Free-jet rotational spectrum and tunneling motion of difluoromethane...xenon

The rotational spectra of three isotopologues of difluoromethane...xenon have been investigated by free-jet millimeter-wave absorption spectroscopy. Only mu(c)-type transitions have been observed, all of them evenly split due to the internal motion of Xe relative to the difluoromethane moiety. The vibrational splitting, 39.1(3) MHz, has been used to estimate the tunneling barrier, V(2) = 109 cm(-1). Information on the dissociation energy has been deduced from centrifugal distortion effects (E(B) = 1.8 kJ mol(-1)). The xenon atom lies in the sigma(v) symmetry plane of difluoromethane containing the hydrogen atoms, at an r(0) distance of 3.816 A from its center of mass (cm), and forms a Xe-cm-C r(0) angle of 118 degrees . The observed conformation is in agreement with the minimum found with a distributed polarizability model.     

N-Methyl inversion and structure of six-membered heterocyclic rings: rotational spectrum of 1-methyl-4-piperidone

The conformational and structural properties of the six-membered heterocyclic ring of 1-methyl-4-piperidone have been observed in a jet-cooled supersonic expansion using Fourier transform microwave spectroscopy (FT-MW). The rotational spectrum evidenced two different conformations originated by nitrogen inversion, with the N-methyl group in either equatorial (most stable) or axial position. Additional observation of the rotational spectra for all possible carbon, nitrogen, and oxygen monosubstituted species (4 × (13)C, (15)N, (18)O) in natural abundance allowed us to determine substitution (r(s)) and effective structures (r(0)) for the equatorial conformer. Additional ab initio and DFT calculations provided comparative rotational parameters, structural data, conformational energies, and the axial-equatorial interconversion barrier. The structural data were compared with the related azabicycle of tropinone, revealing the molecular changes and structural relaxation associated with the presence of the two-carbon bridge in the latter molecule.     

The spectroscopic characterization of the methoxy radical. III. Rotationally resolved Ã2A1-X̃2E electronic and X̃2E submillimeter wave spectra of partially deuterated CH2DO and CHD2O radicals

Rotationally resolved laser induced fluorescence and stimulated emission pumping A?(2)A(1)-X?(2)E spectra, along with pure rotational spectra in the 153-263 GHz region within the E(3/2) component of the ground state in asymmetrically deuterated methoxy radicals CH(2)DO and CHD(2)O have been observed. The combined data set allows for the direct measurement with high precision of the energy separation between the E(1/2) and E(3/2) components of the ground state and the energy separation between the parity stacks in the E(3/2) component of the ground state. The experimentally observed frequencies in both isotopologues are fit to an effective rotational Hamiltonian accounting for rotational and spin-rotational effects arising in a near-prolate asymmetric top molecule with dynamic Jahn-Teller distortion. Isotopic dependencies for the molecular parameters have been successfully implemented to aid the analysis of these very complex spectra. The analysis of the first and second order contributions to the effective values of molecular parameters has been extended to elucidate the physical significance of resulting molecular parameters. Comparisons of measured parameters, e.g., spin-orbit coupling, rotational and spin-rotation constants, are made among the 5 methoxy isotopologues for which data is now available. Comparisons of experimental results, including the derived geometric structure at both the C(3v) conical intersection and at the Jahn-Teller distorted minima, are made with quantum chemistry calculations.     

High-resolution infrared spectrum of the nu1 Ba nd of eta5-C5H5NiNO

Gas-phase rotational constants and distortion constants have been determined for the nu1 (v=1) excited vibrational state of cyclopentadienylnickel nitrosyl (C5H5NiNO) using a high-resolution Fourier transform spectrometer system at Kitt Peak, Arizona. The rotationally resolved lines have been measured for the C-H symmetric stretch vibration (nu1=3110 cm(-1)). In the present analysis, over 150 lines have been assigned and fitted using a rigid-rotor Hamiltonian with centrifugal distortion. The vibrational band center, excited-state rotational constants, and distortion constants derived from the measured spectrum for this prolate symmetric-top molecule are nuo=3110.4129(4) cm(-1), A'=0.14328(8) cm(-1), B'=C'=0.041285(1) cm(-1), DJ'=0.078(1) kHz, DJK'=2.23(4) kHz, and DK'=-2.63(2) kHz, respectively. Several different combination differences, with a common upper state, were calculated for different K stacks for the observed spectra, and the consistency of the lower state rotational constants obtained provided further support for the current assignment. The ground-state rotational constant (B') derived from this combination differences analysis agrees with the previously obtained Fourier transform microwave value to within 0.15%. However, ground-state rotational constants, A' and B', have been fixed in the present analysis to avoid correlation effects and to get more accurate results. The new measured parameters are compared with the previously obtained results from Fourier transform microwave and infrared spectroscopy measurements. The C-H vibration stretching frequency and rotational constants were calculated using density functional theory calculations, and these were quite helpful in resolving ambiguities in the fitting procedure and for initial assignments of measured lines.     

