Microwave spectra of eight isotopic modifications of 1-chloro-1-fluoroethylene

The rotational spectra of eight isotopomers of 1-chloro-1-fluoroethylene in the 6-22 GHz region have been collected and analyzed. Each rotational transition is split into hyperfine components by the chlorine (either ³⁵Cl or ³⁷Cl) nuclear quadrupole coupling interaction and additionally, one or more smaller interactions such as the spin-rotation interaction due to the fluorine atom, hydrogen-hydrogen spin-spin coupling interactions, and in appropriately substituted species, the deuterium nuclear quadrupole hyperfine interaction. The rotational constants derived from these isotopomers allow the determination of average and Kraitchman substitution structures for 1-chloro-1-fluoroethylene, whereas the availability of the diagonal chlorine nuclear quadrupole coupling constants for all the isotopomers provides complete quadrupole coupling tensors for both ³⁵Cl and ³⁷Cl. In the course of this work, the rotational spectrum of an excited vibrational state of the normal isotopomer was observed, which ab initio calculations suggest should be assigned to ν₉=1, an in-plane bending motion at the CFCl end of the molecule.     

The microwave spectrum of cyanoformamide

The spectrum of cyanoformamide, NCCONH₂, has been measured between 18 and 40 GHz using a Hewlett-Packard spectrometer with Stark modulation. The molecule is somewhat unstable and could most conveniently be measured in a flow system. The quadrupole hyperfine structure due to the two nitrogen nuclei has been analyzed for the ground state, and quadrupole coupling constants, rotational constants, and centrifugal distortion constants have been determined for the ground state. A rough determination of the components of the electric dipole moment was possible from the Stark shifts of suitable transitions.     

Quadrupole coupling constants and centrifugal distortion constants of 3-bromopyridine from its microwave spectrum

The microwave spectrum of 3-bromopyridine has been investigated in the frequency range of 12–18 GHz and 30–40 GHz at dry ice temperature. The rotational, centrifugal distortion and quadrupole coupling constants of the two isotopic species ⁷⁹Br and ⁸¹Br have been evaluated. From the quadrupole coupling constants, double bond character in the CBr bond has been found to be 2.2% and the group electronegativity of the pyridyl radical to be 2.34.     

The Rotational Spectrum of Tricarbonyl(trans,trans-2,4-hexadiene) Iron

The microwave spectrum of tricarbonyl(trans,trans-2,4-hexadiene) iron has been recorded in the range from 3 to 22 GHz using a Balle-Flygare type Fourier transform microwave spectrometer. The b- and c-type spectrum for the (56)Fe and the (54)Fe isotopomer could be assigned. The internal rotation of the two equivalent methyl groups results in a fine structure of the rotational transitions which is analogous to that of 2,2-dimethyloxirane [H. Hartwig and H. Dreizler, Z. Naturforsch., A: Phys. Sci. 47, 1063-1066 (1992)] and cis-2,3-dimethyloxirane [H. Hartwig and H. Dreizler, Z. Naturforsch., A: Phys. Sci. 47, 1051-1057 (1992)]. Analyzing this fine structure yields the internal rotation barrier in addition to the rotational constants and the quartic centrifugal distortion constants. Copyright 2000 Academic Press.     

Intramolecular hydrogen bonding in chiral alcohols: The microwave spectrum of the chloropropanols

The microwave Fourier transform spectrum of a mixture of 1-chloropropan-2-ol and 2-chloropropan-1-ol has been recorded in the range 5-18 GHz. The spectrum is extremely dense, as a result of both the large number of species present in the supersonic expansion, and the splitting of rotational transitions due to quadrupole coupling. Two conformational isomers of each molecule have been identified, and both the ³⁵Cl and ³⁷Cl isotopomers have been observed for both conformations of 1-chloropropan-2-ol and the most abundant conformation of 2-chloropropan-1-ol. All four observed conformations have weak intramolecular hydrogen bonds between the hydroxyl hydrogen and the chlorine atom. The basis of our assignment is a series of ab initio calculations, performed using the GAUSSIAN package, good agreement being observed between theoretical and experimental values of the rotational constants. Analysis of isotopic substitution and quadrupole coupling effects has reinforced this assignment. First-order centrifugal distortion coefficients, as well as diagonal and (in some cases) off-diagonal quadrupole tensor components have also been extracted. Diagonalisation of the quadrupole tensors for the ³⁵Cl isotopomers results in a near-cylindrical quadrupole tensor, distorted in the C-Cl-H plane. This distortion, which is somewhat larger than in several comparable systems, supports the assertion that the most prevalent conformations are stabilised by an intramolecular hydrogen bond.     

