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Matrix isolation infrared spectra of the complexes between methylacetate and water or hydrochloric acid
Journal of Molecular Structure, 100 (1983) 305-315 Elsevier Science Publishers B.V., Amsterdam -Printed
in The Netherlands
MATRIX ISOLATION INFRARED SPECTRA OF THE COMPLEXES BETWEEN METHYLACETATE AND WATER OR HYDROCHLORIC ACID
G. MAES* and Th. ZEEGERS-HUYSKENS Department (Belgium)
of Chemistry,
University
of Leuven,
Celestijnenlaan
200F, 3030 Heverlee
(Received 17 January 1983)
ABSTRACT The hydrogen-bonded complexes between methylacetate (MeAc) and water or hydrochloric acid have been studied by infrared spectrometry in a low temperature Argon matrix. The VCQ, VC__Oand VC+ vibrational modes of MeAc show a splitting to the low and to the high frequency side of the free molecule. This suggests that water and HCl interact with the keto and ether oxygens. This conclusion is supported by the appearance of two main absorptions in the vHC1region. These results are discussed as a function of the gas-phase basicity of the two oxygen atoms, derived from the 0( 1s) core electron binding energies and from the ionization potentials. INTRODUCTION
A number of hydrogen-bonded complexes involving oxygen bases and hydrogen chloride have been investigated by the matrix isolation technique [ 1,2] but so far no experimental data are available for the interaction of esters with proton donors. Previous studies performed in solution have shown that aliphatic esters can be considered as good electron donors towards hydroxylic acids [ 3,4] ; moreover, the vcEo, vc-o and uc_c vibrations observed in an infrared [5] or Raman [6] spectrum are very sensitive to complex formation. In this work, the interaction between methylacetate (MeAc) and water or hydrochloric acid has been studied in a low-temperature Argon matrix. The effect of complexation on the mentioned internal modes of MeAc and on the Vncistretching vibration will be discussed in detail. EXPERIMENTAL
The experimental techniques and apparatus used have been described previously [ 71. Gaseous mixtures were deposited at 15 + 1 K with a rate decreasing from an initial high value to a constant smaller one after a few minutes. Matrix ratios (M/R) ranged from 600 to 300. Spectra were scanned on a P.E. 180 spectrophotometer at 12 K, while annealing was performed at 37 K for about 20 min. *Research Associate of the Belgian N.F.W.O. 0022-2860/83/$03.00
o 1983 Elsevier Science Publishers B.V.
306
HCl was a Matheson product (“Electronic Grade”) and Ar was from Air Liquide (99.999%). Methylacetate was distilled over P,O, before use. Bidistilled water was used for the water doped systems. The absorbance at 1592 cm-’ (u, HzO) was taken as a measure of the relative amount of water present in the solid mixtures. RESULTS
AND DISCUSSION
The effect of complex formation on the internal modes of MeAc and on the vHClstretching band are discussed later. Finally, the results of this work will be compared with data available in solutions and in the gas phase. In ternal modes of MeAc
In a Argon matrix, at very high dilution (matrix/MeAc = 3000) the voo, vibrations are observed at 1760,1245 and 848 cm-‘, respecand VCM tively: these bands are attributable to the MeAc monomer. At lower matrix ratios, additional bands appear at 1755,1255 and 851 cm-‘; these bands are assigned to MeAc dimers and possibly to higher polymers [ 71. e+,
Interaction
with water
When an ester molecule interacts with water or with a hydroxylic derivative in the liquid state, one usually observes a frequency lowering of the vczo vibration and a frequency increase of the vco and vc+ bands [ 5,6] ; these effects arise from an enhanced delocalization in the O=C-O group. Figure 1 shows that when water is added to a MeAc/Ar matrix (lMeAc/lO H,0/550Ar), new bands are observed to the low and high frequency sides of the free voo (Fig. la), vco (Fig. lb) and uc+ (Fig. lc) vibrations. The bands observed at 1740, 1266 and 858 cm-’ are ascribed to the formation of a complex on the carbonyl function (structure I). The frequency shifts of these three bands are somewhat lower than those ,H’
O-H
.. I
I*
‘H,
O-H
observed for the interaction between stronger acids (phenol derivatives) and the same base in carbontetrachloride solution [ 61. The bands shifted to the higher-frequency side of the free carbonyl band (1770 cm-‘) and to the lower-frequency side of the free vc. (1228 cm-‘) and vcX (840 cm-‘) absorptions are assigned to the formation of a hydrogen bond at the ether oxygen (structure II) which decreases the electronic delocalization towards the carbonyl group. This attribution is strengthened by the fact that the vczo band of complex II is narrower than in the free MeAc molecule.
