J. Phys. Chem. 1993,97, 43204325
4320
Vibrational Spectrum of the Acetone-Water Complex: A Matrix Isolation FTIR and Theoretical Study Xiaokui K. Zhang, Errol G. Lewars,s Raymond E. March, and J. Mark Pads’ Department of Chemistry, Trent University, Peterborough, Ontario, Canada K9J 7B8, and Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Received: October 14, 1992; In Final Form: January 5, 1993
The FT-infrared absorption spectrum of the hydrogen-bonded acetonewater complex has been investigated in solid argon matrices. Acetone and water vapors were co-condensed with an excess of argon gas at 12 K, giving rise to red- or blue-shifted absorptions near most of those associated with the fundamental transitions of matrix-isolated acetone or water. Vibrational shifts are indicative of a 1:l complex of acetone and water, in which water is hydrogen-bonded to the carbonyl oxygen of acetone. Accordingly, red shifts are observed for the C=O stretching mode of acetone and the 0-H stretching modes of water. Corresponding shifts of similar magnitude were observed when &-acetone was used. Ab initio S C F computations of the equilibrium structure and fundamental vibrational frequencies indicate a cyclic hydrogen-bonded structure involving interactions between water and acetone at both the carbonyl oxygen and one methyl hydrogen. Changes in the net charges and bond orders upon formation of the complex are as would be expected for the early stages of a water-catalyzed keto-enol transformation. Predicted frequency shifts were found to be in excellent agreement with most of the observed complex vibrational frequency shifts, when the 6-3 1G** basis set was employed. The influence of hydrogen bonding on the structure, electronic distribution, and vibrational frequencies of acetone and water is discussed.
Introduction The vibrational spectra of weakly bound binary complexes can offer insight into the subtle changes in structure and electronic distribution that accompany weak chemical perturbations. Weakly bound complexes can be seen as molecular analogues of physi- or chemisorbed molecules at surfaces, offering detailed experimental data and complementary theoretical calculations in thecontext of well-defined systems. We have recently observed such a weakly bound complex in the form of the hydrogen-bonded acetone-water complex and have endeavored to correlate and interpret the observed vibrational frequency shifts with ab initio calculations of structure and electronic distribution perturbations predicted for the acetone-water complex. Thevibrational spectra of a number of related hydrogen-bonded complexes between water or other small acidic molecules and carbonyl-containing organic molecules have previously been reported. Nelander’32 has published detailed studies of the midand far-infrared spectra of the formaldehydewater complex in inert matrices, the results of which parallel the present studies in several ways. The reported red shifts in the carbonyl and 0-H stretching mode absorptions and a blue shift in the water bending mode region are similar in magnitude to those observed for the acetone-water complex. Complementary to these studies is a recent detailed theoretical study of the formaldehyde-water complex by Kumpf and D a m e ~ o o d ,in ~ which many possible configurations have been explored at various levels of sophistication. Nelander has also made reference to the acetone-water complexlJ but includes no quantitative information on the spectroscopic shifts observed in the acetone-water complex, except in the water 0-H stretching mode region, where a doublet at about 3500 cm-l was assigned to the symmetric 0-H stretch of water in the acetone-water complex. Andrews and Johnson4 have formed 1:l complexes between HF or DF and acetone, acetaldehyde, and formaldehyde and have observed similar red- and blue-shifted features near most fundamentals of these molecules, all of which correspond in sign
* Author to whomcorrespondenceshould beaddressed,at Trent University. 2
Trent University.
and relative magnitude to the modes observed in the present work. Nowak et al.5 have observed a similar complex between HCI and acetone. Numerous studies of the dimer and higher oligomers of water have been reported, the most useful of which are the studies of isotopically substituted water and water dimers by Ayers and P ~ l l i n . ” ~
Experimental Section The matrix isolation apparatus used has been described previously.1° The CsI substrate was cooled to 12 K by means of an APD Cryogenics Displex closed-cycle helium refrigeration system. Acetone (Caledon) or d6-acetone (MSD Isotopes) was degassed in a series of freeze-pumpthaw cycles in an all-metal high-vacuum line. Acetone vapor was mixed in a glass bulb with helium gas1](Canox) in a 1:10 (acetone/He) ratio and introduced into the system through a pair of pinholes about 2 cm from the substrate surface. The flow of acetone was controlled by a manually adjusted needle valve for which appropriate settings for acetone/Ar ratios of the order of ( 1:102-103) had previously been determined. Water and DzO (MSD Isotopes) were similarly degassed and mixed with argon (Matheson) in a second glass bulb. Water/argon ratios were of the order of 1:102-103, as determined by comparing the spectra of matrix-isolated water in argon in the absence of acetone with previously reported work.4 The water vapor/argon mixture was introduced through two gas inlets located about 1 cm above the substrate surface, thereby allowing it to be co-condensed with the acetone vapor. The flow of the water vaporfargon gas mixture was controlled by an MKS mass-flow controller, typically set to 0.30 cm3/min. In some cases where only traces of water in the matrix were required, argon gas with no intentionally added water vapor was used. This procedure yielded matrices in which water was present at concentrations corresponding to the normal water impurity level found in all matrices prepared under high-vacuum conditions. Spectra were recorded following a deposition period of about 10 h.
