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G. Bouchoux, F. Penaud-Berruyer and W. Bertrand, Eur. J. Mass Spectrom. 7, 351–357 (2001)

351

Structure, Thermochemistry and Reactivity of Protonated Glycolaldehyde G. Bouchoux, F. Penaud-Berruyer and W. Bertrand, Eur. J. Mass Spectrom. 7, 351–357 (2001)

Structure, thermochemistry and reactivity of protonated glycolaldehydea

Guy Bouchoux Département de Chimie, Laboratoire des Mécanismes Réactionnels, UMR CNRS 7651, Ecole Polytechnique, 91128 Palaiseau cedex, France

Florence Penaud-Berruyer Laboratoire de Chimie Physique, Groupe de Chimie Théorique, UMR CNRS 8000, Bâtiment 490, Université Paris Sud, 91405 Orsay cedex, France

William Bertrand Laboratoire d’Etudes de Traces Organiques, Institut Pasteur de Lille, CNRSSP, 930 boulevard Lahure, 59505 Douai cedex, France

+

Structures and relative energies of various conformers of the simplest sugar, glycolaldehyde, 1, and its protonated form, [1H] , were investigated by ab initio molecular orbital calculations. The 298 K heats of formation of the most stable conformers, deduced from the atomization energies at the G2 level, are equal to ∆fH°(1) = –324.8 kJ mol–1 and ∆fH°[1H]+ = 426.0 kJ mol–1. The corresponding proton affinity value is PA(1) = 779.8 kJ mol–1, in perfect agreement with the experimental determination of 783.3 ± 3.8 kJ mol–1 obtained by the kinetic method. A gas-phase basicity value, GB(1), of 745–748 kJ mol–1 is also deduced from theory and experiment. The exclusive + + dissociation channel of protonated glycolaldehyde, [1H] , is water loss which leads essentially to the acylium ion [CH3CO] . The corre+ sponding potential energy profile, investigated at the MP2/6–31G* level, reveals a route via a [CH3CO] / water complex after an energy determining step involving a simultaneous 1,2-hydrogen migration and C–O bond elongation. The critical energy of the reac–1 + tion, evaluated at the G2(MP2,SVP) level, is 170 kJ mol above the most stable conformation of the [1H] ion. The 298 K heats of for+ + –1 mation of the three most stable [C2H3O] ions have been calculated at the G2 level: ∆fH°[CH3CO] = 655.0 kJ mol , + –1 + –1 ∆fH°[CH2COH] = 833.0 kJ mol , ∆fH°[c-CH2CHO] = 886.2 kJ mol . Keywords: glycolaldehyde, proton affinity, basicity, G2 calculations, dehydration of protonated glycolaldehyde

Introduction Saccharides are a part of the composition of a large variety of biomolecules and biopolymers. Glycolaldehyde 1, is the lowest α-hydroxycarbonyl molecule and, as such, it constitutes the first sugar. This molecule intervenes in numerous biochemical processes and is the building block of various natural products.1 Moreover, its presence in interstellar 2 media has been reported recently. From a fundamental point of view, glycolaldehyde is a good model for studying intramolecular bifunctional interactions and, particularly, their effect upon protonation or complexation energetics.

The present study was intended, first, to provide structural and thermochemical information on the protonation of the molecule of glycolaldehyde, 1, the simplest species which contains both a carbonyl and a hydroxyl groups as basic sites. The second objective was to explore the unimolecular + reactivity of protonated glycolaldehyde [1H] in order to see what kind of chemistry is initiated by the incoming proton. For this purpose, tandem mass spectrometric experiments and ab initio molecular orbital calculations up to the G2 level have been employed.

Experimental and computational a

This paper is dedicated to Jean-Claude Promé who devoted part of his scientific activity to the biochemistry and to the mass spectrometry of various glycosides.

