J . Phys. Chem. 1986, 90, 1073-1076
Analysis of the spectra permit assignment of the transition along the long axis of the molecule. The integrated intensities of the crystal and solution agree within error although there may be a slight exaltation of the crystal intensity.
zoo--
1
150--
'1
1073
Fourier-Transform Infrared Spectra of HF Complexes with Acetic Acid and Methyl Acetate in Solid Argon Kenneth 0. Patten, Jr.,+ and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: August 13, 1985)
Cmndensation of argon gaseous solutions of HF with acetic acid and with methyl acetate at 10 K has produced hydrogen-bonded complexes in which H F ligates to the carbonyl oxygen in the skeletal plane presumably on the same side as the ester oxygen. These complexes exhibit characteristic v,(HF) and u,(HF) modes and a large number of perturbed base submolecule modes, the most important of which are red-shifted C=O and blue-shifted C-0 stretching fundamentals.
Introduction Electrophilic ligation to the carboxylate group is of considerable importance to its reactivity and to its usefulness in solvation. This group presents two plausible sites at which an acidic proton can bind, the carbonyl oxygen or the ester oxygen. Infrared studies of the perturbation of the C=O and C - 0 stretching modes of the carboxylate can identify the site of proton binding. Further information concerning the structure of the complex is available from the perturbations upon other modes, and argon matrix isolation allows the detection of many of these perturbations by controlling the extent of self-association of the base, a source of band broadening which is particularly important for acetic acid.
(approximately 75% enrichment) was prepared by reacting the elements and then diluting with argon to Ar/DF = 150/1 and 300/ 1 ratios. Spectra between 4000 and 400 cm-l were collected on a Nicolet 7199 FTIR spectrometer at 1-cm-' resolution giving a frequency precision of f0.3 cm-I. Samples consisted of 30 mmol of an argon-base mixture codeposited with 30 mmol of argon-HF or argon-DF over periods ranging from 6 to 20 h. Spectra were also recorded after diffusion at 20 K for 10 min and then after diffusion at 26 K for 10 min. A separate experiment where each argon-base mixture was deposited alone and annealed identified the frequencies of the matrix-isolated base and its polymers.
Experimental Section
Results
The methods and apparatus used in these experiments have previously been discussed in detail.'-3 Glacial acetic acid (Mallinckrodt) was outgassed at 77 K under high vacuum, then metered into the deposition manifold, and diluted with Ar to a mole ratio of Ar/CH3COOH = 500/ 1 or 1000/ 1. An identical outgassing procedure was used for methyl acetate (Aldrich), but this compound was diluted to Ar/CH,COOCH, from 200/1 to 600/ 1. Hydrogen fluoride (Matheson) was outgassed and diluted to A r / H F mole ratios of 200/1 or 300/1; deuterium fluoride
A spectrum of Ar/CH,COOH = 1000/1 with Ar/(HF DF) = 100/1 is shown in Figure 2. The strong band at 3406 cm-' with a site splitting at 3417 cm-' was not found in experiments where either acetic acid or HF was deposited alone; this, as well as its proximity to the previously reported v, value for a ~ e t o n e - H F , ~ 3302 cm-I, indicates that this is the u,(HF) vibration in the acetic
'Undergraduate research student
0022-3654/86/2090-1073$01.50/0
+
(1) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1982, 76, 2875. (2) Andrews, L.; Johnson, G.L.; Kelsall, B. J. J . Chem. Phys. 1982, 76, 5767. (3) Andrews, L.; Johnson, G. L. J . Phys. Chem. 1984, 88, 5887.
6Z 1986 American Chemical Society
1074 The Journal of Physical Chemistry, Vol. 90, No. 6, 1986
Patten and Andrews
TABLE I: Acetic Acid-HF Complex Fundamentals (cm-I) in Solid Areon
mode“ u,(site) u,(H-F str) vl(O-H str) u,(C=O str) u6(a’ CH, sym den u,(C-0 str) u,(COH bend) u,,(OCO bend) v16(a” 70H) u,(in-plane) u,(out-of-plane)
acetic acid-HF
acetic acid-DF
3417.4 3406 3549.8 1753.0 1396.7 1287.1 1197.1 650.3 541.9 736 699
2522 3549.5 1751.8 1396.2 1287.4 1198.1 650.7 542.4 530 shb 513
acetic acid
3564.4 1781.2 1379.7 1259.2 1180.0 637.9 534.2
Base submolecule assignments from ref 5. bFrequencyobtained by spectral subtraction.
