3527
J . Phys. Chem. 1991, 95, 3527-3532
Figure 2. Structural parameters obtained from the spectroscopic data (I) and from ab initio calculations (HF/6-31G*) with geometry opti-
mization for conformations with H atom straddling the symmetry plane (11) and lying in the symmetry plane (111). I1 lies 300 cm-I below Ill, and its binding energy was 1542 cm-l. Total energies: 11, -715.146738 hartrees; Ill, -715.145 396 hartrees. have the correct signs and are in fair quantitative agreement with the values estimated above. The lack of quantitative agreement probably arises from several sources including errors in the polarizability of PF3, errors in the electric field from the a b initio calculation, the assumption that a point polarizability at the center of mass of each species can be used to model their induced-dipole moments and the neglect of vibrational averaging effects. The possibility of some charge transfer also cannot be eliminated. Ab initio methods were also employed to examine the conclusion that the water molecule in PF3-Hz0straddles a symmetry plane as opposed to lying in the plane. For these two conformations, calculations with geometry optimization of R , and the tilt angles were performed at the HF/6-31G* level to estimate the structural parameters associated with the lowest energy for the two con-
formations of H20. These structures and energies are illustrated in Figure 2. The H 2 0 out-of-plane conformation (11) is placed -300 cm-l below the H20 in-plane conformation (111) and 1540 cm-l below dissociation and replicates the experimental R , and in I to -0.1 A and -So. However the value for O2 is different by 14'. It has been the experience in our laboratory and verified by recent work on 03.H20'zand the N20-HX systemz2 that calculations at the Hartree-Fock level will often correctly distinguish between conformational choices; it is gratifying that the PF3.Hz0comparison follows this trend. Nevertheless some caution is warranted since it is clear that calculations beyond the HF level must be made, especially when the complexes are quite weak, to reasonably reproduce experimental structures and estimate energy differences between isomers and binding energies. A rough estimate of the energetics of complex formation can be obtained by using Millen's modified pseudodiatomic approximation. In this model,21the centrifugal distortion constant 0, is related to a force constant for stretching of a hypothetical van der Waals bond between the PF3 and HzO, which gives k, = 0.073 mdyn/A. With a Lennard-Jones 6-12 potential, the force constant is related to a well depth by e = k,Rm2/72,resulting in t = 542 cm-I. This can be compared with the values in PF,.Ar and H20-SOZof 0.016 mdyn/& 180 cm-' and 0.084 mdyn/A, 520 cm-' respectively, when the analyses are done in a consistent manner. Clearly, the association of H 2 0 with either of the polar species is a considerably stronger interaction than in PF3-Ar where only dispersion and an induced polarization of the argon contribute to the binding. It is also apparent that considerably larger polarization occurs in the water complexes, as discussed above.
Acknowledgment. This work was supported by Grants CHEM86 14340 and CHEM89 17945 from the National Science Foundation, Washington, DC. We are grateful to Dr. C. W. Gillies for communicating his results on 03-H20prior to publication. (21) Millen, D. Can. J. Chem. 1985,63, 1477. Equation 21, which strictly holds only when the symmetry axes of the HzOand PF, are perpendicular to &,Le., 8 , = 90°, Oz = Oo, was used, resulting in b = B D / A H ~ BD/BpF, and c C D / B H 0 + CD/CpF,. (22) Zeng, P.; Sharpe, S. W.; Reifschneider, D.; Wittig, C.; Beaudet, R. A. J. Chem. Phys. 1990, 93, 183.
+
k7.
