Volkis, V.; Mei, H.; Shoemaker, R. K.; Michl, J. “LiCB₁₁(CH₃)₁₂ Catalyzed Radical Polymerization of Isobutylene: Highly Branched Polyisobutylene and an Isobutylene - Ethyl Acrylate Copolymer”, J. Am. Chem. Soc. 2009, 131, 3132.

ABSTRACT:

In the presence of LiCB₁₁Me₁₂ catalyst and a non-oxidizing radical initiator, isobutylene undergoes radical chain polymerization in an inert solvent at ambient temperature and pressure to give a new highly branched form of polyisobutylene. Copolymerization with ethyl acrylate is also possible.

Piqueras, M. C.; Crespo, R.; Michl, J. “Interpretation of the Electronic Spectra of Four Disilanes”, J. Phys. Chem. A., 2008, 112 13095.

ABSTRACT:

Time-dependent density functional theory (TD-DFT/B3LYP(AC)/cc-pVTZ/cc-pVTZ/6-311G//MP2/cc-pVTZ/cc-pVTZ/6-31G**) has been used to compute vertical excitation energies and oscillator strengths of the six low-lying excited states of four peralkylated disilanes, hexamethyldisilane (1), hexa-tert-butyldisilane (2),1,6-disila[4.4.4]propellane (3), and 1,7-disila[5.5.5]propellane (4). The results provide an accurate interpretation of the reported UV absorption spectra of 1-4 in solution, and for 1 also in the gas phase up to 62 000 cm-1. The excellent agreement of the calculated with the available experimental energies and oscillator strengths, and with magnetic circular (MCD) and linear (LD) dichroism, gives us confidence that the method will be useful for dependable interpretation of the electronic spectra of longer oligosilanes. Although the disilane chromophore finds itself in quite different environments in 1-4, its fundamental characteristics remain the same, with one important exception. In all four compounds, the first valence excited state is due to an electron promotion from the σ1 HOMO to the π1* orbital, and the second valence excited state to a promotion from the σ1 HOMO to the σ1* orbital. Surprisingly, however, it is only in 2, which has an extraordinarily long SiSi bond, that the terminating σ1* orbital is the σ*(SiSi) antibond, as anticipated, and the σσ* transition has the expected very high oscillator strength. In 1, 3, and 4, the σ*(SiSi) antibonding orbital is high in energy and does not play any role in low-energy excitations. Instead, the terminating orbital of the σ1σ1* excitation is represented by Si-alkyl antibonds, combined symmetrically with respect to rotation around the SiSi axis and antisymmetrically with respect to operations that interchange the two Si atoms. The common assumption that the characteristic intense σσ* transitions of longer peralkylated oligosilanes extrapolate to the lowest σσ* transition in common peralkylated disilanes is incorrect, and only the weak σπ* transitions extrapolate simply.

Winston, E. B.; Lowell, P. J.; Vacek, J.; Chocholoušová, J.; Michl, J.; Price, J. C. “Dipolar molecular rotors in the metal-organic framework crystal IRMOF-2”, Phys. Chem. Chem. Phys. 2008, 10, 5188.

ABSTRACT:

Rotating polar linker groups in the cubic metal–organic framework single crystal known as IRMOF-2 were investigated for
freedom of motion, response to an external electric field, and effects of dipole–dipole interactions. The crystals consist of
octahedrally coordinated zinc oxide clusters linked by the bromoterephthalate group, which contains a rotatable bromo-p-phenylene moiety. We confirmed the rotation by dielectric spectroscopy and found a 7.3 kcal mol
1 barrier. The non-polar analog, IRMOF-1, containing terephthalic acid, was used as a control system. DFT and MP2 computations of the rotational barrier yield results in agreement with the observation, with B3LYP/SDD being the best. A Monte Carlo analysis of the equilibrium polarization fluctuations was used to assess the possibility of polar ordering and the potential for electro-optic applications.

Veis, L.; Čársky, P.; Pittner, J.; Michl, J. “Coupled Cluster Study of Polycyclopentanes: Structure and Properties of C5H2n, n = 0 - 4”, Collect Czech. Chem. Commun. 2008, 73, 1525.

ABSTRACT:

The title hydrocarbons have been examined by the CCSD(T)/cc-pVTZ (singlets) and UMP2/cc-pVTZ (triplets) methods. They were confirmed to represent local minima on the singlet potential energy surface, while 1,3-biradical, 1,4-biradical, or carbene structures were found on the triplet surface, including an intermediate for the triplet energy transfer from one to the other double bond of 1,4-pentadiene. Bonding is discussed in terms of Weinhold’s NBO theory and the absence of a simple correlation between bond strength and bond length in these highly strained systems is pointed out. Predictions of NMR, IR, and Raman spectra are provided.

Vacek, J.; Caskey, D. C.; Horinek, D.; Shoemaker, R. K; Stang, P. J.; Michl, J. “Pyridine Ligand Rotation in Self-Assembled Trigonal Prisms. Evidence for Intra-Cage Solvent Vapor Bubbles?”, J. Am. Chem. Soc. 2008, 130, 7629.

