Vinylidene Isomerization Research Articles

Highly vibrationally excited 12C2H2 in the X̃ 1∑+g state: Complementarity of absorption and dispersed fluorescence spectra

Direct absorption and dispersed fluorescence (DF) spectra sample the 12C2H2 X̃ 1Σ+g potential energy surface via profoundly different zero-order bright states. Despite the complementary nature of the data sets, a polyad model based on the approximate quantum numbers, Ns=v1+v2+v3, Nr=5v1+3v2+5v3+v4+v5, l=l4+l5, accounts for the energies (±0.35 cm−1) and relative intensities for all transitions into pure bending levels (Ns=0) at Evib≤12 000 cm−1 and Nr≤15, l=0 and 2. The parameters that define this model are obtained by fitting 41 pure bending levels, of which 9 are J=0 energies derived from components of the [Ns=0, Nr=8, 10, 12, l=0, g] polyads observed in the DF spectrum. This polyad model provides a basis for extrapolating a description of otherwise indescribably complex spectra and dynamics to the Evib≊16 000 cm−1 region of the barrier to acetylene ↔ vinylidene isomerization, and could therefore provide a basis for detecting the pattern-breaking signature of the onset of isomerization.

Silaacetylene: A possible target for experimental studies

The equilibrium geometries and transition states for interconversion of the CSiH2 isomers in the singlet electronic ground state are optimized at the MP2 and CCSD(T) levels of theory using a TZ2P basis set. The heats of formation, vibrational frequencies, infrared intensities, and rotational constants are also predicted. There are three energy minima on the CSiH2 potential energy surface. Energy calculations at CCSD(T)/TZ2P(fd) + ZPE predict that the global energy minimum is silavinylidene (1), which is 34.1 kcal mol−1 lower in energy than trans-bent silaacetylene (2) and 84.1 kcal mol−1 more stable than the vinylidene isomer (3). The barrier for rearrangement 2→1 is calculated at the same level of theory to be 5.1 kcal mol−1, while for the rearrangement 3→2 a barrier of 2.7 kcal mol−1 is predicted. The natural bond orbital (NBO) population scheme indicates a clear polarization of the C(SINGLE BOND)Si bonds toward the carbon end. A significant ionic contribution to the C(SINGLE BOND)Si bonds of 1 and 2 is suggested by the NBO analysis. The C(SINGLE BOND)Si bond length of trans-bent silaacetylene (2) is longer than previously calculated [1.665 Å at CCSD(T)/TZ2P)]. The calculated carbon-silicon bond length of 2 is in the middle between the C(SINGLE BOND)Si double bond length of 1 (1.721 Å) and the C(SINGLE BOND)Si triple bond of the linear form HCSiH (4), which is 1.604 Å. Structure 4 is a higher-order saddle point on the potential energy surface. © 1996 by John Wiley & Sons, Inc.

Symmetry-breaking perturbations in the ν2+3ν3 rovibrational manifold of acetylene: Spectroscopic and energy-transfer effects

Time-resolved infrared–ultraviolet double resonance (IR-UV DR) spectroscopy is used to characterize complex rovibrational levels in the highly perturbed ν2+3ν3 region (∼11 600 cm−1) of gas-phase acetylene, C2H2. Here, very few of the known rovibrational levels have appreciable Franck–Condon factors linking them to accessible excited rovibronic levels, as is needed in the fluorescence-detected IR-UV DR excitation scheme; rovibrational levels that are ‘‘IR-bright’’ tend to be ‘‘UV-dark’’ and vice versa. The rovibrational states detectable by IR-UV DR in this region are strongly perturbed, such that IR-bright (but UV-dark) vibrational basis states are coupled to other states with more favorable Franck–Condon factors. The characterization of these perturbed rovibrational states (and their associated dynamical properties) is facilitated by a novel IR-UV DR technique in which the UV and IR laser frequencies are simultaneously scanned in opposite directions, with their sum held constant. From the observed IR-UV DR spectra, it is inferred that local perturbations tend to break symmetries and spoil quantum numbers (such as l, J, and possibly I) that are usually regarded as ‘‘good’’ in the C2H2 molecule. The most remarkable case entails an apparent collision-induced breaking of g/u symmetry that gives rise to rovibrational energy transfer with odd ΔJ (rather than the usual even-ΔJ situation). This observation is consistent with IR-UV DR kinetic measurements of collision-induced state-to-state energy transfer that are also briefly described. The supposed mechanism relies on Coriolis coupling to cause strong rovibrational perturbations by basis states with dominant bending character, such that the resulting perturbed state is then susceptible to dynamical breaking of g/u symmetry, with odd-ΔJ rovibrational transfer a direct consequence. Other possible mechanisms imply that excitation of C2H2 to a particular perturbed rovibrational level might cause facile interconversion of the ortho and para nuclear-spin modifications. One such interpretation of g/u symmetry-breaking in C2H2 invokes a combination of Coriolis coupling and nuclear hyperfine interaction, thereby mixing basis states that have a very close accidental coincidence in energy. Another (but energetically unlikely) possibility is that g/u symmetry is spoiled photochemically by intramolecular state-mixing involving the vinylidene isomer, thereby destroying the molecule’s center of symmetry.

