Kr Matrixes Research Articles

Unusual blue to red shifting of C-H stretching frequency of CHCl3 in co-operatively P⋯Cl phosphorus bonded POCl3-CHCl3 heterodimers at low temperature inert matrixes.

Heterodimers of POCl3-CHCl3 were generated in Ne, Ar, and Kr matrixes at low temperatures and were studied using infrared spectroscopy. The remarkable role of co-operative pentavalent phosphorus bonding in the stabilization of the structure dictated by hydrogen bonding is deciphered. The complete potential energy surface of the heterodimer was scanned by ab initio and density functional theory computational methodologies. The hydrogen bond between the phosphoryl oxygen of POCl3 and C-H group of CHCl3 in heterodimers induces a blue-shift in the C-H stretching frequency within the Ne matrix. However, in Ar and Kr matrixes, the C-H stretching frequency is exceptionally red-shifted in stark contrast with Ne. The plausibility of the Fermi resonance by the C-H stretching vibrational mode with higher order modes in the heterodimers has been eliminated as a possible cause within Ar and Kr matrixes by isotopic substitution (CDCl3) experiments. To evaluate the influence of matrixes as a possible cause of red-shift, self-consistent Iso-density polarized continuum reaction field model was applied. This conveyed the important role of the dielectric matrixes in inducing the fascinating vibrational shift from blue (Ne) to red (Ar and Kr) due to the matrix specific transmutation of the POCl3-CHCl3 structure. The heterodimer produced in the Ne matrix possesses a cyclic structure stabilized by hydrogen bonding with co-operative phosphorus bonding, while in Ar and Kr the generation of an acyclic open structure stabilized solely by hydrogen bonding is promoted. Compelling justification regarding the dispersion force based influence of matrix environments in addition to the well-known dielectric influence is presented.

Influence of inert matrixes on the conformational switching of trimethyl phosphate at low temperatures through thermal effects

The conformational switching of trimethyl phosphate (TMP) in Ne, Ar, Kr and Xe matrixes was studied using infrared spectroscopy under isolated conditions at low temperatures. In all the inert matrixes, the vapor phase population of ground state C3(G±G±G±) and the higher energy C1(TG±G±) conformers of TMP is preserved during the initial deposition. However, on annealing the inert Ar/Kr/Xe matrixes, the conformational composition of TMP was irreversibly altered. In Ar and Kr matrixes, an interconversion from ground state C3(G±G±G±) to higher energy C1(TG±G±) conformer was observed whereas Xe presents an interesting variation. In Xe matrix, the higher energy C1(TG±G±) conformer completely depopulated with a concomitant increase in the population of the ground state C3(G±G±G±) conformer. In Ne matrix notably, no conformational interconversion was observed. Computations on the conformers of TMP were performed using B3LYP and M06-2X levels of theory with 6-311++G(d,p) and aug-cc-pVDZ basis sets. The effect of inert matrixes was modeled using Self-Consistent Reaction Field (SCRF) methods.

Photochemical Formation of Diazenecarbaldehyde (HNNCHO) and Diazenecarbothialdehyde (HNNCHS) in Low-Temperature Matrices.

The photochemical decomposition of 1,2,4-oxadiazole-3,5-diamine and 1,2,4-thiadiazole-3,5-diamine was investigated in low-temperature Ar and Kr matrixes at different wavelengths. The analysis of matrix-isolation infrared (MI-IR) spectra aided by high-level quantum chemical computations showed not only that these photochemical reactions yield [NH2, C, N, X] (X = O, S) isomers but also that the bands of a novel, formerly unobserved species were observed. The comparison of computed IR spectra of potential products with the observed spectra suggests that these species are the diazenecarbaldehyde (HNNCHO) and diazenecarbothialdehyde (HNNCHS). Neither of the reactive HNNCHO and HNNCHS molecules was observed experimentally before. Both molecules are identified in the matrix as a complex with the other photoproduct, NH2CN. Comparison of the present experiments with former photochemical experiments on 1,2,5-oxadiazole-3,4-diamine and 1,2,5-thiadiazole-3,4-diamine and the analysis of the rate of formation of the different photoproducts indicate that HNNCHO and HNNCHS are formed in a different reaction path than H2NNCX and H2NC(NX) (X = O, S), and not by photoisomerization from these latter products.

Evidence of a Nitrene Tunneling Reaction: Spontaneous Rearrangement of 2-Formyl Phenylnitrene to an Imino Ketene in Low-Temperature Matrixes.

