JOURNAL OF CHEMICAL PHYSICS
VOLUME 110, NUMBER 7
15 FEBRUARY 1999
Structural studies of alkali methylidyne radicals: High resolution
spectroscopy of NaCH and KCH „ X̃ 3 S 2 …
J. Xin and L. M. Ziurys
Department of Chemistry, Department of Astronomy and Steward Observatory, 933 North Cherry Avenue,
The University of Arizona, Tucson, Arizona 85721-0065
~Received 21 August 1998; accepted 30 October 1998!
High resolution spectroscopic measurements have been carried out for alkali methylidyne radicals.
The pure rotational spectra of NaCH and KCH, along with their deuterium isotopomers, have been
recorded in the frequency range 328–529 GHz using millimeter/submillimeter direct absorption
techniques. These molecules were created in a dc discharge by the reaction of metal vapor and CH4
or CD4. These data indicate that KCH and NaCH are linear molecules with 3 S 2 ground electronic
states arising from a p 2 configuration. Spectroscopic constants for KCH and NaCH have been
determined from the data, including rotational, spin–spin, and spin–rotation parameters, as well as
bond lengths. In comparison with other alkali and transition metal-bearing molecules, these results
suggest some degree of covalent bonding in the alkali methylidynes, with carbon atom undergoing
sp hybridization. © 1999 American Institute of Physics. @S0021-9606~99!02605-7#
I. INTRODUCTION
only experimental structural data available for the molecule
until very recently was an x-ray crystal analysis of its tetramer form,15 and low resolution argon matrix infrared spectroscopic measurements.16 The theoretical values for the
lithium–carbon bond length of 1.969–2.021 Å10,11 varied
quite significantly from the x-ray value of 2.31 Å15 and the
matrix value of 2.10 Å.16 These differences were not reconciled for almost thirty years.
High resolution gas–phase spectroscopy offers an avenue by which accurate structures can be obtained for a
given organometallic species in its monomeric form, without
any solvent or neighboring group effects. Such spectroscopy
also can provide information about the bonding in a molecule. Past spectroscopic investigations of organoalkali compounds have been limited, even using optical methods, although many such studies have been carried out for alkaline
earth and transition metal compounds. For example, the Bernath group has measured electronic transitions of various
calcium and strontium alkyl species such as CaCCH, SrCH3,
and SrOCH317–19 using laser-induced fluorescence, and
Miller and collaborators have investigated magnesium, calcium, and transition metal monomethyl and half-sandwich
complexes20–22 with LIF/laser ablation techniques. The Steimle group have also been active in this area, recording spectra of the Ã→X̃ transition of CaNH2, for example Ref. 23.
Very recently, Merer and coinvestigators have obtained electronic spectra of transition metal methylidyne species of the
general formula M–CH, with metals V, Ti, Zr, and Ta.24,25
Although metal carbyne compounds have been known for
some time, they usually exist as large, multiligand
complexes.26 The work of Merer et al. was the first isolation
of the simplest of metal-CR species.
Recently, we have begun investigating simple organoalkali molecules using millimeter/submillimeter direct absorption techniques. In particular, we have focused on obtaining
the pure rotational spectrum of lithium, sodium, and potas-
The interaction of metals with organic molecules is one
of the major themes in modern chemistry. Organometallic
compounds have a wide use in preparatory techniques,1,2
play a major role in many homogeneous and heterogeneous
catalysis processes,3,4 and are fundamental for biological
systems.5 Of particular interest are organic species containing alkali metals, especially those involving lithium, sodium,
and potassium.6 The usefulness of organolithium compounds
as synthetic reageants has long been documented.7 More recently, sodium and potassium organic compounds have become significant in developments in superbase chemistry.8
For example, mixed reageants containing butyllithium and a
potassium or sodium alkoxide have been found to have exceptional metallating ability.
