Ab Initio Thermochemistry of Some Halogenated Cyclopropanes Igor Novak Department of Chemistry, National University of Singapore, Singapore 117543, Singapore
[email protected] Received March 6, 2002
Abstract: The standard enthalpies of formation for a series of chloro- and fluoro-substituted cyclopropanes have been calculated by using high-level ab initio G3/B3LYP methods. The relative stabilities of isomers and the influence of substituents on thermochemistry in several classes of substituted derivatives are discussed.
The purpose of this work is 4-fold. First, we aim to provide accurate ∆Hf0 values for a series of halogenated cyclopropanes for which few data are currently available in the comprehensive literature sources.1 The halogenated cyclopropanes are of interest to organic chemists because they are highly strained compounds.2,3 To assess the “strain” energy of cyclopropanes (via various possible hypothetical reaction schemes), accurate enthalpies of formation are of course crucial. Second, we wish to study relative isomer stability and compare the effects of halogen substitution on enthalpies in different classes of compounds, including cyclopropanes. Third, we wish to establish the (de)stabilization effects in cyclopropane upon halogenation. Fourth, we wish to assess the reliability of some experimental data reported in the NIST database.1 Previous investigations of halogenated cyclopropanes did not use high-level methods to achieve chemical accuracy and did not include all halogenated cyclopropanes. In this work we have used high-level calculations to probe substituent effects in halogenated cyclopropanes. In the absence of experimental data this appears to be the method of choice. The calculation method has sufficient energy sensitivity to reveal interesting trends in relative isomer stabilities. Also, we have shown that the destabilization introduced by fluorine substitution is at least twice as large as that of chlorine’s. Some interesting results concerning relative isomer stability, presented here, may encourage further studies of the complex effects which influence the stability of cyclopropane system. Method of Calculation The ab initio calculations were performed with Gaussian 98 software5 using the G3/B3LYP method,6 whose typical precision for ∆Hf0(298 K) is better than (8.2 kJ mol-1. G3/B3LYP provides total electron energies from which accurate standard enthalpies (1) NIST Chemistry WebBook; Linstrom, P. J., Mallard W. G., Eds; NIST Standard Reference Database No. 69; NIST: Gaithersburg, MD, July 2001. (2) Wiberg, K. B.; Marquez, M. J. Am. Chem. Soc. 1998, 120, 2932. (3) Zeiger, D. N.; Liebman, J. F. THEOCHEM 2000, 556, 83.
can be deduced for a wide range of organic compounds using isodesmic or atomization approaches. The selection of “anchor” compounds in isodesmic reactions was made so that only those with accurate and reliable enthalpies were included. The experimental ∆Hf0(298 K) values for “anchor compounds” are as follows: cyclopropane (53.3 kJ mol-1), CF4 (-933.2 kJ mol-1), CCl4 (-96 kJ mol-1), CH4 (-74.6 kJ mol-1).1,7-10 These are the most reliable and accurate values available at this time. Selection of isodesmic reactions or anchors is not unique, so we have also calculated enthalpies using atomization reaction schemes to provide reference and check on isodesmic values. The standard enthalpies of formation of elements have been taken from the NIST source.1 The comparison of isodesmic values with the atomization method of calculating enthalpies suggests that the former method provides more accurate values.11 This is due to the cancellation of errors in isodesmic reactions. Thus, the enthalpy values used in further analysis in this work shall be those obtained via isodesmic reaction schemes. The following isodesmic reaction schemes (at 298 K) were used for extracting enthalpies:
4C3H6 + CX4 ) 4C3H5X + CH4 (for monohalo derivatives) 2C3H6 + CX4 ) 2C3H4X2 + CH4 (for dihalo derivatives) 4C3H6 + 3CX4 ) 4C3H3X3 + 3CH4 (for trihalo derivatives) C3H6 + CX4 ) C3H2X4 + CH4 (for tetrahalo derivatives) 4C3H6 + 5CX4 ) 4C3HX5 + 5CH4 (for pentahalo derivatives) 2C3H6 + 3CX4 ) 2C3X6 + 3CH4 (for hexahalo derivatives) where (X ) F, Cl) The procedure for obtaining ∆Hf0(298 K) was as follows. Each isodesmic reaction includes “anchor compounds”, with known ∆Hf(298 K) and a single compound whose ∆Hf(298 K) is to be calculated. Enthalpy change in a reaction is given as ∆Hr(298 K) ) ΣEprod - ΣEreact where E refers to the total energies (including vibrational and rotational energy contributions at 298 K) obtained from G3/B3LYP calculations. Subsequently, the unknown ∆Hf(298 K) is calculated from known ∆Hr and ∆Hf(298 K) for “anchor compounds”. The absolute enthalpies thus obtained are presented in Table 1 together with enthalpies (4) Gaussian 98, Revision A.11: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1998. (5) Gaussian 98W, version 5, Gaussian 1995-98. (6) Baboul, A. G.; Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. J. Chem. Phys. 1999, 110, 7650. (7) Novak, I., J. Chem. Inf.Comput. Sci. 2000, 40, 358. (8) Novak, I. J. Org. Chem. 2000, 65, 5057. (9) Novak, I. J. Org. Chem. 2001, 66, 3600. (10) Novak, I. J. Org. Chem. 2001, 66, 9041. (11) Cheung, T.-S.; Law, C.-K.; Li, W.-K., THEOCHEM 2001, 572, 243.
10.1021/jo0201528 CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/30/2002
J. Org. Chem. 2002, 67, 6279-6281
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TABLE 1. Standard Enthalpies of Formation ∆Hf0(g,
298.15 K)/kJ mol-1 for Cyclopropanes C3XnH6-n Where X ) F, Cl F F Cl Cl substitution (atomization) (isodesmic) (atomization) (isodesmic) 11,1(Z)-1,2(E)-1,21,1,2(E)-1,2,3(Z)-1,2,31,1,2,2(E)-1,1,2,3(Z)-1,1,2,31,1,2,2,31,1,2,2,3,3-
-128.4 -340.0 -295.1 -306.9 -505.0 -442.6 -465.2 -701.6 -650.6 -661.6 -846.0 -1030.0
-132.7 -344.6 -299.7 -311.5 -509.9 -447.5 -470.1 -706.8 -655.8 -666.8 -851.4 -1035.8
33.0 14.2 19.9 12.3 1.1 12.7 1.2 -7.5 -6.7 -11.2 -16.7 -20.7
29.1 10.3 16.0 8.4 -2.8 8.8 -2.7 -11.3 -10.5 -15.0 -20.4 -24.5
FIGURE 2. Calculated standard enthalpies of formation for the following series of chlorinated compounds: CClnH4-n (VAR2); C2ClnH4-n (VAR3); C3ClnH4-n (VAR4); C5ClnH4-nO (VAR6); C5ClnH4-nS (VAR5); C3ClnH6-n (VAR7).
FIGURE 1. Calculated standard enthalpies of formation for the following series of fluorinated compounds: CFnH4-n (VAR2); C2FnH4-n (VAR3); C3FnH4-n (VAR4); C5FnH4-nO (VAR6); C5FnH4-nS (VAR5); C3FnH6-n (VAR7). calculated via the atomization scheme. The two sets of values are in good agreement. The decreases of ∆Hf0(298 K) values upon increasing halogenation within six sets of halogenated compounds are shown in Figures 1 and 2. The fluorination leads to a strong fall in enthalpy, while chlorination decreases the enthalpy to a lesser extent (compare the slopes of the curves in Figures 1 and 2). The halomethanes show deviations from the quasilinear trends observed in the rest of the compounds due to steric reasons, as was discussed previously.7 It is interesting to note that the trends persist regardless of whether the halogenated compound is saturated,
