Article
Journal of
Pharmaceutical Sciences
and Pharmacology
Copyright © 2017 American Scientific Publishers
All rights reserved
Printed in the United States of America
Vol. 3, 44–53, 2017
www.aspbs.com/jpsp
GC-MS and IR Studies on the Six Possible Ring
Regioisomeric Dimethylphenylpiperazines
Karim M. Abdel-Hay1 2 , Jack DeRuiter1 , and C. Randall Clark1 ∗
1
2
Department of Drug Discovery and Development, Harrison School of Pharmacy, Auburn University, Auburn, AL 36849, USA
Department of Pharmaceutical Analytical Chemistry, Faculty of Pharmacy, Alexandria University, Alexandria, 21521, Egypt
The complete series of regioisomeric dimethylphenylpiperazines were synthesized and evaluated using GC-MS and
FT-IR. The EI mass spectra show fragment ions characteristic of both the dimethylphenyl and the piperazine portions of
the molecules. These fragments include the dimethylphenyl aziridinium cation at m/z 148 and dimethylphenyl cation at
m/z 105. In addition, the low mass ion at m/z 56 for the C3 H6 N+ was observed in all piperazine EI spectra. Perfluoroacylation of the secondary amine nitrogen for each of the six regioisomers yielded amides whose mass spectra showed some
differences in relative abundance of fragment ions without the appearance of any unique fragments. FTIR provides direct
confirmation for differentiation of the six regioisomeric aromatic ring substituted dimethylphenylpiperazines. GC separation
of this series of compounds was accomplished on an Rtx-200 column and retention appears related to the degree of
steric crowding of the aromatic ring substituents.
KEYWORDS: DMPP, Dimethylphenylpiperazine, GC-MS, FTIR, Regioisomers.
INTRODUCTION
The focus of this study is the differentiation of all six
of the possible regioisomeric dimethylphenylpiperazines
(DMPPs). The ability to distinguish between regioisomers
is extremely important when some of the molecules are
legally controlled drugs or controlled precursor substances
(Aalberg, 2004; Awad, 2006; 2007; 2008; Staack, 2003;
Peters, 2003; Vorce, 2008) and directly enhances the specificity of the analysis for the target molecules. In some
subsets of compounds, the regioisomerism is within that
portion of the molecule which yields the major mass spectral fragment ions and produces mass spectral equivalency.
Furthermore, these closely related substances often show
similar chromatographic elution properties even co-elution
in some cases. Those substances co-eluting in the chromatographic system and having common mass spectral
fragment ions could be misidentified.
Chemical derivatization methods (primarily acylation)
can be applied to modify or improve the analytical
properties for primary and secondary amines (Awad,
2006; 2008). Derivatization can alter major fragmentation
∗
Author to whom correspondence should be addressed.
Email: [email protected]
Received: 11 October 2016
Accepted: 31 October 2016
44
J. Pharm. Sci. Pharmacol. 2017, Vol. 3, No. 1
pathways often providing additional structural information
about an individual isomer as well as producing altered
chromatographic properties (Awad, 2005; 2007). However,
in some cases, derivatization does not alter major fragmentation pathways and does not yield any regioisomer
specific mass spectral fragments.
Structural modifications in designer drugs are often
used to produce new molecules whose chemical structure
place them outside the group of legally controlled substances. Some of the more common structural variations
involve the introduction of aromatic ring substituents such
as alkyl, methoxy, dimethoxy and methylenedioxy groups.
These designer style structural alterations have been
encountered in the piperazine compounds and the modification of the controlled substance N-benzylpiperazine
(BZP) has led to 1-(3,4-methylenedioxybenzyl)-piperazine
(3,4-MDBP), the methylenedioxy analogue of BZP (Federal Register, 2002). Many of these designer piperazine drugs are reported to bind to serotonin receptors
of the human central nervous system (Tsutsumi, 2005;
Haroz, 2006; Yarosh, 2007). The 1-aryl-piperazines
show good binding affinity to serotonin receptors
(Yarosh, 2007) and the affinity is made more selective with the appropriate aromatic ring substituents
(Glennon, 1986). It appears that N-benzylpiperazine and
2333-3715/2017/3/044/010
doi:10.1166/jpsp.2017.1078
Abdel-Hay et al.