Polarized FT- I R Spectra of Water in Dodecylpropyldimethyiammonium Bromide Hemihydrate Noriyuki Kimura Institute for Chemical Research, Kyoto University, Uji, Kyoto-Fu 61 1. Japan (Received: September 25, 1990) Polarized infrared spectra of two kinds of specimens, with the (001) and (1 10) surface planes, of dodecylpropyldimethylammonium bromide hemihydrate (DPDMJ/,H20), deuterated hemihydrate (DPDMJ/2Dz0), and isotopically diluted hemihydrate (DPDM.1/2(958 D 2 0 + 5% H,O)), were observed in the frequency region from 5200 to 250 cm-I. The electric vector of the normally incident polarized radiation was rotated at 10' interval on the crystal planes. From the polychroism observed, the intra- and intermolecular vibration bands of water were classified into three infrared active species (A], B,, and B2). Furthermore, from the band frequency and the isotopic frequency ratios, these bands were assigned to the three fundamental vibrations, two librations, and their overtones and combinations. The directions of the transition moments of the water bands and of the CH2 stretching and scissoring bands of DPDM accord well with those expected from the crystal structure. The OH and OD stretching bands of HOD in isotopically diluted hemihydrate was found to split into two components. This fact was interpreted as due to the presence of the two kinds of configurations, Brl.-HOD-.Br2 and Br,--DOH--Br,, in the crystal, suggesting that the C, symmetry around water was slightly distorted in the crystal.
Introduction The structure and aggregation properties of alkylammonium bromidewater systems have attracted much attention because they exhibit a variety of micellar size and shape depending upon the number and length of the alkyl chains. For example, Kunitake et a1.l have reported that dialkyldimethylammonium bromides
with the longer alkyl chains than the dodecyl group form a well-defined vesicular structure which resembles that of phospholipid h " e s in aqueous solution. Zana et ala2have pointed (1) Kunitake, T.; Okahata, Y.;Tamaki, K.; Kumamaru, F.; Takayanagi, M. Chem. h t r . 1977, 387.
0022-3654/9l/2095-3527%02.50/0 0 1991 American Chemical Society
3528 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
out for a series of C12H2s(C,,,H~l)N+(CH3)2Brhomologues (m = 4-12) that the compounds with m = 1-4,5-10, and >IO form spherical micelles, disk-shaped micelles, and lamellar or bilayer structure, respectively, in aqueous solution. Detailed studies on hydration and packing of the hydrophobic groups in amphiphilic crystalline hydrates may help to understand physicochemical properties of micelles and bilayers. Since the infrared spectra of water are sensitively affected by its surrounding environment, the analysis of spectral features of water in these hydrates may clarify the interaction between the amphiphile and water. Recently, we studied the X-ray analysis of dodecylpropyldimethylammonium bromide hemihydrate ( D P D M S ~ / ~ Hand ~O) found that the crystal has the triclinic form belonging to the space group It was also found that the sample could be prepared as a thin single crystal as well as a well-oriented crystalline film, both of which are suitable for infrared measurements. In the present paper, the polarized infrared spectra of two oriented specimens, with the (001) and (1 10) surface planes, of the hemihydrate (DPDM-'/,H,O), deuterated hemihydrate (DPDM.'/ 2D20).and isotopically diluted hemihydrate (DPDM.'/2(95% D 2 0 + 5% HZO))were measured. From the results obtained, the water bands were assigned to the three fundamental vibrations, two librations, and their overtones and combinations. Furthermore, the slight deviation from the C, symmetry of the H 2 0 molecule in asymmetric crystalline environment was studied by using the vibrationally isolated O H and OD stretching bands of the HOD molecule.
