The Effects of Some Substituents to the Acidity of Monocarba-Closo-Dodecaborates: DFT Study
Total Page：16
File Type：pdf, Size：1020Kb
The superacidity of closo-dodecaborate-based Brønsted acids:
a DFT study
Lauri Lipping*,‖, Ivo Leito‖, Ivar Koppel‖, Ingo Krossing§, Daniel Himmel§
and Ilmar A. Koppel*,‖
*[email protected],* [email protected]
‖University of Tartu, Institute of Chemistry, 14a Ravila St., Tartu 50411, Estonia
§University of Freiburg, Institute for Inorganic and Analytic Chemistry, 21 Albertstr.,
Freiburg D-79104, Germany
1 Abstract
The structures and intrinsic gas-phase acidities (GA) of some dodecaborate acids, the
– derivatives of YB12H11H (Y = PF3, NH3, NF3, NMe3), B12H12H2 and B12H12H (HA, H2A
- and HA , respectively) have been computationally explored with DFT B3LYP method at
6-311+G** level of theory as new possible directions of creating superstrong Brønsted acids. Depending on the nature and number of the substituents different protonation geometries were investigated. Also, pKa values of B12H12H2 , CB11H12H and their perfluorinated derivatives in 1,2-dichloroethane (DCE) were estimated with SMD and cluster-continuum model calculations.
In general, the GA values of the neutral systems varied according to the substituents in the following order: CF3 < F < Cl and in case of anionic acids: CF3 < Cl < F. The dodecatrifluoromethyl derivative of H2A, B12(CF3)12H1H2, emerges as the strongest among the considered acids and is expected to be in the gas phase at least as strong as the undecatrifluoromethyl carborane, CB11(CF3)11H1H. The GA values of the respective mono-anionic forms of the considered acids remained all, but (CF3)11-derivative, higher than the widely used threshold of superacidity. The HA derivatives’ (Y = PF3, NF3) GA’s were approximately in the same range as the H2A acids’. In case Y = NH3 or NMe3 the
GA values were significantly higher.
Also, the pKa values of B12H12H2 , CB11H12H and their perfluorinated derivatives in 1,2- dichloroethane (DCE) were estimated with SMD and cluster-continuum model calculations. The obtained estimates of pKa values of the perfluorinated derivatives are by around 30 units lower than that of trifluoromethylsulfonylimide, making these acids the strongest ever predicted in solution.
2 The derivatives of B12H12H2 are as a rule not significantly weaker acids than the respective derivatives of CB11H12H. This is important for expanding practical applicability of this type of acids and their anions, as they are synthetically much easier
– accessible than the corresponding CB11H12 derivatives.
3 Introduction
Practical and fundamental reasonsLipping, L.; Leito, I.; Koppel, I.; Koppel, I. A. J. Phys.
Chem. A. 2009, 113, 12972-12978.-Koppel, I. A.; Taft, R. W.; Anvia, F.; Zhu, S.-Z.; Hu,
L.-Q.; Sung, K.-S.; DesMarteau, D. D.; Yagupolskii, L. M.; Yagupolskii, Y. L.; Ignat’ev,
N. V.; Kondratenko, N. V.; Volkonskii, A. Y.; Vlasov, V. M.; Notario, R.; Maria, P.-C. J.
Am. Chem. Soc. 1994, 116, 3047-3057. have motivated scientists to search for molecules and molecular systems that are more acidic than known before. Several strategiesKoppel,
I. A.; Burk, P.; Koppel, I.; Leito, I. J. Am. Chem. Soc. 2002, 124, 5594-5600.,Vianello,
R.; Maksić, Z. B. J. Org. Chem. 2010, 75, 7670-7681. have been proposed to design highly acidic molecules. An obvious route is introducing electron withdrawing substituents (e.g. fluorination or trifluoromethylation) into already strong or superstrong
Brønsted acids. Well-known examples are fluorosulfonic and trifluoromethanesulfonic acids, which can be regarded as derivatives of sulfuric acid. Another much used approach is increasing the hydrogen ion donor ability of a Brønsted acid HA by mixing it with a
Lewis acid so that the anion A– formed in the ionization of HA is converted into the highly stabilized complex with this Lewis acid. This principle is operational in e.g. magic acid or HSbF6. Although, strong, both of these acids are prone to form reaction side products by means of fluorination.
The electron withdrawing effects of substituents are especially powerful if synergized with the charge delocalization ability of electron-deficient systems, most notably different spherical boron compounds. Decades of workKoppel. I. A.; Burk, P.; Koppel, I.; Leito, I.;
Sonoda, T.; Mishima, M. J. Am. Chem. Soc. 2000, 122, 5114-5124.-Krossing, I.; Raabe,
I. Angw. Chem. Int. Ed. 2004, 43, 2066-2090. on boron compounds and their substituted derivatives have resulted in a new generation of anions – derivatives of the closo-
4 dodecaborate and monocarba-closo-dodecaborate anions – superweak (i.e. very weakly coordinating), extremely inert anionic bases whose conjugate acids are the strongest
Brønsted acids presently knownLipping, L.; Leito, I.; Koppel, I.; Koppel, I. A. J. Phys.
Chem. A. 2009, 113, 12972-12978.-Stoyanov, E. S.; Hoffmann, S. P.; Juhasz, M; Reed,
C. A. J. Am. Chem. Soc., 2006, 128, 3160-3161.. These anions have been used as counter-ions for strongly electrophilic cationic species that in dilute solution are not only extremely strong acidsReed, C. A. Chem. Commun. 2005, 1669-1677., but also have extremely low nucleophilicity, electrophilicity and oxidizing activity.
The first computational evidenceKoppel. I. A.; Burk, P.; Koppel, I.; Leito, I.; Sonoda, T.;
Mishima, M. J. Am. Chem. Soc. 2000, 122, 5114-5124. that the intrinsic gas-phase superacidity of boron-based acids can exceed that of sulfuric acid the “classical” basis for definition of superacidity by many powers of ten was published in 2000 in the work of some of the present authors with coworkers.Koppel. I. A.; Burk, P.; Koppel, I.;
Leito, I.; Sonoda, T.; Mishima, M. J. Am. Chem. Soc. 2000, 122, 5114-5124. It was followed by a Density Functional Theory (DFT) investigation of the intrinsic gas-phase acidities of some smaller carborane derivativesLipping, L.; Koppel, I. A.; Koppel, I.;
Leito, I. Proceedings of the Estonian Academy of Sciences. Chemistry. 2006, 55, 145 -
154.. Further computational extension and revision of the intrinsic gas-phase super acidity scale was carried out in 2009Lipping, L.; Leito, I.; Koppel, I.; Koppel, I. A. J.