307
b
P I
'i II
’
:i:'
\I 111
II
1
b
1 I
d
'1
M
1760 172i
\ 126
1240
4
v 100 I
660 I
62C1 I
Fig. 1. l/lO(H,0)/600. (a) vC.@ (a’) after annealing;(b) VC__O,(b’) after annealing; (c) VCC, (c’) after annealing. M = monomer; D = MeAc dimer; I, water complex on the carbonyl oxygen; II water complex on the ether oxygen.
The origin of the band at 1748 cm-’ is less clear; its intensity decreases upon annealing (Fig. la’) and at the same time the band at 1740 cm-’ becomes broader. The attribution of these two absorptions to complexes of 1: 1 and 1:2 (H,O) stoichiometry was ruled out because the 1748 cm-’ band increases with the water content of the matrix; moreover, this band is as narrow as in the free MeAc molecule and a frequency shift of only 12 cm-’ seems to be very low; in liquid solution, where the frequency shifts are usually lower than in a low-temperature matrix, the observed AvcGo value is about 20 cm-’ for the 1: 1 complex [5] . The existence of a complex of 2(MeAc):l stoichiometry also seems very improbable since in a very dilute matrix (MeAc/2750), the intensity of the two components at 1748 and 1740 cm-’ is about the same. Finally, the possibility of a Fermi resonance involving the ye0 and 2~~ vibrational modes can be ruled out owing to the great energy difference between the unperturbed levels. Another possibility is the assignment of the 1748 cm-’ band to a “bisolvated” complex. In this species, the vDo band will be less shifted and
303 ,0-H H3C\ H
3
17L8 ,'n*H c=o,
C-o@
?? b
III
‘\ “\O_H
will be narrower than for the I complex. The perturbation of the voo band will also be smaller than in the II complex. This interpretation is strengthened by the appearance of a second uc. band at 1232 cm-’ for a matrix containing a great excess of water (Fig. lb); this band also disappears upon annealing. It thus seems that the thermodynamically less stable III species almost disappears during the annealing stage of matrix warm-up which is in agreement with the theoretical predictions [8] . The existence of this species, although less stable than complexes I and II, is possible because, even in the stronger interaction (with the sodium ion for example) the charge on the ether oxygen remains negative [ 91. On annealing a second band appears at 1711 cm-’ (Fig. la’) and a very strong asymmetry of the vca band with a shoulder at about 1290 cm-’ (Fig. lb’) is observed. These new absorptions are probably attributable to complexes of 1:2(H,O) or l:n(H*O) stoichiometry; in the liquid state, the frequency shift of the 1:2 complex is about twice that of the 1: 1 complex
r51. Interaction with HCL As expected, interaction with HCl brings about a stronger perturbation of the internal modes of MeAc. Some control of the species present in the matrix is obtained by varying the relative concentrations of base and acid (MeAc/HCl/matrix = 1:1/300,1:2/300 and 1:1/600). The spectra obtained in the vczo, vc, and vrange, at these different matrix ratios, are reproduced in Figs. 2,3 and 4, respectively. As can be seen from Fig. 2, new absorptions at the low (1726,1703 cm-‘) and high frequency sides (1776 cm-‘) of the free vcco band are observed, strongly suggesting, as in the case of water, the formation of carbonyl (IV) and ether (V) complexes. The two bands observed at 1726 and 1703 cm” cannot be ascribed to complexes of 1:l and 1:2(HCl) stoichiometry since the band at 1726 cm-’ is characterized by a higher intensity when the matrix ratio is l/2/300 (Fig. 2b). On annealing, the 1703 cm-’ band attributed to a complex of 1: 1 stoichiometry becomes asymmetric and shows a shoulder at 1690 cm-’ ascribed to 1:2 complexes and possibly to higher complexes. This attribution is reinforced by the fact that at higher matrix dilution (Fig. 2c), the band at 1703 cm-’ is approximately symmetric. The origin of the 1726 cm-’ band, which in some cases is split into a doublet, is more questionable. The assignment of this band to a bisolvated complex seems less adequate because the relative concentrations of HCl are much smaller (1: 1 and 1:2) than in the case of the interaction with water (1: 10); moreover, there was no indication of a further splitting on the low-frequency side of the ZJ~~ band (Fig. 3).
