Computational and experimental study of charge distribution in the α-disulfonyl carbanions

Journal of Molecular Structure 1062 (2014) 35–43

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Computational and experimental study of charge distribution in the a-disulfonyl carbanions Iwona Binkowska ⇑, Jacek Koput, Arnold Jarczewski Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan, Poland

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The paper presents combined

experimental and computational study of carbanions of sulfonyl carbon acids  Charge distribution in the alphadisulfonyl carbanions is discussed using electron densities calculated from 13C NMR spectroscopy supported by the NBO electron density.  Very good correlation between measured and computed results were found.

a r t i c l e

i n f o

Article history: Received 19 November 2013 Received in revised form 3 January 2014 Accepted 3 January 2014 Available online 10 January 2014 Keywords: Proton transfer Carbon acids Charge distribution Ab initio computation

10 9

11

8

12

CH3 3 X

CH2

2

4 5

3

SO2 1

6

C H α SO2 CH2

(1 1) X = H 2) X = Cl (2 3) X = CN (3 4) X = NO2 (4

CH3

X

7 CH2

2

α 5

6

(5 5) X = H 6) X = Cl (6 8 (7 7) X = CN 8) X = NO2 9 (8 (9 9) X = CH3O

3

SO2

1

4

X

C H

2 α

SO2

C H H

5

CH2 7 12 11 10

Y 1

4

6

(1 10) X = H; Y = NO2 (1 11) X = NO2; Y = NO2 (1 12) X = H; Y = CN 13) X = NO2; Y = CN (1

a b s t r a c t The electron densities of the disulfonyl carbanions were determined using experimental 13C chemical shifts. The 13C NMR spectra and electron densities for the disulfonyl, nitro, and cyano carbon acids were calculated at the MP2/cc-pVDZ level of theory. The calculated chemical shifts for disulfonyl carbanions show satisfying correlation with our own experimental data. The calculated p electron densities at the Ca atom correspond roughly to the ‘‘experimental’’ p electron densities estimated from the 13C chemical shifts. The natural charges at Ca in disulfonyl stabilized carbanions are significantly more negative than with other types of carbanions, partly because of the significant negative natural charge of the a carbon in parent carbon acids. The calculated increase of the negative charge caused by ionization is larger for sulfonyl carbon acids than for cyano- and nitroalkanes. The 13C chemical shifts d of Ca in disulfonyl stabilized carbanions decrease with more negative calculated negative natural charge at Ca, with a slope of 220 ppm/electron. The influence of phenyl ring para-substitution on the charge distribution in carbanions and relationship between the 13C chemical shifts and charge density have been discussed. It appears that the p electron density in these planar or nearly planar carbanions has a decisive impact on the chemical shifts. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Despite the fact that an effective charge on a particular atom in a molecule is not an observable, thinking in terms of atomic charges appears to be almost indispensible in mechanistic considerations. Equilibrium acidity of carbon acids is affected by charge ⇑ Corresponding author. E-mail address: [email protected] (I. Binkowska). 0022-2860/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2014.01.007

delocalization in the carbanions. It is believed that the ability to delocalize the negative charge is determined by the type of activating group at the a-carbon, such as nitro, cyano or sulfonyl group. It has been noticed that the carbanions containing the cyano or sulfonyl groups are closer to ‘‘true carbanions’’, than carbanions of nitro-activated carbon acids, where the negative charge is largely shifted onto the electronegative oxygen atoms. Kinetic acidity of carbon acids is less dependent on the ability to delocalize the negative charge which makes an impression that cyano or

36

I. Binkowska et al. / Journal of Molecular Structure 1062 (2014) 35–43

sulfonyl-activated carbon acids are intrinsically faster than their nitro analogues. The 13C chemical shifts depend on a number of factors, including the charge on a carbon atom. When sp3–sp2 rehybridization occurs on ionization, it has a large impact on the chemical shift, so the chemical shifts in planar carbanions should be referenced to those in alkenes that have same substituents as the carbanion rather than to the 13C chemical shifts in the parent carbon acid. In these useful correlations, it is important to make sure that the carbon atom in the carbanion has the same sp2 hybridization as that in the reference alkene. The anions of sulfonyl compounds may have different hybridization of Ca, and different degree of pyramidalization [1]. The correlation between charge density at a carbon atom and its 13 C chemical shift has been studied by various authors [2–5]. The charges on the carbon atoms has been assessed for various carbanions including sulfonyl derivatives. We have been studying acidic properties and the structure of carbon acids activated by two sulfonyl groups [6]. Therefore, we decided to analyze 13C NMR spectra to assess the charge distribution in these carbanions. In this work, ab initio calculations have also been employed to determine the effective atomic charges. It is well known that the calculated atomic charges strongly depend on the way a particular method assign electron density to neighboring atoms. The atomic charges obtained by the natural-bond-orbital (NBO) analysis [7,8] are usually considered to be more consistent with chemical intuition than those obtained by the Mulliken analysis. Focusing our interest on charge distribution in disulfonyl carbanions, the changes in chemical shifts after ionization of carbon acids shown in Fig. 1 gave information about electron density in these carbanions. We have determined the structure of the complex of 4-nitrophenyl[bis(ethylsulfonyl)]methane with 1,5,7-triazabicyclo[4.4.0]dec-5-ene [9]. The crystal structure shows the planarity of the carbanionic center (Ca) with the sum of the angles 359.2°. The sum of the analogous angles in 4-nitrophenyl[bis(ethylsulfonyl)]methane was 336.1° [10]. The bond between Ca and the ipso carbon atom of the phenyl ring is shortened in the carbanion, suggesting that the negative charge of the anion is located in this region. The length of the Ca–S bonds are even more affected by ionization of the carbon acid. The energy difference between planar and pyramidal carbanionions stabilized by sulfonyl group is usually small, for instance for lithium salt of dimethylsulfone it is only 0.57 kcal mol 1 [11,12]. In order to gain more information concerning the charge distribution in carbanions under consideration, we have calculated the electron densities from the experimental NMR data using the method described previously [5,13]. On the other hand, the charge densities were obtained in quantum chemical calculations at the MP2/cc-pVDZ level of theory.

