On the Tautomerism of N-Substituted Pyrazolones: 1,2-Dihydro-3H-pyrazol-3-ones versus 1H-Pyrazol-3-ols

The tautomerism of 1-phenyl-1,2-dihydro-3H-pyrazol-3-one was investigated. An X-ray crystal structure analysis exhibits dimers of 1-phenyl-1H-pyrazol-3-ol units. Comparison of NMR (nuclear magnetic resonance) spectra in liquid state (1H, 13C, 15N) with those of “fixed” derivatives, as well as with the corresponding solid state NMR spectra reveal this compound to exist predominantly as 1H-pyrazol-3-ol molecule pairs in nonpolar solvents like CDCl3 or C6D6, whereas in DMSO-d6 the corresponding monomers are at hand. Moreover, the NMR data of different related 1H-pyrazol-3-ol derivatives are presented.


Results and Discussion
In principle, for the title compounds, two tautomeric forms are possible, namely the OH-form and the NH-form ( Figure 1). In Chemical Abstracts such compounds carrying an alkyl or a (hetero)aryl substituent at the pyrazole nitrogen atom are always listed as 3H-pyrazol-3-ones. Hence, as many authors prefer the latter denomination, in the course of this study we investigated which tautomer is really relevant in a solid state and, especially, in solution.

X-ray Analysis of 1-Phenyl-1,2-dihydro-3H-pyrazol-3-one (1-Phenyl-1H-pyrazol-3-ol) (1)
In the solid state an unambiguous determination of structure and, thus, a safe discrimination between OH and NH-form is possible. In view of this fact crystals of 1-phenyl-1,2-dihydro-3Hpyrazol-3-one (1-phenyl-1H-pyrazol-3-ol) (1)-obtained by crystallization from ethanol/water-were subjected to X-ray structure analysis. It turned out that this compound is present in the 1H-pyrazol-3-ol form constituting dimeric units connected by two identical intermolecular hydrogen bonds ( Figure 2). In principle, the formation of similar dimeric structures would be also possible by combination of two identical NH-isomers establishing two intermolecular hydrogen bonds between C=O and the NH of the second molecule. However, the electron density map clearly shows the position of the hydrogen atom at the oxygen and thus excludes the latter alternative ( Figure 2, further details can be found in the Experimental Section).  2.1. X-ray Analysis of 1-Phenyl-1,2-dihydro-3H-pyrazol-3-one (1-Phenyl-1H-pyrazol-3-ol) (1) In the solid state an unambiguous determination of structure and, thus, a safe discrimination between OH and NH-form is possible. In view of this fact crystals of 1-phenyl-1,2-dihydro-3Hpyrazol-3-one (1-phenyl-1H-pyrazol-3-ol) (1)-obtained by crystallization from ethanol/water-were subjected to X-ray structure analysis. It turned out that this compound is present in the 1H-pyrazol-3-ol form constituting dimeric units connected by two identical intermolecular hydrogen bonds ( Figure 2). In principle, the formation of similar dimeric structures would be also possible by combination of two identical NH-isomers establishing two intermolecular hydrogen bonds between C=O and the NH of the second molecule. However, the electron density map clearly shows the position of the hydrogen atom at the oxygen and thus excludes the latter alternative ( Figure 2, further details can be found in the Experimental Section).

Results and Discussion
In principle, for the title compounds, two tautomeric forms are possible, namely the OH-form and the NH-form ( Figure 1). In Chemical Abstracts such compounds carrying an alkyl or a (hetero)aryl substituent at the pyrazole nitrogen atom are always listed as 3H-pyrazol-3-ones. Hence, as many authors prefer the latter denomination, in the course of this study we investigated which tautomer is really relevant in a solid state and, especially, in solution.

X-ray Analysis of 1-
In the solid state an unambiguous determination of structure and, thus, a safe discrimination between OH and NH-form is possible. In view of this fact crystals of 1-phenyl-1,2-dihydro-3Hpyrazol-3-one (1-phenyl-1H-pyrazol-3-ol) (1)-obtained by crystallization from ethanol/water-were subjected to X-ray structure analysis. It turned out that this compound is present in the 1H-pyrazol-3-ol form constituting dimeric units connected by two identical intermolecular hydrogen bonds ( Figure 2). In principle, the formation of similar dimeric structures would be also possible by combination of two identical NH-isomers establishing two intermolecular hydrogen bonds between C=O and the NH of the second molecule. However, the electron density map clearly shows the position of the hydrogen atom at the oxygen and thus excludes the latter alternative ( Figure 2, further details can be found in the Experimental Section).

