Vibrational spectra of phenyl acridine-9-carboxylates and their 10-methylated cations: A theoretical and experimental study

Article abstract of DOI:10.1016/j.saa.2010.02.014

Infrared spectra of phenyl acridine-9-carboxylates and their 10-methylated cationic derivatives were recorded and discussed. Experimental data were compared with theoretically predicted transitions at the DFT level of theory (using the B3LYP functional and 6-31G** basis set) for optimized geometries of molecules. Substitution influences the values of the wavenumbers of characteristic stretching and bending modes, i.e. those corresponding to ester groups and fragments of molecules containing a heterocyclic nitrogen atom. The experimentally determined transitions of selected groups of atoms correlate well with the theoretically predicted values. Interdependences among some theoretically derived physicochemical features of the compounds and IR frequencies of selected bands are also discussed.

Full text of DOI:10.1016/j.saa.2010.02.014

Spectrochimica Acta Part A 75 (2010) 1546–1551
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Vibrational spectra of phenyl acridine-9-carboxylates and their 10-methylated
cations: A theoretical and experimental study
B. Zadykowicz, A. Oz˙ óg, K. Krzymin´ ski∗
Faculty of Chemistry, University of Gdan´sk, J. Sobieskiego 18, 80-952 Gdan´sk, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Infrared spectra of phenyl acridine-9-carboxylates and their 10-methylated cationic derivatives were
recorded and discussed. Experimental data were compared with theoretically predicted transitions at
the DFT level of theory (using the B3LYP functional and 6-31G** basis set) for optimized geometries of
molecules. Substitution influences the values of the wavenumbers of characteristic stretching and bend-
ing modes, i.e. those corresponding to ester groups and fragments of molecules containing a heterocyclic
nitrogen atom. The experimentally determined transitions of selected groups of atoms correlate well with
the theoretically predicted values. Interdependences among some theoretically derived physicochemical
features of the compounds and IR frequencies of selected bands are also discussed.
Received 19 November 2009
Received in revised form 10 February 2010
Accepted 15 February 2010
Keywords:
Phenyl acridine-9-carboxylates
9-Phenoxycarbonyl-10-methylacridinium
cations
Vibrational spectra assignment
Structural features
© 2010 Elsevier B.V. All rights reserved.
Spectral versus physicochemical
parameters relations
1. Introduction
their structures, electronic properties, mechanism of formation and
other properties [8,18].
Nitrogen substituted cationic derivatives of phenyl acridine-9-
carboxylates (acridinium esters) have long been of interest owing
to their numerous applications in immunological assays [1–3]
and chemical, biological, medical or environmental analyses [4–6].
These compounds are easily oxidized with hydrogen peroxide in
alkaline media. Oxidation produces electronically excited 10-alkyl-
9-acridinones and the process is accompanied by relatively efficient
chemiluminescence in aqueous solutions (quantum yields of ca. 2%)
[7–9]. In this context, 9-phenoxycarbonyl-10-methylacridinium
cations and their precursors–phenyl acridine-9-carboxylates
[10–12,13] and the 9-cyano-10-methylacridinium cation were
recently investigated [14].
Vibrational spectroscopy is a useful tool for acquiring a better
understanding of the structure and molecular properties of organic
compounds [15,16]. It is an important method for studying small
molecules as well as biological systems, i.e. the constituents of
nucleic acids, DNA and RNA [17]. FT-IR spectroscopy is particularly
useful, especially when coupled with theoretical methods. It is a
convenient means of obtaining theoretical estimates of the posi-
tion of bands arising from given molecular oscillations so that they
can be recognized in experimental spectra. Computational methods
are increasingly applied to compounds with the aim of elucidating
In this paper we present the results of IR spectroscopy inves-
tigations of selected phenyl acridine-9-carboxylates and their
10-methylated salts (Scheme 1, Table 1) that we think are inter-
esting for the above-mentioned reasons. It is known that the
chemiluminescence parameters of the latter, like decay constants
and emission efficiency, are affected to a greater or lesser extent
when the structure of the aromatic ester fragment is altered [19]. In
order to find out whether this does actually happen, investigations
were undertaken on phenyl 10-methylacridinium-9-carboxylates,
which contain substituents in the benzene ring differing in their
electronic and steric effects [13]. This unique set of experimen-
tal and theoretical data also provided an opportunity to observe
how the structures of 9-phenoxycarbonyl-10-methylacridinium
cations and phenyl acridine-9-carboxylates affect their infrared
spectra, and how band positions are related to the structural and
physicochemical characteristics of this important group of chemi-
luminogenic compounds.
