Nitrogen Matrixes Research Articles

Matrix Isolation Study of Fumaric and Maleic Acids in Solid Nitrogen.

Fumaric and maleic acids ((E)- and (Z)-HOOC-CH═CH-COOH, FA and MA) were studied experimentally by infrared spectroscopy in nitrogen matrixes and theoretically by quantum chemical calculations. The calculations, carried out at the DFT(B3LYP) and MP2 levels of theory, predicted the existence of at least 5 conformers of maleic acid and 10 conformers of fumaric acid. After the deposition of the matrixes, two conformers of maleic acid (I and II) and three conformers (I-III) of fumaric acid were observed and characterized vibrationally. Selective narrowband near-infrared (NIR) excitation of the first OH stretching overtones of the different conformers of maleic and fumaric acids initially present in the matrixes allowed the generation of higher-energy forms, never before observed experimentally. In the case of maleic acid, conformers I (a cis-trans form, where cis and trans designate the conformation of the carboxylic groups of the molecule) and II (cis-cis) were found to generate the novel conformers VI (trans-trans) and VII (cis-trans), respectively. The conversion of conformer II into the most stable conformer I was also observed. For fumaric acid, the cis-cis conformers I-III were found to give rise to the new cis-trans conformers IV-VII, respectively. The tunneling decay of the new conformers produced upon NIR excitation of the lowest-energy conformers initially trapped in the matrixes was observed, and their lifetimes in solid N2 were determined. The increased stability of all of the observed high-energy conformers of the studied acids in the N2 matrix, compared to the argon matrix, where they could not be observed experimentally, demonstrates the stabilizing effect of the interaction between the OH groups of the acids with the matrix N2 molecules, in line with previous observations for other carboxylic acids. In addition, the photochemistry of matrix-isolated maleic and fumaric acids upon broad-band UV irradiation (λ > 235 nm) was also investigated. UV-induced isomerization of both acids around the C═C double bond was observed, together with their decarboxylation to acrylic acid.

Effects of Entangled IR Radiation and Tunneling on the Conformational Interconversion of 2-Cyanophenol.

The conformers of 2-cyanophenol (2CP) and their interconversions were studied by infrared (IR) spectroscopy after trapping the monomers of the gaseous compound into low-temperature (15 K) argon (Ar) and nitrogen (N2) matrixes. To assist in the interpretation of the experimental results, B3LYP, MP2, and QCISD electronic structure calculations were carried out for the 2CP molecule. Two planar conformers, cis and trans (orientation of OH with respect to the cyano group), are predicted with gas-phase populations at the sublimation temperature of ∼98 and ∼2%, respectively. The most stable form (cis) was experimentally identified in both cryomatrixes, whereas the less stable one (trans) was not detected in Ar but could be observed in the N2 matrix with an abundance of ∼15%. Selective and bidirectional conversion between the two identified conformers was achieved upon irradiating the compound trapped in N2 matrix with near-infrared (NIR) laser light tuned at the wavenumbers of the 2ν(OH) transitions of the respective conformers. The conformational composition of 2CP was also found to be affected by the broad-band IR radiation emitted by the spectrometer source. This effect could be suppressed, partially or completely, by using different long-pass IR filters, with cutoff values of approximately 2200, 1590, and 1170 cm-1. The observed conformational changes are rationalized in terms of a competition between the over-the-barrier (light-induced) and through-the-barrier (hydrogen atom tunneling) effects. Very interestingly, both effects occur on the same time scale.

Infrared Studies of the Symmetry Changes of the 28SiH4 Molecule in Low-Temperature Matrixes. Fundamental, Combination, and Overtone Transitions.

Infrared spectra of 28SiH4 in argon and nitrogen matrixes at low temperature 6.5-20 K in the region of overtone and combination transitions were recorded for the first time. Additionally, the high-resolution spectra were obtained in the fundamental region. The frequencies and the relative intensities of all bands were determined. The set of experimental data suggests that the symmetry of molecules studied in the matrixes is different from the symmetry of the free molecules because of an interaction with the environment. The symmetry of 28SiH4 changes from Td to C3v on transition from the gas phase to a nitrogen matrix and to D2d on transition to an argon matrix. A modeling of SiH4 molecule force fields explains the experimental data as a change of a force constant of the selected SiH bond in the case of SiH4 in the nitrogen matrix or force constants of two opposite angles in the case of SiH4 in the argon matrix. In spite of small values of these changes, they result in noticeable spectroscopic effects: the band splitting and appearance of new bands in matrix spectra compared with spectra of free SiH4. The interpretation of transitions in the fundamental and combination regions was performed.