Fourier-transform infrared absorption spectra were recorded using a Bomem MB-102 spectrometer. Minor modificationswere made to the sample compartment to accommodate the vacuum
0022-3654/93/2097-4320%04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4321
Vibrational Spectrum of Acetone-Water
TABLE 1: IR Absorption Frequencies (cm-1) of Uncomplexed Acetone and Acetone Submolecule Modes of the Acetone-HzO and Acetone-HF Complexes in Solid Argon at 12 K acetone
acetone-H2O0
acetone-HFb
1721.3 1429.1 1361.3 1353.9 1216.5 1091.6 882.3 528.9
1715.9 (-5.4) 1420.9 (-8.2) 1368.3 (+6.7) 1358.7 (+4.8) 1230.4 (+13.9) 1094.5 (+2.9) 893.9 (+11.6) 542.9 (+14.0) 341.8
1715 (-6) 1423 (-6) 1374 (+12) 1242 (+25) 1097 (+5) 556 (+27)
TABLE IIk IR Absorption Frequencies (cm-I) of uncomplexed H20 and DzO and Water Submolecule Modes of the Acetone-HzO and Acetone-DZO Complexes in Solid Argon at 12 K
assignmentc v3 ( a l ) C = O s t r
(a!) asym CH3 def (bl) sym CH3 def v5 ( a l ) sym CH3 def VI7 (bl) asym C-C str v22 (b2) CH3 rock (oop) VI8 ( b d CH3 rock (ip) VI9 (bl) C=O def (ip) complex mode
H20°
acetone-H206
D2O"
acetone-DZ0
VI
3638.0
3498.0 (-140.0) 3517.3 (-120.7)
2657.7
2584.8 (-72.9) 2570.8 (-87.8)
v2
1589.1
1613.7 (+24.6) 1590.6 (+1.5)
1174.6
1187.4(+12.4)
VI
3734.3
3705.3 (-29.0)
2771.1
2741.5 (-28.4)
v4
VI6
Frequency shifts with respect to unperturbed acetone are given in parentheses. Data taken from ref 4. Assignments and nomenclature refer to the corresponding unperturbed modes of acetone, as given in ref
Nonrotating monomer fundamental mode frequencies taken from ref 2. Frequency shifts with respect to unperturbed H 2 0 or D20 are given in parentheses.
15.
a .I5
TABLE 11: IR Absorption Frequencies (cm-I) of Uncomplexed d6-Acetoneand Acetone Submolecule Modes of the d6-Acetone-Hz0 and d6-Acetone-HF Complexes h Solid Argon at 12 K &-acetone d6-acetone-H@
1706.3 1239.6 1000.9 962.8 884.7 475.4 a
1708.7 (+2.4) 1251.6(+12.0) 1005.3 (+4.4) 966.2 (+3.4) 891.5 (+6.8) 490.8 (+15.4) 343.8
d6-acetone-HFh
1266 (+26) 969 (+6) 895 (+lo) 506 (+31)
assignment' unassigned feature (b,) asym CD3.def vi7 (b,) asym C-C str v2* (b2) CDI rock u6 ( a l ) CD] rock VI9 (bJ C = O def complex mode
Frequency shifts with respect to unperturbed &-acetone are given
in parentheses. Data taken from ref 4. Assignments and nomenclature refer to the corresponding unperturbed modes of d6-acetone, as given in ref 15.
manifold. A single-beam reference spectrum of the cold substrate was recorded before formation of the matrix. This spectrum was then subtracted from all subsequent spectra to yield simulated double-beam spectra. Typical spectra were taken at 1-cm-I resolution with 20&500 scans averaged per spectrum.
1
u E
e nE: .OS
0
I
3800
37w
sew
35w
1640
1620
16W
1580
Wavenumber (cm-1)
Figure 1. FT-infrared spectra of argon matrices containing water (1: lX103 H20/Ar) and 1:103(A), 1:3X102(B), 1:102 (C),or 1:50 (D) acetone/Ar, in the regions 3800-3450 and 1645-1560 cm-I. Annotated peaks are associated with features that grow in intensity with increasing acetone concentration.