Table 1 The experimentally determined values of Ai and

H

123.3a

C6H5

135.8b 137.0a 135.7a 135.4a 134.8a 136.4a

12.5 13.7 12.4 12.1 11.5 13.1

[5] [17] [17] [18] [17] [18]

136.2b 135.8c 135.1a

12.9 12.5 11.8

[19]

C1 shift (ppm)

Cl–C6H5 CN–C6H5 NO2–C6H5 CH3O–C6H5 (SO2CH2CH3)2 (SO2CH2Ph)2 a

X

2

4

1 5

3

SO2

6

C H α SO2 CH2

(1 1) X = H 2) X = Cl (2 3) X = CN (3 (4 4) X = NO2

CH3

X

c

0.00

Reference [16]

2. Experimental The ethylsulfonyl carbon acids (1–4) and benzylsulfonyl carbon acids (5–9) were prepared according to the procedure described previously [6]. 4-Nitrophenylcyanomethane (13) from Fluka was recrystallized from ethanol. The purity of compounds was confirmed by 1H NMR and melting points. 1 H and 13C NMR measurements were carried out at the operating frequency 300, and 75 MHz, respectively. The chemical shifts are reported in ppm relative to tetramethysilane (TMS) as the internal standard. The disulfonyl carbon acids were dissolved before experiments in NMR tubes as 0.1 or 0.2 M solution, then tetramethylammonium hydroxide (TMAOH, Aldrich) was added. A 20% excess of TMAOH was used to ensure complete deprotonation of the carbon acid. The experimental p-electron densities at Ca were calculated using 13C NMR shifts of anions, shielding contributions of substituents (Ai values of Table 1), which in turn were found from 13C1 shifts in substituted ethylenes, that is our reference molecules for Ca. For the studied disulfonyl carbanions planar geometry of the carbanionic center was assumed. The ab initio calculation yielded the structures in which the a-carbon was not perfectly planar. However, the energy difference between strictly planar structure such as that found in the crystal of the salt of (1), and the slightly pyramidalized carbanion structure resulting from our ab initio calculations is as low as 0.1 kcal mol 1. The calculation of experimental p electron densities q at sp2 hybridized carbons of disulfonyl carbanions was performed according to equation [5]: danion = dref k(q 1), where k = 160. For Ca, dref = 123.3 + RAi, while for other carbons dref is 13C chemical shift for a given (aromatic) carbon in parent carbon acid.

9

11

8

12 7 CH2

2

α 6

(5 5) X = H 6) X = Cl (6 8 (7 7) X = CN 8) X = NO2 9 (8 (9 9) X = CH3O

3

SO2

1

4 5

Ai

CDCl3. DMSO-d6. MeCN-d3.

b

CH3 3

C1 shifts in substituted ethylenes.

Substituent

10

CH2

13

13

X

C H

2 α

SO2

C H H

5

CH2 7 12 11 10

Y 1

4

6

(1 10) X = H; Y = NO2 (1 11) X = NO2; Y = NO2 (1 12) X = H; Y = CN 13) X = NO2; Y = CN (1

Fig. 1. Disulfonyl, nitro and cyano carbon acids with carbon atoms labeling.

37

I. Binkowska et al. / Journal of Molecular Structure 1062 (2014) 35–43 Table 2 The equilibrium angles in ethylsulfonyl carbanions calculated at the MP2/cc-pVDZ level of theory. Anion

CPh–Ca–S angle

S–Ca–S angle

Sum of angles

CPh–Ca–S–C dihedral

S–Ca–S–C dihedral

Piramidalization angle

(1)

122.6 119.8

116.7

359.0

57.8 119.9

133.1 71.3

10.9 11.2

(2)

122.6 119.7

116.7

359.0

58.8 120.6

132.4 71.0

11.2 11.5

(3)

123.2 119.8

116.2

359.3

58.5 119.0

130.9 70.8

9.4 9.8

(4)

123.3 119.9

116.1

359.3

58.3 118.5

130.7 70.8

9.0 9.4

The 13C shifts and p electron densities were also determined in ab initio calculations at the MP2/cc-pVDZ level of theory, using the Gaussian 03 package of programs [14]. The bulk-solvent effects were investigated using the conductor polarizable continuum model (CPCM) [15]. The relative 13C chemical shifts were always calculated by subtracting the absolute chemical shielding from that of a reference compound, TMS. The value of the absolute carbon shielding was determined to be 207.2, 209.9, and 210.2 ppm for the isolated TMS molecule, the TMS molecule in acetonitrile (the gas-phase equilibrium structure), and the TMS molecule in acetonitrile (the optimized equilibrium structure), respectively.

Our ab intio calculations show that the diethylsulfonyl stabilized carbanions 1–4 should be planar (the sum of the angles is 359°, see Table 2). This together with planar carbanionic center of anion 4 in the crystal structure (sum of the angles 359.2°) justifies our assumption of sp2 hybridization of Ca in the disulfonyl stabilized carbanions. Other structural features of 4 and its anion and their changes caused by ionization, such as the twist angle of 4-nitrophenyl relative to the Ca plane in the anion of 34°, compared to 35° in the crystal, Ca–C1 bond shortening by 0.05 Å

Table 3 C shifts (ppm) of C6H5CH(SO2CH2CH3)2 (1) and the related carbanion: calculated at the MP2/cc-pVDZ level of theory and observed in DMSO and MeCN.

