JOURNAL OF MAGNETIC RESONANCE l&471-484 (1975)

Applications of ENDOR Spectroscopy to Radicals of Low Symmetry: Radical Anions of 2-Phenylcyc1[3.2.2]azines*

F. GERSON AND J. JACHIMOWICZ

Physikalisch-Chemisches Institut der Universitiit Base& 4056 Basel, Switzerland

K. MOEBIUS AND R. BIEHL

Institut fiir Molekiilphysik, Ezchbereich Physik der Freien Universittit Berlin, I Berlin 33, Germany

AND

J. S. HYDE AND D. S. LENIART

Instrument Division of Varian Associates, 611 Hansen Way, Palo Alto, California 94303

Received November 26,1974

The radical anions of 2-phenylcyc1[3.2.2]azine:and Gmethyl-2-phenyl-5-azacycl- [3.2.2]azine were prepared by reaction with Na and with Li. Proton hyperfine couplings have been determined using electron-nuclear double resonance (ENDOR) spectroscopy. Assignments of couplings have been made by a combination of four methods: radio frequency coherence effects, calculations of relative ENDOR intensities, computer simulation of the ESR spectra, and MO calculations of n-spin distribution. Successful use of the two first methods has experimentally corroborated the theory of ENDOR line-shalpes in the presence of both saturating and non- saturating nuclear radio frequency fields.

INTRODUCTION

ESR spectroscopy has proved to be a powerful tool in the elucidation of the n-spin distribution in aromatic radicals (see, e.g. (I, 2)). However, use of the ESR method alone is insufficient when complex hyperfine structure defies a reliable analysis. In such cases, which are frequently encountered in extended paramagnetic n-systems of low symmetry, ENDOR spectroscopy presents a valuable complementary technique. The first ENDOR experiment on free radicals in 1iquid;s was performed by Hyde and Maki in 1964 (3), and several recent reviews (4) give access to the liquid phase ENDOR literature in the intervening decade.

Several years ago, we investigated the radical anions of cyc1[3.2.2]azine (I) and its two 2-phenyl derivatives, II and III. The hyperfine coupling constants were reported

* For the sake of simplicity and continuity the cyclazine nomenclature has been retained. According to IUPAC rules cycl[3.2.2]azine (I), 2-phenylcyc1[3.2.2]azine (II) and 6-methyl-2-phenyl-5-azacycl- 13.2.2lazine (III) are pyrido[6,1,2cd]pyrrolizine (or pyrrolo[2,1,5-cdlindolizine) (I), 2-phenylpyrido- [6,l,Zcd]pyrrolizine (II) and 4-methyl-7-phenylpyrimido[& 1,2-cdlpyrrolizine (III). Copyright 0 1975 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

471

472 GERSON ET AL.

only for Io (.5), because the ESR spectra of 110 and III@ could not be completely analyzed. In the present paper we describe an ENDOR study which provides the coupling con- stants of all protons in 110 and III@. Moreover, this paper illustrates the potential of the ENDOR technique in the unravelling of a complex proton hyperfine structure arising from rc-radicals of low symmetry.

EXPERIMENTAL

The 2-phenylcycl[3.2.2]azine (II) (6) and 6-methyl-2-phenyl-5-azacyc1[3.2.2]azine (III) (7) were a gift of Prof. V. Boekelheide. Their radical anions II@ and III@ were pro- duced by reaction of the respective neutral compounds with an alkali metal (Li or Na) in 1,2-dimethoxyethane (DME). Care had to be exercised not to push the reduction too far, since 110 and III@ (green color) were easily converted into the corresponding dia- magnetic dianions II@ and III@ (red color). When prolonged contact with the metal mirror was avoided, the solutions of IIQ and 1110 in DME remained unchanged for hours, even at room temperature.

The ESR spectra were taken on a Varian E-9 and an AEG 20X apparatus, whereas the ENDOR measurements were performed with the aid of a Varian E-700-High Power System and an instrument constructed by the Berlin workers and described elsewhere (8). While these two ENDOR spectrometers can be operated in various modes, the Berlin instrument has generally been used with nuclear radio frequency fields of the order of 20 G in the rotating frame. It employsfrequency modulation of the radio frequency and, as a result, yields first derivative-like ENDOR spectra. The Varian-E-700-System, on the other hand, has most often been configured to yield 10 G in the rotating frame with an amplitude modulation of the radio frequency level; the spectra thus obtained are pure absorption curves. In fact, the present investigation began as a comparison of the two instruments. Although such a comparison is not the principal aspect of this paper, an experience gained from our work is that both experi- mental approaches have considerable merit. An ideal ENDOR spectrometer would have capability to operate over a range of radio frequency powers with both modulation techniques available.

