Thermodynamic properties of the hydrogen bonded complexes between N-substituted anilines and proton acceptors

Journal of Molecular Liquids 109 (2004) 125–135

Thermodynamic properties of the hydrogen bonded complexes between N-substituted anilines and proton acceptors V.E. Borisenkoa,*, Yu.A. Zavjalovaa, T.G. Tretjakovaa, Z.S. Kozlovaa, A. Kollb a

Department of Physics, Tyumen State University, 10 Semakova Str., 625003 Tyumen, Russian Federation b Faculty of Chemistry, University of Wroclaw, { 14 F. Joliot-Curie, 50-383 Wroclaw, { Poland Received 23 October 2002; accepted 25 June 2003

Abstract The influence of temperature, within the range 285–340 K, on spectral characteristics of n(NH) absorption bands in ‘free’ Nmethyl aniline, N-ethyl aniline, diphenylamine and N-methyl-4-nitroaniline in CCl4 as well as in their hydrogen bonded complexes with acetonitrile, tetrahydrofurane, dimethylformamide (DMF), dimethylsulfoxide (DMSO) and hexamethylphosphoramide (HMPA) was studied. Spectral moments of n(NH) absorption bands were determined: M (0) —the zero spectral moment (integrated intensity), M (1)—the first spectral moment (the centre of band gravity), M (2) —the second central moment as well as ‘effective’ half width of absorption band (Dn1y2)effs2(M (2) )1y2 . The coefficients of the linear correlation of these parameters with a temperature variation YsaTqb (YsM (0) , M (1) , 2(M (2) )1y2 ) were calculated for ‘free’ and hydrogen-bonded molecules. It was demonstrated that these spectral characteristics considerably depend on the character of the N-substitute. The difference in the position of absorption bands nm(NH) in the spectra for non-bonded molecules of N-substituted anilines in CCl4 is caused by s– p conjugation of alkyl radicals with the N atom and substitute polarization influence. Thermodynamic parameters yDH and DS of the complex formation process between amines and proton acceptors were determined from the temperature dependence of the equilibrium constants on the basis of van’t Hoff equation. The increase of yDH enthalpy was observed in the rows of proton donors: N-alkyl-substituted anilines, diphenylamine and N-methyl-4-nitroaniline; and proton acceptors: acetonitrile, tetrahydrofur(1) ane, DMF, DMSO and HMPA. The correlations between yDH values and the shift of the first spectral moment DM(c1)sMm y 1y2 1y2 1y2 (1) Mc , on the one hand, and yDH values and the increment of the square root of integrated intensity DB sBc yBm , on the other hand, were found on passing from free to bonded molecules. However, the proportionality factor a in the equation yDHs aDB 1y2 depends on individual characteristics of proton donors and remains the same for each proton donor in the row of proton acceptors. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: N-substituted anilines; Thermodynamic parameters; H bond

1. Introduction The NH bonds of the amine group are responsible for numerous intermolecular interactions like association of anilines, amides or biologically very important purine and pyrimidine bases. It was challenging to understand to which extent two NH bonds in the amine group interact with each other. In this paper, we aim to investigate both in thermodynamic and spectroscopic aspects, the effects of substituting one of the NH bonds by electron donating or accepting constituents. The acidic properties of such NH groups will be compared *Corresponding author. Fax: q7-3452-261798. E-mail address: [email protected] (V.E. Borisenko).

with those in anilines. Topologically the situation is simpler and spectroscopic effects seem to be easier to understand. The thermodynamic function of the complex formation process is also more directly accessible. They are very important characteristics of this process. In Ref. w1x we have studied the temperature dependence of spectral characteristics of n(NH) bands in free and Hbonded molecules with various proton acceptors of pyrrole and indole in CCl4 solutions, within the temperature range 290–350 K. yDH and yDS values and the complex formation constants were determined for the associates with acetonitrile (CH3CN), dimethylformamide (DMF), dimethylsulfoxide (DMSO) and hexamethylphosphoramide

0167-7322/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2003.06.003

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V.E. Borisenko et al. / Journal of Molecular Liquids 109 (2004) 125–135

