimay introduce spurious peaks in spectra which result after the band H ) is clearly seen at nating component of the V I ~ ( C ~ D ~ * H O 1333.5 cm-' even before the benzene absorption has been subbenzene absorption has been subtracted. The band assigned to v ~ ~ ~ ( C ~ H ~ D . is H shifted O H ) enough from the v13a(C6HSD) band tracted Subtraction of the benzene absorption leaves a band with to make it observable as a separate band and the main component one strong and two weaker components, quite similar to the band O H as ) a shoulder D ; A ~ of ) , the band assigned to v ~ ~ ~ ( C ~ H ~ D - isHseen obtained in the C6H6 case (Figure 4). For v ~ ~ ~ ( C ~ H ~all on v ~ ~ ~ ( C ~before H~D subtraction. ) It therefore seems clear that three components of uI3,(C6H5D.HOH)can be seen without 8.35
-8.885
672 688 688 WAVENUM0ERS CM-1 0.35
L
608 616 WAVENUMBERS CM-1
496 584 WAVENUMBERS CH-1
1
-8.005 888 616 496 504 -8.805 672 YAVENUMBERS 680 CM-1 688 : WAVENUMBERS 0 CM-1 : : 0 WAVENUMBERS5 CH-1 k . Figure 3. The u&.) regions of benzene. Left: Upper: Ar/H20 = 150, Ar/C6H6 = 1200, Ar = 35.4 mmol. Lower: Ar/C6H6 = 1189, Ar = 33.4 "01. Middle: C&D. Upper: Ar/C6HsD = 306, Ar/H20 = 146, Ar = 31.8 mmol. Lower: Ar/C6H5D = 292, Ar = 31.0 mmol. Right: C6D6. Upper: AT/C6D6 = 295, Ar/H20 = 152, Ar = 30.7 mmol. Lower: Ar/C6D6 = 306, Ar = 33.3 "01.
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2863
Benzene-Water Interaction
TABLE II: Fundamentals of Benzene, Free and Complexed, with Water (Ar, 17 K) (cm-l)a C6H6 "4
"8 YIO
C6H6.HOH
675.0
679.8 682.0 683.5 684.7
1047.3
1048.4
CsD6 497.2
C6D6.HOH 500.8 503.0 504.2
Vila "llb
3046.8 1483.3
"12 "13
3042.2 1479.9 1480.8 1482.9
(s) (s)
2288.2 1334.7
(s)
(s)
2289.5 (s) 1331.6 (s) 1332.6 (s) 1333.5
1040.8
"14
1035.3 (s) 1036.4 (s)
Y14b Y15b
Y19b
+ h9b + Y19a " l l a + "19b "11s + "19a "1 + "19b "llb
1812.3
1821.3
1956.6
1964.0
"llb
"1
816.3
813.3 (s)
C6HsD.HOH
608.2
610.8 612.8 614.0
698.1 1161.4 847.4 778.3
699.5
1480.2 1452.4
"13b
"1
C6H5D
1036.9 861.1 2277.3 923.9 1701.6 1744.2 1769.8 1811.1 1906.5 1907.1
782.9 784.6 785.6 1476.7 1477.5 1478.2 1450.8 1451.3 1453.1 1032.2 1033.3 858.1 2279.5 928.3 1710.2 1752.7 1779.1 1820.7
1949.5
1957.4
+ Ylh?
1829.7 1830.6
1839.0
1889.1
1897.0
+ "19b?
(s) (s)
(s) (s) (s)
1914.4
+ "19s
"19s
(s)
Os, observable only after the benzene absorption has been subtracted. The a and b labels on the bands are relevant for the last two columns only.
