An infrared spectroscopic study of complex formation between propionitrile and phenol

Spectrochimica Acta. Vol. 48A, No. 8, pp. 1139-1148. 1992

0584-8539192 $5.00+0.00 t~) 1992 Pergamon Press Ltd

Printed in Great Britain

An infrared spectroscopic study of complex formation between propionitrile and phenol J. C. F. NG, Y. S. PARK and H. F. SHURVELL* Department of Chemistry,Queen's University,Kingston,Ontario, Canada K7L 3N6 (Received 22 October 1991; in final form and accepted 9 January 1992)

Abstract--The nitrilestretchingregionof binarysolutionsof phenol in propionitrilehas been studiedby means of infrared (IR) spectroscopy.The number of independentlyvaryingspectral componentsin this region was determinedby FactorAnalysisto be two. The resultsof FactorAnalysisweresupportedby the observationof a pseudo-isosbestic point. The band envelope was fitted with Lorentzian-Gaussianproduct functionsusing a non-linear least squares procedure. The results indicatethe formationof a 1:1 complex.Equilibriumconstants calculated for the complex formationreactionshow a strong concentrationdependence.

INTRODUCTION

FACTOR Analysis and band contour resolution have been used in the study of a variety of equilibria involving free and complexed molecules in hydrogen bonded systems. These include self-association of carboxylic acids in CCL solution [1, 2] and in aqueous solution [3-5] and self-association of phenol i n CCL solution [6, 7]. Complex formation between chloroform and di-n-butyl ether [8] and pentachlorophenol and acetone [9] have both been studied in CCL solution. The spectroscopic analysis depends on the existence of different vibrational frequencies for a functional group in the free and hydrogen-bonded species. The frequency shifts are usually small and this leads to a complex envelope of overlapping bands. Factor Analysis (FA) provides a determination of the number of independently variable components (NC) which contribute to a complex band envelope [10, 11]. The program of JONES and PITHA [12] utilizes the results of FA and fits NC Gaussian-Lorentzian product functions to the envelope. The integrated absorbances (areas) of the isolated component bands together with the Beer-Lambert Law enable equilibrium constants for the system to be calculated. One class of hydrogen bonded system which has been studied extensively is that involving phenols and nitriles [13-15]. Previous studies were performed using ternary systems, containing the nitrile and the phenol dissolved in an inert solvent. The spectroscopic work was focused on the O - H stretching band of the phenol. Studies of these phenol-nitrile-solvent systems demonstrated that a systematic increase in the phenol concentration resulted in the development of an additional band on the low frequency side of the free phenol O - H stretching band. The new band has been assigned to the OH stretching mode of a hydrogen bonded complex formed between the O - H group of the phenol and the C---N group of the nitrile. MITRA [15], and SOUSA-LOPESand THOMPSON [14] have both postulated the exclusive existence of 1:1 complexes between phenols and nitriles dissolved in inert solvents. The analysis of these ternary systems lead to the determination of equilibrium constants and thermodynamic data. We are not aware of any spectroscopic work on simple binary systems of phenols and nitriles. It is of interest to study such systems and to compare them with the corresponding ternary systems. In the present work a binary system involving mixtures of varying compositions of propionitrile and phenol was analyzed by means of the IR absorption band arising from stretching of the nitrile functional group near 2250 cm- 1. The possible existence of more than one complex was investigated through the use of FA and isobestic behavior. Infrared spectra between 2150 and 2400 cm -~ of solutions of various compositions were recorded. The C---N stretching band envelopes were resolved into their *Author to whomcorrespondenceshouldbe addressed. 1139

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J . C . F . NG et al.

component bands and the areas of the bands were obtained. Then, using the B e e r - L a m b e r t law, an equilibrium constant (K) was calculated for the formation of a 1:1 complex for each binary mixture. For reasons that will be discussed below, these values of K show a strong concentration dependence.

