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Design of the novel neutral organic superacids by comprehensive DFT study on organic fluorosulfuric acids
Accepted Manuscript Design of the novel neutral organic superacids by comprehensive DFT study on organic fluorosulfuric acids Bahareh Shams, Hamid Saeidian PII: DOI: Reference:
S2210-271X(18)30177-4 https://doi.org/10.1016/j.comptc.2018.05.011 COMPTC 2797
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Computational & Theoretical Chemistry
Received Date: Revised Date: Accepted Date:
2 March 2018 16 May 2018 17 May 2018
Please cite this article as: B. Shams, H. Saeidian, Design of the novel neutral organic superacids by comprehensive DFT study on organic fluorosulfuric acids, Computational & Theoretical Chemistry (2018), doi: https://doi.org/ 10.1016/j.comptc.2018.05.011
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Design of the novel neutral organic superacids by comprehensive DFT study on organic fluorosulfuric acids Bahareh Shams, Hamid Saeidian * Department of Science, Payame Noor University (PNU), P.O. Box: 19395-4697, Tehran, Iran Email: [email protected] Abstract Density functional theory (DFT) calculations at B3LYP/6-311++G(d,p) level have been carried out on organic fluorosulfuric acids, in order to examine their acidities in gas-phase. Strong acidity effect is predicted for a novel series of fluorosulfuric acids in which an oxygen is substituted by an 1,3cyclohexanedione or 1,3-cyclopentadiene group. The acidity of these acids without any electron withdrawing groups on the ring, was more than fluorosulfuric acid. It is shown that substitution of hydrogen with electron withdrawing groups such as -CN, -F, and -C=O on the ring increases the acidity to superacidic values, 216-306 kcal/mol. Substitution of -CN groups enhanced the acidity to 216.0 kcal/mol. Some prototropic tautomers of proposed fluorosulfuric acids are also investigated by DFT calculations Keywords: Fluorosulfuric acids; organic superacid; DFT calculation; aromaticity; tautomers ; NICS data; HOMA index. 1. Introduction Proton transfer is a chemical phenomenon which can be observed and measured by acidity and basicity of a compound. H+ formation involves in a variety of important and useful chemical and biochemical transformations [1-3]. Generally in Brønsted term, the ability of a substance to lose a proton and
1
accommodate the resulting negative charge, defines as acidity. However, superacids are very useful reagents in various fields including organic synthesis, industrial catalysis, material sciences [4-7]. Considerable efforts have been made to design, synthesis and examine neutral organic molecules as superacid [8-20]. Delocalization of negative charge after deprotonation, polarizability and aromaticity increasing are some of the common strategies which are utilized in the design of superacids. To reach new superacids, four protocols might be found in the literature: In the first, Koppel et al. wielded the intrinsic acidity of characteristic motifs such as CH, NH, OH and SH and subsequently enhanced it by using some substituents leading to favorable field/inductive and π-electron resonance effects [21]. The second strategy is based on using electronic superacceptor substituents such as =NSO2CF3 and =P(SO2F)3 instead of a doubly bonded oxygen in carbonyl, sulfonyl and other similar groups in molecules such as CH3CHO. Superacceptor groups are very polarizable and have an ability to stablish the corresponding anions by formation of an extended conjugated system. The calculated deprotonation enthalpies (ΔHacid) for the proposed acids made through this strategy are at the range of 241-387 kcal/mol, very strong acids in gas phase [22, 23]. The third one is based on aromatic stabilization of negative charge in the corresponding conjugate anions. Maksić et al., and recently Valadbeigi were reported some organic Brønsted superacids bearing cyclopentadiene derivatives. They used fulvene and polycyclic aromatic hydrocarbon derivatives with -CN and -F as electron withdrawing substituents to reach the most stable conjugate bases and design super and hyperacids [28]. The reported ΔHacid values for the strongest superacids are about 220-275 kcal/mol [24-27]. Fourth strategy in designing highly acidic molecules involves systems that benefit acidity through hydrogen bonds that are stronger in conjugate bases than in neutral acids [29, 30]. Fluorosulfuric acid, FSO₃H, is the inorganic compound which is one of the strongest acids commercially available. It is an important reagent in chemical reactions and was used as a laboratory
2
fluorinating agent [31, 32]. The combination of FSO3H and antimony pentafluoride, produces magic acid, which is a far stronger protonating agent and called superacid [33, 34]. For the first time, in the present study new strategy is used to design of the strongest neutral organic acid based on fluorosulfuric acid. Replacing an oxygen atom doubly bonded to sulfur atom at fluorosulfuric acid with cyclopentadienyl, phenyl or combination of them leads to new small interesting organic compounds which can behave as superacids I- XIX (Scheme 1 and 5). 2. Computational details Geometry optimizations and frequency calculations for all molecules I- XIX were performed by the use of Gaussian 09 program [35]. DFT calculations with the Becke three-parameter hybrid functional (DFT-B3LYP) were performed, using 6–311++G (d, p) basis set. Deprotonation reaction enthalpy (reaction 1, ΔHacid) and Gibbs free energy (ΔGacid) of the compounds I- XIX in gas phase can be considered as trustable parameters to compare the acidity of the compounds.
