Experimental andTheoretical Studies ofthe Fragmentation ofthe Precursor Ions of

Nitrophenyl-arylEthers

2Mattai George, 1Daryl Giblin*, 1AmberRussell, 2Joseph T. Moolayil, 3R. Srinivas and

1Michael L. Gross1Washington University, Department of

Chemistry, St. Louis, MO 631302Sacred Heart College, Department of

Chemistry, Cochin, Kerala, India3Indian Institute of Chemical Technology,

Hyderabad, India

*http://www.chemistry.wustl.edu/~msf

Overview

Purpose: To gain a fundamental understanding of the fragmentationmechanisms of the [M + H]+ ions of nitrophenyl-aryl ethers.

Methods:

w High and low-energy CAD provide the patterns of fragmentation.Further characterization by isotopic labeling.

w Theoretical calculations by DFT to characterize the potentialenergy surface.

Results:

w Proximity effects play a major role in directing course offragmentation in ortho and para isomers.

w Proton and hydroxyl migrations along with cyclization give rise tointermediates which lead to the fragmentation products.

IntroductionAs a consequence of proximity, radical cations of substituted aromaticnitro compounds, generated by electron ionization (EI) rearrange anddissociate to give unusual fragment ions, a phenomenon termed the orthoor proximity effect [1]. The rearrangements include hydrogen migration tothe nitro group from nearby substituents [2] and the migration of oxygento nearby acceptors sites [3,4] leading to heterocyclic products. Proximityeffects leading to cyclic products have been observed in the elimination ofacetic acid from protonated biphenyls [5]. However, proximity effects inthe fragmentation of even-electron [M + H]+ precursor ions are lesscommon, likely owing to less facile rearrangements and migrations ineven-electron ions [6].Nitro aromatic compounds are important in explosive monitoring and havebeen identified in environmental samples. Structural characterization ofthese compounds by mass spectrometric methods have been explored [7].

A proximity effect leading to a cyclic product ion has been reported in thepositive-ion EI spectrum of 2,4-dinitrophenyl-phenyl ether. The proposedpathway involves the migration of an H from the 2'-position to the nitrogroup followed by elimination of OH• and concomitant cyclization [8].The negative-ion EI mass spectrum of o-nitrophenyl-phenyl ether ischaracterized by the elimination of NO•, H2NO2

• (NO• + H2O ?), Ph•, andPhO• [9].

O

N

O

O2N

O

O2N NO2H

- OH•

•+ •+

We have discovered that the even-electron [M + H]+ precursor ions ofo-nitrophenyl-aryl ethers exhibit proximity effect in fragmentation. Ourfocus of this investigation involves experimental and theoretical studies ofthe proximity effects in the fragmentation of these precursor ions.

References1. McLafferty, F. W.; and Turecek, F., Interpretation of Mass Spectrometry, 4rd edn., University

Science Books: Sausalito, CA, 1993, 77-78.

2. Bursey, J. T.; Bursey, M. M.; and Kingston, D. G. I., Chem. Rev., 1973, 73, 191.

3. (a) Ramana, D. V.; Sundaram, N.; and George, M., Org. Mass. Spectrom., 1990, 25, 161; (b)Ramana, D. V.; Sundaram, N.; and George, M., Org. Mass. Spectrom., 1989, 24, 63.

4. Meyerson, S.; Puskas, I.; and Fields, E. K., J. Am. Chem. Soc., 1966, 88, 4974.

5. Orlando, M.; George, M.; and Gross, M. L., Org. Mass. Spectrom., 1993, 28, 1184.

6. Bauld, N. L., Radicals, Ion Radicals, and Triplets, Wiley-VCH: New York, NY, 1997, 20-21.

7. Ritter, L. S.; Fraley, D. F.; and Cooks, R. G., J. Am. Chem. Soc., 2000, 110, 33.

8. Jauregui, J. F.; and Lehmann, P. A., Org. Mass. Spectrom., 1974, 9, 58.

9. Wilson, J. C.; and Bowie, J. H., Org. Mass. Spectrom., 1975, 10, 836.

10. (a) Brown, A. R.; and Cope, F. C., J. Chem. Soc., 1954, 873; (b) Mole, J. D. C.; and Tuner, E.E., J. Chem. Soc., 1939, 1720; (c) Okamoto, T.; and Bunnet, J. F., J. Am. Chem. Soc., 1956,78, 5357

11. Tudge, H.; Evans, S.; Crow, F. W.; Lyon, P. A.; Chess, E. K.; Gross, M. L. Int. J. MassSpectrom. Ion Phys. 1982, 42, 243-254.

