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The ground state restricted Hartree Fock (RHF) wave function of C(60) is found to be unstable with respect to spin symmetry breaking, and further minimization leads to a significantly spin contaminated unrestricted Hartree Fock (UHF) solution ( = 7.5, 9.6 for singlet and triplet, respectively). The nature of the symmetry breaking in C(60) relative to the radicaloid fullerene, C(36), is assessed by energy lowering of the UHF solution, , and the unpaired electron number. We conclude that the high value of each of these measures in C(60) is not attributable to strong correlation behavior as is the case for C(36). Instead, their origin is from the collective effect of relatively weak, global correlations present in the π space of both fullerenes. Second order perturbation (MP2) calculations of the singlet triplet gap are significantly more accurate with RHF orbitals than UHF orbitals, while orbital optimized opposite spin second order correlation (O2) performs even better.

Electron capture and electron transfer dissociations are bioanalytical methods for fragmenting cations after reduction by an electron. Previous computational studies based on conventional DFT schemes have concluded that the first step of these processes, the attachment of the electron, leads to extensive delocalization of the spin density in the intermediate radical cation. Here we show that most DFT methods produce unphysical results when studying single electron reduction of a dicationic peptide. This is not the case for post-HF methods and long-range corrected functionals that show satisfying electron affinities, intermolecular interaction energies, and spin density trends. Our results suggest that the charged group with the highest electron affinity on the precursor cation is also the site of spin density in the electronic ground state after electron attachment. These findings have important implications for the interpretation of experimental data from electron-based processes in biomolecules and may guide the development of new functionals.SECTION: Molecular Structure, Quantum Chemistry, General Theory E lectron capture dissociation (ECD) 1,2 and electron transfer dissociation (ETD) 3 are efficient fragmentation techniques, available on Fourier transform ion cyclotron resonance and ion trap or Q-TOF mass spectrometers, respectively. These methods have an important potential for the structural analysis of peptides or proteins. 4À6 Electron capture by, or transfer to, cationic peptides or proteins in the gas phase characteristically results in cleavage of NÀC R bonds, which makes these techniques complementary to collision-induced dissociation (CID) where peptidic bonds are cleaved. ECD and ETD also have some advantages compared with CID: they preserve labile post-translational modifications, and they allow fragmentation of larger peptides and even entire proteins without prior digestion, making these approaches particularly suitable for top-down proteomics.In ECD and ETD, a multiply charged cation is partially reduced by receiving one electron, thereby converting it from a closed-shell specie to an intermediate cation-radical that undergoes fragmentation. The exact mechanism(s) implicated in such a process are still a matter of active discussion from both computational and experimental points of view. 7À12 Although there is consensus that the spin must become localized on a backbone carbonyl carbon to precipitate cleavage of an adjacent NÀC R bond, the pathway the electron takes to reach the carbonyl remains unresolved. Does the electron attach itself directly to an amide π* orbital, which may be an excited state (the UtahÀWashington mechanism), 13,14 or are preliminary structural rearrangement(s) needed (the Cornell mechanism)? 1 Recent theoretical studies indicate that the vertical reduction of protonated peptides lead to a highly delocalized spin density in the ground state. Delocalization could extend to spatially remote charged groups (ammonium, 15,16 guanidinium, 17 histidinium, 18 amide and/or carbonyl...

Results from ab initio electronic structure theory calculations on model systems allow for the detailed comparison of tunneling through covalently bonded contacts, hydrogen bonds, and van der Waals contacts. Considerable geometrical sensitivity as well as an exponential distance dependence of the tunneling is observed for tunneling through various nonbonded contacts. However, the fundamental result from the present study is that at most a modest difference is observed between tunneling mediated by H-bonds and tunneling mediated by van der Waals contacts at typical distances for each type of interaction. These results are considered in relation to the pathways model of Beratan and Onuchic, and implications for understanding long-range tunneling in biological systems are discussed.

Theory and implementation of the analytical nuclear gradient is presented for orbital optimized scaled opposite-spin perturbation theory (O2). Evaluation of the O2 analytical gradient scales with the 4th power of molecular size, like the O2 energy. Since the O2 method permits optimization of the orbitals in the presence of wavefunction-based electron correlation, it is suitable for problems where correlation effects determine the competition between localization and delocalization of an odd electron, or hole. One such problem is the description of a neutral soliton defect on an all-trans polyacetylene chain with an odd number of carbon atoms. We show that the results of the O2 method compare well to benchmark values for small polyenyl radicals. O2 is also efficient enough to be applied to longer chains where benchmark coupled cluster methods are unfeasible. For C(41)H(43), unrestricted orbital O2 calculations yield a soliton length of about 9 carbon atoms, while other unrestricted orbital methods such as Hartree-Fock, and the B3LYP and ωB97X-D density functionals, delocalize the soliton defect over the entire chain. The O2 result is about half the width inferred experimentally.