Reaction Of NCO Research Articles

Theoretical studies on the mechanism and dynamics of the H-abstraction for NCO with C 3H 8 reaction

Using a direct dynamics method, the mechanism and dynamics of hydrogen abstraction for the reaction of NCO with C 3H 8 were studied. The potential energy surface information required for the rate constant calculations of the title reaction was obtained at the G3MP2//BHH/6-311G(d,p) levels of theory. The rate constants for three channels of the title reaction were calculated by canonical variational transition state theory (CVT) with small-curvature tunnelling (SCT) contributions over a wide temperature region 296–2000 K. The results indicate that the methylene-H abstraction channel is dominant at low temperature, while the methyl-H abstraction channel becomes competitive at high temperature region, and the theoretical overall rate constants are in good agreement with the available experimental data.

Study on Moisture-Cured Polyurethane as Undercoating Layer for Metallization

In this work a technique of metallizing substrate via electroless plating using moisture-cured polyurethane (MCPU) system as the undercoat is presented. MCPU prepolymer was prepared by mixing polyethylene glycol divinyl ether and diphenylmethane-4, 4’-diisocyanate in 1:1 ratio. The effects of etching time as well as curing period on the surface characteristic of MCPU undercoating were investigated. Contact angle measurements, FTIR and SEM were employed to study the changes on the surface of the treated MCPU undercoatsprior to electroless nickel plating. Electroless plating was performed using nickel bath and visual inspection was performed after completing the electroless plating cycles. Relative increase in wettability of the treated MCPU was observed. The surface became hydrophilic after subjecting to mild etching for 1 minute. SEM analysis revealed different pitted structures on the treated MCPU that were cured at different periods. FTIR analysis of the treated MCPU showed some chemical changes marked by the presence of free hydroxyl group and decrease of CH (methylene), urethane C=O, urethane amide and ether peaks. FTIR also showed the sign of further NCO reaction, which indicates by the decrease of NCO peak and increase of NH and urea C=O peaks. The standard pull-off testing method (ASTM D 4541) was employed to evaluate the adhesion strength of nickel deposits coated on MCPU undercoating layer.The test results revealed that curing period and etching time influence the adhesion performance.The results also show that at a selected curing period, prolong etching time will decrease the adhesion strength. Meanwhile prolong curing period will improve the adhesion strength.

Effect of polyisocyanate hardener on adhesive force of waterborne polyurethane adhesives

A series of waterborne polyurethane (WBPU)/hardener adhesives were obtained from mixing of WBPU containing different types of polyol as a soft segment with aliphatic and aromatic polyisocyanates hardeners. By characterization of allophanate and biuret bonds formed from the reaction of hardener NCO with urethane/urea groups of WBPU using 1HNMR spectroscopy. It was found that the optimum number ratio (molar ratio) of NCO group of hardener to urethane/urea group of WBPU that shows the highest adhesion force was depended on the type of hardener (aliphatic/aromatic polyisocyanate) and dimethylol propionic acid (DMPA) content (total content of urethane/urea groups); however independent of the type of soft segment (polyol) of WBPU. The optimum number ratio (molar ratio) of NCO group of aromatic polyisocyanate hardener to urethane/urea was higher than that of aliphatic hardener to achieve the highest adhesion force of WBPU. The adhesive force increased with increasing hardener content up to the optimum point and then decreased. Poly(tetramethylene adipate glycol) (PTAd) based WBPUs with aliphatic hardener show higher adhesive force than Poly(tetramethylene oxide glycol) (PTMG) and aliphatic hardener-based WBPUs at the optimum number ratio (molar ratio) of NCO group of hardener to urethane/urea group of WBPU. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3663–3669, 2007

Effect of polyisocyanate hardener on waterborne polyurethane adhesive containing different amounts of ionic groups

Waterborne polyurethane (WBPU) adhesive with varying amounts of dimethylol propionic acid (DMPA) was synthesized by prepolymer process and blended with polyisocyanate hardener. The mean particle size of the WBPU dispersion decreased with increasing DMPA content.1H NMR spectroscopy confirmed the formation of allophanate bonds and biuret bonds due to the reaction of hardener NCO with urethane/urea groups. The optimum NCO content with the greatest adhesive strength was dependent on the total content of urethane/urea groups in the WBPU molecules. The optimum NCO content increased with increasing number of urethane groups (DMPA content). The adhesion strength of WBPU adhesives was maximized at a molar ratio of hardener NCO to urethane/urea of about 0.28.

