5748
J . Phys. Chem. 1992, 96, 5748-5752
stability of the ammonium group in certain ammonium compounds. It can be postulated that even at room temperature, free protons and free electrons are being released by the ammonium group in (NH4)2S04crystals, which then react with Mn04- to produce Mn2+. The rate of this Mn2+production at room temperature is slow but accumulative such that it can be eventually detected by EPR. It has been shownI0 that free protons released by the ammonium group contributes to the electrical conductivity of LiNH4S04crystals in all of its known phases, including the room-temperature and the low-temperature phases. We suggest that these free protons and free electrons participated in the room-temperature reduction of Mn04- into Mn2+in (NH4)2S04:Mn0, crystals. However, crystals of NH4C104:Mn04-and NH4BF4:Mn04-stored in the dark for more than 1 year did not exhibit any EPR spectrum of Mn2+. This tends to suggest that the ammonium groups in these two ammonium compounds are chemically more stable than those in (NH4)2S04.
Science Council (NSC) of the Republic of China under Project NO. NSC8 1-0208-M003-06. Registry No. Mn04-, 14333-13-2; NH4C104,7790-98-9; NH4BF4, 13826-83-0; Mn2+, 16397-91-4.
References and Notes (1) Zimmerman, G. J . Chem. Phys. 1955, 23, 825. (2) Ada", A. W.; Waltz, W. L.;Zinato, E.; Watts, D. W.; Fleischauer, P. D.; Lindholm, R.D. Chem. Reu. 1968,68, 541. (3) Carrington, A.; Symons, M. C. R. J . Chem. SOC.1956, 3373. (4) Wolfsberg, M.;Helmholz, L. J . Chem. Phys. 1952, 20, 837. (5) Klaning, U.;Symons, M. C. R. J . Chem. SOC.1953, 3580. (6) Wyckoff, R. W. G. Crysral Srrucrures, 2nd ed.; Interscience: New York, 1965; Vol. 3. (7) Jacobs, P. W. M.; Whitehead, H. M. Chem. Reu. 1969, 69, 551. (8) Chakraborty, T.; Khatri, S.S.; Verma, A. L. J . Chem. Phys. 1986,84, 7018. (9) Syamaprasad, U.;Vallbhan, C. P. G.Solid Srate Commun. 1981,38, 555.
(10) Syamaprasad, U.;Vallabhan, C. P. G. Phys. Rev. 1982,826,5941. (11) Yu, J. T.; Chou, S.Y. J. Phys. Chem. Solids 1990, 51, 1255.
Acknowledgment. This research is supported by the National
Ab Initio Study of the Identity of the Reaction Product between C3 and Water in Cryogenic Matrices Ruifeng Lib* Xuefeng Zhou, and Peter Pulay* Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701 (Received: October 8, 1991)
Geometry optimization, energy calculation, and ab initio vibrational analysis were carried out on different conformers of singlet and triplet hydroxyethynylcarbene and 3-hydroxypropadienylidene. They are probable products of the reaction C3 + H20. Theoretical methods used include restricted Hartree-Fock, unrestricted Hartree-Fock natural orbital-complete active space, and Moller-Plesset perturbation theory. By comparing the calculated harmonic frequencies (empirically scaled by 0.9) with observed FTIR frequencies of the product of the reaction C3 + H20 in cryogenic matrices, it appears likely that the product is singlet 3-hydroxypropadienylidene instead of the previously proposed hydroxyethynylcarbene.
