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somewhere near 40% of the total, the 1058-cm-' line is definitely missing in a clear region of the spectrum. This latter sample was prepared by a method that did not involve a mercury-photosensitized decarbonylation of furan, and it is possible that a trace of allene-d is responsible for the 1058-cm-' line. The Raman spectrum of this isotope of allene seems not to have been reported. It is also interesting that the cyclopropene -3-d Raman spectrum shows a polarized line a t 1057 cm-l, and allene-d would also be produced as a by-product in the synthesis. Essentially all of the observed vibrations in the infrared and Raman spectra, see Tables 11-XIV, can be explained in terms of the various fundamentals listed in Table XV. These values have been communicated to Professor Wiberg, and he has used them in normal coordinate calculations. Several force fields were found, and they gave good agreement with these experimental values;15 this lends substantial additional support to the assignments that we have suggested here. Since these assignments appear to be essentially correct, we have also calculated the thermodynamic properties of cyclopropene. The results are given in Table XVIII. Acknowledgment. This work was supported in part by the National Science Foundation.
V. F. Kalasinsky, 13. E. Powers, and W. C. Harris
Supplementary Material Available: Tables 11-XIV contain the observed infrared and Raman vibrations for all of the cyclopropenes, along with suggested assignment,s (19 pages). See any current masthead page for ordering information.
References and Notes (1) D. F. Eggers, J. W. Schultz, K. B. Wiberg, E. L. Wagner, L. M. Jackman, and R. L. Erskine, J . Chem. Phys., 47, 946 (1967). (2) R. W. Mitchell, E. A. Dorko, and J. A. Merritt, J . Mol. Spectrosc., 26, 197 (1968). (3) R. Srinivasan, Pure Appl. Chem., 16, 64 (1968). (4) B. Bak, L. Hansen, and J. Rastrup-Andersen, Discuss. Faraday Soc., 19, 30 (1955). (5) M. Rico, M. Barrachina, J. M. Orza, and G.Michel, J. Mol. Spectrosc., 24, 133 (1967). (6) T. Y. Yum, Dissertation, University of Washington, 1969. (7) G. L. Closs and K. D. Krantz, J , Qrg. Chem , 31, 638 (1966). (8) E. A. Dorko and R. W. Mitchell, Tetrahedron Lett., 27, 341 (1968). (9) G. E. Calf arid J. L. Garnett, Austr. J . Chem., 21, 1221 (1968). (10) D. P. Siewarth and 6.H. Cady, Anal. Chem., 31, 618 (1959). (11) R. E. Miller, D. L. Rousseau, and G. E. Leroi, Technical Report No. 22, ONR Contract 1858 (27), May 1967, DDC acquisition No. AD 651646. (12) P. H. Kasai, R. J. Myers, D. F. Eggers, and K. B. Wiberg, J . Chern. Phys., 30, 512 (1959). (13) See paragraph at end of text regarding supplementary material. (14) R. Ettinger, J. Chem. Phys., 40, 1693 (1964). (15) K. B. Wiberg and J. J. Wendoloski, J. Phys. Chom., preceding article in this issue.
Vibrational Spectra and Conformations of Cyclopropylamine V. F. Kalasinsky," Department of Chemistty, Mississippi State University, Mississippi State, Mississippi 39762
D. E. Powers, and W. C. Harris* Department of Chemistry, Furman University, Greenville, South Carolina 296 13 (Received August 14, 1976) Pubiication costs assisted by Mississippi State University and the Petroleum Research Fund
The infrared (4000-150 cm-l) and Raman (4000-50 cm-') spectra of gaseous, liquid, and solid cyclopropylamine have been recorded along with the infrared spectra of matrix-isolated samples. The vibrational spectra of all phases are consistent with the predominance of a conformer having the NH2group trans to the ring C-C bonds as suggested by previous microwave studies. The torsional mode for trans-cycloprapylamine has been ohserved at, 254 cm-l, and hot bands associated with this low-frequencyfundamental are apparent throughout the spectra of the vapor phase. There are a number of weak bands which can be attributed to a second conformer, and the assignment of torsional transitions for gauche-cyclopropylamine has allowed the calculation of a suitable potential function for internal rotation, The derived value for AH is 592 cm-I, and this is consistent with variable temperature studies carried out for the liquid and vapor states. The internal rotational isomerism is discussed in terms of the bonding schemes which are applicable to cyclopropyl ring compounds.
