Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX
pubs.acs.org/JPCA
Stabilization of Saturated Carbocations in Condensed Phases Evgenii S. Stoyanov* Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia Department of Natural Sciences, National Research University−Novosibirsk State University, Novosibirsk 630090, Russia ABSTRACT: Based on the experimentally established mechanism of hyperconjugative stabilization of the simplest saturated carbocations [Stoyanov, E. S.; et al. PCCP, 2017, 19, 7270], the infrared spectra of t-alkyl+ and methyl-cyclo-pentyl+ carbocations were interpreted. This approach allows us to extract new information about the electronic state of (CH3)2C+R cations with R = H, CH3, C2H5, C4H7, and CH(CH3)2, namely, the electron density distribution over the (CH3)2C group and the positive charge dispersion on the H atoms of this group. Thus, donation of the electron density to the empty 2pz orbital of the sp2 C atom occurs not only from one C−H bond oriented parallel to the 2pz orbital but also equally from all other C−H and C−C bonds of the molecular group involved in hyperconjugation. This mechanism preserved the isoelectronic nature of this group toward the corresponding groups of the neutral alkanes. Hyperconjugation and polarization are closely linked in stabilization of carbocations: the strengthening of one effect weakens the second and vice versa without changing the efficiency of scattering of the positive charge in the carbocation. In the condensed phase, carbocations are additionally stabilized by the bulk effect and hydrogen bonding with the environment: increasing H-bonding strength increased hyperconjugation and decreased polarization. The contribution of all the effects on the stabilization of carbocations was evaluated.
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INTRODUCTION The effect of hyperconjugation is believed to play a key role in carbocation stabilization.1 From quantum-chemical calculations, it follows that when the CH3 group of a carbocation is affected by hyperconjugation, one of its CH bonds is aligned in parallel with the empty 2pz orbital of the sp2 carbon atom (Scheme 1),
carbon’s empty 2pz orbital (Scheme 2). As a result, the frequencies of the CH stretch of the CH3 group decrease and Scheme 2a
Scheme 1a a
a
manifest themselves as group vibrations. The hyperconjugated CH3 groups and polarized CH3* group remain isoelectronic to the methyl groups of the neutral alkanes. The theory-versus-experiment discrepancies are valid for all quantum-chemical calculations of carbocations known to date.5 Possibly, potential energy surfaces of the carbocations are more complicated than expected and cannot be analyzed by singlereference quantum-chemical methods. The empirical data indicate that the role of polarization in carbocation’s stabilization is as important as the hyperconjugation effect.4 In contrast, calculations do not show this phenomenon explicitly. Finally, the important role in carbocation stabilization in condensed phases is played by their H-bonding with the nearest counterions or basic molecules.4,6
R = H, CH2, CH3.
providing the best donation of its σ-electron density to the empty 2pz orbital. Such σ−pz hyperconjugation results in weakening of the CH bond and a decrease in its CH stretch. The electron density on the other two CH bonds even slightly increases, increasing their CH stretches, possibly because of the rehybridization effect.2,3 This is the textbook explanation for carbocation stability in vacuum. Nonetheless, calculated infrared (IR) spectra of carbocations, in accordance with this mechanism of hyperconjugation, are completely inconsistent with empirical IR spectra of ethyl, ipropyl, t-butyl, and cyclo-pentyl cations.4,5 Experimental data suggest that CH3 groups of i-propyl+ and t-butyl+ cations are nonequivalent. One group, designated as CH3*, is polarized more strongly and is weakly affected by hyperconjugation. Other CH3 groups are involved in strong hyperconjugation via equal donation of σ-electrons from all their C−H bonds to the © XXXX American Chemical Society
R = H, CH2, CH3.
Received: October 11, 2017 Revised: November 20, 2017
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DOI: 10.1021/acs.jpca.7b10068 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A In the present work, we expanded analysis of the carbocation stabilization in a condensed phase to the heavier cations with C5−C7 carbon atoms, using the strongest known to-date carborane superacids H(CHB11F11) and CH(B11Cl11) (furthermore abbreviated as H{F11} and H{Cl11} respectively), and show that the updated mechanism of a carbocation’s stabilization allows us to properly interpret their IR spectra and extract new information about their molecular state.
