J. Chem. SOC., Faraday Trans. I, 1982, 78, 2059-2072 Silanol and Water on Silica Studied by the CNDO MO Method BY KATSUYUKI TAKAHASHI Department of Mineral Science and Technology, Faculty of Engineering, Kyoto University, Sakyo, Kyoto 606, Japan Received 13th May, 1981 The CND0/2 molecular-orbital method has been applied to silanol or water on silica. Silanols on silica bind strongly to the surface, though they are more acidic than alcohols. Hydrogen-bonding hydroxyls have a lower bond strength than the surface hydroxyls. On the specific adsorption of water or water dimers on silica, two types of electron flow are observed: H,O (donor) -+ SiOH (acceptor) and SiOH (donor) + H,O (acceptor); in the former the oxygen of the water interacts with the hydrogen of SiOH and in the latter the hydrogen of the water interacts with the oxygen of SOH.The empirical shift of the hydroxyl absorption spectra of silanol and water on silica corresponds to the difference of calculated hydroxyl bond energies in the adsorption system and the isolated system. It is also theoretically explained that hydroxyls on silica tend to be centres for water adsorption and that it is difficult for the silica to release protons from its surface. On siloxane formation, the SiOSi bond angle may decrease in order to form a ring, which is the preferential shape for the Kiselev process. Following some recent papers, our understanding of silanol and water molecules adsorbed on a silica surface on a molecular basis has improved. The physicochemical studies of silanol or water sorption on silica surfaces, such as vapour sorption, calorimetry, i.r.and n.i.r., n.m.r., X-ray scattering, LEED or Auger spectroscopy, etc., are explained in several b00ks.l.~ Those studies indicated that the hydroxyls are centres for the adsorption of water on silica. Klier et al.4 looked into the mechanism of water adsorption and agglomeration on silica and proposed a model for the interaction of water with hydroxyls on silica using information obtained from the position and structure of fundamental and overtone H,O absorption bands. However, neither the binding state of silanol on silica nor the interaction of water molecules with hydroxyl groups on silica have been completely elucidated, This paper is an attempt, using the molecular-orbital method, to throw light on the electronic state and the binding state of both silanol and water molecules adsorbed on silica.The LCAO MO method used here is the completely neglect differential overlap (CND0/2) method, originaly developed by Pople et al.5-9 In quantum-chemical calculations of molecular interactions the molecular complex or cluster in question is considered as a supermolecule, for which the CND0/2 calculation in this paper is carried out. The interaction energy (AE) is determined as the difference between the energy of the supermolecule (Ep~Q) and the sum of the energy of the isolated constituent molecules ( E p , EQ) in their equilibrium geometry, eqn (1): AE = Ep*Q - (Ep + EQ). (1) The energy of the supermolecule or the single molecule including nuclear repulsion 67-2 20592060 SILANOL A N D WATER O N SILICA is divided into two parts, the one-centre energy part ( E A ) and the two-centre energy part (EAB), eqn (2): where A and B represent atoms.RESULTS AND DISCUSSION Silica exists in numerous crystalline modifications, the most important forms being quartz, cristobalite and tridymite. Their basic crystal unit is a tetrahedral structure of Si-0, as shown in the molecular diagram of SiOj- (Si-0 = 1.61 A*) in fig. 1. The charges of Si and 0 become ca. 0.5 (3.502 in electron densities) and - 1.1 (7.124 in electron densities) electrons, respectively, and the bonding strength of Si-0 is 01 - 0.409 ii2\, '\ 0. % \ \ \ 0, 0 4 FIG. 1 .-Charge distribution (electron units) and bond energy (atomic units) in SiOf. 08 P9 \ / Si, - 07 07- Si, \ 142.0' 141.06* ( p 5 1 .6 l i ,! 1.61 O1-J Si '\\ 2 \ \ 03 04 FIG. 2.