I. Introduction In the past several years, there has been increased interest in the study of physical and chemical properties of all carbon clusters.'+ While most of the experimental efforts are focused on synthesis and characterization of larger and larger clusters, little is known about their chemical reactivity. Theoretical calculations on the structures, energetics, and reactivities of small carbon clusters have been shown to be very helpful in elucidating the physical and chemical For example, C, was thought to be linear for a long time, but ab initio calculations concluded that it is bent with an equilibrium angle 162' and a small barrier of about 21 cm-'to linear it^.^,^ Recent experimental w0rk8,~confirmed the nonlinear character of the C3structure, and the angle and the barrier to linearity were deduced to be 162.5O and 16.5 cm-I, in excellent agreement with ab initio predictions. Recently, elegant experimental work was conducted on the reactivity of small carbon clusters ranging from CI to C5toward water in cryogenic matrices.1° On the basis of the matrix FTIR spectra, it was concluded that neither ground-state Cl nor C2forms stable adducts or products under the experimental condition. This is in agreement with ab initio prediction and gas-phase experimental conclusion that it is C('D) and not C(3P) which reacts with H 2 0to form CO + H2 and formaldehyde.' Under the same condition in an argon matrix, C3 forms an addrlct with water without activation. This C3(H20) complex undergoes a sequence of photochemical reactions. Upon irradiation with 400-nm light,
a unique intermediate is formed and isolated. This intermediate photorearrangeswith 280-nm ultraviolet light into propynal, which was previously identified by the gas-phase infrared spectrum. I Based on the observed FTIR frequencies and l80isotope shifts, the intermediate was proposed to be hydroxyethynylcarbene (HEC).'O Carbene is one of the most important transient molecules. It has been well studied both theoretically and experimentally.I2 But hydroxyethynylcarbene had not been reported before. To characterize the structure, energetics, and harmonic vibrations, we carried out ab initio calculations and vibrational analysis on stable conformers of both singlet and triplet states. To our surprise, the calculated frequencies are not in reasonable agreement with the recorded matrix FTIR frequencies. To determine the identity of the product of C3with water, stable conformers of the singlet and triplet 3-hydroxypropadienylidene were also studied by the ab initio methods. The latter molecule has not been reported either, but vinylidenecarbene, an isomer of C3H2 and structurally similar to 3-hydroxypropadienylidene,was generated by photolysis of cyclopropenylidene in a matrix and identified by comparing the observed infrared spectrum with results of ab initio calculati~ns.'~ Both hydroxyethynylcarbeneand 3-h ydroxypropadienylidenc have the same stoichiometry, and both become propynal by a single hydrogen migration; therefore, both are probable reaction products. Our calculated results indicated that it is probably singlet 3hydroxypropadienylidenewhich was generated in the reaction of
0022-365419212096-5748$03.00/00 1992 American Chemical Society
Reaction between C3 and H 2 0 in Cryogenic Matrices
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5749
TABLE I: Optimized Structural Parameters' of the Singlet nod Triplet Hydmxyethynylurbew (HEC) and 3-Hydroxypropadienylne RCIC2 RC2C3 RCH
Rco Ron LHCC LCCC LCCO LCOH LCCOH LHCCC
S-HEC- 1 1.194 1.463 1.070 1.306 0.955 179.4 173.9 108.1 108.2 180.0 180.0
S-HEC-2 1.195 1.472 1.07 1 1.305 0.961 179.1 176.6 110.6 114.9 0.0 0.0
T-HEC 1.238 1.375 1.068 1.339 0.956 176.5 168.4 130.4 110.0 106.1 -167.3
S-HPD-1 1.263 1.339 1.077 1.307 0.953 122.3 177.7 125.6 110.4
0.0 0.0
S-HPD-2 1.265 1.333 1.081 1.313 0.948 120.1 174.9 123.6 111.8 180.0 0.0
T-HPD-1 1.244 1.383 1.079 1.354 0.953 122.6 178.0 123.4 110.2 0.0 180.0
(HPD)
T-HPD-2 1.242 1.384 1.081 1.357 0.952 121.5 177.5 119.6 109.6 180.0
0.0
