Spectroscopy 14 (1999) 41–50 41IOS Press

Mass spectrometric fragmentation reactionsof dialkylselenides1

G. Filsak and H. Budzikiewicz∗

Institut für Organische Chemie der Universität zu Köln, Greinstr. 4, D-50939 Köln, Germany

Abstract. Mass spectrometric fragmentation and especially rearrangement reactions of dialkyl selenides after electron ioniza-tion will be discussed.

1. Introduction

In contrast to the ample investigations of the electron impact induced fragmentation behavior ofaliphatic ethers and thioethers little is known about that of the corresponding selenides [2]. So far, themass spectra of several R2Se (R= CH3 to n-C4H9) [3,4] and CH3SeR (R= C2H5 to n-C4H9) [5] rep-resentatives have been published, but suggested fragmentation paths were not confirmed by labellingstudies. These results convey the following general picture:

– The relative abundance of M+· is higher than for the analogous O or S compounds. Due to the lowerionization energy the percentage of molecular species with insufficient amounts of internal energyfor fragmentation is higher.

– Cleavage of one Se–R bond is a preferred process, but for higher alkyl groups the charge stayspreferentially with the alkyl residue (for R2Se with R> C2H5 loss of ·SeH from RSeH+ maycontribute to this ion); only CH3Se+ is of medium to high abundance.

– α-cleavage is not favored (only CH3SeCH+2 is of medium abundance), also (with increasing chain

length)β- and higher cleavages are of minor importance.– RSeH+· is of high abundance and – as appropriatem∗ show – may decompose further by the loss

of ·SeH yielding R+.– H2Se+· from (CH3)2Se (rel. int.> 70%) is (at least partially) formed directly from M+· as appro-

priatem∗ for this compound and for itsd6-analog show. Although the observation of am∗ doesnot necessarily imply that the neutral (C2H4) is lost as an entity, elimination of two CH2-groupsor any other combination of neutrals not comprising a C2-unit would be highly unlikely for ener-getic reasons. Whatever rearrangement may precede the elimination of C2H4 (e.g.,→ CH3SeH+–CH·2 → CH3–CH2SH+· or a seleniran-intermediate) the reaction (CH3)2Se+· → H2Se+·+ C2H4

(983+52−827 = 208 kJ/mol) is energetically more favored than (CH3)2Se+· → CH+2 SeH+ CH·3

(916+146−827= 235 kJ/mol) [6]; regarding the structure of {CH3Se}+ see below. For (C2H5)2SeH2Se+ still has a rel. int. of 44%, but for higher dialkylselenides it becomes neglegible.

1Part XXXVII of the series “Mass Spectrometric Fragmentation Reactions”. For part XXXVI see [1].* Corresponding author: Prof. Dr. H. Budzikiewicz. Fax: +49 221 470 5057; E-mail: h.budzikiewicz@uni-koeln.de.

0712-4813/99/$8.00 1999 – IOS Press. All rights reserved

42 G. Filsak and H. Budzikiewicz / Mass spectrometric fragmentation reactions of dialkylselenides

To get information regarding possible rearrangement processes during fragment formation of sele-noethersn-butyl-methyl andn-butyl-ethyl selenides were synthesized with deuterium labels in everyposition, as well as anα-13C-analog. The results obtained at 70 eV and after collision induced decompo-sition will be presented.

2. Experimental

2.1. Instrumentation

The mass spectra were measured with a H-SQ 30 tandem (BEqQ) instrument (Finnigan-MAT, Bremen,Germany) using a EI/CI ion source (temp. 150◦C, ionization energy 70 eV). All spectra are a mean of 10–15 scans. Collision activation (CA) was effected in the first (rf- only) quadrupole, Ar pressure (measuredat the housing) 6×10−4 Pa resulting in an intensity reduction of the parent ion to about 30%, translationenergy of the ions entering the collision quadrupole 70 eV, averaging of 12–16 spectra.

An indirect inlet system for volatile compounds was not available. To get nevertheless a constantsample stream to the ion source the vacuum lock was used as a storage volume: 1–2µl sample weredeposited in an Al crucible the opening of which was squeezed tight and which was introduced into thevacuum lock with the direct probe. After pumping for 2–3 s residual air had been removed. The pumpvalve was closed and that to the ion source was opened carefully just to maintain a constant stream ofsubstance into the source (3× 10−4 Pa, up to 1 h). After each analysis the vacuum lock was baked out.

