Ether–O-Ylide Equilibria - The

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Solvent Polarity Effects on Carbene/Ether−O-Ylide Equilibria Pablo A. Hoijemberg, 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: p-Nitrophenylchlorocarbene (PNPCC) reacted reversibly with tetrahydrofuran (THF), tetrahydropyran (THP), 1,3-dioxane (1,3-D), and 1,4-dioxane (1,4-D) to form O-ylides 8, 9, 10, and 11, respectively. The O-ylides were visualized by their characteristic UV−vis spectroscopic signatures. Equilibrium constants (K) were determined spectroscopically, and studies of K as a function of pentane/1,2-dichloroethane (DCE) solvent blends illustrated the dependence of K on solvent polarity. Electronic structure calculations based on density functional theory provided carbene/ ether O-ylide structures and energetics, as well as electronic spectroscopic parameters for use in the determination of K. Comparisons of the computed and experimental data were generally satisfactory.

1. INTRODUCTION Using laser flash photolysis (LFP), coupled with UV−visible spectroscopy, we demonstrated equilibration between arylchlorocarbenes, aromatic substrates, and their π-complexes.1 More recently, we found that p-nitrophenylchlorocarbene (1, PNPCC) reacted reversibly in pentane or heptane with diethyl ether, di-n-propyl ether, or tetrahydrofuran (THF) to form Oylides (2), which could be visualized by their UV−visible spectroscopic signatures (eq 1).2,3

2. EXPERIMENTAL DETAILS AND COMPUTATIONAL METHODS 2.1. Experimental Details. The preparation and purification of p-nitrophenylchlorodiazirine (7), precursor of PNPCC, have been fully described.3c Solvents and ethers were commercial materials, used as received, except for THF, which was distilled from sodium/benzophenone.

Equilibrium constants determined at 295 K ranged from 0.1 (di-n-propyl ether) to 7.5 M−1 (THF), and studies of K as a function of temperature afforded thermodynamic parameters for these equilibria. In particular, ΔH° was decidedly favorable for ylide formation (−6.5 to −11 kcal/mol), but was opposed by quite negative values of ΔS° (∼ −30 eu), so ΔG° was small (∼ −1 to +1 kcal/mol).2 In eq 1, two uncharged reactants are in equilibrium with an O-ylide product bearing formal charges on C and O, thus creating an appreciable dipole. For example, the computed gas phase dipole moment of the O-ylide derived from PNPCC and THF is 12.0 D (see below). We would expect K for such an equilibrium to be solvent dependent, increasing with increasing solvent polarity. Here, we determine K for the equilibria of eq 1 with four ethers: THF (3), tetrahydropyran (4, THP), 1,3-dioxane (5, 1,3-D), and 1,4-dioxane (6, 1,4-D). We find that relatively minor changes in the solvent dielectric constant across mixtures of pentane (ε = 1.837)4 and 1,2-dichloroethane (DCE, ε = 10.42)4 augment K by factors of ∼7−46. © 2012 American Chemical Society

LFP experiments employed a XeF2 excimer laser emitting 42−56 ns light pulses at 351 nm with 55−65 mJ power.5 For these experiments, diazirine 7 was dissolved in the desired solvent, and the selected ether was added to the desired concentration (about 0.0062−0.585 M, depending on the ether and type of experiment). The final absorbance of 7 was 0.30− 0.35 at 360 nm. 2.2. Computational Details. Electronic structure calculations were carried out using density functional theory (DFT)6 methodologies implemented in the Gaussian 09 suite of programs.7 Ground state geometry optimizations of the carbene (PNPCC, 1), ethers (3−6), and carbene−ether complexes (Oylides 8−11) were carried out using the hybrid meta generalized gradient approximation (GGA), dispersion-corrected exchange-correlation functionals denoted wB97XD8 and 6-311+G(d)9 basis sets (wB97XD/6-311+G(d)). General Received: March 13, 2012 Revised: April 20, 2012 Published: May 7, 2012 4745

