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On the role of hydrogen bonds in thiourea condensates—the correlation of a quantum-chemical approach with the experimental results
83
J. Electroanal. Chem., 345 (1993) 83-91 Elsevier Sequoia S.A., Lausanne
JEC 02351
On the role of hydrogen bonds in thiourea condensatesthe correlation of a quantum-chemical approach with the experimental results S . Romanowski and T .M. Pietrzak Department of Theoretical Chemistry, University of L6d% ul. Narutowicza 68, 90 136 t6di (Poland)
M. Skompska, M . Jurkiewicz-Herbich and J . Jastrzgbska Department of Chemistry, Warsaw University, al. Pasteura 1, 02 093 Warsaw (Poland) (Received 13 January 1992; in revised form 26 June 1992)
Abstract The semi-empirical quantum-chemical method PCILO 3 was used to estimate the directions, lengths and energies of the intermolecular hydrogen bonds between thiourea (TU) molecules in selected TU-solvent-TU, TU-anion-TU and TU-TU-TU clusters . The results obtained for the hydrogen bond energies support the sequence TU+H 20 < TU+EtOH < TU+CH 3CN and TU+N03 < TU+ C1O4 < TU+F - of increasing stability of thiourea condensates, established by electrochemical studies at the mercury/solution interface .
INTRODUCTION
Thiourea (TU) is one of the neutral organic molecules that is strongly adsorbed at the mercury/solution interface . Its electrosorption properties are due to the high value of the permanent dipole moment (4 .89 D [1]) as well as to the strong interaction of the sulphur in TU with mercury [2], particularly in the range of positive electrode charge densities . Thiourea, like many other organic adsorbates, may undergo phase transitions at the interface, provided that its bulk concentration is sufficiently high and the temperature is lowered to a certain critical value T, [3-5]. In the phase transition process the surface Gibbs excess of thiourea on the mercury changes from 5 .5 x 10 -10 to 10 x 10-10 mol cm -2 [3]. Despite such a high surface excess of adsorbate, the strong repulsive interactions between parallel oriented dipoles are markedly reduced and the condensed films are stable over a wide range of potentials . According to Buess-Herman et al. [3], the attractive 0022-0728/93/$06.00 C 1993 - Elsevier Sequoia S .A. All rights reserved
84
interaction between two adjacent molecules in the film results from the close proximity of the sulphur and the NH 2 pair belonging to two tilted TU molecules . However, a conspicuous influence of the type of supporting electrolyte as well as the nature of the solvent on the stability of compact films, which we found in previous studies [4-7], led us to other conclusions . In our opinion the condensed structure is probably immobilized by the hydrogen bonds formed between thiourea and the other components of the solution - the solvent molecules and the anions of the supporting electrolyte . Furthermore, it was found that a solid condensate is also formed in the bulk of ethanolic solution containing thiourea and KF [8]. X-ray powder diffraction diagrams showed the large interplanar spacings of the solid phase, probably due to the hydrogen-bonded (TU) 3 F`(EtOH)„ structure . Unfortunately, because of the poor crystallinity of the sample we were not able to find the unit cell but a possible structure of the condensate was proposed . The semi-empirical and ab initio quantum-chemical methods are valuable tools for predicting the directions, length and energies of intermolecular hydrogen bonds. The PCILO3 method developed by Boca and Pelican [9] takes into account the correlation effects of electrons and therefore it is one of the best methods, among all the semi-empirical theories, for calculating the hydrogen bond energy . In this work the results of calculations performed using the PCILO3 method were used to check the previous assumptions about the role of hydrogen bonds in the phase transition processes of thiourea . EXPERIMENTAL
Qualitative studies of the phase transition process of thiourea, focussed on the temperature stability of condensed superficial layers, were carried out using differential capacity-potential (C-E) measurements and cyclic voltammetry . The latter method is particularly useful because of its high sensitivity with respect to the nature of the process studied . The shape of the cyclic voltammograms allows us to interpret questionable C-E profiles . The electrochemical measurements were carried out in a three-electrode, water-jacket cell on a stationary hanging mercury drop electrode . The procedure and the electrochemical equipment used have been described previously [4,5]. Thiourea was recrystallized from a water + ethanol mixture and dried at 50°C under reduced pressure . The salts LiC1O4 , LiNO3 and KF (Merck) were used without further purification . All the solvents were purified : water was triply distilled, and ethanol and acetonitrile were dried and then distilled . The quantum-chemical calculations using the PCILO3 method were performed on the complete neglect of differential overlap/2 (CNDO/2) Hamiltonian level . In the first step of the calculations, the geometry and the total energy of single molecules of TU and the solvent and of the anion of the supporting electrolyte were established . The molecular geometry was fully optimized, i .e. the bond lengths and the angles for the structure with minimum total energy were chosen . The interaction energies were then calculated for clusters containing two molecules
85
of thiourea and one molecule of the solvent or a selected anion . The geometries of the monomers were assumed to be rigid and only the intermolecular geometrical parameters were optimized . The assumption of rigidity means that the interaction energy was calculated in the supermolecular manner, i .e . "E(AB) = E(AB) - (E(A) + Ecs>) After optimizing the intermolecular parameters, the energy was always corrected, including correlation, up to second order of many-body perturbation theory (MBPT) (Moller-Plesset type) . Two main criteria for the existence of the hydrogen bond were taken into account : (a) a significant charge transfer between two neighbouring atoms in a cluster with respect to the charge on the same atoms in the single molecules ; (b) the bond length was chosen as the shortest of all theoretically possible hydrogen bonds .
