Structural Characterization of Co-Crystals of Chlordiazepoxide with p-Aminobenzoic Acid and Lorazepam with Nicotinamide by DSC, X-ray Diffraction, FTIR and Raman Spectroscopy

The low water solubility of benzodiazepines seriously affects their bioavailability and, in consequence, their biological activity. Since co-crystallization has been found to be a promising way to modify undesirable properties in active pharmaceutical ingredients, the objective of this study was to prepare co-crystals of two benzodiazepines, chlordiazepoxide and lorazepam. Using different co-crystallization procedures, slurry evaporation and liquid-assisted grinding, co-crystals of chlordiazepoxide with p-aminobenzoic acid and lorazepam with nicotinamide were prepared for the first time. Confirmation that co-crystals were obtained was achieved through a comparison of the data acquired for both co-crystals using differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), Fourier-transform infrared (FTIR) and Raman spectroscopy, with comparisons acquired for the physical mixtures of both benzodiazepines and coformers. The compatibility of PXRD patterns of both benzodiazepines co-crystals with those contained in the base Powder Diffraction File (PDF-4+) suggests that new crystal structures were indeed created under the co-crystallization procedure. Single-crystal X-ray diffraction revealed that a chlordiazepoxide co-crystal with p-aminobenzoic acid and a lorazepam co-crystal with nicotinamide crystallized in the monoclinic P21/n and P21/c space group, respectively, with one molecule of benzodiazepine and one of coformer in the asymmetric unit. FTIR and Raman spectroscopy corroborated that benzodiazepine and coformer are linked by a hydrogen bond without proton exchange. Furthermore, a DSC study revealed that single endothermic DSC peaks assigned to the melting of co-crystals differ slightly depending on the co-crystallization procedures and solvent used, as well as differing from those of starting components.


Introduction
The co-crystallization of active pharmaceutical ingredients (APIs) with coformers offers a unique way to improve key physicochemical properties without changing the chemical nature and pharmacological activity of APIs [1,2]. Solubility, dissolution rate, bioavailability [3][4][5][6][7][8], melting point [9], stability [10][11][12], tabletability [13], taste [6], and other properties can undergo change Everything Added to Foods in the United States (EAFUS), while nicotinamide belongs to the substances Generally Recognized as Safe (GRAS) [21,30]. p-Aminobenzoic acid and nicotinamide belong to vitamin B group. To the best of our knowledge, there are no data reporting on pharmacological interactions between the coformers and benzodiazepines used in this study.
The thermal properties of co-crystals, their crystal structures and spectroscopic characteristics were investigated using differential scanning calorimetry (DSC), powder and single-crystal X-ray diffraction (PXRD and SCXRD), Fourier-transform infrared (FTIR) and Raman spectroscopy. A comparison of the data acquired for both co-crystals using DSC, PXRD, SCXRD, FTIR and Raman techniques with those obtained for physical mixtures of chlordiazepoxide with p-aminobenzoic acid and lorazepam with nicotinamide permits the confirmation of co-crystal formation and an assessment of their basic properties.

Materials
Chlordiazepoxide and lorazepam were provided by Polfa Tarchomin (Warsaw, Poland). p-Aminobenzoic acid and nicotinamide were bought from Sigma Aldrich (St. Louis, MO, USA). Since the purity of materials was above 99%, both were used without further purification. Methanol and ethyl acetate (pure for analysis) were obtained from POCH (Gliwice, Poland) and acetonitrile from J. T. Baker (Phillipsburg, NJ, USA).

Sample Preparation
Binary physical mixtures of chlordiazepoxide with p-aminobenzoic acid and lorazepam with nicotinamide were prepared at 1:1 molar ratios. Components accurately weighed using Mettler Toledo XA105 Dual Range balance (Schwerzenbach, Switzerland) were transferred into micro tubes (Sarstedt, Nümbrecht, Germany) and mixed using a laboratory stirrer (Kamush, Gdansk, Poland) for 15 min at 20 rpm.
Benzodiazepines co-crystals were prepared by two co-crystallization methods, slurry evaporation and liquid-assisted grinding using binary physical mixtures as starting materials.
To obtain co-crystals by slurry evaporation, a 100 µL of solvent, acetonitrile and ethyl acetate, and methanol and ethyl acetate, was added into a mixture of chlordiazepoxide with p-aminobenzoic acid and lorazepam with nicotinamide. The samples were mixed for 30 min using a laboratory stirrer at 20 rpm and left in sealed micro tubes for 24 h. After this time the micro tubes were unsealed and left for free evaporation.
To obtain co-crystals by liquid-assisted grinding co-crystallization, benzodiazepine mixtures were ground in micro tubes with two agate grinding balls 5 mm in diameter (Eqiumed, Cracow, Poland) using a laboratory stirrer for 30 min at 20 rpm. Before grinding, a small amount of a solvent (five drops) was added. Acetonitrile and ethyl acetate were used for mixture of chlordiazepoxide and p-minobenzoic acid, methanol and ethyl acetate for lorazepam mixture with nicotinamide. The ground samples were unsealed and left for evaporation.

