Influence of the Nature of Self-Assembled Monolayers on Their

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Influence of the nature of self-assembled monolayers on their reactivity under ionizing radiation: a solid state NMR study Sophie LeCaer, Francine Brunet, Caroline CHATELAIN, Lionel LADEVIE, Delphine DURAND, Vincent Dauvois, Jean Philippe RENAULT, and Thibault Charpentier J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 Oct 2013 Downloaded from http://pubs.acs.org on October 1, 2013

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The Journal of Physical Chemistry

Influence of the Nature of Self-Assembled Monolayers on their Reactivity under Ionizing Radiation: A Solid State NMR Study 1*

S. Le Caër , F. Brunet2, C. Chatelain1, L. Ladevie1, D. Durand3, V. Dauvois3, J. Ph. Renault1, T. Charpentier2 1

Institut Rayonnement Matière de Saclay Service Interdisciplinaire sur les Systèmes Moléculaires et les Matériaux, UMR 3299 CNRS/CEA SIS2M Laboratoire de Radiolyse, Bâtiment 546 F-91191 Gif-sur-Yvette Cedex, France 2

Institut Rayonnement Matière de Saclay Service Interdisciplinaire sur les Systèmes Moléculaires et les Matériaux, UMR 3299 CNRS/CEA SIS2M Laboratoire Structure et Dynamique par Résonance Magnétique, Bâtiment 129 F-91191 Gif-sur-Yvette Cedex, France 3

CEA/SACLAY DEN/DANS/DPC/SECR/LRMO F-91191 Gif-sur-Yvette Cedex, France * Corresponding author: phone: 33 1 69 08 15 58; fax: 33 1 69 08 34 66; email: [email protected]

Abstract The behavior of different nanoporous grafted silica (-Si(CH3)2R) under ionizing radiation is investigated as a function of the chemical nature of the grafting. The structure and reactivity of the grafting after irradiation is characterized by solid-state NMR spectroscopy. The chemical

reactivity

is

also

investigated

by

mass

spectrometry

gas

analysis

. The major gases detected after irradiation are H2 and/or CH4. Different types of behavior under ionizing radiation are depicted as a function of R. When R contains an aromatic ring or is the cyclopentadienyl moiety, then the overall measured radiolytic yields are low and CH4 is the major gas detected. The aromatic ring acts as an efficient energy trap. When R contains an alkyl chain with a -CN ending, the major gas detected is dihydrogen due to the lysis of the – CH(H) bond, but it is also shown that the –CN group acts as an energy trap, even if it is of course less efficient than an aromatic ring. When R consists of a long alkyl chain, an efficient energy transfer occurs at the interface and leads to a significant H2 production due to the lysis

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of the –CH(H)- bond. Lastly, when R is small (a methyl or an ethyl group), the situation is different and the Si-C bond is preferentially cleaved over the C-H bond.

Keywords SAM, grafted silica, radiolysis, MAS NMR, bond cleavage selectivity

Introduction The modification of silica surfaces using organic molecules and leading to the formation of organosilicas is relevant in many fields of chemistry, including their use in reversed-phase high performance chromatography1,2, catalysis3,4, and the development of hydrophobic materials5. The attached groups change the surface properties, for instance the hydrophobicity, leading to an improved stability of the resulting materials6. Grafting organosilanes on silica surfaces leads to the formation of self-assembled monolayers (SAMs). The understanding of chemical effects induced by irradiation on SAMs is crucial in electron beam lithography. Whereas the behavior of alkanethiolate SAMs on gold under ionizing radiation has been widely studied,7-10 data dealing with irradiation of organosilicon layers remain scarce and have mainly focused on reaction mechanisms induced by radiolysis in silica containing unsaturated oligomers.11-15 In a previous study,16 we have focused our attention on a controlled pore glass (CPG) having a 8 nm pore size diameter and grafted with chlorodimethylsilane (ClSi(CH3)2H). Solid state NMR spectroscopy has proven to be very helpful in characterizing the grafted silica surface before and after irradiation and to unravel the corresponding reaction mechanisms.16 Initially, radiation induces electron-hole pairs (R1) which can recombine via an exciton state (R2). SiO2 radiation  → e − +h + (R1) e− + h+  → 3 exciton (R2)

These excitonic species can then induce efficient energy transfer at the surface of the glass and a radical chemistry. The glass grafted with chlorodimethylsilane has proven to produce dihydrogen with a high yield (3.3 10-7 mol/J) due to the very efficient cleavage of the Si-H bond. The detection of methane in smaller quantities indicates that the Si-H bond is preferentially cleaved over the Si-C bond. Nevertheless, this selectivity cannot be explained through thermodynamic data. The purpose of the present study is to vary the nature of the organosilane to compare the reactivity under irradiation of different grafted groups (Table 1). We have focused our attention on (3-cyanopropyl)dimethylsilane (named hereafter as Cyano), dimethyloctylsilane 2 ACS Paragon Plus Environment

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(named as Octyl) but also on dimethylphenethylsilane (named as Phen) and dimethyl(2, 3, 4, 5-tetramethyl-2,4-cyclopentadien-1-yl)silane (named as Cp) to determine the influence of aromatic groups on the reactivity. Trimethylsilane (TriMet) and triethylsilane (TriEt) were also chosen to allow the comparison of the reactivity under irradiation between methyl and ethyl groups. The grafted glasses were then submitted to gamma-rays to measure radiolytic yields. In order to get more insight on the characterization of the surface before and after irradiation,

we

studied

more

carefully

the

systems

grafted

with

chloro(3-

cyanopropyl)dimethylsilane and with chlorodimethylphenethylsilane using solid-state NMR. To get significant differences before and after irradiation, these two samples were irradiated using accelerated electrons at a total dose of 500 kGy. Experimental section Materials

The controlled pore glass (CPG) used in the present work is commercially available from Millipore and CPG, Inc. We worked here with the glass having the smallest pore size (i.e. 8 nm) and thus the highest surface area (200 m2/g). The borosilicate glass is composed of 96.5 % of SiO2 and 3 % of B2O3.17 The dried material is obtained after baking at 140°C for one hour and then at 500°C during 6 hours in order to remove organic contaminants and adsorbed water molecules. The glass is then stored overnight at 60°C before drying it under vacuum at 200°C for one hour to further remove physisorbed water. This control of the water content of the sample is an important issue as water molecules adsorbed on the surface prevents an efficient grafting of the surface. More details on the used glasses can be found in 16. The different monochlorosilanes were purchased from Sigma-Aldrich and used as received. We chose to work with monochlorosilanes as these compounds prevent the formation of a multilayer on the surface. Silylation of surfaces is often performed in the liquid phase. Here we chose a gas phase method. The silane is vaporized at an appropriate temperature at a pressure of 1 mbar (Table 1). The silanols of the glass are exposed to the chlorosilane vapor for three hours. After reaction, the grafted glass is placed on a glass filter, washed first with toluene, then acetone, a 3:1 acetone/water mixture (3x) and finally with acetone, and then dried in an oven at 60°C.



