ARTICLE pubs.acs.org/JPCA
Dissociation Dynamics of Asymmetric Alkynyl(Aryl)Iodonium Radicals: An ab Initio DRC Approach to Predict the Surface Functionalization Selectivity Claudio Fontanesi,*,† Carlo Augusto Bortolotti,† Davide Vanossi,† and Massimo Marcaccio‡ † ‡
Department of Chemistry, University of Modena and Reggio Emilia, Via Campi 183, 41100 Modena, Italy Department of Chemistry “G. Ciamician”, University of Bologna, Via Selmi 2, 40126 Bologna, Italy ABSTRACT: The dissociation process of neutral open-shell [4-F —(C6H4)—I—CtC—(CH2)4—Cl] and [4-NO2—(C6H4)—I— CtC—(CH2)4—Cl] asymmetric iodonium radicals was studied theoretically. Vertical electron affinities and DRC (dynamic reaction coordinate) results were obtained and compared with experimental evidence. In particular, the fluorine and nitro substituent groups were selected because of (i) their opposite electron-withdrawing/electrondonating effects and (ii) experimental evidence that the grafting ability, in terms of alkynyl/aryl grafting ratio, increases with decreasing electron-withdrawing nature of the para-position substituent on the phenyl ring. DRC results show that the dissociation dynamics of the iodinealkynyl carbon bond, for the nitro-substituted iodonium, occurs on a longer time scale than that of the fluorinesubstituted iodonium. This finding is in agreement with the overall experimental results.
’ INTRODUCTION The functionalization of surfaces by immobilization (physisorption and/or chemisorption) of organic layers on both conductive and semiconductive substrates is a constantly developing field in fundamental and applied chemistry.1 Tailoring of functional and structural properties of the derivatized layer is considered to be of fundamental importance for several applications such as catalysis, molecular electronics, energetics, and biosensing.24 Because of the wide range of different substrates, adsorbates (organic and biological species), and immobilization strategies currently available, the number of possible interfaces that can be assembled is enormous. Within this field, Pinson pioneered the functionalization of electrode surfaces by exploiting the electrochemical formation of radical species in the so-called electrochemically assisted grafting technique.5 The electrochemically based approach features some extremely interesting characteristics such as versatility, mild operating conditions, and tunability.6 In the past two decades, this method has been used to modify both silicon and carbon surfaces through either electrochemical oxidation of alcohols and amines or electrochemical reduction of diazonium salts and halogen-substituted organics.7,8 Despite the number of electrochemically based grafting approaches, none of them can provide a straightforward approach to immobilizing alkane moieties onto carbon substrates. This issue was addressed by Daasbjerg and co-workers,9,10 who proposed the electrochemical reduction of iodonium salts as a direct method of covalently attaching long alkane-chain (alkynyl) moieties to carbon substrates. In particular, iodonium salts can be represented by the general formula [Ar—I—R]+, where Ar is an aromatic group and r 2011 American Chemical Society
R could be an aromatic or alkynyl substituent; note that the formal oxidation state of iodine is 3+. The overall grafting mechanism is sketched in Scheme 1. In the case of an asymmetric iodonium, [Ar—I—R]+, the dissociation can follow two different routes, depending on which carboniodine bond is broken (Scheme 1). Thus, assuming that the reactivities of the two radicals (Ar• and R•) with the surface are of the same order of magnitude, the key factor in the functionalization of the electrode surface is represented by the ability to control the Ar/R grafting ratio. This, in turn, depends on the way the R and Ar moieties are designed so that one of the two possible paths is favored over the other. In the present work, two iodonium cations are considered: [4F—C6H6—I—CtC—(CH2)4—Cl]+ [(4-fluorophenyl)-(6chloro-1-hexynyl)iodonium, labeled as pF] and [4-NO2— C6H6—I—CtC—(CH2)4—Cl]+ [(4-nitrophenyl)-(6-chloro1-hexynyl)iodonium, labeled as pNO2]. Their experimental “grafting” abilities are compared with the dissociation dynamics of the neutral open-shell species, which was calculated within the DRC approach (i.e., a classical dynamic trajectory relying on a density functional theory (DFT) quality potential energy hypersurface).11 As mentioned above, the modeling assumption underlying the present approach relies on the fact that the cleavage ratio is able to account for the experimental grafting results. Within this picture, comparison between theoretical calculations and experimental results proves to be an extremely valuable, if Received: April 7, 2011 Revised: September 6, 2011 Published: September 08, 2011 11715
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Scheme 1. Sketch of the Two Different Paths of Surface Functionalization by the Iodonium Salts
Scheme 2. Electrochemical Mechanism of the Iodonium Species: Reduction Step Followed up by the Two Possible Chemical Reactionsa
a
Para position substituent —X is: —F for compound pF and —NO2 for species pNO2.
