506
Anal. Chem. 1985, 57,596-601
Infrared Spectrometric Studies of Cyanoalkyl Ligands Immobilized on Chromatographic Surfaces B. R. Suffolk and R. K. Gilpin* Department of Chemistry, Kent State University, Kent, Ohio 44242
Long and lntermedlate chalns of cyanoalkyl llgands were attached to porous sllica-base materlal by uslng monoreactive chemlstry, and short cyanoalkyl chains were attached by using mona-, dl-, and trlreactlve chemistry. Subsequently, the mlcrochemlcal structures of these materlals were studied with Fourier transform Infrared (FT-IR) technlques. I n a nonpolar llquld (Le., hexane) Infrared spectra for the nltrlle band were extremely broad and asymmetrical. Deconvolution of these bands produced a doublet whlch suggested the presence of at least two different types of attached ligands. These bands support the Idea of a certain populatlon of nltrlle groups whlch hydrogen bond with free surface silanol groups and a populatlon of nltrlle groups whlch sterlcally cannot hydrogen bond. This Is posslble by assumlng a nonuniform distribution of groups. Groups bonded wlthln a rlch area or cluster would be restricted from surface lnteractlon by surroundlng groups or neighboring groups, whereas groups at the edges of such a cluster or groups bonded In sparcely populated areas could hydrogen bond. I n a polar solvent (Le., 1-butanol) the groups were displaced from the surface and showed ligand-solvent Interactlon.
The utilization of chemically bonded stationary phases for high-performance liquid chromatography has offered great rewards in terms of selectivity and stability over the earlier physically coated packings ( I ) . These stationary phases generally are prepared by immobilizing various organosilane molecules to porous silica-base materials. The most popular chemically bonded phases are those with hydrocarbon ligands (either octyl or octadecyl), which are used typically in the reversed-phase mode. Probably next in popularity are the more polar chemically altered surfaces formed by attaching either an amino or a cyano group via a short hydrocarbon spacer arm. These latter materials are used in both the reversed-phase as well as the normal-phase mode. Over the course of their development, the physical nature of chemically modified surfaces has been a subject of both interest and controversy (1). Although the chemistry to prepare useful materials has progressed to a rather mature state, a detailed understanding of the stereochemistry of the system is emerging only now (2). During the synthesis of bonded phases, the actual degree of modification is dependent upon a number of factors including backbone chemistry, surface area and porosity of the sorbent, and reagent structure (i.e., size and shape). On a macro basis synthetic procedures are usually such that statistically about one-quarter to one-half of the available silanol groups are involved in the reactions. On an actual microchemical basis the extent and uniformity of coverage may be much more complicated. This has been demonstrated recently by several techniques (3-6). Historically, most descriptions of the surface were based solely on chromatographic results (7). More recently, chromatographic surfaces have been probed with a variety of methods. Although initial experiments were concerned with
the degree and type of surface reactivity, the information currently being derived is providing new insights about surface-ligand structure and interaction, bonded layer solvation, and segmental and total chain mobility. Specific examples of these include: 1. investigations of surface homogeneity by examing the fluorescence behavior of chemically attached pyrene probes ( 3 ) ,2. nuclear magnetic resonance studies of the preferential solvation and motional dynamics of immobilized ligands (7-10), and 3. the elucidation of conformational features for silica immobilized dimethyl-n-alkyl (11) and acetoacetamide (12) ligands using FT-IR. By use of monochlorosilanes or monoalkoxysilanes, chemical bonded phases can be prepared which are reproducible and efficient (6, 13). This is because the bonding chemistry is relatively simple, involving the replacement of the surface silanol proton with the silane ligand. However, since di- or trichloro and di- or trialkoxysilanes have multireactive sites, they are inherently more difficult to control and hence they are less defined and reproducible (6,14). For steric reasons it is more likely that the di- or trichloro and di- or trialkoxysilanes react to form cross-linked attached layers rather than multiattached centers (6). Formerly most commercial bonded phases were prepared with either bifunctional or trifunctional silanes, but because of the advantages of reproducibility, they are now generally manufactured with monofunctional silanes (9, 15). In the current study long and intermediate cyanoalkyl chains were attached to silica surfaces by using monoreactive chemistry, and short cyanoalkyl chains were attached by using monoreactive, direactive, and trireactive chemistry. Subsequently the microchemical structures of these materials were studied by FT-IR techniques. Infrared spectrometry was chosen here as a tool to investigate chemically bonded stationary phases, since solvent environments can be made to simulate liquid chromatographic experiments. For the cyanoalkyl phases infrared spectrometry is especially suited, since the C=N stretch band is isolated from interference bands arising from the siloxane structure skid surface silanol/water. Two major points were of interest: 1.the type and uniformity in distribution of the ligand population and 2. the degrees of ligand-surface and ligand-solvent interactions.
