J. Phys. Chem. 1992, 96, 164-171
164
Boyd, Bratsch, and BPIxc) and the three most relevant experimental measurements, EB,F, and uF,show that BPI,c is the m a t quantitatively useful. Acknowledgment. We thank Eugene T. Knight for writing the
subroutine used to carry out the bond polarity index calculations and for many helpful discussions. We also thank Janan A. Wehbeh for careful reading of this manuscript and numerous useful suggestions. We also acknowledge financial support from the Geo-Centers and the U S . Army, ARDEC, Dover, NJ.
Structure and Formation Mechanisms of Methyl- and Dimethylacetylene Dimer Cations: ESR and ab Initio MO Studies Hiroto Tachikawa,+Masaru Shiotani,*,i and Katutoshi Ohtag Faculty of Engineering, Hokkaido University, Sapporo 060, Japan, Faculty of Engineering, Hiroshima University, Higashi- Hiroshima 724, Japan, and Polymer Research Laboratory, Idemitsu Petrochemical Co. Ltd., Ichihara 299-01, Japan (Received: June 25, 1991)
Structure and formation mechanisms of dimer radical cations of methyl- and dimethylacetylene (MA and DMA) have been studied by means of ESR spectroscopy and ab initio MO calculations. Monomer radical cations were generated in halocarbon matrices by ionization radiation at low temperature. It was found that the methylacetylene radical cation, MA+, is stabilized as a complex cation with a chlorine atom of a matrix molecule immediately after irradiation at 77 or 4 K. When the sample was warmed at 105 K in CF2CICFC12,the matrix complex cation was converted into a dimer radical cation of methylacetylene, MA2+,characterized by a 9.0-G hyperfine (hf) splitting due to eight hydrogen atoms. Upon further annealing, the dimer cation apparently dissociated, since the propagyl radical, CH2C-CH, could be detected. The dimethylacetylene radical cation was stabilized in the matrix as an isolated monomer cation, DMA', after irradiation at low temperature. Similar to the MA system, subsequent annealing of DMA' gave rise to a dimer cation of dimethylacetylene, DMA2+(9.0 G, 12 H). Comparison of the experimental hf splittings with the results of ab initio MO calculations leads us to conclude that the structures of the both dimer cations, MA2+ and DMA2+, are cyclic, Le., in the forms of the dimethyl- and tetramethylcyclobutadiene radical cations, respectively. It is proposed that the formation occurs through a [3+4] cycloaddition reaction of the monomer cation with the parent molecule. The proposed electronic ground states of MA2+and DMA2+are 2A2in C, symmetry (two methyl groups in adjacent positions) and *&in D a symmetry, respectively. The formation mechanisms of these two dimer cations are discussed on the basis of ab initio MO calculations.
Introduction It is well-known that cycloaddition of unsaturated organic molecules is difficult, since the concerted suprafacial process (HOMO-LUMO interaction) is symmetry forbidden.' However, it has been demonstrated by several groups that monomer radical cations can react with the parent neutral molecules to form a cyclic dimer cation2 In the previous we reported ESR evidence concerning the cycloaddition of the dimethylacetylene (DMA) cation to a solute molecule. In the present paper the ESR study is expanded to include the methyl- and ethylacetylene systems, and the cycloaddition reactions are studied utilizing ab inito M O calculations. The ESR study was carried out for y-irradiated methylacetylene (MA) in halocarbon matrices a t low temperature. The results show that a complex cation is first formed a t 4 K, consisting of the MA+ monomer cation and one chlorine atom of the matrix molecule; we call it a matrix complex cation. Upon annealing, the matrix complex cation decays with the comcomitant formation of a dimer cation of MA, MA2+. In the DMA system, no matrix complex cation is observed. The dimethylacetylene dimer cation, DMA2+, is detected upon annealing as in the M A case. The structures of the methyl- and dimethylacetylene dimer cations are discussed by comparing the ESR results with those of the ab initio M O calculations, and the [3+4] cycloaddition of methyl-substituted acetylene cation to a solute molecule at low temperature is concluded. Furthermore, the formation mechanisms of the dimer cations are discussed on the basis of theoretical calculations. The emphasis is put on the results of the methylacetylene system. Hokkaido University. *Hiroshima University. ldemitu Petroleum Chemical Inc.
