Two Equilibria of (N-Methyl-3-pyridinium)chlorocarbene, a Cationic

Feb 2, 2016 - Equilibrium constants and the associated thermodynamic parameters are reported for the equilibria established between the cationic carbe...
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Two Equilibria of (N‑Methyl-3-pyridinium)chlorocarbene, a Cationic Carbene Hui Cang, Robert A. Moss,* and Karsten Krogh-Jespersen* Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States S Supporting Information *

ABSTRACT: Equilibrium constants and the associated thermodynamic parameters are reported for the equilibria established between the cationic carbene (N-methyl-3-pyridinium)chlorocarbene tetrafluoroborate (MePyr+CCl BF4−, 3) and 1,3,5trimethoxybenzene (TMB) to form a carbene−TMB complex, as well as between carbene 3 and chloride ion to form the zwitterion, N-methyl-3-pyridinium dichloromethide (10). These equilibrium constants and thermodynamic parameters are contrasted with analogous data for several related carbenes, and the influence of the pyridinium unit in carbene 3 is thereby highlighted. Computational studies augment and elucidate the experimental results.

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INTRODUCTION The most familiar carbene equilibrium is that of eq 1, formulated by Hine to describe the equilibration of the trichloromethide carbanion (1) with dichlorocarbene and chloride.1,2 We subsequently visualized carbanion 1 at 328 nm in a 1:1 CH2Cl2/MeCN solution.3 An analogous equilibrium connects dibromocarbene and the tribromomethide carbanion for which we estimated an equilibrium constant of 2.8 M−1 in MeCN/ THF solution.4 :CCl 2 + Cl− ⇌ CCl3−

Hammett studies of eq 2 with ring-substituted phenylchlorocarbenes showed that both K and k1 are augmented by electron-withdrawing substituents (ρ = +3.18 and +1.05, respectively), which destabilize the carbene but stabilize the carbanion.6 Conversely, k−1 is enhanced by electron-donating substituents (ρ = −2.63), which destabilize the carbanion but stabilize the carbene.6 A related equilibrium involves the formation of π-complexes between electrophilic carbenes and electron-rich aromatic donor molecules. PhCCl, for example, readily forms π-complexes with 1,3,5-trimethoxybenzene (TMB), with both the carbene and the complexes affording characteristic spectroscopic signatures, cf. eq 3.7 Laser flash photolysis experiments gave K = 1260 M−1 at 294 K, with ΔH° = −7.1 kcal/mol, ΔS° = −10.2 eu, and ΔG° = −4.1 kcal/mol.7

(1)

1

To facilitate the spectroscopic analysis of carbene/carbanion equilibria, we utilized the phenylchlorocarbene (PhCCl)− phenyldichloromethide system in which both carbene and carbanion 2 exhibit strong UV−vis signatures; cf. eq 2.5 In 1,2dichloroethane (DCE), PhCCl was observed at 292 nm and carbanion 2 at 404 nm. Appropriate laser flash photolysis (LFP) experiments enabled extraction of the equilibrium constant (K), the forward (k1) and reverse (k−1) rate constants comprising K, and the thermodynamic parameters associated with the equilibrium, viz., K = 4.0 M−1, k1 = 2.0 × 108 M−1 s−1, k−1 = 4.9 × 107 s−1, ΔH° = −5.7 kcal/mol, ΔS° = −16.8 eu, and ΔG° = −0.71 kcal/mol.5 A parallel set of values was also obtained for the analogous equilibrium involving phenylbromocarbene and phenyldibromomethide carbanion (K = 3.0 M−1).5 ̈ + Cl− ⇌ PhCCl 2− PhCCl 2 © 2016 American Chemical Society

̈ + TMB ⇌ PhCCl/TMB PhCCl

(3)

Given the TMB-donor carbene-acceptor nature of the π-complex, it is not surprising that a Hammett study of arylchlorocarbene/TMB complexation gives ρ = +2.48 for the correlation of log K with σp. ArCCl substituted with electronwithdrawing ring substituents feature equilibria that are shifted toward the complexes, with larger values of K.8 In the same vein, the more electron-deficient pentafluorophenylchlorocarbene (F5-PhCCl) affords a larger K (3.2 × 105 M−1) than PhCCl for complexation with TMB at 294 K.9 We have recently Received: November 19, 2015 Revised: January 20, 2016 Published: February 2, 2016

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DOI: 10.1021/acs.jpca.5b11341 J. Phys. Chem. A 2016, 120, 699−708

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The Journal of Physical Chemistry A

adjusted to a reference state of 1 M concentration for each species participating in the reaction (ΔΔG° = −RT ln(24.5) = −1.89 kcal/mol or, equivalently, ΔΔS° = +6.35 eu). As in our past work,7−9 electronically excited state properties [absorption wavelengths (λ) and oscillator strengths (f)] were calculated using the time-dependent DFT formalism26 and the B3LYP hybrid exchange-correlation functional27−29 (TDB3LYP/6-311+G(d)//B97D/6-311+G(d)). Assignment of a particular electronic excitation was based on consideration of the largest transition amplitudes for the excitation and by visualization of the contributing molecular orbitals (MOs) or natural transition orbitals (NTOs).30 To generate the carbene reactivity data in Table 1 and conform with prior practice, geometry optimizations were made at the

reviewed equilibria between halocarbenes and halocarbanions or carbene complexes.10 (N-Methyl-3-pyridinium)chlorocarbene tetrafluoroborate (MePyr+CCl BF4−, 3) is a very strongly electrophilic cationic carbene.11−13 The permanent positive charge imposed on the pyridinium substituent should exert significant electrostatic effects on equilibria involving chloride addition (analogous to eq 2) or π-complex formation (analogous to eq 3). Here, we report experimental and computational studies of these expectations. Comparisons are made to equilibria of the uncharged but highly electrophilic carbenes p-trifluoromethylphenylchlorocarbene (p-F3C-PhCCl) and F5-PhCCl,6,8 as well as the “parent” carbenes, 4-pyridylchlorocarbene (4-PyrCCl, 4)11,14 and phenylchlorocarbene.5,7