All mass spectra are normalized for80Se and corrected for natural13C.NMR spectra were obtained with AM 300, AC 300 or AC 80 spectrometers (Bruker, Rheinstetten,

Germany), 300 or 80 MHz for1H, 75.5 MHz for H-decoupled13C; solvent CDCl3, shiftsδ ppm.

2.2. Syntheses

n-butyl methyl selenide(1). To 50 ml NH3 condensed in a cooled (−78◦C) 100 ml 3-neck flaskequipped with dropping funnel and gas inlet 9.4 g dimethyl diselenide and in small pieces ca. 2.3 g Naare added until the blue color persists. NH3 is allowed to vaporize, under a N2-stream 50 ml degassedH2O and subsequently 13.7 gn-butyl bromide are added. After refluxing for 1 h the selenide is extractedwith 50 ml ether and after drying with MgSO4 purified by distillation over a Vigreux column. Yield8.5 g (56%), bp 138◦C. 1H-NMR: 1.92 (s); 2.48 (t, 7.45 Hz), 1.59 (quin), 1.35 (sex), 0.86 (t, 7.31 Hz).13C-NMR: 3.7; 25.1, 32.8, 22.8, 13.5.s-butyl methyl selenide(3). Synthesis as for 1. Yield 50%, bp 125◦C. 1H-NMR: 1.89; 2.76 (sex), 1.34

(d, 6.89 Hz), 1.47–1.66 (m), 0.92 (t, 7.36 Hz).13C-NMR: 1.9; 37.1, 21.3, 30.3, 12.2.n-butyl ethyl selenide(2). Instead of H2O CH3OH was used as solvent. Yield 29%, bp 156◦C. 1H-

NMR: 2.47 (quart, 7.50 Hz), 1.31 (t); 2.48 (t, 7.47 Hz), 1.56 (quin), 1.32 (sex), 0.83 (t, 7.34 Hz).13C-NMR: 15.6, 16.8; 22.9, 32.5, 22.9, 13.4.

Labelled compounds. The labelled butyl bromides were prepared by standard methods from the cor-responding labelled butanols by reaction with PBr3. 1,1-[2H]2-butanol was obtained by reduction of bu-tyric acid anhydride with LiAlD4. LiAlD 4 reduction of propionic acid anhydride gave 1,1-[2H]2-propanolwhich was treated with PBr3 to give 1,1-[2H]2-propyl bromide, from which by a Grignard reaction 2,2-[2H]2-butyric acid was obtained and subsequently reduced with LiAlH4. LiAlD 4-reduction of acetic acidanhydride gave 1,1-[2H]2-ethanol which was transformed to 1,1-[2H]2-ethyl bromide and to the cor-responding Grignard reagent, which was reacted with ethylene oxide to give 3,3-[2H]2-butanol. In the

G. Filsak and H. Budzikiewicz / Mass spectrometric fragmentation reactions of dialkylselenides 43

Table 1

Molecular ion region of1 and2 and of their labelled analogs (normalized for80Se and corrected fornatural13C)

Compound {M− 2} {M − 1} M +· {M + 1} {M + 2}CH3–Se–CH2CH2CH2CH3 1 0.8 0.5 100.0 1.1 0.9CH3–Se–CD2CH2CH2CH3 1a 1.9 1.4 100.0 0.3 0.3CH3–Se–CH2CD2CH2CH3 1b 2.2 2.0 100.0 2.4 1.0CH3–Se–CH2CH2CD2CH3 1c 3.7 7.0 100.0 26.1 6.6CH3–Se–CH2CH2CH2CD3 1d 2.1 2.3 100.0 15.2 2.0CD3–Se–CH2CH2CH2CH3 1e 2.7 0.9 100.0 0.9 0.5CH3–Se–13CH2CH2CH2CH3 1f 2.0 9.7 100.0 0.4 0.8

C2H5–Se–CH2CH2CH2CH3 2 2.0 0.0 100.0 0.5 0.5C2H5–Se–CD2CH2CH2CH3 2a 1.9 4.8 100.0 1.0 0.7C2H5–Se–CH2CD2CH2CH3 2b 1.8 2.4 100.0 2.8 1.1C2H5–Se–CH2CH2CD2CH3 2c 4.0 7.5 100.0 26.2 6.6C2H5–Se–CH2CH2CH2CD3 2d 2.0 2.7 100.0 16.0 2.4C2D5–Se–CH2CH2CH2CH3 2e 2.0 3.7 100.0 1.1 0.7C2H5–Se–13CH2CH2CH2CH3 2f 2.0 10.7 100.0 0.4 1.3