2-三氟甲基吡啶的转动光谱及分子结构研究

本文测定了2-三氟甲基吡啶在2∽20 GHz频率范围内的高分辨转动光谱. 测定了转动常数、¹⁴N核四极耦合常数及离心畸变常数等一系列光谱参数. 同时还在自然丰度下测定了5个¹³C和1个¹⁴N单取代同位素异数体的光谱数据. 实验结果结合从头算准确地推导出2-三氟甲基吡啶的骨架结构. 实验测得同位素异数体的平面转动惯量P_cc数值均为44.46 u?²,表明此分子具有C_s对称性. 此外,本文计算了吡啶、2-氟吡啶、2-甲基吡啶和2-三氟甲基吡啶的分子表面静电势,以此分析了三氟甲基的取代对电子分布的影响.     

High resolution spectroscopy of the ν3 band of the van der Waals complex Ar—DCOOH

The vibrational-rotational spectrum of the van der Waals complex Ar-DCOOH has been recorded in the spectral region of the carbonyl stretch band. This work represents the first rotationally resolved vibrational spectrum for this complex. A full set of molecular constants could be deduced by fitting 105 lines with a standard S-reduced Watson Hamiltonian. The band origin was determined to be 1725.8440(1)cm^?1 which corresponds to a small red shift of 0.0309cm^?1 compared to the monomer band. The rotational constants deviate only slightly from the rotational constants in the ground state, as determined by microwave spectroscopy (I. I. Ioannou and R. L. Kuczkowski, 1993, J. phys. Chem., 98, 2231).     

High-resolution spectroscopy near the continuum limit: the microwave spectrum of trans-3-bromo-1,1,1,2,2-pentafluoropropane

ABSTRACT

The microwave spectrum of 3-bromo-1,1,1,2,2-pentafluoropropane has been observed using CP-FTMW spectroscopy. Potential energy scans have been performed and confirm the existence of two conformers – trans and gauche – for which further structural optimisations and electric field gradient calculations have been performed in order to get highly accurate nuclear quadrupole coupling constants for assignment purposes. The combination of multiple conformers and large nuclear quadrupole coupling constants produce a very dense spectrum at an estimated 1?transition/MHz, near the continuum limit. This spectral density makes it necessary to have very sophisticated computational approaches in order to get geometric and electronic structures that are very close to experimental observation. Analysis of the spectrum allowed for the assignment of the trans conformer, but the gauche proved to be prohibitive, although it is believed to be present in the current spectrum. Full analysis of the rotational spectroscopic parameters of two isotopologues – the⁷⁹Br and⁸¹Br – have been observed and are reported. Geometric analysis of the experimentally observed conformer is also reported using Kraitchman coordinate and second moments arguments. Further analysis of the spectrum reveals the occurrence of dipole-forbidden, nuclear quadrupole allowed transitions with one forbidden transition possessing the first known x-type forbidden transition linkage pathway.     

The microwave spectrum of bromoacetic acid

The microwave spectrum, rotational constants, centrifugal distortion parameters, and nuclear quadrupole coupling constants of bromoacetic acid are reported. Deviations from the second-order perturbation theory for quadrupole splittings are discussed, and the principal axes of the nuclear quadrupole tensor are determined. From the a-substitution coordinates of the Br atom and the carboxyl H atom the molecular conformation is found to be trans with respect to the atoms BrCCOH.     

