Gas-phase reactivity of protonated 2-oxazoline derivatives: Mass spectrometry and computational studies

Article abstract of DOI:10.1002/rcm.6182

RATIONALE Oxazolines have attracted the attention of researchers worldwide due to their versatility as carboxylic acid protecting groups, chiral auxiliaries, and ligands for asymmetric catalysis. Electrospray ionization tandem mass spectrometric (ESI-MS/MS) analysis of five 2-oxazoline derivatives has been conducted, in order to understand the influence of the side chain on the gas-phase dissociation of these protonated compounds under collision-induced dissociation (CID) conditions. METHODS Mass spectrometric analyses were conducted in a quadrupole time-of-flight (Q-TOF) spectrometer fitted with electrospray ionization source. Protonation sites have been proposed on the basis of the gas-phase basicity, proton affinity, atomic charges, and a molecular electrostatic potential map obtained on the basis of the quantum chemistry calculations at the B3LYP/6-31 + G(d,p) and G2(MP2) levels. RESULTS Analysis of the atomic charges, gas-phase basicity and proton affinities values indicates that the nitrogen atom is a possible proton acceptor site. On the basis of these results, two main fragmentation processes have been suggested: one taking place via neutral elimination of the oxazoline moiety (99 u) and another occurring by sequential elimination of neutral fragments with 72 u and 27 u. These processes should lead to formation of R⁺. CONCLUSIONS The ESI-MS/MS experiments have shown that the side chain could affect the dissociation mechanism of protonated 2-oxazoline derivatives. For the compound that exhibits a hydroxyl at the lateral chain, water loss has been suggested to happen through an E2-type elimination, in an exothermic step.

Full text of DOI:10.1002/rcm.6182

Research Article
Received: 24 November 2011
Revised: 16 January 2012
Accepted: 31 January 2012
Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2012, 26, 1061–1069
(wileyonlinelibrary.com) DOI: 10.1002/rcm.6182
Gas-phase reactivity of protonated 2-oxazoline derivatives: mass
spectrometry and computational studies
1
1
2
*
Ricardo Vessecchi , José Carlos Tomaz , Guilherme Purcote dos Santos ,
Alfredo R. Marques de Oliveira2, Norberto Peporine Lopes1 and Giuliano Cesar Clososki1
*
1Núcleo de Pesquisa em Produtos Naturais e Sintéticos, Departamento de Física e Química, Faculdade de Ciências
Farmacêuticas de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brasil
2Departamento de Química, Centro Politécnico, Universidade Federal do Paraná, Curitiba, PR, Brasil
RATIONALE: Oxazolines have attracted the attention of researchers worldwide due to their versatility as carboxylic acid pro-
tecting groups, chiral auxiliaries, and ligands for asymmetric catalysis. Electrospray ionization tandem mass spectrometric
(ESI-MS/MS) analysis of five 2-oxazoline derivatives has been conducted, in order to understand the influence of the side chain
on the gas-phase dissociation of these protonated compounds under collision-induced dissociation (CID) conditions.
METHODS: Mass spectrometric analyses were conducted in a quadrupole time-of-flight (Q-TOF) spectrometer fitted
with electrospray ionization source. Protonation sites have been proposed on the basis of the gas-phase basicity, proton
affinity, atomic charges, and a molecular electrostatic potential map obtained on the basis of the quantum chemistry
calculations at the B3LYP/6-31 + G(d,p) and G2(MP2) levels.
RESULTS: Analysis of the atomic charges, gas-phase basicity and proton affinities values indicates that the nitrogen atom
is a possible proton acceptor site. On the basis of these results, two main fragmentation processes have been suggested:
one taking place via neutral elimination of the oxazoline moiety (99 u) and another occurring by sequential elimination of
neutral fragments with 72 u and 27 u. These processes should lead to formation of R+.
CONCLUSIONS: The ESI-MS/MS experiments have shown that the side chain could affect the dissociation mechanism
of protonated 2-oxazoline derivatives. For the compound that exhibits a hydroxyl at the lateral chain, water loss has been
suggested to happen through an E2-type elimination, in an exothermic step. Copyright © 2012 John Wiley & Sons, Ltd.