C−C Bond Formation between Vinylidene and Alkynyl Ligands at Ruthenium(II) Leading to either Enynyl or Dienynyl Complexes

The Ru(II) complex mer,trans-(PNP)RuCl2(PPh3) (1) reacts with either phenylacetylene or p-tolylacetylene to give fac,cis-(PNP)RuCl2{CCH(R)} (R = Ph (2), p-tolyl (3)) in refluxing tetrahydrofuran (THF) and mer,trans-(PNP)RuCl2{CCH(R)} (R = Ph (4), p-tolyl (5)) in a refluxing THF/EtOH mixture (1:2 v:v) [PNP = CH3CH2CH2N(CH2CH2PPh2)2]. The fac,cis vinylidene complexes 2 and 3 react with an excess of LiC⋮CPh converting to the σ-alkynyl−η3-enynyl complexes anti,mer-(PNP)Ru(C⋮CPh){η3-PhC3CH(Ph)} (8) and anti,mer-(PNP)Ru(C⋮CPh){η3-PhC3CH(p-tolyl)} (9), respectively. Conversely, treatment of the mer,trans vinylidene isomers with an excess of LiC⋮CPh, followed by addition of a primary alcohol, exclusively gives the σ-alkynyl−η-dienynyl complexes mer-(PNP)Ru(C⋮CPh){η-PhCC(C⋮CPh)CHCH(R)} (R = Ph, 12; R = p-tolyl, 13). Single-crystal X-ray analyses have been carried out on 8 and 13. In both compounds the coordination geometry around the Ru atom approximates an octahedron with three positions taken by a mer PNP ligand and one position taken by a phenylethynyl group. The coordination sphere around the metal center is completed by an η3-1,4-diphenylbut-3-en-1-ynyl ligand in 8 and by a 1-p-tolyl-3-(phenylethynyl)-4-phenylbuta-1(E),3(Z)-dien-4-yl ligand in 13. The latter ligand essentially uses the C1 carbon atom to bind the metal, although a weak bonding interaction may be envisaged also with the alkynyl substituent in the 3-position. A single-crystal X-ray analysis has also been carried out on the octahedral fac,cis-(PNP)RuCl2(CO) complex obtained by treatment of the vinylidenes 2 and 3 with molecular oxygen. The influence of the bonding mode of the PNP ligand on the different reactivity shown by the fac and mer vinylidene complexes toward LiC⋮CPh is discussed in light of a multiform experimental study (X-ray structure determinations, multinuclear NMR spectroscopy, deuterium- and p-tolyl-labeling experiments, independent reactions with isolated compounds).

The Acetylene S0 Surface: From Dispersed Fluorescence Spectra to Polyads to Dynamics

AbstractDispersed Fluorescence spectra of acetylene have been recorded and unzipped, through a fortuitous coincidence of FC active modes and active anharmonic resonances, there by pictorially revealing the key factors controlling the vibrational dynamics on the acetylene S0 surface. When energy is placed into very high excitation of the trans‐bend and moderate excitation of the CC stretch, three approximately conserved quantities survive, i.e., no trace of the expected total breakdown of vibrational constants of motion due to energetic access to the vinylidene isomer is observed for vibrational energies up to at least 16400 cm−1. Unzipping the DF spectrum reveals the FC intensity profiles for the acetylene à ↔ X̃ system with two FC active modes. Also, the unzipped DF spectrum clearly demonstrates that IVR increases within a polyad by increasing trans‐bead, v4, and by decreasing CC stretch, v2. Increasing IVR with increasing v4 or decreasing v2, are both cases where the first few resonance tiers play the dominant role in controlling IVR for the initial states prepared in these experiments, very high excitation in the trans‐bend and moderate excitation in the CC stretch.