AH (g)
A- (g) + H+ (g)
(1)
On the other hand, aromaticity is an important property of conjugated cyclic compounds in determination of their stability. Nucleus-Independent Chemical Shift (NICS) continues to gain popularity as an easily computed, generally applicable criterion to characterize aromaticity of different compounds [36-38]. NICS is computed as the negative magnetic shielding at selected points at the ring center, above or below the ring. Negative NICS values indicate aromaticity, whereas positive values indicate antiaromaticity. It is recommended that the NICS(1) (at points 1 Å above the ring center) to be the best measure of the π-electron delocalization in a cyclic molecule. The NMR shielding tensors were computed, using B3LYP/ 6–311++G (d, p) method, with the Gauge-Independent Atomic Orbital (GIAO) at the center of five and six member rings, NICS(0), and 1 Å above them, NICS(1), using a Bq
3
atom as a probe [39]. It should be mentioned that the NICS index is not applicable in directly determination of aromaticity of a polycyclic conjugated system like anion of XIV- XIX. Schleyer et al., introduced the summation of the NICS values as a global aromaticity index. As a result, the summation of NICS values for a given polycyclic system produces a single quantity called the “total NICS” [40]. Therefore, in this study the sum of the NICS calculated at 1 Å above the rings, Σ NICS(1), is also used to evaluate the aromaticity of anion of XIV- XIX as a whole (Scheme 2). On the other hand, the harmonic oscillator model of aromaticity (HOMA) index was also used to determine the aromaticity of the proposed compounds [41, 42]. The calculated HOMA indices were calculated according to the equation 1: (1) Where, Ri and Ropt are C-C bond length in the ring of the analyzed molecule and the C-C bond length of benzene (1.388 Ǻ), respectively. Based on number of carbon in the ring, n, is 5 or 6 and
is a
normalization factor equal to 257.7. The HOMA index approaches to 1 for aromatic compounds and it approaches to 0 for nonaromatic compounds. 3. Results and discussion As shown at Scheme 1, in order to obtain some insight into the role of various substituents in determining acidity of neutral organic fluorosulfuric acids, different derivatives I-X with various motifs and different nature were considered. The some prototropic tautomers of fluorosulfuric acids I-X are also inserted in Scheme 1 with their relative energies in kcal/mol. As shown in Scheme 1, C-H tautomers are more stable than the corresponding fluorosulfuric acid derivatives I-X, with an exception of derivative III. Also, the ketenimine tautomers are the other possible isomers for the structures shown in Scheme 1 which are 4
also stable than I-X derivatives. These tautomers do not belong to fluorosulfuric acid derivatives. Therefore in the present study, only fluorosulfuric acid isomers were considered and their acidity were explained by DFT calculations. It should be noted that stabilization of C-H tautomer and ketenimine isomer of neutral organic fluorosulfuric acids I-X reduces their gas-phase acidity of these systems by 10-36 kcal/mol than fluorosulfuric acid systems (Table 1).
5
Scheme 1. The chemical structures of the proposed organic fluorosulfuric acids I-X, prototropic tautomers and their relative energies in kcal/mol.
6
Scheme 2 shows stable isomers of the corresponding conjugate bases of considered superacids I-X. The optimized structures of the acid as well as their corresponding conjugate bases have been collected and delivered as supplementary materials. The calculated ΔHacid and ΔGacid data of I-X as well as their some prototropic tautomers are inserted to Table 1. As seen in Table 1, the NICS values and HOMA indices for VI-X indicate that the conjugate bases are more aromatic than the corresponding acids. Replacing a (=O) group with (=C(CN) 2) at fluorosulfuric acid resulted a stable small superacid I with ΔHacid = 265.9 kcal/mol that is 35.4 kcal/mol lower than that of FSO3H. All the proposed acids are more acidic than H2SO4 (ΔHacid = 311.5 kcal/mol) and FSO3H (ΔHacid = 301.3 kcal/mol) [43] with two notable exceptions: compounds II and III bearing nitro and 1,3-cyclohexanedione group, respectively.
Scheme 2. Structures of stable isomer of the conjugate bases of considered superacids I-X. Substitution of more electron-accepting nitro group decreases the acidity, since nitro group leads to strong Coulomb repulsion and induce non-planarity [26]. It should be noted that during structure 7
optimization of acid III, it converted into the more stable enol form (Scheme 3). In this form distance of S=O......HO hydrogen bond is 1.70 Å and partial charge on the H and O atoms, participating in hydrogen bond, are +0.357 and -0.266, respectively. Hydrogen bonding interaction has electrostatic nature, H and O atoms with large and apposite charges form stronger hydrogen bond at short distance. In additional to non-planarity of enol form of III, the presence of a strong hydrogen bond in the enol form, captures hydrogen atom and decreases its acidity. Such phenomena is also seen on structure optimization of acid II and proton migrates on oxygen atom of nitro group. There is a strong intramolecular hydrogen bonding in the rearrangement structure of II (Scheme 3). Table 1. The calculated ΔHacid, ΔGacid, NICS (1) and HOMA values for superacids I-X. Compound
ΔHacida
ΔGacida
NICS(1)b
HOMA
I
265.9
258.8
-
-
I′
288.5
281.4
-
-
I′′
269.8
262.7
-
-
II
286.3
279.8
-
-
II′
296.7
291.5
-
-
III
315.4
306.6
-
-
III′
305.9
298.2
-
-
IV
254.5
247.4
-
-
IV′
272.8
266.1
-
-
IV′′
256.3
264.0
-
-
V
259.5
252.7
-
-
V′
288.5
281.6
-
-
8
a
VI
242.1
234.5
-3.8
-0.06
VI′
261.7
254.4
-1.8
-1.30
(VI-H)-
-
-
-5.3
0.36
VII
261.9
254.2
-7.2
-0.02
(VII-H)-
-
-
-8.2
0.53
VIII
249.8
242.4
-4.3
0.18
(VIII-H)-
-
-
-5.4
0.40
IX
237.7
230.5
-2.7
-0.03
IX′
250.0
242.2
-5.6
-0.80
IX′′
274.2
267.0
11.1
0.89
IX′′
275.2
265.2
10.6
0.97
(IX-H)-
-
-
-5.0
0.57
X
250.2
242.5
-5.5
0.38
(X-H)-
-
-
-7.8
0.81
kcal/mol, at 298.15 K and in gas phase;
b
calculated at GIAO- B3LYP/6-311++G(d,p) level and in
ppm.
9
Scheme 3. Strong intramolecular hydrogen bonding in the rearrangement structures of II and III. By more substituting (=C(CN)2) group into the acid I, compound IV, the acidity increases and the calculated deprotonation enthalpy decreases to 254.5 kcal/mol. We were curious to investigate stabilization of conjugate base using more cyano substituents. Cyano group has an electron-accepting nature and needs very less steric requirements for keeping of negative charge of conjugate base [44, 45]. B3LYP/6-311++G(d,p) calculations reveal that compound V exhibits strong acidity to superacid behavior, ΔHacid = 259.5 kcal/mol. These findings prompted us to investigate effect of aromatic groups doubly bonded to sulfur atom on acidity nature of the proposed compounds. Our first choice was 2,510
cyclohexadienone group bearing four cyano substitutions. Structurally, the compound VI is similar to V, but more rigid than it, due to cyclic form. After deprotonation of compound VI, it is expected the corresponding conjugate base becomes an aromatic phenoxide anion (Scheme 4). The calculated ΔHacid value for VI is 242.1 kcal/mol. After deprotonation of VI, the negative charge is in the resonance with the strong electron- accepting -CN and -C=O groups (Scheme 4).
