Ultrasonic and IR study of molecular association process through hydrogen bonding in ternary liquid mixtures

Fluid Phase Equilibria 215 (2004) 119–127

Ultrasonic and IR study of molecular association process through hydrogen bonding in ternary liquid mixtures Aashees Awasthi, Madhu Rastogi, J.P. Shukla∗ Department of Physics, University of Lucknow, Lucknow 226007, India Received 21 January 2003; accepted 20 August 2003

Abstract Complex formation in ternary liquid mixtures of heterocyclic compounds, viz. pyridine and quinoline with phenol in benzene has been studied through ultrasonic velocity measurements (at 2 MHz) in the concentration range of 0.010–0.090 at varying temperatures of 35, 45 and 55 ◦ C. The ultrasonic velocity and density data are used to estimate adiabatic compressibility, intermolecular free length, molar sound velocity, molar compressibility and specific acoustic impedance. These acoustical parameters, in turn, are used to study the solute–solute interactions in these systems. The ultrasonic velocity shows a maxima and adiabatic compressibility a corresponding minima as a function of concentration for these mixtures. The results indicate the possible occurrence of complex formation between unlike molecules through intermolecular hydrogen bonding between the nitrogen atom of pyridine and quinoline molecules and the hydrogen atom of phenol molecule. Further, the excess values of adiabatic compressibility and intermolecular free length have also been evaluated and discussed in relation to complex formation. The infrared spectra of both the systems, pyridine–phenol and quinoline–phenol, have been also recorded for various concentrations at room temperature (35 ◦ C) and found to be useful for understanding the presence of N · · · HO bond complexes and the strength of molecular association at specific concentrations. © 2003 Elsevier B.V. All rights reserved. Keywords: Ultrasonic velocity; Molecular interactions; Excess functions; Infrared spectrum

1. Introduction Dielectric relaxation studies have been widely used in order to investigate molecular structures in dilute solutions [1,2]. The N · · · HO bond complexes have been conventionally studied by infrared and ultraviolet techniques [3,4]. Recent developments have found use of ultrasonic energy of medicine, engineering, agriculture and industry [5,6]. In chemical industries, ultrasonic energy is found useful in studying the chemical processes and in synthesis of chemical substances. Wong and Zhu [7] have studied the speed of sound in seawater as a function of salinity, temperature and pressure. Skumiel and Labowski [8] have given a theoretical analysis of the effect of an external constant magnetic field on the propagation of ultrasonic waves in electrically conducting liquids as well as the results of measurements carried out in mercury. Hänel [9] has analytically deduced an equation for the longitudinal sound velocity of thin film ∗

Corresponding author. E-mail address: [email protected] (A. Awasthi).

0378-3812/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2003.08.017

samples and the velocities have been calculated. This has also been experimentally applied to thin polyvinylidene fluoride film. Lagemann and Dunbar [10] were the first to point out the sound velocity approach for qualitative determination of the degree of association in liquids. Ultrasonic waves with low amplitude have been used by many scientists to investigate the nature of molecular interactions and physico-chemical behavior of pure, binary, ternary and quaternary liquid mixtures [11–19]. A survey of literature [20–26] indicates that excess values of acoustical parameters are useful in understanding the nature and strength of the molecular interaction in the mixtures. Recently, ultrasonic investigation of molecular association in ternary liquid mixtures of phenol and o-cresol with dimethylsulfoxide in CCl4 has been studied at varying temperatures by Awasthi and Shukla [25]. The complex formation has been interpreted in terms of the acoustical parameters, their excess values and with their infrared stretching frequencies. The N · · · HO bond complexes are of special interest due to their direct relevance to the biological materials, it was

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considered important to study the complex formation of N · · · HO bonded complexes in ternary liquid mixtures of pyridine and quinoline with phenol in benzene using ultrasonic technique and examine the behavior in relation to the earlier results obtained using dielectric methods.

