3,4-dimethylbenzyl radical formed in a corona discharge of 1,2,4-trimethylbenzene

A precursor, 1,2,4-trimethylbenzene, seeded in a large amount of an inert carrier gas, helium, was electrically discharged in a corona-excited supersonic expansion using a pinhole-type glass nozzle. The blue-green colored fluorescence emanating from the downstream jet was recorded with a long path monochromator to observe the vibronic emission spectrum of the benzyl-type radical formed. Analysis of the spectrum suggests that the most dominant product of the corona discharge is the 3,4-dimethylbenzyl radical formed by extracting a hydrogen atom from the methyl group at the 4-position. The electronic energies of the D1 and D2 states and the vibrational mode frequencies of the 3,4-dimethylbenzyl radical were accurately obtained for the first time by comparison with those from an ab initio calculation as well as those of the known vibrational mode frequencies of the precursor.     

Spectroscopic observation of jet-cooled 2-fluoro-m-xylyl radical

The jet-cooled 2-fluoro-m-xylyl radical was generated and vibronically excited in a corona excited supersonic expansion from precursor 2-fluoro-m-xylene seeded in a large amount of carrier gas helium. The well-resolved visible vibronic emission spectrum of the jet-cooled 2-fluoro-m-xylyl radical was recorded using a long-path monochromator. From the analysis of the spectrum, we determine an accurate electronic energy of the D(1) → D(0) transition and the frequencies of vibrational modes in the ground electronic state by comparison with those of ab initio calculations and the known spectroscopic data of 2-fluoro-m-xylene for the first time.     

Reaction dynamics and vibrational spectroscopy of CH3D molecules with both C-H and C-D stretches excited

State-resolved reactions of CH3D molecules containing both C-H and C-D stretching excitation with Cl atoms provide new vibrational spectroscopy and probe the consumption and disposal of vibrational energy in the reactions. The vibrational action spectra have three different components, the combination of the C-H symmetric stretch and the C-D stretch (nu1 + nu2), the combination of the C-D stretch and the C-H antisymmetric stretch (nu2 + nu4), and the combination of the C-D stretch and the first overtone of the CH3 bend (nu2 + 2nu5). The simulation for the previously unanalyzed (nu2 + nu4) state yields a band center of nu0 = 5215.3 cm(-1), rotational constants of A = 5.223 cm(-1) and B = 3.803 cm(-1), and a Coriolis coupling constant of zeta = 0.084. The reaction dynamics largely follow a spectator picture in which the surviving bond retains its initial vibrational excitation. In at least 80% of the reactive encounters of vibrationally excited CH3D with Cl, cleavage of the C-H bond produces CH2D radicals with an excited C-D stretch, and cleavage of the C-D bond produces CH3 radicals with an excited C-H stretch. Deviations from the spectator picture seem to reflect mixing in the initially prepared eigenstates and, possibly, collisional coupling during the reaction.     

Observation of vibronic emission spectrum of jet-cooled duryl radical in corona excitation

The vibronically excited but jet-cooled 2,4,5-trimethylbenzyl (duryl) radical was formed in a corona excitation from precursor 1,2,4,5-tetramethylbenzene (durene) seeded in a large amount of inert carrier gas helium using a pinhole-type glass nozzle. The vibronically resolved emission spectrum of the jet-cooled duryl radical was recorded, for the first time, with a long path monochromator in the visible region. The spectrum was analyzed to obtain an accurate electronic energy of the D1-->D0 transition and vibrational mode frequencies in the ground electronic state by comparing with those of the precursor and those from an ab initio calculation.     

Rovibrational resonance effects in collision-induced electronic energy transfer: I2(E,v=0-2)+CF4

Collisions of I2 in the E(0(g)+) electronic state with CF4 molecules induce electronic energy transfer to the nearby D, beta, and D' ion-pair states. Simulations of dispersed fluorescence spectra reveal collision-induced electronic energy transfer rate constants and final vibrational state distributions within each final electronic state. In comparison with earlier reports on I2(upsilon(E)=0-2) collisions with He or Ar atoms, we find markedly different dynamics when I2, excited to the same rovibronic states, collides with CF4. Final vibrational state distributions agree with the associated Franck-Condon factors with the initially prepared state to a greater degree than those found with He or Ar collision partners and suggest that internal degrees of freedom in the CF4 molecule represent a substantial means for accepting the accompanying loss of I2 vibronic energy. Comparison of the E-->D transfer of I2 excited to the J=23 and J=55 levels of the upsilon(E)=0 state reveals the onset of specific, nonstatistical dynamics as the available energy is increased above the threshold for excitation of the low frequency nu2 bending mode of CF4.     

