On the ESR spectra and bonding of lithium complexes of acetylene, ethylene and benzene: a matrix isolation study

ChemicalPhysics North-Holland

169 (1993)

219-229

On the ESR spectra and bonding of lithium complexes of acetylene, ethylene and benzene: a matrix isolation study L. Manceron, A. Schrimpf, T. Bornemann, R. Rosendahl, F. Faller and H.-J. StGckmann Fachbereich Physik der Philrpps-Universitiit, Renthof 5, W-3550 Marburg, Germany and Laboratoire Spectrochimre Mol&ulawe (CNRS lJA508), VniversitP P. et M. Cune, 4 place Jussieu, 75252 Parrs Cedex 05, France Received

28 July 1992

Electron spin resonance (ESR) and optical absorption spectra of 6LiC2Hz, ‘LKzH2, ‘LKzH4, ‘LiC2D4, 6LiC2D4, LiC6H6 and LiC6Ds complexes embedded in solid argon have been measured at low temperature. Lithium-acetylene and lithium-ethylene complexes exhibit hyperfine structures due to both lithium and equivalent hydrogen nuclear spins. g tensors, alkali-metal atom and proton hyperfine coupling tensors were determined with help of computer simulation of the powder spectra. It is shown that both these complexes have a ‘B2 ground state thus correlating with the Li ‘P atomic state. The polyligand Li( C2H,), and Li(CZH4)J complexes have also been observed and their ESR spectra are consistent with Dzb and DXh symmetry structures. In contrast, LiC6H6 presents no lithium hypertine structure but a hyperfine splitting of unequivalent hydrogen spins. This confirms that LiC6Hs has lost the six-fold symmetry of the benzene ring resulting from the complete or near complete transfer of the lithium valence electron into one of the formerly degenerate unoccupied molecular orbitals of benzene, and has Czv symmetry.

1. Introduction Among the different alkali metals, lithium has the specific property to form spontaneously symmetrical mononuclear complexes with acetylene [ 11, ethylene [ 21 and benzene [ 31 where the Li atom bridges the II system of one (acetylene) or also several (ethylene and benzene) molecules in a manner comparable to transition metals and group III metals [ 4-6 1. This knowledge is derived from vibrational studies. These studies could deliver information about which bonds are the most affected by metal fixation or about the overall geometry. They, however, could not distinguish between several possibilities concerning the nature of the bonding between lithium and unsaturated molecules and, therefore, the ground states of the complexes. Simple unsaturated hydrocarbons unlike molecules with high electron affinities such as 02, 03, NO*, SO2 etc. do not easily form negative ions unless they are in a very polar, perturbing medium. NeverCorrespondence to: L. Manceron, Laboratoire Spectrochimie Moltculaire (CNRS UA508), UniversitC P. et M. Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France. 0301-0104/93/$06.00

0 1993 Elsevier Science Publishers

theless these molecules are established as lI* acceptors as well as possible II donors. Very schematically, the interaction between an alkali atom and a sideways bound molecule with a filled II orbital can lead to different bonding possibilities.

In the first possibility, the interaction between the Li 2p atomic and the II* molecular orbital (MO) is not very stabilizing. It results that the 2a, antibonding combination of the Li 2s and highest occupied molecular orbital (HOMO) will be filled before the lb* bonding combination of the Li 2p and lowest un-

B.V. All rights reserved.

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L. Manceron et al. /Chemical Physics 169 (1993) 219-229

