10 September 1999
Chemical Physics Letters 310 Ž1999. 411–422
www.elsevier.nlrlocatercplett
Millimeter-wave spectroscopy of vibrationally-excited NaCCH
˜ 1 Sq / and MgCCH žX˜ 2 Sq /: the Õ5 bending mode
žX
M.A. Brewster, A.J. Apponi 1, J. Xin, L.M. Ziurys
)
Department of Chemistry, Department of Astronomy and Steward ObserÕatory, The UniÕersity of Arizona, 933 N. Cherry AÕenue, Tucson,
AZ 85721, USA
Received 20 April 1999; in final form 8 July 1999
Abstract
˜ 1 Sq . and MgCCH ŽX˜ 2 Sq . have been recorded in their ground state and Õ5
Pure rotational spectra of NaCCH ŽX
vibrational level, the metal–C–C bend, in the range 315–525 GHz using millimeter-wave direct absorption techniques. This
data set complements previous measurements. For NaCCH, rotational transitions were recorded for Õ5l s 0 0 , 11, 2 2 , 2 0 , 3 3,
and 4 4 levels, and for the 0 0 states of NaCCD and Na13CCH. Transitions originating in the 11, 2 0 , and 2 2 states of MgCCH
were additionally observed. Rotational, l-type doubling, and vibration–rotation parameters have been determined for both
species, as well as estimates of the v 5 bending frequency. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction
The competition between ionic and covalent
bonding in small, metal-containing molecules is illustrated in various chemical systems. One example
is the metal monocyaniderisocyanide group. In the
highly ionic species NaCN and KCN w1,2x, the metal
ion orbits the CNy moiety in a polytopic bond w3x,
resulting in a T-shaped molecule. More covalent
bonding is predicted to result in the linear cyanide
structure, while the linear isocyanide species arises
)
Corresponding author. Fax: q1-520-621-1532; E-mail:
[email protected]
1
Current address: Harvard University, Division of Engineering
and Applied Sciences, 29 Oxford Street, Cambridge, MA 02138.
from a compromise of forces, as found for MgNC
and AlNC w4,5x. For the metal monohydroxide
species, on the other hand, ionic bonding produces a
linear geometry, as found for the alkali monohydroxides w6,7x and most alkaline earth species w8–10x. As
covalent forces begin to dominate the bonding, such
molecules become bent, like the F excited state in
CaOH w11x, or at least become quasilinear, as in
MgOH w12,13x. Quasilinear behavior has also been
found in MgNC w4x and AlNC w5x.
Another system of interest from this aspect are the
metal monoacetylides. These compounds are widely
used in organic synthesis, especially LiCCH and
NaCCH w14x. In order to better understand the mechanisms involved in these reactions, knowing the
nature of the metal–carbon bond in metal acetylides
would be useful. However, in this case both ionically
and covalently bonded molecules would likely take
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 8 1 6 - 7
412
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
on a linear structure, so differentiating between the
bonding types is not as simple as for metal cyanides
and hydroxides.
Because of the importance of metal acetylide
species, we have been measuring the pure rotational
spectra of both alkali metal and alkaline earth compounds of this type in the gas phase in their ground
electronic and vibrational states, including NaCCH
w15x, KCCH w16x, MgCCH w17x, and CaCCH w18x.
Some of this work complements optical studies by
the Bernath group Že.g., w19x. and, more recently, Li
and Coxon Že.g., w20,21x.. We have begun to examine the pure rotational spectra of these species in
their lowest vibrational mode, the Õ5 M–C–C bend,
and have conducted measurements for LiCCH w22x,
which showed this molecule to have a rigid, linear
structure.
As an extension of our study of lithium
monoacetylide, here we present measurements of the
pure rotational spectrum of the Õ5 mode for both
˜ 1 Sq . and MgCCH ŽX˜ 2 Sq .. In the proNaCCH ŽX
cess of carrying out this study, we found that our
past assignments of both these species were in error.
The lines attributed to the ground state Ž00000. in
MgCCH w17x and NaCCH w15x actually arose from
the Õ5l s 11d state of these molecules. In this Letter
we correctly identify the ground vibrational states of
both acetylides, and present measurements of the
transition frequencies in this state and several quanta
of the Õ5 mode in the range 317–525 GHz. For
NaCCH, spectra of the deuterium and Na13 CCH
isotopomers were also obtained and are included in
this data set. Rotational constants have been determined for all species, including l-type doubling and
vibration–rotation interaction terms for the Õ5 state.
Estimates of the bond lengths and metal–C–C bending frequencies have been obtained as well. ŽThe
bond length values for NaCCH were published in a
previous paper in the organic chemistry literature
w23x.. In addition, the properties of alkali metal and
alkaline earth monoacetylides are compared.
