Experimental and theoretical study on cetylpyridinium dipicrylamide – A promising ion-exchanger for cetylpyridinium selective electrodes

Journal of Molecular Structure 1187 (2019) 77e85

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Experimental and theoretical study on cetylpyridinium dipicrylamide e A promising ion-exchanger for cetylpyridinium selective electrodes Maksym Fizer a, *, Oksana Fizer a, Vasyl Sidey b, Ruslan Mariychuk c, Yaroslav Studenyak a a

Faculty of Chemistry, Uzhhorod National University, Pidhirna Str. 46, Uzhhorod, 88000, Ukraine Research Institute for Physics and Chemistry of Solid State, Uzhhorod National University, Pidhirna Str. 46, Uzhhorod, 88000, Ukraine c Faculty of Humanity and Natural Sciences, University of Pre
sov in Pre
sov, 17th November 1, Pre
sov, 08116, Slovak Republic b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 August 2018 Received in revised form 20 February 2019 Accepted 18 March 2019 Available online 21 March 2019

The cetylpyridinium (CP) dipicrylamide (DPA) ion pair has been synthesized and characterized by the FTIR and NMR (1H and 13C) spectroscopy, theoretically studied and tested as ion-exchanger for cetylpyridinium selective electrodes. The molecular dynamics and further DFT optimization indicate face-to-face p-p stacking interaction between pyridinium and 2,4,6-trinitrophenyl rings. The presence of weak interactions between hydrogens of cetyl chain and oxygens of nitro groups of DPA anion was confirmed by RDG function analysis. Numeric reactivity descriptors computed at the B3LYP/6-31þG(d,p) level of theory show that CP-DPA has electrophilic character and can readily react with bases. Two fabricated electrodes with the CP-DPA ion-exchanger showed near-Nernstian responses towards CP chloride in the concentration range from 1  102 to 1  105 mol/L. In the case where dibutyl phthalate was used as plasticizer, the slope equals 57.2 ± 1.3 mV/decade, whereas for the case of dioctyl phthalate the slope is 61.6 ± 1.2 mV/decade. © 2019 Elsevier B.V. All rights reserved.

Keywords: Aromaticity DFT FT-IR Fukui function NMR Potentiometry

1. Introduction Cetylpyridinium (CP) is a quaternary ammonium cation that, in the form of chloride or bromide, is widely used in industrial and commercial detergent formulations, including cosmetics and pharmaceuticals. Moreover, CP salts are successfully used as catalysts in organic synthesis [1]. The usage of CP salts is mainly dictated by its antiseptic [2,3] and surfactant [4e6] properties. Undoubtfully, the development of fast, reliable and inexpensive methods for express determination of cetylpyridinium in the above-mentioned formulations is an actual and important task [7]. From this point of view, potentiometric techniques with the use of ion-selective electrodes (ISEs) seems to be an extremely promising approach. Thus, ISEs with polymeric membranes are widely used for express determination of ions [7,8]. ISEs are often used in clinical studies [9,10] and environmental monitoring [11]. As the main advantages of using ISEs, one can mention high selectivity and sensitivity, short analysis time, reliability and reproducibility. For construction of CP-sensitive ISEs, different ion-pair associates can be used as ion-exchange agents: 1,3-didecyl-2-methyl-

* Corresponding author. E-mail address: max.fi[email protected] (M. Fizer). https://doi.org/10.1016/j.molstruc.2019.03.067 0022-2860/© 2019 Elsevier B.V. All rights reserved.

imidazolium-tetraphenylborate [12], CP-phosphotungstate [13], CP-iodomercurate [14], CP-ferric thiocyanate [15], S-benzylthiuronium-tetraphenylborate [16], CP-Sn(IV)-phosphate [17], CP associated with tetraphenylborate/Reinecke's salt/phosphotungstate/phosphomolybdate/silicotungstate [18]. CP can be used as counter-ion in ion-pair associates which are used as ionophores in ISEs membranes sensitive to: aklylsulfates [19,20], cadmium ion [21], iodide [22], periodate [23], antimony [24]. Moreover, CP salts can be successfully used as a titrant in potentiometric titration techniques [24e26]. Taking into account the above-mentioned long-standing scientific interest in CP-selective membranes, in the present work we will discuss the structure of the ionic associate of CP with dipicrylamine (DPA). The selection of DPA for our investigation was primarily inspired by good results demonstrated by the use of the DPA counter-anion for determination of N-containing drug-like substances [27e30]. A considerable role of structural and electronic effects in the ionophore and ion association parameters [31e34], as well as the lack of experimental structural studies for CP-DPA complex, motivated us to investigate this ion-associate with spectroscopic methods (FT-IR, 1H NMR, 13C NMR) and with quantum chemical computations.

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programs were used for files’ type converting and input files preparation, respectively. For visualization we have used the Jmol [64] and VMD [65] software.