to the water dimer. Further aggregation of water to higher oligomers was also observed with increased water concentrations. Computational Method A series of experiments was performed in which both acetone and water were present in varying concentrations. New absorp Geometry optimizations, bond order, and net atomic charge tions associated with an acetone-water complex are shown in calculations were done with the MONSTERGAUSS program,'* Figures 1 4 , and their corresponding frequencies are summarized and the harmonic oscillator frequencies for the optimized in Tables I and 111. The principal features are new bands geometries were calculated with GAUSSIAN 88 or GAUSSIAN associated with most of the fundamental aborptions of water and 90.13 The minimum energy structure of the acetonewater acetone. These new features simultaneously grow in intensity complex was determined by first performing a molecular with increasing water or acetone concentration. In cases where mechanics'4 calculation, which was then subjected to optimizations the concentration of water in the matrix was small, absorptions with the STO-3G, 3-21G, and 6-31G** basis sets. In all other associated with isolated water were observed to diminish and cases, only the 3-21G basis set was employed as a starting point eventually disappear with increasing acetone concentration, as for geometry optimizations. would be expected if water were consumed in the formation of an acetonewater complex. Results Analogous experiments were done in which D20 was coArgon matrices containing only acetone, d6-acetone,water, or condensed with acetone in an excess of argon gas, as well as isotopically substituted water were prepared, yielding infrared experiments in which &-acetone was co-condensed with HzO. absorption spectra that were similar to those previously reportNew features arising from these experiments are summarized in ed.697J5 Tables 1-111 summarize the observed fundamental Tables I1 and 111. As a result of exchange with surface water absorptions with spectral assignments based upon the work of in the vacuum systems, some HDO was also present in the D 2 0 Dellepiane and OverendI5 (acetone) and Ayers and P ~ l l i n ~ . ~ experiments and resulted in the observation of some new features (water). The spectra of acetone were as expected for the isolated of an extremely weak nature at 3694.3,2649.8, and 1392.4 cm-I, molecule, showing no significant indication of dimerization. When which may be associated with an acetone-HDO complex. the ratio of water/Ar was about 1:103,water spectra showed two sets of bands that increased or diminished in intensity as a group Computational Results with changes in the concentration of water. One set comprised absorptions at 3776.2, 3755.9, and 3711.6 cm-' in the 0-H Ab initio SCF calculations of the equilibrium geometry and stretching mode region and 1623.9, 1608.4, and 1589.6 cm-I in energy of water, acetone, and the acetonewater complex were the bending region assigned by Ayers and Pullin6s7and Andrews performedusing the 3-21Gand 6-31G** basissets. Nostructural and Johnson4 to the isolated water monomer, and a second set constraints were imposed on any of the molecules, except that water was confined to a C20 structure. The results of these comprisedabsorptionsat 3573.2,1610.4, and 1593.Ocm-l assigned
Zhang et al.
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993
4322
.E
.15
.4
8
8
.1
C
0
g
fl
9
2
El
.2
.05
0
1780
1760
1740 1720 Wavenumber ("1)
1700
1680
500 460 Wavenumber (cm- 1)
I
Figure 2. FT-infrared spectra of argon matrices containing acetone (1: 3X102 acetone/Ar) and 1:103 (A) or 1:102 (B) water/Ar, in the region 1785-1678 cm-I. Annotated peak is associated with a feature that grew in intensity with increasing water concentration.
360
Figure 4. FT-infrared spectra of argon matrices containing acetone (1: 3X102 acetone/Ar) and 1:103 (A) or 1:102 (B) water/Ar, in the region 600-300 cm-'. Annotated peaks are associated with features that grow in intensity with increasing water concentration.
TABLE IV: Selected 3-21C and 6-316** Calculated Structural Parameters for Acetone and Water 3-21G
6-31G**
3-21G
6-31G**
0.967
0.943
107.7
106.0
Distance, Angstroms C,-01"
1.211 1.515 1.080 1.085
1.192 1.513 1.081 1.086
HI-C~-CI Cl-C2-H2,j
122.5 109.7 110.1
121.7 109.8 110.3
HI-C2-CI-OI
0.00
c2-c1 C~-HI C2-Hz.3
02-H7,8
Angle, Degrees c2-Cl-01
(1
1340 1250
1200 1100
I
1085
Wavenumber (cm-1 )
Torsion Angle, Degrees 0.00
Numbering scheme refers to Figure 5.
TABLE V: Selected 3-216 and 6-31C** Calculated Structural Parameters for the Acetone-Water Complex 3-21G
Figure 3. FT-infrared spectra of argon matrices containing acetone (1: 3X102 acetone/Ar) and 1:103(A) or 1:102(B) water/Ar, in the regions
6-31G**
3-21G
6-31G**
0.972 0.966 1.903 2.237
0.948 0.942 2.058 2.600
108.1 113.9 143.0 73.6
105.7 119.4 140.3 66.2
Distance, Angstroms 1.219 1.512 1.512 1.085 1.086 1.080 1.085
1.197 1.510 1.511 1.080 1.086 1.086 1.081 1.086
C~-CI-OI C3-CI-01 HI-CZ-CI CI-C~-H~ Ci-C-Hj
122.4 121.5 109.5 110.1 109.4
121.0 122.1 110.6 110.3 109.6
HI-C~-CI-OI H~-CA-CI-OI
-4.1 -2.6
1470-1396, 1380-1340, 1250-1200, and 1100-1085 cm-I. Annotated
peaks are associated with features that grow in intensity with increasing water concentration.
