4787
J . Phys. Chem. 1994, 98, 41814792
Structures and Infrared Spectra of Perfluoroaldehyde and Perfluoroacetaldehyde J. Pacansky' and R. J. Waltman IBM AImaden Research Center, 650 Harry Road, San Jose, California 95120-6099
Y. Ellinger Laboratoire de Radioastronomie URA 336, Ecole Normale Supirieure, 24 rue Lhomond, 75005 Paris, France Received: October 8, 1993; In Final Form: December 17, 1993"
Ab initio calculations using Hartree-Fock and Moller-Plesset MP2 wave functions are reported for carbonyl fluoride, COF2, and perfluoroacetaldehyde, CF3CFO. Optimized geometries, vibrational frequencies and intensities, force constants, and normal mode displacement vectors are calculated. Vibrational frequencies and intensities are compared to experiment and are reproduced to within a few percent using correlated wave functions. The same accuracy is attainable when proper scaling is applied to Hartree-Fock wave functions.
Introduction
Experimental Section
Poly(perfluor0 ethers) are extensively used as industrial lubricants. Consequently,an understanding of their degradation mechanisms and energetics and the subsequent unstable intermediates and stable products formed thereof have been an area of active research. For example, poly(perfluor0 ethers) are widely used in the magnetic recording industry as disk lubricants to provide the desired tribological characteristics between disk and head. Frictional problems during the starting and stopping of the rotary disk causes depletion of the lubricant, attributed to thermally induced degradation. This has initiated a variety of studies on the thermal degradation of poly(perfluoro ethers) in the presence of metals, metal oxides, and Lewis acids,lJ revealing that main chain scissioning is the predominant effect and the stable products ultimately formed are C3F6, CFsCOF, and COF2. The identification and characterization of the reactive and transient chemical intermediates and stable degradation products have largely appealed to infrared spectroscopy, one of the few available techniques for probing the molecular structure nondestructively under such conditions. Spectral interpretation, however, has been hampered because of the generally broad absorption bands which are characteristic of these material^.^ Additionally, fluorine has no isotopes to do a normal coordinate analysis for band assignment. For this reason, ab initio quantum mechanical calculations have become an important tool via which vibrational frequencies may be matched with experiment when suitable model molecules are considered. When computed with adequate basis sets, frequenciescan becalculated which are within 5-15% of the experimental values and even closer after they are properly scaled to account for the lack of correlation treatment and anharmonicity effects. In this report, we present theoretical and experimental results on two of the simpler, stable gases that are formed upon main chain degradationof poly(perfluor0ethers). They are the simplest perfluoroaldehyde,COF2, and perfluoroacetaldehyde, CF3CFO. The calculations were carried out to establish the level of theory which can be considered useful from the point of view of comparison with experimental data for larger systems. COF2 and CF3CFO are small enough molecules such that large basis set calculations and correlated wave functions may be used to compare with experiment. These results show that excellent agreement between experiment and theory can be obtained at the modest HartreeFock 6-31G* level of theory without recourse to larger basis sets or correlated wave functions.
COFz and CF3CFO were obtained from commercial sources and used directly. The infrared spectra of COF2 and CF3CF0 were obtained on a Perkin Elmer PE 983 Infrared Spectrometer equipped with a Model 600 Data Station. The gases were mixed on a vacuum line with argon and filled to 1 atm in a gas cell designed for interrogation via infrared, as described in an earlier p~blication.~
*Abstract publish4 in Aduance ACS Abstracts. April 1, 1994.
