Ab initio total atomization energies of small molecules — towards the basis set limit

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 solar photospheric carbon abundance has been determined from (C ), C , CH vibration-rotation, CH A-X electronic and C2 Swan electronic lines by means of a time-dependent, 3D, hydrodynamical model of the solar atmosphere. Departures from LTE have been considered for the C  lines. These turned out to be of increasing importance for stronger lines and are crucial to remove a trend in LTE abundances with the strengths of the lines. Very gratifying agreement is found among all the atomic and molecular abundance diagnostics in spite of their widely different line formation sensitivities. The mean value of the solar carbon abundance based on the four primary abundance indicators ((C ), C , CH vibration-rotation, C2 Swan) is logC = 8.39 ± 0.05, including our best estimate of possible systematic errors. Consistent results also come from the CH electronic lines, which we have relegated to a supporting role due to their sensitivity to the line broadening. The new 3D based solar C abundance is significantly lower than previously estimated in studies using 1D model atmospheres.

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Line formation in solar granulation VI. [Cl], Cl, CH and C2 lines and the photospheric C abundance

Technical details of a new global mapping technique for finding equilibrium (EQ) and transition structures (TS) on potential energy surfaces (PES), the scaled hypersphere search (SHS) method (Ohno, K.; Maeda, S. Chem. Phys. Lett. 2004, 384, 277), are presented. On the basis of a simple principle that reaction pathways are found as anharmonic downward distortions of PES around an EQ point, the reaction pathways can be obtained as energy minima on the scaled hypersphere surface, which would have a constant energy when the potentials are harmonic. Connections of SHS paths between each EQ are very similar to corresponding intrinsic reaction coordinate (IRC) connections. The energy maximum along the SHS path reaches a region in close proximity to the TS of the reaction pathway, and the subsequent geometry optimization from the SHS maximum structure easily converges to the TS. The SHS method, using the one-after-another algorithm connecting EQ and TS, considerably reduces the multidimensional space to be searched to certain limited regions around the pathways connecting each EQ with the neighboring TS. Applications of the SHS method have been made to ab initio surfaces of formaldehyde and propyne molecules to obtain systematically five EQ and nine TS for formaldehyde and seven EQ and 32 TS for propyne.

Global mapping of equilibrium and transition structures on potential energy surfaces by the scaled hypersphere search method: applications to ab initio surfaces of formaldehyde and propyne molecules.

Atomization energies were computed for 73 molecules, many of them chosen from the GAUSSIAN-2 and G2/97 test sets. A composite theoretical approach was adopted which incorporated estimated complete basis set binding energies based on frozen core coupled cluster theory with a quasiperturbative treatment of triple excitations and three corrections: (1) a coupled cluster core/valence correction; (2) a configuration interaction scalar relativistic correction; and (3) an atomic spin-orbital correction. A fourth correction, corresponding to more extensive correlation recovery via coupled cluster theory with an approximate treatment of quadruple excitations, was examined in a limited number of cases. For the molecules and basis sets considered in this study, failure to consider any of these contributions to the atomization energy can introduce errors on the order of 1–2 kcal/mol. Although some cancellation of error is common, it is by no means universal and cannot be relied upon for high accuracy. With the largest available basis sets (including, in some cases, up through aug-cc-pV6Z), the mean absolute deviation with respect to experiment was found to lie in the 0.7–0.8 kcal/mol range, neglecting the effects of higher order excitations. Worst case errors were 2–3 kcal/mol. Several complete basis set extrapolations were tested with regard to their effectiveness at improving agreement with experiment, but the statistical difference among the various approaches was small.

http://tyr0.chem.wsu.edu/~kipeters/Pages/pdfs/JCP110.8384.1999.pdf

Re-examination of atomization energies for the Gaussian-2 set of molecules

An experimental determination of the heat of formation of C2 and the CH bond dissociation energy in C2H

Observation of the LIF spectra of C2(a 3Πu) and C2(A 1Πu) from the photolysis of C2H2 at 193 nm

Laboratory studies of the secondary photolysis of the C2H radical are summarized and used to explain some discrepancies between models of C2 emission in comets. These studies show that several states of the C2 radicals produced in the photolysis of C2H2 at 193 nm have bimodal rotational distributions when plotted as a Boltzmann diagram. They also establish that the C2 radicals are formed with varying degrees of vibrational excitation, so that if they are formed in a similar manner in comets, the C2 radicals must start out with this initial vibrational excitation.

Implications of C2H photochemistry on the modeling of C2 distributions in comets

Laser-induced flourescence studies of C3 formation and isomerization in the 193 nm photolysis of allene and propyne

A unified picture of the photodissociation of theC2H radical has been developed using the results from the latest experimental and theoretical work. This picture shows that a variety of electronic states ofC2 are formed during the photodissociation of theC2H radical even if photoexcitation accesses only one excited state. This is because the excited states have many avoided corssings and near intersections where two electronic states come very close to one another. At these avoided crossings and near intersections, the excited radical can hop from one electronic state to another and access new final electronic states of theC2 radical. The complexity of the excited state surfaces also explains the bimodal rotational distributions that are observed in all of the electronic states studied. The excited states that dissociate through a direct path are limited by dynamics to produceC2 fragments with a modest amount of rotational energy, whereas those that dissociate by a more complex path have a greater chance to access all of phase space and produce fragments with higher rotational excitation. Finally, the theoretical transition moments and potential energy curves have been used to provide a better estimate of the photochemical lifetimes in comets of the different excited states of theC2H radical. The photochemically active states are the 22∑+, 22II, 32II, and 32∑+, with photodissociation rate constants of 1.0×10−6, 4.0×10−6, 0.7×10−6, and 1.3×10−6s−1, respectively. These rate constants lead to a total photochemical lifetime of 1.4×105 s.

Randall S. Urdahl

Papers

An experimental determination of the heat of formation of C2 and the CH bond dissociation energy in C2H

Observation of the LIF spectra of C2(a 3Πu) and C2(A 1Πu) from the photolysis of C2H2 at 193 nm

Implications of C2H photochemistry on the modeling of C2 distributions in comets

Laser-induced flourescence studies of C3 formation and isomerization in the 193 nm photolysis of allene and propyne

Non-adiabatic interactions in excitedC 2 H molecules and their relationship toC 2 formation in comets