- Email: [email protected]
The study of rearrangement reactions by field ionization mass spectrometry
International
Else&r
Johal
of Mass
Spectrometry
and Ion Physics
Publishing Company, Amsterdam. Printed in the Netherlands
TIJE STUDY OF REARRANGEMENT TION MASS SPECTROMETRY III. .EXPERIMENTAL REACTIONS*
RESULTS
FOR
REACTIONS-
DIFFERENT
’
...
BY FIELD
TYPES
IONIZA‘.
OF
K. LEVSEN AND I-l. D. BECKEY Institut fiir Physikalische
Chemie
der Unicersit&
Bonr, (W_
Germany)
(Received July 2&h, 1971)
ABSTRACT
The kinetic behaviour of rearrangement reactions and direct bond cleavages as derived qualitatively from FI mass spectra is summarized. At low excitation energies (at 12 eY EI, or under R conditions) skeletal rearrangements are slow, and hydrogen migrations relatively fast, decomposition processes (decomposition times corresponding to the peak maxima: several times 10s6 set, and several times lo-l1 set, respectively). Among direct bond fissions alkyl radical eliminations are especially slow processes. An unambiguous differentiation between rearrangements and direct bond cleavages from the kinetic data is difficult. The kinetic study of the tropylium ion formation demonstrates that this rearrangement starts with a field-induced benzyl ion formation in an FI source (in contrast to ~1 conditions).
1. INTRODUCTiON
mass spectrometry has been used extensively for the study of rearrangement reactions of organic ions in the gas phasel-“. The capability of resolving extremely short decomposition times using this ionization mode Lamed to be promising for detecting kinetic differences in the fcrmation of direct and iearranged fragment ions. These studies revealed that most of the skeletal rearrangements cannot bedetected as normal fragment ~eaks~~~*~* but often a&normal metastable ions** indicating slow decomposition processes. Hydrogen rearrangements, how‘ever, were detected even in FI mass spectra as abundant normal fragment ions (like field-induced direct bond fissions). FI
* For parts I and II of this series, see refs. I and II. ** “Normal”
fragment io&, defined previously’.
hi
1. Mass
Spectiom:
“fast metastable” ions and %or&al”
met&able
-.. ionS have been
.. -63
Ion Plzys., 9 (1972)
. .
:.
These results seemed to be inconsistent with a rule, reported by Beckey’, stating that field-induced direct bond fissions should he faster than rearrangement reactions in an R source. These apparent contradictions were resolved by experiments of Schulz and Richter4 and by the present authorslo, demonstrating that many rearrangement peaks are slightly shifted from normal to lower energy (or apparent mass) values in FI mass spectra. These peak shifts demonstrate a delayed decomposition process. The phenomenon has been discussed in detail for valerophenone as an example I. In the present paper all rearrangement reactions studied so far are reviewed and contrasted to direct bond cleavages. Although reactions with low frequency factors show larger peak shifts than those with high frequency factors, in accordance with the rule’, it is not possible to differentiate between direct bond cleavage and rearrangement reactions by the R and Q factors as defined in Section 6. However, if the frequency factor of a reaction is small, the R and Q factors (if precisely defmed) are small too as stated by the rule.
2.
EXPJZRIMENTAL
The experiments were carried out using a Varian CH4-B single focusing mass spectrometer equipped with a combined electron impact &)-field ionization (rr) source (type EFO 4B). This commercial source was replaced, when necessary, by a non-focusing source as described previously’.
3.
HYDROGEN
REARRANGEMENTS
Principally compounds displaying hydrogen rearrangements of the McLafferty type and related reactions (i.e. hydrogen migration via a six- (or five-) membered transition state) have been investigated. They are listed in Table 1. Due to the similarity of the mechanisms similar results are expected for all these processes_ (It should be emphasized that many other types of hydrogen rearrangements are known in EI mass spectrometry, e.g. those termed “hydrogen abstractions” by Damico et al.“). The kinetic behaviour of these hydrogen rearrangements is quite different at room temperature from that at higher temperatures’l. At hi& emitter temperatures (500-700 “C) all hydrogen rearrangements (except some H20 eliminations of low intensity and the elimination of CHzO from acctyleneclicarboxylic acid dimethyl ester) display intense peak maxima near, but not exactly at, the normal mass position. These peak shifts point to a &htly delayed decompbsition process. The peak maximum corresponds to decomposition times between 4 x lo-l2 and 4 x 10mll sec. These shifted m&ma (fast metastable peaks) demonstrate that none of these processes is a pure surface reaction
TABLE 1 HYDROGEN REARRANGEMENTS (AT LOW E M I t tlr..R TEMPERATURE)
Compound (tool.weight)
Process
I (~o) FI
I (~o) Decomposition E I (12 e V ) time (see)
Butyrophenone (148)
M + - C 2 H . (120) (McLafferty K.) M+--H20 (130)
0.3 (150 °) <0.05 (150 °) 2.9 (150 °) 3.1 (150 °) 0.3
49 (250°) 24 (250 °) 100 (150 °) 100 (150 °)
Valeropheaone (162) Caprophenone (176) Heptyl phenyl ketone (204) n-Tridecyl phenyl ketone (288) Menthone (154) Benzoic acid propyl ester (164) Benzoic acid butyl ester (178)
M+ --C3FI6 (120) (McLafferty R.) M + - - C a H a (120) (McLafferty R.) M + - - C 6 H t 2 (120) (McLafferty R.) M+--C12H2,, (120) (McLafferty R.) M + - C 3 H 6 (112) (McLafferty R.) C 6 H s - C O O H 2 + (123) (double H-R.) CsHs " COOH2 + (123) (Double I-I-R.) M + - - O H (161)
Butyric acid butyl ester (144)
C H a C H = C H , ÷ (42) (McLafferty R.) C3H7 " COOH2 + (89) (Double H-R.) M + - - C 2 H ¢ (102) (McLafferty R.) C 2 H s - C H - C H 2 + (56) (McLafferty R.) - "
C3H7
Butyric acid arnyl ester (158)
Acetic acid phenylethyl ester (164) Salicylic acid methyl ester (152) 2-Hydroxybenzyl alcohol (124) Acetylenedicarboxylic acid dimethyl ester (142) p-Cresol 0 0 8 ) Diphenyl sulfoxide (202)
" COOH2
+ (89)
(Double H-R.) M + - - C 2 H , (116) (McLafferry R.) CaH.r-.CHfCH2 + (70) (McI,afferty R.) C3Hv" C O O H 2 + (89) (Double H-R.) M + - - C 2 H , (130) (McLafferty R.) C s H s " C H - C H 2 + (104) (McLafferty R.) M + - - C H a O H (120) (ortho effect) M + - - H 2 0 (106) (ortho effect) M + - - C H 2 0 (112) M + - - H 2 0 (90) M + - - H ~ O (184)
delayed slow
1.30× 10 -11
delayed
1.39 x 10-11
delayed
1.31x10-11
delayed
1.49 × 10- t t
delayed
(<80 °)
0.2 (<80 °)
<0.03 (25 ° )
<0.05 (25 °) <0.05 (25 °) <0.01 ( 9 0 °)
Butyric acid propyl ester (130)
1.40 × 10 - t t
100 (IO0°) 1.3 (100 °) <0.1 (100 °) 100 (9O°) 0.8 (90 °)
<0.1 (90 °)
38 (130 ° ) 2.5 <0.1
100 (150 °) 100 (15o °) I00 (12o °) ioo (120 °) 0.9 (120 ~) g.0 (I I0 °) I00 (110 °)
8.5 (110 °) 77 0000 I00 (100 °)
slow slow slow slow
< 4 × I0 -Z2
delayed (2)
(1.20 × 10- t a) delayed slow < 4 × 10 -x2
delayed (2)
(1.64× 10 - t l ) delayed slow
4.7 (100 °) 100
3.9 × 10- x2
delayed
(150 ° ) 74
2 . 1 4 × 10 -11
delayed slow
2.7
100 (150 °) 0.03 (50 °) 0.06 (100 °) <0.1
100
( 5 o °)
( 1 5 o °)
<0.01 (I 50 °) <0.01
1.0 (160 °) 0.6 (130 °)
1.04 × 10- t x
delayed
( 1 5 o °)
46 (140 °) 65 (1 lO) ° lOO
(-~1.3 × 10 -11) delayed (2.36x 10 T M ) delayed s!ow slow SIOW
(although a preconfiguration at the emitter surface cannot be excluded’). This is surprising in view of the fact that surface processes are commonly very abundant in an Fi source I2 . An analysis of the peak shape illustrates further that these rearrangements are pure statistical processes even in an FI sc~urce~. They may hence be compared with processes under low energy (12 eV) electron impact conditions. The uniform behaviour of hydrogen rearrangements at high emitter temperatures can be explained by two facts. 1. Most of the rearrangements, listed in Table 1, have a similar mechanism of formation (via a cyclic transition state). 2. The majority of these fragments have uniformly low activation energies (below 1.0 eV). At IOWemitter temperatmes (-c 150 “C) the kinetic differences between these hydrogen rearrangements are more pronounced In the FI spectra of benzoic acid butyl ester and propyl ester, menthone, acetylenedicarboxylic acid dimethyl ester, as well as some compounds displaying elimination of OH and OH,, the rearranged ions can mostly be observed, but as normal me astable ion. The presence of only minor decay in the region of the fast me&stable ion indicates that shorter decomposition times are sterically possible but not energetically favourable. These reactions are classified as “slow” processes in Table ! . Hydrogen migrations in other ions (aromatic ketones, aliphatic esters and- withlow intensity - 2-hydroxybenzjl alcohol and salicyclic acid methyl ester) iead to shifted peak maxima (fz_st metastable peaks) even at room temperature. These are classified as Welayed’. The results at low temperatures are summarized in Table 1. If there is a noticeable shifted peak maximum at this temperature, the correspo?Eng deconiposition time is aoted. The absolute precision of this value is limited to about This is due to uncertainty in the + 50 %, as demonstrated for valerophenone’. actual emitter geometry and to the unknown surface structure. (The values relative to each other are much more exact if the same emitter is always used. Therefore decomposition times not obtained with the same emitter have been shown in brackets. All other values can.be compared directly.) It is of interest that comparable decomposition times have been calculated for most shifted peak maxima although the amount of the peak shift may vary considerably (0.1-0.9 m-u. for wire emitters). Only the olefin formation from aliphatic esters reaches a maximum decay within a much shorter time interval (4 x lo-l2 set). These values are in exceilent correspondence with those published -by Schulze and Richter4 as far as the same classes of substances are compared, although these authors applied a different technique and used another type of field ion emitter (blade). According to these authors the rearranged olefin fragment in rz-butyl acetate (rn[e 56) reaches maximum intensity after 3 x lo-" see, and the double hydrogen migration (m/e 61) after 1.6 x lo-” sec. The corresponding 66
ht. J. Mass Spectrom.