Experimental Section A DPDM sample was synthesized by alkylation of dodecyldimethylamine (Aldrich Chemical Co., Inc., purity 97%) using n-propyl bromide: and recrystallized six times from ethyl acetate solution. n-Propyl bromide and ethyl acetate were guaranteed reagents of Tokyo Chemical Industry Co., Ltd. Anhydrous DPDM was obtained by keeping DPDM in a vacuum desiccator with phosphorus pentoxide for several days at 60 "C. Purity of anhydrous DPDM was evaluated by the elemental analysis. Anal. Calcd: C 60.75, H 11.30, N 4.16; found: C 60.69, H 11.27, N 4.08. DPDM.'/,H20, DPDM.'/2D20, and an isotopically diluted hemihydrate were prepared from anhydrous crystal by adding redistilled water, heavy water (E. Merck, Darmstadt, 99.85%), and isotopically diluted water (5% H 2 0 in D20), respectively, in mole ratio of 2:l in sealed vessels. A thin single crystal of DPDM.1/2H20 ( 5 X 3 X 0.02 mm3) was obtained by recrystallization from ethyl acetate solution at room temperature. Well-oriented crystalline films of DPDM.'/2HZ0 and DPDM. 1 / 2 D 2 0were prepared by slow cooling of molten samples sandwiched between two KBr windows (or polyethylene plates for far-infrared measurements) with slight temperature gradient. The crystal axes were determined by a Rigakudenki Model AFC-SRU X-ray diffractometer by using nickel-filtered Cu K a radiation. The surface (3 X 5 mm2) of the single crystal was found to be the (001) plane. For well-oriented crystalline samples, two kinds of specimens were obtained: one had the (001) plane and the other had the (1 10) plane. Infrared spectra between 5200 and 400 cm-l were recorded on a Nicolet Model 6000C Fourier transform spectrophotometer equipped with an MCT detector. Interferograms were accumulated 1000 times with a maximum optical retardation of 0.25 cm to yield the spectra of high S/N ratio with the resolution of 4 an-,. Infrared spectra below 400 cm-' were measured by a Hitachi Model 260-50 grating spectrophotometer. For the polarization measurements, a wire grid polarizer was used. The electric vector of the normally incident polarized radiation was rotated at IOo interval on the sample surface. Infrared spectra of KBr pellets (2) Lianos, P.; Lang,J.; Zana, R.J. Colloid. Interface Sci. 1983, 91, 216. (3) Taga, T.; Machida, K.; Kimura, N.; Hayashi. S.;Umemura, J.; Takenaka, T. Acta Crystallogr. 1987, C43, 1204. (4) Malliaris, A.; Christias, C.; Margomenou-Leonidoplou, G.; Paleos, C. M. Mol. Crysr. Liq. Crysf. 1982, 82, 161.
Kimura
(b) c
molecute 2 e 11'
ecute 1
Figure 1. Projection of the DPDMsI/~H~O molecules on (a) the (001) and (b) (1 10) planes. BIR means the angle between the (I (or c ) axis and the electric vector of polarized infrared beam on the (001) (or (1 10)) plane. The marks 0 and indicate the sites of center of symmetry at position, respectively. the origin and at the TABLE I: Observed Frequencies, emU, and Ox of the CHI Stretching and Scissoring Bands for Single Crystal of DPDM.'/2H20 with the (001) Plane wavenumber,' cm-' assignment BIRmX? deg ex, deg
va(CH2) h(CH2) f4CH2) "Accuracy i O . 1 cm-I. bAccuracy f3'. 2920 2854 1473
170 80 90
172 81 81
were also recorded for reference.
Results and Discussion Figure 1 illustrates the projection of the DPDMa1/2H20 molecule upon the (001) and (1 10) planese3 A unit cell contains four DPDM and two H 2 0 molecules. Two crystallographically independent DPDM molecules (1 and 2) and one H 2 0 molecule are shown in Figure 1. The residual molecules (1' and 2', and the other H 2 0 molecule) are situated at the symmetrical positions with respect to a center of symmetry. Each of the two O H bonds of H 2 0 form hydrogen bondings with Br, and Bri. The O--Brl and O-Br; distances are 3.361 and 3.332 A, respectively. BIR is the angle between the a (or c) axis and the electric vector of the polarized infrared radiation in the (001) (or the (1 IO)) plane. Figure 2 shows polarized infrared spectra of the single crystal of DPDM.'/,HZ0 with the (001) surface plane and of the well-oriented crystalline sample of DPDMJ/2D20 with the same surface plane. Unpolarized infrared spectrum of anhydrous DPDM powder in a KBr pellet is also shown in Figure 2a. No difference in the band frequency was detected among the spectra of the single crystal, oriented crystalline films, and KBr pellets. Figure 3 represents the polarized infrared spectra in the lower frequency range of the oriented crystalline film of DPDM.1/ZD20 with the (001) surface plane. Vibration Bonds of Hydrocarbon. Before we discuss the water bands, absorption bands due to the hydrocarbon chain are examined in this section. The strong bands at 2920,2854, and 1473 cm-' in Figure 2, a and b, have been assigned to the antisymmetric and symmetric CH2 stretching and CH2 scissoring vibrations of the dodecyl group, respectively. The transition moments of these bands are perpendicular to the trans-zigzag hydrocarbon chain, that of the 2320-cm-I band being normal to the zigzag plane and those of the 2854- and 1473-cm-' bands parallel to the plane. In Figure 4 the intensities of these three bands are plotted as a function of OIR in the (001) plane of the D P D M s ' / ~ H ~crystal. O
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3529
Polarized FT-IR Spectra of Water in DPDM 1,5
W
0
21,o
a a:
m 0
u,
m
%,S
0 5200
5000
3600
3400
3200
2800 2150
3000
1950 1700
WAVE NUMBER/
0 N G7 N
1500 1400 700
1700
500
600
400
cm-
e In
N co
0
I
(0
N Ln
I
w
J I
0,s-
u,
m
li d
h
a
h N
--
,-..,\
0 .