calculations are summarized in Tables IV and V, showing the calculated bond lengths and angles for these molecular species. The optimized geometry is illustrated in Figure 5 . The total energies for acetone, water, and the acetonewater complex were -191.972072,-76.023615, and -268.005635 hartrees, respectively, when the 6-31G**basis set was employed. A binding energy of 6.2kcal mol-] (6-31G**) is calculated for the acetonewater complex from these results. This may be partitioned into the energies of the two hydrogen bonds by making the assumption that bond energies are proportional to thecalculated bond orders.16 Using such an approach, bond energies of 4.3 and 1.9 kcal mol-' are calculated for the carbonyl and methyl hydrogen bonds, respectively. The structure given in Figure 5 represents the only relative minimum energy configuration we were able to find, following a search of all structures with a plane of symmetry. The minimum energy configuration has Cl symmetry, possessing only a trivial
H7-02-H~
1.079
02-H7
02-Hs OI-H~ 02-H1
Angle, Degrees H7-02-HB
CI-OI-H7 C2-HI-02 HI-02-H7
Torsion Angle, Degrees -4.5 -2.5
O l - H r 0 2 - H ~ 96.7 C~-HI-O~-HE -100.8
95.9 -97.0
Numbering scheme refers to Figures 5 and 6.
onefold rotation axis, and is chiral. Rotation of the water molecule along the carbonyl hydrogen bond leads to a configuration of C, symmetry, possessing a symmetry plane, which is the transition state for enantiomeric conversion of the minimum energy
Vibrational Spectrum of Acetone-Water
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4323
TABLE VI: Observed and Calculated Vibrational Mode Frequencies and Frequency Shifts (cm-I) for Acetone-Water Complex calcd obsd
3-21G
acetone-
6-31G**
acetone-
Av
H20
H10
acetoneAu
H20
-4.5 -6.3 -21.1 -28.1 +21.1 -2.9 +7.4 -6.3 +20.7 +20.7 +9.5 +17.6 +6.8 +27.7
2993.8 2985.5 2931.1 1702.7 1448.5 1432.7 1406.3 1366.5 1358.1 1229.1 1095.1 890.3 780.0 543.9
-18.6 +5.0 +3.6 -0.6 +4.9 +4.2 +12.6 +3.5 +8.0 -0.6 +15.0
-28.7
3707.6 3593.4
-26.7 44.6
Au
+0.02--'\,
assignmenta
Acetone
3016.3 2972.4 293 1 .O 1721.3 1443.5 1429.1 1406.9 1361.6 1353.9 1216.5 1091.6 882.3 780.6 528.9
1715.9 1420.9 1368.3 1358.7 1230.4 1094.5 893.9 542.9
3011.8 2966.1 2909.9 -5.4 1693.2 1464.6 -8.2 1426.2 1414.3 +6.7 1355.3 +4.8 1374.6 + 13.9 1237.8 +2.9 1101.1 +11.6 899.9 787.4 +14.0 556.6
-22.5 -13.1 +o. 1
Figure 6. Illustration of the optimized geometry of the acetone-water binary complex at the 6-31G** level, with changes in the bond orders following complex formation for all bonds undergoing significant change.
TABLE VII: Vibrational Mode Frequency Shifts (cm-I) for HCl, HF, and H20 Complexes with Acetone, Gas-Phase Acidities (kcal mol-'), and Dipole Moments (Debye) for Each Proton Donor
Water
3734.3 3638.0 1589.1
Macid
3705.3 -29.0 3498.0 -140.0 3517.3 -120.7 1613.7 +24.6 1590.6 +1.5
3705.6 3584.9 1625.9
-53.1
+36.8
(CH3)zCO-HCl (CHj)&O-HF (CH3)2CO-H20
1616.4 +27.3
Acetone-Water Complex
341.8
363.8
279.2
b
Assignments are based upon those given for the most closely corresponding mode of acetone in ref 15 and water in refs 1 and 4. Mode associated with the acetone-water complex due to intermolecular motions, not present in the vibrational spectrum of acetone or water. Predicted frequency has been scaled by a factor of 0.9 for this mode.
1 I
-0.03
Auca
Avcc
-12
+16 +25 +14
-6
-5
A ~ C C = O (XH)"
+30 +27 +I4
1395 1554 1635
fib
1.08 1.82 1.85
ref
5 4 c
Data taken from ref 20. Data taken from Handbook of Chemistry and Pkysics, 70th ed.; CRC Press: Boca Raton, FL, 1989-1990; p E-59. This work.