-Schematic diagrams of quartz and symmetrical silica with their numbering. almost twice the antibonding strength of 0-0. Fig. 2 shows schematic structures of quartz and symmetrical silica with their numbering. Calculations were done for simplified models of silica, which were assumed to be Si,O;- consisting of two SiO, tetrahedra sharing a bridging oxygen atom, with the distance SiO = 1.61 A, the angle OSiO = 109.47' in a tetrahedral structure, 142.0' for quartz, 180.0' for cristobalite and tridymite and 141.06' for symmetrical silica. The bond length and bond angle we have taken for the equilibrium geometry compare well with 1.617 A and 143.3' calculated from the ab initio methodlo and with the experimental values 1.634 A and 144.1O l1 in disiloxane (H,SiOSiH,) 1.61 A and 143.7' in low quartz.12 Table 1 shows * 1 A E 10-10 m = 10-1 nm.K.TAKAHASHI 206 1 TABLE 1 .-VARIOUS VALUES FOR QUARTZ, CRYSTOBALITE, TRIDYMITE AND SYMMETRICAL SILICA quartz cristobalite tridymite symmetrical charge distribution 0, 0 5 0,-Si, Si,-0, Si , bond energy/a.u. total energy/a.u. - energy gap/eV transition energy/eV - 1.139 0.704 -0.816 - 0.490 - 0.347 129.78 12.81 6.64 - 1.097 0.695 - 0.808 - 0.486 -0.359 - - 129.83 12.51 6.28 - 1.096 0.693 - 0.808 -0.485 -0.358 .129.82 - 12.51 6.28 - 1.041 0.695 -0.817 - 0.478 - 0.344 129.77 12.73 6.48 charge distributions, bond energies, energy gaps and transition energies of the four crystalline models. Unbroken Si2-0, and Si,-0, bonds are weaker than other broken Si-0 bonds with the result that the charge of 0, between Si, and Si, becomes smaller than that of other oxygen atoms binding one silicon. As in the formation of a Si-0-Si bridge, electrons from oxgyen are attracted to two silicon atoms, the electron densities between unbroken Si and 0 became smaller than between broken Si and 0.Various values for the four structures of Si,O!- are approximately the same in table 1, and the value of the energy gap, ca. 12 eV, is similar to a value of 11 eV for a-quartz.13 The CND0/2 results for perfect a-quartzl* and Si20,H,15 and the ab initio STO-3G result for Si,07H,14 are -0.6, -0.7 and -0.6, respectively for the formal charge of the central oxygen atom. Our calculations show that the formal charge of the central oxygen is -0.8 (for si20;-, table 1) and -0.7 (for Si207H,, see later, fig.3), close to the above values and to the oxygen charge in quartz of -0.7 inferred from the shift of the Si Kg X-ray emission spectral6 and the charge of -0.7 calculated from the ab initio SCF method for molecular Si02.17 After the supposition18 that the valences of silicon atoms on surfaces must be saturated with silanol groups, Carman19 visualized the structure of a particle of colloidal silica as consisting of a network of interlinked SiO, tetrahedra with hydroxyl groups attracted to the surface due to the tendency of silicon to complete tetrahedral coordination. Boehrnz0 then considered each particle of silica to be a macromolecule of polysilicic acid.Taking symmetrical silica as the basic structure of the various silicates in order to study the electronic state of silanol or water on silica (see later), we first calculated the total energy of the Si,O;- + 2H+ system as a function of angles from the Si-0 axis and fixed the 0-H bond length as 0.96 A. The total energy of the system falls as the angle from the Si-0 axis increases, attaining its lowest total energy, i.e. the most stable state, at an angle SiOH = 137.5', and then rises up again. This angle is different from the angle of ca. 113' for the SiOH bond angle as found by Peri from i.r. spectroscopy.21 However, the difference in the electron densities and bond energies in both systems is only ca. orders of magnitude in the calculation. The total energy (- 133.398 a.u.) in the geminal configuration2 connecting two hydroxyls to one silicon atom is higher than that (-133.501 a.u.) in the vicinal configuration2 connecting one hydroxyl to one silicon atom, because of higher E A B than E A in eqn (2).We next changed the bond length of 0-H and fixed the angle SiOH as 137.5'. The most stable state appears at 0-H = 1.06 A, slightly longer than the normal bond length of 0.96 A. On addition of the six protons to Si207 ,- , five configurations were considered (table 2) and their total energies calculated with the2062 SILANOL AND WATER ON SILICA 0.881 ‘07 Si, 2.383 -0.583 6.536 \ 0, 6.728 -0.596 1 0.884 H1o Si2 2.389 01 ~ -0.76\;\ -0.476 6.551 I j._i ~ o . 