12. Burinsky, D. J.; Cooks, R. G.; Chess, E. K.; Gross, M. L., Anal. Chem., 1982, 54, 295.

13. (a) Stewart, J. J. P., J Comp. Chem. 1989, 10, 209; (b) Stewart, J. J. P., J Comp. Chem. 1989,10, 221.

14. (a) Wittbrodt, J. M.; and Schlegel, H. B.; J. Chem. Phys., 1996, 105, 6574; (b) Baker, J.;Scheiner, A.; and Andzelm, J., J. Chem. Phys. Lett., 1993, 206, 380; (c) Laming, G. J., Hardy,N. C., and Amos, R. D., Mol. Phys., 1993, 80, 1121.

15. Nicolaides, A.; Smith, D. M.; Jensen, F.; and Radom, L. J., J. Am. Chem. Soc., 1997, 119,8083.

16. (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J.R.; Zakrzewski, V. G.; Montgomery, Jr., J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.;Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;Petersson, G. A.;. Ayala, P. Y; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.;Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.;Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;Replogle, E. S.; and Pople, J. A., Gaussian 98, Revision A.6, Gaussian, Inc., Pittsburgh PA,1998; (b) Frisch, M. J.; and Frisch, A. in Gaussian 98, User's Reference, Gaussian, Inc.,Pittsburgh, PA, 1999, and references therein.

17. Scott, A. P.; and Radom, L., J. Phys. Chem., 1996, 100, 16502.

18. Bauld, N. L., Radicals, Ion Radicals, and Triplets, Wiley-VCH: New York, NY, 1997,166-169.

AcknowledgmentsThis research project was supported by the NCRR Mass SpectrometryResearch Resource at Washington University (Grant No. P41RR00954)and the National Centre for Mass Spectrometry at the Indian Institute ofChemistry Technology, Hyderabad, India.

This poster will be available on the web a couple weeks after ASMS.Website: http://www.chemistry.wustl.edu/~msf

MethodsPreparation: Nitrophenyl-aryl ethers, 1 to 5, were synthesized [10]by heating (~130 oC, 4 hr) a mixture of the potassium salt of phenol or acresol with 1-chloro-2-nitrobenzene or 1-chloro-4-nitrobenzene and acatalytic amount of copper power; excess phenol or cresol was used assolvent. The products were taken up in diethyl ether, washed with NaOHsolution followed by water, and dried. The diaryl ethers were purified bycolumn chromatography on silica gel using n-hexane as eluent.All other chemicals were obtained from Aldrich Chemical Co.

O

R1 R2

o-CH3p-NO25p-CH3o-NO24o-CH3o-NO23Hp-NO22Ho-NO21

R2R1Compound

Mass Spectrometry: [M + H]+ precursor ions were generated byfast-atom bombardment (FAB) or electrospray (ES) and analyzed bymetastable-ion (MI) or collisional-activation (CA) tandemmass-spectrometric methods.w Low energy CA (MS/MS and MS3) was performed on a Finnigan LCQ ion

trap. Precursor ions were generated by ES from a 1:1 water/methanol solutionwith 0.1% trifluoroacetic acid.

w MI and high energy CA were performed by using a VG ZAB (B/E scan) or aKratos MS-50TA (MIKES scan) [11]. In addition, normal MS analysis wasperformed on the VG ZAB, and the MS-50TA was used to performhigh-energy MS3 experiments [12]. Precursor ions were desorbed by FABusing 3-nitrobenzyl alcohol (3-NBA) as matrix.

To track fragmentation pathways, [M + D]+ ions were generated by using3-NBA matrix in which the active hydrogen had been exchanged withdeuterium. The [M + D]+ was analyzed by CA and MI methods.

Theoretical Calculations: For elucidation mechanisms ofproximity-effect fragmentations, we selected o-nitrophenyl-phenyl etherfor theoretical modeling of its potential energy surface for economy.w Owing to the large size of the ions, the initial scan of the potential energy

surface was performed by using by the PM3 [13] semi-empirical algorithm.PM3 was part of the Spartan '02 for Linux package (Wavefunction, Inc.).

w Further characterization was by using density functional theory (DFT), whichrequires less computational overhead than of formal ab initio methods and yetincorporates dynamic correlation, has little spin contamination [14] andusually performs adequately giving proper geometries, energies, andfrequencies [15]. DFT was part of the Gaussian 98 suite (Gaussian, Inc.) [16]Minima and transition states were optimized at the level B3LYP/6-31G(d,p)and confirmed by vibrational frequency analysis. Single-point energies werecalculated at level B3LYP/6-311+G(2d,p) and scaled thermal-energy correc-tions for standard conditions were applied [17]. The DFT calculations are awork in progress.

