Periodicity of Quadrilateral Tetra-Atomic Molecules.

We propose a periodic table for quadrilateral tetra-atomic molecules. Each molecule consists of four atoms, and the geometric shapes of these molecules are studied by ab initio calculations. It is found that by increasing the valence electron numbers, strong periodicity appears with many molecular properties. The periodic table for the quadrilateral tetra-atomic molecules is considered as an extension of Mendeleev's periodic table. It can also be used for estimating the unknown properties of tetra-atomic molecules.

Anharmonic Rice-Ramsperger-Kassel-Marcus (RRKM) and product branching ratio calculations for the partially deuterated protonated water dimers: dissociation and isomerization.

Partially deuterated protonated water dimers, H2O·H(+)·D2O, H2O·D(+)·HDO, and HDO·H(+)·HDO, as important intermediates of isotopic labeled reaction of H3O(+) + D2O, undergo direct dissociation and indirect dissociation, i.e., isomerization before the dissociation. With Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations, we have computed their dissociation and isomerization rate constants separately under the harmonic and anharmonic oscillator models. On the basis of the dissociation and isomerization rate constants, branching ratios of two primary products, [HD2O(+)]∕[H2DO(+)], are predicted under various kinetics models with the harmonic or anharmonic approximation included. The feasible kinetics model accounting for experimental results is shown to include anharmonic effect in describing dissociation, while adopting harmonic approximation for isomerization. Thus, the anharmonic effect is found to play important roles affecting the dissociation reaction, while isomerization rates are shown to be insensitive to whether the anharmonic or harmonic oscillator model is being applied.

Ion-pair dissociation dynamics of SO(2) in the photon energy range 14.87-15.15 eV.

The ion-pair dissociation dynamics of SO(2) → SO(+) (X(2)Π,υ) + O(-) ((2)P(J)) in the excitation energy range 14.87-15.15 eV has been studied using the tunable XUV laser and velocity map imaging method. The O(-) yield spectrum, the translational energy distributions, and the angular distributions of the photofragments have been measured. The electronic structures and selected cuts of potential energy surfaces for the ion-pair states have been calculated by employing the quantum chemistry calculation method at the CASSCF/6-311++g** level. The equilibrium structures of the six ion-pair states all have linear geometries. An orbital correlation diagram was drawn to illustrate the ion-pair dissociation mechanism. Combining the experimental and theoretical results, it is concluded that the ion-pair dissociation takes place mainly via the predissociation of Rydberg states (1)A(1) [(C(2)B(1))4db(1)] and (1)A(1) [(D(2)A(1))4sa(1)]. The experimental results confirm the previous theoretical calculation results about the symmetry assignments for the energy sequence of SO(2)(+) as C((2)B(1)) < D((2)A(1)).

Anharmonic RRKM calculation for the dissociation of (H(2)O)(2)H(+) and its deuterated species (D(2)O)(2)D(+).

Investigations on the dissociation kinetics of hydrated protonium ions, (H(2)O)(2)H(+) and their deuterated species (D(2)O)(2)D(+), are reported based on the harmonic and anharmonic oscillator model using the transition state theory and ab initio calculations. We find that the dissociation of (H(2)O)(2)H(+) and (D(2)O)(2)D(+) exhibits a distinct threshold behavior due to the existence of activation energies. Moreover, the deviation between the harmonic and anharmonic dissociation rate constants becomes larger in the high energy or temperature range, with the rate constants becoming unreasonably large under the harmonic oscillator model. The isotope effect is found to become more distinct but only in the case of the anharmonic oscillator model. These results show that the anharmonic Rice-Ramsperger-Kassel-Marcus (RRKM) theory can provide a reasonably good description for the dissociation of (H(2)O)(2)H(+) and (D(2)O)(2)D(+). Furthermore, a theoretical model to demonstrate the principle of vibrational predissociation spectroscopy (VPS) is established from the viewpoint of RRKM theory and applied in determining the experimental conditions and understanding the role of the dissociation rate constant k(E) played in the VPS experiment, using (H(2)O)(2)H(+) and (D(2)O)(2)D(+) as examples.

Neutral dissociation of superexcited oxygen molecules in intense laser fields.

Superexcited states (SESs) of oxygen molecules and their neutral dissociation processes have been studied both experimentally and theoretically using intense femtosecond laser. We find that at the laser intensity of approximately 2 x 10(14) W/cm(2), ultrashort laser pulse causes neutral dissociation of oxygen molecule by way of SESs. The dissociation products are the excited neutral oxygen atoms, which are observed through fluorescence spectroscopy. Laser power dependence of the fluorescence intensity shows that each molecule effectively absorbs an average of ten laser photons. The total energy absorbed is sufficient to stimulate the molecule to many of the SESs. The effect is equivalent to single photon excitation in the extreme-ultraviolet (XUV) region by synchrotron radiation (SR). Morse potential energy curves (PECs) are constructed for the SESs of O(2) molecules. In light of the PECs, predissociation mechanism is proposed for the neutral dissociation. Quasi-classical trajectory (QCT) calculations show that the predissociation time is as short as 100 fs, which is consistent with our experimental measurement using ultrafast pump-probe technique.