2. Experimental details Ultrasonic velocity was measured using the ultrasonic interferometer (model M81) supplied by Mittal Enterprises, New Delhi, that has a reproducibility of ±0.4 m s−1 at 25 ◦ C. The temperature was maintained constant by circulating water from a thermostatically controlled water bath (accuracy ±0.1 ◦ C). The temperature of the cell was measured using a thermocouple (at the crystal) and was found to be accurate to ±0.25 ◦ C. The density of various systems has been evaluated using a sensitive pyknometer with an accuracy of 0.5 kg m−3 . The two solutes (pyridine–phenol and quinoline–phenol) are taken in 1:1 ratio by weight and fixed amount of solvent, benzene (20 ml), is used for preparing mixtures of varying concentration (X) for both the systems. The chemicals used were of AR grade, procured from BDH. All the chemicals were purified by standard procedures discussed by Perrin and Armarego [28] before use. The infrared spectra for the pyridine–phenol and the quinoline–phenol systems have been recorded using FT-IR spectrophotometer (model 8201) supplied by Perkin-Elmer, at Central Drug Research Institute, Lucknow.

expressions: βE = βexp − βideal

(6)

and LEf = Lf,exp − Lf,ideal

(7)

For βideal and Lf,ideal , the densities and the ultrasonic velocities of various components in pure state at the three given temperatures have been measured. Further, the velocities of both the systems at different concentrations and temperatures have been evaluated, theoretically, using volume additive rule [21] as: Uideal = U1 φ1 + U2 φ2 + U3 φ3

(8)

where U1 , U2 and U3 are the velocities of the three components of the ternary liquid mixture in pure state and φ1 , φ2 and φ3 are their volume fractions. Similarly, ideal density is evaluated using: ρideal = ρ1 φ1 + ρ2 φ2 + ρ3 φ3

(9)

Finally, βideal and Lf,ideal are evaluated using following equations: βideal =

1 2 ρ Uideal ideal

(10)

and 1/2

Lf,ideal = Kβideal

(11)

4. Results 3. Theory Various physical parameters [14] were evaluated from the measured values of ultrasonic velocity (U) and density (ρ) using the following standard formulae: adiabatic compressibility (β) =

1 U2ρ

intermolecular free length (Lf ) = Kβ1/2

(1) (2)

where K values for different temperatures were taken from the work of Jacobson [29]; at 35, 45 and 55 ◦ C the K values are 637, 647 and 656, respectively. molar sound velocity (R) = U 1/3 V
 M molar compressibility (B) = β−1/7 ρ

(3) (4)

where V and M are the molar volume and molecular weight of the mixtures, respectively. specific acoustic impedance (Z) = ρU

(5)

The excess adiabatic compressibility (βE ) and excess intermolecular free length (LEf ) are evaluated by the following

Ultrasonic velocity, molar sound velocity, molar compressibility and specific acoustic impedance for the pyridine–phenol and the quinoline–phenol systems have been listed in Tables 1 and 2. The appropriate conversion of CGS units to SI units have been provided in Table 3. The representative graphs of U, β, Lf , βE and LEf as a function of concentration have been presented in Figs. 1–5, respectively. The measured values of standard deviation of velocities (at the peak) have been found to be lesser than 0.11 m s−1 for the mixtures studied at various temperatures for 20 measurements. The infrared spectra for the pyridine–phenol and the quinoline–phenol systems have been presented in Figs. 6 and 7, respectively.

5. Discussion It is seen from Fig. 1 that in the pyridine–phenol and the quinoline–phenol systems, ultrasonic velocity increases initially as a function of increasing concentration for each temperature of the solution, attains a maximum and then decreases in the 0.010–0.090 weight fraction region. As the temperature increases, velocity maxima shifts towards lower

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Table 1 Ultrasonic velocity and related parameters for pyridine–phenol in benzene t (◦ C)

X (weight fraction)

ρ (g cm−3 )

U × 104 (cm s−1 )

R × 103 (cm3 mol−1 (cm s−1 )1/3 )

B × 103 (cm3 mol−1 (dyn−1 cm2 )−1/7 )

Z × 103 (g cm−2 s−1 )

35

0.010 0.020 0.030 0.035 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.080 0.090