Experimental Section and Method of Calculations Halocarbons (>99%) such as CFCl,, CF2C1CFCl2,and CF2ClCF2Cl were commercially obtained from Tokyo Kasei Co. and used as matrices without further purification. Solutes of protiated methylacetylene (MA-h, >99%) and ethylacetylene (EA-h, >99%) were obtained from Takachiho Kagaku Co. Methylacetylene-d, (CH3C=CD, 98 D atom %; MA-d,) and methylacetylene-d, ( C D 3 m H , 99.3 D atom %; MA-dJ were obtained from Merck Sharp t Dohme Co. Solid solutions containing ca. 1 mol 7%of solute were degassed by several freeze and thaw cycles and then irradiated by y-rays at 4.2 or 77 K. This is a well-established method to generate the solute radical cations and stabilize them in the m a t r i ~ . ~The , ~ ESR spectra were recorded with 4.2 K to (1) (a) Fukui, K. Acc Chem. Res. 1971, 4 , 57. (b) Fleming, I. Froritier Orbital and Organic Chemical Reactions; Wiley: New York, 1976. (2) (a) Bellvill, D. J.; Wirth, D.; Bauld, N. L. J. Am. Chem. SOC.1981, 103,718. (b) Pabon, R. A.; Bellvill, D. J.; Bauld, N. L. J . Am. Chem. SOC. 1983, 105, 5158. (c) Bauld, N. L.; Bellvill, D. J.; Pabon, R. A.; Chelsky, R.; Green, G. J . Am. Chem. Soc. 1983,105,2378. (d) Pabon, R. A.; Bauld, N. L. J . Am. Chem. Soc. 1984,106,1145. (e) Pabon, R. A.; Behill, D. J.; Bauld, N. L. J . Am. Chem. Soc. 1984, 106, 2730. (0 Gassman, P. H.; Singleton, D. A. J. Am. Chem. Soc. 1984,106, 7993. (3) Shiotani, M.; Ohta, K.; Nagata, Y.; Sohma, J. J . Am. Chem. SOC. 1985, 107, 2562. (4) Ohta, K.;Shiotani, M.;Sohma, J.; Hasegawa, A.; Symons, M. C. R. Chem. Phys. Lett. 1987, 136,465. ( 5 ) (a) Shida, T.; Haselbach, E.; Bally, T. Acc. Chem. Res. 1984,17, 180. (b) Symons, M.C. R. Chem. SOC.Reo. 1984, 393. (c) Shiotani, M. Mag. Res. Rev. 1987, 12, 333. (d) Lund, A,; Lindgren, M.; Lunell, S.;Maruani, J. In Mo!ecules in Physics, Chemistry and Biology; Maruani, J., Ed.; Kluwer Academic Press: Dordrecht, Holland, 1989; Vol. 111, 259. (6) Lund, A., Shiotani, M.,Ed. Radical Ionic Systems, Kluwer Academic Press: Dordrecht, Holland, 1991.
