7167
J. Pkys. Ckem. 1992, 96, 7167-7169
Reaction of NOn with Cu Atoms in Ar Matrices Dean Worden and David W. Ball* Department of Chemistry, Cleveland State University, Cleveland, Ohio 441 15 (Received: June 12, 1992; In Final Form: July 20, 1992)
As a potential model of corrosion-type interactions, the reactions of Cu atoms with NOz in cryogenic argon matrices were ascertained. A charge-transfer reaction product, Cu+NOZ-,was unambiguously detected, as well as a species having the assigned stoichiometry CU(NO~)~.
Introduction The problems of corrosion affect society directly and indirectly, and the cost of those problems has been estimated' at over $70 billion per year for the United States alone. Defined as "the destruction or deterioration of a material because of reaction with its environment",2 corrosion can be separated into several general mechanisms and/or conditions. While most corrosion requires the presence of a liquid phase-at ambient temperature, wat"osion at high temperatures without a liquid phase takes place via direct interaction between the environmental and metallic comp~nents.~~ Such corrosion occurs in places such as jet engine turbines, high-speed aircraft, pipelines, and refineries. Although much corrosion is ultimately due to atmospheric 02, a growing impact is being made by natural or man-made pollutants in the atmosphere. Species such as SO, ( x = 2,3), NO, ( x = 1, 2), NH3, and H2S are known4 to take part in the corrosion process. Past research has considered the interactions of SO2and SO3,' HzS,6 and NO: with metal and metal oxide surfaces. The emergence of ultrasmall electronics* suggests that such metal systems would be increasingly susceptibleto environmental pollutants. In the ultimate example of a wire composcd of a chain of atoms, corrosive reaction of a single metal atom would open the circuit. While coatings, inhibitors, and passivators might stay a harmful reaction, there are various economic and engineering reawns why such additives cannot be included in certain systems. Study of the interactions between corrosive pollutants and metal or metal oxide systems is therefore necessary in order to better understand the chemistry and prevent economic and operational lOSS.
We inaugurate a series of studies by reporting infrared spectra and chemistry between NOz and vaporized Cu condensed together in cryogenic argon matrices. We plan to continue study of matrix-isolated metal atom/pollutant gas interactions as a method of modeling direct, high-temperaturecorrosion processes occuning in metallic systems.
Experimental Procedure Figure 1 shows the experimental apparatus. The samples were m n d e n s e d in a vacuum chamber onto the cold f q e r of an APD Displex closed-cycle helium refrigerator. An octagonal copper block was attached to the cold finger; each face of the block was polished to maximize reflection of the infrared beam. One of the faces housed a quartz crystal microbalance (Inficon) to monitor disposition rates directly. The seven other surfaces were available to deposit samples. The copper block was covered by an aluminum heat shield on which were mounted eight individual shutters, allowing each surface to be exposed separately. Cu was vaporized from a Ta tube furnace heated resistively to -1100 "C. The furnace was surrounded by a water-cooled Cu heat shield to minimize stray heat. The temperature of deposition was 15-1 8 K. NO2 (W+%,Union Carbide), I5NOz(Isotec, 99.7% I5N),and Ar (AGA, 99.9995%) were used without further purification and were introduced into the vacuum chamber individually via stainless steel tubing. Deposition rates were measured manometrically or directly using the quartz crystal microbalance. Samples were deposited for 30-60 min, then the cold finger was rotated 180°,
freq 1220 1195 1214 1192 1173 a See
text
assgnt
Cu+NO?CU+~~NO~ C~(N0z)z' Cu(N02)2' C~(N0z)z'
fres
assgnt
1191
CU(~~NO,),~
1172
Cu(I5NO;)>
1153
CU(I~NO~)~~
for discussion.
and the Fourier transform infrared spectrum was measured by reflection off the copper block through ZnSe windows with a Nicolet 5DX FTIR spectrometer at 2-cm-' resolution.
