The Possible Intermediacy of Cyclopropane Complexes in the

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

The Possible Intermediacy of Cyclopropane Complexes in the Isomerization of Aliphatic Amine Radical Cations Christian Benedict Orea Nielsen, Anders Holmen Pedersen, and Steen Hammerum J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10523 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 1, 2019

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The Journal of Physical Chemistry

The Possible Intermediacy of Cyclopropane Complexes in the Isomerization of Aliphatic Amine Radical Cations

Christian B. O. Nielsen, Anders H. Pedersen and Steen Hammerum* Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark

Abstract The isomerization of aliphatic amine radical cations via intermediate [cyclopropane−NH3]+. and [cyclopropane−amine]+. ionneutral complexes was studied experimentally with double-focusing mass spectrometers and computationally with composite ab initio methods. The results examine and extend Audier's suggestion that primary amine radical cations with alkyl substituents at the β- and/or γ-carbon atoms isomerize via transient complexes of NH3 and alkyl cyclopropanes; these are formed by ring closure of the easily accessible γ-distonic isomers.

Ionized amines with substituents at the α-carbon may react analogously when trialkyl cyclopropane complexes can be formed. Isomerization via complex intermediates is a major reaction pathway when the internal energy of the amine radical cation is less than that required for simple CC-bond cleavage. Complexes of unsubstituted or monosubstituted ionized cyclopropanes rarely contribute to the isomerization reactions. Secondary and tertiary amine radical cations do not undergo isomerization via cyclopropane intermediates, whereas aliphatic ether radical cations do.

* To whom correspondence should be addressed; [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction The two characteristic unimolecular reactions of aliphatic amine radical cations and alkoxy radicals are intramolecular hydrogen atom abstraction and cleavage of the CC-bond next to the heteroatom (Scheme 1).1-14 The CC-cleavage results directly in loss of alkyl radicals, whereas hydrogen atom abstraction can initiate a variety of reactions, including for amine radical cations skeletal rearrangement processes.15-21

Scheme 1. The common unimolecular reactions of alkoxy radicals and aliphatic amine radical cations: CC-cleavage and intramolecular hydrogen atom abstraction. The unimolecular cleavage and rearrangement processes of amine radical cations are conveniently examined with mass spectrometers. Ions formed by loss of alkyl radicals by simple cleavage give rise to strong signals in the electron ionization mass spectra of aliphatic amines, as do the products of subsequent fragmentation of these ions. The typical fragmentation of less highly energized primary amine radical cations, the metastable molecular ions, is also expulsion of alkyl radicals, but these radicals are in many cases not the same as those expelled from the molecular ion in the ion source.15-22 Isomerization may precede fragmentation when the internal energy of the reacting ion is less than that required for direct cleavage of the CC-bond; the molecular ions that escape the ion source undecomposed may then undergo fragmentation in other regions of the instrument. Audier and coworkers20,21 demonstrated that ionized aliphatic amines can isomerize by 1,2-NH3 migration prior to CC-cleavage.

Different isomerization reactions can take place when the amine carries βand/or γ-alkyl substituents, and the results of isotope labeling experiments led Audier19 to propose that these reactions proceed via intermediate ion-neutral complexes, in which ammonia is coordinated to ionized cyclopropane molecules. The reactions of similar complexes were previously studied by ICR-experiments.23-25 The term 'ion-neutral


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complex' is used here to describe two-body intermediates that correspond to proper minima on the potential energy surface and whose properties account for an array of reactions; the interaction between the charged and neutral components is predominantly electrostatic, and the components are separated by (at least) the sum of their van der Waals radii.26

A particularly thoroughly studied example19 is the metastable 2methylpentylamine radical cation, which reacts by loss of C2H5., C3H6, C3H7. and C4H9. (Figure 1, Scheme 2). Straight-forward simple cleavage of the α-CC bond would lead to loss of C5H11., and this is the predominant reaction of the energized molecular ions that react in the mass spectrometer ion source; however, it contributes very little to the reactions of the lower-energy metastable ions. The isomerization reactions proposed by Audier19 to precede loss of the various smaller alkyl radicals are exemplified in Scheme 2. Intramolecular 1,4-H-atom transfer yields γdistonic isomer(s), which can undergo ring closure to form [alkylcyclopropane−NH3]+. complexes, followed by ring opening with attachment of NH3 to a substituted cyclopropane carbon, forming an

isomeric amine molecular ion by subsequent hydrogen atom transfers. Overall, the process converts the parent molecular ion to an isomeric radical cation. This is in most cases an only marginally exothermic process, but it results in transfer of the amino group to a more highly

branched position. The fragmentation by CC cleavage of the isomerized radical cation therefore requires less energy than CC cleavage of the parent molecular ion. Ring-closure and ring-opening reactions like these have condensed-phase counterparts.27-31



Figure 1. MIKE spectrum of 2-methylpentylamine radical cations (cf. Scheme 2). Straight-forward -cleavage (→ m/z 30) is not important for the metastable molecular ions.

Scheme 2. The isomerization-fragmentation of 2-methylpentylamine radical cations according to Audier.19 In the present study we use isotope labeling experiments together with computational methods not available when these proposals were first advanced, to examine the intermediacy of the putative ion-neutral complexes of ionized cyclopropanes and NH3 or CH3NH2. We explore

the influence that alkyl substituents on nitrogen and on carbon exert on the energetics of complex formation, to examine when reaction via these intermediates may compete successfully with other possible reactions of the initial molecular ion, in particular with simple α-cleavage. The amines chosen for study are primary and secondary with alkyl substituents in the


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β- and/or γ-positions, deuterium labeled as required, to complement the results presented by Audier.19 Methods Mass spectrometry. The spontaneous fragmentation reactions of metastable amine molecular ions formed by 70 eV electron ionization were studied in the gas phase with mass analyzed ion kinetic energy (MIKE) spectroscopy32 in a four-sector double focusing Jeol JMSHX110/HX110A mass spectrometer (EBEB geometry) in three-sector mode; the mass-selected ions react in the field-free region in front of the second electric sector, typically some 50-100 µsec after ionization. Early results were obtained on a very large custom built instrument at the University of New South Wales. The MIKE technique has the particular advantage over conventional mass spectrometry that it makes it possible to monitor the reactions of specific, mass selected ions rather than the collection of reactions of different ionic species that take place in mass spectrometer ion sources. Furthermore, the ion-source reactions remove most high internal energy molecular ions, and the processes studied by the MIKE technique are therefore those of the remaining not-very-highly energized ions. The MIKE spectra of N-deuterated primary and secondary amines were recorded after exchange of surface-bonded active hydrogen in the mass spectrometer inlet system with deuterium by first admitting D2O at

a relatively high pressure for an extended period of time.