Solid State NMR (SSNMR) of 1
The same material as used for the X-ray analysis was subjected to solid state NMR (CP/MAS). As the X-ray analysis revealed the presence of the 1H-pyrazol-3-ol form, the solid state NMR spectra also should exclusively origin from this species. Here, due to its simplicity the 15 N-NMR spectrum is particularly valuable, showing the "pyridine-like" pyrazole N-atom (N-2) at 243.1 ppm, whereas the "pyrrole-like" N-atom (N-1) is resonating at 192.6 ppm referenced against external 15 NH 4 Cl (39.3 ppm with respect to liquid NH 3 ) (Figure 3). The distinct chemical shift difference of N-1 compared to N-2 (∆δ = 50.5 ppm) clearly reflects the fact that the two nitrogen atoms are of different types.

Solid State NMR (SSNMR) of 1
The same material as used for the X-ray analysis was subjected to solid state NMR (CP/MAS). As the X-ray analysis revealed the presence of the 1H-pyrazol-3-ol form, the solid state NMR spectra also should exclusively origin from this species. Here, due to its simplicity the 15 N-NMR spectrum is particularly valuable, showing the "pyridine-like" pyrazole N-atom (N-2) at 243.1 ppm, whereas the "pyrrole-like" N-atom (N-1) is resonating at 192.6 ppm (referenced against external 15 NH4Cl (39.3 ppm with respect to liquid NH3) ( Figure 3). The distinct chemical shift difference of N-1 compared to N-2 (Δδ = 50.5 ppm) clearly reflects the fact that the two nitrogen atoms are of different types. In addition, in 15 N-NQS (non-quaternary suppression) experiments with different pre-scan dephasing delays (20 µs, 100 µs, 200 µs) the intensities of both 15 N-NMR resonances remained constant. This behaviour suggests that the two nitrogen atoms are of the same type regarding their protonation status what is only the case for the OH isomer.

NMR Spectra in Solution
Whereas in the solid state, an unambiguous determination of individual tautomeric forms is smoothly possible by X-ray structure analysis, the situation in solution is much more complex. Here, depending on a plethora of different influencing variables, tautomeric equilibria with the simultaneous presence of several tautomeric forms are possible, whereas time-averaged signals are obtained in case of fast exchange. Amongst the appropriate methods for investigating such tautomeric equilibria in solution NMR spectroscopic methods play a prominent role [8,9,[18][19][20]. A frequently used concept is the comparison of the data obtained in solution with those of "fixed" derivatives (representing the individual "frozen" tautomeric forms) or with the data of the individual tautomeric forms obtained from solid state NMR experiments. Although this approach comes along with some difficulties (i.e., estimating the difference between the tautomer and the model compound) in many cases fairly good results can be obtained, particularly when one tautomeric form is strongly dominating. However, in cases when several forms are present to a significant extent the precise determination of the percentage composition by interpolation is difficult.
As compound 1 is present as 1H-pyrazol-3-ol in the solid state, a comparison of the crucial SSNMR chemical shifts with those in solution should provide valuable information. As outlined in Figure 4, the 13 C and the 15 N chemical shifts at the pyrazole nucleus show a high degree of accordance This behaviour suggests that the two nitrogen atoms are of the same type regarding their protonation status what is only the case for the OH isomer.