Oxazolines have attracted the attention of researchers world-
wide due to their versatility as carboxylic acid protecting
groups,[1,2] chiral auxiliaries, and ligands for asymmetric
catalysis.[3,4] 2-Oxazoline derivatives have been widely studied
because these compounds are versatile monomers for polymer-
ization reactions. Moreover, these derivatives are of significant
scientific interest because they can be used as copolymers and
they can induce micelle formation.[5] For all these reasons,
natural and synthetic compounds that exhibit an oxazole group
are interesting from a chemical viewpoint, due to their
reactivity and potential biological activities.[6]
important to mention that the losses of H2 and •CH3 can
occur with the formation of three-membered rings.[10]
O’Hair et al. have used tandem mass spectrometry (MS/MS)
experiments to understand the fragmentation mechanisms for
serine and its derivatives, during which oxazoline derivatives
may be formed.[11] Recently, the mass spectrometric analysis of
oxazoline derivatives has been employed in metabolomics
studies,[12] in order to determine the double-bond position in
highly unsaturated fatty acids.[13] Schubert and co-workers have
analyzed some poly(2-ethyl-2-oxazoline)s by matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF), and
their results suggest that the fragmentation of these compounds
occurs by H2, CH2CH2, and a-cleavage.[14]
Proton transfer, spectroscopic, and electronic structure
studies have been performed for oxazoline derivatives in
solution, but to date gas-phase studies have not been carried
out. However, investigation of protonated oxazoline is impor-
tant, so that the reactivity of this class of compounds in the
gas phase can be better understood. Electrospray ionization
mass spectrometry (ESI-MS) is an important tool for the ana-
lysis of synthetic, semi-synthetic, and natural products.[15–17]
MS/MS studies with an ESI source have been conducted for
characterization and structural elucidation, because the frag-
mentation pathways can furnish valuable information about
the reactivity of new compounds, and the established
mechanisms can aid the elucidation of other classes of
compounds.[16–19] In this context, computational quantum
chemistry has been successfully employed in mass spectro-
metry fragmentation analysis.[18,20]
Albeit several methods have been developed for the
preparation/identification of 2-oxazolines,[7] reports on their
analysis by mass spectrometry (MS) are scarce in the litera-
ture, thereby making the structural elucidation of oxazoline
derivatives difficult at lower concentration. Early works have
characterized isomeric oxazole derivatives using the electron
ionization (EI)-MS fragmentation pattern.[8,9] EI-MS analysis
accomplished by Osman et al. has shown that the most
intense fragments are due to lateral chain elimination from
+
•
M . The cis and trans isomers can be identified by fragmenta-
tion processes and by the intensity of the fragment ions. It is
* Correspondence to: R. Vessecchi and G. C. Clososki, Univer-
sidade de São Paulo, Faculdade de Ciências Farmacêuticas
de Ribeirão Preto, Departamento de Física e Química, Av.
do café S/N, CEP: 14040–903, Ribeirão Preto, SP, Brasil.
E-mail: vessecchi@bol.com.br; gclososki@fcfrp.usp.br
Rapid Commun. Mass Spectrom. 2012, 26, 1061–1069
Copyright © 2012 John Wiley & Sons, Ltd.

R. Vessecchi et al.
2-Benzyl-4,4-dimethyl-2-oxazoline (3)
N
N
A mixture of phenylacetic acid (5 g, 36.72 mmol) and 2-amino-
2-methyl-1-propanol (3.39 g, 38 mmol) was refluxed for 48 h in
xylene (50 mL) with azeotropic removal of water, using a Dean-
Stark apparatus. The solvent was removed under reduced
pressure, and the residue was dissolved in ethyl acetate. The
organic layer was washed with NaHCO3, dried over Na2SO4,
and concentrated under reduced pressure. Compound 3 was
distilled under vacuum, to afford a colorless oil (5.21 g, 75%
O
O
1
2
S
N
N
O
O
3
4
HO
1
yield). H NMR (400 MHz, CDCl3) d: 1.28 (s, 6H, C1H3); 3.6
N
(s, 2H, C5H2); 3.9 (s, 2H, C3H2); 7.2–7.38 (m, 5H, CH aromatic).
13 C NMR (100 MHz – CDCl3) d: 164.4 (C4); 135.3 (C6 aromatic);
128.8, 128.6, 126.9 (CH aromatic); 79.4 (C3); 67.0 (C2); 34.9 (C5);
28.3 (C1). IR (lmax, film, cm–1): 1663.
O
5
Figure 1. Structures of the 2-oxazoline derivatives analyzed
in this study.
2-Methyl(thiophenyl)-4,4-dimethyl-2-oxazoline (4)
In this work, some 2-oxazoline derivatives (Fig. 1) were
synthesized[21] and studied through ESI-MS/MS experi-
ments, and the reactivity of these compounds was compared
to computational thermochemical parameters obtained from
density functional theory (DFT) calculations. The reactivity
of cyclic compounds containing N, O, and S originated from
protonation and complexation has been evaluated from both
a theoretical and experimental standpoint.[22–26] Therefore,
these compounds can be used as prototypes for new investi-
gations including other cyclic derivatives. The presence of
basic sites such as N, O, and S may promote an interesting
discussion on the reactivity in terms of proton interaction
and gas-phase chemistry.