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106 M−1 s−1 (cf. Figure S-1 in the Supporting Information). Moreover, the ratio of carbene absorption at 316 nm17 to THP O-ylide absorption at 460 nm was relatively constant between 50 and 150 ns after the laser pulse at several concentrations of THP (cf. Figure S-2 in the Supporting Information), suggestive of the equilibrium represented by eq 1. The equilibrium constant for PNPCC + THP ⇌ 9 was determined by the methodology used to obtain the analogous equilibrium constant for THF.2 Thus, K = (εPNPCC/εylide)(1/ slope) = ( f PNPCC/f ylide)(1/slope), where computed oscillator strengths ( f) have been introduced to replace the experimentally unknown extinction coefficients of the carbene (εPNPCC) and O-ylide (εylide),18 and “slope” refers to the slope of the correlation line formed when APNPCC/Aylide is plotted vs 1/[THP]. This correlation is shown in Figure 2, where the

solvent effects were incorporated into the calculations via the polarizable conductor, self-consistent reaction field model (CPCM).10 The model solvents investigated were n-pentane and 1,2-dichloroethane (DCE), and default solvent parameters provided in Gaussian 09 were employed.11 Geometry optimizations and normal-mode calculations were conducted with enhanced numerical integration grid sizes (integral(grid = ultrafine)) for both idealized gas and simulated solution phases.12 We equate the raw differences in free energies obtained from the CPCM calculations with enthalpies and then apply zero point energy (ZPE) corrections and thermal entropic corrections to generate solution phase free energies.13 The reaction free energies for O-ylide formation presented in Table 4 correspond to a reference state of 1 M concentration for each species participating in the reaction and T = 298.15 K. Excited state calculations made use of the time-dependent DFT formalism14 and the hybrid GGA B3LYP functionals (TD-B3LYP/6-311+G(d)//wB97XD/6-311+G(d)) and provided electronic absorption wavelengths (λ) and oscillator strengths (f).15 Assignment of a particular electronic transition (π → p, σ → p, or π → π*) was based on inspection of the largest transition amplitudes for the excitation and visualization of the contributing molecular orbitals.

3. RESULTS AND DISCUSSION 3.1. PNPCC with THF or THP in Pentane. As previously found, PNPCC generated by LFP of diazirine 7 in hydrocarbon solvent displays intense π → p absorption at 308 nm and a weaker σ → p absorption at 628 nm.2 Addition of THF to pentane solutions of 7 quenches the π → p absorption of LFPgenerated PNPCC (kq = 7 × 107 M−1 s−1, monitored at 316 nm), and gives rise to PNPCC/THF O-ylide 8, which absorbs at 452 nm.2 We calculate a strong absorption for O-ylide 8 at 436 nm in simulated pentane (f = oscillator strength = 0.5611).

Figure 2. Average values of A316/A468 vs 1/[THP] (M−1) for PNPCC and O-ylide 9 in pentane with THP at 295 K, 50−150 ns after LFP of diazirine 7. The slope of the correlation line is 0.512 ± 0.014, r = 0.999.19 See Table S-1 in the Supporting Information for the numerical values of the experimental points.

absorption of ylide 9 is measured at 468 nm and that of PNPCC is measured at 316 nm. The slope of the correlation line is 0.512 ± 0.014 and the computed f for PNPCC is 0.5762 (λ = 301 nm), so K = (0.5762/0.5378)(1/0.512) = 2.1 ± 0.06 M−1. A redetermination of K for the PNPCC + THF ⇌ 8 equilibrium in pentane, using a lower concentration of THF than previously employed, leads to K = 4.4 ± 0.1 M−1, significantly lower than our earlier value of 7.5 ± 0.3 M−1.2 The origin of the difference likely resides in the lower polarity of the THF−pentane solvent used here.20 Indeed, the dependence of K on solvent polarity was investigated in detail and was found to be pronounced. 3.2. Polarity Dependence of K with THF and THP. Values of K for PNPCC with THF or THP were determined in mixtures of pentane and DCE; the results appear in Table 1.21 The values of K increase markedly with the mole fraction of DCE in pentane for 0 ≤ XDCE ≤ 0.43; at mole fractions of DCE higher than 0.43, the decays of PNPCC and its THF or THP ylides are no longer well correlated, equilibrium conditions are not maintained, and reliable values of K cannot be determined. Note, however, that the lifetimes of O-ylides 8 and 9 increase by factors of 14 and 7, respectively, as the solvent polarity increases from pentane (ε = 1.837) to DCE (ε = 10.42). Table 1 reveals that K for the formation of the THF and THP O-ylides increases by factors of 35 and 47 as the solvent dielectric constant increases from 1.84 (pentane) to ∼5.4−5.5 (XDCE = 0.42−0.43). Given the sensitivity of K to solvent polarity for the THF and THP substrates, we extended our study to the dioxa analogues of THP, 1,3-dioxane (5, 1,3-D), and 1,4-dioxane (6, 1,4-D).