2. Experimental
The rotational spectra of NaCCH and MgCCH
were recorded using one of the quasi-optical mil-
limeterrsub-mm spectrometers of the Ziurys group.
The spectrometer basically consists of a phase-locked
Gunn oscillatorrvaracter multiplier source, a freespace gas cell in which the molecules are created,
and an InSb hot electron bolometer detector. Data
are collected by scanning the source through the
phase-lock loop and monitoring the absorption of
radiation by the molecules with the bolometer. For
more details, see Ziurys et al. w24x. The acetylide
species were created by the reaction of metal vapor
and HCCH seeded in ; 40 mTorr of argon carrier
gas; a d.c. discharge was used to facilitate the synthesis. The metal vapor was generated by a Broidatype oven attached to the bottom of the free-space
cell. The discharge was typically run at 220 V with
40 mA current from an electrode attached over the
Broida-type oven. The acetylide species could be
generated adding the HCCH through the bottom of
the oven, mixed with the carrier gas, or over the
oven. NaCCD was created by the same method but
using DCCD instead of acetylene. For Na13 CCH,
BrCCH was used as the reactant instead of HCCH,
with no discharge. This precursor enabled Na13 CCH
to be observed in the natural carbon-13 abundance of
about 1:90, relative to carbon-12.
To establish the identities of the ground state and
vibrational satellite lines, large frequency ranges Ž;
30 GHz. were initially scanned. Once the spectra
were assigned, actual frequency measurements were
made using scans covering 5 MHz in frequency.
These data consisted of an average of two such
scans, each lasting 30 s in duration, one increasing
and the other decreasing in frequency. Because of
the low abundance of carbon-13, eight scans were
averaged for the Na13 CCH data. Gaussian profiles
were fit to the spectra to determine the center frequency.
3. Results
The data recorded for MgCCH are listed in Table
1 and those for NaCCH and its isotopomers are
presented in Tables 2 and 3. Table 1 lists the frequencies recorded for twenty-two separate rotational
transitions of MgCCH in its ground vibrational state,
as well as those for vibrational satellite lines in the
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
Table 1
˜ 2 Sq . a
Observed transition frequencies of MgCCH ŽX
Õ5l
Y
N ™N
X
J
Y
Õobs
Õobs y Õcalc
31.5
30.5
32.5
31.5
33.5
32.5
34.5
33.5
35.5
34.5
36.5
35.5
37.5
36.5
38.5
37.5
39.5
38.5
40.5
39.5
41.5
40.5
42.5
41.5
43.5
42.5
44.5
43.5
45.5
44.5
46.5
45.5
47.5
46.5
48.5
47.5
49.5
48.5
50.5
49.5
51.5
50.5
52.5
51.5
317 497.414
317 480.813
327 399.797
327 383.235
337 300.444
337 283.890
347 199.272
347 182.751
357 096.278
357 079.709
366 991.309
366 974.804
376 884.383
376 867.871
386 775.451
386 758.930
396 664.421
396 647.909
406 551.247
406 534.724
416 435.888
416 419.411
426 318.346
426 301.796
436 198.470
436 181.970
446 076.240
446 059.752
455 951.635
455 935.155
465 824.563
465 808.128
475 695.022
475 678.561
485 562.911
485 546.464
495 428.094
495 411.765
505 290.800
505 274.375
515 150.713
515 134.329
525 007.857
524 991.472
0.071
y0.042
0.034
y0.039
0.024
y0.042
0.009
y0.024
0.040
y0.041
0.016
y0.001
0.010
y0.014
0.024
y0.009
0.019
y0.005
0.002
y0.033
y0.015
y0.004
0.022
y0.040
0.015
0.003
y0.003
y0.002
y0.001
0.008
y0.017
0.036
y0.003
0.024
y0.006
0.035
y0.110
0.049
y0.034
0.029
y0.041
0.063
y0.056
0.048
31.5
30.5
32.5
31.5
33.5
32.5
319572.094
319555.690
329537.744
329521.182
339501.272
339484.813
0.022
0.065
0.100
y0.015
y0.023
y0.035
Table 1 Žcontinued.