Scheme 4. Negative charge distribution in conjugate base of VI by the strong electron-accepting cyano and carbonyl groups. It should be pointed out that replacing -CN with -F groups in the structure of VI increases ΔHacid value to 261.9 kcal/mol, 19.8 kcal/mol lower than of VII. On the other hand, 2,5-cyclohexadienone sulfuric acid compound, VIII, is 7.7 kcal/mol less acidic than VI. It shows that electron withdrawing fluorine atom, which bonded directly to sulfur atom, enhances the acidity of VI. There is a strong donor– acceptor interaction between sulfur and fluorine atom which enhances the acidity of VI.
This
interaction will be discussed in following sections. We carried out computational investigation on structure IX, in which the carbonyl group (C=O) in VI was replaced with (=C(CN)2). This strategy results in acidity enhancement by 4.4 kcal/mol. This compound is more acidic than the most poly enol and polycyano compounds which have been reported in literature [24-27]. The calculated ΔHacid value of IX is 237.7 kcal/mol, which shows strong 11
superacid property. This change is also observable through aromaticity of (IX-H)- in comparisons with (VI-H)- (Table 1). (=C(CN)2) group facilitates the conjugation of π-electrons in (IX-H)- structure. Dramatic enhancement in the HOMA index of (IX-H)- compared with IX also supports this finding. Computed gas phase deprotonation enthalpy of tetrafluoride substituted X is 250 kcal/mol. Seeking more acidity improvement, 1,3-cyclopentadiene (Cp) group bearing different substitutions bonded doubly to sulfur atom at fluorosulfuric acid were considered (Scheme 5). Scheme 6 shows stable structures of the corresponding conjugate bases of superacids for XI-XIX. The optimized structures of XI-XIX as well as their corresponding conjugate bases have been presented as supplementary materials. As shown in Scheme 5, there are more stable C-H prototropic tautomers of the fluorosulfuric acids XI-XIX that these reduce the calculated gas phase deprotonation enthalpy of them. ΔHacid and ΔGacid values as well as HOMA and NICS(1) data of these tautomers are inserted in Table 2. As expected, gas-phase acidity of C-H tautomer systems are reduced by 30-48 kcal/mol than fluorosulfuric acid derivatives. As mentioned above only fluorosulfuric acid derivatives were investigated in this paper. Further DFT study of the prototropic tautomers of the fluorosulfuric acids IXIX is in progress
12
Scheme 5. The superacids bearing Cp group, XI-XIX, prototropic tautomers and their relative energies in kcal/mol. The calculated ΔHacid,
ΔGacid, HOMA values for superacids
XI-XIX as well as NICS five(1),
NICSsix(1) and Σ NICS(1) data are provided in Table 2. The subscript of five and six show that NICS values were calculated at 1 Å above the five and six membered rings, respectively. The NICS values for these acids are generally between -1.8 to -12.58 ppm, while these values for the corresponding bases are in the range of -3.4 to -15.9 ppm. The HOMA index for benzene equals to 1, while for phenyl ring of the base (XVI-H)- is 0.96, which shows high aromaticity of (XVI-H)-. The HOMAsix indices of the conjugate base of acids XIV-XIX are in the range of 0.63-0.96. NICS and HOMA values for XI-XIX confirm that the conjugate bases are more aromatic than the corresponding acids.
13
Scheme 6. The structures of corresponding conjugate base of superacids bearing Cp group, XI-XIX. Table 2. The calculated ΔHacid, ΔGacid, HOMA, NICSfive(1), NICSsix(1) and Σ NICS(1) data for XIXIX superacids. Compound
ΔHacida
ΔGacida
NICSfive(1)b
NICSsix(1)b
Σ NICS(1)b
HOMAc
XI
286.3
279.1
-6.1
-
-
0.36
XI′
315.3
307.8
-3.6
-
-
-0.99
XI′′
316.3
309.1
-6.3
-
-
-0.84
(XI-H)-
-
-
-8.1
-
-
0.75
XII
247.2
239.4
-11.0
-
-
0.70
(XII-H)-
-
-
-18.5
-
-
0.83
14
a
XIII
273.5
265.7
-6.8
-
-
0.12
(XIII-H)-
-
-
-8.9
-
-
0.64
XIV
286.9
266.7
-1.8
-3.4
-5.2
0.04 (0.42)
XIV′
304.1
291.2
-1.1
-1.0
-2.1
-0.12 (-0.48)
XIV′′
328.5
308.7
-4.0
-12.1
-16.1
-1.19 (0.98)
(XIV-H)-
-
-
-8.3
-14.9
-23.3
0.61 (0.76)
XV
277.4
270.2
-10.2
-5.3
-15.5
0.41 (0.46)
XV′
300.9
294.6
-3.4
-2.4
-5.8
-0.90 (0.08)
XV′′
325.6
318.1
-4.3
-8.9
-13.2
-1.62 (0.96)
(XV-H)-
-
-
-15.9
-8.3
-24.2
0.66 (0.63)
XVI
233.8
226.7
-6.6
-6.2
-12.9
0.61 (0.71)
(XVI-H)-
-
-
-8.0
-8.7
-16.7
0.74 (0.96)
XVII
240.8
232
-12.5
-8.7
-21.3
0.65 (0.72)
(XVII-H)-
-
-
-11.6
-10.0
-21.7
0.64 (0.73)
XVIII
225.7
218.7
-6.99
-5.63
-12.62
0.42 (0.63)
(XVIII-H)-
-
-
-8.42
-8.86
-17.28
0.76 (0.92)
XIX
216.0
208.7
-5.14
-4.37
-9.51
0.55 (0.61)
(XIX-H)-
-
-
-8.47
-10.55
-19.02
0.78 (0.87)
kcal/mol, at 298.15 K and in gas phase; b calculated at GIAO- B3LYP/6-311++G(d,p) level and in
ppm; c in parenthesis for six membered ring. ΔHacid values for XI, its tetracyano XII and tetrafluoro XIII derivative are 279.1, 247.2 and 273.5 kcal/mol, respectively. The strong acidity of compounds bearing Cp is due to aromatization of the five-membered ring and a strong anionic resonance effect in the corresponding conjugate base. The
15
cyclic compound XII is 12.3 kcal/mol more acidic than its acyclic analog V, which indicates vital role of aromatization of Cp in acidity strength improvement. Valadbeigi has mentioned that the extension of conjugate system increases the acidity of poly enols [27]. An interesting structure improvement of the proposed compounds is made through connection of Cp ring with 1,4-cyclohexadiene group via a double bond or fusion of Cp ring to1,3-cyclohexadiene which resulted XIV-XIX. DFT calculations show that ΔHacid values for the unsubstituted structure of these systems, XIV and XV, are 273.6 and 277.4 kcal/mol, respectively. In continuation, attention was focused on cyano substituted XIV and XV structures (Scheme 5, XVI-XIX). It was seen that the acidity of XIV is increased by substituting cyano groups on Cp ring. ΔHacid for tetracyano substituted XVI is 233.8 kcal/mol, showing a very strong superacid. By more substituting cyano groups (6 CN) on XV and XIV, the acidic property changed which is 47.8 and 36.6 kcal/mol much higher than that of XV and XIV, respectively (Table 2, XVII and XVIII). The most important