Unimolecular dissociations of metastable ions were studied in the second field-free region of a B–E mass spec-

© IM Publications 2001, ISSN 1356-1049

352

Structure, Thermochemistry and Reactivity of Protonated Glycolaldehyde

trometer (VG-ZAB 2F, Micromass) working in the massanalyzed ion kinetic energy (MIKE) mode. A chemical ionization source was used to produce protonated glycolaldehyde and protonated heterodimers with ethanol, butanol and acetonitrile. Typical source conditions were: temperature = 200°C, filament current = 100 µA, ionizing energy = 150 eV, accelerating voltage = 8 keV and reagent gas = methane. Glycolaldehyde was produced by evaporating its dimer directly into the septum inlet system. Standard ab initio calculations have been carried out using the Gaussian-94 series of programs.3 G2,4(a) 4(b) 4(c) G2(MP2) and G2(MP2,SVP) theories employ a geometry optimized at the MP2(full)/6–31G(d) level and an HF/6–31G(d) zero-point energy (ZPE) scaled by a factor 0.893. A base energy calculated at the MP4/6–311G(d,p) or MP2/6–311G(d,p) level is corrected by several additivity approximations to QCISD(T) and to the 6–311+G(3df,2p) basis set. In an attempt to account for residual basis set deficiencies, G2 theories introduce higher-level corrections (HLC) that depend on the number of paired and unpaired electrons. G2 formalisms yield, in general, reliable heats of formation, ionization energies and proton affinities. At these levels of theory, the accuracy on these calculated enthalpic quantities is better than 5 kJ mol–1, as established recently 4(d),4(e) for a set of approximately 150 compounds. Heats of formation have been evaluated from the G2 total energies by considering atomization reactions.5 Using this approach, the heat of formation at 0 K, for a given species X, ∆fH°0(X), is given by Equation (1) and the heat of formation at 298 K is, therefore, given by Equation (2) where the difference between the enthalpy at 298 K and 0 K is represented by the ∆298H° terms (∆298H° = H°298 – H°0). ∆ f H °0 (X) = ∑ ∆ f H °0 (atoms) − ∑ E[G2](atoms + E[G2](X) (1)

∆ f H °298 (X) = ∆ f H °0 + ∆ 298 H °(X) − ∑ ∆ 298 H °(elements) (2)

Note that, for an ionic species, the expression leads to the enthalpy of formation in the so-called “ion convention” (i.e. the enthalpy of formation of the electron is equal to its integrated heat capacity at the considered temperature). For the elements, experimental ∆298H° values have been used –1 (viz 8.468, 1.050 and 8.68 kJ mol for H2(g), C(s) and O2(g), respectively) whereas, for the other species, the translational and rotational contributions were taken to be equal to 3RT and the vibrational contribution estimated from the scaled (by a factor 0.893) HF/6–31G(d) vibrational frequencies. The calculation of third law entropies uses standard statistical thermodynamic formulae. Each vibrational contribution to entropy was computed according to the standard Equation (3) S° = R [ (θ / T) / (eθ / T – 1) – ln (1 – e–θ / T ) ] (3) where h and kB are the Planck’s and Boltzman’s constants, respectively, and θ = hν / kB. The latter term was calculated

using the scaled harmonic vibrational frequencies ν, calculated at the HF/6–31G(d) level. Entropies for internal rotations were computed by using the hindered-rotor model 6 developed by Pitzer. In this approach, the energy levels of a rotor associated with a potential energy barrier of the form V0 / 2 (1 – cos n φ), where φ is the dihedral angle, are found with the help of a one-dimensional Schrödinger equation. The results are presented as a function of two dimensionless variables: V0 / RT and 1 / Qfr (i.e. the reciprocal of the partition function for free rotation). In practice, the entropy of a given rotor is obtained by addition of a corrective term to the entropy calculated under the free-rotor approximation. In the present study, the required rotational energy barriers, V0, were obtained at the MP2/6–31G(d) level using a relaxed rotation approach (i.e. all geometrical parameters were optimized with the exception of the dihedral angle being considered).

Results and discussion The results of the present study will be presented in four parts. A theoretical investigation of the structures and ener+ gies of glycolaldehyde, 1, and its protonated form [1H] will be described in the two first paragraphs. This is followed by the presentation of the protonation thermochemistry of glycolaldehyde, 1, and of the unimolecular reactivity of the [1H]+ ions. Structure and stability of glycolaldehyde 1

The most stable conformation of the glycolaldehyde molecule is the eclipsed structure, 1a, for which the dihedral angles θ1 (OCCO) and θ2 (CCOH) are equal to 0° (Scheme 1). The stability of this eclipsed conformation is obviously due to the existence of an internal hydrogen bond between the hydrogen of the hydroxyl group and the oxygen of the carbonyl moiety. This result is in perfect agreement with gas-phase microwave7 and infrared8 data as well as with earlier molecular orbital calculations on partially optimized 9,10 structures. Two other conformers of neutral glycolaldehyde have been identified and characterized as minima on the potential energy surface, namely 1b (θ1 = 180°, θ2 = 180°) and 1c

Scheme 1.

