Vibrational overtone spectroscopy and intramolecular dynamics of ethene

The first through fourth C-H stretching overtone regions of ethene were measured by photoacoustic spectroscopy of room-temperature molecules and action spectroscopy of jet-cooled molecules. The rotational cooling led to improved resolution in the action spectra, turning these spectra into key players in determining the multiple band appearance in each region, their types, and origins. These manifolds arise from strong couplings of the C-H stretches to doorway states and were analyzed in terms of a simplified joint local-mode/normal-mode (LM)/(NM) model and an equivalent NM model, accounting for principal resonances. The diagonalization of the LM/NM and NM vibrational Hamiltonians and the least-square fittings revealed model parameters, enabling assignment of A- and B-type bands. These bands behave differently through the V = 2-4 manifolds, showing coupling to doorway states for the former but not for the latter. The energy flow out of the fourth C-H overtone is governed by the interaction with bath states due to the increase in the density of states.     

Cavity ring-down spectroscopy of jet-cooled silane isotopologues in the Si-H stretch overtone region

Absorption spectra of silane in the region of the first overtone of the Si-H stretch vibration have been recorded in a seeded supersonic jet expansion by cavity ring-down spectroscopy as well as in a static gas cell at room temperature by photoacoustic spectroscopy. Spectral simplification due to strong rotational cooling in the jet expansion enables us to clearly resolve and assign the rovibrational transitions of the (2000) and (1100) bands of the three isotopologues, (28)SiH(4), (29)SiH(4), and (30)SiH(4), in their natural isotopic abundance. Interconversion between different nuclear spin species of SiH(4) is found to be absent during the jet expansion. Isotope shifts for (29)SiH(4) and (30)SiH(4) relative to (28)SiH(4) are measured and found to be suitable for selective vibrational excitation of any of three silane isotopologues by pulsed laser excitation in a jet expansion.     

Nonstatistical energy flow in the first N–H stretch overtone region of methylamine

Methylamine (CH₃NH₂) molecules, excited to vibrational states lying in the energy window of the first N–H stretch overtone, were studied by room temperature photoacoustic spectroscopy and jet-cooled action spectroscopy. Benefiting from both types of spectra and specifically from the narrowed action spectrum, a multiband structure was revealed. Simulation of the spectral contours allowed retrieving the band origins, band types and transition linewidths. The linewidths indicate that the energy redistribution occurs nonstatistically and that the 2ν₁₀ state (antisymmetric N–H stretch) is relatively longer lived.     

Vibrationally mediated photodissociation of ethene isotopic variants preexcited to the fourth C-H stretch overtone

H and D photofragments produced via vibrationally mediated photodissociation of jet-cooled normal ethene (C2H4), 1,2-trans-d2-ethene (HDCCDH), and 1,1-d2-ethene (CH2CD2), initially excited to the fourth C-H stretch overtone region, were studied for the first time. H and D vibrational action spectra and Doppler profiles were measured. The action spectra include partially resolved features due to rotational cooling, while the monitored room temperature photoacoustic spectra exhibit only a very broad feature in each species. Simulation of the spectral contours allowed determination of the band types and origins, limited precision rotational constants, and linewidths, providing time scales for energy redistribution. The H and D Doppler profiles correspond to low average translational energies and show slight preferential C-H over C-D bond cleavage in the deuterated variants. The propensities toward H photofragments emerge even though the energy flow out of the initially prepared C-H stretch is on a picosecond time scale and the photodissociation occurs following internal conversion, indicating a more effective release of the light H atoms.     

Thermal lens spectroscopy in liquid argon solutions: (Deltav = 6) C-H vibrational overtone absorption of methane

A dual-beam thermal lens technique has been used to obtain the absorption spectrum of the (Deltav = 6) C-H stretch of liquid methane and methane in liquid argon solutions. The lowest concentration detected was 1 x 10(-3) (mole fraction) of CH(4) in liquid Ar with a continuous wave laser power of 20 mW. The thermal lens signal is linear with the mole fraction of methane up to 1 x 10(-2) but not for higher concentrations. Considering the system CH(4)-Ar as an ideal solution, the factors that contribute to the thermal lens signal were calculated as a function of the concentration of methane. A mechanism of energy transfer based on the gas-phase results could explain qualitatively the dependence of the magnitude of the signal on the mole fraction of methane.     