Compared to their long history in synthetic applications,
the investigation of the structure and bonding in organoalkali
compounds and organometallic species in general has been
much shorter. Such information, however, is essential in understanding the physical behavior of such reageants, their
reaction mechanisms, and which bond types are possible.9
The most common methods employed to obtain structural
and bonding information in organometallic compounds are
x-ray crystallography, or theoretical calculations,10,11 although a few species have been investigated in an argon
matrix.12,13 Ab initio computations deal with individual molecules, but studies in the solid or solution state are subject to
subtle neighboring group and solvent effects and therefore
concern oligomeric structures. Evaluating the properties of
even the simplest organoalkali compounds experimentally in
their monomeric and most fundamental form has been difficult.
A prime example of the problems in obtaining accurate
structures is the case of methyllithium, LiCH3. Although
many theoretical calculations of this molecule have been carried out over several decades ~e.g., Refs. 10, 11, and 14!, the
0021-9606/99/110(7)/3360/8/$15.00
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© 1999 American Institute of Physics
J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
J. Xin and L. M. Ziurys
TABLE I. Observed transition frequencies for NaCH and KCH (X̃ 3 S 2 ). a
NaCH
N 8 ←N 9 J 8 ←J 9
n obs
NaCD
n obs2 n calc
n obs
KCH
n obs2 n calc
n obs
KCD
n obs2 n calc
n obs
n obs2 n calc
15←14
16←15 339 264.598
15←14 339 286.033
14←13 339 310.762
0.019
20.015
0.055
-------
-------
-------
-------
-------
-------
16←15
17←16 361 828.480
16←15 361 848.725
15←14 361 871.620
20.024
20.032
0.005
-------
-------
-------
-------
-------
-------
17←16
18←17 384 381.298
17←16 384 400.637
16←15 384 421.894
20.024
0.080
20.047
-------
-------
-------
-------
-------
-------
18←17
19←18 406 922.395
18←17 406 940.848
17←16 406 960.817
0.001
0.081
20.112
-------
-------
-------
-------
-------
-------
19←18
20←19 429 451.093
19←18 429 468.733
18←17 429 487.864
0.027
0.028
0.021
-------
-------
-------
-------
-------
-------
20←19
21←20 451 966.669
20←19 451 983.638
19←18 452 001.991
20.013
20.051
0.031
-------
-------
-------
-------
-------
-------
21←20
22←21 474 468.543
21←20 474 485.057
20←19 474 502.566
20.034
0.020
20.002
-------
-------
-------
-------
-------
-------
22←21
23←22 496 956.056
22←21 496 972.038
21←20 496 988.919
20.029
20.030
20.043
439 155.189
439 169.776
439 185.007
0.008
0.013
0.012
328 736.419
328 746.435
328 757.199
0.050
0.003
0.003
-------
-------
23←22
24←23 519 428.601
23←22 519 444.040
22←21 519 460.524
0.067
20.059
0.083
459 033.609
459 047.800
459 062.537
20.020
20.036
20.025
343 626.845
343 636.584
343 647.034
0.036
20.002
0.058
-------
-------
24←23
25←24
24←23
23←22
-------
-------
478 900.848
478 914.669
478 928.970
0.040
20.014
0.003
358 510.146
358 519.664
358 529.735
20.021
20.027
20.020
-------
-------
25←24
26←25
25←24
24←23
-------
-------
498 756.218
498 769.834
498 783.759
20.017
0.016
0.047
373 386.160
373 395.418
373 405.239
0.019
20.023
0.021
-------
-------
26←25
27←26
26←25
25←24
-------
-------
518 599.418
518 612.768
518 626.265
20.010
0.017
20.036
388 254.381
388 263.517
388 273.045
20.048
20.013
20.009
-------