Resuits and Discussion Spectra of NOz in Ar indicated that not only NOz monomer but NOz dimer, N2O4, was present in all but the most dilute samples. IR absorptions of N2O4 have been previously assigned? Figure 2 shows infrared absorption spectra of the area of interest in these depositions. When Cu vapors were co-condensed with NO2, a new absorption appeared at 1220 cm-I, which shifted to 1195 cm-l when I5NOzwas reacted with Cu. This absorption first appeared at low concentrations (C3 parts per thousand parts Ar) of Cu and varied linearly with Cu concentration and so is attributed to a reaction of NOz with Cu atoms. At higher concentrations of NO2, several new absorptions appeared at 1214, 1192, and 1173 cm-I, again appearing only when Cu was codeposited in the matrix. Figure 3 shows infrared spectra indicating appearance of those absorptions at higher NOz concentration. Upon substitution of "N, these absorptions were shifted to 1191, 1172, and 1153 cm-l. T h e absorptions are assigned to a Cu atom reacting with two NOz molecules to form a product ostensibly having the stoichiometry C U ( N O ~ )A ~ .set of mixed isotopically labeled NO2 (1:l N02:15N02)was attempted, but at the concentrations necessary to detect the C U ( N O ~product, )~ a broad infrared absorption of lSNzO4obscured this region of interest, eliminating the chance of eliciting any useful information. No other new absorptions that could be attributed to an interaction between NOz and Cu were noted. The new absorptions found upon co-condensation of NOz and Cu are listed in Table I. Identification of the 1:l reaction product is simplified by first the several previous studies'*" of matrix-isolated M + NOz systems, and second the lack of ambiguity in the assignments of the reaction product's infrared absorptions. The product absorption at 1220 cm-' is an N-O stretch, as confirmed by the 2 5 a - I red shift upon 15Nsubstitution. Rwiow work by Milligan et al.12 has shown that the nitrite ion, NO2-, absorbs at 1244 cm-l in an Ar matrix and shows a 26-cm-l "N isotopic shift to 1218 cm-'. This vibration has been assigned to the u3 asymmetric stretching mode of NO2- ( C , symmetry).14 We therefore assign the infrared-absorbing product to NO2-, implying that NOz abstracted an electron from an available Cu atom to make the Cu+NOZ-ion pair. This corresponds to the previous similar assignments made in matrix reactions of NOz with Mg, Cd, and Zn,lo Ca, Ba, and Sr," as well as with several of the alkali
0022-365419212096-7167$03.00/0 0 1992 American Chemical Society
716% The Journal of Physical Chemistry, Vol. 96, No. 18, 1992
Letters
-
To
I-
Vacuum
N
N I
;
;
1915.0
Cti(N0z)~
1295.0
1275.0
1255.0 W A V m 1295.0 R S 1CH- 11215.0 I
1195.0
1171.0
1150.0
Figure 3. Fourier transform infrared spectrum of Cu vapors and higher concentrations of NOz in argon matrices: Cu:NOz:Ar = 6:20:1000.
constructed for the vibrations of a nonlinear symmetric XY2 molecule. For the asymmetric stretch
I '*
H
Figure 1. Diagram of the matrix isolation apparatus: A, octagonal copper cold block (Au plated); B, aluminum heat shield, octagonal; C, aluminum vacuum chamber; D, high-temperature furnace (Ta tube suspended from top flange); E, water-cooled heat shield; F, quartz window, for UV-vis spectra; G, quartz windows, for viewing; H, ZnSe windows, for IR spectra.
A
where v3 is the wavenumber of the vibration, mx and my are the masses of atoms X and Y,and a is one-half of the Y-X-Y bond angle. Aa,) is a simple function of the vibrational potential constants ai/,and the form of the function depends on the type of potential used (Le., central force or valence force; see ref 15). Using the above equation along with a value of 2a = 117.5' for gas-phase N02-,16the ratio V ~ ( N O ~ ) / V ~ (is~calculated ~ N O ~ ) as 1.0215, predicting a shift from 1220 to 1194 cm-*. The actual experimental value of 1195 cm-l confirms the assignment made above. It also suggests but does not prove that the bond angle of the matrix-isolated, ion-paired NO< is not much different from its gas-phase value. The exact nature of the CU(NO,)~product is more ambiguous. The absorption at 1214 cm-l shows a 23-cm-l isotopic shift, consistent with an N-O stretch, probably in a bent species (see above). The absorptions at 1192 and 1173 cm-l show a slightly smaller 20 cm-l isotopic shift. An N-N stretch would show an isotopic shift of -40 cm-l; these two absorptions must almost certainly be assigned to