The PASADENA or PHIP effect originates from the breakdown of the parahydrogen symmetry when the two protons are found in magnetically inequivalent positions after hydrogenation. As a result, a signal enhancement of up to 104 in the proton NMR spectra can be observed. We introduce three polarization transfer techniques, namely PH-INEPT, PH-INEPT+, and INEPT(+π/4), that are highly effective to record in situ hetero NMR spectra of parahydrogen-labeled compounds. A complete product operator treatment as well as experimental results for 13C and 29Si have been given. With these sequences we obtained a signal enhancement of 500 which allows in situ NMR investigations of hetero nuclear systems at natural abundance.

Sensitivity and selectivity pose constant challenges in NMR studies of surfaces. 1 The number of nuclei residing on the surface of a material may be too small for NMR observation, and potentially detectable signals may not be discernible from the spectrum of the bulk material. Some progress has been achieved in recent years by the use of laser-polarized xenon, either by freezing the gas onto the solid surface followed by high-field cross-polarization (CP) 2 or by nuclear Overhauser cross-relaxation from adsorbed polarized xenon, 3 a process dubbed SPINOE 4 (spin polarization induced nuclear Overhauser effect). For both approaches, experiments are typically performed with one batch of isotopically enriched laser-polarized Xe, with subsequent polarization decay either by relaxation in the adsorbed states or by radio frequency driven transfer of the spin polarization. In either case, loss of polarization prevents signal accumulation and the multiple pulse manipulation of spins necessary for spectral resolution. A method for rapid production and continuous delivery of polarized xenon gas to surfaces would clearly be advantageous.In this communication, we report the use of a fast optical pumping process, combined with a closed gas-circulation NMR probehead designed to deliver a continuous flow of laserpolarized Xe to an Aerosil surface. The technology for the production of 129 Xe with nuclear spin polarization several orders of magnitude higher than thermal Boltzmann levels is now well established. 5 In the present study, we exploited the pressure broadening of the rubidium D1 transitions, and recently available high-power diode laser arrays, to pump much denser Rb vapors ([Rb] ) 4 × 10 14 cm -3 ), thereby enabling spin-exchange time constants on the order of tens of seconds. 6 The high-pressure optical pumping cell was connected to a gas recirculation system capable of delivering a continuous stream of the polarized xenon gas to the NMR sample, as shown schematically in Figure 1. This new methodology allowed the observation of difference SPINOE's that selectively highlight surface spins, thereby enhancing both sensitivity and selectivity for surface-resolved NMR spectroscopy.Aerosil300 was obtained from Degussa Inc. and is reported to have a surface area of 300 m 2 /g with about 2.5 hydroxyl protons/nm 2 . Sample (150 mg) was packed into a 10 mm U-shaped sample tube placed in a double-tuned probe ( 1 H: 178.02 MHz; 129 Xe: 49.24 MHz) built on the basic principle of lumped circuit elements published previously. 7 The sample tube and the optical pumping cell were constructed of glass and subsequently glued to 1 / 8 in. copper tubing. The optical pumping cell was placed in the 150 G fringe field of a 4.2 T superconducting magnet; this assembly was then attached to the recirculation and vacuum pumps, as well as the sample probe. This entire assembly was evacuated to below 10 -5 Torr for several hours to remove physisorbed water and oxygen; during evacuation, the sample was kept at 50°C. The evacuated system was then fi...

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In this paper we describe the construction of an apparatus for optical pumping of 129Xe in a circulating gas system connected to a probe for static high-field NMR. The optimal working conditions for the optical pumping under flow are explored, yielding a continuous gas flow with a steady-state spin-polarization of about 2%. It is shown that on various types of high surface area materials a steady-state of adsorbed laser-polarized xenon can be reached with polarizations of about 0.3%. NMR spectroscopy of this layer is greatly facilitated due to the renewable magnetization under the continuous-flow conditions allowing unprecedented fast detection of two-dimensional NMR experiments with laser-polarized Xe. The steady-state spin-polarization of adsorbed 129Xe gives rise to cross-relaxation to surface nuclei such as 1H and 29Si. A direct spin-polarization-induced nuclear Overhauser effect (SPINOE) to heteronuclei is observed with an enhancement factor of about 20−30. A method is presented for simple modulation of the spin-polarization direction in the fringe field of the high-field magnet; this is exploited to obtain difference SPINOE spectra to highlight only the surface nuclei. Furthermore, a transfer via cross-polarization (CP) from adsorbed Xe to surface protons is demonstrated to select surface groups with reduced mobility compared to SPINOE detection.

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