0.864 0.867 0.870 0.871 0.872 0.874 0.876 0.878 0.883 0.885 0.887 0.889 0.893

12.465 12.522 12.566 12.590 12.617 12.679 12.751 12.884 12.989 12.948 12.918 12.845 12.772

4.524 4.524 4.522 4.524 4.526 4.527 4.523 4.539 4.529 4.519 4.509 4.499 4.479

2.534 2.535 2.534 2.536 2.547 2.538 2.539 2.544 2.540 2.535 2.521 2.527 2.517

107.70 108.56 109.32 109.66 110.02 110.81 111.70 113.12 114.69 114.59 114.58 114.19 114.05

45

0.010 0.020 0.030 0.035 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.080 0.090

0.855 0.858 0.861 0.862 0.863 0.864 0.866 0.869 0.872 0.874 0.877 0.882 0.885

12.061 12.194 12.221 12.249 12.341 12.368 12.408 12.353 12.298 12.268 12.187 12.076 11.985

4.522 4.531 4.527 4.529 4.539 4.542 4.540 4.522 4.504 4.494 4.473 4.442 4.424

2.533 2.538 2.537 2.538 2.543 2.545 2.544 2.546 2.528 2.523 2.513 2.499 2.491

103.12 104.62 105.22 105.59 106.50 106.86 107.45 107.35 107.24 107.19 106.88 106.51 106.07

55

0.010 0.020 0.030 0.035 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.080 0.090

0.846 0.848 0.852 0.853 0.855 0.856 0.858 0.860 0.863 0.866 0.869 0.871 0.878

11.506 11.562 11.696 11.710 11.749 11.724 11.669 11.638 11.593 11.552 11.505 11.474 11.372

4.499 4.504 4.508 4.509 4.507 4.503 4.490 4.479 4.462 4.446 4.428 4.423 4.382

2.522 2.525 2.528 2.528 2.528 2.526 2.520 2.515 2.508 2.500 2.492 2.490 2.471

97.34 98.05 99.65 99.89 100.45 100.36 100.12 100.09 100.05 100.04 99.98 99.94 99.85

concentration in each system (Fig. 1). The non-monotonous behavior of ultrasonic velocity with concentration indicates occurrence of complex formation between unlike molecules [30]. The molecular association becomes maximum at those concentrations where velocity maxima occurs. This may be interpreted to have occurred due to the formation of strong hydrogen bonding resulting into complex formation producing displacement of electrons and nuclei. The chemical interaction may involve the association due to hydrogen bonding, dipole–dipole interaction or formation of charge-transfer complexes. All these processes may lead to strong interaction of forces [20]. At 35 ◦ C, the occurrence of maximum velocity at 0.060 weight fraction for the pyridine–phenol system and at 0.070 weight fraction for the quinoline–phenol system and subsequent decrease of velocity with further increase in concentration may be explained as follows.

It is probable that at 0.060 and 0.070 weight fractions for respective systems maximum associated pyridine and quinoline molecules are broken into their monomers and the hydrogen bonds are formed between the nitrogen atom of monomers and the hydrogen atom of phenol molecules. However, it is also likely that the pyridine and quinoline molecules in excess of these concentrations may stay in the associated form. These associated molecules are fairly large in size as compared to phenol molecules and have to be accommodated in the system and this may cause some structural changes resulting in the weakening of the intermolecular forces. This is probably the reason for the decrease in velocity above 0060 and 0.070 weight fractions for respective systems [13,25]. Further, it may be seen from Fig. 1 that in the pyridine–phenol system, velocity peak obtained at a specific temperature exists at a lower concentration as compared to

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Table 2 Ultrasonic velocity and related parameters for quinoline–phenol in benzene t (◦ C)

X (weight fraction)

ρ (g cm−3 )

U × 104 (cm s−1 )

R × 103 (cm3 mol−1 (cm s−1 )1/3 )

B × 103 (cm3 mol−1 (dyn−1 cm2 )−1/7 )

Z × 103 (g cm−2 s−1 )

35

0.010 0.020 0.030 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.075 0.080 0.090

