JOURNAL OF MASS SPECTROMETRY, VOL. 31, 749-755 (1996)

Formation of a Stable ‘Proton-bridged Dimer’ bv DecomDosition of ~H~CH(OC~H,)CH(OC,H,)CH,+ in a Mass Spectrometer

Arielle Milliet,? Muriel Rempp and Georges Sozzi DCMR Ecole Polytechnique, URA CNRS 1307, F-91128 Palaiseau, France

A stable ethanal...H+...ethanol proton-bridged dimer (2) by decomposition of CH,CH(OC,H,)CH(OC,H,)CH~ ‘ was formed in the ion source of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer and also in the second field-free region of a reverse-geometry mass spectrometer. Its structure was established with collisional activation and FT-ICR spectra. The formation of this ion is preceded by extensive hydrogen exchanges and carbon rearrangement. These exchanges are the result of reversible isomer- izations between a series of ion-neutral complexes and hydrogen- and proton-bonded dimers. FT-ICR experiments reveal a selective substitution of ethanal by C,D,OH in the proton-bridged dimer 2.

KEYWORDS: electron impact ionization; mass-analysed ion kinetic energy; proton-bridged dimers: carbon rearrangement; 2,3-diethoxybutane

INTRODUCTION

The most important radical cation peaks in classical mass spectra are the result of the simple cleavage of a covalent bond, as is particularly well illustrated by the a-cleavage of alkyl ethers. In contrast, the low-energy spectra exhibit fragment ions resulting from a series of isomerizations involving transient species of different kinds. The most frequent are the following:

(i) an H-shift to the ionized site leading to the forma- tion of a transient distonic ion2 (Scheme l);

(ii) a covalent bond cleavage without separation of the molecular fragments, supplying ion-neutral com- plexes which are held together by ion-dipole and ion-induced dipole attractions3 (Scheme 2);

(iii) an H-migration with subsequent covalent bond cleavage resulting in the formation of a proton- bridged4 dimer (Scheme 3).

These different species have most often been identified as transient entities, lying in potential wells, in the frag- mentation processes. The structures of a few distonic ions have been established as stable fragment ions.5 More rarely, proton-bridged dimers have been charac- terized as the product ions of fragmentationx6

In previous work, we studied 1,2-dialkoxyethane radical cations and observed that they are good b- distonic ion precursor^.^ With the 2,3-diethoxybutane metastable radical cation, ,!?-distonic ion formation is no longer the major fragmentation process. In particular, the 1,Zdiethoxybutane radical cation produces few b- distonic ions but abundant proton-bridged asymmetric dimers, which will be demonstrated here.

EXPERIMENTAL

Unimolecular reactions occurring close to the energetic -x- - -X- threshold were studied by mass-analysed ion kinetic

+- H+ energy (MIKE) spectrometry in the second field-free region (2nd FFR) of a double-focusing, reverse- geometry VG ZAB-2F mass spectrometer, under stan- dard conditions: ionizing electron energy 70 eV and accelerating potential 8 kV. Collision activation (CA) reactions were examined, after collison with helium, in

Scheme 1

- [ i .,“-I H. H Scheme 2

+. o/ - H~c=o--H+--~-cHH,’ u H t Author to whom correspondence should be addressed. Scheme 3

CCC 1076-51 74/96/070749--07 0 1996 by John Wiley & Sons, Ltd.

Received 7 November 1995 Accepted 15 March 1996

750 A. MILLIET, M. REMPP AND G. SOZZI

the collision cell located behind the magnetic field; nor- mally, the collision gas pressure was adjusted so as to reduce the intensity of the main beam signal by 50%. The kinetic energy releases (KER) were determined after correction for the width of the main beam and calcu- lated from the width at half height

Bimolecular reactions and ion source fragmentations were studied with a Bruker CMS-47X Fourier trans- form ion cyclotron resonance (FT-ICR) spectrometer equipped with an external ion source. The neutral reac- tants were introduced through a leak valve (Balzers) at a pressure of 2 x lo-' mbar (1 bar = lo5 Pa) and diluted in an argon bath (total pressure 4 x lo-' mbar). Isolation of the proton-bridged dimer formed in the external ion source, after transfer to the ICR cell, was performed by r.f. ejection of all unwanted ions. When necessary, high-resolution measurements were per- formed to check the isotopic composition of the product ions.