(HMPA). It was found that the temperature dependences of maximum positions (nmax), half widths (Dn1y2) and integrated intensities (B) of absorption bands n(NH) in monomers and complexes are practically linear. The coefficient as≠nm(NH)y≠T of the temperature dependence of nm(NH) band positions for non-bonded molecules of indole and pyrrole equals to 0.05 cmy1 Ky1 in CCl4, while the change of the band half width per Kelvin is two times lower. The increase of temperature leads to a relatively small decrease of the integrated intensity Bm of nm(NH) absorption bands for non-bonded molecules of indole and pyrrole in CCl4. Qualitatively similar changes of nc(NH) bands for the complexes with hydrogen bonds were observed; however, the value of the effects appears to be 5–7 times larger. The temperature dependencies of the spectral characteristics of indole and pyrrole are similar to those previously found for alcohols and phenols w2,3x. Analogous studies were performed for bis(polyfluoroaryl)amines forming complexes with CH3CN, THF, DMF, DMSO and HMPA w4x. Temperature effects on n(NH) bands, equilibrium constants and thermodynamic parameters of H-bonded complexes for the set of different NH proton donors in the same row of proton acceptors were studied in Ref. w5x. The control of the universal applicability of the ‘intensities rule’, based mostly on the study of OH donors, was performed w6 x . According to Ref. w6x, the enthalpy of the complex formation process linearly depends on the increment of the square root of the integrated intensity when passing from ‘free’ molecules (Bm) to H-bonded (Bc). yDHsaw(Bc=10y4)1y2y(Bm=10y4)1y2x sa=DB1y2

(1)

It was assumed w6x that the proportionality coefficient a in Eq. (1) was universal and did not depend on the donor type. In Ref. w5x it is shown that the dependence Eq. (1) exists in reality; however, the proportionality coefficient a depends on individual features of proton donors. Thus, for the alcohols and phenols the coefficient a equals 2.91 w6x, for indole—3.2, for carbazole and Nmethylbenzamide—4.6 w5x. The temperature influence on spectral characteristics of the imino group as well as thermodynamic characteristics of N-substituted perfluorobenzamides with various proton acceptors were investigated in Ref. w7x. The influence of the substitution in the phenyl radical of aniline on dynamic, electro optic and energetic characteristics of the amino group in 1:1 and 1:2 complexes of substituted anilines with proton acceptors was investigated w8–12x. It was shown that complexes of 1:1 composition are stronger than those of 1:2, and

the difference becomes larger with the increase of the strength of the hydrogen bond. The environment can strongly impact the process of the complex formation of donor and acceptor molecules w13–17x. In some cases (CS2, CCl4) a solvent is relatively inert and its influence is carried out through dielectric features, while in other cases the solvent molecules directly interact with the reactant molecules. With reference to these facts, one must account for the interaction between the reactants and the solvent molecules in the study of the complex formation. In the simplest case, when the specific interaction between the solvent and solute molecules does not take place, it is necessary to take into account the distortion of the internal field of light wave w18–21x, which leads to the difference between true (B*) and observed (B) integrated intensity: BUs

9n B Žn q2.2 2

(2)

where n is the refraction index. The aim of the present work is to study the influence of N-substitution in aniline on the spectroscopic characteristics of the n(NH) absorption bands in free and hydrogen-bonded complexes and the determination of thermodynamic parameters of the complexes of Nsubstituted anilines with various proton acceptors in CCl4. 2. Experiment The spectral characteristics of n(NH) absorption bands of ‘free’ N-methyl, N-ethyl aniline, diphenylamine and N-methyl-4-nitroaniline and their complexes with various proton acceptors were determined within the temperature range 285–350 K. The samples of the above mentioned compounds were obtained from Aldrich Chemical Company, Inc. We applied proton acceptors CH3CN, THF, DMF, DMSO and HMPA, the properties of which vary within a wide range. All the substances were previously purified from contaminations and moisture using the methodology described in Ref. w22x. Spectroscopic measurements were performed on modernized Specord 75 IR spectrophotometer (Karl Zeiss, Jena) with automatic recording of spectra on the basis IBM-Pentium. In the experiments dismountable cells with CaF2 windows were applied. The spectra were registered in the optimal conditions with appropriate choice of the spectral width of aperture, the speed of scanning and the time response of detecting unit w23x. The maximal photometric precision was achieved by regulation of the thickness of the absorbing layer. Thermostat unit maintained the required temperature with precision, "0.5 K.

