MOLECULA R PHYSICS, 1998, VOL. 94, NO. 3, 565±569 Anisotropy of molecular reorientation and geometrical information as determined from short and long range 13 C± 1 H spin cross-relaxation rates By P. MUTZENHARDT 1 , O. WALKER 1 , D. CANET 1 , E. HALOUI 2 and I. FURO  3 1 Laboratoire de Me  thodologie RMN, Universite  H. Poincare  , BP 239, 54506-Vandoeuvre-les-Nancy, France 2 De  partement de Chimie, Faculte  des Sciences de Tunis, Campus Universitaire, 1060-Tunis, Tunisia 3 Division of Physical Chemistry, Department of Chemistry, Royal Institute of Technology, SE-10044 Stockholm, Sweden ( Received 2 February 1998; accepted 17 February 1998) This study deals with all available 13 C- 1 H cross-relaxation rates in a rigid medium size mol- ecule including short range (one bond) and remote (long range) dipolar interactions. They are obtained by the heteronuclear Overhauser eect spectroscopy (HOESY) experiment modi®ed in such a way that correlations between equivalent sites becomes accessible. It is shown that the whole set of data provides not only the three rotational diusion elements characterizing a fully anisotropic reorientation but also some interatomic distances which can be compared with those determined from quantum chemistry calculations and/or from crystallographic data. 1. Introduction One of the most useful quantities provided by nuclear magnetic resonance (NMR) is undoubtedly the direct interaction between magnetic moments associated with nuclear spins (also called dipolar coupling). In an aniso- tropic medium, through its dependence in r - 3 ( r being the internuclear distance), it yields directly the relevant piece of structural information. By contrast, in solution, the dipolar couplings are averaged to zero by molecular motions, and thus do not give rise to any splitting in the NMR spectrum. Nevertheless, dipolar interactions are involved in relaxation processes and constitute one of the mechanisms entering classical relaxation times T 1 and T 2 . Although extracting their contributions from the latter parameters is not an easy task, there exists a unique relaxation parameter, the so-called cross-relaxa- tion rate, which depends solely on the dipolar mechanism and which can be accessed by appropriate experimental methods, the most popular ones being related to the nuclear Overhauser eect (NOE). In particular, the two-dimensional experiment, dubbed NOESY, based on proton±proton cross-relaxation rates, opened the way, some twenty years ago [1], to the structural determination of large biomolecules such as proteins. Meanwhile, smaller molecules as well were investigated still by means of homonuclear cross-relaxa- tion rates [2], most of the time via selective 1D experi- ments. Heteronuclear cross-relaxa tion rates (e.g., 13 C± 1 H or 15 N± 1 H) have been widely used also for deriving parameters associated with molecular motions [3]. It must indeed be borne in mind that any cross- relaxation rate can be expressed as r - 6 f ( ¿ eff c ) , f ( ¿ eff c ) being a sometimes rather complex function of an eec- tive correlation time ¿ eff c , describing the motion of the vector joining the two interacting nuclei (the relaxation vector). Most determinations of heteronuclear cross- relaxation rates deal with directly bonded nuclei whose distance is, at least in principle, known well. Less atten- tion has been paid to what we shall denote in the fol- lowing as remote interactions [4], i.e., between heteronuclei several bonds apart. It is the purpose of this paper to show that remote interactions, combined with short range ones, are able to provide additional assessment of anisotropic tumbling and/or essentially structural information if molecular reorientation can be characterized fully by the sole short range inter- actions. The relevant data are obtained by the 13 C± 1 H two-dimensinal HOESY experiment [5±8] without the central carbon-13 p pulse in the evolution period [9]. In the standard experiment, this pulse yields, in the proton dimension, spectra J -decoupled from carbon-13 ( J is the carbon±proton indirect coupling). Omitting this 0026±8976/98 $12 . 00 Ñ 1998 Taylor & Francis Ltd.