J. Phys. Chem. 1990, 94, 8081-8090
808 1
An ESR and Theoretical ab Initio Study of the Structure and Dynamics of the Pyrrolidine Radical Cation and the Neutral 1-Pyrrolidinyl Radicalt M. Shiotani, Faculty of Engineering, Hiroshima University, Higashi, Hiroshima 0724, Japan
L. Sjiiqvist,* A. Lund, Chemical Physics Laboratory, Department of Physics, Linkoping University, S-581 83 Linkoping, Sweden
S. Lunell, L. Eriksson, and M.-B. Huangt Department of Quantum Chemistry, Uppsala University, Box 518, S - 75120 Uppsala, Sweden (Received: February 14, 1990)
The electronic structures and molecular dynamics of the pyrrolidine radical cation, C4H8NH+,and the deprotonated neutral 1-pyrrolidinyl radical, C4H8N,have been investigated by ESR spectroscopy and ab initio UHF, MP2, and SDCI calculations. The ground state of C4H8NH+is proposed to have a twisted C2structure with twist angles 23.1 O and 22.0°, on the UHF/3-21G and UHF/6-31G* levels, respectively. Experimental hyperfine coupling constants observed at 4 K were found to be in good agreement with SDCI calculated values for the 2B electronic state, giving further support for the C2structure. A twisted C2 ground-state structure was also obtained for the C4H8Nradical, in this case with twist angles 18.0' and 17.3', on the UHFI3-2IG and UHF/6-3 1G* levels, respectively. Hyperfine coupling constants for the C4H8Nradical, corresponding to the C2structure calculated from the SDCI wave functions, were found to be in good agreement with experimental values. Both C4H8NH+and the C4H8Nradical exhibit temperature-dependent ESR line shapes which were qualitatively analyzed in terms of ring inversion between two equivalent twisted C2 structures. The activation energies related to the motions were evaluated and were found to be very close to each other, e.g., 1.6 kcal/mol for C4H8NH+in CFCI, and 1.9 kcal/mol for the C4H8Nradical in CF2CICFCI2. In addition, the N-methylpyrrolidineradical cation was studied at 4 K by ESR in several matrices for comparison of the electronic and geometric structure with C4H8NHt.
Introduction
The electronic structure and dynamics of saturated organic radical cations have attracted attention in the past decade. It is of fundamental interest to gain insight and obtain a deeper understanding of the behavior of these intermediate reactive species. Linear and cyclic saturated radical cations of hydrocarbons have been comprehensively investigated both by electron spin resonance (ESR) spectroscopy using the matrix isolation technique and by theoretical methods.' Radical cations can easily be produced and stabilized in halocarbon matrices at low temperature during exposure to ionizing irradiation. Comparison between experimental and calculated ESR parameters usually facilitates a definite assignment of the studied species concerning the geometric and electronic structure. Several radical cations of heterocyclic compounds have been investigated in detail by using the ESR technique. The heterocyclic rings in general undergo ionization by removal of one electron from a lone pair, forming a *-type radical cation. The tetrahydrofuran (THF) radical cation and methyl-substituted analogues were investigated by Shida and co-workers.2 THF+ was assigned a twisted ring structure at low temperature while ring puckering could be observed at higher temperature. The oxetane and T H F cations stabilized in a CF2ClCFClzmatrix were studied by Williams et al., and reaction mechanisms for formation of neutral radicals were ~onsidered.~Furthermore, they have discussed the effect of the ring size on hyperconjugation in heterocyclic molecules in terms of the magnitude of the @-proton hyperfine (hf) coupling ~ o n s t a n t . ~Radical cations of saturated heterocyclic rings containing one nitrogen or one sulfur atom have been studied with emphasis on the structures and reactions in halocarbon matrices.3q5 It was concluded that the three-membered ring containing one nitrogen atom, aziridine, has a ring-opened 'A part of this study was presented at the 2 n d Japanese ESR Symposium, Abstract pp 133-135, 1983. IOn leave from Department of Modern Chemistry, University of Science and Technology of China, Hefei, Anhui, China.
0022-3654/90/2094-808 1$02.50/0
structure at 90 K in CFC1, while the four-membered analogue, azetidine, was ring-~losed.~ Several cyclic radical cations and neutral radicals stabilized in halocarbon matrices exhibit internal motion, observable through line-width variations in the ESR spectra.Z@ Detailed information about the molecular motion can be obtained if electronic structure calculations are combined with an analysis of the alternation of the ESR line width as a function of the temperature. Matrix effects can qualitatively be discussed by a comparison of the dynamical behavior of the cation or the neutral radical stabilized in different matrices! Ion-molecule reactions between the cation and a parent neutral molecule have often been observed in the glasslike matrices such as CFzCICFClz and CFzCICFzCl above 100 K. The reactions of several cations of heterocyclic rings have been studied, and formation of neutral carbon-centered radicals was often ~ b s e r v e d . ~ . ~ Neither pyrrolidine (C4H8NH) nor N-methylpyrrolidine (C4H8NMe) radical cations have previously been investigated by ab initio methods. However, the structure and dynamics of neutral C4H8NHand C4H8NMehave been discussed in detail, employing ab initio theory and gas electron diffraction.loJ' One intention (1) See e.g. for reviews: (a) Lund, A.; Lindgren, M.; Lunell, S.;Maruani, J. Hydrocarbon Radical Cations in Condensed Phases. In Molecules in Physics; Chemistry and Biology; Maruani, J., Ed.; Kluwer: Dordrecht, 1989, Vol. 111, pp 259-300. (b) Shiotani, M. Mugn. Reson. Rev. 1987, 12, 333. (c) Symons, M. C. R. Chem. SOC.Rev. 1984, 393. (2) Kubodera, H.; Shida, T.; Shimokoshi, K. J . Phys. Chem. 1981, 85, 2583. (3) Williams, Ff.; Qin, X.-Z. Rudiat. Phys. Chem. 1988, 32, 299. (4) Qin, X.-2.; Williams, Ff. J . Chem. SOC.,Chem. Commun. 1987, 257. (5) Qin, X.-Z.; Williams, Ff. J . Phys. Chem. 1986, 90, 2292. (6) Iwasaki, M.; Toriyama, K.; Nunome, K. Furuday Discuss. Chem. SOC.
1984, 78, 19. (7) SjGqvist, L.; Lund, A.; Maruani, J. Chem. Phys. 1988, 125, 293. (8) SjGqvist, L.; Lindgren, M.; Lund, A. Chem. Phys. Left. 1989, 156, 323. (9) Matsushita, M.; Momose, T.; Kato, T.; Shida, T. Chem. Phys. Lett. 1989, 161, 461. (IO) Pfafferot, G.; Oberhammer, H.; Boggs, J. E.; Caminati, W. J . Am. Chem. SOC.1985, 107, 2305.