V.E. Borisenko et al. / Journal of Molecular Liquids 109 (2004) 125–135

As the n(NH) absorption bands of monomers and Hbonded complexes overlapped, they were separated graphically. The criteria of proper separation of the absorption bands were the maximum position and half width of the selected nm(NH) band, which in the experimental conditions (low donor and acceptor concentrations in solution) were practically similar to the same parameters of the nm(NH) band in CCl4. The spectrum record was written in the following coordinates. ´sln(I0 yI)n yCD0 d (dm3 (mole cm)y1 ), n (cmy1), y3 where CD ) is the initial proton donor 0 (mole dm concentration in solution and d is the layer thickness in cm. After the separation of nm(NH) and nc(NH) bands of monomers and complexes the following value was experimentally defined:

127

tral moments were applied as the most reliable ones. Therefore, together with usual spectral characteristics— maximum position (nmax), half width (Dn1y2) and integrated intensity B—we used the spectral moments w24,25x: M (0)—zero spectral moment (integrated intensity B):

|

(9)

M0s ´ndn

M (1)—the first spectral moment (gravity centre of the band):

|´ ndn n

M(1)s

(10)

M(0)

M (2)—the second central moment:

|lnŽI yI. 0

Dms

(3)

D 0

C d

M(2)s

|ŽnyM

The integrated intensity of nm(NH) band is

|

(4)

D Cm d

Therefore, D Cm sC0D

Dm Bm

(5)

For H-bonded complexes, Bcs

CD 0 Dc Cc

(6)

which allows to state the integrated intensity of nc(NH) absorption bands. The equilibrium constant monomer-complex is Ks

Cc C CA m A m

where C is the concentration of proton acceptor nonbonded molecules in solution. After the transformation Eq. (7) is as follows: D CD 0 yCm D z C C yŽC0 yCD m.~ x

A 0

(11)

(8)

|

In the case of asymmetric and complicated contours of absorption bands as spectral characteristics, the spec-

(12)

Spectral moments M (0), M (1), M (2) of nm(NH) and nc(NH) absorption bands were calculated on the basis of Eqs. (9)–(11) replacing integration by addition within three half widths on both sides of band maximum with the step along frequency scale Dns2.5 cmy1, using the programs developed by the authors for personal computers. While determining integrated intensities and equilibrium constants, the concentration variations, resulting from a volume dependence on temperature, were properly accounted. This allows the reduction of the error for K(T) and B(T) determination to 5–7%. Thermodynamic characteristics of the complex formation process were determined according to van’t Hoff equation:

(7)

D m

Dw my

M(0)

ŽDn1y2.effs2ŽM(2).1y2

ln K(T)s

Ks

. ´ndn

related to the effective half width by the following relation w24x

lnŽI0yI.

Bms

(1) 2

yDH 1 DS q R T R

(13)

The error in determination of M (1) (nmax), (Dn1y2)eff (Dn1y2) of the nm(NH) band of monomers did not exceed 1–2 cmy1, while for H-bonded complexes it was 3–5 cmy1. Integrated intensity M(m0) (B) of nm(NH) bands was determined with error of 5–7%, while for complexes y10%. All the linear correlations were processed by the least square method.

V.E. Borisenko et al. / Journal of Molecular Liquids 109 (2004) 125–135

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Fig. 1. n(NH) stretching absorption bands of N-substituted anilines in CCl4 (Ts298 K). (1) N-methyl aniline (0.03 mole dmy3); (2) Nethyl aniline (0.04 mole dmy3); (3) diphenylamine (0.02 mole dmy3) and (4) N-methyl-4-NO2-aniline (0.002 mole dmy3).

3. Results and discussion The absorption bands nm(NH) of stretching vibrations in the imino group of N-substituted anilines in CCl4 at 298 K are presented in Fig. 1. The spectral characteristics are given in Table 1. The parentheses in Table 1 contain the values of the maximum positions as well as half widths directly obtained from the spectra. These results are in good agreement with the previously known data for the absorption bands nm(NH) of diphenylamine w26–28x and N-methyl aniline w27x. Table 1 shows that spectral characteristics of the absorption nm(NH) bands substantially depend on the character of the N-substitute. The changes of the integrated intensities are qualitatively in accordance with the values of the inductive Taft constant for substitutes