0022-3654/92/2096- 164$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992 165
Methyl- and Dimethylacetylene Dimer Cations
TABLE I: Summary of the Experimentaland Calculated ESR Parameters for the Cation Radicals and Neutral Radicals Presented in "his Work radical (matrix)
hyperfine coupling (G)
g
temp
II
(K)
I
It
I
9.8 65.9 54.8 17.9 65.9 54.8 9.8 17.9 65.9 54.8
6.0 ('H) 10.0("CI) 8.3 ( 3 7 ~ 1 ) 16.5 (3H) 10.0 (35~1) 8.3 ( 3 7 ~ 1 ) 6.0 ('H) 16.5 (3 H) 10.0 (35~1)
22.0 22.0
19.7 (6 H) 19.7 (3 H)
Comdex Cation Radical with Matrix CI
[(CD$rCH)..*CI-CC12F]+
(CFCIJ)
4.2
2.0015
2.0065
[(CH3C~CD)...CI-CC12F]'
(CFCI3)
4.2
2.0015
2.0065
[(CHyC=CH)***CI-CC12F]'
(CFCl3)
4.2
2.0015
2.0065
8.3 (37~1)
Monomer Cation Radical
CH,CdCH,' (CF2CICFCI2) C H 3 C 4 C D 3 + (CFZCICFCl2) CH3CH2C=CH+ (CF2CICF2CI)
77 77 IO0
Osb
2.0031 2.0031
2.0071 2.0071
15.9 (3 H) 23.8 (2 H) 9.5 (1 H)
2.0025
Dimer Cation Radical
(CDyCWH)2' (CF2CIC FZCI) (CH$sCD)2' (CF2CICF2CI) (CHIC*H)2+ (CF2CICF2CI. CF2CICFCI2) (CH3C*CH3)2'" (CF2CICFCIz) (CH,C=CCH3)2' (CH2CCIz)
105 I05 105 1
9.0 (2 H) 9.0 (6 H) 9.0 (8 H) 9.0 (12 H) 8.75 (12 H)
2.0030 2.0030 2.0030 2.0026 2.0030
os
193 Neutral Radical
2.0029
77
125 71
2.0029
136 130
2.0029
8.0 23.5
14.8 (1 H) 16.5 (2 H)
12.5 ( 1 H) 17.9 (2 H) 24.0 16.0(2 H) 12.5 (3 H) 18.6 (2 H) 12.5 (3 H) 12.0 (1 H) 18.6 (4 H)
Reference 3. Reference 4. Reference 15a.
A(H+ 4 *A(H)
//
-U-
IC -+I M-A(CI)
LLr
A - h+A(cl)* -U- J-L '
35CI
XI Figure 1. ESR spectra of the complex radical cations of three isotopic methylacetylenes with one chlorine of the matrix CFC13molecule recorded at 4 K (A) C H 3 m H (MA-A), (B) C H 3 m D (MA-dI), and (C) C D 3 m H(MA-I]) systems. The dashed curves are simulated spectra calculated by using the ESR parameters listed in Table I. The stick plotting is given for the M A - I , system to show the resonance positions.
temperatures at which all radicals decayed. Geometry optimization was performed by the energy gradient method' using a standard routine of the GAUSSIAN 82 program.8 (7) (a) Schlegel, H. B. J . Comut. Chem. 1982, 3, 214. (b) Schlegel, H. B. J. Chem. Phys. 1982, 77, 3676. (8) Binkley, J. S.;Whitaide, R.A.; Raghavachari, K.; Seeger, R.;DeFres, D. J.; Schlegel, H. B.: Frisch, M. J.; Pople, J. A.; Kahn, L. R. GAUSSIAN 82 Release A; Carnegie-Mellon University: Pittsburgh, PA, 1982.
The theoretical hyperfine (hf) splitting constants were calculated using INDO MO methodg for the optimized geometry. All calculations were carried out on HITAC 680H at the Institute for Molecular Science Computer Center in Okazaki. (9) Pople, J. A.; Beveridge, D. L. Approximate Molecular Orbital Theory; McGraw-Hill: New York, 1970. Modified by Itoh H. and deposited in the Program Library at Hokkaido University Computing Center.