Phys. Chem. A. 2009, 113, 12972-12978..
The practical chemical use of these novel reagents has yet to gather impetus. The main obstacle is the high cost and limited availability of the borate and carborate acids and their salts. The quantities that presently can be prepared via complex and time-consuming synthetic pathways are suitable for obtaining small quantities of valuable substances,
5 enough for small scale experiments, but not for extensive or large-scale use. In the recent reportsKüppers, T.; Bernhardt, E.; Eujen, R.; Willner, H.; Lehmann, C. W. Angew.
Chem. Int. Ed. 2007, 46, 6346-6349.-Stoyanov, E. S.; Stoyanova, I. V.; Tham, F. S.;
Reed, C. A. J. Am. Chem. Soc. 2010, 132, 4062-4063. some experiments of fundamental interest have been reported made possible by the free acids CB11XnR12-nH (X = Cl, F; R =
H, CH3; n = 6, 11). However, the question about the availability of the derivatives of
– CB11H12 remains. Therefore, the quest for anions of similar inertness and low basicity, but easier to prepare, is constantly on. The interest in the derivatives of H2(B12X12) comes
2– from a fact that the salts of the starting compound B12H12 are commercially available at
2– a reasonable price. Furthermore, as B12X12 are dianions, a useful approach to further increase their acidity could be decreasing the availability of the negative charge by using a single positively charged group, which turns the bianionic closo-dodecaborate into a monoanion. Besides a positive charge this group should have electron-withdrawing properties, should not contain any well-defined protonation centers and should be
+ + + reasonably stable. Based on these considerations we have chosen the -PF3 , -NF3 , -NH3 ,
+ -NMe3 groups for this purpose.
In a recent reportAvelar, A.; Tham, F. S.; Reed, C. A. Angew. Chem. Int. Ed. 2009, 48,
3491-3493. the solution-phase superacidities of two diprotic acids, based on the closo- dodecaborate anions H2(B12X12) (X = Cl, Br) have been estimated indirectly by Reed et al. using the anions’ ν(NH) basicity scale based on NH stretching frequency shifts of
+ Oct3NH in CCl4 induced by H-bond formation between the latter cationic proton donor and the superweak anionic base.Stoyanov, E. S.; Kim, K.-C.; Reed C. A. J. Am. Chem.
Soc. 2006, 128, 8500-8508.,Juhasz, M.; Hoffman S. P.; Stoyanov, E. S.; Kim, K.-C.;
Reed, C. A. Angew. Chem. Int. Ed. 2004, 43, 5352 –5355. Based on these results and the
6 ability of the derivatives of H2(B12X12) (X = Cl, Br) to protonate benzeneAvelar, A.;
Tham, F. S.; Reed, C. A. Angew. Chem. Int. Ed. 2009, 48, 3491-3493. by forming
[C6H7]2[B12X12] salts their acid strength – even corresponding to the detachment of the second proton – was considered to be comparable with the respective carborane acids.
These results were explained with the hypothesisAvelar, A.; Tham, F. S.; Reed, C. A.
Angew. Chem. Int. Ed. 2009, 48, 3491-3493. that “halogeno substituents on both anions form an effective screen for negative charge that is delocalized and buried within the icosahedral cage”. Therefore, for accurate experimental and computational estimation of intrinsic acidity the careful analysis of possible protonation sites is necessary.
However, no direct measurement or computational estimation of these acids in any solvent has been published according to our best knowledge.
Therefore, to give an estimation about the acidity of borane and carborane acids in solution, pKa values were calculated for the parent compounds and for their perfluorinated derivatives in 1,2-dichloroethane (DCE). Although, the computational estimations of the pKa values in solution have large practical importance the results have to be addressed carefully because of the imperfection of the computational models and difficulties in validating the results against experiment. Even the measurement of these compounds in the gas phase has not been very successful because of the problems in bringing them into gas phase.
DCE is the one of the least polar and basic solvents where a self-consistent ladder of relative acidities (i.e. ΔpKa) has been measuredKütt, A.; Rodima, T.; Saame, J.; Raamat,
E.; Mäemets, V.; Kaljurand, I.; Koppel, I. A.; Garlyauskayte, R. Y.; Yagupolskii, Y. L.;
Yagupolskii, L. M.; Bernhardt, E.; Willner, H.; Leito, I. J. Org. Chem. 2011, 76, 391–
395.. C2H4Cl2 is a very weak hydrogen bond donor and acceptor by its Kamlet-Taft
7 hydrogen bond acidity (α) and basicity (β) parameters of 0.0 and 0.1, respectivelyMarcus,
Y. Chem. Soc. Rev. 1993, 22, 409-416.. According to its polarity parameter (π*) of 0.81 and dielectric constantReichardt, C. Solvents and Solvent Effects in Organic Chemistry,
3rd ed.; Wiley-VCH: Weinheim, Germany, 2003.Gaussian 09 default value. of
10.1360,@Panin siiski Reichardt’i väärtuse, mis meil on ka varasemas töös. it is a medium polar solvent and can dissolve ionic compounds.
In this paper we shall focus on the study of the closo-dodecaborate-based superacid derivatives with a range of substituents of different nature using mostly high level DFT calculations. In order to obtain reliable results, different possible protonation geometries and the effects of substituents on the protonation site are compared. First time the pKa values of some borane and carborane acid derivatives are calculated in solution.
Methods
2- Unless otherwise indicated, density functional theory (DFT) calculations were carried out on B12XnH12-n (X
- = F, Cl, CF3; n = 0, 1, 6, 11, 12) and YB12XnH11-n (Y = PF3, NH3, NF3, NMe3, X = F, Cl, CF3; n = 0, 1, 6,
11) cages and their protonated forms at B3LYP/6-311+G** level with Gaussian 09 system of programs with full thermal corrections to Gibbs energies at the optimized structuresGaussian 09, Revision A.1,
Frisch, M. J. et al. Gaussian, Inc., Wallingford CT, 2009.. In the largest systems where X = CF3 and n = 12 (n = 11 with the Y-borates), the vibrational analysis at the B3LYP/6-311+G** level failed, so thermal corrections were calculated using the RI-BP86Becke, A. D. Phys. Rev. A 1988, 38, 3098 –
3100.,Perdew, J. P. Phys. Rev. B 1986, 33, 8822 – 8824; erratum: Perdew, J. P. Phys.