NMR Spectra in Solution
Whereas in the solid state, an unambiguous determination of individual tautomeric forms is smoothly possible by X-ray structure analysis, the situation in solution is much more complex. Here, depending on a plethora of different influencing variables, tautomeric equilibria with the simultaneous presence of several tautomeric forms are possible, whereas time-averaged signals are obtained in case of fast exchange. Amongst the appropriate methods for investigating such tautomeric equilibria in solution NMR spectroscopic methods play a prominent role [8,9,[18][19][20]. A frequently used concept is the comparison of the data obtained in solution with those of "fixed" derivatives (representing the individual "frozen" tautomeric forms) or with the data of the individual tautomeric forms obtained from solid state NMR experiments. Although this approach comes along with some difficulties (i.e. estimating the difference between the tautomer and the model compound) in many cases fairly good results can be obtained, particularly when one tautomeric form is strongly dominating. However, in cases when several forms are present to a significant extent the precise determination of the percentage composition by interpolation is difficult.
As compound 1 is present as 1H-pyrazol-3-ol in the solid state, a comparison of the crucial SSNMR chemical shifts with those in solution should provide valuable information. As outlined in Figure 4, the 13 C and the 15 N chemical shifts at the pyrazole nucleus show a high degree of accordance between the solid state and those in CDCl 3 or in C 6 D 6 solution which leads to the conclusion that the 3-hydroxy form is far dominating in these solvents.
Molecules 2018, 23, 129 4 of 14 between the solid state and those in CDCl3 or in C6D6 solution which leads to the conclusion that the 3-hydroxy form is far dominating in these solvents. In DMSO-d6 solution it is noticeable that the signal of pyrazole N-2 is clearly shifted downfield compared to the recordings in CDCl3 or C6D6. A possible explanation for this phenomenon is the fact that in the latter nonpolar solvents 1 is obviously present as a dimer of 1H-pyrazol-3-ols (like in the solid state) and, thus, the pyrazole N-2 atom is involved in an intramolecular hydrogen bond, whereas-in contrast-in the strong acceptor solvent DMSO-d6 these intermolecular hydrogen bonds are broken and now monomers are dominating. It is well-known that involvement of a nitrogen's lone-pair in hydrogen bonds (or-to a larger extent-oxidation, alkylation, or complexation) leads to a marked upfield shift of the corresponding 15 N resonance [21][22][23]. In 3-methoxy-1-phenyl-1Hpyrazole (2) ( Figure 5) the above mentioned dimerization and, thus, participation of pyrazole N-2 into intermolecular hydrogen bonding is not possible, what is reflected by a larger chemical shift of the latter. Hence, δ(N-2) (261.7 ppm) is now comparable to the value in DMSO-d6 (262.1 ppm), whereas δ(N-1) in compound 2 (195.6 ppm) and in 1 in different solvents (191.7-194.5 ppm) is very similar. Hence, it can be concluded that 1 is also present as 1H-pyrazol-3-ol in DMSO-d6 solution, however, not in the dimeric form stabilized by intermolecular hydrogen bonds.
In addition, employing the concept of the fixed derivatives we compared the 1 H-, 13 C-and 15 N-NMR chemical shifts, as well as characteristic spin coupling constants of 1 with its O-methyl (2) and N-methyl derivative (3), the "fixed" OH-and NH-forms, respectively. From the data given in Figure 5 the above conclusions are confirmed, namely that compound 1, which in principle is capable of prototropic tautomerism, is predominantly existing as OH-isomer in CDCl3 solution. This is supported by the fact that the 1 H-, 13 C-, and 15 N-NMR chemical shifts of 1 In DMSO-d 6 solution it is noticeable that the signal of pyrazole N-2 is clearly shifted downfield compared to the recordings in CDCl 3 or C 6 D 6 . A possible explanation for this phenomenon is the fact that in the latter nonpolar solvents 1 is obviously present as a dimer of 1H-pyrazol-3-ols (like in the solid state) and, thus, the pyrazole N-2 atom is involved in an intramolecular hydrogen bond, whereas-in contrast-in the strong acceptor solvent DMSO-d 6 these intermolecular hydrogen bonds are broken and now monomers are dominating. It is well-known that involvement of a nitrogen's lone-pair in hydrogen bonds (or-to a larger extent-oxidation, alkylation, or complexation) leads to a marked upfield shift of the corresponding 15 N resonance [21][22][23]. In 3-methoxy-1-phenyl-1H-pyrazole (2) ( Figure 5) the above mentioned dimerization and, thus, participation of pyrazole N-2 into intermolecular hydrogen bonding is not possible, what is reflected by a larger chemical shift of the latter. Hence, δ(N-2) (261.7 ppm) is now comparable to the value in DMSO-d 6 (262.1 ppm), whereas δ(N-1) in compound 2 (195.6 ppm) and in 1 in different solvents (191.7-194.5 ppm) is very similar. Hence, it can be concluded that 1 is also present as 1H-pyrazol-3-ol in DMSO-d 6 solution, however, not in the dimeric form stabilized by intermolecular hydrogen bonds.
In addition, employing the concept of the fixed derivatives we compared the 1 H-, 13 C-and 15 N-NMR chemical shifts, as well as characteristic spin coupling constants of 1 with its O-methyl (2) and N-methyl derivative (3), the "fixed" OH-and NH-forms, respectively. between the solid state and those in CDCl3 or in C6D6 solution which leads to the conclusion that the 3-hydroxy form is far dominating in these solvents. In DMSO-d6 solution it is noticeable that the signal of pyrazole N-2 is clearly shifted downfield compared to the recordings in CDCl3 or C6D6. A possible explanation for this phenomenon is the fact that in the latter nonpolar solvents 1 is obviously present as a dimer of 1H-pyrazol-3-ols (like in the solid state) and, thus, the pyrazole N-2 atom is involved in an intramolecular hydrogen bond, whereas-in contrast-in the strong acceptor solvent DMSO-d6 these intermolecular hydrogen bonds are broken and now monomers are dominating. It is well-known that involvement of a nitrogen's lone-pair in hydrogen bonds (or-to a larger extent-oxidation, alkylation, or complexation) leads to a marked upfield shift of the corresponding 15 N resonance [21][22][23]. In 3-methoxy-1-phenyl-1Hpyrazole (2) ( Figure 5) the above mentioned dimerization and, thus, participation of pyrazole N-2 into intermolecular hydrogen bonding is not possible, what is reflected by a larger chemical shift of the latter. Hence, δ(N-2) (261.7 ppm) is now comparable to the value in DMSO-d6 (262.1 ppm), whereas δ(N-1) in compound 2 (195.6 ppm) and in 1 in different solvents (191.7-194.5 ppm) is very similar. Hence, it can be concluded that 1 is also present as 1H-pyrazol-3-ol in DMSO-d6 solution, however, not in the dimeric form stabilized by intermolecular hydrogen bonds.