166 The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992
Results and Discussion 1. ESR ResulQ. A. Methylacetyleae. Matrix Complex Cation. y-Irradiation of methylacetylene in a halocarbon matrix at 4 or 77 K did not yield the monomer radical cation of MA, but a complex cation with a chlorine atom of the matrix. This was confirmed by comparing the ESR spectra of the complex cations of three isotopic methylacetylenes, C H 3 C e H (MA-h), CH3m D (MA-dl), and CD3C=CH (MA-d,). The ESR parameters of each matrix-hlorine complex cation are listed in Table I. Figure 1A shows the spectrum observed at 4 K for an irradiated solid solution of MA-h in a CF3Cl matrix at 77 K. The spectrum is obviously an anisotropic one, and too complicated to be analyzed. Using two selectively deuterium (2D) labeled methylacetylenes, MA-d, and MA-d3, the spectrum was unambiguously assigned. The experimental spectra observed for the irradiated MA-d, and MA-d3 systems are shown in Figure 1B,C. The spectra become simpler in the order A (MA-h), B (MA-dl), and C (MA-d3), since the 2D labeling reduces the corresponding 'H hyperfine splitting by a factor of 6.5, the ratio of the magnetic moments of 'H to 2D. Spectrum B was successfully analyzed in terms of axially symmetric hf splittings due to one chlorine atom of the CF3C1 matrix molecule and three equivalent methyl protons of MA-d, as shown by the simulation spectrum (dotted lines): All = 65.9 G and A , = 10.0 G for 3sCl;All = 17.9 G and A , = 16.5 G for 'H (3 H). Anisotropic g factors were also extracted from the spectrum: gll = 2.0015 and g, = 2.0065. The stick plot shown in the lower part of Figure 1B is given to clarify the analysis. In spectrum C, the quartet due to the methyl protons is replaced by a new doublet due to the W H proton of MA-d3whose hf splitting is slightly anisotropic: A,, = 9.8 G and A , = 6.0 G (1 H). Employing the hf splittings and g factors derived from spectra B and C, spectrum A was well reproduced as shown by the dotted-line spectrum. Thus, spectrum A was concluded to consist of three equivalent methyl protons, one CHE proton, and one chlorine atom of the matrix molecule. This result, together with nature of the matrix used,5 strongly suggests that spectrum A is identified as a complex radical cation formed between the solute cation and the solvent molecules, i.e., [(CH3C=CH)--C1CC12F]+,in which the unpaired electron is delocalized over orbitals of MA and a p orbital of one chlorine atom of CFC13: About one-third of the spin density (more exactly, 0.37) is in the p orbital of the chlorine atom. The matrix complex cation was easily bleached by visible light illumination, however, without yielding any new radical species. The complex cation was also formed in two other halocarbon matrices, CF2ClCFC12and CF2ClCF2C1, but with less resolved hf pattern. Furthermore, samples irradiated at 4 K gave essentially the same ESR spectra as the ones irradiated at 77 K. Thus, we conclude that ionization of M A in the halocarbon matrices results in the matrix complex cation formation, but not as an isolated monomer cation of MA. This probably originates from the small differences in ionization potentials between solute and matrix molecules (Ip,s of CH3C=CH, CH3C=CCH3, CFC13, CF2C1CFCl2,and CF2ClCF2Clbeing 10.4,1° 9.9," 11.8,12 12.0,13 and 12.7 eV,I3 respectively). Similar complex cations including a matrix chlorine atom have been observed for "localized" a-type solute cations whose SOMO (singly occupied molecular orbital) is mainly localized on one atom (oxygen, sulfur, or hal~gen).~'"' The present result demonstrates that such a complex cation can be formed with a solute cation having "delocalized" *-type SOMO as well. Dimer Cation. Upon warming, the matrix complex cation was irreversibly converted into the dimer radical cation of MA. The spectrum shown in Figure 2A was recorded at 105 K for the irradiated MA-h/CF2ClCF2Cl system. The spectrum consists of nine lines with an isotropic hf splitting of 9.0 G. The ESR spectra of the MA-d, and MA-d3 systems under the same con(10) Ensslin, W.;Bock, H.; Becker G . J . Am. Chem. SOC.1984, 96, 2757. (1 1) Coats, F. H.; Anderson, R. C. J . Am. Chem. SOC.1957, 79, 1340. (12) Shida, T.; Nosaka, Y.; Kato, T. J . Phys. Chem. 1978, 82, 695. (13) Katumata, S.; Shiotani, M. Unpublished data.
Tachikawa et al.
A
n
B
C
A
?\
-
*
I
*
I /
206 *
Figure 2. ESR spectra of the dimer radical cations of three isotopic methylacetylenes in the CF2C1CF2C1matrix recorded at 105 K: (A) (CH3C=CH)2+,(B) (CH3C=CD)*+, and (C) (CD3C=CH)2+. The background signals marked with an asterisk in (C) are attributed to an unidentified radical species which are probably formed by the reaction between the matrix molecule and (CH3C=CH)2+ or CH2C=CH.
