1940
J. Phys. Chem. 1980, 84, 1940-1944
Vibratlonal Spectra and Conformations of 2,2,2-Trifiuoroethylamine and 2,2,2-Trifluoroet hanol' V. F. Kalasinsky" and H. V. Anjarla* Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762 (Received December 2 1, 1979) Publication costs assisted by Mississippi State University
The infrared and Raman spectra of 2,2,2-trifluoroethylamine(CF3CH2NHz)and 2,2,2-trifluoroethanol (CF3CH20H)have been recorded in the fluid and solid phases. For CF3CH2NH2, the C-C stretch and the -NH2 stretch appear as doublets as a result of the equilibrium mixture of trans and gauche conformers. A value for AH of 2.9 kcal/mol has been determined from a variable temperature study for the gas phase, and the -NH2 torsion for the more stable trans conformer has been observed at 264 cm-l. The Au = 2 transitions for the -CF3 torsion have been observed, and the derived barrier to internal rotation is 3.85 f 0.06 kcal/mol. Microwave transitions for two excited vibrational states have also been assigned for trans-2,2,2-trifluoroethylamine. A similar equilibrium between two conformers exists for CF3CH20H. The -OH and -OD torsions for the more stable gauche conformer have been observed in the far-infrared spectra. The value of the -CF3 barrier has been calculated to be 3.72 i 0.15 kcal/mol.
Introduction The existence of multiple conformations in primary alkyl amines and alcohols has been adequately established. Raman, infrared, and microwave indicate that the trans conformer of ethylamine is the predominant one, but the gauche form is less stable by only about 200 cm-l (0.57 k ~ a l / m o l ) . ~Ethanol also exhibits a conformational equilibrium, and the trans conformer appears to be between 100 and 200 cm-l more stable than the gauche.6p6 A similar isomerism would be expected for substituted derivatives of ethylamine and ethanol, and, in particular, the corresponding conformational possibilities for 2,2,2-trifluoroethylamine and 2,2,2-trifluoroethanol are indicated in Figure 1. 2,2,2-Trifluoroethylamineand its N-deuterio derivatives have been the subjects of microwave,' infrared, and limited Raman8 studies. On the basis of rotational constants and a partial substitution structure, the trans conformer has been identified, and its stability over the gauche form has been attributed to an intramolecular dipole-dipole interaction between the N-H and C-F bondsa7The vibrational data were interpreted in terms of a single trans conformer! Vibrational spectra for CF3CH20Hand CF3CH20D have also been r e p ~ r t e d . ~Using J ~ data from infrared studies of CC14solutions, Krueger and Mettee9 have calculated a value of 3.3 kcal/mol for the enthalpy difference between the trans and more stable gauche conformers. In addition to the asymmetric -NH2 and -OH internal rotors, there will be torsional vibrations of the symmetric -CFB groups at rather low frequencies. Since symmetric and asymmetric torsions for molecules in the gas phase can be studied rather effectively by using far-infrared and Raman spectroscopy,ll we have undertaken such studies for 2,2,2-trifluoroethylamine and 2,2,2-trifluoroethanol. Additionally we have investigated the microwave spectra of CF3CH2NH2in excited vibrational states. Experimental Section The sample of 2,2,2-trifluoroethylamine was obtained from commercially available CF3CHZNH2.HC1 (Aldrich) by using a slight modification of the procedure described by Gillman and Jones.12 CF3CHzNH2.HC1was treated with a saturated solution of KOH, and CF3CHZNHz was isolated by using vacuum distillation through two traps, 0022-3654/80/2084-1940$0 1.OO/O
one of which contained BaO at -196 "C. 2,2,2-Trifluoroethanol was obtained commercially (Aldrich), and the deuterated analogue was prepared by an exchange reaction with excess DzO. Raman spectra were recorded with a Spex Ramalog DUV spectrometer equipped with a Spectra Physics 171 argon ion laser operating at 4880 A. Spectra of solid 2,2,2-trifluroethanol were obtained by using either a steel cell equipped with quartz windows and a KBr substrate maintained at liquid-nitrogentemperatures or a cell similar to one described by Miller and Harney.13 Liquid samples were sealed in Pyrex capillaries, and standard multipass optics and cells were used when recording data for the vapor phases. Representative Raman spectra of the gaseous samples are shown in Figure 2. Infrared spectra of the vapor phases of the three compounds in the mid-infrared region were recorded with a Nicolet 7199 Fourier transform interferometer equipped with a KBr/Ge beam splitter and a liquid-nitrogen-cooled MCT detector. For recording far-infrared spectra, a 6.25-pm Mylar beam splitter and a TGS