Indirect spin-spin coupling in multiple-quantum magic-angle-spinning NMR spectra of quadrupolar nuclei.

In this Communication, we report the observation of J couThe indirect spin–spin coupling constant, J , is an impling in isotropic MQMAS spectra of half-integer quadrupoportant NMR parameter that can provide fundamental inforlar nuclei. In this experiment, the J coupling becomes obmation about molecular structure and chemical bonding (1– servable since the MQ effect not only refocuses the second4) . In general, coupling constants J involving quadrupolar order quadrupole broadening but also amplifies the size of (I ú 2) nuclei are difficult to measure in solution NMR the splitting. We have found that the B (S Å 2) MQMAS studies because efficient nuclear quadrupolar relaxation respectra of a solid borane–triphenylphosphite complex, sults in broad NMR peaks which obscure the fine structure (PhO)3P–BH3, exhibit two isotropic peaks, a doublet. The from J coupling (5) . In particular, molecular tumbling leads B MAS and MQMAS experiments performed at two magto fluctuations in the electric field gradients and hence effinetic fields confirm that the splitting of the doublet arises cient quadrupolar relaxation in isotropic fluids (solutions) . from the one-bond indirect spin–spin coupling between B In solid materials, molecular motion is generally highly reand P nuclei, J(B, P) . stricted, and quadrupolar nuclei consequently have much longer relaxation times than in solution. Thus, it is often For a quadrupolar nucleus with half-integer spin number advantageous to measure J couplings involving quadrupolar S , there are a total of S / 2 symmetric (/m , 0m) transitions nuclei by solid-state NMR. In fact, a large number of recent which are independent of the first-order quadrupole interacsolid-state NMR studies of I Å 2 nuclei have demonstrated tion. The success of the MQMAS method (13) is based the feasibility of measuring J couplings between spin-2 and upon correlations between these symmetric MQ coherences. In a typical MQMAS experiment, a particular symmetric quadrupole spins (6) . MQ coherence is excited and allowed to evolve for a time On the other hand, there is a paucity of reports concerning t1 . This MQ coherence is then transferred to the observable J couplings in solid-state NMR spectra of quadrupolar nuclei single-quantum (1Q) coherence of the central transition. (7, 8) because of the failure of magic-angle spinning (MAS) Since the second-order quadrupole Hamiltonians determinto suppress the second-order quadrupolar broadening. The ing the evolutions of the MQ and 1Q coherences are of resulting NMR spectra exhibit broad peaks with widths rangopposite sign, an echo will be observed at a time t2 after ing from 10 to 10 Hz, making it difficult or impossible to conversion of the MQ to 1Q coherence. The Fourier transdetermine values of J . Both dynamic-angle-spinning (DAS) form of the echo maxima as a function of the total evolution (9, 10) and double-rotation (DOR) (11, 12) techniques are time, t1 / t2 , will yield an NMR spectrum consisting of capable of averaging out the second-order quadrupole interisotropic peaks free of second-order quadrupole broadening. action; however, to the best of our knowledge, values of J Since the total evolution process of the echo consists of have not been reported in either DAS or DOR NMR studies. contributions from both MQ and 1Q coherences, the splitting Another technique capable of yielding isotropic NMR in isotropic MQMAS spectra is not simply the value of the spectra for half-integer quadrupolar spins is the recently deheteronuclear J coupling in the spin system. Using previous veloped multiple-quantum MAS (MQMAS) method (13) . notation (21) , it is readily shown that the observed splitting, Applications of this technique (14–20) have shown that JMQ, in isotropic MQMAS spectra is related to the heteroMQMAS spectra generally exhibit resolution more than an order of magnitude higher than conventional MAS spectra. nuclear J coupling by the equation