not unique, tool for clarifying electrochemical mechanisms at the molecular level.1220 From the experimental point of view, the two iodonium salts show a peculiar difference in the alkynyl/aryl grafting ratio, which is about 50/50 for pF and approximately 10/ 90 for pNO2.21
’ ELECTROCHEMICALLY ASSISTED GRAFTING MECHANISM The grafting process, on a glassy carbon (GC) electrode, proceeds through the electrochemical reduction of the closedshell iodonium cation to its neutral open-shell form. Subsequent surface analysis [electrochemical degrafting potentials, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOFSIMS)] confirmed that the organic moieties of the parent iodonium cation (bound to the iodine atom) were covalently chemisorbed onto the GC surface (Scheme 1). The whole grafting mechanism was modeled assuming that the electrochemical reduction of the iodonium cation salt is followed by the subsequent dissociation of one of the two carboniodine bonds, yielding a neutral closed-shell molecule and a radical, as suggested by Daasbjerg and Pedersen.9,10 In the case of an asymmetric iodonium salt, as pF and pNO2, the dissociation can follow two different pathways, as shown in Scheme 2. Moreover, possible side electroreductions involving the radicals formed by the process following the iodonium cation
reduction must also be considered •
CC—ðCH2 Þ2 —Cl þ e f
ðÞ
CC—ðCH2 Þ2 —Cl
ð1Þ
NO2 —C6 H4 • þ e f NO2 —C6 H4 ð Þ
ð2Þ
F—C6 H4 • þ e f F—C6 H4 ð Þ
ð3Þ
In principle, the three radicals rad-hexCl, rad-NO2, and rad-F, as defined in reactions 13, respectively, can undergo a further electron uptake at the potential corresponding to the reduction of the iodonium cation,22,23 thus inhibiting, or at least worsening, the grafting ability of the iodonium salts.
’ COMPUTATIONAL DETAILS In the present work, the overall calculations on iodonium salts and their parent neutral radicals were performed in the framework of ab initio methods using Firefly QC package,24 which is partially based on the GAMESS (US)25 source code. Unless otherwise indicated, all calculations were performed using C1 symmetry. For the closed-shell cations, we employed a restricted description, whereas the unrestricted formalism was used for the neutral open-shell radicals. First, the molecular structures of the cation and neutral-radical forms were subjected to a full optimization procedure at the DFT level employing two 11716
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Figure 1. Molecular structure of the perpendicular conformers of (a) pF and (b) pNO2. Structures optimized at the B3LYP/6-311G** level of theory.
widely used hybrid functionals: B3LYP and PBE0. To gain confidence in the reliability of the results, we chose the rather large all-electron basis set 6-311G**. Using this basis set, we also performed (on the cation forms only) MP2 structure relaxations, to take the effects of electron correlation into account in a completely different way. Particular attention was paid to the geometry optimization of the pNO2 neutral-radical species, which was also carried out at the CAM/6-311G** and B3LYP/ AUGcc-pVTZ levels of theory. After having selected the DFT B3LYP/6-311G** level of theory as the most suitable one for our purposes, we started the DRC analysis11,26 from the previously obtained optimized structures of the cations. Standard potentials for pF, pNO2, rad-hexCl, rad-NO2, and rad-F were calculated at the B3LYP/AUG-ccpVTZ level of theory. It is worth mentioning that, at the very beginning of this study, several preliminary test calculations (not reported here) were performed at the DFT B3LYP level using three smaller basis sets:27 LanL2DZ, 3-21G*, and the generalized mixed ECP 6-31G*.