achievement of this study is to reach the structural design of XIX which its ΔHacid data is 216.0 kcal/mol, indicating the highest superacid value obtained till date. It is interesting to note that by carefully examining the NICS data, it was revealed that Cp ring in neutral superacids XV-XIX have strong aromaticity. On the other hand less changes were observed in NICS, HOMA data and in bond lengths and angles of XV-XIX compared with their corresponding conjugate bases, except S-F bond length. A possible reason for aromaticity increase for XV-XIX compounds is resonance. In such structures, sulfur atom could form a partial double bond with fluorine atom, as a result of donor–acceptor interaction between two elements [46, 47]. The geometrical analysis on the acids bearing Cp group shows also great conjugation in Cp group. Figure 1 compares the bond lengths of XII and its conjugate base, (XII-H)-. Comparison of S-F bond length in XII and
16
(XII-H)- was confirmed the presence of a donor–acceptor interaction between sulfur and fluorine atoms in XII. S-F bond length in (XII-H)- is longer than that XII.
Fig. 1. The optimized structure of XII and (XII-H)-. The bond distances and angles are in (Å) and (°), respectively. 4. Conclusion In the present study, for the first time, a new category of organic fluorosulfuric acids containing aromatic motifs was designed and their acidities were investigated by DFT calculations in gas phase. These small compounds, without electron withdrawing groups, are more acidic that H2SO4, FSO3H and even CF3SO3H. After deprotonation, the negative charge is delocalized in the aromatic cyclopentadienyl and phenyl groups. This is in harmony with NICS values and HOMA indices of the corresponding conjugate base of acids. Substitution of electron withdrawing groups, CN, on aromatic rings enhanced the acidity to 216.0 kcal/mol. The calculated ΔHacid values of the proposed acids bearing aromatic group were between 216-306 kcal/mol which fall into the defined range of superacidity. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at: 17
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[24] R. Vianello, J.F. Liebman, Z.B. Maksić, In search of ultrastrong Brønsted neutral organic superacids: A DFT study on some cyclopentadiene derivatives, Chem. A Eur. J. 10 (2004) 5751-5760. [25] R. Vianello, Z.B. Maksić, Strong acidity of some polycyclic aromatic compounds annulated to a cyclopentadiene moiety and their cyano derivatives–a density functional B3LYP study, Eur. J. Org. Chem. (2005) 3571-3580. [26] R. Vianello, Z.B. Maksić, Polycyano Derivatives of some Organic Tri- and Hexacyclic Molecules Are Powerful Super- and Hyperacids in the Gas Phase and DMSO: Computational Study by DFT Approach, J. Org. Chem. 75 (2010) 7670-7681. [27] Y. Valadbeigi, Design of the strongest organic Brønsted acids in gas phase, Chem. Phys. Lett. 681 (2017) 50-55. [28] R. Vianello, Z.B. Maksic, Rees polycyanated hydrocarbons and related compounds are extremely powerful Brønsted superacids in the gas-phase and DMSO—a density functional B3LYP study, New J. Chem. 32 (2008) 413–427. [29] A. Shokri, J. Schmidt, X.B. Wang, S.R. Kass, Hydrogen Bonded Arrays: The Power of Multiple Hydrogen Bonds, J. Am. Chem. Soc. 134 (2012) 2094-2099. [30] Z. Tian, A. Fattahi, L. Lis, S.R. Kass, Single-Centered Hydrogen-Bonded Enhanced Acidity (SHEA) Acids: A New Class of Brønsted Acids, J. Am. Chem. Soc. 131 (2009) 16984-16988. [31] E. Tabel, E. Zirngiebl, J. Maas "Fluorosulfuric Acid". In Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. [32] G.A. Olah, G.K. Prakash, Q. Wang, X.Y. Li, "Fluorosulfuric Acid". In Encyclopedia of Reagents for Organic Synthesis. Encyclopedia of Reagents for Synthesis. John Wiley & Sons, 2001. [33] G.A. Olah, O. Farooq, A. Husain, N. Ding, N. Trivedi, J. Olah, Superacidic FSO3H/HF catalyzed butane isomerization [1], Catal. Lett. 10 (1991) 239-247. [34] A.R. Christopher, Acc. Chem. Res. Myths about the Proton. The Nature of H+ in Condensed Media, 46 (2013) 2567-2575. [35] J.M. Frisch, et al. Gaussian, Inc., Wallingford CT, Gaussian 09, Gaussian, Inc. 2013. [36] B. Hou, P. Yi, Z. Wang, Sh. Zhang, J. Zhao, R.L. Mancera, Y. Cheng, Z. Zuo, Assignment of aromaticity of the classic heterobenzenes by three aromatic criteria, Comput. Theor. Chem. 1046 (2014) 20-24. 20
[37] Y. Valadbeigi, Comput. Theor. Chem. NICS values in gas phase, implicit solvents and microsolvated systems, 1102 (2017) 44-50. [38] A. Stanger. J. Org. Chem. Nucleus-Independent Chemical Shifts (NICS): Distance Dependence and Revised Criteria for Aromaticity and Antiaromaticity, 71 (2006) 883-893. [39] K. Wolinski, J.F. Hilton, P. Pulay, Efficient implementation of the gauge-independent atomic orbital method for NMR chemical shift calculations, J. Am. Chem. Soc. 112 (1990) 8251-8260. [40] P.V.R. Schleyer, M. Manoharan, H. Jiao, F. Stahl, The Acenes: Is There a Relationship between Aromatic Stabilization and Reactivity?, Org. Lett. 3 (2001) 3643-3646. [41] S. Ostrowski, J.C. Dobrowolski, What does the HOMA index really measure?, RSC Adv. 4 (2014) 44158-44161. [42] S. Radenković, M. Antić, S. Đorđević, B. Braïda, π-electron content of rings in polycyclic conjugated compounds – A valence bond based measure of local aromaticity, Comput. Theor. Chem. 1116 (2017) 163-173. [43] I.A. Koppel, P. Burk, I. Koppel, I. Leito, T. Sonoda, M. Mishima, Gas-Phase Acidities of Some Neutral Brønsted Superacids: A DFT and ab Initio Study, J. Am. Chem. Soc. 122 (2000) 51145124. [44] R. Vianello, Z.B. Maksić, Towards highly powerful neutral organic superacids—a DFT study of some polycyano derivatives of planar hydrocarbons, Tetrahedron, 61 (2005) 9381-9390. [45] H. Brand, J. F. Liebman, A. Schulz, Cyano-, Nitro- and Nitrosomethane Derivatives: Structures and Gas-Phase Acidities, Eur. J. Org. Chem. (2008) 4665-4675. [46] K. Gholivand, S. Ghadimi, H. Naderimanesh, A. Forouzanfar, Dependence of the long-range phosphorus–hydrogen coupling constant nJP–H (n = 3,6,7) on the bond order between phosphorus and its substituents: preparation and spectroscopic characterization of several phosphoramidates, Magn. Reson. Chem. 39 (2001) 684-688. [47] H. Saeidian, M. Babri, D. Ashrafi, M. Sarabadani, M.T. Naseri, Fragmentation pathways of Oalkyl methylphosphonothionocyanidates in the gas phase: Toward unambiguous structural characterization of chemicals in the Chemical Weapons Convention framework, Anal. Bioanal. Chem. 405 (2013) 6749-6759.
