2526
J. Phys. Chem. 1903, 87, 2526-2535
Vibrational Analysis of Acetate Ion Molecules and Estimation of Equilibrium Constants for Their Hydrogen Isotopic Exchange Reactions Masato Kaklhana, Masahlro Kotaka, and Makoto Okamoto' Research Laboratory for Nuclear Reactors, Tokyo Institute of Technomy, Ookayama. Megureku, Tokyo 152, Japan (Received: October 25, 1982; I n Final Form: December 27, 1982)
The infrared spectra (4000-300cm-') of CH3COONaand its five isotopic substitutionsincluding D-and 13C-labeled modifications suspended in KBr disks were measured at 80 and 290 K. Excellent resolutions were obtained by recording the spectra at low temperature. Complete vibrational assignments were established on the basis of correlations of group modes and the 13C shifts of the fundamentals. From a normal coordinate analysis a general valence force field involving 26 force constants has been determined, which reproduces 82 experimental frequencies with a root-mean-squaresdeviation of 3.3 cm-'. The composition of the normal vibrations from the symmetry coordinates has been given in terms of the potential energy distribution which makes it possible to inspect the relation between the normal modes and the group vibrations. The valence force field was used to predict fundamental frequencieswhich were not available from experiments,and the reduced partition function ratios of acetate ion molecules were calculated by using the spectroscopic data including the predicted values of the fundamentals. Calculated equilibrium constants of some hydrogen isotopic exchange reactions between the acetate ion molecules are reported.
Introduction The equilibrium constants for isotopic exchange reactions between molecules can be estimated from their vibrational spectroscopic data on the basis of statistical mechanics with small quantum corrections. Our interest is mainly focused on hydrogen isotopic exchange reactions especially associated with tritium. In contrast to the large availability of experimental frequencies from deuterium (D)-labeled modifications, fundamental frequencies from tritium (T)-labeled species are not easily obtained because of the experimental difficulties in their observation. Fortunately fundamental frequencies for a species containing rare isotopes like tritium can be approximately estimated by use of the well-known G and F matrix method,l assuming that the potential energy of the system is invariant with respect to isotopic substitution. However, the calculated fundamental frequencies for a species involving hydrogen isotopes are expected to be very sensitive to the molecular force field used, because the substitution of deuterium or tritium for hydrogen causes very large frequency shifts which sometimes lead to misassignments of the fundamentals. It should therefore be emphasized that determination of a reliable molecular force field with careful vibrational analyses must be performed prior to the estimation of the equilibrium constants for isotopic exchange reactions. The present study relates to a vibrational analysis of the acetate ion molecule on which no complete spectroscopic investigation has been carried out. In a recent paper,2 Kakihana et al. measured the infrared spectra of 13Csubstituted species of solid sodium acetate at 80 K and reported a new improved assignment of the spectra on the basis of the qualitative empirical rules available and the 13C shifts of the fundamentals. However, there still remained some ambiguity about the reliable assignment of the asymmetric CH3 deformation modes in the spectra of the isotopic species. Furthermore, in past studies3-6there (1) Wilson, Jr., E. B.; Decius, J. C.; Cross, P. C. 'Molecular Vibrations"; McGraw-Hill: New York, 1955. (2) Kakihana, M.; Kotaka, M.; Okamoto, M. J. Phys. Chen. 1982,86, A2R.5.
(3) Jones, L. H.; McLaren, E. J. Chem. Phys. 1954,22, 1796.
O022-3654/83/2087-2526$01.50/0
exists confusion about the assignment of several fundamentals from D-labeled species (12CD312COONa).In view of these studies, normal coordinate analysis on the acetate ion molecule is required for conclusive assignment of these fundamentals. In the present work, the infrared spectra of 12CH312COONa(parent species), 12CH313COONa(lJ3C), 13CH312COONa (2-13C), 13CH313COONa (l,2-13C), 12CD22COONa(2-d3),and 13CD3'2COONa(2-13C,d3)were measured at 80 and 290 K, and a detailed vibrational analysis based on the data for the six isotopic modifications was carried out in order to establish an exact assignment of the fundamentals and to determine an approximate molecular force field. The major contents discussed in the present paper are outlined as follows: (A) The great advantage of measuring spectra at low temperature and making vibrational assignment based on qualitative empirical rules is discussed. Detailed profiles of the low-temperature spectra are given in order to demonstrate their great advantage in obtaining excellent resolution. Spectra measured at low temperature make it possible to clarify overlapping regions of the room temperature spectra and therefore to locate the fundamental frequencies without ambiguity. Some arguments about the vibrational assignments are briefly advanced in terms of qualitative empirical rules for group vibrations. (B) Normal coordinate analysis and determination of an empirical molecular force field is discussed. On the basis of normal coordinate analysis, the vibrational assignment for the parent species reported in the preceding paper2 is confirmed and a new improved assignment for the 2-d, species is reported. The present assignment is strongly supported by the excellent agreement between the observed and calculated frequencies. Although the experimental data are comprehensive, a complete harmonic force field could not be determined; a large number of insignificant off-diagonal force constants were found to be nonestimable through a least-squares refinement procedure. The set of force constants is therefore reduced by a number of constraints which are expected to be reason(4) Nakamura, K. J. Chen. SOC.Jpn. 1958,79,1420. ( 5 ) Ito, K.; Bernstein, H. J. Can. J. Chem. 1956,34, 170.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
Vibratlonal Analysis of Acetate Ion Molecules
400
114c
'UCC
POC
6
c wa e
213
(IC IO e r
"1
9
60
4"O
2527
IJC
c
Figure 1. Infrared spectrum of 12CH312COONa in KBr pellet at room temperature; 1 mg of sample suspended in 100 mg of KBr.
able from a physical point of view. (C) The reduced partition function ratios and equilibrium constants for hydrogen isotopic exchange reactions are calculated. Using the force constants obtained, we can now calculate fundamental frequencies which are not available from experiments, and consequently the reduced partition function ratios of the isotopic species can be estimated by use of the well-known equation derived by Uref and Bigeleisen and Mayer.' Finally, we report calculated equilibrium constants for several hydrogen isotopic exchange reactions.
