J. Am. Chem. SOC.1985, 107, 7877-7880 predict the preferred structures of ionophores, artificial molecular receptors, proteins, ..., in solution, even when relative conformations of species with identical binding sites are compared. It also makes it clear that the solvation pattern depends on the very nature of the solvent.I6 Particularly, the bridging position found for some water molecules is likely to be a specificity of that solvent. In mixed solvents containing water, or even when traces of water are present, such bridging water molecules a r e expected to become part of some of the structures. It is also clear that H30+,which makes stronger H bonds to ethers than does H,O, should also influence the conformation of crown ethersI0-l7and of other flexible (16) Mosier-Boss,P. A.; P o p , A. I. J . Am. Chem. SOC.1985, 107, 6168. (17) Sharma, R. B.; Kebarle, P. J . Am. Chem. SOC.1984, 106, 3913.
7877
molecules, even in very weakly acid conditions.
Note Added in Proof. For the C1conformer, we have rerun a Monte Carlo simulation on a cluster of 250, instead of 100, water molecules. The energy results, after lo6 moves, are as follows: E,, = -6.7 kcal/mol per water molecule, E,, = -53.3 kcal/mol. They show, as expected, an appreciable improvement of the water-water energy, while the solute-water interaction energy is very close to the previous one, indicating that 100 H 2 0 already account for the major part of the “solvation” of the 18-crown-6. Acknowledgment. W e thank R . Ripp and J. M. Wurtz for setting up programs on the PS300 computer graphics system a t the IBMC, Strasbourg. Registry No. 18-Crown-6, 17455-13-9.
The Infrared Spectrum and Force Field of C 3 0 Ronald D. Brown,* David E. Pullin, Edward H. N. Rice, and Martin Rodler Contribution from the Department of Chemistry, Monash University, Clayton, Victoria, Australia 31 68. Received June 4, 1985
Abstract: The infrared spectra of matrix-isolated tricarbon monoxide and six isotopically substituted species have been recorded. C 3 0 was formed by pyrolyzing fumaroyl dichloride seeded in excess argon. The v,, v2, and v4 bands have been observed. A normal coordinate analysis has yielded a general harmonic force field. This has been compared with the force field from ab initio calculations.
W e recently reported the pyrolytic generation of tricarbon monoxide and its characterization by its microwave spectrum,’ followed by further microwave studies of various isotopic variations of the molecule leading to a substitution geometry.2 The study was substantially assisted by a preceding theoretical investigation by ab initio molecular orbital method^.^ This provides predictions of force constants and vibrational frequencies, the lowest of which, v5, could be estimated approximately from the I-doubling transitions observed in the microwave spectrum.2 It seemed worthwhile to attempt to observe vibrational transitions in the infrared for C 3 0by preparing low-temperature argon matrices, particularly since DeKock and Weltner4 had tentatively proposed the presence of C 3 0 among the products obtained by deposition of CO and carbon vapor, mainly C or C2, in cold argon matrices. Their evidence was an infrared band a t 2244 cm-I, which they assigned as the v1 of C30. W e now report the successful observation of the I R spectrum of matrix-isolated C30,including a number of different isotopic versions yielding a general harmonic force field, and more elaborate a b initio molecular orbital calculations of both vibrational frequencies and intensities that helped in the assignment of the bands.
Experimental Section Apparatus. A closed cycle cryostat (model 21C cryodyne from CTI cryogenics) was used for all experiments. The cold stage of the cryostat, with which the copper block containing the CsI window was in direct contact, was at 9-10 K. Both deposition of the matrices and measurements of the infrared spectra were carried out at the lowest available temperature. The spectra were recorded in the region 4000-300 cm-’ (1) Brown, R. D.; Eastwood, F. W.; Elmes, P. S.;Godfrey, P. D. J . Am. Chem. SOC.1983, 105, 6496-6497. (2) Brown, R. D.; Godfrey, P. D.; Elmes, P. S.; Rodler, M.; Tack, L. M. J. Am. Chem. SOC.1985, 107, 41 12-41 14.
