The Rotational Spectra of H2CCSi and H2C4Si

Journal of Molecular Spectroscopy 211, 228–234 (2002) doi:10.1006/jmsp.2001.8467, available online at http://www.idealibrary.com on

The Rotational Spectra of H2 CCSi and H2 C4 Si M. C. McCarthy and P. Thaddeus Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, and Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, Massachusetts 02138 Received July 26, 2001; in revised form October 3, 2001

The microwave rotational spectra of two singlet chains H2 CCSi and H2 C4 Si, have been observed in a pulsed supersonic molecular beam by Fourier transform microwave spectroscopy. Following detection of the singly substituted rare isotopic species and D2 CCSi, an experimental structure (r0 ) has been determined to high accuracy for H2 CCSi by isotopic substitution. In addition, the rotational transitions of the 29 Si and the two 13 C isotopic species of H2 CCSi exhibit nuclear spin–rotation hyperfine structure. The component of the spin–rotation tensor along the a-inertial axis is abnormally large for each of these isotopic C 2002 Elsevier Science (USA) species, especially for that with 29 Si, where the magnitude of Caa is in excess of 700 kHz.  I. INTRODUCTION

Although silicon analogs of many pure carbon clusters (Cn ) and acetylenic free radicals (Cn H) have recently been characterized by high-resolution spectroscopy (1, 2), only H2 CSi, the silicon analog of vinylidene (H2 CC:), has been reported in the gas phase (3, 4). Because small silicon-bearing molecules such as SiS, SiC, and SiC2 , and the cumulene carbenes H2 C3 (5), H2 C4 (6), and H2 C6 (7) are abundant in the circumstellar shells of evolved carbon stars (8), H2 CCSi and longer silacumulenes are plausible candidates for astronomical detection. In the laboratory, these chains may serve as intermediates in chemical vapor deposition and in the formation of thin films (9), so experimental studies of the spectroscopy and structure of silacumulenes is of general interest. There is presently little experimental or theoretical data on H2 Cn Si clusters beyond H2 CSi. The photochemical interconversion between isomers of H2 C2 Si and H2 C4 Si was studied by matrix isolation spectroscopy (10, 11), and silacyclopropenylidene, a low-lying singlet cyclic isomer of H2 C2 Si, was detected in the gas-phase by millimeter-wave spectroscopy and its molecular structure determined by isotopic substitution (12). The relative stabilities, structures, and dipole moments of the H2 C2 Si isomers have been the subject on several theoretical investigations (13–15), but no calculations are available for H2 C3 Si or larger analogs. We describe here the laboratory detection of the two new silicon–carbon chains H2 C2 Si and H2 C4 Si shown in Fig. 1. Each has a singlet electronic ground state (1 A1 ) and a linear heavy-atom backbone with two equivalent off-axis H atoms. As a consequence the rotational spectrum of each is that of a nearly prolate rotor with C2v symmetry and ortho and para nuclear spin statistics for the K a rotational levels. An effective structure (r0 ) of H2 CCSi has been determined by detection of the singly-substituted isotopic species and the doubly-substituted D2 CCSi. In addition, rotational lines of the 29 Si and both 13 C 0022-2852/02 $35.00  C 2002 Elsevier Science (USA) All rights reserved.

isotopic species exhibit hyperfine structure owing to the interaction between the nuclear spins and the overall rotation of the molecule. The diagonal elements of the nuclear spin–rotation tensor have been derived for all three species; Caa , the element along the a-inertial axis, for each is large relative to that of other closed-shell molecules of similar symmetry. Because this hyperfine interaction is proportional to the radial expectation value of the valence electrons, the determination of Caa at different atomic positions along the carbon chain may provide a sensitive probe of the molecular wave function.

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II. EXPERIMENTAL

Rotational lines of the two molecules here were detected in a supersonic molecular beam at centimeter-wavelengths with a Fourier transform microwave (FTM) spectrometer that has been previously described (16). This instrument operates from 5 to 43 GHz and employs a liquid nitrogen-cooled receiver and cold optics for high sensitivity; computer control provides rapid data acquisition and analysis. A gate valve allows the small discharge source in the throat of the supersonic nozzle to be readily serviced without breaking vacuum, so continuous operation over many tens of hours or days is achieved. The same discharge source used to detect many new silicon carbides SiCn and radicals SiCn H was employed in the present investigation. The best gas mixture and source conditions were similar to those used before (1, 2): silane and diacetylene (0.1% each) heavily diluted in neon, a gas pulse of 200 µsec duration (15–20 sccm flow), a 1000-V dc discharge, and a stagnation pressure behind the nozzle of 2.5 kTorr (3.2 atm). To detect the isotopic species of H2 CCSi, deuterated diacetylene was used in place of normal diacetylene for D2 CCSi, and a statistical mixture of carbon-13 HCCH (25% HCCH, 50%H13 CCH, and 25%H13 C13 CH) was used in place of normal diacetylene to observe the two single 13 C isotopic species; H2 CC29 Si was observed in natural abundance. Carbon-13 HCCH was produced

