A theoretical study of the valence tautomerism of 1H-pyrazolium-4-olates (X = O) and related compounds (X = S, Se, NH): Relative stabilities, protonation effects and tautomerization barriers

doi:10.21203/rs.3.rs-1526069/v1

The valence isomerism of a series of heterocyclic mesomeric betaines (HMBs) belonging to class 5, called pseudo-semi-conjugated HMBs, have been studied theoretically both the neutral and the protonated species. These HMBs are 1H-pyrazol-2-ium-4-olates and related compounds where the oxygen atom has been replaced by S, Se atoms and an NH group. The main conclusion of the present work is that the ring/open valence tautomerism is possible both for neutral and protonated although it has never been observed experimentally.

Pyrazoles

Pyrazolium salts

Mesoionic rings

Heterocyclic Mesomeric Betaines

Bis(arylimino)pentan-3-ones

To facilitate the following discussion, the numbering of the studied structures is reported in Fig. 1; actually, for protonated molecules the number of possible structures is larger but this not alter this introduction.

Mesoionic compounds occupy a singular place amongst the huge field of heterocyclic chemistry. Their structure and complex system of classification is due to Baker [¹,²], Ollis [³,⁴], Potts [⁵,⁶,⁷], and Oziminski [⁸,⁹], amongst others, although the main contribution is that of Ramsden [¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶]. Ramsden classification is reported in Fig. 2.

In 1988 [¹⁷] we used the SOS (Simulated Organic Synthesis) program due to Barone and Chanon [¹⁸,¹⁹,²⁰] to find new ways to prepare pyrazoles, different from the classical ones [²¹,²²,²³,²⁴,²⁵]. In that paper we reported a new method and calculated within the MNDO [²⁶] method the minima and TS of the reaction A1 ⇌ TS1 ⇌ B1 (Fig. 3), corresponding to the present numbering of 1(O)c, 1(O)TS and 1(O)c.

According to the MNDO calculations it appeared that 1,3-diminopropan-2-one B1 was much more stable than 1H-pyrazol-2-ium-4-olate (also called 4-hydroxy-1H-pyrazolium inner salt); the barrier TS1 between A1 and the cyclic compound, B1, was very high.

In 2019 Ramsden and Oziminski studied a series of closely related reactions based on MP2 ab initio calculations [15]; in Fig. 3 we have reported one of their examples concerning their A2 compounds [our n = 2(O) compounds of Fig. 1]. Their results concerning the general solvation by water, PCM model [²⁷], and the specific solvation by a water molecule show that the differences in stability can be strongly modified, to the point that A2·H₂O (PCM) is more stable than B2·H₂O (PCM). The corresponding TSs were not calculated.

A search for 1H-pyrazol-2-ium-4-olates and 2,4-bis(imino)pentan-3-ones in Scifinder [²⁸] afforded compounds 3 to 17 gathered in Fig. 4 and Fig. 5 together with their CAS registered numbers. The simplest compound of Fig. 5 (11, R = H) is only a calculated compound [15].

Note that compounds 8 (Fig. 4) and 11 (Fig. 5) are valence tautomers. No experimental synthetic method that involve the creation of a N-N bond has any relationship with the procedure of Fig. 3; the reported methods correspond to oxidative synthesis using nitriles and creating simultaneously N-N and C-N bonds [²⁹,³⁰,³¹]. The synthesis of 1H-pyrazol-2-ium-4-olates uses hydrazines as starting materials [³²,³³,³⁴].

Concerning the protonated species, only one structure has been reported and always only in patents [³⁵], the perchlorate of 4-hydroxy-1,2-dimethyl-3,5-diphenyl-1H-pyrazol-2-ium 7H⁺ [R¹ = R² = Me, R³ = R⁵ = Ph, 4(O)cH⁺] (Fig. 6); this compound was not prepared by protonation of 7 but by methylation of the corresponding N-methyl pyrazole [34]. These 4-hydroxy-pyrazolium salts were proposed by Begtrup in 1970 as intermediates but were not isolated [³⁶]

The geometry of the molecules has been optimized with the M06-2x [³⁷] functional and the aug-cc-pVDZ basis set [³⁸] in gas phase and with the continuum solvation model PCM [³⁹] using the water parameters. Frequency calculations at the same computational levels were carried out to verify that the structures obtained correspond to energetic minima (no imaginary frequencies) or to transition states (only one imaginary frequency). The IRC of some of the reactions studies have been explored. All these calculations have been carried out using the Gaussian-16 package [⁴⁰].