Table 4 C shifts (ppm) of Cl–C6H4CH(SO2CH2CH3)2 (2) and the related carbanion: calculated at the MP2/cc-pVDZ level of theory and observed in DMSO and MeCN.

13

Compound

Calculated

13

C shifts

Experimental C shifts

3. Results and discussion

13

Compound

Calculated

13

Gas phase

MeCNa,c

MeCNb,c

DMSO

MeCNd

94.7 119.8 126.6 126.3 125.5 125.8 137.8 55.9 51.2 7.9 6.9 0.986 0.988 0.6 0.4

94.5 124.8 120.4 118.6 119.8 117.6 131.6 54.8 49.1 2.8 2.1 0.970 0.972 4.1 5.2

91.5 126.9 121.2 118.0 119.9 117.6 131.6 54.6 49.6 1.4 2.0 0.974 0.976 4.0 5.1

78.8 124.6 133.0

80.5 125.2 133.8

129.1

130.2

135.5 48.6

137.4 49.9

5.6

6.2

84.8 134.6 120.2 124.6 118.5 118.3 122.9 55.6 51.0 10.4 10.0 0.988 0.985 2.3 2.1

84.0 137.9 113.8 115.9 113.3 113.1 119.3 55.3 50.4 1.4 0.9 0.969 0.964 6.4 6.3

82.2 137.5 115.1 119.4 114.3 114.1 120.4 55.8 49.7 1.9 0.8 0.976 0.970 5.9 5.8

73.1 137.2 133.2

73.1 139.3 131.8

126.3

128.3

126.6 51.7

129.1 46.2

8.5

10.1

C shifts

Experimental 13 C shifts

13

Carbon acid (1)

Ca C1 C2,6 C3,5 C4 CH2 CH3 2

r (DMSO) r2(MeCN) MD(DMSO) MD(MeCN) Carbanion (1)

Ca C1 C2,6 C3,5 C4 CH2 CH3 2

r (DMSO) r2(MeCN) MD(DMSO) MD(MeCN)

Gas phase

MeCNa,c

MeCNb,c

DMSO

MeCNd

95.4 122.0 125.3 125.8 125.4 125.2 123.8 55.8 51.1 7.9 6.9 0.987 0.988 0.1 1.0

95.9 126.0 117.6 119.8 122.4 123.6 123.0 55.2 49.6 1.0 0.8 0.974 0.975 2.8 3.9

92.8 128.0 116.8 120.5 122.5 124.9 123.5 55.0 50.2 0.2 0.3 0.978 0.979 2.7 3.8

79.7 125.6 130.3

81.5 126.3 132.2

128.9

130.0

C3,5

131.2 48.4

131.6 49.8

C4 CH2

5.6

6.2

CH3

84.4 136.5 119.4 123.7 118.8 118.2 110.9 51.0 55.7 10.1 10.6 0.986 0.981 3.0 2.8

83.5 138.6 112.7 117.9 118.4 117.9 111.9 50.9 55.6 2.0 2.1 0.971 0.967 5.8 5.6

82.0 138.3 114.3 118.4 119.2 118.8 113.6 50.2 56.2 1.8 2.6 0.978 0.974 5.2 4.9

73.2 137.9 132.5

73.3 140.5 130.0

126.5

128.4

C3,5

122.6 51.6

126.4 46.3

C4 CH2

8.6

10.1

CH3

Carbon acid (2)

Ca C1 C2,6

2

r (DMSO) r2(MeCN) MD(DMSO) MD(MeCN)

r2(DMSO) and r2(MeCN) – the squared correlation coefficient for the experimental data in DMSO and MeCN, respectively. MD(DMSO) and MD(MeCN) – the mean deviation with respect to experimental data in DMSO and MeCN, respectively, in ppm. a Calculated for the gas-phase equilibrium structure. b Calculated for the equilibrium structure in MeCN solution. c Taking into account the MeCN bulk-solvent effect. d Data from ref [6].

Carbanion (2)

Ca C1 C2,6

2

r (DMSO) r2(MeCN) MD(DMSO) MD(MeCN)

r2(DMSO) and r2(MeCN) – the squared correlation coefficient for the experimental data in DMSO and MeCN, respectively. MD(DMSO) and MD(MeCN) – the mean deviation with respect to experimental data in DMSO and MeCN, respectively, in ppm. a Calculated for the gas-phase equilibrium structure. b Calculated for the equilibrium structure in MeCN solution. c Taking into account the MeCN bulk-solvent effect. d Data from ref [6].

38

I. Binkowska et al. / Journal of Molecular Structure 1062 (2014) 35–43

Table 5 C shifts (ppm) of CN–C6H4CH(SO2CH2CH3)2 (3) and the related carbanion: calculated at the MP2/cc-pVDZ level of theory and observed in DMSO and MeCN. 13

Compound

Carbon acid (3)

Ca C1 C2,6 C3,5 C4 CH2 CH3 CN r2(DMSO) r2(MeCN) MD(DMSO) MD(MeCN)

Carbanion (3)

Ca C1 C2,6 C3,5 C4 CH2 CH3 CN r2(DMSO) r2(MeCN) MD(DMSO) MD(MeCN)

Table 6 C shifts (ppm) of NO2–C6H4CH(SO2CH2CH3)2 (4) and the related carbanion: calculated at the MP2/cc-pVDZ level of theory and observed in DMSO and MeCN. 13

Calculated

13

Gas phase

MeCNa,c

MeCNb,c

DMSO

MeCNd

95.0 123.5 127.9 128.0 126.3 126.1 115.7 56.2 51.4 7.9 6.8 121.1 0.985 0.987 0.5 0.4