A mixture of (phenylthio)acetic acid (5 g, 29.72 mmol), 2-amino-
2-methyl-1-propanol (2.85 g, 32 mmol), and a catalytic amount of
boric acid was refluxed in xylene (50 mL) for 48 h with azeotropic
removal of water, using a Dean-Stark apparatus. The solvent was
removed under reduced pressure, and the residue was dissolved
in ethyl acetate. The organic layer was washed with NaHCO3,
dried over Na2SO4, and concentrated under reduced pressure.
Compound 4 was distilled under vacuum, to furnish a slightly
1
green oil (5.13 g, 75% yield). H NMR (400 MHz, CDCl3) d:
1.19 (s, 6H, C1H3); 3.66 (s, 2H, C5H2); 3.9 (s, 2H, C3H2);
7.17–7.32 (m, 3H, CH aromatic); 7.41–7.48 (m, 2H, CH aromatic).
13 C NMR (100 MHz – CDCl3) d: 162.0 (C4); 134.7 (C6 aromatic);
130.3, 128.8, 126.9 (CH aromatic); 79.6 (C3); 67.3 (C2); 31.1 (C5);
28.1 (C1). IR (lmax, KBr, cm–1): 1663.
EXPERIMENTAL
Synthesis of 2-oxazoline derivatives
2-Ethyl-4,4-dimethyl-2-oxazoline (1)
Propionic acid (20 g, 270 mmol) was added to 2-amino-2-
methyl-1-propanol (24.1 g, 270 mmol) under stirring, and
the mixture was refluxed for 4 h. The product was then
azeotropically distilled into 200 mL of hexanes. The organic
layer was separated, and the aqueous phase was extracted with
hexanes (3 ꢀ 50 mL). The combined organic portions were
dried with Na2SO4 and filtered, and the solvent was removed
by distillation. The crude product 1 was distilled from CaH2
under argon atmosphere, to give a colorless oil (27.4 g, 80%
1
yield). H NMR (200 MHz, CDCl3) d: 1.18 (t, J = 7.5 Hz, 3H,
C6H3); 1.27 (s, 6H, C1H3); 2.26 (q, J = 7.5 Hz, 2H, C5H2);
3.9 (s, 2H, C3H2). 13 C NMR (50 MHz – CDCl3) d: 167.0 (C4);
78.9 (C3); 66.8 (C2); 28.4 (C1); 21.6 (C5); 10.4 (C6). IR (lmax, film,
cm–1): 1673. The numbered atoms can be assessed in
Supplementary Fig. S1 (see Supporting Information).
2-Butyl-4,4-dimethyl-2-oxazoline (2)
Compound 2 was prepared as described above in 92% yield
from n-pentanoic acid. 1H NMR (200 MHz, CDCl3) d: 0.92
(t, J = 7.23 Hz, 3H, C8H3); 1.26 (s, 6H, C1H3); 1.36 (m, 2H,
C7H2); 1.59 (m, 2H, C6H2); 2.25 (t, J = 7.15 Hz, 2H, C5H2);
3.90 (s, 2H, C3H2). 13 C NMR (50 MHz – CDCl3) d: 166.2
(C4); 78.9 (C3); 66.9 (C2); 28.5 (C1); 28.2 (C5); 27.9 (C6); 22.3
(C7); 13.8 (C8). IR (lmax, ATR, cm–1): 1668.
Figure 2. Molecular electrostatic potential map (MEP) for
compounds 1–5, mapped on the total electron density
(0.005 e/Bohr3). Red region indicates the most nucleophilic site.
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Copyright © 2012 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2012, 26, 1061–1069

Gas-phase reactivity of protonated 2-oxazoline derivatives
2-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-1-phenylethanol (5)
mode. The accuracy masses were obtained by using sodiated
trifluoroacetic acid as internal standard. Samples were
infused into the ionization source at a flow rate of 10 mL.
min–1. The source block and desolvation temperatures were
set to 150 ꢂC. The optimum energy applied at the capillary
emitter was tested, so that a larger intensity of protonated
molecules would be achieved. The protonated molecule,
[M + H]+, was selected and dissociated by collision-induced
A flask containing 2,4,4 trimethyl-2-oxazoline (1 mmol) and
THF (2 mL) under argon was cooled in a dry ice/acetone bath
at ꢁ78 ꢂC. Then, n-BuLi (1.1 mmol) was added dropwise.
