J. Phys. Chem. 1996, 100, 15805-15809
15805
13C
NMR Spectroscopic and Density Functional Theory (DFT), ab Initio, and IGLO Theoretical Study of Protonated Cycloalkylcarboxylic Acids (Carboxonium Ions) and Their Acyl Cations (Oxocarbenium Ions)1 G. K. Surya Prakash,* Golam Rasul, G. Liang, and G. A. Olah* Loker Hydrocarbon Research Institute and Department of Chemistry, UniVersity of Southern California, UniVersity Park, Los Angeles, California 90089-1661 ReceiVed: June 13, 1996X
A series of protonated cycloalkylcarboxylic acids (carboxonium ions) and their corresponding acyl cations (oxocarbenium ions) were investigated in FSO3H-SbF5-SO2ClF solution by 13C NMR spectroscopy. The ions were also studied by density functional theory (DFT) and ab initio and IGLO theoretical methods. The study shows that the various cycloalkyl groups have little effect on the 13C NMR chemical shift of the carbocationic carbons of the ions. Charge calculations show that the delocalization into the cycloalkyl group is greater for the oxocarbenium ions than for the carboxonium ions. This is consistent with the greater electronic demand in oxocarbenium ions wherein one adjacent oxygen atom effects stabilization.
Introduction Protonated carboxylic acids (carboxonium ions) and their acyl cations (oxocarbenium ions) play important roles as intermediates in acid-catalyzed reactions.2 Many of the alkyl and arylcarboxonium and acyl cations were obtained as stable, longlived ions and directly observed by 1H and 13C NMR spectroscopy.3
ing atomic orbitals),9 and LORG (local origin/localized orbitals)10 related calculations of NMR chemical shifts. We wish now to report the 13C NMR spectroscopic investigation of a series of protonated cycloalkylcarboxylic acids and their corresponding acyl cations. Our study also included density functional theory (DFT),11 ab initio, and IGLO theoretical calculations. Results and Discussion
Olah et al. reported4 a 13C NMR spectroscopy study of the protonated formic, acetic, propionic, and benzoic acids and their corresponding acyl cations in FSO3H-SbF5-SO2 solution. The data demonstrated the linear sp-hybridization of the acyl cations (i.e. oxocarbenium ions) showing more shielded 13C NMR absorptions than those in the sp2-hybridized carboxonium ions. Olah et al. also reported5 the study of the cycloalkyloxocarbenium hexahaloantimonate complexes by infrared and 1H NMR spectroscopy. The ionic nature of the complexes was indicated by the presence of a strong absorption frequency around 22002250 cm-1 in their infrared spectra, which is characteristic for the -CtO+ group. No systematic 13C NMR or theoretical studies have been carried out. Recently we reported6 the preparation and 13C NMR spectroscopic and DFT theoretical study of protonated cubylcarboxylic and cubyldicarboxylic acids and their corresponding acyl cations. The study showed that the strained C-C bonds of the cubyl system are able to engage in hyperconjugative stabilization of adjacent charge to a greater extent than those of unstrained (or less strained) systems. Theoretical calculations have become an integral part of the study of electron deficient intermediates.7 This includes highlevel ab initio calculations of geometries and energies and IGLO (individual gauge for localized orbitals),8 GIAO (gauge-includX
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01676-0 CCC: $12.00
Preparation and NMR Studies. Protonated cycloalkylcarboxylic acids were obtained by dissolving the corresponding acids in excess FSO3H-SbF5-SO2ClF solutions at -80 °C. They subsequently can be dehydrated to their corresponding acyl cations (oxocarbenium ions) by raising the temperature to about -10 °C. The acyl cations were also directly prepared via ionization of corresponding acyl chlorides with SbF5-SO2ClF at -80 °C. The 13C NMR spectrum of protonated cyclopropylcarboxylic acid (1a) consists of three peaks (Table 1). The peak at δ13C 195.8 is assigned to the carboxylic carbon and is only 14.1 ppm deshielded from that of cyclopropylcarboxylic acid. The absorptions at δ13C 14.8 and 20.6 are assigned to the CH and CH2 carbons of the cyclopropyl ring of protonated cyclopropylcarboxylic acid, respectively. The 13C NMR chemi-
cal shifts of CH and CH2 carbons of protonated cyclopropylcarboxylic acid (1a) are deshielded by 1.9 and 11.4 ppm from the corresponding carbons of cyclopropylcarboxylic acid (Table 1). The 11.4 ppm difference between 13C NMR chemical shifts of the CH2 carbon may be due to limited cyclopropyl group © 1996 American Chemical Society
15806 J. Phys. Chem., Vol. 100, No. 39, 1996
Prakash et al.