In addition, employing the concept of the fixed derivatives we compared the 1 H-, 13 C-and 15 N-NMR chemical shifts, as well as characteristic spin coupling constants of 1 with its O-methyl (2) and N-methyl derivative (3), the "fixed" OH-and NH-forms, respectively. From the data given in Figure 5 the above conclusions are confirmed, namely that compound 1, which in principle is capable of prototropic tautomerism, is predominantly existing as OH-isomer in CDCl3 solution. This is supported by the fact that the 1 H-, 13 C-, and 15 N-NMR chemical shifts of 1 From the data given in Figure 5 the above conclusions are confirmed, namely that compound 1, which in principle is capable of prototropic tautomerism, is predominantly existing as OH-isomer in CDCl 3 solution. This is supported by the fact that the 1 H-, 13 C-, and 15 N-NMR chemical shifts of 1 resemble closely to those of the fixed O-methyl congener 2. Especially the 15 N-chemical shifts are valuable indicators, as the N-methyl derivative 3 exhibits two sp 3 -type nitrogen atoms with similar 15 N-NMR chemical shifts, whereas in 1 and 2 the large chemical shift differences between the two nitrogen atoms in the corresponding molecule hint to different types of N-atoms (sp 3 and sp 2 ). For comparison only, with 1-phenylpyrazolidin-3-one, which formally is the dihydro derivative of the NH form of 1, we found 105.1 ppm for N-1 and 151.8 ppm for N-2 in DMSO-d 6 solution. Moreover, 1 and 2 show equal sizes of the vicinal 3 J(H4,H5) coupling constant at the pyrazole nucleus (2.6 Hz), whereas in 3 this coupling is considerably larger (3.6 Hz) ( Figure 5). An additional difference consists in the 13 C-NMR chemical shift of Ph C-2/6 which is akin in 1 (118.6 ppm) and 2 (117.8 ppm). In contrast, 3 shows a markedly larger chemical shift for these carbon atoms (123.0 ppm) which can be attributed to some distorsion of phenyl and pyrazole ring obviously induced by the sterical hindrance of the N-methyl group [24,25]. Additionally, the large differences in 13 C-NMR chemical shifts of pyrazole C-5 (1: 129.1 ppm, 2: 127.7 ppm) in comparison to that of 3 (142.3 ppm) provide an extra confirmation.
Moreover, 1 H-, 13 C-, and 15 N-NMR spectra of compound 1 were additionally taken from C 6 D 6 , DMSO-d 6 and CD 3 OD solutions. As all the significant criteria discussed above were almost similar to those in CDCl 3 solution, it is reasoned that, also in these solvents, the hydroxy form is far more predominant. The regarding data are presented in the Experimental Section.
In the following, congeners of 1 carrying a halogen atom or an acyl moiety at pyrazole C-4 were investigated (compounds 4-8). Again, all these species clearly exist as pyrazol-3-ols in CDCl 3 , as well as in DMSO-d 6 solution based on the 13 C-and 15 N-NMR chemical shift considerations outlined above. Regarding the 4-bromo derivative 5 a "fixed" 3-methoxy derivative 9 has been described already by us, whose data resemble the "free" 1H-pyrazol-3-ol 5 [26]. The same is the case for the pair 7 and 10 ( Figure 6). When switching from CDCl 3 to DMSO-d 6 solution, the 13 C chemical shift of the carbonyl C-atom in 7 receives an upfield shift of 4.0 ppm (195.7 ppm → 191.7 ppm) which hints to the existence of an intramolecular hydrogen bond-but now-between carbonyl O-atom and OH proton in CDCl 3 solution, which is broken in the strong acceptor solvent DMSO-d 6 [27]. In principle, also 3-O-acyl derivatives of 1 (compounds 11-13) can be regarded as fixed 3-OH derivatives, although the 3-O-acyl rest seems to be less comparable to OH than an OCH 3 group. However, despite larger differences regarding the 13 C chemical shifts of the pyrazole C-atoms between 1 and 11-13 appear, the data of the phenyl ring closely resemble as well as the 3 J(H4,H5) coupling constant at the pyrazole nucleus, which for 11-13 is the same as in 1 (2.5-2.6 Hz).  5). An additional difference consists in the 13 C-NMR chemical shift of Ph C-2/6 which is akin in 1 (118.6 ppm) and 2 (117.8 ppm). In contrast, 3 shows a markedly larger chemical shift for these carbon atoms (123.0 ppm) which can be attributed to some distorsion of phenyl and pyrazole ring obviously induced by the sterical hindrance of the N-methyl group [24,25]. Additionally, the large differences in 13 C-NMR chemical shifts of pyrazole C-5 (1: 129.1 ppm, 2: 127.7 ppm) in comparison to that of 3 (142.3 ppm) provide an extra confirmation. Moreover, 1 H-, 13 C-, and 15 N-NMR spectra of compound 1 were additionally taken from C6D6, DMSO-d6 and CD3OD solutions. As all the significant criteria discussed above were almost similar to those in CDCl3 solution, it is reasoned that, also in these solvents, the hydroxy form is far more predominant. The regarding data are presented in the Experimental Section.
In the following, congeners of 1 carrying a halogen atom or an acyl moiety at pyrazole C-4 were investigated (compounds 4-8). Again, all these species clearly exist as pyrazol-3-ols in CDCl3, as well as in DMSO-d6 solution based on the 13 C-and 15 N-NMR chemical shift considerations outlined above. Regarding the 4-bromo derivative 5 a "fixed" 3-methoxy derivative 9 has been described already by us, whose data resemble the "free" 1H-pyrazol-3-ol 5 [26]. The same is the case for the pair 7 and 10 ( Figure 6). When switching from CDCl3 to DMSO-d6 solution, the 13 C chemical shift of the carbonyl C-atom in 7 receives an upfield shift of 4.0 ppm (195.7 ppm → 191.7 ppm) which hints to the existence of an intramolecular hydrogen bond-but now-between carbonyl O-atom and OH proton in CDCl3 solution, which is broken in the strong acceptor solvent DMSO-d6 [27]. In principle, also 3-O-acyl derivatives of 1 (compounds 11-13) can be regarded as fixed 3-OH derivatives, although the 3-O-acyl rest seems to be less comparable to OH than an OCH3 group. However, despite larger differences regarding the 13 C chemical shifts of the pyrazole C-atoms between 1 and 11-13 appear, the data of the phenyl ring closely resemble as well as the 3 J(H4,H5) coupling constant at the pyrazole nucleus, which for 11-13 is the same as in 1 (2.5-2.6 Hz).   The triple 14-16 provides another example of comparing a free 1H-pyrazol-3-ol (14) with the corresponding O-alkyl (15) and N-alkyl derivative (16), respectively. Again, the selected data depicted in Figure 7 clearly hint that 14 predominantly exists as OH-isomer and not as pyrazol-3-one. The triple 14-16 provides another example of comparing a free 1H-pyrazol-3-ol (14) with the corresponding O-alkyl (15) and N-alkyl derivative (16), respectively. Again, the selected data depicted in Figure 7 clearly hint that 14 predominantly exists as OH-isomer and not as pyrazol-3-one. In addition, we investigated 1H-pyrazol-3-ols carrying a methyl (17) and a benzyl substituent (18), respectively, at pyrazole N-1. From the relevant data of these compounds, depicted in Figure 8, the conclusion can be drawn, that also 17 and 18 exist in the 3-hydroxy form in CDCl3, DMSO-d6, and C6D6 solution. Again, as found with 1-phenyl-1H-pyrazol-3-ol 1 and compounds 17, 18, the markedly larger chemical shifts of pyrazole N-2 in DMSO-d6 compared to those in CDCl3 or C6D6 hint to the absence of dimers stabilized by intermolecular hydrogen bonds in this solvent, what is supported by a distinctly smaller 1 H-NMR chemical shift of the OH-proton in DMSO-d6 ( Figure 8).