After 30 min., a solution of benzaldehyde (1 mmol) in dry
tetrahydrofuran (THF) (4 mL) was added to the flask. The
mixture was extracted with ethyl acetate (3 ꢀ 20 mL) and
dried over anhydrous Na2SO4. Compound 5 was isolated
by filtration, followed by solvent removal and vacuum, and
then purified by flash chromatography (ethyl acetate/hexane
dissociation, using N2 as collision gas at specific Elab
.
Energy-resolved plots[27] were obtained for different values
of Elab, in order to understand the energy dependence of the
fragmentation processes.
1
2:1). H NMR (200 MHz, CDCl3) d: 1.26 (s, 3H, C1H3); 1.28
(s, 3H, C1H3); 2.57 (dd, J1 = 6.1 Hz, 1Hb’, C5H); 2.63 (dd,
J1 = 6.6 Hz, 1Hb’’, C5H); 3.90 (d, J = 8.2 Hz, 1H’, C3HH’);
3.92 (d, J = 8.2 Hz, 1H’’, C3HH’’); 5.05 (dd, J1 = 6.1;
J2 = 6.6 Hz, 1H, HC6-OH); 7.25–7.44 (m, 5H, CH aromatic).
13 C NMR (50 MHz, CDCl3) d: 164.55 (C4); 142.83 (C7);
128.41, 127.58, 125.75 (CH aromatic); 78.83 (C3); 70.39
(C6); 67.10 (C2); 37.00 (C5); 28.43 (C1); 28.39 (C1).
Theoretical calculations
In order to obtain the most accurate model for calculation of
the thermochemical parameters of 2-oxazoline derivatives,
proton affinity and gas-phase basicity were computed for
1,3-oxazole employing G2,[28] G2(MP2),[28] and B3LYP/
6-31 + G(d,p),[29] and these results were compared to values
described at the NIST website.[30] On the basis of the results
(see Supplementary Table S1, Supporting Information) and
early studies,[31] the errors obtained by these models as
compared to experimental values can be considered to be
very small. Thus, the B3LYP/6-31 + G(d,p) model was selected
for calculation of the selected parameters.
Mass spectrometry analysis
Stock solutions of the target compounds were prepared for
mass spectrometry analysis from a 0.2 mg.mL–1 solution of
each compound in acetonitrile/water (2:1 v/v).
The high-resolution ESI-MS analyses were performed on an
UltroTOF-Q (Bruker Daltonics, USA) fitted with an electro-
spray ionization (ESI) source operating in the positive ion
HO
S
N
N
N
N
N
OH
1H+_b
OH
5H+_b
OH
4H+_b
OH
2H+_b
OH
3H+_b
(24.48)
(24.66)
(26.35)
(26.11)
(24.45)
(200.06)
(201.24)
[206.70]
(200.86)
[206.76]
(204.30)
[208.21]
(201.73)
[206.85]
(196.43)**
[204.78]
[203.99]**
HO
ring
opening
N
S
N
N
N
N
O
H
O
H
+H+
O
H
+H+
O
H
+H+
O
H
+H+
+H+
HO
S
N
N
N
N
N
O
O
O
O
O
1
+H+
2
+H+
3
+H+
4
+H+
5
+H+
HO
H
N
H
N
H
N
H
N
H
N
S
O
O
O
O
O
1H+_a
2H+_a
3H+_a
4H+_a
5H+_a
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
(224.53)
(222.74)**
[232.09]
[230.15]**
(225.53)
[233.36]
(230.69)
[237.32]
(227.36)
[235.64]
(226.18)
[234.11]
Scheme 1. Gas-phase basicities (bold values), Proton affinities (values in parentheses),
and relative Gibbs energies (italic values) for protonated species of 2-oxazoline com-
pounds obtained by the B3LYP/6-31 + G(d,p) model. **Values obtained by the G2(MP2)
model. All the values are given in kcal.mol–1. Geometries, absolute enthalpies, and Gibbs
energies are given in the Supporting Information.
Rapid Commun. Mass Spectrom. 2012, 26, 1061–1069
Copyright © 2012 John Wiley & Sons, Ltd.
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R. Vessecchi et al.
All the molecules were optimized by means of the B3LYP/
6-31 + G(d,p) model,[29] using Gaussian 03 suite programs.[32]
The vibrational frequencies indicated that all the structures
are minima in the potential energy surface. All the structures
were visualized with the J.Mol 9.0 software.[33]
pathways suggested by enthalpies at 298.15 K. No transition
states were calculated for the fragmentation mechanisms; only
the relative enthalpies/energies were obtained.