Õ5l
Y
N ™N
X
33 ™ 34
34 ™ 35
35 ™ 36
36 ™ 37
37 ™ 38
38 ™ 39
39 ™ 40
40 ™ 41
41 ™ 42
42 ™ 43
43 ™ 44
44 ™ 45
45 ™ 46
46 ™ 47
47 ™ 48
48 ™ 49
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
35 ™ 36
36 ™ 37
37 ™ 38
38 ™ 39
39 ™ 40
40 ™ 41
31 ™ 32
32 ™ 33
33 ™ 34
34 ™ 35
35 ™ 36
36 ™ 37
33 ™ 34
Õobs y Õcalc
34.5
33.5
35.5
34.5
36.5
35.5
37.5
36.5
38.5
37.5
39.5
38.5
40.5
39.5
349462.977
349446.508
359422.576
359406.173
369380.100
369363.604
379335.518
379319.085
389288.658
389272.236
399239.657
399223.271
409188.230
409171.807
0.008
y0.014
y0.030
0.014
y0.049
y0.098
y0.021
y0.007
y0.060
y0.035
0.029
0.090
0.019
0.043
31.5
30.5
32.5
31.5
33.5
32.5
34.5
33.5
35.5
34.5
36.5
35.5
320337.714
320321.234
330326.131
330309.674
340312.552
340296.114
350296.915
350280.465
360279.131
360262.697
370259.215
370242.777
0.042
0.010
0.002
y0.008
y0.018
y0.009
y0.019
y0.022
y0.029
y0.016
0.029
0.038
31.5
30.5
32.5
31.5
33.5
32.5
34.5
33.5
35.5
34.5
36.5
35.5
37.5
36.5
38.5
37.5
39.5
38.5
322494.218
322477.972
332548.023
332531.907
342599.754
342583.581
352649.249
352633.120
362696.337
362680.097
372741.104
372724.867
382783.463
382767.111
392823.280
392806.919
402860.512
402844.233
0.072
0.063
y0.047
0.074
y0.046
0.018
y0.022
0.087
y0.077
y0.080
y0.061
y0.061
0.007
y0.108
0.059
y0.065
0.118
0.076
31.5
30.5
32.5
31.5
322427.363
322410.880
332473.246
332456.730
0.161
0.061
0.070
y0.063
Ž2 0 .
31 ™ 32
32 ™ 33
33 ™ 34
34 ™ 35
35 ™ 36
36 ™ 37
37 ™ 38
38 ™ 39
39 ™ 40
32 ™ 33
Õobs
Ž11d .
Ž11c .
31 ™ 32
Y
Ž1
34 ™ 35
32 ™ 33
J
1c .
Ž0 0 .
31 ™ 32
413
Ž2 2c .
31 ™ 32
32 ™ 33
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
414
Table 1 Žcontinued.
Õ5l
Y
N ™N
X
J
Y
Õobs
Õobs y Õcalc
Ž2 2c .
33 ™ 34
34 ™ 35
35 ™ 36
36 ™ 37
37 ™ 38
38 ™ 39
39 ™ 40
33.5
32.5
34.5
33.5
35.5
34.5
36.5
35.5
37.5
36.5
38.5
37.5
39.5
38.5
342516.437
342499.957
352556.874
352540.386
362594.384
362577.908
372629.046
372612.518
382660.794
382644.201
392689.457
392672.924
402714.936
402698.463
0.023
y0.074
0.026
y0.079
y0.030
y0.123
y0.003
y0.148
0.094
y0.116
0.142
y0.008
0.083
y0.007
31.5
30.5
32.5
31.5
33.5
32.5
34.5
33.5
35.5
34.5
36.5
35.5
37.5
36.5
38.5
37.5
39.5
38.5
322583.041
322566.780
332645.276
332628.988
342705.703
342689.464
352764.423
352748.188
362821.327
362805.058
372876.286
372860.065
382929.413
382913.050
392980.394
392964.236
403029.396
403013.104
-0.044
0.078
0.010
0.105
-0.056
0.088
-0.078
0.071
-0.094
0.020
-0.161
0.001
-0.087
-0.067
-0.102
0.123
0.052
0.144
Table 2
Observed transition frequencies of NaCCH Ž˜1 Sq . a
Y
Õ5l
Ž0
J ™J
Ž2 2d .
