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Elucidation of reactions in the mass spectrometer
169
Anaiytiu Chutuca Actu, 265 (1992) 169-182 Elsevler Science Pubhshers B V , Amsterdam
Elucidation of reactions in the mass spectrometer S Bauerschnndt, Organwh-Chemwdes
W Hanebeck, K-P
Schulz and J Gasterger
Instuut, Techmsche Unrversltat Muden,
W-8046 Garchmg (Germany)
(Recensed 20th Januaxy 1992)
Abstract A system has been developed that can denve from the structure and the mass spectrum of an organic compound the fragmentation and rearrangement reactlons occurrmg m the mass spectrometer Peak mtensltles are used to calculate conversion probabtities for the mdlwdual reactlon steps of the fragmentations scheme These analyses can be performed for all classes of orgamc compounds except saturated hydrocarbons and explam between 70 and 100% of the intensity of quabty mass spectra A study on a senes of C,-ammes dustrates the approach and its results Keywords Mass spectrometry, Butylammes, Computer modelhng, Reaction elucldatlon
Mass spectrometry plays a major role 111the structure elucrdatron of orgamc compounds The fragmentations of an orgamc molecule m the mass spectrometer are charactenstrc of the arrangements of atoms and bonds m the molecule and thus give detarled mformatron about us constitution However, thrs richness of mformatron 1s only rarely used Often the chemist only looks for the peak of the molecular ran as proof of rts molecular formula. No other peaks 111the mass spectrum are analysed and thus all the mformatron they contam IS lost Thrs deplorable srtuatron IS largely due to the poor understanding of the events that take place m the mass spectrometer Even experts have drffrcultres assrgnmg a structure to many of the peaks 111a mass spectrum Drawmg conclusrons about the fragmentation and rearrangement reactions of a molecule m the mass spectrometer IS even more comphcated The hrgh energy used for the romzatron of a molecule by
electron impact m the source of a mass spectrometer creates species that can undergo a multrtude of reaction pathways because of the addrtronal energy content of these radical catrons and catrons On the other hand, the large number of mass spectra that have been measured provrde a rrch source of information for leammg more about the processes that occur m the mass spectrometer Leammg needs a model for the structure of the information that has to be understood The model that was chosen for these studies IS based on concepts that have been used over many years for the interpretatron of mass spectra The large amount of mformatron to be processed and the many alternatives m high-energy species and their reactrons to be consrdered suggest modellmg by computer programs
FOUNDATIONS OF THE APPROACH
Correspondenceto J Gastelger, Orgamsch-Chemlsches Instltut, Techmsche Umversltat Munchen, W-8046 Garchmg (Germany)
From the begmnmgs of the mass spectrometry of orgamc compounds, attempts have been made to descrrbe the structures of the species involved
0003~2670/92/$05 00 0 1992 - Elsewer Science Publishers B V All nghts reserved
170
S Bauerdmdt
by structural formulae Analogously, the reactions m the mass spectrometer were wntten as electron-transfer processes A comprehensive exposition of this approach was gven by McLafferty
Dl Such a descnptlon of the structures and their reactions 1s also the basis of the present model Spectically, tlus means that the chenucal species formed from organic compounds m the mass spectrometer can be represented by one or more valence bond structures In other words, the unpaired electron of the radical and the posltlve charge of the cation are locahzed on an atom or delocalized m a r-system In fact, It will be seen later that this 1s not absolutely necessary The analysis of a mass spectrum will also work when a species can be represented by a series of rearrangmg isomers For example, the C,H,O+ radical catlon can be represented either as CH3CH20+ H, or as CH,-CI-I,O+ H,, or as a
A-B-C
-
A-B
-
Al+;
+C
1-w + A-E
+
‘A
L,J0-c
A-B
\,Jc
+
A-0 -
B-C \3-0
J
OniumReactkm(AnotCdmn) i-B
C-H_
L3J HalogenCIemge @ = C&W)
A-X
i--H
-
A+?
-
iJl+cso
+
B-C
b-3 J
carbony( Etiminatbn R-c-6
Chm Acta 265 (1992) 169-182
species mterconvertmg between these two lsomerlc structures Further, the fragmentation and rearrangement reactions occurring m the mass spectrometer can be represented by shifts of bonds and of electrons m the above valence bond structures Scheme 1 contams all the elementary processes used m the computer model, largely from the book by McLafferty [l] By apphcatlon of such a general reaction scheme to the structure of a specific cation or radical catlon, the reactions of this species are obtamed, e g , for the a-cleavage of ionized methyl ethyl ether A-B-C
-
Me-%-CH,-CH,
A=B+ -
C Me-6=CH,
+ CH,
The elementary processes m Scheme 1 were selected to cover the reactlons observed m electron impact (EI) mass spectra The present approach can also be extended to other types of mass spectra, such as chermcal lomzatlon (CI) mass spectra A series of elementary processes somehow different from those m Scheme 1 then have to be specified The first process m an EI mass spectrum IS lomzatlon Here we consider lomzatlon from free electron pairs and from +orbltals, lomzatlon from a a-orbital 1s not yet included
ESSENTIAL STEPS IN THE ANALYSIS
C
L37J
i--o +
-‘-
et al /Anal
Scheme 1 Elementary processes covermg the reations of orgamc compounds m the mass spectrometer A, B, C, D may be any type of atoms (except m carbonyl elunmatlon), m some reations, the atom types may be restncted to special cases, 3-7 gwes the range for the number of atoms m the cham lmkmg the two atoms specified
The determmatlon of the species and their reactions occurrmg m the mass spectrometer needs two pieces of mforrnatlon the structure of a compound and its mass spectrum The essential steps m the analysis of a mass spectrum to determme which reactions take place m the mass spectrometer are shown m Scheme 2 and are bnefly described below First, all formally possible reactions are generated for the given structure After lomzatlon at all atoms with free electron pairs or from each r-orbital, all elementary reactions m Scheme 1 are applied to these prunary ions and thus lead to all potential secondary ions These are then submitted to the elementary processes glvmg the next level of ions The entire procedure IS repeated untd the ions are so small that no elemen-
S Bauersdwnadt et al. /AnaL Chm. Acta 265 (1992) 169-182
r-l r-l Reduchofl of the ReacUon Network
calculahon
of
Reachon
PrObahbUes
Scheme 2 The function flow m the automatx analysis of a mass spectrum
tary process fits any longer, or pathways are generated that lead to Ions not observed m the mass spectrum (see below) The elementary processes are contained m a separate file outside the program Hence they can easily be modified by changmg the restnctlons for then apphcatlon, or additional processes can be added wlthout having to change the program system Each group of peaks m the mass spectrum IS analysed to see which of the ions generated have a mass that fits these peaks This process is strongly guided by takmg account of the theoretlcal isotope dlstnbutlon of the molecular formula of an ion
171
If an Ion 1s generated that 1s not observed m the mass spectrum, it 1s nevertheless kept and further subnutted to the elementary processes, as It could be that the ion decomposes so rapldly that it does not gve a peak Only If no daughter Ion of this Ion can be detected either IS the entire reaction sequence deleted from the reaction network All those ions and reactions of the network that have been asslgned on the basis of the mass spectrum are used to reconstruct a mass spectrum The reconstructed mass spectrum gwes that part of the mass spectrum which can be explamed by the entire analysis Rearrangement reactions ave lsomerlc ions that have the same mass and isotope patterns Therefore, a peak posItIon 1s msufflaent evldence for decldmg whether one or several lsomers are present and which structure or structures to associate with a peak For the next steps m the analysis of the reaction networks, the lsomers of one or more rearrangement steps are therefore combmed to a smgle node m the reaction network Whether a rearrangement reaction takes place or not can be inferred only later when it 1s seen that the rearrangement of a structure to a different isomer IS needed m order that an observed fragmentation reaction can proceed After a structure, or a series of interconverting lsomerlc structures, has been asslgned to all peaks, the mtensltles of the peaks are used to determme how much material has passed through the vanous branches of the reaction network Thrs 1s performed by starting with the species at the end of the reactlon network that are not further fragmented Their intensity 1s added to the mtenslty of their precursor ion because the entire amount of this Ion has been produced from this precursor ion However, several features of the reaction network make this analysis complicated The sequences of fragmentation and rearrangement reactlons form a network and not only a tree of reactions In other words, an ion m&t be formed from more than one presursor Ion Thrs has the result that two or more sequences of reactions combme m a smgle node of the network