0022-3654f 94/20~8-4181%04.50 f0
Computational Details The calculations reported here have been performed at two levels of theory. First, at the SCF level, we have employed three basis sets of increasing quality; these are standard bases, namely, 3-21G, 6-31G*, and 6-311+G*, as developed by Pople and cow o r k e r ~ .Notation ~ is conventional with * and indicating the polarization and diffuse functions, respectively. While none of the basis sets used can be considered large enough to reach saturation in the one-particle space, they provide a selection of coherent bases which are computationally tractable for mediumsized molecules. The second level of theory takes into account correlation effects by means of the MP2 standard development. Only the extended 6-31G* and 6-311+G* basis sets were employed at this level. The Gaussian system of programs6 were used in this study. All geometries were gradient optimized and the second derivatives computed at each optimized geometry; no imaginary frequencies were computed for structures located at minima on the potential surface. For each level of wave function considered, we have evaluated the IR frequencies and intensities. In order to extract a clear description of the normal modes, we evaluated the appropriate B matrix' which in turn was used to transform the force constant matrix from a Cartesian to a symmetry-adapted internal coordinate representation. This transformation facilitates the assignment of the spectra and allows a comparison of the various types of motions among the five levels of theory.
+
Results and Discussion COFz. The optimized geometries for COFz are given in Table 1, where they are compared with the available experimental data.*-" The HF/6-3 11+G* optimized geometry for COFz is presented in Figure 1 as an illustrative example. The molecule is coplanar with CZ,point group symmetry. The results obtained at the H F level of theory show the usual trend of a shortening of the bond lengths with increasing flexibilityof the basis set. The CO bond is 0.012 A shorter at the 6-31G* level than at 3-21G. The effect is larger for the C F bonds which are 0.032 A shorter 0 1994 American Chemical Society
Pacansky et al.
4788 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994
TABLE 2 COF2
Vibrational Frequencies and IR Intensities for Lpla
Ialcd
scale (l" vexptla (km/ (cm-1) factor mol) (cm-I) mol) V-M
Figure 1. HF/6-3 ll+G*-optimized geometry for COF2.
vibration
level
6 (Ai)
HF/3-21G HF/6-31G* HF/6-311 + G* MP2/6-31GS MP2/6-311 + G* HF/3-21G HF/6-31G* HF/6-3 1 1 + G* MP2/6-31GS MP2/6-311 G* HF/3-21G HF/6-31G* HF/6-311+ G* MP2/6-3 1G' MP2/6-311 + G* HF/3-21G HF/6-31G* HF/6-311 + G* MP2/6-31G* MP2/6-3 1 1 G* HF/3-21G HF/6-31G* HF/6-311 G* MP2/6-31G* MP2/6-311 + G* HF/3-21G HF/6-31G* HF/6-3 1 1 + G* MP2/6-31G* MP2/6-311 + G*
5 (B2)
TABLE 1: Optimized Geometries and Energies for Perfluoroaldebyde rco zero-point (A) LFCF(deg) energy (au) energy (au) level
4 (Ai)
HF/3-21G HF/6-31G* HF/6-311 G* MP2/6-31G* MP2/6-311 G*
3 (BI)
exptl
+ +
1.169 1.157 1.150 1.186 1.177 1.174
1.322 1.290 1.285 1.326 1.318 1.312
108.4 108.3 108.3 107.5 107.5 108.0
-309.904 -311.615 -311.707 -312.265 -312.517
282 306 148 165 539
0.015 333 0.015 787 0.015 692 0.014 185 0.014041
at 6-31G* than at 3-21G. A further shortening of 0.007 8,(CO) and 0.005 8, (CF) is obtained when a polarized basis of triple-{ quality plus diffuse functions (6-311+G*) is used. With the exception of the 3-21G C F bond length, all interatomic distances are smaller than experimental distances. At the MP2 level we note a lengthening of all bonds by 0.026-0.036 8, with respect to the S C F results, leading to values systematically larger than experimental values. Theoretical bond angles are within 0.5' from the reported experimental value and appear insensitive to the level of calculation. A final comment can be made on the geometry of COF2. All calculations give a C O bond length that is 0.03 8, shorter than the analogous COH2; this is also observed experimentally. Additionally, the >CO angle is observed to close by 14' when the hydrogens are replaced by fluorine atoms; Le., the experimental