LriCElWJt’iEPZ
Figure 2. FTIR spectrum of sample prepared by codepositing 45 mmol of Ar/CH,COOCH3 = 300/1 with 54 mmol of Ar/(HF DF) = 150/1 (70% DF enriched) at 10 K for 20 h. Acetic acetate ester bands are marked E; new product bands near base absorptions are marked with
+
arrows. TABLE II: Fundamentals (cm-I) for Methyl Acetate-HF Complexes in Solid Argon
mode“ u,(H-F str) 2u14
Figure 1. FTIR spectrum of sample prepared by codepositing 33 mmol of Ar/CH,COOH = 1000/1 with 30 mmol of Ar/(HF + DF) = lOO/l (90% DF enriched) at 20 K for 6 h. Acetic acid absorptions are marked A; new product bands near base absorptions are marked with arrows.
acid-HF complex. The D F counterpart u,(DF) mode was observed at 2522 cm-I. In each experiment, the intensity of the us bands increased by approximately 20% upon annealing at 20 K for 10 min. As expected for a complex with less than a third-order axis, two nondegenerate HF librations were observed. These bands for the HF complex were found at 736.2 and at 699.4 cm-’, while those for the D F complex appeared at 530.1 and 513.5 cm-I. These bands increased with the Y, motions upon annealing. The 530.1-cm-’ ul(DF) appeared as a shoulder on the 534-cm-’ band of the acetic acid monomer; spectral subtraction of Ar/CH,COOH = 1000/1 from acetic acid + D F clearly shows the new band. Several acetic acid vibrations are strongly perturbed by HF complexation. The motions for which perturbations were observed are listed in Table I and marked with arrows in Figure 1; these motions have a’ symmetry in acetic acid monomer (C, symmetry). The intensities of these bands also increased by 20% on annealing. Of these bands, only the OH stretch and the C=O stretch are reduced in frequency by H F complexation; the other bands are increased in frequency. Unlike the case for acetic acid, complexation of HF with methyl acetate is not nearly so complicated by base self-association, and the production of complexes including two HF molecules per base molecule (1:2 complexes) is much more favorable on sample
us(C=O str) u6(a’, CH,-0-C bend) u20(a”r CH3-O-C bend) v9(a’, CH3-C=O bend) vlo(C-O str) uI4(CH3-Cstr) uj4(a”, 0-C-0 bend) u16(a’, CH,-C=O bend) v,(in-plane) u,(out-of-pIane) u,(H,-F) u,(H,-F) ui(H,-F) ui(H,-F) u I ( Hb-F) h(Hb-F) ‘Reference 6.
methyl acetate-HF
methyl acetate-DF
3351 1783.3 1718.7 1460.1 145 1 . I 1378.1 1290.7 874.0 608.7 438.2 756 728 3576 2800 835 782
2483 1783.9 1716.4 1460.2 1451.1 1378.2 1291.0 874.4 608 435.8 552 531 2634 2151 625 606 477 445
564
methyl acetate 1760.7 1462.5 1448.0 1370.2 1246 847.6 603.7 430.4
annealing. Upon codeposition of Ar/CH3COOCH, = 300/ 1 with A r / H F = 300/1, the most intense band in the H-F stretching region a t 3351 cm-l is due to u,(HF) of the 1:l complex. This band decreased in absorbance by 30% upon annealing at 26 K in favor of weaker bands present upon deposition at 3576, 3229, and 2800 cm-I. A similar experiment using A r / ( H F + DF) = 150/1 produced the D F counterpart us at 2483 cm-l for the 1:l complex, which is illustrated in Figure 2. Weaker bands at 2634, 2410, and 2150 crn-I increased markedly on sample annealing. The uI bands for the 1:l HF complex were observed at 755 and 727 cm-’, while their D F counterparts were observed at 552 and 534 cm-I. Assignments to u, modes for the 1:2’complexare based upon intensity relationships with the observed us bands on sample warming. Both HF and base submolecule modes are summarized in Table 11. As with acetic acid, most perturbations of methyl acetate modes occur upon vibrations with a’ symmetry; however, unlike acetic acid, perturbations on a” modes were also found. Among the
HF-CH3COOH and HF-CH3COOCH3 Complexes observed perturbations, two of the most important were for the C = O stretch ( u s ) and the C - O stretch (vlO), which were red and blue shifted, respectively, along with shoulders due to higher aggregates. Many of the other base submolecule modes, while fairly intense in general, are incompletely resolved from the base modes. An interesting exception is v16 (a’, CH3C=0 bend), which was observed at 438.2 cm-’ with H F and 435.8 cm-I with DF. Perturbed base submolecule modes are marked by arrows in Figure 2.