Protonation sites were indicated on the basis of HOMO
analysis, MK charges, gas-phase basicity, and proton affinity.[34]
The molecular electrostatic potential maps (MEP) were plotted
from MK charges with the aid of the Molekel software.[35]
Gas-phase basicity and proton affinity were obtained by
variations of Gibbs energies and enthalpies for a protonation
reaction M + H+ ! MH+, respectively, where the enthalpy and
Gibbs energy of the proton were considered as 1.48 and
ꢁ6.28 kcal.mol–1, respectively.[36] In this way, it was possible
to propose the most stable ions, as well as the fragmentation
RESULTS AND DISCUSSION
Protonation sites
The first step to understanding the fragmentation mechanism
undergone by the protonated analyte is to find out the most
stable ion formed by association between a proton and a neutral
compound.[20] Several methodologies have been employed to
confirm the most stable protonation site in organic compounds,
and there are some models that accurately describe the protona-
102.0907
Intens.
Intens.
6000
72.0852
1
2
1200
1000
74.0650
128.1082
55.0579
800
72.0837
56.0538
4000
2000
0
600
156.1383
84.0848
400
200
0
57.0693
67.0548
40
50
60
70
80
90
100
110
120
130
140 m/z
50
60
70
80
90 100 110 120 130 140 150 160 m/z
Intens.
Intens.
x105
x104
6
91.0575
123.0243
3
4
1.0
0.8
0.6
0.4
0.2
0.0
5
4
3
2
1
0
113.0815
190.1234
98.0622
136.0748
150.0353
145.1003
118.0647
222.0934
135.0234
73.0682
60 70 80 90 100 110 120 130 140 150 160 170 180 190 m/z
Intens.
60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 m/z
x104
130.0660
5
6
4
2
202.1244
220.1348
103.0554
114.0921
0
100 110 120 130 140 150 160 170 180 190 200 210 220 m/z
Figure 3. ESI-MS/MS spectra for protonated 2-oxazoline derivatives, at 15 eV (N2).
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Gas-phase reactivity of protonated 2-oxazoline derivatives
tion sites in the gas phase.[20,21] The combination between
atomic charges, frontier orbitals, Fukui functions, and energetic
parameters, such as proton affinities (PA) and gas-phase
basicity (GB), have been satisfactorily used by us to indicate
the most probable site for proton attachment.[16–18] However,
the PA and GB parameters have demonstrated to be the most
useful for the identification of protonation sites.
H
N
H
N
H
N
N
O
R1
R1
R1
R1
O
O
O
Scheme 2. Resonance forms for protonated 2-oxazoline
derivatives.
the N atom, as evidenced by GB, PA, and atomic charge values
(Supporting Information). Nevertheless, the protonation at the
N atom leads to an imminium ion, which can be stabilized by
formation of a tertiary carbocation or by the mesomeric effect
of the oxygen lone pair (Scheme 2). As for compound 4, the
protonation at the S atom culminates in proton migration to
the N atom during the optimization steps. On the basis of these
data, mass spectrometry studies have been performed, and a
fragmentation mechanism has been proposed. As observed
during the G2(MP2) model and B3LYP/6-31 + G(d,p) studies,
protonation at the O atom results in opening of the oxazoline
ring. For compound 5, the protonation at the hydroxyl group
provides an ion-neutral complex[37] by lengthening of the
C–OH+2 bond. This process has GB= 204.8 kcal.mol–1.
Analysis of the atomic charges (Supplementary Fig. S1,
Supporting Information) suggests that protonation should occur
at the N atom. HOMO for compounds 1, 2, and 5 is located in the
O-C-N portion. On the other hand, HOMO is located in the
aromatic ring in the case of compounds 3 and 4 (Supplementary
Fig. S2, Supporting Information). Examination of the molecular
electrostatic potential maps (Fig. 2) indicates that the nitrogen
atom is the most nucleophilic atom in the molecule in the
gas phase. Thus, these results suggest that the protonation will
occur at the N atom in higher proportion when compared to
the O atom.
