4 September 1998
Chemical Physics Letters 293 Ž1998. 397–404
˜ 1A/
The pure rotational spectrum of LiCH 3 ž X
M.D. Allen
a
a,b,1
, T.C. Pesch
a,b
, J.S. Robinson b, A.J. Apponi
L.M. Ziurys a,b,)
a,b
, D.B. Grotjahn
b,2
,
Department of Chemistry, Department of Astronomy and Steward ObserÕatory, UniÕersity of Arizona, 933 North Cherry AÕenue,
Tucson, AZ 85721-0065, USA
b
Department of Chemistry and Biochemistry, Arizona State UniÕersity, Tempe, AZ 85287-1604, USA
Received 27 March 1998; in final form 8 July 1998
Abstract
˜ 1A ground electronic state has been measured using
The pure rotational spectrum of lithium monomethyl, LiCH 3 , in its X
millimeterrsub-millimeter direct absorption techniques. Spectra of LiCD 3 and 6 LiCH 3 have been recorded as well. These
species were synthesized in the gas phase by the reaction of lithium vapor, produced in a Broida-type oven, and ŽCH 3 .4 Sn,
ŽCH 3 . 2 Hg or CH 3 I. Transitions were recorded in the 90–505 GHz range for J s 2 § 1 to J s 11 § 10 and K s 0 up to
K s 10. Rotational and centrifugal distortion constants have been determined for each isotopomer, as well as an ro structure
for LiCH 3. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction
Organolithium compounds are widely used in organic synthesis, for example to introduce a CH 3
group, or in the preparation method of Fischer-type
carbene complexes in conjunction with metal carbonyls w1x. Solvated methyllithium is one of the more
common compounds of this type used in organic
chemistry. The exact aggregation of methyllithium is
often unknown in this form, although in ether it is
thought to be a tetramer; therefore, the mechanism of
product formation using this reactant can be quite
vague. Experimental structures of solvent-free
monomeric organolithiums in fact have seldom been
)
Corresponding author. E-mail: [email protected]
Current address: NIST Frequency and Time Division, 325
Broadway, Boulder, CO 80302, USA.
2
Current address: Department of Chemistry, San Diego State
University, San Diego, CA 92182-1030, USA.
1
determined, even though such information is crucial
for the understanding of reaction mechanisms. In
contrast, a large number of theoretical structure calculations for monomers exist Že.g. w2–5x..
Small organometallic species have recently been
the topic of many spectroscopic investigations. For
example, various LIF measurements have been conducted for certain alkaline earth and transition metal
monomethyl radicals, including MgCH 3 w6x, CaCH 3
w7x, SrCH 3 w7x, ZnCH 3 w8x and CaCH 3 w9x, as well as
monoacetylide species such as CaCCH w10x and SrCCH w11x. Also, certain alkali and alkaline earth
cyaniderisocyanide molecules have been investigated by pure rotational spectroscopy using microwave and electron resonance techniques, including KCN w12x, CaNC w13x and MgNC w14x. In our
group, we have recorded the millimeterrsub-mm
spectra of several metal monomethyl, monoacetylide
and isocyanide species. These studies include the
alkaline earth monomethyls magnesium through bar-
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 0 8 1 0 - 0
M.D. Allen et al.r Chemical Physics Letters 293 (1998) 397–404
398
Table 1
˜ 1A. a
Transition frequencies for LiCH 3 , 6 LiCH 3 and LiCD 3 ŽX
X
J §J
2§1
3§2
4§3
5§4
6§5
7§6
8§7
9§8
10 § 9
K
1
0
2
1
0
3
2
1
0
4
3
2
1
0
5
4
3
2
1
0
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
8
7
6
5
4
3
2
1
0
9
8
7
6
5
4
3
6
LiCH 3
LiCH 3
LiCD 3
yobs
yobs y ycalc
yobs
yobs y ycalc
yobs
yobs y ycalc
91 766.738
91 779.012
137 587.508
137 642.652
137 661.027
183 314.016
183 436.179
183 509.581
183 534.084
228 907.485
229 120.407
229 272.929
229 364.563
229 395.158
274 329.995
274 656.882
274 912.040
275 094.841
275 204.657
275 241.314
319 544.773
320 008.063
320 388.824
320 686.022
320 898.873
321 026.827
321 069.507
364 516.102
365 137.470
365 666.018
366 100.304
366 439.338
366 682.182
366 828.091
366 876.766
409 210.181
410 010.057
410 707.622
411 300.934
411 788.477
412 169.070
412 441.637
412 605.496
412 660.146
463 594.748
454 592.433
455 479.061
456 252.233
456 909.962
457 450.491
457 872.297
0.024
0.037
y0.008
0.043
0.038
y0.027
0.023
0.030
0.047
y0.047
y0.033
0.017
0.009
0.031
y0.055
y0.070
y0.054
0.029
0.024
0.042
0.028
y0.038
y0.046
y0.034
y0.013
0.022
0.025
0.011
0.010
0.001
y0.065
y0.041
0.020
0.007
y0.001
0.042
0.039
0.060
0.011
y0.056
y0.044
y0.032
0.010
0.006
y0.015
0.012
0.029
0.009
0.021
0.046
y0.015
351 951.482
352 501.809
352 954.108
353 307.215
353 560.146
353 712.166
353 762.874
401 425.530
402 163.148
402 790.769
403 306.683
403 709.349
403 997.774
404 171.154
404 229.059
450 570.836
451 520.027
452 347.978
453 052.424
453 631.415
454 083.375
454 407.090
454 601.629
454 666.610
499 349.175
500 532.899
501 585.007
502 502.530
503 283.157
503 924.797
504 425.660
y0.093
y0.091
y0.091
y0.036
0.037
0.072
0.073
0.074
y0.056
y0.090
y0.034
y0.036
y0.005
0.028
0.099
0.211
0.086
0.012
y0.010
y0.016
y0.012
0.006
y0.021
0.046
y0.189
0.027
0.121
0.028
y0.048
y0.068
y0.080
268 867.872
269 126.138
269 337.776
269 502.791
269 620.907
269 691.648
269 715.270
306 895.015
307 242.353
307 537.030
307 778.701
307 966.950
308 101.619
308 182.417
308 209.433
344 764.037
345 212.869
345 602.950
345 933.811
346 205.155
346 416.486
346 567.623
346 658.396
346 688.617
0.052
0.031
y0.095
y0.057
0.071
y0.039
y0.044
0.021
y0.043
y0.057
0.003
0.020
0.070
0.030
0.089
y0.025
0.026
0.023
y0.013
0.033
0.004
y0.019
y0.017
y0.065
M.D. Allen et al.r Chemical Physics Letters 293 (1998) 397–404
399
Table 1 Žcontinued.
X
J §J
11 § 10
a
6
K
LiCH 3
LiCH 3
LiCD 3
yobs
yobs y ycalc
yobs
yobs y ycalc
2
1
0
10
9
8
7
6
5
4
3
2
1
0
458 174.418
458 356.017
458 416.609
496 639.184
498 853.262
499 947.638
500 920.323
501 768.640
502 490.254
503 083.190
503 546.032
503 877.489
504 076.721
504 143.189
y0.018
y0.010
y0.001
0.038
0.026
y0.063
y0.044
0.021
0.057
0.003
0.008
y0.003
0.001
0.001
504 784.484
505 000.129
505 072.054
0.008
0.025
0.009
yobs
yobs y ycalc
In MHz.
ium w15–18x, NaCH 3 w19x and AlCH 3 w20x, and
various alkali ŽNaCCH w21,22x, KCCH w23x. and
alkaline earth monoacetylides ŽMgCCH w24x, CaCCH
w25x, SrCCH w26x..
Although organolithium compounds figure prominently in organic chemistry, almost all structural
studies of such species have been X-ray or electron
diffraction measurements of dimers or tetramers; the
only monomers investigated have had extremely large
organic groups w27x. Gas-phase spectroscopy of such
compounds is virtually non-existent. For lithium
monomethyl, the simplest of all these molecules, the
only spectroscopic data available up to the present
have been infrared vibrational measurements of the
species conducted while frozen in an argon matrix
w28x.
Here we report the first high-resolution gas-phase
spectroscopy of LiCH 3 . The pure rotational spectrum
˜ 1A electronic state was
of this species in its ground X
recorded using millimeterrsub-mm wave direct absorption techniques. Frequencies for 10 rotational
transitions in all possible K ladders Ž K ( J . were
measured; several transitions were also recorded for
LiCD 3 and 6 LiCH 3 . From these data, very accurate
rotational and centrifugal distortion constants have
been derived for all three isotopomers. An ro structure was additionally obtained. A preliminary estimate of this structure was presented for organic
chemists in an earlier paper w29x. In this work the
spectroscopy of LiCH 3 is described in detail and
comparison of the bonding is made with other lithium
molecules.
