Robert K. Bohn

The Millimeter- and Submillimeter-Wave Spectrum of the Gt Conformer of n-Propanol (n-CH3CH2CH2OH)

As part of our continuing spectroscopic program of measuring the rotational spectra of large gas-phase molecules that may exist in observable quantities in the interstellar medium, we have studied the millimeter- and submillimeter-wave spectrum of the alcohol n-propanol (n-CH3CH2CH2OH), a straight-chain molecule with one more carbon atom than ethanol, which has already been detected in hot molecular cores. Although the molecule is thought to exist in five different conformers, we have only studied what is most likely to be the Gauche-trans (Gt) conformer, and analyzed over 2800 lines through 375 GHz in frequency. These lines have been added to both previously and newly measured rotational lines at lower frequencies. The global data set has been fitted to experimental accuracy, allowing us to determine accurate spectral parameters so as to predict useful frequencies for many more transitions of the conformer.

Low resolution microwave spectroscopy. I. The structural significance of the band spacing

Compounds which are nearly symmetric tops, have ∼20 mtorr vapor pressure and have a sizeable dipole moment along the a axis display band microwave rotational spectra under low resolution conditions. The frequencies of the band maxima are adequately represented by a symmetric rotor type equation, ν = (B + C)(J + 1). From calculated spectra it is shown that the B + C value defined above exceeds the ground state effective rotational constant B0 + C0. Low resolution B + C values and high resolution B0 + C0 values are compared for nineteen species. B + C differs from B0 + C0 by less than 2% in all cases and less than 0.8% for molecules with κ < −0.8. Empirically it is found that (B + C)/(B0 + C0) − 1 = ν/ν0 − 1 = 0.025(±0.005)(1 + κ), where k is the asymmetry parameter and ν0 is the hypothetical symmetric rotor frequency, (B0 + C0)(J + 1). This empirical relation allows B0 + C0 to be estimated from B + C with an uncertainty of only a few tenths of a percent.

Helical C2 Structure of Perfluoropentane and the C2v Structure of Perfluoropropane

Saturated hydrocarbons have structures with completely staggered bonds and dihedral angles of 180 degrees . Substituting hydrogen by fluorine results in a slight shift from 180 degrees , giving rise to a helical structure. X-ray diffraction studies on fibers and computational studies on perfluoroalkanes estimate a dihedral angle of about 17 degrees from the trans position. The rotational spectra of perfluoropentane and its three (13)C isotopomers have been observed and assigned using a pulsed-jet Fourier transform microwave spectrometer. The rotational constants for the parent species are A 990.6394(3) MHz, B 314.0002(1) MHz, and C 304.3703(1) MHz, respectively. The determination of an exact dihedral angle has been challenging, as the helical twist has proven to be quite sensitive to the structural inputs and constraints. A series of r(0) structures incorporating various model constraints and a Kraitchman analysis gives a range of 13-19 degrees for the torsional angle. An objective approach, which only assumes overall C(2) symmetry, is to scale the principal coordinates from ab initio models by the square root of the ratio of the observed second moments to the computed second moments. The scaled structures of computed models at various levels of theory reproduce the parent second moments exactly and the (13)C second moments very well, giving a dihedral angle of 17 +/- 1 degrees from trans. The microwave spectrum of perfluoropropane has also been observed and assigned. The rotational constants are A 1678.5982(9) MHz, B 900.1968(10) MHz, and C 955.3216(11) MHz, respectively. Unlike longer perfluoroalkanes, perfluoropropane has a nonhelical, C(2v) structure. Computations are in excellent agreement with experimental results.

Low resolution microwave spectroscopy, part IV1: The conformation of isopropyl formate, isopropyl fluoroformate, isopropyl chloroformate, isopropyl cyanoformate, and isopropyl trifluoroacetate

Abstract Low resolution microwave spectra of isopropyl formate, fluorofonnate, chloroformate, cyanoformate, and trifluoroacetate each display strong a-type band series associated with a single rotamer. Experimental B+C values for these compounds are: 4641±5; 3172±4; 2275±1/2226±1 MHz ( 35 Cl/ 37 Cl); 2257±1 MHz; and 1509±4 MHz, respectively. In each case the experimental values agree with a τ - 1 (OCOC) = 0°, τ 2 (COCH) ~ 40° structure indicating a modest internal rotation barrier when the methine C—H bond is syn eclipsed with the carboxyl framework. A potential function for the internal rotation of an isopropyl group extrapolated from the known functions in methyl and ethyl formate and one calculated by ab initio methods support the model derived from the low resolution microwave spectra. The single isopropyl conformer observed can be viewed as a combination of the two most stable forms of ethyl esters.

Microwave studies of the three conformers of butyl cyanide

Abstract Three conformers of n-butyl cyanide have been characterized by microwave spectroscopy. Low and medium resolution studies on static gas samples in Stark-modulated spectrometers revealed the presence of three conformational species and identified them. High resolution pulsed-jet Fourier transform spectra allowed the assignment of individual transitions of each species yielding rotational and centrifugal distortion constants, 14N quadrupole coupling constants, and qualitative relative energies of the species. The anti-anti and gauche(CN end)-anti forms are more stable than the anti-gauche(methyl end) conformer. The 14N quadrupole coupling constants, which appear so different among the different conformers, are consistent with a reference electric field gradient, that in methyl cyanide, rotated to the inertial axes of each conformer of butyl cyanide.

