J . Phys. Chem. 1990, 94, 5041-5048
5041
Reactivity of Molecular Clusters in the Gas Phase. Proton-Transfer Reaction in Neutral Phenol-(NH,), and Phenol-(C,H,NH,), C. Jouvet, C. Lardeux-Dedonder, M. Richard-Viard,+D. Solgadi,* and A. Tramer Laboratoire de Photophysique MolPculaire du CNRS, Bat. 21 3, UniuersitP Paris-Sud, 91 405 Orsay, France (Received: October 20, 1989: In Final Form: January 2, 1990)
Photoionization of phenol(B), (B = NH,, C2H5NH2)clusters formed in a supersonic expansion has been studied by the mass-selected two-color two-photon resonance-enhanced multiphoton ionization technique. This study clearly shows that a reaction of proton transfer takes place in the excited state (SI)of the neutral clusters, depending on the nature of the base molecule (B) and the size of the cluster. The clusters with n 5 3 for ammonia and n 5 2 for monoethylamine show an ionization cross section steeply increasing in the 7.5-7.8-eV range. This corresponds to the vertical ionization potential of the clusters lowered by about 1 eV with respect to free phenol, due to hydrogen bonding. For larger clusters, the onset lies at about 6.8-7.0 eV and corresponds to the ionization of the proton-transferred species. These results are corroborated by fluorescence measurement: an emission similar to that of free phenol is observed for the small clusters while an emission similar to that of phenolate anion is observed for the larger ones. The cluster-size dependence of the proton-transfer reaction is mainly governed by the gas-phase basicity of B, cluster (its proton affinity), Le., by its proton solvation ability.
Introduction Intermolecular charge- and proton-transfer processes, Le., the motion of proton associated with the change of the electronic structure in hydrogen-bonded complexes, are of great interest from the viewpoint of the chemical reactions in solution. A case of special interest is that of the proton transfer induced by electronic excitation of molecules with drastically different acidities in their ground and excited states. The absorption spectrum of the hydrogen-bonded complex AH-B is only slightly modified comparatively to the free AH molecule, but its fluorescence spectrum is close to that of the A- anion, which may be explained by the process AH...B
hv
A H *...B
-+
A*- ...HB+
-hv’
A- ...HB+
-+
AH...B
as has been shown by Weller et al.’ and Mataga et al.* for a number of aromatic acids and bases. A similar approach has been used in their pioneering works by Leutwyler et aL3” to study heterogeneous free clusters AH-B, produced in a supersonic expansion. In these conditions, it is possible to get rid of environment effects and to study selectively the systems with a well-defined number of “solvent” molecules B. They showed that, for a-naphthol microsolvated clusters, the occurrence of the proton transfer (evidenced by the fluorescence spectrum similar to that of naphtholate anion in solutions) AH* ...B, A* -...HB,+
complexes with monoethyl- (MEA), diethyl- (DEA), and triethylamines (TEA). The case of MEA has been studied in detail and completed by fluorescence spectra. The energetics of the reaction is also discussed.
Experimental Section The clusters are formed in a pulsed supersonic expansion of helium seeded with phenol (Aldrich), using either previously prepared N H 3 or MEA (1-5%)/He mixtures or helium saturated with DEA or TEA maintained at temperatures between -30 and -60 “ C . Cluster size control is obtained either by changing the delay between the valve opening and the laser shot or by using different proportions of MEA in helium. Our experimental setup consists of two independently pumped vacuum chambers separated by a skimmer (3 mm diameter). The mixture of products in helium is expanded in the first chamber through a 200-km nozzle (General Valve). This chamber is equipped with a light-collecting system focusing the fluorescence light through appropriate glass filters on the window of a R T C XP2020 photomultiplier. In the second chamber a homemade time of flight mass spectrometer (20 cm) is used for the detection. The same laser beam crosses twice the molecular beam (1 cm from the nozzle for the fluorescence detection and about 15 cm for the ion detection).
--P
depends mainly on the basicity and the number of solvent molecules. In this paper we report a study of heterogeneous clusters of phenol-PhOH (a very weak acid in its ground state, pK, = 9.94,’ and a relatively strong acid in its SI state, 4 I pK,* I 5.78) with ammonia and amines by two-color two-photon ionization, using as the fundamental criterion the changes of ionization thresholds of the clusters. Phenol complexed with proton acceptors has been widely studied in supersonic j e t ~ . ~ - I ’If proton-transfer reaction in the ionized phenol-trimethylamine complex has been clearly evidenced by Mikami and c o - w ~ r k e r s ,this ’ ~ reaction has never been observed in neutral complexes of phenol until recently; in our preliminary communication,I8 we showed that the dependence of ionization thresholds on the number of solvent molecules in the PhOH(NH,), clusters strongly suggests that the proton transfer in the S, state PhOH*(NH3),
-
PhO-***H(NH3),,+
takes place only for n I 4. We extended this study to phenol ‘Permanent address: L.C.A.M., Bat 35 I , Universite Paris-Sud, 91405 Orsay Cedex, France.
(1) Beens, H.; Grellmann, K. H.; Gurr, M.; Weller, A. H.; Discuss. Furaday SOC.1965, 39, 183. (2) (a) Mataga, N.; Kaifu, Y . Mol. Phys. 1964, 7, 137. (b) Mataga, N.; Kawasaki, Y . ;Torihashi, Y . Theor. Chim. Acta 1964, 2, 168. (3) Cheshnovsky, 0.;Leutwyler, S. Chem. Phys. Lett. 1985, 121, 1. (4) Cheshnovsky, 0.;Leutwyler, S. J . Chem. Phys. 1988, 88, 4127. ( 5 ) Knochenmuss, R.; Cheshnovsky, 0.; Leutwyler, S. Chem. Phys. Lett. 1988. 144, 317. (6) Knochenmuss, R.; Leutwyler, S. J . Chem. Phys. 1989, 91, 1268.
(7) Grabner, G.; Kiihler, B.; Zechner, J.; Getoff, N. Photochem. Photobiol. 1977, 26, 449.