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EXPERIMENTAL SECTION All sample handling was carried out in an inert atmosphere (H2O, O2 < 1 ppm) in a drybox. The carborane acids, H{F11}, H{Cl11}, and Et3Si+{F11−} as precursors for the synthesis of carbocation salts were prepared as previously described.7,8 The salt of t-pentyl+{F11−} was obtained by wetting the excess of solid Et3Si{F11} with liquid 1-F-pentane followed by washing out the excess of Et3Si+{F11−} with cold dichloromethane and drying. The salts of methyl-cyclopentyl+ and theptyl+ with {F11−} ions were obtained via a direct interaction of the H{F11} acid with n-hexane or n-heptane for 4 h at room temperature followed by filtering out the solids and drying in vacuum:8 CnH 2n + 2 + H{F11} → CnH 2n + 1+{F11−} + H 2
Figure 1. IR spectra of the salts of t-pentyl+ {F11−} (red) and theptyl+{F11−} (blue) in comparison with t-Bu+{F11−} (green). Spectra are normalized to unit absorption of the {F11−} anion. Bands marked with an asterisk belong to νCH of the {F11−} anion.
{Cl11−} counterions, because the salt of methyl-cyclo-pentyl+ cation is formed predominantly. The salt of t-heptyl+{F11−} may comprise two cationic isomers, 2-methylhexyl+ and 3-methylhexyl+. Its IR spectrum shows the same absorption of the charged C4 core as that of tBu+ (Figure 1, blue). The only difference is the appearance of weak bands at 2960 and 2880 cm−1, typical for νas and νs of the CH3 group, and at 2920 cm−1 typical for νas of the CH2 group of neutral alkanes.12 Their intensities favor an aliphatic −(CH2)2CH3 chain attached to the charged C4 core, indicating presumable formation of the 2-methylhexyl+ cation. One can expect that replacement of two H atoms in one CH3 group of t-Bu+ by methyl groups with formation of 2,3dimethylbutyl+ cation will have a greater impact on the charge redistribution in the C4 core and its local C−H stretches, as compared with t-pentyl+. Nevertheless, the IR spectrum of 2,3dimethylbutyl+ {Cl11−} preserves high similarity to that of tBu+{Cl11−} (Figure 2, Table 1): (a) the three CH stretches from CH bonds involved in hyperconjugation are close in frequencies; (b) the presence of a very broad absorption band from stretch and bent CH vibrations (striped bands in Figure 2, insets) from the CH3* group, which undergo stronger polarization and weaker hyperconjugation; (c) coincidence of the frequencies of CC stretch vibrations at 1323−1262 cm−1 of
(1)
A tertiary cationic center in carbocations arose from the wellknown rearrangement of initially formed primary or secondary carbocation-like species into a more stable tertiary cation via rapid 1,2 shifts.9 The methyl-cyclopentyl+{Cl11−} salt was obtained by direct interaction of the H{Cl11} acid with cyclohexane, but with longer aging: for 2−3 weeks. The white solid was dried in vacuum. The salt of 2,3-dimethylbutyl cation with the {Cl11−} anion was obtained 10 by decomposing chloronium salt (iC3H7)2Cl+{Cl11−} at 80 °C (eq 2), followed by washing out the impurities with cold CH2Cl2 and isolation of a white powdery salt. 80°C (i‐C3H 7)2 Cl+{Cl11−} ⎯⎯⎯⎯⎯⎯→ C6H13+{Cl11−} + HCl
(2)
IR spectra of t-Bu+{F11−}, t-Bu+{Cl11−}, and cyclo-pentyl{F 1 1 − } were obtained earlier, 4 , 5 and those of tBu+[CHB11Me5Br6−] and methyl-cyclopentyl+[CHB11Me5Br6−] were obtained from the samples that we studied earlier.11 IR spectra were recorded on a PerkinElmer Spectrum-100 and Shimadzu IRAffinity-1S spectrometers housed inside the glovebox in the 4000−400 cm−1 frequency range in transmittance and attenuated total reflectance mode (ATR). Spectra were manipulated using the GRAMMS/A1 (7.00) software from Thermo Scientific.
