We have studied the relaxation of conformers and the formation/relaxation of isomeric, weakly bonded dimers in pulsed supersonic expansions of seeded inert gases (He, Ne, Ar, Kr). The relaxation was determined from the intensity of a rotational transition for the higher energy species as a function of carrier gas composition, using the Balle/Flygare Fourier transform microwave spectrometer. Of thirteen molecules with rotational conformers which we examined, those with barriers to internal rotation greater than 400 cm−1 did not relax significantly in any of the carriers. The higher energy forms of ethyl formate, ethanol, and isopropanol, with smaller barriers, were not relaxed by He; those of ethanol and isopropanol were somewhat relaxed by Ne; and all were completely relaxed by as little as 5 to 20 mole percent of Ar or Kr in He or Ne. The relaxation in He or Ne is first order in the concentration of added Ne, Ar, or Kr as well as in the concentration of the high energy conformer. The pseudo first-order rate constants (larger in Ne than in He) increase sharply with Z of the rare gas, roughly in a 0:1:2:4 progression for He, Ne, Ar, and Kr, suggesting that the relaxation involves relatively long-range polarization effects. Similar behavior was found in the formation/relaxation of the weakly bonded dimer pairs: linear OCO–HCN, T-shaped HCN–CO2; linear FH–NNO and bent NNO–HF; and bent HF–DF and DF–HF. The case of the HCN/CO2 dimers is particularly striking. The T-shaped dimer was found first, using Ar as the carrier gas. Five years later the linear form was found with first run neon as carrier, but it could not be detected at all with Ar as the carrier. These results show that in favorable cases high energy species can be studied in supersonic expansions by freezing out a ‘‘high-temperature’’ concentration with a nonrelaxing carrier gas.

We report the deposition of films composed of overlapped and stacked platelets of graphene oxide (G-O) reduced by an electrophoretic deposition (EPD) process. The oxygen functional groups of G-O were significantly removed by the EPD process, and the as-deposited G-O film by EPD showed improved electrical conductivity (1.43 Â 10 4 S 3 m -1 ) over G-O papers made by the filtration method (0.53 Â 10 -3 S 3 m -1 ). This method for reducing G-O without added reducing agents has the potential for high-yield, large-area, low-cost, and environmentally friendly production of films composed of reduced G-O platelets. SECTION Nanoparticles and Nanostructures C

Low J (0–4) rotational transitions have been observed for the benzene–water dimer of which high J (≥4) transitions were reported recently by Blake [Science 257, 942 (1992)]. Our experiments used a modified Balle/Flygare Fourier transform microwave spectrometer, with a pulsed supersonic nozzle as the sample source, and examined a variety of isotopic species in the ground and first excited internal rotor states (m=0 and 1). The dimers of the parent C6H6 benzene with H2O, HDO, D2O, and H218O have symmetric top spectra characteristic of two coaxial rotors with a symmetric top frame and a very low effective V6 barrier. The dimers of H2O and D2O with the 13C and D monosubstituted benzenes have asymmetric top spectra of which only the m=0 state was assigned. However, doublets in the m=1, J=0→1 transitions show that there is a V2 term of ∼0.5 MHz in their barriers. A substitution analysis was made of the rotational constants found for the m=0 state of the dimers with H218O, D2O, and the 13C and D monosubstituted benzenes. It shows that the oxygen is at the a axis of the dimer, well outside (0.48 Å) the hydrogens. However, the C2 axis of the H2O is not coincident with the a axis but is at an angle β of 37° to it, rotated so that the two hydrogens are equivalent. The sixfold axis of the benzene corresponds to the a axis, there is little or no tilt (γ) of the benzene. The c.m. (C6H6) to c.m. (H2O) distance R is 3.329 Å. The closely spaced hyperfine structure from the proton–proton magnetic dipole interaction and the deuterium quadrupole interaction was resolved for several dimers and transitions, principally J=0→1 and 1→2. The results demonstrate effective nuclear equivalence in dimers with H2O and D2O. Also, the symmetries found for their nuclear spin functions correlate with the lowest rotational levels of free water, the m=0 state with 000 and m=1 with 101 and 111. For the m=1, K=0 transitions of C6H6–H2O the correlation is with 111 and for the K=±1, with 101. These assignments are reversed for C6H6–D2O.

Rotational spectra were observed for dimers of argon and krypton with pyridine (Pyr) by using a Flygare-Balle Fourier transform microwave spectrometer. The dimers are prolate near symmetric tops. Rotational constants were determined for several isotopic species of each. For Ar-Pyr, we found A, B, and C to be 2990.327 (7), 1207.862 (1), and 1199.335 (2) MHz, and D¡, D]K, and 0K to be 3.58 (5), 19.6 (1), and -23.5 (15) kHz. The corresponding values for 84Kr-Pyr are 2986.703 (1) , 806.9294 (1), and 803.0235 (2) MHz and 1.370 (5), 8.74 (7), and -9.4 (2) kHz. Nuclear quadrupole coupling constants were determined from the hyperfine structure of the rotational transitions for dimers with one or more of the quadrupole nuclei 14N, D, or 83Kr. Analysis of the data shows that the Ar/Kr is above the pyridine ring on a plane of symmetry containing the pyridine center of mass (cm) and the nitrogen. The pyridine cm to rare gas vector R is 3.545 Á for the dimer with Ar and 3.648 Á for Kr. R is rotated from the vertical by ~3.5°toward the nitrogen (-3.5°). The pyridine axis perpendicular to its plane oscillates about the equilibrium position with an average displacement of ~7°. A simple psepdodiatomic model for the interaction potential gives the stretching force constant to be 0.0270 and 0.0351 mdyn/A for the Ar-Pyr and Kr-Pyr dimers, with well depths of 232 and 322 cm"1. In 83Kr-Pyr the electric field gradient (EFG) at the Kr has near-axial symmetry with , = -4.722 MHz in the ab plane and the z axis rotated away from the nitrogen by 26.7°from the vertical (+26.7°). Calculation of the electric field gradient with an electric, distributed multipole expansion model gives results agreeing semiquantitatively with experiment.

Biomolecules are highly pressure-sensitive, but their dynamics upon return to ambient pressure are often too fast to observe with existing approaches. We describe a sample-efficient method capable of large and very fast pressure drops (<1 nanomole, >2,500 atmospheres and <0.7 microseconds). We validated the method by fluorescence-detected refolding of a genetically engineered lambda repressor mutant from its pressure-denatured state. We resolved barrierless structure formation upon return to ambient pressure; we observed a 2.1 +/- 0.7 microsecond refolding time, which is very close to the 'speed limit' for proteins and much faster than the corresponding temperature-jump refolding of the same protein. The ability to experimentally perform a large and very fast pressure drop opens up a new region of the biomolecular energy landscape for atomic-level simulation.