Ar Matrices Research Articles

Luminescence spectroscopy of matrix-isolated zP6 state atomic manganese

The relaxation of electronically excited atomic manganese isolated in solid rare gas matrices is observed from recorded emission spectra, to be strongly site specific. z 6P state excitation of Mn atoms isolated in the red absorption site in Ar and Kr produces narrow a 4D and a 6D state emissions while blue-site excitation produces z 6P state fluorescence and broadened a 4D and a 6D emissions. MnXe exhibits only a single thermally stable site whose emission at 620 nm is similar to the broad a 6D bands produced with blue-site excitation in Ar and Kr. Thus in Ar(Kr), excitation of the red site at 393 (400) nm produces narrow line emissions at 427.5 (427.8) and 590 (585.7) nm. From their spectral positions, linewidths, and long decay times, these emission bands are assigned to the a 4D72 and a 6D92 states, respectively. Excitation of the blue site at 380 (385.5) nm produces broad emission at 413 (416) nm which, because of its nanosecond radiative lifetime, is assigned to resonance z 6P --> a 6S fluorescence. Emission bands at 438 (440) and 625 (626.8) nm, also produced with blue-site excitation, are broader than their red-site equivalents at 427.5 and 590 nm (427.8 and 585.7 nm in Kr) but from their millisecond and microsecond decay times are assigned to the a 4D and a 6D states. The line features observed in high resolution scans of the red-site emission at 427.5 and 427.8 nm in MnAr and MnKr, respectively, have been analyzed with the W(p) optical line shape function and identified as resolved phonon structure originating from very weak (S=0.4) electron-phonon coupling. The presence of considerable hot-phonon emission (even in 12 K spectra) and the existence of crystal field splittings of 35 and 45 cm(-1) on the excited a 4D72 level in Ar and Kr matrices have been identified in W(p) line shape fits. The measured matrix lifetimes for the narrow red-site a 6D state emissions (0.29 and 0.65 ms) in Ar and Kr are much shorter than the calculated (3 s) gas phase value. With the lifetime of the metastable a 6D92 state shortened by four orders of magnitude in the solid rare gases, it is clear that the probability of the "forbidden" a 6D --> a 6S atomic transition is greatly enhanced in the solid state. A novel feature identified in the present work is the large width and shifted 4D and 6D emissions produced for Mn atoms isolated in the blue sites of Ar and Kr. In contrast, these states produce narrow, unshifted (gas-phase-like) 4D and 6D state emissions from the red site.

Magnetic Circular Dichroism and Absorption Spectra of Phosphinidene in Noble‐Gas Matrices

AbstractFor Abstract see ChemInform Abstract in Full Text.

Role of Conformation and Electronic Structure in the Chemistry of Ground and Excited Stateo-Pyrazolylphenylnitrenes

The chemistry of 2-(1-pyrazolyl)- (2a) and 2-[1-(3,5-dimethylpyrazolyl]phenylnitrene (2b) has been studied in EtOH solution at room temperature, in EtOH glasses at 90 K, and in Ar matrices at 12 K. These nitrenes were chosen as suitable models for clarifying the mechanism of intramolecular reactions because attack at the pyrazole ring can occur according to different modes and the asymmetry of the substituent gives rise to different conformations. Detailed DFT and CASSCF/CASPT2 studies on the conformation and decay paths of both spin states of the nitrenes have been carried out. Ring expansion to dehydroazepines (via benzoazirines) is calculated to be competitive in both nitrenes, but in the dimethyl derivative, 2b, attack onto the N lone pair (which is made more nucleophilic by the methyl groups) is favored. The higher barriers (by 4-8 kcal/mol) in singlet 2a cause 60-70% of this nitrene to decay by intersystem crossing to the triplet. Thus, the seemingly straightforward formation of benzo-fused heterocycles through intramolecular attack of the pyrazoline N lone pair by the singlet phenylnitrene can only overcome ring expansion and intermolecular reactions under favorable circumstances. The comparatively persistent triplet nitrenes are characterized in matrices, and the yields of photocyclization products (mainly pyrazolo[1,5-a]benzimidazole (7) from 2a and 5,6-dihydropyrazolo[1,5-a]quinoxaline (8) from 2b) are shown to depend on the preferred conformation of the starting azide and nitrene.