2. Experimental 2.1. Synthesis of CP-DPA associate Caution! DPA is corrosive and explosive, so must be used with special safety precautions. CPC is toxic if swallowed or inhaled, and it is irritating to skin, eyes, and respiratory system. DPA (0.878 g, 2 mmol) was dissolved in 50 mL of distilled water with NaOH (0.1 g, 2.5 mmol) to obtain dark-red solution (A). CP chloride monohydrate (0.715 g, 2 mmol) was dissolved in 50 mL to obtain clear colorless solution (B). Solutions (A) and (B) were mixed with immediate formation of dark red oil that solidifies upon standing. Dark red precipitate was filtered and washed with ethanol-water (1:9) mixture. The obtained solid was dried upon standing on air. The yield is 96% (1.420 g). The elemental analysis: found (%): C, 53.3; H, 5.8; N, 15.1; calculated for C33H42N8O12 (%): C, 53.4; H, 5.7; N, 15.1. 2.2. Instruments and measurements IR spectra were recorded on a Shimadzu IRPrestige-21 Fourier transform infrared spectrophotometer with ZnSe detector in ATR mode. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a Varian Mercury-400 instrument with the use of (CD3)2SO as deuterated solvent. TMS was used as internal standard. 2.3. Computational software and methods Starting geometries were created using the Avogadro software [35] and pre-optimized with the UFF force field [36]. Tinker 8.4 [37] was used for finding the preferred conformation of CP-DPA through the molecular dynamics simulation with OPLS-AA force field [38]. Parameters for correct modeling of the CP cation were chosen in accordance with [39]. Complete refinements of the geometry of the CP-DPA associate, CP cation, and DPA anion were performed using the PRIRODA 17 code [40] with the PBE functional [41,42] in conjunction with the 6-311þþG(d,p) basis set [43,44]. The computation of reduced density gradient (RDG) [45,46] for analysis of non-covalent interactions (NCI) was performed by using NCIPLOT 3.0 [47]. ORCA 4.0 [48] was used for computation of the electron density, Kohn-Sham orbitals, CHELPG [49], and Hirshfeld [50] partial charges with the B3LYP/6-31þG(d,p) method [51], as it was previously shown that reactivity descriptors computed at this level of theory are fair enough [52,53]. Solvent effect was included by means of the CPCM continuum solvation model [54]. Molecular electrostatic potential (MESP) surface was calculated in Multiwfn 3.3.9 and 3.5(dev) [55,56] analyzer. Natural population analysis (NPA) [57] was performed with the JANPA program [58]. Computations of NMR chemical shifts and NICS(0) aromaticity indexes [59] were performed by gauge-independent atomic orbital (GIAO) method [60], using the PBE functional in a combination with 6e311þþG(d,p), cc-pVTZ or L2 [61] basis sets. NMR chemical shifts were computed in PRIRODA 17 program for the gas-phase states and in ORCA 4.0 program with inclusion of solvation effects through the CPCM continuum model. The OpenBabel [62] and Gabedit [63]

3. Results and discussion 3.1. Structure of the 1:1 CP-DPA associate The ion-exchange reaction between CP chloride and DPA in basic water solution produces the precipitation of CP-DPA ion pair (Scheme 1) with the composition ratio of CP:DPA equal to 1:1. We have used 0.05 mol/L NaOH to neutralize DPA (strong N-H acid [66]) and to prevent possible formation of HCl. In theory, the main force that combines CP and DPA ions must be the electrostatic attraction between formally positively charged nitrogen of pyridinium ring and formally negatively charged amide nitrogen bonded with two 2,4,6-trinitrophenyl moieties, respectively. However, this model is too simplified and should be verified through the analysis of the electronic structure of CP-DPA. For correct modeling of CP-DPA's electronic and structural properties we have to find the optimal relative position of ions in the associate. We already faced with the same task [67] which was successfully solved using molecular dynamics simulation, and this is why we have used this approach for simulation of the CP-DPA structure for 10 ns with OPLS-AA force field. First 1 ns was considered as the relaxation period and next 9 ns were taken as the production period. The conformation with the lowest total energy was obtained at 7693 ps (see Supplementary materials, Fig. S1) and this was used as starting geometry for next DFT optimization at the PBE/6-311þþG(d,p) level of theory. The DFT optimized structure was used for further wavefunction calculation. Analysis of the corresponding Hessian indicates that the geometry obtained is a true local minimum, as there were no any imaginary frequencies. The full list of Cartesian coordinates of the CP-DPA optimized structure and the list of computed frequencies can be found in Tables S1 and S2, respectively (see Supplementary materials). Analysis of the molecular dynamics trajectory indicates that the average distance between the N1 nitrogen atom of the pyridinium ring and the central N61 amide atom of the DPA anion is about 3.8e4.7 Å with the maximum peak of the radial distribution function (RDF) at 4.2 Å (see Supplementary materials, Fig. 2S). A relatively long distance between two formally charged centers is due to steric hindrance of two bulky 2,4,6-trinitrophenyl fragments. The CP cation contains the long flexible alkyl chain which causes the existence of numeric conformations. The distance between the terminal methyl group C57 carbon atom and the whole DPA anion is about 3.4e6.5 Å with the RDF maximum at 4.0 Å. This clearly indicates that the cetyl chain is not linear but folded and this chain deformation is dictated by the dispersion attraction between the alkyl group and bulky DPA anion. Fig. 1 presents the PBE/6-311þG(d,p) optimized geometry of CPDPA. All the bonds distances have usual values; the earliermentioned N1-N61 distance is 4.77 Å. The distance (3.89 Å) between the centroids of pyridinium and 2,4,6-trinitrophenyl rings testifies the presence of the face-to-face p-p stacking interaction

Scheme 1. Synthesis of the CP-DPA associate.