(8) (a) Wehry, E. L.; Rogers, L. B. J . Am. Chem. SOC.1965, 87, 4234. (b) Bartok, W.; Lucchesi, P. J.; Snider, N. S.J . A m . Chem. SOC.1962, 84, 1842. 19) Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1982, 86, 1768. (IO) Abe, H.; Mikami, N.; Ito, M.; Ugadawa, Y . J . Phys. Chem. 1982, 86. 2567. ( I I ) Abe, H.; Mikami, N.; Ito, M.; Ugadawa, Y . Chem. Phys. Lett. 1982, 9.3, 217. (12) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1983.87, 5083. (13) Fuke, K.; Kaya, K. Chem. Phys. Leu. 1983, 94, 97. (14) Gonohe, N.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1985,89, 3642. (15) Mikami, N.; Suzuki, 1.; Okabe, A. J . Phys. Chem. 1987, 91, 5242. (16) Mikami, N.; Okabe, A,; Suzuki, I. J . Phys. Chem. 1988, 92, 1858. ( 1 7 ) Lipert, R. J.; Colson, S.D. J . Chem. Phys. 1988, 89, 4579. (18) Solgadi, D.;Jouvet, C.; Tramer, A. J . Phys. Chem. 1988,92, 3313.
0022-3654 I 9 0 12094-5041$02.50/0 t 2 1990 American Chemical Societv
The Journal of Physical Chemistry. Vol, 94, ,Yo. I.?* 9 90
5042
,
Jouvet et al.
PhOHMEA
I
d
'If'"'
-
.r
+_________..___ +--__
35030
-
36cec
Figure 1. REMPI spectrum of the Sl
phenol-monoethylamine complex The @
~
~
'
So transition of jet-cooled band occurs at 35 593 i= 2
cm-I
Clusters are detected by two-color resonance-enhanced multiphoton ionization (REMPI). Two homemade dye lasers pumped by the second and the third harmonics of a YAG laser (B.M. Industry) are used as light sources. The different frequencies are obtained with the usual dyes, frequency doubling (BBO cristal) or mixing with the 1.06 km of the YAG laser. Both light beams are slightly focused anticollinearly in the ionization region of the T O F mass spectrometer. No signal due to direct ionization of ammonia or one of the amines alone has been detected. SI So spectra for each mass-selected cluster have been recorded either by one-color REMPI or by two-color technique with a variable u1 frequency of the pump laser and a constant ionization u2 frequency. The measurements of the ionization cross sections are carried out in the following way. The pump frequency ( v i ) is fixed at the 0-0 band for complexes with a well-resolved absorption or close to the first absorption maximum for larger clusters with broad, overlapping absorption spectra. The intensity of the pump laser is constant and low (about 5 ~ J / p u l s e )in order to minimize one-color ionization. The ion current for each cluster mass is then measured with the ionization laser (200-500 pJ/pulse) on and off, the relative ionization cross sections being deduced from the enhancement of the ion signal. This operation is repeated at different ionization energies ( u 2 variable). A large range of frequencies is needed to cover the whole domain separating the ionization potentials of the clusters and this can lead to some fluctuations of the signal, which depend strongly on the spatial and temporal overlap of the two laser beams. So, for each wavelength, it was made certain to use similar laser intensities and the overlap of both laser beams was optimized until a maximum enhancement of the ion signal was obtained.
-
Results The results are presented in the following manner: first we report on the REMPI spectra of the clusters, then on the fluorescence of phenol( MEA),, clusters, and finally on the ionization thresholds and dissociation channels of the clusters. ( A ) SI So Spectra of Phenol(R,N),, Clusters. ( a ) Phenol(NH,),,. The REMPI spectra of clusters involving phenol and one, two, or three ammonia molecules have been presented in our previous paper.Ie The main features can be summarized in the following way: Phenol(NH,) presents a dominant sharp @ band located at 35 710 cm-l red-shifted by 6u = -642 cm-I with respect to the 0; band of free phenol (36 352 cm-.' Ih). The REMPI spectrum of phenol(NH,), shows a progression of four sharp peaks (!8 cm-') starting at 35 543 cm-I (&J = -809 cm-'). A broad structureless absorption has been observed for larger clusters (n L 3) with an onset at =35 350 cm-" and a maximum a t ~ 3 5 7 0 0cm-' for n = 3. +-
-
34500
h
J
-- - t,TI
1,
b
35500
cm
Figure 2. (a) REMPI spectrum of the S i SOtransition of jet-cooled phenol(MEA), complex. (b) First bands of phenol-MEA complex in the same energy region.
-
( h ) Phenol(MEA),. Figure 1 shows the one-color REMPI spectrum of phenol(MEA) complex in the region of the Si So transition of free phenol. The first group of intense bands begining at 35 593 cm-I is assigned to the @ transition of the 1-1 complex. The red shift (fiu = -759 cm-l) relative to the 0; band of bare phenol shows that the phenol acts as a proton donor and the amine as a proton acceptor in the hydrogen bonding.' The vibrational structure of this band, corresponding to intermolecular vibrations, is rather complicated. However six narrow peaks separated by 12-16 cm-l are clearly observable. The intensity distribution suggests a slightly different geometry of the complex in its ground and excited states. A second group of absorption bands appears at about 140 cm-' higher in energy than the @ band of the complex. This shift as compared to 182 cm-l obseerved for phenol ammoniaI6 allows us to assign this band to the excitation of a N.-HO stretching mode. The bands begining at 471 and 766 cm-' from the @ of the complex (respectively, -288 and $7 cm-I from bare phenol) observed on the phenol(MEA) spectrum can be assigned respectively to the 6 3 and intramolecular vibrations of phenol with practically unchanged f r e q u e n ~ i e s . 'These ~ ~ ~ ~bands show a similar intermolecular vibrational structure as the @ band. Phenol(MEA), clusters with n L 2 present a broad structureless excitation spectrum (see Figure 2). The onsets of n = 2 and 3 have been found at 34 700 f 100 cm-' and 33 240 f 100 cm-I, respectively, with a very large red shift ( 6 u = -1650 and -31 10 cm-!) as compared to n = 1 . (c) Phenol(DEA) and Phenol( T E A ) . For comparison, it was interesting to look at the excitation spectra of secondary and So spectra of the complex 1-1 of tertiary amines. The SI phenol(DEA) and phenol(TEA) are represented in Figures 3 and 3. The phenol(DEA) spectrum presents relatively sharp bands. The firsr absorption band at 35 5 18 cm-' corresponding to the @ band of the complex presents three peaks ( ( 6 u = -834, -717, and -62.' cm-I). These peaks separated by 117 and 209 cm-' are attributed to excitation of the hydrogen-bond mode. The other bands correspond as for the preceding complexes to the intramolecular modes of phenol. All these bands present very low frequency vibrations as shown in the insert of Figure 2. The intensity distribution suggests a similar geometry of the complex i n its ground and excited states. The phenol(TEA) spectrum is a superposition of two components: the first one with the @ transition at 35 460 cm-' (6v = -892 cm-') composed of a group of bands involving low-frequency
2
-
W .J. J. Mol. Spectrosc. 1985, 109, 60. (20) Bist, H. D.; Brand, J. C. D.; Williams, D. R. J . Mol. Spectrosc. 1966, 21. 76: 1967, 24. 413 (19) Balfour,
Reactivity of Molecular Clusters
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5043
i i
I I
121
400 nm
300 a
-
Figure 3. REMPI spectrum of the SI So transition of jet-cooled phenol-diethylamine complex. The @ b a n d occurs a t 35 518 f 2 cm-l. Inset: vibrational structure of the first band.
