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Notes: An analysis of the electronic structure of the high-spin 3d7 Co(II) ion in the approximately octahedral Co(II)(acac)2(H2O)2 complex is presented in terms of crystal fields of descending symmetry from octahedral to orthorhombic. The energies and wave functions resulting from the interplay of these fields with the spin–orbit coupling are used to obtain zero-field splittings, magnetic moments, magnetic susceptibilities, and g values for the complex. The calculated temperature dependence of the susceptibility is compared to the reported dependence for Co(II)(acac)2(H2O)2, yielding bounds on the strength of the tetragonal component of the crystal field. The calculated anisotropy in the susceptibility is used in an analysis of our observed pseudocontact NMR shifts for methyl and methine protons in the complex. A procedure is outlined for using a crystal field analysis to compute pseudocontact contributions to proton chemical shifts starting from g values extracted from ESR spectra. The relationship between molecular structure and crystal-field splittings is also explored via a series of ab initio electronic structure calculations for the M(II)(acac)2(H2O)2 complexes with M=Mn, Co, Ni, and Zn. © 1999 American Institute of Physics.

Notes: Ab initio computational quantum chemical methods are used to calculate reduced partition function ratios for all isotopomers of CO, HCO+, and HOC+ involving the nuclides 1H, 2H (D), 12C, 13C, 16O, and 18O. The ratios are used to calculate equilibrium constants for the reaction pairs HCO+/CO, HOC+/CO, and C+/CO. Both simple proton transfers and more complex isotopic variants involving the breaking and reforming of CO bonds are considered. The probable pathways for the HCO+/CO and C+/CO exchange reactions are explored in detail using high-accuracy quantum chemical calculations. It appears most likely that the HCO+/CO reaction proceeds through exothermic formation of the linear adduct OCHCO+ with D∞h symmetry. Similarly, the C+/CO reaction proceeds along a spin-allowed pathway with exothermic formation of the linear adduct COC+ with D∞h symmetry. An alternate but higher energy spin-allowed pathway for the C+/CO reaction passes through a transition state with only Cs symmetry and a locally stable intermediate with C2v symmetry. In the ISM these reactions may proceed by these direct pathways or indirectly through coupled exothermic reaction pairs involving other species to achieve 13C/12C isotope exchange. © 1998 American Institute of Physics.

Notes: The enhancement of nuclear magnetic resonance (NMR) relaxation rates produced by paramagnetic solutes is physically rather different for electron spin S=1/2 paramagnetic species than for S≥1 species due to the presence of zero-field splitting interactions in the electron spin Hamiltonians of the latter. When the zfs energy is larger than the electronic Zeeman energy, the electron spin precessional motion is spatially quantized with respect to the molecule-fixed principal axis system (PAS) of the zfs tensor rather than along the external laboratory magnetic field. An analytical theory of the orthorhombic zfs limit has been derived in which the motion of the electron spin variables is described in the zfs-PAS and that of the nuclear spin variables in the laboratory coordinate frame. The resulting theoretical expressions are simple in form and suggest a physically transparent interpretation of the experiment. The NMR relaxation enhancement R1p results from additive contributions, R1x, R1y, and R1z, arising from the molecular-frame Cartesian components of the time-dependent electron spin magnetic moment operator μr(t). Each Cartesian component R1r depends on the dipolar power density at the nuclear Larmor frequency that is produced by the corresponding Cartesian component of μr(t). The theory displays the dependence of the relaxation enhancement on the variables of molecular structure in a very simple and physically transparent form: R1r∝r−6[1+P2(cos θr)], where r is the interspin distance and cos θr is the direction cosine of the interspin vector with the rth principal axis of the zfs tensor. New experimental data are presented for the model S=1 complex [trans-Ni(II)(acac)2(H2O)2] (acac=acetylacetonato) in dioxane solvent. The magnetic field dependence of the proton T1 of the axial water ligands has been measured over the range 0.15–1.5 T, the lower end of which corresponds to the zfs limit. The experimental data have been analyzed using the new analytical theory for the zfs-limit regime in conjunction with spin dynamics simulations in the intermediate regime. Dipolar density power plots are presented as graphical devices which clearly exhibit the physical information in the experiment, and which permit a rapid differentiation of the sensitive and insensitive parameters of theory. The data analysis depends strongly on the zfs parameter |E| and on the electron spin relaxation time τS,z along the zfs-PAS z-axis, but only very weakly on the other parameters of theory. A fit of the data to theory provided the values |E|=1.8±0.1 cm−1 and τS,z=8.0±0.3 ps. © 1997 American Institute of Physics.

