We have estimated the complete basis set limits for the Hartree-Fock, MP2, CASSCF, and CASSCF+1+2 wave functions for the titled molecules and calculated the molecular quadrupole moment as a function of bond length. Our recommended values for Θ (V)0,J)0) compare favorably to the current experimental values and previous high-level calculations. To aid in the analysis of the relationship between the molecule's electronic structure and quadrupole moment, we introduce the concept of a quadrupole moment density that permits one to write the molecular quadrupole moment as a sum of the separated atoms quadrupole moments and a purely molecular contribution. The quadrupole density provides a (reference state dependent) means of determining the contribution to Θ from various regions in the molecule and gives considerable insight into the relationship between the electron density and the magnitude and sign of Θ, and it allows a detailed assessment of the contribution of electron correlation to Θ.
This paper reports the theoretical results of a thorough, state-of-the-art, coupled-cluster, renormalized coupled-cluster, and vibrational study on the molecule imine peroxide, HNOO, in its trans conformation. This molecule is isoelectronic with ozone and presents many of the same difficulties for theory as ozone. We report both the theoretical geometry and the vibrational frequencies, including anharmonic corrections to the computed harmonic vibrational frequencies obtained by calculating the quartic force field at the high levels of coupled cluster theory, including CCSD(T) and its renormalized and completely renormalized extensions and methods including the combined effect of triply and quadruply excited clusters [CCSD(TQf) and CCSDT-3(Qf)]. The motivation behind our study was the disagreement between two previous reports that appeared in the literature on HNOO, both reporting theoretical (harmonic) and experimental (matrix isolation) vibrational spectra of HNOO. Our new theoretical results and our analysis of the previous two papers strongly suggest that the correct assignment of vibrational spectra is that of Laursen, Grace, DeKock, and Spronk (J. Am. Chem. Soc. 1998, 120, 12583−12594). We also compare the electronic structure of HNOO with the isoelectronic molecules HONO and O3. The NO and OO bond lengths are practically identical in HNOO, in agreement with the identical OO bond lengths (by symmetry) in ozone. Correspondingly, the NO and OO stretching frequencies of trans-HNOO are in close proximity to each other, as are the symmetric and antisymmetric OO stretching frequencies in O3. This is in contrast to the electronic structure of HONO, which has a large difference between the two NO bond lengths, and a correspondingly large difference between the two NO vibrational frequencies. These results are readily understood in terms of simple Lewis electron dot structures.
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