The amide protonation of (-)-N-benzoylcytisine in its perchlorate salts

Article abstract of DOI:10.1016/j.saa.2014.02.192

The ¹³C NMR spectrum of (-)-N-benzoylcytisine perchlorate does not show a double set of signals typical of amide compounds, although this effect has been observed for the other diamine derivatives of cytisine. This observation means that in solution there must be the state of equilibrium between two forms of the cation with the protonated amide groups. DFT calculations have indeed indicated two preferred tautomeric forms with protonated oxygen atoms of amide groups. In the solid state however, according to X-ray analysis of perchlorate and perchlorate hydrate of N-benzoylcytisine the oxygen atom of the amide group in the six-membered ring A is preferred protonation site as compared with the oxygen in benzoic moiety. (-)-N-benzoylcytisine salt is the first compound from among the known derivatives of quinolizidine alkaloids that are not N-oxides, in which in solid state only the oxygen atom at cyclic amide is protonated instead of nitrogen atom or oxygen in benzoic moiety.

Full text of DOI:10.1016/j.saa.2014.02.192

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 1–6
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
The amide protonation of (–)-N-benzoylcytisine in its perchlorate salts
⇑
Anna K. Przybył , Maciej Kubicki, Marcin Hoffmann
Faculty of Chemistry, A. Mickiewicz University in Poznan´, Umultowska 89b, 61-614 Poznan´, Poland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
The new salts of (–)-N-
benzoylcytisine were characterized
by spectroscopic methods.
The energy calculations for
tautomeric forms of the salts is
presented.
ꢀ
ꢀ
ꢀ
The structures were established by
crystallographic analysis.
In crystal, the protonation is on
oxygen atom at cyclic amide, not at
benzoic moiety.
ꢀ
N-benzoylcytisinium salts
protonation is at the oxygen atom
instead nitrogen atom.
a r t i c l e i n f o
a b s t r a c t
The 13C NMR spectrum of (–)-N-benzoylcytisine perchlorate does not show a double set of signals typical
of amide compounds, although this effect has been observed for the other diamine derivatives of cytisine.
This observation means that in solution there must be the state of equilibrium between two forms of the
cation with the protonated amide groups. DFT calculations have indeed indicated two preferred tauto-
meric forms with protonated oxygen atoms of amide groups. In the solid state however, according to
X-ray analysis of perchlorate and perchlorate hydrate of N-benzoylcytisine the oxygen atom of the amide
group in the six-membered ring A is preferred protonation site as compared with the oxygen in benzoic
moiety. (–)-N-benzoylcytisine salt is the first compound from among the known derivatives of quinolizi-
dine alkaloids that are not N-oxides, in which in solid state only the oxygen atom at cyclic amide is pro-
tonated instead of nitrogen atom or oxygen in benzoic moiety.
Article history:
Received 23 October 2013
Received in revised form 25 February 2014
Accepted 27 February 2014
Available online 27 March 2014
Keywords:
Quinolizidine alkaloids
N-benzoylcytisine
Crystal structure
NMR
Ó 2014 Elsevier B.V. All rights reserved.
Computational chemistry
Introduction
ring, and the other acyclic, connecting the aromatic substituent
with the cytisine molecule.
Amide bonds continue attracting the attention of the chemists,
biologists etc. because of their profound importance in living sys-
tems; for instance, they determine the interactions of biologically
active structures like peptides or proteins. Here we attempt to ad-
dress the interesting aspect of protonation possibilities of a tertiary
amide, taking as an example the certain cytisine derivative con-
taining two such groups, one cyclic – embedded in a six-membered
(–)-Cytisine (1, Fig 1) is a naturally occurring quinolizidine alka-
loid extracted from seeds of Laburnum anagyroides and other Legu-
minosae plants. (–)-Cytisine has been used as a smoking cessation
aid (TabexÒ), and is also a very promising compound for develop-
ment of new drugs for potential treatment of the central nervous
system disorders, particularly Alzheimer’s and Parkinson’s
diseases. On the basis of the hitherto studies of certain cytisine
derivatives it has been found that introduction of substituents
modifying the molecular structure also changes the pharmacolog-
ical properties of these compounds, that is the affinity and inner
⇑
Corresponding author. Tel.: +48 618291004; fax: +48 618291555.
