Evaporation of dimers from proton-bound acetic acid clusters

Mass Spectrometry

ELSEVIER

and Ion Processes

International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 223 232

Evaporation of dimers from proton-bound acetic acid clusters C h a v a Lifshitz *'l, W a n Y o n g F e n g Department qf Physical Chemistry and The Fritz Haber Research Center [i)r Molecular Dynamics, The Hehrew ~;niversity ql.lerusalem. Jerusalem 91904, Israel

Received 3 November 1994: accepted 2 February 1995

Abstract

Proton-bound clusters of acetic acid and mixed clusters of acetic acid and water evaporate neutral dimers of acetic acid. This attribute is shared with liquid acetic acid. Clusters are considered to bridge the gap between the gas phase and condensed phases. The evaporation of dimers begins to be observed with the heptamer. Cluster binding energies to monomer and dimer units were deduced from kinetic energy releases. The values for the low members of the series are in quite good agreement with previous data from high pressure mass spectrometry. Kinetic energy releases were remeasured and re-evaluated also for proton-bound formic acid clusters. Thermochemical cycles lead to the following bond energies of the evaporated neutral dimers: 0.58 :t: 0.07 eV for acetic acid and 0.66 4- 0.13 eV for formic acid. These values indicate, contrary to previous conclusions, that the neutral dimers evaporate in the ring-closed, two hydrogen-bonded, form. Kevwords: Binding energies; Clusters; Kinetic energy release

1. Introduction

Clusters are considered to bridge the gap between the gas phase and condensed phases. Formic acid and acetic acid are well known to form stable dimers both in the gas phase [1] and in the liquid [2]. An exciting recent finding demonstrated [3] that dimers evaporate as such from the liquid surface into the gas phase. We have studied recently [4] proton-bound formic acid clusters and mixed clusters of formic acid and water and observed evaporation

£ Dedicated to Professor AI Nier for his outstanding contributions to mass spectrometry. * Corresponding author. Archie and Marjorie Sherman Professor of Chemistry.

of neutral dimers of formic acid. The lower members of the cluster series evaporate monomers; however, evaporation of dimers was observed to begin already with the hexamers. Cluster binding energies to the monomer and dimer units were deduced from the analysis of kinetic energy release distributions (KERDs) employing the theoretical approach of Klots [5]. This treats the unimolecular decompositions of clusters as evaporations from small particles having characteristic temperatures. The single-step evaporation of dimer units from the formic acid clusters was demonstrated [4] by the technique of reionization of neutral fragments. One of the tentative suggestions which came out of the research on the formic acid clusters

0168-l176/95/$09.50 ;('~ 1995 Elsevier Science B.V. All rights reserved SSD1 01 68-1 176(95)04183-4

224

C. Lifshitz, W.Y. Feng/International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 223-232

[4] was that while the most stable neutral dimer structure of formic acid is known to be cyclic, the dimers evaporate from the proton-bound clusters in the open chain form. A pronounced drawback of the formic acid study was that there were no independent measurements of binding energies--for example from high pressure mass spectrometry--with which the data deduced from the K E R D s could be compared. Binding energies for the low members of proton-bound acetic acid clusters are known from high pressure equilibria studies [6]. Preliminary studies [4] have demonstrated dimer evaporations of acetic acid from neat protonbound acetic acid clusters. The present study is an extension of those preliminary studies with the following objectives: (1) comparing binding energies deduced from K E R D s with known values where available [6]; (2) repeating and refining the kinetic energy release (KER) studies for formic acid clusters; (3) trying to decide whether the dimers evaporate in the ring-closed or open chain configuration.