0002-7863/85/1507-7877$01.50/0
with a Perkin Elmer 180 spectrophotometer. The instrument was calibrated by measuring reference gases. The absolute frequencies of the absorption maxima are believed to be accurate to within 0.3 cm-l. Generation of C,O. Gas mixtures of fumaroyl dichloride and argon (ratio 1500) were prepared in advance and stored in a 1.6-L bulb at 0.5 atm. The argon used had a stated purity of 99.995%. The matrices were deposited at a rate of 40 mmol of argon per h. The mixtures were pyrolyzed in a silica tube of 16 mm inner diameter. A 30 cm long electric oven was used and the temperature measured with a thermocouple. Under these conditions the optimum temperature for the generation of C30 was 1100-1200 “C. In some experiments a straight quartz tube was used, the oven ending about 25 cm in front of the cryostat. In order to reduce the influx of radiation from the oven to the CsI window, a 90” bend was later introduced between oven and cryostat. This had no noticeable influence on the yield of C30. Chemicals. The preparation of isotopically enriched fumaroyl dichloride (fumaryl chloride, (E)-2-butenedioyl dichloride) has been described earlier.2 The three enriched samples yielded mixtures of C 3 0 with the following isotoptic ratios: 90% CC”C0 + 10% CCCO, 45% C”CC0 + 45% ”CCCO + 10% CCCO, and 16% CCClSO + 84% ccco. Molecular Orbital Calculations. To obtain an improved prediction of the infrared spectrum we extended the previously reported ab initio calculations.’ The 6-31G* basis set was used throughout this work. This basis set includes d-polarization functions on all atoms. The off-diagonal force constants were calculated at the Hartree-Fock level (HF) with the MP3/6-3 1G* geometry reported, while all diagonal force constants were computed at the MP3 level, which includes electron correlation. Due to the regular overshoot of the calculated frequencies a scaling factor of 0.958 was introduced which was obtained from similar calculations on known molec~les.~The derivatives of the dipole moment with respect to the geometrical parameters were calculated numerically at the Har(3) Brown, R. D.;Rice, E. H. N. J . A m . Chem. SOC. 1984, 106,
6475-6478.
(4) DeKoch, R. L.; Weltner, W. J . Am. Chem. SOC.1971,93,7106-7107. ( 5 ) Brown, R. D.; Rice, E. H. N.; Rodler, M. Chem. Phys. 1985, 99, 347-356.
0 1985 American Chemical Society
7878 J . Am. Chem. SOC.,Vol. 107, No. 26, 1985
Brown et al.
Table I. Observed Infrared Transitions (cm-I) of Isotopomers of Tricarbon Monoxide Embedded in an Argon Matrix isotopomer assignment frequency re1 intensity S h(2) 2242.6 ccco
CCCI*O
CC"C0
CWCO
V2(V
1907.2
W
u4(m
579.6
W
v2
2224.2 1888.8
u4
575.3
81
y4
563.9
W
VI
2225.7 1872.8 575.8
W
y4
S
W
S
0.0'
F2.2 F2,3
10.62 (25) 1.05 (12)
F3,3
11.61 (15)
0.00 10.99
0.52 13.12
0.609 (12) 0.653 F4,s 0.014 (8) 0.027 F5S 0.066' 0.066 a Stretching force constants in mdyn.A-', bendin,g constants in mdyn-A. bNumbers in parentheses represent one standard deviation of the fit. 'Not fitted, transferred from the calculated force field.
W
V4
2240.0 1886.5 579.6
C'3C"CO
VI
2 174.0
W
'3CC13CO
V1
2190.5
W
VI
u2
F1,3
F4.4
2194.3 1898.2
y2
'3CCCO
m vw vw
V2
VI
Table 111. Force Constant Matrix of C 3 0 Derived from Experimental Data and ab Initio Calculations" force constant exptlb calcd FIJ 16.82 (16) 19.27 Fl,2 1.85 (16) 1.55
S
W W
Table 11. Observed and Calculated Relative Intensities of the Fundamentals of C,O obsd calcd VI 100 100 y2 2.3 2.6 0.02 u3 u4 3.3 3.4 0.16 US tree-Fock level. This allowed the prediction of the relative intensities of the bands.
