Matrix IR Research Articles

Where is the hidden intramolecular H-bonding vibrational signal in the proline matrix IR spectrum?

The assignment of the hydrogen bonded O-H stretch vibration in the proline matrix IR spectrum has sparked controversy. Employing constrained nuclear electronic orbital methods, we provide a clear assignment that the vibrational frequency drops to near 3000cm-1 as a result of the interplay between electronic effects, nuclear quantum effects, and matrix effects.

Investigation of Isolated IrF5 -, IrF6 - Anions and M[IrF6] (M=Na, K, Rb, Cs) Ion Pairs by Matrix-Isolation Spectroscopy and Relativistic Quantum-Chemical Calculations.

The molecular IrF5 -, IrF6 - anions and M[IrF6] (M=Na, K, Rb, Cs) ion pairs were prepared by co-deposition of laser-ablated alkali metal fluorides MF with IrF6 and isolated in solid neon or argon matrices under cryogenic conditions. The free anions were obtained as well by co-deposition of IrF6 with laser-ablated metals (Ir or Pt) as electron sources. The products were characterized in a combined analysis of matrix IR spectroscopy and electronic structure calculations using two-component quasi-relativistic DFT methods accounting for spin-orbit coupling (SOC) effects as well as multi-reference configuration-interaction (MRCI) approaches with SOC. Inclusion of SOC is crucial in the prediction of spectra and properties of IrF6 - and its alkali-metal ion pairs. The observed IR bands and the computations show that the IrF6 - anion adopts an Oh structure in a nondegenerate ground state stabilized by SOC effects, and not a distorted D4h structure in a triplet ground state as suggested by scalar-relativistic calculations. The corresponding "closed-shell" M[IrF6] ion pairs with C3v symmetry are stabilized by coordination of an alkali metal ion to three F atoms, and their structural change in the series from M=Na to Cs was proven spectroscopically. There is no evidence for the formation of IrF7, IrF7 - or M[IrF7] (M=Na, K, Rb, Cs) ion pairs in our experiments.

The matrix infrared spectra of fulvenone (C5H4=C=O) and its thermal decomposition products

The matrix IR absorbance spectra of fulvenone, a reactive intermediate in the pyrolysis of lignin, and three of its thermal dissociation products were recorded and analysed using a flash pyrolysis microreactor and ab initio calculations. The predicted spectra of fulvenone, penta-1,2-diene-4-yne, penta-1,3-diyne, and n- matched well with the experimentally observed transitions, and are in good agreement with existing literature data. The cyclic isomer, cyclopentadienylidene, could not be conclusively identified, suggesting that fulvenone is the last relatively stable cyclic species in this pyrolytic chain of reactions. The evolution of the IR data with increasing extent of the pyrolysis shows a gradual trend of dissociation of complex organic molecules towards highly unsaturated hydrocarbons. The isomer-specific analysis by IR assignments of these pyrolytic pathways helps pave the way for further study of biomass thermal reactions, towards possible uses of biomass as a renewable source of energy.

Soft photon radiation and entanglement

We study the entanglement between soft and hard particles produced in generic scattering processes in QED. The reduced density matrix for the hard particles, obtained via tracing over the entire spectrum of soft photons, is shown to have a large eigenvalue, which governs the behavior of the Renyi entropies and of the non-analytic part of the entanglement entropy at low orders in perturbation theory. The leading perturbative entanglement entropy is logarithmically IR divergent. The coefficient of the IR divergence exhibits certain universality properties, irrespectively of the dressing of the asymptotic charged particles and the detailed properties of the initial state. In a certain kinematical limit, the coefficient is proportional to the cusp anomalous dimension in QED. For Fock basis computations associated with two-electron scattering, we derive an exact expression for the large eigenvalue of the density matrix in terms of hard scattering amplitudes, which is valid at any finite order in perturbation theory. As a result, the IR logarithmic divergences appearing in the expressions for the Renyi and entanglement entropies persist at any finite order of the perturbative expansion. To all orders, however, the IR logarithmic divergences exponentiate, rendering the large eigenvalue of the density matrix IR finite. The all-orders Renyi entropies (per unit time, per particle flux), which are shown to be proportional to the total inclusive cross-section in the initial state, are also free of IR divergences. The entanglement entropy, on the other hand, retains non-analytic, logarithmic behavior with respect to the size of the box (which provides the IR cutoff) even to all orders in perturbation theory.