Triplet 2-formyl phenylnitrene was generated by photolysis of 2-formyl phenylazide isolated in Ar, Kr, and Xe matrixes and characterized by IR, UV-vis, and EPR spectroscopies. Upon generation at 10 K, the triplet nitrene spontaneously rearranges in the dark to singlet 6-imino-2,4-cyclohexadien-1-ketene on the time scale of several hours. The intramolecular [1,4] H atom shift from the nitrene to the imino ketene occurs by tunneling, on the triplet manifold, followed by intersystem crossing. This case constitutes the first direct evidence of a tunneling reaction involving a nitrene.

Vibrational Shifts of HXeCl in Matrix Environments

The hybrid quantum-classical simulations are performed to investigate the unusual vibrational spectral shifts of a noble-gas hydride HXeCl in matrix environments (in Ne, Ar, Kr, and Xe matrixes). The high-level ab initio calculations at the CCSD(T) level are employed to construct interaction potential energy surfaces between HXeCl and noble-gas atoms (Ne, Ar, Kr, and Xe). The configurations of noble-gas atoms are sampled by the Monte Carlo simulations and the vibrational levels of HXeCl in the presence of the surrounding noble-gas atoms are solved by the DVR approach. It is found that the H-Xe stretching frequencies are blue-shifted from the isolated gas-phase value in all matrix environments and that the relative blue shifts are in good agreement with the experimental results (Ne < Xe < Kr), demonstrating that the explicit treatment of matrix environments around HXeCl is essential to reproduce the observed unusual vibrational shifts.

Near-Infrared Laser Induced Conformational Change of Alanine in Low-Temperature Matrixes and the Tunneling Lifetime of Its Conformer VI

The near- and mid-IR spectra of α-alanine isolated in low-temperature Ar, Kr, and N2 matrixes were measured. Production of the short-lived conformer VI at the expense of the predominant conformer I was observed upon short irradiation with NIR laser light at the first O-H stretching overtone band of conformer I. Conformer VI decays by H-atom tunneling at 12 K with half-lives of 5.7 ± 1 s, 2.8 ± 1 s in Ar (two different sites), 7.0 ± 1 s in Kr, and 2.8 × 10(3) ± 1.2 × 10(3) s in N2. Upon prolonged irradiation, conformer I slowly transformed into conformer IIa. On the basis of these irradiation experiments, the unambiguous vibrational assignments of conformers I, IIa, and VI are given. In contrast to similar experiments for glycine, the irradiation experiments did not lead to the formation of conformer IIIb. This is explained by a very low IIIb → I barrier height computed for alanine, which results in a very fast depletion of conformer IIIb even in low-temperature matrixes.

Chiral and Achiral Fundamental Conformational Building Units of β-Peptides: A Matrix Isolation Conformational Study on Ac-β-HGly-NHMe and Ac-β-HAla-NHMe

The infrared spectra of two model beta-peptides, N-acetyl-3-aminopropionic acid-N'-methylamide (Ac-beta-HGly-NHMe) and N-acetyl-3-aminobutanoic acid-N'-methylamide (Ac-beta-HAla-NHMe), have been recorded in low-temperature Ar and Kr matrixes. The spectra were assigned by the help of electronic structure calculations. The analysis of spectra, in line with the theoretical predictions, revealed that both biocompatible peptide building blocks have a single dominant backbone conformer. Besides this prevalent conformer, which has a six-membered H-bonded pseudoring, conformers with eight-membered H-bonded rings are also observed but in a significantly smaller amount. The calculated conformer distribution is consistent with the experimental findings. The present work along with other recent results supports the concept that the backbone conformation of longer biopolymers, such as alpha- and beta-peptides, can be deciphered using the conformers of their structural building blocks. In this respect, our conformational study on the simplest models for beta-peptides both by IR spectroscopic experiments and quantum chemical studies has significance for the better design and understanding of the backbone conformations of larger beta-peptides with biomedical potential. The present conformational analysis suggests that although beta-peptides, having an "extra" backbone torsion and hence more conformational freedom, should be more flexible than alpha-peptides, fewer backbone conformers are viable based on their relative energies. Thus, from a larger conformational arsenal, only a lower number of backbone conformers can emerge, which possibly had a fundamental effect on their applicability during prebiotic evolution.