Electron spin resonance g tensors for complexes of Ne and Ar with AlO: Theoretical studies related to the large matrix effect observed for AlO

For Ne(n)-AlO (n=2, 4, 6, 8, 10) and Ar(n)-AlO clusters (n=2, 4, 6, 8), the perpendicular (relative to AlO) component of the g tensor was calculated by second-order perturbation theory, using multireference configuration-interaction wave functions. The rare-gas (Rg) atoms were placed axially and/or off axially (one or two rings of four Rg atoms each), and the distance of the Rg atoms from the Al and O atoms, or from the AlO axis, was varied from 4 to 12 bohrs. Rg atoms placed axially mostly increase g(perpendicular), whereas off-axially placed ones lower it below the gas-phase value of AlO. The largest deviations from g(perpendicular) of isolated AlO occur at Ne-Al,O distances of 5-6 bohrs, and Ar-Al,O distances of 6-9 bohrs, with maximal lowerings of about 1600 ppm for Ne and about 2200 ppm (estimated) for Ar in the case of two axial and eight off-axial Rg atoms. Electron spin resonance studies by Knight and Weltner found large matrix effects for AlO, with downshifts of g(perpendicular) observed to be about 450 and 1150 ppm in Ne and Ar matrices, respectively.

Magnetic Circular Dichroism and Absorption Spectra of Phosphinidene in Noble-Gas Matrices

Electronic magnetic circular dichroism and absorption spectra are reported for the A 3pi(i) <-- X 3sigma- transitions of phosphinidene (PH) isolated in Ar, Kr, and Xe matrices at cryogenic temperatures (approximately 1.4-20 K) and over a range of magnetic field strengths (0-5 T). The results are analyzed by the method of moments, and parameters are extracted by fitting the experimental data to a model in which the A 3pi(i) term is split by spin-orbit (SO) coupling interactions, while the X 3sigma- term is split by spin-spin and higher-order SO coupling. The analysis indicates that, unlike the equivalent imidogen (NH) systems, ground-state PH radicals isolated in noble-gas matrices do not behave as free rotors. Trends in excited-state SO coupling constants are attributed to the external heavy-atom effect and guest-host orbital mixing. It is tentatively concluded that librational amplitudes of the guest radical decrease in the order Ar > Xe > Kr, probably as a consequence of competition between stronger guest-host interactions and larger matrix sites in heavier hosts.

Matrix isolation FTIR and DFT studies of methyl isocyanide-boron trifluoride complex

The FTIR spectra of CH 3NC and BF 3 as well as their isotopomers co-deposited in low temperature Ar matrices were recorded. We observed several new infrared bands absent in the spectra of each component. These bands were assigned to the 1:1 EDA complex of CH 3NC and BF 3 based on the DFT calculations at the B3LYP/6-311++G** level. The vibrational assignments and frequencies of the observed bands are the NC stretching at 2319.9 cm −1, the BF 3 degenerate stretching at 1203.8 cm −1, the BF 3 symmetric stretching at 840.4 cm −1 and the BF 3 symmetric deformation at 638.0 cm −1 for the CH 3NC− 11BF 3 isotopomer. Although the BF 3 symmetric stretching is infrared inactive for free BF 3 of D 3 h symmetry, complexation with methyl isocyanide would have induced a structural change in the BF 3 moiety and the activation of this mode. Apart from some numerical discrepancies, the observed frequency shifts on complexation were in good agreement with the results of DFT calculation. Therefore, the structure of CH 3NC-BF 3 in solid Ar would be similar to the optimized C 3 v geometry, which corresponds to the gas phase structure.

Some novel binuclear group 13 metal tin hydrides formed in Ar matrices following the codeposition of the metal vapor with SnH4.

IR measurements show that co-condensation of Al or Ga atoms (M) with SnH4 in a solid Ar matrix at about 12 K results mainly in the spontaneous insertion of the metal into an Sn-H bond to form the M(II) hydride HMSnH3. Simultaneously the Ga2 dimer also reacts with SnH4, possibly to form a nido-type cluster Ga2(mu-H)4Sn, with a metal-deficient cubane-like structure. All of these products are photolabile. Irradiation with visible light causes HMSnH3 to tautomerize to the novel dihydrido-bridged species H3M(mu-H)2Sn, which decomposes in turn under broad-band UV-visible light (lambda=200-800 nm); some H3Al(mu-H)2Sn is formed even on deposition. The data collected from experiments with SnH4 and SnD4 and different reagent concentrations, together with the results of quantum chemical calculations, are used to interpret the results and elucidate the structures and bonding of the new species.