G. Bouchoux, F. Penaud-Berruyer and W. Bertrand, Eur. J. Mass Spectrom. 7, 351–357 (2001)

353

Table 1. Summary of the G2 calculations on the protonated glycoaldehyde system.

Species 1a [1Ha]

+

+

[CH3CO] + H2O +

a

[CH2COH] + H2O +

a

[c–CH2CHO] + H2O

a

G2 (0 K) Hartree

∆298H° kJ mol

– 228.710522

14.9

– 324.8

– 229.005301

15.2

426.0

– 229.013191

22.0

411.9

– 228.945512

22.3

589.9

– 228.924744

21.0

643.1

b

–1

∆H°298c kJ mol

–1

a

G2(0 K) (H2O) = – 76.33205 Hartree, G2 Enthalpy at 298 K for (H2O) = – 76.32826 Hartree ∆298H° = H°298 – H°0 c Calculated using the atomization reactions of the species considered with the G2 energies of the atoms: (H) – 0.5, (C) – 37.78432, (O) – 74.98203 Hartree and the G2 calculated heat of formation of water ∆fH°298(H2O) = – 243.1 kJ mol–1

b

(θ1 = 0°, θ2 = 180°) (Scheme 1). The potential energies of 1b –1 and 1c, with respect to 1a, are 14.5 and 23.8 kJ mol , respectively (MP2/6–31G* results). Assuming a thermal equilibration between these three conformers, component 1a is highly favored: at 300 K the population of 1b plus 1c represents less than 0.5 % of the mixture. A consequence of this situation is that the thermochemical properties of neutral glycolaldehyde can be evaluated from the characteristics of 1a alone. Starting from 1a, the effect of the dihedral angles θ1 and θ2 upon the potential energy of the molecule of glycolaldehyde has been examined. The effect of increasing θ1 is to transform 1a into 1b rather than giving the unstable conformer 1d (θ1 = 180°, θ2 = 0°). In fact, the increase of θ1 is accompanied by a parallel increase of θ2 so as to keep a favorable interaction between the hydrogen of the hydroxyl group, H(8), and the carbonyl oxygen, O(4) and to minimize the electrostatic repulsion between H(8) and the aldehydic hydrogen, H(5). The transition structure 1a → 1b lies 19 kJ mol–1 above 1a and corresponds to the dihedral angles θ1 = 80° and θ2 = 64°. Rotation around the C–O bond allows the transition 1a → 1c. The increase of θ2 is accompanied by a clear increase of potential energy, a maximum is attained –1 26 kJ mol above 1a for θ2 = 135°. This transition structure –1 is only 2 kJ mol above conformer 1c which is thus located in a very shallow well of the potential energy surface. Finally, the variation of the dihedral angle θ2 has been examined for structure 1b also. A maximum of potential energy is –1 observed 15 kJ mol above 1b for θ2 = 0°, i.e. for the eclipsed conformation (which is consequently situated –1 ~ 30 kJ mol above the most stable conformer 1a). The heat of formation of neutral glycolaldehyde has been deduced from the atomization energy of structure 1a at the G2 level. The value, ∆fH°(1) = –324.8 kJ mol–1 (Table 1), is less than the estimate based on the Benson’s incremen11 –1 tal method (–302 kJ mol , assuming CH2(O)(CO) –1 =CH2(C)(CO) = –21.8 kJ mol ). This is in line with the existence of an internal hydrogen bond of approximately

20 kJ mol–1 for 1a, as roughly indicated by the energy difference between the two conformers 1a and 1c. The absolute 298 K entropy of neutral glycolaldehyde has been calculated by considering as hindered rotations the two torsion modes associated with θ1 and θ2 as described in the computational section. Their contribution to the entropy 6 was deduced from the model of Pitzer using the calculated rotational barriers indicated above. The net result is S°(1) = 300.0 J mol–1 K–1 at 298 K; a value which compares

Scheme 2.