3900
I
x2
m
3700 3600
ROO
3200
3000
2800
2600 2400
1700
1600
1500 1400 700
600
500
400
WAVENUMBER/cm-' Figure 2. Polarized infrared spectra of (a) DPDM-'/zHzO crystal with the (001) plane and (b) well-oriented crystalline film of DPDMJ/,D20 with represents unpolarized infrared spectrum of anhydrous DPDM powder in the same surface plane measured at BIR = 40° (---) and 130° (-), a KBr pellet. I
1473
0.3
2920
x2
w
0
z
a
m
[1:
0
v)
m
a
ci'
. ,... /p.......,.. . ..,,..'.,., .... . ,,
0
I
1
1 . . . . .
400 300 WAVENUMBER/cm"
500
Figure 3. Polarized infrared spectra in the lower frequency region of the oriented crystalline film of DPDM.i/2D20 with the (001) surface plane measured at el, = 40° (---) and 130° (-) on the (001) plane. represents unpolarized infrared spectrum of anhydrous DPDM powder in a polyethylene pellet.
The 2920-cm-' band gives the maximum intensity at e i R H 170° and the 2854- and 1473-cm-l bands at e l R = 80°-90°. These e l R
0"
30"
66
90"
120"
156 180
OIR
Figure 4. Intensity variation of the antisymmetric (2920 cm-I) and symmetric (2854 cm-I) CH2 stretching and CH2 scissoring (1473 cm-l) bands against the change in the BIR value on the (001) plane of DPDM*'/ZH20.
values are listed as 01,- in Table I. @Xis the corresponding angle obtained from the crystal structure determined by X-ray difand Ox are in good agreement for the 2920- and f r a ~ t i o n .OiRmaX ~ 2854-cm-l bands. The slight discrepancy between them for the
3530 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
Kimura
TABLE 11: Obaened Frequencies of Water Bands, Isotopic Frequency Ratios, ODu, e,, Symmetry Species, and Band Assignments for Single Crystal with tbe (001) Plane and Well-Oriented Crystalline Sample with tbe (110) Plane of D P D M V ~ / ~ H ~DPDM.1/2DZ0, O, and DPDM.1/2HOD (001) plane (1 10) plane
freq,' cm-l H,O D,O
5028 3526 3429 3366 3196 2075 1614 579 466 3402d 3382d
freq ratio dH,O)/v(D,O)
3727 2630 2560 2475 236 1 1520 1 192c 430 335 25 17e 2504r
1.35 I .34 1.34 I .36 1.35 1.36 1.35 1.34 1.39 1.35, 1.351
OIR'"'',~
den 120 120 120 20 20 120 20 40 120 1 IO 140
8,Rm'X)
OX,
den 127 127 127 18 18 I27 18 44 127 104 140
OX>
den
den
100
96 96 96
100 170 0
SY?
species
assignment
IO IO
100
96
0 50 100 120 70
49 96 121 72
IO
OAccuracy f0.I cm-'. bAccuracy k3O. CEstimatedfrom the frequency of the 2u2 band. See text. "u(OH) of HOD. Iv(0D) of HOD. Ju(OH)/u(OD).
1
0' 3d
60"
96
126
1
(b)
i;'
156 186
OIR
OIR
Figure 5. Intensity variation of the H20 bands against OIR on (a) the (001) and (b) (1 10) planes of DPDM.1/2H20.
