letters to the editor

Letters to the Editor

Dear Sir

Aromatic Ion Attachment Mass Spectrometry: an Ion-Molecule Reaction for Organosulphur Analysis

In the course of recording the electron impact mass spectrum of dibenzyl sulphoxide (1) an unexpected peak at m / z 321 was observed (Fig. 1). The base peak of the spectrum (m/z 91) is due to the benzylic fragment, while the molecular ion [MI+'

(mlz 230) is also observed. The peak at m / z 321, corresponding to [M + 91]+, was therefore assigned as an adduct ion, formed by attachment of the base ion to 1.

An investigation was made into the use of this attachment ion-molecule reaction for the analysis of organosulphur compounds, benzyl chloride' being used as the reagent gas. Table l(a) shows that substantial attachment is observed with diphenyl sulphide (2) (20%) and diphenyl sulphoxide (3) (16%), while attachment occurs to a

lesser extent with diphenyl sulphone (4) (2%). The reversible isomerization of the benzyl cation (a) into the cycloheptatrienyl cation (tropylium ion) ( b ) (Scheme 1) is a well-known phenomenon:+ and it was considered likely that attachment occurs via the formation of the aromatic tropylium ion.

It was decided to investigate if attachment to organosulphur compounds is also observed with the aromatic cyclopropenyl cation (c). Propargyl chloride (5), known to

PhCH,SCH,Ph II * o 1

[M+91]+ 321

[MI+. 181 230

rl I 150 200 250 300 100

m / z

Figure 1. The electron impact mass spectrum of dibenzyl sulphoxide (1).

Table 1. Chemical ionization spectra of organosulphur compounds with (a) benzyl chloride reagent gas and (b) propargyl chloride

(a) Organosulphur compound Base peak Relative intensity ( % I

[ MI+' [ M + 1 I' [ M + 9 1 I' PhSPh (2) [MI*' 100 38 20 PhSPh (3) [MIf' 100 35 16

It 0

0 Ph!Ph (4) mlz 125 3 91 2

I1 0

(b) Organosulphur compound Base peak Relative intensity (%)

[MI+' [ M + 11+ [M + 39]+ [M + 39 + 38l+[M + 39 + 38 + 381' (2) [MI" 100 55 15 4 3 (3) (4) mlr 125 25 14 1 <0.5

[MI+' 100 51 4 <0.5 <0.5 -

0 Wiley Heyden Ltd, 1985 ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 11, 1985 685

LETTERS TO THE EDITOR

b M [M+9fl+

U b

Scheme 1

A HCECCH,CI

5 5 t M c\ [M+39f -> [M+39+38f + [M+39+38+38]

- HCI -HCI

Scheme 2

- H+ -H+ [M+34+ + [M+38] c\ [M+39+3€$ + [M+38+3fj c\ [M+39+38+3Ef

Scheme 3

be an abundant s o ~ r c e ” ~ of c , was used as the reagent gas for this part of the work. Table l(b) shows that, presumably due to the much smaller size of c , multiple as well as single attachments occur with 2-4. The multiple attachment may proceed by reaction of the [M+39]+ adducts with 5 (Scheme 2), or via deprotonation of [M+39]+ to give the neutral [M+38] species and subsequent reaction with c (Scheme 3).

Although detailed ab initio calculations are required to determine the full nature of the bonding, attachments are likely to be formed by C S bonds between the aromatic ions and the organosulphur compounds. This is suggested, rather than an electrophilic attack by the aromatic ions on the phenyl substituents of 2-4, since qualitative measurements were also obtained for attachments to thiophenes, alkyl

mercaptans and alkyl sulphides. In view of the known aryl migrations within aryl sulphoxide and aryl sulphone molecular ion^^.'^ attachments to 4 and multiple attachments to 3 may occur after phenyl migration from S to 0. This is also consistent with the lower intensity of signals observed for these adducts.

Due to the diversity in structure of the compounds for which attachment has been demonstrated, it is hoped that the general utility of these ion-molecule reactions for the selective analysis of organosulphur compounds will be realized.