Fig. 1. Mass spectrum ofn-butyl methyl selenide (normalized for80Se and corrected for natural13C).

same way 4,4,4-[2H]3-butanol was obtained starting from CD3COOD. CD3Br and C2D5Br were syn-thesized from commercial CD3OH and C2D5OH, respectively. 1-[13C]-butanol was prepared by reactionof n-propyl Grignard reagent with13CO2 obtained from Ba13CO3 (90%). The labelled selenides werechecked by their1H-NMR and mass spectra. The molecular ion regions (normalized for80Se and cor-rected for natural13C) are given in Table 1. In can be seen that for the compounds prepared by reactionof labelled ethyl Grignard reagent with ethylene oxide appreciable overdeuteriation is observed whichwas evident also in for the labelled butyl bromides.

3. Results and discussion

3.1. Loss of the smaller alkyl group

The [M–CH3]+-ion of n-butyl methyl selenide (1) (Fig. 1) stemms solely from the loss of the CH3–Se-group; the CD3–Se-group of1e is lost while all the labels of the butyl chain are retained (Table 2).

44 G. Filsak and H. Budzikiewicz / Mass spectrometric fragmentation reactions of dialkylselenides

Table 2

Mass spectra of1 and its labelled analogs (normalized for80Se and cor-rected for13C)

m/z Compound1 1a 1b 1c 1d 1e 1f

157 0.6 1.9 0.5156 0.3 1.0 6.6 14.5 0.9155 0.3 2.4 26.1 95.5 100.0 0.7154 100.0 100.0 100.0 2.2 0.9 0.4153 1.1 0.4 2.0 7.0 2.0 2.7 89.0152 100.0 1.9 2.2 3.7 3.1 2.0 8.6151 0.5 2.2 2.1 2.2 2.4 2.2 1.8150 0.8 1.7 2.2 2.3 1.5149 0.6 1.7148 1.1140 3.9139 3.4 6.6 4.9138 4.5137 4.7 4.1126 0.3 17.5125 16.6 19.1 0.9 0.4124 0.8 7.1 5.6 0.6 15.7123 15.9 1.5 2.3 17.1 15.8 0.3 2.4122 1.0 0.2 0.3 0.9121 0.8 0.9 0.5 0.7 0.7120 0.6 0.1 0.3 0.4119 0.9 0.2114 0.8113 1.7112 29.9 0.4111 27.1 3.0 4.1 2.1 2.0 0.7110 1.9 5.3 7.0 12.1 4.3 1.2 27.3109 32.3 6.7 33.5 27.2 31.7 2.5 7.7108 2.5 2.3 2.9 3.4 1.8 2.7 6.9107 9.7 1.4 3.2 7.3 6.8 6.7 2.6106 0.9 0.2 1.9 2.1 2.2 0.1 1.4105 1.5 1.7 1.7 0.4100 5.899 1.1 0.9 0.4 99.998 1.4 3.0 4.3 2.7 21.6 0.297 4.0 19.2 34.3 40.0 20.0 5.3 5.596 77.1 95.3 70.6 99.9 100.0 8.3 100.095 27.6 22.1 25.0 32.4 27.4 8.0 27.094 10.5 12.3 14.8 16.1 15.4 12.5 13.493 14.1 13.3 16.7 19.6 18.8 6.2 14.592 1.9 2.6 3.1 3.2 3.6 1.1 3.085 1.584 0.6 0.283 2.7 4.8 3.5 3.9 0.3 4.282 0.8 2.2 1.1 1.4 2.5 0.7

G. Filsak and H. Budzikiewicz / Mass spectrometric fragmentation reactions of dialkylselenides 45

Table 2

(Continued)

m/z Compound1 1a 1b 1c 1d 1e 1f

81 2.8 2.2 3.3 4.1 4.3 3.7 3.980 5.5 7.3 7.6 9.5 9.5 9.0 7.962 1.161 5.4 12.360 0.2 2.0 21.7 87.359 83.1 83.2 89.1 57.0 0.258 0.3 21.1 44.4 38.5 21.0 1.3 71.357 97.7 26.5 30.9 49.3 12.7 85.4 54.556 40.1 7.0 9.8 15.5 4.5 58.7 28.355 19.3 2.4 3.0 4.5 3.8 25.4 3.254 1.0 2.0 3.0 3.0 1.4 0.8 5.1

Fig. 2. Mass spectrum ofn-butyl ethyl selenide (normalied for80Se and corrected for natural13C).