Infrared action spectroscopy of the OD stretch fundamental and overtone transitions of the DOOO radical

The DOOO radical has been produced by three-body association between OD and O2 in a supersonic free-jet expansion and investigated using action spectroscopy, an IR-UV double-resonance technique. Partially rotationally structured bands observed at 2635.06 and 5182.42 cm(-1) are assigned to the OD stretch fundamental (nu(OD)) and overtone (2nu(OD)), respectively, of the trans-DOOO radical. Unstructured bands observed in both spectral regions are assigned to cis-DOOO. Nascent OD X(2)Pi product state distributions following vibrational predissociation appear to be nearly statistical with respect to the degree of rotational excitation, but display a marked propensity for Pi(A') Lambda-doublets, which is interpreted as a signature of a planar dissociation. The energetically highest open OD X(2)Pi product channel implies an upper limit dissociation energy D0 < or = 1856 cm(-1) or 5.31 kcal mol(-1). This value allows refinement of the upper limit D0 of the atmospherically important HOOO isotopomer, suggesting that it is marginally less stable than previously thought.     

Usual conversion of phenylthioacetylene into 1,2-di(phenylthio)-ethene in the KOH-DMSO-cyclohexanone oxime system

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, p. 1702, July, 1989.     

Infrared overtone spectroscopy and unimolecular decay dynamics of peroxynitrous acid

Peroxynitrous acid (HOONO) is generated in a pulsed supersonic expansion through recombination of photolytically generated OH and NO(2) radicals. A rotationally resolved infrared action spectrum of HOONO is obtained in the OH overtone region at 6971.351(4) cm(-1) (origin), providing definitive spectroscopic identification of the trans-perp (tp) conformer of HOONO. Analysis of the rotational band structure yields rotational constants for the near prolate asymmetric top, the ratio of the a-type to c-type components of the transition dipole moment for the hybrid band, and a homogeneous linewidth arising from intramolecular vibrational energy redistribution and/or dissociation. The quantum state distribution of the OH (nu=0,J(OH)) products from dissociation is well characterized by a microcanonical statistical distribution constrained only by the energy available to products, 1304+/-38 cm(-1). This yields a 5667+/-38 cm(-1) [16.2(1) kcal mol(-1)] binding energy for tp-HOONO. An equivalent available energy and corresponding binding energy are obtained from the highest observed OH product state. Complementary high level ab initio calculations are carried out in conjunction with second-order vibrational perturbation theory to predict the spectroscopic observables associated with the OH overtone transition of tp-HOONO including its vibrational frequency, rotational constants, and transition dipole moment. The same approach is used to compute frequencies and intensities of multiple quantum transitions that aid in the assignment of weaker features observed in the OH overtone region, in particular, a combination band of tp-HOONO involving the HOON torsional mode.     

The intramolecular dynamics of allene in the region around 6000 cm via eigenstate resolved IR spectroscopy

We have recorded and analyzed the molecular beam spectra of allene in the regions of the ν₁ + ν₅ and 2 ν₈ bands around 5947.5 and 6135.5 cm⁻¹, respectively. The ν₁ + ν₅ band only shows minor perturbations and we suspect the presence of a doorway state that causes parallel Coriolis coupling to the bath states. Perpendicular Coriolis interactions do not seem to play an important role since the size of the matrix elements does not increase systematically with J′. The spectrum in the region of the 2 ν₈ band is more complicated; a total of six sub-bands has been identified with K = 0–2. Based on the lack of any systematic dependence on J′ and an inverse dependence of the coupling on K, we expect that neither parallel nor perpendicular Coriolis coupling is present in this band. The effective lifetime for both bands is calculated to be about 200 ps, which is very similar to the lifetimes of an acetylenic C---H stretch overtone.     

