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We determine for the first time the magnetic dipole moment of a short-lived nucleus with part-permillion (ppm) accuracy. To achieve this 2-orders-of-magnitude improvement over previous studies, we implement a number of innovations into our beta-detected nuclear magnetic resonance (beta-NMR) setup at ISOLDE at CERN. Using liquid samples as hosts, we obtain narrow, subkilohertz-linewidth, resonances, while a simultaneous in situ H-1 NMR measurement allows us to calibrate and stabilize the magnetic field to ppm precision, thus eliminating the need for additional beta-NMR reference measurements. Furthermore, we use ab initio calculations of NMR shielding constants to improve the accuracy of the reference magnetic moment, thus removing a large systematic error. We demonstrate the potential of this combined approach with the 1.1 s half-life radioactive nucleus Na-26, which is relevant for biochemical studies. Our technique can be readily extended to other isotopic chains, providing accurate magnetic moments for many short-lived nuclei. Furthermore, we discuss how our approach can open the path toward a wide range of applications of the ultrasensitive beta-NMR in physics, chemistry, and biology.

We determine for the first time the magnetic dipole moment of a short-lived nucleus with part-permillion (ppm) accuracy. To achieve this 2-orders-of-magnitude improvement over previous studies, we implement a number of innovations into our beta-detected nuclear magnetic resonance (beta-NMR) setup at ISOLDE at CERN. Using liquid samples as hosts, we obtain narrow, subkilohertz-linewidth, resonances, while a simultaneous in situ H-1 NMR measurement allows us to calibrate and stabilize the magnetic field to ppm precision, thus eliminating the need for additional beta-NMR reference measurements. Furthermore, we use ab initio calculations of NMR shielding constants to improve the accuracy of the reference magnetic moment, thus removing a large systematic error. We demonstrate the potential of this combined approach with the 1.1 s half-life radioactive nucleus Na-26, which is relevant for biochemical studies. Our technique can be readily extended to other isotopic chains, providing accurate magnetic moments for many short-lived nuclei. Furthermore, we discuss how our approach can open the path toward a wide range of applications of the ultrasensitive beta-NMR in physics, chemistry, and biology.

An efficient implementation of the generalized energy-based fragmentation (GEBF) approach (Li, W.; Li, S.; Jiang, Y. J Phys. Chem. A 2007, 111, 2193) for treating a wide range of large molecules is presented. In this implementation, the fragmentation process can be automatically done for a general molecule, with only some functional groups defined by users. A new and fast scheme is designed for the generation of various subsystems and the derivation of their coefficients. The newly implemented GEBF approach has been applied to several large molecules including proteins, nucleic acids, and supermolecules with fused aromatic rings. Test calculations within the Hartree-Fock (HF) and density functional theory (DFT) framework demonstrate that the GEBF approach can provide reasonably accurate ground-state energies and optimized structures, which are in good agreement with those from conventional HF or DFT calculations. The GEBF approach implemented in this work can now be employed by nonexpert users to compute energies, optimized structures, and some molecular properties at various ab initio levels for a broad range of large molecules on ordinary PC workstations., QC 20110215

The most striking and counterintuitive consequences of quantum mechanics play out in the strong correlations of many-particle systems. The physics of these phenomena are exponentially complicated and often non-local. In chemistry, these strong correlations are vital to even qualitative pictures of chemical bonding, but they grow intractably more numerous with the number of particles and remain a significant challenge for models of chemical behavior. Luckily, the strong correlations relevant to most chemical situations can be significantly simplified and compressed using the heuristics which have been developed by chemists up to present day: ideas like bonding electron pairs, and resonance. In this thesis we present a convergent and systematically improvable series of approximations to the many-electron Schro ̈dinger equation which exploit these patterns. Two themes dominate the work: the use of bonding electron pairs as local units for developing efficient models, and an exponential parameterization of the many-electron wave-function.