0 1990 American Chemical Society
8082
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
Shiotani et al. C4H8NH' / CF2C1CFC12
a)
I
Figure 1. (a) Experimental and simulated ESR spectra of C4H8NH+stabilized in CFCI, at 4, 77, and 100 K. The sample was irradiated at 77 K and subsequently cooled to 4 K. (b) Experimental and simulated ESR spectra of C4H8NH+stabilized in CF2CICFC12at 4 K. The sample was irradiated and measured at 4 K. (c) Experimental and simulated ESR spectra of C4H8NH+in CF,CCI,, observed at 4 K after irradiation at 77 K. The hf parameters used in all simulated spectra are given in Table I .
of the present study is to utilize ab initio unrestricted Hartree-Fock (UHF) and configuration interaction calculations including singleand double-excitation (SDCI) theory in order to determine the ground-state structure of C4H8NH+. Several halocarbon matrices were employed in the present study, and the hf coupling constants assigned to C4H8NH+at 4 K were evaluated and compared with the calculated SDCI isotropic and anisotropic values. The ESR spectra attributed to C4H8NH+showed a line-width alternation when the temperature was changed, due to dynamics of the cation. This dynamics was analyzed in terms of internal molecular motion by using a model based on results obtained from the calculations and simulations of the ESR spectra. In addition to the study of C4H8NH+,we investigated the structure and dynamics of the neutral C4H8Nradical. This radical was observed in CF2CICFCI, and CF2CICF2CIupon warming the sample of C4H8NH+above 100 K. The experimental hf coupling constants corresponding to the rigid structure were compared with SDCI calculated values for different possible ground states of the neutral radical. The ESR spectra attributed to the C4H8Nradical showed a line-width alternation above 100 K, due to internal motion. The calculated structure of C, symmetry for the C4H8N radical was used to interpret this molecular motion. Furthermore, C4H8NMe+was studied at 4 K and elevated temperatures in different matrices, with the purpose of studying the effect of methyl (Me) group substitution on the nitrogen atom with respect to the electronic structure, dynamics, and reactions of the radical cation. Experimental Section
The halocarbon compounds CFCI, (Alfax), CF,CCI, (Fluka), CFzCICFClz(Fluka), and CF,CICF,CI (Alfax) and the solutes ( I 1) Pfafferot, G.; Oberhammer, H.; Boggs, J. 107, 2309.
E.J . Am. Chem. SOC.1985.
C4H8NH (Merck) and N-C4H8NMe (Fluka) were obtained commercially and used without further purification. All halocarbon compounds were degassed on a vacuum line and stored in glass bulbs before usage. Less than 1 .O mol 76 of the solute was mixed with the halocarbon matrix in an ESR Suprasil sample tube connected to a vacuum line employing standard techniques.' The prepared samples were cooled down to 4 or 77 K and thereafter irradiated 3-5 min with an X-ray tube having a W anode operating at 70 kV and 20 mA. All measurements were performed using a Bruker Er 200D ESR spectrometer. An Oxford Instrument E-9 helium flow cryostat was used in the low-temperature measurements below 77 K. Irradiation at 4 K was performed with a double quartz He Dewar in which the outer part was cooled with liquid nitrogen and the inner with liquid helium. Experimental Results A. Pyrrolidine. CFCI,. An ESR spectrum attributed to C4H8NH+ at 4 K in CFCI, is shown in Figure la. The C4H8NH/CFC13mixture was irradiated at 77 K and subsequently cooled to 4 K. A considerable broadening of the spectrum occurred when the temperature was decreased from 77 to 4 K. Increasing the temperature to 77 K caused an appearance of a rather well-resolved spectrum, which did not change significantly upon further warming to 100 K. This spectrum is complicated due to the hf anisotropy of the 14Nnucleus; the I4N(ml=O) components, however, are well-defined. The experimental spectrum observed at 100 K can be simulated with satisfactory agreement, employing the following hf parameters: a(2 H) = 70.5 G, a(2 H) = 34.0 G, a(l H) = 24.5 G, A,(I4N) = 8.0 G, and A1,(l4N)= 44.0 G, using a Gaussian line shape with line width (Iw) = 8.0 G. In Table 11, the hf parameters of C4H8NH+ are compared with those reported for other related N-centered cations with similar structure. A reversible line-width alternation of the ESR spectra
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8083
Structure and Dynamics of C4HsNH+ and C4HsN
Sim.
Exp. 105
K
Figure 2. (a)-(d) Line-width alternation of the ESR spectra for C4H8NH+in CFCI, in the temperature region 105-160 K. (a')-(d') Simulations of the experimental ESR spectra with given rate constants, k , using a dynamical model where interchange between two C2structures takes place; see text for details. Only isotropic hf coupling constants were used in the simulations (Table I). TABLE I: ESR Parameters of C4HsNH+, C4H8N', and C4HsNMe+ Stabilized in Halocarbon Matrices at L o w Temperature radical matrix T, K g hf couplings, G C4H8NH+ CFCI, 100 2.003 a(2 H) = 70.5,a(2 H) = 34.0,a(l H) = 24.5,A,(14N) = 8.0,AlI(l4N) = 44.0 4 CF,CCI, g, = 2.0045 4 2 H) = 69.1,4 2 H) = 34.5,a(l H) = 24.5,A,(14N) = 10.0, Al1(I4N) = 38.0 gll = 2.0060 4 CF2CICFCI2" g, = 2.0035 4 2 H) = 73.5,4 2 H) = 32.3,a(l H) = 25.5,AL(I4N)= 8.0,AlI(l4N) = 42.5 gll = 2.0065 CF2CICFzCI 4 2.004 4 2 H ) = 71.2,4 2 H) = 34.3,a(l H) = 23.5,A1(I4N)= 7.5,AI1(l4N) = 42.5
C4H8NMe+
C FCI, CF2CICFC12' CFJCCI, C F2CIC F2CI
C4H8N'
CFZCICFC12 CF2CICF2C1 bulk
4 77 4 4 4
2.003
100
2.004
77 77
2.004
Me: 4 2 H) = 14.3,a(l H) = 57.0,4 2 H) = 57.8,4 2 H) = 29.0 a(CH3) = 28.5, ~ ( H) 2 = 57.8,4 2 H) = 29.0,A,(I4N) = 10.0, A11(14N)= 42.5 a(CH3) = 29.4,a(2 H) = 57.3,4 2 H) = 28.5,A,(I4N) = 10.0, All(14N) = 43.0 a(CH3)= 29.4,4 2 H) = 57.3,4 2 H) = 28.5,A,(14N) = 10.0, AIl(I4N) = 43.0 n(CH,) = 29.4.a(2 H) = 58.3,a(2 H) = 29.4,AL(14N)= 10.0,AI1(l4N)= 43.0
a(2 H) = 54.2,4 2 H) = 27.1,A,(I4N) = 1.5, 4(I4N)= 41.0 4 2 H) = 54.2,4 2 H) = 27.1,A,(I4N) = 1.5, AlI(l4N) = 41.0 4 2 H) = 55.3, a(2 H) = 27.0,AL(14N)= 2.0,AI1(l4N) = 38.0
Irradiated and measured at 4 K.
was observed for C4HsNH+ in the temperature region 105-160 K, due to an onset of dynamics, as shown in Figure 2a-d. The anisotropic features of the spectra due to the I4N nucleus are also present in the high-temperature region. CF2CICFCI2. The ESR spectrum assigned to C4HENH' stabilized in CF2CICFCI2at 4 K is shown in Figure 1b. The sample was irradiated and measured at 4 K without annealing. The observed spectrum can be simulated with the hf coupling constants presented in Table I. Annealing of the sample to 77 K did not change the spectrum significantly. At approximately 100 K, an irreversible spectral change occurred and an ESR spectrum assigned to the C4HENradical appeared (Figure 3a). This spectrum was simulated with the following hf coupling constants: a ( 2 H) = 54.2 G, a ( 2 H) = 27.1 G, A,(I4N) = 1.5 G, and A,l('4N) = 41.0 G, using a Gaussian line shape with Iw = 10.0 G. Further increase of the temperature results in an alternation of the line width as depicted in Figure 4a-e. The observed spectral changes in the temperature region 100-145 K were reversible, indicating an onset of dynamics.