Vibrational spectroscopy and intramolecular dynamics of 1-butyne

Photodissociation of jet-cooled vibrationally excited 1-butyne, C(2)H(5)C[Triple Bond]C[Single Bond]H, coupled with mass spectrometric detection of H photofragments, facilitated measurements of action spectra and Doppler profiles, expressing the yield of the ensuing fragments versus the vibrational excitation and UV probe lasers, respectively. Both the action spectra and the simultaneously measured room temperature photoacoustic spectra in the 2nu(1), 3nu(1), and 4nu(1) C[Single Bond]H acetylenic stretch regions exhibit unresolved rotational envelopes with significant narrowing of the former due to temperature-related change in the rotational structure. The narrowing of the action spectrum in the 3nu(1) region exposed a resonance splitting, implying intramolecular vibrational energy redistribution (IVR) time of approximately 1 ps. Asymmetric rotor simulation of the band contours provided the rotational constants and estimates for the homogeneous broadening arising from IVR to the bath vibrational states. The homogenous linewidth of 4nu(1) is anomalously narrower than that of 2nu(1) and 3nu(1), indicating a longer lived 4nu(1) state despite the increasing background state density, suggestive of a lack of low-order resonances or of mode-specific coupling with the bath states. The Doppler profiles indicate that the H photofragments are released with low average translational energies, pointing to an indirect dissociation process occurring after internal conversion (IC) to the ground electronic state or after IC and isomerization to butadiene.     

Synthesis and structural characterization of ruthenium(II) and iron(II) complexes containing 1,2-di-(2-thienyl)-ethene derived ligands as chromophores

A new family of three-legged piano stool structured organometallic compounds containing the η⁵-cyclopentadienylruthenium(II)/iron(II) fragments {M(η⁵-C₅H₅) (DPPE)}⁺, {Ru(η⁵-C₅H₅)(PPh₃)₂}⁺ and {Ru(η⁵-C₅H₅)(TMEDA)}⁺ with coordinated thiophene based chromophores, namely 5-(2-thiophen-2-yl-vinyl)-thiophene-2-carbonitrile (L1) and 5-[2-(5-Nitro-thiophen-2-yl)-vinyl]-thiophene-2-carbonitrile (L2) has been synthesized and fully characterized by ¹H, ¹³C, ³¹P NMR, IR and UV-Vis spectroscopies. Also, electrochemical studies were carried out by cyclic voltammetry and all experimental data are interpreted and compared with related compounds under the scope of NLO properties. Compounds [Ru(η⁵-C₅H₅)(DPPE)(NC(C₄H₂S)C(H)C(H)(C₄H₃S))][CF₃SO₃] (1′Ru) [Fe(η⁵-C₅H₅)(DPPE)(NC(C₄H₂S)C(H)C(H)(C₄H₃S))] [PF₆] (1Fe) and [Ru(η⁵-C₅H₅)(DPPE)(NC(C₄H₂S)C(H)C(H)(C₄H₂S)NO₂)][CF₃SO₃] (4′Ru) were also crystallographically characterized.     

Stereoselective intramolecular 1,3 C-H insertion in Rh(II) carbene reactions

[reaction: see text] 1,3 C-H insertion has been found to be a predominant reaction pathway in the Rh(II)-mediated reaction of beta-tosyl alpha-diazo carbonyl compounds.     

Inter- and intramolecular stereoselective protonation of enols(1,2)

Kinetic control of the stereoselectivity of protonation of enolates and other delocalized species commonly affords the less stable diastereomer as a consequence of the considerable exothermicity and resulting sp(2) transition-state hybridization of the alpha-carbon. Protonation then is from the less hindered face of the enolate. The present study is aimed at reversing this phenomenon by intramolecular delivery of the proton. The approach employed required the synthesis of two enolate precursors, one with a 2-pyridyl group strategically close to the alpha-carbon and the other with a phenyl group in the same location. The synthesis required 15 steps and involved new methodology. Intramolecular proton transfer, reversing the usual stereoselectivity, was successful. The selectivity proved to depend on several factors including the exo versus endo configuration of the diastereomer reacting, the proton donor employed, and the concentration of the proton donor. A kinetic analysis permitted the determination of the relative reaction orders of the protonation on the two faces of the enolate.     

Stereocontrolled concise synthesis of (±)-halosaline through intramolecular C-H amination

Total synthesis of 2-(2-hydroxyalkyl)-piperidine alkaloid (±)-halosaline is described from 7-octen-4-ol using a Rh-catalyzed chemo- and diastereo-selective intramolecular CH amination of sulfamate ester, ring-closing metathesis, and S_N2 displacement reaction of the six-membered ring sulfamidate as the key steps.     