detector were installed in the interferometer. Gaseous samples were housed in 10-cm cells fitted with KBr and polyethylene windows, respectively. The infrared spectra of solid samples were recorded with a Perkin-Elmer Model 283 spectrophotometer in a liquid-nitrogen-cooled cell having a CsI support. The microwave spectra of 2,2,2-trifluoroethylaminewere recorded with a Hewlett-Packard 8460 MRR spectrometer which uses a Stark modulation frequency of 33.33 kHz. Frequency and intensity measurements were made at room temperature. Results 2,2,2-Trifluoroethylamine. The more symmetric of the two possible conformers of 2,2,2-trifluoroethylamineshown in Figure 1 is the trans conformer. It possesses a plane of symmetry and belongs to the C, molecular point group. We expect 14 fundamental vibrations to be of symmetry species A' and 10 fundamentals to belong to species A". All of these vibrations are active in the infrared and Raman spectra. The molecule is very nearly a prolate symmetric top ( K = -0.98),7 and from the structures presented by Warren and Wilson we expect the A" vibrations to exhibit 0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 15, 1980
Spectra of Substltuted Alkyl Amines and Alcohols
1941
-
I:$- CH2 NH2 H
850
3400 3350 trans
800
gauche
F3C,- CHp- OH
"i3;; "TH H
I
P
H
CF3
CF3
trans
gauche
A.-
&L-?-
3670 3650
j -_ L
850
1
810
Flgure 3. Raman spectra of gaseous CF3CHzNHzand CF3CHzOH showing (A) the NHzstretch and (B) the C-C stretch of CF3CHzNHzand (C) the OH stretch and (D) the C-C stretch of CF3CHzOH.
Flgure 1. Newman projecAions of the conformers of 2,2,2-trifluoroethylamlne and 2,2,2-triflu~oroethanol.
I
I
v . 3 d o o
A
',bo'
'
'
'
IO00
I
~
'
I
560 ' '
'
'
'
1
-JL
-A B d - L ' u L '15100'
1
'
1000 I
'
'
' si0 ' '
'
i
400
VYAVENUMBEF~CM-~)
Flgue 2. Raman spectra 01 gaseous (A) CF,CH$H,
C-type band contours in the infrared and depolarized Raman lines. For the A' vibrations, pure A- or pure B-type or hybrid infrared bands and polarized Raman lines are expected. On the other hand, the gauche conformer would have C1symmetry, and all the vibrations would belong to the A symmetry species. We would expect, therefore, polarized Raman lines and complex infrared band contours. The assignments for the fundamentals for gaseous CF3CH2NH2are listed in Table I, and they are essentially in agreement with those proposed by Wolff et a1.* (See paragraph at end of text regarding miniprint material.) We have also duplicated the data for the liquid phase, but these have been omitted from our table since they have been reported previously. Of particular interest in the new data are the two doublets which appear in the Raman spectra of the gas. For the NH2 symmetric stretch we observe a pair of lines at 3382 and 3372 cm-l, and for the C-C stretch at 836 and 822 cm-l. The trans conformer has been clearly identified in the microwave study of the gas phase,7 and it would appear to be the preferred conformation of 2,2,2-trifluoroethylamine. Accordingly we have assigned the more intense components of the doublets (3372 and 836 cm-l) to the trans conformer and thlt? weaker components to the gauche form (see Figure 3, A and B). We have measured the
200
300
and (B) CF3CH,0H.
100
WAVENUMBER(CM-~)
Flgure 4. Far-infrared and low-frequency Raman spectra of gaseous CF3CHzNHp
relative intensities of these pairs of lines in the Raman spectra at five temperatures between 23 and 61 "C. A AH value of 2.9 kcal/mol was calculated from these data by using a slightly modified version of the method described by Hartman et al.14 This is a relatively large value for AH, and it accounts for the failure of previous studies to identify a gauche form. To completely describe the equilibrium between transand gauche-2,2,2-trifluoroethylamineit is necessary to investigate the NH2 torsional vibration. The Q branches observed at 269 and 258 cm-l and the feature at 244 cm-l in the far-infrared spectrum shown in Figure 4 are assigned to the 1 0, 2 1, and 3 2 transitions, respectively, for the NH2 torsional motion in the trans conformer. Unfortunately, though, there are no infrared bands or Raman lines which can be confidently assigned to the fundamental band of the corresponding motion for the gauche conformer. However, a Raman line at 352 cm-l remains unassigned, and if it is a Av = 2 transition for the gauche conformer, a potential function can be generated. The assigned gauche and trans torsional transitions can be reproduced by using a potential function of the form V(e) = (l/2)CVn(l- cos ne) with potential constants V1
-
+-
-
1942
The Journal of Physical Chemistry, Vol. 84, No. 15, 1980
Kalaslnsky and Anjaria
Lfr.r.d 11118
Q c
1
2926 w
907
w
I
827
Y.