172
There 1s not enough mformation m the mass spectrum that allows one to detemune what amount of a wen ion has come from the various preeursor ions In this situation, three altematlves are calculated. an estunated value for the conversion of one precursor ion to a daughter ion IS obtamed by makmg the assumption that all precursor Ions have contnbuted to the common daughter Ion by an equal amount, an upper-bound value 1s calculated by makmg the assumption that all mtenslty of the daughter ion has been produced from the one precursor Ion bemg considered, and a lower-bound value 1s calculated by assummg that the precursor Ion bemg considered has not produced the common daughter Ion at all The mtensltles of the daughter ions are added to the parent ion by gomg through the reaction network from the bottom to the top In this way, the amount of matenal that flows through the vanous reaction pathways can be calculated In the next step, all those species of the reactlon network that have a value of zero for the upper-bound of the amount of matenal are elmunated In addltlon, all reactlon sequences that start from such species are ehmmated Hence the network of all concenrable reactions generated m the fit steps 1s reduced to a network that contams only the reactions that are actually occurrmg Next, probabdltles are assigned to the reactlons of the reduced network The senes of competmg reactions 1s handled by equations havmg the formal charactenstxs of first-order kmetlcs The conversion rates and amounts of matenal allow the calculation of reaction probablhtles for each fragmentation reaction As was done for the converstons, three values are calculated for the reactron probabdltles a lower-bound, an estlmated value and an upper-bound For rearrangement reactions a calculation of reactlon probabdltles 1s not possible for the reasons mentloned above (Isomers have the same peak position) However, one can now determme whether a rearrangement reaction 1s occurrmg or not based on the analysis of the reduced reactlon network The node contammg a senes of Isomers IS expanded mto the mdlvldual isomers and thex rearrangements Only those rearrangements that
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Chm
Acta 265 (1992) 169-182
lead to an ion that reacts further to a fragment that IS observed are kept m the mass spectrum The dfierence between the upper- and lowerbounds of the reaction probabrlltles and the confidence of the amounts of mater& are used to assign a confidence value to the reaction probabrlltles Three levels of confidence are carned very high (11, high (2) and low (3)
IMPLEMENTATION
The system has been wntten m standard FOR1s therefore portable At present it 1s implemented on VAX statlons That part of the system which generated the overall fragmentation and rearrangement scheme consists of about 170 routmes vvlth ca 25 000 lmes of codes The reductlon and analysis of the reactlon network 1s performed by a system wth about 60 routmes and ca 12000 lmes of code Commumcat~on between the two phases of the analysis 1s reahzed through several dn-ect access files The results of an analy~1sare wntten on files The system can also analyse a series of pairs of structures and then associated mass spectra Then, all reactions of a certam type (e g , cr-cleavages) observed m this senes of spectra are collected m a file In fact, each reaction type 1s collected m three different files dependmg on the degree of confidence that has been attnbuted to the conversion probability of the reactlon step consldered In other words, there 1s one file of cw-cleavages that have a very h& level of cotidence m their reaction probabdmes, another one collectmg those with a high degree of confidence and a third one with those with a low level of confidence The same 1s true for all other elementary reactlons m Scheme 1 These files form the basis for systematic statlstrcal analyses of reactlon types to build a knowledge base for mass spectra sunulatlon TRAN 77 and
RESULTS
The approach IS applicable to all those orgamc structures havmg a w-system or a free electron
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Chum. Acta 265 (1992) 169-182
CHxBrCl
I
Observed 0
50
0
50
spectrum 100
150
200
100
150
200
Calculated
lid=Br+ _ ClEC=Br+ u 0 476(l) 5
0 741(l)
lid=Cl+
spectrum
Fig 1 Compmson of the expenmental Hnththe reconstructed mass spectrum of chlorobromomethane The explanation factor IS910%
g
Scheme 3 Fragmentation scheme of chlorobromomethane
pair Two examples from different ends of the range of complex@ of the reaction network are presented first before a more detailed mvestlgatlon of the mass spectra of C,-ammes 1s given All mass spectra were taken from the NIH mass spectral database and were determined with a 70-eV iomzation source Scheme 3 shows the fragmentation scheme of chlorobromomethane deduced from its mass spectrum The neutral species lost m these fragmentations are not shown for clarity of presentation For each reaction the calculated transition probabllrty 1s given together vvlth its level of confidence (1 = very high, 2 = high, 3 = low) In Fig 1, the experimental mass spectrum of chlorobromomethane 1s compared with that reconstructed from the reaction network m Scheme 3 and the reactlon probablhtles contained m this scheme, 910% of the overall intensity m the expernnental mass spectrum can be explained by Scheme 3 The analysis of the fragmentations and rearrangements of ethyl hexanoate leads to a reactlon network wrth has mltlally 1196 ions and 3992 reactions After reduction of the network based on mformatlon m the experimental mass spectrum, 883 reactions remam Because of the large number of reactions the reaction network 1s not
given here All 34 peaks havmg an mtenslty higher than 15% can be assigned to fragments Only four signals with an intensity below 1% cannot be assigned Altogether 99 9% of the intensity of the mass spectrum can be explained (Fig 2) In a more systematic study, all C,-ammes that had spectra m the NIH database were mvestlgated The results are presented m the followmg sections
(Zompomd
spectra
/
ldsnt
1
195279
Observed
spectrum
1000 50
0
50
100;
I 0
I
I
!
I
I
,
50 Calculated
, 100
,
,
,
,
( 150
spectrum
Fig 2 Comparison of the experunental with the reconstructed mass spectrum of ethyl hexanoate The explanation factor IS 99 9%
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174
0 742(3)
Chm Acta 265 (1992) 169-182
0 098 (2)
l-l l
.= .rlcl
Scheme 4 Fragmentation and rearrangement reactmes of n-butylamme obtamed with the elementary processes 111Scheme 1 The numbers gtve the values calculated for the reaction probabthttes, the values m parentheses are the level of confidence Cl= very high, 2 = high, 3 = low) Note that some ams show up for clartty of presentation at several places m the reactlon scheme
n-Butyhmme (I-amuwbutane) The analysis of the mass spectrum of nbutylamme with the elementary reactions m Scheme 1 gave the reaction network m Scheme 4 The pnmary ion undergoes two a-cleavages with the one leading to H, C = NH; glvlng the base peak The prnnary ion has to rearrange to an lsomenc ion by a hydrogen atom m order to be able to explain a senes of other peaks (the bottom three levels m the reactron tree in Scheme
4 Hence we have here a case where a rearrangement reaction 1s found to occur The reaction network m Scheme 4 can explam 90 2% of the mtenslty of the experlmental mass spectrum (see Fig 3) The peak at mass 18 cannot be assigned m the automatic analysis It 1s presumably caused by water contamed as an unpunty m the amme The altematwe, that this peak was caused by the ammomum ion NH:, seems unhkely The analy-
S Jhuembd
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C&m Acta 265 (1992) 169-182
175
-spraun 0 100’
0
25 -sBcbum
50
”
0
75
F@ 3 Companaon of the expertmental mass spectrum of n-butylamme with that r~~~astructed from the reaction network m Scheme 4 The explanatton factor IS 90 2%
a
25 ”
”
50 ”
25 maI=c@Jn
”
”
’
’
50
--
75
b
0
25
50
75
0
25 m-m
50
75
100
SIS IS sometimes able to fmd peaks of nnpuntles and thus can be used in checkmg the quahty of mass spectra
50 0
set-Butylamzne (2-ammbutane) Apphcation of the elementary processes m Scheme 1 to mc-butylamme and compaxxson wrth
50 100
,
Ftg. 4 Cempanson of the expenmental mass spectrmn of xc-batytamme anth (a) that reconstructed from the reactions m S&me 5 (obtamed from the ps m Scheme l), the exphutabcm factor 1s 585%, and (b) that reconstructed from the reactton network obtamed wrth the extended set of elementary processes (Schemes 1 and 6), the explanatton IS factor 75 290
0-m
3 T
Scheme 5 Fragmentatton reacttons of see-butylamme obtamed wtth the processes m Scheme 1