ROTATIONAL SPECTRA OF H2 CCSi AND H2 C4 Si

117.3°

b

Å 99 1.0

H

C

1.321 Å

C

Si

1.703 Å

a

H H C

C

C

C

Si

229

shown in the energy level diagram in Fig. 2, the K a = ±1 rotational ladders of H2 CCSi lie about 14 K above ground, but these are metastable owing to ortho–para nuclear spin statistics, and therefore are well populated in our rotationally cold molecular beam (Trot ∼ 3 K). The rotational spectrum of H2 CCSi as a result consists of fairly tightly spaced triplets with an intensity ratio of 3 : 2 : 3. At least three successive triplets were measured in H2 CCSi and each of its isotopic species (Table 1). The spacing between the triplets is determined by the asymmetry splitting of the K a = ±1 transitions; for the transitions near 20 GHz, the triplet splitting is about 100 MHz. The spectrum of H2 CCSi and its isotopic species were analyzed with Watson’s S-reduced Hamiltonian (19) which reproduces the observed spectra of each species to the measurement

H FIG. 1. Structures of the two new silacumulenes, H2 CCSi and H2 C4 Si. The bond lengths and angle of H2 CCSi were obtained by isotopic substitution (see Section. III.D).

by the hydrolysis of 13 C-enriched Li2 C2 which was prepared at the NIH Stable Isotope Resource, Los Alamos National Laboratory. The lines of H2 CCSi are about twice as weak with acetylene as with diacetylene in the discharge, but lines of the two 13 C species are fivefold stronger with carbon-13 enriched acetylene than with normal acetylene, about the enhancement expected. The laboratory detection of H2 CCSi and H2 CCCCSi benefited from theoretical calculations that significantly narrowed the search range for the rotational transitions. On the basis of several calculations (14, 15), H2 CCSi was detected first, its rotational constants differing by only 0.3% from those predicted by Cooper (14). For D2 CCSi and the singly substituted isotopic species, rotational constants calculated from Cooper’s theoretical structure were scaled by the ratio of the experimental rotational constant to that calculated for the normal species. Such scaling predicts the transition frequencies of the isotopic lines to better than 1% of the frequency shift from the lines of the normal species, so a search of only a few MHz in the vicinity of 20 GHz was required for detection. Because no theoretical geometries are available for H2 C4 Si, laboratory searches were based on a high-level ab initio geometry of SiC4 (17) in which two hydrogen atoms were appended to ˚ the terminal carbon atom of the chain with a C–H bond of 1.06 A length and an internal  HCH angle of 116◦ —a procedure previously used for the detection of four new thiocumulene chains similar in structure to the molecules here (18). Rotational transitions predicted in this way turned out to be accurate to about 1%. III. RESULTS AND ANALYSIS

A. Rotational Spectrum of H2 CCSi The rotational spectrum of H2 CCSi is similar to that of formaldehyde, a molecule with similar structure that also possesses C2v symmetry and two equivalent off-axis protons. As

FIG. 2. Lowest rotational energy levels of H2 CCSi, showing the transitions detected in the normal isotopic species (arrows) and the singly substituted isotopic species and D2 CCSi (dots). Owing to the ortho–para nuclear spin statistics, the K a = ±1 rotational levels are metastable in our rotationally cold molecular beam.