E-mail address: annaprz@amu.edu.pl (A.K. Przybył).
http://dx.doi.org/10.1016/j.saa.2014.02.192
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

2
A.K. Przybył et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 1–6
unsubstituted cytisine contains three heteroatoms, but only two
of them can be considered as potential basic sites: the oxygen
(C2@O) in the pyridone ring (ring A, Fig. 1) and the nitrogen atom
(N12) in the piperidine ring (ring C).
In (–)-N-benzoylcytisine (2, Fig. 1), free electron pairs of both
nitrogen atoms N1 and N12 are strongly involved in the conjugated
bond systems of lactams and amide groups, respectively [12,13].
Owing to this fact, the protonation at either of these atoms is
highly improbable, and for this reason it was expected that the
protonation will take place at either of oxygen atoms O2 or O14.
In our previous paper [13] we reported the chemical shifts of
carbon atoms in the 13C NMR spectra of free bases (–)-N-acetylcyti-
sine (3), (–)-N-propionylcytisine (4) and (–)-N-benzoylcytisine (2)
in DMSO-d6. These derivatives of cytisine occur in solution as mix-
tures of two conformers cis and trans, giving double sets of signals
(both in 1H NMR and 13C NMR).[13] The same difficulties to assign
the chemical shifts have been encountered in the spectrum of the
perchlorate salt of 2 taken in DMSO-d6 at r.t. It was finally deemed
necessary to measure the spectrum at the coalescence temperature
(100 °C) [13]. In contrast to the spectra of free bases 2, 3 and 4 and
also to the spectra of the perchlorate salts of 3 and 4, the 13C NMR
spectrum of the protonated N-benzoylcytisine (2ꢁHClO4) measured
in CD3OD in r.t. reveals only one set of signals of carbon atoms (Ta-
ble 1). Although, in the same conditions (r.t., CD3OD) the 1H NMR
spectrum of 2ꢁHClO4 still shows not completely overlapped signals
of the mixture of isomers, proving that the perchlorate salt of 2 in
solution still occurs in a specific conformational equilibrium.
At the beginning, we supposed that the intermolecular hydro-
gen bond OAHꢁꢁꢁO with the protonated oxygen atom (O2 or O14)
as a hydrogen-bond donor and solvent oxygen atom as an acceptor
or vice versa, had blocked the rotation of the substituent at N12, so
the 13C NMR spectrum of this salt showed only one set of signals.
However, such situation has not been observed in other protonated
cytisine derivatives 3 and 4 in which the small substituents (small
steric hindrance) are not bulky enough to stiffen the structures of
the molecules (Table 1). It should be mentioned that such situation
was observed only in protic solvent – CD3OD (Table 1), while in
aprotic solvent – DMSO-d6 a double set of signals was observed
which suggests that the possibility of the solvent acting as hydro-
gen bond donor is more probable. Unfortunately, the NMR data did
Fig. 1. Cytisine (1) and its derivatives: N-benzoylcytisine (2), where R = COC6H5; N-
acetylcytisine (3), where R = COCH3, and N-propionylcytisine (4), where R = COC2H5.
activity towards certain subtypes of nACh receptors and the
affinity to ganglionic and centric receptors [1–3]. Moreover, among
N-substituted cytisines some compounds with analgesic activities
have been found [2–4]. It is generally accepted that biologically
active compounds are usually polyfunctional derivatives whose
protonation strongly depends on acid–base properties of individ-
ual functional groups, intra- and intermolecular interactions. The
calculations in the gas phase and in water (pKa = 6,11 for cytisine)
have shown – not surprising – that the oxygen is clearly a more ba-
sic site, which is consistent with the results of this study [5,6].