Experimental Section Potassium bromide and 12CH312COONa were obtained commercially (suprapur reagents, E. Merck Co. Ltd., Darmstadt). 12CD22COONa(99% deuterium isotopic purity) and 13CDJ2COONa(98% deuterium and 90% 13C isotopic purity) were purchased from Merck Sharp and Dohme Canada Ltd. The remaining 13C species were obtained from Prochem Co. Ltd., London, with 90% 13C isotopic purity. The sample manipulation and the preparation of KBr disks were the same as those described elsewhere.2 A liquid nitrogen cryostat Model DN70 (Oxford Instrument Co. Ltd.) with KRS-5 windows was used for the low-temperature experiment. Infrared spectra at low-temperature were measured in the region 4000-300 cm-' with a Jasco Model DS701G with slit program 5 (spectral resolutions 1.0-2.4 cm-'). Infrared spectra at room temperature were measured at 0.24-cm-' resolution on a Nicolet 7000 Fourier transform spectrometer purged with nitrogen and equipped with a mercury cadmium telluride detector. All frequencies except for those of the fundamentals observed at room temperature were calibrated against the standard absorption bands of H20,NH3, C02, HC1, and DC1.8 Results and Discussion ( A )The Great Advantage of Measuring Spectra at Low Temperature, and Vibrational Assignment Based on Qualitative Empirical Rules. ( 1 ) Infrared Spectra of the Parent and Its 13C-Labeled Modifications. A typical infrared spectrum of the parent species measured at room temperature is reproduced in Figure 1. Comparison of the spectra measured at 80 K with those obtained at room temperature are made for two interesting spectral regions, 3100-2900 and 1700-1300 cm-l, for the parent and its 13C-labeledmodifications; in these spectral regions, there considerable confusion has existed about the vibrational assignments in the literat~re.~-~.~J" In the other spectral (6) Ufey, H. C. J. Chem. SOC.1947, 562. (7) Bigeleisen, J.; Mayer, M. G. J. Chem. Phys. 1947, 15, 261. (8) International Union of Pure and Applied Chemistry (IUPAC), "Tables of Wavenumbera for the Calibration of Infrared Spectrometers", Pergamon Press: New York, 1977. (9) Wilmshurst, K. J . J . Chem. Phys. 1955, 23, 2463. (10) Spinner, E. J. Chem. SOC.1964, 4217.
uu
3100
3000
2900
3100
3000
2900
Wavenumber/cm-'
Figure 2. Characteristic absorptbn bands due to CH,-stretching modes of '*CH,'%OONa (a) and '3CH,'%OONa (b) at 80 (left) and 290 (right) K; 1.0 mg of sample suspended in 100 mg of KBr.
290 K
Y
(a)
u3100
3000
2900
3100
3000
2900
Wavenumber/cm"
Figure 3. Characterlstic absorption bands due to CH,-stretching modes of '2CH,'3COONa (a) and 13CH,'3COONa (b) at 80 (left) and 290 (right) K; 1.0 mg of sample suspended in 100 mg of KBr.
ranges, no significant changes between the two spectra were observed except that the line widths of the absorption bands at low temperature were found to be much smaller than those at room temperature.
2528
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
1 -
‘
L
-
1
Kakihana et al.
I -
Figure 4. CH,deformation and COO-stretching region of the spectra of ‘2CH3‘2COONa(a) and ‘3CH3’2COONa(b) at 290 and 80 K; 1.0 mg of sample suspended in 100 mg of KBr
--
+
1
A i
290 K
Flgwe 5. CH,deformation and COO-stretching re ion of the spectra of ‘2CH,’3COONa (a) and 13COONa(a) and I3CH3’COONa (b) at 290
f
and 80 K; 1.0 mg of sample suspended in 100 mg of KBr
1 :c
L--q+-4po
1;bc
’
1532
I
.30i
‘ia*;erumStr/o
Figure 2 represents the typical infrared absorption bands of the parent and 2-I3C labeled species at 80 and 290 K in the CH,-stretching region (3100-2900 cm-I), where three fundamentals are expected. The corresponding absorption bands of the l-13Cand l,2-13C labeled modifications are shown in Figure 3. As can be seen from Figure 2 or 3, the middle absorption band, which is shoulderlike in the spectrum measured at room temperature, was clearly resolved in the low-temperature spectrum; the measurement at low temperature makes it feasible to locate the fundamentals with higher confidence. I3C substitution at the methyl carbon caused lower frequency shifts of the three fundamentals, which can be convenient for assignment of these modes to group vibrations. The absorption band centered at the lowest frequency in each spectrum was assigned to the symmetric CH3-stretchingmode and the others to the asymmetric CH3-stretching modes. Figures 4 and 5 represent the infrared spectra of the parent and its three I3C-labeled species in the range 1500-1300 cm-’, where four fundamentals due to one COO-stretching and three CH3-deformation modes are expected to be observed. One strong broad band observed at room temperature for each isotopic species, which covers the frequency region under consideration, was found to split into several bands in each spectrum measured at low temperature. However, the resolution was not sufficient owing to the relatively high concentration of the sample (1.0 mg of sodium acetate suspended in 100 mg of KBr powder) and therefore no explicit indication of assignment was found; this problem can be resolved by measuring the spectra at a lower sample concentrations as described later. On the other hand, the weak absorption band observed at 1330 cm-l in each spectrum measured at room temperature was assigned to the symmetric CHS-deformation mode to which empirical rules for group vibrations can be
-
Figure 6. CH,deformation and COO-stretching region of the spectra of ‘2CH3’2COONa(a) and 12CH3’3COONa(b) at 290 and 80 K; ca. 40 pg of sample suspended in 100 mg of KBr.
