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M. 8. McBride, T. J. Plnnavaia, and M. M. Mortland
Electron Spin Resonance Studies of Cation Orientation in Restricted Water Layers on Phyllosilicate (Smectite) Surfaces Murray B. McBride, T. J. Plnnavaia,*lb and M. M. Mortland*la Departments of Crop and Soil Science and Chemistty, Michigan State University, East Lansing, Michigan 48823 (Received March 4, 1975) Publ/cation costs assisted by Michigan State University
Small amounts of Cu2+ and Mn2+ have been doped into divalent (Mg2+,Ca2+,Zn2+)and monovalent (H+, Li+, Na+) exchange forms of smectite minerals (hectorite and montmorillonite), and the structural nature of the cations in the restricted water layers which are formed under ambient conditions (-40% relative humidity) have been investigated by ESR spectroscopy. In the divalent minerals, which form well-grdered hydration states with interlayers 5.1-5.4 A thick, the Cu(H20)e2+ ions are oriented with their elongated symmetry axis perpendicular to the plane of the silicate sheets. This orientation is markedly different from that found for the ion in the two water layer phase of vermiculite where the ligand axes are inclined near 45" to the silicate sheets. Differences in the position of silicate charge and the hydrogen bonding ability of the silicate oxygens are important in determining cation orientation. Cu2+ and Mn2+ in the monovalent smectites, which are interstratified with one-three molecular layers of water, occupy a distribution of anisotropic environments near two adjacent silicate charge sites. The divalent ions most likely are coordinated to silicate oxygens and to interlayer water molecules which may not be highly structured. Thermal dehydration of Mn2+ in Mg2+-hectorite causes the ions to penetrate hexagonal cavities in the silicate structure as Mn2+ loses most of its ligand water This results in a unique Mn2+ ESR spectrum, unlike that of Mn2+in aqueous environments.
Introduction The swelling phyllosilicate minerals known as smectites possess mica-like structures in which stacked layers of rigid, two-dimensional silicate anions are separated by exchangeable, hydrated cations. Because the thickness of the cation interlayers can be controlled to some degree by regulating the partial pressure of water in equilibrium with the solid phase, the minerals are especially well suited for spectroscopic investigations of hydrated cations in restricted water layers. ESR line width studies2 of Mn(H20)e2+ in interlayers containing four or more molecular water layers have shown that the mean lifetime of the ion prior to collisional relaxation with outer-sphere water molecules approaches the value expected for the ion in dilute bulk solution. Reducing the height of the interlayer to two molecular layers of water causes the correlation time and line width of M ~ I ( H ~ O to ) ~increase ~+ due to increased water structure and reduced mobility of the ion. Related results have been obtained from ESR investigat i o n ~of~ hydrated Cu2+ ions in the intracrystal environment. When a monolayer of water is present, the planar Cu(H20)d2+ is confined to lying flat between the silicate sheets. On the other hand, with two molecular layers of water present, a tetragonally elongated C U ( H ~ O )ion ~ ~ is + formed which is restricted to an orientation with the ligand axes approximately 45O to the silicate sheets. Rapid tumbling of the ion occurs only when the interlayers are fully hydrated by multiple layers of water. It became apparent in the course of the Cu2+ and Mn2+ studies that the structural nature of the hydrated interlayers depends to some degree on the type of exchange cation present. In addition, the position and extent of positive charge deficiency in the silicate sheets probably plays some role in determining the interlayer structure. More detailed knowledge of the importance of these two factors is desirThe Journal of mysiurl chem/stry, vo~.79, NO. 22, 1975
able, in part, because it may be useful in optimizing the conditions necessary for carrying out metal-catalyzed reactions in the intracrystal space of the mineral^.^ In the studies reported herein small amounts of Cu2+ and Mn2+ have been doped into divalent and monovalent cation exchange forms of smectite, and ESR spectroscopy has been used to determine the structural properties of the ions in the restricted intracrystal water layers which are formed under ambient conditions (-40% relative humidity). Also, the ESR spectra of thermally dehydrated Mn2+and Cu2+-doped hectorites establish the nature of the silicate-cation interaction in the absence of intracrystalline water. Experimental Section The smectite used in this study was a hectorite with an approximate anhydrous unit cell formula5 of Nao.66[Mg5.34Li0.66](Sis)O20(oH,F)4 and an experimentally determined2 cation exchange capacity