309
ir a
-
d
a’
Fig. 2. vCzOrange. (a) l/1/300, l/1/600, (c’) after annealing.
b
C’
(a’) after annealing; (b) l/2/300,
(b’) after annealing;(c)
In solution, the ~c.~ absorption of methylic esters involved in hydrogen bonds with hydroxylic acids shows a band splitting attributed to a Fermi resonance between the vDo and 2v,c levels [6, lo]. In the present case, the uc+ vibration is observed at 867 cm-’ and an interaction between its first overtone (- 1730 cm-‘) and the vDo vibration cannot be completely excluded. A Fermi resonance mechanism, however, cannot explain the variation of the relative intensity of the 1726 and 1703 cm-’ bands with the composition of the matrix; further, this mechanism cannot account for the strong decrease of the 1726 cm-’ band upon annealing. We have observed that the intensity of this band strongly depends on the water content of the matrix, roughly estimated from the absorbance of the water band at 1592 cm-‘. This absorbance is 0.03,0.08 and 0.12 for the spectra of Figs. 2a, 2b and 2c, respectively. This suggests that the 1726 cm-’ band can be reasonably ascribed to the interaction of the carbonyl group with the H,O*HCl complex (structure VI); in this last species, the proton donor ability of the
310
1
I 1210
100 a
1 100
I 12&O b
IV 1
VI
a’ C’
\
4 Fig. 3. YC-O range. (a) l/1/300, l/1/600, (c’) after annealing.
(a’) after annealing;(b)
l/2/300,
(b’) after annealing;(c)
hydrogen atom is enhanced by complex formation with an acid [ll] and so the frequency shift Av,., and the bandwidth will be higher than in the case of the interaction with water, but lower than for the HCl complex. This assignment is further confirmed by the study of the ~no bands. In the uoo region (Fig. 3a, b, c) the band at 1224 cm-’ is assigned to the ether complex; the bands lying at the high-frequency side of the free voo absorption are ascribed to the carbonyl complexes. The intensity of the less shifted band at 1273 cm-’ increases with the water concentration and decreases upon annealing; this absorption was therefore assigned to the complex with H20*HCl. The bands at 1288 cm-l and 1300 cm-’ whose intensity varies in the same way as the 1703 and 1690 cm-’ absorption, were ascribed to the 1: 1 and 1:2 species. In the vc_+ region, bands at 867 and 836 cm-’ (Fig. 4a, b) are ascribed to the carbonyl and ether complexes, respectively. The assignments of this work are summarized as follows:
311 H .H’
HC
,HCI
H3C,
3 \
4$=
HCI
A,“ ’ .
1726 0"
0,
4% V
IV
‘HCI
VI
vHa region As indicated in Fig. 5, several absorptions are observed in this range. Besides the band at 2663 cme2 previously assigned to the H20*HCl complex [ 121, two broad absorption bands at 2560 cm-’ (Av~,~= 70 cm-‘) and 2450-2460 cm-’ (Av~,~= 250 cm-‘) are observed. Owing to the greater electronic density on the carbonyl oxygen (-0.329) than on the ether oxygen (-0.229) [9], it is expected that in complex IV more charge will be
300
660
620
900
Fig. 4. VC-C range. (a) l/1/300,
88O’jj 820 (a’) after annealing; (b) l/1/300,
(b’) after annealing.
312 2900
2600
2700
Fig. 5. VHC~range. (a) l/2/300,
2600
2500
2400
2300
(a’) after annealing.
transferred to the HCl molecule; as a consequence, the frequency shift of the vHCiband and its intensity increase should be higher for the carbonyl than for the ether complexes. The assignment of the bands at 2450 and 2560 cm-’ to the complexes IV and V is further supported by considering the proton affinity (PA) of the two oxygen atoms, deduced from the O(ls) core electron binding energies, respectively 537.9 and 539 eV [ 13,141. The PA values are indicated in Table 1 along with the experimental uHCl values relative to other oxygen complexes. From Fig. 6, where VHCl has been plotted against the PA, it appears that the two experimental vnQ values of this work fit, within reason, the straight line when taking for the bands at 2568 and 2450 cm-’ corresponding proton TABLE 1 v~,-J values in complexes of oxygen bases and corresponding PA of the base Base Hz0 CH,OH C,H,OH (C,H, )zO (CH, ),CO (CH,),CHOH (CH, ),COH CH,COOCH,
aFrom ref. 1. bFrom ref. 13.