applied. This band became drastically sharp with significant intensity enhancement in each spectrum obtained at low temperature. One exceptional case was recognized in the spectra of the lJ3C labeled species; an unexpected strong absorption band with a shoulder was observed at 1344.0 cm-l in the room temperature spectrum and the band showed a doublet structure with components at 1343.7 and 1327.5 cm-’ in the spectrum measured at low temperature as shown in Figure 5a. Although no quantitative interpretation of this irregularity can be given at the moment, there exists the possibility of Fermi resonance between the symmetric COO-stretching mode (- 1390 cm-’) and the combination tone of C-C stretching (-920 cm-’) and in-plane OCO-rocking (-470 cm-’) modes; heavy atom modifications such as I3C labeling seem to be useful for identification of Fermi resonance, because such a resonance is generally very sensitive to small isotopic shifts. In view of this, the absorption band appearing at 1343.7 cm-’ was regarded as a counterpart of the Fermi resonance lines and the band centered at 1327.5 cm-l was assigned to the symmetric CH3-deformation mode. Figures 6 and 7 give detailed features of the spectra at a lower sample concentrations (ca. 40 pg of sodium acetate suspended in 100 mg of KBr powder), covering the spectral range from 1700 to 1300 cm-l. The major problem with the vibrational assignment in this region concentrates on one symmetric COO-stretching mode and two asymmetric CH3-deformationmodes which are expected to be observed in the range between 1460 and 1360 cm-l; these fundamentals could not be identified from the spectra meaaured at room temperature because of their complex overlapping band contours. In the spectra measured at 80 K, the
The Journal of Physical Chemisfry, Vol. 87, No. 14, 1983
Vibrational Analysis of Acetate Ion Molecules
L',1;,
,
IC9
2co
2 a0
B 0
I
6 0
*a
e-,un,er
a
,
i
, (0
' ,
,
,
8CC
2529
I , 6 0
,
,]
49c
300
c
Figure 8. Infrared spectrum of '2CD3'2COONain KBr pellet at room temperature; 1 mg of sample suspended in 100 mg of KBr.
u
1700
1500
1300
L
1700
I
U 1500
'I 1- I'ri
I 1300
Wavenumber/cm-'
Flgure 7. CH,deformation and COO-stretchingregion of the spectra of 13CH,'2COONa (a) and 13CH,'3COONa (b) at 290 and 80 K; ca. 40 pg of sample suspended in 100 mg of KBr.
resolutions are high enough to locate the fundamentals without ambiguity. The three fundamentals in question were clearly observed in each spectrum at 80 K as shown in Figures 6 and 7; since the spectra were measured for samples with 90% 13C isotopic purity, some additional absorption bands from several isotopic modifications were found in the spectra. The well-defined absorption bands appearing at -1580 cm-l for the parent and 2J3C labeled species can be assigned to the asymmetric COO-stretching modes for which empirical rules for group modes are available; the corresponding fundamentals for the lJ3C and l,2-13Clabeled modifications were observed at 1540 cm-'. These large shifts of the fundamentals arise from the 13Csubstitutions at the carboxylate carbons. Similarly large shifts are expected for the symmetric COO-stretching modes. On the basis of this argument, the fundamentals of the symmetric COO-stretching modes for the parent and 2J3C labeled species can be reasonably assigned to the frequencies at 1423.7- and 1421.0-cm-' and the corresponding fundamentals for the lJ3C and l,2-13Clabeled modifications were located at 1393.8 and 1390.4 cm-', respectively; the isotopic shifts of the fundamentals of these symmetric COO-stretching modes are comparable with those of the fundamentals due to the asymmetric COOstretching vibrations. The two remaining fundamentals (e.g., 1407.3 and 1447.3 cm-l for the parent species) are apparently due to the asymmetric CH3-deformationmodes. (2)Infrared Spectra of the Deuterated and Its Methyl 13C-LabeledSpecies. A representative spectrum of the deuterated species obtained at room temperature is shown in Figure 8, covering the range from 4000 to 300 cm-'. A similar spectrum with small isotopic shifts of the fundamentals was obtained for its methyl 13C-labeledspecies.
-
290 K
80 K
2300
2200
2100
u
2300
2200
2100
Wavenmber/cr-'
Flgure 0. Characteristic absorption bands of CD,-stretching modes of '2CD3'2COONa(a) and 13CD,1%OONa (b) at 80 (left)and 290 (right) K; 1.O mg of sample suspended in 100 mg of KBr.
Two important portions of the spectra which lie in the regions 2300-2100 and 1100-800 cm-' are discussed. Figure 9 shows typical infrared absorption bands for '2CD3'2COONa (a) and '3CD312COONa(b) at 80 K in the region 2300-2100 cm-', together with the corresponding spectra at 290 K for comparison. In this spectral range, three characteristic fundamentals due to CD3-stretching modes are expected to be observed; the empirical rules applied to the CH3-stretchingmodes are also available for the CD3-stretching modes. Fundamentals for the symmetric CD3-stretching modes, which are expected to be observed around 2100 cm-', were not observed for both isotopic species. The absorption band centered at -2180 cm-' in the low-temperature spectrum of 12CD312COONa wm interpreted as one of overtones or combination tones. The corresponding absorption band for l3CD3l2C0ONa appeared at -2150 cm-' in the low or room temperature spectrum. The remaining two absorption bands experienced downward frequency shifts of about 15 cm-l by 13C substitution at the methyl carbon and were assigned to the asymmetric CD&retching modes, whereas in past studies3t4the lower absorption band of these two fundamentala
2530
Kakihana et ai.
The Journal of Physical Chemistry, Vol. 87,No. 14, 1983
w
I
-290 K
(b)
,.*p
303
8'0
11cc
1001
Y
3co
8'13
INavenuTber I &
Flgure 10. CD,deformation and rocking region of the spectra of '2CD.'%OONa at 80 (a) . , and 290 (b) . . K; 1.0mg of sample suspended in 10'0 mg of KBr. I
I
(b)
1
I
I
290 K
91
P
Figure 11. CD,deformation and rocking region of the spectra of 13CD,'2COONa at 80 (a) and 290 (b) K; 1.0 mg of sample suspended in 100 mg of KBr.
was assigned to the symmetric CD3-stretching mode and the higher one to the asymmetric CD,-stretching mode. The resolutions of the spectra measured at 80 K were found to be higher than those of the spectra obtained at room temperature as shown in Figure 9. Figures 10 and 11represent the infrared spectra of the deuterated and its methyl 13C-labeledspecies in the region 1100-800 cm-l, where six fundamentals should occur. The major problem with the assignment in this region centers on the three fundamentals due to one symmetric and two asymmetric CD,-deformation modes which are expected to be observed in the spectral region 1100-1000 cm-'. In each spectrum measured at 80 K the three fundamentals expected in this CD3-deformation region were clearly observed, whereas in the corresponding spectrum at room temperature only two major bands were detectable, in which the lower vibrational band exhibited a relatively broad contour with a shoulder. The absorption band at 1083.4 cm-' in the low-temperature spectrum of '2CD312COONacan be assigned to the symmetric CD3deformation mode and the corresponding fundamental for 13CD,'2COONa was observed at 1066.6 cm-' with an
Figure 12. Definition of stretching, bendlng, and in-plane rocking coordinates for the acetate ion molecule.