Ion hays.,
9 (1972)
values for n-amyl butyrate obtained by the present authors are 3.9 x 10- l2 set for the olefin fragment and 2.1 x 10-l’ set for the double hydrogen migration. It was shown in a previous paper’ ’ that the rates of hydrogen rearrangement reactions as studied by FI so far could neither be correlated with the degree of sterical complication, nor with the size of a molecule of a homologous series of compounds (see Table 4 in ref. 11). Further, the differences in the kinetic behavionr of hydrogen rearrangements at low temperatures cannot be related to specific chemical properties of the compound nor to differences in the activation energy*. They can hence be attributed only to differences in the energy distribution fuuction. 4.
REARRANGEMENTS
PRIOR
TO iONIZATICN
A rearranged fragment at normal mass position is observed in the spectrum of 2-hydroxybenzyl alcohol (M4 -Hz0 at m/e 106) and salicylic acid methyl ester (MiCH30H at mn/e120) as a very sharp peaklo_ If the resolution of the FI spectrometer is high enough, a shifted peak can be distinguished near this fragment with normal mass position, the tailing of which Cx+snds to the normal metastable peak. The temperature dependence is helpful in explaining the origin of these two peaks. If, on the one hand, the field ion emitter is heated with the ion source at room temperature, the sharp peak at normal mass position is absent and only the shifted peak is observed. If, on the other hand, the ion source is heated, both peaks are observed simultaneously, and the intensity &f the sharp peak is strongly enhanced with rising temperature. One may conclude from this result that the sharp peak at normal mass position is formed by a thermal rearrangement prior to ionization at the hot walls of the ion source and not by a reaction at the emitter surface as assumed earlier*?_ The. shifted peak originates -._ from a statistical decay. There are several possible explanations of the different effects of emitter and ion source heating on the formation of these rearranged ions. 1. The adsorption time at the hot emitter surface is too short for a thermal rearrangement. 2. The thermal rearrangement at the emitter surface is suppressed by the high electric field, forcing the molecule in an unfavourable configuration for a rearrangement. 3. The catalytic properties of the hot stainless steel walls of the ion source are different from those of the&nitter surface coated with organic microneedles. ?i _. 5.
SURFACE
PROCESSES
As stated above, hydrogen rearrangements bf the McLaflerty type and related processes .do not occur at the emitter surface in general, Hoivever, the -*SeeTable5inref,l.
ht. L Mass Spectrom. Ionrphus.;9
(1972)
67
possibility cannot be excluded that surface reactions might occti with other types of rearrangement. Thus some of the rearrangements reported_in the literature (refs. 2, 3, 5-7) may be attributed to such surface mechanisms. This can only be clar&d by careful peak shape analysis. Chait and Kitson for instance reported arearrangementpeak at normal massposition in the spectrumof octanone (m/e58). However, a remeasurementby the present authors revealed a shifted peak maximum aiso for this case*. A real surface rear*-angementwas observed in the spectrum of n-hexyl bromide at mfe I34(M+ -HBr) as an entirely sharp peak (Fig. 1) It is of interest t&t S&s fragment is not observed in the EI spectrum indicating the non-statistical OtigiIL
Surface rearrangements cannot be evaluated kinetically**.
Mf
Br
.,a
85
1..
1 .‘
80
Fig. 1. Surface rearrangement in n-hexyl bromide: part of FI spectrum. The direct bond cleavage at m/e 85 (M* -Br) displays a long peak tailing towards lower masses. An entirely sharp and symmetric rearrangement peak at m/e 84 CM+-HBr) is superimposed upon this peak taiiing and indicates a surface process.
6. SKELZTAL REARRANGEMENTS
Under the energy conditions realized in an FI source, skeletalrearrangements are observed mostly as normal metastable peaks of low abundance-or not at allThey can hence be classiikd as “slow” reactions under FI and low energy EI condi* Thisshift is observed l
both with wire and blade emitters..
* Kinetic studies of hydrogen re&rangemenis tie further c&mplicated by the fact that some
partiallydecomposeif stored for several mo&hs. Bu&ophenone &d valerophenone decomposition products having the same mass as the rearranged ion (m/e 120) after storage = at roomSemp&ature. A purification is-recommended .before_‘&achmeasurement. compounds
f6&
,.
,’
i
o\ ,w
,
_----
(2610”) 11 (17OO)
$OQ)
2240°)
(Y50”) 22 (140”)
(F50”)
c;J20°,
(1840”) 51 (160’)
(&)
(l!&
::35e)
I(%) El (12 eV) ------_
I(%) Fl
0.005 (350”)
(61)
MS-HS
Dimethyl disuhidc (94)
,$
h
M+-CO (142)
(80)
Diphenyl ether (170)
M+-CO
W
G
,‘%
PCresol (108)
(154)
M+-SO
.“y‘04
F
(174)
M+-CO
& .’ Diphenyl sulfoxidc (202) ,zQ
-__-_..--e--
Process
fikf%BTAL REARRANGEhKNTS
‘SABLE 2
Conrpourtd(ntol. weight)
p ” s, --
0.02
I.8
9
16
11
30
<0.15 co.6
60
10 1.4
0.1
8
3,7
0.10
0,lO
0,5
0,5
_I
0.04
0.35
68
<0.5
I.8
0.08
0.5
0.3
0,14
0.11
11
41
55
15
27
8
co.5
1
co.2
cl.3
<0.2
co.2
,0.002
0.11,
<0,05
‘: <0,005
0.02
0.01
<0,007
<0,05
0.02
<0.004
<0.09
co.007
<0.02
~lawn. (%) ~/&lO,nl.(%) I(%) R FI El (12 eV) FI’MetasIable . . .I__ ,--
--.-_I.
_--
<0.005
1 ok
< 0,04
0,24
-
0.03
<0*03
‘.
0,003”
0.06
<0,03
<$I,07
co.1
Q.