Synthesis and computational methods. The unlabeled primary and secondary aliphatic amines were prepared by standard methods, in most cases by reduction of the appropriate nitrile or amide with LiAlH4, or by reductive amination of the appropriate ketone. Primary amines labeled

with deuterium at the α-position were obtained by LiAlD4 reduction of

the corresponding nitrile. Purity and identity was ascertained with GC/MS and 13C NMR.

Heats of formation were derived from total energies calculated with the G3 and G3(MP2) composite ab initio methods,33,34 slightly modified in that the geometry optimization was performed at the UMP2(full)/6-



31+G(d,p) level. These methods have been shown to yield accurate estimates of the thermochemical properties of ions and radicals.35-41 The

energies were obtained with the Gaussian 09 suite of programs42 and

converted to 298 K heats of formation as described by Nicolaides et al.;43 the required auxiliary thermochemical data were taken from the compilation by Chase.44 Additional results were obtained with the

G3//B3LYP and G3(MP2)//B3LYP methods,45 to examine if the results

would be sensitive to the different methods used to determine structure and vibrational frequencies. They were not. Results Mass spectrometry. The outcome of the fragmentation reactions of nearly all metastable primary amine radical cations with up to six carbon atoms is described by their MIKE spectra (included in Table 2). Most product ions arise by loss of alkyl radicals from the molecular ion, directly or after isomerization; other reactions contribute occasionally, such as elimination of NH3 molecules. Metastable molecular ions with a terminal amino group, RCH2NH2+., do not undergo simple cleavage of the initial reactant ion (loss of R.) to any great extent, whereas those molecular ions that have non-terminal amino groups, R1R2CHNH2+., are observed to lose R1 or R2. Previous work has shown that the fragmentation of metastable primary and secondary amine radical cations is preceded by CH/NH hydrogen atom exchange,14-16,19-21,46 demonstrating that γ- and δdistonic isomers of the molecular ions are easily formed by intramolecular hydrogen atom abstraction. The hydrogen atoms bonded to the α-carbon atom do not normally participate in the reversible intramolecular hydrogen atom exchange, which allows us to use deuterium labeling at this site as a marker, to monitor the position of a key molecular fragment after skeletal isomerization. As an example, metastable 2-ethylbutylamine radical cations expell C2H5., C3H7. and C4H9. radicals, to yield product ions of m/z 72, 58, and 44. The reactions of the ,-d2 analog give rise to m/z 74, 58, 44 ions (Scheme 3), showing that the propyl and butyl radicals

expelled incorporate the α-CH2 group whereas the ethyl radical does not;

for this to happen, rearrangement involving migration of the amino group


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necessarily precedes the final CC bond cleavage.

Scheme 3. The three prominent cleavage reactions of the metastable (1,1-D2)-2ethylbutylamine radical cation.

The MIKE spectra of secondary amine molecular ions with substituents at C(2) and/or C(3) show that these undergo straight-forward simple cleavage almost exclusively, by loss of the alkyl group(s) bonded to the αcarbon atom(s). Smaller signals (less that 10% rel int) show that the alkyl group bonded to the β-carbon atom can also be expelled, but loss of other alkyl radicals is not observed, in contrast to the reactions of the primary amine molecular ions. Extensive deutrium labeling was performed (particularly of N-methyl-2-methylpentylamine) in an attempt to tease out even minor products of skeletal rearrangement, but without success. Computational results. High-level ab initio calculations have shown4752 that the structure of ionized cyclopropane is that of an isosceles triangle with one long and two short CC bonds, about 1.85 Å and 1.53 Å, respectively (values vary slightly with method of computation and with basis set). We find that the formation of complexes of cyclopropane radical cations and NH3 or CH3NH2 does not significantly modify the

structure of the hydrocarbon part. A similar situation applies with alkyl substituted cyclopropane radical cations. In the more stable form of the [C3H6−NH3]+. complex, the NH3 is situated over the ring plane, close to the long CC bond. The amino group can occupy a variety of positions with respect to the ring when alkyl substituted cyclopropane radical cations form complexes with NH3 or CH3NH2 (Scheme 4) in a manner reminiscent of that observed for protonated cyclopropanes:53 over the ring plane close to the long CC bond (face), over a hydrogen atom of a ring methylene group (corner), in the ring plane close to the long CC bond (edge). The face position is generally preferred when the NH3 is trans to a



cyclopropane alkyl substituent, and the corner position when the two are cis to each other; the edge position can be favored in complexes such as that between 1,1-dimethylcyclopropane and NH3. However, the heats of

formation of these various isomeric forms are in most cases within af few kJ mol−1 of each other, and interconversion appears to be facile.

Scheme 4. The corner, face and edge complexes of NH3 and ionized cyclopropanes.

Alkyl substitution considerably stabilizes ionized cyclopropanes as well as their complexes with NH3 and CH3NH2. The magnitude of the interaction between the ionized hydrocarbon and NH3 or CH3NH2 is

relatively independent of substitution. It amounts to about 45 kJ mol−1, calculated as ΔHf(complex) − ΣΔHf(components).

It is not a priori obvious if an ion-neutral complex of methyl-

cyclopropane and CH3NH2 would be better described as ionized

methylcyclopropane interacting with CH3NH2 or as neutral methylcyclopropane interacting with CH3NH2+.. The electrostatic interactions in the two limiting forms of the complex are not easily assessed, but the structure determined with MP2 and DFT methods strongly suggests that the hydrocarbon carries the charge (one very long CC bond). There is no corresponding problem with regard to cyclopropane complexes involving NH3 or the complexes of CH3NH2 and

disubstituted cyclopropanes: in these, the charge is on the cyclopropane component. Conversely, the structure of complexes of cyclopropanes and dialkylamines corresponds to that expected of a dialkylamine radical cation interacting with the neutral hydrocarbon, which agrees well with the difference between the ionization energies of the components. The intermediate cyclopropane complexes involved in the isomerization of amine radical cations are formed by cyclization of γ-

distonic isomers of the ionized amines; subsequent ring opening of the cyclopropane complexes produces (different) γ-distonic isomers of amine


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radical cations.19 The facile interconversion of amine molecular ions and their distonic isomers by intramolecular hydrogen atom transfer has been described in detail.14 Many attempts notwithstanding, we have not succeeded in locating transition states for the ring closure and ring opening steps. Our results suggest that regardless of the position of the NH3 with respect to the

cyclopropane ring, the lower-energy pathways will involve the anti (or trans) forms of the γ-distonic ions in both steps, which agrees with the results of Shaik et al.54 These authors also suggest that the ring opening takes place without an appreciable energy barrier when NH3 approaches ionized cyclopropane: that may not be the case when the cyclopropane carries stabilizing substituents. The complexes are well-characterized minima, but in the absence of certainty with regard to the barriers for formation and ring-opening there is no good evidence that they would be particularly long-lived species.