Normal mode vibrational frequencies were calculated for the equilibrium geometries of acetone, water, and the acetone-water complex. The predicted frequencies are summarized in Table VI with the corresponding normal mode assignments and the experimental results for comparison. The predicted frequencies have been scaled on a mode-by-mode basisIg by factors between 0.85 and 0.98 (3-21G) or 0.85 and 0.92 (6-31G**). The magnitude and direction of the predicted frequency shifts are given where appropriate.
Discussion
Figure 5. Illustration of the optimized geometry of the acetone-water binary complex at the 6-31G**level, with changes in the net atomic charges following complex formation for all atoms undergoing significant change. Slight deviations from planarity have been ignored in the representation, which is otherwise a scaled drawing of the calculated geometry.
structure.'' We have calculated the activation energy for racemization of the minimum energy structure to be 0.22 kcal mol-'. Figures 5 and 6 illustrate the change in the magnitude of the net charge on each nucleus and the change in bond orders, respectively, following formation of the hydrogen bond, as calculated using Mulliken population analysis and Mayer bond order calculations,ls respectively. For all nuclei for which a significant change in net charge is found, the net charge becomes greater on formation of the complex, that is, the sign of the change in the net charge is the same as the sign of the calculated charge itself. The most significant changes calculated are those involving the carbonyl carbon and oxygen and the hydrogen-bonded 0-H bondof water. Changes to the bond lengths and angles of acetone and water following complex formation include significant lengthening of the hydrogen-bonded 0-H bond and the C=O bond and a corresponding shortening of the C-C framework bonds.
Spectral Interpretation. The observed spectral features presented above are consistent with a hydrogen-bonded a c e t o n e water complex in which the bonding and vibrational mode frequencies of acetone and water are perturbed as a result of partial proton transfer from water to acetone. This interpretation of the spectrum is supported by the following observations: (1) All new spectroscopic features are closely associated with fundamentals of either nonrotating water or acetone, indicating that the complex is composed of perturbed acetone and water molecules. (2) The vibrational modes associated with stretching of the 0-H bonds of water and the carbonyl oxygen of acetone are red-shifted with respect to free, nonrotating water or acetone, indicating a significant weakening of the 0-H and C = O bonds. (3) The vibrational mode frequencies associated with skeletal motions are all blue-shifted with respect to free acetone, indicating a concomitant increase in bond strength in the framework C-C bonds. (4) A new vibrational mode associated with a binary complex is implied by the observation of an absorption at 341.8 cm-1 which has no counterpart in the spectra of acetone or water; this feature corresponds closely to the most intense absorption of the acetone-water complex predicted by the a b initio calculations (see below). ( 5 ) No perturbation of the C-H stretching modes associated with the acetone methyl groups is observed, thus indicating that there is no significant interaction between water and the acetone methylgroups. Note that the ab initiocalculations do predict a weak hydrogen bond between the water oxygen and the methyl group (see below). ( 6 ) All new modes shift upon deuterium isotope substitution to frequencies close to the appropriate fundamentals of the isotopically substituted molecular constituent. The sign and magnitude of these shifts correlate in
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The Journal of Physical Chemistry, Vol.97,No.17,1993
most cases with the corresponding mode shifts in the acetonewater complex. The new vibrational mode frequencies associated with the acetonewater complex have been assigned in terms of the corresponding vibrational mode in the molecule with which the motion is most closely associated. Thus, there are seven new modes observed which are associated with the acetone submolecule and four new modes associated with the water submolecule. In addition, one new low-frequency mode (341.8 cm-I) has been observed, which is assigned to an intermolecular vibration, perhaps corresponding to the439.2-cm-1 modeof the formaldehyde-water complex assigned by Nelanderl to the out-of-plane shear vibration of water. As noted above, Nelander has also reported2 spectra of the acetonewater complex in the water symmetric stretch region, in which two absorptions are observed a t about 3500 and 3509 cm-I. These absorptions correspond with our own observation ofthe two bands at 3498.0and 3517.3 cm-I, and the present assignment of these modes to the symmetric stretch of complexed water agrees with that of Nelander. The observation of two absorptions in this region has been suggested by Nelander to be due to two conformations of the acetone-water complex, both of which involve hydrogen bonding. We have also observed two new absorptions in the water v2 symmetric bend region. However, the absorption at 1590.6 cm-' may be due to the presence of water dimer, for which a bending mode associated with the 0-bonded water moiety a t 1593.6 cm-I has been observeda8The only other reasonable assignment of this new mode would be to a softened bending mode of water in another conformation of the complex, corresponding to the observation of two stretching modes, as noted above. Recent theoretical results for the formaldehydewater