Similar experiments with THP (4) produce analogous results. LFP of diazirine 7 with 0.102 M THP in pentane affords the spectrum shown in Figure 1, where PNPCC appears at 308 nm and (weakly) at 628 nm, while the PNPCC/THP Oylide 9 absorbs at 460 nm. We calculate a strong absorption for O-ylide 9 at 439 nm in simulated pentane ( f = 0.5378). Successive additions of THP to solutions of diazirine 7 in pentane led to increased quenching of PNPCC with kq = 3.2 ×

Figure 1. Calibrated16 UV−vis spectrum of PNPCC in pentane with 0.102 M THP, 150 ns after the laser pulse. PNPCC absorptions at 308 and 628 nm; PNPCC/THP ylide at 460 nm. 4746

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Table 1. K for PNPCC with THF or THP in Pentane−DCE Solvent Blends ether

XDCEa

εb

fcarbenec

f ylided

τe

K, M−1

r

THF (0.123 M)

0 0.15 0.27 0.43 1 0 0.14 0.27 0.42 1

1.837 3.09 4.14 5.51 10.42 1.837 3.04 4.14 5.44 10.42

0.5762 0.5808 0.5847 0.5898 0.6081 0.5762 0.5807 0.5848 0.5896 0.6081

0.5611 0.6098 0.6502 0.7037 0.8947 0.5378 0.5859 0.6304 0.6818 0.8818

0.094 0.10 0.13 0.21 1.3 0.45 0.53 0.80 1.3 3.3

4.4 ± 0.1 17.5 ± 0.3 71 ± 1 153 ± 6 f 2.10 ± 0.06 11.4 ± 0.5 31 ± 1 99 ± 3 f

0.9991 0.9994 0.9994 0.9978

THP (0.102 M)

0.9988 0.9969 0.9970 0.9984

a

Mole fraction of DCE in pentane. bDielectric constant.21 cf of PNPCC computed in pure pentane or DCE, and assumed to vary linearly with XDCE at intermediate mole fractions of DCE. df of ylide 8 (from THF) or 9 (from THP); treated similarly to f of PNPCC. eLifetime of O-ylide 8 or 9 in μs. fThe carbene and ylide exhibit very different lifetimes when XDCE > 0.43; no equilibrium was observed.

3.3. PNPCC with 1,3-D and 1,4-D. The O-ylides of 1,3-D and 1,4-D are 10 and 11, respectively. Presumably due to the electron withdrawing inductive effect of the second oxygen atom, these species are less stable than THP ylide 9, and they decay more rapidly. The lifetime of 9 in pure THP (ε = 5.66)22 exceeds 3.0 μs, but the lifetimes of 10 and 11 in pure 1,3-D or 1,4-D (ε = 2.22)22 are 0.42 in pentane (ε ∼ 5.43) were we able to visualize ylides 10 and 11.

