3438
J. Phys. Chem. 1995,99, 3438-3443
Ab Initio Calculations of Vibrational Frequencies and Infrared Intensities for Global Warming Potential of CFC Substitutes: CF3CH2F (HFC-134a) Stella Papasavva, Stephanie Tai, Amy Esslinger, Karl H. Illinger,” and Jonathan E. Kenny* Department of Chemistry, Tufts University, Medford, Massachusetts 02155 Received: September 22, 1994; In Final Form: December 29, 1994@
We have investigated the feasibility of using ab initio molecular orbital methods for predicting the global warming potential of the proposed chlorofluorocarbon (CFC) substitute CF3CH2F, HFC-134a. Various levels of theory and basis sets were used to optimize geometry and calculate harmonic vibrational frequencies and infrared intensities for the molecule using the GAUSSIAN 92 software package. In attempting to assess the quality of the computations, we found it necessary to reconsider the vibrational assignments available in the literature. We have remeasured the infrared spectrum of the vapor from 400 to 4000 cm-’ at a resolution (0.08 cm-I) sufficient to resolve some overlapping fundamentals and to assign symmetry species unambiguously for several bands on the basis of their rotational contours. The higher resolution spectra and the results of the computations together permit a fm assignment of all 18 fundamentals to be made. Some bands previously assigned as fundamentals are found to be combination or impurity bands. On the basis of the current assignment, we find that for the highest level calculation, MP2/6-3 1G**, the calculated harmonic frequencies agree extremely well with the experimentally observed ones at frequencies below 800 cm-I, with a systematic error toward higher calculated frequencies becoming apparent above 800 cm-I. At lower levels of theory, the systematic error is apparent at all frequencies. The regularity of the deviation between calculated and observed frequencies makes a b initio calculations of vibrational frequencies much more useful than semiempirical calculations, which tend to show random deviations, as demonstrated with a PM3-UHF calculation in this work. The calculated absolute intensities are in good agreement with the limited experimental measurements previously reported, and the observed relative intensities for the fundamentals are also in approximate agreement with our calculated values.
I. Introduction
As we approach the targeted date for phaseout of the ozonedestroying chlorofluorocarbons (CFCs), the scientific and technological community is expending great efforts toward identifying suitable alternatives. The most immediately available substitutes for many applications are the hydrochlorofluorocarbons, which have smaller but still significant ozone depletion potentials (ODPs) and have themselves been targeted for phaseout early in the next century. Alternatives which do not contain any chlorine atoms are being investigated, and in particular, the hydrofluorocarbons (HFCs) seem to hold great promise. The US EPA has established a clearinghouse called the Significant New Alternatives Program (SNAP) to assist users in finding acceptable substitutes. Besides showing unacceptable ODPs, CFCs rank among the highest atmospheric trace gases in global warming potential (GWP). Thus, despite their low atmospheric concentrations] (about 0.8 ppbv), they contribute significantly to infrared radiative forcing, Le., the greenhouse effect. Accordingly, the GWPs of possible CFC substitutes are receiving close scrutiny. We have begun a research program to assess the GWPs of proposed CFC substitutes. We are examining the utility of ab initio calculations in predicting the frequencies and intensities of the infrared absorption bands of these molecules, from which the GWPs may be calculated. The correlation of these quantities with molecular structure may be facilitated by the information the calculations provide, e.g., potential energy distributions and dipole moment derivatives. Furthermore, the GWPs of molecules not readily available or amenable to direct experimental @
Abstract published in Advance ACS Abstracts, February 15, 1995.