Vibrational overtone spectra of N-H stretches and intramolecular dynamics on the ground and electronically excited states of methylamine

The vibrational pattern and energy flow in the N-H stretch manifolds and the dissociation dynamics of methylamine (CH(3)NH(2)) were investigated via vibrationally mediated photodissociation. Action spectra and Doppler profiles, reflecting the yield of the ensuing H photofragments, versus near infrared/visible vibrational excitation and UV excitation, respectively, were measured. The jet-cooled action spectra and the simultaneously measured room temperature photoacoustic spectra of the first to third N-H stretching overtones exhibit broad features, somewhat narrower in the former, consisting of barely recognized multiple bands. Two phases of fitting of the spectroscopic data were performed. In the first phase, the raw data were analyzed to obtain band positions, types, intensities, and transition linewidths. In the second, the information derived from the first phase was then used as data in a fit to joint local mode/normal mode (LM/NM) and NM Hamiltonian parameters. The derived parameters predicted well band positions and allowed band assignment. The LM/NM Hamiltonian and the extracted Lorentzian linewidths enabled the determination of the initial pathways for energy redistribution and the overall temporal behavior of the N-H stretch and doorway states, as a result of Fermi couplings and interactions with bath states. The results indicate a nonstatistical energy flow in the V=2 manifold region, pointing to the dependence of the coupling on specific low order resonances rather than on the total density of bath states. The Doppler profiles suggest lower average translational energies for the released H photofragments, in particular, for V=3 and 4 as compared to V=1 and 2, implying a change in the mechanism for bond cleavage.     

An experimental value for the B(1u) C-H stretch mode in benzene

We here present experimental infrared spectra on two (C(6)H(6))(C(6)D(6)) benzene dimer isomers in the gas phase. The spectra show that the two benzene molecules in the dimer are symmetrically inequivalent and have distinct IR signatures. One of the two molecules is in a site of low symmetry, which leads to the IR activation of fundamental modes that are IR forbidden by symmetry in the monomer. In the spectra, all four fundamental C-H stretch modes of benzene are observed. Modes in the dimer are shifted up to 3 cm(-1) to the red, compared to the modes that are known for the monomer. For the nu(13) B(1u) C-H stretch fundamental mode of benzene, a first experimental value of 3015(+2) (-5) cm(-1) is determined, in excellent agreement with anharmonic frequency calculations presented here.     

Rovibrational photoionization dynamics of methyl and its isotopomers studied by high-resolution photoionization and photoelectron spectroscopy

High-resolution photoionization and pulsed-field-ionization zero-kinetic-energy photoelectron spectra of CH(3), CH(2)D, CHD(2), and CD(3) have been recorded in the vicinity of the first adiabatic ionization threshold following single-photon excitation from the ground neutral state using a narrow-bandwidth vacuum-ultraviolet laser. The radicals were produced from the precursor molecules methyl-bromide, methyl-iodide, dimethyl-thioether, acetone, and nitromethane by 193 nm excimer photolysis in a quartz capillary and were subsequently cooled to a rotational temperature T(rot) approximately equal to 30 K in a supersonic expansion. Nitromethane was identified as a particularly suitable photolytic precursor of methyl for studies by photoionization and threshold photoelectron spectroscopy. Thanks to the cold rotational temperature reached in the supersonic expansion, the rotational structure of the threshold ionization spectra could be resolved, and the photoionization dynamics investigated. Rydberg series converging on excited rotational levels of CH(3) (+) could be observed in the range of principal quantum number n=30-50, and both rotational autoionization and predissociation were identified as decay processes in the threshold region. The observed photoionization transitions can be understood in the realm of an orbital model for direct ionization but the intensity distributions can only be fully accounted for if the rotational channel interactions mediated by the quadrupole of the cation are considered. Improved values of the adiabatic ionization thresholds were derived for all isotopomers [CH(3): 79 356.2(15) cm(-1), CH(2)D: 79 338.8(15) cm(-1), CHD(2): 79 319.1(15) cm(-1), and CD(3): 79 296.4(15) cm(-1)].     

High resolution spectroscopic study of the first overtone of the N---H stretch and of the fundamentals of the C---H stretches in pyrrole

We have examined the first overtone N---H stretching region and the fundamental C---H stretching region of gas phase pyrrole (C₄H₅N), using high resolution Fourier transform spectra. The first overtone N---H stretch has been rotationally analysed using an asymmetric top model and was found to exhibit two separate perturbations. These perturbations produce line splittings and anomalous intensity patterns in the spectrum which are briefly discussed. Hot band transitions, one of them red-shifted and others likely to overlap the main cold transition are also discussed. Three bands observed around 3100 cm⁻¹ were also rotationally analysed, using a symmetric top Hamiltonian, and assigbed to three of the four closely overlapping fundamentals of the C---H stretch vibrations. Evidence was obtained for the fourth expected C---H fundamental.