Calibration of thermal imaging systems based on matrix IR photodetectors

The article presents a set of calibration techniques for thermal imaging systems calibration. The proposed algorithms allow high-precision calibration of the IR photodetector with finding and subsequent correction of the entire spectrum of defective pixels: by noise, amplitude and sensitivity. Part of the work is devoted to the development of methods for correcting the influence of ambient temperature on the optical system of a thermal imaging channel, which helps to prevent the effect of super saturation of the IR image pixels.

Conformer-specific [1,2]H-tunnelling in captodatively-stabilized cyanohydroxycarbene (NC-C[combining umlaut]-OH).

We report the gas-phase preparation of cyanohydroxycarbene by high-vacuum flash pyrolysis of ethyl 2-cyano-2-oxoacetate and subsequent trapping of the pyrolysate in an inert argon matrix at 3 K. After irradiation of the matrix with green light for a few seconds singlet trans-cyanohydroxycarbene rearranges to its cis-conformer. Prolonged irradiation leads to the formation of cyanoformaldehyde and isomeric isocyanoformaldehyde. Cis- and trans-cyanohydroxycarbene were characterized by matching matrix IR and UV/Vis spectroscopic data with ab initio coupled cluster and TD-DFT computations. Trans-cyanohydroxycarbene undergoes a conformer-specific [1,2]H-tunnelling reaction through a 33.3 kcal mol-1 barrier (the highest penetrated barrier of all H-tunnelling reactions observed to date) to cyanoformaldehyde with a half-life of 23.5 ± 0.5 d; this is the longest half-life reported for an H-tunnelling process to date. During the tunnelling reaction the cis-conformer remains unchanged over the same period of time and the Curtin-Hammett principle does not apply. NIR irradiation of the O-H stretching overtone does not enhance the tunnelling rate via vibrational activation. Push-pull stabilisation of hydroxycarbenes through σ- and π-withdrawing groups therefore is even more stabilizing than push-push substitution.

Dichlorosilylene–hydrogen chloride complex: direct IR spectroscopic detection in argon matrix

A 1: 1 donor–acceptor complex between SiCl2 and HCl was detected by matrix IR spectroscopy. The existence of the complex was previously predicted theoretically in the course of analysis of mechanisms of chemical vapor deposition (CVD) processes involving chlorosilanes. The quantum chemical calculations at the G4(MP2) level confirmed the possibility of formation of only one stable complex upon the reaction of SiCl2 with HCl. In addition to the complex of the simplest composition, complexes of SiCl2 with HCl associates were observed upon matrix annealing. The only product formed upon the photolysis of the complexes of all types was trichlorosilane, a product of silylene insertion into the H–Cl bond.

1,3-Dioxolane-4-ol Hemiacetal Stores Formaldehyde and Glycolaldehyde in the Gas-Phase.

We report the spontaneous gas-phase formation of 1,3-dioxolane-4-ol, a mixed hemiacetal resulting from the addition of glycolaldehyde to formaldehyde. It was spectroscopically characterized by matching matrix IR spectra with coupled cluster computations. The formation of the hemiacetal must be surface-catalyzed owing to the very high computed reaction barrier of 39.8 kcal mol-1. The reaction barrier is lowered by almost 20 kcal mol-1 when a single water molecule acts as a proton shuttle in a favorable six-membered transition state. We characterized the hemiacetal in solution via NMR spectroscopy and followed its decomposition into its constituents within a few hours; it also dissociates upon contact with water. Sugars form in the presence of Ca(OH)2, in line with formose-type reactivity. 1,3-Dioxolane-4-ol may be considered a storage form for formaldehyde and glycolaldehyde that is rather stable in the gas-phase.