VHCl(Cm-l)a
PA (eV)b
2664 2525 2504 2415 2392 2465 2462 2450 2568
7.32 7.91 8.10 8.59 8.44 8.25 a.34 8.47 (doubly-bonded oxygen) 7.81 (singly bonded oxygen)
313
affinities of 7.81 and 8.47 eV, respectively. (The dimethylether.HCl complex shows unusual spectroscopic properties [ 151 and is not taken into account in Fig. 6.) General support for the assignments of the present work comes from Fig. 7, which shows that the vHClvalues of the free molecule, of the water complex and of the carbonyl complexes of 1:l and 1:2 stoichiometry are related to the corresponding vczo or vca values.
8.0 PAleVI
Fig. 6. vHcl as a function of the PA of the oxygen bases.
3000
2800
2600
2400
2200
I 1690
I
I 1710
I
I 1730
I
I 1750
12L.5
y.=Okm')
Fig. 7. vHClasafunctionof 4, HCl (1:2 complex).
vc=o and VC__OI1, freeMeAc;2,
1265
1265
1305
VC_Okm')
H,O*HCI;3, HCI (1:l complex);
314
Hydrogen
bond and protonation
site in esters
In solution, where the equilibrium conditions are reached, hydrogen bonding [ 3,6] and protonation [16-181 clearly takes place on the carbonyl function. Gas-phase measurements have shown that the ether oxygen proton affinity is lower than that of the doubly-bonded oxygen. The ionization potentials of the two oxygen atoms of MeAc are 10.48 eV (C=O) and 11.16 eV (O-CH3), respectively [ 191. Thus, as discussed by Benoit and Harrison [ 131, under conditions of thermodynamic control, protonation at the carbony1 will be favoured. (It must be pointed out, however, that from the principal fragments ions obtained in ion cyclotron resonance spectroscopy measurements, the esters appear to protonate at the ether oxygen [ 20, 211.) Nevertheless the calculated electronic density on the two oxygen atoms, the values of the ionization potential of the O(ls) core electron binding energies and of the proton affinities suggest that the basicity of the carbonyl and ether oxygens does not differ too much. In this sense, it seems reasonable to admit that in a low-temperature matrix where the conditions of thermodynamic control are not reached, hydrogen-bonded attachment, considered as the first step of protonation, will occur on the two basic sites of the ester molecule. Structure
of the complexes
As discussed in this work, the voco band shows at high HCl content a band at 1690 cm-’ attributed to the 1:2 species. The frequency shift from the free molecule (Av = 70 cm-‘) is much less than twice the shift of the 1:l complex (Av = 57 cm-‘). This suggests that in the 1:2 adduct, the two HCl molecules are bonded together by a hydrogen bond. The same conclusion can be drawn from study of the unQ absorptions where a broad band at the low frequency side of the 2460 cm-’ absorption was observed in excess of HCl and upon annealing. This structure agrees with that reported for the acetone*HF complex in the gas phase [22] and with that of the cyclohexanone*HBr complex in solution [ 231. However, in the case of the dimethylether complex, increasing the acid concentration brings about a frequency shift of the vHClband to higher frequencies; this clearly indicates the bifurcated structure. The structure of the 1:2 adducts seems to be different in ether and carbonyl bases; the origin of this difference is not very well understood. ACKNOWLEDGEMENT
The authors are indebted to the University of Leuven and to the Belgian N.F.W.O. for financial support.