-------
27←26
28←27
27←26
26←25
-------
-------
-------
-------
403 114.758
403 123.618
403 132.948
0.031
20.032
20.002
-------
-------
28←27
29←28
28←27
27←26
-------
-------
-------
-------
417 966.782
417 975.511
417 984.608
0.049
0.016
0.012
-------
-------
29←28
30←29
29←28
28←27
-------
-------
-------
-------
432 810.105
432 818.757
432 827.694
20.037
20.003
0.012
-------
-------
30←29
31←30
30←29
29←28
-------
-------
-------
-------
447 644.626
447 653.093
447 661.876
20.024
20.044
20.023
-------
-------
31←30
32←31
31←30
30←29
-------
-------
-------
-------
462 469.927
462 478.339
462 486.936
20.024
0.019
20.001
-------
-------
32←31
33←32
32←31
31←30
-------
-------
-------
-------
477 285.715
477 293.988
477 302.492
20.027
20.014
0.003
423 437.023
423 444.510
423 452.126
20.030
0.022
0.009
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J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
J. Xin and L. M. Ziurys
TABLE I. ~Continued)
NaCH
NaCD
N 8 ←N 9
J 8 ←J 9
n obs n obs2 n calc n obs n obs2 n calc
33←32
34←33
33←32
32←31
-------
-------
-------
34←33
35←34
34←33
33←32
-------
-------
35←34
36←35
35←34
34←33
-------
36←35
37←36
36←35
35←34
37←36
KCH
KCD
n obs
n obs2 n calc
n obs
n obs2 n calc
-------
492 091.721
492 099.889
492 108.201
0.005
0.012
20.044
436 589.688
436 597.086
436 604.612
20.061
20.015
20.017
-------
-------
506 887.577
506 895.702
506 903.896
0.009
0.063
20.003
449 735.073
449 742.374
449 749.725
0.022
0.047
20.037
-------
-------
-------
521 673.009
521 680.973
521 689.168
0.016
20.008
0.026
462 872.727
462 880.003
462 887.277
20.009
0.062
20.016
-------
-------
-------
-------
-------
-------
476 002.579
476 009.713
476 016.983
20.002
20.008
20.013
38←37
37←36
36←35
-------
-------
-------
-------
-------
-------
489 124.322
489 131.483
489 138.677
20.039
0.042
0.031
38←37
39←38
38←37
37←36
-------
-------
-------
-------
-------
-------
502 237.924
502 244.935
502 251.988
0.069
0.055
20.032
39←38
40←39
39←38
38←37
-------
-------
-------
-------
-------
-------
515 342.788
515 349.877
515 356.872
20.050
0.065
20.020
40←39
41←40
40←39
39←38
-------
-------
-------
-------
-------
-------
528 439.061
528 445.993
528 453.006
20.026
20.020
20.032
a
In MHz.
sium bearing species, such as NaCH3, 27 LiCCH,28 and
KCH3. 29 Our recent investigation of LiCH3, for example, has
determined the geometry of the monomer.30 Interestingly,
the structure obtained in this study is in very good agreement
with the theoretical predictions, including the Li–C bond
length, measured to be 1.959 Å.
In this article, we present the pure rotational spectrum of
two new organoalkali species, sodium methylidyne, and potassium methylidyne. This study is the first detection of either molecule by any spectroscopic technique. Spectra of
both deuterium isotopomers were measured as well. Fine
structure splittings were resolved in these spectra, and analysis of the data shows that both alkali radicals are linear with
3
S ground electronic states. The rotational transition frequencies of NaCH were presented previously in the astronomical literature.31 In this article the spectroscopic measurements of both methylidyne molecules, their analysis, and
calculation of their structures are described. A discussion of
the bonding of these unusual radicals and its chemical implications are additionally presented.
