Gradient-induced longitudinal relaxation of hyperpolarized noble gases in the fringe fields of superconducting magnets used for magnetic resonance

When hyperpolarized noble gases are brought into the bore of a superconducting magnet for magnetic resonance imaging (MRI) or spectroscopy studies, the gases must pass through substantial field gradients, which can cause rapid longitudinal relaxation. In this communication, we present a means of calculating this spatially dependent relaxation rate in the fringe field of typical magnets. We then compare these predictions to experimental measurements of (3)He relaxation at various positions near a medium-bore 2-T small animal MRI system. The calculated and measured relaxation rates on the central axis of the magnet agree well and show a maximum (3)He relaxation rate of 3.83×10(-3) s(-1) (T(1)=4.4 min) at a distance of 47 cm from the magnet isocenter. We also show that if this magnet were self-shielded, its minimum T(1) would drop to 1.2 min. In contrast, a typical self-shielded 1.5-T clinical MRI scanner will induce a minimum on-axis T(1) of 12 min. Additionally, we show that the cylindrically symmetric fields of these magnets enable gradient-induced relaxation to be calculated using only knowledge of the on-axis longitudinal field, which can either be measured directly or calculated from a simple field model. Thus, while most MRI magnets employ complex and proprietary current configurations, we show that their fringe fields and the resulting gradient-induced relaxation are well approximated by simple solenoid models. Finally, our modeling also demonstrates that relaxation rates can increase by nearly an order of magnitude at radial distances equivalent to the solenoid radius.     

Probing the structure of CH5+ by dissociative charge exchange

Dissociative charge exchange of CH5+ with Cs, coupled with quasiclassical trajectory calculations on an ab initio PES for CH5, has been used to probe the structure of the CH5+ cation. Product kinetic energy release distributions and branching ratios for CH5 --> CH4 + H and CH5 --> CH3 + H2 have been compared. The agreement of the product branching ratios provides evidence for the fluxional nature of CH5+.     

Ex vivo quantification of mTHPC concentration in tissue: influence of chemical extraction on the optical properties

A method for the quantification of the concentration of the photosensitizer meso-tetra(hydroxyphenyl) chlorin (mTHPC) in tissue samples is presented. The technique is an extension of a previously published method based on alkaline hydrolysis of tissue, using Solvable as a tissue solubilizer. mTHPC quantification was achieved by subsequent fluorescence spectroscopy. Since the original extraction method involved multiple steps in which water dilution of the sample was implemented, we studied the spectral characteristics of mTHPC in different Solvable/water mixtures. Using UV-VIS absorption and fluorescence spectroscopy, it was demonstrated that the spectral characteristics of mTHPC vary for different Solvable concentrations. In the range of 20-100% Solvable, the fluorescence intensity of mTHPC did not change, while dramatic changes in the mTHPC fluorescence intensity were observed for lower Solvable concentrations (< 20%) due to increasing hydrophilicity of the environment, combined with pH alterations. We also demonstrated that the absorption and fluorescence spectra of the dissolved tissue were time-dependent. Longer incubation of the samples resulted in a significant increase of the native tissue chromophore fluorescence. This implies that for the correct quantification of photosensitizer concentrations, the fluorescence of native tissue chromophores must be accounted for.     

Full-dimensional quantum calculations of ground-state tunneling splitting of malonaldehyde using an accurate ab initio potential energy surface

Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcalmol, in excellent agreement with the reported ab initio value. Model one-dimensional and "exact" full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased "fixed-node" diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm(-1) in Cartesian coordinates and 22.6 cm(-1) in normal coordinates, with an uncertainty of 2-3 cm(-1). This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21-22 cm(-1). The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2-3 cm(-1). These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm(-1), and agree well with the experimental values of 21.6 and 2.9 cm(-1) for the H and D transfer, respectively.     

Variational calculation of second-order reduced density matrices by strong N-representability conditions and an accurate semidefinite programming solver

The reduced density matrix (RDM) method, which is a variational calculation based on the second-order reduced density matrix, is applied to the ground state energies and the dipole moments for 57 different states of atoms, molecules, and to the ground state energies and the elements of 2-RDM for the Hubbard model. We explore the well-known N-representability conditions (P, Q, and G) together with the more recent and much stronger T1 and T2(') conditions. T2(') condition was recently rederived and it implies T2 condition. Using these N-representability conditions, we can usually calculate correlation energies in percentage ranging from 100% to 101%, whose accuracy is similar to CCSD(T) and even better for high spin states or anion systems where CCSD(T) fails. Highly accurate calculations are carried out by handling equality constraints and/or developing multiple precision arithmetic in the semidefinite programming (SDP) solver. Results show that handling equality constraints correctly improves the accuracy from 0.1 to 0.6 mhartree. Additionally, improvements by replacing T2 condition with T2(') condition are typically of 0.1-0.5 mhartree. The newly developed multiple precision arithmetic version of SDP solver calculates extraordinary accurate energies for the one dimensional Hubbard model and Be atom. It gives at least 16 significant digits for energies, where double precision calculations gives only two to eight digits. It also provides physically meaningful results for the Hubbard model in the high correlation limit.     

New ab initio potential energy surface and the vibration-rotation-tunneling levels of (H2O)2 and (D2O)2

We report a new full-dimensional potential energy surface (PES) for the water dimer, based on fitting energies at roughly 30,000 configurations obtained with the coupled-cluster single and double, and perturbative treatment of triple excitations method using an augmented, correlation consistent, polarized triple zeta basis set. A global dipole moment surface based on Moller-Plesset perturbation theory results at these configurations is also reported. The PES is used in rigorous quantum calculations of intermolecular vibrational frequencies, tunneling splittings, and rotational constants for (H2O)2 and (D2O)2, using the rigid monomer approximation. Agreement with experiment is excellent and is at the highest level reported to date. The validity of this approximation is examined by comparing tunneling barriers within that model with those from fully relaxed calculations.     