2. Experimental Measurements were performed on the VG ZAB-2F as previously described [4]. Ions were formed by electron impact in a temperature- and pressure-variable source [7]. The metastable fragmentations were studied by MIKES. Metastable ion peak shapes were determined by scanning the ESA (electrostatic analyser) and using single-ion counting. Ion counting was achieved by a combination of an electron multiplier, amplifier/discriminator, and multichannel analyzer [8]. The metastable ion peak shapes obtained were mean values of several hundred accumulated scans. This was done in a computer-controlled experiment, monitoring the main beam scan and correcting for the drift of the main beam [9]. We have recently found an error in the

K E R measurements of the formic acid clusters. This was due to the use of faulty potentiometers in connection with the multichannel analyzer scans. The energy scale was recalibrated prior to the beginning of each of the present set of experiments. The original experiments were corrected through recalculation of the data as well as through repeating the actual experiments until the two correction methods gave the same results. The kinetic energy spread in the parent ion beam was subtracted from the width recorded for each of the fragmentation processes. Kinetic energy release distributions (KERDs) were obtained from the first derivatives of the metastable ion peak shapes [10]. Collisional activation spectra were obtained using air as collision gas. Some of the metastable ion intensities, specifically for dimer loss from the pentamer of formic acid and for the hexamer of acetic acid, were very weak and contamination from a C A D (collisionally activated dissociation) contribution cannot be totally excluded. However the pressure in the second field-free region used in unimolecular MIKES was low enough (~< ~ 10 .8 mbar) to exclude contamination from CAD in all other instances. Acetic acid from Aldrich Chemical Company was of stated 99.8% purity. Some samples were spiked with about 1% water impurity.

3. Results and discussion 3.1. Mass spectra, cluster size distributions and magic numbers

Two major cluster series were observed (Fig. l(a)) for pure acetic acid spectra, (CH3COOH)nH + (n = 1-9) and (CH3COOH)nCH3CO + (n = 1-7). Contrary to the formic acid case, where a minor (HCOOH)£ + series was observed, no ((CH3COOH),~ + clusters were found under

C. L![~'hitz, W.Y. Feng/International Journal ~f Mass Spectrometry and Ion Processes 146,'147 (1995) 223 232

the present conditions. The cluster spectra shift to higher masses with decreasing temperatures in the temperature-variable ion source. Spiking with 1% water produced a spectrum (Fig. l(b)) with the additional I00

cluster series (CH3COOH)n(H20)H + and a very pronounced peak ("magic" number) at (CH3COOH)5(H20)H +. Ions of special stability lead to magic numbers in cluster mass spectra and the latter ion has been

A! ]~

An=(CH3COOH)n H+ +

Bn-(CH 3 COOH)nCH 3 CO

80

a. T=249 K, P=0.024 torr 6O A2 B

40

A6

A5 B2

20

A4

L 0

I

i

50

I

100

i

150

i

i

200

i

i

I

250

]6i7

Ii 5 I

3OO

i

I

l

350

i

i

i

400

i

,

450

I

500

m/z 100

c5 An=(CH3COOH)nH + +

Bn=(CH 3COOH)nCH3CO 80 Cn=(CH3COOH)n(H20)H + b. T=251 K, P=0.030 torr ; ~ 6O

40

20

B2 A3 AI

BI 1 2

50

Ii C l

,

100

A6

A5

C6

B5 B3

i

i

200

A7 C7

4

I

150

225

250

I

I

i

300

350

I

,

I

400

I

I

450

I

l! 500

rn/z Fig. 1. Acetic acid cluster ion mass spectra: (a) a sample of pure acetic acid; (b) a sample containing 1% water.

226

C. Lifshitz, W. Y. Feng/ lnternational Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 223-232

demonstrated previously [11] to be of special abundance. 3.2. Unimolecular reactions

The M I K E spectra of the cluster series (CH3COOH)n H+ demonstrate two major reactions: evaporation of a single acetic acid unit and evaporation of an acetic acid dimer unit (CH3COOH)n H+ ~ (CH3COOH),_I H+ + CH3COOH

(1)

(CH3COOH)~H + ---, (CH3COOH),_2 H+ + (CH3COOH)2

(2)

The low members of the series undergo reaction (1) preferentially while the high members of the series undergo reaction (2) (Fig. 2). Proton-bound formic acid clusters behave

...