CF3CCI3. The experimental ESR spectrum assigned to C4HENH+stabilized in CF3CC13at 4 K is shown in Figure IC. The sample was irradiated at 77 K and thereafter cooled to 4 K. The observed spectrum can be simulated as depicted in Figure IC, employing the hf parameters given in Table I. These values are very close to those observed in CFC13. Annealing of this sample to 77 K did not cause any major changes of the spectrum. At 145 K, an irreversible change of the ESR spectrum occurred, indicating the formation of a new species. The spectrum observed above 145 K could be identified as belonging to the C4HsN radical with hf couplings according to Table I. CF2ClCF2Cl. An ESR spectrum attributed to C4HsNH+was observed at 77 K in this matrix with the hf parameters displayed in Table I. The spectral changes observed between 77 and 11 1 K were reversible. Here, an irreversible spectral change occurred when the temperature was increased above 11 1 K. The appearing spectrum was assigned to the CdHEN radical. In Figure 3b is shown a spectrum of the C4HENradical recooled to 77 K after annealing to 120 K. This spectrum is nearly identical with the
8084
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
TABLE 11: ESR radical
Shiotani et ai.
Parameters of Some Radical Cations and Radicals Containing One Heteroatom Related to the Present Study
matrix CFCI, CFCI, CFCI, CFZCICFC12 CFCI, CFC13 acid s o h acid soh CFCI, CFCI, C FZCIC FC12 adamantane acid soh acid s o h
7, K
P
90 I50 120 90 77 77
2.0038 2.0026 2.0038 2.0040
RTb
2.0036 2.0036
RT 77 155 77 81 228 183 I83
2.014" 2.030 2.0046 2.0044 2.0044
hf couplings. G a(4 H) = 54.2, a ( l H) = 22.6, A,,(I4N)= 41.4, A1(I4N) = 8.0 4 4 H) = 16.1, a ( l H) = 4.3, a,,(I4N) = 7.7 a(Me) = 28.0, 4 2 H) = 28.0, a(2 H) = 57.0, A,(I4N) = f 4 . 0 , A1,(l4N)= 48.0 a(9 H) = 28.6 4 9 H) = 28.3, A1(I4N) 5 4, AlI(l4N)= 47 a(9 H) = 28.2, AI,(I4N)= 47, A1(l4N) = 0 f 4 a,,,(14N) = 20.55, a(9 H) = 28.56 CI,,,(~~N) = 19.28, a(l H) = 22.73, a(6 H) = 34.27 4 2 H) = 89, 4 2 H) = 40 4 4 H) = 65 4 2 H) = 40, a(2 H) = 20 a(2 H) = 37.8, 4 2 H) = 18.9 a(4 H) = 39.1, a,,(14N) = 14.4 also(14N)= 14.78, a(Me) = 27.36 a,,,(I4N) = 14.27, 4 4 H) = 36.90
ref 5 5 23 25 33 23 34 35 2 36 3 28 37 37
'Averaged value. bRoom temperature CqH8N
/ CF2CLCFCl2
100 G L
I
/\
/------
1
Figure 3. (a) Experimental and simulated ESR spectra of the C4H8N radical in CF2CICFCI2a t 100 K (rigid conformation). The ESR parameters used in the simulation are given in Table 1. (b) ESR spectrum corresponding to the C4HENradical in CF2CICF2CI,observed at 77 K after annealing to 120 K.
one observed in CF2ClCFC12at 100 K, but with slightly narrower line width in the high field region. An alternating line-width effect due to internal motion of the C4HENradical is also present in this matrix in the temperature region 11 1-130 K. B. N-Methylpyrrolidine. CFCI,. The ESR spectrum assigned to C4HENMe+exhibits a reversible spectral change in the temperature region 4-77 K. On decreasing the temperature from 77 to 4 K, the number of lines increases in the ESR spectra, as shown in Figure 5a. The spectrum observed at 19 K can be satisfactorily simulated, employing the following hf parameters: for Me, 4 2 H) = 14.3 G and a(l H) = 57.0 G; 4 2 H ) = 57.8 G and 4 2 H) = 29.0 G, neglecting the hf coupling from the I4N nucleus. We used a "three-site" jump model for the restricted rotation of the Me group with the rate constant k = 8.0 X IO6 s-I (see Discussion). The ESR spectrum observed at 77 K (Figure 5a, lower) was simulated with the following hf coupling constants: a(CH3) = 28.5 G, a(2 H) = 57.8 G, 4 2 H) = 29.0 G, A,(14N) = 10.0 G, and A1,(14N)= 42.5 G, using a Gaussian line shape with Iw = 6.0 G. Note that the Me group is freely rotating at this temperature. The signal did not change significantly upon
increasing the temperature to 165 K where the signal decayed. CF2CICFCI2.An ESR spectrum of a C4HENMe/CF2CICFCI2 mixture, irradiated and measured at 4 K, is shown in Figure 5b. This spectrum was attributed to C4HENMe+and could be simulated with the hf couplings given in Table I. The spectrum changed irreversibly at approximately 115 K into a broad doublet with ca. 60 G splitting, probably due to a 19Fnucleus in the matrix. Further increase of the temperature to 130 K caused.the appearance of a new spectrum which was attributed to the CCI2CF3 radical. CF,CC13. The ESR spectrum of C4HENMe+in CF,CCI, a t 4 K is shown in Figure 5c. This spectrum is nearly identical with the one observed in CF2ClCFCll at 4 K and can hence be analyzed with the same hf parameters. The spectrum was not noticeably altered when the temperature was increased. CF2ClCF2Cl.An ESR spectrum attributed to C4HENMe+was observed at 77 K and can be simulated by the hf parameters given in Table 1. The spectrum changed irreversibly into a broad triplet with ca. 49 G splitting, at approximately 100 K. The signal decayed when the temperature was further increased. Theoretical Methods and Results The pyrrolidine radical cation, C4HENH+,and the 1-pyrrolidinyl radical, C4H8N(the deprotonated form of C4HENH),were studied by using ab initio UHF, second-order Mraller-Plesset (MP2) perturbation theory and SDCI methods. For the geometry optimizations (UHF) the program package GAUSSIAN8612was used, and these were performed with the 3-21GI3 and 6-31G*I4 basis sets. The spin properties of CdHENH' were calculated on the optimized geometries (3-21G and 6-31G*), using SDCI calculations, with van Duijneveldt's (1 3s,8p)/[5s,3p] basis set for carbon and nitrogen and his (lOs)/[4s] for hydrogen.I5 The program MELD was used in the SDCI calculation^.^^ On the optimized geometries of the C4H8Nradical using the 6-31G* basis set, we performed single-point MP2/6-3 1 G* calculations using CADPAC." Thereafter, subsequent SDCI calculations were carried out for more accurate spin properties, employing the MELD program with the same basis sets as mentioned above.I5 The results and conclusions based on the calculations are presented as follows. (12) Frisch, M. J.; Binkley, J. S.; Schlegel, H. B.; Ragavachari, K.;Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; DeFrees, D. J.; Seeger, R.; Whiteside, A.; Fox, D. J.; Fluder, E. M.; Pople, J. A. GAUSSIAN 86; Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984. (13) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J . Am. Chem. SOC.1980, 102, 939. (14) Hariharan, P. C.; Pople, J. A. Theor. Chim.Acra 1973, 38, 213. (15) (a) van Duijneveldt, F. IBM Tech. Rep. 1971, No. RJ945. (b) Gianoli, L.; Pavani, R.; Clementi, E. Gazz. Chim.Iral. 1978, 108, 181. (16) MELD program: Davidson, E. R.; et al. Department of Chemistry, Indiana University, Bloomington, IN. (17) CADPAC: Amos, R. D.; Handy, N. C.; Rice, J. E.; et al. Cambridge
University, Cambridge, MA.
