Jan Hrušák

Oxidative addition of mono and bis(carbene) complexes derived from imidazolyl and thiazolyl gold(I) compounds

Abstract A series of cationic imidazolinylidene and thiazolinylidene gold(III) complexes was synthesized by the oxidative addition of halogens to the corresponding bis (carbene) gold(I) compounds. Similar reactions with the thiazolyl-derived mono (carbene) gold(I) complex only give mono (carbene) halogold(I) compounds. Quantum mechanical calculations employing the GAUSSIAN92 series of programs at the DFT/HF (hybrid density functional theory) level were carried out for the model complexes [Au{ovbar|CNHCHue5fbCHNH} 2 Cl 2 ] + and [Au- CNHCHue5f8CHNH ) 2 ) and compared to the molecular structure of the imidazolinylidene gold(III) compound [Au{ CNMeCHue5f8CHN Me} 2 Cl 2 ][CF 3 SO 3 ] as well as an analogous gold(I) complex.

The thiazole ylide: a frequently invoked intermediate is a stable species in the gas phase.

The 1, 2-hydrogen shift isomers of neutral (singlet and triplet) thiazole (1) and its radical cation have been investigated by a combination of mass spectro-metric experiments and hybrid density functional theory calculations. The latter were used to probe the structures and stabilities of selected C3 H3 NS and C3 H3 NS(.+) isomers and transition state structures. Although 3H-thiazole-2-ylidene (2) is less stable than 1, by 31.5 kcalmol(-1) , it is expected to be capable of independent existence, since the 1, 2-hydrogen shift from carbon to nitrogen involves a very large energy barrier of 72.4 kcalmol(-1) . The other 1, 2-hydrogen shift reaction from C(2) leads not to the expected cyclic 1H-thiazole-2-ylidene structure (3), which is apparently unstable, but rather to the ring-opened species HSCHuf8feCHNC (4), which is 34.5 kcalmol(-1) higher in energy than 1. The barrier in this case is lower but still large (54.9 kcalmol(-1) ). The triplet ground states of 1, 2 and 4 are considerably destabilised (69.5, 63.2 and 58.7 kcalmol(-1) ) relative to their singlet states. Interestingly, in addition to 2(.+) and 4(.+) , the cyclic radical cation 3(.+) is predicted to be stable although it is substantially higher in energy than ionised thiazole 1(.+) (by 53.9 kcalmol(-1) ), whereas 2(.+) and 4(.+) are much closer in energy (only 10.2 and 27.0 kcalmol(-1) higher, respectively). Dissuading 2(.+) and 3(.+) from isomerising to 1(.+) are energy barriers of 52.6 and 15.3 kcalmol(-1) , respectively. Experimentally, dissociative ionisation of 2-acetylthiazole enabled the generation of 2(.+) , which could be differentiated from 1(.+) by collisional activation mass spectrometry. Reduction of the ylide ion 2(.+) in neutralisation-reionisation mass spectrometry experiments yielded the corresponding neutral molecule 2. This direct observation of a thiazolium ylide provides support for postulates of such species as discrete intermediates in a variety of biochemical transformations.

The calculation of the vibrational states of SO2 in the C̃1B2 electronic state up to the SO(3Σ−)+O(3P) dissociation limit

Abstract In a previous paper [P. Nachtigall, J. Hrusak, O. Bludsky, S. Iwata, Chem. Phys. Lett. 303 (1999) 441], we reported an investigation of the stationary points along the dissociation path of C 1 B 2 SO 2 , carried out with high-level ab initio methods. Here we calculate the vibrational energy levels up to the SO( 3 Σ − )+O( 3 P ) dissociation limit using a scaled ab initio potential energy surface. The scaled potential energy surface is of near-spectroscopic accuracy below the dissociation limit and has a realistic behaviour along the dissociation path.

Investigation of the potential energy surfaces for the ground X̃1A1 and excited C̃1B2 electronic states of SO2

The stationary points along the dissociation path are investigated by means of high-level ab initio methods and the reliability of different methods is discussed. The multi-reference AQCC method using the ANO-type basis set is shown to give geometrical parameters and relative energies in very good agreement with experiment. At this level of theory, the state has an asymmetrical equilibrium geometry and double-minimum potential with a barrier of 170 cm−1, in good agreement with experimental data.

Heat of formation of the CF2++ dication: a theoretical estimate

Abstract Relative energies and ionization potentials of neutral and ionic fragments of CF 4 were calculated using the coupled cluster single double (triple)/correlation consistent triple zeta basis set ab initio method. The individual values were compared with available experimental and theoretical data to assess the accuracy of this approach for these particular systems. In addition, reaction enthalpies for possible fragmentation reactions were calculated. Maximum errors found were in the order of 5 kcal/mol (0.2 eV). The estimation of the heat of formation of the CF 2 ++ dication, derived from the calculations based on four different reactions, is 687 ± 5 kcal/mol (29.8 ± 0.2 eV). This value is considerably smaller than the one which follows from the appearance potential measurements (732 ± 7 kcal/mol).