100

o

80

n-

6O

01 .-J= 0

40

*" .= m

:M-loss =~ uni

CH,COOH) H°

800

20 D-Io.ss ~ / /

0

similarly [4] but the switching from preferential m o n o m e r evaporation to preferential dimer evaporation occurs for n = 6 in the ( H C O O H ) , H + series and for n = 8 in the (CH3COOH),H + series. C A D changes the ratio between the two channels (Fig. 2) to a limited extent. Equal probabilities of monomer and dimer losses are still observed at n = 7, but lower members (n = 3-6) have enhanced losses of two acetic acid units under CAD, which are most probably consecutive. The cluster series ( C H 3 C O O H ) , ( H 2 0 ) H + undergoes three reactions: H 2 0 evaporation for n = 1-5, CH3COOH evaporation for n = 6 and (CH3COOH)2 evaporation for n = 7 and 8. The general behavior is again similar to that for the ( H C O O H ) , ( H 2 0 ) H + series, but in the latter case H C O O H evaporation competes with H 2 0 evaporation already forn =4. The cluster series (CH3COOH)nCH3CO + undergoes acetic acid m o n o m e r and dimer evaporations as well.

i

_- = = ;

_

'~C C I J: 0 G,

~

100

o

8O

re

60

O~ J= 0=

40

._c .=

m

"~.~loss

/

0

o

640 480 j

320 160 0 5550

5585

5620

20 Energy, oV

0

123456789 Cluster Size, n Fig. 2. Branching ratios (%) for monomer (M) and dimer (D) loss from proton-bound acetic acid clusters as a function of cluster size: upper part, unimolecular evaporations; lower part, collisionally activated dissociations.

Fig. 3. Metastable ion peak shape for dimer evaporation for the protonated heptamer of acetic acid. The reaction takes place in the second field-free region of the ZAB-2F mass spectrometer. The electrostatic analyzer voltage is scanned, and ion counts are accumulated on the multichannel analyzer. Ion counts are plotted as a function of ion energy (in the laboratory frame). The main beam [(CH3COOH)7 H+] had an energy of about 7800 eV.

C. Li#shit=, W.Y. Feng/International Journal of Mass Spectrometry and Ion Processes 146,'147 (1995) ~ 3 .3~

The metastable peak shapes were all pseudo-Gauss±an. A typical example is shown in Fig. 3 for reaction (2) in the protonated acetic acid heptamer, (CH3COOH)7 H+. The K E R D s obtained were Boltzmann like, as is seen in Fig. 4 for the dimer loss from acetic acid heptamer. The average kinetic energy releases (KERs) (~) deduced from the distributions, are summarized in Tables 1 and 2. The ion source pressure and temperature employed in these measurements were 0.03 Torr and 251 K. The KERs found in the formic acid system (Table 1) are higher in the present study than in the previous one [4]. The present data are considered to be more reliable, in view of the recalibration of the energy scale. The average energies for reactions (1) and (2) are plotted as a function of cluster size in Fig. 5. The average KERs for monomer and dimer evaporations are similar for n = 7, the cluster for which the relative abundance of the two evaporations (Fig. 2) are nearly the same as well. This similarity in the KERs suggests that the two evaporations take place competitively and in parallel from the same ion structure [4]. For two parallel reactions to occur in the field-free region, their rate constants have to be nearly equal. The unimolecular decompositions of clusters are considered to be evaporations having universal pre-exponential A factors [5]. As a

1.0

~.

0.8

. m

.Q

.Q O a.

0.6

>

0.4

t~ fl-

0.2

0.0

0.00

0.06 C.M. Kinetic

Energy,

0.12 eV

Fig. 4. Product kinetic energy release distribution for rectastable loss of acetic acid dimer from protonated acetic acid heptamer: O, experimental; - - , model fit.

