J. Phys. Chern. mi, 85, 426-434
426
ferent behavior of PDMPO and MDPPO.
Conclusion It has been clearly brought out by the data presented here that NMR and relaxation studies on paramagnetic complexes lead to a more accurate knowledge of the complexation of phosphine oxides with lanthanide chelates. Simultaneous studies of 13C paramagnetic relaxation in-
duced by Gd(DPM)3and of pseudocontact shifts arising from interactions with other lanthanide ions lead us to a better understanding of the dynamical behavior in these flexible systems. Although our conclusions still remain qualitative, it is nevertheless suggested that the residence time of the ligand in the coordination sphere of the metal as well as the internal mobility in these ligands can be used as criteria characterizing structure and complexation.
Analysis of Torsional Spectra of Molecules with Two Internal C,, Rotors. 20.+ Vibrational Spectra, Torsional Potential Functions, and Conformational and Thermodynamic Properties of 3-Methyl-1-butene J. R. Durlg" and D. J. Gersont Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: July 18, 1980; In Final Form: October 9, 1980)
The Raman spectra of gaseous, liquid, and solid, and the infrared spectra of gaseous and solid 3-methyl-1-butene have been investigated between 3500 and 50 cm-'. Differences between the spectra of the solid phase and those of the fluid phases have been interpreted in terms of an equilibrium between the low-energy s-trans and high-energy gauche conformers, and a vibrational assignment has been proposed for both conformers. The low-frequencyvibrational spectra of the gaseous phase contained bands due to the asymmetric vinyl and coupled methyl torsions; the assignment of these bands was aided by observation of a number of two quantum transitions for the methyl torsional modes. The asymmetric potential function was calculated and resulted in a value for the enthalpy difference between the two conformers of 415 cal/mol. The methyl torsional potential functions were calculated, which led to values for the barrier height to internal rotation of the methyl rotora of 3.39 kcal/mol, for the s-trans conformer, and 3.49 and 3.52 kcal/mol, for the gauche conformer. Values for the ideal gas thermodynamic functions have been calculated over a range of temperatures.
Introduction Our interest in this laboratory in the problems of conformer stability and of barriers to internal rotation has led us to study the vibrational spectra of several molecules with one asymmetric top, such as isopropylamine,l 1butene,2 p r ~ p a n a l2-methyl-l-butene,4 ,~ and methyl vinyl ethera5 The conformational analysis of another member of this series, 3-methyl-1-butene, has been the subject of a recent paper by Creswell et al.6 In their analysis of the microwave spectrum of 3-methyl-l-butene, Creswell et alS6 assigned rotational transitions for the s-trans and gauche rotamers and determined the energy difference between the two conformers, aE, to be 130 f 20 cm-l with the s-trans conformer being more stable. In an earlier nuclear magnetic resonance study, de Haan et al.' had also concluded that the s-trans rotamer was more stable than the gauche rotamer (see Figure 1). Potta and Nyquist8 have recorded the infrared spectrum of 3-methyl-1-butene and assigned the out-of-plane hydrogen deformational modes of the vinyl group to bands in the region from 900 to 1000 cm-l. Later Ziomek and Forretteg recorded the infrared and Raman spectra of 3-methyl-1-butene and assigned all the observed bands to a planar, C,, conformer, although they could not define this conformer further. As in the case of the earlier study by +Forpart 19, see J. R. Durig, D. J. Gerson, and D. A. C. Compton, J . Phys. Chem., 84, 3554 (1980). *Taken in part from the thesis of D. J. Gerson which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. Degree. Presently with Allied Chemical Corp., P.O. Box 1021R, Morristown, NJ 07960. 0022-3654/81/2085-0426$01 .OO/O
Potta and Nyquist? Ziomek and Forretteg assigned "extra bands" in the spectrum to overtones or combinations rather than to a second conformer. The thermodynamic properties of 3-methyl-1-butene have been determined by calorimetriclo and statisticalll methods, as well as approximated from theoretical calculations.12 Values of C,,,S, and AHf at 298 K as determined in the calorimetric studylo and calculated by statisticalll and theoretical12 methods differ by more than 20%. The presence of two methyl groups in this molecule gives rise to two methyl torsional modes which should be coupled motions, because in studies on propane,13 isopropylamine,' and isopropyl chloride,14significant top-top (1)J. R. Durig, G. A. Guirgis, and D. A. C. Compton, J. Phys. Chem., 83, 1313 (1979). (2) J. R. Durig and D. A. C. Compton, J.Phys. Chem., 84,773 (1980). (3) J. R. Durig, D. A. C. Compton, and A. Q. McArver, J. Chem. Phys., 73,719 (1980). (4) J. R. Durig, D. J. Gerson, and D. A. C. Compton, J. Phys. Chem., 84, 3554 (1980). (5) J. R. Durig and D. A. C. Compton,J.Chem. Phys., 69,2028 (1978). (6) R. A. Creswell, M. Pagitsas, P. Shoja-Chaghervand, and R. H. Schwendeman, J.Phys. Chem., 83, 1427 (1979). (7) J. W. de Haan, L. J. M. Van de Ven, A. R. N. Wilson, A. E. van der Hout-Lodder, C. Altona, and D. H. Faber, Org.Magn. Reson., 8,477 (1976). (8) W. J. Potta and R. A. Nyquist, Spectrochim. Acta, 9,679 (1959). (9) J. S. Ziomek and J. E. Forrette, J. Appl. Polym. Sci., 7,1307 (1963). (10) S. S. Todd, G. D. Oliver, and H. M. Huffman, J. Am. Chem. SOC., 69, 1519 (1947). (11) Z. A. Radyuk, G. E. Kabo, and D. N. Andraackee, Neftekhimiya, 13, 356 (1963). (12) J. E. Kilpatrick, E. J. Porsen, K. S. Pitzer, and F. D. Possini, J. Res. Natl. Bur. Stand., 36, 559 (1946). (13) J. R. Durig, P. Groner, and M. G. Griffin,J. Chem. Phys., 66,3061 (1977).