31 ™ 32
32 ™ 33
33 ™ 34
34 ™ 35
35 ™ 36
36 ™ 37
37 ™ 38
38 ™ 39
39 ™ 40
a
Õobs
Õobs y Õcalc
37 ™ 38
38 ™ 39
39 ™ 40
40 ™ 41
41 ™ 42
42 ™ 43
43 ™ 44
44 ™ 45
48 ™ 49
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
54 ™ 55
342 150.745
351 121.005
360 088.580
369 053.498
378 015.682
386 975.023
395 931.487
404 885.084
440 668.632
449 606.458
458 540.972
467 472.060
476 399.705
485 323.817
494 244.330
0.033
0.055
0.009
y0.011
y0.017
y0.051
y0.082
y0.035
0.064
0.040
0.038
0.009
0.001
y0.014
y0.036
37 ™ 38
38 ™ 39
39 ™ 40
40 ™ 41
41 ™ 42
42 ™ 43
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
54 ™ 55
345 000.712
354 041.769
363 079.811
372 114.913
381 146.928
390 175.796
453 283.796
462 284.959
471 282.367
480 275.809
489 265.402
498 251.038
y0.180
y0.083
y0.071
0.002
0.059
0.111
0.204
0.176
0.152
y0.011
y0.132
y0.252
37 ™ 38
38 ™ 39
39 ™ 40
40 ™ 41
41 ™ 42
42 ™ 43
48 ™ 49
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
54 ™ 55
345 958.526
355 022.422
364 083.238
373 140.877
382 195.268
391 246.322
445 478.981
454 504.750
463 526.666
472 544.590
481 558.511
490 568.289
499 573.916
0.175
0.105
0.054
0.002
y0.047
y0.108
y0.161
y0.165
y0.112
y0.068
0.027
0.102
0.220
37 ™ 38
48 ™ 49
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
348 976.150
449 152.038
458 231.221
467 305.458
476 374.631
485 438.749
494 497.772
0.004
y0.047
0.006
0.045
0.017
y0.010
y0.015
Ž11c .
Ž11d .
In MHz.
Õ5 bending mode. The ground electronic state of
MgCCH is 2 Sq; consequently, each rotational transition, labeled by quantum number N, is split into
doublets arising from fine structure interactions, indicated by quantum number J. Therefore, every transition observed consists of two separate lines, as shown
in Table 1. Additional structure is apparent in the
vibrational satellite lines because the Õ5 mode is
doubly degenerate. The Õ5 s 1 state is split into
doublets due to l-type doubling, labeled by 1c and
1d, respectively, and the Õ5 s 2 state separates into
triplets Ž Õ5l s 2 0 , 2 2c , and 2 2d ., arising from l-type
doubling and l-type resonance w25x. Rotational transi-
X
0.
Ž2 0 .
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
Table 2 Žcontinued.
Õ5l
Y
J ™J
415
Table 2 Žcontinued.
X
Õobs
Õobs y Õcalc
Ž2 2c .
Y
Õ5l
J ™J
X
Õobs
Õobs y Õcalc
468 733.346
478 006.590
487 273.984
496 535.615
505 791.343
y0.019
0.079
0.037
0.021
y0.030
Ž4 4d .
37 ™ 38
48 ™ 49
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
54 ™ 55
349 029.389
449 355.880
458 452.298
467 544.384
476 632.003
485 715.093
494 793.603
503 867.499
0.097
y0.056
y0.052
y0.014
y0.001
y0.003
0.004
0.058
37 ™ 38
47 ™ 48
48 ™ 49
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
54 ™ 55
349 186.019
440 553.062
449 670.614
458 784.307
467 894.284
476 999.910
486 101.631
495 199.171
504 292.459
y0.088
y0.022
0.007
y0.003
0.178
y0.001
y0.009
y0.033
y0.060
37 ™ 38
47 ™ 48
48 ™ 49
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
352 905.420
445 077.764
454 270.470
463 458.428
472 641.521
481 819.605
490 992.686
500 160.517
y0.012
0.008
y0.027
y0.010
0.024
0.013
0.045
y0.048
37 ™ 38
47 ™ 48
48 ™ 49
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
352 914.316
445 104.601
454 299.930
463 490.940
472 677.105
481 858.519
491 035.087
500 206.773
0.021
0.005
y0.113
0.050
0.042
0.032
y0.003
y0.027
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
a
In MHz.
Ž2 2d .
Ž3 3c .
Ž3 3d .
Ž4 4c .
tions originating from a total of five different excited
vibrational levels were therefore observed for MgCCH. Six to ten transitions, each consisting of fine
structure doublets, were recorded for every state.
ŽThe l-type doubling was easily distinguished from
the spin–rotation splitting because it is in general
much larger: ) 100 MHz vs. 14–17 MHz.. Several
lines in the 11d state had been previously recorded by
Anderson and Ziurys w17x.
Table 2 lists the transition frequencies measured
for NaCCH. The ground state for this molecule is
1 q
S and therefore each transition is a single line and
the rotational quantum number is J. Fifteen separate
transitions were recorded for the ground vibrational
state and 7 to 13 for each vibrationally-excited state.
