Substituted 3-phenylpropenoates and related analogs: Electron ionization mass spectral fragmentation and density functional theory calculations

Article abstract of DOI:10.1002/jms.1384

Analysis of ethyl 3-(2-chlorophenyl)propenoate by electron ionization mass spectrometry showed the distinct loss of an ortho chlorine. To characterize the structural requisites for the observed mass fragmentation, a series of 30 halogen-substituted 3-phenylpropenoate-related structures were examined. All ester-containing alkene derivatives exhibited loss of the distinctive chlorine from the 2-position of the phenyl ring. Analogous derivatives with the halogen (chlorine or bromine) in the para position did not evidence selective halogen loss. Results demonstrated that substituted 3-phenylpropenoates and their analogs fragment via the formation of a previously reported benzopyrylium intermediate. To understand the correlation between the intramolecular radical substitution and the abundance and selectivity of the chlorine (or other halogen) displacement, density functional theory calculations were performed to determine the charge on the principal cation involved in the chlorine loss (in the ortho, meta, and para positions), the charge for the neutral radical (noncation), the excess alpha-electron density on the relevant atom and the energy to form the cation from the neutral atom (ionization energy). Results showed that the selectivity and extent of halogen displacement correlated highly to the electrophilicity of the radical cation as well as the neutral radical. These data further support the proposed fragmentation mechanism involving intramolecular radical elimination. Copyright

Full text of DOI:10.1002/jms.1384

JOURNAL OF MASS SPECTROMETRY
J. Mass Spectrom. 2008; 43: 1053–1062
Published online 20 February 2008 in Wiley InterScience
(
www.interscience.wiley.com) DOI: 10.1002/jms.1384
Substituted 3-phenylpropenoates and related analogs:
electron ionization mass spectral fragmentation and
density functional theory calculations
C. E. Wheelock,1,2 M. E. Colvin, J. R. Sanborn, and B. D. Hammock
1∗
3
1
1
Department of Entomology and Cancer Research Center, University of California, Davis, California 95616, USA
Division of Physiological Chemistry II, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Scheeles v a¨ g 2 SE-171 77
2
Stockholm, Sweden
3
School of Natural Sciences, University of California, Merced, California 95344, USA
Received 6 September 2007; Accepted 17 December 2007
Analysis of ethyl 3-(2-chlorophenyl)propenoate by electron ionization mass spectrometry showed the
distinct loss of an ortho chlorine. To characterize the structural requisites for the observed mass
fragmentation, a series of 30 halogen-substituted 3-phenylpropenoate-related structures were examined.
All ester-containing alkene derivatives exhibited loss of the distinctive chlorine from the 2-position of
the phenyl ring. Analogous derivatives with the halogen (chlorine or bromine) in the para position did
not evidence selective halogen loss. Results demonstrated that substituted 3-phenylpropenoates and their
analogs fragment via the formation of a previously reported benzopyrylium intermediate. To understand
the correlation between the intramolecular radical substitution and the abundance and selectivity of
the chlorine (or other halogen) displacement, density functional theory calculations were performed to
determine the charge on the principal cation involved in the chlorine loss (in the ortho, meta, and para
positions), the charge for the neutral radical (noncation), the excess alpha-electron density on the relevant
atom and the energy to form the cation from the neutral atom (ionization energy). Results showed that
the selectivity and extent of halogen displacement correlated highly to the electrophilicity of the radical
cation as well as the neutral radical. These data further support the proposed fragmentation mechanism
involving intramolecular radical elimination. Copyright  2008 John Wiley & Sons, Ltd.
KEYWORDS: 3-phenylpropenoate; substituted cinnamic esters; benzopyrylium cation; density functional theory calculations;
electron ionization mass spectrometry
INTRODUCTION
pattern had been previously reported for ˛, ˇ-unsaturated
7
8
esters of propenoic acids and nitriles, cinnamic acids and
Our laboratory reported the synthesis of haptens for the
detection of polyhalogenated dibenzodioxins by enzyme-
linked immunosorbent assay (ELISA). This ELISA has been
successfully used in the analysis of these contaminants in
a variety of environmental matrices.
designing the ELISA, a series of dioxin analogs contain-
ing a rigid propenoic acid side chain were synthesized
for coupling the hapten to immunizing proteins. During
the electron ionization (EI) mass spectral (MS) character-
ization of these haptens and the synthetic intermediates
used to prepare the haptens, an interesting fragmentation
was observed for molecules structurally related to ethyl
9
˛
-phenylcinnamic acids. The fragmentation occurred via
a substituted benzopyrylium intermediate, whose relative
abundance depended upon the position of halogen sub-
stitution on the phenyl ring. In particular, Williams et al.
reported that the ortho-isomer of chlorine-substituted diethyl
1
2
–6
In the process of
žC
benzalmalonate exhibited a relatively abundant ([M] ꢀ Cl)
because of chlorine loss, whereas the meta- and para-isomers
7
did not. These data strongly suggested that the ortho-
isomer underwent a facile chlorine radical elimination via
1
0
cyclization similar to that reported by Ronayne et al. In
addition, Schaldach and Gr u¨ tzmacher reported that m- and
p-chlorocinnamic acids were capable of undergoing the same
3
-(2-chlorophenyl)propenoate. Examination revealed that
8
chlorine elimination process as the ortho analog.
a significant loss of chlorine occurred from the molecu-
lar ion (m/z 384) of ethyl trans-3-(3,7,8-trichlorodibenzo-p-
dioxin-2-yl)propenoate, giving a fragment of m/z 349. A
literature search showed that this type of fragmentation
Therefore, to further investigate this mass spectral frag-
mentation process, a series of ethyl and methyl trans-3-(3,7,8-
trichlorodibenzo-p-dioxin-2-yl)propenoate-related analogs
were prepared and subjected to EI-MS analysis. We analyzed
three dibenzodioxin structures related to ethyl trans-
Ł
Correspondence to: B. D. Hammock, Department of Entomology
3
-(3,7,8-trichlorodibenzo-p-dioxin-2-yl)propenoate and 26
and Cancer Research Center, University of California, Davis,
California 95616, USA. E-mail: bdhammock@ucdavis.edu
3-(substituted phenyl)propenoate-related compounds. This
Copyright  2008 John Wiley & Sons, Ltd.

1054
C. E. Wheelock et al.
Table 1. Synthetic methods for reported compounds
Cl
Cl
O
O
Y
Z
No.
Synthesis
1
5
–4
–6
Methods reported in Sanborn et al., 19981
Esterification of commercially available 3-(2- and
4-chlorophenyl)propenoic acids
X
Figure 1. The structure inside the box shows the general
composition of the ethyl
1
7
8
Synthetic intermediate in Sanborn et al., 1998
trans-3-(3,7,8-trichlorodibenzo-p-dioxin-2-yl)propenoate
analogs examined in this study. X D a series of substituted
halogens or hydrogens. Y D a range of 1 or 2 carbon alkane,
alkene, and alkyne derivatives. Z D ethyl ester, primary alcohol,
methyl ketone, or nitrile.