3.3. Kinetic energy releases

Kinetic energy releases were determined for reactions (1) and (2) in acetic acid and for the analogous reactions (HCOOH)n H+ ~ (HCOOH)n_I H+ + HCOOH

(3)

' H C O O H ) . H + ~ (HCOOH)._2 H+ + (HCOOH)2

227

(4)

m formic acid. Table 1 Kinetic energy releases for unimolecular reactions of ( H C O O H ) . H ~ Cluster size,

Monomer loss (~), meV [ T:;, K

n

2

3

4

5

6

7

8

12 ± 4 0.56 71 ± 3

23 + 1 0.56 167 + 13

27 ± 3 0.57 191 ± 10

27.3 ± 0.9 I).56 204 ± 7

33 ± 2 0.56 227 ± 26

31 ~_ 5 (I.57 241 ± 26

28.0 ± 0.4 0.59 219.5 ± 0.7

93 ± 3 0.47 4 5 0 + 15

27.2 ± 0.3 0.60 204+2

33 ± 2 0.60 251± 14

30 ± 3 0.60 213± I1

Dimer loss @), meV l T ~, K C,,

6

13

21

27

33

39

45

C. Lifshitz, W.Y. Feng/International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 223-232

228

Table 2 Kinetic energy releases for unimolecular reactions of ( C H 3 C O O H ) , H + Cluster size, n

M o n o m e r loss (c), meV l T~,K Dimer loss (c), meV l T~,K Cn

2

3

4

5

6

7

29 4- 1 0.57 222-t-12

26.5 + 1.5 0.58 201+9

24 + 2 0.55 1694-4

28 4- 2 0.57 2144-9

23.7 + 0.6 0.57 178±2

22 + 1 0.58 1554-5

83 ± 1 0.51 596 4- 21

24.7 4- 0.8 0.58 189 + 8

8

16

25

39

32

result, the critical energies of activation of the two parallel reactions of the heptamer of acetic acid have to be nearly equal. For reactions having no reverse activation energy, this leads to similar KERs, since the excess energy required for decomposition in the metastable time window is the same. The K E R for dimer loss from n = 6 (Fig. 5) is quite high. A similar situation exists for the formic acid clusters at n = 5 (Table 1 and Ref. [4]). These reactions are very weak and could be collision induced [4]. Alternatively, dimer evaporation from the acetic acid hexamer (CH3COOH)6 H+ could

8

46

53

be due to an isomer different from that involved in m o n o m e r evaporation [12]. The average KERs for m o n o m e r and dimer evaporations from (CH3COOH)nCH3CO + are plotted as a function of cluster size in Fig. 6.

3.4 Binding energies The unimolecular decompositions of the clusters may be viewed as evaporations from small particles. This process has been treated theoretically by Klots [5]. It has been proposed 50

90

D-loss

72

40

M-loss +

"~ :) O

>' o

54

D-loss

30

E

E

A

v

20 4- 1 0.58 143 4- 5

36

M-loss

A

,,

V

20

10

18

2

3

4

5

6

7

8

9

Cluster S i z e , n Fig. 5. Plot of the average kinetic energy release (~) vs cluster size n for m o n o m e r (M) and dimer (D) loss from (CH3COOH)nH +.

2

3 Cluster

4

5 Size,

6

7

n

Fig. 6. Plot of the average kinetic energy release (~) vs cluster size n for m o n o m e r (M) and dimer (D) loss from ( C H 3 C O O H ) . C H 3 C O ÷.