of 73 mequiv/100 g. Although hectorite was preferred in the ESR studies because of its low iron content, a montmorillonite from Upton, Wyoming with the formula6 Nao.64[A13.06Fe0.32Mg0.66]( A ~ o . ~ o S ~ , . ~ ~ ) Owas ~ Oalso ( O Hused ) ~ in a few instances. Various exchange forms of the minerals were prepared by washing them with an aqueous solution of the appropriate metal chloride, Excess salt was removed by washing the mineral several times with distilled water and decanting the supernatants following centrifugation. H+-hectorite was prepared by passing a suspension of the mineral in water through a column of acid-saturated Dowex-50 resin. Doping the minerals with Cu2+ or Mn2+ was accomplished by allowing them to equilibrate with enough aqueous MnC12 or CuCl2 so that approximately 5% of the exchange sites were replaced. X-Ray basaI spacings (&I) were determined with a No-
ESR Studies of Phyllosilicate Surfaces
2431
relco diffractometer and Ni-filtered Cu radiation. The height of the interlayers was determined by subtracting the c dimension of the silicate sheet (9.6 A) from the observed do01 values. X-band ESR spectra were obtained with a Vary ian E-4 spectrometer. The self-supporting, oriented film samples of the microcrystalline minerals used in the ESR studies were prepared by a method described earlier.3 Dehydration of the mineral was accomplished by heating them in quartz ESR tubes and sealing the tubes immediately to prevent rehydration.
Results and Discussion Structural Considerations. The phyllosilicates discussed in this work, vermiculite, hectorite, and montmorillonite, have related unit cell structures which consist of 20 oxygen atoms and four OH groups, except that some isomorphous replacement of OH by F occurs in hectorite (see Figure 1). However, there are two. important differences between a vermiculite and a smectite which can cause differences in interlayer structure. First, the negative charge on the silicate sheets of vermiculite originates mainly from isomorphous replacement of Si(1V) by Al(II1) in tetrahedral sites, whereas in the smectites the positive charge deficiency arises mainly due to replacement in octahedral sites of Mg(1I) by Li(1) (hectorite) or of AI(II1) by Mg(I1) (montmorillonite). Thus the origin of silicate charge differs in vermiculite and smectite. Secondly, the extent of charge deficiency per unit cell formula is greater in vermiculite than in smectite (-2.0 vs. -0.8 equiv per unit cell formula, respectively). Consequently, the average distance between exchange ions is ca. 1.6 times greater in smectite than in vermiculite. M2+-Smectites. Earlier studies3 of Cu2+-smectite have shown that as the relative humidity is increased from -40% at ambient temperature to loo%, the mineral undergoes a continuous transition from a monolayer phase (dml = 12.4 8) to a multimolecular interlayer water phase (do01 N 20 8) without forming a well-defined intermediate hydration state. Although Mg2+-smectite can also be swelled to a do01 value of -20 8, the initial hydration state is well-ordered with do01 = 15.0 8. The difference in initial hydration states underscores the importance of metal ion hydration in determining interlayer structure. The more weakly held axial water molecules in the distorted Jahn-Teller Cu(H20)e2+ ion are lost more readily than those of symmetric Mn(H20)62f, and, hence, complete replacement of Mg2+ by Cu2+ causes the interlayer to collapse from two or three water layers to a single water layer. When Cu2+ is doped into Mg2+-hectorite a t the 5% level, however, the basal spacing remains a t 15.0 8, under air-dried conditions. Figure 2 shows the ESR spectra obtained for the 15.0-A phase with the magnetic field direction oriented parallel and perpendicular to the plane of the silicate sheets. Equivalent spectra were obtained for C u ( H 2 0 ) ~ ~doped + into Ca2+- and Zn2+-montmorillonite with dml values of 14.7 8. Because gll(2.335) and gl(2.065) is observed for the perpendicular and parallel orientations, respectively, it may be concluded that the symmetry axis of the tetragonal Cu(Hz0)e2+ion is perpendicular to the silicate sheets. The ion is not in a solution-like environment insofar as dynamic Jahn-Teller distortions7 or rapid tumblings do not average the anisotropy or broaden the spectra. The imposition of the silicate sheets on the hydrated ion differentiates the elongated ligand axis (g 11 > g 1) from the r and y axes, and the three equivalent Jahn-Teller states that may be
Flgure 1. Schematic representation of the oxygen network in vermiculite and smectite (adapted from ref 5). Open circles are oxygen, dark circles are OH (and F, as in hectorite). The hydrated Interlayer exchange cations are represented, but the nonexchangeable cations within the octahedral and tetrahedral sites of the silicate sheets have been omitted for clarity.