1473-cm-l band may be due to the coupling of this band with other bands. Vibration Bands of Water. Careful comparison of the spectra of the DPDM.1/2H20crystal, anhydrous DPDM powder (Figure 2a), and DPDM.1/2D20crystalline film (Figure 2b) suggests that the infrared bands are divided into four groups, each being ascribable to the DPDM, H 2 0 , D20, and HOD molecules. The frequencies of the H 2 0 , D20, and HOD bands are listed in Table 11. Since, in DPDM.1/2H20crystal, the hydrogen-bonding O..Brl and O-aBr,' distances are approximately the same (3.361 and 3.332 A), the H 2 0 molecule can virtually be treated as the C, symmetry even under such hydrogen-bonding circumstances. Therefore, the vibrational modes belonging to the AI, B1,and B2 species are infrared active, and the directions of the transition moments for the A,, B1,and B, bands can be considered to be parallel to the HOH bisector, parallel to the H-.H direction, and perpendicular to the HOH plane, respectively. Thus, from the crystal structure the Bx value for the AI, BI, and B2 bands are calculated to be 18O, 44O, and 127O, respectively, on the (001) plane, and loo, 49O, and 96O, respectively, on the (1 10) plane. Intensities of the water bands of DPDM.1/2H20are plotted as a function of BIR in the (001) plane of the single crystal in Figure 5a, and those in the (1 10) plane of the oriented crystalline sample in Figure 5b. From these figures, the water bands are found to be classified into three groups as follows. Those belonging to the first group (3366-, 3 196-, and 1614-cm-, bands) have BIRm around 20' in Figure 5a and around Oo in Figure 5b. The band belonging
to the second group (579-cm-I band) has elRmaX around 40° in Figure 5a and around 50° in Figure 5b. The other bands belonging to the third group (5028-, 3526-, 3429-, 2075-, and 466-cm-' bands) have OIRmaX around 120° in Figure 5a and around looo in Figure 5b. From comparison of these findings with the calculated Ox values mentioned above, it is concluded that the first group corresponds to the A, species, the second group to the B, species, and the third group to the B2 species. The results are summarized in Table 11. The same polarization measurements were also performed for the (001) and (110) planes of the DPDM.1/2D20 crystal, and the same BIRmaX values were obtained for the corresponding bands of D20. Thus they are undoubtedly classified into the A,, BI, and B2 species as also shown in Table 11. The results of the frequency ratios (v(H20)/v(D20)= 1.36 f 0.03) also support the above classification. I . Fundamental Vibrations. The H 2 0 molecule has three fundamental vibrations; the symmetric O H stretching (vI), HOH scissoring (v2), and antisymmetric OH stretching modes ( v J . The first two belong to the A, species and the last one belongs to the B2 species. Thus, the bands at 3366, 1614, and 3429 cm-' are unambiguously assigned to these modes, respectively. The scissoring band at 1614 cm-' appears as a singlet in Figure 2a. This is consistent with the result of the X-ray analysis that there is only one crystallographically equivalent water molecule in a unit cell. For DPDM.1/2D20, the 2475- and 2560-cm-I bands are assigned to the v I and v, modes, respectively. The DOD scissoring band could not be observed because of its weak intensity and overlapping with other bands (not shown in Figure 2b). The
Polarized FT-IR Spectra of Water in DPDM frequency of this band will be discussed later in connection with that of the 2u2 band. 2. Librations. Three librations of the H 2 0 molecule are expected in the frequency range between 700 and 400 cm-'. These are the rotational modes around three principal axes of the moment of inertia of H 2 0 , Le., the wagging (B, species), twisting (A, species), and rocking (B, species) vibrations. The twisting vibration is infrared inactive. Although the translational modes often fall in the same frequency range, the rotational modes can be distinguished from the translational one by the isotopic frequency ratio (u(H,O)/U(D~O)).