geometry12of COHz is CO = 1.203 A, CH = 1.099 8 , , a n d ~ H C H= 121.8'. Thecalculationperformedat theMP2/ 6-311+G* level of theory comes closest to reproducing the experimental results. The vibrational frequencies and infrared intensities are given in Table 2, where they are compared to the values obtained experimentally. The symmetry-adapted representation used for the assignment of the normal modes (Figure 3) is summarized in TabYe 3. An examination of the vibrational frequencies calculated at the SCF level of theory, presented in Table 2, reveals that the split-valence frequencies are often too large by 10-15%. The effects of polarization and diffuse functions do, however, tighten the discrepancy closer to the 10% deviation. Correlated wave functions give results much closer to experiment on the average, the discrepancy being not greater than several percent of the experimentally observed values. The calculated frequencies are harmonic values and thus suffer from the neglect of anharmonic effects. Further, they also suffer from the neglect of correlation effects when they are evaluated a t the H F level. General scaling factors used to match the theoretical to the experimental values can then be derived for every level of wave function: 0.92 for HF/3-21G, 0.90 for HF/6-31G*, and HF/6-31 l+G*, respectively, a t the SCF level. In the same way, scaling factors of 0.98 and 0.99 are obtained for MP2/6-31G* and MP2/6-31 l+G*, respectively. It can be seen also that the scaling factors of the individual vibrations are closer to the average value as the level of the wave function is improved. Experimental and theoretical spectra for COF2 are presented in Figure 2. The computed spectra were simulated using a Lorentzian function for each band, and the band widths were all set equal to 8 cm-1. This procedure is intended only for qualitative arguments, and to match the computed with the experimental
+
+
2 (B2)
+
1 (Ai)
a
2132.7 2186.8 2164.6 1994.9 1973.5 1454.5 1462.3 1416.4 1296.9 1232.9 1053.5 1093.7 1088.0 976.0 965.6 817.8 872.7 886.8 770.2 783.7 674.0 683.4 690.3 613.7 621.0 598.0 630.9 641.8 575.6 586.5
0.91 0.89 0.90 0.97 0.98 0.86 0.85 0.88 0.96 1.01 0.92 0.88 0.89 0.99 1 .00 0.95 0.89 0.87 1.00 0.99 0.93 0.92 0.91 1.02 1.00 0.98 0.93 0.91 1.01 1.00
450 599 699 376 477 407 476 539 407 48 5 35.2 55.1 68.7 55.3 73.8 83.2 70.8 73.6 35.5 37.2 24.9 17.1 16.5 8.5 7.4 9.7 6.6 7.6 6.5 7.4
1942
116
1244
105
965
19
774
7
626
584
This work.
TABLE 3
Symmetry Coordinates for COF2
y-
P
symmetry Ai
(.9
definition
SI = RI S2 = R2
s3 = a B2
+ R3
S4 = R2 - R3 s5
= 81 - 8 2
Bi SS = 7
description CO stretch symmetricCF stretch FCF bending
asymmetricCF stretch in-plane CO wag out-of-planeCO wag
spectrum as closely as possible. The corresponding normal mode displacement vectors are shown in Figure 3. The theoretical spectra at all levels of theory are generally well-matched with the experimental spectrum. Each of the calculated spectra presented in Figure 2 was uniformly scaled so that the highest frequency calculated matched the experimental high-frequency band at 1957 cm-I. It is observed that all levelsof calculationspresent essentially the same pattern, with two intense bands corresponding to the stretching of the CO bond (1 942 cm-I) and the asymmetric stretch of the CF bonds (1244 cm-1). Four bands of lower intensities are also found, the symmetric C F stretch (965 cm-I), the out-ofplane wagging of the CF2 group (774 cm-I), the in-plane rocking of the CF2 group (626 cm-I), and the FCF bending (584 cm-1). The assignment depends upon the types of motions that best describe thedisplacements of theatoms. As readily seen, in simple cases these motions are unique and easily identified, while in
The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4789
Structures and Spectra of COFl and CF3CFO
-
100
1957,
100
40 20 100
40 20
i:rq
100
40
20 0
2200 2000 1800 1600 1400 1200 1000 800 600
Wavenumber (cm-')
Figure 2. Experimental and computed vibrational spectra for COF2: (a) (c) HF/6-31G*, experimental;(b) HF/3-21G,scaleduniformlyby0.918; scaled uniformly by 0.895; (d) MP2/6-31G*,scaled uniformly by 0.98 1.