Treatment of 2-chloro-4-methoxy benzaldehyde with
triethylphosponoacetate under basic conditions
Treatment of 6-chloropiperonylaldehyde, 2- and
3-bromobenzaldehyde, and 2-fluorobenzaldehyde
with triethylphosponoacetate under basic conditions
Treatment of 2-chlorobenzaldealdehyde with triethyl
9–12
1
1
1
1
3
4
5
6
2
-phosphonoproprionate under basic conditions
series of 3-phenylpropenoate-related structures shown in
Fig. 1 was rationally designed to explore the steric and elec-
tronic effects on this EI-MS fragmentation. Earlier studies
focused solely on substituted esters or nitriles and main-
Treatment of 2-chloroacetophenone with
triethylphosphonoacetate under basic conditions
Treatment of 2-chloroacetophenone with triethyl
2-phosphonoproprionate under basic conditions
Commercially available
7
tained the olefin linkage. Follow-up studies expanded this
motif in cinnamic acids, but again maintained the olefin
and did not vary the acid or linker/spacer moiety.8,9 We
17
Treatment of 2-chloro benzaldehyde with cyanoacetic
acid under basic conditions
subsequently expounded upon this theme and synthesized
analogs incorporating a variety of linkers between the phenyl
ring and ester, including alkane (with one or two carbon
atoms), alkene, substituted alkene, and alkyne moieties. We
were particularly interested in the mechanism responsible for
the observed fragmentation patterns and therefore replaced
the ester functionality with a nitrile, primary alcohol, or a
methyl ketone. These distinct and varied chemical moieties
were examined for their ability to affect the selective loss of
a halogen from the ortho position versus meta or para. Taken
together, these groups expand upon the previous work and
extend the studied moiety to probe the effects of a num-
ber of functional groups that could potentially displace the
halogen from the phenyl ring. On the basis of these results,
we demonstrated that a variety of substituents are effec-
tive at selectively displacing the ortho chlorine and that the
fragmentation occurs via an intramolecular radical substitu-
tion to form a benzopyrylium intermediate similar to that
1
8–21 Treatment of 2-chlorobenzaldehyde,
4
3
-chlorobenzaldehyde, 2-bromobenzadehyde and
-bromobenzaldehyde with 3-diethyl
phosphono-2-propanone under basic conditions
2–23 Lithium aluminum hydride reduction of compound 9
in tetrahydrofuran (THF)
2
2
4
Treatment of 2-chlorobenzyl chloride with di-t-butyl
malonate followed by hydrolysis, decarboxylation
and esterification
2
2
2
5
Esterification of commercially available
3
-(3-chlorophenyl)propionic acid
6–28 Esterification of commercially available 2- and
-chlorophenylacetic acids
3
9–30 Prepared according to the method of Newman and
Merrill11
7
–9
quadrapole temperature of 186 °C and a source temperature
of 240 °C at 70 eV.
observed in previous studies.
EXPERIMENTAL
Materials
Density functional theory calculations
The molecular structure of all compounds studied were
All the chemicals were purchased from Aldrich Chemical
Co. (Milwaukee, WI) unless otherwise noted and were used
without further purification. A summary of the synthetic
methods is shown in Table 1. Further synthetic details are
optimized using density functional theory (DFT) with the
Becke 3-parameter hybrid exchange functional and the
Lee-Yang-Parr (LYP) gradient corrected electron correlation
1
2
1
3
Ł
functional (B3LYP) using a 6-31G basis set. The B3LYP
functionalset has been widely demonstrated to yield accurate
chemical structures and reaction energies when used with
1
–6
provided in the original papers.
1
4
Mass spectral analysis
Samples were analyzed on a HP 6890 GC (Agilent Technolo-
gies; Palo Alto, CA) equipped with 30-m DB-17MS column
sufficient basis sets for most molecules. The atomic charges
were calculated from the B3LYP/6-31G wave function at the
B3LYP/6-31G optimized geometries using natural atomic
Ł
Ł
1
5
(
0
J&W Scientific; Folsom, CA), 0.25-mm internal diameter,
.25-µm film thickness and He carrier gas at a flow rate
population analysis (NPA), a method that yields atomic
charges that validate many qualitative chemical concepts and
is much more independent of molecular conformation and
basis set than other methods such as Mulliken populations.
These atomic charges are reported in units of electrons (e).
All calculations were carried out with Gaussian 98 Version
A.11.4 (Gaussian, Inc., Pittsburgh PA, 2002).
of 0.8 ml/min. The injector temperature was 250 °C and
the initial column temperature was 50 °C and was held for
5
.00 min and then ramped at 15 °C/min to 320 °C and held
for 2.00 min. The GC was interfaced with a HP 5973 MS
that was run in full-scan mode from 50 to 550 m/z with a
Copyright  2008 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2008; 43: 1053–1062
DOI: 10.1002/jms

EI mass spectra of substituted 3-phenylpropenoates 1055
bonds has been shown to be a facile process upon EI.19 The
carbonyl oxygen of the ester is now positioned to assist in
the elimination of the halogen from the 2-position of the
phenyl ring through the formation of a transitory 2-ethoxy-
OEt
[EI]
O
OEt
8
Cl
O
benzopyrylium intermediate. This mechanism is consistent
Cl
with the fact that skeletal rearrangements are often assisted
1
9
[
EI]
by the presence of highly unsaturated groups. To further
support this mechanism, many of the compounds examined
-
-
Cl
C H
-35/37 Da
-
63/65 Da
-Cl
2
4
žC
ž
in this study lose 63 Da, ([M] –C
2
H
4
–Cl ; Scheme 1). This
fragment is best explained by chlorine loss and formation of
the 2-ethoxybenzopyrylium intermediate, followed by loss
of ethene (C H ) to form 2-hydroxybenzopyrylium. It is
2 4
also possible that compounds follow a concerted mechanism
to directly form 2-hydroxybenzopyrylium, as many of the
structures examined in this study had high abundance ions
because of loss of 63 Da.
-
28 Da
-
C H
2
4
O
OEt
O
OH
Scheme 1. Proposed mechanism for chlorine loss from
ethyl 3-(2-chlorophenyl)propenoate (compound 5). The olefin
in the spacer arm of the molecule undergoes trans to cis
isomerization, placing the carbonyl oxygen in a position to
displace the chlorine to form the 2-ethoxy-benzopyrylium
intermediate, which was observed as the m/z 175 fragment
in Fig. 3(B). The 2-ethoxy-benzopyrylium intermediate then
loses 28 Da to form 2-hydroxy-benzopyrylium (m/z 147), which
also results from the concerted loss of m/z 63 observed for
many other compounds in this study. The saturated analog
ethyl 3-(2-chlorophenyl)propanoate (compound 24) formed the
analogous benzopyrylium structures as evidenced by the m/z
Dibenzodioxin esters
Table 2 contains EI-MS fragmentation data for four dibenzo-
dioxin esters. The fragmentation of compound 1 is shown
in Fig. 2, displaying the unique loss of chlorine ortho to the
propenoic acid ester. Comparison of 2 and 3 with 1 demon-
strates that significant loss of chlorine (35/37 Da) occurs
only from the 3-position and not from the 7- or 8-positions
of the dibenzodioxin ring system. Compound 4 is interest-
ing because, even though it has a chlorine substituent ortho
to the propenoate ester, significant loss of exclusively chlo-
rine does not occur. The molecular ion (m/z 418) instead
1
77 and 149 fragments in Fig. 3(A).