unexpected large isotopic shift. This large isotopic shift of about 17 cm-' reveals that the normal mode is associated with the skeletal C-C stretching mode. On the other hand, the absorption bands centered at 1027.8 and 1040.5 cm-' in the low-temperature spectrum of 12CD312COONa were assigned to the asymmetric CD3-deformationmodes. The corresponding fundamentals for its methyl 13C-labeled species can be localized at 1024.6 and 1038.9 cm-' in the spectrum measured at 80 K. Finally we discuss the three remaining fundamentals located in the range from lo00 to 800 cm-'. No remarkable difference between the spectrum measured at 80 K and that observed at 290 K can be recognized except that the absorption band centered at about 880 cm-' has a counterpart at about 870 cm-' in the low-temperature spectrum. The assignment of these three fundamentals is less clear because the normal modes observed in this spectral region are expected to be associated with several vibrational modes. In previous works,= the absorption band at 880.7 cm-' in the room temperature spectrum of '2CD,'2COONa was assigned to the C-C stretching mode. However, if the corresponding absorption band for 13CD312COONa,which was observed at 880.4 cm-', is taken into consideration, empirical rules for group vibrations of C-C stretchings cannot be applied to this fundamental. Therefore this normal mode will be described as complex mixtures of several vibrational modes. The ambiguous part of the assignment described in this section will be resolved by the normal coordinate analysis introduced in the next section. (B)Normal Coordinate Analysis and Determination of a n Empirical Molecular Force Field. (1)Internal Coordinates. In the present vibrational analysis, all the molecules concerned are assumed to have structures of C, symmetry; the 15 normal modes may be classified into 10 A' and 5 A" modes. The definition of the stretching, bending, and in-plane rocking coordinates used in this study is illustrated in Figure 12. The in-plane rocking coordinate (7)is defined as the angle between the C-C bond and the line bisecting the angle 8. In addition, one our-of-plane rocking coordinate ( T ) and one torsional coordinate (7)were introduced. The coordinate T is defined as the angle between the C-C bond and the OCO plane. The torsional coordinate T is chosen as a linear combination of four mass-torsional coordinates as follows: = 1 / ( T 5 1 2 3 + T5124 + T6123 + T6124 + T7123 + 77124) where the subscripts refer to the numbering of the atoms as shown in Figure 12. Geometrical parameters of the
The Journal of Physical Chemistty, Vol. 87,
Vibrational Analysis of Acetate Ion Molecules
No. 14,
1983
2531
TABLE I: Structural Parameters of the Acetate Ion* bond lengthb/ nm r,(C-H) = r,fC-H) = r;(C-H j l(C-C) s( c - 0 )
O.10gc
d(C-0)
0.1253
0.1505 0.1257
a,(HCH) = a,(HCH) = a;(HCH) p,(CCH) p,(CCH) = p,(CCH) @(OCO) Y
bond angleb deg 110.56d 113 106
123.7 0.55
Syma Geometry at equilibrium according to ref 18. bols are in accord with the notations given in Figure 12. Estimated values, assuming that the From ref 19. three angles are identical.
acetate ion molecule used in this calculation are listed in Table I, where the symbols shown are according to the notations given in Figure 12. Table I1 gives the definition of the symmetry coordinates which are constructed as appropriate linear combinations of the internal coordinates by taking into account the distorted tetrahedral configuration around the methyl group; correction coefficients for the distortion are introduced into four A‘ symmetry coordinates, one of which is a redundant coordinate. (2) Constraints to the Valence Force Field. Since the acetate ion molecule with a configuration of C, symmetry has 10 A’ and 5 A” normal modes, the general harmonic force field involves 70 parameters. Although a number of experimental data on the fundamentals are available, only 26 force constants were determined through the leastsquares refinement procedure. In the present calculation, the set of force constants was reduced by a number of constraints which were introduced by Hollenstein and Gunthard.” With considerable success, this type of approximation was used on the pyruvic acid molecule.12 The approximation used in this study is given as follows: (i) Interaction force constants of the methyl stretching and torsional coordinates with other internal coordinates are omitted, while the interaction force constants between the adjacent C-H stretchings are taken into consideration. (ii) Off-diagonal force constants associated with the methyl and carboxylate groups are subject to CSuand Czu local symmetry constraints, respectively. (iii) Interaction force constants between coordinates having less than two common atoms are excluded, while off-diagonal constants containing adjacent stretchings, bendings with a common vertex, and stretching-bending having a common appex are taken into account. (iv) Since one redundancy is involved within the six methyl bending internal coordinates, only linear combinations of force constants associated with these coordinates can be determined. We chose these combinations according to the convention presented by Hollenstein and Gunthard. l3 In order to define the force constants, we introduce a potential function which was restricted by the abovementioned assumptions:
(11) Hollenstein, H.; GUnthard, He. H. J. Mol. Spectrosc. 1980, 84, 457. (12) Hollenstein, H.; Akermann, F.; GUnthard, He. H. Spectrochim. Acta, Part A 1978, 34, 1041. (13) Hollenstain, H.; Ghthard, HE. H. Chem. Phye. 1974, 4 , 388.
where the primed quantities follow convention (iv). The potential function involves 27 parameters; the parameter H , referring to the methyl torsion is derived from eq 214
and is kept constant for all isotopic species, where V 3 is a threefold potential barrier for the methyl group. The remaining 26 force constants were adjusted to fit 82 observed fundamental frequencies by the least-squares iteration procedure. The final values of the harmonic force constants associated with the internal coordinates are given in Table 111. By this valence force field the experimental frequencies can be reproduced with a root-mean-squares (rms) deviation of 3.3 cm-’. (3)Discussion. All the fundamentals observed at 80 and 290 K are listed in Tables IV-IX. These tables also give the normal frequencies calculated from the harmonic force field given in Table I11 and the approximate description of the normal modes in terms of main contributions of the symmetry coordinates (group modes) to the potential energy distribution. The excellent agreement between the observed and calculated fundamentals indicates that the proposed vibrational assignment of the spectra is reasonable. Therefore the present approximations to the general valence force field are to be suitable. A few comments are given with respect to the normal modes and valence force field in the following sections. ( a ) Normal Modes and Additional Comments on Vibrational Assignments. The following arguments are made on molecules of the parent and its deuterated species; similar interpretations are available for the 13C-labeled modifications. (i) The three fundamentals centered at 3002.8, 2930.1, and 2974.4 cm-’ in the low-temperature spectrum of the parent species were assigned to the u,(A’), uz(A‘), and vll(A”) modes which can be well described by the group modes of v,(CH3), vs(CH3),and v(CH3),respectively (see Table IV). The corresponding fundamentals of the deuterated species were assigned to the frequencies 2255.3, 2105.9, and 2227.2 cm-’ in which the second one is the value predicted from normal coordinate calculations (see Table VIII). This predicted value is in fair agreement with the value of 2111 cm-l obtained by a Raman spectral measurement of saturated aqueous solutions of sodium a~etate.~ (ii) The v4(A’), u&A’), and v12(A”) normal modes of the parent species were assigned to the frequencies 1423.7, 1407.3, and 1447.3 cm-’. The v4(A’) mode is a complex combination of several group modes, the predominant mode being v,(CO). The u,(A’) mode can be expressed as ~
(14) Fateley, W. G.; Miller, F. A. Spectrochim. Acta 1961, 17, 857.