Introduction Cyclopropane and its derivatives are of considerable interest as a result of the particularly strained bonding of the ring. Various theoretical models have been developed to account for the structure and reactivity of these species. Notably, the Walsh modell and its more recent revision by Hoffman2 suggest a hybridization model in which there exists an electron-deficient, three-center bond within the ring and three electron-rich bonds external to the ring. This model would predict very specific substituent effects, and experimental results concerning conformational 0022-3654/79/2083-0506$0 1.OO/O
problems can be used to test the models. For an electron-rich substituent such as an m i n o group, a trans conformation allows overlap between the nitrogen lone pair and the electron deficient inner orbitals of the ring. In fact, absorption lines associated with transcy~lopropylamine~~~ and trans-cycl~propylphosphine~ have been identified in microwave studies. No evidence for a gauche conformer was found in either case, but recent theoretical studies have indicated that a gauche conformer may be on the order of 3 kcal/mol less stable than the This sharply contrasts results for alkylarninesa Q 1979 American Chemical Society
The Journal of Physical Chemistry, Vol, 83, No. 4, 1979 !507
Vibrational Spectra of Cplopropylamine ..__
-
---
XCLOPROFYLAMINE
I
I
__- - ~ - 1 - 2800 __--
00
- 1 2 1500 1000
i
-L 500
100
Figure 4. Raman spectra of cyclopropylamine in the solid, vapor, and liquid phases.
and a l k y l p h ~ s p h i n e in s ~ which an equilibrium between trans and gauche conformers is readily observable. Vibrational spectroscopy can be useful in identifying conformers that exist in small relative abundance, and it has been our interest to characterize the possible rotational isomers of cyclopropylamine. Since no vibrational data have been previously reported, we have undertaken a detailed infrared and Raman study of cyclopropylamine, and the results are reported herein.
trogen-cooled MCT detector. Infrared spectra between 450 and 100 cm‘-l were obtained for gaseous cyclopropylamine using Digilab FTS-15B and Nicolet 7199 interferometers equipped with 6.25-pm mylar beamsplitters and TGS detectors. The sample a t pressures up to 150 torr were contained in a 10-cm cell fitted with polyethylene windows. Infrared spectra are shown in Figures 2 and 3.
Experimental Section The sample of cyclopropylamine was obtained commercially (Aldrich), and trace amounts of water and ammonia were removed with 3A and 4A Linde molecular sieves. The sample was stored in a previously evacuated tube, and all sample transfers were performed using standard vacuum techniques. Raman spectra of gaseous cyclopropylamine were recorded using either modified Jarrell-Ash Model 500 or Spex RamaKog DUV spectrophotometers equipped with Coherent Radiation Laboratories 52B and Spectra Physics 171 argon ion lasers, respectively, and standard multipass optics and cells. Spectra of the liquid and solid were obtained by using the Jarrell-Ash instrument with the samples sealed in a Pyrex capillary. The sample was cooled by affixing lhe capillary to a CTI Model 21 closed-cycle helium Cryostat. Typical spectra are shown in Figure 1. Infrared spectra between 4000 and 180 cm-l were recorded on a Perkin-Elmer Model 580 spectrophotometer interfaced to an Interdata 6/16 computer. The spectra of the vapor were obtained with a 10 cm cell equipped with CsI or polyelhylene windows while, for the liquid samples, KBr plates with various spacers were used. Spectra of the solid phase were recorded after condensing the sample onto a CsI window maintained a t 18 K by the CTI cryostat. A matrix was prepared by depositing a previously mixed 400:l argon-to-cyclopropylamine mixture onto the CsI plate held at 18 K. Spectra of the vapor were also recorded by using a Nicolet 7199 Fourier transform interferometer equipped with a KBr/Ge beamsplitter and a liquid ni-