'Bond lengths in angstrom and angles in degrees. C3 with water in low-temperature matrices. The details of the calculation are presented in the next section. The results and brief discussions are reported in section 111. Our conclusion is given in section IV. 11. Details of Calculation Both hydroxyethynylcarbene and 3-hydroxypropadienylidene are highly reactive and highly unsaturated molecules. For them, the simple restricted Hartree-Fock (RHF) procedure may not work satisfactorily. For the proper choice of theoretical methods, the recently proposed unrestricted HartreeFock (UHF) natural orbital criteriont4is applied. This is based on the occupancy of UHF natural orbitals to determine whether a multiconfigurational method is desired for proper descriptions of these molecules. Those orbitals with occupancies between 0.02 and 1.98 are considered fractionally occupied and taken as active orbitals in multiconfiguration SCF (MC-SCF) calculation^.'^ By this method, the triplet hydroxyethynylcarbene (T-HEC) and triplet 3-hydroxypropadienylidene (T-HPD) require CAS wave functions generated by distributing six electrons in six active orbitals (6 X 6). The six orbitals are the valence T orbitals on carbons. The singlet hydroxyethynylcarbene (S-HEC) has two fractionally occupied orbitals and thus requires a 2 X 2 CAS wave function. Great effort was focused on a search for UHF solutions of singlet 3hydroxypropadienylidene(S-HPD); the UHF solutions we converged do not differ significantlyfrom those of RHF. The natural orbital occupancies are either very close to 2 or very close to 0. Therefore, by the UHF natural orbital occupancy criterion, RHF should be sufficient for a qualitatively correct description in such a case. For the three multiconfiguration demanding molecules, the recently developed UHF natural orbital-complete active space (UNO€AS)l5 method was applied for the geometry optimization and force field calculations. All the geometries were fully optimized with the gradient techniqueI6 and geometry DIIS algorithm'' working in internal coordinateswith the program T X W . ~ * The UNO-CAS force fields were evaluated by numerical differentiation of the analytic energy gradients. For S-HPD, the geometries were optimized with RHF, and force fields were evaluated via the analytic second derivatives of energy with the program C A D P A C . ~ ~ Our experience shows that the fractionally occupied UHF natural orbitals approximate the complete active space SCF (CAS-SCF) orbitals so well that a full CI in the space of the fractionally occupied UHF natural orbitals gives a good approximation to CAS-SCF. Previous studies2OV2lhave shown that the UNO-CAS potential energy surfaces are closely parallel to CAS-SCF ones, which ensures that the UNO-CAS geometries and force fields are of CAS-SCF quality. For energy calculations, the Moller-Plesset perturbation method was used. The reference wave functions include RHF (MP2, MP3, MP4(SD)), U H F (UMP2 and spin-projected UMP2 (PUMP2)22), and GVB (generalized MP2 (GMP2)z3). These calculations were done with TX90 (for MP2 and GMP2) and CADPAC.
Throughout this study, Dunning's double-l (DZ)24plus sixcomponent d polarization functions on carbon (exponent 0.75)
S-HEC-1
S-HEC-2
T-HEC
S-HPO-1
'k
H'
I
H
sm-2
'k
"\
T-HPO-1
T3RO-2
Figure 1. Schematic structures of hydroxyethynylcarbene and 3h ydroxypropadienylidene.
and oxygen (exponent 0.85) were used. To try to reproduce the observed '*Oisotope shifts, a set of diffuse sp functionsZSon oxygen (exponent 0.0845) and carbon (exponent 0.0438) was added to the DZP basis for S-HPD. 111. Results Geometries. Results of complete geometry optimizations indicate that for these molecules only triplet hydroxyethynylcarbene is nonplanar. The rest are planar, and each has two minima on the potential energy surfaces around the equilibrium geometries. The two minima differ by the relative orientation between the C-C and 0-H bonds (the CCOH dihedral angles). Schematic structures of the molecules are given in Figure 1, The optimized structural parameters are presented in Table I. For the singlet hydroxyethynylcarbene, the bond distances for CC triple bonds (1.195 A) are in the range of a normal carbon-carbon triple bond (1.202 8,as in ethylenG6),and the CC single bond distance (1.46 A) is in the range of a normal carbon-carbon (sp-sp3) distance (compared to 1.46 A as in propynez7). The picture changes qualitatively for the triplet state. The CC distance 1.238 A is significantly longer than the normal carbon-carbon triple bond, and 1.375 A is apparently shorter than normal s p p 3 hybridized CC single bond. This indicates that the triplet state is more appropriately described by the reasonance structuresI0
-
HC=C~OH
HC=C=COH
Compared to hydroxyethynylcarbene, the CC distances for both singlet and triplet 3-hydroxypropadienylidene show stronger resonance effects. The C1-C2bond lengths (numbering of atoms is given in Figure 1) are well in between the normal CC double and triple bonds, and the C2-C3bond lengths are in between the
5750 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
Liu et al.