r
I
355%
I
PhOH-TEA
1-701 1
359m
%?Q3
I
1
Figure 5. Fluorescence spectra of phenol(MEA), clusters. TheSpectra of the clusters (full line) have been obtained by measurement of the fluorescence cut by a series of filters: (a) Fluorescence spectra of phenol(MEA), with n 5 2 (excitation wavelength 280.9 nm = 35 593 cm-I). For comparison the fluorescence spectra in solution of phenol in ethanol (excitation wavelength 280 nm = 35 714 cm-I) is represented by dashed line. (b) Fluorescence spectra for larger phenol(MEA), clusters (excitation wavelength 281.2 nm = 35 562 cm-I). The fluorescence spectra in solution of phenolate anion in N a O H lo4 mol/L in ethanol (excitation wavelength 300 nm = 33333 cm-I) is represented by dashed line. Maxima of these curves have been normalized to unity. The mass spectra of the clusters corresponding to these excitation conditions are given below.
cm-'
Figure 4. REMPI spectrum of the SI So transition of jet-cooled phenol-triethylamine complex. The spectrum shows two different systems ( I and I I ) attributed to two different isomeric forms. The @ bands occur at 35 460 f 2 cm-' (I) and 35 651 f 2 cm-I (11). +-
vibrations as in the phenol(MEA) spectrum, and the second one with the origin at 35651 cm-I (6v = -701 cm-I) contains single narrow bands. Both systems present nearly the same 6ah and @ intramolecular frequencies with a pronounced J = 144 cm-I band in the system I and may be tentatively considered as corresponding to two isomeric forms of the complex. Other experiments (dispersed fluorescence, ionization potential measurements, etc.) are needed to confirm this hypothesis. The DEA and TEA complexes have not been studied in more detail. However, we observed phenol(DEA), complexes with n L 2 and we did not detect clusters of phenol with more than one molecule of TEA: in this case the absence of hydrogen bonds (NH-N) between the TEA molecules seems unfavorable to the formation of the clusters. ( B ) Fluorescence of the Phenol(MEA), Clusters. The total fluorescence of phenol( MEA),, clusters has been measured in two cases (cf. Figure 5): first for n = 1 and 2 (laser excitation = 280.9 nm) and second for 3 In I6 (laser excitation = 281.2 nm). The cutoff of fluorescence lies around 330 nm for small and around 420 nm for the larger clusters. From these data, one can conclude that the small clusters ( n = 1 and 2) are emitting between 280 and 330 nm. For n 2 3 the emission mainly lies above 320 nm and presents a maximum at ~ 3 5 nm. 0 The fluorescence spectra of phenol and phenolate anion (produced in the presence of NaOH 10-4 mol/L in ethanol) in solution are also presented in Figure 5 (in dashed lines). For the small clusters the emission matches the emission of phenol and for the larger clusters the emission is more typical of the emission of the phenolate anion. A similar behavior has been observed by
PhOH (MEA), Figure 6. Ionization threshold measurements: (a) mass spectrum of phenol(MEA), clusters upon one-color, 280.9-nm excitation with a low laser intensity (==5 pJ/pulse) in order to minimize the one-color twophoton-ionization process; (b) the same spectrum when the second laser 0 is switched on. (excitation wavelength: 462.7 nm; ~ 3 0 fiJ/pulse)
Leutwyler et al. for the naphthol cluster^.^-^ The displacement of the emission spectrum of the phenolate anion toward the blue in solution as compared to the gas-phase emission can be easily understood because PhO- is more basic in the ground state than in the excited state, and it is well-known that polar, hydroxylic solvents induce in this case a blue shift of the fluorescence.*' (0Ionization Thresholds. The procedure of measurement of ionization thresholds has been described in the Experimental Section. Figure 6 presents a typical result obtained in the case of phenol(MEA), clusters. The frequency of v I has been fixed (21) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectru, Dekker: New York, 1970.
Jouvet et al.