2592
The Journal of phvslctll Chemlsiry, Vol. 87, No. 14, 1983
Kakhana et al.
TABLE 11: Symmetry Coordinates for t h e Acetate Ion representation C, symmetry coordinate ~
A'
A1
- q A p , - qAp,)'
description
Vas(CH3) u,(CH,) u,(CO) U,(CO) u(C-C) 6 ,(CH,) 6 ,(CH,)
asym CH, stretch sym CH, stretch sym CO stretch asym CO stretch C-C stretch asym CH, deform sym CH, deform in-plane OCO rock in-piane CH, rock OCO bend redundant CH, stretch CH, deform out-of-plane CH, rock out-of-plane OCO rock torsion
in,(oco1
AY
Y
(1/fi)(2pA01- q A P 2 - q A p 3 ) a
Yin(CH3) 6 (OCO)
A0
+ b A a , + b A a , + P A P , + q A p , + qAp3)a
A'
U(CH3) 6 (CH,) n(CH,) R(OC0) r(CH3)
A77 AT a
notation
Correction coefficients for the distorted tetrahedral configuration around the methyl group: a = 0.96645, b = 1.04668,
p = 1.00269, q = 0.96683.
TABLE 111: Harmonic Force Constants for t h e Acetate Iona notationb 107Kr /N nm-' I O ~ K , ~ / nm-1 N 1 0 7 ~ ~ nm-1 fN 107Ks/N nm-I 107Kd/N nm-'
diagonal force constants Stretch 4.891 f 0.011 4.780 t 0.004 4.124 i 0.054 9.358 i 0.228 9.282 i. 0.228
notationb
Stretch-Stretch Interaction 107k,/N nm-' 0.070 t 0.007 107k1,/N nm-I 0.661 t 0.034 107kd/N nm-' 2.015 t 0.063 108f'1alN 1 08fie/N 108fse/N 108fsY/N 1 O 8 f I Y /N
Bend 109H'(u1/Nnm 109H',,/N nm 109H'pl/N nm
0.513 i. 0.003 0.523 t 0.002 0.625 + 0.017
109H'p2/Nnm
0.536 i 2.125 f 2.432 r 0.580 f 0.073
109He/N nm 109HY/Nnm 109H,/N nm 109H,C/Nnm
The errors shown are standard deviations. threefold V, barrier for the acetate ion."
off-diagonal force constants
0.009 0.025 0.028 0.004
Stretch-Bend Interaction -0.216 t -0.491 i 0.450 f -0.622 i -0.102 c
Bend-Bend Interaction 109h'p/Nnm -0.003 109h'ap/N nm -0.021 1O9hply/Nnm -0.367 1O9hpzY/Nnm -0.010 109h8,/N nm 0.244 109hp,/N nm 0.066
0.007 0.014 0.006 0.006 0.095
f
0.013 0.005 0.027 0.022 0.029
f
0.001
?:
i f
f
Primed quantities follow the convention given in ref 13.
Derived from a
TABLE IV: Observed and Calculated Fundamental Frequencies for 1zCH,12COONa obsd freq/cm-' representa- assigncalcd tion C, ment 290 K 80 K freq/cmpotential energy distributiona A'
'1
'2
u3 v4 'S
--
6' u7
U8 v9
A"
lJ
10
v
11
v I2
'
13 14
'15
a
-
3000.9 3002.8b 2933.6 2930.1b 1583.7b 1579.5 1440' 1423.7b 1420 1407.3b 1333.8 1332.5b 1011.8b 1012.8 923.6b 922.8 650.7b 650.3 462.4b 467.9 2983.2 2974.4b 144OC 1447.3b 1044.4b 1045.0 622.1b 625.1 not obsd
Contributions smaller than 10%are omitted.
3006.0 2928.8 1583.6 1427.7 1411.4 1333.7 1013.4 924.9 651.1 465.6 2978.7 1439.2 1045.4 622.5 204.1
99 u,,(CH,) 100 u,(CH,) 86 u,(CO), 1 3 y h ( O C 0 ) 66 v,(CO), 18 6,(CH,), 1 5 S(OCO), 1 2 v(C-C) 74 6,(CH,), 20 u,(CO) 8 9 6,(CH,), lOv(C-C) 70 yin(CH,), 1 6 u,(CO) 5 3 u(C-C), 26 6(0CO), 17 u,(CO) 57 6 ( o c o ) , 25 v(c-c) 98 Y ~ ( O C O )28 , rh(CH,) 99 u(CH,) 97 s(CH,) 59 n(CH,), 25 n(OC0) 78 n(OCO), 39 n(CH,) 1 0 0 ,(CH,)
Frequency used for the determination of the force field.
the methyl deformation mode of 6,(CH3) with a small contribution from v,(CO), whereas a good group vibration of 6(CH3) is valid for the vI2(A") mode. The corresponding fundamentals due to the CD3-deformationmodes of the deuterated species were found at 1027.8 and 1040.5 cm-I; the lower vibration was assigned t o the v6(A') mode and the higher one to the vI2(A") mode. Good group vibrations
Not resolved.
