Klaas Nauta

Spectroscopic Observation of the Resonance-Stabilized 1-Phenylpropargyl Radical

The gas-phase laser-induced fluorescence (LIF) spectrum of a 1-phenylpropargyl radical has been identified in the region 20,800-22,000 cm(-1) in a free jet. The radical was produced from discharges of hydrocarbons including benzene. Disregarding C2, C3, and CH, this radical appears as the most strongly fluorescing product in a visible wavelength two-dimensional fluorescence excitation-emission spectrum of a jet-cooled benzene discharge. The structure of the carrier was elucidated by measurement of a matching resonant two-color two-photon ionization spectrum at m/z = 115 and density functional theory. The assignment was proven conclusively by observation of the same excitation spectrum from a low-current discharge of 3-phenyl-1-propyne. The apparent great abundance of the 1-phenylpropargyl radical in discharges of benzene and, more importantly, 1-hexyne may further underpin the proposed importance of the propargyl radical in the formation of complex hydrocarbons in combustion and circumstellar environments.

Spectroscopic identification of the resonance-stabilized cis- and trans-1-vinylpropargyl radicals.

The cis-1-vinylpropargyl (cis-1VPR, cis-pent-4-en-1-yn-3-yl) and trans-1-vinylpropargyl (trans-1VPR, trans-pent-4-en-1-yn-3-yl) radicals, produced in a supersonically cooled hydrocarbon discharge, have been identified by a synergy of 2-dimensional fluorescence and ionization spectroscopies, revealing their electronic origin transitions at 21,232 and 21,645 cm(-1) respectively. These assignments are supported by an excellent agreement between calculated ground state frequencies of cis-1VPR and trans-1VPR with those obtained by dispersed fluorescence spectroscopy. In addition, high-resolution rotational contours of the two bands are well simulated using calculated X- and A-state trans-1VPR and cis-1VPR rotational constants. Finally, computed origin transition energies of these two isomers are within several hundred wavenumbers of the observed band positions. With the 1-phenylpropargyl radical, the 1VPR isomers are the second 1-substituted propargyl species to have been observed abundantly from a hydrocarbon discharge, while no 3-substituted analogue has been positively identified. This is likely due to the greater resonance stabilization energy of the 1-substituted species, arising from concerted delocalization of the unpaired electron over the vinyl and propargyl moieties.

The dΠg3-cΣu+3 band system of C2

A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the dΠg3←cΣu+3 and dΠg3←aΠu3 excitations. Rotationally resolved excitation spectra of the (4←1), (5←1), (5←2), and (7←3) bands in the dΠg3←cΣu+3 system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the cΣu+3 state. The principal molecular constants for the cΣu+3 state are Be=1.9319(19)cm−1, αe=0.01855(69)cm−1, ωe=2061.9cm−1, ωexe=14.84cm−1, and T0(c−a)=8662.925(3)cm−1. We report also the first experimental observations of dispersed fluorescence from the dΠg3 state to the cΣu+3 state, namely, dΠg3(v=3)→cΣu+3(v=0,1).A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the d (3)Pi(g)<--c (3)Sigma(u) (+) and d (3)Pi(g)<--a (3)Pi(u) excitations. Rotationally resolved excitation spectra of the (4<--1), (5<--1), (5<--2), and (7<--3) bands in the d (3)Pi(g)<--c (3)Sigma(u) (+) system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the c (3)Sigma(u) (+) state. The principal molecular constants for the c (3)Sigma(u) (+) state are B(e)=1.9319(19) cm(-1), alpha(e)=0.018 55(69) cm(-1), omega(e)=2061.9 cm(-1), omega(e)x(e)=14.84 cm(-1), and T(0)(c-a)=8662.925(3) cm(-1). We report also the first experimental observations of dispersed fluorescence from the d (3)Pi(g) state to the c (3)Sigma(u) (+) state, namely, d (3)Pi(g)(v=3)-->c (3)Sigma(u) (+)(v=0,1).

The Optical Spectrum of a Large Isolated Polycyclic Aromatic Hydrocarbon: Hexa-peri-hexabenzocoronene, C42H18

The first optical spectrum of an isolated polycyclic aromatic hydrocarbon large enough to survive the photophysical conditions of the interstellar medium is reported. Vibronic bands of the first electronic transition of the all-benzenoid polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene were observed in the 4080-4530 A range by resonant 2-color 2-photon ionization spectroscopy. The strongest feature at 4264 A is estimated to have an oscillator strength of f = 1.4 × 10−3, placing an upper limit on the interstellar abundance of this polycyclic aromatic hydrocarbon at 4 × 1012 cm−2, accounting for a maximum of ~0.02% of interstellar carbon. This study opens up the possibility to rigorously test neutral polycyclic aromatic hydrocarbons as carriers of the diffuse interstellar bands in the near future.

Spectroscopy of the free phenalenyl radical.