occupied molecular orbital (LUMO). The complex would then have a 2A, ground state. Two electrons are stabilizing and one destabilizing resulting in a net bonding interaction with a charge transfer in direction of the metal. This situation would be comparable to that encountered in Li-OH2 and Li-NH3 complexes [ 781. In the second, reverse possibility, the singly occupied molecular orbital (SOMO) is the bonding combination of II* and Li 2p and would yield a ‘B2 ground state with a charge transfer in direction of the hydrocarbon molecule. After the experimental characterization of the Lihydrocarbon complexes there appeared three independent ab initio studies devoted to the simplest of these systems, the LiC2H2 complex [ 9- 111, whose results are somewhat contradictory. One of them [ 9 ] confirms that formation of a Li-C2H2 II complex is energetically competitive with respect to the separate fragments and calculates its vibrational spectrum which matches well that of the observed species. Unfortunately this study did not define precisely the ground state of Li-C2H2, or the amount of charge transfer between metal and acetylene. A second study [ lo] reported a weakly bonded II complex with a 2A1 ground state, while a third study [ 111 stated that the most stable form corresponds to a 2B2 ground state. As all these complexes are paramagnetic, we have undertaken an ESR study of the lithium-acetylene, ethylene and -benzene complexes, isolated in rare gas matrices.

2. Experimental The cryogenic refrigeration system and ESR vacuum vessel permitting simultaneous ESR and optical absorption studies have been described previously [ 12,13 1. A few minor changes were carried out to enable handling of lithium vapors in the 380-450°C range in a stainless steel crucible, and monitoring the Li atom deposition using a quartz microbalance. Natural ( Alfa Inorganics, 99.9%) and isotopically enriched lithium (6Li 95%, CEA, France and ‘Li 99.9%, ORNL, USA) were used without further purification. Ethylene and acetylene were provided by L’Air Liquide (99.5W) and purified and outgassed by freeze and thaw cycles before use. A sample of benzene (Janssen, reagent grade) was further dehy-

drated over molten potassium before use; this procedure successfully eliminated water contamination. Perdeuterated ethylene and benzene were supplied by MSD (99% D) and CEA (99.5% D), respectively. Simulations of powder spectra were achieved using programs kindly provided by Belford [ 141. X-band ESR spectra were recorded with a ER 200D Bruker spectrometer with 12.5 kHz, 1.4 gauss peak-to-peak modulation and variable low power ( lo-100 uW) microwave radiation to avoid saturation. UV-visible absorption spectra were recorded with = 1 nm resolution on the same samples as those used for ESR studies.

3. Results 3.1. LiC2H, Deposits of lithium atoms and acetylene molecules in argon gave rise to only one ESR active species. The observed spectrum consists of a complex, anisotropic structure, framed by the well-known [ 15 ] unreacted atomic lithium ESR transitions (fig. 1). The structure corresponds to a triplet (with a spacing of about 6.5 mT, corresponding to 180 MHz) with each component further split in a more complex pattern from which emerge roughly four maxima spaced by 0.70.9 mT (20-25 MHz), using ‘Li (I= 3/2, flu= 3.256), and only three maxima spaced by about 0.35 mT ( 10 MHz) using 6Li (I= 1, ~=0.822). This shows that the smaller structure corresponds to a hypertine interaction with the lithium nucleus, and the larger pattern with a stronger central component is due to the superhyperfine interaction with two, magnetically equivalent H nuclei (I= l/2 ). It is thus obvious that the carrier of the spectrum is the LiC2H2 II complex, already identified using IR spectroscopy [ 11. It is to be noted that the spectrum changes very slightly with deposition conditions, but after annealing to x 25 K, results are then consistent from experiment to experiment. These changes can be attributed to slightly different packing sites in the defectrich polycrystalline argon matrices, as previously documented on the simpler, isotropic atomic spectra

t161.

L. Manceron et al. /Chemical Physrcs 169 (1993) 219-229

Fig. 1. Comparison of experimental and calculated X-band ESR spectra of LiCzHz in solid argon (Li/C2H2/Ar%0.4/ l/ 120). Top spectrum: ‘Li+C2H2; bottom spectrum: 6LifC2HZ, after light annealing to 25-28 K. The simulated spectra were calculated with the parameters given in table 1. The free electron resonance position is indicated by an arrow and the unreacted lithium atomic lines (two of the four ‘Li lines in the upper spectrum and the three 6Li lines in the lower one) are in dotted lines in the figure. The microwave frequency is 9283 MHz.