N-O motions. We feel that there are three possible assignments for the C U ( N O ~ )product: ~ Cu+(N204-),Cu+(N02-)(N02),or C U ~ + ( N O ~The - ) ~product Cu+(N204-)presupposes either a simultaneous three-body interaction (highly unlikely) or a stepwise formation of N2O4 and subsequent reduction by a copper atom. One concern regarding the possibility of this structure is the viability of forming NzO,, a species not yet reported but upon consideration of other oxygen-nitrogen molecules expected to have a very low electron affinity. The absorptions at 1192 and 1173 cm-l could be N-O stretches of NzO4-. We now consider the divalent copper species, CU~+(NO;)~The total (gas phase) ionization energy for the reaction Cu Cu2+ 2eis 28.018 eV,17 or 2703.6 kJ/mol, a seemingly prohibitive energy requirement given the lack of solvation energy to compensate. Also, the electron affinity of NO2 has been measured by PES16 as 2.273 eV. The attractive Coulombic potential between positively and negatively charged ions will provide energetic stabilization; using the known ionization energy and electron affity from above, the necessary distance between the ions can be estimated. If the distances from the Cu2+to the two NO2- ions are the same and repulsion between the NO - ions is neglected, a maximum interionic distance of -2.45 is calculated. This distance suggests that the entire C U ~ + ( N O ~species - ) ~ must be located within three or fewer adjacent substitutional sites in the solid Ar lattice. The probability of this occurring increases as the NO2 concentration
-
P 1315 + .o
1295. o
t 275. o
I 255 .a t 235 .a U R V ~ B E A 9I cn- 11
12 1s .o
I
1185.0
Figure 2. Fourier transform infrared spectra of Cu vapors and NOz isolated in argon matrices at -15 K. (A) NOZ:Ar= 7:lOOO; (B) Cu: N02:Ar = 4:7:1000; (C) Cu:ISN02:Ar= 5:5:1000.
metals.12J3 In all cases, the appearance of a new absorption in the range 1205-1244 cm-' has been attributed to the formation of matrix-isolated NOT or M+N02-ion pair. The 25-cm-1 I5N isotopic shift observed for the v3 asymmetric stretch of matrix-isolated NO2-can be compared to the predicted shift based on equations derived15from the secular determinant
2
+
7169
J . Phys. Chem. 1992, 96,7169-7172 in the matrix is increased; the probability of having two substitutional sites about any particular atom in the matrix having dopant species becomes 0.1 at -5 mol % of dopant from purely statistical considerations.’* Given the propensity of NO2 toward self-association,this concentration estimate may be high. The required proximity of all three ions virtually ensures that there will be some interaction between the two NO2- ions which would in turn perturb their infrared absorptions substantially. TOO,the simple estimate of the interionic distance is based on an additive Coulombic potential point charges, and neglects repulsion, shielding, structural effects, polarizabilities, etc. The s ies Cu+(N02-)(N02)requires an interionic distance of 2.64r a n d since only two ions are involved we feel that it is much more probable than the first two possibilities. At high matrix concentrations of NO2, there is a relatively high possibility of a simple perturbation of the &+NO; moiety by a neighboring NO,. The infrared absorptions at 1214 cm-l can be assigned to the asymmetric N-O uj stretch of the perturbed NO;; the absorptions at 1192 and 1173 cm-I would be the same absorption but shifted due to structural isomerism of the NO2 units or due to matrix effects.
Acknowledgment. Funding provided through the Research Challenge Program and a Research Initiation Grant from Cleveland State University is acknowledged.
References and Notes (1) Payer, J. et al. Mater. Performance 1980, May.
(2) Uhlig, H. H.; Revie, R. W. Corrosion and Corrosion Control, 3rd ed.;
