Vinylidene Isomerization Research Articles

Collisional Relaxation of Highly Vibrationally Excited Acetylene Mediated by the Vinylidene Isomer.

Collisional relaxation of highly vibrationally excited acetylene, generated from the 193 nm photolysis of vinyl bromide with roughly 23,000 cm-1 of nascent vibrational energy, is studied via submicrosecond time-resolved Fourier transform infrared (FTIR) emission spectroscopy. IR emission from vibrationally hot acetylene during collisional relaxation by helium, neon, argon, and krypton rare-gas colliders is recorded and analyzed to deduce the acetylene energy content as a function of time. The average energy lost per collision, ⟨ΔE⟩, is computed using the Lennard-Jones collision frequency. Two distinct vibrational-to-translational (V-T) energy transfer regimes in terms of the acetylene energy are identified. At vibrational energies below 10,000-14,000 cm-1, energy transfer efficiency increases linearly with molecular energy content and is in line with typical V-T behavior in quantity. In contrast, above 10,000-14,000 cm-1, the V-T energy transfer efficiency displays a dramatic and rapid increase. This increase is nearly coincident with the acetylene-vinylidene isomerization limit, which occurs nearly 15,000 cm-1 above the acetylene zero-point energy. Combined quasi-classical trajectory calculations and Schwartz-Slawsky-Herzfeld-Tanczos theory point to a vinylidene contribution being responsible for the large enhancement. This observation illustrates the influence of energetically accessible structural isomers to greatly enhance the energy transfer rates of highly vibrationally excited molecules.

Accurate quantum mechanical calculations on deuterated vinylidene isomerization.

We present accurate quantum dynamics calculations on vinylidene-d2 (fully deuterated vinylidene) isomerization. This is achieved by large-scale parallel computations with an efficient theoretical scheme developed by us in which basis functions are customized for the deuterium migration process. Mode-specific permutation tunneling splittings of vinylidene-d2 states are reported here for the first time, and evident isotope effects are revealed. The A1 ', B2 ', A1 ″, and B2 ″ symmetric vinylidene-d2 states are calculated, and the peaks recently observed in the cryo-SEVI spectra are analyzed, with very good agreement between theory and experiment achieved for the energy levels. In addition, a few vinylidene-d2 states beyond the experimental energy region are reported. Our studies are helpful for acquiring a better understanding of isotope effects in the double-hydrogen migration processes.

Encoding of vinylidene isomerization in its anion photoelectron spectrum.

Vinylidene-acetylene isomerization is the prototypical example of a 1,2-hydrogen shift, one of the most important classes of isomerization reactions in organic chemistry. This reaction was investigated with quantum state specificity by high-resolution photoelectron spectroscopy of the vinylidene anions H2CC- and D2CC- and quantum dynamics calculations. Peaks in the photoelectron spectra are considerably narrower than in previous work and reveal subtleties in the isomerization dynamics of neutral vinylidene, as well as vibronic coupling with an excited state of vinylidene. Comparison with theory permits assignment of most spectral features to eigenstates dominated by vinylidene character. However, excitation of the ν6 in-plane rocking mode in H2CC results in appreciable tunneling-facilitated mixing with highly vibrationally excited states of acetylene, leading to broadening and/or spectral fine structure that is largely suppressed for analogous vibrational levels of D2CC.

PAH chemistry at eV internal energies. 2. Ring alteration and dissociation

Recognized as important interstellar constituents, polycyclic aromatic hydrocarbons (PAHs) have been intensively studied in astrochemistry and their spectroscopy, thermodynamics, dynamics, and fragmentations are now amply documented. There exists typical alternatives to the ground-state regular planar structures of PAHs, as long as they bear internal energies in the range 1–10eV. Resulting from intramolecular rearrangements, such high-lying minima on the potential-energy surfaces should be taken into consideration in the studies of PAH processing in astrophysical conditions. Resting upon DFT calculations mainly performed on two emblematic PAH representatives, coronene and pyrene, in their neutral and mono- and multi-cationic states, this second survey addresses the following alternatives: (1) opened forms containing ethynyl or 2-butynyl groups, (2) vinylidene isomers, in which phenanthrene patterns are reorganized into dibenzofulvene ones, (3) “twisted” forms, where external CHCH bonds can be partly twisted, and (4) bicyclobutane forms, in which the latter are integrated in saturated bicyclic forms. A few scenarios for elimination of fragments H, H2, C2H2 and C2H4 are explored. As far as possible, familiar concepts of organic chemistry, such as aromaticity or Clar's rules, are invoked for interpretations.

