Rotational spectra and conformational structures of 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone

Microwave spectra have been recorded for 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone from 11 to 24 GHz using a Fourier-transform microwave spectrometer. Only one spectrum from a single conformational isomer was observed for each species. The rotational transitions in the spectrum of 1-phenyl-2-propanone were split into separate transitions arising from the A- and E-torsional levels of the methyl rotor. The fit of the E-state transitions to a "high-barrier" internal rotation Hamiltonian determines V3 = 238(1) cm-1 and rotor-axis angles of thetaa = 87.7(5) degrees, thetab = 50.0(5) degrees, and thetac = 40.0(5) degrees. Ab initio optimizations (MP2/6-31G**) and single-point calculations (MP2/6-311++G**) were used to model the structures of 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone. The lowest energy conformations of these species were found to be stabilized by weak OH-pi, NH-pi, and CH-pi hydrogen-bonding interactions. Moments of inertia, derived from the model structures, were used to assign the spectra to the lowest energy conformation of each species. A series of MP2/6-31G* partial optimizations along the internal rotation pathway were used to estimate the barrier to methyl rotation to be 355 cm-1 for 1-phenyl-2-propanone.     

Analysis and fit of the Fourier-transform microwave spectrum of the two-top molecule N-methylacetamide

The jet-cooled Fourier-transform microwave spectrum of N-methylacetamide (CH₃NHC(O)CH₃), a molecule containing two methyl tops with relatively low barriers to internal rotation, has been recorded and fit to nearly experimental uncertainty. Measurements were carried out between 10 and 26 GHz, with the nitrogen quadrupole splittings resolved for many transitions. The permutation-inversion group for this molecule is G₁₈ (not isomorphic to any point group), with irreducible representations A₁, A₂, E₁, E₂, E₃, and E₄. One of these symmetry species and the usual three asymmetric rotor quantum numbers J_KaKc were assigned to each torsion-rotation level involved in the observed transitions. F values were assigned to hyperfine components, where . Transitions involving levels of A₁ and A₂ species could be fit to an asymmetric rotor Hamiltonian. The other transitions were first fit separately for each symmetry species using a Pickett-like effective rotational Hamiltonian. Constants from these fits show a number of additive properties which can be correlated with sums and differences of effects involving the two tops. A final global fit to 48 molecular parameters for 839 hyperfine components of 216 torsion-rotation transitions involving 152 torsion-rotation levels was carried out using a newly written two-top computer program, giving a root-mean-square deviation of observed-minus-calculated residuals of 4 kHz. This program was written in the principal axis system of the molecule and uses a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. Such an approach requires quite long computation times, but it is much less prone to subtle programming errors (a consideration felt to be important since checking the new program against precise fits of low-barrier two-top molecules in the literature was not possible). The two internal rotation angles in this molecule correspond to the Ramachandran angles ψ and φ often defined to describe polypeptide folding. Barriers to internal rotation about these two angles were found to be 73 and 79 cm⁻¹, respectively. Top-top coupling in both the kinetic and potential energy part of the Hamiltonian is relatively small in this molecule.     

Rotational Spectrum of Sarin

As part of an effort to examine the possibility of using molecular-beam Fourier-transform microwave spectroscopy to unambiguously detect and monitor chemical warfare agents, we report the first observation and assignment of the rotational spectrum of the nerve agent Sarin (GB) (Methylphosphonofluoridic acid 1-methyl-ethyl ester, CAS #107-44-8) at frequencies between 10 and 22 GHz. Only one of the two low-energy conformers of this organophosphorus compound (C(4)H(10)FO(2)P) was observed in the rotationally cold (T(rot)<2 K) molecular beam. The experimental asymmetric-rotor ground-state rotational constants of this conformer are A=2874.0710(9) MHz, B=1168.5776(4) MHz, C=1056.3363(4) MHz (Type A standard uncertainties are given, i.e., 1sigma), as obtained from a least-squares analysis of 74 a-, b-, and c-type rotational transitions. Several of the transitions are split into doublets due to the internal rotation of the methyl group attached to the phosphorus. The three-fold-symmetry barrier to internal rotation estimated from these splittings is 677.0(4) cm(-1). Ab initio electronic structure calculations using Hartree-Fock, density functional, and Moller-Plesset perturbation theories have also been made. The structure of the lowest-energy conformer determined from a structural optimization at the MP2/6-311G(**) level of theory is consistent with our experimental findings. Copyright 2001 Academic Press.     