unsaturated, or aromatic. This can be taken as an illustration of the inductive nature of halogen substitution. The topology of substitution exhibits an interesting influence on the relative stabilities of different isomers. Among fluorinated cyclopropanes, the most stable isomers have two fluorine substituents on the same carbon; i.e., 1,1-substitution is energetically stabilizing (Table 1). In chloro analogues, no clear trend can be discerned within the uncertainty range of the G3/B3LYP method (Table 1). Why is there this dependence of isomer stability on topology? The effect is not related to nonbonding/ steric interactions, because it is absent in chlorine analogues. The tentative explanation given below follows the principles outlined in ref 12. The cyclopropane ring exhibits surface delocalization of σ-electrons inside the ring, which is the consequence of geminal delocalizations.12 Fluorine atom is an σ-attractor, and when both fluorine atoms are on the same carbon (e.g., C1 carbon) this leads to a very significant increase in the electronegativity of the C1 atom. Subsequently, the p-character of C1 hybrid orbitals that contribute to the ring bonds increases, geminal delocalizations become more bonding, and the ring gains stability.12 The same effect is much less pronounced in other isomers as the example of difluorocyclopropanes below shows. Natural population analysis gives the ring atom charges in difluoro isomers as follows: 0.767, and -0.525 (1,1-difluoro); 0.163, and -0.543 (1,2-difluoro). It is clear from the calculated charges that the electronegativity of substituted ring carbons is much smaller in 1,2 vs 1,1 isomers, hence the greater stabilization of the latter isomer. It had been suggested previously2,3 that fluorination destabilizes the cyclopropane ring, but no accurate analysis comprising all fluoro derivatives had been performed. We report the
TABLE 2. Isodesmic Reaction Enthalpies ∆Hr0(298 K)/kJ mol-1 for Reactions Predicting Effects of Halogenation on Cyclopropane C3H6 substitution
isodesmic reaction
X)F
X ) Cl
11,1(Z)-1,2(E)-1,21,1,2(E)-1,2,3(Z)-1,2,31,1,2,2(E)-1,1,2,3(Z)-1,1,2,31,1,2,2,31,1,2,2,3,3-
CH3CHXCH3 + C3H6 ) C3H5X + C3H8 CH3CX2CH3 + C3H6 ) C3H4X2 + C3H8 CXH2CXHCH3 + C3H6 ) C3H4X2 + C3H8 CXH2CXHCH3 + C3H6 ) C3H4X2 + C3H8 CXH2CX2CH3 + C3H6 ) C3H3X3 + C3H8 CXH2CHXCXH2 + C3H6 ) C3H3X3 + C3H8 CXH2CHXCXH2 + C3H6 ) C3H3X3 + C3H8 CH3CX2CHX2 + C3H6 ) C3H2X4 + C3H8 CXH2CX2CXH2 + C3H6 ) C3H2X4 + C3H8 CXH2CX2CXH2 + C3H6 ) C3H2X4 + C3H8 X2CHCX2CXH2 + C3H6 ) C3HX5 + C3H8 X2CHCX2CHX2 + C3H6 ) C3X6 + C3H8
24.5 55.6 35.0 23.5 61.1 41.2 49.6 82.6 73.2 71.1 93.4 113.3
16.5 27.7 20.1 17.8 30.1 24.2 11.8 32.4 34.6 19.6 22.5 8.5
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results of such an analysis that was based on the appropriate isodesmic reaction schemes (Table 2). The data clearly show that both fluoro and chloro substitution lead to destabilization of cyclopropane. However, in fluorocyclopropanes the destabilization is large and cumulative; i.e., it increases with the number of fluorines present. Chloro substitution, on the other hand, leads to smaller destabilization, without discernible pattern. This lack of a trend suggests that several factors of comparable importance may contribute to the final value of enthalpy. This comment can be related to the results in Figures 1 and 2, which show that (12) Cremer, D.; Kraka, E.; Szabo, K. J. In The chemistry of the cyclopropyl group; Rappoport, Z., Ed.; Wiley: Chichester, 1995; Vol. 2, p 43.
the chloro substitution is less effective than fluoro substitution in producing enthalpy changes. Standard enthalpies for only two halocyclopropanes are available from the NIST source:1 1,1dichlorocyclopropane (6.7 kJ mol-1 listed as uncertain value) and 1,1,2,2-tetrafluorocyclopropane (-155.4 ( 7.0 kJ mol-1). On the basis of our results (Table 1), we suggest that the former value is plausible (i.e., within the accuracy limits of the theoretical method) while the latter is not and should be remeasured.
Supporting Information Available: Geometries and computed total energies. This material is available free of charge via the Internet at http://pubs.acs.org. JO0201528
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