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The grafting of the surface is followed by elemental analysis, by FT-IR spectroscopy and NMR measurements in the case of the Phen and Cyano samples, as the obtained spectra can be then compared with reference literature data.18

Elemental analysis

Before and after grafting, elemental analyses for C were carried out by the Service Central d’Analyse, Vernaison, France. Before grafting, no carbon is detected. The grafting density (ρ, groups/nm2) can be calculated using the following equation:

ρ=

6 × 10 5 (%C ) 1 1200nc − MW (%C ) S ( BET )

Where %C is the carbon percentage in the sample, nc is the number of carbons in the grafted group, MW is the molecular weight of the grafted group and S(BET) is the Brunauer-EmmettTeller (BET) surface are of the glass before grafting (200 m2/g). The results are summarized in Table 2 for the different silanes. Except in the case of the TriEt and Cp compounds, the grafting density is very similar in all cases (around 1.2 group/nm2) and is also consistent with the value obtained using chlorodimethylsilane.16 The smaller grafting densities obtained for TriEt and Cp can be attributed to a greater steric hindrance of the groups. FT-IR spectroscopy

IR spectra of the samples were recorded on a Bruker Vertex 70 FT-IR spectrometer from the glass (1 %) dispersed in potassium bromide (KBr) pellets. The powder was compacted into thin self-supporting discs for IR transmission studies using a pressure of about 109 Pa for five minutes. Spectra were collected over the range of 4000 to 400 cm−1 at 4 cm−1 resolution from 100 scans. The background was subtracted in all cases.

Solid-state NMR measurements 1

H,

13

29

C (see Supporting Information) and

Si experiments were performed at room

temperature on a Bruker Avance 300 WB spectrometer operating at a Larmor frequency of 299.5 MHz for 1H, 75.31 MHz for 13C and 59.59 MHz for 29Si, using a 4 mm Bruker crosspolarization (CP) magic angle spinning (MAS) probe with a spinning frequency of 12.5 kHz. Powdered samples of about 50 mg were quickly introduced in 4 mm o.d. zirconia rotors in order to prevent any further water uptake. To allow comparison between different NMR spectra, the mass sample was carefully weighed and known in each case. All NMR spectra 4 ACS Paragon Plus Environment

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were simulated using a software developed by one of the authors.19,20 The experimental details are carefully described in a previous paper and are briefly given below.16 1

H MAS NMR

Briefly, single-pulse excitation (SPE) spectra were recorded using a 1H 90° pulse length of 1.90 µs. Variable recycle delays (ranging from 2 s to 8 s) were tested to check that no change in the signal intensity was observed. The 1H chemical shifts were referenced using tetrakis (trimethylsilyl) silane (δ = 0.2 ppm). 1

H Hahn echo MAS spectra were acquired with the pulse sequence 90°- τ – 180°- τ -

acquisition with a rotor-synchronized echo delay τ varying from 80 µs to 80 ms. This experiment (also referred to as dipolar dephasing experiment) is helpful in contrasting protons according to their T2, that is their proton neighbourhood and has been successfully applied to silica gels.21,22 Here, as will be discussed in the text, this experiment was found useful to discriminate between the rigid and mobile components of each detected peak. To account for the lineshape of the observed peaks, it was found necessary to introduce a special lineshape that will be discussed in the text. To improve the resolution and to gain more insight into the proton-proton interactions, two-dimensional 1H-1H homonuclear double quantum (DQ) correlations experiments were performed using the back to back (BABA) pulse sequence.23-25 Complementary 2D exchange experiments sequence (90° - t1 – 90° - tExch – 90° - acquisition) were also performed to observe signals arising from more mobile components which are not detected in the 2D DQ correlation experiments. For both experiments, typically 192 FIDs (Free Induction Decays) were acquired in t1 dimension with a 80 µs (i.e. rotor synchronised) increment. 13

C MAS NMR (see Supporting Information)

Single-pulse excitation (SPE) spectra with proton decoupling (80 kHz) were recorded using a 13

C 90° pulse length of 5 µs, with a recycle delay of 4 s and a spinning frequency of 12.5 kHz.

Typically 31000 scans were collected. The

13

C chemical shifts were referenced using L-

alanine enriched in 13C nuclei as an external reference (δ = 19.8 ppm for the methyl group). 29

Si CP MAS NMR

The primary aim of the Cross-Polarization (CP) MAS experiment is to increase the magnitude of the signal to noise ratio of low sensitivity nuclei like 29Si (4.7% natural abundance and long 5 ACS Paragon Plus Environment


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relaxation time) thanks to the polarization transfer from abundant protons to neighbouring silicon sites.26-29 The silicates tetrahedra are usually named Qn where Q represents the SiO4 tetrahedron and n is the number of other tetrahedral to which it is linked. CP-MAS 1H-29Si spectra were acquired using a spinning frequency of 12.5 kHz, a recycle delay of 2 s and contact times of 4 and 19 ms. 12 000 scans were necessary to obtain a good signal to noise ratio in our samples (pristine and irradiated). The

29

Si chemical shifts were

referenced using tetrakis (trimethylsilyl) silane as an external reference (two peaks at -9.9 and -135.3 ppm with respect to liquid tetramethylsilane).

Radiolysis: irradiations with gamma-rays and with 10 MeV electrons

The irradiations were performed in an ampoule under argon atmosphere. To study gas productions, γ-irradiations were performed using a Gammacell 3000

137

Cs

source. The dose rate was 5.85 Gy/min as determined using the Fricke dosimeter.30 In order to get high doses to characterize the changes induced by irradiation by means of spectroscopic techniques, irradiations were also performed using the electron pulses of a Titan Beta, Inc. linear accelerator which delivers electrons of 10 MeV energy. The energy bandwidth of the electrons is ± 15% 31. In the present experiments, 10 ns pulses of 10 MeV electrons were used and we worked at a repetition rate of 10 Hz. A dose of 20 Gray per pulse was determined using the Fricke dosimetry.30 We have checked that the dose determined by this method is analogous to the dose calculated using the SCN- dosimeter in pulse radiolysis. In both irradiation conditions (using gamma-rays or accelerated electrons), considering the stopping powers of electrons in silica and water (ESTAR program32), the dose received by silica was calculated to be the same as the dose determined with the Fricke dosimeter. We checked by measuring the gas yields that the behaviours of the grafted glasses are the same, within the error bars, using these two irradiation conditions. The results we obtain can be then confidently compared.

Gas analysis

After irradiation, gas analysis was performed by mass spectrometry with a quantitative gas mass spectrometer with a direct inlet for chemical and isotopic analysis equipped with an electron impact ionization source, a magnetic sector for mass separation, a Faraday cup and an electron multiplier detector. Glass containers were tightly connected to the gas line. The gas mixture was then expanded to the mass spectrometer. Gas volumes and pressures were measured. Afterwards, GC/MS studies were performed with an Agilent 6890 GC system 6 ACS Paragon Plus Environment

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interfaced with an Agilent 5973 MS system with an electron impact ionization source, a quadrupole analyser for mass separation and an electron multiplier detector. More experimental details can be found in reference 16.

Results and discussion.