’ RESULTS AND DISCUSSION Structural Analysis. A structural analysis was carried out for both the cation (oxidized-state) and neutral (reduced-state) species. In accord with the experimental results, the iodine alkynyl carbon bond length (Calkynyl—I), the iodinearyl carbon bond length (Caryl—I), and the Caryl—Calkynyl dihedral angle (formed between the Calkynyl—I bond and the aromatic ring plane) are considered the three main geometrical parameters in the dissociation process. Thus, the study was essentially focused on a comparison of the relative stability of the two structural arrangements, denoted perpendicular (in which the iodine alkynyl carbon bond forms a 90° dihedral angle with respect to the aromatic ring) and planar (in which the iodinealkynyl carbon bond has a 0° dihedral angle with respect to the aromatic ring). These two spatial arrangements were then used as starting points for the subsequent calculations. Figure 1 shows the perpendicular arrangement for both pF and pNO2. As previously stated, for the perpendicular closedshell cation species, the optimization procedure was performed at the B3LYP/6-311G**, PBE0/6-311G**, and MP2/6-311G**
levels of theory. For both pF and pNO2 perpendicular cations, the standard Hessian analysis confirmed that a geometry corresponding to a true minimum was found. In Table 1, the results for the main geometric parameters are reported for the perpendicular cations. It is worth mentioning that, apart from minor differences, the three computational approaches lead to geometries extremely similar. For example, the largest differences for the Caryl—I bond length and Calkynyl—I—Caryl angle were found to be 0.043 Å and 2.7°, respectively (Table 1). We therefore decided to perform all other calculations at the DFT B3LYP/6311G** level of theory, to limit the computational demands. It is worth noticing that only a few alkynyl(phenyl)iodonium salts (in the solid phase) have been characterized by X-ray diffraction.2831 The results reveal that the Caryl—I and Calkynyl—I bond lengths range between 2.1 and 2.2 Å and the Calkynyl—I—C aryl angle falls within the 90100° range. Such experimental findings are in reasonable agreement with our theoretical predictions. Considering the planar arrangement for the pF and pNO2 cations, the optimization procedure led to a stationary point having the single C—C, triple CtC, and Cl—C bonds coplanar with the aromatic moiety that was characterized, by the usual Hessian analysis, as a transition state (TS). This result was achieved assuming both Cs and C1 symmetries for the starting geometry. The relevant geometrical parameters for the two planar cations (TS) are reported in Table 2. The results obtained by the optimization procedure for pF and pNO2 neutral (open-shell) species were found to be rather different when the perpendicular and planar structures were used as initial guesses. In the case of pF, a stationary point corresponding to a true minimum was found. This geometry (called twisted-planar) is characterized by an almost-planar configuration in which the CtC, I—Caryl, and I—Calkynyl bonds are almost coplanar with the aromatic ring (Figure 2). The alkynyl carbon of the I—Calkynyl bond forms a 1.5° dihedral angle with the aromatic plane (see Table 2 for the main geometric values). For the pNO2 neutral species, the optimization procedure was not able to converge to the selected gradient-threshold criterion, and hence, it was stopped after 400 steps (with the energy practically constant during the last 50 optimization steps). The relevant main geometrical parameters characterizing the 11717
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Table 1. Main Bond Lengths and Angles Corresponding to the Optimized True Minimum of the Perpendicular Cations at Three Levels of Theory, along with Bond Orders in Parenthesesa pF Calk—I (Å)
method
a
Caryl—I (Å)
pNO2
Calk—I—Caryl (deg) dihedral angle (deg)
Calk—I (Å)
Caryl—I (Å)
Calk—I—Caryl (deg) dihedral angle (deg)
B3LYP 1.995 (1.048) 2.146 (0.969)
102.5
90.0
1.991 (1.063) 2.163 (0.954)
101.9
90.0
PBE0
1.980 (1.069) 2.114 (0.996)
102.3
90.0
1.976 (1.081) 2.131 (0.982)
101.7
90.0
MP2
1.985 (0.960) 2.109 (0.935)
99.9
90.0
1.982 (0.963) 2.120 (0.930)
99.2
90.0
6-311G** basis set.
