Symmetric Top Spectra Research Articles

Microwave rotational spectral study of CH3CN–H2O and Ar–CH3CN

The microwave spectrum of the molecular complex of acetonitrile (CH3CN) with water (H2O) has been studied with a pulsed-beam Fourier Transform Microwave Spectrometer (FTMW) from a gas sample of 1% by volume of CH3CN in Ar flowed over a water sample. The frequency coverage was about 9–22GHz for various isotopomers. The CH3CN–H2O complex exhibited a symmetric top spectrum for two tunneling components, whereby the water subunit tunnels between two equivalent forms. The molecular structure was determined with the aid of spectral studies of isotopically substituted monomers containing 13C, 15N and 18O and mono- and di-deuterated water. In the process of measuring the 13CH3CN–H2O species, we observed some nearby transitions for the Ar–13CH3CN complex. Since the molecular structure of the Ar complex in the literature was not completely determined, we also measured several isotopomers of it to improve the molecular structure. The rotational analyses provide the rotational, centrifugal distortion and nuclear quadrupole coupling constants for all of the isotopomers analyzed.

Molecular geometries of H2S⋯ICF3 and H2O⋯ICF3 characterised by broadband rotational spectroscopy

The rotational spectra of three isotopologues of H(2)S···ICF(3) and four isotopologues of H(2)O···ICF(3) are measured from 7-18 GHz by chirped-pulse Fourier transform microwave spectroscopy. The rotational constant, B(0), centrifugal distortion constants, D(J) and D(JK), and nuclear quadrupole coupling constant of (127)I, χ(aa)(I), are precisely determined for H(2)S···ICF(3) and H(2)O···ICF(3) by fitting observed transitions to the Hamiltonians appropriate to symmetric tops. The measured rotational constants allow determination of the molecular geometries. The C(2) axis of H(2)O/H(2)S intersects the C(3) axis of the CF(3)I sub-unit at the oxygen atom. The lengths of halogen bonds identified between iodine and sulphur, r(S···I), and iodine and oxygen, r(O···I), are determined to be 3.5589(2) Å and 3.0517(18) Å respectively. The angle, φ, between the local C(2) axis of the H(2)S/H(2)O sub-unit and the C(3) axis of CF(3)I is found to be 93.7(2)° in H(2)S···ICF(3) and 34.4(20)° in H(2)O···ICF(3). The observed symmetric top spectra imply nearly free internal rotation of the C(2) axis of the hydrogen sulphide/water unit about the C(3) axis of CF(3)I in each of these complexes. Additional transitions of H(2)(16)O···ICF(3), D(2)(16)O···ICF(3) and H(2)(18)O···ICF(3) can be assigned only using asymmetric top Hamiltonians, suggesting that the effective rigid-rotor fits employed do not completely represent the internal dynamics of H(2)O···ICF(3).

Structures of HCN-Mgn (n=2-6) complexes from rotationally resolved vibrational spectroscopy and ab initio theory.

High-resolution infrared laser spectroscopy has been used to determine the structures of HCN-Mgn complexes formed in helium nanodroplets. The magnesium atoms are first added to the droplets to ensure that the magnesium complexes are preformed before the HCN molecule is added. The vibrational frequencies, structures, and dipole moments of these complexes are found to vary dramatically with cluster size, illustrating the nonadditive nature of the HCN-magnesium interactions. All of the complexes discussed here have the nitrogen end of the HCN pointing towards the magnesium clusters. For Mg3, the HCN binds to the "threefold" site, yielding a symmetric top spectrum. Although the HCN-Mg4 complex also has C3v symmetry, the HCN sits "on-top" of a single magnesium atom. These structures are confirmed by both ab initio calculations and measurements of the dipole moments. Significant charge transfer is observed in the case of HCN-Mg4, indicative of charge donation from the lone pair on the nitrogen of HCN into the lowest unoccupied molecular orbital of the Mg4.

Rotational Spectrum and Structure of the Quinuclidine–Water Complex

The rotational spectrum of the quinuclidine–water complex has been observed in the region 6–20 GHz using a pulsed molecular beam Fourier transform microwave (MB-FTMW) spectrometer. In order to obtain detailed structural information on the complex, spectra of quinuclidine, also called 1-azabicyclo[2.2.2]octane (ABCO), combined with both H2O and H218O were examined. The observation of a symmetric top spectrum is consistent with a complex in which water undergoes internal rotation. Using a reasonable model for the interaction potential, it has been possible to estimate the torsional barrier of water. Centrifugal distortion analysis yields a N–H bond stretching force constant of 12.3 N/m, corresponding to a stretching frequency of 116 cm−1for ABCO–H216O.

