The influence of mercury contact angle, surface tension, and retraction mechanism on the interpretation of mercury porosimetry data

The use of a semi-empirical alternative to the standard Washburn equation for the interpretation of raw mercury porosimetry data has been advocated. The alternative expression takes account of variations in both mercury contact angle and surface tension with pore size, for both advancing and retreating mercury meniscii. The semi-empirical equation presented was ultimately derived from electron microscopy data, obtained for controlled pore glasses by previous workers. It has been found that this equation is also suitable for the interpretation of raw data for sol-gel silica spheres. Interpretation of mercury porosimetry data using the alternative to the standard Washburn equation was found to give rise to pore sizes similar to those obtained from corresponding SAXS data. The interpretation of porosimetry data, for both whole and finely powdered silica spheres, using the alternative expression has demonstrated that the hysteresis and mercury entrapment observed for whole samples does not occur for fragmented samples. Therefore, for these materials, the structural hysteresis and overall level of mercury entrapment is caused by the macroscopic (> approximately 30 microm), and not the microscopic (< approximately 30 microm), properties of the porous medium. This finding suggested that mercury porosimetry may be used to obtain a statistical characterization of sample macroscopic structure similar to that obtained using MRI. In addition, from a comparison of the pore size distribution from porosimetry with that obtained using complementary nitrogen sorption data, it was found that, even in the absence of hysteresis and mercury entrapment, pore shielding effects were still present. This observation suggested that the mercury extrusion process does not occur by a piston-type retraction mechanism and, therefore, the usual method for the application of percolation concepts to mercury retraction is flawed.     

Ink-bottle effect in mercury intrusion porosimetry of cement-based materials

Mercury intrusion porosimetry (MIP) is a widely used method for studying porous materials, in particular, cement-based materials. The usual interpretation of such measurements is based on certain assumptions. One of these is that each pore is connected to the sample surface directly or through larger pores. Pores not meeting this assumption are called ink-bottle pores. The effect that sample size has on the MIP characteristics of concrete samples, like the ink-bottle effect and hysteresis, was studied by measuring additional extrusion and intrusion cycles. In order to characterize the extrusion and ink-bottle behavior, the amount of entrapped mercury chi(p) was estimated. Superimposition of extrusion and second intrusion curves is achieved if the contact angle theta is adjusted from theta(i), the intrusion contact angle, to theta(e), the extrusion contact angle. The threshold radius is often assumed to be a dominant pore radius, yet in this study the entrapped mercury content shows no evidence for the presence of a dominant pore radius. Even if characteristic properties of cement-based materials can be estimated with MIP, comparison of results is rendered difficult by the significant effects of sample preparation techniques and sample size and the ink-bottle effect due to randomly present air bubbles.     

Rigid sphere molecular model enables an assessment of the pore curvature effect upon realistic evaluations of surface areas of mesoporous and microporous materials

A gas adsorption rigid spheres model (RSM) was incorporated into the CPSM model (corrugated pore structure model) to correlate the pore surface areas obtained from the BET and CPSM methods. The latter is a method simulating the gas sorption hysteresis loop and enables the evaluation of surface areas S(CPSM) through the integration of the pertinent pore size distributions. Thus, S(CPSM) values are inherently influenced by pore curvature. The new CPSM-RSM version estimates surface areas S(CPSMfs) that are independent of pore curvature and can be compared with the pertinent S(BET) values. The RSM exploits the fact that a curved pore surface accommodates fewer molecules, assumed to behave as rigid spheres, than an equal flat one. Thus, the RSM accounts for a higher molecular surface coverage Ac (nm2/molec.) in pores with marked curvature than that (i.e., Af) on a flat surface. The ratio Ac/Af for nitrogen adsorbed on single pore sizes varies in the range Ac/Af = 1.44-1.03 for pore sizes D = 1.5-15 nm, respectively. Also for D = 1.5-5.0 nm the S(CPSMfs) and S(BET) values are lower by approximately 10-45% than the S(CPSM) estimates. From the application of the CPSM-RSM model on several porous materials exhibiting all known types of sorption hysteresis loops, it was confirmed that S(BET) approximately S(CPSMfs) (+/-5%) and (S(CPSM) - S(BET))/S(BET) = 3-68% for the materials examined. In conclusion, the BET method may produce quite conservative surface area estimates for materials exhibiting pore structures with appreciable pore curvature, whereas the CPSM-RSM model can reliably predict both S(CPSM) and S(CPSMfs) = S(BET) values.     