Table 1. Quantitative Measures of Carbenic Reactivity

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Experimental Details. The precursors of carbenes 3 and 4 are diazirines 5 and 6, respectively. Preparative details for these compounds have been published in full.13,15

carbene

εLUa

εHOa

ωb

ΔEstabc

PhCCl 4-PyrCCl p-F3C-PhCCl F5-PhCCl MePyr+CCl

0.94 0.70 0.37 −0.02 −0.16d

−9.62 −9.89 −10.02 −10.25 −10.58d

0.89 1.00 1.12 1.29 1.38d

44.8 41.6 43.3 34.5 30.2e

εLU and εHO are the LUMO and HOMO energies (in eV), computed at the HF/6-31G(d,p)//MP2/6-31G(d,p) level; see ref 35. bIn eV; ω = global electrophilicity = (εLU + εHO)2/8(εLU − εHO), see refs 33−35. cIn kcal/mol. ΔEstab is defined as the negative of the reaction energy for CH2 + CH3X + CH3Y → CXY + 2CH4. Computed here at the B3LYP/6-311++G(2d,p) level; see refs 36 and 37. dCalculated for the intimate MePyr+CCl BF4− ion pair, see ref 13. eCalculated for the “naked” MePyr+CCl carbene. a

MP2/6-31G(d,p)18,19,31 or B3LYP/6-311++G(2d,p)20−22,27−29 levels, assuming an idealized gas phase. For all other calculations (ground or excited state), the polarizable conductor selfconsistent reaction field model (CPCM) was chosen to incorporate general bulk solvent effects;32 Gaussian 09 default parameters were applied for the model dichloroethane (DCE) solvent.

LFP experiments employed a xenon fluoride excimer laser emitting at 351 nm. A recent detailed description of this installation has appeared.16 Precise temperatures (±0.1 K) were measured at the moment of LFP with a thermocouple immersed in the photolysis solution contained in a quartz cuvette. 2.2. Computational Details. Electronic structure calculations, based on wave function (HF, MP2) or density functional theory (DFT), were carried out with the Gaussian 09 suite of programs.17 We employed the B97D functional18 and 6-311+G(d)19−23 basis sets in geometry optimizations of carbenes, anions, and complexes (B97D/6-311+G(d)); this functional/basis set combination is economical to use and has performed well in analogous prior situations, cf., for example, refs 7−9. The stationary points located on the potential energy surfaces were characterized further by normal-mode analysis, and the (unscaled) vibrational frequencies formed the basis for the calculation of vibrational zero-point energy (ZPE) corrections. Expanded integration grid sizes (pruned (99 590) atomic grids, invoked using the integral = ultrafine keyword), were applied to increase numerical accuracy and stability in both geometry optimizations and normal-mode analysis.24 Standard thermodynamic corrections (based on the harmonic oscillator/rigid rotor approximations and ideal gas behavior) were made to convert from purely electronic energies to enthalpies (ΔH°, ΔZPE included, T = 298.15 K) and Gibbs free energies (ΔG°; T = 298.15 K, P = 1 atm).25 To facilitate comparison with thermodynamic parameters derived from experiment, the free energy changes for complex or anion formation were

3. RESULTS AND DISCUSSION 3.1. Computational Indices of Carbenic Reactivity. In Table 1, we collect a few computational indices of carbenic reactivity for the five carbenes central to the present study. Tabulated are the LUMO (εLU) and HOMO (εHO) orbital energies of the carbenes, as well as their global electrophilicities (ω);33−35 also included are the stabilities of the carbenes relative to methylene, ΔEstab.36,37 On the basis of either the accessibility of its formally vacant p orbital (cf. εLU) or its global electrophilicity (ω), MePyr+CCl (3) is predicted to be the most electrophilic of the carbenes in Table 1, followed by F5-PhCCl and p-F3C-PhCCl. The “parent” carbenes, 4-PyrCCl (4) and PhCCl, lag in these two metrics of electrophilicity. In terms of ΔEstab, MePyr+CCl is predicted to be the least stable of the carbenes, closely followed by F5-PhCCl. The data presented in Table 1 imply that MePyr+CCl is a highly electrophilic and reactive carbene, which should avidly complex with electron-rich aromatic donors and readily add anions. 3.2. The MePyr+CCl−TMB Equilibrium. We first examine the equilibrium between MePyr+CCl BF4−, TMB, and the carbene−TMB complex (7); cf. eq 4. 700

DOI: 10.1021/acs.jpca.5b11341 J. Phys. Chem. A 2016, 120, 699−708

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silent above 300 nm and the weak σ → p absorption of carbene 8 appears around 600 nm,13 we attribute the 500−650 nm band to carbene−TMB complex 7. On the basis of electronic structure calculations (B97D/ 6-311+G(d)) we propose a sandwich structure for complex 7, cf. Figure 1, parts b and c. We have demonstrated previously that sandwich-type complexes are more stable than O-ylides for phenylchlorocarbenes and TMB.7,9 The preference for π−π stacking interactions should only be enhanced by the presence of a positive charge in the pyridine moiety, and consequently, our potential energy surface scans have only considered sandwich-type complexes. The optimized structure of lowest energy for 7 reveals considerable π−π overlap between the aryl rings (ring−ring separation ∼3.2 Å, Figure 1, parts b and c) as well as a direct C(carbene)−O(OMe) interaction (RC−O = 3.0 Å); the short C−O distance suggests favorable overlap between an oxygen lone pair and the LUMO of 8, which has a substantial contribution from the formally vacant 2p orbital on the carbenic center.41 The computed interaction enthalpy between 8 and TMB is substantial, ΔH° = −11.8 kcal/mol, and comparable to the interaction enthalpy computed for the intimate MePyr+CCl BF4− ion pair (8; ΔH° = −13.6 kcal/mol). The computed UV−vis spectrum of 7, shown as Figure 2b, is clearly in excellent agreement with the observed spectrum (Figure 2a) in terms of both absolute transition energies and relative intensities. The intense feature around 300 nm may be assigned to the aforementioned characteristic carbene (free and in complex) π → π* absorption. The feature around 350 nm (not clearly developed in the experimental spectrum) and the broad absorption around 550 nm are brought about by complex