Characterization of the grafting of the silica surface by FT-IR spectroscopy The FT-IR spectra of the grafted glasses are displayed in Figure 1 and the corresponding wavenumbers are given in Table 3. The 370-1600 cm-1 is characteristic of silica and the effect of the grafting can be seen in the 2600-4000 cm-1 spectral region corresponding to CHx (x = 1-3) and OH modes (Figure 1b), and also around 2250 cm-1 in the case of the Cyano compound (CN stretching mode). From Figure 1 it is clear that the surface of the glass has been grafted.

Irradiation effects on the grafted glasses

Gas analysis and dose rate effects The radiolytic yields of production of the main gases obtained after gamma irradiation are displayed in Table 4. The main gases obtained are H2 and CH4. A small formation of CO and CO2 was also observed and attributed to reactions with residual O2 in the atmosphere of the glass container. Various hydrocarbons are also detected in small quantities. To detect the changes induced by irradiation on the grafted glasses by means of spectroscopy, the samples underwent an irradiation at a total dose of 500 kGy using 10 MeV accelerated electrons (high dose rate conditions). The radiolytic yields are measured at lower doses using gamma ray irradiation (low dose rate conditions, Table 4). We have checked that the radiolytic yields arising from the organic moiety (typically CH4) are the same under high or low dose rate conditions. Differences are only observed for the H2 yield measurements. H2 arises from the inorganic grafted part but also from the lysis of silanol or adsorbed water molecules on the surface of the glass. This lysis of silanol or adsorbed water molecules on the surface of the glass is negligible under high dose rate conditions as compared to low dose rate irradiation.33-35 For the present samples (Table 4), a 1.2 10-8 mol.J-1 H2 yield value measured under gamma irradiation corresponds to the radiolysis of non-grafted silanol groups and residual adsorbed water molecules. Therefore this 1.2 10-8 mol.J-1 yield was subtracted from 7 ACS Paragon Plus Environment


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the H2 yields in order to evidence the H2 production arising from the inorganic grafted part (Table 4). This residual H2 production is explained by the fact that silanol groups are indeed trapping sites for excitons (R3).34 The resulting H• atoms dimerize forming H2 (R4). 3

exciton+ ≡ SiOH  → RO • + H • (R3)

H• +H•  → H 2 (R4)

This H2 yield we report here for the radiolysis of surface silanol groups under gamma irradiation (1.2 10-8 mol.J-1) is smaller, but of the same order of magnitude than the one measured in reference35. This is due to the more severe thermal treatment undergone in this study by the glasses prior to irradiation, leaving less trapping sites for excitons.35 Keeping this in mind, the gas measurements and the spectroscopic data can be compared.

The gaseous radiolytic yields presented in Table 4 for the different samples can then be rationalized. The grafting with dimethylchlorosilanes leads to the major formation of H2 and/or CH4. Other hydrocarbons can be detected but in a lesser extent. Similarly, grafting with triethylchlorosilane leads to the preferential formation of ethane (TriEt sample). The Phen, TriMet and Cp systems exhibit the same and smallest H2 radiolytic yields (about 1.2-1.3 10-8 mol/J) (Table 4). As discussed before, these measured H2 yields are attributed to the lysis of – OH groups on the surface under low dose rate irradiation. The major gas detected in these systems and corresponding to irradiation of the inorganic grafted part is then methane. When the alkyl chain lengthens and when strongly energy trapping sites such as the aromatic ring are no longer present (case of the Cyano and Octyl samples), then H2 becomes the major product detected and methane is formed in smaller quantities.

Considering the spectroscopic data, the IR spectra recorded before and after irradiation are quite similar, even if the signals due to the grafting seem to be less intense after a 500 kGy irradiation (see for example Figure 1 in Supporting Information); therefore these spectra will not be discussed below. Nevertheless, the anchoring of organosilane on silica can be carefully described using NMR characterizations. This was performed on two samples: the Phen and Cyano ones whose spectra can be compared with reference literature data.18 Moreover, solid state NMR spectroscopy has proven to be more appropriate to get insight into the fate of the surface after irradiation.16 The NMR spectra obtained for the Phen and Cyano samples before and after a 500 kGy irradiation are then given below. 8 ACS Paragon Plus Environment

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Characterization of the Phen and Cyano samples before and after irradiation by means of NMR spectroscopy

The Phen sample A.

1

H MAS NMR

As shown in Figure 2, the 1H MAS NMR (SPE) spectra of the CPG 8 nm sample grafted with chlorodimethylphenethylsilane (-Si(CH3)2CH2aCH2bC6H5) is characterized by a broad resonance line at about 0.25 ppm (CH3 protons) with a shoulder at 0.60 ppm, (CH2a protons), another line at 2.45 ppm (CH2b protons) and a fine resonance at 6.85 ppm corresponding to aromatic protons. To support our peak assignment and to get additional structural information, 2D 1H Double Quantum (DQ) MAS spectra allowing probing spatial proximities between the protons were acquired. The 2D DQ 1H MAS NMR spectrum presented in Figure 3a is characterized by intense (diagonal) autocorrelation peaks (CH3, CH2b and C6H5) and also by off-diagonal cross-correlation peaks as in the case of aromatic protons. The autocorrelation peaks mean that these protons are near each other and likely in the same molecule. Cross peaks are clearly observed between the aromatic protons (6.85 ppm) and CH3 protons (0.25 ppm), CH2b (2.45 ppm) and also CH2a protons at 0.60 ppm (weaker intensity). Besides the expected presence of correlations between adjacent protons in the alkyl chain, the correlation between C6H5 and CH3 protons could be attributed to the DQ excitation duration which could enable, in the strongly dipolar coupled proton network, multiple-bonds correlation to appear (the minimal duration for the BABA pulse sequence is one rotor period, i.e 80 µs). Other explanations would be the formation of multilayers or folding of the grafted molecules. The formation of multilayers can be rejected because of the chemical route used in this work, the grafting being performed with monochlorosilanes. Assessment of the second mechanism (i.e. folding of grafted molecules) would require further experiments, such as the detailed analysis of the DQ spectra collected at various DQ excitation/reconversion times but the strong limitation of a rotation period as the minimal time-step can be used. It was not possible to observe the initial growth of the DQ coherence with a time-step of 80 µs for this sample, confirming the strength of the dipolar couplings involved and thus supporting the idea that some of the observed correlations could involve more than two protons.