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RESULTS AND DISCUSSION Noncyclic Carbocations with a Quaternary Carbon. tBu+ is the simplest representative of tertiary carbocations with the interpreted IR spectrum.4 It is interesting to trace how the substitution of a hydrogen atom in t-Bu+ by an alkyl groups affects the IR spectrum and the charged core of the carbocation. Replacement of one H atom of t-Bu+ with a methyl group (formation of the t-pentyl+ cation, t-C5H11+) has a very slight effect on the IR spectrum of the cation (Figure 1). Absorption of the new CH3 group, which is not disturbed by hyperconjugation, is not registered with certainty, and adsorption of the charged C4 core remains unchanged. We failed to obtain pure samples of the salts of t-hexyl+ cation with {F11−} and
Figure 2. IR spectrum of 2,3-dimethylbuthyl{Cl11} (red) in comparison with that of t-Bu+{Cl11−} (blue). Spectra are normalized to unit intensity of an anion. The inset shows the proposed separation of the spectrum in the frequency ranges of CH stretch (a) and bent vibrations (b) with shaded strong and broad absorption. Bands marked with an asterisk belong to νCH of the {Cl11−} anion. B
DOI: 10.1021/acs.jpca.7b10068 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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Table 1. Comparison of the C−H and C−C Stretches of the Bonds Forming the Charged C4-Core of 2,3-Dimethylbuthyl+ with Those of the C4-Core of t-Bu+, and Comparison of C−H Stretches of the Free CH3 Groups of 2,3-Dimethylbuthyl+ with Those of Gaseous Isobutene νCH3hypera
salt 2,3-dimethylbuthyl t-Bu+{Cl11−}4 (CH3)3CH, gas12 ratio a
+
{Cl11−}
νCH3free
2853 2830
2797 2791
2737 2746
1.008
1.002
0.997
νCC
2947
2905
2886
2962 0.995
2904 1.000
2894 0.997
1323 1333
1290 1291
1262 1262
0.92
1.00
1.00
C−H stretches of the CH3 groups involved in strong hyperconjugation.
Scheme 4. Schematic Presentation of the cyclo-Pentyl and Methyl-cyclo-pentyl Cations
the charged C4 core. Such high similarity in the vibrations of the C4 core of 2,3-dimethylbutyl+ and t-Bu+ suggests that the −CH(CH3)2 group, via hyperconjugation, donates the same electron density, as the −CH3 group did, to an empty 2pz orbital of the sp2 carbon. Other important findings following from the comparison of the spectra of 2,3-dimethylbutyl+ and t-Bu+ are as follows: (1) intensity of the broad absorption in the CH stretch region of 2,3-dimethylbutyl+ is ∼50% of the total absorption, indicating that in this cation the CH3* group is saved; (2) intensity of the CH stretches involved in hyperconjugation (at ca. 2791 cm−1) for 2,3-dimethylbutyl+ is markedly lower relative to that of tBu+, indicating that in 2,3-dimethylbutyl+ two groups, CH[C2] and CH3, are involved in strong hyperconjugation; (3) the spectrum of 2,3-dimethylbutyl+ showed weak bands at 2946, 2905, and 2886 cm−1, which correlate with CH stretches of the CH3 groups of isobutane (Table 1); this situation proves that they belong to methyls of the −CH(CH3)2 group. Thus, schematically, the 2,3-dimethylbutyl+ should be represented as in Scheme 3.