of 6,(CD3) and 6(CD3)are available for the v6(A') and vl2(A'') modes, respectively. (iii) The C-C stretching mode, u(C-C),is dispersed into several normal modes, v,(A'), v,(A'), vs(A'),and vg(A'),for the parent species. The v(C-C) mode seems to be strongly coupled with S(OC0)mode; the most remarkable case is the vg(A') normal mode (650.7 cm-l) and therefore poor
Vibratbnel Analysis of Acetate
Ion Molecules
The Journal of Physlcal Chemistry, Vol. 87, No. 14, 1983 2533
TABLE V: Observed and Calculated Fundamental Frequencies for 11CH,’3COONa obsd freq/cm-’ representa- assigncalcd tionC, ment 290 K 80 K freq/cm“ potential energy distributiona
A’
v1 v1
v3 ”4
v.5 ’6
v7
v8 v9
v 10
A”
11 11
’
13
v 14 v 1s
3002.2 3002.7’ 2934.8 2928.1’ 1536.1’ 1543.1 1420‘ 1418.9’ 1404 1393.8’ -1330d 1327.5’ 1013.0’ 1013.6 916.8’ 916.8 649.1’ 648.2 462.3’ 468.4 2980.4 2975.4’ 1420‘ 1432.5’ 1031.6’ 1033.5 609.0’ 611.9 not obsd
--
-
3006.0 2928.7 1540.9 1416.5 1391.3 1332.3 1013.2 917.5 650.4 463.9 2978.6 1439.1 1031.7 611.1 204.2
99 Vas(CH3) 1 0 0 v,(CH,) 8 5 v,(CO), 1 2 rh(OC0) 8 2 6,(CH3), 11 6,(CH3) 79 vS(CO), 1 6 6(OCO), 1 5 v(C-C), 1 0 6,(CH,) 87 6,(CH,), 1 2 ~(c-c) 70 rin(CH31, 1 6 vas(CO) 56 v ( C - C ) , 27 S(OCO), 1 4 vS(CO) 58 6(OCO), 25 v(C-C) 98 r h ( o c o ) , 28 r h ( C H 3 ) 99 u(CH,) 97 6 (CH,) 62 n(CH,), 22 n ( O C 0 ) 80 n(OCO), 36 n(CH,) 100 7(CH3)
‘ Frequency used for the determination of the force field.
“ Contributions smaller than 10%are omitted. Shoulder.
Not resolved.
TABLE VI: Observed and Calculated Fundamental Freauencies for 13CH,lZCOONa obsd freq/cm-’ 290 K 80 K
representa- assigntion C, ment A’
v1
v2
v3 v4
v5 ‘6
v7 v8 v9
v 10
A“
v11
v 12
2991.0 2931.0 1581.0’ -1435‘ 1416 1326.9 1003.4’ 917.0’ 639.8’ 459.5’
-
-
LJ 13
v 14 v 15
calcd freq/cm-’
potential energy distributiona
2993.1’ 2925.9’ 1583.3 1421.0’ 1405.2’ 1324.9’ 1004.4 917.2 640.2 467.4
2992.7 2925.4 1582.7 1427.2 1407.8 1323.0 1005.0 917.6 642.0 463.3
99 vas(CH3) 100 u,(CH3) 87 v,(CO), 1 2 ri,-,(OCO) 7 1 v,(CO), 1 6 S(OCO), 1 3 6,(CH,), 80 6,(CH,), 1 5 v,(CO) 9 1 6,(CH,), 10 u(C-C) 7 1 rh(CH,), 1 5 v,(CO) 52 u(C-C), 29 6(OCO), 1 6 v,(CO) 54 6(OCO), 28 v(C-C) 98 rh(oco),27 r h ( C H , )
2971.5 2967.2’ 1435‘ 1441.0’ 1036.5’ 1038.1 619.7’ 622.9 not obsd
2966.0 1436.8 1039.2 622.4 204.2
100 u(CH3) 96 6 (CH,) 5 8 n(CH,), 25 n ( O C 0 ) 78 n(OCO), 39 n(CH,) 100 7(CH3)
“ Contributions smaller than 10% are omitted.
1 3 v(C-C)
’ Frequency used for the determination of the force field.
Not resolved.
TABLE VII: Observed and Calculated Fundamental Frequencies for 13CH,’3COONa representa- assigntionC, ment
--
obsd freq/cm-’ 290 K 80 K
calcd freq/cm-’
potential energy distribution“
2991.2 2990.1’ 2931.1 2924.2’ 1542.2’ 1537.3 1418‘ 1416.6’ 1386 1390.4’ 1323.5 1321.8’ 1003.5’ 1003.7 909.8’ 910.4 639.1’ 1.7 459.2’ 464.5 2970.2 2964.6’ 1418‘ 1429.3’ 1024.4’ 1025.6 608.9’ 611.5 not obsd
2992.7 2925.4 1540.0 1413.0 1390.7 1321.5 1004.9 909.9 641.6 461.5 2966.0 1436.7 1025.4 611.1 204.2
99 v,(CH,) 100 u,(CH,) 85 vas(CO), 1 2 r h ( O C 0 ) 79 6,(CH,), 11 6,(CH,), 1 0 vs(CO) 76 v,(CO), 1 5 S(OCO), 1 5 v(C-C), 1 4 6,(CH3) 8 9 6,(CH3), 11u(C-C) 70 ri,-,(CH3),16 v,(CO) 55 LJ(C-C),30 6(OCO), 1 4 v,(CO) 55 6(OCO), 27 v(C-C) 99 r h ( O C O ) , 27 yh(CH3) 100 u(CH,) 96 6(CH3) 6 1 n(CH,), 23 n(OC0) 80 n(OCO), 37 n(CH,) 100 7(CH3)
-
A”
v 11 v 11 v 13
’
14
v 15 a
-
Contributions smaller than 10%are omitted.
’ Frequency used for the determination of the force field.
group vibration of the S(OC0)is available for this normal mode. Good group vibration of 6,(CH3)mode is applied to the v6(A’)normal mode (1332.5 cm-’), whereas the vs(A’) normal mode (922.8 cm-’) is a complex mixture of several group vibrations in which the v(C-C)is the predominant mode. For the deuterated species, the v(C-C)mode contributes significantly to several n o d modes, v4(A’), v&A’), &A’), a d rdA‘). W o u g h the v4(A’) normal d e (1423.7
‘ Not resolved.
cm-’) is a complex combination of several group modes, this can be approximately described as the group mode of v,(CO). The vs(A’)normal mode (1082.5 cm-’) can be expressed as the group vibration of 6,(CD3),whereas the vg(A’)normal mode (613.1 cm-’) is a mixture of the a(OC0) and v(C-C)modes. The v,(A’) normal mode (880.0 cm-’) is strongly mixed with several group modes none of which seems to be predominant.