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TABLE 1 Measured Transitions of the Isotopic Species of H2 CCSi (in MHz) JK a ,K c → JK a ,K c

H2 CCSi

H13 2 CCSi

H2 C13 CSi

H2 CC29 Si

D2 CCSi

→ 00,0 → 01,1 → 10,1 → 11,0 → 21,2 → 20,2 → 21,1 → 31,3 → 30,3 → 31,2

10565.420 21031.757 21130.786 21228.719 31547.541 31696.040 31842.984 42063.221 42261.128

10209.805 20327.102 20419.559 20510.993 30490.569 30629.214 30766.405

10536.132 20973.778 21072.208 21169.539 31460.574 31608.174 31754.215

10415.729 20735.161 20831.405 20926.584 31102.655 32146.977 31389.788 41470.044

9375.359 18596.498 18750.574 18903.995 27894.614 28125.490 28355.856 37192.566 37499.964 37807.552

10,1 21,2 20,2 21,1 31,3 30,3 31,2 41,4 40,4 41,3

41852.889

Note. Centroid of hyperfine-split line is given; estimated experimental uncertainties (1σ ) are 2 kHz. Observed minus calculated frequencies are 0–2 kHz; the best-fit constants are given in Table 2.

uncertainty (2 kHz) with only four free parameters: the rotational constants B and C, and the two leading centrifugal distortion constants D J and D J K (Table 2). The A rotational constant, however, could not be determined from the present data set; it was instead constrained to the theoretical value derived by Cooper (14). B. Rotational Spectrum of H2 C4 Si The rotational spectrum of H2 C4 Si is very similar to that of H2 CCSi, but the rotational lines are weaker by a factor of 10; five rather than three rotational transitions, each with the predicted triplet structure, were measured between 8 and 20 GHz (Table 3). Owing to the small difference between the B and C rotational constants, the asymmetry splitting in the K a = ±1 transitions is only about 25 MHz for the transitions near 20 GHz. Five spectroscopic constants (Table 2) were again adequate to reproduce the observed transition frequencies of H2 C4 Si. The spectroscopic and chemical evidence for our assignments is extremely good. The rotational constants of each molecule are

within 1% of those predicted, and the centrifugal distortion constants (D J and D J K ) are close to those of other silicon–carbon or hydrocarbon chains of similar size and structure. The absence of lines at subharmonic frequencies indicates that the carriers of the observed lines are not from larger or heavier molecules, and the relative intensity ratio of the triplet components of each transition closely agrees with that expected from the nuclear spin statistics. The observed lines also pass several other tests: they are only found in the presence of an electrical discharge through gas containing SiH4 , as expected for a silicon-bearing molecule, and they vanish when diacetylene is replaced with deuterated diacetylene, indicating a molecule containing hydrogen. The intensities of the observed lines are also unaffected when a permanent magnet is brought near the molecular beam, as expected for molecules with closed-shell, singlet electronic ground states. Crucial conformation of the H2 CCSi assignment is finally provided by isotopic substitution: lines of 29 Si and the 13 C isotopic species were observed within 0.3% of those calculated from the theoretical structure, and those of the doubly deuterated isotopic species of H2 CCSi were observed within 0.4% of those similarly calculated.

TABLE 2 Spectroscopic Constants of H2 CCSi and H2 CCCCSi Isotopic Species (in MHz) H2 CCSi Constant

Measured

A B C D J × 103 DJ K

300000.b 5331.9529(4) 5233.4724(6) 1.27(1) 0.1432(3)

Expecteda 300000 5318. 5225. 0.86c 0.144d

H2 13 CCSi

H2 C13 CSi

H2 CC29 Si

D2 CCSi

H2 CCCCSi

300000.b 5150.8773(5) 5058.9321(5) 1.19(3) 0.1333(3)

300000.b 5317.0086(5) 5219.1281(5) 1.29(3) 0.1435(3)

300000.b 5255.7224(4) 5160.0112(4) 1.21(1) 0.1390(3)

150000.b 4764.5547(4) 4610.8076(4) 1.02(1) 0.1108(2)

300000.b 1391.4106(1) 1384.4756(1) 0.041(1) 0.01901(6)

Note. 1σ uncertainties (in parentheses) are in the last significant digit. From Ref. (14). b Constrained to theoretical A constant of H CCSi or D CCSi (Ref. 14). 2 2 c From C H (Ref. 35). 4 d From H C (Ref. 36). 2 4

a

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TABLE 3 Measured Transitions of H2 CCCCSi (in MHz) JK a ,K c 31,3 30,3 31,2 41,4 40,4 41,3 51,5 50,5 51,4 61,6 60,6 61,3 71,7 70,7 71,6

→

JK a ,K c

TABLE 4 Observed hfs of H2 CCSi Isotopic Species (in kHz) JK a ,K c → JK a ,K c

H2 CCCCSi

→ 21,2 → 20,2 → 21,1 → 31,3 → 30,3 → 31,2 → 41,4 → 40,4 → 41,3 → 51,5 → 50,5 → 31,4 → 61,6 → 60,6 → 61,5