The Rotational Spectrum of the Benzene‐Carbonyl Sulfide Complex. A Contribution to the Theory of Internal Rotation with Heavy Tops

AbstractThe rotational spectra of the four isotopomers normal, d1, d6, 18O of the benzene‐carbonyl sulfide complex, C6H6 ‐ OCS, have been analyzed. The interpretation of the rotational spectra as quasi symmetric top spectra has been deduced from the internal rotation theory. Most probably the carbonyl sulfide is perpendicular to the plane of benzene with sulphur towards benzene. The distance between the centers of mass of the partners is 4.42 Å.

Intermolecular dynamics of benzene–rare gas complexes as derived from microwave spectra

The rotational spectra of three benzene–X complexes, where X=20Ne, 129Xe, or 132Xe, and of the benzene-1,3,5-d3–Ar complex have been observed using pulsed nozzle Fourier transform microwave (FTMW) spectroscopy. Rotational transitions assigned in the 8–18 GHz range have been found to match symmetric top spectra. Rotational constants B and centrifugal constants DJ and DJK were determined from the measured frequencies. Intermolecular motions between benzene and the rare gas atom have been modeled with a rovibrational Hamiltonian. The three-dimensional interaction potential has been assumed of a simple form with three adjustable parameters. These parameters, one of which represents the equilibrium distance of the rare gas atom from the plane of benzene, have been adjusted for all benzene–rare gas complexes in a least-squares fit by direct inversion of the observed rotational transition frequencies. From the potential, the force constants and frequencies of the van der Waals vibrations and the binding energies have been deduced for all benzene–rare gas complexes.

Rotational spectra and structures of Rg–C6H6–H2O trimers and the Ne–C6H6 dimer (Rg=Ne, Ar, or Kr)

Rotational spectra of Rg–C6H6–H2O isotopomers, where Rg=Ne, Ar, or Kr, have been observed with a Balle–Flygare Fourier transform microwave spectrometer. In these trimers the benzene is sandwiched between the rare gas and H2O. Isotopic substitution and inertial analyses show that the Rg–C6H6 distance in the trimer is reduced by about 0.01 Å for Ne, Ar, and Kr compared to the corresponding distance in the Rg–C6H6 dimer. On the other hand, the c.m. (C6H6) to c.m. (H2O) distance in the trimers is increased by only about 0.003 Å from its distance in the dimer. Symmetric top spectra of 20Ne–C6H6 and 22Ne–C6H6 were observed as an aid in the comparison. Hyperfine structure (hfs) and substitution analyses with HDO/D2O containing isotopomers reveal that the m=0 and 1 states of H2O in the trimer are virtually unchanged from those in the C6H6–H2O dimer, including essentially free rotation. In addition, analyses are made of the root-mean square (rms) deviation for the benzene C6 axis from the a axis in the dimers (∼19 deg) and trimers (∼21 deg) and of the displacement of the C2 axis of the water from the a axis in the trimers (∼37 deg).

Ab initio studies on the mixed heterodimers of ammonia and hydrogen cyanide

Ab initio molecular structure calculations have been used to examine the association of NH 3 with HCN to form a heterodimer. Because of the potential importance of electron correlation in weakly associated complexes, the structure and energy of the heterodimers and the associated monomers are reported at both Hartree—Fock and Møller—Plesset levels of calculation. Other calculated properties including harmonic vibrational frequencies and rotational constants are also reported for each system. Several stationary points on the potential surface were located and characterized. In the two most stable configurations, the monomer units are associated through a hydrogen bond with HCN acting as the hydrogen donor. The structure predicted to be the most stable has a linear hydrogen bond and C 3v symmetry in accordance with the experimentally observed symmetric top spectrum. The stability of this structure is 5.28 kcal/mol (with MP2FULL/6-31 + G(3df, 2p) and 5.01 kcal/mol with counter-poise correction). The second structure having only C 1 symmetry is identified as a transition state. A third stable hydrogen bonded structure has HCN acting as the hydrogen acceptor and is the most weakly bound of the three.