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79

Fig. 1. General view of the DFT optimized geometry of the CP-DPA ion-pair.

between these moieties. According to the experimental data, the CN-C angle between two 2,4,6-trinitrophenyl rings in DPA anion can vary widely ( ): 135.30(17) [68], 141.9(3) [68], 141.2(3) [68], 153.0(2) [69]; and the C62-N61-C63 angle of 144.7 computed here is in well agreement with the above experimental data. 3.2. Non-covalent interaction in CP-DPA Analysis of non-covalent interactions in the CP-DPA associate has been performed with the RDG method (see Fig. 2). Green colored areas of the RDG isosurface indicate the predominant presence of weak interaction between pyridinium and 2,4,6trinitrophenyl rings (an additional prove of the p-p stacking) and weak interactions between the cetyl chain and nitro group. Red areas in the rings indicate strong repulsion effect. Bright blue area between ortho-hydrogen H10 of pyridinium ring and O92 atom of a nitro group indicates attraction between these two atoms (the H10O92 distance is 2.22 Å). The same attraction exists between H14/ H15 a-hydrogens of the cetyl chain and O92/O94 atoms of two different nitro groups. The corresponding H14-O92 and H15-O94 distances are 2.33 Å and 2.71 Å, respectively. 3.3. Electrostatic interactions between CP and DPA ions in the associate To understand the above-mentioned interaction between the hydrogens of CP and the oxygens of DPA, we suggested that the attraction is electrostatic in nature; and, in order to verify this suggestion, we have analyzed the electrostatic potential (ESP) of the two separate ions calculated with the B3LYP/6-31þG(d,p) method. Generating and analysis of the ESP isosurface calculated from the CHELPG atomic charges were performed using the Multiwfn program. Fig. 3 shows the color coded ESP over the CP and DPA ions. As expected, the ESP maxima are distributed over the pyridinium ring. To be more precise, the two highest values (þ111 kcal/mol) are located near the H10, H11 ortho-hydrogens and H14, H15 a-hydrogens of the cetyl chain. The ESP minima in the DPA anion are located near the oxygens of nitro groups. The minima values are relatively close, from 78 to 73 kcal/mol, and the absolute minimum is located near the nitro group in ortho-position. In the work [69], it was suggested that in the DPA anion the negative

Fig. 2. Non covalent interactions analyzed through the RGD method. (a) The RDG isosurface computed for CP-DPA [the CP cation and the DPA anion are shown in brown and grey, respectively]; (b) the RDG scatter plot color-graded in accordance with the interaction type: strong attraction (blue), weak interaction (green), and strong repulsion (red).

charge is rather localized on the oxygen atoms of the nitro groups in para positions; however, here we can see that negative ESP is delocalized over the all nitro groups more or less equally. Thus, in the separate DPA anion, the CHELPG partial charges of central N61 nitrogen and all oxygens are from 0.48e to 0.46e (for the full list see Supplementary materials, Table S3). Also, we have calculated the NPA atomic charges which are better for description of electron redistribution in bulky molecules and better predict the reactivity [52,69] and physical property such as logPo/w [70]. The NPA charge of N61 atom in the free DPA anion is 0.41e and the charges of the oxygens are in the range from 0.41e to 0.37e. These two population analyses clearly indicate that anionic formal negative charge is delocalized over the central nitrogen and the oxygens of nitro groups; corresponding resonance structures are shown on Scheme 2. The charge delocalization in the separate CP cation is not as tremendous as in the DPA anion. The sum of the NPA atomic partial charges of pyridinium ring gives only þ0.64e, whereas adding of the a-methylene (b-methylene, g-methylene) group partial charges results in the sum of þ0.920e (þ0.940e, þ0.956e). This obviously testifies that positive charge is delocalized not only over the ring

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Fig. 3. Electrostatic potential isosurface over the CP cation (a), the DPA anion (b) and the CP-DPA associate (c) with the values of ESP in the isosurface extrema in kcal/mol.

Scheme 2. Resonance structures of the DPA anion, as proposed on the base of NPA.

but also over the neighboring methylene groups of the cetyl chain. The farther the methylene group, the lower its impact. The charge-transferring effect, when present, plays an important role in the molecular complexes [71e73]. To study this in CPDPA, we considered the change of the NPA atomic charges of separate ions when isolated and when associated. This approach can clearly show how the electrons are transferred between two ions due to the interaction. The sum of partial charges of the CP cation in the associate is þ0.961e, and the marginal difference from unity clearly denies charge transferring between the CP and DPA ions. As an alternative for the study of the charge-transferring effect, we have considered the electron density difference plots between the ground and excited states of CP-DPA in different media. We have adopted the known theoretical procedure [74] employing the

Fig. 4. FT-IR spectrum (orange) and calculated IR spectrum (blue) of the CP-DPA complex.

6-31þG(d) basis set in combination with a range-separated DFT functional, but in our case the newer and improved uB97X functional was used [75]. As with the analysis of NPA charges, the conclusion made from the electron density difference plots is definite e there is charge transfer effect between the pyridinium ring and a-methylene group of CP and o-nitro group of DPA ion (see Fig. S3 in Supplementary materials). 3.4. FT-IR studies In the CP-DPA IR spectrum (Fig. 4), sharp weak peak(s) at 3086 cmel (the computed modes 278, 277, 276 and 275 with the frequencies of 3162, 3160, 3156 and 3156 cmel, respectively) were assigned to CeH stretching in trinitrophenyl rings of the DPAanion. The asymmetric and symmetric stretching vibration of CeCH2 from the cetyl chain appears as two medium bands at 2913 and 2849 cmel, respectively. The calculated modes 265 and 264, with the frequencies of 3002 and 2997 cmel, respectively, are the most intensive from those corresponding to the asymmetric stretching vibration of the cetyl chain. The most intensive symmetric CeH stretching in the cetyl chain, in the calculated IR spectrum, is presented by the modes 253 and 251 with the scaled frequencies of 2958 and 2951 cmel, respectively. A medium broad peak at 1693 cmel corresponds to the asymmetric stretching CeN-C vibration of the diphenylamide anion. We have to note the presence of a shoulder at 1743 cmel that also corresponds to the same stretching [69]. The computed mode 237 corresponds to this C-N-C vibration with the frequency of 1726 cmel. A medium sharp signal at 1589 cmel is caused by the CC(NO2)(ortho) stretching ring deformation (the computed mode 234 with the frequency of 1592 cmel). A strong sharp peak at 1577 cmel is caused by the C-C(NO2)(para) bend, and the related computed modes are 231 and 232 with frequencies of 1552 and 1562 cmel, respectively. Asymmetric O-N-O bends appear as strong sharp signal at

