Volume 5, number 8 CHEhIICAL PHYSICS LETTERS 1 June 1570

HYPERFINE INTERACTIONS WITH CARBON-13

IN PARAMAGNETIC COMPLEXES

R. Z. SAGDEEV and Yu. N. MOLIN Institute of Chemical Kinetics and Combustion.

Siberian Division of the USSR Academy of Sciences, Novosibirsk, USSR

Recei-ied 16 January 1970

The isotropic contact shifts of carbon-13 in series of paramagnetic complexes of nickel acetylacetonate \vith organic lisands have been measured b.y means of double l&I-13C heteronuclear resonance. The con- clusions hbout the mechanisms of hyperfine interactions with 1% have been made from the analysis of ob- _ . tained data.

The NMR method has been widely applied to the investigation of paramagnetic complexes. Isotropic contact shifts give valuable information about the nature of metal-ligand bonds and spin density delocalisation onto the ligands. Most re- search in this area has been performed using proton magnetic resonance. However, in many cases it is necessary to have information about hyperfine interactions with “other” nuclei. In this letter we report our data on I3C contact shifts in the complexes of acetylacetonate Ni(I1) with some organic ligands (see table 1). Until now there have been no reports of NMR data on carbon-13 hyperfine interactions in paramagnetic species.

All of the experiments were performed using heteronuclear double resonance ‘H-(13C)equip- ment with a commerical JNY-4H-100 spectrom- eter. Tinis equipment has been described in [l].

The paramagnetic shifts 6p in the complexes were determined by means of the extrapolation of the observed contact shifts to a 1:2 metal to ligand ratio. This method can be used in the case of fast ligand exchange between the first coordina- tion sphere and solution. The fast exchange con- dition has been demonstrated in all cases in our work by means of variable temperature measure- ments. The measurement of 13C -paramagnetic shifts (fig 1) has been made for methyl carbon atoms with 13C in natural abundance.

The extrapolated values of paramagnetic Shifts in complexes op, coupling constants aC and spin densities pC are given in table 1. The coupling constants QC for ligands 1 and 2 can be discussed in terms of the theory of carbon-13 hyperfine splittings developed by Karplus and Fraenkel [2,3].

+6@-

2 u +60-

I , I , ,

2 4 6 8 10 I I ,

12 14 La.103

D-1

Fig. 1. The observed contact shifts piotteci against the metal to ligand molar ratio.

CH3 CH3 A comparison of the constants ac and uEI for these ligands shows that the hyperconjugation model appears to give good agreement with the experimental distribution of spin density on the methyl group. This is in agreement with the re- sults of Strauss and Fraenkel [3] obtained from ESR data on ii -radicals. From our experiments we have fou$d a value of the spin-polarisation constant Q,,, N - 14G that is different from that

471

Volume 6, number 8 CHFXICRL PHYSICS LETTERS 1 June 1970

Table I

P~ra~~~etic shifts. coupling constoots and spin densities for methyl carSon atoms in complexes of Ni(acaq2 with some organic ligands (T= 27’9

I - 36 - 0.046 - 3.3

&

,CH3

4 H,N-C -CH,

’ CH3

+ 20

- 100

+ 200

+ 66

40.026

- 0.13

+ 0.26

+ 0.007

+ 1.86

- 3.4

-1-18.8

+ 5.5

calculated by KarpIus and Fraenkel (Q&J% =

= 2 20.9G f3j). The negative value of Q& ob-

tained from our results is the first reported ex- perimental verification of the theoretical calcula- tions 124.

It is very interesting to note that for all the systems +&died a linear relationship exists between the spin density PC on 1% and pa on the W in the same molecular position (for example

on methyl - 1% and N-H-hydrogen in 3)” BY means of the linear relationship between PH and pc which we have obtained (fig. 21, it is possible to estimate values far crc from IH NMR data.

Our results indicate, that the‘mechanisms of hyperfine interaction with carbon-13 are the same as for 1H. The negative spin density on carbon N-13C can be explained by means of the spin polarisation mechanism and the positive pc on N-C-l% by hyperconjugation. In our opinion the similarity of the spin densities derived from

472

Volume 5. number 8 CHEMICAL PHYSICS LETTERS 1 June 2970

1~ and 136 NMR data (Pa a PC) means. that only one of the Sp3-orbit& oi the carbon atom main- &ins its aptn density, whHe the other three or- bitals have no resulting upaired de&ran&

REFERENCES

[I) J. H. Allen, J.K. Beeconsali and D. W. Turner, J. Sci. Instr. 41 (1964) 673.

[2] M.Karplus and G. K. Fraenkel, J, Chem. Phys. 35 f1961) 1312.

p] EL L, Strauss and G. EC. Fraenkel. J. Chcm. Ph_p-s_ 35

fl361) 1738.

Fig.2. The relationship between the spin density pc on 13C andpH on 1H in the same molecular position.

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