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The ionic isomerization [HCOH]+ → [CH2O]+: proton-transport catalysis by CO and CO2
Chemical Physics Letters 387 (2004) 204–208 www.elsevier.com/locate/cplett
The ionic isomerization [HCOH]þ ! [CH2@O]þ : proton-transport catalysis by CO and CO2 Cathy Y. Wong a, Paul J.A. Ruttink b, Peter C. Burgers c, Johan K. Terlouw
a,*
a
b
Department of Chemistry, McMaster University, 1280 Main St. W., Hamilton, Canada ON L8S 4M1 Theoretical Chemistry Group, Department of Chemistry, University of Utrecht, 3584 CH Utrecht, Netherlands c Hercules European Research Center, Hercules B.V., P.O. Box 252, 3770 AG Barneveld, Netherlands Received 9 December 2003; in final form 29 January 2004
Published online:
Abstract Mass spectrometry based experiments show that CH2 @Oþ is cleanly generated in the spontaneous decarbonylation of low energy glyoxal ions, O@CHACH@Oþ . Likewise, glyoxilic acid ions, O@CHAC(OH)@Oþ , decarboxylate into CH2 @Oþ , but in admixture with some HCOHþ . A mechanistic analysis using the CBS-QB3 model chemistry shows that the above ions first rearrange into hydrogen bridged radical cations, HCOAHþ C@O and HCOAHþ O@C@O, respectively. Solitary ions HCOHþ do not isomerize into CH2 @Oþ but the CO or CO2 molecule in the above complexes catalyzes this transformation prior to dissociation. CO is the better catalyst because its proton affinity is closer to the criterion for efficient proton-transport catalysis. 2004 Elsevier B.V. All rights reserved.
1. Introduction Removal of an electron from a closed-shell neutral molecule (M), as it occurs in electron ionization (EI) mass spectrometry, yields a radical cation (Mþ ). Such ions are intriguing in that isomers which for the neutral system are high-energy transient species only, may become more stable upon ionization [1]. For example, the carbene HCOH is only a transient species lying 52 kcal mol1 above CH2 @O [2], but upon ionization the energy difference is reduced to a mere 6 kcal mol1 [1,3]. For the system HCOOH/HOCOH, the ionized carbene is even lower in energy [1]. Following the seminal study of Berkowitz [4], it was shown [5] that HCOHþ (generated by loss of H2 from ionized methanol) and CH2 @Oþ (formed by EI of formaldehyde) yield characteristically different collisioninduced dissociation (CID) mass spectra: see items (a) and (b) of Fig. 1. These isomers can be easily distinguished because a large 1,2-H shift barrier prevents interconversion: *
Corresponding author. Fax: +1905-522-2509. E-mail address: [email protected] (J.K. Terlouw).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.02.020
HCOHþ —//→ CH2 @Oþ . The barrier exceeds the energy required for loss of H by direct bond cleavage, so even high energy ions will dissociate rather than interconvert. However, when a neutral molecule is allowed to interact with HCOHþ , a process called proton-transport catalysis (PTC) [6] may promote its isomerization into the lower energy CH2 @Oþ isomer. The molecule may act as a base (B) and accept a proton from one site of the HCOHþ substrate, and then donate it back to a different site, as illustrated in the following equation HCOHþ þ B ! HCOAHþ B ðaÞ ! HC@O HBþ ! O@CðHÞ HBþ
! O@CðHÞAHþ B ðbÞ ! CH2 @Oþ þ B ð1Þ
ð1Þ
Several examples of ion–molecule reactions in the gas phase are now available [6–10], where interaction of the radical cation with a single, judiciously chosen, ÔsolventÕ molecule leads to its transformation into a more stable isomer. Key intermediates in these reactions are hydrogen-bridged radical cations (HBRCs) of the type (a) and (b) in Eq. (1).
C.Y. Wong et al. / Chemical Physics Letters 387 (2004) 204–208
Criteria for successful proton-transport catalysis have been developed by Radom and co-workers [7]. The most important criterion states that a facile isomerization for the reaction of Eq. (1) occurs if the proton affinity (PA) of the base B lies between the PA of HCO at O and at C. If PA(B) is too low, proton abstraction may not take place. If PA(B) is too high, the incipient ion BHþ will not release the proton: dissociation to HCO + BHþ will ensue instead. Here we report on the isomerization HCOHþ ! CH2 @Oþ , catalyzed by CO or CO2 as the base in dissociation reactions of ionized molecules. The reactions chosen were: (i) decarbonylation of ionized glyoxal, O@CHACH@Oþ (1) ! CH2 @Oþ + CO and (ii) decarboxylation of ionized glyoxilic acid, O@CHA C(OH)@Oþ (2) ! CH2 @Oþ + CO2 . HBRCs of the type HCOAHþ B (a) could be formed from 1 and 2 by simple rearrangement; proton-transport catalysis may promote their conversion into O@C(H)AHþ B (b), the immediate precursor to the formation of CH2 @Oþ . Mass spectrometric experiments combined with ab initio calculations provide evidence that proton-transport catalysis indeed features in these reactions. The gasphase synthesis of the glyoxal radical cation O@CHACH@Oþ by a collision encounter of CH2 @Oþ and CO is also feasible. Such a reaction may play a role in interstellar space chemistry.