~-' Tayal et al. have pointed out that the ratios for the wagging and rocking vibrations of water are 1.34 and 1.39, respectively, but that for the translational modes is 1.05.5 The 579-cm-I band which is strong for the broken line in Figure 2a and shows elRmnX of 40' in Figure 5a and 50' in Figure 5b can be assigned to the wagging mode (uw) belonging to the B, species. This band shifted to 430 cm-I on deuteration (Figure 2b), resulting in the isotopic frequency ratio of 1.34. This is in complete agreement with the above-mentioned theoretical value for this vibration. The intense band at 466 cm-l for the solid line in Figure 2a can be attributed to the rocking vibration (uR) belonging to the B2 species. For DPDM.'/2Dz0, the 335-cm-' band is assigned to the corresponding vibration, because it shows the same polarization as the 466-cm-l band of DPDM.1/2Hz0 (see Figure 3). This assignment is also supported by the frequency ratio of 1.39. The intensity ratio of the wagging to rocking bands was found to be 3.1 in the KBr pellet spectrum, being in satisfactory agreement with the theoretical value (2.7) by Miyazawa's model? No translational modes was observed here. 3. Overtone and Combinations. The 5028-cm-' band of H20 has been assigned to the combination of the u j and u2 modes (3429 1614 = 5043 cm-I) belonging to the B2 specie^.^ This assignment is supported by the result of the polarization measurements in this study; elRmaX is 120' on the (001) plane. The corresponding D 2 0 band is found at 3727 cm-' which shows the same elRmaX value. These results lead to a reasonable value of the isotopic frequency ratio (1.35). The 5028-cm-' band could not be observed on the ( I IO) plane probably because of its weak intensity. A weak H 2 0 band at 3526 cm-l exhibits the B2 polarization characteristic. The same feature is also seen for the 2630-cm-' band of DPDMe1/2D2O. The frequency ratio of 1.34 is reasonable for the B2 species band. Heyns has assigned the band to the combination of the 2u2 and vR.lo But his assignment leads to the calculated frequency of 3662 cm-' (=3196 466) for H20, which is too high as compared with the observed value of 3526 cm-I. A possible assignment is the combination of the v 1 band and some B2 species band near 160 cm-1.11*12This low-frequency band may be attributed to the translational mode ( u t ) along the Ha-H direction, although it was not observed here. Another support for this assignment is that the calculated istropic frequency ratio for the ut band (u,(H20)/u,(D20) = (3526 - 3366)/(2630 - 2475) = 1.03) is close to the theoretical value of 1.05 for the translational mode.5 A weak band at 3196 cm-l of DPDM.'/2H20 shows the A I polarization characteristic. For DPDMJ/,D20, the corresponding band is observed at 2361 cm-I, resulting in the isotopic frequency ratio of 1.35. These bands can be assigned to the first overtone of ut. On the basis of this assignment, the frequency of ca. 1192 cm-I for the u2 band of D 2 0 is obtained from the frequency ratio of the 1614-cm-I band (4 to 3196cm-l band (2u2) for H,O. This frequency also gives the reasonable calculated value (3752 cm-I)
+
+
(5) Tayal, V. P.; Srivastava, B. K.; Khandelwal. D. P.; Bist, H. D. Appl. Spectrosc. Rev. 1980, 16, 43. (6) Eriksson, A.; Lindgren, J. J . Mol. Srrucr. 1978, 48, 417. (7) Lutz, H. D.; Kluppel, H.-J.; Pobitschka, W.; Baasner, B. Z . Naturforsch. 1974, 298 723. (8) Miyazawa, T. Bull. Chem. Soc. Jpn. 1961, 34, 202. (9) Bonner, 0. D.; Choi, Y . S.J . Phys. Chem. 1974, 78, 1723. (IO) Heyns, A. M. Specrroehim. Acta 1977, 3 3 4 315. (1 I ) Draegert, D. A.; Williams, D. J . Chem. Phys. 1968, 48, 401. (12) Walrafen, G. E. J . Chem. Phys. 1966.44, 1546.
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3531
W
0
z
a
m K
0
m
m
a
W A VENUMB E R / cm-' Figure 6. Changes in O H stretching band of H O D in an isotopically diluted DPDM.1/2HOD (5% H 2 0 in D 2 0 ) at various OIR values on (a) the (001) and (b) ( 1 10) planes. Maximum intensities are marked by arrows.