RESULTS AND DISCUSSION
žC
ž
fragments with a loss of 63 Da ([M] –C
2
H
4
–Cl ).
Benzopyrylium ion formation
Analogs of the right hand side of the dibenzodi-
oxin moiety were used to further probe the fragmen-
tation patterns (shown in the boxed region in Fig. 1).
Table 3 contains 11 structures that are ethyl esters of
substituted 3-phenylpropenoic acids. They differ in the
position, type and number of ring halogens and other
substituents (e.g. methylenedioxy and methoxy) on the
phenyl ring as well as methyl substitution on the olefin
between the phenyl ring and the ester functionality. Mul-
tiple fragmentation pathways were reported by Schaldach
The distinct loss of a halogen observed for many of the
compounds in this study occurs via a benzopyrylium inter-
mediate as shown in Scheme 1. Formation of this ion in
1
6
EI-MS has been observed for cinnamic acid esters, ˛,
7
ˇ-unsaturated esters of propenoic acids and nitriles, cin-
8
17
18
namic acids, methyl cinnamates, ˛-fluorocinnamates and
˛
9
-phenylcinnamic acids. In this mechanism, the trans olefin
isomerizes to place the phenyl ring and ester function in
1
0
a cis configuration. The cis–trans isomerization of double
Table 2. Mass spectral data for chlorinated dibenzodioxin derivatives
8
1
Cl
O
2
3
Cl
O
7
No.
1
2
3
Formula
MMa Ion (% abundance)b
1
H
CH CHCO2C2H5 Cl C17H11Cl3O4 383.97 384/386/388/390(20.8/20.0/6.5/1.0);
3
49/351/353(37.0/24.5/4.5); 321/323/325(100/66.2/12.1)
2
H
CH CCH3CO2C2H5
H
H
C18H14Cl2O4 364.03 364/365/366(100/20.5/66.3);
2
90/291/292(72.6/36.8/53.7); 115(56.9)
3
4
H
CH CHCO2CH3
Cl
C15H9Cl2O4 336.00 336/338(100/69.3); 305/307(59.0/34.1)
CH CHCO2C2H5
Cl C17H10Cl4O4 417.93 418/420/421/422(19.4/24.4/5.1/12.2); 383/385(6.4/6.9);
55/357/358/359(100/95.0/15.7/30.8)
3
a
MM, molecular mass (Da), monoisotopic.
Ions are given in Thompson (Th, mass to charge ratios), with the relative abundance of each ion (normalized to the base peak) shown
b
in parentheses.
Copyright  2008 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2008; 43: 1053–1062
DOI: 10.1002/jms

1056
C. E. Wheelock et al.
2
or 3 was examined by varying the halogen substitution as
•+
O
well as the ester moiety. Displacement of a weakly bonded
Cl
O
O
ortho substituent can occur either directly or via a short-lived
intermediate. Mass spectra of ortho isomers are therefore
OC H 321
2
5
7
Cl
Cl
more likely to display reduced abundance of the molecular
•
•
(
-Cl )
(-OC2H5 )
žC
C
ion ([M] ), but exhibit relatively abundant [M ꢀ X] peaks
7
(
pathway 1). However, in the case of meta or para sub-
m/z 349/351/353 m/z 339/341/343/345
-C H )
3
49
stituents, additional rearrangement steps are required for
cleavage. These extra steps can lead to an increase in the
(
2
4
žC
m/z 321/323/325
abundance of the [M] ion and to a subsequent decrease
3
84
2
76
in the [M ꢀ X] peak intensities,8 which are indicative of
whether the fragmentation has proceeded via pathway 1 or
via pathways 2 and 3.
C
9
4
9
155 184 213 247
304 339
300 360
7
5
0
123
120
6
0
180
240
m/z
Figure 2. Mass spectral fragmentation of the dioxin hapten
ethyl trans-3-(3,7,8-trichlorodibenzo-p-dioxin-2-yl)propenoate
compound 1) showing the characteristic chlorine loss from
Halogenated ethyl substituted
3-phenylpropenoates
(
The halogen-dependent loss was examined by compounds
5, 10, and 12 containing the substituents, chlorine, bromine
and fluorine, respectively in the 2-position of the phenyl
ring of ethyl 3-phenylpropenoate. Compounds 5 and 10, like
the dioxin derivatives discussed above, lose a halogen in
the 2-position (35/37 and 79/81 Da, respectively) to give
a base peak of m/z 147. This fragment was recognized
as the 2-hydroxybenzopyrylium intermediate reported by
m/z 384 to 349.
and Gr u¨ tzmacher8 for substituted cinnamic acids, which
depended upon the position of the halogen substitution on
the phenyl ring (Scheme 2). Compounds with ortho sub-
stitution formed a 2-hydroxybenzopyrylium ion in high
abundance (pathway 1), while those with meta or para
substitution fragmented through a number of different path-
ways that could produce the 2-hydroxybenzopyrylium ion
in a lower abundance (pathway 1), the halogen-substituted
7
,8
other researchers (pathway 1). However 12, with a 2-
fluoro substituent, did not undergo significant loss of 19
Da from the molecular ion (m/z 194). Rather, loss of 45
žC
ž
Da ([M] ꢀ C H O ) preferentially occurred to yield a base
2
5
8
2
-hydroxybenzopyrylium ion (pathway 2), or the halogen-
peak of m/z 149. Similarly, Schaldach and Gr u¨ tzmacher
substituted cinnamoylium ion (pathway 3). In this study, the
propensity of the fragmentation to proceed via pathway 1,
reported that o-fluorocinnamic acid did not produce the
benzopyrylium intermediate, but instead fragmented via
Table 3. Mass spectral data for substituted ethyl 3-phenylpropenoates
α
O
5
6
2
4
OC H
2
5
β
3
No.
