No abstract

The N–O degenerate stretching band ν3 of the NO3 radical has been studied in the gas phase by infrared tunable diode laser spectroscopy. The NO3 radical was generated by the reaction of NO2 with an excess of O3. Zeeman modulation was employed to observe the paramagnetic absorption lines of 14NO3 and 15NO3 in the wavelength regions 1480–1500 and 1463–1479 cm−1, respectively. Only K′′=3n (n denoting an integer) transitions were observed, and the N′′=even members were missing from the K′′=0 manifold. These observations indicate that the NO3 radical belongs to D3h symmetry in the 2A2′ ground electronic state. The observed spectrum was analyzed using a symmetric-top vibration-rotation Hamiltonian including the spin-rotation interaction. The main parameters thus obtained for 14NO3 are B3=0.455 22(11), C3=0.227 13(6), Cζ3=0.044 79(11), q3=0.001 624(33), t3= 0.000 000 458 0(43), B0=0.457 46(12), C0=B0/2 (fixed), εbb=0.0280(27), and εcc=0.1197(36) for v3=1, εbb=0.0277(28), and εcc=0.1117(34) for v=0, and ν0=1492.3929(9), all in cm−1 with one standard error in parentheses. Although these parameters well reproduced the observed spectrum, the following anomalous features were noted: (1) a large εcc spin-rotation interaction constant was required to explain the spin splittings for both the ν3 and ground states, (2) a higher-order vibration-rotation interaction term having Δk=±4 and Δl=±2 needed to be included, with the corresponding interaction constant t3 larger than that of CHF3, and (3) the centrifugal distortion constants and the first order Coriolis coupling constant which were derived did not agree with those calculated assuming a reasonable force field. These anomalies were ascribed to the interaction with a low-lying excited electronic state and, to some extent, with a combination or overtone state. The N–O bond length was calculated from the B0 rotational constant to be 1.240 Å, in good agreement with an ab initio calculated value.

The reactions of Cl atoms with XCH2I (X = H, CH3, Cl, Br, I) have been studied using cavity ring-down spectroscopy in 25-125 Torr total pressure of N2 diluent at 250 K. Formation of the XCH2I-Cl adduct is the dominant channel in all reactions. The visible absorption spectrum of the XCH2I-Cl adduct was recorded at 405-632 nm. Absorption cross-sections at 435 nm are as follows (in units of 10(-18) cm2 molecule(-1)): 12 for CH3I, 21 for CH3CH2I, 3.7 for CH2ICl, 7.1 for CH2IBr, and 3.7 for CH2I2. Rate constants for the reaction of Cl with CH3I were determined from rise profiles of the CH3I-Cl adduct. k(Cl + CH3I) increases from (0.4 +/- 0.1) x 10(-11) at 25 Torr to (2.0 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) at 125 Torr of N2 diluent. There is no discernible reaction of the CH3I-Cl adduct with 5-10 Torr of O2. Evidence for the formation of an adduct following the reaction of Cl atoms with CF3I and CH3Br was sought but not found. Absorption attributable to the formation of the XCH2I-Cl adduct following the reaction of Cl atoms with XCH2I (X = H, CH3, Br, I) was measured as a function of temperature over the range 250-320 K.

The rate constant of the reaction of BrO with CH(3)O(2) was determined to be k1 = (6.2 +/- 2.5) x 10(-12) cm3 molecule(-1) s(-1) at 298 K and 100-200 Torr of O2 diluent. Quoted uncertainty was two standard deviations. No significant pressure dependence of the rate constants was observed at 100-200 Torr total pressure of N2 or O2 diluents. Temperature dependence of the rate constants was further investigated over the range 233-333 K, and an Arrhenius type expression was obtained for k1 = 4.6 x 10(-13) exp[(798 +/- 76)/T] cm3 molecule(-1) s(-1). The product branching ratios were evaluated and the atmospheric implications were discussed.

Ishiwata et al. [J. Chem. Phys. 82, 2196(1985] have recently observed an infrared diode laser spectrum of N0 3 in the 1492 cm -I region and have assigned it to the V3 band in the X 2 A i state. However, some of the derived constants such as the Coriolis coupling and spin-rotation constants did not conform well with expected values. In the present study, the observation was extended so as to take combination differences, which led us to revise the previous assignment slightly and to remove all the anomalies in the lower (i.e., ground) state. A most important result of the present study is that a spin-orbit interaction constant a eff I (L z ) I = 0.17 cm -I is indispensable to explain the spin splitting observed for the upper state. The first-order Coriolis coupling constant of the upper state (; = 0.19) remains essentially the same as in the previous study and differs considerably from the value calculated for the V3 state (; = 0.7). Possible explanations of these data are discussed in some detail to obtain more information on the molecular structure of the N0 3 radical.