The electron impact mass spectrum of 1 was recorded on a VG Micromass 305F mass spectrometer with an electron energy of 70eV, and zero repeller potential. Chemical ionization mass spectra were recorded on a VG Micromass 7070H mass spectrometer. The reagent gases were

leaked into the source through the liquid inlet so as to maintain a pressure of 2-3 x 10K3 Pa (read from the ion gauge positioned outside the source).

Yvours HOWARD HILL DQA/TS, Materials Centre, Royal Arsenal East, Woolwich , London SE18 6TD, UK

June 1985

References

1. F. W. McLafferty, Anal. Chem. 34, 16 (1962).

2. K. L. Rinehart, A. C. Buchholz, G. E. Van Lear and H. L. Cantrill, J. Am. Chem. SOC. 90,2983 (1968).

3. S. Meyerson, H. Hart and L. C. Leicht, J. Am. Chem. SOC. 90, 3419 (1968).

4. A. Siegel, J. Am. Chem. SOC. 96, 1251 (1974).

5. F. W. McLafferty and J. Winkler, J. Am. Chem. SOC. 96, 5182 (1974).

6. C. Cone, M. J. S. Dewar and D. Landrnan, J. Am. Chem. SOC. 99, 372 (1977).

7. J. L. Holmes and F. P. Lossing, Can. J. Chem. 57, 250, (1979).

8. J. K. Terlouw, P. C. Burgers and H. Hornmes, Org. Mass Spectrom. 14, 574 (1979).

9. J. H. Bowie, D. H. Williams, S. 0. Lawesson, J. 0. Madsen, C. Nolde and G. Schroll, Tetrahedron 22, 3515 (1966).

10. S. Meyerson, H. Drews and E. K. Fields, Anal. Chem. 36, 1294 (1964).

Dear Sir

Mass Spectrometric Studies on Cyclo- and Poly-phosphazenes: &On the Gas Phase Ion-Molecule Reaction Between Hexachlorocyclophosphazene (NPC12)3 and Amines

Condensed phase aminolysis reactions of chlorine-containing cyclophosphazenes have attracted considerable interest in the past.’ In this context, it can be demonstrated that nucleophilic replacement of the halogen atom in these molecules occurs basically according to an SN2 mechanism, while the concurrent influence of both steric and basicity factors of the substituents plays an important role in determining the overall substitution pattern in the phosphonitrilic compound.’.’ Very recently this problem has been reinvestigated by Katti and Krishnam~rthy~ who showed a changeover of the chlorine substitution mechanism in (NCPl& from SN2 to sN1, increasing the

t loot

Q m / r 345 B m / r 3 1 0 m m/z 346 A m / r 327

m / z 2 9 1

1 x lO- ’ 2 x l O - l 3x10.’ 4 x l O - I 5x10.’ 6 x l O - l

P (Torr) Figure 1

686 ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 11, 1985

LEITERS TO THE EDITOR

degree of aminolysis in the trimer. In line with this investigation, and looking at the interesting results obtained with the gas phase oligo-/polymerization processes of different cyclophosphazenes induced lYy electron impact,68 we have now undertaken the study of gas phase, ion-molecule reactions between hexachlorocyclophosphazene and different amines.

The mass spectrometric measurements have been performed with a VG-ZAB-2F instrument operating in the chemical ionization (CI) mode (100eV, 2mA). During the experiments, the partial pressure of the reacting amines was always kept one or two orders of magnitude higher than that of hexachlorocyclophosphazene: under these conditions, in fact, it is reasonable to assume that the reacting species are the molecular ions of the amine and the neutral molecules of cyclophosphazene.

On introducing hexachlorocyclophosphazene, (NPCI,),, together with NH3 into the ionization chamber of the mass spectrometer, besides the peaks at mlz 345 ([M+') and at mlz 310 ([M - C1]+) already described: three other ionic species became detectable at mlz 346, 327 and 291, which correspond to [M + HJ', [M - C1+ NH3]+ and [M - C1- HCl + NH3]+, respectively. The close correlation existing between the increase in the absolute abundance of these species (see Fig. 1) with the increase in the NH, pressure in the chamber strongly suggests that these ions are products derived from gas phase, ion-molecule reactions between [NH4]+ and hexachlorocyclophosphazene.

