Low-temperature Ar Matrix Research Articles

Cis−trans Formic Acid Dimer: Experimental Observation and Improved Stability against Proton Tunneling

We prepared the first cis-trans dimer of formic acid and measured its vibrational spectrum in a low-temperature Ar matrix. This preparation was done by selective vibrational excitation of the trans-trans noncyclic dimer. It was found that the stability of the cis-trans dimer against proton tunneling is strongly improved compared to the monomer, especially at elevated temperatures (>30 K). This surprising phenomenon was explained by differences in dynamical, energetic, and vibrational properties of the dimer and monomer. The obtained results show that the proton tunneling reactions can be strongly modified in the hydrogen-bonded solid network compared to the monomeric species.

UV-Induced Oxo → Hydroxy Unimolecular Proton-Transfer Reactions in Hypoxanthine

Monomers of hypoxanthine isolated in low-temperature Ar matrixes were studied using Fourier transform infrared spectroscopy. Two most stable tautomeric forms of hypoxanthine: oxo-N(9)-H and oxo-N(7)-H as well as a very small amount of the minor hydroxy-N(9)-H tautomer were observed in Ar matrixes directly after their deposition. UV irradiation of the matrixes induced conversion of the oxo-N(9)-H and oxo-N(7)-H tautomers of the compound into the hydroxy-N(9)-H and hydroxy-N(7)-H forms, respectively. Upon exposure of the matrixes to the UV (lambda > 270 nm) light, the oxo-N(9)-H --> hydroxy-N(9)-H phototautomeric reaction dominated strongly over the oxo-N(7)-H --> hydroxy-N(7)-H phototransformation. The latter phototautomeric reaction occurred effectively when matrix-isolated hypoxanthine was irradiated with shorter-wavelength (lambda > 230 nm) UV light. Thanks to this wavelength dependency, it was possible to clearly distinguish the oxo --> hydroxy photoreaction within the N(9)-H tautomers from the analogous phototautomeric process within the N(7)-H tautomers. All of the observed isomers of hypoxanthine (substrates and products of the photoreactions) were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-31++G(d,p) level of theory.

Ab initio geometry and anharmonic vibrational spectra of PbX 2 halides and PbX 2⋯L (X=F, Cl, Br, I; L=CO, N 2) complexes

Equilibrium geometries of PbX 2 halides and PbX 2⋯L (X=F, Cl, Br, I; L=CO, N 2) complexes have been determined at ab initio level with second order perturbative (MP2) treatment of electron correlation using Stevens–Basch–Krauss–Jasien–Cundari (SBKJC) relativistic effective core potentials (RECP) and basis sets. Optimized structures of PbX 2⋯L complexes have C s symmetry with end-on coordination of the ligand, and are characterized by Pb⋯C bond length within 279.9–281.6 pm, Pb⋯N distance of 273.9–278.0 pm, and Pb⋯L–L angle of 163.3–173.5 o. Anharmonic vibrational spectra of PbX 2 halides and PbX 2⋯L complexes have been determined from ab initio MP2/SBKJC+(d) potential energy surfaces using vibrational self-consistent field (VSCF) and correlation-corrected vibrational self-consistent field (CC-VSCF) methods. Sensitivity of ν(Pb–X) and ν(L–L) stretching modes to complex formation, established from CC-VSCF anharmonic spectra, closely follows that observed in low temperature Ar matrix infrared spectra of these complexes. Anharmonic estimates of ν(Pb⋯C) and ν(Pb⋯N) stretching modes were computed at 137–131 and 149–150 cm −1, respectively.

