Hyperfine splitting in the ESR spectra of random oriented alkali earth biradical chelates

Volume 39, number 2

HYPERFINE SPUITING

IN THE ESR SPECTRA

OF RANDOM ORIEI’dTED ALKALI EARTH BiRADICAL

of Pbysicd

Chknristry,

University

of Padova,

CHELATES”

.

Carlo CORVAJA and Luigi PASIMENI Institute

1.5 April 1976

CHEMICAL PHYSICS LETTERS

ItQIy

Received 14 November 1975

The ESR spectra of riejd ,$ass solutions of biradicals of glyosal diimine anions and alkali earth cations were recorded. The spectra couId be computer simulated only by adding to the spin hamiitonian 3C=g@ N-S+ D[S$--$S(S+l)] a hyperfine term due to the anisotropic interaction of the nitrogen nuclei_ Two different modeis were tested: the first one with the two radicals in the same plane and the second one with the two radicals perpendicular; agreement with the experiment was found for the htter.

1. Kntroduction

By reaction of alkali metals with aromatic compounds in ethereal soIvents, free radical ion pairs are formed, whose structure is usually investigated by ESR [I]. Structural information is mainly extracted from the hyperfine structure due to the magnetic nuclei of the aromatic part and to the alkali counter ion. A fairly good description of the unpaired electron distribution is thus obtained. The analogous reaction of alkali earth metals gives rise to triplet biradical species: ionic aggregates having two radical anions bonded to a metal dipositive cation [3,3] _These species give an ESR spectrum when dissolved in a rigid glass matrix. The ESR

spectra of randomly oriented triplet state molecules [4] give valuable structural information through the measure of the zero field splitting parameters D and E, which are directly related respectively to the mean distance of the two unpaired electrons and to the geometry of the aggregate. Usually the hyperf’ine structure is unfortunately lost in the large linewidth. In the present paper we present a case in which this does not occur. Triplet biradicals produced by al* This work was supported by the italian National Rescarcb Council (C.N.R.) tbrougb its Centro di Studi sugli Stati Molecolari radicalici ed eccitati.

klili earth reduction of bis N-t-buty1 glyoxaldiimine (CLI) give an ESR spectrum in the Am = +- 1 region which cannot be computer simulated using the spin

hamiltonian JC=gflWS’t

DS;

iE(S&S$).

(1)

An additional hyperfine term is necessary with which the agreement between calculeted and experimental spectra is excellent. It should be pointed out Lhat hyperfine structure in the APIA= I 1 lines has been observed only in a very few cases [S, 61.

2. Experimental Glyoxaldiimine (GLI) was prepared by reacting a 30% aqueous solution of glyoxal aldehyde with tbutyl ammine in water. The white precipitate was filtered and recrystallized from ETOH-Hz0 in 1: 1 ratio [7]. The solvent, 2 methyl-tetrahydrofuran (MTHF), was distilled from calcium hydride and stored under vacuum over potassium anthracenide. Solutions of triplet biradicals (GLI),Me were obtained by shaking the appropriate alkali earth mercury amalgam with an approximately IO-* M solution of GLI. The reaction is fast with SKand Ca, while with Mg and Zn it requires a period of several hours. With Ba amalgam it does not occur at all. All the manipula-. 261

d

il JE*EJES.

*i*h

th variable tempers-

3. The

,~~~,$ifliX,‘,~a~d Z,the principal 8xxcsof the cfecfron/:‘z?+$+Oii’ dipoiar. interaction. :. Aldompitet’simulation, with the absorption first I’;-: l’,‘~~:; ,,:y,, der!ytivq curve norrr$lzed fn a way to fit the peak ’ t;i, pea~.‘!intensity’.~ of the inner lines, is shown at the ;‘: .&p.of.fi’g 1. The ~~n~d~ was taken from the ex,,..,.~t&iaal lines. Clearly the inner @es h8Ve to be much ~~,,~~~~de~~,,~g~fiiies ar&duc to triplet rno~~ul~ ~~&&&&I with the magnetic field perpendicular to the ,’ ~~i~cip~ tiis Z of the dip&r ~tera~ti~~. .The;,inclusion,of a term E(S$-G$) in the hamil1: t&an does not improve the fit, w;t$e a large one 1,carises the internal Lines to be split into two compo;‘:r&ts, ‘. ’ ~~o~denjng of the absorption lines of triplet specios could be caused by a residual motion of the moleides 19, tOjj. However the effect of this motion is to bic$sdcn the external lines more than the inner ones _. [9]‘1+ corifrast with our observations. Thus a~otro~~~ ~~filrps!finc splitting is the only cause left for the expla’riatfon. ‘? .,,.p compfete hamiltonian has the form @+W%s+

asi

+ A??(&-$2,) + x,,

f

(31

“’ ‘,,ln order to write the explicit form of XW some ad:d~#~~;c?nsjderations are neces~~.

