13C and 15N fractionation of CH4/N2 mixtures during photochemical aerosol formation: Relevance to Titan.

The ratios of the stable isotopes that comprise each chemical species in Titan's atmosphere provide critical information towards understanding the processes taking place within its modern and ancient atmosphere. Several stable isotope pairs, including 12C/13C and 14N/15N, have been measured in situ or probed spectroscopically by Cassini-borne instruments, space telescopes, or through ground-based observations. Current attempts to model the observed isotope ratios incorporate fractionation resulting from atmospheric diffusion, hydrodynamic escape, and primary photochemical processes. However, the effect of a potentially critical pathway for isotopic fractionation - organic aerosol formation and subsequent deposition onto the surface of Titan - has not been considered due to insufficient data regarding fractionation during aerosol formation. To better understand the nature of this process, we have conducted a laboratory study to measure the isotopic fractionation associated with the formation of Titan aerosol analogs, commonly referred to as 'tholins', via far-UV irradiation of several methane (CH4) and dinitrogen (N2) mixtures. Analysis of the δ13C and δ15N isotopic signatures of the photochemical aerosol products using an isotope ratio mass spectrometer (IRMS) show that fractionation direction and magnitude are dependent on the initial bulk composition of the gas mixture. In general, the aerosols showed enrichment in 13C and 14N, and the observed fractionation trends can provide insight into the chemical mechanisms controlling photochemical aerosol formation.

Photochemistry of benzylallene: ring-closing reactions to form naphthalene.

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S(0)-S(1) origin at 37,483 cm(-1). UV-UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump-probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact with the gas pulse in a short tube that extended the collisional region of the expansion. The products were cooled during expansion of the gas mixture into vacuum, before being interrogated using mass-selective resonant two-photon ionization. The UV-vis spectra of the photochemical products were compared to literature spectra for identification. Several wavelengths were chosen for photoexcitation, ranging from the S(0)-S(1) origin transition (266.79 nm) to 193 nm. Comparison of the product spectral intensities as a function of photoexcitation wavelength provides information on the wavelength dependence of the product yields. Photoexcitation at 266.79 nm yielded five products (benzyl radical, benzylallenyl radical, 1-phenyl-1,3-butadiene, 1,2-dihydronaphthalene, and naphthalene), with naphthalene and benzylallenyl radicals dominant. At 193 nm, the benzylallenyl radical signal was greatly reduced in intensity, while three additional C(10)H(8) isomeric products were observed. An extensive set of calculations of key stationary points on the ground state C(10)H(10) and C(10)H(9) potential energy surfaces were carried out at the DFT B3LYP/6-311G(d,p) level of theory. Mechanisms for formation of the observed products are proposed based on these potential energy surfaces, constrained by the results of cursory studies of the photochemistry of 1-phenyl-1,3-butadiene and 4-phenyl-1-butyne. A role for tunneling on the excited state surface in the formation of naphthalene is suggested by studies of partially deuterated benzylallene, which blocked naphthalene formation.

The excited states and vibronic spectroscopy of diphenyldiacetylene and diphenylvinylacetylene.

Laser induced fluorescence (LIF) excitation scans and dispersed fluorescence (DFL) spectra have been recorded for two four-carbon α,ω-diphenyl systems, diphenyldiacetylene (DPDA, φ-C≡C-C≡C-φ) and trans-diphenylvinylacetylene (DPVA, φ-CH≡CH-C≡C-φ) as isolated molecules cooled in a supersonic expansion. While these molecules have similar conjugation length, they exhibit strikingly different vibronic spectroscopy and photophysics. The near-UV LIF excitation spectrum of diphenyldiacetylene has its electronic origin at 32,158 cm(-1), and a strong progression in the C≡C stretch (2156 cm(-1)). All transitions are inherently broad, with widths of ~30 cm(-1) fwhm or greater. The S(1) origin DFL spectrum is composed of sharp transitions with Franck-Condon activity mirroring that in the excitation spectrum, and broad emission shifted well to the red ascribable to phosphorescence on the µs timescale. Using ab initio calculations, it is possible to show that DPDA exists as a single, planar conformer with D(2h) symmetry. In contrast, trans-diphenylvinylacetylene shows intense sharp transitions in both LIF and DFL spectra with an S(0)-S(1) origin of 31,183.2 cm(-1) and long progressions involving the in-plane fundamentals ν(53) (bridge-phenyl bending) and ν(51) (bridge-phenyl stretch). A sharp reduction in fluorescence yield in DPVA occurs within 300 cm(-1) of the S(1) origin. Possible causes for the photophysical processes occurring in the two molecules are discussed.