The reactions of iodine monoxide radical, IO, with alkyl peroxide radicals, RO(2) (R = CH(3), C(2)H(5), and CF(3)), have been studied using cavity ring-down spectroscopy. The rate constant of the reaction of IO with CH(3)O(2) was determined to be (7.0 +/- 3.0) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 100 Torr of N(2) diluent. The quoted uncertainty is two standard deviations. No significant pressure dependence of the rate constant was observed at 30-130 Torr total pressure of N(2) diluent. The temperature dependence of the rate constants was also studied at 213-298 K. The upper limit of the branching ratio of OIO radical formation from IO + CH(3)O(2) was estimated to be <0.1. The reaction rate constants of IO + C(2)H(5)O(2) and IO + CF(3)O(2) were determined to be (14 +/- 6) x 10(-11) and (6.3 +/- 2.7) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, 100 Torr of N(2) diluent, respectively. The upper limit of the reaction rate constant of IO with CH(3)I was <4 x 10(-14) cm(3) molecule(-1) s(-1).

The primary photochemistry of gas phase dichlorine monoxide (Cl2O) and of hypochlorous acid (HOCl) following excitation at 235 nm has been investigated using photofragment ion imaging to obtain the recoil velocity and angular distributions of the ground (2P3/2) and spin-orbit excited (2P1/2) atomic chlorine products. In the case of Cl2O, both Cl spin-orbit products exhibit angular distributions characterized by an anisotropy parameter, β=1.2±0.2, consistent with previous interpretations of the ultraviolet (UV) absorption spectrum of Cl2O which associate the broad intense absorption feature peaking at λ∼255 nm with excitation to a (bent) dissociative state of B21(C2v) symmetry. The recoil velocity distributions of the two Cl spin-orbit products are markedly different. The ground state atoms (which constitute &gt;90% of the total Cl atom yield) are partnered by ClO fragments carrying significantly higher average levels of internal excitation. The slowest Cl atoms are most readily understood in terms of three body fragmentation of Cl2O to its constituent atoms. These findings are rationalized in terms of a model potential energy surface for the 1 1B2 state, which correlates diabatically with ClO(X) radicals together with a spin-orbit excited Cl atom, with efficient radiationless transfer to one (or more) lower energy surfaces at extended Cl-O bond lengths accounting for the dominance of ground state Cl atom fragments. The image of the ground state Cl atoms resulting from photolysis of HOCl at 235 nm is consistent with parent excitation via a transition for which the dipole moment is closely aligned with the Cl-O bond, followed by prompt dissociation (β=1.7±0.2) with the bulk of the excess energy partitioned into product recoil. Such conclusions are consistent with the results of laser induced fluorescence measurements of the OH(X) products resulting from 266 nm photodissociation of HOCl which reveal OH(X) products in both spin-orbit states, exclusively in their zero-point vibrational level, and carrying only modest levels of rotational excitation (well described by a Boltzmann distribution with Trot∼750±50 K).

An artificial lipid bilayer in planar form, well known as bilayer lipid membrane ͑BLM͒, spontaneously forms from a lipid droplet ͑L-␣-phosphatidylcholine in n-decane and chloroform in this work͒ in an aperture of a thin partition in aqueous solution. The thinning dynamics of the lipid droplet or membrane has been studied by simultaneous capacitance and image recording, because the lipid membrane sandwiched by aqueous solutions can be considered as a parallel-plate capacitor. The simultaneous measurements have revealed the two-step thinning of the lipid membrane from its specific capacitance value: first, the initial droplet thins to yield a membrane of about 100 nm thickness (0.02 F/cm 2 ), and second, within this thin lipid membrane, a lipid bilayer of 4 nm thickness (0.42 F/cm 2 ) suddenly emerges and grows, keeping a bilayer structure. In addition, the simultaneous measurements have a time stamp, and thus can determine the trigger moment of the bilayer formation. The revealed dynamics provides the first quantitative support for a ''zipper'' mechanism ͓H. T. Tien and E. A. Dawidowicz, J. Colloid Interface Sci. 22, 438 ͑1966͔͒; in the mechanism, the first thinning results in a sandwich consisting of the organic solvent between two adsorbed lipid monolayers whose distance is the order of 100 nm, and then a chance contact of both monolayers initiates the formation and growth of a lipid bilayer in a zipper-like manner. However, because of the existence of the two solvent-water interfaces containing surface-active molecules, phospholipids, this work claims that the zipper mechanism should be modified in view of the Marangoni effect. The simultaneous measurements have also revealed the adjacency effect, namely, that only the prebilayer region just adjacent to the bilayer changes into a bilayer. The present formation and growth of a lipid bilayer, including the adjacency effect, can be explained by the classic nucleation theory of two-dimensional crystallization. BLM systems with the simultaneous measurements can be considered as a useful environment for the study of soft-matter chemical physics.