III. Formation of Carbon Clusters 2317 IV. Thermodynamic Properties 2319 V. C2 2319 A. Structure and Spectroscopy 2319 B. C2 2320 C. C2 2321 VI. C3 2321 A. Early Experimental Work 2321 B. Mid-IR and Far-IR Spectroscopy 2321 C. Electronic Spectroscopy 2323 1. The Σu + Ground State 2323 2. The AΠu State 2324 3. The Σu + State 2324 4. The aΠu and bΠg States 2325 5. Other Band Systems 2326 D. C3 in the Interstellar Medium 2326 E. C3 2326 F. C3 2327 VII. C4 2327 A. Theory 2327 B. Experiment 2328 1. The Rhombic Isomer 2328 2. The Linear Isomer 2328 C. Excited Electronic States 2330 D. C4 2331 E. C4 2332 VIII. C5 2332 A. Theory 2332 B. Experiment 2332 C. Excited Electronic States 2334 D. C5 2334 E. C5 2335 IX. C6 2335 A. Theory 2335 B. Experiment 2336 1. The Cyclic Isomer 2336 2. The Linear Isomer 2336 C. Excited Electronic States 2337 D. C6 2338 E. C6 2338 X. C7 2338 A. Theory 2338 B. Experiment 2338 C. Excited Electronic States 2340 D. C7 2340 E. C7 2340 XI. C8 2341 A. Theory 2341 B. Experiment 2342 1. The Cyclic Isomer 2342 2. The Linear Isomer 2342 C. Excited Electronic States 2342 D. C8 2342 E. C8 2342 XII. C9 2343 A. Theory 2343 B. Experiment 2343 C. Excited Electronic States 2345 D. C9 2345 E. C9 2345 XIII. C1

http://www.cchem.berkeley.edu/rjsgrp/publications/papers/1998/200_orden_1998.pdf

Small carbon clusters: spectroscopy, structure, and energetics.

The importance of the selective activation of alkanes for science and technology in the forthcoming decades does not need to be explicitly pointed out in this thematic issue of Chemical ReViews. Instead, we would like to summarize the development and the state-of-art of experimental and theoretical methods for the investigation of model reactions for alkane activation in the gas phase.1 However, before doing so let us address the question, how other scientists, both in academia as well as industry, can profit from such model studies in the gas phase, usually involving very small, charged species under conditions in a mass spectrometer which are very far from real catalysis. To this end, we refer to the synthesis of HCN as a reasonably simple example of how large-scale technical processes can be investigated in microscopic models. † In memoriam of Dr. Andreas Fiedler. * To whom correspondence should be addressed. E-mail: detlef.schroeder@ uochb.cas.cz, E-mail: roithova@natur.cuni.cz. ‡ Charles University in Prague. § Academy of Sciences of the Czech Republic. Jana Roithová (right), Ph.D., leads a group on the topic of reaction mechanisms at the Faculty of Science at the Charles University in Prague. Her research is based on the synergy of gas-phase experiments and theoretical calculations. She received her Ph.D. degree in the group of Prof. Zdenek Herman (J. Heyrovsky Institute of Physical Chemistry, Prague) on the topic of reactivity of small molecular dications in 2003. Since then she has made key contributions to superelectrophilic chemistry in the gas phase, with particular attention toward the activation of nonreactive substrates such as rare gases, nitrogen, and methane. Recently, she concentrates her research on the properties of redox-active molecules, their interactions with transition metals, and mechanisms of organometallic reactions. She is author of more than 80 papers and received, among others, a “L’Oréal for Women in Science” stipend and the Hlavka prize.

Selective activation of alkanes by gas-phase metal ions.