suggests that this band consists of three components, as expected for CH3 group vibrations (Figure 3a). The presence of these three components is confirmed by the fact that they are clearly detectable in the spectrum of the salt with the CHB11Me5Br6− anion (Figure 3c). The ratios of C−H stretches of the methyl group in methylcyclo-pentyl+{F11−} to those of the methyl groups in tBu+{F11−} (denoted as νCH3hyper in Table 2) almost coincide, thus proving that in both carbocations the CH3 groups are isoelectronic. The spectrum of methyl-cyclo-pentyl+ depends on the basicity of a counterion (Figure 3) because carbocations form hydrogen bonds with the surrounding anions.4,6 The greater the anion basicity, the larger are red shift frequencies of CH stretches. Figure 4 shows that the frequency dependences of the most intense bands of CH3, CH2(α), and CH2(β) groups of CH3C5H9+ on the anion basicity are linear. Their extrapolation to zero basicity yields the frequencies of a “free” cation. Its νasCH2β at 2980 cm−1 is higher than that of the gaseous methylcyclo-pentane (2963 cm−1).14 Thus, the CH2β groups undergo some polarization. The C−H stretches of CH2α (2893 cm−1) and CH3 groups (2830 cm−1), which formed the C4 charged core, are red-shifted relative to those of the neutral methylcyclo-pentane because the effect of hyperconjugation overrides the effect of polarization. The slope of dependences shown in Figure 4 characterizes the strength of the H-bonds formed by CH2/CH3 groups with neighboring anions: the steeper the slope, the stronger the H-bonds. The CH3 group most affected by hyperconjugation forms the strongest H-bonds. This finding confirms the conclusion made earlier about t-Bu+:4 the more strongly the CH3/CH2 group is involved in hyperconjugation, the greater its ability to form H-bonds with neighboring anions.
Scheme 3. Schematic Presentation of a 2,3-Dimethylbutyl+ Cation with Charged C4-Core (Marked Red) Comprising a Polarized CH3* Group (Marked with an Asterisk) and Two Hyperconjugated CH3 and CH[C2] Groups (Marked with Empty Circles)
Methyl-cyclo-pentyl Cation. IR spectra of this cation in different salts are given in Figure 2. Comparison the spectra of methyl-cyclo-pentyl+ and cyclo-pentyl+ reveals that νCH2 frequencies of the pentyl ring of both cations correlate well (Table 2, Scheme 4). The only difference is a very intense lowfrequency band at 2757 cm−1 in the spectrum of methyl-cyclopentyl+, which should be attributed to the methyl group. Breaking this spectrum down into Gaussian components
Table 2. Comparison of the Frequencies of CH Stretches of the Methyl-cyclo-pentyl Cation with Those of cyclo-Pentyl and t-Bu+ Cationsa νCH2β
salts CH3C5H8+{F11−} (1) cyclo-pentyl+{F11−} (2) t-Bu+{F11−}4 (3) ratio (1)/(2) ratio (1)/(3) a
νCH2α
νCH3hyper
2960 2956
2896 2904
2852 2836
2813 2765
∼1
∼1
1.005
1.017
2784
2757
2723
2880
2839
2793
0.967
0.971
0.975
The most intense bands are boldfaced. C
DOI: 10.1021/acs.jpca.7b10068 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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weaker effects on the chemical shifts: the solvent media and currents. To eliminate the influence of the medium, the proton spectra given in Table 3 were obtained using the same solvent with comparable concentrations of a solute. Currents are set up in the molecule, when it is placed in a magnetic field. They initiate an additional magnetic field in the nucleus causing additional shifts. In our case, the shifts of currents are weak and do not prevent some general conclusions. The 1H NMR spectrum of cyclo-pentyl+ shows only one singlet at −4.75 ppm. Only through rapid hydride shifts around the ring can all hydrogen atoms become magnetically equivalent.17 In the spectrum of methyl-cyclo-pentyl+, the signal from CH2(α) groups is very close to that of the methyl group, whereas the chemical shift of CH2(β) groups is much smaller (Table 3). The same is valid for 1H NMR of t-pentyl: the signals from CH2(α) and CH3 are very close. Therefore, the positive charges on the H atoms of these