Matrix Isolation Spectroscopy Research Articles

Reactivity of Atomic Silicon: A Playground for Matrix Isolation Spectroscopy

AbstractFor Abstract see ChemInform Abstract in Full Text.

A matrix isolation and ab initio study of the hydrogen bonded complexes of acetylene with pyridine

Hydrogen bonded complexes of acetylene and pyridine were studied using matrix isolation spectroscopy and ab initio computations. The adduct was formed by depositing acetylene and pyridine in an argon matrix and a 1:1 C 2H 2–NC 5H 5 complex was identified using infrared spectroscopy. Formation of the adduct was evidenced from the shifts in the vibrational frequencies of C 2H 2 in the complex compared with that of free C 2H 2. The molecular structure, vibrational frequencies and stabilization energies of the complex were computed at the HF/6-31++G** and B3LYP/6-31++G** levels. We located one minimum on the potential surface, corresponding to a strongly bound C 2H 2–NC 5H 5 n–σ complex. Both experimental and computational data indicated that C 2H 2 acts as a proton donor and C 5H 5N as a proton acceptor.

Intra- and intermolecular hydrogen bonding in formohydroxamic acid complexes with water and ammonia: infrared matrix isolation and theoretical study

Abstract The complexes of formohydroxamic acid with water and ammonia have been studied using FTIR matrix isolation spectroscopy and MP2 calculations with a 6-311++G(2d,2p) basis set. The analysis of the experimental spectra of the HCONHOH/H 2 O(NH 3 )/Ar matrixes indicates formation of strongly hydrogen-bonded complexes in which the NH group of formohydroxamic acid acts as a proton donor toward the oxygen atom of water or the nitrogen atom of ammonia. The NH stretching vibration of formohydroxamic acid exhibits 150 cm −1 red shift in the complex with water and 443 cm −1 red shift in the complex with ammonia as compared to the NH stretch of the HCONHOH monomer. The theoretical calculations indicate stability of five isomers for the water complex and three isomers for the ammonia complex. The most stable are the cyclic structures in which the water or ammonia molecules are inserted within the intramolecular hydrogen bond of the formohydroxamic acid molecule and act as proton donors for the CO group and proton acceptors for the OH group of the formohydroxamic acid molecule. In spite of their stability the cyclic structures have not been observed in the matrixes which indicates high energy barrier for their formation, the reaction of complex formation is under kinetic and not thermodynamic control.

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.

Matrix isolation spectroscopy of H2O, D2O, and HDO in solid parahydrogen

We present infrared (IR) absorption spectra over the 800–7800 cm−1 region of cryogenic parahydrogen (pH2) solids doped with H2O, D2O and HDO molecules. Analysis of the rovibrational spectra of the isolated H2O, D2O and HDO monomers reveals their existence as very slightly hindered rotors, typically showing only 2–5% reductions in rotational constants relative to the gas phase. The nuclear spin conversion (NSC) of metastable J=1 ortho-H2O (oH2O) and para-D2O (pD2O) molecules follow first order kinetics, with single exponential decay lifetimes at T=2.4K of 1900±100 s, and 860±50 s, respectively. We report without discussion some absorptions of water clusters produced during sample annealing. We report and assign a number of absorptions to oH2–water pairs or ‘complexes’. The main features of the oH2–H2O and oH2–D2O spectra are explained qualitatively by assuming a semi-rigid C2v structure with the oH2 acting as a proton donor to the O atom. Surprisingly, NSC of oH2–water complexes proceeds at very nearly the same rate as for the corresponding water monomer. We report unassigned spectra of larger (oH2)n–water clusters, and the even more surprising observation of the prolonged survival of oH2O and pD2O molecules clustered with several oH2 molecules. We report and assign a number of water dopant-induced IR absorption features of the pH2 host, along with cooperative water–pH2 transitions in which the vibrational excitation of the pH2 solid is accompanied by a pure rotational transition of the water dopant.