Insertion of Noble Gas Atoms into Cyanoacetylene: An ab Initio and Matrix Isolation Study

A computational and experimental matrix isolation study of insertion of noble gas atoms into cyanoacetylene (HCCCN) is presented. Twelve novel noble gas insertion compounds are found to be kinetically stable at the MP2 level of theory, including four molecules with argon. The first group of the computationally studied molecules belongs to noble gas hydrides (HNgCCCN and HNgCCNC), and we found their stability for Ng = Ar, Kr, and Xe. The HNgCCCN compounds with Kr and Xe have similar stability to that of previously reported HKrCN and HXeCN. The HArCCCN molecule seems to have a weaker H-Ar bond than in the previously identified HArF molecule. The HNgCCNC molecules are less stable than the HNgCCCN isomers for all noble gas atoms. The second group of the computational insertion compounds, HCCNgCN and HCCNgNC, are of a different type, and they also are kinetically stable for Ng = Ar, Kr, and Xe. Our photolysis and annealing experiments with low-temperature cyanoacetylene/Ng (Ng = Ar, Kr, and Xe) matrixes evidence the formation of two noble gas hydrides for Ng = Kr and Xe, with the strongest IR absorption bands at 1492.1 and 1624.5 cm(-1), and two additional absorption modes for each species are found. The computational spectra of HKrCCCN and HXeCCCN fit most closely the experimental data, which is the basis for our assignment. The obtained species absorb at quite similar frequencies as the known HKrCN and HXeCN molecules, which is in agreement with the theoretical predictions. No strong candidates for an Ar compound are observed in the IR absorption spectra. As an important side product of this work, the data obtained in long-term decay of KrHKr+ cations suggest a tentative assignment for the CCCN radical.

Photodissociation of Formyl Fluoride in Rare Gas Matrixes

Photodissociation of formyl fluoride (HCOF) is studied in Ar, Kr, and Xe matrixes at 248 and 193 nm excitation by following spectral changes in the infrared absorption spectra. In all matrixes, the main photodissociation products are CO/HF species, including CO-HF and OC-HF complexes and thermally unstable CO/HF species (a distorted CO/HF complex or a reaction intermediate), which indicate negligible cage exit of atoms produced via the C-F and C-H bond cleavage channels. However, the observation of traces of H, F, CO, CO(2), F(2)CO, FCO, and HRg(2)(+) (Rg = Kr or Xe) in Kr and Xe matrixes would imply some importance of other reaction channels too. The analysis of the decay curves of the precursor shows that dissociation efficiency of HCOF increases as Ar < Kr < Xe, the difference being the factor of 10 between Ar and Xe. Moreover, HCOF dissociates 20-50 times faster at 193 nm compared to 248 nm. Interestingly, whereas the CO/HF species are stable with respect to photolysis in Ar, they photobleach in Kr and Xe matrixes at 248 and 193 nm, even though the first excited states of CO and HF are not energetically accessible with 193 and 248 nm photons. In krypton matrix, the photodissociation of CO/HF species at 248 nm is observed to be a single photon process. Quantum chemical calculations of electronic excitation energies of CO-HF and OC-HF complexes show that the electronic states of HF and CO mostly retain their diatomic nature in the pair. This clearly demonstrates that photodissociation of CO/HF complexes is promoted by the surrounding rare gas lattice.

Rare Gas Effects on Hyperfine Coupling Constants of BO, AlO, and GaO

Using density functional theory methods and large basis sets, we calculated hyperfine coupling constants (HFCCs) for the (11)B, (17)O, (27)Al, and (69)Ga nuclei of the radicals BO, AlO, and GaO (XO), embedded in 2-14 rare gas (Rg) Ne and Ar atoms. Kr atoms were included for AlO. The distance of the Rg atoms from XO was varied from 4 to 12 bohr. Matrix effects cause A(iso)(X) to increase, accompanied by decreases in A(dip)(X) and A(dip)(O), while A(iso)(O) remains close to zero. Changes are largest for AlO, slightly smaller for GaO, and very small for BO, in line with the molecular polarizabilities. Observed changes of A(iso)(X) and A(dip)(X) for BO in Ne matrixes and for AlO in Ne, Ar, and Kr matrixes are reproduced in complexes with 12 Rg atoms at distances of 5-6 bohr or 14 Rg atoms at distances of 6-7 bohr. For GaO, experimental data are available only in Ne matrixes. Theoretical results obtained for HFCCs of (17)O could not be verified due to insufficient experimental information. Estimates of HFCCs in matrixes not yet experimentally studied and for GaO in the gas phase have been made. Due to the interaction with rare gas atoms, p-spin density on the X and O atoms of XO is converted into s-spin density on X, thereby causing an increase (in magnitude) of A(iso)(X), accompanied by decreases in A(dip) of X and O. The higher polarizability of XO along the bond axis is reflected in complexes that have axial Rg atoms showing larger changes in HFCCs than comparable complexes without axial Rg atoms.