0 1981 American Chemical Society
The Journal of Physlcal Chemlstty, Vol. 85, No. 4, 198 1 427
Vibrational Spectra of 3-Methyl-1-butene
I
H
S-TRAYS
GAUCHE
Flgure 1. Newman projection of s-trans- and gauche-3-methyl-lbutene.
interaction was found. In our laboratory, a more rigorous approach has been used to explain the rich torsional spectra for a number of molecules containing two internal C, rotors, such as dimethyl ether,16 propane,13isopropyl chloride,14 and isopropylamine.' The data from these studies have been satisfactorily explained by using a two-dimensional treatment of the torsional potential function. Substitution of a hydrogen, in propane, by a vinyl group may result in a complication of the torsional spectrum by not only the addition of an asymmetric torsion but also by changing the methyl torsional potential function, along with the possible presence of two conformers. Vibrational spectroscopy and the calculation of the asymmetric potential function from observed torsional transitions have proved to be useful in the determination of conformer stability and in the calculation of statistical-thermodynamic properties of molecules which exist as a mixture of conformers. In view of the discrepancies among the reported values for the thermodynamic functions, as well as the lack of a full vibrational assignment for 3-methyl-l-butene, it has been found necessary to record the Raman and infrared spectra of the fluid and solid phases, with special attention being paid to the torsional data and the depolarization values in the Raman effect. It should be mentioned that our depolarization data differ somewhat from those reported earlierg and only the strong lines observed in our spectra were observed with the Toronto arc excitation.
Experimental Section The sample of 3-methyl-1-butene was obtained from Chemical Samples Co., Columbus, OH, and purified by low-temperature sublimation. The estimated purity by gas-phase chromatography was 99%. Trace amounts of water were removed by condensation over 3-A molecular sieves (Linde) a t 10 "C. Far infrared spectra were recorded on a Digilab FTS15B Fourier transform interferometer equipped with either a 6.25- or 12.5-pm Mylar beam splitter and mercury-arc source. Spectra of the gaseous phase were examined by using a vapor pressure of up to 400 torr in a cell of 1 m pathlength, fitted with concentrically wedged polyethylene windows. Spectra of the solid phase were obtained by condensing the vapor onto a silicon plate maintained at -20 K by a Cryogenic Technology, Inc. Spectrim cryostat equipped with a Lake Shore Cryotronics Model DTL-500 high-precision temperature controller. Solid samples, annealed at 100 K for 2 h, showed significant reduction or removal of the bands of one conformer. Interferograms were recorded 3000 times at an effective resolution of 0.25 cm-' for gaseous samples, and 200 times at an effective resolution of 1cm-' for solid samples. A boxcar apodiza(14) J. R. Durig and G. A. Guirgis, Chem. Phys., 44, 309 (1979). (15) P. Groner and J. R. Durig, J. Chern. Phys., 66, 1856 (1977).
tion function with no artificial spectral enhancement was used to compute the spectra. Raman spectra were recorded by using a Cary Model 82 spectrophotometer equipped with a Spectra Physics Model 171 argon ion laser tuned to the 514.5-nm line. Gaseous phase samples were examined by utilizing the standard Cary multipass accessories with sample pressures up to 700 torr. The laser power at the sample was approximately 2 W, and the spectra were measured with resolutions varying from 2.5 cm-' for measuring bands below 100 cm-' to 4 cm-l for observing higher frequency fundamentals. Spectra of the liquid samples at reduced temperatures, down to 140 K, were recorded by using a Miller-Harney apparatus.16 Solid phase samples were obtained by condensing the vapor onto a copper block maintained at -20 K and annealed in a cryostat as described above. Mid-infrared spectra were recorded by using a PerkinElmePModel 621 grating spectrophotometer. Gaseous samples were contained in a 10-cm cell equipped with CsI windows, while solid samples were obtained by using the techniques described above with CsI optical material.
-
Vibrational Assignment The s-trans conformer has C, point group symmetry and the 39 vibrational modes fall into the symmetry species 24A' + 15A". The A' modes should be polarized in the Raman spectrum and show A/C hybrid gaseous phase band contours in the infrared spectrum. The A" modes should be depolarized in the Raman spectrum and possess B-type infrared band contours. The gauche conformer is of C1 symmetry with all 39 modes of A' symmetry species and, thus, polarized in the Raman effect and possessing A/B/C hybrid band contours in the infrared spectrum. The Raman and infrared spectra are shown in Figures 2 and 3, respectively, and the assignments which are listed in Tables I and I1 were selected in the following manner. First, bands which increased in intensity upon vooling the liquid sample were assigned to the s-trans conformer. Second, these s-trans bands were assigned to the two different symmetry species on the basis of the depolarization data. Third, infrared band contours were used when they were well defined. Finally, bands which decreased in intensity on cooling were assigned to the gauche conformer, whereas those which changed little were assumed to be due to overlapping s-trans and gauche bands of the same fundamental vibration. The final assignment of the fundamental vibrations, given in Table 11, generally follows that for 1-butene? isopropylamine,' and the earlier assignment of 3-methyll - b ~ t e n e .We ~ have chosen to present our assignment of the observed frequencies rather than indicate how our assignment differs from the earlier onegsince in the earlier study no bands below 300 cm-l were reported and several bands were previously reportedgwhich were not observed in this study. The frequencies listed in Table I1 are taken from the Raman spectrum, unless noted otherwise, in which case they are taken from the infrared spectrum, since they are then better defined than their Raman counterparts. Assignment of the CH stretching modes is rather straightforward. The isopropyl group assignments follow those of isopropylamine;' the CH, stretching modes occur at 2962 and 2875 cm-' and the CH stretching mode occurs at 2931 cm-'. Assignment of the C-H stretching vibrations of the vinyl group is very similar to that of l-butene;2 the =CH2 stretching modes occur at 2978 and 3085 cm-' and the =CH stretch occurs a t 3001 cm-'. (16) F. A. Miller and B. M. Harney, AppE. Spectrosc., 24,291 (1970).