However, in order to confirm these results, the gas-phase
basicity and proton affinities have been computed for all the
compounds (Scheme 1). The most stable protonation site is
(A)
(B)
100
100
m/z 128
m/z 74
80
m/z 156
m/z 102
m/z 84
80
m/z 72
m/z 57
60
60
m/z 72
m/z 56
m/z 55
m/z 67
m/z 57
40
20
0
40
20
0
-5
0
5
10
15
20
25
30
35
40
0
10
20
30
40
Elab (eV)
Elab (eV)
(C)
(D)
100
80
60
40
20
0
100
80
60
40
20
0
m/z 190
m/z 145
m/z 136
m/z 118
m/z 91
m/z 222
m/z 150
m/z 123
m/z 113
m/z 98
m/z 73
m/z 70
0
10
20
30
40
0
10
20
30
40
Elab (eV)
Elab (eV)
(E)
(F)
100
80
60
40
20
0
100
80
60
40
20
0
m/z 202
m/z 130
m/z 103
m/z 77
m/z 220
m/z 202
m/z 130
m/z 103
-5
0
5
10
15
20
25
30
35
40
0
10
20
30
40
Elab (eV)
Elab (eV)
Figure 4. Energy-resolved plots for [M + H]+ ions: (A) [1 + H]+; (B) [2 + H]+; (C) [3 + H]+, (D)
[4 + H]+, (E) [6 + H]+, and (F) MS/MS of m/z 202 from [5 + H]+.
Rapid Commun. Mass Spectrom. 2012, 26, 1061–1069
Copyright © 2012 John Wiley & Sons, Ltd.
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R. Vessecchi et al.
The differences between the PA and GB parameters obtained
by G2(MP2) and G2 are lower than 3 kcal.mol–1, as compared to
B3LYP/6-31 + G(d,p) (see Scheme 1 for compound 1). Thus, on
the basis of these results and of those described for 1,3-oxazole
(see Supplementary Table S1, Supporting Information), it can
be concluded that the B3LYP/6-31+ G(d,p) level can be an use-
ful tool for the determination of the thermochemical parameters
of these compounds.
The stability of N protonated species may also be attributed
to the resonance forms and the electron delocalization of
these ions (Scheme 2), as evidenced in ion-molecule reactions
with other heterocyclic compounds (tioxane and dioxane).[38]
in combination with mass spectrometry analysis may be a
useful tool for the establishment of the role of the lateral chain
present in the 2-oxazole ring.
Four main fragmentation pathways have been suggested
on the basis of the ions observed in the MS/MS spectra
(Figs. 3 and 4) of the protonated molecules (Scheme 3). Other
ions have also been detected, and the mechanisms through
which they are generated have been proposed. However,
these ions occur with lower intensity as compared to those
described in Scheme 3.
At an Elab of 20 eV, the most intense fragment ions for
[1 + H]+ are m/z 72 and 74 (Fig. 3(A)). The m/z 72 ion can be
formed by neutral loss of 56 u via a ring contraction in which
an isobutene is eliminated as a neutral fragment (see Scheme 3,
red arrows). The enthalpy necessary for this reaction has been
computed as being approximately 126 kcal.mol–1.
The m/z 72 fragment ion occurs for compounds 1–4, and its
intensity decreases in this sequence (Fig. 3). Thus, another
mechanism is suggested here for m/z 72 ion formation (see
Scheme 4). The higher intensity of this ion for compounds 1
and 2 as compared to the other compounds can be ascribed
to the presence of an aliphatic hydrogen at the lateral chain,
which culminates in ketene loss through hydrogen migration.
The formation of m/z 74 is triggered by elimination of 54 u
(see Scheme 5). The enthalpy for this process has been deter-
mined as being 69 kcal.mol–1. The lowest energy necessary
for production of the m/z 74 ion as compared to the m/z 72
ion (69 kcal.mol–1) accounts for its higher intensity after
ESI-MS/MS and gas-phase ion chemistry
In order to confirm the molecular formulae of the compounds,
high-resolution analysis (HR-ESI-MS) has been conducted on
an electrospray Q-TOF mass spectrometer. The results can be
found in Supplementary Figs. S3–S7 and Table S2 (Supporting
Information).
All the protonated molecules [M + H]+ have been selected
and dissociated by CID (Fig. 3), from which the energy-
resolved plots were obtained, aiming at a better understanding
of the influence of the collision-energy (Elab) on the fragmenta-
tion processes (Fig. 4). The fragmentation profiles of the
assessed compounds are different, so it is necessary to compre-
hend the effect of the side chain on the reactivity of these
protonated molecules. In this sense, computational chemistry
H
N
O
(- 99u)
(-56 u)
HN
O
R
R+
R
N
O
1: m/z 29 (n.o.)(94.40)
1: m/z 128
2: m/z 156
3: m/z 190
4: m/z 222
1: m/z 72
(126.66)
(68.98)
(60.39)
(55.30)
2: m/z 57
3: m/z 91
4: m/z 123
2: m/z 100 (n.o.)
3: m/z 134 (n.o.)
4: m/z 166 (n.o.)
