INTERNAL MIXING, PHENYL RING TORSION AND EXCITONIC INTERACTION IN DIPHENYLMETHANE

Author Institution: Department of Chemistry, Purdue University, West Lafayette, IN 47907-2084The close proximity of two identical ultraviolet chromophores render diphenylmethane~(\mbox{DPM}) an interesting case for the study of the dependence of excitonic coupling on the distortion along low-frequency large-amplitude vibrational coordinates, in particular the phenyl ring torsional coordinates present in~\mbox{DPM}. We have studied the fluorescence excitation spectrum and several single vibronic level fluorescence~(\mbox{SVLF}) spectra of the $\tilde{A}^{1}B(\mathrm{S}_{1})$\,$\leftarrow$\,$\tilde{X}^{1}A(\mathrm{S}_{0})$ and the $\tilde{B}^{1}A(\mathrm{S}_{2})$\,$\leftarrow$\,$\tilde{X}^{1}A(\mathrm{S}_{0})$ transition of~\mbox{DPM} cooled in a supersonic jet. The band in the excitation spectrum blue-shifted by~$123\,\mathrm{cm}^{-1}$ from the $\mathrm{S}_{1}$\,$\leftarrow$\,$\mathrm{S}_{0}$ origin was assigned to the $\mathrm{S}_{2}$\,$\leftarrow$\,$\mathrm{S}_{0}$ origin. Its \mbox{SVLF} spectrum shows two-region fluorescence reminiscent of that previously observed in the gas-phase and supersonic jet spectra of naphthalene$^{1}$ and~ovalene$^{2}$. The high-energy region of this $\mathrm{S}_{2}$~$0^{0}$~\mbox{SVLF} spectrum was tentatively assigned to transitions originating in vibrationally excited levels of the $\mathrm{S}_{1}$~state. This finding indicates the presence of efficient internal mixing of the $\mathrm{S}_{2}$~$0^{0}$ level with the sparse manifold of $\mathrm{S}_{1}$ vibronic background levels. The quantum number changes in the non-totally symmetric torsional mode~(see\,figure) upon internal mixing suggest that additional to the internal conversion transitions accounted for by the Jortner-Berry $\Delta v$\,$=$\,$\pm 1$ rule higher order vibronic mechanisms$^{3}$ have to be considered as well. \vspace*{0.2cm} (1) S.~M.~Beck, D.~E.~Powers, J.~B.~Hopkins and R.~E.~Smalley, \textit{J.~Chem. Phys.}, \textbf{1980}, \textit{73}, 2019.\\ (2) A.~Amirav, U.~Even and J.~Jortner, \textit{J.~Chem. Phys.}, \textbf{1981}, \textit{74}, 3745.\\ (3) B.~Scharf, \textit{Chem. Phys.}, \textbf{1975}, \textit{7}, 478

Isomer -specific spectroscopy and conformational isomerization energetics of flexible benzene derivatives

My thesis research investigated the spectroscopy and dynamics of molecules involved in key steps leading toward soot formation in combustion environments. A critical feature of models of combustion is something shared by many areas of science; namely, the need to assess and characterize the role played by structural isomers in chemistry. In this thesis structural isomers of benzene derivatives, some of which lead directly to polyaromatic hydrocarbon formation, were studied. A supersonic jet was used to cool these molecules into their zero-point levels. Then, various laser-based spectroscopies were used to provide the infrared and ultraviolet spectral signatures of individual structural isomers free from interference from one another. Unique infrared and ultraviolet spectral signatures of ortho-, meta-, and para-ethynyl styrenes (oES, mES, and pES), para- and meta-divinylbenzene (pDVB and mDVB), 3-benzyl-1, 5-hexadiyne (BHD), 5-phenyl-1-pentyne (PP), and 4-phenyl-1-butyne (PB) were obtained. These studies showed that mES, mDVB, BHD, PP, and PB all have multiple conformations present in the expansion. Assignments of individual conformations were made for BHD, PP, and PB based on the observed and calculated frequencies in the alkyl and acetylenic CH stretch regions, S0-S1 electronic origin shifts, and rotational band contour analysis. High resolution fluorescence excitation studies were used to assign the two conformations of m ES. Armed with this foundation of spectroscopic data on individual conformational isomers, detailed studies of the conformational isomerization dynamics were carried out on mES and mDVB. These studies used the method of stimulated emission pumping-population-transfer spectroscopy (SEP-PT) as a means to place narrow bounds on the energy thresholds for isomerization between individual reactant-product isomer pairs. This method utilizes selective excitation of a single conformation by means of SEP in the early portions of the gas-phase expansion, followed by collisional re-cooling of the vibrationally excited population into the conformational minima for subsequent conformation-specific detection. These studies were used to determine the thresholds for isomerization, the relative energies of the different conformations in the ground state, and the pathways for isomerization. In addition, single vibronic level fluorescence studies were used to fit the vinyl torsional levels observed in m DVB to obtain the barrier to isomerization in a second manner