M. Fizer et al. / Journal of Molecular Structure 1187 (2019) 77e85

1563 cmel and as medium sharp signal at 1487 cmel. The most intensive computed modes directly corresponding to these vibrations are 224 (1470 cm1) and 229 (1501 cm1). A weak sharp signal at 1429 cmel corresponds to the CeC stretching ring deformation in the DPA anion, the relevant computed modes being 214 and 208 with the frequencies of 1437 and 1423 cm1, respectively. The symmetric O-N-O vibrations appear at 1317 cmel, and the most intensive corresponding computed modes are 180 and 175 with the respective frequencies of 1280 and 1269 cm1. The C-N in-plane bend in the phenyl rings of the DPA anion appears at 1257 cmel, and the computed frequency is 1262 cm1 (mode 173). A strong peak at 1238 cm1 corresponds to the CeNO2(ortho) stretching, and corresponding modes 168 and 170 have the frequencies of 1241 and 1248 cm1, respectively. 3.5. NMR studies and aromaticity The NMR spectra of the CP-DPA associate were recorded in (CD3)2SO and, for comparison, we have recorded the NMR spectra of starting CP chloride and DPA. Fig. S4 presents the 1H NMR and 13C NMR spectra of CP-DPA (see Supplementary materials). 3.5.1. 1H NMR The four protons, H70, H71, H80, and H81, of the DPA anion produce an intensive singlet at 8.76 ppm, and this chemical shift is the same as in the neutral DPA molecule (see Table 1). As expected, the NH proton of the DPA molecule (10.56 ppm) is absent in the associate spectra. Signals of the pyridinium ring protons H7, H8/H9, and H10/H11 appear at 8.59 ppm (triplet), 8.15 ppm (triplet), and 9.07 ppm (doublet), respectively. A signal of the a-methylene group appears as a triplet at 4.58 ppm. Then, in the high field, the signal of the b-methylene group protons appears at 1.90 ppm as a broad multiplet. The signal of the terminal methyl group of the cetyl chain appears at 0.84 ppm, whereas the signals of other protons of the chain overlap and form the broad intensive peak at 1.22 ppm. The largest difference between the associate's and separate ions' spectra is observed for the ortho-hydrogens (0.20 ppm) and amethylene hydrogens of the cetyl chain (0.10 ppm). As shown above, these atoms are in close contact to the DPA anion; so we can assume that this small change in chemical shifts is due to interaction with DPA anion that causes shielding and upfield shifts. Moreover, these observable differences clearly testify that CP-DPA is present in a (CD3)2SO solution in an associated form rather than as separate ions. This conclusion is in agreement with the literature data reported for similar ionic associates of lipophilic anions such as tetraphenylborate and its fluorinated analog [76]. Comparing the calculated data with the experimental values, we

81

have to notice that inclusion of solvation effect in DFT calculations is mandatory. At first, we have studied the NMR chemical shifts of CPDPA without the solvents consideration, and the PBE/L2 method showed more correct results (R2 ¼ 0.997) as compared with PBE/6311þþG(d,p) (R2 ¼ 0.996) or PBE/cc-pVTZ (R2 ¼ 0.996). In the case of the L2 basis set, the linear equation that describes the correlation between the experimental and calculated chemical shifts can be written as d(exp.) ¼ 1.018  d(calc.) e 0.78. Despite this, the results obtained with inclusion of the CPCM model are more accurate. However, as the L2 basis set is not implemented in the ORCA 4.0 program and neither explicit solvation model is implemented in PRIRODA 17, we had to exclude the L2 basis set from modeling with the CPCM model. The accuracy of the CPCM-PBE/6-311þþG(d,p) and CPCM-PBE/cc-pVTZ methods in modeling of CP-DPA is very similar, with the correlation coefficients R2 of 0.999 and 0.998, respectively. However, taking into account that in the linear equation d(exp.) ¼ slope  d(calc.) þ intercept the slope is closer to unity and the intercept is closer to zero in the case of the cc-pVTZ basis set, we can conclude that the CPCM-PBE/cc-pVTZ method is better for GIAO modeling of chemical shifts of CP-DPA. We had chosen the H7 triplet at 8.59 ppm in CP-DPA and at 8.63 ppm in CP cation as the reference signals for computed chemical shifts in the case of CPDPA and separated ions, respectively. It must be noted that the experimentally observable upfield shifts in the NMR spectra of the CP-DPA associate and separate ions is not reproducible by DFT computations. The suspicion that the disagreement is due to the use of GGA functional forced us to try a hybrid functional. It was shown that good correlation between DFT-computed NMR shifts and experimental values can be achieved at the B3LYP/aug-ccpVDZ level of theory [77]. Also, quite good correlation was reported for much cheaper B3LYP/6-31G(d,p) method. For the above reason, we have additional computed NMR shifts in DMSO with these two methods by using ORCA 4.1 package. However, despite the superior correlation in the case of 6-31G(d,p) basis set, the experimentally observable upfield shifts are still not reproduced by DFT. 3.5.2. 13C NMR The assignment of the signals of the 13C NMR spectra is presented in Table 2. The para-carbon C6 of the pyridinium ring (145.43 ppm) was chosen as the reference signal for computed chemical shifts. We have to note that the difference in chemical shifts of the CP-DPA associate and the CP cation (in the form of chloride) is only marginal (less than 0.07 ppm) for majority of carbons. However, the signals of the C2/C3, C12, and C13 carbons differ from the signals of the CP cation by 0.16, 0.28, and 0.14 ppm, respectively. The largest difference for C12 carbon can be explained

Table 1 1 H NMR spectra of CP-DPA ion-pair (IP) and corresponding chemical shifts in CP chloride and DPA separate reagents (SR). GIAO chemical shifts computed in DMSO and in gas phase at different levels of theory: 1 e PBE/6-311þþG(d,p); 2 e PBE/cc-pVTZ; 3 e B3LYP/6-31G(d,p); 4 e B3LYP/aug-cc-pVDZ; 5 e PBE/L2. All numerical values are in ppm. Phase

DMSO

Hydrogens in IA

Exp.

