X-ray powder diffraction, differential scanning calorimetry, infrared absorption spectroscopy, and Raman spectroscopy have been used to study the phenomenon of cocrystal formation in the molecular complexes formed by 5-nitrobarbituric acid with four cinchona alkaloids. The cocrystal products were found to contain varying degrees of hydration, ranging from no hydration in the nitrobarbiturate-quinidine cocrystal up to a 4.5-hydrate species in the nitrobarbiturate-cinchonine cocrystal. For the nitrobarbiturate cocrystals with cinchonine, cinchonidine, and quinidine, the predominant interaction was with the quinoline ring system of the alkaloid. However, for quinine, the predominant interaction was with the quinuclidine group of the alkaloid. These properties serve to demonstrate the utility of 5-nitrobarbituric acid as a preferred reagent for chemical microscopy, since the differing range of hydrate and structural types would serve to easily differentiate the cinchona alkaloids from each other, even when different compounds contained the same absolute configurations at their dissymmetric centers.
The realization that cocrystallization of drug substances with other compounds could lead to modification of physical properties has led to a vitalization of the crystal engineering community and to substantial interest in the part of the pharmaceutical industry [
However, the subject area of cocrystals extends much further back in time than the recent activity [
Another source of unrecognized cocrystals is contained within the extensive scope of analytical chemistry conducted under the general category of chemical microscopy, where analysts would prepare derivatives or adducts of an analyte species and then identify the analyte on the basis of the morphology of the crystalline products [
One of the premier chemical microscopic reagents was 5-nitrobarbituric acid (also known as dilituric acid), whose adduct complexes with basic compounds were superior to surpass other nitroenolic reagents owing to the high degree of crystallinity and ease of recrystallization of the products of [
In the present work, the phenomenon of cocrystal formation in the molecular complexes of 5-nitrobarbituric acid has been further investigated through studies of adducts formed by this reagent with four cinchona alkaloids (quinine, quinidine, cinchonine, and cinchonidine). Structures of the 5-nitrobarbituric acid reagent and the alkaloids studies in this work are found in Figure
Structures of 5-nitrobarbituric acid and the four cinchona alkaloids used in this study.
5-Nitrobarbituric acid trihydrate, quinine dihydrate, quinidine, cinchonine, and cinchonidine were purchased from Sigma-Aldrich and were recrystallized from 70% aqueous isopropanol before use. The products studied in this work were prepared using solvent-drop-mediated solid-state grinding [
X-ray powder diffraction (XRPD) patterns were obtained using a Rigaku MiniFlex powder diffraction system, equipped with a horizontal goniometer operating in the
Measurements of differential scanning calorimetry (DSC) were obtained on a TA Instruments 2910 thermal analysis system. Samples of approximately 1-2 mg were accurately weighed into an aluminum DSC pan and then covered with an aluminum lid that was crimped in place. The samples were then heated over the range of 20–175°C, at a heating rate of 10°C/min.
Measurements of total volatile content were made using an Ohaus model MB45 system. The samples were heated isothermally at a temperature of 125°C for a period of 15 minutes. Invariably, samples had desolvated to constant weight after approximately 10 minutes of heating time.
Infrared absorption spectra were obtained at a resolution of 4 cm−1 using a Shimadzu model 8400S Fourier-transform infrared spectrometer, with each spectrum being obtained as the average of 25 individual spectra. The data were acquired using the attenuated total reflectance sampling mode, where the samples were clamped against the ZnSe crystal of a Pike MIRacle single reflection horizontal ATR sampling accessory.
Raman spectra were obtained in the fingerprint region using a Raman Systems model R-3000HR spectrometer, operated at a resolution of 5 cm−1 and using a laser wavelength of 785 nm. The data were acquired using front-face scattering from a thick powder bed kept in an aluminum sample holder.
