Lowest Triplet Potential Energy Surfaces Research Articles

The unimolecular dissociation of H2CO on the lowest triplet potential-energy surface

The unimolecular dissociation reaction of H2CO on the triplet potential-energy surface has been studied via ab initio electronic structure theory. The stationary point geometries for the equilibrium and transition state are determined employing the configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with large basis sets up to the correlation consistent (cc)-pVQZ basis. With the best method, cc-pVQZ CCSD(T), the first excited triplet (ã 3A″) state lies 72.2 kcal/mol (25 260 cm−1) above the ground (X̃ 1A1) state of H2CO, which is in excellent agreement with the experimental observation of 72.03 kcal/mol (25 194 cm−1). The dissociation limit (H⋅+HCO⋅) is located at 86.3 kcal/mol (30 170 cm−1) above the ground state of H2CO, which is again in good agreement with the two experimentally determined values of 86.57 kcal/mol (30 280 cm−1) and 86.71 kcal/mol (30 328.5 cm−1). With the same method the triplet dissociation transition state lies 92.4 kcal/mol (32 300 cm−1) above the ground state. Consequently, the activation energy for the dissociation reaction of H2CO on the triplet surface is determined ab initio to be 18.9–20.1 kcal/mol (6620–7040 cm−1) (including an estimated error bar of 1.2 kcal/mol or 420 cm−1). The zero-point vibrationally corrected exit barrier height is predicted to be 4.9–6.1 kcal/mol (1710–2130 cm−1). These newly predicted energies are consistent with the recent experimental observations by the Moore group at University of California-Berkeley (1987) and by the Wittig group at University of Southern California (1997).

Electron‐Energy‐Loss Spectroscopy of Spin‐Forbidden Transitions with Vibronic Resolution: The Lowest Triplet State of Fluorobenzenes

AbstractLow‐energy electron‐energy‐loss spectra in the range of the lowest triplet state are presented for solid benzene, fluorobenzene, and p‐difluorobenzene deposited at a temperature of 30 K. Vibronic structure within the triplet bands of fluorobenzene and p‐difluorobenzene is observed for the first time. The spectrum of fluorobenzene closely resembles that of benzene, whereas p‐difluorobenzene shows characteristic differences. The spectra of all three molecules are dominated by pure progressions of the totally symmetric ring‐breathing vibrational mode v1. In addition, less intense transitions to vibronic levels relating to the e2g modes in benzene are observed and their assignment is given. The occurrence of e2g modes in all three spectra underlines that the vibronic coupling pathways between the lowest and the second triplet state are similar in benzene and its derivatives. In p‐difluorobenzene a splitting of the extremely low frequency mode that relates to the v8 (e2g) vibration in benzene is observed. This is in agreement with theoretical predictions of an asymmetric shape of the lowest triplet potential energy surface along a symmetry coordinate relating to v8 in substituted benzenes.

Condensed phase electron-energy-loss spectroscopy of the low-lying triplet states of acetylene

Low-energy electron-energy-loss spectra of solid acetylene were recorded in the range of the low-lying valence excited triplet states at a temperature of 17 K. The spectra show that the first band, which was previously assigned to a single transition, actually contains several overlapping singlet–triplet transitions. The transition Σg+1→3Au starts with an extrapolated lowest vibronic transition at 4.89 eV and is dominated by the bending vibrational mode with frequency 86 meV (694 cm−1). Below this band the spectra show a short progression with vibrational spacings of 100 meV (806 cm−1) starting at 4.62 eV and a weak band with irregular vibrational spacings close to the energy-loss threshold. Both are assigned to the transition to the lowest triplet potential energy surface, which correlates to the linear Σu+3 state and the two bent B2 and Bu isomer states.