10,1 → 00,0

8317.138 8327.652 8337.942 11089.513 11103.533 11117.254 13861.884 13879.408 13896.557 16634.246 16655.278 16675.858 19406.608 19431.142 19455.152

20,2 → 10,1 21,1 → 11,0 31,3 → 21,2 30,3 → 20,2 31,2 → 21,1 41,4 → 31,3 41,3 → 31,2

Spin–rotation hfs (hyperfine structure) has also been observed in the lowest-J rotational transitions of the three singly substituted isotopic species of H2 CCSi; it is a consequence of the interaction of the 29 Si or 13 C nuclei with the small magnetic field produced by molecular rotation. Other possible hyperfine interactions such as H-H spin–spin, H spin–rotation, or 13 C-H spin–spin have been ruled out because the additional hfs is only observed when 13 C is present, and the magnitude of the observed splittings generally becomes larger as the distance between the two nuclei increases. The nuclear spin–rotation Hamiltonian can be written as

g

g

n

Bgg an0

0

0

0

−12 36 0

−13 37 0

— −57 0

−15 31 −9 12 0

−13 36 −7 12 0

— −60 28 −62 0

−9 16

−7 14

30 −55 18 −26 13 −29

C gg = 4Bgg



an0

0|L g |n2 + effects of nuclear charges, En − E0 [3]

[1]

where Ci is the spin–rotation coupling tensor of the ith nuclei (20). The measured hfs for the 29 Si and the two 13 C species of H2 CCSi is given in Table 4, and the diagonal elements of the nuclear spin–rotation tensor are given in Table 5. Sample lines of H2 CC29 Si with resolved hfs are shown in Fig. 3. Owing to lower signal to noise, it has not been possible to detect the rare isotopic species of H2 C4 Si. The theory of nuclear spin–rotation has been developed by Ramsey and co-workers, (20, 21), Flygare and co-workers, (22, 23), and others (24). General expressions for the coupling constants can be written as 

H2 CC29 Si

where the summations are over the principal inertial axes (g = a, b, c), all electrons i, and excited n; Bgg is the rotational  states −3 1 constant; an0 = hc µB gN µN i ri  is an off-diagonal radially dependent hfs constant; and the orbital angular momentum matrix elements are between the ground and excited states. Since hyperfine interactions are generally very small, normally it is only the diagonal elements of the spin–rotation tensor that are important. If the orbital angular momentum matrix elements are with respect to each atomic nucleus rather than with respect to the center of mass, and if centrifugal and vibration effects are neglected, Eq. [2] simplifies to

n

Ii · Ci · J,

i

C gg =

H2 C13 CSi

Note. Experimental uncertainties (1σ ) are 2 kHz.

C. Nuclear Spin–Rotation Hyperfine Structure in H2 CCSi



H13 2 CCSi

3/2 → 1/2 1/2 → 1/2 5/2 → 3/2 3/2 → 1/2 5/2 → 3/2 3/2 → 1/2 5/2 → 3/2 3/2 → 1/2 7/2 → 5/2 5/2 → 3/2 7/2 → 5/2 5/2 → 3/2 7/2 → 5/2 5/2 → 3/2 9/2 → 7/2 7/2 → 5/2 9/2 → 7/2 7/2 → 5/2

21,2 → 11,1

Note. Estimated experimental uncertainties (1σ ) are 2 kHz. Observed minus calculated frequencies are 0–2 kHz; the best fit constants are given in Table 2.

Hnsr =

F  → F 

0|L  g |n n|L g |0 , En − E0

[2]

where g = a, b, c. The contribution to Eq. [3] from the rotation of the nuclear frame of the molecule is readily calculated from the molecular structure. For H2 CCSi it is of order 10 kHz or less, and can be neglected in the present analysis. TABLE 5 Diagonal Elements of the Nuclear Spin–Rotation Tensor for H2 CCSi Isotopic Species (in kHz) Isotopic Species

Caa

Cbb

Ccc

H13 2 CCSi H2 C13 CSia H2 CC29 Sia

112(14) 142(14) −723(30)

−8(4) — —

−6(4) — —

Note. The 1σ uncertainties (in parentheses) are in the last significant digit. a C and C constrained to zero. bb cc

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FIG. 3. Sample spectra of H2 CC29 Si showing nuclear spin–rotation hyperfine splitting in rotational transitions near 31 GHz in the K a = 0 and the two K a = ±1 ladders. Each hyperfine transition has a double peaked line profile as a result of the Doppler shift between the fast-moving molecular beam and the two traveling waves that compose the confocal Fabry–Perot mode. Typical integration times were 8 min per spectrum.

