The rigidly fixed cis arrangement of the substituents in 1,2-disubstituted o-carboranes makes possible their mutual influence through space. A number of papers [1-6] describe qualitatively the steric influence of a substituent on one carbon atom on a reaction center situated at the other carbon atom in o-carboranes. To estimate the efficiency of this type of interaction of the substituents, we studied the dynamic :3C NMR spectra of 2-substituted derivatives of l-(ortho-isopropylphenyl)-o-carboranes, in which the isopropyl group provides an indicator test (see [7]) for the hindered character of the rotation of substituents on the second carbon atom of the o-carborane structure. Interesting results were obtained in the study of the 13C--{~H} DNMR of l-(ortho-isopropylphenyl)-2-benzyl-o-carborane (I) (see Fig. i). At +32~ theNMR spectrum of compound I in CH2CIz shows singlet signals from each carbon atom with chemical shifts (ppm) relative to TMS: 24.60(CH~), 24.82(CH), 41.22(CH2) and 84.9 and 85.8(Ccb). With decrease in temperature, the greatest changes in the spectrum take place in the range corresponding to the CH3 groups of the isopropyl fragment. At 0, --5, and --i0 ~ broadening of the signal is observed, and at--15 ~ two signals due to CH3 groups appear, with 6~ = 24.77 and 6~ = 24.26 ppm (A6 = 0.51 ppm). A6 did not show a strict temperature dependence: at--30 and--50 ~ A~ = 0.84 ppm, and at--60 ~ A~ = 0.71 ppm. An approximate estimate of the free energy of activation, carried out on the basis of the values of the dynamic broadening (A~/2 = 11.3 Hz, T = 263~ due to the retarded rotation in the molecule, gives the value AG~ = 13.6 kcal/mole. On the basis of a mechanical model and an approximate calculation by the method of atom-atom potentials [8], it may be suggested that this quantity characterizes the barrier to rotation about the bond of the Ccb substituent. The presence of two signals due to the CHs groups in the NMR spectrum at --15 ~ indicates unambiguously that the benzyl group spends a large proportion of time in the asymmetric position, and this makes the molecule chiral, and the CH3 groups anisochronous.