s
= 758.3,V , = 453.9,V3 = 798.4,and v6 = -51.7 cm-l, and the derived value for AH (910cm-l) is within experimental error of that determined in the variable-temperature studies. This potential function must be considered very tentative because it depends very heavily on the assignment of the 352 cm-l line, and this may instead be the CF3 rocking mode for gauche-2,2,2-trifluoroethylamine.Its intensity would be anomalously large if this is in fact the case, but the corresponding rock (A") for the trans conformer is assigned to the infrared Q branch at 374 cm-l and to the ccB-type"feature in the Raman spectrum having
maxima at 379 and 366 cm-l. As mentioned earlier, the molecule also possesses a threefold CFs internal rotor, and the fundamental band for this vibration has been observed as a broad feature centered a t about 107 cm-l. Also the 2 0,3 1, and 4 2 transitions have been observed at 218,212,and 206 cm-l. Using the double jumps we have generated a torsional potential function which utilizes only the V3 and v6 terms in the cosine potential given above. The results are shown in Table 11, and the barrier to internal rotation is 3.85 f 0.06kcal/mol. This barrier is comparable to similar
-
- -
The Journal of Physical Chemistry, Vol. 84, No. 15, 1980 1943
Spectra of Substituted A,lkyl Amines and Alcohols
TABLE 11: Observed and Calculated Frequencies (ern-' ) for the CF, Torsion in 2,2,2-Trifluoroethylamine and 2,2,2-Trifluoroetllanol transition obsd calcd wt 2+0 3-1 4-2 l e 0
2,2,2-'l'rifluoroethylamine 220 219.V 213.5 213.8 208 207.7 (107) 111.4
1.0 1.0 0.5 0.0
2-0 3-1 1-0
2,2,2-Trifluoroethanol 221 221b 215.6 (106) 111.8
1.0 0.0 0.0
a Calculated by using V , = 1348.7 i 22.4 ern-', V, = Calculated by 41.1 i 7.5 c m - ' , and F = 0.9592 cm-'. using V , = 1303.3 crn1-l and F = 1.116 c m - ' .
-
quantitites determined for the trifluroethyl halides.15 Also listed in Table I1 is the calculated 1 0 transition, and this corresponds to the high-frequency side of the broad line in the Raman spectrum. To further study the low-frequency vibrations of trans-2,2,2-trifluoroc3thylamine, we have also recorded its microwave spectrum and assigned two vibrational satellites. The frequencies and assignments are presented in Table 111. The relative intensities are consistent with vibrational intervals of 240 f 30 cm-l and 113 f 30 cm-l. The latter clearly corresponds to the CF3torsion, and on the basis of the inttensities and the planar moment the former is assigned to the NH2torsional excited state. The lack of splitting for the NH2 torsional excited-state lines is indicative of relatively large barriers to internal rotation and inversion; however, the slight broadening of the lines f selection rules for inis probably related to the f version. There are idso other weak lines in the spectrum which arise from other excited states and probably the gauche form as well, but these have not been assigned. 2,2,2-Trifluoroethanol. As in the case of 2,2,2-trifluoroethylamine, there is also the possibility of two conformers for 2,2,2-trifluoroethanol as indicated in Figure 1. The gauche conformer belongs to the C1 point group, and all 21 vibrations belong to the A species, while the trans conformer possesses C, symmetry and should give rise to 13 A' vibrations and 8 A" vibrations. The band contours are effectively the same as those described for 2,2,24rifluoroethylamine. Additionally, the sharp Q branches in the Raman spectra arise from totally symmetric vibrations, while the broad features are due to the nontotally symmetric vibrations. Evidence for two {conformersof 2,2,2-trifluoroethanol is also found in the Raman spectra of the vapor. The C-C stretch near 830 cm-' appears as a doublet, while the OH stretch exhibits a shoulder on the high-frequency side (see Figure 3, C and D). Additional evidence for a second conformer is found in the spectra of the amorphous solid where a pair of lines at 828 and 819 cm-l is observed for the C-C stretch. After annealing, only a single line remains for this vibration (830 cm-'), while a number of other bands become doublets. The latter splitting5 can be attributed to factor group effects or the presence of two molecules per unit cell rather than the presence of two molecular forms in the crystalline solid state. That the gauche conformer is the more abundant can be ascertained from an inspection of the Raman spectra. For example, the CH2 antisymmetric stretch at 3003 cm-l in the spectrum of the vapor appears as a polarized Q branch, and this is only possible for the C1 symmetry of gauche-2,2,2-trifluoroethanol.The depolarization ratios further support the contention that the gauche form is
-
jG
r
rni
h *
, 500
,
100 WAVENUMBER(CM-~)
300
Figure 5. Far-Infrared and low-frequency Raman spectra of gaseous CFSCHpOH.