rts experimental mass spectrum leads to the reactlon tree m Scheme 5 Three a-cleavages are observed for the prnnary Ion, the loss of an ethyl ra&cai 1s the preferred process gwmg the base peak at m/z = 44 The loss of a methyl radical to give the peak at m/z = 58 1s less favourable and the loss of a hydrogen atom occurs to only a very small extent The reactlon network m Scheme 5 can only explain a dlsappomtmgly low 58 5% of the overall mtenslty m the expernnental mass spectrum (Fw 4a) What IS the reason for ths poor agreement7
176
S Baumdwdt
Are there some rmportant processes lntssmg m Scheme 1 or must they be modtied? Hydrogen rearrangements are allowed to occur m this set of processes only d the transition state for the transfer of a hydrogen atom forms at least a sm-membered rmg (n = 3 m Scheme 6) This IS by far the most favourable situation However, m secbutylamme such a rmg cannot be formed, the rmg size for transferrmg a hydrogen atom from the most distant carbon atom to the mtrogen atom m the prunary Ion 1s only five
If we set IZ111Scheme 6 to a lower hrmt of 2 to allow this process to occur m the network, the number of reactions increases from 7 to 30 This more complex reaction network can now explam twelve mstead of eight peaks and 70 2% of the overall mtenslty m the experunental mass spectrum Settmg n m Scheme 6 to a lower lmt of 1 and thus allowmg a hydrogen atom to be transferred to mtrogen also m a transltlon state wth a fourmembered rmg further increases the number of steps m the network to 56 Now, SIXaddItiona peaks and 75 2% of the intensity m the mass spectrum can be explamed (Fig 4b) Using the extended set of elementary processes (n extended from 3 down to 1 111Scheme 6) m the mvestlgatlon of the mass spectrum of n-butylamme mcreases the number of reactions from 16 to 44 and the explanation factor from 902%to934% What can be learned from this investigation7 Hydrogen transfer 1s indeed highly preferred for transition states with rmgs of s1x and more atoms (see n-butylamme) However, if an Ion has no
A
“\
&q/B-
/”
*
t,_,;
n=l,2,3
Scheme 6 Hydrogen rearrangement
et aL /AnaL Chm. Acta 265 (1992) 169-182
posslbtity of attammg such a sm-membered transltlon state It wdl also follow reaction channels of hydrogen transfer through transition states Hrlth five- or four-membered rmgs Any extension of the elementary processes m Scheme 1 nught result 111a drastic mcrease m the complextty of the reaction networks These more complex fragmentation and rearrangement schemes often have several alternatives for the conversion of one mto another spec&c Ion This reduces the level of confidence that can be assigned to the reaction probabdttles Care should therefore be taken to work with the mmunum set of elementary processes Just necessary to explam as much of the mtenslty m the mass spectrum as possible Retummg to the example of set-butylamme 75 2% of explamed mtenslty leaves still somethmg to be desired, although the mcrease m the explanation factor by 16 7% 1s remarkable A further increase m the explanations potential can be expected by allowmg additional rearrangements m the alkyl groups We are currently workmg to find an appropnate extension of the elementary prcesses that keeps the number of rons wlthm a reasonable lmut and holds the confldence level for the reaction probablhtles high enough Again, an unpurlty of water was present m the amme glvmg the peak at m/z = 18 Dlsregardmg the mtenslty of this peak mcreases the explanation factor of the mtensltles of the mass spectrum to 82 0% Isobutyiamme Cl-ammo-2methy~ropane) Analysis of the mass spectrum of lsobutylamme wth the standard set of elementary processes (Scheme 1) leads to the fragmentation sequences m Scheme 7 This fragmentation scheme and the associated reaction probabllltles can explain 68 7% of the intensity m the mass spectrum Workmg wrth the extended set of elementary processes (Schemes 1 and 6, IZ= 1) mcrease the number of peaks that can be explained from 4 to 8, the number of reactions m the network from 4 to 28, and the explanation factor from 68 7% to 76 5% Figure 5 shows how the reconstructed mass spectrum fits the experunental spectrum
S Bawm&m&
et aL/AnoL chun Acta 265 0992) 169-182
177
Scheme 7 Fragmeotatloo sequences of lsobutyhumoe obtamed with the processes IOScheme 1
tert-Butylmm.e (2-ammo-2methylpropane) The standard set of elementary processes (Scheme 1) can explam four peaks and four reactlons (Scheme 8) and grves an agreement between the expermental and reconstructed mass spectrum of 55 4% based on the overall mtenslty The extended set of processes (Schemes 1 and 6, n = 1) mcreases the number of explamed peaks from 4 to 7 vvlth 11 reactlons (mstead of 4) m the network and an explanation factor of 65 8% (Fig 6)
Experimsntd
0
Scheme 8 FragmeotaUoo scheme of twt-butylamme with the standard set of elementary processes m Scheme 1
Only the extended set (n = 1) can explam the formation of the ally1 catlon by two alternative pathways (Scheme 9) The additional peaks not yet assigned m the spectrum of tert-butylamme, and also m the spectra of set- and tsobutylamme, results from rearrangements and fragmentations m the alkyl groups that are stdl under mvestlgatlon
Spctrm
25
50
75
0
25
50
75
0
25
50
75
25
50
75
100
50
0
50
0
Recmsmcied slmchum
F@ 5 Comparison of the expenmeotal mass spectrum of lsobutylamme wth that reconstructed from the fragmeotatloo scheme obtamed wth the extended set of elemenw processes The explaoatlon factor m 76 5%
msmchum
FIN 6 Comparison of the experunental mass spectrum of tert-butylamme with that reconstructed from the fragmentatloo scheme obtluoed wth the extended set of processes The explanatloo factor IS65 8%
178
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Chun Acta 265 (1992) X69-182
EqmdmentalSpectmm
0
Scheme 9 Formation of ally1 catlon m the mass spectrum of tert-butylamme wrth the extended set of elementary processes
Lkthylamme The standard set of processes gives the reactlon sequences m Scheme 10 mth eight reactlons, assms seven peaks and explams 73 0% of the intensity The extended set of processes mcreases the assmed peaks to 9, Hnth 30 reactions m the fragmentation scheme and an explanation factor of 80 9% (Fig 7) Note that the mtensltles of the molecular Ion at m/z 73 and of the peak at m/z 30 are not completely reproduced This 1s due to
n T
Scheme 10 Fragmentation sequences obtamed for dlethylamme mth the standard set of processes m Scheme 1
25
50
75
Fig 7 Comparison of the expenmental mass spectrum of dlethylamme with that reconstructed from the fragmentation scheme obtamed by the extended set of processes The explanatlon factor 1s 80 9%
the fact that the correspondmg Isotope peaks at m/z 74 and 31 are either completely mlssmg or too small because the experimental mass spectrum 1s contamed m the database by glvmg mtensltles only m umts of percent If the mtensltres of those two peaks were completely reproduced (and they would be if the expetunental spectrum was of higher quality), 88 8% of the mtenslty m the experrmental mass spectrum could be explamed Isopropylmdhylamu The standard set of processes can explain eight peaks with eleven reactions m the network
Scheme 11 Fragmentation scheme obtamed for ~sopropylmethylamme wth the standard set of processes m Scheme 1
179
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(Scheme 11) and an assignment of the mtenslty The extended set (n = 1 m creases the number of assigned and the explanation factor to 84 a network of 35 reactlons
factor of 75 2% Scheme 6) mpeaks to twelve 1% (Fig 8) with
Dunethylethylamme The standard set of processes assigns seven peaks wth seven reactions (Scheme 12) and gwes an explanation factor of 63 3% The extended set can assqn 10 peaks by 27 reactions and mcreases the explanation factor to 76 2% (Fig 9) Scheme 13 shows fragmentations that can only be obtamed by the extended set Comparison The results of the analyses of the mass spectra of C,-ammes wtth the three dd’ferent sets of elementary processes are compared 111Table 1 They differ only m the restrictions reposed on the hydrogen rearrangement (Scheme 6) If the transition state for the hydrogen transfer from a C-H bond to the nitrogen atom must form a rmg with a mmunum srze of SIXatoms, we have the standard set If the tram&Ion state 1s also allowed to form a five- and a four-membered rmg we have the extended set This study shows that hydrogen transfer from an alkyl group to the nitrogen atom through
Scheme 12 Fragmentation scheme of dlmethylethylamme obtamed with the standard set of processes m Scheme 1
tranatlon states vvlth a five- or a four-membered rmg has to be taken mto account m the molecular ion of ammes with small alkyl groups as long as alkyl rearrangements are not explicitly allowed. Only when the alkyl group IS large enough to form a transitIon state wth a SE-membered rmg (m n-butylamme) does hydrogen transfer through five- and four-membered rmgs become ummportant Expmimenh!