3.2. Lithium-ethylene complexes Lithium and ethylene deposits in argon produced a more complicated system. At least four ESR active species could be detected by varying the C2H4/Ar molar ratio from 0.2 to lo%, while keeping the lithium content relatively low (~0.25%) to prevent clustering. Three complexes corresponding to Li ( C2H4)” n = 1,2, 3 had been previously characterized in comparable conditions [ 21. The different species are presented on fig. 2 for either ‘Li or 6Li complexed with C2D,. Non-deuter-

221

ated ethylene produced an additional superhyperfine structure which could be only partially resolved for the simplest complex, LiC2H4. For the larger complexes, the signals were broadened to the point of masking any detail. Care had to be taken to choose extreme ethylene dilutions, or else the measured spectrum resulted in a mixture of the spectra presented in fig. 2. The carrier of spectrum 2a is dominant in samples diluted in ethylene (C,H,/ Ar
222

L. Manceron et al. /Chemical Physics169 (1993) 219-229

600

800

1000

WAVELENGTH / IUn

Fig. 2. Comparison of X-band ESR spectra (left) and electronic absorption spectrum (right) for the Li ( C2D,) _, n = 1,2,3 complexes. (Full line) ‘Li; (dotted line) 6Li, recorded on the same samples (‘Li isotope). (a) Li/C2D4/Ar= 0.4/0.25/ 100; (b) Li&D,/Ar zO.2/ 5/100; (c) Li/C2D4/Ar~0.2/10/100 after annealing to z 32 K. The arrow indicates the free electron resonance position (g=2.0023), the asterisks the unreacted 6Li atom rn,= + 1 and 0 transitions. The microwave frequency is 9279 MHz.

possible to detect some differences in the relative intensity of the two low-field features, with the g= 2.080 feature favored by ethylene concentration increase. In a neat ethylene sample a simple, broad, axiallysymmetric spectrum results with g values corresponding to g,, ~2.079, g, x2.0065 and with about 1.8 mT-broad linewidths. It therefore appears that spectrum c in fig. 2 is itself composed by the sum of two spectra of species with axially-symmetrical g tensors. One (c’) presents Li hyperfine splitting in the g, region ( ~0.54 mT spacing), and the other (c”) has positively shifted g values but is too broadened to present Li hyperfme structure. Only one species containing more ethylene was previously identified: lithium-tris( ethylene). Given the high ethylene content of the spectra in which spectrum 2c was observed, it seems likely that c’ and c” are in fact the same Li ( C2D4) 3 species, but isolated in argon or in large ethylene clusters. The optical spectrum 2c consists of an intense, broad absorption centered around

750 nm, matching for Li(C,H,),. 3.3. Lithium-benzene

the previously

reported value [ 2 ]

complexes

In metal-diluted samples, two species were identified corresponding to LiCbH6 and Li( CsH,), [ 3 1. In conditions favoring the Li(C6H6)2 complex, i.e. in concentrated or well-annealed samples a broad feature (linewidth x 1.6 mT) centered around the free electron resonance position was observed. It presented some traces of hyperfine structure, but not resolvable, and this species will not be discussed here. The spectrum of the lithium-monobenzene species obtained in benzene-diluted matrices is presented in fig. 3 for ‘LiC6H6 and ‘LiC6Ds. The spectrum for LiC6H6 exhibits a strongly asymmetrical, partially resolved, triplet hyperline structure (spaced by roughly 1 mT) in which each component is further split into at least live submaxima (spaced by about 0.2 mT).

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L. Manceron et al. /Chemical Physics 169 (1993) 219-229

4. Discussion

-

,

2mT B

* i I

7LiC6D6

Fig. 3. X-band ESR spectra for ‘LiCsH6 and ‘LiC6D6 complexes (Li/C6H6/Ar% 1 /l/250). The arrow indicates the free electron resonance position (g=2.0023). The microwave frequency is 9284 MHz.

