ISSN 0012-5008, Doklady Chemistry, 2007, Vol. 417, Part 2, pp. 289–291. © Pleiades Publishing, Ltd., 2007.Original Russian Text © V.V. Korol’kov, B.N. Tarasevich, G.V. Lisichkin, 2007, published in Doklady Akademii Nauk, 2007, Vol. 417, No. 6, pp. 774–777.

289

It is traditionally believed that a homogeneous solu-tion as a sample is necessary to obtain informativehigh-resolution NMR spectra [1]. Therefore, to studythe structure of grafted layers of surface-modifiedmaterials, cross-polarization magic angle spinningNMR spectroscopy is usually used [2]. Because of theknown limitations, this method has not become a rou-tine technique in study of grafted surface compounds.NMR spectroscopy of suspensions of surface-modifiedmaterials in a suitable liquid medium is a good candi-date for a more readily available and experimentallysimpler method.

Only a few high-resolution NMR spectra of suspen-sions have been reported. The first such spectra were

31

P NMR spectra of hydroxyapatite (Ca

10

(OH)

2

(PO

4

)

6

)suspensions in water [3]. However, the hydroxyapatitestudied in [3] was not modified.

31

P NMR spectra havealso been obtained for suspensions of surface-modifiedoxide substrates in organic solvents [4, 5]. In theseworks, dealing with the synthesis of functionalizedmineral substrates for heterogeneous metal complexcatalysts, grafted surface compounds based onPh

2

P

(

CH

2

)

3

Si

(

OEt

)

3

(immobilized on TiO

2

,

Al

2

O

3

,

o

r

SiO

2

) [4] and

(

Ph

2

P

)

2

N

(

CH

2

)

3

Si

(

OEt

)

3

(immobilizedon SiO

2

) [5] were studied. These data were used forestimating the mobility of the grafted molecules. Themobility of grafted layers was also estimated from

1

HNMR spectra of suspensions of silica modified withchlorodimethyl(octadecyl)silane and trichlorooctade-cylsilane [6]. The spectra showed the signals of theCH

3

–

and

−

CH

2

–

protons in octadecyl radicals in con-centrated suspensions of modified silicas in CD

3

OD(10 mg/100

µ

L). At the same time, the spectra showedrather strong extra signals, assigned in [6] to adsorbedbyproducts. The

1

H and

13

C NMR spectra of suspen-

sions of gold clusters (with core diameters of 1.5–5.2 nm) with grafted dodecathiol groups were reportedin [6]. It is worth noting that, in all cited works exceptthe first one, NMR studies were not the main objective.

Judging from the available literature data, no tar-geted and systematic NMR studies have hithertobeen carried out on suspensions of surface-modifiedmaterials.

In this paper, we report the

1

H NMR spectra of sus-pensions of detonation-synthesis nanodiamonds withgrafted alkyl groups (

n

-C

4

H

9

–

,

n

-C

6

H

13

–,

n

-C

16

H

33

–,

cyclo

-C

6

H

11

–, C

6

H

5

–). Modified samples were pre-pared by treatment of the initial nanodiamond withhydrogen at 800

°

C for 5 h followed by photochemicalchlorination with molecular chlorine in CCl

4

and theinteraction of the chlorinated nanodiamond with corre-sponding organolithium reagents. The procedure wasdescribed in detail in [7]. Before recording spectra, thesamples were kept for 5–6 h at 160–170

°

C and a pres-sure <0.1 mmHg to remove adsorbed impurities. Nano-diamond (ZAO Almaznyi tsentr, St. Petersburg) with

S

sp

= 284

±

1 m

2

/g and a primary diamond particle sizeof

~

4–5 nm treated with hydrogen at 800

°

C for 5 h wasused as the reference. It is important that both the initialand modified nanodiamond particles are paramagnetic[8], which is believed to be unfavorable for recordingNMR spectra.