There are several points worth emphasizing. First, the Caa values determined here (Table 5) are all large. In other closed-shell molecules where nuclear spin–rotation hfs has also been observed, coupling constants are typically on the order of a few kilohertz (25, 26). One notable exception is difluorcarbene, CF2 , where Caa is comparable to that here, presumably because of the presence of a low-lying excited electronic state (27). The large Caa values here, however, arise for a different reason: as Eqs. [2] and [3] show, the magnitude of the diagonal elements is proportional to the rotational constant along that axis. Because the A rotational constant is more than 50 times larger than either B or C in H2 CCSi, the hfs splitting in the K a = ±1 levels is much larger than the hfs splitting in the K a = 0 level, and consequently Caa is much larger than either Cbb or Ccc . The sign of Caa for H2 CC29 Si is the opposite of that of the two 13 C isotopic species, because of the opposite sign of the nuclear magnetic moments (28). The three Caa values in Table 5 differ considerably in magnitude because an0 in Eq. [2] is proportional to r −3 , and this expectation value apparently changes considerably along the carbon chain.

D. The Structure of H2 CCSi An experimental (r0 ) structure of H2 CCSi was obtained by a least-squares adjustment of the three bonds and the HCH bond angle in Fig. 1 to reproduce the measured rotational constants in Table 2, i.e., those of the three rare isotopic species and D2 CCSi, plus that of the normal species. Our calculation assumes that H2 CCSi is planar and possesses C2v symmetry. The bond lengths and angle are compared in Table 6 with equilibrium re structures calculated ab initio using different basis sets at the self-consistent field (SCF) level of theory. Uncertainties in the r0 structure are derived on the assumption that the largest source of error is zero-point vibration. The magnitude of this error and how it is partitioned among the three moments of inertia are unknown, but it was estimated by assigning to each rotational constant an uncertainty which yields a χ 2 of 7, the most probable value for six degrees of freedom (29). The same uncertainty in B and C was assumed for each isotopic species: 0.018 MHz. One advantage of the approach used here and elsewhere (30) is that since the present

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ROTATIONAL SPECTRA OF H2 CCSi AND H2 C4 Si

TABLE 6 H2 CCSi Structures

Parameter r (H-C(1) )/A˚ r (C(1) -C(2) )/A˚ r (C(1) -Si)/A˚  HC(1) H/(deg)

Theoretical

Experimental (r0 )

DZ SCFa

DZ+P SCFa

CASSCFb

1.099(1) 1.321(1) 1.703(1) 117.3(1)

1.079 1.324 1.709 116.0

1.083 1.318 1.684 116.4

1.078 1.310 1.716 116.4

Note. Structure that best reproduces the observed rotational transitions of the five isotopic species (see Sect. III.D). Estimated uncertainties in the last significant digit are given in parentheses. a From Ref. (13). b From Ref. (14).

molecules possess large vibrational–rotational coupling constants (owing to the light mass of hydrogen) zero-point vibration is explicitly taken into account in a simple and systematic way. The ro structure here is in reasonable agreement with the re structures of H2 CCSi at the SCF level of theory (13), calculated with a double-ζ basis set either alone (DZ SCF) or augmented with polarization functions (DZ + P SCF), or at the complete active-space SCF level of theory (14) using the TZVP basis set (CASSCF). Not surprisingly, the H–C bond differs by the largest ˚ but the amount between the r0 and re structures (∼0.02 A), ˚ of experimental C–C and C–Si bond lengths are within 0.015 A the theoretical values using the three basis sets, and with the DZ basis set—for example—the theoretical and experimental bonds ˚ The experimental  HC(1) H bond differ by no more than 0.006 A. ◦ angle of 117.3 ± 0.1 also closely matches that from theory, the difference between the two amounting to less than 1.3◦ for either of the three re structures. The C–C and C–Si bond lengths are close to the standard value for double bonds, indicating that the bonding in the heavy atom backbone of H2 CCSi is cumulenic. IV. DISCUSSION