The 4051-Angstroms band of C3 (A 1Piu-X 1Sigmag +, 000-000): perturbed low-J lines and lifetime measurements

Rotational analyses have been carried out at high resolution for the 000-000 and 000-100 bands of the A (1)Pi(u)-X (1)Sigma(g) (+) transition of supersonic jet-cooled C(3). Two different spectra have been recorded for each band, using time gatings of 20-150 and 800-2300 ns. At the shorter time delay the spectra show only the lines observed by many previous workers. At the longer time delay many extra lines appear, some of which have been observed previously by [McCall et al.Chem. Phys. Lett. 374, 583 (2003)] in cavity ring-down spectra of jet-cooled C(3). Detailed analysis of these extra lines shows that at least two long-lived states perturb the A (1)Pi(u), 000 state. One of these appears to be a (3)Sigma(u) (-) vibronic state, which may possibly be a high vibrational level of the b (3)Pi(g) state, and the other appears to be a P = 1 state with a low rotational constant B. Our spectra also confirm the reassignment by McCall et al. of the R(0) line of the 000-000 band, which is consistent with the spectra recorded towards a number of stars that indicate the presence of C(3) in the interstellar medium. Fluorescence lifetimes have been measured for a number of upper-state rotational levels. The rotational levels of the A (1)Pi(u) state have lifetimes in the range of 230-190 ns, decreasing slightly with J; the levels of the perturbing states have much longer lifetimes, with some of them showing biexponential decays. An improved value has been obtained for the nu(1) vibrational frequency of the ground state, nu(1) = 1224.4933 +/- 0.0029 cm(-1).     

Isotopic and internal CX3 (X = D,H) rotational motion effects in the Ba...FCX3 + h nu --> BaF + CX3 intracluster reactions

Photodepletion and action spectra of the laser-induced Ba...FCD3 fragmentation have been measured over the 16 075-16 380 cm(-1) range. The observed band and peak structures allowed us to estimate the vibrational and rotational structures of the excited complex at the transition state configuration. The relative reaction probability P(R)(E) for the intracluster Ba...FCD3 + h nu --> BaF + CD3 reaction has been determined over the cited energy range. P(R)(E) shows a peak structure with an energy spacing of 8.9 cm(-1) which was attributed to an internal rotation of the CD3 group in the intermediate state. A comparison with previous Ba...FCH3 photofragmentation spectra reveals the dynamical role of the internal CX3 (X = H,D) motion which is manifested by the presence of rotational resonances in the laser-induced intracluster reaction.     

Infrared, and Raman spectra, conformational stability, normal coordinate analysis, ab initio calculations, and vibrational assignment of 1-chlorosilacyclobutane

The infrared spectra (3500–40 cm⁻¹) of gaseous and solid and the Raman spectra (3500–30 cm⁻¹) of liquid and solid 1-chlorosilacyclobutane, c-C₃H₆SiClH, have been obtained. Both the axial and equatorial conformers with respect to the chlorine atom have been identified in the fluid phases. Variable temperature (−105 to −150°C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 211±17 cm⁻¹ (2.53±0.21 kJ/mol), with the equatorial conformer being the more stable form and the only conformer remaining in the annealed solid. At ambient temperatures, approximately 26% of the axial conformers are present in the vapor phase. A complete vibrational assignment is proposed for the equatorial conformer, and many of the fundamentals of the axial conformers have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been determined for both rotamers by ab initio calculations employing the 6-31G(d) basis set at the levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. Structural parameters have also been obtained using MP2/6-311+G(d,p) ab initio calculations. The r₀ parameters for both conformers are obtained from a combination of the ab initio predicted values and the twelve previously reported microwave rotational constants. The results are discussed and compared to those obtained for some similar molecules.     