ditions have seven and three hf lines, respectively, with the same splitting; see Figure 2B,C. Recall that the 2D labeling essentially turns off the corresponding IH hf splitting. The seven and three lines can be attributed to six magnetically equivalent protons of the methyl groups and a pair of protons of the CH= groups, respectively. Thus, the nine hf lines observed for the MA-h system can be attributed to all protons of the dimer cation, where the splittings of CH3 and CHE protons are accidentally coincident in magnitude. The formation of MA2+is summarized as follows: [CH3C=LH-.matrix]+
+ CH3C=CH
-+ AT (IO5 K)
(CH$ECH)~+
matrix (1)
Here, it is assumed that the solute MA molecule and/or the complex cation can diffuse to become in the vicinity of each other in the softened matrix upon warming. There are two possible structures of the dimer cation, either a charge resonance (CR) type or a cycloaddition type to form the dimethylcyclobutadiene cation. The most probable structures will be discussed in a following section, which presents the results of the ab initio M O calculations. JXsmciition of the L)imer Cation. When sampla were annealed further to ca. 115 K using CF2C1CFCl2or CF2C1CF,C1 as matrix, the MA2 dimer radical cation was irreversibly converted to the propargyl radical, CH2C=CH.I4 The ESR spectral line shape
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 167
Methyl- and Dimethylacetylene Dimer Cations of CH2C=CH showed temperature dependence, changing from an anisotropic spectrum observed at 77 K, to become a more isotropic one with increasing temperature. The associated ESR parameters observed are given in Table I. The reaction is summarized as follows:
AT(115 K)
(CH3C~CH)2+
CH2C=CH
+ H+ + CH3C=CH
A 110.4 &=&=@ 1.466 1.188
1.050
MA
1.084
(2)
The formation of the propargyl radical was also o ~ b ~ for r v ~ ~ the dg$ximiLl-MA systems, giving the spectra due to CD2C*H and CH,C=CD radicals from CD3C=CH and CH3C=CD, 89.9 respectively. Furthermore, the propargyl radical was observed in the CF3Cl matrix, too, but with a low relative intensity. B. Ethylacetylene (EA) in CF2ClCF2CI.In order to investigate how the structure and reactions of the EA cation differ from those of the MA cation, EA in a CF2ClCF2C1matrix was irradiated 1.189 1.467 and subjected to ESR study. The spectrum recorded at 77 K shows a broad spectrum with less resolved hf structure. The spectral resolution was much improved upon warming of the 180.0 sample to ca. 100 K, enough to allow an analysis of the spectrum. This spectrum reveals three different isotropic hf splittings, 9.5 G (1 H), 23.8 G (2 H), and 15.9 G (3 H), which are attributed DMA+ to the CH=, CH2, and CH3 hydrogen atoms, respectively. No hf splittings due to chlorine atoms were observed. Thus, we conclude that, in contrast to the MA system, the EA cation is Figure 3. Optimized geometrical structures for neutral methylacetylene stabilized as an isolated one in the CF2C1CF2Clmatrix. (MA) and dimethylacetylene(DMA) and their monomer radical cations When the sample was warmed further to ca,-1_1_5_ K, the EA (MA+ and DMA') calculated at HF/3-21G level: A, MA and MA+; cation was converted into a neutral radical, HC=CCHCH3, B, DMA and DMA'. without formation of a dimer cation. This neutral radical has isotropic hf splittings of alp= 12.0 G (1 H) and u2, = 18.6 G (4 a similar but intramolecular cycloaddition was reported for irH ) at 130 K. In analogy with the spin density distribution of the radiated deca-2,8-diene in CFC1, at 77 K.16 As in the MA system, propagyl and methylpropagyl radicals, the associated hf splittings the c-C4(CH >4 cation decomposed into the methylpropagyl of al, and a,, can be attributed to one proton of the CH= group radical, H 2 r 8 3 c H 3 9 upon further annealing of the sample to and four protons of the freely rotating CH3 group and the C H ca. 125 K in the m a t r i ~ . ~The . ~ reactions in the CF2C1CFCl2 group, respectively, the latter being accidentally of the same matrix can be summarized as follows: magnitude. A cycloaddition reaction of EA+ with EA was not AT(I05K) observed, differing from the behavior of MA+ and DMA+ in the CH-,C=CCH3+ CH3C=CCH3 c-C4(CH3)4+ (3) same matrix. This may imply that the dimer cation of EA is too unstable to be observed. An alternative explanation is that the ethyl group may play an important role in preventing the cyclization; Le., EA may not diffuse enough in the solid matrix to meet each other because of its bulkiness. 