As was pointed out previously benzene molecules are difficult to isolate, due to their large size [ 3 1, and it is impossible to exclude a small contribution to the spectra from Li ( C6Hs) *. Spectra recorded with higher dilution ( C6H6/Ar = 1 / 1000) showed a poorer signal-to-noise ratio than that presented in fig. 3, but were not essentially different. The narrowing of the signal when going from LiC6H6 to LiC6D6 is most conspicuous, and proves that the observed hyperfine structure is in fact entirely due to coupling of the unpaired electron magnetic moment with the H nuclei magnetic moments. Indeed, experiments run with 6Li did not show any change with respect to 7Li, thus indicating that any lithium hyperline structure would be within the signal bandwidth and therefore could not be greater than about 0.04 mT ( x 1 MHz).

As the observed spectra correspond to randomly oriented radicals, the different magnetic parameters had to be estimated using computer simulation via trial-and-error procedures. We used the powder spectrum simulation program of Nilges and Belford [ 141 which diagonalizes for a number of orientations the spin Hamiltonian of a radical with spin l/2 coupled to magnetic nuclei with hyperfine tensors A, which may be rotated with respect to the reference frame in which the g tensor is diagonal. All the species discussed here have at most a cylindrical symmetry and so the Zeeman and hypertine interactions have to be described by tensor coupling constants. This implies a large number of unknown parameters considering that the geometries, as well as linewidths and lineshapes are not precisely known. We have therefore guided our spectral simulations with three hypotheses which we detail below in order of importance. (a) The lithium hyperfine interaction tensor ALi was supposed to be of axial symmetry. This assumption is based on the following considerations. As explained in the introduction the bonding schemes invoked imply in terms of LCAO an interaction of Li 2s and 2p orbitals with the bonding or antibonding combinations of the C 2p orbitals. If we assume that the SOMO of the complex can be approximated in the close vicinity of the Li nucleus by a combination of Li 2s and 2p, and if, as it is the case here, the g tensor deviates little from the free electron resonance value, then the hyperfine interaction tensor will be the sum of an isotropic (due to Li 2s contribution) and axial (due to Li 2p contribution) tensor and will have axial symmetry overall with A, =Aim-Adi, and A ,,=Aim + 2A,+,, where Ai,,=g~P~g*P~(g~/3)1~(0)1*,

(1)

and &p=g&gnPn(

(3cos2@-

l)/2r3>

.

(2)

Here I@( 0) ( * is the Fermi contact term and represents the spin density at the nucleus, r and 8 the unpaired electron polar coordinates with respect to the magnetic nucleus. For ground state Li *S atom, the Ai, parameter is to equal to x 401 MHz for 7Li (corresponding

224

L. Manceron et al. /Chemical

1@(O) ]‘=0.2309 for a unit spin). For Li 2P unfortunately no experimental value exists and we can only have an order of magnitude for Adip for a unit spin using the early theoretical estimate of ( ( 3 cos2B- 1) / 2r3) ~0.0387 by Barnes and Smith [ 171 which yields an A+ value (probably largely underestimated) of 3.5 MHz. Hence, the lithium hyperfke structure should be very much dominated by the isotropic term. (b) The lineshapes are assumed to be Lorentzian. (c) The Li quadrupole interaction is neglected since both lithium isotopes have very small quadrupole moments [ 18 ] .

Physics 169 (1993) 219-229

ge 3mT

77

1 1

4.1. LiC,H, The LiC2Hz species is the only one of these complexes for which the geometry could be relatively precisely estimated [ 1,9].

The 40” rotation of the z and y axes of the hydrogen hyperfine interaction tensor was therefore included in the model. Trial-and-error procedures show that the anisotropy of the hydrogen and lithium hyperfke structures can be modeled separately, as they enter in different ways into the spectra, they can therefore be estimated separately (rig. 4 ). g and ALitensor anisotropies affect the shapes and intensities of the smaller hype&me structure and can be first fitted on the central component of the larger triplet structure as it is unaffected by the An tensor values; secondly, AH tensor anisotropy affects the relative intensity and contour of the larger triplet structure. Comparison of traces 4a and 4b shows that AL, and g tensor anisotropies must be included to approach the asymmetric pattern of the triplet central component. Isotropic interaction with the two H nuclei would yield a symmetrical triplet with a 1: 2: 1 relative intensity pattern. Differences in the relative intensities appear when an additional slight anisotropy is introduced in the AH tensor (see trace 4c and 4d). The fit was also