TABLE 1: Experimental, IGLO II/B3LYP/6-31G* [Given in Brackets], and IGLO II/MP2/6-31G* (Given in Parentheses) 13C NMR Chemical Shifts of 1-8a
a Experimental chemical shifts are referenced to TMS; calculated chemical shifts are referenced to TMS calculated at the same level (the calculated absolute shielding (σ) for TMS are 196.8 at IGLO II//B3LYP/6-31G* and 197.4 at IGLO II//MP2/6-31G* level). b Average of C2 and C4. c Average of C2 and C5. d Average of C3 and C4. e Average of C2 and C6. f Average of C3 and C5. g Average of C2 and C7. h Average of C3 and C6. i Average of C4 and C5. j For 7a, expt C5 28.1, C6 30.4, C7 35.7; for 7b, expt C5, C6 30.4, C7 36.3. k For 8a, expt C5 25.1, C6 29.0, C7 33.3; for 8b, expt C5 27.4, C6 28.5, C7 38.2.
participation in stabilizing the carbocationic center in protonated cyclopropylcarboxylic acid (1a).
The 13C NMR chemical shifts of the carboxylic carbons of protonated cyclobutyl (2a), cyclopentyl (3a), cyclohexyl (4a), cycloheptyl (5a), and adamantyl (6a) carboxylic acids are δ13C 196.6, 198.5, 197.1, 199.6, and 199.9, respectively. They are 14.4, 15.0, 14.1, 15.7, and 15.5 ppm, respectively, deshielded from those of the corresponding carboxylic acids. This limited degree of deshielding, including that of protonated cyclopropylcarboxylic acid (1a), shows that the various cycloalkyl groups have little effect on the 13C NMR chemical shift of cationic carbons of carboxnium ions. It has been shown experimentally12 that the cyclopropyl group in some cases is capable of stabilizing an adjacent carbocation center to even a greater degree than a phenyl group. However, the narrow range of deshielding due to various cycloalkyl groups in protonated cycloalkylcarboxylic acids compared to their unprotonated forms suggests that the
cycloalkylcarboxonium ions are primarily stabilized by delocalization involving neighboring oxygen atoms. On the other hand, the 13C NMR chemical shift4 of the carboxylic carbon of protonated benzoic acid is only 7.7 ppm deshielded compared to that of benzoic acid, indicating that the positive charge is better delocalized in protonated benzoic acid than protonated cyclopropylcarboxylic acid. In the highly strained cubane system the carboxylic carbon of protonated cubylcarboxylic acid is even shielded by 2.5 ppm from that of the unprotonated acid.6 This is due to the strained C-C σ-bonds of cubane, which are capable of hyperconjugative stabilization of the adjacent positive charge better than less strained cyclic systems.
NMR studies of carboxylic acids in HF-SbF5 or HSO3FSbF5 (Magic Acid) have shown that the protonated acids may exist in two different conformers (syn-anti and anti-anti) and that these conformers can be frozen out at low temperatures.13
Carboxonium and Oxocarbenium Ions
J. Phys. Chem., Vol. 100, No. 39, 1996 15807 TABLE 2: Energies (-au) and ZPE (kcal/mol) of 1-8a B3LYP/6-31G*
Of the two possible isomers of protonated carboxylic acids, except for formic and acetic acids, only the syn-anti isomer is observed. For an example, we have calculated the syn-anti and anti-anti conformers of protonated cyclopropylcarboxylic acid and found the syn-anti conformer is 6.1 kcal/mol more stable than the anti-anti conformer at the B3LYP/6-31G* level (see the Calculations section of this paper for the level of calculations). In agreement with this only one isomer (in all probability the syn-anti) was observed in the 13C NMR spectra of all protonated cycloalkylcarboxylic acids in FSO3H-SbF5SO2ClF solutions at -80 °C. The 13C NMR chemical shift of the oxocarbenium carbon of corresponding cycloalkyloxocarbenium ions (acyl cations) is considerably more shielded than either that of the starting cycloalkylcarboxylic acids or their protonated forms. However, similar to protonated carboxylic acids, the range of the observed 13C NMR chemical shifts of the oxocarbenium carbons is also very narrow (152.8-155.7). Olah et al. suggested4a that the substantial shielding of oxocarbenium carbon is probably due to the anisotropic effect caused by the CtO triple-bond character of the oxocarbenium ions. This has been confirmed by 17O NMR spectroscopy.4b The shielding to some extent could
also be due to ketene-like resonance contributions in oxocarbenium ions.