0.847 0.850 0.854 0.858 0.859 0.860 0.862 0.865 0.867 0.870 0.871 0.872 0.874

12.500 12.531 12.590 12.635 12.655 12.678 12.694 12.715 12.728 12.762 12.742 12.720 12.690

4.637 4.651 4.663 4.674 4.685 4.696 4.700 4.700 4.705 4.707 4.713 4.718 4.732

2.590 2.598 2.606 2.615 2.620 2.626 2.630 2.631 2.634 2.636 2.640 2.644 2.653

105.87 106.51 107.52 108.41 108.71 109.03 109.42 109.98 110.35 111.02 110.98 110.92 110.91

45

0.010 0.020 0.030 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.075 0.080 0.090

0.837 0.840 0.843 0.848 0.850 0.852 0.854 0.856 0.857 0.859 0.860 0.861 0.864

12.150 12.188 12.224 12.252 12.270 12.289 12.307 12.343 12.327 12.297 12.277 12.255 12.173

4.648 4.663 4.678 4.681 4.686 4.691 4.695 4.703 4.709 4.708 4.714 4.720 4.721

2.595 2.604 2.614 2.617 2.620 2.624 2.627 2.632 2.636 2.637 2.641 2.644 2.647

101.70 102.38 103.05 103.90 104.30 104.70 105.10 105.66 105.64 105.63 105.58 105.52 105.17

55

0.010 0.020 0.030 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.075 0.080 0.090

0.829 0.832 0.835 0.837 0.840 0.842 0.844 0.847 0.848 0.850 0.851 0.853 0.855

11.536 11.580 11.622 11.682 11.736 11.795 11.765 11.720 11.704 11.670 11.655 11.625 11.597

4.613 4.628 4.644 4.668 4.672 4.682 4.680 4.671 4.678 4.676 4.682 4.681 4.694

2.578 2.587 2.592 2.611 2.614 2.620 2.620 2.617 2.621 2.621 2.625 2.626 2.634

95.63 96.35 97.04 97.78 98.58 99.31 99.30 99.27 99.25 99.20 99.18 99.16 99.15

the quinoline–phenol system. This may probably would be caused from the fact that the pKα value (where Kα is the acidity constant) of pyridine is greater than that of quinoline which suggests that pyridine is more basic than quinoline. Moreover, the resonating structures of pyridine and quino-

line indicate that the unshared pair of electrons are involved more in the structure of quinoline than pyridine. Thus, in pyridine, the availability of the unshared pair of electrons for sharing with acids is more than for quinoline and hence pyridine is more basic than quinoline. Due to the above,

Table 3 Conversion of CGS units to SI units No. 1 2 3 4 5 6 7 8

Parameter

CGS units

SI units

Ultrasonic velocity (U) Density (ρ) Adiabatic compressibility (β) Intermolecular free length (Lf ) Molar sound velocity (R) Molar compressibility (B) Specific acoustic impedance (Z) ¯ Wave number (λ)

1 cm s−1

10−2 m s−1 103 kg m−3 10 N−1 m2 10−10 m 10−20/3 m3 mol−1 (m s−1 )1/3 10−43/7 m3 mol−1 (N−1 m2 )−1/7 10 kg m−2 s−1 102 m−1

1 g cm−3 1 dyn−1 cm2 1Å 1 cm3 mol−1 (cm s−1 )1/3 1 cm3 mol−1 (dyn−1 cm2 )−1/7 1 g cm−2 s−1 1 cm−1

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Fig. 2. Adiabatic compressibility (β) vs. concentration (X) at: (䊊) 35 ◦ C; ( ) 45 ◦ C; (䊐) 55 ◦ C.

Fig. 1. Ultrasonic velocity (U) vs. concentration (X) at: (䊊) 35 ◦ C; ( ) 45 ◦ C; (䊐) 55 ◦ C.

pyridine reacts with phenol more quickly, giving rise to the velocity peak at a lower concentration than with quinoline. The adiabatic compressibility (β) and intermolecular free length (Lf ) both have an inverse relationship with ultrasonic velocity (Figs. 2 and 3). The decrease in β with increase in concentration in an indicative of the fact that intermolecular forces are increasing which brings the molecules to a closer packing resulting into a decrease in Lf . The interdependence of Lf and U has been evolved from a model for sound propagation proposed by Eyring and Kincaid [31]. Occurrence of U maxima, and β and Lf minima at the same concentrations further strengthens the process of complex formation through H-bonding between the two solute molecules [15,19].