2. Methods
2.1. Experimental
Compounds 1a–1d (Scheme 1, Table 1) were synthesized
as described previously [12,20,21]. Briefly, 9-(chlorocarbonyl)-
acridine was esterified at room temperature with the appropriate
hydroxybenzene (phenol, 2-fluorophenol, 2-methoxyphenol,
∗
Corresponding author. Tel.: +48 58 523 54 67; fax: +48 58 345 04 64.
E-mail address: karolk@chem.univ.gda.pl (K. Krzymin´ ski).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.02.014

B. Zadykowicz et al. / Spectrochimica Acta Part A 75 (2010) 1546–1551
1547
Scheme 1. Canonical structures of phenyl acridine-9-carboxylates (1) and 9-phenoxycarbonyl-10-methylacridinium cations (2) (Table 1).
2-nitrophenol) in anhydrous dichloromethane in the presence
of N,N-diethylethanamine and N,N-dimethyl-4-pyridinamine.
The crude products, chromatographically purified on SiO2
(cyclohexane/ethyl acetate, 3:2 v/v), gave yields in the range
of 60–80%. The esters obtained were subjected to elemental
analyses (EAGER 200, Carlo Erba Instruments) and TLC tests in
the above-mentioned system. The % of elements found/calculated,
retention factors (Rf) and melting points (m.p., in K) were
51.63/51.98, H 2.92/2.85, N 2.64/2.64, Rt = 4.20, m.p. = 506–508.
Structural, NMR and mass spectrometry data for the compounds
presented here will be discussed in a separate paper. The crystal
structure of compound 2d was determined by the X-ray method
[11].
The FT-IR spectra of all compounds were recorded at room tem-
perature in the 5000–400 cm−1 range (resolution 4 cm−1) using a
Bruker IFS 66 FTIR spectrometer and the KBr pellet technique.
as follows (1a):
C
79.79/80.25,
H
4.44/4.38,
75.46/75.70,
4.26/4.41, Rf = 0.59, m.p. = 436 − 437; (1c):
4.55/4.59, 4.22/4.25, Rf = 0.56, m.p. = 460–462; (1d):
N
H
4.25/4.68,
3.98/3.81,
Rf = 0.38, m.p. = 463 − 465; (1b):
C
2.2. Theoretical
N
H
C 76.45/76.58,
N
C
Unconstrained geometry optimizations of isolated molecules
were carried out at the density functional level of theory (DFT) [22]
using the gradient technique [23], the 6–31G** basis set [24,25]
and the Becke 3LYP (B3LYP) non-local spin density exchange func-
tional [26,27]) supplemented with the Lee–Yang–Parr non-local
correlation functional [28]. After completion of each optimization,
the Hessian (second derivatives of the energy as a function of
nuclear coordinates) was calculated and checked for positive defi-
niteness in order to assess whether the structures were true minima
[29]. The harmonic vibrational frequencies (wave numbers) were
subsequently derived from the numerical values of these second
derivatives. These wavenumbers were then scaled by multiplying
them by 0.96 [30].
69.71/69.76, H 3.46/3.51, N 8.10/8.14, Rf = 0.52, m.p. = 421 − 422.
