The study of rearrangement reactions by field ionization mass spectrometry

ion

-. IrltertitioRQi Journd of iuass spe&tIehy c&d P&uics ElsevierF-ublishiigC.o&xzmy~Amsterdam. P&ted -& the Nethe&nds

l-FE

OF REARRANGE&IENT MA!23 SPECTROMETRY

.STu?Y

TION ti.

TEMPERATURE

REACTIONS

01.

:

BY .FIELti.

IaNIzA;

DEPENDENCE

K. LEVSEN +ND H. D. BECKEY Institut ftir Physikalische Chemie der Unicersitfit Bonn (W. Germany)

(Received July 12th, 1971; in revised form August SOth, 1971)

ABSTRACT

The influence of the internal energy of organic ions in the gas phase on their decomposition rate constant is studied by means of the temperature dependence of field ionization (FI) mass spectra. An increase of the average rate constant with increasing internal energy is found, in agreement with the quasi equili’orium theory (QET) of mass spectra.

INlRODUCTiON

The temperature dependence of FI mass spectra has already been studied by Knijppel and Beckey’*‘. The investigations included fast field dissociations as well as surface processes and were for the most part limited to the evaluation of peaks at the ncrmal mass position. The present investigations include the study of ‘fast” and ‘normal” metastable ions* and are mainly concerned with the discussion of statistical fmgmexitation processes and therefore the results apply inimediately to fragmentation processes. under low-energy electron impact conditions. As demonstrated. in an earlier pz$er3. by means of bqth peak shift. and peak Gape, McLaf&erty-type hydrogen rearrangements and related reactions are often pure statistical processes, even in ati n source, aiid thus fall within the scope of the WT.

-‘A single-focus*mg; Nier-type-mass spectrometer eq&ed with a no@-focusing field‘ iod source, a& already d&cribed3; Was &ed for .till experiments: The. : l

I.e.-ionsdecornpo&g betweenthe field ion emitterand.the counter.eiebrode;-or-in-thkfield-free

$pac$kkti& .

the counter electrodeand themagnetic field, ke&kti$~. .:

.-

I+. J-~~~~~~~~~~.-ion.Phys.j9~(~g72j.~~:;1:

,-

:

.-._

;

.__. ~’ .. .:

:

.f Y -.~. .-

.‘..I-

...;

-. i. _I 1 ::-- -.- :; .::I.--:~-t ._ ; _..; :.

._

.I-.-

:_ ‘;-- :L,. -_-._....

Lemperature could be varied by eiectrical heating of the wire emitter and was measured pyrometrically or by means of the resistance variation. TWEORETICAL

According to the QI# the rate constant k is a function of the total energy &,,, transferred to the molecular ion. This total energy is the sum of the therm6 energy, &, and the excitation energy transferred by the ionization processes themselves, E ionsTb.USE = Ee+Eionw While in EI mass spectrometry the total excitation ener,y transferred to the molecuIe during the ionization act can be varied simpiy by altering the electron energy, in R mass spectrometry this total energy can only be varied slightly by altering the field strength (especially with wire emitters). Hence, the affect of the tota! transferred energy on the rate function in this case can only be studied by varying the thermal energy. The influer.ce of the thermal ener,7 can be illustrated by interpretation of the k vs. E curves. For this purpose one has to discuss the energy distribution function P(E) in more detail. According to Ehrhardt and Osberghaus’ the distribution function P,,(E) for the thermal energy can be calculated as a function of the thernzalenergy and the temperature in the following way: (1

4h(&, 0 = exp(-GW - F(E) n

_e-hvi/kT)si-l _

cs

l)!chv)s’-l

z

where E is the total thermal energy of the molecule and F(E) is a poiynomial of (s- lyh degree, the coefficients of which are determined largely by the zero point energies. Fig. 1 represents this thermal energy distribution as calculated by Ehrhardt and Osberghaus for propane. As the vibrational frequencies of rather complicated molecules (as, for example, those studied here) are not known, P,,(E, T) cannot be calculated in this case. But the shape of the thermal energy distribution will be similar to that of propane for all other molecules. However, the maximum tiill be shifted to higher energies with increasing molecular size6. Using the form

Fig. 1. The distribution function 6f the thermal energy, Pa(Ej, as calculated for propane at differentgas temperatures(after Ehrhardt and 0sberghaus’).