II. EXPERIMENT
The rotational transitions of the alkali methylidyne radicals were measured using one of the millimeter/
submillimeter spectrometers of the Ziurys group.32 This instrument consists of a phase-locked Gunn oscillator/varacter
multiplier source, a double-pass reaction chamber, and a
helium-cooled InSb bolometer for a detector. Radiation is
propagated through the system quasioptically using a series
of Teflon lenses and a polarizing grid. FM modulation of the
source enables phase-sensitive detection. For more details,
see Ziurys et al.32
The alkali methylidyne radicals were created in a dc discharge by the reaction of metal vapor and CH4. The vapor
was produced in a Broida-type oven attached to the bottom
of the reaction cell. For both NaCH and KCH, about 20–25
mTorr of argon carrier gas was mixed with about 8 mTorr of
methane, and flowed into the cell through the bottom of the
oven. The metal vapor/argon/CH4 mixture was then discharged using a current of 500–700 mA at 200 V. In the case
of sodium, the discharged gas glowed bright yellow–orange,
likely resulting from sodium D-line emission. For potassium,
the discharge plasma was a pale purple in color, mostly due
to argon. Almost identical conditions were used to create the
deuterium isotopomers, except a slightly lower pressure of
the precursor gas, CD4, was used ~;6 mTorr! because of its
higher cost.
The discharge conditions to produce the methylidynes
were established from previous tests done with NaCH3. It
was found that weak signals from sodium monomethyl could
be produced from the reaction of sodium vapor and methane
using about 10–30 mA at 200 V. ~In our original study of
this molecule,27 dimethyl mercury was used as the source of
the methyl group.! Hence, under these conditions, one hydro-
J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
J. Xin and L. M. Ziurys
3363
gen atom was apparently being removed from the CH4 molecule. Increasing the current to over 200 mA caused the
NaCH3 signal to disappear altogether; it therefore appeared
that more than one H atom was coming off methane. On
increasing the current to 500 mA, triplet features arising
from NaCH became visible.
All transition frequencies were determined from an average of two scans, each 5 MHz in width and 5 s in duration,
and one recorded in increasing frequency and the other in
decreasing frequency. The line profiles were fit with Gaussian functions to establish their center. Typical linewidths
were 900 kHz, and estimated experimental accuracy is better
than 6100 kHz.
III. RESULTS
FIG. 1. Spectra of the N531←30 transition of KCH near 462.5 GHz and
the N535←34 transition of KCD at 462.9 GHz recorded in this work. A
closely spaced triplet pattern, indicating a 3 S ground state with a near case
~b! coupling scheme, is readily visible in these data. The individual fine
structure components are indicated by quantum number J. Each spectrum
covers 90 MHz in frequency and was recorded in approximately 50 s.
The transition frequencies recorded for NaCH, KCH,
and their deuterium isotopomers are listed in Table I. As is
evident from the table, nine transitions were measured for
NaCH and KCD, while 14 were observed for KCH, and five
for NaCD. Every transition was found to consist of relatively
closely spaced triplets which were resolved in the measurements. The splittings are largest for the lowest N transitions
~;46 MHz for NaCH:N515←14!, and decrease with increasing frequency and N quantum number to as small as 14
MHz for the N540←39 line of KCD. This regular triplet
pattern had to arise from fine structure interactions and indicated that the alkali methylidynes have 3 S ground electronic
states that follow a case ~b! coupling scheme. Hence, the
preferred rotational quantum number is N while J labels the
fine structure components, and J5N1S such that J5N21,
N, N11. The individual fine structure energy levels are indicated by F 1 (N11), F 2 (N), and F 3 (N21). The triplet
pattern arises because the most favorable electric dipole transitions are for DN5DJ561. Hyperfine splittings attributable to the metal nuclei (I53/2) or the H or D nuclei (I
5 21 , I51) were not observed.
Figure 1 displays representative spectra for KCH and
KCD. In this figure, the N531←30 transition of KCH near
462.5 GHz and the N535←34 line of KCD at 462.9 GHz
are shown. The triplet pattern is readily apparent in both
spectra, and the fine structure components are labeled by
quantum number J. These spectra cover 90 MHz in frequency and were recorded in approximately 50 s. A baseline
has also been removed from the data.