2. Experimental and theoretical methods The experiments were performed with the VG Analytical ZAB–R mass spectrometer of BEE geometry (B ¼ magnet, E ¼ electric sector) as described in [11]. The spontaneous and collision induced dissociations of the mass selected precursor ions were monitored in the second field free region (2ffr) by recording their metastable ion (MI) and CID mass spectra. The m/z 30 product ion structures were probed by CID in the 3ffr using O2 as the target gas. The spectra of Fig. 1 all refer to ions of ca. 4 keV translational energy. The calculations were performed with the CBS-QB3 model chemistry [12], using GA U S S I A N 98, Rev A.11.3 [13]. The resulting enthalpies of formation (298 K) for minima, transition states and dissociation levels are presented in Fig. 2 for the glyoxal system, and in Fig. 3 for the glyoxilic acid system. Table 1 lists the CBS-QB3 (0 K) total energies for the principal species. Spin contaminations were within an acceptable range. The G3 model chemistry [14] was also used to analyse the two systems. Agreement with the CBS-QB3 method was quite satisfactory (within 1–2 kcal mol1 ). The complete set of computational results including optimized geometries of the structures is available from the authors upon request.
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3. Results and discussion As mentioned above, the unassisted isomerization HCOHþ ! CH2 @Oþ has a prohibitively high barrier, 37 kcal mol1 (Fig. 2). RadomÕs criterion for efficient proton-transport catalysis [7] prescribes a base with a PA of 146–151 kcal mol1 for this system. Such a narrow range of PA values severely limits the choice of B. However, the criterion refers to an isomerization whose barrier has become vanishingly small. The criterion can be relaxed, considering that the isomerization may occur as long as its barrier remains below the threshold for formation of CH2 @Oþ + B. Thus, the lower PA(B) limit can be significantly extended downwards, but the upper limit remains: if PA(B) is too high, the abstracted proton stays with BHþ . Computational studies [15–17] provide further insight, notably a study of the transformation HCOAHþ B (a) fi O@C(H)AHþ B (b) with HF as the base. PA(HF) is only 116 kcal mol1 [18], 30 kcal mol1 below the PA criterion. The low PA causes the proton to remain on the substrate side in the equilibrium structures HCOAHþ FH (a) and O@C(H)AHþ FH (b). However, it moves to the HF side in the transition a ! b with a barrier of only 21.4 kcal mol1 , roughly half of that of the unassisted reaction. Thus, although HF poorly matches the PA criterion, it still acts as a fairly effective catalyst. We therefore expected that CO and CO2 , whose PAs (142 and 129 kcal mol1 , respectively, [18]) are higher, would reduce the isomerization barrier even further. Note also that CO and CO2 lack H atoms and so no complications
Fig. 1. Partial CID mass spectra of m/z 30 [C,H2 ,O]þ ions. Items (a) and (b) are the spectra of reference ions CH2 Oþ and HCOHþ , respectively [5]. Spectra (c) and (d) refer to the collision induced decarbonylation of glyoxal (1) and hydroxy ketene (1c) ions. Spectra (e) and (f) refer to the spontaneous and collision induced decarboxylation of glyoxilic acid ions (2).
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C.Y. Wong et al. / Chemical Physics Letters 387 (2004) 204–208
Fig. 2. Proton transport catalysis in ionized glyoxal. Energy diagram based on CBS-QB3 calculations.
