J . Phys. Chem. 1988, 92, 6908-6913
6908
Far-Infrared Spsctra, Vibrational Asrrlgnrnent, and Conformational Stability of
J. R. Durig,* S. E. Godbey,* and J. F. Sullivan Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: March 7, 1988) The Raman (3200-20 cm-I) and infrared (3200-50 cm-I) spectra of gaseous and solid l-iodo-2-methylpropane, as well as the Raman spectrum of the liquid, have been recorded. The trans and gauche asymmetric torsions have been observed at 102.5 and 98.25 cm-I, respectively, in the infrared spectrum of the gas, along with three additional transitions for the gauche conformer. From these data, the asymmetric potential function is calculated to be V, = -379 10, V, = 595 f 10, and V3 = 1999 f 6 cm-' with the trans conformer being more stable than the gauche conformer by 133 f 26 cm-' (380 f 55 cal/mol). A variable-temperature Raman study of the liquid was carried out and the enthalpy difference was found to be 118 f 19 cm-' (338 55 cal/mol) with the trans conformer being more stable. A complete vibrational assignment is proposed and compared with the previous assignments for the 1-fluoro-, 1-chloro-, and 1-bromo-2-methylpropane molecules.
*
*
Introduction We have undertaken an investigation of the Vibrational spectrum and conformational preference of 1-iodo-2-methylpropane as an extension of our recently completed studies of the other 1-halo2-methylpropane molecules.'*2 Like the analogous fluoride, chloride, and bromide, the 1-iodo-2-methylpropane molecule is known to exist as a mixture of trans and gauche conformers, where the planar C, form has the halogen atom trans to the hydrogen on the central carbon, and the Cl or gauche conformation has the halogen atom gauche to this hydrogen and approximately trans to a CH3 group. We found that the gauche rotamer is the preferred form in the fluid and solid phases of the fluoride and chloride molecules and in the solid of the bromide. However, the trans conformation was found to be more stable in the gaseous and liquid phases of the bromide molecule. Previous investigators have utilized molecular mechanics calculation~,~ and infrared,@' Raman," and N M R spectroscopySto study the conformational preference of 1-iodo-2-methylpropane. From molecular mechanics calculations3 the most stable conformation was predicted to be the trans form, with the gauche form being 84 cm-' (0.24 kcal/mol) less stable. The barriers to internal rotation of the asymmetric rotor were also calculated, the relative magnitudes of which are comparable to those obtained in our study of the corresponding bromide molecule,' but the shape of the potential function is different in that the highest barrier is the one in which the iodine atom is eclipsing the hydrogen atom, Le., the gauche/gauche barrier. Intuitively, it would seem the barriers in which the halogen atom eclipses a methyl group should be higher, and indeed the potential function we have obtained for the bromide molecule appears to support this expectation. However, the (CH3)2CHCH21molecule is not totally analogous to the (CH3),CHCH2Br molecule, as the iodide has been shown to exist in the trans form in the annealed The previous vibrational assignment^^^^ of the normal modes of the (CHJ2CHCH21 molecule have been based primarily on infrared data of the condensed phases; in one of these studies problems of decomposition on exposure to the laser utilized in an attempt to obtain Raman data were reported. The far-infrared spectrum of this molecule has not been reported.4 We have therefore undertaken a complete investigation of the vibrational spectrum of the 1-iodo-2-methylpropane molecule with special emphasis on the low-frequency spectral region.