H7 H8, H9 H10, H11 H14, H15 H16, H18 H19eH56 H58, H59, H60 H70, H71, H80, H81 R2 Slope Intercept

Gas 1

2

3

4

1

5

2

IP

SR

IP

SR

IP

SR

IP

SR

IP

SR

IP

SR

IP

SR

IP

SR

8.59 8.15 9.07 4.58 1.90 1.22 0.84 8.76

8.63 8.18 9.27 4.68 1.90 1.21 0.83 8.76

8.59 8.26 9.34 4.85 2.28 1.59 1.15 9.19 0.999 1.017 0.37

8.63 8.23 8.66 4.51 2.05 1.35 0.96 9.21 0.993 1.018 0.11

8.59 8.29 9.33 4.83 2.18 1.53 1.09 9.30 0.998 1.003 0.27

8.63 8.25 8.66 4.56 2.02 1.34 0.92 9.31 0.993 1.010 0.08

8.59 8.19 9.33 4.69 2.17 1.50 1.08 9.14 0.999 1.009 0.25

8.63 8.18 8.69 4.48 1.98 1.33 0.93 9.14 0.995 1.017 0.08

8.59 8.13 9.29 4.71 1.91 1.30 1.00 9.60 0.995 0.970 0.01

8.63 8.03 8.60 4.36 1.69 1.06 0.78 9.64 0.987 0.968 0.25

8.59 8.41 9.68 5.29 2.60 1.95 1.53 9.49 0.996 1.038 0.78

8.63 8.23 8.45 4.44 2.12 1.45 1.05 9.19 0.990 1.036 0.21

8.59 8.40 9.58 5.18 2.40 1.77 1.34 9.43 0.997 1.018 0.55

8.63 8.22 8.42 4.41 1.98 1.29 0.87 9.10 0.991 1.019 0.03

8.59 8.45 9.70 5.29 2.50 1.87 1.40 9.45 0.996 1.023 0.65

8.63 8.22 8.45 4.48 2.06 1.39 0.96 9.17 0.991 1.028 0.14

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Table 2 13 C NMR spectra of CP-DPA ion-pair (IP) and corresponding chemical shifts in CP chloride and DPA separate reagents (SR). GIAO chemical shifts computed in DMSO and in gas phase at different levels of theory: 1 e PBE/6-311þþG(d,p); 2 e PBE/cc-pVTZ; 3 e B3LYP/6-31G(d,p); 4 e B3LYP/aug-cc-pVDZ; 5 e PBE/L2. All numerical values are in ppm. Phase

DMSO

Carbons in CP-DPA

Exp.

C2, C3 C4, C5 C6 C12 C13 C17 C21 C23 C27 C29 C33 C35 C39 C41 C45 C47 C51 C53 C57 C62, C63 C64, C65, C74, C75 C66, C67, C76, C77 C68, C78 R2 Slope Intercept

Gas 1

2

3

4

1

5

2

IP

SR

IP

SR

IP

SR

IP

SR

IP

SR

IP

SR

IP

SR

IP

SR

144.69 128.06 145.43 60.78 30.68 25.38 28.68 29.02 29.02 29.02 29.02 29.02 28.98 28.88 28.75 28.35 31.27 22.06 13.90 141.93 138.40 124.09 131.68

144.85 128.03 145.44 60.50 30.82 25.38 28.69 29.04 29.04 29.04 29.04 29.04 29.00 28.93 28.81 28.42 31.27 22.07 13.89 141.78 138.39 124.17 131.82

144.93 129.87 145.43 66.28 32.42 22.09 28.77 22.88 22.69 29.76 27.25 31.41 33.86 36.28 34.04 35.07 35.99 26.13 12.26 140.38 139.04 127.68 135.89 0.995 0.995 1.14

144.64 130.59 145.44 66.59 38.81 30.72 33.78 34.17 34.38 34.49 34.49 34.49 34.47 34.43 34.42 34.33 36.11 26.14 12.58 140.48 140.00 127.92 135.69 0.999 1.032 6.07

144.93 129.94 145.43 66.83 32.59 22.52 28.96 23.27 23.08 29.92 27.72 31.70 34.25 36.68 34.39 35.42 36.32 26.49 12.77 141.11 139.26 127.28 135.66 0.995 0.998 1.57

144.25 130.38 145.44 66.97 38.78 31.00 33.80 34.25 34.41 34.50 34.52 34.51 34.50 34.49 34.49 34.49 36.19 26.24 12.61 140.74 139.67 126.76 134.92 0.999 1.036 6.27

144.98 128.06 145.43 68.63 36.30 27.73 33.69 28.55 28.47 34.36 32.52 36.00 38.07 40.37 38.15 38.94 39.52 30.96 20.16 143.47 140.55 130.03 135.02 0.996 1.037 7.39