Table 2. Main Bond Lengths and Angles Calculated at the DFT 6-311G**/B3LYP Level for the Optimized Planar Cations and Corresponding Neutral Radicals, along with Bond Orders in Parentheses Calk—I (Å)
Caryl—I (Å)
Calk—I—Caryl (deg)
dihedral angle (deg)
pF cation
1.982 (1.132)
2.191 (0.865)
100.8
0.0
pNO2 cationa
1.982 (1.140)
2.194 (0.843)
101.9
0.0
pF neutral radicalb
2.509 (0.453)
2.150 (0.961)
89.4
1.1
pNO2 neutral radicalc
∼2.0
>5.8
0.0
molecule a
a
Starting planar arrangement that leads to an optimized planar TS. b Twisted-planar stationary point corresponding to a true minimum. c No stationary point was found; geometrical parameters refer to a structure obtained with 400 optimization steps.
Figure 2. Molecular structure of the pF neutral open-shell optimized species, twisted conformer, at the B3LYP/6-311G** level of theory.
Table 3. Absolute Energies Calculated at the DFT 6-311G**/ B3LYP Level for the Structures Reported in Tables 1 and 2a molecule
energy (au)
pF perpendicular cation
7943.944894
pF planar cation
7943.939901
pNO2 perpendicular cation pNO2 planar cation
8049.223619 8049.220848
pF twisted/planar neutral radical
7944.166695
a
Energy of the pNO2 neutral radical not reported as the corresponding structure was not optimized.
nonequilibrium geometry are those one reported in Table 2. Owing to this peculiar result, the same optimization procedure was followed for only the pNO2 neutral species using both a DFT model that is particularly suitable for systems featuring large interatomic distances (CAM3LYP/6-311G**) and including diffuse functions in the basis-set (B3LYP/AUGcc-pVTZ). Again, no stationary point was found, even after 400 optimization steps. Values of Calkynyl—I = 2.021 and Caryl—I = 4.951 Å were obtained at the CAM3LYP/6-311G** level of theory, whereas values of Calkynyl—I = 2.001 and Caryl—I = 4.542 Å were obtained at the B3LYP/AUG-ccpVTZ level of theory. For this reason, no Hessian analysis was carried out for the neutral pNO2 species.
Figure 3. Unrelaxed scan of the PES for the rotation of the aryl moiety around the CAryl—I bond for the cation closed-shell species. The 90° geometry corresponds to that of the true minimum, perpendicular conformer. (O) pF, (0) pNO2. B3LYP/6-311G** level of theory.
Note that a comparison of bond order values (Table 1) suggests that the Caryl—I bond in pF is “stronger” than the Caryl—I bond in pNO2; the opposite holds for the Calkynyl—I bond. These theoretical findings are reinforced by the structural 11718
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Scheme 3. Thermodynamic Cycle Used to Calculate E°theo Valuesa
Figure 4. Unrelaxed scan of the PES for the rotation of the aryl moiety around the CAryl—I bond for the neutral open-shell species. (O) pF, (0) pNO2. UB3LYP/6-311G** level of theory.