Discussion A planar complex is formed when H F ligates with aldehydes and ketones, as determined in gas-phase microwave4 and matrix infrared3 studies. The carboxylate group can be expected to bind analogously; however, other possibilities for binding exist. The carboxylate group includes not only the nonbonded electrons of the carbonyl oxygen, but also those of the ester oxygen, which can be involved with electrophiles in certain circumstances. Additionally, if H F were to bind the carbonyl oxygen, the complex could be weakened by electrostatic repulsions with the base skeleton. Both acetic acid5 and methyl acetate6 are known to be most stable as an s-trans conformer: 0
! A
CH3
/R 0
RsHorCH3
so that H F binding in the plane of the base at the carbonyl oxygen could interact with either the carbonyl methyl group or the substituent R. Still another possibility for acetic acid is a complex in which the acetic acid proton binds to the fluorine of HF; an appreciable extent of complexation in this manner, however, is doubtful because of the considerably greater basicity of the carboxylate group over fluoride and the stronger acidity of H F relative to acetic acid. The 1:l H F complex with acetic acid gives rise to a u,(HF) absorption at 3406 cm-’, markedly lower than the induced Q branch of matrix-isolated HF7 at 3919.5 cm-l, two H F librations at 736 and at 699 cm-l, and a 28-cm-’ decrease in the C=O stretching and 28-cm-’ increase in the C-0 stretching frequencies of the base submolecule in the complex. A decrease in carbonyl fundamental was observed for acetone-HF, indicating complexation of H F at the carbonyl oxygen in the plane of the base s~bmolecule.~ The increase in C-0 fundamental in the complex is inconsistent with attachment to the ester oxygen since C - 0 fundamentals in the dimethyl ether-HF complex decreased.* The isotopic substitution ratio uH/uD is found to be 1.350 for the us absorption, consistent with a strong complex between H F and nonbonded electrons. This, as well as the high uI frequencies and isotopic ratios (1.389 for in-plane, 1.362 for out-of-plane), rule out interaction of H F with the C=O T bond, as does the large magnitude of the C=O stretch displacement. The increase of 28 cm-I in the C - 0 stretch, however, does indicate an increased donation of the nonbonded electrons of the hydroxyl oxygen to the carbonyl system under the inductive influence of the H F ligand. Further, donation of electron density from the methyl substituent may be manifested in an increase of 17 cm-I in the a’ symmetric CH, deformation. No perturbation in the a” out-of-plane C H 3 motions was observed, however, indicating that H F binds in the plane of the acetic acid skeleton. An analogous structure to acetone-HF, in which the hydrogen bond forms an angle of approximately 120’ with the C=O bond, is expected for acetic acid-HF, but the two possible sites under (4) Baiocchi, F. A,; Klemperer, W. J . Chem. Phys. 1983, 78, 3509. (5) (a) Wilmshurst, J . K. J . Chem. Phys. 1956, 25, 1171. (b) Berney, C. V.; Reddington, R. L.; Lin, K. C. J . Chem. Phys. 1970, 53, 1713. (6) Wilmshurst, J. K. J . Mol. Specfrosc. 1957, I , 201. (7) Andrews, L.; Johnson, G. L. J . Phys. Chem. 1984, 88, 425. (8) Andrews, L.; Johnson, G . L.; Davis, S. R. J . Phys. Chem. 1985, 89, 1710.