ESR SPECTRA

Assuming a pairwise equivalence of the ortho- as well as of the meta-protons in the 2-phenyl substituent, both radical anions 110 and 1110 contain 10 sets of magnetic nuclei. In II@, these sets are formed by the six nonequivalent cyc1[3.2.2]azine protons, the one phenyl para-proton, the two pairs of phenyl ortho- and meta-protons, and the

ENDOR OF 2-PHENYLCYCL[3.2.2[AZINE ANIONS 473

single 14N nucleus, whereas the corresponding sets in 1110 consists of four non- equivalent cyc1[3.2.2]azine protons, the one phenyl para-proton, the two pairs of phenyl ortho- and meta-protons, the set of three methyl protons, and the two single 14N nuclei. The numbers of hyperfine lines thus amount to 26 x 2 x 3’ x 3 = 3456 and 24 x 2 x 3’ x 4 x 32 = 10368 for 110 and III@, respectively.

The ESR spectra of II@ and III@, shown in Figs. 1 and 2, represent the two extreme cases characteristic of n-radicals with complex hyperfine structure. In the first case (II@),

10 / I H

GAUSS

FIG. 1. ESR spectrum of the radical anion IIQ. Solvent: DME; counterion: Na@; temp. : -6OT.

FIG. 2. ESR spectra of the radical anion IIP. Solvent: DME; counterion: Na@; temp. : -60°C (top) or +40°C (bottom).

the spectrum taken at -60” (Fig. 1) is well resolved and displays almost 300 lines, i.e., 10 % of those expected for 110 (3456). In the second case (IIP), the spectrum observed at low temperatures (Fig. 2, top) consists of a single S-shaped band with only an indication of hyperfine structure. Although the resolution is improved at higher tem- peratures, the total number of discernible components does not exceed 50, even at 1-40” (Fig. 2, bottom). This number is less than 0.5 “/, of that calculated for III@ (10368).

According to our experience, a well-resolved ESR hyperfine structure arising from up to seven different coupling constants can be completely analyzed by means of repeated

17

474 GERSON ET AL.

computer simulation (see, e.g. (9)). In some favorable cases, where accidental degeneracy occurs and/or some of the splittings are too small to be resolved, complete analysis of a hyperfine structure due to even more than seven different coupling constants is still possible (see e.g. (10)). A s verified by the ENDOR study described below, neither of these simplifications holds for the radical anion II@. It is therefore unlikely that 10 different coupling constants could be determined unambiguously from its ESR spec- trum, even with the extensive use of computer simulation.

Since the hyperfine structure of III@ is poorly resolved, its analysis must be considered a hopeless venture from the very beginning. An additional feature, which does not make the job easier in this case, is the sensitivity of the ESR spectrum to changes in tempera- ture and the choice of alkali metal as reducing agent. The obvious presumption that such a sensitivity is due to ion pairing of the radical anion IIIo with its counterions, is supported by the ENDOR studies and MO models (see below).

ENDOR SPECTRA Figures 3 and 4 show ENDOR spectra of II@ and III@, respectively. These spectra

have been obtained with the Berlin instrument, whereas a spectrum of 110 presented in Fig. 5 has been taken on a Varian E-700-System. In contrast to the spectra of 110 which are relatively insensitive to the nature of the counterion, those of III@ exhibit a

4 VP t i

I 15

r

I) I I

20 MHz

I’ ii\ \ \ % n

r 0

FIG. 3. ENDOR spectrum of the radical anion IF. (E%erlin instrument; 20 G in the rotating frame.) Solvent: DME; counterion: Na@; temp. : -90°C. The cross marks the frequency v, of the free proton.

marked dependence on the choice of the associated alkali metal cation, Na@ or Lie. Table 1 lists the pertinent coupling constants of the protons in IIo and III@, along with the 14N hyperfine splittings determined from the ESR spectra. The corresponding values obtained by previous ESR investigations of the parent radical anion Io (56) are also given for comparison.