EXPERIMENTAL SECTION Materials. Tetrahydro-2-furancarbinol was purchased from Pfaltz and Bauer, Inc. Tetrahydropyran-2-methanol (98%), 10-undecen-1-01(99%), and acrylonitrile (99+%) were purchased from Aldrich Chemical Co., Inc. Dimethylchlorosilane, (2cyanoethyl)methyldichlorosilane, (2-cyanoethyl)trichlorosilane, (3-cyanopropyl)dimethylchlorosilane,and (3-cyanopropy1)trichlorosilane were obtained from Petrarch Systems, Inc. Alkylnitrile groups were bonded onto Li Chrosorb Si 60 silica (mean particle size 10 gm and surface area 550 m2/g) from E Merck. HPLC grade hexane and 1-butanol were obtained from MCB Manufacturing Chemists, Inc., and spectrograde chloroform (0.75% ethanol preservative) was obtained from Fisher Scientific co. Synthesis of the Noncommercially Available. (Cyanoalky1)dimethylchlorosilanes. Tetrahydrofuran-2-methyl chloride and tetrahydropyran-2-methylchloride were synthesized
0003-2700/85/0357-0596$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
597
Scheme I
STEP 3
STEP 2
STEP 1
I
Bo% aqueous
CH,=CH-(CH,)m--C!
N
4
Fgl
CH,=CH(CH&,OTs pyridine STEP4
STEP 5
CH, STEP6
c
Table I. Surface Coverage for the Chemically Bonded Alkylnitriles surface coverageb normalized carbon carbon
% bound
from tetrahydrc-2-furancarbinoland tetrahydropyran-2-rnethano1, respectively, as described by Kirner (16). From the tetrahydrofuran-2-methyl chloride and the tetrahydropyran-2-methyl chloride, Cpenten-1-01and 5hexen-1-01were prepared by an initial reaction with sodium and subsequent hydrolysis according to the procedure of Baubert, Linstead, and Rydon (17). Tosylates were synthesized from the alcohols, as described by Gangoda and Gilpin (18). Tosylates were converted to nitriles using a method similar to Wilt, Massie, and Dabek (19). To carry out the final coupling step in the synthesis of the monochlorosilanes, 50 drops of 0.1 M hydrogen hexachloroplatinate in 2-propanol was added to a 1:1.15 ratio of an alkenylnitrile to dimethylchlorosilane mixture. The contents were heated in a 50-mL stainless steel tube at 100 "C for 2 days. Purification of the product was obtained by vacuum distillation. Structures of the synthesized silanes were verified by 'H and 13CNMR. Preparation of the Alkylnitrile Bonded Phases. A 1.5-g portion of silica was slurried with water, and the excess water was decanted away. The silica was dried at 110 "C for 2 h and then conditioned with water-saturated toluene overnight to establish the degree of surface hydration. The silica was refluxed with 55 mL of a 10% reaction mixture of the (cyanoalky1)dimethylchlorosilane, (cyanoalkyl)methyldichlorosilane, or (cyanoalky1)trichlorosilane monomer in water-saturated toluene. While conditioning the silica surfaces and also during the reaction, dried nitrogen was bubbled through the bottom of the reaction vessel to provide mild agitation of the reaction mixture and to expel HC1 generated by the silane reaction. Subsequently, the chemically modified silica was exhaustively washed with four 50-mL portions of dry toluene, two 50-mL portions of water-saturated toluene, and two 50-mL portions of ether, dried overnight in a 110 "C oven, and then stored in a desiccator. The stabilized (3-cyanopropyl)trimethylsilanewas synthesized from methylmagnesium iodide and (3-cyanopropyl)trichlorosilane in sodium-dried ether. Spectrometry. Fourier transform infrared spectra were scanned with a Nicolet 7199 FT-IR spectrometer equipped with a MCT detector and operated to give 1 cm-l resolution. The cyanoalkylsilanized silica was placed between two KCl windows of a Connecticut Instrument Corp. demountable liquid cell equipped with a 0.1- or 0.2-mm spacer. After the demountable cell was assembled, the appropriate liquid medium was added through the two liquid ports via a dropper. Ratios of 250 sample scans vs. 100 nitrogen-atmosphere background scans were taken. The absorbances of the C s N band maxima varied from 0.42 to 0.99. No base line corrections or subtractions were employed, In a 0.1074-mm Perkin-Elmer liquid cell, a 1:20 dilution of (3-cyanopropyl)trimethylsilane in hexane or 1-butanol was scanned. The measured absorbance readings of the C=N band maxima in hexane and 1-butanol were 0.14 and 0.21, respectively.