Structure and Dynamics of C4H8NH+and C4H8N
The Journul of Physical Chemistry, Vol. 94, No. 21 1990 8085 I
Exp. h,
Figure 4. (a)-(e) Alternation of the ESR line width for the C4H8N radical in CF2ClCFCll in the temperature region 110-145 K. (a')-(e') Simulations of the experimental spectra with given rate constants, k , employing a dynamical model where interchange between two C, structures occurs (see text). The hf coupling constant from the I4N nucleus was neglected in (a') and (b'), and only the isotropic part was included in (c')-(e') (Table I). TABLE 111: Optimized Geometries and Energies for the C2and Cki Structures of C4H8NH+Using (UHF/3-21G) and UHF/6-31G* Basis Sets"
CZ
c2u
1.5370 (1.5490) 1.5327 (1.5432) 1.4661 (1.5019) 2.4587 (2.5014) 1.0834 (1.0815) 1.0804 (1.0792) 1.0817 (1.0796) I .0865 (1.0841) 1.0087 (1.0141) 22.0 (23.1) 109.0 (110.1) 110.8 (110.5) 111.3 (111.0) 112.7 (112.1) 109.2 (109.0) 116.1 (115.6) 106.5 (106.3) 112.5 (113.2)
1.5524 (1.5663) 1.5384 (1.5489) 1.4591 (1.4913) 2.4722 (2.5168) 1.0803 (1.0789) 1.0844 (1.0824) 1.0088 (1.0145)
EUHF
-210.904926 (-209.743 921)
-210.900947 -209.739 496)
(S2)
0.765 (0.765)
0.765 (0.765)
RI R2 R3 R4 R5 R6 R7 R8 R9 a a1
a2 a3 a4 a5 a6 a7 a8
TABLE IV: Calculated and Experimental Isotropic and Anisotropic Hyperfine Couplings (in G) for the C2Structure of C4H8NH+and the C2and C, Structures of the C4H8N Radical"
ais0
H6
H9 N
B(NY 111.0 (110.4) 110.0 (109.9) 114.5 (113.8) 107.4 (107.3)
HI HZ H5
32.3 73.5 25.5d 19.5 23.0
c,
c2 ais0
HI H6
N
Pyrrolidine Radical Cation. Two possible structures of C4H8NH+,with C2and C, symmetry, were investigated. The C, structure was obtained in the UHF/3-21G and UHF/6-31G* optimization calculations. The optimized geometrical parameters are summarized in Table 111. The definitions of the parameters are depicted in Figure 6. The twist angle a (Figure 6) of the molecular framework was predicted to be 23.1O and 22.0° respectively on the 3-21G and 6-31G* levels. The supposed C, structure does not seem to exist. Optimizations with an initial geometry of C, symmetry starting from a bending angle, b, of 130° (Figure 7 ) converged to planar C,, geometries on both the UHF/3-21G and UHF/6-3 I G* levels. The optimized geometrical parameters for the C,, symmetry are given in Table 111. These parameters are defined i n Figure 7 . The electronic states are
expb
C,HnN
H5
" Distances in A, angles in deg, and energies in au; see Figures 6 and
CI -0.09 -0.54 22.0 41.1 -24.9 10.3 31.6
B,I
HZ
7 for notation, 1 au = 27.21 eV.
CdHnNH' UHF/3-21G UHF/6-31G* -0.9 -1.1 -1.8 -1.7 21.6 27.5 39.4 49.9 -43.8 -46.7 40.1 37.0
B(NIC
Bl,
UHF/6-31G*
CI
-1.1 -1.3 24.6 46.4 29.1
-0.2 -0.4 18.9 36.0 3.0 30.8
UHF/6-31G* -1.4 -1.4 14.8 47.7 28.5
CI -0.3 -0.4 10.4 36.9 2.6 30.6
expb
27.1 54.2 14.7 26.3
"For notations, see Figures 6, 8, and 10. *Experimental data obtained from measurements in CF2CICFCI2 (Table 1). e B denotes the dipolar coupling. dThe sign of the hf coupling has not been determined, however, considering that the main part of the spin density is confined to the 2p, orbital on the nitrogen atom a negative value is expected.
,B and 2B, for the C2 and C2, structures, respectively. In both cases the lone pair on the N atom gives a dominating contribution to the singly occupied molecular orbital (SOMO). The SDCI isotropic hf coupling constants aimon the H and N atoms for the C2structure and the anisotropic hf coupling constant B(N) on the N atom were calculated on the UHF/6-31G* optimized geometry. The results are presented in Table IV together with the experimental values. The calculated hf coupling constants (isotropic) from the UHF/3-21G and UHF/6-31G* wave functions are also displayed in Table IV. The calculated values are in qualitative agreement with the experimental ones. For the isotropic hf coupling constants with positive sign, the SDCI values are smaller than the corresponding experimental values. This
Shiotani et al.
8086 The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 C,,II~NM~'
I/ C F C l 3
--
100 C
I
C4H8NMe
/ CF2C1CFClZ 100
c
A
Sim
A
Ii Ilil 'Ill I '
Figure 5. (a) Experimental and simulated ESR spectra of C4H8NMe stabilized in CFCI, at 4, 19, and 77 K. The dynamical model and the h. xuplings used in the simulations are described in the text. The given rate constant, k = 8.0 X IO6 s-', is related to a restricted rotation of the Me group. (b) Experimental and simulated ESR spectra of C4H8NMe+in CF2CICFCl2at 4 K. The sample was X-irradiated and measured at 4 K. (c) Experimental ESR spectrum attributed to C4H8NMe* in CF,CCI, observed at 4 K. The hf parameters used in all the simulations are given in Table I.