The spectra were recorded on a Bruker AVANCE-400 spectrometer (400 MHz, 600 scans). The delaytime was chosen individually for each sample. For allsamples except the sample with grafted butyl groups,D

2

O (98 wt %) was used as the dispersion medium. Forthe sample with grafted butyl groups, CDCl

3

(99 wt %)was used. A weighed portion (5–10 mg) of modifiednanodiamond was suspended in D

2

O (0.5 mL) directlyin an NMR tube and sonicated for 5 min in an ultrasonicbath (50 W, 35

±

10% kHz). This treatment gave a sta-ble suspension.

Previously [9], we showed that the high-temperaturetreatment of nanodiamond with hydrogen leads to theformation of a bifunctional surface at which CH and

The Use of

1

H NMR to Study Suspensions of Surface-Modified Diamond Nanoparticles

V. V. Korol’kov, B. N. Tarasevich, and G. V. Lisichkin

Presented by Academician Yu.A. Zolotov June 5, 2007

Received

July

7, 2007

DOI:

10.1134/S001250080712004X

Moscow State University, Vorob’evy gory, Moscow, 119992 Russia

CHEMISTRY

290

DOKLADY CHEMISTRY

Vol. 417

Part 2

2007

KOROL’KOV et al.

hydroxyl groups are mainly located. The procedure [9]makes it possible to graft alkyl groups on the nanodia-mond surface, which was reliably demonstrated by IRspectroscopy.

The

1

H NMR spectrum of the initial reduced nano-diamond has no signals that could be assigned to sur-face groups, which allows us to use this spectrum forcomparison. A broad strong signal at

~

4.7 ppm, causedby HOD, was suppressed by a routine method.

Figure 1a shows the

1

H NMR spectrum of a suspen-sion of nanodiamond with grafted

n

-butyl groups. Thespectrum shows broad signals at 0.91, 1.30, and1.58 ppm. Based on standard chemical shift values,these signals can be assigned, respectively, to the CH

3

protons, the protons of the two intermediate CH

2

groups, and the protons of the CH

2

group adjacent tothe nanodiamond surface. The integrated intensity ratioof these signals is 1 : 2.2 : 1.5 (CH

3

: CH

2

: CH

2

(sur-face)), which is rather close to the ratio calculated fromthe number of corresponding protons, 1 : 2 : 1.5. Thedifference falls within the error of signal integration.

Figure 1b shows the spectrum of the nanodiamondwith grafted

n

-hexyl groups at 25

°

C. The spectrumshows two broad signals at 0.73 and 1.13 ppm. Thesesignals are shifted downfield by

~

0.3 ppm as the tem-perature increases to 50

°

C, their Lorentzian shape andhalf-widths being unaltered. It is worth noting that thespectrum also shows a very weak broadened signal at1.4 ppm. For this sample, the observed integrated inten-sity ratio is inconsistent with the value calculated fromthe number of protons. In our opinion, this can be dueto conformational effects in the longer

n

-hexyl chain.Figure 1c shows the

1

H NMR spectrum of the sam-ple with hexadecyl groups at 25

°

C. The spectrum dis-plays signals at 0.85, 1.27, and 1.53 ppm. The upfieldsignal has a poorly resolved triplet structure, which istypical of the methyl group. In the above spectra, split-ting was not observed. These signals are shifted down-field by ~0.3 ppm as the temperature increases to 50

°

C,as in the case of nanodiamond with grafted

n

-hexylgroups. The integrated intensity ratio for the protons ofmethylene and methyl groups is also inconsistent withthe calculated ratio. We may assume that this is associ-ated with the ratio of the carbon chain segments locatedin the dispersion medium and at the surface. It shouldbe noted that, for these samples, the distance betweenthe signals tentatively assigned to the methyl and meth-ylene protons is roughly the same and is

~

0.4 ppm.Figure 2 shows the

1

H NMR spectra of the samplewith grafted cyclohexyl groups at 25 and 50

°

C. Thespectrum is represented by overlapping signals at 0.90and 1.07 ppm and a signal at 1.61 ppm (Fig. 2a). Withincreasing temperature, the signals not only are shifteddownfield by

~

0.3 ppm, as observed for the other sam-ples, but also change their relative intensities andshapes. In particular, the broad signal at 1.61 ppm has arather symmetric shape at 25