428
Durig and Gerson
The Journal of Physical Chemistry, Vol. 85, No. 4, 7981
1
A
11 1; \
400
350
'
100
I
1
cl
!
I
I
I
50
I I
D
l
100
l
50
WAVE NUM6 E R Cc 6') Figure 4. Low-frequency vibrational spectra of 3-methyl-1-butene in the region of uSB: (A,B) far-Infrared spectrum of the gaseous phase; (C) Raman spectrum of the gaseous phase. 1500
3Ooo
K)oo
500
0
WAVENUMBER (CM")
Figure 2. Raman spectra of 3-methyl-l-butene: (A) gaseous phase: (B) liquid phase; (C) unannealed solid; (D) annealed solid.
x
A
!
I
1
3000
2500
2000
!
1
1500
1000
I
1
500
0
WAVENUMBER Ccm')
Flgure 3. Infrared spectra of 3-methyl-l-butene: (A) gaseous phase: (B) solid phase. Asterisks indicate change in instrumental optics.
Assignments for the region from 1700 to 600 cm-l, as with the assignments presented above, are analogous to those given for the similar modes in isopropylaminel and l-butene? Of particular interest are the CH8deformations, the C=C stretch, and the C-C (isopropyl) stretches. The in-phase and out-of-phase CH, symmetric deformations occur a t 1383 and 1333 cm-l, respectively, which are in close agreement with those for isopropylamine (1375 and
1343 cm-l).l The asymmetric deformations a t 1473,1464, 1456, and 1444 cm-l are in the same general region as those in isopropylamine.' The vinyl C - C stretch occurs at 1644 cm-l, the same frequency as the C=C stretch in l-butene? Finally, the isopropyl C-C in-phase and out-of-phase stretches a t 920 and 1170 cm-l occur at slightly higher frequencies than the corresponding modes in isopropylamine (919 and 1170 cm-l). The assignment for the spectral region below 600 cm-' is quite similar to that proposed by Durig et al." for (CH3)&HX, X = halogen, compounds. The C-C-C deformation occurs at 380 cm-l in 3-methyl-l-butene, which is quite similar to its frequency in is~propylbromide~'(400 cm-'), isopropyliodide" (393 cm-l), and isopropylaminel (404 cm-I). The A' and A" skeletal bends occur at 321 and 292 cm-l, respectively, in 3-methyl-l-butene. These are in the same general range as in isopropyl chloride17(342 and 328 cm-l) and isopropyl bromide17 (297 and 289 cm-l). The vinyl group bend, C=C-C, occurs at 347 cm-l, which compares well with the value of 311 cm-l for this mode in l-butene.2 We have also observed the methyl and frame torsional vibrations and will discuss their assignments in the following sections. Asymmetric Torsion Several sharp bands have been observed in the Raman spectrum of gaseous 3-methyl-l-butene below 150 cm-l, as shown in Figure 4. Additionally, a broad band centered at approximately 100 cm-' is observed in the infrared spectrum of the gas. These bands have all been assigned to the asymmetric torsional mode of this molecule, as the only other unassigned low-frequency modes are the methyl torsions, which occur above 200 cm-' in most isopropyl (17) J. R. Durig, C. M. Player, Jr., Y. S. Li, J. Bragin, and C. W. Hawley, J. Chem. Phys., 57,4544 (1972).
Vibrational Spectra of 3-Methyl- 1-butene
The Journal of Physical Chemistty, Vol. 85,No. 4, 1981 429
c o m p ~ u n d s . ~ J ~The J ~ Jseries ~ of bands observed in the Raman spectrum between 100 and 60 cm-I can only be assigned to the gauche conformer (Cl symmetry) because these bands were observed to be polarized. The C, symmetry of the s-trans conformer results in the asymmetric torsional mode being depolarized in the Raman effect and having a B-type band contour in the infrared spectrum. From experience we have found that the torsions which give rise to depolarized Raman lines are usually very weak or unobservable for the gaseous phase, and that B-type bands in this frequency region exhibit broad nondescript absorptions in the infrared spectrum because of the population of several excited states. Thus, we could only assign the broad band at 100 cm-l, shown in Figure 4A, as overlapping B-type envelopes of the s-trans asymmetric torsion. Upon closer analysis of the infrared spectrum below 400 cm-’, several C-type bands were observed on top of the R-branch envelope of the band at 380.5 cm-l, shown in Figure 4B. These Q branches, with frequencies of 398.5, 394.2, 390.2, and 386 cm-l, are believed to arise from a combination of the skeletal bend at 293.0 cm-I and the asymmetric torsion at -100 cm-’ of the s-trans isomer. From these data we have calculated the torsional fundamental (1 0) to be at 105.5cm-l with higher transitions falling to lower frequencies. No correction has been made for the vibrational anharmonicity. The potential function for the asymmetric torsion was calculated by using the torsional transitions listed in Table 111. The reduced rotational constant was calculated as a function of the angle of internal rotation, a F , = Fo CFi cos ia
-0001
+180
-180 DIHEDRAL ANGLE
Flgure 5. Asymmetric potential function of %methyl-1-butene. Dihedral angle of *18O0 corresponds to the s-trans conformer. Observed torsional levels are indicated by solid lines; the dashed line Is the predicted 3* gauche split level (see text for discussion).