General Information
Melting points were determined on a Büchi M-565 melting point apparatus (Büchi Labortechnik AG, Flawil, Switzerland) and are uncorrected. IR (infrared) spectra (KBr pellets) were recorded on a Bruker Tensor 27 spectrometer (Bruker Optik GmbH, Ettlingen, Germany) and are reported in wave In addition, we investigated 1H-pyrazol-3-ols carrying a methyl (17) and a benzyl substituent (18), respectively, at pyrazole N-1. From the relevant data of these compounds, depicted in Figure 8, the conclusion can be drawn, that also 17 and 18 exist in the 3-hydroxy form in CDCl 3 , DMSO-d 6 , and C 6 D 6 solution. Again, as found with 1-phenyl-1H-pyrazol-3-ol 1 and compounds 17, 18, the markedly larger chemical shifts of pyrazole N-2 in DMSO-d 6 compared to those in CDCl 3 or C 6 D 6 hint to the absence of dimers stabilized by intermolecular hydrogen bonds in this solvent, what is supported by a distinctly smaller 1 H-NMR chemical shift of the OH-proton in DMSO-d 6 ( Figure 8). The triple 14-16 provides another example of comparing a free 1H-pyrazol-3-ol (14) with the corresponding O-alkyl (15) and N-alkyl derivative (16), respectively. Again, the selected data depicted in Figure 7 clearly hint that 14 predominantly exists as OH-isomer and not as pyrazol-3-one. In addition, we investigated 1H-pyrazol-3-ols carrying a methyl (17) and a benzyl substituent (18), respectively, at pyrazole N-1. From the relevant data of these compounds, depicted in Figure 8, the conclusion can be drawn, that also 17 and 18 exist in the 3-hydroxy form in CDCl3, DMSO-d6, and C6D6 solution. Again, as found with 1-phenyl-1H-pyrazol-3-ol 1 and compounds 17, 18, the markedly larger chemical shifts of pyrazole N-2 in DMSO-d6 compared to those in CDCl3 or C6D6 hint to the absence of dimers stabilized by intermolecular hydrogen bonds in this solvent, what is supported by a distinctly smaller 1 H-NMR chemical shift of the OH-proton in DMSO-d6 ( Figure 8).