Excited and ground state vibrational dynamics revealed by two-dimensional electronic spectroscopy

Broadband two-dimensional electronic spectroscopy (2DES) can assist in understanding complex electronic and vibrational signatures. In this paper, we use 2DES to examine the electronic structure and dynamics of a long chain cyanine dye (1,1-diethyl-4,4-dicarbocyanine iodide, or DDCI-4), a system with a vibrational progression. Using broadband pulses that span the resonant electronic transition, we measure two-dimensional spectra that show a characteristic six peak pattern from coherently excited ground and excited state vibrational modes. We model these features using a spectral density formalism and the vibronic features are assigned to Feynman pathways. We also examine the dynamics of a particular set of peaks demonstrating anticorrelated peak motion, a signature of oscillatory wavepacket dynamics on the ground and excited states. These dynamics, in concert with the general structure of vibronic two-dimensional spectra, can be used to distinguish between pure electronic and vibrational quantum coherences.     

Confirmed assignments of isomeric dimethylbenzyl radicals generated by corona discharge

The controversial vibronic assignments of isomeric dimethylbenzyl radicals were clearly resolved by using different precursors. By employing corresponding dimethylbenzyl chlorides as precursors, we identified the origins of the vibronic bands of the dimethylbenzyl radicals generated by corona discharge of 1,2,4-trimethylbenzene. From the analysis of the spectra observed from the dimethylbenzyl chlorides in a corona excited supersonic expansion, we revised previous assignments of the 3,4-, 2,4-, and 2,5-dimethylbenzyl radicals. Spectroscopic data of electronic transition and vibrational mode frequencies in the ground electronic state of each isomer were accurately determined by comparing them with those obtained by an ab initio calculation and with the known vibrational data of 1,2,4-trimethylbenzene.     

Vibronic spectroscopy of benzyl-type radicals: observation of mesityl radical in the gas phase

We observed, for the first time, the vibronic emission spectrum of the jet-cooled mesityl radical that was formed from mesitylene seeded in a large amount of inert carrier gas helium using a pinhole-type glass nozzle in a corona excited supersonic expansion. The well-resolved vibronic emission spectrum was recorded in the visible region with a long path monochromator. The spectrum was analyzed to identify the origin of the D(2) --> D(0) and D(1) --> D(0) transitions as well as the frequencies of the vibrational modes in the ground electronic state of the mesityl radical by comparison with those of the known data of the precursor and an ab initio calculation.     

Formation Mechanism and Emission Spectrum of AlO Radicals in Reaction of Laser－ablated Al Atom and Oxygen

IntroductionLaser ablation has been applied broadly toplasma production,cluster formation,nanometermaterial and thin film material preparation[1,2 ] .Us-ing spectroscopic technique and mass spectrometryto identify the transient species formed in the pro-cess could provide much useful information aboutthe prosperity ofplasma,the mechanism of materi-al growth[3 ,4] .Aluminum is an important metal material andwidely used in industry and daily life. To study thereaction mechanism of Al atoms with…     

The D Parameter (EPR Zero-Field Splitting) of Localized 1,3-Cyclopentanediyl Triplet Diradicals as a Measure of Electronic Substituent Effects on the Spin Densities in Para-Substituted Benzyl-Type Radicals

The zero-field splitting parameters D of the symmetrically disubstituted and unsymmetrically monosubstituted 1,3-diaryl-1,3-cyclopentanediyl triplet diradicals 1, 2 (X = p-MeO, p-Me, p-Cl, p-NH(2), p-CO(2)Me, p-CN, p-NO(2)), and 5 were determined in 2-methyltetrahydrofuran glass at 77 K. The linear plot (m = 0.558, r(2) = 0.993) of the experimental D values for the symmetrically disubstituted derivatives versus the corresponding monosubstituted ones reveals that the electronic substituent effects are additive and implies (except for the magnetic dipolar interaction) that each benzyl-type radical site acts independently in the localized diradicals. This additivity permits us to view these triplet diradicals as a composite of the two separate monoradical components and allows us to assess valuable electronic properties of benzyl-type monoradicals from the D parameter of the triplet diradical species. A theoretical analysis shows that the D parameter is a measure of the spin density rho at the benzylic positions and the inter-radical distance d in localized diradicals. A good correlation exists between the D parameter of these triplet diradicals (constant inter-radical distance d) and the EPR hyperfine coupling constants of the corresponding benzyl-type monoradicals, which establishes that the observed electronic substituent effects reflect changes in the spin densities at the radical sites. The novel DeltaD scale allows us to quantify spectroscopically the para substituent effect on the spin delocalization at the benzylic position.     

