B-type Rotational Transitions Research Articles

Pure Rotational Spectrum of Benzophenone Detected by Broadband Microwave Spectrometer in the 2–8 GHz Range

The investigation on microwave spectrum of benzophenone was conducted with a recently constructed broadband chirped-pulse Fourier transform microwave spectrometer with a heating nozzle in the 2–8 GHz range. In this work, 138 b-type pure rotational transitions were assigned to bridge the measuring gap in the microwave region. The rotational constants for benzophenone were accurately determined by a combined microwave data fitting with frequency coverage between 2–14 GHz and have the following values: A = 1692.8892190(119) MHz, B = 412.6446602(43) MHz and C = 353.8745644(43) MHz.

Millimeter-wave spectroscopy of HDC=CH.

Rotational transitions of the mono(β)-deuterated vinyl radical, HDC=CH, produced in a supersonic jet expansion by the ArF excimer laser photolysis, were observed by millimeter-wave spectroscopy. The b-type rotational transitions together with the weak a-type transitions were observed only for the lower component of the tunneling doublet, and no tunneling-rotation transitions connecting the lower and upper components were observed, suggesting that state mixing between the two components is negligibly small. The derived molecular constants such as the A rotational constant, Fermi contact interaction constants, and magnetic dipolar interaction constants indicate that the carrier of the observed spectrum is the trans-form of HDC=CH isomers, where the α-proton is located on the opposite side of the β-deuteron. The present conclusion of the trans-form of HDC=CH was also supported by the ab initio calculation in the CCSD(T)/cc-pVTZ level since the trans-form is calculated to be located by 30.04 cm-1 lower than the cis-form due to the difference in the zero point energy. As a result, the tunneling components in the ground state of HDC=CH behave as two different isomers localized at the trans- and cis-wells of the asymmetric double minimum potential. Observed hyperfine constants for HDC=CH were compared with those for H2C=CH to be consistent with each other.

The microwave spectrum of phenylpropiolic acid

The microwave spectrum for phenylpropiolic acid was measured in the 4.8–10 GHz frequency range using a Flygare-Balle type pulsed-beam Fourier transform microwave spectrometer. 34 a-type and 11 b-type rotational transitions were measured and assigned for the most abundant isotopologue. Based on the measured transitions, the rotational constants were determined to be A = 3859.823(33) MHz, B = 443.54379(10) MHz, and C = 398.09128(13) MHz. The centrifugal distortion constants were determined to be DJ = 0.00286(66) kHz and DJK = 0.1030(82) kHz. DFT(B3LYP) and MP2 calculations were performed with aug-cc-pVTZ basis set and the calculated rotational constants compare well with experimentally determined values.

Laboratory detection of the rotational-tunnelling spectrum of the hydroxymethyl radical, CH2OH

Context. Of the two structural isomers of CH3O, methoxy is the only radical whose astronomical detection has been reported through the observation of several rotational lines at 2 and 3 mm wavelengths. Although the hydroxymethyl radical, CH2OH, is known to be thermodynamically the most stable (by ~3300 cm-1), it has so far eluded rotational spectroscopy presumably because of its high chemical reactivity. Aims. Recent high-resolution (~10 MHz) sub-Doppler rovibrationally resolved infrared spectra of CH2OH (symmetric CH stretching a-type band) provided accurate ground vibrational state rotational constants, thus reviving the quest for its millimeter-wave spectrum in laboratory and subsequently in space. Methods. The search and assignment of the rotational spectrum of this fundamental species were guided by our quantum chemical calculations and by using rotational constants derived from high-resolution IR data. The hydroxymethyl radical was produced by hydrogen abstraction from methanol by atomic chlorine. Results. Ninety-six b-type rotational transitions between the v = 0 and v = 1 tunnelling sublevels involving 25 fine-structure components of Q branches (with Ka = 1 ← 0) and 4 fine-structure components of R branches (assigned to Ka = 0 ← 1) were measured below 402 GHz. Hyperfine structure alternations due to the two identical methylenic hydrogens were observed and analysed based on the symmetry and parity of the rotational levels. A global fit including infrared and millimeter-wave lines has been conducted using Pickett’s reduced axis system Hamiltonian. The recorded transitions (odd ΔKa) did not allow us to evaluate the Coriolis tunnelling interaction term. The comparison of the experimentally determined constants for both tunnelling levels with their computed values secures the long-awaited first detection of the rotational-tunnelling spectrum of this radical. In particular, a tunnelling rate of 139.73 ± 0.10 MHz (4.6609(32) × 10-3 cm-1) was obtained along with the rotational constants, electron spin-rotation interaction parameters and several hyperfine coupling terms. Conclusions. The laboratory characterization of CH2OH by millimeter-wave spectroscopy now offers the possibility for its astronomical detection for the first time.