315 REFERENCES 1 A. J. Barnes, in H. Ratajczak and W. J. Orville-Thomas (Eds.), Molecular Interactions, Vol. 2, John Wiley, 1980, pp. 273-299. 2 J. P. Perchard, in A. J. Barnes, W. J. Orville-Thomas, A. Mtiller and R. Gaufres (Eds.), Matrix Isolation Spectroscopy, NATO Advanced Institute Series, Vol. 76, D. Reidel, Dordrecht, 1981, pp. 551663. 3 T. Gramstad, Spectrochim. Acta, 19 (1963) 497. 4 E. M. Arnett, L. Joris, E. Mitchell, TSSR Murthy, T. M. Gorrin and PvR. Schleyer, J. Am. Chem. Sot., 92 (1970) 2365. 5 R. M.Moravie, J. Corset, M. L. Josien, G. Nee, G. Leny and B. Tchoubar, Tetrahedron, 32 (1976) 693. 6 L. Vanderheyden and Th. Zeegers-Huyskens, J. Mol. Liquids, 12 (1983) 1. 7 G. Maes, Spectrosc. Lett., accepted. 8 S. Cradock and A. J. Hinchcliffe, Matrix Isolation, Cambridge University Press, 1975. 9 P. V. Kostetsky, V. T. Ivanov, Yu. Avchinnikov and G. Schchembelov, FEBS Lett., 30 (1973) 205. 10 R. M. Moravie, J. Corset and A. Burneau, J. Chem. Phys., 79 (1982) 119. 11 P. Huyskens, J. Am. Chem. Sot., 99 (1977) 2578. 12 A. Schriver, B. Silvi, D. Maillard and J. P. Perchard, J. Phys. Chem., 81 (1977) 2095. 13 F. M. Benoit and A. G. Harrison, J. Am. Chem. Sot., 99 (1977) 3980. 14 F. M. Benoit and A. G. Harrison, Org. Mass. Spectrom., 13 (1978) 128. 15 L. Schriver, A. Louteiller, A. Burneau and J. P. Perchard, J. Mol. Struct., 95 (1982) 37. 16 G. Fraenkel, J. Chem. Phys., 33 (1961) 1466. 17 G. A. Olah, D. H. O’Brien and A. M. White, J. Am. Chem. Sot., 89 (1967) 5694. 18 C. F. Wells, J. Phys. Chem., 77 (1973) 1994. 19 D. A. Sweigert and D. W. Turner, J. Am. Chem. Sot., 94 (1972) 5592. 20 C. V. Pesheck and S. E. Buttrill Jr., J. Am. Chem. Sot., 96 (1974) 6027. 21 P. Ausloos, S. G. Lias and J. R. Eyler, Int. J. Mass. Spectrom. Ion Phys., 18 (1975) 261. 22 M. Couzi, J. Le Calve, P. Van Huongand J. Lascombe, J. Mol. Struct., 5 (1970) 3633. 23 F. Geleyn, R. Thijs and Th. Zeegers-Huyakens, Adv. Mol. Relax. Interaction Processes, 21(1981) 259.
in The Netherlands
MATRIX ISOLATION INFRARED SPECTRA OF THE COMPLEXES BETWEEN METHYLACETATE AND WATER OR HYDROCHLORIC ACID
G. MAES* and Th. ZEEGERS-HUYSKENS Department (Belgium)
of Chemistry,
University
of Leuven,
Celestijnenlaan
200F, 3030 Heverlee
(Received 17 January 1983)
ABSTRACT The hydrogen-bonded complexes between methylacetate (MeAc) and water or hydrochloric acid have been studied by infrared spectrometry in a low temperature Argon matrix. The VCQ, VC__Oand VC+ vibrational modes of MeAc show a splitting to the low and to the high frequency side of the free molecule. This suggests that water and HCl interact with the keto and ether oxygens. This conclusion is supported by the appearance of two main absorptions in the vHC1region. These results are discussed as a function of the gas-phase basicity of the two oxygen atoms, derived from the 0( 1s) core electron binding energies and from the ionization potentials. INTRODUCTION
A number of hydrogen-bonded complexes involving oxygen bases and hydrogen chloride have been investigated by the matrix isolation technique [ 1,2] but so far no experimental data are available for the interaction of esters with proton donors. Previous studies performed in solution have shown that aliphatic esters can be considered as good electron donors towards hydroxylic acids [ 3,4] ; moreover, the vcEo, vc-o and uc_c vibrations observed in an infrared [5] or Raman [6] spectrum are very sensitive to complex formation. In this work, the interaction between methylacetate (MeAc) and water or hydrochloric acid has been studied in a low-temperature Argon matrix. The effect of complexation on the mentioned internal modes of MeAc and on the Vncistretching vibration will be discussed in detail. EXPERIMENTAL
The experimental techniques and apparatus used have been described previously [ 71. Gaseous mixtures were deposited at 15 + 1 K with a rate decreasing from an initial high value to a constant smaller one after a few minutes. Matrix ratios (M/R) ranged from 600 to 300. Spectra were scanned on a P.E. 180 spectrophotometer at 12 K, while annealing was performed at 37 K for about 20 min. *Research Associate of the Belgian N.F.W.O. 0022-2860/83/$03.00
o 1983 Elsevier Science Publishers B.V.
