NO Dimer Research Articles

Microwave spectrum of the 15N16O—15N16O dimer

The microwave transitions for the (15NO)2 dimer were reinvestigated following a report by McKellar et al. (1995, Molecular Physics, 86, 273) of a discrepancy between a previously measured microwave transition frequency and the corresponding frequency calculated from their molecular constants. The present microwave measurements confirm the previous observation of a rotational transition at 11159•297(4) MHz, but this transition is apparently not due to the cis N—N bonded NO dimer, since a mixture of 15NO in argon gas is required for observation of the 11159•297 MHz transition. A new doublet was located at 11128•434(4) MHz and this transition is now assigned to the 303 ← 212 transition of the 15N—15NO dimer, in agreement with predictions from the rotational constants of McKellar et al. This transition can be observed with either argon or neon as the expansion gas. New structural parameters obtained from the microwave data are in excellent agreement with those based on the FTIR data.

Microwave spectrum of the 15N16O-15N16O dimer

The microwave transitions for the (15NO)2 dimer were reinvestigated following a report by McKellar et al. (1995, Molecular Physics, 86, 273) of a discrepancy between a previously measured microwave transition frequency and the corresponding frequency calculated from their molecular constants. The present microwave measurements confirm the previous observation of a rotational transition at 11159•297(4) MHz, but this transition is apparently not due to the cis N—N bonded NO dimer, since a mixture of 15NO in argon gas is required for observation of the 11159•297 MHz transition. A new doublet was located at 11128•434(4) MHz and this transition is now assigned to the 303 ← 212 transition of the 15N—15NO dimer, in agreement with predictions from the rotational constants of McKellar et al. This transition can be observed with either argon or neon as the expansion gas. New structural parameters obtained from the microwave data are in excellent agreement with those based on the FTIR data.

Enhanced photodesorption of NO on roughened silver surfaces

The photodesorption of NO at 458 nm on roughened silver is contrasted with that on Ag(111). The data for Ag(111) are discussed in terms of recent studies of the complex adsorption behaviour of this system; it is concluded that the most likely mechanism involves a substrate mediated dissociative electron attachment to the NO dimer followed by monomer NO desorption. A five-fold enhancement of the photodesorption cross section on the roughened surface is found on irradiation at 458 nm. It is suggested that this effect arises from an enhancement of the substrate-mediated process. Specifically it is proposed that surface plasmon excitation on the rough surface results in an enhanced hot electron yield, with the consequence of an enhanced cross section for photodesorption.

Molecular assembly formation in a solid nanospace

The molecular assembly formation of a simple gaseous molecule such as N2, H2O, S02, NO and O2 in graphitic slit micropores is described. The strong molecular potential profile of a molecule in the graphitic slit pore is calculated and it is shown that the micropore can be used to control intermolecular structures and molecular reactivities. Examples of the molecular assembly formation in the micropore are described: vapor and supercritical N2 form the dimer in the micropore. H2O molecules in the micropore have an ordered structure showing short-range structure different from that in the bulk liquid, while S02 molecules produce a two-dimensional dipole-oriented structure in the micropore. Supercritical NO can be easily adsorbed through dimerization with the assistance of a magnetic perturbation, and highly concentrated NO dimers give rise to a high-pressure disproportionation reaction in the micropore. The structure of O2 molecules in the micropore is different from that on the two-dimensional graphite surface. Furthermore, NO and H2O form a clathrate compound in the micropore even at sub-atmospheric pressures, although the bulk molecular hydrate is presumed to be produced above 100 MPa in the bulk phase.

The photolysis of NO dimers, adsorbed on an MgF 2 surface, at 248 nm

The dynamics of the photolysis of NO dimers, and subsequent desorption of NO, from a single crystal MgF 2 surface have been investigated using resonance-enhanced multi-photon ionisation spectroscopy. The rotational distribution of NO( ν″ = 2) is correlated with its velocity. For a velocity of 1500 m s −1 there is a Gaussian distribution of rotational states, peaking at J = 35.5; lower velocities show progressively colder distributions. It is suggested that these changes are due to collisional deactivation of rotation and translation, but not of vibration, of NO( ν, J) on the surface prior to desorption.