Irreversible rearrangement of half-sandwich ruthenium hydrido-alkynyl complexes to their vinylidene isomers

The complex [(C5Me5)RuCl(dippe)](dippe = l,2-bis(diisopropylphosphino)ethane) reacts with alk-1-ynes in MeOH in the presence of NaBPh4 yielding the metastable hydrido-alkynyl derivatives [(C5Me5)Ru(H)(CCR)(dippe)][BPh4](R = CO2Me, SiMe3 or Ph), intermediates in the formation of the corresponding vinylidene complexes, to which these compounds rearrange both in solution and in the solid state.

Die Darstellung sterisch anspruchsvoller Phosphinoester und ihre Verwendung zur Synthese von Rhodium(I)-und Rhodium(III)-Komplexen [1] / The Preparation of Sterically Demanding Phosphinoesters and their Use for the Synthesis of Rhodium(I) and Rhodium (III) Complexes

Two routes for the preparation of sterically demanding phosphinoesters RR′P(CH2)nCO2Me (n=1,2 or 3) and RR′P(CH2)nCO2Et (n=1 or 2) have been developed: (1) reaction of trimethyl-silyl phosphines RR′PSiMe3 (R=R′=/Pr. rBu) with ω-chloroalkylesters, and (2) hydrophos- phination of secondary phosphines HP(R)Ph (R = Me, Et. Pr. iPr, iBu) with unsaturated carboxylic esters. From iPr2PCH2CO2Me (3) and iPr2PCH2CO2Et (4), a variety of rhodium(I) and rhodium(III) complexes, mainly with alkyne and vinylidene ligands, have been prepared. Compound [RhCl(C8H14)2]2 (19) reacts with four equiv. of 3 to give [RhCl(iPr2PCH2CO2Me)2] (20) which on further treatment with CO affords the carbonyl complex trans-[RhCl(CO)(iPr′>PCH2CO2Me)2] (21). The related ethylene derivative trans-[RhCl(C2H4)(iPr2PCH2CO2Me)2] (23) is obtained from [RhCl(C2H4)2]2 (22) and 3. Compound 20 reacts with H2 and HCl by oxidative addition to yield [RhHXCl(iPr2PCH2CO2Me)2] (24, 25) and with RC≡CR′ to give the alkyne complexes trans-[RhCl(RC≡CR′)(iPr2PCH2CO2Me)2] (26-28). The analogous compounds trans- RhCl(HC≡CR)(iPr2PCH2CO2Me)2] (29, 30), equally prepared from 20 and HC≡CR, both rearrange thermally and photochemically to produce the vinylidene isomers trans-[RhCl(=C=CHR)(iPr2PCH2CO2Me)2] (31, 32). In contrast, propyne reacts with 20 to give the allene complex trans-[RhCl(CH2=C=CH2)(iPr2PCH2CO2Me)2] (33). The synthesis of [RhCl(iPr2PCH2CO2Et)2] (34) and the corresponding alkyne and vinylidene adducts 35 and 36, obtained with HC≡CCO2Me as the substrate, is also described.

Stationary points on the S1 potential energy surface of C2H2

Recent theoretical developments that facilitate characterization of excited-state potential energy surfaces are applied to study five stationary points on the S1 surface of C2H2. Although previous calculations have focused on the acetylenic trans and cis forms, the present research predicts that the vinylidene isomer is the global minimum on the potential surface. However, a high activation barrier calculated for rearrangement to the trans isomer suggests that S1 vinylidene is not likely to be formed via photoexcitation of ground-state acetylene. The trans and cis acetylenic forms of the S1 state are shown to interconvert along an in-plane reaction coordinate with an activation energy of ∼4000 cm−1, a value which is significantly lower than usually assumed in spectroscopic analyses.