˛
ˇ
2
3
4
5
6
Formula
MMa
Ion (% abundance)b
5
H
H
Cl
H
H
H
H
C11H11ClO2
C11H11ClO2
210.04 210/212(12.0/4.4); 176(7.0); 175(49.0); 165/167(36.0/11.5);
47(100)
210.04 210/212(34.4/11.6); 182/184(21.7/7.1); 165/167(100/32.6)
1
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
Cl
H
CH3O
H
H
H
H
H
H
Cl Cl
Cl
H
Cl C11H9Cl3O2 277.97 278/280(8.6/8.6); 243/245(32.9/21.1); 215/217(100/62.2)
H
Cl C12H11ClO4
H
H
H
H
C12H13ClO3
240.06 240/242(6.9/2.3); 205(61.8); 195/197(28.3/9.6); 177(100)
254.03 254/256(13.2/4.8); 219(48.8); 191/193(100/11.6)
253.99 254/256(4.8/4.9); 209/211(17.2/17.6); 176(4.3); 175(34.1);
OCH2O
1
0
1
Br
H
C11H11BrO2
C11H11BrO2
1
48(10.5); 147(100)
253.99 254/256(42.0/41.0); 226/228(28.6/27.8);
09/211(92.3/90.5); 181/183(21.0/21.9); 102(100)
1
H
H
H
Br
H
H
2
1
1
1
1
2
3
4
5
H
CH3
H
H
H
F
Cl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C11H11FO2
C12H13ClO2
C12H13ClO2
C13H15ClO2
194.07 194(31.1); 195(3.4); 166(13.2); 149(100)
224.06 224/226(3.3/1.1); 189(11.0); 161(100)
224.06 224/226(3.0/1.0); 189(52.0); 179/181(28.0/8.0); 161(100)
238.08 238(0.7); 203(59.9); 193/195(22.5/7.4); 175(100)
c
c
CH3 Cl
CH3 Cl
CH3
a
MM, molecular mass (Da), monoisotopic.
Ions are given in Thompson (Th, mass to charge ratios), with the relative abundance of each ion normalized to the base peak shown
b
in parentheses.
c
Compounds are a mixture of cis and trans isomers, which had identical fragmentation.
Copyright  2008 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2008; 43: 1053–1062
DOI: 10.1002/jms

EI mass spectra of substituted 3-phenylpropenoates 1057
Cl
O
CH CH COC H
2
5
m/z 210
[
C11H11O Cl]
2
-
H
-
-
Cl
Pathway 1
Pathway 2
-C H
Pathway 3
-OC H
2 5
2
4
C H
2
4
Cl
Cl
CH CH
C
O
O
OH
O
OH
m/z 147
m/z 181
[C H O Cl]
m/z 165
[C9H6OCl]
[
C H O ]
9 6 2
9 7 2
Further Fragmentation
Scheme 2. Fragmentation pathways for the chlorine-substituted ethyl 3-(2-chlorophenyl)propenoate. Pathway 1 is mainly observed
for ortho-substituted compounds, whereas Pathways 2 and 3 are for meta- or para-substituted compounds.).
a coumarin. We did not observe a coumarin for 3-
2-fluorophenyl)propenoate ethyl ester (m/z 146), which
contains a mixture of electron-donating (para) and electron-
1
,3
(
withdrawing (meta) groups in the benzo[d] dioxole sub-
stituent. The benzo[d]1,3dioxole ring in 9 provides similar
substituent electronic effects as occurs in the dichlorodiben-
zodioxin in 1. Hence, it is not unexpected that 9 shows
significant loss of ortho chlorine from the molecular ion (m/z
254) to give m/z 219 (48.8%). Again, as in the previous
structure, there is also loss of 63 Da to provide the base
peak at m/z 191 (pathway 1). However, the lack of the
electron-withdrawing oxygen in the meta position in 8 did
not prevent the loss of chlorine from the molecular ion (m/z
240) to give m/z 205. This observation demonstrated that
the mixture of electron-donating and electron-withdrawing
oxygens present in the dioxin structure (1–4) were not nec-
essary for the characteristic loss of the chlorine. Similar
results were observed with 5, which does not contain an oxy-
gen substituent, and had essentially the same fragmentation
pattern as molecules containing either one or two oxygen
substituents on the phenyl ring. Both 8 and 9 produced
the equivalent substituted benzopyrylium ion as the base
peak (m/z 177 and 191, respectively), showing that benzopy-
rylium ion formation is variable and does not depend upon
an unsubstituted phenyl ring.
instead fragmented similar to the m- and p-chlorocinnamic
8
acids reported by Schaldach and Gr u¨ tzmacher (pathway 3),
producing a peak at m/z 166. This peak was most likely
the 2-fluorocinnamic acid ion, which further fragments to
form the 2-fluorocinnamoylium ion (m/z 149). This lack of
fragmentation is most likely due to the stronger C–F bond
2
0
energy compared to C–Cl or C–Br.
Compound 6 with a chlorine in the 4-position did
not exhibit significant chlorine loss, instead losing a 45
žC
ž
Da fragment ([M] ꢀ C
2 5
H O ) via pathway 3 to give the
2
-chlorocinnamoylium ion base peak (m/z 165). The chloro-
substituted benzopyrylium ion was not observed (m/z 181)
and the compound instead lost C (28 Da) to give the
-chlorocinnamic acid ion (m/z 182). The mass spectrum of
provides an interesting comparison to the data discussed
2
H
4
2
7
previously for 4. Both of these compounds have chlorine
substituents in the 2- and 3-positions of the phenyl ring
žC
ž
and exhibit simultaneous loss of 63 Da ([M] ꢀ C
2
H
4
ꢀ Cl )
to form their respective base peaks (pathway 1). However,
the mass spectrum of 7, in contrast to 4, shows selective
chlorine loss from the molecular ion (m/z 278) to give m/z
2
43 (32.9%) as opposed to 4 for which no selective chlorine
Structures 13–15 were prepared to investigate whether
steric effects, resultant from the replacement of the hydrogens
on the olefinic side chain with methyl groups, affected the
mass spectral fragmentation. All three structures lost the
characteristic ortho chlorine to some extent. Compound 14
(ˇ-methyl) and 15 (˛,ˇ-methyl) had higher abundance ions
from loss of 35 Da (ˇ-methyl, 52%; ˛,ˇ-methyl 60%) than 13
(˛-methyl, 11%). There are minimal steric effects on chlorine
loss. These results agree with those of Madhusudanan
loss was observed. The fragmentation of compound 7 is
consistent with compound 5 in that the base peak consists
of a benzopyrylium ion, which in this case is the dichloro-
substituted 2-hydroxybenzopyrylium cation (pathway 2).
This fragmentation pattern is particularly interesting in
that the presence of multiple halogen substituents does
not interfere with the previously observed fragmentation
pathway in which ortho-substituted compounds fragment
via pathway 1.
9
et al. who showed that phenyl substitution on the olefin
Compounds 8 and 9 are structurally related; however, 8
has a single electron-donating methoxy substituent while 9
of cinnamic acid (˛-phenylcinnamic acid) also resulted
in the phenyl-substituted benzopyrylium ion (m/z 223).
Copyright  2008 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2008; 43: 1053–1062
DOI: 10.1002/jms

1058
C. E. Wheelock et al.
Table 4. Mass spectral data for structures related to substituted ethyl 3-phenylpropenoates
5
6
4
Y
Z
3
2
No.
