Synthesis, experimental characterization and theoretical calculation of novel charge transfer complex between (S, S)-bis-N,N-sulfonyl bis –L-phenylalanine dimethylester and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)

Article abstract of DOI:10.1016/j.molstruc.2019.126890

Charge transfer complex (CTC) formed between (S, S)-bis-N, N-sulfonyl bis-L-phenylalanine dimethylester as donor and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as acceptor, has been studied spectrophotometrically. Benesi-Hildebrand and molar ratio meth

Full text of DOI:10.1016/j.molstruc.2019.126890

Journal Pre-proof
Synthesis, experimental characterization and theoretical calculation of novel charge
transfer complex between (S, S)-bis-N,N-sulfonyl bis –l-phenylalanine dimethylester
and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
Messaouda Mohamdi, Nadjia Bensouilah, Nadia Trad, Mohamed Abdaoui
PII:
S0022-2860(19)30981-0
DOI:
https://doi.org/10.1016/j.molstruc.2019.126890
MOLSTR 126890
Reference:
To appear in: Journal of Molecular Structure
Received Date: 5 January 2019
Revised Date: 7 July 2019
Accepted Date: 1 August 2019
Please cite this article as: M. Mohamdi, N. Bensouilah, N. Trad, M. Abdaoui, Synthesis, experimental
characterization and theoretical calculation of novel charge transfer complex between (S, S)-bis-N,N-
sulfonyl bis –l-phenylalanine dimethylester and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ),
Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.126890.
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Synthesis, experimental characterization and theoretical calculation of novel
charge transfer complex between (S, S)-bis-N,N-sulfonyl bis –L-phenylalanine
dimethylester and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
Messaouda Mohamdi1, Nadjia Bensouilah1,2,*, Nadia Trad1, Mohamed Abdaoui1
1Laboratory of applied chemistry (LAC), University of May 8th, 1945, BP 401, 24000
Guelma, Algeria.
2 laboratory of applied organic chemistry , faculty of chemistry (LAOC), USTHB, BP 32, El-Alia, 16111 Bab-
Ezzouar, Algiers, Algeria
Abstract:
Charge transfer complex (CTC) formed between (S, S)-bis-N, N-sulfonyl bis-L-
phenylalanine dimethylester as donor and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ)
as acceptor, has been studied spectrophotometrically. Benesi-Hildebrand and molar ratio
methods were applied in order to determine association constant (KCT), molar extinction
coefficient (εCT) and stoichiometric ratio, respectively. The solid CT complex has been
synthesized and its structure was characterized via spectroscopic techniques (UV–vis and FT-
IR). In order to determine the equilibrium geometries, DFT/CAM-B3LYP has been
employed. Moreover, TD-DFT method was used to investigate the electronic structures of
donor, acceptor and their complex. Natural bond orbital (NBO) analysis has been used to
study the atomic charges and molecular bonding interactions of the interesting complex.
These studies show that non-bonding interactions are important in charge transfer complex.
Keywords: Sulfonamides, DDQ, CTC, DFT-CAM-B3LYP, TD-DFT, NBO.
Corresponding author:
Doctor Bensouilah Nadjia
Tel : 00213 0661 49 56 39
*laboratory of applied organic chemistry , faculty of chemistry (LAOC), USTHB, BP 32, El-Alia, 16111 Bab-
Ezzouar, Algiers, Algeria
Email: nadjia.bensouilah@yahoo.fr

1. Introduction:
Charge transfer complexes CTC are held together by weak intermolecular forces such
as hydrogen bonding and van der Waals forces. Hydrogen bond plays an extremely significant
role on determining the structural stability of many chemical complexes and biological macro
molecules [1, 2]. The hydrogen bond is observed when a hydrogen atom related to an
electronegative atom (HX; where X is an element with an electronegativity greater than H) is
approximate to a free electron pair which is generally carried by another electronegative atom
(acceptor) [3].
These CTC-interactions have been the subjects of many chemical and biological
studies, in particular with acceptors of biological interests like DDQ [4, 5].
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a strong electron acceptor as mild
oxidizing agent and its extraordinary potency as a dehydrogenating agent is discovered [6].
It is structurally characterized by the presence of potentially three different halogen bonding
acceptor sites; the carbonyl oxygen atoms, the nitrogen atoms and the chlorine atoms [7].
The studies of interaction of drug as electron donor with different acceptors are
important in the area of research [8-11]. In fact, it is shown that the drug-acceptor complexes
can enhance the activity of antibacterial agents especially against both Gram-positive and
Gram-negative bacteria as well as fungi [12-15].
Compounds including N–SO2–N fragment possesses antiviral action [16] .The most
important biological activity of sulfamide derivatives is due to the fact that the sulfamide
belongs to important essential structures which are used in human medicine
with various types of pharmacological agents having antibacterial properties [17,18], Protease
inhibitors such as HIV Protease [19, 20], as nonhydrolyzable components in peptidomimetics
[21] and certain analogues of symmetric N, N'-substituted sulfamides show anticonvulsant
activity [22]. According to these researchs, it is interesting for us to invest the characteristics
of its electron donor nature to form stable complexes of the CTC type.
The presence of the ion pairs of oxygen and nitrogen electrons and the aromatic ring in
(S, S)-bis-N, N-sulfonyl bis –L-phenylalanine dimethylester facilitate the hydrogen-bonding
and charge transfer interaction with acceptors. The results show a tremendous increase in the
stability of their Tetracyanoethylene TCNE and chloranil after complexation [23]. The
purpose of the present study is to explore a definite relation between the oxidation-reduction
potentials of DDQ and their reactivity in charge-transfer reaction with (S, S)-bis-N, N-
sulfonyl bis –L-phenylalanine dimethylester.