Spectroscopic Evidence for Aminomethylene (H−C̈−NH2)—The Simplest Amino Carbene

AbstractAlthough N‐heterocyclic carbenes have been well‐studied, the simplest aminocarbene, aminomethylene H−C̈−NH2, has not been spectroscopically identified to date. Herein we report the gas‐phase preparation of aminomethylene by high‐vacuum flash pyrolysis of cyclopropylamine and subsequent trapping of the pyrolysate in an inert argon matrix at 12 K. Aminomethylene was characterized by matching matrix IR and UV/Vis spectroscopic data with ab initio coupled cluster computations. After UV irradiation of the matrix aminomethylene rearranges to its isomer methanimine (formaldimine) H2C=NH. Based on our experimental results and computations aminomethylene has a singlet ground state with a reaction barrier of almost 46 kcal mol−1 to methanimine so that H‐tunneling is excluded.

Spectroscopic Evidence for Aminomethylene (H-C̈-NH2 )-The Simplest Amino Carbene.

Although N-heterocyclic carbenes have been well-studied, the simplest aminocarbene, aminomethylene H-C̈-NH2 , has not been spectroscopically identified to date. Herein we report the gas-phase preparation of aminomethylene by high-vacuum flash pyrolysis of cyclopropylamine and subsequent trapping of the pyrolysate in an inert argon matrix at 12 K. Aminomethylene was characterized by matching matrix IR and UV/Vis spectroscopic data with ab initio coupled cluster computations. After UV irradiation of the matrix aminomethylene rearranges to its isomer methanimine (formaldimine) H2 C=NH. Based on our experimental results and computations aminomethylene has a singlet ground state with a reaction barrier of almost 46 kcal mol-1 to methanimine so that H-tunneling is excluded.

Complexes of dichlorosilylene with allyl chloride and allyl bromide: matrix IR spectroscopy and quantum chemical studies

Formation of donor-acceptor complexes between dichlorosilylene, SiCl2, and allyl halides, AllHal(Hal= Cl, Br) was detected in Ar matrixes using matrix IR spectroscopy. In agreement with the predictions of the performed quantum chemical calculations, only broad unstructured absorption bands contributed by different conformers of the 1 : 1 complexes between SiCl2 and AllHal were observed in IR spectra of matrixes after deposition in the regions of characteristic vibrations of starting reactants. Annealing of matrixes resulted in strong narrowing the bands due to conversions of different conformers into the most stable structures. The predominantly formed conformers in both the reaction systems were those of complexes with SiCl2 coordinated to the Hal atoms of AllHal in the gauche conformations. At the same time, according to the calculations, the complexes with SiCl2 coordination to the double bonds of AllHal can be only slightly less stable than the complexes with coordination to the Hal atoms, and all these basic centers can be considered as comparable in their activity in the complexation. The only products revealed upon photolysis of complexes were the products of silylene insertion into the C–Hal bonds, viz., AllSiCl3 and AllSiCl2Br. Theoretical study of thermal transformations in the SiCl2 + AllHal systems showed that formal insertion of SiCl2 in the C–Hal bonds and its addition to the double bonds of AllHal have low activation barriers of 3–8 kcal mol–1. However, these barriers are too high for these reactions to occur under the matrix isolation conditions.

Matrix Infrared Spectra of Insertion and Metallacyclopropane Complexes [CH3CH2-MH and (CH2)2-MH2] Prepared in Reactions of Laser-Ablated Group 3 Metal Atoms with Ethane.

CH3CH2-MH and (CH2)2-MH2 were identified in the matrix IR spectra from reactions of laser-ablated group 3 metal atoms with ethane, and they were characterized via theoretical investigations. The observed products are the most stable in the proposed reaction path. Because of the small number of valence electrons, the group 3 metal high oxidation-state complexes are less stable. The C-C insertion product [(CH3)2M], which was predicted to be more stable than the observed ones, was not observed probably because of the high energy barrier and a likely slower rate for insertion into one C-C bond than one of six C-H bonds. The C-C bond of the metallacyclopropanes is the shortest among the early transition-metal analogues, and its stretching frequencies are the highest, revealing the weakest interaction between the metal dihydride and ethylidene groups. The undetected ethylidene is not agostic, parallel to the previously examined methylidene.