Elimination of the terminal methyl from the butyl chain plays apparently no role. This is not the casefor the competition between ethyl loss from ethyl-n-butyl selenide (2) (Fig. 2) (m/z 137) by cleavage ofthe C,Se-bond and by degradation of the butyl chain (see below) which occur in a ratio of∼ 2 : 3 (seeTable 3,2e). Extensive studies with thioethers led to the conclusion [7] that alkyl loss is not a simpleC,S-bond cleavage, but rather a two-step process initiated by a 1,2-hydrogen shift with subsequent lossof the other alkyl group yielding the more stable protonated thioaldehyde (R–CH=SeH+) rather thanRCH2S+. It is, therefore, likely that the [M–·R]+-ions of 1 and 2 have the structure of a protonatedbutaneselenal (cf. also below).

The loss of C2H5 from the Se-ethyl group is accompanied to a minor extent by that of C2H4 (m/z 138)probably giving ionized butaneselenol. The absence ofm/z 143 and the presence ofm/z 139 for 2edemonstrates that the Se-ethyl is involved (cf. below the loss of C4H8) in accordance with the observationthat1 does not lose C2H4, while for diethyl selenide [M–C2H4]+· is the main process [3].

46 G. Filsak and H. Budzikiewicz / Mass spectrometric fragmentation reactions of dialkylselenides

Table 3

Mass spectra and2 and its labelled analogs (normalized for80Se and cor-rected for13C)

m/z Compound2 2a 2b 2c 2d 2e 2f

173 0.4172 0.6171 0.6 1.2 54.7170 0.4 0.9 3.6 7.8 2.0169 0.6 2.3 14.3 48.9 1.1 0.6168 0.6 58.5 82.9 54.6 1.3 1.3 0.2167 0.3 2.8 2.0 4.1 1.0 1.4 46.0166 55.8 1.1 1.5 2.2 1.7 4.9165 0.0 1.1 1.6 1.1 1.5 0.9164 1.1 1.3 2.1 1.4 1.0163 1.4 1.0162 1.4 0.1142 11.0141 0.1 1.2 1.0 2.9 0.0140 2.5 4.8 4.4 7.0 0.5 0.4139 0.3 19.6 31.4 8.2 1.2 1.1 1.7138 2.8 1.7 2.4 5.1 2.9 0.1 16.4137 17.8 1.3 1.9 10.2 8.9 7.4 2.7136 0.2 0.5 1.1 0.3 0.2 0.0 0.6135 0.6 0.5 1.4 0.6 0.7 0.4 0.4134 0.8 0.2 0.7128 10.3127 0.0126 0.4125 9.6 2.2 0.4 0.2124 0.8 2.1 3.2 8.6123 10.9 1.8 15.5 10.1 10.3 2.3122 0.0 0.3 0.0 0.7121 0.9 0.2 0.8 0.4 0.9 0.3116 2.7115 100.0114 10.3113 0.4 1.1 0.8 0.6 3.6112 0.1 0.4 1.5 1.8 1.4 2.6111 2.9 19.6 53.2 42.8 12.3 4.1 2.4110 100.0 100.0 100.0 100.0 100.0 6.1 100.0109 14.0 11.5 19.1 16.6 14.4 1.1 10.1108 4.7 5.8 8.2 7.4 5.9 1.4 6.3107 13.5 5.5 7.8 8.4 7.0 2.9 5.4106 3.2 2.3 2.8 2.7 2.8 0.9 2.5105 0.5 0.3 0.3 0.198 0.7 0.3 0.197 8.4 2.2 1.7 0.7 3.3 0.896 1.0 7.1 7.1 5.0 3.4 7.0 8.395 11.8 3.7 14.6 10.4 10.5 4.2 6.2