Since the intensities of ENDOR lines are not direct indications of the relative numbers of nuclei contributing to the lines, assignment of coupling constants presents a serious problem. Several methods have been used in the past either singly or in combination, namely: (1) selective deuteration (II) ; (2) computer simulation of the ESR hyperfine

ENDOR OF %PHENYLCYCL[3.2.2]AZINE ANIONS 475

FIG. 4. ENDOR spectra of the radical anion IIIQ. (Berlin instrument; 20 G in the rotating frame.) Solvent: DME; counterion: NaQ (top) or LP (bottom); temp. : -90°C. The crosses mark the frequency v, of the free proton.

15 ZOMHz

FIG. 5. ENDOR spectra of the radical anion IF. (Varian E-700-System; 10 G in the rotating frame.) Solvent: DME; counterion: Li @‘; temp. : -9O’C. The arrow marks the frequency vP of the free proton.

structure with the aid of proton coupling constants determined by ENDOR spectro- scopy (12); (3) correlation of the experimental data with the rc-spin populations calcu- lated by means of MO models (13); (4) ENDOR nuclear-nuclear coherence effects (14); and (5) analysis of ENDOR intensities, which makes use of the relaxation proper- ties of free radicals in solution (1.5,16). In the present work the latter four methods have been employed to justify the assignment of the proton coupling constants made in Table 1.

COHERENCE EFFECTS As has been shown by Freed et al. (14), high radio frequency (NMR) fields can

influence the ENDOR line-shape by inducing multiquantum transitions within equi- distant nuclear energy levels. Since the total spin quantum number I for the detected

476 GERSON ET AL.

TABLE 1

PROTON AND 14N COUPLING CONSTANTS (IN G)” FOR THE RADICAL ANIONS I@, II0 AND IIF. SOLVENT DME

Radical anion (counterion)

IIQ (Na@)

IF (Li@)

1110 (Na@)

IIIB (Lie)

Position (nuclei) l(llH) 2(1’H) 3(l’H) 4(1’H) 5(11H) 6(11H) 7(1’H)

2’,6’ = ortho (2lH) 3’,5’ = meta (2lH) 4’=para(11H) 6-methyl(3lH)

8 (114N) 5 (1 14N)

1.13b 2.89’ 2.88” 2.02’ 1.95” 5.34b - - - - 5.34b 3.91” 3.96c 5.17” 5.64” 1.13b 1.29” 1.3oc 1.21” 0.96” 6.02’ 4.12” 4.77” - - 1.20b 1.17” 1.20” - - 6.02’ 5.00’ 5.02’ 3.63” 3.11” - 2.00’ 2.00” 2.12c 2.05’ - 0.57” 0.5lC 0.66” 0.61” - 2.38” 2.33’ 2.73c 2.54’ - - - 0.88” 0.61”

0.60b 0.34d 0.34d 0.7 * 0.1e 0.8 + 0.1’ - 2.5 + I* 2.5 k I*

a Experimental error in the coupling constants for 110 and 1110: +O.Ol G, except if explicitly noted. 1 G = 1O-4 Tesla.

b Determined by ESR at -60°C (5b). ’ Measured by ENDOR at -90°C. d Obtained from ESR spectrum at -60°C. e Obtained from ESR spectrum at +4O”C. f Estimated from the total extension of the ESR spectrum.

set of equivalent nuclei must be greater than l/2, at least two equivalent nuclei are required in the case of protons. The phenomenon, known as nuclear-nuclear coherence effect, may be distinguished from the electron-nuclear coherence effect which occurs at high microwave powers. Although the latter influences the ENDOR line-shape, it is independent of the nuclear spin, i.e., it broadens all ENDOR lines. However, if one keeps the microwave power sufficiently low, complications due to the electron-nuclear coherence effect are easily avoided. Increase of the radio frequency field (at rather low microwave power) leads initially to general saturation broadening. At even higher fields, however, the nuclear-nuclear coherence effect additionally broadens only those ENDOR lines which belong to I> l/2. As a result, these lines deviate from the Lorentzian line-shape and finally exhibit a splitting pattern that depends in a complex way on the various relaxation rates, the radio frequency field, and the number of equivalent protons. In general, such coherence splitting is resolved only if one can independently saturate the individual MI components in the ESR spectrum. There are three examples in the literature where the nuclear-nuclear coherence effect has been applied to the assignment of ENDOR lines (14,17,8).