In Fig. 2, the N521←20 transition of NaCH near 474.5
GHz and the N524←23 line of NaCD at 479.0 GHz, are
displayed. Again, the three fine structure lines within each
transition are visible. These data were produced from 50 s,
80 MHz scans, with a baseline removed for presentation.
IV. ANALYSIS
FIG. 2. Spectra of the N521←20 transition of NaCH near 474.5 GHz and
the N524←23 lines of NaCD at 479.0 GHz, measured in this study. Again,
the fine structure triplets for each transition are visible in the data, evidence
that the ground electronic state for sodium methylidyne is 3 S. These spectra
cover 80 MHz in frequency and the scan duration was about 50 s.
The data for NaCH, KCH, and their deuterium isotopomers were modeled with a 3 S Hamiltonian, which consists
of three major terms: rotation and the two fine structure interactions, spin–spin and spin–rotation, i.e.,
Ĥ eff5Ĥ rot1Ĥ ss1Ĥ sr .
~1!
3364
J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
J. Xin and L. M. Ziurys
Implicit in these three symbols are their centrifugal distortion
corrections. The individual terms in this effective Hamiltonian are:33
Ĥ rot5BN2 2DN4 ,
~2!
Ĥ ss52/3l ~ 3S 2Z 2S2 ! 11/3l D @ N2 ~ 3S 2Z 2S2 !
1 ~ 3S 2Z 2S2 ! N2 # ,
Ĥ sr5 g N–S1 g D N2 ~ N–S! ,
~3!
~4!
where B and D are the rotational and centrifugal distortion
constants, l and l D are the spin–spin parameter and its centrifugal distortion correction, and g and g D the spin–rotation
and corresponding distortion terms. Here Ĥ ss is written in
case ~a! notation, where S Z is defined, because the case ~b!
form is extremely complicated when expressed in Cartesian
coordinates. Using this effective Hamiltonian, a computer
program was constructed modeled in a case ~b! basis, uNSJ&,
using spherical tensor algebra. Eigenvalues were determined
from diagonalization of the matrix representation of Ĥ eff .
In order to carry out this analysis, the ordering of the fine
structure energy levels in NaCH and KCH had to be established. For a good case ~b! molecule, the J5N(F 2 ) level is
highest in energy for l.0, which is usually the situation for
a ground 3 S state. Either of the other two levels, J5N
21(F 3 ) and J5N11(F 1 ), may be lower in energy depending on the relative values and signs of l and g and the rotational quantum number N ~e.g., Ref. 34!. At the same time,
in a 3 S 2 ground electronic state, for fine structure levels
with the same quantum number J, the V50 ~e parity! component must lie lower in energy than the V51 ~e and f
parity! components. This behavior occurs because of interac-
tions with the isoconfigurational 1 S 1 state, which perturbs
only the V50 level. The effect on rotational levels is illustrated in Fig. 3, which shows the 3 S 2 energy level ordering
in case ~a! and case ~b! coupling schemes and their correlation. The 3 S 2
0e level @case ~a! scheme# lies lower in energy
3 2
2
relative to the 3 S 2
1 f and S 1e components. ~A p configuration, which is found for the alkali methylidynes, gives rise to
a 3 S 2 , not 3 S 1 state, as will be discussed later.!
To actually fit the data, J quantum numbers were randomly assigned to the observed transitions and then energy
levels predicted on that basis and compared with the pattern
in Fig. 3. ~The signal-to-noise ratio was not sufficient to
assign quantum numbers on the basis of relative intensity.!
The J assignment was changed and the procedure repeated
until the energy level ordering fit the theoretically expected
one and a satisfactory rms was achieved. For KCH, KCD,
and NaCD, sufficiently high N transitions were recorded
such that F 1 was always the lowest energy level. In contrast,
for NaCH, the lowest level was F 3 for the N515←14 transition, but changed to F 1 at the N516←15 line and higher.