4 7 9 0 4 / ,’ ,’ * $:. 7 63 H13 H 12 0.884 FIG. 3.-Electron density (electron units) and bond energy (atomic units) in the adsorption system Si,O:- + 6H+.TABLE 2.-TOTAL ENERGY OF FIVE CONFIGURATIONS OF THE ADSORPTION SYSTEM si,o!- -k 6H+ configuration 1-2 cis 1-2 trans 5-2 cis 5-2 trans 5-2 trans 7-6 cis 7-6 trans 5-6 cis 5-6 trans 5-6 cis total energy/a.u. - 1 3 8 . 1 0 4 - 1 3 8 . 1 0 6 - 1 3 8 . 1 0 4 - 1 3 8 . 1 0 6 - 1 3 8 . 1 0 7 fixed angle SiOH = 1 37S0 and the fixed 0-H bond length of 1.06 A. With reference to fig. 3, 5-2 trans, 5-6 cis in table 2 means that H,, (or H13) locates itself at a trans position against 0, on the Si2-03 (or Si2-04) axis, and Hi4 (or HIS) locates itself at a cis position against 0, on the Si,-0, (or Si,-0,) axis. The other expressions have similar meanings. The optimal state among the five Si,O$- + 6H+ systems could be obtained for 5-2 trans, 5-6 cis, whose molecular diagram is illustrated in fig.3. When six protons add to Si,O;-, hydrogen has a slight positive charge, +0.12 (0.88 in electron densities), due to electron flow from silicate to hydrogen. Consequently, the positive charge of Si increases as the negative charge of 0 decreases due to the decreasing electron densities of both Si and 0. The large value of the bond energy of hydroxyl groups, e.g. 0,-H,, or 07-H11, means that it is difficult to remove silanol from the silica surface. Note, however, the weak hydrogen-bonding strength of O7-HlO (- 0.01 6 a.u.), which is approximately equivalent to one-fiftieth that of O,--H,o.K. TAKAHASHI 2063 When the silanol groups of silica are compared with the hydroxyl group of alcohol, the former are considered more acidic than the latter when measuring the shifts of the infrared 0-H stretching bands22 upon admixture with the bases ether and mesitylene and the acid phenol, respectively.The energetic calculations (AE), using a configuration given by ref. (23), for reactions to release a proton, eqn ( 3 ) : difference bond energy of energy of leaving H CH,OH -+ CH,O-+ H+ AE = 0.898 -0.741 a.u. Si20,6H + Si20,5H-+ H+ AE = 0.853 - 0.736 a.u. } ( 3 ) result in silanol on silica being more acidic than the hydroxyl group in alcohol. Furthermore, the bond energy of the hydroxyl groups is lower in silica than in alcohol, so the silanol on silica may be more acidic. The CND0/2 calculations for a series of compounds containing the Si-0 bond indicated that the experimental oxygen bond angles in these compounds are reproduced reasonably well without the use of Si ( 3 4 functions.24 However, we will investigate the effect of the Si 3d orbital on the electron densities of each atom as well as on the bond energies of Si-0 and 0-H or the total energy of the Si20!-+6H+ system, making use of the work of Santry and SegaP on the effect of 3d functions of various second-row elements on the charge distributions, bond angles and dipole moments TABLE 3.-EFFECT OF si d ORBITALS ON ELECTRON DENSITY, BOND ENERGY AND TOTAL ENERGY OF THE ADSORPTION SYSTEM, Si,O!- + 6H+ bonding electron density 0, Si, 0 5 HI0 bond energy/a.u. 0,-Si, Si,-0, O,--H,, E'4 E m total energy/a.u.total 6.551 2.389 6.728 0.884 0.476 - 0.596 - 0.763 - 138.107 - 130.509 - 7.599 6.326 3.418 1.861 (sp) 1.557 ( d ) 6.267 0.828 -0.849 -0.817 -0.746 - 140.883 - 129.750 - 11.133 of molecules.The results of the sp and spd methodsa are shown in table 3. The Si 3d orbital causes not only a large increase in the electron densities of Si (the positive charge of Si decreases), but also a decrease in the electron densities of 0 (negative charge of 0 decreases) and H (positive charge of H increases). Note that the Si 3d orbitals are to play a great role in electron populations on Si. Regarding the bonds, the Si-0 bond strength increases as the 0-H bond strength decreases because of the Si 3d orbital. The total energy of the system becomes lower, i.e. the system is stabilized due to mutual interaction energies (,TAB), including the Si 3d orbitals, between different atoms.2064 SILANOL A N D WATER ON SILICA The structure and physicochemical properties of water are explained in a few book^.