Jürgen Bernardab,
Eva-Maria Köckab,
Roland G. Huber†
c,
Klaus R. Liedlc,
Ludwig Calld,
Robert Schlöglb,
Hinrich Grothe
e and
Thomas Loerting
*a
aInstitute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria. E-mail: thomas.loerting@uibk.ac.at
bMax-Planck-Institut für chemische Energiekonversion, D-45470 Mülheim an der Ruhr, Germany
cInstitute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria
dInstitute of Organic Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria
eInstitute of Materials Chemistry, TU Wien, A-1060 Vienna, Austria
First published on 21st April 2017
The monoesters of carbonic acid are deemed to be unstable and decompose to alcohol and carbon dioxide. In spite of this, we here report the isolation of the elusive carbonic acid monoethyl ester (CAEE) as a pure solid from ethanolic solutions of potassium bicarbonate. The hemiester is surprisingly stable in acidic solution and does not experience hydrolysis to carbonic acid. Furthermore, it is also stable in the gas phase, which we demonstrate by subliming the hemiester without decomposition. This could not be achieved in the past for any hemiester of carbonic acid. In the gas phase the hemiester experiences conformational isomerism at 210 K. Interestingly, the thermodynamically favored conformation is only reached for the torsional movement of the terminal ethyl group, but not the terminal hydrogen atom on the millisecond time scale. Accordingly, IR spectra of the gas phase trapped in an argon matrix are best explained on the basis of a 5:
1 mixture of monomeric conformers. Our findings necessitate reevaluation of claims of the formation of a carbonic acid polymorph in methanolic solution, which is the subject of a forthcoming publication.
While the occurrence in solution as intermediates seems to be established now, isolation of carbonic acid monoalkyl esters in pure solid form has remained a challenge. In their review Shaikh and Sivaram specify that “Hemicarbonic acid can only be isolated as salts (with inorganic metal such as sodium and potassium), simple anhydrides, or mixed anhydrides with carboxylic acids.”.10 We want to emphasize that the term “hemicarbonic acid” is a short, but incorrect form of what should be named more precisely “hemiesters of carbonic acid”. Furthermore we want to emphasize that there is in fact one single report of a successful isolation of a pure hemiester of carbonic acid, namely CAME (HO2COCH3) by Behrendt et al. in the 1970s.11,12 This was achieved via synthesis of alkali metal monoalkyl carbonates M[O2COR],13 following the protocol of Dumas and Peligot established in 1840.14 The protonated form could then be formed by interaction between a suspension of the salt Na[O2COCH3] in dimethyl ether and a solution of HCl in dimethyl ether under cryoconditions, namely at 223 K.11,12 After distilling off dimethyl ether at 193 K, CAME could be isolated as a pure, colourless solid of melting point 237 K. Its IR spectrum and acid dissociation constant pKa were also determined by Behrendt et al.11,12 The IR spectrum led them to conclude that CAME exists as a monomer. Furthermore it is instable in contact with water, so the technique for its formation requires protonation in non-aqueous solution.
To the best of our knowledge, no hemiesters of carbonic acids other than CAME could be isolated so far, neither in the gas phase nor as a pure solid. In this study we demonstrate that CAEE (HO2COC2H5) can be prepared as a pure solid using a cryotechnique and characterized by FT-IR spectroscopy, and furthermore that this solid evaporates without decomposition at 210 K by trapping the vapor above the solid in an argon matrix at 6 K.