Additional calculations at domain based local pair natural orbital coupled cluster with single-, double- and perturbative triple excitation level [⁴¹], DLPNO-CCSD(T)/aug-cc-pVDZ, have been performed with the Orca5 program [⁴²].

The electron density of the molecules has been analyzed with the quantum theory of atoms in molecules (QTAIM) [45] using the AIMAll program [⁴³]. The presence of (3,-1) critical points associated to interatomic interactions allows to classify such contact in covalent and non-covalent.

The analysis of the IRC profile has been carried out with the Eyring program [⁴⁴].

Neutral molecules, comparison between n(X)c and n(X)o

Energetic comparisons between closed (pyrazolium-olates and related compounds) and open structures [2,4-bis(arylimino)pentan-3-ones and related compounds] are reported in Fig. 7 and Table 1 and the geometries are listed in the Supplementary data.

Table 1 Energies (in kJ·mol^–1) of the valence tautomerization between n(X)c and n(X)o. Relative energies to the open structures n(X)o.

R¹ = R²	R³ = R⁵	X	Code c	Gas phase M06-2x		Gas Phase-CCSD(T)		PCM(H₂O)-M06-2x
				n(X)c	TS	n(X)c	TS	n(X)c	TS
H	H	O	1(O)c	68.1	179.5	107.6	190.9	3.1	168.1
H	H	S	1(S)c	19.3	155.8	67.8	168.2	–75.3	138.8
H	H	Se	1(Se)c	–4.1	142.7	56.7	163.9	–102.2	125.6
H	H	NH	1(NH)c	157.7	245.6	195.6	255.6	90.3	227.8

CH₃	H	O	2(O)c	61.7	173.8	93.2	181.0	–2.7	156.9
CH₃	H	S	2(S)c	7.3	146.8	47.2	155.6	–86.5	122.3
CH₃	H	Se	2(Se)c	–16.6	133.2	35.3	152.9	–113.6	108.3
CH₃	H	NH	2(NH)c	144.9	235.5	176.6	240.8	79.6	214.3

CH₃	CH₃	O	3(O)c	46.3	156.0	74.4	159.8	–6.7	144.7
CH₃	CH₃	S	3(S)c	–30.8	112.5	10.0	119.3	–108.5	99.4
CH₃	CH₃	Se	3(Se)c	–57.9	97.8	–6.1	111.6	–137.9	85.3
CH₃	CH₃	NH	3(NH)c	117.4	204.3	146.1	208.7	66.0	190.8

CH₃	C₆H₅	O	4(O)c	15.8	141.2	53.1	150.4	–28.2	128.9
CH₃	C₆H₅	S	4(S)c	–37.5	118.2	9.2	126.3	–106.2	102.5
CH₃	C₆H₅	Se	4(Se)c	–61.6	105.5	–3.4	120.2	-–132.9	90.4
CH₃	C₆H₅	NH	4(NH)c	72.7	175.7	111.9	188.4	41.0	173.0

C₆H₅	CH₃	O	5(O)c	64.8	160.1	78.0	153.8	23.8	188.0
C₆H₅	CH₃	S	5(S)c	–7.1	108.8	17.19	106.9	–66.1	97.1
C₆H₅	CH₃	Se	5(Se)c	–33.8	91.3	3.4	101.6	–94.4	80.7
C₆H₅	CH₃	NH	5(NH)c	134.6	205.6	146.5	196.8	96.2	193.7

Several of the 20 values of Table 1 columns are linearly related and the simple linear regression equations, y = a + bx, obtained from these values are reported in Table 2.