Rev. B 1986, 34, 7406./def-TZVPSchäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992,
97, 2571. level with default RI-J auxiliary basisEichkorn, K.; Treutler, O.; Oehm, H.; Häser, M.;
Ahlrichs, R. Chem. Phys. Lett. 1995, 242, 652 – 660. on the corresponding optimized structures using the Turbomole 6.4TURBOMOLE V6.4 2012, a development of University of
8 Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989–2007, TURBOMOLE GmbH, since 2007, see http://www.turbomole.com.-Treutler, O.; Ahlrichs, R. J. Chem. Phys.
1995, 102, 346 – 354. program system (see the discussion for details).
Figure 1. The numbering of closo-dodecaborate’s vertexes.
The starting position of the substituent insertion for the borates without Y-group was considered as position
1 (Figure 1). In case of the Y-substituted borates the position 1 was the vertex with Y-group. Replacement of the hydrogen atoms with substituents was done subsequently in the following groups of vertexes (belts):
1, 2…6, 7…11 and 12.
For most of the acids several input geometries of protonated forms with different protonation sites were composed to determine the most stable one. Full geometry optimizations as well as vibrational analyses were carried out for all acids and anions.
The intrinsic gas-phase acidity (Gacid = GA) of a neutral acid HA was calculated according to the following thermodynamic heterolysis equilibrium:
HA ⇄ A– + H+, (1)
The Gacid values (at 298 K) were calculated taking into account the zero-point energies, finite temperature
(0 to 298 K) and entropy correction and the pressure-volume work term pV. The absence of imaginary
9 frequencies (NImag = 0) was taken as the criterion of finding geometry corresponding to true energy minimum.
By definition, the gas-phase acidity of a neutral acid HA is equal to the gas-phase basicity (toward the proton) of its conjugate base, A–. The lower numerical values of GA’s (in kcal mol-1) mean stronger/higher acidities.
For the calculations in the solution phase the SMD modelMarenich, A. V.; Cramer, C. J.; Truhlar,
D. G. J. Phys. Chem. B. 2009,113 , 6378-6396. with structural relaxation at SMD/B3LYP/6-
311+G** level was used. Gibbs solvation energies were corrected by 1.9 kcal mol-1 (= RT ln 24.47) to obtain 1 bar ideal gas (denoted as (g)) to 1 molar ideal solution (denoted as (solv)) standard values. An additional correction by 1.5 kcal mol-1 (= RT ln 12.6) was applied for DCE, as the pure (i.e. 12.6 molar) solvent (denoted as (l) was used as standard state. The SMD model is parametrized for calculating Gibbs solvation energies with the inclusion of H-bond and nonelectrostatic interactions. Using a cluster- continuum modelPliego, J. R. Jr.; Riveros, J. M. J. Phys. Chem. A. 2001, 105 , 7241-7247.,
Gibbs standard solvation energy (-210.5 kcal mol–1) was calculated (i.e. an absolute chemical standard potential) for the proton which is somewhat lower (by 5.1 kcal mol–1) than previously calculated by some of usHimmel, D.; Goll, S. K.; Leito, I.; Krossing, I. Chem. Eur. J. 2011, 17, 5808−5826.
(more details in SI). Tissandier et al have reported the absolute chemical standard potential for the proton in water -264.0 kcal mol–1.Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen,
M. H.; Earhart, A. D.; Coe, J. V. J. Phys. Chem. A. 1998, 102, 7787-7794. According to this calculation DCE as a bulk solvent is by 53.45 kcal mol–1 less basic than water. This means that in DCE at a pH of 39.2 the proton has the same acidity than in water at pH 0!
Results and Discussion
2– The computational Gacid values of the conjugate acids of the borate anions B12XnH12-n are presented in Table 1. The respective results of the Y-substituted borate acids are presented in Table 2. More detailed information about the results of the DFT calculations is available in the SI or from the authors upon request.
10 - For the unsubstituted (parent) compounds H2A and HA the calculations resulted in the
-1 Gacid values 267.5 and 359.8 kcal mol , respectively. As can be seen, the GA of
-1 B12H12H2 (Figure 2) is within 2 kcal mol range from that of the respective carborane acid
-1 CB11H12H (GA = 266.5 kcal mol ).Lipping, L.; Leito, I.; Koppel, I.; Koppel, I. A. J. Phys.
Chem. A. 2009, 113, 12972-12978. The protonation sites of the neutral acid, H2A were positioned antipodally to each other.
Figure 2. Geometry of the neutral acid B12H12H2.
The derivatives of HA had the most stable protonation site placed on the spherical boron
cage diametrically opposite to the positively charged Y-group and the Gacid values in
-1 -1 case of PF3- and NF3-derivatives were 266.6 kcal mol and 269.2 kcal mol , respectively.
-1 The respective NH3- and NMe3-acids had the GA’s around 281 kcal mol . These results show that in the gas phase the electrostatically bound proton is well able to act as a partly covalently interacting and positively charged substituent. On Figure 3 we introduce a
11 scale of computational gas-phase acidities of some borate anions’ conjugate acids supplemented by some Brønsted acids as landmarks.
12 13 Figure 3. A scale of gas phase acidities from a selection of dodecaborate derivatives
2– accompanied with some Brønsted acids. Blue color denotes the derivatives of B12XnH12-n
– (X = F, Cl, CF3; n = 0, 1, 6, 11, 12) and YB12XnH11-n (Y = PF3, NH3, X = F, Cl, CF3; n =
0, 1, 6, 11), purple denotes carborane derivatives.
– The most favorable protonation site of the unsubstituted carborane anion CB11H12 is
determined by the anisotropyKörbe, S.; Schreiber, P. J.; Michl, J. Chem. Rev. 2006, 106,
5208-5249. of the electrostatic potential throughout the molecule and is the boron atom
antipodal to the carbon atom. Protonation of its substitution derivatives is additionally
influenced by the placement and nature of substituents.Lipping, L.; Leito, I.; Koppel, I.;
– Koppel, I. A. J. Phys. Chem. A. 2009, 113, 12972-12978. The same is true for YB12H11 .
2– When a positively charged substituent is added to the spherical B12H12 anion then the
charge anisotropy is created and this causes the relocation of the negative charge density
– in a way similar to the case of the carborane anion CB11H12 . Further addition of
substituents makes the interplay of substituents and protonation sites more complex.
Below we will present an overview of the most stable protonation sites of theses
derivatives and their gas-phase acidities.