2534
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
Kakihana et al.
TABLE VIII: Observed and Calculated Fundamental Frequencies for '2CD,'2COONa obsd freqicm~l representa- assigncalcd tion C,q ment 290 K 80 K freq/cmpotential energy distributionn A'
2255.2 2255.3b not obsd 1568.4b 1566.1 1423.2b 1425.4 1082.5b 1083.4 1031.2 1027.8b 880.0 880.7b 827.Bb 828.7 613.4b 613.1 412.6b 409.8
2251.5 2105.9 L' 3 1564.9 4' 1420.3 " 5 1081.8 '6 1027.3 879.3 V , "8 823.9 "9 609.3 "IO 405.6 A" 1'11 2231.0 2227.2b 2223.8 1040.2 1O4Oc 1040.5b "I2 924.8 " 13 928.2' 929.8 521.0 " I4 F~25.5~ 529.0 not obsd 149.8 1) 1 5 Contributions smaller than 10% are omitted. Frequency used for C l
"2
-
a
97 uas(CD3) 98 US(CD3) 94 v,(CO), 1 3 y h ( O C 0 ) 88 u,(CO), 21 .(C-C), 20 S(OC0) 76 S,(CD,), 30 u(C-C) 88 Sas(CD3) 37 S(OCO), 25 u(C-C), 1 5 &,(CD,), 1 4 u,(CO) 57 r h ( C D , ) , 11 yj,,(OCO), 1 0 u,(CO) 4 6 S(OCO), 28 Y(C-C) 88 rh(OCO), 44 rh(CD3) 98 u(CD,) 98 S(CD,) 49 n(OCO), 36 n(CD,) 65 n(CD,), 5 3 n(OC0) 100 7(CD3) the determination of the force field.
Shoulder.
TABLE IX: Observed and Calculated Fundamental Frequencies for 13CD,1ZCOONa obsd frey/cm-'
representa- assigntion C, ment
290 K
80 K
calcd freq/cm-'
potential energy distributionn
2240.0 2240.0b 2232.0 97 "a(CD3) not obsd 2100.2 9 9 us(CD3) 1564.0 "3 1 564.7 1564.5 94 uas(CO), 1 3 ri,(OCO) " 4 1424.1b 1426.1 1419.8 89 u,(CO), 21 u(C-C), 20 &(OCO) " 5 1065.2 1066.6 1063.6 75 6,(CD,), 27 "(C-C), 1 2 S,(CD,) "6 1028.2 1024.6b 1020.5 8 4 6 as(CD3) v7 880.4b 879.5 878.6 38 S(OCO), 26 .(C-C), 1 4 &,(CD,), 1 4 u,(CO) CS 819.1b 820.3 817.3 5 8 r b ( C D , ) , 11 rh(OCO), 10 v,(CO) 9 605.8b 606.6 603.2 45 S(OCO), 30 v(C-C) li 10 411.9b 417.6 405.1 88 rh(OCO), 4 3 r h ( C D , ) A" " 11 2216.5 2212.8b 2205.8 98 "(CD,) 1O3jc 1038.gb 1036.8 98 S(CD,) 1'12 922.'ib 923.7 920.9 50 n(OCO), 35 n(CD,) " 13 524.0b 528.2 519.9 65 n(CD,), 5 3 n(OC0) " 14 149.8 100 T(CD,) not obsd 1' 15 Contributions smaller than 10% are omitted. Frequency used for the determination of the force field. Shoulder, A'
"I
"a
-
a
(iv) There exists strong coupling between the out-ofplane CH, and OCO rocking (r(CH3)and r(OC0)) in the normal modes of ~13(A") and ulq(A"). This coupling is more remarkable for the deuterated species; both group vibrations of r(CD3) and r(OC0) contribute significantly to the normal modes. ( b ) Valence Force Field. There has been a considerable lack of vibrational analyses of fatty acid salts which contain hybrid links, being neither double bonds nor single bonds. Therefore the present result gives valuable information especially on the force constants associated with carboxylate group (-COO-). The following views are worth discussing (see Table 111). (i) The difference between K,, and K , was small but significant; the C-H stretching force fiela of the methyl group deviates slightly from local C3" symmetry. On the other hand, no remarkable difference between the force constants K, and Kd were found; this suggests that the Czu symmetry constraint for the interaction constants associated with the carboxylate group is a reasonable approximation. (ii) The HCC-bending force field associated with the methyl group deviates significantly from local CSusymmetry in contrast to the HCH-bending force field (see the values of HCC-bending (H,) and HCH-bending (Ha) force constants in Table 111). A similar trend has been already found for several molecules containing methyl groups, e.g., a~etaldehyde,'~ pyruvic acid,12and acetic acid."
(iii) Inclusion of data from appropriate heavy atom ( W ,
'Q, etc) modifications serves to determine important force constants associated with the heavy atom skeleton with considerable reliability. As shown in Table 111, most of the off-diagonal force constants associated with the C-C and COO- groups are evidently different from zero. These values of the force constants are expected to be physically acceptable, because in the present study the characteristic frequencies from the 13C-substitutedspecies were taken into consideration. (iv) Among the bend-bend interaction constants, h', and h, turned out to be indeterminable from our experimental data; they can be approximated to zero. (v) It should be finally pointed out that the interaction N constant ksd has a relatively large value of 2.015 X nm-I. The large value of this off-diagonal constant can be interpreted to be a consequence of the resonance between two possible electronic structures related to the delocalization of the molecular orbitals of the two CO bonds. A similar large value for this type of interaction constants has been found for the formate ion mo1ecule.l6 (C) Calculation of Reduced Partition Function Ratios and Equilibrium Constants of Hydrogen Isotopic Erchange Reactions. The well-known expression for the reduced partition function ratio of isotopic molecules was (15) Hollenstein, H.; Giinthard, Ha. H. Spectrochim. Acta, Part A 1971,27, 2027. (16) Kidd, K. G.; Mantach, H. H. J. Mol. Spectrosc. 1981, 85, 375.