Table 2 Statistical analyses of Table 1 data.

y	x	method	intercept a	slope b	R²	Eq.
Cyclic minimum	TS	M06-2x	133.6±2.8	0.63±0.04	0.94	1
Cyclic minimum	TS	CCSD(T)	114.1±3.5	0.68±0.04	0.94	2
Cyclic minimum	TS	PCM(H₂O)	159.6±3.8	0.54±0.05	0.88	3
Cyclic minimum	Cyclic minimum	CCSD(T)/M06-2x	41.8±2.7	0.88±0.04	0.97	4
Cyclic minimum	Cyclic minimum	PCM(H₂O)/M06-2x	–71.4±4.9	1.14±0.07	0.95	5
TS	TS	CCSD(T)/M06-2x	15.3±6.2	0.95±0.04	0.97	6
TS	TS	PCM(H₂O)/M06-2x	–11.4±9.2	0.99±0.06	0.94	7

The square correlation coefficients are not very good and that is also apparent in the relatively large errors of the coefficients. The first three equations are a consequence of the Hammond postulate, which states that the transition state resembles the structure of the nearest stable species, in this case the cyclic compounds. Equation 4 corresponds to the fact that CCSD(T) calculations increase considerably the differences in stability between open and cyclic structures compared with M06-2x ones. General water solvation stabilizes the mesoionic compounds compared to the neutral open forms due to their high dipole moments (see Supplementary data), equation 5. Equations 6 and 7 have slopes near 1.00; thus, the intercepts can be directly compared; the values are relatively small, +15.3 for the CCSD(T)/M06-2x pair, positive like in equation 4 but lower, and –11.4 for the PCM(H₂O)/M06-2x comparison, again similar and lower to that of equation 5.

The solvent effect produces an elongation of the N-N distance in the TS between 0.03 and 0.12 Å (see Supplementary data, Table S2).

A clear relationship is found between the energetic values of the TS structures in gas phase and their N-N distances as can be observed in Fig. 8. The shorter the N-N distance in the TS, the larger is the barrier. Linear correlations between these two parameters show R² values larger than 0.98 for each family of compounds (1-5) and a R² value of 0.92 considering all the points together. A linear correlation is only the simplest model, for instance, a second order polynomial affords an R²= 0.936. A similar relationship is obtained when the values in PCM(H₂O) are considered (see Supplementary data, Fig. S1):

The analysis of the electron density of the structures within the QTAIM methodology [⁴⁵] shows the presence of a bcp between the nitrogens involved in the reaction in the closed and TS structures but not in the open ones. The ρ_BCP in the closed structures range between 0.387 and 0.331 au with negative values of ∇²ρ_BCP and H_BCP that are large. In contrast, the ρ_BCP for the TS structures range between 0.175 and 0.071 au with positive values of ∇²ρ_BCP and negative values of H_BCP as indication of partial covalent character of this contact in the TS [⁴⁶].

The IRCs (Intrinsic Reaction Coordinate) of some selected transformations have been calculated. Using these values, the reaction force profile along the reaction has been derived. One example is given in Fig. 9. The reaction force divides the IRC in three regions: i) for the reactant to the minimum (ξ₁) which is associated to structural reordering, ii) from ξ₁to the maximum ξ₂where the electronic variation dominates and iii) between ξ₂and the products thatcorresponds to a structural relaxation. It should be noted that the TS (ξ_TS) is located between ξ₁and ξ₂allowing to divide the reaction in four regions. Table 3 shows the energetic values between the critical points in the IRC and reaction force along the reaction coordinate for some of the reactions. The structural reorderings (W1 and W4) are larger in the processes of reaching the TS from the products and the reactants than the electronic reorganization (W2 and W3). Good linear correlations (R² > 0.96) of each term with the corresponding TS have been obtained when the two families (1 and 3) are considered separately.

Table 3 Energy decomposition (kJ·mol^–1) using the reaction coordinate and the reaction force critical points.

				W1	W2	W3	W4
R¹ = R²	R³ = R⁵	X	Code c	R→ξ₁	ξ₁→ξ_TS	ξ_TS→ξ₂	ξ₂→P
H	H	O	1(O)c	59.4	51.9	–61.1	–118.4
H	H	S	1(S)c	72.8	63.6	–50.6	–105.5
H	H	Se	1(Se)c	78.7	68.2	–44.8	–97.9
H	H	NH	1(NH)c	49.0	38.9	–97.5	–48.1

CH₃	CH₃	O	3(O)c	70.7	41.0	–28.0	–145.6
CH₃	CH₃	S	3(S)c	88.7	51.0	–26.4	–120.5
CH₃	CH₃	Se	3(Se)c	96.2	53.6	–25.9	–107.5
CH₃	CH₃	NH	3(NH)c	59.4	31.4	–37.2	–173.6

Protonated molecules, comparison between n(X)cH⁺ and n(X)oH⁺/n(X)o'H⁺

Se situation for cations is much more complex than in neutral molecules because instead of two minima now they are five. In Fig. 10 are represented the five minima of protonated structures and in Table 4 the corresponding energy results.