Matrix isolation spectroscopy of H 2O, D 2O, and HDO in solid parahydrogen

We present infrared (IR) absorption spectra over the 800–7800 cm −1 region of cryogenic parahydrogen ( pH 2) solids doped with H 2O, D 2O and HDO molecules. Analysis of the rovibrational spectra of the isolated H 2O, D 2O and HDO monomers reveals their existence as very slightly hindered rotors, typically showing only 2–5% reductions in rotational constants relative to the gas phase. The nuclear spin conversion (NSC) of metastable J=1 ortho-H 2O ( oH 2O) and para-D 2O ( pD 2O) molecules follow first order kinetics, with single exponential decay lifetimes at T=2.4 K of 1900±100 s, and 860±50 s, respectively. We report without discussion some absorptions of water clusters produced during sample annealing. We report and assign a number of absorptions to oH 2–water pairs or ‘complexes’. The main features of the oH 2–H 2O and oH 2–D 2O spectra are explained qualitatively by assuming a semi-rigid C 2 v structure with the oH 2 acting as a proton donor to the O atom. Surprisingly, NSC of oH 2–water complexes proceeds at very nearly the same rate as for the corresponding water monomer. We report unassigned spectra of larger ( oH 2) n –water clusters, and the even more surprising observation of the prolonged survival of oH 2O and pD 2O molecules clustered with several oH 2 molecules. We report and assign a number of water dopant-induced IR absorption features of the pH 2 host, along with cooperative water– pH 2 transitions in which the vibrational excitation of the pH 2 solid is accompanied by a pure rotational transition of the water dopant.

Matrix-isolation and solid state low temperature FT-IR study of 2,3-butanedione (diacetyl)

2,3-Butanedione (diacetyl) was studied by matrix-isolation and low temperature solid state FT-IR spectroscopy, supported by molecular orbital calculations undertaken at the DFT(B3LYP) and MP2 levels of theory with the 6-311++G(d,p) basis set. Both in the crystalline phase and in the matrices, the compound exists in the C 2h symmetry trans conformation (OC–CO dihedral angle of 180°). This form corresponds to the single conformational state predicted by the theoretical calculations for the compound in vacuum. However, in the low temperature amorphous state, obtained by fast deposition of the vapour of the compound onto a suitable cold (9 K) substrate, as well as in the liquid and gaseous phases, spectroscopic features are observed that can only be explained by assuming that conformations without an inversion centre ( C 2 symmetry) do also contribute to the spectra. These results are in agreement with the experimental evidence that diacetyl has a permanent dipole moment (ca.1 Debye) in the vapour phase at room temperature and are here explained taking into consideration the influence of the low frequency large amplitude torsional vibration around the central C–C bond on the molecular properties.

The Characterization and Reactivity of Photochemically Generated Phenylene Bis(diradical) Species as Revealed by Matrix Isolation Spectroscopy and Computational Chemistry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Matrix Isolation of Electron Bombarded Gases Containing Fe(CO)5: An FTIR Absorption Study of Neutral and Anion Decomposition Products

Fe(CO)5 was used as a probe of the efficiency of cation and anion trapping in cryogenic matrices under electron-bombardment, matrix-isolation spectroscopy (EBMIS) conditions. Ar gas containing Fe(CO)5 (1:1000−3000; Fe:Ar) was subjected to electron bombardment with 150−300 eV electrons above a 16 K spectroscopic window. FTIR spectroscopic analysis of the matrix-isolated products demonstrates the presence of the known neutral and anionic subcarbonyl species, Fe(CO)n (n = 2, 3, and 4) and Fe(CO)m- (m = 3 and 4). Yields for both neutrals and anions diminish significantly with decreasing carbonyl ligand number. Annealing studies in Ar indicate facile conversion of Fe(CO)4 and Fe(CO)4- to higher nuclearity iron polycarbonyls, including Fe2(CO)9. Irradiation with visible light of samples of electron bombarded Fe(CO)5 in Ar was found to stimulate loss of Fe(CO)4 to form Fe(CO)5 by photoinduced CO recombination. Further irradiation with UV−visible light resulted in additional loss of Fe(CO)5 and Fe(CO)4-, and loss of features attributed to CO perturbed by Fe(CO)4-, with concomitant production of Fe(CO)4. The mechanism for formation of these products and the efficiency of ion trapping was further investigated through electron bombardment of Fe(CO)5 in Ar containing small amounts of CH4 or N2, and in pure Kr, Xe, CH4, and N2. Essentially the same distribution of neutral and anionic products was observed in Kr and Xe, but with somewhat diminishing yields. The presence of small amounts of CH4 (4%) or N2 (2%) in Ar resulted in decreased yields of all products (<50%), with preferential loss of features associated with Fe(CO)4 compared with Fe(CO)4-. Experiments in pure methane gave very small product yields (<10%), and in pure N2, no products were observed. Anion product yields are consistent with gas-phase studies of the dissociative capture of low-energy electrons by iron pentacarbonyl, and therefore, the matrix-isolated products appear to be qualitatively representative of the gas-phase product distribution for such a process. The absence of cationic species is in contrast to the efficient production of all cationic subcarbonyls in gas-phase EI experiments. This difference is believed to be due to the low efficiency with which Ar, Kr, or Xe trap cationic species, as well as the high concentration of scattered secondary electrons in the EBMIS environment, resulting in the isolation of the neutral counterparts of cationic species.