306
HCl was a Matheson product (“Electronic Grade”) and Ar was from Air Liquide (99.999%). Methylacetate was distilled over P,O, before use. Bidistilled water was used for the water doped systems. The absorbance at 1592 cm-’ (u, HzO) was taken as a measure of the relative amount of water present in the solid mixtures. RESULTS
AND DISCUSSION
The effect of complex formation on the internal modes of MeAc and on the vHClstretching band are discussed later. Finally, the results of this work will be compared with data available in solutions and in the gas phase. In ternal modes of MeAc
In a Argon matrix, at very high dilution (matrix/MeAc = 3000) the voo, vibrations are observed at 1760,1245 and 848 cm-‘, respecand VCM tively: these bands are attributable to the MeAc monomer. At lower matrix ratios, additional bands appear at 1755,1255 and 851 cm-‘; these bands are assigned to MeAc dimers and possibly to higher polymers [ 71. e+,
Interaction
with water
When an ester molecule interacts with water or with a hydroxylic derivative in the liquid state, one usually observes a frequency lowering of the vczo vibration and a frequency increase of the vco and vc+ bands [ 5,6] ; these effects arise from an enhanced delocalization in the O=C-O group. Figure 1 shows that when water is added to a MeAc/Ar matrix (lMeAc/lO H,0/550Ar), new bands are observed to the low and high frequency sides of the free voo (Fig. la), vco (Fig. lb) and uc+ (Fig. lc) vibrations. The bands observed at 1740, 1266 and 858 cm-’ are ascribed to the formation of a complex on the carbonyl function (structure I). The frequency shifts of these three bands are somewhat lower than those ,H’
O-H
.. I
I*
‘H,
O-H
observed for the interaction between stronger acids (phenol derivatives) and the same base in carbontetrachloride solution [ 61. The bands shifted to the higher-frequency side of the free carbonyl band (1770 cm-‘) and to the lower-frequency side of the free vc. (1228 cm-‘) and vcX (840 cm-‘) absorptions are assigned to the formation of a hydrogen bond at the ether oxygen (structure II) which decreases the electronic delocalization towards the carbonyl group. This attribution is strengthened by the fact that the vczo band of complex II is narrower than in the free MeAc molecule.
307
b
P I
'i II
’
:i:'
\I 111
II
1
b
1 I
d
'1
M
1760 172i
\ 126
1240
4
v 100 I
660 I
62C1 I
Fig. 1. l/lO(H,0)/600. (a) vC.@ (a’) after annealing;(b) VC__O,(b’) after annealing; (c) VCC, (c’) after annealing. M = monomer; D = MeAc dimer; I, water complex on the carbonyl oxygen; II water complex on the ether oxygen.
The origin of the band at 1748 cm-’ is less clear; its intensity decreases upon annealing (Fig. la’) and at the same time the band at 1740 cm-’ becomes broader. The attribution of these two absorptions to complexes of 1: 1 and 1:2 (H,O) stoichiometry was ruled out because the 1748 cm-’ band increases with the water content of the matrix; moreover, this band is as narrow as in the free MeAc molecule and a frequency shift of only 12 cm-’ seems to be very low; in liquid solution, where the frequency shifts are usually lower than in a low-temperature matrix, the observed AvcGo value is about 20 cm-’ for the 1: 1 complex [5] . The existence of a complex of 2(MeAc):l stoichiometry also seems very improbable since in a very dilute matrix (MeAc/2750), the intensity of the two components at 1748 and 1740 cm-’ is about the same. Finally, the possibility of a Fermi resonance involving the ye0 and 2~~ vibrational modes can be ruled out owing to the great energy difference between the unperturbed levels. Another possibility is the assignment of the 1748 cm-’ band to a “bisolvated” complex. In this species, the vDo band will be less shifted and
303 ,0-H H3C\ H
3
17L8 ,'n*H c=o,
C-o@
?? b
III
‘\ “\O_H
will be narrower than for the I complex. The perturbation of the voo band will also be smaller than in the II complex. This interpretation is strengthened by the appearance of a second uc. band at 1232 cm-’ for a matrix containing a great excess of water (Fig. lb); this band also disappears upon annealing. It thus seems that the thermodynamically less stable III species almost disappears during the annealing stage of matrix warm-up which is in agreement with the theoretical predictions [8] . The existence of this species, although less stable than complexes I and II, is possible because, even in the stronger interaction (with the sodium ion for example) the charge on the ether oxygen remains negative [ 91. On annealing a second band appears at 1711 cm-’ (Fig. la’) and a very strong asymmetry of the vca band with a shoulder at about 1290 cm-’ (Fig. lb’) is observed. These new absorptions are probably attributable to complexes of 1:2(H,O) or l:n(H*O) stoichiometry; in the liquid state, the frequency shift of the 1:2 complex is about twice that of the 1: 1 complex
r51. Interaction with HCL As expected, interaction with HCl brings about a stronger perturbation of the internal modes of MeAc. Some control of the species present in the matrix is obtained by varying the relative concentrations of base and acid (MeAc/HCl/matrix = 1:1/300,1:2/300 and 1:1/600). The spectra obtained in the vczo, vc, and vrange, at these different matrix ratios, are reproduced in Figs. 2,3 and 4, respectively. As can be seen from Fig. 2, new absorptions at the low (1726,1703 cm-‘) and high frequency sides (1776 cm-‘) of the free vcco band are observed, strongly suggesting, as in the case of water, the formation of carbonyl (IV) and ether (V) complexes. The two bands observed at 1726 and 1703 cm” cannot be ascribed to complexes of 1:l and 1:2(HCl) stoichiometry since the band at 1726 cm-’ is characterized by a higher intensity when the matrix ratio is l/2/300 (Fig. 2b). On annealing, the 1703 cm-’ band attributed to a complex of 1: 1 stoichiometry becomes asymmetric and shows a shoulder at 1690 cm-’ ascribed to 1:2 complexes and possibly to higher complexes. This attribution is reinforced by the fact that at higher matrix dilution (Fig. 2c), the band at 1703 cm-’ is approximately symmetric. The origin of the 1726 cm-’ band, which in some cases is split into a doublet, is more questionable. The assignment of this band to a bisolvated complex seems less adequate because the relative concentrations of HCl are much smaller (1: 1 and 1:2) than in the case of the interaction with water (1: 10); moreover, there was no indication of a further splitting on the low-frequency side of the ZJ~~ band (Fig. 3).