Fig. 1. Experimental &~~ter B) and computer simulstcd ESR spectra of random otkntcd Mg(CrLIR)~tripkt birzdicals. ‘f?~e spectrum $t the top (Al was calclllated wlthout the nitrogen splitting, spectrum C was c44nted with an anisbrropic nitrogen couplfng as g&en in the text, Both spectrtt bavz been nOrMa&Cd in Such a W3y tbt the iRRer trip&t lines have the same peak to peak “intensity” as the experimental ORC.The centml fine is due ta S = f mol+cuIcss.

anions and an alkali earth cation, the unpaired electrons are mainly focalized one for each organic radical, the spin distribution in the latter being approx~mafely that one has in the alkaIi metal ion pair. A recent investigation showed that the latter possess a plrsnar structure with the metal ion chelated by the two nitrogsns, moreover the distribution of the unpaired electron on the A system was inferred from the proton hyperfine splitting [ 1 I J. The McConnell equation gives the spin density on the carbon ITorbit&, white that on the nitrogen is obtained by taking into account that z:p,” = f. The values thus obtained for RGL,l using Qc~ = -23.7 f 121 are & = 0.191 and p$ = 0.309. Small differen~e~,fro~ the& values arc ex$cte+! inour system because: df th+iff&nt perturbation of the doubly ” _:,_.: “.,_ “. ._

).”:

.:.

,‘

!

:, .‘. ,. .., : ” ,. .,,“.

: .,.‘. .’

.‘. ,::: .‘

..‘

Volume 39, number 2

-CHEMICALPHYSICSLETTERS

-positive cation with respect to the monopositive one, on the P system. The 14N isotropic hypcrfine splitting aN for the alkali metar ion pairs is in the range 5.3-5.7 G and a similar value is expected for MgGLI. Concerning the anisotropic coupling of the 14N nuclei, when there is a high spin density on the nitrogen, the main contribution is the ‘“Iocal” one. It has axial symmetry around the axis of the rr orbital and its principal values are f:I>, 141: T,N= 33.6 pa ,

T_p =-16.8~;

1

zc$ = s, ii [T,q,(3cos%-1)

f 4 [T&(3cos%l) T&)

sin2rPcos2p] 1!3 +

(a)

(4)

We therefore assumed for the 14N hyperfine tensor components tentative values of Ap = tzN + Tt = 15 G and A? = 0. The indexes 11and 1 refer of course to the direction of the rr orbital. For the protons the anisotropic coupling can be calculated according to the McConnell and Strathdee formula [ 1S] from the foregoing spin densities. The calculated principal values are: 3.3, 2.11 and -1.58 G. Since the anisotropy is here mther smaller than that of the nitrogens, we have neglected it, by assuming that its contribution may be included in the linewidth parameter. For a-magnetic field in a direction z with polar angles 9 and fp with respect to X, Y.2 the anisotropic hyperfine hamiltonian can be written as:

f (Tj&-

15 April 1976

x’ ,

where T&J and T&J (U= X, KZ) are the principal values of the nitrogen tensor for the nuclei in the two radicals and If, 1: the z component of their total nuclear spin. In Jt’ all the pseudo and non-secuiar terms are collected. Because of the smallness of Xhf with respect to 3CZeemanand ZKDwe can consider it in the frame of first order perturbation theory and so the effect of ?C’ was neglected. Furthermore we have to point out the relative orientations of the principal axes of the hyperfme tensors of the 14N nuclei of the two radicals and their orientations in the system. Both of them depend on the structure of the dimer.

Pig. 2. MolecuIar models for M(GLIR)2 systems. (I) All the atoms lie in the YZ plane. The direction X is paraIlel_to the x orbitals of both radicals A and B. (II) The atoms of radical A lie in the YZ plane; those of radical B in the X2 plane. The direction X is parallel to the TC orbitals of radical A and perpendicufar to those of radical B. The reverse is true for Y. 2 is perpendicular to thu inorbitals of both radicals A and B.

Two models are worth to be considered for Me (GLI), : one with the two radicals in the same plane

(model I) and a second one with the two radicals in two perpendicular planes with a tetrahedral coordination of the metal cation (model II). In both cases the principal axes of the nitrogen hyperfine tensor coincide with those of the electron-electron dipolar interaction. It should be noted that model II possesses a twofold screw axis and therefore E = 0 while model 1 could have a non-vanishing E. Wowever since the latter depends on the relative distance of the two radicals it could be accidentally smaI1 for some distances and by inspection of the spectrum we cannot rule out ‘
253

V&ma 39,

hble I >?og~tic

pxlramctcrs of the M(GLIR)z

cation

-7 %I ti Sl while

systems (E$USS)

D

l/T?

T

222.0 207.6 187.5

4.31 10.0 10.8

11.0 11.5 Il.9

for I1 one has:

T)x = Ts *

T”XX-q?-

@Y=

B TYY-TU,

T,.

T$‘.. = Tl,

A _ T2.z - TL -

were computer simulated for the two cases by changing the two parameters l/T2 and T. The simulations were performed by calculating the expectation value for each value of the magnetic field with the Alexander’s formalism of density matrix [ 161 according to the-method outiined in refss. [17, 181. The curve at the bottom of fig. 1 was obtained by usin; model II with the nitrogen hyperfine couplings which give the best fit. Model 1 @es il rather poor fit. The magnetic parameters obtained from‘the calculations are collected in table 1 and they show that there is an increase in Tas the size of the metal ion increases. A simiIar increase is observed for the ;ilkaIi metal GUR ion pairs [I I ] _Tllis is due to a larger polarisation of the II electron distribution by the smaller ions, Spectra

whose effect is to increase the charge density on the nitrogen and to decrease its spin density. Asimilareffect has been observed for the fluorenone

ketyls through the variation of the carbonyl 13C hyperfine coupling and of the g factor [ 191. Finally,

264

I.5 April 1976

CHEMICAL PHYSICS LETTERS

number 2 3

the decrease in d is due to the fact that the larger the cation size, the more distant are the two radicals.

References J-L. Sommcrdijk and E. de Boer, in: Ions and ion pairs in organic reactions. Vol. 1, cd. $1. Szwwc (Wiley, New York. 1972) ch. 8. 121 1-M. Brown, S.I. Weissman and LC. Snyder, J. Chem. Phys. 42 (1965) 1105. [31 M. Brustolon, L. Pasimeni and C. Corvzja, Chem. Phys. Letters 7-l (1973) 194. l41 E. Wasserman, L.C. Snyder and W.A. Yager, J. Chem. Phys. 41 (1964) 1763. r51 E. Wasserman, A.hl. TrozzoIo, W.A. Yager and R.W. Murray, 3. Chem. Phys. 40 (1964) 2408. I61 3. hlispelter, J.Ph. Grivct and 3-M. Lhoste, Mol. Phys. 21 (1971) 1015. [71 P. Qopath and A. von Zelewsky, Helv. Chim. Acta 55 (1972) 52. PI P. Ciapath and A. von Zelewsky. Helv. Chim. Acta 56 (1973) 980. PI J. Norris and S.I. Weissman, J. Phys. Chem. 73 (1969) 3119. 1101 S-I. Weissman. Accounts Chem. Res. 6 (1973) 233. IllI L Pasimeni and C. Corvajia, to be published. rm hf.M. McConneU, 5. Chem. Phys. 24 (1956) 762. [13l E. Kochansky and G. Berth&. in: La structure hyperfine des atomes et des molecules, Colloque C.N.R.S., Paris, 1966. 1141 8. Lamotte, Thesis, University of Grenoble. France (1968). [ 151 H.Sf. McConnell and Strathdee, Mol. Phys. 3 (1959) 129. [ 161 S. Alexander, J. Chem. Phys 37 (1962) 967. [ 171 F-W. Pijpcrs. hi. van Willigen and J.J. Th. Gerdjng, Rec. Trav. Chim. 86 ( 1967) 5 1 l_ [ 181 M. van Wihipen, Thetis, University of Amsterdam, The Netherlands. [I91 T. Takeshim and N. Hirota, 3. Am. Chem. Sot. 93 -[iI

(1971)

6421.