EXPERIMENTAL

Infrared spectra were recorded on a Perkin-Elmer 983G IR spectrophotometer. Solutions were contained in a Wilks Scientific variable pathlength cell with BaF2 windows. The pathlength was calibrated using the standard absorber method [16] with the 845 cm -I band of benzene. A constant pathlength of 0.0234 mm was used for the recording of all spectra. The temperature was 23°C. Solid phenol was obtained from BDH Chemicals Company Inc. and propionitrile was purchased from Aldrich Chemical Company Inc. Infrared spectra of both chemicals confirmed their purity. Mixtures were prepared from these compounds, by weight in a glove-box through which there was a continuous flow of dry nitrogen. Prior to experimental operations, the relative humidity inside the box was monitored in order to ensure the high degree of dryness of phenol and it never excceded 20%. The mole fraction of phenol in the solutions prepared ranged from 0.167 to 0.600. All solutions were scanned from 2400 to 2150cm -1 using a resolution of 2cm-L A higher resolution was not justified, because no change in the structure of the band envelopes was observed using smaller spectral slit widths. Replicate spectra were digitized at 0.5 cm -I intervals and transferred to an IBM-PC compatible computer via an RS232 serial interface for subsequent Factor Analysis, band fitting and plotting. Band resolution of the IR spectra was performed using the program PCl16 of JONES and PITHA [12]. Prior to band resolution a baseline was removed from each spectrum. The fitted band envelopes together with the component bands were obtained using the programs PC122 [12] and PDP11 [10] and plotted on a Hewlett-Packard 7475A graphic plotter. Factor Analysis was carried out using an expanded version of the program CHISQ [10]. All data manipulations were performed on a Zenith Z286/12 computer.

COMPUTATIONS

Factor Analysis Factor Analysis (FA) or Principal C o m p o n e n t Analysis is a computational technique that determines the n u m b e r of principal components or factors present in a set of data. The technique has been described recently in monographs by MALINOWSKI and HOWERY [11] and HARMAN [17]. It is particularly useful when applied to systems with constitutive properties such as I R absorbance or Raman intensity [4, 10-12, 18, 19]. The m e t h o d involves forming a data matrix from a set of experimental band envelopes. The eigenvalues and eigenvectors of this matrix are then calculated. These are used to determine the number of linearly independent components (NC) in the band envelope. In applying F A to analysis of IR spectra, a matrix Q is f o r m e d from the product of the NS x N W IR absorbance matrix (A) and its transpose (AT), where N W is the number of wavenumber data points and NS is the number of solutions of varying concentration. It is assumed that every point in the intensity matrix can be expressed as a linear sum of products: NC

Aij = ~', eik Ckj

(1)

k=l

where Aij is the absorbance at the ith data point in the spectrum of the jth solution, Ckj is the concentraton of the kth species in the jth solution and eik is the molar absorption coefficient of the kth species at the ith data point [10]. The number of independently varying components (NC) contained within the system is equivalent to the number of significant non-zero eigenvalues (m) of Q. Several statistical tests have been developed to

•

IR study of complex formation between propionitrile and phenol

J,i ~)

1141

~_0.000

L//J\'

o,o.

Q3 n-0 oo nq

22'50 WAVENUMBERS

2200 (CM--1)

Fig. 1. IR spectra of seven mixtures of phenol and propionitrile; mole fractions of phenol are noted on each curve.

determine the value of NC [10, 20-23]; these will be described in the Results and Discussion section. Band resolution

The program P C l l 6 [12] was utilized to resolve the IR band envelopes into component bands. The individual bands are represented by products of Lorentzian and Gaussian functions. When this number of absorbing components NC is known, the fitting procedure consists of varying the peak position, height, and width parameters for all components until a satisfactory fit is obtained. Knowledge of the quantity NC is therefore essential for the band resolution procedure.

RESULTS AND DISCUSSION

Comprehensive studies of the IR spectrum of propionitrile, in both its liquid and gaseous states have been reported [24]. The spectrum of pure liquid propionitrile contains two absorptions between 2400 and 2150 cm-L These are a strong band centred at 2249cm -~, which is assigned to the C~-N symmetric stretch (A') and a very weak shoulder at 2263 cm -~, which is assigned to the combination of the cn3 deformation (1429 cm -~) and the C - C - C stretch (834 cm-~). The distinctive feature of phenol/nitrile systems noted by W.rrE and T.OM~ON [13] was the upward frequency shift of the C - N stretching vibration in the associated molecules. Similar effects have been noted in hydrogen bonded systems involving alcohols and nitrogen-containing rings [27]. Figure 1 shows a montage of eight IR spectra recorded from solutions of increasing mole fraction of phenol in propionitrile. It can be seen that the absorbance of the band at 2249 cm- ~decreases with addition of phenol. A new band appears at first as a shoulder at low concentrations of phenol. As the mole fraction of phenol increases, this shoulder develops into a band centred at 2255 cm -~. Finally the 2255 cm -1 band dominates the spectrum at high concentrations of phenol, leaving the band due to the C---N strtetch of free propionitrile as a shoulder on the low frequency side. At all concentrations a second shoulder is evident at 2263 cm -~. This feature is present in the spectrum of pure propionitrile and appears to be independent of the concentration of phenol. Thus three

