Frank E. Hruska

Some molecular orbital computations of the internal rotational barrier heights in benzaldehyde and its 4-fluoro, 4-cyano and 4-hydroxy derivatives

Abstract Some molecular orbital calculations on the title molecules imply that correlation-gradient computations will be necessary if the internal barriers are to agree with those deduced from torsional frequencies. For example, the energy difference between the planar and perpendicular conformers of benzaldehyde is 34.7 kJ mol−1 for an MP2/6-311G∗//6-31G∗(5D) computation. A calculation of the difference of the zero point energies of the two conformers, using somewhat less flexible basis sets, suggests that the enthalpy difference of the two conformers will not be substantially less than 32.5 kJ mol−1. The barrier height deduced from torsional frequencies is 19.3 kJ mol−1. Computations of the torsional frequencies, using STO-3G and 6-31G bases, suggests that the ensuing two-fold barriers are substantially lower than the computed energy and enthalpy differences of the two conformers for all the compounds. Comparisons are made with the measured barriers in condensed media. The computed dipole moments of the planar and perpendicular conformers imply that the solvent effects on the internal barriers differ in sign among the compounds. For 4-hydroxybenzaldehyde this sign depends on the orientation of the hydroxyl group relative to the Cue5fbO bond. The need for certain new measurements and computations is emphasized.

Metastable ion studies with a secondary ion time-of-flight mass spectrometer. Enhanced distinction between isomers of O-alkylated thymidines

Abstract First-order rate constants and half-lives have been measured for the decomposition of several metastable ions in the secondary ion time-of-flight mass spectra of 4- and 2- O -alkylthymidines. Some of the factors which influence the accuracy of these measurements have been analyzed. Relative values of the kinetic parameters appear to be a more reliable indicator of isomeric form than the “normal” mass spectra, which are sensitive to the method of sample preparation. In particular, the [M + Na] + ions of 4- O -alkylthymidines are significantly more stable than those of 2- O -alkylthymidines, an effect attributed to chelation of Na + by the 2, 4′, and 5′ oxygens of the former isomers. An interesting rearrangement involving loss of an alkene from the [B + 2H] + ion (B = pyrimidine base) has also been confirmed. The rate of this decomposion is much greater for O -isopropyl- than for O -ethylthymidines, an effect which is rationalized in terms of conformational preferences of the O -alkyl group.

Hydrogen ion compartmentation during and following cerebral ischemia evaluated by 31P NMR spectroscopy

Two peaks were observed in the inorganic phosphate region in 31P NMR spectra obtained during and shortly after incomplete forebrain ischemia induced in rats under normoglycemic conditions. Pre-ischemia, a single Pi peak was observed at a chemical shift of 2.5 ppm (pH 7.2). This peak shifted upfield to 1.6 ppm (pH 6.5) and, following reperfusion, returned to its pre-ischemia value. During ischemia and for a short time following reperfusion, a second, smaller peak was observed which we assigned to a second pool of inorganic phosphate. The data support the proposal by Kraig and coworkers of pH compartmentation between neurons and glia during and following transient forebrain ischemia.

Molecular orbital estimates of the structures and internal rotational barriers in 2-fluorotoluene and 2-chlorotoluene

Abstract The internal rotational barriers in 2-fluorotoluene and 2-chlrrotoluene are computed with the ab initio basis sets, 6-31G and 6-31G*, as well as by single-point MP2/ / 6-31G* calculations. For the fluoro derivative, good agreement with the barrier from rotational spectra is obtained. The computed barrier for 2-chlorotoluene agrees with that derived from dipolar couplings in solution but is possibly smaller than the value implied by the absence of splittings in its rotational spectrum. The 6-31G* structures are given.

Time-of-flight secondary ion mass spectrometry of isomeric O-alkylthymidines

Abstract Positive ion mass spectra of the title compounds and their parent molecule, thymidine, show prominent [M + H]+ and [M + Na]∗ ions, as well as base fragment ions resulting from cleavage of the N-glycosyl bond. In the case of alkylated nucleosides the base fragments can undergo further elimination of an alkene. In the negative ion spectra the prominent ions for all nucleosides are B− and NCO−. Molecular mass can be verified for thymidine by a prominent [M - H]− ion, which is not, or barely, detectable for the alkylated molecules. In addition, the NCO− ion produces a prominent [M + 41]− adduct with a (dT - H)• radical, but not with the alkylated species. Trends in the relative abundances of the ions are discussed.

A simplified strategy for the synthesis of dideoxyribonucleotide blocks

Abstract The rapid synthesis of dideoxyribonucleotide blocks in 60–85% isolated yields has been achieved by combining the phosphorylation and condensation steps in a sequential reaction series which also allows the recovery of unreacted nucleotides.