Observation and Characterization of CH3CH2-MH, (CH2)2-MH2, CH2═CH-MH3, and CH3-C≡MH3- Produced by Reactions of Group 5 Metal Atoms with Ethane.

The primary products in reactions of laser-ablated group 5 metal atoms with ethane were identified in argon matrix IR spectra and characterized via density functional theory computations. The second- and third-row transition metals Nb and Ta produced insertion, metallacyclopropane, vinyl trihydrido, and anionic ethylidyne complexes (CH3CH2-MH, (CH2)2-MH2, CH2═CH-MH3, and CH3C≡MH3-), while the first-row transition metal V yielded only the insertion and metallacyclopropane products. The energetically higher ethylidenes and neutral ethylidynes (CH3CH═MH2 and CH3C[Formula: see text]MH3) were not detected. The unique anionic ethylidynes are the most stable anionic species in the Nb and Ta systems. Evidently back-donation from the metal center to the C-C π* orbital is stronger than that in the group 6 metal analogue but weaker than that in the corresponding group 4 metal complex. The C-M bond for the Nb and Ta ethylidyne anions is a true triple bond.

Observation and Characterization of CH3CH2–MH, (CH2)2–MH2, and CH3–C≡MH3 Prepared in Reactions of Ethane with Laser-Ablated Group 6 Metal Atoms

Reactions of laser-ablated group 6 metal atoms with ethane have been carried out, and the products identified in argon matrix IR spectra on the basis of deuterium shifts, correlation with computation results, and previous related works. Mo and W generated the insertion, metallacyclopropane, and ethylidyne complexes [CH3CH2–MH, (CH2)2–MH2, and CH3C≡MoH3], whereas Cr gave only the insertion product. The higher oxidation-state complex from ethane becomes relatively more stable in the heavier metal system, parallel to those from methane. The unobserved ethylidene CH3CH═MH2 is energetically higher than (CH2)2–MH2, and the energy barrier from CH3CH2–MH to the ethylidene is also substantially higher than that for the observed cyclic product. NBO calculations indicate that the C–M bonds of the group 6 metal ethylidenes and ethylidynes are true double and triple bonds.

The effects of conformation and zwitterionic tautomerism on the structural and vibrational spectral data of anserine

In this study, the stable conformers of neutral anserine were searched by molecular dynamics simulations and energy minimization calculations using the MM2 force field. Thermochemical calculations at B3LYP/6-31G(d) level of theory followed these preliminary calculations. The results confirmed that neutral anserine has quite a flexible structure and many stable gauche and trans conformers at room temperature. Nevertheless, two are considerably more favourable in energy than the others and expected to dominate the gas-phase and matrix IR spectra of the molecule. The corresponding structural and vibrational spectral data for these two conformers of neutral anserine, whose relative stabilities were also examined by high-accuracy energy calculations carried out using G3MP2B3 method, and for the most stable conformer of anserine in zwitterion form were calculated at B3LYP/6-311++G(d,p) level of theory. The calculated harmonic force constants were refined using the Scaled Quantum Mechanical Force Field (SQM-FF) method and then used to produce the refined wavenumbers, potential energy distributions (PEDs) and IR and Raman intensities. These refined data together with the scaled harmonic wavenumbers obtained using another method, Dual Scale factors (DS), enabled us to correctly analyse the observed IR and Raman spectra of anserine and revealed the effects of conformation and zwitterionic tautomerism on its structural and vibrational spectral data.

Cell of the silicon integrated reading circuit with built-it analog-digital converter

This paper describes the project of an accumulative cell of a silicon integrated signal reading circuit with a built-in analog-digital converter for matrix IR photodetectors based on the Hg1-xCdxTe solid solutions with the sensitivity in the spectral range from 8 to 10 µm. The cell is designed according to the silicon technology HCMOS8D of JSC “NIIME i Mikron” (Moscow) with a project norm of 0.18 µm. The presented project of the cell has the size of 20×2020 µm, and the number of bits in the built-in analog-digital converter is 15. When the average photocurrent is 7 nA and the integration time is 7.5 ms, the estimated value of the noise equivalent delta temperature is 4.6 mK.