Microwave spectra and structure of an isoxazole-water complex

The rotational spectrum of a hydrogen-bonded isoxazole-water complex has been measured between 6–18 GHz with a pulsed nozzle Fourier transform microwave spectrometer. In addition to isoxazole-H₂O, the complexes with HDO and D₂O as well as isoxazole-¹⁵ N-H₂O have been investigated in order to determine the structure of the complex. Rotational constants, quartic centrifugal distortion constants and quadrupole coupling constants, where applicable, have been fitted to the measured transition frequencies of the isotopomers. Structural data, which have been deduced from the planar moments of inertia and the quadrupole coupling constants of the isotopomers, have established conclusively that water binds to nitrogen in the ring plane of isoxazole. Ab initio calculations have revealed that complexes with a hydrogen-bond to nitrogen or to oxygen are both stable. The complex with water attached to nitrogen has been found to be more strongly bound than that with water attached to oxygen. Small splittings of the rotational transitions of the two complexes with H₂O have been interpreted as being the result of an internal rotation of water with respect to isoxazole.     

Hyperfine spectrum of NaF

The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectrum of ²³Na¹⁹F. Coupling constants for the nuclear electric quadrupole interactions, the spin-rotation interactions, the tensor and scalar spin-spin interactions, and their dependence on vibrational and rotational state have been determined.     

FTMW spectroscopy of jet-cooled 9-cyanoanthracene

Rotational spectrum of jet-cooled 9-cyanoanthracene has been observed in the 4-8 GHz region with a Fourier-transform microwave spectrometer. The present observation of 25 low-J transitions with J^′′?11 has confirmed the previous results on the rotational constants of the ground state determined by rotational coherence spectroscopy [J. Phys. Chem. A. 105 (2001) 1131] and provided the values with significantly improved precision. An accurate set of hyperfine splitting constants is also reported for the ¹⁴N nuclear quadrupole coupling. The electric dipole moment was determined from Stark effect measurements on several split components: μ_b(=μ)=4.406(7) D.     

Rotational Spectrum, Hyperfine Structure, and Internal Rotation of Methyl Carbamate

Methyl carbamate, an isomer of glycine, is a possible candidate for interstellar detection. It might be more abundant than glycine and its dipole moment is much larger. Furthermore, using high-level quantum chemical calculations, it is shown that syn methyl carbamate has a lower energy than glycine. The quadrupole hyperfine structure due to ¹⁴N has been measured using microwave Fourier transform spectroscopy. The rotational spectrum of the ground vibrational state has been measured from 8 to 240 GHz and accurate spectroscopic constants have been determined for the A internal rotation components of the rotational lines. Finally, the internal rotation splittings have been analyzed.     

Fourier transform microwave spectroscopy of N,N-dimethylacetamide

The jet-cooled Fourier-transform microwave spectrum of N,N-dimethylacetamide was recorded in the region of 12-24 GHz, and was analyzed to determine rotational constants and nuclear quadrupole coupling constants. Coriolis-like coupling parameters characterizing interaction between internal rotation of methyl groups and the overall rotation were also determined from internal-rotation tunneling splittings of the rotational transitions. The Coriolis-like coupling parameters permitted determination of the barrier heights to internal rotation of the three methyl groups, which were found to be 677, 237, and 183 cm⁻¹ for the C-methyl top, the trans-N-methyl top and the cis-N-methyl top, respectively.     

A molecular beam Fourier transform microwave study of 2-methylpyridine and its complex with argon: structure,methyl internal rotation and 14N nuclear quadrupole coupling

The rotational spectrum of 2-methylpyridine (α-picoline, CH₃C₅H₄N) in the two lowest levels of methyl internal rotation (m=0, ±1) has been recorded in the frequency range from 2 to 15 GHz using a molecular beam Fourier transform microwave spectrometer. The high resolution and sensitivity of this spectroscopic technique allowed resolution of hyperfine structures due to ^l4N nuclear quadrupole coupling with high accuracy and detection of the spectra of the ¹⁵N- and all ¹³C-isotopomers. These investigations considerably extend the results from an earlier study on the normal species (Dreizler, H., Rudolph, H. D., and Mader, H., 1970, Z. Naturforsch., 25a, 25); improved rotational and centrifugal distortion constants as well as all components of the ¹⁴N quadrupole coupling tensor have been obtained. Analysis of the spectra of the isotopomers yielded the ^I4N quadrupole coupling constants χ _cc and χ _aa – χ _bb (for the ¹³C species), the potential parameter V ₃ for the barrier hindering the internal rotation of the methyl group, and, in particular, r_o, r_s r _m ⁽¹⁾ and r _m ⁽²⁾ structural parameters for the molecule. In addition to the microwave studies on the monomer, we have also investigated the rotational spectrum of the weakly bound dimer of normal 2-methylpyridine with Ar. The results obtained for the quadrupole coupling constants and the hindering potential for the internal methyl rotation show that the corresponding parameters are not significantly, or only slightly, changed in the complex.     