G. Filsak and H. Budzikiewicz / Mass spectrometric fragmentation reactions of dialkylselenides 47

Table 3

(Continued)

m/z Compound2 2a 2b 2c 2d 2e 2f

94 4.5 4.0 7.3 5.0 4.6 4.4 4.093 5.4 3.9 7.6 6.0 5.5 3.4 3.592 0.3 0.2 1.1 0.5 0.5 0.2 0.486 3.285 0.8 0.284 1.0 3.1 1.1 0.8 3.483 5.4 6.1 11.0 9.4 6.6 12.2 5.382 12.1 11.8 12.4 13.4 12.1 1.9 12.381 2.7 2.7 3.5 3.4 3.0 1.6 2.980 1.9 2.3 3.2 2.7 2.4 2.0 2.261 2.3 5.160 0.4 1.4 9.1 34.5 0.959 39.0 54.5 36.5 14.8 4.258 0.3 15.3 18.2 14.3 12.1 4.4 29.457 36.5 21.4 29.3 20.7 8.0 31.4 11.556 14.8 7.9 12.1 11.3 3.2 13.0 21.555 24.3 2.7 3.5 8.7 7.4 16.6 3.154 0.1 1.6 2.7 1.9 0.4 0.2 2.9

3.2. Cleavage of the Se-butyl bond

The main processes for1 and 2 involve the cleavage of the C(butyl),Se-bond. Loss of C4H8 is ofhigh abundance for1 (m/z 96) and the main process for2 (m/z 110). That the smaller alkyl group isretained in these reactions follows from the appropriate mass shifts for1e (m/z 99) and2e (m/z 115).The analogs labelled in the butyl chain show that a hydrogen is preferentially transferred back from theβ- andγ-positions yielding distonic ions which lose C4H8 and result in ionized selenols.

In contrast to the elimination of the smaller alkyl group loss of the entire butyl chain is of secondaryimportance. From the spectra of1eand2e it follows that H-losses from the smaller alkyl group (furtherdecomposition of the selenol formed by the loss of C4H8) do not contribute to a major extent.

As mentioned in the introduction for higher alkyl groups the charge stays to an appreciable extent withthe alkyl group (m/z 57) after cleavage of the C,Se-bond. This ion is accompanied bym/z 56 formedby the loss of RSeH from M+·. Mass shifts of the labelled analogs show that the hydrogen stems fromvarious positions of the butyl chain. An exact evaluation of the shift data is not possible sincem/z 55can be formed from different precursors (cf. below) and thus the contributions to the masses from thevarious processes overlap.

48 G. Filsak and H. Budzikiewicz / Mass spectrometric fragmentation reactions of dialkylselenides

3.3. Cleavages of the butyl chain

3.3.1. α-cleavage (loss of propyl)In contrast to the O- and S-analogsα-cleavage is not the main process for Se ethers. The spectra of

the labelled analogs show that this process occurs essentially without any H migrations.

3.3.2. β-cleavage (loss of ethyl)The almost complete shifts in the spectra of1a and1b from m/z 123 to 125 and for1f to m/z 124

show that theα- andβ-CH2-groups are retained. The contribution tom/z 124 in the case of1c and1d are due to the over-deuteriation mentioned in Section 2. Thus theγ-, δ-ethyl group is lost withoutH-scrambling. For2 the results are the same as far as the butyl group is concerned, but superimposed isthe loss of the Se-ethyl group (see above).

For the [M–·C2H5]+-ions several structures can de discussed, such asa−d/d′. Some information canbe derived from the CA spectra, though one must keep in mind that by this method long-lived low-energyspecies are selected which may be the result of internal rearrangement processes. They possibly differ intheir structure from species obtained in the ion source from high-energy ions fragmenting further beforereaching the collision region. C2D5SeC4H9 (2e) offers a unique possibility to differentiate between twoisomeric structures: The ions formed by the loss of C2D5 (m/z 137) and of C2H5 (m/z 142) are farenough apart so that there is now interference with ions containing heavy isotopes of Se. From Table 4 itcan be seen that [M–·C2D5]+ (which possesses most probably structurea) yieldsm/z 55 (C4H+

7 ) as themain fragment accompanied by C2H+

5 and C2H+3 (m/z 29 and 27). For [M–·C2H5]+ in contrast C2D+

5(m/z 34) is the most abundant fragment accompanied by C2D+

3 (m/z 30) while the C4-region comprisesmainly C4H4D3 to C4H2D5 (m/z 58–60). The two ions clearly differ in their structure.

From the C3H7Se+-ions of different mechanistic origin theα-cleavage productc of 2 (see above)shows a distinct fragmentation pattern upon CA, which can be correlated with its assumed structure:Both, loss of C2H4 (m/z 95, “onium”-reaction) and C2H+

5 (m/z 29) are of high abundance. The genesisof the main fragment C3H+

5 (m/z 41) implies the loss of H2Se and hence a rearrangement (cf. thediscussion in the introduction).

