J . Phys. Chem. 1992, 96, 1691-1696
1691
Transition-State Geometries and Intrinsic Reaction Pathways for a Series of Formyl and Thioformyl Compounds James Tyrrell* and Wyn Lewis-Bevan Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 62901 (Received: September 12, 1991; In Final Form: November I , 1991)
Ab initio calculations, including electron correlation, have been used to determine the equilibrium and transition-stategeometries and the harmonic vibrational frequencies and intensities for both geometries for a series of formyl and thioformyl compounds. The intrinsic reaction coordinate method was employed to determine the unimolecular decomposition pathways. Activation and reaction energies corrected for zero-point vibrational energy were determined for each molecule. The molecules studied were formyl cyanide and thioformyl cyanide, propynal and thiopropynal, and formyl chloride and thioformyl chloride.
Introduction Formyl cyanide (HCOCN), thioformyl cyanide (HCSCN), and formyl chloride (HCOCl) are currently the subject of high-resolution infrared spectroscopic investigation by one of us (W.L.-B.). As part of that investigation, it was considered desirable to determine reasonable estimates of their equilibrium geometries and the corresponding vibrational frequencies and intensities. It was also desired to have some indicator of the stability of these molecules and the nature of the possible decomposition products, if any. This latter information could be determined by first identifying their transition-state geometries and from these finding their intrinsic reaction pathways. Formyl cyanide has been prepared by the flash vacuum pyrolysis of methoxyacetonitrile,’ (allyloxy)acetonitrile,2 and (cinnamyloxy)acetonitrile3 with the latter method of preparation giving a pure and apparently stable product. Thioformyl cyanide has been prepared by the flash vacuum thermolysis of allylcyanomethyl sulfide4 but not in the amounts or level of purity necessary for detection and analysis of its infrared spectrum. Formyl chloride is prepared by passing formic acid over phosphorus pentachloride at room temperature.5 Several vibrational frequencies of formyl cyanide in its A ‘A’ excited state and in its X ’A’ ground state were obtained from a partial analysis of the vibrational structure of the r* n electronic transition with origin at 380.5 nm.’ The gas-phase FTIR spectrum of formyl cyanide has also been investigated at low resolution,6 yielding three of the stretching frequencies and the symmetric CCN bending frequency. The millimeter wave spectrum of formyl cyanide has also been investigated,2 yielding rotational and centrifugal distortion constants but, in the absence of more than one isotopic species, no geometry. The millimeter wave spectrum of thioformyl cyanide has also been a n a l y ~ e d , ~ again yielding rotational and centrifugal distortion constants but no geometry. The low-resolution infrared spectrum of formyl chloride has been studied,’ giving a complete set of vibrational fundamentals. The microwave spectrum of formyl chloride has also been analyzed,* giving among other information re and Be parameters. Ab initio calculations on the equilibrium structure of formyl cyanide have been carried out9 at the HF/SCF level using 3-21G and 6-31G* basis sets and at the MP2 level using a 6-31G* basis
-
( I ) Judge, R. H.; Moule, D. C.; Biernacki, A,; Benkel, M.: Ross, J. M.; Rustenburg, J. Mol. Spectrosc. 1986, 116, 364. (2) Bogey, M.; Destombes,J.-L.; Vallee, Y.; Ripoll, J.-L. Chem. Phys. Lett. 1988, 146, 227. (3) Lewis-Bevan,W.; Gaston, R. D.; Tyrrell, J.; Stork, W. D.; Salmon, G. L. J . Am. Chem. SOC.,accepted for publication. (4) Bogey, M.; Demuynck, C.; Destombes, J.-L.; Gaumont, A,; Denis, J.-M.; Vallee, Y.; Ripoll, J.-L. J . Am. Chem. SOC.1989, 111, 7399. ( 5 ) Takeo, H.; Matsumura, C. J . Chem. Phys. 1976, 64,4536. ( 6 ) Clouthier, D. J.; Moule, D. C. J . Am. Chem. SOC.1987, 109, 6259. (7) Hisatsune, C.; Heicklen, J. Can. J . Spectrosc. 1973, 18, 77. (8) Davis, R. W.;Gerry, M. C. L. J. Mol. Spectrosc. 1983, 97, 117. (9)Goddard, J. D. Chem. Phys. Lett. 1986, 132, 483.