na
bonding chemistry
2
monochloro monochloro monochloro monochloro monochloro
5.8
2
2
3 5
6 11
3
6.6 8.6
1.2 1.1 1.1
9.2 9.9
0.7
dichloro
8.0
2.0
trichloro trichloro
6.3
2.1
7.9
1.0
I
2.0
Length of the alkyl spacer arm (Le., >SiO-Si-(CH,),C=N), bManufacturer listed surface area of 550 m2/e. 1
A FORTRAN computer program was written to access the file on the hard disk where each spectrum was stored and then to select the band maxima and to compute the asymmetry ratio, a/ b, of the C=N stretch bands. For bonded phases in chloroform and hexane, base lines between 2295 and 2162 cm-' were used. For bonded phases in 1-butanol, base lines between 2299 and 2210 cm-' were used. Fourier self-deconvolution (20) was performed with the Nicolet 1RDCON.FTN program (21). RESULTS AND DISCUSSION The reaction scheme which was used to synthesize the (cyanoalky1)silanized silicas from (cyanoalky1)dimethylchlorosilanes involved the steps shown in (Scheme I). When m = 3 or 4,m = 9, m = 0, or m = 1,the syntheses began at steps 1,3,5, or 6, respectively. Likewise, the reaction schemes utilized in the preparation of (cyanoalky1)silanizedsilicas from the (cyanoalky1)dichloromethylsilane and the (cyanoalkyl)trichlorosilanes ( m = 0 or 1)are shown in (Scheme 11). Surface coverages for the various cyanoalkyl bonded phases prepared in the current study are shown in Table I. Results are listed in terms of both weight percent and normalized values. For the bonded phases synthesized from either mono-, di-, or trichlorosilanes, the normalized carbon was found by dividing the percent bound carbon by n + 3, n + 2, and n + 1,respectively. In this relationship n is equal to the number of carbons (i.e., the length of the spacer arm) the cyano group is removed the silane atom. Except for the longer cyanoundecyl chain, the normalized carbon values were nearly the same for all monochloro modified materials. In the cases of the di- and the trichloromodified materials, coverage was about twice that obtained with the (cyanoalky1)monochlorosilanes. (3-, 5-, 6-, and 11-Cyanoalky1)silanized Silicas. The infrared spectra of the C=N stretch region for each of the
598
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
Table 11. Band Maxima and Asymmetry for the C=N Stretch Band of Various Immobilized Cyanoalkyl Ligands in 1-Butanol, Chloroform and Hexane albb
vmn,
cm-’ hexane
nu
bonding chemistry
1-butanol
chloroform
hexane
1-butanol
chloroform
3 5 6 11 3
monochloro monochloro monochloro monochloro trichloro
1.3 1.6 1.8 1.6 1.5
2.5 1.8 2.0 1.8 1.8
3.1 2.9 2.7 1.9 2.0
2251.4 2250.0 2248.5 2249.5 2250.4
2248.5 2249.0 2247.6 2248.5 2249.5
2248.0 2248.5 2248.0 2251.9 2250.0
2256.9, 2247.1 2258.0, 2247.8 2257.8, 2246.8 2258.9, 2248.8 2257.4, 2248.1
2 2 2
monochloro dichloro trichloro
1.6 1.5 1.1
2.4 1.6 1.2
3.0 1.9 1.0
2228.8 2251.9 2255.8
2226.3 2251.9 2256.2
2226.3 2250.9 2257.7
2234.5, 2226.1 2259.7, 2250.1 2258.4, 2253.1
a
hexanec
See Table I. *Band asymmetry. Deconvoluted doublet.