The Journal of Physical Chemistry, Vol. 94, No. 12, I990
5044
PhCH (NH,),
PhOH(MEA),
I
;L! 1
sz
0 1 2 3 4 5 1U 23456 (NH3)mH+ PhOH(NH3)+,
M
7 8 I (eV) Figure 7. Ionization cross sections versus h(u, + u 2 ) of phenol(NH,), clusters ( n = 1-4)18 of phenol(MEA), clusters ( n = 1-6) and phenol(MEA),.H,O clusters (4 5 n 5 6). The arrow a t 8.5 eV represents the ionization potential of free phenol. TABLE I: Ionization Thresbolds ( I , ) of Clusters of Phenol with Ammonia, Monoethylamine, and Monoethylamine-(H,O), (in eV) n
0 1
2 3 4 5 6
phenol(NH,), 8.50” 7.85 f 0.Ib 7.69 f 0.1 ?.SI f 0.1 6.89 f 0.1 6.89 f 0.1
phenol(MEA),
phenol(MEA),(H@)I
8.50°
7.60 f 0.1 7.5 f 0.1
6.93 f 0.1 6.68 f 0.1 6.5-6.6 f 0.1 6.5-6.6 f 0.1
6.8 f 0.1 6.7 f 0.1 6.7 f 0.1
Reference 15. bThis ionization threshold has also been measured by Mikamia and co-workers.’5 at 35 593 cm-I (0-0 band of phenol-MEA) and v2 = 21 612 cm-I (v, u2 = 57205 cm-I, i.e., 7.09 eV). At this energy the ion current corresponding to phen~l(MEA),,,,,~ is not affected by the second laser, while a small enhancement of the signal for n = 3 and a large one for n 2 4 are found. The dependence of the ionization cross sections (0,)on the total energy I = hul + hu2 (in eV) deduced from similar measurements in an extended hu2 range for phenol(MEA), is represented in Figure 7 . In this figure, only the points around the ionization thresholds of each cluster are shown. The ionization cross section and the ionization thresholds for mixed phen0l(MEA),,~.H~0 clusters measured at the same time are also represented (smaller mixed clusters have not been detected). These ionization thresholds and those previously reported for phenol(NH3), are summarized in Table I . These values correspond to vertical ionization potentials and do not take into account the vibrational excitation in the S I level of clusters showing a structureless absorption. So these values constitute an upper limit of the ionization potential. From these results the following observations can be made: (i j All clusters present an ionization threshold lower than the ionization potential of free phen01.I~ (ii) The clusters may be roughly divided into two groups: (a) the smallest ones ( n = 1-3 for N H 3 and n = 1, 2 for MEA) with ionization thresholds ( I o ) lowered by ca. 0.6-1 eV with respect to the bare phenol show a slow increase of the ionization cross section with I . (b) The larger ones ( n L 4 for ammonia and n I3 for MEA) with a further decrease of Io by 0.8-1 eV and show a rapid increase of the cross section with energy. The PhOH-
+
Figure 8. Dissociation of the ionic PhOH(NH,),+ clusters: (a) fragmentation of clusters with n = 1 and 2 (one-color two-photon ionization: excitation wavelength, 280 nm);(b) fragmentation of clusters with n = 1-6 (one-colortwo-photon ionization; excitation wavelength, 280.8 nm). The mass of PhOH’ is marked by an asterisk.
(MEA), presents an intermediate character: a slow increase of crrlike small clusters. (iii) The addition of one water molecule to PhOH(MEA), clusters leads to an increase of the ionization threshold of about 0.1 eV. (D) Dissociation of Ionized Clusters. On some mass spectra we observed peaks which are clearly coming from dissociation of ionic clusters. Such a process may be due to two different mechanisms: (i) an excess of internal energy in the ion due to the one-color twophoton ionization process: 2h(v1)- I , (where I, is the adiabatic ionization potential which may be lower than the vertical ionization threshold Io);(ii) absorption of a third photon (hvl or hu2)exciting the ion to dissociative or predissociated states. The respective roles of both mechanisms may be estimated by a study of the dissociation yield either as a function of u I + u2 or as a function of the laser power. Dissociation of PhOH(NH,),+ corresponds probably to the first mechanism. No dissociation is observed for the PhOH(NH3)+ ion in two-color experiments when h(u, + u2) - Io does not exceed = l eV, while upon one-color ionization with 2hul - /, = 1 eV, the ion dissociates into PhOH+ NH,, the PhO’+ NH4’ channel being closed16 (Figure 8a). On the other hand, in larger ionic clusters the latter channel is open in similar ionization conditions (Figure 8b), and we observe the protonated ammonia clusters (H(NH,),)+ with m = 1-5. The fragmentation pattern are probably complex involving the
+
-
PhOH(NH3),,+
-
PhOH(NH3),+ PhOH(NH3),,,+
PhO’
+ H(NH3),,,+ + (n - m)NH,
+ ( n - m)NH3
(where 0 5 m 5 n j
channels. The excess energy is clearly sufficient to allow an “evaporation” of a number of ammonia molecules: we never observed the H(NH3),+ clusters with m > 6 even when the mass spectrum shows strong lines of large ( n up to 20) PhOH(NH3),,+clusters. From purely thermodynamical considerations, we cannot completely exclude a possible intracluster ion-molecule reaction of the charged ammonia cluster as stated in the case of phenylacetylene-ammonia cluster by Breen et aLZ2also leading to the (22) Breen, J. J.; Tzeng, W. B.; Kilgore, K.; Keesee, R.G.: Castleman, A . W . In J . Chem. Phys. 1989. 90, 19.
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5045
Reactivity of Molecular Clusters
-
TABLE 11: 0; Transition of Phenol-B Complexes (B is the Proton Acceptor), Spectral Shifts Relative to the Origin of Free Phenol (6v) and Absolute Proton Affinities of B 0-0
proton affinity (PA), eV (kcal/mol) ref
B
H20
-7.21 (-166.5) -8.88 (-205) -9.75 (-225) -9.40 (-217) -9.70 (-224) -1 0.00 (-23 1) -9.79 (-226) -10.02 (-231.2)
NH3 C2H5NH2
(CHd3N (C2HS)lNH
I
24 24 25 26 25 25 26 25
transition, shift 6w, cm-' cm-' 35 996" 35710 35 593
-356 -642 -759
35 532b 35 518
-820 -834
34 46OC 3565Ic
-892 -701
IONIC STATE
(C,HS)3N
EXCITED STATE
"Reference 12. bReference 15. 'Isomers 1 and I1 (see text).
1 AHB, AHB,'
Figure 9. Double-minimum potential energy curves for the proton motion in the ground, excited, and ionic states. formation of protonated ( n - 1) ammonia cluster ions. A similar picture is found for PhOH(MEA), clusters, the PhOH(MEA)+ dissociating at low laser powers into PhOH+ + MEA while for larger clusters the H(MEA),+ fragments are abundant. On the other hand, when PhOH(NH,) and PhOH(MEA) are ionized with an increased laser power, NH4+ and MEAH' fragments are observed as well as the PhOH' one, which suggests a dissociation induced by absorption of a third photon.