Figure 18 presents the C-O stretching vibrational frequencies of the first-row transition-metal monocarbonyl cations, neutrals, and anions in solid neon; similar diagrams have been reported for neutral MCO species in solid argon, but three of the early assignments have been changed by recent work and one new assignment added. The laser-ablation method produces mostly neutral atoms with a few percent cations and electrons for capture to make anions; in contrast, thermal evaporation gives only neutral species. Hence, the very recent neon matrix investigations in our laboratory provide carbonyl cations and anions for comparison to neutrals on a level playing field. Several trends are very interesting. First, for all metals, the C-O stretching frequencies follow the order cations > neutrals > anions with large diagnostic 100-200 cm-1 separations, which is consistent with the magnitude of the metal d to CO pi * donation. Second, for a given charge, there is a general increase in C-O stretching vibrational frequencies with increasing metal atomic number, which demonstrates the expected decrease in the metal to CO pi * donation with increasing metal ionization potential. Some of the structure in this plot arises from the extra stability of the filled and half-filled d shell and from the electron pairing that occurs at the middle of the TM row; the plot resembles the "double-humped" graph found for the variation in properties across a row of transition metals. For the anions, the variation with metal atom is the smallest since all of the metals can easily donate charge to the CO ligand. Third, for the early transition-metal Ti, V, and Cr families, the C-O stretching frequencies decrease when going down the family, but the reverse relationship is observed for the late transition-metal Fe, Co, and Ni families. In most of the present discussion, we have referred to neon matrix frequencies; however, the argon matrix frequencies are complementary, and useful information can be obtained from comparison of the two matrix hosts. In most cases, the neon-to-argon red shift for neutral carbonyls is from 11 to 26 cm-1, but a few (CrCO) lie outside of this range. In the case of FeCO and Fe(CO)2, it appears that neon and argon trap different low-lying electronic states. In general, the carbonyl neutrals and anions have similar shifts but carbonyl cations have larger matrix shifts. For example, the FeCO+ fundamental is at 2123.0 cm-1 in neon and 2081.5 cm-1 in argon, a 42.5 cm-1 shift, which is larger than those found for FeCO- (11.7 cm-1) and FeCO (11.7 cm-1). It is unusual for different low-lying electronic states to be trapped in different matrices, but CUO provides another example. The linear singlet state (1047.3, 872.2 cm-1) is trapped in solid neon, and a calculated 1.2 kcal/mol higher triplet state is trapped in solid argon (852.5, 804.3 cm-1) and stabilized by a specific interaction with argon. The bonding trends are well described by theoretical calculations of vibrational frequencies. Table 5 compares the scale factors (observed neon matrix/calculated) for the C-O stretching modes of the monocarbonyl cations, neutrals, and anions of the first-row transition metals observed in a neon matrix using the B3LYP and BP86 density functionals. Most of the calculated carbonyl harmonic stretching frequencies are within 1% of the experimental fundamentals at the BP86 level of theory, while calculations using the B3LYP functional give frequencies that are 3-4% higher as expected for these density functionals and calculations on saturated TM-carbonyls. For second- and third-row carbonyls using the BP86 density functional and the LANL effective core potential in conjunction with the DZ basis set, the agreement between theory and experiment is just as good. For example, the 16 M(CO)1-4 neutral and anion and 2 MCO+ cation (M = Ru, Os) carbonyl frequencies are fit within 1.5%. The 16 species (M = Rh, Ir) are fit within 1%, but the Rh(CO)1-4+ calculations are 2-3% too low and Ir(CO)1-4+ computations are 1-2% too low. In addition to predicting the vibrational frequencies, DFT can be used to calculate different isotopic frequencies, and isotopic frequency ratios can be computed as a measure of the normal vibrational mode in the molecule for an additional diagnostic. For diatomic CO, the 12CO/13CO ratio 1.0225 and C16O/C18O ratio 1.0244 characterize a pure C-O stretching mode. In a series of molecules such as RhCO+, RhCO, and RhCO-, where the metal-CO bonding varies, the Rh-C, C-O vibrational interaction is different and the unique isotopic ratios for the carbonyl vibration are characteristic of that particular molecule. Table 6 summarizes the isotopic ratios observed and calculated for the RhCO+,0,- species. Note that RhCO+ exhibits slightly more carbon-13 and less oxygen-18 involvement in the C-O vibration than CO itself and that this trend increases to RhCO and to RhCO- as the Rh-C bond becomes shorter and stronger. Note also how closely the calculated and observed ratios both follow this trend. In a molecule with two C-O stretching modes, for example, bent Ni(CO)2 exhibits a strong b2 mode at 1978.9 cm-1 and a weak a1 mode at 2089.7 cm-1 in solid neon, and these two modes involve different C and O participations. The symmetric mode shows substantially more C (1.0242) and less O (1.0217) participation than does the antisymmetric mode with C (1.0228) and O (1.0238) involvement, based on the given isotopic frequency ratios, which are nicely matched by DFT calculations (a1 1.0244, 1.0224 and b2 1.0232, 1.0241, respectively). These investigations of vibrational frequencies in unsaturated transition-metal carbonyl cations, neutrals, and anions clearly demonstrate the value of a close working relationship between experiment and theory to identify and characterize new molecular species.