Sub-Doppler, infrared laser spectroscopy of the propyne 2ν1 band: Evidence of z-axis Coriolis dominated intramolecular state mixing in the acetylenic CH stretch overtone

The eigenstate-resolved 2ν1 (acetylenic CH stretch) absorption spectrum of propane has been observed for J′=0–11 and K=0–3 in a skimmed supersonic molecular beam using optothermal detection. Radiation near 1.5 μm was generated by a color center laser allowing spectra to be obtained with a full-width at half-maximum resolution of 6×10−4 cm−1 (18 MHz). Three distinct characteristics are observed for the perturbations suffered by the optically active (bright) acetylenic CH stretch vibrational state due to vibrational coupling to the nonoptically active (dark) vibrational bath states. (1) The K=0 states are observed to be unperturbed. (2) Approximately 2/3 of the observed K=1–3 transitions are split into 0.02–0.25 cm−1 wide multiplets of two to five lines. These splittings are due to intramolecular coupling of 2ν1 to the near resonant bath states with an average matrix element of 〈V2〉1/2=0.002 cm−1 that appears to grow approximately linearly with K. (3) The K subband origins are observed to be displaced from the positions predicted for a parallel band, symmetric top spectrum. The first two features suggest that the coupling of the bright state to the bath states is dominated by parallel (z-axis) Coriolis coupling. The third suggests a nonresonant coupling (Coriolis or anharmonic) to a perturber, not directly observed in the spectrum, that itself tunes rapidly with K; the latter being the signature of diagonal z-axis Coriolis interactions affecting the perturber. A natural interpretation of these facts is that the coupling between the bright state and the dark states is mediated by a doorway state that is anharmonically coupled to the bright state and z-axis Coriolis coupled to the dark states. Z-axis Coriolis coupling of the doorway state to the bright state can be ruled out since the ν1 normal mode cannot couple to any of the other normal modes by a parallel Coriolis interaction. Based on the range of measured matrix elements and the distribution of the number of perturbations observed we find that the bath levels that couple to 2ν1 do not exhibit Gaussian orthogonal ensemble type statistics but instead show statistics consistent with a Poisson spectrum, suggesting regular, not chaotic, classical dynamics.

Low-J rotational spectra, internal rotation, and structures of several benzene–water dimers

Low J (0–4) rotational transitions have been observed for the benzene–water dimer of which high J (≥4) transitions were reported recently by Blake [Science 257, 942 (1992)]. Our experiments used a modified Balle/Flygare Fourier transform microwave spectrometer, with a pulsed supersonic nozzle as the sample source, and examined a variety of isotopic species in the ground and first excited internal rotor states (m=0 and 1). The dimers of the parent C6H6 benzene with H2O, HDO, D2O, and H218O have symmetric top spectra characteristic of two coaxial rotors with a symmetric top frame and a very low effective V6 barrier. The dimers of H2O and D2O with the 13C and D monosubstituted benzenes have asymmetric top spectra of which only the m=0 state was assigned. However, doublets in the m=1, J=0→1 transitions show that there is a V2 term of ∼0.5 MHz in their barriers. A substitution analysis was made of the rotational constants found for the m=0 state of the dimers with H218O, D2O, and the 13C and D monosubstituted benzenes. It shows that the oxygen is at the a axis of the dimer, well outside (0.48 Å) the hydrogens. However, the C2 axis of the H2O is not coincident with the a axis but is at an angle β of 37° to it, rotated so that the two hydrogens are equivalent. The sixfold axis of the benzene corresponds to the a axis, there is little or no tilt (γ) of the benzene. The c.m. (C6H6) to c.m. (H2O) distance R is 3.329 Å. The closely spaced hyperfine structure from the proton–proton magnetic dipole interaction and the deuterium quadrupole interaction was resolved for several dimers and transitions, principally J=0→1 and 1→2. The results demonstrate effective nuclear equivalence in dimers with H2O and D2O. Also, the symmetries found for their nuclear spin functions correlate with the lowest rotational levels of free water, the m=0 state with 000 and m=1 with 101 and 111. For the m=1, K=0 transitions of C6H6–H2O the correlation is with 111 and for the K=±1, with 101. These assignments are reversed for C6H6–D2O.

Optothermal-detected microwave-sideband CO 2-laser spectroscopy of NCH-NH 3

A microwave-sideband CO 2 laser is used together with an electric-resonance optothermal spectrometer to measure the infrared spectrum of the NH 3 umbrella vibration in NCH-NH 3 at a resolution of ≈ 3 MHz. The infrared radiation is produced by mixing Lamb-dip-stabilized CO 2 laser radiation with synthesizer-derived microwave radiation in a CdTe-buffered GaAs stripline electrooptic waveguide modulator. For NCH-NH 3 a symmetric top spectrum is observed with a band origin at 1041.7 cm −1, blue-shifted ≈ 91.8 cm −1 from the hypothetical inversion-free ν 2 band origin of free NH 3, which indicates a decrease in the van der Waals zero-point binding energy, D 0, for the excited state. The observed Δ B of −14.3 MHz, implying a hydrogen-bond extension, is consistent with this blue shift. The vibrationally excited complex does not predissociate within the ≈ 1 ms transit time between the laser excitation region and the bolometer detector, implying that D 0 is greater than the laser frequency, ≈ 1042 cm −1.