95.1 128.6 122.3 124.0 121.1 123.5 116.4 55.3 50.0 1.6 0.7 111.6 0.973 0.976 2.8 3.8

91.9 130.8 121.8 124.6 121.2 123.8 116.4 55.1 50.6 0.2 0.6 111.5 0.978 0.980 2.7 3.7

78.9 130.8 132.1

80.6 131.4 132.9

132.8

133.7

C3,5

113.1 48.8

115.0 50.1

C4 CH2

5.6

6.1

CH3

118.2

118.2

90.0 137.2 120.0 123.5 119.9 119.2 99.5 55.3 50.7 10.1 9.8 122.5 0.990 0.984 1.8 2.4

89.2 141.8 115.7 117.0 118.0 117.2 103.5 55.3 50.7 2.1 2.0 113.0 0.980 0.968 4.5 5.1

87.4 141.7 117.1 120.3 118.7 117.9 104.4 55.4 50.0 2.7 2.0 113.0 0.984 0.973 4.1 4.7

80.3 144.6 127.0

79.9 145.0 130.6

130.2

132.5

100.1 51.8

102.9 46.2

8.0

10.1

120.7

120.0

C shifts

Experimental 13 C shifts

Compound

Carbon acid (4)

Ca C1 C2,6

r2(DMSO) r2(MeCN) MD(DMSO) MD(MeCN) Carbanion (4)

Ca C1 C2,6 C3,5 C4 CH2 CH3

r2(DMSO) and r2(MeCN) – the squared correlation coefficient for the experimental data in DMSO and MeCN, respectively. MD(DMSO) and MD(MeCN) – the mean deviation with respect to experimental data in DMSO and MeCN, respectively, in ppm. a Calculated for the gas-phase equilibrium structure. b Calculated for the equilibrium structure in MeCN solution. c Taking into account the MeCN bulk-solvent effect. d Data from ref [6].

(0.04 Å in the solids), Ca–S bond shortening by 0.1 Å (0.11 Å in the solids) and C4–NO2 distance shortening by 0.02 Å (0.01 Å in the solids) are well reproduced by our ab initio calculations. Apparently crystal forces do not distort the structure of the anion 4 to any significant extent. The structural changes indicate considerable strengthening of the C–S bonds, whereas very small changes in the 4-nitro group and Ca–C1 bond lengths suggest that resonance stabilization of the negative charge by this twisted nitrophenyl group is not very strong. The shortening of the C–S bonds can be qualitatively explained by more effective overlap of the enlarged p orbital of Ca in the carbanion with the large 3p orbital of sulfur, leading to stronger C–S r bond. A p-p – d-p overlap cannot be effective because the 3d orbitals of S atoms are too diffuse and high in energy. Ab initio calculated structures of carbon acids 1–3 and their carbanions show similar features as discussed above for 4. The 13C chemical shifts of disulfonyl carbon acids and their carbanions measured in dimethyl sulfoxide and acetonitrile are shown in Tables 3–6. These results correlate well with the quantum chemical calculated shifts in vacuum and acetonitrile. As apparent from data of Tables 3–6, the experimental 13C shifts are very similar in MeCN and DMSO. The differences between the experimental and calculated shifts are of a few ppm and do not exceed 10 ppm, which is an accuracy expected typically for ab initio

r2(DMSO) r2(MeCN) MD(DMSO) MD(MeCN)

Calculated

13

Gas phase

MeCNa,c

MeCNb,c

DMSO

MeCNd

94.7 123.4 129.6 129.7 117.7 117.5 145.7 56.2 51.4 7.9 6.8 0.986 0.987 0.1 1.1

94.8 128.8 124.5 126.2 110.4 112.7 143.8 55.3 49.9 1.9 1.2 0.975 0.977 3.5 4.5

91.6 131.1 124.1 126.9 110.5 113.0 143.4 55.1 50.5 0.5 0.9 0.980 0.981 3.4 4.4

78.6 132.6 132.6

80.4 133.4 133.2

124.0

124.9

148.6 48.8

150.2 50.2

5.6

6.2

92.2 137.8 120.0 123.5 112.5 112.0 134.5 50.7 55.2 9.7 10.0 0.99 0.99 2.3 3.4

91.7 143.2 116.5 117.6 108.5 107.8 132.9 50.5 55.0 1.4 1.6 0.98 0.97 5.1 6.2

89.6 143.0 117.7 120.8 109.0 108.2 134.0 49.8 55.2 1.5 2.1 0.99 0.98 4.8 5.8

85.2 147.8 124.3

84.3 148.5 130.8

122.4

123.6

138.1 51.6

140.4 46.2

7.7

10.1

C shifts

Experimental 13 C shifts

r2(DMSO) and r2(MeCN) – the squared correlation coefficient for the experimental data in DMSO and MeCN, respectively. MD(DMSO) and MD(MeCN) – the mean deviation with respect to experimental data in DMSO and MeCN, respectively, in ppm. a Calculated for the gas-phase equilibrium structure. b Calculated for the equilibrium structure in MeCN solution. c Taking into account the MeCN bulk-solvent effect. d Data from ref [6].

calculations at the MP2/cc-pVDZ level of theory applied in this work. In Tables 7 and 8, changes in chemical shifts occurring upon ionization of disulfonyl carbon acids are given. The variation of the chemical shifts could be helpful in evaluation of the structural reorganization during formation of carbanion. In general, it is known that high-field shifts (DdC < 0) are caused by the presence of negative charge, while the sp3 ? sp2 rehybridization of the a carbon atom, contributes to the low-field shifts (DdC > 0). A comparison of DdC of carbanionic center (Ca) shows that the changes are not large, ranging between 7.9 and 6.5 ppm, but they correlate with the electron withdrawing/donating nature of substituent attached to the phenyl ring, e.g. with substituent constant (Fig. 2). The delocalization of the negative charge from Ca (carbon acids 1–9) results always in upfield shifts of the para carbon atom Table 7 Changes in the experimental 13C chemical shifts (DdC, ppm) after ionization of bis(ethylsulfonyl)methanes (1–4) in DMSO.