Conformational and structural studies of 1-fluorosilacyclobutane from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

The infrared spectra (3500 to 40 cm⁻¹) of gaseous and solid and the Raman spectra (3500 to 30 cm⁻¹) of liquid and solid 1-fluorosilacyclobutane, c-C₃H₆SiFH, have been obtained. Both the axial and equatorial conformers with respect to the fluorine atom have been identified in the fluid phases. Variable temperature (−105 to −150 °C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 282 ± 27 cm⁻¹ (3.37 ± 0.32 kJ/mol), with the equatorial conformer the more stable form and the only conformer remaining in the annealed solid. At ambient temperature there is approximately 21 ± 2% of the axial conformer present in the vapor phase. From isolated Si–H stretching frequencies the Si–H (r₀) distances are calculated to be 1.484 and 1.485 Å for the equatorial and axial conformers, respectively. Structural parameters have been predicted from MP2/6-311 + G(d,p) ab initio calculations and the adjusted r₀ parameters for both conformers were obtained from a combination of the ab initio predicted values and the six previously reported microwave rotational constants. Along with the Si–H bond distance, the Si–C, and C–C distances of 1.865(5), and 1.571(5) Å, respectively, for the equatorial conformer are significantly different from the values for these parameters previously reported from an election diffraction study. Both the SiC and CC distances and the SiF distance are longer by 0.002 and 0.004 Å, respectively, for the axial conformer. Structural parameters have also been obtained for silacyclobutane, c-C₃H₆SiH₂ and ethylsilylfluoride, CH₃CH₂SiH₂F, from combined ab initio predicted values and previously reported rotational constants. Several of these newly determined parameters are significantly different from those previously reported for both molecules. Complete equilibrium geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been determined for both rotamers by ab initio calculations employing the 6-31G(d) basis set at the level of Moller–Plesset (MP) to second order. A complete vibrational assignment supported by normal coordinate calculations is proposed for the equatorial conformer, and several of the fundamentals of the axial conformer have also been identified. The results are discussed and compared to corresponding quantities for some similar molecules.     

1 Pi<--X1 Sigma+ band systems of jet-cooled ScCo and YCo

Rotationally resolved resonant two-photon ionization (R2PI) spectra of ScCo and YCo are reported. The measured spectra reveal that these molecules possess ground electronic states of (1)Sigma(+) symmetry, as previously found in the isoelectronic Cr(2) and CrMo molecules. The ground state rotational constants for ScCo and YCo are B(0)(")=0.201 31(22) cm(-1) and B(0) (")=0.120 96(10) cm(-1), corresponding to ground state bond lengths of r(0) (")=1.812 1(10) A and r(0) (")=1.983 0(8) A, respectively. A single electronic band system, assigned as a (1)Pi<--X (1)Sigma(+) transition, has been identified in both molecules. In ScCo, the (1)Pi state is characterized by T(0)=15,428.8, omega(e)(')=246.7, and omega(e)(')x(e)(')=0.73 cm(-1). In YCo, the (1)Pi state has T(0)=13 951.3, omega(e)(')=231.3, and omega(e)(')x(e) (')=2.27 cm(-1). For YCo, hot bands originating from levels up to v(")=3 are observed, allowing the ground state vibrational constants omega(e)(")=369.8, omega(e)(")x(e)(")=1.47, and Delta G(12)(")=365.7 cm(-1) to be deduced. The bond energy of ScCo has been measured as 2.45 eV from the onset of predissociation in a congested vibronic spectrum. A comparison of the chemical bonding in these molecules to related molecules is presented.     

Pure rotational spectra of LuF and LuCl

The pure rotational spectra of two isotopic species of LuF and three of LuCl have been measured in the frequency range 5-17 GHz using a cavity pulsed jet Fourier transform microwave spectrometer. The samples were prepared by laser ablation of Lu metal in the presence of SF(6) or Cl(2), and stabilized in supersonic jets of Ar. Spectra of molecules in states having v= 0, 1, and 2 have been measured, to produce rotational constants and centrifugal distortion constants, along with hyperfine constants for all the nuclei. Dunham-type fits for LuCl produced a Born-Oppenheimer breakdown parameter for Cl. Although a theoretical calculation showed that Lu in LuCl should have a significant field shift effect parameter, it could not be determined from the spectrum. Equilibrium internuclear distances, r(e), and dissociation energies have been evaluated for both molecules. The nuclear quadrupole coupling constants are discussed in terms of the molecular electronic structure.     

Interactions between organic molecules and water: rotational spectrum of the 1:1 oxetane-water complex

The 1:1 molecular complex between oxetane and water has been investigated by using free-jet millimeter-wave spectroscopy. The rotational spectra of five isotopomers (with H(2)O, D(2)O, DOH, HOD and H(2) (18)O) have been assigned. Partial r(0) and r(s) structures of the complex have been derived. The water moiety lies in the plane of symmetry of oxetane, with the "free" hydrogen E with respect to the ring. The oxetane ring appears to be slightly nonplanar, with the C(beta) carbon tilted on the opposite side of the water unity. The three atoms involved in the hydrogen bond adopt a linear arrangement with an O(ring).H distance of about 1.86 A, and the angle between the COC bisector and the O(ring).H bond being congruent with 106 degrees. Additionally, quantum-chemical calculations for the complex were performed and were found to be in agreement with the experimental results.     