can arise from alternative base assisted mechanisms, such as the Quid-pro-Quo mechanism [9,11]. Ionized glyoxal O@CHACH@Oþ (1), spontaneously undergoes one reaction in the ls time-frame, viz. loss of CO to produce m/z 30 ions. CID experiments on m/z 30 show that these ions are CH2 @Oþ to the exclusion of HCOHþ [19]. Direct bond cleavage to HCOþ + HCO does not take place, showing that the barrier for formation of CH2 @Oþ + CO lies below the threshold for dissociation to HCOþ + HCO , see Fig. 2. The MI spectrum shows that formation of CH2 @Oþ + CO is associated with an exceedingly small kinetic energy release [20], T0:5 ffi 0:007 kcal mol1 [19]. The reaction clearly takes place at threshold so that there is no acti-
vation energy for the reverse reaction CH2 @Oþ + CO ! 1. The CH2 @Oþ ion is also formed (to the exclusion of HCOHþ ) when 1 is induced to dissociate by a keV collision with oxygen, see Fig. 1e, and this rules out any formation of HCOHþ amongst ions formed in the higher energy regime. The calculations indicate that ions formed by vertical ionization of glyoxal collapse to the long bonded species O@(H)C C(H)@Oþ having a CAC bond of 1.86 A. This is not an ion–dipole complex but a one-electron bonded species [21]. The CH@Oþ part can easily donate a proton to produce the HBRC, HCOAHþ C@O (1a). The barrier for the PTC reaction, HCOAHþ C@O (1a) ! O@C(H)AHþ C@O (1b) is 8.5 kcal mol1 , compared to 37 kcal mol1 for the unassisted reaction, see Fig. 2. Note that the stabilization energy of 1b (10.3 kcal mol1 ) is significantly smaller than that of 1a (20.3 kcal mol1 ) as expected for a C H C vis a vis an O H C bonded species [21]. The isomerization barrier 1a ! 1b lies below the threshold for dissociation to CH2 @Oþ + CO making loss of CO the reaction of lowest energy requirement. Thus our calculations provide a rationale for the experimental observation that 1 produces CH2 @Oþ + CO at threshold. The key step in this decarbonylation is the transformation HCOHþ ! CH2 @Oþ catalyzed by CO. Our derived mechanism for the spontaneous decarbonylation of low energy ions 1 involves the sequence 1 ! 1a–PTC ! 1b ! CH2 @Oþ + CO, depicted in Fig. 2. A direct 1,2-H shift in the long bonded species O@CH CH@Oþ would also yield ions CH2 @Oþ . However, see Fig. 2, the associated barrier lies well above that for rearrangement to 1a, and also above the dissociation threshold. This pathway, therefore, does not pertain to the spontaneous decarbonylation of 1 in the ls time-frame. The glyoxal isomer HOACH@C@Oþ (1c), ionized hydroxyketene, also spontaneously decarbonylates into CH2 @Oþ [22]. This isomer is some 20 kcal mol1 lower in energy than 1, see Fig. 2. Its CID spectrum differs
Table 1 Enthalpies of formation (kcal mol1 ) and total energies (Hartree) derived from CBS-QB3 calculations Ionized glyoxal
Ionized glyoxilic acid
Isomer
Etotal (0 K)
DH 0f (298 K)
Isomer
Etotal (0 K)
DH 0f (298 K)
1 1a 1b 1c 1d
)227.14910 )227.14564 )227.14034 )227.17979 )227.13354
181.6 184.1 187.5 161.9 191.3
2 2a 2b 2c
)302.31515 )302.33109 )302.32929 )302.32938
127.9 118.4 119.6 119.6
)227.13211 )227.12341 )227.13211 )227.12130 )227.10752
192.3 198.2 192.6 199.4 207.3
TS TS TS TS
)302.30343 )302.29583 )302.29977 )302.30595
135.2 140.5 137.7 134.0
TS TS TS TS TS
1 ! 1a 1 ! 1b 1a ! 1b 1a ! 1c 1d
2 ! 2a 2 ! 2b 2 ! 2c 2a ! 2b
C.Y. Wong et al. / Chemical Physics Letters 387 (2004) 204–208
from that of 1 by the presence of a unique peak at m/z 41 (loss of H2 O) which constitutes 20% of the base peak at m/z 30. The decarbonylation of 1c begins with stretching of the C@C bond which requires as much as 37.5 kcal mol1 , after which HBRC 1a is formed. Next, the sequence of events depicted in Fig. 2 for the decarbonylation of glyoxal takes place. TS 1c ! 1a represents the highest energy level in the decarbonylation pathway of 1c, lying 1.6 kcal mol1 above CH2 @Oþ + CO. Consistent with this theoretical picture, the kinetic energy release associated with 1cÕs decarbonylation, T0:5 ¼ 0:36 kcal mol1 , is larger than that of 1. Ions 1c lie in a deep potential well with a high barrier for isomerization into 1a. It is therefore not surprising, see Fig. 1d, that the direct bond cleavage 1c ! HCOHþ competes with the rearrangement 1c ! 1a ! CH2 @Oþ when ions 1c are collisionally energized. When CH2 @Oþ and CO are allowed to interact under chemical ionization conditions [10], we find that the CID spectrum of the encounter complex at m/z 58 corresponds with mostly 1 and some 1c. This is precisely what the calculations of Fig. 2 predict. Such an encounter does not generate the stable distonic isomer CH2 –OACþ @O (1d): a high activation energy (9.5 kcal mol1 , Table 1 and Fig. 2) prevents this reaction. In line with this, the spontaneous decarbonylation of 1d (generated by loss of CH2 @O from ionized ethylene carbonate [22]) displays a large kinetic energy release (T0:5 ¼ 7 kcal mol1 ).