D- and I3C-labelled 172-diethoxybutanes were pre- pared by standard methods detailed elsewhere.8

RESULTS

The EI mass spectrum of 2,3-diethoxybutane obtained by ionization of 2,3-diethoxybutane (1) in the external source of an FT-ICR mass spectrometer presents two major peaks at m/z 45 (base peak) and m/z 73 (53% of base peak (Fig. 1). The formation of CH3CH+OC2H5 oxonium ion (m/z 73) is the result of the expected cleav- age of the weakest bond. The main fragment ion, CH,CH+OH (m/z 45), may originate from the CH,CH+OC,H, fragment ion by loss of ethylene. A [C,H, ,02]+ fragment ion (m/z 91) (2), corresponding to loss of 55 mass units, which may be the butenyl radical, is also detected (4% of the base peak) (Fig. 1). In the spectrum of its isotopomer la, CH,CH(O '3CH,CH,)CH(0 13CH,CH,)CH,, the m/z 45 ion remains unchanged, the CH3CH+OC2H5 ion is shifted to m/z 74 and the loss of butenyl radical is spread at m/z 91,92 and 93 (Table 1).

In the second field-free region of the ZAB 2F mass spectrometer, metastable 1 dissociates, leading to C,H1102+ (m/z 91) as the major fragment ion (90%) accompanied by small amounts of [M - C2H60]+' (m/z 100) and [M - C2H40]+' (m/z 102) ions (Fig. 2). The MIKE spectrum of the 13C labelled compound l a

ao so 70 I I D

d Z

Figure 1. El mass spectra of (a) 2,3-diethoxybutane (1) and (b) its isotopomer (la) recorded using an FT-ICR mass spectro- meter.

reveals that all fragmentations are preceded by a rearrangement of the molecular ion (Table 1).

The MIKE spectrum of the deuterium-labelled com- pound [CH,CH(OC2D,)CH(OC2D,)CH3]+* (lb) shows that the fragmentations are also preceded by extensive hydrogen exchanges (Table 2).

Table 1. External ion source of FT-ICR mass spectrometer (YO of base peak) and MIKE spectra (% of the sum of all peaks) of 2,Methoxybutane (1) and its "C isotopomer (la) with To.5 (meV) in parentbeses

m/z 45 73 74 91 92 93 100 101 102 103 104

- - - - - - - 1 (source) 100 53 - 4 - - - - - 7 - 3 - - 90

(1 8) l a (source) 100 - 72 i l 2.5 3

1 (MIKE)

- - - (1 9) - (1 4)

- la(MIKE) - _ - 14.5 38.5 37.2 - 4.5 2.4 1.7 1.2

(18) (19) (17)

FORMATION OF A STABLE PROTON-BRIDGED DIMER 75 1

/

m/z

Figure 2. MIKE spectrum of 2,3-diethoxybutane (1).

Structure of C4H1102+ ion 2

The MIKE spectrum of the [C,H,102]+ fragment ion 2 generated in the ion source presents three peaks at m/z 45, 47 and 73, corresponding to the loss of ethanol, ethanal and of a water molecule, respectively. The inten- sity of the m/z 73 peak is observed to be much smaller in the CA spectrum (Table 3). No peak corresponding to the loss of methanol was detected. This observation rules out structures containing a methoxy group for this ion. The possible remaining candidates are protonated 2-ethoxyethanol (3) and 1-ethoxyethanol (4), and an ethanal-ethanol proton-bridged dimer (5) (Scheme 4). Protonated 1-ethoxyethanol (4) is obtained by loss of ethylene from protonated 1,l-diethoxyethane (Scheme 4). The proton-bridged ion 5 is generated by intro- ducing ethanal and ethanol into the chemical ionization (CT) ion source. Both the MIKE and CA spectra of protonated 2-ethoxyethanol (3) are different from those of 2, 4 and 5, which permits this structure to be dis- carded. The MIKE spectra of 2,4 and 5 are not signifi- cantly different from each other, which suggests that at low energy 4 and 5 may isomerize into one another before fragmenting. In contrast, the relative intensities of the peaks are substantially different in the CA spectra of 4 and 5. The CA spectrum of 2 is nearly identical with that of 5, which suggests that these two ions have the same structure (Table 3).

\'-OH + H+ __t

7 ,*H< 0-0"

3

'IH* + CH,=CH, OH 4

-i

Scheme 4

Bimolecular reactions of [ C4HI1O2] + ion 2

Another way to test the hypothetical structure of an ion is to study its bimolecular reactivity.