C. L(]~hitz, W.Y. Feng/lnternational Journal Of Mass Spectrometry and lon Processes 146/147 (1995) 223 232

that the average kinetic energy with which a monomeric unit leaves the surface of an aggregate can measure the temperature for the transition states, T~. This assumption holds, provided the decomposition reaction does not demonstrate a reverse activation energy. The pseudo-Gaussian metastable peaks obtained for all the present decompositions (equivalent to evaporations) suggest the absence of reverse activation energies. This idea was developed further by Klots [5] by considering the full K E R D . It allows one to extract the vaporization energies (i.e. binding energies) of the clusters from the KERDs. In the model-free approach, the K E R D is written in the form

p(e) ~ eZexp(-~/kBT{)

0 ~< l ~< 1

(5)

where e is the kinetic energy, kB is Boltzmann's constant, T~ is the transition state temperature, and l is a parameter. The K E R D s for all the reactions studied could be fitted by Eq. (5). An example of the quality of the fit is shown in Fig. 4. The parameters T~ and l were extracted from the fits, as previously explained [4,13], and are included in Tables 1 and 2. Once T~ is extracted from the K E R D , T b may be calculated [14] from Th = T $ e x p ( 7 / C ) '7/c

1

(6)

where T b is the isokinetic temperature to which a heat bath must be set to yield a thermal rate constant k(Tb) equal to the microcanonical rate coefficient k(E) characteristic of the cluster decomposition; "7 is the universal Gspann parameter related to the pre-exponential A factor, '7 = 23.5 + 1.5 [15], and C is the cluster heat capacity in units of k~ minus one. The cluster vaporization energy AEva p is calculated from Trouton's rule [4,5,13-15]: AEvap kBTb -- '7

(7)

229

The parameter which is least well known is the heat capacity. The values employed in the present calculations of AEvap are included in Tables 1 and 2. They are calculated assuming open chain clusters for low n members of the series, and open chain or cyclic structures for higher members of each series. This leads to contributions of 6 ( n - 1) or 6n for the intercluster modes in the open chain or cyclic configurations, respectively. In addition, contributions for some of the soft modes, e,g. methyl and peripheral OH internal rotations, were added. The calculated vaporization energies are presented in Table 3 and those for acetic acid clusters are plotted in Fig. 7. The values for (CH3COOH)n H+, n = 2 5, are compared with Meot-Ner's results [6]. Equilibria measurements on acetic acid clusters presented a problem [6] in that the neutral vapor dimerizes, leading to unreasonable results below 240 K. The value for n =- 5 was based on a AG: value at a single temperature and on an estimate of A S '~. The proton-bound dimer binding energy deduced from the K E R D is obviously unreliable, but the values deduced for n --- 3-5 are in quite good agreement with Meot-Ner's values. The dimer is usually the exception when applying the theoretical approach of Klots [5]; this has been observed in the case of ammonia clusters [14] and binding energies were derived from KERs [14,16] only tbr n >~ 4. The agreement between the high pressure mass spectrometry data for n = 3-5 in acetic acid and the present binding energies deduced from the K E R D s is gratifying and leads us to believe that the present values for n > 5 are also reliable. Our results indicate that the binding energy of the cluster to the m o n o m e r is still dropping, for n > 5. Furthermore, neutral dimer evaporation takes over for higher n values. It is thus questionable whether a limiting value for AEvav has been reached, which can be compared with the bulk vaporization energy, as

C. Lifshitz, W.Y. Feng/International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 223-232

230

Table 3 Monomer and dimer binding energies, ALva p (eV) Cluster

n ~ n - 1

(HCOOH),H +

2 ---, 1

1.8 ± 0.08

3 ~ 2

0.95 + 0.08

4 ~ 3

0.72 ± 0.08

5 ---* 4

0.66 4- 0.04

5~3

1.454-0.1

6 --, 5

0.67 4- 0.08

6 ~ 4

0.60 :i: 0.02

7 ~ 5 8 ---, 6

0.70 ± 0.05 0.57 + 0.04

6 --~ 4 7 ---, 5 8 4 6

1.66 ± 0.09 0.50 4- 0.03 0.36 4- 0.02

(CH3COOH)nH +

AEvap

7 4 6

0.67 4- 0.08

8 ~ 7

0.58 4- 0.01

2 ~

1.22 a 2.7±0.2 0.80 a

1

3 ---* 2

n ~ n - 2

AEwp

0.93 ± 0.05 4 --+ 3 5 4 4

0.57 a 0.57 4- 0.04 0.52 a

6 4 5 7 4-4 6

0.49 + 0.02 0.41 4- 0.02

0.64 + 0.05

847

a Data from Ref. [6]; the notation here is different from that in Ref. [6] where dimerization is 0 ~ 1, formation of the trimer from the dimer is 1 --* 2, etc.

