Quartic Centrifugal Distortion Research Articles

Structural and Dynamic Properties of a Hydrogen Bond from the Study of the CH3Cl–HCl Complex and Isotopic Species

The microwave (4-20 GHz range) and infrared (HCl and DCl stretch ranges) spectra of six isotopic species of the CH3Cl-HCl hydrogen bond complex have been recorded for the first time and analyzed with the support of high level ab initio calculations (MP2 and CCSD(T) levels). Accurate molecular parameters, including rotational, quartic centrifugal distortion, and nuclear-quadrupole coupling constants, vibrational frequencies, and anharmonic coupling constants, are presented in this paper. These parameters have then been used to estimate the hydrogen bond geometry and confirm the strong coupling between intramolecular and low frequency intermolecular modes. Experimental and theoretical evidence, in agreement with each other, tend to point out a free rotation of the CH3Cl unit in the complex, emphasizing the very peculiar dynamical properties of a hydrogen bond and, consequently, the necessity of taking those effects into account to correctly model the intra- and intermolecular interactions.

AB INITIOSTUDY OF SPECTROSCOPIC CONSTANTS AND ANHARMONIC FORCE FIELD OFAsH2RADICAL

The equilibrium structure, spectroscopic constants and anharmonic force field of AsH2have been investigated at B3LYP, B3PW91 and MP2 methods employing the basis sets of cc-pVNZ and aug-cc-pVNZ (N ∈ { T , Q }), respectively. The computed geometries, rotational constants, part of vibrational frequencies, quartic and sextic centrifugal distortion constants are compared with the available experimental data or theoretical results. The other vibrational frequencies, equilibrium rotational constants, anharmonic constants, vibration–rotation interaction constants, cubic and quartic force constants of AsH2are also predicted for the first time. Furthermore, the calculated results show that the DFT method is superior to MP2 at the calculations of geometries, spectroscopic constants and force constants. The B3PW91/aug-cc-pVQZ results are more reliable. Our predictions can provide useful data for the experimental studies of the corresponding spectroscopic constants of AsH2.

ChemInform Abstract: Anharmonic Vibrational Properties of CH2F2: A Comparison of Theory and Experiment.

Ab initio theoretical chemistry is used to provide a complete understanding of the infrared spectroscopy of CH2F2. Second‐order Mo/ller–Plesset perturbation theory (MP2) with a 631G extended basis set is used to provide a quartic expansion of the potential energy surface and a cubic expansion of the dipole surface. Standard perturbation theory is then used to determine effective vibrational and rotational Hamiltonians for fundamentals, selected overtones, and combination bands. Effects of Fermi resonance, Darling–Dennison resonance, and Coriolis resonance are included by matrix diagonalization. Empirical (x,K) relations are used to demonstrate that the anharmonic constants for C–H are in good agreement with those determined from CH2Cl2. The local mode nature of the CH overtones is demonstrated. Important resonances are found to be (ν3,2ν4), (ν8,ν4+ν9), and (ν1,2ν2,2ν8,ν4+ν8+ν9, 2ν4+2ν9,ν3+2ν9). Rotational constants, quartic and sextic centrifugal distortion constants, vibration rotation interaction consta...

Gas-phase detection of the FCO2 radical by millimeter wave and high resolution infrared spectroscopy assisted by ab initio calculations

Low pressure pyrolysis at 600 K of bis(fluoroformyl) peroxide, FC(O)OOC(O)F, yields the fluorocarboxyl radical, FCO2, in a concentration high enough to allow its detection by millimeter wave and infrared spectroscopy. The radical was first identified from its high resolution infrared spectrum obtained using a Fourier transform infrared interferometer. Observation and identification of its millimeter wave (MMW) spectrum were made possible by reliable ab initio calculations at the level of open-shell coupled cluster theory using large basis sets. The excellent agreement between the experimental and theoretical results confirms the structure of the FCO2 radical and the efficiency of the synthesis. The analysis of the MMW spectrum has given a set of ground state parameters including rotational, quartic centrifugal distortion, fine and hyperfine constants.