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Table 1. Neutral and ionized cyclopropanes, and the ionic complexes of cyclopropanes and NH3 or CH3NH2.a

cyclopropane neutral

1

2

3

4

5

c-C3H6

CH3C3H5

C2H5C3H5

C3H7C3H5

isoC3H7C3H5

ΔHf

CCb

56

1.50

26

3

‒18

‒28

1.50

1.50

1.50

1.50

ion ΔHf

CCb

complex with NH3 ΔHf CCb N-posc

complex with CH3NH2 ΔHf CCb N-posc

1009

1.83

914

933

1.82

face

935

1.82

corner

858d 859e

1.82

face

1.83

corner

830d 833e

1.82

face

1.83

corner

928

898

869

861

1.84

1.84

1.83

1.84

1.82

face

838d 841e

1.82

face

1.83

corner

811d 813e

1.82

face

1.83

corner

783d 785e

1.82

face

1.82

corner

775d 776e

1.82

face

1.83

corner

6

cis-1,2-(CH3)2C3H4

1

1.51

858

1.86

770f

1.84

face

791g

1.84

face

7

trans-1,2-(CH3)2C3H4

−4

1.51

856

1.84

772

1.83

corner

791

1.83

corner

8

1,1-(CH3)2C3H4

−10

1.50

856

1.85

794

1.84

edge

9

1-C2H5-1-CH3C3H4

−31

1.50

828

1.85

772

1.84

corner

777

1.83

edge

745

1.84

corner

750

1.84

edge

10 trans-1-C2H5-2-CH3C3H4 −27

1.50

828

1.84

745

1.83

corner

765

1.83

corner

11 cis-1-C2H5-2-CH3C3H4

−22

1.51

829

1.86

744

1.85

face

764

1.84

face

12 1,1,2-(CH3)3C3H3

−35

1.51

792

1.87

710

1.86

corner

730

1.86

face

716

1.87

edge

13 trans-1,2,3-(CH3)3C3H3

−28

1.51

812

1.85

730

1.83

face

14 1-C2H5-1,2-(CH3)2C3H3

−56

1.51

765

1.88

684

1.87

corner

a

G3 heats of formation in kJ mol−1 (298 K), CC-distances in Å

b

Length of the long CC bond of the ring

c

Position of the nitrogen atom with respect to the ring (see Schme 4).

d

Alkyl group and N trans

e

Alkyl group and N cis

f

NH3 trans to the methyl groups

g

CH3NH2 trans to the methyl groups


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Table 2. Heats of formation of primary amine radical cations, of the NH3 complexes of ionized cyclopropanes, and of the products of -cleavage of the initial molecular ion (G3, kJ mol−1, 298 K).

amine

molecular simple αion cleavage

CH3CH2CH2NH2

778

873

complexa

914 (1)

-α-d2 CH3CH2CH2CH2NH2

751

852

838c (2)

-α-d2 (CH3)2CHCH2NH2

740

842

838c (2)

-α-d2 CH3CH2CH2CH2CH2NH2

726

832

811 (3)

(CH3)2CHCH2CH2NH2

718

826

772 (8)

--d2 CH3CH2(CH3)CHCH2NH2

717

821

811 (3)

-α-d2 CH3CH2(CH3)CHCH2NH2

717

821

770 (6,7)

695

807

772 (8)

--d2 CH3CH2CH2CH2CH2CH2NH2

703

811

783 (4)

--d2 CH3CH2CH2(CH3)CHCH2NH2

693

800

783 (4)

-α-d2 CH3CH2CH2(CH3)CHCH2NH2

693

800

745 (9)

-α-d2 CH3CH2(CH3)CHCH2CH2NH2

isomerization via complex

30

30

no

30,32

32

44

30,44

44

32,44,46

44

30,44

44

32,46

58

44,45,58w

possibly

696

805

745 (9)

(CH3)2CH(CH3)CHCH2NH2 688

794

775 (5)

-α-d2 (CH3)2CH(CH3)CHCH2NH2 688

794

710 (12)

possibly

yes

no

58

58br

58

58br

58

44,58

no

44

44,60d 44,58

44,60d

yes

44 58

32

no

58

34

58,72

44

58,74

45,46

58,72

no

44,58,72

44,58,60,74e

no

60,72 44,58,72

44,58,72

yes

yes

44,58,74

-α-d2

-α-d2

product ions observedb,c

58,60

-α-d2 (CH3)3CCH2NH2

product ions if complexb

44,58,60,74e

58

58br

58,60

58br,60br

58,72

44,58

60,72

44,58

44,58

44,58

44,58

44,58

(continued)


no

yes


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Table 2 (continued). Heats of formation of primary amine radical cations, of the NH3 complexes of ionized cyclopropanes, and of the products of -cleavage of the initial molecular ion (G3, kJ mol−1, 298 K).

amine

molecular simple αion cleavage

CH3CH2(CH3)2CCH2NH2 -α-d2

671

786

CH3CH2(CH3)2CCH2NH2

671

786

complexa

product ions if complexb

product ions observedb,c

745 (9)

72 72,74

72d (br)(44w,58w) no 74d (br)(44w,58w)

710 (12)

44,58

72d (br)(44w,58w) yesf

-α-d2 (CH3CH2)2CHCH2NH2

44,58 693

801

744 (10,11)

-α-d2


74d (br)(44w,58w)

44,58,72

44,58,72

44,58,74

44,58,74

yes

CH3CH2(CH3)CHNH2

734

788

838c (2)