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Decarboxylation is the only spontaneous reaction observed in the ls timeframe for glyoxilic acid ions, O@CHAC(OH)@Oþ (2). The resulting product ions, see Fig. 1e, are largely CH2 @Oþ but the enhanced intensities of the peaks at m/z 13 and m/z 17 reveal that some HCOHþ is also present. When stable ions 2 are collisionally energized, the decarboxylation remains the dominating reaction, but the product ions are now largely HCOHþ , see Fig. 1f. These observations are entirely compatible with the computational results of Fig. 3. As with 1, the optimized geometry of 2 corresponds to a long bonded structure, O@CH C(OH)@Oþ , having a CAC bond of 1.83 A. A formal 1,3-H shift in this ion generates HBRC 2c, HOCOHþ C@O, which would then shed CO. The barrier for this reaction, see Fig. 3, lies above the TS for the 1,4-H shift, and this may provide a rationale why decarbonylation is not observed. A formal 1,4-H shift in 2 produces HBRC 2a, HCOHþ O@C@O. This rearrangement, (TS 2 ! 2a) requires only 7.3 kcal mol1 . The resulting reactive ions 2a can either dissociate by direct bond cleavage into HCOHþ or else rearrange by PTC into HBRC 2b, O@C(H)AHþ O@C@O, and then lose CO2 . The calculated energy requirement for the PTC route lies only marginally below that for direct bond cleavage of 2a. Within the error margin of the calculations, 1–2 kcal mol1 [12], theory clearly supports experiment. The spontaneous decarboxylation may occur via the competing pathways 2 ! 2a ! HCOHþ and 2 ! 2a ! 2b ! CH2 Oþ . On the other hand, collisionally energized ions 2 of higher energy will preferentially dissociate by the kinetically more attractive direct bond cleavage of 2a. Thus the spontaneous decarboxylation of 2 involves PTC by CO2 . The barrier for the CO2 assisted reaction, 15.6 kcal mol1 , is much larger than that for the CO assisted PTC reaction in 1, 8.5 kcal mol1 . This clearly reflects that the PA of CO2 is more out of line with RadomÕs criterion. Nevertheless a significant reduction of the barrier is still achieved.
Acknowledgements J.K.T. thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for continuing financial support, and C.Y.W. thanks SHARCNET for a research award.
References
Fig. 3. Proton transport catalysis in ionized glyoxilic acid. Energy diagram based on CBS-QB3 calculations.
[1] R. Flammang, M.T. Nguyen, G. Bouchoux, P. Gerbaux, Int. J. Mass Spectrom. 202 (2000) A8. [2] CBS-QB3 calculations, this work. [3] S. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin, W.G. Mallard, J. Phys. Chem. Ref. Data 17 (Suppl. 1) (1988).
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[4] J. Berkowitz, J. Chem. Phys. 69 (1978) 3044. [5] P.C. Burgers, A.A. Mommers, J.L. Holmes, J. Am. Chem. Soc. 105 (1983) 5976. [6] D.K. Bohme, Int. J. Mass Spectrom. Ion Processes 115 (1992) 95. [7] J.W. Gauld, L. Radom, J. Am. Chem. Soc. 119 (1997) 9831. [8] G. van der Rest, P. Mourgues, H. Nedev, H.E. Audier, J. Am. Chem. Soc. 124 (2002) 5561. [9] M. Haranczyk, P.C. Burgers, P.J.A. Ruttink, Int. J. Mass Spectrom. 220 (2002) 53. [10] M.A. Trikoupis, P.C. Burgers, P.J.A. Ruttink, J.K. Terlouw, Int. J. Mass Spectrom. 217 (2002) 97, and references cited therein. [11] L.N. Heydorn, P.C. Burgers, P.J.A. Ruttink, J.K. Terlouw, Int. J. Mass Spectrom. 227 (2003) 453. [12] J.A. Montgomery Jr., M.J. Frisch, J.W. Ochterski, G.A. Petersson, J. Chem. Phys. 110 (1999) 2822; J. Chem. Phys. 112 (2000) 6532. [13] M.J. Frisch et al., GA U S S I A N 98, Revision A.11, Gaussian Inc., Pittsburg, PA, 1998.