DdCa (1) (2) (3) (4)

6.50 5.64 1.37 6.51

DdC = dCanion

DdCCH2

DdCCH3

DdC1

DdC2,6

DdC3,5

3.21 3.09 2.98 2.81

3.04 2.93 2.44 2.13

12.28 12.58 13.75 15.20

2.14 0.18 5.06 8.28

2.43 2.86 2.66 1.56

dCC-acid, ppm.

DdC4 8.67 8.95 13.00 10.56

39

I. Binkowska et al. / Journal of Molecular Structure 1062 (2014) 35–43 Table 8 Changes in the experimental

DdCa (5) (6) (7) (8) (9)

5.97 5.39 0.12 4.15 7.94

DdC = dCanion

13

C chemical shifts (DdC, ppm) after ionization of bis(benzylsulfonyl)methanes (5–9) in DMSO.

DdC1

DdC2,6

DdC3,5

11.85 12.08 12.83 14.29 39.21

0.41 5.98 4.83 6.90 4.55

3.15 3.75 3.79 2.95 2.96

DdC4 8.82 3.02 13.23 10.43 26.67

DdCCH2

dC7

dC8,12

3.63 2.69 2.33 2.21 2.27

6.55 5.55 5.68 5.28 6.67

2.15 2.27 2.05 2.06 2.27

dC9,11 0.99 0.76 0.78 0.71 0.88

dC10 4.83 6.01 5.02 5.43 3.99

dCC-acid, ppm.

8

Table 10 C shifts (ppm) of cyano carbon acids: phenylcyanomethane (12) and (4-nitrophenyl)cyanomethane (13) and the related carbanions: calculated at the MP2/ccpVDZ level of theory and observed in DMSO. 13

6 4

0

13

Δδ C

13

Compound

2

Ca C1 C2,6 C3,5 C4 CN

27.6 127.4 122.8; 122.6 124.3; 123.5 122.8 112.2

22.6a – – – – –

Carbanion (12)

Ca C1 C2,6 C3,5 C4 CN r2 MD

38.3 144.2 111.4; 112.5 116.4; 117.2 100.0 129.3 0.988 5.7

33.6a 148.9a 115.9a 127.6a 109.0a –

Carbon acid (13)

Ca C1 C2,6 C3,5 C4 CN r2 MD

26.9 128.8 126.4; 126.7 116.3; 117.0 144.8 111.5 0.992 4.4

22.4 139.1 129.4 124.0 147.0 118.4

Carbanion (13)

Ca C1 C2,6 C3,5 C4 CN r2 MD

53.0 143.1 113.0; 113.0 114.6; 114.0 126.6 125.8 0.978 3.4

61.8 152.8 115.2 117.3 125.8 125.0

-4 -6 benzylsulfonyl series ethylsulfonyl series

-10 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

σ− Fig. 2. 13Ca shifts changes (Dd13C, ppm) associated with ionization of disulfonyl carbon acids as a function of Hammett substituent constants.

Table 9 C shifts (ppm) of nitro carbon acids: phenylnitromethane (10) and (4-nitrophenyl)nitromethane (11) and the related carbanions: calculated at the MP2/cc-pVDZ level of theory and observed in DMSO. 13

13

Compound

C shifts (ppm)

Calculated gas phase

Experimental DMSO

Carbon acid (10)

Ca C1 C2,6 C3,5 C4

78.7 129.1 123.7; 123.7 124.8; 125.1 123.4

79.3a – – – –

Carbanion (10)

Ca C1 C2,6 C3,5 C4 r2 MD

96.0 137.8 112.8; 115.6 116.8; 118.8 105.7 0.885 9.6

109.2a 135.7a 123.9a 127.5a 123.0a

Carbon acid (11)

Ca C1 C2,6 C3,5 C4 r2 MD

77.8 130.1 127.7; 128.1 117.3; 117.8 145.5 0.992 4.1

77.6b 137.0b 132.1b 123.0b 148.0b

Carbanion (11)

Ca C1 C2,6 C3,5 C4 r2 MD

103.2 136.4 114.6; 115.8 112.6; 114.6 130.6 0.972 8.1

109.8b 143.3b 121.9b 123.9b 140.1b

r2 – correlation with experimental data in DMSO. MD – mean deviation with respect to experimental data in DMSO, ppm. a Chemical shifts from Ref. [5]. b Chemical shifts from Ref. [26].

Experimental DMSO

Carbon acid (12)

-2

-8

C shifts (ppm)

Calculated gas phase

r2 – correlation with experimental data in DMSO. MD – mean deviation with respect to experimental data in DMSO, ppm. a Chemical shifts from Ref. [5].