Differential Scanning Calorimetry (DSC)
The DSC curves of the samples examined were acquired by a heat-flux Mettler Toledo DSC device, model 822e (Schwerzenbach, Switzerland), coupled with STARe software, ver. 15.00. The experiments were run in dynamic nitrogen atmosphere at a flux rate 70 mL/min and at 5 °C/min heating rate over a range of 25-300 °C. About 4 mg of samples were weighed in flat-bottomed pans, which then were closed with perforated lids.

Powder X-Ray Diffraction (PXRD)
Powdered samples were measured using modernized and computer controlled horizontal TUR M-62 device (VEB TUR, Dresden, Germany). A PXRD technique with CuKα radiation at 1.5418 Å was used to collect X-ray diffraction patterns. Instrumental parameters were as follows: 2θ angle range 5-50°, counting time 3 s per step, counting step (2θ) 0.04°. Measurements were performed with nickel filtering. The diffraction patterns of starting components, physical mixtures and co-crystals were obtained using standard software. Identification of the compounds tested was carried out using the XRAYAN program (software for material identification by PXRD technique) [31]. This program is connected to the database Powder Diffraction File (PDF-4+) ICDD (International Centre for Diffraction Data, Newtown Square, PA, USA).

Single-Crystal X-Ray Diffraction (SCXRD)
Single crystals for SCXRD investigations were prepared by the solvent evaporation co-crystallization method using physical mixtures of benzodiazepines with coformers at a 1:1 molar ratio. Accurately weighed components were placed in a beaker consisting of 10-20 mL of solvent (acetonitrile, methanol or ethyl acetate) and mixed for 15 min using a magnetic stirrer. Afterwards the solution was filtered through paper filters (Bionovo, Legnica, Poland) and left for slow evaporation of solvent. Crystals suitable for SCXRD study were obtained using acetonitrile for chlordiazepoxide co-crystals with p-aminobenzoic acid, and acetonitrile and ethyl acetate for lorazepam co-crystals with nicotinamide.
The SCXRD experiments for single-crystals of chlordiazepoxide co-crystals with p-aminobenzoic acid and lorazepam co-crystals with nicotinamide were recorded using Oxford Diffraction Gemini R ULTRA Ruby CCD diffractometer (T = 295(2) K and λMo = 0.71073 Å) ( Table 1). Software packages used were as follows: data collections-CrysAlis CCD [32]; cell refinement, data reduction and multi-scan absorption corrections-CrysAlis RED [32]; crystal structure solving and refining-SHELX [32,33]; calculations of intermolecular interactions-PLATON [34]; preparing of molecular graphics-ORTEPII [35], PLUTO-78 [36] and Mercury [37]. The chlorobenzene ring (C14-C19 and Cl20 atoms) in lorazepam co-crystals with nicotinamide has orientation disorders with refined site-occupancy factors of the disordered parts 0.897 and 0.103 (the disordered benzene rings were refined as rigid ideal hexagons with C-C = 1.  FTIR analyses of the co-crystals under study were conducted using a Thermo Fischer Scientific FTIR spectrometer, model Nicolet 380 (Madison, WI, USA) coupled with a DTGS KBr detector and OMNIC software. A hydraulic press (Specac, Orpington, UK) was used to prepare samples containing 1 mg of sample and 100 mg of KBr (Merck, Darmstadt, Germany). FTIR spectra were collected over a spectral range of 4000-400 cm −1 , with 4 cm −1 resolution at ambient temperature. The background spectrum was checked prior to each measurement.

Raman Spectroscopy
A Thermo Fisher Scientific DXR Smart Raman spectrometer (Madison, WI, USA) was used to record Raman spectra. The device was equipped with OMNIC software, a 15-mW DXR 780 nm laser with a slit width of 25 µm, CCD detector and Raleigh filter. The Raman spectra were collected over a range of 3413-99 cm −1 with a resolution of 2 cm −1 . Exposure time was 1 s (twice).