Ionization of 2,3-diethoxybutane in the external source of the FT-ICR mass spectrometer yields ion 2, which is isolated into the cell. Its energy-controlled CA spectrum (Par = 1.4 x lo-' mbar, excitation time 90 ps) shows only two major peaks at m/z 45 and 47, as does the ZAB-2F CA spectrum (Table 3).

The reactivity of the relaxed ion 2 with C,D,OH (P = 2 x mbar) was studied. In the 300 ms reac- tion time, the only observed reaction is a selective sub- stitution of the ethanal molecule by a deuterated ethanol molecule. At longer reaction times, other product ions appear but the substitution reaction keeps its total selectivity.

The proton-bridged dimer 2 contains an ethanal mol- ecule [proton affinity (PA) = 186.6 kcal mol-' (1 kcal = 4.184 kJ)] and an ethanol molecule ( P A = 188.3 kcal mol-') bonded by a proton. A substitution of ethanal by deuterated ethanol C,D,OH within the complex is not surprising but its total selectivity is unforeseen considering the small difference in the PAS (1.7 kcal mol-I). This total selectivity may be explained by geometrical considerations (Scheme 5).

The entering C,D,OH presents its oxygen atom towards the accessible hydroxylic hydrogen Hb within the complex, and the exothermically favoured dissociation leads to the observed [C,D,OH. - . H + - . .HOC,H,] ion formation (m/z 98). The study of this phenomenon is in progress.

This result corroborates the structure established with the ZAB-2F experiments, but is not a proof.

Table 2. Shifts of ion 2 in the FT-ICR ion source and MIKE spectra for l a and precursor l b

m P 91 92 93 94 95 96 97 98 99 100 101

- - _ _ - _ _ - l a (source) 10 43 47 l a (MIKE) 16 43 41 -

Stat. 2 13C/2 "C -+ 2 "C or 13Ca 16.7 66.6 16.7 - - _ _ _ _ _ - 1 b (MIKE) _ Stat. 10 D/8 H - + I 1 H or Do

- - _ _ _ - -

_ _ 7 5 2 1 2 4 7 7 9 20 - _ 0.5 5.5 22 37 26 8 1 <0.1 _

a Calculated statistical distribution of the isotopes, see text.

752 A. MILLIET, M. REMPP AND G. SOZZI

m/z 91 d z 98

Scheme 5

butenyl radical. Dissociation of D gives m/z 91, frag- ment ion 2.

Although mechanism 1 has been easily rejected by Table 3. CA and (in parentheses) MIKE spectra of isomeric

m/z 91 ions (% of the sum of all peaks) . .

mlz experimental proofs, there is no argument to make a Precursor Conditions 45 47 73 choice between mechanisms 2 and 3.

2,3-Diethoxybutane (1) + (2) El ZAB 33 65 2 M I K E (*) (60) (32) Thermochemistry El FT-ICR 28 72 -

2- Ethoxyethanol-H + (3)

1,l -Diethoxyethane-H' -

EthanoCH+-ethanal (5)

CI ZAB 53 7 40 (96) In complex C, the charge has been located on butene M I K E (4) -

+(4) CI ZAB 14 20 66 (C,), as it has the lowest ionization potential ( IF): M I K E (<I) (67) (33) but-2-ene 9.1 eV < ethanal 10.23 eV < ethanol 10.47 CI ZAB 25 71 4 eV. However, the loss of one electron in the . . MIKE (5) (71) (23) CH,CH=O. ..H-OC,H, part may result in great sta-

bilization of the C complex (C2), and this possibility must be considered. A comparative evaluation of the thermochemistry of these C, and Cz complexes would be helnful.

Fo;complex D, AH,(D) is estimated as follows : DISCUSSION

AHf(D) = AHf(CH3CH=CHCH,')

+ AHdC2H50H. * .H+- * *OCHCH3) + EstabD Larson and McMahong measured the hydrogen bond

strength of the asymmetric proton-bridged dimer 2 (C,H,OH- - .H+ . . OCHCHJ as 30.5 kcal mol-'.

Formation of (C,Hl,02]+ ion 2

Three hypothetical mechanisms, described below, may account for the formation of [C,H, 2.