rification was accomplished by low-temperature vacuum fractionation. The far-infrared spectrum of the gas was recorded on a Nicolet Model 200 SXV interferometer equipped with a vacuum bench and a liquid helium cooled Ge bolometer containing a wedged sapphire filter and polyethylene windows. A 6.25-pm Mylar beam splitter was employed and the sample was contained in a 1-m cell fitted with polyethylene windows. The cell was filled to the maximum vapor pressure at ambient temperature. The mid-infrared spectrum of the gas was obtained by using a Digilab Model FTS-14C Fourier-transform interferometer equipped with a Ge/KBr beam splitter and a TGS detector. The far-infrared spectrum of the solid, as well as the infrared spectrum of the gas from 500 to 360 cm-', was obtained by using a Digilab Model FTS- 15B Fourier-transform interferometer equipped with a 6.25-pm Mylar beam splitter and a TGS detector. The Raman spectra were recorded on a Cary Model 82 spectrophotometer equipped with a S ra-Physics Model 171 argon ion laser operating on the 5145- line. The spectrum of the gas was recorded with a standard Cary multipass accessory, and the laser power at the sample was 2-3 W. The vapor pressure of the sample at ambient temperature was used. Reported frequencies are expected to be accurate to at least f2 cm-I. The spectrum of the solid was obtained by depositing the sample onto a blackened brass plate, cooled by liquid nitrogen and contained in a cell fitted with quartz windows, and annealed until no further changes were noted in the spectrum. A Pyrex capillary tube filled under vacuum was used to obtain the Raman spectrum of the liquid. Copper was utilized to stabilize the 1-iodo-Zmethylpropane molecule, although at low laser power and without the presence of copper, no problems were evident in the spectrum of the liquid. In the absence of the stabilizing copper, however, the sample was observed to decompose over a period of time as evidenced by the change of the sample color from clear to amber. This change was not observed when the sample was stored over copper filings.
Experimental Section The 1-iodo-2-methylpropane sample used in the present study was obtained from Aldrich Chemical Co., Milwaukee, WI. Pu-
213.
'Taken in part from the thesis of S.E. Godbey which was submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree, August 1987. ... .
*Present address: US. Army Missile Command, Attn: AMSMI-RDRE-QP, Redstone Arsenal, AL 35898-5248.
r
Enthalpy Difference From molecular mechanics calc~lations,~ it has been predicted that the trans conformation of 1-iodo-2-methylpropane is 84 cm-' (240 cal/mol) more stable than the gauche conformation. Additionally, the conformational energy difference of this compound ( I ) Durig, J. R.; Sullivan, J. F.; Godbey,S.E.J . Mol. Srrucr. 1984, 246, (2) Durig, J. R.; Godbey, S . E.; Sullivan, J. F., to be submitted for publication. (3) Meyer, A. Y. J. Mol. Struct. 1983, 94, 95. (4) Crowder, G. A,; Jalilian, M. J. Mol. Srrucr. 1978, 49, 287. (5) Houeix, A,; Martin, G.; Queneudec, M.; Martin, G. J . Mol. Strucr. 1968, 2, 369.
( 6 ) Gates, P. N.; Mooney, E. F.; Willis, H. A. Spectrochim. Acta, Parr A 1967, 23, 2043. (7) T'Kint de Roodenbeke, M.; Meinnel, J.; Martin, G. J . Chim. Phys. Phys.-Chim. Biol. 1971, 68, 6.
0022-3654/88/2092-6908$01.50/00 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6909
Spectra of 1-1odo-2-methylpropane TABLE I: Temperature and Intensity Ratios (582/602-cm-l Lines) for the Conformational Study of Liauid 1-Iod0-2-methYlDrOpaf~? T,O C lOOO(l/T), K-I K = IJI. -In K -0.5425 22 3.388 0.5813 -0.4496 -12 3.832 0.6379 -0.3394 -43 4.348 0.7122 -70 4.927 0.7758 -0.2539 -87 5.382 0.8131 -0.2069 ~~
in the liquid phase has been investigated by Houeix and cow o r k e r ~ .These ~ investigators found that the trans conformer is more stable in the liquid by 133 f 14 cm-I (380 f 40 cal/mol) from a variable-temperature experiment utilizing infrared spectroscopy, and 126 f 10 cm-' (360 f 30 cal/mol) by N M R methods. We have attempted to obtain the value of the enthalpy difference between conformers using variable-temperature Raman experiments in order to compare the results from this technique with those reported previously. The lines used for this study were the 602 and 582 cm-' lines assigned to the C-I stretching fundamental of gauche and trans l-iodo-2-methylpropane, respectively. For the study of the liquid phase, measurements were made over a range of temperatures from 22 to -87 "C, at five different temperatures, with three measurements made at each temperature (Table I). Utilizing the equation In K = -(AH/RT) (ASfR),the enthalpy change was evaluated by plotting In K versus 1/T, which gives AH/R as the slope of the line. For this equation K is the ratio Z-/Zssuchn where Z is the intensity of the corresponding trans or gauche Raman line, and it is assumed that AH is not a function of temperature. The results indicate that the trans form is 118 f 19 cm-' (338 f 55 cal/mol) more stable than the gauche form in the liquid phase. A similar experiment was carried out for the gaseous phase of 1-iodo-2-methylpropane. Over a temperature range of 26-93 O C we observed no measurable change in the relative intensity between the conformer peaks, and we were thus unable to determine the AH value in this phase. However, on the basis of our experience in measuring AH values in the gas phase, we can postulate that it is small, perhaps on the order of 100 cm-I.
