Fourier transform multiple quantum nuclear magnetic resonance

Abstract

The excitation and detection of multiple quantum transitions in systems of coupled spins offers, among other advantages, an increase in resolution over single quantum n.m.r. since the number of lines decreases as the order of the transition increases. This paper reviews the motivation for detecting multiple quantum transitions by a Fourier transform experiment and describes an experimental approach to high resolution multiple quantum spectra in dipolar systems along with results on some protonated liquid crystal systems. A simple operator formalism for the essential features of the time development is presented and some applications in progress are discussed.The energy level diagram of a system of coupled spins 1/2 in high field is shown schematically in fig. I. The eigenstates are grouped according to Zeeman quantum number mi with smaller differences in energy within a Zeeman manifold due to the couplings between spins and the chemical shifts. For any eigenstate 10 of the spin Hamiltonian H (in frequency units)The single quantum selection rule 1 of the low power c.w. experiment and the one dimensional Fourier transform experiment arises because <i I lx I j) vanishes unlessSimple combinatorial considerations show that the number of eigenstates decreases as lmi I increases and the number of transitions decreases as lqiil increases. The highest order transition possible is the single transition with lql = 21 where I is the total spin. For a system of N spins 1/2, transitions up to order N are possible. Detection of multiple quantum transitions in c.w. experiments is well known. -4Extension to high order transitions is not promising, since the transitions observed are a sensitive function of r.f. field strength. This leads to difficult spectral simulations and experimental problems of saturation and sample heating. The alternative time domain experiment is the determination of multiple quantum transition frequencies by following the time development of multiple quantum coherences point by point. 5 -7 This work treats a class of such multidimensional

Curcacycline A — a novel cyclic octapeptide isolated from the latex of Jatropha curcas L

“…SSDI 0014-5793(94)01405-1 tional phase increment (TPPI) method [12,13] was used for t~ amplitude modulation, and 512 free induction decays (FIDs) of 1,024 complex data points each were collected. The spectral width was 5,000 Hz in both dimensions.…”

Time-Domain Multiple-Quantum NMR

“…These illustrative examples are, as a matter of convenience, largely from the work of the Berkeley group directed by Alex Pines. Figure 1 is a proton multiple quantum spectrum of benzene partially oriented in a liquid crystal (14)(15)(16). It is presented here to serve as a point of reference for introduction of the sections to follow.…”

Multiple-quantum NMR studies of spin clusters in liquid crystals and zeolites

“…Spin labeling which is independent of variations in resonance offset may be achieved by use of a phase shift of the rf carder frequency during the evolution period [33]. Such a phase shift is equivalent to an rf pulse along the z axis and contributes a time independent term to the Hamiltonian:…”

“…Fourier transformation of S(tl,t 2) along the t1 dimension and projection to the t2---0axis generates the multiple-quantum spectrum [33]. A MQ spectrum contains many more lines than a standard single-quantum spectrum, ali overlapping one another without the use of TPPI or a resonance offset to separate the transitions according to the order n. This is due to detection occurring within the rotating frame, which removes the frequency differences caused by the Zeeman interaction.…”