General Information
Melting points were determined on a Büchi M-565 melting point apparatus (Büchi Labortechnik AG, Flawil, Switzerland) and are uncorrected. IR (infrared) spectra (KBr pellets) were recorded on a Bruker Tensor 27 spectrometer (Bruker Optik GmbH, Ettlingen, Germany) and are reported in wave

General Information
Melting points were determined on a Büchi M-565 melting point apparatus (Büchi Labortechnik AG, Flawil, Switzerland) and are uncorrected. IR (infrared) spectra (KBr pellets) were recorded on a Bruker Tensor 27 spectrometer (Bruker Optik GmbH, Ettlingen, Germany) and are reported in wave numbers (cm −1 ). High-resolution ESI-TOF mass spectra were measured on a Bruker maXis spectrometer (Bruker Daltonik GmbH, Bremen, Germany). Elemental analyses were performed at the Microanalytical Laboratory, University of Vienna. 1 H and 13 C-NMR spectra were recorded on a Bruker Avance 500 spectrometer (500. 13 6 ), δ 7.16 ppm ( 1 H in C 6 D 6 ), δ 3.31 ppm ( 1 H in CD 3 OD), δ (δ 77.0 ppm ( 13 C in CDCl 3 ), δ 39.5 ppm ( 13 C in DMSO-d 6 ), δ 128.06 ppm ( 13 C in C 6 D 6 ) and δ 49.00 ppm ( 13 C in CD 3 OD). The digital resolutions were 0.20 Hz/data point in the 1 H and 0.33 Hz/data point in the 13 C-NMR spectra. 15 N-NMR spectra were obtained on Bruker Avance 500 (50.69 MHz) and Bruker Avance III 400 (40.56 MHz) spectrometers (both equipped with "direct" detection broadband z-gradient observe probes) or on a Bruker Avance III 700 (70.96 MHz) equipped with a 5 mm TCI 1 H-13 C/ 15 N/D z-gradient cryoprobe, and were measured against external nitromethane (coaxial capillary) and recalculated to liquid ammonia. Solid-state NMR spectra (CP/MAS, MAS: 10 kHz) were recorded on a Bruker Avance III 500 instrument with a broadband MAS-probe for 3.2 mm rotors. CP contact times were 2 ms for ( 1 H, 13 C) and 3 ms for ( 1 H, 15 N). 1 H RF of 100 kHz was used for spinal64 broadband decoupling. 1 H, 13 C-HETCOR spectra were recorded using FSLG homonuclear decoupling during t 1 -evolution and mixing times of 50 µs and 200 µs. 15 N-NMR spectra were referenced to 15 NH 4 Cl and recalculated to the liquid ammonia scale (δ 15 NH 4 Cl 39.3 ppm). 13 C spectra were referenced to the methylene carbon signals of adamantane and recalculated to the TMS scale (δ 13 CH 2 38.5 ppm). 1 H chemical shifts were referenced to the NH 3 + resonance in α-Glycine and recalculated to the TMS scale (δ 15 NH 3 + 8.5 ppm).
Product yields were not optimised.