Upon protonation of quinolizidine and bisquinolizidine com-
pounds their monosalts can be easily formed. The aim of this study
is to explain the influence of the structure of N-cytisine derivatives
(with additional proton-accepting groups) on the preferred proton-
ation site. The explanation of the hierarchy of protonation sites in
cytisine derivatives can help in understanding the mechanisms of
binding these molecules to the nAChRs. Up to now, some of the
N-substituted cytisines have been docked into a rat and human
nAChR models based on the extramolecular domain of a molluscan
acetylcholine binding protein and the results agreed with the bind-
ing data [7,8].
In this paper, we are presenting the results of the NMR spectros-
copy, DFT calculations, and X-ray studies of the perchlorate salt of
N-benzoylcytisine.
Results and discussion
It is obvious that protonation of amides can take place at the
oxygen or nitrogen atoms [6]. The calculation of molecular orbitals
of strained amides and their N- and O-protonated forms reported
by Greenberg indicated that the N-protonated form was favoured
over the O-protonated one [10].
not bring
a decisive answer as to the protonation site in
N-benzoylcytisine (2). The signals of the lactam carbon atoms ap-
peared as expected at low fields in the range 162–173 ppm, but
However, from the thermodynamical point of view the oxygen
atom is much preferred position in this process [9,11]. Free,
insignificant changes in chemical shift (
D
d) values of C10, C11
Table 1
13C NMR chemical shifts [ppm] of N-benzoylcytisine (2) and the salts of 2 and N-acetylcytisine (3), N-propionylcytisine (4).
1
1ꢁHClO4
2 [13]
CD3OD trans/cis
2ꢁHClO4
3ꢁHClO4
4ꢁHClO4
At C
CD3OD
CD3OD
CD3OD
CD3OD trans/cis
CD3OD trans/cis
2
3
4
5
6
7
8
9
10
11
13
14
15
16
10
20/60
30/50
40
165.9
116.8
141.3
108.2
153.1
36.5
26.7
28.9
51.1
53.1
165.9
118.5
142.2
110.1
148.3
32.9
24.1
26.4
50.8
49.8
165.4/165.3
117.4/117.2
141.5/141.8
108.6/109.1
150.8/151.0
36.4/ 36.0
26.4/26.5
29.1/29.3
49.6/50.2
49.1/50.3
55.9/54.7
172.8/173.3
–
165.1
116.8
142.2
109.8
151.3
36.4
26.5
29.2
50.9
49.9
55.9
173.3
–
163.5/163.1
114.8/115.6
144.6/144.9
114.5/113.9
153.0/153.5
36.5/35.9
20.7/21.2
28.8/28.9
52.6/52.5
49.4/48.5
54.4/53.0
172.5/172.4
25.9/25.7
–
162.9/162.5
116.5/117.1
145.5/145.7
113.9/113.3
153.7/154.2
36.4/35.9
25.8/25.7
28.8/28.7
53.5/52.95
a
49.6
54.2
49.6
53.0/52.2
175.8/175.7
26.7/27.0
9.7/9.8
–
–
136.1/136.5
129.4/130.0
127.4/127.6
131.1/130.0
136.2
131.1
127.5
129.5
a
Two overlapped signals.

A.K. Przybył et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 1–6
3
Table 2
and C13 were observed. The protonation of the nitrogen atom N1
Relative energies of two tautomeric forms A and B of the molecular cation studied.
or N12 should result in a shift of the corresponding signals (C6,
C10, C11 and C13) towards lower field, while the protonation of
the oxygen atom O2 or O14 should shift the signals assigned to
C2 and C14 towards higher field.
As the NMR analysis did not bring an expected answer, we
decided to use molecular modelling (DFT) and these data provided
a very interesting result. DFT level of protonated cytisine confirms
that the conjugation effect (O@CAN< and ꢂOAC@N<+) strongly re-
duces the basicity of the pyridone nitrogen (ring A) and increases
the basicity of the oxygen atom [6].