As previously discussed,13 LFP of diazirine 5 at 351 nm in DCE solution affords MePyr+CCl BF4− as an intimate ion pair. Here, the BF4− unit interacts with the positively charged nitrogen atom in a tridentate manner (∠BNC(ring or methyl) ∼ 90°; N−F ∼ 2.8−3.0 Å; N−B ∼ 3.1 Å), viz., structure 8 in Figure 1. The UV spectrum of MePyr+CCl BF4− shows broad and intense absorption in the 290 nm region with a (calibrated) maximum at 284 nm.13 This absorption feature is characteristic of phenylhalocarbenes and their derivatives and provides a suitable spectral “marker” for these carbenes.38,39 The elementary excitations giving rise to this absorption are π → π* in nature and involve frontier orbitals delocalized over the aryl ring, the carbene center, and the halogen. For 8, we compute electronic transitions (TD-B3LYP/6-311+G(d)// B97D/6-311+G(d) in simulated DCE) at 274 nm (f = oscillator strength = 0.117) and 292 nm ( f = 0.274). We thus consider that the absorption observed around 290 nm includes contributions from both computed transitions at 274 and 292 nm, with a combined oscillator strength ( f = 0.391).40 In the presence of 0.188 mM TMB, the MePyr+CCl BF4− absorption maximum was observed at 292 nm, while a new, very broad absorption feature of moderate intensity appeared at 500−650 nm; cf. Figure 2. Since TMB is spectroscopically

Figure 1. Minimum energy structures of the intimate MePyr+CCl BF4− ion pair (8) and carbene−TMB sandwich complex 7: (a) 8, side view; (b) 7, side view; (c) 7, perpendicular view through aryl rings; BF4− unit omitted for clarity.

Figure 2. (a) Calibrated UV−vis absorption spectrum acquired 88 ns after LFP generation of MePyr+CCl BF4− in the presence of 0.188 mM TMB in DCE. (b) Computed (TD-B3LYP/6-311+G(d)) UV−vis spectrum of complex 7 (cf. Figure 1) in simulated DCE. 701

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The Journal of Physical Chemistry A formation between carbene 8 and TMB. The excited-state calculations suggest that the spectral feature around 350 nm is composed of absorption to several energetically close-lying states, viz., computed transitions and oscillator strengths are at 355 nm (0.028), 376 nm (0.012), and 381 nm (0.015). The only transition for complex 7 in the region of 400−600 nm emerges at 568 nm ( f = 0.114). All absorption observed in the 350−600 nm region for 7 has its origin in charge-transfer transitions involving electron transfer from electron-rich TMB to carbene 8. This is most easily illustrated by examination of the NTOs.30 Figure 3

Figure 4. Ratio of calibrated absorption intensities at 292 and 564 nm (A292/A564) vs 1/[TMB] in DCE at 298 K. The slope of the correlation line (Ka) is 1.20 × 10−4 M (r = 0.992), leading to K = 2.86 × 104 M−1.

constant as K = (f 3/f1)(1/Ka); cf. Figure 4.44 In the present case of MePyr+CCl BF4− and TMB, Ka = 1.20 × 10−4 M, f 3 = 0.391 (MePyr+CCl BF4− ion pair 8 absorption peaking at 292 nm), and f1 = 0.114 (564 nm absorption of complex 7) affording K = 2.86 × 104 M−1 for the equilibrium described by eq 4 at 298 K. The change in Gibbs free energy for formation of complex 7 at 298 K is thus ΔG° = −RT ln K = −6.10 kcal/mol. To obtain the enthalpic and entropic components we determined, in an entirely analogous manner, the equilibrium constant K at four additional temperatures between 282 and 313 K. These results appear in graphical form in the Supporting Information. Figure 5 illustrates the correlation between ln K

Figure 3. Natural transition orbitals for the excitations computed in 7 at 568 and 355 nm. (a) “Hole” NTO for the 568 nm transition. (b) “Particle” NTO for the 568 nm transition. (c) “Hole” NTO for the 355 nm transition. (d) “Particle” NTO for the 355 nm transition.

displays the NTO from which the excitation computed at 568 nm originates (the “hole” NTO) and also the NTO in which the excitation terminates (the “particle” NTO). The former NTO is clearly localized on TMB, while the latter is concentrated on the carbene moiety. In the effective oneelectron picture adopted by the NTO formalism, the excitation computed in 7 at 355 nm involves an electron transferred from the TMB localized “hole” NTO visualized in panel c of Figure 3 to a “particle” NTO centered on the carbene and visualized in panel d. The amplitudes (“coefficients”) are larger than 0.95 for both excitations, a → b and c → d. Figure 4 displays a correlation of the quotient of the calibrated absorption intensities at 292 and 564 nm (A292/A564) as a function of 1/[TMB] at 298 K.42 A Beer−Lambert-type analysis allows us to write A564 = ε1[complex]

and

A 292 = ε2[complex] + ε3[carbene] (5)

Figure 5. Correlation of ln K vs 1/T for the equilibrium of eq 4. The slope (5566) affords ΔH° = −11.08 ± 0.88 kcal/mol, and the intercept (−8.41) gives ΔS° = −17 ± 3 eu. The correlation coefficient r = 0.990.

where the ε’s represent extinction coefficients. The extinction coefficients are not obtainable from our experiments, however; hence, we replace the ε’s in eq 5 by computed oscillator strengths (f),43 i.e. A564 = f1 [complex]

and

and 1/T (r = 0.990) from which the slope and intercept afford ΔH° = −11.1 ± 0.9 kcal/mol and ΔS° = −17 ± 3 eu, respectively. For complex 7, the favorable enthalpy of formation term (ΔH°) is essentially twice as large as the unfavorable entropy term (−TΔS°). The computed interaction enthalpy ΔH° = −11.8 kcal/mol (B97D/6-311+G(d)) is in remarkably good agreement with the experimental value (−11.1 kcal/mol); however, the agreement

A 292 = f2 [complex] + f3 [carbene] (6)

Rearrangement of eq 6 and introduction of the equilibrium constant for complex formation (K = [complex]/[carbene][TMB]) finds, as detailed in the Supporting Information, that the slope of the correlation line between (A292/A564) and 1/[TMB] (defined as Ka) is simply related to the equilibrium 702