,,’
tions. In Table 2 all the skeletal rearrangement studies so far are listed. Beside the relative intensity of the rearranged peak in the FI and 12 eV EI spectra and the reltitive intensity of the normal metastabIe peak in the FI spectra, corresponding R ‘and Q, factors are given (following the practice in previous papers”). They are defined as:
I, is the intensity at exact normal mass position. If the tailing of a shifted peak extends to the exact normal mass position, its contri’oution is disregarded Small R and Qr factors point to slow reactions (small rate constants). Fig. 2 represents the FI and EI spectra of benzzi alcohol’ 3. Here the elimination of HCO is observed only as metastable ion in the FI spectrum. In this case an enhanced zero Iine of the recorder plot indicates that shorter decomposition times are sterically possible, in principle, but energetically unfavourable. This demonstrates that the low reaction rate is mainly due to the high activation energy of these processes, which is substantiated by appearance potential measurementsl. A small frequency factor can further contribute to the slow reaction rate, especially if the rearrangement is sterically complicated. The discussion of the rate constant k as a function of the iuremal energy, as reported previously*, clarifies the strong influence of the activati .EI energy on the rate constant.
7. FORMATION OF TROPYLIUM IONS Ring expansions are special cases of skeletal rearrangements. Among these ring expansions the formation of the tropylium ion and ions of the homologues is of particular importance in EI mass spectrometry14. Ifit is assumed that the tropylium ion is formed by a two step process, two different reaction paths are possible:
0/ 1 -
cl+-x
.:
(0) -e-x
0/ \
I=-
-
Cm/e=91)
cii;
+ Cl,--, .;+I
._I
(m/.=91)
On the one hand-(a), the rearrangement is assumed to start with the ehmination of the subst&eirt -X and is’foUoW& by the ring expansion. If in this case the eliminatiorz of the substituent occurs with a.h.ighreactjon rate,.neither.the intensity ratio of t&e normal ftigment peak to -the normal meta&aWc peak (the Qi factor) 70
ht. 3. Mass Spectrcw. hn Phys., .9. :(1972)
-_..
loo
t
10
.
E 7
5 a
1
0.01 40
50
60
70
80
100
90
110
-m/e
loo f
80
.
E -
1108)
Isi
El-12eV (250°C)
60
40 d-H-CO (79)
I
I! 40
50
60
70
8Q;
c
C7J47+ 1911
II
II 90
I
100
k
,
110
-m/e
-179) M+-H-CO llO8) hi'
t
100 80
& I= 5 tE
6(J
E-L-?7Ck!J (25Q°C 3
40 w+3+ 151)
20
! IL. 40
50
C7'47+ 191) c5w5+ 165) +I
.
I,,I I
~60
70
80
II I
90
II I
100
,
110
CHS_OH Fig. 2. FI, 13 eV and. $0 eV 61 spectra of benzyl alcohol. (Note the logarithmic intensity scale of the FI spectrum. Metastable ions are plotted as dashed lines at their apparent-n=.)
HO61
loo
M+
F_L(3OO*C)
If
1 :
m5-91; I : : : : !
d+
1 t911*
0.1’
. 40
I
.
50
60
7ci
60
90
I
m,
I
.
110 b
loo
I1061
120
*
m/e
d
12eV (300°C)
t
El-
-80 E A d6a 40 (911 1C,H7+
20 178)
4p
50
60
70
80
I. .
so
100
.I_ 110
# 120
’
I+
E-1. 12eV
!250°C)
50
60
70
80
90
no
100
120
130
loo-
(105)
140
m/e
&*-C2H5
t
EL-
--. 5
70eV
(250°Cl
a SO.LO20.
cin3+ 1511
I
1‘1 m
50
CpnH;' (65) I 60-m
c7eP 1911
113i) I
I 80
M+
90
100
* 110
120 130 _-m/e
140 _
..
w5 \ .c’w .\ 0 CH3 Fig. 4. Fl,12 eV tid 70 eV EIspectraof sec.-butylbekene. (FI spectrti with logarithmk i&n&y ‘. scale. Metastabli ions a& pldtte$. as dashed lines at th&kpparent II&S_)
nor the peak shape reflects the kinetics of the second step (the actual rearrangement) as the fragment mass does not change during this second step. On the other hand in (b), the ring expansion is assumed to occur as the tist step followed by the elimination of the substituent. In this case the peak shape reflects the decomposition time of the overall reaction. In EI mass spectrometry the second mechanism is assumed to be correct for most compounds l5 _ The a-C-atom is inserted between the ring C-atoms andsimultaneously one c&ydrosen atom migrates to the Cl-atom, followed by the elimination of the substituent. Moreover, randomization of the hydrogen atoms is observed with most compounds. Fig. 3 represents the FI and EI spectra of ethylbenzene (with the tropylium ion at m/e 91), Fig. 4 those of sec.-but$lbenzene, where a methyltropylium ion is observed (m/e 105). In both cases the rearranged ions are observed with relatively high intersity even in FI mass spectra. Moreover, the fragment peak is located at the normal mass position (in contrast to the shifted hydrogen rearrangement peaks). Similar results are observed with all the other compounds (summarized in Table 3) except with m-xylene and tetralin. Ring expansions are skeletal rearrangements and therefore have relatively high activation energies (the activation energy for the formation of C,H,+ from ethylbenzene is 2.9 eY and from m-xylene 3.1 eYf6). Thus the reaction rate shouId be low. Assuming the second mechanism applies also to FI conditions, only a metastable ion and no normal fragment should be expected with all tropylium ions. The intense no_rmal fragment ions demonstrate, however, that the first reaction path (a) is correct for FI in contrast to EL The ring expansion occurs after the elimjnation of the substituent. Such differences in the mechanism between R and EI conditions must be caused by either the high electric field or the adsorption at the emitter surface_ Variation of the field strength (which can easily be realized using a tip emitter) demonstrates that alkylbenzenes undergo field dissociaticn as a first step of the tropylium ion formation: An increase of the tip potential from 5 kY to 6.5 kY raises the intensity of the methyl tropylium ion at .nz/e105 in the FI spectrum of sec.-butylbenzene from 2.1 % to 18 %, which clearly indicates fieldinduced dissociation. In addition to this field dissociation a statistical decay leads to a normal metastable ion and intense decay within the region of the fast metastable peak. With m-xylene and tetralin only this statistical decay is observed. Field dissociation is obviously not possible with these compounds.
8. DIRECT BOND CLEAVAGES The rate constants as functions of the internal energy (k(E) curves) have been discussed iti detaii previously I. These considerations demonstrate that rearrange74
ht.
J. Mass Spectrom;
Ion Pi13%, 9 (1972)
loeiglll)
wXylcnc (106)
Dibcnzyl sulfoxide (230)
Bcnzylalcohol (108)
/I-Cresol (108)
(91)
(Tropyliumion)
C7H,+
(Tropylium ion)
($1)
C7H7+
6.6
Bcnzyl chloride (126)
(105)
(hi{
0.008 (250°)
<0.005
H
I (%)
Benzyl propyl ketone (162)
M+-C2Hd
M” -CHJ (91) (Tropyliumion) C7H7+ (91) (Tropyliumion) M’-CNJ (I 19) (subst, Tropylium)
M*-CHB (91) (Tropylium ion)
..--.--___-__-.__
Process
(subst. Tropylium) (250’) Me-H (107) (Hydroxytropylium) (1:G) C7H,+ (91) (Tropylium ion) M+-OH (91) (Tropyliumiou)
scc.~Butylbcnzcnc (I 34)
w.Butylbcnzcnc
(134)
_I__ Ethylbenzene (106)
ContporcNd (mol.
IONS
---__..-..L~_-_--__I_____
FORMATIONOF ‘~ROPYLIIJM
$.j Tctrulin (132)
kJ
Ci
W
* ”
a
36
‘?