complex suggest two stable forms, one involving a normal linear hydrogen bond and one involving a cyclic geometry in which some bonding occurs between the water oxygen and the hydrogen of formaldehyde. Such an interaction may also be possible for acetone and water, since the acidity of the methyl hydrogens of acetone is likely to be higher than that of the hydrogens of formaldehyde, and is suggested by the weak hydrogen bond predicted in the a b initio results discussed above. However, as noted above, no significant perturbation of the C-H stretching modes of complexed acetone was observed. One might expect to see a small red shift in some of these modes, on the order of the 13-cm-l shift predicted for Y ~ O if , the water oxygen were associated with a methyl group. These new absorptions may, however, simply be too weak to observe, as analogous modes have not been seen in any other studies of binary complexes to date involving acetone. The spectroscopic data obtained for the d6-acetonewater and the D20-acetone complexes clearly confirm that the carrier of the new vibrational frequencies is an acetone-water complex. This is clear, since all new modes shift in the expected fashion to frequencies close to those of the isotopically shifted fundamentals of either d6-acetone or D20. The data do show some unusual features, including the fact that no absorption associated with the C=O stretching mode of complexed acetone (u3) is found. Andrews and Johnson4 have also failed to observe this mode with d6-acetone. There does not appear to be an obvious explanation for the absence of a shifted C=O stretching mode in these cases. Other modes that do appear in complexes of d6acetone with water and HF but do not appear in the spectra of h6-acetone complexes with these molecules are v I S and V6. It would therefore appear that the presence or absence of spectral features in such complexes is determined by relatively complex or subtle factors. The observations made in the present study closely parallel those of Andrews and Johnson4for the hydrogen-bonded complex between HF and acetone. In Tables I, 11,and VI1 are summarized the results of the latter study, for which analogous trends in the relative magnitude and direction of shift are observed for the frequencies associated with the acetone-HF complex. However, the absolute magnitude of the vibrational frequency shifts for
Zhang et al. each of the blue-shifted vibrational frequencies of the acetone and d6-acetone complexes with HF are approximately twice those of the corresponding vibrational modes of the acetonewater complex. Red shifts in both the acetone-water and acetoneHF complexes are of approximately the same magnitude. The corresponding frequency shifts observed for the acetone-HCI complex5 are interesting, in that the C-C stretching mode is blue-shifted with a magnitude similar to that observed for acetonewater, but the C=O stretching and in-plane deformation modes of the acetone-HC1 complex are shifted by about twice as much as either of the complexes of acetone with HF or water (see Table VII). The differing magnitudes of the red and blue shifts in the HF and HCl complexes have been interpreted by Andrews and Johnson4 as being due to (1) the higher acidity of HC1 over HF, resulting in a greater red-shifting of the C=O stretching mode and a blue-shifting of the C=O deformational mode frequencies of the HC1 complex, and (2) the greater polarity of HF over HCl, giving rise to greater polarization of the entire acetone molecule and a greater blue shift in the antisymmetric C-C stretching mode of the HF complex. As an extension of this reasoning, we have compared the gasphase acidity [AHacid(XH)] of HCl, HF, H20,and D2O2Owith the observed frequency shifts for the three frameworkvibrational modes of acetone noted above (see Table VII). Thedata suggest a reasonably linear correlation between these two parameters for two of the chosen vibrational modes, v3 and ~ 1 9 ,whose shift magnitudes are associated above with the degree of proton donation. This correlation indicates a direct relationship between the degree of protonation expected upon complexation and the magnitude of the frequency shift and suggests that predictions of shift magnitudes could be made for other binary complexes involving acetone. No correlation can be made between the gasphase acidity and the observed shift in the antisymmetric C-C stretching mode, q 7 , in agreement with Andrews and J o h n ~ o n . ~ Instead, we have attempted to correlate the shifts in this mode with the polarity of the proton donor, as measured by its dipole moment. In Table VI1 are shown the dipole moments of HCl, HF, and water. It is clear that, while a trend is suggested by the data associated with the former two acids, the water data are in sharp contradiction since, while water has a greater dipole moment than HF, the observed shift in VI7 is less than that observed in the HCl complex. Thus, there appears to be no useful correlation between polarity of the perturber and the observed shifts of this mode. Computational Work. The results of our a b initio SCF calculations on the acetone-water complex support most of the above observations. The minimum energy structure obtained is essentially the same with both 3-21G and 6-31G** basis sets, being a cyclic complex involving two hydrogen bonds (see Figures 5 and6). Thestrongeroftheseis thebondat thecarbonyloxygen for which bond orders of 9% (3-21G) and 5% (6-31G**) are found, while the weaker is between the water oxygen and one hydrogen nucleus of an acetone methyl group, with bond orders of 8% (3-21G) and 2.4% (6-31G**) calculated.21 