144.62 128.41 145.44 68.85 41.79 34.97 37.77 38.09 38.22 38.28 38.28 38.29 38.24 38.29 38.21 38.25 39.43 30.94 20.01 143.17 141.00 129.76 134.40 0.999 1.071 11.73

146.73 126.89 145.43 65.18 33.28 24.02 30.19 26.60 25.17 31.57 28.33 32.62 34.23 36.26 35.36 36.99 38.28 28.07 16.71 144.36 138.55 128.91 134.01 0.996 1.010 3.31

147.19 128.56 145.44 66.33 39.96 33.42 36.01 37.60 37.54 37.77 37.72 37.52 37.57 37.18 36.85 36.77 38.21 29.65 15.14 145.77 140.97 130.34 135.74 0.998 1.045 9.23

148.04 131.07 145.43 70.30 35.98 26.67 32.90 26.76 26.66 33.89 31.24 36.21 37.74 41.21 37.76 39.54 40.59 30.21 16.79 143.00 140.96 127.84 136.06 0.994 1.021 6.19

142.62 131.03 145.44 69.15 43.39 33.22 35.71 35.72 35.95 36.05 36.18 36.19 36.25 36.21 36.26 36.21 38.14 28.16 14.42 140.69 139.08 125.14 132.98 0.998 1.061 9.20

146.63 130.52 145.43 69.11 33.23 23.67 29.65 23.25 22.98 30.43 27.90 32.62 34.74 37.90 34.59 35.82 36.77 26.96 12.30 140.78 140.90 126.19 136.65 0.994 0.996 2.00

141.35 130.41 145.44 67.99 40.63 30.51 32.49 32.41 32.46 32.50 32.56 32.56 32.59 32.60 32.63 32.64 34.24 24.66 9.55 137.95 138.52 122.72 133.41 0.997 1.035 4.65

147.69 131.12 145.43 70.20 35.18 26.15 32.01 26.14 26.04 32.87 30.67 35.38 37.01 40.35 36.94 38.40 39.35 29.27 16.50 143.43 141.19 127.51 135.91 0.995 1.014 5.20

144.93 129.87 145.43 66.28 32.42 22.09 28.77 22.88 22.69 29.76 27.25 31.41 33.86 36.28 34.04 35.07 35.99 26.13 12.26 140.38 139.04 127.68 135.89 0.998 1.054 7.91

by the attraction of the neighboring H14/H15 hydrogens to the DPA anion. Moreover, a slight difference in the signals of the DPA anion and molecule can be observed: chemical shifts of the C62/C63 and C68/C78 atoms differ by 0.15 and 0.14 ppm, respectively, from the signals in the DPA molecule. This can be explained by deprotonation of the DPA molecule in the CP-DPA associate. Chemical shifts computed with the proposed DFT methods well correlate with the experimental values. In the case of PBE/6311þþG(d,p), the linear equation d(exp.) ¼ 1.021  d(calc.) e 6.19 can be used for predicting the experimental 13C NMR shifts, with the coefficient of determination R2 ¼ 0.994. The liner equation d(exp.) ¼ 0.996  d(calc.) e 2.00 can be used in the case of PBE/L2, with R2 ¼ 0.994. In the case of cc-pVTZ basis set, the correlation (R2 ¼ 0.995) between the experimental and calculated CP-DPA chemical shifts can be written as d(exp.) ¼ 1.014  d(calc.) e 5.20. Despite the fact that R2 is marginally better in the case of the ccpVTZ basis set, the coefficients in the linear equation are much closer to ideal values for the L2 basis set. Taking this fact into consideration, we recommend the use of the PBE/L2 level of theory for the gas-phase computation of the NMR spectra in similar associates. In the case of B3LYP/6-31G(d,p) computations, the correlation is highest: R2 ¼ 0.996 for CP-DPA and R2 ¼ 0.999 for separate ions; however, intercepts are too far from zero: 7.39 and 11.73, respectively. Moreover, the use of solvation model changes the picture only marginally. Thus, for the 6-311þþG(d,p) and cc-pVTZ basis sets the correlations are very similar (R2 ¼ 0.995 in the case of CP-DPA) and the coefficients are not much closer to the ideal values in comparison to the gas-phase computations. These observations can be explained by lower solvent's effect sensitivity of the 13C chemical shifts in comparison to the 1H chemical shifts.

3.5.3. Aromaticity Aromaticity is one of the basic concepts in the chemistry of planar rings with conjugated p-system. The CP-DPA associate

contains three aromatic rings: the pyridinium ring and two 2,4,6trinitrophenyl rings. The NICS(0) indexes are negative for aromatic rings and are positive for anti-aromatic rings. As shown above, the PBE/L2 combination is better for computation of chemical shifts in gas phase; so we have used this method for calculation of the NICS(0) values in gas phase. For the pyridinium ring in CP-DPA, the NICS(0) value is 7.98 ppm, and this is close to the aromaticity of benzene (8.08 ppm) at the same level of theory. The aromaticity of the free CP cation is slightly lower, with NICS(0) ¼ 7.72 ppm. In the cases of C62, C64-C68 and C63, C74C78 rings, the aromaticity indexes are 6.18 ppm and 6.74 ppm, respectively. In turn, the NICS(0) values for the aforementioned 2,4,6-trinitrophenyl rings are 5.84 ppm and 5.88 ppm, respectively. Considering aromaticity as a stability descriptor, we can assume that formation of the CP-DPA associate is preferable, since the aromaticity is generally increased. Very similar behavior of NICS(0) values can be observed in the case of the use of CPCM solvation model (see Table S4 in Supplementary materials).

