Organic Mass Spectrometry, 1974, Vol. 8, pp. 279 to 287. Heyden & Son Limited. Printed in Northern Ireland

APPARENT CHARGE LOCALIZATION EFFECTS IN THE McLAFFERTY REARRANGEMENT OF ACETATE ESTERS :

FIELD IONIZATION KINETIC MEASUREMENTS GORDON WOOD," A. M. FALICK? and A. L. BURLINGAME

Space Sciences Laboratory, University of California, Berkeley, California 94720, USA

(Received 11 October 1972; accepted (revised) 2 July 1973)

Abstract-The relative rates of the McLafferty rearrangement in n-butyl acetate and several related molecules have been examined over the time range 10-l1 to s by means of a field ionization mass spectrometric technique. It was found that the introduction of a substituent group of low ionization potential (phenyl, dimethylamino) a t sites remote from the ester oxygens reduces the rate of the McLafferty rearrangement and that the rate becomes lower as the substituent is moved further away from the ester group. The results suggest that for the cases examined the charge on the molecular ion may be localized on the functional group of lowest ionization potential.

I N T R O D U C T I O N

PREVIOUS work from this laboratory utilizing field ionization mass spectrometry (FIMS) has shown that n-butyl acetate undergoes a rapid, site-specific, gas phase McLafferty type rearrangement1 which may be represented as follows :

The electron-impact (EI) mass spectra of many compounds containing functional groups related to esters show abundant 6-center rearrangement ions corresponding to the McLafferty rearrangement. Most of these molecules, however, give small or undetectable rearrangement peaks in normal field ionization (FI) mass spectra$ taken with a double focusing instrument. Since only fragment ions formed at times less than about 10-l1 s after ionization (and ions formed at the emitter surface) are normally detected in such spectray1s2 this implies that the McLafferty rearrangement in aliphatic esters proceeds at an unusually rapid rate.3

In an attempt to provide some further insight into the nature of this rearrangement we have employed the field ionization kinetic (FIK) m e t h ~ d . ~ , ~ . ~ , ~ This technique was used because it can afford much more detailed kinetic information about mass spectrometric fragmentation processes than conventional EI or FI spectra (e.g. see Refs. 7 to 9). It provides the capability of measuring the rates of unimolecular decomposition of primary ions at times of 10-l1 to s after ionization. The fact that the field ionization process imparts comparatively little excitation energy to the molecular ions tends to increase the relative contribution of low frequency factor, low activation energy processes (e.g. rearrangements) to the total fragmentation.'*lO*ll

* Present address: Department of Chemistry, University of Windsor, Windsor, Ontario, Canada. 7 Author to whom correspondence should be addressed. 3 A number of amides, ethers, ester vinylogs and nitrogen and sulfur compounds closely related

to esters were examined in this laboratory. None was found to have a significant peak in its FI spectrum corresponding to a McLafferty type rearrangement.

279

280 G. WOOD, A. M. FALICK and A. L. BURLINGAME

FIK would therefore appear to be an excellent method for study of the effects of structural changes at a site remote from the carbonyl group in Reaction (1).

RESULTS The compounds chosen for study all contain the hydrogen atom at C-2 which had

been implicated in the butyl acetate rearrangement1 and in addition, a functional group (either phenyl or dimethylamino) further along the alcohol chain.

Figure 1 shows the phenomenological rate constants R ( t ) plotted as a function of parent ion lifetime for the loss of a neutral fragment of mass 60 from compounds I to

I I I I I I I -l n - butyl acetate (m/e 561, I

2 - p h eny let hy I acetate ( m / e 1041, II

- I I -9 -7 -5 log t ( sec 1

FIG. 1. Log-log plot of phenomenological rate constant L ( t ) (plotted as a function of parent ion lifetime) for the loss of a neutral fragment of m/e 60 from compounds I to IV

following field ionization.

Apparent charge localization effects in the McLafferty rearrangement of acetate esters 281

100

>- I- - 80- v, z W t- 6 0 - Z n

IV following field ionization. The rate of this reaction for compound V was so slow that no product ion (m/e 85) resulting from gas phase decomposition was detectable in any portion of the time range covered in Fig. 1. The data clearly show that the rate of the rearrangement for compounds I to V is in the order I > I1 > IV > I11 > V over the entire time range shown in the figure. The maximum rates appear to fall in the same order, although in the case of compounds I and I1 these maxima lie at times shorter than - 10-l1 s and could not be directly measured.