Note: The calculations of theoretical modeling yield information aboutthe potential-energy surface, but fragmentation patterns are dependentupon kinetic processes.

Results and DiscussionMass Spectrometry Results: Major source-produced fragmentions from FAB-produced [M + H]+ precursors are summarized in Table 1.(Fragments reflecting losses of H• and NO• not included). We note:w Abundances of these fragments correlate with the presence of an o-nitro group.

This observation implies that the o-nitro group participates in the generation ofthese fragment ions.

w Losses of H2O, ArOH, and the elements of HOOH (2 x OH•) requirerearrangement. For ether 1, losses of ArOH and 2 x OH• have been verified byhigh-resolution characterization of the corresponding fragments ions (Table 1).

w The loss of the elements of CH3OH occurs in ethers 3-5. That this loss instrongest in 4 where the CH3 group is distant at the para position indicateseither (1) a consecutive loss of OH• and CH3

•, (2) migration of OH or CH3

about the phenyl ring to a position where formation of CH3OH is possible, or(3) formation of an ion-dipole complex with OH• or CH3

• that decomposes withformation of CH3OH.

Tandem Mass Spectrometry Results: MI fragmentation of the[M + H]+ ion of o-nitrophenyl-p-cresol ether, 4 (Spectra 1), yields majorfragments at m/z 229 (-H•), 213 (-OH•), 212 (-H2O), 184 (-NO2

•), 122(-ArOH), 108 (ArOH+), and 107. Formation of m/z 196 (2x -OH•) is aminor process for MI, but it becomes a major fragment upon high-energyCA (Spectra 2), which also produces m/z 182 (-H2O, -NO•) and other lessinformative fragment ions. Low energy MS/MS does not produce the m/z196 fragment whereas, high-energy MS3 through the m/z 213 intermediateproduct confirms that m/z 213 also loses OH•. This evidence indicates thatm/z 196 is formed by consecutive OH• losses. Similarly, low-energy MS3

through the intermediate m/z 212 indicates that m/z 182 is formed byconsecutive losses of H2O and NO•.Product ions from the precursor ion of o-nitrophenyl-phenyl, 1, ether forman analogous series to 4, whereas p-nitrophenyl-phenyl ether, 2, yields m/z199 (-OH•) as sole major fragment.To aid the elucidation of fragmentation mechanism, ether 4 was ionizedby deuteration as described in methods. MI fragmentation of the [M + D]+

ion from 4 yields major fragments at m/z 230 (-H•), 214 (-OH•), 213

(-OD•), 212 (-HDO), 185 (-NO2•), 122 (-ArOD), 109 (ArOD+), and 108

(Spectra 3). Also present is fragment at m/z 196 (-OH•, -OD•), whichincreases in abundance upon high-energy CA.Comparison of fragment patterns of the [M + H]+ ion with the [M + D]+

ion indicates that:w There is no general H/D scrambling.

w The source of the hydrogen in the H• loss from the precursor does not arisefrom the ionizing H+ or D+.

w Based of relative fragment abundances, the losses of OH• and OD• involvestatistically the deuterium, whereas the H2O loss becomes essentially an HDOloss from the [M + D]+ ion. In both cases, the ionizing H+ or D+ is incorporatedin the losses.

w Fragments at m/z 122 and 108 (109) constitute a complimentary pair of ionsderived from a common intermediate with the ionizing H+ (D+) retained on thelatter. This implies that the ionization potentials of m/z 122 and 108 (109) mustbe similar.

Theoretical modeling results: The potential energy surface of theprecursor ion of o-nitrophenyl-phenyl ether has been calculated asdescribed in Methods. Mechanisms for the proximity-effectfragmentations are proposed consistent with the experimental results.w On the basis of calculated relative heats of formation, the initial protonation

site of o-nitrophenyl-phenyl ether is on an O of the nitro group such that anintramolecular hydrogen bond with the ether O is formed (Figure 1).

w The initially-formed ion undergoes ring closure via C-N bond formation.Hydrogen transfer to the vacant O forms the N,N-dihydroxy-phenoxazinecation which is the progenitor of pathways leading to loss of water andconsecutive losses of OH radical (Figure 2). This mechanism is consistent withthe observed fragment abundances from the [M + D]+ precursor.An alternate pathway to water loss starts with rotation of the ONOH groupfollowed by cyclization and H transfer.

w A 1,6 OH transfer within the initially-formed ion followed by a 1,2 H transferproduces most directly an intermediate which gives rise to both the m/z 122and 94 product ions depending on charge retention (Figure 3). The productions have similar ionization potentials (∆IP = 0.30 eV), and the ionizing agentremains on the latter product, consistent with experimental results.

