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C4n cyclacenes exhibit strong bond-alternation in their equilibrium geometry. In the two equivalent geometries, the system keeps an essentially closed-shell character. The two energy minima are separated by a transition state suppressing the bond-alternation, where the wave function is strongly diradical. This paper discusses the physical factors involved in this energy difference and possible evaluations of the barrier height. The barrier given as the energy difference between the restricted density functional theory (DFT)/B3LYP for the equilibrium and the broken symmetry DFT/B3LYP of the transition state is either negative or small, in contradiction with the most reliable Wave Function Theory calculations. The minimal (two electrons in two molecular orbitals) Complete Active Space self-consistent field (CASSCF) overestimates the barrier, and the subsequent second-order perturbation cancels it. Due to the collective character of the spin-polarization effect, it is necessary to perform a full π CASSCF + second-order perturbation to reach a reasonable value of the barrier, but this type of treatment cannot be applied to large molecules. DFT procedures treating on an equal foot the closed-shell and open-shell geometries have been explored, such as Mixed-Reference Spin-Flip Time-dependent-DFT and a new spin-decontamination proposal, namely, DFT-dressed configuration interaction, but the results still depend on the density functional. M06-2X without or with spin-decontamination gives the best agreement with the accurate wave function results. [ABSTRACT FROM AUTHOR]

Broken-symmetry calculations of diradicals exploit the mean-field energies of determinants that are not eigenfunctions of the Ŝ 2 operator, the mean value of which is close to 1 for the ms = 0 solution. This spin contamination must be corrected. Two different contributions affect ⟨ Ŝ 2 ⟩ , namely, the mixing between neutral and ionic valence bond components, the so-called kinetic exchange, which decreases ⟨ Ŝ 2 ⟩ , and the spin polarization of the supposedly closed shell orbitals, which increases ⟨ Ŝ 2 ⟩. The popular Yamaguchi formula is valid for the first effect but irrelevant for the second one. From a few constrained broken-symmetry calculations, one may treat separately the two contributions and apply their specific spin decontamination correction. This work proposes a consistent spin-decontaminated procedure for the evaluation of singlet–triplet gaps in diradicals. [ABSTRACT FROM AUTHOR]

Spin-polarization effects may play an important role in free radicals and in the magnetic coupling between radical centers. Starting from restricted open-shell calculations, i.e., a closed-shell description of the non-magnetic core electrons, a low-order perturbation expansion identifies the spin-polarization contribution to the energy of mono-radicals and to singlet–triplet energy differences in diradicals. Broken-symmetry (BS) single-determinant calculations introduce only a fraction of spin-polarization effects, and in a biased manner, since BS determinants are not spin eigenfunctions. We propose a simple technique to correctly evaluate spin-polarization energies by taking into account the effect of spin-flip components on one-hole one-particle excited configurations. Spin-decontamination corrections are shown to play a non-negligible role in the BS evaluation of bond energies. The importance of spin decontamination is illustrated in cases for which spin polarization is the leading contribution to the singlet–triplet gap, which characterizes twisted conjugated double bonds and disjoint diradicals. [ABSTRACT FROM AUTHOR]

Taking as an example the simple CH3 radical, this work demonstrates the cooperative character of the spin-polarization phenomenon of the closed-shell core in free radicals. Spin polarization of CH σ bonds is not additive here, as spin polarization of one bond enhances that of the next bond. This cooperativity is demonstrated by a series of configuration interaction calculations converging to the full valence limit and is rationalized by analytic developments. The same phenomenon is shown to take place in those diradicals where spin polarization plays a major role, as illustrated in square planar carbo-cyclobutadiene C12H4. The treatment of cooperativity represents a challenge for usual post-Hatree–Fock methods. [ABSTRACT FROM AUTHOR]

Optimal geometries, harmonic vibrational frequencies, and relative energies of different sets of metal complexes are calculated using the effective group potentials (EGP) method. All of the systems under consideration contain the cyclopentadienyl (Cp) ligand.