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As explained in our previous paper,16 2D 1H-1H exchange experiments (EXSY) are more sensitive to mobile components and give therefore supplementary information with respect to 2D DQ 1H MAS NMR experiments. The 2D 1H exchange spectrum at 8 ms exchange time (Figure 3c) shows the intense autocorrelation peaks (CH3, C6H5, CH2b and also CH2a) as already observed in the 2D DQ 1H MAS NMR spectrum (Figure 3a), but also a supplementary broad resonance extending from about 2.5 ppm to 6 ppm. This broad resonance corresponds to mobile hydroxyl protons Si-OH which are not visible in the 2D DQ 1

H MAS NMR spectrum Rotor-synchronized spin echo

1

H MAS NMR (Figures 2a-b in Supporting

Information) spectra evidences that the narrow resonance at 6.85 ppm attributed to aromatic protons is characterized by a longer T2 value than the other resonances. Close examination of the variation of the 1H MAS NMR spectra with the echo delay (see Figure 2b in Supporting Information) confirms the chemical shifts observed on the MAS NMR SPE spectrum but also allows to extract the SiOH band (resolved for a long echo delay of 8 ms). This latter band is then used for the deconvolution of the MAS NMR SPE spectrum. An important feature is also observed for each peak of the grafted molecule: its linewidth decreases (Figure 2b in Supporting Information for example) when the echo delay increases. Nevertheless, the position remains at the same isotropic chemical shift value. The SPE spectrum has therefore to account for a distribution of grafted molecule mobility, thus of T2. Using Inverse Laplace Transform, as done in our previous work,16 revealed a rather continuous but clear distribution of T2 (Figure 4). Because of their good resolution, the C6H5 and CH3 lines could be used to assess new lineshapes that we introduce in this work for fitting the MAS spectrum. A proper choice of a lineshape will be shown to be even more critical in the case of the Cyano sample. After numerous attempts, it was found that, in order to simultaneously account for the broad component that characterize the rigid (short T2, observed in DQ spectra) and more mobile (long T2, observed in EXSY spectra) components but also for the continuous distribution of these components (i.e. distribution of T2), an exponential distribution convoluted by a Lorentzian function

was found to yield the most satisfactory results, as shown in Figure

3 in Supporting Information. A single alpha exponent (α) and a single Lorentzian broadening (lb) parameter were used for all bands (except the SiOH ones for which Gaussian lineshapes were found to provide the best result):


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 1  ν − ν iso  α  f (ν ) = exp −    ∗ Lλ (ν ) (Eq. 1)  α  σ  

Note that for α = 2, one recovers the Gaussian distribution. The features of the exponential distribution (convoluted by a Lorentzian function, see Eq.1) for an improved fit of the peak are the following i) sharper at the top and ii) broader at the bottom. For SiOH, the line was determined from the fit of the spectrum acquired at a long echo delay (i.e. four components from 2.8 ppm to 5.6 ppm with ~0.8 ppm interval and a Gaussian lineshape). For purpose of comparison, Figure 3 in Supporting Information presents the deconvolution using the popular mixed Lorentzian/Gaussian lineshape and the model presented above that, in the case of the phenyl sample only, leads to very close results. Using Equation 1, populations could then be extracted from the MAS spectrum and were found to be in good agreement with the theoretical percentages, yielding 2% as an estimate of the accuracy of the populations (Table 5). This simulation (Equation 1) of data was used to fit the NMR spectra throughout this work, as this model takes into account rigid and more mobile components. It enabled to perform satisfactory fits for both the Phen and Cyano samples before and after irradiation and then to extract properly the different populations.

The

1

H

MAS

NMR

spectrum

of

the

CPG

8

nm

grafted

with

chlorodimethylphenethylsilane and irradiated at 500 kGy is presented in Figure 2 after normalization to the same sample mass as the pristine sample. The important feature concerns the decrease of all the resonance lines, particularly the C6H5 and CH3 resonances at 6.85 ppm and 0.25 ppm. The global loss of signal intensity is estimated to be about 17%. This loss is easily explained by the production of gas like methane and benzene. No influence of the irradiation on the 1H relaxation time T1 is observed as in the case of the non-grafted CPG 8 nm36 meaning that if paramagnetic species are formed after irradiation, they have no effect on the spin-lattice relaxation time. As shown in Figure 4 in the Inverse Laplace Transform data, no significant change is observed after irradiation except a slight shortening of the global T2 in agreement with a broadening of the spectrum which suggest an overall reduction of the grafted molecule mobility after irradiation.



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After irradiation only weak modifications are visible on the 2D 1H DQ MAS spectrum as shown in Figure 3b. There is a loss of intensity and of resolution in the 2D spectrum in agreement with the 1H MAS NMR spectrum (Figure 2) and with the production of gas after irradiation. The intensities of autocorrelation peaks have slightly decreased like those of the cross peaks between C6H5 and CH3 and CH2a. Let’s point out that the overall number of C6H5 and of CH3 groups has decreased, which implies that irradiation has damaged these groups (leading to the formation of CH4 or benzene). Considering the overall loss of signal intensity (17%) of the NMR spectrum after irradiation and the error bars in the population estimation (2%), the number of CH2 sites remains constant after irradiation, meaning that these sites are hardly affected by ionizing radiation. These NMR data are in agreement with the gas measurements which evidence that methane and benzene are the major gases formed after irradiation, whereas ethylbenzene is detected in smaller quantities (Table 4). This means also that the Si-Cprimary (Si-CH3) bond, and to a lesser extent the CH2-C6H5 bond, are more prone to be cleaved than the Si-Csecondary one (SiCH2-). The more resistant bond is of course the C-C bond in the aromatic ring.

B.

13

C MAS NMR 13

C MAS NMR spectra are shown in Figure 4 in Supporting Information. A global

decrease of 21 % of the

13

C signal is observed after irradiation. No additional 13C species is

observed: this can be explained either by the fact that the formed products are in too small quantity to be observed or that their chemical shift is close to the one of an initial species.

C.

29

Si CP MAS NMR

The

29

Si CP MAS NMR spectra of the Phen sample have been recorded at different

CP contact times (19 ms and 4 ms) and are presented on Figure 5 along with the simulations. Three main resonances at -92.8, -101.0 and -110.0 ppm are clearly observed. They are respectively attributed to Q2, Q3 and Q4 sites in CPG 8, as previously reported.36 Another fine and intense resonance is also observed at 13.8 ppm and attributed to the monomer M (OSi(CH3)2-CH2R)37 proving that the silane is covalently bounded to the surface of silica38. At short contact time (4 ms) the relative intensities of M or Q2 are optimized (meaning that these sites are close to protons), whereas the intensities of Q4 are optimized at longer contact time (19 ms) due to further polarizing protons. The relative population of the various sites is given 12 ACS Paragon Plus Environment

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in Table 6. Comparatively to the results obtained for the non-grafted CPG 836 we observe that the Q4/Q3 ratio has greatly increased (from about 1.25 to 2.05 after grafting) and that the population of the Q2 site is much weaker (about 1% instead of 11%) in agreement with the fact that the grafting reaction has taken place at the surface of silica. Moreover, the near disappearance of the Q2 sites indicates that they are more reactive towards grafting than the Q3 sites, as already observed.16,39 Lastly, the increase of the Q4/Q3 ratio means a greater degree of polymerization in the sample after grafting. The 29Si CP MAS NMR spectra of the Phen sample (Figure 5) show modifications after irradiation: first, the monomer M (OSi(CH3)2(CH2R)) has decreased whereas new sites appear at -7.6 ppm and -17.2 ppm respectively attributed to D1 (-OSi(CH3)2OH) and D2 (-(O)2Si(CH3)2) species.37,40,41 The formation of these new species can account for the additional peaks evidenced after irradiation in the box in dashed line in Figure 4. Moreover, an increase of the Q4/Q3 ratio from 2.08 to 2.37 (Table 6) is observed after irradiation.