Photooxidation of Trimethyl Phosphite in Nitrogen, Oxygen, and para-Hydrogen Matrixes at Low Temperatures.

Trimethyl phosphite (TMPhite) was photooxidized to trimethyl phosphate (TMP) in N2, O2, and para-H2 matrixes at low temperatures to correlate the conformational landscape of these two molecules. The photooxidation produced the trans (TGG)-rich conformer with respect to the ground state gauche (GGG) conformer of TMP in N2 and O2 matrixes, which has diverged from the conformational composition of freshly deposited pure TMP in the low-temperature matrixes. The enrichment of the trans conformer in preference to the gauche conformer of TMP during photooxidation is due to the TMPhite precursor, which exists exclusively in the trans conformer. Interestingly, whereas the photooxidized TMP molecule suffers site effects possibly due to the local asymmetry in N2 and O2 matrixes, in the para-H2 matrix owing to the quantum crystal nature the site effects were observed to be self-repaired.

Degeneracy Lifting Effect in the FTIR Spectrum of Fluoroform Trapped in a Nitrogen Matrix. An Experimental and Car–Parrinello Molecular Dynamics Study

The FTIR spectra of fluoroform trapped in argon and nitrogen matrixes are studied at T ∼ 10-30 K. The bands of E symmetry show the splitting effect in a nitrogen matrix, which is absent in an argon matrix. The effect is the most prominent in the case of the ν4 CH bending vibration. It decreases slightly with increasing temperature. Both static and Car-Parrinello molecular dynamic simulations suggest that the degeneracy lifting is due to C3v symmetry lowering caused by interactions between fluoroform and all neighbor N2 matrix molecules.

Conformers of Kojic Acid and Their Near-IR-Induced Conversions: Long-Range Intramolecular Vibrational Energy Transfer.

Conformational transformations were investigated for molecules of kojic acid trapped in low-temperature argon and nitrogen matrixes. Two conformers, differing from each other by 120° rotation of the hydroxymethyl (-CH2OH) moiety, were found to be populated in freshly deposited matrixes, prior to any irradiation. Matrixes containing isolated monomers of kojic acid were irradiated with narrowband, tunable near-infrared (near-IR) laser light. Excitations at wavenumbers corresponding to the overtone of the stretching vibration of the OH bond of the hydroxymethyl group led to conversion of one of the observed conformers into another. The direction of this conformational transformation depended on the wavenumber (within the 7126-7115 cm(-1) range) used for irradiation. The same conformational photoconversion was also observed to occur upon narrowband irradiation at much lower wavenumbers (from the 6468-6447 cm(-1) range). Near-IR light from this range selectively excites overtone vibrations of the OH group directly attached to the heterocyclic ring. Such an observation provides a convincing evidence of a long-range vibrational energy transfer from the initially excited OH group (directly attached to the ring) to the remote hydroxymethyl fragment which changes its orientation. Structural changes, occurring in matrix-isolated molecules of kojic acid upon near-IR excitation, were monitored by FTIR spectroscopy.

Conformational Changes in Thiazole-2-carboxylic Acid Selectively Induced by Excitation with Narrowband Near-IR and UV Light.

Conformers and photoinduced conformational transformations were studied for monomers of thiazole-2-carboxylic acid (TCA). The matrix-isolation technique and excitations with narrowband near-IR and UV light, tuned in an optical parametric oscillator, were used for this purpose. Form I, with the carboxylic moiety in the trans orientation and with the hydrogen atom of the OH group directed toward the nitrogen atom of the ring, was the most abundant in low-temperature argon or nitrogen matrixes. Conformer II, differing from I by 180° rotation of the OH group around the C-O bond, was also trapped in the matrixes, but in much smaller amount. The abundance of form II was experimentally determined as ∼6% of the total amount of TCA molecules. Selective excitation of I with narrowband near-IR laser light resulted in I → II transformation. This near-IR-induced conformational change was photoreversible: form II converted back to I upon selective excitation of II with near-IR light of different wavelength. Conformational conversions of I into II, or vice versa, were also induced in TCA monomers by narrowband UV excitations at 300 nm (for I → II transformation) and at 305 nm (for II → I transformation). A spontaneous conversion of photogenerated II into the most stable form I was observed for the compound trapped in the matrix at 15 K and kept in the dark. This process was very slow; the estimated half-life time of conformer II was longer than 50 h. Finally, TCA was shown to thermally decompose at room temperature, yielding CO2 and thiazole.

