Unique para-effect in electron ionization mass spectra of bis(perfluoroacyl) derivatives of bifunctional aminobenzenes

Article abstract of DOI:10.1002/rcm.4661

A new kind of 'para-effect' under electron ionization (EI) conditions has been discovered for a series of bis(perfluoroacyl) derivatives of o-, m- and p-phenylenediamines, -hydroxybenzeneamines and -mercaptobenzeneamines of a common structure RCOX-C₆H₄-NHCOR (X=NH, S, O; R=CF₃, C₂F₅, C₃F₇). Only the para-isomers showed successive loss of a radical RCO^? and a molecule RCN, leading to very intense peaks in the EI spectra. The composition and the origin of the [M-COR-NCR]⁺ ions were confirmed by exactmassmeasurements and linked scan experiments. The proposedmechanism of their formation takes into account likely para-quinoid structures of the precursor ions. A similar rearrangement has not been observed for para-isomers in the series of bis(perfluoroacyl) derivatives of benzenediols, mercaptophenols and dimercaptobenzenes.

Full text of DOI:10.1002/rcm.4661

RAPID COMMUNICATIONS IN MASS SPECTROMETRY
Rapid Commun. Mass Spectrom. 2010; 24: 2529–2532
Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.4661
Unique para-effect in electron ionization mass spectra of
bis(perfluoroacyl) derivatives of bifunctional
aminobenzenesy
1
1
2
2
*
Kirill V. Tretyakov , Nino G. Todua , Roman S. Borisov , Vladimir G. Zaikin ,
Stephen E. Stein1 and Anzor I. Mikaia1
1National Institute of Standards and Technology, Gaithersburg, Maryland
2Topchiev Institute of Petrochemical Synthesis RAS, Moscow, Russia
Received 21 April 2010; Revised 5 June 2010; Accepted 7 June 2010
A new kind of ‘para-effect’ under electron ionization (EI) conditions has been discovered for a series of
bis(perfluoroacyl) derivatives of o-, m- and p-phenylenediamines, -hydroxybenzeneamines and -mer-
captobenzeneamines of a common structure RCOX–C6H4–NHCOR (X ¼ NH, S, O; R ¼ CF3, C2F5, C3F7).
Only the para-isomers showed successive loss of a radical RCOꢀ and a molecule RCN, leading to very
intense peaks in the EI spectra. The composition and the origin of the [M–COR–NCR]R ions were
confirmed by exact mass measurements and linked scan experiments. The proposed mechanism of their
formation takes into account likely para-quinoid structures of the precursor ions. A similar rearrange-
ment has not been observed for para-isomers in the series of bis(perfluoroacyl) derivatives of benze-
nediols, mercaptophenols and dimercaptobenzenes. Published in 2010 by John Wiley & Sons, Ltd.
Commonly, electron ionization mass spectra cannot be reli-
ably predicted, although a large number of mass spectro-
metric rules describing the behavior of organic molecules
under electron ionization (EI) are known. As a result the
spectra of compounds to be identified are often treated
simply as molecular fingerprints. This is particularly true for
compounds undergoing fragmentation through hydrogen
migration or skeletal rearrangements. Our long-term project
on the quality improvement and the extension of the Nat-
ional Institute of Standards and Technology/National
Institute of Health/Environmental Protection Agency
(NIST/NIH/EPA) Mass Spectral Library includes computer
assisted ‘manual’ evaluation of every spectrum.1 This
requires a knowledge of many fragmentation pathways,
and the recognition of new unexpected reactions may be
particularly helpful in this mission.
of o-, m- and p-phenylendimaines, o-, m- and p-hydroxyanilines
and o-, m- and p-mercaptoanilines presented as structures
1a–c to 3a–c:
The EI spectra of the derivatives under investigation were
obtained to extend the NIST/EPA/NIH Mass Spectral
Library. During the course of this work a ‘para-effect’ for
EI-induced dissociation of diacyl derivatives of bifunctional
aminobenzenes was found. This is a novel process, not
predictable with the known mass spectrometry rules. Unlike
‘ortho-effects’ (see2,3, for example), far fewer ‘para-effects’ are
known in organic mass spectrometry (see4, for example).
The main objects of our investigation were the bis-acyl
derivatives (acyl ¼ perfluoro-acetyl, -propionyl and -butyryl)
EXPERIMENTAL
Initial o-, m- and p- amino-, hydroxy-, -mercaptobenzenea-
mines as well as o-, m- and p- benzenediols, -benzenedithiols
and -hydroxybenzenethiols and derivatizing agents (tri-
fluoroacetyl, pentafluoropropionyl and heptafluorobutyryl
anhydrides or chlorides) were commercially available
(Sigma-Aldrich, St. Louis, MO, USA)a.
*Correspondence to: K. V. Tretyakov, National Institute of Stan-
dards and Technology, 100 Bureau Drive, Stop 8320, Gaithers-
burg, MD 20899, USA.
aCertain commercial materials are identified in this paper in
order to specify the experimental procedure adequately. Such
identification is not intended to imply recommendation or
endorsement by the National Institute of Standards and Tech-
nology, nor is it intended to imply that the identified materials
are necessarily the best available for the purpose.
E-mail: kirill.tretyakov@nist.gov
yThis article is a U.S. Government work and is in the public
domain in the U.S.A.
Published in 2010 by John Wiley & Sons, Ltd.