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 1075 these conditions, shown in structures 1 and 2, are not equivalent. F H‘
.. “0
II
c
/H, \ CH3 1
0
2\
CH3
0/ H 2
The large blue shifts observed for the in-plane C O H bend and out-of-plane 0 - H torsion suggest that the hydroxyl proton is interacting slightly with the HF proton, supporting structure 2 instead of structure 1. This may, in fact, rationalize the small 14.9-cm-I red shift in the 0-H stretch in the complex, which stands in marked contrast to the 313-cm-’ red-shifted 0 - H stretching fundamental in acetic acid The small 14.9-cm-’ red shift and the lack of perturbation in the a” (out-of-plane) C O H bend show that hydrogen bonding of the hydroxyl proton to fluorine does not occur in the complex. Since diffusion of acetic acid apparently occurs at temperatures comparable to those at which H F diffuses readily, higher H F complexes were not observed with acetic acid. Methyl acetate-HF exhibits perturbations of several of the same vibrational modes as does acetic acid-HF indicating an analogous complex. Its C=O stretch is decreased in frequency by 42 cm-’ while its C-0 stretch is increased by 45 cm-l, demonstrating a greater extent of electron rearrangement in the carboxylate system than for acetic acid and indicating H F binding to the carbonyl oxygen in the carboxylate plane. Furthermore, the greater v, shift to 3351 cm-I indicates stronger H F complexation to methyl acetate. The frequency and uHF/vDF ratio of 1.350 for u, are typical of carbonyl-HF complexes, and the vI frequencies and u H F / u D F ratios (1.370 for each) confirm the lone-pair hydrogen bond. Both methyl groups contribute electron density to the carboxylate system: both the a‘ symmetric C H 3 bend of the acetate methyl group and the CH3-C stretch are strongly perturbed by hyperconjugative electron donation, and the a’ and a” antisymmetric bends of the ester methyl group undergo much smaller perturbations arising mostly from electron removal from the ester oxygen. The observation of small blue shifts for the u 1 6 (a’, C H 3 C = 0 bending) mode suggests a planar complex, as found in like manner for the acetone-HF c ~ m p l e x .That ~ these small blue shifts showed an H F and D F isotopic dependence is due to interaction with the vi (in-plane) modes of H F and D F in the complexes. Likewise, the 2-cm-I red shift for the v5(C=O) fundamental complexed to D F as compared to H F verifies hydrogen bonding to the carbonyl oxygen and reflects interaction with the strong us mode of the isotopic acid submolecules. A similar interaction has been characterized for the acetone c ~ m p l e x e s . ~ Following the acetone-HF ~ y s t e m the , ~ 3576- and 2800-cm-’ bands are assigned to the two H-F stretching modes in the methyl acetate- -Ha-F- -Hb-F complex. The 3329-cm-’ band that increased markedly on sample warming is believed to be due to an unidentified higher aggregate. Some comment on the possibility of H F binding to the ester oxygen of methyl acetate is warranted, particularly in view of the matrix spectroscopic evidence attributed to this second site of attachment in similar HC1 s t ~ d i e s . ~First, the only additional H-F stretching fundamental observed above the 3351-cm-’ value assigned here to the carbonyl--HF complex, the band at 3576 cm-I, is substantially higher than the dimethyl ether- - H F v, value’ of 3351 cm-I, and the 3576-cm-’ band exhibited a higher order dependence on HF concentration, which led to its above assignment to the 1:2 complex. Second, the only band observed here that is even suggestive of an ester oxygen complex is a very sharp 1783.3-cm-’ absorption above the C=O precursor mode, which is likely the HF counterpart of the sharp 1776-cm-’ band assigned to the ester complex with HCL9 In the present HF experiments the sharp 1783.3-cm-’ band maintained a constant relative intensity of 1.7 with the 1718.7-cm-’ band in several experiments (9) Maes, G.; Zeegers-Huyskens, Th. J . Mol. Strucr. 1983, 100, 305.
1076 The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 TABLE III: Carbonyl-HF Complex HF Submolecule Mode Fundamentals (cm-') in Solid Argon base u, din-plane) ul(out-of-plane) 3570 612 602 H2CO 3416 122 712 CH3CHO" isomer 3369 78 1 766 (CH,LCO 3302 778 756 3406 736 699 CH3COOH 3351 756 728 CH3COOCH3 "On the matter of diastereomeric CH,CHO- - H F complexes, discussed in ref 3, it is significant that the anisotropy in u1 associated with us = 3416 cm-I is the same as that for H 2 C O - - H F , whereas the anisotropy for u, associated with us = 3369 cm-l is comparable to that for (CHj)*CO- - H F .