Conformations of n-butyl imidazole: Matrix isolation infrared and DFT studies

Conformations of n-butyl imidazole (B-IMID) were studied using matrix isolation infrared spectroscopy by trapping in argon, xenon and nitrogen matrixes using an effusive nozzle source. The experimental studies were supported by DFT computations performed at the B3LYP/6-311++G(d,p) level. Computations identified nine unique minima for B-IMID, corresponding to conformers with tg±tt, tg±g∓t, tg±g±t, tg±tg±, tg±tg∓, tg±g∓g∓, tg±g±g±, tg±g∓g± and tg±g±g∓ structures, given in order of increasing energy. Computations of the transition state structures connecting the higher energy conformers to the global minimum, tg±tt structure were carried out. The barriers for the conformer inter-conversion were found to be ∼2kcal/mol. Natural Bond Orbital (NBO) analysis was performed to understand the reasons for conformational preferences in B-IMID.

Infrared Spectra and Structures of SiH4and GeH4Dimers in Low-Temperature Nitrogen Matrixes

IR absorption spectra of monoisotopic silane (28)SiH4 and germane (76)GeH4 are studied in nitrogen matrixes at T = 11 K. It is shown that the absorption spectra of silane and germane are similar in the regions of the ν3 stretching and ν4 bending vibrations. A significant influence of a nitrogen matrix on the absorption spectra of XH4 molecules was established. The bands in the ν3 region of silane and germane in a nitrogen matrix are blue-shifted. The spectra of monomer molecules of SiH4 and GeH4 contain a triplet in the stretching region and a doublet in the bending region, which is explained by the change in the molecular symmetry of an XH4 molecule from T(d) to C3 on passage from the gas phase to solid nitrogen. The calculations of the matrix structure were made by the Monte Carlo method with the ensemble of particles containing the SiH4 molecule and 863 N2 molecules. The calculations predicted a set of equilibrium structures with the C3 symmetry. The structure of the absorption bands of the dimer is analyzed in the ν3 stretching and ν4 bending regions. The intensity of these absorption bands increases quadratically with concentration. The contribution of resonance dipole-dipole interactions in the formation of the absorption bands of (XH4)2 dimers in the ν3 stretching and ν4 bending regions is discussed.

Conformational switching in pyruvic acid isolated in Ar and N₂ matrixes: spectroscopic analysis, anharmonic simulation, and tunneling.

Monomers of pyruvic acid (PA) isolated in cryogenic argon and nitrogen matrixes were characterized by mid- and near-infrared spectroscopy. Interpretation of the experiments was aided by fully anharmonic calculations of the fundamental modes, overtones, and combinations up to two quanta, including their infrared intensities. The initially dominating PA conformer (Tc) has a cis CCOH arrangement and is stabilized by a strong intramolecular H-bond. Selective near-infrared excitation of Tc at the first OH overtone (6630 cm(-1) in Ar, 6643 cm(-1) in N2) induced a large scale conformational conversion to the higher-energy conformer (Tt) with trans CCOH arrangement. Tt was then converted back to Tc by selective NIR irradiation at the first Tt OH overtone (6940 cm(-1) in Ar, 6894 cm(-1) in N2). In N2 matrix, the Tt form was stabilized due to interaction between the OH group and the matrix molecules. This stabilization manifested itself in the absence of Tt → Tc relaxation and in a considerable change of the vibrational Tt signature upon going from argon to nitrogen matrix. In argon, the Tt form spontaneously decayed back to Tc in the dark (characteristic lifetime +16 h). In the presence of broad-band near-infrared light, the Tt → Tc relaxation speed considerably increased. The decay mechanisms are discussed.

Tunable Diode Lasers as a Tool for Conformational Control: The Case of Matrix-Isolated Oxamic Acid