Simulation of nonwetting phase entrapment within porous media using magnetic resonance imaging

Models representing the pore structures of amorphous, mesoporous silica pellets have been constructed using magnetic resonance images of the materials. Using magnetic resonance imaging (MRI), maps of the macroscopic (approximately 0.01-1 mm) spatial distribution of porosity and pore size were obtained. The nature and key parameters of the physical mechanism for mercury retraction, during porosimetry experiments on the silica materials, were determined using integrated gas sorption experiments. Subsequent simulations of mercury porosimetry within the structural models derived from MRI have been used to successfully predict, a priori, the point of the onset of structural hysteresis and the final levels of mercury entrapment for the silicas. Hence, a firm understanding of the physical processes of mercury retraction and entrapment in these amorphous silica materials has been established.     

The "lotus effect" explained: two reasons why two length scales of topography are important

Surfaces containing 4 x 8 x 40 microm staggered rhombus posts were hydrophobized using two methods. One, using a dimethyldichlorosilane reaction in the vapor phase, introduces a smooth modified layer, and the other, a solution reaction using methyltrichlorosilane, imparts a second (nanoscopic) length scale of topography. The smooth modified surface exhibits contact angles of thetaA/thetaR = 176 degrees /156 degrees . Arguments are made that the pinning of the receding contact line by the post tops (with thetaA/thetaR = 104 degrees /103 degrees ) is responsible for the hysteresis. The second level of topography raises the contact angles of the post tops and the macroscopic sample to theta(A)/theta(R) = >176 degrees />176 degrees and eliminates hysteresis. The increase in Laplace pressure due to the increase in the advancing contact angle of the post tops is a second reason that two length scales of topography are important.     

Mercury porosimetry in mesoporous glasses: a comparison of experiments with results from a molecular model

We present results from experiments and molecular modeling of mercury porosimetry into mesoporous Vycor and controlled pore glass (CPG) solid materials. The experimental intrusion/extrusion curves show a transition from a type H2 hysteresis for the Vycor glass to a type H1 hysteresis for the CPG. Mercury entrapment is observed in both materials, but we find that the amount of entrapped mercury depends on the chosen experimental relaxation time. No additional entrapment is found in a second intrusion/extrusion cycle, but hysteresis is still observed. This indicates that hysteresis and entrapment are of different origin. The experimental observations are qualitatively reproduced in theoretical calculations based on lattice models, which provide significant insights of the molecular mechanisms occurring during mercury porosimetry experiments in these porous glasses.     

Porous structure of natural and modified clinoptilolites

The evaluation of the pore-size distribution (PSD) of natural and modified mesoporous zeolites, i.e., clinoptilolites is presented. We demonstrate the SEM results showing that the pores of fracture-type from 25-50 nm to 100 nm in size between clinoptilolite grains, as well as pores between crystal aggregates up to 500 nm in size are present in the studied material. The detailed distribution of pore sizes and tortuosity factor of the above-mentioned materials are determined from the adsorption-desorption isotherms of nitrogen measured volumetrically at 77 K. To obtain the reliable pore size distribution (PSD) of the above-mentioned materials both adsorption and desorption branches of the experimental hysteresis loop are described simultaneously by recently developed corrugated pore structure model (CPSM) of Androutsopoulos and Salmas. Evaluated pore size distributions are characterized by well-defined smooth peaks placed in the region of the mesoporosity. Moreover, the mean pore diameter calculated from the classical static measurement of nitrogen adsorption at 77 K correspond very well to the pore diameters from SEM, showing the applicability of the CPSM for characterization of the porosity of natural zeolites. We conclude that classical static adsorption measurements combined with the proper modeling of the capillary condensation/evaporation phenomena are a powerful method which can be applied for pore structure characterization of natural and modified clinoptilolites.     

Theoretical contact angles on a nano-heterogeneous surface composed of parallel apolar and polar strips

Neumann-Good's parallel strip model (J. Colloid Interface Sci. 1972, 38, 341) was used to analyze the contact angle hysteresis for a liquid on a heterogeneous surface composed of alternatively aligned horizontal apolar (theta = 70 degrees ) and polar (theta = 0 degree ) strips. The critical size of the strip width, below which the contact angle hysteresis disappears, was determined on the basis of the analysis of the activation energy for wetting to be from 6 to 12 nm. This calculated value of the critical strip size is 1 order of magnitude smaller than that of 0.1 microm, which has been commonly considered as the limit of heterogeneity size causing the appearance of the contact angle hysteresis.     