N
-
+
i
where a is defined as 180’ for the s-trans conformer. The structural parameters utilized were those proposed for propylenels and propanelg from their respective microwave studies. The observed frequencies were fitted to the potential function varying in a
v,
= ‘/2ZcVb(1- cos ia) 1
For initial calculations, the enthalpy difference between the lowest s-trans and gauche energy levels, AH, and the values of Vz and V3 taken from the microwave study by Creswell et a1.6 were utilized. The torsional assignment was chosen by assuming that both series had their fundamental (1 0) transitions at the highest frequency with higher transitions falling at successively lower frequencies. After each series of iterations further transitions were added into the calculations and additional potential constants were added. A reasonable fit could be obtained with four potential constants, Vl, V,, V,, and v6 (values listed in Table IV), and V4 and V5 were found to be insignificant. Sufficient transitions were observed in the gauche series to define the energy levels up to the height of the gauche/gauche barrier, at which point the series abruptly ends. As is usually found near the top of the barrier, the highest pairs of energy levels were calculated to be split; however, the very weak intensity of the observed transitions made observation of the split 3* 2f transition pair impossible. The potential function shown in Figure 5 has values for the s-trans/gauche, gauche/s-trans, and gauche/gauche barriers and gauche dihedral angle of 877, 726, and 347 cm-I and 127O, respectively. The value of the s-trans/gauche barrier, 2.5 kcal/mol, and the gauche dihedral angle of 127O, are in good agreement with the values
-
-
(18)I). R. Lide, Jr., and D. Christensen, J. Chem. Phys., 35, 1374 (1961). (19)D. R.Lide, Jr., J. Chem. Phys., 33, 1514 (1960).
of -2.1 kcal/mol and - 1 2 2 O , respectively, obtained in the microwave study.6
Methyl Torsions The s-trans conformer of 3-methyl-1-butene has two methyl torsions, one symmetric (A’) and the other asymmetric (A”) to the molecular A/C plane. However, because the methyl tops are in equivalent symmetry environments, the barrier to internal rotation of each top is equal, and the system can be labeled C,(e), in the notation of Groner and Durig,15 where e indicates the tops are symmetry equivalent. In the case of the gauche conformer, the two methyl tops are not symmetry equivalent because one top is closer (2.9 vs. 3.6 A) to the vinyl group. This system is labeled Cl(n), where the n indicates nonequivalent tops.15 The gauche methyl torsions are both A’ modes and may have different barrier heights. Analysis of the far-infrared spectrum of gaseous 3methyl-1-butene (Figure 6) reveals two distinct C-type bands at 234.0 and 224.0 cm-l. As mentioned earlier, the A‘ methyl torsion should exhibit C-type bands, and thus we have assigned these two bands to the first and second A’ methyl torsional transitions. The A” torsion is expected to give rise to a B-type band in the infrared spectrum and a depolarized line in the Raman effect. Thus, as in the case of isopropyl chloride,14they are expected to be very broad and weak, if at all observable in the infrared spectrum. We did not observe in the torsional fundamental region any B-type transitions in the infrared spectrum, as is evident in Figure 6. However, the two quanta or “double jump” transitions are often observable in the Raman effect. Examination of the Raman spectrum, shown in Figure 7, reveals several lines, all of which can be assigned to two quanta transitions of either the gauche conformer torsions or the A’ s-trans torsion. The A” torsion of the s-trans conformer is expected to fall at higher frequency than the A’ torsion, which would place the “double jumps” for this mode above 480 cm-l. The polarized Raman lines at 242,235, and 230 cm-l are assigned to one of the methyl torsions of the gauche conformer. The corresponding two quanta transitions are observed at 477 and 466 cm-’. The Raman lines at 438 and 425 cm-l are assigned as the two quanta transitions of the other methyl torsional mode also for the gauche conformer. Therefore, unlike the s-trans conformer, both methyl torsional modes can be assigned for the gauche conformer. The internal rotational Hamiltonian for the methyl torsions of the s-trans C,(e) molecule has been derived15 as HI= 1/2[$4p02+ g45p41+ g55p12] + 1/2[V30(1- cos 3 ~ +~V60(1 ) - cos 670) + v60/ sin 670 + Vo3(l - cos 37J + VM(l - cos 671) + Vas/ sin 671 + V3,(cos 3~~cos 3~~- 1) + V3i sin 3~~ sin 3~~ + V33”sin 3~~cos 3~~ + V33/11cos 3~~sin 3 ~ ~ 1
430
Durig and Gerson
The Journal of Physical Chemistry, Vol. 85, No. 4, 1981
TABLE I : Observed Frequencies (crn-' ) and Approximate Vibrational Assignment for 3-Methyl-1-butene Raman re1 int? gas polrzn
3092 3087 3085 3013 3001 2982 2970 2962
Raman liquid
re1 inty polrzn
mw
3082 3060
m vw
3001 2990
S,P S,P
3001
S
2963 2935
2891 2875
2910 2880 2862
ms,dp m,p ms,p m,p VS,P
2840 2762 2722 1728 1644
2840 2748 2710 1724 1643
w, P
1590
1580
vw,p
1458 1456
w,p vw,p vs,p
m,p br, d p
1415
m,p
1383
1377
w,p
1288 1190 1170 1162 1132 1100 1092
2971 2962 2960 2945 2931 2914 2880 2862
ms ms ms
1333 1308 1300 1285 1190 1182 1160
w,p sh m,p ms,p vw,p vw,p
1121 1101 1090 1020
Ww,p mw,dp mw,p w,p
995 985 955
w,dp sh,p mw,dp
VW,P
sh W
vs
2705
W
1639
vs
1468 1460 1452 1444 1440 1411 1409 1378
m ms m m sh ms sh m
1361 1358 1335
W W
1304 1285 1195 1178 1160 1135 1120 1100 1088 1020 1006
m
'1
t
VI
t g
s, sh
t, g t, g
S S
' 2
Vl '3 'I
'4,
2958
s
t,g
'35, ' 1 6
2933
sh
2933
ah
t
v6
g
vs
2895 2873
m m
g t
'79
t, g
Vl?