Newly assigned microwave transitions and a global analysis of the combined microwave/millimeter wave rotational spectra of 9-fluorenone and benzophenone

Microwave spectra of 9-fluorenone and benzophenone have been observed using a broadband chirped-pulse Fourier Transform Microwave (cp-FTMW) Spectrometer. An analysis of the microwave spectra allowed for the assignment of 178 b-type rotational transitions for 9-fluorenone in the 8.0–13.0GHz region, the assignment of 166 b-type transitions for benzophenone in the 8.0–14.0GHz region, and effectively quadrupled the total number of pure rotational transitions observed for these molecules. This new microwave data and the previously published millimeter wave data of Maris et al. have been analyzed together in a global fit, where the resulting rotational constants accurately reproduce the spectra over the entire 8–80GHz region for both molecules. In addition, the resulting constants have been found to be consistent with the expected planar C2v structure for 9-fluorenone and the paddle-wheel like C2 structure of benzophenone. The rotational constants of the combined global fit have allowed for a more precise determination of the inertial defects (Δ) and second moments of inertia (Pcc) for 9-fluoreneone and benzophenone. Specific focus has been paid to the second moment of benzophenone, which when used in conjunction with theory strongly suggests an ∼32.9° torsional angle out of the ab-plane for the paddle-wheel structure of the gas-phase molecule.

Microwave measurements of the tropolone-formic acid doubly hydrogen bonded dimer.

The microwave spectrum was measured for the doubly hydrogen bonded dimer formed between tropolone and formic acid. The predicted symmetry of this dimer was C2v(M), and it was expected that the concerted proton tunneling motion would be observed. After measuring 25 a- and b-type rotational transitions, no splittings which could be associated with a concerted double proton tunneling motion were observed. The calculated barrier to the proton tunneling motion is near 15,000 cm(-1), which would likely make the tunneling frequencies too small to observe in the microwave spectra. The rotational and centrifugal distortion constants determined from the measured transitions were A = 2180.7186(98) MHz, B = 470.873 90(25) MHz, C = 387.689 84(22) MHz, DJ = 0.0100(14) kHz, DJK = 0.102(28) kHz, and DK = 13.2(81) kHz. The B3LYP/aug-cc-pVTZ calculated rotational constants were within 1% of the experimentally determined values.

Rovibrational Spectra of the Polar and Nonpolar Nitrous Oxide Dimers

The extended measurement of rovibrational spectrum of nonpolar nitrous oxide dimer has been carried out using a tunable infrared laser to probe a pulsed supersonic slit jet. Almost 400 new transitions have been assigned covering (J, K a ) values up to (28, 10). Besides the transitions with and 1, the perturbations with much larger magnitude are widely observed in this band. Excluding the transitions with perturbations in a least-square fit, the assigned transitions together with the data observed previously in this region are analyzed using a Watson A-reduced asymmetric rotor Hamiltonian and a set of improved molecular parameters is determined for both ground and excited vibrational states. Furthermore, the infrared spectrum of a polar isomer, which has been studied by Berner et al. (2009) in the ν3 region of nitrous oxide monomer, is also observed and analyzed in this region. Forty-five rotational transitions are assigned as a hybrid band with both a- and b-type rotational transitions. The vibrational origin is determined to be 1280.0027(2) cm−1, which shows a red-shift of 4.9 cm−1 from the band origin of N2O monomer.

Spectroscopic characterization of the complex between water and the simplest Criegee intermediate CH2OO

The hydrogen-bonded complex between water and the simplest Criegee intermediate CH2OO was detected by Fourier-transform microwave spectroscopy under a jet-cooled condition. Both a-type and b-type rotational transitions were observed for H2O-CH2OO and D2O-CH2OO. The determined rotational constants enable us to conclude that the complex has an almost planar ring structure with the terminal oxygen atom of CH2OO being a strong proton acceptor.