Heat of formation of the SiF2++ dication: a theoretical prediction

Abstract Energetics of the SiF2++ dication and fragments related to it, SiF++ and SiF+, was calculated using different ab initio approaches, including the semiempirically corrected G2, the complete basis set method, and the coupled cluster method up to the CCSD(T)/aug-cc-pVQZ level. The calculated values of bond energies and ionization potentials were carefully compared with the available experimental data to assess the accuracy of these approaches. In addition, reaction enthalpies for possible fragmentation reactions were calculated. The heat of formation of the SiF2++ dication, resulting from these calculations, is ΔHf(SiF2++) = 546 ± 2 kcal/mol and a theoretical estimate of ΔHf(SiF++) is 649 ± 2 kcal/mol.

STRUCTURE AND STABILITY OF THE CF32+ DICATION

Abstract The structure of the CF 3 2+ dication was studied by means of different ab initio methods. The previously reported D 3h structure was found to be a minimum only at the Hartree–Fock level, while it corresponds to a higher order saddle point on correlated potential energy surfaces. The calculated adiabatic ionization energy for CF 3 2+ leading to a C 2 v symmetric dication of 26.80 eV is in very good agreement with the experimental value of 26.30 ± 0.4 eV.

Some remarks on the stability of the ground and excited electronic states of the CF2++ dication

Abstract The dissociation path of the ground state of CF 2 ++ ( 1 Σ g + ) is investigated by means of the multi-reference averaged quadratic coupled cluster (AQCC) method using an ANO type basis set. Low lying electronically excited states involved in the dissociation are also calculated. The CF 2 ++ ( 1 Σ g + ) dication has to be considered as metastable ( ΔE Diss =−8.5 kcal/mol). However, the dissociation into the ground states of the products (i.e., CF + ( 1 Σ + ) and F + ( 3 P ) ) is spin forbidden. Further, the surface crossing of the ground-state potential energy surfaces (PES) with the lowest triplet surface ( 3 A ′ ) occurs around the C–F distance of 1.85 A. The related barrier is calculated to be ∼5 eV.

A theoretical study of the ground and excited states of the CHCl2+ dication and the CHCl+ cation

Abstract Ground and excited states of the dication CHCl2+ and the corresponding cation CHCl+ were investigated using the CASSCF-AQCC method with moderate ANO basis sets. Both isomers of the dication and cation were investigated: HCCl2+ and HCCl+ (with H bonded to C), and CClH2+ and CClH+ (with H bonded to Cl). The energetics and geometries of stable configurations were characterized and energies of both adiabatic and vertical transitions between the dication and the cation were determined. The data were found essential to interpret correctly experimental results on charge transfer collision of the dication with noble gases and to assess the role of the CClH+ isomer in them.

Identification of photofragmentation patterns in trihalide anions by global analysis of vibrational wavepacket dynamics in broadband transient absorption data

Trihalide anions are linear molecules that can be photodissociated with ultraviolet (UV) light. Whereas deep-UV excitation leads to three-body dissociation, for near-UV excitation just one molecular bond is cleaved, which notionally opens up the possibility for different fragmentation patterns. Here, we explore whether the dihalide fragment is formed as an anionic or neutral species and whether heteronuclear trihalides can lead to two different dihalides. The analysis is based on pronounced wavepacket dynamics induced by femtosecond UV pulses and associated both with the initial trihalide and the nascent dihalide species. For the trihalide anions I3-, Br3-, IBr2-, and ICl2- (point group D∞h), as well as for I2Br- and I2Cl- (point group C∞v) in dichloromethane solution, we identify dihalide fragments by their characteristic vibrational wavenumbers, which we achieve from globally fitting the vibrational wavepacket oscillations, considering a wavelength-dependent phase. No signature from neutral species is found right after excitation, hence there is only one diatomic product in D∞h trihalides. For the investigated C∞v trihalides, which could allow a homonuclear and a heteronuclear product, only the homonuclear one is observed. Since dihalide anions are unstable intermediates, their absorption and the ground-state bleach of the trihalide anion show a biexponential recovery for all samples due to recombining fragment pairs. The rate of the electron transfer yielding a neutral dihalogen and an atomic anion, a prerequisite for the recombination, gives rise to the biexponential behavior; fast recombination is mediated by vibrational excess energy, while slow recombination occurs for cooled-down dihalogens. These data reveal the fragmentation and recombination dynamics from a time-domain approach rather than frequency-domain vibrational spectroscopy and contribute to the in-depth comprehension of these versatile model molecules.