The present comment formulates some recommendations regarding the use of broken-symmetry Unrestricted Density Functional Theory (UDFT) solutions in those polyradical architectures predicted to be of ground-state singlet character according to Ovchinnikov's rule. It proposes a procedure to identify the number of open shells, to reach the relevant Ms = 0 solution, and to estimate the low-energy spectrum of the states which keeps this number of open shells. [ABSTRACT FROM AUTHOR]

Analysis of the parent phosphinoamide/iminophosphide anion, H2PNH- and its fluoro derivatives, involved ab initio computations. A saddle point connects the pair of real minima observed for the geometries, optimized at the SCF level. The pair of minima match cis and trans conformations. The saddle point, in a majority of instances, matches the rotational barrier about the P-N bond.

The ab initio calculations of the potential energy surfaces for all X2H(sub 5)(super +) identified the stationary points, the isomeric possibilities, the intramolecular rearrangement saddle points and the dissociation products of several analogue structures. These ethyl cation forms featured nearly identical energies with relatively small distance barriers. Moreover, modelling studies were used to derive the values of the observed minimum energy barriers and relative stabilities.

The double exchange is a well-known and technically important phenomenon in solid state physics. Ionizing a system composed of two antiferromagnetically coupled high-spin units, the ground state of which is a singlet state, may actually produce a high-spin ground state. This work illustrates the possible occurrence of such a phenomenon in organic chemistry. The here-considered high-spin units are triangulenes, the ground state of which is a triplet. Bridging two of them through a benzene ring produces a molecular architecture of singlet ground state. A careful exploitation of a series of unrestricted density functional calculations enables one to avoid spin contamination in the treatment of the doublet states and shows that under ionization the system becomes of quartet multiplicity in its ground state. The possibility to align more than three spins from conjugated hydrocarbon polyradicals is explored, considering partially hydrogenated triangulenes. A dramatic example shows that ionization of a singlet ground state molecule may generate a decuplet. [ABSTRACT FROM AUTHOR]

The measurement of the steric repulsion generated by two facing side chains at a cis-peptide bond linking two (L-) amino acids is presented. Analogous treatments on larger radicals as seen in leucine or phenylalanine dimers have not changed the increment, which has defined the basic reference per-plaque quota to be overcome along all-cis chains.

The theoretical exploration of an all-cis L-homochiral [alpha]-aminoacid open chain, namely all-cis tridecaalanine structure is presented.

The structural and energetic issues associated with the peculiar arrangements of amide bonds are examined through quantum calculations. The frame of radiating cis-amide bonds can be considered as defining a new kind of peptidic material, endowed with specific self-assembling properties.

1,2,4-triazole-3,5-diylidene was used as a building block for organometallic polymer synthesis. Ab initio calculations were performed on bis(carbenes) to confirm its rigid pi-conjugated system. Bis(carbenes) was calculated to be planar as a result of the interaction between the vacant carbene p-orbital and the nitrogen lone pairs. It was also observed to exhibit several aromatic characteristics.

Electron-deficient single bridges occur in charged species like B2H7(super 1-), in organic cations such as the nonclassical form of the ethyl cation and sometimes in neutral systems such as Sn2H4 and Pb2H4. The formation of an X-H-Y electron-deficient connectivity from X-H+Y fragments involves the charge displacement of the X-H bonding electron pair. Migration in symmetrical systems occur without any disproportionation. On the other hand, electron transfer may be beneficial for ionic systems, as long as the starting fragments are either X-H(super 1-) or Y1+.

Some conjugated alternant hydrocarbons, of singlet ground state according to Ovchinnikov's rule, may exhibit strong polyradical character, despite admitting complete pairing of electrons in bond orbitals between adjacent atoms. Typical organizations of this kind are encountered in polycyclic frames supporting two or more extracyclic methylene groups. Lewis bond pairing would require quinonization of six-membered rings, whereas safeguarding aromaticity proves sufficient to impose ground-state open-shell character, that is, the existence of unpaired electrons, providing the number of benzene rings to be quinonized is larger than two. Several examples built as variations around para-polyphenylene frames are examined through unrestricted DFT (UDFT) calculations, using various methods for spin decontamination of wavefunctions, geometries, and singlet-triplet energy gaps. They all illustrate how it is possible to conceive architectures that can be written with a closed-shell bond pairing, although they exhibit a large number of unpaired electrons. The same analyses also apply to systems in which quinonization would not kill but only reduce the number of unpaired electrons. [ABSTRACT FROM AUTHOR]