In DCE solvent, it is possible to quantitate both PNPCC and the O-ylides derived from 1,3-D or 1,4-D. Moreover, Figures S6 and S-7 in the Supporting Information show that the ratio of carbene absorption at 316 nm to O-ylide absorption at 500 nm (for both 10 and 11) remains relatively constant 50−400 ns after the generation of PNPCC in the presence of 1,3-D or 1,4D in pentane−DCE mixtures with XDCE ≥ 0.42. Here, equilibrium conditions prevail for PNPCC + (1,3-D or 1,4D) ⇌ (10 or 11). Table S-2 in the Supporting Information documents the time intervals during which Acarbene/Aylide is relatively constant, thus enabling the determination of K for all four ether substrates under all of the selected solvent conditions. The equilibrium constants for O-ylide formation between PNPCC and 1,3-D or 1,4-D were determined in several pentane−DCE solvent blends with XDCE ≥ 0.42. We correlated the carbene/O-ylide absorbance ratio (A316/A500) with the inverse concentration of 1,3-D or 1,4-D. The slopes of these correlations were then inserted into K = (f PNPCC/f ylide)(1/ slope) to obtain K. Figure 4 illustrates correlations of A316/A500 vs 1/[1,3-D] or 1/[1,4-D] in DCE. The derived K values are 2.6 ± 0.09 M−1 for PNPCC + 1,3-D ⇌10 and 6.1 ± 0.4 M−1 for PNPCC + 1,4-D ⇌11. 3.4. Polarity Dependence of K with 1,3-D and 1,4-D. K values for PNPCC with 1,3-D and 1,4-D were similarly

The UV−vis spectrum of PNPCC/1,3-D O-ylide 10 in DCE appears in Figure 3, where 10 absorbs at 500 nm. We calculate this absorption at 438 nm (f = 0.7741). The π → p PNPCC absorption is observed at ∼310 nm.

Figure 3. Calibrated16 UV−vis spectrum of PNPCC in DCE with 0.117 M 1,3-D 100 ns after the laser pulse. O-Ylide 10 absorbs at 500 nm; PNPCC is at ∼310 nm.

A very similar UV−vis spectrum was obtained for the PNPCC/1,4-D O-ylide 11 in DCE, with λmax at 500 nm; cf. Figure S-3 in the Supporting Information. We calculate this absorption at 450 nm (f = 0.6456). Due to their rapid decay in less polar solvents, O-ylides 10 and 11 could not be quantified in pentane or in pentane−DCE blends with XDCE < 0.42. However, 1,3-D and 1,4-D do quench PNPCC in pentane with kq = 3.9 × 107 and 5.6 × 107 M−1 s−1, respectively; cf. Figures S-4 and S-5 in the Supporting Information.

Figure 4. Average values of A316/A500 vs 1/[1,3-D] or 1/[1,4-D] (M−1) for PNPCC and O-ylides 10 and 11 in DCE at 295 K, 200−500 ns (1,3-D) and 230−1200 ns (1,4-D) after LFP of diazirine 7. The slopes of the correlation lines are 0.297 ± 0.010, r = 0.998 (1,3-D), and 0.154 ± 0.010, r = 0.994 (1,4-D).23 See Table S-1 in the Supporting Information for the numerical values of the experimental points. 4747

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Table 2. K for PNPCC with 1,3-D or 1,4-D in Pentane−DCE Solvent Blends ether

XDCEa

εb

fcarbenec

f ylided

τe

K, M−1

1,3-D (0.117 M)

0 0.42 0.60 0.82 1 0 0.43 0.60 0.82 1

1.837 5.44 7.00 8.89 10.42 1.837 5.51 7.00 8.89 10.42

0.5762 0.5897 0.5952 0.6025 0.6081 0.5762 0.5898 0.5955 0.6023 0.6081

0.5268 0.6311 0.6745 0.7307 0.7741 0.5055 0.5654 0.5902 0.6202 0.6456

f 0.24g 0.32h 0.25i 0.26 f 0.090 0.089 0.12j 0.16

f 0.263 ± 0.009 0.63 ± 0.02 1.26 ± 0.04 2.64 ± 0.09 f 0.77 ± 0.03 1.45 ± 0.03 3.8 ± 0.1 6.1 ± 0.4

1,4-D (0.117 M)

r 0.9983 0.9981 0.9988 0.9982 0.9973 0.9995 0.9988 0.9936

a

Mole fraction of DCE in pentane. bDielectric constant.21 cf of PNPCC computed in pure pentane or DCE, and assumed to vary linearly with XDCE at intermediate mole fractions of DCE. df of ylide 10 (from 1,3-D) or 11 (from 1,4-D); treated in the same manner as f of PNPCC. eLifetime of Oylide 10 or 11 in μs. fYlide signal is not visualized in pentane; see text. gXDCE = 0.41. hXDCE = 0.61. iXDCE = 0.85. jXDCE = 0.79.