Vibronic structure of jet-cooled 2,6-dimethylbenzyl radical: revisited

We reexamined the vibronic structure of the jet-cooled 2,6-dimethylbenzyl radical that was generated from 1,2,3-trimethylbenzene seeded in a large amount of inert carrier gas helium using a pinhole-type glass nozzle in a corona excited supersonic expansion, from which the vibronically resolved emission spectrum was recorded with a long path double monochromator in the visible region. The spectrum exhibited bands arising from not only the D1 --> D0 transition but also the D2 --> D0 transition, in which transitions the accurate electronic energies of the D2 and D1 states and the revised vibrational mode frequencies in the ground electronic state were obtained by comparison with those from the known data of the precursor and an ab initio calculation.     

Dissociation channels of the 1-buten-2-yl radical and its photolytic precursor 2-bromo-1-butene

The work presented here is the first in a series of studies that use a molecular beam scattering technique to investigate the unimolecular reaction dynamics of C(4)H(7) radical isomers. Photodissociation of the halogenated precursor 2-bromo-1-butene at 193 nm under collisionless conditions produced 1-buten-2-yl radicals with a range of internal energies spanning the predicted barriers to the unimolecular reaction channels of the radical. Resolving the velocities of the stable C(4)H(7) radicals, as well as those of the products, allows for the identification of the energetic onset of each dissociation channel. The data show that radicals with at least 30.7 +/- 2 kcal/mol of internal energy underwent C-C fission to form allene + methyl, and radicals with at least 36.7 +/- 4 kcal/mol of internal energy underwent C-H fission to form H + 1-butyne and H + 1,2-butadiene; both of these observed barriers agree well with the G3//B3LYP calculations of Miller. HBr elimination from the parent molecule was observed, producing vibrationally excited 1-butyne and 1,2-butadiene. In the subsequent dissociation of these C(4)H(6) isomers, the major channel was C-C fission to form propargyl + methyl, and there is also evidence of at least one of the possible H + C(4)H(5) channels. A minor C-Br fission channel produces 1-buten-2-yl radicals in an excited electronic state and with low kinetic energy; these radicals exhibit markedly different dissociation dynamics than do the radicals produced in their ground electronic state.     

A stimulated emission pumping study of the first excited singlet state of germylidene (H2C=Ge)

The A (1)A(2) states of H(2)CGe and D(2)CGe have been explored for the first time by A-X laser-induced fluorescence (LIF) spectroscopy of the orbitally forbidden S(1)-S(0) transition and stimulated emission pumping (SEP) and wavelength resolved fluorescence studies of the allowed B-A electronic transition. Medium-resolution SEP studies gave the excited A state nu(2), nu(3), nu(4), and nu(6) vibrational frequencies for H(2)C(74)Ge and D(2)C(74)Ge. The 4(1) and 6(1) levels and higher combination and overtone states are strongly Coriolis coupled, which perturbs the rotational subband structure, limiting the accuracy of the determination of the vibrational frequencies. High-resolution SEP studies of the B-A 0(0) (0) band have allowed us to determine the rotational constants of the A state of H(2)C(74)Ge, from which we were able to calculate an approximate r(0) structure with the CH bond length constrained to the ground state value. The zero-point level of D(2)C(74)Ge is substantially perturbed, most plausibly by interaction with an excited vibrational level of the nearby triplet (a (3)A(2)) state.     

Time‐Resolved FTIR Emission Spectroscopy of Transient Radicals

Fourier Transform spectroscopy with 10^?8 second time resolution for recording IR emission spectra has been developed as an efficient means for detecting previously unknown vibrational modes of transient radicals. 193 nm photodissociation of a precursor molecule is used to generate vibrationally excited radicals, from which IR emission is recorded with time and spectral resolution. Assignment of the spectra is performed using information obtained through multiple precursors, isotopic substitution, time dependence of emission intensity, theoretical calculations, and 2‐dimensional cross‐spectra correlation analysis. The radicals vinyl, cyanovinyl, and OCCN have been studied with many vibrational modes identified.     