The phenyl acridine-9-carboxylates were quaternalized using
an excess of methyl trifluoromethanesulphonate dissolved
in dichloromethane at room temperature. 10-Methyl-9-
(phenoxycarbonyl)-acridinium salts 2a–2d (Scheme 1, Table 1)
were purified using repeated precipitation with an excess
of ethyl ether from ethanolic solution and subjected to ele-
mental analysis and HPLC chromatography (Waters 600
E
Multisolvent Delivery System, Waters 2487 Dual ␭ Absorbance
Detector; mobile phase: acetonitrile/0.1 M TFA = 1/1.5 (40%/60%);
stationary phase: C-8 column, 3 × 150 mm, ‘Symmetry’). Ele-
mental analyses (% of elements found/calculated), retention
times (Rt, in minutes) and melting points (m.p., in K) were
The calculations were carried out on computers of the Tri-City
Academic Network Computer Centre in Gdansk (TASK) using the
GAUSSIAN 03 program package [31]. Structural data and dipole
moments were extracted directly from files following the geometry
optimizations. The atomic partial charges were those reproduc-
as follows (2a):
C
56.27/57.02,
H
3.42/3.48,
55.73/54.89,
2.82/2.91, Rt = 3.72, m.p. = 501–503; (2c):
3.16/2.97, 5.36/5.51, Rt = 3.79, m.p. = 514–516; (2d):
N
H
2.72/3.02,
3.23/3.14,
Rt = 0.38, m.p. = 500 − 502; (2b):
C
N
H
C
52.94/51.97,
N
C
Table 1
The compounds investigated (Scheme 1).
Compound no.
Compound name*
R
Phenyl acridine-9-carboxylates (1)
1a
1b
1c
1d
Phenyl acridine-9-carboxylate
H
F
OCH3
NO2
2-Fluorophenyl acridine-9-carboxylate
2-Methoxyphenyl acridine-9-carboxylate
2-Nitrophenyl acridine-9-carboxylate
9-Phenoxycarbonyl-10-methylacridinium cations (2)
2a
2b
2c
2d
9-Phenoxycarbonyl-10-methylacridinium
H
F
OCH3
NO2
9-(2-Fluorophenoxy)carbonyl-10-methylacridinium
9-(2-Methoxyphenoxy)carbonyl-10-methylacridinium
9-(2-Nitrophenoxy)carbonyl-10-methylacridinium
*
For 1, the compound is named; for 2, the cation is named.

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B. Zadykowicz et al. / Spectrochimica Acta Part A 75 (2010) 1546–1551
Fig. 1. DFT-optimized geometries of the entities shown in Scheme 1.
ing molecular electrostatic potential around molecules (MEP fitted
charges) [32].
the central ring in 2a–2d is more or less convex at the nitrogen
atom site and concave at the ester group site, as the values of the
above dihedral angles lie between −8.6 and −8.9◦ and 7.4 − 7.9◦
respectively; values are highest for the 2-nitro cation (2d). Simi-
lar conclusions can be drawn from an analysis of the values of the
3. Results and discussion
C
(12)C(11)C(9)C(14)/C(11)C(9)C(14)C(13) dihedral angles.
As our recent X-ray investigations showed, the acridine nucleus
in phenyl acridine-9-carboxylates containing an alkyl group in the
benzene ring is practically flat, being distorted from planarity by
less than 2◦ [10,33,34]. The C(9)C(15) bond lengths are typical, reach-
The (9)C(15) bond lengths in the uncharged derivatives
C
(1a–1d) are almost identical, having values of around 1.50 Å.
The analogous lengths for 2a–2d are comparable–ca 1.51 Å. The
N(10)C(12)/N(10)C(13) bond lengths are almost the same for a given
entity. They are somewhat greater in cations but generally typical
of aromatic heterocyclic systems. The length of the N(10)C(24) bond
is practically constant in all of the cations, having a value of around
1.48 Å.
ing values of about 1.49 Å. The recently published crystal structure
of 2d shows that distortion from planarity is negligible in this
cation, with values of <2◦ [11]. The acridine ring in the cation is
somewhat twisted along the C(9)N(10) axis, as suggested by the
different dihedral angles within the central ring. The C(9)C(15) and
The carboxyl group is twisted at angles from 51.3◦ (for 1a) to
55.9◦ (for 2d) relative to the acridine skeleton. The acridine and
benzene ring systems are oriented at about 54◦–86◦ in 1a–1d and
48◦–83◦ in cations 2a–2d. No relationships were found between
the substituents’ size parameters (Charton’s size values or Taft size
parameters [35]) and the theoretically established orientation of
the benzene ring in both series of acridine derivatives.
The dipole moments, calculated for the uncharged esters
(1a–1d), are also presented in Table 2. As expected, the 2-nitro
derivative (1d) has the largest dipole moment, and the unsubsti-
tuted ester 1a the lowest.