0022-3654/92/2096-1691$03.00/0
set. The same paper9 also reports vibrational frequencies obtained at the HF/SCF level using a 3-21G and a 6-31G* basis and then scaled using a constant scaling factor. Optimized geometries for both formyl cyanide and thioformyl cyanidelo have been determined at the HF/4-21G, HF/6-31G1*, MP2/6-31G**, and MP2/6-31G** levels, and scaled quadratic force fields along with vibrational frequencies calculated using them have been deter: mined at the HF/4-21G and HF/6-31G** levels. Ab initio determination of the equilibrium geometry of formyl chlorideII has been carried out at the HF level using double-f quality basis sets but without polarization functions. No previous calculation of the vibrational frequencies of formyl chloride is known to the authors as are no previous calculations on the transition states or intrinsic reaction pathways of any of the molecules reported on herein. The unimolecular dissociation of formyl fluoride (HFCO) has been the subject of investigation,12 yielding a transition-state geometry and a barrier height of approximately 47 kcal mol-’. For comparison purposes we have also investigated the equilibrium and transition-state geometries and their respective vibrational frequencies as well as the intrinsic reaction pathways of propynal (HCOCCH), thiopropynal (HCSCCH), and thioformyl chloride (HCSCI). Propynal and thiopropynal are the acetylenic, isoelectronic equivalents of formyl cyanide and thioformyl cyanide, respectively. Propynal has been studied extensively by a variety of spectroscopic methods. A complete set of bond lengths and angles have been obtained from the analysis of the microwave spectrum of several isotopic species of propynal,13 and a complete set of vibrational fundamentals is also a~ai1able.l~ Thiopropynal, prepared by the flash pyrolysis of dipropargyl sulfide, has been studied using millimeter wave spectroscopy, providing a set of rotational and centrifugal distortion constants but no geometry.15 No vibrational spectrum has been obtained for thiopropynal. Ab initio calculations at the HF/SCF level using 3-21G and 6-31G* basis sets and at the MP2 level using a 6-31G* basis set have provided optimized geometries for propyna19 as well as vibrational frequencies determined at the HF/3-21G and HF/6-31G* levels and scaled by a constant factor. Ab initio geometry optimizations and vibrational frequency calculations at the MP2 level using a D95** basis set have also been reported for propynal and thiopropynal.16 To the authors knowledge no experimental or theoretical studies have been reported on thioformyl chloride. (10) Csaszar, A. G . Chem. Phys. Lett. 1989, 162, 361. ( I I ) Tyrrell, J. J . Phys. Chem. 1979, 83, 3276. (12) Goddard, J. D.; Schaefer, H. F.,111 J. Chem. Phys. 1990, 93, 4907. (13) Costain, C. C.; Morton, J. R. J . Chem. Phys. 1959, 31, 389. (14) Brand, J. C. D.; Callomon, J, H.; Watson, J. K. G. Cun. J. Phys. 1961.39, 1508. Brand, J. C. D.; Watson, J. K. G. Trans. Faraday Soc. 1960, 56, 1582. ( I 5 ) Brown, R. D.; Godfrey, P. D.; Champion, R.; Woodruff, M. Ausr. J. Chem. 1982, 35, 1747. (16) Tyrrell, J. J. Mol. Struct. (Theochem) 1991, 231, 87.
0 1992 American Chemical Society
Tyrrell and Lewis-Bevan
1692 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 TABLE I: Formyl Cyanide Optimized Geometriesa ground state MP2/D95** MP2/6-31G** RC-c 1.481 1 1.4689 1.2202 1.2230 RC4 1.0952 1.0980 RC-H 1.1817 1.1892 RC=N 121.8594 122.2238 LCCO 180.4309 LCCN 180.933 1 114.4584 115.01 17 LCCH -206.195 20 1 -206.265 739 EHCOCN -1 13.028 180 -1 13.068 770 Eco -93.174 366 -93.204 097 EHCN
~~~
MP2/6-311G** 1.4733 1.2092 1.1003 1.1756 122.133 180.515 114.4918 -206.336 167 -1 13.1 11 416 -93.242 684
~
~
transition state MP2/6-31G** 2.2178 1.1541 1.0954 1.1920 1 16.9249 161.6197 52.97 17 -206.077 993
MP2/D95** 2.2277 1.1577 1.0997 1.2004 117.389 166.0462 52.607 1 -206.152 924
~~~~~
MP2/6-311G** 2.2438 1.1411 1.0979 1.1858 117.846 163.8337 52.7689 -206.226 623
Bond lengths are in angstroms, angles are in degrees, and energies are in hartrees.