Mechanism 1. 1,5-H transfer in the molecular ion 1 leads to the distonic ion 6. A l74-ethano1 migration to the radical site gives an isomeric distonic ion 7, in which a

yields an m/z 91 fragment ion having a protonated 1- ethoxyethanof structure (4) (Scheme 6).

spectra Of [C,H, ,O,] + isomeric ions (see above) (Table 3).

fragment ion AH,(D) = AH,(CH,CH=CHCH,')'O

+ AH&H50H2+)10 + AHf(CH,CHO)'O

- 30.5 kcal mol-I - EstabD

A ~ ~ ( ~ ) = 31.7 + 121 - 39.6 - 30.5 - E~~~~~

1,4-H transfer with a simultaneous C-0 cleavage = 82.5 kcal mOl-' - EstsbD

EstabD is rather low since the dipole moment of but-2- ene is zero and one can postulate that the dipole moment of the butenyl radical is of the same order of magnitude.

This mechanism is Out by the

Mechanism 2. After the cleavage in 6 of the C-0 bond leading to carbonyl group formation (Scheme 7), a 1,4- H shift concomitant with the last C-0 bond cleavage leads to the formation of an ethanol-ethanal proton- bridged ion with the loss of a butenyl radical.

This mechanism explains the formation of a proton- bridged species, the structure of which agrees with the obtained CA spectrum.

gor the C , complex:

AHdC1) = AHdCH,CH=CHCH:')

Mechanism 3. The cleavage of the two C-0 bonds in 6 yields the ion-neutral complex C, a Cbut-Zene, hydrogen-bonded ethanol-ethanal] radical cation (Scheme 8). A reversible hydrogen migration (see below) from but-2-ene to the ethanol oxygen of the H-bonded ethanal-ethanol entity yields complex D, composed of a proton-bridged ethanol-ethanal ion and a neutral

1

+ OH ;-8 2 3 r\ + 6 4 5 a4

1 *4 4

Scheme 6

FORMATION OF A STABLE PROTON-BRIDGED DIMER 753

6

? I-

\

Scheme 7

2

6

H H

C D

1 2

Scheme 8

The hydrogen bond strength" between the ethanol

AHAC,) = AHdCH3CH=CHCHi *)lo

and ethanal neutrals is known to be - 5 kcal mol-'.

+ AH&2HSOH)10 + AHf(CH3CHO)10

- 5 kcal mol-' - EstabCl

AHf(C1) = 207 - 56.1 - 39.6 - 5 - EstabCl

= 106.3 kcal m01-l - EstabCl

EstabCl is much higher than EstabD because the dipole moments of ethanol and ethanal add to give an impor- tant resulting dipole moment (pCthano, = 1.69 D and pethana, = 2.69 D) of 4.4 D in the H-bonded neutral part

The thermochemistry of Cz is less accessible. Griffin et al." computed the stabilization energy of a proton bridged dimer consisting of a protonated methanal and a methoxy radical (CH2=0. . .H+- - -'OCH3) at the MP3/6-31G(d,p) + ZPVE level. If one postulates that this stabilization energy (15 kcal mol-l) is of the same order of magnitude as that of C,H,O'. . . H + - . .O=CHCH,, then it is possible to cal- culate an approximate value for AH,&,).

AHAC,) = AHdCH3CH=CHCH3)'*

of c,.

+ AHf(C2H50')13 + AHf(CH3CH0H+)'O

- 15 kcal mol-I - EstabC2

AHI(C2) = - 12 - 4 + 139 - 15 - EstabCz

= 108 kcal mol- - Es,abC2

The value of EstabC2 must be as low as that of EstabD because the neutral part of the complex is butene instead of the butenyl radical in D.

Finally,

AHf(Cz) 4- = 108 > AHf(C1) + EstabCl = 106.3

and with EstabC2 < EstabCl then C, is less stable than C, and is rejected.

It may be surprising to observe an abundant peak corresponding to a proton-bridged dimer formation in 70 eV mass spectrum that can be ascribed to a molecu-

lar ion dissociating in the ion source. Its presence in the high-energy spectrum may be attributed to the strength of a proton bond which is weaker than covalent bonds of neutral molecules but not always than some weakened covalent bonds of radical cations. The AHf of the final state (FS2) corresponding to 2 formation is AHXFS2) = 82.5 kcal mol-' (see above).