TABLE II: Total and Relative Energies of the Singlet and Triplet Hydroxyetbynylcarbene (HEC) and 3-Hydroxypropdienylidene (HPD) method S-HEC- 1 S-HEC-2 T-HEC S-HPD- 1 S-HPD-2 T-HPD-1 T-HPD-2 Total Energies [-(E + 189.0)] (hartrees) RHF 0.502 00 0.495 41 0.501 75 0.492 57 MP2 1.079 45 1.073 78 1.080 49 1.071 02 MP3 1.097 9 1 1.092 22 1.097 98 1.089 45 MP4(SD) 1.01048 1.004 50 1.009 35 1.000 65 GVB 0.51793 0.511 91 GMPZ 1.084 00 1.078 66 UHF 0.503 34 0.497 24 0.506 79 0.478 70 0.475 40 PUHF 0.51038 0.504 86 0.527 69 0.501 08 0.498 07 UMP2 1.073 39 1.066 96 1.029 88 0.991 93 0.987 32 0.01459 PUMP2 1.08 1 92 1.076 49 1.050 08 0.01032 RHF MP2 MP3 MP4(SD) GVB GMPZ UHF PUHF UMP2 PUMP2
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Relative Energies (kcal/mol) 0.2 -0.6 0.0 0.7
4.1 3.6 3.6 3.8 3.8 3.3 3.8 3.5 4.0 3.4
-2.2 -10.9 27.3 20.0
5.9 5.3 5.3 6.2 15.5 5.8 51.1 42.2
17.5 7.1 54.0 44.9
TABLE 111: Calculated and Observed Frequencies' and "0 Isotope Shiftsbof Hydroxyetbynylcarbene and Triplet 3-Hydroxypropdienylidene S-HEC-1 S-HEC-2 T-HEC T-HPD- 1 T-HPD-2 observede 3674, (-12) 3635, (-12) 3699, (-12) 3553, (-12), [39] 3646, (-12), [190] 3090, (-1) 3244, (0) 3057, (0) 3259, (0). 1431 1781, (-1) 1809, (0) 1992.8, (-0.7) 1797, (0) 1315, (-9),- [170] 1410, (-7) 1345, (-15) 1423, (-3) 1459.6, (-5.3) 1337, (-8),-[150] 1290, (-28), [274] 1252, (-8) 1186, (-10) 1240, (-13) 1252.5, (21.6) 1294, (-28), [305] 1164, (-12) 834.3, (-9) 1183, (-13) 1221.7, (-6.3) 850.0, (-l), [13] 868.7, (0), [85] 920.2, (-10) 525.4, (-6) 912.6, (-5) 1016.1, (-12.3) 823.0, (0), [81] 816.2, (O), [37] 509.8, (-7) 502.2, (-9) 756.6, (-9,[ 1891 472.9, (-2) 770.2, (-2), [28] 315.1, (-0) 684.4, (0), [66] 425.0, (0) 326.0, (0) 674.0, (0), [72] 285.9, (-1) 190.0, (-2) 258.4, (0) 621.0, (-8), [17] 606.2, (-lo), [2] 127.2, (-1) 140.0, (-0) 225.2, (-1) 263.9, (0), [14] 274.4, (01, [81 125.4, (-1) 77.2, (0) 36.1, (0) 218.6, (-3), [6] 233.0, (-3), [9] "The calculated frequencies have been scaled by 0.9; unit is cm-'. intensities. Reference 10.