The Cyano sample A. 1H MAS NMR

The 1H MAS NMR (SPE) spectra (Figure 6) of the CPG 8 nm sample grafted with chloro(3cyanopropyl)dimethylsilane (Cyano sample) is characterized by a broad resonance at about 0.1 ppm with a shoulder at 0.8 ppm, two others resonance lines at 1.8 ppm and 2.3 ppm and also a large resonance attributed to Si-OH protons which give rise to the shoulder from 3-4 to 6 ppm. The resonances are respectively assigned to CH3 protons (0.1 ppm), CH2a (near CH3) protons (0.8 ppm) and the others to CH2b (1.8 ppm) and CH2c (2.3 ppm) protons near the CN group.42 In contrast to the phenyl sample, the MAS spectrum is here much broader. This is confirmed by the 2D DQ 1H MAS NMR displayed in Figure 7a which is characterized by a poor resolution of the resonance lines. Intense autocorrelation peaks particularly for CH3 and CH2c protons are observed that reflect the existence of strong dipolar interactions between these protons which are spatially near each other (d < 5Å). As the width of the lines is characteristic of stronger interactions as compared to the sample grafted with chlorodimethylphenethylsilane, cross peaks are much less well defined and correlation peaks between the CH3 and different types of CH2 protons are more difficult to characterize. These data suggest that dipolar couplings in the Cyano proton network probed by DQ NMR (i.e. the rigid lattice) are greater than in Phen. The 2D 1H exchange spectrum at 8 ms exchange time (Figure 7c) shows exchange between CH3 protons, CH2c and CH2b protons. A broad 13 ACS Paragon Plus Environment


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resonance extending from about 3 to 6 ppm is observed and attributed to hydroxyl protons SiOH which are mobile species and hence not visible in the 2D DQ 1H NMR experiment. Rotor-synchronized spin echo 1H MAS NMR (Figure 5 in Supporting Information) also shows the highly heterogeneous variation of the peak widths with the echo delay, as already observed for the phenyl sample but here in a much more pronounced manner. Moreover, the SiOH broad peak appears much more clearly. Inverse Laplace Transform of these data is given in Figure 6 in Supporting Information. In comparison to the phenyl sample, the larger width of the peak and the shorter T2 behaviour suggest that the Cyano grafted molecules are globally much less mobile than the phenyl ones. For this sample, an exponential distribution convoluted by a Lorentzian function

was the only way to

satisfactorily fit the spectra (fit shown in Figure 7 in Supporting Information). Combining the constraints from all these data, the relative populations of the various protons can then be estimated and are given in Table 7. Because of the poor resolution of the spectra, the population of the different types of CH2 had to be constrained to be in the expected ratio (1/1/1). The

spectrum

of

the

CPG

8

nm

sample

grafted

with

chloro(3-

cyanopropyl)dimethylsilane and irradiated at 500 kGy is presented in Figure 6 after normalization to the same sample mass as the glass before irradiation. The most striking feature concerns the strong decrease of the intensity of the signal (37%). An important feature concerns also the very net loss of resolution of all the resonance lines, which can be ascribed either to an increase of structural disorder or to a decrease of grafted molecule mobility (more rigid proton network leads to an increase of the widths) after irradiation. This stronger loss intensity measured here (37%) than in the preceding case (17%) is consistent with the fact that the gases are released in greater quantities with the Cyano sample than with the Phen one (Table 4) and evidence also the radiation resistance effect of the aromatic ring as compared to other chemical groups. The behaviour versus the relaxation time T2 is shown on Figure 6 in Supporting Information. Comparatively to the pristine sample, the resonance lines of the CH3 and CH2c protons have nearly disappeared at 3.2 ms meaning a shorter T2 value. This could be due to the interactions with Si-OH groups which are clearly visible between 3 and 6 ppm. Unfortunately, due to a lack of resolution, SiOH/CH2 correlations cannot be observed in Figure 7. As expected, the 2D DQ 1H MAS NMR (Figure 7b) presents a very broad contour plot and shows a large distribution of protons signals with a resolution still much weaker than for the pristine sample: indeed except for the CH3 and CH2c protons the other signals are not clearly 14 ACS Paragon Plus Environment

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visible. The decrease of the T2 value supports the idea that the observed broadening is more likely due to an increased rigidity, or at least to the increase of the rigid population. The 2D 1H exchange spectrum at 8 ms exchange time (Figure 7d) shows that the various groups of protons are in exchange with a loss of resolution more important than for the pristine sample. The 2D map shows the spatial proximity between the CH3 protons and Si-OH hydroxyl protons which are mobile and are characterized by a large distribution of environments. The 1H MAS NMR spectrum has been then simulated from the same types of protons as in the case of the pristine sample and the population of the various protons has been quantified. The results presented in Table 8 with an uncertainty of ± 2% indicate a stable percentage of all the CH2 protons, and more generally considering the error bars, of all protons. Nevertheless, considering the strong decrease of the signal intensity (37%), the overall number of CH2 and CH3 protons has decreased after irradiation. This is consistent with the fact that H2 and CH4 are produced upon irradiation.

B.

13

C MAS NMR 13

C MAS NMR spectra are shown in Figure 8 in Supporting Information. A global

decrease of 24% of the

13

C signal is observed. As in the preceding case, no additional

species could be observed. Interestingly, the global decrease of the

13

13

C

C is very similar in the

Phen and Cyano samples, whereas the decrease of the proton signals is clearly enhanced for Cyano. This can be linked to the fact that the H2 production due to the organic moiety is greater in this case, whereas the production of carbon-containing molecules is of the same order of magnitude in the two cases.

C.

29

Si CP MAS NMR

The 29Si CP MAS NMR spectra of the Cyano sample have been recorded at different CP contact times (19 ms and 4 ms) and are presented on Figure 8 along with the simulations. The populations of the various sites are given in Table 8 after simulation of the spectra. As in the Phen sample case, the Q4/Q3 ratio is equal to 2.08 (greater than the 1.25 value found for the non-grafted CPG 8 nm) and the population of Q2 sites is much weaker (1% instead of 11%) after grafting, meaning an increased reactivity of Q2 sites towards grafting as compared to the Q3 ones. The Q4/Q3 ratio is very close to the value obtained in the Phen case (2.05). This can be linked to the fact that the two grafted glasses have similar grafting densities (Table 2). 15 ACS Paragon Plus Environment


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After irradiation, as in the Phen case, two new sites appear around -8 ppm and -16 ppm and are formed in weak quantity (few %). The same increase of the Q4/Q3 ratio as in the Phen sample is also observed. This indicates a higher degree of polymerization after irradiation as already observed previously.16