N(10)C(24) bond lengths have respective values of about 1.51 Å and
1.47 Å. The carboxyl group is twisted at an angle of 65.8◦ or 67.0◦
relative to the acridine skeleton, while the acridine and benzene
ring systems in the cation are respectively oriented at 3.0◦ or 11.1◦
to each other.
The structures of compounds 1a–1d and 2a–2d obtained from
DFT optimizations are shown in Fig. 1. According to the theoret-
ical predictions, their structural features depend only minimally
on the type of substituents present in the phenoxycarbonyl frag-
ment (Table 2); differences are more pronounced if uncharged
and charged derivatives are compared. The acridine skeleton is
almost planar in 1a–1d, as values between −0.6◦ and −1.4◦ were
obtained for the dihedral angle C(12)N(10)C(13)C(14) and between
0.5◦ and −0.2◦ for the dihedral angle C(11)C(9)C(14)C(13). In contrast,
To the best of our knowledge, no systematic experimental
or quantum mechanical vibrational data have been reported for
phenyl acridine-9-carboxylates and their 10-methylated deriva-
tives. However, spectral data, including infrared spectra have been
Table 2
Theoretically predicted structural and physicochemical characteristics of phenyl acridine-9-carboxylates and their cationic derivatives.
Compound
Bond length (Å)
Angle (◦)
Dipole
LCAO coefficient of the MEP fitted charge
moment (D)
pz LUMO orbital at
C(9)C(15)
N(10)C(12)/N(10)C(13)
Aa
Ba
Ca
C(9)
C(15)
C(9)
C(15)
1a
1b
1c
1d
2a
2b
2c
2d
1.501
1.498
1.502
1.496
1.515
1.513
1.514
1.515
1.341/1.341
1.341/1.341
1.341/1.341
1.341/1.341
1.379/1.379
1.379/1.379
1.379/1.378
1.378/1.378
−0.1/−1.1
0.5/−1.4
−0.2/−0.8
−0.2/−0.6
7.4/−8.6
7.9/−8.7
7.6/−8.8
7.9/−8.9
−1.8/0.8
−2.5/1.8
−1.1/0.2
−0.9/0.2
−4.7/3.6
−5.2/4.4
−4.7/3.5
−4.7/3.7
85.5
54.4
54.8
75.1
83.0
48.0
48.4
66.3
1.99
2.76
3.09
4.58
0.291
0.286
0.280
0.256
0.314
0.177
0.293
0.299
0.0832
0.0726
0.0501
0.0680
0.0256
0.0249
0.0263
0.0473
−0.163
−0.150
−0.0650
−0.151
−0.201
−0.218
−0.221
−0.163
0.734
0.730
0.655
0.748
0.695
0.699
0.682
0.719
a
A, dihedral angle C(11)C(12)N(10)C(13)/C(12)N(10)C(13)C(14); B, dihedral angle C(12)C(11)C(9)C(14)/C(11)C(9)C(14)C(13); C, dihedral angle between the mean plane of the acridine
nucleus and the mean plane of the benzene ring.

B. Zadykowicz et al. / Spectrochimica Acta Part A 75 (2010) 1546–1551
1549
Table 3
Selected vibrational transitions in phenyl acridine-9-carboxylates and their cationic derivatives.
Compound no.
Mode
No.
