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Supplement F2: The Chemistry of Amino, Nitroso, Nitro and Related Groups.

Edited by Saul Patai Copyright 1996 John Wiley & Sons, Ltd.

ISBN: 0-471-95171-4

CHAPTER 6

Mass spectrometry of nitro and nitroso compounds

HELGE EGSGAARD

Environmental Science and Technology Department, Risø National Laboratory,

DK-4000 Roskilde, Denmark

Fax: +45-42-37-04-03; e-mail: Helge.Egsgaard@Risoe.dk

and

LARS CARLSEN

National Environmental Research Institute, Department of Environmental Chemistry, DK-4000 Roskilde, Denmark

Fax: 45-46-30-11-14; e-mail: mklc@hami1.dmu.dk

I. INTRODUCTION .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

250

II. MASS SPECTROMETRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

250

A. Unimolecular Dissociations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

251

B. Collision Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

253

C. Neutralization/Reionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

254

III. IONIZATION . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

255

A. Radical Cations

Ionization Energy . . . . . . . . . . . . . . . . . . . . . . . .

255

B. Radical Anions

Electron Affinity . . . . . . . . . . . . . . . . . . . . . . . . .

256

C. Chemical Ionization Proton Affinity . . . . . . . . . . . . . . . . . . . . . . .

258

IV. NITRO COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

258

A. Generalized Fragmentation Processes . . . . . . . . . . . . . . . . . . . . . . .

258

B. Simple Cleavages

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

259

C. Rearrangements .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

259

1.

Tautomerism: Nitro/aci-nitro radical cations . . . . . . . . . . . . . . . . .

259

2.

N O isomerization: Nitro/nitrite radical cations . . . . . . . . . . . . . .

262

3.

Aliphatic nitro compounds: Cyclization reactions . . . . . . . . . . . . .

262

D. Hydrogen Transfer Reactions: The ortho-Effect . . . . . . . . . . . . . . . . .

264

1.

Rearrangements due to ortho-effects . . . . . . . . . . . . . . . . . . . . . .

267

2.

ortho-Effects in anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

269

249

250

Helge Egsgaard and Lars Carlsen

 

E. Remote Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

270

1.

Remote oxidation of multiple carbon bonds . . . . . . . . . . . . . . . . .

270

2.

Remote oxidation of imine and ketenimine functionalities . . . . . . . .

273

3.

Competing oxidation of sulphur and carbon . . . . . . . . . . . . . . . . .

276

F. Polynitro Aromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

281

1.

Nitroadamantanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

281

2.

Diand tri-nitroaromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

281

G. Nitro-heteroaromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

281

1.

Nitroazoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

281

2.

3-Nitro-2H-chromenes and 3-nitrochromanes . . . . . . . . . . . . . . . .

284

H. Annelation Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

286

I. Protonated Nitro Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

287

V. NITROSO COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

290

VI. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

291

I. INTRODUCTION

During the last decade knowledge of the ion chemistry of nitro compounds in the gas phase has increased significantly, partly due to the more widespread use of specialized techniques. Thus various ionization methods, in particular electron impact ionization and chemical ionization, have been used extensively. In addition, structure investigations as well as studies on fragmentation pathways have involved metastable ion dissociations, collision activation and neutralization/reionization studies, supplementary to studies carried out in order to disclose the associated reaction energetics and reaction dynamics. In general, the application of stable isotopes plays a crucial role in the in-depth elucidation of the reaction mechanisms.

The rationalization of mass spectrometric investigations of nitro compounds has benefited significantly from numerous studies applying techniques adopted from photochemistry, such as photodissociation, photoionization and photoelectron photoion coincidence spectroscopy.

The extensive research on nitro compounds confirms a rich gas-phase ion chemistry. It is, however, noteworthy that Porter, Beynon and Ast in the classical review, The Modern Mass Spectrometer A Complete Chemical Laboratory, were able to demonstrate the capabilities of mass spectrometry with no less than thirty different experiments involving a single compound, i.e. nitrobenzene1.

A brief introductory section on advanced mass spectrometry is given with special emphasis on nitro and nitroso compounds, followed by a discussion addressing the reaction mechanisms characteristic for these classes of compound.

This report primarily covers the literature from the period 1980 1994 and is therefore a follow-up of the report by Schwarz and Levsen2.