THE INFRARED AND ULTRAVIOLET SPECTROSCOPY OF JET-COOLED ORTHO-, META-, and PARA-ETHYNYLSTYRENE

Author Institution: Department of Chemistry, Purdue UniversityVibronic spectroscopy of ortho-, meta-, and para-ethynylstyrene (OES, MES, and PES) was studied by resonant two photon ionization (R2PI). The origins of the OES and PES are $32369 cm^{-1}$ and $33417 cm^{-1}$, respectively. In the MES, there are two possible conformations that occur close in energy. In the R2PI spectrum of MES the two most prominent peaks occur at $32683 cm^{-1}$ and $32936 cm^{-1}$. UV-UV holeburning (UVHB) spectroscopy confirmed that these transitions are due to the two different conformations of MES. There are also two possible conformations in OES, but calculations suggest that the conformation with the substituents pointing towards each other should only be about $5$ of the room temperature population. The UVHB spectrum of OES confirms that only one conformer is present. In the R2PI spectra of each of these isomers there is evidence of vibronic coupling of the $S_{1}$ state to an excited electronic state. Ground state infrared spectra in the C-H stretch region $(3000-3300 cm^{-1})$ were obtained using resonant ion-dip infrared spectroscopy (RIDIRS). In all three isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance. Infrared spectra were also recorded in the excited electronic state using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. Experiments designed to measure the barrier to isomerization of the vinyl group will also be discussed

CONFORMATION SPECIFIC SPECTROSCOPY OF 4-PHENYL-1-BUTYNE, 5-PHENYL-1-PENTYNE, AND 3-BENZYL-1, 5-HEXADIYNE

Author Institution: Department of Chemistry, Purdue University, West Lafayette, IN 47907 U.S.A.4-Phenyl-1-butyne, 5-phenyl-1-pentyne, and 3-benzyl-1,5-hexadiyne were studied by a combination of methods, including resonant two photon ionization, UV-UV hole-burning spectroscopy, resonant ion-dip infrared spectroscopy, and rotational band contour studies. There are two conformations of 4-phenyl-1-butyne observed in the expansion with their S$_{1} \leftarrow$ S$_{0}$ origins occurring at 37617 and 37620 \wn. {\it Ab initio} calculations show that there are also only two low energy conformations (with the chain anti or gauche with respect to the ring). The experimental rotational band contours of the origin bands were compared to {\it ab initio} calculations to make conformational assignments. The gauche and anti conformations are assigned to the red and blue-shifted conformers, respectively. Three conformations of 5-phenyl-1-pentyne are observed in the expansion with their S$_{1} \leftarrow$ S$_{0}$ origins at 37538, 37578, and 37601 \wn. {\it Ab initio} calculations predict four low energy structures (two gauche and two anti). Rotational band contour analysis was used to assign the three conformations observed. The two red-shifted conformations are assigned to gauche structures. In 3-benzyl-1, 5-hexadiyne, 5 conformations are observed in the expansion with their electronic S$_{1} \leftarrow$ S$_{0}$ origins spread over about 100 \wn. DFT calculations predict six low energy conformations. Conformational assignments have been made by comparison of the experimental infrared spectra in the hydride stretch region to DFT frequency calculations. The electronic origin shifts of 3-benzyl-1,5-hexadiyne compare favorably to the origin shifts of 5-phenyl-1-pentyne with the exception of one conformation. This conformation is unique in that it is the only structure with both acetylenic groups in the gauche position over the ring. This gauche-gauche conformation produces extensive vibronic coupling typical of symmetric mono-substituted benzenes. The conformational isomerization energetics of 4-phenyl-1-butyne, 5-phenyl-1-pentyne, and 3-benzyl-1,5-hexadiyne will also be discussed. \end{document} % ------ end electronic abstract submission template -----

CONFORMATION-SPECIFIC ELECTRONIC SPECTROSCOPY OF JET-COOLED 5-PHENYL-1-PENTENE

{Chang-Dar D. Ho and Harry Morrison, \textit{J. Am. Chem. Soc.Author Institution: Department of Chemistry, Purdue University, West Lafayette, IN 47907The electronic spectroscopy of jet-cooled 5-phenyl-1-pentene was studied by resonant two photon ionization (R2PI). This molecule is of interest because intramolecular photochemistry occurs from intermediate conformations that bring the vinyl group up over the phenyl ring.}, \textbf{127}, 2114 (2003).} Five conformations were confirmed by UV-UV hole-burning (UVHB) spectroscopy. The $S_{0}$-$S_{1}$ origins of these conformations were found between 37507 and 37585\wn. The origin transitions clearly separate into two groups split by about 50\wn (three to the red and two to the blue). According to existing literature, the interaction of the methylene gamma hydrogen with the pi cloud of the ring in gauche conformations causes a red shift in their origins. Therefore, the three origins to the red are anticipated to be gauche conformations while the origins to the blue are anti conformations. Tentative structural assignments have been made by comparing rotational band contours with calculated contours based on transition moments and rotational constants from \textit{ab initio} calculations. These assignments and experimental techniques will be discussed. A study was also conducted to look for conformation-specific lifetime shortening in vibronic levels above the origin, but no evidence of this was found