After deposition of the glassy bicarbonate solution layers the cryoplate was heated in vacuo to 290 K, which results in evaporation of the solvent and a solid precipitate remaining on the cryoplate. This step of evaporating the solvent from the dissolved salt was not part of the technique employed by Hage, Hallbrucker and Mayer for preparing carbonic acid polymorphs from either aqueous or methanolic solution.15,16 Thus, comparison of our results with the results obtained by them is only possible to a limited extent. Upon dissolving in ethanol the bicarbonate may transform to the monoethyl carbonate, and upon evaporating ethanol the reaction may be reversed to form the bicarbonate again. The IR spectrum of the precipitate shown in Fig. 1a provides evidence for the precipitate to be the monoethyl carbonate. Bands indicating presence of the ethyl group are marked in red. That is, by evaporating the solvent at cryoconditions the back-reaction to bicarbonate is much slower than the evaporation of the solvent, i.e., the reaction product is kinetically controlled. This precipitate was treated with 1.5 M aqueous HCl at 180 K to afford CAEE, first in solution and after heating and removal of water as a pure solid. The reaction progress was monitored in situ by FT-IR spectroscopy using a Varian Excalibur 3100, in which the beam of light passes through optical discs (KBr) into the vacuum chamber, through the thin film sample and out of the vacuum chamber to the detector. FT-IR spectra were recorded with a resolution of 4 cm−1 and by accumulating 100 scans. Fig. 1b shows the IR spectrum of the solid assigned to be CAEE.
A two-dimensional relaxed PES scan was performed around the torsions A:4-3-2-1 and B:4-3-5-6 simultaneously (see Fig. 5). Torsion A was sampled for 345 degrees in 15 degree intervals and torsion B was sampled from 0 to 180 degrees in 15 degree intervals for a total of 312 points. Torsions A and B were frozen at each point and a geometry optimization at the MP2/aug-cc-pVTZ level was performed. The resulting energies were plotted as a two-dimensional map and the key minima, maxima and saddle points were labeled with their respective energies (Fig. 6). In contrast with Fig. 5, PES energies do not include zero-point corrections as this would require a frequency calculation at each point. The ethyl torsion 3-5-6-9 was unrestrained for the PES scan.
Zero-point energies were calculated, but did not significantly alter the energy landscape without entertaining a zero-point energy correction. Specifically, the zero-point energy correction does not change the stabilities by more than ±0.1 kcal mol−1.
The formation of CAEE was effected from an ethanolic solution of potassium bicarbonate (KHCO3, Sigma Aldrich, >99.5%). Subsequent sample preparation/isolation was performed with a hyperquenching method directly in the cryo-FT-IR vacuum chamber (more details see in the Experimental section). Residual solvent was evaporated and pumped off inside the chamber, which results in the pure solid of first the salt of CAEE and second – after protonation – the hemiester itself. With the cryo-FT-IR setup we can directly control this preparation procedure. In addition the resulting solid can be transferred to another laboratory, which we did to gather high resolution IR-spectra of single molecules trapped in an inert matrix.
We obtain CAEE purely in both the solid and the gaseous state since we are able to sublime the solid without appreciable decomposition at cryoconditions in the vacuum. CAEE was previously neither known in crystalline form nor as gas phase species. For CAME either the solid state11,12 or the gaseous state (in a mixture with isobutene)9 were accessible. Thus, no inferences about their stability and phase transitions could be made, which we now establish in our manuscript. This provides us with the unique opportunity to investigate the conformational isomerism of this molecule at low temperatures, close to the ground state. So far, the knowledge about conformational isomerism in carbonic acid hemiesters is solely based on the high-temperature pyrolysis experiment by Reisenauer et al., who have established two conformational isomers for CAME in the gas-phase at a pyrolysis temperature of about 1000 K.9