^^.^^ As far as water clusters are concerned, recent quantum-mechanical calculations were carried out from dimer to hexamer or with water-ion interactions.The dimers of water and their configurations, stabilization and hydrogen bonding have been discussed using ab initio method^.^^-^^ Dimeric water was here considered as only a linear dimer in its most stable state, though other bifurcated and cyclic dimers may also be present. Fig. 4 shows molecular diagrams of the water molecule and of the 0.142 0.155 H2 "@qJ -0.060 -0.737 -0.326 H3- 02 Y.755 A 0 .7 5 5 v.756 01-0.2 84 ,Qpoo;;----- 0.1 76 H1 U . H, O . l i 2 0.155 0.;; 6 (a) ( b ) FIG. 4.-Charge distribution (electron units) and bond energy (atomic units) in water and a linear dimer of water. linear dimer of water in the most stable configuration obtained using the CND0/2 calculation, with the bond length of 0,-H, (or 0,-H,) taken as 0.9572 A and the angle H,O,H, as 104*52', in common with the values for water. In this water dimer the bond length of 0,-0, for the lowest energy was determined to be 2.53 A, different from the value of 3.00 A obtained in an ab initio calculation by Popkie et ~ 1 . ~ ~ On going from water dimer to the water molecule, the gross electron densities of H,O,H, decrease slightly as those of H,O,H, increase slightly, so that the linear dimer may form a molecular complex as H,O,H, becomes a donor and H,O,H, an acceptor.The new hydrogen bond in the water dimer is calculated to have a strength of -0.06845 a.u. (-42.95 kcal mol-l), one-tenth the strength of the 0-H bond of water, resulting in weakening of its nearest-neighbour bond. If the 0-H binding energy in isolated water molecule is estimated to be 109 kcal mol-l, the H,-O, bond energy is ca. 10 kcal mol-1 in the range of empirical hydrogen-bond energies. In water oligomers and tetrahedral clusters the hydrogen-bond energies obtained using the CND0/2 method (8.7-10.1 kcal m01-l)~~ are in fairly good agreement with those obtained using the ab initio method (6.1-8.2 kcal m ~ l - l ) . ~ ~ The stabilization energy for water dimer formation was calculated to be 6.5 kcal mol-l, larger than the value of 5.1 kcal mol-1 when including its correlation energy,35 and close to the calorimetric heat of a single hydroxyl-water bond, 6 kcal m01-l.~~ In addition, the values of dissociation energy of the linear water dimer obtained by different methods are similar, e.g.CND0/2 (5.0-8.7 kcal mol-l), INDO (4.0-6.9 kcal mol-l), STO-3G (6.0 kcal mol-l), 6-31G (5.6 kcal mol-l) and extended +polarization (4.7-5.1 kcal m ~ l - l ) . ~ ' 39 Therefore, the CND0/2 calculation is reliable for hydrogen-bonded systems. Five schematic models of water adsorbed on silica using the sp method are illustrated in fig. 5. In structure (1) oxygen (Ole) of water strongly interacts with hydrogen (H,,,) of silanol, while in structure (11) hydrogen (H,,) of water strongly interacts with oxygen (0,) of silanol.Structure (111) shows that another water molecule behaves like the water molecule in structure (I). In both structure (IV) and structure (V) another water molecule adsorbs and forms a linear water dimer, analogously to structures (I) and (11),K. TAKAHASHI 2065 (IV) (V) FIG. 5.-Five schematic models of water adsorbed on silica and the change of electron density and bond energy of the adsorption system as compared with the isolated system. AElkcal mo1-l. (I) -7.84, (11) -8.10, (111) - 16.04, (IV) - 13.79, (V) -7.35. respectively. The most stable configuration of water on silica was first determined for structure (I). Fixing 0,-0,, = 2.53 A on an extension of the 0,-H,, line, we rotate water H,,O,,H,,, taking O,, as the centre and angles XO,,H,, as 8, where X means a horizontal axis through O,,.The total energy of the system (Si20,6H + H,O) as a function of angles (0) indicates the most stable state at an angle XO,,H,, = 25O, a bisector of H,,O,,H,, being located at 1 1 . 