Table 3. Comparison of Equilibrium Constants for O-Ylide Formation with PNPCC and Ethers

determined in pentane−DCE blends with XDCE = 0.42−1.0. The results appear in Table 2. As with ylide formation from PNPCC and THF or THP (Table 1), values of K for the reactions of PNPCC with 1,3-D or 1,4-D increase as solvent polarity increases. As XDCE increases from 0.42 to 1.0, K increases by factors of 10 and 7.9 for 1,3-D and 1,4-D, respectively. Note, however, that the lifetimes of O-ylides 10 and 11 (Table 2) do not vary greatly with solvent polarity for 0.42 ≤ XDCE ≤ 1.0, because the solvent is already reasonably polar at XDCE = 0.42 (ε ∼ 5.44).21 The lifetimes of the THF and THP O-ylides, 8 and 9, appear to be more sensitive to solvent polarity (Table 1), because the solvent is varied from pure pentane (ε = 1.84) to pure DCE (ε = 10.4). The dependence of K on XDCE is portrayed graphically in Figure 5a (PNPCC + THF or THP) and Figure 5b (PNPCC +

ether

O-ylide

XDCE

ε

K, M−1

Krel

THF THP 1,3-D 1,4-D

8 9 10 11

0.43 0.42 0.42 0.43

5.51 5.44 5.44 5.51

153 ± 6 99 ± 3 0.26 ± 0.09 0.77 ± 0.03

588 381 1.0 3.0

effect of the second oxygen, operating on the positively charged ylidic oxygen of 10 or 11, destabilizes these ylides relative to the O-ylides of THF or THP (8 or 9), which lack the additional oxygen atom. In fact, this qualitative explanation is supported by the computational studies presented in section 3.5, where the order of experimental K values of Table 3 (THF > THP ≫ 1,4-D > 1,3-D) is paralleled nicely by the computed values of K, ΔH°, and ΔG° for PNPCC/ether O-ylide formation. In addition, the observed ordering of K is also consistent with (inter alia) basicity values measured (in methanol) for the effects of these cyclic ethers on the cationic polymerization of isobutyl vinyl ether,24 and their degrees of association with trityl cation,25 or p-fluorophenol.26 3.5. Computational Studies. The thermodynamic parameters (ΔH°, ΔS°, ΔG°) for O-ylide formation under idealized gas phase and simulated n-pentane and DCE solvent conditions are presented in Table 4. The rank order of O-ylide stability is established already in the gas as 8 > 9 ≫ 11 > 10, in agreement with the relative magnitudes of the experimental equilibrium constants reported above. The computed enthalpies of stabilization (gas phase) are −11.9, −11.2, −8.9, and −7.3 kcal/mol for 8, 9, 11, and 10, respectively, whereas the reaction entropies are uniformly large and negative (−39 to −41 eu). The enthalpic stabilization provided by even a low strength dielectric medium such as pentane (ε = 1.84) is 1−2 kcal/mol for O-ylides 8−11. The significant increase in solvent polarity afforded by DCE (ε = 10.4) provides substantial additional increases in enthalpic stability, ranging from a low of 2.2 kcal/ mol for 11 to 3.4 kcal/mol for 8.27 The solvation enthalpies correlate modestly with the magnitude of the solute O-ylide dipole moment, being larger (and similar) for ylides 8−10 than for 11. The dipole moments for 8−10 grow from gas phase values of approximately 12 D to 20−22 D in DCE; for 11, the change in dipole moment from gas phase to DCE solution is less than 5 D (Table 4). We also note that the increased thermodynamic stability of the O-ylides is accompanied by a reduction in the length of the polar carbene−ether C−O bond

Figure 5. K (M−1) vs XDCE in pentane−DCE solvent blends for equilibria of PNPCC with (a) THF (◇) or THP (△) and (b) 1,3-D (○) or 1,4-D (□).