2
3
4
5
6
Y
Z
Formula
MMa
Ion (% abundance)b
1
1
1
1
2
6
7
8
9
0
Cl
H
Cl
H
H
H
H
H
H
H
Cl
H
Cl
H
H
H
H
H
H
H
H
H
H
H
CH CH
CH CH
CH CH C(O)CH3
CH CH C(O)CH3
CH CH C(O)CH3
CNc
CNc
C9H6ClN
C9H6ClN
C10H9ClO
C10H9ClO
C10H9BrO
163.01 163/165(53.8/17.3); 136/138(8.3/2.8); 128(100)
163.01 163/165(100/33.0); 136/138(20.2/6.9); 128(97.0)
180.03 180/182(9.4/3.1); 165/167(28.9/9.7); 145(100)
180.63 180/182(28.3/9.9); 165/167(100/33.0); 145(37.6)
223.98 224/226(4.7/4.7); 209/211(13.1/12.6); 181/183(10.1/9.7);
Br
1
45(100); 102(40.3)
223.98 224/226(21.8/21.1); 209/211(61.4/59.9);
81/183(20.3/20.9); 145(68.7); 102(100)
2
1
H
Br
H
H
H
CH CH C(O)CH3
C10H9BrO
1
2
2
2
2
2
3
4
5
H
H
Cl
H
OCH2O H Cl CH CH CH2OH
OCH2O H Cl CH2CH2 CH2OH
C10H9ClO3 212.02 212/214(31.1/13.3); 196/198(30.5/9.3); 169/171(100/32.1)
C10H9ClO3 214.04 214/216(37.2/12.8); 169/171(100/32.8)
H
H
H
H
H
H
H
CH2CH2 CO2C2H5 C11H13ClO2 212.06 212/214(1.1/0.4); 177(100); 167(20.6); 149(76.7)
CH2CH2 CO2C2H5 C11H13ClO2 212.06 212/214(36.5/12.4); 167/169(14.1/4.5);
Cl
1
38/140(100/35.4); 141(44.4)
2
2
2
2
3
6
7
8
9
0
Cl
H
Cl
Cl
H
H
Cl
H
H
H
H
H
H
H
Cl
H
H
H
H
H
H
H
H
H
H
CH2
CH2
CH2
CO2C2H5 C10H11ClO2 198.04 198/200(1.6/0.5); 163(35.4); 135(8.3); 125(100)
CO2C2H5 C10H11ClO2 198.04 198/200(73.4/31.6); 153/155(11.2/3.1); 125/127(100/35.7)
CO2CH3
C9H9ClO2
184.62 184/186(5.2/1.7); 149(80.7); 125(100)
C
C
C
CO2C2H5 C11H9ClO2 208.03 208/210(10.1/2.8); 163/165(87.5/30.6); 136/138(100/33.4)
CO2C2H5 C11H9ClO2 208.03 208/210(14.6/4.8); 163/165(91.9/31.3); 136/138(100/33.4)
C
a
MM, molecular mass (Da), monoisotopic.
Ions are given in Thompson (Th, mass to charge ratios), with the relative abundance of each ion normalized to the base peak shown
b
in parentheses.
c
Compounds are a mixture of cis and trans isomers, which had identical fragmentation.
The reason for the lower abundance of the ion resulting
from chlorine loss from 13 compared to 14 or 15 is
unknown. It would be expected that steric congestion
is greatest for 15, which contains methyl substituents
at both the ˛- and ˇ-positions. However, the data for
In compounds 18–21, the ester functionality was replaced
with a methyl ketone to test if other carbonyls could cause the
characteristic halogen loss. It was speculated that the ketone
in a substituted 4-phenylbut-3-en-2-one, with an oxygen in a
2
similar sp hybridization as the carbonyl oxygen of the ester,
1
5 are most similar to those for 14, which contains a
might facilitate the loss of the halogen. Compound 18, with a
chlorine substituent located in the 2-position of 4-phenylbut-
3-en-2-one, underwent significant loss of 35/37 Da. The loss
of 15 Da from the molecular ion (m/z 180) provided an
ion at m/z 165 suggesting loss of a methyl group, which
is to be expected from a terminal methyl ketone. These
data agree with the ester-containing 5, which also lost the
2-chloro fragment. The methyl ketone was, therefore, able
to effect the same unique fragmentation as an ester moiety.
However, compound 19, which had the chlorine substituent
in the 4-position, also lost a significant amount of chlorine
unlike 6. Interestingly, 19 exhibited less chlorine loss then 18
(37.6% vs 100% respectively). The phenomenon was further
examined with compounds 20 and 21, which contain bromine
substituents in the 2- and 3-positions of 4-phenylbut-3-
en-2-one. Both compounds selectively lost the bromine
constituent (79/81 Da), concurring with that observed for
the ester-containing 10, which also lost bromine from the
2-position. The observed amount of halogen loss was similar
to compounds 18 and 19, in that halogen substitution in
the 2-position resulted in 100% loss (base peak), whereas
substitution in the 3-position (21) or 4-position (19) had
significantly less halogen loss. All four compounds 18–21
methyl in the ˇ-position. All three of these structures
have a base peak that results from the simultaneous
loss of 63 Da ([M] –C
equivalent methyl-substituted benzopyrylium ions formed
žC
ž
2
H
4
–Cl ), corresponding to their
from pathway 1 (m/z 161 for 13 and 14, m/z 175 for 15).
Other analogs related to ethyl 3-phenylpropenoates
Compounds 16 and 17, which have a nitrile in place of
the ester group, contain the chlorine substituent in the
2
- and 4-positions, respectively (Table 4). The mass spectra
for these two structures were similar, with the molecular
ion (m/z 163) losing chlorine to give (m/z 128), which
was the base peak for 16 and 97.0% abundant for 17. A
similar fragmentation pattern was reported for 1-cyano-2-
phenylethylene methyl ester, which also produced the m/z
1
compounds in this study that exhibited 100% chlorine loss
in the para position. Another interesting observation is that
compound 17 fragmented less than 16, with similar trends
repeated for compounds 18 and 19 as well as 5 and 6. These
data consistently show that less fragmentation is observed
when chlorine is in the para versus ortho position.
7
28 fragment. These nitrile-substituted olefins were the only
Copyright  2008 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2008; 43: 1053–1062
DOI: 10.1002/jms

EI mass spectra of substituted 3-phenylpropenoates 1059
formed the methyl benzopyrylium ion (m/z 145), which
was either the base peak or second most abundant peak
from the National Institute of Standards and Technology
(NIST) EI-MS library, and neither one showed selective loss
of chlorine. Examination of the acid derivatives of halogen-
displacing compounds in this study showed that the acid
forms also lost the halogen (data not shown) as had been
(
pathway 1).
Compounds 22 and 23 contain a 1,3-dioxole substituent
and are similar to 9 in structure. However, both of these
compounds contain a primary alcohol moiety instead of
the ester. Compound 22 still contains the olefin (molecular
ion m/z 212) and did not readily lose the characteristic
chlorine. Compound 23 is the saturated analog of 22 and
did not lose the 2-chloro substituent either, instead losing
8
earlier reported. Therefore, the fact that 2-chlorobenzoic
acid did not lose chlorine suggests that compounds lacking
at least one carbon spacer are unable to displace the halogen.