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The Journal of Physical Chemistry A Table 2. Thermodynamic Parameters for Carbene−TMB Complexation exptl

a

computed

carbene

K, M−1

ΔG°a

ΔH°a

ΔS°b

ΔG°a

ΔH°a

ΔS°b

PhCCl PyrCCl F3C-PhCCl F5-PhCCl MePyr+CCl

1450 c 25800 321000 28600

−4.1 c −6.0d −7.4 −6.1

−7.1 c

−10.2 c

−10.2 −11.1

−9.5 −17

3.9 2.8 3.5 1.1 −0.5

−5.9 −7.0 −8.4 −10.6 −11.8

−32.8 −32.9 −40.3 −39.3 −37.6

In kcal/mol. bIn eu. Corrected to a standard state corresponding to concentrations of 1 M. cNot determined due to self-ylide formation; see text. Calculated from ΔG° = −RT ln K.

d

favorable ΔΔG° of 1.3 kcal/mol, an advantage arising from a less unfavorable entropy of complexation (by ∼7.5 eu). The enthalpy of complexation, however, appears to favor MePyr+CCl by ∼1 kcal/mol. Given that electron-withdrawing features in the carbene favor complexation with electron-rich TMB,8 we see that the positively charged pyridinium of MePyr+CCl is comparable in this regard to the p-trifluoromethyl substituent of F3C-PhCCl or the pentafluorophenyl group of F5-PhCCl. In Table 2, we observe excellent agreement between computed and experimental values of ΔH° for TMB complexation by PhCCl, F5-PhCCl, or MePyr+CCl (experimental values are not available for PyrCCl and F3C-PhCCl). As stated above, the computed values of ΔS°, however, are much more negative than the observed values, which renders comparisons of experimental and computed free energies or equilibrium constants fruitless. 3.4. The PyrCCl−PyrCCl2− Equilibrium. Although we were unable to study the complexation of TMB by PyrCCl due to the formation of ylide 9, this reaction did not prevent determination of the equilibrium constant for chloride addition to PyrCCl; cf. eq 7. Figure 6 shows the UV−vis spectrum

between the computed entropy change for formation of 7 (ΔS° = −37.6 eu) and the experimental value (−17 eu) is decidedly poor. We ascribe this variance primarily to the different physical phases serving as references for the calculations (idealized gas phase with continuum dielectric correction) versus experiment (condensed phase).7,45−49 In solution, molecular translation and rotation is markedly restricted prior to complex formation and specific solute−solvent interactions also appear inevitable in the moderately polar and ionic solutions used in the experiments. The agreement between computed (−0.5 kcal/mol) and measured (−6.1 kcal/mol) free energies (or equilibrium constants) is consequently also poor. We attempted to determine K for the formation of a complex between parent carbene 4 (PyrCCl) and TMB, but this proved impossible. LFP generation of PyrCCl from diazirine 6 in DCE resulted in the appearance of strong absorption at 468 nm due to the formation of “self-ylide” 9, stemming from attack of PyrCCl on its precursor diazirine. This reaction is well-known; in isooctane, ylide 9 absorbs at 480 nm.11,14

The formation of 9 and its concomitant absorption precluded complexation experiments with PyrCCl and TMB. 3.3. Comparisons of Carbene−TMB Complexation Equilibria. In Table 2 we gather experimental and computed thermodynamic parameters for carbene−TMB equilibria of MePyr+CCl, PhCCl,7 PyrCCl, F3C-PhCCl,8 and F5-PhCCl.9 Although it is not possible to experimentally compare MePyr+CCl with PyrCCl, comparison to PhCCl indicates that the positively charged pyridinium unit incorporated into MePyr+CCl enhances K for TMB complexation by a factor of ∼20, relative to PhCCl. The enhancement is enthalpy-driven: ΔH° for TMB complexation is 4 kcal/mol more favorable for MePyr+CCl than for PhCCl. This difference is partially offset by a 7 eu less favorable entropy for the MePyr+CCl−TMB complexation so that ΔΔG° is only 2 kcal/mol in favor of MePyr+CCl, relative to PhCCl. MePyr+CCl is comparable to F3C-PhCCl in TMB complexation; the two carbenes exhibit very similar values of K and ΔG°. F5-PhCCl, however, is the most effective carbene for TMB complexation among those shown in Table 2. Its K exceeds that of MePyr+CCl by a factor of ∼11 due to a more

Figure 6. Calibrated UV−vis spectrum 200 ns after LFP of diazirine 6 (A = 0.5) in DCE. Carbene 4 (PyrCCl) is at 292 nm, ylide 9 at 468 nm.

obtained upon LFP of diazirine 6 in DCE. PyrCCl absorption forms a sharp peak at 292 nm, while ylide 9 absorbs at 468 nm. When this experiment is repeated in the presence of 0.5 M tetrabutylammonium chloride (TBACl), a prominent new absorption is observed at 372 nm, which we attribute to carbanion 10 (PyrCCl2−); cf. Figure 7.

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the pyridine plane by ca. 10°, see Figure 8a. The C−Cl bonds are long (1.89 Å), and the bond angles are close to tetrahedral (∠Cl−C−C = 112.1°, Cl−C−Cl = 105.7°). We compute electronic transitions in PyrCCl2− (above 350 nm, Figure 8b) at 420 nm ( f = 0.174) and 402 nm ( f = 0.028), well to the red of the observed absorption peak (372 nm, Figure 7). Analysis of the NTOs for the more intense transition (420 nm) shows that it may be viewed as a one-electron excitation (amplitude = 1.00) from an orbital concentrated on the carbanion center (Figure 8c) into a widely delocalized orbital which contains contributions from not only the π-system of the aryl ring and the carbanion center but also from the C−Cl σ-bonds (Figure 8d). Figure 9 depicts the correlation between the absorbance ratio A292/A372 for PyrCCl/PyrCCl2− as a function of 1/[TBACl] at Figure 7. Calibrated UV−vis spectrum 200 ns after LFP of diazirine 6 in 0.50 M TBACl in DCE: PyrCCl at 292 nm, PyrCCl2− at 372 nm, ylide 9 at 468 nm.