In this case lines originating in the Õ5 s 1, 2, 3, and
4 levels were observed. Again, l-type doubling and
l-type resonance interactions are present in these
Table 3
Observed transition frequencies of NaCCH isotopomers Ž Õ5l s 0 0 . a
Na13 CCH
Y
J ™J
46 ™ 47
47 ™ 48
48 ™ 49
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
440 879.303
450 169.030
459 452.984
468 731.422
478 004.176
487 271.395
496 532.750
505 788.153
0.038
0.185
0.043
y0.042
y0.152
y0.051
0.014
0.037
46 ™ 47
47 ™ 48
48 ™ 49
440 880.327
450 170.271
459 454.608
y0.171
y0.003
0.017
Ž4 4d .
X
49 ™ 50
50 ™ 51
51 ™ 52
52 ™ 53
53 ™ 54
54 ™ 55
55 ™ 56
56 ™ 57
57 ™ 58
58 ™ 59
59 ™ 60
60 ™ 61
a
In MHz.
NaCCD
Õobs
Õobs y Õcal
Õobs
Õobs y Õcalc
447 543.167
456 436.801
465 326.939
474 213.700
483 096.949
491 976.575
500 852.687
–
–
–
–
–
0.047
0.048
y0.048
y0.054
y0.040
y0.048
0.096
–
–
–
–
–
–
–
–
–
450 137.469
458 419.206
466 697.932
474 973.692
483 246.392
491 515.942
499 782.328
508 045.529
–
–
–
–
0.078
0.033
y0.046
y0.059
y0.045
y0.042
y0.008
0.089
416
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
˜ 2 Sq . electronic state near 387 GHz.
Fig. 1. Spectrum of the N s 38 ™ 39 rotational transition of MgCCH in its ground vibrational and ŽX
The spin–rotation doublets, indicated by quantum number J, are readily resolved in the spectrum. This scan covers 100 MHz in frequency
and was acquired in ; 50 s.
data. The l-type doublets were recorded for the
Õ5l s 11 and 2 2 states, as well as lines arising from 2 0
level; for the higher quanta, only the 3 3 and 4 4 l-type
doublets were measured. No data were obtained for
˜ 1 Sq . in its ground and Õ5l s 2 0 states,
Fig. 2. Spectrum of the J s 52 ™ 53 and J s 51 ™ 52 rotational transitions of NaCCH ŽX
respectively, observed in this work near 476 GHz. This spectrum covers 80 MHz in frequency and was obtained in ; 30 s.
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
the 31, 4 0 , and 4 2 states. Table 3 presents the
transitions recorded for Na13 CCH and NaCCD. Seven
lines were measured for the 13 C isotopomer, and
eight for the deuterated form.
A typical spectrum obtained for the MgCCH radical is shown in Fig. 1, which presents the N s 38 ™
39 rotational transition near 387 GHz. The fine
structure doublets, arising from spin–rotation interactions and indicted by quantum number J, are
clearly resolved in this spectrum, which covers 100
MHz in frequency. The separation of the doublets is
; 16.6 MHz. Fig. 2 presents the J s 51 ™ 52 line
for the 2 0 excited vibrational state and the J s 52 ™
53 transition of the ground state of NaCCH near 476
GHz. This species is closed-shell; hence, each transition is a single line. Frequency coverage in this
spectrum is 80 MHz. In Fig. 3, data for one of the
13
C isotopomers of sodium monoacetylide, Na13 CCH,
are shown. This spectrum is the J s 49 ™ 50 transition near 447 GHz. Here the frequency scale is
greatly expanded, showing only 5 MHz total coverage. Hence, the line is much broader. This spectrum
was recorded with 13 C in its natural abundance,
using BrCCH as the acetylide donor.
417
4. Analysis
NaCCH has a 1 Sq ground electronic state, and
hence analysis of this molecule concerns only molecular frame rotation and its centrifugal distortion corrections, namely B, D, and H parameters, except for
the Õ5 data. In these cases, l-type doubling had to be
considered. For the 11 state, the l-type doubling was
modeled with the following additional term in the
energy level expression w25x:
D E Ž l-type . s " 12 qJ Ž J q 1 . y q D J 2 Ž J q 1 .
2
,
Ž 1.
where q is the l-type doubling constant and q D its
centrifugal distortion correction. As discussed in Apponi et al. w22x, for the other vibrationally excited
states 2 2 , 3 3 , and 4 4 , the l-type doubling takes on a
more complicated form because it includes the effects of l-type resonance, i.e. interactions with the
other l-type components in that particular Õ5 level.
To model these states, the energy differences between these l-type levels must be known. Such
Fig. 3. A spectrum of the J s 49 ™ 50 transition of the sodium acetylide isotopomer Na13 CCH observed near 447 GHz in the natural
abundance. This spectrum covers 5 MHz in frequency is an average of eight 30 s scans.