This observation is most likely due to the ring being sterically
unable to form a 4-membered structure. Compound 28 (the
methyl ester of 26) shows that the carbonyl of the methyl
ester is also efficient at effecting chlorine loss in the 2-position.
However, compound 28 did not evidence direct loss of the
methyl ester (seen as loss of methanol as was observed with
4
5 Da to give a base peak at m/z 169. A significant ion
was observed at m/z 171 indicating loss of the saturated
ethylene spacer (43 Da) from the molecular ion (m/z 214).
Both 22 and 23 have the same base peak (m/z 169), which
2
2
retains the chlorine substituent and may be a stable benzyl or
o-hydroxy cinnamic acid methyl esters. These data further
support the hypothesis that the loss of halogens, and chlorine
in particular, occurs through a concerted mechanism.
C
rearranged tropylium cationic species [C
8
H
6
ClO
2
] . Neither
compound formed the benzopyrylium ion intermediate.
The mass spectra of four additional structures were used
in the investigation of the effect of two (24, 25) and one
The alkane analogs with 1 or 2 carbons between the
phenyl ring and ester function both underwent significant
halogen loss, indicating that the length of the spacer can
be somewhat flexible. These analogs are less rotationally
constrained compared to the esters with a trans olefinic
linkage and it is not necessary to undergo the initial olefin-
isomerization step. This observation may be reflected in the
relative ion abundances of different fragments. Compound
5 contains an olefin and has loss of 63 Da as the base
(
26, 27) carbon-saturated spacers between the ester and the
phenyl ring on the fragmentation pattern. Comparison of the
mass spectral data for 24 and 26 again shows that loss of
3
5/37 Da occurs to a significant extent when the chlorine is
in the 2-position of the phenyl ring, but not in the 3-position
compounds 25 and 27). This observation shows that it is
(
possible to have a variable length spacer between the ester
and phenyl ring and still achieve chlorine loss. Compound
žC
ž
peak ([M] ꢀ C
2
H
4
–Cl ) as opposed to 24, the saturated
2
4 formed the saturated analog of the benzopyrylium ion of
analog, which has loss of 35/37 Da as the base peak
žC
ž
compound 5 (m/z 149). Compound 25 fragmented similar
to the m-chlorocinnamic acid reported by Schaldach and
([M] ꢀ Cl ). The less constrained alkane can more easily
effect sole loss of chlorine, whereas the trans olefin must
undergo isomerization first thus producing more of the 63 Da
fragment. The intermediate proposed in Scheme 1 may also
form a 5-membered ring as suggested by the results for 26.
8
Gr u¨ tzmacher and 26 formed the benzofuran analog (m/z
2
1
1
35), which had also been previously reported. The mass
spectra of 2- and 3-chlorobenzoic acid were downloaded
1
77
1
36
-
Cl
1
63
1
49
1
25
1
03
139
-OC H
2
5
7
7
9
9
7
4
1
67
5
1
89
6
3
43 62
89 109123
208
3
9
152 180
160
2
12
4
0
80
120
160
200
240
40
80
120
m/z
200
240
(A)
m/z
(C)
1
47
O
1
01
OC H
2 5
1
75
7
5
-Cl
1
63
1
36
5
1
Cl
210
6
3
8
9
3
9
118
2
80
4
0
80
120
160
200
240
(B)
m/z
(D)
Figure 3. Mass spectral fragmentation of (A) ethyl 3-(2-chlorophenyl)propanoate (compound 24), (B) ethyl
-(2-chlorophenyl)propenoate (compound 5), and (C) ethyl 3-(2-chlorophenyl)propynoate (compound 29). Spectra A and B show the
3
characteristic chlorine loss from the molecular ion to give m/z 177 and 175 respectively, while spectra C does not show loss of
chlorine. The 2-hydroxybenzopyrylium cation is observed in spectra B (m/z 147) and the saturated analog is observed in spectra A
(
m/z 149), whereas no corresponding ion is observed in spectra C. Figure (D) gives the base structure of compounds 24, 5 and 29,
and the boxed area highlights the region of variable saturation.
Copyright  2008 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2008; 43: 1053–1062
DOI: 10.1002/jms

1060
C. E. Wheelock et al.
Table 5. Atomic charges and molecular ionization potentials calculated using density functional theory. The atomic charges listed
are for the atom presumed to be involved in halogen displacement (usually the carbonyl oxygen)
Orthoa
Meta
Para
R
Cl
Chargeb Charge Excess
No. (neutral) (cation) ˛ spinc
Charge Charge Excess
(neutral) (cation) ˛ spin
Charge Charge Excess
(neutral) (cation) ˛ spin
R
IEd
IE
IE
O
O
31
ꢀ0.61
ꢀ0.54
0.10
ꢀ186.1
ꢀ0.61
ꢀ0.54
0.09 ꢀ187.9
ꢀ0.61
ꢀ0.55
0.09 ꢀ183.7
Et
O
O
3
2
ꢀ0.60
ꢀ0.54
0.10
ꢀ187.1
ꢀ0.61
ꢀ0.54
0.09 ꢀ188.8
ꢀ0.61
ꢀ0.54
0.09 ꢀ184.6
Me
3
3
3
4
ꢀ0.75
ꢀ0.30
ꢀ0.71
ꢀ0.12
0.03
0.26
ꢀ177.8
ꢀ194.0
ꢀ0.75
ꢀ0.30
ꢀ0.71
ꢀ0.12
0.03 ꢀ179.2
0.23 ꢀ195.5
ꢀ0.75
ꢀ0.31
ꢀ0.72
ꢀ0.13
0.02 ꢀ175.2
0.24 ꢀ191.1
OH
N
O
35
ꢀ0.55
ꢀ0.40
0.28
ꢀ186.4
ꢀ0.55
ꢀ0.40
0.27 ꢀ188.3
ꢀ0.56
ꢀ0.42
0.23 ꢀ184.4
3
3
6
7
ꢀ0.75
ꢀ0.60
ꢀ0.63
ꢀ0.52
0.17
0.13
ꢀ189.2
ꢀ192.5
ꢀ0.75
ꢀ0.60
ꢀ0.63
ꢀ0.52
0.16 ꢀ189.8
0.13 ꢀ192.2
ꢀ0.75
ꢀ0.60
ꢀ0.64
ꢀ0.53
0.15 ꢀ186.2
0.11 ꢀ189.3
OH
O
Me
O
3
3
8
9
ꢀ0.59
ꢀ0.60
ꢀ0.54
ꢀ0.51
0.09
0.15
ꢀ193.3
ꢀ191.5
ꢀ0.60
ꢀ0.60
ꢀ0.54
ꢀ0.52
0.15 ꢀ191.3
0.15 ꢀ191.2
ꢀ0.61
ꢀ0.60
ꢀ0.55
ꢀ0.53
0.07 ꢀ190.5
0.13 ꢀ188.3
O
O
Me
Et
O
O
O
40
ꢀ0.61
ꢀ0.53
0.11
ꢀ191.9
ꢀ0.61
ꢀ0.53
0.17 ꢀ190.2
ꢀ0.61
ꢀ0.53
0.10 ꢀ189.3
Et
O
a
Ortho, meta and para refer to the position of the chlorine substitution on the phenyl ring.