N-chlorodifluoromethanimine, CF2=NCl: Quadrupole coupling and r structure

The microwave spectra of CF212=15N35Cl, CF212=15N37Cl, and CF213=14N35Cl have been recorded and analyzed. Rotational, quartic centrifugal distortion and quadrupole coupling constants were determined. The off-diagonal element of the chlorine quadrupole coupling tensor, χab, and the principal element of that tensor, χz, were obtained as 63.7 and −103.5 MHz, respectively, for Cl35 (average), and 51 and −82 MHz, respectively, for Cl37 in CF212=15N37Cl. During the analysis of the spectrum of the C13 species, it was necessary to diagonalize the complete exact Hamiltonian of the asymmetric rotor with two quadrupole nuclei because of irregular hyperfine patterns of some transitions caused by previously neglected matrix elements from the N14 quadrupole. Earlier data for the normal isotopic species were re-evaluated because neglected off-diagonal matrix elements of the Hamiltonian led to data for χab and χz inconsistent with the results obtained during this investigation. The rotational constants of all five isotopic species were used to determine the r0 structure. All structural parameters for the planar molecule except for the difference between the CF distances could be determined. The results for the distances r(C=N) and r(NCl) and the angle α(CNCl) were 1.254(27) Å, 1.713(17) Å, and 113.62(26)°, respectively. The experimental data for the structure, the inertial defect, and the centrifugal distortion constants are compared to results from ab initio calculations.

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 atomrsstructure.

Observation and Properties of the Hydrogen-Bonded Heterodimer Tetrahydrothiophene···HCl

The ground-state rotational spectra of five isotopic species, C4H832S···H35Cl, C4H832S···H37Cl, C4H832S··· D35Cl, C4H832S···D37Cl, and C4H834S···H35Cl, of a hydrogen-bonded dimer between tetrahydrothiophene and hydrogen chloride have been measured in the frequency range 6−18.5 GHz using a molecular beam Fourier transform microwave spectrometer. Spectral analysis gave rotational, quartic centrifugal distortion and Cl−nuclear quadrupole coupling constants for each isotopomer. The rotational and quadrupole coupling constants have been interpreted in terms of a geometry in which hydrogen chloride lies on the plane bisector to the CSC angle of tetrahydrothiophene. The angle between the S···Cl internuclear line and the line bisecting the CSC angle was found to be 86.6(7)° and the distance r(S···Cl) = 3.48(3) Å. The deviation of the atoms S···H−Cl involved in the hydrogen bond from a collinear arrangement (θ = 0°) was estimated to be θ ∼ 14°.

Ab initio study and millimeter-wave spectroscopy of P2O

The millimeter-wave spectrum of P2O produced by reacting P4 vapor with atomic or molecular oxygen directly inside the absorption cell has been observed in 13 vibrational states. Reliable identification of the various vibrationally excited states has been achieved with the help of high-level ab initio calculations resulting in accurate geometric data and an anharmonic force field. Careful analysis of the vibrational levels observed resulted in 13 sets of rotational and centrifugal distortion parameters, which have been used, in turn, to determine the equilibrium rotational, quartic centrifugal distortion, and rovibrational interaction constants of P2O. The 2ν1−ν3 Fermi resonance has been analyzed.

Rotational Spectrum and Internal Rotation Barrier of 1-Chloro-1,1-difluoroethane

The rotational spectra of 1-chloro-1,1-difluoroethane (HCFC-142b) has been investigated in the frequency region 8–115 GHz with Stark, waveguide Fourier transform (FTMW), and millimeter-wave spectrometers. Assignments in large frequency regions with the corresponding frequency measurements have been made for the ground andv18= 1 (CH3torsion) vibrational states of the35Cl isotopomer and for the ground state of the37Cl species. Accurate rotational, quartic centrifugal distortion, and quadrupole coupling constants have been determined from global fits considering all these states. SmallA–Einternal rotation splittings have been observed for thev18= 1 vibrational state using FTMW spectroscopy. The barrier height for the internal rotation of the methyl group has been determined to be 3751 (4) cal mol−1, in disagreement with the previous microwave value of 4400 (100) cal mol−1reported by G. Graner and C. Thomas [J. Chem. Phys.49,4160–4167 (1968)].