52

ht.

J. Mau

Sperrron. Ion P&w., 9 (1972)

P(E)

MjF*

F

“7’

---hot

f f log k

T

-

Etot

Fig. 2. Dependence of the P(E) function on the temperature for field ionization (schematically) M = energy range of the molecular ion; .J = decay within the magnetic analyzer; F = energy range of the fast metastable ion; F* = energy range of the normal metastable ion. In the lower part an arbitrarily taken k(E) curve is drawn. The horizontal dashed lhes in&de k-values, contributing preferentially to the normal metastable ion.

of the P(E)-function

f or Fi for n-heptane at room temperature as derived by Tenschert and Beckey’ and considering the results of Ehrhardt and Osberghaus, the temperature dependence of the P(E)-function for FI can be estimated qualitatively as indicated in the upper part of Fig. 2. The lower part of this figure represents an arbitrarily taken k(E) curve with a maximum rate constant of 2 x 1Ol.f see-‘. Depending on their excitation energy, molecular ions decompose either within the fast metastable region, the normal metastable region, the magnetic analyzer, or not at all. This is indicated in Fig. 2. RiSULl-s AND DISCUSSION The hydrogen rearrangement reactions can be divided according to their temperature dependence: Int. J. Mass Spectrom.

Ion Phys., 9 (1972)

into two

groups

53

1. Menthone and benzoic acid esters; 2. Aromatic ketones, aliphatic acid esters, oriho effect.

1. Menthone and benzoic acid esters The temperature dependence of menthone will be discussed in more detail. A rearrangement at m/e = 112 is observed with this compound**‘:

McLafferty

-e

(m/e

= 112)

Considering fist the qualitatively estimated P(E) function of Fig. 2, the following predictions on the temperature dependence of the menthone decomposition reaction can be made: at room temperature the maximum of the P(E) function is obvio-.-sly still located at energies below the normal metastable region as illustrated in Fig. 2 for t, . If ihe temperature is subsequently raised to t2, t3 and t4 the maximum of the P(E) function is shifted into the energy range of the normal metastable ion and &rally reaches the range of the fast metastable ion.

100

200

300

400

$00 -

500

L”d

Fig. 3. Temperaturedependenceof menthone (M+-&&). Relative abundanceof the molecuku ion, the integratedfast metastableion, and the normal metastabb ion.

Thus as the area of the P(E) function in Fig. 2 characterizing the molecular peak diminishes with increasing temperature the intensity of the molecular peak should decrease continuously_ The intensity of the “normal metastable” ion, however, should reach a maximum value as soon as the maximum of the P(E) function is shifted into the metastable region and should decrease at higher temperatures. Finally,- it is expected from Fig. 2 that the “fast metastable” intensity increases .. Inr. J. Mass Spectrom. Ion P&s.., 9 (1972)

(al

f

m/e

-

Fig. 4a. Temperature dependence of menthone (M +- C3H6). Part of the

FI

spectra.

steadily with temperature rising. This expected result can be verified experimentally as illustrated in Fig. 3 for-the McLafferty rearrangement of menthone. In this figure the intensity of the integrated “fast metastable” ion, normal metastable ion and molecular ion cutie-nts relative to the total ion current at the collector are seperately plotted as a futiction of the tempera&e. Fig. 3 has a pattern similar to

breakdown diagrams used in the Q&~. One must, however, take intd account that the temperature scale is not a monoenergetic scale. In principkthis resuit can also be obtained using an electron-impact (EI) sake. However;-it is an advantage of lni. J. Mass ~pe&om.

Ton Phys., 9 (1972)

55.

(b)

_A‘

La33-l 155

AO

x5

l&o

l35

Do

l25

lm

ns

110

105

im

90

95 -;

Fig. 4b. For legends seeFig.

