Millimeter-wave spectroscopy of vibrationally-excited NaCCH (X̃1Σ+) and MgCCH (X̃2Σ+) : the v5 bending mode

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, ˚A. 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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