groups are also very similar. At the same time from IR spectra of methyl-cyclopentyl+ it follows that the CH bonds of the strongly hyperconjugated CH3 group are weaker than those of the less hyperconjugated and more strongly polarized CH2(α) group. Thus, both effects, hyperconjugation and polarization, are closely linked: the weakening of one of them reinforces the other and vice versa, keeping the positive charge on the H atoms almost unchanged. This situation leads to magnetic equivalency of CH3 and CH3* groups in 1H NMR spectra: they yield a doublet signal for i-propyl (−5.03 ppm) and a triplet for t-pentyl (−4.50 ppm). If they were magnetically nonequivalent, the spin−spin coupling would not be manifested due to rapid exchange, as in cyclo-pentyl+. Another observation is that replacement of the H atom of the C+−H group in i-Pr+ with a CH3 or C2H5 group (formation of t-Bu+ or t-pentyl+, respectively) has only a small effect on the 1 H signals of CH3 and CH2α groups forming the C4 core (Table 3). That is, the value of the δ+ charge concentrated on the strongly polarized H atom of the C+−Hδ+ group of i-Pr+ is comparable with that on the strongly hyperconjugated CH3/ CH2α groups forming the C4 core. Finally, the downfield shift of the signals of the CH3β group in t-pentyl (at −2.27 ppm) and of CH2β groups in methyl-cyclopentyl+ (at −2.47 ppm) suggests that the positive charge is still distributed up to these groups due to polarization. Specific Features of Stabilization of Carbocations. Combining the results of the present work with those from our previous publications,4,5 that is, summing up the data for all the analyzed C1−C7 carbocations, we can draw the following conclusions. Stabilization of carbocations is mediated by two competing intramolecular effects, polarization and hyperconjugation, and in a condensed phase and additionally by intermolecular Hbonding with the basic surroundings. The polarization effect in carbocations is always the case at the site where the positive charge is located. This phenomenon is most clearly evident in the simplest CH3+ cation5 because of the increase in the frequencies of C−H stretches (by 60 cm−1 for ν as CH 3 as compared with that for CH 3 Cl) and strengthening of the C−H bonds. Replacement of one H atom in CH3+ by the CH3 group with formation of an asymmetric C2H5+ (in the condensed phase) results in the appearance of a strong hyperconjugation effect: the transfer of electron density from the CH3 group to the empty 2pz orbital of the sp2 C atom, predominantly via the C−H bond oriented parallel to the 2pz orbital (Scheme 2). The weakening of C−H
Figure 3. IR spectra of the methyl-cyclo-pentyl cation in its salts with {F11−} (a), {Cl11−} (b), and CHB11Me5Br6− (c) anions in the CH stretch frequency region. The spectrum (c) is shown initial (dashed line) and after subtraction of the absorption from methyl groups of the CHB11Me5Br6− anion using a spectrum of Cs(CHB11Me5Br6) salt (bold green line). The region of subtraction of strong bands is shaded; CH stretches of the anions are marked with asterisks.
Figure 4. Dependences of some frequencies of CH3/CH2 groups of methyl-cyclo-pentyl+ (shown in bold in Table 2 for the {F11−} salt) on the anion basicity on the NH scale.13
It is useful to compare the IR spectra of carbocations in the {F11−} salts with their 1H NMR spectra in a liquid matrix of SbF5 + HF superacids15−17 (basicity of a carbocation’s surroundings in both cases is comparable4). The main feature of the 1H NMR spectra of carbocations is substantial deshielding of the protons of the C4/C3(H) charged core as compared with the corresponding neutral alkanes (Table 3). There is no doubt that local charges have the most important effect on hydrogen shifts. Nonetheless, there are some other D
DOI: 10.1021/acs.jpca.7b10068 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Table 3. 1H NMR Shifts (δTMS ppm) of Some Carbocations in SbF5 + HF Solutions at −60 to −80 °Ca
a
The C3/C4 charged core in the schemes of carbocations is marked in red. bQuick exchange.