has been done for the limited acetic acid cluster series studied previously [6]. Mixed proton-bound formic acid and acetic acid clusters evaporate formic acid preferentially [4]. Even (HCOOH)(CH3COOH)4 H+ evaporates formic acid. Furthermore, the

3.0 o

2.4 0 >, 0 I11 O) C "0

1.8

M-Iss

•

1.2

0.6

0.0

1

2

3

4

5

6

7

8

9

Cluster Size, n Fig. 7. Plot for binding energies of(CH3COOH)nH+: ©, present calculated results for monomer (M) loss; II, present calculated results for dimer (D) loss; e , experimental high pressure mass spectrometry data from Ref. [6].

bulk vaporization energy /kHv°ap of acetic acid is higher (12.8 kcal m o l - b than that of formic acid (11.0 kcal tool -1) [6,17,18]. Nevertheless, the binding energies we deduce for the n=6-8 members of the (HCOOH)nH + cluster series are higher than for the corresponding (CH3COOH)nH + series (Table 3). Whether this is a real effect or an experimental error is an open question. We are nevertheless in a position to decide whether the dimers evaporate in the open chain or ring-closed configuration. Acetic acid and formic acid dimers are well known to exist in the closed (two hydrogen bond) as well as in the open (one hydrogen bond) forms [19,20]. The energies of dissociation to monomers of the closed form dimers are about equal for formic and acetic acids, the preferred values being 0.64 eV [1]. However, sequential breaking of the cyclic structure was proposed [19,21]. The first step-ring opening of the dimer--requires 0.17 eV (3.94 kcal mol -i) [19]. The structures of the proton-bound formic acid and acetic acid clusters are unknown and ab initio

C. Lifshitz, W.Y. Feng/International Journal of Mass Spectrometry and Ion Processes 146/147 (19953 223 232

calculations have not yet been carried out. Long chain or ring forms are possible, from which the neutral monomers or dimers evaporate. However, it is also feasible that beyond a certain critical size the dimers are present as such in the proton-bound cluster. What we can presently perform are thermochemical calculations based on our experimental results for (CH3COOH)7 H+ and for ( H C O O H ) n H + ( n = 6-8) given in Table 3 from which we can deduce the binding energies of the neutral dimers being evaporated. For example for acetic acid (Ac) Ac7 H+ ---~Ac6 H+ + Ac AEvav = 0.41 :t: 0.02eV

(8)

231

While the error limits are quite large, our present results (Eqs. (123 and (133) indicate that the neutral dimers evaporated from the acetic acid heptamer and from the formic acid hexamer, heptamer and octamer are in the ring-closed, two hydrogen-bonded, forms.

4. Conclusion Binding energies deduced from K E R D s for proton-bound acetic acid clusters are in good agreement with results from high pressure mass spectrometry equilibria studies [6]. Neutral dimers evaporate from protonbound acetic acid and formic acid clusters in the ring-closed, two hydrogen-bonded, form.

Ac7 H+ ~ Ac5 H+ + Ac2 AEva p = 0.50 + 0.03 eV

(9)

Acknowledgment

therefore AHf(Ac5 H+) + AHf(Ac)2 = AHf(AcoH +) + AHf(Ac) + 0.09 4- 0.05 eV

This research was funded by The James Franck Research Center.

