Lech Pszczółkowski

Rotational spectra of quinoline and of isoquinoline: spectroscopic constants and electric dipole moments

Abstract Rotational spectra of quinoline and of isoquinoline have been observed in the centimeter- and millimeter-wave regions. The spectra were assigned on the basis of bands formed by high-J transitions, which were measured up to J″⩽128 and ν⩽234 GHz. Complementary measurements were also made on low-J, centimeter-wave spectra observed in supersonic expansion and with fully resolved nuclear quadrupole hyperfine structure. Accurate rotational, centrifugal distortion and hyperfine splitting constants for the ground states of both molecules are reported. The electric dipole moments for the two molecules were also determined from Stark effect measurements and are μa=0.14355(19), μb=2.0146(17), μtot=2.0197(17) D for quinoline, and μa=2.3602(21), μb=0.9051(14), μtot=2.5278(20) D for isoquinoline. The experimental observables were found to be rather accurately predicted by MP2/6-31G** ab initio calculations, and corresponding molecular geometries are also reported.

The rotational spectra, electric dipole moments and molecular structures of anisole and benzaldehyde

The rotational spectra of anisole and of benzaldehyde were investigated in supersonic expansion at frequencies up to 41 GHz, and at room temperature in the millimetre-wave region, from 170 to 330 GHz. Accurate spectroscopic constants for the parent isotopomers in the ground vibrational state and for the first excited torsional state were determined for both molecules. The supersonic expansion spectrum allowed measurement, in natural abundance, of all singly substituted 13C isotopomers, as well as of the 18O isotopomer for both anisole and benzaldehyde. The rotational constants were used to determine the r(s) and the r(m)(1) gas-phase geometries, which are found to be consistent with prediction of bond length alternation in the phenyl ring induced by the asymmetric substituent. Stark measurements were made on the supersonic expansion spectrum resulting in electric dipole moment determination, /mu a/ = 2.9061(22) D, /mu b/ = 1.1883(10) D, /mu tOt/ = 3.1397(24) D for benzaldehyde and /mu a/ = 0.6937(12) D, /mu b/ = 1.0547(8) D, mu tOt = 1.2623(14) D for anisole. During the investigation it was found that use of a carrier gas mixture consisting of 30% Ar in He carries significant advantages for studies of weak lines, and pertinent experimental details are reported.

Laboratory characterization and astrophysical detection of vibrationally excited states of vinyl cyanide in Orion-KL

Context. We perform a laboratory characterization in the 18–1893 GHz range and astronomical detection between 80–280 GHz in Orion-KL with IRAM-30 m of CH2CHCN (vinyl cyanide) in its ground and vibrationally excited states. Aims. Our aim is to improve the understanding of rotational spectra of vibrationally excited vinyl cyanide with new laboratory data and analysis. The laboratory results allow searching for these excited state transitions in the Orion-KL line survey. Furthermore, rotational lines of CH2CHCN contribute to the understanding of the physical and chemical properties of the cloud. Methods. Laboratory measurements of CH2CHCN made on several different frequency-modulated spectrometers were combined into a single broadband 50–1900 GHz spectrum and its assignment was confirmed by Stark modulation spectra recorded in the 18–40 GHz region and by ab-initio anharmonic force field calculations. For analyzing the emission lines of vinyl cyanide detected in Orion-KL we used the excitation and radiative transfer code (MADEX) at LTE conditions. Results. Detailed characterization of laboratory spectra of CH2CHCN in nine different excited vibrational states: 11 = 1, 15 = 1, 11 = 2, 10 = 1 ⇔ (11 = 1,15 = 1), 11 = 3/15 = 2/14 = 1, (11 = 1,10 = 1) ⇔ (11 = 2,15 = 1), 9 = 1, (11 = 1,15 = 2) ⇔ (10 = 1,15 = 1) ⇔ (11 = 1,14 = 1), and 11 = 4 are determined, as well as the detection of transitions in the 11 = 2a nd 11 = 3 states for the first time in Orion-KL and of those in the 10 = 1 ⇔ (11 = 1,15 = 1) dyad of states for the first time in space. The rotational transitions of the ground state of this molecule emerge from four cloud components of hot core nature, which trace the physical and chemical conditions of high mass star forming regions in the Orion-KL Nebula. The lowest energy vibrationally excited states of vinyl cyanide, such as 11 = 1 (at 328.5 K), 15 = 1 (at 478.6 K), 11 = 2 (at 657.8 K), the 10 = 1 ⇔ (11 = 1,15 = 1) dyad (at 806.4/809.9 K), and 11 = 3 (at 987.9 K), are populated under warm and dense conditions, so they probe the hottest parts of the Orion-KL source. The vibrational temperatures derived for the 11 = 1, 11 = 2, and 15 = 1 states are 252 ± 76 K, 242 ± 121 K, and 227 ± 68 K, respectively; all of them are close to the mean kinetic temperature of the hot core component (210 K). The total column density of CH2CHCN in the ground state is (3.0 ± 0.9) × 10 15 cm −2 . We report the detection of methyl isocyanide (CH3NC) for the first time in Orion-KL and a tentative detection of vinyl isocyanide (CH2CHNC). We also give column density ratios between the cyanide and isocyanide isomers, obtaining a N(CH3NC)/N(CH3CN) ratio of 0.002. Conclusions. Laboratory characterization of many previously unassigned vibrationally excited states of vinyl cyanide ranging from microwave to THz frequencies allowed us to detect these molecular species in Orion-KL. Column density, rotational and vibrational temperatures for CH2CHCN in their ground and excited states, and the isotopologues have been constrained by means of a sample of more than 1000 lines in this survey.

