Association between lithium biphenylide and biphenyl

Volume 7. number2

CHEMICALPHYSICSLETTERS

15

October I.970

Received 2 September 1970

Addition of biphenyl to a dilute sdution of fithium biphonylide in meeirgltetrahyzkofuran produces a

second species with IitMbiumhyperfine couptingconstantaboutt;urlcethatof the first. Dependence ou the

relative intensities of the spectra of the tw species on concentration of biphenyi suggests an equitibrium between monomeric ion pair and its dimer with bipbenyi. Hyperfine splittings by ffie protons in ORly one biphenyt of the dimer are observed.

Recently Adams et al. El] have presented data

which suggest the ion pair of sodium with duros~~~quinon~ forms a dimer with dur~u~none in which the sodium hyperfinr toupling constant is

larger than in the monomeric ion pair. Their conclusions are based on the observation that in the presence of a high concentration (2M) of duroquirmne the spectrum coilapses to one in which the surviving sodium splitting is larger than in the monomer. We have observed a related phenomenon in our studies of electron transfer between biphenylide and biphenyl. A dikte (k:10-5 M) solution of lithium biphenyiide in methyl tetrahydrofuran at 2OoC exhibits the well known proton hyperfine chimings and further splittings into quartets by 7Li. The latter splitting is 0.068 gauss, Addition of neutral biphenyl produces a decrease in

intensity of the original spectrum, a broadening of the surviving part, and growth of a new spectrum with 0.132 gauss hyperfine s~$i&g

forms of the ion pair exist, the A form with aLi = 0.068 @USS, the 3 form tiritha~i = 0.132 gauss. A reacts with neutral ~~p~e~~~ with lrte constant z 5 x 103 ~-1 se&, the constant is the sum of the r&e constants for eLectran

transfer to biphenyi and for associatton ta the 3 form. B dissociates to biphenyl with rate constant less than I.5 x 106 ~ec-~. The ~~Lib~iu~ constant for assaciation between A and biphenyl to form B is about 200 M-l. The change in TLi

&

Fig, I.

[a) Part

of the spectra

of the

by

?Li. With increasing concentration of biphenyl the original spectrum (A) disappears and the lines of the second one (B) broaden. Portions of the spectra are shown in fig. P, The data are consistent with the following scheme. Two

a4iw.m

rt

A form. (bj Parr of the spectrumof the 3 form.

Volume 7, number2

CHEMICAL PHYSICS LETTERS

coupling constant with addition of biphenyl is not produced by bulk changes in the dielectric properties of the solvent; addition of large concentration Q2M) of benzene or cyclohexane to the

A form produces no change in spectrum.

The concentration dependence appeaFr: to req&e that the B form contains two biphenyl mo-

lecules; yet it exhibits hyperfine splittings from the protons in only one biphenyl. The time for passage of spin from one biphenyt to another in the complex must be longer than the reciprocal of the smallest hyperfine splitting which is pre-

212

15 October3970

served: t > lo-7 seconds.

We acknowledge the support of this work by the X-&or& Science Foundation, the National Institute of Health, and the Petroleum Research Fund of the American Chemical Society.

REFEREXCE fif RF. Adams, T. I,. Staplesand M. Szwarc, Chem.

Phys. Letters 5 Q970) 474.