Gas-phase fragmentations of anions derived from N-phenyl benzenesulfonamides

Article abstract of DOI:10.1007/s13361-013-0671-4

In addition to the well-known SO₂ loss, there are several additional fragmentation pathways that gas-phase anions derived from N-phenyl benzenesulfonamides and its derivatives undergo upon collisional activation. For example, N-phenyl benzenesulfonamide fragments to form an anilide anion (m/z 92) by a mechanism in which a hydrogen atom from the ortho position of the benzenesulfonamide moiety is specifically transferred to the charge center. Moreover, after the initial SO₂ elimination, the product ion formed undergoes primarily, an inter-annular H₂ loss to form a carbazolide anion (m/z 166) because the competing intra-annular H₂ loss is significantly less energetically favorable. Results from tandem mass spectrometric experiments conducted with deuterium-labeled compounds confirmed that the inter-ring mechanism is the preferred pathway. Furthermore, N-phenyl benzenesulfonamide and its derivatives also undergo a phenyl radical loss to form a radical ion with a mass-to-charge ratio of 155, which is in violation of the so-called "even-electron rule." [Figure not available: see fulltext.]

Full text of DOI:10.1007/s13361-013-0671-4

B American Society for Mass Spectrometry, 2013
J. Am. Soc. Mass Spectrom. (2013) 24:1280Y1287
DOI: 10.1007/s13361-013-0671-4
RESEARCH ARTICLE
Gas-Phase Fragmentations of Anions Derived
from N-Phenyl Benzenesulfonamides
John A. Hibbs, Freneil B. Jariwala, Carl S. Weisbecker, Athula B. Attygalle
Center for Mass Spectrometry, Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute
of Technology, Hoboken, NJ 07030, USA
Abstract. In addition to the well-known SO2 loss, there are several additional
fragmentation pathways that gas-phase anions derived from N-phenyl
benzenesulfonamides and its derivatives undergo upon collisional activation.
For example, N-phenyl benzenesulfonamide fragments to form an anilide
anion (m/z 92) by a mechanism in which a hydrogen atom from the ortho
position of the benzenesulfonamide moiety is specifically transferred to the
charge center. Moreover, after the initial SO2 elimination, the product ion
formed undergoes primarily, an inter-annular H2 loss to form a carbazolide
anion (m/z 166) because the competing intra-annular H2 loss is significantly
less energetically favorable. Results from tandem mass spectrometric
experiments conducted with deuterium-labeled compounds confirmed that the inter-ring mechanism
is the preferred pathway. Furthermore, N-phenyl benzenesulfonamide and its derivatives also undergo
a phenyl radical loss to form a radical ion with a mass-to-charge ratio of 155, which is in violation of
the so-called “even-electron rule.”
Key words: Sulfonamides, ESI-MS/MS, Substituent effects, Fragmentation, Even-electron rule
Received: 25 February 2013/Revised: 6 May 2013/Accepted: 9 May 2013/Published online: 19 June 2013
amide residues in food and environmental samples are widely
sought. Liquid chromatography in conjunction with tandem
mass spectrometry (LC-MS/MS) is the preferred technique for
Introduction
any consider that the present era of chemotherapy
started with the discovery of antibacterial activity of
detection and quantification of these compounds [8–10].
Although older studies have primarily focused on the mass
spectrometry of cationic adducts derived from sulfonamides [9,
10], some of the newer investigations have initiated the use of
anions generated under negative-ion conditions [11, 12].
M
sulfonamides in the mid-1930s [1]. Today, sulfonamides are
extensively utilized in animal husbandry as antibacterial
agents. In addition, they are widely deployed as intermedi-
ates for drug synthesis in the pharmaceutical industry and as
pesticides in agriculture [2–4]. However, the widespread use
of sulfonamides in farming leads to their accumulation in
meat and dairy products, as well as in landfill leachates [5,
6]. The presence of sulfonamides in food has been carefully
monitored and regulated by the USFDA since the 1970s [7].
Thus, efficient analytical techniques for identifying sulfon-
To characterize sulfonamides by mass spectrometry, the
fragmentation mechanisms of gas-phase ions derived from
these compounds should be well understood. In addition to
classic EI-MS studies, several investigations have been carried
out on fragmentations of protonated and deprotonated sulfon-
amides [11–13]. A loss of a neutral sulfur dioxide molecule by
an intramolecular rearrangement process has been reported for
both cationic and anionic derivatives [11, 12, 14]. The proposed
mechanisms indicate that the fragmentation proceeds along a
specific pathway as shown in Scheme 1 [11, 12, 14], which has
been shown to be energetically favorable by computa-
tional studies [12, 14]. However, most of the previous
studies carried out on “tandem-in-time” instruments have
Electronic supplementary material The online version of this article
(doi:10.1007/s13361-013-0671-4) contains supplementary material, which
is available to authorized users.
Correspondence to: Athula Attygalle; e-mail: aattygal@stevens.edu