2530 K. V. Tretyakov et al.
Published in 2010 by John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 2529–2532
DOI: 10.1002/rcm

Unique para-effect in EI mass spectra of bis(perfluoroacyl) derivatives 2531
Figure 2. EI mass spectrum (70 eV) of N-trifluoroacetyl-
S-heptafluorobutyryl-p-mercaptoaniline (Agilent 6890/5973).
Derivatization of the bifunctional compounds was accom-
plished by reaction with trifluoroacetyl, pentafluoropro-
pionyl and heptafluorobutyryl anhydrides or chlorides in the
presence of pyridine.5
The EI mass spectra were measured using gas chromatog-
raphy/mass spectrometry (GC/MS) systems with quadrupole
(Agilent 6890/5973: Agilent Technologies, Santa Clara, CA, USA;
ionization energy 70 eV; ion source temperature 2308C) and
magnetic sector (Finnigan MAT 95XL: Thermo Scientific, Bremen,
Germany; ionization energy 70 eV; temperature of ionization
chamber 2208C) analyzers. Derivatized analytes were introduced
into the mass spectrometer via GC. In all cases a fused quartz
capillary column (stationary phase polydimethylsiloxane
þ5% phenyl groups; 30 m ꢁ 0.25 mm i.d.) was used (injection
port temperature was 250 to 2708C; temperature of the oven
increased from 608C to 200–2708C at a rate of 58C/min).
Exact mass determinations and linked-scan (collision-
induced dissociation gas: helium) experiments were per-
formed on a GCmate II magnetic sector mass spectrometer
(JEOL, Tokyo, Japan).
RESULTS AND DISCUSSION
The structure of the bis(trifluoroacetyl), bis(pentafluoro-
propionyl) and bis(heptafluorobutyryl) derivatives of the
o-, m- and p-phenylenediamines, -hydroxybenzeneamines
and -mercaptobenzeneamines under investigation may
be represented as RCOX–C6H4–NHCOR (X ¼ NH, S, O;
R ¼ CF3, C2F5, C3F7). Complete EI mass spectra of the
trifluoroacetyl derivatives of all the compounds are
depicted in Fig. 1 and partial spectra, revealing the most
characteristic ions for all the derivatives under study, are
given in Table 1. As can be seen, all the spectra contain
rather abundant Mþꢀ ions and the intensities of these
ions are especially high for the meta- and para-isomers. As
expected, the primary decompositionꢀ of the molecular
ꢀ
ions was due to the loss of the R and COR radicals. For
the ortho-isomers, the loss of RCOXH and RCONH2 was
also expected. Some other characteristic ions are given
in Table 1. It should be noted that the elimination of the
first acyl radical occurs from the RCOX group, and the
resulting ions are particularly characteristic for para-
isomers. The spectrum of a mixed derivative prepared
from para-mercaptoaniline by successive N- and S-acyla-
Published in 2010 by John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 2529–2532
DOI: 10.1002/rcm