The most striking and counterintuitive consequences of quantum mechanics play out in the strong correlations of many-particle systems. The physics of these phenomena are exponentially complicated and often non-local. In chemistry, these strong correlations are vital to even qualitative pictures of chemical bonding, but they grow intractably more numerous with the number of particles and remain a significant challenge for models of chemical behavior. Luckily, the strong correlations relevant to most chemical situations can be significantly simplified and compressed using the heuristics which have been developed by chemists up to present day: ideas like bonding electron pairs, and resonance. In this thesis we present a convergent and systematically improvable series of approximations to the many-electron Schro ̈dinger equation which exploit these patterns. Two themes dominate the work: the use of bonding electron pairs as local units for developing efficient models, and an exponential parameterization of the many-electron wave-function.

Quantum chemical calculations have been performed to investigate radical cation rearrangement, radical carbonylation, and homolytic substitution reactions of organic molecules. The rearrangement of the bicyclopropylidiene radical cation to the tetramethyleneethane radical cation is predicted to proceed with stepwise disrotatory opening of the two rings. Each ring opening is found to be combined with a striking pyramidalization of a carbon atom in the central bond. The isomerization of the norbornadiene radical cation to the cycloheptatriene radical cation (CHT.+), initialized by opening of a bridgehead–methylene bond, is investigated. The most favorable path involves concerted rearrangement to the norcaradiene radical cation followed by ring opening to CHT.+. The barrier of this channel is found to be significantly reduced upon substitution of the methylene group with C(CH3)2. Stepwise mechanisms are predicted to be favored over concerted isomerization for the McLafferty rearrangement of the radical cations of butanal and 3-fluorobutanal. The barrier for the concerted rearrangement is found to be lowered by 17.2 kcal/mol upon substitution, a result which is rationalized by the calculated dipole moments and atomic charges. Recent experiments showed that photoinitiated carbonylation of alkyl iodides with [11C]carbon monoxide may be significantly enhanced by using small amounts of ketones that have nπ* character of their excited triplet state. DFT calculations show the feasibility of an atom transfer type mechanism, proposed to explain these observations. Moreover, the computational results rationalize the observed differences in yield when using various alcohol solvents. Finally, following photolysis of methyliodide, recent electron spin resonance spectroscopy experiments demonstrated that the SH2 reaction •CD3 + SiD3CH3 → CD3SiD3 + •CH3 proceeds with high selectivity over the energetically more favorable D abstraction. The role of geometrical effects, especially the

Quantum chemical calculations have been performed to investigate radical cation rearrangement, radical carbonylation, and homolytic substitution reactions of organic molecules. The rearrangement of the bicyclopropylidiene radical cation to the tetramethyleneethane radical cation is predicted to proceed with stepwise disrotatory opening of the two rings. Each ring opening is found to be combined with a striking pyramidalization of a carbon atom in the central bond. The isomerization of the norbornadiene radical cation to the cycloheptatriene radical cation (CHT.+), initialized by opening of a bridgehead–methylene bond, is investigated. The most favorable path involves concerted rearrangement to the norcaradiene radical cation followed by ring opening to CHT.+. The barrier of this channel is found to be significantly reduced upon substitution of the methylene group with C(CH3)2. Stepwise mechanisms are predicted to be favored over concerted isomerization for the McLafferty rearrangement of the radical cations of butanal and 3-fluorobutanal. The barrier for the concerted rearrangement is found to be lowered by 17.2 kcal/mol upon substitution, a result which is rationalized by the calculated dipole moments and atomic charges. Recent experiments showed that photoinitiated carbonylation of alkyl iodides with [11C]carbon monoxide may be significantly enhanced by using small amounts of ketones that have nπ* character of their excited triplet state. DFT calculations show the feasibility of an atom transfer type mechanism, proposed to explain these observations. Moreover, the computational results rationalize the observed differences in yield when using various alcohol solvents. Finally, following photolysis of methyliodide, recent electron spin resonance spectroscopy experiments demonstrated that the SH2 reaction •CD3 + SiD3CH3 → CD3SiD3 + •CH3 proceeds with high selectivity over the energetically more favorable D abstraction. The role of geometrical effects, especially the