more stable. Although experimental difficulties have precluded our measuring relative stabilities in the gas phase, Krueger and Metteeg have reported a AH value of 3.3 kcal/mol in dilute CCll solution and have concluded that the gauche conformer is the more stable one. The torsional vibrations for the gauche conformer have been observed, and Figure 5 shows the low-frequency regions of the infrared and Raman spectra. The Q branch at 282 cm-l in the far-infrared spectrum of CF3CH20H is assigned to the u = 1 0 transition for the OH torsion. The corresponding OD transition is observed at 199 cm-'. The fundamental for the threefold CF3internal rotor has been observed in the Raman spectrum as a broad feature centered at approximately 106 cm-l, and u = 2 0 transition is assigned at 220 cm-l. Using the double-jump frequency and only a V3 term, we estimate the barrier to internal rotation of the CFB group to be 3.72 f 0.15 kcal/mol, and it is very close to the value calculated for
-
-
2,2,2-trifluoroethylamine. Tables IV and V list the observed vibrational frequencies and their assignments for the normal and deuterated gauche-2,2,2-trifluoroethanol.The proposed vibrational assignments are supported by the results of the TellerRedlich product rule. The theoretical T value is 2.70, and the observed shift is 2.58.
Discussion Our results indicate that 2,2,2-trifluoroethylamineand 2,2,2-trifluoroethanolexist as mixtures of conformers, with the trans form more stable for the former molecule and the gauche more stable for the latter. The stability of trans-2,2,2-trifluoroethylamine has been discussed by Warren and Wilson' in terms of a dipolar attraction which tends to align the N-H bonds with the C-F bonds. The internuclear distances are such that a true hydrogen bond cannot form, but in other respects the interactions certainly approach such a condition. In the trans conformer there are two such interactions, whereas there is only one for the gauche form. This difference would account for the rather large AH which we have measured for the conformational equilibrium in 2,2,2-trifluoroethylamine. A similar dipole interaction in 2,2,2-trifluroethanol can account for the predominance of the gauche conformer. In the gauche orientation the 0-H bond is aligned with a C-F bond, but there would be no such alignments for the trans conformer. As a result of this dipole interaction, the magnitudes of the energy separations between gauche and trans conformers are larger for CF3CH20H and
1944
J. Phys. Chem. 1980, 84, 1944-1950
CF3CH2NH2than for CH3CH20H and CH3CH2NH2.3’5 The barriers to internal rotation for the CF, groups in CF3CH2F (3.58 kcal/mol),15 gauche-CF3CH20H, and trans-CF3CHzNHz follow the same trend as the CH, barriers in CH3CHzF (3.33 kcal/mol),16 gaucheCH3CH20H (3.62 kcal/mol),15 and trans-CH3CH2NH2 (3.74 kcal/m01),~but the barriers for the CF, groups are somewhat higher. A dipolar interaction such as the one described above cannot exist for ethanol and ethylamine, and we must conclude that, while the positions of the C-F bonds are important for stabilizing certain conformations, the CF3 barriers are not appreciably affected by the conformational preferences that exist for the OH and NH2 groups. Acknowledgment. The authors thank the Mississippi State University Office of Research for partial support of this work. Acknowledgment is also extended to the Mississippi Imported Fire Ant Authority for the funds to purchase the Nicolet and Spex instruments. We also extend our thanks to the Langley Research center, Hampton, VA, for the loan of the microwave spectrometer and to Professor R. L. Cook for providing easy access to the instrument. Miniprint Material Available: Full-sized photocopies of Tables I, 111, IV, and V (9 pages). Ordering information