+
Asymmetric Torsion
The structure of the 1-iodo-2-methylpropane molecule in the trans conformation necessitates that out-of-plane modes, such as the asymmetric torsion, exhibit B-type infrared band contours. Also, due to the C, symmetry of this rotamer, the torsional mode is of A" symmetry; hence this vibration yields a depolarized line in the Raman effect. The gauche conformer has C, symmetry and, consequently, the out-of-plane modes yield polarized lines in the Raman spectrum and A/B/C hybrid infrared band contours. On these bases, the bands observed in the far-infrared spectra (Figures 1 and 2) can be readily assigned. The minimum at 102.5 cm-' must arise from the asymmetric torsion of the trans conformer, and the series of Q branches beginning at 98.25 cm-I must arise from the gauche conformer. These frequencies were used to fit a potential function, similar to the one described by Lewis et a1.,8 in which the torsional potential is represented by a Fourier cosine series in the internal rotation angle 0 6
v(e) = F~ + C(vi/2)(1
-COS
io)
i= 1
where i is the foldness of the barrier. The potential coefficients ( i = 1-6) are calculated from the torsional transition frequencies and the reduced internal rotation constant F(B), which varies as a function of 0 for an asymmetric potential function. The dihedral angle dependence is approximated by another Fourier series: F(0) = Fo +
6
CF, cos io
i s1
1912, 12, 421.
1
300
Id0
200
WAVEN UMBER (cm -1)
Flgure 1. Far-infrared spectra of gaseous (upper trace), unannealed solid (middle trace), and annealed solid (lower trace) 1-iodo-2-methylpropane. The region from 360 to 80 cm-' was recorded on a Nicolet Model 200 SXV spectrometer. TABLE 11: Observed and Calculated Asymmetric Torsional Transitions (cm-'), Potential Constants, and Barriers for 1-Iodo-2-methylpropane transitions conformer assignt obsd calcd" A -0.02 102.5 102.52 trans 1 0 98.77 -0.52 gauche l+O 98.25 97.30 -0.15 2+1 97.15 0.48 96.25 95.77 3+2 +
Potential Constants (cm-l)b
V3 = 1999 f 6 AH = 133 i 26
VI= -379 f 10
v, = 595 f 10
Barriers (cm-l) gauche/gauche dihedral angle, deg
gauche/trans trans/gauche
2190 2340
1470 124
"Calculated by using the potential constants above and Fo = 0.585515,Fi -0.018963,Fz -0.060766,F3 = -0.009606,F4 = 0.005684,F5 = 0.001 157, and F6 = -0.000412. bThe uncertainties are the statistical uncertainties but the actual uncertainties are probably much larger since the data are not sufficient to give independent values for the VIand V, terms.
A potential function governing the asymmetric internal rotation has thus been obtained for l-iodo-2-methylpropane, based on the observed torsional data and structural parameters extrapolated and iodoethane (for the CI from 1-chloro-2-methylpr~pane~ ~~
(8) Lewis, J. D.; Malloy, T. B.Jr.; Chow, T. H.; Laane, J. J. Mol. Struct.
I
400
~
~
(9)Brooks, W. V. F.; Gosselin, J. A.; Mohammadi, M. A.; Thibault, J. D. J. Mol. Struct. 1981, 72, 11.
Durig et al.