The spectroscopic parameters derived from the analysis
for NaCH, KCH, and their deuterated forms are given in
Table II. B, D, l, and g were needed to fit the data for all
four species, as well as l D for NaCH and KCH. The centrifugal distortion correction to the spin–rotation constant,
g D , was not found necessary. The constants in general are
well-determined except for l, which has large errors associated with it. These errors are not unexpected because only
higher N transitions with DJ511 were recorded. On the
other hand, the rms error of the analyses are quite small ~47
kHz or less!, and the residuals within experimental uncertainty, with n obs2 n calc<83 kHz for all data with the exception of one measurement.
V. DISCUSSION
FIG. 3. Energy level diagram for a 3 S 2 electronic state showing level
ordering and the correlation between case ~b! and case ~a! coupling
schemes. In a 3 S 2 state, the V50 ~e! sublevel must lie lower in energy
relative to the V51 ~e and f! levels, for a given J quantum number. ~In this
figure, the case of J5N is indicated.!
Metal methylidyne species have been known in organic
chemistry for over two decades since Fischer’s discovery of
transition–metal alkylidyne molecules in 1973.35 However,
these molecules generally involve octahedral complexes with
the CH or CR group being one out of six other ligands. As
mentioned, experimental investigations of metal methylidyne
compounds with the formula M–CH have been confined to
gas–phase optical spectroscopy of the Merer group.24,25
These authors have examined species of this type with transition metals and found them to be linear molecules, with a
triple bond between the carbon and the metal atoms. The
triple bond occurs in these cases because of the existence of
d orbitals in the metals.
A similar triple bond structure for alkali methylidynes is
not possible. For sodium, any d orbitals are much higher in
energy than the 3s or 3 p, and for both NaCH and KCH there
are not enough valence electrons to form triple bonds. It is
not obvious then that the alkali methylidynes would be linear
in their ground electronic state, or that they would have two
unpaired electrons. From the covalent viewpoint, their structure suggests that the carbon atom in the alkali methylidyne
species is sp hybridized, with the unpaired electrons residing
in the remaining two p orbitals ~or p! orbitals. A 3 S 2 elec-
J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
J. Xin and L. M. Ziurys
3365
TABLE II. Molecular constants for NaCH and KCH (X̃ 3 S 2 ). a
Parameter
~MHz!
NaCH
NaCD
KCH
KCD
B0
D0
g
l
lD
11 322.3175~34!
0.0284078~41!
210.76~13!
9120~460!
0.0013~17!
10 000.8136~43!
0.0203355~36!
29.75~24!
9000~2900!
0.0032~11!
7483.87069~94!
0.01276949~51!
26.550~55!
8240~740!
---
6635.4223~23!
0.00932723~86!
25.98~19!
8100~2000!
---
rms
of fit
0.047
0.025
0.027
0.037
a
All uncertainties are 3-sigma and apply to the last quoted decimal places.
tronic state is the result. A bent geometry would result from
sp 2 hybridization, with a lone pair filling the third orbital in
a trigonal planar structure. The carbon could be s p 3 hybridized as well, but again this would mean a bent molecule.
On the other hand, a linear structure can also be interpreted as arising from ionic bonding. A recent theoretical
calculation by Tyerman et al.36 suggests a linear structure
and 3 S 2 ground states for the alkali methylidynes. Their
predictions are based on the assumption that the methylidynes are ionic molecules, and bonding is achieved through
electron transfer from the alkali atom into the 3s orbital of
the CH radical in its a 4 S 2 state. The addition of the electron
creates a 3 S 2 state.