Discussion A. General Remarks. The processes under study may be schematically represented as in Scheme I. It is well-known that the ionic character of the hydrogen bond between A H acid and B, basic cluster depends on the acidity of AH modified by electronic excitation (AH*) or ionization (AH') and the basicity of B, which depends on the nature of B and the cluster size. The resulting structural changes are currently discussed in terms of double-minimum potential surfaces with minima corresponding to the neutral (AH-B,) and ion pair (A-.-HB,+) structures (Figure 9). Phenol is a very weak acid in its ground state and we can assume that in all AH-B, clusters the ground state corresponds to the neutral form (I). The red shift in the absorption (excitation) spectra of the complex with respect to the absorption of the bare phenol molecule is thus the measure of the increase of the bonding energy in the excited neutral form AH*-B, of the cluster. In absence of proton transfer, we expect a quasi-resonant fluorescence of AH*-B, (IV) with a red shift similar to that found in absorption and formation of the AH+-B, ion after absorption of a second photon (VI). On the other hand, when a proton transfer takes place (111), the emission will be that of a hydrogen-bonded phenolate anion (A*--HB,+, process V) with a large Stokes shift with respect to the AH-B, absorption and the second photon ionization will prepare the A'-.HB,+ ion (VII). Since the ionization potential (or rather the photodetachment threshold) of the free gas-phase phenolate anion is drastically lowered as compared to phenol,*, one can expect the A*---HB,+ A-HB,' + e-
-
(23) Richardson, J. H.; Stephenson, L. M.; Brauman, J. I. J . A m . Chem. SOC.1975, 97, 2967. (24) (a) Aue, D. H.; Bowers, M . T. In Gas Phase Ion Chemistry; Bowers, M . T., Ed.; Academic Press: New York, 1979; Vol. 2. (b) Walder, R.; Franklin, J. L. Int. J . Mass Spectrom. Ion Phys. 1980. 36, 8 5 . (25) Lias, S. G.;Liebman, J. F.; Levin, R. D. J . Phys. Cbem. Ref. Data 1984. 13, 695
/
I
I
-
Figure 10. Relation between gas-phase proton affinities (PA in eV) of base molecules B or B, clusters and spectral shifts of the S , So state of phenol(B) complexes or phenol(B), clusters (in cm-I). (MEA = monoethylamine, DEA = diethylamine, TEA = triethylamine, TMA = trimethylamine).
-
ionization to occur at lower energies than the AH*-.B, AH*-.B, + e- process, as actually observed. B. S, So Transition in Fluorescence Excitation and REMPI Spectra. In Table I1 we list the literature data on the gas-phase proton affinities of different proton acceptors (PA), the energy of the 0; bands in the S,-So spectra of correspondin 1 1 complexes, and their spectral shifts with respect to the 0, transition of free phenol. The correlation between these spectral shifts and the proton affinities of the B molecules and B, clusters is given in Figure 10. A linear dependence is observed for proton affinities lower than 10.3 eV (237 kcal/mol). Above this limit, the spectral shift is much larger than expected, as was previously found by Leutwyler et aL3" for a-naphthol complexes. Such a large spectral shift cannot be related to the occurrence of the proton transfer in the excited state (which does not occur in PhOH(MEA)* cluster, see below) but to a strong hydrogen bonding in the PhOH*(MEA), systems. It has to be also noticed that the intensity distribution within the progressions involving low-frequency (intermolecular) modes varies in the same time. For the PhOH(NH,) complex the @ band is predominant, while in the spectra of PhOH(NH,)* and PhOH(MEA) complexes, the strongest bands correspond to higher members of this progression (cf. Figure 1 and Figure 1 of ref 18). The shape of unresolved bands of larger clusters suggests vertical transitions to high members of this intermolecular progression with a missing @ band. Such a behavior corresponds to a difference between So and S , equilibrium configurations for the AH-B, neutral form increasing with the basicity of B and with the cluster size. From this point of view, it is interesting to compare the intensity distributions in the two systems of bands observed in the spectrum of the PhOH(TEA) complex and assigned to two isomeric forms 1 and 11. The spectrum of form I (well aligned in the 6 w vs PA
-
0 -
(26) Bisling, P. G . F.; Riihl, E.; Brutschy, B.; Baumgartel, H. J . Phys. Cbem. 1987, 91, 4310.
5046
The Journal of Physical Chemistry, Vol. 94. No. 12. I990
Jouvet et al.
TABLE 111: Energetics (in eV) of the Pbenol(B), Clusters (B = NH, or MEA) in Their Ground and Excited States" (PA), So ground state S, excited state n (4. 26) aH,,,, f PA(n) ?JH,,,~*4- PAln) NH3 1 2 ,I
-9 8 7 -10 3 2
6.09 5.14 4.65
4
-10.66
4.31
3.38 (3.99)
5
-10.94
4.03
3.08 (3.71)
-9.75 -10.22
5.22 4 75
4.27 (4.90) 3.80 (4.43)
-8.88
PhOH
5.15 (5.77) 4.19 (4.82) 3.70 (4.33)
--6H,"* -* 4.05
-6H,"* - * 3.71
(4.68) (4.34)
ME.4 1 2
-6H,"*
-+
-
4.15
(4.78) 3
-!0.35
4.62
3.67 (4.30)
4H,"
4.02
( 4 65)
(PA), is the proton affinity of B, clusters. The gas-phase acidifities of phenol are 6HaCid= 14.97 eV for the ground stae and 6Haad* = 14.02 eV for the excited stated. Values in parentheses correspond to 6Hacid* = 14.65 eV (see text). The complexation energy between phenol and E, for n Z 2 is approximated to 6HcN* = -0.35 eV (see text).