[14] L.A. Curtiss, K. Raghavachari, P.C. Redfern, V. Rassolov, J.A. Pople, J. Chem. Phys. 109 (1998) 7764. [15] E.L. Coitino, A. Lledos, R. Serra, J. Bertran, O.N. Ventura, J. Am. Chem. Soc. 115 (1993) 9121. [16] A. Pereira, E.L. Coitino, O.N. Ventura, J. Mol. Struct. (Theochem) 314 (1994) 31. [17] V. Barone, C. Adamo, Chem. Phys. Lett. 224 (1994) 432. [18] E.P.L. Hunter, S.G. Lias, J. Phys. Chem. Ref. Data 27 (1998) 413. [19] P.J.A. Ruttink, in: R. Naaman, Z. Vager (Eds.), Ion and Cluster Ion Spectroscopy and Structure, Plenum Press, New York, 1989, p. 243. [20] J.L. Holmes, J.K. Terlouw, Org. Mass Spectrom. 15 (1980) 383. [21] P.C. Burgers, J.K. Terlouw, in: N.M.M. Nibbering (Ed.), Encyclopedia of Mass Spectrometry, vol. 4, Elsevier, Amsterdam, 2004, in press. [22] J.K. Terlouw, P.C. Burgers, B.L.M. van Baar, Th. Weiske, H. Schwarz, Chimia 40 (1986) 357.
The ionic isomerization [HCOH]þ ! [CH2@O]þ : proton-transport catalysis by CO and CO2 Cathy Y. Wong a, Paul J.A. Ruttink b, Peter C. Burgers c, Johan K. Terlouw
a,*
a
b
Department of Chemistry, McMaster University, 1280 Main St. W., Hamilton, Canada ON L8S 4M1 Theoretical Chemistry Group, Department of Chemistry, University of Utrecht, 3584 CH Utrecht, Netherlands c Hercules European Research Center, Hercules B.V., P.O. Box 252, 3770 AG Barneveld, Netherlands Received 9 December 2003; in final form 29 January 2004
Published online:
Abstract Mass spectrometry based experiments show that CH2 @Oþ is cleanly generated in the spontaneous decarbonylation of low energy glyoxal ions, O@CHACH@Oþ . Likewise, glyoxilic acid ions, O@CHAC(OH)@Oþ , decarboxylate into CH2 @Oþ , but in admixture with some HCOHþ . A mechanistic analysis using the CBS-QB3 model chemistry shows that the above ions first rearrange into hydrogen bridged radical cations, HCOAHþ C@O and HCOAHþ O@C@O, respectively. Solitary ions HCOHþ do not isomerize into CH2 @Oþ but the CO or CO2 molecule in the above complexes catalyzes this transformation prior to dissociation. CO is the better catalyst because its proton affinity is closer to the criterion for efficient proton-transport catalysis. 2004 Elsevier B.V. All rights reserved.
1. Introduction Removal of an electron from a closed-shell neutral molecule (M), as it occurs in electron ionization (EI) mass spectrometry, yields a radical cation (Mþ ). Such ions are intriguing in that isomers which for the neutral system are high-energy transient species only, may become more stable upon ionization [1]. For example, the carbene HCOH is only a transient species lying 52 kcal mol1 above CH2 @O [2], but upon ionization the energy difference is reduced to a mere 6 kcal mol1 [1,3]. For the system HCOOH/HOCOH, the ionized carbene is even lower in energy [1]. Following the seminal study of Berkowitz [4], it was shown [5] that HCOHþ (generated by loss of H2 from ionized methanol) and CH2 @Oþ (formed by EI of formaldehyde) yield characteristically different collisioninduced dissociation (CID) mass spectra: see items (a) and (b) of Fig. 1. These isomers can be easily distinguished because a large 1,2-H shift barrier prevents interconversion: *
Corresponding author. Fax: +1905-522-2509. E-mail address: [email protected] (J.K. Terlouw).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.02.020
HCOHþ —//→ CH2 @Oþ . The barrier exceeds the energy required for loss of H by direct bond cleavage, so even high energy ions will dissociate rather than interconvert. However, when a neutral molecule is allowed to interact with HCOHþ , a process called proton-transport catalysis (PTC) [6] may promote its isomerization into the lower energy CH2 @Oþ isomer. The molecule may act as a base (B) and accept a proton from one site of the HCOHþ substrate, and then donate it back to a different site, as illustrated in the following equation HCOHþ þ B ! HCOAHþ B ðaÞ ! HC@O HBþ ! O@CðHÞ HBþ
! O@CðHÞAHþ B ðbÞ ! CH2 @Oþ þ B ð1Þ
ð1Þ
Several examples of ion–molecule reactions in the gas phase are now available [6–10], where interaction of the radical cation with a single, judiciously chosen, ÔsolventÕ molecule leads to its transformation into a more stable isomer. Key intermediates in these reactions are hydrogen-bridged radical cations (HBRCs) of the type (a) and (b) in Eq. (1).
