theoretical investigation of polyfluorinated allyl cations

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  • THEORETICAL INVESTIGATION OF POLYFLUORINATED ALLYL CATIONS

    Yu. A. Borisov, M. V. Galakhov, and V. I. Bakhmutov UDC 541.124:547.413.5- 128.4:546.16

    The problem of the existence of polyfluorinated al!yl cations was solved as a result of their direct observation by 19F and 13C spectroscopy [1-3]. The experimental studies [1-3] made it possible to represent the structures of cations I and II and the distribution of the charge density in them and to obtain

    X CF3 F a 121F F a I21F N~%/ \ . / '%/

    F F F X (I) (II)

    X = H (Ia), F (Ib), C1 (Ic), CFs (Id); X = C1 (IIa), Br (IIb), I (IIc).

    the first data on the effect of substituents on the stabilities of such particles [4]. The scantiness of information on the geometrical structure that can be obtained from the NMR spectra makes it necessary to study fluorine-containing allyl cations by quantum-chemical methods and to compare the theoretical results with the experimental data.

    In the present research, we calculated the geometries of allyl cations la-d, as well as the Z and E isomers of cation lla, by the semiempiricai MO LCA0 SCF method within the MND0 approximation [5].* It should be noted that the geometries of the investigated cations were optimized completely in all cases, except for the lla system, for which the geometry of the CF~ group was fixed in the calculations. For this reason, in the work with complete optimi- zation we calculated the geometry of a homolog of lla, viz., l-chloroperfiuoroallyl cation !II, which has not previously been experimentally studied. The results are presented in Fig. 1 and Tables 1-3.

    Planar structures for la-d and lla were proposed in [3] on the basis of an analysis of the J~3C_19 F spin-spin coupling constants (SSCC) through three bonds. This conclusion is in good agreement with the calculations, which showed that the energy minima of all of the investigated cations are reached only when atoms 1-8 lie in the same plane. Thus, the 5 ~ deviation of the F 4 and F s atoms from the plane for the Ib cation (see Fig. i) upon rotation about the C:---C 2 bond, according to our calculation by the MO LCAO SCF method in the ST0- 3GF basis, increases the energy by 3.1 kcal/mole, which is in agreement with the results of calculation of the perfluoroallyl cation by the INDO method [6]. It is apparent from Fig. 1 that arrangement of atoms 1-8 in the same plane is achieved with substantial deviations of the bond angles at the C I, C =, and C 3 atoms from 120~ ~ is considerably less than 120 ~ . The deformation of these angles makes is possible to retain the favorable (from an energy point of view) planar structure of the cations also in the case of an increase in the effective radii of the substituents in the ! and 2 positions (see Fig. i). Of course, in this case the F6--F 4 internuclear distances in the series of la-d, Z-lla, and Z-Ill cations are shortened. It is interesting to compare the changes in the ~JF6-F~ SSCC and the F6...F ~ internuclear distances with one another as the substltuents in the i and 2 positions are varied. In [i] it was proposed that "pushing apart" of the adjacent F atoms by the sub- stituent in the 2 position gives rise to a decrease in the F6--F 4 internuclear distance and leads to an increase in ~JF6-F4 (spin--spin coupling through "space" [7]). A dependence ob- tained in the present research is presented in Fig. 2, from which it is apparent that the SSCC decreases monotonically with an increase in the internuclear distance taken from our calculations. Thus, good agreement between the experimental aJF~-F 4 values and the theoreti- cal F6--F ~ distances is observed.

    *The calculations were made with an ES-1060 computer.

    A. N. Nesmeyanov Institute of Heteroorganic Compounds, Academy of Sciences of the USSR, Moscow. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1498- 1501, July, 1986. Original article submitted February 19, 1985.

    0568-5230/86/3507-1357512.50 9 1987 Plenum Publishing Corporation 1357

  • }I 8

    le7 ~ ~5

    ~,6/ 2,97" jz~,o ~\F 4

    ];8 /

    C17 12_3,o ~ ~,fi

    Fs/-. , , L~,,~ ~3"~,6 F 4

    F 8

    l~ 7 ~ F 5

    ~6/ 2,91 9 124,1-- \ F 4

    c1 $

    ~. 1 ~'F5 (Ic) ~ " 125,1 ~ '~5!11"3

    .F 6 / 2,85 124,3--'\F4

    ~8

    6 119 3 ~ i24 I .5 s , ' ~ ~-(m)

    (317 F 4 FI! F 10 F 9 \!/

    7 u99 ~ :247-s CI ~ ~ Z- (IIa) F 6/,..~ ,zv,~ 2:64 124,7~'F4

    17 Io

    \ c l /

    F7 ~ - ,F 5

    F 6/ 2,78 tz~,o -- \F.4

    dO r"\!/r 9

    "~6 125,4 ~ . ~ ]24,7 ~5

    /~-(Ila)

    Fig. i. Structures of i- and 2-substituted derivatives of the perfluoroallyl cation calculated by the MNDO method (r, ~; ~, deg).

    ~%t~,Hz o

    170

    ~0 o

    o 1 L-

    Fig. 2. Dependence of the SSCC of spatially drawn-together F 4 and F 6 nuclei on the internuclear distance calculated by the MNDO method.