w22x. However, no correlation of nm(NH) frequencies and polar Taft s constants is observed. It is assumed that together with the inductive influence of the substitutes, in N-alkyl substituted anilines there is another mechanism that is able to influence electron density distribution in the area of the imino group localisation. Electron configuration of the N atom in basic state is 1s22s22p3 at which the valence p-orbits are located in three perpendicular directions as in the NH3 molecule. In most organic compounds the electron configuration of the N atom corresponds to 1s22s2p4 when sp2hybridisation takes place and a non-shared electron pair is on the 2p-orbit. The hybridised sp2-orbits are located in one plane, while the non-shared pair is in the perpendicular plane. Aniline molecule has Cs symmetry at which the C–N bond constitutes the angle us43.68 with the plane of the NH2-group w29x, therefore sp2 hybridisation prevails in electron structure of the N atom in the aniline molecule. In Ref. w30x there is an assumption that the electron pairs of C–H bonds of alkyl groups are considerably less localised than C–C bonds and they have the ability similar to a non-shared pair to conjugate with unsaturated systems. The theoretic explanation of this effect named s–p conjugation or hyper conjugation is given in Ref. w31x. Some aspects of this phenomenon are presented in Ref. w32x. Quantum-mechanical calculations of charge distribution on the atoms of the C–NH2 fragment in the molecules of anisidines w33x and aminopyrimidines w34x show that redundant negative charge is localised on the N atom of the amino group, while H atoms have the positive charge. The substitution of one H atom to the methyl group will lead as a result of s–p conjugation to the polarization increase of the N–H bond in Nmethyl aniline. While passing from N-methyl aniline to N-methyl-4-nitroaniline, one should expect a further increase of negative charge on the N atom of the imino group due to the inductive influence of the nitro group. Therefore, it will cause an increase of nm(NH) frequency. The effect of s–p conjugation when passing from methyl radical to ethyl radical decreases w30–32x, thus nm(NH) frequency of N-ethyl aniline is lower than that of N-methyl aniline.

Table 1 Spectral characteristics of nm(NH) absorption bands of the free molecules of N-substituted anilines in CCl4 (Ts298 K) Compounds

M(m1) (nmax) (cmy1)

2(M(m2))1y2 (Dn1y2) (cmy1)

(1) (2) (3) (4)

3442 (3439.6) 3426.4 (3419.1) 3429.7 (3434.2) 3455.1 (3451.7)

36.7 46.7 31.2 32.7

a b

N-methyl amine N-ethyl aniline Diphenylamine N-methyl-4-NO2-aniline

(26.6) (35.2) (24.9) (27.6)

For direct spatial interaction of the substitute with the reaction center (s*). For the substitutes at para-position in phenyl ring (s0n).

Bm=10y3 (dm3 moley1 cmy2)

s (constant Taft) w22x

2.7 2.3 3.6 8.4

(–CH3) 0a (–C2H5) y0.1a (–C6H5) 0.6a (–NO2) 0.82b

V.E. Borisenko et al. / Journal of Molecular Liquids 109 (2004) 125–135

In diphenylamine electron density distribution on the imino group is caused by the inductive influence of the phenyl radical, but this influence is rather lower than the influence of s–p conjugation in N-methyl aniline, which defines the spectral position of the nm(NH) band of diphenylamine. The maximum position (nmax) and the first spectral moment M(m1), Table 1, differ by 3–6 cmy1, while the (1) 1y2 difference in half widths (Dn1y2)m and 2(Mm ) reachy1 es 6–10 cm . These differences are connected with serious discrepancies between the real shape of nm(NH) bands of N-substituted anilines and the Gauss profile. Within the range of 285–340 K the spectral charac(1) (1) 1y2 teristics M(m0)sBm, Mm , 2(Mm ) of nm(NH) bands of N-substituted anilines in CCl4 show practically linear correlation with temperature. The parameters of linear (2) 1y2 regression YsaTqb (YsM(m1), 2(Mm ) , Bm) are presented in Table 2. The temperature shift of the first spectral moment as ≠M(m1) y≠T (cmy1 Ky1) and the coefficient of the change of effective half width as≠(2M(m2))1y2 y≠T (cmy1 Ky1) are of the same order as for other secondary amines w4– 7x. The integrated intensity Bm(T) of the nm(NH) band decreases with the temperature increase. The temperature sensitivity of the integrated intensity as≠Bm y≠T (dm3 moley1 cmy2 Ky1) grows in the row of compounds: N-alkyl anilines, diphenylamine, N-methyl-4nitroaniline. The typical situation observed in the area of n(NH) absorption bands at partial association of N–H donors with proton acceptors is presented in Fig. 2. The values of the shifts of the nc(NH) band for Hbonded complexes in relation to the nm(NH) band for free molecules of N-substituted anilines in CCl4 increase in the row of proton acceptors CH3CN, THF, DMF, DMSO and HMPA. It is practically impossible to separate nm(NH) and nc(NH) absorption bands in case of partial association of N-methyl aniline (Fig. 2) and N-ethyl aniline with CH3CN, due to strong overlapping of the bands. However, for the stronger proton donors—diphenylamine (Fig. 3) and N-methyl-4-nitroaniline—such resolution is possible. We also studied the influence of temperature on spectral characteristics of the absorption bands nc(NH) of H-bonded complexes of N-substituted anilines with different proton acceptors. Fig. 4 presents the temperature effects observed on the nc(NH) bands of N-methyl-4-nitroaniline at partial association with the weakest proton acceptor CH3CN. The temperature increase results in evident decrease of the intensity of nc(NH) bands (a low frequency component of a doublet), while the intensity changes of the nm(NH) band are much less pronounced. The intensity of the latter band depends on two factors—the intensity should increase owing to the increase of the number of