Flgure 2. First derivative ESR spectra at 25’ of oriented film tamples of the dool = 15.0-A hydration state of Mg*+-hectorite doped with Cu2+: (a) silicate sheets parallel to the applied field, H; (b) silicate sheets perpendicular to H. Position of a standard pitch signal is shown with g = 2.0028.
achieved in bulk solution or in frozen glasses are no longer equivalent in interlayer space. Segregation of the copper ions as planar Cu(H20)d2+into interlayers with a basal spacing of 12.4 A can be excluded as a possible explanation of the observed spectra. The spectra clearly exhibit copper hyperfine splitting of both g 11 ( A h = -0.0156 cm-l) and g l ( B / c = -0.0022 cm-l). Splitting of gl is not observed when Cu(H20)d2+ions exclusively occupy an interlayer because of Cu2+-Cu2+ dipolar b r ~ a d e n i n g .Thus ~ , ~ the Cu(&0)s2+ ions must be homogeneously dispersed along with Mg(H20)62+ions in the interlayers. The restricted perpendicular orientation of the C U ( H ~ O ) ions ~ ~ +most likely involves partial penetration and hydrogen bonding of axial water ligands into the hexagonal cavity of oxygen atoms in the silicate surface as illustrated in Figure 3b. Presumably, the Mg(H2O)e2+ ions responsible for forming the 5.4-A thick interlayers also adopt the same orientation. This orientation, however, is markedly different from that adopted by divalent hexaaquo ions, including Mg2+ l o and C U ~ +in, the ~ 14.3-A hydration phase of vermiculite. In this latter mineral, the hexaaThe Journal of Physical Chemistry, Vol. 79, No. 22, 1975
M. B. McBride, T. J. Pinnavaia, and M. M. Mortland
2432
a
a . VERMICULITE
b . HECTORITE
C
38
,
Figure 3. Orientation of Cu(H20)e2+ in (a)the 14.3-A hydration state of vermiculite and (b) the 15.0-A state of hectorite. Open circles rep-
resent surface oxygen atoms of the silicate structure and the ligand water molecules of Cu(H20)e2+.Thickness of the interlayers was determined by subtracting the height of the silicate sheet (9.6 A) from the observed door spacings. quo ions are part of a double water layer structure with ligand axis inclined a t or near 45O to the silicate sheets (Figure 3a). The difference in cation orientation is undoubtedly related to the difference in silicate charge position (see above) and hydrogen bonding ability of silicate oxygen atoms in the two types of minerals. It is known that hydrogen bonding of water to the surface oxygens is stronger in vermiculite" than in smectite because of the greater negative charge arising from tetrahedral rather than octahedral substitution in the silicate structure. The inclined orientation is well suited for vermiculite as it allows all six water ligands to hydrogen bond to surface oxygens while a t the same time allowing the cation to lie near the tetrahedral charge site. Although the perpendicular orientation in smectite allows less overall ligand water-silicate interaction, it places the axial water ligands closer to octahedral charge sites in the silicate framework. Thus the difference in cation orientation seems to fit well with the hydration model of Farmer and Russell12 wherein it is recognized that the water molecules serve not only to solvate the interlayer cations, but they also function as dielectric links between the cations and the origin of negative charge in the silicate structure. The anisotropic ESR signal of Cu2+ doped into Mg2+hectorite is lost upon solvation of the mineral in liquid water or ethanol. Rapid tumbling or Jahn-Teller distortion of the Cu(H20)$+ complex in the expanded interlayers (do01 u 20-22 A) averages the anisotropy, and rapid relaxation13J4 due to modulation of the anisotropic g tensor and spin rotation interactions results in a broad, almost undetectable signal analogous to that observed for the ion in water and methanol solution. Hectorite fully exchanged with Cu2+ and fully swelled by water to a do01 value of 22 8, exhibits a single signal with a width near 120 G. The line width is appreciably smaller than that observed for Cu2+-doped Mg2+-hectorite or for The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
Figure 4. ESR spectra (25') for oriented film samples of Cu2+doped Na+-smectite with H perpendicular and arallel to the silicate sheets: (a) airdried hectorite, do01 = 13.6 (b) after exposure of
1;
air-dried hectorite to pyridine; (c) airdried montmorillonite, dool= 12.4 A.
