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Notes: Rovibrational absorption spectra of weakly bonded complexes of N2O with HF, DF, HCl, and HBr were recorded in the ν3 region of N2O by using pulsed, slotted nozzle expansions and tunable diode lasers. A fast-scan technique was used that takes advantage of the rapid tuning capabilities of diode lasers; i.e., 4000 resolution elements were recorded with a single opening of the nozzle. Of the two known NH- and OH-bonded isomers of N2O–HF, we detected only linear ONN–HF; the ground-state rotational constants are in excellent agreement with previous microwave and IR results. Deuteration resulted in ONN–DF linewidths that are much narrower than those of ONN–HF, as observed previously in studies of the analogous CO2–H(D)F system. Vibrational band origins for ONN–HF and ONN–DF are blue shifted 21.8 and 23.4 cm−1, respectively, relative to uncomplexed N2O. The additional blue shift upon deuteration is attributed to enhanced hydrogen bonding in a highly anharmonic potential. High-resolution spectra of NNO–HCl and NNO–HBr are presented for the first time. The average NNO–HCl geometry is asymmetric, with the separation between the N2O and HCl centers-of-mass Rcm equal to 3.51 A(ring). The angle between Rcm and the NNO principal axis θ1 is 72°–76°. NNO–HBr complexes are also asymmetric (θ1=75°–82°) with Rcm =3.62 A(ring). Linear ONN–HCl(Br) isomers were not observed. Blue shifts in the NNO–HCl and NNO–HBr band origins are 2.44 and 1.86 cm−1, relative to uncomplexed N2O. The qualitative changes observed in the NNO–HX geometries and force fields are attributed to competing effects arising from hydrogen–bonding and dispersion forces, as were observed with CO2–HF(Cl) and CO2–HBr. The experimental geometries and vibrational frequencies are compared to ab initio calculations; agreement with N2O–HF is good, CO2–HCl less so. Although the H atom position cannot be determined experimentally with NNO–HCl(Br), ab initio estimates suggest it is localized near the O atom. Implications for photoinitiated reactions in weakly bonded complexes are discussed.

Notes: Infrared absorption spectra associated with the CO2 asymmetric stretch vibration have been recorded for weakly bonded gas-phase complexes of CO2 with HF, DF, HCl, DCl, and HBr, using tunable diode laser spectroscopy and a pulsed slit expansion (0.15×38 mm2) that provides 〉20 MHz overall resolution. Results obtained with CO2–HF are in agreement with earlier studies, in which the HF-stretch region near 3900 cm−1 was examined. In both cases, broad linewidths suggest subnanosecond predissociation. With CO2–DF, the natural linewidths are markedly narrower than with CO2–HF (e.g., 28 vs 182 MHz), and this difference is attributed to slower predissociation, possibly implicating resonances in the case of CO2–HF. Both CO2–HF and CO2–DF exhibited overlapping features: simple P and R branches associated with a linear rotor, and P and R branches containing doublets. As in earlier studies, the second feature can be assigned to either a slightly asymmetric rotor with Ka=1, or a hot band involving a low-frequency intermolecular bend mode.Results obtained with CO2–HCl are in excellent agreement with earlier microwave measurements on the ground vibrational state, and the vibrationally excited state is almost identical to the lower state. Like CO2–DF, linewidths of CO2–HCl and CO2–DCl are much sharper than those of CO2–HF, and in addition, CO2–HCl and CO2–DCl exhibited weak hot bands, as were also evident with CO2–HF and CO2–DF. Upon forming complexes with either HF or HCl, the asymmetric stretch mode of CO2 underwent a blue shift relative to uncomplexed CO2. This can be understood in terms of the nature of the hydrogen bonds, and ab initio calculations are surprisingly good at predicting these shifts. Deuteration of both HF and HCl resulted in further blue shifts of the band origins. These additional shifts are attributed to stronger intermolecular interactions, i.e., deuteration lowers the zero-point energy, and in a highly anharmonic field this results in a more compact average structure. While both HF and HCl complexes exhibit nearly linear geometries,CO2–HBr is asymmetric, with the Br–C symmetry line essentially perpendicular to the CO2 axis, and the H atom probably localized near one of the oxygens. Although the moments of inertia are insensitive to the location of the H atom in CO2–HBr, Bose–Einstein statistics require that odd K‘a states are missing for C2v symmetry, as is observed with T-shaped CO2–(rare gas) complexes. However, we observe a full complement of odd and even Ka states, indicating that the H atom is not located symmetrically about the C2v axis on the time scale of the measurement. With CO2–HBr, the low gas-phase acidity of HBr and the high Br-atom polarizability encourage a qualitative change in the geometry relative to CO2–HCl and CO2–HF. This has valuable implications for photoinitiated reactions in such complexes.

Notes: Gelling temperatures of gellan solutions with the addition of Na+ and K+ ranging from 15 to 450 mM or Ca++ and Mg++ from 2 to 40 mM were determined by dynamic rheological testing at four polymer concentrations between 0.4 and 2.0% (w/w). Gelling temperatures were much higher for gellan solutions containing divalent cations than for those containing the same amount of monovalent cations. Solutions containing K+ gelled at higher temperatures than those containing Na+. Effects of Ca++ and Mg++ on gelling temperatures were not significantly different. A general model was developed to predict the gelling temperature of gellan solutions as functions of cation and polymer concentrations.

Notes: Gelling temperatures were measured using a dynamic rheological testing method, a spectrophotometric method and direct visual examination combined with use of a thermocouple. Results from the dynamic testing method were most consistent. Gelling temperatures of gellan solutions increased from 30 to 72°C, as polymer concentrations increased from 0.4 to 2.0% w/v and Ca++ concentrations increased from 2 to 40 mM. A mathematical model was developed based on the van't Hoff equation to relate gelling temperatures to composition and polymer molecular weight. The heat of cross-linking in Ca-gellan gels was estimated to be 428.6 kJ/mole, about 1.4 to 2.1 times greater than for gelatin gels.