The structures of donor and acceptor are shown in scheme 1. The new complex formed
between (S, S)-bis-N,N-sulfonyl bis-L-phenylalanine dimethylester and DDQ was
synthesized and characterized using elemental analyses, electronic absorption spectroscopy
and Fourier transform infrared FT- IR technique. The geometrical parameters, electronic
spectroscopic properties, the Natural population analysis (NPA) for the investigated donor,
DDQ and their complex and the Natural bond orbital (NBO) were performed by CAM-
B3LYP level of theory.
O
O
O
O
CH3
H3C
O
S
O
N
N
Cl
Cl
NH
NH
O
O
Scheme 1. Chemical structures of (S, S)-bis-N,N-sulfonyl bis –L-phenylalanine dimethylester and 3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ).
2. Experimental:
2.1. Chemicals and materials:
(S, S)-bis-N, N-sulfonyl bis –L-phenylalanine dimethylester was prepared according
to the known method [21, 24]. 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) was
obtained from Aldrich Chemical. Chlororm and acetonitrile of analytical grade obtained
respectively from: Merck and Fluka groups were used without any further purification. The
UV–vis spectrum was recorded on UV Probe PC software within 190 nm – 1100 nm range
using a Perkin-Elmer Precisely Lambda 25 UV/Vis double beam Spectrometer fitted with a
quartz cell of 1.0 cm path length. The FT-IR of the compounds was measured on JASCO/FT-
IR Model 410 spectrophotometer in the range of: 400–4000 cm-1 with 30 scans and 2 cm-1
1
resolution using KBr pellet technique. The H NMR spectrum of the donor was recorded on
JEOL EX-400MHz device (400 MHz 1H).

2.3. General procedure for the synthesis of the donor:
The synthesis of (S, S)-bis-N,N-sulfonyl bis –L-phenylalanine dimethylester in two
steps was outlined in Scheme 2. The synthesis consists of an esterification reaction followed
by sulfamoylation. Starting by the preparation of L-phenylalanine methyl hydrochloride, the
synthesis of aminoester is usually performed as follows: 1 equivalent of phenylalanine
(C9H11O2N) dissolved in anhydrous methanol at 0°C was added dropwise to 2 equivalents of
thionyl dichloride (SOCl2) in the same solvent, after 30 min the reaction was followed to
more than 4 h at at room temperature. Once the reaction is complete the solvent and the
excess thionyl dichloride were evaporated in vacuum, the crude product is taken up in ether to
obtain a white powder of amino ester hydrochloride. The second step begins by the
deprotection of the amino ester hydrochloride : at 0°C, 1 equivalent of triethylamine (TEA)
(C6H15N) was added dropwise to 1 equivalent of amino ester hydrochloride.
The reaction mixture is left under magnetic stirring for 15 min. After this step, the
amino ester in dichloromethane was added dropwise (at 0°C) to a mixture of 0.5 equivalent
sulfuryl chloride (SO2Cl2) in the presence of triethylamine (3 equivalents). The mixture was
stirred for 45 min at room temperature. The resulting mixture was accomplished by treatment
with HCl 0.1N and water. The organic layer was separated and dried over anhydrous Na2SO4.
The obtained organic solution was filtered, concentrated in vacuum and purified by
chromatography on silica gel to obtain the final product: (S, S)-bis-N, N-sulfonyl bis –L-
phenylalanine dimethylester. Spectroscopic evidences concerning illustration of the structure
of the donor are given as follows: FT-IR (KBr,
ν
en cm-1): 3329 cm-1(NH), 1736 cm-1(C=O)
1
et 1362 cm-1 (SO2). (See Fig.6 (a)) H RMN (400MHz, DMSOd6, δ : ppm): 7.08-7.21 (m,
10H, 2ArH), 4.45 (d, 2H, NH), 3,84 (m, 2H, CH*), 3.67 (s, 6H, OCH3), 3.29-3.04 (m, 4H,
2CH2Bn).(See Fig.1 )