Observations The intensity of the band at 2243 cm-l, which was assigned as u1 of C 3 0 by deKoch and W e l t ~ ~ ewas r , ~ used to optimize the experimental conditions. (These authors called this highest stretching frequency u3 rather than v,.) Figure 1 shows the infrared spectrum of the pyrolysis of unlabeled fumaroyl dichloride under the optimum condition described above. With the use of the ab initio force field the two weak peaks visible besides the strong band a t 2243 cm-' could be readily assigned to I3C isotopes of C 3 0 . The definite assignment of other bands to C 3 0 was not possible without further information. Thus three samples of isotopically enriched fumaroyl dichloride were prepared and their spectra measured after pyrolysis. They allowed the unequivocal assignment of the u2 and v4 bands which was mainly based on relative intensities and the agreement between observed and calculated frequency shifts. Table I lists all the measured absorption maxima of bands assigned to the different isotopomers of tricarbon monoxide. None of the bands was found to be split due to different crystal fields in different matrix sites, but the intense u1 bands show an unresolved tail on the high frequency side. The full width a t half maximum of all other bands was 1.0-1.2 cm-I. Table I1 contains the measured and calculated relative intensities of the fundamentals of C 3 0 . The excellent agreement between theory and experiment must be considered fortuitous because the small calculated values result from differences between rather large numbers. The very low intensity calculated for the lowest stretching mode u3 explains why it could not be observed. The energy of the lower bending mode was estimated from microwave measurements2 to be about 150 cm-'and was thus outside the frequency range used. General Valence Force Field The 17 vibrational frequencies listed in Table I were used for the force-field refinement. It was done by fitting the force constants to the observed frequencies of the parent isotopic species and to the frequency shifts between the other isotopomers and the parent species. Due to the different kinds of data the following
weight was attached to each datum: for the frequencies 2% of their value and for the shifts 10% of their value plus 0.2 cm-l. The reported substitution structure2 was used for the calculation. The symmetry coordinates were defined as follows:
SI = r(O-Cl)
S2 = r(CI-C2)
S3= r(C2-C3) s 4
= P(OCIC2)
sS
=
P(clc2c3)
with the atomic numbering scheme C3C2C10. The ab initio force constants listed in Table 111 have been used as starting values for the refinement. Due to the absence of any experimental data for the lowest stretching mode u3, not all 6 force constants of the 2 block could be determined. We therefore decided to fix the interaction constant F1,3because it was least determined by the observed data. It was set to zero as predicted by the molecular orbital calculations. A similar situation was found in the II block, where the absence of precise data for the low-lying CCC bending vibration did not allow the determination of Fs+s. Its value was also transferred from the calculated force field. This seemed justified by the fairly good agreement between the calculated and estimated2 frequency of the CCC bending mode. The resulting force constant matrix is shown in Table 111; Table IV compares the calculated frequencies and shifts from the experimental and ab initio force field with the observed values.
Discussion and Conclusions The study of the products of the thermal decomposition of fumaroyl dichloride by matrix isolation infrared spectroscopy has led to the unequivocal observation of the spectrum of tricarbon monoxide. The assignment of the u1 band by deKoch and Weltner4 in 1971 proves to be correct. Besides C 3 0 ,C 0 2 ,CO, and HC1 the pyrolysis yielded many bands which could not easily be assigned to know substances. Of special interest in this context is the potential intermediate C,O2. Two possible reaction paths may lead from fumaroyl dichloride to C 3 0 :
o=c=c=c=c=o p o
-HCI/
,c=c H
-HCI , o=c=c=c
YH
CIOC,
COCl
H / \
c=c=c=o COC I
-co\ HC=C-COCI
-