Electronic Spectroscopy of C2 in Solid Rare Gas Matrixes

Electronic spectroscopy of the C(2) molecule is investigated in Ar, Kr, and Xe matrixes in the 150-500 nm range. In the Ar matrix, the D ((1)Sigma(u)(+)) <-- ((1)Sigma(g)(+)) Mulliken band near 240 nm is the sole absorption in the UV range, whereas in the Kr matrix additional bands in the 188-209 nm range are assigned to the Kr(n)()(+)C(2)(-) <-- Kr(n)()C(2) charge-transfer absorptions. Because of the formation of a bound C(2)Xe species, the spectral observations in the Xe matrix differ dramatically from the lighter rare gases: the Mulliken band is absent and new bands appear near 300 and 423 nm. The latter is assigned to the forbidden B'((1)Sigma(g)(+)) <-- X ((1)Sigma(g)(+)) transition, but the origin of the former remains unclear. The spectral assignments are aided by electronic structure calculations at the MCSCF, CCSD(T), and BCCD(T) levels of theory and correlation consistent basis sets. A significant presence of multireference character of the C(2)Xe system was noted and a linear ground-state structure is predicted. The computational results contradict previous density functional studies on the same system.

H/D isotope effects on formation and photodissociation of HKrCl in solid Kr

The HCl (DCl)/Kr matrixes were irradiated at 193 nm, which produced matrix-isolated H (D) and Cl atoms as permanent photolysis products. The annealing-induced formation of HKrCl and DKrCl was used as a measure of atomic hydrogen (H and D) mobility at various annealing temperatures (from 24 to 30 K). The temperature dependencies of the HKrCl and DKrCl formation rates yield ∼64 and 68 meV for the corresponding activation energies estimating the isotope effect on atomic hydrogen mobility in solid Kr (D slower than H). The difference in mobility of H and D atoms allowed us to demonstrate a reaction between D atoms and HKrCl molecules, and the suggested kinetic model is in good agreement with experiment. In addition, the H/D isotope effects on the solid-state photodissociation of HCl and HKrCl are studied and discussed.

Perylene and terrylene in rare gas matrixes: spectroscopic and computational studies of inclusion sites

The low-temperature (20 K) electronic absorption and emission spectra of terrylene (C 30H 16) and perylene (C 20H 12) isolated in Kr and Xe matrixes are presented. The spectra of both chromophores in Kr matrix reveal similar multiplet structure, and in each spectrum, six different sites were identified. In order to understand the origin of observed site—splitting, the theoretical calculations of complex system—chromophore embedded into Kr matrix—were carried out by means of MM+ method. The optimization of system structure yielded six different types (sites) of embedding chromophore molecules into Kr crystal lattice. As calculations have shown, the nearest environment of each site distinguishes by its specific density of rare gas atoms, on which basis the relationship between theoretically calculated sites and observed spectral features was established.

Spectroscopy and Relaxation Kinetics of Matrix-Isolated CH/D Radicals

The A2Δ−X2Π and B2Σ-−X2Π transitions of CH/D were examined for radicals trapped in Ar and Kr matrixes. Excitation spectra yield further evidence that CH/D(B) rotates in solid Ar and Kr. Vibrational relaxation of CH/D(B) is faster for the heavier isotope, indicating that vibration to rotation energy transfer is the dominant mechanism. The decay of CH(B), v = 0 is primarily radiative in both Ar and Kr, with small contributions from B → A nonradiative transfer. Fluorescence was not detected from CD(B), v = 0 as the B → A transfer process was much faster than radiative decay for this isotope. The proximity of the CD(B), v = 0 and (A), v = 1 levels is responsible for the accelerated transfer rate. Spectra for the A−X transition of CH−Arn clusters were recorded for comparison with the matrix data. Relative to free CH, the transition is blue-shifted in the cluster and red-shifted in an Ar matrix. This contrast suggests that the clusters consist of CH bound to the surface of Arn.