0022-3654/95/2099-3438$09.00/0
measurement, and possible degradation products of these compounds, including unstable and metastable species, may be estimated. To assess the value of the ab initio calculations for these purposes, we have begun a comparison of experimental and theoretical infrared spectra for key compounds in the SNAP listing. Our first results, for the substituted ethane CF3CH2F, also known as HFC-I34a, are presented in this paper. This molecule has already found wide use as a CFC substitute, in applications such as industrial chillers and air conditioning, including automobile air conditioning.2 The experimental investigation of the vibrational modes of CF3CH2F began in 1959 with the far-infrared spectrum reported by Danti and Wood,3 which allowed an estimation of the torsional frequency and the barrier to internal rotation. In the 1960s, two groups, Edge11 et aL4 and Nielsen and H a l l e ~ , ~ measured the vapor-phase infrared and liquid-phase Raman spectra of the molecule. Each group made an attempt to assign the 11 a’ and 7 a” fundamentals of the C, molecule, with considerably different results. Harnish and Hirschmann6 remeasured the infrared spectra of the vapor and the solid and provided yet a third set of assignments. In 1972, Crowder7 reassigned the spectra on the basis of force field calculations. Assignments resulting from these early studies are shown in Table 1. In 1977, Lopata and Durig8 reported vapor-phase Raman spectra but only analyzed the low-frequency structure. The basic reason for the disagreement among assignments is that the infrared spectrum does not show 18 separate strong bands, of distinct symmetry species (Le., rotational contour), that may be assigned to the 18 fundamentals. Part of the problem is that the a’ bands of this accidentally near-symmetric 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 11, 1995 3439
Global Warming Potential of CF3CH2F
TABLE 1: Previous Assignments of the Fundamental Frequencies (cm-') of CFJCH~F(Vapor State Values Given Except Where Noted) experimental (vapor state) force field infrared, Raman" infrared, Ramanb infrared' calculatedd A' VI
v2 v3 v4
v5 v6
v7 V8
v9 VI0
VI1
2984 1431 1296 1096 1067 908 843 666 550 358' 20 1
2984 1464 1427 1298 1103 972 843 665 549 408 225
2986 1427 1296 1186 1103 1070 846 666 557 410 222
2984 1431 1371 1298 1191 1096 853 661 546 385 246
3015 1374 1189 972 541 407' 124'
3013 1182 665 539 352 225 120
3015 1463 1296 97 1 542 358 112
3015 1302 1199 967 545 358 139
A" VI2
VI3 VI4
VI5 VI6 VI7
Vi8
Reference 4. Reference 5. Reference 6. Reference 7. e Frequency assigned from Raman spectrum taken in the liquid state. top have hybrid rotational structure, and only those that are nearly pure parallel transitions show clear-cut (P,Q,R) structure, while the a" bands are perpendicular. Limited resolution and overlap of bands exacerbate this problem in the infrared and hamper polarization measurements in the Raman spectrum. Thus, different choices were made as to symmetry type (e.g., of the 408, 972, 1374, and 1463 cm-' bands) and as to which intensity region might represent two overlapping fundamentals. In addition, various relatively weak bands have sometimes been selected by investigators to fill out the list of fundamentals. This lack of consensus necessitated a careful reexamination of the spectrum before a meaningful comparison of calculated and measured frequencies and intensities could be attempted. In fact, our final assignment of fundamentals relies on both the higher-resolution spectra we recorded and the ab initio calculations, and we propose the general utility of ab initio calculations in facilitating the assignment of vibrational spectra.
11. Materials and Methods A. Experimental Spectra. Spectra were recorded on a Nicolet Model 800 FTIR spectrometer at maximum instrument resolution of 0.08 cm-' from 400 to 4000 cm-I. The gas was contained in a 10 cm path length temperature-controlled cell with KBr windows held at 300 K. Each spectrum shown in Figure la, taken at gas pressures of 3.5 and 18.5 Torr, was the average of 32 scans. B. Computational Procedure. We calculated the equilibrium geometry, molecular dipole moment, harmonic vibrational frequencies, and absolute infrared intensities with the GAUSSIAN 92 software p a ~ k a g eusing , ~ the Tufts University Convex supercomputer. Calculations were carried out at the HartreeFock (HF) level using the following basis sets: STO-3G, 6-31G**, 6-311G**, 6-31+G**, and 6-311-t-G**. Calculations were carried out at the second-order Moller-Plesset ( M E ) level using the 6-3 1G** basis set. Semiempirical calculations were also carried out, using the HyperChem 4 software package. An unrestricted HF calculation was done using PM3, a reparametrization of AM1.Io Fundamental frequencies and infrared intensities were obtained after geometry optimization.