129

monomer molecules and it should decrease owing to temperature changes of integrated intensity Bm(T). Fig. 5 presents the pattern of temperature changes of the spectrum while associating N-methyl-4-nitroaniline with the strongest of the applied proton acceptors— HMPA. Temperature dependence is observed both for the nc(NH) and nm(NH) bands, as in this case the effect of the increase in the intensity of the nm(NH) band caused by the increase of the number of free molecules pronouncedly exceeds the effect of the decrease in the intensity due to the temperature changes of the absorption integrated coefficient. Similarly, like in the case of nm(NH) of monomers, the spectral characteristics of the nc(NH) absorption band of H-bonded complexes change practically linearly within the studied temperature range. The parameters of the equations of linear regression, YsaTqb, where Ys M(c1), 2(Mc(2))1y2, Bc, are presented in Table 3. The temperature impact on the nc(NH) bands are a few times higher than that for the free molecules of Nsubstituted anilines in CCl4 (Tables 2 and 3). The temperature sensitivity factor as≠Yy≠T of the first spectral moment M(c1) and the ‘effective’ half width (2M(c2))1y2 for N-alkyl- and N-phenyl-substituted anilines are of similar values (Table 3). The exclusion is Nmethyl-4-nitroaniline, for which the factor as ≠(2M(c2))1y2 y≠T is in most cases negative; however, the tendency to its increase in the row of proton acceptors CH3CN, THF, DMF, DMSO and HMPA remains. The similar effect as the decrease of half width of nc(NH) absorption bands in H-bonded complexes with the temperature increase was observed earlier in Ref. w5x studying the association of N-methylphoramide with CH3CN, THF and DMF as well as studying the temperature effects on nsm(NH2) and nas m (NH2 ) absorption bands in free molecules of 2-aminopyrimidine in CCl4 and on nc(NH) bands in 1:1 complexes of 2- and 4-aminopyrimidine with DMF, DMSO and HMPA w34x. At present, we cannot explain this effect in one way. Nevertheless, the fact of the decrease in the half width of nm(NH) absorption bands in free molecules of 2aminopyrimidine in CCl4 allows to produce an idea about possible partial modification of N atom hybridisation at the change of the temperature in solution. However, the mechanism of this effect is not quite clear. The more complicated situation takes place in case of nc(NH) bands in H-bonded complexes. At that we should not exclude the possibility of complex formation with charge transfer at the account of the free pair of pelectrons in the N atom. Table 4 contains the values of spectral characteristics of the nc(NH) absorption bands of the complexes of Nsubstituted anilines, estimated on the basis of the linear equation YsaTqb, at room temperature. The values of relative shifts of the first spectral moment DM (1)s M(m1)yMc(1) and the increment of integrated intensity

130

Proton donors

M(m1) (nmax), (cmy1)

2(M(m2))1y2 (Dn1y2), (cmy1)

Bm (dm3 moley1 cmy2)

a (cmy1 Ky1)

b (cmy1)

r

a (cmy1 Ky1)

b (cmy1)

r

a (dm3 moley1 cmy2 Ky1)

b (dm3 moley1 cmy2)

r

(1) N-methyl aniline

0.052 (0.055)

3426.6 (3423.2)

0.95 (0.96)

0.072 (0.085)