GC-MS and IR Studies on the Six Possible Ring Regioisomeric Dimethylphenylpiperazines
NH
CH3
NH
CH3
N
N
H3C
CH3
(1)
(4)
(1) 2,3-DMPP
(4) 2,6-DMPP
2,3-Dimethylphenylpiperazine
2,6-Dimethylphenylpiperazine
H3C
NH
NH
CH3
N
H3C
(2)
H3C
N
(5)
EXPERIMENTAL DETAILS
(2) 2,4-DMPP
(5) 3,4-DMPP
2,4-Dimethylphenylpiperazine
3,4-Dimethylphenylpiperazine
NH
CH3
N
NH
N
H3C
(3)
(6)
CH3
CH3
(3) 2,5-DMPP
(6) 3,5-DMPP
2,5-Dimethylphenylpiperazine
3,5-Dimethylphenylpiperazine
Figure 1. Structures of the six dimethylphenylpiperazines in
this study.
3-trifluoromethylphenyl piperazine (3-TFMPP) are among
the most commonly abused compounds of this group
(Glennon, 1987). These piperazine-derived compounds
likely represent a new category of designer drugs.
Recently, our group has published a report describing
the differentiation of 3-TFMPP from the regioisomeric
2- and 4-trifluoromethylphenylpiperazines by GC-IRD
H3C
and GC-MS (Maher, 2009). Similar studies on the
dimethoxyphenylpiperazines (Abdel-Hay, 2015) and the
dimethoxybenzylpiperazines (Abdel-Hay, 2013) as well as
the bromodimethoxybenzyl-piperazines (Abdel-Hay, 2014)
have been reported from our laboratory. Analytical differentiation of the regioisomeric dimethylphenylpiperazines
(Compounds 1–6 in Fig. 1) is the focus of this study. These
isomers represent possible designer analogues in this series
and methods to differentiate them have not been reported.
The synthetic precursors for the preparation of these six
regioisomeric substances are commercially available. The
aim of this study is to evaluate analytical methods using
GC-MS and FTIR to characterize and differentiate among
this set of ring regioisomeric compounds.
Instrumentation
GC-MS analysis was performed using an Agilent Technologies (Santa Clara, CA) 7890A gas chromatograph and
an Agilent 7683B auto injector coupled with a 5975C VL
Agilent mass selective detector. The GC was operated
in splitless mode with a helium (grade 5) flow rate of
0.7 mL/min and a column head pressure of 10 psi. The
MS was operated in the electron ionization (EI) mode
using an ionization voltage of 70 eV, a scan rate of
2.86 scans/s and a source temperature of 230 C. The GC
injector was maintained at 250 C and the transfer line
at 280 C.
GC-MS chromatographic analyses were carried out on
a 30 m × 0.25 mm i.d. column coated with 0.5 m film
of 100% trifluoropropyl methyl polysiloxane (Rtx-200)
purchased from Restek Corporation (Bellefonte, PA). The
separation of the heptafluorobutyryl derivatives was performed using a temperature program consisting of an initial hold at 70 C for 1.0 min, ramped up to 250 C at a
rate of 30 C/min then held at 250 C for 15.0 min. Samples were dissolved and diluted in high-performance liquid chromatography-grade acetonitrile (Fisher Scientific,
Fairlawn, NJ) and introduced via the auto injector using
an injection volume of 1 L.
H3C
Cl
NH2
NH2Cl–
+
K2CO3
N
NH
H3C
H3C
Cl
Scheme 1.
Synthesis of the dimethylphenylpiperazines in this study.
H3C
O
H3C
C3F7
N
NH
+
H3C
N
C3F7
O
Scheme 2.
O
O
H3C
N
C3F7
Synthesis of HFBA derivatives of the dimethylphenylpiperazines.