Water microdroplets on molecularly tailored surfaces: correlation between wetting hysteresis and evaporation mode switching

The evaporation of water microdroplets from solid surfaces was studied using digital contact angle analysis techniques. An inclusive trend for the evaporation process, that is, a switch from the initial constant contact area to the subsequent constant contact angle mode was observed for all surfaces examined, including mixed self-assembled monolayers (SAMs) on gold and "conventional" surfaces such as silicon wafers, polycarbonate, and Teflon. More importantly, it has been shown that the change in contact angle during the evaporation process (i.e., evaporation hysteresis, delta theta(evap), the difference between the initial and "equilibrated" contact angle) correlates well with the wetting hysteresis determined directly (i.e., measuring the advancing and receding contact angles on these surfaces by changing the drop volume). The comparison between mixed SAM surfaces and conventional solids revealed that the evaporation/wetting hysteresis is dominated by the roughness (from nanometer to micrometer scale) rather than the chemical heterogeneity of the surface. The evaporation rates of water microdroplets on these surfaces were also monitored and modeled.     

Wetting of regularly structured gold surfaces

In this study we report results for a systematic study of the wetting of structured gold surfaces formed by electrodeposition through monolayer templates of close-packed uniform submicrometer spheres. Removal of the template after deposition leaves a regular hexagonal array of sphere segment pores where the depth of the pores and, thus, the topography of the surface are controlled by the thickness of gold deposited through the template. We find that, as the thickness of the porous film increases up to the radius of the pores, the apparent contact angle for water on the surface increases from 70 degrees on the flat surface to more that 130 degrees , and then with increasing thickness above the radius of the pores the apparent contact angle decreases back toward 70 degrees . We show that these changes in the apparent contact angle agree with the model of Cassie and Baxter for nonwetted surfaces even though the gold itself is hydrophilic. We also show that the apparent contact angle is independent of the diameter of the pores over the range 400-800 nm. This is the first reported example showing the change of a hydrophilic surface (theta; < 90 degrees ) into a hydrophobic surface (theta; > 90 degrees ) purely by control of the surface topography. The role of the pore shape and size in stabilizing the nonwetting (Cassie-Baxter) droplet on the surface is discussed.     

Observation of a new phosphorus-containing reactive intermediate: electronic spectroscopy and excited-state dynamics of the HPBr free radical

The A (2)A(')-X (2)A(") electronic spectra of jet-cooled HPBr and DPBr have been obtained for the first time using the pulsed electric discharge technique with a precursor mixture of PBr(3) and H(2)/D(2). Laser-induced fluorescence and single vibronic level emission spectra gave the bending and P-Br stretching frequencies in the ground and excited states of both isotopomers. Rotational analyses of the HPBr and DPBr 0(0) (0) bands showed small spin splittings characteristic of a doublet-doublet transition of an asymmetric-top molecule. From the ground- and excited-state rotational constants, effective (r(0)) structures were derived with r(")(PH)=1.4307(86) A, r(")(PBr)=2.2021(9) A, and theta(")=95.2(8) degrees, and r(')(PH)=1.434(31) A, r(')(PBr)=2.1669(26) A, and theta(')=115.5(16) degrees . In a few favorable cases, further hyperfine splitting of the spin-rotation energy levels has been observed, due to the excited-state Fermi contact interaction of the unpaired electron with the spin magnetic moment of the (31)P nucleus, with a(F) (')=0.064(9) cm(-1) for HPBr. Fluorescence depletion spectroscopy and lifetime measurements indicate that higher vibrational levels of the A (2)A(') state are predissociated by a X (2)A(") dissociative continuum. CCSD(T)/aug-cc-pVTZ calculations predict that the most likely dissociation process is HPBr (X (2)A("))-->PH((3)Sigma(-))+Br((2)P(u)).     

