Mulcahy, M. E.; Magnera, T. F.; Michl, J. “Molecular Rotors on Au(111): Rotator Orientation from IR Spectroscopy”, J. Phys. Chem. C 2009, 113, 20698.

ABSTRACT:

Gold surfaces carrying altitudinal molecular rotors firmly attached through sulfur and/or mercury atoms have been examined by IR spectroscopy. The presence of an intact rotator has been confirmed and its average orientation with respect to the gold surface determined by single reflection thin-layer attenuated total reflection (ATR) spectroscopy, using metal surface selection rules. The requisite IR polarization directions were obtained from IR linear dichroism of a model rotator oriented in stretched polyethylene. The results are compatible with those of prior differential barrier height imaging measurements and molecular mechanics calculations.

Bande, A.; Michl, J. “Conformational Dependence of σ-Electron Delocalization in Linear Chains: Permethylated Oligosilanes”, Chem. Eur. J. 2009, 15, 8504.

ABSTRACT:

The effects of σ-electron delocalization on optical properties of saturated linear chains of permethylated oligosilanes are strongly conformation dependent. We analyze the origin of the conformational dependence of the energies of molecular orbitals and of electronic excitations in simple intuitively understandable terms using a first-order approximation to the Hückel version of the Ladder C model. The analysis is supported by comparison with results of numerical calculations by the time-dependent density functional theory, which agree well with experiment. To facilitate the comparison, a simple procedure has been developed that defines the overall and local fractional σ and π characters of a backbone molecular orbital and a fractional overall and local σσ* and σπ* characters of an excited state for any conformation of a linear chain.

Michl, J.; Sykes, E. C. H. “Molecular Rotors and Motors: Recent Advances and Future Challenges”, ACS Nano 2009, 3, 1042.

ABSTRACT:

At the “Molecular Rotors and Motors” symposium of the Spring 2009 ACS National Meeting in Salt Lake City (March 22-26) a diverse mix of talks addressed many current issues in the field. Speakers described topics that varied from single-molecule rotors and nanomachines to exquisite synthetic approaches towards building functional materials, and mathematical and computational methods aimed at uncovering design opportunities and highlighting the fundamental limitations of molecular motors. While the realization of building useful nanomachines remains far off, a general consensus abounded that investigating biological systems and understanding the implications of the laws of thermodynamics and quantum mechanics for the behavior of nanostructures will help drive important advances in the quest for molecular machinery. Molecular rotors were demonstrated to have practical applications as probes for microviscosity and many speakers presented experimental studies that indicated that highly directed translation and rotation of individual molecules, as well as interacting dipolar arrays, are just around the corner. While this Nano Focus is not intended to be a comprehensive review of the subject, it will focus on several key advances that were presented at the ACS meeting and highlight future challenges for the field of molecular rotors and motors.

Oliva, J. M.; Serrano-Andrés, L.; Havlas, Z.; Michl, J. “On the Electronic Structure of a Dianion, a Radical Anion, and a Neutral Diradical Carborane Dimer”, J. Mol. Struct. 2009, 912, 13.

ABSTRACT:

The electronic structure of a neutral, a radical anion, and a dianion carborane dimer connected via an acetylenic bridge unit (HB)11C-CC-C(BH)11 is analyzed by quantum chemical methods. Geometries, relative stabilities, and singlet-triplet gaps are determined in the neutral and dianion species for the lowest-lying singlet and triplet states and for the doublet ground state in the radical anion. As for the recently studied biradical compounds derived from o-carborane, mcarborane
and p-carborane [J. Chem. Theory and Comput. 4 (2008) 1338] via double hydrogen abstraction, the neutral dimeric compound displays a biradical ground-state structure in which both singlet and triplet state are practically degenerate, with the singlet state lying slightly lower in energy ( 0.005 eV) at both DFT broken-symmetry and ab initio CASPT2 levels of theory. The singlet-triplet splitting is therefore close to k_B·T at room temperature, approaching the microwave region of the electromagnetic spectrum. The neutral dimer biradical becomes then a strong candidate to behave as a molecular magnet in molecular architectures based on carborane units. It is also shown that the system is a powerful electron acceptor with increasing stability from the neutral to the radical anion and dianion systems.

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/ccpVTZ (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. The natural 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.