J. Pharm. Sci. Pharmacol. 3, 44–53, 2017
45
GC-MS and IR Studies on the Six Possible Ring Regioisomeric Dimethylphenylpiperazines
Abdel-Hay et al.
Average of 7.198 to 7.327 min.: 130326–01.D\data.ms
Abundance
148.1
6500000
6000000
NH
CH3
5500000
190.2
N
H3C
5000000
4500000
(1)
4000000
3500000
3000000
132.1
2500000
2000000
1500000
1000000
77.1
56.1
500000
117.1
91.1
105.1
42.1
160.1
0
m/z–>
30
40
50
60
70
80
90
100
110
120
130
140
150
160
174.1
170
180
Average of 7.012 to 7.152 min.: 130326–02.D\data.ms
Abundance
148.1
7500000
190.2
7000000
NH
CH3
6500000
6000000
N
5500000
5000000
(2)
H3C
4500000
4000000
132.1
3500000
3000000
2500000
2000000
1500000
77.1
56.1
1000000
117.1
91.1
500000
105.1
42.1
160.1
0
m/z–>
30
40
50
60
70
80
90
100
110
120
130
140
150
160
173.1
170
180
Average of 7.152 to 7.222 min.: 130326–03.D\data.ms
Abundance
190
190.2
148.2
8000000
NH
CH3
7000000
N
6000000
(3)
5000000
CH3
4000000
132.1
3000000
2000000
56.1
1000000
77.1
117.1
91.1
42.1
105.1
160.1
0
m/z–>
Figure 2.
46
30
40
50
60
70
80
90
100
110
120
130
140
150
160
174.1
170
180
190
Continued.
J. Pharm. Sci. Pharmacol. 3, 44–53, 2017
Abdel-Hay et al.
GC-MS and IR Studies on the Six Possible Ring Regioisomeric Dimethylphenylpiperazines
Average of 7.024 to 7.134 min.: 130326–04.D\data.ms
Abundance
148.2
8000000
7500000
7000000
6500000
6000000
5500000
5000000
4500000
4000000
3500000
3000000
2500000
2000000
1500000
1000000
500000
0
m/z–>
190.2
N
(4)
CH3
117.1
40
105.1
91.1
42.1
30
132.1
77.1
56.1
160.1
175.2
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
Average of 7.787 to 7.822 min.: 130326–05.D\data.ms
Abundance
148.2
8000000
7500000
7000000
6500000
6000000
5500000
5000000
4500000
4000000
3500000
3000000
2500000
2000000
1500000
1000000
500000
0
m/z–>
NH
CH3
NH
N
H3C
190.2
(5)
H3C
133.1
105.1
77.1
56.1
91.1
118.1
42.1
30
40
50
60
70
80
90
100
110
120
160.1
130
140
150
160
174.1
170
180
190
Average of 7.595 to 7.688 min.: 130326–06.D\data.ms
Abundance
148.2
8000000
7500000
7000000
6500000
6000000
5500000
5000000
4500000
4000000
3500000
3000000
2500000
2000000
1500000
1000000
500000
0
m/z–>
Figure 2.
190.2
NH
N
H3C
(6)
CH3
133.1
105.1
77.1
56.1
91.1
119.1
42.1
30
40
160.1
50
60
70
80
90
100
110
120
130
140
150
160
175.2
170
180
190
EI mass spectra of the six dimethylphenylpiperazines.
J. Pharm. Sci. Pharmacol. 3, 44–53, 2017
47
GC-MS and IR Studies on the Six Possible Ring Regioisomeric Dimethylphenylpiperazines
Abdel-Hay et al.
amine in 50 L of ethyl acetate, followed by addition of
a large excess (250 L) of the derivatizing agent (HFBA,
neat), and the resulting reaction mixtures were incubated
in capped tubes at 70 C for 20 min. Following incubation,
each sample was evaporated to dryness under a stream
of air at 55 C and reconstituted with 200 L of ethyl
acetate and 50 L of pyridine. A portion of each final
solution (50 L) was diluted with HPLC grade acetonitrile
(200 L) to give the working solutions.