Laboratory Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbon Molecules

Infrared spectroscopy of a variety of interstellar sources shows strong mid-IR emission bands, which are generally attributed to emission from highly vibrationally excited polycyclic aromatic hydrocarbon molecules (PAHs) in the neutral and, particularly, cationic states. Over the past decade, various experimental methods have been developed to record the infrared spectra of cationic PAHs in the laboratory. In this paper, we discuss available experimental spectra obtained with matrix isolation spectroscopy (MIS), infrared multiple-photon dissociation of trapped ions (MPD), dissociation spectroscopy of ionic PAH van der Waals clusters (VDW), and infrared emission (IRE). Moreover, we compare these experimental spectra to density functional theory (DFT) calculations. The main body of experimental data relies on MIS and MPD spectra, and we present a detailed comparison of results from these methods, providing linear and multiple-photon absorption data, respectively. The effects of multiple-photon absorption, as encountered in MPD, and multiple-photon emission, occurring in interstellar spectra, are carefully assessed with the use of mathematical models, which include the effects of vibrational anharmonicity. We show that an analysis of the multiple-photon and linear data can provide important information on the anharmonicity parameters, which is otherwise difficult to attain. This is illustrated with a detailed comparison of the linear and multiple-photon absorption spectra of the naphthalene cation, yielding experimental anharmonicity parameters for the IR-active modes in the 500-1700 cm-1 range.

Conformational structures and vibrational spectra of isolated pyrimidine nucleosides: Fourier transform infrared matrix isolation study of 2-deoxyuridine

The conformational structures of 2-deoxyuridine (dU) were investigated using Fourier transform infrared (FTIR) matrix isolation spectroscopy. For the first time the FTIR spectra of dU in Ar matrices were obtained in the range 4000–200 cm−1. The stabilities of conformers were estimated by the methods HF/3-21G (p), HF/6-31G (d,p) and MP2/6-31G (d,p). Ab initio calculations of the infrared spectra were performed by the methods HF/3-21G (p) and HF/6-31G (d,p). The actual occupancy of conformational isomers in matrix samples was determined. It was shown that anti-conformers of dU are dominant. The ribose rings of the main anti-conformers dU _a0, dU _a1 are in the C2′-endo conformation, but the ribose rings of minor anti-conformers dU_a2, dU_a3 have the C3′-endo conformation, stabilized by intramolecular hydrogen bonds O3′H⋯O5′ and O5′H⋯O3′, accordingly. Syn-conformers of dU are stabilized by the intramolecular hydrogen bond O5′H⋯O2 and the dominant conformation of the ribose ring is C2′-endo.

Intense, hyperthermal source of organic radicals for matrix-isolation spectroscopy

We have incorporated a pulsed, hyperthermal nozzle with a cryostat to study the matrix-isolated infrared spectroscopy of organic radicals. The radicals are produced by pyrolysis in a heated, narrow-bore (1-mm-diam) SiC tube and then expanded into the cryostat vacuum chamber. The combination of high nozzle temperature (up to 1800 K) and near-sonic flow velocities (on the order of 104 cm s−1) through the length of the 2 cm tube allows for high yield of radicals (approximately 1013 radicals pulse−1) and low residence time (on the order of 10 μs) in the nozzle. We have used this hyperthermal nozzle/matrix isolation experiment to observe the IR spectra of complex radicals such as allyl radical (CH2CHCH2), phenyl radical (C6H5), and methylperoxyl radical (CH3OO). IR spectra of samples produced with a hyperthermal nozzle are remarkably clean and relatively free of interfering radical chemistry. By monitoring the unimolecular thermal decomposition of allyl ethyl ether in the nozzle using matrix IR spectroscopy, we have derived the residence time (τnozzle) of the gas pulse in the nozzle to be around 30 μs.