Li
r
i
H
Figure 7. Geometry and labeling used for the C , structure of C4H8NH+.
problem has been discussed by Davidson and Feller in earlier studies of the hyperfine structure for smaller radical^.'^*'^ On the other hand, UHF calculations mostly predict too large values for isotrqpic hf coupling constants with negative sign, due to spin (18) Feller, D.: Davidson, E. (19) Feller, D.; Davidson, E.
R. J . Cfiem.Phys. 1984, 80, 1006. R. Tfieor. Chim. Acta 1985, 68,57
L-i
Figure 9. Geometry and labeling used for the C, structure of the C4H8N radical.
contamination in the UHF wave functions. This seems to be true also in this case; see, for example, the aiwvalue on H9. We propose that C4H8NH+observed in the present ESR experiments does not have the C , (the supposed C,) but the C, structure due to the following reasons. (1) The ESR hf coupling
Structure and Dynamics of C4H8NH+and C4H8N
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8087 TABLE V: Structural Parameters for the Optimized Geometries and Energies of the C b C, and C, Symmetries of the C4H8N Radical, Using the UHF/6-31G* and (UHF/f21G) Basis Sets'
\
Y
Figure 10. Geometry and labeling used for the C, structure of the C4H8N
radical. constants calculated for the C2,structure do not agree with the experimental values while the agreement is much better for the C2structure. (2) The energy of the C,, structure is higher than the C, structure by 2.8 kcal/mol on the UHF/3-21G level and 2.5 kcal/mol on the UHF/6-31G* level. C4H8NRadical. Three structures of the C4H8Nradical have been studied: the twisted structure of C2symmetry (Figure 8), the bent C, structure (Figure 9), and the planar C2, structure (Figure IO). It is found in the U H F calculations that the C, geometry is the most stable one. On the UHF/6-31G* level, the 2B state of C, symmetry lies about 2.2 kcal/mol lower than the supposed 2A' state of C,symmetry and about 2.8 kcal/mol below the 2BIstate of C , symmetry. The differences in energies obtained with the 3-21G basis set were found to be 2.5 and 2.7 kcal/mol for C,and C2,, respectively. From Table V, we also obtain the corresponding differences from the MP2 calculations; 2.8 and 3.6 kcal/mol for C2and C2,. It should be remarked that the SDCI energies, quite contrary to the above-mentioned results, predict that C2,structure (Figure 10) to be lowest in energy. This illustrates the fact that the energies evaluated from truncated SDCI expansions are less suitable for comparison purposes. The reason is that the number and types of included configurations differ from calculation to calculation (Table V) and consequently also the remainder term. If we instead look at the values for the extrapolated full CI limit20 (extrap in Table V), we can obtain a consistent ordering between the different geometries which is in accordance with the U H F and MP2 energies. In Table V, we have also given the number of spin-adapted configurations (s-a config) used in the SDCI calculations. The geometrical parameters of the 2B ground state of C2symmetry, as obtained from the UHF/3-21G and UHF/6-31G* optimization calculations, are summarized in Table V. These parameters are defined in Figure 8. In this set of calculations, the predicted twist angle cy (Figure 8) was 18.0' (3-21G) and 17.3' (6-31G*). These values can be compared with the twist angles 23.1' (3-21G) and 22.0' (6-31G*) obtained for C4H8NH+. Hence, the calculations predict the molecular geometry of the neutral ClH8N radical being less twisted than the radical cation, C4H8NH+.The neutral radiczl has decreased values of the angles a2, a3, 06, and a8 (Figure 8 and Table V) and a general shortening of the C-C and C-N bonds and the C3-C4 distance than the cation. The supposed C, structure does exist as a stable minimum, opposite to what was found for C4H8NH+. Starting the optimization with an initial bending angle, 0,of 130' (Figure 9), a final bending of 151.5' (6-31G*) and 155.4' (3-21G) was obtained. The results for the 2A' state of C, symmetry are given in Table V and Figure 9. Note that the parameters (e& R5, R6, C Y ] , etc.) lying on the same side of the carbon framework as the nitrogen and the corresponding parameters (e.&, R5', R6', cyl', etc.) lying on the opposite side are given in the same rows of Table V (e.g., R5/R5', R6/R6' etc.) for easy comparison with the CZL' symmetry. The UHF/6-31G* optimized parameters for the C, structure are presented in Table V and Figure IO. The experimental ESR hf coupling constants cannot, however, be explained by a C,, (20) Davidson, E. R.; Silver, W . Chem. Phys. Lett. 1977, 52, 403
RI R2 R3 R4 R5 R6 Rl R8 a al
a2 a3 a4 a5 a6 a7 a8
P
c2
cs
1.5310 (1.5446) 1.5354 ( I ,5468) 1.4534 (1.4977) 2.3456 (2.4029) 1.0865 (1.0839) 1.0839 (1.08 14) 1.0844 (1 .0805) 1.0852 (1.0894) 17.3 (18.0) 109.9 (110.0) 107.7 (108.8) 104.0 (102.5) 113.6 (112.9) 109.7 (109.1) 107.8 (107.5) 107.3 (106.9) 110.9 (110.7)
1.5458 1.5422 1.4486 2.3172 1.0804/ 1.0837 1.0831/ 1.0902
1.5438 1.5443 1.4474 2.3626 1.0839 1.0867
110.8/112.4 110.5/111.6 113.4/111.1 109.8/107.2
111.3 111.1 112.3 108.0
151.5
180.0
c 2 c
EUHF
-210.538 664 (-209.366 849)
-210.535 217 (-209.362 938)
-210.534251 (-209.362 480)
(S2)
0.762 (0.762)
0.762
0.762
-21 1.208 I96 -210.931 025 26 243 -211.208 196
-21 1.203 671 -210.947 140 29851 -211.206866
-21 1.202496 -210.982979 28218 -211.195838
ESDCl
s-a config extrap
'Distances in A, angles in deg, and energies in au; see Figures 8-10 for notation; for doubly valued parameters in the C, symmetry, see text.