Notes: An ab initio approach to the theoretical description of centrifugal distortions in molecules is presented. The method is based on the calculation of the electronic energy changes and gradients accompanying rotationally induced distortions. A centrifugal distortion pathway in nuclear coordinate space is defined. Results are presented at the HF/6-31G@B|@B| level for NH3, CH4, BF3, and SF6, and at the Guillemin–Zener level for H+2. Quartic centrifugal distortion parameters are computed for each molecule, with distortion anisotropies presented in the form of tensor coefficients. Centrifugally induced electric dipole moment coefficients are presented for CH4 and BF3 together with dipole moment changes for NH3. Agreement with experiment and with conventional theoretical descriptions is generally quite satisfactory, particularly for the tensor coefficients representing the cubic anisotropy in the distortions of the spherical tops CH4 and SF6.

Notes: Potential energy surfaces for the ground and excited electronic states responsible for the Hartley continuum of ozone are used to obtain quadratic, cubic, and quartic force constants. Vibrational dependence of rotational constants to sixth order is calculated by perturbation theory. The spectroscopic constants enable computation of rovibronic energy levels. Overlap of ground state and excited state perturbed vibrational wave functions yield Franck–Condon factors. Electric dipole allowed rovibronic transitions are generated under the Ir representation. The entire set of results generate the ultraviolet absorption spectrum. It is shown that inclusion of anharmonic terms in the vibrational Hamiltonian has a small effect upon the final spectrum, whereas rotational broadening plays a greater role in achieving agreement with experiment.

Notes: The emission properties of HCP excited to the A, B, and d electronic states have been studied. Lifetimes and quenching rates have been measured. By spectrally resolving the emission spectrum, the energy of 94 vibrational levels of the ground electronic state have been measured to an accuracy of (approximate)5 cm−1. These energy levels were fit to experimental accuracy by a rigid bender Hamiltonian thereby determining the bending potential over a range of bending angle from 0 to 100° (0–17 500 cm−1). An ab initio bending potential has been computed for HCP and found to be in excellent agreement with the experimentally fitted one over the range that the experimental data span. This potential predicts that HPC has an energy maximum with respect to the bending coordinate. The bending potential decreases monotonically by about 30 000 cm−1 in going from HPC to HCP.

Notes: We have obtained analytic expressions, parametric in centrifugal displacement coordinates, which provide exact classical descriptions of the rotational energy dispersions, that is, the dependence of the combined rotational and "electronic'' (vibrational potential) energies on the rotational angular momenta, for small molecular clusters bound by van der Waals interactions modeled by pairwise additive Lennard-Jones 6–12 potential energies. The clusters considered consist of three (equilateral triangle), four (tetrahedron), and six (octahedron) units and serve as models for small clusters of rare-gas atoms such as argon. This work represents an extension of our recently published study of analytic rotational energy dispersions for diatomic molecules bound by harmonic oscillator, Morse, or Lennard-Jones potentials [J. Mol. Spectrosc. 155, 205 (1992)]. A parallel set of studies were made using an angular momentum-conserving simulation program. The physical properties of the clusters that are addressed using our results include calculation of quartic and higher-order spectroscopic constants, location of rotational instabilities, and characterization of the "cubic'' anisotropies for the spherical top clusters A4 and A6. Of particular interest is the result that for each of these cluster types the preferred direction of the rotational angular momentum is parallel to a molecular fourfold axis, leading to reduced symmetries of D2d for tetrahedral A4 and D4h for octahedral A6.

Notes: The complexes of PF3 with Ar and Kr, were studied by Fourier transform microwave spectroscopy. The force constants and amplitudes of vibration for the van der Waals modes of the complexes and the average moments of inertia and structural parameters were estimated from the centrifugal distortion constants. The distance (Rc.m. )ave between the rare-gas atom and the center of mass of PF3 is 3.959 A(ring) for the Ar complex and 4.077 A(ring) for Kr while the angle (θc.m. )ave between the Rc.m. vector and the C3 axis of the PF3 is 69.30° and 67.25°, respectively. The dipole moments of both complexes and of free PF3 were determined. The induced dipole components estimated for the rare gas using electric fields from ab initio calculations of PF3 agree with the experimental values for a conformation with the rare gas over a PF2 face. The PF2 face conformation is also consistent with the observed and ab initio estimates of the 83 Kr nuclear quadrupole coupling constant for the 83 Kr–PF3 species.