21
Highlights 1. DFT calculations were done on fluorosulfuric acids, in order to examine their acidities. 2.
These small compounds, without electron withdrawing groups on the rings, are more acidic than FSO3H.
3. Substitution of CN groups, on the aromatic rings enhanced the acidity to 216.0 kcal/mol.
22
Design of the novel strongest neutral organic superacids by comprehensive DFT study on organic fluorosulfuric acids Bahareh Shams, Hamid Saeidian
23
S2210-271X(18)30177-4 https://doi.org/10.1016/j.comptc.2018.05.011 COMPTC 2797
To appear in:
Computational & Theoretical Chemistry
Received Date: Revised Date: Accepted Date:
2 March 2018 16 May 2018 17 May 2018
Please cite this article as: B. Shams, H. Saeidian, Design of the novel neutral organic superacids by comprehensive DFT study on organic fluorosulfuric acids, Computational & Theoretical Chemistry (2018), doi: https://doi.org/ 10.1016/j.comptc.2018.05.011
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Design of the novel neutral organic superacids by comprehensive DFT study on organic fluorosulfuric acids Bahareh Shams, Hamid Saeidian * Department of Science, Payame Noor University (PNU), P.O. Box: 19395-4697, Tehran, Iran Email: [email protected] Abstract Density functional theory (DFT) calculations at B3LYP/6-311++G(d,p) level have been carried out on organic fluorosulfuric acids, in order to examine their acidities in gas-phase. Strong acidity effect is predicted for a novel series of fluorosulfuric acids in which an oxygen is substituted by an 1,3cyclohexanedione or 1,3-cyclopentadiene group. The acidity of these acids without any electron withdrawing groups on the ring, was more than fluorosulfuric acid. It is shown that substitution of hydrogen with electron withdrawing groups such as -CN, -F, and -C=O on the ring increases the acidity to superacidic values, 216-306 kcal/mol. Substitution of -CN groups enhanced the acidity to 216.0 kcal/mol. Some prototropic tautomers of proposed fluorosulfuric acids are also investigated by DFT calculations Keywords: Fluorosulfuric acids; organic superacid; DFT calculation; aromaticity; tautomers ; NICS data; HOMA index. 1. Introduction Proton transfer is a chemical phenomenon which can be observed and measured by acidity and basicity of a compound. H+ formation involves in a variety of important and useful chemical and biochemical transformations [1-3]. Generally in Brønsted term, the ability of a substance to lose a proton and
1
accommodate the resulting negative charge, defines as acidity. However, superacids are very useful reagents in various fields including organic synthesis, industrial catalysis, material sciences [4-7]. Considerable efforts have been made to design, synthesis and examine neutral organic molecules as superacid [8-20]. Delocalization of negative charge after deprotonation, polarizability and aromaticity increasing are some of the common strategies which are utilized in the design of superacids. To reach new superacids, four protocols might be found in the literature: In the first, Koppel et al. wielded the intrinsic acidity of characteristic motifs such as CH, NH, OH and SH and subsequently enhanced it by using some substituents leading to favorable field/inductive and π-electron resonance effects [21]. The second strategy is based on using electronic superacceptor substituents such as =NSO2CF3 and =P(SO2F)3 instead of a doubly bonded oxygen in carbonyl, sulfonyl and other similar groups in molecules such as CH3CHO. Superacceptor groups are very polarizable and have an ability to stablish the corresponding anions by formation of an extended conjugated system. The calculated deprotonation enthalpies (ΔHacid) for the proposed acids made through this strategy are at the range of 241-387 kcal/mol, very strong acids in gas phase [22, 23]. The third one is based on aromatic stabilization of negative charge in the corresponding conjugate anions. Maksić et al., and recently Valadbeigi were reported some organic Brønsted superacids bearing cyclopentadiene derivatives. They used fulvene and polycyclic aromatic hydrocarbon derivatives with -CN and -F as electron withdrawing substituents to reach the most stable conjugate bases and design super and hyperacids [28]. The reported ΔHacid values for the strongest superacids are about 220-275 kcal/mol [24-27]. Fourth strategy in designing highly acidic molecules involves systems that benefit acidity through hydrogen bonds that are stronger in conjugate bases than in neutral acids [29, 30]. Fluorosulfuric acid, FSO₃H, is the inorganic compound which is one of the strongest acids commercially available. It is an important reagent in chemical reactions and was used as a laboratory
2
fluorinating agent [31, 32]. The combination of FSO3H and antimony pentafluoride, produces magic acid, which is a far stronger protonating agent and called superacid [33, 34]. For the first time, in the present study new strategy is used to design of the strongest neutral organic acid based on fluorosulfuric acid. Replacing an oxygen atom doubly bonded to sulfur atom at fluorosulfuric acid with cyclopentadienyl, phenyl or combination of them leads to new small interesting organic compounds which can behave as superacids I- XIX (Scheme 1 and 5). 2. Computational details Geometry optimizations and frequency calculations for all molecules I- XIX were performed by the use of Gaussian 09 program [35]. DFT calculations with the Becke three-parameter hybrid functional (DFT-B3LYP) were performed, using 6–311++G (d, p) basis set. Deprotonation reaction enthalpy (reaction 1, ΔHacid) and Gibbs free energy (ΔGacid) of the compounds I- XIX in gas phase can be considered as trustable parameters to compare the acidity of the compounds.