Fig. 4. Hypothetical 7LiC2H2ESR spectra using various parameters (to be compared with the experimental spectrum shown in fig. 1). (a) Isotropic g (2.0025); Ati axial 26 and 18 MHz; isotropic AH= 175 MHz. (b) Axial g and A,., tensors (g,=gs= 2.0039, g,=2.006 and A,,=26 MHz, Ati,=A,,=17.5 MHz), isotropic An = 175 MHz. Only g and A, tensors affect the central component, both must be anisotropic. The outer components are not correctly reproduced by an isotropic&. (c) Same as (b) but with anisotropic AH with AHI =AHt= 182 MHz, AH~= 165 MHz. The relative intensities of the outer components are not correctly reproduced. (d) Same as (b) and (c), but with A~,=l67, AH*= 185, AN3= 180 MHz. The AH tensor component along axis 1 (larger g and A,_,) must be the smaller.

based on 6LiCzH2 data (identical g and An tensors, but different nuclear spin and ALi values). The best set of magnetic parameters is presented in table 1 and the corresponding spectral simulations are compared in fig. 1 with the experimental spectra. We could not find a set of parameters which provided an exact fit, but the main trends were nevertheless reproduced (multiplicity, spacings, intensity patterns) in spite of the coarseness of the model and the possible complications arising from site or orientation effects [ 17 1.

L. Manceron et al. /Chemical Physics 169 (1993) 219-229

Table 1 Proposed ESR parameters for the LiC2H2complex isolated in solid argon. The angle between the AU and A, tensor principal axis is taken 40”. Values for 6LiC2H2can be deduced from the 6Li/7Li nuclear gyromagnetic ratio Axis

g tensor l&I (MHz) l&l (MHz)

1=x

2

3

2.0039 26 167

2.0039 17 185

2.0006 18 180

Our purpose is to roughly assess the most important parameters for structure and bonding. From the trialand-error simulations, we can state that the absolute values of the AL, and An tensor elements are certainly within 20% of the values given here. The IA,, I and IAl ( values assessed here for the lithium coupling tensor enable an estimate of the A,, and&p for lithium using the relationships: A3o=~f(~_L

&p=f(A,,

+A[,)

-A,,

3

>

(3) (4)

with additional assumptions for the relative signs of A,, and A,. (i) If A,, and Al have the same sign, then Ai, and Adipwill be equal to 20.5 ? 1.5 and 2.8 + 1.3 MHz, respectively, as at least one of them must be positive. (ii) If A,, and Al are of opposite signs, then Ai, and Adip take the values + 4 and T 14.5 MHz, which would imply a very large value for Adip compared to the 3.5 MHz theoretical estimate for a unit spin density in a Li 2p orbital. In any case, the reduction in Ai, is drastic with respect to the free atom value (40 1 MHz) and corresponds to a Li 2s spin population of at most 5%. This is certainly not compatible with a ‘Ai ground state (scheme 1). Partial Li 2s character in the SOMO of the complex would yield, as in the case of Li-OH2 or Li-NH3 complexes [ 7 ] a reduction in Ai, of the order of 40-50°h instead of 95-98Oh as here. On the other hand, a positive value of 2.8 f 1 MHz for AdIp would be compatible with substantial Li 2p spin population, if compared with the theoretical value for a unit spin population, as expected for a ‘B2 ground state. Further quantitative interpretation of the ALI

