HeH+ Collisions with H2: Rotationally Inelastic Cross Sections and Rate Coefficients from Quantum Dynamics at Interstellar Temperatures.

We report for the first time an accurate ab initio potential energy surface for the HeH+-H2 system in four dimensions (4D) treating both diatomic species as rigid rotors. The computed ab initio potential energy point values are fitted using an artificial neural network method and used in quantum close coupling calculations for different initial states of both rotors, in their ground electronic states, over a range of collision energies. The state-to-state cross section results are used to compute the rate coefficients over a range of temperatures relevant to interstellar conditions. By comparing the four dimensional quantum results with those obtained by a reduced-dimensions approach that treats the H2 molecule as an averaged, nonrotating target, it is shown that the reduced dimensionality results are in good accord with the four dimensional results as long as the HeH+ molecule is not initially rotationally excited. By further comparing the present rate coefficients with those for HeH+-H and for HeH+-He, we demonstrate that H2 molecules are the most effective collision partners in inducing rotational excitation in HeH+ cation at interstellar temperatures. The rotationally inelastic rates involving o-H2 and p-H2 excitations are also obtained and they turn out to be, as in previous systems, orders of magnitude smaller than those involving the cation. The results for the H2 molecular partner clearly indicate its large energy-transfer efficiency to the HeH+ system, thereby confirming its expected importance within the kinetics networks involving HeH+ in interstellar environments.

Photoenolization via excited state double proton transfer induces "turn on" fluorescence in diformyl diaryl dipyrromethane.

A light triggered enolization in diformyl diaryl dipyrromethane by excited state dual proton transfer (ESDPT) induces "turn on" fluorescence. The role of diaryl and diformyl groups in the enolization process was confirmed by photophysical and theoretical studies.

Theoretical investigation of mechanisms for the gas-phase unimolecular decomposition of DMMP.

All species involved in the multichannel decomposition of gas-phase dimethyl methylphosphonate (DMMP) were investigated by electronic structure calculations. Geometries for stationary structures along the reaction paths, were fully optimized with the MP2 method and the B3LYP and MPW1K DFT functionals, and the 6-31G*, 6-31++G**, and aug-cc-pVDZ basis sets. The geometries determined by the B3LYP and MPW1K functionals are in very good agreement with the MP2 values. Increasing the basis set size from 6-31G* to aug-cc-pVDZ does not significantly alter this result. Single point energy calculations were carried out with highly accurate but computationally more expensive CBS-QB3 theory. DMMP has three conformers, which lead to the four primary product channels, (O)P(CH(2))(OCH(3)) + CH(3)OH, (O)P(CH(3)) (OCH(3))(OH) + CH(2), c-(O)P(CH(3))OCH(2) + CH(3)OH, and (O)P(CH(3))(OCH(3))(OCH) + H(2). The first channel has the lowest energy barrier and is expected to be the most important pathway. It occurs via C-H and P-O bond cleavages accompanied by O-H bond formation. The other three channels have higher and similar energy barriers, and are expected to have smaller and similar rates. The product (O)P(CH(3))(OCH(3))(OCH) undergoes a secondary decomposition to form (OH)P(CH(3))(OCH(3)) + CO.

Imaging nucleophilic substitution dynamics.

Anion-molecule nucleophilic substitution (S(N)2) reactions are known for their rich reaction dynamics, caused by a complex potential energy surface with a submerged barrier and by weak coupling of the relevant rotational-vibrational quantum states. The dynamics of the S(N)2 reaction of Cl- + CH3I were uncovered in detail by using crossed molecular beam imaging. As a function of the collision energy, the transition from a complex-mediated reaction mechanism to direct backward scattering of the I- product was observed experimentally. Chemical dynamics calculations were performed that explain the observed energy transfer and reveal an indirect roundabout reaction mechanism involving CH3 rotation.

Ab initio quantum chemical investigation of the ground and excited states of salicylic acid dimer.

The structure and stability of different forms of salicylic acid dimer have been examined by Hartree-Fock and density functional theoretic calculations using 6-31G(d,p) and 6-311++g(d,p) basis sets. Vertical excitation energies for the monomer as well as the dimer have been computed using the time-dependent density functional theory using 6-311++G(d,p) basis set. The predicted absorption maxima for the first excited singlet state of salicylic acid monomer and the dimer of the primary form are in reasonable agreement with the experimental result. There is a slight red shift (approximately 6 nm) in the absorption maximum in going from the monomer to the dimer, in accord with the experimental observation. Configuration-interaction calculations including single excitation have been carried out to map the potential-energy profile for the intra- as well as the intermolecular proton transfer in different forms of the dimer. The barrier for proton transfer in the ground state as well as the excited states makes it clear that most of the processes take place in the primary form and largely by intramolecular proton transfer.