It is interesting to note that the 13C NMR chemical shift of the oxocarbenium carbon of highly strained cubyloxocarbenium ion is δ13C 151.4,6 slightly more shielded than those of other cycloalkyloxocarbenium ions, because it is probably more influenced by the anisotropic effect of the CtO triple bond rather than the electron rich C-C bonds of the cubyl system. The same conclusion also holds for the benzoyl cation (phenyloxocarbenium ion), where the δ13C 153.6 NMR chemical shift4 of the oxocarbenium carbon is probably less influenced by the phenyl group than by the anisotropic effect of CtO triple bond, although ketene-like resonance forms have been invoked in the case of benzoyl cations. Calculations To further investigate the nature of interactions of the cycloalkyl group with the adjacent carbocationic center of carboxonium and oxocarbenium ions, we have calculated their geometries, charge distribution, and 13C NMR chemical shifts. Density functional theory (DFT) and ab initio calculations were carried out by using the GAUSSIAN-9414 package of programs. Optimized geometries were obtained with the DFT B3LYP15/ 6-31G* and ab initio HF/6-31G* and MP2/6-31G* levels. Vibrational frequencies at the HF/6-31G*//HF/6-31G* level were used to characterize stationary points as minima and to evaluate zero-point vibrational energies (ZPE), which are scaled by a factor of 0.89.16 The energies and ZPE of the ions are listed in Table 3. IGLO calculations were performed according
MP2/6-31G*
no.
a
b
a
b
1 2 3 4 5 6 7 8
306.796 11 (65.9) 346.107 04 (83.5) 385.451 09 (101.4) 424.773 36 (118.9) 464.076 61 579.625 50 462.864 26 (123.3) 462.862 94 (123.1)
230.329 60 (48.2) 269.647 32 (65.8) 308.993 00 (83.6) 348.315 19 (101.2) 387.619 42 503.171 22 386.407 77 (105.3) 386.407 91 (105.4)
305.863 71 345.026 97 384.226 86 423.402 06
229.619 98 268.790 24 307.990 91 347.166 48
a Zero-point vibrational energies (ZPE) at HF/6-31G*//HF/6-31G* are scaled by a factor of 0.89.
to the reported method8 at the IGLO II level using B3LYP/631G* and MP2/6-31G* optimized geometries. Huzinaga17 Gaussian lobes were used as follows: basis II, C, O, 9s5p1d contracted to [51111, 2111, 1]; d exponent, 1.0; H, 5s1p contracted to [311, 1]; p exponent, 0.70. Both experimental and calculated 13C NMR chemical shifts are referenced to TMS and are listed the Table 1 and Table 2. Selected B3LYP/631G* and MP2/6-31G* parameters are described throughout the text. We have fully optimized the structure of protonated cycloalkylcarboxylic acids and their corresponding oxocarbenium ions at the DFT B3LYP/6-31G* and ab initio HF/6-31G* and MP2/6-31G* levels (for simplification we calculated only the more stable syn-anti conformers of protonated cycloalkylcarboxylic acids; the anti-anti conformers are expected to give similar results but are not important for the topics discussed in this paper). Overall the DFT methods give reliable geometries compared to the MP2 calculations, as shown in Figure 1. In general the calculated C1-C2 bond length (for numbering see Table 1) of the cycloalkyl ring is longer for oxocarbenium ions than that of carboxonium ions. Thus the C1-C2 bond length of the cyclopropyl group of oxocarbenium ion 1b is 1.571 Å, 0.1 Å longer than that of carboxonium ion 1a at the B3LYP/ 6-31G* level. This suggests that the C1-C2 bond of 1b is more involved in hyperconjugative stabilization of the adjacent carbocationic center than that of 1a. The calculated Mulliken18 partial charges for C+ atoms, -COOH2+ or -CO+ groups, and cycloalkyl groups (C + H) are listed in Table 3. In general the cycloalkyl group of protonated cycloalkylcarboxylic acids accommodate about 33% of the