Hydrogen promotes the selective catalytic reduction of NO x by ethanol over Ag/Al 2O 3

To understand the effect of H 2 on the selective catalytic reduction of NO x with C 2H 5OH over Ag/Al 2O 3, surface intermediates were examined using in situ DRIFTS spectra, and by-products were identified using GC–MS. Results showed that H 2 addition promoted the partial oxidation of C 2H 5OH to form enolic species, and enhanced the reaction of NCO with NO + O 2 at low temperature. We propose that the enhancement of the enolic species was the main contributor in accelerating NO x reduction under the presence of H 2 over Ag/Al 2O 3 at low temperatures.

Reduction of stored NO x on Pt/Al 2O 3 and Pt/BaO/Al 2O 3 catalysts with H 2 and CO

In situ Fourier transform infrared spectroscopy, coupled with mass spectrometry and time-resolved X-ray diffraction, were used to study the efficiency of nitrate reduction with CO and H 2 on Pt/Al 2O 3 and Pt/BaO/Al 2O 3 NO x storage reduction (NSR) catalysts. Surface nitrates were generated by NO 2 adsorption, and their reduction efficiencies were examined on the catalysts together with the analysis of the gas-phase composition in the presence of the two different reductants. H 2 was found to be a more effective reducing agent than CO. In particular, the reduction of surface nitrates proceeds very efficiently with H 2 even at low temperatures (∼420 K). During reduction with CO, isocyanates form and adsorb on the oxide components of the catalyst; however, these surface isocyanates readily react with water to form CO 2 and ammonia. The NH 3 thus formed in turn reacts with stored NO x at higher temperatures (>473 K) to produce N 2. In the absence of H 2O, the NCO species are stable to high temperatures and are removed from the catalyst only when they react with NO x thermal decomposition products to form N 2 and CO 2. The results of this study point to a complex reaction mechanism involving the removal of surface oxygen atoms from Pt particles by either H 2 or CO, the direct reduction of stored NO x with H 2 (low-temperature NO x reduction), the formation and subsequent hydrolysis of NCO species, and the direct reaction of NCO with decomposing NO x (high-temperature NO x reduction).

Ab initio quantum chemical studies of reaction mechanism for CH 2CO with NCO

An extensive quantum chemical study of the potential energy surface (PES) for all possible isomerization and dissociation reactions of CH 2CO with NCO is reported at the DFT (B3LYP/6-311++G(d,p)) and CCSD(T)/cc-pVDZ//B3LYP/6-311++G(d,p) levels of theory. For the CH 2CO+NCO reaction, the formation of CO+CH 2NCO via an addition–elimination mechanism is the dominant channel on the doublet surface. While the formation of CO+CH 2OCN via bimolecular substitution reaction is in the secondary. Meanwhile, the isomerization and dissociation reactions of the products, CH 2NCO and CH 2OCN, also have been investigated using the same theoretical approach. It can be concluded that these reaction channels are not feasible kinetically at low or fairly high temperatures. On the basis of the ab initio data, the total rate constants for the CH 2CO+NCO reaction in the T=296–560 K range have been computed using conventional transition state theory with Wigner tunneling correction and fitted by a rate expression as k=2.14×10 −12 (cm 3 molecule −1 s −1) exp(654.29/ T). The calculated total rate constants with Wigner tunneling correction for the CH 2CO+NCO reaction are in good agreement with the available experimental values.

DFT study on the mechanisms of the CH 2CO+NCX (X=O, S) reactions

A detailed theoretical survey on the potential energy surface for the CH 2CO+NCX(X=O, S) reaction is carried out at the QCISD/6-311+G(d, p)//B3LYP/6-311+G (d, p) level. The geometries, vibrational frequencies and energies of all stationary points involved in the reactions are calculated at the B3LYP/6-311+G(d, p) level. And the more accurate energy information is provided by single point calculations at the QCISD(T)/6-311+G(d, p) level. Relationships of the reactants, transition states, intermediates and products are confirmed by the intrinsic reaction coordinate (IRC) calculations. The calculations suggest that the channel producing CH 2NCO+CO is the dominant one, and CH 2NCO+CO is the major product for the CH 2CO+NCO reaction. The results are in good agreement with experiments. And similarly, product CH 2NCS+CO is the most important product for the CH 2CO+NCS reaction.