Ground and excited states of the monomer and dimer of certain carboxylic acids.

The ground-state properties of the monomer and the dimer of formic acid, acetic acid, and benzoic acid have been investigated using Hartree-Fock (HF) and density functional theory (DFT) methods using the 6-311++G(d,p) basis set. Some of the low-lying excited states have been studied using the time-dependent density functional theory (TDDFT) with LDA and B3LYP functionals and also employing complete-active-space-self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) methodologies. DFT calculations predict the ground-state geometries in quantitative agreement with the available experimental results. The computed binding energies for the three carboxylic acid dimers are also in accord with the known thermodynamic data. The TDDFT predicted wavelengths corresponding to the lowest energy n-pi* transition in formic acid (214 nm) and acetic acid (214 nm) and the pi-pi* transition in benzoic acid (255 nm) are comparable to the experimentally observed absorption maxima. In addition, TDDFT calculations predict qualitatively correctly the blue shift (4-5 nm) in the excitation energy for the pi-pi* transition in going from the monomer to the dimer of formic acid and acetic acid and the red shift (approximately 19 nm) in pi-pi* transition in going from benzoic acid monomer to dimer. This also indicates that the electronic interaction arising from the hydrogen bonds between the monomers is marginal in all three carboxylic acids investigated.

Determination of stability and degradation in polysilanes by an electronic mechanism.

Polysilanes are potential candidates for active materials in light emitting diodes because of possible emission in the near-ultraviolet to blue region. Unfortunately, they degrade rapidly upon exposure to light because of scission of sigma bonds. Relative stability of four polysilanes, for example, poly(di-n-butylsilane) (PDBS), poly(di-n-hexylsilane) (PDHS), poly(methylphenylsilane) (PMPS), and poly[bis(p-butylphenyl)silane] (PBPS), which have been reported as active materials in light emitting diodes, have been investigated theoretically through semiempirical (AM1) and ab initio (HF/6-31g) methods and density functional theory using B3LYP parametrization. The AM1 level of calculation predicts the absorption maxima reasonably, but it fails to explain the relative stabilities of the four polysilanes in the excited state. However, calculations based on configuration interaction with single excitation and time-dependent density functional theory suggest additional stabilization in the excited states through intersystem crossing to triplets for PMPS and PBPS, consistent with the experimental observation. In contrast, no such stabilization is predicted for PDBS and PDHS. Furthermore, the existence of a stable triplet state in PMPS may also explain the visible emission observed experimentally in PMPS.

Conformational control and photoenolization of pyridine-3-carboxaldehydes in the solid state: stabilization of photoenols via hydrogen bonding and electronic control.

We have investigated the solid-state photobehavior of a broad set of pyridine-3-carboxaldehydes 1-5. The introduction of a heteroatom into mesitaldehydes as in aldehydes 1 raises the question of conformational preference in the solid state. The preferred conformations have been unequivocally established from X-ray crystal structure analyses of two of the aldehydes, 1c and 2c; it is shown that intramolecular hydrogen bonding could be utilized to achieve conformational control. In contrast to mesitaldehydes, which undergo efficient photocyclization to benzocyclobutenols in the solid state, the heteroatom analogues 1b and 1c exhibit a perceptible color change (from colorless to pale yellow for 1b and yellow-orange for 1c) upon UV irradiation; the color attributed to (E)-enols is persistent for several hours. Continued irradiation leads to an intractable polymeric material. The AM1 calculations, which have been reliably applied to the thermal cyclization of xylylenols to benzocyclobutenols, reveal that the (E)-enols of 1 are more stable than those of the mesitaldehydes relative to their corresponding benzocyclobutenols. The stabilization is interpreted as arising from the possibility of engaging the heteroatom in resonance delocalization. That the contribution from such a role of the nitrogen atom is so pronounced is elegantly demonstrated by forming the fluoroborate salts; 1a-HBF(4) and 1b-HBF(4) readily exhibit highly red-shifted absorption upon exposure to UV radiation as a result of stabilization of the photoenols. Notably, such a remarkable stabilization via electronic control of the photoenols is unprecedented. All of the 2-methoxy- and 2-chloro-substituted aldehydes 2-5 exhibit photochromism. Ab initio calculations show that the methoxy group in aldehydes 2 and 3 stabilizes the (E)-enols via O[bond]H...O hydrogen bonding as compared to those of 1 by 5-6 kcal/mol relative to their corresponding benzocyclobutenols. Thus, the presence of methoxy and halo groups at position 2 serves not only to direct the formyl oxygen toward the methyl group for H-abstraction but also to stabilize the (E)-enols.