(124.44)
(123.70)
(118.82)
O
(-72 u)
HN
R
(70.76)
1: m/z 56
2: m/z 84
3: m/z 118
4: m/z 150
(69.14)
(71.14)
(70.57)
(-27 u)
-HCN
R+
1: m/z 29 (n.o.) (127.35)
(101.94)
(93.35)
(88.25)
2: m/z 57
3: m/z 91
4: m/z 123
Scheme 3. Fragmentation pathways for the 2-oxazoline derivatives presented
in Fig. 1. Values in parentheses are relative enthalpies for reactions at 298 K.
All the values correspond to the protonated form. Geometries, absolute
enthalpies, and energies are given in the Supporting Information. (n.o.) indicates
the non-occurrence.
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Copyright © 2012 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2012, 26, 1061–1069

Gas-phase reactivity of protonated 2-oxazoline derivatives
20 eV (Fig. 3(A)). As for [2 + H]+, the most intense fragment
ion (m/z 102) occurs due to a mechanism similar to the ones
described for formation of m/z 74 from 1H+.
m/z 56 and 84 ions may also be due to the formation of
nitrilium ions. However, for [1 + H]+ and [2 + H]+, this process
does not generate the most intense ions in the MS/MS spec-
tra, which can be attributed to the lower energy necessary
for formation of the m/z 74 and 102 ions, as discussed before.
In order to understand the influence of the lateral chain on
the fragmentation channels, compound 5 and the most
intense fragment ion observed in its tandem mass spectrum
have been fragmented via CID experiments, as illustrated in
Figs. 4(E) and 4(F). For [5 + H]+, the elimination of H2O may
occur through in-source dissociation under CID conditions.
This consideration is based on the NMR spectrum of
[5 + H]+ (see Experimental section). Thus, it can be concluded
that the m/z 202 ion observed in the MS/MS spectrum of
[5 + H]+ is a result of water loss (Figs. 3 and 4(E)). The MS3
spectra for m/z 202 have been registered (Fig. 4(F)), and it
can be assumed that this ion is a precursor of the m/z 130
and 77 ions identified in the MS/MS spectra of [5 + H]+. On
the basis of the protonation sites obtained by analysis of GB
for [5 + H]+, it can be stated that the protonation at N atoms
or hydroxyl can promote water loss via direct elimination or
an E2-type mechanism (Scheme 6).
Concerning [3 + H]+ and [4 + H]+, elimination of the lateral
chain takes place in higher proportion as compared to the
other fragmentation processes, thereby producing the m/z 91
and 123 ions, respectively (Figs. 3 and 4). These ions can be
formed via two alternative ways: (i) elimination of 99 u, as a
neutral loss; (ii) ring contraction with elimination of 72 u
(epoxide), which yields a nitrilium ion with consecutive
elimination of -HCN (27 u) (Scheme 3). The second pathway
generates the observed ions m/z 118 and 150 for [3 + H]+ and
[4 + H]+, respectively. The intensity of the ions m/z 91 and
123 indicates the influence of the lateral chain on the fragmen-
tation of 2-oxazoline derivatives, and the energies calculated
for these ions corroborate with experimental observations.
For compounds [1 + H]+ and [2 + H]+, the formation of the
H
H
N
H
N
R
R
R
NH2
NH
O
O
O
The formation of m/z 202 from [5 + H]+_1 occurs in an
exothermic step, as compared to the enthalpy variation for
the generation of m/z 202 from [5 + H]+_3. Thus, it is possible
to conclude that the in-source dissociation of m/z 220 via the
mechanism indicated in pathway (a), to furnish the m/z 202
m/z 72
(0.0)
1: (26.45)
2: (26.77)
3: (30.00)
4: (30.01)
5: (24.51)
1: (50.51)
2: (51.14)
3: (49.31)
4: (49.02)
5: (52.43)
Scheme 4. Mechanism proposed for the formation of m/z 72
from [M + H]+. All the values are relative enthalpies at
298.15 K, in kcal.mol–1.
H2O
HO
H
N
O
N
H
O
(5)H+_3
(5)H+_1
m/z 220
(0.0)
H
m/z 220
(26.60)
H
N
R1
O
-H2O
(b)
(a) -H2O
H
N
O
N
O
m/z 202
m/z 202
(-3.67)
(33.65)
H2N
O
H
R1
- 99 u
(a1)
(a2)
H
HN
m/z 103
(69.41)
m/z 130
(62.22)
H3N
O
R1
+
-HCN
H
1: m/z 74
2: m/z 102
3: m/z 136
(69.12)
(68.97)
(66.49)
m/z 103
(102.36)
4: m/z 168 (n.o.)(63.98)
Scheme 6. Fragmentation for [5 + H]+. All the values are
relative enthalpies at 298.15 K. Values in kcal.mol–1 calculated
at the B3LYP/6-31 + G(d,p) level of theory.