°

C, whereas, at 50

°

C, ithas a clearly pronounced two-component structure withmaxima at 1.94 and 1.82 ppm (Fig. 2b). At the sametime, in the upfield two-component signal, the relativeintensity of the low-field component increases withincreasing temperature. Interpretation of the spectrum

1.6 1.4 1.2 1.0 0.8

Chemical shift, ppm

0

Inte

nsity

1.53

1.27

0.85

(c)

1.2 1.0 0.6 0.2 00

Inte

nsity

1.41

1.13

(b)

0.73

0.40.81.41.6

0

Inte

nsity

2.02

1.58

(a)

1.30

2.02.5 1.5 1.0 0.5

0.91

Fig. 1.

1

H NMR spectra at 25

°

C of a suspension of nanodi-amond with grafted (a)

n-

butyl groups in CDCl

3

, (b)

n-

hexylgroups in D

2

O, and (c)

n

-hexadecyl groups in D

2

O.

DOKLADY CHEMISTRY

Vol. 417

Part 2

2007

THE USE OF 1H NMR TO STUDY SUSPENSIONS 291

of grafted cyclohexyl groups is most challenging sincethe spectrum depends on the conformation of thegrafted molecule and its orientation to the surface,which is demonstrated by recording of spectra at differ-ent temperatures.

For the sample of nanodiamond with grafted phenylgroups, we did not observe signals that could beassigned to the benzene ring protons. Taking intoaccount that the fact of grafting phenyl groups wasproved by IR spectroscopy [9], we can conclude thatthe lack of signals in the 1H NMR spectrum is related tostructural features of the grafted layer, which should beelucidated.

Thus, the main result of our work is the demonstra-tion of the applicability of NMR to the study of the finestructure of grafted layers of surface-modified diamondnanoparticles.

ACKNOWLEDGMENTS

We are grateful to Prof. Yu.A. Ustynyuk for helpfuldiscussions.

REFERENCES

1. Derome, A.E., Modern NMR Techniques for ChemistryResearch, Oxford: Pergamon Press, 1987. Translatedunder the title Sovremennye metody YaMR dlyakhimicheskikh issledovanii, Moscow: Mir, 1992.

2. Lisichkin, G.V., Fadeev, A.Yu., Serdan, A.A., et al.,Khimiya privitykh poverkhnostnykh soedinenii (TheChemistry of Grafted Surface Compounds), Moscow:Fizmatlit, 2003.

3. Yesinowski, J.P., J. Am. Chem. Soc., 1981, vol. 103,pp. 6266–6267.

4. Merckle, Ch. and Blumel, J., Oxides Chem. Mater.,2001, vol. 13, pp. 3617–3623.

5. Posset, T., Rominger, F., and Blumel, J., Chem. Mater.,2005, vol. 17, pp. 586–595.

6. Ansarian, H.R., Derakhshan, M., Rahman, M., et al.,Anal. Chim. Acta, 2005, vol. 547, pp. 179–187.

7. Hostetler, M.J., Wingate, J.E., Zhong, Ch-J., et al., Lang-muir, 1998, vol. 14, pp. 17–30.

8. Belobrov, P.I., Gordeev, S.K., Petrakovskaya, E.A., andFalaleev, O.V., Dokl. Phys., 2001, vol. 46, no. 7, pp. 459–462 [Dokl. Akad. Nauk, 2001, vol. 379, no. 1, pp. 38–41].

9. Lisichkin, G.V., Korol’kov, V.V., Tarasevich, B.N., et al.,Izv. Ross. Akad. Nauk, Ser. Khim., 2006, no. 12, p. 2130.

0

Inte

nsity

1.94

1.82

(b)

1.41

2.0 1.5 1.0 0.5

1.17

Chemical shift, ppm

0

Inte

nsity

1.61

(‡)

1.07

1.5 1.0 0.5

0.90

Fig. 2. 1H NMR spectra of a suspension of nanodiamondwith grafted cyclohexyl groups in D2O at (a) 25 and(b) 50°C.