Table 1. Results of acidity calculations with DFT B3LYP method at 6-311+G** level. protonation protonation
a b a b acid sites Gacid acid site Gacid ― B12H12H2 B1 & B12 267.5 B12H12H 1 - 2 - 3 359.8 ― B12(CF3)1H11H2 B2 & B10 259.0 B12(CF3)1H11H 2 - 3 - 7 7 - 8 - 12 349.2 ― B12(CF3)6H6H2 B7 & B9 230.1 B12(CF3)6H6H B12 308.4 ― B12(CF3)11H1H2 1 - 2 - 3 & B12 177.5 B12(CF3)11H1H B12 283.3 c ― c B12(CF3)12H2 1 - 2 - 3 & 9 - 10 - 12 170.8 B12(CF3)12H 1 - 2 - 3 253.9 ― B12F1H11H2 B2 & B10 265.2 B12F1H11H 2 - 3 - 7 7 - 8 - 12 356.6 ― B12F6H6H2 B7 & B9 243.1 B12F6H6H 7 - 8 - 12 339.6 ― B12F11H1H2 1 - 2 - 3 & B12 220.0 B12F11H1H B12 317.4 ― B12F12H2 F1 → F2 & F10 → F12 213.4 B12F12H 1 - 2 - 3 310.7
B12F12H2 F1→ F2 & 9 - 10 - 12 212.3 ― B12Cl1H11H2 B2 & B10 261.1 B12Cl1H11H 2 - 3 - 7 7 - 8 - 12 353.2
14 ― B12Cl6H6H2 Cl1 → Cl2 & B12 246.6 B12Cl6H6H 7 - 8 - 12 B12 323.3 ― B12Cl11H1H2 Cl2 → Cl3 & Cl9 → Cl10 238.8 B12Cl11H1H Cl1 → Cl2 Cl2 → Cl3 304.0 ― B12Cl12H2 Cl1 → Cl2 & Cl10 → Cl12 236.8 B12Cl12H Cl1 → Cl2 302.5 a The sites of protonation for the most stable forms. Bx denotes a boron vertex with proton arranged to it symmetrically with the substituent. X - Y- Z
denotes a facet of the boron cage. Ax → Cy denotes a geometry where proton is on a substituent A in the position x having HB interaction with substituent C in the position y. The "" mark denotes that there are two protonation sites with approximately the same Gacid value. b c Gacid values given in kcal/mol at 298 K, calculated at 6-311+G** level if not noted differently. The acidity is obtained by combining B3LYP/6-311+G** SCF energy and BP86 thermal correction.
– The monosubstituted derivatives of B12XH11H2 and B12XH11H where X = F, Cl, CF3
The computational acidity predictions of the H2A with a single substituent placed on the
-1 B12 vertex ranked the systems according to the GA values as follows: F (265.2 kcal mol )
-1 -1 → Cl (261.1 kcal mol ) → CF3 (259.0 kcal mol ). The derivative with more
electronegative fluorine is less acidic than its more polarizable chlorine counterpart. The
monosubstituted F- and Cl-derivatives’ least acidic (most stable) forms have very similar
protonation geometry: the protons are interacting with B2-6 and B7-11 and are placed
diametrically opposite to each other, they are equidistant (1.354 – 1.359 Å) from the
respective B’s, they are placed 0.825-0.829 Å from the H on the same boron vertex. The
geometrical positioning of the protons in CF3-derivative was similar to the F- and Cl-
derivative. The notable exception was the protonation site near the CF3-group where
H/H+ distances from the B were 1.373 and 1.357 Å. The longer B-H distance resulted in
the bond nearer to the CF3. The small distance between the hydrogen nuclei supports the
idea of some charge transfer(a) Leito, I.; Raamat, E.; Kütt, A.; Saame, J.; Kipper, K.;
Koppel, I. A.; Koppel, I.; Zhang, M.; Mishima, M.; Yagupolskii, L. M.; Garlyauskayte,
R. Y.; Filatov, A. A. J. Phys. Chem. A. 2009, 113, 8421. (b) Zhang, M.; Sonoda, T;
Mishima, M.; Honda, T.; Leito, I.; Koppel, I. A.; Bonrath, W.; Netscher, T. J. Phys. Org.
Chem. 2014, 27, 676–679.-Umeyama, H.; Morokuma, K. J. Am. Chem. Soc. 1977, 99,
15 1316-1332. (covalent) character of the formed H-bond besides the electrostatic component.
– The same acidity order applies to the monoprotic anionic acids B12X1H11H with intrinsic
-1 2– gas-phase acidities around 350 kcal mol . Interestingly, in the B12X1H11 anions the most favorable protonation site is not on B12 vertex as one could expect, but on the facets 2 – 3
– 7, 3 – 7 – 8 and 7 – 8 – 12, all in the range of 1.4 kcal mol-1. That refers to a certain
“surplus” of negative charge across the anion that has not been significantly diminished by the size of the system nor the substituent.
– The hexasubstituted derivatives of B12X6H6H2 and B12X6H6H where X = F, Cl, CF3
In terms of protonation site geometries the largest variations occurred in hexasubstituted borates. In the case of the diprotic Cl-substituted system one proton is attached on the substituent in the position 1 and chelated by the substituent in the position 2. The second proton is bound to the B12 vertex. In terms of negative charge distribution, the neutral acid
B12F6H6H2 represents a system with unique features. Although, in similarKoppel. I. A.;
Burk, P.; Koppel, I.; Leito, I.; Sonoda, T.; Mishima, M. J. Am. Chem. Soc. 2000, 122,
5114-5124.,Lipping, L.; Koppel, I. A.; Koppel, I.; Leito, I. Proceedings of the Estonian
Academy of Sciences. Chemistry. 2006, 55, 145 - 154. systems the one-atom halogen substituents, in general, appear to be the most favorable protonation sites in the form of intramolecular hydrogen bond, low polarizability of fluorine atom makes the proton interaction with the fluorine-shield somewhat less favorable. This can be observed as one protonation site appears on the B7 while the second is on the B9 vertex at the opposite side of the cage. In the hexakis-CF3 derivative the most favorable protonation sites are the same, B7 and B9 vertexes, which is probably the nearest placement of the protons to each other across the systems, resulting in the gas-phase acidity of 230.1 kcal mol-1 vs 243.1
16 -1 kcal mol for the B12F6H6H2 acid. The initial geometries where proton is placed near the
CF3 substituent during the geometry optimization result in abstraction of HF or HCF3 and the formation of two neutral molecules e.g. HCF3 + B12(CF3)5H6H. Based on calculations of monocarba-closo-borane derivatives the resulting geometries with the leaving group can have lower energies compared with the most stable protonated form beginning from
-1 few up to tens of kcal mol , mostly depending on the number of CF3-groups on the vertexes. However, when HF separated from the PF3B12H11H the resulting system (HF +
+ - + PF2 + B12H11 , also PF2 moved from its position above the boron in the position 1 and formed a system where P was above the B – B bond of positions 1 and 2) was by 48.2 kcal mol-1 less stable.