The CAD MIKE spectrum of the ionic species at rn l z 327 provides interesting information on their structure, showing losses of Cl', HCl and NH, only. In addition, B'IE linked scans demonstrate that the ionic species at rn l z 291 originate from the ions at mlz 327 through HCI loss. On the basis of the previously reported results, the structure a may be proposed for the ions at miz 327, in which a [NH,]+ group is bonded to the cyclophosphazene ring:

C I C I 0

These ions may originate in principle through either of the following two pathways:

M + [NH4]+-* [M + NH4]+-

[M - C1+ NH,]+ (1)

(2) [M - Cl]' + NH, --z [M - C1+ NHs]+

but reaction (1) seems to be more reasonable for the following reasons.

(i) The concentration of the [NH4]+ ions

C and /or

- C I ' f--

C I C I b

d Scheme 1

is, as above described, higher than that of

(ii) The absolute abundance of the species [M - C1+ NH3]+ increases with the increase of the NH, pressure in the chamber (see Fig. 1), i.e. increasing [NH4]+ produc- tion. In addition to this trend, a corresponding decrease of the abundance of the [M - Cl]' ions is also observed.

We may therefore conclude that introducing into the ion source of a mass spectrometer hexachlorocyclophosphazene, (NPCl,),, and ammonia, a gas phase reaction takes place between [NH4]+ ions and neutral molecules of (NPCI,),, which leads to the formation of ions at rnlz 327 whose structure may be reasonably considered that of a . The formation of a short-lived intermediate [M + NH4]+, not detectable in the mass spectrum, can be postulated.

The behaviour in the gas phase, ion- molecule reaction between (NPCI,), and aniline is also interesting. In fact, on introducing (NPCI,), and aniline into the ion chamber of the mass spectrometer, the formation of ionic species at rn l z 402 has been detected, which shows losses of CI' and C6H5NH' radicals.

For these species, formed according to the following reaction mechanism:

M + [C,H,NH]+' --z [M + C6H5NH2]+'

[M - Cl]+.

--HCI\[M - C1+ C6HSNH]+' (3)

the structure b can be proposed:

C I C I

6, m/z 402

Both the intermediate product and the ions at mlz 402 are odd-electron species and the latter can be considered as the molecular ion of a genuine substitution product.

It has to be emphasized that in this case no unimolecular loss of HCI has been observed from the ionic species at mlz 402, but only C1' and C,H5NH' losses leading to the formation of the highly stable cations c , d and e .

c i

GI C I e

C I

Experiments similar to those described above, involving hexachlorocyclophosphazene and pipendine, show behaviour identical to that in the case of NH3 + (NPCI,),. In fact, with this amine also, abundant even-electron ions, of composi- tion [M - C1+ C5HloN]+ are present, which show further loss of C5HION, CL' and HCI moieties.

Quite surprisingly, gas phase reactions between (NPQ) , and morpholine do not occur, probably due to different charge localization resulting in a different reactivity in the molecular species.

One of us (MG) is indebted to the 'Progetto Finalizzato per la Chimica Fine e Secondaria del CNR' for financial support.

Yours MARIO GLERIA Istituto di Fotochimica e Radiazioni d' Alta Energia del CNR, Sez. di Legnaro, Via Romea 4, 35020 Lggnaro, Padova, Italy

SERGIO DAOLIO, ANNA MARIA MACCION1,t ENRICO VECCHI and PIETRO TRALDI Istituto di Polarografia ed Elettrochimica Preparative del CNR, Corso Stati Uniti 4, 35100 Padova, Italy

June 1985

~

References 1. H. R. Allcock, Phosphorus-Nitrogen

compounds, Academic Press, New York (1972).

2. S. S. Krishnarnurthy, A. C. Sau and M. Woods, Adv. Inorg. Chern. Radio- chem. 21,41 (1978).

3. K. V. Katti and S. S. Krishnamurthy, Phosphorus and Sulphur 14, 157 (1983).

?On leave from Istituto di Chimica Farmaceutica e Tossicologica, Universita di Cagliari, Cagliari, Italy.

ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 11, 1985 687


4. M. Gleria, G. Audisio, S. Daolio, P. 6. S. Daolio, P. Traldi, E. Vecchi and M. 8. M. Gleria, G. Audisio, S. Daolio, E. Traldi and E. Vecchi, Macromolecules Gleria, Org. Mass Specirom. 20, 498 Vecchi, P. Traldi and S. S. Krish- 17, 1230 (1984). (1985). namurthy, J. Chem. SOC., Dalton

5. M. Gleria, G. Audisio, S. Daolio, P. 7. M. Gleria, G. Audisio, S. Daolio, E. Trans., submitted. Traldi and E. Vecchi, J. Chem. SOC., Vecchi and P. Traldi, Org. Mass Chern. Commun. 1380 (1983). Spectrom. 20,492 (1985).

Dear Sir

Ferrocene Compounds: 15-Electron Impact Mass Spectra of Some Bridged Ferrocene Derivatives?

Continuing our research in the field of ferrocene chemistry, we report and discuss here the electron impact (EI) induced fragmentation pattern of compounds 1-4.

FcCO(CH2),COFc 1

FcCOCHZCOCOCH~COFC 2

FcCO(CHZ)4COFnCO(CHz)4COFc 3

FcCHzOCHzFc 4

Fc = C5H5FeC5H4-, Fn = 1,l’-ferrocenylene

Table 1. Relative intensities (YO) of some principal ions in the mass spectra of compounds 1-4”

mls 1 2 3 4

186 27.5 98.0 55.3 52.5 185 17.6 39.2 44.8 35.3 171 1.9 0.9 5.6 1.0 160 1.0 2.0 1.7 0.5 130 3.9 4.9 8.6 0.8 129 33.3 42.5 68.9 1.5 128 7.8 11.8 17.6 2.5 121 53.9 100.0 100.0 41.5 95 3.9 12.3 6.8 2.8 81 3.9 18.1 5.1 1.8 65 3.9 34.3 15.9 3.3 56 31.4 78.5 36.2 17.0

213 8.3 25.5 25.8 0.0 199 4.9 19.5 10.7 47.4

aNot corrected for 13C, 54Fe and 57Fe content. Ions are supported by the appropriate metastable ions.

A

FcCOCH2CH= C% + LFcC(6H) = C%j [FeC5H4CO(CH;Z),CO] + [FeC5H4COC2H3 3 +

m / z 254(3.4) m / z 228 (5-9) m / z 232 (0.9) m/z 175 (2.5)

Scheme 1. Mass spectral fragmentation pathways for compound 1.

Most of the fragmentation pathways originate in the cleavages of the ferrocence moiety. The typical fragmentations of the ferrocene moiety give rise to the pro- nounced peaks in all the spectra: rnlz 186-56. These fragments are shown in Table 1, together with other basic fragments such as [FcCO]+ and [FcCH2]+.’,* the molecular ion constitutes the base peak in the spectra of compounds 1 and 4. The base peak of compounds 2 and 3 is due to [C,H,Fe]+ (mlz 121). The proposed rnech- anisms for the formation of the principal ions in the mass spectra are presented in Schemes 1-4 together with their relative intensities (%) (in parentheses).

The fragmentation of compound 1 begins with the cleavage on both sides of the CO groups characteristic of carbonyl com-

?Part 14: V. RapiC and J. Lasinger, Croat. Chem. Actu, in press.

CM-c,H,] + [M-CO] +. [H-2CO] + *

m / z 445(2.5) m / z 482(1) m / z 454(0.8)

* t /

K

[H ] +. +- [FcCOC~COCOC~]+ & [PeCOC~COCOCH, ] +

m / z 510(88) m / z 297C2.5) m / z 232(1.5)

* [FcCOCH$O 1 + - [FcCoC%] +

m / z 255(rc8.5) m / z 227(2)


pounds. The metastable-supported frag- 2 is mainly governed by sequential losses of ments mlz 254 and rn lz 228 suggest the CO and the concurrent cleavages common McLafferty rearrangement resulting from to carbonyl compounds (Scheme 2). the CO group and y-hydrogen3 (Scheme 1). Compound 3 partially parallels the

The fragmentation pattern of compound behaviour of compound 1, because of the




[MI +* ---.-- [FCCH,WH~] + 4050 * [C5H5FeC5H4 - CH2] +

m / z 414(100) m / z 229(1) m / z 199 (47.4)


primary cleavage to m/z 482 in very high abundance (99%) (Scheme 3).