4a. (The

spectrum

at 600 “C is recorded

85

80

m/e

with a 3 x higher

sensitivity).

using an FI source that the -fast metastable ion can be further distinguished with respect to time. This is illustrated in Tig. 4, which represents a part of the original FI spectrum of menthone. At room temperature the McLafferty rearrangement displays principally an abundant normal metastable ion and only minor decay in the region of the “‘fast metastable” peak due to the tailing of the.@) function to higher energies. If the temperature is’raised to 300 “C the intensity in the region of the “fast metakble” fragment is-greatly enhanced, and the maximum is shifted

towards. higher ma& numbers with jncreasing. temperature. 56

hr.

J.

Mass Specrrotn. I+2 Phys., 9. (1972)

2

L

6

8

10

12

1C

16

I6

Fig. 5. Temperature dependence of menthone (M+-C3Hs). rearranged ion and the decomposition time.

20

22

2L

26

28

30

50

Relation between the intensity of the

In Fig. 5 the “‘fast” metastable region of the spectrum is redrawn with the mass scale of Fig. 4 transformed into a scaie of decomposition time*_ A shift of the decay maxima to shorter decomposition times is observed. Simultaneously, the decay at shorter decomposition times increases at the expense of that at longer times. The reaction is becoming faster. The menthone spectrum offers still more important kinetic information. Careful observation of Fig. 4 shows two. further metastable ions** -at. 400 “C: M+-CH3 at nz/e -:25.46 and M’-C3H, at WI/~80.00. Both processes repres&t * Using eqn. (4) of ref. 3. ** Neglecting the metastable Hz0 elimination at m/e = 120.10. Int. J. Mass S&ctrom. Ion Phys., 9 (1972)

57

d i r e c t b o n d fissions a n d display n o d e c a y in t h e "'fast metastable'" r e g i o n a n d n o n o r m a l f r a g m e n t . W i t h increasing t e m p e r a t u r e t h e i n t e n s i t y o f these m e t a s t a b ! e i o n s increases rapidly, w h e r e a s t h e m e t a s t a b l e i o n intensity o f t h e r e a r r a n g e m e n t process decreases. A t 500 °C t h e m e t a s t a b l e C H a e l i m i n a t i o n passes its m a x i m u m value a n d n o w starts t o display a decay in t h e "fast m e t a s t a b l e " region. ( N o t e t h a t the intcnsity is r e c o r d e d w i t h a 3 x h i g h e r sensitivity at 600 °C.)* M e a n w h i l e at 500 °C a t h i r d m e t a s t a b l e i o n a p p e a r s at m a s s n u m b e r 101.46 ( f o r m e d by t h e rearrangement reaction M+-C2Hs). T h u s t h e t e m p e r a t u r e d e p e n d e n c e o f t h e m e n t h o n e s p e c t r u m offers a n instructive e x a m p l e o f t h e kinetics o f f o r m a t i o n o f field-ion m a s s s p e c t r o m e t r i c fragments. It is o f i m p o r t a n c e , t h a t the main rearrangement reaction is faster than the comparable direct bond cleavages in this case. As discussed in P a r t I, this is d u c to t h e smaller a c t i v a t i o n energy o f t h e r e a r r a n g e d i o n ( E o = 0.08+__0.10 eV as co,np a r e d with 0.98 + 0.10 eV for t h e C H 3 e l i m i n a t i o n ) . F i e l d dissociation is o b v i o u s l y n o t possible w i t h this c o m p o u n d . I n this specific case differences b e t w e e n t h e a c t i v a t i o n energies a n d n o t yet differences b e t w e e n t h e f r e q u e n c y factors d e t e r m i n e t h e rate co-nstant a. T h i s result is illustrated in Fig. 6, w h e r e t h e rate c o n s t a n t s are p l o t t e d in

,

~ N * - C H 3

. °

S/,/,,

,,.-

(Eo=0,gBev}

"

._

ill

,

i I

E1lt=300°C) Ezlt=B00°C|

. Etot ={Em÷Eion)

Fig. 6. M e n t h o n e : rate constant as f u n c t i o n o f the excitation energy f o r the three d o ~ a n t fragments (schematically); I n the upper part t w o simplified P ( Lr) functions f o r 300 °C and 600 °C are plotted. * A t high e m i t t e r t e m p e r a t u r e s (600 °C) o n e c o u l d expect a n e n o l i z a t i o n o f the m e n t h o n e m o l e cule, which m a y influence the kinetic b g h a v i o u r . H o w e v e r , a s i m u l t a n e o u s inlet o f D 2 0 does n o t affect the spectrum. T h e a d s o r p t i o n t i m e a t the h o t e m i t t e r is o b v i o u s l y t o o short f o r e n o l i z a t i o n .