It is interesting to note that in the case of I1 and I11 the rate constants are reduced despite an apparent opportunity for charge delocalization in the product ion. For example, the charged fragment from I1 is formally equivalent to ionized styrene:

-

Compound IV no longer contains this apparent advantage and here one should be looking at a more clear-cut example of introduction of a group remote from the presumed site of ionization. Here the maximum rate is much lower than for n-butyl acetate and occurs at a time at least 2 orders of magnitude longer.

Figure 2 shows the low energy (- 18 eV) EI spectrum of n-butyl acetate (see Ref. 1 b for the FI spectrum). Figures 3 to 6 contain the low energy EI spectra and the normal FI spectra of compounds I1 to IV. The FI spectra must be interpreted with

43

I

56

61

EI ( 1 8 e V )

0 20 40 60 80 100 120

m/e FIG. 2. Low energy (- 18 eV) EI mass spectrum of n-butyl acetate (I).

282

I

G. WOOD, A. M. FALICK and A. L. BURLINGAME

l o o t

60 8ol > 401 I- - i W 20t c 4[ 2 w o u

100

80

60

> 40- I- v) z 2 0 - W I-

-

5 0 -

I

92

I04 -

-

-

I64 48

I h I I

FI

W > F too- a

a J w 8 0 -

60

40

20

-

-

-

L 140 160 180

4

EI ( I 8eV)

x 100

164

w

I I 0

20 40 60 80 100 120 140 \60 180

m/e FIG. 3. Low energy (- 18 eV) EI and normal FI mass spectra of 2-phenylethyl

acetate (11).

caution, because they represent fragments produced via surface and high field proc- esses5*12 as well as those produced in the gas phase at times up to - s following ionization.2 For example, the small m/e 85 peak observed in the FI mass spectrum of V is very probably due to a surface andlor high field process since no evidence of gas phase reaction was observed at times slightly longer than 10W1 s. Thus, peaks in these FI spectra must be regarded as upper limits to the amount of gas phase reaction which occurs during the first s of the parent ion lifetime.

DISCUSSION

The differences in the rates of the McLafferty rearrangement* in the molecules studied are most readily interpreted in terms of the preferential location of charge on

* It should be noted that the particular rearrangement examined here differs slightly from the usual McLafferty rearrangement which results in a charged acid fragment. In the present case, a sort of ‘inverse’ McLafferty process (in which the charge is retained on the olefin fragment) is operative. Recent evidence (P. J. Derrick, A. M. Falick and A. L. Burlingame, to be published) suggests that these are two distinct reactions and that the latter reaction has the lower activation energy.

Apparent charge localization effects in the McLafferty rearrangement of acetate esters 283

x 1/10 I )-----3

80

60

40

I

l o o t -

-

-

> 100 w I - - 58 I

I

FI

I. 140

EI (18eV) J 8 0 - W

60 -

71

40 -

20 - 42

131 0. I II I. ( I

I I

$0 ' 40 I $0 I 80 IbO 120 I 140

m /e FIG. 4. Low energy (- 18 ev) EI and normal FI mass spectra of 2-dimethyl-aminoethyl

acetate (111).

the group of lower ionization potential. Typical values of the first ionization potential for aliphatic esters are about 10.0 eV (Ref. 13), whereas the ionization potential of benzene is 9.25 eV (Ref. 13) and a typical value for aliphatic amines is 8.7 eV (Ref. 13). Thus our results are consistent with the expectation that a charge localization effect will be present in compounds I1 and IV, and more pronounced in compounds I11 and V.

As Howe and W i l l i a m ~ l ~ . ~ ~ have pointed out, however, interpretation in these terms is not necessarily in conflict with a quasi-equilibrium theory (QET)16*17 approach. Ions formed by FI have relatively small internal energies.l* If it is assumed that the accessible electronic states are in equilibrium, then those molecular ions occupying the lowest lying electronic state will also have the highest vibrational energies and therefore this electronic state will have the greatest degeneracy. Decompositions are most likely to occur from the lower electronic states on both statistical and energetic grounds. The lowest electronic state of a molecular ion corresponds to the removal of one

284

> - 5 i oo - J w lx 8 0 -

60

G. WOOD, A. M. FALICK and A. L. BURLINGAME

-

FI

I

20 40 60 8U IOU I Z U 14U IbU IUU w

8

L

118 I

E I (18eV)

I II

2 0 4U bU UV IUV I C V I 4 V IbU I U V

m /e FIG. 5. Low energy (- 18eV) EI and normal FI mass spectra of 3-phenylpropyl

acetate (IV).

electron from the highest occupied molecular orbital. It is precisely this state which would be expected to have the greatest amount of charge concentrated at the part of the molecule having the lowest ionization potential.