The mass spectrometric behaviour of compound 4 proves to be affected by the presence of the ether oxygen. It might be proposed that the loss of the ferrocenyl ion accompanied with loss of CHzO could lead to the formation of mlz 198 (5.7%) as a

result of 9 rearrangment process (Scheme

The mass spectrometric measurements were performed on CEC 21-llOB and Kratos MS 25, Kratos DS 50 S instruments operating in the EI mode (70eV, 150- 300pA) with a source temperature of 150-180 "C. Samples were introduced under

4).

direct EI CL ions. The samples were purified by recrystallization and preparative thin-layer chromatography and their structure were additionally confirmed by IR and 'H NMR spectroscopy. All the compounds studied have been reported previously: 1 and 3,4 2,' 4.6

Yours VLADIMIR RAPIC and NEVENKA FILIPOVl~-MARlNlC Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottieva 6, Croatia, Yugoslavia June 1985

References

1. A. Mandelbaum and M. Cais, Tetrahedron Lett. 3847 (1964).

2. M. Cais, M. S. Lupin and J. Sharvit, lsrael J. Chem. 7, 73 (1969).

3. V. RapiC and N. FilipoviC-MariniC, Org. Mass Spectrom. 20, 104 (1985).

4. M. LaCan, V. RapiC and A. Brbot- SaranoviC, Croat. Chem. Acta 49, 857 (1977).

5. M. LaCan and V. RapiC, Croat. Chem. Acta 42,411 (1970).

6. V. RapiC and I. HabuS, Croat. Chem. Acta 57, 265 (1 984).

Dear Sir

On the Ionization of Oligosaccharides

The oligosaccharides, sucrose, raffinose and stachyose, have been extensively used as test compounds in mass spectrometry to ex- amine the potentialities of new ionization techniques for the ionization of thermally labile and non-volatile compounds. Sucrose and raffinose are still volatile enough to form molecular ions such as [M+H]+ and [M-HI- uner appropriate conditions of chemical ionization (CI) in the gas phase, whereas stachyose is not amenabIe to CI techniques. Furthermore, in contrast to glucose, which gives [MI+' signals in field desorption (FD) spectra, the ionization behaviour of oligosaccharides is characterized by the fact that [MI" ions have never been observed and therefore these ions are considered to be unstable.

Recently, in contrast to this view, the formation under FD conditions of [MI" ions of sucrose and other di- and trisac- charides were reported.' This was con- cluded from the observation of a pe,tk at

mlz 342 in the spectra of these compounds. However, this conclusion is not correct for the following reasons.

(i) The FD spectra were recorded by using a single focusing mass spectrometer with an energy-dependent mass scale. Hence the ap- pearance of these peaks is rather easily explained by energy deficits and energy dis- tribution effects arising from a field induced extraction and desolvation of [M + H]+ ions from transient protrusions of viscous sample deposits.'

(ii) From previous experiments indepen- dently carried out some time ago in Bonn and Darmstadt, strong evidence was obtained that [MI" ions of sucrose, stable enough to reach the detector, are not formed in FD mass spectrometry. These experiments were directed toward the detection of [MI+' ions of sucrose in FD spectra resulting from the field ionization of sucrose molecules on the tip of field- enhancing micro-needles of activated emitters. A quadrupole and a double focusing instrument were employed and, up to a signal-to-noise ratio of about lo4, no ion signal at m/z 342 could be detected.

HOCH, HOCH,

@o*HzoH HOCH, HO H o & ~ c H z o H + 'CHZOH (1) OH H OH H

mlz 311

(iii) By comparison of FD spectra of sucrose obtained under varying experimental conditions, it was discovered that a fragment ion of mass 311 was formed on activated emitters, but not on bare wire emitters. Further, this fragment ion became very intense at elevated emitter temperatures if activated emitters with long microneedles and a high field ionization efficiency were utilized. The fragment at m/z 311 was up to a factor of 10 more abundant than the [M+ HI+ signal inten~ity.~ Taking field ionization of intact sucrose molecules into account, this fragment is easily explained by a dissociative ionization process leading to a resonance stabilized ion of mass 311 (reaction (1)).