over a range of reagent concentrations and during sample annealing so it is due to a similar or the same complex. The former band did not exhibit the extensive red shoulders that grew with the 1718.7-cm-' band on annealing characteristic of higher-order complexes, so the 1783.3-cm-' band cannot be due to a combination with the 17 18.7-cm-' fundamental nor is it likely to contain a significant portion of C=O stretching character. Maes and Zeegers-Huyskens consider but reject possible contribution of 2vI4 in Fermi resonance with us to the spectrum of the HCI c ~ m p l e x . ~ We believe that the 1783.3-cm-' band is best assigned to 2v14 in Fermi resonance with v s rather than postulating another complex with attachment to the ester oxygen. The 1783.3-cm-I overtone is double the intensity of the 874.0-cm-' fundamental, which, of course, arises from interaction with the very strong 1718.7-cm-I fundamental. In the HF complex, 2 X vi4 = 2 X 874 = 1748 cm-I, which is sufficiently close to the expected position of v5 in the complex (taking the acetic acid complex as a model predicts v5 in the complex to be 1733 cm-I) to allow Fermi resonance interaction to displace 2vI4to higher and vs to lower frequencies. Finally, conjugation between the ester and carbonyl oxygen atoms in methyl acetate may reasonably aid the H F ligand in finding the most stable basic site for attachment and preclude trapping the less stable arrangement of the complex. It is in fact the perturbing effect of the acid ligand that makes possible the observation of 2vI4 of the methyl acetate submolecule in the complex, which is not clearly observed in the matrix spectrum of methyl acetate. The v14(C-C) fundamental is blue shifted and the u5(C==O) fundamental is red shifted in the complex such that v 5 and 2vI4 are almost coincident and the resonance interaction follows. Table 111 shows that the anisotropy of the two vI motions for methyl acetate-HF is similar to that for acetic acid-HF, but not for acetone-HF complex despite its greater strength. The vI anisotropy is related to differences in base submolecule structure presented to the HF proton when it librates in the base submolecule plane from when it librates perpendicular to the base submolecule plane, so that the structure of methyl acetate-HF must be more analogous to that of acetic acid-HF than to that of acetone-HF. The stabilization of this structure is once again explained by net dipole minimization for the complex due to the lack of polarizability of the argon matrix.
Patten and Andrews
As illustrated by the increased vI anisotropy for carboxylate-HF complexes relative to carbonyl-HF complexes, the substituent on the ester oxygen contributes significant electrostatic repulsion against the HF ligand, reducing the strength of the hydrogen bond. The presence of the ester oxygen also weakens the hydrogen bond by inductive withdrawal of electron density from the carbonyl system, reducing the nonbonded electron density available at the carbonyl oxygen. The acid hydrogen bound to the ester oxygen of acetic acid-HF is less capable of donating electron density to the oxygen than is the methyl group of methyl acetate, so that the acetic acid ester oxygen withdraws more electron density from the carbonyl oxygen than is the case for methyl acetate. This effect contributes to the weakness of the acetic acid-HF complex relative to methyl acetate-HF and affects another aspect of the differences between carbonyls and carboxylates in hydrogen bonding. Donation of nonbonded electron density from the ester oxygen to the C=O system through conjugation tends to partially cancel the inductive effects of the ester oxygen and the repulsive effects of the substituent. This conjugation is observed in the increase of the C - 0 stretching frequency upon HF complexation for the carboxylate-HF complexes. The acetic acid C-0 stretch undergoes a 28-cm-' increase and the C=O stretching frequency decreases 28 cm-' on H F complexation, but methyl acetate, in which the nonbonded electron density at the ester oxygen is more available, exhibits a 45-cm-' increase in the C-0 stretch and a 42-cm-I decrease in the C=O stretching frequency. This contribution to the greater strength of the methyl acetate-HF hydrogen bond than that of acetic acid-HF would be more pronounced in amides, which are of extreme importance in biology as hydrogen bond electron and proton donors.I0
Conclusions The complexes acetic acid-HF, methyl acetate-HF, and methyl acetate-(HF), have been observed by FTIR spectroscopy and argon matrix isolation. Perturbations of the vibrations of both submolecules are caused by the transfer of electron density associated with hydrogen bond formation, by electron density rearrangement within the base submolecule, and by repulsions between the two submolecules. For acetic acid-HF, a red shift in the C=O mode and blue shift in the C-O mode, large increases in the bending modes us and vl and a lack of change in modes with a" symmetry indicate a complex in which H F ligates to the carbonyl oxygen on the same side as the -OH group; methyl acetate-HF presumably has a similar structure due to the similarities in observed vibrations, particularly the H F librational modes vl, with those of acetic acid-HF. Based on the matrix spectra these complexes are almost as strong as the acetone- - H F complex.
Acknowledgment. The authors acknowledge financial support from the National Science Foundation, Grant CHE82-17749, for this research. Registry No. HF, 7664-39-3; CH,COOH, 64- 19-7; CH,COOCH,, 79-20-9; Ar, 7440-37-1. (10) Rossmann, M . G.; Argos, P. Annu. Reti. Biochem. 1981, 50, 497.
