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Density functional calculations are used to generate a series of the maximum-spin lithium clusters n+1 Li n (n)2-12). These clusters do not possess any electron pairs and have formally a bond order of zero but are nevertheless strongly bound by what we describe here as "ferromagnetic bonding" (FM bonding). The FM bonding energy rises from 1.7 kcal mol -1 for the dimer to 145 kcal mol -1 for the dodecamer, and the bond energy per atom converges for cluster sizes of n ) 11-12 reaching values of 11-12 kcal mol -1 atom -1 . In line with previous studies of such clusters (Isr.

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A key step in cytochrome P450 catalysis includes the spin-state crossing from low spin to high spin upon substrate binding and subsequent reduction of the heme. Clearly, a weak perturbation in P450 enzymes triggers a spin-state crossing. However, the origin of the process whereby enzymes reorganize their active site through external perturbations, such as hydrogen bonding, is still poorly understood. We have thus studied the impact of hydrogen-bonding interactions on the electronic structure of a five-coordinate iron(III) octaethyltetraarylporphyrin chloride. The spin state of the metal was found to switch reversibly between high (S=5/2) and intermediate spin (S=3/2) with hydrogen bonding. Our study highlights the possible effects and importance of hydrogen-bonding interactions in heme proteins. This is the first example of a synthetic iron(III) complex that can reversibly change its spin state between a high and an intermediate state through weak external perturbations.

We have investigated the model system H2S∴SH2 +, i.e., the sulfur−sulfur bound dimer radical cation of H2S, using both density functional theory (LDA, BP86, PW91) and traditional ab initio theory (up to CCSD(T)). Our purpose is to better understand the nature of the three-electron bond. The S−S bond length is 2.886 Å and the bond enthalpy (for 298.15 K) amounts to −40.7 kcal/mol at the BP86/TZ2P level. The best ab initio estimates for the S−S bond strength (our CCSD(T)/6-311++G(2df,2pd)//MP2(full) and literature values) are some 10 kcal/mol weaker than those from nonlocal DFT. It is shown, using an energy decomposition scheme for open-shell systems, that the sulfur−sulfur bond (ΔE = ΔE 2c-3e + ΔE elst) is nearly 60% provided by the three-electron bond (ΔE 2c-3e) between the unpaired sulfur 3p x electron on H2S+• and the sulfur 3p x lone pair on H2S; electrostatic attraction (ΔE elst) is important, too, with a contribution of somewhat more than 40%. We show furthermore that the three-electron bond (ΔE 2c-3e = ΔE 2c- 1e + ΔE Pauli) can be conceived as and quantitatively analyzed in terms of a one-electron bond (ΔE 2c-1e), arising from the β-electron of the H2S lone pair interacting with the corresponding empty β-spin orbital of H2S+•, opposed by the Pauli repulsion (ΔE Pauli) between the α-electrons of the H2S lone pair and H2S+• SOMO.

High-spin lithium clusters ( n +1Li n , n = 2−6) have been studied using several density functional methods. Although these high-spin clusters have no bonding electron pairs, they are stable with respect to isolated lithium atoms. Full geometry optimizations have been performed with alternatives under a variety of symmetry constraints which led to local minima. In general, the most stable structure is the one with the maximum coordination number for the lithium atoms. The agreement between B3P86/cc-pVDZ and B3PW91/cc-pVDZ density functional methods with UQCISD(T)/6-31G* and UCCSD(T)/cc-pVDZ calculations is excellent. Trends of bond dissociation energies are discussed as a function of total number of “bonds” and number of atoms.

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