PyrCCl can exist in both syn and anti conformations. The free energy difference between the two conformers is calculated to be just 0.3 kcal/mol, in favor of anti-PyrCCl (B97D/6311+G(d)); in the following, we will thus assume that the two conformers are isoenergetic. For syn-PyrCCl, we calculate the characteristic transitions at 311 nm ( f = 0.164) and 276 nm (f = 0.291), while for anti-PyrCCl, we calculate transitions at 304 nm (f = 0.219) and 279 nm (f = 0.266). We consider that these transitions, all π → π* in nature, will not be resolved, and together constitute the observed PyrCCl absorption at 292 nm to which we assign a total oscillator strength f = 0.470 (= f 3, see below) by averaging the f-values over the two conformers. Figure 9. Calibrated absorption intensity ratios A292/A372 for PyrCCl/ PyrCCl2− vs 1/[TBACl] in DCE at 298 K. The slope of the correlation line (Ka) is 7.94 × 10−2 M (r = 0.996), leading to K = 29.4 M−1.

−

298 K. The slope of the correlation line is 7.94 × 10−2 M, leading to K = (f 3/f1)(1/Ka) = [0.470/(0.174 + 0.028)](1/7.94 × 10−2 M) = 29.4 M−1.

PyrCCl2−

is There is only one conformer for PyrCCl2 . pyramidal at the carbanion center, which has moved away from

Figure 8. (a) Minimum energy structure of PyrCCl2−; side view. (b) Computed (TD-B3LYP/6-311+G(d)) UV−vis spectrum of PyrCCl2− (cf. Figure 7) in simulated DCE. (c) “Hole” NTO for the 420 nm transition. (d) “Particle” NTO for the 420 nm transition. 704

DOI: 10.1021/acs.jpca.5b11341 J. Phys. Chem. A 2016, 120, 699−708

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The Journal of Physical Chemistry A Next, we determined K at four additional temperatures ranging from 278 to 308 K; the pertinent correlations of A292/A372 as a function of 1/[TBACl] appear in the Supporting Information. Figure 10 shows a correlation of ln K versus 1/T for the

Figure 11. Calibrated UV−vis spectrum of MePyr+CCl BF4− in DCE, 200 ns after LFP of 1.67 mM diazirine 5.

BF4− in DCE, as generated by LFP of diazirine 5. The absorption of the carbene appears at 284 nm. Repetition of this experiment in the presence of 0.542 mM TBACl gives the spectrum shown in Figure 12, where the new absorptions at 348 and 580 nm are attributed to the zwitterion, MePyr+CCl2− (11); cf. eq 8.

Figure 10. ln K vs 1/T for the equilibrium of eq 7. The slope (3317) affords ΔH° = −6.6 ± 0.6 kcal/mol, and the intercept (−7.64) leads to ΔS° = −15 ± 2 eu. The correlation coefficient r = 0.988.

equilibrium of eq 7, from which the thermodynamic parameters can be extracted. We find ΔH° = −6.6 ± 0.6 kcal/mol, ΔS° = −15 ± 2 eu, and thus ΔG° = −2.1 kcal/mol. We also determined the rate constant at 298 K for the forward direction of eq 7, i.e., the addition of Cl− to PyrCCl, by following the apparent rate of formation of PyrCCl2− at 372 nm as a function of [Cl−]. Two determinations (see Supporting Information) gave k1 = 3.6 ± 0.1 × 108 M−1 s−1. Given that Keq = 29.4 M−1 at 298 K, k−1 = 1.2 × 107 s−1 for the reversion of PyrCCl2− to PyrCCl. The computed enthalpy change for the model reaction PyrCCl + Cl− → PyrCCl2− is ΔH° = −7.5 kcal/mol (B97D/ 6-311+G(d); continuum model DCE solvent), in good agreement with the experimental value (−6.6 kcal/mol); as expected, the computed entropy change is too negative compared to experiment, ΔS° = −22.8 eu (calcd) versus −15 eu (exptl), and hence the computed free energy change, ΔG° = −0.7 kcal/mol at T = 298 K, is slightly higher than the experimental value determined above (ΔG° = −2.1 kcal/mol). Although good numerical agreement is obtained between the computed and measured chloride affinity of PyrCCl, this result must be considered somewhat fortuitous because the electronic structure calculations oversimplify the complex nature of the experimental solution. In particular, specific interactions with the TBA+ and Cl− ions, present in appreciable concentrations in the DCE solvent, are completely ignored.50 However, the observation that specific interactions between the solute and dissolved ions perhaps need not be treated explicitly is consistent with our previous work in which such simple computational models led to good agreement between calculated and measured enthalpies for addition of halide resulting in carbanion formation.3,5,6 The presence of ions significantly increases the complexity of computational modeling, and the extent to which PyrCCl/PyrCCl2− are “solvated” by TBACl or DCE probably belongs more in the domain of molecular dynamics than electronic structure. 3.5. The MePyr + CCl−MePyr + CCl 2 − Equilibrium. Figure 11 depicts the calibrated UV−vis spectrum of MePyr+CCl

As in the case of PyrCCl (above), there are also two conformations of MePyr+CCl BF4− (3), syn and anti, and we anticipate that 3 exists as intimate ion pairs, viz., 8 (Figure 1a). The two conformers are essentially isoenergetic, and they absorb at similar wavelengths but with somewhat different intensities. The syn conformer is calculated to absorb at 274 nm ( f = 0.075) and 295 nm (f = 0.206), while the anti conformer should absorb at 274 nm ( f = 0.117) and 292 nm ( f = 0.274). We expect that these transitions cannot be resolved and, together, may be identified as the observed carbene absorption at 284 nm, to which we assign an aggregate oscillator strength of f = 0.337 by averaging. We find only a single conformer of MePyr+CCl2−. It is likely that, as with 3, a BF4− anion is associated with MePyr+CCl2−, piggybacking on top of the zwitterion near the positively charged N atom (viz., 8). This intimate ion pair MePyr+CCl2− BF4− species is computed to absorb at 309 nm with f = 0.719, and we identify this absorption as that observed in Figure 12 at 348 nm.51

A correlation between the absorbance ratio A284/A348, representing MePyr+CCl BF4−/MePyr+CCl2− BF4− versus 1/[TBACl] appears in Figure 13. The slope of the correlation line is 1.05 × 10−4 M. After correction for the computed intrinsic intensities of MePyr+CCl BF4− (f 3 = 0.337) and MePyr+CCl2− (f1 = 0.719) absorptions, we find K = 4.46 × 103 M−1 for the equilibrium of eq 8. 705

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Figure 12. Calibrated UV−vis spectrum of MePyr+CCl BF4− in 0.542 mM TBACl in DCE solution 160 ns after LFP of diazirine 5.