Results and Conformation For cyclopropylamine the designation “trans” refers to the relative orientation of the N-H bonds and the ring C-C bonds, and this conformer has been identified from its microwave ~ p e c t r u m . ~ trans-Cyclopropylamine ,~ possesses a plane of symmetry and we expect A- or C-type infrared bands to correspond to polarized Raman lines, while B-type contours will be coincident with depolarized lines. A rotation of 120° about the C-N bond defines gauchecyclopropylamine, and this species should give rise to polarized Raman lines, and infrared contours are expected to be complex hybrids. The observed infrared and Raman frequencies are given in Table I, and the data are consistent with the predominance of the trans conformer. The assignments for most of the fundamental vibrations for trans-cyclopropylamine are rather straightforward, but certain assignments warrant some comment. In the C--H stretching region, it is clear that the CH2 out-of-phase symmetric stretch must be assigned to the B-type infrared band whose central minimum is at 3023 cm-l. For the other vibrations in this region, our preference is to assign the CH2 symmetric stretch to the Raman Q branch a t 3032 cm and the C-Ha stretch a t 2975 cm-l. This assignment implies that the two symmetric CH2 stretches are nearly degenerate, as is generally found.1° The C-H, stretch is somewhat lower than usual, but this vibration seems to be very sensitive to the kind of substituent on the ring. In this particular case the C-H bond is parallel to the C axis, so the assignment of the stretching mode to a C-type
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The Juurnal uf Physlcal Chemlstry, Vu/, 83, Nu. 4, 1873
V, F. Kalaeinsky, D, E. Powere, and W.
C,Harrle
Figure 2. Infrared spectra of cyclopropylamlne In the eolld, vapor, and liquld phase8 and trapped In an Ar matrlx.
infrared band is straightforward. An interesting feature of the Raman spectra of the vapor is that there is a relatively strong doublet (3036 and 3021 cm-l) which corresponds to an A" vibration which we have assigned to a B-type infrared band. The same contours are also observed for the A" ring deformation for which a B-type
infrared band and a Raman doublet are centered a t 884 cm-l in the vapor phase. The ring deformation mentioned above and the other ring vibrations of cyclopropylamine fall near the ranges suggested as group frequencies for these vibrational modes.1° The ring breathing motion and the A' ring
The Journal of Physical Chemistry, Vol. 83, No. 4,
Vibrational Spectra of Cyclopropylamine
7979 509
TABLE 111: Observed and Calculated Frequencies ( c m - l ) for the Torsion in Cyclopropylamine transition l t 0 24-0 3-1 4 t 2
obsd
calcd'
obsd calcd
weight
254 500 482 452
Trans 251.8 500.5 485.3 449.8
2.2 -0.5 -3.3 2.1
1.0 1.0 1.0 1.0
Gauche 1 - + 0' 188 187.6 0.4 1.0 1+4-0176 175.8 0.2 1.0 ' Calculated using a potential function with VI = 279.5 + 46.4, V,=575.9 ?r 22.9, V,=7 2 8 . 4 + 9.6, V 6 = - 8 0 . 3 ?: 4.6 cm-', and F = 9.80 cm-'.
I
L.
400
WAVENUMBER
200
Figure 3. Far-infrared spectra of cyclopropylamine vapor recorded on a Digilab FTS-156 with effective resolution of 1 cm-'.