Table 4. Most Intense νasCH3 Band of the CH3 Group of Carbocations Involved in Strong Hyperconjugation with the Empty 2pz Orbital of the sp2 Carbon Atoma
bonds of the methyl group manifests itself in a significant decrease (by ∼200 cm−1) in the C−H stretch frequencies as compared with neutral ethane.5 Thus, the weakening of C−H bonds under the influence of hyperconjugation significantly exceeds their strengthening because of polarization. Replacement of the two H atoms in CH3+ with CH3 groups can result in the involvement of both CH3 groups in hyperconjugation with the 2pz orbital of the sp2 C atom forming symmetrical i-Pr+sym. Its νCH3 expectantly decreases by a half of the decrease for C2H5+ (by ∼100 cm−1).5 It should be noted that an asymmetric isomer, i-Pr+as, can form as well: with one hyperconjugated CH3 group and another presumably polarized group, CH3*. For both isomers, the C−H stretches of hyperconjugated CH3 groups actually coincide (νasCH3 = 2831sym and 2825as cm−1);5 this means that energies of i-Pr+sym and i-Pr+as are very similar, and positive charge dispersion over hyperconjugated CH3 and polarized CH3* groups is equal. Replacement of the H atom in the C+−H group of i-Pr+as with a CH3 group to form the C4 charged core of t-Bu+ preserves the polarized CH3* group and does not change CH stretches of the methyl groups (Table 4). This means that the δ+ charge on the polarized H atom of the C+−H group and the charge on the hyperconjugated CH3 groups have nearly the same values, as we have concluded above from 1H NMR data. This feature of identical charge distribution over the C3H and C4 cores of i-Pr+ or t-Bu+, respectively, is retained in the transition to t-pentyl+ and t-heptyl+: CH stretches of the C4 core are nearly constant (Table 4). Only transition to 2,3dimethylbutyl+ results in some visible changes (Figure 2): intensity absorption from hyperconjugated CH3 groups noticeably decreases because one of these groups is converted to the CH[C2] group; but the changes in the frequencies of the remaining CH3 and CH3* groups are insignificant (Tables 1 and 4). Thus, replacement of the CH3 group in t-Bu+ by the CH2(CH3), CH2(C3H7), or CH(CH3)2 group has only a small effect on the electron density and charge distribution in the entire part representing the C4 core (Scheme 5). This means that three groups, CH3, CH2[C], and CH[C2], approximately equally donate electron density to the 2pz orbital of the sp2 C atom, and therefore, the C−H and C−C bonds equally contribute to hyperconjugative stabilization. Let us compare IR spectra of i-Pr+sym and cyclo-pentyl+ with the C3H charged core. The νas values of the CH3 and CH2α groups constituting these cores are very close (2831 and 2836 cm−1, respectively5) even though spatial orientation of the CH2α groups in cyclo-pentyl+ is less favorable for hyper-
a
The charged C3(H)/C4 core of carbocations is highlighted in red. Frequency difference relative to vasCH3 of t-Bu+. cCation does not exist in this salt (is covalently linked with anion). dNot obtained. b
conjugation than that of CH3 groups in i-propyl+. Thus, (i) rotation of the CH3/CH2α groups by some angle differing from the optimal angle for the best hyperconjugation has no appreciable influence on the strength of this effect; (ii) replacement of the CH bonds in CH3 groups of i-Pr+sym with a CC bond (cyclo-pentyl+ formation) does not affect the electron density distribution over the C3(H) charged core. The charged C4 core of the methyl-cyclo-pentyl+ differs from that of t-alkynes+ because of the markedly stronger hyperconjugated CH3 group (see its Δν red shift, Table 4). This feature can be explained as follows: when two CH2α groups do not have a spatial orientation optimal for hyperconjugation and are more polarized, hyperconjugation of the optimally oriented CH3 group is strengthened. In a condensed phase, carbocations are additionally stabilized by intermolecular H-bonding. The H-bonds formed by C−Hδ+ with a counterion decrease the δ+ charge on the H atoms, thereby weakening the effect of polarization but affecting hyperconjugation differently. The latter phenomenon can be seen in the example of the salts of methyl-cyclo-pentyl+ (Figure 3). The stronger the H-bonding, the greater the slope of νCH E
DOI: 10.1021/acs.jpca.7b10068 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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Scheme 5. Similarity of the Electron Density and Charge Distribution (Highlighted with the Dashed Rectangle) in the Charged Core (Marked in Red) of Some Carbocationsa
a
Methyl groups are shown as black circles.