Charge units are electrons (e).
Units are electrons (e).
b
c
d
Ionization energy, units are kcal/mole.
Compounds 29 and 30 are the 2- and 4- chlorine
and propanoic acid esters (Fig. 3). Examination of the
mass spectrum of ethyl 3-phenylpropiolate from the NIST
library showed loss of 45 Da as the base peak, indicating
that cleavage of the ester linkage is the major route of
fragmentation for these structures. The observation that the
2-chloro substituted alkyne ester does not preferentially lose
chlorine is likely related to the sp geometry of the alkyne
substituted 3-phenyl alkynoate esters, which have nearly
identical mass spectra. The molecular ion (m/z 208) loses
a 44 Da fragment, suggesting loss of the ester moiety
to provide a m/z 163 ion with ¾90% abundance for
both the compounds. This fragment does not undergo
the characteristic chlorine loss seen for the propenoic
Copyright  2008 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2008; 43: 1053–1062
DOI: 10.1002/jms

EI mass spectra of substituted 3-phenylpropenoates 1061
linkage between the ethyl ester and the phenyl ring. This
rigid spacer prevents the carbonyl of the ester moiety from
interacting with the halogen in the 2-position (Fig. 3).
the nitrile-containing compounds 16 and 17) was found to
be correlated with both the selectivity and % of halogen
displacement. The only compound not capable of effecting
chlorine displacement was the primary alcohol, which was
the most negatively charged proposed electophilic atom
(ꢀ0.71 e), as opposed to the nitrile (ꢀ0.12 e) that effected
100% chlorine loss in both ortho- and para-substituted
compounds. Both the unsaturated and saturated ethyl 3-
(4- or 3-chlorophenyl)propenoate compounds (6 and 25) did
not evidence any chlorine loss as would be expected, given
that the halogen was not in the ortho position. However,
the ketone (18) contained sufficient electrophilicity to effect
some chlorine loss (37%) and the nitrile exhibited 100% loss
for para chlorine. Results from these calculations indicate
that there is a correlation between the net charge on the
presumed halogen-displacing atom and the loss of chlorine,
and therefore provide support for the proposed electrophilic
mechanism for the formation of the benzopyrylium ion.
Density functional theory calculations
A particularly interesting observation was the position-
dependent loss of chlorine from the different structures
examined. Both the chemical moiety attached to the olefin
as well as the position of the halogen on the phenyl ring
appeared to affect the overall percentage of halogen loss. To
further examine this effect, we calculated a number of chemi-
cal properties that may be associated with the fragmentation
process. These properties include the electronic charge on the
atom assumed to be involved in halogen displacement – in
most cases the carbonyl oxygen – for both the neutral and
cationic forms of the molecule. We also calculated the excess
˛-spin on the relevant atom (for the cationic form) and overall
ionization energy of the molecule. These calculations were
performed with the halogen (chlorine) in the ortho, meta, and
para positions (Table 5). There were essentially no differences
between charges calculated on the presumed reactive atom
as the chlorine position was varied from ortho to meta and
para, and only small differences were observed in the excess
CONCLUSIONS
The observation of an interesting mass spectral fragmenta-
tion with loss of 35/37 Da for substituted ortho-chlorinated
dibenzodioxin propenoate esters lead to an exploration
of the structural fragmentation requisites using a series
of substituted ethyl 3-(substituted phenyl)propenoates. A
proposed mechanism, based upon previous studies, sug-
gests that an ester moiety linked to a phenyl ring facil-
itates halogen loss from the 2-position. With respect
to halogen substitution, chlorine and bromine, but not
fluorine (substituents on phenyl) in the 2-position are
lost. Methyl substituents on the olefin of ethyl 3-(2-
chlorophenyl)propenoate did not preclude chlorine loss.
Saturation of ethyl 3-(2-chloropheny)propenoate to give
ethyl 3-(2-chloropheny)propanoate also resulted in prefer-
ential ortho chlorine loss. In addition, reduction in the spacer
length between the ester and phenyl ring to form ethyl
˛
-spin and ionization energy. This suggests that any differ-
ences in halogen displacement for these different isomers are
because of nonelectronic effects.
The predicted atomic charges for neutral and cationic
2
forms of the molecules are strongly correlated (R D 1.000)
and, therefore, only the cation data are shown in Fig. 4. In
the following analysis, we will assume that the net charge
on an atom is indicative of its electrophilicity, and that an
atom with a more negative charge is less electrophilic. The
net atomic charge of the key atom proposed to be involved
in the halogen displacement (usually the oxygen, except for
Ortho
Meta/Para
2
-chlorophenyl acetate also provided the characteristic chlo-
Sat Ester
100
rine loss. Positioning of the halogens in the 3- or 4-position
of the phenyl ring resulted in other fragmentations, such as
cleavage of the ester with a loss of 45 Da. The replacement of
the ester moiety in ethyl 3-(2-chlorophenyl)propenoate with
a methyl ketone, but not a primary alcohol, also resulted in
selective chlorine loss. The substitution of a nitrile for the
ester moiety resulted in isomer nonspecific chlorine loss in
both the 2- and 4-positions of the phenyl ring. Finally, ethyl
Ketone
Nitrile
7
5
2
0
5
0
5
Unsat Ester
Ketone
Alcohol
0.8
Sat &Unsat Ester
-
-0.6
-0.4
-0.2
0
(
2-chlorophenyl)propynoate did not lose chlorine, indicating
Cation Charge (e)
that the rigid alkyne spacer with sp bond geometry between
the phenyl ring and the ester prevents the ester-facilitated
loss of chlorine from the 2-positon of the phenyl ring. These
results suggested that geometric constraints are important
because rigid sp-hybridized groups are too inflexible to allow
the carbonyl oxygen to be in the required position to effect
the loss of the halogen. However, these data show that the
selective chlorine loss from the 2-position of the phenyl ring
Figure 4. Relationship between % of chlorine loss and the
charge on the cation of the chemical moiety attached to the
phenyl ring. Cation charges are from Table 5 using either the
ortho, meta or para values as appropriate for the following
compounds: Unsat ester (31), alcohol (33), nitrile (34) and
ketone (35), and sat ester (39). The % chlorine loss values are
from Tables 3–5 and for ortho values include compounds 5,
2
16, 18, 22 and 24; for meta values include compound 25; for
is only achieved by the ester moiety and that other sp
para values include compounds 6, 17 and 19. Nitrile-containing
compounds exhibited 100% chlorine loss with substitution in
either the ortho (16) or para (17) position. Sat, saturated; Unsat,
unsaturated.
hybridized atoms or moieties in the spacer are unable to
effect the chlorine loss.