The AHf of the final state corresponding to the simple cleavage leading to CH3CH+OC2H2 (m/z 73) formation (FS1) is

AHf(FS1) = AHf(CH3CH+OC2H5)'O + AH,(CH ,CH'OC,H,)

AHf(FS1) = 125 - 20 = 105 kcal mol-'

Hence AH,(FS2) lies 22.5 kcal mol-' below AHf(FS1). As CH3CH+OC2Hs is not formed at low energy (MIKE), the rate-determining step leading to 2 formation is lower than that leading to CH3CH+OC,H5 formation. Therefore, any ion pro- cessing enough internal energy to dissociate into CH3CH+OC2Hs + CH3CH'OC2H, will fragment and the remaining ions able to form 2 will not have SUE- cient internal energy to undergo proton bond fragmen- tation (30.5 > 22). Hence the presence of a large peak at m/z 91 in the high-energy spectrum is elucidated.

Hydrogen exchanges and carbon rearrangement in 1

Hydrogen exchanges and carbon rearrangement prior to fragmentation of metastable 1 may be rationalized by the following pathway (Scheme 9): cleavage of the C-2-C-3 bond yields the ion-neutral complex A, which dissociates in the ion source leading to the rn/z 73 ion; reversible hydride migration within ion-neutral complex A leads to complex B and in this complex the l2C-0 and 13C-0 carbons of the diethyl ether moiety are indistinguishable; a hydride transfer from diethyl ether to the ethyl vinyl ether radical cation leads to A and finally 1' by a covalent recombination in which the I3C has been incorporated into the butane chain.

By this process, all hydrogens lose their original posi- tions. In Table 2, the statistical distribution of the four

154 A. MILLET, M. REMPP AND G. SOZZI

r _.

1 L

A 'Id +q I k

B

1' A '

* = 13c

Scheme 9

oxygen-bonded carbon atoms (two ''C and two 13C) into an ion keeping two of these is reported. One can observe that the reversible reactions described in Scheme 9 do not reach equilibrium for l a either in the 2nd FFR or in the FT-ICR external ion source. The formation of an abundant product ion at m/z 93 may correspond to the fragmentation of the molecular ion in its initial structure. The proportions of ions at m/z 91, 92 and 93 seem not to be time dependent (Table 2). One can postulate that there are two discrete populations of molecular ions, both of which, however, have the same metastable path. The major part of 1 goes through the isomerization pathway 1 e A $ B e A s 1' before the 1,5-H transfer in 1 leading to all fragment ions. The rel- eased kinetic energies (To,5) are the same for m/z 91, 92 and 93 (Table l), the interpretation of which may be that the two populations go through the same rate- determining step, most probably the 1,5-H transfer 1 +6.

Similarly, the statistical distribution of ten D and eight H in an ion containing eleven of these is given in Table 2.

The experimental results show that the statistical dis- tribution of hydrogens and deuteriums is not reached in metastable lb (CH,CH(OC,D,)CH(OC,D,)CH~ '). The important intensity of the m/z 101 and 100 peaks for l b reveals that two exchanges are competing, one involving only one deuterium scrambling with eight hydrogens (m/z 100 and 101, Scheme lo), corresponding to the population of unchanged molecular ions, and another slower, which is described in Scheme 9, involving all deuteriums and hydrogens of the molecular ion.

The C and H exchanges observed in metastable l a and l b do not allow the adoption of one of the mecha- nisms 2 and 3. Indeed, the C exchanges and the exchange of ten H and eight D occur before the 1,5-H transfer from 1 leading to the distonic ion 6 in any case. The exchange of a single D with eight hydrogens described in Scheme 10 may be explained by mecha- nism 2 if one supposes that ion 2 does not dissociate from the butenyl radical but forms complex D reversibly isomerizing with complex C before fragmen- ting (Scheme 11).

CONCLUSION

Usually asymmetric proton-bridged dimers are pre- pared in the cell of an FT-ICR spectrometer by intro- ducing the two corresponding neutrals. The study of its reactivity with another neutral is complicated because three different neutrals are present together in the cell. Here we propose a reliable method for the preparation of an asymmetric proton-bridged dimer which can be generated in the EI external ion source of the FT-ICR spectrometer, in order to study its reactivity with a single neutral. However, the scrambling of hydrogens and carbons restrict the utilization of labelled com- pounds.

4 2 101

I rnh 100

Scheme 10 H

D C Scheme 11

FORMATION OF A STABLE PROTON-BRIDGED DIMER 755

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