354

Structure, Thermochemistry and Reactivity of Protonated Glycolaldehyde

11

–1

–1

correctly with the incremental estimate of 308 J mol K which ignores the entropy lowering due to the internal hydrogen bond. Structure and stability of protonated glycolaldehyde [1H]+

Protonation of the glycolaldehyde molecule may occur on the carbonyl oxygen atom or on the hydroxyl oxygen + + atom. Two low lying structures, [1Ha] and [1Hb] , have indeed been located at the MP2/6–31G* level (Scheme 2). + The most stable structure, [1Ha] , corresponds to carbonyl protonation and is characterized by a dihedral angle OCCO (θ1) equal to 3.4°. This situation is reminiscent of that of the neutral molecule 1 and, again, the stability of such an eclipsed conformation is due to the existence of an internal hydrogen bond. In the present case, it involves the incoming proton H(9) and the oxygen O(3) of the hydroxyl group. In the second structure, [1Hb]+, the proton is covalently bonded to the oxygen O(3). An internal hydrogen bond is also responsible of the quasi-planar arrangement of the atoms H(9)O(3)C(2)C(1)O(4). Hydroxyl-protonated glycolaldehyde [1Hb]+ is less stable than carbonyl-protonated + –1 glycolaldehyde [1Ha] by 12.4 kJ mol (MP2/6–31G* level). This energy difference reflects the proton affinity difference of the two basic sites as expected between a carbonyl and a hydroxyl oxygen [compare, for example, the proton –1 affinity values for acetaldehyde (768 kJ mol ) and for meth–1 anol (760 kJ mol ) in Reference 12]. The energy difference of 12.4 kJ mol–1 calculated between [1Ha]+ and [1Hb]+ corresponds to more than 99% of the former in an equilibrium mixture of both conformers at 298 K. Consequently, the thermochemical characteristics of protonated glycolaldehyde can be calculated by considering exclusively the con+ former [1Ha] . + Starting from [1Ha] and increasing the dihedral angle θ1 from 0° to 180°, the potential energy of the system increases markedly until it attains a maximum value of –1 60 kJ mol for θ1 ≈ 80° and then decreases slightly to attain a –1 second minimum of 51.9 kJ mol for θ1 = 180°. This point + corresponds to the stable structure [1Hc] presented in Scheme 2. At this stage, the internal hydrogen bond is clearly broken and, thus, the energy difference between + + [1Ha] and [1Hc] may be considered as a rough assessment of the internal hydrogen bond energy. The value of –1 51.9 kJ mol is far from the hydrogen bond energy of the + –1 13 complex [CH3CHOH] / CH3OH which is ≈ 130 kJ mol , and which is expected to reflect the ideal case of an unconstrained hydrogen bonding. The difference is accounted for by ring strain and by non-linearity of the O–H...O moiety, effects which weaken the internal hydrogen bond in [1Ha]+. + Rotation of the hydroxyl group in [1Ha] is accompanied by a considerable increase of the potential energy. As expected, starting from θ2 = 180°, an energy maximum is observed for θ2 = 0°; the barrier height for the overall rota+ + –1 tion [1Ha] → [1Ha] is 82 kJ mol . The heat of formation of protonated glycolaldehyde, as calculated from the atomization energy of structure [1Ha]+

at the G2 level, is equal to ∆fH°[1Ha] = 426.0 kJ mol + (Table 1). The absolute 298 K entropy of [1Ha] has been calculated by considering only the torsion mode associated with θ1 as hindered rotation, its contribution to the entropy 6 was deduced from the model of Pitzer using the calculated –1 rotational barriers of 60 kJ mol indicated above. All the other vibrational contributions have been evaluated using the harmonic oscillator approximation. Under these conditions, the third law entropy of protonated glycolaldehyde is equal to S°[1Ha]+ = 291.6 J mol–1 K–1 at 298 K. +

