Infrared spectrum of the propargyl peroxyl radical, HC[triple bond]C-CH2OO X 2A''.

When the propargyl radical, HCCCH(2), and O(2) are codeposited onto a cold argon matrix, a chemical reaction ensues; infrared absorption spectra reveal the formation of the propargyl peroxyl radical: HC=C-CH(2) X (2)B(1) + O(2) --> trans-HC=C-CH(2)OO X (2)A. We do not observe the isomeric adduct, CH(2)=C=CHOO X (2)A''. The propargyl radicals are produced by a hyperthermal nozzle while a second nozzle alternately fires bursts of O(2)/Ar at the 20 K matrix. The absorption spectra of the radicals are measured using a Fourier transform infrared spectrometer. We observe 13 of the 18 fundamental infrared bands of the propargyl peroxyl radical in an Ar matrix at 20 K. The experimental frequencies (cm(-1)) of trans-HC[triple bond]C-CH(2)OO X (2)A'' are assigned. The a' modes are nu(1) = 3326, nu(2) = 2960, nu(3) = 2148, nu(4) = 1440, nu(5) = 1338, nu(6) = 1127, nu(7) = 982, nu(8) = 928, nu(9) = 684, and nu(10) = 499 cm(-1), while the a'' modes are nu(14) = 1218, nu(15) = 972, and nu(16) = 637 cm(-1). Linear dichroism spectra were measured with photo-oriented HCCCH(2)OO radical samples to establish the experimental polarizations of several vibrational bands. The experimental frequencies (nu) for the propargyl peroxyl radical are compared to the anharmonic frequencies (upsilon) resulting from electronic structure calculations. We have used CBS-QB3 electronic structure calculations to estimate the peroxyl bond energies: DeltaH(298)(trans-HC[triple bond]CCH(2)-OO --> CH(2)CCH X (2)B(1) + O(2)) = 19 +/- 1 kcal mol(-1) and DeltaH(298)(trans-CH(2)=C=CH-OO --> CH(2)CCH X (2)B(1) + O(2)) = 21 +/- 1 kcal mol(-1). The experimental thermochemistry for C(3)H(3) reacting with oxygen has been reanalyzed as Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(2)=C=O + HCO) = -83 +/- 3 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(3)CO + CO) = -111 +/- 3 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(2)CHO + CO) = -106 +/- 4 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> HCHO + HCCO) = -67 +/- 4 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(2)CH + CO(2)) = -105 +/- 3 kcal mol(-1).

Electronic spectra of the MgC4H and MgC6H radicals.

Electronic transitions of the linear MgC 4H and MgC 6H radicals have been observed in the gas phase using laser-induced fluorescence spectroscopy. The species were prepared in a supersonic expansion by ablation of a magnesium rod in the presence of acetylene, diacetylene, or methane gas. The transitions were recorded in the 445-446 nm region and assigned to the A (2)Pi- X (2)Sigma (+) systems ( T 0 = 22 431.978(7) and 22 090.08(7) cm (-1)) based on previously reported mass-selective resonance-enhanced ionization spectra and the rotational structure. A spectral fit in MgC 4H yields the rotational constants B'' = 0.04619(19) cm (-1) for the X (2)Sigma (+) state and B' = 0.04748(20) cm (-1) for A (2)Pi. Astrophysical implications are briefly addressed.

Electronic spectroscopy of carbon chains.

Investigators have recorded the electronic spectra of assorted carbon-chain systems in the gas phase using a variety of methods, ranging from direct cavity ringdown absorption spectroscopy to photofragmentation techniques that utilize the cooling capabilities of an ion trap. We summarize the results from these studies and compare them with astronomical measurements of the diffuse interstellar band (DIB) absorptions. Although carbon chains comprising up to a handful of carbon atoms cannot be the carrier species, we explore which chains remain viable. In particular, the (1)Sigma(u)(+)-Chi(1)Sigma(g)(+) transitions of the odd-numbered carbon chains (n = 17,19,...) possess large oscillator strengths and lie in the 400-900-nm DIB range. The origin bands of larger bare carbon rings, such as C(18), have also been observed, with striking similarities to some DIB measurements at high resolution, although at other wavelengths. Finally, we consider recently obtained electronic spectra of metal-containing carbon chains.

Oxidation of organic films by beams of hydroxyl radicals.