Circular dichroism of cobalt(III) complexes: dependence on the outer sphere configuration

The single crystal axial CD of three Co(III) completes has been measured between 295 and 9 K. For Λ-Co (sepulchrate) (NO₃)₃ the rotational strength R(¹T₁(E) ← ¹A₁) decreases from ≈11 × 10^?40 c.g.s. units at 295 K to ≈0 at 80 K. The main features of the single crystal CD spectra can be explained by a static coupling model of optical activity.     

E.p.r. studies on dichloromono(1-phenylamidino-o-alkylurea)copper(II) complexes. Evidence for field induced partial ordering in the solid state and some unusual features in solution

E.p.r. and optical absorption studies have been conducted on dichloromono(1-phenylamidino-o-alkylurea)copper(II) complexes (Alkyl = Me, Et, Pr, Bu or Pe). The e.p.r. spectra of solids, recorded at 300 K, confirmed the square-planar geometry with the unpaired electron in the dx ²–y ² orbital of copper. Magnetic field-induced partial molecular alignment has been observed in some of the polycrystalline samples when cooled in a magnetic field of 1 T at 77 K. E.p.r. spectra at 77 K in pyridine and DMF has shown axial ligation of solvent molecules (pyridine and DMF), whereas in MeOH and DMSO at least three structurally different Cu^II species have been identified. These features are consistent with differences in electronic absorption spectra in the powder and in solution.     

Spectroscopic and theoretical studies on axial coordination of bis(pyrrol-2-ylmethyleneamine)phenyl complexes

Metal (M=Zn(II), Ni(II), Cu(II)) complexes with tetradentate Schiff base ligand, bis(pyrrol-2-ylmethyleneamine)phenyl, has been synthesized and characterized by elemental analyses, (1)H NMR, mass spectra and UV-vis spectra. The standard association constants (K(theta)) and the thermodynamic parameters (Delta(r)H(m)(theta),Delta(r)S(m)(theta),Delta(r)G(m)(theta)) for axial coordination of imidazole derivatives with these Shiff base complexes were measured with UV-vis spectrophotometric titration. The decrease of enthalpy is found to be the drive of the axial coordination. Our Schiff base complexes can incorporate two axial ligands, except 2-Et-4-MeIm with two big substituents of great steric bulk according to stoichiometry of 1:1. ZnL displays high selectively binding to imidazole due to the steric bulk effect. Supporting density functional theory (DFT) calculations have been undertaken on B3LYP/6-31G(d) level.     

Structure, dipole moment and large amplitude motions of 1-benzofuran

The rotational spectrum of 1-benzofuran has been investigated by three different rotational spectroscopy techniques: (i) millimeterwave absorption free jet spectroscopy, useful for a fast assignment of the spectrum; (ii) molecular beam Fourier transform microwave spectroscopy, sensitive to detect less abundant isotopic species in natural abundance; (iii) waveguide conventional microwave spectroscopy, useful for the study of intramolecular dynamics, through the rotational spectra of the vibrational satellites of low energy modes. Besides the rotational spectrum of the ground state of the normal species, the spectra of 9 singly substituted 13C and 18O isotopomers in natural abundance, and of 6 vibrational satellites, have been measured. Precise structural parameters for the molecule, as well as information on the potential energy surface of the low energy vibrations, have been obtained. The dipole moment components have been determined to be micro(a)= 0.216 (2) and micro(b)= 0.720 (3) D, respectively.     

Chirped pulse Fourier transform microwave study of 2,2,2-trifluoroethyl formate

The rotational spectra of 2,2,2-trifluoroethyl formate and its three (13)C isotopologues have been measured with a molecular-beam-based, chirped-pulsed Fourier transform microwave spectrometer in combination with a conventional Balle-Flygare-type instrument up to 18 GHz. Although ab initio calculations predict the presence of two low-energy conformers (analogous to the trans and gauche forms of ethyl formate), the trans isomer was the only stable conformer observed. The r(s) geometry of the molecular main carbon frame was precisely derived based on a coplanar heavy-atom backbone of this conformer. ESPs of the two lowest energy conformers were calculated to obtain information about the role of through-space effects on their structures and relative stability.     