Communication: Observation of local-bender eigenstates in acetylene.

We report the observation of eigenstates that embody large-amplitude, local-bending vibrational motion in acetylene by stimulated emission pumping spectroscopy via vibrational levels of the S1 state involving excitation in the non-totally symmetric bending modes. The N(b) = 14 level, lying at 8971.69 cm(-1) (J = 0), is assigned on the basis of degeneracy due to dynamical symmetry breaking in the local-mode limit. The level pattern for the N(b) = 16 level, lying at 10 218.9 cm(-1), is consistent with expectations for increased separation of ℓ = 0 and 2 vibrational angular momentum components. Increasingly poor agreement between our observations and the predicted positions of these levels highlights the failure of currently available normal mode effective Hamiltonian models to extrapolate to regions of the potential energy surface involving large-amplitude displacement along the acetylene ⇌ vinylidene isomerization coordinate.

Quantum Dynamics of Vinylidene Photodetachment on an Accurate Global Acetylene-Vinylidene Potential Energy Surface.

Vinylidene is a high-energy isomer of acetylene, and the rearrangement of bonds in the two species serves as a prototype for isomerization reactions. Here, a full-dimensional quantum mechanical study of the vinylidene vibration is carried out on a recently developed global acetylene-vinylidene potential energy surface by simulating the photodetachment dynamics of the vinylidene anion. Several low-lying vibrational levels of the anion were first determined on a new ab initio based potential energy surface, and their photoelectron spectra were obtained within the Condon approximation. The vibrational features of the vinylidene isomer are found to agree well with the experiment in both positions and intensities, validating the global acetylene-vinylidene potential energy surface.

Simplified Cartesian basis model for intrapolyad emission intensities in the bent-to-linear electronic transition of acetylene.

The acetylene emission spectrum from the trans-bent electronically excited à state to the linear ground electronic X̃ state has attracted considerable attention because it grants Franck–Condon access to local bending vibrational levels of the X̃ state with large-amplitude motion along the acetylene ⇌ vinylidene isomerization coordinate. For emission from the ground vibrational level of the à state, there is a simplifying set of Franck–Condon propensity rules that gives rise to only one zero-order bright state per conserved vibrational polyad of the X̃ state. Unfortunately, when the upper level involves excitation in the highly admixed ungerade bending modes, ν4′ and ν6′, the simplifying Franck–Condon propensity rule breaks down--as long as the usual polar basis (with v and l quantum numbers) is used to describe the degenerate bending vibrations of the X̃ state--and the intrapolyad intensities result from complicated interference patterns between many zero-order bright states. In this article, we show that, when the degenerate bending levels are instead treated in the Cartesian two-dimensional harmonic oscillator basis (with vx and vy quantum numbers), the propensity for only one zero-order bright state (in the Cartesian basis) is restored, and the intrapolyad intensities are simple to model, as long as corrections are made for anharmonic interactions. As a result of trans ⇌ cis isomerization in the à state, intrapolyad emission patterns from overtones of ν4′ and ν6′ evolve as quanta of trans bend (ν3′) are added, so the emission intensities are not only relevant to the ground-state acetylene ⇌ vinylidene isomerization, they are also a direct reporter of isomerization in the electronically excited state.

Full dimensional Franck-Condon factors for the acetylene à (1)Au-X̃ (1)Σ(g)(+) transition. II. Vibrational overlap factors for levels involving excitation in ungerade modes.