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absorption bands make up the C--N stretching band envelope. Factor Analysis was used to determine the number of independent absorbing components.

Factor Analysis results The output data of the CHISQ program for the system of solutions is summarized in Tables 1 and 2. Statistical tests developed to determine NC may be divided into those which depend upon an a priori knowledge of the experimental error contained within the data and those which are extensions of the Theory of Errors for abstract Factor Analysis. Three tests from the latter group are the imbedded error (IE) [11, 20], the indicator function (IND) [11, 20], and the f(m) function [21]. When the value of rn is equivalent to NC, the values of IE and IND should have their minimum values. In the present case IND becomes a minimum at m = 3 , while IE fails by a factor of 10 between m = 1 and m = 2 and then levels off. The f(m) was introduced as an additional test for cases where there is a broad minimum in the IE value. In the present case this occurs at m = 2. Four statistical tests have been developed for the determination of NC based on an estimation of the experimental error in the absorbance measurements, the value of which in the present case varies between 0.002 and 0.005 absorbance units. This experimental error is greater than the residual standard deviation (RSD) [22] for all eigenvalues after m--1. HUGUS and EL-AWADY derived a test which involves the comparison of each eigenvalue of Q (Am) with its standard error [23]. It is seen in Table 1 that only the first two eigenvalues are greater than the standard error in ;tin. This indicates that NC = 2. A second test devised by HcGus and EL-AWADY involves the comparison of the generated values of x 2 ( m ) with the calculated expectation value ( N C - m ) ( N S - m ) [23]. In the present c a s e z 2 ( m ) becomes less than the expectation value when m = 2. A further indication of the value of NC can be obtained from a comparison of the experimental data matrix with the matrix regenerated using the first m eigenvectors. NC is assumed to be the smallest value of m which regenerates the absorbance matrix with most elements within three times the estimated error (o) of the absorbance measurements [23]. For experimental errors of 0.004 and 0.005, this criterion is satisfied at m -- 2. For the present data, five of seven tests conclusively indicate NC -- 2 and the remaining tests did not yield contradictory results. The existence of two species in equilibrium supports the unique 1:1 complex structure proposed by WHITE and THOMPSON [13] and MITRA [15].

Isosbestic points The previous discussion indicates that the two forms of propionitrile (free and complexed) are present in equilibrium. The C--=N stretching frequency of the free species is 6 cm-~ lower than that of the complex. This results in two overlapping bands in the spectrum. If there are only two species present there may be a frequency between the two bands at which the absorbance is constant for all compositions of the system. This point at which the band envelopes all intersect is known as an isosbestic point [28, 29]. However, an isosbestic point will only be observed if the total concentration of the C = N Table 1. Results of Factor Analysis of IR spectra of the phenol-propionitrile system

rn

3.,,

Standard error in ~.,,

RSD x 102

IE x 102

IND x 103

f(m) × 10 -1

1 2 3 4 5 6 7

357.2737 19.7208 0.0878 0.0076 0.0045 0.0016 0.0011

0.1309 0.1372 0.1144 0.1010 0.1655 0.1246 0.1306

1.190 0.086 0.033 0.023 0.014 0.009 .

4.497 0.458 0.213 0.171 0.118 0.080

3.305 0.343 0.203 0.252 0.349 0.974 .

0.348 1.982 1.019 0.078 0.324 0.024

.

.