Hibbs et al.: Fragmentation of N-Phenyl Benzenesulfonamides
1281
Scheme 1. The mechanism that has been proposed to rationalize the loss of sulfur dioxide from N-phenyl
benzenesulfonamide [11, 12, 14]
focused only on the SO2 elimination because resonance
excitation methods used in ion storage devices generate
only fragments that originate directly from the precursor
ion [12]. In contrast, it is known that “tandem-in-space”
instruments activate ions by multiple collisions and
generate several generations of product ions [11].
However, attempts have not been made to rationalize
fragmentations that take place either subsequent to the
initial SO2 loss, or simultaneously in competition with
the SO2 elimination. In this paper, we present results
from a detailed investigation on gas-phase fragmentation
of anions derived from N-phenyl benzenesulfonamides.
residue (see Table 1 for a list of compounds that were
synthesized).
Purification of N-Phenyl Benzenesulfonamide
To purify N-phenyl benzenesulfonamide, the residue
obtained from the dichloromethane extract was dissolved in
methanol and streaked on a preparative TLC plate (silica gel
on aluminum, 5×10 cm, 0.2 mm coat thickness, 60 Å pore
size). The plate was developed using ethyl acetate-hexane
(1:5) as the mobile phase, and the silica gel of the area that
corresponded to N-phenyl benzenesulfonamide was scraped
and extracted with methanol. The mixture was filtered
through a cotton wool plug, and the filtrate was evaporated
to afford N-phenyl benzenesulfonamide as white solid.
Experimental
Materials
Acetonitrile (HPLC grade) and ammonium hydroxide
(ACS grade) were purchased from Pharmco-AAPER
(Brookfield, CT, USA). Benzenesulfonyl chloride,
[2H5]benzenesulfonyl chloride, mesitylenesulfonyl chlo-
ride, p-toluenesulfonyl chloride, p-nitrobenzenesulfonyl
chloride, and p-bromobenzenesulfonyl chloride, aniline,
and TLC plates were purchased from Aldrich Chemical
Co. (St. Louis, MO, USA) and used as obtained.
ESI-MS/MS
A Micromass Quattro Ultima mass spectrometer equipped
with an electrospray ion source was used to obtain CID mass
spectra. The source and desolvation gas temperatures were
maintained at 100 and 200 °C, respectively. Capillary
voltage was held at 3000 V. The collision energy was kept
Synthesis of N-Phenyl Benzenesulfonamide
Derivatives
N-phenyl benzenesulfonamide, N-phenyl [2H5]benzenesulfonamide,
N-phenyl mesitylenesulfonamide, N-phenyl p-toluenesulfonamide,
N-phenyl p-nitrobenzenesulfonamide, and N-phenyl p-
bromobenzenesulfonamide were synthesized by adding
aniline (20 mg) to solutions of benzenesulfonyl chloride,
[2H5]benzenesulfonyl chloride, mesitylenesulfonyl chloride, p-
toluenesulfonyl chloride, p-nitrobenzenesulfonyl chloride, or
p-bromobenzenesulfonyl chloride (40 mg), respectively, in
dichloromethane (2 mL) and pyridine (0.1 mL). After 5 h, the
mixture was neutralized with concentrated hydrochloric acid
(1 mL) and the aqueous phase was removed. The organic phase
was then evaporated to obtain the desired compound as a solid
Table 1. N-phenyl benzenesulfonamide derivatives synthesized
Compound
Nominal mass
R1
R2
R3
1
2
3
4
5
6
233
238
247
275
311/313
278
H
D
CH3
CH3
Br
H
D
H
H
H
H
H
D
H
CH3
H
NO2
H