M^|^ouml;ssbauer Investigation into the Reactions of Laser-evaporated Iron with Solid Oxygen at Low Temperatures

Laser-ablation has been studied extensively and has been applied to many fields including the surface treatment of solids and the formation of thin films. Laser-evaporation is a very useful method for vaporizing various kinds of materials without causing radiant heating of surrounding materials. This makes it a convenient method for vaporizing materials near a cold head in a cryostat in order to study low-temperature reactions. We have previously reported reactions of laser-evaporated iron atoms with various reactant gases using a low temperature matrix isolation technique and Mossbauer spectroscopy. 1-4 The reactions of laser-evaporated iron atoms with oxygen produced FeO, Fe(O2), FeO3, (O2)FeO2, and OFeO which are reaction products of the gas phase and are trapped in a low temperature Ar matrix. 5 We have also reported laserdeposition of Fe metal onto Al or Si substrates at various temperatures, 6 and the formation of Fe-Al alloy or Fe-Si compounds was observed at the boundary between the Fe-films and the Al or Si substrate. Laser-deposition of hematite or magnetite onto Al substrates produced iron oxide films whose composition varied depending on the substrate temperature. 7 Thereby laser-deposition has the possibility of producing functional films whose chemical compositions and physical properties can be controlled. Here, we report the reactions of laser-evaporated Fe with solid oxygen at 20 K. While the experimental setup is very similar to that employed in our previous matrix-isolation studies, 5 in this study the Fe atoms and O2 gas are introduced alternately to the cold substrate in order to investigate the reaction of Fe with solid oxygen rather than the gas-phase reactions. This study is also a useful cross reference for the 57 Mn in-beam Mossbauer study of solid oxygen, 8 although the densities of 57 Fe and 57 Mn atoms are very different. Furthermore the reac

UV-Induced Generation of Rare Tautomers of 2-Thiouracils: A Matrix Isolation Study

Unimolecular photoisomerization reactions were studied for 2-thiouracil, 6-aza-2-thiothymine, 1-methyl-2-thiouracil, and 3-methyl-2-thiouracil isolated in low-temperature Ar matrixes. The IR spectra have revealed that before UV irradiation all the matrix-isolated compounds adopted exclusively the oxo-thione tautomeric form. Upon UV (lambda > 320 nm) irradiation of the matrixes, two oxo-thiol photoproducts were generated for monomeric 2-thiouracil as well as for monomeric 6-aza-2-thiothymine. Generation of these products corresponds to transfer of a proton from either the N(1)-H or N(3)-H group to the sulfur atom of the C(2)=S thiocarbonyl moiety. The first of the above reactions was photoreversible. As a consequence, after prolonged UV irradiation most of the material was transformed into the oxo-thiol-N(1)H form. The hydroxy-thiol tautomers of 2-thiouracil and 6-aza-2-thiothymine were also photogenerated as minor products. For 1-methyl-2-thiouracil and 3-methyl-2-thiouracil, thione --> thiol phototautomeric reactions yielded the oxo-thiol isomers of the compounds. Since these reactions were photoreversible, the final stages of the photoinduced processes corresponded, for both methylated 2-thiouracils, to photostationary states. All the products of the investigated photoreactions were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-311++G(2d,p) level.

Evidence for a non-planar conformer and conformational isomerization of o-fluoroanisole in a low-temperature Ar matrix

FT-IR spectra of o-fluoroanisole were measured in a low-temperature Ar matrix with and without UV irradiation. The observed bands, without irradiation, were assigned to the trans conformer with the aid of the quantum chemical calculations at the B3LYP/cc-pVTZ level. By comparing the IR spectrum, after irradiation, with the calculated spectra, the formation of a non-planar conformer was established. The non-planar conformer gradually decayed by a unimolecular process under dark conditions at 16 K, while the trans conformer increased. The back-reaction rate was estimated, and the reaction dynamics between the trans and non-planar conformers was discussed.

Pair Association and Complexation with Water of Matrix-Isolated Pyridine N-Oxide: A Theoretical Study of Their Manifestations in the IR Spectrum

The structure and the IR spectra of three types of pair associates of pyridine N-oxide, as well as of three different complexes of this compound with one or two water molecules, are calculated in terms of the supramolecular approach by the B3LYP/6-311+G(d, p) hybrid density functional method. The experimentally observed IR spectrum of pyridine N-oxide in a low-temperature Ar matrix is theoretically interpreted in detail. The agreement in sign and magnitude between all the calculated and observed frequency shifts of fundamental vibrations shows that the hydrogen-bonded complexes make the main contribution to the formation of complex bands of self-associates.