As the present work demonstrates, cumulenic chains terminated with oxygen, sulfur, and now silicon are readily produced in certain gas discharges, and can be detected in the laboratory by FTM spectroscopy. It is likely that the rotational spectra of other cumulenic chains, including those terminated with phosphorus, germanium, etc. are also stable and detectable. Phosphorus molecules such as H2 CCP and H2 CCCP are especially good candidates for laboratory detection because the isovalent nitrogen chains have been well characterized in the laboratory (31) and because phosphorus-bearing molecules can be readily produced through discharges containing PH3 (32). The present silacumulenes are candidates for astronomical detection for several reasons: (1) they are similar in structure and composition to known astronomical silicon-bearing chains,

233

(2) H2 CCSi is isovalent to H2 C3 , which has been detected in several astronomical sources, including the circumstellar envelope of the evolved carbon star IRC + 10216, and (3) astrochemical models of dense molecular clouds (33) predict that the simplest silacumulene chain, H2 CSi, has a fractional abundance comparable to SiC2 , which is quite abundant in IRC + 10216. Preliminary radioastronomical searches by Saito and co-workers (3) failed to detect H2 CSi toward several astronomical sources, but this failure may be the result of its small dipole moment, estimated to be only 0.3 D. The dipole moment of H2 CCSi is calculated by Cooper (14) to be considerably larger (µ = 1.21 D, i.e., a factor of 4), so H2 CCSi may be much easier to detect in space than H2 CSi, even if less abundant. With the spectroscopic constants listed in Table 2 the astronomically most interesting lines of H2 CCSi can be predicted to better than 1 ppm up to 150 GHz, and those of H2 C4 Si can be predicted to the same level of accuracy up to 75 GHz. Other isomers of H2 C2 Si may be detectable with our present spectrometer because rotational lines of silacyclopropenylidene (c-SiC2 H2 ) and vinylidenesilene, the cumulene isomer detected here, are fairly strong. The next best candidate is silylenylacetylene, a low-lying isomer that is calculated to possess a nearly linear Si-C-C backbone in which a hydrogen atom is attached to each end of the chain. It is calculated ab initio (13) to lie 22 kcal/mol above the cyclic ground state isomer, but only 5 kcal/mol higher in energy than the cumulene isomer. Because silylenylacetylene is a nearly prolate, asymmetric top, with transitions that are harmonically related in frequency by integer quantum numbers, its rotational spectrum should be fairly easy to identify. The determination of nuclear spin–rotation coupling constants at different atomic positions along a carbon chain may be useful in elucidating the electronic structure and bonding in closed-shell molecules such as H2 CCSi with an even number of electrons and a 1 A1 electronic ground state, and hence no magnetic interactions between the nuclei and electrons in the absence of molecular rotation. Isotopic substitution combined with rotation gives rise to a perturbation which slightly excites higher orbitals of the valence electrons and produces a small internal magnetic field. Because the coupling constants that describe the magnitude and projection of this interaction onto the molecular axes are proportional to the average distribution of valence electrons in the vicinity of each atomic nucleus, these constants may prove to be a sensitive probe of the molecular wave function—a key feature for molecules which otherwise lack magnetic hyperfine interactions. Although coupling constants apparently have not yet been calculated ab initio, it may be worthwhile doing so now that experimental data is available. The spin–rotation coupling constant Caa at each nucleus along a chain can be compared with little ambiguity because isotopic substitution has a negligible effect on excited state energies, on the radial and angular expectation values of the valence electrons, and on the A rotational constant. Our results, for example,

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suggest that the unpaired electron density is highest on the silicon atom of H2 CCSi, owing to the pair of singlet-coupled electrons which are formally located on this atom. This interpretation is in good agreement with the 13 C spin–rotation coupling constants of H2 C3 , H2 C4 , and H2 C5 , which were measured by the same technique and are described in the accompanying paper (34). For each of these molecules, the largest coupling constant (by a factor of 2 or more) was at the carbene carbon terminating the chain. ACKNOWLEDGMENTS The authors thank C. A. Gottlieb and J. K. G. Watson for helpful discussions, A. J. Apponi for assistance with some of the early experiments, J. Dudek for the synthesis of diacetylene, and E. S. Palmer for help with the microwave electronics and cryogenic system. We also gratefully acknowledge L.A. Silks III of the NIH Stable Isotope Resource at The Los Alamos National Laboratory for assistance in the production of the carbon-13 enriched lithium carbide. The Los Alamos Isotope Resource was supported by USPHS Grant RR02231 and by the U.S. Department of Energy. The present research is supported by NASA Grant NAG5-9379 and NSF Grant AST-9820722.

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