1282
Hibbs et al.: Fragmentation of N-Phenyl Benzenesulfonamides
Calculated structures were first optimized to obtain stationary
states using a 6-31G(d) split-valence basis set. Electronic
energies of structures were then recalculated using a 6-
311++G(d,p) basis set incorporating diffuse functions.
Frequency analysis of optimized structures was used to
distinguish energy minima from saddle points. Transition
states were identified as first-order saddle points with
one imaginary frequency. Zero-point energy corrections
were made to the energies of stationary states, and the
corrections were scaled by a factor of 0.9804 [18]. The
structures of diradical triplet states were calculated using
the unrestricted method.
Results and Discussion
Figure 1. CID Mass Spectrum of m/z 232 ion derived from
N-phenyl benzenesulfonamide (1)
Sulfonamides derived from primary and secondary amines are
sufficiently acidic to generate gaseous anions under ESI
conditions. For example, N-phenyl benzenesulfonamide (1)
undergoes deprotonation and produces an m/z 232 ion upon
electrospray ionization. Figure 1 shows a collision-induced
dissociation mass spectrum of the m/z 232 ion obtained under
ESI-MS/MS conditions. The peak at m/z 168 represents a SO2
elimination from the parent ion. This fragmentation step has
been well investigated previously [11, 12], and a mechanism
for the loss of the SO2 (Scheme 1) has been proposed [12, 14].
However, the CID spectra of N-phenyl benzenesulfonamides
show many other significant peaks, which had not been
thoroughly scrutinized. For example, the m/z 168 peak is
accompanied by a significant peak at m/z 166. A product ion
spectrum recorded from the m/z 168 ion generated in-source
at 25 eV (unless otherwise stated), and argon was used as the
collision gas. Samples were infused into the ESI source as
acetonitrile-water-ammonia (50:50:0.01) solutions at a flow
rate of 15 μL/min.
Quantum Mechanical Calculations
All calculations were performed using the Gaussian 03 W
program package [15]. Calculations were performed using
density functional theory. The B3LYP functional was used,
incorporating the Becke three-parameter non-local-exchange
functional [16] and the correlation functional of Lee et al. [17].
Figure 2. Product ion mass spectra of in-source generated [M – H – SO2]– ions derived from N-phenyl benzenesulfonamide (1)
(a), N-phenyl [2H5]benzenesulfonamide (2) (b), N-phenyl p-toluenesulfonamide (3) (c), and N-phenyl mesitylenesulfonamide (4)
(d). Spectra were recorded at a collision energy setting of 17 eV

Hibbs et al.: Fragmentation of N-Phenyl Benzenesulfonamides
1283
Scheme 2. Proposed mechanisms for the loss of H2 from the diphenylamine ion
established that the m/z 166 ion is the outcome of a H2 loss
from the m/z 168 ion (Figure 2a).
To determine the precise origin of the two hydrogen atoms
involved in the dihydrogen elimination, we synthesized N-
phenyl [2H5]benzenesulfonamide (2) and recorded a product
ion spectrum from its m/z 173 fragment ion generated in the ion
source. The spectrum of the m/z 173 ion, which bears five
deuterium atoms on the sulfonamide ring, showed a significant
peak at m/z 170 for a loss of a 3-Da H-D molecule (Figure 2b),
indicating that each ring contributes one hydrogen atom for the
Figure 3. Energy values and structures of proposed intermediates, transition states, and products for the diphenylamine ion
fragmentation