plot) shows a well-developed intermolecular-mode progression, while that of isomer 11, with an anomalously small red shift, is composed of relatively narrow bands. One can presume that the form 1 is similar to other complexes, while isomer I1 forms a weaker bond, in the ground as well as in the excited state, maybe because of steric hindrance effects. C. Ionization Thresholds and Fluorescence Spectra. The results of the two-color REMPI experiments may be treated in terms of double-minimum energy surfaces in the following way: I n small clusters, where the proton affinity of B, is low, the ionization takes place by absorption of the second photon by the neutral, excited PhOH*-.B, system. The ionization threshold of phenol is lowcred by the iarge bonding energy of the PhOH+-B,, ion. The spectra of these clusters show a slow decrease of the ionization cross section with decreasing energy of the second photon (see Figure 7) which may be assigned to nonvertical transitions between excited and ionized states. As a matter of fact, if there is no (or a very low) barrier between the more stable PhO'-.HB,+ and less stable PhOH+-.B, forms of the ion, the nonvertical transition to higher level of the latter form is allowed but with rapidly decreasing Franck-Condon factors. I n large clusters, the ionization takes place after a rapid transition from PhOH*-B, to PhO* -.-.HB,+ by proton transfer. A steep increase of the ionization cross section cI with u2 for this group of clusters suggests an almost vertical transition to the bottom of the ion PhO'-HB,+ energy well, Le., similar geometries in the excited and ionked states. A particular form of the cI = f ( v 2 ) dependence for PhOH(MEA), complex may be tentatively explained by a quasi-equilibrium between nearly isoenergetic neutral and ionic forms of the complex in the S, statc. In conclusion, the proton transfer in the S, state occurring for MEA clusters with n L 3 is in good agreement with our fluorescencc measurements: a drastic increase of the Stokes shift for n = 3 as compared to n = 1 and 2 which corresponds to the appearance of the phenolate anion emission expected for the ionic PhO*--HB,+ form of the excited cluster. D . Energetics ofthe Reaction. One can expect a correlation between the proton-transfer reaction and the proton affinity of the solvent cluster associated with the phenol molecule. I n this hypothesis, it is assumed that the structure of the clusters consists in a cluster of 6 , bound with the phenol. In regard to the experimental results obtained when the clusters dissociate, this assumption seems reasonable: if n molecules B were distributed all over the phenol moiety one should not obtain B,H+ fragments but smailer ones (see Figure 8). Moreover, it can be expected that the hydrogen-bond energies in ammonia or in amine clusters (except for TEA clusters in which these hydrogen bonds do not exist) will be larger than the energies of interaction between N H I
B,
*
8bT
I&'
Pho-.**Bd*
or amines and the aromatic ring in phen01.~~~** The proton affinities of clusters of ammonia and MEA evaluated or compiled by Cheshnowsky and Leutwyler4 and Bisling and co-workersZ6are summarized in Table 111. The protontransfer reaction occurs for n 2 4 for (NH,), and n 2 3 for (MEA)n and thus seems to correspond to an affinity of about 10.33 eV (238 kcal/mol). This value is to be compared with the It value of 10.52 eV (243 kcal/mol) obtained for ~u-naphthol.~ can be noticed that in the case of phenol-DEA or -TEA (PA = 9.9 f 0.1 and 10 f 0.1 eV, respectively) the proton transfer should not occur in the 1-1 complex. Another approach, also proposed by Leutwyler and co-workers, consists in estimation of the complexation energies of PhOH(B), and PhO-H+B,. The energetics of the system can be summarized by the thermodynamic cycle in Scheme 11, where dHcNis the complexation energy between phenol and B,, 6Hahdthe gas-phase acidity of phenol. PA(n) the gas-phase proton affinity of B,, 6H,' the complexation energy between PhO- and B,H+, and 6HpTthe enthalpy of proton-transfer reaction. The value of 6HaCid in the ground state is deduced from the following scheme for which all data are known experimentally:
PhOH
-
PhO'
+e
H' net:
-+
+ H'
PhO'
-
PhO-
-2.36 eV (ref 23)
H+ + e
PhOH -* PhO-
3.73 eV (ref 29)
13.60 eV (ref 30)
+ H+
14.97 eV (345 kcallmol)
For the excited state the evaluation of this term is more difficult because the value of the energy of the phenolate ion in its first excited electronic state in the gas phase is not accurately known. The photodetachment spectrum of the gaseous phenolate anion measured by Richardson et al.23 shows a maximum at 340 nm (3.65 eV) which may correspond to the autoionized PhO*- state. Another experimental value of this excitation energy (4.32 eV) has been reported in the work of Nishimoto and Forster3' but without information on the conditions in which it has been measured. Then may be evaluated by using the following scheme,
-
PhOH
-4.6 eV
PhO-
+ H+
14.97 eV
PhOH*(S,) PhOH PhO-net:
PhO*-
PhOH*
-
-
3.65 eV (ref 23) or 4.32 eV (ref 31) PhO*- + H+ 14.02 eV (323 kcal/mol) or 14.65 eV (338 kcal/mol)
This value can be compared with the 14.16 eV (327 kcal/mol) estimated for the a-naphthol in solution! In solution, from Forster expression A = 6H*ac,d- 6Ha,,, = 2.303RT(pKa* - pKa), we find a difference between -0.30 and -0.45 eV depending on the value of pK,*. Our estimation gives a A between -0.32 and -0.95 eV. However, although the value of 14.65 eV leads to a A close to 118. ( 2449. 7 ) Bombach, R.; Honegger, E.; Leutwyler, S. Chem. Phys. Lerr. 1985,
(28) Honegger, E.; Bombach. R.; Leutwyler, S. J . Chem. Phys. 1986, 85, 1234. (29) DeFrees. D. J.; McIver, R. T.; Hehre, W. J. J . Am. Chem. Soc. 1980, 102, 3334. (30) Robinson, J . W. Handbook of Spectroscopy; CRC Press. Boca Raton, FL, 1974; Vol. 1 . (31) Nishimoto, K.; Forster. 1. S J . Phy.r. Chem. 1968. ??. 914.