Attenuated total reflection infrared (ATR FTIR) spectra
were obtained on a Shimadzu IRAffinity-1 Fourier Transform Infrared Spectrophotometer (Kyoto, Japan) equipped
with a DLATGS detector with temperature control system
at a resolution of 4 cm−1 with an aperture of 3.5 mm and
scan rate of 10 scans per second. The FTIR spectrophotometer was equipped with MIRacle Single Reflection
Horizontal ATR Accessory purchased from Pike Technologies (Madison, WI). The single-reflection sampling plate
of the accessory has a 1.8 mm round crystal surface allowing reliable analysis of small samples. FTIR spectra were
recorded in the range of 4000–520 cm−1 . The samples
were prepared by dissolving the solid or oily compounds
in acetonitrile and introducing the resulting solutions in
small volumes to the center of the single-reflection sampling plate. The IR spectra were recorded for the residue
following evaporation of the acetonitrile solvent.
RESULTS AND DISCUSSION
Mass spectrometry is the primary method for confirming
the identity of drugs in forensic samples. Figure 2 shows
the EI mass spectra of the six regioisomeric dimethylphenylpiperazines (Compounds 1–6). The mass spectra
in Figure 2 indicate that very little structural information is available for differentiation among these isomers
since all the major fragment ions occur at equal masses.
The common fragment ions observed for the regioisomeric
dimethyl group substitution on the aromatic ring likely
indicate that the piperazine ring is the source for most
of the fragmentation. The structures for the fragmentation product ions are summarized in Figure 3. The ions of
significant relative abundance common to all these regioisomers arise from fragmentation initiated at the nitrogen
atom of the piperazine ring. The dimethylphenyl aziridinium cation at m/z 148 is the base peak in all these
spectra. The fragment ions in the unsubstituted aromatic
ring for phenylpiperazine have been described by de Boer
et al. (De Boer, 2001). Equivalent fragmentation pathways
for the dimethylphenylpiperazines (DMPPs) yield the fragment ions at m/z 148, 134, 133, 132, 105 and 56 as
shown in Figures 2 and 3. The elemental composition for
the base peak at m/z 148 was confirmed in this study
by exact mass (TOF-MS) analysis as C10 H14 N based on
the observed mass of 148.1123 compared to the calculated mass of 148.1126 (2 ppm). These data indicate that
mass spectrometry provides data to identify this set of
Drugs and Reagents
The general procedure for the synthesis of these six
regioisomeric dimethyl-phenylpiperazines begins with the
appropriate aniline, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and
3,5-dimethylaniline, as starting materials. All six of
these precursor anilines were obtained from Alfa-Aesar
Chemical Company (Ward Hill, MA). The desired
regioisomeric dimethylphenylpiperazines were individually synthesized in approximately 300 mg quantities using
bis(2-chloroethyl) amine hydrochloride, anhydrous potassium carbonate, the appropriately substituted dimethylaniline in diglyme heated at reflux (Scheme 1). All laboratory
reagents and chemicals were purchased from SigmaAldrich, Inc (Milwaukee, WI) or Fisher Scientific
(Atlanta, GA).
The derivatizing agent heptafluorobutyric anhydride (HFBA) was purchased from Sigma-Aldrich, Inc
(Milwaukee, WI). Each perfluoroamide was prepared individually (Scheme 2) from each of the six regioisomers by
dissolving approximately 0.3 mg (1.30 × 10−6 mol) of each
H3C
N
N
H
H3C
Mwt190
H3C
H3C
CH2
N
CH2
N
H2C
m/z = 56
H
m/z = 148
CH2
H3C
H3C
N
H3C
m/z = 134
H
H3C
CH2
H3C
N
H3C
H3C
H3C
N
m/z = 105
CH
m/z = 132
m/z = 133
Figure 3.
48
EI mass spectral fragmentation pattern of the underivatized dimethylphenylpiperazines.
J. Pharm. Sci. Pharmacol. 3, 44–53, 2017
Abdel-Hay et al.