Results and Discussion
The aim of this study was to prepare and examine newly obtained benzodiazepine co-crystals, viz. chlordiazepoxide with p-aminobenzoic acid and lorazepam with nicotinamide, prepared by different methods (slurry evaporation and liquid-assisted grinding) with selected solvents. The thermal profile and crystal structure of the co-crystals in question were studied using DSC, PXRD and SCXRD techniques, while the hydrogen bonding formation between APIs and coformers was assessed using spectroscopic techniques, i.e., FTIR and Raman. Co-crystal formation was fully verified by comparing their thermal, diffractometric and spectroscopic data with those for physical mixtures and starting components.

DSC
The DSC curves for chlordiazepoxide co-crystals with p-aminobenzoic acid prepared by different methods and solvents revealed a single endothermic peak followed by an exothermic one ( Table 2). The endothermic effect is associated with the melting of co-crystal and differs slightly, depending on the co-crystallization procedure and solvent used. For the slurry evaporation method, the peaks were at 219.6 °C ( Figure 2a) and 221.7 °C for the sample prepared using acetonitrile and ethyl acetate, respectively. The liquid-assisted grinding procedure, in turn, leads to peaks at slightly higher temperatures, i.e., 220.4 °C for acetonitrile and 223.1 °C for ethyl acetate. This indicates that the melting point of co-crystals differs from those of starting components, 242.3 °C for chlordiazepoxide and 187.2 °C for p-aminobenzoic acid. A mixture of the two components heated at higher rate (10 °C/min) displays an endothermic peak at 180.1 °C due to eutectic melting followed by a co-crystal recrystallization at 185.1 °C (Figure 2b). An exothermic effect immediately after the endothermic is characteristic of physical mixtures whose components co-crystallize under heating to form co-crystals [38,39]. Thus, the DSC method confirms chlordiazepoxide co-crystallization with p-aminobenzoic acid. It is noteworthy that melted co-crystals undergo decomposition, as proved by the second, exothermic, DSC peak. Thermogravimetric examination confirms that chlordiazepoxide decomposes immediately after melting with ~60% mass loss at ~300 °C (data not published).
Melting of lorazepam co-crystals with nicotinamide also depends on the preparation method and solvent used but regardless of co-crystallization procedure, DSC curves indicated endothermic peaks above 177 °C (Table 2). Co-crystals prepared by slurry evaporation revealed higher melting points, at 177.5 °C and 178.9 °C for co-crystallization from methanol and ethyl acetate, respectively. The co-crystals obtained by liquid-assisted grinding melted at ~176 °C (Figure 2c). An additional, slight endothermic DSC peak at 127.2 °C was found for co-crystals prepared by this procedure with ethyl acetate as a solvent. This is probably attributable to the melting of a slight quantity of coformer (Tp for nicotinamide = 128.3 °C). The melting point of lorazepam co-crystals coincides with that of lorazepam (Tp = 177.0 °C). The DSC curve of mixture for both components indicated three peaks (Figure 2d), the first was endothermic at 127.3 °C, followed by an exothermic one at 130.1 °C. The last, an endothermic effect, can be assigned to the melting of co-crystals at 171.5 °C. Like chlordiazepoxide co-crystals, an exothermic effect immediately after the endothermic one confirms co-crystallization, whereas melted lorazepam co-crystals undergo decomposition, as proved by the last exothermic DSC peak. Table 2. Thermal data acquired from differential scanning calorimetry (DSC) curves for benzodiazepines co-crystals prepared using different co-crystallization methods and solvents, and for physical mixtures of benzodiazepines and coformers.