Structure
Relative energy (kcal molꢂ1
)
B3LYP/6-31G(d)
B3LYP/6-311++G(d,p)
PBE/6-311++G(d,p)
A
B
0.0
9.6
0.0
11.4
0.0
11.4
The results of crystallographic analyses supplied the informa-
tion about the preferred protonation sites in the solid state, that
could not be transferred to the results of analyses in solution
(NMR). However, similar result of amide protonation has been ob-
tained in dutasteride hydrochloride salt with two secondary amide
moieties. It was found that the favoured site of protonation de-
pends on the resonance contribution and acceptor strength of
amide oxygen [9] Although, for quinolizidine and bisquinolizidine
alkaloids such a protonation in solid state has been observed for
the first time. It has been only reported the N-oxide oxosparteine
derivatives have an extremely strong proton-acceptor centre on
oxygen atom, but – to the best of our knowledge – there are no
published X-ray data of the salts of dioxo-bisquinolizidine alka-
loids like 2,15-dioxosparteine or 2,17-dioxosparteine, that contain
two amide moieties. Upon protonation of 15-oxosparteine-N1-oxi-
deꢁHCl (5) and 17-oxosparteine-N1-oxideꢁHClO4 (6) (Fig. 4) the
N1+AOAH group is formed and in the crystal structure the short
intermolecular hydrogen bonds between the lactams and N1-oxy-
gen atoms (N1+AO1AHꢁ ꢁ ꢁO15 and N1+AO1AHꢁ ꢁ ꢁO17, respec-
tively) are formed. Such bonds (N16+AO16AHꢁ ꢁ ꢁO2) have been
also observed in the structures of 2-oxosparteine-N16-oxide HClO4
Therefore, the oxygen atom should be the favoured site of pro-
tonation in molecules with an amide group. It seems that this cal-
culation can be applied to (–)-N-benzoylcytisine salt in which
oxygen atoms (O2 and O14) are available to a proton.
For protonated N-benzoylcytisine the relative energies of the
possible isomeric forms A–B (Fig. 2) of the studied molecular cation
calculated at different basis sets and exchange correlation func-
tionals [14] are presented in Table 2. The results for other struc-
tures are presented in Supplementary Information. The lowest
energy structure is A in which proton is attached to the carbonyl
oxygen atom (C2AO2), where it favours phenol-like tautomeric
form. The next tautomer in the energetic sequence, higher in en-
ergy by ca. 10 kcal molꢂ1, is the B structure in which proton is
bound to the oxygen atom (O14) from the benzoic moiety. The tau-
tomeric forms with protonated N atoms that show much higher
energy than O-protonated tautomeric forms (20–40 kcal molꢂ1
,
Supplementary Table 1).
In the crystal structures of both (–)-N-benzoylcytisine perchlo-
rate (2ꢁHClO4) its monohydrate (2ꢁHClO4ꢁH2O) protonation
undoubtedly took place on O2 atom (Figs. 3 and 4). The positions
of the appropriate hydrogen atoms were determined, in either
case, by (i) localization of the hydrogen atom in the difference Fou-
rier map, (ii) successful refinement of this atom without any con-
straints and (iii), by the geometry of the neighbourhood, for
instance C2AO2 bond lengths are 1.3156(18) Å in 2ꢁHClO4 and
(7) and 17-b-methyl-a-isolupanine-N16-oxide HClO4 0.5H2O (8)
(Fig. 5) [16–20].
Experimental details
Procedure
1.299(2) Å in 2ꢁHClO4ꢁH2O, as compared with app0ropriate values
0
N-benzoylcytisine (2) was obtained according to literature [13]:
2 was dissolved in methanol and 60% HClO4 solution in methanol
were added to pH = 6.0. A yellow powder of perchlorate salt (2)
was precipitated. Recrystalization from ethanol (yield 72%), m.p.