C.Y. Wong et al. / Chemical Physics Letters 387 (2004) 204–208
Criteria for successful proton-transport catalysis have been developed by Radom and co-workers [7]. The most important criterion states that a facile isomerization for the reaction of Eq. (1) occurs if the proton affinity (PA) of the base B lies between the PA of HCO at O and at C. If PA(B) is too low, proton abstraction may not take place. If PA(B) is too high, the incipient ion BHþ will not release the proton: dissociation to HCO + BHþ will ensue instead. Here we report on the isomerization HCOHþ ! CH2 @Oþ , catalyzed by CO or CO2 as the base in dissociation reactions of ionized molecules. The reactions chosen were: (i) decarbonylation of ionized glyoxal, O@CHACH@Oþ (1) ! CH2 @Oþ + CO and (ii) decarboxylation of ionized glyoxilic acid, O@CHA C(OH)@Oþ (2) ! CH2 @Oþ + CO2 . HBRCs of the type HCOAHþ B (a) could be formed from 1 and 2 by simple rearrangement; proton-transport catalysis may promote their conversion into O@C(H)AHþ B (b), the immediate precursor to the formation of CH2 @Oþ . Mass spectrometric experiments combined with ab initio calculations provide evidence that proton-transport catalysis indeed features in these reactions. The gasphase synthesis of the glyoxal radical cation O@CHACH@Oþ by a collision encounter of CH2 @Oþ and CO is also feasible. Such a reaction may play a role in interstellar space chemistry.
2. Experimental and theoretical methods The experiments were performed with the VG Analytical ZAB–R mass spectrometer of BEE geometry (B ¼ magnet, E ¼ electric sector) as described in [11]. The spontaneous and collision induced dissociations of the mass selected precursor ions were monitored in the second field free region (2ffr) by recording their metastable ion (MI) and CID mass spectra. The m/z 30 product ion structures were probed by CID in the 3ffr using O2 as the target gas. The spectra of Fig. 1 all refer to ions of ca. 4 keV translational energy. The calculations were performed with the CBS-QB3 model chemistry [12], using GA U S S I A N 98, Rev A.11.3 [13]. The resulting enthalpies of formation (298 K) for minima, transition states and dissociation levels are presented in Fig. 2 for the glyoxal system, and in Fig. 3 for the glyoxilic acid system. Table 1 lists the CBS-QB3 (0 K) total energies for the principal species. Spin contaminations were within an acceptable range. The G3 model chemistry [14] was also used to analyse the two systems. Agreement with the CBS-QB3 method was quite satisfactory (within 1–2 kcal mol1 ). The complete set of computational results including optimized geometries of the structures is available from the authors upon request.
205
3. Results and discussion As mentioned above, the unassisted isomerization HCOHþ ! CH2 @Oþ has a prohibitively high barrier, 37 kcal mol1 (Fig. 2). RadomÕs criterion for efficient proton-transport catalysis [7] prescribes a base with a PA of 146–151 kcal mol1 for this system. Such a narrow range of PA values severely limits the choice of B. However, the criterion refers to an isomerization whose barrier has become vanishingly small. The criterion can be relaxed, considering that the isomerization may occur as long as its barrier remains below the threshold for formation of CH2 @Oþ + B. Thus, the lower PA(B) limit can be significantly extended downwards, but the upper limit remains: if PA(B) is too high, the abstracted proton stays with BHþ . Computational studies [15–17] provide further insight, notably a study of the transformation HCOAHþ B (a) fi O@C(H)AHþ B (b) with HF as the base. PA(HF) is only 116 kcal mol1 [18], 30 kcal mol1 below the PA criterion. The low PA causes the proton to remain on the substrate side in the equilibrium structures HCOAHþ FH (a) and O@C(H)AHþ FH (b). However, it moves to the HF side in the transition a ! b with a barrier of only 21.4 kcal mol1 , roughly half of that of the unassisted reaction. Thus, although HF poorly matches the PA criterion, it still acts as a fairly effective catalyst. We therefore expected that CO and CO2 , whose PAs (142 and 129 kcal mol1 , respectively, [18]) are higher, would reduce the isomerization barrier even further. Note also that CO and CO2 lack H atoms and so no complications
Fig. 1. Partial CID mass spectra of m/z 30 [C,H2 ,O]þ ions. Items (a) and (b) are the spectra of reference ions CH2 Oþ and HCOHþ , respectively [5]. Spectra (c) and (d) refer to the collision induced decarbonylation of glyoxal (1) and hydroxy ketene (1c) ions. Spectra (e) and (f) refer to the spontaneous and collision induced decarboxylation of glyoxilic acid ions (2).