n
r 2500
2580
2260 22’iO WFlVENUflBERS
2220
2iOO
Flgure 1. FT-IR spectra of @Sstretch I bands for various immobilized cyanoalkyl ligands. Surfaces prepared from (A) (3-cyanopropy1)trichlorosllane, (B) (3-cyanopropyl)dimethylchlorosllane,(C) (5-cyanopentyl)dlmethylchlorosilane, (0) (6-cyanohexyl)dimethylchlorosilane, and (E) (1 1-cyanoundecyl)dimethylchlorosllane.Liquid medium was 1-butanol.
\I
2300
2280
2260 2290 UFIVCNUNEC’AS
I ” 2iOO
2220
Flgure 3. C# stretch bands of the same (cyanoa1kyl)silanizedsilicas (A-E) shown in Figure 1. Liquid medium was hexane.
WWJENUMEERS
Figure 4. Asymmetty ratio a lb of each E N stretch band, measured by dividing the distance a by the distance b .
2280
2k60
2240
2220
2500
WAVENUMBERS
stretch bands of the same (cyanoalky1)silanlzedsilicas (A-E) shown in Flgure 1. Llquid medium was chloroform.
Flgure 2. C#
cyanoalkyl bonded phases using 1-butanol, chloroform, or hexane as liquid media are shown in Figures 1, 2, and 3, respectively. Corresponding band asymmetry ratios, a/ b, were
calculated as illustrated in Figure 4 and are summarized in Table 11. Generally for a given solvent, changes in length of the alkyl spacer arm did not affect band contour significantly. However, comparing results among solvent types, the asymmetry increased in the order of decreasing polarity of the media in contact with the surface (i.e., 1-butanol < chlorofrom < hexane). In hexane (Figure 3), the bands for the immobilized groups were extremely asymmetrical and distorted to the point that shoulders were noted. The Fourier self-deconvolution of these,
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
599
.A 2400
zFao
2260
2Fno
zizo
2200
WAVENUMBERS
Figure 7. FT-IR spectra of (A) (3-cyanopropyl)silanized silica and (B) (3-cyanopropyl)trimethylsilane (Le.,stabilized analogue) in 1-butanol.
UFIVENUtlBERS
Figure 5. Fourier selfdeconvolution of the E
N stretch bands shown
In Figure 3.
Figure 6. FT-IR spectra of (A) (3-cyanopropyl)sllanlzedsilica and (B) (3-cyanopropyl)trimethylsilane (Le.,stabilized analogue) in hexane.