IR study of complex formation between propionitrile and phenol

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Table 2. X2 test and regeneration of the absorbance matrix m

(NW-m)(NS -m)

X2

3or

1 2 3 4 5 6 7

8436 7025 5616 4209 2804 1401 0

218,975 2320 566 272 166 70 0

662 69 0 0 0 0 0

group is constant and the molar absorption coefficients of the group in the two forms of the compound are equal. Previous work on hydrogen bonded systems has demonstrated that the molar absorption coefficient associated with the vibration of an acceptor group often increases significantly upon association [30, 31]. In the present case, the superimposition of the raw spectra in Fig. 2 shows no isosbestic point. However, BULMER [32] has demonstrated the existence of a pseudo-isosbestic point for such a system when the absorption bands are normalized to unit area. This procedure differentiates between systems for which the absence of an isobestic point is due to inequality of absorption coefficients and systems in which the absence of an isosbestic point is an indication of more than two absorbing species. In the present system the total area of the three bands between 2400 and 2150cm -~ was used to normalize each spectrum. Seven areanormalized phenol-propionitrile spectra are shown in Fig. 3. A pseudo-isosbestic point is dearly seen near 2252 cm- ~. This independently verifies the value of NC = 2 obtained from FA.

Band resolution The program PCl16 [12] was utilized for resolution of the observed bands into components due to the free propionitrile, the associated propionitrile, and the 1429+ 834 cm-~ combination band. The results of the band resolution procedure are summarized in Table 3. Sample resolved spectra are shown in Figs 4, 5 and 6. General trends are observed for the resolved bands with increasing mole fraction of phenol. A significant growth in the intensity of the C--N band of the associated molecule is noted together with a corresponding, more marked, decrease in the intensity of the C--=N band of the free molecule;

2300

22'50

WAVENUMBER (CM

2200 --I

)

Fig. 2. IR spectra of seven mixtures of phenol and propionitrile; spectra plotted as recorded.

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2300

22'50 WAVENUMBER

2200 (CM

--1

Fig. 3. IR spectra of seven mixtures of phenol and propionitrile; spectra normalized to unit area.

Calculation of the equilibrium constant The calculation of equilibrium constants from spectroscopic data has been discussed by BULMER and SHURVELL[10] and MCBRYDE [33]. In the present system the solutions vary with respect to the known total concentration of propionitrile, CT. The concentration of free, CF, and associated propionitrile, CA, must add up to CT, thus:

(2)

C T = C F --I-C A.

Using the Beer-Lambert Law, A = eCd (where A = absorbance, C = concentration, d=pathlength and e = m o l a r absorption coefficient), the above equation may be rewritten: AF

CT=~

AA

~.Ad,

(31

which can be rearranged to give: AA CT d

eA AF eA

(4)

eF CTd"

If AA/Cvd is plotted against AF/CTd, a straight line should be obtained with a slope of -eA/eF, and an intercept eA. Values of the two integrated molar absorption coefficients were obtained from Eqn (4) by means of a linear least squares fit of the data. The calculated values for the free Table 3. Integrated absorbances (areas) of band resolved spectra Mole fraction phenol 0.167 0.200 0.250 0.333 0.400 0.556 0.600

Area of combination band

Area of complexed C - N band

Area of free C---N band

239.5275 251.0904 266.1218 269.5261 314.2881 283.2258 304.3526

131.0616 139.8968 206.1856 247.1188 351.7204 388.8963 411.9347

957.2953 943.4022 827.0833 666.8773 565.5728 234.8122 212.0217

IR study of complex formation between propionitrile

22’50

23’00

and phenol

1145

2: -1

WAVENUMBER

(CM

)

Fig. 4. Band resolved IR spectrum of 0.167 mole fraction of phenol in propionitrile.

the complexed C=N stretch were 4.65 respectively. For the association reaction: and

x 104

I mol cm-’ and 5.39 x 104 I mol cm-‘,

C6H50H + CzHsCN = C6H50H - - - NCC2H5, the equilibrium

1

constant K is given by:

where C, is the concentration of phenol (monomer) at equilibrium. It is difficult to calculate C,, because there are other equilibria competing for phenol monomers. These are the self-association equilibria: 2C6HSOH = (C6H50H)2 and nC,H,OH

n=3,4,...

