Electron paramagnetic resonance spectra of ... - ACS Publications


by David W.Pratt,* John J. Dillon,. Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15213. Roger V. Lloyd, and David E. Wo...
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D. PRATT, J. DILLON,R. LLOYD,AND D. WOOD

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Electron Paramagnetic Resonance Spectra of Pyrrolidino and Pyrrolino Free Radicals. The Structure of Dialkylamino Radicals1 by David W. Pratt," John J. Dillon, Department of Chemistrv, University of Pittsburgh, Pittsburgh, Pennsylcania

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Roger V. Lloyd, and David E. Wood Department of Chemistry and Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pennsylvania 16818 (Receined December 7 , 1970) Publication costs assisted by the Merck Foundation

The nitrogen-centered free radicals pyrrolidino and pyrrolino have been prepared by X irradiation of the corresponding amines in an adamantane matrix at 77°K. At temperatures near 200°K, the radicals undergo rapid reorientation in the matrix and exhibit isotropic epr spectra. Comparison of the 1% hyperfine splittings and g values of these radicals with those found for several open-chain dialkylamino radicals iiidicates that all of these species appear to have similar electronic structures and RNR bond angles of less than 120'. This view is supported by steric considerations in the case of the cyclic radicals and by INDO calculations on the dimethylamino radical which show that the eclipsed form with a CNC bond angle of 117" is the most stable conformation. These calculations also indicate that (CHJzN. is a T radical with very little r character and predict hyperfine splittings in good agreement with the experimental values.

I. Introduction The NHZ radical has sevcn valence electrons and therefore, according to Walsh's rulesj2the ground electronic state should be characterized by an HNH angle of less than 180". This prediction has been verified by careful analysis of the so-called cy bands of ammonia, ~ of S H 2 in the which showed that the ground 2 B state gas phase has a bond angle of 103.4".3 Extensive optical4 and epr5 spectral studies of the amino radical in solid matrices have also been reported, but interpretation of these results is complicated by the influence of the environment on radical motion and structure. For example, the isotropic 14N hyperfine splitting (his) constant of NH2 appears to vary from 10.4 to 31 G in different mat rice^.^ Symons has suggested that these variations might be attributed to effects of the local environment on the HNH bond anglej6but no additional information supporting this view has been published. Similar difficulties have been encountered in interpretation of epr spectra of higher homologs of the aminoradical. Alkylamino radicals of the general formula R 8 R ' may be produced by high-energy irradiation of precursor amines at low temperatures, but the resulting complex powder spectra have not yielded a great deal of structural inf~rmation.'-~ On the other hand, if the radicals can be produced in an environment which allows them them to tumble freely, then detailed structural information can only be obtained by comparison of the resulting isotropic epr parameters with careful theoretical calculations. It was of interest in the present study to explore T h e Journal of Physical Chemistry, Val. 76, N o . 22, 1971

furt'her thc possible relationships between these parameters and the CSC bond angles in several members of the dialkylamino radical family. In particular, use of the recently described adamantane matrix techniquelO has enabled us to prepare and characterize the

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pyrrolidino (I) and pyrrolino (11) free radicals in an environment which allows them to undergo rapid reorientation. While this work was in progress, Danen and Iiensler" obtained the epr spectra of dimethyl-, (1) Taken in part from the M.S. Thesis of J. J. Dillon, 111, University of Pittsburgh, 1970. (2) A . D. Mialsh, J . Chem. Soc., 2260 (1953). (3) G. Herzberg and D. A. Ramsay, J. Chem. Phys., 20, 347 (1952) ; K. Dressler and D. A. Ramsay, Phil. Trans. Roy. Soe., 251A, 553 (1959). (4) See G. TV.Robinson, A d t a n . Chem. Ser., No. 36, 10 (1962), and references contained therein. (5) See D . R. Smith and W. A . Seddon, Can. J . Chem., 48, 1938 (1970), and references contained therein. (6) M. C. R. Symons, Adcan. Chem. Ser., No. 82, 1 (1968). (7) G. V. Pukhal'skaya, A . G. Kotov, and S. Ya. Pshezhetskii, Dokl. A k a d . N a u k SSSR, 171, 1380 (1966). (8) S. G. Hadley and D. H. Volnian, J . A m e r . Chem. Soc., 89, 1053 (1967). (9) D . Cordischi and R . DiBlasi, Can. J . Chem., 47, 2601 (1969). (10) D. E. Wood and R . V. Lloyd, J . Chem. Phys., 52, 3840 (1970); 53, 3932 (1970). (11) W. C. Danen and T. T. Kensler, J . A m e r . Chem. Soc., 92, 5235 (1970).

EPRSPECTRA OF FREERADICALS ESRSYMPOSIUM.

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diethyl-, and diisopropylamino radicals in solution by in situ photolysis of the corresponding tetrazines. Since the cyclic radicals I and I1 would be expected to have CNC bond angles of less than 120” from steric considerations alone, a comparison of our results with those obtained for the open-chain dialkylamino radicals and wiih INDO calculations12 on the prototype dimethylamino radical would be expected to shed considerable light on the geometry and electronic structure of this class of free radicals.