Refinement of the spectroscopic constants of cyclopropyl bromide via perturbations in the Br quadrupole hyperfine structure in its microwave spectrum

The microwave spectra of two isotopic species of cyclopropyl bromide have been measured in the frequency region 8–90 GHz. Since the molecule is a slightly asymmetric prolate rotor with strong a-type and weak c-type transitions, it was necessary to use perturbations in the Br quadrupole hyperfine structure of the a-type transitions to improve the accuracy of A 0 and reduce the search region for the c-type lines. All rotational, quartic centrifugal distortion, and Br nuclear quadrupole coupling constants have been evaluated. The rotational constants are compared with those calculated from published structures.

The Microwave Spectrum of Isopropyl Iodide: The Iodine Hyperfine Structure and a Centrifugal Distortion Analysis

Abstract The ground stafe microwave spectrum of isopropyl iodide has been investigated by microwave Fourier transform (MWFT) spectroscopy. 212 hyperfine components of 39 mainly a-type transitions have been assigned from the measured spectra in the frequency range between 4 and 30 GHz. With a MWFT double resonance modulation (MWFTDR) spectrometer a weak c-type transition and a "forbidden" ΔJ = 2 line could be observed. The spectra were analysed using a diagonalization procedure of the complete Hamiltonian matrix and a simultaneous least squares fit of the rotational, quartic centrifugal distortion and the iodine quadrupole and spin rotation coupling constants directly to the measured frequencies. A set of 14 constants could be obtained with high accuracy. The rotational constant A and some of the centrifugal distortion constants could be improved using some large higher order quadrupole perturbations and the results of the MWFTDR measurements for the analysis.

Theoretical prediction of vibrational and rotational spectra. Formyl cyanide, HCOCN, and thioformyl cyanide, HCSCN

Optimized geometries and complete, scaled quadratic force fields of HCOCN and HCSCN have been determined at different theoretical levels (HF/4–21, HF/6–31G **; geometries also at MP2/6–31G ** and MP2/6–311G **). Frequencies calculated from the force fields confirm, with one exception, the assignment of the vibrational spectrum of HCOCN. The vibrational fundamentals calculated for HCSCN (accurate within about 50 cm −1) could direct a spectroscopy study aimed at determining them. Calculated rotational and quartic centrifugal distortion (QCD) constants are in good agreement with the experimental data for both molecules, but QCD constants only after scaling of the force fields.

14N-and D-Hyperfine Structure in the Rotational Spectrum of Pyrrolidine

Abstract We present an analysis of the rotational spectra of the normal and the N-deuterated pyrrolidine measured by microwave Fourier transform spectroscopy. The quartic centrifugal distortion con­ stants and the 14N coupling constants have been determined with higher accuracy. In addition the D hyperfine structure could be analyzed.

Spectra and structure of organophosphorus compounds. XXXIV. The r s and r structures of t r a n s and g a u c h e ethylphosphine

The microwave spectra of 11 chemically different isotopic species of ethylphosphine (CH3CH2PH2) including all 13C and D single substituted species were recorded and assigned for the vibrational ground state. The b- and c-type transitions of the gauche conformers with symmetrically substituted ethyl groups and a PH2 group were found to be split due to tunneling between the equivalent forms. The spectra of these molecules were fit to a Hamiltonian containing Coriolis interaction terms. All other spectra were fit to asymmetric rotors with quartic centrifugal distortion. Complete rs structures were obtained for both the trans and gauche conformations. The r0 structures were derived from 45 and 48 rotational constants for the trans and gauche forms, respectively. Significant differences in the structures were found with the most notable one in the CCP angle which changes from 110.5° in the gauche form to 116° in the trans conformer. The methyl group in the gauche conformation is rotated away from a position symmetric with respect to the heavy atom plane. The structural differences are attributed to nonbonded repulsive interactions between the methyl and phosphino hydrogen atoms.