GC-MS and IR Studies on the Six Possible Ring Regioisomeric Dimethylphenylpiperazines
Average of 9.128 to 9.245 min.: 130326–07.D\data.ms
Abundance
386.2
6500000
O
6000000
5500000
N
CH3
5000000
N
H3C
4500000
C3F7
(1)
4000000
189.2
3500000
3000000
2500000
160.1
132.1
2000000
215.0
1500000
77.1
1000000
500000
241.0
293.0
105.1
42.1
266.1
0
m/z–>
40
60
80
100
120
140
160
180
200
220
240
260
319.1
280
300
320
344.1
340
360
380
Average of 8.866 to 8.965 min.: 130326–08.D\data.ms
Abundance
386.2
O
7500000
7000000
N
CH3
6500000
C3F7
N
6000000
5500000
(2 )
5000000
H3C
4500000
189.2
4000000
3500000
132.1
3000000
160.1
2500000
2000000
1500000
56.1
215.0
1000000
105.1
500000
253.0
0
m/z–>
40
60
80
100
120
140
160
180
200
220
240
260
293.0
280
319.0
300
320
344.1
340
360
380
Average of 8.866 to 9.023 min.: 130326–09.D\data.ms
Abundance
386.2
8000000
7500000
7000000
6500000
6000000
5500000
5000000
4500000
4000000
3500000
3000000
2500000
2000000
1500000
1000000
500000
0
m/z–>
Figure 4.
O
N
CH3
C3F7
N
189.2
132.1
(3)
CH3
160.1
56.1
105.1
215.0
253.1
40
60
80
100
120
140
160
180
200
220
240
260
293.0
280
318.1
300
320
344.1
340
360
380
Continued.
J. Pharm. Sci. Pharmacol. 3, 44–53, 2017
49
GC-MS and IR Studies on the Six Possible Ring Regioisomeric Dimethylphenylpiperazines
Abundance
Abdel-Hay et al.
Scan 1179 (8.959 min): 130326–10.D\data.ms
386.2
6500000
O
6000000
5500000
CH3
N
5000000
C3F7
N
4500000
(4)
4000000
CH3
189.2
160.1
3500000
3000000
132.1
2500000
2000000
1500000
56.1
1000000
215.0
105.1
500000
m/z–>
40
60
80
100
Abundance
120
140
160
180
200
220
240
260
280
319.1
300
347.2
320
340
360
380
Scan 1219 (9.192 min): 130326–13.D\data.ms
386.2
8000000
7500000
7000000
6500000
6000000
5500000
5000000
4500000
4000000
3500000
3000000
2500000
2000000
1500000
1000000
500000
0
m/z–>
293.0
253.0
0
O
N
H3C
C3F7
N
(5)
H3C
189.2
160.2
132.1
56.1
215.0
241.0
105.1
40
60
80
100
Abundance
120
140
160
180
200
220
240
260
330.1
293.0
266.1
280
300
320
357.1
340
360
380
Scan 1262 (9.443 min): 130326–12.D\data.ms
386.2
8000000
7500000
7000000
6500000
6000000
5500000
5000000
4500000
4000000
3500000
3000000
2500000
2000000
1500000
1000000
500000
0
m/z–>
Figure 4.
50
O
N
H3C
C3F7
N
(6)
189.2
CH3
133.1
160.1
56.1
215.0
105.1
240.0
40
60
80
100
120
140
160
180
200
220
240
267.1
260
293.0
280
319.1
300
320
344.1
340
360
380
MS spectra of heptafluorobutyryl derivatives of the six dimethylphenylpiperazines.
J. Pharm. Sci. Pharmacol. 3, 44–53, 2017
Abdel-Hay et al.
GC-MS and IR Studies on the Six Possible Ring Regioisomeric Dimethylphenylpiperazines
compounds but does not provide confirmation of identity
for an individual DMPP with the exclusion of the other
regioisomers.
The second phase of this study involved the preparation
and evaluation of acylated derivatives of the regioisomeric
dimethylphenyl piperazines, in an effort to individualize
their mass spectra and identify marker ions that would
allow discrimination between these compounds. Acylation
often allows other fragmentation pathways to play a more
prominent role in the resulting mass spectrum (Awad,
2006; 2007; 2008).
