Ilya N. Ioffe

Two Isomers of C60F48: An Indented Fullerene

Deflated buckyballs: The single-crystal structure of C60 F48 ⋅2 (mesitylene) revealed the presence of both D3 and S6 isomers in the same crystal. C(sp2 )-C(sp2 ) bonds (1.30 Å) are much shorter than C(sp3 )-C(sp3 ) bonds (1.54-1.63 Å). The C60 cage is characterized by concave areas in the regions of six double bonds. Each double bond is effectively shielded by four F atoms, which accounts for the low reactivity of C60 F48 .

Organometallic complexes of graphene: toward atomic spintronics using a graphene web.

Graphene|metal|ligand systems open a new realm in surface magnetochemistry. We show that by trapping metal atoms in the two-dimensional potential lattice of a graphene-ligand interface it is possible to build a chemical analogue of an optical lattice, a key setup in quantum information and strongly correlated systems. Employing sophisticated first-principles calculations, we studied electronic and dynamic properties of graphene|metal|ligand assemblies and showed that there is a general principle--spin-charge separation in π-d systems--that underlies the possibility of synthesizing and controlling such systems. We find that ligands can work as a local gate to control the properties of trapped metal atoms and can impose bosonic or fermionic character on such atomic nets, depending on the ligands nature. Remarkably, the magnetization energy in such systems reaches record-high values of ca. 400 meV, which makes the respective magnetic phenomena utilizable at room temperature. Accompanied by spin polarization of the graphene π-conjugated system it leads to spin-valve materials and brings the realization of quantum computing one step closer.

Photoisomerization dynamics and pathways of trans- and cis-azobenzene in solution from broadband femtosecond spectroscopies and calculations.

The photoisomerization of azobenzene in solution was studied experimentally and by calculations. trans-to-cis and cis-to-trans dynamics are described through broadband transient absorption, fluorescence, and stimulated Raman spectroscopy. Transient absorption was extended to cover not only the nπ* band but also the ππ* band in the ultraviolet. Isomerization yields are used for a quantitative comparison of trans and cis transient spectra under different excitation. For the trans-to-cis path upon nπ*(S(1)) excitation, the evolution develops with 0.3, 3, and 16 ps. The first two times reflect population relaxation to a local minimum S(1t )(L) and subsequent transition to a dark intermediate S(1t)(D) over an 8 kJ/mol barrier. The existence of stationary points S(1t)(L) and S(1t)(D) is confirmed by quantum-chemical calculations. The third time corresponds to S(1t) (D) → S0 relaxation to the ground state via an S1/S0 conical intersection over a 12 kJ/mol barrier. Thus, the 16 ps time constant is attributed to the isomerization process and not to vibrational cooling, contrary to the current view and in line with the previous interpretation by Lednev et al. (J. Phys. Chem. 1996, 100, 13338). The decay of the long-lived intermediate S(1t)(D) is consistent with the hula twist rather than with the inversion mechanism. For the cis-totrans reaction following nπ* excitation, signal decay is strongly nonexponential, with 0.1 and 1 ps. The latter (1 ps) is much shorter than the 16 ps decay of the trans isomer, implying different S1/S0 conical intersections and relaxation paths for the cis-totrans and trans-to-cis reaction. New results are also obtained with ππ*(Sn) excitation. Thus, for trans-azobenzene, 50% of the population relaxes to an S1 region, which is not accessible under nπ* excitation. For cis-azobenzene, up to 30% of the excited species isomerize to trans via an Sn/S1 intersection, resulting in a mixed cis/trans S1 population. The isomerization kinetics of azobenzene shows no viscosity dependence, putting into question the torsion mechanism and suggesting the hula-twist isomerization mechanism.

Fusing Pentagons in a Fullerene Cage by Chlorination: IPR D2‐C76 Rearranges into non‐IPR C76Cl24

As is well known, fullerenes obtained by conventional arcdischarge synthesis obey the isolated pentagon rule (IPR). Unless fullerene molecules are directly subjected to “fullerene surgery”, exohedral functionalization does not affect the connectivity of their carbon networks. Non-IPR fullerene isomers have been available through appropriate modifications of the arc-discharge methodology to synthesize an already chemically derivatized molecule in which the derivatization stabilizes the pentagon–pentagon junctions. In particular, non-IPR cages are quite common in endohedral metallofullerenes, as encapsulated metal atoms are likely to stabilize the fused pentagon fragments by charge-transfer binding to them. More recently, a number of unconventional exohedral fullerene derivatives, including C50Cl10, [5, 6] C56Cl10, [7] C66H4, [8] C68Cl4, [6] and non-IPR C60Cl8 and C60Cl12, [9] have been obtained by means of an arc-discharge process in presence of additives such as CCl4, Cl2, and CH4. Rare examples of more classical chemical approaches to nonIPR fullerenes are indirectly confirmed transformation of dodecahedrane into C20 [10] and synthesis of a C62 derivative with four-membered cycle in its carbon cage from C60. [11]

Photoisomerization of Stilbene: The Detailed XMCQDPT2 Treatment.

We report the detailed XMCQDPT2/cc-pVTZ study of trans-cis photoisomerization in one of the core systems of both experimental and computational photochemistry-the stilbene molecule. For the first time, the potential energy surface (PES) of the S1 state has been directly optimized and scanned using a multistate multiconfiguration second-order perturbation theory. We characterize the trans-stilbene, pyramidalized (phantom), and DHP-cis-stilbene geometric domains of the S1 state and describe their stationary points including the transition states between them, as well as S1/S0 intersections. Also reported are the minima and the activation barriers in the ground state. Our calculations correctly predict the kinetic isotope effect due to H/D exchange at ethylenic hydrogens, the dynamic behavior of excited cis-stilbene, and trans-cis branching ratio after relaxation to S0 through a rather unsymmetric conical intersection. In general, the XMCQDPT2 results confirm the qualitative adequacy of the TDDFT (especially SF-TDDFT) picture of the excited stilbene but also reveal quantitative discrepancies that deserve further exploration.