the same shape for all water and benzene concentrations used and for different isotopomers their shapes are rather similar (Figure 4). We therefore believe that these bands are real and assign them to v13(Be.aq). In the benzene spectra, it is mostly the out-of-plane hydro-0.005 -0.005 gen-bending fundamentals that are affected by complex formation 1460 1468 1328 1336 and we therefore suggest that the water molecule interacts mainly WRVENUMBERS CM-I WRVENUMBERS CM-1 with the a-electron system of benzene. The complex shifts of all 0.15 0.15 the out-of-plane fundamentals of C6HsD, including shifts calculated from observed combinations, are close to 4 cm-I, perhaps an indication that the water molecule has no fiied orientation with respect to the CD bond. This is, of course, expected sihce the somewhat smaller amplitudes of the D(-C) motions, compared -0.005 -0.005 to the H(-C) motions, should have a very slight directing influence 1472 1480 1440 1458 on the water molecule. For C6HsD, all bands are nondegenerate WRVENUMBERS CM-1 WAVENUMBERS CM-I and therefore have fixed orientations relative to the CD bond. The Figure 4. The v I 3 regions of benzene after subtraction of the benzene situation is different for the C6-symmetric benzene molecules. absorption. upper left: VI,(C~H~.HOH).Upper right: VIJ(C~D~*HOH). Lower left: u13,(C6H5D.HOH).Lower right: Y ~ ~ ~ ( C ~ H ~ D - HThe O H ) . Here a complex-forming water molecule may remove the degeneracies of E-symmetric bands and split them into components main benzene peak did not subtract perfectly but left scars to the right whose orientations are dependent on the position of the water of the main peak in the upper panels, at ca. 1480 cm-l in the lower left molecule. The similarity of the vl3(be.aq) bands of C6H6, C@6, panel and to the left of the main peak in the lower right panel. and C6HsD (Figure 4) may therefore suggest that the benzenewater interaction is too weak to remove the "13 degeneracies of the two bands assigned to V , ~ ( C ~ H ~ D . H O are H )real. For c6D6.HOH, the minor components of the band remaining after C6H6 and C6D6 or that the water molecule moves over the benzene subtraction were hidden by the ( c ~ D 6 absorption )~ and could, ring in such a way that effective c6 symmetry is retained in the complex. therefore, in principle be due to water-induced changes in the polymer C6D6distribution but the main peak was observed before The red shifts of the v1 bands of H 2 0 , HDO, and D 2 0 in the subtraction. For C6H6, the main component of the band assigned benzene complex indicate that water forms a weak hydrogen bond with benzene. Note that the red shift of v~(HOH.C,&), 21 cm-', to V13(C&HOH) iS uncomfortably Close to the V13(C,5&) band is larger than the red shift of ul(H20.S02), 14 cm-1,8,10in spite but the presence of the minor components was obvious even before the,benzene absorption had been subtracted. In an experiment of the fact that H,O.SO2 is relatively strong non-hydrogen-bonded complex. For H D O only the OD-stretching fundamental is with Ar/C6H6 = 146, A r / H 2 0 = 150, also a component of ~ 1 3 ( 13CC5H6-HOH)was observed after subtraction of the benzene red-shifted, showing that only D bonding occurs. The complex absorption. Its shape is identical with that of the band assigned shifts of all isotopomers of water are very close to those observed for the water-ethylene complex.1° It therefore seems reasonable to v13(C6H6.HOH). For a given isotopomer of benzene, the bands to assume that these two complexes have similar energies of appearing after subtraction of the free benzene absorption have
I A
d!!d%S
2864
J. Phys. Chem. 1985, 89, 2864-2869
formation. For the ethylene-water complex, the complex shifts of the water and ethylene fundamentals were used together with the assumption of a linear relation between complex shift and energy of formation to estimate an energy of formation of -2 kcal/mol.'O The calculated global energy minimum of the water-benzene interaction energy given in ref 1 is -3 kcal/mol. When the zero-point vibration energyI5 of the external vibrations of the complex are added to this value, the result is expected to be close to the experimental estimate. The calculated potential energy surface' was found to be rather insensitive to the precise orientation of the water molecule around the energy minimum. For instance, it was not possible to decide whether the complex has a single or a bifurcated hydrogen bond.I6 This observation may provide an explanation for the tine structure of the complex adsorption bands. While most water complexes studied in matrices have OHstretching bands with a single maximum, it is not uncommon to find bands with two maxima, but in these cases the shape of the band is identical for HOH--, HOD-., and DOD-. The v l (HOH-be) and vl(DOD.be) bands are similar in shape but v l (HOD-be) is entirely different (Figure 1). A matrix perturbation should produce similar structures in all three bands and is therefore not a likely source of the structure. Another possible cause of (15) See L. A. Curtiss, D.J. Fruip, and M . Blander, J. Chem. Phys., 71, 2703 (1979). for an estimate of the zero-point vibration energy of the water dimer. (16) Personal communication from the authors of ref 1.