Spectroscopic and theoretical investigations of vibrational frequencies in binary unsaturated transition-metal carbonyl cations, neutrals, and anions.

3.1. Sc Group 6772 3.2. Ti Group 6773 3.3. V Group 6775 3.4. Cr Group 6776 3.5. Mn Group 6777 3.6. Fe Group 6779 3.7. Co Group 6780 3.8. Ni Group 6782 3.9. Cu Group 6782 3.10. Zn Group 6784 3.11. Lanthanide Group 6784 3.12. Actinide Group 6785 3.13. Periodic Trends on Bonding and Reactivity 6785 4. Ionic Mononuclear Transition Metal Oxide Species 6787 4.1. Cations 6788 4.2. Anions 6790 4.2.1. Monoxide Anions 6790 4.2.2. Dioxide Anions 6791 4.2.3. Oxygen-Rich Anions 6792 5. Multinuclear Transition Metal Oxide Clusters 6792 5.1. Sc Group 6793 5.2. Ti Group 6793 5.3. V Group 6793 5.4. Cr Group 6797 5.5. Mn Group 6798 5.6. Fe Group 6798 5.7. Co Group 6798 5.8. Ni Group 6798 5.9. Cu Group 6799 6. Summary 6800 7. Acknowledgments 6800 8. References 6800

Spectroscopic and theoretical studies of transition metal oxides and dioxygen complexes.

Understanding the reactivity of transition metal complexes involving multiple spin states

1. Electron Spin Resonance 2116 2. Infrared and Optical Absorption 2117 3. Laser-Induced Fluorescence 2119 III. Quantum Hosts 2122 IV. Matrix Isolation Spectroscopy of Molecular Ions 2124 A. Ion Sources for Matrix Isolation Spectroscopy 2124 B. Spectroscopic Methods 2126 1. Electron Spin Resonance 2126 2. Infrared and Optical Absorption 2126 3. Laser-Induced Fluorescence 2127 V. Deposition of Mass-Selected Species 2128 VI. Summary 2130

New Developments in Matrix Isolation Spectroscopy.

The spectra and relaxation of CN radicals are studied by FTIR absorption emission spectroscopy. The relaxation of excited A 2Π vibrational levels proceeds via efficient intramolecular internal conversion processes between the A 2Π and X 2Σ level. The intensity distribution in the observed vibrational CN fluorescence is explained by the occurrence of rapid intermolecular vibrational energy transfer and “ladder climbing” processes. The effect of isotopic substitution and medium effects on the relaxation dynamics are also discussed.

Jiirgen Agreiter

Papers

New Developments in Matrix Isolation Spectroscopy.

Infrared fluorescence and absorption studies of CN: spectra and relaxation in solid rare gases

Laser-induced fluorescence of matrix-isolated C6N2+ and of C8N2+

Infrared emission and nonradiative relaxation of C2 in solid neon

New Developments in Matrix Isolation Spectroscopy