A full-dimensional Franck-Condon calculation has been applied to the à (1)Au-X̃ 1Σg+ transition in acetylene in the harmonic normal mode basis. Details of the calculation are discussed in Part I of this series. To our knowledge, this is the first full-dimensional Franck-Condon calculation on a tetra-atomic molecule undergoing a linear-to-bent geometry change. In the current work, the vibrational intensity factors for levels involving excitation in ungerade vibrational modes are evaluated. Because the Franck-Condon integral accumulates away from the linear geometry, we have been able to treat the out-of-plane component of trans bend (ν4('')) in the linear X̃ state in the rotational part of the problem, restoring the χ Euler angle and the a-axis Eckart conditions. A consequence of the Eckart conditions is that the out-of-plane component of ν4('') does not participate in the vibrational overlap integral. This affects the structure of the coordinate transformation and the symmetry of the vibrational wavefunctions used in the overlap integral, and results in propensity rules involving the bending modes of the X̃ state that were not previously understood. We explain the origin of some of the unexpected propensities observed in IR-UV laser-induced fluorescence spectra, and we calculate emission intensities from bending levels of the à state into bending levels of the X̃ state, using normal bending mode and local bending mode basis sets. Our calculations also reveal Franck-Condon propensities for the Cartesian components of the cis bend (ν5('')), and we predict that the best Ã-state vibrational levels for populating X̃-state levels with large amplitude bending motion localized in a single C-H bond (the acetylene↔vinylidene isomerization coordinate) involve a high degree of excitation in ν6(') (cis-bend). Mode ν4(') (torsion) populates levels with large amplitude counter-rotational motion of the two hydrogen atoms.

Counteranion and Solvent Assistance in Ruthenium-Mediated Alkyne to Vinylidene Isomerizations

The complex [Cp*RuCl((i)Pr2PNHPy)] (1) reacts with 1-alkynes HC≡CR (R = COOMe, C6H4CF3) in dichloromethane furnishing the corresponding vinylidene complexes [Cp*Ru═C═CHR((i)Pr2PNHPy)]Cl (R = COOMe (2a-Cl), C6H4CF3 (2b-Cl)), whereas reaction of 1 with NaBPh4 in MeOH followed by addition of HC≡CR (R = COOMe, C6H4CF3) yields the metastable π-alkyne complexes [Cp*Ru(η(2)-HC≡CR)((i)Pr2PNHPy)][BPh4] (R = COOMe (3a-BPh4), C6H4CF3 (3b-BPh4)). The transformation of 3a-BPh4/3b-BPh4 into their respective vinylidene isomers in dichloromethane is very slow and requires hours to its completion. However, this process is accelerated by addition of LiCl in methanol solution. Reaction of 1 with HC≡CR (R = COOMe, C6H4CF3) in MeOH goes through the intermediacy of the π-alkyne complexes [Cp*Ru(η(2)-HC≡CR)((i)Pr2PNHPy)]Cl (R = COOMe (3a-Cl), C6H4CF3 (3b-Cl)), which rearrange to vinylidenes in minutes, i.e., much faster than their counterparts containing the [BPh4](-) anion. The kinetics of these isomerizations has been studied in solution by NMR. With the help of DFT studies, these observations have been interpreted in terms of chloride- and methanol-assisted hydrogen migrations. Calculations suggest participation of a hydrido-alkynyl intermediate in the process, in which the hydrogen atom can be transferred from the metal to the β-carbon by means of species with weak basic character acting as proton shuttles.

Accurate partition function for acetylene, 12C2H2, and related thermodynamical quantities

The internal partition function (Q(int)) of ethyne (acetylene), (12)C(2)H(2), is calculated by explicit summation of the contribution of all individual vibration-rotation energy levels up to 15,000 cm(-1). The corresponding energies are predicted from a global model and constants reproducing within 3σ all 18,415 published vibration-rotation lines in the literature involving vibrational states up to 8900 cm(-1), as produced by Amyay et al. [J. Mol. Spectrosc. 267, 80 (2011)]. Values of Q(int), with distinct calculations for para and ortho species are provided from 1 to 2000 K, in step of 1 K. The total internal partition function at 298.15 K is 104.224387(47) or 416.89755(19), with the nuclear degeneracy spin factors taken as 1/4:3/4 (astronomer convention) or 1:3 (atmospheric convention), respectively, for para:ortho species. The Helmholtz function, Gibbs enthalpy function, entropy, and specific heat at constant pressure are also calculated over the same temperature range. Accuracies as well as the missing contribution of the vinylidene isomer of acetylene in the calculations are discussed.