–1

Protonation energetics of glycolaldehyde 1

No information on the basicity of α-hydroxycarbonyl compounds is presently available in the literature. The present paragraph describes our experimental and theoretical data concerning the first member of the series; a more exten14 sive examination of these molecules will appear soon. The monomeric form of glycolaldehyde has been found to be amenable to experiments in the chemical ionization source of the ZAB-2F mass spectrometer. We thus decided to determine its proton affinity by the so called “kinetic method”.15 In its most traditional formulation, the method assumes a linear relationship between the logarithm of the branching ratio for the dissociation of a proton-bound dimer between an unknown M and a reference B and the difference in proton affinities of the two species. No special account has been taken for a possible entropy effect since, in the present 16 case, it is expected to be hardly detectable. Three bases (B) were used to produce the protonated mixed species [1HB]+ (B = ethanol, acetonitrile and n-butanol). Competitive dis+ + sociations of these complexes into [1H] + B and [BH] + 1 were studied in the metastable time frame by the MIKE method. The linear correlation observed between + + ln([1H] / [BH] ) and the proton affinities of the bases B, PA(B), allows us to assess the value of PA(1). Using + + ln([1H] / [BH] ) = 2.2, 1.7 and –2.1 and PA(B) = 776.4, –1 12 779.2 and 789.2 kJ mol , for B = ethanol, acetonitrile and –1 n-butanol, respectively, we obtain PA(1) = 783.3 kJ mol . The statistical uncertainty on this measurement (the “90% confidence limit” as defined in Reference 16) is ± 3.8 kJ mol–1. The proton affinity of 1 may also be obtained from molecular orbital calculation. If H°298[G2] represents the G2 enthalpy, then PA(1) is given by PA(1) = H°298[G2](1) – H°298[G2]([1H]+) + H°298([H]+) with the enthalpy of the proton H°298([H]+) = 5 / 2 –1 RT = 6.19 kJ mol . The preceding equation leads to –1 PA(1) = 779.8 kJ mol in perfect agreement with the experi–1 mental value of 783.3 ± 3.8 kJ mol [note that, according to –1 Reference 4(d), an accuracy of ± 6 kJ mol is expected for proton affinities estimated at the G2 level]. + Since the entropy of 1 and [1H] have been estimated previously, the gas-phase basicity of glycolaldehyde can be deduced from:

G. Bouchoux, F. Penaud-Berruyer and W. Bertrand, Eur. J. Mass Spectrom. 7, 351–357 (2001)

+

+

GB(1) = PA(1) – T [S°([H] ) + S°(1) – S°([1H] )] i.e. GB(1) = PA(1) – 34.9 kJ mol–1 at 298 K Thus, we have GB(1) = 748.4 ± 3.8 kJ mol–1 from experi–1 ment and GB(1) = 744.9 kJ mol (with a probable error of –1 ± 6 kJ mol ) from the G2 calculations. Reactivity of protonated glycolaldehyde [1H]+ +

The MIKE spectrum of the [1H] ions (m/z 61) presents + only one peak at m/z 43 corresponding to [C2H3O] ions. This unimolecular dehydration reaction is accompanied by a simple Gaussian metastable peak (T0,5 = 41 meV; Taverage = 140 meV). Metastable ions of m/z 63, produced by chemical ionization of glycolaldehyde with D2O and, thus, probably of + structure [HCO(D)CH2OD] , dissociate almost entirely by loss of D2O pointing to a negligible (less than 5%) hydrogen exchange before the specific elimination of a water molecule involving the hydrogen of the hydroxyl group and the incoming proton. A large amount of data concerning the mass spectral characterization17 and the structures and stabilities18 of + [C2H3O] ions has been published earlier. Briefly, the three + identified structures, acylium a [CH3CO] , hydroxyvinyl b + + [CH2COH] and oxiranyl c [cyclo–CH2CHO] can be distinguished by examining their metastable decompositions and their collision-induced dissociations. Metastable peak shapes for the m/z 43 → m/z 15 transition are all Gaussian but with different T0.5 values.17(c),17(d) A low value for the abundance ratio of m/z 29 to m/z 28 is a reliable criterion for assigning structure a, whereas the reverse is true for the oxiranyl cation c.17(c–e) Considering these results, we made use of the MIKE and collision-induced dissociation (CID)/MIKE spectra of m/z 43 ions produced from [1H]+ to + characterize the corresponding [C2H3O] ion structures. The MIKE spectrum exhibits only one signal, m/z 43 → m/z 15 with a peak shape nearly identical to that observed in the case of [C2H3O]+ ions produced by electron ionization of ace-

Scheme 3.