100
95
90
85
%T 80
NH
CH3
75
N
70
CH3
(4)
65
60
4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750
FTIR Measurement
100
95
1/cm
100
90
95
85
90
%T 80
85
H3C
%T
NH
CH3
75
N
70
80
(1)
NH
75
65
70
60
4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000
FTIR Measurement
H3C
N
H3C
(5)
65
750
4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750
1/cm
FTIR Measurement
1/cm
100
97.5
95
90
90
85
82.5
%T 80
%T
NH
75
H3C
75
70
(6)
67.5
NH
CH3
CH3
N
65
60
(2)
H3C
N
60
52.5
4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750
FTIR Measurement
1/cm
4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750
FTIR Measurement
100
Figure 5.
1/cm
FTIR spectra of the six dimethylphenylpiperazines.
95
90
85
%T
80
NH
CH3
75
N
(3)
70
CH3
65
60
4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750
FTIR Measurement
Figure 5.
Continued.
J. Pharm. Sci. Pharmacol. 3, 44–53, 2017
1/cm
The heptafluorobutryl derivatives were evaluated for
their ability to individualize the mass spectra of the
DMPPs. The mass spectra for the six heptaflurobutryl
amides are shown in Figure 4. From these spectra, a
common peak with high relative abundance occurs at
m/z 386 which corresponds to the molecular ions for
HFBA amides. Fragment ions occurring at m/z 134, 133,
132, 105 and 56 seen in all MS spectra of piperazine
acyl amides are due to different patterns of cleavage reactions in the piperazine ring, analogous to those found in
the underivatized compounds. Fragment ions at m/z 189
seen in all derivatized spectra are likely formed by the
51
GC-MS and IR Studies on the Six Possible Ring Regioisomeric Dimethylphenylpiperazines
at 1240 cm−1 compared to a peak of strong intensity at
1263 cm−1 in the 3,5-isomer, a strong singlet at 1139 cm−1
in the 2,5 isomer and two bands of weak intensity at 1253
and 1211 cm−1 in the 2,6-isomer.
These results indicate that FT-infrared spectra provide useful data for differentiation among these regioisomeric piperazines of mass spectral equivalence. Mass
spectrometry establishes these compounds as having an
isomeric relationship of equal molecular weight and equivalent major fragment ions. Infrared absorption bands then
provide further distinguishing and characteristic information to individualize the regioisomers in this set of
uniquely similar compounds. Thus, FTIR readily discriminates between the members of this limited set of regioisomeric dimethylphenylpiperazine compounds.
Gas chromatographic separation of the HFBA derivatives of the six dimethylphenylpiperazines was accomplished using an Rtx-200 (100% trifluoropropyl methyl
polysiloxane) stationary phase in a capillary column
(30 m × 0.25 mm) of 0.5-m film thickness. Several temperature programs were evaluated and the most efficient
program (see experimental details) was used to generate
the representative chromatogram in Figure 6. This chromatogram shows the separation of the six regioisomers
in this study. The elution order appears related to the
degree of substituent crowding on the aromatic ring. Compounds 1 and 4 elute first and these two isomers contain
substituents arranged in a 1, 2, 3-pattern on the aromatic
ring. Three isomers (Compounds 2, 3 and 5) have two
groups substituted 1, 2 with one isolated substituent. The
1, 3, 5-trisubstituted pattern in Compound 6 provides minimum intramolecular crowding and elutes last in this group
of compounds. The two compounds with maximum crowding substituted in a 1, 2, 3 arrangement on the aromatic
ring show the 2,3-dimethyl substitution pattern to elute
first followed by the 2,6-dimethyl isomer eluting second.
The relative position of the methyl groups appears to
determine the elution order in the three compounds having
two groups substituted in a 1, 2 pattern. Within this group
elimination of the acyl moiety from the corresponding
derivative. Those occurring at m/z 215 are formed as a
result of the elimination of heptafluoropropyl moiety from
the HFBA amides. Thus, even acylation of the six piperazines does not yield characteristic MS fragments that help
to discriminate among these regioisomeric compounds.