AH (g)
A- (g) + H+ (g)
(1)
On the other hand, aromaticity is an important property of conjugated cyclic compounds in determination of their stability. Nucleus-Independent Chemical Shift (NICS) continues to gain popularity as an easily computed, generally applicable criterion to characterize aromaticity of different compounds [36-38]. NICS is computed as the negative magnetic shielding at selected points at the ring center, above or below the ring. Negative NICS values indicate aromaticity, whereas positive values indicate antiaromaticity. It is recommended that the NICS(1) (at points 1 Å above the ring center) to be the best measure of the π-electron delocalization in a cyclic molecule. The NMR shielding tensors were computed, using B3LYP/ 6–311++G (d, p) method, with the Gauge-Independent Atomic Orbital (GIAO) at the center of five and six member rings, NICS(0), and 1 Å above them, NICS(1), using a Bq
3
atom as a probe [39]. It should be mentioned that the NICS index is not applicable in directly determination of aromaticity of a polycyclic conjugated system like anion of XIV- XIX. Schleyer et al., introduced the summation of the NICS values as a global aromaticity index. As a result, the summation of NICS values for a given polycyclic system produces a single quantity called the “total NICS” [40]. Therefore, in this study the sum of the NICS calculated at 1 Å above the rings, Σ NICS(1), is also used to evaluate the aromaticity of anion of XIV- XIX as a whole (Scheme 2). On the other hand, the harmonic oscillator model of aromaticity (HOMA) index was also used to determine the aromaticity of the proposed compounds [41, 42]. The calculated HOMA indices were calculated according to the equation 1: (1) Where, Ri and Ropt are C-C bond length in the ring of the analyzed molecule and the C-C bond length of benzene (1.388 Ǻ), respectively. Based on number of carbon in the ring, n, is 5 or 6 and
is a
normalization factor equal to 257.7. The HOMA index approaches to 1 for aromatic compounds and it approaches to 0 for nonaromatic compounds. 3. Results and discussion As shown at Scheme 1, in order to obtain some insight into the role of various substituents in determining acidity of neutral organic fluorosulfuric acids, different derivatives I-X with various motifs and different nature were considered. The some prototropic tautomers of fluorosulfuric acids I-X are also inserted in Scheme 1 with their relative energies in kcal/mol. As shown in Scheme 1, C-H tautomers are more stable than the corresponding fluorosulfuric acid derivatives I-X, with an exception of derivative III. Also, the ketenimine tautomers are the other possible isomers for the structures shown in Scheme 1 which are 4
also stable than I-X derivatives. These tautomers do not belong to fluorosulfuric acid derivatives. Therefore in the present study, only fluorosulfuric acid isomers were considered and their acidity were explained by DFT calculations. It should be noted that stabilization of C-H tautomer and ketenimine isomer of neutral organic fluorosulfuric acids I-X reduces their gas-phase acidity of these systems by 10-36 kcal/mol than fluorosulfuric acid systems (Table 1).
5
Scheme 1. The chemical structures of the proposed organic fluorosulfuric acids I-X, prototropic tautomers and their relative energies in kcal/mol.
6
Scheme 2 shows stable isomers of the corresponding conjugate bases of considered superacids I-X. The optimized structures of the acid as well as their corresponding conjugate bases have been collected and delivered as supplementary materials. The calculated ΔHacid and ΔGacid data of I-X as well as their some prototropic tautomers are inserted to Table 1. As seen in Table 1, the NICS values and HOMA indices for VI-X indicate that the conjugate bases are more aromatic than the corresponding acids. Replacing a (=O) group with (=C(CN) 2) at fluorosulfuric acid resulted a stable small superacid I with ΔHacid = 265.9 kcal/mol that is 35.4 kcal/mol lower than that of FSO3H. All the proposed acids are more acidic than H2SO4 (ΔHacid = 311.5 kcal/mol) and FSO3H (ΔHacid = 301.3 kcal/mol) [43] with two notable exceptions: compounds II and III bearing nitro and 1,3-cyclohexanedione group, respectively.