Discussion: reaction mechanism of the grafted glasses under ionizing radiation The 1H MAS NMR measurements evidence that the loss of signal intensity after irradiation is smaller in the Phen system (17%) than in the Cyano one (37%). The NMR measurements indicate also that irradiation systematically increases rigidity, or at least increases the rigid population. More generally, the glasses grafted with aromatic groups are rather resistant towards ionizing radiation (as shown by NMR and gas measurements). Table 4 evidences that the Phen glass has the overall smallest radiolytic yields. This radiation resistance of aromatic compounds is well documented43,44 and is explained by the fact that energy is transferred from aliphatic parts into aromatic ones which act as energy traps. This energy in aromatic molecules is then dissipated mainly by heat or light emission.45 It was reported that, under gamma irradiation in polystyrene, the gaseous benzene yield is 8 10-10 mol.J-1.46 The benzene yield we report here for the gamma irradiation of the Phen compound is the same within the error bars (10-9 mol.J-1, Table 4). This benzene formation can be explained by the cleavage of the CH2-C6H5 bond, leading to the formation of the highly reactive phenyl radical (C6H5●) which will then easily abstract one hydrogen atom. Due to aromaticity, the phenyl radical is more stabilized than alkyl radicals, explaining why benzene is produced in higher quantity than ethylbenzene (Table 4) after energy transfer has occurred from the matrix to the surface. The Cp glass is also very resistant towards radiation (Table 4). The bond between the silicon atom and the carbon of the ring is not cleaved, as no cyclopentadienyl compound was detected. This cleavage would indeed lead to the formation of a cyclopentadienyl radical which is not stable. Therefore methane is the major compound detected under irradiation. With a SiCl(CH3)2R silane and with a R alkyl group, when the alkyl chain lengthens, the C-H cleavage is preferred over other bond cleavages (see second H2 column in Table 4, after having subtracted the contribution of the radiolysis of surface silanol groups) and the H2 production becomes significant. This yield is 2.2 10-8 mol.J-1 with R being –CH2CH2CH2CN and 1.6 10-7 mol.J-1 with R being –(CH2)7CH3 (Table 4). This H2 production arises then from the radiolysis of the R group, as the –CH3 groups do not contribute to it (see the radiolysis of 16 ACS Paragon Plus Environment

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the TriMet compound, Table 4). In the Cyano glass, there are 6 H atoms in the R group whereas there are 17 in the Octyl one. The increase in the H2 production when going from the Cyano to the Octyl group (7 times) do not then correspond to the increase in the H content of the R group (3 times). The CN group may then act as an energy trap, of course less efficient than an aromatic ring. NMR measurements confirm that in the case of the Cyano sample, the –CH2– groups are affected by ionizing radiation (Figures 6 and 7), which leads then to the production of H2. This is also particularly efficient with the Octyl compound for which the major bond cleavages are the –C(H)-H ones under irradiation. In the case of Octyl, the yields are indeed of the same order of magnitude, although lower, as the yields measured in octane (G(H2) = 5.0 10-7 mol.J-1 and G(CH4) = 8 10-9 mol.J-1)47, indicating an efficient energy transfer at the interface. We can also point out that the H2 yield measured using the –Si(CH3)2H grafting is much higher (3.3 10-7 mol.J-1)16, implying that the Si-H bond is much more efficiently cleaved than the –CH(H)- bond.

Considering the samples which exhibit a significant H2 production (i.e., the Cyano and Octyl

ones),

the

following

reaction

mechanisms

can

be

proposed:

− (CH 3 ) 2 Si (CH 2 CH 2R ' ) → −(CH 3 ) 2 Si (CH • CH 2 R ' ) + H • (R5) with R’ being –(CH2)5CH3 and –CH2CN for the Octyl and Cyano samples respectively (other H atoms can of course be removed). Dihydrogen can be produced through H● atom abstraction: − (CH 3 ) 2 Si (CH 2 CH 2R ' ) + H • → −(CH 3 ) 2 Si (CH • CH 2 R ' ) + H 2 (R6) − (CH 3 ) 2 Si (CH • CH 2R ' ) → −(CH 3 ) 2 Si (CH = CHR ' ) + H • (R7) This new species can be theoretically observed in

13

C NMR spectra. Unfortunately, due to a

low sensitivity, the signal to noise ratio was too weak in such experiments to account clearly for the formation of this species (see Figures 4 and 8 in Supporting Information).

Considering the methane production, the major source of methane is obviously the – CH3 groups linked to the silicon atom. Indeed, in all compounds bearing two methyl groups, the methane production under irradiation is of the same order of magnitude. For the TriMet sample (bearing three methyl groups), the methane is the major product formed upon irradiation and its production is clearly enhanced (Table 4) as compared to the other samples.



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− (CH 3 ) 2 Si − R → −(CH 3 )( R) Si • + CH 3• (R8) The methyl radicals can then react by abstracting an H● atom to form methane. The methane yields are rather similar (Table 8) for all the –(CH3)2SiR graftings, about a few 10-9 mol.J-1. Lastly, NMR measurements suggest the increase of the D1 and D2 species after irradiation. The increase of the D1 species after irradiation suggests that active ≡Si● radical sites react with water molecules leading to the formation of H● radicals (R9) as previously reported.15,16

− (CH 3 )( R ) Si • + H 2 O → −(CH 3 )( R) SiOH + H • (R9) The increase of D2 species by condensation of D1 with a surface silanol group can then be written as follows (R10): O

OSi(CH3)(R)OH +

OSi(CH3)(R) + H2O (R10)

SiOH O

D2

D1

In the case of polymers, Postolache and Matei48 have proposed that radiolytic effects occur through the ionization of the molecule and then neutralization by free electron capture and fragmentation. The authors have then calculated the homolytic dissociation energies (HDE) in the neutral and ionized states. For instance, they have shown in the case of polyethylene that the C-C HDE is smaller than the C-H HDE in the neutral case and that this is the opposite in the ionized state. This accounts for the preferential cleavage of C-H bond as compared to C-C ones observed in polyethylene. The same kind of calculations performed on polydimethylsiloxanes48 ((Si(CH3)2O)n) evidence that the HDE in the ionized state is 30.32 kcal/mol for Si-C and around 100 kcal/mol for C-H bond in CH3 groups and also for the Si-O bond. This suggests that the Si-C bond cleavage is favored over the other bond cleavages. These calculations are in agreement with our experimental observations that the Si-C bond is preferentially cleaved over the –CH ones when the alkyl chains are small (TriMet and TriEt samples, Table 4). This trend becomes inverted in –(CH3)2SiR samples when the R chain lengthens: the –CH(H)- bond is then preferentially cleaved over the Si-C bond (Cyano and Octyl compounds). Moreover, in these latter systems, the C-H bond cleavages are preferred over the C-C ones. Supplementary quantum chemical calculations could help rationalize this behavior. Nevertheless, other reasons can be invoked to account for this observation. One of them is that, as previously reported16, the silica glass and the grafted layer on it can act as an exciton propagation medium allowing the preferential localization in –CH bonds as compared 18 ACS Paragon Plus Environment

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to –CC- ones. The generated H• radical is very small and mobile, which makes its recombination reaction with the other radical to form the parent species less likely than with other radicals, and then leads to higher observed CH bond cleavages than CC ones.