Examination of Figure
Quinine and quinidine are also a diastereomeric pair and differ from cinchonine and cinchonidine by the presence of a methoxy group attached to the quinoline ring. The more systematic name for quinidine is (9
Although the vibrational spectra of a large number of substituted barbituric acids have been reported and analyzed [
It is known that the vibrational motions of the three carbonyl groups on the barbituric acid moiety are observed as transitions into three-group frequency ring modes, and the atomic numbering shown in Figure
The highest frequency band (observed at an energy of 1705 cm−1 in the infrared absorption spectrum of 5-nitrobarbituric acid) is identified as the 4,6-CO symmetric mode, where the vibrational motion is more localized on the carbonyl groups located at carbons 4 and 6. The lowest frequency mode corresponds to the 2-CO stretching mode and is observed at an energy of 1614 cm−1, while the 4,6-CO antisymmetric mode is observed at an intermediate energy of 1651 cm−1. Other important absorption bands in the infrared absorption spectrum of 5-nitrobarbituric acid are the symmetric stretching mode associated with the nitro group (1381 cm−1), the C–N ring deformation mode (1246 cm−1), and the carbonyl out-of-plane bending mode (779 cm−1).
A number of important bands were observed in the Raman spectrum of 5-nitrobarbituric acid, with these being primarily associated with motions of the carbonyl groups. An in-plane bending mode was observed at an energy of 379 cm−1, while out-of-plane bending modes were observed at 625 and 695 cm−1. Other band assignments that will be important to succeeding discussion are an N–H out-of-plane bending mode (846 cm−1), a C–N stretching mode (1150 cm−1), and a C–N deformation mode (1255 cm−1).
While the vibrational modes of 5-nitrobarbituric acid are dominated by ring character, the vibrational modes of the cinchona alkaloids consist mainly of ring modes associated with the quinoline ring and C–C and C–H vibrational modes associated with the vinylquinuclidine group and the aliphatic linkage between the two. Since the vibrational spectroscopy of both quinoline [
Unfortunately, were the protonated cinchona bands to be present in the absorption spectrum of a cocrystal product, they would be overwhelmed by the more intense absorption bands associated with the 5-nitrobarbituric acid. Fortunately, the Raman spectrum of the cinchona alkaloids is not so compromised, and one may readily assess the interactions existing in the cocrystal by tracking the energies of the key quinoline and quinuclidine vibrational modes.
XRPD patterns obtained for 5-nitrobarbituric acid, cinchonine, and the nitrobarbiturate-cinchonine cocrystal product are shown in Figure
X-ray powder diffraction patterns of 5-nitrobarbituric acid, cinchonine, and the 1 : 1 nitrobarbituric-cinchonine cocrystal.
The nitrobarbiturate-cinchonine cocrystal product was found to evolve 14.8% of its mass when heated isothermally at 125°C, which would correspond to the existence of a 4.5-hydrate form of the cocrystal. The DSC thermogram of the cocrystal product consisted of a strong desolvation endothermic transition, characterized by a temperature maximum of 80°C and an enthalpy of fusion equal to 246 J/g. The desolvated compound formed upon completion of this thermal reaction did not exhibit a melting endothermic transition, indicating that an amorphous substance formed upon desolvation. The only other thermally induced transition observed in the DSC thermogram was the onset of decomposition, which began at a temperature of approximately 220°C.
The infrared absorption spectra obtained for 5-nitrobarbituric acid, cinchonine, and the 1 : 1 nitrobarbituric-cinchonine cocrystal within the fingerprint region are shown in Figure
Assignments of the major bands in the fingerprint region of the infrared absorption spectra of 5-nitrobarbituric acid, cinchonine, and their 1 : 1 cocrystal product.
Assignment | Nitrobarbituric (cm−1) | Cocrystal (cm−1) | Cinchonine (cm−1) | Assignment |
---|---|---|---|---|
Carbonyl out-of-plane bending mode | 779 | 756 | ||
C–N ring deformation mode | 1246 | 1286 | ||
Obscured and not observed | 1360 | Nonprotonated quinuclidine mode | ||
Nitro group symmetric stretching mode | 1381 | 1377 | ||
Obscured and not observed | 1566 | Nonprotonated quinoline mode | ||
2-CO stretching mode | 1614 | 1622 | ||
4,6-CO antisymmetric mode | 1651 | Not observed | ||
4,6-CO symmetric mode | 1705 | 1701 |
Infrared absorption spectra within the fingerprint region of 5-nitrobarbituric acid, cinchonine, and the 1 : 1 nitrobarbituric-cinchonine cocrystal.
The Raman spectra obtained for 5-nitrobarbituric acid, cinchonine, and the 1 : 1 nitrobarbituric-cinchonine cocrystal within the fingerprint region are shown in Figure
Assignments of the major bands in the fingerprint region of the Raman spectra of 5-nitrobarbituric acid, cinchonine, and their 1 : 1 cocrystal product.