structure because of the following reasons. (1) The C , structure has two groups of four equivalent protons which are not observed experimentally. (2) The energy of the optimized C , structure lies about 2.8 kcal/mol (UHF/6-31G*) above the C, structure and 0.6 kcal/mol above the C, structure. For the MP2 single-point calculations the corresponding differences are 3.6 and 0.8 kcal/mol for C, and C, respectively. The isotropic hf coupling constants, um, for the H and I4N nuclei of the molecules with C2 and C, structures, obtained in the UHF/6-31G* and SDCI calculations, are presented in Table IV. This table also displays the anisotropic hf coupling B(N) for the I4N nucleus as obtained from the SDCI calculations. We can observe that the isotropic hf coupling for the 14Nnucleus, obtained from the SDCI calculations, is about 10 G too low compared with the experimental value for both geometries taken into consideration. This discrepancy is then also reflected in the total hf coupling, including both the anisotropic B(N) and isotropic part. If we, however, subtract the isotropic part from the total hf coupling, we get the purely dipolar hf couplings of I4N: Bll= 30.8, 30.6, and 26.3 G for the C2, C, and experimental values, respectively ( B , = -1/2Bll).These dipolar couplings on the nitrogen nucleus are thus in very good agreement with experiment. The reason for the poor result in calculating the isotropic coupling on the nitrogen is probably that the selection routine in the SDCI program uses single and double excitations out of a single reference determinant and hence may neglect some wave functions that are highly localized on the nitrogen atom but correspond to triple and higher excitations with respect to the Hartree-Fock determinant. To overcome this error, one would probably need to perform large MRSDCI calculations including single and double excitations from a multideterminental reference function,2',22where the multireference functions are chosen such that they have large amplitude on the nitrogen atom. In summary, one can say that for both geometries we have a good agreement with the experimental data. The isotropic proton hf coupling constants with positive sign (21) Engels, B.; Peyerimhoff,S. D.; Karna, S. P.; Grein, F. Chem. Phys. Lett. 1988, 152, 397. (22) Engels, B.; Peyerimhoff, S. D. Mol. Phys., in press.
8088 The Journal of Physical Chemistry, Vol. 94, No. 21, 19'90
Figure 11. Schematical picture showing interchange between the two twisted structures of C, symmetry passing via a bent C, transition state.
obtained in the SDCI calculations are as usual smaller than the experimental value^.^^^'^ Based on the calculated results that the structure with C2 symmetry lies ~ 2 . kcal/mol 2 lower in energy than the corresponding lowest electronic state of C, symmetry and that the obtained hf coupling constants show a slightly better agreement with experimental results for the C, than for the C, case (at least for H5), we conclude that the ground state of the C4H8Nradical is of C2symmetry. Most likely, the radical also has a C, transition state between the two C , structures which are mirror images of each other, as shown in Figure 11. Since the energy difference between the structures of C, and C2, symmetry is rather small, the exchange between the two structures may also pass via a transition state of planar geometry having C,, symmetry.
Discussion Structure of C4H8NH+and C4H8NMe+.The C4H8NH+cation is assigned a twisted C2 structure in all matrices employed at 4 K . Although slightly different hf coupling constants are observed in the different matrices (Table I), the matrix effects are too small to alter the electronic ground state of C4H8NH+.The UHF/SDCI calculations support this assignment. The hf coupling constants obtained from the simulations of the experimental data and the theoretical calculated values are in qualitative agreement, as noted in the previous section. Note that the N-Ha proton is assigned a purely isotropic hf coupling constant while a small anisotropic part is expected due to the highly localized SOMO. This fact might account for the slight differences between the simulated and experimental spectra. N o essential changes were observed in the ESR spectra between 4 and 77 K, suggesting that C4HaNH+ has a rigid structure also at 77 K. In CFCI3, an extreme broadening of the ESR line shape occurs at 4 K (Figure la), probably reflecting the rigidity of this matrix or a microwave power saturation effect due to a very long relaxation time ( T I )of the cation in CFC13. If one compares the anisotropic hf coupling constants extracted from the experimental data with those of cations having a similar local structure (Table 11), C4H8NH+can be assigned to have a planar structure in the vicinity of the I4N atom (Le., sp2 hybridized). The spin density residing in the 2p, orbital on the 14N atom (see experimental values, Table I) is estimated to p2p, Z= 0.67-0.71; in CF3CCI3one finds a slightly smaller value, pZp,= 0.55, at 4 K. This reduction of the spin density in the 2p, orbital might be due to a distortion induced by the matrix causing a slight deviation from a planar local structure of the nitrogen atom. Furthermore, we can compare the twisted C2structure of C4H8NH+with the isoelectronic radical cations, THF+ and c-C5Hlo+,which also have distorted ground-state s t r ~ c t u r e s .THF+ ~ ~ ~ was studied by Shida and co-workers2 in CFC13, and the rigid structure was proposed to be a twisted C2 structure similar to that of C4HaNH+. None of the radical cations or neutral radicals of the saturated five-membered rings seem to have a planar ground-state s t r u ~ t u r e . ~ * ~ ~ ~ - ~ Due to the inequivalent hf coupling constants assigned to the two pairs of 0 protons observed at 4 K (Table I), the Nmethylpyrrolidine cation is proposed to reside in a twisted C2 structure. The fact that averaging of the P-proton hf coupling constants does not occur at elevated temperatures suggests a rigid conformation of the ring. C4HaNMe+has previously been studied in CFCI, at 77 K, and hf parameters similar to those observed at 4 K in this study have been reported.23 The activation energy for ring inversion was estimated to be higher than 4 kcal/mol. Replacing the amine proton with a Me group causes a slight reduction of the spin density in the nitrogen 2p, orbital to pZp, (23) Eastland, G . W.; Ramakrishna, D. N.; Symons, M. C. R. J . Chem. Soc., Perkin Trans. 2 1984, 1551.