charge. This can be compared with 48% of the phenyl ring of the protonated benzoic acid, which shows more charge delocalization into the phenyl ring. The cubyl group of protonated cubylcarboxylic acid accepts 44% of the charge.6 However, the delocalization into the cycloalkyl group is greater in the acyl cations (oxocarbenium ions), as the cycloalkyl group of the acyl cations accepts about 54% of the charge compared to the case of the carboxonium ions, consistent with the greater electronic demand due to there being only one neighboring oxygen atom in the former to effect stabilization. Whereas the alkyl group in cycloalkylcarboxonium ions accept about 54% of the charge, the phenyl group in benzoyl cation accepts as much as 70%. We also reproduced the 13C NMR chemical shifts of protonated cycloalkylcarboxylic acids and their corresponding oxocarbenium ions at the IGLO II//B3LYP/6-31G* and IGLO II/MP2/6-31G* levels with reasonable accuracy. In general the calculated 13C NMR chemical shifts of the carbocationic center of protonated cycloalkylcarboxylic acids and cycloalkyloxocarbenium ions are 11-15 and 17-20 ppm, respectively, more deshielded than the experimentally observed results. However, the deviation appears to be systematic (Table 1). It is known19 that IGLO performs poorly in calculations of 13C NMR chemical
Figure 1. Selected B3LYP/6-31G* optimized parametes (MP2/6-31G*) of 1-8. Bond lengths are in angstroms.
15808 J. Phys. Chem., Vol. 100, No. 39, 1996 Prakash et al.
Carboxonium and Oxocarbenium Ions
J. Phys. Chem., Vol. 100, No. 39, 1996 15809
TABLE 3: Calculated (B3LYP/6-31G*//B3LYP/6-31G*) Mulliken Chargea protonated acids (a) +
no.
C
1 2 3 4 5 6 7 8 Ph-
0.682 0.660 0.637 0.662 0.648 0.660 0.651 0.659 0.586
a
oxocarbenium ions (b)
-COOH2
cycloalkyl
C+
-CO+
cycloalkyl
0.662 0.689 0.670 0.690 0.679 0.658 0.668 0.676 0.542
0.338 0.311 0.330 0.310 0.321 0.342 0.332 0.324 0.458
0.623 0.629 0.622 0.632 0.624 0.608 0.619 0.621 0.486
0.457 0.471 0.459 0.471 0.462 0.428 0.449 0.446 0.303
0.543 0.529 0.541 0.529 0.538 0.572 0.551 0.554 0.697
+
Charge of hydrogens summed into the charge of the heavy atoms.
shifts of tertiary carbons (such as C1 of 1b, C1 and C2 of 8b). The agreement between experimental and calculated values may be improved by using correlated level calculations such as GIAO-MP2 method. However, GIAO-MP2 calculations using the ACES II program20 are presently limited to only small size molecules (limits strongly dependent on molecular symmetry). Conclusions Our study shows that the various cycloalkyl groups have little effect on the 13C NMR chemical shift of cationic carbons of carboxonium and oxocarbenium ions. Cycloalkylcarboxonium and oxocarbenium ions are primarily stabilized by delocalization involving oxygen atom(s). Charge calculations show that the delocalization into the cycloalkyl group is greater for the oxocarbenium ions than for the carboxonium ions. This is consistent with the greater electronic demand in oxocarbenium ions, wherein only one neighboring oxygen atom effects stabilization of the positive charge. Carboxonium and oxocarbenium ions are important classes of ionic reaction intermediates. The carbocationic nature of these ions, as shown in this study, is limited due to their strong delocalization. As a result, reactions at the carbocationic center are limited to those by efficient nucleophiles. However, their reactivity in superacidic systems can be greatly enhanced21 by further protonation (or protosolvation) or Lewis acid activation resulting in dipositive superelectrophiles.21 These dicationic species are not observed by NMR spectroscopy due to their expected very low equilibrium concentrations. Experimental Section All cycloalkylcarboxylic acids were purchased from the Aldrich Chemical Co. and used as received. The acid chlorides were prepared by treating the acids with thionyl chlorides. 13C NMR spectra were obtained at 300 MHz using a variable temperature probe. Carbocation Preparation. To a slurry of the appropriate precursor (ca. 30 mg) in SO2ClF (0.5 mL) in a 5 mm NMR tube and cooled to -80 °C (dry ice/acetone slurry) was added