239 °C.; 1H NMR (300 MHz, MeOD-d6, ppm); d 8.07 (1H, dd,
C4AH, J = 8.8; 7.4 Hz), phenyl ring: 7,54–7.42 (5H, m, C20AH,
C30AH0, C40AH, CA50AH, C60AH), 7.18 (1H dd, C3AH, J = 7.4;
0.8 Hz); 7.11 (1H, dd, C5AH, J = 8.9, 1.3 Hz); 8.81–3.53 (6H, m,
for C14AO14 bonds, of 1.2467(17) ÅA and 1.248(2) ÅA.
Overall geometries of both cations 2 in the crystal structures are
similar (Table 3), and close to the typical ones. The cytisine skele-
ton is stiff. The rings A are planar, rings B are close to the sofa con-
formation; and finally the rings C are almost ideal chairs close to
the D3d symmetry.
In the crystals of 2ꢁHClO4 the cations are connected by
O2AHꢁ ꢁ ꢁO14 intermolecular hydrogen bonds into infinite chains,
and the cations and anions are connected by Coulombic interac-
tions and weak CAHꢁ ꢁ ꢁO hydrogen bonds (Fig. 3), while in 2ꢁHClO4-
ꢁH2O the water molecule acts as kind of a ‘spacer’: it accepts the
OAH(cation)ꢁ ꢁ ꢁO(water) hydrogen bonds and acts as a donor in
OAHꢁ ꢁ ꢁO(anion) (Fig. 4) and OAHꢁ ꢁ ꢁO14 ones.
C10AH
3.30 (1H + CD3OD, m, C7AH); 3.01 (1H, b.s., C9AH); 2.30–2.17
(2H, m, C8AH , C8AHb).
a, C10AHb, C11AHa, C11AHb, C13AHa, C13AHb); 3.33–
a
N-acetylcytisine (3) was obtained according to literature [13,21]:
3 was dissolved in methanol and 60% HClO4 solution in methanol
were added to pH = 6.0. A yellow powder of salt of 3 was precipi-
tated. Recrys. from ethanol (yield 71%), m.p. 187–188 °C.
N-propionylcytisine (4) was obtained according to lit. [13,21]: 4
was dissolved in methanol and 60% HClO4 solution in methanol
were added to pH = 6.0. A yellow powder of the salt of 4 was pre-
cipitated. Recrys. from ethanol (yield 75%), m.p. 167–169 °C.
NMR spectra
1D correlation spectra were recorded on a Bruker AVANCE 600
(600.31 MHz for 1H and 150.052 MHz for 13C) spectrometer, with
a 5 mm triple – resonance inverse probe head (1H/31P/BB) with ac-
tively shielded z gradient coil (90_ 1H pulse width 90 ls, 13C pulse
width 13.3 ls). 2D spectra were acquired and processed using stan-
dard Bruker software. Spectral width of 6313.13 and 25,000 Hz
were used for 1H and 13C, respectively. Relaxation delays of 2.0 s
Fig. 2. Molecular structures of (–)-N-benzoylcytisine (2) with different protonation
patterns. A – with protonated oxygen at amide group in ring A. B – with protonated
oxygen at amide group at ring C.

4
A.K. Przybył et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 1–6
Fig. 3. (a) Perspective view of the (–)-N-benzoylcytisine perchlorate (2ꢁHClO4) as seen in the crystal structure [15]. The ellipsoids are drawn at 50% probability level, hydrogen
atoms are shown as spheres of arbitrary radii; dashed line denotes the weak hydrogen bond. (b) A part of infinite chain of hydrogen-bonded cations.
Fig. 4. Perspective view of the (–)-N-benzoylcytisine perchlorate hydrate (2ꢁHClO4ꢁH2O) as seen in the crystal structure [15]. The ellipsoids are drawn at 50% probability level,
hydrogen atoms are shown as spheres of arbitrary radii; dashed lines denote the hydrogen.
Table 3
Selected geometrical parameters (Å, °) with esd’s in parentheses.