1,3-D or 1,4-D), and a convenient overview of the variation of K for the four ether substrates at a common XDCE is presented in Table 3, with data extracted from Tables 1 and 2. We observe that K for THF or THP is more than 2 orders of magnitude larger than K for 1,3-D or 1,4-D. A qualitative explanation for this disparity focuses on the additional oxygen atom in 1,3-D or 1,4-D. The electron withdrawing inductive 4748

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Table 4. Computed Properties of O-Ylidesa K (×103), M−1

ether/O-ylide

ΔH° b

ΔS° b

ΔG° b

THF/8 THP/9 1,3-D/10 1,4-D/11

−11.9 −11.2 −7.3 −8.9

−41.0 −39.8 −39.4 −39.0

0.34 0.67 4.4 2.8

THF/8 THP/9 1,3-D/10 1,4-D/11

−13.9 −13.0 −8.8 −10.1

−42.3 −40.6 −40.3 −40.7

−1.3 −0.93 3.2 2.0

904 484 4.6 33.2

THF/8 THP/9 1,3-D/10 1,4-D/11

−17.3 −15.7 −11.2 −12.3

−44.9 −41.1 −40.0 −41.8

−3.9 −3.5 0.76 0.20

728 346 0.278 0.712

Krel

μ, D

Cc−Oe,c Å

Oe−C,d Å

961 549 1.0 16

12.0 11.7 11.9 9.5

1.473 1.491 1.494 1.505

1.487, 1.481, 1.501, 1.477,

1.474 1.464 1.460 1.459

1.357 2.412

1962 1052 1.0 7.2

13.9 13.5 13.9 11.0

1.451 1.470 1.472 1.484

1.489, 1.487, 1.510, 1.482,

1.479 1.470 1.466 1.464

1.353 2.412

2523 1245 1.0 2.6

22.1 22.0 19.8 14.1

1.400 1.412 1.424 1.452

1.501, 1.496, 1.531, 1.488,

1.494 1.494 1.479 1.473

1.345 2.413

C−O,e Å

Gas Phase 565 322 0.588 0.914 n-Pentane

DCE

a

Calculations were made at the wB97XD/6-311+G(d) level for thermodynamics and the TD-B3LYP/6-311+G(d)//wB97XD/6-311+G(d) level for electronic spectroscopy (UV−vis); see section 2.2. bThe units for ΔH° and ΔG° are kcal/mol; units for ΔS° are cal/(mol K). cC (carbene)−O (ether) distance. dO (ether)−C (THF, THP, 1,3-D, or 1,4-D) distances. eC−O (1,3-D or 1,4-D) distance.

bonds. We find that the ylidic C (carbene)−O (ether) bond is appropriately short at 1.424 Å (Figure 6); however, the O

by more than 0.05 Å from gas phase to DCE solution (Table 4). The computed entropies for O-ylide formation remain very negative throughout the calculations including model solvent (∼ −40 to −45 eu; Table 4). Their magnitudes are most likely overestimated, since continuum dielectric models are not able to capture the reductions in rotational and translational entropy resulting from the restricted molecular motion in a condensed phase (relative to gas phase). The large positive −TΔS° terms cause the computed Gibbs free energies to be generally unfavorable in the gas phase, and only moderately favorable for 8 and 9 in pentane and DCE. Consequently, the computed equilibrium constants for ylide formation are consistently smaller than the experimental values by approximately 3 orders of magnitude, corresponding to an underestimation in the free energy of ylide formation by ca. 4 kcal/mol or, equivalently, an overestimation of the change in entropy by ca. 14 eu.28 However, the relative K values nicely parallel the observed trend: compare, for example, the relative values computed in DCE (Krel = 2500:1250:1.0:2.6 for 8, 9, 10, and 11, respectively; Table 4) with the experimental values in Table 3 (Krel = 600:400:1.0:3.0 for 8, 9, 10, and 11, respectively; XDCE ∼ 0.4). The presence of two highly electronegative atoms in the dioxanes creates competition for electron density, and electrostatic induction reduces the driving force for O-ylide formation (relative to THF and THP). The inductive effect operates through bonds and through space; indeed, the through-space interaction appears to be particularly pronounced in determining the stability of O-ylide 11. We note that the dipole moment of 11 does not increase as rapidly with increasing solvent polarity as that of 9 (Table 4). The difference in dipole moments between the two species is 2.2 D in the idealized gas phase and 7.9 D in simulated DCE solvent,29 demonstrating that the presence of the electron-withdrawing second O atom of 1,4-D, despite being situated three bonds away from the ylidic O atom, exerts a substantial destabilizing effect on the highly polar C (carbene)−O (ether) bond. The optimized geometry of 10, especially in DCE model solvent, illustrates the particularly weak binding between PNPCC and 1,3-D arising from electrostatic induction through

