No abstract

Vertical excitation energies for electronic transitions from the ground state to the first two excited states of phenol, mono- and disubstituted methoxyphenols and methyl-substituted phenols have been characterized with the Time-Dependent Density Functional Theory (TD-DFT), the Complete Active Space Self-Consistent Field method (CASSCF) and the Coupled Cluster with Single and Double Excitations Equation-of-Motion approach (CCSD-EOM) to simulate and interpret experimental ultraviolet absorption spectra. While CASSCF excitation energies for the first two transitions either are grossly overestimated or exhibit a weak correlation with experimental data, both TD-DFT and CCSD-EOM perform very well, reproducing the spectral shifts of both the primary band and secondary band observed upon substitution. The conformational dependence of the calculated excitation energies is generally smaller than the shifts caused by substitution.

A combination of density-functional theory and natural resonance theory has been used to show that a complete description of the electronic structure of nitrilimines, R(1)CNNR(2), requires four resonance structures (propargylic, allenic, 1,3-dipolar and carbenic); appropriate substituents were shown to enhance the carbene character of nitrilimines to the point where they may be considered stable carbenes.

Calculated geometries are presented for five aromatic N-sulfinylamines, RNSO. All carry at least one hydrogen atom on the aromatic ring ortho to the NSO substituent, and all show a preference for a planar isomer in which the substituent's oxygen atom is oriented toward an ortho hydrogen. All these isomers exhibit a shortening of the corresponding CH bond. Using the quantum theory of atoms in molecules (AIM), it can be shown that a CH···O bonding interaction exists that meets all the characteristics of an anti-hydrogen bond.

Calculations at Becke3LYP, MP2, and QCISD levels of theory are presented for different molecular structures of the 2-norbornyl cation (1). With the aid of the quantum theory of atoms in molecules (AIM) we show that 1 has a T-structure at the C6−C−C face (we view it as a π-complex with a tetracoordinate carbon atom C6) and that the (“nonclassical”) bridged structure with a pentacoordinate C6, which is not a minimum on the potential energy surface, is only reached when the C6−C1 and C6−C2 internuclear distances are shortened to 165 pm. At the Becke3LYP/6-31G(d,p) level in the gas phase this bridged species is 7.25 kcal mol-1 higher in energy than the equilibrium optimized geometry. NMR calculations show that 13C chemical shifts of C1, C2, and C6 of the bridged 165 pm species differ significantly from the experimental values.