Theverysmall energy barrier to enantiomeric interconversion of 0.22 kcal mol-' calculated in the present study suggests that the second hydrogen bond between the water oxygen and the acetone methyl hydrogen may be fluctional, since this is on the order of the energy available at 12 K, about 0.2 kcal mol-1. Formation of these hydrogen bonds causes a corresponding shortening of C-C framework bond lengths and a lengthening of the C 4 bond, which account for the red shift in the C - 0 stretching mode (4 frequency and the blue shifts in the C-C asymmetric stretching ( ~ ~ 7and ) C - 0 framework deformation mode (Yl9) frequencies. Both of the latter two modes involve extensive motion of the methyl carbon nuclei and would be expected to blue shift following strengthening of the C-C bonds. Consideration of the decrease in bond order of the hydrogenbonded 0-H and C-H bonds in the context of the above changes suggests that the complex is representative of the early stages of
Vibrational Spectrum of Acetone-Water a water-catalyzed, keto-enol transformation, in which one 0-H bond of water is broken at the carbonyl oxygen and a second is formed simultaneously a t a methyl group. In Figure 5 are shown the values of the change in the net electronic charge on the constituent atoms of acetone and water following the formation of the hydrogen bond, relative to the computed values for the free molecules. Hydrogen bonding causes a significant shift of electron density toward the carbonyl oxygen, the water oxygen, and the carbon nucleus bearing the hydrogen nucleus involved in the weaker hydrogen bond. This shift results in increases in the magnitude of the calculated net charge on all constituent atoms of acetone and water in the ring, reflecting an increase in the polarization of both water and acetone following complex formation. The computed harmonic oscillator vibrational frequencies for the acetone-water complex agree remarkably well with experimental observations when the 6-31G** basis set is employed and when the calculated frequencies are corrected on a modeby-mode basis.19 Most vibrational frequencies of the complex are predicted to within 3 cm-I, with the notable exception of the two stretching modes most closely associated with the carbonyl hydrogen bond. Thus, the agreement is less satisfying for the C=O stretching mode of acetone and the stretching mode of the hydrogen-bonded 0-H bond of the water submolecule, although the direction of the predicted frequency shift is in agreement with the observed shift in both cases. For both modes, the calculated frequency shift is larger than that observed by a factor of about 3. It is likely that this difference reflects the limitation associated with a b initio computations that do not account for dispersion interactions due to neglect of electron correlation.22 It is worthwhile noting that the shifts predicted when the 3-21G basis set is employed are in much greater error for these important modes and that the 6-31G** basis set results are of considerably greater value as a predictive tool even for these vibrational modes in which the agreement is modest. The results also indicate that the 3-21G basis set is useful for the prediction of the direction of the vibrational shifts upon complex formation but that it is inadequate for the prediction of shift magnitudes. Thus, the predicted shifts are generally significantly larger than the observed shifts, but with no easily predicted or correctable pattern. Surface Analogues. The spectroscopic perturbations induced in acetone upon formation of the hydrogen bond with water resemble the influence of q1 binding of acetone to pretreated metal surfaces, such as oxygen-doped Ru, Rh, and Pd23-25and untreated Pt.26 Such binding involves end-on electron donation from the lone pairs of the carbonyl oxygen of acetone to electronaccepting metal surfaces and is manifest by red or blue shifting in most vibrational modes of acetone as observed by HREELS and EELS.2’ For several vibrational modes, the direction of the frequency shift is the same for acetone adsorbed to these surfaces as it is in the acetonewater complex. For example, shifts in the carbonyl stretching mode, ~ 3of, -20 (Ru), -25 (Pd), -45 (Rh), and -80 cm-I (Pt) have been associated with vi binding of acetone to metal surfaces, reflecting a significant weakening of the C=O bond on adsorption, as do the shifts of -5.4 and -6 cm-I for this mode in the acetone-water and acetone-HF complexes. Shifts of +10 (Ru) and +45 cm-I (Pt) are found for the C=O deformation mode, ~ 1 9 compared , with the observation of +14.0and +27-cm-I shifts in the acetonewater and acetone-HF complexes. The direction of shifts in the frequency of modes associated with methyl deformations appears to be in the opposite sense, such as for y4, for which values of -8.2 and -6 cm-’ are observed for the acetone-water and acetone-HF complexes, while shifts of +30 (Ru), +15 (Pd), and +20 cm-I (Pt) are found on metal surfaces. Such differences may reflect significant differences in the interaction between the methyl groups of acetone with the hydrogen-bonding molecule on the one hand and the metal surface on the other, perhaps due to a secondary interaction between the methyl groups of acetone and the proton donors in the case of the binary complexes, such as has been discussed
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4325 above for the acetone-water complex. The clear trend toward greater shift magnitude observed for metal-surface-bound acetone with respect to the water or HF complexes reflects the greater strength of interaction and extent of electron transfer in the former case.