75
0 loo--
50
’
t
’
’
’
1111 11’1
75
I
’
’
’
75 ’
’
’
’
I
II I
-
100
3
50 ’
-
0
50
Spectrum
25
I,IJ,I,IJ,IIII0
25
50
75
Remmmadn
Fu 8 Comparison of the expe.nmental mass spectrum of lsopropyhnethylamme with that reconstructed f&n the reactlon network obtamed by the extended set of processes The explanation factor IS84 1%
Fig 9 Comparison of the experunental mass spectrum of dlmethylethylamme mth that reconstructed from the network of 27 reactions obtamed by the extended set of processes The explanation factor IS76 2%
S Bauerschmdt et a&/And
180
J6.0444
P.0,~ l
mhn Scheme 13 Ad&tional rearrangements and fragmentations obtamed wth the extended set of processes
This suggests adoptmg the extended set of elementary processes m the automatic analysis of mass spectra However, Table 1 also shows that the complexity of the fragmentation and rearTABLE 1 Comparison of the performances of the three sets of elementary processes m explammg the mass spectra of C,-ammes ’ Compound
-NH,
Param- 6-rmg TS 5-nng TS eter (n = 3) (n-2)
4-rmg TS (n = 1)
f(%) NS E%)
902 7 585 16
912 7 32
934 9 44
NS NR
8 7
702 12 30
75 2 14 54
f(%) NS NR
687 3 4
763 8 14
765 8 27
f(%) NS ;;%)
554 4 730 4
55 4 4 4
65 8 7 11
NS NR
7 8
73 0 7 8
809 9 30
f(%) NS ;%)
752 8 633 11
75 2 8 11
84 1 11 35
NS NR
8 7
633 8 7
76 2 10 27
a 6-rmg TS = transition state havmg at least a swmembered rmg (Scheme 6, n = 31, etc , f = explanation factor, NS = number of peaks assigned m the mass spectrum, NR = number of reations m the fragmentation scheme
Chm Acta 265 (1992) 169-182
rangement scheme mcreases drastxally when also allowmg hydrogen transfer through five- and four-membered transltlon states Conconutant Hrlth this mcrease m complex@, the assignment of probabllltles to the m&wdual steps m the reactlon network becomes less strmgent because of the various alternatives to achieve a certain conversion of ions This decreases the level of confidence that can be given to the probability value of many restrlctlons Table 2 illustrates this for the various a-cleavages occurrmg 111 the molecular ions The standard set of elementary processes (SIXmembered transition state, n = 3 m Scheme 6) can asslgn probabllltles to the cu-cleavages m the molecular ions of the C,-ammes with a very high degree of certainty The values obtamed reflect m a quantltatlve manner the rankmg an expert would assign on a more mtultlve basis In many cases the upper and lower hmlts m the probablhties are the same, gMng the asslgned value a strong basis wrth mcreasmg flexlblhty m the reactlon altematlves, the results of the cY-cleavages can also be obtamed by a sequence of other processes, lowermg the lower lmt of the probablhtles of the a-cleavages and thus makmg their values less certam The upper llrmts of the reaction probablhtles for the a-cleavages remam fairly constant, mdlcatmg that they are not much influenced by addltlonal reactions In three cases the upper hmlt even increases on gomg from the standard to the extended set of reaction types This 1s caused by addltlonal reactions that open for the daughter ion, mcreasmg the amount of material that passes through that ion In some cases, no estunated values are given when workmg Hrlth the extended set Alternative processes through isomers are open for the conversion of the mother ion to the daughter Ion, grvrng the estimated value a high degree of uncertainty Hence obtammg a high degree of assignment of peaks m a mass spectrum and obtammg a high level of certainty m the probabdltles assigned to the steps m the fragmentation scheme are two mutually not completely reconcdable requlrements The choice of the appropnate set of elementary reactions and the restnctlons unposed
S Bauerschnudt et aL /AnaL
181
Chm. Acta 265 (1992) 169-182
TABLE 2 Comparison of a-cleavages m the molecular ions of C,-ammes ’ Compound
Neutral IOSS
Probablllty
LL H C3H7
0088
CH,
0 362 0 983 0021 0951 0 811 0 156 0689 0000 0 161 0606 0000 0 054 0744
H
C3H7 a3
H
a3 $
CH,CH$N+
KHz)32
23
HZ a3
’ LL = lower hut,
0000 0886
H
C2H5
5-rmg TS
hlllg Ts E 0006 0886 0088 0 362 0 983 0021 0951 0811 0 169 0699 0 102 0211 0609 0 065 0 083 0744
UL 0 012 0886 0088 0 362 0 983 0 021 0951 0 811 0 196 0 718 0 215 0268 0 616 0125 0 112 0744
LL 0000 0 885 0000 0 321 0000 OMMI 0000 0 811 0 156 0 689 0000 0 161 0606 OOOO 0 054 0744
4-rmg TS E 0885 0 026 0 321 0 828 0009 0 752 0811 0 169 0 699 0 102 0211 0609 0065 0 083 0744
UL 0 012 0885 0 075 0 321 0 970 0 019 0 939 0 811 01% 0 718 0 215 0268 0 616 0125 0 112 0744
LL
E
UL
0000 0881 0000 OOOO 0000 0000 0000 OCHKI 0000 0 428 0000 0000 0000 0000 0000 0000
-
0 011 0881 0 036 0 307 0964 0 019 0 938 0764 0 332 0 691 0106 0351 0 588 0 111 0268 0 701
0 881 0 115 0 810 0 752 0 514 0 589
E = estimated, UL = upper hmlt, -, value too uncertam to be asslgned
on them must depend on the prnnary emphasis that 1s desired m a study a high correspondence between expernnental and reconstructed mass spectrum by generatmg many reactlons and assigning many peaks, or a high degree of certainty m the reaction probabllltles of the major fragmentation steps Concluswns It has been shown that It 1s possible to derive the steps of fragmentations and rearrangements occurrmg m the mass spectrometer from an automatic analysis of a mass spectrum The representation of the species m the mass spectrometer by explicit valence bond structures and then- reactions by shifts of bonds and electrons forms a basis that explains most of the peaks 111a mass spectrum The assumptions made m the chemical and mathematical models are sufflclently robust to allow the assignment of conversion probablhties to the mdrvldual steps of a fragmentation scheme The values of these reaction probablhtres quantitatively reflect the more mtultlve and quahtatlve knowledge of experts
The system should be of interest m elucldatmg the events takmg place m the mass spectrometer For the expert it will give quantitative mformatlon that has to be verified and exploited to Increase the knowledge on mass spectral reaction patterns We are currently comparing the results obtained from the system with mformatlon gathered by tandem mass spectrometnc experunents For the organic chemist it will provide an easy access to peak assignment and thus increase mterest m the use and integration of mass spectra Apphcatlon of the system to a series of mass spectra is a way of automatically extracting knowledge from mass spectral data This allows a knowledge base to be built that can be used m the prediction of mass spectra [2] In fact, this was the major objective m developmg this system and 1s being actively pursued
The authors thank the Forschung und Technologle research and development with Dr W Bremser, ROW,
Bundesmmlster fur for supportmg this Helpful dlscusslons Dr H Kubmyl and
182
Dr R Neudert, both BASF, Dr V Schubert, GMD-PTF, Professor K. Varmuza, TU Wien, and Dr M Weller, Chemical Concepts, are acknowledged
S Bawxchmuit
et IrL/AnaL Chvn Acta 265 (1992) 169-182
REFERENCES 1 F W McLafferty, Interpretation of Mass Spectra, Umversity Saence Rooks, Mill Valley, CA, 1980 2 W Hanebeck, K. Rafemer, K-P Schulz, P Rose and J Gasteqger, m J Gastelger (Ed ), SotIwrre Development m Chenustry 4, Sprmger, Berlm, 1990, p 187
Anaiytiu Chutuca Actu, 265 (1992) 169-182 Elsevler Science Pubhshers B V , Amsterdam
Elucidation of reactions in the mass spectrometer S Bauerschnndt, Organwh-Chemwdes
W Hanebeck, K-P
Schulz and J Gasterger
Instuut, Techmsche Unrversltat Muden,
W-8046 Garchmg (Germany)
(Recensed 20th Januaxy 1992)
Abstract A system has been developed that can denve from the structure and the mass spectrum of an organic compound the fragmentation and rearrangement reactlons occurrmg m the mass spectrometer Peak mtensltles are used to calculate conversion probabtities for the mdlwdual reactlon steps of the fragmentations scheme These analyses can be performed for all classes of orgamc compounds except saturated hydrocarbons and explam between 70 and 100% of the intensity of quabty mass spectra A study on a senes of C,-ammes dustrates the approach and its results Keywords Mass spectrometry, Butylammes, Computer modelhng, Reaction elucldatlon