58

Int. ,Jr. M a s s Spectrom. Ion Phys., 9 ( 1 9 7 2 )

such a way as to agree with the exp.erimental results. One should note that these curves are plotted in this manger only as an aid to interpretation and that they may deviate considerably on an absolute scale from the actual curves. +; x5. 0

/;H

AH

t

rtL1

/N&

-

2

‘6% 1 m/e

‘SF’s

’ -t -CHR-CH

-C,OH =

=CH2

_

123)

mass spectra of benzoic acid butyl es-ter and propyl ester Z%double hydrogen rearrangement is observed at m/e 123 (ref. 10). This fragmetit has a similar temperature dependence to that observed with menthone. Fig. 7 represents a part of the original R mass spectrum of benzoic acid butyl ester from the metastable peak of the rearrangement reaction up to the normal mass position. Again the increase of the average rate constant with rising temperature can be derived directly from the spectrum in a very instructive way. (At higher temperatures the tailing of the rearrangement peak is superimposed upon the direct bond -fission at m/e 105 (C,H,CO+).) In

the

FI

(b>

k I

2.~.10~,-5.1o-5* I

5. 3.!(

1 i

)j

1123 5-104SC

J -

m/e

I123

i

/ 1

.

8

I25 120 115 110

.

105

*

100

95

90

85

-m/e

125 120 115 1x1

-

mle

105

100

95

90

85

Fig. 7. Temperature dependence of benzoic acid butyl ester (formation of CsHsCOOHz+ (m/e = 123). Part of FI spectra. At high temperatures the direct bond cleavage C6HsCO* (m/e = 1.05) is srrperimposed upon the peak tailing of the rearranged ioc. Int. .T. Ma&

Spectrom.

ion Phys.,

9 (1972)

59

69-'to"~ec • t

(e)

(d) -ID

t

•

;,7-,0 sec _m I 3,7-10sec

C,~HSCO1,05)

CsHsCO °

/

_" 605) I Vl-lo'sec

,}

t

I

~111 ~111

Iz

t

ti

-, it

,

i

i

I"x!

!

,.,-m

1

\

2.~ - 10--s

-350

°.

\

5-10~ec I

!

i

2~-~D"~-

i

]

t

ds rio,is ~io 16s ~6o 6s 60 8"5

,6o 9"s

rn,'e

--,,------ rnle

F o r legend see fig. 7a

2. Aromatic ketones, aliphatic acid esters, ortho effect I f the H spectra already display a peak m a x i m u m near the normal fragment mass at l o w temperatures, the temperature dependence cannot be observed in the same instructive way. In this case the ratio o f the fast to the normal metastable ion intensity reflects the impact o f the thermal energy o n the rate constant. This ratio is defined as Qs - Ir*/I~* (ref. 11). Table 1 demonstrates for a series o f h o m o l o g o u s aromatic ketones that Qs always increases with rising temperature. The same result is observed with all other hydrogen rearrangements studied so far. A relatively slight increase o f the Qs factor with rising temperature indicates a relatively s l o w rise o f the k(E) function, as expected with all decomposition processes o f high activation energy. TABLE

1"

RATIO OF THE FAST TO THE lqORMAL

METASTABLE

25 °C

(0 mA) Butyrophenone Valerophenone Caprophenone Heptyl phenyl ketone Tridecyl p h e n y l k e t o n e

(0.01) 0.035 0.057 0.087 0.036

PEAK INTENSITIES,

300 °C (I0 mA)

450 °C (13 mA)

(0.018) 0.23 0.30 0.39 0.78

(0.16) 1.54 3.81 5.00 --

Qs

* T h e s a m e 5 p m Pt w i r e w a s used for all e x p e r i m e n t s except for b u t y r o p h e n o n e .