RESULTS AND DISCUSSION
In previous papers on the condensation of thiourea at the mercury/solution interface [4-6], which were mainly qualitative, the following hypotheses were presented . (1) The structure of the quasi-solid condensate I, which forms at non-charged or slightly positively charged mercury, is stabilized by the hydrogen bonds between thiourea and the solvent molecules (both adsorbed at the electrode) . (2) The interactions between the amino groups of TU, pointing out towards the bulk of solution, and the anions of the supporting electrolyte and/or the solvent molecules from the second monolayer may be another stabilizing force for the condensed layer I . (3) An alternative structure for the quasi-solid film which could be also taken into account may be a layer of antiparallel oriented TU molecules stabilized by the hydrogen S . . . H-N bonds . (4) The condensed structure II is probably a mixed TU + anion film formed at the positively charged mercury and stabilized not only by the hydrogen bonds between TU and the coadsorbing anions but also by electrostatic electrode-dipole interactions, leading to the formation of covalent bonds between sulphur and mercury [2], and by electrostatic interactions between the positive end of the TU dipole and anions from the diffuse layer . As the examination of the temperature stability of film II is rather difficult because of the onset of the electrochemical oxidation of mercury at about - 0 .4 V/(SCE), the experimental results presented concern only layer I . In order to verify the hypothesis of the crucial role of hydrogen bonds in the phase transition process, two types of experiment were performed . Firstly, C-E curves and cyclic voltammograms were recorded for a wide range of temperatures (from 10 to 40°C) in solutions of various solvents saturated with TU and containing
86
T/ C (b)
(a)
30
30
25
25
20
20
B 15
15
•
0.5
0.6
0 .7
•
0. 8
-E
iv
Fig. 1 . Phase transition diagrams for the mercury electrode : (a) thiourea-saturated solutions containing 0.2 M LiC104 in the solvents H 2 O, EtOH and CH 3 CN; (b) thiourea-saturated ethanolic solutions of 0.16 M KF, 0 .2 M LiC10 4 and 0.2 M NaN0 3 . The phase transition potentials in 0 .16 M KF are given only in a limited temperature range because below 23°C condensation also takes place in the bulk of the solution . The curves denoted by A and B indicate the borders between non-condensed and condensed regions.
