|
1. |
Ab initioGaussian calculations on the CH3and CH2F cations |
|
Journal of the Chemical Society, Perkin Transactions 2,
Volume 1,
Issue 11,
1976,
Page 1193-1195
James Burdon,
Preview
|
PDF (333KB)
|
|
摘要:
1976 P 1193 Ab lnitio Gaussian Calculations on the CH, and CH,F Cations By James Burdon,' D. W. Davies, and Guillermo del Conde, Department of Chemistry, University of Birming-ham, P.O. Box 363,Birmingham B15 2TT Ab initio Hartree-Fock calculations with minimal basis sets of uncontracted Gaussian functions have been made for the planar and pyramidal conformations of CH3+ and CH2F+. Both cations are more stable in the planar form and the difference in energy between the equilibrium planar form and any pyramidal form was found to be greater for CH,F+ than for CH3+ : single calculations with extended basis sets lead to the same result, which is in agreement with simple qualitative arguments and is consistent with experimental data. The C-H bond lengths in the two cations are about the same and the value for CH,+ is in agreement with literature calculations in which extended basis sets were used.The optimum HCH angle in CH2F+ is larger than 120". Ab initio calculations have shown that CH3+is planar,l compared with planar CH,+. There is experimental and a potential energy surface has been obtained.24 evidence,' on systems for which CH3+ and CH2F+can It is interesting to consider the effect of replacing one be regarded as models, to support the contention that of the hydrogen atoms of CH3+by a fluorine atom to give a-fluorine substituents do stabilize carbenium ions in CH,F+. Like most cations of this type, CHP+ should molecules where the C+ centre is likely to be planar. be planar and the fluorine atom should stabilize the In @ramidaZ CH2F+,the lone pair stabilizing effect conformation by interaction between the lone pair on would be much less, and the electron-withdrawing TABLE1 Basis set A exponents 1s 2s 2P < 7C 0.302 1.53 7.77 39.6 2003 0.178 0.923 4.72 F 0.735 3.73 19.0 96.6 488.0 0.457 2.36 12.2 H 0.158 0.584 3.207 In the extended basis set B calculations, the following additional exponents were included: C, d, 0.6; F, d, 0.6; H, p, 0.75. Basis set C exponents and coefficients a - C F H Type S Exp.9.409 00 3.500 02 C0efi.l 0.426 95 0.357 90 l, Exp. 23.370 5 8.623 72 7Coeff. 0.421 81 0.375 64 Type --h------tExp. 0.123 317 0.453 757 Coeff. 0.508 07 0.474 49 2.013 3 0.134 24 9 470.52 0.000 45 23 342.2 0.000 41 13.361 5 0.019 06 1397.56 0.003 58 3 431.25 0.003 27 S 307.539 0.019 34 757.667 0.017 54 S 0.079 83 1.o 84.541 9 26.911 7 0.007 36 0.226 79 209.192 66.7261 0.070 80 0.213 00 P 0.75 1.0 S 1.068 03 0.400 16 0.085 02 0.606 89 2.691 63 1.008 75 0.097 10 0.607 56 S 0.135 124 1.o 0.331 15 1.o 0.657 707 0.358 71 1.731 93 0.361 14 P 1.787 29 5.776 36 0.182 63 0.054 79 4.788 19 15.218 7 0.192 16 0.057 99 23.365 5 0.008 75 65.659 3 0.008 80 P 0.091 063 8 0.248 046 0.432 16 0.203 47 0.620 64 0.206 99 0.422 68 0.224 02 d 0.6 1.0 0.6 1.o Contracted as indicated by brace.the fluorine atom and the vacant 2p orbital on the carbon. inductive effect of the fluorine would be destabilizing; Ab ilzitio calculations have been carried out by Baird in the planar form this inductive effect is offset by the and Datta and by Kollmann and his co-workers on lone pair effect.596 It is (possible to interpret the de- plavtar CH2F+and these did indeed show that fluorine celeration of some SN1reactions by a-fluorine substituents substitution stabilizes the ion, probably by a x-effect, N.C. Baird and R. K. Datta, Canad. J. Chem., 1971, 49, L. Radom and J. A. Pople in ' MTP International Review of 3708. Science, Vol. 1 (Theoretical Chemistry),' ed. W. Byers Brown, 6 P. A. Kollmann, W. F. Trager, S. Rothenberg, and J. E. Butterworths, London, 1972. Williams, J. Amer. Chem. SOC.,1973, 95, 458. R. E. Kari and I. G. Csizmadia, J. Chem. Phys., 1969, 50, R. H.Martin and R. W. Taft, J. Amer. Chey. SOC.,1966, 1443. $8, 1353; R. 0. C. Norman and R., Taylor, ElectrophilicF. Driessler, R. Ahlrichs, U. Staemmler, and W. Kutzelnigg, Substitution in Benzenoid Compounds, Elsevier, Amsterdam, Theor. Chim. Acta, 1973, 30, 315. 