Electronic Absorption Spectra of Neutral Perylene (C20H12), Terrylene (C30H16), and Quaterrylene (C40H20) and Their Positive and Negative Ions: Ne Matrix-Isolation Spectroscopy and Time-Dependent Density Functional Theory Calculations

We present a full experimental and theoretical study of an interesting series of polycyclic aromatic hydrocarbons, the oligorylenes. The absorption spectra of perylene, terrylene and quaterrylene in neutral, cationic and anionic charge states are obtained by matrix-isolation spectroscopy in Ne. The experimental spectra are dominated by a bright state that red shifts with growing molecular size. Excitation energies and state symmetry assignments are supported by calculations using time dependent density functional theory methods. These calculations also provide new insight into the observed trends in oscillator strength and excitation energy for the bright states: the oscillator strength per unit mass of carbon increases along the series.

Thermal reactivity of HNCO with water ice: an infrared and theoretical study

Abstract The structure and energy of the 1:1 complexes between isocyanic acid (HNCO) and H 2 O are investigated using FTIR matrix isolation spectroscopy and quantum calculations at the MP2/6-31G(d,p) level. Calculations yield two stable complexes. The first and most stable one (Δ E =23.3 kJ/mol) corresponds a form which involves a hydrogen bond between the acid hydrogen of HNCO and the oxygen of water. The second form involves a hydrogen bond between the terminal oxygen of HNCO and hydrogen of water. In an argon matrix at 10 K, only the first form is observed. Adsorption on amorphous ice water at 10 K shows the formation of only one adsorption site between HNCO and ice. It is comparable to the complex observed in matrix and involves an interaction with the dangling oxygen site of ice. Modeling using computer code indicates the formation of polymeric structure on ice surface. Warming of HNCO, adsorbed on H 2 O ice film or co-deposited with H 2 O samples above 110 K, induces the formation of isocyanate ion (OCN − ) characterized by its ν as NCO infrared absorption band near 2170 cm −1 . OCN − can be produced by purely solvation-induced HNCO dissociative ionization. The transition state of this process is calculated 42 kJ/mol above the initial state, using the ONIOM model in B3LYP/6-31g(d,p).

The ground state and electronic spectrum of CUO: a mystery.

Results are presented from a theoretical study of the lower electronic states of the CUO molecule. Multiconfigurational wave functions have been used with dynamic correlation added using second order perturbation theory. Extended basis sets have been used, which for uranium were contracted including scalar relativistic effects. Spin orbit interaction has been included using the state-interaction approach. The results predict that the ground state of linear CUO is phi2 with the closed shell sigma(+)0 state 0.5 eV higher in energy. This is in agreement with matrix isolation spectroscopy, which predicts phi2 as the ground state when the matrix contains noble gas atoms heavier than Ne. In an Ne matrix, the experiments indicate, however, that CUO is in the sigma(+)0 state. The change of ground state due to the change of the matrix surrounding CUO cannot be explained by the results obtained in this work and remains a mystery.

Photolysis of regioisomeric diazides of 1,2-diphenylacetylenes studies by matrix-isolation spectroscopy and DFT calculations.

A series of diazides of 1,2-diphenylacetylenes was photolyzed in matrices at low temperature and transient photoproducts were characterized by using IR, UV/vis methods combined with ESR studies. Theoretical calculations were also used to understand the experimental findings. The introduction of phenylethynyl groups on phenyl azides has little effect on the photochemical pathway. Thus, upon photoexcitation, (phenylethynyl)phenyl azides afforded the corresponding triplet nitrene, which is in photoequilibrium with the corresponding azacycloheptatetraene. In marked contrast, azidophenylethynyl groups exhibited a dramatic effect not only on the photochemical pathway of phenyl azides but also on the electronic and molecular structure of the photoproducts. The patterns of the effect depended upon the relative position of azide groups in the diphenylacetylene unit. Whenever two azide groups were situated in a conjugating position with respect to each other, as in p,p'-, o,o'-, and p,o'-bis(azides), the azides always resulted in the formation of a quinoidal diimine diradical in which unpaired electrons were extensively delocalizedin the pi-conjugation. The situation changed rather dramatically when azide groups were introduced in the meta position. Thus, the formation of azacycloheptatetraene was noted in the photolysis of the m.m'-isomer. ESR studies indicated the generation of a quintet state that was shown to be a thermally populated state with a very small energy gap of ca. 100 cal mol(-1). The m,p'-isomer was shown to be an excellent precursor for the high-spin quintet dinitrene. The IR spectra of the photoproduct showed no bands ascribable to azacycloheptatetraene. The observed spectra were in good agreement with that calculated for the quintet state. Strong EPR signals assignable to the quintet state were observed, along with rather weak signals due to mononitrenes. Moreover, the quintet bis(nitrene) was rather photostable under these conditions.