ConclusionsWe have investigated proximity effects in fragmentations ofnitrophenyl-aryl ethers. Experimental results indicate:w For [M + H]+ of o-nitrophenyl-p-cresol ether, the major fragmentations

exhibiting proximity effect are: losses of H•, OH•, 2x OH• (consecutive), H2Oand (H2O + NO2

•), NO2• and formation of m/z 122 (-ArOH), 108 and 107.

w Fragmentation from [M + D]+ shows little evidence for H/D scrambling,statistical distribution of D in OH• loss, loss of HDO, loss of ArOD andformation of m/z 109 and 108. Loss of H• and NO2

• is same as from [M + H]+.

From the calculation of the potential-energy surface of the [M + H]+ ofo-nitrophenyl-p-phenyl ether, the following mechanisms were proposed:w Losses of OH• and H2O originate from a cyclic intermediate formed by

cyclization and H transfer from the initially formed ion.w Formation of m/z 108 and 94 arise from a common intermediate that results

from consecutive OH and H transfers in the initial [M + H]+ ion.

Table 1: Partial FAB mass spectral data [m/z(%Rel. Abund.].

122(1)182(10)198(4)212(2)196(4)230(100)5122(40)182(25)198(25)212(14)196(64)230(100)4122(30)182(10)198(4)212(8)196(35)230(100)3122(1)168(2)198(1)182(1)216(100)2122(63)‡168(12)198(12)182(60)†216(100)1

*ArOHH2O+NO•CH3OHH2O(OH•)2

Loss from [M + H]+[M + H]+Compound

Mass spectral data was obtained on a VG ZAB. *Ar denotes the phenyl group bearing theCH3 moiety.High-resolution data:†Measured mass, 182.0605; calculated mass, 182.0606 for C12H8NO‡Measured mass, 122.0249; calculated mass, 122.0242 for C6H5NO2

Spectra 1: MI MIKES spectra of 4 [M + H]+ (m/z 230.1).

m/z

Rel

ativ

e A

bund

ance

108 (ArOH+)

107122 (-ArOH)

-NO2•

-H2O

-OH•

-(OH•)2

Spectra was obtained on the Kratos MS-50TA.

Spectra 2: CA MIKES spectra of 4 [M + H]+ (m/z 230.1).

m/z

Rel

ativ

e A

bund

ance

108 (ArOH+)

107

122 (-ArOH)

-NO2• -H2O -OH•

-(OH•)2

-(H20 + NO•)

Spectra was obtained on the Kratos MS-50TA: 30% He CA.

Spectra 3: MI MIKES spectra of 4 [M + D]+ (m/z 231.1)

m/z

Rel

ativ

e A

bund

ance

108

109 (ArOD+)

122 (-ArOD)

-NO2•

-HDO-(H2O, OD•)

-OH•

-H•

-(OH• + OD•)

Spectra was obtained on the Kratos MS-50TA.

Figure 1: Relative heats of formation for protonation sites.

Most stable protonated formPlanes of phenyl rings perpendicular

O

NOO

H1.01 Å

1.57 Å+

13.732.9

O

NOO

31.7

15.76.9

16.6

4.5

16.77.59.719.7*

0.00.0Unstable

3.39.2

22.8 42.0

27.6

26.2*

In kcal/mol (PM3 and DFT)

Figure 2: Proposed mechanism for OH• and H2O losses.

ON

OO H+

N

O

HOO

+

N

O

HOHO+

N

O

HO+• N

O +

N

O

O

+

N

O

OO

H

H

+

0.00.0

23.930.6

-16.0 0.9

1.8*13.9*

34.1*39.4*

-44.7*-44.0*

N

O

OHO

+ No TS

- H20

TS38.8

TS32.2

TS32.2

TS12.0

- OH• - OH•

TS29.6

TS59.1

TS 7.3

3.39.2

27.234.3

ON

OOH

+

Relative heats of formation or *reaction, by PM3 and DFT.Location of ionizing agent in bold.

Figure 3: Proposed mechanism for formation of m/z 122 and 94.

ON

OO H+

N

O

HOO

+

0.00.0

23.930.6

TS32.2

ON

OHO+

ON

OHO +

HO+

N

O

HOO

+

N

O

HOO

+

ON

OOH+

+HO

•+

ON

O

•

ON

O

+

m/z 122

m/z 94

-1.912.4

-4.7 7.5

-15.0 -2.2

-22.1 -7.1

26.7Unstable at DFT

TS28.5

TS20.6

TS17.2

TS-8.2

TS35.3

TS48.3

13.1*32.1*

9.7*25.1*

TS25.4

TS25.4

Relative heats of formation or *reaction, by PM3 and DFT.Location of ionizing agent in bold.