    It follows from the data in Table 2 that the positive charge of the I cations is distri- buted over the terminal C 1'3 atoms; the charge of the C 2 atom is either close to zero or nega- tive. This distribution corresponds to the observation of C 1'3 resonance at considerably weaker field as compared with C 2 [1-3]. It is apparent from Table 2 that the F atoms attached to the C ~'3 atoms that bear a positive charge are characterized by virtually zero charges. It It is useful to examine the diagonal density elements corresponding to the Pz orbitals (~z) for the Ib, c and E-III cations (the Pz orbitals are perpendicular to the plane in Fig. .

    4,5 and 8 (see Table 3) that the q-electron densities It follows from a comparison of Cpz Cpz This on the F atoms attached to C x'a are decreased with respect to the F atom attached to C 2. result constitutes evidence for feedback of electron density and corresponds to the observa- tion of the signals of ~gF attached to.CZ'~ at weak field [1-3]. It is interesting to note that, although the charges of the F 4'516'7) atoms in cations I are close to zero, they are nevertheless different (see Table 2). This is evidently also one of the reasons for the different ~Jx3c_,gF~,S(6,7) SSCC 1-3 in the NMR spectra.

    It is interesting to compare the charge density distributions in the Ib and III cations, as well as in the Id and IIa cations, i.e., to examine the changes introduced by a substituent

    1358

  • TABLE i. Bond Lengths of Polyfluorinated Allyl Cations Cal- culated by the MNDO Method

    Bond length,

    Cation C~--C 2 C~_C 3 C~--F C~--F C~--CI C~--A ~ *

    (ta) (ib) (ic) (Id) Z-(IIa) E-(Ila) E-(III) Z-(III)

    t,420 t,442 tA30 t,442 1,429 1,429 1,435 t,433

    t,420 t ,442 1,430 1,442 1,447 1,440 t,447 t,450

    t,299 t,295 1,298 t,296 1,298 t,290 1,291 t,294

    t,299 t,295 t,298 1,296 t,296 t,296 t,296 1,295

    1,727 t,736 t,729 1,724

    1,i02 t,32t t,735

    $0

    **

    1,323 t,323

    *The letter "A" pertains to the corresponding atom in Fig. I. fOptimization was not carried out.

    TABLE 2. Charges (q) on the Atoms in Polyfluorinated Allyl Cations

    q, electron-charge units Atom No. (I~ (Ib) (Ic) (~) z-(iI~ E-(II~ Z-(IID E-(HD

    t 2 3 4 5 6 7 8 9

    t0 t1

    _0, 33 10

    ol 93 - 74 -~ 57

    74 ,57 .85

    _o

    _o o

    71 60 7t 61 ~35 ~61 ~35 )9t

    _~ 99 ;8t ;99 )69 )43

    -I )69 )43

    -I 108

    0,733 -0,429

    0,725 -0,062 -0,035 -0,063 -0,035

    0,724 -0,t83 -0,187 -0,t87

    0,514 -0,385

    0,7t6 -0,061 -0,040 -0,062

    0,t76 0,702

    -0,184 -0,t88 -0,188

    0,530 -0.394

    01724 -0.066 -01039 -0.023

    01128 0,697

    -0,t83 -0,t88 -0,185

    55 - )42 i ~66 - )60

    )38 -~ )54

    [72 -( )98

    0,409 -0,047

    0,669 -0,065 -0,037 -0,024

    0A35 -0A00

    TABLE 3. Electron Densities in the Pz Orbitals of the Atoms of Cations Ib, E-III, and Ic

    Cp z

    (Ib) E-{In) (h) tt . tom NO. 84 Cp z

    {Ib) E - ( I I I ) (Ic}

    Atom No.

    0,6676 t,3236 0,6676 1,8526

    0,6742 t,2832 0,6639 t,85i8

    0,6467 5 1,3377 6 0,6467 7 i,8529 8

    1,8414 i.8525 L8414 i,9532

    18410 18438 L8331 1,8529 1.9027 i,8438 1195o0 1,9755

    in the 1 position. It follows from Table 2 that the charge on the C I atom decreases on pas- sing from Ib to III and from Id to IIa; this is accompanied by the development of a positive charge on the C1 atom. A comparison of the Cpz 7 value for E-III and the Cpz 8 value for Ic (Table 3) makes it possible to conclude that the T-electron density of the C! atom changes little when its position is changed. In other words, the decrease in the positive charge on the C I atom is due to C ~ C1 a polarization. It should be noted that the calculated distri- butions of the positive charge on the C ~'a atoms in i-chloro-substituted cation IIa are not in agreement with the ~3C NMR data, for which the signal of the C ~ atom is recorded at sub- stantially weaker field as compared with C 3. This contradiction cannot be explained by a significant diamagnetic contribution to the laC chemical shift (evaluation of the diamagnetic contribution by the method in [8] and correction of the ~3C:'3 chemical shifts by the method in [9]).

    In conclusion, it is interesting to compare the energies of formation of the Z and E isomers of the IIa cation. It follows from the calculations that the AHf values for the Z (--40.30 kcal/mole) and E (--40.61 kcal/mole) isomers are virtually the same. This is also evidently the reason for the absence of a dependence of the ratio of the integral intensities of the 19F signals of the Z and E isomers IIa, b on the temperature over the --30~ to 0~ range.

    1359

  • CONCLUSIONS

    i. The structures of the i- and 2-substituted derivatives of the perfluoroallyl cation were c

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