The Microwave Spectrum and Quadrupole Coupling Constants of cis-2-Chlorophenol

The rotational spectrum of cis-2-chlorophenol was observed from 5 to 12 GHz by molecular beam Fourier transform microwave spectrometer (MB-FTMW). The rotational and quadrupole coupling constants for the ³⁵Cl of the molecule were determined: A=2985.4479(22) MHz, B=1549.8591(3) MHz, C=1020.1915(1) MHz, χ_aa=−68.2429(83) MHz, χ₋=−0.922(20) MHz, χ_ab=−11.4(19) MHz. The quadrupole coupling constants of the chlorine nuclei of cis-2-chlorophenol were nearly equal to that of the chlorobenzenes. We concluded that the electric field gradient of the chlorine atom in cis-2-chlorophenol is similar to that of other chlorobenzenes, although the suggestion of intramolecular hydrogen bonding.     

Fourier-transform microwave spectroscopy of N-methylaniline

The jet-cooled Fourier-transform microwave spectrum of N-methylaniline (C₆H₅-NHCH₃) was recorded in the region of 10-26 GHz, and was analyzed to determine rotational constants and nuclear quadrupole coupling constants. Furthermore, a Coriolis-like coupling parameter characterizing an interaction between an internal rotation of a methyl group and an overall rotation was also determined from A-E splittings observed in pure rotational transitions with high K_a values. The Coriolis-like coupling parameter permitted the determination of the barrier to internal rotation of the methyl group which was found to be 975 cm⁻¹.     

Microwave and submillimeterwave spectra of CH2DI and CHD2I

The pure rotational spectra of CH₂DI and CHD₂I have been measured by microwave Fourier transform spectroscopy, millimeterwave spectroscopy and submillimeterwave spectroscopy. The quadrupole hyperfine structure due to iodine has been analyzed by direct diagonalization of the quadrupole tensor. For the J = 1-0 transition of the ground state of CH₂DI, the quadrupole hyperfine structure due to deuterium could be resolved and the quadrupole coupling constant eQq_aa(D) determined.Accurate rotational and centrifugal distortion constants (up to sextic terms) have been determined. They are compared to the constants derived from the ground state combination differences (GSCD). A good agreement is observed but it is also found that the two kinds of data (GSCD and microwave) are complementary and a combined fit allows us to significantly improve the accuracy of the constants.     

Internal rotation and chlorine nuclear quadrupole coupling of o-chlorotoluene studied by microwave spectroscopy and ab initio calculations

The microwave spectrum of o-chlorotoluene has been reinvestigated using molecular beam Fourier transform microwave (MB-FTMW) spectrometers in the frequency range of 4–23 GHz. Due to the high resolution of this molecular beam technique the analysis yielded improved rotational constants, centrifugal distortion constants, and, for the first time, the complete chlorine nuclear quadrupole coupling tensor. From the torsional fine structure the barrier to internal rotation of the methyl group was found to be 5.5798(52) kJ mol⁻¹. Experimental results and ab initio calculations are compared.     

The Molecular Structure of Morpholine and the Morpholine–H2O Complex Determined by FT Microwave Spectroscopy

The rotational spectrum of the morpholine–H₂O complex was measured and assigned using a Balle–Flygare type FT microwave spectrometer. Rotational, quartic centrifugal distortion, and¹⁴N quadrupole coupling constants were determined, and a N … H–O hydrogen-bonded structure was found to be consistent with the derived molecular parameters. Additionally, the rotational spectrum of the¹³C and¹⁵N isotopomers of the morpholine monomer were measured in natural abundance to determine itsr₀structure and a partial heavy atomr_sstructure.