Embedding Impurities into Solid Helium

We have developed a new technique to imbed impurities in solid 4He. It consists in injecting helium gas mixed with the impurity on the top surface of Helium liquid-solid interface, with the solid continuously moving downwards by pumping at the bottom of the cell. Due to the large temperature gradient, sedimentation occurs through the thin (1–2 mm) upper layer of liquid helium. We achieved a guest-particle density of 3 ⋅ 1019 per cm3 and a doped crystal growth rate of 0.05 mm/s. The possibilities of this novel method for matrix isolation spectroscopy as well as for quantum crystal studies are discussed.

Chlorine oxide radicals ClOx (x = 1-4) studied by matrix isolation spectroscopy.

Low pressure flash thermolysis of different precursor molecules containing-ClO, -ClO3 or -OClO3 yield, when highly diluted in Ne or O2 and subsequent quenching of the products in a matrix at 5 or 15 K, ClOx (x = 1, 3, 4) radicals, respectively. If Ne or O2 gas is directed over solid ClO2 at -120 degrees C and the resulting gas mixtures are immediately deposited as a matrix, a high fraction of (OClO)2 is trapped. This enables recording of IR and UV spectra of weakly bonded (OClO)2 dimers and detailed studying of their photochemistry. For Ne or O2 matrix isolated ClO radicals the vibrational wavenumbers and electronic transitions are only slightly affected compared with the gas phase. In this study strong evidence is found for long lived ClO in the electronically excited 2 [symbol: see text] 1/2 state. A comprehensive IR study of Ne matrix isolated ClO3 (fundamentals at 1081, 905, 567, 476 cm-1) yield i) a reliable force field; ii) a OClO bond angle of alpha e = 113.8 +/- 1 degrees and iii) a ClO bond length of 148.5 +/- 2 pm in agreement with predicted data from quantum chemical calculations. The UV/Vis spectrum of ClO3 isolated in a Ne matrix (lambda max at 32,100 and 23,150 cm-1) agrees well with the photoelectron spectrum of ClO3- and theoretical predictions. The origin of the structured high energy absorption is at 22,696 cm-1 and three fundamentals (794, 498, 280 cm-1) are detected in the C2E state. By photolysis of ClO3 with visible light the complex ClO.O2 with ClO in the 2 [symbol: see text] 1/2 state is formed. In an extended spectroscopic study of the elusive ClO4 radical, isolated in a Ne or O2 matrix, three additional IR bands, a complete UV spectrum and a strong interaction with O2 are found. This leads to the conclusion that ClO4 exhibits C2v or Cs symmetry with a shallow potential minimum and forms with O2 the previously unknown peroxy radical O3ClO-O2. All these results are discussed in the context of recent developments in the chemistry and spectroscopy of the important and interesting ClOx (x = 1-4) family of radicals.

Transition Metal—Noble Gas Complexes.

Publisher Summary This chapter discusses transition metal-noble gas complexes. Transition metal-noble gas complexes containing a d 8 -metal center have been characterized using matrix-isolation spectroscopy. Matrix isolation spectroscopy has proved to be an invaluable technique for the isolation and characterization of transition metal-noble gas complexes. In the study of transition metal-noble gas bonding, the use of supercritical noble gases as solvents is a natural extension of the use of liquefied noble gases. Experimental research into transition metal-noble gas complexes has provided a plethora of spectroscopic information, allowing their characterization and their reactivities to be determined. Making the metal center as electron-deficient as possible would increase the strength of the transition metal-noble gas bond in some species. An attractive medium in which to perform the synthesis of a transition metal-noble gas complex is scXe itself.

Hydrogen atoms in impurity-helium solids.

Electron spin resonance (ESR) is employed to study atomic impurities (H and D) stabilized in impurity-helium (Im-He) solids at 1.35-1.5 K. The kinetics of the low temperature tunneling exchange reactions (D+H2-->H+HD, D+HD-->H+D2) are investigated in Im-He samples containing several different mixtures of hydrogen and deuterium impurities. The ESR line structures help determine the local environment of atoms trapped in Im-He solids. High concentrations of atomic hydrogen stored in Im-He solids may ultimately find applications in energy storage, matrix-isolation spectroscopy, and studies of different quantum statistical effects.