Synthese von Alkin-, Alkinyl-und Vinyliden-Rhodium-Komplexen ausgehend von Alkinyltriphenylsilanen [1] / Synthesis of Alkyne, Alkynyl and Vinylidene Rhodium Complexes from Alkynyltriphenylsilanes [1

Abstract Compound [RhCl(PiPr3)2]n (1) reacts with RC≡CSiPh3 to give the rhodium(I) complexes trans-[RhCl(RC≡CSiPh3)(PiPr3)2] (2-5) of which 2 (R = CH3) and 5 (R = C6H5) rearrange thermally to yield the vinylidene isomers trans-[RhCl(=C=C(SiPh3)(R)(PiPr3)2] (6,7). The reaction of 2 and 4 (R = CH2OMe) with NaC5H* affords the cyclopentadienyl complexes [C5H5Rh(RC≡CSiPh3)(PiPr3)] (8,9). The square-planar alkynylrhodium(I) compounds trans-[Rh(C≡CSiPh3)(L)(PiPr3)2] (12,13) are obtained from 10 or 11 on treatment with NaN(SiMe3)2; they react with cyclopentadiene to give [C5H5RhH(C≡CSiPh3)(PiPr3)] (14). Five- and six-coordinate bis(alkynyl)hydridorhodium(III) complexes [RhH(C≡CSiPh3)(C≡CSiR3)(PiPr3)2] (15,16) and [RhH(C≡CSiPh3)(C≡CSiR3)(py)(PiPr3)2] (17,18) are prepared from 12 and 13 by oxidative addition of HC≡CSiR3 (R = CH3, C6H5). The reaction of 12 with Me3SiC≡CCO2Et leads to the formation of trans-[Rh(C≡CSiPh3)(Me3SiC≡CCO2Et)(PiPr3)2] (19).

Theoretical studies of organometallic compounds. 5. Alkyne and vinylidene complexes of molybdenum and tungsten in high-oxidation states.

The geometries of the acetylene complexes MX 4 C 2 H 2 and the vinylidene isomers MX 4 CCH 2 (M=W, Mo; X=F, Cl) are theoretically predicted using quantum mechanical ab initio methods at the Hartree-Fock level of theory and relativistic effective core potentials for the transition metals. The optimized geometries and energies of the anionic complexes MX 5 C 2 H 2 - are also reported. The optimization of WX 5 C 2 H 2 - gives geometries for the chloro and fluoro complexes, which are in good agreement with experiment

ChemInform Abstract: Striking Similarities Between Elementary Silicon and Aluminum Compounds: Monobridged, Dibridged, Trans‐Bent, and Vinylidene Isomers of Al2H2.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Acetylene: Synergy between theory and experiment

Six anomalous vibronic feature states [∼2 cm−1 full-width at half-maximum (FWHM), each consisting of ∼20 partially resolved eigenstates] have been observed in stimulated emission pumping (SEP) spectra of C2D2. Of the two plausible assignments for these features, the one most consistent with spectroscopic observations would imply that the lowest energy cis-bent triplet state of acetylene has T0≤25 820 cm−1, which is inconsistent with previous ab initio predictions. New higher level ab initio quantum mechanical methods have been used to predict the energy difference between X̃ 1Σg+ ground state and the cis-bent ã 3B2 lowest triplet state of acetylene. In conjunction with a triple zeta plus double polarization plus f function (TZ2Pf) basis set, the coupled cluster including single, double, and linearized triple excitations CCSD(T) method yields T0=ΔE(ã 3B2–X̃ 1Σg+)=30 500 cm−1. The true value of T0 for the ã 3B2 state is estimated to be ∼500 cm−1 higher. At the same level of theory the zero-point levels of the lowest triplet state of the trans-bent (ã 3Bu) and vinylidene (ã 3B2) isomers lie at still higher energies. This result conclusively rules out any triplet assignment for the anomalous feature states. The alternative assignment, as highly excited vibrational levels of the X̃ 1Σg+ state, is surprising in view of the Franck–Condon selectivity, dynamical stability, and nonselective relaxation of this special class of ‘‘bright states’’ observed in the SEP spectra. Such an assignment would be implausible in the absence of the present ab initio calculations. Previous experimental observations [Lisy and Klemperer, J. Chem. Phys. 72, 3880 (1980) and Wendt, Hippler, and Hunziker, J. Chem. Phys. 70, 4044 (1979)] of acetylene triplet states are discussed and shown to be completely consistent with each other and with the present ab initio ordering of the cis and trans isomeric minima on the T1 potential energy surface: cis ã 3B2 below trans ã 3Bu .

Ab initio potential energy surfaces for unimolecular reactions during thermal conversion of acetylene

Unimolecular reactions involved in the pyrolysis of acetylene have been studied using ab initio molecular orbital-self-consistent field (MO-SCF) methods. A multiconfiguration (MC-SCF) method has proved adequate to describe the reaction studied. In the case of acetylene → vinylidene isomerization we also performed multireference double configuration interaction (MRDCI) calculations: we found that vinylidene is a real minimum on the surface and it could play an important role in other, more endothermic, reactions.