High-resolution EPR and the origin of the spectrum anisotropy of CH3 radicals in Ar, Kr, and CO matrices at liquid helium temperatures

High-resolution EPR spectra of CH3 radicals have been observed in Ar, Kr, and CO matrices in the temperature range 1.6–4.2K. These spectra showed an anisotropy of both the A-tensor and the g-tensor. It is shown that the experimental spectrum can be simulated as a spectrum of a powder sample with axially symmetric g and A tensors. Comparing the spectra in various matrices and studying samples of both pure matrices and mixture matrices, we found a significant contribution of the radical–matrix interaction to the anisotropy. The contribution of the radical rotation at helium temperatures to the CH3 spectrum appearance and the role it plays in the spectrum averaging are also discussed.

Low-lying electronic states of the Ti2 dimer: electronic absorption spectroscopy in rare gas matrices in concert with quantum chemical calculations.

Absorption spectra were measured for Ti2 in Ne and Ar matrices. The spectra give evidence for several electronic transitions in the region between 4000 and 10 000 cm(-1) and provide important information about some excited electronic states of Ti2 in proximity to the ground state. The vibrational fine structure measured for these transitions allowed to calculate the force constants and the anharmonicity of the potential energy curves of the excited states, and to estimate changes in the internuclear Ti-Ti distances relative to the electronic ground state. The quantum chemical studies confirm the previously suggested (3)Delta(g) state as the ground state of Ti2. The equilibrium bond distance is calculated to be 195.4 pm. The calculated harmonic frequency of 432 cm(-1) is in good agreement with the experimental value of 407.0 cm(-1). With the aid of the calculations it was possible to assign the experimentally observed transitions in the region between 4000 and 10 000 cm(-1) to the 1 (3)Pi(u)<--(3)Delta(g), 1 (3)Phi(u)<--(3)Delta(g), 2 (3)Pi(u)<--(3)Delta(g), 2 (3)Phi(u)<--(3)Delta(g), and (3)Delta(u)<--(3)Delta(g) excitations (in the order of increasing energy). The calculated relative energies and harmonic frequencies are in pleasing agreement with the experimentally obtained values, with deviations of less than 5% and 2%, respectively. The bond distances estimated on the basis of the experimental spectra tally satisfactorily with the predictions of our calculations.

Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis-->trans tunneling rate.

Rotational isomerization of acetic acid (CH3COOH) is studied in Ar, Kr, and Xe matrices. The light-induced trans-->cis reaction is promoted using resonant excitation of a number of modes in the 3500-7000 cm(-1) region, and the quantum yields for this process are measured for various acetic acid isotopologues and matrix materials. For excitation of acetic acid at energies above the predicted isomerization energy barrier (> or =4400 cm(-1)), the measured quantum yields are in average 2%-3%, and this is one order of magnitude smaller than the corresponding values known for formic acid (HCOOH). This difference is interpreted in terms of the presence of the methyl group in acetic acid, which enhances energy relaxation channels competing with the rotational isomerization. This picture is supported by the observed large effect of deuteration of the methyl group on the photoisomerization quantum yield. The trans-->cis reaction quantum yields are found to be similar for Ar, Kr, and Xe matrices, suggesting similar energy relaxation processes for this molecule in the various matrices. The IR-induced cis-->trans process, studied for acetic acid deuterated in the hydroxyl group, shows reliably larger quantum yields as compared with the trans-->cis process. For pumping of acetic acid at energies below the predicted isomerization barrier, the trans-->cis reaction quantum yields decrease strongly when the photon energy decreases, and tunneling is the most probable mechanism for this process. For the cis-->trans dark reaction, the observed temperature and medium effects indicate the participation of the lattice phonons in the tunneling-induced process.