0.2 M LiC1O4 as the supporting electrolyte . Similar measurements were performed in aqueous and ethanolic solutions containing two other electrolytes : KF and LiNO 3 . The abrupt changes observed in the C-E and I-E curves, discussed by us previously [4,5], are associated with phase transition processes at the mercurysolution interface . The potentials corresponding to these abrupt changes in the experimental curves were taken as the phase transition potentials EB . The temperature dependence of the phase transition potentials, the so-called phase transition diagrams, obtained from both sets of experiments are presented in Fig . 1 . The quasi-solid films I exist within the potential region located between lines A and B . It follows that the stability of condensed layer I increases in the order TU + H 2O < TU + EtOH < TU + CH 3CN and TU + N03 < TU + C104 < TU + F -. If the assumption about the role of hydrogen bonds between TU and the solvent and between TU and the anions is true, it should also be confirmed by the results of calculations of hydrogen-bond energy using PCILO3 . However, it should be stressed that the calculated geometry of clusters relates to the structures in the bulk of the solution but not those on the charged electrode and therefore the results presented can only be used to confirm a particular tendency but not to establish the exact geometry of the surface condensate . The geometry of the thiourea molecule and a map of the electrostatic potential distribution in the molecule are presented in Fig . 2 . The bond lengths are given in
87
(b) -5 .00 -4 .00 -3 .00 500
0 3C
00 2 .00 3 00 4 00 5 00 1 1 5 00 1 1 1
(a)
4 00 3 00
N
tits •
3 (-0.17) fZ6> ;,`
2 .00
2 00
00
- 1 00
0 .00
0 00
N
1
H~ C2 (029)
S, (-0 .44)
(0 .14)
-1 .00
-1 00
-2 00
-2 00
-3 00
-300
-4 00
-4 00
1 1 1 1 __ -5 .00 -5 .00-4 .00-300-2 .00-1 CC
_j__ I 1 I I
._2
- 500
100 2 .00 3 .00 400 500
Fig. 2 . (a) The geometry and (b) a map of electrostatic potential distribution of the thiourea molecule obtained using the PCILO3 method . The length of bonds in Fig . 2(a) is given in angstroms, and the net charges of the valence orbitals (in parentheses) are given in e scale and related to those for isolated atoms . The scale in (b) is in angstroms .
Angstroms and the net charges of the valence orbitals (in parentheses) are related to those of the isolated atoms . The dihedral angle between the planes S 1C 2 N3 and C2 N3H 5 is 16.7° and that between S 1 C2 N3 and C 2 N3 H 4 is -23.9°. The total molecular energy of TU with and without second-order MBPT corrections is ETU1 = - 1206 .5211 eV and E SCF _ - 1203 .3844 eV, where SCF and PCI denote the self-consistent field Hartree-Fock method and the perturbation correlation interactions method of energy calculations respectively . Similar calculations were performed for the other isolated molecules (H 20, EtOH and CH 3 CN) and for the N03 anion (Fig. 3). In the next step of the calculations, the energy and geometry of the molecular trimers TU-H 20-TU, TU-EtOH-TU, TU-CH 3 CN-TU and TU-TU-TU and the ionic clusters TU-F - -TU and TU-N03-TU were determined using the procedure described in the Experimental section . Examples of maps of the electrostatic potential distribution of the cluster TU-F --TU and the geometric structure of the trimer TU (down) TU(up)_TU(doK,,,) are presented in Figs . 4 and 5 respectively . In the case of the other trimers studied, the TU (up) molecule from the TU(up)-TU(down) cluster was replaced by the appropriate molecule or an TU(down) ion . The oxygen atom in the TU-EtOH-TU cluster lies between two thiourea amino groups and interacts with them through two hydrogen bonds . The carbon chain of EtOH points down with respect to the up-oriented NH 2 groups. The calculations for the TU-CH 3 CN-TU trimer were performed for two possible orientations of the acetonitrile molecule with the -CH 3 chain pointing up or
88
(b) Ethanol
(a) Water
tocr
Ht (0 .13)
1 .03
(-024)
H 9 (0.01)
H 3 10 .13)
n
(0.01`'CI-a02 ) 146 C1
0 2( - 027)
1
03
1
103 H 4 10.12)
#0 .391
M39 H6(-0 .04)
H7 ( .0)
HS (-0.04)
F
=-1014.343eV
E S F =-541 .234eV
Ee
E PC I =- 541 .876 eV
EEC I _- 1016 .529 eV
(d) N03
(c) Acetonitrite
(-0.53)
H 6 (0.03) 119 H6 10 (-001) 1 .42 (0.03)1"? 0, T- C2 110.7* (0.09) /_
N 3 (- 0.17)
0,
tzo~03 • N2 10.6)
1
(-053)'
H4
04(-0.53)
(0.03) EEN F =-759 .130eV EPC' - 762 703 eV
AN
E SCF =-1823 .688 eV
s E PCI =-1827.129eV NO3
Fig. 3 . Optimized geometry of the molecules : (a) water; (b) ethanol ; (c) acetonitrile ; (d) N03 anion.