Spectroscopic and thermochemical consequences of site-specific H-atom addition to naphthalene.

Vibronic spectra of doublet-doublet transitions of 1-hydronaphthyl (1HN), 2-hydronaphthyl (2HN), and 1,2,3-trihydronaphthyl (THN, tetralyl) radicals have been recorded under jet-cooled conditions. Transitions due to the two C(10)H(9) isomers were identified and assigned based on the choice of radical precursor, visible-visible hole-burning spectroscopy, comparison of observed vibronic transitions with calculation, and photoionization efficiency scans. The latter provided accurate ionization potentials for the three free radicals (IP(1HN) = 6.570 eV, IP(2HN) = 6.487 eV, IP(THN) = 6.620 eV, errors +/-0.002 eV). A thermochemical cycle is used to extract from these ionization potentials the C-H bond dissociation energy (BDE) of 1HN at the 1-position of 121.2 +/- 2 kJ/mol. Using proton affinities of 2HN and THN calculated at the G3(MP2, CC)//B3LYP/6-311G** level of theory, the corresponding C-H BDEs of 2HN at the 2-carbon (103.6 +/- 2 kJ/mol) and of THN at the 3-position (168 +/- 3 kJ/mol) are derived. The possible role played by these hydronaphthyl radicals in Titan's atmosphere, the interstellar medium, and combustion are briefly discussed.

Isomer specific spectroscopy of C10Hn, n = 8-12: exploring pathways to naphthalene in Titan's atmosphere.

Laboratory investigations of the isomer-specific spectroscopy of several C10Hn isomers with n = 8-12 are described, focusing on structures of relevance to the formation or subsequent reaction of naphthalene. The photochemical models of Titan's atmosphere have now progressed to the point that further development of the large-molecule end of the model must recognize and explicitly incorporate the unique spectroscopy, photochemistry, and reactivity of structural isomers. Mass-resolved, resonant two-photon ionization (R2PI) was used to record ultraviolet spectra of specific C10Hn composition, while hole-burning methods were used to resolve the spectra of different structural and conformational isomers under jet-cooled conditions. The R2PI spectrum of a new C10H8 isomer, 1-phenyl-1-butyne-3-ene, is described and contrasted with other C10H8 isomers. The anticipated role for resonance-stabilized radicals is illustrated by studies of the visible spectroscopy of two hydronaphthyl radical isomers, 1-C10H9 and 2-C10H9, and the trihydronaphthyl radical 1,2,3-C10H11. Conformation-specific spectra of an anticipated C10H12 recombination product of benzyl and allyl radicals is also reported. A reaction scheme that fleshes out the experimental data surrounding naphthalene and its hydrogenated radicals and ions is proposed as a basis for future modeling under Titan's conditions.

Structure and dynamics of phthalocyanine-argonn (n = 1-4) complexes studied in helium nanodroplets.

Van der Waals clusters of phthalocyanine with 1-4 argon atoms formed inside superfluid helium nanodroplets have been investigated by recording fluorescence excitation spectra as well as emission spectra. The excitation spectra feature a multitude of sharp lines when recorded in superfluid helium droplets in contrast to the respective spectra measured in a seeded supersonic beam (Cho et al. Chem. Phys. Lett. 2000, 326, 65). The pickup technique used for doping of the phthalocyanine and the argon into the droplets allows for nondestructive analysis of the cluster sizes. Alternation of the pickup sequence gives information on the binding site of the argon atoms. The investigation of dispersed emission spectra in helium droplets can be used as a special tool for the identification of 0(0)0 transitions within the variety of sharp lines seen in the excitation spectra. Thus, different isomers of the clusters can be distinguished. Moreover, the emission spectra reveal information on dynamic processes such as vibrational predissociation of the van der Waals complexes and interconversion among isomeric species. The binding energy of the phthalocyanine-argon1 complex in helium droplets was estimated to be at most 113 cm-1.