TABLE 11: Fundamental Vibrations (cm-' ) for trans-Cwlopropvlamine
A' Species symmetric stretch antisymmetric stretch symmetric stretch u 4 CH stretch u 5 NH, deformation u6 CH, deformation u 7 CH bend (in-plane) u g Ring breathing u 9 CH, twist ul,, C-N stretch u L 1CH, wag ul, Ring deformation u I 3 N11, wag v 1 4 CH, rock u1 C-N bend (in-plane) u 1 NH, u1 CH, u j CH,
3348 3100 3032 2975 1617 1456 1374 1214 1168 1150 1020 989 805' 162 408
A" Species u l 6 NH, antisymmetric stretch
3412 CH, antisymmetric stretch (3100) u1 CH, symmetric stretch 3023 u I 9 CH, deformation 1424 u z 0 CH, twist 1104 u , , CH, wag 1045 u , , CH bend (out-of-plane) 1026' u , N11, ~ twist 940' u Z 4 Ring deformation 884 u Z 5CH, rock 830 u , C-N ~ bend (out-of-plane) 396 v z 7 N11, torsion 254 a Frequency taken from spectra of the solid phase; all others are for the vapor phase. u1
deformation are assigned to strong, polarized Raman lines a t 1214 and 989 cm-I, respectively. Also a number of other vibrational frequencies are characteristic for mono-substituted cyclopropaneslOJ1and a summary of the fundamentals for trans-cyclopropylamine is given in Table 11. The vibrational spectra provide evidence for the coexistence of two conformers insofar as a number of features appear as "doublets". The assignments for these are indicated in Table I, and while some of the secondary peaks are probably hot-band transitions for the trans conformer, others arise from gauche-cyclopropylamine. In particular, doublets are clearly observed a t 759 and 750 cm-l and 1465 and 1458 cm-l in the spectra of the vapor. The relative intensities of these pairs of lines in the Raman
spectra have been studied as a function of temperature from 22 to 51 "C, and we c a l c ~ l a t e the ~ ~ Jvalue ~ of AH separating the two conformers to be 480 f 150 cm'l. A similar treatment has been applied to data obtained from a variable temperature study (24 to -93 " C ) of the pair of lines a t 988 and 966 cm-l in the Raman spectrum of the liquid phase. The overlap of lines in this region makes it somewhat difficult to determine relative intensities, but the data are consistent with a lower limit of 1 kcal for AH in the liquid phase. Additionally, the features which disappear upon solidification reappear when the sample is allowed to melt. To properly describe the equilibrium between trans- and gauche-cyclopropylamine it is necessary to investigate the NH, torsional vibration. The trans conformer gives rise to the complex B-type bands in the far-infrared spectrum shown in Figure 3. The v = 1 0 transition is assigned a t 254 cm-l, the highest frequency central minimum for the B-type band, but it is difficult to choose the upper-level Av = 1transitions. This assignment is consistent, however, with the relative intensity measurements reported in the microwave study3 in which a vibrational interval of 239 f 35 cm-I has been indicated. Also apparent in the farinfrared spectra are Q branches at 201,188, and 176 cm-', which may arise from the torsional motion in gauchecyclopropylamine. The assignments of these frequencies to specific energy-level transitions is anything but straightforward. The only coincidence in the Raman spectrum is a very weak feature a t 187 cm-l, and we feel 0 transition for gauchethat this represents a 1 cyclopropylamine. We have tried to deduce the frequencies of upper level transitions for both conformers by examining regions of the spectra of the vapor where we expect to find overtone and combinations bands involving the torsion. In fact, we observe a number of C-type Q branches in the infrared between 550 and 450 cm-l, while the Raman spectra only exhibit transitions a t 560 and 529 cm-l. The bands a t 539 and 527 cm-l appear to be combination tones for the gauche conformer involving the C-N bending mode (352 cm-!) and the torsional vibration. The series in the infrared spectrum which begins at 500 cm-l may arise from double quantum transitions for trans-cyclopropylamine even though it is uncommon to observe these in the infrared. Using the above assignments we have generated a potential function of the forrnl4 V(0) = (1/2)CVn(1 - cos ne)
-
-
-
n
which we feel is consistent with the available data. The results of the calculation and the values for the potential constants are given in Table 111,and the potential function is shown graphically in Figure 4. Varying the weighting factors for the transitions in Table I11 will cause changes
510
The Journal of Physical Chemistry, Vol. 83,
r\
t3
b 7 4
1;-j
V(0) 500
d
o\e
L lj0 A-
0
V. F. Kalasinsky, D. E. Powers, and W. C. Harris
7 T I
I
1000
No. 4, 1979
p
, '\-,I \
12r
1007
'
1
592
'' I --._LA I 180
360
0 Figure 4. Potential function governing internal rotation in cyclo0 and the propylamine. The trans conformation corresponds to 0 = ' gauche minima are approximately f 133' from trans.
