Higher Order Centrifugal Distortion Research Articles

Conformational stability of cyclopropanecarboxaldehyde is ruled by vibrational effects

Cyclopropanecarboxaldehyde (CPCA) has two main conformers, syn and anti, that are renowned for being very close in energy. The stability order of these two main species is constantly reversed by changing the level of theory or the experimental technique employed in its determination. The anti conformer is predicted to be the most favoured in condensed states of matter, but uncertainty still remains on the relative stability in the gas phase. To gain further insights into this issue, the investigation of the rotational spectrum of both syn- and anti-CPCA has been extended in the 246–294 GHz frequency region and complemented by a detailed computational study of both conformers. A fit incorporating the recorded rotational transitions as well as those reported in the literature led to the accurate determination of the rotational parameters, also including high-order centrifugal distortion constants. Accurately computed vibrational frequencies were used to re-analyse the infrared spectrum of both conformers, thereby allowing a re-assignment of two vibrational bands, namely of anti-CPCA and of the syn conformer. While our state-of-the-art computations favour the anti conformer in the stability order, estimates from analysis of rotational spectra are rather controversial and are strongly affected by several factors, such as the zero-point vibrational correction.

Improvement and determination of higher-order centrifugal distortion constants of the [formula omitted] electronic transition of NO

The highly rotationally excited states in the A2Σ+-X2ΠΩ band of NO were investigated by laser-induced fluorescence and multi-photon ionization spectroscopy. The rotational states with J up to 80.5 were prepared by CH3ONO photodissociation in the S1 and S2 excited states for spectroscopic measurements. In total, 3890 lines of nine vibronic bands were analyzed. The observed line positions deviated from the values calculated using the previous molecular constants as J increased in the J > 50.5 region. The sextet-order centrifugal distortion constants were determined for the A2Σ+and X2ΠΩ states so that the transition frequencies associated with the high-J states could reproduce the observed results within the experimental accuracy. The band origin values, whose errors have been discussed in previous studies, were refined in the present analysis. The experimental condition to suppress the rotational relaxation of the high-J states was examined, enabling us to determine the higher-order centrifugal distortion constants.

MILLIMETER AND SUBMILLIMETER REST FREQUENCIES FOR NaCN (X̃1A′): A REMARKABLY ABUNDANT CIRCUMSTELLAR MOLECULE

The pure rotational spectrum of NaCN has been recorded in the millimeter/submillimeter region using direct absorption techniques, the first experimental study of this molecule at frequencies above 40 GHz. The species was produced in the gas phase in an AC discharge by the reaction of sodium vapor with cyanogen. Twelve rotational transitions of NaCN have been measured in the range of 180-530 GHz, extending the previous microwave data to millimeter/submillimeter frequencies. Multiple Ka asymmetry components were recorded in each rotational transition up to Ka = 5 or 6, a total of 131 individual lines. These data have been analyzed with a standard asymmetric top Hamiltonian, combined with prior microwave measurements. Higher-order centrifugal distortion terms were clearly needed to model the millimeter-wave transitions of this floppy molecule. From this revised set of spectroscopic constants, accurate frequency predictions can now be made up to 600 GHz for NaCN for the lower-value Ka components (Ka ≤ 4). Based on the new laboratory data and past spectral surveys, a revised abundance for NaCN in IRC+10216 has been estimated. For a 5'' source, the fractional abundance for this molecule was found to be f (NaCN/H2) ~ 1 × 10–8, comparable to that of c-C3H2. These new frequency measurements should aid in line identification in surveys in the 0.8 mm band and at shorter wavelengths.