Figure 6. Optimized bond lengths of 10 in DCE model solvent and contributing resonance structure 10a.

(ether)−C (ether) bonds are asymmetric in length: one is normal at 1.479 Å but the other is elongated at 1.531 Å. The C−O bond adjacent to the longer O (ether)−C (ether) bond is correspondingly short at 1.345 Å, showing partial double bond character. These structural features reveal that the electronic structure of 10 contains a significant contribution from zwitterionic resonance structure 10a. As previously described, the signature PNPCC/ether O-ylide absorption in the 450−500 nm range primarily represents electronic excitation from a lone pair on the ylidic carbon (Oylide highest occupied molecular orbital) to a phenyl π* orbital (lowest unoccupied molecular orbital) with substantial contribution from the p-NO2 substituent; hence, the transition contains considerable charge-transfer character.2 The sizable increases in intensities computed for the 450 nm O-ylide band with increasing solvent dielectric constant (cf. f ylide values in Tables 1 and 2) reflects the electrostatic stabilization of the donor lone pair and the acceptor π* orbital, which localizes the electronic excitation, enhances the effective donor and acceptor strengths, and therefore magnifies the transition densities and the transition moment for the charge transfer.30

4. CONCLUSIONS p-Nitrophenylchlorocarbene (PNPCC) reacts reversibly with THF, THP, 1,3-D, and 1,4-D to form O-ylides, which can be visualized by their UV−vis spectroscopic signatures. Equilibrium constants (K) could be determined spectroscopically, 4749

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The Journal of Physical Chemistry A

Article

(11) Ribeiro, R. F.; Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2011, 115, 14556−14562. (12) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995−2001. (13) McQuarrie, D. A. Statistical Thermodynamics; Harper and Row: New York, 1973. (14) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108, 4439−4449. (15) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev.B 1988, 37, 785−789. (16) Calibration corrects the raw UV−vis absorptions for wavelength-dependent variations in sample absorptivity (including precursor diazirine), xenon monitoring lamp emission, and detector sensitivity. (17) The intensity of the PNPCC absorptions was measured at 316 nm, where our detector is 40% more sensitive than at 308 nm. Although the addition of THF quenches the PNPCC absorption at 316 nm, the resulting PNPCC−THF ylide absorption at 452 nm has a tail that overlaps the weak PNPCC absorption at 628 nm, making it difficult to observe PNPCC quenching at this wavelength. However, when the PNPCC forms an O-ylide with THP, carbene quenching can be observed at both 316 and 628 nm. (18) Formally, the computed oscillator strength is proportional to the integrated molar absorptivity. Although the widths of the ylide and carbene absorption bands are quite different, we did not attempt to correct for this. We conservatively estimate that the ratio fcarbene/f ylide accurately reproduces the desired (εcarbene/εylide) ratio to within a factor of 2. (19) Taking into account the standard deviation of each experimental point leads to r = 0.894. (20) In the present experiments, the concentration of THF varied from 0.0062 to 0.0555 M, corresponding to ε = 1.841−1.873 (where ε = 1.837 for pure pentane). In our previous work, [THF] varied from 0.056 to 1.12 M, corresponding to ε = 1.874−2.549. It is shown that K is quite sensitive to the solvent electric permittivity. (21) The dielectric constants of pentane and DCE are 1.837 and 10.42, respectively.4 The effective dielectric constants of solvent blends were estimated by linear interpolation using mole fractions. Note that the absorptions of the ylides shift as the polarity of the solvent changes. Calculations of K evaluated each ylide at the wavelength corresponding to its maximum absorption for the corresponding solvent composition. (22) For the dielectric constants of THP and 1,4-D, see ref 4, pp 6142 and 6-140. (23) Taking into account the standard deviation of each experimental point lowers the r values to 0.945 (1,3-D) and 0.972 (1,4-D). (24) Aoshima, S.; Fujisawa, T.; Kobayashi, E. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1719−1728. (25) Slomkowski, S.; Penczek, S. J. Chem. Soc., Perkin Trans. 2 1974, 1718−1722. (26) Bertholet, M.; Besseau, F.; Laurence, C. Eur. J. Org. Chem. 1998, 925−931. (27) Although distinct parameters are applied for each model solvent, the magnitude of the dielectric constant is clearly the decisive single solvent property determining the solute−solvent interaction strength. (28) We know from past studies that electronic structure calculations reproduce enthalpies for O-ylide formation much more accurately than entropies; see, e.g., ref 2. (29) The dipole moment vectors are effectively oriented in the same manner in all the O-ylides (8−11). (30) Similar considerations also apply to the pentane−DCE changes computed for fcarbene (Tables 1 and 2), since the reference PNPCC transition is a π (phenyl) → p (carbene) transition with chargetransfer character.2