Fourier-transform microwave and submillimeter-wave spectroscopy of chloroiodomethane, CH 2ICl

Guided by a previous microwave study (9–35 GHz), the rotational spectrum of both chlorine isotopologues of chloroiodomethane in its vibrational and electronic ground state has been re-investigated in the microwave region and extended to the millimeter/submillimeter-wave region. Weak a-type transitions have been recorded by Fourier transform microwave spectroscopy below 20 GHz whilst strong b-type rotational transitions have been recorded between 15 and 646 GHz, corresponding to energy levels with J″ ≤ 108 and K a ″ ⩽ 12 . Molecular constants including those describing the hyperfine structures owing to the two halogen atoms were accurately determined for both species from the least-squares analysis of a total of 1475 distinct transition frequencies (of which 906 belong to the CH 2I 35Cl isotopologue). The two sets of rotational constants allowed us to derive an r 0 structure of CH 2ICl.

Rotational Spectra and Computational Analysis of Two Conformers of Leucinamide

Rotational spectra were recorded for two isotopic species of two conformers of the amide derivative of leucine in the range of 10.5-21 GHz and fit to a rigid rotor Hamiltonian. Ab initio calculations at the MP2/6-311++G(d,p) level identified the low energy conformations with different side chain configurations; the rotational spectra were assigned to the two lowest energy ab initio structures. We recorded 16 a- and b-type rotational transitions for conformer 1; the rotational constants of the normal species are A = 2275.6(2), B = 1033.37(2) and C = 911.71(5) MHz. We recorded 23 a- and b-type rotational transitions for conformer 2; the rotational constants of the normal species are A = 2752.775(8), B = 843.502(1) and C = 796.721(1) MHz. The rotational spectra of the (15)N(amide) isotopomer of each conformer were recorded and the atomic coordinates of the amide nitrogen were determined by Kraitchman's method of isotopic substitution. The experimentally observed structures are significantly different from the crystal structures of leucinamide and the gas-phase structures of leucine, and a natural bond orbital analysis revealed the donor-acceptor interactions governing side chain configuration.

Millimeter-wave spectroscopy of titanium dioxide, TiO 2

The millimeter-wave rotational spectrum of TiO 2 in its ground vibrational state has been recorded in the frequency range from 248 to 345 GHz using the Cologne Supersonic Jet Spectrometer for Terahertz Applications (SuJeSTA). Forty-two b-type rotational transitions of the main isotopologue 48TiO 2 and five transitions of 46TiO 2 in natural abundance have been measured up to J = 22 and K a = 8 which corresponds to excitation temperatures of 170 K. TiO 2 was formed by laser ablation and adiabatically cooled in a supersonic jet of helium to rotational temperatures of 20 K. The new transitions have been analyzed together with previously reported data obtained from Fourier-transform microwave spectroscopy in the frequency range from 7 to 42 GHz. The improved and extended set of spectroscopic parameters provides accurate transition frequencies for future astronomical searches in the millimeter-wave region.

Rotational spectra of o-, m-, and p-cyanophenol and internal rotation of p-cyanophenol

Rotational spectra of p-, m-, and o-cyanophenol have been measured in the range of 10.5-21 GHz and fit using Watson's A-reduction Hamiltonian coupled with nuclear quadrupole coupling interaction terms for the (14)N nuclei. Ab initio calculations at the MP2/6-311++G(d,p) and CCSD(T)/6-311++G(d,p) levels predict the cis conformers of m- and o-cyanophenol to be more stable than the corresponding trans conformers. A natural bond orbital analysis of the hydrogen bonding interaction in o- and m-cyanophenol revealed an intramolecular hydrogen bond that preferentially stabilizes the cis conformer of o-cyanophenol but there was no evidence of hydrogen bonding interactions in cis m-cyanophenol. We recorded 25 a- and b-type rotational transitions for cis o-cyanophenol; the rotational constants are A = 3053.758(2) MHz, B = 1511.2760(3) MHz, and C = 1010.7989(2) MHz. The trans conformer of o-cyanophenol was not observed. We recorded 14 a- and b-type rotational transitions for cis m-cyanophenol and 16 a- and b-type rotational transitions for trans m-cyanophenol. The rotational constants are A = 3408.9200(2) MHz, B = 1205.8269(2) MHz, and C = 890.6672(1) MHz and A = 3403.1196(3) MHz, B = 1208.4903(2) MHz, and C = 891.7241(2) MHz for the cis and trans species, respectively. Rotational transitions of the p-cyanophenol monomer are split due to the internal rotation of the hydroxyl group with respect to the aromatic ring. We recorded 25 a- and b-type rotational transitions for p-cyanophenol; the b-type transitions are split by 40 MHz. The rotational constants are A = 5612.96(2) MHz, B = 990.4283(6) MHz, and C = 841.9363(6) MHz. The ground state spitting DeltaE is 20.1608(6) MHz and the barrier to internal rotation, V(2), is 1413(2) cm(-1) from a fit of the rotational transitions to an internal axis system Hamiltonian. The barrier to internal rotation was modeled at the MP2/6-311++G(d,p) level and the effects of substituents on the phenolic ring and the barriers to internal rotation are discussed.

