Molecular Clusters of π-Systems: Theoretical Studies of Structures, Spectra, and Origin of Interaction Energies

Kraitchman has shown that a single isotopic substitution on an atom is sufficient to determine directly the coordinates of that atom with respect to the principal axes of the original molecule. Kraitchman's formulas represent exact solutions of the equations for the equilibrium moments of inertia. However, the effects of the zero‐point vibrations are such that the coordinates obtained by substitution from the ground state moments of inertia I0 are systematically less than r0. These coordinates have here been called r (substitution) or rs, and it is found that rs≃(r0+re)/2, and Is= ∑ imirsi2≃(I0+Ie)/2.In the usual method of solution, the coordinate of one atom is determined from the equation for I0, and therefore the difference I0—Is must be made up by this one coordinate. This introduces a large error in the structures normally determined from ground state constants, and results in variations of 0.01 A in structures determined from different sets of isotopic species. If instead, we obtain the structure on...

A new criterion for the determination of molecular structures from ground state rotational constants

The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe0) were studied in batch reactors for a range of Fe0 surface area concentrations and solution pH values (5.5−9.0). At pH 7.0, there was only a modest difference (2−4×) in first-order rate coefficients (kobs) for each contaminant among the three Fe0 types investigated (Fisher, Peerless, and Connelly). The kobs values at pH 7.0 for both nitrite and Cr(VI) reduction were first-order with respect to Fe0 surface area concentration, and average surface area normalized rate coefficients (kSA) of 9.0 × 10-3 and 2.2 × 10-1 L m-2 h-1 were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe0 surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe0, which reduced nitrate at slower rates at higher Fe0 surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe0 decreased with increasing pH with apparent reaction orders of 0.49 ± 0.0...

Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal.

Recent studies of the anomeric effect

Iron(II) is one of the most important reductants of chromium(VI), a severe, toxic contaminant of natural waters, sediments, and soils. We studied the reaction kinetics between pH 2 and pH 7.2 with UV−VIS and multicomponent fitting, a method without the interference of added reagents. The reaction rate was minimal around pH 4. A rate increase with decreasing pH below 4 is documented in the literature. However, a pH-dependent kinetic expression for environmentally relevant, higher pH conditions has not been reported yet. For pH 4.4−7.2 (solutions buffered with acetate, MES, and PIPES, initial 10−20 μM Cr(VI) and 30−60 μM Fe(II), I = 0.01 M KCl, 23 ± 3 °C), we derived the following rate law:  −d[Cr(VI)]/dt = kobs(pH)[Fe(II)][Cr(VI)], with kobs = (0.34 ± 0.47) M-1 s-1 + (3.29 ± 0.66) × 109 M-2 s-1 [OH-] + (4.82 ± 1.53) × 1016 M-3 s-1 [OH-]2 (standard deviations), where Cr(VI) to Cr(V) is the rate-determining step. The equivalent expression −d[Cr(VI)]/dt = (k1[Fe2+] + k2[FeOH+] + k3[Fe(OH)20])[Cr(VI)], with k1...

Kinetics and pH Dependence of Chromium(VI) Reduction by Iron(II)

Pyrrole-argon: microwave spectrum, structure, dipole moment, and nitrogen-14 quadrupole coupling constants

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/26836/0000396.pdf;sequence=1

The microwave spectrum of argon-phosphorus trifluoride

The microwave spectra of the complex between ethylene and sulfur dioxide and nine of its isotopic species have been observed in a Fourier transform microwave spectrometer. The spectra exhibit a and c dipole selection rules; transitions of the normal species and several of the isotopically substituted species occur as tunneling doublets. The complex has a stacked structure with Cs symmetry; the C2H4 and SO2 moieties both straddle the mirror plane with the C2 axis of SO2 crossed at 90 ° to the carbon–carbon bond axis (i.e., only the S atom lies in the symmetry plane). The distance between the centers of mass (Rcm) of C2H4 and SO2 is 3.504(1) A and the deviation of their planes from perpendicular to Rcm is 21(2) ° and 12(2) °, respectively. The tunneling splittings arise from a rotation of the ethylene subunit in its molecular plane. The barrier to internal rotation is 30(2) cm−1. The dipole moment of the complex is 1.650(3)D. The deuterium nuclear quadrupole coupling constants for C2H3D⋅SO2 are χaa=−0.119(1) MHz, χbb=0.010(1) MHz, and χcc=0.109(1) MHz. The binding energy is estimated to be 490 cm−1 from the pseudo‐diatomic approximation. A distributed multipole electrostatic model is explored to rationalize the structure and binding energies.

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/70892/JCPSA6-93-10-7030-1.pdf;sequence=2

Microwave spectrum, structure, barrier to internal rotation, dipole moment, and deuterium quadupole coupling constants of the ethylene–sulfur dioxide complex

The microwave spectrum of (SO2)2 has been reinvestigated using a pulsed beam Fourier‐transform microwave spectrometer. Several new a‐type transitions for the normal species and the a‐type spectra of eight isotopically substituted species were measured. The spectra indicate that the SO2 dimer undergoes a high‐barrier tunneling motion. Based on the analysis used for (H2O)2 by Coudert and Hougen [J. Mol. Spectrosc. 130, 86 (1988)], the internal motion is identified as a geared interconversion motion similar to that displayed by (H2O)2. From the analysis of the moments of inertia of the various isotopic species, an ac plane of symmetry is established for the dimer and the tilt angles of the C2 axes of each subunit relative to the line joining their centers of mass were determined. From Stark effect measurements, μa was redetermined and μc was shown to be nearly zero. Electrostatic calculations using distributed multipoles were carried out to explore the structure of this dimer.

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/70674/JCPSA6-94-11-6956-1.pdf;sequence=2

The microwave spectrum, structure, and tunneling motion of the sulfur dioxide dimer

The microwave spectrum of the benzene⋅SO2 complex was observed with a pulsed beam Fourier‐transform microwave spectrometer. The spectrum was characteristic of an asymmetric‐top with a‐ and c‐dipole selection rules. In addition to the rigid‐rotor spectrum, many other transitions were observed. The existence of a rich spectrum arose from torsional–rotation interactions from nearly free internal rotation of benzene about its C6 axis. Transitions from torsional states up to m=±5 were observed. The principal‐axis method (PAM) internal rotation Hamiltonian with centrifugal distortion was used to assign the spectrum. Assuming six‐fold symmetry for the internal rotation potential, the barrier height was determined as V6=0.277(2) cm−1. The spectrum of C6D6⋅SO2 was also assigned. Analysis of the moments of inertia indicated that the complex has a stacked structure. The distance Rcm separating the centers of mass of benzene and SO2, as well as the tilt angles of the benzene and SO2 planes relative to Rcm were determ...

/pdf/microwave-spectrum-of-benzene-so2-barrier-to-internal-2m3qmgziga.pdf

Microwave spectrum of benzene⋅SO2: Barrier to internal rotation, structure, and dipole moment

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