In Table 4 are reported the same results as in Section 2.1. for protonated species but only for X = O, the remaining X groups are reported in the Supplementary data. Here the most stable is the cyclic compound and no longer the open compound like in neutral molecules. Note that open compounds a and c are related by a proton transfer, while structure d present a new N–H···N HB.

Table 4 Energies (in kJ·mol^–1) of the valence tautomerization between n(O)cH⁺ and n(O)oH⁺ (open a and open d); the TS-co correspond to the ring opening of the closed structure to yield the open compound of type a. Relative energies to the cyclic n(O)cH⁺ structures.

Code	Gas phase M06-2x			Gas Phase-CCSD(T)			PCM(H₂O)-M06-2x
	n(O)oH⁺ open a	n(O)oH⁺ open d	TS-co	n(O)oH⁺ open a	n(O)oH⁺ open d	TS-co	n(O)oH⁺ open a	n(O)H⁺ open d	TS-co
1(O)H⁺	204.0	75.1	258.2	163.4	29.1	219.0	203.5	76.0	271.0
2(O)H⁺	196.5	75.9	246.0	172.4	36.4	218.7	196.3	71.9	253.8
3(O)H⁺	209.0	60.1	247.4	185.0	29.8	220.4	205.0	61.2	253.0
4(O)H⁺	200.9	77.0	242.0	168.8	44.3	211.6	206.9	77.0	252.1
5(O)H⁺	185.9	55.1	194.6	189.6	45.7	187.7	134.7	49.0	198.2

There are five minima and four TS. TS-co corresponds to the N-N bond breaking, TS-ab to the rotation about a CC single bond, TS-bc to a intramolecular proton transfer between X and N, and TS-cd to a second rotation about a CC single bond. In the case of X = O, these barriers (from left to right, Fig. 11a) correspond to 246.0, 12.9, 7.7 and 27.3 kJ·mol^–1. The direct migration of a proton between a and d being too far away cannot take place without the assistance of water or solvent molecules or counterions.

There are two groups of structures, before proton transfer, a and b, and after proton transfer, c and d. Taking into account that some b structures have not been localized because they evolve spontaneously to type c structures, b is always more stable than a (mean = 39.6 kJ·mol^–1, extreme values 6.5 kJ·mol^–1, 4(Se)H⁺ and 55.5 kJ·mol^–1, 5(O)H⁺; even for 4(NH)H⁺ the differences is much lower, 13.0 kJ·mol^–1, respectively than the mean (see Table S5).

The different character of 4 series is probably due to a hydrogen bond between the acidic OH and the adjacent N atom (Fig. 12); the three other X⁺–H groups are less acidic, NIST gas basicity values in kJ·mol^–1 H₂O (660.0), H₂S (673.8), H₂Se (676.4) and H₃N (819.0) [⁴⁷].

After proton transfer, d is always more stable than c due to the very favorable structure for a strong hydrogen bond, pseudo-six membered ring. The mean value is small than in the preceding case (10.4 kJ·mol^–1).

Statistical analyses of Table 4 data similar to those of Table 2 are much more complicated due to the existence of four open cations. Only the correlation relating the TSs calculated by two methods was acceptable (Table 5)

Table 5 Statistical analyses of Table S5 data. The corresponding data for neutrals from Table 2 has been added for comparison.

Comp.	y	x	method	intercept a	slope b	R²	Eq.
cations	TS	TS	CCSD(T)/M06-2x	25.1±18.1	0.79±0.08	0.86	8
neutrals	TS	TS	CCSD(T)/M06-2x	15.3±6.2	0.95±0.04	0.97	6

When the four profiles of Fig. 11, illustrated for the a series, are compared it appear that those of X = O (Fig. 11a), X = S (Fig. 11b) and X = Se (Fig. 11c) are similar but that of X = NH (Fig. 11d) is different. A closer look indicated that the amino profile has two parts both similar to the other profiles but different between them (Fig. 13).

The three values of the left side (blue) and the two values of the right side (red) are correlated with the black values of the 2(O)oH⁺ series: 2(NH)oH⁺ = (1.07±0.07) 2(O)oH⁺ – (68±13) , n = 5, R² = 0.9995; The value –68 kJ·mol^–1 corresponds to the difference between both sides, the red minus the blue, and it is related to the much larger basicity of NH compared with that of O (see previously NIST data [47]).