2 4 O anticlockwise from the line of O,H,,O,,. Secondly, by changing the 0,-0,, distance whilst fixing the angle XO,,H,, as 2 5 O , we find the total energy of this system indicates the system is at its most stable when 0,-0,, = 2.53 A. For structure (11) we changed the angles XO,H,, = 8, where X means a horizontal axis through 0,, keeping O,H,,O,, in a straight line and fixing 0,-O,, as 2.53 A. The lowest energy for this system was obtained at an2066 SILANOL A N D WATER ON SILICA angle XO1Hl7 = 60'.By changing the distance between 0, and 0,, at the definite angle XO1Hl7 = 60Othe most stable state exists at 0,-O,, = 2.53 A. In the next step, the water molecule H1701,H,, was rotated around OI6, in the plane of the paper. The total energy of the system is found as a function of angles, ZO,,H,, = 8, where Z means a vertical axis through O16. The optimal angle becomes ZO,,H,, = 26O, O,,H,, deviating slightly from the 0,01, line by 4' in a counterclockwise direction. The interaction energy (AE) for adsorption of a water molecule on disiloxane (H,SiOSiH,) at its equilibrium geometry is -5.43 kcal mol-l by the STO-3G method4, and -5.35 kcal mol-l by the 4-31G method.40 Also, the adsorption energy of a water molecule on a surface cluster of silica was calculated to be -9.95 kcal mot1 by the CNDO MO method,41 while the experimental enthalpy of a water molecule on silica is ca.6 kcal m01-l.~~ Our calculations for :he adsorption of a water molecule on silica (Si2O76H), using a different model from those mentioned above for disiloxane and silica, give -7.8 kcal mol-l for structure (I) and -8.1 kcal mol-l for structure (11), in good agreement with the above values. Fig. 5 illustrates that the plus and the minus signs for the electron density and bond strength are increased and decreased on adsorption of water on silica, in comparison with those in the adsorption and isolated systems. When water adsorbs on silica in structure (I), the electron densities of 0,, in adsorbed water and of 0, in silanol increase as those of H,, (or H18) in water and H,, in silanol decrease.Electrons flow from the adsorbed water molecules to silica, so that the gross electron densities of water may decrease by -0.039, as shown in table 4. On the contrary, for structure (11) electrons flow from silica to the adsorbed water, the gross electron densities of water increasing by +0.032. For structures (111) and (IV), electrons flow from two water molecules to silica, as in the case with structure (I), decreasing the gross electron densities of dimeric water by - 0.044 and - 0.029, respectively. On the other hand, for structure (V) electrons flow from silica to water Hl70,,Hl,, as in the case with structure (II), so the gross electron densities of this water molecule increase by + 0.027. Another water molecule H,OO,gH,, scarcely changes in its gross electron densities. In short, we can classify two types of electron movement: one is H,O (a donor) + silica (an acceptor), for structures (I), (111) and (IV), and the other is silica (a donor) -, H,O (an acceptor), for structures (11) and (V).As for bonding states, if a new hydrogen bond is formed between 0,, and Hlo, as shown for structure (I), the bond strength of the nearest-neighbour bond H,,-0, decreases and the next nearest neighbour bond 0,-Si, increases to a lesser extent. The successive alteration of bond strength which occurs for silica and water also occurs when a new hydrogen bond is formed. We note that the frequency of the hydrogen- bonded OH should be lowered. It is already known that the frequency of hydrogen- bonded OH is lower than the frequency of the surface OH43344 and also that the frequency of the surface hydroxyl absorption band is lowered due to the interaction between surface OH and adsorbed molecules such as 0,, N2,45 acetone, ammonia,46* 47 toluene, ethylben~ene,~, e t ~ .