Shiotani et al. 0.63-0.64. The P-proton hf coupling constants are reduced ( 10%) in the presence of the Me group, probably due to a reduced hyperconjugative effect. Reactions. The C4HaN radical was formed in CF,CICFCl, and CF2CICF2CI upon warming the sample containing the C4HaNH+cation above 100 K, probably through an ion-molecule reaction. These two matrices have been utilized in the study of reactions involving radical cations, because of the high translational mobility of molecules within the m a t r i ~ .Reactions ~ of cations of several heterocyclic compounds have been studied extensively by using these matrices. Williams et aL3 have reported the formation of neutral radicals from heterocyclic rings with the unpaired electron in a secondary or tertiary position with respect to the heteroatom. They proposed both proton transfer and hydrogen atom abstraction as possible reaction mechanisms. The ESR spectra attributed to the C4HaN radical having a rigid structure are shown in Figure 3. No traces of neutral radicals with the unpaired electron in a secondary or tertiary position relative to the N nucleus were observed in the present investigation, suggesting that the C4HaN radical is formed through deprotonation from the N-Ha bond of the cation. Another possible reaction mechanism might be hydrogen atom abstraction from the same bond of a neutral C4HaNH molecule by the cation. Contrary to the observation made for C4H8NH+,no formation of neutral aminyl radicals from C4HaNMe+ was observed in CFzClCFC12or CF,CICF,CI at elevated temperatures ( T > 100 K). The appearance of a doublet at 115 K in CF2CICFCI2,a(19F) Z= 60 G, indicates the presence of the CFCICF2C1 radical. When the temperature further increased to 130 K, a spectrum attributed to the CCI2CF3radical emerged.24 This observation suggests a rearrangement of the initially formed chlorofluoroalkyl radical, CFC1CF2C1, to the secondary radical, CC12CF3. A pathway for this isomerization has been suggested by Qin et al.,25which involves a rearrangement of the CF,CICFCI carbanion. They concluded that this type of isomerization was possible if the solute had a low ionization potential, Le., was capable of acting as a good electron donor. Interesting to notice is the distinct difference in reactions between C4HaNH+and C4HaNMe+in CF2CICFCI2. The formation of aminyl radicals from C4HaNH+as well as the absence for C4H8NMe+in CF2ClCFC12might be due to the different ionization potentials of the two neutral species, C4HaNH (8.77 eV)26 and C4H8NMe (8.41 eV).26 Similar reactions as the above-mentioned ones have also been observed for the piperidine and N-methylpiperidine radical cations stabilized in CFzC1CFC12.27 The broad triplet observed in CF2CICF2C1decayed monotonically upon increasing the temperature above 115 K. The matrices CF2C1CFCI2and CF2C1CF2C1seem thus to behave differently with respect to their ability to undergo rearrangement in the presence of amino solutes. Structure of the C4HaNRadical. The hf parameters assigned to the C4H8N radical are given in Table I. These values were obtained from simulations of the experimental spectra attributed to the C4HaN radical with a rigid structure (Figure 3). The C4HaN radical has previously been studied in an adamantane matrix at higher temperature (247 K), and the reported hf coupling constants (isotropic) are very close to those obtained in CF2ClCFC12and CF2ClCF2Cl(comparing the averaged P-proton hf couplings).28 One can further notice that the isotropic part of the 14N hf coupling constant, aim= 14.7 G, has the same magnitude as the isotropic values observed for neutral aminyl radicals with similar local structure of the I4N atom (Table 11). Hence, the spectra observed in CFzCICFClz (100 K) and CFzCICF2C1 (77 K) can unambiguously be attributed to the C4HaN radical. The spin density in the 2p, orbital on the nitrogen atom (24) Walther, B. W.; Williams, Ff. J . Chem. Phys. 1983, 79, 3167. (25) Qin, X.-Z.; Guo, Q.-X.; Wang, J.-T.; Williams, Ff. J . Chem. SOC., Chem. Commun. 1987, 1553. (26) Rozeboom, M. D.; Houk, K. N.; Searles, S.; Seyedrezai,S. E. J . Am. Chem. SOC.1982, 104, 3488. (27) SjMvist, L.; Shiotani, M. Unpublished results. (28) Pratt, D. W.; Dillon, J . J.; Lloyd, R. V.; Wood, D. E. J . Phys. Chem. 1971, 75, 3486.
The Journal of Physical Chemistry, Vol. 94, No. 21. 1990 8089
Structure and Dynamics of C4H8NH+and C4H8N TABLE VI: Dynamical Motion and Energy Barriers for Some Cyclic Radical Cations and Radicals Stabilized in Halocarbon Matrices radical C-CjH6'
matrix CF3CCI3 CF2CICFCI2 CFCI3
s F6
c-C,HI,+
CF3CCI3
C-C6H12+
CFCI3
c-CSH~' TH F+ C4H8NH+
CF2CICFC12 CFCI3 CFCI, CF2ClCFzCI C~HBN. CF2CICFCI2 CF2CICF2CI Me-d,-c-C6+ CF3-c-C6F,, Mez-l ,I-c-c6+ CF3-c-C6FII
T,K
model
E,, kcal/mol
0 C4H0NH+
C4H0NHt
.
ref
A
4-102 4-102 4-102 4-102 4-1 IO 18-37 30-1 40 100-135 77-1 55 75-165 74-1 11 105-150 1 1 1-1 30 4-173 4-77
three-site three-site three-site three-site ten-site two-motion two-site three-site two-site two-site two-site two-site two-site two-site two-site two-site
0.68 0.54 0.05 0.034 1.2 3.6 0.17 0.24 2.8 1.65 1.6 2.0 1.9 1.3 0.2 0.3
6
/ CF2C1CF2Cl / CFCl3
C4H01:
/ CF2ClCFCl2
C4H8N
/ CF2C1CF2Cl
7 6
8 2 U
a a a
32 32
'This study.
is estimated to be approximately pZp = 0.77, using the experimental values in Table I . This is slightly higher than the values estimated for the radical cation. According to the calculations mentioned in the previous section, the C4H8N radical can be assigned a ground state with a twisted C, conformation (Figure 8). A comparison between the C4H8Nradical and the isoelectronic c-C5H9radical! both studied in CF2CICFCI2,suggests a structural resemblance; i.e., both radicals are proposed to have rigid structures described as twisted C2conformations. Dynamics. The observed line-width alternation of the ESR spectra at higher temperatures ( T > 77 K) in CFCI3 is due to internal motion of C4H8NH+. The onset of this motion begins at approximately 100 K, as shown in Figure 2a-d. In order to reveal qualitative information about the dynamics, we have performed simulations of the experimental ESR spectra. All simulations were carried out using a program developed by H e i n ~ e r ?which ~ calculates the exchange-broadened isotropic ESR spectra using the equation of motion for the density matrix within the Liouville formalism. Using the results from the SDCI calculations presented in the previous section, we assumed a dynamical model for C4H8NH+ where interchange between two equivalent twisted C2structures takes place. The two equivalent C,structures (mirror images) are obtained by reflecting the two carbon atoms C3 and C4 in the Cl-N-C2 plane in Figure 6. This process results in an averaging of the hf coupling constants belonging to the two pairs of nonequivalent @-protons(i.e., H5/H6 and H,/H,) and a corresponding line-width alternation of the ESR spectrum.30 Note that the N-H, proton hf coupling constant is not affected by this motion. The hf parameters used in the simulations are displayed in Table I . Both the isotropic and anisotropic I4N hf coupling constants were omitted in the simulations. Only the relative intensity and the shape of the I4N(mI=O) transitions were considered. The experimental and simulated spectra agreed rather satifactorily employing this model as shown in Figure Za-d,a'-d'. An Arrhenius plot, In k vs TI,reveals a straight line as depicted in Figure 12. The activation energy evaluated for the motion is 1.6 kcal/mol. This value is of the same order of magnitude as the activation energies observed for the ring inversion of other five-membered cations such as THF' and c-C5Hlo+(Table VI). The activation energy related to the motion of C4H8NH+ in CFzC1CF2CI is estimated to 2.0 kcal/mol, which is slightly higher than the value in CFC13. Here we used the same dynamical model but with different hf parameters (Table I). Interesting to notice is the difference between these values and those reported for the internal (29) (a) Heinzer, J. Mol. Phys. 1971, 22, 167. (b) Heinzer, J. QCPE 1972, 209. (30) Sullivan, P. D.; Bolton, J. R. The Alternating Linewidth Effect. In Advances in Magnetic Resonance; Waugh, J. S., Ed.; Academic Press: New York, 1980: Vol. 4, p 39.