355

tone. Following the conclusions of References 17(c–e), the + major component of the m/z 43 ions produced from [1H] is + the acetyl cation [CH3CO] . However, a more detailed examination of the m/z 43 → m/z 15 peak shape reveals a small contribution (approximately 5%) of a process corresponding to a larger T value which may correspond to a small amount of structure b or c or both. In the CID/MIKE spectrum, the abundance ratio of m/z 29 to m/z 28 is slightly greater for the m/z 43 ions produced from [1H]+ than from acetone, thus demonstrating the presence of structure c. In conclusion, the m/z 43 ions produced in the ion source from glycolaldehyde, in the chemical ionization mode, appear to be a mixture mainly comprising acylium ions a [CH3CO]+ (≈ 95%) with a + minor amount of the oxiranyl ion c [cyclo–CH2CHO] and, + possibly, ion b [CH2COH] . The dissociation of metastable + ions [1H] , which appears to be a single process, is also expected to produce acylium ions a, since this latter is the + most stable [C2H3O] ion. This proposal may be corroborated by examination of the mechanistic and energetic aspects of + the dehydration of [1H] . + + We investigated the reactions [1H] → [C2H3O] + H2O depicted in Scheme 3 by means of ab initio molecular orbital calculations at the MP2/6–31G* level. We find, indeed, that the lowest energy route corresponds to the dissociation leading to [CH3CO]+ + H2O as products. This pathway involves, + + as a first step, the [1Ha] → [1Hb] isomerization, i.e. a 1,4hydrogen migration between the two oxygen atoms. The transition structure TSa/b is situated 16.7 kJ mol–1 above + –1 [1Ha] and thus only 4.3 kJ mol above the hydroxyl+ protonated form [1Hb] (Scheme 2). It may be noted that, in the structure TSa/b, the migrating hydrogen is closer to the oxygen of the hydroxyl group (1.204 Å) than to that of the carbonyl (1.287 Å). This is in agreement with the Hammond + + postulate since [1Hb] is higher in energy than [1Ha] . The second step of the reaction involves a 1,2-hydride ion migration from the aldehydic position to the methylene group. This migration is accompanied by a simultaneous C–O bond elongation which, in turn, leads to the

356

Structure, Thermochemistry and Reactivity of Protonated Glycolaldehyde

–1

Figure 1. MP2/6-31G* potential energy profile for the dehydration reaction of protonated glycoaldehyde [in parentheses: relative H°298 calculated at the G2(MP2,SVP) level; in brackets: relative H°298 calculated at the G2 level].

+

+

[CH3CO] / H2O complex [1Hd] (Scheme 2). The corre–1 sponding transition structure TSb/d is situated 208 kJ mol + + above [1Ha] . The intermediate complex [1Hd] is in a deep –1 potential energy well (– 71 kJ mol with respect to its com+ ponents [CH3CO] and H2O). This is not unexpected for an ion–dipole complex involving an ion with a strongly localized positive charge at the central carbon atom and a molecule having a large dipole moment. It is interesting to note + that, in fact, [1Hd] is hydroxyl-protonated acetic acid. This ion has been claimed to be the origin of the loss of water 19 observed from protonated acetic acid and identified during 20 a study of 2,4-dihydroxy-2-methylpentane. We observe + that, starting from [1Hd] , the CO bond elongation leads to + the dissociation products [CH3CO] + H2O without the involvement of a reverse energy barrier. This is in line with the observation of an extraordinarily small kinetic energy release during the loss of H2O from metastable ions [1Hd]+ –1 20 (T0.5 ≈ 0.1 kJ mol ). The potential energy profile associated with the overall + + reaction [1Ha] → [CH3CO] + H2O is presented in Figure 1. The concerted 1,2-H migration / CO bond elongation is clearly the energy determining step of this process. The transition structure TSb/d lies 208 kJ mol–1 above the reactant and the products (MP2/6–31G* potential energy). These dif-