for the u3 + u2 band (3727 cm-I) of DzO. The elRmaX values (120' for the (001) plane, and 100' for the (1 10) plane) of the 2075-cm-l band of DPDM.'/2H20 is characteristic of the Bz species (Table I). For DPDM.'/,DZ0, the corresponding band was found at 1520 cm-I (Figure 2b), resulting in the isotopic frequency ratio of 1.36. Thus, this band can be ascribed to the combination band of the u2 and uR vibrations, although it has been tentatively assigned by other investigators1g15 to various combination bands involving the librational modes. The combination of the uR and u1 or u3 modes and that of the uw and twisting modes is also infrared active. However, they were not detected in the expected frequency ranges. Environment of Water. So far, considering that the O-Br, and O.-Bri distances are approximately the same, the water molecule has been treated as the C, symmetry. In this section, however, more rigorous discussion will be given on the slight difference in these two distances. Figure 6 represents spectral change in the OH stretching region of HOD in an isotopically diluted hemihydrate (5% H 2 0 in D20) at various 6 l R values in the (001) and (110) planes. Doublet structures were apparent in both figures. Maximum intensities of the respective components are shown by arrows. The elRmaX values for the 3402- and 3382-cm-I bands areca. 110' and 140°, respectively, in the (001) plane, and about 120' and 70°, respectively, in the (1 10) plane. The OD stretching bands of HOD in the isotopically diluted hemihydrate are observed at 25 17 and 2504 cm-I, showing the same polarization behavior as the OH stretching band. The results are summarized in Table 11. The isotopic frequency ratios are 1.35 for both components. It has been known that the 0-H bond length of water increases with decreasing the hydrogen bond distance, Le., with strengthening of the hydrogen bonding. The stronger hydrogen bonding gives rise to the lower O H stretching and the higher wagging and rocking frequencies, except for the case of extremely strong hydrogen bondings.16J7 Therefore, the O H stretching band a t the lower frequency side (3382 cm-I) can be ascribed to the stron er hydrogen bonding, i.e., the shorter O-.Bri bonding (3.332 ). Contrarity, the higher frequency band (3402 cm-I) is ascribed to the longer hydrogen bonding, that is the longer O-Br, bonding (3.361 A). These considerations for the OH and the OD stretching bands of HOD suggest that there are two different configurations, Brl--HOD--Brz' and Brl-DOH...Bri . In other words, two hydrogen atoms of HzOare nonequivalent and the C, symmetry
w
(13) Giguere, P. A.; Harvey, K. B. Con. J . Chem. 1956, 34, 798. (14) Bertie, J. E.; Whalley, E. J . Chem. Phys. 1964, 40, 1637. (15) Brink, G.; Falk, M. Can. J . Chem. 1970, 48, 2096. (16) Thomas, G. H.; Falk, M.; Knop, 0. Can. J . Chem. 1974, 52, 1029. (17) Chidambaram, R. J . Chem. Phys. 1962, 36, 2361.
3532
J. Phys. Chem. 1991, 95, 3532-3538
is slightly distorted in this crystal. The OH stretching vibration of HOD is vibrationally isolated, and therefore the direction of the transition moment is approximately parallel to either the 0-Br, or 0 - B i bond. The Ox values for the two hydrogen bonds are 104O and 140° on the (001) plane, and 121' and 72O, on the (1 10) plane. These values agree well with the observed OIRm values as shown in Figure 6 and in Table 11.
In addition, the relations between the OD stretching frequency and hydrogen-bonding distance and also the other one between the isotopic frequency ratio v(H20)/v(D20)and the OH stretching frequency of the HOD molecule have been proposed.'*-20 By use of the former relation, the O.-Br distances of 3.332 and 3.361 A result in OH frequencies of 251 1 and 2525 cm-I, respectively, in accordance with the observed values of 2504 and 2517 cm-l within the standard deviation of 8 cm-I. The isotopic frequency (18) Mikenda, W. J . Mol. Srrucr. 1986, 147, 1 . (19) Lutz, H. D. Srrucr. Bonding 1988, 69, 97. (20) Berglund, B.: Lindgren, J.; Tegenfeldt, J. J . Mol. Sfrucr. 1978, 43, 169.
ratios of 1.352 and 1.353 were obtained from the OH frequencies of 3382 and 3402 cm-I, respectively, by the latter relation, agreeing with the experimental values within the standard deviation of 0.002. In conclusion, from the intensity variation of the water bands against the change in the OlR value they were classified into the A,, BI, and B2 species, under the assumption of the C , symmetry. Furthermore, the water bands were assigned to the fundamental vibrations, librations, combinations, and overtones, with the aid of the band frequencies, the isotopic frequency ratios, and the results of the X-ray analysis. The OH (OD) stretching bands of HOD was found to be a doublet, indicating the asymmetric crystal environment as expected from the crystal structure.