Figure 14. Correlation of ln K vs 1/T for the equilibrium of eq 8. The slope (8177) affords ΔH° = −16.3 ± 1.3 kcal/mol, and the intercept (−19.1) gives ΔS° = −38 ± 4 eu. The correlation coefficient r = 0.991.

continuum model DCE solvent), in fair agreement with the experimental value (−16.3 kcal/mol). As expected, the computed entropy change is quite different from the experimental value, ΔS° = −20.8 eu (calcd) versus −38 eu (exptl).52 The experimental entropy change is by far the more negative quantity, and as a result, the computed free energy change, ΔG° = −16.5 kcal/mol at T = 298 K, shows only a limited relationship to the experimental value determined above (ΔG° = −5.0 kcal/mol). The MePyr+CCl2− BF4− species 8 has the BF4− unit piggybacking on top of an essentially planar MePyr+CCl2− zwitterion in which the exocyclic C−C bond is noticeably short at 1.384 Å, indicating considerable doublebond character. 3.6. Comparisons of Carbene−Chloride Addition Equilibria. Table 3 collects experimental and computed thermodynamic parameters for the carbene−chloride addition equilibria of PhCCl,5 PyrCCl, and MePyr+CCl. Compared to the phenyl group of PhCCl, the 3-pyridyl unit of PyrCCl enhances K for the equilibrium of eq 5 by a factor of ∼7. We attribute this to the greater electron-withdrawing effect of 3-Pyr versus Ph, viz., the Hammett σ constants of 3-Pyr and Ph are 0.44−0.5553,54 versus 0.05.55 Greater electron withdrawal destabilizes the carbene relative to the carbanion and increases the equilibrium constant. In the present case, the effect is mainly due to about a 1 kcal/mol more favorable ΔH° for the PyrCCl equilibrium. A much greater enhancement of K attends the imposition of a positive charge on the carbene. Thus, K for the addition of chloride to MePyr+CCl is 150 times greater than for the analogous process with PyrCCl, and 1100 times greater compared to PhCCl. The major component of the enhancement is enthalpic: ΔH° is approximately 10 kcal/mol more negative for addition of Cl− to MePyr+CCl than to PyrCCl. However, this advantage is opposed by a 23 eu less favorable ΔS° with MePyr+CCl, equivalent to 11.3 kcal/mol at 298 K56 so that ΔG° is only ∼3 kcal/mol more favorable for Cl− addition to the cationic carbene than to PyrCCl. We also note that the rate constant for chloride addition is ∼4.5 times greater with MePyr+CCl than PyrCCl. No doubt this results from the favorable influence of the positively charged nitrogen of the cationic carbene. The greater reactivity of MePyr+CCl is not likely to come

Figure 13. Calibrated absorption intensities at 284 nm/348 nm vs 1/[TBACl] M−1 in DCE at 298 K. The slope of the correlation line is 1.05 × 10−4 M (r = 0.995).

We next determined K at four additional temperatures ranging from 278 to 308 K; these results appear in the Supporting Information. A correlation of ln K versus 1/T for the equilibrium of eq 8 appears in Figure 14. The extracted thermodynamic parameters are ΔH° = −16.3 ± 1.3 kcal/mol, ΔS° = −38 ± 4 eu, and ΔG° = −5.0 kcal/mol (at 298 K). The rate constant for the formation of MePyrCCl2− by the addition of Cl− to MePyr+CCl was measured by LFP at 298 K, following the appearance of the zwitterion at 348 nm as a function of [Cl−]. Two determinations afforded k1 = 1.68 ± 0.08 × 109 M−1 s−1 (see Supporting Information). Given that K = 4.46 × 103 M−1 s−1, k−1 = 3.77 × 105 s−1 for the reversion of MePyr+CCl2− to MePyr+CCl. Computational studies of the MePyr+CCl−chloride system are more complicated than for the MePyr+CCl BF4− plus TMB equilibrium or PyrCCl−chloride system discussed above, because two anions may, in principle, be involved. It turns out that, as in the case of chloride addition to PyrCCl, a straightforward computational approach may be adequate. Thus, the computed enthalpy change for the model reaction anti-MePyr+CCl BF4− (i.e., structure 8) + Cl− → MePyr+CCl2− BF 4 − is ΔH° = −22.7 kcal/mol (B97D/6-311+G(d); 706

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The Journal of Physical Chemistry A Table 3. Thermodynamic Parameters for Carbene−Chloride Addition computeda

exptl carbene PhCCld PyrCCle MePyr+CCle,f

K, M

−1

4.0 29.4 4460

ΔG°

b

−0.71 −2.1 −5.0

b

ΔH°

ΔS°

−5.7 −6.6 −16.3

−17 −15 −38

c

b

ΔG°

ΔH°b

ΔS°c

2.5 −0.7 −16.5

−5.4 −7.5 −22.7

−27 −23 −21

a f

Calculated at the B97D/6-311+G(d) level with continuum model solvent (DCE) corrections. bIn kcal/mol. cIn eu. dData from ref 5. eThis work. As the BF4− salt.

from stabilization of the MePyr+CCl2− product, which is a zwitterion, not a carbanion. The formal ion exchange of TBA+Cl− to TBA+BF4− in eq 8 is unlikely to significantly contribute to the overall ΔG°. The calculations of Fry (B3LYP/6-31+G(d)) find the two ion pairs differing by only ∼0.4 kcal/mol in computed ΔG(association) in simulated acetonitrile,45 and the ion exchange is essentially thermoneutral. Our own calculations (B97D/6-311+G(d), DCE solvent) produce a similar result, ΔG° = 0.7 kcal/mol, for the exchange reaction TBA+Cl− + BF4− → TBA+BF4− + Cl−.