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C.Y. Wong et al. / Chemical Physics Letters 387 (2004) 204–208
Fig. 2. Proton transport catalysis in ionized glyoxal. Energy diagram based on CBS-QB3 calculations.
can arise from alternative base assisted mechanisms, such as the Quid-pro-Quo mechanism [9,11]. Ionized glyoxal O@CHACH@Oþ (1), spontaneously undergoes one reaction in the ls time-frame, viz. loss of CO to produce m/z 30 ions. CID experiments on m/z 30 show that these ions are CH2 @Oþ to the exclusion of HCOHþ [19]. Direct bond cleavage to HCOþ + HCO does not take place, showing that the barrier for formation of CH2 @Oþ + CO lies below the threshold for dissociation to HCOþ + HCO , see Fig. 2. The MI spectrum shows that formation of CH2 @Oþ + CO is associated with an exceedingly small kinetic energy release [20], T0:5 ffi 0:007 kcal mol1 [19]. The reaction clearly takes place at threshold so that there is no acti-
vation energy for the reverse reaction CH2 @Oþ + CO ! 1. The CH2 @Oþ ion is also formed (to the exclusion of HCOHþ ) when 1 is induced to dissociate by a keV collision with oxygen, see Fig. 1e, and this rules out any formation of HCOHþ amongst ions formed in the higher energy regime. The calculations indicate that ions formed by vertical ionization of glyoxal collapse to the long bonded species O@(H)C C(H)@Oþ having a CAC bond of 1.86 A. This is not an ion–dipole complex but a one-electron bonded species [21]. The CH@Oþ part can easily donate a proton to produce the HBRC, HCOAHþ C@O (1a). The barrier for the PTC reaction, HCOAHþ C@O (1a) ! O@C(H)AHþ C@O (1b) is 8.5 kcal mol1 , compared to 37 kcal mol1 for the unassisted reaction, see Fig. 2. Note that the stabilization energy of 1b (10.3 kcal mol1 ) is significantly smaller than that of 1a (20.3 kcal mol1 ) as expected for a C H C vis a vis an O H C bonded species [21]. The isomerization barrier 1a ! 1b lies below the threshold for dissociation to CH2 @Oþ + CO making loss of CO the reaction of lowest energy requirement. Thus our calculations provide a rationale for the experimental observation that 1 produces CH2 @Oþ + CO at threshold. The key step in this decarbonylation is the transformation HCOHþ ! CH2 @Oþ catalyzed by CO. Our derived mechanism for the spontaneous decarbonylation of low energy ions 1 involves the sequence 1 ! 1a–PTC ! 1b ! CH2 @Oþ + CO, depicted in Fig. 2. A direct 1,2-H shift in the long bonded species O@CH CH@Oþ would also yield ions CH2 @Oþ . However, see Fig. 2, the associated barrier lies well above that for rearrangement to 1a, and also above the dissociation threshold. This pathway, therefore, does not pertain to the spontaneous decarbonylation of 1 in the ls time-frame. The glyoxal isomer HOACH@C@Oþ (1c), ionized hydroxyketene, also spontaneously decarbonylates into CH2 @Oþ [22]. This isomer is some 20 kcal mol1 lower in energy than 1, see Fig. 2. Its CID spectrum differs
Table 1 Enthalpies of formation (kcal mol1 ) and total energies (Hartree) derived from CBS-QB3 calculations Ionized glyoxal
Ionized glyoxilic acid
Isomer
Etotal (0 K)
DH 0f (298 K)
Isomer
Etotal (0 K)
DH 0f (298 K)
1 1a 1b 1c 1d
)227.14910 )227.14564 )227.14034 )227.17979 )227.13354
181.6 184.1 187.5 161.9 191.3
2 2a 2b 2c
)302.31515 )302.33109 )302.32929 )302.32938
127.9 118.4 119.6 119.6
)227.13211 )227.12341 )227.13211 )227.12130 )227.10752
192.3 198.2 192.6 199.4 207.3
TS TS TS TS
)302.30343 )302.29583 )302.29977 )302.30595
135.2 140.5 137.7 134.0
TS TS TS TS TS
1 ! 1a 1 ! 1b 1a ! 1b 1a ! 1c 1d
2 ! 2a 2 ! 2b 2 ! 2c 2a ! 2b
C.Y. Wong et al. / Chemical Physics Letters 387 (2004) 204–208