44

43,44

no

CH3CH2CH2(CH3)CHNH2

707

768

770 (6,7)

44

44

no

(CH3)2CHCH2(CH3)CHNH2

672

742

710 (12)

44,58

44,58

yes

CH3CH2(CH3)CH(CH3)CHNH2 673

737

730 (13)

44

43

no

CH3CH2(CH3)CHCH2(CH3)CHNH2 650

721

684 (14)

44,72,86

44,72,86

yes

a

See Table 1.

b

Ring opening and fragmentation of a complex formed by hydrogen abstraction from and ring closure to the boldfaced position; w indicates weak signal (other reactions more important), br indicates broad signal

(significant translational energy release). c

MIKE spectrum (peak intensity > 5% rel int). In a few instances also significant peaks corresponding to loss of

NH3; the intensity of these signals varies unsystematically. d Loss C2H5 involves exclusively the intact C2 substituent. e Loss C3H7 involves exclusively the intact C2 substituent, loss C3H5D2 occurs via complex.

f

Reaction via trimethylcyclopropane complex 20%, via dissociation-recombination 80% [57].


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Discussion In a sector mass spectrometer, the internal energy of the metastable molecular ions is delimited by the energetics of the ion-source fragmentation reactions. The predominant unimolecular reaction of longchain primary amine radical cations, RCH2NH2+., is α-cleavage, which typically requires some 100 kJ mol-1. The rate constant, k(E), for this reaction, a simple bond fission, will rise steeply with increasing internal energy, E, and the internal energy of those molecular ions that did not decompose in the ion source will, in turn, be upwardly delimited by the α-cleavage critical energy. These are the molecular ions that may be examined in the mass spectrometer field-free regions and give rise to metastable peaks. They will to a considerable extent be present as distonic isomers of the initial species, because primary amine radical cations undergo exothermic, reversible, intramolecular hydrogen atom abstraction ractions with five-membered or larger cyclic transition states.14 The γ-distonic isomers may undergo cyclization as described by

Audier,19 an intramolecular radical substitution,55,56 forming a loosely bonded complex of NH3 and an ionized cyclopropane (Scheme 5).

Scheme 5. Isomerization of the 2-methylbutylamine radical cation to an NH3complex of the 1,2-dimethylcyclopropane radical cation.

Ring opening with concurrent attachment of the NH3 to a substituted

cyclopropane carbon atom gives rise to a different γ-distonic ion, and in turn to an isomeric amine molecular ion (Scheme 6).



Scheme 6. Ring-opening of the complex of the 1,2-dimethylcyclopropane radical cation and NH3. CC cleavage in aliphatic amine radical cations requires considerably less energy when the amino group is at a branching point, and the cyclopropane isomerization opens the way for simple cleavage reactions unavailable to the parent molecular ion. This will be important for reactant amine radical cations with insufficient internal energy for straight-forward simple cleavage, that is, for metastable amine molecular ions. The potential energy profile in Figure 2 illustrates the interconversions.

Figure 2. Potential energy profile describing the isomerization of the 2-methylbutylamine radical cation to the 2-pentylamine radical cation and subsequent fragmentation (G3 energies of stationary points, y-axis, kJ mol-1). Molecular ions with sufficient internal energy to undergo simple cleavage do so rapidly in the mass spectrometer ion source (loss of C4H9.). Molecular ions with less may isomerize and then undergo different simple cleavage reactions (here: loss of C3H7.).


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Fragmentation demonstrates isomerization. The isotope labeling results presented by Audier et al.19-21 form the core evidence for the proposal

that [cyclopropane–NH3]+. complexes mediate the isomerization of many primary amine radical cations, and show that the ring-opening and

subsequent fragmentation reactions can be predicted with confidence. The results included in Table 2 compare the observed fragmentation reactions of each metastable amine radical cation to those expected if a (substituted) [cyclopropane-NH3]+. complex would be an intermediate. The predictions are particularly exacting when the results also include the reactions of the appropriate -labeled analog. The fragmentation of 2ethylbutylamine shown in Scheme 3 illustrates this. Often, isomerization of the molecular ion may give rise to more than one γ-distonic isomer and, in turn, to more than one possible complex. In Table 2, bold-face indicates the particular γ-position involved in ring closure. Cyclopropanes stabilized by substitution. The formation of a [cyclopropane–NH3] intermediate from ionized propylamine is for

thermochemical reasons unlikely to precede fragmentation: simple

cleavage requires less energy (Table 2). Thermochemically, complexes of NH3 and monosubstituted cyclopropanes would be possible

intermediates when longer-chain primary amines react, even though the barriers involved to ring-closure and ring-opening are not known (v.s.); however, the fragmentation of straight-chain metastable molecular ions suggests that their reactions in most cases do not proceed via complexes (Table 2). Formation of NH3-complexes of disubstituted cyclopropanes requires

less energy than simple CC-cleavage for primary metastable amine

molecular ions with substituents at C2 and/or C3 (Table 2, Figure 2), and the fragmentation reactions confirm that intermediate complexes are in all likelihood involved. A 1,1,2-trisubstituted cyclopropane‒NH3 complex is even sufficiently stable to allow α-branched amine radical cations such as 1,3-dimethylbutylamine and 1,3-dimethylpentylamine to undergo fragmentation after isomerization via an intermediate complex, even though the presence of the α-substituent significantly reduces the energy required for simple cleavage of the initial molecular ion (Figure 3). A related thermochemical argument explains why the 1,2-



dimethylbutylamine radical cation does not react via an NH3-complex of

1,2,3-trimethyl cyclopropane. This would not be adequately stabilized

(only two substituents at the long CC-bond; Table 2); also, cyclopropane isomerization would not give rise to a molecular ion isomer with lower αcleavage requirements.

Figure 3. Potential energy profile for the reactions of the 1,3-dimethylbutylamine molecular ion. No ring closure in primary γ-distonic ions. Amine radical cations branched at C2 have two γ-distonic isomers (Scheme 7). We find, in disagreement with Audier's suggestions,19 that complex formation by ring closure involving a primary C3' position does not take place to any appreciable extent. Cyclization via the C3' -distonic isomer would in all cases result in molecular ion isomers that would undergo other CC cleavage reactions than those observed (Table 2).