Microwave spectrum of acetylene-d2

Eight P-branch transitions from the $\text{ν}{}_5{}^1\text{-ν}{}_4{}^1$ difference band of C₂D₂ have been observed in the microwave region. Significant improvements in the spectroscopic constants for the two states involved in the difference band have been obtained by combining infrared and microwave data. The Stark shifts for the observed C₂D₂ lines are discussed in some detail. The vibrational transition moment is found to be μ_vib = 0.0358 ± 0.0020 D.     

Microwave-based structure and four-dimensional morphed intermolecular potential for HI-CO(2)

(Microwave spectra of the four isotopologue/isotopomers, HI-(12)C(16)O(2), HI-(12)C(18)O(2), HI-(12)C(18)O(16)O, and HI-(12)C(16)O(18)O, have been recorded using pulsed-nozzle Fourier transform microwave spectroscopy. In the last two isotopomers, the heavy oxygen atom tilted toward and away from the HI moiety, respectively. Only b-type Ka = 1 <-- 0 transitions were observed. Spectral analysis provided molecular parameters including rotational, centrifugal distortion, and quadrupole constants for each isotopomer. Then, a four-dimensional intermolecular energy surface of a HI-CO2 complex was generated, morphing the results of ab initio calculations to reproduce the experimental data. The morphed potential of HI-(12)C(16)O(2) had two equivalent global minima with a well depth of 457(14) cm(-1) characterized by a planar quasi-T-shaped structure with the hydrogen atom tilted toward the CO2 moiety, separated by a barrier of 181(17) cm(-1). Also, a secondary minimum is present with a well depth of 405(14) cm(-1) with a planar quasi-T-shaped structure with the hydrogen atom tilted away from the CO2 moiety. The ground state structure of HI-(12)C(16)O(2) was determined to have a planar quasi-T-shaped geometry with R = 3.7717(1) A, thetaOCI = 82.30(1) degrees , thetaCIH = 71.55(1) degrees . The morphed potential obtained is now available for future studies of the dynamics of photoinitiated reactions of this complex.     

The microwave spectrum of the C3 sugars: glyceraldehyde and 1,3-dihydroxy-2-propanone and the dehydration product 2-hydroxy-2-propen-1-al

The triose sugars with empirical formula, C₃H₆O₃, consist of the aldehyde form, glyceraldehyde, and the ketone form, 1,3-dihydroxy-2-propanone. Recently, the simplest sugar, glycolaldehyde (CH₂OHCHO) was identified in the Sgr B2(N) molecular cloud by Hollis et al. (Astrophys. J. (Lett.) 540 (2000) L107), providing the incentive to pursue the present study. The microwave spectra of the triose sugars were obtained with the NIST Fourier-transform pulsed-nozzle microwave spectrometers equipped with heated nozzles. A few tenths of a gram of solid sample was placed in the nozzle base, which was heated to between 105 and 135 °C and pressurized with inert carrier gas. Broad spectral survey scans were carried out from 10 to 20 GHz for samples of both compounds. In the glyceraldehyde sample study, three conformers of the parent species were identified as well as 1,3-dihydroxy-2-propanone. In addition, three decomposition products were also identified: formic acid, trans-methyl glyoxal, and a previously experimentally unknown compound: 2-hydroxy-2-propen-1-al. Ab initio calculations were carried out with the Gaussian 98 program at the MP2/6-311++G** level to aid in the identification of each of the new species. The survey scan of the 1,3-dihydroxy-2-propanone sample confirmed the identification of this species initially assigned in the glyceraldehyde study. The 1,3-dihydroxy-2-propanone survey also exhibited spectra from the same three decomposition products initially observed in the glyceraldehyde work. However, the three conformers of glyceraldehyde were not present in the spectra of 1,3-dihydroxy-2-propanone.     

Sorption of inorganic nanoparticles in woven cellulose fabrics

Titanium dioxide was deposited from aqueous suspension onto cellulosic surfaces.Titania was sourced from Degussa (P25TM,70:30 anatase:rutile).Dry uptake of particles was shown to be rapid and dominant with one-third of the deposition occurring in less than 30 s and over one-half in the first minute.Isotherms were recorded to compare the rate of titanium deposition on dry and pre-wetted cotton.In the dry case uptake reached a maximum in 30 min whereas in the pre-wetted case the uptake was seen to continue beyond 180 min.A broad trend of higher deposition occurring at lower pH was seen,corresponding to the region where surface charges were opposite and thus attractive.Dry pickup was less significant at high pH.The response to varying ionic strength was complex and was attributed to the combined effect of charge screening,particle aggregation and consequent particle entrapment or occlusion.Titania deposition into the interstices of woven cotton sheets resulted in the formation of inorganic,nanoparticulate skeletons which could be isolated by controlled combustion of the cellulose and thus cotton was suggested to have potential for the templated synthesis of high surface area semiconductor materials.     