P
P
n
9
",--s.+ 5
-
7
4
6
Figure 3. HF/6-3 11+G* normal mode displacement vectors for COF2. The vibration numbers correspond to the definition presented in Table 2. The molecular orientation corresponds to the presentation in Figure 1.
others they are complicated linear combinations of several types of atomic motions. In order to avoid ambiguity, we have chosen the descriptions on the basis of the largest contributions to the normal coordinate displacement vectors in our most reliable calculation. At a qualitative level, the major effect of electron correlation is to shift the intensity from the out-of-plane wagging (774 cm-I) to the symmetric C F stretch (965 cm-I; see Table 2 and Figure 2). Concerning infrared intensities,comparison of the experimental and calculated infrared spectra (Figure 2) shows the remarkable ability of the most elaborate MP2 calculation to reproduce the relative intensities in the experimentaldata. Once again, however, we observe that the efficient and cost-effective HF/3-21G calculation provides more than adequate qualitative results that compare favorably with the much more sophisticated calculations and, thus, are appropriate for larger and analogous systemswhere more sophisticated calculations are not readily applicable. Finally, the linearly independent force constants for COFl at the various levels of theory are summarized in Table 4 s
(supplementary material), according to the definition of the force field presented in Figure 4s. The C-0 bond stretching force constant is by far the largest in COFZ, 15-1 9 mdyn/A, depending upon basis set, with the C-F stretches having force constants of =6.6-9. The effect of electron correlation is to reduce the bond stretching force constants by as much as -20%. CFsCFO. The optimized geometries for CF3CFO are summarized in Table 4. The HF/6-31G*-optimized structure is shown in Figure 4 as an illustrative example. The stable conformation of the perfluoromethyl group is one in which a C F bond eclipses the CO bond; it is thus analoguous to acetaldehyde. The rotation barrier is also very low, 1.06 and 1.23 kcal/mol at the 3-2 1G and 6-3 lG* levels of basis sets, respectively. A barrier height of 1.07 kcal/mol has previously been reported at the HF/ 3-21G level of theory,l3 and 1,090 f 0.003 kcal/mol from an analysis of far-infrared spectroscopicdata.14 It is essentially the same as that of CH3CH0, 1.16 kcal/mol,ls confirming that a sixfold or pseudo-sixfold barrier is always very low whatever the substituents are. The perfluoromethyl group is slightly distorted away from a local C3, symmetry. A comparison of the geometry for CF3CF0, at the HF/3-21G and HF/6-31G* levels of theory, shows a general shortening of bondlengthsupon increasing the basisset from3-21G to6-31G*. The CO bond is 0.01 3 8, shorter, and the C F bonds are 0.0270.032 8, shorter in going from the 3-21G to the 6-31G* basis set. Only the CC bond is observed to increase by 0.019 8, in going from 3-21G to6-31G*. At theMP2/6-31G1 levelofcalculation, the bond lengths increase by -0.03 8, except for the C2-C3 length which remains unchanged at 1.527 A. Theoretical bond angles are within l o at all calculated levels. The way the CF3 group rearranges during rotation can be seen by