In the present paper, we followed the ENDOR lines due to the phenyl ortho- and meta-protons in 110, the only ones that form sets of equivalent nuclei. The experimental

ENDOR OF 2-PHENYLCYCL[3.2.2]AZINE ANIONS 477

result was quite convincing : whereas at relatively low radio frequency fields all ENDOR lines displayed almost equal linewidths, at fields as high as 40 G (rotating frame) two lines in the spectrum broadened nearly by a factor of 2 as compared to the remaining ones. These lines lie close to frequencies v = 15 and 17 MHz (Figs. 3 and 5) and can be assigned to the phenyl meta- and ortho-protons, respectively. A coherence splitting was not observed here because, as a consequence of the gross overmodulation of the ESR spectrum, many MI components contributed to the ENDOR enhancement.

ENDOR INTENSITIES In addition to coherence effects, we have attempted to analyze the ENDOR spectra

in terms of the signal intensities. By incorporating an “understanding” of the relative intensities of the signals from different sets of protons in the radical, we should be able to assign correctly each coupling constant to a particular set. Since these intensities are complex functions of the various nuclear, electronic, and cross-relaxation processes, they are usually handled by computer simulations of the general matrix formalism (18). Nevertheless, under certain experimental conditions, it is possible to simplify the com- puter solutions in order to obtain an analytical set of expressions which theoretically describe the problem at hand. These theoretical expressions are preferably developed in the expansion parameter b = W,,/ W, which represents the ratio of nuclear and electronic transition probabilities induced by spin-lattice relaxation. In experimental work, the parameter b is most conveniently controlled by changing temperature and/ or solvent viscosity (16). When an expansion of the general solution is carried out to the lowest-order terms in b, one can derive relatively simple formulas yielding the shapes and average intensities of ENDOR lines. These lines have a simple appearance of saturated Lorentzian curves and are characterized by a single average width and saturation parameter.

According to the paper of Freed, Leniart and Connor (16), it is possible to obtain a complete solution for the average ENDOR enhancement. (Such a solution results when Eqs. [2.45 a,b,c] of that paper are substituted into Eq. [2.54] of (16).) Although the absolute enhancements can be determined experimentally (cf. Eq. [2.48] of (16)), it is generally the relative enhancements, i.e., the ratios of the intensities of the ENDOR signals from different sets of equivalent protons, which are most readily observed.

The analysis of ENDOR spectra in terms of average signal intensities requires the validity of the criterion

3n,b, < 1, Dl where n, denotes the number of equivalent protons in the vth set. (The limits of such a validity may be studied by examination of Fig. 3 in (I6).)

It has been shown (16) that, if the values of b, are small, the relative ENDOR enhance- ment (measured as the ratio of the ENDOR signal heights) is given, to the lowest order in b,, by the relative change in the saturation parameter

The probability rate W,(O) of electronic transitions (number of spins per second) refers here to the central ESR hyperfine line which is associated with the magnetic spin

478 GERSON ET AL.

quantum number iW1 = 0. The transition moment, d,,“, induced by the radio frequency (NMR) field, B,, for the vth set of equivalent protons, is specified by the formula

[31

where L, Y,,,, ms and B. have their usual meanings. The proton coupling constant a,,” is the average hyperflne interaction (in G) for the vth set, and the sign of m, = i-3 depends on the particular electronic quantum number at which the induced NMR transition occurs.

Equations [l] and [2] assume that exchange processes are negligible, such an assump- tion being adequate for the conditions under which the experiments were run in this work.