The Rotational Spectrum of TiO2

The rotational spectrum of titanium dioxide (TiO2) has been detected by laser-ablation molecular beam Fourier transform microwave spectroscopy. Thirteen b-type rotational transitions up to J = 9 and Ka = 4 were measured for the most abundant isotopic species 48TiO2 in the frequency range 7-42 GHz with accuracies of 1-10 kHz, allowing for the precise determination of rotational and centrifugal distortion parameters. In addition, eight and six rotational transitions of the rare isotopic species 46TiO2 and 50TiO2, respectively, have been recorded in the same frequency range. From the derived spectroscopic parameters, rest frequencies of TiO2 can now be calculated to better than 1 km s−1 in equivalent radial velocity up to fairly high J and Ka at millimeter wavelengths, enabling radioastronomical searches for this stable, highly polar transition metal dioxide in space. Preliminary results of two searches for TiO2 in space are presented.

Rotational spectra of the van der Waals complexes of molecular hydrogen and OCS

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2-OCS, ortho-H2-OCS, HD-OCS, para-D2-OCS, and ortho-D2-OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2-OCS relative to ortho-H2-OCS increases exponentially with decreasing normal H2 component in H2He gas mixtures, making the observation of para-H2-OCS in the presence of the more strongly bound ortho-H2-OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2-OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2-OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD-OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2-OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2-OCS.

Millimeter-wave spectrum of Ne–CO: new measurements

The b-type rotational transitions of the van der Waals complex, Ne–CO have been measured using the intracavity OROTRON jet spectrometer in the frequency range of 80–115 GHz. The high sensitivity of this technique enabled us to detect all three Ne isotopic modifications of the complex, 20Ne–CO, 22Ne–CO, and 21Ne–CO in natural abundance. The observed and assigned transitions belong to the Q-branch of the K = 1–0 subband and include also R (0) and P (2) lines. The newly obtained data were analysed together with previously observed millimeter-wave b-type and microwave a-type rotational transitions.

Microwave spectrum of PHD: hyperfine structure

The rotational spectrum of the monodeuterated PH 2 radical was studied using a source-modulated submillimeter-wave spectrometer. The PHD radical was generated in a free space absorption cell by a dc-glow discharge in a gas mixture of PH 3 and D 2. Six a-type and 20 b-type rotational transitions were observed in the frequency region of 170–670 GHz. Hyperfine structure due to the deuterium nucleus was resolved only in the rotational transitions of 1 11–0 00 and 1 10–1 01 and in the low F 2 components of N=2–1 transitions. A total of 219 spectral lines were measured of which 145 were analyzed by least-squares methods. These yielded 34 precise molecular constants including the hyperfine coupling constants of phosphorus, hydrogen, and deuterium. The principal axes and principal values of the magnetic dipole coupling tensors of hydrogen in PH 2 and deuterium in PD 2 were derived from the observed values of PHD, PH 2 and PD 2. The principal axis of the hydrogen magnetic dipole coupling tensor in PH 2 makes an angle of 2.29° with the PH bond and its T σ and T ⊥ principal values are determined to be 12.93 and −18.39 MHz, respectively.

OECD report outlines water management improvements

Microwave spectra of 9-fluorenone and benzophenone have been observed using a broadband chirped-pulse Fourier Transform Microwave (cp-FTMW) Spectrometer. An analysis of the microwave spectra allowed for the assignment of 178 b-type rotational transitions for 9-fluorenone in the 8.0–13.0 GHz region, the assignment of 166 b-type transitions for benzophenone in the 8.0–14.0 GHz region, and effectively quadrupled the total number of pure rotational transitions observed for these molecules. This new microwave data and the previously published millimeter wave data of Maris et al. have been analyzed together in a global fit, where the resulting rotational constants accurately reproduce the spectra over the entire 8–80 GHz region for both molecules. In addition, the resulting constants have been found to be consistent with the expected planar C2v structure for 9-fluorenone and the paddle-wheel like C2 structure of benzophenone. The rotational constants of the combined global fit have allowed for a more precise determination of the inertial defects (Δ) and second moments of inertia (Pcc) for 9-fluoreneone and benzophenone. Specific focus has been paid to the second moment of benzophenone, which when used in conjunction with theory strongly suggests an ∼32.9° torsional angle out of the ab-plane for the paddle-wheel structure of the gas-phase molecule.

