11 December 1998
Chemical Physics Letters 298 Ž1998. 161–169
˜ 1 S / in its ground and Õ5
Rotational spectroscopy of LiCCH žX
vibrational states
A.J. Apponi 1, M.A. Brewster, L.M. Ziurys
)
Department of Chemistry, Department of Astronomy, and Steward ObserÕatory, The UniÕersity of Arizona, 933 North Cherry AÕenue,
Tucson, AZ 85721-0065, USA
Received 3 August 1998; in final form 16 October 1998
Abstract
˜ 1 S . in its ground and Õ5 vibrational states has been recorded using direct
The pure rotational spectrum of LiCCH ŽX
absorption, millimeterrsub-millimeter spectroscopy in the range of 105–538 GHz. For the main isotopomer, 21 transitions
were measured for the ground state and 10 transitions each for the excited levels Õ5 s 1, 2, and 3, where l-type doubling
was resolved. Several transitions were also recorded for 6 LiCCH, Li13CCH, LiC13CH, and LiCCD. All were observed in
their natural abundances except for LiCCD. Rotational, l-type doubling and vibration–rotation constants were determined
for LiCCH, as well as rs and ro structures. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction
Synthesis techniques have widely utilized organolithium species to produce new and exotic compounds Že.g. w1x.. For example, LiCCH is used in
nucleophilic substitution reactions in the creation of
large alkynes. t-Butyl-lithium, a strong base, is employed to extract protons from certain molecular
sites. Many such compounds, however, come in
solvated form and their exact structure and degree of
aggregation in this medium is almost always unknown w2,3x. Their role in reaction mechanisms is
consequently quite uncertain. Since understanding
)
Corresponding author. E-mail: [email protected]
Current address: Harvard University, Division of Engineering
and Applied Sciences, 29 Oxford Street, Cambridge, Massachusetts 02138.
1
such mechanisms is vital for the creation of new
chemical species, obtaining detailed information
about the structure and bonding in organolithium
compounds is desirable.
Accurate geometries for individual organolithium
species for the most part are sparse. The usual
methods used to obtain such data involve either
X-ray crystallography w4x, or IR vibrational measurements while the species is frozen in an argon matrix
w5x. Both techniques probe oligomeric structures that
are influenced by interactions with the matrix or
neighboring species. For example, the X-ray crystal
structure of LiCH 3 applies to either the tetramer or
the hexamer form w6x. Only rarely has a structure
been obtained for a monomer of an organolithium
compound, and these results have invariably involved a very large ligand attached to the Li atom,
such as CHŽSiMe 3 . 2 w7x. However, almost all ab
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 2 0 6 - 8
A.J. Apponi et al.r Chemical Physics Letters 298 (1998) 161–169
162
Table 1 Žcontinued.
Table 1
Observed transition frequencies of LiCCH a
Y
Õ5l
l
Õ5
J ™J
nobs
nobs – ncalc
Ž0 0 .
4 ™5
5™6
6 ™7
7™8
8™9
9™10
10 ™11
11™12
12 ™13
13™14
14 ™15
15™16
16 ™17
17™18
18™19
19™ 20
20 ™ 21
21™ 22
22 ™23
23™ 24
24 ™ 25
15™16
16 ™17
17™18
18™19
19™ 20
20 ™ 21
21™ 22
22 ™23
23™ 24
24 ™ 25
15™16
16 ™17
17™18
18™19
19™ 20
20 ™ 21
21™ 22
22 ™23
23™ 24
24 ™ 25
15™16
16 ™17
17™18
18™19
19™ 20
20 ™ 21
21™ 22
22 ™23
23™ 24
24 ™ 25
15™16
16 ™17
105 435.229
126 519.255
147 601.676
168 682.163
189 760.482
210 836.348
231 909.480
252 979.625
274 046.480
295 109.785
316 169.284
337 224.735
358 275.807
379 322.241
400 363.805
421 400.219
442 431.149
463 456.409
484 475.691
505 488.729
526 495.255
339 469.030
360 657.855
381 841.608
403 020.008
424 192.728
445 359.467
466 519.970
487 673.939
508 821.067
529 961.077
341 160.423
362 451.977
383 738.046
405 018.155
426 291.967
447 559.248
468 819.669
490 072.923
511 318.617
532 556.546
343 485.894
364 881.275
386 264.129
407 633.963
428 990.287
450 332.813
471 661.175
492 975.157
514 274.604
535 559.394
343 598.372
365 038.978
0.006
y0.011
0.005
y0.004
0.001
0.007
0.006
0.016
0.006
y0.012
y0.024
0.001
0.000
y0.014
y0.003
0.022
y0.003
0.005
0.005
0.001
y0.008
y0.062
y0.039
y0.013
0.028
0.049
0.039
0.034
0.024
y0.010
y0.059
0.068
y0.009
0.026
0.022
y0.036
y0.059
y0.055
y0.014
y0.008
0.074
y0.276
y0.001
0.166
0.225
0.135
0.017
y0.135
y0.221
y0.134
0.216
0.191
0.011
Ž11c .
Ž11d .
Ž2 0 .
Ž2 2c .
Ž2 2d .
Ž3 3c .
Ž3 3d .
a
Y
J ™J
nobs
nobs – ncalc
17™18
18™19
19™20
20 ™21
21™22
22 ™23
23™24
24 ™25
15™16
16 ™17
17™18
18™19
19™20
20 ™21
21™22
22 ™23
23™24
24 ™25
15™16
16 ™17
17™18
18™19
20 ™21
21™22
22 ™23
23™24
15™16
16 ™17
17™18
18™19
20 ™21
21™22
22 ™23
23™24
386 473.385
407 901.290
429 322.285
450 736.006
472 142.141
493 540.345
514 930.277
536 311.571
343 882.241
365 378.003
386 873.670
408 369.291
429 864.324
451 358.524
472 851.542
494 342.885
515 832.050
537 318.461
347 220.786
368 895.881
390 565.937
412 230.590
455 542.108
477 188.194
498 827.254
520 458.911
347 235.880
368 916.350
390 593.158
412 266.158
455 600.550
477 261.723
498 918.789
520 571.650
y0.114
y0.128
y0.088
y0.026
0.065
0.134
0.109
y0.140
0.091
0.002
y0.130
y0.042
y0.018
0.010
0.058
0.064
0.024
y0.065
0.037
0.030
0.017
0.007
y0.030
y0.026
y0.025
0.036
y0.054
y0.015
0.004
y0.022
0.018
0.031
0.031
y0.038
In MHz.
initio calculations of geometries of organolithium
compounds have involved monomers, and theories of
mechanisms usually concern reactions of individual
molecules rather than large complexes.
A clear approach to accurate structural determinations of organolithium compounds is gas-phase
high-resolution spectroscopy. In the past, such investigations have been carried out for lithium-bearing
molecules, but have been restricted primarily to
halides w8x, hydride w9x, and hydroxide w10,11x species.
In fact, spectroscopic data of any kind of molecule
where lithium is attached to an organic ligand are
virtually non-existent.
A.J. Apponi et al.r Chemical Physics Letters 298 (1998) 161–169
In order to provide accurate geometries for
organolithium monomers, we have begun a study of
the gas-phase millimeterrsub-millimeter spectrum of
such species. Recently, we have recorded the rotational spectrum of LiCH 3 , including its deuterium
and 6 Li isotopomers, and have established a structure
for this molecule w12,13x. Such work is an extension
of measurements we have carried out for other
metal-alkyl compounds, including NaCH 3 w14x,
KCCH w15x, and NaCCH w16,17x, as well as of LIF
electronic spectroscopy of alkaline earth and transi-
163
tion metal species conducted by the Bernath group
and others w18,19x.
In this Letter we report the observation of the
˜ 1 S ground
pure rotational spectrum of LiCCH in its X
state. To our knowledge, this molecule has never
been studied before by any spectroscopic technique.
LiCCH was created by the reaction of lithium vapor
with HCCH or HCCBr. Transitions were recorded
for this species in its ground vibrational state and
several quanta of the low-lying Õ5 bending mode.
Rotational transitions were also measured for the
Table 2
Observed transition frequencies of LiCCH isotopomersa
Ž Õ5l .
Ž0
0.
Ž11c .
Ž11d .
Ž2 0 .
Ž2 2c .
Ž2 2d .
a
Y
J ™J
X
17 ™ 18
18 ™ 19
19 ™ 20
20 ™ 21
21 ™ 22
22 ™ 23
23 ™ 24
24 ™ 25
25 ™ 26
26 ™ 27
27 ™ 28
18 ™ 19
19 ™ 20
20 ™ 21
21 ™ 22
22 ™ 23
18 ™ 19
19 ™ 20
20 ™ 21
21 ™ 22
22 ™ 23
18 ™ 19
19 ™ 20
20 ™ 21
21 ™ 22
22 ™ 23
18 ™ 19
19 ™ 20
20 ™ 21
21 ™ 22
22 ™ 23
18 ™ 19
19 ™ 20
20 ™ 21
21 ™ 22
22 ™ 23
In MHz.
6
LiCCH
Li13 CCH
LiCCD
LiC13 CH
nobs
nobs –Õcalc
nobs
nobs – ncalc
nobs
nobs – ncalc
nobs
nobs – ncalc
–
438 364.104
461 395.148
484 419.928
507 438.165
530 449.521
–
–
–
–
–
441 210.873
464 387.712
487 557.692
510 720.507
533 875.766
443 569.180
466 865.011
490 153.198
513 433.408
536 705.276
446 228.515
469 588.324
492 931.197
516 256.945
539 565.591
446 671.750
470 125.435
493 570.688
517 007.092
540 434.302
447 346.061
470 904.403
494 462.627
518 020.071
541 576.022
–
0.019
y0.003
y0.024
y0.009
0.017
–
–
–
–
–
0.040
y0.013
y0.044
y0.013
0.031
0.064
y0.013
y0.067
y0.047
0.064
y0.019
0.030
0.013
y0.041
0.017
0.017
y0.028
y0.001
0.021
y0.009
y0.006
0.005
0.008
y0.011
0.004
–
–
–
–
–
442 233.437
461 422.136
480 605.857
499 784.460
518 957.688
538 125.393
–
–
–
–
–
0.026
0.010
y0.028
y0.021
0.020
0.034
–
–
–
442 223.525
463 239.297
484 249.046
505 252.608
526 249.780
–
–
–
–
–
–
y0.010
0.034
y0.010
y0.036
0.023
–
–
–
370 101.543
390 632.528
–
431 680.026
452 195.620
472 705.901
493 210.012
513 708.093
–
–
–
0.080
y0.070
–
0.036
y0.113
0.069
y0.019
0.020
–
–
–
164
A.J. Apponi et al.r Chemical Physics Letters 298 (1998) 161–169
isotopically substituted species LiCCD, 6 LiCCH,
Li13 CCH, and LiC13 CH. Spectroscopic parameters
were determined for all five isotopomers and the Õ5
bending mode, from which the geometry of LiCCH
was calculated. Preliminary structural results for the
ground vibrational state of LiCCH were presented in
a previous paper in the organic chemistry literature
w20x. Here we present the complete data set, and
discuss the spectroscopic parameters derived in terms
of bonding and rigidity in the molecule. We additionally compare the Õ5 vibrational data to those of
similar metal-bearing species with low-lying bending
modes.
2. Experimental
The transition frequencies for LiCCH and its isotopomers were measured using a millimeterrsub-mm
direct absorption spectrometer, the details of which
are given elsewhere w21x. To summarize briefly, the
instrument consists of a Gunn oscillatorrSchottky
diode multiplier source, a reaction chamber in which
the molecules to be studied are created, and a helium-cooled InSb detector. The radiation is propagated quasi-optically through the cell, which is a
double-pass system. FM modulation of the source
enables phase-sensitive detection.
The LiCCH molecules were generated by the
reaction of acetylene and lithium vapor. The metal
vapor was produced in a Broida-type oven, which
had to be slightly modified to accommodate lithium.