Lit.13 | Expt., Fig. 1a | Assign.13 |
---|---|---|
a vw, very weak; w, weak; m, medium, s, strong; vs, very strong; sh, shoulder, νs and νas, symmetric and asymmetric stretching mode; δs and δas, symmetric and asymmetric bending in-plane; δoop, out of plane bending mode; ω, wagging mode. | ||
2967 (s); 2907 (m) | 2978 (m); 2943 (w); 2907 (w) | ν(CH) |
1689 (s) | 1659 (vs) | νas(CO2) |
1484 (w); 1445 (w); 1389 (m); 1370 (s) | 1447 (sh); 1400 (m); 1371 (m) | δ(CH) |
1295 (s) | 1304 (s) | νs(CO2) |
1274 (m) | δs(CH3) | |
1172 (w); 1114 (m); 1075 (sh) | 1165 (vw) | ν(CO) |
1059 (s) | 1074 (s) | ν(CO) |
966 (s) | 980 (m) | ν(CC) |
876 (s) | 878 (m) | ω(CH2) |
821 (s); 812 (w) | 829 (m) | δoop(CO3) |
701 (w) | 710 (vw) | δas(CO2) |
584 (w) | 579 (w) | δs(CO2) |
Fig. 3 compares the spectra of the new species (bottom red trace, rescaled from Fig. 1b) obtained here with β-H2CO3 (top black trace) as taken from literature,27 and Table 2 presents assignments to CAEE. Apparently, the new species obtained in the present work shows more IR active bands than β-H2CO3. Thus, some bands cannot satisfactorily be explained on the basis of a pure solid of carbonic acid H2CO3. In particular, there is no explanation for the bands at 1466, 1310, 1163, 1121 and 1009 cm−1 if one assumed the new solid species to be a polymorph of carbonic acid, in analogy to the assignments done earlier by Hage, Hallbrucker and Mayer.16,28 However, assuming the new species to be CAEE (HO2COC2H5), these frequencies can easily be explained on the basis of stretching and deformation modes of the ethyl group. On this basis, the band at 1466 cm−1 is assigned to δ(CH), at 1310 cm−1 to δs(CH3) and at 1009 cm−1 to ν(CC), cf. Table 2. The band at 1163 cm−1 and 1121 cm−1 can be assigned as ν(CO) modes of the ethyl ester. Also the bands at 2994 and 2909 cm−1 can be explained as CH-stretching modes rather than as combination bands. This implies that only the protonation (step 1 in Fig. 2), but not the hydrolysis of the hemiester (steps 2–4 in Fig. 2) take place at cryoconditions in acidic solution.
β-H2CO3 | CAEE | CAEE |
---|---|---|
Lit.28 | Expt., Fig. 1b | Assign. |
a νs and νas, symmetric and asymmetric stretching mode; δip and δoop, in-plane and out-of-plane bending mode. | ||
3030 | 2994 | ν(CH) |
2969 | 2909 | ν(CH) or 2 × νas[EtOCOH] |
2830 | 2723 | νas[EtOCOH] + δip(COH) or 2 × νas[EtOCOH] + 2 × δip(CO3) |
2652 | ||
2613 | 2569 | 2 × δip(COH) |
1698 | 1730 | ν(C![]() |
1504 | 1487 | νas[EtOCOH] |
1466 | δ(CH) | |
1302 | 1379 | δip(COH) |
1310 | δs(CH3) | |
1163; 1121 | ν(CO) | |
1038 | 1082 | νs[EtOCOH] |
1009 | ν(CC) | |
910; 881 | 928; 901 | δoop(COH) |
813 | 800 | δoop(CO3) |
686; 664 | 583 | δip(CO3) |
The crystal structure of the new species CAEE is not known, and could also not be determined in this work because the thin film geometry did not allow for a good quality X-ray diffractogram to be recorded. Hence, the intermolecular coupling between individual molecules is not entirely clear, and unexpected shifts of bands could occur, e.g., if CAEE formed dimers in the solid state. For this reason the assignment of the unexplained bands in Table 2 to the vibrational modes of the ethoxy-group does not represent a rigorous proof of the isolated solid to be CAEE. The comparison merely shows that the assignment is more complete and better than based on solid carbonic acid. One way of improving the reliability of assignments would be isotope substitution, e.g., repeating the experiment using deuterated or 13C reagents. However, the low solubility of bicarbonates in ethanol and the rather large amount of solvent required to form a thin film solid did not allow us to successfully repeat the experiment using ethanol isotopically labeled at the ethyl group (e.g., ethanol-d2) as the solvent.
In order to sort out the question of the nature of the new solid species we rely on matrix isolation, i.e., we evaporated the solid and investigated the gas phase above by trapping it in a frozen noble gas matrix. Since IR spectroscopy of this matrix is a highly sensitive technique it allows us to carefully assess the nature, including conformational isomerism, and purity of our products.
