Jaime A. Stearns

Exploration of the potential energy surface of C4H4 for rearrangement and decomposition reactions of vinylacetylene: A computational study. Part I

The potential energy surface (PES) of C4H4 was explored using quantum chemical methods (DFT, MP2, MP4, GVB-MP2, CCSD(T), G2M, CBSQ/APNO) and 43 different structures located at global and local minima were identified. The majority of these structures correspond to carbenes, a minority to closed shell systems and biradicals (carbyne structures were not investigated). Whereas the chemistry of the closed shell systems such as vinylacetylene (1), butatriene (2), methylenecyclopropene (3), cyclobutadiene (5) or tetrahedrane (15) is well known, the carbenes represent unusual structural entities. 2-Methyl-cycloprop-2-en-1-ylidene (4) (DeltaDeltaH(298) = 36.2 kcal mol(-1) relative to 1) in its sigma2pi0 electron configuration at the carbene C of the 1A ground state is of comparable stability to cyclobutadiene (5) (DeltaDeltaH(298) = 33.4 kcal mol(-1); exp. value: 32.1 kcal mol(-1) as a result of aromatic 2pi-delocalization; carbene 3-vinylidenecyclopropene (13) (DeltaDeltaH(298) = 53.9 kcal mol(-1) does not possess C(2v) symmetry but has the vinylidene group bent toward the three-membered ring (C(s)-symmetry) thus representing a frozen path point of the chelotropic addition of :C=C: to ethene. Allenyl carbene (14) has a triplet ground state and two low lying excited singlet states of closed shell (2.5 kcal mol(-1) higher) and open shell character (14.1 kcal mol(-1)). Carbene 14 is a crossing point on the C4H4 PES connecting closed-shell systems with each other. Because of the stability of 1, its rearrangement reactions are all connected with high activation enthalpies requiring 66 up to 92 kcal mol(-1) so that they energetically overlap with the activation enthalpies typical of decomposition reactions (from 90 kcal mol(-1) upward). The possible rearrangement reactions of 1 are investigated with a view to their relevance for the chemical behavior of the molecule under the conditions of Titans atmosphere.

Rotationally resolved studies of S0 and the exciton coupled S1/S2 origin regions of diphenylmethane and the d12 isotopologue

Rotationally resolved microwave and ultraviolet spectra of jet-cooled diphenylmethane (DPM) and DPM-d(12) have been obtained in S(0), S(1), and S(2) electronic states using Fourier-transform microwave and UV laser/molecular beam spectrometers. The S(0) and S(1) states of both isotopologues have been well fit to asymmetric rotor Hamiltonians that include only Watson distortion parameters. The transition dipole moment (TDM) orientations of DPM and DPM-d(12) are perpendicular to the C(2) symmetry axes with 66(2)%:34(2)% a:c hybrid-type character, establishing the lower exciton S(1) origin as a completely delocalized, antisymmetric combination of the zero-order locally excited states of the toluene-like chromophores. In contrast, the rotational structures of the S(2) origin bands at S(1)+123 cm(-1) and S(1)+116 cm(-1), respectively, display b-type Q-branch transitions and lack the central a-type Q-branch features that characterize the S(1) origins, indicating TDM orientations parallel to the C(2)(b) symmetry axes as anticipated for the upper exciton levels. However, rotational fits were not possible in line with expectations from previous work [N. R. Pillsbury, J. A. Stearns, C. W. Muller, T. S. Zwier, and D. F. Plusquellic, J. Chem. Phys. 129, 114301 (2008)] where the S(2) origins were found to be largely perturbed through vibronic interactions with the S(1) symmetric, antisymmetric torsional, and butterfly levels in close proximity. Predictions from a dipole-dipole coupling model and ab initio theories are shown to be in fair agreement with the observed TDM orientations and exciton splitting. The need to include out-of-ring-plane dipole coupling terms indicates that in-plane models are not sufficient to fully account for the excitonic interactions in this bichromophore.