13
C
418
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
information does not exist for NaCCH, nor for
MgCCH. Hence, states with Õ5 s 2 and higher could
be fit with only an effective l-type doubling constant, qeff and its centrifugal distortion corrections,
q H, eff and q D, eff . Explicit expressions for this effective doubling interactions are given in Apponi et al.
w22x and Yamada et al. w26x.
˜ 2 Sq . ground state, addiFor MgCCH in its ŽX
tional complications arise because of spin–rotation
interactions, which generate two fine structure components per rotational level described by the following energy expressions w27x:
F1 Ž N . s Bv N Ž N q 1 . y l 2
y Dv N Ž N q 1 . y l 2
2
q 12 gv N ,
Ž 2.
5. Discussion
F2 Ž N . s Bv N Ž N q 1 . y l 2
y Dv N Ž N q 1 . y l 2
31 ™ 32 line up to the N s 52 ™ 53 transition. The
spectroscopic constants are in general well-determined, as indicated by the 3s errors listed in
Table 4, which are based on the statistics of the fit.
The rms values are quite good: 83 and 60 kHz,
respectively, for the analysis of all vibrational levels
of NaCCH and MgCCH. The less extensive data sets
for NaCCD and Na13 CCH had rms values of 55 and
57 kHz, respectively. The residuals of the analysis,
given in Tables 1–3, are less than 165 kHz for all
MgCCH measurements, less than 96 kHz for all
ground state lines of NaCCH and its isotopomers,
and typically smaller than 200 kHz for the vibrationally excited states of NaCCH, with a few exceptions Žsee Table 2..
2
y 12 gv Ž N q 1 . ,
Ž 3.
where gv is the spin–rotation constant. Again, the
effects of the l-type doubling are readily modeled in
the 11 levels using the q and q D parameters, i.e. w27x
D E Ž l-type . s " 12 q v N Ž N q 1 .
" 12 q vD N Ž N q 1 .
2
,
Ž 4.
where the plus sign refers to the F1Že. and F2 Žf.
levels, and the minus sign to the F1Žf. and F2 Že.
levels. The same difficulties arise for Õ5 ) 1 states as
for those of NaCCH, and hence the l-type doubling
in these levels can only be characterized by effective
l-type parameters qeff , q D, eff , and q H, eff , as described.
The spectroscopic parameters resulting from these
analyses are given in Table 4, as well as the rms
values of the individual fits. As the table shows,
higher-order centrifugal distortion corrections H and
q D were often found necessary to obtain a good data
fit, and in one instance Ž Õ5l s 2 2 for MgCCH., q H, eff
as well. These parameters were needed because of
the high J Žor N . transitions recorded, and the wide
frequency range of the data set. For example, the fit
for the 0 0 state of MgCCH extended from the N s
One of the important results in this study was the
identification of the correct ground state transitions
for NaCCH and MgCCH. Both of our original investigations of these species by Li and Ziurys w15x and
Anderson and Ziurys w17x had incorrectly identified
the Õ5l s 11d state as the 0 0 state. The ground state of
NaCCD was similarly misidentified. However, both
studies were the first observation of these species by
any spectroscopic method. MgCCH has been subsequently detected by LIF by Corlett et al. w28,29x;
NaCCH, to our knowledge, has only been studied in
our group. Our new B0 value for MgCCH of 4.965
GHz is now closer to the theoretical value for Be of
4.95 GHz, calculated by Woon w30x. Woon had noted
that our previous value of 5.01 GHz appeared to be
somewhat higher than his calculated constant.
Observation of the ground and excited vibrational
lines additionally enables the rigidity of a given
molecule to be examined. As found for the alkaline
earth monohydroxide series w27x, deviation from a
linear geometry to a floppy, quasilinear structure
results in an altered pattern in the satellite lines as
the excited vibrational states start to correlate with
the energy levels of an asymmetric top. One obvious
feature is the large shift of the 2 0 transitions away
from the 2 2 doublets towards the 0 0 lines, as found
in MgOH rotational spectra w13x. As illustrated in
Fig. 4, which shows the vibrational progression of
the J s 52 ™ 53 transition of NaCCH, such devia-
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
419
Table 4
˜ 1 Sq . and MgCCH ŽX˜ 2 Sq .
Rotational constants for NaCCH ŽX
Constant Ž Õ5l .
NaCCH
NaCCD
Na 13 CCH
MgCCH
B Ž0
D Ž0 0 .
H Ž0 0 .
g Ž0 0 .
4 510.116Ž10.
0.0028240Ž48.
3.63Ž70. = 10y9
–
4 181.0949Ž59.
0.00225585Ž88.