For the purpose of this analysis, it is reasonable to assume that both the electronic saturation parameter, Qe, and the probability rate, T/v,(O), are identical for the hyperfine lines arising from the phenyl o&o- and para-protons in II@. Moreover, if the NMR transitions are not saturated, then dn, Q b, We{O}, and the ratios of the ENDOR peak heights for these protons should vary as

AIIIP,,~~, = ‘mtho d,“,,,,, 20 - 0-2&2,,tim ms>’ Amppara %ara npara d2 = l(1 - 0.2tiPTparamS)2 *

In the case of organic radicals exhibiting proton hyperhne splitting 4, such that j~~tiJ < v,, those ENDOR lines which occur above the free proton frequency, v,, obey the relation &,m, -C 0, whereas the opposite, &“rn, > 0, holds for their counterparts below v,. Use of aoZortho = 2.00 and aPara = 2.33 G (II@-LP; Table 1) yields 1.89 and 2.18 as the ratios Amp,,,,,/Amp,,,, on the high and low frequency sides, respectively. These values, calculated by means of Eq. [4], compare favorably with the ratios 2.2 -t 0.2 (at 17.15/17.64 MHz) and 1.9 + 0.3 (at 11.56/11.05 MHz) measured for thecorrespond- ing peak heights in that ENDOR spectrum which is unsaturated with respect to the radio frequency field (B, = 10 G; Fig. 5).

On the other hand, if the condition of Eq. [l] is satisfied, but the NMR transitions are strongly saturated, then dz” 9 b, W,(O), and the pertinent ENDOR peak heights are related by the expression

Amportho/Amppara = n,,thobortholnparabgara = 2(~ortho/bra)2~ [51

Making use of the z-spin populations portho = 0.075 and pPara = 0.089 (Table 2), one obtains Amp,,tho/Amppara = 1.42, a value which agrees well with the ratio 1.3 + 0.1 found for the peak heights at 16.92/17.43 MHz in the “saturated” ENDOR spectrum of II@ (B, = 20 G; Fig. 3).

The surprisingly good agreement between the predicted and experimental ratios of the average ENDOR peak heights, in both cases of saturated and unsaturated NMR transitions, corroborates the assignment of the two signals to the ovtho- and para- protons of the phenyl group in II@.

Equation [3] can be also used to calculate the ratio of the ENDOR amplitudes expected for the ortlzo-protons on the high and low frequency sides in the “unsaturated” spectrum. Under the obvious assumption of the same radio frequency power on either

ENDOR OF 2+HENYLCYCL&!.2)AZINE ANIONS 479

side, the predicted value of 2.25 may be compared with the ratio 2.5 rt 0.5 observed for the peak heights at 17.15/11.56 MHz (Fig. 5).

At the end of this section, a brief discussion of a remarkable feature displayed by the “unsaturated” ENDOR spectrum seems appropriate. In this spectrum (Fig. 5), a rather broad signal, which certainly arises from dipolar couplings to matrix protons (19), is produced at the free proton frequency (v,). The presence of such a signal can be rationalized by a model of molecules tumbling more or less freely in cages. In accordance with experiment, the dipolar couplings to the protons in the radical, but not to those in the matrix, would be averaged to zero by this kind of motion. The fact that the pertinent signal does not occur in the spectra obtained at more intense frequency fields (Figs. 3 and 4) is consistent with the longer relaxation times of the matrix protons and the con- comitant ease of saturation. One should also consider the possibility that the absence of the “matrix” protons in these spectra could be due to the frequency modulation employed, since such a modulation tends to suppress the intensity of broad lines.

COMPUTER SIMULATION

The proton coupling constants provided by an ENDOR study can be used for a straightforward computer simulation of the ESR spectrum if the number of protons giving rise to each of the ENDOR signals is known, and the hyperfine splittings due to magnetic nuclei other than protons have been determined.

FIG. 6. Low-field part of the ESR spectrum of the radical anion IP. Top : Experimental spectrum (cf. Fig. 1). Bottom: Spectrum simulated with the aid of the following’coupling constants (in G = 10m4 Tesla): 2.893 (llH), 3.975 (l’H), 1.290 (l’H), 4.704 (llH), 1.165 (I’H), 4.965 (llH), 2.008 2lH), 0.582 (2lH), 2.380 (llH) and 0.338 (114N). Lineshape: Gaussian; linewidth: 0.055 G.

480 GERSON ETAL.

As shown in the preceding sections, the first condition is fulfilled in the case of the radical anion II@, Also met in this case is the second requirement, since the small 14N coupling constant for the central nitrogen atom in II@ can be exactly measured from the distance of the two outermost lines in the ESR spectrum. In Fig. 6 the low-field part of this spectrum is compared with the first derivative of the computed Gaussian curves. Even if the coupling constants used in the simulation are precisely those provided by the ENDOR studies, the agreement is satisfactory. The fit can still be improved by the use of very slightly modified values which are indicated in the caption to Fig. 6. (It is known that slight variations of the coupling constants frequently have a striking effect on the appearance of a multiline ESR spectrum; see, e.g. (20).)