Acknowledgment. The generous financial assistance of NSERC’s Operating Grants Program is gratefully acknowledged. X.K.Z. acknowledges Queen’s University for a Graduate Fellowship Award. References and Notes ( I ) Nelander, B. Chem. Phys. 1992, 159, 281. (2) Nelander, B. J. Chem. Phys. 1980, 72, 77. (3) Kumpf, R. A.; Damewood, J. R., Jr. J. Phys. Chem. 1989,93,4478. (4) Andrews, L.; Johnson, G. L. J . Phys. Chem. 1984,88, 5887. ( 5 ) Nowak, M. J.; Szczepaniak, K.; Baran, J. W. J. Mol. Srrucr. 1978, 47, 307. (6) Ayers, G. P.; Pullin, A. D. E. Chem. Phys. Lett. 1974, 29, 609. (7) Ayers, G. P.; Pullin, A. D. E. Spectrochim. Acta. 1976,32A, 1629. (8) Ayers, G. P.; Pullin, A. D. E. Spectrochim. Acta. 1976, 32A, 1641. (9) Ayers, G. P.; Pullin, A. D. E. Spectrochim. Acra. 1976, 32A, 1695. (10) (a) Parnis, J. M.; Ozin, G. A. J. Phys. Chem. 1989, 93, 1204. (b) Zhang, X. K.; Parnis, J. M.; March, R. E. Proc. 40th Ann. Conf.,Amer. Soc. Mass Spectrom., Washington, DC, June 1-5, 1992. (1 1) The present study was done during work in our laboratory involving ionization of acetone in which helium gas was required as a component of the acetone vapor source. The presence of helium was not found to be of any significance, since test experiments in which argon was used in place of helium as the acetone carrier gas, and where no carrier gas was present, yielded essentially identical spectra of acetone, water, and the acetone-water complex. (12) MONSTERGAUSS: Peterson, M.; Poirier, R. A copy of this program may be obtained from Dr. M. Peterson of the University of Toronto. The optimizations utilized the optimally conditioned gradient method of Davidon and Nazareth [Technical Memos 303 and 306, 1977, Applied Mathematics Division, Argonne National Laboratories, Argonne, IL 60439; Davidon, W. C. Math. Program. 1975,9, 1, and this routine was taken from Gaussian 82 (Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Raghavachari, K.; Whiteside, R. A.; Schlegel, H. B.; Fluder, M. J.; Pople, J. A,)]. (13) (a) GAUSSIAN 88: Frisch, M. J.; Head-Gordon, M.; Schlegel. H. B.; Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A.;Seeger, R.; Melius, C. F.; Baker, J.;Martin, R. L.;Kahn, L. R.; Stewart, J. J. P.; Fluder, E. M.; Topiol, S.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA. (b) GAUSSIAN 90: Revision J.; Frisch, M. J.; Head-Gordon, M.;Trucks,G. W.;Foresman, J.B.;Schlegel,H. B.;Raghavachari,K.;Robb, M.; Binkley, J. S.; Gonzalez, C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A,; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.;Pople, J. A. Gaussian, Inc., Pittsburgh, PA. (14) PCMODEL: SERENA Software, P.O. Box 3076, Bloomington, IN 47402-3076. (15) Dellepiane, G.; Overend, J. Specrrochim. Acra 1966, 22, 593. (16) See, for example: Jones, W. H. J. Phys. Chem. 1992, 96, 594. (17) The results of this and other related calculations on the acetonewater complex are beyond the scope of the present work and will be presented in a separate publication in the future. (18) Mayer, 1. Int. J. Quantum Chem. 1986, 29, 477. (19) The calculated vibrational mode frequencies for the acetonewater complex have been scaled in the following way: The frequency for each mode of water or acetone observed in this study was divided by the frequency obtained for this same mode from the ab initio calculations on unperturbed water or acetone. The latter were calculated with geometry optimization using the same basis set as with the corresponding acetone-water complex calculation. The calculated frequencies of the corresponding perturbed modes of the acetone-water complex were then multiplied by these factors to obtain a corrected frequency. These frequencies have therefore been adjusted to obtain the most realistic values for the spectral shift of any particular mode of the complex with respect to the free acetone or water molecule, as well as the most useful predictions of the positions of perturbed modes that were not observed in the spectra. (20) Lias, S.G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref.Data 1988, 17, Suppl. 1. (21) For comparison, the calculated bond orders for the water dimer in its open, acyclic conformation are 11% (3-21G) and 5.5% (6-31G**). (22) Lee, E. P. F.; Dyke, J. M. J. Chem. Soc., Faraday Tram. 1992,88, 21 1 I. (23) Anton, A. B.; Avery, N. R.; Toby, B. H.; Weinberg, W. H. J . Am. Chem. SOC.1986, 108, 684. (24) Houtman, C.; Barteau, M. A. J. Phys. Chem. 1991, 95, 3755. (25) Davis, J. L.; Barteau, M. A. Surj. Sei. 1989, 208, 383. (26) Avery, N. R. Surf. Sei. 1983, 125, 771. (27) Ibach, H.; Mills, D. L. EIecfronEnergyLossSpecrroscopyandSurface Vibrations; Academic Press: New York, 1982.