Reactivity of NO with Anionic Water Clusters A-·(X2O)n (A = O, OX, O2, and XO2; X = D and H). Formation of (NO)2 and N2O3 and Their Reactions

The reactivity of NO toward anionic water clusters, A-·(X2O)n, A = O, OX, O2, and XO2; X = D and H), at several temperatures within the range 157−298 K, and at helium buffer gas pressures between 0.28 and 0.43 Torr, was studied using a flow-tube apparatus. The O-·(X2O)n≤5 and O2-·(X2O)n≤3 clusters were observed to react quite efficiently with NO, and the rate constants for these reactions were determined. The observed rate constants for O2-·(X2O)n show a positive temperature dependence. This observation is explained in terms of an energy barrier to the isomerization of the peroxy structural form, ONOO-·(X2O)n, to the conventional nitrate anion structure, NO3-·(X2O)n. For OX-·(X2O)n≤5, no detectable reaction with NO was observed to occur. Evidence for formation of the NO dimer and N2O3 in the flow-tube apparatus is presented. Under the flow-tube conditions employed, the N2O3 species is found to react efficiently with OX-·(X2O)n≤5, XO2-·(X2O)n≤4 and NO2-·(X2O)n≤2 via a ligand switching mechanism. The implic...

Inducible nitric oxide synthase: identification of amino acid residues essential for dimerization and binding of tetrahydrobiopterin.

Nitric oxide synthases (NOSs) require tetrahydrobiopterin (BH4) for dimerization and NO production. Mutation analysis of mouse inducible NOS (iNOS; NOS2) identified Gly-450 and Ala-453 as critical for NO production, dimer formation, and BH4 binding. Substitutions at five neighboring positions were tolerated, and normal binding of heme, calmodulin, and NADPH militated against major distortions affecting the NH2-terminal portion, midzone, or COOH terminus of the inactive mutants. Direct involvement of residues 450 and 453 in the binding of BH4 is supported by the striking homology of residues 448-480 to a region extensively shared by the three BH4-utilizing aromatic amino acid hydroxylases and is consistent with the conservation of these residues among all 10 reported NOS sequences, including mammalian NOSs 1, 2, and 3, as well as avian and insect NOSs. Altered binding of BH4 and/or L-arginine may explain how the addition of a single methyl group to the side chain of residue 450 or the addition of three methylenes to residue 453 can each abolish an enzymatic activity that reflects the concerted function of 1143 other residues.

The NO dimer:15N isotopic infrared spectra, line-widths, and force field

The v 1 fundamental bands of (15N16O)2 and 14N16O-15N16O isotopes of the NO dimer have been studied using a long-path (200 m) low temperature (84 K) gas cell and Fourier transform infrared spectrometer. The spectra, obtained with a resolution of 0·005 cm-1, were analysed to give accurate rotational and quartic centrifugal distortion parameters for the ground and the v 1 = 1 vibrational states. The predissociation-limited v 1 band line width for (14N16O)2 was measured to be 0·0056(3) cm-1, which is slightly less than previous determinations. The (15N16O)2 width was found to be 0·0044(2) cm-1, and the significant isotope dependence will help to constrain models of the predissociation process. A much larger width of 0·014(2) cm-1 was measured for the mixed isotope, 14N16O-15N16O. This, and the mixed isotope vibrational frequency, may be understood on the basis of a simple coupled-oscillator model. The present results were used to derive a refined structure for the NO dimer: r(NO) = 1·1515(3) A, r(NN) = 2·263...

Vector correlations in the 193 nm photodissociation of the NO dimer

Vector correlations in the 193 nm photodissociation of the NO dimer

Infrared spectroscopy, infrared photoconversion, and ultraviolet photodissociation of NO dimers in neon and nitrogen matrices

The nitric oxide dimers isolated in neon or nitrogen matrices have been irradiated in the infrared by selected lines of a cw CO laser and in the UV at 193 nm by an ArF excimer laser. IR excitation of the antisymmetric mode of the cis dimer in neon induces a conversion to a special van der Waals dimer. At T<6 K UV irradiation dissociates the dimers mainly to NO monomers and, in smaller proportion, to NO2 in the neon matrix or to N2O in the nitrogen matrix. NO2 is formed exclusively from the cis dimer and N2O from the trans dimer. At higher temperature in nitrogen, the oxygen atoms produced in the N2O formation migrate and react with NO to form NO2. The vibrational spectroscopy in nitrogen reveals several new features. The symmetrical vibrational mode of planar trans-(NO)2, normally IR inactive, becomes observable at 5.8 K owing to the matrix strain. cis-(NO)2 is trapped in two sites and the energy difference between them changes abruptly around 14 K, suggesting a structural transformation in the neighborhood of the sites. Cooling at 5.8 K induces a reversible decrease of the cis dimer bands, correlated with the appearance of vdW dimer bands. In both matrices, the antisymmetric band of the cis dimer is broadened by a predissociation effect as in the gas phase.