which are shown in Figure 5, demonstrate that the asymmetrical bands obtained in hexane were actually unresolved doublets. The observed frequencies of each of the resulting deconvoluted peaks are consistent with the work of Rochester et al., which is discussed below. Infrared spectra of the nitrile region obtained in hexane and 1-butanol, respectively, for the (3-cyanopropy1)dimethylimmobilized ligand and for the corresponding stabilized monomer, (3-cyanopropyl)trimethylsilane,are shown in Figure 6 and 7. Due to the inherent problems of reactivity with the chlorosilane monomers, (3-cyanopropyl)trimethylsilanewas used as the unbound control compound. These latter spectra illustrate the presence of two types of band distortion, asymmetry and broadening. Referring to Figure 6, changes in band contour in an apolar solvent (i.e., hexane) easily can be seen by comparing the control compound in the absence of silica to the similar immobilized ligand. In contrast to the extremely asymmetric CEN stretch band of the chemically immobilized form ( a / b = 3.1), the C=N stretch band of the nonimmo-
bilized, free form was very sharp and nearly symmetric ( a / b = 0.9). In 1-butanol (Figure 7) the asymmetrical distortions in the nitrile band for both the free ( a / b = 1.0) and immobilized ( a / b = 1.3) forms were not present. However, the C=N stretch bands were much broader in 1-butanol (Figure 7) than the free form in hexane (Figure 6). Bellamy (22)has stated that band broadening and asymmetry have been found in both OH and NH hydrogen-bonded systems. Likewise, the above changes in CEN band symmetry are consistent with the idea of hydrogen bonding. In the present investigation the observed changes in band shape between free and immobilized nitrile groups acquired in an apolar vs. polar solvent are indicative of two types of hydrogen bonding: 1. ligand-surface and 2. ligand-solvent. The extremely asymmetric C r N stretch for the modified materials in contact with hexane results from hydrogen bonding between a portion of the chemical immobilized groups and free surface silanol groups and will be considered in greater detail below. In 1-butanol, both the free silane and the chemically modified silica exhibited broad bands indicative of hydrogen bonding with the solvent. In the absence of hydrogen bonding (i.e., stabilized control silane in hexane) the C z N stretch band was sharp and symmetric. The idea of specific ligand surface interactions has been suggested for another system. Leyden et al. (22) have suggested that hydrogen bonds are probably formed with unreacted surface silanols in their investigations of immobilized acetoacetamide, which was synthesized by reacting diketene with either (3-aminopropyl)trimethoxysilane or ( N - ( 2 aminoethyl)-3-aminopropyl)trimethoxysilanebound to silica. Leyden et al. suggested that since the silica surface was polar, it could function as a donor in hydrogen bonding with the bound amide groups. The current data are also consistent with nitrile adsorption studies. Rochester et al. have investigated changes in the infrared spectra for the adsorption of propionitrile on silica (23,24). Self-supporting disks of Aerosil silica were placed between two silica windows of an infrared cell, and the assembly was evacuated at 873 K. Subsequently, various binary solutions of propionitrile in heptane were introduced into the cell. As the concentration of propionitrile was increased, the intensities of the 3705 cm-l free Si-OH stretch band decreased linearly as a broad 3395 cm-l band and a band at 2260 cm-I increased. The authors rationlized these spectroscopicchanges in terms of the formation of hydrogen bonds between the surface silanol groups and the cyano groups of adsorbed propionitrile molecules. The broad band at 3395 cm-l was attributed to hydrogen-bonded Si-OH groups, and the band at 2260 cm-l was attributed to hydrogen-bonded C=N groups.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
n w
U
w u z a m
a m d
0 Cn
a
m
D
c
v)
m
U
-
I
2300
2280
Zk60
2290
2220
ZiOO
UaVENUflBERS
Flgure 8. FT-IR spectra of C E N stretch bands for (2-cyanoethy1)silanized silicas prepared using different backbone chemistry. Surface was prepared from (A) (2-~yanoethyl)trichlorosilane,(B) (2-cyanoethyl)methyldichlorosilane, and (C) (2-cyanoethyl)dimethylchlorosilane. Liquid medium was I-butanol.