60

Int. J. Mass Spectrom. Ion Phys., 9 (1972)

Rearrangements in the spectra of 2-hydroxybenzyl aicohol (M’iHzO) and salicylic acid methyl ester (M+-CH,OH) demonstrate this weak dependence of the Q, factor *_ Both are reactions displaying the mass spectrometric ortho effect (refs. 12, 13). Salicylic acid methyl ester has indeed a relatively high-activation energy (1.83 _+ 0.12 eV)3, whereas the activation energy of the process in 2hydroxybenzyl alcohol is not yet known. Table 2 represents the temperature dependence of the Qs factor of 2-hydroxybenzyl alcohol. TABLE 2 rrm -r~hwmtmuruz DE~E~~ENCEANDTHEQ~-FACTOROFTHEREARRAE;GEDI~N~N)-HYDR~XYPEN~YL ALCOHOL

25 “C

mle M+(124) 106

300 “C

550 “C

6.50 “C

100 0.06

100 1.6

100 2.9

100 2.2

m+(90.61)

1.4

26.7

33.4

35.0

QS

0.04

0.05

0.08

0.06

CONCLUSIONS

The temperature dependence of FI maSs spectra can be used to t=st some basic assumptions of the CET in a qualitative way. In good agreement with this theory the average rate constant of decomposition processes studied so far increases with increasing internal ener,y. Hydrogen rearrangement reactions are especially suitable for this purpose as they are often pure statistical fragmentation processes even in an 51 source. ACXNOWLEDGEMENT

We thank the Deutsche Forschungszemeinschaft

for financial support.

REFERENCES

1 H. KN~PFEL .~ND H. D. BEC~EY, 2. Narurfirscfi., 21A (1966) 1930. 2 H. KN~PPEL, Int. J. Mass Spectrom. Ion Phys., 4 (1970) 97. 3 K. LEVSENAW 5%.D. BECKEY, Int. L Moss Specfrom. 10.7 Phys., 7 (1971) 341. 4 H. M. ROSENST~~K AND M. KRAUSS, in R. M. ELLIOIT (Editor), Adcancer in ~WCX~Spectramefry, Vol. 2, Pergamon Presb, Oxford, 1963, p. 251. 5 H. EHRHARDTAND0. GSBERGHAUS, 2. Nuturforsch., 1Sa (1960) 575. 6 B. STEINER,C.F.GIESEAND M.G.INGHRAM, J. Chem.Phys.,34 (1961) 189_ 7 G. TENSCHERT AND H. D. BECKEY, Inf.J. Muss Specfrom. Ion Phys., 7 (1971) 97. 8 3. SEIBL AND T. GXUKA~W, 2. Anal. Chem., 197 (1963) 33. * This is only the case if one neglects the thermal rearrangement prior to ionization observed with these two colmpounds (seePartIIIIO).The thermal rearrangement causes an additionaLsharp peak at the exact mass position. Originally it was assumed that this peak is due to a surface reaction. The temperature dependence has been reportedll. hf.

J. Mass Spectrom..Ion Phys., 9 (1972)

6i

9 B. WILLHAM AND A. F. THOW, 10 c. D JERASI AND C. FEN-U, J. Amer. Chem. Sot., 87 (1965) 5756. 11 H. D. BEAND K. LEVSEN, io R. I. REEK (Editor), Recent Topics in Mass Spectiometrj, Gordon and Breach, New York, 1971. 12 J. S:S~NNON, Amt. J. Ckem., 15 (1962) 265. 13 E. M. EERY, And Chem., 32 (1960) 1495. 14 K. LEVSEN AND H. BECKEY, Inr. 6. Mass Spectrom. Ion Pkys., 9 (19’12) c.3. .I

.(j2

..-

:

..-

.:

.-..

-

__

&t. % -Mass &ectrom.

Zon Phys., 9 i1972)

: