Infrared spectra and structures of lithium-ethylene complexes [Li(C2H4

J . Phys. Chem. 1986, 90, 4514-4528

4514

Infrared Spectra and Structures of Li(C2H4), ( n = 1, 2, 3) and Li2C2H4in Solid Argon Laurent Manceron*+ and Lester Andrews* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: January 23, 1986)

Condensation of ethylene molecules and alkali metal atoms at high dilution in argon produced very different results, depending on the nature of the alkali metal. While heavy alkali metal atoms (Na and Cs) yielded only a very weak complex with ethylene virtually unperturbed, lithium gave evidence for Li2C2H4and Li(C2H4), (n = 1,2, 3) displaying large perturbations in the ethylene ligand. Isotopic substitutions (6Li, 'Li, C2H4, I3C2H4,C2D4, and CHzCD2)showed that the metal center forms a A complex with equivalent CH2 groups in each structure and that the ligands are equivalent in lithium di- and triethylene. Simplified normal-coordinate analyses show that the C=C bond force constant is lowered by approximately 30%, 21%, and 16% in Li(C2H4),with n = 1, 2, and 3, respectively, whereas the CH2 bending force constant is very little altered relative to ethylene. An analysis of the infrared intensities shows that the strong IRactivity of the three Li(C,H,), species is likely to be due to a large variation in charge flow between metal and C=C groups upon variation of the C=C bond distance and, to a lesser extent, upon CH, bond angle variation. The spectrum of the dilithium species indicates nonequivalent metal atoms and is best explained by fixation of the second metal on the first metal center, which is thought to account for the strengthening of the C-Li bonds and the further puckering of the C2H4 group.

Introduction

Structure and stabilities of the primary intermediate species involving interactions between olefins and alkali metal atoms are of importance in order to understand the difference in stabilities of the organoalkali reagents and also failure to obtain a,@or a , y metalations.1,2 Another interest is, of course, the relevance of such interactions to the mechanisms of heterogeneous catalysis. For example, trace lithium promotes catalysis in ethylene oxide preparation3 and substantially increases the heat of desorption of benzene from a transition metal ~ u r f a c e . ~ In the course of our investigation of the alkali metal atom interaction with unsaturated hydrocarbons, the different products of lithium and heavy alkali metal atom complexation with acetylene were c h a r a c t e r i ~ e d ; ~the . ~ next logical step was comparison with ethylene. An early theoretical estimate predicted that interaction of lithium atoms with ethylene was unlikely to give anything else than an ethyl radical like adduct.' Moreover, the potential energy surface of 1,2-dilithioethane has been the subject of a thorough investigation2s8 but has not, so far, been confronted by experimental data. We present here an infrared study of the products of lithium and heavy alkali metal atom matrix reactions with ethylene molecules trapped in solid argon and solid ethylene. Experimental Section

The cryogenic refrigeration system, vacuum vessel, alkali metal source, and experimental techniques have been described earlier.9 Isotopically enriched samples of lithium metal (99.99% 'Li and 95.6% 6Li, O.R.N.L.) and natural sodium metal (Baker and Adamson, 99.9%) were vaporized directly from the liquid phase, whereas cesium vapor was generated by reaction of molten lithium and cesium chloridelo (99.9% Fisher). High-purity ethylene (Matheson), argon (Air Products, 99.995%), 13C2H, (90% 13C), CH2CD2(98% D), and C2D4 (99% D) (Merck Sharp and Dohme) were used without further purification. Gaseous mixtures were deposited at 2-3 mmol/h simultaneously with the alkali metal beam on a CsI (mid-IR) or sapphire (visible-near-IR) window maintained at constant temperatures ranging between 10 and 18 K. The lithium concentration was modified by varying the source temperature, inducing a 1/20 ratio change in metal vapor pressure. The highest Li/Ar concentration used here can be estimated at 1/300. In a typical argon experiment, about 40 mmol of gaseous sample was injected, and the total amount of matrix trapped on the window is estimated to be 0.75 f 0.05mmol by interference fringe measurement. Comparison with blank experiments run Present address: Laboratoire de Recherches de Spectrochimie Moleculaire, CNRS UA508, Universitd Pierre et Marie Curie, Bat. F, 4 Place Jussieu, 75230 Paris Cedex 05. France.

without metal shows that 90% or more of ethylene is left unreacted; Le., the amount of product is then about 180 nmol. Mid-IR (4000-200 cm-I) spectra were recorded with a Perkin-Elmer 983 and data station. The spectra presented here were processed against a background spectrum collected with the base CsI window and then base line corrected to compensate for matrix light scattering. Frequency accuracy is f 0 . 5 cm-I with spectral slit widths of 1-4 cm-I in the product band regions. Visible and near-IR spectra (30 000-4000 cm-I) were recorded with a Cary 17, with slit widths of 5 cm-I (-0.05 nm) in the visible and 4-10 cm-I in the near-IR. Results

When alkali metal atoms were codeposited with ethylene molecules, distinctly different products were obtained with lithium and heavier alkali metals. The Na and Cs reactions gave new bands in the infrared spectrum (Figure 1) with respect to blank samples, Le., the appearance of weak absorptions at 1615, 1338, and 1223 cm-I with C,H4 and 1510, 1005,981,787, and 684 cm-' with C2D4 In each case the first three values are positioned within 5 cm-I of the Raman values of the C=C stretching (Ag), CH2 symmetric (Ag), scissoring, and CHz symmetric rocking (B,J modes of normal and deuterated free ethylene." With CzD4 the 78 1-cm-l absorption corresponds exactly to the symmetric CHz wagging (BZg)mode, but the close proximity of this mode (938 cm-') to the strong v, (antisymmetric wagging mode, 948 cm-') accounts for the failure to observe this band with CzH4. All these absorptions display the same behavior when the experimental conditions are varied and are therefore attributed to the same species, in which the ethylene submolecule shows a very small perturbation. (1) Shimp, L. A.; Lagow, R. J. J. Org. Chem. 1979, 44, 231 1. (2) Schleyer, P. V. R.;Kos, A. J.; Kaufmann, E. J . Am. Chem. Sac. 1983, 105,7617. ( 3 ) Joyne, R. L. Spec. Period. Rep.: Catalysis (London) 1977, 5, 19 and references therein. (4) Garfunkel, E. L.;Farias, M. H.; Somorjai, J. J. Am. Chem. Soc. 1985, 107,349. (5) Manceron, L.; Andrews, L. J . Am. Chem. SOC.1985, 107, 563. (6) Manceron, L.; Andrews, L. J. Phys. Chem. 1985, 89, 4094. (7) Trenary, M.;Schaefer 111, H. F.; Kollman, P. A. J. Chem. Phys. 1978, 68, 4047. (8) Kos, A. J.; Jemmis, E. D.; Schleyer, P.V. R.; Gleiter, R.; Fischbag, H.; Pople, J. A. J. Am. Chem. SOC.1981, 103,4996. (9) Andrews, L.J. Chem. Phys. 1968, 48, 972. (IO) Andrews, L.;Hwang, J. T.; Trindle, C. J . Phys. Chem. 1973, 77, 1065. ( 1 1) Duncan, J. L.; McKean, D. C.; Mallinson, P. D. J. Mol. Spectrosc. 1973, 45, 221.

0022-3654/86/2090-45 14%01.50/0 , 0 1986 American Chemical Society I

,

The Journal of Physical Chemistry, Vol. 90, No. 19, 1986 4515

Li(C2H4), and Li2C2H4in Solid Argon

TABLE I: Observed Frequencies (cm-') for Products of Lithium-7 and Isotopic Ethylene Reaction' species C2H4 C2H4 + C2D4 C2D4 "C2H4 111 additional signals with 'Li 3049 vw 3055 vw 2223 vw 1428 (1428) w 1176.5 (1179) s

112

1312 (1314) s 931 (933.5) m (543.4) vw

700 (710) vw

369 (387) s

360 (374) s

366 (384) s

1189 (1191) s 722 (725) vw

1217 940

389 (413) s 355 vw

w 129901 w ii543j vs [1513], 1511 (1513) s vs 114971 w [mij vs [1243], 1247 (1249) s vs [1225] vw [1131] vw [805] m [781.5], 788 (92) w s [416.5], -408 (430) m

1330 (1327) s

1448 (-1447)

936 (937.5) m

1163 (1165.5) s

554 (557) vw

718 (721)

386 (404) s 352 vw

386 (409) s 347 vw

2234 m 1533 1528 1421 1406 1281 1260

a

1179 (1183) w 990( (991.5) m

a ? a sym wag CH2 (CD,)

3044 vw 2998 vw 2218 vw 2200 vw

2225 w

1484 -1375

2209 vw 1437 (1438) m 1252 (1254) s

574 vw 364.5 (381.5) s

3070

-

m

1453 (1453) m 1265 (1263) w 1202 (1204.5) s 992 (992) m 590 vw 388 (409) s

X

v(CH/CD)

a a a wag u,(LiC) u(C-H)

1388 (1391) s

1491 s

1492 m

a

953 (953) m

1228 s

1302 s 1003 m

a a

784 (790) w

792 vw 640.5 vw

CH2/CD2 wag

602 (?) vw 399 (424) m

-406 (428) m

404 (426) m

u,(LiC2) Ua(LiC2)

2288 w 2154 w

3022 w W

2973 w 2270 w 2173 vw

uCH VCD

1216 (1208) w 972.5 w 656 (661) s 577 (584) m 548 (548) vw 482 (485) m 362 (373) w 303 (307) m

UCC

-962

3036 w 2964 vw

695.5 (698) s 583 (583) m 551 (585.7) w 364 (383) w

wag A, + wag B2 "CC + W H 2 ) UCH2) wag A, v,(LiC) wag B2 uI(LiC) + wag AI v,(LiC)

915 595 477 487 351

360.5 (363) m

tilting

279 (282) m

1240 vw 1162 (1159.5) w

v,(LiC)

2209 w

w [290], 277 (280) vw 211

assignt "(C-HIC-D)

1421 (1421) 1150 (1152.5) s

704 (714) vw

1463 (1464.5) m

1/3

CHzCD2

+

-

1230 vw 1129 w (916) w (620) s (477.5) m (496) (362) w

-3942 1281 1278

3947 1174.5 1167

884 (889.5) 862 (883) 604.5 (607.5) (575) 504 (510.5) 478 356 (373)

359.5 (377) 355 -3948 1143

w(CH2) - u,(LiC2) u,(LiC,) + w,(CH,) rock wag CD, u,(LiC2) tilting

-3945 1224 broad

1098 (1100) 1083 (1088) 927.5 875 (886.5) 867 (876.5) 505 (506) 476 (483.5)

1098 (1103) 1084 (1091)

691 (693) 578 (578) 549 (583)

1110 1096 1083.5 (1088)

1096.5 (1102.5) 1082 (1090)

880 (886.5) 86 1 602 (601) 556 502 (509)

875 (886) 867 (879) 526 480 470

+

'a designates the two components of the [b,(CH,) ucC] mixture (see text). Relative intensities are indicated by s = strong, m = medium, w = weak, vw = very weak. Lithium-6 values are given in parentheses, and lithium-6 in solid C2H4values are given in brackets.

On the other hand, when Li was substituted for N a and Cs, a surprising wealth of reaction products is characterized by absorptions in different regions of the IR spectrum (see Table I). Figure 2 illustrates the variation in relative intensities of the different absorptions when the Li/C2H4 molar ratio and the deposition temperature are changed. All the observed absorptions

can be attributed to eight different species, four of which (labeled 2/1, 1/ 1, 1/2, and 1/3) contain only lithium and ethylene, respectively, and will be discussed in the present paper along with another (labeled X) for which no definite conclusion can be drawn; the remaining three (designated with asterisks) contain lithium C2H4, and nitrogen and will be discussed in a following publi-

4516

Manceron and Andrews

The Journal of Physical Chemistry, Vol. 90, No. 19, 1986

1 i

F

c

2000

3000

4000

1600

1200

BOO

490

CM-'

Figure 1. IR spectra for normal and deuterated ethylene in an argon matrix (1/400) deposited with heavy alkali metal atoms. (a) Na atoms with C2H4,(b) C2D4without any metal, or (c) Cs atoms CzD4. The IR-active fundamentals and combinations of ethylene are indicated (in cm-I), along with the position of the lines observed with metal reaction (framed). W designates water impurity absorption.

+

cation.12 Figures 3-5 present the isotopic shifts observed in the C=C stretching (v& CH2 bending (6(CHz)) and wagging (y(CHz)), and Li-C stretching mode (v(LiC)) regions for various isotopic precursors (6Li, 7Li, C2H4, CzD4, 13CzH4,and CH,CD,) and some of their mixtures (CzH4 + CzD4, 6Li + 7Li). The various product species absorptions are differentiated by variation in the Li/C2H4molar ratio as well as by photolysis, which destroys the photosensitive species or enhances the high Li stoichiometry products by light-induced diffusion of metal atoms in the matrix,13and by annealing, which favors the most stable species and the ultimate level of aggregation. Furthermore, in the case of the 211, 111, 112, and 113 species, stoichiometries have been unambiguously established by observation of clear-cut patterns with the use of isotopic mixtures. In this section, we describe some of the characteristics of the different species and the observations which led to their identification. The frequencies of their IR absorptions and various isotopic shifts reported in Table I are grouped by product; their vibrational assignment as well as structural information is discussed in the following section. 211 Species. The absorptions attributable to the 211 species are observed distinctly only for ethylene concentrations lower than 11400. They increase markedly in relative intensity to become the most predominant in the spectrum when the lithium concentration is raised to the maximum value still compatible with a good transparency of the sample, and the ethylene concentration is kept as low as 1/1500. Visible light irradiation with a medium-pressure mercury arc (Pyrex filter) also produced an increase in the product yield of this species. No new feature appeared in experiments'run with CzH4 CzD4 mixtures in addition to the one observed with CzH4 or C2D4 separately. Therefore, for all the above reasons, this molecule certainly contains only one ethylene group but is very likely to contain more than one lithium atom, which suggests the Li2CzH4identification. It is characterized (see Table I) by absorptions in the C-H stretching and CH, bending regions and by four strong absorptions below 700 cm-I (five with CH2CD2)all displaying H I D and 6Li/7Li shifts of various magnitudes. However, experiments run with both 6Li and 7Li effusing from the oven (Figure 5 ) give a spectrum identical with the sum of the spectra obtained with 6Li or 7Li taken sep-

+

Manceron, L.; Hawkins, M.; Andrews, L., unpublished results. (13) Welker, T.; Martin, T. P. J. Chem. Phys. 1979, 70, 5683. (12)

arately, which means that if this species contains more than one lithium atom, the lithiums are inequivalent and that the isotopic splittings induced in the mixed 6Li-7Li species are small vs. the band widths of the signals (1.5 cm-') which are therefore coalesced with the pure 6Liz- or 7Li2-CzH4 ones. Annealing the sample above 30 K destroys this species while 1/1, 1/2, and 1/3 absorptions grow rapidly. Z I Z Species. This species is predominant when both lithium and ethylene are kept at low concentrations (Li/C2H4/Ar 1/ 11800); it decreases and progressively disappears from the spectrum upon annealing, especially when the ethylenelargon molar ratio is greater than or equal to 1/400 (Figure 6 ) . This product does not display additional features with isotopic mixtures of the precursors (C2H4 C2D4, 6Li 7Li); this fact, along with the predominance of the species at low Li and C2H4 concentration, identifies this molecule as LiCzH4, lithium monoethylene. 1 /2 Species. The absorptions corresponding to the 112 complex start to appear for an ethylene concentration of 1/800 (when the deposition temperature is about 15 K) and dominate the product spectrum when the ethylene concentration is around 11300 but decrease again for values of 1/ 100 or more. Likewise, annealing the matrix above 30 K only produces a distinct increase for samples very diluted in ethylene (see Figure 6 ) ;otherwise, thermal diffusion is difficult to control and leads almost directly to the following product, which has a higher stoichiometry and enjoys a greater stability. However, with isotopic mixtures C2H4 C2D4, there appears one new absorption, not present in the pure CzH4 or CzD4 case, in each region (v%, 6(CHz), or 6(CD2)) shifted upward with respect to the spectra of the pure C2H4 or C2D4 112 species. This indicates clearly that the pure isotopic complex contains two equivalent ethylene molecules, whose v, and 6(CHz) modes are coupled and combined in two sets of vibrations (in-phase and out-of-phase), the higher of which is not IR active or too weak to be seen with the present experimental conditions. The fact that this species can be formed from LiC2H4by diffusion of a second ethylene molecule or is seen in samples with C2H4/Li molar ratio much greater than one identifies it as Li(C2H4)2,lithium diethylene. Another property of this molecule is that it can be photolyzed quite efficiently by using a medium-pressure mercury arc (Figure 4) or the radiation of a Nernst glower. We have observed in the near-IR spectrum a broad absorption ( A Y , , ~N 50 nm) centered around 810 nm (12300 cm-I) which displays the

+

+

+


Li(C2H4)"and Li2C2H4in Solid Argon

c

4000

3000

2000

1600

1LOO

wo

400

CM"

Figure 2. IR spectra recorded after codeposition of lithium and ethylene-argon mixtures: (a) C2H4/Ar = 1/100, deposition temperature (Td) N 16 K,lithium temperature (TLi)N 430 OC (Li/C2H431 X);(b) C2H4/Ar= 1/250, Td N 16 K, T L N~ 430 OC (Li/C2H4e 2.5X); (c) C2H4/Ar= 1/800, rdN 440 OC (Li/C2H4 N 12X); (d) C2H4/Ar = 1/1500, Td N 10 K,TLiN 455 "C (Li/C2H4N 54X). E designates parent 62H4 absorptions, W

designates water impurity absorptions, and asterisks designate nitrogen-containingcomplexes. Each of these samples corresponds to approximately 0.75 mmol of an ethylene-argon mixture. The metal concentration is approximately 1/800 to 1/1000. same photolytic behavior and concentration dependence as the I R absorptions corresponding to this species and therefore might correspond to an electronic transition of this complex. 1/3 Species. This species is observed upon deposition only with sample concentrated in ethylene (more than 1/300), grows markedly upon annealing at the expense of the other species, and is the only product observed in an experiment run in pure ethylene with lithium effusing a t its lowest rate used here (370 OC cell). Moreover, with C2H4 + C2D4 isotopic mixtures, the 1/3 species gives rise to two new sets of absorptions, not seen with C2H4 or C2D4 as precursors, the relative intensity of which varies with the CzH4/CzD4ratio (Figure 4). A first set (absorptions a t 1533, 1406, 1281, and about 962 cm-I) is predominant for CzH4/C2D4 molar ratios greater than one; meanwhile, the second set (absorptions at 1528, 1421, and 1260 cm-') appears more clearly for CZH4/C2D4smaller than one. Note that the 1281-cm-' band is overlapped by another sharp absorption belonging to another product (X,discussed hereafter) but may be observed in different

experimental conditions (after photolysis, with very high C2H4/Li ratio). The appearance of two new sets of absorptions with CzH4 + CzD, mixtures identifies this species as Li(C2H4)3,lithium triethylene. As in the case of the Li(C2H4), species, the different vibrators of each equivalent ligand are coupled with in-phase and out-of-phase motions, the higher of which is IR inactive or too weak to be observed in the pure isotopic species because the new absorptions of the mixed isotopic species are blue-shifted from the pure isotopic values. The broad (Avll2= 110 nm), very intense absorption a t 755 nm (13250 cm-') observed in the visiblenear-infrared spectrum can be confidefltly assigned to an electronic transition of this species, presumably a perturbed Li 2p 2s type transition, since its concentration dependence and thermal stability match those of the corresponding mid-IR absorptions. This strong band is the only visible feature observed with Li atoms in pure ethylene. The 1/3 species enjoys a particular thermal stability relative to the lower stoichiometry ones since it grows on warming until the matrix breaks down around 50 K in argon experiments,

-

4518



c c

c

t

112

t

!-

c c

112

tc t c

1650

1*03

I zoo

1300

eo0

600

400

CM-'

ZOO

Figure 3. IR spectra recorded after codeposition of lithium-7 with various isotopic ethylenes diluted in argon: (a) I2C2H4/Ar = 1/250; (b) I3C2H,/Ar = 1/400; (c) C2D4/Ar = 1/400; (d) CH2CD2/Ar = 1/400.

and in pure ethylene the decrease in these product bands accompanies the loss of solid ethylene at around 95 K. X Species. This species is characterized by a broad absorption at 3945 cm-I (AvlI2 = 25 cm-]) which presents very little isotopic shift (5-7 cm-' within the reproducibility of its position), as well as other IR absorptions quoted in Table I, some displaying anomalously low isotope effect. (The 1098cm-' band has a 5-cm-' 6Li/7Li shift, no H / D shift, and a 2-cm-' l2C/I3Cshift.) Although its concentration dependence in both lithium and ethylene is not completely clarifid, it seems to be present in samples concentrated both in ethylene and lithium and its yield is enhanced when the deposition of the sample is made with the support maintained at 15-18 K. Furthermore, it is never seen when Li(C2H4), is not detected, and it grows at the expense of Li2C2H4,as Li(C2H4), does at the expense of LiC2H4,when the sample is formed at 15-18 K as opposed to 10-12 K. For these reasons, Li2(C2H4), could be advanced. On the other hand, it has not been possible to produce an increase on annealing (although Li(C2H4)" ( n = 1, 2, 3) grows) nor has an extra line been observed in the 13001100-cm-l region with C2H4/C2D4mixtures, corresponding to the 1281- (C2D4) or 1173.5-cm-l (C2H4) sharp line. However, due to the complexity of the observed pattern in that region and

possible overlapping for such mixtures, a definite conclusion cannot be drawn; it is noteworthy that the CH2CD2counterpart of this absorption is much broader, which could be indicative of a complex isotopic pattern. To rule out the possibility of contamination, experiments were conducted with triple-doping Li ethylene N2, O,,CO,, or H 2 0 without producing an increase in the X species, which was also produced consistently with ethylene samples coming from very different sources but not in blanks with metal or ethylene alone in argon.