After benzene and naphthalene, the smallest polycyclic aromatic hydrocarbon bearing six-membered rings is the threefold-symmetric phenalenyl radical. Despite the fact that it is so fundamental, its electronic spectroscopy has not been rigorously scrutinized, in spite of growing interest in graphene fragments for molecular electronic applications. Here we used complementary laser spectroscopic techniques to probe the jet-cooled phenalenyl radical in vacuo. Its spectrum reveals the interplay between four electronic states that exhibit Jahn-Teller and pseudo-Jahn-Teller vibronic coupling. The coupling mechanism has been elucidated by the application of various ab initio quantum-chemical techniques.

Two roaming pathways in the photolysis of CH3CHO between 328 and 308 nm

The correlated speed and rotational energy distributions of the CO fragment from photodissociation of CH3CHO have been measured at a range of wavelengths from 308 to 328 nm. The distributions are bimodal, showing low J, slow speed, and high J, fast speed components. The cold component disappears for λ > 325 nm. This threshold corresponds to C–H bond cleavage and we assign these CO products as arising from roaming of a H-atom about a CH3CO core. We attribute the hot component to CO formed through CH3-roaming. No evidence was observed for the presence of a transition state mechanism. This is the first time two distinct roaming channels have been observed from the same electronic state. The results support the growing understanding that roaming can be significant in chemical reactions and outweigh traditional pathways.

Fully state-resolved photodissociation of formaldehyde, H2CO→H+HCO: K conservation and a rigorous test of statistical theories

The photodissociation dynamics of the reaction H2CO+hnu --> H + HCO have been investigated in the range 60-400 cm(-1) above the reaction threshold. Supersonically cooled formaldehyde was excited into 15 specific J, K(a), K, rotational states i n two vibrational lev el s 2(1) 4(1) 6(1) and 2(2) 4(1) in the A(1A2) state. The laser-induced fluorescence spectra of the nascent HCO fragment provided detailed product state distributions (PSDs), resolved by N, K(a), K(c), and J. When just the overall molecular rotation N is considered the PSDs are in remarkable agreement with calculations based on phase space theory (PST). However, when the projection of N onto the molecular frame (K(a),K(c)) is included the distributions show consistent deviations from PST. In particular, there is a tendency to preserve the initial parent rotational motion about the a and b axes. The effect is that states with higher initial K(a) in H2CO produce higher final K(a) in the HCO fragment. There is also a tendency for the upper/lower members of the asymmetry doublets in H2CO to map onto the same upper/lower set of product state asymmetry doublets. Finally, there are oscillations in some of the detailed PSDs that remain unexplained.

Observation of the dΠg3←cΣu+3 band system of C2

A new band system of C2, dΠg3←cΣu+3 is observed by laser induced fluorescence spectroscopy, constituting the first direct detection of the cΣu+3 state of C2. Observations were made by laser excitation of cΣu+3(v″=0) C2, produced in an acetylene discharge, to the dΠg3(v′=3) level, followed by detection of Swan band fluorescence. Rotational analysis of this band yielded rotational constants for the cΣu+3(v″=0) state: B0=1.9218(2)cm−1, λ0=−0.335(4)cm−1 and γ0=0.011(2)cm−1. The vibrational band origin was determined to be ν3−0=15861.28cm−1.

Excitation and Emission Spectra of Jet-Cooled Naphthylmethyl Radicals

Gas phase excitation and emission spectra of three naphthylmethyl radical chromophores are presented. These resonance-stabilized species, 1-naphthylmethyl, 2-naphthylmethyl, and α-acenaphthenyl, each possessing an sp(2) carbon adjacent to a naphthalene moiety, are studied by resonant two-color two-photon ionization, laser induced fluorescence, and dispersed fluorescence spectroscopy. Identification of the radicals is made through a combination of dispersed fluorescence and density functional theory calculations. All three species possess spectra in the 580 nm region. The possible relevance to unidentified spectroscopic features such as the diffuse interstellar bands and emission from the Red Rectangle nebula is discussed.

Hydroxyl addition to aromatic alkenes: resonance-stabilized radical intermediates.

The spectra of 1-indanyl-based resonance-stabilized radicals containing a hydroxyl group are identified in an electrical discharge containing indene and its alkylated derivatives. It is argued that such species form by addition of a discharge-nascent hydroxyl radical, formed from trace water, to the π bond on the five-membered ring of the parent molecule. The spectral carriers are identified by analysis of their excitation and emission spectra guided by the results from quantum chemical calculations. All three hydroxylated radicals are found to exhibit origin bands in the 21300 cm(-1) region: the 2-hydroxy-indan-1-yl radical at 21364 cm(-1), the 2-hydroxy-2-methyl-indan-1-yl radical at 21337 cm(-1), and the 2-ethyl-2-hydroxy-indan-1-yl radical exhibiting two origins of similar intensity at 21287 and 21335 cm(-1).