Structure and liquid crystalline properties of 2-hydroxyazobenzenes. X-ray diffraction, infra-red and DFT theoretical studies

The family of 2′-hydroxy-4′-alkyloxyazobenzenes containing in position 4 either CH 3 or Cl group with varying number of carbon atoms in alkyl group was analysed from the point of view of liquid crystalline properties and intramolecular OH⋯N hydrogen bonding. The phase transition diagrams show that insertion of the OH group in 2′ position leads to a marked extention of mesophases as compared with parent azobenzenes and appearance in addition to nematic of the smectic SmA phase in the case of 4-chloro-derivatives. The role of chelate conjugating OH⋯N hydrogen bonds in modification of mesophases is discussed based on DFT theoretical and X-ray as well as infrared studies. The importance of the molecule pairing is emphasized. Most important feature of mesophases and crystalline state of compounds under study is appearance of a continuous absorption down to 500 cm −1 which is absent in solutions and Ar matrices.

Reaction of the C3 carbon cluster with benzene

Abstract Benzene and C 3 are both known to exist in the interstellar medium (ISM). Their reaction has been studied here in cryogenic matrices using Fourier transform infrared (FT-IR) absorption spectroscopy. The reaction produced a weakly bonded complex for which a C–C asymmetric stretch vibrational frequency in the C 3 unit is shifted to lower energy (vs. uncomplexed C 3 ) by 4.5 and 1.6 cm −1 in Ar and Kr matrices, respectively. The singlet potential surface of the C 3 + C 6 H 6 reaction was partially mapped using density functional theory (DFT) theory (B3LYP with 6-31G** and 6-311 + G**). Although thirteen stable products are predicted, only two agree satisfactorily with the observed small shift and the isotopomer frequencies of 13 C-substituted C 3 · C 6 H 6 complexes. Products formed by the rearrangement of the complex's components apparently have no (or very low) entrance barriers. Finally, a low energy reaction pathway has been computed for the production of a dehydrogenated PAH (with an indene framework) from the reaction of the complexed C 3 and C 6 H 6 moieties. Indene has been observed previously in the discharge reactions of benzene where C 3 has also been produced.

Spectroscopy of the XeC2 molecule in xenon, argon, and krypton matrices

A self-igniting DC-electric discharge of C2H2 in Xe (matrix gas) or C2H2 and Xe in Ar or Kr (matrix gas) is used to produce and study the XeC2 molecule in these various rare gases at 12 K. Unlike in Ar and Kr, the well-known electronic spectra of C2 is completely absent in a Xe matrix. This together with annealing experiments in Ar matrices indicate that ground state Xe and C2 react uniquely and without a barrier to form the XeC2 molecule. The IR-active C-C stretch of this compound is found to be close to the C-C stretching frequency of the C2 anion, in excellent agreement with our density functional theoretical (DFT) calculations, which yield a XeCC singlet species bent by 148.6° and with substantial charge separation approaching Xe+C2– and a notably short (2.107 Å) Xe—C bond. The spectra of the Xe–13C–12C, Xe–12C–13C, and Xe–13C–13C species are also obtained and the isotopic shifts are in excellent agreement with the DFT predictions, although not sufficient to distinguish a bent from a linear structure. Numerous broad absorptions centered near 423 nm (in Xe) are observed, which are clearly due to the XeC2 molecule. Laser-induced fluorescence studies reveal a near-IR emission likely due to XeC2 but not yet understood. Infrared spectra in the Xe matrix reveal also formation of the HXeCCH molecule.Key words: matrix-isolation spectroscopy, rare gas compounds, charge transfer compounds, xenon–carbon bonds.

Ion pair state emission from I2 in rare gas matrices: effects of solvent induced symmetry breaking†

In this paper, we employ a diabatic model Hamiltonian based on the DIM (diatomics-in-molecules) and DIIS (diatomics-in-ionic-systems) methods to describe the valence and ion-pair states of the iodine molecule, with the objective of characterizing the influence of Ar and Kr rare gas matrices on the ion-pair states. In particular, we combined this model Hamiltonian with classical simulations to calculate the shifts of the ion-pair state emission spectra due to solvation environment. The computed main emission peaks in Ar and Kr solids appear at 378 and 417nm, in excellent agreement with the experimental emission peaks of 380 and 418nm recently observed by Chergui et al. with the identical transition assignments. Our study also shows that these calculated bands correspond to redshifts from the gas phase of 2600 and 5100 cm−1, compared with shifts of 2900 and 4700 cm−1, respectively, observed in experiments, and the equilibrium bond lengths of the D′ (2g) state in Ar and Kr extend to 3.70 and 3.75Å compared to the gas-phase value of 3.594Å, consistent with experimental implications. Emission bands originating from the δ(2u) state are also computed, as well as the counterparts of these bands in the visible region and the results are shown to be in reasonable agreement with experiments. We present a detailed study of the ion-pair state symmetry breaking which mixes the symmetrical gas-phase states resulting in charge localization, and the establishment of permanent molecular dipole moments in these solvated ion-pair state molecules due to local distortion of the polarizable matrix. Finally we use our calculations to explore model defect solvation sites in solid Ar which have been proposed as the source of various ambiguous emission bands in experimental spectra.