1965. R. E. Kari and I. G. Csizmadia, J. Chem. Phys., 1967, 46, 8 G. Kohnstam, D. Routledge, and D. L. H. Williams, 1517. Chem. Comm., 1966, 113. 1194 J.C.S. Perkin I1 as being due to a destabilizing effect of fluorine on calculations were then carried out on the Atlas Labora- pyramidal carbenium ions. With CH3+ none of these tory IBM 370/195 computer, using extended basis sets stabilizing or destabilizing effects can occur. It follows (set B, Table 1, footnote, and set C, Table 1) and the that the difference between planar and pyrimidal forms ATMOL suite of programs; these were all carried out on should be greater for CH,F+ than for CH,+, and it is the geometries optimised with the minimum basis set.Set main purpose of this paper to show that ah initio Hartree-B is merely the minimum set A extended by the addition Fock calculations support this point. of polarisation functions for C, H, and F; set C is TABLE2 Total energies and orbital energies for CH,+ Y?" 0.0 0.0 b 0.0 3.6 5.7 HCH (O) 120.0 120.0 120.0 119.6 119.0 Energy (a.u.) d -38.986 46 -39.028 01 -39.221 74 -38.984 61 -38.981 94 YX) 8.2 16.8 19.5 19.5 19.5 HCH (") 118.0 112.0 109.5 109.5 109.5 Energy (a.u.) -38.977 45 -38.948 69 -38.935 56 -38.977 19 -39.171 66 Orbital symmetry Orbital energies (eV) d le -25.9 -26.0 -25.1 -25.9 -25.9 -25.9 -25.6 -25.5 -25.4 -25.5 2% -34.7 -34.8 -34.9 -34.8 -34.8 -34.9 -35.0 -35.1 -34.9 -35.0 1% -317.1 -316.9 -318.0 -317.3 -317.2 -317.4 -3117.4 -317.4 -317.1 -318.1 For Y = 0 and 19.5'. CH = 1.08 A; otherwise CH = 1.09 (1 a.u.= 0.529 167 A); basis set A unless otherwise stated. Basis s& B. Basis set C. d 1a.u. = 27.211 65 eV = 627.52 kcal mol-I = 2 625.5 kJ mol-1. The columns (left to right) of orbital energies correspond to the increasing values of y listed above. Non-planar form. TABLE3 Total energies and orbital energies for CH,F+ Y (") a 0.0 0.0 ll 0.0 1.5 3.1 H~H(0) 126.6 126.6 126.6 127.1 127.3 Energy (a.u.) d -137.233 58 -137.339 37 -139.090 91 -137.232 83 -137.231 78 5.9 16.3 19.5 19.5 19.5 YQ"HCH (") 128.0 118.0 113.0 113.0 113.0 Energy (a.u.) d -137.227 92 -137.193 73 -137.177 01 -137.278 59 -138.031 30 Orbital symmetry Orbital energies (eV) f 2a" -25.2 -24.8 -25.1 -25.2 -25.2 -25.2 -24.8 -24.6 -23.8 -24.0 6a' -28.1 -27.9 -28.1 -28.0 -28.1 -28.0 -27.7 -27.7 -27.6 -27.8 5a' -29.4 -29.5 -29.5 -29.3 -29.4 -29.3 -29.3 -29.0 -29.2 -29.2 1a" -30.7 -30.3 -30.4 -30.6 -30.7 -30.7 -30.4 -30.4 -29.7 -29.8 4a' -35.2 -35.0 -35.2 -35.2 -35.2 -35.2 -35.2 -35.3 -34.7 -34.9 3a' -55.3 -54.6 -54.9 -55.2 -55.3 -55.3 -55.0 -55.4 -54.4 -54.6 2a' -320.1 -319.3 -320.5 -320.1 -320.2 -320.2 -320.2 -320.0 -319.5 -320.6 la' -724.3 -724.2 -725.6 -724.3 -724.4 -724.3 -724.1 -724.1 -724.2 -725.5 a For y = 0", CH = 1.08, CF = 1.278 A; for y # Oo, CH = 1.09, CF = 1.292 A; basis set A unless otherwise stated.Basis A set B. 6 Basis set C. "Minimum energy (interpolated) is -137.177 21 a.u.for HCH = 115.1". Non-planar form. f The columns (left to right) of orbital energies correspond to the increasing values of y listed above. The calculations have been mainly carried out with completely different and consists of minimum sets taken minimal basis sets (set A, Table 1) of uncontracted Gaus- from Huzinaga l2as contracted by Clementi and Davis l3 sian orbitals, for both CH3+ and CH2F+, on the Univer- sity of Birmingham KDF9 and 1906A computers using POLYATOM.g The optimised exponents for the carbon and fluorine atoms are those given by Csizmadia et aZ.,1° and for the hydrogen atoms by Hehre et aZ.ll These basis sets were used to minimise energies with respect to bond lengths and angles (see below).Eight further Quantum Chemistry Program Exchange, Indiana Univer- sity, QCPE 47.1. lo I. G. Csizmadia, M. C. Harrison, J. W. Moskowitz, and B. T. Sutcliffe, Theor. Chim. Acta, 1966, 6, 191. l1 W. J. Hehre, R. F. Stewart, and J. A. Pople, Faraday SOC. Symposium No. 2, 1968, p. 15. l2 S. Huzinaga, J. Chem. Phys.. 1965, 43, 1293. I l3 E. Clementi and D. R. Davis, J. Chem. Phys., 1966, 45, H 2593. Geometrical parameters €or CH,F+ 1976 (C,H) [or by following their scheme (F)]again extended by the addition of polarisation functions. In the Figure, the geometrical parameters varied in the minimum set calculations on CH2F+ are shown.The A CF and CH bond lengths, the HCH angle, 2p, and the pyramidal angle y define a given conformation. For the planar conformation y = 0", and for the tetrahedral, A y = 19.5'. Let a be the HCF angle, then cosa = sinzy -cosy (cos2y -sin2(3)+. RESULTS AND DISCUSSION The total energies and orbital energies for CH3+ are shown in Table 2. They confirm that CH,+ is planar. The energy difference between the planar and the tetra- hedral conformations is 31.9 kcal mol-l. The optimised l4 CH bond lengths of 1.08 A for y = 0" and 1.09 A for y = 19.5' may be compared with the values 1.078 A for y = 0"and 1.09 A for y = 19.5"obtained by Kari and Csizmadia,2 and the 1.082 A for y = 0" obtained by Driessler et aL3 The other CH bond lengths shown in Table 3 were not optimised.The total energies and orbital energies for CH2F+ are shown in Table 3, and the former confirm that the ion is planar with an optimised l4geometry of CH = 1.08 and ACF = 1.278 A, and HCH = 126.6" (Baird and Datta5 found CF = 1.26 A, but they did not carry out a com- plete optimisation). The bond lengths obtained for y = 19.5" were used in the other calculations for y > 0" A to optimise the HCH angle. The presence of the fluorine atom does not have any appreciable effect on the CH bond length, but the HCH 14 G. del Conde, Ph.D. Thesis, University of Birmingham, 1974. 15 J. K. Tyler and J. Sheridan, Trans. Faraday SOC.,1963, 59, 2661. 1195 angle is larger in CH2F+ than in CH3+, which suggests that H.H repulsion may be more important than H . F repulsion. As Tyler and Sheridan l5have pointed out, CF bond lengths show considerable variation in different mole- cules, from 1.38in methyl fluoride to 1.34 in vinyl fluoride and 1.28 A in fluoroacetylene. The value of 1.278 A found here, and the 1.26 A by Baird and Datta,5 fit well into this picture. For CH,F+, the energy difference between the pyra- midal (y = 19.5') and the planar conformations is 35.5 kcal mol-l, which is 3.6 kcal mol-l larger than for CH3+. This is consistent with the qualitative arguments and experimental evidence mentioned earlier. Tables 2 and 3 also show that this energy difference is greater in CH2F+ than in CH3+ for all pyramidal conformations.The difference, 3.6 kcal mol-l, is rather small, however, and it was to lend further credence to its reality that we carried out the extended basis set (B and C) calculations. These were performed for y = 0 and 19.5" for both CH3+ and CH2F+, using in every case the optimised geometries found by the minimum set calculations. The 3.6 kcal mol-l difference became 6.3 with set B and 6.0 for the very different set C. Our conclusions, then, concerning the effect of fluorine on the stabilities of planar and pyramidal carbenium ions, are therefore very probably correct. We have carried out Mulliken population analyses on planar and pyramidal CH3+ and CH2F+ for all three basis sets used. No clear trends emerged on passing from a planar to a pyramidal form, changes in both gross and overlap populations being generally very small. One of us (G. del C) thanks the British Council for a scholarship and the U.N.A.M. for a loan. [5/009 Received, 3rd January, 19751
ISSN:1472-779X
DOI:10.1039/P29760001193
出版商:RSC
年代:1976
数据来源: RSC
|
|