8 2’
8 _--
84
93
28
30
63
26
26
0.3
79
99
24
0,5
.62
93
98
12
86
0.2
0.4
2.4
0.94
0,l.l
1.5
1.06
0.94
* 1,17
‘60
I,0
0.28
0.26
-
0,003
4.0
Q
>>l
5s
>350
I*3
2,8
2‘0
0,07
>>I
R I (%I WLlgnr, (%) mh7,m (%) 1 (%I Ef (12 cl’) Fl E1 (12 e V) FI Mcrasrnble ~-,-~I_--~~._~ ----w-m_ 33 92 0.63 0,3G
ment reactions may have larger average rate constants-thandirect bond cleavages at low excitation energies (12 eV El or FI) if field dissociations are excluded. At higher excitation energies (70 eV n), however, the rate constants of direct bond tisions dominate. This is due to the fact that a’.low excitation energiesthe reaction rate is mainly determined by the activation energy, and at high excitation energies by the frequency factors (i.e. by the maximum rate constant which is sterically possible). Whether in a specific case the direct bond rupture or the rearrangement has a higher average rate constant depends on the crossover of the k(E) curves of thesetwo fragmentation mechanisms(assumingthat only two.competing decomposition processesare observed). If, on the one hand, this crossover occurs within or below the regime of rate constants, contributing preferentially to the normal metastable peak, the rate constant of the direct bond fission will be higher than that of the rearrangement.If, on the other hand, the crossover is situated above the metastablerange, the rearrangementwill dominate at low excitation energies17. According to these theoretical considerations direct bond cleavages should often be observed, but as normal metastable ions in FI mass spectra, assuming the decay occurs statistically. However, in contrast to rearrangement reactions direct bond cleavages may occur as a field-induced or a surface process, especially with aliphatic compounds. In thesecasesthey are observed as sharp peaks at the exact normal mass position* and are accompanied by no, or only a very weak, normal metastable ion peak. Hence it is obvious that most of the direct bond cleavages cannot be interpreted kinetically. Some exceptions will be discussed.
9. ELIMNATION OF ALKYL RADICALS
Pure statistical decomposition processes are observed with those fragments where neutral alkyl radicals (preferentially methyl radicals) are eliminated. These fragmentation processeshave a uniformly low reaction rate at room temperature and are mostly observed only as normal metastableions. In Table 4 the intensity of the normal fragment ion and the normal metastable ion, as well as the above defined Q factors, are listed. Peak shifts are indicated. Similar resultsare obtained with n-para5s studied by Wanless and Glock” and by Beckey et al.‘. The elim%nationof alkyl radicals (M+- CnH2n+l)+ is predominantly observed as the normal metastable ion with these compounds a!so. The low reaction rate can be explained by the relativeIy high activation energy of these processes.It is possible that a low frequency factor contributes further to the slow reaction rate. * Field-inducedfragments may display a small peak tailing if a tip emitteris used, corresponding t6 a decay in the range of 10-12-10-s1 sec.
76
Int. J. _%-~LG Spixtroni Ion Phys.; 9 (1972)
:
TABLE 4 ELIMINATION
oF.EiEuTRAL
(M+-CHJ.
M+--CzHs
Compound
ALKYL
etc.)
Prccess
Bromohexane (164)
RADICALS
M+ -C2HS (135)
Temp. i”C)
I(%) FI
I(%) FI’ 18.0 0.2
Bond
a
< 50’ 300°
0.5 0.5
M+--CH3 methyl (131) sulfide (146)
< 50” 250°
? ?
0.38 1.9
to.4 to.4
S-CHB
Menthone (154)
M’--C’H3 (39)
t50' 300”
to.01 to.01
0.5 1.1
to.02 to.009
\-C-CHs /
Dimethyl disulfide (94)
M’-CHs (79)
t50” 3O!Y
n-Heptyl
N,N-Dimethyl- M + - CH 3 N’-phenyl(133) formamidine (148) p-Anisidine (1231
M+-CHs (108)
Acetylacetone (loo)
M+---CHs (85)
n = peak shifts.
10.
PEAR
SHIFTS WlTH
(50’ 300”
150” t50” >700°
y
+-&H.
0.11 0.07
0.05 I
0.003 to.Oc2
0.25 1.78
0.012 to.001
0.097
0.36
0.02
-O-CH3
0.005
0.80
0.006 0.38
-&-CH3
13.2
A
-S-CHB
0.006 0.07
5.5
&.
\N-CH /
3
0 a
* = normal metastable peak.
DIRECT
BOND
CLEAVAGES
Shifted peak maxima pointing to a statistical decay are not only observed with rearrangements but in some cases also with direct bond cleavages. The experimental results are summarized in Table 5. The amount of the peak shift is smaller than with rearrangements. With valerophenone a direct bond cleavage at ‘nz/e 105 (C&CO+) can be observed simultaneously as field induced fragmentation (sharp peak at exact normal mass position) and as statistical decay (shifted peak maximum) if very high field strengths are applied by using a tip emitter. Thus field dissociations and statistical decompositions may compete in some cases.
11. CONCLUS!ONS
At low excitation energies, realized in an FI or in a l&w-energy (12 eV) JZI source, the rate constants of statistical -fragmentation processes reflect more the
differences between the activation energies than the differences between the freIJU.J_ Mass Speetrbm. Ion Phys-,
9. il972)
77
TABLE
5
PEAK SHWI-S
WITH DIRECT
Compound
Valerophenone
BOND
CLEAVAGES*
Fragment
C6H5COC
(105)
Potential
Am
&VI
(m-u_)
+5/-5
Acetic
C6HSCH2- CH1+!105)
j-5/--2.5
Butyric acid amyl ester
M+ -C2HS
(129)
+5/-s
Butyric acid
ti+--CnH,
(101)
+--71-3
acid phenylethyl ester
$;&0.05 ._
butyl ester Acetylacetone
M+-CH3
* 2.5 pm platinum wire,
(85)
+5/-5
0.09+o.a5
activatedwith henzonitrile.
quency factors. Therefore, skeletal rearrangements are mostly observed with lower reaction rates than hydrogen migration reactions under low energy conditions. In general, hydrogen rearrangements reach a maximum intensity after several 10S1l set and are thus relatively fast decomposition processes, under FI conditions.They are, however, considerably delayed as compared with the fast-field dissociations
observed with direct bond cleavages in many cases (occuring within about lo-l3 set). As far as these direct bond cleavages can be interpreted kinetically (i.e. if surface reactions are excluded), they differ greatly in their kinetic behaviour under FI conditions. Besides the fast field dissociations mentioned above, extremely slow, direct bond fissions are observed with ions eliminating alkyl radicals_ As neither rearrangement reactions nor direct bond ruptures show a uniform kinetic behaviour, an unambiguous differentiation between these two decomposition mechanisms by analysis of peak shape, shift and intensity is difficult.
REFERENCES 1 XC.LAFXN AND H. D. BECKEY, ht. J. Mass Spectrom. Ion Phys_, 7 (1971) 341. 2 E. M. CHAIT AND F. G. KI'ISON,Org. Mass Spectrom., 3 (1970) 533. 3 J. N. DAMICQR. P. BARRON AND J. A. SPHON, ht. J. Mass Spectrom. Ion Phy.s_, 2 (1949) 141. 4 P. SCHULZE AND W. 3. RICHTER, ht. Mass Spectrom. Ion Phys., 6 (1971) 131. 5 H. K&PPEL, Int. J. Mass Spectrom. Ion Phys., 4 (1970) 97. 6 P. BRO~,G.R.FEXTITAND R.K.Rosm, Org_MassSpectrom.,2 (!.969)521. 7 P. BROWN AND G. Rl PET-I-IT,Oig. Mass Spectrom., 3 (197a) 67. 8 H. D. BECKEY, Int. J. Mass Spectrom. Ion Phys., 1 (1968) 93; 5 (1970) 182. 9 H. D. BECKEY, H. HEY, K. LEVSFN AND G. TENSCHERT,ht. J. Muss Spectrom. Ion Phjs., 2 (1969) 101. 10 p. D.:BECKEY AND K. LEVSE&,ix-R. L REED (Editor), Recent Topics in Mass Spectrometry, Gordon and Breach, New York, 1971, p_ 169. Int. J_ dfasS Spectr.Jmi Ion Phys., 9 (1972)
-.
:
11 12 13 14 I5 16
17 18
KI LEV~ENAND H. D. BECKEY,Int. J. Mass Spectrum. font Phys., 9 (1972) 51.. F. W. R&LGEN AND H. D. BECKEY,Surface Sci., 23 (1970) 69. J. D AKMEN, Dissertation, Bonn, 1968. H. M. GRUBB AND S. MEY~PSON, in F. W. MCLAFFERYY (Editor), M& speckokeriy of Organic Ions, Academic Press,New York, 1963, p. 516. S. MEYERXEJ, H. HART AXD L. C. LEITH, J. Amer. Chem. Sot., 90 (1968) 3419. J. L. FRANKLIN,.J. G. DILLARD, H. M. ROSEN~TOC~,J. T. HERRON,K. DRAXL arm F. fi. FIETJ), IokTz@on Potenfials, Appearance Potentials, and Heats of Formation of Gaseous .Positire Ions, National Bureau of Standards, Washington, 1969. P. BROWN, Org. Mass Spectrom., 3 (1970) 1175. G. G. WAXAND G. A. Gmcx, JR., Ad. Chem., 39 (1967) 2.
Int. J..Mass.Spectrom.