+

+

Discussion From the first glance the products can be divided into two groups: the first group (LiC2H4,Li(C2H4)2,Li(C2H4),) is mainly characterized by two lines in the vicinity of the methylene group scissoring and C==C stretching mode, both having substantial 13C shifts and forming a kind of monotonic progression toward the positions of these two modes in free ethylene, and a second group (Li2C2H4and X) which does not fit exactly in that trend but displays hydrogen bending modes shifted way below any fundamental of ethylene and therefore shows signs of much larger perturbation. The discussion will thus be divided into two parts, each pertaining to one of these two product groups.


Li(C2H4), and Li2C2H4in Solid Argon i

'I

I-

112

% I,,

.n 1

i

0.66 0.33

0.5 0.5 Figure 5. IR spectra of the 700-200-cm-' region of lithium concentrated samples containing different isotopic ethylene precursors highly diluted in argon: (a) 6Li + C2H4/Ar(1/1500); (b) 6Li + CH2CD2/Ar(1/800); (c) 6Li + 'Li + CH2CD2/Ar (1/600); (d) 'Li + CH2CD2/Ar;(e) 'Li + C2D4/Ar(1/500). In all cases the Li/Ar molar ratio is believed to be E 1/300. All sharp product bands (except those marked 1/1) are due to the Liz-C2H4complex.

0.33 0.66

'

0

1 00 '

I

I

1500

,

I

I

s

I

1

1300

1

,,bo

fCM.'

+ various C2H4 + C2D4 mixtures diluted in argon: solid line, after deposition; dotted line, Figure 4. IR spectra of the 158C-1050-cm-' region of Li

after photolysis with medium-pressure Hg lamp. (a) C2H4/C2D4/Ar= 1.5/0/4OO. (b) C2H4/C2D4/Ar= 0.66/0/33/400. (c) C2H4/C2D4/Ar = 0.5/0.5/400. (d) C2H4/C2D4/Ar= 0.33/0.66/400. (e) C2H4/ C2D4/Ar = 0/1/400. E designates parent molecule absorptions, and asterisks designate nitrogen-containing complexes (Td N 16 K). The lithium concentration is believed to be -1/800.

Part I . LiC2H4,Li(C2H4)2,and Li(C2H4)3.The fact that the Li(C2H4), species have IR absorptions showing large I2C/I3C and H / D shifts in the 1520-1180-cm-' region shows that metal complexation activates both vcc and 6,(CH2) modes and, as for transition metal c o m p l e ~ e s ,that ' ~ the two methylene groups remain equivalent (since their scissoring motions remain coupled as in-phase and out-of-phase motions). This is confirmed by the fact that each of these complexes has only one signal in the CD2 (or the CHI region) when CH2CD2is used. Another confirmation can be found considering that the low-frequency metal-carbon stretch (in the 380-cm-' region) shows some mixing with a C-H bending mode belonging to the same symmetry (probably the wagging motion) by its H / D shift (9 cm-I in 'LiC2H4 for instance), but with CH2CD2one single absorption was observed halfway in between the C2H, and C2D4lines (instead of a doublet if the Li was not symmetrical with respect to the two carbons). The same observation can be made for the different species of this group, and it is therefore clear that all three Li(C2H4), species are symmetrical ?r complexes in which the lithium atom bridges the C-C bond(s). However, some observations which are not trivial to interpret need to be emphasized: (14) Herbcrhold, M. Metal r-Complexes: Elsevier: Amsterdam, 1974; Vol. 11, Parts 1, 2.

(1) Although the 1/1, 1/2, and 1/3 bands at 1428, 1463, and 1511 cm-' seem to be comparable and to form a progression similar to the 1176.5, 1 1 8 9 5 , and 1247-cm-' bands, the 1428-cm-I 1/1 band shifts by only 7 cm-' on 13C/'2Csubstitution compared to 15 and 20 cm-I for the 1/2 and 1/3 upper lines. Moreover, it does not show any 6Li/7Li shifts as do all the others. (2) The position of the lower component of the vcc, 6,(CD2) mixture in Li(C2D4), is quite close to the position it has in free ethylene (985 cm-I), which is not the case for the protonated products. (3) Relative intensities and bandwidths vary widely when comparing the two components of the ucc and 6,(CH2) mixed modes when going from C2H4 to C2D4 or when comparing the different lithium mono-, di-, and triethylenes (see Figure 6). All this indicates that assignments are not straightforward, and in order to give a more quantitative view of the relative perturbation of each vibrator involved here ( C = C bond, CH, scissoring) and also to try to bring out a rationale for all the above-mentioned points, we have performed for each complex calculations based on simplified harmonic models involving mechanical couplings between the C=C stretching and CH2 bending oscillators using the Schachtschneider GMAT and FADJ programs. LiC2H4Species. It is assumed that couplings with the other high- and low-frequency modes of the complex (C-H stretch, CH2 wag, and v,(Li-C) stretch) can be neglected; the relative position of the different 6(CH2) scissors and vcc vibrators can be described in a simple 3 X 3 harmonic approximation involving the C-C bond length variation and two H C H angle variations which combine in a C, structure into symmetric in-phase (6,) and antisymmetric out-of-phase (6,) for the symmetrical X2Y4 species (X = lZCor I3C, Y = H or D) but not in the H2C=CD2 case. The starting geometry is derived from similar estimates of M(C2H4) complexes15 and from preliminary results of ab initio calculations,'6 but other calculations not detailed here have shown that small changes in the geometry affect the relative variation of the isotopic (15) Ozin, G. A.; Powder, W. J.; Upton, T. H.; Godd $rd111, W. A. J . Am. Chem. SOC.1978, 100,4751. (16) Trindle, C., private communication.

4520


t


W""

0.0;

I

,

I

I

,

,

,

.-v

1400

I500

,

r

,

I

1200

1300

1100

1000

CM-'

Figure 6. IR spectra recorded in the 1600-900-~m-~ region after annealing 30-32 K samples containing various ethylene-argon mixtures and 6Li: (a) C2H4/Ar = 1/1500; (b) CH2CD2/Ar = 1/500; (c) C2D4/Ar = 1/200. In all cases the lithium concentration is believed to be -1/300. E designates unreacted ethylene absorptions, and asterisks designate nitrogen-containing complexes. TABLE 11: Observed and Calculated Frequencies for the Scissoring and CC Stretching Modes of 1/1 7Li(12C2H4) 7Li(I3C2H4) 7Li('2C2D,) mode mode mode C2, V,,bsd &lcd character uObd vcalcd character v , ~uCalcd character 1312 1309 91% vcc Ai 1453 34% YCC + 1436 25% YCC 92% 6,(CHz) 95% 6,(CH2) 47% 6,(CD2) B2 1428 1426.5 d,(CH2) 1421 1420 6,,(CH2) 1063 da,(CD2)

+

+

AI

1176.5 1183.0

66%~c-8% 6,(CH2)

1150 1151.5 75%vcc5% 6,(CH2)

931

930 9% YCC 53% 6,(CD2)

'LiCzH4a

vow vcaid 1437 1442

1250 1257 990

983

'LiC2H2D2 mode character c, 22% uCC 93% 6(CH2) A' 1.5% d(CD2) 69% vcc - 6.5% 6(CH2) + A' 35% 6(CD2) 9% YCC - 0.5% 6(CH2) A' 63.5% 6(CDz)

+

+

"FCC= 5.77 mdyn/A, FCHl = 0.65 mdyn A, and F c ~ , c H = ~-0.248 mdyn. The percentage of each internal coordinate used to define the mode character of the ith vibration is defined by ( & j ) 2 / ( x k L j k L k t ) for the j t h internal coordinate.

TABLE 111: Observed and Calculated Frequencies for Scissoring and CC Stretching Modes of Free CZH/ I2C2H4 I3C2H, "C2D4 "C2H2D2 mode mode mode mode vow ucalcd character vobd valcd character vow vald character vcald character D2,, vobd 1587 1585.5 48.5% Y C C + 1518 1532 95% vcc + 1585.5 1585.1 66% YCC + 55% A, 1623 1619.1 59% YCC + 73% 6,(CH2) 81% 6,(CH2) 40% 6,(CD2) WH2) + 11.5% 6(CD2) B3" 1443.5 1443.6 6,(CH2) 1438 1437.5 6,(CH2) 1078 1076 ba,(CD2) 1384.0 1384.7 30.5% YCC - 45% 6(CH2) + 13% d(CD2) A, 1342.2 1339.5 41% u C C 1328 1315.0 51.5% vCC 985 1000 5% vCC 1031.0 1036.3 -3.5% YCC + 0.01% 27% 6,(CH2) 19% 6,(CH2) 60% 6,(CD2) 6(CH2) + 75.5% 6(CD2)

C2" A1 A1 AI

= =Geometry and observed values (cm-l) taken from ref 11. FCC = 8.1 mdyn/A, F C H=~ 0.695 mdyn A, FCC,CH> -0.272 mdyn, and FCH+H, 0.048 mdyn A. Mode character is defined as ( L 2 , ) * / ( ~ k L z k L%kof , ) thejth internal coordinate of the ith normal mode, with L being the amplitudes (eigenvectors) of vibrations.

shifts very little, much less than variation in the force constants. The HCH angle a is taken equal to 120°, the title angle 6' is equal to loo,and C-H and C-C internuclear distances are equal to 1.09 and 1.36 A, respectively. I

,L

'\(

nY H

bH

,L

' \

--!-C-C

___-

HY

\H

I

\

H ,?\\\ c L c I / / , H

\

In order to give an order of the significance and accuracy of such a model, we have performed a similar calculation on free ethylene itself using the standard unperturbed geometry.'' The results are presented in Tables I1 and 111. Such a simple model applies reasonably well to free ethylene (5.5 cm-' or 0.45% average error), although it is not possible to fit the experimental frequencies as well as it is for the complex (4 cm-I or 0.33% average error). This could be due to the fact that neglecting the coupling with the A, symmetry C-H(D) stretch is a better approximation in

--


Li(C2H4), and LiZC2H4in Solid Argon SCHEME I free C2H4:

LiC2H4: positions of uncoupled oscillators

k.(CHz) .- .'..

C=C] *,

"'

Loo

'

l:ool

'

1;oo

e .

q

'

'do0

free CpH4:

'

'

I

'

1

1 , l . l

1200

1400

1600

1000

800

LiC2D4: C=CJ

v , cm-'

C=C '.,

l,(CHzZ)]

coupled modes

P.(CDz)

C=C

i 1600

1400

1200

1000

*

"

le00

"

J

( f * ( ~ ~ p )

"

1400

1200

I

1000

1,

I

800

TABLE I V Observed and Calculated Frequencies (cm-I) for LiC2H4for the AI Block Motions of LiC2H4' 6LiC2H4 7LiC2H4 6Li13C2H4 Li C H 6LiC2D4 "OW

1179 714 387

",Id

1451.5 1189.5 702.0 391

YObSd

1176.5 704 369

",led

1451.5 1187.3 701.7 368.5

"OM

1152.5 710 384

"calcd

1433.4 1159.2 697.3 388.9

"OM

1150.0 700 366

YC8lCd

YOM

1433.4 1156.9 697.1 366.1

1313 933.5

'FS,= 5.51 mdyn/A, Fs2 = 0.652 mdyn A, Fs3 = 0.172 mdyn A, Fs4 = 0.33 mdyn/A, F,,,, (with the preceding geometry and LCLiC = 45O). this case than for free C2H4 since "vCCn and "6,(CH2)" are further down. The comparison with other force fields also shows that the C=C bond force constant is underestimated by about 10% vs. the literature value derived from gas-phase data taking into ac~ um interaction. We think, however, count the symmetric U C - and that meaningful comparison between free and "lithium complexed" ethylene can be done within the framework of the same model. (i) When trying to fit the observed frequencies, it is clear that the observed band at 1428 cm-' cannot correspond to the upper mode of the ucc, 6,(CH2) mixture but its 7-an-' l2C/I3Cdownshift strongly suggests its assignment to the 6,,(CH2) mode (B2 symmetry). It also appears that the upper component of the (vcc and &(CHI)) mode would come around 1450 cm-I. This could explain why it is not observed experimentally for it could be masked by the strong 1440-cm-' band of unreacted C2H4 (see also the intensities discussion). (ii) It is immediately obvious from the amplitudes of the different vibrations that vCc and 6,(CH2) vibrations are heavily mixed in both C2H4 and LiC2H4. However, it is also obvious that the high-frequency component of the mixture has, in the complex, less uCc character than 6,(CH2) character (for LiC2H4), Le., exactly the opposite situation of free C2H4. On the other hand, the lowest component has, in LiC2H4,2.3% 12C/13Cshift, that is to say, a bit more than the high component of the mixture in free C2H4 (2.2%). The highest component is not observed in the case of LiC2H4(see intensities discussion), but for the Li(C2H4)2 complex, one will note that it has less 12C/13Cshift (15 cm-') than the lowest component (24.5 an-'). One can illustrate the situation in two simple schemes (see Scheme I). This pictures the difference between the modes of free and complexed ethylene and shows how, although the C=C bond is strongly perturbed by the metal atom and is weakened to such a point that the C=C stretching frequency comes below the methylene group deformation in CzH4, the mixing still produces an upper component in the 1450-cm-' region. This also illustrates how the v m is pushed upward in free C2H4 (only slightly in C2D4); in contrast, vCc is pushed downward in LiC2H, and upward in LiC2D4. In conclusion, we must emphasize that, in the case of such intimate mixing between two oscillators, there is no simple way to appreciate the degree of perturbation of each without isotopic substitutions and subsequent calculation of the force constants. This observation characterizes a perturbation of about 30% on the force constant of C = C bond but only 6% on the CH2 bond angle deformation. In order to obtain the possible amount of mode mixing with the v,(Li-C) stretching mode, we have calculated a more extended

314

",led

1312.8 931.2 543.4 376.4

7LiC2D4 YOLXd

"calcd

1312 931.5 541 360

1312.0 929.4 541.8 355.0

= 0.33 mdyn, Fsis4= 0.50 mdyn, FsZs4= 0.10 mdyn

force held for LiC2H4including all the observed motions of A, symmetry, and the results are summarized in Table IV.

(dihedral angles)

These calculations merit a number of comments: (1) The reproduction of the 2-cm-l 6Li/7Li isotopic effect on the lowest component of the vcc 6,(CH2) mode has necessitated large FCc,Lc and FCH2,LiCinteraction constants, whose signs have been chosen arbitrarily positive and negative (to reflect a shortening of rcc and opening of the H C H angle when rLiCincreases). However, the nature of the modes is not drastically different from that obtained previously in the first simplified model (see Table 11). (2) The proximity of the wagging mode and lithium symmetric stretching mode induces some mixing in the normal modes involving mainly these two coordinates. Note, for example, the 14-cm-' H / D shift on the v,(LiC2) stretch easily reproduced without introducing large interactions. Nevertheless, it is not possible to reproduce the very large lithium effect observed on the hydrogen wagging mode in the C2H4 case with the present hypothesis. Resonance wtih 2u(LiC2) could be the reason for this discrepancy. (3) It is also possible to give a measure of the perturbation of the wagging motion. The calculated force constant is 0.172 compared to that of free ethylene (0.193, from ref 11) or a decrease of about 10% (more than FCHl scissors which is affected by about 6%). (4) We could not include the CHzCD2isotope in this model since the wagging modes were not observed, which also prevented extending the intensities discussion to this model. (5) The model overestimates the magnitude of the lithium isotopic effects and underestimates the I2C/l3C effect on the u,(LiC2) mode. We do not think that this is due to a still weaker interaction with the wagging mode since the H / D shift on the u,(LiC2) is correct. We do not see any other easy explanation but a slightly closer approach of the Li atom, leading to a wider value of the CLiC angle than the one chosen here. Li(C2H4)2 and Li(CZH4)3Species. In order to give a quantitative estimate of the effect of each added ethylene to LiC2H4

+

4522



TABLE V Observed and Calculated Frwuencies (em-') for Scissoring and CC Stretching Modes of Li(C,H.), witb Epuivalent Ligands"

D2h or D2d Li( "C2H4),

Li(C2H4)2 ",bod

Y cld,

VOtd

A,' B"

1507.5 1465.0 1445.8 1445.8 1257.8 1195.2

1463

:,6

1448

cZu Or c2h

Li(C2H2D2)2b

Li(C2D4)2

Ycalcd

l'calcd

YObd

1476.9 1446.2 1439.6 1439.6 1234.3 1164.0

1327

"calcd

YOtd

1420.5 1325.1 1077.7 1077.7 945.2 935.7

Li(C2H4)(C2D4) uobsd

1490.2 1457.3 1337.0 1273.7 1000.3 991.6

1453 1263

(1484) 1375 1217

Ucalcd

1492.7 1445.8 1373.5 1219.6 1077.7 940.2

A,' 1189.5 1165 936 992 (940.0) B" 'The geometry of each C2H4 unit is taken the same as in the preceding model. The frequencies are reproduced by raising Fcc to 6.41 mdyn/A, FcH2= 0.655 mdyn A, F C C , C H ~= 0.249 mdyn, FCH2,CH2 0.005 mdyn A, and adding an interaction force constant, Fcc,cc = 0.494 mdyn/A, in the the hypothesis of a D2h structure, Li(C2H2D2)2 would have two possible isomers of hypothesis of a structure with equivalent ligands DZhor D2,+ C2, and C2, symmetry; in the first case, the Ag modes would keep their symmetry, and in the second, they would not be IR forbidden but would probably be quite weak. In the hypothesis of a Du structure, there would be only one isomer of C2 symmetry. CNotIR active. dOverlapped by parent molecule absorption. TABLE VI: Observed and Calculated Frequencies (em-') for Scissoring and CC Stretching Modes of Li(C,H,), with Equivalent Ligands'

Li(C2H4)3 D3h

"absd

AI'

E'

1512

A; Elfb A,'b

E'

1246

vcalcd

Li(I3C2H4)) Yobsd

1548.5 1508.2 1508.2 1491 1461.2 1438.0 1438.0 1285.4 1245.8 1246 1245.8

Ycalcd

Li(C2D4)3 uabd

1519.1 1483.7 1483.7 1389 1455.1 1431.9 1431.9 1259.7 1217.5 954.5 1217.5

ucalcd

Li(C2H2D2)3c

c$ A' A' A"

yobed

Ycalcd

c3"

A,

1449.7 1393.0 1393.0 1089.2 1071.8 1071.8 972.3 954.3

A' A' A" A' A'

(1008)

1520.1 1491.2 1485.4 1348.5 1323.5 1316.5 1017.6 1008.9

954.3

A"

1003

1004.5

1492 (1480) 1301

E A,

E AI

Li(C2H4)2(C2D4) ucalcd

uabsd

1524.1 (1533) 1485.4 (1511) 1485.4 1356.8 1316.5 1406 1316.5 1281 1022.6 (1246) 1004.5

ucalcd

1538.3 1508.2 1453.8 1438.0 1410.8 1270.8 1245.8 1077.4

Li(C2H4)(C2D4)2 uabd

A,

1528

B2 BI

1421 A?* (1389) A1 1260 AI

B2 B,

Vcalcd

c2,

1526.0 1446.1 1428.0 1393.5 1257.8 1083.2 1071.8 966.1

A,

954.8

B2

B, AI B2 AI

B, A2* A,

E 1004.5

(962)

960.3 AI

(954.5)

"The geometry of each C2H4 unit is taken to be the same in the preceding model. The frequencies are reproduced by raising Fcc to 6.77 mdyn/A, FCH2 = 0.699 mdyn A, FCCICH2 = -0.255 mdyn, FcH2,CH2 = 0.007 mdyn A, and adding intermolecular interaction force constants Fcc,cc = 0.19 mdyn/A, FCC,cH2 = -0.002 mdyn, F c H ~ , c H=~0.007 mdyn A between adjacent CH, groups. bNot active. CForLi(C2H2D2)3 two isomers are possible of C,, and C, symmetry, having a relative statistical weight of 1:3. CHART I

free ethylene LiC2H4 Li(C2H4)2 Li(C2H4)3 our model lit. Fcc, mdyn/A FCH2, mdyn A Fcc,cH,,mdyn

5.77 0.643 -0.248

6.41 0.655 -0.249

6.77 0.669 -0.255

8.1 0.694 -0.272

9.1 0.654 -0.34

in terms of metal perturbation of each ligand as well as in terms of intermolecular coupling, we have extended the simple model employed for lithium monoethylene to these species. In each case, we have attempted to reproduce all isotopic shifts as well as relative positions of the new bands due to mixed isotopic species within the same framework of harmonic force fields based on ucc and 6(CHz) oscillators. The strategy used was to increase the intramolecular force constants until reaching the correct regions and then to add the smallest possible number of intermolecular interactions until the correct trends for the mixed isotopic species were qualitatively reproduced. The results presented in Tables V and VI for models compatible with DZhor D2dfor Li(C2H4)2and D3hstructures for Li(CzH4), (Figure 7) call for a few comments. (1) The calculated intramolecular force constants vary monotonically from lithium monoethylene to triethylene toward the free ethylene values, a result which is quite satisfying since one would expect to have the greatest pepturbation when the metal atom perturbs only one ethylene, the same effect being shared by two and three molecules in the 1/2 and 1/3 complexes (Chart I). (2) In the case of the 1/2 and 1/3 species, the fact that the mixed (C2H4)x(C2D4),,-x bands come above the pure species requires unambiguously that the equivalent oscillators in the pure isotopic species are coupled by positive interaction force constants. Moreover, the relative magnitudes of the mixed species shifts with respect to pure species positions on the high- and low-frequency components indicate that the intermolecular coupling is produced mainly between the C C bonds. For instance, in the Li(C2H4)(C2D4)mixed species the upper [vCc, S,(CD,)J mode (92%

D2h

D2d

Figure 7. Possible structures of the Li(C2H4)2 and Li(C2H4)3complexes.

character) shifts by 48 cm-l vs. Li(C2D& whereas the lower [6,(CDz), uccJ mode shifts only 4 cm-l; on the other hand, the upper [6,(CHz), ucc] mode (35% ucc character) shifts by 21 cm-I vs. the Li(C2H4)2position when the lower [ucc, 6,(CH2)] mode (having the other 65% of vcc character) shifts by 28 cm-'. This explains why the observed spectroscopic trends are well reproduced by introducing mainly a positive interaction between the two (three with Li(C2H,),) C-C bond stretches. This means that the modes with the two (three) molecules in phase (not observed, not IR active in Dzhor DU structures) are higher in energy than the modes with the two (three) ethylene subunits out of phase. Physically, this implies that a shortening of one of the C-C bonds favors a ucc


Li(C2H4),,and Li2C2H4in Solid Argon

literature data and compared our estimate of the C-C force constants for the 1/1, 1/2, and 1/3 complexes in Figure 8. Comparison of the Fcc values for the 1/ 1, 1/2, and 1/ 3 species relative to that of free ethylene yields approximate C-C distances equal to 1.44 f 0.02, 1.42 f 0.02, and 1.39 f 0.02 A, respectively. A few experimental facts merit reexamination in light of the above simplified normal-coordinate analysis: the bandshapes of the two [6,(CH2) vcc] signals and their relative intensities. (i) One can readily notice that the broadness of the bands varies for the different isotopes (4-cm-' fwhm for the 1176-cm-I band of LiC2H4 (65% vcc character)) while the Li13C2H4signal is broader (IO-cm-' fwhm and 75% vcc character). Furthermore, the 1312-cm-' band of LiC2D4is very broad (15-cm-' fwhm and 92% vCc character); meanwhile, the 6,(CD2) is very sharp (1-cm-' fwhm). The same trend is observed in LiC2H2D2from 1435 cm-' (5-cm-' fwhm and 22% vcc character) to 1258 cm-' (7-cm-' fwhm and 69% vcc character) to 990 cm-l (2 cm-' and 9% vCc character). It seems that the more vCc character a mode has, the broader the corresponding absorption. Similar observations can be made for the Li(C2H4)2and Li(C2H4)3absorptions. (ii) When comparing the band intensities for different isotopes of a given species or comparing corresponding bands in the Li(C2H4), Li(C2H4),, and Li(C2H4), series, it is quite surprising to note that the more vcc character a mode has, the more IR intensity it gains. One would have expected, rather, that the vcc mode would gain IR intensity from mixing with the 6,(CH2) mode, which is activated by puckering of the methylene groups in the complexes. This second observation prompted us to undertake a more quantitative analysis of the relative intensities, in view of quantifying approximately the relative contribution of the C-C bond stretch and CH2 bond angle variation to the variation of dipole moment of the molecules. In the case of the LiC2H4species, where the 6,,(CH2) mode is observed, it is possible to conduct the discussion quite rigorously. For the Li(C2H4)2and Li(C2H4),species this mode is not observed (more than likely due to overlapping with parent molecule absorption), and the arguments will be restricted to qualitative considerations. Intensities. For the ith vibration of a given molecule I 0: ( d P / a Q J 2where P represents the dipole moment of the molecule and Q, the normal coordinate associated with the ith vibration. ( a P / a Q J 2= ~,=,,,,,(aP,/dQ,)2can be split into the three contributions of P along the three Cartesian axes. If the axis system is chosen properly, then some of the aP/aQ will be zero by reason of symmetry. LiC2H4is of C , symmetry. For the Al symmetry motions only dP,/aQ, is nonzero. For the B, symmetry motions only dP,/aQ, is nonzero when z is the C2axis, the x axis is along the C = C bond, and they axis is perpendicular to the C = C bond. Experimentally, one has only access to the ( a P / d Q J 2and, in the case of the Li(C2H4), complexes where the exact amount of product is un+ , the ith and j t h known, only to the ratio ( a P / a Q , / a P / a Q J ) ~for vibrations of a given species. The normal coordinates are defined with rJbeing the internal coordinates previously by rJ = &LJ&Q& defined, and the eigenvectors L]&are the ones approximated by the force field of the preceding section. The change in dipole moment for the ith vibration can be developed as a function of the different internal coordinates: aP/aQ, = ~ J L , ( d P / a r J ) . Provided that assumptions on the signs of the aP/aQ, are made for the different modes, it is possible to reach the relative values of aP/arJ and therefore discuss the relative contribution of each internal coordinate to the total change in dipole moment of the complex. Hence, we seek the set of assumptions on the signs of the various aP/aQ, which will give the most consistent result for aP/ar over the different isotopes (C2H4, CzD4, and CH2CD2). In the case of C2D4 only the symmetric modes v , and v3 as defined in Chart I1 were observed, with the following intensity ratio

+

201

1.2

I .3

1.4

1.5

7

(ii)

Figure 8. Comparison of some C-C bond distances with calculated

values of the corresponding force constants (mdyn/A). (a) acetylene: Strey, G.; Mills, I. J . Mol. Spectrosc. 1976 59, 103. (b) diacetylene: Sibos, P. A.; Phibbs, M. K. Can. J . Spectrosc. 1974, 19, 159; ValenceJones, A. J. Chem. Phys. 1952, 20, 860. (c) tetrachloroethylene: Mann, D. E.; Fano, L.; Metal, J. H.; Shimanouchi,T. J . Chem. Phys. 1957, 27, 52. (d) cyclopropene: Mitchell, R. W.; Dorko, E. A.; Merritt, J. A. J . Mol. Spectrosc. 1968, 26, 197. (e) butadiene: Huber-Walchli, P.; Gunthard, H. Spectrochim. Acta, Part A 1981,37A, 285. (f) ethylene: Duncan, J. L.; McKean, D. C.; Mallinson; P. D. J. Mol. Spectrosc. 1973, 45, 221. ( 8 ) benzene: Duinker, J. C.; Mills, I. M. Spectrochim. Acta, Parr A 1968, 24A, 417. (h) styrene (C-C): Marchand, A.; Quintard, J. P. Spectrochim. Acta, Part A 1980, 36A, 941. (i) ethylene oxide: Nakaraga, T. J . Chem. Phys. 1980, 73, 5451; Cant, N. W.; Armstead, W. J. Spectrochim. Acta 1975,31A, 839. 'toluene: Lalau, C.; Snyder, R. G . Spectrochim. Acta, Part A 1971, 27A, 2073. (k) cyclopropane: Duncan, J. I.; Burns, G. R. J . Mol. Spectroc. 1969,30, 253. (1) ethane: Pulay, P.; Meyer, W. Mol. Phys. 1974, 27, 473 and references therein. (m) Zeise's salt: ref 24 and 28.

u)

lengthening of the other(s). This is intuitively very satisfying if one considers that when the lithium atom complexes one ethylene molecule, the C-C bond lengthens; therefore, in the 1/2 and 1/3 complexes, a lengthening of the C-C bond of one molecule means more charge flux from the metal center and therefore less interaction possible for the other ligand(s) leading to a shortening of their C-C distance toward that of free ethylene. (3) To reproduce, at least qualitatively, the trends in the Li(C2H4),(C2D4)and Li(C2H4)(C2D4),spectra,it has been necessary between adjacent to add small interactions +Fcc,cH2and -FCH,CH~ molecules in addition to the large Fcc,cc. The rationale for this is not obvious although one could say that in a 1/3 D3h structure, the ligands are brought closer to each other than in a 1/2 DZh structure and weak electrostatic interactions are more likely to be found than in the 1/2 complex where the intermolecular Fcc,CH2 and FCH2,CH2 were successfully constrained to zero. To conclude this section, it is tempting to relate the weakening of the C-C bond to the decrease in C-C stretching force constant by comparison with other hydrocarbons. Many authors have already compared the C-C bond distance to the Fcc force constant or used empirical relationship^.'^ We have compiled some of the

I1312/1933

(17) Hollenstein, H.; Gunthard, H. H. Spectrochim. Acta, Part A 1971, 27A, 2027.

= (dP/dQ1)~/(aP/aQ= 3 ) 2.9 ~ f 0.5

But in the case of C2H4, the only modes observed were v2 and v3 with 11176/11428 = ( C ~ P / ~ Q ~ ) ~ /=( 18 ~ Pf /4.~ There Q ~ )are ~ two

4524 The Journal of Physical Chemistry, Vol. 90,No. 19, 198'6


CHART 11" amplitudes of vibrations

L=

LiC2H4

c

QI

Q2

Q3

-0.3324 0.2370 0 0.9342 0.9731 0.2724 0.9342 -0.973 1 0.2724 0.3901 0 0.4279 0.7253 0.4279 -0.7253

LiC2D4

LiCH2CD2

L=

free CH2CD2

L=

yields with the same method: for free CH2CD2 with only a P x / a a C H z 2 o

[

0.1937 -0.3387 1.3294 0.3480 0.1 149 -0.6097

ro.3324 -0.2249 11.0198 0.9239 0.3478 -0.3693

-0.1201 0.5274 0.5274 -0.1198 0.0728 0.8 168

1 1 1

-0.07437 -0.01521 0.8915

re1 int calcd I, a 0.45 l2 a 1.67 I , a 0.76

.2x EXPERIMENTAL

exptl 0.21 1.68 0.58

c d

"Q,, Q2, and Q3are the three vCc, 6,,(CH2), and 6,(CH2) modes arranged by decreasing frequencies. rl = C I C 2bond distance vibration. r2 = H C I H bond angle. r3 = HC2H bond angle variation.

CHART 111 set I,

++

set 11,

+-

aPz/aaHcH= 1.15 f 0.20 au aPz/aaHCH= -0.35 0.05 au ap,/arcc = 1.50 0.25 au apz/arcc = 5.3 f 0.8 au aPx/aaHCH= f0.012 f 0.005 au a P x / a a H C H = f0.21 f 0.08 au

TABLE VII: Observed and Calculated Relative Intensities for the CH2Scissoring end C=C Stretching Modes of LiCzH4 set I set I1 exptl LiC2H4 Iul (6,(CH2) + U C C ) 500 1.4 a I v 2 (6,(CH2)) 1 1 1 18 18 Ivi ( ~ c + c 6ACH2)) 15 LiC2D4

Ivl Iu2 Is3

2.9 3 X lo4 1

2.9 0.09 1

a

2.9

4.75 1 1.05

1 5.1 1.1

1.5 1.7 1

1

"Not observed.

+

possibilities: either aP/dQI and aP/aQ2 have the same sign (+ or --, set I) or they have opposite signs (-+ or +-, set 11). These will provide two alternate sets of (aP,,/dr,) that we can test using the three observed normal modes for LiCH2CD2to discriminate between the two possible sets (Chart III). The results (sets of (aPxJari),calculated intensities for LiCH2CD2, and the missing modes of LiC2H4 and LiC2D4) are presented along with experimental intensity ratios in Table VI1 and compared with the experimental spectrum in Figure 9. It is quite obvious that, when comparing synthetic stick spectra with experiment, set I1 is closer to reality than set I. Thus, the most striking feature of this estimate is the existence of a very large variation of the complex dipole moment during the C-C stretching mode (relative to that associated with HCH bond angle variation), and this occurring perpendicularly to the C-C bond. In other words, this means that fluctuation of the C-C distance has a large influence on the amount of possible charge transfer between ligand and metal atom, which is also probably the cause of the vibrational coupling between vCc and v,(LiC2). Another remark is that the dipole moment change along the C2(z) axis over H C H angle variation is of opposite sign and more than 1 order of magnitude smaller than the dipole moment change by C=C distance fluctuation. This leads to two pieces of information: firstly, it is not necessary to invoke a large puckering of the ethylene ligand upon complexation to explain the IR activation of the (6,(CH2) + vcc)

Figure 9. Comparison of calculated stick spectra vs. experiment for LiCH2CD2 in the 6(CH2) ucc 6(CD2) region.

+

+

modes which is largely accounted for by charge fluctuations between metal and a system; secondly, since a lithium atom and an ethylene molecule have no dipole moment at infinite distance and assuming that the Li close approach to the C=C creates a dipole moment along with the C=C lengthening, an increase in the H C H angle would have a reverse effect since the dipole moments derivatives dPz/drcc and aPz/aancHhave opposite signs, which is to say that the H C H angle should have slightly decreased in LiC2H4vs. its free ethylene value. In the case of Li(C2H4)2 and Li(C2H4)3,the complexity as well as the questionable significance of the calculations when the number of approximations increases deterred us from making more than a few qualitative remarks: (i) One can notice that the intensity ratio between the upper and lower components of the vcc/G,(CHz) mixture increases from the 111 to the 113 species, consistent with an increase in the mixture between ucc and 6,(CH2) when the vcc goes back up toward the free ethylene value. (In the case of L I C ~ H both ~)~ modes have an about equal amount of vcc character (see Tables V and VI). (ii) Some simple numerical comparison between the pure Li(C2H4)3 and Li(C2D4)3lead to the same conclusion of opposite signs for aP,/arcc and dP,/daeH,as well as dP,/arCc > d P , / d ~ H 2 by at least 1 order of magnitude. However, the relative intensities in the case of C2H2D2(Z1492/Z1302/11004 = 312.511) indicate that dP,/dacn, must be at least equal to dPz/dacH,(which was not the case in the LiC2H, species). This could be partially explained by a C2H4 group closer to planarity. Comparison and Bonding. One of the most interesting features of the lithium mono-, di-, and triethylene complexes, besides their very existence, is the resemblance of their spectra with that of transition metal-ethylene complexes. In fact, these complexes have been synthesized by various r o ~ t e s , ~and * - ~their ~ spectroscopic properties have been studied by UV-visible, IR, Raman,19vzo and ESRZ'spectroscopies. In the case of M = Ni, Cu, Pd, Co, and Ag, the IR spectra have been assigned to M(C2H4),, where n = 1, 2, 3, by Ozin et al.I9 and are all characterized by features very similar to these observed here in the 1520-1450- and 1230-1 150-cm-I regions, along with weaker features around 900-800 cm-' assigned to wagging modes (see Table VIII). Ozin (18) Fischer, K.; Jonas, K.;Wilke, G. Angew. Chem. 1973, 85, 629. (19) Ozin,G. A. Coord. Chem. Reo. 1979, 28, 147. (20) Bouchareb, S.These de 3%ycle, Universite Bordeaux, 1983. Tranquille, M.,private communication. (21) Kasai, P. H.; McLecd, D.; Watanabe, T. J . Am. Chem. SOC.1980, 102, 179. (22) Ozin, G. A.; Huber, H.; McIntosh, D. Inorg. Chem. 1977,16, 3071. (23) Huber, H.; Ozin, G. A,; Power, W. J. Inorg. Chem. 1977, 16, 979.


Li(C2H4), and Li2C2H4in Solid Argon

TABLE VIII: Comparison of Infrared Absorptions (cm-') of Li(C2H4), with Ethylene Complexes of Some Group VIIIB (10) and IB (11) Metals"

mode assignt v,(CHZ)

Nie 111

112

Pdd 113

111

113 1918 1524 (25)

vcc

2961 2945 2917 2952 (20) 1497 1465 1513 1502 (42)

2943 1463

6,(CH&

1159

1242 1255 (22)

wagCH2

Li

cue

112

111

112

113

111

11 2

113

mode

1475 (15)

1505 (17)

3055 1525 1453b (13)

1463 (15)

1511 (20)

V,,(CHZ) v,(CH,) +

115611138 (11)

1228 (24)

1252 1176 (25)

1189 (24)

1247 (19)

vCC

vcc 901

1225 1245 906

911

1223 (30) 913

906

904

840

862

810

704 (5) 369 (3)

725 (4) 389 (3)

788 (4) 408 (3)

+

&(CH,) wagCH; v(LiC2)

" C- 13 isotopic shifts (cm-I) given in parentheses. Estimated position (see text). Reference 15. Reference 23. e Reference 22. et al. assigned the 1520-145O-cm-' features to vcc and the lower 1230-1 150 cm-'to 6,(CH2) in all the observed complexes without benefit of H / D substitution and concluded that, with Ni for instance, "little perturbation of the ethylene molecule would be expected. Experimentally, this appears as a reduction in the vCc from 1612 cm-' for pure ethylene in Ar to the observed value of 1496 cm-I for Ni(C2H4)and a corresponding reduction of 6CH2 from 1342 to 1158 cm-I". W e hope that after the preceding discussion, the reader is now convinced that, in such an intricate system, one cannot designate one band as "vCC"because it is the closest to the position of the mode that is mainly a vcc vibration in free ethylene. Extensive isotopic substitutions or Raman data and vibrational analysis are necessary to identify vibrational coordinates and to reveal the extent of perturbation of the C C bond in ethylene-metal complexes, and conclusions drawn on a single isotopic species19are misleading. Clearly, without isotopic studies one cannot be sure of the nature of the normal mode involved, and this can lead to substantial underestimate of the C-C bond weakening. However, reports of deuterium shifts20 close to that observed here provide further support for the conclusion that the perturbation of C2H4 in Li(C2H4), species is similar to the case of transition metal complexes. In fact, similar mode mixing in the internal motions of the ethylene submolecule in Zeise's salt and other coordination compounds like (C2H4-,J,,)Fe(CO), and (C2H4)CuClhave been In Zeise's salt, the C-C bond force constant24 (6.55 mdyn A) and crystallographic measurements of the C-C bond length (1.37 f 0.03 A) constitute a precise point of comparison. The observed C = C bond elongation caused by Li in the LiC2H4 species can arise from charge transfer to the A* molecular orbital or by electron density withdrawal from the ethylene A system to form a three-membered metallocycle. On the other hand, the apparent large red shift in the lithium 2p 2s type electronic transition in these species (from 15 300 cm-I for Li in solid argon29 to 13 250 cm-' for Li(C2H4)3to 12 300 cm-' for Li(C2H4)2to an extrapolated position of 11 500 f 500 cm-' for Li(C2H4))also demonstrates a considerable interaction for lithium in the excited state with the A* molecular orbital of ethylene. Further experiments with butadiene,30 where more A delocalization is possible, produced a very strong 9300-cm-I absorption for a similar electronic transition, which shows even more A interaction for lithium in the butadiene complexes. Part 2. Li2C2H4.Comparison between the Li(C2H4)ngroup spectral characteristics and the Li2C2H4absorptions reveals two similarities: the position of the two vC+ stretching modes seen in this case and, more importantly, the sharp band at 1162 cm-I displaying a large red "C shift (to 1129 cm-I) and an even larger blue D shift (to 1258 cm-I) is, therefore, likely due to a C=C stretch coupled with a CH2 scissors motion like in the preceding

b

cases, but shifted further down. However, significant differences are found in the observation of two LiC stretches-the upper one (585/551 cm-I with 6Li/7Li) is much higher than the vs(LiC2) of LiC2H4(387/369 cm-I with 6Li/7Li)-and of several hydrogen bending modes at 695, 583, and 360 cm-I indicating larger perturbation of the C2H4 group. These three modes and their C2D4 and CH2CD2counterparts (see Table I) exhibit various 6Li/7Li shifts, particularly in the case of the deuterated species, which make the analysis intricate but provide useful structural information: (i) The upper bending mode (at 695.5 cm-' with 7Li2C2H4, 595 cm-' with 7Li2C2D4)couples with the higher v(LiC) stretch, whereas the intermediate bending mode (583 cm-' with C2H4,477 cm-' with C2D4) does not in either the C2H4 or C2D4 case, but it does couple when the symmetry between the two methylene groups is broken by using CH2CD2. This means that the upper mode has the same symmetry as v,(LiC2) while the other one has not, but this clear-cut distinction does not exist for CH2CD2. (ii) The fact that the lowest bending mode (360.5 cm-I with C2H4, 279 cm-' with C2D4) yields one single intermediate line (303.5 cm-')with CH2CD2(when only 6Li or 7Li is employed) shows that the two CH2 groups have equal contributions in this mode which can therefore be either a torsion or a tilting around the C-C axis. For these reasons, as well as because of the observed coupling between vcc and 6,(CH2) mentioned above, it is very likely that this species has kept at least a C2 axis or a symmetry plane or an inversion center bisecting the C-C bond. A certain number of structures meeting these requirements have been investigated by a b initio methods for the Li2C2H4 molecule.8 Structures 1 and 2 can be rejected immediately for they would H H Li

L i,

-

(24) Hiraishi, J. Spectrochim. Acta, Part A 1969, 25A, 749. (25) Andrews, D. C.; Davidson, G.; Duce, D. J. Organomet. Chem. 1975,

101, 113. (26) Wardell, J. A. In Comprehensive Organometallic Chemistry: Pergamon: New York, 1982. (27) Bigorgne, M. J . Organomet. Chem. 1978, 160, 345. (28) Jarvis, J. A.; Kilbourn, B. T.; Owston, P. G. Acta Crystallogr., Sect B 1971, B27, 366. (29) Andrews, L.; Pimentel, G. C. J. Chem. Phys. 1967, 47, 2905. (30) Manceron, L.; Andrews, L., unpublished results.