Scheme 5. Elimination of 54 u from protonated 2-oxazoline
derivatives. (n.o.) indicates the non-occurrence.
Rapid Commun. Mass Spectrom. 2012, 26, 1061–1069
Copyright © 2012 John Wiley & Sons, Ltd.
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R. Vessecchi et al.
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ion (Scheme 6). Also, the formation of m/z 202 must occur via
this pathway, because the ion product can be stabilized by
resonance, which has more canonical forms than the
product obtained via pathway (b). The calculated enthalpy
for this step is ꢁ3.6 kcal.mol–1, which indicates an exothermic
process. Thus, the occurrence of the m/z 202 ion through
in-source fragmentation can be observed by mass spectra
(see Figs. 3 and 4(E)).
With respect to [5 + H]+, the formation of m/z 130 and
103 should take place through a similar pathway for sequen-
tial elimination of 72 u or by elimination of 99 u from
m/z 202 (pathways (a1) and (a2), respectively, Scheme 6).
The lowest energy required for production of m/z 130
(ΔH298 = 62.22 kcal.mol–1) as compared to the other com-
pounds (see Scheme 3) accounts for the highest intensity
of this ion in the ESI-MS/MS spectrum.
CONCLUSIONS
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1060.
Analysis of PA, GB, and the ESI-MS/MS spectra indicates that
the nitrogen atom is the protonation site of all the studied
compounds, because it is the most nucleophilic site in the
gas phase. Thus, it can be concluded that the reactivity of
2-oxazoline derivatives regarding the protonation does not
sustain influence of the side chain. The ESI-MS/MS experi-
ments have shown that the side chain could affect the dissocia-
tion mechanism of protonated 2-oxazoline derivatives. For
the compound that exhibits a hydroxyl at the lateral chain,
water loss has been suggested to happen through an E2-type
elimination, in an exothermic step. This explains the occur-
rence of in-source dissociation for this molecule. The theore-
tical calculations in combination with ESI-MS/MS studies
are important for a better understanding of the reactivity
of these compounds under CID conditions.
[14] A. Baumgaertel, C. Weber, K. Knop, A. Crecelius, U. S. Schubert.
Characterization of different poly(2-ethyl-2-oxazoline)s via
matrix-assisted laser desorption/ionization time-of-flight tan-
dem mass spectrometry. Rapid Commun. Mass Spectrom. 2009,
23, 756.
SUPPORTING INFORMATION
[15] A. E. M. Crotti, R. Vessecchi, J. L. C. Lopes, N. P. Lopes.
Electrospray ionization mass spectrometry: Chemical pro-
cesses involved in the ion formation from low molecular
weight organic compounds. Quím. Nova 2006, 29, 287.
[16] R. Vessecchi, C. A. Carollo, J. N. C. Lopes, A. E. M. Crotti,
N. P. Lopes, S. E. Galembeck. Gas-phase dissociation of 1,4-
naphthoquinone derivative anions by electrospray ionization
tandem mass spectrometry. J. Mass Spectrom. 2009, 44, 1224.
[17] R. Vessecchi, F. S. Emery, S. E. Galembeck, N. P. Lopes. Frag-
mentation studies and electrospray ionization mass spectro-
metry of lapachol: Protonated, deprotonated and cationized
species Rapid Commun. Mass Spectrom. 2010, 24, 2101.
[18] R. Vessecchi, J. N. C. Lopes, N. P. Lopes, S. E. Galembeck.
Application of the atoms in molecules theory and computa-
tional chemistry in mass spectrometry analysis of 1,4-
naphthoquinone derivatives. J. Phys. Chem. A 2011, 115,
12780.
[19] O. Sekiguchi, M.C. Letzel, D. Kuck, E. Uggerud. The
unimolecular dissociation of protonated glyoxylic acid:
Structure and dynamics of a step-by-step process. Int. J.
Mass Spectrom. 2006, 255/256, 177.
[20] R. Vessecchi, S. E. Galembeck, N. P. Lopes, P. G. B. D. Nascimento,
A. E. M. Crotti. Application of computational quantum
chemistry to chemical processes involved in mass spectro-
metry. Quim. Nova 2008, 31, 840.
Additional supporting information may be found in the
online version of this article.
Acknowledgements
The authors thank the Brazilian Foundations FAPESP,
CAPES, and CNPq for financial support. R. Vessecchi thanks
CAPES (PNPD) for a scholarship.
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