A tunable diode laser was applied as a source of narrowband near-infrared light used to manipulate the structure of the molecule of oxamic acid. Monomers of the most stable conformer I of the molecule, with the trans orientation of the O═COH group and the trans orientation of the O═CC═O fragment, were trapped from the gas phase in low-temperature argon, neon, and nitrogen matrixes. Monomers of oxamic acid, isolated in argon or neon matrixes, were then irradiated with narrowband near-IR light from the diode laser tuned at 6833 (Ar) or 6840 cm(-1) (Ne). Upon such irradiation another conformer, II, of oxamic acid was generated, with cis orientation of the O═COH group and trans orientation of the O═CC═O fragment. Both forms were identified by comparison of their experimental mid-IR spectra with the spectra theoretically calculated for I and II. Subsequent irradiation of the matrix at 6940 (Ar) or 6991 cm(-1) (Ne), where absorption appeared in the near-IR spectrum of the photoproduct, led to photoconversion of conformer II into form I. In a series of subsequent irradiations at 6833(Ar)/6840(Ne) cm(-1) and at 6940(Ar)/6991(Ne) cm(-1), the population of oxamic acid molecules was selectively shifted several times from I to II and vice versa. As far as we know, this is the first reported study where a tunable diode laser source of narrowband near-IR light was used to manipulate the structure of a molecule. Spontaneous II → I transformation was observed for Ne and Ar matrixes kept in the dark and at cryogenic temperature.

Amine inversion effects on the IR spectra of aniline in the gas phase and cold inert gas matrixes

The mid-infrared spectrum of aniline vapour is reinvestigated, and the IR spectrum of the compound in solid nitrogen matrix under a cryogenic condition is measured for the first time. A thorough assignment of many features of the vapour spectrum, particularly those related to amine inversion mode, is suggested. By use of PGOPHER fitting of the rotational band contours in the fingerprint region, the amine inversion tunnelling splitting at v = 1 level of NH2 scissoring and C–N bending vibrations in the vapour spectrum are identified. Our analysis reveals that excitation of one quantum of NH2 scissoring results in lowering of the tunnelling barrier, whereas, the barrier is increased upon excitation of the C–N bending. Comparison of the spectra recorded in nitrogen and argon matrixes reveal that the former hinders the NH2 inversion and consequently, transitions corresponding to higher quanta of this mode are absent.

Conformations of dimethoxymethylphenylsilane: A matrix isolation infrared and ab initio study

Conformations of dimethoxymethylphenylsilane (DMMPS) were studied using matrix isolation infrared spectroscopy and ab initio computations. DMMPS was trapped in both argon and nitrogen matrixes, using an effusive nozzle source and a supersonic jet source. In order to corroborate our experimental observations, the structure, energy and vibrational frequencies of the conformers of DMMPS were computed at HF and B3LYP levels of theory, using a 6–31++G ∗∗ basis set. The vibrational spectra, obtained in the matrix isolation experiments, were found to be assignable to the lowest energy conformer, with a G ±G ∓ structure.

Conformations of Dimethoxydimethylsilane: Matrix Isolation Infrared and ab Initio Studies

Conformations of dimethoxydimethylsilane (DMDMS) were studied using matrix isolation infrared spectroscopy, by trapping the silane in argon and nitrogen matrixes. The matrix was deposited using both an effusive and a supersonic jet source. The effusive source was maintained at two different temperatures, viz. 298 and 433 K, during deposition to alter the conformational population of the silane. The experimental results were supported by computations performed at both the HF and B3LYP levels, using 6-31++G** basis set. Vibrational frequency calculations were carried out to assign the experimental features and also to ensure that the computed structures did indeed correspond to minima. A conformer with a G+/-G-/+ structure was found to be the ground state, while G+/-T and G+/-G+/- structures were the next higher energy conformers with energies of 1.32 and 1.48 kcal/mol, respectively. Natural bond orbital analysis was carried out at both HF/6-31++G** and B3LYP/6-31++G** level which indicated that the charge-transfer hyperconjugative interactions largely determine the conformational preferences in this molecule. This interaction appears to be smaller in DMDMS than in the corresponding carbon analogue, dimethoxypropane (DMP).

Matrix isolation infrared and ab initio studies of the conformations of dimethoxymethylsilane

Conformers of dimethoxymethylsilane were studied using matrix isolation infrared spectroscopy, by trapping it in nitrogen and argon matrixes. In addition to a room temperature effusive source, supersonic and hot nozzle effusive sources were also used to deposit the matrix. These experiments were done to aid assigning the vibrational features, by altering the population of the various conformers in the matrix. As a result of these experiments, the infrared spectral features have been assigned to the ground state conformer, G ±G ∓ and higher energy conformers, G −G − and G −T. Our experimental observations were supported by computations performed at HF and B3LYP levels using 6-31++G** basis set.