In the order of the intrinsic gas-phase acidities the neutral clusters with six F- and Cl- substituents switched their places compared with the monosubstituted systems: CF3 < F <
Cl < H. However, the acidity order of the anionic acids remained the same as in case of the single substituent systems. This could be the result of higher polarizability of the Cl- substituent over F. In case of the F6-derivative the most favorable protonation site was
-1 again on the 7 – 8 – 12 facet leaving the B12-protonated system by 1.4 kcal mol less stable. Similar chloroborate had the GA’s of 7 – 8 – 12 and B12-protonated derivatives,
-1 both, in 0.6 kcal mol range. The (CF3)6-derivative’s acidities of the respective
-1 protonation sites have already 4 kcal mol difference in favor of B12. In comparison with monosubstituent systems this change of protonation sites illustrates well the behavior of the substituents in terms of ability to delocalize the negative charge in the anions and the importance of considering all possible geometries to obtain correct interpretation about the acidity ranking.
- The derivatives of B12X11HH2 and B12X11HH where X = F, Cl, CF3
17 The B12(CF3)11HH2 acid has the most stable protonation sites above the 1 – 2 – 3 facet and
-1 B12 vertex of the boron cage. The acidity of the system is 177.5 kcal mol , that is about 5 kcal/mol less acidic than the corresponding carborane derivativeLipping, L.; Leito, I.;
Koppel, I.; Koppel, I. A. J. Phys. Chem. A. 2009, 113, 12972-12978., the most acidic molecule in the gas phase, predicted in this work. Similar protonation geometry is visible
-1 in the neutral F11-system with GA 220.0 kcal mol . Cl11-derivative, in turn, had the most stable protonation sites, both, at the opposite sides of the molecule placed between the chlorines of the vertexes 2 – 6 and 7 – 11. The Cl – H distances of each protonation site were 1.814/1.439 Å and 1.746/1.471 Å.
– – The monoanionic acids B12F11H1H and B12(CF3)11H1H have the most favorable protonation site on B12 vertex while Cl11-derivative protonates with almost equal energies on the chlorine atoms on all vertexes. The acidity order of both levels of protonation remained the same compared with the respective X6-derivatives.
- The derivatives of B12X12H2 and B12X12H where X = F, Cl, CF3
Several attempts failed to calculate the vibrational frequencies of the (CF3)12-borate derivatives with B3LYP/6-311+G**, so that it remained unclear if the obtained structures were true minima or not. To minimize the risk of e.g. running into a transition state during optimization, these systems were at first optimized at the BP86/def-TZVP level.
True minima without imaginary frequencies were found at this level with the GA I and
GA II values 181.1 kcal mol-1 and 263.0 kcal mol-1, respectively. That is somewhat higher than could be expected if compared with B3LYP/6-311+G** computations of (CF3)11- systems that have one electron withdrawing group less. To have a better comparability with the existing DFT B3LYP/6-311+G** scale the optimised geometries from
BP86/def-TZVP computations were used as input structures for subsequent B3LYP/6-
18 311+G** optimization. The BP86 thermal corrections were applied to these SCF energies after the verification that no significant changes of geometries had taken place during this procedure. This resulted in GA values 170.8 kcal mol-1 and 253.9 kcal mol-1, respectively.
The same method was applied also to the computations of CB11(CF3)12H that has eluded the efforts to calculate it’s frequencies with Gaussian 09, as well. These calculations resulted in GA of 172.3 kcal/mol that is in the same range with the respective dodecaborane derivative.
The B12F12H2 had the most favorable protonation sites on the opposite sides of the molecule (GA = 212.3 kcal mol-1). The energy of the system where one proton was on the boron facet and another on the fluorine chelated with the neighbouring fluorine was in a
0.9 kcal/mol range of the conformer where both protons were between the fluorines placed antipodally to each other. Similar result was also observed in case of the respective Cl-derivative (GA = 236.8 kcal mol-1).
The most favorable proton locations for the anionic F12- and Cl12-acids were on the 1 – 2
– 3 facet (GA = 310.7 kcal mol-1) and between the substituents (GA = 302.5 kcal mol-1), respectively.
The derivatives of YB12XnH11-nH where Y = PF3, NH3, NF3, NMe3 and X = F, Cl, CF3 and n = 1; 6; 11)
We report about a series of monoanionic dodecaborate derivatives that have gas-phase acidities in the same range as the respective monocarba-closo-dodecaborates. The reduction of charge in the anions was obtained by the use of +Y-group as a substituent.
However, the lower charge of the anions compared to the respective B12H11-derivatives does not have a significant effect on increasing the gas phase acidity.
19 – The anion PF3B12H11 is characterized by significantly lower dipole moment compared to
– the respective carborane anion CB11H12 (1.2596 D vs 2.7345 D).
– – – Figure 4. Mulliken atomic charges of the CB11H12 , PF3B12H11 and B12H12H .
In the Y-borate the largest positive partial charge resides on the Y-atom hence the most favorable protonation site for the unsubstituted system is the B12-vertex. Because of this and also for the steric reasons one could expect for the insertion of electron withdrawing atoms and groups the most favorable position is also B12. However, comparing the
- energies of two PF3B12F1H10 isomers with fluorine placed on positions 12 and 2, the former anion is by only about 1.9 kcal mol-1 more stable. Nevertheless, the derivatives in the Table 2 follow substitution levels starting from the vertex antipodal to the Y-group.
- The monosubstituted dodecaboranes: H2A, HA , the PF3-derivatives of HA and carboranesLipping, L.; Leito, I.; Koppel, I.; Koppel, I. A. J. Phys. Chem. A. 2009, 113,
12972-12978. follow the same GA order: F > Cl > CF3, meaning the F-derivative has the weakest GA. In the case of the singly substituted PF3-halogen derivatives the F-derivative is by about 1.7 kcal mol-1 weaker acid than the respective Cl-derivative and results in the
GA value of 260.4 kcal mol-1. This acidity order refers to the additional destabilisation of
20 the anion by stronger resonance donor effect of the fluoro substituent (compared to the
chloro substituent). This is by 3.2 kcal mol-1 weaker than the respective carborane
acidLipping, L.; Leito, I.; Koppel, I.; Koppel, I. A. J. Phys. Chem. A. 2009, 113, 12972-
-1 12978. and by 4.8 kcal mol stronger than the B12FH11H2. The most favorable protonation
site for the monosubstituted PF3-anion where X = F, Cl, CF3 was B7.