Wavenumber (cm−1
Experimental
)
Type
Description
Theoretical
1a
ꢀ89
ꢀ73
ꢀ66
ꢀ37
ꢀI
C
O stretching
1748
1352
1196
752
1777
1346
1203
777
ꢀII
ꢀIII
ꢀIV
Molecular deformation involving N(10)
C(15)–O(17)–C(18) in plane bending
Out of plane C–H bending
1b
1c
1d
ꢀ90
ꢀ74
ꢀ61
ꢀ37
ꢀI
C
O stretching
1757
1350
1142
753
1795
1348
1139
754
ꢀII
ꢀIII
ꢀIV
Molecular deformation involving N(10)
C(15)–O(17)–C(18) in plane bending
Out of plane C–H bending
ꢀ99
ꢀ80
ꢀ68
ꢀ42
ꢀI
C
O stretching
1751
1352
1158
746
1792
1348
1162
772
ꢀII
ꢀIII
ꢀIV
Molecular deformation involving N(10)
C(15)–O(17)–C(18) in plane bending
Out of plane C–H bending
ꢀ96
ꢀ91
ꢀ80
ꢀ65
ꢀ41
ꢀI
C
O stretching
1754
1591
1347
1133
766
1797
1592
1373
1130
769
NO2 bending (asymmetrical) + molecular deformation
Molecular deformation involving N(10)
C(15)–O(17)–C(18) in plane bending
ꢀII
ꢀIII
ꢀIV
Out of plane C–H bending
2a
2b
2c
2d
ꢀ98
ꢀ80
ꢀ72
ꢀ38
ꢀI
C
O stretching
1762
1376
1211
752
1783
1371
1224
753
ꢀII
ꢀIII
ꢀIV
Molecular deformation involving N(10)
C(15)–O(17)–C(18) in plane bending
Out of plane C–H bending
ꢀ99
ꢀ81
ꢀ72
ꢀ41
ꢀI
C
O stretching
1762
1377
1203
767
1804
1372
1208
762
ꢀII
ꢀIII
ꢀIV
Molecular deformation involving N(10)
C(15)–O(17)–C(18) in plane bending
Out of plane C–H bending
ꢀ107
ꢀ86
ꢀ77
ꢀ43
ꢀI
C
O stretching
1762
1376
1206
754
1787
1370
1214
758
ꢀII
ꢀIII
ꢀIV
Molecular deformation involving N(10)
C(15)–O(17)–C(18) in plane bending
Out of plane C–H bending
ꢀ105
ꢀ99
ꢀ86
ꢀ77
ꢀ44
ꢀI
C
O stretching
1764
1582
1375
1197
768
1802
1585
1376
1206
762
NO2 bending (asymmetrical) + molecular deformation
Molecular deformation involving N(10)
C(15)–O(17)–C(18) in plane bending
ꢀII
ꢀIII
ꢀIV
Out of plane C–H bending
presented as structural proof for some acridinium esters employed
as chemiluminogenic tracers [36,37]. The detailed vibrational
assignments of selected modes of phenyl acridine-9-carboxylates
and their 10-methylated cationic derivatives (Scheme 1), along
with the calculated normal mode descriptions, are reported in
Table 3. Infrared spectra of selected compounds (1d, 2d), together
with the theoretically determined vibrational wavenumbers are
illustrated in Fig. 2. Thorough analysis of the spectra of all the com-
pounds revealed that they reflect features of both acridinium ester
moiety andsubstituents. Thetransitions relating tothe substituents
are unique to a given compound. On the other hand, the transitions
relating to the acridine ester fragment display numerous similari-
ties. In the latter case, band positions differed only insignificantly
from compound to compound, being related to some extent to the
structural and physicochemical properties of the molecules under
investigation.
We compared the calculated wavenumbers for phenyl acridine-
9-carboxylates and their cationic forms methylated at the
endocyclic N atom with the experimental values. When analysing
the spectra with the aid of ‘animations’ of molecular vibrations, we
selected four of them that were characteristic of all the compounds
investigated. These are the C O stretching vibration (ꢀI), the molec-
ular deformation involving N(10) (ꢀII), the C(15)–O(17)–C(18) in plane
bending vibration (ꢀIII) and the out of plane C–H bending vibration
(ꢀIV). All the experimental and predicted characteristic transitions
and the specific vibrations relating to the presence of substituents
are given in Table 3.
The theoretically predicted transitions for isolated molecules
sometimes corresponded to the experimentally determined char-
acteristics, but in most cases they differed from them by <43 cm−1
.
The differences between the experimental and theoretical values
were relatively large in the case of the C O stretching vibrations.
Fig. 3 illustrates the relationships between the calculated and
experimental wavenumbers for 1a and 2a. Both correlations are
linear and are characterized by high correlation coefficients. These
relationships confirm that the computational results correspond
well to the experimental ones. These types of correlations were
investigated for the other derivatives, and in all cases the linear
correlation coefficients were 0.99 or higher.