Ion Phys., 9 (i972)
79
Else&r
Johal
of Mass
Spectrometry
and Ion Physics
Publishing Company, Amsterdam. Printed in the Netherlands
TIJE STUDY OF REARRANGEMENT TION MASS SPECTROMETRY III. .EXPERIMENTAL REACTIONS*
RESULTS
FOR
REACTIONS-
DIFFERENT
’
...
BY FIELD
TYPES
IONIZA‘.
OF
K. LEVSEN AND I-l. D. BECKEY Institut fiir Physikalische
Chemie
der Unicersit&
Bonr, (W_
Germany)
(Received July 2&h, 1971)
ABSTRACT
The kinetic behaviour of rearrangement reactions and direct bond cleavages as derived qualitatively from FI mass spectra is summarized. At low excitation energies (at 12 eY EI, or under R conditions) skeletal rearrangements are slow, and hydrogen migrations relatively fast, decomposition processes (decomposition times corresponding to the peak maxima: several times 10s6 set, and several times lo-l1 set, respectively). Among direct bond fissions alkyl radical eliminations are especially slow processes. An unambiguous differentiation between rearrangements and direct bond cleavages from the kinetic data is difficult. The kinetic study of the tropylium ion formation demonstrates that this rearrangement starts with a field-induced benzyl ion formation in an FI source (in contrast to ~1 conditions).
1. INTRODUCTiON
mass spectrometry has been used extensively for the study of rearrangement reactions of organic ions in the gas phasel-“. The capability of resolving extremely short decomposition times using this ionization mode Lamed to be promising for detecting kinetic differences in the fcrmation of direct and iearranged fragment ions. These studies revealed that most of the skeletal rearrangements cannot bedetected as normal fragment ~eaks~~~*~* but often a&normal metastable ions** indicating slow decomposition processes. Hydrogen rearrangements, how‘ever, were detected even in FI mass spectra as abundant normal fragment ions (like field-induced direct bond fissions). FI
* For parts I and II of this series, see refs. I and II. ** “Normal”
fragment io&, defined previously’.
hi
1. Mass
Spectiom:
“fast metastable” ions and %or&al”
met&able
-.. ionS have been
.. -63
Ion Plzys., 9 (1972)
. .
:.
These results seemed to be inconsistent with a rule, reported by Beckey’, stating that field-induced direct bond fissions should he faster than rearrangement reactions in an R source. These apparent contradictions were resolved by experiments of Schulz and Richter4 and by the present authorslo, demonstrating that many rearrangement peaks are slightly shifted from normal to lower energy (or apparent mass) values in FI mass spectra. These peak shifts demonstrate a delayed decomposition process. The phenomenon has been discussed in detail for valerophenone as an example I. In the present paper all rearrangement reactions studied so far are reviewed and contrasted to direct bond cleavages. Although reactions with low frequency factors show larger peak shifts than those with high frequency factors, in accordance with the rule’, it is not possible to differentiate between direct bond cleavage and rearrangement reactions by the R and Q factors as defined in Section 6. However, if the frequency factor of a reaction is small, the R and Q factors (if precisely defmed) are small too as stated by the rule.
2.
EXPJZRIMENTAL
The experiments were carried out using a Varian CH4-B single focusing mass spectrometer equipped with a combined electron impact &)-field ionization (rr) source (type EFO 4B). This commercial source was replaced, when necessary, by a non-focusing source as described previously’.
3.
HYDROGEN
REARRANGEMENTS
Principally compounds displaying hydrogen rearrangements of the McLafferty type and related reactions (i.e. hydrogen migration via a six- (or five-) membered transition state) have been investigated. They are listed in Table 1. Due to the similarity of the mechanisms similar results are expected for all these processes_ (It should be emphasized that many other types of hydrogen rearrangements are known in EI mass spectrometry, e.g. those termed “hydrogen abstractions” by Damico et al.“). The kinetic behaviour of these hydrogen rearrangements is quite different at room temperature from that at higher temperatures’l. At hi& emitter temperatures (500-700 “C) all hydrogen rearrangements (except some H20 eliminations of low intensity and the elimination of CHzO from acctyleneclicarboxylic acid dimethyl ester) display intense peak maxima near, but not exactly at, the normal mass position. These peak shifts point to a &htly delayed decompbsition process. The peak maximum corresponds to decomposition times between 4 x lo-l2 and 4 x 10mll sec. These shifted m&ma (fast metastable peaks) demonstrate that none of these processes is a pure surface reaction
TABLE 1 HYDROGEN REARRANGEMENTS (AT LOW E M I t tlr..R TEMPERATURE)
Compound (tool.weight)
Process
I (~o) FI
I (~o) Decomposition E I (12 e V ) time (see)
Butyrophenone (148)
M + - C 2 H . (120) (McLafferty K.) M+--H20 (130)
0.3 (150 °) <0.05 (150 °) 2.9 (150 °) 3.1 (150 °) 0.3
49 (250°) 24 (250 °) 100 (150 °) 100 (150 °)
Valeropheaone (162) Caprophenone (176) Heptyl phenyl ketone (204) n-Tridecyl phenyl ketone (288) Menthone (154) Benzoic acid propyl ester (164) Benzoic acid butyl ester (178)
M+ --C3FI6 (120) (McLafferty R.) M + - - C a H a (120) (McLafferty R.) M + - - C 6 H t 2 (120) (McLafferty R.) M+--C12H2,, (120) (McLafferty R.) M + - C 3 H 6 (112) (McLafferty R.) C 6 H s - C O O H 2 + (123) (double H-R.) CsHs " COOH2 + (123) (Double I-I-R.) M + - - O H (161)
Butyric acid butyl ester (144)
C H a C H = C H , ÷ (42) (McLafferty R.) C3H7 " COOH2 + (89) (Double H-R.) M + - - C 2 H ¢ (102) (McLafferty R.) C 2 H s - C H - C H 2 + (56) (McLafferty R.) - "
C3H7
Butyric acid arnyl ester (158)
Acetic acid phenylethyl ester (164) Salicylic acid methyl ester (152) 2-Hydroxybenzyl alcohol (124) Acetylenedicarboxylic acid dimethyl ester (142) p-Cresol 0 0 8 ) Diphenyl sulfoxide (202)
" COOH2
+ (89)
(Double H-R.) M + - - C 2 H , (116) (McLafferry R.) CaH.r-.CHfCH2 + (70) (McI,afferty R.) C3Hv" C O O H 2 + (89) (Double H-R.) M + - - C 2 H , (130) (McLafferty R.) C s H s " C H - C H 2 + (104) (McLafferty R.) M + - - C H a O H (120) (ortho effect) M + - - H 2 0 (106) (ortho effect) M + - - C H 2 0 (112) M + - - H 2 0 (90) M + - - H ~ O (184)
delayed slow
1.30× 10 -11
delayed
1.39 x 10-11
delayed
1.31x10-11
delayed
1.49 × 10- t t
delayed
(<80 °)
0.2 (<80 °)
<0.03 (25 ° )
<0.05 (25 °) <0.05 (25 °) <0.01 ( 9 0 °)
Butyric acid propyl ester (130)
1.40 × 10 - t t
100 (IO0°) 1.3 (100 °) <0.1 (100 °) 100 (9O°) 0.8 (90 °)
<0.1 (90 °)
38 (130 ° ) 2.5 <0.1
100 (150 °) 100 (15o °) I00 (12o °) ioo (120 °) 0.9 (120 ~) g.0 (I I0 °) I00 (110 °)
8.5 (110 °) 77 0000 I00 (100 °)
slow slow slow slow
< 4 × I0 -Z2
delayed (2)
(1.20 × 10- t a) delayed slow < 4 × 10 -x2
delayed (2)
(1.64× 10 - t l ) delayed slow
4.7 (100 °) 100
3.9 × 10- x2
delayed
(150 ° ) 74
2 . 1 4 × 10 -11
delayed slow
2.7
100 (150 °) 0.03 (50 °) 0.06 (100 °) <0.1
100
( 5 o °)
( 1 5 o °)
<0.01 (I 50 °) <0.01
1.0 (160 °) 0.6 (130 °)
1.04 × 10- t x
delayed
( 1 5 o °)
46 (140 °) 65 (1 lO) ° lOO
(-~1.3 × 10 -11) delayed (2.36x 10 T M ) delayed s!ow slow SIOW
(although a preconfiguration at the emitter surface cannot be excluded’). This is surprising in view of the fact that surface processes are commonly very abundant in an Fi source I2 . An analysis of the peak shape illustrates further that these rearrangements are pure statistical processes even in an FI sc~urce~. They may hence be compared with processes under low energy (12 eV) electron impact conditions. The uniform behaviour of hydrogen rearrangements at high emitter temperatures can be explained by two facts. 1. Most of the rearrangements, listed in Table 1, have a similar mechanism of formation (via a cyclic transition state). 2. The majority of these fragments have uniformly low activation energies (below 1.0 eV). At IOWemitter temperatmes (-c 150 “C) the kinetic differences between these hydrogen rearrangements are more pronounced In the FI spectra of benzoic acid butyl ester and propyl ester, menthone, acetylenedicarboxylic acid dimethyl ester, as well as some compounds displaying elimination of OH and OH,, the rearranged ions can mostly be observed, but as normal me astable ion. The presence of only minor decay in the region of the fast me&stable ion indicates that shorter decomposition times are sterically possible but not energetically favourable. These reactions are classified as “slow” processes in Table ! . Hydrogen migrations in other ions (aromatic ketones, aliphatic esters and- withlow intensity - 2-hydroxybenzjl alcohol and salicyclic acid methyl ester) iead to shifted peak maxima (fz_st metastable peaks) even at room temperature. These are classified as Welayed’. The results at low temperatures are summarized in Table 1. If there is a noticeable shifted peak maximum at this temperature, the correspo?Eng deconiposition time is aoted. The absolute precision of this value is limited to about This is due to uncertainty in the + 50 %, as demonstrated for valerophenone’. actual emitter geometry and to the unknown surface structure. (The values relative to each other are much more exact if the same emitter is always used. Therefore decomposition times not obtained with the same emitter have been shown in brackets. All other values can.be compared directly.) It is of interest that comparable decomposition times have been calculated for most shifted peak maxima although the amount of the peak shift may vary considerably (0.1-0.9 m-u. for wire emitters). Only the olefin formation from aliphatic esters reaches a maximum decay within a much shorter time interval (4 x lo-l2 set). These values are in exceilent correspondence with those published -by Schulze and Richter4 as far as the same classes of substances are compared, although these authors applied a different technique and used another type of field ion emitter (blade). According to these authors the rearranged olefin fragment in rz-butyl acetate (rn[e 56) reaches maximum intensity after 3 x lo-" see, and the double hydrogen migration (m/e 61) after 1.6 x lo-” sec. The corresponding 66
ht. J. Mass Spectrom.