down . It is found that the latter orientation is more favourable energetically . The water molecule in the TU-H 2 0-TU cluster is oriented with the oxygen atom pointing down and two hydrogen atoms interacting with two sulphur atoms of the thiourea molecules . The resulting average lengths of hydrogen bonds in all the clusters studied and the binding energies calculated per hydrogen bond are listed in Table 1 . It is clear that the sequence of binding energy strength for the molecular clusters is similar to that found from experiments . It may be somewhat surprising that the binding energy in the TU(d ,)_TU(„p)_TU(do,,,,) trimer is only slightly more favourable than that in TU (down) _H 2O-TU(d , ). This energy is probably not sufficient to maintain the antiparallel orientation of TU molecules at the non-charged or positively charged mercury. The resulting order of the binding energies for the ionic clusters is also consistent with that given above . The particularly strong and relatively short bonds between amino groups of thiourea and fluorides are probably responsible for the formation of the quasi-solid condensate in the bulk of the ethanolic- KF + TU solution [8]. In fact, addition of the third molecule of TU to the trimer TU-F --TU does not change the energy of the single hydrogen bond . Optimization of the
89
(a)
IH ,
u)urea +
F( -!
+
TI
'L .)
urea,
Fig. 4 . (a) Planar and (b) spatial maps of the electrostatic potential distribution for the ionic cluster TU-F - -TU.
TU,do,mi
Tuu;
TU, . . .
H (ai)
E3TIJ-3610 .88eV E3TU'-362Q49eV H (031) H (0 .11) Fig. 5 . The geometry of the molecular cluster TU(dO,,.,,)-TU(„P)-TU(d,f) after optimization of the intermolecular distances and angles. The geometry of the single TU molecule is the same as that presented in Fig . 2.
90 TABLE 1 Characteristics of the hydrogen bonds in some thiourea clusters calculated using PCILO 3 . Type of cluster
Type of hydrogen bond.
Number of hydrogen bonds in cluster
Average length of a single hydrogen bond/A
Molecular clusters TU-W-TU TU-TU-TU TU-Et-TU TU-AN-TU
S...H S...H 0...H N...H
2 4 2 2
2.22 2.19 1.62 1.53
-21.00 -22.16 -29.79 -67.84
Ionic clusters TU-F- -TU TU-N03 -TU
F. . . H O...H
2 4
1.25 1.44
-276.27 -113.22
F...H
3
1 .29
-272.17
F...H S...H
3 1
1 .29 2.18
-272 .17 -40.43
TU I FTU TU TU FI \ Et TUTU
Energy per hydrogen bond /kJ mol -1
TU, thiourea ; W, water; Et, ethanol; AN, acetonitrile.
intermolecular geometry of the cluster (TU) 3 F - yields some corrections to the structure proposed previously [8]. It is found that each thiourea molecule is bonded to the central fluoride ion by only one hydrogen bond of average length 1 .29 A. The distance between F - and the hydrogen atom from the second amino group of each TU molecule is too long (2 .8 A) to form an effective hydrogen bond . It also follows from the calculations that the ethanol molecule incorporated in the system does not change the geometry of the (TU) 3 F - cluster but is joined to it by S . . . H-O hydrogen bonds of length 2 .18 A and binding energy - 40.43 kJ mol -t . CONCLUSIONS
It follows from the experimental results that the stability of the quasi-solid condensates, obtained in various solvents and in the presence of various supporting electrolytes, increases in the order TU + H 2O < TU + EtOH < TU + CH 3 CN and TU+NO3
91
fluoride ion. The molecule of ethanol is joined to the cluster by an S . . . H-O hydrogen bond . ACKNOWLEDGEMENT
Partial financial support from grant B 545 is gratefully acknowledged . REFERENCES 1 W.D. Kumler and G.M . Fohler, J . Am. Chem. Soc ., 64 (1942) 1944 . 2 R. Parsons, Proc . R. Soc . London, Ser . A, 261 (1961) 79. 3 CI . Buess-Herman, L. Gierst, M. Gonze and F . Silva, J . Electroanal . Chem ., 226 (1987) 267 . 4 M. Skompska and K. Jaszczynski, J . Electroanal . Chem., 272 (1989) 207 . 5 M. Skompska and K. Jaszczynski, J . Electroanal . Chem., 291 (1990) 217 . 6 M . Jurkiewicz-Herbich and J . Jastrzgbska, J . Electroanal . Chem ., 199 (1986) 201 . 7 M . Jurkiewicz-Herbich, A . Muszalska and J . Jastrzgbska, Colloids Surf., 41 (1989) 169 . 8 M . Skompska, K . Jaszczynski and M . Wolcyrz, J . Electroanal . Chem., 313 (1991) 341 . 9 R. Boca and P . Pelican, Theor. Chim. Acta, 50 (1978) 11 .
