On Cross Sections for Electron Impact Ionization of Multicharged Ions

Electron impact ionization cross sections calculated by means of two semi-empirical expressions (Lotz, Seaton), a semiclassical model (Burgess: "ECIP"), and a simple quantum approximation (Seaton) are compared with results of more elaborate quantum (Coulomb-Born) calculations and measurements for several multicharged ions (N2+, Mg2+, C3+, O3+, C5+, O5+, Fe14+). While in some cases the comparisons are not definitive, the simple formula of Lotz appears to represent the C-B and experimental results more consistently than the other methods.     

A DFT study of the interresidue dependencies of scalar J-coupling and magnetic shielding in the hydrogen-bonding regions of a DNA triplex.

Scalar coupling constants and magnetic shieldings in the imino hydrogen-bonding region of Hoogsteen-Watson-Crick T.A-T and C(+).G-C triplets have been calculated as a function of the distance between proton donor and acceptor nitrogen atoms. The Fermi contact contributions to (h2)J((15)N-H...(15)N), (1)J((15)N-(1)H), and (h1)J((1)H...(15)N) were computed using density functional theory/finite perturbation theory (DFT/FPT) methods for the full base triplets at the unrestricted B3PW91/6-311G level. Chemical shifts delta((1)H) and delta((15)N) were obtained at the same level using the gauge including atomic orbital (GIAO) method for magnetic shielding. All three scalar couplings and all three chemical shifts are strongly interrelated and exhibit monotonic changes with base pair separation. These correlations are in conformity with experimental data for a 32-nucleotide DNA triplex. The results suggest that both chemical shifts and coupling constants can be used to gain information on H-bond donor-acceptor distances in nucleic acids. In addition to the DFT/FPT calculations, a simple three-orbital model of the N-H...H bond and a sum-over-states analysis is presented. This model reproduces the basic features of the H-bond coupling effect. In accordance with this model and the DFT calculations, a positive sign for the (h2)J(NN) coupling is determined from an E.COSY experiment.     

Theoretical studies of solvent effects on nuclear spin-spin coupling constants

A theoretical study is presented of the effects of solvent molecular motion on nuclear spin-spin coupling constants. Solvent molecules are treated as point dipoles arranged around the solute molecule in a cubic closest packed arrangement. Average dipole moment vectors are given by a rotational Boltzmann average and the resulting perturbation is included in the Fock matrices. Calculations of nuclear spin-spin coupling constants were performed by self-consistent perturbation theory in the INDO (intermediate neglect of differential overlap) approximation of self-consistent-field molecular orbital theory. Calculated results are compared with the experimental data as well as the results of previous models.     

Semiempirical molecular orbital calculations of anisotropic 1H, 13C and 19F hyperfine coupling constants in hydrocarbon and fluorocarbon radicals

The anisotropic hyperfine coupling constants (AHCC) from the electron spin resonance (E.S.R.) spectra of a variety of atoms in organic radicals have been calculated by means of semiempirical molecular orbital wavefunctions in the INDO approximation. Hyperfine tensors involving ¹H, ¹³C and ¹⁹F nuclei are obtained for the ?H, ?H₃, CH₃?H₂, (CH₃)₃? hydrocarbon radicals, malonic acid radical, ?H₂F, ?F₂H, ?F₃ and CF₃?H₂ radicals. The calculated values are compared with available experimental, non-empirical and semiempirical values for these radicals. All integrals of the operator entering the electronic contributions have been evaluated over Slater type orbitals. The introduction of deorthogonalized wavefunctions gives generally better calculated results. In particular, the tensor components of the ¹⁹F AHCC are in good agreement with the experimental results without the necessity of readjusting the effective nuclear charges.     

Evidence for the two-body nature of theE1 transition operator in the sdf-interacting boson model

Two new absolute transition rates are reported for the nucleus¹⁴⁴Sm following an (α, α′) Coulomb excitation study. They are B(E3; 3^?→ 0⁺)=(38±3) W.u. and B(E1;3^? → 2⁺)=(2.8±0.4)×10^?3 W.u. This large E1 matrix element, along with the previously known B(E1; 1^? →⁺) value support the interpretation of the 1^? state in this nucleus as 2-phonon 2⁺ × 3^? excitation. In the frame of the IBM-1 +f-boson model we show the need for a two-body term in the E1 transition operator. Estimates for the strengths of the one and two-body parts of the E1 transition operator are obtained from these experimental data.     

DFT studies of the conformational/structural dependencies of geminal 1H,1H scalar coupling 2J(H,H') in substituted methanes

A study is presented of the structural dependencies for scalar, interproton J-coupling across two bonds in a series of substituted methanes. The coupled perturbed, density functional theory method with a B3PW91 functional and aug-cc-pVTZ-J basis sets is used to examine coupling between geminal protons (2)J(H,H') in methane and a series of substituted compounds CH(3)X (X = CH3, CH(2)CH(3), CH=CH2, CH=O, and NH2) as functions of the dihedral angle phi measured about the C1-X2 bonds. All four contributions are obtained but all conformational effects are dominated by the Fermi contact term. Simple linear combination of atomic orbitals (LCAO)-molecular orbital (MO) sum-over-states methods are used to examine the relationships of the coupling constants with dihedral angles as well as internal H-C-H and H-C1-X2 angles. This study explores some novel aspects of geminal H-H coupling including an analysis of the asymmetry in the conformational dependencies arising from non-next-nearest neighbor interactions. For each of the substituted methanes, explicit trigonometric/exponential expressions are given and these accurately reproduce the (2)J(H,H') structural dependencies with standard deviations usually less than 0.03 Hz. The molecular structures for representative bicyclic molecules were fully optimized, and DFT results for (2)J(H,H') reproduce all the trends in the experimental data. A discussion is given on the applicability of the equations for H--H coupling in the substituted methanes to coupling in the bicyclic molecules.     

Reasons for the nonequivalence of the exo-exo and endo-endo vicinal NMR coupling constants in norbornanes

In a series of norbornanes, benzonorbornenes, and norbornenes, the vicinal cis couplings ³J_exo,exo and ³J_endo,endo are determined. A trend is recognized in which J_exo,exo steadily decreases in this series while J_endo,endo remains relatively constant; in norbornenes J_exo,exo and J_endo,endo are about the same. These observations are understood by means of theoretical calculations performed for representative compounds of the series. This study indicates that interactions of the C₇-methylene bridge with the bonds of the C₂C₃ ethylene bridge are responsible for the nonequivalence of J_exo,exo and J_endo,endo in norbornanes, and that in norbornenes interaction of the olefin functionality with bonds of the ethylene bridge is responsible for bridging J_exo,exo back fortuitously close to J_endo,endo.