For raffinose and other oligosaccharides a similar field ionization induced fragmentation reaction can be postulated. It is known that the same mechanism causes the molecular ions of ketals to be negligible or absent in electron impact spectra. The fragment ion of mass 311 cannot arise from decomposition of [M+Hl+ ions which typi- cally yield the fragment ion at m/z 163. This conclusion is supported by the lack of a fragment ion at m/z 311 in FAB mass spectra of sucrose.

Whereas [M+H]+ ions of sucrose and raffinose are fairly stable, protonated stachyose ions had not been observed in FD spectra. Therefore, the authors considered these ions to be completely unstable in the gas phase. However, recently [M+H]+ ions of stachyose have been observed in Secon-



dary ion (SI) and (fast atom bombardment) FAB mass spectras and have also been found to appear in FD spectra of a mixture of stachyose with tartaric acid. Yours H. J. VEITH (to whom correspondence should be addressed) Institut fur Organische Chemie und Biochemie, Technische Hochschule Darmstadt, Petersenstrasse 22, D-6100 Darmstadt, FRG

F. w . ROLLGEN Institut fur Physikalische Chemie der Universitat Bonn, Wegelerstrasse 12, D-5300 Bonn, FRG

March 1985

~~~

References 1. D. E. C. Rogers and P. J. Derrick, Org.

Mass Spectrom. 19, 490 (1984). 2. U. Giessrnann and F. W. Rollgen, Int.

J. Mass Spectrom. Ion Phys. 38, 267 (1981); S. S. Wong, U. Giessmann, M. Karas and F. W. Rollgen, Int. J. Mass Spectrom. Ion Proc. 56, 139 (1984).

3. H. J. Veith, Annual Meeting Arbeits- gemeinschaft Massenspektrometrie, Heidelberg (1981).

4. K. Harada, M. Suzuki and H. Karnbara, Org. Mass Spectrom. 17, 386 (1982).

5. F. W. Rollgen and S. S. Wong, Pro- ceedings Lamma Workshop, Borstel (1 983).

Dear Sir

Concerning the Mechanism of Ion Formation in Field Desorption

We recently reported results of a detailed study of energy deficits of gaseous ions formed from glucose and other saccharides,’ which we had undertaken with the aim of providing experimental evidence as to the nature of the ionization processes involved in field desorption. Through comparison of the energy deficit distribu- tions of the molecular ion [MIC’ and quasi-molecular ion [M + HI’ of glucose, we were able to alleviate significantly the difficulties in interpretation generally en- countered in field desorption (FD) studies as a result of the poor knowledge of fields and temperatures at the emitter. We were able to conclude that the formation of gaseous quasi-molecular ions [M + H]+ occurs by what we now call’ field evaporation, that it is most probable at the ionization sites at which the field strength is highest and that molecular ions [MI+‘ are formed by field ionization of saccharide vapour above the emitter surface. Results for sucrose were important in that the observation of a molecular ion [MI+’ supported an essential plank in the argu- ment, namely that glucose and sucrose are volatile.

Veith and Rollged report that they are unable to detect the molecular ion [MI+’ of sucrose using FD techniques. Yet they agree with our proposal that sucrose is volatile. Using a double focusing mass spectrometer4 and fairly ordinary FD conditions (resistively heated wire emitter, low heating current, emitter potential 12kV, pure sample), we found quite a number of years ago, and have confirmed recently, that the intensity of rnlz 342 from sucrose (mol. wt = 342) was 1-2% relative to rnlz 343. It could be argued that the structure of the rnlz 342 ion has not been investigated and that it has not been shown

to be the molecular ion [MI+’; however this does not appear to be the point at issue. By taking special measures, in particular with the control of the emitter heating current, and given a highly sensitive instrument and a careful and patient experimentalist, we found that the intensities of mlz 342 in the case of sucrose and rnlz 180 in the case of glucose could be enhanced significantly relative to mlz 343 and rnlz 181, respectively.