To estimate protonation effects, i.e., the basicity of the compounds of Table 1, the latter and Table 4 and Supplementary data should be compared; some relationships could be found. For instance for the M06-2x calculations between n(X)oH⁺ type c = (188.6±3.5) – (0.63±0.04) n(X)c – (101.6±6.3) b series, = 20, R² = 0.98 (eq. 10). Equation 10 shows the energetic difference between the a (0) and the b (1) series, the b series being in average 102 kJ·mol^–1 more stable than the a series.

The TS ring opening show barriers bigger than in the neutral systems and N-N distance systematically larger (0.35 Å in average) in the protonated ones. As in the cases, of the neutral systems, linear correlations are obtained between the barriers and the interatomic N-N distances for each family of compounds with R² > 0.97.

The analysis of the electron density between both nitrogen atoms in the TS(c-o) shows a BCP like in the case of the neutral molecules. The representation of rBCP and Ñ²rBCP vs. the interatomic N-N distance for all the closed and TS(c-o) complexes (neutral and protonated) is shown in Fig. 14. The rBCP values follow an almost perfect exponential relationship with the distance in agreement with previous reports for covalent and non-covalent interactions [⁴⁸,⁴⁹,⁵⁰,⁵¹]. The figure with Ñ²rBCP shows the changing nature of the bond. Negative values are obtained for covalent N-N bonds in the closed systems while the positive values are obtained for the TSs but at the shortest distances they tend towards negative values.

The IRC analysis of the ring opening through the TS-co for molecules 1 and 3, Table 6, show that in the latter ones, 3(O)H⁺ and 3(S)H⁺, the structural reordering (W1 and W4) is larger in absolute value than the corresponding electronic reorganization (W2 and W3) in analogy to what was observed for the neutral systems. In contrast, 1(O)H⁺ and 1(S)H⁺shows a larger electronic reorganization W2 than the geometrical reordering W1.

Table 6 IRC analysis of the ring opening through the TS-co for molecules 1 and 3.

				W1	W2	W3	W4
R¹ = R²	R³ = R⁵	X	Code c	R→ξ₁	ξ₁→ξ_TS	ξ_TS→ξ₂	ξ₂→P
H	H	O	1(O)cH⁺	119.7	138.5	–21.8	–32.6
H	H	S	1(S)cH⁺	122.2	141.8	–18.0	–27.2

CH₃	CH₃	O	3(O)cH⁺	142.7	102.9	–13.8	–36.0
CH₃	CH₃	S	3(S)cH⁺	149.4	109.2	–11.3	–26.4

Comparison with geometries calculated and experimental X-ray data

Unfortunately there are no experimental geometries of 1H-pyrazol-2-ium-4-olates and related compounds (CSD) [⁵²]. However, of the structures reported in Table 5 that of compound 16 has been determined (COPLAV) [⁵³] and COPLAV01 [⁵⁴]), this compound is related to 5(O)o but the presence of two i-propyl at positions 2 and 6 of the phenyl rings should modify the conformation (Fig. 15).

The calculated structure has a symmetry axis (C₂) that is lost in the experimental structures that are different; the main difference between experimental and calculated geometries concerns one of the OCCN angles that are rotated to values (105.3 and 76.6º) much lower than the other N-phenyl group (169.6 and 176.2º) and to our calculated angles (178.0º).

Our conclusions agree with those of Ramsden and Oziminski [15] taking into account that protonation produces the same effect that hydrogen bonds with a water molecule but stronger.

Selecting for the conclusion the two experimentally studied X atoms or groups, X = O and X = NH (X = S and X = Se were not included in Fig. 16) it is possible to summarize the calculated energy of the stationary states, minima and transitions states, in Fig. 16. The values for the "open" protonated compounds correspond to "open-d".

While in the neutral molecules the open structure is the most stable, in the cations it is the opposite, the pyrazoliums are the most stable; although nobody has reported an example of ring/chain isomerization in these series it should be possible by protonation/deprotonation, i.e., by simply changing the pH. The values reported in Fig. 16 indicate that solvation by water decreases the difference in stability between valence isomers so the experiments should be carried in the gas phase (by mass spectrometry, the open structure should be transformed in the cyclic structure of the cation) or in aprotic non-dipolar solvents, like benzene or other aromatic solvents.

