The Jahn-Teller plus pseudo-Jahn-Teller vibronic problem in the C3 radical and its topological implications

The combined Jahn-Teller plus pseudo-Jahn-Teller [(E'+A1')⊗e'] problem is discussed for the tricarbon radical (C3) by means of ab initio calculations at the multireference configuration interaction level of theory. For the (1)E' electronic state arising from a e'(2) valence configuration, three additional symmetry-equivalent C2v seams are found to lie in close proximity to the D3h symmetry-required seam over the entire range of the breathing coordinate here considered. As the perimeter of the molecule increases, the C2v disjoint seams approach the D3h one almost linearly and ultimately coalesce with it at Q1 = 5.005 a0, thence forming an intersection node or confluence. By further increasing the size of the molecular triangle, the C2v seams get rotated by ±π in the g-h plane. A three-state vibronic Hamiltonian is also proposed to model locally the title system and shown to accurately mimic the calculated data over the region close to the minimum energy crossing point. No net geometric phase effect is observed when the associated electronic wave functions are adiabatically transported along closed paths encircling the four singularity points. For all paths enclosing the intersection node, the sign reversal criterion is shown to be not fulfilled, even for infinitesimal loops. The results so obtained are expected to be valid for other ring systems experiencing similar topological attributes.

High-resolution spectroscopy of C3 around 3 μm

We report on the detection of the (10(0)1) ← (00(0)0) vibrational band of gas-phase C3 and the two of its mono (13)C substituted isotopologs in the infrared region around 3200 cm(-1). Additionally, the associated hot band (11(1)1) ← (01(1)0) has been assigned for the parent isotopolog. Spectra have been recorded using a supersonic jet spectrometer with a laser ablation source in combination with a continuous-wave optical parametric oscillator as radiation source. High-level quantum-chemical ab initio calculations have been performed and used to assist the assignment. A combined fit for the vibrational states of C3 found in this study has been done together with previously reported high-resolution data to increase the accuracy of the molecular parameters, especially for the ground state. The vibrational energies are 3260.126, 3205.593, and 3224.751 cm(-1) for the (10(0)1) state of C3, (12)C(13)C(12)C, and (13)C(12)C(12)C, respectively. The (11(1)1) state of C3 has been found to be at 3330.509 cm(-1).

Studies of cyclization reactions of linear cumulenes and heterocumulenes using the neutralization-reionization procedure and/or ab initio calculations

A number of linear cumulenes and heterocumulenes have been made by charge stripping of anions of known bond connectivity in the source of a mass spectrometer. Some of these reactive molecules have been identified in interstellar molecular clouds. The structures of these neutrals may be investigated by reionization to a decomposing positive ion [the neutralization-reionization technique ((-)NR(+))], and/or by ab initio calculations. Energized linear cumulenes and heterocumulenes may undergo cyclization to form stable cyclic isomers. To cite a selection of the examples described in this review: (i) four-atom systems CCCC and some heterocumulenes CCCX (X=B, N, Al, Si, P) involve the formation of stable four-membered ring rhombic (also called kite and fan) structures. One of the cyclic molecules, cyclo-C(3) Si, has been detected in interstellar molecular clouds, (ii) five-atom cumulene and heterocumulene systems are more complex. Linear CCCCC rearranges the carbon skeleton by forming a C substituted rhomboid system, CCCCO forms a three-membered cyclic isomer, while nitrogen containing five-atom cumulenes effect nitrile to isonitrile interconversion via three-centered cyclized intermediates, and (iii) CCCCCC and CCCCBO cyclize to give unique six-membered ring systems.

Investigation of energy deposited by femtosecond electron transfer in collisions using hydrated ion nanocalorimetry

Ion nanocalorimetry is used to investigate the internal energy deposited into M (2+)(H 2O) n , M = Mg ( n = 3-11) and Ca ( n = 3-33), upon 100 keV collisions with a Cs or Ne atom target gas. Dissociation occurs by loss of water molecules from the precursor (charge retention) or by capture of an electron to form a reduced precursor (charge reduction) that can dissociate either by loss of a H atom accompanied by water molecule loss or by exclusively loss of water molecules. Formation of bare CaOH (+) and Ca (+) by these two respective dissociation pathways occurs for clusters with n up to 33 and 17, respectively. From the threshold dissociation energies for the loss of water molecules from the reduced clusters, obtained from binding energies calculated using a discrete implementation of the Thomson liquid drop model and from quantum chemistry, estimates of the internal energy deposition can be obtained. These values can be used to establish a lower limit to the maximum and average energy deposition. Not taking into account effects of a kinetic shift, over 16 eV can be deposited into Ca (2+)(H 2O) 33, the minimum energy necessary to form bare CaOH (+) from the reduced precursor. The electron capture efficiency is at least a factor of 40 greater for collisions of Ca (2+)(H 2O) 9 with Cs than with Ne, reflecting the lower ionization energy of Cs (3.9 eV) compared to Ne (21.6 eV). The branching ratio of the two electron capture dissociation pathways differs significantly for these two target gases, but the distributions of water molecules lost from the reduced precursors are similar. These results suggest that the ionization energy of the target gas has a large effect on the electron capture efficiency, but relatively little effect on the internal energy deposited into the ion. However, the different branching ratios suggest that different electronic excited states may be accessed in the reduced precursor upon collisions with these two different target gases.