Honing in on HONO 2 Modeling air pollution requires knowledge of all the interrelated reactions occurring in the atmosphere. Among the most significant is the formation of nitric acid (HONO 2 ) from OH and NO 2 radicals. One sticking point in the study of this reaction has been the uncertainty in how often radicals link through an O-O rather than an O-N bond. Mollner et al. (p. 646 ) measured the partitioning coefficient, as well as the overall consumption rate of the radicals, with an array of highly sensitive spectroscopic techniques in the laboratory. The measurements yielded a well-defined rate constant for nitric acid formation, which was applied to the prediction of ozone levels in atmospheric simulations of the Los Angeles basin.

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The primary products of n-butoxy and 2-pentoxy isomerization in the presence and absence of O(2) have been detected using pulsed laser photolysis-cavity ringdown spectroscopy (PLP-CRDS). Alkoxy radicals n-butoxy and 2-pentoxy were generated by photolysis of alkyl nitrite precursors (n-butyl nitrite or 2-pentyl nitrite, respectively), and the isomerization products with and without O(2) were detected by infrared cavity ringdown spectroscopy 20 μs after the photolysis. We report the mid-IR OH stretch (ν(1)) absorption spectra for δ-HO-1-C(4)H(8)•, δ-HO-1-C(4)H(8)OO•, δ-HO-1-C(5)H(10)•, and δ-HO-1-C(5)H(10)OO•. The observed ν(1) bands are similar in position and shape to the related alcohols (n-butanol and 2-pentanol), although the HOROO• absorption is slightly stronger than the HOR• absorption. We determined the rate of isomerization relative to reaction with O(2) for the n-butoxy and 2-pentoxy radicals by measuring the relative ν(1) absorbance of HOROO• as a function of [O(2)]. At 295 K and 670 Torr of N(2) or N(2)/O(2), we found rate constant ratios of k(isom)/k(O(2)) = 1.7 (±0.1) × 10(19) cm(-3) for n-butoxy and k(isom)/k(O(2)) = 3.4(±0.4) × 10(19) cm(-3) for 2-pentoxy (2σ uncertainty). Using currently known rate constants k(O(2)), we estimate isomerization rates of k(isom) = 2.4 (±1.2) × 10(5) s(-1) and k(isom) ≈ 3 × 10(5) s(-1) for n-butoxy and 2-pentoxy radicals, respectively, where the uncertainties are primarily due to uncertainties in k(O(2)). Because isomerization is predicted to be in the high pressure limit at 670 Torr, these relative rates are expected to be the same at atmospheric pressure. Our results include corrections for prompt isomerization of hot nascent alkoxy radicals as well as reaction with background NO and unimolecular alkoxy decomposition. We estimate prompt isomerization yields under our conditions of 4 ± 2% and 5 ± 2% for n-butoxy and 2-pentoxy formed from photolysis of the alkyl nitrites at 351 nm. Our measured relative rate values are in good agreement with and more precise than previous end-product analysis studies conducted on the n-butoxy and 2-pentoxy systems. We show that reactions typically neglected in the analysis of alkoxy relative kinetics (decomposition, recombination with NO, and prompt isomerization) may need to be included to obtain accurate values of k(isom)/k(O(2)).

We report vibrational and electronic spectra of the hydroxy-methylperoxy radical (HOCH 2 OO • or HMP), which was formed as the primary product of the reaction of the hydroperoxy radical, HO 2• , and formaldehyde, HCHO. The ν 1 vibrational (OH stretch) spectrum and the A ̃← X ̃electronic spectrum of HMP were detected by infrared cavity ringdown spectroscopy (IR-CRDS), and assignments were verified with density functional calculations. The HMP radical was generated in reactions of HCHO with HO 2• . Free radical reactions were initiated by pulsed laser photolysis (PLP) of Cl 2 in the presence of HCHO and O 2 in a flow reactor at 300−330 Torr and 295 K. IR-CRDS spectra were measured in mid-IR and near-IR regions over the ranges 3525−3700 cm −1 (ν 1 ) and 7250−7800 cm −1 (A ̃← X ) respectively, at a delay time 100 μs after photolysis. The ν 1 spectrum had an origin at 3622 cm −1 and exhibited partially resolved P-and R-branch contours and a small Q-branch. At these short delay times, spectral interference from HOOH and HCOOH was minimal and could be subtracted. From B3LYP/6-31+G(d,p) calculations, we found that the anharmonic vibrational frequency and band contour predicted for the lowest energy conformer, HMP-A, were in good agreement with the observed spectrum. In the near-IR, we observed four well spaced vibronic bands, each with partially resolved rotational contours. We assigned the apparent origin of the A ̃← X ̃electronic spectrum of HMP at 7389 cm −1 and two bands to the blue to a progression in ν 15 ′, the lowest torsional mode of the A ̃state (ν 15 ′ = 171 cm −1 ). The band furthest to the red was assigned as a hot band in ν 15 ″, leading to a ground state torsional frequency of (ν 15 ″ = 122 cm −1 ). We simulated the spectrum using second order vibrational perturbation theory (VPT2) with B3LYP/6-31+G(d,p) calculations at the minimum energy geometries of the HMP-A conformer on the X ̃and A ̃states. The predictions of the electronic origin frequency, torsional frequencies, anharmonicities, and rotational band contours matched the observed spectrum. We investigated the torsional modes more explicitly by computing potential energy surfaces of HMP as a function of the two dihedral angles τ HOCO and τ OOCO . Wave functions and energy levels were calculated on the basis of this potential surface; these results were used to calculate the Franck−Condon factors, which reproduced the vibronic band intensities in the observed electronic spectrum. The transitions that we observed all involved states with wave functions localized on the minimum energy conformer, HMP-A. Our calculations indicated that the observed near-IR spectrum was that of the lowest energy X ̃state conformer HMP-A, but that this conformer is not the lowest energy conformer in the A ̃state, which remains unobserved. We estimated that the energy of this lowest conformer (HMP-B) of the A ̃state is E 0 (A , HMP-B) ≈ 7200 cm −1 , on the basis of the energy difference E 0 (HMP-B) − E 0 (HMP-A) on the A ̃state computed at the B3LYP/6-31+G(d,p) level.

A scheme is presented for stabilizing hydrophobic molecules and releasing them into aqueous solution via DNA hybridization. A tetradecyl hydrophobic tail is covalently attached to synthetic oligomers, and the resulting amphiphilic molecules take up substantial amounts of orange OT and pyrene dyes in aqueous environments. The resulting structures do not affect the surface tension and are predominantly spherical as shown by light scattering and TEM, and the pyrene fluorescence is consistent with a hydrophobic environment. It is concluded that the amphiphilic DNA creates vesicular domains upon which the hydrophobic dyes reside and are stabilized in solution. Upon exposure to the complementary strand, the pyrene dye is released from the structures, showing that the scheme can be used for unlabeled or DNA-mediated drug delivery.