11. Experimental Section Details of the adamantane matrix technique have been described previously. l o Samples were X-irradiated for 15 min a t 77°K by immersion in a foam dewar containing liquid nitrogen, and epr spectra were recorded on a Varian V-4502 spectrometer equipped with a V-4547 variable temperature accessory and rectangular TEo12 cavity. Temperatures were measured with a calibrated copper-constantan thermocouple. Both pyrrolidine and pyrroline were obtained from Aldrich Chemical Co. and were used as received. Samples prepared on a vacuum line with degassed and trap-totrap distilled pyrrolidine gave the same results as those prepared in air. N-Hydroxypyrrolidine was prepared by the peroxide oxidation of pyrrolidine.

111. Results and Discussion Figure 1 shows the epr spectrum obtained a t 247°K following X irradiation of pyrrolidine in adamantane a t 77°K. The spectrum consists of a 1 :4 :6: 4 : 1 quintet of 1 : 1 : 1 triplets which would be expected for four equivalent p protons and a single I4N. The spectrum from pyrroline is identical except for the larger P-proton hfs. Both spectra were fit by computer simulation, and the resulting epr parameters are shown in Table I. Also shown in this table are the results for NH2 in solid a m m ~ n i a ,for ~ the open-chain radicals obtained by Danen and Kensler, l1 and for di-tert-butylamino radical.13 It is seen that all species listed in this table have nearly identical 14Nhfs and g values (where measured). The 14N hfs are also similar to those reported for the corresponding nitroxide radicals; however, g values and P-proton hfs for the nitroxides are quite different. For example, the nitroxide radical corresponding to radical I, which was prepared from N-hydroxypyrrolidine, has U N = 13.5 G, UH’ = 17.6 G, and g = 2.0060 in an adamantane matrix a t 215°K. The hfs of the cyclic dialkylamino radicals are independent of temperature up to 270°K. Above this temperature, the epr spectrum of each radical decreases rapidly and irreversibly in intensity to reveal a new spectrum with a lower g value. The slight distortion of the spectrum shown in Figure 1 is caused by the presence of this as yet unidentified free radical, which could also be obtained exclusively by X irradiation of pyrrolidine in adamantane a t room temperature. Whereas

l c - - 5 0 G 4

Figure 1. Second-derivative epr spectrum of pyrrolidino radical in adamantane a t 247°K.

Table I: Isotropic Epr Parameters of NRz Radicals Radical

a N ,G

aH,G

NH2 in ammonia* (CHs)&. (CHaCH2)zN. [(CHa)zCHInN* [(CHs)aClzN* d Pyrrolidino ( 1 ) ~ Pyrrolino (1I)f

15.2 14.8 14.3 14.3 14.2 14.4 14.4

25.4 27.4 36.9 (4) 14.3 (2)

a Compared to DPPH ence 11. d Reference 13. work, T = 241°K.

= e

...

39.1(4) 44.7(4)

g

valuesa

2.0046

... ... ... 2.0045 2.0046 2.0046

2.0036. Reference 5. c ReferThis work, T = 228°K. f This

the spectrum of the latter radical was easily powersaturated, as is usually observed for carbon-centered radicals, the cyclic dialkylamino radical spectra could not be saturated with the full output of a 100-mW klystron. The similarities of the g values and I4N hfs in these radicals to those found for the open-chain dialkylamino radicals suggest either that the CNC angles are not very different for the two groups of radicals or that the values of both g and a~ are not very sensitive t o the bond angle. That t,he g value should be sensitive to the bond angle is expected on theoretical grounds because the magnitude of the deviation from the free spin value depends on the hybridization of both the lone-pair and unpaired electron orbitals. l 4 This expectation has received recent experimental confirmation from the distribution of g values observed for Si-N-Si color centers in sodium silicate glasses’j which are known to exhibit a distribution of SiOSi bond (12) J. A. Pople, D. L. Beveridge, and P. A. Dobosh, J. Amer. Chem. SOC.,90,4201 (1968).

(13) Obtained in neat di-tert-butylamine at 203’K; D. E. Wood, unpublished results. (14) P. W. Atkins and M. C. R. Symons, “The Structure of Inorganic Radicals,” Elsevier, Amsterdam, 1967. (15) J. H.Mackey, J. W. Boss, and M.Kopp, Phys. Chem. Glasses, 11, 205 (1970).

The Journal of Physical Chemistry, Vol. 76, No. $3, 1971

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D. PRATT, J. DILLON, R. LLOYD,AND D. WOOD

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1 6.0

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15.0

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Sz 14.0

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0

13.0 -

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angles.I6 As the g values for the two groups of dialkylamino radicals are the same within experimental error, and since the CNC angle in the pyrrolidino and pyrrolino radicals is probably somewhat less than 120" because of the ring strain which would otherwise ensue, it seems reasonable to conclude that the CNC angle in the open-chain dialkylamino radicals is also less than 120". This proposal is supported by INDO calculationsl2 on the dimethylamino radical. In these calculations, the optimum geometry was first determined by minimizing the energy of both the staggered and eclipsed conformations of (CH3)2y* with respect to the parameters TCH, TCN, and