Reactivity of Molecular Clusters
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5047 SCHEME 111 H
PhOH
/
0
\
H
3 i S ' b + 1 ' ' ' "5 1 3 2 Figure 11. Schematic representation of potential energy curves along the PhOH-N (PhO--.H+N) coordinate and dissociation limits for the PhOH(NH3) (a) and PhOH(NH3)4(b) neutral (full line) and ionic (dashed line) complexes. The depths of energy wells are determined experimentally or evaluated by using the crude model described in the text. '
2
the value in solution, the other value of 14.02 eV cannot be disregarded because in solution one should observe a difference with the gas phase due to the solvent effects. The second term is the complexation energy of the neutral clusters (6HcN). The energy of dissociation of the complex phenol-NH, in its ground state was determined by Mikami et a1.I6 from the dissociation threshold of electronically excited complexes: This method cannot be so easily applied to larger clusters: 6HcN= -0.22 eV. We will take an approximate value of 6HcN;r. -0.25 eV for all clusters with n 1 2. In fact this term is small compared to the uncertainty in the other thermodynamic terms and does not play a determinant role. From the spectral red shifts 6 v of about -800 cm-I one obtains the complexation energy in the excited state of PhOH(NH3),: 6HCN*= -0.32 to -0.35 eV. I n a first approximation we take the same value for all the PhOH(MEA), clusters. We do not take into account the variations of this shift for MEA clusters with n L 2. Energetic values and the proton affinities for each clusters are summarized in Table 111. The values of complexation energy of ionic species (SH:) cannot be directly measured. For the excited state it may be roughly estimated from the thermodynamic cycle assuming that the proton transfer takes place when SH, I 0. This condition is not fulfilled for clusters involving three N H 3 or two MEA molecules but is for four NH, or three MEA. The result is that -4.05 < 6Hc1* < -3.71 eV (or -4.68 C SH,'* < -4.34 eV) for ammonia and -4.15 C 6Hc'* C -4.02 eV (or -4.78 C 6Hc'* < -4.65 eV) for MEA deduced from complexes, where the first set corresponds to 6Hacid* the gas-phase spectra of the phenolate ion23and the second one is from ref 31. Since the phenolate ion is more basic in its ground state than in its excited state, one can expect 6Hc' < 6Hc'*. I f the second set of data is chosen we would expect the proton transfer to occur in the ground state of large clusters, in disagreement with our experimental results. The gas-phase data seem thus more accurate for the estimation of complexation energies. The estimation value of 6H,'* corresponds to the potential energy of two elementary point charges distant by 3.5-4 8, the distance between 0 and N atoms involved in the PhO--.HN+ is much shorter, of the order of 2.8-3.0 A.32 However, such a picture assuming complete localization of charges on two atoms is obviously oversimplified. Finally from these estimations possible potential curves for hydrogen bonding and ion-pair formation process can be drawn. They are presented in Figure 1 1 for the phenol(NH,) (Figure 1 la) and phenol(NH3), (Figure 1 lb) systems with the different limits of dissociation. For the ionic pair V ( r ) is approximated by the (32) Matsuyama. A.; Imamura, A. Bull. Chem. SOC.Jpn. 1972, 45, 2196.
NhH,
.NR,H,
**
Coulombic e2/ReN interaction. For the small neutral complex the ionization is obtained directly by two-photon excitation (Figure 1 la). It may be noticed that the dissociation limit for the NH4+ production (8.46 f 0.1 eV) lies very close to the limit for the PhOH+ generation. However, no dissociation leading to NH4+has been observed experimentally. Mikami et all6 propose that this preferential generation of PhOH+ may be related to the shape of the potential energy surface of dimer ion which might present a potential barrier (>0.25 eV) separating both dissociation pathways. For the cluster where the proton transfer takes place (Figure 1 Ib), both neutral and ion-pair forms may be ionized with a branching ratio depending on the laser power and proton-transfer s pulses) the proton transfer rate. In our conditions ( ~ 1 5 - n laser is probably more efficient than the direct ionization of the initially excited neutral form. Even if this model seems reasonable to justify the proton transfer in the first excited state, some critical comments are needed. (i) We have no information on the energy barrier for the proton transfer. Its height certainly depends on the coupling between the neutral and ionic states at different values of the protontransfer coordinate. Time-dependent experiments in the picosecond scale would be essential to solve this problem. (ii) We implicitly assumed that the binding energy 6Hc' of PhO---HB,+ clusters does not depend on n. Such a dependence cannot be deduced from our experiments. This problem is closely related to that of the structure of B, clusters. The observed increase of the ionization threshold of heterogeneous PhOH. B,-H20 clusters with respect to that of Ph0H.B" ones by 0.1 eV is of interest. This effect seems too small to assume the structure given in Scheme 111. A further study of heterogeneous clusters involving determination of their dissociation pathways may be interesting if associated with some model calculations. (iii) The nonradiative deactivation channels from the SI state are neglectcd. The corresponding rates may depend on the cluster sizes. As can be seen in Figure 1 la, potential energy curves of the ground ion-pair state PhO-.-HB,+ and of the excited neutral state PhOH*-B,, are crossing but this crossing takes place at a large N.-0 distance ( > l o A) so that an efficient internal conversion can be expected only for high vibronic levels of the PhOH*-B, state. On the other hand, the rate of the intersystem crossing to the triplet state33is probably nonnegligible and may be affected by complex formation as evidenced by drastic changes in fluorescence lifetimes of the jet-cooled bare phenol molecule and its water complex.
Conclusion The study of chemical reactivity of molecules in clusters produced in a supersonic expansion opens a new field of investigation: in this kind of approach it is possible to obtain information on the reaction mechanism at a microscopic level. In this work we have studied the proton-transfer reaction in clusters of phenol with ammonia and monoethylamine. The main results can be summarized as follows: The proton-transfer reaction does not occur for the small clusters ( n 5 3 for N H 3 and n 5 2 for MEA); their ionization thresholds are lowered by about 0.8 eV from that of free phenol, this lowering being due to the strengthening hydrogen bond in the ionic clusters. Their fluorescence spectra are only slightly displaced with respect to that of bare phenol molecule. The proton-transfer reaction in the excited state of large clusters can be evidenced by the following features: (i) the emission spectra are strongly shifted to the red and are similar to that of phenolate anion; (ii) the ionization thresholds are sensibly lowered (by 0.6-1 ( 3 3 ) Kimura, K.; Tsubomura, H . Mol. Phys. 1966, 11, 349
5048
J . Phys. Chem. 1990. 94, 5048-5051
e\') as compared to the small neutral clusters. i.e.,, by I .6-2.0 eV as compared to free phenol.
Acknowledgment. We are highly indebted to Dr. S. Leutuyler for communicating his results prior to publication and to Dr. I.
Dimicoli, P. Ceraolo, and 0. Benoist d'Azy for their help in setting up this experiment. This work has been supported by the GRECO "Dynamique riactionnelle des rdactions mol6culaires". Registry No. MEA, 141-43-5; PhOH. 108-95-2; NH3, 7664-41-7.