ferences reduce to 169 and 182 kJ mol , respectively, on an enthalpy scale at the G2(MP2,SVP) level. This large energy barrier explains why the kinetic energy release for the loss of H2O from metastable ions [1Ha]+ (T0.5 = 41 meV; Taverage = 140 meV) is considerably higher than that from + + [1Hd] . The second consequence is that the ions [1Hd] orig+ inating from [1Ha] possess a considerable excess of vibrational energy and are only transient complexes which spontaneously dissociate once produced. The two other explored reaction channels (Scheme 3) + lead to hydroxyvinyl b [CH2COH] or oxiranyl c + [cyclo–CH2CHO] product ions. Since no precise heats of formation of these structures were available we calculate these quantities from the G2 energies of atomization. These results are summarized in Scheme 4. The calculated ∆fH°298[CH3CO]+ value, 655.0 kJ mol–1, is in excellent agree21 ment with the recent reevaluation by Traeger and Kompe of –1 655.5 ± 0.8 kJ mol and only slightly above the tabulated –1 22 value of 653.0 kJ mol . For hydroxyvinyl and oxiranyl ions, the G2 calculated heats of formation, + + ∆fH°298[CH2COH] and ∆fH°298[cyclo–CH2CHO] , are 30 to –1 40 kJ mol above the values estimated from appearance 22 energy determinations which clearly need reexamination. Returning now to the reaction mechanisms depicted in Scheme 3, a brief comment on the MP2/6–31G* results can be made. First, the formation of hydroxyvinyl ion b from [1H]+ may be accounted for by a 1,3-hydrogen migration from the aldehydic position to the oxygen of the hydroxyl + group from the conformer [1Hc] (Scheme 2). It is found that this hydrogen migration is the energy determining step and –1 the transition structure, TSb, is situated 259 kJ mol above + [1Ha] (Figure 1). Second, the oxiranyl ion c may be pro+ duced from structure [1Hb] by a simultaneous ring closure and CO bond elongation (Scheme 3). At the MP2/6–31G* level, it is observed that the various steps of the process [1Ha]+ → [cyclo–CH2CHO]+ + H2O involve transition structures which have a potential energy below the separated product values. Thus, the potential energy requirement for the reaction is 242 kJ mol–1, or 220 kJ mol–1 on an enthalpy scale (Figure 1). It appears, consequently, that the two reac+ tion channels leading to [C2H3O] ions with structures differ+ ent from [CH3CO] are energetically unfavorable. This conclusion is in line with that deduced from the MIKE and + CID/MIKE data and confirms that the [C2H3O] ions pro-

Scheme 4.

G. Bouchoux, F. Penaud-Berruyer and W. Bertrand, Eur. J. Mass Spectrom. 7, 351–357 (2001)

duced by chemical ionization of glycolaldehyde are essentially of acylium structure a. The formation of structures b and c, alongside a, is only marginal in the ion source (i.e. ≈ 5%) and probably negligible in the metastable time frame. Conclusion The present study of the first sugar, glycolaldehyde 1, under gas-phase acidic conditions offers the following results: • The 298 K heats of formation of 1 and its protonated form + [1H] , as calculated at the G2 level of theory, are –1 –1 – 324.8 kJ mol and 426.0 kJ mol , respectively. • The proton affinity and the gas-phase basicity of the –1 and molecule 1 are PA(1) = 783.3 ± 3.8 kJ mol –1 GB(1) = 748.4 ± 3.8 kJ mol from experiment and –1 –1 PA(1) = 779.8 kJ mol and GB(1) = 744.9 kJ mol (with a –1 probable error of ± 6 kJ mol ) from G2 calculations. • The dehydration mechanism of the protonated molecule + [1H] has been deduced from MP2/6–31G* potential energy calculations and tandem mass spectrometry experiments. Thus, the fragment ions, essentially [CH3CO]+, are produced after an energy determining step which consists of a 1,2-hydrogen migration coupled with a CO bond elongation. • The 298 K heats of formation of the three most stable + + [C2H3O] ions, namely acylium [CH3CO] , hydroxyvinyl + [CH2COH] and oxiranyl [cyclo–CH2CHO]+ (655.0, 833.0 and 886.3 kJ mol–1, respectively) have been calculated at the G2 level of theory.

5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15.

16. 17.

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