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Scheme 7. -Distonic isomers of the 2-methylbutylamine radical cation. Cyclization involving the primary radical (C3') in this and similar -distonic ions does not contribute significantly to the rearrangement-fragmentation. The fragmentation of the α-labeled 2,2-dimethylbutylamine radical cation provides illustration: reaction via a primary γ-distonic ion and the corresponding intermediate cyclopropane would produce a symmetric species (Scheme 8), but the isotopic substitution establishes that the isomerization-fragmentation does not involve an intermediate with two equivalent ethyl groups. Only the ethyl group present in the initial molecular ion is lost by fragmentation (Table 2). 2-Methylbutylamine ions react analogously, only expelling the original ethyl group. It has been demonstrated elsewhere57 that the mechanism of this reaction involves dissociation-recombination (Scheme 9).

Scheme 8. Cyclopropane mediated isomerization of the 3-methylpentylamine radical cation yields the symmetrical 3-methyl-3-hexylamine isomer. 2,2Dimethylbutylamine ions do not react in this manner. Bullet indicates CD2. The reactions of the 3-methylpentylamine radical cation confirm that isomerization via the 1-ethyl-1-methylcyclopropane complex does indeed result in an amine radical cation with two equivalent ethyl groups (Scheme 8): The final loss of an ethyl radical is accompanied by a wide, flattopped metastable peak, demonstrating the presence of an enthalpy barrier in the exit channel (probably isomerization58). The reactions of the -labeled analog give rise to two wide MIKE peaks of equal size, loss of . . C2H5 and C2H3D2 , confirming the intermediacy of a symmetrical

species (Scheme 8).



Scheme 9. Expulsion of an ethyl radical from 2,2-dimethylbutylamine radical cations does not involve a cyclopropane intermediate; instead, CC cleavage is followed by CN bond formation and -cleavage of the resulting isomerized molecular ion.57 Formation of cyclopropane complexes by cyclization of primary distonic ions does not compete efficiently with other isomerization reactions because the precursor, the primary C3' distonic isomer, is not formed to any great extent. That would require initial hydrogen atom abstraction from a primary position, which is less facile and less favorable than abstraction from secondary or tertiary positions,14 and would often give rise to less stable cyclopropane complexes (Table 1). Audier's observation19 that hydrogen atom exchange between NH3 and the 2methyl group in 2-methylpentylamine is not very extensive provides direct support of this interpretation. Competing reactions. When the alkyl chain is sufficiently long, sequential intramolecular hydrogen atom abstraction reactions can give rise to distonic isomers, and isomerization by 1,2-migration of NH3 can become a competing reaction.20-22 Cyclopropane-isomerization can also take place in direct competition with dissociation-recombination.57

Secondary amines. The unimolecular reactions of the metastable molecular ions of simple secondary amines with substituents at C2 and/or C3 appear not to involve intermediate cyclopropane complexes. Fragmentation is almost exclusively by straight-forward simple cleavage (Figure 4), which agrees well with the thermochemistry; complex formation would require more energy than simple cleavage (if only just59) (Table 3, Figure 5). Extensive CH/NH-exchange prior to fragmentation indicates that the γ-distonic isomers required for the possible cyclization are indeed accessible (Scheme 10).


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Scheme 10. CH/NH-exchange precedes fragmentation of metastable secondary amine molecular ions..



Figure 4. MIKE spectrum of N-methyl-2-methylpentylamine. Fragmentation via intermediate cyclopropane complexes does not contribute materially; it would have given rise to product ions of m/z 85 by loss of C2H5..

Figure 5. Potential energy profile for the possible isomerization-fragmentation of N-methyl-2-methylbutylamine radical cations. However, ions with sufficient internal energy to form the cyclopropane complex will react more rapidly by simple cleavage (loss of C4H9. ).


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Table 3. Heats of formation of secondary amine radical cations, of the hypothetical CH3NH2 complexes of ionized cyclopropanes, and of the products of -cleavage of the molecular ions (G3, kJ mol−1, 298 K).

amine

molecular ion

simple complexa α-cleavage


CH3CH2CH2NHCH3

721

831

933 (1b)

no

CH3CH2CH2CH2NHCH3

696

811

858 (2b)

no

(CH3)2CHCH2NHCH3

687

801

858 (2b)

no

CH3CH2(CH3)CHNHCH3

679

754cc

858 (2b)

no

CH3CH2(CH3)CHCH2NHCH3

663

780

830 (3b)

no

CH3CH2(CH3)CHCH2NHCH3

663

780

791 (6b,7b)

no

(CH3)2CHCH2CH2NHCH3

665

785

794 (8b)

no

CH3CH2(CH3)2CCH2NHCH3

625

745

730 (12b)

possiblyb

(CH3)2CH(CH3)CHCH2NHCH3 636

753

730 (12b)

possiblyb

a See Table 1. b Cf ref. 59.

Related systems; metastable ether molecular ions Rearrangement reactions resulting in modification of the heavy-atom connectivity via cyclopropane-type isomerization occurs not only for primary amine radical cations; closely related reactions are encountered also for ionized ethers. As suggested by Audier55 and later supported by results reported by Bowen and colleagues,60-63 the loss of alkyl radicals or alkane molecules from metastable ether molecular ions can be preceded by ring-closure/ring-opening via intermediate [cyclopropane— alcohol]+. complexes. These reactions take place much as described above for ionized primary amines. Isotope labeling demonstrates that the methylene groups of the C3-



chain in ionized methyl and ethyl propyl ethers lose their positional identity before fragmentation by CC-cleavage (expulsion of C2H5.) of the metastable molecular ion,55,56 strongly suggesting reversible interconversion of the γ-distonic isomers, by cyclization-ring opening (Scheme 11).

Scheme 11. Methylene group permutation before fragmentation of metastable propyl ether molecular ions. The metastable molecular ions of ethers of longer-chain primary alcohols eliminate ethyl radicals after skeletal rearrangement.64-68

Colburn, Derrick and Bowen63 recently provided deuterium labeling results directly supporting the suggestion55,56 that this transformation involves γ-distonic molecular ion isomers that undergo ring-closure/ringopening via [cyclopropane–alcohol]+. ion-neutral complexes (Scheme 12, cf. Schemes 5 and 6). Hudson and McAdoo65 earlier reported similar labeling results, interpreted differently.

Scheme 12. The fragmentation of isotope labeled metastable pentyl and 2-pentyl ether molecular ions demonstrate the presence of symmetrical intermediates; 50% label incorporation in the alkyl or alkane fragment eliminated. Double arrows indicate intermediate hydrogen atom transfer steps.