(C4) signals. The 13C resonances of ortho-carbon atoms also experience such low field shifts except for carbanions 1 and 2, for which minor positive shifts are observed. A similar pattern of 13C chemical shifts was found for metoxy-substituted benzyl carbanions [20]. The changes of the p electron density at Ca with the para substituent on the phenyl ring are moderate (Table 12) which together with discussed above relatively small high field shifts of the para and orto 13C resonances shows that stabilization of negative charge by the aromatic ring is in these carbanions, which are rich in electron withdrawing groups, is of secondary importance. The twist angle between the phenyl ring and the planar carbanionic center both in the solid and in the ab initio calculated structures of anions of 1–4 is large enough to hamper resonance stabilization of the negative charge. The twist is forced by two bulky tetrahedral sulfonyl groups at Ca. The proton transfer reactions between nitroalkanes or cyanoalkanes with strong nitrogenous bases such as guanidines and amidines have been extensively studied [21–25]. Therefore

40

I. Binkowska et al. / Journal of Molecular Structure 1062 (2014) 35–43

Table 11 The natural atomic charges and p electron densities (in e) for nitro- (10, 11) and cyanoalkanes (12, 13) calculated at the MP2/cc-pVDZ level of theory. Natural atomic chargea

Compound

C-acid

Calculated C-acid gas phase

Calculated anion gas phase

Experimental anion DMSO

(10)

Ca C1 C2,6 C3,5 C4 Phc NO2

0.27 0.07 0.21 0.20 0.22 1.11 0.23 0.45 (N), 0.34 (O)

0.23 0.08 0.24 0.25 0.29 1.35 0.66 0.42 (N), 0.54 (O)

– 1.02 1.00 0.98 1.00 5.98 1.18 (N), 1.62 (O)

1.27 0.98 1.02 1.03 1.08 6.16 1.25 (N), 1.57 (O)

1.308b – – – 1.04b 6.12b 0.57b

(11)

Ca C1 C2,6 C3,5 C4 Phc NO2 (–Ca)

0.20 0.06 0.20 0.17 0.07 1.02 0.29 0.61 0.45 0.28 0.62 0.45

0.25 0.03 0.29 0.15 0.04 0.89 0.58 0.58 0.58 0.42 0.62 0.52

– 1.00 0.99 0.94 1.08 5.94 1.15 (N), 1.70 (O)

1.34 0.85 1.10 0.91 1.24 6.11 1.05 (N), 1.64 (O)

1.30d 0.96d 1.06d 1.00d 1.05d 6.13d –

1.04 (N), 1.51 (O)

1.02 (N), 1.56 (O)

–

NO2 (–Ph)

c d

(N), (O) (N), (O)

(N), (O) (N), (O)

Ca C1 C2,6 C3,5 C4 Phc CN

0.54 0.04 0.22 0.21 0.22 1.12 0.02 0.30 (C), 0.32 (N)

0.60 0.05 0.25 0.25 0.31 1.36 0.23 0.28 (C), 0.51 (N)

– 1.01 0.99 0.99 0.99 5.96 0.92 (C), 1.12 (N)

1.40 0.98 1.05 1.04 1.11 6.27 0.96 (C), 1.23 (N)

1.55b – – – 1.12b 6.17b 0.28b

(13)

Ca C1 C2,6 C3,5 C4 Phc CN

0.50 0.00 0.23 0.16 0.06 0.72 0.03 0.35 0.38 0.28 0.62 0.45

0.59 0.08 0.32 0.14 0.06 0.90 0.18 0.38 0.56 0.47 0.61 0.54

– 0.96 1.01 0.92 1.11 5.93 0.90 (C), 1.13 (N)

1.41 0.84 1.13 0.90 1.27 6.17 0.85 (C), 1.30(N)

1.41 0.91 1.09 1.04 1.13 6.30 –

1.03 (C), 1.50 (O)

1.03 (C), 1.58 (O)

–

(C), (N) (N), (O)

(C), (N) (N), (O)

Total atomic charge determined by the natural-bond-orbital (NBO) population analysis. Ref. [5]. For the whole phenyl ring (C1–C6). Calculated using the chemical shifts from Ref. [26].

charge distribution in the anions of these carbon acids is of interest. In Tables 9–11, the 13C NMR data and the distribution of charge in carbanions of nitroalkanes (10, 11) and cyanoalkanes (12, 13) are presented. The results obtained for disulfonyl carbon acids show that these compounds have most of the negative charge present on the a carbon atom, and are more similar to those obtained for previously studied cyanoalkanes than nitroalkanes. The results obtained by Moutiers et al. [26] show that a significant delocalization of negative charge in the carbanion of (4-nitrophenyl)nitromethane occurs onto the exocyclic nitro group and over the nitrophenyl ring. The ionization of this nitroalkane results in relatively little change in the resonances of the phenyl protons and carbon atoms, which indicates that the role of nitrophenyl ring in the delocalization of the negative charge is of secondary importance. Regarding the large downfield shifts of the Ca atom upon ionization of (4-nitrophenyl)nitromethane, it is clear that rehybridization of the carbanionic center occurs [26]. Our results underline differences between the disulfonyl carbanions and nitroalkane anions. The NMR data presented for nitromethanes show that a large downfield shifts of Ha (DdHa = 0.99 ppm) and Ca

1.60

experimental π electron density

a

Anion

(12)

NO2

b

p electron density

(1) (2)

1.55

(3)

1.50

(4)

1.45 (13)

1.40 1.35 1.30 1.25 1.25

(10) (11)

1.30

1.35

1.40

1.45

1.50

1.55

1.60

1.65

NBO π electron density Fig. 3. Correlation between experimental (in DMSO) and calculated (MP2/cc-pVDZ, the NBO analysis) p electron density at a carbanionic atom for disulfonyl (1–4), nitro (10, 11) and cyano (13) carbon acids.