Mass spectrometry plays a major role 111the structure elucrdatron of orgamc compounds The fragmentations of an orgamc molecule m the mass spectrometer are charactenstrc of the arrangements of atoms and bonds m the molecule and thus give detarled mformatron about us constitution However, thrs richness of mformatron 1s only rarely used Often the chemist only looks for the peak of the molecular ran as proof of rts molecular formula. No other peaks 111the mass spectrum are analysed and thus all the mformatron they contam IS lost Thrs deplorable srtuatron IS largely due to the poor understanding of the events that take place m the mass spectrometer Even experts have drffrcultres assrgnmg a structure to many of the peaks 111a mass spectrum Drawmg conclusrons about the fragmentation and rearrangement reactions of a molecule m the mass spectrometer IS even more comphcated The hrgh energy used for the romzatron of a molecule by
electron impact m the source of a mass spectrometer creates species that can undergo a multrtude of reaction pathways because of the addrtronal energy content of these radical catrons and catrons On the other hand, the large number of mass spectra that have been measured provrde a rrch source of information for leammg more about the processes that occur m the mass spectrometer Leammg needs a model for the structure of the information that has to be understood The model that was chosen for these studies IS based on concepts that have been used over many years for the interpretatron of mass spectra The large amount of mformatron to be processed and the many alternatives m high-energy species and their reactrons to be consrdered suggest modellmg by computer programs
FOUNDATIONS OF THE APPROACH
Correspondenceto J Gastelger, Orgamsch-Chemlsches Instltut, Techmsche Umversltat Munchen, W-8046 Garchmg (Germany)
From the begmnmgs of the mass spectrometry of orgamc compounds, attempts have been made to descrrbe the structures of the species involved
0003~2670/92/$05 00 0 1992 - Elsewer Science Publishers B V All nghts reserved
170
S Bauerdmdt
by structural formulae Analogously, the reactions m the mass spectrometer were wntten as electron-transfer processes A comprehensive exposition of this approach was gven by McLafferty
Dl Such a descnptlon of the structures and their reactions 1s also the basis of the present model Spectically, tlus means that the chenucal species formed from organic compounds m the mass spectrometer can be represented by one or more valence bond structures In other words, the unpaired electron of the radical and the posltlve charge of the cation are locahzed on an atom or delocalized m a r-system In fact, It will be seen later that this 1s not absolutely necessary The analysis of a mass spectrum will also work when a species can be represented by a series of rearrangmg isomers For example, the C,H,O+ radical catlon can be represented either as CH3CH20+ H, or as CH,-CI-I,O+ H,, or as a
A-B-C
-
A-B
-
Al+;
+C
1-w + A-E
+
‘A
L,J0-c
A-B
\,Jc
+
A-0 -
B-C \3-0
J
OniumReactkm(AnotCdmn) i-B
C-H_
L3J HalogenCIemge @ = C&W)
A-X
i--H
-
A+?
-
iJl+cso
+
B-C
b-3 J
carbony( Etiminatbn R-c-6
Chm Acta 265 (1992) 169-182
species mterconvertmg between these two lsomerlc structures Further, the fragmentation and rearrangement reactions occurring m the mass spectrometer can be represented by shifts of bonds and of electrons m the above valence bond structures Scheme 1 contams all the elementary processes used m the computer model, largely from the book by McLafferty [l] By apphcatlon of such a general reaction scheme to the structure of a specific cation or radical catlon, the reactions of this species are obtamed, e g , for the a-cleavage of ionized methyl ethyl ether A-B-C
-
Me-%-CH,-CH,
A=B+ -
C Me-6=CH,
+ CH,
The elementary processes m Scheme 1 were selected to cover the reactlons observed m electron impact (EI) mass spectra The present approach can also be extended to other types of mass spectra, such as chermcal lomzatlon (CI) mass spectra A series of elementary processes somehow different from those m Scheme 1 then have to be specified The first process m an EI mass spectrum IS lomzatlon Here we consider lomzatlon from free electron pairs and from +orbltals, lomzatlon from a a-orbital 1s not yet included
ESSENTIAL STEPS IN THE ANALYSIS
C
L37J
i--o +
-‘-
et al /Anal
Scheme 1 Elementary processes covermg the reations of orgamc compounds m the mass spectrometer A, B, C, D may be any type of atoms (except m carbonyl elunmatlon), m some reations, the atom types may be restncted to special cases, 3-7 gwes the range for the number of atoms m the cham lmkmg the two atoms specified
The determmatlon of the species and their reactions occurrmg m the mass spectrometer needs two pieces of mforrnatlon the structure of a compound and its mass spectrum The essential steps m the analysis of a mass spectrum to determme which reactions take place m the mass spectrometer are shown m Scheme 2 and are bnefly described below First, all formally possible reactions are generated for the given structure After lomzatlon at all atoms with free electron pairs or from each r-orbital, all elementary reactions m Scheme 1 are applied to these prunary ions and thus lead to all potential secondary ions These are then submitted to the elementary processes glvmg the next level of ions The entire procedure IS repeated untd the ions are so small that no elemen-
S Bauersdwnadt et al. /AnaL Chm. Acta 265 (1992) 169-182
r-l r-l Reduchofl of the ReacUon Network
calculahon
of
Reachon
PrObahbUes
Scheme 2 The function flow m the automatx analysis of a mass spectrum
tary process fits any longer, or pathways are generated that lead to Ions not observed m the mass spectrum (see below) The elementary processes are contained m a separate file outside the program Hence they can easily be modified by changmg the restnctlons for then apphcatlon, or additional processes can be added wlthout having to change the program system Each group of peaks m the mass spectrum IS analysed to see which of the ions generated have a mass that fits these peaks This process is strongly guided by takmg account of the theoretlcal isotope dlstnbutlon of the molecular formula of an ion
171
If an Ion 1s generated that 1s not observed m the mass spectrum, it 1s nevertheless kept and further subnutted to the elementary processes, as It could be that the ion decomposes so rapldly that it does not gve a peak Only If no daughter Ion of this Ion can be detected either IS the entire reaction sequence deleted from the reaction network All those ions and reactions of the network that have been asslgned on the basis of the mass spectrum are used to reconstruct a mass spectrum The reconstructed mass spectrum gwes that part of the mass spectrum which can be explamed by the entire analysis Rearrangement reactions ave lsomerlc ions that have the same mass and isotope patterns Therefore, a peak posItIon 1s msufflaent evldence for decldmg whether one or several lsomers are present and which structure or structures to associate with a peak For the next steps m the analysis of the reaction networks, the lsomers of one or more rearrangement steps are therefore combmed to a smgle node m the reaction network Whether a rearrangement reaction takes place or not can be inferred only later when it 1s seen that the rearrangement of a structure to a different isomer IS needed m order that an observed fragmentation reaction can proceed After a structure, or a series of interconverting lsomerlc structures, has been asslgned to all peaks, the mtensltles of the peaks are used to determme how much material has passed through the vanous branches of the reaction network Thrs 1s performed by starting with the species at the end of the reactlon network that are not further fragmented Their intensity 1s added to the mtenslty of their precursor ion because the entire amount of this Ion has been produced from this precursor ion However, several features of the reaction network make this analysis complicated The sequences of fragmentation and rearrangement reactlons form a network and not only a tree of reactions In other words, an ion m&t be formed from more than one presursor Ion Thrs has the result that two or more sequences of reactions combme m a smgle node of the network
172