Mononuclear and dinuclear iridium and heterodinuclear iridium–rhodium complexes derived from 1,4-C 6H 4(C [tbnd]CSiMe 3) 2 as precursor

The labile iridium(I) precursor trans-[IrCl(C 8H 14)(P iPr 3) 2] ( 2), prepared in situ from [IrCl(C 8H 14) 2] 2 ( 1) and P iPr 3, reacted with equimolar amounts of 1,4-C 6H 4(C CSiMe 3) 2 ( 3) at 60 °C to give the mononuclear vinylidene complex trans-[IrCl( C C(SiMe 3)C 6H 4C CSiMe 3)(PiPr 3) 2] ( 4). From 2 and 3 in the molar ratio of 2:1, the dinuclear compound trans,trans-[(P iPr 3) 2ClIr( C C(SiMe 3)C 6H 4C(SiMe 3) C )IrCl(P iPr 3) 2] ( 5) was obtained. Reaction of 4 with [RhCl(P iPr 3) 2] 2 ( 6) at room temperature afforded the heterodinuclear alkyne(vinylidene) complex trans,trans-[(P iPr 3) 2ClIr( C C(SiMe 3)C 6H 4C CSiMe 3)RhCl(P iPr 3) 2] ( 7), which on heating at 45 °C was converted to the bis(vinylidene) isomer trans,trans-[(P iPr 3) 2ClIr( C C(SiMe 3)C 6H 4C(SiMe 3) C )RhCl(P iPr 3) 2] ( 8).

ChemInform Abstract: Structural Characterization of Alkyne and Vinylidene Isomers of (Ru( C2H2)(PMe2Ph)2(Cp))(BF4).

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Density Functional Theory Investigation into the Mechanism for η2-Alkyne to Vinylidene Isomerization by the Addition of Phenylacetylene to [(η3-C3H5)Rh(PiPr3)2

The mechanism for the formation of the alkynyl, vinylidene complex [(PiPr3)2Rh(CCPh)(CC(H)(Ph))] (2) by the addition of 2 equiv of phenylacetylene to [(η3-C3H5)Rh(PiPr3)2] (1) was studied through DFT calculations. At the B3LYP/DZP level of theory, the formation of 2 is exothermic and exergonic by 25.6 and 18.1 kcal·mol−1, respectively. The experimentally observed η2-phenyacetylene, alkynyl complex [(PiPr3)2Rh(η2-HCCPh)(CCPh)] (Ia), is exothermic and exergonic by 22.4 and 12.5 kcal·mol−1, respectively. In the lowest energy pathway from Ia to 2, the η2-phenylacetylene of Ia slips to bind through the σ-C−H bond to form the intermediate [(PiPr3)2Rh(σ-HCCPh)(CCPh)]. Then, a single transition state (TS) (ΔH⧧ = 24.6 kcal·mol−1) connects this intermediate to 2. In an alternative route, oxidative splitting of the σ-C−H bond results in the experimentally observed bis-alkynyl rhodium hydride complex [(PiPr3)2Rh(H)(CCPh)2] (Ib), which was observed experimentally to be in equilibrium with Ia. Species Ib is isoenthalpi...

Multisubstituted N-fused heterocycles via transition metal-catalyzed cycloisomerization protocols

Two complementary protocols for assembly of multisubstituted N-fused heterocycles have been developed. It was demonstrated that 1,3-disubstituted N-fused heterocycles, including indolizines, pyrroloquinoxalines, and pyrrolothiazoles can easily be synthesized via an exceptionally mild and efficient method involving a novel silver-catalyzed cycloizomerization of propargyl-containing heterocycles. Alternatively, 1,2-disubstituted heterocycles can be accessed through the novel cascade transformation involving an alkyne–vinylidene isomerization with concomitant 1,2-shift of hydrogen, silyl, stannyl, or germyl groups. This mild and simple method allows for selective and highly efficient synthesis of indolizines, pyrroloisoquinolines, pyrroloquinoxalines, pyrrolopyrazines, and pyrrolothiazoles.

A mechanistic study into the interconversion of rhodium alkyne, alkynyl hydride and vinylidene complexes

The conversion of the alkyne complex RhCl(PPr(i)3)2(eta2-PhC[triple bond]CH) into its vinylidene isomer RhCl(PPr(i)3)2(=C=CPhH) has been monitored via NMR spectroscopy in a range of solvents. In THF and hexane solution the reaction was observed to proceed via the alkynyl hydride complex Rh(H)Cl(PPr(i)3)2(-C[triple bond]CPh). The kinetic profile of the reaction demonstrates that this species is an intermediate in the formation of the vinylidene complex.