and studies of K as a function of pentane−DCE solvent blends illustrate the dependence of the equilibrium constants on solvent polarity. Electronic structure calculations based on density functional theory provided carbene/ether O-ylide structures and energetics, as well as electronic spectroscopic parameters for use in the determination of K. Comparisons of the computed and experimental data are generally satisfactory.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S-1 to S-7; Tables S-1 and S-2; complete reference to Gaussian 09; wB97XD/6-311+G(d) optimized geometries and absolute energies of PNPCC, THF, THP, 1,3-D, 1,4-D, and Oylides 8−11 in idealized gas phase, model n-pentane and DCE solutions; TD-B3LYP/6-311+G(d)//wB97XD/6-311+G(d) electronic excitation energies and oscillator strengths for PNPCC and 8−11 in idealized gas phase, model n-pentane and DCE solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.A.M.); krogh@ rutchem.rutgers.edu (K.K.-J.). Notes

The authors declare no competing financial interest.

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

(1) Moss, R. A.; Wang, L.; Odorisio, C. M.; Krogh-Jespersen, K. J. Am. Chem. Soc. 2010, 132, 10677−10679. (2) Hoijemberg, P. A.; Moss, R. A.; Krogh-Jespersen, K. J. Phys. Chem. A 2012, 116, 358−363. (3) For other reports of O-ylide formation from PNPCC and ethers, see: (a) Tsao, M. L.; Zhu, Z.; Platz, M. S. J. Phys. Chem. A 2001, 105, 8413−8416. (b) Celebi, S.; Tsao, M. L.; Platz, M. S. J. Phys. Chem. A 2001, 105, 1158−1162. (c) Moss, R. A.; Wang, L.; Weintraub, E.; Krogh-Jespersen, K. J. Phys. Chem. A 2008, 112, 4651−4659. (4) CRC Handbook of Chemistry and Physics, 87th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, USA, 2006; pp 6-142, 6-138. (5) For more details, see: Moss, R. A.; Tian, J.; Sauers, R. R.; Ess, D. H.; Houk, K. N.; Krogh-Jespersen, K. J. Am. Chem. Soc. 2007, 129, 5167−5174. The current pulse width is broader than reported in the earlier study. (6) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; University Press: Oxford, 1989. Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional Theory; Wiley-VCH: Weinheim, Germany, 2000. (7) Frisch, M. J.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Pittsburgh, PA, USA, 2009. See the Supporting Information for the complete reference. (8) Chai, J.-D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10, 6615−6620. (9) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724−728. (b) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209−214. (c) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650−654. (d) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639−5648. (e) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294− 301. (10) Frisch, Æ.; Frisch, M. J.; Clemente, F. R.; Trucks, G. W. Gaussian 09 User’s Reference; p 244. 4750

dx.doi.org/10.1021/jp302426t | J. Phys. Chem. A 2012, 116, 4745−4750