Structural and spectroscopic studies on some chloropyrimidine derivatives by experimental and quantum chemical methods

Ab initio quantum chemical and experimental spectroscopic studies in the infrared (4000-60 cm(-1)) and UV spectral regions are being reported on 4-chloro-2,6-dimethylsulfanyl pyrimidine-5-carbonitrile and 4-chloro-2-methylsulfanyl-6-(2-thienyl) pyrimidine-5-carbonitrile. Optimized geometries, electronic charge distribution, dipole moments and three-dimensional molecular electrostatic potential surfaces have been obtained. These have been used to understand the structure and spectral characteristics of the two compounds. A complete assignment of vibrational spectra on the basis of DFT/6-311G** and electronic spectra on the basis of TD-DFT/6-31+G* 5D calculations have been attempted for the two molecules.     

Decomposition of excited electronic state s-tetrazine and its energetic derivatives

Decomposition of excited electronic state s-tetrazine and its energetic derivatives, such as 3-amino-6-chloro-1,2,4,5-tetrazine-2,4-dioxide (ACTO), and 3,3(')-azobis (6-amino-1,2,4,5-tetrazine)-mixed N-oxides (DAATO(3.5)), is investigated through laser excitation and resonance enhanced multi photon ionization techniques. The N(2) molecule is detected as an initial product of the s-tetrazine decomposition reaction, through its two photon, resonance absorption transitions [a(") (1)Σ(g)(+) (v(') = 0) ← X (1)Σ(g)(+) (v(") = 0)]. The suggested mechanism for this reaction is a concerted triple dissociation yielding rotationally cold (～20 K) ground electronic state N(2) and 2 HCN molecules. The comparable decomposition of excited electronic state ACTO and DAATO(3.5) yields an NO product with a cold rotational (～20 K) but a hot vibrational (～1200 K) distribution. Thus, tetrazine and its substituted energetic materials ACTO and DAATO(3.5) evidence different decomposition mechanisms upon electronic excitation. N(2)O is excluded as a potential intermediate precursor of the NO product observed from these two s-tetrazine derivatives through direct determination of its decomposition behavior. Calculations at the CASMP2∕CASSCF level of theory predict a concerted triple dissociation mechanism for generation of the N(2) product from s-tetrazine, and a ring contraction mechanism for the generation of the NO product from the energetic s-tetrazine derivatives. Relaxation from S(n) evolves through a series of conical intersections to S(0), upon which surface the dissociation occurs in both mechanisms. This work demonstrates that the substituents on the tetrazine ring change the characteristics of the potential energy surfaces of the derivatives, and lead to a completely different decomposition pathway from s-tetrazine itself. Moreover, the N(2) molecule can be excluded as an initial product from decomposition of these excited electronic state energetic materials.     

Photoproduct channels from BrCD2CD2OH at 193 nm and the HDO + vinyl products from the CD2CD2OH radical intermediate

We present the results of our product branching studies of the OH + C(2)D(4) reaction, beginning at the CD(2)CD(2)OH radical intermediate of the reaction, which is generated by the photodissociation of the precursor molecule BrCD(2)CD(2)OH at 193 nm. Using a crossed laser-molecular beam scattering apparatus with tunable photoionization detection, and a velocity map imaging apparatus with VUV photoionization, we detect the products of the major primary photodissociation channel (Br and CD(2)CD(2)OH), and of the secondary dissociation of vibrationally excited CD(2)CD(2)OH radicals (OH, C(2)D(4)/CD(2)O, C(2)D(3), CD(2)H, and CD(2)CDOH). We also characterize two additional photodissociation channels, which generate HBr + CD(2)CD(2)O and DBr + CD(2)CDOH, and measure the branching ratio between the C-Br bond fission, HBr elimination, and DBr elimination primary photodissociation channels as 0.99:0.0064:0.0046. The velocity distribution of the signal at m/e = 30 upon 10.5 eV photoionization allows us to identify the signal from the vinyl (C(2)D(3)) product, assigned to a frustrated dissociation toward OH + ethene followed by D-atom abstraction. The relative amount of vinyl and Br atom signal shows the quantum yield of this HDO + C(2)D(3) product channel is reduced by a factor of 0.77 ± 0.33 from that measured for the undeuterated system. However, because the vibrational energy distribution of the deuterated radicals is lower than that of the undeuterated radicals, the observed reduction in the water + vinyl product quantum yield likely reflects the smaller fraction of radicals that dissociate in the deuterated system, not the effect of quantum tunneling. We compare these results to predictions from statistical transition state theory and prior classical trajectory calculations on the OH + ethene potential energy surface that evidenced a roaming channel to produce water + vinyl products and consider how the branching to the water + vinyl channel might be sensitive to the angular momentum of the β-hydroxyethyl radicals.     