comparing the geometry of the stable conformation of Table 4 with that of the saddle point corresponding to the top of the rotation barrier (Table 5). The in-plane C F bond (CF5) is smaller (-0.004 8, at HF/6-31G*) when it is eclipsed with the CO bond than when it is trans to that bond, as it is in the saddle point. The angle this bond makes to the CC bond is smaller in the eclipsed than in the saddle point geometry. This rearrangement of the CF bonds is coupled to a relaxation of the CC bond which varies by 0.006 A during rotation. Thus, at the top of the barrier, there is a lengthening of the C2-C3 bond length by 0.006 8, to accomodate the change in the dihedral from 180 to Oo. There is a closing of the LOlC2C3 bond angle by 1.2O in concert with an opening of the LC2C3F5 bond angle by 0 . 7 O . The vibrational frequencies and infrared intensities are given in Table 6, where they are compared to the values obtained in experiment (Figure 5). The symmetry-adapted representation used for the assignment of the normal modes is based on the internal coordinate system presented in Table 7. An examination of the vibrational frequencies calculated at the SCF level of theory, presented in Table 6, reveals that the split-valence frequencies can deviate from experimental values by as much as 15%, even when polarization functions are used. Correlated wave functions at 6-31G* give results much closer to experiment on the average, the discrepancy being not greater than -7% of the experimentally observed values, except for the 288-cm-1vibration which deviates here by -20%. General scaling factors used to match the theoretical to the experimental values can then be derived for every level of wave function: 0.91 for HF/3-21G, 0.87 for HF/6-31G*, and 0.98 for MP2/6-31G*, respectively. It can be seen also that the scaling factors of the individual vibrations are closer to the average value as the level of the wave function is improved. Experimentaland theoreticalspectra for CF3CFO are presented in Figure 5. The correspondingnormal mode displacementvectors are shown in Figure 6. The theoretical spectra at all levels of theory are generallywell-matchedwith the experimentalspectrum.
4790 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994
Pacansky et al.
TABLE 4: Optimized Geometries and Energies for Perfluoroacetaldehyde geometrical param bond lengths (A) 01-422 C2-C3 C2-F4 C3-F5 C3-F6 C3-F7 bond angles (deg) LOl-C2-C3 LO 1-C2-F4 LC2-C3-F5 LC2-C3-F6 LC2-C3-F7 dihedral angles (deg) energy (au) zero-point energy (au)
HF/3-21G
HF/6-31G*
MP2/6-31GS
1.174 1.508 1.339 1.334 1.341 1.341
1.161 1.527 1.307 1.305 1.314 1.314
1.193 1.527 1.344 1.333 1.342 1.342
1.158 1.522 1.324
127.13 123.50 109.77 110.00 1 10.00
126.13 124.18 110.48 109.57 109.57
126.76 124.48 110.33 109.61 109.61
129.0 121.5
180.00 59.55 -59.55 -545.359 188 0.028 657
180.00 59.59 -59.59 -548.357 351 0.029 564
180.00 59.66 -59.66 -549.482 772 0.026 755
Vibrational Frequencies and IR Intensities for
vibration
level
15 (A')
HF/3-21G HF/6-31G* MP2/6-31G* HF/3-21G HF/6-31GS MP2/6-31G* HF/3-21G HF/6-31GS MP2/6-31G1 HF/3-21G HF/6-31G* MP2/6-31GS HF/3-2lG HF/6-31G* MP2/6-31G* HF/3-21G HF/6-31G* MP2/6-31G* HF/3-21G HF/6-31G* MP2/6-31GS HF/3-21G HF/6-31GS MP2/6-31Gt HF/3-21G HF/6-31GS MP2/6-31G* HF/3-21G HF/6-31GS MP2/6-31GS HF/3-21G HF/6-31G* MP2/6-31G* HF/3-21G HF/6-31GS MP2/6-31G* HF/3-21G HF/6-31G* MP2/6-31GS HF/3-21G HF/6-31G* MP2/6-31G* HF/3-21G HF/6-31GS MP2/6-31G*