Striking similarities between elementary silicon and aluminum compounds: monobridged, dibridged, trans-bent, and vinylidene isomers of aluminum hydride (Al2H2)

Ab initio quantum mechanical methods five been used to study the singlet potential energy surface of Al 2 H 2 . Optimum geometries and harmonic vibrational frequencies were obtained for four geometrical isomers using the self-consistent-field (SCF), configuration interaction (CI), and coupled cluster (CC) methods. Both correlation methods including single and double excitations (CISD, CCSD) were employed, and all wave functions were determined with both DZP and TZ2P basis sets. Final energy predictions are obtained using large atomic natural orbital basis sets, and including the effects of triple excitations perturbatively using CCSD(T) methods. We found the planar dibridged structure to be the global minimum, as predicted earlier by Baird

Structural characterization of alkyne and vinylidene isomers of [Ru(C2H2)(PMe2Ph)2(Cp)][BF4

The reaction of [RuCl(PMe 2 Ph) 2 (Cp] with ethyne and TlBF 4 in dichloromethane leads to [Ru(η 2 -HC≡CH)-(PMew 2 Ph)(Cp)][BF 4 ] (2), an unusual η 2 -alkyne complex of a d 6 metal center. This ethylene complex smoothly rearranges to its vinylidene isomer,[Ru(C=CH 2 )(PMe 2 Ph) 2 (Cp)][BF 4 ] (4), above ca. 60 o C in acetone solution. The η 2 -ethyne to vinylidene conversion can also be carried out by deprotonation of 2 to give [Ru(C≡CH)(PMe 2 Ph) 2 (Cp)] (3), followed by protonation of 3 to give exclusively vinylidene isomer 4

The vinylidene-acetylene rearrangement. A phantom minimum on the MP2 potential energy surface

At the MP2 level of theory there exists a third minimum on the C 2H 2 potential energy surface in addition to acetylene and vinylidene. This new hydrogen-bridged structure, however, is only an artifact of the MP2 procedure; it disappears at higher levels of theory. High-level calculations (QCISD(T)/6-311++G(3df,2pd)//QCISD(T)/6-311G (d,p)) predict a classical barrier for the isomerization of vinylidene to acetylene of 11.0 kJ mol −1 and an energy difference between vinylidene and acetylene of 185 kJ mol −1. After the approximate incorporation of zero-point vibrational energy vibrational energy corrections, these energies become 3 and 177 kJ mol −1, respectively.

Vinylidene transition-metal complexes. Part 13. The reactivity of [IrH5(PPri3)2] and [IrH2Cl(PPri3)2] toward Alk-1-ynes: synthesis of four-, five- and six-co-ordinate iridium complexes containing alkynyl, vinyl and vinylidene ligands

Reaction of [IrH5(PPri3)2]2 with excess of phenylacetylene in pentane gives the tris(alkynyl) complex [Ir(C2Ph)3(PPri3)2]3, whereas with acetylene and propyne the bis(alkynyl) vinyl compounds [Ir(C2R)2(CRCH2)(PPri3)2](R = H, 4; or Me 5) are obtained. Complexes 3–5 react smoothly with H2 to give 2 and the corresponding alkene and thus are supposed to be intermediates in the hydrogenation of alk-1-ynes catalysed by 2. Reaction of [IrH2Cl(PPri3)2]8 with PhC2H occurs stepwise leading to the formation of the mono-, bis- and tris-(alkynyl) iridium(III) derivatives [IrH(Cl)(C2Ph)(PPri3)2]9, [IrCl(C2Ph)2(PPri3)2]10 and 3. From 8 and acetylene, the vinylidene complex [IrCl(CCH2)(PPri3)2]11 is obtained. Treatment of 8 with an equimolar amount of HC2CO2Me gives first the alkynyl hydrido compound [IrH(Cl)(C2CO2Me)(PPri3)2]12 which on warming to 80 °C in benzene rearranges to give the substituted vinylidene isomer [IrCl(CCHCO2Me)(PPri3)2]13 in virtually quantitative yield. The reaction of 8 with 2 equivalents of HC2CO2Me leads to the formation of the bis(alkynyl) complex [IrCl(C2CO2Me)2(PPri3)2]14.

ChemInform Abstract: High Resolution Spectroscopic Detection of Acetylene‐Vinylidene Isomerization by Spectral Cross Correlation.