5 0
10 0
T-I
(10.3)
K-1
Figure 12. Arrhenius plot. In k is plotted versus T I for C4H8NH+and the C4H8N radical. The activation energies are given in Table VI.
motion O f C-CjHg', C-C6Hl2+, Me-d3-C-C6Hl I + , and Me2-l,1-CC6Hlo+(Table VI). The latter cations have much lower activation energies, suggesting that different types of motion are involved for the cations of the three- and six-membered rings. The C4H8N radical also exhibits an internal motion in CF2ClCFCl, and CF2C1CF2C1. We only present the simulations of the C4H8N radical in CF2CICFCI2 here, but the results for CFzCICFzClwere obtained by an analogous procedure. As shown in Figure 4a-e, a reversible change of the ESR spectra can be observed when the temperature is altered in the temperature region 110-145 K. According to the SDCI calculations, it is reasonable to assume that an exchange occurs between two twisted C2 structures of the C4H8N radical. The hf parameters employed in the simulations are given in Table I. Here it should be noted that the I4N hf coupling constant is completely neglected in the calculated spectra shown in Figure 4a',b', while only the isotropic coupling is included for the spectra shown in Figure 4c'-e'. However, the simulated and experimental spectra are in satisfactory agreement. The activation energies corresponding to the dynamics of the C4H8Nradical in CF,CICFCI, and CF2ClCF2Cl are 1.9 and 1.3 kcal/mol, respectively (Figure 12 and Table 111). These values are close to the results obtained for C4H8NH+(1.6 kcal/mol in CFCI3 and 2.0 kcal/mol in CF2C1CF2CI). The activation energy attributed to the internal motion of the C4H8N radical is slightly lower than the observed value reported for the isoelectronic c-C5H9radical in CF2ClCFCIz(2.8 kcal/mol). This might be due to a larger distortion of the c-C5H9ring. If we consider the calculated ground-state structures for C4H8NH+and the C4H8N radical, Le., twisted C, structures with twist angles 22.0' and 17.3' (6-31G* level), activation energies with the same order of magnitude can be expected for both isoelectronic species. However, slight differences between the employed matrices can be expected due to matrix-dependent effects such as for example the rigidity of the matrix, the size of the trapping cavity, etc. It should also be mentioned that the fast limit, i.e., a complete averaging of the @-protonhf coupling constants, cannot be observed in the matrices used due to the limited temperature region. As mentioned above, the hf anisotropy from to the I4N nucleus was not included in the analysis of the dynamics. An improved dynamical model should include A (and g) anisotropy and take reorientation of the I4N hf tensor due to the internal motion into consideration as well as a possible rotation of the molecule as a whole. The effect of the hf anisotropy is particularly apparent in the case of C4H8NH+(Figure 3a-d), where abnormal intensities of the high field component of the N-Ha lines can be observed. As discussed in the earlier section (see Theoretical calculations),
8090 The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 both C4H8NH+and the C4H8N radical may interchange conformations between the two identical C2structures via a C, planar transition state. The existence of a C,state for the C4H8Nradical further suggests the presence of a C,transition state (Figure 11). Therefore, neither the C, nor the C,state can be excluded as a transition state because of the small energy differences between the two states. The ESR spectra assigned to C4H8NMe+in CFCI, change reversibly in the temperature region between 4 and 77 K, as shown in Figure 5a. We propose that this alternation of the line width is due to a restricted rotation of the Me group. The ESR spectrum observed at 19 K (Figure Sa) was simulated by a "three-site" jump model for the protons on the Me group, attached to a rigid ring with a twisted conformation. The protons were assumed to interchange positions by consecutive 120" jumps. The spectrum was simulated employing the hf coupling constants given in Table I. (The hf coupling constants due to the 14N nucleus were neglected.) Two different models for the restricted rotation of the Me group were investigated (the figures below show a Newman projection along the CH3-N bond):
Model I gave a considerably better agreement between the experimental and simulated spectra. From this result, we draw the conclusion that the ground-state structure of C4H8NMe+has one of the C-H bonds on the Me group located in a parallel position with respect to the 2p, orbital on the nitrogen atom. This conformation (I) will thus be more stable than the conformation (11) due to an increased hyperconjugative effect. In the present study several halocarbon matrices were utilized in order to reveal information about matrix effects. As described above, both CF2ClCFC12and CF2CICF2Clhave high translational mobility and are suitable in the study of reactions involving cation^.^ Interesting to note is the rigidity of CFCI, observed at 4 K compared with the other employed matrices. This observation agrees with a recent ESEM investigation of c-C6HI2+stabilized in different halocarbon matrices at low temperature where it was concluded that CFCI, was more rigid at low temperature.,'
Shiotani et al. Conclusions Theoretical UHF ab initio calculations predict C4H8NH+and the C4H8N radical to have twisted ground-state structures of C, symmetry in 2B electronic states, with twist angles 22.0" and 17.3" (6-3 1G* level), respectively. Experimental hf coupling constants were found to be in good agreement with SDCI calculated values, giving further support for this assignment. Using the theoretical results, it was possible to analyze the dynamics of both the cation, C4H8NH+,and the neutral radical, C4H8N,in several halocarbon matrices at low temperature. The internal molecular motions were described in terms of interconversion of the ring between two twisted, but equivalent, C2 structures, for both C4H8NH+and the C4H8N radical. Activation energies related to the internal motions were evaluated to be 1.6 kcal/mol in CFCl3 and 2.0 kcal/mol in CF2CICF2CIfor C4H8NH+and 1.3 and 1.9 kcal/mol in CF2CICF2CI and CF2CICFCI2for the C4H8Nradical, respectively. These values are of similar magnitude if they are compared with other isoelectronic radical species such as THF+ and c-C5HIo+. Furthermore, C4H8NMe+is proposed to have a ground-state structure that resembles the structure proposed for C4H8NH+, Le., a twisted ring structure. The Me group is oriented so that one of the C-H bonds is parallel to the 2p, orbital on the nitrogen, if a projection along the N-CH, bond is considered. Replacing the N-H, proton with a Me group in the positively charged five-membered heteroring results in an increased rigidity of the ring, probably due to sterical effects.
Acknowledgment. This work was supported by the Swedish Natural Science Research Council (NFR). L.S. gratefully acknowledges the Bank of Sweden Tercentenary Foundation (RJ) for a grant. M.B.H. expresses his gratitude to the Swedish Institute for a guest research scholarship. A grant from the Goran Gustafsson Foundation is gratefully acknowledged. We also acknowledge the National Super Computer Center (NSC) at Linkoping University for the use of their Cray X-MP/48 supercomputer. We also thank Mr. Y . Nagata for the help during the preliminary experiments in this investigation. (31) Westerling, J.; Lund, A. Chem. Phys. 1990, 140, 421. (32) Sjijqvist, L.; Lindgren, M.; Shiotani, M.; Lund, A. J. Chem. SOC., Faraday Trans. 1, in press. (33) Belevskii, V. N.; In Khvan, 0.; Belopushkin, S. I.; Fel'dman, V. I. Dokl. Akad. Nauk SSR 1984, 281, 869. (34) Fessenden, R. W.; Neta, P. J. Phys. Chem. 1972, 76, 2857. (35) Danen, W. C.; Rickard, R. C. J. Am. Chem. SOC.1972, 94, 3254. (36) Rao, D. N. R.; Symons, M. C. R.; Wren, B. W. J. Chem. SOC.,Ferkin Trans. 1984, 2, 1681. (37) Danen, W. C.; Kensler, T. T. J. Am. Chem. SOC.1970, 92, 5235.