This modification consisted of lining the usual alumina crucible used to contain the metal in the oven
with a machined iron insert, which prevented lithium
from attacking the crucible. Approximately 30 mTorr
of acetylene was used for the reaction, which was
mixed with an equivalent amount of argon carrier
gas. These gases were flowed into the cell over the
top of the oven, and the whole mixture discharged
using a 750 mA current at 20–40 V. For the 6 Li
isotopomer, the signal-to-noise was sufficient to observe its spectrum in natural abundance, but pure
lithium 6 metal was used for most of the measurements to save time. Li13 CCH and LiC13 CH were
studied only in their natural abundance Ž12 Cr13 C ;
90r1.. For the synthesis of the deuterated isotopomer, D 2 C 2 was used as a precursor, which was
produced by the reaction of D 2 O and CaC 2 followed
by cryogenic distillation. Finally, it was found that
use of BrCCH instead of HCCH as a reactant gave
much stronger signals, in fact, as much as a factor of
Fig. 1. Spectra of the J s 22 ™ 23 rotational transition of LiCCH and the J s 20 ™ 21 line of 6 LiCCH in the natural lithium 6 abundance
observed near 485.5 GHz. Also visible in these data is one of the l-type doublets the J s 22 ™ 23 transition originating in the Õ4 s 1 state.
This spectrum is a composite of two separate 100 MHz scans, each 30 s in duration.
A.J. Apponi et al.r Chemical Physics Letters 298 (1998) 161–169
Fig. 2. Spectrum of the J s 24 ™25 rotational transition of
LiCCD recorded near 480.6 GHz. This scan covers 100 MHz in
frequency and was acquired in about 30 s.
165
given by l s "1 and l s "3. In this case, only
lines for l s "3 were recorded. Although 7 Li has a
nuclear spin of 3r2, no quadrupole splitting was
observed in any transition. However, it might be
resolved in lower rotational lines not studied in this
work.
In Table 2, the frequencies recorded for the deuterium, lithium 6, and 13 C isotopomers are presented,
which lie in the range of 370–542 GHz. For 6 LiCCH,
five transitions of the ground vibrational state were
measured, as well as five lines of the first and second
quanta of the Õ5 bending mode. Again, l-type doublets were observed in the Õ5 s 1 state and l s 0,
"2 components for the Õ5 s 2 data. For LiCCD,
LiC13 CH, and Li13 CCH, six, seven, and five transitions were recorded, respectively, in the ground vibrational state only.
50 in signal enhancement. No discharge was required
in this case, and only ; 1 mTorr of BrCCH was
needed. However this precursor also produced many
lines of Li79 Br and Li81 Br Žground and excited vibrational states. and hence was not as suitable as HCCH
for the LiCCH measurements. BrCCH was synthesized by the dehydrohalogenation of 1,2 dibromoethene by a strong base ŽNaOH..
Center frequencies were determined from an average of two 5 MHz scans, one in increasing and the
other in decreasing frequency, except for the 13 C
isotopomers. In this case, eight such scans were
averaged. Experimental uncertainty is estimated to
be "50 kHz.
3. Results
The observed rotational transitions of LiCCH in
its ground and Õ5 bending modes are presented in
Table 1. Twenty-one separate transitions were
recorded for the ground state in the range of 105–527
GHz and 10 transitions were measured for the Õ5 s
1,2, and 3 states between 339 and 538 GHz. For the
Õ5 lines, l-type doubling and l –pe resonance was
observed, splitting the Õ5 s 1 state into doublets 11c
and 11d , where ‘c’ denotes lower-frequency component and ‘d’ the upper one, and the Õ5 s 2 state into
three separate levels 2 2c , 2 2d , and 28. The Õ5 s 3
state should in principle be split into four levels,
Fig. 3. Spectra of the J s 23™24 transition of Li13 CCH and the
J s 22 ™23 line of LiC13 CH, near 505.2 and 472.7 GHz, respectively, observed in the natural 13 C abundance. These spectra here
span 5 MHz in frequency and consists of an average of 8 scans,
each 30 s in duration.
A.J. Apponi et al.r Chemical Physics Letters 298 (1998) 161–169
166
Representative spectra are shown in Figs. 1–3.
Fig. 1 displays the J s 22 ™ 23 transition of LiCCH
and the J s 20 ™ 21 transition of 6 LiCCH in their
ground vibrational states near 484.5 GHz. The 6 Li
feature is recorded here in the natural lithium abundance. Another line is present in the spectrum, which
arises from the Õ4 bending mode of the main isotopomer. This spectrum covers 160 MHz in frequency and is a composite of two scans, each 100
MHz in scan length, and 30 s in duration. Fig. 2
shows the J s 24 ™ 25 transition of LiCCD near
480.6 GHz, created using DCCD. This 100 MHz
scan was taken in 30 seconds. Fig. 3 displays the
J s 22 ™ 23 line of LiC13 CH at 472.7 GHz and the
J s 23 ™ 24 line of Li13 CCH near 505.2 GHz, observed in the natural 13 C abundance. The frequency
coverage in these data is 5 MHz; they are each an
average of eight, 30 s scans.
4. Analysis
1
Because the ground electronic term of LiCCH is
S, only molecular frame rotation needs to be con-
sidered in the analysis of the ground vibrational state
for all isotopomers. These data were consequently fit
with only rotational Ž B . and centrifugal distortion
Ž D and H . terms. For the Õ5 bending mode measurements, however, l-type doubling and l-type
resonance must be considered in the Hamiltonian. To
account for the splitting in the Õ5 s 1 state, a term
was added to the rotational Hamiltonian with the
form w22x:
D E l - type s " 12 qJ Ž J q 1 . y q D J 2 Ž J q 1 .
2
4
where q is the l-type doubling constant and q D is
its centrifugal distortion correction. To model the
splittings in the Õ5 s 2 state, where l-type doubling
and l-type resonance both play a role, the energy
difference between the Õ5l s 2 0 and 2 2 states must
be known. For LiCCH, this quantity has not yet been
determined. Therefore, only effective l-type doubling parameters could be established for the Õ5 s 2
levels, which reflect the local sub-state splittings. A
similar situation is found for the Õ5 s 3 data. For the
Table 3
Molecular constants of LiCCH, 6 LiCCH, LiCCD, LiC13 CH and Li13 CCH a
Constant Ž Õ5l .
LiCCH
6
B Ž0.
D Ž0.
H Ž0.
B Ž11 .
D Ž11 .
H Ž11 .
q Ž11 .
q D Ž11 .
B Ž2 0 .
D Ž2 0 .
H Ž2 0 .
B Ž2 2 .
D Ž2 2 .
H Ž2 2 .
qeff Ž2 2 .
q D,eff Ž2 2 .
q H,eff Ž2 2 .
B Ž3 3 .
D Ž3 3 .
H Ž3 3 .
qeff Ž3 3 .
q D,eff Ž3 3 .
rms of fit
10 544.0909Ž46.
0.011373Ž14.
2.7Ž1.3. = 10y8
10 641.545Ž13.
0.013186Ž30.
7.4Ž2.2. = 10y8
53.5077Ž66.
0.0012808Ž70.
10 751.250Ž19.
0.035019Ž43.
3.162Ž32. = 10y6
10 745.059Ž13.
0.005800Ž30.
y1.342Ž22. = 10y6
0.017917Ž36.
9.7Ž1.0. = 10y7
1.12Ž82. = 10y9
10 856.192Ž17.
0.010780Ž43.
1.05Ž34. = 10y7
y2.450Ž20. = 10y6
1.18Ž29. = 10y10
0.078
a
LiCCD
LiC13 CH
Li13 CCH
11 545.322Ž12.
0.013054Ž13.
–
11 652.7529Ž86.
0.0151548Ž95.
–
63.241Ž17.
0.001636Ž19.
11 774.63Ž11.
0.04730Ž24.
6.06Ž18. = 10y6
11 766.998Ž74.
0.00361Ž17.
y2.70Ž13. = 10y6
0.02628Ž17.
2.02Ž53. = 10y6
2.12Ž44. = 10y9
9 622.8736Ž92.
0.0086047Ž69.
–
10 287.5226Ž67.
0.0106889Ž66.
–
10 539.046Ž12.
0.011241Ž11.
–
0.030
0.024
0.066
0.025
LiCCH
Ž 1.
In MHz; errors quoted are 3 sigma and apply to the last quoted decimal places.