Conformations of Trimethoxymethylsilane: Matrix Isolation Infrared and Ab Initio Studies

Conformations of trimethoxymethylsilane were studied using matrix isolation infrared spectroscopy and ab initio computations. Trimethoxymethylsilane was trapped in both argon and nitrogen matrixes using heated nozzle effusive sources and a supersonic jet source, in an effort to alter the conformational population in the matrix. Ab initio calculations were carried out at the HF and B3LYP level using 6-31++G basis set to support our experimental observations. The frequencies computed at the B3LYP level was found to fit well with our experimental data. A conformer with a C1(g(+/-)g(+/-)t) structure was predicted by our computations to be the ground state conformer.

Hydrogen Bonding in ROH:R‘OH (R, R‘ = H, CH3, C2H5) Heterodimers: Matrix-Dependent Structure and Infrared-Induced Isomerization

The structure of ROH:R‘OH heterodimers (R, R‘ = H, CH3, C2H5) trapped in argon and nitrogen matrixes was examined through two approaches: 16O/18O isotopic substitution in water and methanol and infrared-induced isomerization. Isotopic substitution clearly shows that in N2 matrix the bigger molecule plays the role of proton donor (Type-I structure) and in Ar matrix that of proton acceptor (Type-II structure). This difference can be rationalized by considering the existence of weak OH···N2 hydrogen bonds stabilizing the less stable structure. Selective irradiations in the νOH region were carried out for the CH3OH:H2O and C2H5OH:H2O dimers. Type-I → Type-II interconversion was observed in N2 matrix while no effect was detected in Ar. Finally the hydrogen bond strength evolution within the nine ROH:R‘OH homo- and heterodimers was examined on both experimental and theoretical grounds. Experimentally this evolution was followed by considering the νOH frequency shifts of the proton donor subunit. Theoretically a...

Photolysis of Matrix Isolated HONO/SO2 System. Identification and Infrared Spectra of Nitrososulfonic Acid HO(NO)SO2 and Hydroxysulfonyl HOSO2 Radical

The HONO/SO2/N2 and HONO/SO2/Ar matrixes were subjected to UV radiation (λ > 340 nm) from a medium pressure mercury lamp. The products of the photolysis were studied experimentally by means of FTIR spectroscopy and theoretically by the ab initio MP2 method. Two species are formed in nitrogen matrixes: the hydroxysulfonyl radical HOSO2 and identified for the first time nitrososulfonic acid HO(NO)SO2. In solid argon only, the latter species was observed. The identities of the products have been confirmed by carrying out experiments with deuterated nitrous acid DONO in place of HONO, performing concentration and annealing studies, comparison with results of quantum chemical calculations, and reference to the spectral data of related molecules. The mechanism of the photolytic reaction in the studied HONO/SO2/N2 and HONO/SO2/Ar matrixes is discussed on the basis of the observed kinetic profiles of the precursor species and the products.

Matrix Isolation Spectra and ab Initio Calculations of Isothiocyanic Acid Complexes with Carbon Monoxide

The complexes formed by isothiocyanic acid HNCS and its deuterated analogue DNCS with carbon monoxide have been observed and characterized in argon and nitrogen matrixes. The product bands and their shifts relative to the corresponding monomer absorptions prove that the complexes are hydrogen bonded with the carbon-attached structure OC···HNCS. The structure, energetics, and vibrational properties of the complexes have been calculated by ab initio at the MP2 level. Two stable minima were localized on the potential energy surface. Both involved an almost linear hydrogen bond from the NH group of the isothiocyanic acid molecule to either the carbon or the oxygen atom of the CO molecule. The OC···HNCS and CO···HNCS complexes were found to be weakly bound by −2.84 and −1.33 kcal/mol, respectively.

Photochemical Double-Proton-Transfer Reactions in 2,6-Dithiopurine. A Matrix Isolation Study

It was found that 2,6-dithiopurine isolated in low-temperature argon and nitrogen matrixes adopts only one tautomeric form: dithione-N7H. The UV (λ > 345 nm) induced reactions converting this tautomer into dithiol-N3H and dithiol-N7H isomers were observed in Ar and N2 matrixes. The first of these reactions represents a novel type of photoinduced double-proton transfer, whereas the second is analogous to that previously observed for 2,4-dithiouracil (Lapinski, L.; Nowak, M. J.; Kolos, R.; Kwiatkowski, J. S.; Leszczynski, J. Spectrochim. Acta A 1998, 54, 685). The assignment of the form initially present in the matrix to the dithione-N7H isomer and of the photoproducts to the dithiol-N3H and dithiol-N7H isomers was based on the good agreement between the experimental IR matrix spectra and the spectra theoretically simulated at DFT(B3LYP)/6-31G(d,p) level.