Characterization and orientation of adsorbed NO dimers on Ag{111} at low temperatures

Reflection-absorption infrared spectroscopy (RAIRS) and near-edge x-ray absorption fine structure (NEXAFS) have been used, with isotopic 14NO/15NO mixtures, to determine the structure and orientation of the monolayer species formed by NO adsorption on Ag{111} at 40 to 60 K. The adlayer is composed of NO dimers bonded with the N–N axis in the surface plane and with the molecular plane tilted away from the surface normal by about 30°. This structure provides a simple basis for understanding the facile reaction to adsorbed N2O and O which occurs on heating to 70 to 90 K.

LEED and resonance EELS study of the (NO) 2 dimer on graphite

The structure of physisorbed (NO) 2 films on graphite has been investigated using resonance electron energy loss spectroscopy in conjunction with low energy electron diffraction (LEED). Various ordered phases were resolved for the first time by the LEED technique as a function of coverage. Transient negative ion resonance formation was manifested in strong enhancements of the ν = 0−1 vibrational cross section in the incident electron energy range 11–16 eV. A splitting of this resonance by 2.6 ± 0.2 eV indicates formation of the dimer configuration on the surface. The resonance splitting was observed for all adsorbate coverages studied. Detailed angular distributions have been recorded, which showed distinct features for different adsorbed phases. Ab initio self-consistent field calculations allow a qualitative analysis of the orientation of the NO dimer. Our results are consistent with an orientation of the NO dimer plane approximately parallel to the surface in the γ phase and perpendicular in the β phase.

Very Low Temperature Surface Reaction: N2O Formation from NO Dimers at 70 to 90 K on Ag{111}

When Ag{111} is exposed to NO at 40 K, monolayer adsorption is followed by multilayer (NO)(2) formation. Heating to 60 K results in the desorption of the dimer multilayer, leaving a monolayer on the surface. Using isotopes, the monolayer has also been characterized as dimers, which have the N-N axis coplanar with the surface. At 70 to 90 K, N2O is formed from (NO)(2) on the Ag{111} surface. Further heating to 120 K results in the desorption of the N2O. The reactive species is thus shown to be the adsorbed dimer, which cannot be formed at higher temperatures on exposure to NO because the surface coverage is too low. The monomer, adsorbed NO, is thus weakly adsorbed, in agreement with expectation.

Photodissociation dynamics of (NO)2 on LiF(001): Characterization of vibrationally excited NO fragments

The photodissociation of adsorbed NO dimers on LiF(001) was studied in the monolayer regime at 248 nm using resonantly enhanced multiphoton ionization (REMPI) and Fourier transform infrared (FTIR) absorption spectroscopy. Vibrationally excited NO photofragments were observed in v′=0–9. The vibrational energy distribution was found to have a maximum at v′=0 and a second region of substantial population between v′=2 and 9. The rotational and translational energy distributions of the photofragments showed no major change with vibrational excitation. By contrast, the translational energy displayed a systematic increase with increasing rotational excitation. Photodissociation at 1 ML (NO)2 coverage yielded average vibrational, rotational, and translational energies of 0.48, 0.05, and 0.13 eV, respectively, in the NO fragments. The vibrational and rotational energy distributions of the fragments were unchanged for 0.06 ML, while the translational energy increased by approximately 30% in going to this lower coverage. The angular distribution was peaked in the normal direction at both coverages. The results are interpreted in terms of an excitation/deexcitation mechanism, for which the observed energy distributions can be rationalized by assuming differing equilibrium geometries between the ground and excited states of the adsorbed dimer.