1284
Hibbs et al.: Fragmentation of N-Phenyl Benzenesulfonamides
Scheme 3. The loss of a phenyl radical with subsequent SO2 elimination from N-phenyl benzenesulfonamide (1) anions
H2 loss. The m/z 170 peak was, however, accompanied by two
smaller peaks at m/z 169 and 171. In other words, in addition to
the primary mechanism, which eliminates a H-D molecule,
there are other fragmentation pathways which lead, to a lesser
extent, to D2 or H2 losses. Furthermore, peaks for an analogous
H2 loss were also observed in the spectrum of N-phenyl p-
toluenesulfonamide (3) (Figure 2c). This observation confirmed
that the presence of a hydrogen atom at the para position is not
required for the H2 elimination. Moreover, the spectrum of the
N-phenyl mesitylenesulfonamide (4) anion showed a peak at m/z
210 for a SO2 elimination. However, the peak at m/z 208
indicating a H2 loss in the product ion spectrum of m/z 210 was
not as intense as that for the H2 loss from the m/z 168 anion
derived from N-phenyl benzenesulfonamide (1) (Figure 2d).
Thus, it is rational to propose that the primary contribution to the
H2 loss involves the hydrogens at the ortho positions. The
intensity of the peak for the H2 loss is significantly reduced when
hydrogen atoms are unavailable at either of the ortho positions.
From these results we conclude that for the H2 loss, one
hydrogen atom originates from the anilide moiety while the
other hydrogen atom comes from the phenyl ring of the sulfonyl
moiety (Scheme 2, Pathway ‘b’). However, there are other
pathways in which both hydrogen atoms originate entirely from
either the anilide or the phenyl moiety of the sulfonyl group.
From the experimental results, we can envisage several
mechanistic pathways for the dihydrogen loss from the
diphenylamine anion. A computational chemistry study
revealed that an inter-annular loss of H2 is an exothermic
process, and the product ion formed in this way was
7.5 kcal/mol lower in energy than its precursor ion
(Figure 3; and Supplementary Data 1). At least for the
particular intra-annular H2 loss that was considered for
computations, the elimination process was found to be
endothermic by 81.6 kcal/mol. In other words, the compu-
tational data is congruent with the experimental data, which
showed that the intra-annular H2 loss pathway is not the
primary fragmentation pathway.
one of the bonding π orbitals undergoes conversion to a triplet
state (Scheme 2, 2a; Supplementary Data 1) by a charge
remote mechanism. In the diradical anion formed in this
way, one of the C–H bonds then undergoes a bond
elongation to form a transition state (2a-TS) in which a
hydrogen radical and a vinylidene radical anion are held
together. From this arrangement, a dihydrogen molecule
is then eliminated to generate the final product with a
benzyne moiety (although Figure 3 shows only one
arrangement (3a), other benzyne arrangements may also
be possible). For the more energetically favorable
pathway (Scheme 2, Pathway ‘b’), the diphenylamine
anion is first converted to a 4a,4b-dihydrocarbazole triplet
state (3b; Supplementary Data 1). From this triplet state,
a dihydrogen molecule is eliminated to give the carbazole
anion (4b) as the final product (m/z 166). In fact,
diphenylamine has been shown to undergo oxidation to
carbazole under photo-induced conditions, and a mecha-
nism similar to our postulations has been proposed to
rationalize the formation of carbazole from diphenylamine
Figure 4. Precursor ion mass spectrum of the m/z 92 ion
derived from N-phenyl benzenesulfonamide (1) recorded at a
collision energy setting of 17 eV and a cone voltage setting of
80 V
For the more energetic intra-annular pathway, a dihydrogen
molecule is eliminated from one of the phenyl rings by a 1,2-
diabstraction mechanism (Scheme 2, Pathway ‘a’). Initially,

Hibbs et al.: Fragmentation of N-Phenyl Benzenesulfonamides
1285
[19]. Even 2,6-dichlorodiphenylamine is known to elim-
An intense peak at m/z 92 is observed in the mass
inate HCl under photochemical conditions to form
carbazole derivatives [20]. Both proposed pathways pass
through high-energy transition states in which a C–H
bond is elongated and broken (Figure 3, 3b-TS and 2a-
TS). Thus, a strong kinetic isotope effect should be
expected when a C–D bond is broken. The observation
that the intensity of the m/z 171 peak, corresponding to
the H2 loss, is nearly twice that for the D2 loss (m/z 169;
Figure 2b) supports the influence of a kinetic isotope
effect (computed atomic coordinates for all structures are
given in Supplementary Data 2).
Moreover, the mass spectrum of gaseous anions derived
from N-phenyl benzenesulfonamide (1) showed an intense
peak at m/z 155 (Figure 1), which corresponded to a loss
of the phenyl group attached to the sulfonyl moiety by a
homolytic cleavage. This fragmentation is particularly
interesting because it violates the so-called “even-electron
rule” [21]. Previous studies have shown that this rule is
often violated [22–25], even during the fragmentation of
positive ions derived from N-phenyl benzenesulfonamide
derivatives, albeit it is the S–N bond that is cleaved under
positive ion conditions [26] (because there are so many
exceptions, the authors of reference [21] have pointed out
as early as 1980 that the use of the term “rule” is a
misnomer. However, some books continue to advocate it
as a thermodynamically rational generalization [27]). The
m/z 155 anion generated in this way fragments further by
losing SO2 to give a fragment anion of m/z 91 (Scheme 3).
spectrum of N-phenyl benzenesulfonamide (1) (Figure 1).
Presumably, the m/z 92 ion originates from two different
fragmentation pathways. Either the m/z 168 ion formed by
the initial SO2 loss eliminates a benzyne, or the m/z 156 ion
formed by the initial benzyne loss then loses a SO2 molecule.
Although the intensity of the m/z 156 peak is low, a precursor-
ion tandem mass spectrometric experiment conducted using a
purified sample of N-phenyl benzenesulfonamide (1) (Figure 4)
demonstrated that both m/z 156 and 168 ions are the parents of
the m/z 92 ion.
For the benzyne elimination from the m/z 232 ion of N-
phenyl benzenesulfonamide (1), a hydrogen atom from the
phenyl ring of the sulfonyl moiety must be removed. The
hydrogen atoms that are available for the transfer from the
phenyl moiety are represented as R1, R2, and R3 (Table 1).
In order to determine which hydrogen is specifically trans-
ferred, the mass spectra of N-phenyl benzenesulfonamide (1),
N-phenyl p-toluenesulfonamide (3), and N-phenyl
mesitylenesulfonamide (4) were considered. In N-phenyl
mesitylenesulfonamide (4) molecules, both the R1 and R3
positions are “blocked” with methyl groups (Table 1).
Consequently, the intensity of the m/z 92 peak in the mass
spectrum of N-phenyl mesitylenesulfonamide (4) (Figure 5b)
is dramatically reduced (by 98 %) compared with that of the
mass spectrum of N-phenyl benzenesulfonamide (1)
(Figure 1). This result indicated that the R2 hydrogens
(Table 1) are not the preferred source for the hydrogen
atom that is lost during the benzyne elimination process.
Figure 5. CID mass spectrum of m/z 246 ion derived from N-phenyl p-toluenesulfonamide (3) (a) and that of m/z 274 from N-
phenyl mesitylenesulfonamide (4) (b)