Co-Crystals Physical Mixtures Slurry Evaporation
Liquid-Assisted Grinding

PXRD
PXRD patterns confirmed that both the benzodiazepines and coformers used in this study were in crystalline form. The compliance of the diffraction pattern for chlordiazepoxide mixture with p-aminobenzoic acid into those contained in the powder base corroborated that only crystalline forms of benzodiazepine and coformer can be found in the mixture. Hence, the diffraction pattern of the mixture is the sum of the diffraction maxima for chlordiazepoxide and p-aminobenzoic acid, which suggests that there was no interaction between ingredients after simple mixing by 15 min using a laboratory stirrer at 20 rpm (Figure 3a). The mixture displayed sharp diffraction peaks at 2θ of 11.45°, 13.88°, 14.77°, 15.33°, 17.22° and 21.90°. A comparison of these data with those for chlordiazepoxide co-crystals with p-aminobenzoic acid prepared by slurry evaporation method ( Figure 3b) and liquid-assisted grinding method (Figure 3c) revealed that diffraction patterns of co-crystals and the physical mixture are disparate. The most intensive new diffraction peaks were observed in the pattern of co-crystals prepared by liquid-assisted grinding method at 10 The compliance of the diffraction pattern for chlordiazepoxide co-crystals with p-aminobenzoic acid prepared by slurry evaporation method with those contained in the base PDF-4+ did not display diffraction peaks characteristic of benzodiazepine and coformer ( Figure S1, Supplementary Materials). Since the co-crystals' diffraction pattern did not match those for components alone, it may be concluded that a new crystal structure is created in the co-crystallization process.  Figure S2, Supplementary Materials). On the other hand, the diffraction pattern of lorazepam co-crystals with nicotinamide prepared by the liquid-assisted grinding and slurry evaporation methods differ significantly from that of lorazepam mixture with nicotinamide ( Figure 4). Co-crystals prepared by the slurry method have new diffraction peaks at 9.98°, 13.63°, 14.07°, 15.56°, 19.88°, 23.81°, 24.75° and 26.47°, indicative of a new crystalline phase being created during the co-crystallization process. A comparison of co-crystals prepared by the liquid-assisted grinding and slurry evaporation methods with those contained in the bases showed that the diffraction maxima of lorazepam and nicotinamide did not overlap with those for the co-crystals. Thus, the PXRD study confirms that a new crystal phase is obtained during co-crystallization. Differences in the peak positions between patterns of co-crystals obtained by different methods were slight, proving that, irrespective of preparation methods and solvents used, the same co-crystal structure can be obtained. Moreover, the same diffraction maxima at 2θ values for the lorazepam mixture with nicotinamide and both components excludes co-crystallization after lorazepam is mixed with nicotinamide. It follows that the simple mixing of both components did not lead to co-crystal formation.

SCXRD
SCXRD measurements show that the chlordiazepoxide co-crystal with p-aminobenzoic acid and the lorazepam co-crystal with nicotinamide crystallized in the monoclinic P21/n and P21/c space group, respectively, with one molecule of benzodiazepine and one molecule of coformer in the asymmetric unit (Table 1). In the crystal structure of chlordiazepoxide co-crystals with p-aminobenzoic acid, the geometric parameters, i.e., bond lengths and angles, are typical for those observed in crystals containing the chlordiazepoxide molecule [40,41]. In the crystal of chlordiazepoxide compounds with p-aminobenzoic acid, the C-O bond lengths [1.217(2)-1.334(2) Å] in the COOH group of the p-aminobenzoic acid molecule show that no proton transfer has occurred but that a co-crystal is formed, where the chlordiazepoxide and p-aminobenzoic acid molecules are linked via N12-H12···O30 and O29-H29···O14 hydrogen bonds to form heterodimer (Table 3, Figure  5). The neighboring heterodimers are connected through a N31-H31A···N1 hydrogen bond to produce chains along the [0 0 1] direction (Table 3, Figure 6a). Adjacent chains are connected by a N31-H31B···O14 hydrogen bond to create layers along the a-axis (Table 3, Figure 6b). These layers are, in turn, linked via a weak C20-H20···Cl21 hydrogen bond to form a 3D framework. Table 3. Hydrogen bonding interactions in the crystal structure of chlordiazepoxide co-crystals with p-aminobenzoic acid. Symmetry codes: (i) x, y, 1+z; (ii) −½+x, ½−y, ½+z; (iii) 2−x, 1−y, −z.  In the crystal structure of lorazepam co-crystals with nicotinamide, the geometric parameters are similar to those observed in the crystals containing the lorazepam molecule (Table 4, Figure 7) [42,43]. In the crystal of lorazepam compounded with nicotinamide, the C-O bond length [1.395(2) Å] in the hydroxyl group, as well as N-C bond lengths [1.351(2) and 1.409(2) Å], involving the endocyclic N-atom in the lorazepam molecule, show that proton transfer is absent and that a co-crystal is formed. On this occasion, the lorazepam and nicotinamide molecules in the co-crystal are connected by O13-H13···O30 and N29-H29A···O12 hydrogen bonds to form a heterodimer (Table 4, Figure 7). Adjacent heterodimers are linked via N1-H1···N22, N29-H29B···O13 and C25-H25···N4 hydrogen bonds to create tapes along the [0 1 0] direction (Table 4, Figure 8a). The neighboring tapes are connected by π···π interactions (with centroid···centroid distances 3.568(3) Å) Å to form a 3D framework (Figure 8b). Table 4. Hydrogen bonding interactions in the crystal structure of lorazepam co-crystals with nicotinamide. Symmetry codes: (i) 1−x, ½+y, ½−z.