normal CC single and double bonds. These bond distances can be explained by the following resonance structures:
:C=c-c I
P Cc:C=cLc/OH \ H
\ H
I1
:c=c=c
P"
OH
./
CC
*c=c-c
\H
111
\ H
rv
Since electrons with the same spin do not pair, only resonance structure I11 and IV contribute to the stability of the triplet states, while all of the above resonance structures contribute to the stabilization of the singlets. Therefore, the singlet states are expected lower in energy than the triplet states. This agrees with the calculated relative energy ordering between the singlet and triplet states of this molecule. Energies. Total and relative energies of the singlet and triplet hydroxyethynylcarbene and 3-hydroxypropadienylidene at the optimized geometries are presented in Table 11. If the singlets are compared at the RMP2 level to the RMP2 calculation for S-HEC-1 and the triplets at the PUMP2 level calculation for S-HEC-1, the relative energy ordering is as follows: S-HEC-1 (0.0 kcal/mol), S-HPD-1 (-0.6 kcal/mol), S-HEC-2 (3.6 kcal/mol), S-HPD-2 (5.3 kcal/mol), T-HEC (20 kcal/mol), T-HPD-1 (42 kcal/mol), T-HPD-2 (45 kcal/mol). It should be mentioned that although both UHF and projected UHF energies of T-HEC are lower than those of S-HEC- 1 and S-HEC-2, we believe the energy of the former is higher than the
Numbers in ( ) are '*O isotope shifts; numbers in [ ] are SCF/DZP IR latter. This is because UHF and PUHF recover more electron correlation for the triplet than for the singlets. Harmonic Vibrations. As mentioned earlier, the force fields were evaluated with SCF/DZP and UNO-CAS/DZP. These methods do not recover dynamic electron correlation, and together with basis set truncation errors, the harmonic frequencies calculated at these levels are expected systematically higher than the observed fundamental frequencies. Therefore, we scaled the calculated frequencies by 0.9. The results of the triplet molecules (T-HEC, T-HPD- 1, T-HPD-2) and singlet hydroxyethynylcarbenes (S-HEC-1 and S-HEC-2) are presented in Table 111. Those of S-HPD-1 and S-HPD-2 are in Table IV. Also, in these tables are the calculated l8O isotope shifts (in parentheses) and experimental results. For the singlet 3-hydroxypropadienylidenes and hydroxyethynylcarbene, the calculated IR intensities by RHF are also presented. For S-HEC-1 and S-HEC-2, the scaled carbon-carbon triple-bond stretching frequencies are 2140 and 2128 cm-l, which are in the range of normal C-C triple-bond stretching vibrations. The observed FTIR frequency closest to them in 1992 cm-I, more than 100 cm-' lower. There is essentially no calculated frequency within calculation accuracy that corresponds to the observed ones at 1459.6 and 1016.1 cm-I. The agreement between the scaled frequencies of T-HEC and observed results is even worse. The C-C triple-bond stretching is more than 200 cm-' off, and there are no frequencies close to those at 1459.6, 1221.7, and 1016.1 cm-I. On the basis of the comparison, it seems that both the singlet and triplet hydroxyethynylcarbenes are not the reaction product of C3 with water in matrices.I0 Since the triplets of 3-hydroxypropadienylideneare about 40 kcal/mol higher in energy than those of the singlets (Table II), the possibility for them to be the reaction product in cryogenic
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5751
Reaction between C3 and HzO in Cryogenic Matrices
TABLE I V Calculated and Observed Frequencies, Infrared Intemities, and Isotope Shifts', of Singlet 3-Hydroxypropadienyldeae
S-HPD-1
S-HPD-2 DZP+diffuse
DZP
DZP+diffuse
DZP
3674, (-12), [131] 3075, (O), VI
3669, (-13), [127] 3080, (0). [61
3724, (-14), [222] 3025, (-0), [20] 1952, (-0), [1214]
3714, (-14), [221] 3020, (0), [17] 1951, (-0), [1284]
1940, (0), [1214]
1940, (O), [1274] 1494, (-4), [lo91
[116] , 1495, (4)
1481, (-lo), (3611 1311, (-ll), [91]
1482, (-91, [3551 1309, (-11), [lo51 1285, (-18), [617] 1240, (-12), [118]
1281, (-18), [628] 1240, (-12), [lo41