The purpose of the work reported in this paper is to determine the unimolecular dissociation pathways and barrier heights for the molecules previously mentioned. To achieve this, it was necessary to determine the optimum ground-state geometries and transition-state geometries of the molecules as well as the ground-state energies of the optimum structures of the products of the decomposition. Since a frequency calculation was necessary to confirm that the transition-state geometry corresponded to a true saddle-point frequency calculations for the ground state were included for completeness and to aid in the assignment of the transition-state frequencies. Beginning from the transition state, the intrinsic reaction pathway was determined in both directions providing a step-by-step view of the mechanism of the unimolecular decomposition of these molecules.
Method All calculations were carried out using the GAUSSIAN 8817 package of programs. Geometry optimizations and harmonic vibrational frequency calculations for both ground and transition states were carried out at both the HF/SCF and MP218 levels. All electrons were included in the correlation energy calculations. In the geometry optimization the Berny optimization procedure19 was used. In some instances the "tight" option was used to improve convergence and the resulting reliability of the computed frequencies. The 3-21G and 6-31G** basis sets were used in geometry optimization and frequency calculations on all molecules studied. The D95**basis set, a Dunning-Hay20 double-t basis set with polarization functions on all atoms was also used in calculations on formyl cyanide, thioformyl cyanide, propynal, and thiopropynal. The 6-31 1G** basis set was used only on the formyl cyanide series of calculations. The intrinsic reaction pathway21 was determined at the HF/3-21G level using transition-state geometries optimized at that level. Tables I, 11, IV, V, and VI1 list the optimized geometries and total energies of the ground and transition states of formyl cyanide, thioformyl cyanide, propynal, thiopropynal, formyl chloride, and thioformyl chloride, respectively. Also presented in these tables are the total energies of their appropriate decomposition products. Tables 111, VI, and VI11 list the harmonic vibrational frequencies (unscaled) and intensities for the ground- and transition-state geometries of these molecules. Figure 1 illustrates the geometry of propynal at various points along the intrinsic reaction pathway determined at the HF/3-21G level. Figure 2 plots the unimolecular decomposition potential of formyl cyanide as determined using the intrinsic reaction coordinate method. Figure 3 compares the geometries of the ground and transition states for all the species studied, and Figure 4 summarizes the energy differences between (17) GAUSSIAN 88; M. J. Frisch, M. Head-Gordon, H. B. Schlegel, K. Raghavachari, J. S. Binkley, C. Gonzalez, D. J. Defrees, D. J. Fox, R. A. Whiteside, R. Seeger, C. F. Melius, J. Baker, R. Martin, L. R. Kahn, J. J. P. Stewart, E. M. Fluder, S. Topiol, J. A. Pople; Gaussian Inc.: Pittsburgh, PA, 1988. (18) Merller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618. (19) Schlegel, H. B. J. Comp. Chem. 1982, 3, 214. (20) Dunning, T. H.; Hay, P. J. Modern Theoretical Chemistry; Plenum: New York, 1976; Chapter 1, pp 1-28. (21) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989,90,2154. Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
GROUND STATE
@ CARBON
0
OXYGEN
0
HYDROGEN
n
TRANSITION STATE
0
Y
0
0
''PRODUCTS''
Figure 1. Optimum geometries of propynal determined for the ground state, the transition state, and several points along the unimolecular decomposition path as determined by the IRC method. All geometries are determined at the HF/3-21G level. 0
-20
-40
-60
-80
-100
'
I
I
I
I
-2.00 -1.50 -1.00 -0.50 0.00
I
8
I
I
I
0.50
1.00
1.50
2.00
2.50
REACTION CooRDINATE(au)
Figure 2. Unimolecular decomposition path for formyl cyanide determined by the IRC method at HF/3-21G level.
reactants, transition states, and products along the unimolecular decomposition path for all six molecules.
Results and Discussion Formyl Cyanide. Table I lists the optimized geometries for both the ground and transition states of formyl cyanide using three different basis sets, D95**,6-31G**, and 6-31 lG**. The ge-
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1693
Formyl and Thioformyl Compounds
TABLE 11: Thioformvl Cvanide ODtimized Geometries" ground state MP2/D95** MP2/6-3 1G** RC-C 1.4457 1.4336 Rc=s 1.6322 1.6244 RC-H 1.0903 1.0858 1.191 1 1.1838 LCCS 122.849 123.4929 LCCN 179.923 180.6485 LCCH 1 15.066 115.1319 EHCSCN -528.900075 -528.795 21 1 Ecs -435.637 852 -435.557 681 EHCN -93.204 097 -93.174 366
transition state MP2/D95* * MP2/6-31G** 2.2904 2.2886 1.5298 1 S375 1.1056 1.1084 1.1926 1.201 1 116.801 1 1 16.4964 155.523 156.0897 46.6561 47.2209 -528.657 762 -528.770 425
"Bond lengths are in angstroms, angles are in degrees, and energies are in hartrees.