3.6. Ion-selective electrodes PVC ion selective membrane matrix was fabricated by mixing of 2.5 mg CP-DPA, 150.0 mg dibutyl phthalate (or dioctyl phthalate), 100.5 mg PVC þ 2.0 mL THF. After homogenizing and dissolution of all components, the resulting clear dark-red solution was transferred to a glass well (d ¼ 3 cm), covered with filter paper and left for 48 h to full evaporation of the solvent. The disks with a diameter of 0.5e1 cm were cut off and glued to PVC tubes of corresponding diameters. The PVC electrodes were filled with the 1 mol/L NaCl solution and few drops of 102 mol/L of CP chloride. Sensors calibration was carried out by measuring the potential of solutions having CP chloride concentration from 107 to 102 mol/L (Fig. 5). The fabricated electrodes show near-Nernstian responses towards CP chloride in the concentration range from

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83

Table 3 Ionization energy (I) and electron affinity(A) of CP-DPA and global reactivity descriptors: electronic chemical potential (m), absolute hardness (h), absolute softness (S), global electrophilicity (u), electron acceptor (uþ), electron donator (ue) and net electrophilicity (Du±) computed in gas phase, water and PVC membrane at B3LYP/631þG(d,p) level of theory. All values are in eV.

I A

h m S Fig. 5. Calibration of two PVC ion selective electrodes with dioctyl phthalate (DOP) and dibutyl phthalate (DBP).

1  102 to 1  105 mol/L with the slopes of 57.2 ± 1.3 mV/decade (dibutyl phthalate) and 61.6 ± 1.2 mV/decade (dioctyl phthalate). The lower limit of detection (LOD) was calculated as cross-section of both extrapolated linear portions of the calibration curve of the electrodes, and it equals 5.7  106 M for DBP-electrode and 8.0  106 M for DOP-electrode. For both electrodes, the response time is about 30 s in the case of 105e102 M solutions, whereas in the case of the concentration range of 107e105 M the response time is about 60 s. During 3-week period, the electrodes showed Nernstian slopes, which changed only by about ±2 mV/decade; however, at the end of this period, the detection limit changed to 103 M of CP chloride. We explain a relatively short lifetime of the electrodes by possible leaching of the CP-DPA associate and by the relatively high reactivity of this ion pair, which is discussed in the next section. Potentiometric selectivity coefficients for a few common interfering ions are presented in Table S5 (see Supplementary materials). 3.7. CP-DPA's reactivity descriptors and stability In our previous works, we have used computational approaches for prediction and explanation of regio-selectivity [78e80], reactivity [81,82] and photo-stability [83]. To understand the reactivity and thus stability of CP-DPA, we have calculated numeric reactivity descriptors in terms of the conceptual DFT theory [84e91]. Thus, for computation of electronic chemical potential (m), absolute hardness (h), absolute softness (S), global electrophilicity (u), electron acceptor (uþ), electron donator (ue) and net electrophilicity (Du±), we have used ionization energy (I) and electron affinity(A), according to equations (1)e(7):

h ¼ ðI  AÞ=2

(1)

m ¼ ðI þ AÞ=2

(2)

S ¼ 1=2h

(3)

u ¼ m2 =2h

(4)

uþ ¼ ðI þ 3AÞ2 =16ðI  AÞ

(5)

u ¼ ð3I þ AÞ2 =16ðI  AÞ

(6)

Du± ¼ uþ þ u :

(7)

To understand the influence of an outer medium, we have performed calculations in gas phase, water and PVC-membrane. The CPCM continuum approach was used. It was shown that in the range 10e100 Hz most PVC-dibutyl(or dioctyl) phthalate

u uþ ue Du±

Gas

Water

Membrane

7.68 2.47 2.60 5.07 0.38 4.95 2.74 7.81 10.55

6.35 3.52 1.42 4.93 0.71 8.60 6.31 11.24 17.55

6.47 3.39 1.54 4.93 0.65 7.88 5.60 10.54 16.14

mixtures’ dielectric constants are about 8e13 [92], and this is why we have modeled PVC-membrane medium as continuum with ε equal to 10. Considering absolute hardness h as a characteristic of the resistance of a molecule to changes (stability descriptor), we have to note that stability of CP-DPA decreases with an increase of polarity of outer medium e the lowest stability is predicted for water solution (Table 3). High values of jmj and u are common for molecules of electrophiles (which readily react with bases). Hence, as jmj and u are relatively high, the CP-DPA can readily react with bases, and its electrophilic character increases considerably in water: thus, alkali solutions can decompose the ion-selective agent and decrease the analytical characteristics of an electrode. The uþ and ue descriptors measure the propensity of a studied system to accept and donate electrons, respectively. By means of these two descriptors, the electron-accepting/donating powers (reactivity) increases from gas phase to membrane and then to water solution. Net electrophilicity Du± is the electrophilicity of a system relative to its own nucleophilicity, and again this indicates that the electron-accepting power of CP-DPA is highest in water solution and lowest in gas phase; but anyway CP-DPA shows electrophilic properties and thus must be sensitive to bases. The condensed Fukui function (CFF) [93] was used for finding the most reactive site in the CP-DPA molecule. The Fukui function indexes were calculated as the difference between the Hirshfeld atomic partial charges in different states (neutral, cation or anion). The full list of the indexes can be found in Tables S5eS7 (see Supplementary materials). Electrophilic attack is most probable on the N61 atom of the DPA anion. The CFF indexes for N61 are 0.0873, 0.0855 and 0.0857 in gas phase, water and membrane, respectively. Different electrophilicity descriptors show that the CP-DPA molecule is quite electrophilic and thus can readily react with nucleophiles/bases. In gas phase, the CFFþ(C6) ¼ 0.0733 is the highest value of the Fukui function; it indicates that the C6 atom of the pirydinium ring is the most reactive under the action of bases. However, the reactivity is different in membrane and, even more, in water. Thus, in water solution, the CFFþ indexes are high for atoms of nitro groups, about 0.02e0.07 for oxygens and about 0.01e0.04 for nitrogens. Considering that the oxygens are negatively charged (see Table S3 in Supplementary materials), the nitrogens in nitro groups must be considered as main reactive sites in reaction with bases. Obviously, high CFFþ values of the oxygens represent not the direct attraction to a base, but high electronic response of oxygens when neighboring nitrogen forms a bond. In the case of radical attack, the oxygens of nitro groups and the N61 atom are most reactive centers, and this trend is maintained for the gas phase, PVC-membrane, and water solution.