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
Vibrational Analysis of Acetate Ion Molecules
TABLE Xi Calculated Reduced Partition Function Ratios o f Acetate Ion Molecules with respect t o Isotope Substitution of Deuterium for Hydrogen isotopic speciespairs CHD,COO-/ CH .DCOOCH,DCOO-/ CH,COOCD,COO-/ CHD,COOCDT,COO-/ CHT,COO-
temp/K 300
350
400
450
500
10.368
6.720
4.883
3.827
3.162
10.530
6.829
4.961
3.886
3.208
10.821
6.959
5.027
3.923
3.230
10.954
7.016
5.055
3.938
3.240
TABLE XI: Calculated Reduced Partition Function Ratios o f Acetate Ion Molecules with respect to Isotope Substitution o f Tritium for Hydrogen isotopic speciespairs CHT,COO-/ CH,TCOOCH,TCOO-/ CH,COOCD,TCOO-/ CH D ,CO 0CT,COO-/ CHT,COO-
temp/K 300
350
400
450
500
26.020
14.086
8.974
6.370
4.874
26.366
14.326 9.141
6.490
4.964
27.428
14.719
9.315
6.578
5.012
27.909
14.890
9.389
6.614
5.032
2535
TABLE XII: Calculated Reduced Partition Function Ratios o f Acetate Ion Molecules with respect to Isotope Substitution of Tritium for Deuterium temp/K isotopic speciespairs 300 350 400 450 500 CH,TCOO-/ 2.504 2.098 1.843 1.670 1.547 CH,DCOOCDT,COO-/ 2.510 2.096 1.838 1.664 1.541 CD,TCOOCD,TCOO-/ 2.535 2.115 1.853 1.677 1.551 CD,COOCT,COO-/ 2.548 2.122 1.857 1.679 1.553 CDT,COO‘
of the values listed in Tables X-XII. As examples, we introduce the following isotopic exchange reactions together with their equilibrium constants at 300 K: (i) H-D exchange reactions K
CH3COO- + CHDzCOO-& 2CHzDCOOK1 = (s/s’)~(CH~DCOO-/CH~COO-)/ (S /s’)f( CHDzCOO-/CHZDCOO-) = 1.016
.&CH,DCOO- + CHD2COO-
CH3COO- + CD3COO-
K2 = (S/S?~(CH~DCOO-/CH~COO-)/
derived by Uref and Bigeleisen and Mayer7 in the harmonic oscillator, rigid rotor approximation: 3 ~ 7 - 6 ui exp(-ui/2) 1 - exp(-u’J (3) (s/s’)f = exp(-ufi/2) 1 - exp(-ui) where the primed symbols are for the lighter isotopic species, s is the symmetry number, ui is equal to hcvi/kT, vi is the i-th normal vibrational frequency in cm-l, h is Planck’s constant, c is the velocity of light, k is Boltzmann’s constant, and T is the temperature. Equation 3 describes the deviation of the ratio of the vibrational partition functions for a pair of isotopic molecules from the classical one. The contribution of the nonclassical rotation to the quantum-mechanical partition function for small molecules containing hydrogen should be generally taken into account even at room temperature. The deviation of the quantum-mechanical rotational partition function from the classical one, (Qsm/QcJrot, can be calculated by use of the formulas summarized by Herzberg.” For the acetate ion molecules, CH3COO- and CD3COO-, these ratios have been estimated to be 1.00030and 1.00027 a t 300 K, respectively. This suggests that the moments of inertia of the acetate ion molecules are high enough so that corrections for the nonclassical rotations need not be made. In the present calculation, the rotational contribution was assumed to be negligible for all isotopic species concerned. The calculated reduced partition function ratios of the acetate ion molecules with respect to isotopic substitutions of deuterium for hydrogen, tritium for hydrogen, and tritium for deuterium are given in Tables X-XII, respectively, where fundamental frequencies which are not known were calculated with use of the G and F matrix method.’ Now one can estimate equilibrium constanb for isotopic exchange reactions with the appropriate selection
E
(17) Herzberg, G. “Infrared and Raman Spectra of Polyatomic Molecules”; Van Noetrand New York, 1955; Chapter 5. (18)Wei, K. T.; Ward, D. L. Acta Crystallogr., Sect. B 1977,33, 522. (19) Chen, D. M.; Reeves, L. W.; Tracey, A. S.; Tracey, M. M. J. Am. Chem. SOC. 1974, %, 5349. (20) Kakihana, M.; Kotaka, M.; Okamoto, M. to be submitted for publication.
(S/S?~(CD~COO-/CHDZCOO-) = 0.973 (ii) H-T exchange reactions K 2 2CHzTCOO-
CH3COO- + CHT2COOK3
= (s/s’)~(CH~TCOO-/CH~COO-) / (s/s’)~(CHT~COO-/CH~TCOO-) = 1.013
CH3COO- + CT3COO-
2CH,TCOO- + CHTzCOO-
K4 = (s/s’)~(CH~TCOO-/CH~CO~-)/ (S /s’)f(CT3COO-/CHTZCOO-) = 0.945 (iii) D-T exchange reactions
& 2CDzTC00-
CD3COO- + CDT2COO-
K5 = (s/s’)~(CD~TCOO-/CD~COO-)/
(s/s’)~(CDT~COO-/CD~TCOO-) = 1.010 CD3COO- + CT3COO-
2CD,TCOO- + CDTzCOO-
K6 = (s/s’)~(CD~TCOO-/CD~COO-) / (s/s’)~(CT~COO-/CDT&OO-) = 0.995 Although many equilibrium constants for isotopic exchange reactions can be calculated with a certain accuracy in this manner, it is, naturally, desirable to verify the validity of the calculations by experiments for as many systems as possible.
Acknowledgment. We heartily thank Dr. M. Akiyama of Rikkyo University for his helpful discussions and the use of his computer program for determining force constants. Thanks are also due to Dr. E. Miki of Rikkyo University for the use of the Oxford Instrument Model DN70 cryostat. We are greatly indebted to Mr. H. Fujishima and Mr. T. Ootake of Nicolet Japan Corporation for the use of the Nicolet 7000 FT infrared spectrometer. Registry No. CH3COONa,127-09-3; CH$2OONa, 23424-284; 13CH3COONa,13291-89-9; 13CH,’3COONa,56374-56-2; CD3COONa, 14044-94-1; 13CD3COONa,85355-10-8; H2, 1333-74-0.