The isomerization barriers lie in the range 162.1-237.6 (M06-2x), 167.2-218.1 [CCSD(T)] and 157.3-245.5 kJ·mol^− 1 (PCM water). These are high barriers that correspond to slow processes, in the order of 10^− 3 to 10^− 2 s^− 1 []. Even so, a 1H-pyrazol-2-ium-4-olate like 8 = 5(O)c in an aprotic solvent (e.g. 1,4-dichlorobenzene-d₄) and heating the solution at 400 K for two weeks, or shorter in a microwave oven, should open it (the reaction can be followed by NMR).

Acknowledgements This work was carried out with financial support from the Ministerio de Ciencia, Innovación y Universidades (PGC2018-094644-B-C22 and PID2021-125207NB-C32), Comunidad de Madrid (P2018/EMT-4329). Thanks, are also given to the CTI (CSIC) for their continued computational support.

Funding Ministerio de Ciencia, Innovación y Universidades (Project PGC2018-094644-B-C2 and PID2021-125207NB-C32) and Dirección General de Investigación e Innovación de la Comunidad de Madrid (PS2018/EMT-4329 AIRTEC-CM).

Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Availability of data and material The data that support this research are available in the article and supporting information material

Code availability Not applicable

Author contribution I.A did the calculations. J.E. wrote the first draft of the article. Both authors have read and agreed to the published version of the manuscript.