II. MASS SPECTROMETRY (MS)

The development of tandem MS techniques has undisputably contributed significantly to the basic understanding of fundamental aspects of nitro compounds. Further possible analytical applications have benefited from an advanced use of ion chemistry. In particular, the MS studies on nitroarenes (nitro-PAHs), which are known to be highly mutagenic, should be emphasized as fast and reliable analytical strategies.

MS has proven to be a unique technique, since ionic reactions can be studied under strictly controlled conditions even collision-free conditions can be achieved. Thus, MS may provide fundamental information on highly reactive systems.

6. Mass spectrometry of nitro and nitroso compounds

251

A. Unimolecular Dissociations

The ions observed as a result of unimolecular dissociations (metastable ions) in the mass spectrometer correspond to reactions in the low-ms time frame3 5. Due to the duality of the reaction rate and the available energy, only processes with rather low-energy requirements are observed3 5, which is nicely reflected in the metastable ion spectrum of nitromethane (Figure 1)6. Two dominant processes, i.e. the loss of OHž (m/z 44) and CH3Ož (m/z 30), respectively, are observed leading directly to the conclusion that these reactions have critical energies within a few hundredths of meV7. Recent literature reviews8,9 offer excellent discussions on the metastable ion dissociations. Nevertheless, it appears reasonable to summarize the more important aspects.

It is generally assumed that similar product ion abundances indicate that the metastable ions exhibit identical structures. It should, however, be emphasized that the abundance ratios can be highly sensitive to variations in internal energies. Hence, based on variation in the product ion abundance ratios only, it cannot unambiguously be concluded that the parent ion structures are different3 5.

Closely related to the metastable ion structure is the determination of kinetic energy release, which appears as a rather sensitive probe to reaction dynamics3 5. However, a comprehensive analysis of the kinetic energy release is in general a complicated process and requires detailed information regarding the ion-optic system3 5.

Nevertheless, even used as a qualitative probe, kinetic energy release determinations are rather useful. Thus, two isomeric metastable ions showing identical fragmentation as well as the same peak shape, i.e. identical kinetic energy release, most likely orignate from the same precursor3 5. On the other hand, if significant differences are observed in the kinetic energy release (visualized in the peak shape), it has to be concluded that the

44

60

30

FIGURE 1. Metastable ion spectrum of the molecular ion of nitromethane6

252

FIGURE 2. Peak shapes of the [M NO]C ions of (a) p-nitro-aniline (kinetic energy release 1240 meV) and (b) p-nitrobenzaldehyd (kinetic energy release 70 meV) Adapted from Reference 11

6. Mass spectrometry of nitro and nitroso compounds

253

fragmentation apparently takes place via different structures/transition states10,11. Finally, it should be noted that the kinetic energy release reflects qualitatively the strain of the TS involved in the fragmentation process, i.e. tight systems show the highest release displayed by broad peaks. This may be illustrated by two closely related reactions, namely the NO loss from p-nitro-aniline and p-nitrobenzaldehyd, respectively (Figure 2)11. The peak shape for the fragmentation of p-nitro-aniline ion shows a very high kinetic energy release pointing to a very tight TS11. In addition, analysis of the energy partitioning of the system has revealed that a large fraction of the reverse activation energy appears as kinetic energy11.

B. Collision Experiments

The above-mentioned sensitivity of metastable ion spectra to variations in internal energy can be surmounted by excitation of the ion of interest with a target gas at low pressure (ca 10 6 mbar)3,5,12 14. Collision activation has been used extensively in ion structure analyses3,5. Differences observed in collision activation MS can be related directly to structural differences in the parent ions. However, the ions giving rise to metastable ion spectra can make up quite a large fraction of the decomposing ions following collision activation5. The effect may be of particular importance when using instruments operating with moderate translational energies5. However, focussing on the endothermic processes, i.e. the processes observed in the collision activation mass spectra only, the effect of initial energy may be reduced. In Figure 3, the collision activation mass spectrum of nitromethane is shown. The difference compared to the metastable spectrum shown in Figure 1 is striking. The feature in the collision-induced spectrum is the simple cleavage of a C N bond giving rise to NO2C (m/z 46) and, to a minor extent, CH3C (m/z 15). This direct reaction is virtually absent in the metastable spectrum as dominated by rearrangement processes (Figure 1).