225

tensor could be misleading given the uncertainty of the theoretical estimate and the absence of correction for the large positive charge transfer effect [ 19 1. Indeed the IA, I tensor element values are large ( z 177 MHz) and close to each other, indicating the existence of a large isotropic component in the hypertine interaction with the H nuclei which could not be accounted for by polarization effects. From the H atom hyperfine splitting value of 1420 MHz [ 20 ] one can derive a Hi, spin population of the order of 12.5%. This denotes a rehybridization of the carbon atoms and participation of Hi, wavefunction in the SOMO of LiC2H2. The SOMO must also lie in the molecular plane. In his recent ab initio study [ 111, Nguyen has reported the atomic orbital expansion coefficients for the SOMO of the LiC2H2 species in the ‘B2 state (0.3 1 for Li 2p; 0.315,0.285,0.323 for C 2p, 3s, 3p respectively ). These values correspond to an unpaired spin population of 0.66 on Li and C2 which is well matched by the 2 x 0.125 = 0.25 residual spin density on the hydrogens. All this produces consistent evidence in favor of a ‘B2 ground state and therefore stabilization of the metal-II system bonding through the Li 2pC2H2 II* interaction. The comparison with a bona fide ion pair is interesting. In Li+CO, the isotropic part of the lithium hyperfine interaction is of comparable magnitude (A,,= 32 MHz), while the anisotropic part is more than one order of magnitude smaller (A+=0.17 MHZ) [21]. 4.2. Li-ethylene

4.2.1. Lithium-monoethylene For the lithium-monoethylene species for which data are available for several isotopic species, we searched a set of parameters providing the best fit for ‘LiC2D4, ‘LiC2H4 and 6LiC2D, (table 2 ) . Fig. 5 compares the spectra calculated using the parameters of table 2 and the experimental spectra. Here again the fit gives a general picture but is not entirely satisfactory. Both g and AL tensor anisotropies must be included and are of magnitude comparable with those in LiC2H2. The main difference with the former species is the much smaller superhyperfine structure due to the H atoms. With C2D4 the signals are conspicuously sharpened and any superhyperfine structure is within the linewidth. The determination of g and ALi

226 Table 2 Proposed

L. Manceron et al. /Chemical Physics 169 (1993) 219-229

ESR parameters

for Li(C2H4)”

n= 1,2,3

complexes

in argon and ethylene matrices Axis

‘LiCzH4

g tensor I&l l&l

(MHz) (MHz)

g tensor

‘Li(CZD4)z

l4_il l&l

‘Li ( CID,) 3 in argon

(MHz) (MHz)

‘Li ( CzD,) s in neat ethylene

(MHz)

g tensor I&I

1 *

2=x

3

2.0039 15.5 23.5

2.0035 21.5 Z5.5

2.0022 15.5 x3.5

2.0033 r 12.5 >1

2.0025 =21 21

2.0017 z 12.5 >1

g tensor l&l

ge

1

Li C2D4

(MHz)

9e 1

Y

2.033 >I

2.0010

2.0010

=I5 2.080 within linewidth

Zl5 2.0065

2.0065

‘LiC2H4, a small additional superhyperfine splitting of about 3-5 MHz complicates the spectrum. The A, and Adi, for ‘Li are thus estimated from (3) and (4) to be 20 and 4f 1 MHz, respectively, if they are of same signs, and - 1.6 and 14.5 MHz, if they are of opposite signs. Here again the complex certainly has a *B2 ground state. By symmetry, in LiC2H4 the SOMO has no Hi, character as it lies in the xz plane perpendicular to the xy plane containing the four H nuclei.

Y

expertmenta(

calculated

Fig. 5. Comparison of experimental (left) and simulated (right) ESR powder spectra for various isotopic species of LiCzH+ (Full line) ‘Li isotope, (dotted line) 6Li. The arrow indicates free electron resonance position (g= 2.0023), and the asterisk unreacted atomic 6Li m,=O transition. The simulated spectra are calculated using the parameters in table 2 and corresponding values for 6Li and D atoms and Lorentzian bandshapes with 3 MHz fwhm.

parameters is hence more convenient and is facilitated by the fact that both 6Li and ‘Li data are available. The 6LiCzD4 species gives a limit of the g tensor anisotropy and the ‘LiCzD4 species enables proper assessment of the IALl] tensor elements. With

The small hydrogen superhyperfine structure is thus likely due to polarization effects of the C-H o electrons. It is here somewhat less than in A1C2H4 which has a comparable structure (7.5 MHz [ 5 ] ), but is of the same order of magnitude as in the allene anion

[221. The observed electronic absorption at about 1.3 eV can therefore be attributed to the *A, c2B2 electronic transition. It is 0.5 eV less than the Li 2pt2s transition which has an energy of about 1.8 eV. The LiCzH4 binding energy in the *A, excited state should be very small [ 10,111, but the Li-C2H4 interaction energy in the *B2 state was calculated to be of this order of magnitude (0.43 eV).