Matrix Infrared Spectra and Density Functional Calculations of CH2Cl–Cl and CH2Br–Br Produced by Laser‐ablated Metal Plume Irradiation

CH2Cl–Cl and CH2Br–Br, photoisomers of CH2Cl2 and CH2Br2 , were identified in the matrix IR spectra from the precursors exposed to plume radiation of a laser‐ablated transition metal. On the other hand, the corresponding photoisomers of CH2F2 and CH2FCl (CH2F–F, CH2F–Cl, and CH2Cl–F) were not observed because of their instabilities. The CX bond of the product is unusually strong, leading to a high CX stretching frequency. Natural bond orbital (NBO) analysis reveals that it is a true carbon–halogen double bond; in contrast, the XX bond is largely ionic (H2CXδ+ ∙∙∙Xδ− ), similar to the previously studied analogs from tri‐ and tetrahalomethanes. Intrinsic reaction coordinate (IRC) computation reproduces the smooth interconversion between the precursor and the energetically higher photoisomer, consistent with the disappearance of the product during visible photolysis.

Matrix Infrared Spectra and DFT Computations of 2H-Azirine Produced from Acetonitrile by Laser-Ablation Plume Radiation

Han-Gook Cho Department of Chemistry, University of Incheon, Incheon 406-772, Korea. E-mail: hgc@incheon.ac.krReceived March 2, 2014, Accepted March 18, 20142H-azirine, a less known acetonitrile isomer, is observed in matrix IR spectra from the precursor exposed toradiation from laser ablation of transition-metals. Its vibrational characteristics confirm the previous results,and those for the deuterated and

Trifluoromethylphenylcarbenes. Carbene-Carbene Interconversion on the Singlet Energy Surface and Rearrangement to Trifluorobenzocyclobutene, Trifluorostyrene, and Trifluoromethylfulvenallenes

The four isomeric α-, ortho-, meta-, and para-trifluoromethylphenylcarbenes were generated by photolysis of the corresponding 3-phenyl-3-trifluoromethyldiazirene 1 or the four isomeric trifluoromethylphenyldiazomethanes 2 and 4–6. The four corresponding triplet trifluoromethylphenylcarbenes 3 and 7–9 were observed by electron spin resonance (ESR) spectroscopy in Ar matrices at 14 K. The α- and ortho-carbenes 3 and 7 and the ortho- and para-carbenes 7 and 9 interconvert partially when generated by short-wavelength photolysis (350 nm) before intersystem crossing to the triplet states. The triplet states do not undergo further Carbene-Carbene interconversion. The interconversions are assumed to take place via the meta-trifluoromethylphenylcarbene 8. When the ortho- and para-carbenes are generated by long-wavelength photolysis (>450 nm), the discrete, non-interconverting triplet carbenes are observed in the ESR spectra. Flash vacuum thermolysis of the diazirene 1 at 500°C afforded a mixture of bis(trifluoromethyl)heptafulvalenes 11, bis(trifluoromethyl)stilbenes 12, and bis(trifluoromethyl)anthracenes 13, and the presence of their likely precursor(s), trifluoromethylcycloheptatetraene(s), was confirmed by a peak at 1830 cm–1 in the Ar matrix IR spectrum. In addition, at 700°C, four monomeric carbene rearrangement products were isolated and characterised, viz. 1,1,2-trifluorobenzocyclobutene 14, 1′,2′,2′-trifluorostyrene 15, and 1- and 2-trifluoromethylfulvenallenes 16 and 17.

New evidence in an old case: the question of chromium hexafluoride reinvestigated.

The question of whether or not the chromium hexafluoride molecule has been synthesized and characterized has been widely discussed in the literature and cannot, in spite of many efforts, yet be answered beyond doubt. New matrix-isolation experiments can now show, together with state-of-the-art quantum-chemical calculations, that the compound previously isolated in inert gas matrixes, was CrF5 and not CrF6. New bands in the matrix IR spectra can be assigned to the Cr2F10 dimer, and furthermore evidence was found in the spectra for a photodissociation or reversible excitation of CrF5 under UV irradiation. However, even if CrF6 is not stable at ambient conditions, its formation under high fluorine pressures in autoclave reactions cannot be excluded completely.