t,g
v9
2868
m
V7r V I V I
m mw mw
rn
330
br
292
298
w,p
300
W
277 258
vw
150 122 115
1640
m
1466
m
W
m m
t, B t, 8 t, g t, g
VI0
vI1 v u ' 2 9
1421
W
1411
w
t, g
vI2
1393,1383, 1372
m
1375
mw
t, g
uI3
1358
W
1309 1301
mw mw
vI,
1099
br
1092
w
g t, 8 t,g
1003 999 985
sh m sh
996
m
t, €! t
uM
g
' 3 4
t, g
'16
v32
v33
'18
mw W
m mw mw ms br W
917 911 684,671 663
sh ms W
W
906 678 658
ms
W W
m
W W
'19
t
'19
t
'
t,g g
w
'10 20
g
vJ6
t
V Y
g
'11
t
380 350,349, 347 328,320, 315 295,293, 291
m m
386 365
m mw
t,g
m
329
mw
g, t
v13
mw
304
w
t,g
vg,
234 100
vw
vw
ms
g
w
mw
240 m,dp
m
vw
w,p
ms,p m,p
W
W
328
798 780
1720 1655,1649, 1644,1635 1570 1473 1464
S
321
800
114
g
vw mw vw m
380 347
mw,p mw,dp
507
w
vibrn
m
br, p w,p m,p w,p w,p
925 918
663 531
3077
conformera
m
660 532 505 382 350
928 920 783
2985 2978 2965
m, sh m m,sh
re1 IR solid inta
W
959 933 925 918 908 798 779 670 660 533 505 384,381 364
945
IR gas 3094 3089 3078
re1 inta
W,P
1422
1305
re1 inta
3080
2945 2931 2920
1460
Raman solid
br
v 11
The Journal of Physical Chemistry, Vol. 85, No. 4, 1981 431
Vibrational Spectra of 3-Methyl-1-butene
TABLE I (Continued) Raman re1 int,a gas polrzn 92
Raman liquid
re1 int,a polrzn
m , ~
Raman solid
re1 int“
103 95
W
90
W V? W W
84 75 70 59
re1 inta
IR gas
re1 IR solid inta
g
m
mw
48 40
m
vibrn lm
m
52
conformera
vw
v39
lm Im lm lm lm lm lm lm
a s, strong; m, medium; v, very; w, weak; sh, shoulder; p, polarized; dp, depolarized; br, broad; t, s-trans; g, gauche. denotes lattice mode.
A
lm
/’. 500
B
400
WAVENUMBER (cni’) Flgwe 7. Raman spectrum of the gaseous phase of %methyl-1-butene in the region of 2v,,.
rl’ 250
200
+-
WAVENUMBER Ccm’l) Flgure 6. Low-frequency vibrational spectra of the gaseous phase of 3-methyl-1-butene in the region of vu and va8: (A) far-infrared spectrum; (8)Raman spectrum.
where Vm = Vo3,Vm.= V,, V,l = -Vw’, V3< = -V3”’, and g“A = $5. The kinetic coefficients $4, etc., were calculated by using structural parameters transferred from propylene18 and propanel9 and are given in Table V. Initial calculations using bands at 234.0,224.0,537.0, and 522.0 cm-’ as the 10 00,20 10,02 00, and 03 01 transitions, respectively, required large negative sinesine and cosine-cosine coupling V33,V33/and V33,to obtain a satisfactory fit under the symmetry restriction of C,(e). Since previous studies on other isopropyl compounds1J3J4 have shown V33, when needed, to be large and positive, it was concluded that this initial assignment was unreason-
- -
+ -
able. Thus, only the bands a t 234.0 and 224.0 cm-’ have been assigned as torsional transitions, which prohibits the evaluation of the coupling terms, V3,and V33/.However, since under a C,(e) Hamiltonian, V,, = Vo3, the barrier to internal rotation can be and has been determined. The values of the potential coefficients and their dispersions are given in Table V. The gauche methyl torsional barriers were computed by using the Hamiltonian under the Cl(n) restrictions, $4 # g55,V30 # Vo3, Ve0# Vo&In the initial assignment, the bands at 242, 235, and 438 cm-l were assigned as the 01 00,02 01, and 20 00 transitions. This allowed for the determination of an initial fit by using V30, Vo3, and V33/.The bands at 230.0 and 425 cm-’ were then assigned as the 03 02 and 30 10 transitions. The final potential coefficients and their dispersions are given in Table V.
-
-
-
+-
Thermodynamic Functions The thermodynamic properties of 3-methyl-1-butene have been measured by Todd et al.1° and subsequently calculated by Radyuk et al.ll The calculationsll were based upon rather poor vibrational data and lacked experimental values for the torsional barriers, but qualitatively their values were similar to the experimentallo ones. Since we have now obtained more reliable vibrational data, we present new values for the thermodynamic functions and they are listed in Table VI. A method has been derived by Compton20to calculate
The Journal of Physical Chemistry, Vol. 85,No. 4, 1981
432
TABLE 11: Frequencies (cm-l ) and Approximate Descriptions of the Normal Modes of Gaseous 3-Methyl-1-butenea description
s-trans
Durig and Gerson
TABLE 111: Observed Bands (cm-') and Proposed Assignments of the Vibrational Spectrum of Gaseous 3-Methyl-1-butene below 550 cm-'
gauche
Raman
A'
14
'15
=CH, asymmetric stretch =CH stretch =CH2 symmetric stretch CH, asymmetric stretch (ip) CH, asymmetric stretch (ip) CH stretch CH, symmetric stretch (ip) CH, symmetric stretch (ip) C=C stretch CH, asymmetric deformation (ip) CH, asymmetric deformation =CH, deformation CH, symmetric deformation (ip) = CH bend (in-plane) CH deformation (in-plane ) CH, rock (ip) =CH, rock CH, rock (ip) C-C stretch (ip) C-C(=C) stretch C-C-C symmetric deformation skeletal bend (vinyl) skeletal bend (isopropyl) CH, torsion (geared)
3085 3001
3087
cm-l
2982
531 527 522 507 477 466 458 438 425
2978 2962 2962 2931 2875
2920 2891
2875
2891
cm"
m w w w w w
g, V Z I g, 02 + 00 g, 0 3 + 0 1
398.5
vw
394.2
vw vw
t,
v37
+4.15 -0.42
+ 0.04
+ v 3 9+
101.5, 2
t, v 3 7 +
+1
+0.54
Vgg+
97.3. 3 -+ 2
vw
t, v
+
380
1309 1121b 1092 945 928 800 507 347 320 221d
A' ' CH, asymmetric 2960' stretch (op) CH, asymmetric 2960' stretch (op) v,, CH, symmetric 2873 stretch (op) v18 CH, antisymmetric 1452' deformation (op) uZ9 CH, antisymmetric 1444' deformation (op) vw CH, symmetric 133SC deformation (op) v , ~ C-C stretch (op) 1170 1162 v,, CH, rock (op) 1100 v , ~ CH, rock (op) 1003 v y ) HC=CH trans wag 995b 985 v , ~ =CH, wag 911 HC=CH cis wag 5 31 663 v,, skeletal bend 292 (isopropyl) ug8 CH, torsion 242 v , ~ asymmetric torsion -100 92 All frequencies are measured from the gaseous phase spectrum unless otherwise noted; ip, in-phase; op, out-ofphase. Liquid phase. ' Solid phase. Calculated from "double-jump" transitions. v15
the thermodynamic functions of an ideal gas which exists as a mixture of conformers. This method uses the enthalpy difference between the two conformers, AH, as obtained from the asymmetric potential function, to calculate the concentration of the high-energy conformer, XB, at each (20) D.A. C. Compton, J . Chem. SOC.,Perkin Trans. 2,1307 (1977).