C u ( H 2 0 ) ~ ~in' dilute solution. Apparently, the close approach of Cu2+ ions in the fully exchanged mineral (average Cu2+-Cu2+distance -14 A) gives rise to spin exchange narrowing. Although a single line is observed under these conditions, magnetic anisotropy in the interlayer environment can still be deduced from oriented film spectra. The values of ( g ) and the peak-to-peak line widths, respectively, are 2.174 f 0.002 and 115 f 2 G for I3 parallel to the silicate sheets and 2.183 f 0.002 and 127 f 2 G for the perpendicular orientation. Similar anisotropy has been observed and discussed in greater detail for Mn(H20)e2+in fully expanded interlayem2 M+-Srnectites. Under ambient conditions H+-, Li+-, and Na+-hectorite do not form a well-ordered hydration state. Instead, they exhibit broad 001 X-ray reflections, centered in the do01 range 13.6-14.5 A, indicative of randomly interstratified interlayers containing one to three layers of water. The minerals remain interstratified upon doping with Cu2+ or Mn2+ions. Figure 4a illustrates the broad, poorly resolved ESR signals obtained at 25' for an oriented film of Cu2+-doped Na+-hectorite (do01 N 13.6 A). Analogous spectra are obtained also for doped samples of the air-dried H+ and Li+ exchange forms and for Na+-montmorillonite (Figure 412). Mn2+-doped forms also exhibit broad, indistinct ESR signals. A typical example of a spectrum for an air-dried powder sample of Mn2+-doped Na+-smectite is provided in Figure 5a.
ESR Studies of
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Phyllosilicate Surfaces
H, 7
100 GAUSS
Figure 6. ESR spectra (25’) for an oriented film sample of Cu2+doped Mg2+-hectorite dehydrated at 210’ with the silicate sheets (a)perpendicular to H and (b) parallel to H.
A n
i
V’
1
Figure 5. ESR spectra (25’) for powder samples of Mn2+dopedNa+hectorite (a) airdried, door = 13.6 A; (b) heated at 215’ for 16 hr; and (c)fully hydrated, dool> 20 A (interstratified).
The poorly resolved Cu2+ and Mn2+ signals indicate that, unlike the well-ordered initial hydration states of M2+-smectites, the divalent probe ions in the interstratified M+-smectites occupy sites of ill-defined symmetry. It is unlikely that the lack of well-resolved g components for the Cu2+ resonance is caused by tumbling of the ion because the spectra are independent of the average interlayer thickness (compare Figure 4a for Na+-hectorite at dml = 13.6A with Figure 4c for Na+-montmollonite a t dml = 12.4 A) and the spectral resolution is not appreciably improved upon cooling the samples to -145’. Moreover, the Mn2+ signals in the air-dried state are much weaker and more diffuse than those observed for rapidly tumbling Mn2+ in fully hydrated interlayers (see Figure 5c). The lack of well-defined symmetry for the probe ions is attributed to their preferred association with two adjacent negative charge sites within the silicate structure. These doubly charged sites, which in hectorite are expected simply on the basis of random substitution of Li(1) for Mg(I1) in octahedral positions, would be better neutralized by a single Cu2+or Mn2+ ion than by two closely associated monovalent host ions. Glaeser and Mering15 have provided independent evidence for the preferred association of Ca2+ to doubly charged sites when the ion is exchanged into Na+hectorite below the 30% level. They suggested an association mechanism in which the divalent ions are dehydrated and reside in hexagonal cavities (cf. Figure 1)near the doubly charged sites. We propose that the divalent probe ions associated with doubly charged sites in air-dried Na+-hectorite are partial-