Ti2: Accurate Determination of the Dissociation Energy from Matrix Resonance Raman Spectra and Chemical Interaction with Noble Gases.

UV-visible and resonance Raman spectra of Ti(2) isolated in Ar, Kr, and Xe matrices at temperatures of 10 K were measured by using the 514 nm line of an Ar ion laser. The data show that the Ti(2) molecule interacts strongly with Xe, leading to a significant weakening of the Ti[bond]Ti bond strength. The f(Ti[bond]Ti) force constant decreases in the series Ar>Kr>Xe, from 232.8 Nm(-1) in Ar and 225.5 Nm(-1) in Kr to 199.7 Nm(-1) in Xe. Additional experiments in an Ar matrix containing 2 % of Xe indicate the formation of a molecule of the formula Ti(2)Xe. Our spectra for Ti(2) in an Ar matrix give evidence for several previously not observed members of the Stokes progression. The sum of experimental data allows for an improved estimation of the dissociation energy on the basis of a LeRoy-Bernstein-Lam analysis. A dissociation energy of 1.18 eV was derived from this analysis. The UV-visible data give evidence of the vibrational levels of an excited state of Ti(2).

Ti2: accurate determination of the dissociation energy from matrix resonance Raman spectra and chemical interaction with noble gases.

UV-visible and resonance Raman spectra of Ti(2) isolated in Ar, Kr, and Xe matrices at temperatures of 10 K were measured by using the 514 nm line of an Ar ion laser. The data show that the Ti(2) molecule interacts strongly with Xe, leading to a significant weakening of the Ti[bond]Ti bond strength. The f(Ti[bond]Ti) force constant decreases in the series Ar>Kr>Xe, from 232.8 Nm(-1) in Ar and 225.5 Nm(-1) in Kr to 199.7 Nm(-1) in Xe. Additional experiments in an Ar matrix containing 2 % of Xe indicate the formation of a molecule of the formula Ti(2)Xe. Our spectra for Ti(2) in an Ar matrix give evidence for several previously not observed members of the Stokes progression. The sum of experimental data allows for an improved estimation of the dissociation energy on the basis of a LeRoy-Bernstein-Lam analysis. A dissociation energy of 1.18 eV was derived from this analysis. The UV-visible data give evidence of the vibrational levels of an excited state of Ti(2).

Infrared absorption spectroscopy of the CnXe (n = 2, 3, 5, 7, 9) species

CnXe (n = 2, 3, 5, 7, 9) compounds, generated by pulsed laser evaporation of a carbon pellet or by the electrical discharge of acetylene, have been deposited in an argon matrix at 12 K and studied by Fourier transform infrared absorption spectroscopy. Five new vibrational bands at 1774.2, 2033.3, 2161.4, 2126.6 and 1997.2 cm−1 have been observed. Using ab initio MP2 theoretical methods, the first two bands have been assigned to the fundamental stretching modes of C2Xe and C3Xe, respectively. These assignments are supported by 13C isotopic shift data. The C–Xe bond in C2Xe is predicted to be stronger than in C3Xe. Although bent C3Xe is a little more stable than linear C3Xe, both species have very flat potential surfaces and both may exist in Ar matrices. The latter three bands are tentatively assigned to C–C asymmetric stretches in C5Xe, C7Xe, and C9Xe, respectively. Despite the lack of isotopic data for these species, this assignment is reasonable based on the smooth frequency shift for the CnXe species compared to the Cn species.

Synthesis of 1,3-diazepines and ring contraction to cyanopyrroles.