309
ir a
-
d
a’
Fig. 2. vCzOrange. (a) l/1/300, l/1/600, (c’) after annealing.
b
C’
(a’) after annealing; (b) l/2/300,
(b’) after annealing;(c)
In solution, the ~c.~ absorption of methylic esters involved in hydrogen bonds with hydroxylic acids shows a band splitting attributed to a Fermi resonance between the vDo and 2v,c levels [6, lo]. In the present case, the uc+ vibration is observed at 867 cm-’ and an interaction between its first overtone (- 1730 cm-‘) and the vDo vibration cannot be completely excluded. A Fermi resonance mechanism, however, cannot explain the variation of the relative intensity of the 1726 and 1703 cm-’ bands with the composition of the matrix; further, this mechanism cannot account for the strong decrease of the 1726 cm-’ band upon annealing. We have observed that the intensity of this band strongly depends on the water content of the matrix, roughly estimated from the absorbance of the water band at 1592 cm-‘. This absorbance is 0.03,0.08 and 0.12 for the spectra of Figs. 2a, 2b and 2c, respectively. This suggests that the 1726 cm-’ band can be reasonably ascribed to the interaction of the carbonyl group with the H,O*HCl complex (structure VI); in this last species, the proton donor ability of the
310
1
I 1210
100 a
1 100
I 12&O b
IV 1
VI
a’ C’
\
4 Fig. 3. YC-O range. (a) l/1/300, l/1/600, (c’) after annealing.
(a’) after annealing;(b)
l/2/300,
(b’) after annealing;(c)
hydrogen atom is enhanced by complex formation with an acid [ll] and so the frequency shift Av,., and the bandwidth will be higher than in the case of the interaction with water, but lower than for the HCl complex. This assignment is further confirmed by the study of the ~no bands. In the uoo region (Fig. 3a, b, c) the band at 1224 cm-’ is assigned to the ether complex; the bands lying at the high-frequency side of the free voo absorption are ascribed to the carbonyl complexes. The intensity of the less shifted band at 1273 cm-’ increases with the water concentration and decreases upon annealing; this absorption was therefore assigned to the complex with H20*HCl. The bands at 1288 cm-l and 1300 cm-’ whose intensity varies in the same way as the 1703 and 1690 cm-’ absorption, were ascribed to the 1: 1 and 1:2 species. In the vc_+ region, bands at 867 and 836 cm-’ (Fig. 4a, b) are ascribed to the carbonyl and ether complexes, respectively. The assignments of this work are summarized as follows:
311 H .H’
HC
,HCI
H3C,
3 \
4$=
HCI
A,“ ’ .
1726 0"
0,
4% V
IV
‘HCI
VI
vHa region As indicated in Fig. 5, several absorptions are observed in this range. Besides the band at 2663 cme2 previously assigned to the H20*HCl complex [ 121, two broad absorption bands at 2560 cm-’ (Av~,~= 70 cm-‘) and 2450-2460 cm-’ (Av~,~= 250 cm-‘) are observed. Owing to the greater electronic density on the carbonyl oxygen (-0.329) than on the ether oxygen (-0.229) [9], it is expected that in complex IV more charge will be
300
660
620
900
Fig. 4. VC-C range. (a) l/1/300,
88O’jj 820 (a’) after annealing; (b) l/1/300,
(b’) after annealing.