the fine structure is the presence of a relative motion of the two complex components. Normally the fine structure resulting from, for instance, hindered rotation depends strongly on the matrix temperature, since the populations of the motional states vary with temperature and different relaxation processes become faster at higher temperatures. We lowered the temperature from 17 to 12 K in one experiment and observed some changes in the relative intensities of the components of v ~ ( H O H - C ~ H (Figure ~) 2). Unfortunately, the cryostat used did not allow us to lower the temperature further. It should be noted that if the motional states which are accessible at 20 K are within a few wavenumbers in energy, the changes in the populations in the 10-20 K interval may be obscured by changes in shape and overlap of the different components of the band. The dissimilarity in the structure of vl(HOD.be) and q(HOH-be) may be understood if H,O (or D20) is able to switch rapidly between the two possible hydrogen bonds. This would be consistent with the flatness of the calculated potential energy surface. In conclusion we suggest that water is hydrogen bonded to the flat surface of the benzene ring in such a way that two or more states of relative motion are accessible in the 11-17 K interval. The water molecule is not locked into a fixed position relative to a specific benzene carbon but moves over the ring in such a way that the interaction has effective C, symmetry and the two hydrogens of H 2 0 (or D 2 0 ) are equivalent. Registry NO. C6H6, 71-43-2; C6D6, 1076-43-3; CbHsD, 1120-89-4; H,O, 7732-18-5; DZO, 7789-20-0; HDO, 14940-63-7.
Equilibria of Nitric Acid in Sulfuric and Perchloric Acid at 25 O C by Raman and UV Spectroscopy M. Sampoli, A. De Santis, N. C. Marziano,* F. Pinna, and A. Zingales Facoltci di Chimica Industriale, Universitii, Dorso Duro 21 37, 301 23 Venezia, Italy (Received: June 1 1 , 1984; In Final Form: December 12, 1984)
Solutions of nitric acid in aqueous sulfuric (1-98 wt %) and perchloric acid (1-70 wt %) are studied by Raman and UV spatroscopy, and the concentrations of HN03,NO3-, and NOz+species are determined vs. medium composition. The variations of [HNO,] / [NO3-] and [NO,+]/ [HNO,] are analyzed to evaluate the corresponding dissociation constants. Different assumptions about the behavior of the activity coefficients of the species involved in the equilibria are tested. The M,activity coefficient function is found able to describe the protonation and the protonation-dehydration equilibria of nitric acid.
Introduction Equilibria of nitric acid in aqueous sulfuric acid have been extensively studied by different spectroscopic techniques.'-7 Raman,'-, UV,fs IR,6 and NMR7 spectroscopy, for instance, have been used as suitable means to identify molecular and ionic species present in the solutions. It is well-known that the solute dissociation gives rise to different ionic species which are described (1) J. Chbdin, C. R. Acad. Sci., 200, 1397 (1935); Ann. Chim., 8, 243-315 (1937); MEm.Seru. Chim. h a r , 31, 113 (1944); J. Chain, S . Ftntant, M6m. Seru. Chim. E r a , 40, 292 (1955). (2) C. K. Ingold, D.J. Millen, and H. G. Poole, J . Chem. SOC.,2576 (1 950). (3) N. C. Marziano, P. G. Traverso, A. De Santis, and M. Sampoli, J . Chem. SOC.,Chem. Commun., 873 (1978). (4) N. C. Deno, M. J. Peterson, and E. Secher, J . Phys. Chem., 65, 199 (1961). ( 5 ) N. S. Bayliss and D. W. Watts, Austr. J. Chem., 16, 943 (1963). (6) R. A. Marcus and J. M. Frescoe, J . Chem. Phys., 27, 564 (1957). (7) F. Seel, V. Hartman, and W. Gombler, Z . Naturforsch. B, 278, 325 (1972); D. S. Ross, K. F. Kulmann, and R. Malhotra, J . Am. Chem. SOC., 105, 4299 (1983).
0022-3654/85/2089-2864$01.50/0
by equilibria (1) and (2) in going from diluted to concentrated sulfuric acid.'-9 Direct evidence for the nitronium ion in the
+ NO3s NOz+ + H,O
HNO, s H" HN03
+ H"
(2)
concentrated acidity range (equilibrium 2) is of importance in nitration studies of aromatic compounds by sulfuric acid-nitric acid mixtures, since NO2+is the reactive species of the Moreover, the estimation of its concentration is particularly relevant and indeed necessary before the mechanism and kinetics can be interpreted. As for the evaluation of the concentrations of the solute species, few studies are a ~ a i l a b l e . l - ~Our $ ~ .purpose ~ is to determine these concentrations as a function of medium composition by Raman and UV spectroscopy as well as possible. ~~
(8) P B D De la Mare and J H Ridd, 'Aromatic Substitution, Nitration and Halogenation", Butterworths, London, 1959 (9) K Schofield, "Aromatic Nitration", Cambridge University Press, London, 1980
0 1985 American Chemical Society