Several tetrazolo[1,5-a]pyridines/2-azidopyridines undergo photochemical nitrogen elimination and ring expansion to 1,3-diazacyclohepta-1,2,4,6-tetraenes, as well as ring cleavage to cyanovinylketenimines, in low temperature Ar matrices. 6,8-Dichlorotetrazolo[1,5-a]pyridine/2-azido-3,5-dichloropridine undergoes ready exchange of the chlorine in position 8 (3) with ROH/RONa. 8-Chloro-6-trifluoromethyltetrazolo[1,5-a]pyridine undergoes solvolysis of the CF(3) group to afford 8-chloro-6-methoxycarbonyltetrazolo[1,5-a]pyridine. Several tetrazolopyridines/2-azidopyridines afford 1H- or 5H-1,3-diazepines in good yields on photolysis in the presence of alcohols or amines. 5-Chlorotetrazolo[1,5-a]pyridines/2-azido-6-chloropyridines and undergo a rearrangement to 1H- and 3H-3-cyanopyrroles and, respectively. The mechanism of this rearrangement was investigated by (15)N-labelling and takes place via transient 1,3-diazepines. The structures of 6,8-dichloro-tetrazolo[1,5-a]pyridine, 6-chloro-8-ethoxytetrazolo[1,5-a]pyridine, dipyrrolylmethane, and 2-isopropoxy-4-dimethylamino-5H-1,3-diazepine were determined by X-ray crystallography. In the latter case, this represents the first reported X-ray crystal structure of a 5H-1,3-diazepine.

Luminescence spectroscopy of P13 and P3 state atomic mercury isolated in solid Ar, Kr, and Xe

Multicomponent emission bands are recorded for the P13→S01 transition of atomic mercury isolated at single sites in solid Ar, Kr, and Xe matrices. A blueshift observed at elevated temperatures on the 273 nm emission of Hg/Xe is identified in line shape analysis as arising from decreasing intensity of the central component in the band profile. The origin of the multiple components in the emission bands is ascribed to the existence of several vibronic modes which lead to excited state stabilization in the Hg(3P1)/RG matrix systems. A detailed description of these modes and their energetics is presented in the paper directly following. Photoexcitation of the P13 state also yields small amounts of P03 state emission. Hg atom P13 to P03 state intramultiplet relaxation (IMR) is most efficient in Hg/Xe where the ratio of this relaxation channel to P13 state radiative decay is 1/200 as established in time-integrated emission spectra. Despite the weakness of IMR, pulsed laser excitation combined with photon counting detection provide time-gated P03 state emission spectra largely free of the more intense P13 state emission. Such emission spectra recorded under high resolution for the P03→S01 transition of atomic mercury isolated in solid Xe provide the first example of the occurrence of a zero-phonon lines for a metal atom isolated in a rare gas matrix. Wp line shape analysis conducted on the emission bands recorded at specific temperatures, confirm this assignment. The electron–phonon coupling strength (Huang-Rhys, S factor) extracted in the line shape fits for the Hg/Xe transition is 1.3. Slightly stronger coupling is identified in Kr (S=2.2) and stronger still in Ar (S=3.3). Analysis of the diatomic Hg⋅RG potential energy curves reveal that the origin of the weak electron–phonon coupling lies primarily in the similarity in the ground and excited states, but also indicates the site size offered by the host solid plays a role.

Intrinsic lifetime of metastable excited C 4H 2: implications for the photochemistry of C 4H 2 in Titan's atmosphere

A better understanding of the complex organic chemistry occurring in the methane-rich atmosphere of Titan can be achieved by comparing observational data with the results of photochemical models. Until now, these models failed to reproduce the observed concentrations of C 4H 2. This may be due to the lack of kinetic data with regard to polyynes chemistry. In particular, no data were available for the intrinsic lifetime of the metastable excited state of C 4H 2. This state, probably a triplet excited state, seems to be a precursor in the photochemical reactions occurring in the atmosphere of Titan. Standard matrix isolation technique associated with time resolved spectroscopy is well adapted to characterize metastable states. Absorption and emission spectra (phosphorescence) of C 4H 2 in rare gas matrices are presented and a vibrational analysis is conducted. We also report the first experimental intrinsic lifetime of C 4 H 2 ∗ , measured in Ar and Kr matrices. A value of about 100 ms in the gas phase is deduced from matrices results ( ∼45 ms in Ar and ∼20 ms in Kr). The determined lifetime of C 4H 2, despite being two orders of magnitude higher than the value currently used in the models, cannot explain the discrepancy between models and observations. Nevertheless, it can account for an increase in the production of heavier compounds.