Fourier transform microwave spectroscopy and Fourier transform microwave-millimeter wave double resonance spectroscopy of the ClOO radical

Pure rotational spectra of the ClOO radical for the (35)Cl and (37)Cl isotopomers have been observed using Fourier transform microwave and Fourier transform microwave-millimeter wave double resonance spectroscopy. The rotational, centrifugal, spin-rotation coupling, and hyperfine coupling constants have been determined by least-squares fits of the observed transition frequencies. The molecular constants indicate that the electronic ground state is 2A". The r(0) structure is determined to be r(0)(ClO)=2.075 A, r(0)(OO)=1.227 A, and theta;(0)(ClOO)=116.4 degrees . Several highly accurate ab initio calculations have also been performed. Some of them turned out to be inaccurate because it is necessary to take into account both static and dynamic electronic correlations. Only multireference (single and double) configuration interaction calculations with large basis sets reproduce the present experimental results. The anharmonic force constants obtained by the ab initio calculations are used to determine the r(e) structure, r(e)(ClO)=2.084(1) A, r(e)(OO)=1.206(2) A, and theta;(e)(ClOO)=115.4(1) degrees . Unique features of the ClOO radical have become clear by the present experiment and the ab initio calculations.     

Line energy and the relation between advancing, receding, and young contact angles

The line energy associated with the triple phase contact line is a function of local surface defects (chemical and topographical); however, it can still be calculated from the advancing and receding contact angles to which those defects give rise. In this study an expression for the line energy associated with the triple phase contact line is developed. The expression relates the line energy to the drop volume, the interfacial energies, and the actual contact angle (be it advancing, receding, or in between). From the expression we can back calculate the equilibrium Young contact angle, theta0, as a function of the maximal advancing, thetaA, and minimal receding, thetaR, contact angles. To keep a certain maximal hysteresis between advancing and receding angles, different line energies are required depending on the three interfacial energies and the drop's volume V. We learn from the obtained expressions that the hysteresis is determined by some dimensionless parameter, K, which is some normalized line energy. The value of K required to keep a constant hysteresis (thetaA-thetaR) rises to infinity as we get closer to theta0 = 90 degrees.     

On equilibrium structures of the water molecule

Equilibrium structures are fundamental entities in molecular sciences. They can be inferred from experimental data by complicated inverse procedures which often rely on several assumptions, including the Born-Oppenheimer approximation. Theory provides a direct route to equilibrium geometries. A recent high-quality ab initio semiglobal adiabatic potential-energy surface (PES) of the electronic ground state of water, reported by Polyansky et al. [ ibid. 299, 539 (2003)] and called CVRQD here, is analyzed in this respect. The equilibrium geometries resulting from this direct route are deemed to be of higher accuracy than those that can be determined by analyzing experimental data. Detailed investigation of the effect of the breakdown of the Born-Oppenheimer approximation suggests that the concept of an isotope-independent equilibrium structure holds to about 3 x 10(-5) A and 0.02 degrees for water. The mass-independent [Born-Oppenheimer (BO)] equilibrium bond length and bond angle on the ground electronic state PES of water is r(e) (BO)=0.957 82 A and theta e (BO)=104.48(5) degrees , respectively. The related mass-dependent (adiabatic) equilibrium bond length and bond angle of H2 (16)O is r(e) (ad)=0.957 85 A and theta e (ad)=104.50(0) degrees , respectively, while those of D2 (16)O are r(e) (ad)=0.957 83 A and theta e (ad)=104.49(0) degrees . Pure ab initio prediction of J=1 and 2 rotational levels on the vibrational ground state by the CVRQD PESs is accurate to better than 0.002 cm(-1) for all isotopologs of water considered. Elaborate adjustment of the CVRQD PESs to reproduce all observed rovibrational transitions to better than 0.05 cm(-1) (or the lower ones to better than 0.0035 cm(-1)) does not result in noticeable changes in the adiabatic equilibrium structure parameters. The expectation values of the ground vibrational state rotational constants of the water isotopologs, computed in the Eckart frame using the CVRQD PESs and atomic masses, deviate from the experimentally measured ones only marginally, especially for A0 and B0. The small residual deviations in the effective rotational constants are due to centrifugal distortion, electronic, and non-Born-Oppenheimer effects. The spectroscopic (nonadiabatic) equilibrium structural parameters of H2 16O, obtained from experimentally determined A'0 and B'0 rotational constants corrected empirically to obtain equilibrium rotational constants, are r(e) (sp)=0.957 77 A and theta e (sp)=104.48 degrees .     