Theoretical Study of NCO and RCCH (R = H, CH3, F, Cl, CN) [3 + 2] Cycloaddition Reactions

We carried out a theoretical study of the radical [3 + 2] cycloaddition reaction of NCO + RCCH (R = H, CH3, F, Cl, CN), which produced a five-membered ring heterocyclic oxazole. An asynchronous two-bond formation mechanism was found, which led to a certain regioselectivity in the products when the substituted alkyne was used as a reactant. The preferable reactive sites of RCCH in various substituents are calculated by employing the Fukui functions and HSAB theory, and the results are in good agreement (except R = F) with the calculated energy barriers of the transition states in the potential energy surfaces. The N atom of NCO attacks the unsubstituted carbon atom of RCCH first, followed by the ring closure of the O atom with the other carbon atom to form the substituted oxazole. The order of the calculated first transition barriers (uts1) in the substituted alkynes (RCCH) is R = H > F > CN > Cl > CH3, and that for the second transition barriers (uts2), R = H > CH3 > CN > Cl > F. The reason for the decreased transition barriers of the substituted alkynes is analyzed.

AM1 study of the reaction between NCO and NO 2 yielding N 2O and CO 2

AM1 molecular orbital method using the unrestricted Hartree–Fock (UHF) calculation has been applied to investigate the mechanism of NCO reaction with NO 2. The geometries of the reactant, transition states, intermediates and product have been optimized and verified by frequency analysis. Zero-point energies are also corrected. The results show that this is a multi-step mechanism. Along the reaction process, there are three intermediates and four transition states. The step from IM1 to IM2 is the rate-controlling step, whose energy barrier is 193.24 kJ mol −1. The whole reaction is exothermic, with energy difference −285.22 kJ mol −1.

Three‐group type mechanism in the curing behavior of polyacrylate and blocked toluene diisocyanate

AbstractAbstract:The curing behavior of two resins, five blocked toluene diisocyanate (TDI) crosslinkers and polyacrylate, was studied using Torsional Braid Analysis (TBA) to examine their dynamic mechanical properties. The curing process was clearly divided into two stages. Associated with data from in situ FTIR at the molecular level, the first stage was the reaction of deblocked NCO and hydroxyl in polyacrylate. In the second stage, the deblocked NCO attacked the hydrogen in NH group of the urethane bond to conduct deep curing. Computer modeling was used to investigate the lowest‐energy configuration of the crosslinker molecule. The space hindrance effect in the molecule causes this type of three‐group crosslinking curing mechanism in the blocked TDI crosslinker. © 2002 John Wiley & Sons, Inc. J Appl Polym Sci 83: 112–120, 2002

The Influence of14N(e−, ν)14C(α, γ)18O Reaction on He Ignition in Degenerate Physical Conditions

The importance of NCO chain on the onset of the He-flash in degenerate physical conditions has been reevaluated. We find that low-mass, metal-rich (Z $\ge$ 0.001) structures climbing the Red Giant Branch do never attain the physical conditions suitable for the onset of this chain, while at lower metallicities the energy contribution provided by NCO reaction is too low to affect the onset of the central He-flash. At the same time, our evolutionary models suggest that for a Carbon-Oxygen White Dwarf of mass M_{WD}=0.6 M_sun accreting He-rich matter, directly or as a by-product of an overlying H-burning shell, at rates suitable for a dynamical He-flash, the NCO energy contribution is not able to keep hot enough the He-shell and in turn to avoid the occurrence of a strong electron degeneracy and the ensuing final explosion.

Synthesis and properties of novel thermoplastic poly(urethane-imide)s

A new class of poly(urethane-imide)s (PUI) thermoplastic elastomers were synthesized via the reaction of NCO terminated polyurethane with imide containing chain extender diols. The prepared chain extenders and polymers were characterized by conventional methods, and physical properties such as solution viscosity, solubility property, thermal stability and thermal behavior were studied. Compared to typical polyurethanes, these polymers exhibited better thermal stabilities due to the presence of the imide groups.

Reactivity of surface isocyanate species with NO, O2 and NO+O2 in selective reduction of NOχ over Ag/Al2O3 and Al2O3 catalysts

Reactivity of surface isocyanate (NCO(a)) species with NO, O2 and NO+O2 in selective reduction of NOχ over Ag/Al2O3 and Al2O3 catalysts was studied by a pulse reaction technique and an in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The NCO(a) species on Ag/Al2O3 reacted with O2 or NO+O2 mixture gas to produce N2 effectively above 200°C, while the reaction of NCO(a) with NO hardly produced N2 even at 350°C. In the case of Al2O3 alone, less N2 was detected in the reaction of NCO(a) with NO+O2, indicating that silver plays an important role in the N2 formation from NCO(a). These behaviors of the reactivity of NCO(a) species with reactant gases were in good agreement with the changes in NCO(a) bands shown by in situ DRIFT measurements. Based on these findings, the role of NCO(a) species in the selective reduction of NOχ on Ag/Al2O3 and Al2O3 catalysts is discussed.

Interfacial studies of crosslinked polyurethanes; Part I. Quantitative and structural aspects of crosslinking near film-air and film-substrate interfaces in solvent-borne polyurethanes

One of the reactions leading to the formation of polyurethane (PU) crosslinked networks is the reaction of NCO and OH functionalities. In this study, we examined how crosslinking reactions of hexamethylene diisocyanate isocyanurate and polyacrylate near the film-air (F-A) and film-substrate (F-S) interfaces in urethane coatings may affect crosslink density as well as other network properties. While at the initial stages of the crosslinking reactions, solvent evaporation competes with the urethane network formation and isocyanate consumption changes at various depths from the F-A and F-S interfaces. Quantitative analysis of the NCO consumption as a function of depth showed that the NCO concentrations change from 2.35×10−5 to 2.09×10−5 M, while going from 0.27 to 1.14 µm. During reaction times not exceeding two to three hours, the NCO consumption at the F-A and F-S interfaces is consumed more rapidly. At low relative humidity conditions, excessive amounts of unreacted NCO exists at both the F-A and F-S interfaces. However, at the extended reaction times, NCO concentration levels at the F-S are greater than at the F-A interface, and the NCO concentration differences can be as high as 3×10−5 M. In this study we also examined how crosslinking reactions of hexamethylene diisocyanate (HDI) isocyanurate and polyacrylate near the F-S interfaces in urethane films may affect orientation and distribution of urethane functionalities.

Theoretical ab Initio Study of CN2O3Structures: Prediction of New High-Energy Molecules

Structures and energies of CN2O3 isomers have been investigated theoretically at the ab initio CCSD(T)/TZ2P//MBPT(2)/6-31G* level in search of new high-energy molecules. The most energetically favorable isomer, five-membered cyclic nitrous carbonate, has only a 16 kcal/mol dissociation barrier toward decomposition into CO2 and N2O. Nitroisocyanate, O2N−NCO, a collision complex in the gas-phase reaction of NCO and NO2 radicals, is 29 kcal/mol higher in energy than nitrous carbonate and has a 11 kcal/mol barrier in the exothermic decomposition into CO2 and N2O, which involves formation of an intermediate four-membered cyclic form. Nitrofulminate has a higher CN bond dissociation energy and is expected to be a more stable molecule than the earlier studied nitrosofulminate (Korkin, A. A.; et al. J. Phys. Chem. 1996, 100, 19840). Regarding its high exothermicity in decomposition (O2N−CNO → CO2 + N2 + 1/2O2; ΔE = −165 kcal/mol), nitrofulminate is suggested as a potential energetic oxidizer. Another candidate is its six-membered cyclic trimer, 2,4,6-trinitro-1,2,3-triazine 1,2,3-trioxide. The estimated gaseous heats of formations of nitrofulminate and trinitrotriazine trioxide are 71 and 108 kcal/mol, respectively. Trinitrotriazine trioxide and two other cyclic trimers, trinitroisocyanurate and 2,4,6-tri(nitrosooxy)-1,3,5-triazine, have been optimized at the HF/6-31G* and at the MBPT(2)/6-31G* levels and characterized by HF analytical harmonic frequencies. Another CN2O3 isomer, dinitrosocarbonyl, OC(NO)2, has a low stability toward decomposition into CO and NO radicals.