Scheme 2. Structures of stable isomer of the conjugate bases of considered superacids I-X. Substitution of more electron-accepting nitro group decreases the acidity, since nitro group leads to strong Coulomb repulsion and induce non-planarity [26]. It should be noted that during structure 7
optimization of acid III, it converted into the more stable enol form (Scheme 3). In this form distance of S=O......HO hydrogen bond is 1.70 Å and partial charge on the H and O atoms, participating in hydrogen bond, are +0.357 and -0.266, respectively. Hydrogen bonding interaction has electrostatic nature, H and O atoms with large and apposite charges form stronger hydrogen bond at short distance. In additional to non-planarity of enol form of III, the presence of a strong hydrogen bond in the enol form, captures hydrogen atom and decreases its acidity. Such phenomena is also seen on structure optimization of acid II and proton migrates on oxygen atom of nitro group. There is a strong intramolecular hydrogen bonding in the rearrangement structure of II (Scheme 3). Table 1. The calculated ΔHacid, ΔGacid, NICS (1) and HOMA values for superacids I-X. Compound
ΔHacida
ΔGacida
NICS(1)b
HOMA
I
265.9
258.8
-
-
I′
288.5
281.4
-
-
I′′
269.8
262.7
-
-
II
286.3
279.8
-
-
II′
296.7
291.5
-
-
III
315.4
306.6
-
-
III′
305.9
298.2
-
-
IV
254.5
247.4
-
-
IV′
272.8
266.1
-
-
IV′′
256.3
264.0
-
-
V
259.5
252.7
-
-
V′
288.5
281.6
-
-
8
a
VI
242.1
234.5
-3.8
-0.06
VI′
261.7
254.4
-1.8
-1.30
(VI-H)-
-
-
-5.3
0.36
VII
261.9
254.2
-7.2
-0.02
(VII-H)-
-
-
-8.2
0.53
VIII
249.8
242.4
-4.3
0.18
(VIII-H)-
-
-
-5.4
0.40
IX
237.7
230.5
-2.7
-0.03
IX′
250.0
242.2
-5.6
-0.80
IX′′
274.2
267.0
11.1
0.89
IX′′
275.2
265.2
10.6
0.97
(IX-H)-
-
-
-5.0
0.57
X
250.2
242.5
-5.5
0.38
(X-H)-
-
-
-7.8
0.81
kcal/mol, at 298.15 K and in gas phase;
b
calculated at GIAO- B3LYP/6-311++G(d,p) level and in
ppm.
9
Scheme 3. Strong intramolecular hydrogen bonding in the rearrangement structures of II and III. By more substituting (=C(CN)2) group into the acid I, compound IV, the acidity increases and the calculated deprotonation enthalpy decreases to 254.5 kcal/mol. We were curious to investigate stabilization of conjugate base using more cyano substituents. Cyano group has an electron-accepting nature and needs very less steric requirements for keeping of negative charge of conjugate base [44, 45]. B3LYP/6-311++G(d,p) calculations reveal that compound V exhibits strong acidity to superacid behavior, ΔHacid = 259.5 kcal/mol. These findings prompted us to investigate effect of aromatic groups doubly bonded to sulfur atom on acidity nature of the proposed compounds. Our first choice was 2,510
cyclohexadienone group bearing four cyano substitutions. Structurally, the compound VI is similar to V, but more rigid than it, due to cyclic form. After deprotonation of compound VI, it is expected the corresponding conjugate base becomes an aromatic phenoxide anion (Scheme 4). The calculated ΔHacid value for VI is 242.1 kcal/mol. After deprotonation of VI, the negative charge is in the resonance with the strong electron- accepting -CN and -C=O groups (Scheme 4).
Scheme 4. Negative charge distribution in conjugate base of VI by the strong electron-accepting cyano and carbonyl groups. It should be pointed out that replacing -CN with -F groups in the structure of VI increases ΔHacid value to 261.9 kcal/mol, 19.8 kcal/mol lower than of VII. On the other hand, 2,5-cyclohexadienone sulfuric acid compound, VIII, is 7.7 kcal/mol less acidic than VI. It shows that electron withdrawing fluorine atom, which bonded directly to sulfur atom, enhances the acidity of VI. There is a strong donor– acceptor interaction between sulfur and fluorine atom which enhances the acidity of VI.
This
interaction will be discussed in following sections. We carried out computational investigation on structure IX, in which the carbonyl group (C=O) in VI was replaced with (=C(CN)2). This strategy results in acidity enhancement by 4.4 kcal/mol. This compound is more acidic than the most poly enol and polycyano compounds which have been reported in literature [24-27]. The calculated ΔHacid value of IX is 237.7 kcal/mol, which shows strong 11
superacid property. This change is also observable through aromaticity of (IX-H)- in comparisons with (VI-H)- (Table 1). (=C(CN)2) group facilitates the conjugation of π-electrons in (IX-H)- structure. Dramatic enhancement in the HOMA index of (IX-H)- compared with IX also supports this finding. Computed gas phase deprotonation enthalpy of tetrafluoride substituted X is 250 kcal/mol. Seeking more acidity improvement, 1,3-cyclopentadiene (Cp) group bearing different substitutions bonded doubly to sulfur atom at fluorosulfuric acid were considered (Scheme 5). Scheme 6 shows stable structures of the corresponding conjugate bases of superacids for XI-XIX. The optimized structures of XI-XIX as well as their corresponding conjugate bases have been presented as supplementary materials. As shown in Scheme 5, there are more stable C-H prototropic tautomers of the fluorosulfuric acids XI-XIX that these reduce the calculated gas phase deprotonation enthalpy of them. ΔHacid and ΔGacid values as well as HOMA and NICS(1) data of these tautomers are inserted in Table 2. As expected, gas-phase acidity of C-H tautomer systems are reduced by 30-48 kcal/mol than fluorosulfuric acid derivatives. As mentioned above only fluorosulfuric acid derivatives were investigated in this paper. Further DFT study of the prototropic tautomers of the fluorosulfuric acids IXIX is in progress
12
Scheme 5. The superacids bearing Cp group, XI-XIX, prototropic tautomers and their relative energies in kcal/mol. The calculated ΔHacid,
ΔGacid, HOMA values for superacids
XI-XIX as well as NICS five(1),
NICSsix(1) and Σ NICS(1) data are provided in Table 2. The subscript of five and six show that NICS values were calculated at 1 Å above the five and six membered rings, respectively. The NICS values for these acids are generally between -1.8 to -12.58 ppm, while these values for the corresponding bases are in the range of -3.4 to -15.9 ppm. The HOMA index for benzene equals to 1, while for phenyl ring of the base (XVI-H)- is 0.96, which shows high aromaticity of (XVI-H)-. The HOMAsix indices of the conjugate base of acids XIV-XIX are in the range of 0.63-0.96. NICS and HOMA values for XI-XIX confirm that the conjugate bases are more aromatic than the corresponding acids.