41

I. Binkowska et al. / Journal of Molecular Structure 1062 (2014) 35–43 Table 12 The natural atomic charges and p electron densities (in e) of disulfonyl carbon acids (1–4) and their anions calculated at the MP2/cc-pVDZ level of theory. Compound

(1)

Ca C1 C2,6 C3,5 C4 Phc CH2CH3 SO2

(2)

Ca C1 C2,6 C3,5 C4 Phc CH2CH3 SO2

a b c

C-acid

Calculated C-acid gas phase

Calculated anion gas phase

Exp.b anion DMSO

Exp.b anion MeCN

– 1.09 1.13 1.14 1.05 6.68 – 2.00 (S), 1.57 (O)

1.58 1.00 1.03 1.05 1.05 6.21 – 2.00 (S), 1.71 (O)

1.552 0.923 0.986 1.015 1.054 5.979 – –

1.547 0.911 1.014 1.010 1.032 5.991 – –

–

–

0.234

0.231

– 1.09 1.14 1.12 1.08 6.69 – 2.00 (S), 1.57 (O)

1.58 1.00 1.03 1.06 1.10 6.28 – 2.00 (S), 1.71 (O)

1.552 0.921 0.999 1.018 0.944 5.899 – –

1.548 0.912 1.012 1.012 1.052 6.012 – –

– – –

– – –

– – 0.549

– – 0.440

– 1.09 1.14 1.13 1.10 6.73 – 2.00 (S), 1.57 (O)

1.55 0.99 1.01 1.04 1.10 6.19 – 2.00 (S), 1.71 (O)

1.506 0.939 1.032 1.016 1.081 6.116 – –

1.503 0.915 1.014 1.006 1.091 6.046 – –

– 0.92 (C), 1.08 (N) –

– 0.92 (C), 1.11 (N) –

– –

– –

0.378

0.447

– 1.09 1.14 1.14 1.08 6.73 – 2.00 (S), 1.58 (O)

1.54 0.99 1.00 1.03 1.08 6.13 – 2.00 (S), 1.71 (N)

1.472 0.905 1.052 1.01 1.066 6.095 – –

1.472 0.906 1.014 1.008 1.061 6.011 – –

– 0.97 (N), 1.41 (O) –

– 1.18 (N), 1.42 (O) –

– –

– –

0.434

0.516

Anion 0.91 0.06 0.23 0.24 0.26 1.26 0.17 0.21 2.11 (S), 0.95 (O) 0.04

0.42 0.11 0.20 0.22 0.04 0.99 0.08 0.28 2.08 (S), 0.90 (O) 0.20 0.01 0.41

0.91 0.06 0.22 0.26 0.06 1.08 0.17 0.21 2.11 (S), 0.95 (O) 0.04 0.05 0.05

SO2CH2CH3 CN

0.42 0.10 0.19 0.17 0.16 0.98 0.08 0.28 2.08 (S), 0.90 (O) 0.20 0.00

0.88 0.06 0.22 0.21 0.21 1.13 0.16 0.23 2.11 (S), 0.94 (O) 0.07 0.06

(SO2CH2CH3)2 + CN

0.42

0.08

Ca C1 C2,6 C3,5 C4 Phc CH2CH3 SO2

SO2CH2CH3 NO2

0.42 0.10 0.18 0.21 0.09 0.79 0.08 0.28 2.08 (S), 0.90 (O) 0.20 0.21

0.87 0.06 0.20 0.24 0.04 0.90 0.16 0.23 2.11 (S), 0.94 (O) 0.07 0.31

(SO2CH2CH3)2 + NO2

0.21

0.17

SO2CH2CH3 Cl (SO2CH2CH3)2 + Cl

(4)

p electron density

0.42 0.10 0.20 0.20 0.22 1.12 0.08 0.29 2.09 (S), 0.90 (O) 0.21

SO2CH2CH3

(3)

Natural atomic chargea

Ca C1 C2,6 C3,5 C4 Phc CH2CH3 SO2

The total atomic charge determined by the natural-bond-orbital (NBO) population analysis. p electron densities of carbanion calculated according the method described in experimental. For the whole phenyl ring (C1–C6).

(DdCa = 29.9 ppm) obtained for phenylnitromethane is evidence for resonance stabilization of nitronate anion [27]. The NMR analysis for carbanions of benzyltriflones reports a strong upfield shift of Ha and small downfield shift of Ca. The para-substitution in the phenyl ring (CN, NO2, CF3 or SO2CF3) does not change significantly the values of DdHa and DdCa. These results suggest that charge delocalization through the phenyl ring has a minor role in the stabilization of benzyltriflone carbanions [27]. Delocalization of negative charge, especially onto electronegative atoms, provides a stabilization of the carbanionic center. How the delocalization of negative charge occurs in studied carbanions? In this regard, the negative charge in disulfonyl carbanions (1–4) resides on the carbanionic center (Ca) and only part of it is delocalized to two

sulfonyl groups and phenyl ring (Table 12). The p electron density on the atom carbon Ca is very similar to the result obtained for unsubstituted benzyl anion [28] (Table 13). The presence of the additional sulfonyl group attached to carbanionic carbon atom does not change substantially the value of the p electron density of this atom (1.552 and 1.527 for PhCH(SO2C2H5)2 and PhCH(SO2CH2C6H5)2, respectively, in comparison with 1.539 and 1.555 for PhCH2SO2CH3 and PhCH2SO2C6H5 (see Table 13). The ab initio calculated p electron densities (Table 11 and 12) obtained for the compounds under consideration correlate unexpectedly well with experimental results, derived from 13C NMR data, with the correlation coefficient of 0.94 and the standard error of 0.03 (see Fig. 3). The total natural charges were also calculated

42

I. Binkowska et al. / Journal of Molecular Structure 1062 (2014) 35–43

Table 13 Comparision of changes in the chemical shifts (DdCa, ppm) and experimental p electron densities in benzyl carbanions stabilized with sulfonyl, nitro and cyano groups in DMSO. System: PhCH3, XPhCHY2 or XPhCH2Y

DdCa

qCa

qC4

qY

qPh

Reference

PhCH3 PhCH(SO2C2H5)2 PhCH(SO2CH2C6H5)2 PhCH2NO2 p-NO2PhCH2NO2 PhCH2CN p-NO2PhCH2CN PhCH2SO2CH3 PhCH2SO2C6H5 CH2(SO2CH3)2 CH2(SO2C6H5)2

15.4 6.5 5.97 29.9 32.21a 11.0 39.4 4.6 3.6 – –

1.52 1.552 1.527 1.308 1.296b 1.547 1.410 1.539 1.555 – –

1.19 1.054 1.055 1.040 1.050b 1.115 1.132 1.107 1.101 – –

– 0.234 0.223 0.567 – 0.283 – 0.269 0.282 0.225 0.206

6.32 5.979 6.026 6.125 6.138b 6.170 6.307 6.192 6.163 – –

[30,28] This work This work [5] [26] [5] this work [5] [5] [13] [13]

qPh – calculated p electron densities on the whole phenyl ring. qY – negative charge transferred from carbanionic carbon to the Y substituent. a Data from Ref. [26]. b Calculated using NMR data from Ref. [26].