OH
O
MeOH,0°C
4h, T amb
CH3
O
O
SO2
(2 eq)
HCl
SOCl2
+
+
+
H2N
HCl.NH 2
(0,5eq)SO2Cl2
O°C,45mn,3h,Tamb
TEA(3eq),CH2Cl2
O°C,15mn
O
O
O
O
H3C
O
S
CH3
NH
NH
O
Scheme 2. Synthesis of (S, S)-bis-N,N-sulfonyl bis –L-phenylalanine dimethylester
Fig.1.The 1H NMR spectra of (S, S)-bis-N,N-sulfonyl bis –L-phenylalanine dimethylester.
2.4. Photometric measurements:
Solutions for spectroscopic measurements (1.00 10-4 mol /l) were prepared by
mixing an appropriate constant volume of donor and acceptor stock solutions with
acetonitrile. Acetonitrile which was chosen as a principal reaction medium was found to be an
ideal solvent for the study of CT complexes [25]. For highlighting the formation of a stable
complex, the stoichiometry of the CT complex in solution was determined by molar ratio
method [26] and the association constant was determined using Benesi- Hildebrand equation

[27-29] (The concentration of DDQ is kept constant (1.00 x 10-5 mol /l) whereas, the donor
concentration is varied from (1.00 x10-5 to 1.00 x10-4 mol /l)).
3. Theoretical calculations:
Quantum chemical calculations were carried out using the Gaussian-09 software
package [30] with the help of Gauss View 5.0 [31]. The molecular structures of bis-L-
aminoesters sulfones, DDQ and their complex have been optimized by using DFT/ CAM-
B3LYP method [32] with 6-31++G (d, p) basis set in gas phase and in acetonitrile as solvent
using PCM model [33]. The bond length and bond angle of the optimized structures are
classified. The absorption spectra of DDQ, donor and their complex were calculated using the
time-dependent density functional theory (TD-DFT) method using the PCM model on
optimized geometries. The calculation of different HOMO and LUMO energies was also
performed at the same level of theory. NBO calculations were employed to calculate the
second-order perturbation stabilization energy (E2) of DDQ complex. The atomic charges in
free molecule and in their complex were calculated by the NPA. Finally, the natural NBO
atomic charges [34-36] were analyzed to correlate the properties of both partners before and
after complexation.
4. Results and discussion:
4.1. Determination of molar ratio:
Representative of spectrophotometric titration plot based on the characteristic
absorption bands are depicted in Fig. 2. In order to obtain the stoichiometry of the resulting
complex, the varying molar ratio method (Skoog, 1985) was followed. Then, 0.20, 0.40, 0.80,
1.00, 1.20, 1.60, 2.00, 2.40 ml of a solution of 1.00 x 10-4 M of the donor was added to 1.00
ml of a solution of 1.00 x 10-4 M of the acceptor. The absorbance obtained in the spectra
assigned to the formed CT complexes were measured and plotted as a function of the [D]/[A]
ratio according to the known method. As can be seen, the spectrophotometric titration plot
confirmed the formation of donor-DDQ complex at a ratio of 1:1.

1,2
1,0
0,8
0,6
0,4
0,2
0,0
0,5
1,0
1,5
2,0
2,5
[D]/[A]
Fig. 2. Spectrophotometric titration curve of bis-L-aminoester sulfone with DDQ in
acetonitrile at 298 K.
4.2. Formation constants and energy (ECT) of CT-complex:
The electronic absorption spectra of DDQ (kept fixed at 1.00 x 10-5 M) in the presence
of various donor concentrations has been followed by an increase in absorbance at 208 nm
(see Fig. 3). An important observation from Fig. 3 is the unchanging of the maximum of
wavelengths of the complex bands upon going from: 4.00 x10-5 to10-4 M of donor. This is
consistent with the appearance of stable complex. The band maxima as obtained in the present
study indicate the formation of electron donor acceptor complex between DDQ and donor in
acetonitrile. The formation constant for the complexation process can be obtained by a proper
analysis of the absorbance data. Benesi–Hildebrand equation [37-39] was used to calculate
both formations constant (KCT), and extinction coefficient (εCT) of the CT complex in
acetonitrile. The equation is written as follows:
[ꢀ]
ꢃ
ꢃ
ꢃ
=
.
+
ꢇ
ꢅꢆ
(1)
ꢀꢁꢂ
ꢄ
.ꢇ
ꢅꢆ ꢅꢆ
[ꢈ]
ABS is the absorbance of the detected CT bands while [D] and [A] are the initial
molar concentrations of the donor and acceptor, respectively. From the previous equation, and
by plotting the values of [A]/ABS versus 1/[D], straight line (with a regression coefficient : R
= 0.99) was obtained with a slop of (1/KCT.εCT) and intercept of 1/ εCT as shown in Fig. 4. The
Values of KCT and ƐCT of the formed CT complex are respectively equal to 9.56.103 L.mol−1
and 1.36.105 L .mol-1.cm-1.