Proton transfer processes in selenourea: UV-induced selenone→selenol photoreaction and ground state selenol→selenone proton tunneling

Abstract Infrared spectrum of selenourea isolated in low temperature Ar matrix indicates that monomers of this compound adopt exclusively the selenone tautomeric form. UV irradiation ( λ >345 nm) of matrix-isolated selenourea leads to generation of the selenol tautomer (isoselenourea). Two conformers of the selenol tautomer with imino N–H bond oriented anti or syn with respect to the Se–H group were photoproduced. For the matrix kept at 10 K and in darkness, a proton tunneling transforming the photoproduced selenol anti form back into the initial selenone tautomer was observed. The time constant of this process was 16 h. The asymmetric barrier for the selenol → selenone proton tunneling was estimated at MP2/6-311++G(2d,p) level to be as high as 95 kJ mol −1 (7940 cm −1 ). The relation between the height of the barrier and the time constant of proton tunneling was discussed by comparing the current case with that previously studied for thiourea [J. Phys. Chem. A 107 (2003) 6373].

Combined Matrix-Isolation FT-IR and Theoretical Study of the Intrinsic Tautomeric, Vibrational, and H-Bonding Properties of N4-Methoxycytosine

The tautomeric, vibrational, and H-bonding characteristics of N4-methoxycytosine (N4MC) are investigated by a combined experimental, matrix-isolation Fourier transform infrared spectroscopic and computational study. For the theoretical part of the study, the methods RHF, MP2, and DFT(B3LYP) are used with the 6-31++G** basis set. N4MC occurs in a low-temperature Ar matrix exclusively in the oxo-imino form. Two isomers of this tautomer exist. Using IR intensities of characteristic bands, an approximate value of 2.3 × 10-2 is obtained for the isomerization constant KI (anti/syn). Theoretically, two closed H-bonded complexes with water are found for both isomers, i.e., N1−H...O−H...OC2 and N3−H...O−H...OC2, as well as one less stable open complex, N7...H−O and O8...H−O, for the syn and anti isomer, respectively. In the experimental FT-IR spectra, both closed H-bonded complexes are clearly identified based on characteristic H-bonding absorptions predicted by the calculations. Trace amounts of the open complexe...

A soft-landing experiment on organometallic cluster ions: infrared spectroscopy of V(benzene) 2 in Ar matrix

Vanadium (V)–benzene cluster ions, produced by laser ablation with reaction toward benzene vapor, were size-selected and deposited into a low-temperature Ar matrix. Infrared spectrum of V 1(benzene) 2 in the Ar matrix was measured after one-hour deposition with the deposition energy of 20 eV. The spectrum was in agreement with both the reported spectrum and our theoretical calculations, showing that (1) V 1(benzene) 2, prepared in the gas-phase reaction, takes a sandwich structure and that (2) the ions were soft-landed onto the Ar matrix and were neutralized by charge transfer from a metal substrate without fragmentation.

Geometrically Strained Carbenes: Interdependence among Geometry, Spin Multiplicity, and Reactivity.

The singlet and triplet states of cyclobutenylidene 1 and benzocyclobutenylidene 2 have been studied computationally (using ab initio and DFT methods) in order to assess the effect of angle strain on the S-T gap of vinyl- and phenylcarbenes. It is found that both carbenes have a singlet ground state. Cyclobutenylidene 1 hasa more significant singlet-triplet gap (-25 kcal mol-1) than benzocyclobutenylidene 2 (-14.5 kcal mol-1). The strong preference of 1 for a singlet ground state may be understood if it is viewed as bicyclobut-1-ene with a considerably puckered ring. Singlet benzocyclobutenylidene (12) is computed to have also a puckered cyclobutene ring albeit less pronounced. The lowest isomerization path available for singlet cyclobutenylidene (11) is the formation of vinylacetylene, which is predicted to have a barrier of around 9 kcal mol-1. Our calculations suggest that singlet benzocyclobutenylidene (12) lies in a rather deep potential well and should be observable under suitable experimental conditions. However, under low-temperature Ar matrix conditions no evidence for the formation of carbene 2 was obtained from the photolysis of precursor 15. On the other hand, laser flash photolysis (LFP) of precursor 16 enables us to observe benzocyclobutenylidene at room temperature. The lifetime of carbene 12 is determined to be 0.6-0.7, us in cyclohexane. The mCXY value for carbene 12 is determined to be 0.43, revealing a strong electrophilic nature and ranking 12 second among the electrophilic carbenes listed in the carbene philicity spectrum.