is given on any current masthead page. References and Notes (1) Presented in part at the Sixth International Conference on Raman Spectroscopy, Bangalore, India, Sept 1978. (2) Taken in part from the thesls of H. V. Anjarla to be submitted to the Department of Chemistry. (3) J. R. Durig and Y. S. Li, J. Chem. Phys.,63,41 10 (1975); M. Tsubol, K. Tamagake, A. K. Hirakawa, J. Yamaguchi, H. Nakagawa, A. S. Manocha, E. C. Tuazon, and W. G. Fateley, ibid., 63, 5177 (1975). (4) Y. S. Li and V. W. Laurle, paper presented at the 24th Symposium on Molecular Structure and Spectroscopy, Columbus, OH, 1969. (5) J. R. Durig, W. E. Bucy, C. J. Wurrey, and L. A. Carrelra, J. Phys. Chem., 79,988 (1975); A. J. Barnes and H. E. Hallam, Trans. Farahy Soc., 66, 1932 (1970). (6) R. Kakar and P. J. Seibt, J. Chem. Phys., 57, 4060 (1972), and references therein. (7) I. D. Warren, and E. B. Wilson, J. Chem. Phys., 56, 2137 (1972). (8) H. Wolff, D. Horn, and H. G. Rollar, Spectrochim. Acta, Part A , 29, 1835 (1973). (9) P. J. Krueger and H. 3. Mettee, Can. J . Chem., 42, 340 (1964). (10) J. Travert and J. C. Lavalley, Spectrochim. Acta, Part A , 32, 637 (1976). (11) C. J. Wurrey, L. A. Carreira, and J. R. Durig in “Vibrational Spectra and Structure”, J. R. Durig, Ed., Elsevier, Amsterdam, 1976, Chapter 1. (12) H. Gillman and R. G. Jones, J. Am. Chem. Soc., 65, 1458 (1973). (13) F. A. Miller and B. M. Harney, Appl. Spectrosc., 24, 291 (1970). (14) K. 0. Hartman, G. L. Carison, R. E. Witkowski, and W. 0. Fateley, Spectrochim. Acta, Part A , 24, 157 (1968). (15) A. D. Lopata and J. R. Durlg, J . Raman Spectrosc., 6, 61 (1977). (16) G. Sage and W. Klemperer, J . Chem. Phys., 39, 371 (1963).
Interpretation of NMR Relaxation Data for Reorientation in Brittle and Plastic Phases of Organic Solids David W. Larsen” Chemistry Depaflment, University of Missouri, St. Louis, Missouri 63 12 1
and John H. Strange Physics Laboratory, University of Kent, Canterbury, Kent, CT2 7NR. United Kingdom (Received January 14, 1980)
NMR relaxation behavior which is not accounted for by simple BPP theory has been observed for many organic solids which exhibit high-temperature plastic phases. The motional processes in question involve anisotropic molecular reorientation in the brittle phase and endospherical reorientation in the plastic phase. Models for these motions are presented from which NMR relaxation times T1and T1,are calculated. The model for plastic-phase motion involves reorientation about a molecular axis with simultaneous independent changes of position between molecular centers. The model for brittle-phase motion involves reorientation about a molecular axis, characterized by wells of unequal depth. Both threefold and fourfold rotations are considered. The results of the calculations me in good agreement with experimental data for azabicyclononane. Calculations of this type could provide a basis for understanding the detailed nature of these motions.
Introduction A class of molecular solids with globular structures is known to form a plastic crystalline high-temperature phase.l In general, one observes a phase transition below which the substance exists in a brittle phase. Substances of this type have recently been the subject of NMR relaxation s t u d i e ~ . ~ -Three l~ types of motional processes have been consistently found in these studies. In the brittle phase at low temperature, anisotropic molecular reorientation is observed, while in the plastic phase isotropic molecular reorientation is observed at lower temperatures and translational diffusion is observed at higher 0022-3654/80/2084- 194450 1.OO/O
temperatures. Since these substances have molecules with almost spherical shape, they are solids for which the simplest theories for NMR relaxation should apply. Two features of the NMR data for reorientation cannot be explained15in terms of the usual Bloembergen-PurcellPound (BPP) type expressions.16 (i) Above the Tl minimum for isotropic reorientation in the plastic phase, Tl f Tlpis observed in some cases, and where T1 and T1,can be observed over a temperature range, the usual Arrhenius plot shows that the gradients of both Tl and Tlpare identical within experimental error. Departure from linearity, described as a kink, in the 0 1980 American Chemical Society