The results show that the [D→DDQ] complex shows a higher formation constant. This high
KCT value indicates a strong interaction between both partners. Therefore, it confirms the
expected high stability of the formed CT-complex as a result of the expected high donating
power of donor and high electron affinity of DDQ.
1,2
1,0
4.10-5mol/L
5.10-5mol/L
6.10-5mol/L
0,8
0,6
0,4
0,2
0,0
7.10-5mol/L
8.10-5mol/L
10-4mol/L
200
205
210
215
220
225
230
nm
Fig.3. Electronic absorption spectra of 10-5 M DDQ with different
concentrations of donor ranging from: 4x10-5 to 10-4 M in acetonitrile as solvent.
Fig.4. Benesi–Hildebrand plots for (S, S)-bis-N,N-sulfonyl bis-L-phenylalanine dimethylester donor with DDQ.
(Solvent: acetonitrile)

4.3. Preparation of solid complex:
Literature research revealed that the color changes observed upon the mixture of donor
with various electron acceptors is due to formation of CT complex [40, 41]. The solid CT-
complex (1:1 according to the obtained ration molar complex) is formed by the addition of a
saturated solution of the donor to a saturated solution of acceptor in CHCl3 at room
temperature with stirring. When they are mixed together, they give no immediate color
change. However, a dark yellow color started to develop after 15–30 min and became bluish
dark orange with time. The resulting precipitate was filtered and analyzed with FT-IR and
UV-Vis measurements.
4.4. Spectral analysis:
4.4.1. Observation of CT bands:
The absorption spectra of donor, acceptor and its obtained CT complex in CH3CN are
presented in Fig. 5(a). The electron absorption spectra of donor in acetonitrile solution exhibit
three bands. Two strong bands are at 196, 208 nm accompanied by week intensity at 258 nm,
while that of DDQ shows four strong absorptions (208, 226, 272, 280 nm) and week intensity
at 348 nm.
Interestingly, when donor and DDQ are mixed together, hyperchromic and
hypochromic effect can be assigned to the formation of charge transfer complex. It is
associated with the electronic transitions at 196, 208, 234, 258, 270, 280 and 346 nm. In
parallel, the band observed at 226 nm and associated to free DDQ was shifted to 237 nm in
CTC complex (bathochromic shift). This red shift at longer wavelength confirms
intermolecular charge transfer between the two partners. This observed behavior suggests
strong intermolecular charge transfer ability. Then, it is clear from the characteristic data
presented in Fig. 5(b) that the absorption spectrum of solid complex in acetonitrile is in
perfect agreement with that observed in Fig. 5(a). This result confirms the formation of CTC
complex.

(a)
196 nm
208 nm
complex
donor
ddq
2,0
1,5
1,0
0,5
0,0
234 and 258 nm
270 and 280 nm
346 nm
200
250
300
Wavelength (nm)
350
(b)
200 nm
208 nm
2,0
1,5
1,0
0,5
0,0
the solid complex
232 and 258 nm
270 and 280 nm
346nm
200
250
300
350
400
wavelength (nm)
Fig.5. Electronic absorption spectra: a) [bis –L-aminoesters sulfones- DDQ] charge-transfer systems and free
donor, acceptor in acetonitrile solvent, b) the solid complex in acetonitrile.
4.4.2. Infrared spectra:
As seen in Fig. 6, we note a shift in positions and changes in intensities of some
characteristic peaks. This is due to the complexation of the donor with DDQ, where changes
in both electronic structure and symmetry are expected. Fig. 6 shows the infrared absorption
spectra of donor, DDQ and the formed complex (in solid state). In Table 1, we have dressed
the most characteristic bands of acceptor, donor and their corresponding charge-transfer
complex. Some of the significant shifts are:

When the donor interacts with DDQ acceptor, the characteristic band that results from
the ѵ (C≡N) in free DDQ occurs at 2232 ꢉꢊꢋꢌ, whereas, in its complex ѵ (C≡N) occurs at
lower wavenumber values at 2031 ꢉꢊꢋꢌ. It is known that (C≡N) group is an electron
withdrawing group in DDQ. This observation clearly indicates that the (C≡N) group in the
DDQ participates in the complexation process. Similarly for DDQ, the CO stretching
frequencies that appeared at 1670 ꢉꢊꢋꢌ of the free acceptor was shifted to 1694 ꢉꢊꢋꢌ upon
complex formation. In parallel, the CO stretching frequencies that appeared at 1735 ꢉꢊꢋꢌ of
the free donor was shifted to 1730 ꢉꢊꢋꢌ. The characteristic bands of (SO2) group at:
1362, 1342 and 1313 ꢉꢊꢋꢌ in the free donor are also shifted and their intensities are affected.
Additionally, the significant shift of the C-Cl stretching frequency was detected from
802 ꢉꢊꢋꢌ towards lower frequency at 794 ꢉꢊꢋꢌ after complexation.
Table 1 : Characteristic bands of infrared spectra of the acceptor, donor and their corresponding charge-transfer
complex.
Donor
DDQ
complex
Assignments
3453w
3329s
3302s
3477br
3415br
Ѵ (N-H) donor
2232m
2031w
1730s
1617m
1637m
1694s
ѵ(C≡N) DDQ
ѵ(C=O) donor
1735vs,br
1603w
1670vs,br
ѵ (C=O) DDQ
1362vs
1342s
1313s
1356m
1324w
Ѵ(SO ) donor
2
802vs
794m
Ѵ(C-Cl) DDQ
s: strong, w: weak, m: medium, v: very, vs: very strong, br: broad.

(a)
101.5
3453.84
90
80
458.84
496.62
1962.56
1583.82
1603.59
3726.80
70
2857.01
3086.75
3061.31
1028.99
60
3007.53
684.97
767.73
%T 50
40
3030.61
2956.31
2943.62
558.55
1496.68
1010.02
813.25
854.05
933.79
3302.89
745.03
603.19
1431.82
977.34
30
3329.08
1443.35
1457.03
1342.53
1254.75
1215.49
1202.481098.53
1313.11 1180.37
1165.92
20
705.40
1362.38
10
3.2
4000.0
3600
3200
2800
2400
2000
1800
cm-1
1600
1400
1200
1000
800
600
450.0
(b)
100.8
505.83
457.22
90
80
70
60
50
40
30
20
1754.83
1338.50
957.01
930.69 837.11
1009.37
1402.19
764.43
621.17
2724.05
1072.09
3543.01
2232.81
1216.60
3466.15 3234.38
340373.4905.48
1453.69
1515.78
%T
722.42
896.66
1269.07
1172.17
1553.27
10
802.35
3.2
4000.0
3600
3200
2800
2400
2000
1800
cm-1
1600
1400
1200
1000
800
600
450.0

(c)
100.8
523.11
3935.37
90
80
70
60
50
40
30
20
3027.80
2956.96
915.03
887.99
860.64 764.77
743.39
2031.76
1033.04
1496.43
1483.89
991.38
574.67
479.93
1455.74
705.94
1324.93
1263.23
794.16
1121.36
1218.80
1564.76
1356.74
%T
619.53
1159.74
1172.83
1694.44
1637.46
1617.80
1730.37
3477.15
3551.98
10
3.2
4000.0
3600
3200
2800
2400
2000
1800
cm-1
1600
1400
1200
1000
800
600
450.0
Fig.6. Experimental FT-IR spectra of donor (a), DDQ (b) and CTC complex (c).
4.5. Theoretical calculations:
We compare the optimized structural parameters (binding lengths and angles) of free
molecule (donor and acceptor: DDQ) and those of its complex in vacuum and in solvent (see
Tables 2 and 3). Fig.7 illustrates the most important intermolecular interactions. The binding
energy ∆E of a complex formed [42, 43] by donor (D) and DDQ (A) can be obtained by the
following equation:
∆E = ED-A – (ED + EA)
(2)
Where in our system ED-A is the minimized energy of the (D-A: Donor-Acceptor) complex. ED
and EA are the energies of donor and DDQ molecules, respectively. The values of binding
energy for complex obtained at CAM-B3LYP/631G++(d, p) in the vacuum and in acetonitrile
are -9.91 and -3.75 Kcal/mol respectively. It should be noted that the value of the binding
energy of the complex in acetonitrile is very low compared to vacuum.

Fig.7. Optimized geometries of the complex in gas phase (a) and in acetonitrile (b) at CAMB3LYP/631G++(d,p)
level of theory.