0
HCOCCH: AH = -4.39 kcal/mol
HCSCCH: AH = 39.13 kcal/mol
HCOCI: AH = -6.54 kcal/mol
HCSCI: AH = 27.85 kcal/mol
0
/ \
tIt
70.22 'Or2
0
46.74
778.54 8i4
HCOCN: AH = -8.32 kcal/mol
HCSCN: AH = 35.78 kcal/mol
Figure 4. Schematic energy diagrams for unimolecular decomposition determined a t MP2/6-3 1G** level and including zero-point vibrational corrections.
0 Figure 3. Ground- and transition-state structures determined at MP2/ 6-3 1G** level: (a) propynal; (b) thiopropynal; (c) formyl chloride; (d) thioformyl chloride; (e) formyl cyanide; (f) thioformyl cyanide.
ometry optimizations were carried out at the MP2 level. While no experimental data are available on the bond lengths and angles, a set of rotational constants has been obtained ( A = 67.473 54 GHz, B = 5.010 19 GHz, C = 4.65660 G H z ) , ~and the best agreement between these experimental values and the calculated ones is found using the MP2/6-311G** calculation ( A = 65.199 89 GHz, B = 4.968 89 GHz, C = 4.617 03 GHz). However the results of all three calculations are in close agreement. Fixedgeometry CISD calculations using the D95** basis were carried out for the equilibrium geometry, the transition-state geometry, and several geometries along the reaction path. Little variation in the coefficient of the SCF reference was noted for all the geometries studied. The transition-state geometries are characterized by substantially longer C-C bonds, significantly smaller CCN and CCH angles, and smaller changes in the other parameters. The transition state is found to be planar. From the total energies of formyl cyanide, its transition state and its decomposition products, hydrogen cyanide and carbon monoxide, all listed in Table I, it is possible to determine the activation energy for the intrinsic reaction and the energy difference between formyl cyanide and its decomposition products. In the MP2/6-3 1G** calculation the activation energy is 73.55 kcal mol-' and the
products - reactant energy difference is -4.61 kcal mol-', i.e., the intrinsic decomposition of formyl cyanide is quite unlikely and would be slightly exothermic if it did occur. This is in agreement with our experimental observations3 where the only decomposition products observed are from the reaction of the formyl cyanide with traces of water vapor to form formic acid and hydrogen cyanide. No trace of carbon monoxide is detected. The vibrational frequencies and corresponding intensities for formyl cyanide are given in Table I1 for both ground and transition states with the former being compared with the experimentally observed values. The calculated values use a harmonic force field approximation and are expected to show significant deviations from the experimental frequencies. There is good qualtitative agreement between the intensities of the fundamentals observed by us3 and the calculated intensities. The only fundamental not experimentally observed so far is the out-of-plane C-H wag which is predicted from our calculations to be about 50 times weaker than the next weakest fundamental, the CCO rock. In the transition state the imaginary frequency correlates with the inplane C-H rock corresponding to the large change in the HCC angle between the ground- and transition-state geometries. The C=N and the C=O stretching vibrations are highly coupled in the transition state with the higher frequency representing a C = O stretch coupled with a C=N contraction while the lower frequency represented both bonds stretching. The lowest lying frequency in the transition state involves substantial motion of all atoms in the molecule. The vibrational frequency calculations for both ground and transition states allowed determination of the zeropoint vibrational corrections to the energy differences and allowed
1694 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
Tyrrell and Lewis-Bevan
TABLE III: Formyl Cyanide and Thioformyl Cyanide Vibrational Frequencies and Intensities" formyl cyanide exutl
ground state MP2/6-3 1G**
transition state MP2/6-3 1G**
thioformyl . cyanide . ground state transition state MP2/6-31G** MP2/6-3 lG**
2892 2230 1716 1383 914 626 230
3122.7 (42.8) 2189.9 (51.1) 1732.4 (78.7) 1446.4 (9.0) 949.9 (87.0) 618.5 (1.7) 215.0 (11.7)
3180.5 (89.1) 2040.9 (21.9) 1984.3 (91.4) 15621' 581.6 (228.1) 329.7 (97.2) 83.7 (1.8)
3219.7 (1.9) 2187.7 (17.3) 1153.0 (8.8) 1386.6 (1 2.4) 917.3 (7.8) 526.3 (0.17) 201.6 (5.2)
3027.5 (69.1) 2026.2 (95.6) 1293.4 (366.3) 1287.51' 483.9 (191.7) 282.8 (89.5) 78.1 (6.8)
278
1009.1 (0.03) 315.9 (1.7)
923.3 (17.2) 143.5 (9.4)
864.8 (24.1) 363.1 (0.88)
876.7 (34.4) 138.6 (0.35)
a' C-H str C=N str C=O(S) str HCC rock C-C str CCO(S) rock CCN bend a"
HCC wag CCN bend
"Frequencies are in cm-I. Intensities are in km mol-'. Experimental frequencies are taken from refs I , 3, and 6.
