Devin N. Sears

Xe NMR lineshapes in channels of peptide molecular crystals.

To further an understanding of the nature of information available from Xe chemical shifts in cavities in biological systems, it would be advantageous to start with Xe in regular nanochannels that have well known ordered structures built from amino acid units. In this paper, we report the experimental observation of Xe NMR lineshapes in peptide channels, specifically the self-assembled nanochannels of the dipeptide l-Val-l-Ala and its retroanalog l-Ala-l-Val in the crystalline state. We carry out grand canonical Monte Carlo simulations of Xe in these channels to provide a physical understanding of the observed Xe lineshapes in these two systems.

The 129Xe nuclear shielding tensor surfaces for Xe interacting with rare gas atoms

The shielding tensor surfaces for the Xe–Xe, Xe–Kr, Xe–Ar, and Xe–Ne dimers are calculated as a function of separation, using gauge-including atomic orbitals (GIAO) at the Hartree–Fock level, and also using density functional theory with the B3LYP hybrid functional. Since the highest quality potential energy functions are available for these systems, the available experimental data (temperature dependent second virial coefficients of the nuclear magnetic resonance chemical shifts) are from measurements on well-defined physical systems (Xe at low mole fraction in the gas phase), and the relation between the observed quantity and the shielding function is well-defined, these systems provide a means by which the dispersion component of the isotropic shielding function of Xe–Rg can be determined. The parallel component of the intermolecular shielding tensor is small and nearly independent of the method of calculation. Therefore, the dispersion component of the perpendicular component of the shielding function...

Molecular dynamics averaging of Xe chemical shifts in liquids.

The Xe nuclear magnetic resonance chemical shift differences that afford the discrimination between various biological environments are of current interest for biosensor applications and medical diagnostic purposes. In many such environments the Xe signal appears close to that in water. We calculate average Xe chemical shifts (relative to the free Xe atom) in solution in eleven liquids: water, isobutane, perfluoro-isobutane, n-butane, n-pentane, neopentane, perfluoroneopentane, n-hexane, n-octane, n-perfluorooctane, and perfluorooctyl bromide. The latter is a liquid used for intravenous Xe delivery. We calculate quantum mechanically the Xe shielding response in Xe-molecule van der Waals complexes, from which calculations we develop Xe (atomic site) interpolating functions that reproduce the ab initio Xe shielding response in the complex. By assuming additivity, these Xe-site shielding functions can be used to calculate the shielding for any configuration of such molecules around Xe. The averaging over configurations is done via molecular dynamics (MD). The simulations were carried out using a MD technique that one of us had developed previously for the simulation of Henrys constants of gases dissolved in liquids. It is based on separating a gaseous compartment in the MD system from the solvent using a semipermeable membrane that is permeable only to the gas molecules. We reproduce the experimental trends in the Xe chemical shifts in n-alkanes with increasing number of carbons and the large chemical shift difference between Xe in water and in perfluorooctyl bromide. We also reproduce the trend for a given solvent of decreasing Xe chemical shift with increasing temperature. We predict chemical shift differences between Xe in alkanes vs their perfluoro counterparts.

Theoretical calculations of the Xe chemical shifts in cryptophane cages

Toward an understanding of the factors that affect the chemical shift in the Xe nuclear magnetic resonance spectrum of Xe atoms trapped in cages which may have applications as biosensors, we carry out calculations of Xe nuclear magnetic shielding using Hartree–Fock and density functional methods. The resulting values for various Xe positions within the cage can be described by an analytical function of Xe and cage atom coordinates. This shielding function is used in Monte Carlo canonical averaging of a Xe atom within cryptophane cages to investigate the dependence of the Xe chemical shifts on cage size (cryptophane-A versus cryptophane-E), isotopic substitution, and temperature. We compare our theoretical average Xe chemical shifts with the experimental values in four types of cryptophane cages, and with the temperature and isotopic dependence of Xe chemical shifts in cryptophane-A, and achieve a quantitative understanding of the factors that influence the Xe chemical shifts in these cages. The predicted ...

The Xe shielding surfaces for Xe interacting with linear molecules and spherical tops

The 129Xe nuclear magnetic resonance spectrum of xenon in gas mixtures of Xe with other molecules provides a test of the ab initio surfaces for the intermolecular shielding of Xe in the presence of the other molecule. We examine the electron correlation contributions to the Xe-CO2, Xe-N2, Xe-CO, Xe-CH4, and Xe-CF4 shielding surfaces and test the calculations against the experimental temperature dependence of the density coefficients of the Xe chemical shift in the gas mixtures at infinite dilution in Xe. Comparisons with the gas phase data permit the refinement of site-site potential functions for Xe-N2, Xe-CO, and Xe-CF4 especially for atom-Xe distances in the range 3.5-6 A. With the atom-atom shielding surfaces and potential parameters obtained in the present work, construction of shielding surfaces and potentials for applications such as molecular dynamics averaging of Xe chemical shifts in liquid solvents containing CH3, CH2, CF3, and CF2 groups is possible.

Nuclear magnetic shielding and chirality IV. The odd and even character of the shielding response to a chiral potential

We investigate the odd and even character of the shielding response in a chiral molecule (modeled by a Ne8 helix) when subjected to a chiral potential. We establish that the diastereomeric splittings are a measure of odd powers of Vodd. Implications for diastereomeric, splittings of Xe in handed cages with handed tethers are discussed.

Calculation of the 129Xe chemical shift in Xe@C60

We report, for the first time, a reasonably good calculation of Xe shielding in a fullerene. We find the 129Xe intermolecular shielding value [σ(129Xe@C60)−σ(Xe atom)]=−181.58 ppm (B3LYP), in very good agreement with the value observed for 129Xe@C60 dissolved in liquid benzene.

Nuclear magnetic shielding and chirality. III. The single electron on a helix model

In this third paper of the triptych we use the Tinoco–Woody model of an electron on a helix as our chiral system [I. Tinoco, Jr. and R. W. Woody, J. Chem. Phys. 40, 160 (1964)]. Diastereomerism is achieved by varying the pitch of the helix. The full nuclear magnetic shielding tensor of a naked spin is determined with various subtleties explicated. The results are compared to the Ne helix diastereomers.In this third paper of the triptych we use the Tinoco–Woody model of an electron on a helix as our chiral system [I. Tinoco, Jr. and R. W. Woody, J. Chem. Phys. 40, 160 (1964)]. Diastereomerism is achieved by varying the pitch of the helix. The full nuclear magnetic shielding tensor of a naked spin is determined with various subtleties explicated. The results are compared to the Ne helix diastereomers.

Nuclear magnetic shielding and chirality. II. The shielding tensor of a naked spin in Ne helices

We continue our investigation of the nuclear magnetic shielding tensors of Xe@Nen helix complexes (I). Here we replace Xe by a naked spin. The full shielding tensor of Ne8 helix diastereomers is calculated and compared with the equivalent Xe@Ne8 diastereomers.

Nuclear magnetic shielding and chirality. I. The shielding tensor of Xe interacting with Ne helices

Chirality and, in particular, induced chirality is investigated using Xe interacting with chirally perturbed Ne helices. The full nuclear magnetic shielding tensors are calculated and physical implications are discussed.