2. Experimental
The measurements of lithium monomethyl and the
Li and deuterium isotopomers were taken using one
of the millimeterrsub-mm-wave direct absorption
spectrometers of the Ziurys group, which will be
described in detail elsewhere w30x. This spectrometer
uses phase-locked Gunn oscillators as the radiation
source, which range in frequency from 65 to 140
GHz. Schottky diode varacter multipliers are used to
double, triple and quadruple the Gunn frequency to
give an overall coverage of 65–525 GHz. The reaction chamber is double pass cell 0.7 m in length with
a polystyrene foam-backed mylar window at one end
and a rooftop reflector at the other. The radiation
from the source is propagated quasi-optically to the
chamber by two offset spherical mirrors. The radiation is focused to a waist at the rooftop reflector,
where the plane of polarization is rotated by 908, and
is subsequently reflected back through the chamber
and propagated into the detector by the two mirrors
and a wire grid. The detector used is a liquid
helium-cooled InSb bolometer. Phase-sensitive detection is achieved by FM modulation of the radia6
400
M.D. Allen et al.r Chemical Physics Letters 293 (1998) 397–404
tion source. Demodulation at 2f gives the spectra
their second derivative form. After demodulation, the
signal is processed using a PC computer through an
IEEE-488 interface.
7
LiCH 3 was synthesized by reacting tetramethyltin Ž; 3 mTorr. with lithium vapor Ž; 1 mTorr.,
produced by heating metal shot in a Broida-type
oven. The tetramethyltin was added through the bottom of the oven and acted both as the reactant and
the carrier gas. Unlike our usual synthetic methods,
an inert gas was not used in the reaction mixture. No
dc discharge was found necessary for LiCH 3 production. Several other methyl donating precursurs were
used to confirm the identity of LiCH 3 , such as
dimethylmercury ŽHgŽCH 3 . 2 . and methyliodide
ŽCH 3 I.. The deuterated forms of these two compounds were used for the production of 7 LiCD 3 .
HgŽCD 3 . 2 was not readily commercially available
and hence was synthesized by the reaction of
CD 3 MgBr and HgCl 2 in diethyl ether. A slightly
higher pressure of ; 7 mTorr had to be used for
CH 3 I and CD 3 I. 6 LiCH 3 was produced under simi-
lar conditions using 98% enriched 6 Li ingot ŽCambridge Isotopes. in the Broida oven.
The frequency measurements were accomplished
by fitting Gaussians to the line profiles from scans 5
MHz in coverage. The 5 MHz scans were averages
of two or four scans, taken in pairs of increasing and
decreasing frequency. The linewidths ranged from
200 to 900 kHz over the range of 90–505 GHz.
3. Results and analysis
The methyl lithium series have a C 3v symmetry
axis and are prolate symmetric top molecules with 1A
ground electronic states. Hence, the rotational quantum number is J and the quantized projection of the
rotational angular momentum along the symmetry
axis results in K-ladder structure Ž K s J, J y
1, . . . , 0. for every transition. Since all the species
are closed-shell, fine and hyperfine structure do not
exist, except for I˜P R˜ hyperfine interactions which
are generally too small to be resolved. On the other
˜ 1A ground state near 504 GHz, showing the four lowest energy
Fig. 1. Spectrum of the J s 11 § 10 rotational transition of 7 LiCH 3 in its X
K-components, K s 0–3. The splitting of K-components clearly indicates a symmetric top species. This spectrum was created from a
composite of seven 100 MHz scans, each 1 minute in duration.
M.D. Allen et al.r Chemical Physics Letters 293 (1998) 397–404
hand, the 7 Li, 6 Li and deuterium nuclei have spins of
3r2, 1 and 1, respectively. Hence, nuclear quadrupole
interactions may be detectable in the lower rotational
transitions, which were recorded for 7 LiCH 3 only.
However, none were observed, although they still
may be resolved in the J s 1 § 0 line, whose frequency was out of the range of our spectrometer.