–
–
4 489.3191Ž72.
0.0027776Ž13.
–
–
4 965.3346 Ž38.
0.0022324 Ž20.
1.44 Ž34. = 10y9
16.488 Ž45.
B Ž11 .
D Ž11 .
H Ž11 .
g Ž11 .
q Ž11 .
q D Ž11 .
4 555.2517Ž81.
0.0033040Ž38.
1.089Ž56. = 10y8
–
y13.1245Ž28.
0.00018225Ž58.
–
–
5 004.2501 Ž36.
0.0024847 Ž14.
–
16.447 Ž53.
y12.2106 Ž71.
0.0001212 Ž29.
B Ž2 0 .
D Ž2 0 .
H Ž2 0 .
g Ž2 0 .
4 605.374Ž15.
0.0048274Ž69.
5.73Ž10. = 10y8
–
–
–
5 044.5166 Ž35.
0.0027697 Ž13.
–
16.237 Ž71.
B Ž2 2 .
D Ž2 2 .
H Ž2 2 .
g Ž2 2 .
qeff Ž2 2 .
q D, eff Ž2 2 .
q H, eff Ž2 2 .
4 603.5764Ž86.
0.0035002Ž38.
4.28Ž56. = 10y9
–
y0.0007843Ž13.
3.205Ž32. = 10y8
–
–
–
–
–
5 044.7100 Ž25.
0.00279170 Ž93.
–
16.383 Ž50.
y0.000952 Ž16.
y2.26 Ž16. = 10y7
5.17 Ž44. = 10y11
B Ž3 3 .
D Ž3 3 .
H Ž3 3 .
qeff Ž3 3 .
q D, eff Ž3 3 .
4 655.8188Ž96.
0.0043356Ž44.
2.834Ž66. = 10y8
y2.046Ž45. = 10y8
9.5Ž1.3. = 10y13
B Ž4 4 .
D Ž4 4 .
H Ž4 4 .
qeff Ž4 4 .
4 712.746Ž52.
0.005294Ž20.
4.74Ž26. = 10y8
y3.04Ž10. = 10y13
0.055
0.057
0.
rms of fit
a
0.083
0.060
In MHz; errors quoted are 3 s and apply to the last quoted decimal places.
tions are not significant for this molecule. The 0 0
line and the centers of the l-doublets of the 11 , 2 2 ,
3 3, and 4 4 states appear at regular frequency intervals relative to each other, and the 2 0 transition is
are only slightly shifted to lower frequency from that
of the 2 2 level. An identical progression has been
found for LiCCH w23x. A similar pattern is also
observed for MgCCH, except in this case the 2 0
lines appear in between the 2 2 doublets. Such regular
progressions indicate a rigid molecule, as opposed to
a floppy one. Therefore, both NaCCH and MgCCH
appear to be very linear, tightly bound species.
Another insight into the floppiness of these
molecules can be obtained by calculating the vibrational dependence of Bv , as described by Lide and
Matsumura w7x in the following relationship:
2
Bv s Be y a 5 Ž Õ5 q 1 . q g 55 Ž Õ5 q 1 . q g l l l 2 .
Ž 5.
In this expression, Be s Be y Ý4is1 a i Ž Õi q 12 d i ., i.e.
it includes the vibration–rotation interaction terms of
the other four modes. Using the data for the 0 0 , 11 ,
2 0 , and 2 2 states, this vibrational dependence was
calculated for MgCCH; for NaCCH, the 3 3 and 4 4
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
420
Fig. 4. A stick diagram illustrating the vibrational progression of the Õ5 mode of NaCCH for the J s 52 ™ 53 transition, including the
positions of the l-type doublets.
states were considered as well in this computation.
The results are given in Table 5, along with parameters for LiCCH w22x and CaCCH w21x. As is evident
from the table, the non-linear vibrational dependence
of Bv for both NaCCH and MgCCH is small; g 55 is
on the order of F 2 MHz and g l l is - 0.5 MHz.
These values are comparable to those found for
LiCCH and CaCCH, and noticeably smaller than
those of quasilinear MgOH w13x, which has g 22 s
21.3 MHz and g l l s y18.3 MHz. Another interesting point is that a 5 in all four species listed in Table
5 is negative. As described by Lide and Matsumura
w7x, a negative a value indicates that harmonic terms
dominate the bending potential. A positive a value,
as found for the bending mode in MgOH w13x,
suggests a predominately anharmonic contribution.