Ion hays.,
9 (1972)
values for n-amyl butyrate obtained by the present authors are 3.9 x 10- l2 set for the olefin fragment and 2.1 x 10-l’ set for the double hydrogen migration. It was shown in a previous paper’ ’ that the rates of hydrogen rearrangement reactions as studied by FI so far could neither be correlated with the degree of sterical complication, nor with the size of a molecule of a homologous series of compounds (see Table 4 in ref. 11). Further, the differences in the kinetic behavionr of hydrogen rearrangements at low temperatures cannot be related to specific chemical properties of the compound nor to differences in the activation energy*. They can hence be attributed only to differences in the energy distribution fuuction. 4.
REARRANGEMENTS
PRIOR
TO iONIZATICN
A rearranged fragment at normal mass position is observed in the spectrum of 2-hydroxybenzyl alcohol (M4 -Hz0 at m/e 106) and salicylic acid methyl ester (MiCH30H at mn/e120) as a very sharp peaklo_ If the resolution of the FI spectrometer is high enough, a shifted peak can be distinguished near this fragment with normal mass position, the tailing of which Cx+snds to the normal metastable peak. The temperature dependence is helpful in explaining the origin of these two peaks. If, on the one hand, the field ion emitter is heated with the ion source at room temperature, the sharp peak at normal mass position is absent and only the shifted peak is observed. If, on the other hand, the ion source is heated, both peaks are observed simultaneously, and the intensity &f the sharp peak is strongly enhanced with rising temperature. One may conclude from this result that the sharp peak at normal mass position is formed by a thermal rearrangement prior to ionization at the hot walls of the ion source and not by a reaction at the emitter surface as assumed earlier*?_ The. shifted peak originates -._ from a statistical decay. There are several possible explanations of the different effects of emitter and ion source heating on the formation of these rearranged ions. 1. The adsorption time at the hot emitter surface is too short for a thermal rearrangement. 2. The thermal rearrangement at the emitter surface is suppressed by the high electric field, forcing the molecule in an unfavourable configuration for a rearrangement. 3. The catalytic properties of the hot stainless steel walls of the ion source are different from those of the&nitter surface coated with organic microneedles. ?i _. 5.
SURFACE
PROCESSES
As stated above, hydrogen rearrangements bf the McLaflerty type and related processes .do not occur at the emitter surface in general, Hoivever, the -*SeeTable5inref,l.
ht. L Mass Spectrom. Ionrphus.;9
(1972)
67
possibility cannot be excluded that surface reactions might occti with other types of rearrangement. Thus some of the rearrangements reported_in the literature (refs. 2, 3, 5-7) may be attributed to such surface mechanisms. This can only be clar&d by careful peak shape analysis. Chait and Kitson for instance reported arearrangementpeak at normal massposition in the spectrumof octanone (m/e58). However, a remeasurementby the present authors revealed a shifted peak maximum aiso for this case*. A real surface rear*-angementwas observed in the spectrum of n-hexyl bromide at mfe I34(M+ -HBr) as an entirely sharp peak (Fig. 1) It is of interest t&t S&s fragment is not observed in the EI spectrum indicating the non-statistical OtigiIL
Surface rearrangements cannot be evaluated kinetically**.
Mf
Br
.,a
85
1..
1 .‘
80
Fig. 1. Surface rearrangement in n-hexyl bromide: part of FI spectrum. The direct bond cleavage at m/e 85 (M* -Br) displays a long peak tailing towards lower masses. An entirely sharp and symmetric rearrangement peak at m/e 84 CM+-HBr) is superimposed upon this peak taiiing and indicates a surface process.
6. SKELZTAL REARRANGEMENTS
Under the energy conditions realized in an FI source, skeletalrearrangements are observed mostly as normal metastable peaks of low abundance-or not at allThey can hence be classiikd as “slow” reactions under FI and low energy EI condi* Thisshift is observed l
both with wire and blade emitters..
* Kinetic studies of hydrogen re&rangemenis tie further c&mplicated by the fact that some
partiallydecomposeif stored for several mo&hs. Bu&ophenone &d valerophenone decomposition products having the same mass as the rearranged ion (m/e 120) after storage = at roomSemp&ature. A purification is-recommended .before_‘&achmeasurement. compounds
f6&
,.
,’
i
o\ ,w
,
_----
(2610”) 11 (17OO)
$OQ)
2240°)
(Y50”) 22 (140”)
(F50”)
c;J20°,
(1840”) 51 (160’)
(&)
(l!&
::35e)
I(%) El (12 eV) ------_
I(%) Fl
0.005 (350”)
(61)
MS-HS
Dimethyl disuhidc (94)
,$
h
M+-CO (142)
(80)
Diphenyl ether (170)
M+-CO
W
G
,‘%
PCresol (108)
(154)
M+-SO
.“y‘04
F
(174)
M+-CO
& .’ Diphenyl sulfoxidc (202) ,zQ
-__-_..--e--
Process
fikf%BTAL REARRANGEhKNTS
‘SABLE 2
Conrpourtd(ntol. weight)
p ” s, --
0.02
I.8
9
16
11
30
<0.15 co.6
60
10 1.4
0.1
8
3,7
0.10
0,lO
0,5
0,5
_I
0.04
0.35
68
<0.5
I.8
0.08
0.5
0.3
0,14
0.11
11
41
55
15
27
8
co.5
1
co.2
cl.3
<0.2
co.2
,0.002
0.11,
<0,05
‘: <0,005
0.02
0.01
<0,007
<0,05
0.02
<0.004
<0.09
co.007
<0.02
~lawn. (%) ~/&lO,nl.(%) I(%) R FI El (12 eV) FI’MetasIable . . .I__ ,--
--.-_I.
_--
<0.005
1 ok
< 0,04
0,24
-
0.03
<0*03
‘.
0,003”
0.06
<0,03
<$I,07
co.1
Q.