w
347 321 292 242.0 240.0 235.0
w w w w sh w
230.0
w
221.0
sh
381.0 350.0 348.5 347.0 320.5 293.0
m mw mw mw m mw
234.0
vw
224.0
vw
220.5 100
vw br
+0.83
, ~ v39+
92.8, 4
1288
783 380
+ 00, 2v, g, 30 + 1 0 t , v 3 , + v 3 g-+ 105.5, 1+ 0
g, 20
386.0
918b
-2.61
+ 2.68 0.00 + 0.53
t, 20 +- 00
1460
1132
assignmenta
obsd calcd, cm-I
VS,S~
390.2
1304'
re1 inta
mw w,sh
1644 1468'
1422 1383
234
re1 inta
infrared
+
3
t, v z 1 hot band of v,, hot band of v 1 2 vll V~
v3,
g, 0 1 + 0 0 vSl g, 11 + l o b g, 02 + 01 t, 1 0 + 0 0 , V , g, 0 3 + 02 t, 2 0 + 1 0 g, 11 -+ O l b
-1.66 +1.62 -0.95 -0.91 + 2.63 +0.91 -1.42
t, v , ~ g, 1 f + 0 +, v 3 9 +0.37 g, 2 f + 1 +, v J 9 -0.39 vw g, 3 f + 2 +, v , ~ -0.04 a See Table I for abbreviations used. Not used in the potential function calculations. 92.0 83.0 76.0
vw w
TABLE IV: Potential Function Coefficientsa for the Asymmetric Torsion of 3-Methyl-1-butene with Dispersions and Standard Deviation of the Frequency fit coefficient Vl V,
v3 V6 AH 0
value
dispersion
-129 361 627 30 145 0.41
4 7 11 5
F, = 1.658, F, = -0.024, F, = -0.064, F, = -0.003, F4= 0.003. All values are in cm-'. a Calculated by using
temperature, T. From these data, values of the thermodynamic functions are calculated, allowing for the mixing of the two conformers. One source of error in these calculations results from the asymmetric or methyl (or both) torsional vibrations. We have used the barriers to the methyl rotation calculated from the coupled-basis set and the moments of inertia obtained from the microwave study! The values of the methyl barriers as calculated by =I 4.2 kcal/mol, and Radyuk et al." from 2-butene, VCH~ as obtained in this study, VCHs= 3.1 kcal/mol, differ significantly. We therefore expect the largest difference between these two studies to be associated with these data. Furthermore, we have considered the asymmetric torsional motion to be a separate harmonic vibration for each conformer, which is a definite oversimplification. Thus, we expect the largest error in our calculations to arise from
Vibrational Spectra of 3-Methyl-1-butene
The Journal of Physical Chemistty, Vol. 85,No. 4, 1981 433
TABLE V: Potential Coefficients for the Methyl Torsions of 3-Methyl-1-butene, with Calculated Dispersions, in cm-' a coefficient
value
dispersion
=
s-Trans 1189.30
19.56
10.947 -0.010 Gauche 1222.19 1230.84
21.22 35.14
'30
'03
Vm = VLm -244 = 2-5 5
d5 v 3 0
V,,
vi:; v,
-118.88 16.47 10.880 11.051 -0.061 u (standard deviation of the frequency f i t ) = 2.45. v3,
g" gS5 g45
CI
TABLE VI: Calculated Values for the Gaseous Phase Thermodynamic Functionsa (in J K-I mo1-l) of 3-Methyl-l-butene, with the Concentration of the High-Energy Gauche Conformer Indicated by X B
-(GHG,)/T H,/T S T,K e~ XB 100 184.35 66.40 250.75 59.87 0.2184 82.98 0.5117 200 237.11 64.90 302.02 273.15 271.25 59.99 331.24 102.79 0.5852 298.15 280.62 60.00 340.63 110.05 0.5993 300 281.31 60.00 341.32 110.59 0.6002 313.44 63.82 377.27 139.53 0.6323 400 500 341.01 70.29 411.31 165.28 0.6497 600 367.36 76.08 443.45 187.12 0.6628 700 393.52 80.24 473.77 205.80 0.6739 800 419.26 83.06 502.33 221.95 0.6829 900 443.64 85.67 529.31 236.11 0.6908 1000 467.52 87.30 554.83 248.47 0.6969 a Calculated by using a value of 1734.7 J mol-' (414.6 cal/mol) for AH in the standard state.
the anharmonicity associated with the asymmetric torsional mode; nevertheless, we expect that the presented values are accurate to f l J/mol K.