results obtained for the neutral molecules (Table 2): Neutral pNO2 shows a plain dissociative behavior (see the large value characterizing the Caryl—I bond), whereas for neutral pF, a stationary point corresponding to a minimum was found. For the sake of comparison, Table 3 summarizes the absolute energies relevant to the structures reported in Tables 1 and 2. Our structural analysis was completed by considering the unrelaxed scan of the potential energy surface (PES) along the torsional coordinate that connects the two geometrical arrangements (perpendicular and planar) considered previously. Figure 3 reports the scan for the cation species, showing that the 90° geometry corresponds to a true minimum. The energy increases going from 90° to 0°. In Figure 4, which explores the same geometrical arrangements as generated for Figure 3, the neutral open-shell species are considered. The energy surface somewhat mirrors the results obtained for the cations. In fact, neutral openshell species show that the planar structures are more stable than the perpendicular ones. Furthermore, the neutral pF species features an energy profile that is essentially barrierless, whereas the pNO2 energy goes through a maximum of about 0.2 kcal mol1 at 45°. Side Reactions. To assess the possible interference of intermediates in the overall electrochemical mechanism, vertical electron affinities (EAv) and theoretical standard reduction potentials (E°theo) corresponding to the reduction of the iodonium cations (Scheme 2) were compared to those pertaining to the reduction of the possible radical intermediates (reactions 13). In particular, vertical electron affinities were obtained as EA v ¼ ½Eðreduced species in unrelaxed geometryÞ Eðoxidized species in relaxed geometryÞ
ð4Þ
Experimentally, pF and pNO2 were found to be reduced in the range between 1.0 and 0.5 V vs SCE (in agreement with previously reported data on the reduction of related iodoniums compounds21,22), giving EAv values of 4.88 and 5.34 eV, respectively. The EAv values calculated for the three radicals rad-hexCl, rad-NO2, and rad-F in reactions 13 are smaller than those of pF and pNO2, being 2.67, 0.91, and 1.49 eV, respectively. Thus, the comparison of electron affinity values indicates that the reaction intermediates are not electrochemically reduced at the potentials suitable for the iodonium cations reduction. E°theo values were determined using the thermodynamic cycle proposed by Cramer and Truhlar,19 as sketched in Scheme 3. (Both
a ΔG(IV)° = ΔG(I)° + ΔG(III)°(solv) ΔG(II)°(solv), ΔG°(A/A1 vs NHE) = ΔG(IV)° + ΔG°(H). and ΔG(H) = 4.44 eV.33
Figure 5. SOMO surfaces of the neutral open-shell perpendicular species: (a) pF, (b) pNO2.
electronic and solvation energies were calculated at the B3LYP/ AUG-cc-pVTZ level of theory, which yields quite reliable E°theo values.15,16,32). Values of 1.66, 0.56, 1.21, 1.08, and 0.98 V (vs SCE) were obtained for pF, pNO2, rad-hexCl, rad-NO2, and rad-F, respectively. As a consequence, the radhexCl species should be found in the reduced state within the experimental reduction potential range, typical of iodonium salts. The same also holds for the species rad-F [compare E°theo(rad-F) = 1.08 V with the E°theo(pF) = 1.66 V); thus, the species rad-F should be reduced as soon as it is formed. This implies that the reduced species rad-F should not be able to graft onto the GC surface, a picture that is not consistent with the experimental results (vide supra). It is worth noting, however, that the E°theo values were calculated as differences between two equilibrium (solvated) states, whereas the picture that is consistent with the experimental evidence, obtained using the (nonequilibrium, solvation-independent) EAv values, suggests that the grafting process occurs on a dramatically shorter time scale than equilibration (involving geometry relaxation, subsequent solvation, and diffusion steps). Thus, dynamics-based methods appear to provide a better tool for rationalizing the grafting process. 11719
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Figure 6. Time dependence of the carboniodine bond distance for the perpendicular pF neutral open-shell species. (a) v1 DRC trajectory: solid line (black), CAryl—I bond versus time; dotted line (blue), CAlkynyl—I bond versus time. (b) v2 DRC trajectory: solid line (black), CAryl—I bond versus time; dotted line (blue), CAlkynyl—I bond versus time.