(4) Moss, R. A.; Tian, J. Concurrent Cyclopropanation by Carbenes and Carbanions. J. Am. Chem. Soc. 2005, 127, 8960−8961. (5) Wang, L.; Moss, R. A.; Krogh-Jespersen, K. Directly Observed Halocarbene−Halocarbanion Equilibration. J. Am. Chem. Soc. 2012, 134, 17459−17461. (6) Wang, L.; Moss, R. A.; Krogh-Jespersen, K. Hammett Analyses of Halocarbene−Halocarbanion Equilibria. Org. Lett. 2013, 15, 2014− 2017. (7) Moss, R. A.; Wang, L.; Odorisio, C. M.; Krogh-Jespersen, K. A Carbene−Carbene Complex Equilibrium. J. Am. Chem. Soc. 2010, 132, 10677−10679. (8) Wang, L.; Moss, R. A.; Thompson, J.; Krogh-Jespersen, K. Hammett Analysis of a Family of Carbene−Carbene Complex Equilibria. Org. Lett. 2011, 13, 1198−1201. (9) Wang, L.; Moss, R. A.; Krogh-Jespersen, K. Trimethoxybenzene Complexes of Pentafluorophenylchlorocarbene. J. Phys. Chem. A 2011, 115, 8113−8118. (10) Moss, R. A.; Wang, L.; Hoijemberg, P. A.; Krogh-Jespersen, K. Photochem. Photobiol. 2014, 90, 287−293. (11) Moss, R. A.; Jang, E. G.; Kim, H.-R.; Ho, G.-J.; Baird, M. S. Pyridylhalocarbenes and Pyridiniumhalocarbenes. Tetrahedron Lett. 1992, 33, 1427−1430. (12) Moya-Barrios, R.; Fregeau, B. M.; Cozens, F. L. Reactivity of Halo(pyridinium)carbenes. J. Org. Chem. 2009, 74, 9126−9131. (13) Cang, H.; Moss, R. A.; Krogh-Jespersen, K. Absolute Reactivity of (N-Methyl-3-pyridinium)chlorocarbene. J. Phys. Chem. A 2015, 119, 3556−3562. (14) Moya-Barrios, R.; Cozens, F. L.; Schepp, N. P. Absolute Reactivity of Halo(pyridyl)carbenes. J. Org. Chem. 2009, 74, 1148− 1155. (15) Baird, M. S.; Bruce, I. Generation and Trapping of 2- and 3Pyridylhalogenocarbenes from Diazirines. J. Chem. Res., Miniprint 1989, 2852−2871. (16) Moss, R. A.; Wang, L.; Krogh-Jespersen, K. Reactivity of Chlorotrifluoromethylcarbene. Activation Parameters for Halocarbene−Alkene Addition Reactions. J. Org. Chem. 2013, 78, 11040− 11044. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. The complete author list is available in Supporting Information. (18) Grimme, S. Semiempirical GGA-type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (19) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent Molecular Orbital Methods. IX. An Extended Gaussian-Type Basis for Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724−728. (20) Hariharan, P. C.; Pople, J. A. Accuracy of AHn Equilibrium Geometries by Single Determinant Molecular Orbital Theory. Mol. Phys. 1974, 27, 209−214. (21) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. SelfConsistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wavefunctions. J. Chem. Phys. 1980, 72, 650−654. (22) McLean, A. D.; Chandler, G. S. Contracted Gaussian-Basis Sets for Molecular Calculations. I. Second Row Atoms, Z = 11−18. J. Chem. Phys. 1980, 72, 5639−5648.

4. CONCLUSION As illustrated in Tables 2 and 3, the positively charged pyridinium unit of MePyr+CCl BF4− (3) enhances the equilibrium constants for the formation of a carbene−TMB complex (7), or for the addition of chloride to form the pyridinium dichloromethide zwitterion (11); cf. eqs 4 and 8. Supporting these conclusions are quantitative comparisons to analogous equilibria of PhCCl, PyrCCl (4), p-F3C-PhCCl, and F5-PhCCl. The thermodynamic parameters underlying the various equilibria are extracted and compared, while computational studies augment the analysis and discussion.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b11341. Analysis and graphical representations of various equilibrium constant and rate constant determinations, computed geometries and ground-state energetics, excited-state transition energies, amplitudes, and oscillator strengths (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We are grateful to the National Science Foundation and to Rutgers University for financial support. REFERENCES