Photochemistry of hydrogen peroxide in Kr and Xe matrixes

UV photolysis of hydrogen peroxide (H2O2) in various rare-gas matrixes is comparatively studied. The photorecovery of H2O2 from the tight H2O⋯O complex is observed in Kr and Xe matrixes, in addition to this reaction in an Ar matrix found previously. The similarity of spectral position and efficiency of the photorecovery reaction in various rare-gas solids indicates its fundamental character, supports charge-transfer excitation of H2O⋯O as its origin, and preserves promises to find this photoreaction in media of environmental importance. In UV photolysis of H2O2, the relatively small concentration of isolated OH radicals is found in a Kr matrix, and no OH radicals appear in a Xe matrix, and this trend is discussed in terms of delayed cage exit. Moreover, additional species photogenerated from H2O2 in a Xe matrix as well as the absence of OH radicals might be connected with participation of some hidden intermediates (HOXeOH, HXeOOH, etc.) in the dynamics, thus, catalyzing new photodissociation channels. Among the photolysis products, the loose H2O//O complex is suggested to be stabilized in Kr and Xe matrixes. This loosely bound complex is quasistable and decomposes at relatively low temperatures (below 20 K) quantitatively forming the known tight H2O⋯O structure. This low-temperature process offers one additional example of short-range atomic mobility introduced recently in the literature.

Infrared Spectroscopy of H2S and SH in Rare-Gas Matrixes

Infrared (IR) spectra of hydrogen sulfide (H2S), some of its complexes, and their photolysis products are studied in rare-gas matrixes (Ar, Kr and Xe) at 7.5 K. In all of these matrixes, H2S is found to undergo hindered rotation. The vibration−rotation structures of monomeric H2S are observed at 2651−2615, 2640−2600, and 2625−2580 cm-1 in Ar, Kr, and Xe matrixes, respectively. Furthermore, the H2S dimer and the H2S···H2O complex are observed in various matrixes. Upon UV-induced decomposition of H2S, IR absorptions of SH radical appear at 2607 cm-1 (Ar) and 2594 cm-1 (Kr). The SH radical is also observed to be complexed with H2S (2563, 2553, and 2550 cm-1 in Ar, Kr, and Xe, respectively) and water (2556 cm-1 in Kr).

Photochemical Insertion Reaction of Hg in SiH4/SiD4 in Low-Temperature N2, Ar, and Kr Matrixes. Formation of Radicals in Kr

The products of the insertion reaction of Hg(3P1) in the SiH (SiD) bond of h- or d-silane in nitrogen and rare gas matrixes are observed by FTIR spectroscopy. This study has shown that the reaction takes place without activation energy. Disilane molecules are formed from the dimers of parent molecules close to mercury or from the complexes of the insertion products with the parent molecules. In this last case a weak isotopic effect shows that the reaction probably takes place through a small barrier. In the krypton matrix the formation of SiH2 and SiH radicals is observed by a mercury-sensitized two-photon excitation. The SiH3 radical is produced upon annealing. A mixed CH4/SiH4 experiment gives the ratio of the insertion rates in these two molecules.

Theoretical Investigations of Vinyl Bromide Dissociation in Xe and Kr Matrixes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTheoretical Investigations of Vinyl Bromide Dissociation in Xe and Kr MatrixesParas M. Agrawal, Dan C. Sorescu, Lionel M. Raff, and Samuel A. AbrashCite this: J. Phys. Chem. 1995, 99, 41, 14959–14966Publication Date (Print):October 1, 1995Publication History Published online1 May 2002Published inissue 1 October 1995https://doi.org/10.1021/j100041a009RIGHTS & PERMISSIONSArticle Views26Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (869 KB) Get e-Alerts

Mechanisms of the Electronic Relaxation of the Hg(1P1) State in Low-Temperature Matrixes

The fluorescence-excitation and fluorescence spectra as well as the fluorescence decay times were recorded for Hg atoms excited to the 61P1 state by synchrotron radiation in Ar, Kr, Xe, N2, and CH4 matrixes and in Ar matrixes with a low NH3 content. Sequential decays from the 61P1 to different 63PJ levels involve the intersystem crossing and intramultiplet relaxation, the dependence of the relaxation rate on the properties of the matrix being different for the 2 mechanisms. [on SciFinder (R)]

Cage effect on the photodissociation of water in xenon matrixes

Photodissocn. of H2O in Xe matrixes was investigated by excitation in the first continuum ( A1B1) by using dispersed synchrotron radiation. The dissocn. threshold near 6.4 eV corresponds to a barrier of .apprx.1.3 eV due to repulsive H-Xe pair interactions and is attributed to a prompt cage exit of the H atoms. The dissocn. efficiency for cryst. samples does not depend on temp. in the 5-40 range. An increase of the dissocn. efficiency with temp. for non-cryst. samples up to the value of cryst. ones is caused by partial annealing. The dependence of dissocn. efficiency on excess kinetic energy is well rationalized in terms of a model based on the mass dependence of energy exchange between the H fragment and the cage atoms that was derived for the case of Ar and Kr matrixes (Schriever, R. et al., 1990). [on SciFinder (R)]