There 1s not enough mformation m the mass spectrum that allows one to detemune what amount of a wen ion has come from the various preeursor ions In this situation, three altematlves are calculated. an estunated value for the conversion of one precursor ion to a daughter ion IS obtamed by makmg the assumption that all precursor Ions have contnbuted to the common daughter Ion by an equal amount, an upper-bound value 1s calculated by makmg the assumption that all mtenslty of the daughter ion has been produced from the one precursor Ion bemg considered, and a lower-bound value 1s calculated by assummg that the precursor Ion bemg considered has not produced the common daughter Ion at all The mtensltles of the daughter ions are added to the parent ion by gomg through the reaction network from the bottom to the top In this way, the amount of matenal that flows through the vanous reaction pathways can be calculated In the next step, all those species of the reactlon network that have a value of zero for the upper-bound of the amount of matenal are elmunated In addltlon, all reactlon sequences that start from such species are ehmmated Hence the network of all concenrable reactions generated m the fit steps 1s reduced to a network that contams only the reactions that are actually occurrmg Next, probabdltles are assigned to the reactlons of the reduced network The senes of competmg reactions 1s handled by equations havmg the formal charactenstxs of first-order kmetlcs The conversion rates and amounts of matenal allow the calculation of reaction probablhtles for each fragmentation reaction As was done for the converstons, three values are calculated for the reactron probabdltles a lower-bound, an estlmated value and an upper-bound For rearrangement reactions a calculation of reactlon probabdltles 1s not possible for the reasons mentloned above (Isomers have the same peak position) However, one can now determme whether a rearrangement reaction 1s occurrmg or not based on the analysis of the reduced reactlon network The node contammg a senes of Isomers IS expanded mto the mdlvldual isomers and thex rearrangements Only those rearrangements that
S Bauemhmdt
et aL /Ad
Chm
Acta 265 (1992) 169-182
lead to an ion that reacts further to a fragment that IS observed are kept m the mass spectrum The dfierence between the upper- and lowerbounds of the reaction probabrlltles and the confidence of the amounts of mater& are used to assign a confidence value to the reaction probabrlltles Three levels of confidence are carned very high (11, high (2) and low (3)
IMPLEMENTATION
The system has been wntten m standard FOR1s therefore portable At present it 1s implemented on VAX statlons That part of the system which generated the overall fragmentation and rearrangement scheme consists of about 170 routmes vvlth ca 25 000 lmes of codes The reductlon and analysis of the reactlon network 1s performed by a system wth about 60 routmes and ca 12000 lmes of code Commumcat~on between the two phases of the analysis 1s reahzed through several dn-ect access files The results of an analy~1sare wntten on files The system can also analyse a series of pairs of structures and then associated mass spectra Then, all reactions of a certam type (e g , cr-cleavages) observed m this senes of spectra are collected m a file In fact, each reaction type 1s collected m three different files dependmg on the degree of confidence that has been attnbuted to the conversion probability of the reactlon step consldered In other words, there 1s one file of cw-cleavages that have a very h& level of cotidence m their reaction probabdmes, another one collectmg those with a high degree of confidence and a third one with those with a low level of confidence The same 1s true for all other elementary reactlons m Scheme 1 These files form the basis for systematic statlstrcal analyses of reactlon types to build a knowledge base for mass spectra sunulatlon TRAN 77 and
RESULTS
The approach IS applicable to all those orgamc structures havmg a w-system or a free electron
S Bauemhmdt
et al /Anal
173
Chum. Acta 265 (1992) 169-182
CHxBrCl
I
Observed 0
50
0
50
spectrum 100
150
200
100
150
200
Calculated
lid=Br+ _ ClEC=Br+ u 0 476(l) 5
0 741(l)
lid=Cl+
spectrum
Fig 1 Compmson of the expenmental Hnththe reconstructed mass spectrum of chlorobromomethane The explanation factor IS910%
g
Scheme 3 Fragmentation scheme of chlorobromomethane
pair Two examples from different ends of the range of complex@ of the reaction network are presented first before a more detailed mvestlgatlon of the mass spectra of C,-ammes 1s given All mass spectra were taken from the NIH mass spectral database and were determined with a 70-eV iomzation source Scheme 3 shows the fragmentation scheme of chlorobromomethane deduced from its mass spectrum The neutral species lost m these fragmentations are not shown for clarity of presentation For each reaction the calculated transition probabllrty 1s given together vvlth its level of confidence (1 = very high, 2 = high, 3 = low) In Fig 1, the experimental mass spectrum of chlorobromomethane 1s compared with that reconstructed from the reaction network m Scheme 3 and the reactlon probablhtles contained m this scheme, 910% of the overall intensity m the expernnental mass spectrum can be explained by Scheme 3 The analysis of the fragmentations and rearrangements of ethyl hexanoate leads to a reactlon network wrth has mltlally 1196 ions and 3992 reactions After reduction of the network based on mformatlon m the experimental mass spectrum, 883 reactions remam Because of the large number of reactions the reaction network 1s not
given here All 34 peaks havmg an mtenslty higher than 15% can be assigned to fragments Only four signals with an intensity below 1% cannot be assigned Altogether 99 9% of the intensity of the mass spectrum can be explained (Fig 2) In a more systematic study, all C,-ammes that had spectra m the NIH database were mvestlgated The results are presented m the followmg sections
(Zompomd
spectra
/
ldsnt
1
195279
Observed
spectrum
1000 50
0
50
100;
I 0
I
I
!
I
I
,
50 Calculated
, 100
,
,
,
,
( 150
spectrum
Fig 2 Comparison of the experunental with the reconstructed mass spectrum of ethyl hexanoate The explanation factor IS 99 9%
S Bauerschmtdt et at!/And
174
0 742(3)
Chm Acta 265 (1992) 169-182
0 098 (2)
l-l l
.= .rlcl
Scheme 4 Fragmentation and rearrangement reactmes of n-butylamme obtamed with the elementary processes 111Scheme 1 The numbers gtve the values calculated for the reaction probabthttes, the values m parentheses are the level of confidence Cl= very high, 2 = high, 3 = low) Note that some ams show up for clartty of presentation at several places m the reactlon scheme
n-Butyhmme (I-amuwbutane) The analysis of the mass spectrum of nbutylamme with the elementary reactions m Scheme 1 gave the reaction network m Scheme 4 The pnmary ion undergoes two a-cleavages with the one leading to H, C = NH; glvlng the base peak The prnnary ion has to rearrange to an lsomenc ion by a hydrogen atom m order to be able to explain a senes of other peaks (the bottom three levels m the reactron tree in Scheme
4 Hence we have here a case where a rearrangement reaction 1s found to occur The reaction network m Scheme 4 can explam 90 2% of the mtenslty of the experlmental mass spectrum (see Fig 3) The peak at mass 18 cannot be assigned m the automatic analysis It 1s presumably caused by water contamed as an unpunty m the amme The altematwe, that this peak was caused by the ammomum ion NH:, seems unhkely The analy-
S Jhuembd
et d/And
C&m Acta 265 (1992) 169-182
175
-spraun 0 100’
0
25 -sBcbum
50
”
0
75
F@ 3 Companaon of the expertmental mass spectrum of n-butylamme with that r~~~astructed from the reaction network m Scheme 4 The explanatton factor IS 90 2%
a
25 ”
”
50 ”
25 maI=c@Jn
”
”
’
’
50
--
75
b
0
25
50
75
0
25 m-m
50
75
100
SIS IS sometimes able to fmd peaks of nnpuntles and thus can be used in checkmg the quahty of mass spectra
50 0
set-Butylamzne (2-ammbutane) Apphcation of the elementary processes m Scheme 1 to mc-butylamme and compaxxson wrth
50 100
,
Ftg. 4 Cempanson of the expenmental mass spectrmn of xc-batytamme anth (a) that reconstructed from the reactions m S&me 5 (obtamed from the ps m Scheme l), the exphutabcm factor 1s 585%, and (b) that reconstructed from the reactton network obtamed wrth the extended set of elementary processes (Schemes 1 and 6), the explanatton IS factor 75 290
0-m
3 T
Scheme 5 Fragmentatton reacttons of see-butylamme obtamed wtth the processes m Scheme 1
rts experimental mass spectrum leads to the reactlon tree m Scheme 5 Three a-cleavages are observed for the prnnary Ion, the loss of an ethyl ra&cai 1s the preferred process gwmg the base peak at m/z = 44 The loss of a methyl radical to give the peak at m/z = 58 1s less favourable and the loss of a hydrogen atom occurs to only a very small extent The reactlon network m Scheme 5 can only explain a dlsappomtmgly low 58 5% of the overall mtenslty m the expernnental mass spectrum (Fw 4a) What IS the reason for ths poor agreement7