Table 5. Decrease in the CH Stretch of the Methyl Group of Carbocations under the Influence of Hyperconjugation and after Additional Effects of the Solid Phase and Formation of H-Bonds with the Environment νasCH3
compound 1 2 3 4 5 6 7 8 a
(CH3)3CH (CH3)3C+ (CH3)3C+{F11−} (CH3)3C+{Me5Br6−} CH3−C5H9 CH3−C5H8+ CH3−C5H8+{F11−} CH3−C5H8+{Me5Br6−}
gas gas solid solid gas “free” solid solid
296218 283919 2822 2741 296320 2830b 2757 2680
difference
ΔνasCH3
comments
(1) − (2) (2) − (3) (3) − (4)
123 51a 81
hyperconjugation transfer gas/solid additional H-bonding effect
(5) − (6) (6) − (7) (7) − (8)
133 73 77
hyperconjugation transfer “free”/solid additional H-bonding effect
On average for all CH3 groups: two hyperconjugated (17 cm−1) and one CH3* (120 cm−1). bAt zero basicity of the environment (see Figure 4).
hyperconjugation that decreases the difference of this group from the others. Hyperconjugation and polarization lead to equal dispersal of the positive charge onto the H atoms of C−H bonds of the charged core of carbocations and, at the same time, to a different distribution of electron density over these C−H bonds. Hyperconjugation decreases the σ-electron density of the C−H bonds, thus decreasing their stretch vibrations. Polarization does the opposite: strengthens the CH bonds and increases their stretch vibrations. As a result, the polarized CH3* group and hyperconjugated groups are distinguishable by IR spectroscopy but are not distinguishable by 1H NMR analysis because the positive charge is equally dispersed over the H atoms. It is expected that the polarized CH3* group and hyperconjugated CH3/CH2(C)/CH(C2) groups are energetically equivalent. The efficiency of the intermolecular solid-phase effects at stabilizing carbocations is comparable with that of intramolecular hyperconjugation and polarization effects.
dependence on anion basicity. For the CH3 group with the strongest hyperconjugation, the slope is >2-fold that for the CH2α group. Thus, formation of the H-bonds and their further strengthening enhances the effect of hyperconjugation. That is, they mutually reinforce each other. To evaluate the significance of H-bonding relative to hyperconjugation for carbocation stabilization in a solid phase, we measured three steps of the increase in the ΔCH3 red shift for CH3hyper groups. They are given in Table 5. In the absence of environmental influence, the hyperconjugation decreases the CH stretches of the CH3 group by 124 cm−1 (t-Bu+) or 133 cm−1 (CH3−C5H8+). The bulk effect of the solid phase (in the absence of H-bonding) further decreases ΔνasCH3 by 51 cm−1 (t-Bu+) or 73 cm−1 (CH3−C5H8+). Finally, formation of the H-bonds with neighboring anions results in the subsequent decrease of ΔνasCH3: in the case of the most basic {Me5Br6−} anion, by 81 cm−1 (t-Bu+) or 77 cm−1 (CH3−C5H8+). Thus, the bulk effect of the solid phase and H-bonding formation make a significant contribution to carbocation stabilization. Taking into account that they enhance the hyperconjugation effect as well, one can say that H-bonding with the immediate environment plays an important, but nevertheless, supporting, role in carbocation stabilization.
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AUTHOR INFORMATION
ORCID
Evgenii S. Stoyanov: 0000-0001-6596-9590
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Notes
The author declares no competing financial interest.
CONCLUSIONS Two effects are responsible for stabilization of carbocations in the gas phase: polarization and hyperconjugation. The feature of the t-alkynes+ of (CH3)2C+R type (with R = CH3, CH2R′, CHR2′) is the ability of only two CH3 groups (or CH3, and CH2R′ or CHR2′ group) to donate σ-electrons to the up and down lobes of the empty 2pz orbital of the sp2 C atom. The third group, CH3*, remains polarized. In the solid phase, two additional effects emerge: the bulk effect of the condensed phase and H-bonding with the neighboring bases, which contributes to scattering of the positive charge in the immediate surroundings. These effects reinforce hyperconjugation resulting in involvement of the polarized CH3* group in weak
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ACKNOWLEDGMENTS This work was supported by grant # 16-13-10151 from the Russian Science Foundation. The author thanks Irina V. Stoyanova for providing the carborane acids and technical assistance that was supported by the Russian Foundation for Basic Research (grant # 16-03-00357).
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REFERENCES
(1) Alabugin, I. V.; Gilmore, K. M.; Peterson, P. W. Hyperconjugation. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2011, 1, 109− 141.
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DOI: 10.1021/acs.jpca.7b10068 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpca.7b10068 J. Phys. Chem. A XXXX, XXX, XXX−XXX