Spectroscopic results for the adsorption of propionitrile on silica immersed in 2,2,4-trimethylpentane or toluene were similar, except that the maxima of the silanol groups perturbed by toluene was at 3595 cm-' instead of 3395 cm-l. In the absence of silica, infrared symmetric CEN stretch bands occurred at 2249 cm-' for propionitrile in heptane and 2248 cm-' for propionitrile in 2,2,4-trimethylpentane or toluene. The 2260 cm-' band arising from hydrogen-bonded propionitrile adsorbed on silica appeared at the same frequency for all three liquid media. The frequencies of the band maxima,,,,v for the spectra shown in Figure 1-3 are summarized in Table 11. Within a given solvent v- was nearly independent of spacer arm length for the 3-, 5-, 6-, and 11-cyanoalkyl immobilized ligands. In both 1-butanol and chloroform the current ,v values are similar to those reported by Rochester et al. for propionitrile in heptane, 2,2,4-trimethylpentane, or toluene in the absence of silica. As such, these data suggest the absence of significant ligand-surface interaction via hydrogen bonding when the immobilized groups are solvated by polar liquidslike 1-butanol or chloroform. Values of ,,v for the deconvoluted spectra obtained in hexane (Figure 5) are also listed in Table 11. The lower frequency values of the deconvoluted doublets are similar in frequency to the band maxima for the modified silica in contact with eiher 1-butanol or chloroform and also we similar in frequency to the non-hydrogen-bonded C e N stretch bands obtained by Rochester et al. Therefore it seems reasonable to suggest that the bands a t the lower v were the result of a certain percentage of cyano groups which did not hydrogen bond with the surface. However, since the bands at the higher Y correspond closely in frequency with the hydrogen-bonded form of propionitrile reported by Rochester et al., it seems reasonable also to suggest the existence of hydrogen bonding between the attached cyano groups and unreacted silanol groups on the silica surface. West and Baney (25) have shown that for hydrogen bonding systems, silanols are more associated than carbinols at any given concentration, but the differences are not large. We also found that hydrogen bonding between immobilized nitrile groups and the silanols resulted in larger band shifts than did the nitrile-carbinol hydrogen-bonding interaction. In hexane, the lower frequency non-hydrogen-bonded bands and the higher frequency hydrogen-bonded bands (Figure 5) were separated by 8 to 11 cm-l. With the exception of the 11-
2300
2280
2260 22'tO UFIVEMUfleERS
2220
2kOO
Flgure 9. C E N stretch bands of the same (2-cyanoethyl)silanized silicas shown in Figure 8. Liquid medium was chloroform.
n
UaVCNUHBERS
Flgure 10. C B N stretch bands of the same (2-cyanoethyl)silanized
silicas shown in Figure 8. Liquid medium was hexane. cyanoundecyl surface, for a given spacer arm in 1-butanol the values of urn= were shifted 0.5 to 3 cm-l higher in frequency than when there was no ligand-solvent hydrogen bonding. (2-Cyanoethy1)silanized Silicas. The infrared spectra of the CEN stretch region for the (2-cyanoethy1)silanized silicas in 1-butanol, chloroform, and hexane are shown in Figures 8,9, and 10, respectively. The asymmetry and band broadening arguments presented above for the longer cyanoalkyl immobilized ligands also hold for the 2-cyanoethyl groups attached via either mono- or dichloro chemistry (Table 11). For these backbone chemistries the asymmetry of the nitrile bands increased in the order of decreasing polarity of the liquid medium (i.e., 1-butanol < chloroform < hexane). In the case of the immobilized 2-cyanoethyl ligands synthesized from trichlorosilane, broadened symmetrical C s N stretch bands were observed regardless of solvent type. Each of the spectra of the (2-cyanoethy1)silanizedsilicas in hexane yielded two bands after undergoing Fourier self-deconvolution (Figure 11). These results also are consistent with the above data obtained from the longer immobilized groups. In comparison to the (3-, 5-, 6-, and 11-cyanoalky1)silanized silicas and the (2-cyanoethy1)silanizedsilica synthesized from (2-cyanoethyl)methyldichlorosilane,,v values for the 2cyanoethyl surface synthesized from the corresponding monochlorosilane and trichlorosilane were about 22 cm-l lower and about 7 cm-' higher, respectively. These shifts are reasonable based on changes in the electronic environment
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
801
Based on luminescence studies of (3-(3-pyrenyl)propyl)dimethylchlorosilane chemically bonded to microparticulate silica, Lochmuller et al. (3) also have proposed that surface immobilized ligands do not bond evenly. Rather the synthesis process results in an inhomogeneous distribution with high density regions and molecular aggravation or “clustering”. Likewise, clustering has been suggested by Gilpin et al. to explain solvent entrapment in a chemically bonded decyl HPLC stationary phase ( 6 ) .
LITERATURE CITED
Wf3VENUflBERS
Figure 11. Fourier selfdeconvolution of the P in Figure 10.
N stretch bands shown
surrounding the B-carbon. For (2-cyanoethy1)silanizedsilica, made with the mono-, di-, and trichlorosilanes, two methyl groups and one oxygen atom, one methyl group and two oxygen atoms, and three oxygen atoms, respectively, were attached to the silicon atom. Kitson and Griffith (26) have shown previously that band shifts take place when substitutions are made in the p-position of alkylnitriles. They observed that when electronegative groups were substituted on &carbon atoms, shifts to slightly higher frequencies occurred.