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Ethers of secondary alcohols also isomerize via cyclopropane intermediates, resulting in modification of the carbon skeleton prior to fragmentation.55,56 The putative complex intermediates are not much higher in energy than the initial ether molecular ions; the over-all rate of isomerization is limited by the hydrogen atom abstraction steps, which typically involve energy barriers of 50-60 kJ mol-1 (Schemes 13 and 14). However, the final fragmentation, by loss of alkyl radicals or of alkane molecules, reflects the energetics of the ion-source reactions, which provide an upper limit to the internal energy of the metastable ions. As first pointed out by Traeger et al.,67 molecular ions with internal energy in excess of the critical energy of the prominent ion-source reactions will react rapidly in the ion source and not contribute to the reactions of the metastable ions. Simple CC-cleavage in the intact molcular ion of ethers of primary alcohols such as methyl pentyl ether requires some 130 kJ mol-1, whereas cleavage after rearrangement (Scheme 13) to a more highly branched molecular ion isomer requires considerably less. Most of the molecular ions of ethers of primary alcohols that escape undecomposed from the ion source will react in this manner, expelling an ethyl radical.

Scheme 13. Stationary points involved in the rearrangement-fragmentation of methyl pentyl ether molecular ions. 298 K heats of formation in kJ mol-1 derived from G3//B3LYP energies. Fragmentation after isomerization by loss, instead, of an ethane molecule would be thermochemically favored (Scheme 13); however, this reaction is apparently not sufficiently rapid to compete effectively with what is, after ring-opening, a simple cleavage. Photoionization results for related systems indicate that the critical energy of loss of ethane will be 25 kJ mol-1 lower than the critical energy of loss of ethyl.67,69



Conversely, the ion-source simple cleavage of ethers of secondary alcohols takes place at a branching point and requires only about 60 kJ mol-1, which limits the internal energy of the metastable molecular ions much more severely, to the point where simple alkyl radical loss no longertakes place to a significant degree. Cyclopropane isomerization requires less energy than CC-bond cleavage, and systematic isotope labeling55,56 demonstrates that it occurs prior to fragmentation (Scheme 12). However, isomerization does not provide a route to alternative simple CC-cleavage reactions; less demanding reaction pathways dominate, such af loss of alkane molecules (Scheme 14). Loss of an alkane molecule is expected to have a slightly lower critical energy than loss of the corresponding alkyl radical.67,69

Scheme 14. Loss of propyl from ethyl 2-pentyl ether radical cations and of propane from the metastable molecular ions. 298 K heats of formation in kJ mol-1 derived from G3//B3LYP energies. Concluding remarks. The isomerization-fragmentation of metastable primary amine molecular ions via intermediate cyclopropane complexes described here is not exceptional. The very closely related reactions of ionized ethers suggest that saturated aliphatic radical cations would often react in a similar manner. For the amine isomerization to compete sucessfully with straight-forward simple cleavage, the cyclopropane must be stabilized by substitution, which is why only amines with substituents at C2 or C3 react in this manner. The thermochemistry of the molecular-ion/cyclopropanecomplex interconversion of ether radical cations does not introduce similar constraints. The isomerization of the molecular ion is particularly important for radical cations with insufficient internal energy to undergo straightforward simple cleavage, the metastable molecular ions. Ring closure to


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form the intermediate cyclopropane complex takes place in -distonic molecular ion isomers; those formed by H-abstraction from CH3-groups do not contribute significantly. Secondary and tertiary amine radical

cations do not isomerize in a similar manner; the -distonic isomers can be formed, but the intermediate cyclopropane complexes are too high in energy to allow competition with simple cleavage of the molecular ions. The thermochemical properties of the putative intermediate substituted [cyclopropane–NH3]+. complexes account successfully for the skeletal rearrangement of many amine radical cations, and, as importantly, for the absence of rearrangement for closely related ions. Acknowledgement. We are grateful to professor Peter. J. Derrick for the opportunity to perform early studies on an unusually large double focusing custom built mass spectrometer (MMM63) at the University of New South Wales. Supporting information. Tabulated MIKE spectra of D- and 13C-labeled ethyl 2-pentyl ether radical cations.



Literature references 1

Greene, F. D.; Savitz, M. L.; Osterholtz, F. D.; Lau, H. H.; Smith, W. N.; Zanet, P. M. Decomposition of tertiary alkyl hypochlorites. J. Org. Chem. 1963, 28, 55-64.

2

Walling, C.; Padwa, A. Positive halogen compounds. VI. Effects of structure and medium on the β-scission of alkoxyradicals. J. Am. Chem. Soc. 1963, 85, 1593-97.

3

Heusler, K.; Kalvoda, J. Intramolecular free-radical reactions. Angew. Chem. Int. Ed. 1964, 3, 525-96.

4

Choo, K. Y.; Benson, S. W. Arrhenius parameters for the alkoxy radical decomposition reactions. Int. J. Chem. Kinet. 1981, 13, 833-44.

5

Walling, C.; Clark, R. T. Reactions of primary and secondary alkoxyradicals derived from hypochlorites. J. Am. Chem. Soc. 1974, 96, 4530-34.

6

Hornung, G.; Schalley, C. A.; Dieterle, M.; Schröder, D.; Schwarz, H. A study of the gas-phase reactivity of neutral alkoxy radicals by mass spectrometry: α-cleavages and Barton-type hydrogen migrations. Chem. Eur. J. 1997, 3, 1866-83.

7

Fossey, J.; Lefort, D.; Sorba, J. Free Radicals in Organic Chemstry. Wiley, Chichester, 1995.

8

McLafferty, F. W.; Gohlke, R. S. Mass spectrometric analysis. Aliphatic amines. Anal. Chem. 1962, 34, 1281-87.

9

Chow, Y. L.; Danen, W. C.; Nelsen, S. F.; Rosenblatt, D. H. Nonaromatic aminium radicals. Chem. Rev. 1978, 78, 243-74.

10

Sozzi, G.; Denhez, J. P.; Audier, H. E.; Vulpius, T.; Hammerum, S. Intramolecular hydrogen atom abstraction with an eight-membered cyclic transition state in openchain aliphatic aminium radicals. Tetrahedron Lett. 1985, 26, 3407-8.