Li

H

H

4, C2"

not predict IR-active vcc stretching modes. Structures 3 and 4 have to be discarded for an indirect reason: since the upper hydrogen bending mode shows a noticeable 6Li/7Li shift (2.5 cm-I with C2H4, 5 cm-' with C2H2D2),these structures would display a triplet pattern for this absorption with the 6Li + 7Li experiment which is clearly not observed in spite of the sharpness of the absorption (fwhm = 1.5 cm-I); moreover, structure 4 would not account for the very large isotopic shift of the 551-cm-' lithiumcarbon stretch, which implies at least a bridging position of the lithium and the assignment of this mode to the symmetric lithium stretching mode. Hence, the nonobservation of a triplet pattern means that the two lithium atoms are not equivalent and the only possible structure satisfying all the conditions described above is 5 where the second metal atom is added on the first metal center, enhancing the perturbation of the ethylene group (the vcc is lowered but not to the value of a C-C single bond, while the two v(LiC2) go up) and forming a weak Li-Li bond. This could explain the very low thermal stability of this species which dis-

4526

The Journal of Physical Chemistry, Vol. 90, No. 19, 1986 LI

I

5,GV

appears as soon as thermal diffusion of ethylene takes place in the argon lattice.

+

-

Li2C2H4 C2H4

2LiC2H4

Consequently, the upper bending mode (695.5 cm-I) is assigned to the symmetric (A,) wagging mode since it must belong to the same symmetry class as the symmetric LiCz stretch. The intermediate bending mode is assigned to the antisymmetric (B2) wagging mode rather than the rocking (B,) because of the relative magnitude of the I3C/l2C shift ( 5 cm-I, instead of 1-2 cm-l expected for the B1 rock) and the small 6Li/7Li shift observed with CHzCDz. The lowest bending mode would be identified as the ethylene tilt (BJ, roughly depictable as an ethylene libration around the C-C axis. In order to give some support to these propositions, we have calculated the different H/D,l2C/I3C, and 6Li/7Li shifts expected for model 5 using a harmonic model and the GMAT and FADJ programs. Since both A I and Bz wagging modes are observed with similar intensities, it was assumed that the CH2 pyramidalization is enhanced relative to LiC2H4. Eighteen force constants were used to fit 75 isotopic frequencies. The assignments of vCHare quite arbitrary based only on IR data and are made by analogy with free ethylene; also, the very tentative estimate of the CzH4 group rocking and twisting motion is made on the basis of the very weak but reproducible bands at 762 cm-' with CZH4, 541 cm-' with CzD4, and 773 and 548 cm-' with CHzCD2. The Li-Li stretching mode frequency (not observed) has been set arbitrarily a t half the value of that of the Liz molecule3' and below the spectrometer limit; likewise, the two LiLiC bond angle deformations have been set at very low values. Table IX shows that this model reproduces very correctly, within small discrepancies easily accountable by anharmonicity of hydrogen motions, the observed isotopic shifts and therefore the mode mixing trend between vs(LiCz) and the wagging motion with successive deuterations. The presence of the second Li atom definitely helps to reach the very high 6Li/7Li isotopic shifts (for instance, 35 cm-' for the symmetric mode when 33 cm-' is expected for a CLiC triatomic model with the same geometry and without the mixing with the A, wagging model); however, the calculated splitting in the case of 6Li7LiCzH4and 'Li6LiC2H4is well below what could be detected (0.3-0.4 cm-l predicted on absorptions having 1.52.5-cm-I bandwidths). Likewise, it is interesting to note that the C-Li force constant tends toward that of the C-Li bond in methyllithium monomer32(0.64 vs. 0.78 mdyn/%r) which could be regarded as a standard unbridged C-Li bond. The weak 1240cm-' (1 230 cm-l with I3C2H4)broad absorption is not assigned to any fundamental in this model, but we note that it is not observed with C2D4and that a combination of the two wagging modes would have the correct position and isotopic shift. In conclusion, we cannot prove that LiZC2H4has the supposed structure, but the model shows that this structure is very realistic and accounts for the observed spectrum. Such a result is quite thought-provoking in the sense that it does not correspond to what common chemical intuition would have suggested (a 1,2-dichloroethane-like structure of some sort). It might be that this structure is not the most thermodynamically stable but is kinetically stabilized in the experimental conditions of this study. However, the fact that addition of the second metal atom takes place preferably on the metal center rather than on the other side of the C-C bond could also provide some clue to explain why synthesis of an a@-dilithioalkanecompound has not been, to our knowledge, so far reported.26 (31) Loomis, F. W.; Nausbaum, R. E. Phys. Rev. 1913, 38, 1447. (32) Andrews, L. J Chem. Phys. 1961, 47, 4834.


Species X . As given in the Results section, no positive identification can be made for this molecule except that concentration and temperature effects suggest aggregation of both lithium and ethylene. The observation of the doublets at 1174 cm-' with C2H4, 1281 cm-' with CzD4, and 1143 cm-' with 13C2H4suggests a C2H4 unit very similar to that of the LiC2H4;on the other hand, the nature of the doublets at 1090 and 884 cm-I (little or no H / D shift, very small lZC/l3Cand 6Li/7Li shifts) remains mysterious. Likewise, the position, relative broadness, and lack of isotopic shift of the 3945-cm-' absorption seem to indicate that it involves an electronic transition, but shifted down a long way from anything of that sort in both lithium and ethylene or even from the transitions measured for Li(CZH4),and Li(CZH4)2.It may also be that the presence of such a low-lying excited state (0.49 eV) could induce more interactions between electronic and nuclear motions and cause the appearance of weak additional bands.

Conclusions Reaction of lithium atoms with ethylene molecules and isolation of the products in an inert medium over a wide range of experimental conditions led to the characterization of two types of products by their visible-near-IR and mid-IR spectra. The first type involves symmetrical n complexes Li(C2H4),, where n = 1, 2, 3, for which extensive isotopic substitutions have shown that each ligand has equivalent CH2 groups and that ligands are equivalent in Li(C2H4)zand Li(CzH4)3. An analysis of the isotopic shifts in the case of 6Li, '3C2H4, CzD4, and CH2CD2provides estimates of changes in the C = C bond and CH2 bond angle from calculated approximate force constants. The perturbation of the C=C bond force constant is found to be the most important, decreasing monotonically from 30% in LiC2H4 to 21% in Li(CzH4)2 and 16% in Li(C2H4)3. In comparing these values to well-known data for other hydrocarbons, it is possible to estimate C-C bond lengthening of about 0.1,0.08, and 0.05 A, respectively, in the 1/1, 1/2, and 1/3 complexes. An analysis of the infrared intensities of the main absorptions showed that it is possible to estimate the order of magnitude and relative signs of the different nonzero partial derivatives of the dipole moment of the complex with respect to C=C bond lengthening and bond angle deformation. It has thereby been possible to show that C==C bond length variation produces a very large variation of dipole moment perpendicular to the C-C axis. Such a variation is also found for the H C H bond angle variation (but 1 order of magnitude smaller and of opposite sign). These results can be interpreted as a consequence of charge flow from Li toward the a* orbital of C2H4, leading to C-C distance lengthening and CHz group pyramidalization, or else by an electron density withdrawal from the n system produced by formation of a loose metallocycle. The extent of intermolecular coupling between the different ligands of Li(C2H4)zand Li(C2H4)3can be estimated. The main interaction force constant is found to be between C-C bond stretchings in different ligands, and its positive sign indicates that a shortening of one C=C bond (decrease of charge flow) favors a lengthening of the other(s) C=C bond(s) (increase of charge flow). The second type of product corresponds to species containing more than one metal atom and in which the ethylene is more strongly perturbed. Li2C2H4has been identified: the spectrum presents evidence for two equivalent CH2 but nonequivalent Li atoms and a vcc frequency only slightly decreased from what it is in LiC2H4. It could then be described as a LiC2H4a complex itself complexed by fixation of a second lithium onto the first metal center. Its spectrum is correctly reproduced in the framework of a harmonic model by using a structure with amplified pyramidalization of the methylene groups relative to LiC2H4,strong C-Li bonds, and a weak Li--Li interaction. Another product is tentatively identified as (LiC2H4)2. The specificity of lithium in all these interactions must be emphasized, since other alkali metals gave only weak van der Waals complexes. This fact alone should, in our opinion, cast serious doubt on a simple ionic model where lithium transfers its


Li(C2Hs), and Li2C2H4in Solid Argon

TABLE I X Comparison between Observed and Calculated Frequencies for Li2C2H4and Isotopic Species'

5i 2

~ 2 ~ 4

-

CALC .F R E O ,

08s SFREP. (CM-1)

""

(A21 (B1)

7Li6LiC2H4

(CM-1)

TFKK 3040.9 2973.3 2965.9 141P.8 1355.9 1174.3 1019.6 780.4 702.0 623.8 586.3 584.9 381a 1 363.4

3036.0 2964.0

762.0 698.0 5R5.5 5P3.0 383.0 363.0

175.6 32.2

3022.0

1127.0

691.0 578.0 549.0 359.5 355.0

CALCeFRFO. CALCeFREQ. (CM-11 (CC-1) 3048.6 3048 6 3040.9 3040.9 2973.3 2973.3 2965.9 2965.9 1418.R 1418.8 1355.9 1355.9 1174.2 1174.3 1010.6 1019.6 7R6.4 780.6 701.9 698 2 623.8 623.8 586.0 \ a 5p4.9 , 181.1 269.5 363.4 359.6 164.7 175.5 3n.2 32.2

CALr.FRE9. (CM-1) 3034.9 3028.3 2967 8 2960.3 1410.3 1'41.7 1141.6 1007.1 779.4 bQ2.R 623.8 580.4 550.9 36105 358.7 163.6 30.0

ORSeFREO. (CM-1) 3022.0

1129.0 693.0 584.5 578.0 377.0 362.5

6Li2C2H2D2 ORS.FPEQ. ICM-I) 297?.0 2270.0 2173.0 1217.0 972.5 661 a

5

770.0 584.4 54n.O 485.5 373.0 307.0

7 L i 2C2H4

OB S FRFO. (CM-1) 3036.0 2964.0 1162.0 762.0 695.5

;;;:;

6Li213C2H4

7Li213C2H4 04S.FREQ. (CN-1 I

6 L i 7 L i C2H4

6Li2C2D4

- -

CALCmFREO. (C"-1) 3034.9 302P e3 2967.n 2960.3 1416.3 1341.7 1141.8 1007.1 779.5 696.2 623.8 585.1 581.2 373.1 362.5 174.7 32.0

583.0 551.0 364.0 360.5

OSS-FRFP. (CM-1)

7Li2C2D4 CALC.FREQ. (CM-1) 2282.9 2263.9 2157.3 ?153.5 1223.5 1078.3 901.3 814.4 625.3 560.1 490.6 452.8 441.4 367.1 281.5 173.6 31.7

22RR.n 2154.0 1258.0

916.0 620.5 541.0 496.0 477.5 362.7 283.5

7Li6LiC2H2D2 6 L i 7Li,C2H2D2

CALCeFREO. CALC.FRE0. (CM-1) (Cr-1) 3044.7 3044.7 2069.6 2969.6 2273.7 2273.7 2155.5 2155.5 138610 13PE.Q 1217.5 1217.5 972.7 972.7 9 5 1 e3 951.3 667.4 667.4 643 5 643 5 584.6 584.4 549.4 545.4 46P.5 4 68.. 5 374.0 374.0 30C $1 705.1 174.6 163.7 22.0 30.0