Baker W, Ollis WD, Poole VD (1950) Cyclic meso-ionic compounds. Part III. Further properties of the sydnones and the mechanism of their formation. J Chem Soc 1542–1551
Baker W, Ollis WD, Meso-ionic compounds. Q Rev Chem Soc 11 (1957) 15–29
Ollis WD, Ramsden CA, Meso-ionic compounds. Adv Heterocycl Chem 19 (1976) 1–122
Ollis WD, Stanforth SP, Ramsden CA, Heterocyclic mesomeric betaines. Tetrahedron 41 (1985) 2239–2329
Potts KT, Murphy PM, Kuehnling WR, Cross-conjugated and pseudo-cross-conjugated mesomeric betaines. 1. Synthesis and characterization. J Org Chem 53 (1988) 2889–2898
Potts KT, Murphy PM, DeLuca MR, Kuehnling WR, J Org Chem 53 (1988) 2898–2910.
Potts KT, Rochanapruk T, Padwa A, Coats SJ, Hadjiiarapoglou L, Intramolecular 1,4-dipolar cycloadditions of cross-conjugated heteroaromatic betaines. Synthesis of hexahydrojulolidines and related peri- and ortho-fused ring systems. J Org Chem 60 (1995) 3795–3805
Oziminski WP, Ramsden CA, A DFT and ab initio study of conjugated and semi-conjugated mesoionic rings and their covalent isomers. Tetrahedron 71 (2015) 7191–7198
Oziminski WP, Ramsden CA, Valence tautomerism and recyclization of type B mesoionic tetrazoles: a computational study. Struct Chem 32 (2022) 229–235
Newton CG, Ramsden CA, Meso-ionic heterocycles. Tetrahedron 38 (1982) 2965–3011
Ramsden CA, Non-bonding molecular orbitals and the chemistry of non-classical organic molecules. Chem Soc Rev (1994) 111–118
Ramsden CA, Heterocyclic mesomeric betaines: the recognition of five classes and nine sub-classes on connectivity-matrix analysis. Tetrahedron 69 (2013) 4146–4159
Ramsden CA, Oziminski WP, A DFT study of isomeric conjugated, cross-conjugated and semiconjugated six-membered heterocyclic mesomeric betaines. Tetrahedron 70 (2014) 7158–7165
Ramsden CA, Semi-conjugated heteroaromatic rings: a missing link in heterocyclic chemistry. Prog Heterocycl Chem 28 (2016) 1–25
Ramsden CA, Oziminski WP, An ab initio study of the valence tautomerism of type B mesoionic rings. Tetrahedron Lett 60 (2019) 150876
Ramsden CA, Oziminski WP, The influence of exocyclic lone pairs on the bonding and geometry of type A mesoionic rings. Struct Chem 32 (2021) 2075–2081
Elguero J, Goya P, Martínez A, Rozas I, Síntesis Orgánica Simulada: aplicación a la síntesis de pirazoles. An Quim 84 (1988) 285–289
Barone R, Chanon M, Ordinateur et synthèse organique. Représentation des molécules et des réactions. Propositions de synthèse pour l'aza-6-uracile. New J Chem 2 (1978) 659–663
Barone R, Chanon M, Metzger J, Ordinateur et synthèse organique. Application d'un programme non interactif à la synthèse du thiazole. Chimia 32 (1978) 216–219
Barone R, Camps P, Elguero J, Synthèse Organique Assistée par Ordinateur: application aux indazoles. An Quim 75 (1979) 736–738
Elguero J, Pyrazoles and benzo derivatives, Comprehensive Heterocyclic Chemistry, 1984, Volume 5 pp. 167-304, Pergamon, Londo
Elguero J, Pyrazoles and benzo derivatives, Comprehensive Heterocyclic Chemistry II, 1996, Volume 3 pp. 1-76, Pergamon, London
Fustero S, Simón-Fuentes A, Sanz-Cervera JF, Recent advances in the synthesis of pyrazoles. A review. Org Prep Proc Int 41 (2009) 253–290
Fustero S, Sánchez-Roselló M, Barrio P, Simón-Fuentes A, From 2000 to mid-2010: a fruitful decade for the synthesis of pyrazoles. Chem Rev 111 (2011) 6984–7034
Elguero J, Silva AMS, Tomé AC, Five-membered heterocycles: 1,2-Azoles. Part 1. Pyrazoles, Modern Heterocyclic Chemistry, 2011, 2, 635–725
Dewar MJS, Thiel W, Ground state of molecules. 38. The MNDO method. Approximations and parameters. J Am Chem Soc 99 (1977) 4899–4907
Cossi M, Barone V, Cammi R, Tomasi J, Ab initio study of solvated molecules: a new implementation of the polarizable continuum model. Chem Phys Lett 255 (1996) 327–335
SciFinder; Chemical Abstracts Service: Columbus, OH
Neumann JJ, Suri M, Glorius F, Efficient synthesis of pyrazoles: oxidative C–C/N–N bond-formation cascade. Angew Chem Int Ed 49 (2010) 7790–7794
Hyvl J, Agrawal D, Pohl R, Suri M, Glorius F, Schröder D, Electrospray ionization mass spectrometry reveals an unexpected coupling product in the copper-promoted synthesis of pyrazoles. Organometallics 32 (2013) 807–816
Pearce AJ, Harkins RP, Reiner BR, Wotal AC, Dunscomb RJ, Tonks IA, Multicomponent pyrazole synthesis from alkynes, nitriles, and titanium imido complexes via oxidatively induced N−N bond coupling. J Am Chem Soc 142 (2020) 4390–4399
Nye MJ, Tang WP, Tautomerism of 4-hydroxypyrazoles and 4-hydroxyisoxazoles – I – Spectroscopic evidence. Tetrahedron 28 (1972) 455–462