The Rotational Spectra, Structure, Internal Dynamics, and Electric Dipole Moment of the Argon–Ketene van der Waals Complex

Pulsed-beam Fourier transform microwave spectroscopy was used to observe and assign the rotational spectra of the argon–ketene van der Waals complex. Tunneling of the hydrogen or deuterium atoms splits the a- and b-type rotational transitions of H2CCO–Ar, H213CCO–Ar, H2C13CO–Ar, and D2CCO–Ar into two states. This internal motion appears to be quenched for HDCCO–Ar where only one state is observed. The spectra of all isotopomers were satisfactorily fit to a Watson asymmetric top Hamiltonian which gave A=10 447.9248(10) MHz, B=1918.0138(16) MHz, C=1606.7642(15) MHz, ΔJ=16.0856(70) kHz, ΔJK=274.779(64) kHz, ΔK=−152.24(23) kHz, δJ=2.5313(18) kHz, δK=209.85(82) kHz, and hK=1.562(64) kHz for the A1 state of H2CCO–Ar. Electric dipole moment measurements determined μa=0.417(10)×10−30 C m [0.125(3) D] and μb=4.566(7)×10−30 C m [1.369(2) D] along the a and b principal axes of the A1 state of the normal isotopomer. A least squares fit of principal moments of inertia, Ia and Ic, of H2CCO–Ar, H213CCO–Ar, and H2C13CO–Ar for the A1 states give the argon–ketene center of mass separation, Rcm=3.5868(3) Å, and the angle between the line connecting argon with the center of mass of ketene and the C=C=O axis, θcm=96.4°(2). The spectral data are consistent with a planar geometry with the argon atom tilted toward the carbonyl carbon of ketene by 6.4° from a T-shaped configuration.

Laboratory Detection of the Ring‐Chain Carbenes HC4N and HC6N

The highly polar ring-chain carbenes HC4N and HC6N, formed by substituting either CN or CCCN for a hydrogen atom in cyclopropenylidene (c-C3H2), were detected in a supersonic molecular beam with a Fourier transform microwave spectrometer. Seven a- and four b-type rotational transitions of HC4N and 11 a-type transitions of HC6N, each with resolved nitrogen nuclear quadrupole hyperfine structure, were measured between 6 and 21 GHz, yielding precise values for the three rotational constants, the leading centrifugal distortion constants, and the quadrupole coupling constants. Like the hydrocarbon carbenes C5H2, C7H2, and C9H2, both new molecules have a planar ring-chain structures and singlet electronic ground states. The strongest lines of HC4N can be detected with a signal-to-noise ratio exceeding 10 in a total integration time of less than 1 s, but the lines of HC6N were nearly 100 times weaker.

The rotational spectrum and nuclear quadrupole hyperfine structure of CO2–N2O

The microwave spectrum of CO2–N2O has been obtained in the 7–19 GHz region using a Fourier transform microwave spectrometer. The nuclear quadrupole hyperfine structure in 26 a- and b-type rotational transitions has been analyzed using the Watson S-reduced Hamiltonian with the inclusion of nuclear quadrupole interactions. The rotational constants and six centrifugal distortion constants (in MHz) are A=8843.4133(1), B=1738.777 37(6), C=1449.807 41(5), DJ=6.510(3)×10−3, DJK=−3.7405(8)×10−2, DK=2.3459(3)×10−1, d1=−1.3751(4)×10−3, d2=−8.3(1)×10−5, and HJ=−1.3(4)×10−7. The nuclear quadrupole coupling constants (in MHz) for the terminal nitrogen nucleus are χaa=−0.0966(4), χbb=−0.3111(4), and χcc=0.4077(4), and those for the central nitrogen nucleus are χaa=−0.0411(6), χbb=−0.0968(6), and χcc=0.1380(6). The spectroscopic constants are consistent with an approximately slipped parallel structure where the distance between the centers of mass of the subunits is 3.472 Å, the acute angle between the CO2 molecular axis and the intermolecular axis is 62.8°, and the acute angle between the N2O axis and the intermolecular axis is 58.1°. The experimental data cannot identify whether the terminal nitrogen or the oxygen in N2O is closest to the C in CO2. The nuclear quadrupole coupling constants show that the electric field gradients at the nitrogen nuclei are perturbed to differing extents.