We have studied the oxidation of self-assembled monolayers (SAMs) of alkanes and alkenes with a thermal beam of OH radicals. The target films were produced by bonding alkane thiols and alkene thiols to a gold surface and the SAMs are mounted in a vacuum chamber at a base pressure of 10-9 Torr. Hydroxyl radicals were produced by a corona discharge in an Ar/H2O2/water mixture. The resultant molecular beam was scanned by an electrostatic hexapole and the OH radicals [4 (+/- 1) x 1011 OH radicals cm-2 sec-1] were focused onto the target SAM. All of the hydroxyl radicals impinging on the SAM surface are rotationally (J' '

Electronic spectrum of the AlC(2) radical.

An electronic transition of the AlC2 radical (C2v structure) has been observed using laser-induced fluorescence spectroscopy. The molecule was prepared in a supersonic expansion by ablation of an aluminum rod in the presence of acetylene gas. A spectrum was recorded in the 451-453 nm region and assigned to the C 2B2-X 2A1 system (T0 = 22,102.7 cm(-1)) based on a rotational analysis and agreement with calculated molecular parameters and excitation energies. Ab initio results obtained using couple cluster methods are in accord with previous theoretical work which concludes that ground-state AlC2 possesses a T-shaped C2v 2A1 geometry, with the linear 2Sigma+ AlCC isomer 0.70 eV higher in energy. A fit of the experimental spectrum yields rotational constants in the ground and electronically excited states that are in reasonable agreement with the calculated values: A'' = 1.7093(107), B'' = 0.4052(50), C'' = 0.3228(49) cm(-1) for the X 2A1 state, and A' = 1.5621(137), B' = 0.4028(46), C' = 0.3201(54) cm(-1) for C 2B2. Variation in individual fluorescence lifetimes suggests that the emitting C 2B2 state undergoes rovibronic mixing with lower lying electronic states.

Electronic gas-phase spectra of larger polyacetylene cations.

The origin bands of the A 2Pi-X 2Pi electronic transition for three new linear polyacetylene cation chains, HC12H+, HC14H+, and HC16H+, have been recorded in the gas phase at approximately 30 K, located at 924.7, 1034.6, and 1144.0 nm. The absorption spectra were observed using a two-color two-photon ion-photodissociation experiment that utilizes the cooling capabilities of a 22-pole ion trap. Such spectra allow a direct comparison between laboratory and astrophysical data; however, no matches were found between the experimentally determined origin bands and the known diffuse interstellar bands.

Electronic absorption spectra of the protonated polyacetylenes HC2nH2+ (n=3,4) in the gas phase.

A new approach has been developed for the purpose of measuring the electronic transitions to bound exited states for cations that have been collisionally relaxed to low vibrational and rotational temperatures. This has been used to obtain the first gas phase electronic spectra of the protonated polyacetylenes using a two-color ion-photodissociation approach. Specifically, the origin bands in the B (1)A(1)<-- X(1)A(1) transitions of HC(6)H(2) (+) and HC(8)H(2) (+) (C(2v) geometry) were observed at 26,403.3 and 21,399.8 cm(-1). Data on such cooled systems allow a direct comparison between laboratory and astrophysical measurements.

Propargyl radical: ab initio anharmonic modes and the polarized infrared absorption spectra of matrix-isolated HCCCH2.

The propargyl radical has twelve fundamental vibrational modes, gamma(vib)(HCCCH2) = 5a1 [symbol: see text] 3b1 [symbol: see text] 4b2, and nine have been detected in a cryogenic matrix. Ab initio coupled-cluster anharmonic force field calculations were used to help guide some of the assignments. The experimental HC=:C-:CH2 matrix frequencies (cm(-1)) and polarizations are a1 modes--3308.5 +/- 0.5, 3028.3 +/- 0.6, 1935.4 +/- 0.4, 1440.4 +/- 0.5, 1061.6 +/- 0.8; b1 modes--686.6 +/- 0.4, 483.6 +/- 0.5; b2 modes--1016.7 +/- 0.4, 620 +/- 2. We recommend a complete set of gas-phase vibrational frequencies for the propargyl radical, HC=:C-:CH2 2 X (2)B1. From an analysis of the vibrational spectra, the small electric dipole moment, mu(D)(HCCCH2) = 0.150 D, and the large resonance energy (HCCCH2), roughly 11 kcal mol(-1), we conclude that propargyl is a completely delocalized hydrocarbon radical and is best written as HC=:C-:CH2.