13 (A') 12(A")
Figure 4. HF/6-3 IC*-optimized geometry for CF3CFO. 11 (A')
TABLE 5 Geometries and Energies for CF3CFO at the Top of the Rotational Barrier bond lengths (A) 01x2 C2-C3 C2-F4 C3-F5 C3-F6 C3-F7 bond angles (deg) LOl-C2-C3 LO 1-C2-F4 LC2-C3-F5 LC2-C3-F6 dihedral angles (deg) energy (au)
HF/3-21G
HF/6-31G1
1.176 1.514 1.335 1.344 1.314 1.314
1.161 1.533 1.304 1.309 1.312 1.312
124.94 123.30 111.14 109.47
123.74 124.03 111.82 109.08
0.00 120.76 120.76 -545.357 497
0.00 120.58 120.58 -548.355 237
10 (A') 9 (A") 8 (A') 7 (A')
I t is observed in Figure 5 that all levels of calculations considered here reproduce the experimental pattern of absorption bands, with the single exception occurring in the HF/3-21G-calculated spectrum whereby the 1455- and 1449-cm-I bands have overlapped. The characteristic bands for CF3CFO have been assigned previously from experiment and a b initio calculations at the HF/ 3-21G levelof theory." Themajor bandsobservedin experiment have the CO stretching vibration a t 1897 cm-l and the various CFstretchesoccurringat 1334,1253,1213,1200,and 1098cm-1. The small absorption band a t 805 cm-I is attributed to a CC stretch, and the 692-cm-l band to a deformation of the CF3 group. Many of these absorption bands are complicated linear combinations of several other atomic motions; here we have chosen the best description for each vibration on the basis of the largest
109.5 109.5
TABLE 6 CFKFO
14(A')
geometrical Daram
exumtl3
6 (A") 5 (A') 4 (A') 3 (A")
2 (A') 1 (A')
v0lcd v scale IC.lcd vcxptl Iexptl (cm-I) factor (km/mol) (cm-1) (km/mol)
2095.0 2175.5 1933.3 1454.5 1537.8 1403.7 1449.3 1447.2 1311.3 1408.1 1393.2 1259.9 1246.5 1256.8 1137.9 850.0 897.1 816.0 825.8 855.1 764.0 723.8 759.6 685.2 620.0 647.5 595.2 533.3 561.6 512.0 451.3 466.8 424.5 402.7 419.7 389.9 248.1 260.8 236.7 226.1 245.6 226.9 44.7 53.0 47.4
0.91 0.87 0.98 0.92 0.87 0.95 0.86 0.87 0.96 0.85 0.86 0.95 0.88 0.87 0.96 0.95 0.90 0.99 0.92 0.88 0.91 0.96 0.91 1.01 0.96 0.92 1.00 0.97 0.92 1.01 0.94 0.91 1.00 0.97 0.93 1.00 1.16 1.10 1.22 1.07 0.99 1.07 1.12 0.94 1.05
184 282 151 309 116 96 51 356 293 257 311 272 296 345 265 1.7 5.7 6.6 15.9 26.9 12.3 75 67 47 5.2 2.3 1.8 22 14 8.4 5.7 3.3 2.3 0.2 0.2 0.1 11
9 6 7 5 4 0.4 0.9 0.6
1897" 18996
48"
1334" 13406
300
1253' 12546
86O
12000 12146
75"
1098' 10996
800
805" 8066
6"
7600 7616
14"
692" 6926 5956 5196 426b 3906 2886
2426 506
This work. Reference 13.
contributions to the normal coordinate displacement vectors in our most reliablecalculation. We also note that the experimental carbonyl stretching frequency in CF3CFO occurs approximately 50 cm-I higher than in COF2. While the results of the
The Journal of Physical Chemistry, Vol. 98, No. 18. 1994 4791
Structures and Spectra of COF2 and CF3CFO
N 3
* 4
2200 2000 1800 1600 1400 120 100 800 600
Wavenumber (cm-')
Figure 5. Experimentaland computed vibrational spectra for CFsCFO
(a) experimental; (b) HF/3-21G, scaled uniformly by 0.906; (c) HF/ 6-31G*,scaled uniformly by 0.872; (d) MP2/6-31G*, scaled uniformly by 0.981.