Conclusions Past spectroscopic studies on the structure of 3methyl-1-butene have lead to varying results as to the structure of the high-energy ~onformer.6J9~J~ As mentioned earlier, both Ziomek and Forretteg and Radyuk et a1.l1 could not identify a high-energy conformer; however, in were assumed the latter study,l' the values of AH and V, to be the same as those in 1-butene. Later de Haan et al.7 and Creswell et al.6 observed the presence of a high-energy form and concluded that the structure of this form was gauche. The Raman spectrum of 3-methyl-1-butene recorded at various temperatures and in several phases has enabled us to characterize the more stable conformer of this molecule. Analysis of the Raman spectrum of the liquid phase showed a predominance of the gauche conformer over the temperature range of 0 to -120 OC, and the bands which could be assigned to the s-trans conformer (predominantly depolarized) grew in relative intensity upon cooling the sample. This strongly indicated that the s-trans conformer was more stable, and that the enthalpy difference between the two conformers is very small, as concluded in the microwave study.6 We must also point out that we could not remove all the bands resulting from the gauche conformer by freezing the sample, although a comparison of spectra of the annealed and unannealed
TABLE VII: Values of So and C D 0at 298 K ref 12'"
this studyC
ref l l b
this studyd
78.91 81.41 79.57 79.70 26.08 26.15 26.30 Cpo 28.35 a Calculated by using approximate theoretical methods from available spectroscopic data of 1-butene. Calculated by using measured values of AH,K, and e,, early spectroscopic data,9 and VCH, = 4.2 kcal/mol. ' Calculated by using the data presented in this study as treated by the conformer mixing method2@ with VCH, = 4.2 kcal/mol. Calculated by using the data presented in this study as treated by the conformer mixing method" with VCH, 2 3.1 kcal/mol. S"
solids (see Figure 2) reveals the intensity of the gauche bands to be reduced significantly. The asymmetric potential function, shown in Figure 5, approximates a threefold system. Examination of the potential coefficients, listed in Table IV, shows that the values of Vl through V, are significant, as is v6, and that V, is approximately twice V,. The barriers to internal rotation around the asymmetric bond have been calculated to be s-trans/gauche, 877 cm-l (2.50 kcal/mol); gauche/ gauche, 347 cm-' (0.99 kcal/mol); and gauche/s-trans, 726 cm-' (2.08 kcal/mol). From a comparison of the asymmetric potential function barriers with those in isopropylaminel and isopropylphosphine,21several similarities are observed. In all three molecules, isopropylamine, isopropylphosphine, and 3methyl-1-butene, V3is -2V2 (or V4),AH is small and on the order of 200-450 cal/mol, and the s-trans conformer is more stable. It was hoped that a comparison of the potential function of 3-methyl-1-butene with those of other l - b u t e n e ~and ~~~ isopropyl~arboxaldehydes~~-~~ would provide the link between these two related species. However, the more stable conformer in 3-methyl-l-butene, s-trans (trans hydrogens across the Cisopropyl-Cvinyl bond, see Figure 11, is equivalent to the gauche form of l-butene,2 which is not the lower energy conformer in any of the l - b u t e n e ~ .Also, ~ ~ ~comparison of the structures of the stable rotamers of 3methyl-1-butene with the structure of the lower energy rotamers of several isopropylcarboxaldehydes22-24reveals that the structure expected for the low-energy rotamer of 3-methyl-1-butene is gauche and not s-trans. Thus, although the results of the potential function calculations for 3-methyl-1-butene do not agree with those expected on the basis of organic structure theory25or by comparison with related mo1ecu1es,24~22-24 it is reassuring to note that the microwave spectroscopic,6 nuclear magnetic resonance,' and vibrational spectroscopic data are in accord. The values reported in Table VI for the gaseous phase standard state thermodynamic functions are quite different from those previously presented." The differences arise from the inclusion of conformer mixing as well as a lower (by -1.1 kcal/mol) value of the methyl torsional barrier in the present study. Todd et al.lohad reported the values of C and S at 298.15 K. However, from a later study by Mcdullough and Scottz6 of 2-methyl-l-butene, it was shown that the study by Todd et al.1° was incorrect. In ~
~~~~
(21)J. R. Durig and A. W. Cox, Jr., J.Phys. Chem., 80, 2493 (1976). (22)0.L.Stiefvater, Colloquim on High Resolution Molecular Spectroscopy, Dijon, 1971. (23)0.L. Stiefvater, Information Brief, No. 49 (1976). (24)J. P. Guillory and L. S. Bartell, J. Chem. Phys., 43,654 (1965). (25)L. Pauling, "The Nature of the Chemical Bond, Cornell University Press, New York, 1960. (26)J. P.McCullough and D. W. Scott, J. Am. Chem. SOC.,81,1331 (1959).
J. Phys. Chem. 1981, 85, 434-439
434
an attempt to answer the question as to which value of S (298 K) is correct, we have calculated the statistical thermodynamic functions of 3-methyl-1-butene using both VCH3= 4.2 kcal/moP’ and VCHsas calculated in this study. The values of S and C, calculated by using these different barriers, given in Table VII, indicate that the values of the thermodynamic functions, as reported in Table VI1 and in ref 11,differ by the contribution of the methyl torsional barriers. As previous studies of 1-butene2and 2-methyl1-butene4 have yielded a good agreement between the calorimetric thermodynamic functions and the statistical functions calculated by using methyl potential function barriers, we feel that the values of the thermodynamic
functions given in Table VI are probably correct. The calculated concentration of the gauche conformer, XB, 59.9% at 298 K, agrees well with that reported in the microwave study! at least 50% at 298 K, and explains the predominance of polarized bands in the Raman spectrum of the gaseous phase.