Information about the unimolecular acetylene (HC 3/4 CH)↔vinylidene (H2C=C:) isomerization on the S0 energy surface has been extracted from vibrationally unassigned high resolution stimulated emission pumping (SEP) spectra of acetylene. The combination of a new pattern recognition scheme, spectral cross correlation (SCC) with complete nuclear permutation‐inversion (CNPI) group theory is shown to be a powerful new technique for characterizing bond rearrangement in highly vibrationally excited normally rigid polyatomic molecules. SCC detects isomerization ‘‘resonances’’ which destroy an approximate vibrational symmetry (e.g., the number of cis‐bending quanta). The energies (relative to the zero point level of the stablest isomer) and widths of such resonances provide information about the ‘‘energies’’ of isomer rovibrational levels and the isomer‐level‐specific isomerization rate. Vinylidene isomerization resonances may be distinguished from ordinary acetylene Fermi or Coriolis perturbations by a unique rot...

A study of the singlet and triplet states of vinylidene by photoelectron spectroscopy of H2C=C−, D2C=C−, and HDC=C−. Vinylidene–acetylene isomerization

The X̃ 1A1, ã 3B2, and b̃ 3A2 states of vinylidene are observed in the ultraviolet (351.1–364.0 nm) photoelectron spectra of X̃ 2B2 H2CC−, X̃ 2B2 D2CC−, and X̃ 2A′ HDCC−. The X̃ 1A1 state exhibits vibrational structure well above the barrier for isomerization to acetylene. A strict lower bound to the lifetime of the singlet state against rearrangement is τ>0.027 ps, with an estimate of τ≊0.04–0.2 ps based on a simulation of the line shapes including rotational broadening. A vibrational analysis of the singlet and lower triplet state bands provides vibrational frequencies and estimates of the changes of molecular geometries between the anion and the neutral species. A qualitative potential energy surface for the CH2 rock mode, which closely corresponds to the reaction coordinate for isomerization, is extracted from the experimental data. The adiabatic electron affinity is EA(X̃ 1A1 H2CC)=0.490±0.006 eV and the triplet term energies are T0(ã 3B2 H2CC)=2.065±0.006 eV and T0(b̃ 3A2 H2CC)=2.754±0.020 eV. Experimental values for the bond dissociation energy of vinyl radical, D0(H2CC–H)=80.0±5.0 kcal/mol, and the acetylene–vinylidene isomerization energy, ΔHI=46.4±5.5 kcal/mol, are derived. Combining the latter value with the upper limit of Field and co-workers, ΔHI≤44.1–44.7 kcal/mol, yields ΔHI≊41–45 kcal/mol.

High resolution spectroscopic detection of acetylene–vinylidene isomerization by spectral cross correlation

Information about the unimolecular acetylene (HC 3/4 CH)↔vinylidene (H2C=C:) isomerization on the S0 energy surface has been extracted from vibrationally unassigned high resolution stimulated emission pumping (SEP) spectra of acetylene. The combination of a new pattern recognition scheme, spectral cross correlation (SCC) with complete nuclear permutation-inversion (CNPI) group theory is shown to be a powerful new technique for characterizing bond rearrangement in highly vibrationally excited normally rigid polyatomic molecules. SCC detects isomerization ‘‘resonances’’ which destroy an approximate vibrational symmetry (e.g., the number of cis-bending quanta). The energies (relative to the zero point level of the stablest isomer) and widths of such resonances provide information about the ‘‘energies’’ of isomer rovibrational levels and the isomer-level-specific isomerization rate. Vinylidene isomerization resonances may be distinguished from ordinary acetylene Fermi or Coriolis perturbations by a unique rotational symmetry dependence due to the correlation between acetylene [D∞h(M)] and vinylidene [C2v(M)] levels in the CNPI group G8. An SCC map of the HCCH S0 15 000–15 900 cm−1 energy region above the zero point level was obtained by comparing SEP spectra recorded via S1(Ã 1Au)33 and 2162 Ka=1 intermediate levels. The predicted rotational symmetry dependence of the SCC was found between 15 410–15 640 cm−1, but the vinylidene resonance line shape was obscured by Franck–Condon interference effects from well known perturbations between the 33 and 2162 SEP intermediate levels. It was therefore not yet possible to determine the height of the isomerization barrier (or even whether a barrier exists) but an upper bound for the vinylidene zero point level of 15 525 cm−1 was inferred from the SCC data.