L. Manceronet al. /Chemical Physics 169 (1993) 219-229 4.2.2. Lithium-bis(ethylene) The lithium-bis( ethylene) produces an ESR spectrum with a very symmetrical quartet structure using ‘Li, but collapsing to a pattern which is almost axially symmetric using 6Li isotope (fig. 2b). The spectrum seems at first highly isotropic but in fact, some features (difference in linewidths within the quartet, relative intensities in the ‘Li hyperfine splitting) cannot be accounted for by species with spherical g and AL1 tensors. In the spectrum of the 6Li-substituted species where the lithium hyperfine splitting is drastically reduced, a small g factor anisotropy can be detected (fig. 2b) in spite of partial overlapping with the 6Li ml= 0 atomic line. An acceptable simulation can be obtained with a uniaxial ALi tensor whose principal axis is the one having the intermediate value in the g tensor, of orthorhombic symmetry. The values of IA,, I and 1A I 1 for AL are determined with a somewhat higher uncertainty, but do not seem to differ largely from those in LiC2H4 (see table 2) _ From vibrational studies it was deduced that Li( C2H4)* should have one of the two structures sketched on scheme IV. Z

/= CHz= CH> /

T CH2

‘32 II

LI’

CH2

_x II

/I’

LI II

CH2

_X

CH2

CH2

D2d

D2h

(1)

(2)

Structure ( 1) of Did symmetry would imply the existence of S4 symmetry axes and therefore axially symmetric g and A tensors, but structure (2) of DZh symmetry is compatible with our simulation results. 4.2.3. Lithium-tris(ethylene) The spectrum of Li(C2D4)3 (see fig. 2c) corresponds clearly to that of a species with g and ALi tensors of cylindrical symmetry, as expected for a species of Dg,, symmetry. The g,, value is substantially shifted with respect to free electron resonance posi-

221

tion (see table 2) and the correspondingd,, hyperfine splitting is not detectable. On the other hand g, is little shifted, but shows a marked ‘Li splitting (A, z 15 MHz). From relations (3 ) and (4) one can estimate A,, and Aalp, which must have opposite signs to satisfy the experimental values namely !Z 10 and f 5 MHz, respectively. A small negative A,, due to polarization of occupied ethylene II orbitals and a larger, positive Adlp would be consistent with our model. 4.3. Lithium-benzene In contrast to the lithium-acetylene and -ethylene molecules, the lithium-benzene species shows no lithium hypefine structure at all in the ESR spectrum. This indicates that the lithium valence electron charge transfer to the C6H6 molecule must be complete or nearly complete and that the bonding scheme is highly ionic, as the SOMO of LiC6H6 must have negligible participation of Li 2s or 2p orbital. On the other hand, a prominent anisotropic hyperfine structure due to an interaction with several hydrogen nuclei can be observed. This means that the LiC6H6 molecule is static here on the ESR time scale, in contrast to the Li+C6Hc ion pair solvated in polar media [23,24] or the AlC6H6 species in hydrocarbon matrices [ 2 5 1. The species is however distinct from the free C6H; anion with an equal interaction of 0.375 mT (10.5 MHz) with six equivalent protons [23,24]. It is also certainly not a lithium-cyclohexadienyl analogue of some sort since a proton hypefine interaction about live times larger would result [ 261. In a previous IR study [ 31 the LiC6Hs was found to correspond to a species in which the Li atom occupies an axial position and the six fold symmetry of the benzene ring is lost. Data were inconclusive concerning the overall symmetry: CZv or C3”. Two features stand out of the LiC6H6 and LiC6Db ESR spectra: (i) LiC6H6 presents an asymmetrical but clear triplet of quintets revealing a primary interaction, quite anisotropic in nature, with two H nuclei of about 30 MHz and a second, much smaller interaction of about 5 MHz probably involving four H nuclei. (ii) This structure disappears completely using C6D, and the spectrum consists of a quite symmetri-