Acknowledgment. The authors gratefully acknowledge the financial support of this study by the National Science Foundation by Grant CHE-79-20763. D.J.G. gratefully acknowledges the help of Dr. S. D. Hudson during the methyl torsion potential function calculations and Dr. D. A. C. Compton for suggesting the project.
Formation of Adsorbed Oxygen and Its Reactivity with Ethylene over Silver Catalysts Shulchl Kagawa, * Masakazu Iwamoto, Hlroshl Morl, Department of Industrial Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan
and Tetsuro Seiyama Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Fukuoka 8 12, Japan (Received: July 3, 1980)
The temperature-programmed desorption of oxygen from silver catalysts showed a single peak in the range 413-573 K with a maximum around 503 K. This temperature was not affected by the differences in the methods of preparation of silver samples. The amount of adsorbed oxygen increased in proportion to the logarithm of the adsorption pressure. This oxygen adsorbate led to the formation of ethylene oxide and carbon dioxide by reaction with ethylene, which were desorbed at 298-393 and 373-573 K, respectively. Both the amounts of produced ethylene oxide and carbon dioxide showed a bell-shaped dependence on the concentration of adsorbed oxygen; that is, the excess amount of adsorbed oxygen inhibited the productions of both ethylene oxide and carbon dioxide. In a similar manner the adsorption of carbon dioxide or ethylene oxide on oxygen-preadsorbed silver was investigated. On the basis of the results obtained, it was concluded that ethylene and oxygen adsorb competitively on silver atoms on which a slightly positive charge had been induced by the neighbor oxygen adsorbates. The mechanism for the oxidation of ethylene to ethylene oxide is also suggested, including the reaction of molecular oxygen adsorbates with ethylene adsorbed linearly on a silver atom.
Introduction In efforts to clarify a comprehensive mechanism for the silver-catalyzed oxidation of ethylene to produce ethylene oxide, several models have been suggested as active species.‘I2 A lot of evidence has been presented demonstrating the existence of molecular oxygen species by using EPR,4 IR,S FEM,G and isotopic exelectron diffra~tion,~ change reaction.'^^ Atomic oxygen species have also been proposed on a silver surface by a variety of technique^.^^^^^^^ (1) Hucknall, D.J. “Selective Oxidation of Hydrocarbons”;Academic Press: London, 1974;p 6. (2)Kilty, P.A.; Sachtler, W. M. H. Catal. Rev. 1974,10, 1. (3)Vol. Yu, Ts.;Shishakov, N. A. Izu. Akad. Nauk SSSR, Otd. Khim. Nauk 1962,586;Izu. Akad. Nauk. SSSR Ser. Khim. 1963,1920.Kagawa, S.;Tokunaga, H.; Seiyama, T. Kogyo Kagaku Zasshi 1968,71,775. (4)Clarkson, R. B.; McClellan, S. J. Phys. Chem. 1978, 82, 294. Abou-Kais. A.: Jarioui, M.: Verine. J. C.: Gravelle, P. C. J. Catal. 1977, 47,399. Tanaka, 8.; Yamashina, T. Ibid. 1975,40,140. Shimizu, N.; Jpn. 1973,46,2929. Shimokoshi, K.; Yasumori, I. BulZ. Chem. SOC. (5)Kilty, P. A,; Rol, N. C.; Sachtler,W. M. H. “Catalysis”;Hightower, J. W., Ed.; North Holland Publishing Co.: Amsterdam, 1973;p 929. (6)Czanderna, A. W.; Frank, 0.; Schnidt, W. A. Surf. Sci. 1973,38, 129. (7)Meisenheimer, R. G.; Ritchie, A. W.; Schissler, D. 0.;Stevenson, D. P.; Voge, H. H.; Wilson, J. N. Proc. Int. Congr. Surf. Act., 2nd,1957, 1957,2,337. Malgolis, L. Y. Izu. Akad. Nauk SSSR 1959,225. (8)Kagawa, S.; Iwamoto, M.; Morita, S.; Seiyama, T., unpublished results. (9)Kagawa, S.;Kono, K.; Futata, H.; Seiyama, T. Kogyo Kagaku Zasshi 1971, 74, 819. Wachs, I. E.;Kelemen, S. R. Proc. Int. Congr. Catal., 7th 1980,Preprint A-48. 0022-3654181 l2085-0434$01.0010
At the present, however, many disagreements exist concerning the oxygen species active for the catalytic oxidation and the reaction mechanism. Considerable studies have suggested a mechanism in which the molecular oxygen species reacts with ethylene to give epoxide and the atomic species leads to formation of carbon dioxide and water.l-1° Based on this mechanism, a model for the intermediate giving ethylene oxide was proposed by Kilty and S a ~ h t l e rto ~ ,be ~ ethylene adsorbed symmetrically on the top of adsorbed molecular oxygen. However, Cant and Hallll claimed that such an intermediate forms carbon dioxide as well as ethylene oxide. Force and BelP2 proposed an intermediate containing only one oxygen atom which is common to both epoxide formation and combustion. Recently, Kuczkowski et al.13 have discussed a variety of possible mechanisms. In addition, an interesting kinetic study by Harriott et suggested competitive adsorption of ethylene and oxygen on a partially oxygenated silver surface. (10)Herzog, W. Ber. Bunsenges. Phys. Chem. 1970, 74,216. Carra,
S.;Forzatti, P. Catal. Reu. 1977,15,1.
(11) Cant, N. W.; Hall, W. K. J. Catal. 1978,52,81. (12)Force, E. L.; Bell, A. T. J. Catal. 1975,38, 440; 1975,40,356. (13)Larrabee, A. L.; Kuczkowski, R. L. J. Catal. 1978, 52, 72. Egashira, M.; Kuczkowski, R. L.; Cant, N.W. Ibid. 1980,65,297. (14)Klugherz, P.D.;Harriott, P. AIChE J. 1971,17,856.Metcalf, P. L.; Harriott, P.Ind. Eng. Chem. Process Des. Deu. 1972,11, 478.
0 1981 American Chemical Society