46

30

60

44

15

31

FIGURE 3. Collision activation mass spectrum of the molecular ion of nitromethane15

254

Helge Egsgaard and Lars Carlsen

C. Neutralization/Reionization

The technique of neutralization/reionization mass spectrometry (NRMS), originally introduced by McLafferty, invokes the formation of fast neutrals from a preselected ion beam, any residual ions being deflected, followed by collision-induced reionization of the neutrals and a subsequent mass spectrometric analysis16 21.

The NRMS technique has been used to distinguish between ion structures, which by other means are not distinguishable, i.e. in situations where fast isomerization takes place after collision activation, but prior to fragmentation5. Thus, NRMS experiments have been used to disclose, e.g., the site of protonation of nitroarenes22.

The fast neutrals may well originate from ordinary fragmentations, i.e. metastable ions. Thus, NRMS experiments enable studies on the ‘neutral-half’ of the MS. In the case of nitromethane, the dominant generation of NOC is associated with a simultaneous formation of a ‘CH3O’ species. Using the NRMS technique it is possible unambiguously to distiguish between the possible CH2OHž and CH3Ož structures23; See Scheme 1. The fragmentation of nitromethane apparently gives rise to the latter7.

In addition, the NRMS technique is an excellent tool to generate and characterize otherwise non-achievable compounds. Thus, the elusive aci-nitromethane was successfully

NO+ + CH3 O

CH3 NO2 +

NO+ + CH2 O

SCHEME 1

FIGURE 4. Neutralization/reionization mass spectrum of [M C2H4]C ion of 1-nitropropane15

6. Mass spectrometry of nitro and nitroso compounds

255

generated by neutralization of the [M C2H4]C ion of 1-nitropropane as demonstrated by the dominant recovery signal at m/z 61. The identity of the survivor is given by the fragments in comparison with that of nitromethane; cf Figure 315. The NRMS spectrum is shown in Figure 4. It appears that the loss of OHž (m/z 44) is the characteristic feature. In the case of nitromethane, this process is apparently only significant for long-lived ions with an appropriate internal energy; cf Figure 1.

III. IONIZATION

Ionization may take place by the interaction with a particle sufficiently high in energy, e.g. an electron or a photon, or by the addition of charged species, e.g. an electron or a proton. The thermochemistry associated with the ionization process provides information on ion structures, since a structure may be assigned based on heat of formation when compared to data of reference ions. Thus, the determination of ionization energy, electron affinity and proton affinity plays a central role in mass spectrometry.

A. Radical Cations Ionization Energy

The heat of formation of a positive ion in the gas phase is obtained by taking the heat of formation of the corresponding neutral species and adding the energy required to remove an electron from the molecule, i.e. the ionization energy24:

IE

M ! MC C e

Protocols for the estimation of heat of formation of cations have been developed25 28. It appears that heat of formation of positive ions in homologous series is well represented by the equation:

fH MC D A Bn C C n

where A, B and C are constants for a given homologous series and n is the total number of atoms in the molecule. The success of this approximation is due to the fact that the ionization energy within homologous series varies linearly with n 1. The terms A and Bn reflect the additive nature of heats of formation for the corresponding neutrals.

Nitro compounds were not included in the comprehensive list given elsewhere24. However, taking the data available for the C1 to C4 species24 the constants A D 237, B D 1.8 and C D 84 can be derived for 1-nitroalkanes29. It is apparent that the ionization energies of nitroalkanes are significantly lower than those for the corresponding alkanes. Thus, the ionization energy of nitromethane is 11.02 eV30 compared to 12.51 eV for methane24. The prototypical organic nitro compounds, nitromethane and nitrobenzene, have been studied by high resolution photoelectron spectroscopy; cf Figure 531. The weakest bound molecular orbital in nitromethane has been identified as the a1 bonding orbital31,32. The first and second ionization bands of nitromethane show extensive vibrational structure. The vibrational progression was assigned to the symmetric NO2 bending mode31. However, the spacing was approximately 565 cm 1, which is significantly lower than in the molecular ground state (647 cm 1) and implies that the ONO angle of the ionized state is considerably different from the molecular ground state31. Ab initio calculations are in complete agreement, as the ONO angle is calculated to be 137.8° and 125.8° for ionized and neutral forms, respectively32,33.