Electronic structure and short-range recombination dynamics of S2 in solid argon

Potential energy curves for 13 lowest electronic states of S2 and 6 lowest states of ArS are computed at the MRCI level utilizing the CASSCF orbitals. The electronic structure of S2 is described by the correlation consistent cc-pVQZ basis set, whereas for ArS the augmented version of this basis is combined with ten electron-core pseudopotential basis set for S and Ar, respectively. Thermal and shock wave induced recombination dynamics of sulfur atoms trapped in Ar lattice are investigated by classical Molecular Dynamics simulations. It is observed that atoms separated by nearest neighbor distance of the lattice do immediately recombine even at 1 K with no thermal activation. While separated by one lattice constant, the S atoms stay stable up to 80 K and no recombination is observed in the classical trajectories. Consequently, the simulation was able to reproduce the experimental S+S glow curve only by lowering the reaction barrier by introducing lattice vacancies in the four atom plane separating the S–S pair. Local 1.1 eV kinetic excitation of a lattice atom next to the trapped S–S pair or artificial shock waves initiated by more distant excitation at 2 eV or more greatly enhanced the probability for recombination. Nonradiative D1–3P relaxation of sulfur provides a mechanism for such phonon emission and is discussed as a potential reason for the observed photoinduced recombination in low temperature Ar matrix.

Density functional theory and ab-initio computational study of the 2-hydroxypyridine/2-pyridone system: a comparison with FT-IR data from matrix isolation experiments

The tautomeric equilibrium 2-hydroxypyridine⇔2-pyridone was observed in a low-temperature Ar matrix. The infrared spectra of the tautomers were analysed and assigned using DFT/(B3-LYP and B3-PW91)/6-31++G** and RHF/6-31++G** methods. The molecular parameters (rotational constants and dipole moments) are calculated with different methods and compared with recent gas-phase microwave data. The rotational constants and dipole moments predicted using the DFT method agree much better with the experimental results than the parameters predicted at the HF level. The energy difference for the tautomeric equilibrium and the tautomerization constant were calculated and these parameters are compared with recent theoretical and experimental predictions.

Visible light induced oxygen atom transfer from NO2 to (CH3)3N in a cryogenic ar matrix

Visible-light-induced oxygen-atom transfer from NO2 to trimethylamine has been investigated in a low-temperature Ar matrix. The bimolecular reaction with the threshold wavelength of 610 nm is tentatively interpreted by a mechanism to form the intermediate products of (CH3)2NONO and CH3, or the final products of (CH3)2NOCH3 and NO. Further irradiation at shorter wavelength (457.9–514.5 nm) seems to cause the secondary photolysis of (CH3)2NONO into (CH3)2NO and NO, which is followed by the recombination of (CH3)2NO with CH3 coexisting in a cryogenic Ar matrix cage to produce N,N,O-trimethylhydroxylamine, (CH3)2NOCH3. The reaction mechanism proposed is different from those reported for CH3NH2/NO2 and (CH3)2NH/NO2, the reason of which is discussed in comparison of the potential energy surfaces of three amine/NO2 photolytic systems.

Conformational Behavior of α-Alanine. Matrix-Isolation Infrared and Theoretical DFT and ab Initio Study

Two conformers of the nonionized α-alanine and its isotopomer N,N,O-d3-alanine have been observed in low-temperature Ar matrixes. Their infrared spectra have been analyzed and assigned using DFT/B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ geometry and frequency theoretical calculations. Two different intramolecular H-bonds, bifurcated NH2···OC and N···H−O, were found in the observed α-alanine conformers, I and IIa. We found that the DFT/B3LYP/aug-cc-pVDZ method yields vibrational frequencies of the α-alanine conformers in excellent agreement with the experimental data.