Rotational spectra of methyl ethyl and methyl propyl nitrosamines. Conformational assignment, internal rotation and quadrupole coupling

Abstract A structural determination of two carcinogenic nitrosamines, methyl ethyl and methyl propyl nitrosamine, was performed. Microwave spectra were gathered from both a Stark cell spectrometer and a pulsed jet Fabry-Perot Fourier transform microwave spectrometer. Each rotational transition is split into quadrupole hyperfine components by two nitrogen nuclei. This quadrupole pattern is doubled by a low barrier methyl rotor which produces resolvable A and E states. Rotational spectra were assigned for one conformer of methyl ethyl nitrosamine and two conformers of methyl propyl nitrosamine. The lowest energy conformers of each compound, according to empirical force field calculations, were assigned. The structure found for methyl ethyl nitrosamine has the nitrosyl oxygen on the methyl side with the terminal methyl group of the ethyl chain in the gauche position (OMG). Both conformers of methyl propyl nitrosamine have the same skeletal structure as the methyl ethyl compound; one conformer has the terminal methyl of the propyl group in the anti position (OMGA) while the other conformer has this methyl in the gauche position (OMGG − ). Rotational constants and quadrupole coupling constants are reported for each assigned species. A barrier to internal rotation of the N -methyl group in each compound is also reported.

Soil bed scrubbing of fugitive gas releases

Abstract Accidental and occasional discharges of hazardous and pollutant gases to the atmosphere could be greatly diminished if inexpensive, effective, large capacity, and fail‐safe gas scrubbers were available. Soil bed scrubbers often satisfy all of these criteria.

Rotational Spectrum, Tunneling Motions, and Potential Barriers of Benzyl Alcohol

The rotational spectra of benzyl alcohol and of its OD isotopologue have been assigned and measured in a supersonic expansion, either with pulsed-jet Fourier transform microwave or free jet absorption millimeter wave spectroscopy. The spectrum is consistent with a gauche conformation of the oxygen atom, characterized by a theta (OC(7)-C(1)C(2)) dihedral angle of approximately 55 degrees. Such a configuration is 4-fold degenerate, corresponding to minima with theta approximately +/-60 degrees, +/-120 degrees. The four equivalent minima are separated by two kinds of barrier, corresponding to theta = +/-90 degrees, and 0 or 180 degrees. Only the theta = +/-90 degrees barriers are low enough to generate a tunneling splitting, which has been measured in a spectrum strongly perturbed by tunneling interactions. The observed splittings diminish considerably upon deuterium substitution. The tunneling splittings are consistent with a barrier about 280 cm(-1) and high level ab initio calculations predicting a 320 cm(-1) barrier.

ROTATIONAL SPECTRA OF BENZYL CYANIDE ASSIGNMENT OF THE PLANAR CONFORMER AND EVIDENCE OF A LOW BARRIER TO INTERNAL ROTATION

Abstract Rotational spectra of benzyl cyanide were obtained at a very low temperature in a pulsed beam Fourier transform microwave spectrometer, and over a range of much higher temperatures in a continuous wave spectrometer. The ground state rotational constants, A = 4346.815 (5) MHz, B = 1057.373(3) MHz and C =856.642(3) MHz, and five centrifugal distortion constants were derived from transitions measured between 7.5 and 17 GHz and are only consistent with a heavy-atom planar structure. The 14 N quadrupole coupling constant, X aa = −1.313(2) MHz and X bb = −0.657 (3) MHz, is also only consistent with a planar heavy-atom structure if the electric field gradient in the CN group is assumed cylindrically symmetrical with X zz = −3.9 MHz. Temperature dependent low resolution microwave spectra between 18 and 26.5 GHz display R -branch a -type bands from two types of species. The planar ground state produces a band series with B + C = 1927(2) MHz. At 300 K three series of bands of steadily diminishing intensity are displaced to higher frequency from the ground state bands; they are assigned to three torsionally excited species. Relative intensities of these satellites indicate a low internal rotation barrier. Superposed on this satellite pattern are broader bands of another species whose characteristics are incompatible with another conformational isomer. Its intensity relative to the series assigned to the ground state increases from 0.5 to 4 when the temperature is increased from 280 to 360 K. This series may originate from many weakly bound and/or freely rotating torsional satellites with similar rotational constants.

Low resolution microwave spectroscopy: Part 14. Conformations of S-isopropyl thioesters and a high resolution study of S-isopropyl thiofluoroformate

Abstract A high resolution microwave assignment of S -isopropyl fluorothioformate yielded the following rotational constants: A 0 = 4196(1) MHz, B 0 = 1374.23(3) MHz and C 0 = 1159.80(4 MHz, compatible with a syn-gauche (τ 1 (OCSC) = 0°, τ 2 (CSCH) ∼ 40°) conformer. Low resolution microwave spectra of S -isopropyl trifluorothioacetate, cyanothioformate and chlorothioformate each display strong a -type band series compatible with either syn-syn (τ 1 (OCSC) = 0°, τ 2 (CSCH) = 0°) or syn-gauche (τ 1 (OCSC) = 0°, τ 2 (CSCH) ∼ 60°) conformers. Experimental B+C values are 1211.7(6), 1839.6(7) and 1814.6(5), (1774.4(5), 37 Cl) MHz for the trifluorothioacetate, cyanothioformate and chlorothioformate, respectively. S -isopropyl trifluorothioacetate also displays a weak band series with a B+C value of 1310.2(5) MHz compatible with a syn-anti (τ 1 (OCSC) = 0°, τ 2 (CSCH) = 180°) structure. Torsionally excited species freely rotating about τ 2 have been previously observed in S -ethyl thioesters but are not observed in S -isopropyl thioesters indicating a higher torsional barrier for the S -isopropyl compounds.