86 84

0.0

(b) (a)

82

(10)

(11)

-0.2

NBO charge

78

13

δ Cα

80

76

(12) (13)

-0.4

-0.6 74 72

-0.8

(4) (3) (2) (1)

70 68 1.46

1.48

1.50

1.52

1.54

1.56

1.58

1.60

1.62

π electron density 13

Fig. 4. C shifts of the a carbanionic atom for disulfonyl carbon acids (1–4) as a function of p electron density at Ca: (a) experimental, in DMSO, (b) natural, calculated at the MP2/cc-pVDZ level of theory.

86 84 82

13

δ Cα

80 78 76 74 72 70 -0.92

-0.91

-0.90

-0.89

-0.88

-0.87

-0.86

charge at Cα Fig. 5. 13C shifts of the a carbanionic atom in diethylsulfonyl carbon acids (1–4) as a function of the natural charge calculated at the MP2/cc-pVDZ level of theory.

by ab initio method for the ethylsulfonyl series of carbon acids and their carbanions. For Ca in sulfonyl stabilized carbanions 1–4, the natural charges fall between 0.8e and 1.0e, thus suggesting very little charge delocalization from Ca. Fig. 5 shows that the sulfonyl stabilized carbanions are exceptional with respect to charge distribution when compared to cyano and especially to nitro group sta-

-1.0 1.25

1.30

1.35

1.40

1.45

1.50

1.55

1.60

1.65

π electron density Fig. 6. Total natural charge at Ca as a function of p electron density (the NBO analysis) for disulfonyl (1–4), nitro (10, 11) and cyano (12, 13) carbanions.

bilized carbanions. In the latter, the natural charge at Ca is close to zero. The total natural charges at Ca in the anions 1–4 are more negative than those calculated from p electron densities alone, which would be equal 1 q  0.55e. It appears that either the substantial r-electron charge resides at Ca and the electronic structure is close to that of a ‘‘true carbanion’’ (that is a carbanion with negative charge at Ca close to 1e) or the NBO method fails to predict partitioning the r electron density between Ca and neighboring sulfur atoms [29]. It is alarming that the natural charges of Ca in the carbon acids 1–4 are about 0.42e (Table 12), which are very different from the corresponding charges in nitroalkanes 10 and 11 ( 0.27e and 0.20e) and cyanoalkanes 12 and 13 ( 0.54e and 0.50e) (Table 11). We note that the increase of the natural negative charge at Ca on ionization of disulfonyl carbon acids, which ranges between 0.49e for 1 and 0.45e for 4, is not far from the excess of the p charge, (1 q)  0.55e. It is plausible that the changes in the natural charge at Ca on ionization are more reliable than the natural charges in carbanions (and carbon acids) themselves. Taking this into account, the exceptional position of sulfonyl activated carbon acids would be now characterized by the extremely large change in charge at Ca,  0.5e, as opposed to about ( 0.2e) to ( 0.3e) for nitroalkanes, on the other hand. In this view, it is unlikely that the sulfonyl group stabilized carbanions are nearly ‘‘true carbanions’’ as suggested by the natural charge at Ca in the carbanions alone, although they may have more negative charge at Ca than anions activated by other popular activating groups. If the whole negative charge was localized on Ca, it would be hard to understand why the carbanion should be planar,

I. Binkowska et al. / Journal of Molecular Structure 1062 (2014) 35–43

or nearly planar. This picture is consistent with our analysis of p electron density in these carbanions, which indicates substantial shift of p electron density from Ca. The obtained results suggest that there is indeed a relationship between the chemical shift of carbon atom, either experimental or ab initio calculated, and p electron density (see Fig. 4). Furthermore, the 13C chemical shifts for Ca in the ethylsulfonyl stabilized carbanions decrease with increasing negative total natural charge at the carbanionic center. Even if we may consider the natural charges to be unreliable, the changes in these charges within a series of structurally similar compounds, differing only by the remote substituent, are by far more trustworthy. The slope of the linear fit shown in Fig. 5 is about (260 ± 30) ppm/electron. Wiberg et al. [4] have discussed the correlation between the 13C chemical shift and charge density in monocyclic aromatic compounds and carbenes. It has been concluded that the chemical shifts for studied compounds are not directly related to the charge at carbon, but they depend on the difference in the occupancies of the p orbitals perpendicular to the field direction. This is in line with our finding that p electron density has dominant impact on the chemical shifts. The total natural negative charge at Ca strongly increases with p electron density in the carbanions under consideration (Fig. 6), which substantiates the observed correlation of the chemical shifts with the ab initio calculated overall charge at Ca in the carbanions shown in Fig. 5. We believe that the atomic charges for Ca in carbanions 1–4 are considerably less negative than the natural charges shown in Fig. 6, as discussed above. Therefore, there is probably even closer correspondence between the overall charge and p electron density at Ca in these planar carbanions. Acknowledgement We gratefully acknowledge for the calculations at the Poznan Supercomputer Center PCSS.

[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

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