Biomedical subjects

Gerard S Harbison

Publications and source records attributed to Gerard S Harbison.

7 recordsLinked to original sources

PubMed2006

Deuterium quadrupolar tensors of L-histidine hydrochloride monohydrate-d7.

A deuterium NMR study at 14 T of a single crystal of L-histidine hydrochloride monohydrate has determined the deuteron quadrupole coupling constants CQ, asymmetry parameters eta, and electric field gradient orientation for the imidazolium and primary ammonium groups and for a water of crystallization. The imidazolium deuterons, which have very long relaxation times, have quite different coupling constants, reflecting different hydrogen bonding but nearly identical orientations, with the most distinct principal axis in both cases nearly parallel to the N-D vector. The -ND3 groups undergo 3-fold hops about the C-N bond axis and have typical quadrupole couplings; the D2O undergoes 2-fold hops, leading to a tensor with a large asymmetry parameter. With appropriate corrections for vibrational averaging, density functional cluster calculations give an excellent fit to the imidazolium tensor magnitudes and orientations.

Deuterium↗

PubMed2006

Helical chirality in hexamethylene triperoxide diamine.

The primary explosive hexamethylenetriperoxide diamine has previously been found to exist in the solid state as a racemic mixture of helically chiral, threefold symmetric enantiomers; another enantiomeric pair of low-energy conformers has been predicted, but has never been observed. We show by solution 2D NMR at 14 T, in achiral solution and by addition of chiral shift reagents, that all four optically isomeric conformers coexist at slow equilibrium on the NMR timescale at room temperature, and can be observed. Calculations of the 1H and 13C NMR chemical shifts using gauge-including atomic orbital methods are in excellent agreement with experiment; thermochemical calculation of the free energies in solution are in somewhat worse agreement, but correctly predict the relative stability of the conformers. Analysis of the effects of chiral shift reagents on the NMR spectra suggests that discrimination between chiral isomers is primarily around the molecular equator, around which the enantiomeric gauche O--O linkages are arrayed.

Bridged Bicyclo Compounds, Heterocyclic↗

PubMed2006

Origins of isotopomeric polymorphism.

The complex formed between 4-methylpyridine and pentachlorophenol (4MPPCP) crystallises in a triclinic space group. If the same complex is synthesized from deuterated pentachlorophenol, it crystallizes in an entirely different monoclinic polymorph. Using solid-state NMR of samples synthesized with a full range of deuteration levels, crystallized from solution or the melt, and in the presence or absence of seeds, we have confirmed that the isotopomers indeed have different thermodynamically stable crystal structures. The roots of this phenomenon of isotopomeric polymorphism apparently lie in the differences in hydrogen bonding between the polymorphs. The triclinic form has a relatively short hydrogen bond. High-field solid-state NMR shows both the 1H chemical shift and the 2H electric quadrupole coupling of the hydrogen involved in the bond to be strongly temperature-dependent, indicating a low-lying excited state of the hydrogen bond longitudinal vibration. Inelastic neutron scattering of isotopomers of 4MPPCP has allowed us to identify the three orthogonal vibrational modes of the hydrogen in the hydrogen bond, at 29.7, 145, and 205 meV (240, 1168, and 1651 cm(-1)). The longitudinal mode is the lowest in energy, and it indicates a slightly asymmetric low-barrier double-well potential. Intrinsic to such potentials is a very small difference in zero-point energies (ZPEs) between the protonated and deuterated forms. As a contrast, the monoclinic form has a comparatively normal hydrogen bond, in which the proton and deuteron ZPEs should be different by approximately 500 cm(-1). A scenario can be envisaged where the triclinic protonated form is lower in energy than the monoclinic protonated form, but the triclinic deuterated form is higher in energy than the monoclinic deuterated form. This evidently accounts for the difference in relative stabilities of the two forms upon isotope substitution.

Crystallization↗

PubMed2005

Orientation of single crystals using linear approximations to NMR transits.

We have derived analytical expressions for determining the orientation of high-symmetry single crystals from line-crossings in a single rotation plot. We demonstrate the utility of the method using the strontium-87 resonance in strontium nitrate. Employing our new method, which we call orientation of single crystals using linear approximations to NMR transits (OSCULANT), in combination with fourth-order perturbation theory, we obtain a highly accurate value for the quadrupole coupling constant, and an estimate for the chemical shielding anisotropy.

Crystallization↗

PubMed2004

Isotopomeric polymorphism.

The complex formed between 4-methylpyridine and pentachlorophenol (4MPPCP) crystallizes in a triclinic space group. If the same complex is synthesized from deuterated pentachlorophenol (4MPPCP-d1), it crystallizes in an entirely different monoclinic polymorph. Solid-state NMR of samples synthesized with a full range of deuteration levels, crystallized from solution or the melt, and in the presence or absence of seeds confirms that the isotopomers indeed have different thermodynamically stable crystal structures. The phenomenon is apparently due to very different hydrogen bond strengths between the two polymorphs.

Deuterium↗

PubMed2002

The electric dipole polarity of the ground and low-lying metastable excited states of NF.

NF (nitrogen monofluoride, fluoroimidogen) is isoelectronic with O2, and, like O2, it has a triplet configuration in the ground state, with two low-lying metastable singlet excited states. The dipole moment of the a 1Delta excited state was measured in 1973 to be 0.37 +/- 0.06 D; at the time its polarity was assumed to be normal (i.e., with the negative charge on the fluorine). However, high-level electronic structure calculations, which reproduce with high accuracy the known spectroscopic constants of the ground and excited states of NF, predict a dipole moment of -0.388 D for a 1Delta NF, indicating that, despite the electronegativities, this molecule carries a positive charge on fluorine. The other singlet state is predicted to have an even larger negative dipole moment; the ground-state triplet should have a very small positive moment. Singlet NF resembles in this respect CO and BF, from the N2 isoelectronic series, both of which also have negative dipole moments.

Journal Article↗

PubMedDate not supplied

Deuterium NMR and ab initio studies of strongly hydrogen-bonded molecules.

Two newly synthesized alkylated cyanomalonaldehyde derivatives possess very short intramolecular hydrogen bonds, which are studied by deuterium NMR. Both the dimethyl and di-tert-butyl derivatives have small deuterium quadrupole coupling constants and large asymmetry parameters. In addition, the di-tert-butyl compound, which has one of the shortest O D...O hydrogen bonds yet measured, has a quadrupole coupling which anomalously increases with temperature. The significance of these phenomena is explored using a theoretical model which employs vibrationally averaged ab initio electric field gradient tensors calculated with large basis sets and electron correlation via the multiconfigurational self-consistent field method.

Computer Simulation↗