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(43) The oscillator strength is proportional to the dipole strength. The dipole strength is proportional to εmax if a Gaussian band shape is assumed, see ref 40. (44) Computational results suggest that an absorption of complex 7 at 286 nm ( f = f 2 = 0.178) also contributes to the broad absorption observed near 300 nm. Note, however, that the analysis demonstrates that the correlation of A292/A564 with 1/[TMB] will afford a value of K that is uncontaminated by the potential contribution of complex 7 at 292 nm; cf., Supporting Information. (45) Lau, J. K.-C.; Deubel, D. V. Hydrolysis of the Anticancer Drug Cisplatin: Pitfalls in the Interpretation of Quantum Chemical Calculations. J. Chem. Theory Comput. 2006, 2, 103−106. (46) Mammen, M.; Shakhnovich, E. I.; Deutch, J. M.; Whitesides, G. M. Estimating the Entropic Cost of Self-Assembly of Multiparticle Hydrogen-Bonded Aggregates Based on the Cyanuric Acid-Melamine Lattice. J. Org. Chem. 1998, 63, 3821−3830. (47) Steinberg, I. Z.; Scheraga, H. A. Entropy Changes Accompanying Association Reactions of Proteins. J. Biol. Chem. 1963, 238, 172− 181. (48) Cooper, J.; Ziegler, T. A Density Functional Study of SN2 Substitution at Square-Planar Platinum(II) Complexes. Inorg. Chem. 2002, 41, 6614−6622. (49) Wertz, D. H. Relationship Between the Gas-Phase Entropies of Molecules and Their Entropies of Solvation in Water and 1-Octanol. J. Am. Chem. Soc. 1980, 102, 5316−5322. (50) Fry, A. J. Computational Studies of Ion Pairing. 10. Ion Pairing Between Tetrabutylammonium Ion and Inorganic Ions. A General Motif Confirmed. J. Org. Chem. 2015, 80, 3758−3765. (51) It seems possible that, in the presence of excess TBACl, Cl− may replace BF4− in an essentially thermoneutral reaction. Our calculations (B97D/6-311+G(d)) indicate that, rather than piggybacking on top of the zwitterion, Cl− seemingly prefers to hydrogen bond. We find that a low-energy Cl− “solvated” species, in which Cl− is hydrogen-bonded to both a methyl and a ring proton, would absorb at 306 nm (f = 0.634), and thus would be less likely to be observed near 350 nm. (52) As described in the text and Supporting Information, oscillator strengths (f) computed via TD-DFT enter into the expression for the experimentally derived equilibrium constant, K = ( f 3/f1)(1/Ka). This ratio of oscillator strengths is rather insensitive to the specific choice of density functional for the TD-DFT calculations. Specifically, for the MePyr+CCl BF4−/MePyr+CCl2− BF4− species we find f 3/f1 = 0.337/ 0.719 = 0.469 at the B3LYP/6-311+G(d) level. Applying another functional generally considered suitable for TD calculations such as CAM-B3LYP (and 6-311+G(d) basis sets) we find f 3/f1 = 0.370/0.805 = 0.460 for the MePyr+CCl BF4−/MePyr+CCl2− BF4− pair, i.e., virtually no change to the premultiplicative ratio of oscillator strengths and hence no change to the equilibrium constant or quantities derived from it. Thus, we infer that the substantial discrepancy between computed and measured entropy change for the addition of Cl− to MePyr+CCl BF4− is not related to our specific choice of the B3LYP functional for the TD-DFT calculations. (53) Radeglia, R.; Kim, D. G.; Boedeker, J. Hammett-Type Correlations Between Carbon-13 NMR Shifts and Substituent Constants of Anils of p-Nitrocinnamaldehyde. J. Prakt. Chem. (Leipzig) 1984, 326, 505−510. (54) Blanch, J. H. Determination of the Hammett Substituent Constants for the 2-, 3-, and 4-Pyridyl and−Pyridinium Groups. J. Chem. Soc. B 1966, 937−939. (55) Smith, M. B.; March, J. March’s Advanced Organic Chemistry, 6th ed.; Wiley-Interscience: New York, 2007; p 404. (56) The very negative ΔS° may indicate the recruitment of additional chloride anions (beyond BF4−) to provide dielectric screening of product MePyr+CCl2−.

(23) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. Efficient Diffuse Function-Augmented Basis Sets for Anion Calculations. III. The 3-21+G Basis Set for First-Row Elements, Li−F. J. Comput. Chem. 1983, 4, 294−301. (24) Frisch, Æ.; Frisch, M. J.; Clemente, F. R.; Trucks, G. W. Gaussian 09 User’s Reference; Gaussian, Inc.: Wallingford, CT, 2009; p 167. (25) McQuarrie, D. A. Statistical Thermodynamics; Harper and Row: New York, 1973. (26) Casida, M. E. Time-Dependent Density Functional Response Theory of Molecular Systems: Theory, Computational Methods, and Functionals. In Recent Developments and Applications of Modern Density Functional Theory; Seminario, J. M., Ed.; Elsevier: Amsterdam, The Netherlands, 1996; pp 391−439. (27) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (28) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (29) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627. (30) Martin, R. L. Natural Transition Orbitals. J. Chem. Phys. 2003, 118, 4775−4777. (31) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. MP2 Energy Evaluation by Direct Methods. Chem. Phys. Lett. 1988, 153, 503−506. (32) Barone, V.; Cossi, M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. J. Phys. Chem. A 1998, 102, 1995−2001. (33) Parr, R. G.; Szentpaly, L. v.; Liu, S. Electrophilicity Index. J. Am. Chem. Soc. 1999, 121, 1922−1924. (34) Perez, P. Theoretical Evaluation of the Global and Local Electrophilicity Patterns of Singlet Carbenes. J. Phys. Chem. A 2003, 107, 522−525. (35) Moss, R. A.; Krogh-Jespersen, K. Carbenic Philicity and the ‘Intrinsic Reactivity Index’. Tetrahedron Lett. 2013, 54, 4303−4305. (36) Rondan, N. G.; Houk, K. N.; Moss, R. A. Transition States and Selectivities of Singlet Carbene Cycloadditions. J. Am. Chem. Soc. 1980, 102, 1770−1776. (37) Zhang, M.; Moss, R. A.; Thompson, J.; Krogh-Jespersen, K. Evolution of Structure and Reactivity in a Series of Iconic Carbenes. J. Org. Chem. 2012, 77, 843−850. (38) Pliego, J. R., Jr.; De Almeida, W. B.; Celebi, S.; Zhu, Z.; Platz, M. S. Singlet Triplet Gap, and the Electronic and Vibrational Spectra of Chlorophenylcarbene: A Combined Theoretical and Experimental Study. J. Phys. Chem. A 1999, 103, 7481−7486. (39) Moss, R. A.; Wang, L.; Weintraub, E.; Krogh-Jespersen, K. The Solvation of Carbenes: π and Ylidic Complexes of p-Nitrophenylchlorocarbene. J. Phys. Chem. A 2008, 112, 4651−4659. (40) The oscillator strength ( f) may be considered as representing the total intensity of the transition. It is proportional to the extinction coefficient integrated over the entire absorption band (or “the area under the curve”). See, for example, Stephens, P. J.; Harada, N. ECD Cotton Effect Approximated by the Gaussian Curve and Other Methods. Chirality 2010, 22, 229−233. (41) Intimate ion pair 8 can exist in nearly isoenergetic syn and anti conformations. We find (B97D/6-311+G(d)) that the anti conformer of 8 is ca. 0.2 kcal/mol more stable than the syn conformer. The lowest energy conformer for the complex with TMB favors the anti conformer of the carbene by an even smaller amount (ΔΔG° ∼ 0.1 kcal/mol). The computed electronic spectroscopy data are extremely similar for both conformers (see section 3.5), and we do not expect that experimental resolution would be possible. Rather than averaging nearly indistinguishable values, we prefer to present in detail only data as they relate to the anti conformer. (42) For instrumental reasons, the absorption of complex 7 was followed at 564 nm rather than at the computed maximum of 568 nm. 708

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