176
S Baumdwdt
Are there some rmportant processes lntssmg m Scheme 1 or must they be modtied? Hydrogen rearrangements are allowed to occur m this set of processes only d the transition state for the transfer of a hydrogen atom forms at least a sm-membered rmg (n = 3 m Scheme 6) This IS by far the most favourable situation However, m secbutylamme such a rmg cannot be formed, the rmg size for transferrmg a hydrogen atom from the most distant carbon atom to the mtrogen atom m the prunary Ion 1s only five
If we set IZ111Scheme 6 to a lower hrmt of 2 to allow this process to occur m the network, the number of reactions increases from 7 to 30 This more complex reaction network can now explam twelve mstead of eight peaks and 70 2% of the overall mtenslty m the experunental mass spectrum Settmg n m Scheme 6 to a lower lmt of 1 and thus allowmg a hydrogen atom to be transferred to mtrogen also m a transltlon state wth a fourmembered rmg further increases the number of steps m the network to 56 Now, SIXaddItiona peaks and 75 2% of the intensity m the mass spectrum can be explamed (Fig 4b) Using the extended set of elementary processes (n extended from 3 down to 1 111Scheme 6) m the mvestlgatlon of the mass spectrum of n-butylamme mcreases the number of reactions from 16 to 44 and the explanation factor from 902%to934% What can be learned from this investigation7 Hydrogen transfer 1s indeed highly preferred for transition states with rmgs of s1x and more atoms (see n-butylamme) However, if an Ion has no
A
“\
&q/B-
/”
*
t,_,;
n=l,2,3
Scheme 6 Hydrogen rearrangement
et aL /AnaL Chm. Acta 265 (1992) 169-182
posslbtity of attammg such a sm-membered transltlon state It wdl also follow reaction channels of hydrogen transfer through transition states Hrlth five- or four-membered rmgs Any extension of the elementary processes m Scheme 1 nught result 111a drastic mcrease m the complextty of the reaction networks These more complex fragmentation and rearrangement schemes often have several alternatives for the conversion of one mto another spec&c Ion This reduces the level of confidence that can be assigned to the reaction probabdttles Care should therefore be taken to work with the mmunum set of elementary processes Just necessary to explam as much of the mtenslty m the mass spectrum as possible Retummg to the example of set-butylamme 75 2% of explamed mtenslty leaves still somethmg to be desired, although the mcrease m the explanation factor by 16 7% 1s remarkable A further increase m the explanations potential can be expected by allowmg additional rearrangements m the alkyl groups We are currently workmg to find an appropnate extension of the elementary prcesses that keeps the number of rons wlthm a reasonable lmut and holds the confldence level for the reaction probablhtles high enough Again, an unpurlty of water was present m the amme glvmg the peak at m/z = 18 Dlsregardmg the mtenslty of this peak mcreases the explanation factor of the mtensltles of the mass spectrum to 82 0% Isobutyiamme Cl-ammo-2methy~ropane) Analysis of the mass spectrum of lsobutylamme wth the standard set of elementary processes (Scheme 1) leads to the fragmentation sequences m Scheme 7 This fragmentation scheme and the associated reaction probabllltles can explain 68 7% of the intensity m the mass spectrum Workmg wrth the extended set of elementary processes (Schemes 1 and 6, IZ= 1) mcrease the number of peaks that can be explained from 4 to 8, the number of reactions m the network from 4 to 28, and the explanation factor from 68 7% to 76 5% Figure 5 shows how the reconstructed mass spectrum fits the experunental spectrum
S Bawm&m&
et aL/AnoL chun Acta 265 0992) 169-182
177
Scheme 7 Fragmeotatloo sequences of lsobutyhumoe obtamed with the processes IOScheme 1
tert-Butylmm.e (2-ammo-2methylpropane) The standard set of elementary processes (Scheme 1) can explam four peaks and four reactlons (Scheme 8) and grves an agreement between the expermental and reconstructed mass spectrum of 55 4% based on the overall mtenslty The extended set of processes (Schemes 1 and 6, n = 1) mcreases the number of explamed peaks from 4 to 7 vvlth 11 reactlons (mstead of 4) m the network and an explanation factor of 65 8% (Fig 6)
Experimsntd
0
Scheme 8 FragmeotaUoo scheme of twt-butylamme with the standard set of elementary processes m Scheme 1
Only the extended set (n = 1) can explam the formation of the ally1 catlon by two alternative pathways (Scheme 9) The additional peaks not yet assigned m the spectrum of tert-butylamme, and also m the spectra of set- and tsobutylamme, results from rearrangements and fragmentations m the alkyl groups that are stdl under mvestlgatlon
Spctrm
25
50
75
0
25
50
75
0
25
50
75
25
50
75
100
50
0
50
0
Recmsmcied slmchum
F@ 5 Comparison of the expenmeotal mass spectrum of lsobutylamme wth that reconstructed from the fragmeotatloo scheme obtamed wth the extended set of elemenw processes The explaoatlon factor m 76 5%
msmchum
FIN 6 Comparison of the experunental mass spectrum of tert-butylamme with that reconstructed from the fragmentatloo scheme obtluoed wth the extended set of processes The explanatloo factor IS65 8%
178
S Bauerschmtdt et aL /Anal
Chun Acta 265 (1992) X69-182
EqmdmentalSpectmm
0
Scheme 9 Formation of ally1 catlon m the mass spectrum of tert-butylamme wrth the extended set of elementary processes
Lkthylamme The standard set of processes gives the reactlon sequences m Scheme 10 mth eight reactlons, assms seven peaks and explams 73 0% of the intensity The extended set of processes mcreases the assmed peaks to 9, Hnth 30 reactions m the fragmentation scheme and an explanation factor of 80 9% (Fig 7) Note that the mtensltles of the molecular Ion at m/z 73 and of the peak at m/z 30 are not completely reproduced This 1s due to
n T
Scheme 10 Fragmentation sequences obtamed for dlethylamme mth the standard set of processes m Scheme 1
25
50
75
Fig 7 Comparison of the expenmental mass spectrum of dlethylamme with that reconstructed from the fragmentation scheme obtamed by the extended set of processes The explanatlon factor 1s 80 9%
the fact that the correspondmg Isotope peaks at m/z 74 and 31 are either completely mlssmg or too small because the experimental mass spectrum 1s contamed m the database by glvmg mtensltles only m umts of percent If the mtensltres of those two peaks were completely reproduced (and they would be if the expetunental spectrum was of higher quality), 88 8% of the mtenslty m the experrmental mass spectrum could be explamed Isopropylmdhylamu The standard set of processes can explain eight peaks with eleven reactions m the network
Scheme 11 Fragmentation scheme obtamed for ~sopropylmethylamme wth the standard set of processes m Scheme 1
179
S Bauersdwmdt et aL /AnaL Chum Acta 265 (1992) 169-182
(Scheme 11) and an assignment of the mtenslty The extended set (n = 1 m creases the number of assigned and the explanation factor to 84 a network of 35 reactlons
factor of 75 2% Scheme 6) mpeaks to twelve 1% (Fig 8) with
Dunethylethylamme The standard set of processes assigns seven peaks wth seven reactions (Scheme 12) and gwes an explanation factor of 63 3% The extended set can assqn 10 peaks by 27 reactions and mcreases the explanation factor to 76 2% (Fig 9) Scheme 13 shows fragmentations that can only be obtamed by the extended set Comparison The results of the analyses of the mass spectra of C,-ammes wtth the three dd’ferent sets of elementary processes are compared 111Table 1 They differ only m the restrictions reposed on the hydrogen rearrangement (Scheme 6) If the transition state for the hydrogen transfer from a C-H bond to the nitrogen atom must form a rmg with a mmunum srze of SIXatoms, we have the standard set If the tram&Ion state 1s also allowed to form a five- and a four-membered rmg we have the extended set This study shows that hydrogen transfer from an alkyl group to the nitrogen atom through
Scheme 12 Fragmentation scheme of dlmethylethylamme obtamed with the standard set of processes m Scheme 1
tranatlon states vvlth a five- or a four-membered rmg has to be taken mto account m the molecular ion of ammes with small alkyl groups as long as alkyl rearrangements are not explicitly allowed. Only when the alkyl group IS large enough to form a transitIon state wth a SE-membered rmg (m n-butylamme) does hydrogen transfer through five- and four-membered rmgs become ummportant Expmimenh!