,,’
tions. In Table 2 all the skeletal rearrangement studies so far are listed. Beside the relative intensity of the rearranged peak in the FI and 12 eV EI spectra and the reltitive intensity of the normal metastabIe peak in the FI spectra, corresponding R ‘and Q, factors are given (following the practice in previous papers”). They are defined as:
I, is the intensity at exact normal mass position. If the tailing of a shifted peak extends to the exact normal mass position, its contri’oution is disregarded Small R and Qr factors point to slow reactions (small rate constants). Fig. 2 represents the FI and EI spectra of benzzi alcohol’ 3. Here the elimination of HCO is observed only as metastable ion in the FI spectrum. In this case an enhanced zero Iine of the recorder plot indicates that shorter decomposition times are sterically possible, in principle, but energetically unfavourable. This demonstrates that the low reaction rate is mainly due to the high activation energy of these processes, which is substantiated by appearance potential measurementsl. A small frequency factor can further contribute to the slow reaction rate, especially if the rearrangement is sterically complicated. The discussion of the rate constant k as a function of the iuremal energy, as reported previously*, clarifies the strong influence of the activati .EI energy on the rate constant.
7. FORMATION OF TROPYLIUM IONS Ring expansions are special cases of skeletal rearrangements. Among these ring expansions the formation of the tropylium ion and ions of the homologues is of particular importance in EI mass spectrometry14. Ifit is assumed that the tropylium ion is formed by a two step process, two different reaction paths are possible:
0/ 1 -
cl+-x
.:
(0) -e-x
0/ \
I=-
-
Cm/e=91)
cii;
+ Cl,--, .;+I
._I
(m/.=91)
On the one hand-(a), the rearrangement is assumed to start with the ehmination of the subst&eirt -X and is’foUoW& by the ring expansion. If in this case the eliminatiorz of the substituent occurs with a.h.ighreactjon rate,.neither.the intensity ratio of t&e normal ftigment peak to -the normal meta&aWc peak (the Qi factor) 70
ht. 3. Mass Spectrcw. hn Phys., .9. :(1972)
-_..
loo
t
10
.
E 7
5 a
1
0.01 40
50
60
70
80
100
90
110
-m/e
loo f
80
.
E -
1108)
Isi
El-12eV (250°C)
60
40 d-H-CO (79)
I
I! 40
50
60
70
8Q;
c
C7J47+ 1911
II
II 90
I
100
k
,
110
-m/e
-179) M+-H-CO llO8) hi'
t
100 80
& I= 5 tE
6(J
E-L-?7Ck!J (25Q°C 3
40 w+3+ 151)
20
! IL. 40
50
C7'47+ 191) c5w5+ 165) +I
.
I,,I I
~60
70
80
II I
90
II I
100
,
110
CHS_OH Fig. 2. FI, 13 eV and. $0 eV 61 spectra of benzyl alcohol. (Note the logarithmic intensity scale of the FI spectrum. Metastable ions are plotted as dashed lines at their apparent-n=.)
HO61
loo
M+
F_L(3OO*C)
If
1 :
m5-91; I : : : : !
d+
1 t911*
0.1’
. 40
I
.
50
60
7ci
60
90
I
m,
I
.
110 b
loo
I1061
120
*
m/e
d
12eV (300°C)
t
El-
-80 E A d6a 40 (911 1C,H7+
20 178)
4p
50
60
70
80
I. .
so
100
.I_ 110
# 120
’
I+
E-1. 12eV
!250°C)
50
60
70
80
90
no
100
120
130
loo-
(105)
140
m/e
&*-C2H5
t
EL-
--. 5
70eV
(250°Cl
a SO.LO20.
cin3+ 1511
I
1‘1 m
50
CpnH;' (65) I 60-m
c7eP 1911
113i) I
I 80
M+
90
100
* 110
120 130 _-m/e
140 _
..
w5 \ .c’w .\ 0 CH3 Fig. 4. Fl,12 eV tid 70 eV EIspectraof sec.-butylbekene. (FI spectrti with logarithmk i&n&y ‘. scale. Metastabli ions a& pldtte$. as dashed lines at th&kpparent II&S_)
nor the peak shape reflects the kinetics of the second step (the actual rearrangement) as the fragment mass does not change during this second step. On the other hand in (b), the ring expansion is assumed to occur as the tist step followed by the elimination of the substituent. In this case the peak shape reflects the decomposition time of the overall reaction. In EI mass spectrometry the second mechanism is assumed to be correct for most compounds l5 _ The a-C-atom is inserted between the ring C-atoms andsimultaneously one c&ydrosen atom migrates to the Cl-atom, followed by the elimination of the substituent. Moreover, randomization of the hydrogen atoms is observed with most compounds. Fig. 3 represents the FI and EI spectra of ethylbenzene (with the tropylium ion at m/e 91), Fig. 4 those of sec.-but$lbenzene, where a methyltropylium ion is observed (m/e 105). In both cases the rearranged ions are observed with relatively high intersity even in FI mass spectra. Moreover, the fragment peak is located at the normal mass position (in contrast to the shifted hydrogen rearrangement peaks). Similar results are observed with all the other compounds (summarized in Table 3) except with m-xylene and tetralin. Ring expansions are skeletal rearrangements and therefore have relatively high activation energies (the activation energy for the formation of C,H,+ from ethylbenzene is 2.9 eY and from m-xylene 3.1 eYf6). Thus the reaction rate shouId be low. Assuming the second mechanism applies also to FI conditions, only a metastable ion and no normal fragment should be expected with all tropylium ions. The intense no_rmal fragment ions demonstrate, however, that the first reaction path (a) is correct for FI in contrast to EL The ring expansion occurs after the elimjnation of the substituent. Such differences in the mechanism between R and EI conditions must be caused by either the high electric field or the adsorption at the emitter surface_ Variation of the field strength (which can easily be realized using a tip emitter) demonstrates that alkylbenzenes undergo field dissociaticn as a first step of the tropylium ion formation: An increase of the tip potential from 5 kY to 6.5 kY raises the intensity of the methyl tropylium ion at .nz/e105 in the FI spectrum of sec.-butylbenzene from 2.1 % to 18 %, which clearly indicates fieldinduced dissociation. In addition to this field dissociation a statistical decay leads to a normal metastable ion and intense decay within the region of the fast metastable peak. With m-xylene and tetralin only this statistical decay is observed. Field dissociation is obviously not possible with these compounds.
8. DIRECT BOND CLEAVAGES The rate constants as functions of the internal energy (k(E) curves) have been discussed iti detaii previously I. These considerations demonstrate that rearrange74
ht.
J. Mass Spectrom;
Ion Pi13%, 9 (1972)
loeiglll)
wXylcnc (106)
Dibcnzyl sulfoxide (230)
Bcnzylalcohol (108)
/I-Cresol (108)
(91)
(Tropyliumion)
C7H,+
(Tropylium ion)
($1)
C7H7+
6.6
Bcnzyl chloride (126)
(105)
(hi{
0.008 (250°)
<0.005
H
I (%)
Benzyl propyl ketone (162)
M+-C2Hd
M” -CHJ (91) (Tropyliumion) C7H7+ (91) (Tropyliumion) M’-CNJ (I 19) (subst, Tropylium)
M*-CHB (91) (Tropylium ion)
..--.--___-__-.__
Process
(subst. Tropylium) (250’) Me-H (107) (Hydroxytropylium) (1:G) C7H,+ (91) (Tropylium ion) M+-OH (91) (Tropyliumiou)
scc.~Butylbcnzcnc (I 34)
w.Butylbcnzcnc
(134)
_I__ Ethylbenzene (106)
ContporcNd (mol.
IONS
---__..-..L~_-_--__I_____
FORMATIONOF ‘~ROPYLIIJM
$.j Tctrulin (132)
kJ
Ci
W
* ”
a
36
‘?
8 2’
8 _--
84
93
28
30
63
26
26
0.3
79
99
24
0,5
.62
93
98
12
86
0.2
0.4
2.4
0.94
0,l.l
1.5
1.06
0.94
* 1,17
‘60
I,0
0.28
0.26
-
0,003
4.0
Q
>>l
5s
>350
I*3
2,8
2‘0
0,07
>>I
R I (%I WLlgnr, (%) mh7,m (%) 1 (%I Ef (12 cl’) Fl E1 (12 e V) FI Mcrasrnble ~-,-~I_--~~._~ ----w-m_ 33 92 0.63 0,3G
ment reactions may have larger average rate constants-thandirect bond cleavages at low excitation energies (12 eV El or FI) if field dissociations are excluded. At higher excitation energies (70 eV n), however, the rate constants of direct bond tisions dominate. This is due to the fact that a’.low excitation energiesthe reaction rate is mainly determined by the activation energy, and at high excitation energies by the frequency factors (i.e. by the maximum rate constant which is sterically possible). Whether in a specific case the direct bond rupture or the rearrangement has a higher average rate constant depends on the crossover of the k(E) curves of thesetwo fragmentation mechanisms(assumingthat only two.competing decomposition processesare observed). If, on the one hand, this crossover occurs within or below the regime of rate constants, contributing preferentially to the normal metastable peak, the rate constant of the direct bond fission will be higher than that of the rearrangement.If, on the other hand, the crossover is situated above the metastablerange, the rearrangementwill dominate at low excitation energies17. According to these theoretical considerations direct bond cleavages should often be observed, but as normal metastable ions in FI mass spectra, assuming the decay occurs statistically. However, in contrast to rearrangement reactions direct bond cleavages may occur as a field-induced or a surface process, especially with aliphatic compounds. In thesecasesthey are observed as sharp peaks at the exact normal mass position* and are accompanied by no, or only a very weak, normal metastable ion peak. Hence it is obvious that most of the direct bond cleavages cannot be interpreted kinetically. Some exceptions will be discussed.