111. Results A. Experimental Spectra. The vapor-phase infrared spectrum of CF3CH2F is shown for pressures of 3.5 and 18.5 Torr in Figure la. Measured fundamental frequencies reported in this work were taken from these two spectra; the agreement between the measured values for each maximum was excellent. These 0.08 cm-I resolution spectra allow some definitive conclusions to be drawn. The bands at 408, 666, 1104, 1428, and 1462 cm-I are clearly parallel bands, confirming their assignment as a' species. The separation of the P-R branch maxima averages 15.5 cm-', in good agreement with the approximate 15 cm-' separation calculated by Nielsen and H a l l e ~ . ~The regions around 550, 1200, and 1300 cm-' definitely show multiple intensity maxima, suggesting the presence of two overlapped fundamentals in each case. B. Computations. 1. Ab Initio Calculations. a. Geometry. The best HF and the MP2 geometries are compared to the experimental" and earlier HFI2 and MP2I3 ab initio calculated geometries in Table 2. In the convention of ref 11, the unprimed F atom is the unique fluorine on the CF3 group, lying in the symmetry plane of the molecule, while F' refers to the two symmetry-equivalent fluorines on CF3 and F" to the fluorine on CH2F. b. Dipole Moment. The equilibrium dipole moment is overestimated by about 20-30% at every level. The values are shown in Table 2. c. Harmonic Vibrational Frequencies. With few exceptions, and those only at low frequency, the calculated harmonic frequencies are all higher than the experimentally observed frequencies, as is generally true with ab initio calculations. The changes in individual frequencies as the calculation is improved are depicted in Figure 2. It can be seen that relatively small changes, usually to lower frequency, are observed as the basis set is improved within the HF methodology; a larger drop is seen in each case on going from HF to MP2. GAUSSIAN provides the symmetry species when the calculation is constrained with appropriate symmetry input. In the case of the MP2 calculation, constraints were removed, and we obtained the symmetry type of each normal mode by visualizing the vibrational motion as determined by the MP2 calculation on a computer monitor using HyperChem. Within a symmetry species we simply list modes in order of increasing frequency; the frequencies calculated at the MP2 level are listed in Table 3. d. Infrared Intensities. The absolute infrared intensities for the 18 fundamentals are reported in Table 3 for the MP2 calculation. A stick spectrum derived from the MP2 results is superimposed on the experimental spectrum in Figure lb, with the most intense band arbitrarily depicted as full scale on the vertical axis. Comparison of calculated absolute intensities with reported measured absolute intensities is deferred until after discussion of the reassignment of the experimental spectra. 2. Semiempirical Calculations. The calculated fundamental frequencies and absolute infrared intensities obtained using HyperChem are listed in Table 3. The same procedure as described above was used for determining symmetry labels and arranging the order of the fundamentals.
IV. Discussion A. Geometry. Our HF and MP2 calculations agree with the previous ones to better than 2 ppt, providing a check on our implementation of the GAUSSIAN software. The two computed equivalent C-F bonds are not sensitive to the addition of diffuse functions to the G** basis sets, but the nonequivalent
Papasavva et al.
3440 J. Phys. Chem.. Val. 99,No. 11, I995
1600
1400
1200
1000
600
800
400
Wavenumber
'f
0.8
3200
3000
2800
Figure 1. (a. top) Experimental FTlR decadic absorbance spectrum of CF3CH2Fvapor measured in this work at 0.08 cm-' resolution: upper trace, 18.5 Torr; lower trace, 3.5 Torr. (b) Ab initio frequencies and intensities of fundamental bands of CFCHzF, calculated at the MP2/6-31G** level using GAUSSIAN 92. A relative intensity of 1.00 corresponds to an integrated band intensity of 262 km/mol.