TABLE IV: ProDvnal ODtimized Geometries" exptl geom RC-c RC=C R=C-H RC-H RC-0 LCCO LC-c=c LCzC-H LHCC EHCWCH Eco EHCCH
1.4444 1.2091 1.0552 1.1057 1.2144 123.9 181.6 180 113.9
ground state MP2/D95**
MP2/6-31G**
1.4627 1.2302 1.0681 1.1005 1.2286 123.5037 182.1441 179.0395 114.7861 -190.184 119 -1 13.068 770 -77.116111
1.4496 1.2214 1.0628 1.0985 1.2257 123.25 17 18 1.5047 178.8839 115.2959 -190.1 19098 -113,028 180 -77.091 458
transition state MP2/D95** MP2/6-3 1G** 2.1243 1.2496 1.0703 1.1055 1.1671 116.1055 158.0276 18 1S573 52.9394 -190.054 129
2.106 1.2397 1.0647 1.1027 1.164 115.7918 152.0369 182.9805 53.1929 -1 89.984 040
"Bond lengths are in angstroms, angles are in degrees, and energies are in hartrees. The experimental geometry is taken from ref 13.
confirmation that the transition state geometries were true saddle points (one imaginary frequency). Figure I11 summarizes the intrinsic reaction pathway data for formyl cyanide obtained using a 3-21G basis set at the HF/SCF level. While this represents a less rigorous treatment, we have ascertained by spot-check calculations along the pathway using higher level calculations that it does reasonably represent both the energetics and geometries of the intrinsic reaction. The initial changes in the formyl cyanide are dominated by a lengthening of the C-C bond in conjunction with a decrease in the HCC angle. This is followed at a later stage by a change in the CCN angle. As the molecule proceeds from its ground state to the transition state, the C=O bond shortens, the W N bond lengthens, and the C-H bond first shortens then lengthens though it is still close to its ground state value a t the transition state. The substantial change in the C-C bond length and in the CCH angle, while the C-H bond length has hardly altered, could perhaps be anticipated from a comparison of the ground-state vibrational frequencies associated with these molecular parameters. The frequency associated with the C-H stretch is 2-3 times that of the frequencies associated with the other two parameters. The H-C(N) distance of 1.787 8, in the transition state implies the likelihood of significant vibrational excitation involving the H-C stretch of H C N as it is formed in this process. The reaction has proceeded well beyond the transition state before the hydrogen is equidistant between the two carbons. Once the hydrogen transfers from the o;-O to the e N group, the C 4 bond lengthens and the CCO angle increases. The product molecules HCN and CO are oriented at about 109O to each other with the hydrogen end of the H C N toward the carbon end of the CO. As the reaction progresses from the ground state to the transition state, there is a substantial transfer of electron density from the HCO group to the C N group to give essentially HCO+ and CN- followed by a transfer of the hydrogen as a proton. Thioformyl Cyanide. Tables I1 and 111 summarize the results of our calculations on thioformyl cyanide which suggest a very similar structure for the ground state as compared to formyl
cyanide and a similar process for its unimolecular decomposition. The principal difference in the ground-state structure other than the C=S bond is a noticeably shorter C-C bond in the thio compound, and this is a feature observed also in the C - C bond of propynal and thiopropynal and in the C-CI bond of formyl and thioformyl chloride. This shortening effect is presumably due to the reduced capability of the sulfur to attract electron density leaving more in the *-bonding orbital associated with the C-C bond. A particularly noticeable feature of the vibrational data for thioformyl cyanide is the change in the intensities