CALC .FRFO. (CM-1 1 3048 6 3040.9 2 9 7 3 e3 2965.9 1418.8 1355.9 1174.2 1019.6 780.4 69P .2 673.13 584.2 552.7 369.5 359.6 164.5 30.2

CALCeFPFO. (CM-1) 3044.7 2969.6 2273.7 2155.5 1286.9 1217.4 972.7 951.3 660.4 643.4 556.9 545.2 465.9 361 6 300.8 174.5 3 1e 9

O8S.FREO. (CH-l)

CALCaFPEO. (CM-1) t2PZ.9 4 ) 2263.9 (B1) 2157.3 (AI) VCD ?153.5 (B2) 1 7 2 3 . 4 V C (A 1 0 7 8 . 2 6,802 b 2 1 901.2 GsCDZ ( A i ) P14.4 rsCD2 (A21 597.7 v,LiCz wa (A11 560.0 raCD2 (BlY 4 8 0 . 6 wag vsLiC2 (AI) 4 5 2 . ~ wag (B2) 441.4 twist (A21 354.0 vgLiC (B2) 276.6 tilt PB1) 162.6 v L i L i 29.7 GLiLiC2

2288.0 2154.0 1258.0 015.0

487.0 477.0

351 5 279.0

7Li2C2H2D2

'ISS.FREO. (CM-1)

2973.0 2270.0 7173.0 1216.0

979-05 656.5 710.0 557.5 548.0 482.5 762.0 303.5

CALC . F R E P (CW-1) 3044.7 2969.6 ?273.7 2153.5 1386.9 1 2 17 e 4 972.7 951.3 660.4 643.4 556.6 545.2 465.8 361.6 ?oo. R 163.6 30.0

vaCH2

VSCHZ vaCD2 vsCD2 -6CH2

-"CC -6CD2

-rCHz wag CHpvSLiC2

-

-rCDZ

L12

twist wag CH2

-rzD2+twist wag CD2 vaLi2

t i It .vLiLi

OMolecular parameters taken as follows: f C H = 1.09 A, rCc = 1.45 A, rcLi = 1.95 A, rLiLi= 3 A, LHCH = 1 1 5 O , 8CC = 150' with 8 being the = 0.641; FLiLi = 0.10; Fc,H,cIH = 0.05; FLiC,Lic = -0.064, bisector of the HCH angle. Force constants (in mdyn/A): FCC = 5.75, FCH = 4.95; FL~C FcH,cc = 0.18; FCCH v 0.80; FHcLi = 0.227; F ~ i ~ i=c 0.010; F L ~= ,0.010; ~ ~ FCCH,CC ~ = 0.30; FL~C,L~CH = 0.030; FcCH,CcH(adjacent)= 0.380; = 0.031; FLicH,LicH(opposite)= -0.035; FLicH,LicH(alternate) = 0.084. FccH,ccH(opposite) = -0.010; FLicH,LicH(adjacent)

valence electron to form a (Li+)(C2H4-)ion pair, since there is no reason not to observe the same with heavier alkali metal atoms whose ionization potentials are lower. As suggested by Schleyer et al. in a series of publications?f' lithium has p orbitals relatively close in energy which can stabilize partially covalent interactions yielding unexpected bridging structures. Similar studies on the analogous group IIA (2) and IIIA (13) metal complexes with ethylene are in progress;33these species are of particular interest (33) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the gblock elements comprise group 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e&, 111-3 and 13.)

in view of the different electron configurations of the metal. The succession of reactions leading to the different products characterized here can be summarized in the following scheme:

fi:H4"t(h", +LI

L i,C,H4

red light)

LiC2H4 *

+CZH4

Li(C2H4),

-

Li(CzH4)3

+c2H4

very stable

(LiC2H4),

Similar alkali metal complexes with benzene and other conjugated A systems are under investigation. These results open the possibility that lithium functions as a catalyst promoter by forming

4528

J . Phys. Chem. 1986, 90, 4528-4533

direct lithium atommolecule complexes, of the type characterized here, which undergo subsequent reactions with other molecules.

Hawkins for preliminary experiments, and the N S F for financial support.

Acknowledgment. We thank C . Trindle and M. Tranquille for helpful discussions and communication of preliminary results, M.

Registry No. Li, 7439-93-2; 'Li, 13982-05-3; 6Li, 14258-72-1; C2H4, 74-85-1; I3C2H4, 51915-19-6; C2HZD2, 6755-54-0;C2D4, 683-73-8.

A Vibrational Study of Lithium Sulfate Based Fast Ionlc Conducting Borate Glasses E. I. Kamitsos,* M. A. Karakassides, and G. D. Chryssikost Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation, Athens 1 1 6-35, Greece (Received: January 27, 1986)

The influence of the sulfate anion on the structure of the xLizO.yLi2SO4-B2O3 fast ionic conducting glasses has been studied by Raman and Fourier-transform infrared spectroscopies, for compositions probing the whole glass-forming region (x = 0.20, 0.56; y = 0-0.50). Difference spectra are presented to elucidate the S042--inducedstructural changes. Thus, for the x = 0.20 series, combined Raman and infrared results showed that Li2S04additions induce the formation of BO4 tetrahedra. However, for high lithium oxide content ternary glasses (x = 0.56), while Raman spectra did not show obvious changes of the glass structure upon increasing y , infrared spectra clearly indicated that the numbers of BO4 tetrahedra and nonbridging oxygen-containing borate groups both increase. It was concluded that the presence of SO:- anions, in the glass melt, favors the formation of borate groups, which are more polar than those of the corresponding binary glasses.

Introduction Studies of inorganic glasses with high ionic conductivity have attracted much interest due to their advantageous characteristics, for energy conversion and storage applications, over the crystalline conductive solids.' Fast ionic conducting glasses (FIC) are usually presented by the formula xM20.yM,X.A,,0,, where ApOqis the glass former (Le., Si02,B203,etc.), M is a metal, usually Li, Na, Ag, and X" is an anion, usually halogen, S2-,S042-,MOO^^-, etc. Although fast ion conduction in glasses is a general phenomenon, the mechanism of ion transport in glasses is not well understood, contrary to transport in crystalline solids. This is partly because no detailed knowledge is available about the local glass structure and the way this is affected by the anion, X". It is obvious that understanding the structural characteristics of such technologically important FIC glasses is critical not only in explaining the mechanism of ionic conduction but also in designing systems with improved characteristics appropriate for the current needs. There are contradictory reports for the effect of X" on the glass structure and, to an extent, on the mechanism of conduction. For instance, on the basis of Raman, infrared r e f l e ~ t a n c e ,and ~ , ~I'B N M R 4 data, it was concluded that the only factor affecting N4, the fraction of 4-coordinated boron atoms, in ternary xLi20. yLiC1.B203 glasses, is the O / B ratio. However, Uhlmann and co-workers were able to conclude on the basis of density, conductivity, and glass transition temperature measurements that the halide indeed causes major network modification^.^-' These conclusions are supported in part by new Raman results based on careful control of the O/B ratio8 and by N M R results as we1L9 While halogens can either sit in an interstitial position and/or coordinate boron atoms, there are anions, like that can only occupy interstitial sites. A very interesting FIC glass system based on Sod2-, Le., xLi20.yLi2S04.B203,was reported by Levasseur and c o - w ~ r k e r s . ~ Very ~ ' ~ stable, easily prepared, high lithium content glasses were obtained in this system. The high lithium content of these glasses is an important feature, since binary xLi20.B20, glasses with lithium content up to ca. x = 1.8 are very difficult to obtain, besides the fact that they are very hygroscopic." The effect of the S042-anion on the glass structure was studied by Raman spectroscopy in the systems 0.7 I Li20-yLi2S04.B203.'o 'On leave of absence from the Chemistry Department, Brown IJniversity, Providence, RI 02912.

TABLE I: Compositions of Ternary xLi20.yLi2S04.B203Glasses Studied in This Work X

V

A

0.20

B C D

0.20 0.20 0.20

0 0.01 0.02 0.03

X

V

E

0.56

F G H

0.56 0.56 0.56

0 0.15 0.30 0.50

It was concluded that SO:- anions are diluted in the glass network without any detectable interactions with it. These observations are very reasonable, since for such a large x value (x = 0.71) the boron-oxygen network is greatly affected by Li20. Large interstices are created which can easily accommodate the SO?-ions, without any important interactions, at least to a degree detectable by Raman spectroscopy. What remained to be investigated is whether the large and ionic SO:- groups can induce changes to borate networks modified to a lesser extent by Li20 and thus more compact. Besides Raman spectroscopy, such changes could effectively be probed by infrared spectroscopy, which, due to differences in selection rules, is known to be complementary to Raman, especially when N4 values are to be estimated.12 Thus, we have studied by both infrared and Raman spectroscopy the system 0.56Li2O.yLi2SO4.B2O3and compared the spectra. For (1) For review articles in fast ionic conducting glasses, see: (a) Tuller, H. L.; Button, D. P.; Uhlmann, D. R. J . Non-Cryst. Solids 1980, 40, 93. (b) Tuller, H. L.; Barsoum, M. W. J . Non-Cryst. Solids 1985, 73, 331. (c) Ravaine, D. J. Non-Cryst. Solids 1985, 73, 287. (d) Minami, T. J. Non-Cryst. Solids 1985, 73, 273. (2) Irion, M.; Couzi, M.; Levasseur, A.; Reau, J. M.; Brethous, J. C. J. Solic State Chem. 1980, 31, 285. (3) Levasseur, A,; Brethous, J. C.; Reau, J. M.; Hagenmuller, P.; Couzi, M. Solid State Ionics 1980, I , 177. (4) Geisberger, A. E.; Bucholtz, F.; Bray, P. J. J. Non-Cryst. Solids 1982, 49, 117. ( 5 ) Button, D. P.; Tandon, R. P.; Tuller, H. L.; Uhlmann, D. R. J. NonCryst. Solids 1980, 42, 297. (6) Button, D. P.; Tandon, R. P.; Tuller, H. L.; Uhlmann, D. R. Solid State Ionics 1981, 5, 655. (7) Button, D. P.; Tandon, R. P.;King, C.; Velez, M. H.; Tuller, H. L.; Ulhmann, D. R. J. Non-Cryst. Solids 1982, 49, 129. (8) Turcotte, D. E., Risen, Jr., W. M.; Kamitsos. E. I. Solid Srate Commun. 1984, 51, 313. (9) Kline, D.; Bray, P. J. Phys. Chem. Glasses 1966, 7, 41. (10) Levasseur, A.; Kbala, M.; Brethous, J. C.; Reau, J. M.; Hagenmuller, P.; Couzi, M. Solid State Commun. 1979, 32, 839. (11) Martin, S. W.; Angell, C . A. J . Non-Crysr. Solids 1984, 66, 429. (12) Glass Structure by Spectroscopy; Wong, J., Angell, C. A,, Eds.; Marcel Dekker: New York, 1976.

0022-3654/86/2090-4528$01.50/00 1986 American Chemical Society

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