Submillimeter-wave spectrum of in the extended negative glow and hollow-anode discharges

Abstract The pure rotational transitions of C 3 N - have been observed in the frequency range of 359–504 GHz. An extended negative glow discharge and a hollow-anode discharge in a gas mixture of C 2 N 2 (0.3–2 mTorr) and C 2 H 2 (2–3 mTorr) diluted in Ar or Ne buffer gas of about 15 mTorr pressure prove to be an excellent source of the negative ion. Throughout the experiments, the cell was cooled down to about −45 °C. In the present study, we have determined the accurate molecular constants including a higher order centrifugal distortion constant H . The number density of C 3 N - in the discharge cell is estimated to be about 3.6 × 10 6 cm−3. The chemical processes related to this anion and other neutral species are briefly discussed.

High Order Centrifugal Distortion Corrections to Energy Levels of Asymmetric Top Molecules

High order centrifugal distortion terms have been derived and added to the effective Hamiltonian of asymmetric top molecules. Based on this Hamiltonian, a program in Fortran 77 has been developed for spectral analysis of asymmetric top molecules. The high order centrifugal distortion terms are found to be non-negligible even for the low-lying rotational transitions of molecules, such as H2 18O, subjected to severe centrifugal distortion effect, and for the high-lying rotational transitions of molecules, such as 14N16O2 subjected to the moderate centrifugal distortion effect.

Laboratory Detection and Rotational Rest Frequencies of N[CLC]a[/CLC]SH

The first laboratory spectrum of sodium hydrogensulfide, NaSH, is observed in the millimeter- and submillimeter-wavelength regions by direct absorption spectroscopy. The NaSH molecule is produced in a DC glow discharge plasma of a gaseous mixture of sodium, hydrogensulfide, and argon. A total of 100 a-type rotational transitions with Ka<10 in its ground state is measured in the frequency range 230-395 GHz. The spectrum of an isotopic species, NaSD, is also observed to confirm our identification of the species. The observed spectral lines are analyzed with a Hamiltonian for asymmetric top molecules, and the rotational, centrifugal distortion, and higher-order centrifugal distortion parameters for NaSH and NaSD are accurately determined. The r0 structure of NaSH is derived from the observed rotational constants of NaSH and NaSD: r0(Na—S) = 2.478(2) Å, r0(S—H) = 1.396(7) Å, bond angle Na—S—H = 92(1) degrees, with 1 s.d. values in parentheses. The observed molecular constants produce accurate transition frequencies for NaSH in the millimeter-wavelength region and thereby enable the searches for these lines in interstellar and circumstellar sources with radio telescopes.

Fourier-Transform Microwave Spectroscopy of the Argon–Diacetylene van der Waals Complex

The rotational spectrum of the argon–diacetylene van der Waals complex, produced in a supersonic molecular beam at 1 K, has been observed with a Fourier-transform microwave spectrometer. We observed 22a-type rotational transitions withKaup to 3. Three rotational constants, five centrifugal distortion constants, and one higher-order centrifugal distortion constant were determined precisely by least-squares analysis. The complex is shown to have a planer T-shaped structure withC2vsymmetry. The structural analysis provides that the Ar atom is located 3.68 Å from the center of mass of diacetylene. Force constants for the van der Waals vibrations were determined from the centrifugal distortion constants. It has been found that this complex has a much steeper and more harmonic intermolecular potential than the argon–acetylene complex.

Fourier transform microwave study on 2-methyloxetane and 3-methyloxetane

Abstract The rotational spectra of 2-methyloxetane and 3-methyloxetane have been reinvestigated using Fourier transform microwave spectroscopy (FTMW). A,E splittings due to internal rotation of the methyl group have been observed in the ground and several ring-puckering and methyl torsion excited states for both molecules. Internal rotation barriers of V 3 = 3.332(14) kcal mol −1 (13.941(58) kJ mol −1 ) for 2-methyloxetane and V 3 = 3.284(12) kcal mol −1 (13.740(50) kJ mol −1 ) for 3-methyloxetane have been determined from the ground vibrational state spectra. Furthermore, the rotational, quartic and some higher-order centrifugal distortion constants are reported. The experimental set-up of a waveguide Fourier transform microwave spectrometer is also presented.