17.6 (1.3)

0.96 (0.96)

y7.77

5003

0.95

(2) N-ethyl aniline

0.051 (0.076)

3411.0 (3394.6)

0.88 (0.89)

0.104 (0.082)

15.7 (10.8)

0.90 (0.93)

y10.51

5424

0.88

(3) Diphenylamine

0.056 (0.065)

3412.9 (3412.9)

0.91 (0.89)

0.072 (0.077)

9.8 (2.0)

0.99 (0.94)

y23.20

10 609

0.93

(4) N-methyl-4-NO2-aniline

0.047 (0.076)

3441.2 (3429.1)

0.97 (0.97)

0.045 (0.078)

19.2 (4.4)

0.87 (0.97)

y63.51

24 279

0.85

V.E. Borisenko et al. / Journal of Molecular Liquids 109 (2004) 125–135

Table 2 The parameters of the linear regression Ysa=Tqb of the spectral characteristics of nm(NH) transitions in N-substituted anilines in CCl4

V.E. Borisenko et al. / Journal of Molecular Liquids 109 (2004) 125–135

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The decrease of equilibrium constants while passing from aniline to N-alkyl substituted anilines can be explained by steric obstacles created by alkyl radicals of N-substituted anilines. However, it should be noted that there is no direct dependence between yDH and equilibrium constant in general case. Such dependence should be expected in the row of proton acceptors with the fixed proton donor, which is observed in our case, Table 5. Diphenylamine constitutes stronger complexes with proton acceptors than N-alkyl substituted anilines. The equilibrium constants of complex formation increase at the same time. For the diphenylamine–THF complex the obtained equilibrium constants K298s1.1 dm3 moley1 appeared to be higher than the values given in Ref. w35x (0.40 dm3 moley1). The introduction of the –NO2 group in the paraposition of the phenyl radical of N-methyl aniline leads to substantial (more than two times) increase of the enthalpy of the complex formation, and a few times increase of equilibrium constants K298, Table 5. The inductive effect of the nitro group in para-position of the phenyl radical on the imino group has the same sign Fig. 2. n(NH) absorption bands of N-methyl aniline (0.03 mole dmy3) in complexes with proton acceptors in CCl4 (Ts298 K). (1) HMPA (0.07 mole dmy3); (2) DMSO (0.25 mole dmy3); (3) DMF (0.25 mole dmy3); (4) THF (0.3 mole dmy3 ); (5) CH3CN (0.5 mole dmy3) and (6) CCl4.

(DBsBcyBm) when passing from free to H-bonded molecules depend on the character of the N-substitute. Table 5 contains the values of thermodynamic characteristics of the complexes between N-substituted anilines and proton acceptors in CCl4 obtained using van’t Hoff Eq. (13) and equilibrium constants K298 monomercomplex. The correlation coefficient r changed within 0.9443–0.9900 while defining yDH enthalpy. For the complexes of aniline with DMF, DMSO, HMPA in CCl4 we earlier obtained w9x the values of yDH1 (for 1:1 complexes), which are equal to 2.2, 3.0 and 3.6 kcal moley1, respectively. The equilibrium constants K298 appeared to be 3.5, 4.3 and 5.5 dm3 moley1, respectively. For N-alkyl substituted anilines (Table 5) some increase of the enthalpy and decrease of K298 equilibrium constants is observed compared to aniline. The yDH values for N-alkyl substituted anilines appeared to be close to those obtained for N-methyl substituted amides w5x. The increase of the enthalpy of H-bond in the same row of proton acceptors while passing from aniline (1:1 complexes) to N-alkyl substituted anilines is likely to be caused by partial modification of the N atom hybridisation and by the increase of the NH bond polarity.

Fig. 3. The n(NH) absorption bands of diphenylamine (0.02 mole dmy3) in complexes with proton acceptors in CCl4 (Ts298 K). (1) HMPA (0.04 mole dmy3); (2) DMSO (0.2 mole dmy3); (3) DMF (0.2 mole dmy3); (4) THF (0.3 mole dmy3 ); (5) CH3CN (0.5 mole dmy3) and (6) CCl4.