We would point out that a quadrupole mass filter, as used for key experiments in Bonn,’ is not an ideal instrument for the detection of ions of low abundance formed by FD. Combinations of FD sources and quadrupole mass filters suffer from low resolution and sensitivity relative to mag- netic deflection instruments, basically because of the need to retard the ions prior to analysis. In our experience with FDI quadrupole combinations, it is not always that easy to observe a small peak separated by one mass unit from a much more intense peak in the range mlz 300-400. In the case of sucrose, the difficulties are compounded by the difference in kinetic energy between rnlz 342 and rnlz 343, stemming directly from the difference in energy deficits which we reported.’ This difference in kinetic energy also has consequences in the case of double focusing mass spectometers, in that mlz 342 and rnlz 343 require marginally different operating conditions.

Veith and Rollgen state that it is impossible to detect the molecular ion [MI+’ of sucrose, because it decomposes completely to a rnlz 311 fragment ion. There is a previously unexplained peak at miz 311 in the FD mass spectrum of sucrose, the relative intensity of which, like those of all the other peaks, depends upon experimental conditions. To propose, in the absence of supporting evidence, that the mlz 311 ion is formed by decomposition of the molecular ion [MI+’ is, however, speculative. Even given that rnlz 311 were formed from [MIC’, it would still not follow

that the molecular ion [MI+’ is not detectable. This would only be the case if there were no energy barrier to the reaction.

In conclusion, the observation of a peak at mlz 342 in the FD mass spectrum of sucrose provides support for the contention that both glucose and sucrose are volatile, and that gaseous [M + H]+ ions are formed by field evaporation’ and [MI+’ ions by field ionization.

Yours PETER J. DERRICK (to whom correspondence should be addressed) and

School of Chemistry, University of New South Wales, PO Box 1, Kensington, New South Wales 2033, Australia

D. E. C . ROGERS National Institute for Materials Research, CSIR, PO Box 395, Pretoria 0001, South Africa

June 1985

NGUYEN THAN-TRONG

References 1. D. E. C. Rogers and P. J. Derrick, Org.

Mass Spectrorn. 19,490 (1984). 2. (a) S. C. Davis and P. J. Derrick, paper

presented at the Australia and New Zealand Mass Spectrometry Society, Canberra (1984); (b) S. C. Davis and P. J. Derrick, in preparation.

3. H. J. Veith and F. W. Rollgen, Org. Mass Spectrom. 20, 689 (1985).

4. P. G. Cullis, G. M. Neumann, D. E. Rogers and P. J. Derrick, Adv. Mass Spectrom. 8, 1729 (1980).

5. F. W. Rollgen, private communication (1 985).



Dear Sir

The Structure of [C2H3S]+

In a recent letter Paradisi et al.' have reported that in collisionally activated decomposition mass-analysed ion kinetic energy spectroscopy the ions [qH3S]+ have an almost identical fragmentation pattern regardless of the ion precursor, and this has led to speculation that one common ion is produced from a wide variety of organosulphur compounds. Six plausible structures ( a - f ) have been considered and from the limited theoretical data available structure d appears to be lower in energy than c and e,* while there is little or no barrier for the collapse of b into c . ~ We are carrying out a systematic theoretical study of carbenium ions containing Group VI elements and the publication of the experimental results on the [&H,S]' ion has prompted us to report our calculations on the energy surface for this ion.

H\ + H'"Ic- c = s H

H 1

S' / \

H HNC= \

C

H, +

' H

C-C-H / S

b

H, + ' H

H / c = c = s

d

# H

f

All our calculations were performed with the MONSTERGAUSS p r ~ g r a m . ~ Structu- ral parameters were optimized at both the SYO-3G5 (not reported here) and the 6-31G* levels using gradient techniques.' Ion b and e were assumed to be planar, c , d and f were assumed to have a plane of symmetry and a was assumed to belong to the C3" point group. Structure b was optimized in two conformations, with the SH group either eclipsing the C-H (as shown for b ) or eclipsing the double bond (given the label b ' ) . The cyclic structure c was found to be pyramidal at S with the planar ion c' being by far the highest energy species (see Table 1).