Fitting the Rovibrational Spectra with an Effective Rotational–Large Amplitude Motion Hamiltonian

A general method of solution for the rovibrational problem for a single large amplitude motion coupled to the overall rotation is presented. The numerical construction of the effective rovibrational Hamiltonian is applicable to any semirigid or flexible model for a large amplitude motion. High-order centrifugal distortion and Coriolis interaction terms generated by the small amplitude vibrations of normal modes are effectively included into the Hamiltonian. The method is illustrated by performing a global fit of the rotational transitions observed in the ground and excited states of some typical large amplitude modes.

Hyperfine structure in high spin multiplicity electronic states: Analysis of the B 4Π–X 4Σ− transition of gaseous NbO

The (0,0) band of the B 4Π–X 4Σ− transition of NbO, near 6600 Å, has been analyzed from spectra taken at sub-Doppler resolution. The transition is notable for the great width of its Nb nuclear hyperfine structure, which is caused principally by the unpaired 5sσ electron in the ground state interacting with the large magnetic moment of the 4193Nb nucleus (I=9/2). A fit to the ground-state combination differences, including four very precise microwave lines measured by Suenram et al. [J. Mol. Spectrosc. 148, 114 (1991)], has given a comprehensive set of rotational, spin, and hyperfine parameters. Prominent among these are the third-order spin–orbit distortions of the spin-rotation interaction and the Fermi contact interaction, which are large and well determined, reflecting different degrees of spin–orbit contamination of the the 4Σ1/2− and 4Σ3/2− components of the ground state. The δ 2π B 4Π state was hard to fit, for a number of reasons. First, its spin–orbit structure is asymmetric, because of strong perturbations by a 2Π state which has been identified in this work, from among the various weak bands in the NbO spectrum near 7000 Å; the result is that many high order centrifugal distortion terms are needed in an effective Hamiltonian model for the rotation. Second, the hyperfine structure is perturbed, not only by this 2Π state, but by distant Σ and Δ states at higher energy. The δ 2σ* C 4Σ− state at 21 350 cm−1 appears to be one of these. The distant states generate large apparent nuclear spin-rotation interactions, both within and between the Λ components of the Π state, as a result of cross terms between matrix elements of the operators −2BJ⋅L and aI⋅L. Similar cross terms arising from the operators AL⋅S and aI⋅L produce corrections to the Fermi contact matrix elements and are responsible for the unexpected negative sign of the magnetic hyperfine parameter d. The ‘‘off-diagonal’’ quadrupole parameter e2Qq2 is very large, and causes some of the higher J line shapes of the B–X system to be noticeably asymmetric at Doppler limited resolution; its value is consistent with the electron configuration of the B 4Π state being δ 2π.

Infrared and microwave study of angular–radial coupling effects in Ar–HCN

Microwave and infrared spectra of Ar–HCN have been obtained using an electric-resonance optothermal spectrometer. The microwave measurements extend to higher J the previous results of Leopold et al. and Klots et al., allowing the determination of higher-order centrifugal distortion constants for this quasilinear, highly nonrigid complex. A Padé approximant fit to the microwave data indicates a significant rotation-induced asymptotic increase in the zero-point center-of-mass separation between the Ar and the HCN, above that expected from pure radial distortion. This results from the large coupling between the angular and radial degrees in the intermolecular potential forcing the centrifugal alignment of the HCN. Infrared spectra are reported for the C–H streching fundamental ν1 and the combination band ν1+ν15, where ν5 is the van der Waals bending vibration. The band-origin difference between these two bands gives ν5=7.8 cm−1, in rough agreement with the 10 cm−1 harmonic value predicted from the microwave-determined nuclear quadrupole coupling constant. The complexation-induced red shift of the C–H stretching vibration is 2.69 cm−1 and the vibrational predissociation linewidths Γ are &amp;lt;10 MHz (FWHM). The vibrationally excited complex predissociates before striking the bolometer detector, implying that the predissociation lifetime τ&amp;lt;1 ms.