401
The transition frequencies measured for 7 LiCH 3 ,
LiCH 3 and LiCD 3 are listed in Table 1. As the
table shows, all possible K components for each
rotational transition were recorded. For the main
isotope, 10 transitions were measured, as well as
four transitions for 6 LiCH 3 and three for LiCD 3 .
The splittings between the K-components within a
6
Fig. 2. Spectra of the J s 10 § 9 rotational transition of 6 LiCH 3 near 505 GHz Žtop panel. and the J s 9 § 8 transition of LiCD 3 near 346
˜ 1A ground states. Again, the K s 0,1,2 and 3 components are shown in each spectrum. Several weak
GHz Žbottom panel., both in the X
contaminant lines arising from the precursor material are visible in the LiCD 3 data. Each spectrum is a composite of several 100 MHz scans,
each 1 minute in duration.
402
M.D. Allen et al.r Chemical Physics Letters 293 (1998) 397–404
transition followed the expected ratio of 1:3:5:7, etc.,
for K s 0,1,2,3,4, . . . . Small deviations from this
pattern are accounted for by the centrifugal distortion
parameters DJK , HK J and H JK . The intensity ratios
also were typical for a symmetric top, with the
K s 3,6,9, . . . 3n components being stronger by about
a factor of two as opposed to nearby K / 3n lines,
as expected from spin statistics of the methyl hydrogens or deuteriums.
Typical data for the methyl lithium species are
shown in Figs. 1 and 2. In Fig. 1, the K s 0,1,2 and
3 components of the J s 11 § 10 transition of LiCH 3
near 504 GHz are displayed. The typical symmetric
top pattern is very apparent in this data, and K s 3
line is noticeably stronger than the others. This
spectrum is a composite of seven separate scans,
each one minute in duration and 100 MHz in frequency coverage. A baseline was subtracted from
each individual scan before the composite was made.
Fig. 2 presents the K s 0–3 components of the
J s 9 § 8 transition of LiCD 3 and the J s 10 § 9
transition of 6 LiCH 3 . The same pattern is visible as
in Fig. 1. The spectrum of the deuterium isotopomer
has many weak, unidentified features which likely
arise from contaminants in the deuterated dimethyl
mercury, which was difficult to purify. The LiCD 3
spectrum is a composite of four 100 MHz scans,
each lasting one minute, and the lithium isotopomer
consists of seven such scans.
The data for the three isotopomers in Table 1
were separately analyzed using a 1A Hamiltonian,
which involves only molecular frame rotation and its
centrifugal distortion corrections. The resulting spectroscopic parameters are presented in Table 2 for all
three species. Because no cross-ladder transitions
were measured, constants depending solely on quantum number K could not be determined, including
DK and rotational parameter A. Five constants were
found necessary to fit the data, including two
higher-order centrifugal distortion terms H JK and
HK J . All constants appear to be well-determined and
reproduce the observed frequencies with residuals of
nobs y ncalc - 100 kHz, with the exception of three
measurements of high K components. There are no
other experimental constants available in the literature for comparison. However, a theoretical value of
B ; 21800 MHz has been calculated for the rotational constant of LiCH 3 w2x which compares favor-
Table 2
˜ 1A. a
Molecular constants for LiCH 3 , 6 LiCH 3 and LiCD 3 ŽX
Parameter
LiCH 3
6
Bo
DJ K
DJ
HJ K
HK J
22 945.7401Ž22.
3.06741Ž12.
0.124548Ž12.
0.00018804Ž60.
0.0006587Ž12.
25 283.346Ž15.
3.64665Ž63.
0.148718Ž96.
0.0002440Ž41.
0.0008065Ž54.
LiCH 3
LiCD 3
19 272.878Ž16.
1.69696Ž72.
0.07652Ž11.
0.0000935Ž55.
0.0001740Ž67.
a
In MHz; all errors quoted are 3s and apply to the last quoted
decimal places.
ably with the actual value of Bo s 22945.7401Ž22.
MHz.