ABSTRACT:

The rate of interconversion of the two inequivalent edges of the pyridine rings in the trigonal prism 3c, self-assembled from 3 equiv of the star connector, tetrakis[4-(4-pyridylethynyl)phenyl]cyclobutadienecyclopentadienylcobalt, and 6 equiv of a platinum linker, cis-(Me₃P)₂Pt²⁺2TfO^-, was determined by DNMR in nitromethane. It exhibits a highly unusual bilinear Eyring plot. In the low temperature regime, the activation enthalpy ΔH‡ is ∼12 kcal/mol and an activation entropy ΔS‡ ranges from ∼-15 to ∼0 cal/mol ·K as a function of the nature and concentration of the anions present. The reaction is attributed to hindered rotation of the pyridine rings about the Pt-N bond, facilitated by a tight pairing with a counterion. Above a counterion-dependent limiting temperature, ΔH‡ and ΔS‡ change abruptly to ∼35 kcal/mol and ∼60 cal/mol · K, respectively. The changes largely compensate, such that the reactions have comparable rates in the two regimes, both amenable to DNMR measurement, but their mechanisms clearly differ. Several kinetic models for the involvement of ion pairing equilibria fit the observed data nearly equally well, and they all contain a reaction step with high ΔH‡ and ΔS‡ values in the high-temperature regime. Its mechanism is proposed to involve a counterion-assisted reversible dissociation of one or two adjacent Pt-N bonds, followed by nearly free rotation of the terminal pyridine ring or rings and subsequent bond reclosure, which is similar to the last presumed step in the initial prism assembly. An interpretation of the very high ΔS‡ value is suggested by molecular dynamics calculations: at equilibrium, there is a bubble of gaseous nitromethane solvent inside the prism, and it collapses when the prism opens as the transition state is reached. A simple calculation of the entropy of cavitation provides quantitative support for this tentative proposal. The presence of such voids might be generally important for the formation and properties of self-assembled cages.

Caskey, D. C.; Yamamoto, T.; Addicott, C.; Shoemaker, R. K.; Vacek, J.; Hawkridge, A. M.; Muddiman, D. C.; Kottas, G. S.; Michl, J.; Stang, P. J. “Coordination-Driven Face-Directed Self-Assembly of Trigonal Prisms. Face-Based Conformational Chirality”, J. Am. Chem. Soc. 2008, 130, 7620.

ABSTRACT:

The coordination-driven self-assembly of four different trigonal prisms from 3 equiv of one of four different tetrapyridyl star connectors and 6 equiv of a platinum linker dication in nitromethane is presented. This face-directed approach affords high yields without template assistance. The prisms have been characterized by multinuclear and DOSY NMR and dual ESI-FT-ICR mass spectrometry. The use of a conformationally chiral star connector leads to a conformationally chiral prism when connector arm ends attached to a vertex have a strongly correlated twist sense and chirality is communicated across polyhedral faces, edges, and vertices. Molecular mechanics results suggest that in the smallest prism 3d collective effects dominate and the all-P and all-M conformers are strongly favored. NMR data prove that the two edges of the pyridine rings in the triflate salts of 3a-3d are distinct. An Eyring plot of rates obtained from line-shape analysis and 1-D EXCHSY NMR yields an activation enthalpy ΔH‡ of ∼12 kcal/mol and activation entropy ΔS‡ of ∼-15 cal/mol ·K for the edge interconversion process, compatible with pyridine rotation around the Pt-N bond. For 3c, this behavior is observed only up to ∼318 K. At higher temperatures, the Eyring plot is again linear but follows a very different straight line, with a ΔH‡ of ∼35 kcal/mol and ΔS‡ of ∼60 cal/mol · K. This highly unusual result is further investigated and discussed in the following companion paper.

Michl, J. “Chemistry of the Three-Dimensionally Aromatic CB11 Cage”, Pure Appl. Chem. 2008, 80, 429.

ABSTRACT:

After a brief introduction to the electronic structure of the three-dimensionally aromatic icosahedral closo-monocarbadodecaborate anion CB₁₁H₁₂^–, some recent results for its permethylated version, CB₁₁H₁₂^– and three highly reactive electroneutral analogs are presented and discussed. These are the radical CB₁₁H₁₂^•, the boronium ylide CB₁₁Me₁₁ with a naked boron vertex, and the isomeric carbonium ylide with a naked carbon vertex. These ylides are probably better viewed as unusual types of singlet borylene and carbene, respectively.

Michl, J.; Nozik, A. J.; Chen, X.; Johnson, J. C.; Rana, G.; Akdag, A.; Schwerin, A. F. “Toward singlet fission for excitonic solar cells”, in Organic Photovoltaics VIII, Kafafi, Z. H. and Lane, P. A., Eds. 2007 Proc. of SPIE Vol. 6656, p. 66560E1.

ABSTRACT:

Sensitizer dyes capable of producing two triplet excited states from a singlet excited state produced by the absorption of a single photon would allow an increase of the efficiency of photovoltaic cells by up to a factor of 1.5, provided that each triplet injects an electron into a semiconductor such as TiO₂. Although singlet fission in certain crystals and polymers was reported long ago, little is known about its efficiency in dyes suitable for use as sensitizers of photoinduced charge separation on semiconductors surfaces. Biradicaloids and large alternant hydrocarbons are desirable parent structures likely to meet the requirement Ei(T₂), E(S₁) > 2E(T₁) for the excitation energies of the lowest excited singlet (S₁) and the two triplet (T₁, T₂) states. We report results for 1,3-diphenylisobenzofuran, a model compound of the biradicaloid type. Its energy levels satisfy the desired relation, and in solution it shows no triplet formation by intersystem crossing. In the neat solid state, it forms triplets efficiently, and indirect evidence suggests that this is due to singlet fission. This appears to be the first compound displaying SF by design. When two such chromophores were combined into dimers, triplet formation yields of up to 9% were observed in polar solvents, possibly due to singlet fission, but possibly due to intersystem crossing. The triplet formation occurs in two steps, via an intermediate assigned as an intramolecular charge-transfer state and responsible for most of the observed excitation loss.