1001, (-71, 121 958.3, (-2), [2] 585.1, (-ll), [14] 487.0, (-l), [118] 200.5, (01,1191 189.0, (-2), [2]
1001, [ll 956.3, (-2), [2] 594.5, (-11), [15] 494.0, (-l), [lo61 267.8, (0), [32] 213.5, (-2), [2]
1215, (-12), [285] 1016, (-12), [92]
1216, (-12), [284) 1018, (-ll), [91]
989.8, (0), [0.8] 652.3, (-4), [205] 583.5, (-9), [47] 198.1, (-01, [I1 181.9, (-2), [2]
995.0, (-l), [ l ] 657.5, (-5), [193] 596.0, (-9), [46] 267.2, (01, [ll 211.4, (-2), [3]
observedb
assignment
OH str CH str 1999.8, (-0.7) 1992.8, (-0.7) 1459.6, (-5.3)
CC str
CCH, HCO bend COH bend, CO str
w,
1252.5, (-21.6) 1221.7, (-6.3) 1016.1, (-12.3)
CCH, HCO bend CC, CO str CH wagging (a") HOCC, HOCH tors (a)') CCO bend CCCO bend (ar/) CCC (in-plane) bend
'The calculated frequencies have been scaled by 0.9; unit is cm-I. The numbers in ( ) are '*Oisotope shifts, and numbers in [ ] are ab initio infrared intensitiea in km/mol. bobserved results are from ref 10. matrices is also low. This is confmed by noting the dissimilarity between the scaled frequencies of T-HPD-1 and T-HPD-2 with the observed frequencies. Table I11 shows the scaled frequencies of the triplets corresponding to the observed one at 1992.8 c m - I are about 200 cm-l lower, which is out of the calculation accuracy. On the other hand, the scaled frequencies of both S-HPD-1 and S-HPD-2 (Table IV) are all within 50 cm-'of the o k e d results. However, neither S-HPD-1 nor S-HPD-2 itself could explain the o k e d I8O isotope shifts. Considering that the charge separation resonance structures I and I1 may be important resonance structures as indicated by the large dipole moment (about 4 D at the RHF/DZP level), the diffuse sp functions of Pople et alaz5 were added to the DZP basis to try to improve the description for the charge separation. This basii set was denoted DZP+diffw in Table IV. Contrary to our expectation, the calculated geometries, frequencies, and I8Oisotope shifts with the augmented basis set do not differ significantly from those of DZP. To bring the observed and calculated data into agreement, a plausible assumption may be that a mixture of S-HPD-1 and S-HPD-2 was observed in the experiment. With this assumption, the observed IR shift of the vibration at 1992.8 cm-I is in agreement with the calculated results of both S-HPD-1 and SHPD-2. The observed shift (-5.3 cm-') of the frequency 1459.6 cm-I agrees with the calculated shift of S-HPD-2 (-4.4 cm-I) within 1 cm-I, but this disagrees with the predicted intensities which are higher for S-HPD-1. The observed shift 21.6 cm-' of the vibration at 1252.5 cm-' is in reasonable agreement with the calculated shift 18 cm-I of S-HPD-2. The observed shift (-12 cm-l) of the vibration at 1016 cm-I is in agreement with the calculated result of S-HPD-1 (-1 1 cm-I). However, the observed shift of the frequency at 1221.7 cm-I does not agree with any of the calculated results for the two singlet HPD conformers. The difference between them is more than 5 cm-I. Moreover, if the predicted intensities are approximately correct, the observed band at 1459.6 cm-I should correspond to S-HPD-1 rather than to S-HPD-2. At present, the source of these discrepancies is not clear. In view of the closed-shell character of S-HPD, it is unlikely that higher levels of electron correlation or larger basis sets would change the qualitative nature of the predicted spectra. We have searched for other low-energy structures of this system, but we found only propadienal, HzC=C=C==O, which has a grossly different infrared spectrum. With the assumption that both conformers of S-HPD are present in the experiment, the observed doublet at 1992.8 and 1999.8 cm-I, which was assumed to be a result of matrix site splitting, could also be explained as absorptions of the C-C asymmetric stretchings of both S-HPD-1 and S-HPD-2. The calculated IR intensities of this mode are the strongest in both isomers. In fact, these calculated intensities were not anticipated, and it may be the highest IR intensities we have known so far.