Computing vibrational energy relaxation for high-frequency modes in condensed environments

In this paper we consider vibrational relaxation of high-frequency impurity modes in condensed environments as a computational problem. Linear response theory provides convenient routes for this computation: The vibrational relaxation rate is obtained as a Fourier transform of a force–force time correlation function. However, numerical difficulties arise for processes characterized by a direct relaxation of high-frequency modes into an environment characterized by a relatively low cutoff frequency. It is shown that modern signal processing procedures can significantly enhance the efficiency and accuracy of the needed computation. Since the relevant “signal” can be very small, the computation can be very sensitive to boundary conditions, and care must be taken to avoid artifacts. The computation may be facilitated by using the expected functional form, exponential dependence on the impurity frequency for high frequency, and fitting the parameters of this form from the simulation. It is emphasized that this exponential dependence seems to be the correct functional form, in spite of theoretical arguments in favor of a Gaussian dependence. The main difficulty in the numerical evaluation of the relaxation rate of high-frequency modes results from the fact that at low temperature the dynamical behavior of such modes is essentially quantum mechanical. We demonstrate this issue by considering vibrational relaxation of an impurity CO molecule in a low-temperature Ar matrix. The results obtained for this system by estimating the quantum correction to the classical force–force correlation function are consistent with experimental results, which indicate that under these conditions the relaxation of the vibrationally excited CO is dominated by radiative decay.

Infrared Matrix Isolation and Theoretical Studies on Glutarimide

Infrared spectra of glutarimide isolated in low-temperature Ar and N2 matrixes are reported. The molecular structure, vibrational frequencies, and infrared intensities of glutarimide are calculated with ab initio Hartree−Fock and second-order Moller−Plesset perturbation (MP2) methods as well as with density functional theory (DFT) using the nonlocal gradient corrected functional (BP86). The best overall agreement between the calculated and experimental spectra has been obtained at the MP2/D95V** level. Unequivocal assignment of the experimental infrared bands is performed on the basis of the potential energy distribution (PED). A striking similarity is noted for frequencies of the corresponding CO and NH vibrations in glutarimide and in uracil, thymine, and their methyl derivatives. Furthermore, the significant flattening of the glutarimide ring, predicted by calculations, indicates its structural resemblance to pyrimidine bases. It is suggested that some glutarimide drugs are able to intercalate between ...

Infrared matrix isolation studies of amino acids. Molecular structure of proline

Abstract IR spectra of proline and deuteroproline isolated in low temperature Ar matrixes have been obtained. It is shown that in the isolated state proline exists in the molecular form. The spectra are interpreted using normal coordinate analysis. It is found that band splitting in the spectra is caused by the occurrence of two proline conformations. The structures of these conformation differ in the position of the COOH group with respect to pyrrolidine ring. The conformations are found to be stabilized by the intramolecular hydrogen bond. It has been shown that conformational equilibrium results in the splitting of most bands in the IR spectra of proline and deuteroproline. This splitting is maximal for CO stretching bands (23 cm −1 ). The structure and relative energies of the conformations are determined by the AM1 quantum chemical method.

Hydrogen bonded complexes of methyl thiolacetate with proton donors: an infrared investigation in CCl 4 solution and in Ar matrices

Hydrogen bonded complexes of methyl thiolacetate (MTA) with several proton donors have been studied using IR spectrometry. In CCl 4 solution the complexation of MTA with phenols occurs at the carbonyl oxygen lone pairs, and the interactions is weaker than that with methyl acetate (MA). On the other hand, in a low temperature Ar matrix both the carbonyl oxygen and the thiolester sulphur atoms are involved in hydrogen bonding with H 2O, HCl and HBr. The relative strength of both types of complexes is discussed, along with the relative basicity of esters and thiolesters.

ChemInform Abstract: INFRARED SPECTRUM OF PROPARGYLAMINE

Abstract Infrared absorption spectra of propargylamine in the gas phase and in the low-temperature Ar matrix were observed. The vibrational analysis was made with the help of an ab initio MO calculation. A set of effective force constants was derived which reproduces the observed frequencies. The spectrum indicated the occurrence of one rotational isomer, namely trans around the CN bond.