Experimental and ab Initio Results on the Geometry and Dipole Moment e)in dehye of C F K H P this work (ab initio) ref I I (experimental) ref 12 ref 13 MP2/6-31G** set II set I11 HF/6-31G** MP2/6-3I G** HF/6-31G* cc 132s 1525 I .sm i .sox 1.508 1.506 .. ~.~ 1.352 1.324 CF 1.336 1.336 1.323 1.353 1.344 1.318 CF' 1.336 1.336 1.317 1.346 1.380 1.382 1.355 CF" 1.345 1.345 1.355 CH 1.090 1.090 1.081 1.088 1.080 1.087 109.1 109.1 CCF 108.9 109.3 109.1 109.1 111.7 111.6 111.6 CCF' 112.1 111.0 111.7 108.7 108.6 CCF" 109.7 110.4 108.7 108.7 109.2 109.0 CCH 112.9 112.9 109.1 108.9 108.0 108.0 108.0 FCF' 107.8 107.8 107.9 109.7 110.0 F"CH 106.1 105.6 109.7 109.9 110.4 110.3 HCH 108.9 108.9 110.3 110.3 108.2 108.1 FCF' 108.1 108.2
TABLE 2
~~~
~
~
~~
~
~~
P
I .80
2.14
2.28
"The experimental microwave data are from ref 11. The notation given for the bond lengths (CF, CF'. CF") and bond angles (CCF, CCF', CCF") is explained in the text and is according to ref 11. C-F bonds are. In the best representation, MF'2/6-31G**, the equilibrium geometry agrees to within 1% with the experimental geometry from microwave spectroscopy." The staggered conformation is obtained, as in the microwave analysis. A salient feature of the calculation is the asymmetry of the CF3 group, as reflected in the C-F bond lengths (CF' = 1.346 8, and CF = 1.353 8,) and, to a smaller extent, in the FCF bond angles (FCF' = 108.0° and FCF' = 108.2'). It is interesting to note that the microwave data were insufficient to provide a unique structure determination,and the authors actually provided three sets of structural parameters for the molecule, each based on slightly different constraints. Set I, not reported here, assumed symmetrical CF3 with its symmetry axis collinear with the CC bond and produced an unsatisfactory fit to the observed moments of inertia. Two additional sets were determined, set
I1 by tilting the CF3 axis by 2.14' with respect to the CC bond and set 111 by allowing asymmetry in the bond angles on CF,. These two sets, both of which assumed equal carbon-fluorine bond lengths within the CF3 group, provided moments of inertia in nearly equally good agreement with the experimental values. The theoretical calculations suggest that a better way to solve the structure from the microwave data might have been to hold the angles equal and let the C-F bond lengths vary, although the largest differences between the geometries reported in the microwave analysis and in our work are found in the values of the bond angles about the carhon atom of the CH2F group. B. Assignment of Fundamentals. As can be seen from Table 1, previous workers had achieved consensus on fewer than two-thirds of the 18 fundamentals. In particular, the two lowest and two highest frequency modes were not in question,
Global Warming Potential of CF3CH2F 3400
J. Phw. Chem., Vol. 99, No. 11, 1995
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7
3200 3000 2800
+G"
.HF/6-31Ge*
OHF/6-311G'*
BHF/6-31
mHF/6-311 + G "
OMP2/6-31Ge*
Ed EXPERIMENTAL
2600 7
2400
v Lu
2200
3
2000
3
1800
E
1600
z w
1400
2
d w
E
1200 1000 800
600 400 200 0 18
II
17
16
10
9
7
8
15
6
5
14
13
I
3
2
I
12
NORMAL MODE Figure 2. Calculated and experimental vibrational frequencies (cm-I) for the 18 normal modes of CFCH2F. arranged in order of increasing frequency.
TABLE 3: Vibrational Frequencies (cm-') and Absolute Infrared Intensities (kmlmol) of CF3CH2FDetermined in This Work (Except for Experimental Values