9. ELIMNATION OF ALKYL RADICALS
Pure statistical decomposition processes are observed with those fragments where neutral alkyl radicals (preferentially methyl radicals) are eliminated. These fragmentation processeshave a uniformly low reaction rate at room temperature and are mostly observed only as normal metastableions. In Table 4 the intensity of the normal fragment ion and the normal metastable ion, as well as the above defined Q factors, are listed. Peak shifts are indicated. Similar resultsare obtained with n-para5s studied by Wanless and Glock” and by Beckey et al.‘. The elim%nationof alkyl radicals (M+- CnH2n+l)+ is predominantly observed as the normal metastable ion with these compounds a!so. The low reaction rate can be explained by the relativeIy high activation energy of these processes.It is possible that a low frequency factor contributes further to the slow reaction rate. * Field-inducedfragments may display a small peak tailing if a tip emitteris used, corresponding t6 a decay in the range of 10-12-10-s1 sec.
76
Int. J. _%-~LG Spixtroni Ion Phys.; 9 (1972)
:
TABLE 4 ELIMINATION
oF.EiEuTRAL
(M+-CHJ.
M+--CzHs
Compound
ALKYL
etc.)
Prccess
Bromohexane (164)
RADICALS
M+ -C2HS (135)
Temp. i”C)
I(%) FI
I(%) FI’ 18.0 0.2
Bond
a
< 50’ 300°
0.5 0.5
M+--CH3 methyl (131) sulfide (146)
< 50” 250°
? ?
0.38 1.9
to.4 to.4
S-CHB
Menthone (154)
M’--C’H3 (39)
t50' 300”
to.01 to.01
0.5 1.1
to.02 to.009
\-C-CHs /
Dimethyl disulfide (94)
M’-CHs (79)
t50” 3O!Y
n-Heptyl
N,N-Dimethyl- M + - CH 3 N’-phenyl(133) formamidine (148) p-Anisidine (1231
M+-CHs (108)
Acetylacetone (loo)
M+---CHs (85)
n = peak shifts.
10.
PEAR
SHIFTS WlTH
(50’ 300”
150” t50” >700°
y
+-&H.
0.11 0.07
0.05 I
0.003 to.Oc2
0.25 1.78
0.012 to.001
0.097
0.36
0.02
-O-CH3
0.005
0.80
0.006 0.38
-&-CH3
13.2
A
-S-CHB
0.006 0.07
5.5
&.
\N-CH /
3
0 a
* = normal metastable peak.
DIRECT
BOND
CLEAVAGES
Shifted peak maxima pointing to a statistical decay are not only observed with rearrangements but in some cases also with direct bond cleavages. The experimental results are summarized in Table 5. The amount of the peak shift is smaller than with rearrangements. With valerophenone a direct bond cleavage at ‘nz/e 105 (C&CO+) can be observed simultaneously as field induced fragmentation (sharp peak at exact normal mass position) and as statistical decay (shifted peak maximum) if very high field strengths are applied by using a tip emitter. Thus field dissociations and statistical decompositions may compete in some cases.
11. CONCLUS!ONS
At low excitation energies, realized in an FI or in a l&w-energy (12 eV) JZI source, the rate constants of statistical -fragmentation processes reflect more the
differences between the activation energies than the differences between the freIJU.J_ Mass Speetrbm. Ion Phys-,
9. il972)
77
TABLE
5
PEAK SHWI-S
WITH DIRECT
Compound
Valerophenone
BOND
CLEAVAGES*
Fragment
C6H5COC
(105)
Potential
Am
&VI
(m-u_)
+5/-5
Acetic
C6HSCH2- CH1+!105)
j-5/--2.5
Butyric acid amyl ester
M+ -C2HS
(129)
+5/-s
Butyric acid
ti+--CnH,
(101)
+--71-3
acid phenylethyl ester
$;&0.05 ._
butyl ester Acetylacetone
M+-CH3
* 2.5 pm platinum wire,
(85)
+5/-5
0.09+o.a5
activatedwith henzonitrile.
quency factors. Therefore, skeletal rearrangements are mostly observed with lower reaction rates than hydrogen migration reactions under low energy conditions. In general, hydrogen rearrangements reach a maximum intensity after several 10S1l set and are thus relatively fast decomposition processes, under FI conditions.They are, however, considerably delayed as compared with the fast-field dissociations
observed with direct bond cleavages in many cases (occuring within about lo-l3 set). As far as these direct bond cleavages can be interpreted kinetically (i.e. if surface reactions are excluded), they differ greatly in their kinetic behaviour under FI conditions. Besides the fast field dissociations mentioned above, extremely slow, direct bond fissions are observed with ions eliminating alkyl radicals_ As neither rearrangement reactions nor direct bond ruptures show a uniform kinetic behaviour, an unambiguous differentiation between these two decomposition mechanisms by analysis of peak shape, shift and intensity is difficult.
REFERENCES 1 XC.LAFXN AND H. D. BECKEY, ht. J. Mass Spectrom. Ion Phys_, 7 (1971) 341. 2 E. M. CHAIT AND F. G. KI'ISON,Org. Mass Spectrom., 3 (1970) 533. 3 J. N. DAMICQR. P. BARRON AND J. A. SPHON, ht. J. Mass Spectrom. Ion Phy.s_, 2 (1949) 141. 4 P. SCHULZE AND W. 3. RICHTER, ht. Mass Spectrom. Ion Phys., 6 (1971) 131. 5 H. K&PPEL, Int. J. Mass Spectrom. Ion Phys., 4 (1970) 97. 6 P. BRO~,G.R.FEXTITAND R.K.Rosm, Org_MassSpectrom.,2 (!.969)521. 7 P. BROWN AND G. Rl PET-I-IT,Oig. Mass Spectrom., 3 (197a) 67. 8 H. D. BECKEY, Int. J. Mass Spectrom. Ion Phys., 1 (1968) 93; 5 (1970) 182. 9 H. D. BECKEY, H. HEY, K. LEVSFN AND G. TENSCHERT,ht. J. Muss Spectrom. Ion Phjs., 2 (1969) 101. 10 p. D.:BECKEY AND K. LEVSE&,ix-R. L REED (Editor), Recent Topics in Mass Spectrometry, Gordon and Breach, New York, 1971, p_ 169. Int. J_ dfasS Spectr.Jmi Ion Phys., 9 (1972)
-.
:
11 12 13 14 I5 16
17 18
KI LEV~ENAND H. D. BECKEY,Int. J. Mass Spectrum. font Phys., 9 (1972) 51.. F. W. R&LGEN AND H. D. BECKEY,Surface Sci., 23 (1970) 69. J. D AKMEN, Dissertation, Bonn, 1968. H. M. GRUBB AND S. MEY~PSON, in F. W. MCLAFFERYY (Editor), M& speckokeriy of Organic Ions, Academic Press,New York, 1963, p. 516. S. MEYERXEJ, H. HART AXD L. C. LEITH, J. Amer. Chem. Sot., 90 (1968) 3419. J. L. FRANKLIN,.J. G. DILLARD, H. M. ROSEN~TOC~,J. T. HERRON,K. DRAXL arm F. fi. FIETJ), IokTz@on Potenfials, Appearance Potentials, and Heats of Formation of Gaseous .Positire Ions, National Bureau of Standards, Washington, 1969. P. BROWN, Org. Mass Spectrom., 3 (1970) 1175. G. G. WAXAND G. A. Gmcx, JR., Ad. Chem., 39 (1967) 2.
Int. J..Mass.Spectrom.
Ion Phys., 9 (i972)
79