![]() | ||
Fig. 4 (a) Matrix isolation spectrum of the new species CAEE in argon matrix. The calculated line spectrum assuming a 5![]() ![]() |
Major differences between the two sets of spectra are found for the C–H stretching region, which is featureless for carbonic acid, but which contains several bands for CAEE. This region is complicated by the presence of anti- and gauche-ethanol in the matrix,29 which reflects either unreacted ethanol or ethanol as decomposition product from CAEE. Ethanol bands are marked by stars, and some of them overlap with the bands assigned to the ethoxy group of CAEE. However, the four most intense bands of the most stable isomer (structure I in Fig. 5) can be identified, and are labelled in Fig. 4a. Even though the scaled calculated frequencies do not match the experimental observations, the calculated shifts between the four individual bands are exactly reproduced in the experiment, i.e., the pattern matches. It is also clear that the labelled bands for the other spectral regions (Fig. 4a) do not coincide with carbonic acid bands (Fig. 4b) observed in the matrix isolation experiment, e.g., the most intense band is shifted by about +50 cm−1 to 1177 cm−1, and the CO stretching band by −18 cm−1 to 1774 cm−1. These shifts are so large that carbonic acid monomers cannot account for the observations. Similarly, the carbonic acid dimer20 is not suitable at all to explain the bands. Our suggestion of CAEE to explain the observations is based on a comparison with calculated bands for the six isomers shown in Fig. 5, which are energy minima in gas phase calculations at MP2/aug-cc-pVTZ level of theory.
Cis–cis H2CO3 | HOCO2C2H5 | ||||
---|---|---|---|---|---|
Theo. | Assign. | Expt., Fig. 4a | Structure I | Structure II | Assign. |
Theory | Theory | ||||
3726 (15) | νs(OH) | ||||
3725 (206) | νas(OH) | 3611 | 3722 (111) | 3722 (110) | ν(OH) |
1797 (516) | ν(C![]() |
1774 | 1775 (410) | 1773 (377) | ν(C![]() |
1438 (144) | νas(C(OH)2) | 1419 | 1419 (68) | 1413 (93) | ω(CH2), δip(COH), δs(CH3) |
1389 | 1394 (108) | 1389 (85) | δs(CH3), δip(COH) | ||
1263 (28) | δip(COH) | 1360 | 1357 (160) | 1359 (125) | ω(CH2), δip(COH) |
1143 (434) | ν(O–C–O) | 1181 | 1190 (537) | 1190 (514) | δip(COH) |
963 (19) | δ(CO3) | 887 | 872 (5) | 865 (5) | δas(CH3), δip(COC) |
786 (41) | δoop(COH) | 793 | 785 (26) | 785 (25) | δoop(CO3), r(CH2) |
HOCO2C2H5 | |||||
---|---|---|---|---|---|
Expt., Fig. 4a | Structure III | Structure IV | Structure V | Structure VI | Assign. |
Theory | Theory | Theory | Theory | ||
3614 | 3724 (88) | 3722 (93) | 3722 (107) | 3752 (69) | ν(OH) |
1821 | 1821 (376) | 1819 (343) | 1800 (626) | 1831 (567) | ν(C![]() |
1414 | 1410 (22) | 1402 (21) | 1409 (8) | 1404 (11) | ω(CH2), δip(COH), δs(CH3) |
1384 | 1383 (57) | 1381 (53) | 1388 (52) | 1382 (11) | δs(CH3), δip(COH) |
1309 | 1312 (639) | 1320 (281); 1311 (334) | 1346 (204) | 1297 (532) | ω(CH2), δip(COH) |
1177 | 1185 (121) | 1185 (114) | 1172 (317) | 1171 (56) | δip(COH) |
868 (26) | 862 (29) | 848 (1) | 845 (7) | δas(CH3), δip(COC) | |
784 | 775 (18) | 775 (8) | 774 (30) | 762 (15) | δoop(CO3), r(CH2) |