Tautomerism and microsolvation in 2-hydroxypyridine/2-pyridone

The Fourier transform microwave spectra of the hydrated forms of the tautomeric pair 2-pyridinone/2-hydroxypyridine (2PO/2HP) have been investigated in a supersonic expansion. Three hydrated species, 2PO-H?O, 2HP-H?O, and 2PO-(H?O)?, have been observed in the rotational spectrum. Each molecular complex was confidently identified by the features of the 1?N quadrupole hyperfine structure of the rotational transitions. The presence of water affects the tautomeric equilibrium -N═C(OH)- ? -NH-C(═O)-, which is shifted to the enol form for the bare molecules 2PO/2HP but to the keto tautomer for the hydrated forms.     

Conformational stability from variable temperature FT-IR spectra of krypton solutions, r0 structural parameters, vibrational assignment, and ab initio calculations of 4-fluoro-1-butene

Variable temperature (-115 to -155 degrees C) studies of the infrared spectra (3200-400 cm-1) of 4-fluoro-1-butene, CH2=CHCH2CH2F, dissolved in liquid krypton have been carried out. The infrared spectra of the gas and solid as well as the Raman spectra of the gas, liquid, and solid have also been recorded from 3200 to 100 cm-1. From these data, an enthalpy difference of 72 +/- 5 cm-1 (0.86 +/- 0.06 kJ x mol-1) has been determined between the most stable skew-gauche II conformer (the first designation refers to the position of the CH2F group relative to the double bond, and the second designation refers to the relative positions of the fluorine atom to the C-C(=C) bond) and the second most stable skew-trans form. The third most stable conformer is the skew-gauche I with an enthalpy difference of 100 +/- 7 cm-1 (1.20 +/- 0.08 kJ x mol-1) to the most stable form. Larger enthalpy values of 251 +/- 12 cm-1 (3.00 +/- 0.14 kJ x mol-1) and 268 +/- 17 cm-1 (3.21 +/- 0.20 kJ x mol-1) were obtained for the cis-trans and cis-gauche conformers, respectively. From these data and the relative statistical weights of one for the cis-trans conformer and two for all other forms, the following conformer percentages are calculated at 298 K: 36.4 +/- 0.9% skew-gauche II, 25.7 +/- 0.1% skew-trans, 22.5 +/- 0.2% skew-gauche I, 10.0 +/- 0.6% cis-gauche, and 5.4 +/- 0.2% cis-trans. The potential surface describing the conformational interchange has been analyzed and the corresponding two-dimensional Fourier coefficients were obtained. Nearly complete vibrational assignments for the three most stable conformers are proposed and some fundamentals for the cis-trans and the cis-gauche conformers have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared, and Raman intensities have been predicted from ab initio calculations and compared to the experimental values when applicable. The adjusted r0 structural parameters have been determined by combining the ab initio predicted parameters with previously reported rotational constants from the microwave data. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.     

Equatorial and axial hydrogens in heterocyclic six-membered rings: the rotational spectrum of piperazine

The rotational spectrum of piperazine has been investigated by free jet absorption millimeter wave spectroscopy. The spectrum of the polar conformer with axial–equatorial orientations of the two amino hydrogens was only observed. This assignment was confirmed by the spectra of the two monodeuterated and the bideuterated species.Ab initio and density functional calculations predict the observed conformer to have an energy intermediate between the equatorial–equatorial and axial–axial non-polar forms, the former species being the global minimum.     

Vibrational overtone spectroscopy of phenol and its deuterated isotopomers

We have measured the OH- and OD-stretching fundamental and overtone spectra of phenol and its deuterated isotopomers under jet-cooled conditions using nonresonant ionization detection spectroscopy and vapor-phase infrared (IR) and near-infrared (NIR) spectra at room temperature using conventional and photoacoustic spectroscopy. The OH- and OD-stretching bands in the jet-cooled spectra are about 1-10 cm(-1) wide and generally show a few Lorentzian shaped peaks. The bands in the room-temperature spectra have widths of 20-30 cm(-1) and display clear rotational profiles. The band profiles in the jet-cooled spectra arise mostly from nonstatistical intramolecular vibrational redistribution (IVR) with specific coupling to "doorway" states, which are likely to involve CH- and CD-stretching vibrations. The transition dipole moment that determines the rotational structure is found to rotate significantly from the fundamental to the third overtone and is not directed along the OH(D) bond. We use these calculated transition dipole moments to simulate the rotational structure. We determine the rotational temperature in the jet-cooled spectra to be about 0.5 K. Anharmonic oscillator local mode calculations of frequencies and intensities of the OH- and OD-stretching transitions are compared with our measured results. The calculated intensities are in good agreement with the absolute intensities obtained from conventional spectroscopy and with the relative intensities obtained from the room-temperature laser spectroscopy.