The structure and electric dipole moment of camphor determined by rotational spectroscopy

The rotational spectrum of camphor was investigated for the first time. Accurate spectroscopic constants for the parent isotopomer were determined from measurements in supersonic expansion at 6.7–18.5 GHz, and on a room-temperature sample at 173–222 GHz. All three types of allowed dipole moment transitions, μa, μb, μc, were observed. The supersonic expansion spectrum allowed measurement, in natural abundance, of all ten singly substituted 13C isotopomers, as well as of the 18O isotopomer. The rotational constants were used to determine the rs, r0, and the r(1)m gas-phase geometries of the camphor molecule. The electric dipole moment of camphor was determined from Stark effect measurements, and is |μa| = 2.9934(23) D, |μb| = 0.7298(6) D, |μc| = 0.0804(7) D, μtot = 3.0821(22) D. The experimental results are compared with predictions from ab initio calculations and with the geometry of solid camphor.

The Millimeter- and Submillimeter-Wave Spectrum of the trans-trans Conformer of Diethyl Ether (C2H5OC2H5)

Since dimethyl ether is found to have a high abundance in hot cores, it is reasonable to search among such sources for detectable abundances of the more complex analogs ethyl methyl ether (C2H5OCH3) and diethyl ether (C2H5OC2H5). Indeed, preliminary detections of both complex ethers have been reported. Definitive interstellar searches do require laboratory spectra, however, but for many years only low-frequency data have been available for both of these species. Following our recent study of the millimeter-wave and submillimeter-wave spectrum of ethyl methyl ether, we report here the study of the millimeter- and submillimeter-wave spectrum of the lowest energy conformer of diethyl ether. With four different spectrometers, over 1000 new spectral lines have been measured and analyzed at frequencies up to 350 GHz. Fitting the data to a set of spectroscopic parameters from the Watson A-reduced form of the asymmetric-top Hamiltonian has allowed us to predict the frequencies and intensities of many more transitions through 400 GHz. A more precise determination of the electric dipole moment of diethyl ether was also carried out, resulting in μ = μb = 1.0976(9) D.

The anomeric effect in 1,3-benzodioxole: additional evidence from the rotational, vibration–rotation and rovibronic spectra

The millimetre wave pure rotation and vibration–rotation spectrum of 1,3-benzodioxole has been studied and the vibrational spacing between the two lowest vibrational states in the five membered ring puckering potential has been determined with considerable accuracy, at ΔE01 = 259 726.035(10) MHz or 8.663 5280(4) cm−1. In addition, rotationally resolved analysis of two hot bands near the reported origin band in the laser-induced fluorescence spectrum has been carried out and was found to rule out the current literature assignment of this spectrum. The new information has been used to reassign the lowest vibrational transitions in the far-infrared and the Raman spectrum of 1,3-benzodioxole in terms of a one dimensional potential with a central barrier of 108 cm−1, which successfully accounts for all states up to υ = 5, and for the observed variation in rotational constants. Considerable reassignment of the published spectra, as well as new experimental work is still necessary for a confident determination of the low-energy region of the molecular potential of 1,3-benzodioxole.

The High-Frequency Rotational Spectrum of 1,1 -Dichloroethylene

Abstract The b-type rotational spectrum of 1,1-dichloroethylene was investigated up to 450 GHz and was found to be dominated by type-II R-type bands. All constants in the sextic Hamiltonian for the ground states of the common isotopic species and of the 37C1 isotopomer were determined from measurements on transitions with J up to 95. Quartic and sextic planarity defects were evaluated and are compared and discussed with those for several recently investigated planar molecules

ROTATIONAL SPECTROSCOPY OF NEWLY DETECTED ATMOSPHERIC OZONE DEPLETERS: CF3CH2Cl, CF3CCl3, AND CF2ClCCl3

Three of the four compounds have non-zero dipole moments and are amenable to study by rotational spectroscopy, establishing the basis for analytic applications. Relatively limited studies have been reported for CF3CH2Cl and CF3CCl3, while CF2ClCCl3 has not yet been studied by this technique. We presently report extensive results obtained for all three compounds, resulting from concerted application of supersonic expansion FTMW spectroscopy in chirped pulse and cavity modes, and room-temperature MMW spectroscopy. Among the plentiful results, we have been able to resolve and fit the complex nuclear quadrupole hyperfine splitting.