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N-methyl-4-nitroaniline: yDHs0.0420DM(1)q1.600;

ns5, rs0.9487 (17)

The general linear regression for all N-substituted anilines, Fig. 6, has the following form: yDHs0.0448DM(1)q0.977;

ns18, rs0.9349 (18)

The as≠(yDH)y≠DM (1) parameter for N-alkyl substituted anilines (Eqs. (14) and (15)) appears to be close to the same parameter for OH-donors (aOHs0.024 kcal moley1 cm w36x), while for diphenylamine (Eq. (16)) and N-methyl-4-nitroaniline (Eq. (17)) it turns out to be almost two times larger. The average value of the a parameter for all N-substituted anilines equals to 0.0448 kcal moley1 cm. Table 5 gives the comparison of the experimental yDH values and yDHcalc values obtained using the ‘intensity rule’ w6x. The a factor from Eq. (1) appears to be substantially larger for N-substituted anilines than that for OH donors (aOHs2.91 w6x). For phenyl-substituted anilines w9x as2.4–3.3, for N-substituted perfluoFig. 4. Influence of temperature on the n(NH) absorption bands of the complex of N-methyl-4-NO2-aniline (0.004 mole dmy3) with CH3CN (0.5 mole dmy3) in CCl4. (1) 289 K; (2) 298 K; (3) 305 K; (4) 316 K and (5) 328 K.

that it has at the direct polar conjugation of the imino group with the phenyl radical in diphenylamine, Table 1. However, the influence of the nitro group on the imino group is a little higher than the influence of the phenyl radical. Thus, it results in the increase of yDH enthalpy and equilibrium constants of the complex formation process while passing from diphenylamine to N-methyl-4-nitroaniline. There exists a linear correlation between yDH and the shift of the first spectral moment DM (1), which is individual for each proton donor: N-methyl aniline: yDHs0.0232DM(1)q1.973;

ns4, rs0.9411 (14)

N-ethyl aniline: yDHs0.0221DM(1)q2.919;

ns4, rs0.8327 (15)

Diphenylamine: yDHs0.0520DM(1)q0.360;

ns5, rs0.9811 (16)

Fig. 5. The temperature effect on n(NH) absorption bands in complexes of N-methyl-4-NO2 -aniline (0.004 mole dmy3 ) with HMPA (0.011 mole dmy3) in CCl4. (1) 289 K; (2) 298 K; (3) 306 K; (4) 317 K and (5) 327 K.

Compounds

Proton acceptors

M(c1) (cmy1)

2(M(c2))1y2 (cmy1)

a (cmy1 Ky1)

b (cmy1)

r

CH3CN THF DMF DMSO HMPA

– 0.10 0.32 0.20 0.21

– 3359.3 3290.5 3301.8 3265.2

– 0.92 0.94 0.95 0.90

(2) N-ethyl aniline

CH3CN THF DMF DMSO HMPA

– 0.13 0.14 0.19 0.25

– 3328.7 3325.1 3290.4 3237.3

(3) Diphenylamine

CH3CN THF DMF DMSO HMPA

0.11 0.14 0.15 0.16 0.18

(4) N-methyl-4-NO2-aniline

CH3CN THF DMF DMSO HMPA

0.21 0.27 0.38 0.46 0.22

(1) N-methyl aniline

a (cmy1 Ky1)

Bc (dm3 moley1 cmy2)

b (cmy1)

r

a (dm3 moley1 cmy2 Ky1)

b (dm3 moley1 cmy2)

r

– 0.18 0.44 0.29 0.23

– 12.69 214.64 170.53 143.26

– 0.98 0.96 0.89 0.83

– y67.7 y117.6 y235.6 y242.2

– 28 396 46 670 92 156 96 676

0.89 0.88 0.87 0.96

– 0.97 0.90 0.88 0.88

– 0.12 0.13 0.16 0.25

– 14.71 27.31 40.50 12.11

– 0.92 0.94 0.85 0.86

– y75.7 y95.1 y209.6 y215.7

– 32 859 44 339 79 761 82 479

– 0.92 0.94 0.89 0.96

3355.4 3297.1 3288.8 3260.6 3210.3

0.96 0.96 0.99 0.96 0.83

0.15 0.18 0.20 0.22 0.25

9.52 11.60 12.38 15.47 19.77

0.95 0.92 0.97 0.95 0.97

y105.5 y138.4 y263.4 y271.3 y280.5

43 512 60 510 102 708 107 130 118 462

0.91 0.90 0.91 0.90 0.97

3335.9 3275.6 3227.2 3162.2 3199.7

0.87 0.91 0.91 0.92 0.98

y0.86 y0.39 y0.49 y0.27 0.17

337.71 180.47 239.39 200.33 48.48

0.92 0.85 0.77 0.91 0.95

y360.9 y440.6 y177.1 y215.8 y375.7

131 574 155 212 93 290 101 992 171 940

0.88 0.96 0.78 0.88 0.54

V.E. Borisenko et al. / Journal of Molecular Liquids 109 (2004) 125–135

Table 3 Linear regression of temperature dependence of the spectral characteristics YsaTqb of nc(NH) bands for complexes of N-substituted anilines with various proton acceptors in CCl4

