Journal of Molecular Spectroscopy 270 (2011) 98–107 Contents lists available at SciVerse ScienceDirect Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms Spectroscopic characterization of structural isomers of naphthalene: 1-Phenyl-1-butyn-3-ene ⇑ Joshua A. Sebree a, David F. Plusquellic b, Timothy S. Zwier c, a Goddard Space Flight Center, Greenbelt, MD 20771, USA b Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA c Department of Chemistry, Purdue University, West Lafayette, IN 47909, USA article info abstract Article history: Laser induced fluorescence (LIF), single vibronic level dispersed fluorescence (DFL) spectra, and high res- Received 14 September 2011 olution rotationally resolved scans of the S0–S1 transition of the C10H8 isomer 1-phenyl-1-butyn-3-ene Available online 19 October 2011 À1 have been recorded under jet-cooled conditions. The S0–S1 origin of PAV at 34922 cm is very weak. 1 A vibronic band located 464.0 above the origin, assigned as 30 0, dominates the LIF excitation spectrum, Keywords: with intensity arising from vibronic coupling with the S2 state. High resolution scans of the S0–S1 origin Naphthalene isomers 1 1 and 30 0 vibronic bands determine that the former is a 65:35 a:b hybrid band, while 30 0 is a pure a-type Vibronic coupling band, confirming the role for vibronic coupling and identifying the coupled state as the S state. DFL spec- Phenylvinylacetylene 2 tra of all vibronic bands in the first 800 cmÀ1 of the spectrum were recorded. A near-complete assignment of the vibronic structure in both S0 and S1 states is obtained. Herzberg–Teller vibronic coupling is carried by two vibrations, m28 and m30, involving in-plane deformations of the vinylacetylene side chain, leading to Duschinsky mixing evident in the intensities of transitions in excitation and DFL spectra. Extensive Dus- chinsky mixing is also present among the lowest five out-of-plane vibrational modes, involving motion of the side chain. Comparison with the results of DFT B3LYP and TDDFT calculations with a 6-311+G(d,p) basis set confirm and strengthen the assignments. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction [7,8]. It is therefore important to establish pathways that lead from benzene and its simple derivatives to naphthalene. Among the From combustion to planetary atmospheres, the formation and pathways worth exploring in some detail are those that involve growth of polyaromatic hydrocarbons (PAHs) is of great interest. formation of singly- or doubly-substituted aromatic derivatives, PAH formation during the combustion of fossil fuels is known to which themselves either photolyze or photoisomerize to naphtha- be a critical first step in the process of soot formation [1–4]. lene. As a precursor to photochemical studies, it is important first Recently, the Cassini orbiter has provided evidence of complex to characterize the spectroscopy of key aromatic derivatives that organic chemistry taking place in the atmosphere of Titan, will act as intermediates in pathways to naphthalene. To that Saturn’s largest moon. Data from the Cassini Plasma Spectrometer end, we have recently carried out systematic studies of the vibronic (CAPS) shows intriguing peaks near the masses of naphthalene spectroscopy of a series of C10H8 isomers, including o-, m-, and (C10H8, m/z 128) and anthracene (C14H10, m/z 178), the simplest p-ethynylstyrene and phenylvinylacetylene (PVA) [9–11]. PAHs [5]. The formation of these simple fused ring aromatics is In a previous benzene discharge study by Güthe et al., four postulated to be the first step in the formation of large organic unknown m/z 128 isomers were found [12]. While not identified compounds called ‘‘tholins’’ [5,6]. While it is tempting to assign in the study, it was confirmed that none of them were naphthalene. species with M = 128 amu to naphthalene, it is important to keep Later, studies by our group identified two of the species to be in mind that there are many other C10H8 structural isomers pos- e- and z-phenylvinylacetylene [10,11]. These C10H8 species were sible, calling for the need for isomer-specific studies. also identified in a photochemistry experiment looking at the reac- Ã The chemistry of Titan’s atmosphere is driven by solar radiation, tion of excited diacetylene (C4H2) with styrene [13]. We have with poly-ynes and aromatic hydrocarbons as important ultravio- recently reported in a preliminary way on the spectroscopy of let absorbers. Current photochemical models lack much in the way 1-phenyl-1-butyn-3-ene (PAV) [14]. Here we provide a more de- of structural details beyond formation of the first aromatic ring tailed account of its vibronic spectroscopy, including high resolu- tion studies with rotational resolution. ⇑ Corresponding author. Structurally, 1-phenyl-1-butyn-3-ene, can be viewed as a close E-mail address: [email protected] (T.S. Zwier). analog of