It is seen from Tables 1 and 2 that the variation in molar sound velocity (R), molar compressibility (B) and specific acoustic impedance (Z) is non-monotonous with concentration at various temperatures for both systems. The specific acoustic impedance is governed by the inertial and elastic properties of the medium. Therefore, it is important to examine specific acoustic impedance in relation to concentration and temperature. The non-linear behavior of R, B and Z further supports the possibility of molecular interactions due to the H-bonding as well as due to the formation of charge-transfer complex between solute molecules [13,16,19,22,25]. When the temperature is increased, the velocity maxima shifts towards lower concentration in each system (Fig. 1). This is because of the thermal energy which facilitates the breaking of bonds between the associated molecules of pyridine and quinoline, resulting into creation of their

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Fig. 3. Intermolecular free length (Lf ) vs. concentration (X) at: (䊊) 35 ◦ C; ( ) 45 ◦ C; (䊐) 55 ◦ C.

monomers. The hydrogen bonds are then formed between the nitrogen atom of pyridine and quinoline monomers and the hydrogen atom of phenol molecules. This may probably be the reason for the observed shift in velocity maxima (β and Lf minima) towards lower concentrations [14]. The increase in thermal energy weakens the molecular forces and hence decrease in velocity is expected [17]. In order to proceed further with the investigations, the excess values of acoustical parameters, βE and LEf were also calculated. It is expected that the dispersion forces should make positive contributions to excess values while dipole–dipole, dipole-induced dipole, charge-transfer interaction and hydrogen bonding between unlike components should make negative contributions [20]. In the case of pyridine–phenol system, the positive values of the excess parameters, βE and LEf both tend to decrease toward negative values indicating a strong interaction between unlike molecules. The change from positive to increasingly negative excess values shows greater strength of interaction between the components and may be qualitatively interpreted

Fig. 4. Excess compressibility (βE ) vs. concentration (X) at: (䊊) 35 ◦ C; ( ) 45 ◦ C; (䊐) 55 ◦ C.

in terms of closer approach of unlike molecules leading to reductions in compressibility and volume. In the case of quinoline–phenol system, the positive values of βE and LEf tend to decrease toward the less positive values showing the weaker interaction as compared to pyridine–phenol system [20,22,23,26]. The βE and LEf minima occur at the same concentrations where U maxima (β and Lf minima) is indicated for both the systems which further strengthens the occurrence of molecular association [25]. The observed acoustical parameters and their variation with concentration and temperature clearly indicate the formation of complexes between unlike molecules through hydrogen bonding. The complexation becomes maximum at those concentrations where the maxima or the minima, corresponding to the respective parameters, occur for both the systems. Saxena and Saxena [27] have studied the dielectric relaxation in N · · · HO bond complexes of pyridine and quinoline with phenol in benzene at microwave frequency of 9.8 GHz and at temperature of 25 ◦ C. The complex formation has

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system. This could be reasonable because pyridine is more basic than quinoline thus nitrogen atom of pyridine molecule could interact with hydrogen atoms of more than one phenol molecules resulting into a bulkier complexed species which give rise to the higher τ 2 values for pyridine–phenol system than quinoline–phenol system [1]. The most probable relaxation time (τ OH ) was found to be 20.8–53.4 ps for pyridine–phenol system and 17.7–63.1 ps for quinoline–phenol system. The observed values of τ OH for pyridine, quinoline and phenol are 3.35, 8.6 and 8.9 ps, respectively. The τ OH values for both the systems are much larger than their individual values which suggests the occurrence of complex formation in both the systems. The dipole moment was found to be 3.96–5.06 D for pyridine–phenol system and 3.56–4.65 D for quinoline– phenol system. The values of dipole moment for pyridine, quinoline and phenol are 2.23, 2.08 and 1.77 D, respectively. The dipole moment for both the systems are much larger than their individual values which clearly indicates the presence of molecular interaction between the solutes molecules in the ternary liquid mixture [1]. This is in agreement with our observed results in the above systems using ultrasonic technique.