Ion-pair dissociation dynamics of HCl: fast predissociation.

We have studied the ion-pair dissociation dynamics of HCl --> Cl(-) ((1)S(0)) + H(+) in the 14.41-14.60 eV using tunable XUV laser and the velocity map imaging method. The measured ion-pair yield spectrum has P- and R-branch resolved vibrational structure, which indicates a predissociation mechanism for the ion-pair dissociation. All of the anisotropy parameters for the angular distribution of the fragments have the limiting values of beta = 2, which suggests that the predissociation occurs via (1)Sigma(+) Rydberg states, and is fast in comparison with the rotational period of HCl. To understand the predissociation dynamics, the diabatic potential energy curve of the ion-pair state has been calculated at the MRCI/CAS/vtz level. The experimental and theoretical results obtained in this work have provided a solid foundation for the previously proposed mechanism that the ion-pair dissociation occurs via predissociation of Rydberg states converging to HCl(+) (A(2)Sigma(+)).

Multichannel reaction of C2Cl3 + O2 studied by time-resolved Fourier transform infrared emission spectroscopy.

The multichannel reaction of the C(2)Cl(3) radical with O(2) has been studied thoroughly by step-scan time-resolved Fourier transform infrared emission spectroscopy. Vibrationally excited products of Cl(2)CO, CO, and CO(2) are observed and three major reaction channels forming respectively ClCO + Cl(2)CO, CO + CCl(3)O, and CO(2) + CCl(3) are identified. The vibrational state distribution of the product CO is derived from the spectral fitting, and the nascent average vibrational energy of CO is determined to be 59.9 kJ/mol. A surprisal analysis is applied to evaluate the vibrational energy disposal, which reveals that the experimentally measured CO vibrational energy is much more than that predicted by statistical model. Combining previous ab initio calculation results, the nonstatistical dynamics and mechanism are characterized to be barrierless addition-elimination via short-lived reaction intermediates including the peroxy intermediate C(2)Cl(3)OO* and a crucial three-member-ring COO intermediate.

Explosive photodissociation of methane induced by ultrafast intense laser.

A new type of molecular fragmentation induced by femtosecond intense laser at the intensity of 2 x 10(14) W/cm2 is reported. For the parent molecule of methane, ethylene, n-butane, and 1-butene, fluorescence from H (n = 3-->2), CH (A 2Delta, B 2Sigma-, and C 2Sigma+-->X 2Pi), or C2 (d 3Pi g-->a 3Pi u) is observed in the spectrum. It shows that the fragmentation is a universal property of neutral molecule in the intense laser field. Unlike breaking only one or two chemical bonds in conventional UV photodissociation, the fragmentation caused by the intense laser undergoes vigorous changes, breaking most of the bonds in the molecule, like an explosion. The fragments are neutral species and cannot be produced through Coulomb explosion of multiply charged ion. The laser power dependence of CH (A-->X) emission of methane on a log-log scale has a slope of 10 +/- 1. The fragmentation is thus explained as multiple channel dissociation of the superexcited state of parent molecule, which is created by multiphoton excitation.

Computational study of the reaction of chlorinated vinyl radical with molecular oxygen (C2Cl3 + O2).

Eight exothermic product channels of the reaction of chlorinated vinyl radical (C2Cl3) with molecular oxygen (O2) have been investigated using ab initio quantum chemistry methods. The energetics of the reaction pathways were calculated at the second-order Moller-Plesset Gaussian-3 level of theory (G3MP2) using the B3LYP/6-311G(d) optimized geometries. It has been shown that the C2Cl3 + O2 reaction takes place via a barrierless addition to form the chlorinated vinylperoxy radical complex, which can decompose or isomerize to various products via the complicated mechanisms. Two major reaction routes were revealed, i.e., the three-member-ring reaction mechanism leading to ClCO + CCl2O, CO + CCl3O, CO2 + CCl3, Cl + (ClCO)2, etc., and the OO bond cleavage mechanism leading to O(3P) + C2Cl3O. The other mechanisms are shown to be unimportant. The results are validated by the calculations using the restricted coupled cluster theory [RCCSD(T)] with the complete basis set extrapolation. Variational transition state theory was employed to calculate the individual and total rate coefficients as a function of temperature and pressure (helium). The theoretical rate coefficients are in good agreement with the available experimental data. It was found that the total rate coefficients show strong negative temperature dependence in the range 200-2000 K. At room temperature (297 K), the total rate coefficients are shown to be nearly pressure independent over a wide range of helium pressures (1-10(9) Torr). The deactivation of the initial adduct, C2Cl3O2, is only significant at pressures higher than 1000 Torr. The three-member-ring reaction mechanism is always predominant over the OO bond cleavage.