~ ~ Klier et aL4 classified the interaction of partially hydrophobic silicas with water and discussed the structures and frequency shift in SOH-H,O on a donor-acceptor basis. As shown in table 4, for structures (I), (11) and (111) the shift of OH bond energies of silanol and water, determined by the difference of OH bond energies both in the adsorption system and in the isolated system, is likely to correspond to the spectroscopic shift of H20 and SOH. The shift of OH bond energies for structures (IV) and (V) tends to be similar to the shift for structures (I) and (11), respectively.The new hydrogen bond formed between water and silanol was calculated to be at ca. -0.09 a.u., an actual hydrogen-bonding value being ca. 13 kcal mol-1 by analogy with the 0-H binding energy in isolated water. The bond strength (-0.059 a.u.) between water far from silica and silanol isK. TAKAHASHI 2067 TABLE 4.-vARIOUS CHANGES IN THE FIVE ADSORPTION SYSTEMS AS COMPARED WITH EACH ISOLATED SYSTEM configuration difference of electron density of H,O electron flow difference of bond energy of OH in silica/a.u. shift difference of bond energy of OH in H20/a.u . shift new bond of OH/a.u. - 0.039 H20 -+ silica -0.019 large - 0.006 (O16H17) - 0.01 1 (O16H18) small - 0.096 + 0.032 silica -+ H20 -0.001 small - 0.023 (O16H17) + 0.001 (O16H18) large - 0.090 H,O + silica -0.019 (OIHlo) - 0.024 (O,Hll) large - 0.006 (O16H17) -0.014 (O16H18) - 0.006 (019H20) - 0.005 (O19Hzl) small - 0.095 (O16H10) - 0.092 (O19Hll) -0.010 (O16H11) configuration difference of electron density of H,O electron flow difference of bond energy of OH in silica/a.u.shift difference of bond energy of OH in H,O/a.u. shift new bond of 0Hla.u. -0*04 (H17016H18) -o*029 (H20019H21) H,O -+ silica -0.020 (OIHlo) - 0.01 7 (07H11) large - 0.005 (O16H17) -0.010 (O16H18) small - 0.102 (H10016) - 0.059 (Hl1019) - 0.03 1 (0,H18) ~ ~~ +o'027 (H17016H18) +O*Oo2 (H20019H21) silica + H,O 0.000 (07Hll) - 0.002 (OIHlo) small -0.019 (O16H17) + 0.004 (O16H18) large - 0.084 (0,H17) -0.016 (OlH18) - 0.005 (HloO19) approximately half the bond strength (-0.102 a.u.) between water near silica and silanol with structure (IV) and with structure (V) the difference is of the order of a fifth ( - 0.0 16 and - 0.084 a.u.). We will now see how an oxygen atom removing a proton and a silanol group com- pare as an adsorption centre from a molecular point of view.Fig. 6 shows the main bond energies in the adsorption system of structure of fig. 5 without the H,, and H,, hydrogen atoms. The oxygen-oxygen bonds, especially the 0,-0,, bond, have relatively large antibonding values which are different from the relatively large bonding values of the Hlo-Ol6 bond of structure (I). The hydrogen-oxygen bonds2068 SILANOL A N D WATER ON SILICA F15 -0.004 0 " 1 4 ; H FIG.6.-Main bond energies (atomic units) for structure (I) of fig. 5 without the H,, and H,, hydrogen atoms. also have comparatively smaller bond energies. The energy difference of this water adsorption was calculated to be AE = 2.53 kcal mol-l, so that the adsorption system may not be theoretically stabilized by water adsorbed on silica. Although there is the possibility that hydrogen Hi7 or H,, interacts strongly with 0, or 07, we shall not discuss that here as it is similar to structure (11) of fig. 5 . On the other hand, the five structures of fig. 5 are stabilized by water adsorption on silica, as indicated by the values of AE. The isolated hydroxyl groups were far more resistant to removal from the surface during evacuation as the hydroxyl groups already involved in hydrogen bonding were more favourably situated for This is supported by the fact that although a narrow absorption band at 3770 cm-l due to free hydroxyl groups and a broad band at 3450 cm-1 due to hydrogen-bonded hydroxyls were observed in the i.r.spectrum of silica gel evacuated at 350 O C , the low-frequency band disappeared after evacuation at 800 O C while