In the case of III@, computer simulation of the ESR spectra is more problematic. The lack of experimental evidence with regard to the number of protons responsible for the ENDOR signals has to be noted first. An even more serious handicap is the absence of a resolved ESR hyperfine structure at low temperatures required by the ENDOR studies. Thus not only must the two 14N coupling constants be determined from the ESR spectra taken at higher temperatures, but also the computed derivative curves must be fitted to these spectra. The poor agreement achieved by tentative simula- tions is not surprising, in view of the observed sensitivity of the hyperfine structure to changes in temperature.

TABLE 2

EXPERIMENTAL" AND CALCULATED~ Z-SPIN POPULATIONS IN II@ AND IIF

Radical anion (counterion)

IIIQ III0 (Na9 (Lie)

, , (hk = 0.4) (h; = 0.6)

Center 1 2 3 4 5 6 7 8 2a 4a 7a 1’

2',6' (ortho) 3',5' (meta) 4' (para)

0.108 0.109 0.075 0.066 0.073 0.048 0.140 - 0.160 - - 0.169

0.149 0.137 0.193 0.165 0.211 0.178 0.048 0.055 0.045 0.045 0.036 0.037 0.177 0.192 0.189 - 0.178

(-)0.044" -0.051 (-)0.039” -0.037 (-)0.027" -0,021 0.187 0.200 0.136 0.152 0.116 0.128

- -0.008 - -0.008 - -0.009 - -0.021 - -0.024 - -0.026

- - 0.025 - 0.056 0.073 - 0.011 - 0.048 - 0.067

0.027 - 0.014 - - 0.009 0.075 0.064 0.079 0.064 0.077 0.063

(-)0.021’ -0.017 (-)o.oz5c -0.017 (-)0.023' -0.017 0.089 0.090 0.102 0.082 0.095 0.079

n Obtained from the observed proton coupling constants (Table 1) by means of Eqs. [6] and [7]. b Computed by means of HMO-McLachlan procedure (see text). ’ Sign suggested by theory.

ENDOR OF .%PHENYLCYCL[3.2.2]AZINE ANIONS 481

i-c-SPIN DISTRIBUTION

In Table 2, MO theoretical n-spin populations ,r” at the centers p of II@ and III@ are compared with their “experimental” counterparts ~7”. The latter values were obtained from the observed coupling constants a,, by means of the McConnell equation1

a&,, = QcHpy with 1 QcHl = 26.7 G Fl

for the ring protons (a) or an analogous relation’

a& = QccH, pp” with 1 QccH,I = 22.4 G [71

for the methyl protons (p). The assignment of aHp is such as to achieve an optimal correlation between py and pp”.

In the HMO-McLachlan modlel (21) used in the calculation of pp”, a Coulomb integral aN = a + 1.5p was adopted for the central nitrogen atom3 in II@ and III@, whereas for the 5-azanitrogen atom in 1110 the value of h, in the integral CI~ = CI + h&P has been varied between 0 and I .O (22). Figure 7, in which the n-spin populations calcu- lated for the centers ,LL = 1 to 7 are plotted vs. the parameter h,, demonstrates the high sensitivity of pplc to changes in &. Comparison of p:‘” with py suggests larger values h, for the radical anion III@ associated with Li@ (hA = 0.6) than for that having Na@ as counterion (h, = 0.4). All remaining Coulomb and bond integrals were not modified relative to their standard a and fi, respectively.4 The value of 1.0 was taken for the McLachlan parameter /z (21).

For II@, the good correlation between pp” and ~7” (standard deviation 0.010 gauss) supports the present assignment of the proton coupling constants. Although the corresponding correlation is still satisfactory for IIIe (standard deviations 0.014 and 0.013 gauss for IIIO-Na@ and III@-Li@, respectively), additional experimental and/or MO theoretical evidence would be desirable to corroborate the assignment made in Tables 1 and 2. Nevertheless, it is gratifying to note that this assignment allows one to rationalize the trends in the n-spin distribution on passing from 110 to IIIQ-Na@ and IIIe-Li@ .