L. Manceron et al. /Chemical Physics 169 (1993) 219-229

228

ium hypertine interaction (Adip for free Al is x 178 MHz [ 29 1, much larger than for Li). According to a recent ab initio study [ 30 1, Al&H6 was calculated to have also CzVoverall symmetry, with a very low spin density on Al (~~0.04). lb,)

ets

lb,)

(a,)

e,”

la,)

Fig. 6. II-type molecular orbitals of benzene. The symmetries refer to Dbb C.& and those indicated in parentheses to CZvdis torted benzene in LiC6Hs.

cal single line of z 15 MHz linewidth. This means that the g tensor anisotropy must be quite small (g,=2.0030f0.003). The first observation is only compatible with a CIV symmetry for LiC6H6 (one group of two and one group of four equivalent H atoms). Following the familiar Hiickel description of the II MO of benzene sketched on fig. 6 one could further specify that the unpaired electron has been transferred into the formerly degenerate ezU symmetry LUMO of benzene, now of symmetry a,. Transfer into the other LUMO, now of symmetry a2, would have produced an interaction with four equivalent H atoms, yielding a spectrum with an evenly spaced quintet structure, as for the p-xylene anion [ 271. With a ‘A, ground state, a SOMO of Ai symmetry indeed does not correlate with either Li pX or pYorbitals and should have zero overlap with the Li 2s orbital, thus explaining the absence of lithium hypertine interaction in the ESR spectrum. Occupancy of the partially antibonding al orbital will cause a distortion of the benzene ring [ 28 ] (the two parallel C-C bonds are shortened, the other four lengthened) which will destabilize the b2 symmetry level and bring it closer in energy to Li 2p,. In addition to the electrostatic attraction, it may therefore be possible that some non-negligible stabilization can be brought out by back-donation of electron density from these filled levels into the empty Li 2p orbitals. Presumably, a small effect of this sort would further stabilize the lithium closer to the benzene and help the quenching of the ring distortion which is here observed. This situation should be very comparable to that of the Al&H6 complex, recently isolated in hydrocarbon matrices [ 25 1. A primary hyperfine interaction with two protons was also detectable, but further proton hyperfine splitting was masked by larger signal linewidths and a small residual alumin-

5. Conclusions The ESR spectra of lithium complexes of acetylene, ethylene and benzene have been measured at low temperature in solid argon. Analyses of the 6Li/7Li hypertine structures show in all cases a drastic reduction of the isotropic term of the hyperfine interaction of the unpaired electron with the lithium nucleus, hence almost negligible Li 2s participation in the SOMO of all species. However, for lithium-acetylene and -ethylene complexes, a substantial dipolar term can be deduced in the lithium hyperfme interaction tensor. It is interpreted as stemming from an appreciable contribution of Li 2p atomic orbital in the SOMO of LiC2H2 and LiC2H4, both having a 2B2 ground state. For LiC6H6 in contrast, absence of observable lithium hyperfine structure indicates total transfer of unpaired electron to benzene. The observed pattern of the hyperline structure due to interaction with the six hydrogen nuclei evidences an overall maximum CzV symmetry for LiC6Hs. The unpaired electron occupies an a, (formerly ezu) molecular orbital of benzene which does not correlate with the Li 2p atomic orbitals parallel to the molecular plane, which explains the absence of litbium hyperfine structure and evidences a ‘Ai ground state.

Acknowledgement One of us (LM) would like to thank the W.H. Hereaus Stiftung for a fellowship. This work was also funded by the Deutsche Forschungsgemeinschaft and the Centre National de la Recherche Scientifique.

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