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4. Conclusions An ion-pair of CP with DPA is synthesized, and its structure is determined by the FT-IR and NMR techniques. Under the basic reaction conditions, the DPA molecule and CP chloride form the ion pair, which is insoluble in water. Analysis of non-covalent interactions testifies the presence of very weak hydrogen bonding between the CP's alkyl chain and the oxygen atoms of the DPA anion. The pyridinium ring coordinates near the 2,4,6trinitrophenyl ring, which is due to the presence of the face-toface p-p stacking interaction between these moieties. A medium broad band at 1693 cm-l in the FT-IR spectrum of the crystalline CPDPA associate is assigned to the asymmetric stretching CeNeC vibration of the dipicrylamide anion. The presence of a shoulder at 1743 cm-l corresponds to the same stretching. The NMR studies explain the predicted weak interactions between the CP and DPA ions through the analysis of small variation in chemical shifts of the associate and free molecules. The NICS(0) indexes show that aromaticity is generally increased during the association reaction. The pyridinium ring in the CP cation is more aromatic than the DPA rings. The fabricated ion-selective electrodes with CP-DPAcontaining PVC membrane show good analytical characteristics. The calculated reactivity descriptors indicate that CP-DPA is electrophilic compound and thus not stable in basic medium. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2019.03.067. References [1] O. Paley, Cetyltpyridinium chloride, Synlett 25 (2014) 599e600. [2] P.K. Sreenivasan, V.I. Haraszthy, J.J. Zambon, Antimicrobial efficacy of 0.05% cetylpyridinium chloride mouthrinses, Lett. Appl. Microbiol. 56 (2012) 14e20. [3] J. Latimer, J.L. Munday, K.M. Buzza, S. Forbes, P.K. Sreenivasan, A.J. McBain, Antibacterial and anti-biofilm activity of mouthrinses containing cetylpyridinium chloride and sodium fluoride, BMC Microbiol. 15 (2015) 169. [4] P.K. Khatua, S. Ghosh, S.K. Ghosh, S.C. Bhattacharya, Characterization of binary surfactant mixtures (cetylpyridinium chloride and Tween 60) in an aqueous medium, J. Dispersion Sci. Technol. 25 (2004) 741e748. [5] V. Karikalan, A. Panneerselvam, K. Vallalperuman, Physico-chemical analysis on cetylpyridinium chloride (CPC) with alcohol solution at different temperatures e ultrasonic, UV and FTIR analysis, Digest J. Nanomater. Biostruct. 13 (2018) 115e128. [6] L.R. Harutyunyan, R.S. Harutyunyan, Micellar parameters of cationic surfactant cetylpyridinium bromide in aqueous solutions of amino acids at different temperatures: conductometric, surface tension, volumetric and viscosity study, Tenside Surfactants Deterg. 54 (2017) 141e159. [7] K.N. Mikhelson, Ion-selective Electrodes, Springer, Berlin, 2013. [8] G. Dimeski, T. Badrick, A. St John, Ion selective electrodes (ISEs) and interferences e a review, Clin. Chim. Acta 411 (2010) 309e317. [9] V.K. Thakur, M.K. Thakur, Handbook of Polymers for Pharmaceutical Technologies, Processing and Applications, vol. 2, Scrivener Publishing LLC, USA, 2015. [10] R. Yan, Sh Qiu, L. Tong, Y. Qian, Review of progresses on clinical applications of ion selective electrodes for electrolytic ion tests: from conventional ISEs to graphene-based ISEs, Chem. Speciat. Bioavailab. 28 (2016) 72e77. [11] A. Radu, T. Radu, C. McGraw, P. Dillingham, S. Anastasova-Ivanova, D. Diamond, Ion selective electrodes in environmental analysis, J. Serb. Chem. Soc. 78 (2013) 1729e1761. [12] D. Madunic-Cacic, M. Sak-Bosnar, O. Galovic, N. Sakac, R. Matesic-Puac, Determination of cationic surfactants in pharmaceutical disinfectants using a new sensitive potentiometric sensor, Talanta 76 (2008) 259e264. [13] A.F. Shoukry, S.S. Badawy, R.A. Farghali, Hexadecylpyridinium-phosphotungstate ion association in construction of a hexadecylpyridinium cation selective electrode, Anal. Chem. 60 (1988) 2399e2402. [14] M.N. Abbas, G.A.E. Mostafa, A.M.A. Homoda, Cetylpyridiniumeiodomercurate PVC membrane ion selective electrode for the determination of cetylpyridinium cation in Ezafluor mouth wash and as a detector for some potentiometric titrations, Talanta 53 (2000) 425e432. [15] G.A.E. Mostafa, PVC matrix membrane sensor for potentiometric determination of cetylpyridinium chloride, Anal. Sci. 17 (2001) 1043e1047. [16] G.A.E. Mostafa, , s-Benzylthiuronium PVC matrix membrane sensor for potentiometric determination of cationic surfactants in some pharmaceutical

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