FTIR and Raman Spectroscopy
The FTIR data compiled in Table 5 indicate discrepancies between the spectra of chlordiazepoxide co-crystals and the physical mixture. It is significant that the spectra of co-crystals prepared by various procedures showed no significant divergence, nor did the spectrum of physical mixture reveal major changes in band position compared with starting components. The spectra of benzodiazepine and coformer are consistent with the literature data [30,44]. It therefore follows that chlordiazepoxide and p-aminobenzoic acid do not interact after mixing. Characteristic bands at 3460, 3363, 3196 and 3057 cm -1 due to stretching vibrations of the amine group in the physical mixture were shifted to lower or higher values at 3443, 3322, 3216 and 3122 cm -1 in the spectra of co-crystals, suggesting a hydrogen bonding formation between the amine group of chlordiazepoxide and p-aminobenzoic acid. Table 5. Characteristic FTIR and Raman bands of chlordiazepoxide co-crystals with p-aminobenzoic acid, prepared using different co-crystallization methods and solvents. The formation of hydrogen bonding can also be confirmed by consecutive changes in the co-crystal spectra. Two new bands at ~2490 and 2934 cm -1 were identified, which were over the spectral range of 2300-3000 cm -1 assigned to hydrogen bonding formation [30]. A new band at 1675 cm -1 was also created, probably due to the shift of a band at 1665.6 cm -1 , attributed to the stretching vibrations of the C=O group of p-aminobenzoic acid. However, a strong band at ~1283 cm -1 assigned to the C-OH stretching vibration of coformer disappeared, which confirms the participation of this group in the formation of hydrogen bonding with chlordiazepoxide. Furthermore, the band at 1625 cm -1 in the physical mixture, due to bending vibrations in the plane of the amine group of coformer and the methylamine group of chlordiazepoxide, is shifted to 1621 cm -1 in the co-crystal spectra. In addition, bands observed at 1170, 1150 and 1134 cm -1 in the spectra of physical mixture are shifted to lower values at ~1161, 1146 and 1115 cm -1 in the co-crystal spectra. Hence, the FTIR study confirms the participation of the carboxyl and amine groups of coformer and the methylamine group of chlordiazepoxide in the creation of hydrogen bonding.

Vibrations
As was the case with chlordiazepoxide co-crystals, discrepancies were observed between the FTIR spectra of lorazepam co-crystals and the physical mixture ( Figure 9, Table 6). Characteristic bands at 3459 and 3363 cm -1 , assigned to stretching vibrations of amine and hydroxyl groups of lorazepam and asymmetric stretching of the amide group of nicotinamide [24,28,29,45], were shifted to lower values at ~3418 and 3313 cm -1 in the co-crystal spectra, indicating the formation of hydrogen bonding between benzodiazepine and coformer. The participation of C=O stretching vibrations in hydrogen bonding is confirmed by the shift in physical mixture bands at 1703 and 1686 cm -1 to lower values at 1695 and 1662 cm -1 . Moreover, distinctive bands at 1143 and 1133 cm -1 assigned to -OH stretching vibrations of lorazepam are invaluable for co-crystal identification. The shift of the band assigned to -NH2 deformation vibrations of nicotinamide from 1618 cm -1 in the physical mixture spectrum to 1602 cm -1 in the co-crystal spectra confirms the participation of the amide group in hydrogen bonding. Table 6. Characteristic FTIR and Raman bands of lorazepam co-crystals with nicotinamide prepared using different co-crystallization methods and solvents.  Overall, the FTIR study confirms that the amide group of nicotinamide and hydroxyl and carbonyl groups of lorazepam participate in the formation of hydrogen bonding. Additionally, changes in the band positions of -NH of lorazepam imply hydrogen bonding by the N1 ring of diazepine. As lorazepam is known to form dimers in the solid state [24], the dimer could be created between the N1 ring of diazepine from one molecule and an oxygen atom of the carbonyl group from the second molecule of lorazepam. Moreover, it is possible to create a hydrogen bond between the carbonyl and hydroxyl groups of the diazepine ring in two adjacent molecules of lorazepam. The FTIR study shows that lorazepam can create hydrogen bonds with nicotinamide in the same manner as dimers are formed in the solid state.