Nye MJ, Tang WP, Tautomerism of 4-hydroxypyrazoles and 4-hydroxyisoxazoles – II – Zwitterionic tautomers. Tetrahedron 28 (1972) 463–470
Fagan PJ, Neidert EE, Nye MJ, O'Hare MJ, Tang WP, Cycloadditions and other chemistry of 4-oxygenated pyrazoles. Can J Chem 57 (1979) 904–912
Scifinder CAS register number 60613-72-1
Begtrup M, Reactions between azolium salts and nucleophilic reagents. III. Base-catalyzed interhalogenation and cine-substitution of bromopyrazolium salts with formation of 1,2-dimethyl-pyrazol-4-in-3-ones. Acta Chem Scand 24 (1970) 1819–1835
Zhao Y, Truhlar DG, The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor Chem Acc 120 (2008) 215–241
Kendall RA, Dunning Jr. TH, Harrison RJ, Electron affinities of the first-row atoms revisited. Systematic basis sets and wave functions. J Chem Phys 96 (1992) 6796–6806
Tomasi J, Persico M, Molecular interactions in solution: An overview of methods based on continuous distributions of the solvent. Chem Rev 94 (1994) 2027–2094
Gaussian 16, Revision A.03, FrischMJ, Trucks, GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Petersson GA, Nakatsuji H, Li X, Caricato M, Marenich AV, Bloino J, Janesko BG, Gomperts R, Mennucci B, Hratchian HP, Ortiz JV, Izmaylov AF, Sonnenberg JL, Williams-Young D, Ding F, Lipparini F, Egidi F, Goings J, Peng B, Petrone A, Henderson T, Ranasinghe D, Zakrzewski VG, Gao J, Rega N, ZhengG, Liang W, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Throssell K, Montgomery JA, Peralta JE, Ogliaro F, Bearpark MJ, Heyd JJ, Brothers EN, Kudin KN, Staroverov VN, Keith TA, Kobayashi R, Normand J, Raghavachari K, Rendell AP, Burant JC, Iyengar SS, Tomasi J, CossiM, Millam JM, KleneM, Adamo C, Cammi R, Ochterski JW, Martin RL, Morokuma K, Farkas O, Foresman JB, Fox DJ (2016) Wallingford: Gaussian Inc
Riplinger C, Pinski P, Becker U, Valeev EF, Neese F, Sparse maps – A systematic infrastructure for reduced-scaling electronic structure methods. II. Linear scaling domain based pair natural orbital coupled cluster theory. J Chem Phys 144 (2015) 024109
Neese F, Wennmohs F, Becker U, Riplinger C, J Chem Phys 152 (2020) 224108
AIMAll (Version 19.10.12), Todd A. Keith, TK Gristmill Software, Overland Park KS, USA, 2019 (aim.tkgristmill.com).
Dzib E, Quintal A, Ortiz-Chi F, Merino G, Eyringpy 2.0, Cinvestav, Mérida, Yucatan 2021
Bader R, Atoms in Molecules: A Quantum Theory, Oxford University Press, 1984
Rozas I, Alkorta I, Elguero J, Behavior of ylides containing N, O, and C atoms as hydrogen bond acceptors. J Am Chem Soc 122 (2000) 11154–11161
NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. Linstrom PJ and Mallard WG
Sánchez-Sanz G, Alkorta I, Elguero J, Theoretical study of the HXYH dimers (X, Y = O, S, Se). Hydrogen bonding and chalcogen–chalcogen interactions. Mol Phys 109 (2011) 2543−2552
Mata I, Alkorta I, Molins E, Espinosa E, Universal features of the electron density distribution in hydrogen-bonding regions: a comprehensive study involving H···X (X = H, C, N, O, F, S, Cl, π) interactions. Chem Eur J 16 (2010) 2442−2452
Alkorta I, Solimannejad M, Provasi P, Elguero J, Theoretical study of complexes and fluoride cation transfer between N2F+ and electron donors. J Phys Chem A 111 (2007) 7154−7161
Ferrer M, Alkorta I, Elguero J, Oliva-Enrich JM, Sequestration of carbon dioxide with Frustrated Lewis Pairs based on N-heterocycles with silane/germane groups. J Phys Chem A 125 (2021) 6976–6984
Allen FH, CSD database version 5.42 (2020), The Cambridge Structural Database: a quarter of a million crystal structures and rising. Acta Crystallogr Sect. B 58 (2002) 380–388. Allen FH, Motherwell WDS, Applications of the Cambridge Structural Database in organic chemistry and crystal chemistry. Acta Crystallogr Sect B 58 (2002) 407–422
Azulay JD, Rojas RS, Serrano AV, Ohtaki H, Galland GB, Wu G. Bazan GC, Nickel α-keto-β-diimine initiators for olefin polymerization. Angew Chem Int Ed 48 (2009) 1089–1092
Zhang J, Zhang Z, Chen Z, Zhou X, Oxidation and coupling of β-diketiminate ligand in lanthanide complexes: novel eight-nuclear lanthanide clusters with μ-, μ3-Cl, and μ4-O bridge. Dalton Trans 41 (2012) 357–359
El Seoud OA, Baader WJ, Bastos EL, Practical Chemical Kinetics in Solution, in Encyclopedia of Physical Organic Chemistry, First Edition. Edited by Z. Wang, John Wiley & Sons, 2016, 1–68

A theoretical study of the valence tautomerism of 1H-pyrazolium-4-olates (X = O) and related compounds (X = S, Se, NH): Relative stabilities, protonation effects and tautomerization barriers

Status:

Version 1

Abstract

Figures

Introduction

Computational Methods

Results And Discussion

Conclusions

Declarations

References

Status:

Version 1