11

Hammerum, S.; Norrman, K.; Sølling, T. I.; Andersen, P. E.; Jensen, L. B.; Vulpius, T. Competing simple cleavage reactions: the elimination of alkyl radicals from amine radical cations. J. Am. Chem. Soc. 2005, 127, 6466-75.

12

Mandeville, S. J.; Bowen, R. D.; Trikoupis, M. A.; Terlouw, J. K. The influence of the size and structure of a spectator alkyl group on the relative rates of alkyl radical elimination from ionised tertiary amines. Europ. Mass Spectrom. 1999, 5, 339-51.

13

McLafferty, F. W.; Tureček, F. Interpretation of Mass Spectra. 4th ed. University Science Books. Sausalito, CA, 1993.

14

Hammerum, S.; Nielsen, C. B. Intramolecular hydrogen bonding and hydrogen atom abstraction in gas-phase aliphatic amine radical cations. J. Phys. Chem. A 2005, 109, 12046-53.

15

Hammerum, S. Rearrangement and hydrogen abstraction reactions of amine cation radicals; a gas-phase analogy to the Hofmann-Löffler-Freytag reaction. Tetrahedron Lett. 1981, 22, 157-60.

16

Hammerum, S.; Christensen, J. B.; Egsgaard, H.; Larsen, E.; Derrick, P. J.; Donchi, K. F. Slow alkyl, alkene, and alkenyl loss from primary alkylamines; isomerization of the low-energy molecular ions prior to fragmentation in the microsecond timeframe. Int. J. Mass Spectrom. Ion Phys. 1983, 47, 351-54.

17

Sozzi, G.; Audier, H. E.; Denhez, J. P.; Milliet, A. Réarrangement des cations radicaux aminés en phase gazeuse, le cas de l'isopentylamine. Nouv. J. Chim. 1983, 7, 735-40.


Page 26 of 31

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18

Audier, H. E.; Milliet, A.; Sozzi, G.; Denhez, J. P. The isomerization mechanism of alkylamines: structure of the C2H6N+ and C3H8N+ fragment ions. Org. Mass Spectrom. 1984, 19, 79-81.

19

Audier, H. E.; Denhez, J. P.; Milliet, A.; Sozzi, G. Transposition des cations radicaux aminés en spectrométrie de masse. Can. J. Chem. 1984, 62, 931-38.

20

Audier, H. E.; Milliet, A.; Denhez, J. P. Fragmentation of linear primary amines: origin of the m/z 44 fragment ions. Org Mass Spectrom. 1983, 18, 131-2.

21

Audier, H. E.; Sozzi, G.; Denhez, J. P. Réactions unimoléculaires des cations radicaux [CH3(CH2)n-1NH2]+.. Tetrahedron 1986, 42, 1179-90. Hammerum, S. The 1,2-migration of NH3 in protonated β-aminoalkyl radicals. Int. J. Mass Spectrom. 2000, 199, 71-78. Gross, M. L.; McLafferty, F. W. Identification of C3H6+. structural isomers by ion cyclotron resonance spectroscopy. J. Am. Chem. Soc. 1971, 93, 1267-68. Gross, M. L. An ion cyclotron resonance study of the structure of C3H6+. and the mechanism of its reaction with ammonia. J. Am. Chem. Soc. 1972, 94, 3744-48.

22 23 24 25

Gross, M. L.; Lin, P.-H. Analytical applications of ion-molecule reactions. Reactions of various substituted cyclopropane molecular ions with ammonia. Org. Mass Spectrom. 1974, 9, 1194-99.

26

Hammerum, S. in Jennings, K. R. (ed), Isomers and isomerization of molecular ions: the formation of stable ion-neutral complexes during unimolecular dissociation. Fundamental Aspects of Gas Phase Ion Chemistry, Kluwer, Dordrecht, 1991; pp 379-90.

27

Drury, R. F.; Kaplan, L. 3-Iodopropyl radical. Closure to cyclopropane. J. Am. Chem. Soc. 1973, 95, 2217.

28

Curran, D. P.; Gabarda, A. E. Formation of cyclopropanes by homolytic substitution reactions of 3-iodopropyl radicals: preparative and rate studies. Tetrahedron 1999, 55, 3327-36.

29

Dinnocenzo, J. P.; Simpson, T. R.; Zuilhof, H.; Todd, W. P.; Heinrich, T. Three-electron SN2 reactions of arylcyclopropane cation radicals. J. Am. Chem. Soc. 1997, 119, 987. Dinnocenzo, J. P.; Zuilhof, H.; Lieberman, D. R.; Simpson, T. R.; McKechney, M. W. Three-electron SN2 reactions of alkylcyclopropane cation radicals. 2. Steric and electronic effects of substitution. J. Am. Chem. Soc. 1997, 119, 994.

30

31

Rao, V. R.; Hixson, S. S. Arylcyclopropane photochemistry. Electron-transfer-mediated photochemical addition of methanol to arylcyclopropanes. J. Am. Chem. Soc. 1979, 101, 6458.

32

Cooks, R. G.; Beynon, J. H.; Caprioli, R. M.; Lester, G. R. Metastable Ions. Elsevier, Amsterdam, 1973.

33

Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.; Pople, J. A. Gaussian-3 (G3) theory for molecules containing first and second-row atoms. J. Chem. Phys. 1998, 109, 7764-76.

34

Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Rassolov, V.; Pople, J. A. Gaussian-3 theory using reduced Møller-Plesset order. J. Chem. Phys. 1999, 110, 4703-09.

35

Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Pople, J. A. Assessment of Gaussian-3 and density functional theories for a larger experimental test set. J. Chem. Phys. 2000, 112, 7374-83.



36

Curtiss, L. A.; Raghavachari, K. Gaussian-3 and related methods for accurate thermochemistry. Theor. Chem. Acc. 2002, 108, 61-70.

37

Hammerum, S. Heats of formation of gas-phase ions calculated by composite ab initio procedures. Int. J. Mass Spectrom. Ion Proc. 1997, 165, 63-69.

38

Hammerum, S. Heats of formation and proton affinities by the G3 method. Chem. Phys. Lett. 1999, 300, 529-32.

39

Deakyne, C. Proton affinities and gas-phase basicities; theoretical methods and structural effects. Int. J. Mass Spectrom. 2003, 227, 601-16.

40

Simmie, J. M.; Somers, K. P. Benchmarking compound methods (CBS-QB3, CBSAPNO, G3, G4, W1BD) against the active themochemical tables: A litmus test for costeffective molecular formation enthalpies. J. Phys. Chem. A 2015, 119, 7235-46.