Micellized Sequestered Silver Atoms and Small Silver Clusters E. Borgarello, ENI Ricerche Spa, San Donato Milanese, I t a h
D. Lawless, N. Serpone,* Department of Chemistr.y, Concordia University, Montrtal. Qutbec, Canada H3G I M8
E. Pelizzetti, Dipartimenlo di Chimica Fisica Applicara, Unifiersit2 di Parma. Parma, Italy
and D. Meisel" Chemistry Difiision, Argonne National Laboratory, Argonne, Illinois 60439 (Receioed: October 20, 1989; In Final Form: January 29, 19901
Pulse radiolysis was used to examine the nature of the silver species obtained when an aqueous solution containing sequestered Ag' ions was reduced by hydrated electrons in the presence of a surfactant macrocyclic crown ether, labeled L, and/or a maltoside surfactant. The initially formed product is the ,4g0(L) species which rapidly loses its ligand (half-life 5.5 p ) and reacts with another .4g'(L) ion to form Ag2+(L). The latter species decays by a bimolecular process to form the Agtt(,L), species at a faster rate than its ligand free analogue. Ultimately, colloidal metallic silver, (Ag),, forms which is stabilized by the surfactant moieties. N o long-term stability to the reduced monomolecular species could be obtained.
Introduction Silver atom cluster, Ag,,", are the essential components (a) in latent image specks (Ag4) that catalyze the silver halide photographic processla and (b) in heterogeneous catalytic silver ion reduction processes where tetrameric silver has also been implicated.'~~Early studies of Henglein and co-workers concentrated on the absorption ~ p e c t r a ~and . ' ~chemical reactivity" of small aggregates of silver atoms in aqueous solutions. Well-defined absorption bands in the UV region were observed for small nuclearity species such as Ago, and Agz', Ag42+,which eventually lead to the formation of colloidal metallic, Ag,. particles. Much effort has been expended in the preparation and identification of similar species i n the gas phase'? or in cryogenic mat rice^.'^-^' ( I ) Mitchell, J. W . Photogr. Sci. Eng. 1981, 25, 170. (2) Moisar, E. Photogr. Sci. Eng. 1982, 26, 124. (3) (a) Moisar, E.; Granzer, F. In Growth and Properties o/ Meral Clusters; Bourdon, J . , Ed.: Elsevier: Amsterdam, 1980; pp 331-343. (b) Malinowski, J. Ibid. pp 303-320. (c) Sahyun. M. R . V . Ibid. pp 379-385. (d) Levy, B.: Chang, K. C.; Chen, F. P. Ibid. pp 393-397. (4) Hamilton, J F.; Baetzold, R. C. Photogr. Sei. Eng. 1981, 25, 189. (5) (a) Sahyun, M. R . V . Photogr. Sei Eng. 1982, 26, 21 I . (b) Mitchell. J . W . Photogr. Sei. Eng. 1982, 26, 2 1 1 , (6) Marquardt, C. L.; Gliemeroth, G. J . Appl. Phys. 1979, 50. 4554 (7) Proceedings of the International Easr- West Symposium I I on the Factors Influencing Efficiency of Pholographic Imaging, October 30, 1 988, Kona, Hawaii; The Society for Imaging Science and Technology: Springfield VA.
(8) Hamilton, J . F.; Logel, P. C. Photogr. Sei. Eng. 1974, 18, 507. (9) Pukies, J.; Roebke, W.; Henglein, A . Ber. Bunsen-Ges Phps. Chem. 1968, 72, 842.
( I O ) Pukies, J.; Roebke, W. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 1101. ( 1 I ) (a) Henglein, A Ber. Bunsen-Ges. Phys. Chem. 1977. 81, S56. (b) Tausch-Treml, R.; Henglein. A.: Lilie. J. Ber. Bunsen-Ge.r. Phjs. Chrm 1978, 82, 133.5. (12) Hilpert, K.: Gingerich, K 4. Ber. Busen-Ges. Phys. rhein. 1980. 84. 139.
002~-3654/90!2094-50JX$02.50/'0
The work of Ozin and c o - ~ o r k e r sindicates '~ striking similarities between the UV-visible spectra of Ag; and Ag,q+ species, with the degree of aggregation ( n ) of these clusters poorly defined. In aqueous solutions, a spectral band observed on photochemical reduction of silver ions, at ca. 410 nm, has been attributed to surfactant macrocyclic stabilized Ago a t ~ m s . ~ ~The J ' well-known spectra of classical colloidal silver sols, on the other hand, may have a strong absorption band anywhere from 370 nm and above depending on particle size. Such radiolytically produced sols have been thoroughly ~ t u d i e d . ' ~ . ' ~ The formation and kinetics associated with the generation of silver atoms/clusters by laser flash photolysis of silver iodide (size -5 100 A) were recently examined in the picosecond time d o m a h a Attempts to identify the silver atom/cluster species in this study, by comparing the transient visible spectra with the earlier data (13) Stevens, A. D.; Symons, M. c. R. J . Chem. Soc., Faraday Trans. I 1989, 85, 1439.
(14) (a) Schulze. W.; Frank. F.; Charle, K. P.; Tesche, B. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 263. (b) Charle, K. P.; Frank, F.; Schulze, W. Ber. Bunsen-Ges. Phys. Chem. 1984,88,350. (c) Bennemann, K. H.; Reindl, S. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 278. ( 1 s ) (a) Ozin, G.A.; Hughes, F. J . Phys. Chem. 1983,87, 94. (b) Ozin, G. A.: Huber, H. Inorg. Chem. 1978, 17, 155. (c) Ozin, G. A,; Hughes, F.; Mattar, S. M.; McIntosh, D.F . J . Phys. Chem. 1983, 87, 3445. (d) Ozin, G . A . ; Hughes, F.; Mattar, S. M. J . Phys. Chem. 1985, 89, 300. (16) (a) Monserrat, K.; Gratzel, M.; Tundo, P. J . A m . Chem. SOC.1980, 102,5527. (b) Jao, T.-C.; Beddard, G. S.: Tundo, P.; Fendler, J. H. J . Phys. Chem. 1981, 85, 1963. ( I 7) Humphry-Baker, R.;Gratzel, M.; Tundo, P.; Pelizzetti. E. Angew. Chem., I n t . Ed. Engl. 1979, 18, 630. (18) (a) Henglein, A.; Tausch-Treml, R. J . Colloid InterJace Sci. 1981, 80. 84. (b) Henglein, A. J . Phys. Chem 1979, 83, 2858. ( 19) (a) Vucemilovic, M. 1.; Micic, 0. 1. Radial. Phys. Chem. 1988, 32, 7 9 . !b) Lee, P. C.; Meisel, D. J . Phys. Chem. 1982, 86, 3391. 120) Micic, 0. I.; Meglic, M.: Lawless. D.; Sharma, D.K.; Serpone. N. Langnruir 1990. 6.487.
9 1990 American Chemical Society