Comparison of the new spectrum (Fig. 4a) with calculated frequencies and intensities yields the same result as the comparison between the two experimental spectra in Fig. 4 does. It becomes immediately clear that carbonic acid does not satisfactorily explain the observed matrix spectrum. The matrix isolation spectrum of a mixture of carbonic acid monomers and the cyclic dimer was reported in ref. 20. None of the carbonic acid monomeric conformers nor the dimer explain the spectrum in Fig. 4a. Most notably, the rather strong band at 1389 cm−1 cannot be explained, the band at 1360 cm−1 can only be matched if a shift of about 100 cm−1 is accepted. Similarly, the weak band at 887 cm−1 cannot be explained unless a shift of 75 cm−1 is accepted. The calculated CO stretching vibration is also shifted by about 25 cm−1, whereas in earlier work we found agreement between calculated gas phase and measured matrix bands of better than 5 cm−1.20
By contrast to carbonic acid, there is an excellent match for the CAEE conformers shown in Fig. 5. In terms of calculated IR spectra structures I and II are very similar (see Table 3) so that it seems out of practical reach to distinguish them experimentally. Furthermore, also structures III and IV show practically identical IR spectra, so that they also cannot be distinguished in experiments. We do not find evidence for structures V and VI in the matrix spectrum (see Fig. 4a). This is expected for structure VI because it is disfavored by 10.2 kcal mol−1. The best match between observed and calculated spectrum is found by assuming a 5:
1 mixture of structures I/II and III/IV. The matching is demonstrated in Fig. 4a by showing the calculated line spectrum of such a mixture. In the frequency range up to 1800 cm−1 the calculated band positions and intensities match in an excellent way with the observed bands. Above 2900 cm−1 the frequency scaling does not reproduce exact band positions especially since hydrogen bonding is involved, as known from many previous studies about frequency scaling. However, the calculated patterns still show a good match. In particular there is an excellent match between calculated band splitting and experimentally observed splitting. However, this region is also affected by water and ethanol in the matrix.
To make the case for the 5:
1 mixture, Table 3 is split in two parts, where the top part lists the matrix bands assigned to structure I/II and the bottom part lists the bands assigned to structures III/IV. As explained below, this reproduces the measured spectrum in an excellent way.
All calculated bands (see Fig. 4, lines) are actually observed in the spectrum, and all of them agree to better than 10 cm−1. The location of the CO stretching band is even matched to an accuracy of 1 cm−1. The only exception is the OH-stretching band, which is well known to be overestimated.20 Furthermore, the calculated intensity pattern also matches the experimentally observed pattern well. The bands at 1774 and 1181/1177 cm−1 are the most intense ones both in calculation and in experiment, and the bands at 3614/3611, 1419 and 1389 cm−1 are about 3–7 times less intense than the former two ones. Structure I and II match the experimental spectrum well. Both of them show a weak interaction between the OH-group as well as the OC2H5-group with the C
O bond. Whereas in structure I the stabilization arises from interaction of the methylene group with the C
O group, it is from the methyl group in structure II. The former is calculated to be imperceptibly more stable, namely by 0.1 kcal mol−1 according to MP2/aug-cc-pVTZ.