Identification, Structure, and Vibrational Assignment of the NO Dimer Cation

Identification, Structure, and Vibrational Assignment of the NO Dimer Cation

NO diffusion and dimer formation in a nitrogen matrix studied by FTIR spectroscopy

The formation of nitric oxide dimers in a nitrogen matrix at 24.5 and 25.2 K was monitored by Fourier transform infrared spectroscopy and the results are interpreted by a numerical simulation. The diffusion coefficient is fitted by D = D 0 exp (− E/kT), with D 0 = 90 cm 2 s −1 and E = 716 cm −1. The dissociation coefficient, K c = 2.2 × 10 19 molecule cm −3, does not change between 24 and 32 K. A C 3 rotation of a NO, N 2, N 2 cluster is proposed as the most probable elementary step for diffusion. The cis- and trans-(NO) 2 band intensities are evaluated relative to the monomer.

Photodissociation of the NO dimer at 193 nm. The rotational alignment of the NO (A) fragments

The rotational alignment A (2) 0 of the NO(A) fragments in the photodissociation fo teh NO dimer at 193 nm has been determined based on the polarization response of the laser-induced fluorescence in the E 2Σ +-A 2Σ + transition. The observed alignment A (2) 0 is negative, with increasing magnitude in higher rotational levels, though it does not reach the limiting value of - 0.4. The negative value shows that the rotational angular momentum of the fragment is mostly perpendicular to the electric transition dipole vector, implying that the cis planar (NO) 2 dissociates while keeping its planarity. This tendency is stronger for fragments of high rotational quantum number.

Isotopic studies of the reaction of NO on silver surfaces

The isotopomers 14N 16O and 15N 18O were used to obtain information on the mechanisms of NO reaction on silver layers on Ru(001), by observation of the isotopic distributions of the products. NO adsorption on Ag in the 75–100 K range produces adsorbed N 2O and oxygen atoms. Very different isotopic distributions of product N 2O are observed, depending on whether the NO isotopomers are dosed simultaneously or sequentially. The observed N 2O distributions favor a non-dissociative reaction pathway with a NO dimer as the reaction intermediate. The isotopic distribution of a high temperature NO desorption state can be explained by dissociation of a NO 2 surface species that is formed at a low temperature. The N 2O which is produced by reaction of NO on the surface is found to have a higher desorption temperature than N 2O adsorbed on either the clean Ag surface or on an oxygenated Ag surface. This is attributed to a strong binding interaction with surface NO 2.

Isotopic studies of the reaction of NO on silver surfaces

Abstract The isotopomers 14 N 16 O and 15 N 18 O were used to obtain information on the mechanisms of NO reaction on silver layers on Ru(001), by observation of the isotopic distributions of the products. NO adsorption on Ag in the 75–100 K range produces adsorbed N 2 O and oxygen atoms. Very different isotopic distributions of product N 2 O are observed, depending on whether the NO isotopomers are dosed simultaneously or sequentially. The observed N 2 O distributions favor a non-dissociative reaction pathway with a NO dimer as the reaction intermediate. The isotopic distribution of a high temperature NO desorption state can be explained by dissociation of a NO 2 surface species that is formed at a low temperature. The N 2 O which is produced by reaction of NO on the surface is found to have a higher desorption temperature than N 2 O adsorbed on either the clean Ag surface or on an oxygenated Ag surface. This is attributed to a strong binding interaction with surface NO 2 .

Thev1band of (NO)2

The v 1 band of the NO dimer has been studied using a long-path (84 m) low-temperature (83·5 K) gas cell and a Fourier transform infrared spectrometer. Over 440 lines of (NO)2 have been measured in the 1840 to 1900 cm-1 region, with a resolution of 0·008 cm-1, and assigned to perpendicular (Δka = ± 1) transitions with J and Ka values up to 35 and 16, respectively. These measurements were analysed, along with the four previous microwave transitions, using an s-reduced asymmetric rotor Hamiltonian in order to obtain accurate rotational and quartic centrifugal distortion parameters for (NO)2 in its ground and v 1 = 1 vibrational states. As far as this experiment is concerned, the NO dimer behaves as a normal, planar, semi-rigid molecule, with no observed perturbations, only modest centrifugal distortion effects, and only small changes in parameters due to vibrational excitation. The observed intensities of (NO)2 lines were used to estimate a value for the dissociation energy of the dimer of D 0 = 639 ± 35 cm-1, where the indicated uncertainty depends mostly on an assumption that the vibrational transition moment of NO changes by 40% or less upon complex formation.