Assignment | Nitrobarbituric (cm−1) | Cocrystal (cm−1) | Cinchonine (cm−1) | Assignment |
---|---|---|---|---|
Carbonyl in-plane bending mode | 379 | 357 | ||
Carbonyl out-of-plane bending mode | 625 | 609 | ||
Carbonyl out-of-plane bending mode | 695 | 681 | ||
N–H out-of-plane bending mode | 846 | 838 | ||
C–N stretching mode | 1150 | 1137 | ||
C–N deformation mode | 1255 | 1248 | ||
1361 | 1360 | Nonprotonated quinuclidine mode | ||
1572 | 1566 | Nonprotonated quinoline mode |
Raman spectra within the fingerprint region of 5-nitrobarbituric acid, cinchonine, and the 1 : 1 nitrobarbituric-cinchonine cocrystal.
The XRPD patterns obtained for 5-nitrobarbituric acid, cinchonidine, and the nitrobarbiturate-cinchonidine cocrystal product are shown in Figure
X-ray powder diffraction patterns of 5-nitrobarbituric acid, cinchonidine, and the 1 : 1 nitrobarbituric-cinchonidine cocrystal.
The nitrobarbiturate-cinchonidine cocrystal product was found to evolve 5.5% of its mass when heated isothermally at 125°C, which would correspond to the existence of a 1.5-hydrate form of the cocrystal. The DSC thermogram of the cocrystal product consisted of a strong desolvation endothermic transition, characterized by a temperature maximum of 101°C and an enthalpy of fusion equal to 97 J/g. The desolvated compound formed upon completion of this thermal reaction was found to exhibit a melting endothermic transition having a temperature maximum of 219°C, but as this transition was coupled with an exothermic decomposition reaction no estimation of enthalpy could be made.
The fingerprint region infrared absorption spectra obtained for 5-nitrobarbituric acid, cinchonidine, and the cocrystal product are shown in Figure
Assignments of the major bands in the fingerprint region of the infrared absorption spectra of 5-nitrobarbituric acid, cinchonidine, and their 1 : 1 cocrystal product.
Assignment | Nitrobarbituric (cm−1) | Cocrystal (cm−1) | Cinchonidine (cm−1) | Assignment |
---|---|---|---|---|
Carbonyl out-of-plane bending mode | 779 | 762 | ||
C–N ring deformation mode | 1246 | 1256 | ||
Obscured and not observed | 1352 | Nonprotonated quinuclidine mode | ||
Nitro group symmetric stretching mode | 1381 | 1358 | ||
Obscured and not observed | 1568 | Nonprotonated quinoline mode | ||
2-CO stretching mode | 1614 | 1595 | ||
4,6-CO antisymmetric mode | 1651 | 1666 | ||
4,6-CO symmetric mode | 1705 | 1691 |
Infrared absorption spectra within the fingerprint region of 5-nitrobarbituric acid, cinchonidine, and the 1 : 1 nitrobarbituric-cinchonidine cocrystal.
The fingerprint region Raman spectra obtained for 5-nitrobarbituric acid, cinchonidine, and the 1 : 1 nitrobarbituric-cinchonidine cocrystal are shown in Figure
Assignments of the major bands in the fingerprint region of the Raman spectra of 5-nitrobarbituric acid, cinchonidine, and their 1 : 1 cocrystal product.
Assignment | Nitrobarbituric (cm−1) | Cocrystal (cm−1) | Cinchonidine (cm−1) | Assignment |
---|---|---|---|---|
Carbonyl in-plane bending mode | 379 | 373 | ||
Carbonyl out-of-plane bending mode | 625 | 617 | ||
Carbonyl out-of-plane bending mode | 695 | 681 | ||
N–H out-of-plane bending mode | 846 | 843 | ||
C–N stretching mode | 1150 | 1132 | ||
C–N deformation mode | 1255 | 1252 | ||
1358 | 1359 | Nonprotonated quinuclidine mode | ||
1591 | 1564 | Nonprotonated quinoline mode |
Raman spectra within the fingerprint region of 5-nitrobarbituric acid, cinchonidine, and the 1 : 1 nitrobarbituric-cinchonidine cocrystal.
The XRPD patterns obtained for 5-nitrobarbituric acid, quinine, and the nitrobarbiturate-quinine cocrystal product are shown in Figure
X-ray powder diffraction patterns of 5-nitrobarbituric acid, quinine, and the 1 : 1 nitrobarbituric-quinine cocrystal.