Structure of the benzene–Ar2 cluster from rotationally resolved ultraviolet spectroscopy

Rotationally resolved spectra of the two vibronic bands 610 and 1620 and a vibronic van der Waals band of the benzene–Ar2 cluster are presented, whose vibronic assignments are based on the analysis of their rotational structures. A fit to the rotational line positions in the symmetric top spectra yields an accurate set of rotational constants in the ground and the excited electronic state and the exact values for the band origins of the bands. From these values the spectral shift between corresponding cluster and monomer bands as well as the frequency of the van der Waals symmetric stretching vibration in the excited electronic state are precisely determined. The structure of the cluster is identified to be symmetric with one Ar atom located on the C6 axis on each side of the benzene ring at a distance of 3.58 Å in the S0 state and 3.52 Å in the S1 state. These bond lengths exactly agree with our recent values for benzene–Ar. From the result that the bond lengths are equal for the dimer and the trimer we conclude that there is no Ar–Ar interaction through the intermediate benzene ring plane.

The microwave spectrum and structure of the neon-phosphorus trifluoride complex

Symmetric top spectra were observed for the 20Ne·PF 3 and 22Ne·PF 3 van der Waals dimers using a Fourier-transform microwave spectrometer. The center-of-mass distance between Ne and PF 3 is 3.373(3) Å. The experimental data, in conjunction with the van der Waals radii of the atoms and with ab initio calculations, are consistent with the neon atom on the symmetry axis over the F 3 face of the PF 3. Normal mode analysis of the van der Waals vibrations based on the centrifugal distortion constants yields force constants f R = 0.00647 mdyn Å −1 and f Θ = 0.00464 mdyn Å for the 20Ne·PF 3 isotopic species.

Rotational spectrum and structure of the HCN–(CO2)3 tetramer

Microwave rotational transitions have been observed for HCN–(CO2)3, DCN–(CO2)3, H13CN–(CO2)3, HC15N–(CO2)3, HCN–(13CO2)3, HCN–(18OCO)(CO2)2, and HCN–(CO2)(C18O2)2 with the pulsed Fourier transform, Flygare/Balle Mark II spectrometer. A symmetric top spectrum was observed for the parent isotopic species with rotational constants of B0=861.6392(1) MHz, DJ =0.681(5) kHz, and DJK =0.821(12) kHz. The results for isotopic substitution indicate a zero-point, vibrationally averaged geometry having the C3 symmetry of a cyclic (CO2)3 structure with the HCN along the symmetry axis and the N end closest to the (CO2)3. The C3 symmetry is confirmed by the observation of states limited to K=±3n, with n=0,1,2,..., as predicted for threefold symmetry generated by bosons only. The (CO2)3 has a pinwheel configuration, as in the free trimer, and the three carbons lie in a plane R=2.758 Å below the center of mass (c.m.) of the HCN. The C-C distance in this subunit is 3.797 Å which is 0.241 Å shorter than that found in the free (CO2)3 trimer. The individual CO2’s in the (CO2)3 pinwheel are rotated out of the C–C–C plane by γ=−6.9°, as determined from an inertial analysis, with the inner oxygens rotated away from the HCN. The HCN has an average torsional angle of 10.3°, as determined by isotopic substitution, and an observed χcc value of −3.891 MHz for the 14 N. The c.m.(HCN) to C distance is 3.525 Å, compared to 3.592 Å in the HCN-CO2 T-shaped dimer. The isotopic substitution by 18O perturbs the structure of the symmetric top clusters by a remarkable amount, decreasing γ to −28.9° and increasing R and RCC to 2.797 and 3.814 Å, respectively. In the 18O substituted species, the CO2’s are rotated in the C–C–C plane from C3v symmetry by the pinwheel angle β=∼32.5°.

Rotational spectrum and structure of CF3H–NH3

The rotational spectrum of CF3H–NH3 has been obtained using a pulsed nozzle Fourier transform microwave spectrometer. A symmetric top spectrum is observed that is consistent with free internal rotation of the NH3 subunit against the CF3H subunit. Rotational transitions have been measured for both the ground and first excited internal rotor state of the complex. The spectroscopic constants which have been obtained include: B0=1996.903(2) MHz, DJ =3.46(12) kHz, and eQqN =−3.186(8) MHz. From the quadrupole coupling constant of the nitrogen nucleus, eQqN, the bending amplitude of the NH3 unit is determined to be 22.57(10)°. The hydrogen bond length is 2.314(5) Å and the weak bond stretching force constant is 0.066(2) mdyn/Å. The bond length and stretching force constant for CF3H–NH3 are similar in value to those determined for HCCH–NH3 (2.33 Å and 0.070 mdyn/Å, respectively).