As can be seen from fig.7 the optimized structure of complex by a DFT method in gas
phase, shows that O7...C54 inter-atomic distance is 2.793 Å and the O7…C56 distance was
2.969 Å, where the C54-C56 and C54-C55 bond lengths in the DDQ decrease slightly by
0,002 and 0,003 Å respectively in the complex. In addition the inter-atomic distances
O7…C55 and O7…C58 as noted in complex are 3.017 Å and 3.294 Å respectively; witch the
bond length for C58-C59 is decreased by 0,002 Å.
Apart from that, we noted an elongation of S3-O7 bond (1.462 Å) compared to the
isolated donor molecule (1.452 Å) and a decrease in the bond angles O6S3O7 by 1.3° in
complex (see Table 3). On the other hand, as displayed in Fig7 three bonding interactions are
observed between O60…H39C14 with a bond distance of 2.549 Å. This bond distance is
shorter than the N63…H38C12 and O60…H32 from (2.720 Å, 3.281 Å) respectively.
Whereas, the C54-C60 and N4-H32 distances: 1.013 Å in acceptor and 1.200 Å in
donor increased with 0.002 and 0,001 Å respectively when they are compared to the complex.
The most remarkable feature is that the charges on H39, H38 and H32 atoms are +0.158 e,
+0.194 e and +0.425 e respectively. They are found to increase after complexation to be:
+0.215 e, +0.242 e and +0.464 e respectively.
In the case of the solvent effect on the molecular structures modeled by PCM
procedure; as displayed in Fig7; the most important point to be mentioned here is that the:
O7...C54, O7…C56, O7…C55, O7…C58, N63…H38C12, O60…H32 interaction distances
are increased by 0.035, 0.033, 0.023, 0.035, 0.24, 0.034 Å in comparison with the complex in
vacuum, except the O60…H39C14 distance which decreases slightly by 0.006 Å. To
elucidate these results and assess the solvent effect, we have investigated the charge NBO in
vacuum and in CH3CN as solvent.
Table 2: Selected bond lengths (Å) of donor, of DDQ and of CTC complex calculated by using CAM-B3LYP/6-
31G++(d,p) method.
Parameters
S3O7
S3O6
Donor
acceptor
complex
1.452 (1.458)
1.461 (1.463)
1.660 (1.654)
1.462 (1.462)
1.458 (1.462)
1.653 (1.650)
S3N2
S3N4
N4H32
1.644 (1.637)
1.013 (1.015)
1.638 (1.636)
1.015 (1.016)
N2H31
C54C55
C54C56
1.016 (1.017)
1.017 (1.017)
1.490 (1.489)
1.498 (1.498)
1.493 (1.490)
1.500 (1.499)

C55C57
C56C58
C58C59
1.346 (1.345)
1.347 (1.344)
1.500 (1.499)
1.344 (1.344)
1.345 (1.343)
1.498 (1.497)
C57C59
C54C60
C59O61
1.498 (1.490)
1.207.208)
1.207 (1.208)
1.492 (1.488)
1.209 (1.209)
1.207 (1.210)
All values between parenthesis correspond to the bond lengths in acetonitrile medium
Table 3: Selected bond angles of donor, of DDQ and of CTC complex calculated by using CAM-B3LYP/6-31G
++(d, p) method.
Parameters
O6S3O7
N2S3O6
N2S3O7
Donor
acceptor
complex
119.4 (117.9)
113.1 (113.5)
105.2 (105.4)
118.1 (117.5)
114.1 (114)
104.8 (105.1)
O6S3N4
O7S3N4
S3N2C1
C14C12C13
H38C12H37
C5N4H32
C16C14H39
C13C14H39
105.4 (106)
106.2 (106.3)
109.9 (110.2)
119.4 (119.7)
120.6 (120.6)
107.9 (107.8)
117.5 (117.5)
119.5 (119.5)
119.8 (119.7)
110.3 (110.2)
118.8 (119.6)
120.5 (120.6)
108.1 (107.7)
118.1 (117.6)
119.7 (119.6)
119.4 (119.5)
C1C3C9
C9C3C5
C3C5C6
116.3 (116.2)
122.5 (122.7)
121.2 (121.2)
115.9 (116.1)
122.7 (122.6)
120.9 (121.1)
All values between parentheses correspond to the bond lengths in acetonitrile medium
4.5.1. Electronic spectroscopic properties:
The comparison of absorption wavelengths λmax between the experimental findings
and theoretical results of DDQ, of the donor and of their complex in acetonitrile is presented
in Table 4. While, experimental peaks maximum of the free molecules of Donor appeared at
196 nm and 208 nm TD-DFT predicted the calculated peaks maximum in acetonitrile at
204.79 nm, 205.20 nm and 210.90 nm. In parallel, the experimental values of ʎmax(nm) of
DDQ are as follows : 208, 226, 272, 280 and 348 nm and their corresponding calculated
values are : 204.83, 224.64, 271.19, 276.90 and 370.73 nm. For CTC complex, the computed
absorption maximum obtained at about 220.08 nm, 224.36 nm, 237.38 nm, 251.79 nm,
278.65 nm, 292.24 nm, 342.59 nm and at 347.66 nm are compared with the corresponding
experimental values : 196, 208, 234, 258, 270, 280 and 346 nm respectively. Hence, the
reasonable agreement between the theoretical and experimental results reflects well the
suitability of CAM-B3LYP/6-311++G (d,p) method for this type of study.