2532 K. V. Tretyakov et al.
Table 2. High-resolution data for characteristic ions derived from various bis(perfluoroacyl) derivatives of p-phenylenediamines,
p-hydroxyanilines and p-mercaptoanilines
Compounds 3a–c
Ion
Elemental composition
Calculated mass
Measured mass
Error (ppm)
X ¼ NH, R ¼ CF3
X ¼ NH, R ¼ C2F5
X ¼ NH, R ¼ C3F7
X ¼ O, R ¼ CF3
X ¼ O, R ¼ C2F5
X ¼ O, R ¼ C3F7
X ¼ S, R ¼ CF3
m/z 108
m/z 108
m/z 108
m/z 109
m/z 109
m/z 109
m/z 125
m/z 125
m/z 125
C6H6NO
C6H6NO
C6H6NO
C6H5O2
C6H5O2
C6H5O2
C6H5OS
C6H5OS
C6H5OS
108.0449
108.0449
108.0449
109.0289
109.0289
109.0289
125.0061
125.0061
125.0061
108.0450
108.0453
108.0442
109.0286
109.0295
109.0281
125.0060
125.0066
125.0072
0.9
3.7
ꢂ6.5
ꢂ2.7
5.5
ꢂ7.3
ꢂ0.8
4.0
X ¼ S, R ¼ C2F5
X ¼ S, R ¼ C3F7
8.8
tion with different derivatizing agents (Fig. 2) unambigu-
ously confirms the above statement:
In principle, a similar rearrangement could be expected for
the ortho-isomers, where ortho-quinoid ion structures may
also be formed after elimination of an RCO radical from Mþꢀ
,
but the spectra demonstrate the almost complete absence of
such a reaction. It is likely that the competitive ortho-effect,
noted above, suppresses the fragmentation route character-
istic of para-isomers.
Further fragmentation of the primary ions differed for the
The possible elimination of the stable perfluoroalkane-
nitriles from a para-quinoid cation [M–COR]þ is likely to be
the driving force of the described para-effect. This can lead to
the ‘para-effect’ only in derivatives of bifunctional anilines. In
fact, no similar process was observed for the para-isomers in
the series of bis(perfluoroacyl) derivatives of benzenediols,
mercaptophenols and dimercaptobenzenes. Mass spectro-
metric investigation of the complete series of homologous
compounds will be published elsewhere.
different isomeric compoundꢀs. For the ortho-isomers the
ꢀ
successive loss of the R and COR radicals was especially
prominent as the expected consequence of the ortho-effect:
The possible driving force for the process is the formation
of stable cation radicals and this fragmentation pattern is
another example of exceptions to the ‘even-electron’ rule
since the resulting ions can be formed, at least partly, through
the step-wise loss of the noted radicals.
CONCLUSIONS
The study of a series of bis(perfluoroacyl) derivatives of o-,
m- and p-phenylenediamines, hydroxyanilines and mer-
captoanilines allowed the discovery of a very prominent
para-effect consisting of successive losses of an acyl radical
and a molecule of perfluoroalkanenitrile. The driving
force of the two-stage process is the formation of a para-
quinoid cation, its rearrangement and decomposition to
form protonated para-benzoquinone, -thiobenzoquinone
or -iminobenzoquinone, and a very stable molecule of
perfluoroalkanenitrile. This process was only characteristic
of perfluoroacyl derivatives of bifunctional aminoben-
zenes and no similar reaction was observed for the same
derivatives of para-benzenediols, -mercaptophenols and -
dimercaptobenzenes.
For the meta- and ortho-isomers the elimination of the ꢀCOR
radical proceeds more easily than for the para-isomers. The [M–
COR]þ ions become predominant for the para-isomers. The
most interesting fragmentation of these ions includes a
rearrangement process accompanied by the loss of a RCN
molecule. The resulting [M–COR–NCR]þ ions appear as very
intense peaks and in some cases they are the base peak.
The compositions and the origins of the ions are confirmed by
exact mass measurements (Table 2) and linked scan exper-
iments. The analysis of precursor ion scan spectra clearly
demonstrates that the [M–COR]þ ion was the only source of
the [M–COR–NCR]þ ion. Without doubt, this fragmentation
mode is a manifestation of a ‘para-effect’, and the suggested
mechanism of the reaction takes into account the likely para-
quinoid structures of the precursor ions:
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Published in 2010 by John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 2529–2532
DOI: 10.1002/rcm