However, like the nitrobarbiturate-cinchonidine cocrystal, the nitrobarbiturate-quinine cocrystal product was also found to be a 1.5-hydrate form, since it evolved 5.2% of its mass when heated isothermally at 125°C. The DSC thermogram of the nitrobarbiturate-quinine cocrystal product consisted of a strong desolvation endothermic transition, characterized by a temperature maximum of 98°C and an enthalpy of fusion equal to 74 J/g. The thermal characteristics of this desolvation endotherm are fairly similar to those of the nitrobarbiturate-cinchonidine cocrystal. Also akin to its stereochemical equivalent, the desolvated nitrobarbiturate-quinine cocrystal formed after completion of the desolvation thermal reaction exhibited a melting endothermic transition (temperature maximum of 217°C) that was accompanied by a strong exothermic decomposition reaction.
The fingerprint region infrared absorption spectra obtained for 5-nitrobarbituric acid, quinine, and the cocrystal product are shown in Figures
Assignments of the major bands in the fingerprint region of the infrared absorption spectra of 5-nitrobarbituric acid, quinine, and their 1 : 1 cocrystal product.
Assignment | Nitrobarbituric (cm−1) | Cocrystal (cm−1) | Quinine (cm−1) | Assignment |
---|---|---|---|---|
Carbonyl out-of-plane bending mode | 779 | 791 | ||
C–N ring deformation mode | 1246 | 1234 | ||
1364 | 1371 | Nonprotonated quinuclidine mode | ||
Nitro group symmetric stretching mode | 1381 | 1359 | ||
1575 | 1576 | Nonprotonated quinoline mode | ||
2-CO stretching mode | 1614 | 1620 | ||
4,6-CO antisymmetric mode | 1651 | 1661 | ||
4,6-CO symmetric mode | 1705 | 1691 |
Infrared absorption spectra within the fingerprint region of 5-nitrobarbituric acid, quinine, and the 1 : 1 nitrobarbituric-quinine cocrystal.
Raman spectra within the fingerprint region of 5-nitrobarbituric acid, quinine, and the 1 : 1 nitrobarbituric-quinine cocrystal.
Even more interesting was the behavior of the two key absorption bands of the quinuclidine and quinoline moieties of quinine. While these were obscured by the nitrobarbiturate absorption bands in the infrared absorption spectra of the cinchonine and cinchonidine cocrystal products, they were observed in the spectrum of the quinine cocrystal. However, the energy of the nonprotonated quinoline vibrational mode was only barely shifted in the spectrum of the cocrystal (1576 cm−1 for quinine versus 1575 cm−1 for its cocrystal), while the energy of the nonprotonated quinuclidine vibrational mode was definitely shifted (1371 cm−1 for quinine versus 1364 cm−1 for its cocrystal). This finding would imply that while the dominant interaction between 5-nitrobarbituric acid and cinchonine or cinchonidine was between the acid and the quinoline ring, the dominant interaction between 5-nitrobarbituric acid and quinine was between the acid and the quinuclidine ring nitrogen.
Quinine also contains a methoxy group bound on the quinoline ring that is not present for either cinchonine or cinchonidine. In the spectrum of the free base, the vibrational bands associated with the methoxy group were observed as a strong doublet of peaks (having energies of 1227 and 1240 cm−1) that each underwent significant shifting in the spectrum of the cocrystal (1234 and 1252 cm−1). This observation indicates that while the quinoline ring nitrogen was not strongly involved in the intermolecular interactions of the synthon, its methoxy group certainly was.
The fingerprint region Raman spectra obtained for 5-nitrobarbituric acid, quinine, and the 1 : 1 nitrobarbituric-quinine cocrystal are shown in Figure
Assignments of the major bands in the fingerprint region of the Raman spectra of 5-nitrobarbituric acid, quinine, and their 1 : 1 cocrystal product.
Assignment | Nitrobarbituric (cm−1) | Cocrystal (cm−1) | Quinine (cm−1) | Assignment |
---|---|---|---|---|
Carbonyl in-plane bending mode | 379 | 368 | ||
Carbonyl out-of-plane bending mode | 625 | 610 | ||
Carbonyl out-of-plane bending mode | 695 | 679 | ||
N–H out-of-plane bending mode | 846 | 834 | ||
C–N stretching mode | 1150 | 1125 | ||
C–N deformation mode | 1255 | 1242 | ||
1359 | 1366 | Nonprotonated quinuclidine mode | ||
1586 | 1585 | Nonprotonated quinoline mode |
The XRPD patterns obtained for 5-nitrobarbituric acid, quinidine, and the nitrobarbiturate-quinidine cocrystal product are shown in Figure
X-ray powder diffraction patterns of 5-nitrobarbituric acid, quinidine, and the 1 : 1 nitrobarbituric-quinidine cocrystal.