PVA, differing in the ordering of the ethynyl and vinyl 0022-2852/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2011.10.001 J.A. Sebree et al. / Journal of Molecular Spectroscopy 270 (2011) 98–107 99 groups relative to the phenyl ring. To emphasize this reordering, external resonant cavity using a BBO crystal. The molecular beam we have labeled this molecule PAV. In much the same way that was skimmed and interrogated by the laser 18 cm downstream. PVA was shown to have similar characteristics to styrene, it is ex- Fluorescence was collected using two spherical mirrors and de- pected that PAV will have many properties similar to that of tected using a PMT and computer-interfaced photon counter. This phenylacetylene. allowed for a Doppler-limited resolution of 21(1) MHz. As a prototypical ethynylbenzene, phenylacetylene has received PAV was synthesized via a published procedure [25]. Cuprous considerable attention over the years [15–20]. The inherent oscil- iodide was added to a mixture of bis(triphenylphosphine) palla- lator strength of the S0–S1 transition of phenyl acetylene (PA) is dium dichloride and phenylacetylene in ethylamine under a nitro- very weak due to the near cancellation of the electronic transition gen atmosphere. Vinylbromide was bubbled through the flask and dipole moment (TDM) contributions from the two orbital compo- the solution was stirred for 6 h at room temperature. The solvent, nents involved in the transition [15]. As a result, the vibronic spec- and unreacted starting material, was removed under reduced pres- troscopy of PA is dominated by vibronic coupling. In particular, the sure and the remaining residue was extracted using a water/hex- strongest transition in the excitation spectrum of PA is a vibronic ane extraction. GC/MS showed the final product to contain 95% À1 band 490 cm to the blue of its S0–S1 origin [15,17]. This band 1-phenyl-1-butyn-3-ene with a 5% impurity of diphenyldiacety- gains its intensity by Herzberg–Teller vibronic coupling to a higher lene. The sample was purified using column chromatography. electronic state (S2) with large oscillator strength [19]. Other phen- The ground state geometries of PAV, PVA and PA were opti- ylacetylene derivatives have similar vibronic intensity patterns mized using density functional theory (DFT) with a Becke3LYP reflecting closely analogous effects [16,19]. In the case of PA, two [26,27] functional and 6-311+G(d,p) basis set. To obtain predic- separate vibrational modes are strongly involved in the coupling, tions for the TDM, time-dependent excited-state (TDDFT) optimi- leading to a mixing of the modes upon excitation [19]. This mode zations were carried out at the same level of theory. To obtained mixing, or Duschinsky mixing [21], can lead to a break down in accurate ordering of the excited states, CASSCF(10,10) optimiza- the mirror symmetry generally seen between fluorescence excita- tions [28] at 6-31G(d) level of theory were carried out. These cal- tion and dispersed fluorescence spectra [19]. It was shown by culations were performed using Gaussian 09 [29]. Simulations of Small et al., that comparison of the intensities on the mixed modes the high resolution rotational structure of key bands in the spec- in the ground and excited states can give insight on the relative trum were carried out with JB95 software [24]. All spectra were ini- directions of the TDM components induced by each normal mode tially fit to an asymmetric rotor Hamiltonian (representation Ir) [18]. Later work by Chang and co-workers proved that inclusion using a distributed parallel version of the genetic algorithm (GA) of vibronic coupling involving both totally symmetric and non- program [24] beginning with ground state rotational constants totally symmetric vibrations was essential to quantitative predic- estimated from ab initio theory. Parameter uncertainties from lin- tion of the vibronic spectrum of PA [22]. ear least-squares fits of the bands (type A, k =1) [33] were con- In this work, a detailed study of the jet-cooled vibronic and firmed from comparisons with results from repeated GA runs on rovibronic spectroscopy of the S0–S1 transition of PAV is presented. the same band. Using a combination of laser induced fluorescence (LIF), dispersed fluorescence (DFL), and high-resolution UV techniques, it is seen that the S0–S1 transition has more in common with the corre- 3. Results and analysis sponding transition in phenylacetylene than it does with PVA. In particular, the spectrum is dominated by Herzberg–Teller vibronic 3.1. LIF excitation and high resolution UV spectroscopy coupling involving in-plane modes that also show evidence for substantial Duschinsky mixing. As will be shown, the spectrum In previous work, we reported the mass-resolved resonant two- of PAV also provides a match with the unassigned transitions in photon (R2PI) spectrum of PAV [14].