of the fundamentals relative to formyl cyanide. In particular the outof-plane CCH wag, which is the weakest fundamental in formyl cyanide and so far undetected experimentally, becomes the most intense fundamental in the thioformyl cyanide. The structure of the transition state in thioformyl cyanide is similar to that of formyl cyanide and the imaginary frequency again can be correlated with the CCH in-plane rock. The activation energy for unimolecular decomposition is (MP2/6-31G**) 86.25 kcal mol-', and the energy difference between reactant and products, in this case H C N and CS, is 39.64 kcal mol-', suggesting that thioformyl cyanide is even more stable to unimolecular decomposition than formyl cyanide and that, if it did occur, the reaction would be endothermic as opposed to the exothermic result for formyl cyanide. This latter difference is due to the difference in stability of CO and CS. The IRC calculations show that the geometry changes during the unimolecular decomposition closely mimic those determined for formyl cyanide. hopynal. There is good agreement between the experimental and calculated geometries for the ground state of propynal (Table IV) and between the calculated and observed vibrational frequencies, allowing for the fact that the calculated values are based on a harmonic force field (Table V). The calculated intensities for the vibrational fundamentals of propynal are also in good agreement with the qualitative estimates assigned to the observed fundamentalsi4 The structural changes observed for the transition state of propynal are in line with those observed for formyl and thioformyl cyanide with substantial changes in the C-C bond
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1695
Formyl and Thioformyl Compounds TABLE V ThioDroDvnal ODtimized Geometries"
ground state MP2/D95* * 1.4355 1.2321 1.0678 1.0915 1.6366 124.1327 181.183 179.2354 115.297 -512.816 625 -435.637 852 -77.116 111
RC-c R m Re-H RC-H Rc-s LCCS
cc=c
L
LCEC-H LCCH EHCSCCH Ecs EHCCH
transition state
MP2/6-3 1G** 1.4212 1.2228 1.0626 1.0873 1.629 1 124.7744 18 1.9306 178.0755 115.5407 -512.718 140 -435.557 681 -77.091 458
MP2/D95** 2.1797 1.2482 1.0704 1.1193 1.5516 115.0679 154.0235 183.5304 46.5528 -512.670964
MP2/6-3 lG** 2.1634 1.2382 1.0647 1.1186 1.5444 115.0686 150.8474 183.802 46.0583 -512.563 229
Bond lengths are in angstroms, angles are in degrees, and energies are in hartrees. TABLE VI: Propynal and Thiopropynal Vibrational Frequencies and Intensities"
propynal ground state
transition state
thiopropynal . .. ground state transition state
exptl
MP2/6-3 1G**
MP2/6-31GS*
MP2/6-3 1G**
MP2/6-3 1G**
3326 (sb) 2858.2 (s) 2106 (w,b) 1696.9 (vs) 1389 (m) 943.7 (vs) 650.0 (sh) 613.7 (m) 205.3
3537.9 (51.97) 3077.7 (80.0) 2157.7 (61.7) 1727.8 (98.96) 1455.6 (10.8) 983.4 (99.1) 646.2 (42.8) 610.9 (5.0) 205.1 (4.2)
3504.3 (17.4) 3105.7 (57.4) 1931.9 (487.0) 1944.8 (86.0) 1760.4i 628.9 (333.9) 674.9 (34.9) 354.8 (1 19.8) 98.1 (6.4)
3538.6 (68.8) 3196.4 (9.3) 2165.2 (31.1) 1173.4 (25.2) 1378.2 (16.8) 928.9 (12.1) 628.2 (43.3) 508.7 (0.9) 198.3 (1.9)
3504.5 (23.3) 2904.7 (29.3) 1941.4 (31.2) 1250.1 (308.4) 1465.1i 530.6 (259.1) 627.2 (47.7) 287.3 (102.2) 88.8 (2.9)
981.2 (sh) 692.7 (s) 260.6
1017.4 (0.46) 627.4 (38.9) 279.7 (3.9)
887.1 (23.9) 644.9 (26.97) 74.6 (43.0)
877.1 (14.5) 586.2 (45.4) 324.5 (3.7)
844.9 (46.7) 605.1 (23.3) 63.5 (11.6)
a' IC-H str C-H str C=C str C=O(S) str C-H rock C-C str =C-H rock CCO(S) rock CCC bend
a" C-H wag IC-H wag CCC bend
"Frequencies are in cm-l and intensities are in km mol-'. Experimental frequencies are taken from ref 14. bApparent emax:>lo0 (vs), 10-100 shoulder.
(s), 1-10 (m),