Structures III–VI in Fig. 5 are characterized by the fact that there is only one weak intramolecular bond to the CO group, either from the OH-terminus (structure V) or from the OC2H5-terminus (structures III and IV) or none at all (structure VI). Therefore, the latter is the least stable one, compared to structure I disfavored by 10.2 kcal mol−1. The other candidates are disfavored by only about 1.3–3.2 kcal mol−1, and therefore also likely to be observed in the vapor trapped at 210 K. Inspection of Table 3 reveals that the best marker band to discriminate the conformational isomers is the C
O stretching band, which is shifted by +30 to 50 cm−1 in structures III–VI compared to structures I and II. Indeed we find a band at 1821 cm−1 of about 15–20% of the intensity of the band at 1774 cm−1. This exactly and hence best matches the position calculated for the very similar structures III and IV. In these structures there is also a shift of about −10 cm−1 compared to the band at 792 cm−1 observed in experiment and assigned here to structures I/II. Indeed a band of about 15–20% of the intensity is found at 782 cm−1, thereby suggesting that the matrix spectrum can be rationalized on the basis of a 5
:
1 mixture of structures I/II and structures III/IV. Structure VI can be ruled out due to the absence of a band near 762 cm−1 or 1292 cm−1, structure V can be ruled out based on the absence of the relatively strong band predicted at 1346 cm−1. By contrast there are bands in the experimental spectrum near 1312 cm−1, in accordance with the idea of 15–20% of structures III/IV present in the gas phase mixture. Assuming this type of a mixture of monomeric conformers we are able to explain extremely well all bands observed experimentally both in terms of intensity and in terms of position. The only bands we do not explain are the very weak band at 1710–1760 cm−1 and the bands at 1090–1120 cm−1, which are less than 10% of the intensity of the most intense bands. Fig. 4a shows the calculated line spectrum assuming a 5
:
1 mixture of structures I and III in the gas phase (see Fig. 5), and can be directly compared to the experimental spectrum in argon matrix. The very good agreement between the two demonstrates that we have isolated CAEE both in its pure solid form and after sublimation in Ar matrix at a purity of >99%. In the gas-phase we see evidence for a 5
:
1 ratio of two conformers, but no evidence for dimers, by contrast to the case of carbonic acid.20 There is no evidence for the formation of a carbonic acid polymorph after dissolving bicarbonate in ethanolic solution, evaporating the solvent at cryo-conditions in the vacuum and protonation.
In order to shed light on this question we have studied the kinetics of the isomerization process and compare the time scales for switching between isomers with the time scale the CAEE molecules spend in the gas phase. At the temperature of 210 K the velocity v of CAEE molecules can be calculated from kinetic gas theory. Using v = (2 × RT/M)1/2 and the molecular weight for CAEE (M = 90 × 10−3 kg mol−1) the most likely velocity in the Boltzmann distribution of velocities is v = 19600 cm s−1. The distance between the surface of the thin film and the mirror, on which the matrix freezes, is a few centimeters. Thus, the CAEE molecules take about 0.5 ms between desorption and landing in the matrix.
The time scale for isomerization can be estimated from the potential energy landscape shown in Fig. 6. In this 2D landscape the stable structures I, III, V and VI are indicated based on two dihedral angles. Please note that structures II and IV are very close to structures I and III, respectively, but not found on this landscape because these structures are not encountered in the scans.
If a molecule desorbs from the surface it may be in an unfavorable conformational state because the crystal field might force molecules into such orientations. Thus, it is necessary to know how long it takes for them to convert to the most stable isomer. For structure V a barrier of 7.06 kcal mol−1 (see Fig. 6, 10.39–3.33 kcal mol−1) needs to be overcome in order to convert to structure I. This represents the torsional movement of the ethyl group, specifically from 3–5 s-cis to 3–5 s-trans conformation. Based on Eyring theory and the quasi-equilibrium assumption of transition state theory this barrier can be converted to a reaction rate constant k using the equation k = kBT/h × exp(−ΔG′/RT), using Planck's constant h, the Boltzmann constant kB and the barrier ΔG′. This equation assumes the absence of quantum tunneling and corner cutting effects on the rate constant. This seems plausible for a gas-phase temperature of 210 K, whereas in the matrix itself at much lower temperatures of 10 K tunneling may play an important role.33 The half-life against isomerization τ can then be calculated from τ = ln2/k. Approximating ΔG′ with the energy difference, an energy barrier of 7.06 kcal mol−1 results in the half-life of isomer V, τ(V → I) = 9 μs. This time is shorter than the residence time in the gas-phase by about a factor of 50. In other words, 50 half-lives have passed before the molecule lands in the matrix. Accordingly, all of the isomer V molecules initially sublimed to the gas phase have already converted to isomer I before landing. For comparison, the half-life of isomer III towards isomer I is much longer, τ(III → I) = 600 μs based on the higher barrier of 8.72 kcal mol−1. In this case the residence time in the gas-phase is similar to the half-life. That is, only half of the isomer III molecules transform to isomer I. Isomer VI very rapidly transforms to isomer III, since the barrier only amounts to 2.15 kcal mol−1: its half-life is about 0.1 μs, and so this isomer cannot persist long enough in the gas-phase to be detectable in the matrix.