A.J. Apponi et al.r Chemical Physics Letters 298 (1998) 161–169
Õ5 s 2 state, the l s "2 lines were fit with an
effective l-doubling constant qeff defined by w23x:
D E s " 12 qeff J Ž J q 1 . J Ž J q 1 . y 2
4.
Ž 2.
For the Õ5 s 3 data, the l s "3 lines were modeled
with a similar effective term of the form w23x:
D E s " 12 qeff J Ž J q 1 .
3
.
Ž 3.
A second-order correction q D,eff was needed to fit
the data in every case and in some instances a
third-order term q H,eff , as well.
The results of this analysis are given in Table 3.
As the table shows, the second-order centrifugal
distortion parameter H was necessary to analyze the
main isotope data, including all excited vibrational
states, likely because of the wide range of transitions
recorded. H was not needed to fit the spectra of the
other isotopomers except the Õ5 s 2 state of 6 LiCCH.
A second-order correction to qeff , q H,eff , was needed
for the Õ5 s 2 analyses as well. In general the spectroscopic parameters are well-determined. The rms
of the fits for the individual isotopomers, which
include ground and excited vibrational states, are 78
kHz or better. The residuals from the fits are - 25
kHz for the LiCCH ground state transitions, less than
167
80 kHz for the isotopomer data Žwith one exception.,
and somewhat higher for the vibrationally excited
lines of the main isotope Ž( 200 kHz..
Certain ro-vibrational terms were also determined
for LiCCH and 6 LiCCH for the Õ5 mode. The vibrational satellite lines arising from this excited state
were found to follow a very regular pattern, as
shown in Fig. 4. This figure presents a stick diagram
of the vibrational progression of the J s 21 ™ 22
transition for the ground and Õ5 states with approximate relative intensities. The Õ5 s 1, 2, and 3 lines
appear at regular intervals relative to that of the
ground state, and the 2 0 state is not widely separated
from the 2 2 state, indicating a more rigid as opposed
to quasilinear molecule. This pattern was fit using
Eq. Ž4., based on an expression used for triatomic
molecules from Lide and Matsumura w24x:
2
BÕ s B˜e y a 5 Ž Õ5 q 1 . q g 55 Ž Õ5 q 1 . q g l l l 2 ,
Ž 4.
where B˜e s Be y Ý4is1 a i Ž Õi q d ir2.. The resulting
parameters B˜e , a 5 , g 55 and g l l are presented in
Table 4. Also listed in this table are similar terms
derived for the Õ 2 bending modes of LiOH and
Fig. 4. A stick figure showing the vibrational progression of the Õ5 mode of LiCCH in the J s 21 ™ 22 transition and the approximate
relative intensities. The Õ5 s 1, 2, and 3 states exhibit a fairly regular pattern with relatively small l-type doubling and l-type resonance
interactions. For the Õ5 s 3 state, only lines for l s 3 Žand not l s 1. were identified.
A.J. Apponi et al.r Chemical Physics Letters 298 (1998) 161–169
168
Table 4
Vibrational dependence of BÕ for LiCCH and related moleculesa
Species
B˜eb
a5
g 55
gl l
LiCCH
LiCCH
LiOH
MgOH
10 455.35Ž20.
11 446.61Ž50.
35 496.90
14 912.21
y83.905Ž50.
y93.396Ž40.
176.03
111.00
4.91Ž10.
5.31Ž50.
21.98
21.31
y1.49Ž50.
y1.91Ž50.
y36.68
y18.26
6
a
b
In MHz.
B˜e s Be y Ý4is1 a i Ž n i q d ir2..
MgOH. Although the parameters listed in Table 4
are mass dependent, such that their values must be
compared with some caution, it is noteworthy that
the second-order correction g i i and l-dependent g l l
term are considerably smaller for LiCCH than MgOH
or LiOH. This effect is particularly noticeable for
MgOH, which has similar B˜e and a i values to
LiCCH but a substantially larger g i i parameter Ž21.31
vs. 4.91 MHz. and g l l constant Žy18.26 vs. y1.49
MHz..
5. Discussion
The regular pattern exhibited by the Õ5 satellite
lines and the relatively small values of the secondorder ro-vibration parameters indicate that LiCCH is
a fairly rigid molecule. A negative g l l constant also
suggests absence of Fermi resonance interactions.