Acknowledgment. The author thanks Professors T. Takenaka and S. Hayashi and Dr. J. Umemura of this laboratory for their invaluable guidance and encourangement during this work. The author's thanks are also due to Professor T. Kobayashi of this institute for the use of the X-ray diffractometer, and to Professor K. Machida and Dr. T. Taga of the Faculty of Pharmaceutical Science, Kyoto University, for the crystal structure analysis.
Investigation of Dynamic Processes in Low-Lying Ionic States of o-Hydroxybenzaldehyde Umpei Nagashima,* Institute for Molecular Science, Okazaki 444, Japan
Shin-ichi Nagaoka,*3ti* Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790, Japan
and Shunji Katsumatat Department of Fundamental Science, Iwaki Meisei University, Iwaki 970, Japan (Received: December 27, 1989; In Final Form: October 19, 1990)
The spectroscopic properties and the dynamic processes of the low-lying ionic states of o-hydroxybenzaldehyde have been studied by means of photoelectron spectroscopy and ab initio calculations. The photoelectron spectrum of OHBA indicates that the potential surface of the first ionic state is distorted from that of the ground state. A classical trajectory following vertical excitation to the first ionic state has been calculated by the Runge-Kutta method. In the trajectory, the amplitude of the 0-0 and 0-H oscillations is quite large, and the two oscillations are strongly coupled to the oscillation of the benzene skeleton. Finally, the 0-H bond distance lengthens owing to the Coulomb attraction between the proton and the electron cloud located between the proton and the carbonyl oxygen. The lengthening is regarded as having proton-transfer character. As a result, the first ionic state is bound at a longer 0-H distance, and the two 0-Hdistances become close to each other.
Introduction The dynamic process of the excited states of intramolecularly hydrogen-bonded molecules is a topic of current interest.'-3 We have investigated the dynamic processes in various electronic states (The numbering of ehydroxybenzaldehyde (OHBA) in detail."' system for the atoms of OHBA is shown in structure 1. OHBA H1
1
'Also at Research Institute of Applied Electricity, Hokkaido University, Sapporo 060, Japan. IAlso at Institute for Molecular Science, Okazaki 444, Japan.
is the simplest aromatic molecule with intramolecular hydrogen bonding involving a carbonyl group and is readily amenable to ( I ) Barbara, P. F.; Walsh, P. K.; Brus, L. E. J. Phys. Chem. 1989,93,29. (2) Spectroscopy and Dynamics of Elementary Proton Transfer in Polyatomic Systems; Barbara, P. F., Trommsdorff, H. p., Eds. In Chem. Phys. (Special Issue) 1989, 136. 153-360. (3) References cited in refs 4-1 I . (4) Nagaoka, S.;Hirota, N.; Sumitani, M.; Yoshihara, K. J . Am. Chem. Soc. 1983, 105,4220. ( 5 ) Nagaoka, S.;Hirota, N.; Sumitani, M.; Yoshihara, K.; LipczynskaKochany, E.; Iwamura, H. J . Am. Chem. Soc. 1984, 106, 6913. (6) Nagaoka, S.; Fujita, M.; Takemura, T.; Baba, H. Chem. Phys. Lett. 1986, 123,489. (7) Nagaoka, S. J . Phorochem. Photobiol. A 1987, 40, 185. (8) Nagaoka, S.;Nagashima, U.; Ohta, N.; Fujita, M.; Takemura, T. J . Phys. Chem. 1988, 92, 166. (9) Nagaoka, S.;Nagashima, U. In Specrroscopy and Dynamics of Elementar) Proron Transfer in Polyaromic Systems; Barbara, P. F., Trommsdorff, H. P., Eds. In Chem. Phys. (Special Issue) 1989, 136, 153.
0022-3654/91/2095-3532$02.50/00 1991 American Chemical Society