2. Ab Initio Calculations. A. Methylacetylene. Structure of C. Dimethylacetylene. The ESR results of the DMA formed Monomer Cation. The optimized geometries of methylacetylene radiolytically in halocarbon matrices were reported by us preand its monomer radical cation are depicted in Figure 3. As a v i o ~ s l y . ~Here, + ~ the experimental results are summarized for the result of one-electron removal from the *-bonding orbital, sake of convenience in discussions of structure and reaction the bond length of MA+ becomes slightly longer than that of MA: mechanisms. 1.237 A (MA+) vs 1.188 A (MA). Both MA' and M A have a An irradiated solid solution of DMA in CF2C1CFCl2gave an linear carbon skeleton. The total energy calculated for MA+ is ESR spectrum consisting of seven IH hf lines with slightly an9.24 eV higher than that of M A at the HF/3-21G 1 e ~ e l . I ~The isotropic splittings of All = 22.0 G and A , = 19.7 G at 77 K. They unpaired electron of MA+ mainly distributes in the 2 p r orbitals could be attributed to the six equivalent methyl protons of DMA+. of the C, and C2 atoms with slightly higher unpaired electron The g values were found to be gll = 2.003 1 and g, = 2.007 1. The density on the latter atom: 0.397 (C,) vs 0.548 (C2). The num4 K spectrum of DMA+ was less resolved with a complicated hf bering is given in the figure. pattern, and a distortion due to the Jahn-Teller effect was sugStructure of the Dimer Cation. There are two possible geogested. The monomer cation was found to be transformed into metrical structures of MA2+, depending on locations of the methyl the dimer radical cation, DMA2+, upon warming of the sample groups, i.e., the two methyl groups being located either in opposite to ca. 105 K in the same matrix. The ESR spectrum of DMA2+ positions (a head to tail form) or in adjacent positions (a head resolved an isotropic hf splitting of 9.0 G due to twelve protons to head form). Here the former is called trans-MA2+ and the of the four methyl groups and could be identified as the tetralatter &-MA2+. The calculations revealed that each geometrical methylcyclobutadiene radical cation, c - C ~ ( C H ~ )formed ~ + , by a structure has two energy minima, reflecting the type of bonding [3+4] cycloaddition reaction of the DMA+ to a solute DMA orbital between two MA units: u type or a type. Thus, four molecule. This cycloaddition reaction was concluded from the structures are taken into account as candidates for the observed magnitude of the 'H splitting (9 G) which compares almost equally MA2 dimer cation. They are summarized in Figure 4: The cis) ~solution, + 8.75 G.15 with the experimental one of c - C ~ ( C H ~ in and trans-MA2+ with the u types correspond to the 2B2(in C2, This is the first report3 to show direct ESR evidence for cyclosymmetry) and 2B, (in C2,J electronic states, respectively, and addition initiated by an alkyne radical cation. After this report, those with the a types to the 2A2 (in C2Jand 2B, (in C2h)states.
+
(14) (a) Fessenden, R. W.; Schuler, R . H. J . Chem. Phys. 1963,39, 2147. (b) Kasai, P. H. J . Am. Chem. Soe. 1972, 94, 5950. ( 1 5) (a) Broxterman, Q.B.; Hogeveen, H.; Kok, D. M. Tetruhedron Lett. 1981, 22, 173. (b) Broxterman. Q.B.; Hogeveen, H. Tetrahedron Lett. 1983,
24, 639.
-
(16) Courtneige, J. L.; Davies, A. G.; Tollerfield, S. M.; Rideout, J.; Symons, M. C. R. J . Chem. Soc., Chem. Commun. 1985, 1092. (17) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; DeFrees, D. J.; Pople, J. A,; Binkley, J. S. J . Am. Chem. Soc. 1982, 104, 5039.
