No abstract

Nuclear magnetic shielding tensors computed by the gauge including atomic orbital (GIAO) method in the Hartree–Fock self-consistent-field (HF-SCF) framework are partitioned into magnetic contributions from chemical bonds and lone pairs by means of natural chemical shielding (NCS) analysis, an extension of natural bond orbital (NBO) analysis. NCS analysis complements the description provided by alternative localized orbital methods by directly calculating chemical shieldings due to delocalized features in the electronic structure, such as bond conjugation and hyperconjugation. Examples of NCS tensor decomposition are reported for CH4, CO, and H2CO, for which a graphical mnemonic due to Cornwell is used to illustrate the effect of hyperconjugative delocalization on the carbon shielding.

An experimental and theoretical study of the deuterium quadrupole coupling constants of glycine J. Chem. Phys. 82, 3516 (1985); 10.1063/1.448931 Solvent dependence of the deuteron quadrupole coupling constant of CDCl3 determined by 2H spinlattice relaxation and Raman line shape studies J. Chem. Phys. 81, 4790 (1984); 10.1063/1.447504On the deuteron quadrupole coupling constant in hydrogen bonded solids Quadrupole coupling constants, Q , for the deuteron and the oxygen nuclei in neat, liquid water were determined by both theoretical and experimental methods. The theoretical values of D ϭ264 kHz and O ϭ8.4 MHz obtained from ab initio calculations at the MP2/6-31ϩG* level in combination with a quantum cluster equilibrium model for liquids are in good agreement with results from NMR relaxation time experiments. Both theory and experiment show no observable temperature dependence of the quadrupole coupling constants. The theoretical values reported here for the oxygen quadrupole coupling constant and both the oxygen and deuterium asymmetry parameters are different from values obtained from ab initio calculations of clusters using molecular dynamics methods. This may be due to the use of pairwise additive potentials in the molecular dynamics simulations which do not take into account many-body or polarizability effects.

Temperature-dependent infrared spectra for liquid N-methylacetamide are calculated by the ab initio quantum cluster equilibrium (QCE) and Gaussian-94 methods and compared with experimental measurements. The calculations are based on standard ab initio self-consistent-field (SCF) methods at the 3-21G levels for five different molecular clusters. The cluster sizes vary from the monomer up to a five-membered linear structure. Strong cooperative effects are found in the molecular clusters and are reflected in the geometries and vibrational spectra for each species. The equilibrium populations of the clusters were calculated for the entire liquid range. At low temperatures the linear pentamer is the dominant species. At higher temperatures these clusters are replaced, primarily, by linear dimers and monomers. The calculated frequencies are in excellent agreement with the temperature behavior found in FT-IR experiments.

Proton spin–lattice relaxation times, T1, have been measured as a function of temperature for KBH4, NaBH4, and LiBH4. For NaBH4 and KBH4, 23Na and 11B relaxation measurements were also made. In all cases, the magnetization recovery is approximately exponential. Correlation times, τc, derived from the T1 data were used to calculate activation energies, V, for BH4− ion reorientations. For the cubic phase of KBH4, V = 14.8 ± 0.4 kJ/mole (3.55 ± 0.1 kcal/mole) (± always refers to rms error) from measurements on proton and 11B. For NaBH4, V was found to be 11.2 ± 0.5 and 14.8 ± 0.7 kJ/mole (2.7 ± 0.1 and 3.5 ± 0.2 kcal/mole) for the high- (cubic) and low-temperature (tetragonal) phases; an anomaly in τc was observed at temperatures slightly below the phase transition, and may be interpreted as a relatively sudden change in V associated with the phase transition. In LiBH4, a rather broad minimum was observed for the proton T1 vs temperature; this has been interpreted as due to two inequivalent BH4− tetrahedra with activation energies of 20 ± 1 and 16 ± 1 kJ/mole (4.7 ± 0.3 and 3.8 ± 0.3 kcal/mole). The proton and 11B nuclei are relaxed by magnetic dipolar interactions, but quadrupolar fluctuations are the dominating relaxation mechanism for 23Na in the cubic phase of NaBH4.