Table 4: Experimental and computed UV-vis data of the free molecules and their complex in acetonitrile.
ʎmax(nm) Theoretical
ʎmax(nm) Experimental
Donor
210.90
DDQ
complex Donor DDQ complex
208
370.73 347.66
342.59
348
346
/
276.90 292.24
280
280
205.20
204.79
/
271.19 278.65
224.64 251.79
204.83 237.38
190.19 224.36
220.08
272
226
208
/
270
258
234
208
196
196
The computed electronic values, such as absorption wavelength (λmax), the transition
energies, and oscillator strengths of electronic transitions for complex in acetonitrile are given
in Table S1 (in supplementary data). The theoretical absorption spectra of DDQ, of the donor
and of the complex are displayed in Fig 8. The primary strong band at 224.36 nm with the
greater oscillator strength value at 0.3518 are assigned to the predominant transitions:
HOMO-6 → LUMO+1
(59.15 %), HOMO-5 → LUMO+1 (26.50 %). The secondary
strong band appeared at 278.65 nm (f = 0.3436) is mainly caused by the following transitions:
HOMO -8 →LUMO (78.18 %) and HOMO -5 ->LUMO (8.65 %). The absorption at 292.24
nm has up to five configurations for electronic excitations: HOMO -6 → LUMO (29.90 %),
HOMO -5 → LUMO (51.85 %), HOMO -14 →LUMO (3.12%), HOMO -8 → LUMO
(4.72%), HOMO -7 → LUMO (2.68%).
In addition, the TD-DFT analysis has provided other maximum absorptions which have very
small oscillator strength, the corresponding wavelengths are as follow: 347.66 nm (f =
0.0021), 342.59 nm (f = 0.0022), 251.79 nm (f=0.0082), 237.38 nm (f= 0.0191) and 220.08
nm (f=0.0099). Their transition contributions are also reassembled in Table S1. Then, we
presented in Figure 9 some of the molecular orbitals and their corresponding energies based
on the geometries optimized with CAM-B3LYP/6-31++G (d,p) method. The strong value of
excitation energy was obtained at 224.36 nm. The strong value of Energy was obtained at

HOMO-6 and LUMO+1 at 224.36 nm: -9.55 eV and -1.50 eV and the calculated difference in
energy ∆E for complex was: -8.05 eV.
a)
b)
c)
Fig.8. TD-DFT calculated absorption spectra of donor (a), DDQ (b) and their CTC complex (c).

Fig.9. Theoretical electronic absorption transitions for complex in acetonitrile
.
4.5.2. Charge analysis Natural bond orbital (NBO):
The NBO method is a valuable approach for the investigation of molecular bonding
interactions, atomic charges, and to analyze the charge transfer process [44]. In the NBO
analysis of X-H…Y hydrogen bonded systems, the interaction between the lone pair (LP) (Y)
of the proton acceptor and σ* (X-H) anti-bond of proton donor is characterized by a
significant E(2) stabilization energy [45]. The results of the NBO charges for donor, for DDQ
and for their complex (in gas phase and in acetonitrile) were been performed by the B3LYP/6-
31++G (d, p) method.

As apparent in the table S2, the NBO results show that the complexation increases the
positive electronic charge of the H39, H38 and H32 atom and increases the negative
electronic charge of the O60 and N63 atoms involved in hydrogen bonds. In addition, the
NBO charge on the O61, N65, Cl66 and Cl67 atoms of complex are found to be more
negative than that obtained for the individual DDQ molecule, suggesting that a charge is
transferred from donor to DDQ. In addition, in the gas phase, the electronic charge NBO on
the S3 atom of the complex increases from 2.503 e to 2.512 e. The result is opposite in
acetonitrile where the charge of the S3 atom in the complex decreases by passing from 2.521e
to 2.515 e. A comparison between the calculated NBO charge in vacuum and in acetonitrile as
a solvent for DDQ complex was listed in Table S2. Other slight changes have also been
observed.
In Tables 5, 6 we present some of the significant donor–acceptor interactions and their
stabilization energies (E²) in vacuum and in solvent. As shown in Table 5, the greater
interaction is formed between LP(1)O7…BD*(2)C54-O60 (0.92 kcal/mol) and LP(3)O 7….
BD*(2)C54-O60 (1.47 kcal/mol). In case of DDQ and bis –L-aminoester sulfone, the NBO
analysis clearly shows the existence of two inter-molecular hydrogen bonds between lone
pairs : LP(1)O60, LP(2)O60 and LP(1)N63 and BD*(1)C14-H39, BD*(1)C14-H39 and
BD*(1)C12-H38 respectively.
The corresponding E² values are 0.63, 0.37 and 0.35 kcal/mol in vacuum and 0.69,
0.30 and 0.08 kcal/mol in solvent. Consequently NBO analysis confirms the charge transfer
nature between the donor molecule bis –L-aminoester sulfone and DDQ.
Table 5 : Donor–acceptor interactions and stabilization energies E² (kcal/mol) in vacuum.
Donor
Acceptor
E² (kcal/mol) CAM-B3LYP
6-31++G (d,p)
complex in vacuum
bis –L-aminoester sulfone Donor and DDQ Acceptor
BD(1)C12-H38
BD(1)C12-H38
LP(1)O7
LP(1)O7
LP(1)O7
LP(1)O7
LP(2)O7
LP(2)O7
LP(3)O7
BD*(2)C62-N63
BD*(3)C62-N63
BD*(1)C54-O60
BD*(2)C54-O60
BD*(2)C55-C57
BD*(2)C56-C58
BD*(2)C55-C57
BD*(2)C56-C58
BD*(2)C54-O60
BD*(2)C55-C57
BD*(2)C56-C58
0.05
0.08
0.06
0.92
0.09
0.09
0.08
0.23
1.47
0.13
0.22
LP(3)O7
LP(3)O7
DDQ Donor and bis –L-aminoester sulfone Acceptor
BD(2)C54-O60
BD(2)C55-C57
BD*(1)C14-H 39
BD*(1)S3-O7
0.07
0.05