41

Somers, K. P.; Simmie, J. M. Benchmarking compound methods (CBS-QB3, CBSAPNO, G3, G4, W1BD) against the active thermochemical tables: Formation enthalpies of radicals. J. Phys. Chem. A 2015, 119, 8922-33.

42

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B. et al., Gaussian 09, Gaussian Inc, Wallingford CT, 2013.

43

Nicolaides, A.; Rauk, A.; Glukhovtsev, M. N.; Radom, L. Heats of formation from G2, G2(MP2), and G2(MP2,SVP) total energies. J. Phys. Chem. 1996, 100, 17460-64.

44

Chase, M. W. jr. J. Phys. Chem. Ref. Data, Monograph No. 9 (1998).

45

Baboul, A. G.; Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. Gaussian-3 theory using density functional geometries and zero-point energies. J. Chem. Phys. 1999, 110, 765057.

46

Hammerum, S.; Petersen, A. C. Isomerization and unimolecular fragmentation of straight-chain secondary amine radical cations in the gas phase. Acta Chem. Scand. 1998, 52, 1045-51.

47

Hudson, C. E.; Giam, C. S.; McAdoo, D. J. Structure of the cyclopropane cation radical and its energy barrier to pseudorotation. J. Org. Chem. 1993, 58, 2017-19.

48

Lunell, S.; Yin, L.; Huang, M.-B. Ab initio SCF and CI study of the cyclopropanetrimethylene radical cations. Chem. Phys. 1989, 139, 293-99.

49

Liu, Y.-J.; Huang, M.-B. Hyperfine structure of some hydrocarbon radical cations: a B3LYP and MP2 study. J. Mol. Struct. Theochem. 2001, 536, 133-42.

50

Skancke, A. Conversion of cyclopropane radical cation to propene radical cation. J. Phys. Chem. 1995, 99, 13886-89.

51

Krogh-Jespersen, K.; Roth, H. D. Computational study of Jahn-Teller type distortions in radical cations of methyl-substituted cyclopropanes. J. Am. Chem. Soc. 1992, 114, 8388-94.

52

Fokin, A. A.; Schreiner, P. R. Selective alkane transformations via radicals and radical cations: Insights into the activation step from experiment and theory. Chem. Rev. 2002, 102, 1551-94.

53

Vogel, P. Carbocation Chemistry. Elsevier, Amsterdam, 1985; pp 331.

54

Shaik, S.; Reddy, A. C.; Ioffe, A.; Dinnocenzo, J. P.; Danovich, D.; Cho, J.K. Reactivity paradigms: Transition state structure, mechanisms of barrier formation, and stereospecificity of nucleophilic substitutions on σ-cation radicals. J. Am. Chem. Soc. 1995, 117, 3205-22.


Page 28 of 31

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


55

Audier, H. E.; Sozzi, G.; Milliet, A.; Hammerum, S. Isomerization and fragmentation of aliphatic ether radical cations: Interconversion of distonic ions and cyclopropane intermediates. Org. Mass Spectrom. 1990, 25, 368-74.

56

Audier, H. E.; Milliet, A.; Sozzi, G.; Hammerum, S. Skeletal rearrangement of ether radical cations – 1,3-ROH migration or 1,3-CC interaction? Adv. Mass Spectrom. 1988, 11, 922-3. See also Supporting Information

57

Pedersen, A. H.; Nielsen, C. B. O.; Bojesen, G.; Hammerum, S. Isomerization of metastable amine radical cations by dissociation-recombination. Europ. J. Mass Spectrom. 2015, 20, 635-39.

58

Kalman, J. R.; Fairweather, R. B.; Williams, D. H.; Hvistendahl, G. Methyl radical expulsion occuring with kinetic energy release. J. Chem. Soc. Chem. Commun. 1976, 6045.

59

The thermochemical results (Table 3) indicate that fragmentation of 2,3-dimethyl-Nmethylbutylamine molecular ions could well involve a CH3NH2 complex of the 1,1,2trimethylcyclopropane radical cation, but this reactant was not examined, and our current instrumentation does not permit us to rectify this omission.

60

Bowen, R. D.; Suh, D.; Terlouw, J. K. Reactions of ionized dibutyl ether. Org. Mass Spectrom. 1994, 29, 791-805.

61

Bowen, R.D.; Derrick, P. J. Unimolecular reactions of ionized 4-methoxyheptane. J. Chem. Soc., Perkin 2, 1992, 1041-47.

62

Bowen, R. D.; Maccoll, A. Unimolecular reactions of ionized ethers. J. Chem. Soc., Perkin 2, 1990, 25, 147-55.

63

Colburn, A. W.; Derrick, P. J.; Bowen, R. D. Peter J. Derrick and the grand scale 'Magnificent Mass Machine' mass spectrometer at Warwick. Europ. J. Mass Spectrom., 2017, 23, 319-26.

64

Bernasek, S. L.; Cooks, R. G. The -cleavage reaction in ethers. Org. Mass Spectrom. 1970, 3, 127-29 Hudson, C. E.; McAdoo, D. J. Origin of the [C4H9O]+ ions in the mass spectrum of nbutyl ethyl ether. Org. Mass Spectrom. 1979, 14, 109-13.

65 66

Hudson, C. E.; Lerner, R. D.; Aleman, R.; McAdoo, D. J. Loss of alkanes from ionized branched chain ethers. Metastable decomposition from an electronically excited state? J. Phys. Chem. 1980, 84, 155-57.

67

Traeger, J. C.; Hudson, C. E.; McAdoo, D. J. Energy dependence of ion-induced dipole complex mediated alkane elimination from ionized ethers. J. Phys. Chem. 1990, 94, 5714-17.

68

Audier, H. E.; Bouchoux, G.; Hoppilliard, Y.; Milliet, A. The mechanism of formation of [C4H9O]+ ions from isobutyl ethyl ether. Org. Mass Spectrom. 1982, 17, 382-85. McAdoo, D. J.; Hudson, C. E.; Traeger, J. C.; Grose, A.; Griffin, L. L. Size effects in ionneutral complex-mediated alkane elimination from ionized aliphatic ethers. J. Am. Soc. Mass Spectrom. 1991, 2, 261-6

69



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The Possible Intermediacy of Cyclopropane Complexes in the

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