Generation and interrogation of a pure nuclear spin state by parahydrogen-enhanced NMR spectroscopy: a defined initial state for quantum computation.

We describe a number of studies used to establish that parahydrogen can be used to prepare a two-spin system in a pure state, which is suitable for implementing NMR quantum computation. States are generated by pulsed and continuous-wave (CW) UV laser initiation of a chemical reaction between Ru(CO)(3)(L(2)) [where L(2) = dppe = 1,2-bis(diphenylphosphino)ethane or L(2) = dpae = 1,2-bis(diphenylarsino)ethane] with pure parahydrogen (generated at 18 K). This process forms Ru(CO)(2)(dppe)(H)(2) and Ru(CO)(2)(dpae)(H)(2) on a sub-microsecond time-scale. With the pulsed laser, the spin state of the hydride nuclei in Ru(CO)(2)(dppe)(H)(2) has a purity of 89.8 +/- 2.6% (from 12 measurements). To achieve comparable results by cooling would require a temperature of 6.6 mK, which is unmanageable in the liquid state, or an impractical magnetic field of 0.44 MT at room temperature. In the case of CW initiation, reduced state purities are observed due to natural signal relaxation even when a spin-lock is used to prevent dephasing. When Ru(CO)(3)(dpae) and pulsed laser excitation are utilized, the corresponding dihydride product spin state purity was determined as 106 +/- 4% of the theoretical maximum. In other words, the state prepared using Ru(CO)(3)(dpae) as the precursor is indistinguishable from a pure state.

Preparing high purity initial states for nuclear magnetic resonance quantum computing.

Here we demonstrate how parahydrogen can be used to prepare a two-spin system in an almost pure state which is suitable for implementing nuclear magnetic resonance quantum computation. A 12 ns laser pulse is used to initiate a chemical reaction involving pure parahydrogen (the nuclear spin singlet of H2). The product, formed on the micros time scale, contains a hydrogen-derived two-spin system with an effective spin-state purity of 0.916. To achieve a comparable result by direct cooling would require an unmanageable (in the liquid state) temperature of 6.4 mK or an impractical magnetic field of 0.45 MT at room temperature. The resulting spin state has an entanglement of formation of 0.822 and cannot be described by local hidden variable models.

Dynamics of beta-cyclodextrin hydrate: solid state 2H NMR studies.

The 2H NMR spectra and spin-lattice relaxation times of powder samples of C-deuterated beta-cyclodextrin (CD) hydrate (beta-CD-2,3,6,6-d28.11H2O) 1 and O-deuterated beta-CD hydrate (beta-CDd21.11H2 O-d22) 2 were measured in the temperature range from 160 to 295 K at 46 MHz. Information about the degree and time scales of dynamic disorder processes was obtained by comparing the powder pattents with computer simulations and analyzing the multi-exponential recovery curves. The data on 1 provide evidence for small-angle reorientations of the C-D groups with a correlation times tau c approximately 10(-7)s at 295 K and suggest that the beta-CD macrocycles do not move as a rigid entity. The NMR responses of 2 at 295 K show two components and are consistent with the hydroxyl O-D groups undergoing 2-fold reorientational jumps with tau c approximately 10(-8) s and with the water molecules exchanging between all sites and a significant fraction performing reorientational jumps with tau c approximately 10(-8)s. Below 235 K, at least three components are evident two were associated with slow reorientations (tau c approximately 10(-4)s at 60 K) of hydroxyl and water O-D groups, and the third with a small number of water molecules for which rapid reorientations (tau c < 10(-7)s) persist at 160 K. These dynamic features are considered in terms of the known structure and the findings of other techniques. These results suggest that the reorientation jumps of the hydroxyls and water molecules are lower than previously thought.

A 2H NMR spin-lattice relaxation study of -NH+3 group dynamics in polycrystalline L-alanine.

The 2H spin-lattice relaxation times T1Z and T1Q of the -ND+3 group in polycrystalline L-alanine were measured over the temperature ranges 250-425 K and 300-425, respectively. The relaxation data were fitted to analytical expressions for three-fold reorientation, giving a correlation time tau(c) = 1.5 x 10(-14) s exp(40.2 kJ mol-1/RT), which is in good agreement with the results of a limited lineshape study. T 1Z values derived from numerical simulations, taking into consideration geometrical distortion of the -ND+3 group and experimental imperfections, are compared with values given by an analytical expression for three-fold jumps and with the experimental results. The extraction of average site quadrupolar coupling constants is discussed.

Study of the interaction of 4-bromomercuriocinnamic acid with alpha-chymotrypsin by 79 Br and 81Br pulsed nuclear-magnetic resonance.

Br- has been used as a nuclear magnetic resonance (NMR) probe to study the reversible association of alpha-chymotrypsin and an Hg-labelled substrate (4-bromomercuriocinnamic acid, BrHgCin) which rapidly exchanges Br-. T1 was measured for 79Br and 81Br, using a pulse spectrometer. Values of the parameters that determine T1, Obs in aqueous solutions of KBr (pH=5.5) containing alpha-chymotrypsin and BrHgCin are reported. It is found that the rate of Br exchange is diffusion-limited and faster than the rate of reorientation of the BrHgCin-alpha-chymotrypsin complex. The rate constant for the formation of the covalent BrHgCin-alpha-chymotrypsin complex determined by this technique agrees well with previously published data. The rapid rate of Br exchange with the complex, however, is incompatible with the side chain of BrHgCin being entirely buried in a nonpolar pocket on the enzyme but compatible with the side chain being exposed to the solution. The contribution to the NMR signal from the non-covalent complex is negligible.