Infrared diode laser kinetic spectroscopy of the ν3 band of C3

Infrared diode laser kinetic spectroscopy was applied to reaction intermediates generated by the photolysis of diacetylene and allene at 193 nm. A strong band was observed around 2040 cm−1 and was assigned to the ν3 band of C3 . The observed spectrum was analyzed by using an energy level expression for a linear molecule; higher-order centrifugal distortion terms were necessary to be included in the least-squares fit. The band origin was determined to be 2040.0198 (8) cm−1. The rotational and centrifugal distortion constants obtained suggest the presence of a potential barrier at the linear configuration in both the ground and ν3 states.

Analysis of the B2Σ + ∼ A2Π i perturbations in the CN( B2Σ +- X2Σ +) main band system : I. Molecular constants for B2Σ + and A2Π i

The CN( B 2Σ +- X 2Σ +) main band transitions (0 ≤ v′ ≤ 9) observed by H. S. Uhler and R. A. Patterson were assigned, and a number of local perturbations between the B 2Σ +, v = 0–9 and A 2Π i , v = 11–24 levels occurring at 30 ≲ J′ ≲ 140 were analyzed. The molecular constants for the B 2Σ + and A 2Π i states including the higher-order centrifugal distortion constants, E v , B v , D v , H v , Δ L v , Δ M v , and Δ N v , were determined, where Δ represents the difference between the constants for the B 2Σ + and X 2Σ + states. The CN( B 2Σ +- X 2Σ +) 0-0 band observed by R. Engleman was also reanalyzed, and the perturbation constants for the B 2Σ +, v = 0 ∼ A 2Π i , v = 10 perturbation were redetermined. The electronic terms of the interaction matrix elements, H el SO and H el RE, determined in the present study were found to be nearly constant and in good agreement with those determined in our previous analysis of the CN( B 2Σ +- X 2Σ +) system.

The millimeter wave spectrum of cyanoallene, CH2CCHCN, using a new digital lock-in technique

The ground state rotational spectrum of cyanoallene, CH2CCHCN, has been observed in the frequency region 87–178 GHz. Over 210 a-type and 10 b-type transitions have been identified. The rotational constants and all quartic, sextic, and several higher-order centrifugal distortion constants have been determined using Watson's S-reduced Hamiltonian. The millimeter wave spectra were recorded with the frequency-modulated output of a phase-locked klystron source, followed by a frequency multiplier. The detector signal was demodulated with a newly constructed digital lock-in amplifier.

Heterodyne frequency measurements on carbonyl sulfide near 1050 cm −1

Heterodyne techniques have been used to measure the frequency differences between carbonyl sulfide (OCS) absorption lines and CO 2 laser transitions. A tunable diode laser was used both to scan the OCS absorption spectrum and to provide a beat signal against a CO 2 laser. Frequency differences as great as 8.6 GHz were measured. Many different OCS hot-band transitions were measured near 1050 cm −1, and the measurements on the 02 00-00 00 band have been extended to such high J levels ( J′ = 86) that higher-order centrifugal distortion terms are needed to fit the data.

Higher-order centrifugal distortion effects in ONCl

One hundred and nine new rotational transitions of ONCl in the ground vibrational state have been measured. A centrifugal distortion analysis of these transitions yields the values of six sextic and one octic centrifugal distortion constants. A comparison between experimental and calculated sextic distortion constants is reported.

The effect of centrifugal distortion on CO&amp;lt;inf&amp;gt;2&amp;lt;/inf&amp;gt;-laser frequencies

An analysis of the effects of higher order centrifugal distortion terms on C12O 2 16-laser frequencies is presented. This analysis, coupled with recent high-accuracy measurements of C12O 2 16-laser beat frequencies by Petersen et al. at the National Bureau of Standards, points out the weakness of existing frequency tables for high J lines. Expected values for both H and L centrifugal distortion coefficients are calculated, and J values for which these terms become important are discussed.