Table 2. Results of acidity calculations with DFT B3LYP method at 6-311+G** level. protonation protonation
a b a b acid site Gacid acid site Gacid
PF3B12H11H B12 266.6 PF3B12Cl6H5H Cl11 → Cl12 245.0
PF3B12(CF3)1H10H B7 254.9 PF3B12Cl11H Cl11 → Cl12 235.2
PF3B12(CF3)6H5H B2 217.3 NH3B12H11H B12 280.7 c PF3B12(CF3)11H 7 - 8 - 12 180.9 NH3B12F11H 7 - 8 - 12 232.2
PF3B12F1H10H B7 260.4 NF3B12H11H B12 269.2
PF3B12F6H5H 2 - 3 - 7 237.8 NF3B12F11H 7 - 8 - 12 F12 → F11 221.3
PF3B12F11H 7 - 8 - 12 F12 → F11 221.0 NMe3B12H11H B12 281.9
PF3B12Cl1H10H B7 258.7 NMe3B12F11H 7 - 8 - 12 236.6 a The sites of protonation for the most stable forms. Bx denotes a boron vertex with proton arranged to it symmetrically with the substituent. X - Y- Z
denotes a facet of the boron cage. Ax → Cy denotes a geometry where proton is on a substituent A in the position x having HB interaction with substituent C in the position y. The "" mark denotes that there are two protonation sites with approximately the same Gacid value. b c Gacid values given in kcal/mol at 298 K, calculated at 6-311+G** level if not noted differently. The acidity is obtained by combining B3LYP/6-311+G** SCF energy and BP86 thermal correction.
The gas-phase acidities of the hexakis-substituted PF3-borate acids where X = F, CF3 are
both by ca 5 kcal mol-1 weaker than the respective carborane acids. With the chlorine
-1 derivatives the difference is about 3 kcal mol . The comparison with H2A (B12X6H6H2, X
= F, Cl, CF3) shows rather interesting results. The fluorine derivative of the H2A is by 5.3
-1 -1 kcal mol less acidic than the respective PF3-borate (GA = 237.8 kcal mol ), the CF3-
derivative is by 12.8 kcal mol-1 weaker (GA = 230.1 kcal mol-1) and Cl-derivative only by
1.6 kcal mol-1 weaker (GA = 246.6 kcal mol-1).
The GA computations of PF3B12(CF3)11H required the same procedure as the respective
H2A derivative. The SCF energies from B3LYP 6-311+G** calculations with BP86-
21 thermal corrections resulted in gas-phase acidity of 180.9 kcal mol-1. That is about 10
-1 kcal mol less acidic than the respective derivative of H2A.
In case of the perfluorinated and perchlorinated systems of B12X12H2 and PF3B12X11H the
-1 -1 fluorinated H2A was 1 kcal mol stronger and chlorinated H2A 3.6 kcal mol weaker than the respective HA. The acidities of YB12F11H (Y = PF3, NF3) where about the same. The
-1 respective systems where Y = NH3 and NMe3 were about 11 – 15 kcal mol weaker than
Y = PF3, NF3 derivatives.
Acidity calculations in solution
We carried out estimation of pKa values in 1,2-dichloroethane (DCE) would like to report about for B12H12H2, CB11H12H and their perfluorinated derivatives’ pKa calculation results in DCE. For comparison also the pKa of bis-trifluoromethylsulfonylimide (HNTf2) was calculated as HNTf2 is one of the strongest(a) Leito, I.; Raamat, E.; Kütt, A.; Saame, J.;
Kipper, K.; Koppel, I. A.; Koppel, I.; Zhang, M.; Mishima, M.; Yagupolskii, L. M.;
Garlyauskayte, R. Y.; Filatov, A. A. J. Phys. Chem. A. 2009, 113, 8421. (b) Zhang, M.;
Sonoda, T; Mishima, M.; Honda, T.; Leito, I.; Koppel, I. A.; Bonrath, W.; Netscher, T. J.
Phys. Org. Chem. 2014, 27, 676–679. common acids with experimentally determined GA
–1 of 286.5 kcal mol and pKa -11.9 in DCE (against picric acid).Kütt, A.; Rodima, T.;
Saame, J.; Raamat, E.; Mäemets, V.; Kaljurand, I.; Koppel, I. A.; Garlyauskayte, R. Y.;
Yagupolskii, Y. L.; Yagupolskii, L. M.; Bernhardt, E.; Willner, H.; Leito, I. J. Org.
Chem. 2011, 76, 391–395..
It was The calculations show ed that in the gas phase HCB11F12 has strong interaction with dichloroethane DCE molecule via a hydrogen bond with ΔH° = –19.6 kcal mol –1
–1 (ΔG° = –9.5 kcal mol ). Similarly, H2B12F12 exothermically binds two C2H4Cl2 DCE molecules in the gas phase with overall ΔH° = –30.8 kcal mol –1 and ΔG° = –12.6 kcal
22 mol –1 . As very strong hydrogen bonds with dichloroethane DCE are very uncommon and may not have been adequately parametrizised in SMD, also a cluster-continuum model was applied in which the DCE adducts were used for the Gibbs solvation energy calculation (Born-Fajans-Haber Cs cylces in the SI).
Furthermore, it was noticed that contrary to the gas phase where the proton is located on a
– B1B2B3 facet, in solution the Cs-symmetric tautomer of HB12F12 with an F – H bond is more stable. This tautomerization energy was included in the Gibbs solvation energy (see
SI).
There is presented on the Figure 5 presents the Born-Fajans-Haber protolysis cycle
(BFHC) of HNTf2. Mainly due to the very low basicity of dichloromethane DCE as bulk solvent the calculations resulted in a very high absolute pKa of 32.6. From this it could be concluded that also other common Brønsted acid molecules like H2SO4, HSO3F or HClO4 may have absolute pKa values in DCE around 30 or above.
-1 Figure 5. BFHC for HNTf2 protoysis in DCE. ΔG° values in kcal mol .
Using first-year knowledge, one can calculate a pH of 17.8 and a protolysis degree of
–15 1.6*10 for a 1 millimolar solution of HNTf2 in DCE. At first sight this appears not very acidic, but remembering the (above discussed) shift of 39.2 units between the aqueous and the DCE pH scales, the proton´s chemical potential is by 29.2 kcal mol –1 higher compared to water at pH 0Himmel, D.; Goll, S. K.; Leito, I.; Krossing, I. Chem. Eur. J.
2011, 17, 5808−5826. and by absolute acidity this solution would correspond to an aqueous solution with pH of –21.9.