4. Discussion
Measuring the pure rotational spectrum and determining accurate rotational constants for LiCH 3 in its
ground electronic and vibrational states lends some
insight into the bonding characteristics of organolithium compounds. Using the data from all three
isotopomers of lithium monomethyl, an ro structure
can be determined, which consists of two bond
distances Ž r Li – C and rC – H . and the H–C–H bond
angle Ž u H – C – H .. The results of this analysis are
shown in Table 3, along with values from ab initio
calculations of Streitweiser et al. w2x and Wiberg and
Breneman w3x. As the table shows, the theoretical
studies predict a slightly longer lithium–carbon bond
length than the experimental work Ž2.001–2.021 vs.
˚ ., and a somewhat shorter C–H bond dis1.959 A
˚ .. On the other hand,
tance Ž1.089–1.093 vs. 1.111 A
the theoretical and experimental bond angles agree
extremely well Žto within 0.48., and the overall
Table 3
Comparison of structures for LiCH 3
Molecule
Source
r Li-C
˚.
ŽA
rC-H
˚.
ŽA
u H-C-H
Ž8.
LiCH 3
this work
ab initio w2x a
ab initio w3x b
ab initio w5x c
1.959
2.021
2.001
1.884
1.111
1.089
1.093
-
106.2
105.8
106.2
-
LiC
a
SSqd basis set.
6-31 G) basis set.
c
MRCIqCPP method.
b
M.D. Allen et al.r Chemical Physics Letters 293 (1998) 397–404
403
Table 4
Lithium–ligand bond lengths
Species
r Li-Ligand
˚.
ŽA
Method
Reference
LiH
LiF
LiOH
LiC
LiCCH
LiCH 3
LiCHŽSi Me 3 . 2
1.595
1.564
1.594
1.884
1.888
1.959
2.03
millimeter-wave, re structure
millimeter-wave, re structure
millimeter-wave, ro structure
ab initio; MRCIq CPP
millimeter-wave, rs structure
millimeter-wave, ro structure
electron diffraction
w32x
w33x
w34x
w5x
w35x
this work
w27x
agreement for the LiCH 3 structure is good. In contrast, past estimates of the lithium–carbon bond
length from argon matrix work w28x and that found
for the LiCH 3 tetramer w31x are significantly longer:
˚ respectively. Our work suggests a
2.10 and 2.31 A,
˚ as found by theory. The
value very close to 2.0 A,
lengthening of the Li–C bond in the argon matrix
data may result from stabilization of the dipole in
that medium.
Also given in Table 3 is the ab initio value of the
lithium–carbon bond distance in LiC, which is predicted to have a 4 Sy ground electronic state. This
comparison is of interest because both LiC and
LiCH 3 are thought to have predominantly ionic
bonding w3,5x. Curiously, the bond length in lithium
˚ which is even
carbide is calculated to be 1.884 A,
shorter than that of LiCH 3 . LiC has three unpaired
electrons, w ith an electron configuration
1s 2 2 s 11p 2 . The presence of single electrons in the
three carbon orbitals, as opposed to bond pairs,
likely explains the somewhat shorter bond for lithium
carbide. The lithium atom is able to get closer to the
single carbon atom as opposed to the methyl group.
In Table 4, a comparison of lithium-ligand bond
lengths is presented. Most of the bond distances in
this table were derived from millimeter-wave rotational data so this comparison is consistent, although
some values come from re and rs structures as
opposed to ro ones w32–35x. A trend is clearly
visible in these bond lengths. The highly ionic
molecules LiF and LiOH have fairly short bond
˚ Changing the ligand to a group
lengths near 1.60 A.
that is less ionic than fluorine or hydroxide but with
a similar amount of steric hindrance, such as carbon
atom or the acetylide group, lengthens the bond to
˚ Finally, as the steric hindrance innear 1.89 A.
creases with the substitution of methyl and bis-Žtrimethylsilyl. methyl as ligands, the bond distance to
˚ It is interestlithium further increases to near 2.0 A.
ing to note that even with as large a group as
CHŽSiMe 3 . 2 , the lithium-ligand bond distance is still
much shorter than that found in the LiCH 3 tetramer
˚ .. Generalization from oligomeric structures
Ž2.31 A
to individual molecules must be done with some
caution.
Our work on LiCH 3 suggests that the structures
of other organolithium compounds might be accessible by gas-phase high-resolution spectroscopy. Very
recently we have measured the mm-wave spectrum
of LiCCH w35x, as well. Studies of LiC and LiCH 2
are in progress.
Acknowledgements
This research was supported by NASA Grant
NAG5-3785 and NSF Grant CHE95-31244.
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