I
I,
I
I
2200 2000 1800 1603 1400 1200 1000
rim
( ;&
'
-,>;I
'
S-UX-1
,
,
ado
,~
F i p e 2. (a) Observed FTIR spectrum of the product of the reaction C3 H20, from ref 10. (b) Calculated IR spectrum of S-HEC-2.(c) Calculated IR spectrum of S-HEC-1.
+
It should be pointed out that the calculated IR intensities are only qualitatively reliable. This is because the basis set which was optimized by energy criterion is perhaps not diffuse enough for dipole moment derivative calculations. For discussions on basis sets for intensity calculations readers are referred to ref 28. Based on the calculated frequencies and IR intensities of the singlet species, the predicted IR spectra arc compared with the observed spectrum (Figure 4 of ref 10) in Figures 2 and 3. In these figures, the bar height is proportional to the calculated intensity. They illustrate clearly the dissimilarity between the theoretical spectra of the singlet hydroxyethynylcarbeneand the observed spectrum of C3H20and the closer but still imperfect similarity between the observed spectrum and the theoretical spectra of the singlet 3-hydroxypropadienylidenes. The structure of the transition state of the interconversion between S-HPD-1 and S-HPD-2was also located at the
Liu et al.
5152 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
c,
,I,,
(Hp)
,
!
I
,
2200 2000 1800 1603 1400 1200 1000
I
with those of the scaled ab initio frequencies of the singlet 3hydroxypropadienylidene. Most of the observed '*Oinfrared isotope shifts, however, have to be explained by the presence of both conformers. On the basis of the stoichiometry and the rearranged product of this intermediate, we conclude that the reaction product is definitely not hydroxyethynylcarbene;instead, it is a molecule with a C=C=C: structural unit and probably a mixture of S-HPD-1 and S-HPD-2. 4. The predicted infrared intensity of the antisymmetric C= C = C stretching in the singlet hydroxypropadienylidenes is extraordinarily high, possibly the highest IR intensity encountered so far. We hope that these results will stimulate further work in this field; the frequency region below 1000 cm-' in the infrared spectrum would be particularly useful for positive identifications.
Acknowledgment. This research was supported by the US. National Science Foundation under Project No. CHE-8814143. Most of the calculations were done on an RS6530 workstation donated by IBM Co. Helpful discussions with Dr. G. Fogarasi on the internal coordinate definition and ab initio force field calculations are gratefully acknowledged.
I
Registry No. C, 7440-44-0; H 2 0 , 7732-18-5; 3-hydroxypropadienylidene, 141708-56-7; hydroxyethynylcarbene, 141708-57-8.
References and Notes I
I
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(24) Dunning Jr., T. H. J . Chem. Phys. 1970, 53, 2823. (25) The exponents of the diffuse functions are from those of 3-21+G and 6-31+G* basis sets; see: Hehre, W.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. In Ab initio Molecular Orbital Theory; John Wiley & Sons: New York, 1986; p 87. (26) Kostyk, E.; Welsh, H. L. Can. J . Phys. 1980, 58, 912. (27) Dubrulle, A.; Boucher, D.; Burie, J.; Demaison, J. J. Mol. Spectrosc. 1978, 72, 158. (28) Van Duijneveldt, J. G. C. M.; Van Duijneveldt, F. B. J . Mol. Strucf. 1982.89, 185. Pulay, P.; Fogarasi, G.; Zhou, X.;Taylor, P. W. Vib.Spectrarc. 1990, 1, 159.
