Gas phase infrared spectra of small polyatomic ions as a benchmark for theory

Infrared multiple photon dissociation (IRMPD) provides a method for examining vibrational spectroscopy of ions in the gas phase. Comparison of such experiments with the IR spectra of neutral molecules shows that the strength of a bond between a trigonal center and an attached methyl group tends to diminish after fluoride attacks to make the central atom tetrahedral. For selected main group species, the change becomes more pronounced as the central atom becomes more electropositive. Trimethylboron displays a more dramatic alteration than do silicon analogs, which indicates an effect beyond that of electronegativity by itself. In comparing gaseous trimethylboron with the negative ions derived from it, CH hyperconjugation ought to operate to a much greater extent in the neutral than in the anions. Anharmonic vibrational calculations using Density Functional Theory (DFT) match the experimental vibrational spectra and confirm that interpretation, providing evidence in favor of substantial hyperconjugative stabilization. However, further simplification of vibrational spectroscopy in terms of “atomic force constants” does not show much promise. Atoms in molecules (AIM) comparison of C-C bond orders in cations with B-C bond orders in neutral boranes is consistent with hyperconjugation.     

Linear free energy relationship for the anomeric effect: MP2, DFT and ab initio study of 2-substituted-1,4-dioxanes

The anomeric effect of 2-substituted 1,4-dioxane derivatives was calculated and compared with the values for substituted cyclohexane. The bond lengths, bond angles, torsion angles, and relative energies of axial and equatorial conformers of 2-substituted 1,4-dioxanes were calculated by the second-order Møller–Plesset (MP2), density functional theory (DFT/B3LYP), and Hartree–Fock (HF) methods using 6-31G^∗ basis set. The energy differences between the axial and equatorial conformers, endo and exo-anomeric effects, repulsive non-bond and H-bonding interactions were investigated. A linear free energy relationship (LFER) between calculated (MP2/6-31G^∗) anomeric effect and inductive substituent constants (σ_I) was obtained for 2-substituted-1,4-dioxanes (slope = 6.19 and r² = 0.967). The calculated energy differences indicate lower equatorial orientation for 2-substituted-1,4-dioxanes compared to the 2-substituted-tetrahydropyrans. The contribution of resonance, hyperconjugation, inductive, steric, hydrogen bonding, electrostatic interaction, and level of theory influences the anomeric effect.     

Differential deuterium isotope shifts and one-bond 1H−13C scalar couplings in the conformational analysis of protein glycine residues

Summary The one-bond deuterium isotope shift effect for glycine C resonances exhibits a conformational dependence comparable to that of the corresponding ¹J_HC scalar coupling in both magnitude (11 Hz at 14.1 T) and dihedral angle dependence. The similarity in the conformational dependence of the ¹J_HC and deuterium isotope shift values suggests a common physical basis. Given the known distribution of (,) main-chain dihedral angles for glycine residues, the deuterium isotope shifts and the ¹J_HC scalar couplings can determine conformations in the left-and right-handed helical-to-bridge regions of the (,) plane to an accuracy of approximately 13°. In the absence of stereochemical assignments, the differential deuterium isotope shifts and the ¹J_HC scalar couplings can be combined with limited independent structural information (e.g., the sign of ) to determine the chirality of the deuterium substitution.