6910 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 TABLE III:
(cm-') and Vibrational Assignments of l-Iodo-2-methylpropane Raman re1 re1 int and re1 int and re1 int gas depol liq depol solid int
obge~edFrequencies'
infrared
re1 gas 3013 R 3009 Q,A 3006 P 2915 Q? 2969 Q? 2959 2955 2943 2922 2911 Q?
int w ws ws sh, vs s, sh s, sh
sh, s m
2884 R 2879 min, B s 2876 P
1476 1473 Q 1471 Q 1467 P
sh, m m m
1457
m
1436
solid 3011 w 2999 w 2966 s 2959 vs 2955 vs 2934 2923 2904 2983 2978 2971
3006
w
2997
w
3011 mw
CH,(I) antisym str
2972 2968 2958
m,sh vs ws
2955 2946
s, sh, p
ws, p
m vs ms
2946
ws
2923
s, p
2967 2162 2954 2946
s
CH, antisym str (gauche) CH, antisym str CH, antisym str CH, antisym str CHI antisym str
2923 2909
s
2907 2892
s, p
2905 2896 2886 2880
m vw
CH,(I) sym str CH str
m s
m
s
s, sh, p
s, sh
sh 2882
vs
2865
s,p
2861 vs
CH, sym str
2956 m, sh 2821 w
2864
s
2855
m, sh, p
2855 w s
CH, symstr
2843
w
2833
w, sh, p
2710
vw
2756
w, p
2732
w
2720
w, p
2127 2723 2115 1483
w w w w
1469 1466 1461 1458 1456 1451 1446 1441 1425
m m sh, ms vs vs sh, ms sh, m m m
1383 m
1371 Q
m
1381 m 1319 m 1366 m
1319 R 1316 ctr, B 1311 P
m
1320 1317 1304 1289
vw sh,vw vs m, sh
1456
-1430
mw,bd
sh
1459
w
1444
w
1423
w
2162 w w 2755 w w 2115 w 1482 vw
1197 vs
1382?
-1365 1355
1311
1195
mw
1306
ww ww
mw
1036 969 R 965 Q 960 P
s
CH, antisym defn CH2(I) defn CH,(I) defn (gauche)
1385 w w
-
CH, sym defn
1380 w w 1364? w w
CH3 sym defn CH bend (gauche)
1318 w
CH bend
1306 w
CH bend CH,(I) wag (gauche)
1192
ms, p
1199 m 1196 sh 1185 w
CH,(I) wag CH,(I) twist
w
(1167)
1168
w
(1168)
CH, rock (gauche) or CH2(I) twist (gauche)
w
(1101)
1098
w
1121 w w (1100)
CC2 antisym str CC, antisym str (gauche)
W
1055 1040 1036 968 964
W
ww m mw m ms
1041
vw, bd
1035
962
vw, bd
962
w, p
955
w, p
942
mw, p
958 mw, sh 949 R 945 Q 943 Q 940 P
CH, antisym defn
ms, sh
1107 R
1102Q 1098 P
1462 w 1456 w 1445 w 1437 w 1425 w
ww
1184 m 1175 R 1170Q 1167 P
1469 w
CH, antisym defn CH, antisym defn (gauche) CH, antisym defn
1419 w
-1203
sh vs
ww?
2965 s
m
1209 1200 R 1196 Q 1192 P
vw
s, sh
1420 mw 1394 R 1390Q,A/C 1385 P
assignment (approx descripn)
(942)
944
mw
vw
1053 1041 1036 969 965
ww vw vw w w
961 sh,vw 958 sh, vw (942)
CH3 rock CH3 rock CH,rock C-C(1) str C-C(I) str (gauche)
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6911
Spectra of 1-1odo-2-methylpropane
TABLE 111 (Continued) gas 927 R 922 Q
infrared re1 int solid 925
re1 int W
gas
re1 int and depol
-922
vw, bd
Raman re1 int and liq depol 922
919 P 889 874 835 R 832 Q 827 P 796 R 792 Q 787 P 614 R 610 Q 605 P 589 R 584 Q,w 580 P 427 bd
W
793
S
ms
(600)
366
ww
bd, vw
291 R 284 Q 278 P
W
-250
vw
103 min, B
w, bd
830
CC2 sym str (gauche)
(832) 818
vw
CH21rock
m
CC2 sym str
781
ms
791
794
609
VS
602
(600)
C-I str (gauche)
vs
583
ms
582
579
vs
C-I str
428 425 (415) (389) 377 366 363
vs
43 1
m
429
428
ws
CC2 wag
41 1 388
vw
415 388
(416) (388)
sh
CC2 twist (gauche) CC2 wag (gauche)
37 1 360
377 364
W
CC2 twist CC2 defn
285
(289)
s, sh
ms ms 284
(288)
170 162 127 117
vvw
S
ms
mw
S
m, sh
CH, rock
sh, ww
578
241 174 R 171 Q 164 P 161
831
(832)
mw
sh, vw
vw
W
ms
414 390
925 92 1
assignment (approx descripn)
vw
815
W
re1 int
solid
W
243 170
mw
175
158
w
165
m m m
-
106
vw
CC2 defn (gauche)
264
ww
methyl torsion
240
vvw
methyl torsion CCI bend (gauche)
171 162 126 116
W
vw vw
asym torsion asym torsion (gauche)
98 94 86
CCI bend
m
ms
mw
84 64 60 56 49 42 40 30 23
ww W
sh, w sh, vw W
lattice modes
W
vw, sh vw vw
“Abbreviations used: v, very; s, strong; m, moderate; w, weak; bd, broad; sh, shoulder; p, polarized; dp, depolarized;A, B, and C refer to infrared gas phase band contours; R, Q, and P refer to rotational-vibrational branches; ctr, center of a B-type band. The assignment has been made on the basis of C, symmetry for the more stable trans conformer. Frequencies in parentheses are taken from the spectra of the unannealed solid. distance only).1° The results are summarized in Table 11, and since only four transitions were observed, only the first three terms, VI through V3,were retained in the potential function. Also, it should be noted that the V, and V2terms were highly correlated. The AH value of 133 f 26 cm-’ is consistent with experimental values obtained for the liquid.
Vibrational Assignment The vibrational spectrum of 1-iodo-2-methylpropane is very similar to the spectra of the other 1-halo-2-methylpropanes we A number of bands, present in the have reported spectra of the gas, liquid, and amorphous solid, disappear upon annealing (Figures 3 and 4), and these must arise from the gauche conformer. However, most of the vibrations give rise to similar frequencies for both conformers. Thus,in a number of cases, bands that retain a component in the annealed solid are polarized in the Raman spectrum of the liquid, even though a depolarized line (10) Kasuya, T.; Oka, T. J. Phys. SOC.Jpn. 1959, 15, 296.
would be expected for the trans conformer for many of the vibrations. Overall, the assignment of the vibrational spectrum of 1iodo-2-methylpropane is straightforward and is based upon our previous assignments’s2 for I-fluoro-, 1-chloro-, and l-bromo-2methylpropane. The assignment is summarized in Table 111. If one tabulates our assigned frequencies for these four l-halo-2methylpropanes, several trends are obvious. For example, the CH2X deformation, wag, and twist show a slight downward shift in frequency upon substitution of the heavier halogen atom. On the other hand, the CH2X rock does not shift appreciably among the four molecules, nor does the gauche C-C(X) stretch which is assigned at about 945 cm-I in all four molecules. The gauche C-X stretch shifts down from 1038 cm-’ in the fluoride to 609 cm-I in the iodide. A similar trend is observed for the trans C-X stretch where it is assigned at 697 cm-’ in the chloride, 627 cm-I in the bromide, and 583 cm-’ in the iodide. The vibrations associated with the isopropyl moiety also warrant some comment. The frequencies of the CH3 symmetric defor-
Durig et al.
6912 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988
t
120
I
a
I
I
I
I
I
80
100 WAVENUMBER ( c d )
Figure 2. Far-infrared spectrum of gaseous 1-iodo-2-methylpropanein the region of the asymmetric torsional transitions.
I
1
1
Wavenumber (cm-1)
Mid-infrared sDectra of l-iodO-2-methVlDrODane: .. . gaseous (upper), amorphous solid {middle),annealed solid (lower).