DRC Calculations. The DRC analysis was anticipated by a check of the electronic density, corresponding to the singly occupied molecular orbital (SOMO) of the neutral species, to elucidate the electronic effect of the substituent in the phenyl para position. Such an analysis can also help clarify the results of the DRC trajectories discussed later. Parts a and b of Figure 5 show the SOMOs of the perpendicular neutral open-shell species pF and pNO2, respectively. For both molecules, the SOMO has antibonding character between the alkynyl/aryl carbons and iodine. However, a difference between pF and pNO2 exists in the extent of the associated electronic density. In pNO2, the SOMO is clearly spread over the phenyl group and the substituent —NO2 (Figure 5b), in contrast to the corresponding neutral pF molecule (Figure 5a). Because of the electron-withdrawing character of the nitro substituent and its proximity to Caryl, this group should cause electron depletion that is more evident for the Caryl—I bond than the Calkynyl—I bond. The calculation of DRC trajectories for the neutral radicals requires the definition of the initial conditions as a starting point under which the classical equations of motion are integrated.11,26 In our DRC study, we always used the atomic coordinates of the true minimum geometry corresponding to the perpendicular arrangements for the cations pF and pNO2. Using the Hessian matrix calculated for this minimum, we set up two different initial velocity vectors for each species. The first velocity vector, v1, was constructed to distribute 0.1 kcal mol1 of kinetic energy over all of the vibrational modes; the second, v2, was obtained by assigning the 0.1 kcal mol1 of kinetic energy to the Calkynyl—I stretching vibrational normal mode. Using this strategy, two DRC trajectories were calculated for each neutral species pF and
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Figure 7. Time dependence of the carboniodine bond distance for the perpendicular pNO2 neutral open-shell species. (a) v1 DRC trajectory: solid line (black), CAryl—I bond versus time; dotted line (blue), CAlkynyl—I bond versus time. (b) v2 DRC trajectory: solid line (black), CAryl—I bond versus time; dotted line (blue), CAlkynyl—I bond versus time.
pNO2. It is worth mentioning that the formal justification for employing the optimized structures and corresponding Hessian matrices of the cations for the DRCs of the neutral species relies on the assumption that electron transfer occurs on a shorter time scale than nuclear relaxation. This approach has led to consistent results (in comparison to studies both in ultra-high-vacuum and condensed phase) in the analysis of the electrochemically induced dissociation of organic compounds.13 To gain insight into the dissociation dynamic process of the neutral radicals, plots of the Caryl—I and Calkynyl—I bond distances as functions of time were employed. Figure 6 reports the bond distances corresponding to the v1 (Figure 6a) and v2 (Figure 6b) initial velocity vectors for the radical pF as a function of time. The solid line represents the Caryl—I distance, whereas the dotted line describes the time evolution of the Calkynyl—I bond length. The same type of DRC data are presented in Figure 7 for the radical pNO2. From a qualitative point of view, Figures 6 and 7 share rather similar features. For both radicals, the v1 initial conditions (Figures 6a and 7a) lead to the dissociation of the Caryl—I bond (solid lines), whose distance increases almost linearly with time, whereas the Calkynyl—I bond distance (dotted lines) oscillates about an effective mean value. Thus, the v1 initial conditions set the dynamics toward the formation of the aryl radical (see Scheme 2), which can act as the relevant grafting species. A linear fit of the solid-line data (Figures 6a and 7a) suggests that the dissociation of the Caryl—I bond proceeds with similar speeds in the two species, as average slopes of about 5.5/ 300 and 5/300 Å 3 fs1 were obtained for pF and pNO2, respectively. 11720
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can clearly see that, for pF, a deeper energy minimum than the one at t = 0 fs was achieved around 35 fs and that, after about 40 fs, the energy profile was, on average, almost constant. On the other hand, in the time interval between 0 and 50 fs, the v2 DRC trajectory obtained for pNO2 (Figure 8b, black line) shows a single high-energy peak, whereas the v1 DRC trajectory goes through four energy minima (Figure 8b, red line). It is interesting to note that, for pNO2 (at variance with the results found for pF), the v2 DRC trajectory also leads, on average, to a downhill pattern for the energy profile. The DRC results reported in Figure 8a,b suggest that the switching-on of the Calkynyl—I stretching vibrational normal mode (v2 initial conditions), which can lead the system toward the dissociation of the Calkynyl—I bond, is energetically more favorable for pF than for pNO2. On a longer time scale (>200 fs), a comparison of the DRC energy trajectories shows that (i) for pF, the v1 and v2 DRC energy values are, on average, similar, whereas (ii) for pNO2, the v2 initial conditions lead to an uphill DRC energy pattern (Figure 8b, for time > 150 fs).