ly hydrated and a t the same time coordinated to silicate oxygen atoms. This ill-defined hydration structure within the interstratified interlayers would give rise to a distribution of Cu2+and Mn2+ environments in accord with the observed spectra. Evidence against the divalent cations being completely dehydrated and residing within hexagonal cavities is provided by the spectra for Cu2+- doped Mg2+-hectorite dehydrated a t 210’. Under these conditions the Cu2+ ions are known9 to be exclusively within the hexagonal cavities in a rhombic environment, and well resolved g components are readily observed in the ESR spectrum (see Figure 6). These spectra are markedly different from those obtained for Cu2+ in air-dried Na+-hectorite (cf. Figure 4a). It may also be concluded that the divalent ions are not irreversibly bonded to the doubly charged sites as the Mn2+ ions are lifted from the silicate surface and tumble rapidly in the interlayer space upon swelling the minerals to do01 values 220 A (cf. Figure 5c). Also, the introduction of pyridine into the interlayer of Cu2+- doped Na+-hectorite results in the formation of a tetragonal Cu2+-pyridine complex which is restricted to an orientation with the symmetry axis perpendicular to the silicate sheets and with gll = 2.23, g l = 2.05, and A / c = 0.0167 cm-’ (see Figure 4b). The lower gll and larger A / c values relative to Cu(H20)e2+ reflect the greater covalent character of the complex.16 It is noteworthy that a rather intense “free electron” signal appears near g = 2.00 in the spectrum of Cu2+-doped Mg2+-hectorite which has been heated to 210’ (cf. Figure 6). The signal is also present in the spectrum of heated Mn2+-doped Na+-hectorite (see Figure 5b). The signal may be due to a defect formed in the silicate structure upon heating, but further studies are needed to better define its origin. Thermal Dehydration. When Mn2+-hectorite is heated to -210’, almost all of the interlayer water is lost and the Mn2+ ions enter hexagonal cavities in the silicate structure.2 In these positions, the Mn2+ ions may form monohydrates12 as a single H2O remains associated with each Mn2+ ion. Because of dipolar Mn2+-Mn2+ interactions which lead to line broadening, fine structure has not been detected in the ESR spectra of heated Mn2+-saturated minerals previously investigated.2 In an attempt to eliminate dipolar broadening, we have doped small amounts of Mn2+ into Na+- and Mg2+-hectoThe JOlJrnd of Physical Chemistry, Vol. 79, No. 22, 1975
M. B. McBride, T. J. Pinnavaia, and M. M. Mortland
2434
rite prior to heating the minerals to -215' for 16 hr. Apparently, the Na+-hectorite entraps some water under these conditions because an Mn2+ spectrum is observed which is similar to that obtained for the air-dried mineral (cf. Figure 5a and 5b). It is likely that the Na+ ions lose their water of hydration more readily than the Mn2+ ions and, consequently, the interlayers collapse around the Mn2+ ions before their dehydration is complete. However, in doped Mg2+-hectorite, where the two divalent ions have similar hydration energies, the interlayers dehydrate uniformly and the ions move into the hexagonal cavities. This is indicated by the ESR spectra shown in Figure 7 in which a set of six intense lines due to allowed transitions with Am1 = 0 and a set of weak "forbidden" lines (Am1 = 1) is observed. The presence of both allowed and forbidden transitions is not uncommon for Mn2+ in solid matrices. Rehydration of the mineral restores the solution-like Mn2+ ESR spectrum, and essentially all of the Mn2+ can be exchanged by washing with aqueous MgC12 solution. The six allowed transitions of Mn2+ in the dehydrated mineral are split by ca. 14 and 10 G, respectively, when the silicate sheets are perpendicular and parallel to H (see Figure 8). Although through space dipolar coupling of Mn2+ with structural lH or '9F might be expected to split spectral lines into doublets, more detailed analysis of the spectra eliminates this mechanism as a simple explanation. The dipolar interaction energyI7 between an electron dipole, pe, and a nuclear dipole, p", a t a distance r is given by
The splitting should decrease to zero for 8 = 5 5 O , but the observed splitting a t this angle is ca. 11.8 G. In addition, the splitting is the same a t X-band and Q-band frequencies. Therefore, the splitting must be due to a mixture of isotropic and anisotropic coupling. It is tentatively suggested that the coupling reflects some degree of covalence between Mn2+ and F in F-containing cavities (cf. Figure l), though the contribution due to Mn-HO interactions in OH-containing cavities cannot be precluded in absence of ENDOR studies. Nonetheless, the hyperfine coupling unequivocally establishes the position of Mn2+ in hexagonal cavities.