Vibrations
The co-crystals prepared by different methods show no significant changes in their band spectral positions. The spectrum of the physical mixture was, in turn, a simple overlap of the benzodiazepine and coformer spectra and does not reveal major changes to band position when compared with starting components. This excludes the interaction of components after mixing.
Raman spectra displayed merely minor changes in band intensity and position (up to 2 cm -1 ) for the co-crystals of both chlordiazepoxide and lorazepam prepared using different methods and solvents. Moreover, the Raman spectra of physical mixtures of chlordiazepoxide with p-aminobenzoic acid and lorazepam with nicotinamide do not reflect any distinctive changes in band position or intensity compared with the starting components; hence, it can be asserted that no co-crystallization occurs when benzodiazepines with coformers are mixed. However, comparing the Raman spectra of co-crystals with those of physical mixtures revealed new unique bands at ~1672, 1552, 1255 and 1162 cm -1 , and at ~1692, 1506 and 1383 cm -1 , for chlordiazepoxide and lorazepam co-crystals, respectively. These changes in Raman spectra and those specified below reveal that new structures are created, thereby confirming co-crystal formation.
Raman spectra of chlordiazepoxide co-crystals with p-aminobenzoic acid prepared by different methods and of the physical mixture of both components are shown in Figure 10. Raman bands at 1181.0 and 1133.0 cm -1 found in the chlordiazepoxide physical mixture spectrum disappeared in the co-crystal spectra, while another at 1515.9 cm -1 is shifted to 1521 cm -1 . Moreover, the band at 1287cm -1 in the mixture spectrum assigned to the hydroxyl stretching of p-aminobenzoic acid [46] changes position slightly to ~1284 cm -1 and a new strong band at 1255 cm -1 , near the hydroxyl band, is created in the co-crystal spectra. Figure 10. Raman spectra of: (a) chlordiazepoxide and p-aminobenzoic acid physical mixture, (b) chlordiazepoxide co-crystals with p-aminobenzoic acid prepared by grinding assisted acetonitrile, and (c) chlordiazepoxide co-crystals with p-aminobenzoic acid co-crystals prepared by slurry evaporation method and ethyl acetate as a solvent.
In the lorazepam co-crystal spectra, a band at 1674.3 cm -1 assigned to the C=O stretching vibration of nicotinamide [47] is absent. The intensity of the band at 1167.9 cm -1 increased in relation to that in the mixture spectrum. Moreover, additional weak peaks at ~1196, 1132, 722 and 696 cm -1 appeared, confirming the interaction of lorazepam with nicotinamide due to co-crystallization.

Conclusions
The outcomes of this research reveal that two new co-crystals, i.e., chlordiazepoxide with p-aminobenzoic acid and lorazepam with nicotinamide, were prepared using liquid-assisted grinding and slurry evaporation procedures. Both co-crystallization methods could be used for further preparation of benzodiazepine co-crystals, since they are environmentally benign procedures. However, in the case of lorazepam co-crystals with nicotinamide, prepared by a liquid-assisted grinding procedure with ethyl acetate as a solvent, a DSC study revealed a minute amount of unreacted starting component; hence, the usage of ethyl acetate in this procedure should be reconsidered.
Crystal structures, spectroscopic characteristics and thermal profiles of both benzodiazepines co-crystals were described using powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SCXRD), Fourier-transform infrared (FTIR), Raman spectroscopy, and differential scanning calorimetry (DSC). The study revealed that benzodiazepine co-crystals are crystalline materials, where the compliance of co-crystal patterns with those contained in the base PDF-4+ show that new structures were obtained. The chlordiazepoxide co-crystal with p-aminobenzoic acid and the lorazepam co-crystal with nicotinamide crystallized in the monoclinic P21/n and P21/c space group at a 1:1 molar ratio. New, unique bands and the shifting of band positions observed in the Raman spectra of co-crystals verify that new structures were obtained after the co-crystallization process.

Supplementary Materials:
The following are available online at www.mdpi.com/1999-4923/12/7/648/s1, Figure  S1. Identification cards of chlordiazepoxide and p-aminobenzoic acid (from the database PDF-4+ ICDD) and the diffraction pattern of chlordiazepoxide co-crystals with p-aminobenzoic acid prepared by slurry evaporation method, Figure S2. Identification cards of lorazepam and nicotinamide (from the database PDF-4+ ICDD) and the diffraction pattern of lorazepam mixture with nicotinamide.