Quasiclassical trajectory calculations of acetaldehyde dissociation on a global potential energy surface indicate significant non-transition state dynamics

A recent experimental study [Houston, P. L.; Kable, S. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16079] of the photodissociation of acetaldehyde (CH(3)CHO) has suggested two distinct mechanisms for the production of the molecular products CH(4) + CO. One corresponds to the traditional transition state mechanism and the other to a transition state-skirting path similar to the roaming channel previously reported in formaldehyde. To investigate this theoretically, a full-dimensional potential energy surface (PES) has been constructed. The PES was fit with permutationally invariant polynomials to 135,000 points calculated using coupled cluster theory with single and double excitations and a perturbative treatment of triple excitations [CCSD(T)] and correlation consistent basis sets of double- and triple-zeta quality. To test the accuracy of the PES additional CCSD(T) and multireference configuration interaction calculations were carried out. Quasiclassical trajectory calculations were run on the PES starting at the acetaldehyde equilibrium geometry and also at the conventional transition state (TS) for the molecular products CH(4) + CO. The former calculations agree well with the experimental results of Houston and Kable; however, those from the TS do not. The implications for a non-transition state, roaming mechanism in this molecule are discussed.     

Effectiveness of cell adhesive additives in different supramolecular polymers

Supramolecular motifs in elastomeric biomaterials facilitate the modular incorporation of additives with corresponding motifs. The influence of the elastomeric supramolecular base polymer on the presentation of additives has been sparsely examined, limiting the knowledge of transferability of effective functionalization between polymers. Here it was investigated if the polymer backbone and the additive influence biomaterial modification in two different types of hydrogen bonding supramolecular systems, that is, based on ureido-pyrimidinone or bis-urea units. Two different cell-adhesive additives, that is, catechol or cyclic RGD, were incorporated into different elastomeric polymers, that is, polycaprolactone, priplast or polycarbonate. The additive effectiveness was evaluated with three different cell types. AFM measurements showed modest alterations on nano-scale assembly in ureido-pyrimidinone materials modified with additives. On the contrary, additive addition was highly intrusive in bis-urea materials. Detailed cell adhesive studies revealed additive effectiveness varied between base polymers and the supramolecular platform, with bis-urea materials more potently affecting cell behavior. This research highlights that additive transposition might not always be as evident. Therefore, additive effectiveness requires re-evaluation in supramolecular biomaterials when altering the polymer backbone to suit the biomaterial application.     

Enantioselective liquid-liquid extraction of (R,S)-phenylglycinol using a bisnaphthyl phosphoric acid derivative as chiral extractant

This study demonstrates that enantioseparation by liquid-liquid extraction can be done in a continuous flow mode on both laboratory and industrial scale and is a promising technique that could become a competitive alternative for crystallization or chromatographic approaches. We studied the enantioselective liquid-liquid extraction of phenylglycinol (Pgl) using a bisnaphthyl phosphoric acid extractant. Batch experiments were performed to estimate extraction model parameters. The system was described using an extraction mechanism with homogeneous organic phase complexation. The complexation constants were very large, in the order of 10⁸-10¹⁰ L/mol in the temperature range 279<T<303 K. The developed model was then used to design a multistage countercurrent extraction process with Centrifugal Contactor Separator (CCS) equipment. This study demonstrates that high purity (70% ee) with a reasonable yield (36%) can be obtained for a moderately selective system (α=1.7) with only six extraction stages. The technology is potentially applicable to a wide range of racemates.     

The dynamics of allyl radical dissociation

Dissociation of the allyl radical, CH(2)CHCH(2), and its deuterated isotopolog, CH(2)CDCH(2), have been investigated using trajectory calculations on an ab initio ground-state potential energy surface calculated for 97,418 geometries at the coupled cluster single and double and perturbative treatment of triple excitations, with the augmented correlation consistent triple-ζ basis set level (CCSD(T)/AVTZ). At an excitation energy of 115 kcal/mol, corresponding to optical excitation at 248 nm, the primary channel is hydrogen loss with a quantum yield of 0.94 to give either allene or propyne in a ratio of 6.4:1. The total dissociation rate for CH(2)CHCH(2) is 6.3 × 10(10) s(-1), corresponding to a 1/e time of 16 ps. Methyl and C(2)H(2) are produced with a quantum yield of 0.06 by three different mechanisms: a 1,3 hydrogen shift followed by C-C cleavage to give methyl and acetylene, a double 1,2 shift followed by C-C cleavage to give methyl and acetylene, or a single 1,2 hydrogen shift followed by C-C cleavage to give methyl and vinylidene. In this last channel, the vinylidene eventually isomerizes to give internally excited acetylene, and the kinetic energy distribution is peaked at much lower energy (6.4 kcal/mol) than that for the other two channels (18 kcal/mol). The trajectory results also predict the v-J correlation, the anisotropy of dissociation, and distributions for the angular momentum of the fragments. The v-J correlation for the CH(3) + HCCH channel is strongest for high rotational levels of acetylene, where v is perpendicular to J. Methyl elimination is anisotropic, with β = 0.66, whereas hydrogen elimination is nearly isotropic. In the hydrogen elimination channel, allene is rotationally excited with a total angular momentum distribution peaked near J = 17. In the methyl elimination channel, the peak of the methyl rotational distribution is at J ≈ 12, whereas the peak of the acetylene rotational distribution is at J ≈ 28.