2ꢁHClO4
2ꢁHClO4ꢁH2O
N1AC2
N1AC6
N1AC10
C2AO2
C11AN12
N12AC13
N12AC14
C14AO14
C7AC13AN12AC14
C9AC11AN12AC14
C11AN12AC14AC15
C13AN12AC14AC15
C11AN12AC14AO14
C13AN12AC14AO14
N12AC14AC15AC16
N12AC14AC15AC20
1.3591(19)
1.3807(17)
1.4953(18)
1.3156(18)
1.4721(18)
1.4641(18)
1.3421(19)
1.2467(18)
ꢂ108.37(15)
113.39(15)
167.98(12)
ꢂ26.9(2)
1.368(2)
1.378(2)
1.495(2)
1.299(2)
1.468(2)
1.470(2)
1.345(2)
1.248(2)
ꢂ111.89(19)
113.85(18)
176.79(16)
ꢂ16.1(2)
ꢂ2.5(3)
ꢂ9.3(2)
155.81(13)
ꢂ50.0(2)
164.69(17)
ꢂ50.2(2)
134.36(17)
Fig. 5. The structure of 15-oxosparteine-N1-oxideꢁHCl (5), 17-oxosparteine-N1-
oxideꢁHClO4 (6), 2-oxosparteine-N16-oxideꢁHClO4 (7) and 17-b-methyl-
nine-N16-oxideꢁHClO4ꢁ0.5H2O (8).
a-isolupa-
133.29(15)

A.K. Przybył et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 1–6
5
were used for all 2D experiments and the mixing time 0.8 s for
Conclusions
1HA1H NOESY spectrum was applied. All 2D spectra were col-
lected with 2 K points in F2 and 256 increments (F1) with 4 (g-
COSY) and 64 (g-HSQC) transients each and zero filling in F2 to
2048 _ 1024 data matrix.
In conclusion, we present many aspects of protonation possibil-
ities in the perchlorate salt of N-benzoylcytisine, the cation with
two tertiary amide moieties, one cyclic, in the six-membered ring
A and the other acyclic with a benzoic group. The energy calcula-
tions for the isolated cations of (–)-N-benzoylcytisine (DFT level
of theory) showed that the lowest energy tautomeric form of the
cation had protonated oxygen atom in the six-membered cyclic
ring (A), and this form is favoured over the tautomeric form with
protonated oxygen atom in benzoic moiety (F) and both these
O-protonated forms have lower energy the structures with proton-
ated nitrogen atoms in amide moieties B–E. It turned out that we
X-ray diffraction
Data were collected at 130(1) K by the
Agilent Technologies Xcalibur four-circle diffractometer with Eos
CCD detector and graphite-monochromated Mo radiation
x-scan technique on an
K
a
(k = 0.71069 Å). The data were corrected for Lorentz-polarization
as well as for absorption effects. Precise unit-cell parameters were
determined by a least-squares fit of 19,545 (2ꢁHClO4) and 3249
(2ꢁHClO4ꢁH2O) reflections of the highest intensity, chosen from
the whole experiment. The calculations were mainly performed
within the WinGX program system. The structures were solved
with direct methods (SIR92) and refined with the full-matrix
least-squares procedure on F2 (SHELXL97). The scattering factors
obtained
two
crystal
forms
containing
this
cation:
(–)-N-benzoylcytisinium perchlorate (2ꢁHClO4) and (–)-N-ben-
zoylcytisinium perchlorate monohydrate (2ꢁHClO4ꢁH2O). The X-
ray data showed that also in the periodic conditions, in the crystal
lattice, in both structures the protonation takes place at the O2
atom. This may indicate that the protonation pattern observed in
the crystal structure results from energetic preferences of the iso-
lated molecular cation rather than from the influence of the crystal
lattice and intermolecular hydrogen bonds.