Conclusions The ESR spectra of Cu2+ and Mn2+ ions doped into the interlayers of smectite minerals containing divalent and monovalent exchangeable cations are useful in deducing the structural nature of the ions in the restricted water layers formed under ambient conditions (-40% relative humidity). Hydrated C U ( H ~ O )in ~ ~the + do01 = 15.0-A phase of divalent cation exchange forms of hectorite and the 14.7-A phase of the divalent exchange form of montmorillonite are restricted to an orientation in which the elongated ligand axis is perpendicular to the silicate sheets. The divalent host ions responsible for forming these well-order hydration states presumably adopt the same perpendicular orientation. This is in marked contrast to the inclined orientation of the ion in the 14.3-A hydration state of vermiculite where the ion is part of a two-layer water structure and the ligand axes are -45' to the silicate sheets. The inclined orientation is best suited for vermiculite because it optimizes electrostatic interaction with the tetrahedral site of silicate charge and allows hydrogen bonding of all six ligand waters The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
-
H ,
a
/I 1
100GAUSS
Figure 7. ESR spectra (25') for an oriented film of Mn2+doped Mg2+-hectorite dehydrated at 215' for 16 hr: (a) silicate parallel to H, (b) silicate sheets perpendicular to H.
100 GAUSS
+$ H,
-y2 line (second lowest field line) of Mn2+ in dehydrated Mg2+-hectorite with the silicate sheets (a)parallel to H and (b)perpendicular to H. Figure 8. Splitting of the allowed M, =
to silicate oxygen atoms, while in smectite the perpendicular orientation permits the axial water ligands to serve as dielectric links between the cation and the site of octahedral charge deficiency in the silicate structure. Cu2+ and Mn2+ in interstratified monovalent cation exchange forms of smectites containing one to three molecular water layers occupy a distribution of anisotropic environments. Under these conditions the divalent ions are preferentially associated with doubly charged sites in the silicate framework. An association mechanism involving simultaneous coordination of the divalent ion to silicate oxygens and water molecules in disordered interlayers is proposed. Total dehydration of the ions in hexagonal cavities is precluded by the spectral data. Dehydration of Mn2+-doped Mg2+-hectorite a t 215' causes the ion to move into hexagonal cavities, and a distinct solid matrix Mn2+ spectrum is obtained. A combination of isotropic and anisotropic coupling of Mn2+ to structural F and/or OH causes splitting of the allowed Am1 = 0 transitions. Acknowledgment. This research was supported by the National Science Foundation, Grant No. MPS74-18201. We thank Dr. Hideo Kon for obtaining the Q-band spec-
2435
Lewis Orbital Models of B2Hs, CH3BH2, and CH3CH2’
trum of thermally dehydrated Mn2+-dopedMg2+-hectorite and for many helpful discussions.
References and Notes (1) (a) Department of Crop and Soil Science. (b) Department of Chemistry. (2) M. B. McBride, T. J. Pinnavaia, and M. M. Mortland, Am. Mineral., 60, 66 (1975). (3) D. M. Clementz, M. M. Mortland, and T. J. Pinnavaia. J. Phys. Chem., 77, 196 (1973). (4) T. J. Pinnavaia and P. K. Welty, Abstracts, 169th National Meeting of the American Chemical Society, Philadelphia, Pa., April, 1975. (5) R. E. Grim, “Clay Mineraloqy”, _. 2nd ed, McGraw-Hill, San Francisco, Calif., 1968, p 86.. (6) G. J. Ross and M. M. Mortland. Soil Sci. Soc. Am., Proc., 30, 337
(1966). (7) A. Hudson, Mol. Phys., 10, 575 (1966). (8) B. R. McGarvey in “Transition Metal Chemistry”, Vol. 3, R . L. Carlin, Ed., Marcel Dekker, New York, N.Y., 1966. (9) M. B. McBride and M. M. Mortland, Soil Sci. SOC.Am., Proc., 38, 408 (1974). (10) G. F. Walker, Clays Clay Miner., 4, 10 1 (1956). (11) J. Hougardy, J. M. Serratosa, W. Stone, and H. van Olphen, faraday Spec. Dscuss. Chem. Soc., No. 1, 187 (1971). (12) V. C. Farmer and J. D. Russell, Trans. faraday Soc., 67, 2737 (1971). (13) P. W. Atkins and D. Kivelson, J. Chem. Phys., 44, 169 (1966). (14) R. Poupkoand 2 . Luz, J. Chem. Phys., 57, 3311 (1972). (15) R. Glaeser and M. J. Mering, C.R. Acad. Sci., 246, 1569 (1958). (16) D. Kivelson and R. Neiman, J. Chem. Phys., 35, 149 (1961). (17) J. E. Wertz and J. R. Bolton, “Electron Spin Resonance”, McGraw-Hill, New York, N.Y., 1972, p 40.