Gold‐Catalyzed 1,2‐Migration of Silicon, Tin, and Germanium en Route to C‐2 Substituted Fused Pyrrole‐Containing Heterocycles.

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Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.

Infrared Spectra and Density Functional Calculations for Three Pt−C2H2Reaction Product Isomers: PtCCH2, HPtCCH, and Pt−η2-(C2H2)

Laser-ablated Pt atoms react with C2H2 upon co-condensation in excess argon and neon to form the vinylidene PtCCH2, the insertion product HPtCCH, and the strong complex or metallacyclopropene Pt−η2-(C2H2). These species are identified through 13C2H2, C2D2, and C2HD isotopic substitutions and density functional theory isotopic frequency calculations. The global energy minimum PtCCH2 is identified at 3022.4 cm-1 (C−H stretching), 1716.5 cm-1 (C−C stretching), and 707.9 cm-1 (CH2 deformation) in an argon matrix. The C−C stretching modes for Pt−η2-(C2H2) and HPtCCH are observed at 1653.7 and 2016.2 cm-1, respectively. In addition, the insertion product HPtCCH is identified by a Pt−H stretching mode at 2350.8 cm-1 and a C−C stretching mode at 2010.4 cm-1 in the neon matrix. The most stable PtCCH2 vinylidene isomer is the favored primary product. The strong complex Pd−η2-(C2H2) rearranges by 1,2 hydrogen atom migration to form the vinylidene PtCCH2 on 240−290 nm photolysis and on reaction during annealing solid...

Electronic Interactions in Dinuclear Platinum and Palladium Ethynylferrocene and Ferrocenylvinylidene Complexes

AbstractThe complex [Pd2(dppm)2(μ‐η1:η1‐H−C=C−Fc)Cl2] (2) and its vinylidene isomer [Pd2(dppm)2(μ‐C=CH−Fc)Cl2] (3) [dppm = bis(diphenylphosphanyl)methane, Fc = ferrocenyl unit, CpFeC5H4] have been synthesized and characterized by single‐crystal X‐ray diffraction. Both 2 and 3 are A‐frame structures with long Pd−Pd distances of 3.2055(9) Å (2) and 3.1002(2) Å (3), respectively, and square‐planar metal coordination. The cyclic voltammograms (CV) of the complexes are measured and compared with that of [Pt2(dppm)2(μ‐η1:η1‐H−C=C−Fc)Cl2] (1). The CV of 2 and 3 consist of a nearly reversible Fc‐oxidation, an irreversible oxidation and a small reversible couple. The E1/2 values (vs. Cp2Fe+/Cp2Fe) of the Fc‐groups in complexes 1−3 follow the order 2 (−75±8 mV) > 1 (−108±10 mV) > 3 (−124±8 mV). The difference in the E1/2 values is explained by the orbital interactions between the acetylene and vinylidene groups with Pt2 and Pd2 cores. Our analysis shows that the metal‐to‐ligand π back‐bonding and ligand‐to‐metal σ‐donation have determining effects on the electronic interactions between the dimetallic centers and the bridgehead ligands. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Further investigation of the HCl elimination in the photodissociation of vinyl chloride at 193 nm: a direct MP2/6-31G(d,p) trajectory study

Direct MP2/6-31G(d,p) classical trajectories were performed to further explore the photodissociation of vinyl chloride at 193 nm. In general, the results are in good agreement with those obtained previously using a semiempirical AM1 Hamiltonian supplemented with specific reaction parameters (SRPs). The calculations support the earlier proposal that the three-center elimination of HCl and the isomerization of vinylidene to acetylene take place in a concerted, nonsynchronous fashion. On the other hand, the results indicate that the four-center channel produces rotationally hot HCl molecules while low rotational states of HCl are essentially populated when vinyl chloride dissociates through the three-center channel. This result is at odds with a previous conclusion inferred from fully resolved vibration–rotation emission spectra of HCl and simple theoretical models.