= (C,H,OH),

I

I 2300

2: 30

2250 -1 WAVENUMBER

(CM

>

Fig. 5. Band resolved IR spectrum of 0.333 mole fraction of phenol in propionitrile. U(A) 48:8-6

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23.00

22.50

2:

30

-1 WAVENUMBER Fig. 6. Band resolved

However, equation:

IR speclrum

if self-association

(CM

)

of 0.400 mole fraction of phenol in propionitrilt.

of phenol is neglected,

C, may be obtained

from the

c,-co,-c, Ot-

where CF is the initial concentration of phenol. The values calculated for K for the phenol-propionitrile equilibrium [using Eqn (6) to obtain Cr] are summarized in Table 4. These values of K show a strong concentration dependence. This apparent anomaly is almost certainly due to the neglect of the selfassociation of phenol. Also, the high concentrations of phenol used in the present binary system negate the assumptions generally made in the calculation of equilibrium constants for dilute ternary systems. The assumption that activities may be replaced by concentrations of the species in equilibrium is unsatisfactory for the high concentrations used in the present binary system in which the deviations of the activity coefficients from unity are likely to be large. The equations utilized in the determination of the equilibrium constant assume the validity of the Beer-Lambert Law, which involves a linear relationship between absorbance and concentration. However, it is known that at high concentrations the relationship between absorbance and concentration often becomes non-linear. Previous work on this hydrogen bonded system was carried out at low concentrations in Ccl4 solution [13], or in C&f4 solution [14]. Calculations of equilibrium constants, were based on values of the extinction coefficient obtained from the peak maximum of Table 4. Equilibrium constant calculation (values of C in mol 1-l) CT

10.42 9.92 9..51 8.6.5 7.69 5.26 5.10

CF=(A&d) 8.90 8.67 7.60 6.13 5.20 2.16 1.95

c*= (AJE,d) 1.04 1.11 1.63 1.96 2.79 3.08 3.24

C,=Co,-C, 1.04 1.37 1.54 2.37 2.34 3.94 4.39

K (1mol-‘) 0.11 0.09 0.14 0.13 0.23 0.36 0.38

IR study of complex formation between propionitrile and phenol

1147

the free O H stretching band. Integrated areas of free and associated bands were not used. SOUSA-LoPES and THOMPSON [14] reported a value of K of 7.25 for the formation of the 1:1 phenol-propionitrile complex in C2C14 at 25°C and 4.65 at 43°C. WHITE and THOMPSOrq [13] gave a value of 4.81 for K for the system in CC14 solution at about 40°C. These previous studies were carried out using low phenol concentrations, in which no self association was detectable in the O H stretching region of the infrared spectrum.

CONCLUSIONS

The band resolution technique has been applied to IR spectra of binary solutions of propionitrile and phenol. Analysis of the spectra suggests that the formation of a single 1:1 hydrogen bonded complex between propionitrile and phenol occurs. Support for the formation of a 1:1 complex is provided by both Factor Analysis and the observation of a pseudo-isosbestic point. The band corresponding to the C ~ N stretch of free propionitrile is centred at 2249 cm -1. The associated C = N group gives rise to a band centred at 2255 cm- t. Resolved spectra showed a decrease in the integrated absorbance of the band assigned to the free nitrile molecule with increasing mole fraction of phenol. This was accompanied by a more rapid increase in the intensity of the band assigned to the associated molecule. Equilibrium constants for the system were calculated utilizing the integrated absorbances of the component bands obtained by band resolution. The values for K ranged from 0.11 1mol-1 for low concentrations of phenol to 0.381 mo1-1 for high concentrations of phenol. This apparent concentration dependence of K is probably due to neglect of self-association of phenol. Also, in the calculation of K it was assumed that all activity coefficients are unity and that the Beer-Lambert law is obeyed. Acknowledgements--The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support.

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P. Venkateswarly, J. Chem. Phys. 19, 293 (1950). H. Takahashi, K. Mamola and E. K. Plyler, J. Molec. Spec. 21,217 (1966). M. D. Cohen and E. Fisher, J. Chem. Soc. 3044 (1962). T. Nowick-Jankowska, J. Inorg. Nucl. Chem. 33, 2043 (1971). S. N. Vingogeadov and R. H. Lindell, Hydrogen Bonding. D. Van Nostrand Reinhold Co., New York (1971). [31] G. C. Pimentel and A. L. McClellan, The Hydrogen Bond. W. H. Freeman and Sons, San Francisco (1960). [32] J. T. Bulmer, Ph.D. Thesis, Queen's University, Kingston, Ont. (1973). [33] W. A. E. McBryde, Talanta Review 21,979 (1974).