from that of 1 by the presence of a unique peak at m/z 41 (loss of H2 O) which constitutes 20% of the base peak at m/z 30. The decarbonylation of 1c begins with stretching of the C@C bond which requires as much as 37.5 kcal mol1 , after which HBRC 1a is formed. Next, the sequence of events depicted in Fig. 2 for the decarbonylation of glyoxal takes place. TS 1c ! 1a represents the highest energy level in the decarbonylation pathway of 1c, lying 1.6 kcal mol1 above CH2 @Oþ + CO. Consistent with this theoretical picture, the kinetic energy release associated with 1cÕs decarbonylation, T0:5 ¼ 0:36 kcal mol1 , is larger than that of 1. Ions 1c lie in a deep potential well with a high barrier for isomerization into 1a. It is therefore not surprising, see Fig. 1d, that the direct bond cleavage 1c ! HCOHþ competes with the rearrangement 1c ! 1a ! CH2 @Oþ when ions 1c are collisionally energized. When CH2 @Oþ and CO are allowed to interact under chemical ionization conditions [10], we find that the CID spectrum of the encounter complex at m/z 58 corresponds with mostly 1 and some 1c. This is precisely what the calculations of Fig. 2 predict. Such an encounter does not generate the stable distonic isomer CH2 –OACþ @O (1d): a high activation energy (9.5 kcal mol1 , Table 1 and Fig. 2) prevents this reaction. In line with this, the spontaneous decarbonylation of 1d (generated by loss of CH2 @O from ionized ethylene carbonate [22]) displays a large kinetic energy release (T0:5 ¼ 7 kcal mol1 ).
207
Decarboxylation is the only spontaneous reaction observed in the ls timeframe for glyoxilic acid ions, O@CHAC(OH)@Oþ (2). The resulting product ions, see Fig. 1e, are largely CH2 @Oþ but the enhanced intensities of the peaks at m/z 13 and m/z 17 reveal that some HCOHþ is also present. When stable ions 2 are collisionally energized, the decarboxylation remains the dominating reaction, but the product ions are now largely HCOHþ , see Fig. 1f. These observations are entirely compatible with the computational results of Fig. 3. As with 1, the optimized geometry of 2 corresponds to a long bonded structure, O@CH C(OH)@Oþ , having a CAC bond of 1.83 A. A formal 1,3-H shift in this ion generates HBRC 2c, HOCOHþ C@O, which would then shed CO. The barrier for this reaction, see Fig. 3, lies above the TS for the 1,4-H shift, and this may provide a rationale why decarbonylation is not observed. A formal 1,4-H shift in 2 produces HBRC 2a, HCOHþ O@C@O. This rearrangement, (TS 2 ! 2a) requires only 7.3 kcal mol1 . The resulting reactive ions 2a can either dissociate by direct bond cleavage into HCOHþ or else rearrange by PTC into HBRC 2b, O@C(H)AHþ O@C@O, and then lose CO2 . The calculated energy requirement for the PTC route lies only marginally below that for direct bond cleavage of 2a. Within the error margin of the calculations, 1–2 kcal mol1 [12], theory clearly supports experiment. The spontaneous decarboxylation may occur via the competing pathways 2 ! 2a ! HCOHþ and 2 ! 2a ! 2b ! CH2 Oþ . On the other hand, collisionally energized ions 2 of higher energy will preferentially dissociate by the kinetically more attractive direct bond cleavage of 2a. Thus the spontaneous decarboxylation of 2 involves PTC by CO2 . The barrier for the CO2 assisted reaction, 15.6 kcal mol1 , is much larger than that for the CO assisted PTC reaction in 1, 8.5 kcal mol1 . This clearly reflects that the PA of CO2 is more out of line with RadomÕs criterion. Nevertheless a significant reduction of the barrier is still achieved.
Acknowledgements J.K.T. thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for continuing financial support, and C.Y.W. thanks SHARCNET for a research award.
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Fig. 3. Proton transport catalysis in ionized glyoxilic acid. Energy diagram based on CBS-QB3 calculations.
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