As a result of these considerations, half of the isomer III that has sublimed from the thin film or that has emerged from the isomer VI → isomer III conversion will persist and enter the matrix. By contrast, isomer V and isomer VI do not survive for 0.5 milliseconds, which would be required to reach the matrix. In other words, the flipping of the terminal H-atom is too slow for the conformer population to reach thermodynamic equilibrium in 0.5 milliseconds, whereas the flipping of the terminal ethyl-group is fast enough. As a result, the matrix contains only molecules, for which the ethyl group is on the same side as the CO group, whereas the terminal H-atom can be on either side of the C
O group. That is, only the ester group of CAEE is under thermodynamic control, whereas the acid group of CAEE is not fully equilibrated. For this reason, much more of structure III, about 15% of the mixture, is observed experimentally, as compared to complete thermodynamic control, which would result in only about 4.4% of isomer III in the matrix.
The spectrum obtained after treatment of potassium monoethyl carbonate with acid is shown in Fig. 1b. This spectrum is not identical to either the spectrum of crystalline β-H2CO3 (see Fig. 3), or to the spectrum of its amorphous precursor.27 Therefore, the alkyl group remains covalently linked after protonation at cryoconditions rather than being cleaved by hydrolysis. In other words, steps 2–4 in Fig. 2 are too slow to be of practical importance at cryoconditions. That is, the hydrolysis step does not take place, which would transform the ester into H2CO3. In this case, CAEE (HOCOOC2H5) rather than H2CO3 is the final product obtained after treatment with HCl and removal of solvent at ∼210 K. From the much better qualitative match of the assignments we conclude that the observed solid state spectrum indicates that in fact we were able to isolate pure CAEE.
In order to answer the question whether we have indeed isolated pure solid CAEE with high confidence we have here undertaken a matrix isolation study of the thin solid film, which avoids the ionization step and instead lands the neutral gas phase molecules in an inert matrix. The IR-spectrum of the vapor collected in vacuum at 210 K and deposited together with solid argon can be explained in an excellent manner assuming the presence of conformers of CAEE, whereas there is no match with carbonic acid matrix spectra reported earlier.20 If carbonic acid is present at all, it is an impurity of <1%. We suggest a 5:
1 mixture of the conformers I/II and III/IV to be present in the gas phase above the solid compound from the comparison with spectra calculated at MP2/aug-cc-pVTZ level of theory. Again, isotope substitution would be desirable to back up the interpretation, but is very hard to be realized in practice.
One interesting aspect of the conformational isomerism in CAEE is that according to the thermodynamic stabilities we would expect a matrix that is much more depleted of conformer III, namely at a ratio of 22:
1 rather than the experimental observation of 5
:
1. We explain the difference based on the energy barriers for conformational isomerism depicted in Fig. 6. These barriers are low enough for isomer VI to convert to isomer III and for isomer V to convert to the most stable isomer I within 0.5 ms, which is the time each CAEE molecule resides in the gas phase before entering the matrix. However, the barrier for the conversion of isomer III to isomer I is high enough so that only part of the isomer III molecules transform to isomer I, thereby avoiding thermodynamic equilibrium. In turn, this implies that a significant fraction of the molecules subliming from the thin solid CAEE film are initially in the unfavorable structure III (or structure VI, which rapidly transforms to structure III).
We conclude that the evidence we provide here is compelling and clearly proves that we have isolated CAEE in its pure form and evaporated it without significant decomposition in the vacuum, in spite of the lack of further desirable experiments. This defies claims of the impossibility to isolate the pure solid and demonstrates how powerful cryotechniques can be in isolating compounds, which are deemed to be instable.1,10
Footnote |
† Present address: Bioinformatics Institute (BII), Agency for Science, Technology and Research (A*STAR), Matrix 07-01, 30 Biopolis Street, 138671 Singapore. |
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