For the dimethylamino substituted series (I, 111, V) the relative intensities of the rearrangement ions from Reaction (1) in the EI spectra (Table 1) suggest that charge localization or its equivalent is operative. Thus both 70 eV and 18 eV spectra show an apparent small reduction in the rearrangement ion upon introduction of the dimethyl- amino group at C-2 (111 us. I) and a further reduction of about one order of magnitude when the dimethylamino group is moved to C-3 (V us. 111). By comparison, the most energetic ions detected in the FIK measurements (those decomposing at N 10-11 s after ionization) show a difference in decomposition rate of about 3 orders of magni- tude between I11 and I. In the case of the phenyl substituted series (IV us. I1 us. I), no charge localization effect can be discerned from the EI spectra, whereas the FIK data show successive changes in decomposition rate of one to two orders of magnitude

Apparent charge localization effects in the McLafferty rearrangement of acetate esters 285

I I .

100

0 3

60

>- 40 I- v)

W I- z H O

W > I

- = 20

I41 I . 85

F 100

I

‘“I 20

I03

x 1/10

145 w

m*( I45+ 58)

20 4 0 60 80 100 120 140 ’ I60

58 E I ( I 8 e V )

FT

I)

m /e FIG. 6. Low energy (- 18 eV) EI and normal FI mass spectra of 3-dimethylamino-

propyl acetate (V).

The greatly reduced ability to distinguish the charge localization effect following EI ionization as compared to FI is very likely the result of the increased internal energy imparted to the ions in this case. The fact that the effect is substantial in the spectra of the dimethylamino compounds lends support to this hypothesis in view of

TABLE 1. INTENSITIES OF FRAGMENT IONS PRODUCED IN REACTION (1) (AS A PERCENTAGE OF TOTAL IONIZATION) FOLLOWING 70 eV AND 18 eV EI IONIZATION. THE COMPOUNDS ARE LISTED IN DECREASING

ORDER OF RATE OF REACTION (1) AS MEASURED BY FIK

Compound mle 70 eV 18 eV

286 G. WOOD, A. M. FALICK and A. L. BURLINGAME

the respective ionization potentials for these groups. An additional factor which may contribute to the lack of a clear-cut trend in the series I , 11, IV is the expected strong activation of the rearranging hydrogen by the adjacent benzene ring in compound 11. The larger amount of energy available following EI ionization, particularly by 70 eV electrons, as compared to F P causes a variety of higher activation energy processes (e.g. direct cleavage reactions) to compete successfully with Reaction (I), thereby depleting the supply of molecular ions at an unknown rate. Furthermore, the addi- tional available internal energy undoubtedly enhances secondary fragmentation of the McLafferty product ion to an unknown but almost certainly significant degree. Unfortunately, the kinetic details of processes which occur at times less than - s after ionization cannot be examined by means of conventional EI techniques. The intensities of particular peaks in EI mass spectra represent only the integrated result after - s of all of the competing and consecutive reactions leading to and from the product of interest.

The FIK measurements, on the other hand, begin with a lower energy ionization process which suppresses competing reactions of the molecular ion and subsequent reactions of the product ion. The extent of depletion of the molecular ion can be calculated since only one or two fragmentations occur to a significant extent and the rates of these decompositions can be measured as functions of time.

A good deal of study has gone into the question of charge localization in EI mass spectrometry.lg The present work suggests that FIK measurements can be a valuable complementary tool in clarifying mechanistic questions of this kind. In particular, it has been shown that when the fragmentations of field ionized molecules are examined in such a way that surface and high field effects are eliminated, the results are consis- tent with reasonable chemical expectations.