Theory of mercury intrusion in a distribution of unconnected wedge-shaped slits

Effective mercury intrusion in a wedge-shaped slit is gradual, the intruded depth increasing with applied pressure. The Washburn equation must be modified accordingly. It relates the distance, e, separating the three-phase contact lines on the wedge faces to the hydrostatic pressure, P, wedge half-opening angle alpha, mercury surface tension gamma, and contact angle theta: e=(-2gamma/P)cos(theta-alpha) if theta-alpha>pi2. The equations relating the volume of mercury in a single slit to hydrostatic pressure are established. The total volume of mercury V(Hg)(tot)(E(0),e) intruded in a set of unconnected isomorphous slits (same alpha value) with opening width, E, distributed over interval [E(0),0], and volume-based distribution of opening width, f(V)(E), is written as where G(X)=(sin(-1)X-X1-X(2))/X(2) and X(E,e)=-cos(theta-alpha)Ee. The exact relation between total internal surface area and integral pressure work is.     

Application of thin-shielded mercury microelectrodes in anodic stripping voltammetry

The performance in anodic stripping voltammetry (ASV) of hemispherical mercury microelectrodes, fabricated by electrodeposition of liquid mercury on the surface of Pt microdisks which were surrounded by a rather thick or thin insulating shield, was compared. The Pt microdisks were produced by sealing a wire of 25 μm diameter into a glass capillary, and by coating the cylindrical length of the Pt wire with a cathodic electrophoretic paint. The ratio of the overall tip radius b, to the basal radius of the electrode a, so-called RG = b/a, was equal to 110 ± 10 and 1.52 ± 0.01 for the thick- and thin-shielded microdisk, respectively. The mercury microelectrodes were characterized by cyclic voltammetry at 1 mV s⁻¹, in 1 mM Ru(NH₃)₆³⁺ aqueous solution. The steady-state voltammogram recorded with the thin-shielded mercury microelectrode displayed less hysteresis, while the steady-state current was about 30% higher than that of the thicker one. This was a consequence of the additional flux due to diffusion from behind the plane of the electrode. The flux enhancement, which was operative at the thin-shielded mercury microelectrode during the deposition step in the ASV experiments, allowed recording stripping peaks for Cd and Pb, which resulted about 32% larger than those recorded at the thicker shielded mercury microelectrode, under same experimental conditions.The usefulness of the thin-shielded mercury microelectrode for ASV measurements in real samples was verified by determining the content of heavy metal ions released in the pore water (pH 4.5) of a soil slurry.     

Alpha-gamma hybrid peptides that contain the conformationally constrained gabapentin residue: characterization of mimetics of chain reversals

The crystal structures of four dipeptides that contain the stereochemically constrained gamma-amino acid residue gabapentin (1-(aminomethyl)cyclohexaneacetic acid Gpn) are described. The molecular conformation of Piv-Pro-Gpn-OH (1), reveals a beta-turn mimetic conformation, stabilized by a ten atom C[bond]H...O hydrogen bond between the Piv CO group and the pro S hydrogen of the Gpn CH(2)[bond]CO group. The peptides Boc-Gly-Gpn-OH (2), Boc-Aib-Gpn-OH (3), and Boc-Aib-Gpn-OMe (4) form compact, folded structures, in which a distinct reversal of polypeptide chain direction is observed. In all cases, the Gpn residue adopts a gauche,gauche (g,g) conformation about the C(gamma)[bond]C(beta) (theta(1)) and C(beta)[bond]C(alpha) (theta(2)) bonds. Two distinct Gpn conformational families are observed. In peptides 1 and 3, the average backbone torsion angle values for the Gpn residue are phi=98 degrees, theta(1)=-62 degrees, theta(2)=-73 degrees, and psi=79 degrees, while in peptide 2 and 4 the average values are phi=-103 degrees, theta(1)=-46 degrees, theta(2)=-49 degrees, and psi=-92 degrees. In the case of 1 and 3, an intramolecular nine-membered O[bond]H...O hydrogen bond is formed between the C[double bond]O of the preceding residue and the terminal carboxylic acid OH group. All four alpha-gamma dipeptide sequences yield compact folded backbone conformations; this suggests that the Gpn residue may be employed successfully in the design of novel folded structures.     