in the potential constants which are within the stated error limits. The derived value of AH can serve as a useful check since AH has been determined independently in the variable temperature study. The potential function indicates a mixture of approximately 90% trans- and 10% gauche-cyclopropylamine in the vapor phase, and this represents a somewhat larger amount of the gauche conformer than would be predicted on the basis of theoretical potentia1 curve^.^,^ One of the most interesting aspects of the conformation of cyclopropylamine is related to the bonding schemes which describe cyclopropane ring systems. Harmony and c o - ~ o r k e r shave ~ , ~ presented a substitution structure for the ring and found CI-C2 and Cz-C3 distances of 1.535and 1.513 A, respectively. This is not inconsistent with the predictions of Hoffman's theory, since, in the trans configuration, the electron pair of the NH2 group can overlap with the intra-annular ring orbitals. For a gauche orientation of the amine group a similar interaction is not possible, and one might expect a very different ring structure for gauche-cyclopropylamine. Our data indicate the presence of such a conformer and, while the exact structure can in principle be obtained from microwave spectra, there are a number of factors which would make these experiments very difficult.
Acknowledgment. The authors acknowledge partial support by the Donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the Research Corporation. W.C.H. also acknowledges the support of the Camille and Henry Dreyfus Foundation provided by a Teacher-Scholar Grant. V.F.K. acknowledges the use of the facilities of the Thomas E. Tramel Computing Center, with particular thanks to Robert P. Russum for his personal assistance in initiating the calculations. Acknowledgment is also extended to the Mississippi Imported Fire Ant Authority for the funds to purchase the Nicolet and Spex instruments. We are also grateful to Professor James R. Durig of the University of South Carolina for providing access to the Digilab FTS- 15B. Miniprint Material Available: Full-sized photocopies of Table I (4pages), Ordering information is available on any current masthead page. References a n d Notes A. D. Walsh, Trans. Faraday SOC., 45, 179 (1949). R. Hoffman, Tetrahedron Lett., 33, 2907 (1970). D. K. Hendricksen and M. D. Harmony, J . Chem. Phys., 51, 700 (1969). M. D. Harmony, R. E. Bostrom, and D. K. Hendricksen, J . Chem. Phys., 62, 1599 (1975). L. A. Dinsmore, C. 0. Britt, and J. E. Boggs, J. Chem. Phys., 54, 915 (1971). A. R. Mochel, J. E. Boggs, and P. N. Skancke, J . Mol. Struct., 15, 93 (1973). M. Pelissier, C. Leibovici, and J. F. Labarre, Tetrahedron, 28, 4825 (1972). J. R. Durig and Y. S. Li, J. Chem. Phys., 63, 4110 (1975); M. Tsuboi, K. Tamagake, A. K. Hirakawa, J Yamaguchi, H. Nakagawa, A. S. Manocha, E. C.Tuazon, and W. G. Fateley, ibid., 63, 5177 (1975). J. R. Durig and A. W. Cox, Jr., J . Chem. Phys., 63, 2303 (1975); 64, 1930 (1976). C. J. Wurrey and A. B. Nease in "Vibrational Spectra and Structure", Vol. VII, J. R. Durig, Ed., Elsevier, New York, 1978, Chapter 1; C. J. Wurrey, R. B. Blatt, and A. B. Nease, J. Phys. Chem., 81, 2279 ( 1977). T. Hirokawa, M. Hayashi. and H. Murata, J . Sci. Hiroshima Univ., Ser A , 37, 301 (1973). K. 0. Hartman, G. L. Carison, R. E. Witkowski, and W. G. Fateley, Spectrochim. Acta, Part A , 24, 157 (1968). W. C. Harris, J. R. Holtzclaw, and V. F. Kalasinsky, J . Chem. Phys., 67, 3330 (1977). J. D. Lewis, T. B. Malloy, Jr., T. H. Chao, and J. Laane, J. Mol. Struct., 12, 427 (1972): L. A. Carrerra, J . Chem. Phys., 62, 3851 (1975).