Kinetics of the Reactions of NCO Radicals with NO and NH3

AbstractThe rate constants for the reaction of NCO (X2Π) radicals with NO and NH3 were measured at 20 Torr total pressure in temperature ranges of 290–1098 K and 295–882 K, respectively, using ClNCO excimer laser photolysis for NCO radical formation in combination with laser‐induced fluorescence detection of NCO. In the temperature range investigated the NCO + NO reaction exhibits a negative temperature dependence which is described by the following three parameter Arrhenius equation: , with E0 in units of kJ/mol.For the NCO + NH3 reaction the measurements exhibit a positive temperature dependence over the temperature range investigated with a slight upwards curvature. A modified three parameters Arrhenius fit provides a good description of the experimental data: , with E0 in units of kJ/mol. In addition, the rate constant of the NCO + NH3 reaction was found to be pressure independent in the range 10–193 Torr at 295 K.

Kinetics of the N+NCO reaction at 298 K

R. A. Brownsword, G. Hancock and D. E. Heard, J. Chem. Soc., Faraday Trans., 1997, 93, 2473 DOI: 10.1039/A702542D

Theoreticalab InitioStudy of CN2O2Structures: Prediction of Nitryl Cyanide as a High-Energy Molecule

Structures, energies, and harmonic vibrational frequencies of CN2O2 isomers have been investigated theoretically at the ab initio CCSD(T)/TZ2P//MBPT(2)/6-31G* level in search of new high-energy molecules and in a study of the mechanism of the reaction between NCO and NO radicals. Nitrosoisocyanate, ONNCO (1), earlier studied as a collision complex in the reaction of NCO and NO (Lin, M. C.; Melius, C. F. J. Phys. Chem. 1993, 97, 9124) is the most energetically favorable CN2O2 isomer, but its 18 kcal/mol unimolecular dissociation barrier is very low. Thus 1 can only be observed as a short-lived intermediate. However, nitrosofulminate, ONCNO (8), and nitryl cyanide, NCNO2 (12), higher energy isomers (69 and 38 kcal/mol above trans-1a, respectively), are more stable than 1 toward decomposition. This offers species 8 and 12 as interesting molecules for experimental study. Moreover, 12 can be a reasonably stable molecule as its C−N bond dissociation energy (59 kcal/mol) and the barrier to decomposition into N2 and CO2 (54 kcal/mol) are rather high, being comparable to those of nitromethane. The estimated large values of the heat of formation (ΔHf°300 = 60 kcal/mol) and of the decomposition energy of 12 (12 → N2 + CO2; ΔE = 150 kcal/mol) make this species potentially interesting as a high-energy molecule. Our study also includes four- (2) and three-membered (17) cyclic and bicyclic (3) isomers. The Cs cyclic isomers, 2 and 17, are extremely unstable, but the bicyclic C2v form (3) has a 29 kcal/mol dissociation barrier and should be observable.

Comparison of species profiles between O 2 and NO 2 oxidizers in premixed methane flames

The profiles of the species H, OH, CH, NH, CN, NCO, NO 2, and CH 3O are compared in a series of five premixed stoichiometric 15-torr CH 4/O 2/NO 2/N 2 flames with NO 2 comprising between 0% and 40% of the oxidizer. Relative species concentrations were measured by laser-induced fluorescence (LIF) and these results are compared with calculations using measured temperature profiles. The reaction mechanism of Miller and Bowman incorrectly predicts the standoff from the burner in flames containing more than 20% NO 2; addition of several reactions involving NO 2 and HONO produces excellent agreement with experiment for most species. The reaction CH 3 + NO 2 → CH 3O + NO is found to be particularly important in the reaction mechanism. LIF profiles of CH 3O show this species to be present in far larger quantities in the NO 2 supported flames than in the CH 4/O 2 system. The nitrogen-containing intermediates CN, NCO, and NH are all overpredicted by a factor of two in the 40% NO 2 flame relative to the 10% NO 2 flame. This indicates an inaccuracy in either the reburn reactions or the fuel nitrogen chemistry when large amounts of NO are present. The kinetic modeling shows that in the 40% NO 2 flame, the dominant pathway to N 2 formation is through N 2O, which is produced primarily by the reaction of NCO with NO. Comparison of emission profiles of NO 2∗ for the various flames indicates that the appearance of an orange-yellow luminous zone at the base of NO 2 supported flames is caused by thermal excitation of NO 2, not by a chemiluminescence mechanism.