J. Electroanal. Chem., 345 (1993) 83-91 Elsevier Sequoia S.A., Lausanne
JEC 02351
On the role of hydrogen bonds in thiourea condensatesthe correlation of a quantum-chemical approach with the experimental results S . Romanowski and T .M. Pietrzak Department of Theoretical Chemistry, University of L6d% ul. Narutowicza 68, 90 136 t6di (Poland)
M. Skompska, M . Jurkiewicz-Herbich and J . Jastrzgbska Department of Chemistry, Warsaw University, al. Pasteura 1, 02 093 Warsaw (Poland) (Received 13 January 1992; in revised form 26 June 1992)
Abstract The semi-empirical quantum-chemical method PCILO 3 was used to estimate the directions, lengths and energies of the intermolecular hydrogen bonds between thiourea (TU) molecules in selected TU-solvent-TU, TU-anion-TU and TU-TU-TU clusters . The results obtained for the hydrogen bond energies support the sequence TU+H 20 < TU+EtOH < TU+CH 3CN and TU+N03 < TU+ C1O4 < TU+F - of increasing stability of thiourea condensates, established by electrochemical studies at the mercury/solution interface .
INTRODUCTION
Thiourea (TU) is one of the neutral organic molecules that is strongly adsorbed at the mercury/solution interface . Its electrosorption properties are due to the high value of the permanent dipole moment (4 .89 D [1]) as well as to the strong interaction of the sulphur in TU with mercury [2], particularly in the range of positive electrode charge densities . Thiourea, like many other organic adsorbates, may undergo phase transitions at the interface, provided that its bulk concentration is sufficiently high and the temperature is lowered to a certain critical value T, [3-5]. In the phase transition process the surface Gibbs excess of thiourea on the mercury changes from 5 .5 x 10 -10 to 10 x 10-10 mol cm -2 [3]. Despite such a high surface excess of adsorbate, the strong repulsive interactions between parallel oriented dipoles are markedly reduced and the condensed films are stable over a wide range of potentials . According to Buess-Herman et al. [3], the attractive 0022-0728/93/$06.00 C 1993 - Elsevier Sequoia S .A. All rights reserved
84
interaction between two adjacent molecules in the film results from the close proximity of the sulphur and the NH 2 pair belonging to two tilted TU molecules . However, a conspicuous influence of the type of supporting electrolyte as well as the nature of the solvent on the stability of compact films, which we found in previous studies [4-7], led us to other conclusions . In our opinion the condensed structure is probably immobilized by the hydrogen bonds formed between thiourea and the other components of the solution - the solvent molecules and the anions of the supporting electrolyte . Furthermore, it was found that a solid condensate is also formed in the bulk of ethanolic solution containing thiourea and KF [8]. X-ray powder diffraction diagrams showed the large interplanar spacings of the solid phase, probably due to the hydrogen-bonded (TU) 3 F`(EtOH)„ structure . Unfortunately, because of the poor crystallinity of the sample we were not able to find the unit cell but a possible structure of the condensate was proposed . The semi-empirical and ab initio quantum-chemical methods are valuable tools for predicting the directions, length and energies of intermolecular hydrogen bonds. The PCILO3 method developed by Boca and Pelican [9] takes into account the correlation effects of electrons and therefore it is one of the best methods, among all the semi-empirical theories, for calculating the hydrogen bond energy . In this work the results of calculations performed using the PCILO3 method were used to check the previous assumptions about the role of hydrogen bonds in the phase transition processes of thiourea . EXPERIMENTAL
Qualitative studies of the phase transition process of thiourea, focussed on the temperature stability of condensed superficial layers, were carried out using differential capacity-potential (C-E) measurements and cyclic voltammetry . The latter method is particularly useful because of its high sensitivity with respect to the nature of the process studied . The shape of the cyclic voltammograms allows us to interpret questionable C-E profiles . The electrochemical measurements were carried out in a three-electrode, water-jacket cell on a stationary hanging mercury drop electrode . The procedure and the electrochemical equipment used have been described previously [4,5]. Thiourea was recrystallized from a water + ethanol mixture and dried at 50°C under reduced pressure . The salts LiC1O4 , LiNO3 and KF (Merck) were used without further purification . All the solvents were purified : water was triply distilled, and ethanol and acetonitrile were dried and then distilled . The quantum-chemical calculations using the PCILO3 method were performed on the complete neglect of differential overlap/2 (CNDO/2) Hamiltonian level . In the first step of the calculations, the geometry and the total energy of single molecules of TU and the solvent and of the anion of the supporting electrolyte were established . The molecular geometry was fully optimized, i .e. the bond lengths and the angles for the structure with minimum total energy were chosen . The interaction energies were then calculated for clusters containing two molecules
85
of thiourea and one molecule of the solvent or a selected anion . The geometries of the monomers were assumed to be rigid and only the intermolecular geometrical parameters were optimized . The assumption of rigidity means that the interaction energy was calculated in the supermolecular manner, i .e . "E(AB) = E(AB) - (E(A) + Ecs>) After optimizing the intermolecular parameters, the energy was always corrected, including correlation, up to second order of many-body perturbation theory (MBPT) (Moller-Plesset type) . Two main criteria for the existence of the hydrogen bond were taken into account : (a) a significant charge transfer between two neighbouring atoms in a cluster with respect to the charge on the same atoms in the single molecules ; (b) the bond length was chosen as the shortest of all theoretically possible hydrogen bonds .