We also examined the relationships between selected experi-
mental frequencies (Table 3) and tabularized constants character-
izing the electronic or steric properties of substituents [38]. The
most pronounced correlation (Fig. 4) was found between the fre-
quency of C(15)–O(17)–C(18) in plane bending (ꢀIII) and the inductive
sigma constant (ꢁI) of the substituents (H, F, OCH3, NO2) [35]. Gen-
erally, the frequency of this vibration is inversely proportional to
the rise in value of ꢁI in the series of compounds 2a–2d. Interest-
ingly, the correlation for phenyl acridine-9-carboxylates (1a–1d) is
multinomial. From the data in Fig. 4 it emerges that the energy of
the C(15)–O(17)–C(18) vibrations falls in the order: 1a > 1c > 1b > 1d
and 2a > 2c > 2b > 2d. The C(15)–O(17)–C(18) frequencies characteris-
tic of cations are generally higher than for those of esters and seem
to be less sensitive to changes in the polarity of the substituent in
the ester fragment.

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B. Zadykowicz et al. / Spectrochimica Acta Part A 75 (2010) 1546–1551
Fig. 4. Relationships between wavenumbers of C(15)–O(17)–C(18) in plane bending
vibration (ꢀIII, Table 3) and the inductive sigma constant (ꢁI) of the substituents.
Fig. 2. FT-IR spectra of 2-nitrophenyl acridine-9-carboxylate (1d; A); calculated
FT-IR spectra of 1d (B); 9-(2-nitrophenoxy)carbonyl-10-methylacridinium (2d; C);
calculated FT-IR spectrum of 2d (D).
In the case of compounds 2a–2d we also noted a trend between
the frequency of the ꢀI, ꢀIII vibrations (Table 3) and the MEP fitted
atomic partial charge at C(9) (Table 2). These relationships, pre-
sented in Fig. 5, are linear. The MEP fitted atomic partial charge
may be one of the parameters related to the chemiluminogenic
properties of acridinium esters, as C(9) is the site of the primary
nucleophilic attack of OOH− in the step initiating the ‘light path
of reaction’ [7,8]. However, it must be noted that the values of the
MEP fitted charge at C(9) are lower than the ones characterizing
C(15). On the other hand, molecular centres sensitive to nucleophilic
attack are also determined by LUMO distribution in an electrophilic
species (cations), as indicated by the Frontier Orbital Theory [38].
The values of the LCAO coefficients of the pz atomic orbital in 2a–2d
LUMO are ca 6–12 times higher at the endocyclic C(9) than at the
carbonyl group carbon atom (Table 2). This implies that C(9) rather
than the carbon atom of the carbonyl group is the site of the pri-
mary nucleophilic attack of the above-mentioned anions. On the
other hand, the trends of both MEP fitted charges and LCAO coef-
ficients of the pz atomic orbital indicate that 2a and 2d should be
Fig. 5. C O stretching wavenumbers for 2a–2d (ꢀI, Table 3) versus MEP fitted
charges at the C(9) and C(15) atoms (Table 2).
the most susceptible to nucleophilic attack at C(9); this has been
confirmed experimentally.
4. Concluding remarks
In summary, four new phenyl acridine-9-carboxylates and their
10-methylated cationic derivatives were synthesized, examined in
terms of their purity and structure, and subjected to advanced
infrared spectroscopic investigations. The experimental results
were compared with the theoretical ones obtained at the level of
density functional theory. Comparison of the experimental and cal-
culated vibrational frequencies indicates that the B3LYP results are
Fig. 3. Correlation between the calculated and experimental wavenumbers for phenyl acridine-9-carboxylate (1a, left) and the 9-phenoxycarbonyl-10-methylacridinium
cation (2a, right) (the data represent transitions in the 490–1700 cm−1 range). The equations are: ꢀtheor. = 0.989 ꢀexp. + 19.51 (correlation coefficient, 0.996) (1a) and
ꢀtheor. = 1.008 ꢀexp. + 2.72 (2a) (correlation coefficient, 0.998).

B. Zadykowicz et al. / Spectrochimica Acta Part A 75 (2010) 1546–1551
1551
in reasonable agreement with the experimental ones. Additionally,
the DFT structures and structural parameters found in the work
are generally in good agreement with the crystallographic data
reported earlier.
The correlations found between the experimental values of
wavenumbers of selected bands and the inductive constants of sub-
stituents, as well as the MEP fitted charges at the C(9) atom reveal
the existence of structural-physicochemical property trends in this
group of compounds. These findings provide a good starting tool
for the search for new acridinium esters with practical applications
as chemiluminogenic tracers.
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