The relative energies at the more reliable 6-31G*//6-31G* level are a < d < f < c < e < b < b' < c ' . At the STO-3G//STO-3G level and in the calculations reported previously2 c was found to be higher in energy than e , due to the inadequate description of small-ring compounds by small basis sets. This problem is corrected by using the larger basis set and particularly by including polarization functions.

Some of the structures can be eliminated as candidates for the ion observed in the gas phase. The planar thiirenium ion (c') is the transition state to inversion at sulphur. Structures b and b' are both at minima on the surface. Transition-state structures were optimized for the rearrangement b + c and the barriers for this reaction are low (11.7kJmol-' for b and 5.5kJmol-' for b') leading us to the conclusion that the P-thiovinyl cation will collapse into the cyclic ion c under the conditions in the mass spectrometry experiments. The transition state for the interconversion of e and f is only 6 kJ mol-' above e at the 6-31G*//6- 31G* level. The rearrangement of e to a by a 1,2-hydride shift from the central carbon atom to the terminal carbon has a transition state 81 kJ mol-' above e , while rearrangement of e to d by a 1,2-hydride shift to sulphur has an even higher transition state,

Table 1. Relative and total energies of ions and transition states Energy relative to aa

Ion STO-BG//STO-BG 6-31G*//6-31GX Total energy at 6-31 *G//6-31G*/Eh

a 0 0 -474.68974 b 297 249 -474.59507 b' 299 250 -474.59471 C 219 171 -474.62472 C ' 61 7 540 -474.48395 d 150 107 -474.64907 e 210 180 -474.621 11 f 154 128 -474.64093

Transition states

e-, d 384 357 -474.55385 e-. f 250 186 -474.61 883 e-, a - 261 -474.5901 5

"Relative energies in kJ mol-'.

177 kJmol-I above e . Clearly then e , if initially formed in the fragmentation of the organosulphur compounds, will only be converted into the cyclic isomer f unless the ion contains a large amount of excess energy.

Experimentally it was noted that very little [CH,]+ was produced in the fragmentation of [C2H3S]+ and this was taken as evidence that the common gas phase ion did not have the thioacetyl structure ( a ) . This leaves three possible structures, ions c, d and f . Attempts at optimizing the structure for the transition state for interconversion of the two cyclic structures were unsuccess- ful, although all cyclic structures containing a bridging hydrogen were much higher in energy than either c or f , indicating that the two ions do not easily interconvert. Attempts at finding the transition-state structures for the interconversion of the cyclic ions with d also failed. In this context we note with interest that in a comprehen- sive study' of the [&H30]+ surface the oxygen analogues of the transition-state structures for interconversion of c, d and f were not reported. Yours C. F. RODRIQUEZ and A. C. HOPKINSON (to whom correspondence should be addressed) Department of Chemistry, York University, Downsview, Ontario, Canada May 1985

References

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C. Paradisi, G. Scorrano, S. Daolio and P. Traldi, Org. Mass Spectrom. 19, 198 (1984). P. Kollman, S. Nelson and S. Rothen- berg, J. Phys. Chem. 82, 1403 (1978). (a) I. G. Csizrnadia, F. Bernardi, V. Lucchini and G. Modena, J. Chem. SOC., Perkin Trans. 2 542 (1977); ( b ) 1. G. Csizmadia, V . Lucchini and G. Modena, Gazz. Chim. Ital. 108, 543 (1 978). Program MONSTERGAUSS, M. R. Peterson and R. A. Poirier, Depart- ment of Chemistry, University of Toronto, Toronto, Ontario, Canada. W. J. Hehre, R. F. Stewart and J. A. Pople, J . Chem. Phys. 51, 2657 (1969). P. C. Hariharan and J. A. Pople, Theor. Chim. Acta 28, 213 (1973). H. B. Schlegel, Ph.D. Thesis, Queen's University, Kingston, Ontario, Canada (1975). R. H. Nobes, W. J. Bouma and L. Radom, J . Am. Chem. SOC. 105, 309 (1 983).


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