Lewis Orbital Models of B2H6,CH3BH2,and CH3CH2+ Carl Trindle” and Lucy Cline Weiss Department of Chemistry, University of Virginia, Charloftesville, Virginia 2290 1 (Received August 8, 1974; Revised Manuscript Received Ju/y 15, 1975)
The Lewis orbital model introduced by Frost predicts accurate geometries for hydrocarbons, though it is less accurate for systems with lone pairs. In this report we show that the Lewis orbital model also produces useful estimates of geometries and describes certain aspects of the rearrangement dynamics in the systems B2H6, CH3BH2, and CH3CH2+ where proton bridging and three-center bonding play an important role. The electronic repulsion is seen to parallel the total energy in a rough way, which behavior provides theoretical support for the valence shell electron-pair repulsion (VSEPR) Gillespie-Nyholm model.
Introduction The simplest ab initio computation suitable for moderate sized molecules is the floating spherical gaussian (FSG) model or the Lewis orbital method introduced by Fr0st.l In this model each pair of electrons is constrained to occupy a single spherical gaussian space orbital; since a 2N electron system requires only N spherical gaussians in the basis, the position and scale of each gaussian can be fully optimized in a brief time. The virial and Hellmann-Feynman theorems hold in this model, and predictions of geometry have proved to be remarkably accurate. Inadequacies in the simplest form of the model, that is, the tendency of orbitals to coalesce when a molecule contains double bonds or two lone pairs on a single atom, have been remedied by such small extensions as defining lobe functions2 or improving the description of cores.3 The Lewis orbital model is a quantum mechanical transcription of the classical bonding ideas of Lewis and Langmuir4 and bears some resemblance to the valence shell electron pair repulsion (VSEPR) model of Gillespie and Nyholm.5 It is of course a local orbital wave function, but the connection with symmetry adapted MO’s is easily established;6 according to a recent discussion,2 SCF orbitals localized by the criterion of Edmiston and Ruedenberg7 are very similar in form to the floating spherical gaussian orbitals, suitably orthogonalized. The advantages of Lewis local orbitals, namely, intuitive significance and approximate transferability, have been exploited in Lewis-type computations on very large systems reported by Christofferson.8 Recently Lipscomb and others have described local orbitals for boron hydrides: obtained from SCF MO’s by
transformation. These systems, in which proton bridging is a striking feature of the bonding, can in most cases by represented well by two-center and three-center bonds. In this report we show that Frost’s Lewis orbital model can accommodate three-center bonding and provides a fair estimate of geometries in bridged and potentially bridging systems. To illustrate these points we consider the molecules C2H6, CH3BH2, and CH3CH2+.
Bridging in A2H6 Systems The fact that C2H6 assumes a D3d staggered geometry, while B2H6 takes up the D2h proton-bridged structure can be traced to the presence of 14 valence electrons in the hydrocarbon, while diborane has 12 valence electrons. Gimarclo explained the influence of the extra pair of electrons in ethane by qualitative molecular orbital arguments. A detailed computation of the formation of D2h B2H6 from a pair of BH3 molecules was reported by Lipscomb,” and a description of the same process in the language of intramolecular CI was provided by Fukui.I2 Unambiguously localized orbitals for diborane are available, and the three-center nature of the bonding in the bridge is well established. The fact that B2H6 is well represented by two- and three-center localized orbitals explains the successful geometry predictions by Frost; ethane has been thoroughly studied as well, both by the localization of canonical MO’sL3and in Frost’s m0de1.l~We have computed Lewis orbital wave functions for C& and B2H6 for a number of structures ranging from D3,j to D2h (Figure 1, Table I). As other computations have shown, ethane is computed to resist distortion toward a bridged structure, while diborane The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