256

Helge Egsgaard and Lars Carlsen

FIGURE 5. The lowest energy ionization bands of nitromethane. The vibrational progressions are labelled. The argon lines are shown to the right. Reproduced by permission of the American Institute of Physics from Ref. 31

B. Radical Anions Electron Affinity

The electron affinity (EA) for a molecule is a quantity which is analogous to the ionization energy for cations. Thus, the electron affinity is defined as the negative of the enthalphy change for the electron attachment reaction:

EA

M C e ! M

Electron affinities of molecules are of interest not only in gas-phase reactions, e.g. in negative chemical ionization mass spectrometry, but also in the field of condensed-phase chemistry. It is characteristic that negative ions are by far not studied to the same level of detail as the corresponding positive ions. However, during the last decade a large number of EA determinations based on measurements of electron transfer equilibria utilizing pulsed high-pressure mass spectrometry have been reported34,35.

A C B ! A C B

6. Mass spectrometry of nitro and nitroso compounds

257

The equilibrium concentration of the ions A and B participating in the equlibrium can be directly observed by mass spectrometry. Thus, the free-energy change can be derived from the equilibrium constant, since the concentrations of the neutral species are known in advance. Similarly, by measuring the temperature dependence of the equilibrium constants, the associated enthalpy and entropy can be obtained from van’t Hoff plots. By measuring a series of interconnecting equlibria, an appropriate scale can be established. The primary standard in such work has frequently been SO2 whose electron affinity is well established by electron photodetachment36.

 

30

 

20

≈LUMO) Energy

10

10

 

0

1

 

(kcalmol

 

° a

20

G

 

 

30

 

40

 

50

C6H6

C6H5CN

C6F5H

C6H5NO2

C6H5NO2

C6F5NO2

p-CNC6H4NO2

p-C6H4(NO2)2

CH3NO2

FIGURE 6. Schematic plot of LUMO energies of aromatic compounds and the resulting LUMO from the interaction of the aromatic LUMO and the substituent LUMO37. Energy for the NO2 substitutent LUMO is approximated by the EA of nitromethane37. Reprinted with permission from Reference 37. Copyright (1989) American Chemical Society

258

Helge Egsgaard and Lars Carlsen

Aromatic compounds have a lowest unoccupied molecular orbital (LUMO), sufficiently low in energy to lead to stable radical anions upon electron capture. The presence of electron-withdrawing substituents such as NO2 lowers the energy of the LUMO and, hence, increases the EA leading to rather stable anions. This is illustrated in Figure 6, where the G° is used as an approximate value for the energy levels of the various orbitals37. This figure shows that the LUMO of the nitro group is much lower than the LUMO of benzene. Thus, when benzene is substituted with a nitro group, the LUMO of the nitro group will make a dominant contribution to nitrobenzene LUMO causing much

of the LUMO to be located on the nitro group37. Thus, the extra electron apparently enters a Ł -orbital37.

Electron affinities for 35 substituted nitrobenzenes have been reported and provided a comprehensive data set for the examination of substituent effects38. The data were used to derive Taft gas-phase substituent parameters and discussed qualitatively based on frontier orbital molecular theory38. The rate constants for the exo-energetic electrontransfer reactions were found to be close to those predicted by the ADO (average dipole orientation) theory38.

C. Chemical Ionization Proton Affinity

The proton affinity is defined in terms of the hypothetical reaction

A C HC ! AHC

as the negative of the enthalpy change. Since entropy associated with transfer reactions of atomic particles in general is low, the proton affinity can be used directly to predict the equilibrium between a protonated species and neutral molecule

A C BHC ! AHC C B

since

RT ln Keq D rG D rH T rS ³ rH

It appears that gas-phase basicity of nitro compounds has been studied only scarcely. Thus, only the values of the parent compounds, nitromethane (179.2 kcal mol 1) and nitrobenzene (193.4 kcal mol 1), are found in the comprehensive listing given in Reference 39. The rather high PA values for nitro compounds suggest protonation by common chemical ionization reagent systems, such as hydrogen (H3C ) and methane (CH5C ).

IV. NITRO COMPOUNDS

A. Generalized Fragmentation Processes

The fragmentation of the radical cations of nitro compounds may be initiated by a surprisingly high number of different mechanisms. The following reaction mechanisms appear as particularly important and will be discussed comprehensively:

žSimple cleavages, e.g. the rupture of the C N bond.

žTautomerism, i.e. the nitro/aci-nitro radical cations.

žIsomerization of the nitro functionality, i.e. the nitro/nitrite radical cations.

žHydrogen transfer prior to fragmentation, e.g. the ortho effect.

žExtensive rearrangement, e.g. electro-cyclic ring-closure.

žRemote oxidation.

Соседние файлы в папке Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups