J. Phys. Chem. zyxwvu 1994, 98, zyxwvu 12849-12855 12849 zyxwvutsrqp Multiple-Pulse COSY NMR Spectroscopy of Oriented Molecules in Thermotropic Cholesterics P. Lesot, F. Nielsen, J. M. Ouvrard, and J. Courtieu* Laboratorie de Chimie Structurale Organique, ICMO, URA CNRS no. 1384, Universitg de Paris-Sud, 91405 Orsay, France Received: June 4, 1994; zyxwvuts In Final Form: September 22, 1994@ The coherent reduction of anisotropic NMR interactions in molecules oriented in a thermotropic cholesteric phase, using a multiple-pulse COSY experiment, is described. The technique, which directly modifies the spin part of the nuclear spin Hamiltonian, allows the transformation of second-order spectra into first-order spectra. An experimental study of the evolution of the chemical shift and dipolar reduction factors is presented. The effects of the finite pulse duration in the multiple-pulse sequence are discussed, and explicit equations for the anisotropic interactions which take into account the pulse widths are obtained. The technique in cholesteric solvents is illustrated by using two specific examples for AB and ABC type of spectra: 2,6- dichloro-3-nitropyridine and 2-cyanofuran. Introduction Cholesteric solvents are optically active liquid crystals which allow for the NMR visualization of enantiomer^.'-^ The observation of each enantiomer is obtained through a doubling of the spectra, from which detailed structural information can be extracted. Unfortunately, the observed 'H-NMR spectra are generally second-order spectra and very complicated to decipher even at high magnetic Therefore, it is interesting to find an effective method for simplifying the proton spectra of molecules dissolved in such media, in order to fully exploit their analytical potential. The fist approach to simplify the analysis of the anisotropic 'H-NMR spectra was achieved by averaging the anisotropic interactions, by rotation of the sample around an axis perpen- dicular to Later this approach evolved to the variable angle sample spinning (VASS) technique, introduced by Hom- reich,s in which the sample is rotated quickly around an axis tilted at an angle 0 to the magnetic field. This technique showed great potential with uniaxial fluid liquid crystal such as nematics?-" Nevertheless, this technique fails in cholesteric or chiral smectic liquid crystal solvents due to the particular behavior of the director field in the magnetic field BO.l2J3 Indeed, cholesterics are known to be twisted nematics, which under the influence of chiral centers give rise to a director field of helical ~tructure.'~J~ When the samples are spun about an axis tilted from Bo, the helicity axis h tends to align either parallel (Axm zyxwvutsrqpo < 0) or perpendicular (Axm > 0) to the rotation axis. Consequently, the directors n are distributed in planes which are perpendicular or parallel to the spinning axis and are not homogeneously oriented relatively to zyxwvut BO, as shown in Figure 1. In both cases, the spectra for the dissolved molecules have the appearance of a powder pattem.16-18 Only few experiments, where the anisotropic interactions were averaged to zero by using a fast magic angle spinning (MAS) technique, have been reported.lg Another way to reduce the anisotropic spin interactions consists of using a multiple-pulse sequence which results in a coherent reduction of the spin Hamiltonian through spin reorientation.20,21Recently, this laboratory has reported the coherent reduction of dipolar splittings through a 16-pulse @ Abstract published in Advance ACS Abstracts, November 1, 1994. 0022-365419412098- 12849$04.50/0 a b h A h Bo R z+ ,y X zyxwvutsrqponmlkjihgfedcbaZYXW X J Figure 1. Orientational behavior of director n of a Ax,,, 0 cholesteric phase in the magnetic field upon rotation around an axis R parallel to Bo (a) and tilted by an angle zyxwv B from Bo (b). h is the cholesteric helical axis. sequence, named flip-flop-16 (FF-16).22 Furthermore, it was demonstrated that the technical difficulties generally encountered in multiple-pulse experiments synchronized with the sampling of the magnetization can be avoided if the pulse train is applied during the tl period of a simple COSY experiment, as diagrammed in Figure 2. Under these conditions a normal one- dimensional spectrum is expected in zyx YZ and a reduced spectrum along VI, following a tl evolution under the theoretical zero- order FF-16 average Hamiltonian expressed in eq 1: . %& represents the chemical shift Hamiltonian, the scalar coupling Hamiltonian, and zyxw 6 the dipolar coupling Hamiltonian. KCS and KD are the chemical shift and dipolar reduction factors, respectively. In the ideal case of 6 pulses, these reduction factors can be expressed as 0 1994 American Chemical Society