Conclusion The behaviour under ionizing radiation of different groups grafted on nanoporous silica was investigated, mainly by means of mass spectrometry gas analysis and solid-state NMR spectroscopy. NMR is indeed a powerful tool to characterize the structure of the inorganic-organic interface. For instance, these spectroscopic measurements indicate that Q2 sites are more reactive towards grafting than Q3 sites and that irradiation systematically increases rigidity, or at least increases the rigid population. The major gases detected after irradiation are always H2 and/or CH4. After irradiation, the grafting groups can be divided into three groups according to gas measurements. The first group concerns radiation resistant species with the Phen and Cp compounds, with the aromatic ring and the cyclopentadienyl moiety acting as energy traps. For these samples, the major gas detected after irradiation is methane, due to the preferential lysis of the Si-CH3 bond. NMR experiments evidence also that, in the case of the Phen sample, the Si-Cprimary (SiCH3) bond, and to a lesser extent the CH2-C6H5 bond, are more prone to be cleaved than the Si-Csecondary one (SiCH2-). The second group consists of the Cyano sample, with the –CN group being also an energy trap, but less efficient than an aromatic ring. The major product obtained under irradiation is dihydrogen (a few 10-8 mol.J-1) and NMR measurements prove that the –CH(H)- bonds are the good candidates to account for this H2 production. Lastly, the third group consists of the Octyl sample, with a very efficient energy transfer at the interface. No energy dissipation pathway is created upon grafting, leading to significant amounts of dihydrogen, due to the cleavage of –CH(H)- bonds.

We could also analyze the chemical selectivity of the surface radiolysis: when the alkyl chains of the grafted groups are small (TriMet and TriEt samples), the Si-C cleavage is preferred over the C-H cleavage. This can not be understood with the number of available sites, as there are more C-H bonds than Si-C bonds. Moreover, we have already evidenced that thermodynamical data of excitation energy bonds are not appropriate to understand this phenomenon.16 An alternate explanation can arise from quantum chemical calculations of 19 ACS Paragon Plus Environment


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homolytic dissociation energy in the ionized state48. Supplementary quantum chemical calculations in the ionized state would be helpful to understand fully the different behaviours that we observe when the length of the alkyl chain increases, for example when going from the TriMet and TriEt samples to the Octyl one and to understand when the cleavage of the – CH(H)- bond becomes preferred over the other bond cleavages.

Of course, the presented results are only valid for an organo-silica interface. It would be interesting to study the influence of the nature of the oxide on the reactivity at the inorganic/organic interface.

Associated content Supporting Information Infrared spectra of the TriEt sample before and after irradiation; spin echo spectrum of the Phen sample; deconvolution of the 1H MAS NMR spectrum of the Phen sample;

13

C MAS

NMR spectra of the Phen sample; spin echo spectrum of the Cyano sample; Inverse Laplace Transform of the spin echo decay experiments for the Cyano sample; deconvolution of the 1H MAS NMR spectrum of the Cyano sample and 13C MAS NMR spectra of the Cyano sample.

Acknowledgements Dr Michel Heninger and Julien Leprovost are gratefully acknowledged for helpful discussion and preliminary mass spectrometry experiments.


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Table captions Table 1: The different silanes used in the present study. The temperatures at which the gas phase reactions at 1 mbar take place are also given. Tamb stands for ambient temperature. Table 2: Grafting densities ρ (in group/nm2) as determined by elemental analysis for the different grafted glasses obtained. nc is the number of carbons in the grafted group, %C is the carbon percentage in the sample, and MW is the molecular weight of the grafted group. Table 3: IR wavenumbers and the corresponding vibration band assignments for the grafted and non-grafted glass. Values are taken from reference 34 and references therein for the nongrafted glass; the assignments corresponding to the grafting are taken from reference 49. When the grafted chains become small, then the assignments of the bands can be less clear, as in the case of the TriEt sample. Ab initio calculations are then required to assign properly the bands, as was done for example in the case of the ethylsilane molecule in reference 50. Table 4: Radiolytic yields measured after γ-irradiation of the Phen, Cyano, Octyl, Cp and TriMet glasses. The error bars are estimated to be ± 10%. The second H2 column corresponds to the H2 production due to the grafted groups, the contribution due to the radiolysis of surface silanol groups being subtracted (1.2-1.3 10-8 mol.J-1). Table 5: Chemical shifts δ (ppm), σ values (line widths in ppm) and population (%) of the different proton sites obtained from simulation using Equation (1) of the 1H MAS NMR spectra of pristine and irradiated CPG 8 nm pore size samples grafted with chlorodimethylphenethylsilane. The SiOH band was fitted with three Gaussian bands centered at 2.8, 3.6 and 4.4 ppm and with σ = 1.2-1.5 ppm. The percentage of the whole SiOH band is given in the table. Table 6: Chemical shifts δ (ppm), gb values (line widths in ppm) and population (%) of the different silicon sites obtained from simulation of the 29Si CP MAS NMR spectra of pristine and irradiated CPG 8 nm pore size samples grafted with chlorodimethylphenethylsilane. Table 7: Chemical shifts δ (ppm), σ values (line widths in ppm) and population (%) of the different proton sites obtained from simulation using Equation (1) of the 1H MAS NMR spectra of pristine and irradiated CPG 8 nm pore size samples grafted with chloro(3cyanopropyl)dimethylsilane. The population of the different types of CH2 had to be constrained to be in the expected ratio (1/1/1). The SiOH band was fitted with three Gaussian bands centered at 2.8, 3.6 and 4.4 ppm and with σ = 1.2-1.5 ppm. Only the results concerning the whole SiOH band are given in this table (the deconvolution into three Gaussian bands is not presented here for the sake of clarity). Table 8: Chemical shifts δ (ppm), gb values (line widths in ppm) and population (%) of the different silicon sites obtained from simulation of the 29Si CP MAS NMR spectra of pristine and irradiated CPG 8 nm pore size samples grafted with chloro(3cyanopropyl)dimethylsilane.



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Table 1 Name of the silane/Name used in this study

Nature of the grafting at the surface CH3

Chlorotrimethylsilane/ TriMet

Si

O

Si

Temperature used at 1 mbar Tamb

CH3

CH3 CH2CH3 Chlorotriethylsilane/ TriEt

Si

O Si

Tamb

CH2CH3

CH2CH3 Chloro(3cyanopropyl)dimethylsilane/ Cyano

CN

CH3

54°C

Si O Si CH3

Chlorodimethylphenethylsilane/ Phen

102°C CH3 Si O Si CH3

C H2

C H2

H3C C H3

Chlorodimethyl(2, 3, 4, 5tetramethyl-2,4-cyclopentadien-1yl)silane/ Cp

Si

O

C H3

75°C

Si C H3

CH3

H3C

Chlorodimethyloctylsilane/ Octyl

CH3

CH3 Si O Si CH3

Table 2 Nature of the grafted silica TriMet TriEt Cyano Phen Cp Octyl

nc

%C

MW (g.mol-1)

ρ(group/nm2)

3 6 6 10 11 10

1.23 0.75 2.99 4.15 2.3 4.66

73.2 115.3 126.3 163.4 179.4 171.4

1.1 0.3 1.3 1.1 0.5 1.2


50°C

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Table 3

ν (cm-1)