The geometry of organophosphonates: Fourier-transform microwave spectroscopy and ab initio study of diethyl methylphosphonate, diethyl ethylphosphonate, and diisopropyl methylphosphonate

The rotational spectra of diethyl methylphosphonate (DEMP), diethyl ethylphosphonate (DEEP), and diisopropyl methylphosphonate (DIMP) in supersonic expansions have been acquired using Fourier-transform microwave spectroscopy. Spectroscopic constants have been determined for five distinct conformers of the three molecules. Experimental data have been compared to ab initio calculations performed for each species. For both DEMP and DEEP, the calculations indicate the presence of several low-energy conformers (i.e., ?∼400 cm⁻¹ above the ground state) may be present at room temperature (300 K) for both DEMP and DEEP. When entrained in a supersonic expansion, the rotational temperatures of the samples are much colder (∼2 K); nonetheless, spectra from three conformers of DEEP are still observed experimentally, whereas only one conformer of DEMP is observed. In contrast, only a single low-energy conformer of DIMP is predicted by theory, and is present in the molecular beam. The relative abundance of low-energy conformers of DEMP and DEEP is attributed to the flexibility of the ethoxy groups within each molecule. The presence of multiple DEEP conformers in the supersonic beam indicates a more complex potential energy surface for this molecule that is directly related to conformational shifts of the PCH₂CH₃ group. Conversely, the absence of low-energy conformers of DIMP is attributed to steric hindrance between isopropoxy groups in the molecule. The internal rotation barrier for the PCH₃ group in DEMP and DIMP is compared to that found in DMMP and several phosphonate-based chemical weapon agents.     

The a-Type K = 0 Microwave Spectrum of the Methanol Dimer

The rotational spectrum of (CH₃OH)₂ has been observed in the region 4-22 GHz with pulsed-beam Fabry-Perot cavity Fourier-transform microwave spectrometers at NIST and at the University of Kiel. Each a-type R(J), K_a = 0 transition is split into 15 states by tunneling motions for (CH₃OH)₂, (¹³CH₃OH)₂, (CH₃OD)₂, (CD₃OH)₂, and (CD₃OH)₂. The preliminary analysis of the methyl internal rotation presented here was guided by the previously developed multidimensional tunneling theory which predicts 16 tunneling components for each R(J) transition from 25 distinct tunneling motions. Several isotopically mixed dimers of methanol have also been measured, namely ¹³CH₃OH, CH₃OD, CD₃OH, and CD₃OD bound to ¹²CH₃OH. Since the hydrogen bond interchange motion (which converts a donor into an acceptor) would produce a new and less favorable conformation from an energy viewpoint, it does not occur and only 10 tunneling components are observed for these mixed dimers. The structure of the complex is similar to that of water dimer with a hydrogen bond distance of 2.035 Å and a tilt of the acceptor methanol of 84° from the O-H-O axis. The effective barrier to internal rotation for the donor methyl group of (CH₃OH)₂ is ν₃ = 183.0 cm⁻¹ and is one-half of the value for the methanol monomer (370 cm⁻¹), while the barrier to internal rotation of the acceptor methyl group is 120 cm⁻¹.     

Syntheses and reactions of some fluorinated dienes,cyclobutenes and furans

There are many reports¹ of the pyrolysis of fluorinated organic compounds, including the defluorination of cyclic fluorocarbons over iron to give aromatic compounds. Extending this technique we have investigated the flow pyrolysis of some readily accessible unsaturated fluorocarbons, such as I, II, and III, and found these to be synthetically

useful routes to fluorinated dienes, cyclobutenes, and furans. Pyrolyses were carried out using a nitrogen flow over platinum, iron or caesium fluoride heated at 430–700°. The various products can all be rationalized in terms of intermediate allylic radicals, and the solid substrate influences which allylic radicals are formed.We are also investigating the chemistry of those now accessible compounds, such as IV, V, and VI, and some of the preliminary results are described.

For example the fluoride ion induced dimerisation of IV gave two major products VII and VIII via a particular interesting mechanism.