CONCLUSION The above data thus support the idea that in hexane a certain population of immobilized groups can hydrogen bond with surface silanols and a certain population of attached groups sterically cannot hydrogen bond. For this to occur the ligands which do not hydrogen bond with the surface must be sterically restricted. Such restriction is most likely possible through formation of a nonuniform distribution of immobilized surface groups. During the synthesis process areas which are organically rich and organic areas which are sparsely populated form. Groups bonded within rich areas or clusters sterically cannot hydrogen bond with free silanols because of other surrounding cyanoalkyl chains. In sparsely populated organic areas or at the edges of clusters, cyano groups can hydrogen bond.
(1) Gilpin, R. K. Am. Lab. (Fairfield, Conn.) 1982 (March), 103-108. (2) Gllpin, R. K. J. Chromatogr Sci. 1984, 22, 371-377. (3) Lochmuller, C. H.; Colborn, A. S.; Hunnicutt, M. L.; Harris, J. M. Anal. Chem. 1983, 55, 1344-1348. (4) Lochmuller, C. H.; Wilder, D. R. J. Chromatogr. Sci. 1979, 77, 574-579. (5) Marshall, D. B.; Stutler, K. A.; Lochmuller, C. H. J. Chromatogr. Sd. 1984, 22, 217-220. (8) Giipin, R. K.; Gangoda. M. E.; Krishen, A. E. J. Chromatogr. Sci. (l982), 20, 345-348. (7) Gllpin, R. K.; Gangoda, M. E. J. Chromatogr. Sci. 1983, 27,352-361. (8) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. SOC. 1983, 705, 1848-1851. (9) Gllpln, R. K.; Gangoda, M. E. Anal. Chem. lQ84, 56, 1470-1473. (10) Slotfeldt-Ellingsen, D.; Resing, H. A. J. Phys. Chem. 1980, 6 4 , 2204-2209. (11) Sander, L. C.; Callis, J. B.; Field, L. R. Anal. Chem. 1983, 55, 1068-1 075. (12) Leyden, D. E.; Kendall, D. S.; Burggraf, L. W.; Pern, F. J.; De Bello, M. Anal. Chem. 1982, 5 4 , 101-105. (13) Berendsen, G. E.; Pikaart, K. A.; De Galan, L. J. Liq. Chromatogr. 1980, 3, 1437-1464. (14) Berendsen, G. E.; De Galan, L. J. Liq. Chromatogr. 1978, 7 , 561-586. (15) Grushka, E.; Kikta, E. J., Jr. Anal. Chem. 1977, 4 9 , 1004A-1014A. (16) Kirner, W. R. J. Am. Chem. SOC. 1930, 52, 3251-3256. (17) Gaubert, P.; Linstead, R. P.; Rydon, H. N. J. Chem. SOC. 1937, 1971-1979. (18) Gangoda, M. E.; Gllpln, R. K. J. Labelled Compd. Radiopharm . 1982, 79, 1051-1055. (19) Wilt, J. W.; Massie, S. N.; Dabek, R. B. J. Org. Chem. 1970, 35, 2803-2805. (20) Kauppinen, J. K.; Moffatt, D. J.; Mantsch, H. H.; Cameron, D. G. Appl. Spectrosc. 1981, 35, 271-276. (21) Compton, D. A. C. Spectral Lines 1983, 5 , 4-7. Reprlntedas Nicolet FT-IR Application Note 8311. (22) Bellamy, L. J. “The Infrared Spectra of Complex Molecules”, 2nd ed.; Chapman and Hall: New York, 1980; p 241. (23) Rochester, C. H. Trebilco, D. A. Chem. Ind. 1978, 127-128. (24) Rochester, C. H.; Yong, G. H. J. Chem. SOC., Faraday, Trans. 7 1980, 76, 1466-1475. (25) West, R.; Baney, R. H. J. Am. Chem. SOC. 1959, 8 1 , 6145-6148. (26) Kitson, R. E.; Griffith, N. E. Anal. Chem. 1952, 24, 334-337.
RECEIVED for review September 28,1984. Accepted December 3, 1984.