The nitrobarbiturate-quinidine cocrystal product proved to be unique in that it barely evolved 0.25% of its mass when heated isothermally at 125°C and was therefore determined to be a nonsolvated solid-state form. This type of cocrystal product is totally different from that of its stereochemical analogue, as the nitrobarbiturate-cinchonine cocrystal product was characterized by the highest degree of hydration among the cocrystals studied. The DSC thermogram of the nitrobarbiturate-quinidine cocrystal product did not contain a desolvation endotherm and instead consisted of a weak melting endothermic transition having a temperature maximum of 196°C and an enthalpy of fusion equal to 15 J/g. At a temperature of 230°C the cocrystal was found to undergo a strong exothermic decomposition reaction.
The fingerprint region infrared absorption spectra obtained for 5-nitrobarbituric acid, quinidine, and the cocrystal product are shown in Figure
Assignments of the major bands in the fingerprint region of the infrared absorption spectra of 5-nitrobarbituric acid, quinidine, and their 1 : 1 cocrystal product.
Assignment | Nitrobarbituric (cm−1) | Cocrystal (cm−1) | Quinidine (cm−1) | Assignment |
---|---|---|---|---|
Carbonyl out-of-plane bending mode | 779 | 756 | ||
C–N ring deformation mode | 1246 | 1240 | ||
Obscured and not observed | 1360 | Nonprotonated quinuclidine mode | ||
Nitro group symmetric stretching mode | 1381 | 1362 | ||
Obscured and not observed | 1562 | Nonprotonated quinoline mode | ||
2-CO stretching mode | 1614 | 1622 | ||
4,6-CO antisymmetric mode | 1651 | Not observed | ||
4,6-CO symmetric mode | 1705 | 1690 |
Infrared absorption spectra within the fingerprint region of 5-nitrobarbituric acid, quinidine, and the 1 : 1 nitrobarbituric-quinidine cocrystal.
The fingerprint region Raman spectra obtained for 5-nitrobarbituric acid, quinidine, and the 1 : 1 nitrobarbituric-quinidine cocrystal are shown in Figure
Assignments of the major bands in the fingerprint region of the Raman spectra of 5-nitrobarbituric acid, quinidine, and their 1 : 1 cocrystal product.
Assignment | Nitrobarbituric (cm−1) | Cocrystal (cm−1) | Quinidine (cm−1) | Assignment |
---|---|---|---|---|
Carbonyl in-plane bending mode | 379 | 365 | ||
Carbonyl out-of-plane bending mode | 625 | 608 | ||
Carbonyl out-of-plane bending mode | 695 | 681 | ||
N–H out-of-plane bending mode | 846 | 836 | ||
C–N stretching mode | 1150 | 1144 | ||
C–N deformation mode | 1255 | 1243 | ||
1362 | 1363 | Nonprotonated quinuclidine mode | ||
1570 | 1561 | Nonprotonated quinoline mode |
Raman spectra within the fingerprint region of 5-nitrobarbituric acid, quinidine, and the 1 : 1 nitrobarbituric-quinidine cocrystal.
X-ray powder diffraction and thermal analysis have been used to demonstrate the existence of new solid-state forms generated by the cocrystallization of 5-nitrobarbituric acid with four cinchona alkaloids. The cocrystal products were found to contain varying degrees of hydration, ranging from no hydration in the nitrobarbiturate-quinidine cocrystal up to a 4.5-hydrate species in the nitrobarbiturate-cinchonine cocrystal. Raman spectra of the products were used to determine the predominant mode of interaction in the nitrobarbiturate-cinchona synthon. For the nitrobarbiturate cocrystals with cinchonine, cinchonidine, and quinidine, the predominant interaction was with the quinoline ring system of the alkaloid. However, for quinine, the predominant interaction was with the quinuclidine group of the alkaloid.
Previous papers in this series [
The author declares that there is no conflict of interests regarding the publication of this paper.