BD(2)C62-N63
BD(3)C62-N63
LP(1)O60
LP(2)O60
LP(1)N63
BD*(1)C12-H38
BD*(1)C12-H 38
BD*(1)C14-H39
BD*(1)C14-H39
BD*(1)C12-H38
BD*(1)S3-O7
0.15
0.21
0.63
0.37
0.35
0.06
BD*(2)C54-O60
Table 6 : Donor–acceptor interactions and stabilization energies E² (kcal/mol) in acetonitrile.
Donor
Acceptor
E² (kcal/mol) CAM-B3LYP
6-31++G (d,p)
complex in CAN
bis –L-aminoester sulfone Donor and DDQ Acceptor
BD(1)C14-H39
LP(1)O7
LP(1)O7
LP(1)O7
LP(1)O7
LP(2)O7
LP(2)O7
LP(3)O7
LP(3)O7
LP(3) O7
BD*( 2)C54-O60
0.05
0.06
0.79
0.08
0.09
0.09
0.20
1.32
0.13
0.20
BD*( 1)C54-O60
BD*( 2)C54-O60
BD*( 2)C55-C57
BD*( 2)C56-C 58
BD*( 2)C55-C57
BD*( 2)C56-C58
BD*( 2)C54-O60
BD*( 2)C55-C57
BD*( 2)C56-C58
DDQ Donor and bis –L-aminoester sulfone Acceptor
BD(2)C54-O60
BD(2)C62-N63
BD(3)C62-N63
LP(1)O60
LP(2)O60
LP(1)N63
BD*(1)C14-H39
0.09
0.07
0.09
0.69
0.30
0.08
0.05
BD*(1)C12-H38
BD*(1)C12-H38
BD*(1)C14-H39
BD*(1)C14-H39
BD*(1)C12-H38
BD*(1)S3-O7
BD*(2)C54-O60
Conclusion:
In this study, we have synthesized a novel charge transfer complex CTC ((S, S)-bis-N,
N-sulfonyl bis –L-phenylalanine dimethylester-DDQ). Elemental analyses, electronic
absorption, FT- IR were used to characterize the donor, the acceptor and the new product.
The binding ratio of complexation was determined to be 1:1 according to the photometric
titrations. The obtained data show that the CT-complex has high values of KCT and ƐCT. The
confirmation of these results was also checked by theoretical calculations. The molecular
geometrical parameters, the electronic properties, the natural population analysis (NPA) and
Natural bond orbital (NBO) of the compounds have been calculated by using DFT (CAM-
B3LYP) method with 6-31++G(d,p) basis set. The spectral data in acetonitile as a solvent
were computed and compared to the experiment, indicating that TD-DFT/PCM calculations
provide good results. Consequently, theoretical and experimental results have indeed
confirmed the formation of charge transfer complex.

Acknowledgments:
This study was supported by the Algerian minister of higher education and scientific research through
the CNEPRU research grants.
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Highlights
The Highlights of research are as follows:
1- The new CTC complex formed between (S, S)-bis-N,N-sulfonyl bis-L-phenylalanine
dimethylester and DDQ was synthesized.
2- The molar ratio method and Benesi-Hildebrand methods were applied to the
determination of stoichiometric ratio and association constant (K).
3- The CTC was theoretically investigated using DFT:CAM/B3LYP and TD-DFT
methods.
4- The NBO methodology provides a qualitative picture of the charge-transfer process
between the acceptor and donor partners.