6. Infrared spectrum

Fig. 5. Excess intermolecular free length (LEf ) vs. concentration (X) at: (䊊) 35 ◦ C; ( ) 45 ◦ C; (䊐) 55 ◦ C.

been discussed in terms of distribution parameter, relaxation times and dipole moment. The distribution parameter (α) has sufficiently high values (0.28–0.58) for pyridine–phenol system and (0.28–0.51) for quinoline–phenol system which suggests the highly flexible nature of N · · · HO bonding. It is well known fact that if system has a high value of α then the relaxation mechanism may be resolved into two separate processes, i.e., the group relaxation (τ 1 ) and the molecular relaxation (τ 2 ) processes, where τ 1 corresponds to the rotation of one of the interacting solute molecules and τ 2 corresponds to the rotation of the complex molecule as a whole. The observed group relaxation time (τ 1 ) was in the range 3.1–11.9 ps for pyridine–phenol system and 6.8–15.1 ps for quinoline–phenol system. The larger values of τ 1 for quinoline–phenol system are perhaps due to the bulkier quinoline molecule taking part in the rotational process. The molecular relaxation time (τ 2 ) was 29.9–66.2 ps for pyridine–phenol system and 32.4–58.5 ps for quinoline–phenol system. The τ 2 values for pyridine–phenol system are higher than quinoline–phenol

Further, in order to examine the presence of N · · · HO bond complexes and the strength of molecular association at specific concentrations (at and near the ultrasonic velocity peaks) for both the systems, infrared spectra were recorded for various concentrations at room temperature (35 ◦ C). It is seen from the observed infrared spectra that two sharp bands appear at frequencies (3293.6, 3235.5), (3266.9, 3204.3) and (3285.9, 3233.1) cm−1 for 0.055, 0.060 and 0.065 weight fractions, respectively, for the pyridine–phenol system (Fig. 6) and at frequencies (3273.0, 3214.2), (3258.3, 3196.8) and (3275.6, 3218.3) cm−1 for 0.065, 0.070 and 0.075 weight fractions, respectively, for the quinoline–phenol system (Fig. 7). Due to much weaker tendency to form hydrogen bond, the associated N–H stretching absorption is usually sharp [32]. It is seen from Figs. 6 and 7 that in the pyridine–phenol system the N–H bands appear at frequencies 3266.9 and 3204.3 cm−1 for 0.060 weight fraction and in the quinoline–phenol system the N–H bands appear at frequencies 3258.3 and 3196.8 cm−1 for 0.070 weight fraction but with further increase or decrease in concentration in both the systems the N–H bands shift toward higher frequency which indicates the weakening of molecular association through intermolecular hydrogen bonding. Thus, the study of infrared spectra shows that the strength of complex formation becomes maximum at 0.060 weight fraction for the pyridine–phenol system and at 0.070 weight fraction for the quinoline–phenol system which is also indicated through peaks of ultrasonic velocities as discussed earlier.

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Fig. 6. Observed N–H stretching bands in infrared spectrum of pyridine–phenol system at various concentrations.

Fig. 7. Observed N–H stretching bands in infrared spectrum of quinoline–phenol system at various concentrations.

A. Awasthi et al. / Fluid Phase Equilibria 215 (2004) 119–127

Thus, the pattern, position and intensity of the N–H band as per infrared data strongly supports the conclusions drown from the ultrasonic data that there is possibility of complex formation between unlike molecules through hydrogen bond [19,25]. It may be concluded that ultrasonic studies of liquid mixtures provide for a comprehensive investigation of molecular association arising from the hydrogen bonding between the nitrogen atom of pyridine and quinoline molecules and the hydrogen atom of phenol molecule. The non-linear variations of acoustical parameters with concentration exhibits the complex formation which is also strongly supported by the excess parameters and IR data in these ternary liquid mixtures.

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