Further work could examine the effects of increased
spacer length and ring saturation upon halogen loss. The
Copyright  2008 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2008; 43: 1053–1062
DOI: 10.1002/jms

1062
C. E. Wheelock et al.
increased electrophilicity of the cation involved in halogen
displacement resulted in decreased selectivity of halogen loss
6. Denison MS, Nagy SR, Ziccardi M, Clark GC, Chu M, Brown DJ,
Shan G, Sugawara Y, Gee SJ, Sanborn JR, Hammock BD. In
Bioanalytical Approaches for the Detection of Dioxin and Related
Halogenated Aromatic Hydrocarbons, Wilson SH Suk WA (eds).
Lewis Press: Boca Raton, 2002; 483.
7. Williams DH, Cooks RG, Bowie JH, Madsen P, Schroll G,
Lawesson S. Studies in Mass Spectrometry-XX: Bond-forming
reactions occurring in the fragmentation of some ˛, ˇ-
unsaturated esters and nitriles upon electron impact. Tetrahedron
(
in terms of ortho versus meta or para) and increased percentage
of halogen loss. Examination of the structural requirements
for the preferential loss of a phenyl ring halogen ortho
to a variable spacer arm showed that this process can
only be selectively achieved by an ester moiety. Results
showed that a number of different chemical moieties were
capable of effecting the intramolecular radical substitution to
form a substituted benzopyrylium ion. These observations
greatly expand earlier studies that had focused on only
esters and acids. The correlations between cation charge
1967; 23: 3173.
8
. Schaldach B, Gr u¨ tzmacher H-F. The fragmentations of
substituted cinnamic acids after electron impact. Organic Mass
Spectrometry 1980; 15: 175.
9
. Madhusudanan KP, Murthy VS, Fraisse D, Becchi M. Re-
arrangements in the mass spectra of a-phenylcinnamic acid
and its derivatives. Organic Mass Spectrometry 1991; 26: 505.
(electrophilicity) and selectivity of halogen loss were very
strong, providing support for the proposed intramolecular
radical substitution.
1
0. Ronayne J, Williams DH, Bowie JH. Studies in Mass
Spectrometry. XIX. Evidence for the Occurrence of Aromatic
Substitution Reactions upon Electron Impact. Journal of the
American Chemical Society 1966; 88: 4980.
Acknowledgements
The authors thank Jozsef Lango, A. Daniel Jones and Roger Mercer
for advice and technical assistance. C.E.W. was supported by an
EU Sixth Framework Programme (FP6) Marie Curie International
Incoming Fellowship (IIF). This work was supported in part by
NIEHS Grant R37 ES02710, NIEHS Superfund Grant P42 ES04699,
and NIEHS Center for Environmental Health Sciences Grant P30 ES
11. Newman MS, Merrill SH. Synthesis of a series of substituted
phenylpropiolic acids. Journal of the American Chemical Society
1955; 77: 5549.
12. Becke AD. Density-functional thermochemistry. III. The role of
exact exchange. Journal of Chemical Physics 1993; 98: 5648.
13. Lee C, Yang W, Parr RG. Development of the Colle-Salvetti
correlation-energy formula into a functional of the electron
density. Physical Review B 1988; 37: 785.
05707. The chemical modeling was funded by the U.S. Department of
Energy, Office of Science, Offices of Advanced Scientific Computing
Research and Biological & Environmental Research (DE-FG02-
14. Jensen F. Introduction to Computational Chemistry. John Wiley &
Sons: New York, 1999; 446.
04ER25625), through the U.C. Merced Center for Computational
Biology.
15. Reed AE, Weinstock RB, Weinhold F. Natural population
analysis. Journal of Chemical Physics 1985; 83: 735.
1
1
1
6. Emery EM. Mass spectra of aromatic esters. Analytical Chemistry
REFERENCES
1960; 32: 1495.
7. Zalewski RI. The mass spectra of some substituted methyl
cinnamates. Organic Mass Spectrometry 1981; 16: 52.
8. Dass C, Robinson CN, Pflug JL, Desiderio DM. Mass spectral
fragmentation of ˛, ˇ-unsaturated esters: a study on the meta
and para derivatives of ethyl ˛-fluorocinnamate. Organic Mass
Spectrometry 1988; 23: 853.
1
. Sanborn JR, Gee SJ, Gilman SD, Sugawara Y, Jones AD, Rogers J,
Szurdoki F, Stanker LH, Stoutamire DW, Hammock BD. Hapten
synthesis and antibody development for polychlorinated
dibenzo-p-dioxin immunoassays. Journal of Agricultural and Food
Chemistry 1998; 46: 2407.
2
. Sugawara Y, Gee SJ, Sanborn JR, Gilman SD, Hammock BD.
Development of a highly sensitive enzyme-linked immunosor-
bent assay based on polyclonal antibodies for the detection of
polychlorinated dibenzo-p-dioxins. Analytical Chemistry 1998; 70:
1
9. Bowie JH, Williams DH, Madsen P, Schroll G, Lawesson SO.
Studies in mass spectrometry - XVII. Rearrangement processes
in some esters containing unsaturated linkages - the elimination
of CO2 from esters. Tetrahedron 1967; 23: 305.
1092.
20. Sanderson RT. Chemical Bonds and Bond Energy. Academic Press:
New York, 1976; 218.
3
4
5
. Shan G, Sanborn JR, Gilman SD, Gee SJ, Stoutamire DW,
Mercer RS, Jones AD, Stanker LJ, Denison MS, Hammock BD.
Surrogates for dioxin analyses. Analytical, ELISA and
toxicological aspects. Organohalogen Compounds 2000; 45: 188.
. Denison MS, Nagy SR, Clark GC, Chu M, Brown DJ, Murata H,
Shan G, Sanborn JR, Hammock BD. Bioanalytical approaches
for the detection of dioxin and related halogenated aromatic
compounds. Organohalogen Compounds 2001; 54: 8.
21. G u¨ sten H, Klasinc L, Kramer V, Marsel J. Mass spectra of
monosubstituted trans-stilbenes. Organic Mass Spectrometry 1974;
8: 323.
2
2. Kuster T, M a¨ ndli H, Robbiani R, Seibl J. Vergleichende unter-
suchung von reaktivit a¨ ten und ionenstrukturen beim mono-
molekularen abbau isomerer hydroxyzimst a¨ uremethylester
unter elektronenbeschuss. Helvetica Chimica Acta 1978; 61: 1017.
. Shan G, Leeman WR, Gee SJ, Sanborn JR, Jones AD, Chang DPY,
Hammock BD. Highly sensitive dioxin immunoassay and its
application to soil and biota samples. Analytica Chimica Acta
2001; 444: 169.
Copyright  2008 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2008; 43: 1053–1062
DOI: 10.1002/jms