a small quantity of neat FSO3H, Magic Acid, or a 50% v/v solution of SbF5 in SO2ClF, as required (see text). The ensuing mixture was vigorously stirred (Vortex agitator) with periodic cooling prior to transfer to a precooled NMR probe. Acknowledgment. Support of our work by the National Science Foundation is gratefully acknowledged. References and Notes (1) Part 300 of the series Stable Carbocations. For Part 299 see: Olah, G. A.; Rasul, G.; Heiliger, L.; Prakash, G. K. S. J. Am. Chem. Soc. 1996, 118, 3580. (2) Olah, G. A. Chem. Eng. News. 1967, 45, 13. (3) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; John Wiley & Sons: New York, 1985. (4) (a) Olah, G. A.; White, A. M. J. Am. Chem. Soc. 1967, 89, 7072. (b) Olah, G. A.; Iyer, P. S.; Prakash, G. K. S.; Krishnamurthy, V. V. J. Org. Chem. 1984, 49, 4317. Olah, G. A.; Berrier, A. L.; Prakash, G. K. S. J. Am. Chem. Soc. 1982, 104, 2373. (5) Olah, G. A.; Comisarow, M. B. J. Am. Chem. Soc. 1966, 88, 4442. (6) Head, N. J.; Rasul, G.; Mitra, A.; Heshemi, A. B.; Prakash, G. K. S.; Olah, G. A. J. Am. Chem. Soc. 1995, 117, 12107. (7) Bremer, M.; Schotz, K.; Schleyer, P. v. R.; Fleischer, U.; Schindler, M.; Kutzelnigg, W.; Koch, W.; Pulay, P. Angew. Chem., Int. Ed. Engl. 1989, 28, 1042; Bremer, M.; Schleyer, P. v. R.; Schotz, K.; Kausch, M.; Schindler, M. Angew. Chem., Int. Ed. Engl. 1987, 26, 761. Schleyer, P. v. R.; Carneiro, J. W. M.; Koch, W.; Forsyth, D. J. Am. Chem. Soc. 1991, 113, 3990. (8) Kutzelnigg, W. Isr. J. Chem. 1980, 19, 193. Schindler, M.; Kutzelnigg, W. J. Chem. Phys. 1982, 76, 1919. Schindler, M. J. Am. Chem. Soc. 1987, 109, 1020. Kutzelnigg, W.; Fleischer, U.; Schindler, M. NMR 1991, 91, 651. Sieber, S.; Schleyer, P. v. R.; Gauss, J. J. Am. Chem. Soc. 1993, 115, 6987. (9) Gauss, J. J. Chem. Phys. Lett. 1992, 191, 614. Gauss, J. J. Chem. Phys. 1993, 99, 3629. (10) Hansen, A. E.; Bouman, T. D. J. Chem. Phys. 1985, 82, 5035. (11) Ziegler, T. Chem. ReV. 1991, 91, 651. (12) Deno, N. C.; Richey, H. G.; Liu, J. S.; Lincoln, D. N.; Turner, J. O. J. Am. Chem. Soc. 1965, 87, 4533. (13) Olah, G. A.; White, A. M.; O’Brien, D. H. Chem. ReV. 1970, 70, 561. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Peterson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; HeadGordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94 (Revision A.1); Gaussian, Inc.: Pittsburgh, PA, 1995. (15) Becke’s Three Parameter Hybrid Method Using the LYP Correlation Functional: Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (16) Here, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley-Interscience: New York, 1986; p 226. (17) Huzinaga, S. Approximate Atomic WaVe Function; University of Alberta: Edmonton, Alberta, 1971. (18) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833. (19) Sieber, S.; Schleyer, P. v. R.; Gauss, J. J. Am. Chem. Soc. 1993, 115, 6987. (20) Stanton, J. F.; Gauss, J.; Watts, J. D.; Lauderdale, W. J.; Bartlett, R. J. ACES II, an ab initio program system; Quantum Theory Project: University of Florida, 1991 and 1992. (21) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767. Hartz, N.; Rasul, G.; Olah, G. A. J. Am. Chem. Soc. 1993, 115, 1277.
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