The electronic spectrum of the C2P free radical and a Renner-Teller analysis of the 2Delta and X 2Pi electronic states

Subsequent to our spectroscopic detection of the C(2)X(X=P,As) free radicals [F. X. Sunahori et al., J. Am. Chem. Soc. 129, 9600 (2007)], we have studied the electronic spectrum of the (2)Delta(i)-X (2)Pi(r) system of the jet-cooled C(2)P free radical in the 490-630 nm region. The high-resolution laser-induced fluorescence spectrum of the two spin components of the 0(0) (0) band of (12)C(2)P has been recorded, and the rotational and spin-orbit coupling constants have been determined for both electronic states. The Renner-Teller effect has been observed in both the (2)Pi and the (2)Delta states, and the vibrational structure has been assigned. For the ground state, all of the observed levels up to 3500 cm(-1) were fitted with a standard Renner-Teller model. The excited (2)Delta state vibrational levels were successfully fitted using literature energy level expressions derived from perturbation theory, yielding vibrational and Renner-Teller parameters for both (12)C(2)P and (13)C(2)P. The molecular structure of C(2)P in the ground and excited states has also been estimated and compared to ab initio calculations and the geometries of similar molecules.     

Electronic spectroscopy and excited state dynamics of the Al-H2/D2 complex

The electronic spectra of the Al-H2 and Al-D2 complexes are investigated in a collaborative experimental and theoretical study. The complexes were prepared in a pulsed supersonic beam and detected with laser fluorescence excitation spectroscopy. Transitions to bound vibrational levels in electronic states correlating with the excited-state Al(3d, 4p, 4d) + H2/D2 asymptotes were observed by monitoring emission from lower excited Al atomic levels, formed in the non-radiative decay of the excited complex. Fluorescence depletion has also been used to verify that the observed Al-H2 bands all involve the same molecular carrier. The bands have been assigned to the more strongly bound Al-oH2 and Al-pD2 nuclear spin modifications. In contrast to our previous observations for Al(5s)-H2 [X. Yang and P. J. Dagdigian, J. Chem. Phys., 1998, 109, 8920], for which only one potential energy surface (PES) emanates from the dissociation asymptote, the Lorentzian widths of the different vibrational bands in the 3d, 4p, 4d<--3p transitions vary widely, in some cases allowing resolution of the rotational structure of the bands. With the help of the calculated Al(3p)-oH2/pD2 dissociation energies, binding energies of the observed excited vibronic levels are reported. The mechanism of predissociation is investigated theoretically through ab initio calculation of C2 nu cuts of the excited PESs. It is concluded that predissociation occurs through coupling with the repulsive Al(4s)-H2 PES. With these calculations, a qualitative interpretation of the observed bands could be made.     

Solvent-dependent photoacidity state of pyranine monitored by transient mid-infrared spectroscopy.

We investigate with femtosecond mid-infrared spectroscopy the vibrational-mode characteristics of the electronic states involved in the excited-state dynamics of pyranine (HPTS) that ultimately lead to efficient proton (deuteron) transfer in H2O (D2O). We also study the methoxy derivative of pyranine (MPTS), which is similar in electronic structure but does not have the photoacidity property. We compare the observed vibrational band patterns of MPTS and HPTS after electronic excitation in the solvents: deuterated dimethylsulfoxide, deuterated methanol and H2O/D2O, from which we conclude that for MPTS and HPTS photoacids the first excited singlet state appears to have charge-transfer (CT) properties in water within our time resolution (150 fs), whereas in aprotic dimethylsulfoxide the photoacid appears to be in a non-polar electronic excited state, and in methanol (less polar and less acidic than water) the behaviour is intermediate between these two extremes. For the fingerprint vibrations we do not observe dynamics on a time scale of a few picoseconds, and with our results obtained on the O-H stretching vibration we argue that the dynamic behaviour observed in previous UV/Vis pump-probe studies is likely to be related to solvation dynamics.