Terahertz Absorption Spectroscopy of a Liquid Using a Polarity Probe: A Case Study of Trehalose/Water Mixtures

Water is important for the structure, stability, and function of biomolecules. It can simplify the energy landscape for molecular recognition or protein folding and often controls the native stability. 2] Proton transfer through local water networks requires correlated movement of H-bonds, and conformational changes of proteins appear to be coupled to the dynamics of bulk and hydration water. To understand such processes the vibrational absorption spectrum of the biomolecule–water interface must be observed. The underlying dynamics are widely distributed in time, from fast vibrational modes to slow diffusive reorientation. In lowviscosity liquids, like water, the diffusive regime comprises processes on the picosecond to nanosecond timescale (corresponding to wavenumbers n< 1.5 cm ), which are captured by microwave dielectric spectroscopy. On the high-frequency side (in water above 1000 cm ) intramolecular vibrations are observed by infrared absorption spectroscopy. What is usually lacking is information on the intermolecular vibrational and librational dynamics, which are reflected in the intermediate segment of the spectrum, in the THz (up to 30 cm ) and farinfrared (FIR, 30–250 cm ) regions. It is just this intermediate regime in which processes associated with H-bond dynamics are expected. Unfortunately, the generation and detection of light is difficult here. Furthermore, the response of the biomolecule–water interface is generally not so different from that of bulk water. There is thus a clear need to develop local spectroscopic schemes which avoid contributions from the bulk and confine absorption measurements to the interfacial region. Using the polarity probe N-methyl-6-quinolone (MQ, inset Figure 3) we recently showed that the time-resolved Stokes shift (TRSS) of fluorescence reflects the infrared spectrum of the surrounding liquid. The effective distance for the interaction ranges up to approximately 15 ; spatial resolution of this size may therefore be achieved by linking the probe to the supramolecular structure of interest. What is missing to date is how to extract the (unknown) dielectric properties from a measured TRSS curve. This key step is introduced herein and tested with aqueous trehalose solutions. The disaccharide trehalose (inset Figure 1) is synthesized by some organisms in dry climates for protection against osmotic pressure and freezing; it alters the Hbonding structure of water and modifies the collective dynamics. Trehalose is therefore an intriguing biomolecular model solute for demonstrating the practical use and spectroscopic potential of MQ.

Femtosecond Raman spectra of cis-stilbene and trans-stilbene with isotopomers in solution

Femtosecond stimulated Raman spectra of trans-stilbene (D0), its isotopomers D2, D10, D12, (13)C2 and of cis-stilbene in hexane are measured in the ground (S(0)) and excited (S(1)) electronic states. The ground (13)C2 and excited D12 spectra are presented for the first time; the excited cis-spectra differ substantially from previously published ones. S(1) Raman bands of trans-stilbene are 20 cm(-1) wide corresponding to ~1 ps vibrational dephasing. For cis-stilbene the bands are broadened to 40 cm(-1) reflecting a short excited-state lifetime of 0.3 ps, in agreement with transient absorption data. From a dynamic shift of the 1569 cm(-1) band, pump-induced intramolecular cooling is estimated to be less than 20 K. Many S(1) Raman lines are detected for the first time. Vibrational spectra are calculated at MP2/cc-pVTZ (for S(0)) and XMCQDPT2/cc-pVTZ (for S(1)) levels of theory. Experimental and computational results can be used for a re-evalution of Rice-Ramsberger-Kassel-Marcus (RRKM) predictions for this famous photoisomeration reaction.

Trifluoromethylated [60]Fullerenes: Synthesis and Characterization

Abstract The high temperature reaction of C60 with silver(I) trifluoroacetate followed by 500°C sublimation and HPLC purification has led to the characterization of the trifluoromethyl‐[60]fullerenes 1,4‐C60(CF3)2, C s‐C60(CF3)4, C 1‐C60(CF3)4, and C 1‐C60(CF3)6 by EI‐MS and 19F NMR. The compounds C 1‐C60(CF3)4 and C 1‐C60(CF3)6 were obtained with 90+% compositional purity. A sample of C60(CF3)2 also contained ca. 15–20% of a C s‐symmetry isomer of C60(CF3)4. The structural assignments are based on calculations at the AM1 and DFT levels of theory.

Electron affinity of some trimetallic nitride and conventional metallofullerenes

Abstract The electron affinity of a series of discandium endohedral fullerenes and new trimetallic nitride endohedral molecules Sc x Er 3-x N@C 80 was determined by means of the Knudsen cell mass spectrometry—ion-molecular equilibria method. Some considerations were made concerning the influence of the charge transfer from the endohedral atoms to the carbon cage on the electron affinity of the endohedral molecule.

Cage Shrinkage of Fullerene via a C2 Loss: from IPR C90(28)Cl24 to Nonclassical, Heptagon-Containing C88Cl22/24

A new case of chlorination-promoted fullerene cage shrinkage is reported. Chlorination of C90 (isolated pentagon rule isomer no. 28) with VCl4 afforded C88Cl22 with a nonclassic carbon cage (NCC) containing 1 heptagon and 13 pentagons including 2 fused pairs flanking the heptagon. The pathway of C2 abstraction from the C90 cage is suggested on the basis of density functional theory calculations.