EXPERIMENTAL The experiments were performed with a modified2 Du Pont (CEC) 21-1 10B double focusing mass

spectrometer equipped with a combination FI/EI ion source. Uncoated Schick stainless steel razor blades were used as FI emitters. All spectra were produced at 80°C. Decompositions occurring in the 10-l' to lo-* s region ('fast metastables') were detected by means of a defocusing technique essentially similar to the 'metastable defocusing technique',20~21 in which the accelerating potential was raised to permit ions formed at varying distances from the blade surface to bedetected. lb.' The resulting curves of ion current at a particular mass against accelerating potential were transformed to yield rates of dissociations as a function of parent ion lifetime.2 Dissociations occurring in the first field free region (lifetimes - s) were detected by the metastable defocusing technique. 20.21 Ions formed by dissociations in the second field free region (- s) appeared as normal metastable peaks in the FI spectrum.

The low energy EI spectra were obtained on a Varian MAT 311 mass spectrometer coupled to an XDS Sigma 2 computer.22

The compounds used in this study were obtained commercially with the exception of 3-dimethyl- aminopropyl acetate. n-Butyl acetate (I) and 2-dimethylaminoethyl acetate (111) were obtained from Eastman. 2-Phenylethyl acetate (11) was obtained from K & K. 3-Phenylpropyl acetate (IV) was obtained from Aldrich.

Purity of all sampleswas checked by gas liquid chromatography (g.1.c.) and EI mass spectrometry. 3-Dimethylaminopropyl acetate (V) was prepared from acetic anhydride and 3-dimethylamino

propanol by heating on a steam bath. The product was isolated by g.1.c..

Acknowledgements-The authors are indebted to Dr Robert Cox for obtaining the low energy EI spectra, and to the National Aeronautics and Space Administration for financial support (Grant NGL 05-003-003). One of us (GW) wishes to thank the University of Windsor for a sabbatical leave during which this work was initiated.

Apparent charge localization effects in the McLafferty rearrangement of acetate esters 287

R E F E R E N C E S 1. (a) A. L. Burlingame, A. M. Falick, G. W. Wood, P. Schultze and W. J. Richter, American

Society for Mass Spectrometry, 18th Annual Conference on Mass Spectrometry and Allied Topics, San Francisco, California, June 1970, p. 416; (b) P. Schultze and W. J. Richter, Znt. J. Mass Spectrom. Zon Plzys. 6, 131 (1971).

2. A. M. Falick, P. J. Derrick and A. L. Burlingame, Znt. J. Mass Spectrom. Zon Phys. in press. 3. K. Levsen and H. D. Beckey, Znt. J. Mass Spectrom. Zon Phys. 7, 341 (1971). 4. H. D. Beckey, H. Hey, K. Levsen and G. Tenschert, Znt. J. Mass Spectrom. Zon. Phys. 2, 101

5. H. D. Beckey, Field Ionization Mass Spectrometry, Pergamon Press, New York, 1971. 6. P. J. Derrick and A. J. B. Robertson, Proc. Roy. SOC. (A) 324,491 (1971). 7. P. J. Derrick, A. M. Falick and A. L. Burlingame, J. Amer. Chem. SOC. 94,6794 (1972). 8. P. J. Derrick, A. M. Falick and A. L. Burlingame, J. Amer. Chem. SOC. 95,437 (1973). 9. P. J. Derrick, A. M. Falick, S. Lewis and A. L. Burlingame, Org. Mass Spectrom. 7, 887 (1973).

(1 969).

10. A. N. H. Yeo, R. G. Cooks and D. H. Williams, Chem. Commun. 1269 (1968). 1 1 . A. N. H. Yeo and D. H. Williams, J. Amer. Chem. SOC. 91, 3582 (1969). 12. H. D. Beckey and F. W. Rollgen, Naturwissenschaften 58,23 (1971). 13. J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron, K. Draxl and F. H. Field, Zoniza-

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19. For a recent paper dealing specifically with the McLafferty rearrangement see: J. R. Dias,

20. M. Barber and R. M. Elliott, American Society for Mass Spectrometry, 12th Annual Conference

21. P. Schultze and A. L. Burlingame, J . Clzem. Phys. 49,4870 (1968). 22. D. H. Smith, R. W. Olsen, F. C. Walls and A. L. Burlingame, Anal. Chem. 43, 1796 (1971).

London, p. 80.

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Petroleum, London, 1968, p. 523.

University Park Press, Baltimore, 1970, p. 1154.

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