168 The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
A
Tachikawa et al. TABLE II: Total Energies Calculated at tbe HF/IZlG Level molecule (state in symmetry) total energy (au) neutral monomer MA ('Al in,C3,)" -1 15.225 39 DMA ('A,.
in
monomer cation
C2,+)b
-154.053 65
-1 14.88565 -153.738 90
MA' (,A'' in C,) DMA' (2A" in C,)
neutral dimer rrans-MA, ('ABin C2h)
-230.41965
dimer cation trans-MA2+ ('B, in C2,+)' trans-MA,+ (2B, in C,,)" cis-MA,' (,A2 in C,)c cis-MA2' (,B2 in C,)d DMA2+ ('B,, in D2,,)< DMA2+ (2B2uin &h)d
1
-230.224 96 -230.128 83 -230.205 03 -230.145 88 -307.880 81 -307.806 1 1
" MA = methylacetylene. DMA = dimethylacetylene. addition type. dCR (charge-resonance) type.
B
Cyclo-
TABLE IU Theoretical Proton Hyperfine Splittings Compared with the Experimental Ows (Calculations Carried out Using the INDO Method for the HF/121G Optimi~edGeometries)
radicalec state (symmetry) MA' 'A" (C,) DMA' A" (C,) 2B2u
Figure 4. Optimized geometrical structures for the dimer radical cations of methylacetylene (MA,') and dimethylacetylene (DMA,') calculated at HF/3-21G level: A, MA2' with four different structures; B, DMA2' with two different structures. For details see the text.
31.7
-13.3
31.1 {20.5}8*h
EA'
2A" (C,) MA2+ (C2h)d
2BU(c2,+)e
For the trans-MA2+,the intramolecular C-C distances in the 2B, and 2B, states are r(CI-C2)= r(CI-C2,)= 1.445 and 1.214 A, respectively. The former value is close to the intermolecular distances in the same state; r(C1-C2,)= r(C2-C1,)= 1.468 A. This suggests that the 'Bg state can be regarded as the electronic ground state of the dimethylcyclobutadiene radical cation with two methyl groups in the opposite positions (i.e., the trans conformation). On the other hand, the intramolecular distance in the 2B, state, 1.214 A, is close to that of the monomer cation; r(Cl-C2)= 1.237 A. The intermolecular distances of the 2B, state are r(CI-C2.)= r(C2-C1,)= 2.691 A. This value is far beyond the normal C - C singlebond distance, 1.54 A. Therefore, the 2Bustate is considered to be a cation with weak interaction between MA+ and neutral M A (i.e., a charge-resonance type of complex cation). The 2A2structure in the cis-MA2+has the intra- and intermolecular distances of 1.379 and 1.519 A, respectively, which are rather close to each other. Similar to the 2B structure, the 2A2 structure can be regarded as the dimethylcyciobutadiene radical cation with two methyl groups in the adjacent positions (Le., the cis conformation). In the 2B2 state the intramolecular distance is 1.244 A, which is very close to that of the neutral molecule. One of the intermolecular C - C distances, the C2-C2?, is 1.8 16 A, short enough to form a CR-type interaction. The other intermolecular C I - C I tdistance is 3.164 A, too long for such an interaction. Therefore, the ,B2 can be regarded as a C R complex cation with interaction only a t the C2-C2,bond (see Figure 4). The total energies of each of the four structures are summarized in Table 11. The 2B, state is energetically the most favorable one. The 2A2state is 0.54 eV less stable than the 2B,. The sum of energies for the noninteracting monomer cation and neutral molecule, Le., E(MA+) added to E ( M A ) , is -230.1 11 04 au at the HF/3-31G level. Therefore, the 2B, and 2A2structures are 3.10 and 2.56 eV, respectively, more stable than the noninteraction MA+ and M A . Singly Occupied Molecular Orbital. Figure 5 shows the molecular orbital contour maps of the SOMOs for the four structures
theoretical isotropic 'H hf (G) lexptl value) CHI =CH CH,
2A2 (C2,Jd
,B2 (+Cz,.)'
4.9 ll5.9
-12.3 (-)9.5
-2.9 9.9 8.1 (9.0 17.0
-18.5 -6.6 -6.5 (-)9.0Ih -18.4
76.3 23.8}h
DMA2
'B2g (Dzh)d
--
'82" (D2h)'
8.8 19.01 12.1
H C=C-C HC H 3 ,A" (C,) 24.1 -10.0 -17.Y 2A" (C,) (18.6 (-)12.0 (-)18.qh "MA = methylacetylene. bEA = ethylacetylene.