In solution the salt occurs in equilibrium between two forms
and protonation can take place on oxygen atom, at either lactam
or amide moieties. 1D and 2D NMR spectra (300 MHz and
600 MHz) of N-benzoylcytisine were analyzed and the chemical
shifts have been assigned to the corresponding atoms of the cyti-
sine derivatives salts (2–4). Surprisingly, to the best of our knowl-
edge, N-benzoylcytisinium perchlorate (2) is the first compound
among the salts of quinolizidine alkaloids derivatives other than
N-oxides, in which the protonation in the solid state has been ob-
served at the oxygen atom but not at nitrogen atom. These obser-
vations of the protonation of amide moieties can help e.g. in the
explanation of intermolecular interactions in binding affinity of
cytisine derivatives to nACh receptors.
incorporated in SHELXL97 were used.
P
The function
w(|Fo|2–|Fc|2)2 was minimized, with wꢂ1
=
[
r
2(Fo)2 + (A.P)2 + B.P] (P = [Max (Fo2, 0)+2F2c]/3). All non-hydrogen
atoms were refined anisotropically, all hydrogen atoms in 1 were
found in the difference Fourier map and isotropically refined; in 2
only OH hydrogens were treated in this way, all other were placed
in idealized positions and refined as ‘riding model’ with isotropic
displacement parameters.
Crystal data: 2ꢁHClO4: C18H19N2O+2ꢁClO4ꢂ, Mr = 394.80, ortho-
rhombic,
P212121,
a = 7.8331(3) Å,
b = 14.6905(6) Å,
c = 15.6218(6) Å, V = 1797.63(12) Å3, Z = 4, dx = 1.46 g cmꢂ3
,
F(000) = 824,
l
= 0.25 mmꢂ1, 36,002 reflections collected, 3904 un-
ique (Rint = 0.072), Final R = 0.027, wR2 = 0.071, S = 0.95,
Dq in the
final
D
F map 0.20/ꢂ0.21 e Åꢂ3. 2ꢁHClO4ꢁH2O: C18H19N2O2+ꢁClO4ꢂꢁH2-
O, Mr = 412.82, monoclinic, P21, a = 7.3699(7) Å, b = 10.7915(8) Å,
c = 12.2632(11) Å,
b = 105.853(8),
V = 938.22(14) Å3,
Z = 2,
dx = 1.46 g cmꢂ3, F(000) = 432,
l
= 0.25 mmꢂ1, 5645 reflections col-
Acknowledgements
lected, 3568 unique (Rint = 0.011), Final R = 0.030, wR2 = 0.083,
S = 1.08,
D
q
in the final
D
F map 0.46/ꢂ0.36 e Åꢂ3
.
The study was supported by Norway Grants and the National
Centre for Research and Development of Poland (NCBiR) as a part
of Polish–Norwegian Research Programme: Superior bio-friendly
systems for enhanced wood durability. No. Pol-Nor/203119/32;
DURAWOOD). This research was also supported in part by PL-Grid
infrastructure. The authors wish to express their appreciation to
Computational methods
Initial tautomers of molecular cations with different proton-
ation patterns were generated with the help of Marvin [14] and
were subjected to full geometry optimization using the popular
Becke [22] three parameter hybrid exchange and Lee–Yang–Parr
correlation density functional (B3LYP) [23]. Full geometry optimi-
zation was performer at 6-31G(d) basis set followed by single point
energy calculations at 6-311++G(d,p) basis set because it contains
both polarized and diffusion functions focused on all atoms. Partic-
ularly, diffuse functions have a serious effect on the systems where
electrons are relatively far from the nucleus including molecules
with lone pairs [24]. The optimized structures were characterized
by harmonic frequency calculations at B3LYP/6-31G(d) level of
theory. The analytical harmonic vibrational wave numbers were
positive values, confirming that the local minima on the potential
energy surface had been found. Finally, to verify performance of
B3LYP method calculations with PBE exchange correlation func-
tional [25] were carried out. Complete optimized structures in
the Cartesian coordinate format are available in Supplementary
data. Electronic Supplementary Information (ESI) available: CCDC
862379 (2) and 862378 (2ꢁH2O) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif or e-mail: deposit@ccdc.ca-
m.ac.uk See DOI:10.1039/b000000x/.
_
M.Sc. Bozena Wyrzykiewicz for help in NMR experiments.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.02.192.
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