RESULTS AND DISCUSSION
In previous papers on the condensation of thiourea at the mercury/solution interface [4-6], which were mainly qualitative, the following hypotheses were presented . (1) The structure of the quasi-solid condensate I, which forms at non-charged or slightly positively charged mercury, is stabilized by the hydrogen bonds between thiourea and the solvent molecules (both adsorbed at the electrode) . (2) The interactions between the amino groups of TU, pointing out towards the bulk of solution, and the anions of the supporting electrolyte and/or the solvent molecules from the second monolayer may be another stabilizing force for the condensed layer I . (3) An alternative structure for the quasi-solid film which could be also taken into account may be a layer of antiparallel oriented TU molecules stabilized by the hydrogen S . . . H-N bonds . (4) The condensed structure II is probably a mixed TU + anion film formed at the positively charged mercury and stabilized not only by the hydrogen bonds between TU and the coadsorbing anions but also by electrostatic electrode-dipole interactions, leading to the formation of covalent bonds between sulphur and mercury [2], and by electrostatic interactions between the positive end of the TU dipole and anions from the diffuse layer . As the examination of the temperature stability of film II is rather difficult because of the onset of the electrochemical oxidation of mercury at about - 0 .4 V/(SCE), the experimental results presented concern only layer I . In order to verify the hypothesis of the crucial role of hydrogen bonds in the phase transition process, two types of experiment were performed . Firstly, C-E curves and cyclic voltammograms were recorded for a wide range of temperatures (from 10 to 40°C) in solutions of various solvents saturated with TU and containing
86
T/ C (b)
(a)
30
30
25
25
20
20
B 15
15
•
0.5
0.6
0 .7
•
0. 8
-E
iv
Fig. 1 . Phase transition diagrams for the mercury electrode : (a) thiourea-saturated solutions containing 0.2 M LiC104 in the solvents H 2 O, EtOH and CH 3 CN; (b) thiourea-saturated ethanolic solutions of 0.16 M KF, 0 .2 M LiC10 4 and 0.2 M NaN0 3 . The phase transition potentials in 0 .16 M KF are given only in a limited temperature range because below 23°C condensation also takes place in the bulk of the solution . The curves denoted by A and B indicate the borders between non-condensed and condensed regions.