Reaction of the C2H radical with 1-Butyne (C4H6): Low Temperature Kinetics and Isomer-Specific Product Detection

The rate coefficient for the reaction of the ethynyl radical (C{sub 2}H) with 1-butyne (H-C{triple_bond}C-CH{sub 2}-CH{sub 3}) is measured in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by laser photolysis of acetylene (C{sub 2}H{sub 2}) at 193 nm and detected via chemiluminescence (C{sub 2}H + O{sub 2} {yields} CH (A{sup 2}{Delta}) + CO{sub 2}). The rate coefficients are measured over the temperature range of 74-295 K. The C{sub 2}H + 1-butyne reaction exhibits no barrier and occurs with rate constants close to the collision limit. The temperature dependent rate coefficients can be fit within experimental uncertainties by the expression k = (2.4 {+-} 0.5) x 10{sup -10} (T/295 K)-(0.04 {+-} 0.03) cm{sup 3} molecule{sup -1}s{sup -1}. Reaction products are detected at room temperature (295 K) and 533 Pa using a Multiplexed Photoionization Mass Spectrometer (MPIMS) coupled to the tunable VUV synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory. Two product channels are identified for this reaction: m/z = 64 (C{sub 5}H{sub 4}) and m/z = 78 (C{sub 6}H{sub 6}) corresponding to the CH{sub 3}- and H-loss channels, respectively. Photoionization efficiency (PIE) curves are used to analyze the isomeric composition of both product channels. The C{sub 5}H{sub 4} products are found to be exclusively linear isomers composed of ethynylallene and methyldiacetylene in a 4:1 ratio. In contrast, the C{sub 6}H{sub 6} product channel includes two cyclic isomers, fulvene 18({+-}5)% and 3,4-dimethylenecyclobut-1-ene 32({+-}8)%, as well as three linear isomers, 2-ethynyl-1,3-butadiene 8({+-}5)%, 3,4-hexadiene-1-yne 28({+-}8)% and 1,3-hexadiyne 14({+-}5)%. Within experimental uncertainties, we do not see appreciable amounts of benzene and an upper limit of 10% is estimated. Diacetylene (C{sub 4}H{sub 2}) formation via the C{sub 2}H{sub 5}-loss channel is also thermodynamically possible but cannot be observed due to experimental limitations. The implications of these results for modeling of planetary atmospheres, especially of Saturn's largest moon Titan, are discussed

EXCITONIC COUPLING AND PHENYL RING TORSION IN DIPHENYLMETHANE

Author Institution: Department of Chemistry, Purdue University, West Lafayette, IN, 47907; National Institute of Standards and Technology, Gaithersburg, MD, 20899Diphenylmethane is a simple bichromophore in which two phenyl rings are connected only by a methylene group, giving considerable flexibility in the ring torsion coordinates as well as substantial electronic coupling between the rings. According to DFT calculations, the minimum energy structure has C$_{2}$ symmetry with a ring torsion angle near 60$^{o}$. There are two such minima connected by a C$_{2v}$ structure 176 cm$^{-1}$ higher in energy. The rotationally-resolved fluorescence excitation spectrum of the S$_{0}$-S$_{1}$ origin indicated a 70\% a-type, 30\% c-type transition moment, suggestive of an excitonic state in which the zero-order transition moment is rotated significantly from that of toluene. CIS underestimates the a-type character of the band, while time-dependent DFT overestimates it. The ground state experimental rotational constants are consistent with those predicted by DFT. A Franck-Condon progession in the torsional coordinate was observed in the experimental jet-cooled resonant two-photon ionization and dispersed fluorescence spectra. This progression was well-fit using a harmonic Franck-Condon analysis indicating a 3$^{o}$ change in the ring torsion angle upon excitation to S$_{1}$. The S$_{0}$-S$_{2}$ origin was assigned to a vibration 123 cm$^{-1}$ above the S$_{0}$-S$_{1}$ origin. The dispersed fluorescence spectrum from this transition showed a great deal of activity in low frequency vibrations which were not present in the excitation spectrum, indicating that a vibronic band of the S$_{1}$ state may be resonant with the S$_{0}$-S$_{2}$ origin. To test the effect of asymmetry on the excitonic coupling, the spectroscopy of 4-methyldiphenylmethane has also been studied. Addition of a methyl group to one chromophore completely localizes the electronic excitation, demonstrated by the fact that the two electronic origins are very near those of toluene and {\it para}-xylene. Dispersed fluorescence from the S$_{0}$-S$_{2}$ origin showed only S$_{0}$-S$_{1}$ origin-like emission broadened by IVR, indicative of extremely rapid electronic energy transfer