Figure
Figure 3. Raman spectra of 1-iodo-2-methylpropane:gaseous (upper),
liquid (middle), and solid (lower). mations remain virtually unchanged among the four molecules; the C H bending frequencies are also nearly constant, although one of the gauche CH bends in the iodide is about 15 cm-' higher than the corresponding vibration in the other three molecules. The CC2antisymmetric and symmetric stretches show slight downward shifts upon substitution of the heavier halogens, where the former vibration is assigned in the 1130-1 102-cm-l region, and the CC2 symmetric stretch spans the range of 908-831 cm-'. According to our assignments, three of the four CH2rocks show little change in frequency among the four molecules where they can be found near 1170,965, and 925 cm-'; the remaining CH3 rock appears to vary in frequency where it has been assigned at 1124 cm-' in the fluoride and 1055 cm-' in the iodide. In accordance with the results of our vibrational and ab initio and the present investigation study of 1-fluoro-2-methylpropane
of the iodide, it becomes apparent that some of our previous assignments' for 1-chloro-2-methylpropaneand l-bromo-2methylpropane should be revised. In the previous assignments' the gauche CC2vibrations were assigned such that the deformation was the highest frequency, followed by the twist, and then the CC2 wag. However, our present results indicate that the order of these gauche vibrations should be amended as the CC2 twist, wag, and deformation. Additionally, for the trans conformer of 1-chlor+2-methylpropane, the assignment of the CC2 deformation and wag should probably be reversed. Similarly, for the bromide, the trans CC2 wag, rather than the deformation, should be assigned to the 475-cm-' band. If one now tabulates the low-frequency vibrations of the four 1-halo-2-methylpropanes it is clear that the trans CC2 twist is the only vibration which is not significantly affected by substitution of heavier halogen atoms. Also, in general, the gauche CC2 wag, CC2 twist, the CCX bend in the fluoride molecule show a greater percent change upon substitution of a chlorine atom than do the corresponding vibrations in the C1, Br or I molecules. One can, for instance, compare the frequencies of 482,426,416, and 41 1 cm-' for the gauche CC2 twist in the F, C1, Br, and I molecules, respectively, and observe that the greatest difference in frequency is between the fluoride and chloride. The methyl torsions in the 1-iodo-2-methylpropane molecule could only be assigned with any confidence from the spectra of the solid phase where they occur at 264 and 240 cm-I. If one compares the assignments among the four 1-halo-2-methylpropanes, it appears that the torsions in the trans conformer lie at slightly lower frequencies than do the corresponding gauche vibrations. Discussion We have carried out variable-temperature Raman experiments in the liquid and gaseous phases of 1-iodo-2-methylpropane in an effort to determine the relative stability of the trans and gauche
Spectra of 1-1odo-2-methylpropane conformers. We were unable to observe a measurable change in the relative intensity of the conformer peaks over the range of temperatures studied (26-90 "C) for gaseous 14odo-2-methylpropane, which indicates that the enthalpy difference is relatively small in this phase. However, we were able to obtain the value of the enthalpy difference in the liquid for comparison with previously reported values. Our value of 118 f 19 cm-' (338 f 55 cal/mol), determined from a variable-temperature Raman experiment, is in excellent agreement with the previously reported5 values of 133 f 14 cm-' (380 f 40 cal/mol) and 126 f 10 cm-' (360 f 30 cal/mol) from variable-temperature infrared and NMR experiments, respectively. All results indicate that the trans conformer is more stable in the liquid phase. The potential function governing the internal rotation of the asymmetric CHzI rotor in 1-iodo-2-methylpropane has been calculated based upon the observed torsional data and assumed structural parameters for 1-iodo-2-methylpropane. The resulting potential function is very similar to the one we obtained previously for 1-bromo-2-methylpropane.' The trans conformer is predicted to be about 133 f 20 cm-' (380 f 57 cal/mol) more stable than the gauche conformer. This energy difference is slightly larger than was found for the corresponding bromide and close to the value of 118 f 19 cm-' (338 f 55 cal/mol) determined for the liquid phase of the iodide. The overall shape of the potential function calculated herein is different from that obtained from molecular mechanics calculation^,^ although the overall magnitudes of the barriers are the