0.2 M LiC1O4 as the supporting electrolyte . Similar measurements were performed in aqueous and ethanolic solutions containing two other electrolytes : KF and LiNO 3 . The abrupt changes observed in the C-E and I-E curves, discussed by us previously [4,5], are associated with phase transition processes at the mercurysolution interface . The potentials corresponding to these abrupt changes in the experimental curves were taken as the phase transition potentials EB . The temperature dependence of the phase transition potentials, the so-called phase transition diagrams, obtained from both sets of experiments are presented in Fig . 1 . The quasi-solid films I exist within the potential region located between lines A and B . It follows that the stability of condensed layer I increases in the order TU + H 2O < TU + EtOH < TU + CH 3CN and TU + N03 < TU + C104 < TU + F -. If the assumption about the role of hydrogen bonds between TU and the solvent and between TU and the anions is true, it should also be confirmed by the results of calculations of hydrogen-bond energy using PCILO3 . However, it should be stressed that the calculated geometry of clusters relates to the structures in the bulk of the solution but not those on the charged electrode and therefore the results presented can only be used to confirm a particular tendency but not to establish the exact geometry of the surface condensate . The geometry of the thiourea molecule and a map of the electrostatic potential distribution in the molecule are presented in Fig . 2 . The bond lengths are given in
87
(b) -5 .00 -4 .00 -3 .00 500
0 3C
00 2 .00 3 00 4 00 5 00 1 1 5 00 1 1 1
(a)
4 00 3 00
N
tits •
3 (-0.17) fZ6> ;,`
2 .00
2 00
00
- 1 00
0 .00
0 00
N
1
H~ C2 (029)
S, (-0 .44)
(0 .14)
-1 .00
-1 00
-2 00
-2 00
-3 00
-300
-4 00
-4 00
1 1 1 1 __ -5 .00 -5 .00-4 .00-300-2 .00-1 CC
_j__ I 1 I I
._2
- 500
100 2 .00 3 .00 400 500
Fig. 2 . (a) The geometry and (b) a map of electrostatic potential distribution of the thiourea molecule obtained using the PCILO3 method . The length of bonds in Fig . 2(a) is given in angstroms, and the net charges of the valence orbitals (in parentheses) are given in e scale and related to those for isolated atoms . The scale in (b) is in angstroms .
Angstroms and the net charges of the valence orbitals (in parentheses) are related to those of the isolated atoms . The dihedral angle between the planes S 1C 2 N3 and C2 N3H 5 is 16.7° and that between S 1 C2 N3 and C 2 N3 H 4 is -23.9°. The total molecular energy of TU with and without second-order MBPT corrections is ETU1 = - 1206 .5211 eV and E SCF _ - 1203 .3844 eV, where SCF and PCI denote the self-consistent field Hartree-Fock method and the perturbation correlation interactions method of energy calculations respectively . Similar calculations were performed for the other isolated molecules (H 20, EtOH and CH 3 CN) and for the N03 anion (Fig. 3). In the next step of the calculations, the energy and geometry of the molecular trimers TU-H 20-TU, TU-EtOH-TU, TU-CH 3 CN-TU and TU-TU-TU and the ionic clusters TU-F - -TU and TU-N03-TU were determined using the procedure described in the Experimental section . Examples of maps of the electrostatic potential distribution of the cluster TU-F --TU and the geometric structure of the trimer TU (down) TU(up)_TU(doK,,,) are presented in Figs . 4 and 5 respectively . In the case of the other trimers studied, the TU (up) molecule from the TU(up)-TU(down) cluster was replaced by the appropriate molecule or an TU(down) ion . The oxygen atom in the TU-EtOH-TU cluster lies between two thiourea amino groups and interacts with them through two hydrogen bonds . The carbon chain of EtOH points down with respect to the up-oriented NH 2 groups. The calculations for the TU-CH 3 CN-TU trimer were performed for two possible orientations of the acetonitrile molecule with the -CH 3 chain pointing up or
88
(b) Ethanol
(a) Water
tocr
Ht (0 .13)
1 .03
(-024)
H 9 (0.01)
H 3 10 .13)
n
(0.01`'CI-a02 ) 146 C1
0 2( - 027)
1
03
1
103 H 4 10.12)
#0 .391
M39 H6(-0 .04)
H7 ( .0)
HS (-0.04)
F
=-1014.343eV
E S F =-541 .234eV
Ee
E PC I =- 541 .876 eV
EEC I _- 1016 .529 eV
(d) N03
(c) Acetonitrite
(-0.53)
H 6 (0.03) 119 H6 10 (-001) 1 .42 (0.03)1"? 0, T- C2 110.7* (0.09) /_
N 3 (- 0.17)
0,
tzo~03 • N2 10.6)
1
(-053)'
H4
04(-0.53)
(0.03) EEN F =-759 .130eV EPC' - 762 703 eV
AN
E SCF =-1823 .688 eV
s E PCI =-1827.129eV NO3
Fig. 3 . Optimized geometry of the molecules : (a) water; (b) ethanol ; (c) acetonitrile ; (d) N03 anion.