(10)

or

References

AHf(Ac5 H+) + AHf(Ac)2 + AHf(Ac) = AHf(Ac6 H+) + 2AHf(Ac) + 0.09 ± 0.05 eV which means that O(Ac)2 = O(Ac6 H+) + 0.09 + 0.05eV

(11)

but the binding energy of the hexamer is known (Table 3) to be D(Ac6 H+) = 0.49 + 0.02eV, therefore the dissociation energy of the acetic acid dimer to two monomers which we deduce from our measurements is D(Ac)2

= 0 . 5 8 4-

0.07eV

(12)

In a similar fashion we obtain from the formic acid (F) measurements D(F)2 = 0.66 i 0.13eV

(13)

[1] J. Chao and B.J. Zwolinski, J. Phys. Chem. Ref. Data, 7 (1978) 363. [2] A. Ben-Naim, Hydrophobic Interaction, Plenum, New York, 1980. [3] M. Faubel and Th. Kisters, Nature, 339 (1989) 527. [4] W.Y. Feng and C. Lifshitz, J. Phys. Chem., 98 (1994) 6075. [5] C.E. Klots, J. Chem. Phys., 83 (1985) 5854; Z. Phys. D, 5 (1987) 83: J. Phys. Chem., 92 (1988) 5864; Z. Phys. D, 20 (1991) 105; Z. Phys. D, 21 (19913 335: J. Chem. Phys., 98 (1993) 1110. [6] M. Meot-Ner (Mautner), J. Am. Chem. Soc., 114 (19923 3312. [7] P.A.M. van Koppen, P.R. Kemper, A.J. Illies and M.T. Bowers, Int. J. Mass Spectrom. Ion Processes, 54 (19833 263. [8] C. Lifshitz and F. Louage, J. Phys. Chem., 93 (1989) 5633. [9] N.J. Kirchner and M.T. Bowers. J. Phys. Chem., 91 (19873 2573. [10] J.L. Holmes and A.D. Osborne, Int. J. Mass Spectrom Ion Phys., 23 (1977) 89. C. Lifshitz and E. Tzidony, Int. J. Mass Spectrom. Ion Phys., 39 (1981) 181. M.F. Jarrold, W. Wagner-Redeker, A.J. lilies, N.J.

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Kirchner and M.T. Bowers, Int. J. Mass Spectrom. Ion Processes, 58 (1984) 63. [11] M. Tsuchiya, S. Teshima, T. Kaneko and T. Harano, J. Chem. Soc. Jpn., 6 (1993) 687. S. Teshima, T. Kaneko, Y, Yokoyama and M. Tsuchiya, 12th International Mass Spectrometry Conference, 26-30 August 1991, Amsterdam, Book of Abstracts. [12] S. Wei, W.B. Tzeng and A.W. Castleman, Jr., J. Phys. Chem., 94 (1990) 6927. [13] C. Lifshitz, P. Sandler, H.°Fr. Grtitzmacher, J. Sun, T. Weiske and H. Schwarz, J. Phys. Chem., 97 (1993) 6592. [14] C. Lifshitz, in C.Y. Ng, T. Baer and 1. Powis (Eds.), Cluster Ions, Wiley, New York, 1993, pp. 121-164. [15] C.E. Klots, Int. J. Mass Spectrom. Ion Processes, 100 (1990) 457.

[16] S. Wei, W.B. Tzeng and A.W. Castleman, Jr., J. Chem. Phys., 93 (1990) 2506. [17] D.R. Stull, E.F. Westrum and G.S. Sinke, The Chemical Thermodynamics of Organic Compounds, Wiley, New York, 1969. [18] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney and R.L. Nuttal, J. Phys. Chem. Ref. Data, 11 (1982) Suppl. 2. [19] R.D. Corsaro and G. Atkinson, J. Chem. Phys., 54 (1971) 4090. [20] K.I. Lazaar and S.H. Bauer, J. Am. Chem. Soc., 107 (1985) 3769. [21] D. Borchardt, J.F. Caballero and S.H. Bauer, J. Am. Chem. Soc., 109 (1987) 6651.