Ab initio study of the potential energy surfaces for the reaction N(4Su +CH(X 2IIr) → CN(X 2Σ+, A 2IIi + H(2Sg)

Abstract The three lowest triplet potential energy surfaces of HCN have been investigated with the complete active space multiconfigurational self consistent field (CAS/MCSCF) method. Two surfaces are issued from the ground state reactants, while the third one is issued from the first excited state of the reactants, and is coupled to the others by a conical intersection. Of the two surfaces issued from the ground state reactants, one possesses a very small potential barrier in the entrance channel, while the other surface has none, thus making the reaction possible at very low temperature. For each surface, two potential wells have been found, corresponding to bent structures HCN and HNC. These potential wells are separated by high isomerisation saddle points. The approach of H toward CN shows a potential barrier for each of the three surfaces. Some parts of the conical intersection have been calculated and found energetically lower than the ground state reactants. Thus, transitions between surfaces should play an important role in the reaction dynamics.

Quasiclassical trajectory study of the effect of reactant rotation on the reaction B + + HD → BH + (BD +) + D(H)

Quasiclassical trajectories were calculated for the lowest triplet potential energy surface of the title reaction at different rotational states of the HD molecule and at two values of the relative translational energy of reactants. The opacity function, reaction cross sections, rotational and vibrational excitation of products, intramolecular isotopic effect, symmetry of backward-forward scattering and lifetime of the BHD+ collision complex are discussed in connection with the initial conditions defined. Results are compared with similar systems taken from the literature.

Quasiclassical trajectory calculation of the B ++H 2→BH ++H reaction

Quasiclassical trajectory calculations were done for the lowest triplet potential energy surface of the reaction B ++H 2→BH ++H. The analytical form of the surface is based on ab initio points fitted by an extended LEPS function with modification for intersection of the two surfaces. The angular and translational energy distributions calculated are compared with the results of crossed molecular beam experiments. Vibrational and rotational excitation of BH + molecules, reaction cross sections, and mean lifetimes of collision complex BH 2 + are discussed as well.

Unusual structures of dilithiosilanes and disodiosilanes. Ionicity of the silicon-alkali-metal bond

Both dilithiosilanes (SiH 2 Li 2 ) and disodiosilanes (SiH 2 Na 2 ) favor unusual structures. The lowest singlet and lowest triplet potential energy surfaces (PES) were searched with the 3-21G and 3-21G(•) basis sets and a number of stationary points were located. The «normal» tetrahedral-based singlet, 3s (C 2v ), was the lowest energy structure at HF but not at correlated levels of theory. The most interesting singlet SiH 2 Li 2 structures were reoptimized at HF/6-31G••, and the final energies were calculated at MP4SDTQ/6-31G•• and corrected for the zero-point vibrational energy

Theoretical study of the lowest triplet potential energy surface of silasilene

Bent silasilene, SiSiH/sub 2/ (C/sub s/, /sup 3/A''), which has been reported as a global minimum on the lowest triplet potential energy surface of Si/sub 2/H/sub 2/, is found to be higher in energy than planar silasilene (C/sub 2v/, /sup 3/A/sub 2/) by about 2 kcal/mol at the MP4SDTQ/6-31++G(d,p) level, suggesting that triplet silasilene (still the global minimum) must have a planar geometry. Trans-bent HSiSiH (C/sub 2h/, /sup 3/A/sub u/) should be the most stable triplet disilyne, instead of the twisted structure reported earlier, while the cis-bent structure (C/sub s/, /sup 3/A'') might be observable.

Ab initio potential energy surface of the lowest triplet BH +2 collision complex

Important parts of the lowest triplet potential energy surface of the BH + 2 molecular system are studied at the ab initio level. Obtained topological parameters of this surface and regions of its crossing with excited surfaces are compared with results of the DIM method and with previous ab initio studies.

An ab initio study of the reaction Be(3P) +H2(1Σg+) → BeH(2Σ+)+H(2S)

The lowest triplet potential energy surface for the reaction of metastable Be(3P) with H2 has been studied by ab initio SCF–MO and CI calculations. The stationary points along the reaction coordinate were obtained by automatic gradient optimization. Broadside (C2v) attack is found to be favored over the collinear C∞v approach and a stable intermediate of C2v symmetry is found for 3BeH2. We discuss experimental implications for the dynamics of this interesting exothermic process.