Hydrogen atom dislocation in the excited state of anthranilic acid: probing the carbonyl stretch fundamental and the effects of water complexation

This paper describes further efforts to understand the excited state hydrogen atom dislocation of anthranilic acid. Resonant ion-dip infrared spectroscopy was used to probe the carbonyl stretch fundamental in both the ground and excited states in an effort to observe the excited state behavior of the heavy atoms surrounding the displaced hydrogen. A small peak in the excited state infrared spectrum was tentatively assigned to the carbonyl stretch fundamental, shifted 80 cm−1 to the red of its position in the ground state, indicative of a significant weakening of the CO bond. CASSCF calculations on a prototypical system, 3-amino-2-propenoic acid, were carried out to aid interpretation of vibrational frequencies and intensities. The effects of water complexation on the excited state hydrogen atom dislocation were also investigated. The vibronic spectrum, acquired by resonant two-photon ionization, displayed similar features as the monomer spectrum, as well as a progression in a low frequency intermolecular vibration. The infrared spectrum of the water complex, supported by density functional theory calculations, established that the water binds between the carbonyl oxygen and the acid hydrogen. The NH stretch fundamentals of the water complex in the ground and excited state were quite similar to those of the monomer, indicating complexation to water has little effect on the hydrogen atom dislocation

Experimental and computational study of the ultraviolet photolysis of vinylacetylene. Part II.

The ultraviolet photochemistry of vinylacetylene (C4H4) was studied under temperature and pressure conditions similar to Titans atmosphere by exciting the molecule in a constrained expansion that opens into the ion source region of a time-of-flight mass spectrometer. The primary dissociation products detected by vacuum-ultraviolet ionization were found to be C4H3 and C4H2, in a ratio of 3-10 : 1. Subsequent reaction of the C4H3 radicals with the parent C4H4 produced two major secondary products: C8H6 and C6H4. The former was spectroscopically identified as phenylacetylene, confirming that photochemical reactions of C4H4 can produce aromatic molecules. The primary dissociation reaction was also studied computationally. The results were consistent with the experimental findings for C4H2 and C4H3. However, the major product is C2H2, which is undetected by 118 nm photoionization in the present experiment but should account for roughly two-thirds of the products. Simulations were also performed to confirm that the present experiment accurately represents the 220 nm photochemistry of vinylacetylene at the temperature and pressure of Titans atmosphere, with a product yield of C2H2 : C4H2 : C4H3 of 66 : 7 : 27. Accounting for the wavelength dependent solar flux on Titan, the estimated absorption cross section of vinylacetylene in the ultraviolet, and the slightly wavelength dependent product distribution, the overall product yield predicted by the simulations for ultraviolet photolysis of vinylacetylene on Titan is C2H2 : C4H2 : C4H3 = 65 : 8 : 27. Finally, a simulation was performed under conditions of a shock tube experiment to examine the differences between thermal and photochemical dissociation. The product yield of this simulation was C2H2 : C4H2: C4H3 = 61 : 1 : 38.

Collision Dynamics of O(3P) + DMMP Using a Specific Reaction Parameters Potential Form

Starting from previous benchmark CBS-QB3 electronic structure calculations (Conforti, P. F.; Braunstein, M.; Dodd, J. A. J. Phys. Chem. A 2009, 113, 13752), we develop two global potential energy surfaces for O((3)P) + DMMP collisions, using the specific reaction parameters approach. Each surface is simultaneously fit along the three major reaction pathways: hydrogen abstraction, hydrogen elimination, and methyl elimination. We then use these surfaces in classical dynamics simulations and compute reactive cross sections from 4 to 10 km s(-1) collision velocity. We examine the energy disposal and angular distributions of the reactive and nonreactive products. We find that for reactive collisions, an unusually large amount of the initial collision energy is transformed into internal energy. We analyze the nonreactive and reactive product internal energy distributions, many of which fit Boltzmann temperatures up to ~2000 K.