same. From the molecular mechanics calculations the barriers to internal rotation were calculated to be as follows: gauche/gauche of 1958 cm-', gauche/trans of 1679 cm-', and trans/gauche of 1763 cm-'. Thus, the largest barrier from the molecular mechanics calculations arises from eclipsing the hydrogen atom with the iodine atom. The calculated AH value of 84 cm-' from molecular mechanics is in excellent agreement with the value of 133 f 26 cm-' which we obtained from the fit of the asymmetric torsional transitions. Thus, the molecular mechanics calculations appear to have yielded a good value for AH but the barrier values do not appear to be in the correct order. The potential function calculated from the torsional data seems more reasonable since the highest barrier is the one in which the halogen atom eclipses a methyl group. One expects the steric interaction between the iodine atom and the methyl group to be considerably larger than that between the iodine and hydrogen atom. Therefore, the trans/gauche barrier value of 2340 cm-' obtained from the torsional data, which is 870 cm-' higher than the gauche/gauche barrier where the iodine atom eclipses the hydrogen atom, is consistent with intuitive expectations. There is a very weak generalized absorption around 250 cm-' in the infrared spectrum of the gas which corresponds to two rather nondescript bands at -248 and -232 cm-I in the infrared spectrum of the unannealed solid (see Figure 1). These absorptions are undoubtedly due to the methyl torsional modes and apparently arise from the gauche conformer since in the annealed solid, where
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6913 only the trans conformer remains, a pronounced band is observed at 241 cm-'. This frequency agrees well with the lower frequency methyl torsional mode observed in the Raman spectrum of the solid. The other torsional mode, which is probably the A' mode, is observed at 264 cm-I in the Raman spectrum of the solid. The barriers for the gauche conformer need not be the same, but for the trans conformer the two methyl rotors are equivalent so there is only one methyl barrier for this conformer. Since the torsional modes always shift to higher frequency with solidification, a higher limit to the barrier can be calculated from the torsional frequencies observed in the spectra of the solid. Using the theoretical treatment" of two coupled rotors of C3, symmetry and a frame of C, symmetry, the barrier to internal rotation was calculated for the methyl rotors of the trans conformer. The barrier and the sine-sine coupling term were calculated to be V30 = Vo3 = 1443 cm-I (4.13 kcal/mol) and V5, = 156 cm-I (0.45 kcal/mol), respectively, where the 1443-cm-I value is the threefold barrier. This value is essentially the same value as the barrier of 4.11 kcal/mol found for (CH3)$HI in the solid phase.l2 Assuming a 10-15% shift in going from the gas to the solid, the value of 1228 cm-' for 1-iodo-Zmethylpropane is consistent with the gas-phase values of 1130 and 1141 cm-' found for the methyl barriers in the trans conformers of 1-chloro- and 1-bromo-2methylpropane, respectively.' A complete vibrational assignment has been proposed, based upon infrared data for the gas and solid phases, and Raman data of the gas, liquid and solid phases. The similarity of the frequencies of the normal modes among the 1-halo-2-methylpropanes is apparent, although the iodide differs in that the trans rotamer is in higher abundance and the only conformer present in the solid phase. Many of the normal modes appear as doublets in either the infrared or Raman spectra of the solid. Therefore, there are at least two molecules per primitive cell. In the Raman spectrum of the solid there are at least eight lattice mades observed which may indicate that there are more than two molecules per primitive cell since the maximum observable number for two molecules is nine and one seldom observes nearly the permitted number of lattice modes in the Raman effect. Alternatively, some of the higher frequency shoulders in the Raman spectrum may be two phonon bands. Nevertheless, there are at least two molecules per unit cell and the intermolecular forces must be relatively weak since the frequencies for the normal modes do not shift appreciably in going from the gas to the solid state.
-
Acknowledgment. We gratefully acknowledge the financial support of this study by the National Science Foundation through Grant CHE-83-11279. Registry No. l-Iodo-2-methylpropane, 51 3-38-2. (11) Groner, P.; Durig, J. R. J. Chem. Phys. 1977, 66, 1856. (12) Durig, J. R.; Player, C. M. Jr.; Li, Y.S.; Bragin, J.; Hawley, C. W. J. Chem. Phys. 1972, 57, 4544.