13
Scheme 6. The structures of corresponding conjugate base of superacids bearing Cp group, XI-XIX. Table 2. The calculated ΔHacid, ΔGacid, HOMA, NICSfive(1), NICSsix(1) and Σ NICS(1) data for XIXIX superacids. Compound
ΔHacida
ΔGacida
NICSfive(1)b
NICSsix(1)b
Σ NICS(1)b
HOMAc
XI
286.3
279.1
-6.1
-
-
0.36
XI′
315.3
307.8
-3.6
-
-
-0.99
XI′′
316.3
309.1
-6.3
-
-
-0.84
(XI-H)-
-
-
-8.1
-
-
0.75
XII
247.2
239.4
-11.0
-
-
0.70
(XII-H)-
-
-
-18.5
-
-
0.83
14
a
XIII
273.5
265.7
-6.8
-
-
0.12
(XIII-H)-
-
-
-8.9
-
-
0.64
XIV
286.9
266.7
-1.8
-3.4
-5.2
0.04 (0.42)
XIV′
304.1
291.2
-1.1
-1.0
-2.1
-0.12 (-0.48)
XIV′′
328.5
308.7
-4.0
-12.1
-16.1
-1.19 (0.98)
(XIV-H)-
-
-
-8.3
-14.9
-23.3
0.61 (0.76)
XV
277.4
270.2
-10.2
-5.3
-15.5
0.41 (0.46)
XV′
300.9
294.6
-3.4
-2.4
-5.8
-0.90 (0.08)
XV′′
325.6
318.1
-4.3
-8.9
-13.2
-1.62 (0.96)
(XV-H)-
-
-
-15.9
-8.3
-24.2
0.66 (0.63)
XVI
233.8
226.7
-6.6
-6.2
-12.9
0.61 (0.71)
(XVI-H)-
-
-
-8.0
-8.7
-16.7
0.74 (0.96)
XVII
240.8
232
-12.5
-8.7
-21.3
0.65 (0.72)
(XVII-H)-
-
-
-11.6
-10.0
-21.7
0.64 (0.73)
XVIII
225.7
218.7
-6.99
-5.63
-12.62
0.42 (0.63)
(XVIII-H)-
-
-
-8.42
-8.86
-17.28
0.76 (0.92)
XIX
216.0
208.7
-5.14
-4.37
-9.51
0.55 (0.61)
(XIX-H)-
-
-
-8.47
-10.55
-19.02
0.78 (0.87)
kcal/mol, at 298.15 K and in gas phase; b calculated at GIAO- B3LYP/6-311++G(d,p) level and in
ppm; c in parenthesis for six membered ring. ΔHacid values for XI, its tetracyano XII and tetrafluoro XIII derivative are 279.1, 247.2 and 273.5 kcal/mol, respectively. The strong acidity of compounds bearing Cp is due to aromatization of the five-membered ring and a strong anionic resonance effect in the corresponding conjugate base. The
15
cyclic compound XII is 12.3 kcal/mol more acidic than its acyclic analog V, which indicates vital role of aromatization of Cp in acidity strength improvement. Valadbeigi has mentioned that the extension of conjugate system increases the acidity of poly enols [27]. An interesting structure improvement of the proposed compounds is made through connection of Cp ring with 1,4-cyclohexadiene group via a double bond or fusion of Cp ring to1,3-cyclohexadiene which resulted XIV-XIX. DFT calculations show that ΔHacid values for the unsubstituted structure of these systems, XIV and XV, are 273.6 and 277.4 kcal/mol, respectively. In continuation, attention was focused on cyano substituted XIV and XV structures (Scheme 5, XVI-XIX). It was seen that the acidity of XIV is increased by substituting cyano groups on Cp ring. ΔHacid for tetracyano substituted XVI is 233.8 kcal/mol, showing a very strong superacid. By more substituting cyano groups (6 CN) on XV and XIV, the acidic property changed which is 47.8 and 36.6 kcal/mol much higher than that of XV and XIV, respectively (Table 2, XVII and XVIII). The most important achievement of this study is to reach the structural design of XIX which its ΔHacid data is 216.0 kcal/mol, indicating the highest superacid value obtained till date. It is interesting to note that by carefully examining the NICS data, it was revealed that Cp ring in neutral superacids XV-XIX have strong aromaticity. On the other hand less changes were observed in NICS, HOMA data and in bond lengths and angles of XV-XIX compared with their corresponding conjugate bases, except S-F bond length. A possible reason for aromaticity increase for XV-XIX compounds is resonance. In such structures, sulfur atom could form a partial double bond with fluorine atom, as a result of donor–acceptor interaction between two elements [46, 47]. The geometrical analysis on the acids bearing Cp group shows also great conjugation in Cp group. Figure 1 compares the bond lengths of XII and its conjugate base, (XII-H)-. Comparison of S-F bond length in XII and
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(XII-H)- was confirmed the presence of a donor–acceptor interaction between sulfur and fluorine atoms in XII. S-F bond length in (XII-H)- is longer than that XII.
Fig. 1. The optimized structure of XII and (XII-H)-. The bond distances and angles are in (Å) and (°), respectively. 4. Conclusion In the present study, for the first time, a new category of organic fluorosulfuric acids containing aromatic motifs was designed and their acidities were investigated by DFT calculations in gas phase. These small compounds, without electron withdrawing groups, are more acidic that H2SO4, FSO3H and even CF3SO3H. After deprotonation, the negative charge is delocalized in the aromatic cyclopentadienyl and phenyl groups. This is in harmony with NICS values and HOMA indices of the corresponding conjugate base of acids. Substitution of electron withdrawing groups, CN, on aromatic rings enhanced the acidity to 216.0 kcal/mol. The calculated ΔHacid values of the proposed acids bearing aromatic group were between 216-306 kcal/mol which fall into the defined range of superacidity. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at: 17
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Highlights 1. DFT calculations were done on fluorosulfuric acids, in order to examine their acidities. 2.
These small compounds, without electron withdrawing groups on the rings, are more acidic than FSO3H.
3. Substitution of CN groups, on the aromatic rings enhanced the acidity to 216.0 kcal/mol.
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Design of the novel strongest neutral organic superacids by comprehensive DFT study on organic fluorosulfuric acids Bahareh Shams, Hamid Saeidian
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