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V.E. Borisenko et al. / Journal of Molecular Liquids 109 (2004) 125–135

134

Table 4 Spectral characteristics of nc(NH) bands of N-substituted anilines in complexes with various proton acceptors in CCl4 (Ts298 K) Compounds

Proton acceptors

M (1) (cmy1)

(1) N-methyl aniline

CH3CN THF DMF DMSO HMPA

– 3388.8 3386.1 3361.7 3325.6

– 66.0 82.1 82.5 73.5

– 8.2 11.6 22.0 24.5

(2) N-ethyl aniline

CH3CN THF DMF DMSO HMPA

– 3369.7 3364.5 3346.6 3311.8

– 52.5 68.1 89.2 87.6

– 10.3 16.0 17.3 18.2

(3) Diphenylamine

CH3CN THF DMF DMSO HMPA

3386.7 3340.2 3333.1 3308.7 3266.2

54.2 65.2 71.9 81.0 94.3

12.1 19.3 24.3 26.3 34.9

(4) N-methyl-4-NO2-aniline

CH3CN THF DMF DMSO HMPA

3401.4 3358.8 3342.5 3299.7 3265.3

54.4 63.5 91.5 118.3 99.1

24.0 23.9 40.3 46.1 60.0

robenzamides as1.97–3.89 w7x. The increase of the proton donor ability of N–H bonds results, as a rule, in the increase of a parameter; however, there is a tendency to its limitation. For the majority of the studied NHdonors, within quite wide range of proton donor abilities of the partners in the H-bond w5,7,8x, the a factor in Eq. (1) does not exceed the value of 6.5.

2(M (2))1y2 (cmy1)

B=10y3 (dm3 moley1 cmy2)

Thus, the proton donor ability of the imino group of N-substituted anilines, the spectral characteristics of n(NH) absorption bands in free and H-bonded molecules, enthalpy (yDH) of H bond and equilibrium constants of the complex formation process pronouncedly depend on individual characteristics of N-substitutes. The hyper conjugation effects of alkyl substitutes

Table 5 Thermodynamic characteristics of the complexes between N-substituted anilines and various proton acceptors in CCl4 K298 (dm3 moley1)

Compounds

Proton acceptors

a

yDHcalc (kcal moley1)

yDHexp (kcal moley1)

yDS (cal moley1 Ky1)

N-methyl aniline (ras0.95–0.98)

CH3CN THF DMF DMSO HMPA

4.8

– 1.8 2.7 4.6 5.0

– 2.9 3.5 4.0 4.6

– 9.1 10.3 11.9 13.7

– 1.3 2.0 2.1 2.3

N-ethyl aniline (ras0.97–0.98)

CH3CN THF DMF DMSO HMPA

6.0

– 3.2 4.7 5.0 5.2

– 3.7 4.6 5.0 5.3

– 12.3 14.1 15.1 15.9

– 1.1 2.0 2.3 2.6

Diphenylamine (ras0.94–0.98)

CH3CN THF DMF DMSO HMPA

6.4

3.2 5.0 6.0 6.5 8.1

2.3 4.8 5.8 7.2 8.4

7.9 15.9 17.7 20.6 25.1

1.0 1.1 2.5 5.8 6.6

N-methyl 4-NO2-aniline (ras0.97–0.98)

CH3CN THF DMF DMSO HMPA

6.2

4.0 3.9 6.8 7.7 9.6

4.1 4.5 7.1 8.5 9.3

12.3 13.1 18.8 22.5 23.9

2.1 3.0 12.1 21.4 39.1

a

r, correlation coefficient in van’t Hoff equation.

V.E. Borisenko et al. / Journal of Molecular Liquids 109 (2004) 125–135

Fig. 6. Correlation of the complexes formation enthalpy (yDH) the absorption band shift (DM (1)).

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