Effect of temperature on the adsorption of water in porous carbons

We report experimental and simulation studies to investigate the effect of temperature on the adsorption isotherms for water in carbons. Adsorption isotherms are measured by a gravimetric technique in carbon-fiber monoliths at 378 and 423 K and studied by molecular simulation in ideal carbon pores in the temperature range 298-600 K. Experimental adsorption isotherms show a gradual water uptake, as the pressure increases, and narrow adsorption-desorption hysteresis loops. In contrast, simulated adsorption isotherms at room temperature are characterized by negligible uptake at low pressures, sudden and complete pore filling once a threshold pressure is reached, and wide adsorption-desorption hysteresis loops. As the temperature increases, the relative pressure at which pore filling occurs increases and the size of the hysteresis loop decreases. Experimental adsorption-desorption hysteresis loops are narrower than those from simulation. Discrepancies between simulation and experimental results are attributed to heterogeneities in chemical composition, pore connectivity, and nonuniform pore-size distribution, which are not accounted for in the simulation model. The hysteresis phase diagram for confined water is obtained by recording the pressure-density conditions that bound the simulated hysteresis loop at each temperature. We find that the hysteresis critical temperature, i.e., the lowest temperature at which no hysteresis is detected, can be hundreds of degrees lower than the vapor-liquid critical temperature for bulk model water. The properties of confined water are discussed with the aid of simulation snapshots and by analyzing the structure of the confined fluid.     

Interpretation of water isotherm hysteresis for an activated charcoal using stochastic pore networks

Water vapor adsorption equilibria on activated carbons typically exhibit hysteresis. The size and shape of the hysteresis loop which separates the adsorption and desorption branches is a strong function of the pore size and interconnectivity of the pores. Neither conventional pore filling models nor statistical thermodynamics approaches provide a means for predicting the extent of hysteresis from only adsorption measurements. This work uses the Kelvin Equation in conjunction with the structural concept of a stochastic pore network to describe measured water isotherms on BPL carbon. Using a pore segment distribution function determined from the adsorption branch, it is shown that totally random assemblies underestimate the extent of hysteresis. It is possible, however, to closely fit the measured BPL-water hysteresis loop using a patchy heterogeneity in which a proportion of the larger pores are preferentially located on the exterior, mid-range pores are concentrated in a sub-surface layer and some large pores form shielded voids behind much smaller pores.Nomenclature p vapor phase partial pressure of sorbate - p _sat saturation vapor pressure of sorbate - R gas constant - r pore radius - T absolute temperature - t adsorbed layer thickness - V _L molar volume of adsorbed phase - surface tension - contact angle     

In search of definitive signatures of the elusive NCCO radical

Previous experimental assignments of the fundamental vibrational frequencies of NCCO have been brought into question by subsequent unsuccessful attempts to observe IR signatures of this radical at these frequencies. Here we compute the fundamental vibrational frequencies by applying second-order vibrational perturbation theory to the complete quartic force field computed at the all-electron (AE) coupled cluster singles, doubles, and perturbative triples level [CCSD(T)] with the correlation-consistent, polarized core-valence quadruple-zeta (cc-pCVQZ) basis set, which has tight functions to correctly describe core correlation. The AE-CCSD(T)/cc-pCVQZ geometric parameters are r(e)(N-C)=1.1623 A, r(e)(C-C)=1.4370 A, r(e)(C-O)=1.1758 A, theta(e)(N-C-C)=168.55 degrees , and theta(e)(C-C-O)=132.22 degrees . Our CCSD(T)/cc-pCVQZ values of the characteristic stretching frequencies nu(1) and nu(2) are 2171 and 1898 cm(-1), respectively, in stark contrast to the experimentally derived values of 2093 and 1774 cm(-1). Finally, focal-point extrapolations using correlation-consistent basis sets cc-pVXZ (X=D,T,Q,5,6) and electron correlation treatments as extensive as full coupled cluster singles, doubles, and triples (CCSDT) with perturbative accounting of quadruple excitations [CCSDT(Q)] determine the vibrationless barrier to linearity of NCCO and the dissociation energy (D(0)) of NCCO-->NC+CO to be 8.4 and 26.5 kcal mol(-1), respectively. Using our precisely determined dissociation energy, we recommend a new 0 K enthalpy of formation for NCCO of 50.9+/-0.3 kcal mol(-1).