Measuring nitrogen quadrupolar coupling with C detected wide-line

Nitrogen is one of the most important elements in nearly all branches of chemistry. While N NMR has been widely used with the favorable NMR properties of a spin-1/2, the highly abundant N has been rarely studied. N NMR is capable of measuring quadrupolar coupling, which provides unique information on nitrogen electric-field-gradients (EFG) inaccessible through the spin-1/2 N. N quadrupolar coupling has been mostly measured in the past by NQR at zero magnetic fields. The measurements can benefit greatly from high magnetic fields to enhance spectral resolution and sensitivity. Nevertheless, the N energy levels of a spin-1 lack a central transition that half-integer quadrupolar nuclei have with zero first-order quadrupolar broadening. As a result, N spectra at high magnetic fields are often over several MHz wide making direct NMR observation and resolution among multiple nitrogen sites very difficult. Ingenious NMR techniques using overtone transitions 8–13 have been developed to avoid the first-order quadrupolar broadening at a trade off of low signal intensity and excitation efficiency of the 1 2 1 double-quantum transition. This communication introduces an indirect detection technique that overcomes the sensitivity and resolution limitations of N wide-line NMR for measuring nitrogen quadrupolar coupling under the high resolution magic-angle spinning (MAS) condition. The proposed experiment extends the recent development of HMQC type experiment for N–C or N–H correlation using a combination of J, second-order quadrupolar–dipolar and direct dipolar couplings. It replaces the pair of N frequency-encoding pulses by a long pulse and measures the C signal response as a function of scanning N radiofrequency (rf) over the range of the first-order quadrupolar coupling. This type of wide-line experiment was briefly discussed in the development of the C–N distance measurement technique, the rotational echo adiabatic passage double resonance (REAPDOR), but was not explored further. It will be shown here that by choosing the N pulse length and rf field properly such an indirect continuous wave experiment yields wide-line N spectra from which both the quadrupolar coupling constant and the asymmetry factor can be precisely determined. Fig. 1a shows the indirect N experiment through C detection. The pulse sequence does not have the conventional evolution time t1 of a two-dimensional (2D) experiment. Instead, wide-line N spectra are acquired point by point with a scanning N frequency uN. The C part of the pulse sequence consists of C–N dipolar recoupling with a spinecho after the cross-polarization (CP). We use here rotary resonance for recoupling because it is less susceptible to spinning frequency fluctuation than other multiple p-pulse recoupling sequences. With a rf field matching the spinning frequency u1 = ur, rotary resonance reintroduces both the C–N dipolar interaction and C chemical shift anisotropy under MAS. The rotor-synchronized spin-echo segment in the middle refocuses the C CSA part. Any N spin state change induced by the N pulse results in incomplete refocusing of the C–N dipolar interaction. Therefore this experiment maps out the frequency response of N spin population hSzi to the N pulse through the C signal intensity of the rotary resonance echo. Simulations in Fig. 1b show that the hSzi frequency response under MAS and the static N powder lineshape are very similar. The most obvious feature is the two edges near uzz and uxx. This feature can lead to the determination of all three principal components of the EFG tensor by utilizing uxx + uyy + uzz = 0. The indirect wide-line experiment is demonstrated here with a naturally abundant tripeptide Ala-Gly-Gly in Fig. 1c. The N spectra were obtained by measuring the difference ratio (S0 S)/S0 where S and S0 are the C signal intensities with and without the N pulse, respectively, in a manner similar to the rotational echo double resonance (REDOR) experiment. The dipolar evolution time t was fixed at 800 ms for one-bond C–N dipolar coupling between an amide nitrogen and nearby CO. The resulting spectra shows clearly the two expected edges near uzz and uxx and a fitting of this feature yields quadrupolar coupling parameters Cq = 3.48 MHz, Z = 0.368 for Ala1 and Cq = 3.35 MHz, Z = 0.373 for Gly2. The results have a higher precision than the ones obtained from second-order effects and are in good agreement with previous studies in ref. 14 and 29. The main advantage of this frequency-sweep method using a long pulse is the high efficiency over the HMQC-type experiment. Magic-angle spinning modulates the first-order quadrupolar coupling and brings the N rf irradiation onresonance during the long pulse. The rf action during the level crossing is brief but effective and it covers all spins despite that National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, FL 32310, USA. E-mail: [email protected]; Fax: +01-850-644-1366; Tel: +01-850-64-4662