Wiley-Interscience: New York, 1985. (3) Fontana, M. G. Corrosion Engineering, 3rd ed.; McCraw-Hill: New York, 1986. (4) Chawla, S.K. Ph.D. Thesis, Case Western Reserve University, 1990 and references therein. (5) Baxter, J. P.; Grunze, M.; Kong, C. W. J . Vac. Sci. Techno/. A 1988, 6, 1123. &Berry, D. W.; Aludek, K. J. Can. J . Chem. Eng. 1971,49,781. Lipfert, F. W. JAPCA 1989, 39, 446. Sahoo, P. K.; Sircar, S.C.; b e , S. K. TransJnst. Min. Merall. Sect. C 1984, 94, C1-3. Kent, S.A.; Katzer, J. R.; Manogue, W. H. Ind. Eng. Chem. Fundam. 1977, 16, 443. (6) Sharma, S.P. J . Electrochem. Soc. 1980, 127, 21. Sansregret, J. L. J . Elecrrochem. Soc. 1980,127, 2083. (7) Judeikis, H. S.; Siegel, S.;Stewart, T. B.; Hedgpath, H. R.; Wren, A. G. In Nitrogenous Air Pollutants; Grosjean, D., Ed.;Ann Arbor Science: Ann Arbor, MI, 1979; pp 83-1 10. Gimzewski, E.; Hawkins, S.H. Thermochim. Acta 1986, 99, 379. (8) For example: Science 1992, Mar 20,255,1513-5. Marrian, C. R. K.; Dobisz, E. A.; Glembocki, 0. J. Res. Den 1992, Feb, 123. (9) St. Louis, R. V.; Crawford, Jr., B. J . Chem. Phys. 1965, 42, 857. (10) McDonald, S. A.; Andrews, L. J . Mol. Specrrosc. 1980, 82, 455. (11) Tevault, D. E.; Andrews, L. Chem. Phys. Lerf. 1977,48. 103. See also: Tevault, D. E.; Andrews, L. Specfrochim. Acta 1974, 3OA, 969. (12) Milligan, D. E.; Jacox, M.; Guillory, W. A. J. Chem. Phys. 1970,52, 3864. (13) Milligan, D. E.; Jacox, M. J. Chem. Phys. 1971, 55, 3404. (14) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley-Interscience: New York, 1986. (1 5) Herzberg, G. Infrared and Raman Spectra; Van Nostrand Reinhold Company: New York, 1945, .. pp 160, 169. Dennison, D. M. Philos. Mag. 1926; 1,-195. (16) Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Phys. Chem. 1988, 92, 5405. (17) Lide, D. R., Ed.CRC Handbook of Chemistry and Physics, 71st ed.; CRC Press: Boca Raton, FL, 1990. (18) Rytter, E.; Gruen, D. M. Spectrochim. Acra 1979,35A, 199.
Pulse Radiolysis Studies of the Dynamics of Acid-Base Equilibration in Phenolic Systems’ R.H.Schuler Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: June 15. 1992;In Final Form: July 20, 1992)
Pulse radiolysis of nitrous oxide saturated solutions has been used to generate a transient population of OH- sufficiently rapidly to allow the direct observation of the deprotonation of a-naphthol in neutral and moderately acidic solutions. It is shown that below pH 6 the a-naphtholate anion protonates by reaction with H+ with a rate constant of 5.9 X 1Olo M-I s-I. This value, together with the pKa for a-naphthol of 9.3,establishes the rate constant for its spontaneous dissociation as 30 s-l. The approach used here should be generally applicable to the examination of the equilibration dynamics of weak acids and to the determination of their rates for spontaneous dissociation.
Pulse radiolysis of nitrous oxide saturated solutions provides a convenient way to inject a nonequilibrium population of hydroxide ion into neutral and moderately acidic aqueous solutions on the nanosecond time scale. At the micromolar concentrations typical of many pulse radiolysis experiments, these OH- ions in neutral solutions have lifetimes in excess of microseconds. We show here that it is readily possible to use timeresolved absorption spectrophotometry in conjunction with nanosecond pulse radiolysis to examine the dynamics of the deprotonation and reprotonation processes involved in establishing the acid-base equilibria of phenols in aqueous solutions down to pHs as low as 4. The approach used is possible because the rate constants for deprotonation of phenols by OH- are of the magnitude of 1OIo M-I s-’ so that at millimolar substrate concentrations reaction is essentially complete on the 100-ns time scale. Given appropriate kinetic and spectroscopic characteristics, it should be possible to use pulse radiolysis to study the kinetics of acid-base reactions of other weak acids. Since the classic work of Eigen in the 1960~,~ a variety of relaxation methods have, of course, been used to examine acid-
base reactions in aqueous solutions. Phenols have, however, been studied only in a few special cases such as the nitrophenols which have relatively low pKas.’ In the case of the phenols direct timeresolved absorption methods are particularly attractive since the principal absorption band in the 300-nm region shifts to the red on ionization. Previously Kabakcki, Zansokhova, and Pika& have used microsecond pulse radiolysis methods to demonstrate that the OH- produced directly in the radiolysis of oxygen-saturated neutral water rapidly deprotonates thymol blue. The shorter time scale of the present experiments allows one to examine the kinetics of the reactions in more acidic solutions and in considerably greater detail. Nitrous oxide has been used by radiation chemists for many years to convert e-aqto OH radicals, via the reactions N 2 0 + e-,,
0’+ H20
k l = 9.1
X
lo9 M-I s-15
*Nz+O.k2 = 1.8 X IO6 M-l s-I6
- I
~
+ OH-
(2)
In contrast to most radiation chemical studies which focus on the
0022-365419212096-7 169S03.00/0 , 0 1992 American Chemical Society I
‘OH
(1)