These trends (an increase in py for p= 3 and a decrease for p = 1, 4, 6 and 7) are nicely reflected by j~ga’~l by changing h, from 0 (II@) to 0.4 (IIIO-Na@) and 0.6 (IIIO- Lie). Of special interest is the finding that the effect of replacing the counterion Na@ by Lie for III@ can be simulated by an enlargement of h,. It indicates that the alkali metal catitin is closely associated with the lone pair of the 5-azanitrogen atom. The introduc- tion of this atom into the cyc1[3.2.2]azine n-system thus accounts for the observed sensitivity of the n-spin distribution in the 1110 to changes in counterion and tempera- ture, as contrasted with the relative insensitivity found for 110. Moreover, since the proximity of an alkali metal cation enhances the electronegativity of the azanitrogen

1 [&I = s UHJ~]~~~‘~I = 26.56/0.996 = 26.67 G, where p is a proton-bearing center of II@.

’ /&cH3J = 22.4 G was derived from an ESR study of dimethyl-naphthalene radical anions (~a). 3 The choice of MN = K + 1.5/3 proved to be adequate in the series of cyclazine radical ions (90, I&). 4 This holds in particular for the Coulomb integral GC~ of the methyl substituted center 6 in III”.

Preliminary calculations showed that small modifications of a6 have a negligible effect on ,;I,.

482 GERSON ET AL.

atom, the larger value of hN for IIIo-Li@ than for IIP-Na@ points to a stronger associa- tion in the former than in the latter ion pair. This result is in accordance with the experience that solvation is unimportant for counterions attached to nonbonding electrons of heteroatoms and thus the association becomes closer in the order

0.X

0.1:

0.10

t

P 0.05

0.0

- 0.05

FIG. 7. HMO-McLachlan z-spin populations &‘I” plotted vs. the parameter h; of the 5-azanitrogen atom (cf. text).

Cs@ < K@ < Nao < Lie. (A reverse order is usually found for hydrocarbon radical anions (2b).)

The observed 14N coupling constants for the central nitrogen atom of 110 and 1110 can be used along with the z-spin populations ,Plc to test the relationship

a NO = QNP~ + QCN C PY, PI Y for which the parameters QN = 39 k 4 and Q cN = -6.5 k 2 G have been recently proposed in the cyclazine series (10b). Insertion of the pertinent values of Table 2 (,u = 8 : v = 2a, 4a and 7a) yields a N~=-O.4+0.1,-0.9+O.3,and-l.1~0.3asthe 14N coupling constants for the central nitrogen atom of II@, III@-Na@ and III@-Lie, respectively, in good agreement with the experimental data (Table 1).

ENDOR OF 2-PHENYLCYCL[3.2.2]AZINE ANIONS 483

CONCLUSION

The present work demonstrates how ENDOR spectroscopy contributes to an analysis of a complex ESR hyperfine structure. The information gathered from an ENDOR spectrum can be used not only to determine the number and exact magnitude of coupling constants, but also allows one to assign some of these values to specific sets of nuclei in the radical. It is then possible to confirm and/or complement the ENDOR data by a computer simulation of the ESR spectrum. Finally the experimental results can be compared with the spin populations calculated by means of conventional MO models.

Hopefully, the use of ENDOR coherence effect and intensity analysis will become a more generally applicable technique and thus a complex hyperfine structure will be even more readily unravelled by a combined ESR-ENDOR spectroscopy.

ACKNOWLEDGMENTS We thank Prof. V. Boekelheide, Eugene, for the samples of the 2-phenylcycl[3.2.2]azines II and III.

We are also obliged to Drs. K. P. Dinse and M. Plato for many helpful discussions. Last but not least, support of the Schweizerische Nationalfonds (project Nr. SR 2.824.73) and the Deutsche Forschungs- gemeinschaft is gratefully acknowledged.

REFERENCES 1. A. CARRINGTON, Quart. Rev. 17,67 (1963). 2. F. GERSON, “High Resolution E.S.R. Spectroscopy”, (a) p. 107; (b) p. 139. Wiley, New York, and

Verlag Chemie, Weinheim, 1970. 3. J. S. HYDE AND A. H. MAKI, J. Chem. Phys. 40, 3 117 (1964). 4. A. L. KWIRAM, Ann. Rev. Phys. Chem. 22, 133 (1971); K. MOEBIUS AND K.-P. DINSE, Chimia 26,

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