312 2900
2600
2700
Fig. 5. VHC~range. (a) l/2/300,
2600
2500
2400
2300
(a’) after annealing.
transferred to the HCl molecule; as a consequence, the frequency shift of the vHCiband and its intensity increase should be higher for the carbonyl than for the ether complexes. The assignment of the bands at 2450 and 2560 cm-’ to the complexes IV and V is further supported by considering the proton affinity (PA) of the two oxygen atoms, deduced from the O(ls) core electron binding energies, respectively 537.9 and 539 eV [ 13,141. The PA values are indicated in Table 1 along with the experimental uHCl values relative to other oxygen complexes. From Fig. 6, where VHCl has been plotted against the PA, it appears that the two experimental vnQ values of this work fit, within reason, the straight line when taking for the bands at 2568 and 2450 cm-’ corresponding proton TABLE 1 v~,-J values in complexes of oxygen bases and corresponding PA of the base Base Hz0 CH,OH C,H,OH (C,H, )zO (CH, ),CO (CH,),CHOH (CH, ),COH CH,COOCH,
aFrom ref. 1. bFrom ref. 13.
VHCl(Cm-l)a
PA (eV)b
2664 2525 2504 2415 2392 2465 2462 2450 2568
7.32 7.91 8.10 8.59 8.44 8.25 a.34 8.47 (doubly-bonded oxygen) 7.81 (singly bonded oxygen)
313
affinities of 7.81 and 8.47 eV, respectively. (The dimethylether.HCl complex shows unusual spectroscopic properties [ 151 and is not taken into account in Fig. 6.) General support for the assignments of the present work comes from Fig. 7, which shows that the vHClvalues of the free molecule, of the water complex and of the carbonyl complexes of 1:l and 1:2 stoichiometry are related to the corresponding vczo or vca values.
8.0 PAleVI
Fig. 6. vHcl as a function of the PA of the oxygen bases.
3000
2800
2600
2400
2200
I 1690
I
I 1710
I
I 1730
I
I 1750
12L.5
y.=Okm')
Fig. 7. vHClasafunctionof 4, HCl (1:2 complex).
vc=o and VC__OI1, freeMeAc;2,
1265
1265
1305
VC_Okm')
H,O*HCI;3, HCI (1:l complex);
314
Hydrogen
bond and protonation
site in esters
In solution, where the equilibrium conditions are reached, hydrogen bonding [ 3,6] and protonation [16-181 clearly takes place on the carbonyl function. Gas-phase measurements have shown that the ether oxygen proton affinity is lower than that of the doubly-bonded oxygen. The ionization potentials of the two oxygen atoms of MeAc are 10.48 eV (C=O) and 11.16 eV (O-CH3), respectively [ 191. Thus, as discussed by Benoit and Harrison [ 131, under conditions of thermodynamic control, protonation at the carbony1 will be favoured. (It must be pointed out, however, that from the principal fragments ions obtained in ion cyclotron resonance spectroscopy measurements, the esters appear to protonate at the ether oxygen [ 20, 211.) Nevertheless the calculated electronic density on the two oxygen atoms, the values of the ionization potential of the O(ls) core electron binding energies and of the proton affinities suggest that the basicity of the carbonyl and ether oxygens does not differ too much. In this sense, it seems reasonable to admit that in a low-temperature matrix where the conditions of thermodynamic control are not reached, hydrogen-bonded attachment, considered as the first step of protonation, will occur on the two basic sites of the ester molecule. Structure
of the complexes
As discussed in this work, the voco band shows at high HCl content a band at 1690 cm-’ attributed to the 1:2 species. The frequency shift from the free molecule (Av = 70 cm-‘) is much less than twice the shift of the 1:l complex (Av = 57 cm-‘). This suggests that in the 1:2 adduct, the two HCl molecules are bonded together by a hydrogen bond. The same conclusion can be drawn from study of the unQ absorptions where a broad band at the low frequency side of the 2460 cm-’ absorption was observed in excess of HCl and upon annealing. This structure agrees with that reported for the acetone*HF complex in the gas phase [22] and with that of the cyclohexanone*HBr complex in solution [ 231. However, in the case of the dimethylether complex, increasing the acid concentration brings about a frequency shift of the vHClband to higher frequencies; this clearly indicates the bifurcated structure. The structure of the 1:2 adducts seems to be different in ether and carbonyl bases; the origin of this difference is not very well understood. ACKNOWLEDGEMENT
The authors are indebted to the University of Leuven and to the Belgian N.F.W.O. for financial support.
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