23 The calculated protolysis cycle for H2B12H12 (Figure 6) reveals an interesting result. The large gain from solvation energy overcomes the GA difference between H2B12H12 and
– HB12H12 , thus, the second pKa is calculated to be lower than the first one!
-1 Figure 6. BFHC for H2B12H12 protoysis in DCE. ΔG° values in kcal mol .
We would like to point out that as dianions were not included in the parametrization of
2– SMD. Thus it is possible that the calculated Gibbs solvation energy of the B12H12 dianion (-145.8 kcal mol -1 ) may have a considerably less negative value than calculated, . so aAt this time it is unclear whether this result is an artefact of the applied model or not.
The HCB11H12 is calculated to be somewhat more acidic than its borane counterpart with absolute pKa in DCE of 20.4 (Figure 7).
-1 Figure 7. BFHC for HCB11H12 protoysis in DCE. ΔG° values in kcal mol .
The Born-Fajans-Haber cycle (BFHC) for the protolysis of H2B12F12 is shown in Figure 8.
-1 Figure 8. BFHC for H2B12F12 protoysis in DCE. ΔG° values in kcal mol .
As H2B12F12, is much more acidic in the gas phase compared to the previously discussed compounds, we calculated a pKa around zero for the first dissociation step which would classify it as a “strong acid” in DCE. Again, as discussed above, the pKa for the second
2– dissociation step should be taken with caution, as the ΔsolvG°(B12F12 ) is questionable. An
24 even slightly lower pKa of –1.6 was calculated for the perfluoro carborane acid whose protolysis BFHC is shown in Figure 9.
-1 Figure 9. BFHC for HCB11F12 protoysis in DCE. ΔG° values in kcal mol .
These estimated pKa values can be compared with the published experimental values in
DCEKütt, A.; Rodima, T.; Saame, J.; Raamat, E.; Mäemets, V.; Kaljurand, I.; Koppel, I.
A.; Garlyauskayte, R. Y.; Yagupolskii, Y. L.; Yagupolskii, L. M.; Bernhardt, E.; Willner,
H.; Leito, I. J. Org. Chem. 2011, 76, 391–395. , if one takes into account that the experimental values in Kütt, A.; Rodima, T.; Saame, J.; Raamat, E.; Mäemets, V.;
Kaljurand, I.; Koppel, I. A.; Garlyauskayte, R. Y.; Yagupolskii, Y. L.; Yagupolskii, L.
M.; Bernhardt, E.; Willner, H.; Leito, I. J. Org. Chem. 2011, 76, 391–395. and have been arbitrarily anchored to picric acid (HPi) with pKa taken as zero. The experimental pKa of
HNTf2 relative to HPi is -11.9.Kütt, A.; Rodima, T.; Saame, J.; Raamat, E.; Mäemets, V.;
Kaljurand, I.; Koppel, I. A.; Garlyauskayte, R. Y.; Yagupolskii, Y. L.; Yagupolskii, L.
M.; Bernhardt, E.; Willner, H.; Leito, I. J. Org. Chem. 2011, 76, 391–395. Based on the pKa of HNTf2 32.6 computed in this work the pKa values of the acids investigated in this work are relative to HPi as follows: H2B12H12 -17.8, HCB11H12 -24.1, H2B12F12 -44.6 and
HCB11F12 -46.1. The most acidic acid on the experimental scaleKütt, A.; Rodima, T.;
Saame, J.; Raamat, E.; Mäemets, V.; Kaljurand, I.; Koppel, I. A.; Garlyauskayte, R. Y.;
Yagupolskii, Y. L.; Yagupolskii, L. M.; Bernhardt, E.; Willner, H.; Leito, I. J. Org.
Chem. 2011, 76, 391–395. is CF3SO(=NTf)NHTf with pKa -18 relative to HPi. Thus even though the pKa values found in this work are estimates, it is clear that the borate- and
25 carborate-based acids can offer acidities by many orders of magnitude stronger than any class of acids with pKa values thus far measured in DCE or any other solvent.
Due to the enormous acidities of the more acidic boranes and carboranes, it can be concluded that DCE is not suited as superacid chemistry solvent for these compounds, not only because of its leveling effect, but also because of the rearrangement and decomposition reactions of the protonated DCE may limit its practical use.
Conclusion
In this investigation two new directions of designing possibly more available superstrong
Brønsted acids were studied. The acidities of, both, B12H12H2 and PF3B12H11H-acids are very similar to the CB11H12H. However, when the substituents are inserted the derivatives
-1 of B12H12H2 are in most cases about 5 – 8 kcal mol less acidic than the respective derivatives of the carborane family. In the case of B12(CF3)6H6H2 the difference is about
-1 -1 18.4 kcal mol , and in case of the B12F12H2 only -0.3 kcal mol .
Although, for the PF3B12H11H derivatives the differences in acidities are less the decrease of the anion’s charge with a positively charged substituent group does not have a significant effect on the gas-phase acidity. The intrinsic gas-phase acidities from these computations were in good accordance with the existing knowledge about acidities in gas phase. According to model pKa calculations, the most acidic borane and carborane derivatives may be strong acids (with absolute pKa < 0) and fully dissociated even in very low basic 1,2-dichloroethane, while common “strong” acids like HNTf2 may have absolute pKas pKa values around 30 or higher.
26 Acknowledgment
This work was supported by the Grant 8162 from the Estonian Science Foundation and also by the Centre of Excellence HIGHTECHMAT (SLOKT117T), by the targeted financing SFO180089008 as well as the institutional funding IUT20-14 (TLOKT14014I) from the Ministry of Education and Research of Estonia and by the ERC in the Advanced
Grant UniChem.
Supporting Information Available: Full details of quantum chemical calculations of
Table 1 and Table 2 (S1, S2, S4); complete ref. Gaussian 09, Revision A.1, Frisch, M. J. et al. Gaussian, Inc., Wallingford CT, 2009.23; supporting information for the pKa calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
27 References
1. Lipping, L.; Leito, I.; Koppel, I.; Koppel, I. A. J. Phys. Chem. A. 2009, 113,
12972-12978.
2. Olah, G. A.; Prakash, K. S.; Sommer, J. Science 1979, 206, 13-20.
3. Strauss, S. H. Chem. Rev. 1993, 93, 927-942.
4. Koppel, I. A.; Taft, R. W.; Anvia, F.; Zhu, S.-Z.; Hu, L.-Q.; Sung, K.-S.;
DesMarteau, D. D.; Yagupolskii, L. M.; Yagupolskii, Y. L.; Ignat’ev, N. V.;
Kondratenko, N. V.; Volkonskii, A. Y.; Vlasov, V. M.; Notario, R.; Maria, P.-C.