Further insight into the bonding question can be gained
by examining bond lengths. Experimental and theoretical
values for bond lengths for the alkali methylidyne radicals
are presented in Table III, along with those of transition
metal species, for comparison. In this table, r 0 structures for
NaCH and KCH are given, derived from the millimeter–
wave data. ~Because only the deuterium was substituted,
meaningful r s structures cannot be calculated.! R 0 structures
for TiCH and VCH from the optical studies are also presented. As is shown, the C–H bond lengths for KCH, TiCH,
and VCH are virtually identical ~1.080–1.085 Å!, while that
of NaCH is only a little shorter ~1.070 Å!. In contrast, the
theoretical values for the C–H bond for the alkali species are
longer than any experimental ones ~1.09–1.1 Å!. As is also
evident from the table, the experimental metal–carbon bond
lengths vary considerably from transition to alkali metal
compounds. The metal–carbon distances for vanadium and
titanium methylidyne are 1.703 and 1.728 Å, respectively,
while r Na–C52.21 Å and r K–C52.53 Å for NaCH and KCH.
The calculated alkali metal bond lengths are slightly longer
than the measured values. Hence, the M–C bond in the transition metal methylidynes is shorter by 0.5 Å or more relative to the alkali species. Such variations do not directly
correlate with atomic size. The atomic radii37 of sodium and
titantium differ by 0.41 Å ~1.86 and 1.45 Å!, and vanadium
is smaller ~1.31 Å! and potassium larger ~2.27 Å!. The shortening of the M–C bond is likely a result of triple bond formation for the transition metal species as opposed to a single
bond in the alkali molecules, pointing to some degree of
covalent character.
Evidence for an sp hybridized carbon atom in the alkali
methylidynes is additionally given in Table IV, which lists
metal–carbon bond distances for various organosodium and
organopotassium species. These bond lengths were all established from millimeter–wave data r 0 structures. As is found
in the table, the bond lengths for the metal acetylide molecules, where the carbon atoms must be sp hybridized, are
very close to those of the methylidynes. NaCCH, for example, has r Na–C52.221 Å, versus r Na–C52.207 Å for
NaCH, while r K–C52.540 Å in KCCH and r K–C52.526 Å
for KCH.38 Interestingly, the M–C bond distance in both
methylidynes is slightly shorter than in the acetylides, perhaps a result of p backbonding of the two unpaired electrons,
although calculations for isovalent NaN suggest this effect is
small.39 For the monomethyl molecules, the metal–carbon
bond distances are significantly longer (NaCH3 :
2.299 Å;KCH3 :2.634 Å). This lengthening is due to sp 3 hybridization of the carbon atom, as expected in a covalently
bonded molecule. Unfortunately, geometries of the equivalent metal carbenes are not known to examine the sp 2 case.
Another class of alkali compounds that are thought to be
highly ionic are the cyanide species. In fact, they are even
more ionic than the methylidyne analogs because of the
higher electron affinity of the CN vs. the CH moiety.36,40 If
the same bonding arguments as used by Tyerman et al.36 for
TABLE III. Molecular geometries for metal methylidyne molecules.
NaCH
KCH
TiCH
VCH
a
r M–C ~Å!
r C–H ~Å!
Reference
2.207a
2.227b
2.526a
2.586b
1.7277a
1.7025a
1.073a
1.091b
1.082a
1.105b
1.085a
1.080a
This work
36
This work
36
25
24
Experimental value, r 0 structure.
Theoretical ~SCF! calculations.
b
TABLE IV. Metal carbon bond lengths for organosodium and organopotassium species.
Molecule
MCH3
MC[CH
MCH
a
r Na–C ~Å!
2.299
2.221b
2.207
Assumes r C–H51.091 Å.
Assumes r C–H51.06 Å.
b
a
r Ka–C ~Å!
Reference
2.634
2.540b
2.526
27, 29
38
This work
3366
J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
J. Xin and L. M. Ziurys
the alkali methylidynes are applied to the cyanides, then they
should be linear as well. ~Bonding would be achieved
through electron transfer from the alkali atom into the unfilled 6s orbital of the CN radical.! Yet, both NaCN and
KCN are bent T-shaped molecules.40,41 In this case, the metal
cation ‘‘orbits’’ the CN2 group, evidently attracted to the p
electron cloud of the anion. The CH2 moiety does not have
the same p electrons, so a direct analogy perhaps cannot be
made. However, the Na–C bond length in NaCN is significantly longer than in NaCH, with a value of 2.379~15! Å40 as
opposed to 2.207 Å.
The 3 S 2 ground states in the alkali methylidynes result
from a p 2 configuration. The electron configuration of
NaCH is therefore KKL~5s!2~6s!2~2p!2, while that of KCH
is KKL~5s!2~6s!2~2p!4~7s!2~8s!2~3p!2. The p 2 configuration also gives rise to 1 S 1 and 1 D electronic states, but the
3 2
S term lies lowest in energy, as might be expected from
Hund’s rules, since it is the only triplet state.
Measurement of the spectra of these two 3 S 2 molecules
has allowed for an estimate of l, the spin–spin constant.
There are two contributions to this effective parameter, the
dipolar spin–spin interaction and the second order spin–orbit
term,42 i.e.,
l eff5l ss1l so .
~5!
For most molecules, the major contribution to the effective
spin–spin parameter is l SO . In fact, for heavier molecules,
l SS is virtually negligible. @The l SO term follows the selection rule DV50 and arises from perturbations of the nearest
1 1
S state, which affect the V50( 3 S 2
0e ) fine structure level
only.# In O2, only 38% of l eff arises from the actual spin–
spin interaction, and about 5% in SO.42 The spin–orbit contribution to l eff can be calculated from the spin–orbit constant A of the nearest 3 P state which arises from a sp 3
electron configuration, and the energy difference between the
3 2
S ground state (V50 component! and the nearest 1 S 1
state, namely:42
l so.
2A 2 ~ 3 P !
E ~ 1 S 1 ! 2E ~ 3 S 2
0 !
.
~6!
Conversely, the energy above ground state for the nearest
S 1 state can be estimated if l SO and the spin–orbit constant of the lowest 3 P state are known.
To our knowledge, no data are currently available for the
excited electronic states of KCH and NaCH, including spin–
orbit constants. However, A for the 3 P state can be estimated
from the spin–orbit constants z (np) of the atoms making up
the given molecule, weighted by the percent electron density
of the unpaired electrons on those atoms.42 For NaCH and
KCH, the lack of observable alkali atom hyperfine structure,
coupled with the molecule geometry and carbon hybridization, suggest that most of the unpaired electron density resides on the C atom. Therefore, to a first order approximation, A( 3 P); z (C@ 2p 2 # );29 cm21 for NaCH and KCH.
Moreover, KCH is similar in mass to SO, and hence almost
all of the contribution to l is probably due to l SO , or l eff
;lSO . NaCH is a lighter molecule more similar to O2, and,
in analogy, it might be estimated that about 60% of l eff
comes from l SO . If these assumptions are made, then the
1
energy of the nearest 1 S 1 state above ground state can be
estimated. For KCH and NaCH, these values are DE( 1 S 1
2X̃ 3 S);6100 and 9200 cm21, respectively.
VI. CONCLUSION
This study presents the first experimental structural information on alkali methylidyne molecules. Measurements
of the pure rotational spectra of NaCH and KCH have shown
that they are linear species with 3 S ground electronic states
arising from a p 2 configuration. This geometry suggests a
fair degree of ionic bonding, in comparison with some theoretical predictions, but could also indicate sp hybridization of
the carbon atom, as well. Bond lengths determined from the
deuterium isotopomer measurements suggest a single bond
to the alkali atom, in comparison to triply bonded transition
metal analogs. The methylidyne species consequently may
exhibit more covalent character than alkali species with other
ligands such as CN.
ACKNOWLEDGMENTS
This research was supported by NSF Grants Nos. AST95-03274, CHE-95-31244, and NASA Grant No. NAG53785. We thank the referee for bringing several theoretical
papers concerning metal methylidyne species to our attention.
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