Quasilinear behavior does not appear to be particularly prominent in LiCCH, unlike MgOH w25x and
LiOH w11x. Rigidity in lithium monoacetylide likely
results from partial covalent bonding character and
sp hybridization of the carbon atom. On the other
hand, the bonding in the molecule certainly has a
considerable ionic component. Due to the acidic
nature of acetylenic hydrogens in HCCR, their
alkynes are known to form carbanions in solution
when treated with a strong base, such as lithium
metal in liquid ammonia. The resulting complex is
often written as Liq:CCRy w1x. However, this proposed structure is in solution phase and hence will be
surrounded by stabilizing solvent molecules. The
monomer in the gas-phase is in a different environment.
If LiCCH were totally an ionic species, a non-linear T-shaped structure might be expected. The highly
ionic alkali cyanide species such as KCN and NaCN
are actually triangular in shape w26,27x, exhibiting a
nondirectional polytopic bond where the Mq ion
orbits the CNy moiety w28x. These species are also
floppy molecules, meaning that the potential surface
around the minimum is flat in the bending direction.
Lithium cyanide is floppy as well, but in this case
the geometry is linear with the isocyanide configuration, LiNC. For this molecule, the long range attraction term manages to overcome the short-range repulsion, unlike KCN and NaCN w29x. Long-range
attractions favor the LiCN configuration, and the
isocyanide is a compromise between the forces. In
analogy to LiNC, LiCCH might be linear and still
very ionic; however, it should also be more floppy
than our data indicate.
Because various isotopic substitutions were carried out, ro , and rs structures w30x could be derived
for LiCCH. An ro geometry is calculated by using
the differing moments of inertia relationships found
for each isotopomer; rs structure use Kraitchman’s
equations that partially account for the difference in
moment of inertia when substituting a particular
atom in a given structure. Hence, the rs method
gives more reliable results because zero-point vibrations largely cancel. For both calculations, however,
the Li13 CCH isotopomer data could not be used,
because the substituted atom, 13 C, lies very close to
the center of mass, as determined from the centerof-mass equation. The change in moment is therefore
very small and is substantially affected by zero-point
vibration effects. ŽUsing this substituted structure in
Table 5
Bond lengths for LiCCH
Structure
r Li – C
˚.
ŽA
rC – C
˚.
ŽA
rC – H
˚.
ŽA
ro
rs
1.886
1.888
1.230
1.227
1.060
1.062
A.J. Apponi et al.r Chemical Physics Letters 298 (1998) 161–169
the calculations gave results that deviated significantly from other combinations of substitutions.. The
calculated bond lengths for LiCCH are presented in
Table 5. As is seen in these data, the ro and rs
˚
structures disagree to within only 0.002–0.003 A,
again evidence that this molecule is rigid. The C–H
bond length is virtually identical to that of acetylene,
˚ w31x. The C[C bond
which has rC – H s 1.060 A
distance, on the other hand, is longer in LiCCH
˚ . than in HCCH Ž1.204 A˚ .. This lengthening
Ž1.227 A
may result from having a larger atom Žlithium. and
hence electron cloud attached to the CCH group;
also, LiCCH must be partially ionic and the resonance structure LiqCCHy has some contribution,
again resulting in more electron density on the
acetylide group. The C[C bond therefore lengthens
to accommodate the extra density.
A final interesting discovery in this study is that
BrCCH is a more effective reactant than HCCH in
the gas phase as a source of the CCH group. Certainly in solution chemistry bromine is a much better
leaving group than a hydrogen atom. In fact, preliminary results in the work here suggest that similar
amounts of LiBr and LiCCH are formed when reacting lithium and BrCCH. Such data supports a mechanism for lithium acetylide formation that involves
extraction of Br by an Li atom, followed by the
reaction Li q CCH ™ LiCCH. An alternative process
could be insertion of lithium into the Br–C bond,
and then extraction of Br by an additional Li atom to
form LiBr q LiCCH.
Acknowledgements
This research is supported by NASA Grant
NAG5-3785 and NSF Grant CHE95-31244.
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