75
0 loo--
50
’
t
’
’
’
1111 11’1
75
I
’
’
’
75 ’
’
’
’
I
II I
-
100
3
50 ’
-
0
50
Spectrum
25
I,IJ,I,IJ,IIII0
25
50
75
Remmmadn
Fu 8 Comparison of the expe.nmental mass spectrum of lsopropyhnethylamme with that reconstructed f&n the reactlon network obtamed by the extended set of processes The explanation factor IS84 1%
Fig 9 Comparison of the experunental mass spectrum of dlmethylethylamme mth that reconstructed from the network of 27 reactions obtamed by the extended set of processes The explanation factor IS76 2%
S Bauerschmdt et a&/And
180
J6.0444
P.0,~ l
mhn Scheme 13 Ad&tional rearrangements and fragmentations obtamed wth the extended set of processes
This suggests adoptmg the extended set of elementary processes m the automatic analysis of mass spectra However, Table 1 also shows that the complexity of the fragmentation and rearTABLE 1 Comparison of the performances of the three sets of elementary processes m explammg the mass spectra of C,-ammes ’ Compound
-NH,
Param- 6-rmg TS 5-nng TS eter (n = 3) (n-2)
4-rmg TS (n = 1)
f(%) NS E%)
902 7 585 16
912 7 32
934 9 44
NS NR
8 7
702 12 30
75 2 14 54
f(%) NS NR
687 3 4
763 8 14
765 8 27
f(%) NS ;;%)
554 4 730 4
55 4 4 4
65 8 7 11
NS NR
7 8
73 0 7 8
809 9 30
f(%) NS ;%)
752 8 633 11
75 2 8 11
84 1 11 35
NS NR
8 7
633 8 7
76 2 10 27
a 6-rmg TS = transition state havmg at least a swmembered rmg (Scheme 6, n = 31, etc , f = explanation factor, NS = number of peaks assigned m the mass spectrum, NR = number of reations m the fragmentation scheme
Chm Acta 265 (1992) 169-182
rangement scheme mcreases drastxally when also allowmg hydrogen transfer through five- and four-membered transltlon states Conconutant Hrlth this mcrease m complex@, the assignment of probabllltles to the m&wdual steps m the reactlon network becomes less strmgent because of the various alternatives to achieve a certain conversion of ions This decreases the level of confidence that can be given to the probability value of many restrlctlons Table 2 illustrates this for the various a-cleavages occurrmg 111 the molecular ions The standard set of elementary processes (SIXmembered transition state, n = 3 m Scheme 6) can asslgn probabllltles to the cu-cleavages m the molecular ions of the C,-ammes with a very high degree of certainty The values obtamed reflect m a quantltatlve manner the rankmg an expert would assign on a more mtultlve basis In many cases the upper and lower hmlts m the probablhties are the same, gMng the asslgned value a strong basis wrth mcreasmg flexlblhty m the reactlon altematlves, the results of the cY-cleavages can also be obtamed by a sequence of other processes, lowermg the lower lmt of the probablhtles of the a-cleavages and thus makmg their values less certam The upper llrmts of the reaction probablhtles for the a-cleavages remam fairly constant, mdlcatmg that they are not much influenced by addltlonal reactions In three cases the upper hmlt even increases on gomg from the standard to the extended set of reaction types This 1s caused by addltlonal reactions that open for the daughter ion, mcreasmg the amount of material that passes through that ion In some cases, no estunated values are given when workmg Hrlth the extended set Alternative processes through isomers are open for the conversion of the mother ion to the daughter Ion, grvrng the estimated value a high degree of uncertainty Hence obtammg a high degree of assignment of peaks m a mass spectrum and obtammg a high level of certainty m the probabdltles assigned to the steps m the fragmentation scheme are two mutually not completely reconcdable requlrements The choice of the appropnate set of elementary reactions and the restnctlons unposed
S Bauerschnudt et aL /AnaL
181
Chm. Acta 265 (1992) 169-182
TABLE 2 Comparison of a-cleavages m the molecular ions of C,-ammes ’ Compound
Neutral IOSS
Probablllty
LL H C3H7
0088
CH,
0 362 0 983 0021 0951 0 811 0 156 0689 0000 0 161 0606 0000 0 054 0744
H
C3H7 a3
H
a3 $
CH,CH$N+
KHz)32
23
HZ a3
’ LL = lower hut,
0000 0886
H
C2H5
5-rmg TS
hlllg Ts E 0006 0886 0088 0 362 0 983 0021 0951 0811 0 169 0699 0 102 0211 0609 0 065 0 083 0744
UL 0 012 0886 0088 0 362 0 983 0 021 0951 0 811 0 196 0 718 0 215 0268 0 616 0125 0 112 0744
LL 0000 0 885 0000 0 321 0000 OMMI 0000 0 811 0 156 0 689 0000 0 161 0606 OOOO 0 054 0744
4-rmg TS E 0885 0 026 0 321 0 828 0009 0 752 0811 0 169 0 699 0 102 0211 0609 0065 0 083 0744
UL 0 012 0885 0 075 0 321 0 970 0 019 0 939 0 811 01% 0 718 0 215 0268 0 616 0125 0 112 0744
LL
E
UL
0000 0881 0000 OOOO 0000 0000 0000 OCHKI 0000 0 428 0000 0000 0000 0000 0000 0000
-
0 011 0881 0 036 0 307 0964 0 019 0 938 0764 0 332 0 691 0106 0351 0 588 0 111 0268 0 701
0 881 0 115 0 810 0 752 0 514 0 589
E = estimated, UL = upper hmlt, -, value too uncertam to be asslgned
on them must depend on the prnnary emphasis that 1s desired m a study a high correspondence between expernnental and reconstructed mass spectrum by generatmg many reactlons and assigning many peaks, or a high degree of certainty m the reaction probabllltles of the major fragmentation steps Concluswns It has been shown that It 1s possible to derive the steps of fragmentations and rearrangements occurrmg m the mass spectrometer from an automatic analysis of a mass spectrum The representation of the species m the mass spectrometer by explicit valence bond structures and then- reactions by shifts of bonds and electrons forms a basis that explains most of the peaks 111a mass spectrum The assumptions made m the chemical and mathematical models are sufflclently robust to allow the assignment of conversion probablhties to the mdrvldual steps of a fragmentation scheme The values of these reaction probablhtres quantitatively reflect the more mtultlve and quahtatlve knowledge of experts
The system should be of interest m elucldatmg the events takmg place m the mass spectrometer For the expert it will give quantitative mformatlon that has to be verified and exploited to Increase the knowledge on mass spectral reaction patterns We are currently comparing the results obtained from the system with mformatlon gathered by tandem mass spectrometnc experunents For the organic chemist it will provide an easy access to peak assignment and thus increase mterest m the use and integration of mass spectra Apphcatlon of the system to a series of mass spectra is a way of automatically extracting knowledge from mass spectral data This allows a knowledge base to be built that can be used m the prediction of mass spectra [2] In fact, this was the major objective m developmg this system and 1s being actively pursued
The authors thank the Forschung und Technologle research and development with Dr W Bremser, ROW,
Bundesmmlster fur for supportmg this Helpful dlscusslons Dr H Kubmyl and
182
Dr R Neudert, both BASF, Dr V Schubert, GMD-PTF, Professor K. Varmuza, TU Wien, and Dr M Weller, Chemical Concepts, are acknowledged
S Bawxchmuit
et IrL/AnaL Chvn Acta 265 (1992) 169-182
REFERENCES 1 F W McLafferty, Interpretation of Mass Spectra, Umversity Saence Rooks, Mill Valley, CA, 1980 2 W Hanebeck, K. Rafemer, K-P Schulz, P Rose and J Gasteqger, m J Gastelger (Ed ), SotIwrre Development m Chenustry 4, Sprmger, Berlm, 1990, p 187