TABLE I: Symmetry Coordinates for CFJCFO R I 01-CZ R2 C2-F4 R3 CZ-C3 R4 C3-F5 R5 CSF6 R6 C3-F7 61 C3-C2-F4 QI FW3-n a2 F5-C.3-F7 y C3-CZ-F401 TI 01.CtC3-FS U 01-C2-C3-F6 i) 01-CtC3-F7
svmmetrv
definition
description
~~
At Si = R1
S2 = R2 S3 = R3 S, R4 Ss Rs + & s6 = 61
s, = 62 - 83 ss = 81
S9 = - a 2 - a3 + 82 + 43 s i 0 = a1
A'
= Rs - & Si2 = 412 - 413 + 82 - 83
CO stretch single CF stretch CC stretch in-plane CF stretch symmetricCF2 stretch carbonyl FCC bending in-plane CO wag in-plane CF deformation symmetric CF2 wag symmetricCF2 bending
asymmetric CF2 stretch asymmetric CF2 twist S13 = 412 - 413 - 82 + 8 3 asymmetric CF2 rock out-of-plane CO wag SI4 = Y' si5 = T I + T2 + T3 CF3 torsion computations predict a longer C O bond for CF3CFO a t all levels of theory considered here, in accord with the ordering of the experimental CO stretching frequencies, attempts a t determining theCO bond lengths experimentally"l1J3 have resulted in slightly longer CO bonds for COF2 instead of CF3CFO. Finally, the linearly independent force constants for CF3CFO at the various levels of theory are summarized in Table 9S, according to the definition of the force field presented in Figure 8s. The C - 0 bond stretching force constant is by far the largest, -15-19 mdyn/A,dependingupon basisset, with theC-Fstretches having force constants of ~ 6 . 6 - 9 . The effect of electron correlation is to reduce the bond stretching force constants by as much as -20%. Sii
Figure 6. HF/6-3 lG* normal mode displacement vectors for CF3CFO. The vibration numbers correspond to the definition presented in Table 6. The molecular orientation corresponds to the presentation in Figure 4.
Conclusion Experimental and theoretical vibrational spectra were compared for COF2 and CF3CFO a t the HF and MP2 levelsof theory, using 3-21G, 6-31G*, and 6-311+G* basis sets. Excellent agreement between experiment and theory for vibrational frequencies and relative intensities have been attained at the MP2 level of theory with average deviations of approximately several percent. However, SCF calculations using flexible basis sets, 6-31G* and 6-31 1+G*, and even the cost effective split-valence 3-21G have also provided excellent agreement with experiment after scaling. Thus, in consideration of larger perfluorinated systems like polyperfluorinated ethers,3 SCF calculations are expected to provide an excellent description of chemical structure and vibrational spectra, to facilitate interpretation of the broad infrared absorption bands characteristic of these materials.
SupplementaryMaterial Available: Table 4s of force constants for COFz in mdyn/A; Table 9 s of force constants for CF3CFO in mdyn/& Figure 4s giving the definition of the force field for COF2; and Figure 8s giving the definition of the force field for
4792 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994
CFJCFO ( 5 pages). Ordering information is given on any current masthead page.
References and Notes (1) Sianesi, D.; Zamboni, V.; Fontanelli, R.; Binaghi, M. Wear 1971,18,
85. (2) Kasai, P. H.; Tang, W. T.; Wheeler, P. Appl. Surf.Sci. 1991, 51, 201. -. (3) Pacansky, J.; Miller, M.; Hatton, W.; Liu, B.; Scheiner, A. J . Am. Chem. SOC.1991, 113, 329. (4) Pancansky, J.; Waltman, R. J. J. Phys. Chem. 1991, 95, 1512. (5) Raghavachari, K.; Pople, J. A. Inr. J . Quantum Chem. 1981, 20, ~
1I "nl.7 ",.
(6) Gaussian 90, Revision I; Frisch, M. J.; Head-Gordon, M.;Trucks, G. W.; Foreman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.;
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