X-ray Crystal Structure Analysis
The X-ray intensity data was measured on a Bruker X8 APEXII equipped with multilayer monochromators, with a Mo K/a INCOATEC micro focus sealed tube, and a Kryoflex II cooling device. The structure was solved by direct methods and refined by full-matrix least-squares techniques. Non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen located at O1 was refined without any restraints or constraints. All other hydrogen atoms were inserted at calculated positions and refined with a riding model. The following software was used: Frame integration, Bruker SAINT software package [35] using a narrow-frame algorithm, Absorption correction, SADABS [36], structure solution, SHELXL-2013 [37], refinement, SHELXL-2013 [37], OLEX2 [38], SHELXLE [39], molecular diagrams, OLEX2 [38]. Experimental data and CCDC-Code [40] can be found in Table 1. Crystal data, data collection parameters, and structure refinement details are given in Tables 2 and 3. Molecular structures in "Ortep View" are displayed in Figures 2 and 9. Bond length details are given in Table 4.  Figure 9. Asymmetric unit of 1, drawn with 50% displacement ellipsoids. Figure 9. Asymmetric unit of 1, drawn with 50% displacement ellipsoids.  The difference electron density map ( Figure 2) gives very detailed information about the position of the searched hydrogen atom. The electron density distance of the latter to N2 -as displayed in Figure 2-excludes the possibility of a single bond to the nitrogen for the concerning hydrogen atom. Additionally, its distance to O1 for the electron density proves the position of the hydrogen is located at the oxygen. The free refinement of the hydrogen position without using any restraints or constraints clears all doubts about the non-tautomeric geometry of the molecule in the solid state. Furthermore, at least two very close molecules [41,42] were already measured and interpreted in the same way. In these samples also two identical hydrogen bonds build up molecule pairs because of symmetry reasons.
Pyrazole is well known in crystallography and its different bonds are well characterized by the Handbook of Chemistry and Physics [43]. Table 4 compares the results from compound 1 with corresponding bonds in pyrazole. In detail the double bond N2=C7 in 1 is with 1.329 Å identical to the unweighted mean of the table value for pyrazoles N2=C3 from the Handbook of Chemistry and Physics. The single bond C7-O1 with 1.339 Å is also in strong correlation to expected values like in enols, 1.333 Å, given. Double bond values like in lactams, 1.240 Å, and benzoquinones, 1.222 Å are in contrast to the measured 1.339 Å too small. Finally we proved the position of the searched hydrogen at O1 and we can exclude the NH form in the solid crystalline state. d is the unweighted mean in Å of all the values for that bond length found in the sample; m is the median in Å of all values; σ is the standard deviation in the sample; q l is the lower quartile for the sample; q u is the upper quartile for the sample.