1286
Hibbs et al.: Fragmentation of N-Phenyl Benzenesulfonamides
–
Figure 6. Plot of ln(I(M
–/I(M
–) versus the σp substituent constants [from reference 28] for N-phenyl p-
–
H)
–
(C6H3R)
–
H)
substituted benzenesulfonamides (1,3,5,6)
Furthermore, the hydrogen transfer was not affected in
any significant manner when the R3 position was
blocked, which was revealed by the intense peak that
was observed at m/z 92 in the spectrum of N-phenyl p-
toluenesulfonamide (3) (Figure 5a).
differences in collision energies on ions with very close nominal
masses is not very dramatic. For example, a comparison of
breakdown curves of N-phenylbenzenesulfonamide and N-phe-
nyl-p-toluenesulfonamide showed that the profiles are very
–
similar. See Supplementary Data 3). The σp substituent
To find out the effect of inductive and resonance
properties of substituent groups on the benzyne elimination,
several N-phenyl derivatives were synthesized from para-
substituted benzenesulfonyl chlorides and their spectra were
recorded. A semi-logarithmic plot of relative intensity ratios of
the product ion [(M – H) – (benzyne)]– and precursor ion [(M –
constants were selected because the resonance effect of the R1
substituent group was considered to be the predominant factor
that affects the overall electron distribution of the ion. The
experimental data indicate that electron-withdrawing groups
(such as NO2) facilitate the transfer of a hydride from the R3
position to the sulfone moiety as is indicated in Scheme 4.
Although, N-phenyl p-methoxybenzenesulfonamide was avail-
able, data from this compound were not used for our substituent
effect study because this compound exhibited a strong preference
to fragment by another pathway and lose a methyl radical. In fact,
to date, no σp– values have been reported for the p-oxygen radical
group.
–
H)]– versus the substituent constant values (σp ) as described by
Hansch [28] showed that electron-withdrawing groups at the R1
position of the phenyl ring facilitate the benzyne elimination
process (Figure 6) (although different collision energies are
required to afford a similar degree of fragmentation of ions of
different mass, a detailed study showed the effect of small
Scheme 4. Proposed mechanism for the loss of a substituted benzyne and subsequent sulfur dioxide elimination from anions
derived from N-phenyl benzenesulfonamides (1,3,5,6)

Hibbs et al.: Fragmentation of N-Phenyl Benzenesulfonamides
1287
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Conclusions
N-phenyl benzenesulfonamide anions were shown to exhibit
several fragmentation channels beyond sulfur dioxide elim-
ination. The anion derived from the initial SO2 loss, then
eliminates either a H2 molecule to form a carbazolide
derivative, or a benzyne to generate an anilide anion. The
ion generated by the benzyne elimination then loses a SO2
molecule while transferring the hydrogen atom, which was
attached to the ortho position of the precursor ion, to the
nitrogen atom of the anilide anion. Lastly, the homolytic
cleavage that fragments the S–C bond of anions of N-phenyl
benzenesulfonamide and its derivatives is in violation of the
so-called “even-electron rule.”
Acknowledgments
The authors thank Jason Bialecki for the assistance provided
with preparative TLC work. They are grateful to Bristol-
Myers Squibb Pharmaceutical Company (New Brunswick,
NJ, USA) for the donation of the Waters Quattro Ultima
mass spectrometer.
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