The rotational spectrum and structure of NH3–HCN

The microwave spectrum of H3N–HCN has been measured using the molecular beam electric resonance technique. A symmetric top spectrum is observed and the following spectroscopic constants were obtained::[RW2:B0(MHz):3016.756 1(24)]

High Si [sbnd]H local mode overtones in SiHD 3

Spectra for SiHD/sub 3/ obtained using a nonresonant photoacoustic cell mounted within the cavity of a CR490 tunable CW laser are reported herein. The symmetric top spectra exhibit partial rotational resolution. A relation for determining the Si-H bond distance is reported, and the Si-D bond distance is taken to be the same as the Si-H distance in the ground vibrational state. The bond angle is assumed to remain tetrahedral in both situations. The noted spectral vibrational band widths arise only from rotational structure with contributions from fast vibrational relaxation not being evident. 10 references, 2 figures, 1 table.

ChemInform Abstract: THE SPECTRUM OF ALLENE NEAR 5 μM

Abstract The infrared bands of allene between 1920 and 2028 cm −1 were measured on a large grating spectrometer with effective resolution of 0.006–0.007 cm −1 after deconvolution. The main band in this region is the antisymmetric CC stretching fundamental ν 6 of symmetry B 2 , accompanied on the high-frequency side by the overtone band 2 ν 9 of the degenerate CH 2 rocking vibration. The overtone has three components of species B 2 , B 1 , and A 1 , of which B 2 is in Fermi resonance with ν 6 , B 1 has second-order Coriolis interaction with ν 6 , while A 1 is in l -type interaction with the B 1 and B 2 components. Additional perturbations by other unobserved states were detected in the spectrum. The observed transitions were analyzed with the aid of a general computer program system SYMTOP designed for analysis of symmetric-top spectra. The main program effects a least-squares refinement of the spectroscopic constants for a set of bands whose upper states interact through arbitrary matrix elements. Two sets of spectroscopic constants for the ν 6 , 2 ν 9 states were calculated. One, based only on those lines which appear to be unperturbed by unknown and unobserved states, and a second, obtained after inclusion in the H matrix of two perturbing states designed to simulate the effect of the perturbations on all of the data. Comparison indicates that the lower-order constants based merely on the “unperturbed” data are quite satisfactory. Molecular constants for the ν 9 vibration of C 3 H 4 are reported.

Microwave spectrum of alleged P 4O 7

By microwave detection (26.5–39.5 GHz) of the products of the pyrolysis of methyl-dichlorophosphite, CH 3OPCl 2, the fragments CH 2PCl, HCP, CH 3Cl and HCHO have been readily identified by their already known spectra. In addition, a hitherto unreported symmetric top spectrum has been detected (species XST) with rotational constant B o = 827.863 MHz. After the exclusion of C, H, Cl and Si (wall) as elements of XST, P 4O 7 is suggested as the source of the new spectrum since earlier predicted and experimental structures of P 4O 7 (mostly referring to crystalline P 4O 7) have rotational constants B close to B o = 827.863 MHz. The possibility of XST being P 4O 8 or P 4O 9 is excluded.

The microwave spectra of symmetric top polyacetylenes: 1,3,5-Heptatriyne, CH 3(CC) 3H and 1-cyano-2,4-pentadiyne, CH 3(CC) 2CN

The rotational spectra of 1,3,5-heptatriyne, CH 3(CC) 3H and 1-cyano-2,4-pentadiyne, CH 3(CC) 2CN have been studied in detail between 26.5 and 40.0 GHz. The molecules have long linear chains of heavy atoms and show characteristic C 3 v symmetric top spectra consisting of groups of R-branch lines at regular intervals separated by approximately 2 B 0. Six isotopic modifications of CH 3(CC) 2CN have been detected in natural abundance allowing r s substitution structural data to be derived for this species. Long linear polyacetylenic chains are quite flexible and this dynamic property manifests itself in the appearance of extended sequences of complex vibrational satellites associated with the bending of the chain. The vibrational ground state spectra as well as several low frequency vibrational satellites have been analyzed yielding various vibration-rotation parameters. For CH 3(CC) 3H B 0 = 778.2445 ± 0.0007 and for CH 3(CC) 2CN B 0 = 778.040 ± 0.001 MHz.