The microwave spectrum, harmonic force field, and structure of N-chlorodifluoromethylenimine, CF 2NCl

The microwave spectra of two isotopic species of N-chlorodifluoromethylenimine (CF 2NCl) have been measured in the frequency region 12–54 GHz. Rotational, quartic centrifugal distortion, and Cl and 14N nuclear quadrupole coupling constants have been evaluated. The distortion constants have been combined with vibrational wavenumbers from the literature to calculate a harmonic force field. Effective ( r 0) and ground state average ( r z ) parameters have been evaluated, and have provided evidence that the structure of CF 2NCl is a hybrid of those of CF 2NF and CCl 2NCl. Bonding information has been obtained from the quadrupole coupling constants.

Microwave spectrum of chlorine dioxide in excited vibrational states

Abstract Microwave spectra of 35ClO2 and 37ClO2 in the excited vibrational states, ν1, ν2, ν3, and 2ν2, were observed. The rotational, quartic centrifugal distortion, and spin-rotation interaction constants were reduced to the equilibrium values. It was confirmed that the equilibrium bond lengths and valence angles of the 35ClO2 and 37ClO2 species are identical in at least six significant figures. The equilibrium structure was determined as follows; r e =1.469 839 ± 0.000 013 A and θ e =117.403 3 ± 0.002 7° The harmonic and anharmonic force constants were derived from the normal frequencies, inertia defects, centrifugal distortion constants, and vibration-rotation constants.

Microwave Spectrum of 1-Butene Oxide

The microwave spectrum of 1-butene oxide in the gas phase has been studied in the frequency region 18.0-39.0 GHz. The spectrum observed arose from a rotamer with a dihedral H-C2-C3-C4 angle of 59° ± 1°. In addition to several Q-branch progressions the spectrum contained several long perpendicular RP and PR progressions. However, of the ground state lines, only the intermediate PR transitions showed internal rotation splittings that could be resolved to yield a barrier height of 3.02 kcal mol-1. The value derived from the line splittings of the first excited methyl torsional state was slightly higher (3.17 kcal mol-1) but must be regarded as being less reliable. The components of the dipole moment, the rotational constants, and the quartic and sextic centrifugal distortion coefficients for the ground state and three vibrationally excited states were determined.

The high resolution rotational spectrum of methyl iodide using a molecular beam spectrometer

The hyperfine structure of the J = 5 ← 4 transition of the ground-state rotational spectrum of methyl iodide is investigated with a molecular beam-absorption spectrometer. Combining these measurements with the results of conventional spectroscopy allows us to determine with great accuracy the B rotational constant, some of the quartic and sextic centrifugal distortion constants, the two spin-rotation constants of iodine and the quadrupole coupling constant. Moreover, it is found necessary to take into account the interaction of centrifugal distortion with quadrupole coupling.

Infrared – Radio-frequency Double Resonance Observations of Δ J = 0 ‘Forbidden’ Rotational Transitions of SiH4

The radio-frequency and microwave spectrum of SiH4 corresponding to ΔJ = 0 ‘forbidden’ rotational transitions between the centrifugal distortion multiplets of levels has been studied by using the method of infrared–microwave double resonance in a laser cavity. Coincidences between the v4 fundamental band of SiH4 and N2O infrared laser lines in the 10 μm region have been used. Seven double resonance signals have been found and four of them were assigned to the J = 5, and J = 9 'forbidden' rotational transitions. From the measured frequencies, the quartic and sextic centrifugal distortion constants were determined as D1 = 74 861 ± 15 Hz, H4t = −7.14 ± 0.3 Hz, and H6t = 6.57 ± 1.1 Hz.