Establishing a pseudo-Be constant, Be, along with
the l-type doubling parameter q for the 11 state,
enables the v 5 bending frequencies to be estimated
using the approximate relationship
qv f y
2 Be2
v5
,
Ž 6.
which neglects the Coriolis term. This interaction,
which is proportional to v 52rv i2 y v 52 , is expected to
be negligible, since the v 5 bending frequency is
likely to be considerably less than the other vibra-
Table 5
Vibrational dependence of B va
Molecule
Be
a5
g 55
gll
Ref.
LiCCH
NaCCH
MgCCH
CaCCH
10 455.35Ž20.
4 469.95Ž19.
4 927.915Ž30.
y83.905Ž50.
y37.896Ž93.
y36.695Ž5.
y32.05
4.91Ž10.
2.415Ž69.
0.724Ž5.
1.7
y1.49Ž50.
y0.45Ž15.
0.048Ž5.
y0.087
w22x
this Letter
this Letter
w21x
a
Constants in MHz.
M.A. Brewster et al.r Chemical Physics Letters 310 (1999) 411–422
Table 6
Estimated M–C–C bending frequencies
Molecule
v5
Žcmy1 .
Ref.
LiCCH
NaCCH
MgCCH
CaCCH
136
102
133
102.9
w22x
this Letter
this Letter
w20x
tional frequencies, as calculations for MgCCH have
shown w30x. Using Eq. Ž6. with Be and q for the 11
state, values for v 5 have been calculated and are
listed in Table 6, along with those of LiCCH and
CaCCH, for comparison. The value of v 5 found for
MgCCH is ; 133 cmy1 , in good agreement with the
theoretical estimate of v 5 f 150–160 cmy1 calculated by Woon w30x and with the experimental value
of 143 cmy1 , measured by Corlett et al. w29x using
dispersed fluorescence. The v 5 frequency found for
NaCCH is ; 100 cmy1 , and thus far is the only
estimate of this quantity. Consequently, for MgCCH
the frequency of the second quantum of the bend is
near 2 v 5 f 260 cmy1 , far in energy from the both
the lowest energy stretch, v 3 , the Mg–C stretch near
500 cmy1 , and the other bending mode, v 4 , the
C–C–H bend, near 660 cmy1 w29,30x. Hence, no
Fermi resonance interactions are expected for magnesium monoacetylide, which could also alter the
vibrational satellite pattern, in addition to quasilinear
effects. The NaCCH vibrational structure suggests
lack of Fermi resonance as well.
Because two isotopic substitutions were carried
out for NaCCH, both r 0 and rs bond lengths were
calculated. For the r 0 structure, the C–H bond dis˚ and the both isotopomers
tance was fixed to 1.06 A
were used to calculate average values. The resulting
˚ and rC – C s
bond distances are r Na – C s 2.221 A
˚
1.217 A, very close to ab initio values, as reported in
a previous paper w23x. The rs calculation yielded
˚ rC – C s 1.192 A,
˚ and rC – H s 1.072
r Na – C s 2.239 A,
Å. This structure is probably less reliable than the r 0
one because it results in a C–C bond length shorter
˚ .. ŽSubstituting sodium
than that of acetylene Ž1.204 A
for hydrogen should increase the electron density at
the carbon atoms, and hence lengthen the carbon–
carbon bond.. For MgCCH, no isotopomer spectra
were recorded. Therefore, only the Mg–C bond dis-
421
tance was calculated, holding the other two lengths
fixed to those of NaCCH. The Mg–C bond length
˚ close to
was in this case calculated to be 2.039 A,
˚ w30x. Hence, in
the theoretical value of 2.041 A
going from sodium to magnesium acetylide, the
metal–carbon bond distance shortens. This effect can
be accounted for by the difference in the atomic radii
˚
of sodium and magnesium, which vary by ; 0.2 A.
Finally, what can be said about the metal–carbon
bond in NaCCH and MgCCH? The spectra of these
two species are quite similar, suggesting that the
bonding does not appreciably change on substituting
an alkaline earth for an alkali metal atom, even with
the addition of an unpaired electron in MgCCH.
Both molecules appear to be very linear systems.
Unlike the metal monohydroxide species, a more
covalent metal–carbon bond does not drive the
MCCH series to a bent structure. Instead, increased
covalent character likely induces backbonding of the
p acetylenic system into open orbitals on the metal
atom, which still favors linearity. Such backbonding
is thought to reduce the dipole moment of MgCCH
w30x. It would be interesting to know if the dipole
moment of NaCCH is comparable, or is larger, due
to a greater degree of ionic character. On the bases
of these data, the bonding of both molecules appears
very similar, and it is difficult to ascertain relative
amounts of ionic vs. covalent character.
Acknowledgements
This research is supported by NASA Grant
NAG5-3785 and NSF Grant CHE95-31244.
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