Assignment

469

O-Si-O bending

676

Si-O-B deformation

812

[SiO4] tetrahedron rings

915

Si-O-B stretching

1095

Asymmetric Si-O stretching

1200

Asymmetric Si-O stretching

1407

B-O stretching

1631

H-O-H deformation

2250

CN stretching

2858

CH2 symmetric stretching

2879

CH3 symmetric stretching

2930-2940

CH2 asymmetric stretching

2964

CH3 asymmetric stretching

3030-3070

Aromatic CH stretching

3400-3500

O-H stretching of adsorbed water molecule

3540-3550

SiO-H stretching of adjacent pairs of SiOH groups with hydrogens bonded to each other

3650-3660

SiO-H stretching of isolated pairs of adjacent SiOH groups (vicinal) mutual hydrogen bonded

3745-3750

Isolated SiO-H groups (stretching)

Table 4

Yield (mol.J-1)

H2

Grafting

Phen

1.4 10-8

H2 after subtraction of the contribution of surface silanol groups ≈0

CH4

3 10-9

Hydrocarbons

Benzene: 1.10-9 Ethylbenzene: 3.10-10



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Cyano

3.4 10-8

2.2 10-8

6.10-9

Alkanes: 3.10-10

Octyl

1.7 10-7

1.6 10-7

4.10-9

Alkanes: 1.10-9

Cp

1.3 10-8

≈0

7 10-9

1 10-11

TriMet

1.2 10-8

≈0

2.5 10-8

Ethane: 2 10-9

No radiolytic yield was measured with TriEt but ethane was found to be the major product after irradiation.

Table 5

Sample

Chemical shift (ppm)

Attribution

σ (ppm)

Population (± 2%)

Pristine α = 0.71 and

-0.2

CH3

1.1

36%

lb = 0.25 ppm

0.7

CH2a

1.0

12%

2.4

CH2b

1.0

14%

6.9 2.8 3.6 4.4 5.6

C 6 H5 Si-OH Si-OH Si-OH Si-OH

0.5 1.4 1.2 1.5 1.5

33% 6% (whole SiOH band)

Irradiated 500 kGy

-0.2

CH3

1.1

31%

α = 0.52 and

0.7

CH2a

1.0

21%

lb = 0.25 ppm

2.4

CH2b

1.0

18%

6.9

C 6 H5

0.6

2.8 3.6 4.4 5.6

Si-OH Si-OH Si-OH Si-OH

1.4 1.2 1.5 1.5


27% 4% (whole SiOH band)

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Table 6

Sample Pristine

Population (± 3%) 4 ms 19 ms

Q2

3.8

2%

2%

Q

3

3.7

27%

22%

-110

Q

4

3.9

48%

51%

+ 13.8

M

1.3

23%

25%

-92.2

Q2

3.9

3%

0%

3

3.7

23%

25%

-92.8 -101

Irradiated 500 kGy

gb (ppm)

δ (ppm)

-101

Q

-110

Q4

4

51%

55%

+13.8

M

1.3

17%

18%

D

1

2.8

3%

1%

D

2

2.8

3%

1%

-7.6 -17.2



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Table 7

Sample

Chemical shift (ppm)

Attribution

σ (ppm)

Population (± 2%)

0.0

CH3

0.9

44%

0.7

CH2a

1.0

15%

1.6

CH2b

1.0

15%

2.2 3

CH2c Si-OH

1.0 2

15% 12%

0.0

CH3

1.6

46%

0.7

CH2a

1.8

15%

1.6

CH2b

1.8

15%

2.3 3

CH2c Si-OH

1.8 2

15% 9%

Pristine α = 0.45 and lb=0.29 ppm

Irradiated 500 kGy α = 0.51 and lb=0.35 ppm

Table 8 δ (ppm)

Sample Pristine

Population (± 5%) 4 ms 19 ms

Q2

3.8

2%

1%

Q

3

3.8

24%

27%

-110

Q

4

3.9

50%

52%

+ 13.9

M

1.3

23%

22%

-92.2

Q2

3.9

2%

0%

-101

Q3

3.9

24%

24%

-110

Q

4

3.9

52%

58%

+13.4

M

1.6

16%

14%

D

1

2.4

2%

2%

D

2

2.4

4%

2%

-92.8 -101

Irradiated 500kGy

gb (ppm)

-8 -16


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Figure captions Figure 1: FT-IR spectra of the grafted glasses: (a) in the 370-2300 cm-1 spectral region for the Cyano glass. The 370-1600 cm-1 spectral region is given as a typical example of the signal arising from silica. The CN stretching mode of the cyano group is detected at 2250 cm-1; (b) in the 2600-4000 cm-1 spectral region for all the studied compounds. Figure 2: 1H MAS NMR spectra of the CPG 8 nm pore size sample grafted with chlorodimethylphenethylsilane (pristine) and the same sample irradiated at 500 kGy. The spectra are normalized to the same sample mass. Figure 3: 1H double-quantum (DQ) MAS spectra (τ = τR) of the CPG 8 nm pore size sample grafted with chlorodimethylphenethylsilane: a) pristine sample b) irradiated sample (500 kGy). Resonances along the diagonal (solid line) correspond to autocorrelation peaks resulting from dipolar interactions between protons with the same chemical shift whereas off-diagonal resonances (dashed line) correspond to protons with different chemical values. Two-dimensional exchange 1H-1H correlation spectra of the CPG 8 nm pore size sample grafted with chlorodimethylphenethylsilane: c) pristine sample d) irradiated sample (500 kGy). Figure 4: Inverse Laplace Transform of the spin echo decay experiments for the pristine and the 500 kGy irradiated Phenyl samples. The box in dashed line highlights the additional peaks observed after irradiation but those species were found to be in a small quantity. Figure 5: Experimental (solid lines) and simulated (dashed lines) 29Si CPMAS spectra acquired with 19 ms and 4 ms contact times for the CPG 8 nm pore size sample grafted with chlorodimethylphenethylsilane: pristine sample a) and c); irradiated sample (500 kGy) b) and d). The fit parameters are given in Table 6. Figure 6: 1H MAS NMR spectra of the pristine CPG 8 nm pore size sample grafted with chloro(3-cyanopropyl)dimethylsilane (pristine) and of the irradiated sample (500 kGy). The spectra are normalized to the same sample mass. Figure 7: 1H double-quantum (DQ) MAS spectra (τ = τR) of the CPG 8 nm pore size sample grafted with chloro(3-cyanopropyl)dimethylsilane: a) pristine sample b) irradiated sample (500 kGy). Two-dimensional exchange 1H-1H correlation spectra of the CPG 8 nm pore size sample grafted with chloro(3-cyanopropyl)dimethylsilane: c) pristine sample d) irradiated sample (500 kGy). Figure 8: Experimental (solid lines) and simulated (dashed lines) 29Si CPMAS spectra acquired with 19 ms and 4 ms contact times for glass grafted with chloro(3-cyanopropyl)dimethylsilane a) and c); the irradiated sample b) and d); The fit parameters are given in Table 8. 27 ACS Paragon Plus Environment


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Figure 1

(a)

(b)


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Figure 2

CH3 Si

CH2a

CH2b

C6H5

CH3



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Figure 3


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Figure 4



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Figure 5


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Figure 6

CH3 Si

CH2a

CH2b

CH2c

CH3


CN


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Figure 7


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Figure 8



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Influence of the Nature of Self-Assembled Monolayers on Their

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