J. Am. Chem. Soc. 1994, 116, 3047-3057.
5. Koppel, I. A.; Burk, P.; Koppel, I.; Leito, I. J. Am. Chem. Soc. 2002, 124, 5594-
5600.
6. Vianello, R.; Maksić, Z. B. J. Org. Chem. 2010, 75, 7670-7681.
7. Koppel. I. A.; Burk, P.; Koppel, I.; Leito, I.; Sonoda, T.; Mishima, M. J. Am.
Chem. Soc. 2000, 122, 5114-5124.
8. Stoyanov, E. S.; Hoffmann, S. P.; Juhasz, M; Reed, C. A. J. Am. Chem. Soc.,
2006, 128, 3160-3161.
9. Meyer, M. M.; Wang, X.-B.; Reed, C. A.; Wang, L.-S.; Kass, S. R. J. Am. Chem.
Soc. 2009, 131, 18050-18051.
10. Krossing, I.; Raabe, I. Angw. Chem. Int. Ed. 2004, 43, 2066-2090.
11. Reed, C. A. Chem. Commun. 2005, 1669-1677.
12. Lipping, L.; Koppel, I. A.; Koppel, I.; Leito, I. Proceedings of the Estonian
Academy of Sciences. Chemistry. 2006, 55, 145 - 154.
13. Küppers, T.; Bernhardt, E.; Eujen, R.; Willner, H.; Lehmann, C. W. Angew.
Chem. Int. Ed. 2007, 46, 6346-6349.
28 14. Stoyanov, E. S.; Gunbas, G.; Hafezi, N.; Mascal, M.; Stoyanova, I. V.; Tham, F.
S.; Reed, C. A. J. Am. Chem. Soc., 2012, 134, 707-714.
15. Nava, M.; Reed, C. A. Organometallics, 2011, 30, 4798-4800.
16. Tsurumaki, E.; Hayashi, S.; Tham. F. S.; Reed, C. A.; Osuka, A. J. Am. Chem.
Soc. 2011, 133, 11956-11959.
17. Stoyanov, E. S.; Stoyanova, I. V.; Reed, C. A. J. Am. Chem. Soc. 2011, 133,
8452-8454.
18. Stoyanov, E. S.; Stoyanova, I. V.; Tham, F. S.; Reed, C. A. J. Am. Chem. Soc.
2010, 132, 4062-4063.
19. Avelar, A.; Tham, F. S.; Reed, C. A. Angew. Chem. Int. Ed. 2009, 48, 3491-3493.
20. Stoyanov, E. S.; Kim, K.-C.; Reed C. A. J. Am. Chem. Soc. 2006, 128, 8500-
8508.
21. Juhasz, M.; Hoffman S. P.; Stoyanov, E. S.; Kim, K.-C.; Reed, C. A. Angew.
Chem. Int. Ed. 2004, 43, 5352 –5355.
22. Marcus, Y. Chem. Soc. Rev. 1993, 22, 409-416.
23. Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3 rd ed.; Wiley-
VCH: Weinheim, Germany, 2003.Gaussian 09 default value.
24. Kütt, A.; Rodima, T.; Saame, J.; Raamat, E.; Mäemets, V.; Kaljurand, I.; Koppel,
I. A.; Garlyauskayte, R. Y.; Yagupolskii, Y. L.; Yagupolskii, L. M.; Bernhardt,
E.; Willner, H.; Leito, I. J. Org. Chem. 2011, 76, 391–395.
25. Gaussian 09, Revision A.1, Frisch, M. J. et al. Gaussian, Inc., Wallingford CT,
2009.
26. Becke, A. D. Phys. Rev. A 1988, 38, 3098 – 3100.
29 27. Perdew, J. P. Phys. Rev. B 1986, 33, 8822 – 8824; erratum: Perdew, J. P. Phys.
Rev. B 1986, 34, 7406.
28. Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
29. Eichkorn, K.; Treutler, O.; Oehm, H.; Häser, M.; Ahlrichs, R. Chem. Phys. Lett.
1995, 242, 652 – 660.
30. TURBOMOLE V6.4 2012, a development of University of Karlsruhe and
Forschungszentrum Karlsruhe GmbH, 1989–2007, TURBOMOLE GmbH, since
2007, see http://www.turbomole.com.
31. Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kølmel, C. Chem. Phys. Lett. 1989,
162, 165-169.
32. Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346 – 354.
33. Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B. 2009,113 , 6378-
6396.
34. (a) Leito, I.; Raamat, E.; Kütt, A.; Saame, J.; Kipper, K.; Koppel, I. A.; Koppel, I.;
Zhang, M.; Mishima, M.; Yagupolskii, L. M.; Garlyauskayte, R. Y.; Filatov, A.
A. J. Phys. Chem. A. 2009, 113, 8421. (b) Zhang, M.; Sonoda, T; Mishima, M.;
Honda, T.; Leito, I.; Koppel, I. A.; Bonrath, W.; Netscher, T. J. Phys. Org. Chem.
2014, 27, 676–679.
35. Pliego, J. R. Jr.; Riveros, J. M. J. Phys. Chem. A. 2001, 105 , 7241-7247.
36. Himmel, D.; Goll, S. K.; Leito, I.; Krossing, I. Chem. Eur. J. 2011, 17,
5808−5826.
37. Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. H.;
Earhart, A. D.; Coe, J. V. J. Phys. Chem. A. 1998, 102, 7787-7794.
38. Körbe, S.; Schreiber, P. J.; Michl, J. Chem. Rev. 2006, 106, 5208-5249.
30 39. Umeyama, H.; Morokuma, K. J. Am. Chem. Soc. 1977, 99, 1316-1332.
40. Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565-573.
41. Kollman, P. A.; Allen, L. C. Chem. Rev. 1972, 72, 283-303.
42. Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.; Koetzle, T.
F. Acc. Chem. Res. 1996, 29, 348-354.
43. Remer, L. C.; Jensen J. H. J. Phys. Chem. 2000, 104, 9266-9275.
44. Gao, J.; Garner, D. S.; Jorgensen W. L. J. Am. Chem. Soc. 1986, 108, 4784-4790.
45. Raamat, E.; Kaupmees, K.; Ovsjannikov, G.; Trummal, A.; Kütt, A.; Saame, J.;
Koppel, I.; Kaljurand, I.; Lipping, L.; Rodima, T.; Pihl, V.; Koppel, I. A.; Leito, I.
J. Phys. Org. Chem. 2013, 26, 162-170.
31 TOC Graphic
32