the band at 3770 cm-l remained.51 Our calculations also showed that hydrogen-bonded OH is less strongly bound than free OH. Little et al.43 discussed the removal of water from the neighbourhood of the hydroxyl groups, and Young52 has pointed out that this reaction is reversible at low temperatures and is irreversible at high temperatures. The CND0/2 calculation results in AE = 151.46 kcal mol-1 as the energy difference for eqn (4) from which we deduce that we need to heat silica to a very high temperature in order to obtain siloxane by removal of H20.Fig. 7 is a molecular diagram of siloxane with 0, located at the symmetrical position against 0, on the Si,-Si, line. In comparison with fig. 3 the Si,-O, and Si,-0, bonds become weaker. Furthermore, the 01-Si2 bond or the 0,-Si, bond is antibonding while the 0,-0, bond is strongly bonding,K. TAKAHASHI 2069 1.069 H12 / I 6.539 / / 6.418 O j O5 6.251 0 , 1 3 4 /0.007 l,,?i\3. 466 -0.109 I / FIG. 7. 1.051 1.034 FIG. 8. 66 0 FIG. 7.-Electron density (electron units) and bond energy (atomic units) in the Si,O:-+ 4H+ system-for the formation of siloxane. FIG. 8.-Electron density (electron units) and bond energy (atomic units) in the Si,OE-+ 4H+ system without formation of siloxane.-0.418 -0.418 0.108 0 7 6.785 I I 6.785 0, eqn (6) (the lower part is symmetrical). FIG. 9.-Electron density (electron units) and bond energy (atomic units) of the upper part of Si,Oy; from2070 SILANOL A N D WATER ON SILICA the opposite to the case of the 0,-0, bond in fig. 3. We now consider the case where one silicon atom is missing a tetrahedral bond while the other SiO, has an exact tetrahedral structure, eqn ( 5 ) : A calculated result is shown in fig. 8. Looking at both fig. 7 and 8, we see that the 0,-Si, and 0,-0, bond energies are different: in fig. 8 the 0,-Si, bond is weakly bonding and the 0,-0, bond strongly antibonding, while in fig.7 the opposite is the case. These calculations result in the formation of a strained Si-0-Si bridge, similar to that found by Hocky et af.53 on the irreversible removal of hydroxyls from silica. In fact, the angle of Si,O,Si, will decrease on the formation of siloxane. Another calculation for siloxane formation using Kiselev’s prop~sition~~ was done for eqn (6) : - 0 * +H,O 0 0 I / o O,,\ 1 0 kSi\,/ k0 Si ‘ The molecular diagram of the product compound is given in fig. 9. The bonds between silicon and oxygen become strongly bonding to form a ring, in spite of their antibonding in fig. 7. This indicates that the ring formation of siloxane in fig. 9 is rather easier than that in fig. 7 : the Kiselev process tends to be preferential for ring formation of siloxane.CONCLUSION CNDO/2 calculations show that silanols bound to silica need to be heated to high temperatures in order to remove them from the silica surface, although they are more acidic than alcohol groups. The bond energies of hydrogen-bonding hydroxyls have a lower frequency than those of the surface hydroxyls, in agreement with the empirical results. On specific adsorption of water or dimeric water on silica, two types of electron movement occur: one is H,O (a donor) -+ silica (an acceptor) and the other is silica (a donor) -+ H20 (an acceptor). The empirical shift of the OH absorption spectra of silanol and water due to their mutual interactions on silica corresponds to the difference of their hydroxyl bond energies in the adsorption system and in the isolated system.Hydroxyls on silica are, theoretically, centres for water adsorption, while silica which is releasing protons from its surface is not. In siloxane formation, the Si-0-Si bridge leads to a strained bridge and the Kiselev process becomes the preferential one.K. TAKAHASHI 207 1 I am grateful for permission to use the FACOM 230-75 computer of the Data Processing Center, Kyoto University. I also thank a referee for helpful comments. J. Texter, K. Klier and A. C. Zettlemoyer, in Progress in Surface and Membrane Science, ed. D. A. Cadenhead and J. F. 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