down . It is found that the latter orientation is more favourable energetically . The water molecule in the TU-H 2 0-TU cluster is oriented with the oxygen atom pointing down and two hydrogen atoms interacting with two sulphur atoms of the thiourea molecules . The resulting average lengths of hydrogen bonds in all the clusters studied and the binding energies calculated per hydrogen bond are listed in Table 1 . It is clear that the sequence of binding energy strength for the molecular clusters is similar to that found from experiments . It may be somewhat surprising that the binding energy in the TU(d ,)_TU(„p)_TU(do,,,,) trimer is only slightly more favourable than that in TU (down) _H 2O-TU(d , ). This energy is probably not sufficient to maintain the antiparallel orientation of TU molecules at the non-charged or positively charged mercury. The resulting order of the binding energies for the ionic clusters is also consistent with that given above . The particularly strong and relatively short bonds between amino groups of thiourea and fluorides are probably responsible for the formation of the quasi-solid condensate in the bulk of the ethanolic- KF + TU solution [8]. In fact, addition of the third molecule of TU to the trimer TU-F --TU does not change the energy of the single hydrogen bond . Optimization of the
89
(a)
IH ,
u)urea +
F( -!
+
TI
'L .)
urea,
Fig. 4 . (a) Planar and (b) spatial maps of the electrostatic potential distribution for the ionic cluster TU-F - -TU.
TU,do,mi
Tuu;
TU, . . .
H (ai)
E3TIJ-3610 .88eV E3TU'-362Q49eV H (031) H (0 .11) Fig. 5 . The geometry of the molecular cluster TU(dO,,.,,)-TU(„P)-TU(d,f) after optimization of the intermolecular distances and angles. The geometry of the single TU molecule is the same as that presented in Fig . 2.
90 TABLE 1 Characteristics of the hydrogen bonds in some thiourea clusters calculated using PCILO 3 . Type of cluster
Type of hydrogen bond.
Number of hydrogen bonds in cluster
Average length of a single hydrogen bond/A
Molecular clusters TU-W-TU TU-TU-TU TU-Et-TU TU-AN-TU
S...H S...H 0...H N...H
2 4 2 2
2.22 2.19 1.62 1.53
-21.00 -22.16 -29.79 -67.84
Ionic clusters TU-F- -TU TU-N03 -TU
F. . . H O...H
2 4
1.25 1.44
-276.27 -113.22
F...H
3
1 .29
-272.17
F...H S...H
3 1
1 .29 2.18
-272 .17 -40.43
TU I FTU TU TU FI \ Et TUTU
Energy per hydrogen bond /kJ mol -1
TU, thiourea ; W, water; Et, ethanol; AN, acetonitrile.
intermolecular geometry of the cluster (TU) 3 F - yields some corrections to the structure proposed previously [8]. It is found that each thiourea molecule is bonded to the central fluoride ion by only one hydrogen bond of average length 1 .29 A. The distance between F - and the hydrogen atom from the second amino group of each TU molecule is too long (2 .8 A) to form an effective hydrogen bond . It also follows from the calculations that the ethanol molecule incorporated in the system does not change the geometry of the (TU) 3 F - cluster but is joined to it by S . . . H-O hydrogen bonds of length 2 .18 A and binding energy - 40.43 kJ mol -t . CONCLUSIONS
It follows from the experimental results that the stability of the quasi-solid condensates, obtained in various solvents and in the presence of various supporting electrolytes, increases in the order TU + H 2O < TU + EtOH < TU + CH 3 CN and TU+NO3
91
fluoride ion. The molecule of ethanol is joined to the cluster by an S . . . H-O hydrogen bond . ACKNOWLEDGEMENT
Partial financial support from grant B 545 is gratefully acknowledged . REFERENCES 1 W.D. Kumler and G.M . Fohler, J . Am. Chem. Soc ., 64 (1942) 1944 . 2 R. Parsons, Proc . R. Soc . London, Ser . A, 261 (1961) 79. 3 CI . Buess-Herman, L. Gierst, M. Gonze and F . Silva, J . Electroanal . Chem ., 226 (1987) 267 . 4 M. Skompska and K. Jaszczynski, J . Electroanal . Chem., 272 (1989) 207 . 5 M. Skompska and K. Jaszczynski, J . Electroanal . Chem., 291 (1990) 217 . 6 M . Jurkiewicz-Herbich and J . Jastrzgbska, J . Electroanal . Chem ., 199 (1986) 201 . 7 M . Jurkiewicz-Herbich, A . Muszalska and J . Jastrzgbska, Colloids Surf., 41 (1989) 169 . 8 M . Skompska, K . Jaszczynski and M . Wolcyrz, J . Electroanal . Chem., 313 (1991) 341 . 9 R. Boca and P . Pelican, Theor. Chim. Acta, 50 (1978) 11 .