Figure 8. UB3LYP/6-311G** DRC trajectories of perpendicular (a) pF and (b) pNO2 neutral species. Red curve: dissociation of the CAryl—I bond. Black curve: dissociation of the CAlkynyl—I bond.
The analysis of DRC data in Figures 6b and 7b clearly shows that the v2 initial conditions favor Calkynyl—I bond dissociation for both pF and pNO2 (the corresponding bond distances increase almost linearly with time, as indicated by the dotted lines) and, hence, the subsequent formation of the alkynyl radical (see Scheme 2). In this case, however, two major differences can be observed: (i) The dotted line in Figure 7b (i.e., the Calkynyl—I bond length of pNO2) is the only one that shows, in the first tens of femtoseconds, a decrease of the bond length, even though (for v2) the bond is eventually forced to dissociate. (ii) The linear fit of this set of data (i.e., the dotted-line data in Figure 7b) leads to an average slope of 2/300 Å 3 fs1, which is one-half of the value obtained by the linear fit of the dotted-line data reported in Figure 6b. This theoretical prediction can be used to understand the experimental evidence for the different alkynyl/aryl grafting ratios that characterize the pF and pNO2 iodonium salts. In Figure 8a, DRC data relevant to the neutral radical pF are reported, where the red and black curves represent the energy versus time profiles obtained using v1 and v2, respectively, as the initial velocity vectors. Figure 8b shows the same type of DRC trajectories for neutral pNO2. After the initial “thermalization” transient, the v1 DRC trajectories calculated for both pF and pNO2 show energy profiles that, on the average, display quite similar downhill patterns. Thus, using this set of initial conditions, one can conclude that there are no significant differences in the dissociation dynamics of the two neutral radicals. In contrast, a rather distinct behavior is exhibited for the v2 initial conditions. Focusing on the black curve of Figure 8a, one
’ CONCLUSIONS DRC calculations show that the neutral pF and pNO2 iodonium radicals feature qualitatively different dissociation dynamics. In all of the calculated DRC trajectories, the “lifetime” of the radical did not exceed the time corresponding to a single oscillation of the dissociating bond. Thus, the dissociation mechanism, following the electron transfer, can be described as a dissociative rather than a concerted mechanism, as was previously found for benzyl chloride radical anion dissociation.13 On the whole, the theoretical DRC results are consistent with the experimental evidence of an almost-complete grafting of the electrode surface by the aryl moiety in the case of the pNO2 derivative, whereas this at variance with the 1:1 aryl/alkynyl grafting ratio found in the case of the pF compound.21 Thus, calculation of DRC trajectories allows the reliable prediction of the alkynyl/aryl grafting ratios in asymmetric iodonium compounds and the assessment of the relevant dissociation mechanism. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by the Italian Ministry of University and Research (MIUR) through PRIN 2008 (Project 2008N7CYL5) and by the Fondazione Cassa di Risparmio di Modena (Progetto Prot. N. 1297.08.8C). ’ REFERENCES (1) Buriak, J. M. Chem. Rev. 2002, 102, 271. (2) de Villeneuve, C. H.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415. (3) Buriak, J. M. Chem. Commun. 1999, 12, 1051. (4) Weissmann, M.; Baranton, S.; Coutanceau, C. Langmuir 2010, 26, 15002. (5) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757. (6) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429. (7) Liu, J.; Cheng, L.; Liu, B.; Dong, S. Langmuir 2000, 16, 7471. (8) Adenier, A.; Chemini, M. M.; Gallardo, I.; Pinson, J.; Vila, N. Langmuir 2004, 20, 8243. 11721
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