Attenuated total reflection Fourier transform infrared
spectroscopy (ATR FTIR) was evaluated for differentiation
among the six regioisomeric dimethylphenylpiperazines.
Infrared detection could provide compound specificity
without the need for chemical modification of the drug
molecule. The ATR-FTIR spectra for the six underivatized piperazines are shown in Figure 5. Each compound
shows an IR spectrum with absorption bands in the regions
700–1700 cm−1 and 2700–3100 cm−1 . In general, variations in the ring substitution pattern with no change in
the side chain composition results in variations in the IR
spectrum in the region 700–1700 cm−1 (Kempfert, 1988).
Because the six piperazines share the same side chain,
they share almost the same IR features in the region
2700–3100 cm−1 . However, they can be easily differentiated by the positions and intensities of several IR peaks in
the region of 750–1750 cm−1 .
The 2,3-DMPP regioisomer is characterized by the
medium intensity band at 1579 cm−1 which is shifted to
1504 cm−1 in the 2,5-DMPP regioisomer. This isomer also
has a doublet at 1471 and 1452 cm−1 shifted to a doublet at 1498 and 1450 cm−1 in the IR spectrum of the
2,4 isomer. Finally, the IR spectrum of 2,3-DMPP shows a
strong band at 1236−1 which is shifted to 1220 cm−1 and
1139 cm−1 in the 2,4-DMPP and 2,5-DMPP, respectively.
The 3,5-DMPP regioisomer can be distinguished by the
relatively strong IR band at 1593 cm−1 which is shifted
to a strong intensity doublet at 1504 and 1481 cm−1 in
the 3,4-regioisomer, a strong intensity doublet at 1504 and
1446 cm−1 in the 2,5-regioisomer and a strong intensity
doublet at 1593 and 1473 cm−1 in the 2,6-regioisomer.
The IR spectrum of the 3,4-DMPP regioisomer can be
distinguished by a singlet of strong intensity appearing
Abundance
Abdel-Hay et al.
TIC: 130424–01.D\data.ms
3
3500000
3000000
2500000
2000000
4
1500000
2
1000000
1
6
5
500000
Time-->
7.50
8.00
8.50
9.00
9.50
10.00 10.50 11.00 11.50 12.00 12.50 13.00 13.50
Figure 6. Gas chromatographic separation of the heptafluorobutyryl derivatives of the six dimethylphenylpiperazines on an
Rtx-200 column.
52
J. Pharm. Sci. Pharmacol. 3, 44–53, 2017
Abdel-Hay et al.
GC-MS and IR Studies on the Six Possible Ring Regioisomeric Dimethylphenylpiperazines
of three compounds the first to elute is the 1,4-relationship
for the two methyl groups in compound 3. This is followed by the 1,3-pattern for compound 2 and lastly the
1, 2 pattern for compound 5. A similar GC elution pattern has been observed in other series of ring substituted
piperazine derivatives (Abdel-Hay, 2013).
CONCLUSIONS
The six regioisomeric dimethylphenylpiperazines yield the
same fragment ions in their mass spectra. Perfluoroacylation of the secondary amine nitrogen for each of the six
regioisomers was done in an effort to individualize their
mass spectra. The resulting derivatives were resolved by
GC and their mass spectra showed some differences in relative abundance of fragment ions without the appearance
of any unique fragments for specific confirmation. ATR
FTIR analysis yields unique and characteristic infrared
spectra for these regioisomeric piperazines. These spectra
allow discrimination among the six regioisomeric compounds included in this study. This differentiation was
accomplished without the need for chemical derivatization.
Mixtures of the dimethylphenylpiperazines were successfully resolved via capillary gas chromatography using a
relatively polar stationary phase and temperature programming conditions. The elution order appears related to the
degree of substituent crowding on the aromatic ring with
the most crowded 1, 2, 3 substitution patterns eluting first
and the highest retention for the compound with minimum intramolecular crowding (the 1, 3, 5-trisubstitution
pattern).
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