Clearly different from the CA pattern ofc, but unexpectedly identical are those of the [M–·C2H5]+ ionof 1 and ofsec-butyl methyl selenide (3). The latter one should be formed byα-cleavage and hence havethe structured. The formation of the main fragment C3H+

5 (m/z 41) would have been anticipated fromthe behavior ofc (rearrangement accompanying the loss of H2Se), but the loss of C2H4 (m/z 95) is notself-evident unless one assumes a loss of CHCH3 with a concomitant H-migration to give CH2=SH+. Inany case, an equilibrium betweend andc can be excluded due to the absence ofm/z 29.

For theβ-cleavage product of the S-analog of1 a thiacyclopropane structure corresponding toe wassuggested [8]. Structured′ would explain the loss of C2H4 and the identical CA spectra of1a and1b(the α- andβ-CD2-groups would have become identical). However, the loss of C2H3D (m/z 96) in

G. Filsak and H. Budzikiewicz / Mass spectrometric fragmentation reactions of dialkylselenides 49

Table 4

CA spectra of selected ions

m/z Compound: 3 1 1a 1b 2 2e 2eParent ion: m/z 123 m/z 123 m/z 125 m/z 125 m/z 123 m/z 137 m/z 142Possiblestructure: d d,d′ d,d′ d,d′ c a b,b′

114 3113 1110 3 4 5109 1 1 1108 4 4 1 1107 2 2 197 4 496 17 1995 65 65 59 67 55 594 1 1 1 180 1 1 1 160 1359 5158 8 2257 4 256 2 155 10044 1 143 1 1 100 100 442 2 1 48 48 141 100 100 5 6 100 940 3 2 2 139 2 1 1 234 10033 232 231 130 1 1 2 2829 1 3 3 73 22 128 1 3 3 2 2 327 5 4 1 1 8 16 4

addition to C2H2D2 (m/z 95) and the formation of C3H4D (m/z 42) in addition to C3H3D2 (m/z 43)requires H-rearrangements preceding or accompanying the fragment formation. An explanation wouldbe an isomarization to a common structure/a mixture of common structures prior to CA fragmentation(as it is discussed for various {C3H7S}+ ions [9,10]) or an isomerisation of1 to 3 as it was shown forn-butyl amine [11] (Scheme 1). In this case theβ-cleavage product of2 would have the structureb ratherthanb′. Why the loss of C2H4−nDn is of low abundance only, cannot be explained.

4. Conclusions

α-cleavage dominant for N-, O- and S-analogs is of minor importance an consequently the subsequentreactions as “onium” or McLafferty decomposition are essentially absent. The prominent processes ofSe-ethers involve cleavage of the C,Se-bonds where the charge may remain to an appreciable extent with

50 G. Filsak and H. Budzikiewicz / Mass spectrometric fragmentation reactions of dialkylselenides

Scheme 1. Possible rearrangement of the molecular ion ofn-butyl methyl selenide.

the hydrocarbon fragments. Rearrangements which result in the recombination of (parts of) the two alkylresidues are especially prominent in CA spectra of long-lived low energy ions.

References

[1] D. Lange and H. Budzikiewicz,Org. Mass Spectrom.29 (1994), 432.[2] G.D. Sturgeon and M.L. Gross, in:The Chemistry of Selenium and Tellurium, Vol. 1, S. Patai and Z. Rappoport, eds,

Wiley, New York, 1986, p. 243.[3] G.M. Bogolyubov, N.N. Grishin and A.A. Petrov,Zh. Obshch. Khim.39 (1969), 2244.[4] E. Rebane,Acta Chem. Scand.27 (1973), 2861.[5] H. Budzikiewicz and R. Pesch,Org. Mass Spectrom.9 (1974), 861.[6] S.H. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and G. Mallard,J. Phys. Chem. Rev. Data17 (1988),

passim.[7] R.H. Nobes, W.J. Bouma and L. Radom,J. Am. Chem. Soc.106(1984), 2774.[8] K. Levsen, H. Heimbach, Ch.C. Van de Sande and J. Monstrey,Tetrahedron33 (1977), 1785.[9] B. van de Graaf and F.W. McLafferty,J. Am. Chem. Soc.99 (1977), 6810.

[10] W.J. Broer and W.D. Weringa,Org. Mass Spectrom.14 (1979), 36.[11] T. Bjørnholm, S. Hammerum and D. Kuck,J. Am. Chem. Soc.110(1988), 3862.

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